Effects of operating conditions and fuel properties on emission performance and combustion ef ciency of a swirling  uidized-bed combustor  red with a biomass fuel Vladimir I. Kuprianov a, * , Rachadaporn Kaewklum b , Songpol Chakritthakul a a School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, P.O. Box 22, Thammasat Rangsit Post Of  ce, Pathum Thani 12121, Thailand b Department of Mechanical Engineering, Faculty of Engineering, Burapha University, 169 Long-Hard Bangsaen Road, Chonburi 20131, Thailand a r t i c l e i n f o  Article history: Received 29 October 2009 Received in revised form 10 April 2010 Accepted 15 May 2010 Available online 12 June 2010 Keywords: Swirling  uidized bed Rice husk Temperature Gas concentrations Combustion ef ciency a b s t r a c t This work reports an exp erimental study on  rin g 80 kg/ h ric e husk in a swi rli ng  uidized-bed combustor (SFBC) using an annular air distributor as the swirl generator. Two NO  x  emission control techniques were investigated in this work: (1) air staging of the combustion process, and (2)  ring rice husk as moisturized fuel. In the  rst test series for the air-staged combustion, CO, NO and C  x H  y  emissions and combustion ef ciency were determined for burning  as-received  rice husk at xed excess air of 40%, while secondary-to-primary air ratio (SA/PA) was ranged from 0.26 to 0.75. The effects of SA/PA on CO and NO emissions from the combustor were found to be quite weak, whereas C  x H  y  emissions exhibited an apparent inuence of air staging. In the second test series, rice husks with the fuel-moisture content of 8.4% to 35% were red at excess air varied from 20% to 80%, while the  ow rate of secondary air was xed. Radial and axial temperature and gas concentration (O 2 , CO, NO) proles in the reactor, as well as CO and NO emissions, are discussed for the selected operating conditions. The temperature and gas concentration proles for variable fuel quality exhibited signi cant effects of both fuel-moisture and excess air. As revealed by experimental results, the emission of NO from this SFBC can be substantially reduced through moisturizing rice husk, while CO is effectively mitigated by injection of secondary air into the bed splash zone, resulting in a rather low emission of CO and high (over 99%) combustion ef ciency of the combustor for the ranges of operating conditions and fuel properties.  2010 Elsevier Ltd. All rights reserved. 1. Introduction For man y years , rice husk hasbeen an imp ort antenerg y resource in most Asian countries. The uidized-bed combustion technology with its apparen t econ omica l and environ ment al bene ts is proven to be t he most effec tive tech nolog y fo r energ y prod ucti on fr om this agricultural residue. However, the combustion of rice husk, gener- ally characte rized by elevated fuel-N and fuel-ash contents, is acco mpani ed by substantia l NO  x and CO emi ssions the rat e of those depends on fuel properties, as well as on the design features and operating conditions of a combustion system used [1e4]. For typical ranges of the bed temp erat ure and exc ess air in a  uidized-bed combustion system (combustor or boiler furnace) burning biomass, NO  x  are known to originate mainly from fuel-N via homogeneous oxidation of the dominant nitrogenous volatile spec ies, NH 3  and HCN, to fue l-NO, sin ce the con tri but ions of thermal-NO and prompt-NO are insignicant  [3,5]. The relevant studies reveal that NO  x  emissions from the reactor are in a quasi- linear correlat ion with fuel- N  [4,6,7]. Wi th r is in g of the be d temperature, NO  x  emissions are weakly increased or stay constant [6,8,9], but exhibit a substantial increase with rising excess air in both conventional and air -stag ed combu stion syst ems [1,6e12] . Howe ver, some studies repo rt insign icant eff ect s of the air sta ging on NO  x  emissions when burning biomass [1,8,9,12]. The emission of CO from a biomass-fuelled system is affected by several operating factors: excess air, combustion temperature, resi- dence time of reactants, fuel-ash content and particle size, and, also, fueleair mixing conditions. During the combustion, formation of CO is known to include several sources: CO released with fuel volatiles, oxida tion of volatile hydroc arbons by oxyg en, as well as oxidation of char-C by O 2 , H 2 O and CO 2  on the char surface  [3,6]. As any other unbu rned poll utan t, CO is effectivel y con tro lled (re duc ed) by * Correspondin g author . Tel.: þ66 2 986 9009x2208; fax: þ66 2 986 9112. E-mail address:  [email protected] (V.I. Kuprianov). Contents lists available at  ScienceDirect Energy journal homepage:  www.elsevier.com/locate/energy 0360-5442/$ e see front matter   2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.05.026 Energy 36 (2011) 2038e2048
11
Embed
(KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion Efficiency of a Swirling Fluidized-bed Combustor Fired With a Biomass Fuel
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Effects of operating conditions and fuel properties on emission performance and
combustion ef 1047297ciency of a swirling 1047298uidized-bed combustor 1047297red with
a biomass fuel
Vladimir I Kuprianov a Rachadaporn Kaewklum b Songpol Chakritthakul a
a School of Manufacturing Systems and Mechanical Engineering Sirindhorn International Institute of Technology Thammasat University PO Box 22 Thammasat Rangsit Post Of 1047297ce
Pathum Thani 12121 Thailandb Department of Mechanical Engineering Faculty of Engineering Burapha University 169 Long-Hard Bangsaen Road Chonburi 20131 Thailand
a r t i c l e i n f o
Article history
Received 29 October 2009
Received in revised form
10 April 2010
Accepted 15 May 2010
Available online 12 June 2010
Keywords
Swirling 1047298uidized bed
Rice husk
Temperature
Gas concentrations
Combustion ef 1047297ciency
a b s t r a c t
This work reports an experimental study on 1047297ring 80 kgh rice husk in a swirling 1047298uidized-bed
combustor (SFBC) using an annular air distributor as the swirl generator Two NO x emission control
techniques were investigated in this work (1) air staging of the combustion process and (2) 1047297ring rice
husk as moisturized fuel In the 1047297rst test series for the air-staged combustion CO NO and C xH y emissions
and combustion ef 1047297ciency were determined for burning ldquoas-receivedrdquo rice husk at 1047297xed excess air of 40
while secondary-to-primary air ratio (SAPA) was ranged from 026 to 075 The effects of SAPA on CO
and NO emissions from the combustor were found to be quite weak whereas C xH y emissions exhibited
an apparent in1047298uence of air staging In the second test series rice husks with the fuel-moisture content
of 84 to 35 were 1047297red at excess air varied from 20 to 80 while the 1047298ow rate of secondary air was
1047297xed Radial and axial temperature and gas concentration (O2 CO NO) pro1047297les in the reactor as well as
CO and NO emissions are discussed for the selected operating conditions The temperature and gas
concentration pro1047297les for variable fuel quality exhibited signi1047297cant effects of both fuel-moisture and
excess air As revealed by experimental results the emission of NO from this SFBC can be substantiallyreduced through moisturizing rice husk while CO is effectively mitigated by injection of secondary air
into the bed splash zone resulting in a rather low emission of CO and high (over 99) combustion
ef 1047297ciency of the combustor for the ranges of operating conditions and fuel properties
2010 Elsevier Ltd All rights reserved
1 Introduction
For many years rice husk hasbeen an importantenergy resource
in most Asian countries The 1047298uidized-bed combustion technology
with its apparent economical and environmental bene1047297ts is proven
to be the most effective technology for energy production from this
agricultural residue However the combustion of rice husk gener-ally characterized by elevated fuel-N and fuel-ash contents is
accompanied by substantial NO x and CO emissions the rate of those
depends on fuel properties as well as on the design features and
operating conditions of a combustion system used [1e4]
For typical ranges of the bed temperature and excess air in
a 1047298uidized-bed combustion system (combustor or boiler furnace)
burning biomass NO x are known to originate mainly from fuel-N
via homogeneous oxidation of the dominant nitrogenous volatile
species NH3 and HCN to fuel-NO since the contributions of
thermal-NO and prompt-NO are insigni1047297cant [35] The relevant
studies reveal that NO x emissions from the reactor are in a quasi-
linear correlation with fuel-N [467] With rising of the bed
temperature NO x emissions are weakly increased or stay constant
[689] but exhibit a substantial increase with rising excess air inboth conventional and air-staged combustion systems [16e12]
However some studies report insigni1047297cant effects of the air staging
on NO x emissions when burning biomass [18912]
The emission of CO from a biomass-fuelled system is affected by
several operating factors excess air combustion temperature resi-
dence time of reactants fuel-ash content and particle size and also
fueleair mixing conditions During the combustion formation of CO
is known to include several sources CO released with fuel volatiles
oxidation of volatile hydrocarbons by oxygen as well as oxidation of
char-C by O2 H2O and CO2 on the char surface [36] As any other
unburned pollutant CO is effectively controlled (reduced) by Corresponding author Tel thorn66 2 986 9009x2208 fax thorn66 2 986 9112
E-mail address ivlaanovsiittuacth (VI Kuprianov)
Contents lists available at ScienceDirect
Energy
j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e e n e r g y
0360-5442$ e see front matter 2010 Elsevier Ltd All rights reserved
doi101016jenergy201005026
Energy 36 (2011) 2038e2048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
a rather low level basically due to the secondary air injection into
the bed splash zone As can be concluded based on the results from
Ref [12] and present study excess air (or percentage of total air) is
an important factor in controlling the CO emission in this SFBC
whereas SAPA shows quite weak effects
With increasing SAPA within the selected range 026e075 the
NO emission exhibited a slight reduction from about 150 to140 ppm (see Fig 2a) mainly due to the diminishing of the bed
temperature and reduction of the O2 concentration in the bed the
latter being occurred because of the lowering of PA Thus the air
staging does not seem to be an effective measure to control the NO
emission in this combustor 1047297ring rice husk
It can be seen in Fig 2b that at relatively small proportions of
secondary air the C xH y emissions were at a quite low level
However at SAPA gt 04 these emissions showed a signi1047297cant
increase from 120 to 1400 ppm which can be explained by the
sub-stoichiometric conditions in the bed region Under such
conditions more volatiles were carried over from the combustor
bottom causing the above increase in the C xH y emissions Thus
primary air should be supplied to the SFBC at a 1047298ow rate ensuring
the oxidation conditions in the bed ie with some excess withregard to the theoretical (stoichiometric) air For instance at
EA frac14 40 the combustor should be operated at 026 lt SAPA lt 04
(see Fig 2b) for avoiding high C xH y emissions from this SFBC 1047297ring
rice husk As can be generally concluded the lower limit of SAPA is
speci1047297ed with the aim to provide the reliable coolingof the start-up
burner whereas the upper one is selected taking into account that
EA should be somewhat greater than SA
312 Heat losses and combustion ef 1047297ciency
The analyses of 1047298y ashes for unburned carbon for this test series
indicated the high rate of fuel burnout in this conical SFBC
Depending on SAPA the unburned carbon content in the 1047298y ashes
varied from 081 to 24 the minimum value being found at the
highest SAPA ratio
Table 3 shows the heat losses with unburned carbon (quc) and
owing to incomplete combustion (qic) together with the combus-
tion ef 1047297ciency (all as the percentage of LHV) for 1047297ring 80 kgh rice
husk at excess air of about 40 for different values of SAPA An
increase in SAPA led to a noticeable reduction in the heat loss with
unburned carbon basically due to the higher rate of fuel burnout
which waslikelycaused by an increase in the residence time of char
particles in the combustor However the exponential rise of qic can
be explained by the above behavior of CO and C xH y emissions
Due to the opposite trends exhibited by the heat losses the
combustion ef 1047297ciency was found to have a maximum 991 for1047297ring rice husk at SAPA of 04e06 and EA of about 40
32 Emission and combustion characteristics of SFBC for variable
fuel moisture
As revealed by the experimental results the temperature and
gas concentrations (O2 CO and NO) in this conical SFBC were rep-
resented by three-dimensional patterns (1047297elds) showing the
effects of combustor hydrodynamics fuel quality and operating
conditions on the radial and axial pro1047297les of the temperature and
chemical species Note that at a given excess air SA raised with
increasing fuel moisture because of the reduction in the theoretical
air while the 1047298owrate of secondary air was 1047297xed at the above
constant value changing from 29 (for ldquoas-receivedrdquo fuel) to 41(for rice husk with the highest moisture content) which resulted in
corresponding diminishing of PA Due to the reduction in the
combustion temperature and also in the theoretical air and PA
the residence time of char particles in the bottomregion of the SFBC
was substantially greater when burning rice husks with higher
moisture content leading to the higher rates of devolatilization and
burnout of fuel particles in this region and thus affecting signi1047297-
cantly the behavior of all variables in the reactor
At EA frac14 40 or higher oxidizing conditions were basically
provided in the combustor bottom which justi1047297ed the ignorance of
C xH y emissions in this test series
321 Radial and axial temperature and gas concentration pro 1047297les
in the SFBC Fig 3 shows the radial temperature and O2 concentration
pro1047297les at 1047297ve levels ( Z ) above the air distributor in the SFBC 1047297ring
80 kgh rice husk at excess air of about 40 for variable fuel prop-
erties (Options 1e5 in Table 2) As seen in Fig 3 the variables
exhibitedquitesimilar behaviorsat different levels ( Z ) above the air
distributor The radial temperature pro1047297les were found to be rather
uniform indicating the highly intensive heat-and-mass transfer
along the radius With increasing fuel moisture (at a 1047297xed excess air
level) the temperature at all points in the combustor volume was
found to be reduced (despite the above increase in the residence
time) because of the apparent in1047298uence of the latent heat of water
evaporation Similar results are reported in some studies on
conventional 1047298uidized-bed and 1047297xed-bed combustion systems
1047297ring biomass fuels with variable fuel moisture [4203132]
Fig 2 Effects of secondary-to-primary air ratio (SAPA) on the (a) CO NO and (b) C xH y emissions from the SFBC 1047297ring rice husk at excess air of 40
Table 3
Heat losses and combustion ef 1047297ciency of the SFBC 1047297ring 80 kgh rice husk at excess
air of 40 for variable secondary-to-primary air ratio
SAPA quc () qic () Combustion ef 1047297ciency ()
026 094 024 988
040 051 035 991
056 047 043 991
075 031 212 976
VI Kuprianov et al Energy 36 (2011) 2038e 20482042
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 3 Radial (a) temperature and (b) O2 concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-)
W frac14 84 () W frac1415 () W frac14 20 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2043
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Implementation of air staging seems to have a signi1047297cant impact
on the behavior of gaseous species in both radial and axial direc-
tions Due to the tangential injection of secondary air into the bed
splash zone the radial O2 concentration pro1047297les were characterized
by a positive radial gradient (the most signi1047297cant being observed at
Z frac14 101 m) which resulted in the higher O2 concentration near the
combustor wall than that at the centerline (ie at r R frac14 0)
However the radial gradient of O2 was gradually attenuated along
the bed height Note that the injection of secondary air affected the
radial O2 concentration pro1047297les not only in upper regions of the
reactor but also at levels below the injection point (in Fig 3b see
the pro1047297les at Z frac14 047 m) since the secondary air was injected at
the negative angle In the meantime with increasing the fuel-
moisture content the O2 concentration at all the points across the
combustor was found to be reduced and this reduction was caused
by some physical and chemical factors as addressed below in the
discussion of axial O2 concentration pro1047297les
The axial temperature and O2 concentration pro1047297les in the SFBC
are shown in Fig 4 for the same fuel options and operating
conditions as in Fig 3 At 1047297xed excess air a positive axial temper-
ature gradient was found to occur in the lower part of the reactor
for all the fuels (see Fig 4a) likely due to the diminishing of heat
release in the dense bed (caused by air staging) and (ii) in1047298uence of secondary air injected into the bed splash zone at the ambient
temperature However with higher fuel moisture (ie with dete-
riorating fuel quality) the temperature attained its maximum at
lower levels (Z) above the air distributor which can be explained by
the effects of the residence time The maximum temperature for
burning ldquoas-receivedrdquo rice husk (with W frac14 84) was rather high
about 980 C however it was reduced to 850e860 C when
increasing the fuel-moisture content to 25e30
However with raising fuel moisture the rate of oxygen
consumption in the bottom region of the SFBC ( Z lt 08 m) was
apparently higher which despite the above reduction in the bed
temperature resulted in the lower O2 concentration at all locations
in this region (as seen in Fig 4b) This phenomenon can be
explained by the greater residence time (leading to a higher yield of CO and other combustibles with fuel volatiles) and also greater
contribution of the ldquowetrdquo oxidation of char-C by OH radicals both
leading to higher rates of CO formation and consequently O2
consumption in this region The next region (08 lt Z lt 10 m) was
characterized by a noticeable regaining (rise) of O2 as the response
to secondary air injection However in the freeboard of the
combustor ( Z gt 10 m) the O2 concentration was diminished along
the reactor height at a rather low rate and this reduction was
accompanied by the gradual converging of the axial pro1047297les as all
the tests in Fig 4 for variable fuel moisture were conducted at
(nearly) the same EA
Fig 5 shows the radial CO and NO concentration pro1047297les at
different levels above the air distributor for the same fuels and
operating conditions as in Fig 3 Both CO (Fig 5a) and NO (Fig 5b)
were signi1047297cantly affected by fuel moisture and showed negative
gradients along the radius (different numerically) at all the levels in
the SFBC For the 1047297xed fuel-moisture content due to the effects of
secondary air the CO concentration in the peripheral zone across
the combustor was much lower compared to that at the centerline
thus forming the above radial gradient of CO However the NO
concentration varied weakly along the radius except at Z frac14 047 m
The occurrence of the NO maximum at the centerline indicated
higher rates of both fuel devolatilization and oxidation in the
central zone of the reactor (compared to those at the combustor
wall) despite the uniformity of the temperature and the opposite
trends of the O2 concentration pro1047297les across the SFBC (see Fig 3b)
In the freeboardof the reactor the radial CO and NO gradients were
found to be gradually attenuated with higher Z
Fig 6 depicts the axial CO and NO concentration pro1047297les in this
combustor As seen in Fig 6a the pro1047297les exhibited four sequent
regions along the combustor height With increasing fuel moisture
the CO concentration in the 1047297rst region (0lt Z lt 08 m) was
apparently higher at all the levels above the air distributor
particularly at the centerline (see Fig 5a) mainly due to (i) longerresidence time (leading to greater yield of CO with volatiles) (ii)
lower PA (reducing the rate of CO oxidation) (iii) higher contri-
bution of ldquowetrdquo oxidation of char-C by OH radicals and (iv) lower
bed temperature causing an increase in the COCO2 ratio in the
products of fuel-char oxidation [33]
In the second region (08 lt Z lt 10 m) the CO concentration
along the combustor axis was found to be drastically reduced
mainly due to the effects of secondaryair the greater rate of the CO
reduction being observed at a higher level of fuel moisture (ie at
higher SA) In the third region (10 lt Z lt 18 m) the CO concentra-
tion regained substantial values along the centerline mainly due to
oxidation of unburned hydrocarbons and fuel-C carried over from
the bed region to CO However in the fourth (upper) region the CO
concentration was found to be gradually reduced along thecombustor height likely via homogeneous reaction of CO with
residual O2 and OH [333]
Like for CO four speci1047297c regions can be distinguished in the
axial NO concentration pro1047297les as can be seen in Fig 6b In the 1047297rst
region (0 lt Z lt 08 m) NO was mainly formed from NH3 in volatiles
(a major precursor of NO in biomass combustion) via the fuel-NO
formation mechanism [343435] With increasing fuel moisture
despite the reduction in bed temperature the NO concentration at
the reactor centerline showed a trend to increase at any given Z
mainly due to (i) greater residence time promoting a higher yield
of nitrogenous species with fuel volatiles and (ii) enhanced
a b
500
600
700
800
900
1000
1100
0 1 2 3
Height above air distributor (m)
) C deg ( e r u t a r e p m e T
0
5
10
15
20
0 1 2 3Height above air distributor (m)
O 2
) l o v ( n o i t a r t n e c n o c
Fig 4 Axial (a) temperature and (b) O2 concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415
(
) Wfrac14
20 (6
) Wfrac14
25 and (A
) Wfrac14
30
VI Kuprianov et al Energy 36 (2011) 2038e 20482044
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
Fig 5 Radial (a) CO and (b) NO concentration pro1047297les at different levels in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 ()
W frac14 15 () W frac1420 (6) W frac1425 and (A) W frac1430
VI Kuprianov et al Energy 36 (2011) 2038e 2048 2045
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
oxidation of NH3 by OH [36] In the second region (08 lt Z lt 10 m)
the chemical reactions responsible for NO decomposition such as
catalytic reduction of NO by CO on the char surface [3] as well as
reaction of NO with NH3 at oxygen de1047297ciency [34] were predom-
inant This resulted in a signi1047297cant reduction of the NO concen-
tration along the combustor height and the NO reduction was more
apparent when burning rice husks with higher moisture contentsHowever the injection of secondary air promoted a slight increase
in NO in the third region (10 lt Z lt 16 m) of the reactor as a result
of oxidation of the nitrogenous species carried over from the
combustor bottom In the fourth (upper) region the pro1047297les
exhibited some diminishing of the NO concentration along the
reactor height because of the catalytic reduction of NO by CO
occurred however at a rather low rate
322 Emissions
Fig 7 depicts the CO and NO emissions (on 6 O 2 dry gas basis)
from the combustor 1047297ring 80 kgh rice husk at variable EA for the
whole range of fuel moisture (Options 1e6 in Table 2) It can be
concluded from analysis of data in Fig 7a that at EA greater than
40 the CO emission from the SFBC can be controlled ata comparatively low level below 350 ppm regardless of the fuel
quality On the contrary at excess air lower than 40 the CO
emission exhibited quite strong effects of both fuel quality and EA
Note that at EA frac14 20 the CO emission was extremely high
3000e7000 ppm for the whole range of fuel moisture However
with increasing the fuel-moisture content from 84 (in ldquoas-
receivedrdquo rice husk) to 25 the CO emission at this lowest EA
exhibited some reduction roughly from 4000 to 3000 ppm basi-
cally caused by (i) the higher rate of chemical reaction between CO
and OH [333] and (ii) higher rate of CO decomposition in the
freeboard (due to enhanced SA and greater residence time) Similar
trend is reported in some studies on effects of fuel moisture on the
CO emission from the 1047297xed-bed combustion systems [3132]
However with further increase in the fuel-moisture content (from
25 to 35) the CO emission from the SFBC was found to rise from
about 3000 to 7000 ppm likely due to the signi1047297cant contribution
of ldquowetrdquo oxidation of char-C occurred at the lowered combustion
temperatures This trend is in concordance with the behavior of theCO emission during the conventional 1047298uidized-bed combustion of
some biomass fuels with variable moisture [420]
It can be seen in Fig 7b that at EA frac14 40 (the ldquocriticalrdquo value for
the CO emission) the NO emission diminished from about 170 to
130 ppm (or by some 25) when changing the fuel-moisture
content from 84 to 25 A further increase in fuel moisture up to
30e35 resulted in some more emission reduction Note that this
positive result was accompanied by deterioration of combustion
stability (likely caused by inconsistency in fuel properties) which
showed itself by the noticeable time-domain 1047298uctuations of the
temperature and gaseous species particularly in the vicinity of the
fuel injection As revealed by experimental data from this study
through moisturizing of ldquoas-receivedrdquo rice husk the NO emission
from the SFBC can be substantially reduced However thisachievement was accompanied by the conventional effects of EA
leading to the increase in the NO emission with higher EA [16e12]
323 Optimal excess air and fuel-moisture content
Fig 8 shows the emission costs US$ per 1 ton of fuel for 1047297ring
rice husk in the SFBC at variable fuel moisture and excess air pre-
dicted using the above CO and NO emissions and the speci1047297c
ldquoexternalrdquo costs P NO x frac14 3000 US$=t and P CO frac14 600 US$t (ie for
P NO x=P CO frac14 5) At low values of EA the contribution of CO to the
emission costs was predominant whereas the effects of NO were
substantial at higher EA values It can be seen in Fig 8 that the
Fig 6 Axial (a) CO and (b) NO concentration pro1047297les in the SFBC 1047297ring rice husk at excess air of about 40 for variable fuel-moisture content (-) W frac1484 () W frac1415 ()
W frac1420 (6) W frac1425 and (A) W frac1430
Fig 7 Effects of the fuel-moisture content and excess air on the (a) CO and (b) NO emissions from the SFBC 1047297ring 80 kghr rice husk (-) W frac1484 () W frac1415 () W frac1420 (6)
Wfrac14
25 (A
) Wfrac14
30 and (A
) Wfrac14
35
VI Kuprianov et al Energy 36 (2011) 2038e 20482046
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048
8122019 (KUPRIANOV 2011) Effects of Operating Conditions and Fuel Properties on Emission Performance and Combustion hellip
where sD is the error in physical measurement of inner diameter of the primary air pipe sV is the measurement error of the velocity
(dependent mainlyon the type of a selected probe) sR is therelative
error associated with imperfect1047297xing of the probe in air1047298ow and sP
is the uncertainty in the velocity pro1047297le across the pipe
AssumingsD frac14 1 sV frac14 15 sR frac14 1 and sP frac14 15(as advised in
Ref [38]) the uncertainty in the 1047298owrate of primary air was
calculated using Eq (A1) to be about 3
Appendix B Determining heat losses and combustion
ef 1047297ciency
In this work the combustion ef 1047297ciency of the SFBC 1047297ring rice
husk was calculated using the the heat-loss method [18]
For this combustor with no ashremoval through thebottompartthe heat loss with unburned carbon quc () was determined by
quc frac14 32866
LHV
C fa
100 C fa
A (B1)
where C fa is the unburned carbon content in the1047298y ash wt and A
is the fuel-ash content wt on ldquoas-receivedrdquo basis
The heat loss owing to incomplete combustion qic () was
quanti1047297ed based on theconcentrations (vol)of COand C xH y (asCH4)
measured in the 1047298ue gas at the cyclone outlet at actual excess air
qic frac14 eth1264 CO thorn 3582 CH4THORN V dgeth100 qucTHORN
LHV (B2)
where V dg is the volume of dry 1047298ue gas at the cyclone outlet Nm3
kg calculated by Refs [1822] using the fuel ultimate analysis onldquoas-receivedrdquo basis and excess air ratio at this point
The combustion ef 1047297ciency of the 1047298uidized-bed combustor hc
() was then determined by
hc frac14 100 ethquc thorn qicTHORN (B3)
References
[1] Natarajan E Nordin A Rao AN Overview of combustion and gasi1047297cation of rice husk in 1047298uidized bed reactors Biomass Bioenergy 199814533e46
[2] Bhattacharya SC Abdul Salam P Sharma M Emissions from biomass energyuse in some selected Asian countries Energy 200025169e88
[3] Werther J Saenger M Hartge EU Ogada T Siagi Z Combustion of agriculturalresidues Prog Energy Combust Sci 2000261e27
[4] Janvijitsakul K Kuprianov VI Similarity and modeling of axial CO and NO
concentration pro1047297les in a 1047298uidized-bed combustor (co-)1047297ring biomass fuelsFuel 2008871574e84
[5] Demirbas A Combustion characteristics of different biomass fuels ProgEnergy Combust Sci 200430219e30
[6] Leckner B Karlsson M Gaseous emissions form circulating 1047298uidized bedcombustion of wood Biomass Bioenergy 19934379e89
[7] Spliethoff H Hein KRG Effect of co-combustion of biomass on emissions inpulverized fuel furnaces Fuel Process Technol 199854189e205
[8] Lyngfelt A Leckner B Combustion of wood-chips in circulating 1047298uidized bedboilers e NO and CO emissions as functions of temperature and air-stagingFuel 1999781065e72
[9] Chyang CS Wu KT Lin CS Emission of nitrogen oxides in a vortexing 1047298uidizedbed combustor Fuel 200786234e43
[10] Armesto E Bahillo A Cabanillas A Veijonen K Otero J Combustion behavior of rice husk in a bubbling 1047298uidised bed Biomass Bioenergy 200223171e9
[11] Kuprianov VI Janvijitsakul K Permchart W Co-1047297ring of sugar canebagasse with rice husk in a conical 1047298uidized-bed combustor Fuel 200685434e42
[12] Fang M Yang L Chen G Shi Z Luo Z Cen K Experimental study on rice huskcombustion in a circulating 1047298uidized bed Fuel Process Technol2004851273e82
[13] Permchart W Kouprianov VI Emission performance and combustion ef 1047297-ciency of a conical 1047298uidized-bed combustor 1047297ring various biomass fuelsBioresour Technol 20049283e91
[14] Sirisomboon K Kuprianov VI Arromdee P Effects of design features oncombustion ef 1047297ciency and emission performance of a biomass-fuelled 1047298uid-ized-bed combustor Chem Eng Process Process Intens 201049270e7
[15] Leckner B Amand LE Lucke K Werther J Gaseous emissions from co-combustion of sewage sludge and coalwood in a 1047298uidized bed Fuel200483477e86
[16] Eaimsa-ard S Kaewkohkiet Y Thianpong C Promvonge P Combustionbehavior in a dual-staging vortex rice husk combustor with snail entry IntCommun Heat Mass Transfer 2008351134e40
[17] Madhiyanon T Lapirattanakun A Sathitruangsak P Soponronnarit S A novelcyclonic 1047298uidized-bed combustor (J-FBC) combustion and thermal ef 1047297-ciency temperature distribution combustion intensity and emission of pollutants Combust Flame 2006146232e45
[18] Basu P Cen KF Jestin L Boilers and burners New York Springer 2000[19] Kaewklum R Kuprianov VI Experimental studies on a novel swirling 1047298uid-
ized-bed combustor using an annular spiral air distributor Fuel20108943
e52
[20] Kouprianov VI Permchart W Emission from a conical FBC 1047297red witha biomass fuel Appl Energy 200374383e92
[21] Kaewklum R Kuprianov VI Douglas PL Hydrodynamics of airesand 1047298ow ina conical swirling 1047298uidized bed a comparative study between tangential andaxial air entries Energy Convers Manage 2009502999e3006
[22] Bezgreshnov AN Lipov YM Shleipher BM Computations of steam boilersMoscow Energoatomizdat 1991 [in Russian]
[23] Kuprianov VI Application of a cost-based method of excess air optimizationfor the improvement of thermal ef 1047297ciency and environmental performance of steam boilers Renew Sustain Energy Rev 20059474e98
[24] Streimikiene D Roos I Rekis J External cost of electricity generation in BalticStates Renew Sustain Energy Rev 200913863e70
[25] Zhang Q Weili T Yumei W Yingxu C External costs from electricity gener-ation of China up to 2030 in energy and abatement scenarios Energy Pol2007354295e304
[26] Hainoun A Almoustafa A Aldin MS Estimating the health damage costs of Syrian electricity generation system using impact pathway approach Energy
201035628e
38[27] Nguyen KQ Internalizing externalities into capacity expansion planning thecase of electricity in Vietnam Energy 200833740e6
[28] Kitou E Horvath A External air pollution costs of telework Int J Life CycleAssess 200813155e65
[29] Wei X Zhang L Zhou H Evaluating the environmental value of pollutants inChina power industry In Proceedings of the international conference onenergy and the environment Shanghai China 2003
[30] Salisdisouk N The concept of integrated resource planning In Proceeding of the workshop on electric power quality safety and ef 1047297ciency of its useselectric power system management Pathum Thani Thailand 1994
[31] Yang YB Shari1047297 VN Swithenbank J Effect of air 1047298ow rate and fuel moisture onthe burning behaviours of biomass and simulated municipal solid wastes inpacked beds Fuel 2004831553e62
[32] Zhao W Li Z Zhao G Zhang F Zhu Q Effect of air preheating and fuel moistureon combustion characteristics of corn straw in a 1047297xed bed Energy ConversManage 2008493560e5
[33] Tillman DA Rossi AJ Kitto WD Wood combustion New York AcademicPress 1981
[34] Winter F Wartha C Hofbauer H NO and N2O formation during thecombustion of wood straw malt waste and peat Bioresour Technol19997039e49
[35] Sun Z Jin M Zhang M Liu R Zhang Y Experimental studies on cotton stalkcombustion in a 1047298uidized bed Energy 2008331224e32
[36] Smart JP Robert PA De Soete GG The formation of nitrous oxide in a large-scale pulverized-coal 1047298ames J Inst Energy 199063131e5
[37] Abu-Qudais M Fluidized-bed combustion for energy production from olivecake Energy 199621173e8
[38] Trembovlya VI Finger ED Avdeeva AA Thermo-technical tests of boiler unitsMoscow Energoatomizdat 1991 [in Russian]
VI Kuprianov et al Energy 36 (2011) 2038e 20482048