7/30/2019 Simulation of Circulating Fluized Bed Reactors Using Aspen Plus http://slidepdf.com/reader/full/simulation-of-circulating-fluized-bed-reactors-using-aspen-plus 1/11 ELSEVIER PII: SOO16-2361(97)00211-l Fuel Vol. 77, No. 4, 327-337, 1998 p. 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/98 $19.00+0.00 Simulation of circulating fluidized bed reactors using ASPEN PLUS R. Sotudeh-Gharebaagh, R. Legros*, J. Chaouki and J. Paris Department of Chemical Engineering, &o/e Polytechnique, PO Box 6079, Station ‘Centre-Vi//e’, Montrkal, PO, Canada H3C 3A7 (Received 4 April 1995; revised 18 August 1997) A comprehensive model is developed for the combustion of coal in a circulating fluidized bed combustor (CFBC). The proposed model integrates hydrodyn amic parameters, reaction model and kinetic subroutines necessary to simulate coal combustion in a CFBC . Kinetic expressions were developed for the char combustion rates and the SO2 absorption in the bed using data from the literature. The reaction model, which considers only the important steps of coal combustion, was simulated using four ASPEN PLUS reactor models and several subroutines. The developed subroutines were then nested in the ASPEN PLUS input file, so that the CF’EK may be represented. The validity of the model was demonstrated using 14 different sets of operating conditions for the CANMET 0.8 MWth CFBC pilot plant. 0 1998 Elsevier Science Ltd. All rights reserved. (Keywords: ASPEN PLUS; CFBC; circulating fluidized bed reactors; fluidized bed combustion) INTRODUCTION Circulating fluidized bed combustors (CFBCs) are consid- ered as an improvement over the traditional methods associated with coal combustion. The CFBC exhibits several advantages over conventional coal combustion methods, especially when high sulfur coal is used’. Operation of CFBCs at industrial levels has confirmed many advantages that include fuel flexibility, broad turn- down ratio, high combustion efficiency, low NO, emissions and high sulfur capture efficiency. These characteristics assure an ever-increasing number of successful commer- cializations of CFBC in power generation applications. Although CFBC technology is becoming more common from these commercial applications, there are some significant uncertainties in predicting their performance in large-scale systems. Technical knowledge about design and operation of CFBC is widely available for pilot plant and large scale units. However, little has been done in the field of mathematical modeling and simulation of combustion in CFB Cs. This might be attributed to the fact that the combustion process occurring in a CFBC involves complex phenomena including chemical reactions, heat and mass transfer, particle size reduction due to combustion, attrition, fragmentation and other mechanisms, gas and solid flow structure, etc. Weiss et al.* introduced a CFBC model by dividing it into 11 blocks, each corresponding to a CSTR reactor for both gas and solid phase. Five of these blocks related to the CFBC riser. Basu et aL3 developed a CFBC model in which a plug flow regime for both the gas and solids is assumed. Lee and Hypanen4 presented a CFBC *Corresponding author model which considers the riser as a plug flow reactor for the gas phase and a CSTR reactor for the solid phase. The model also considers the feed particle size distribution and the attrition phenomena. Using a lumped-modeling approach, Arena et aL5 introduced the means for predictive calculation by dividing the CFBC riser into four blocks, each corresponding to a separate reactor. Three of these blocks related to the CFBC riser. The hydrodynamic parameters were considered uniform within each section and were used in various kinetic models to predict char conversion. Wong6 proposed a model for the hydrody- namics of CFBC risers to characterize the effect of the internal flow structure within the riser, the particle size distribution and the operating conditions on CFBC behav- ior. To estimate the axial voidage profile, a core-annulus model was developed. The predictive hydrodynamic model was then applied to a CFBC design. A comprehensive review of relevant work on the hydrodynamics of circulat- ing fluidized bed risers is presented by Berruti et aL7 Moreover, Senior’ conducted some theoretical and experimental investigations to improve the understanding of the fluid and particle mechanics in the CFBC riser and to develop mathematical models to represent riser suspension flows. On the other hand, a few CFBC modeling efforts have been based on extension of bubbling AFBC hydrodynamic concepts’-’ ‘. Beyond those mentioned above, some modeling work have been developed using ASPEN (advanced system for process engineering). ASPEN was developed at the Massachusetts Institute of Technology (MIT) under a United States Department of Energy project to simulate coal conversion processes. It has now become a powerful tool for engineers to model chemical, power generation and other processes. The work of Young’* entails the modeling and simulation of AFBC using ASPEN. 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7/30/2019 Simulation of Circulating Fluized Bed Reactors Using Aspen Plus
0 1998 E lsev ie r Sc ience L td . A l l r i gh ts reserved
Pr in ted i n Grea t B r i ta in
0016-2361/98 $19.00+0.00
Simulation of circulating fluidizedbed reactors using ASPEN PLUS
R. Sotudeh-Gh arebaagh, R. Legros*, J. Chaou ki an d J. ParisDepartment of Chemical Engineering, &o/e Polytechnique, PO Box 6079, Station ‘Centre-Vi//e’,
Montrkal, PO, Canada H3C 3A7
(Received 4 April 1995; revised 18 August 1997)
A comprehensive model is developed for the combustion of coal in a circulating fluidized bed combustor (CFB C).
The proposed model integrates hydrodyn amic parameters, reaction model and kinetic subroutines necessary to
simulate coal combustion in a CFBC . Kinetic expressions were developed for the char combustion rates and the
SO2 absorption in the bed using data from the literature. The reaction model, which considers only the important
steps of coal combustion, was simulated using four ASP EN PLU S reactor models and several subroutines. T he
developed subroutines w ere then nested in the ASPEN PLUS input file, so that the CF’EK may be represented. The
validity of the model was demonstrated using 14 different sets of operating conditions for the CAN ME T
0.8 MW th CFBC pilot plant. 0 1998 Elsevier Science Ltd. All rights reserved.
(Keywords: ASPEN PLUS; CFBC ; circulating fluidized bed reactors; fluidized bed com bustion)
INTRODUCTION
Circulating fluidized bed combus tors (CFB Cs) are consid-ered as an improvement over the traditional metho ds
associated with coal combustion. The CFB C exhibits
several advantages over conventional coal combustion
metho ds, especially when high sulfur coal is used’.
Operation of CFB Cs at industrial levels has confirmed
many advantages that include fuel flexibility, broad turn-
down ratio, high combustion efficiency, low NO , emissions
and high sulfur capture efficiency. These chara cteristics
assure an ever-increasing number of successful comm er-
cializations of CFB C in powe r generation applications.
Although CFBC technology is becoming more common
from these comm ercial applications, there are some
significant uncertainties in predicting their performa nce inlarge-scale systems.
Technical knowledge about design and operation of
CFB C is widely available for pilot plant and large scale
units. Ho weve r, little has been done in the field of
mathem atical modeling and simulation of combustion in
CFB Cs. This might be attributed to the fact that the
combustion process occurring in a CFB C involves complex
phenomena including chemical reactions, h eat and mass
transfer, particle size reduction due to combustion, attrition,
fragmentation and other mechanism s, gas and solid flow
structure, etc. Weiss et al.* introduced a CFBC model by
dividing it into 11 blocks, each corresponding to a CST R
reactor for both gas and solid phase. Five of these blocksrelated to the CFBC riser. Basu et aL3 developed a CFBC
model in which a plug flow regime for both the gas and
solids is assum ed. Lee and Hypanen4 presented a CFB C
*Corresponding author
model which considers the riser as a plug flow reactor for
the gas phase and a CSTR reactor for the solid phase. Themodel a lso considers the feed particle size distribution and
the attrition phenomena. Using a lumped-modeling
approach, Arena et aL5 introduced the means for predictive
calculation by dividing the CFB C riser into four blocks,
each corresponding to a separate reactor. Three of these
blocks related to the CFB C riser. The hydrodynamic
param eters were considered uniform within each section
and were used in various kinetic models to predict char
conversion. Wong6 prop osed a model for the hydrody-
namics of CFBC risers to characterize the effect of the
internal flow structure within the riser, the particle size
distribution and the operating conditions on CFB C behav-
ior. To estimate the axial voidage profile, a core-annulus
model was developed. The predictive hydrodynamic model
was then applied to a CFBC design. A comprehensive
review of relevant work on the hydrodynamics of circulat-
ing fluidized bed risers is presented by Berruti et aL7
Moreover, Senior’ conducted some theoretical and
experimental investigations to improve the understanding
of the fluid and particle mechanics in the CFB C riser and to
develop m athematical models to represent riser suspension
flows. On the other hand, a few CFBC modeling efforts have
been based on extension of bubbling AFBC hydrodynamic
concepts’-’ ‘.
Beyond those mentioned above, some modeling work
have been developed using ASPEN (advanced system for
process engineering). ASPEN was developed at theMas sachuse tts Institute of Technology (MIT) under a
United S tates Department of Energy project to simulate
coal conversion process es. It has now become a powerful
tool for engineers to model chem ical, powe r generation and
other processes. The work of Young’* entails the modeling
and simulation of AFB C using ASPEN . Herein, the ‘black
Fuel 1998 Volume 77 Number 4 32 7
7/30/2019 Simulation of Circulating Fluized Bed Reactors Using Aspen Plus
Circulating fiuidized bed reactors: R . Soutdeh-Gharebaagh et al.
box’ approach with one ASPEN PLUS stoichiometricreactor was used to calculate the mass balances based ongiven combustion and sulfur capture efficiencies. CFBCsimulation work was also initiated at CERCHAR13 toprovide the technical information required for the evalua-
tion and optimization of CFBCs under steady-state con-dition in power generation applications. The study of
pollutant emissions such as SO*, NO, and N20, as well asthe ash composition leaving the CF BC w as not included inthis work; instead several ASPEN PLUS user subroutines
were used in order to study the hydrodynam ic, comb ustionand heat transfer phenom ena in the bed. The approach usedat CERCHAR is similar to that of Young’s, but wasextended to cover CFBCs. Combustion Engineering Inc.14also used ASPEN in modeling a Lurgi circulating fluid bed.The approach, similar to that of Young’s’* used here, has alow level of complexity since the goal was the calculation of
the mass and energy balances for the CFBC. Up to now,modeling of CFBCs using a process simulator (such asASPEN PLUS) has been limited to simple mass and energy
balances, without predictive capabilities.ASPEN PLUS is widely accepted in the chemicalindustry as a design tool because of its ability to simulate
a variety of steady-state processes ranging from single unitoperation to complex processes involving many units.Consequently, ASPEN PLUS w as chosen as a frameworkfor the developmen t of a CFB C process simu lation. Sincethere is no CFBC model provided by ASPEN PLUS, wemust develop our own using the tools offered by ASPENPLus15,16. In addition to its conventional reactor m odels,ASPEN PLUS has the flexibility to allow the insertion ofFortran blocks and user kinetic subroutines into thesimulation.
In this work, several ASPEN PLUS reactor modelsinteract with their corresponding user-written kineticsubroutines to perform calculation during the simulation.This flexible structure of ASPEN PLUS permits handling ofcomplex processes, such as those occurring in a CFBC.Hence, an attempt is made to develop a model whichincludes several features that were neglected or simplified in
the previous studies as outlined above, in order to produce apredictive tool. This paper presents the details of themodeling approaches taken to obtain a process simulationprogramme for coal combustion in a CFBC .
MODELING APPROACHES
In a typical CFBC used for coal combustion, crushed coaltogether with limestone or dolomite and ash particles arefluidized by the combustion air entering at the bottom of thebed and at one or several secondary air injection points. Alarge portion of the bed particles exits the riser of the CFBC
with the flue gas due to the high superficial gas velocitiesutilized. The particles are then separated from the exhaustgas in a gas/solid separator (often a cyclone) and recycledinto the riser to promote complete combustion of the coal.Because coal combustion in a CFBC is directly affected byits hydrodynam ic param eters, both hydrodynam ic andcombustion models must be treated simultaneously toyield a predictive model for the CFB C. The description of
the method followed in developing the hydrodynam ic andreaction models is given below.
Hydrodynamic model
The hydrodynamic model enables the variation of thevoid fraction with height in the riser to be determined. The
32 8 Fuel 1998 Volume 77 Number 4
general hypotheses of the hydrodynam ic model along withthe modeling procedure are presented below.
General hypotheses of the hydrodynamic model. Forsteady state conditions, the assumptions regarding the
hydrodynam ic model are the following:
(1) The CFBC is naturally divided into two hydrody-
namic regions:
(i) a lower region-turbulent fluidized bed (dense bed);(ii) an upper region (dilute bed).
The boundary between the two regions is defined by theheight of the secondary air injection point.(2) There is perfect mixing of solids (individual ash, charparticles and sorbents) in the lower region and in eachzone of the upper region17. This assu mp tion is justified bythe high internal and external recirculation of solids in thebed.(3) Plug flow regime for gas is assumed in the bed. This isconsistent with the results of gas backmixing ex erimentsin the CFB C risers as reported in the literature p7.
(4) The gas velocity througho ut the bed is uniform andconstant for each region of the bed.(5) For a given superficial gas velocity, the mean voidagein the lower region of the CFBC is constant. This assum p-tion is justified by the results of experiments ofChehbouni et al . l8 for group B particles considering thelower region to be operated under the turbulent fluidiza-tion condition.(6) In the upper region of the CFBC, the voidage
decreases with the vertical position along the riser.
M odeling procedure. The model considers that theCFBC is divided into two regions: a dense lower regionwith a constant suspension density (turbulent fluidizedbed) and a more dilute upper region with a decaying suspen-sion density with height. Detail related to the gas-solidstructures chosen to represent two regions of the riser aregiven below:
Lower region of the CFBC. The lower region is fluidizedby the primary air supply. Kunii and Levenspiel’, Saraiva et
al. ‘O ,and Kwaulk et al. ’ ’ treated the lower region of CFBCusing the models developed originally for bubbling flui-dized beds. This is inconsistent with the fact that the gas
superficial velocity in this region is usually higher than acertain critical value, U,, where the region becomes turbu-lent18. At this condition, solid velocity, bubble diametersand velocities are quite different from the bubblingregime7,18. However, for simulation purposes, perfectmixing between the solids and the gas phases is assumedin this region. Under these conditions, the mean voidage ofthe dense region is considered constant and may be obtainedusing the correlation proposed by Kunii and Levenspiel’.
U pper region of the CFBC. The upper region is suspendedboth by the combustion gases from the lower region and thesecondary air supply which determines the boundary
between the two regions. Hydrody namic mod els, as pro-posed in most CFBC literature regarding the upper region,are classified into three broad groups’: (1) those predictingthe axial profile of the solids suspension density but failingto predict the radial variation; (2) those assuming two ormore regions considering either the core annulus or the
7/30/2019 Simulation of Circulating Fluized Bed Reactors Using Aspen Plus
Circulating fiuidized bed reactors: R. Soutdeh-Gharebaagh et al.
Reactor number.j................................
I
I
I l-e3I
- - -MepnsolidfktionUinthekm#tion @
I
Upper&on @
I
_--
1 l-e,) ;I
ILowetregion 0
III + b
Solid fraction,(1 )
Figure 1 Variation of void fraction with height in the riser
Devolatilization and volatile combustion. When coal isintroduced into a CFBC, it decomposes into two parts:hydrogen-rich volatile and char. The char rema ins in the
bed and burns slowly. Based on the plume model, coaldevolatilization and complete comb ustion of the volatileoccur at the feed entry poin t23. Two steps will then be con-sidered in the simulation : decompo sition and volatilecombustion.
Decom position. In this step, coal is converted into itsconstituting com ponents such as carbon, hydrogen, sulfur,nitrogen and ash. This step occurs in the lower region reac-tor only and RYTELD (ASPEN PLUS yield reactor) is usedto model this process by speciying the yield distributionvector according to the coal ultimate analysis.
Volatile comb ustion. To simulate the volatile comb us-tion step, three reactions are considered in the model:
c+$**co
s+o**so*
H2 + $02 * H20
These reactions occur in the lower region reactor only wherethe coal is introduced and RSTOIC (ASPEN PLUS Stoi-chiometric reactor) is used to model the volatile com bustionprocess. The combustion of the volatile matter is based onthe following hypotheses:
(i) Considering that the volatile matter (VM) in the coal,(obtained from a proximate analysis) cons ists exclusivelyof carbon, hydrogen and sulfur, the fraction of total coalcarbon associated to volatile combustion is given by X, =VM - H - S, where H and S are the fraction of hydrogenand sulfur in the coal. This supposes that the entire
hydrogen content of the coal is found in the volatilematter. The volatile carbon fraction (X,) reacts to formCO only during the volatile combustion process becauseof the oxygen depletion in the lower region of the riser.(ii) The coal hydrogen content is entirely consumedduring the volatile combustion process.
(iii) The coal sulfur content is assumed to be converted
completely to SO2 during the volatile com bustion proce ss.
Char combustion kinetic model. The char particles result-
ing from the devolatilization process co nsist of the remain-ing carbon fraction (1 - X,) and ash only. These particlesare then burned to produce a mixture of CO and CO *. Threemain reactions for char combustion are considered here 24:
c+@**co
c+co*=+2co
These reactions occur in the entire riser, hence in the fourCSTR reactors, and RCSTR (ASPEN PLUS CSTR reactor)is used to model this process. This block requires the knowl-edge of the reaction kinetic mod el which is presented below.
The first and third reactions are heterogeneous andthe second is hom ogeneous. Since the temperature of theburning particles in the CFBC is not sufficiently high, theeffect of the third reaction on the combustion rate is 10w*~,and this reaction is neglected in the model. For theremaining two reactions, the reaction rate expressions
mu st be developed. Th e first reaction is a gas-solid reactionand the chemical changes take place on both the externaland the internal surface of the char particles’. The following
expression for the char combustion rate, to form CO, perunit volume of the ith interval is obtained25.
rl,i =
3v02kcrFchar, i( 1 - fi)
PcharrC( 1 - EC)Fsolid, iCO,
k, , can be expressed by an Arrhenius form as follows:
k,, = kolexp
(5 )
The following equation is used to calculate the mean p ar-ticle radius based on the experimen tal particle size
distribution:1
rc= m (7)
t r,(k)
Carbon monoxide produced during the heterogeneous com-bustion of char reacts with 02 in the homogeneous gas phasereaction to form CO* . Factors con tributing to CO emissionlevels are the bed temperature, 02, C O and Hz0 concentra-tion. The following ex ression is used for the CO comb us-tion rate in the model 2g
:
rco,i= 1.18 10’3f&6~~o()exp
x (-F)CEi (8 )
NO, formation. Staged combustion remains an attractive
33 0 Fuel 1998 Volume 77 Number 4
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Circulating fluidized bed reactors: R. Soutdeh-Gharebaagh et al.
Q8
90 92 94 Q8 Q8 100
Expeflmental Combustion Eff i c i ency W
Figure 3 Betw een the predicted and experimental combustion
efficiency
35 0
30 0
25 0
g 200
8
t
15 0
k10 0
50
0 L
0 Equation24 (proposed orrelation)
0 Equation8 (original orrelation)
0 50 100 150 200 250 300 350
mntal co @pm)
Figure 4 Comparison between the predicted and experimental
co
approximately 10 times the carbon fraction in the coal feedrate. This procedure ensured a rapid and stable convergenceprocess.
Model validation
In order to validate the proposed model, 14 different setsof operating data from various CANMET runs*’ were usedto validate the simulation (see Table 5). A detailed
description of the CANME T 0.8 MW th CFBC pilot plantis presented by Desai et ~1.~~.The predicted simulationresults in terms of comb ustion efficiency, emission levels ofCO, SO 2 and NO, and 2O and CO concentration profiles arecompared with the experimental data. The results aredetailed below.
!z1000
B2 800
F.fi 800
40 0
0 I I 1 I I I I
0 200 400 800 800 1000 1200 1400
Waa so2 @pm)
Figure 5 Comparison between the predicted and experimental
so 2
Combustion eficiency. In order to estimate combustionefficiency, the two outlet streams, S 20 and S22 (seeFigure 2), are used. These streams contain small amountof unburnt char particles that controls the combustionefficiency (vc), wh ich is defined as
vc=l-(
Total rate of carbon in the outlet stream
Total rate of carbon in the feed stream 1
(23)Thirteen sets of expenmental data reported values for the
comb ustion efficiency were used to compare with the modelpredictions. In Figure 3, it is found that the model consis-tently overpredicts the combustion efficiency. The differ-
ences between the predicted values and experimental data,which are less than 3%, are related to the value calculatedfor the cyclone efficiency. ASPEN calculated 99.99% effi-ciency for the cyclone used in the pilot plant, which isgenerally higher than that reported experim entally. Conse-quently, the carbon content of the fly ash predicted by themodel becomes substantially smaller than the experimentalvalue. This smaller amount of carbon in flyash causes the
model to overestimate the combustion efficiency.
CO emission levels. Although CO combustion rateshave been widely studied1*6 *24,2 the extension of theseexpressions to CFB C conditions is limited. The validity ofthe proposed CO combustion rates from the literature wasexamined by inserting them into the simulation program.Following the simulation res ults, a new correlation, similarto the Robinson’s expression26, with two adjustable par-ameters wa;prop osed to predict the CO range reported byDesai et al. :
with(24)
/3, = 1.8 * lOI
p* = 0.21
33 4 Fuel 1998 Volume 77 Number 4
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Circulating fluidized bed reactors: R. Soutdeh-Gharebaagh et al.
These parameters were obtained by fitting the CANMET
experimental data. However, more data from varioussources are required in order to confirm this correlation.
The CO emission levels predicted by the model rangedfrom 119 to 271 ppm, while the experimental data variedbetween 112 and 3 16 ppm. Therefore, the model based onthe new correlation estimates CO emission levels relatively
well. The comparison between predicted and experimentalCO levels is presented in Figure 4.
SO* emission levels. The Ca/S molar ratio rangedbetween 1.6 and 2.3 for the various runs considered. Suchvalues of Ca/S are usually considered sufficient to achievereasonably high sulfur capture efficiencies. Without using
any fitting parameters, Figure 5 shows the close agreement
ii‘: 200
8
%150
%100
0
0 50 100 150 200 250 300 350
Ew=t m~t NO, ( ppm
Figure 6 Comparison between the predicted a nd experimental
NO ,
observed between p redicted an d experimen tal SO* concen-tration in the flue gas.
NO, emission levels. As mentioned earlier, two forma-tion mechanisms were considered in the modeling of NO,formation in the CFBC: thermal generation and fuel nitro-gen oxidations. Th ermal generation was calculated con-
sidering equilibrium conditions, while NO, formationfrom fuel nitrogen oxidation was calculated using an overallconversion factor (cY~)rom the literature. Since the aim ofthis work was not to study the NO, formation and reductionprocesses in detail, this overall approach was taken to simu-late the fuel nitrogen oxidation. Fuel nitrogen conversionentails relatively complex reactions schemes involving sev-eral heterogeneous reaction step s and therefore attains alower overall conversion26. Typical values of fuel N2 toNO , conversion factors, as reported by Legros et CZZ.*~ndBecker et d3’, vary approximately between 0.05 to 0.25,
depending on coal properties, feed particle size distribution,excess air level and operating conditions. In our simulation,
a value from that interval, which gave the best agreementbetween predicted and experimental NO, was chosen as theoverall conversion factor. As reported in mos t CFB Cliterature, the results also confirmed that thermal NO, for-
mation, which leads to between 18 and 65 ppm of NO,, isunimportant compared to that of fuel nitrogen oxidationwhich approximately lies between 84 and 104 ppm ofNO ,. The predicted em ission levels ranged from 130 to267 ppm, while NO, emissions for the experimentalCFBC ranged from 107 to 309 ppm.27. Figure 6 appearsto indicate a reasonable agreemen t between predicted andexperimental NO,. The difference between experimentaldata and those of the simulation prediction is attributable
to the fact that a constant value of CY= 0.05 is used through-out the entire simulation.In recent years, several compreh ensive studies have been
reported6.3 -34 regarding NO, formation and reduction
processes. These were conducted to develop an improvedunderstanding of the fundamental nature of NO, chemistryand underlying physical processes in CEB Cs, and to supportthe needs for experimental work in this field. Em phasis is
9
6- 08 Redi *edCOConcart rati on( ppm 2500 p
. g 8- -
E 2000 i
! '
E
- 1500 E-
0"0 - 1000 "
2-
.
l -* . -. . Q 500
_ ..0 . - . . . . _ . _ . .
I I I I I I 00
0 1 2 3 4 5 6
BedHei ght
Figure 7 O2 and CO concentration profiles within the CFB C predicted by the model
Fuel 1998 Volume 77 Number 4 335
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Circulating fluidized bed reactors: R. Soutdeh-Gharebaagh et al.
also given to develop reliable techniques to control NO,emissions from fluidized bed combustion. For example,many published studies suggest that NO, emissions could becontrolled by adding chemical comp onents such as carbonmonoxide, char hydrogen, ammonia, unburnt hydrocarbonsand limestone due to the catalytic reactions found inCFBC32V33.
02 and CO concentration profiles. The emission data
from the CANMET CFBC pilot plant used to validate themodel co nsisted only of flue gas concentration and did notinclude the various gas concentration profiles along theriser. However, since the model can predict these concen-
tration profiles within the riser, oxygen and carbon monox-ide profiles were chosen to be compared qualitatively withdata from the literature. The overall trends observed inexperimental concentration profiles along the riser heightare in close agreement with those predicted by the model.In the lower region a significant change in the oxygen con-centration is found, while in the upper region there is a
gradu al decrease in the oxygen con centration. The COconcentration is constantly high in the lower region, wh ileit sharply decreases in the upper region due to the injection
of secondary air. Typical O2 and CO concentration profilesprovided by Hansen et a1.35and experimental data reportedby Brereton et al .33 and Grace et al .34 are similar with those
predicted by the present model. In Figure 7, the predictedprofiles are presented.
Due to the relatively high dense b ed found at the bottomof the CFBC reactor, Brereton et al.33 and Grace et aZ.34
have not measured the O2 and CO concentrations. There-fore, the experimental data have only been reported for theupper region. Since the operating and bed design datareported in those references differ with that of CA NM ET,the predicted concen tration profiles have been comparedqualitatively with the trend reported in these references.
CONCLUSION 10
A mod el was developed for the comb ustion of coal in acirculating fluidized bed using the ASPEN PLUS simulator.To provide such a CFBC model, several ASPEN PL US unitoperation blocks were combined and, where necessary,kinetic expressions and hydrodynam ic model were devel-oped using data and models from the literature. Thedeveloped models were then inserted into the flowsheet toprovide a complete representation of the CFB C. The
resulting m odel was used to predict the performance ofthe CANMET CFBC pilot plant in terms of combustionefficiency, emission levels of CO , SO2 and NO,, an d 02 andCO concentration profiles. The predictions of CO and NO,were achieved using two and one fitting parameters,respectively. The agreement between the model prediction
and experimental data is satisfactory b ut more experim entaldata are still required to confirm the proposed CFBC modelin order to mak e it more com prehensive and reliable. Themodel can now be used to represent a CFBC unit in variousprocess simulation flowsheetsplants.
such as power generation
ACKNOWLEDGEMENTS
This project was supported by CANMET, part of Energy,Mines and Resources, Canada. This financial assistance isgratefully acknowledged. Special thanks are due to the
Ministry of Culture and Higher Education of Iran for
providing a scholarship to Mr R. Sotudeh-Gharebaagh.Helpful discussions from F. Preto and E. J. Anthony are alsoappreciated. We greatly acknowledge Aspen Technology
for having granted special permission for the use of theASPEN PLUS under the condition of the academiclicensing agreement.
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