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Chapter 35
Clean Combustion of Low Quality Fuel
in Fluidized Bed Combustor
Rami S. El-Emam, Farouk M. Okasha, and Salah H. El-Emam
Abstract Combustion characteristics for rice straw and mazut in a fluidized bed
combustor have been investigated. Rice straw has been prepared as pellets in order
to increase its bulk density and control feeding flow rate. Rice straw pellets have
been burnt in bubbling fluidized combustor operating at atmospheric pressure.
Over-bed fuel feeding of fuel is applied to provide steady condition of performance.
Mazut combustion in the fluidized bed has been also investigated. In-situ desulfur-
ization is considered for the case of mazut combustion. It is concluded that post-
combustion of volatiles in the fluidized bed combustor results in a peak temperature
values in the freeboard zone. The peak temperature value and position shifts based
on the operating condition of the fluidized bed. Carbon monoxide and nitrogen
oxides emissions are measured for the presented cases of fuel combustion. Nitrogen
oxides emission measurements are reported as 175–270 ppm which is considered
relatively low. The effect of fluidization velocity, static bed height and excess air on
emissions of carbon monoxide and nitrogen oxides is also investigated. Improve-
ment in combustion of mazut is achieved with the increase in bed temperature,
static bed height, and with excess air. Adding limestone particles to the fuel caused
sulfur retention up to 90 %.
Keywords Fluidized bed • Biomass • Rice straw • Mazut • Emissions
35.1 Introduction
Abundance of agriculture waste and residues are burnt haphazardly in Egypt,
without energy recovery, causing severe pollution levels in the environment, espe-
cially after rice harvesting season which is the second most cultivated crop in Egypt.
R.S. El-Emam (*)
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology,
Oshawa, Canada
Department of Mechanical Power Engineering, Mansoura University, Mansoura, Egypt
e-mail: [email protected]
F.M. Okasha • S.H. El-Emam
Department of Mechanical Power Engineering, Mansoura University, Mansoura, Egypt
© Springer International Publishing Switzerland 2014
I. Dincer et al. (eds.), Progress in Sustainable Energy Technologies Vol II:Creating Sustainable Development, DOI 10.1007/978-3-319-07977-6_35
531
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Alternatively, utilizing agriculture waste as biomass driven fuel preserves the
diminishing conventional fossil fuels and alleviates the growing waste disposal
problem and results in a remarkable reduction in the emissions that have negative
impact on the environment. According to the Egyptian Agriculture Engineering
Researches Institute, the annual potential of agriculture wastes in Egypt is about
22.5 million ton. About 35% of them are utilized as animal feeding and fertilizer and
65% are available for energy production which is 7 million ton oil equivalent (TOE).
Due to their simplicity, fuel flexibility and higher efficiency, fluidized bed
combustors are good candidates for handling agricultural residues and biomass
driven fuel into useful energy. It is also a good candidate for heavy liquid fuel.
Fluidized bed combustors operate at relatively low temperature values, compared
with other combustion (FBC) technologies, which helps in minimizing the emis-
sions for nitrogen oxides. A detailed case study provides insights to the technical
specifications of the various equipment, systems and cost economics of fluidized
bed combustion technology for cogeneration systems have been reported [1].
Co-generation through fluidized bed combustion boiler using biomass is considered
a renewable clean technology which can also help in mitigation of greenhouse gas
emissions.
Under similar operating condition, combustion process of rice straw is not
experiencing different emissions and operating issues compared with wheat straw
and rice husk. Pretreatment of biomass fuel is utilized to enhance the fuel proper-
ties. The two fundamental properties of rice straw analyzed by Kargbo et al. [2] are
calorific and density values. They tested sizing and compression as utilized as
pretreatment technologies. Results show that both physical and chemical properties
of rice straw are improved significantly by the pretreatment technologies.
Srinath et al. [3] studied the combustion characteristics of rice husk in a
rectangular fluidized bed and reported that maximum carbon monoxide concentra-
tion occurs at active combustion zone. Based on CO emission and unburned carbon
content in fly ash, the combustion efficiency of the fluidized bed combustor was
calculated for the rice husk fired under different operating conditions. The maxi-
mum combustion efficiency of the rice husk is found to be 95 %. Effect of adding
rice straw to wood fuel on the combustion and emission characteristics is investi-
gated by Thy et al. [4]. They reported that there is experimental evidence that the
addition of straw to conventional biomass boiler fuels in some instances may reduce
potassium fouling. Naik et al. [5] carried out an investigation to highlight the
common biomass available in Canada such as wheat straw, barley straw, flax
straw, timothy grass and pinewood. They studied the calorific values, ash, cellulose
and hemicellulose contents in the studied samples. Their analyses showed that
pinewood, wheat and flax have greater potential for bioenergy production.
Liquid biofuel combustion in fluidized bed is investigated experimentally by
Miccio et al. [6]. They performed combustion of biodiesel and sunflower oil in a
lab-scale internal circulating fluidized bed reactor (ICFB) for co-gasification of
biomass and waste fuels or incineration of liquid wastes. The combustion of fuel
vapors was managed to occur with a limited residence time by feeding the fuel the
riser. They investigated the occurrence of the micro-explosive behaviour that was
532 R.S. El-Emam et al.
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observed in the combustion process. Combustion efficiency and carbon monoxide
emissions were observed to be a little different between biodiesel and sunflower oil.
A new possibility is envisaged for applications in which the released heat is directed
at producing high temperature, high pressure fluid streams, taking advantage of the
extremely high heat transfer coefficients in fluidized bed [7].
As rice straw represents a challenging issue of agriculture wastes in Egypt, the
current study is concerned with investigating the combustion of rice straw in
fluidized bed. It also presents combustion of mazut, as common heavy oil utilized
in Egypt, in fluidized bed combustor. Mazut has relatively high sulfur content, as
listed in Table 35.1, which causes negative environmental impact when combusted
in conventional combustor. Hence, alternative combustion technologies should be
implemented to limit the emissions level. The main objectives of this study are
preparation of rice straw in adequate form of pellets, and assessment of FBC at
different operating conditions with observation of the emissions of carbon monox-
ide and nitrogen oxides. Also sulfur retention is considered with adding calcium
(limestone) when combustion of mazut is investigated.
35.2 Experimental Work
Atmospheric bubbling fluidized bed is considered in the proposed work. Figure 35.1
shows a schematic of the experimental test system. A detailed description of the test
system, auxiliary components and pellets preparation can be found elsewhere [8, 9].
The combustor is a cylindrical column of 300 mm inner diameter and 3300 mm
height. Primary air, which serves in fluidizing bed materials and burning fuel,
provided through a nozzle type distributer. Continuous over-bed feeding is
achieved using a paddle shaft which is driven by variable speed electric motor. A
hopper on top of the combustor column is used for feeding the fluidized bed with
sand particles. Flue gases pass through a cyclone to collect the entrained particu-
lates. All parts of the fluidized bed column are insulated using blankets of thermal
Table 35.1 Fuel analysis
and propertiesRice straw Mazut
Ultimate analysis (dry basis, %)
Carbon 42.04 84.8
Hydrogen 6.26 11.59
Oxygen 39 –
Sulfur 0.64 3.21
Nitrogen 1.23 –
Ash 10.83 0.07
Properties
Density, kg/m3 0.9 (pellet) 946.1 (15.5 �C)LHV, MJ/kg 19.441 40.820
Moisture 8.9 % 0.2 %
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wool. Silica sand with of 0.25–0.5 mm is considered as bed material, which has
minimum fluidization velocity of 5.6 cm/s at 850 �C. Pellets of rice straw are
prepared by pressing chopped straw under 200 bar inside a die. The pellet are
produced in 12 mm diameter and 10 mm length of cylindrical shape with bulk
density of 0.9 g/cm3 compared with 0.05 g/cm3 as initial bulk density.
When liquid fuel is injected into a fluidized bed, residence time experienced with
the fuel is short due to immediate evaporation. Consequently, rapid mixing of
droplets and air is important. The fuel injector is fixed at the bottom of the
combustor. It passes through the centerlines of the plenum chamber and the
distributor plate to reach the fluidized bed. A detailed description of the mazut
injector can be found elsewhere [10]. Analysis and properties of the used rice straw
and mazut fuels are listed in Table 35.1. Calcium based sorbent is utilized with the
combustion of mazut to facilitate the removal of sulfur dioxide from the combus-
tion emissions. This is one of the advantages of using fluidized bed instead of
conventional combustion technologies. Limestone particles with size of 0.5–
0.8 mm are utilized in different molar ratios described by Ca/S ratio.
Fig. 35.1 Diagram of the test system equipped for pellets or mazut combustion
534 R.S. El-Emam et al.
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35.3 Results and Discussion
The experimental measurements and results of combustion of rice straw pellets and
mazut are illustrated in this section. The temperature profiles through the fluidized
bed height at different cases are also presented. Emissions and combustion effi-
ciency are also presented. Effects of varying fluidization velocity, static bed height
and excess air ratio on the combustion process are studied. A comparative analysis
of combustion of both fuels is introduced based on the efficiency and emissions at
different operating conditions.
35.3.1 Combustion of Rice Straw Pellets
The results representing the axial temperature profile show a uniform temperature
through the bed zone. The temperature then starts increasing till a peak temperature
is reaching in the freeboard zone. The position and degree of overheating is
controlled by the operating parameters of the fluidized bed combustor. It is noticed
that part of the volatile get into complete combustion in the freeboard zone where
occurrence of flame is observed. This may have occurred because of lack of mixing
with oxygen. In the following subsections, studies on effect of fluidized bed
operating parameters are performed. When effect of one parameter is investigated,
other parameters are kept as in the base case condition.
It is also noticed in the presented results that nitrogen oxides are relatively low.
Through the combustion process, carbon monoxide reacts with the formed nitrogen
monoxide forming elemental nitrogen. At the same time, with proceeding in the
combustion process, the reduction of nitrogen oxide through reacting with char is
getting lower [11, 12]. The results also report fixed carbon losses which is calcu-
lated as the rate of collected carbon, using the cyclone, to the total rate of fixed
carbon feed in the fuel.
35.3.1.1 Effect of Fluidization Velocity
The effect of varying the fluidization velocity on temperature profile, emissions and
combustion performance is presented in Figs. 35.2, 35.3, and 35.4. The results in
Fig. 35.2 discuss the effect of fluidization velocity on the axial temperature profile
of rice straw pellets combustion. It is noticed that at higher velocity, a more uniform
temperature profile is achieved through bed and splashing zones. This is because of
the higher rigorous bed particles mixing. The overheating is reported as 47.8, 68.9
and 87.120C at 0.3, 0.5, 0.7 m/s, respectively. From the temperature profiles, a shift
in the peak temperature along freeboard zone height is noticed with increasing the
velocity. The shorted gas residence time with higher velocity is probably respon-
sible for this shift.
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Fluidization velocity has a noticeable effect on carbon monoxide emissions as
seen in Fig. 35.3. At lower fluidization velocity, better combustion occurs where the
mass transfer between the two phases, i.e.; bubble and emulsion, is enhanced with
Fig. 35.2 Effect of
fluidization velocity on the
axial temperature profile for
straw pellets combustion
Fig. 35.3 Carbon monoxide and nitrogen oxides emissions of straw pellets combustion at
different fluidization velocity values
536 R.S. El-Emam et al.
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the smaller bubbled produced at lower rising velocity. Also the residence time in
bed and freeboard zone is longer with slower fluidization, this result in more
reduction of carbon monoxide to form carbon dioxide. The results in Fig. 35.3
also show the change in nitrogen oxides with fluidization velocity. Nitrogen oxides
are supposed to decrease with increasing the fluidization velocity as the more
carbon monoxide formation, the more nitrogen oxides reduction. However, as a
result of the less time available for the reduction reaction at high velocity, it appears
that nitrogen oxides increase. Figure 35.4 shows the effect of fluidization velocity
on the combustion efficiency and carbon loss. Efficiency values drop at higher
velocity where more carbon loss is indicated where coarser particulates are dragged
with the flue gases at higher velocity.
35.3.1.2 Effect of Excess Air Ratio
Results in Figs. 35.5, 35.6, and 35.7 show the influence of changing the excess air
ratio over the combustion performance of rice straw pellets in the fluidized bed.
Figure 35.5 illustrates the effect of excess air on the axial temperature profile. Three
different excess air ratios are considered. Increasing the available oxygen with more
excess air, results in a higher combustion reaction rate. This means that most of
the combustion occurs inside the bed zone at higher excess air ratio. The results
agree with this as peak temperature value is achieved closed to the bed zone with
higher excess air. It can also be noticed that the lower the air, the hotter the gases at
the end of the fluidized bed height. The reason for this is the combustion of volatiles
that escape to the freeboard zone at low excess air ratio, causing more heat release
along the combustor height by the extended flame of combustion.
Fig. 35.4 Effect on fluidization velocity on straw pellets combustion efficiency and carbon loss
percentage
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Figure 35.6 elucidates that for excess air less that 20 %, carbon monoxide in the
flue gases seems to be really high where it reaches 1,550 ppm at 10 % of excess air.
It also shows that increase of excess air over 25 % doesn’t produce more impact on
carbon monoxide concentration reduction. With regards to nitrogen monoxide
Fig. 35.5 Effect of excess
air on the axial temperature
profile for straw pellets
combustion
Fig. 35.6 Carbon monoxide and nitrogen oxides emissions of straw pellets combustion at
different excess air ratios
538 R.S. El-Emam et al.
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concentration, excess air increases the chances of nitrogen oxides formation as it
increases from 175 to 276 ppm with increase of excess air from 10 to 30 %. Also,
the enhancement of combustion process with the increase of excess air, causes
lower nitrogen oxides reduction reactions rates. Results in Fig. 35.7 show the
increase of combustion efficiency and reduction of carbon loss with increasing
excess air. This is a result of the lower carbon monoxide formation which means
enhanced combustion.
35.3.1.3 Effect of Static Bed Height
Static bed height influences the fluidized bed combustor performance. Figure 35.8,
35.9, and 35.10 shows these effects on the axial temperature profile, carbon
monoxide and nitrogen oxides emissions. Also changes in efficiency and carbon
loss are investigated. The results presented in Fig. 35.8 show that at higher bed
height, the peak temperature value shifts more into the freeboard zone where more
volatiles escape the bed zone without combustion. It is also noticed that the
freeboard zone temperature decreases with the bed height increase as overheating
with changing the static bed height from 20, 30 to 40 cm is measured as 94.5, 71.7
and 47.1 �C, respectively.Increasing static bed height results in a longer residence time in the bed zone.
This cause a slightly enhancement in the combustion which appeases in the limited
reduction in carbon monoxide in Fig. 35.9, which is accompanied by an increase in
nitrogen oxides from 210 to 250 ppm. The improvement in combustion process can
be seen in Fig. 35.10 as well as the efficiency slightly increase with the increase of
static bed height.
Fig. 35.7 Effect on excess air on straw pellets combustion efficiency and carbon loss percentage
35 Clean Combustion of Low Quality Fuel in Fluidized Bed Combustor 539
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Fig. 35.8 Effect of static
bed height on the axial
temperature profile for
straw pellets combustion
Fig. 35.9 Carbon monoxide and nitrogen oxides emissions of straw pellets combustion at
different static bed height values
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35.3.2 Combustion of Mazut
The performance of mazut combustion in fluidized bed combustor is presented in
comparative form with rice straw pellets combustion. Mazut is utilized with
preheated temperature of 100oC. Figure 35.11 shows a comparison of the temper-
ature profile through the fluidized bed column with bed temperature of 850oC and
static bed height of 50 cm. Mazut flow rate is 10 kg/h with fluidization velocity of
1 m/s is considered for the presented results. Occurrence of post combustion in the
freeboard zone is observed with a peak temperature that is also reported to vary in
value and position with the operating and fluidization conditions.
Carbon monoxide and nitrogen oxides of mazut combustion and rice straw are
presented Table 35.2 and 35.3 considering the effect of static bed height and excess
air ratio on the emission levels measured in ppm. Two static bed heights are
considered and three different excess air ratios are tested. Also the calculated
combustion efficiency values are presented for both fuels. Generally, bed height
and excess air causes have a positive effect on reducing of carbon monoxide and
nitrogen oxides emissions.
From the results presented in Fig. 35.12, it is clear that increasing the temper-
ature cause a noticeable reduction in carbon monoxide formation as it is can be seen
in Fig. 35.12a. This is because of the increase in reaction rate of combustion and the
enhancement of gases diffusion at higher temperature values. However, for rice
straw pellets, reduction in carbon monoxide from 850 to 900 �C is insignificant.
These results are reflected on the nitrogen oxides formation and combustion
efficiency as indicated in Fig. 35.12c and d, respectively. For combustion of
mazut, generally, lower emissions are shown compared with combustion of rice
Fig. 35.10 Effect on static bed height on straw pellets combustion efficiency and carbon loss
percentage
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straw. The performance enhancement with increasing the bed temperature is sig-
nificant at with stepping up from 750 to 850oC. Limited improvement is achieved
with increasing the temperature from 850 to 900oC.
The results in Fig. 35.13 show the concentration of sulfur dioxide in the
emissions of mazut combustion case considering no attempt of sulfur retention
with limestone addition. The effect of excess air on sulfur dioxide is shown. Excess
air causes lower emissions of sulfur dioxide, It can be seen from this figure that
sulfur dioxide in flue gases is reduced from 2017 ppm to 1686 ppm by increasing
excess air from 10 % to 30 %. In Fig. 35.14 and 35.15, sulfur retention with
limestone addition is presented. Five different mole ratios of calcium to sulfur are
Fig. 35.11 Comparative
analysis of axial
temperature profile
Table 35.2 Effect of static
bed height on combustion
emissions and efficiency
Straw pellets Mazut
Static bed height¼ 40 cm
Carbon monoxide 181.7 ppm 430.5 ppm
Nitrogen oxides 250.0 ppm 101.9 ppm
Combustion efficiency 98.4 % 99.0 %
Static bed height¼ 30 cm
Carbon monoxide 245.8 ppm 1070 ppm
Nitrogen oxides 218.3 ppm 94.2 ppm
Combustion efficiency 98.2 % 97.8 %
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Table 35.3 Effect of excess
air ratio over the combustion
emissions and efficiency
Straw pellets Mazut
10 % Excess air
Carbon monoxide 1544.7 ppm 515.6 ppm
Nitrogen oxides 174.1 ppm 71.4 ppm
Combustion efficiency 97.4 % 98.3 %
20 % Excess air
Carbon monoxide 262.1 ppm 419 ppm
Nitrogen oxides 229.1 ppm 87.0 ppm
Combustion efficiency 98.2 % 99.2 %
30 % Excess air
Carbon monoxide 131.4 ppm 144.2 ppm
Nitrogen oxides 280.0 ppm 96.1 ppm
Combustion efficiency 98.8 % 99.7 %
Fig. 35.12 Comparison between straw pellets and mazut combustion in fluidized bed (a) carbon
monoxide emissions, (b) nitrogen oxides emissions, (c) combustion efficiency
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tested, presented in 5 steps from no limestone addition to 1, 2, 3, 4, and 5 of Ca/S
molar ratios. Sulfur retention in Fig. 35.14 is presented as percentage of steps of
Ca/S ratio, so it shows the enhancement achieved in sulfur retention by increasing
Ca/S with respect to the previous step. Results in Fig. 35.15 show sulfur retention
for each step of Ca/S as percentage of no limestone addition case. It can be noticed
that the effect of step one results in about 40–48 % reduction in sulfur retention,
where sulfur dioxide is reduced to 1210, 961 and 872 ppm for 10, 20 and 30 %
excess air. The enhancement in retention decreases with higher Ca/S ratios. How-
ever, sulfur dioxide emission is reduced to 282, 202 and 151 ppm for 10, 20 and
30 % excess air when adding Ca/S of ratio 5. This gives around 87 to 91 % of sulfur
retention as can be seen in Fig. 35.15. These results also show a better sulfur
retention with higher excess air.
35.4 Conclusions
Rice straw and mazut are successfully burned in a bubbling fluidized bed. Different
operating conditions are tested and their effect on the temperature profile and
combustion emissions are investigated. Sulfur retention for the case of mazut
combustion is also considered. The following conclusions of the presented work
can be drawn:
Fig. 35.13 Sulfur dioxide emissions of mazut combustion with no added calcium (limestone) at
different excess air ratios
544 R.S. El-Emam et al.
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• Post-combustion of volatiles is observed and it causes peak temperature values
in the freeboard zone. The peak temperature value and location are dependent on
operating and fluidization conditions.
Fig. 35.14 Sulfur retention percentage calculated per step of calcium (limestone) ratio for mazut
combustion at different excess air ratios
Fig. 35.15 Sulfur retention in mazut combustion, calculated as percentage of zero-added
limestone case
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• High combustion efficiency over a wide range of operating conditions is
achieved. Combustion efficiency increase with increasing bed temperature,
static bed height and excess air ratio.
• Increase of excess air and static bed height cause improvement in the efficiency
of rice straw combustion with reduction in carbon loss, however, fluidization
velocity has a negative impact on combustion efficiency.
• The Nitrogen oxides emissions from 175 to 270 ppm are measured for combus-
tion of rice straw.
• Combustion of mazut achieved combustion efficiency of up to 99.8 %. Bed
temperature, static bed height and excess air causes an increase in the combus-
tion efficiency.
• Excess air helps in sulfur retention in mazut combustion.
• Sulphur retention is enhanced by adding limestone. Reduction of sulfur dioxide
to 151 ppm at excess air of 30 % is achieved compared with 1667 ppm when no
limestone is added.
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