clean combustion of low quality fuel in fluidized bed combustor

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

mailto:[email protected]

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.

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 %

35 Clean Combustion of Low Quality Fuel in Fluidized Bed Combustor 533

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


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.


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


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


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


different excess air ratios


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


Fig. 35.8 Effect of static

bed height on the axial

temperature profile for

straw pellets combustion


different static bed height values


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


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




Table 35.3 Effect of excess

air ratio over the combustion

emissions and efficiency

Straw pellets Mazut

10 % Excess air




20 % Excess air

Carbon monoxide 262.1 ppm 419 ppm



30 % Excess air




Fig. 35.12 Comparison between straw pellets and mazut combustion in fluidized bed (a) carbon

monoxide emissions, (b) nitrogen oxides emissions, (c) combustion efficiency


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


• 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


• 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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clean combustion of low quality fuel in fluidized bed combustor

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