F U EL P RO C ES S IN G TE CH N O L O G Y 8 9 ( 2 0 08 ) 7–1 2
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Effect of co-combustion of chicken litter and coal on emissions
in a laboratory-scale fluidized bed combustor
Songgeng Li⁎, Andy Wu, Shuang Deng, Wei-ping Pan
Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, KY, 42101 USA
AR TIC LE I N FO
ABS TR ACT
Article history:
Co-combustion of chicken litter (CL) with coal was performed in a laboratory-scale fluidized
Received 1 October 2006
bed combustor to investigate the effect of CL combustion on pollutant emissions. The
Received in revised form 20 May 2007
emissions of major gaseous pollutants including CO, SO2, H2S and NO and temperature
Accepted 20 June 2007
distribution along the combustor were measured during the tests. Effects of CL fraction and
secondary air on combustion characteristics were studied. The experimental results show
Keywords:
that CL introduction increases CO emissions and reduces the levels of SO2. The ratio of H2S/
Chicken litter
SO2 increases with increasing fraction of CL. NO emissions either increase or decrease
Co-combustion
depending on the percentage of CL in the mixed fuels. The temperature in the freeboard
Fluidized bed
region increases with increasing the fraction of CL while the reverse is true for the bed
Emissions
temperature.
© 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Litter from poultry farms is traditionally used in land applications as a fertilizer because it is rich in nutrients. But overapplication of this material could lead to an overabundance of
water nutrients resulting in eutrophication of water bodies, the
spread of pathogens, the production of phytotoxic substances,
air pollution and emission of greenhouse gases [1–3]. One of the
alternative disposal methods is direct combustion with the
potential to provide a cost-effective, environmentally benign
disposal route for the litter while providing for both space
heating of poultry houses and large-scale schemes involved
power generation or combined heat and power. However, the
high moisture and ash contents as well as low heating value of
the poultry litter could arouse problems on maintaining steady
and complete combustion of poultry litter alone, as indicated in
the literatures [1,4]. Therefore, co-firing poultry litter with coal
is considered as a feasible means.
Several investigations on this aspect have been reported
[4–6]. These research efforts have shown that co-combustion
could address the energy supply issues and aid in the solution
of air pollution control problems.
Compared with the conventional combustion technology,
the fluidized bed combustion is considered as an optimal
technology to dispose the animal waste with energy
recovery due to its ability to accept fuels with a relatively
high ash and moisture content, the low cost associated with
fuel preparation, operational flexibility with regard to ash
collection and easy control for pollutant emissions [7].
However, the use of poultry litter as a secondary fuel in a
fluidized bed combustor has not been yet investigated
extensively.
The purpose of this paper is to present the results obtained
from an experimental study on co-combustion of chicken
litter (CL) and coal in an atmospheric bubbling fluidized bed
combustor with respect to the emissions characteristics of
⁎ Corresponding author. Current address: Department of Chemical and Biomolecular Engineering, Ohio State University, 140 W 19th Ave.
Columbus, OH43210, USA. Tel.: +1 614 292 4935; fax: +1 614 292 3769.
E-mail address: [email protected] (S. Li).
0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2007.06.003
8
F U EL P RO CE S S IN G TE CH N O L O GY 8 9 ( 2 0 08 ) 7–1 2
carried out on the gas stream exiting from the cyclone.
Concentrations of SO2, CO and H2S are measured with on-line
infrared gas analyzers. NOx concentration is measured with an
on-line chemiluminescence detector. O2 concentration is monitored using a paramagnetic sensor. All data is displayed and
logged on a PC via data acquisition unit. A K-type thermocouple
is employed to measure the temperature within the combustor.
2.2.
Experimental procedure
To attain the steady-state combustion and stable bubbling
mode, it is essential to heat up the inert bed material above the
ignition temperature of the fuel. After the required temperature
was reached, the fuel was slowly fed into the bed. The duration
of the test run was about 4 h, out of which 1–1.5 h period was
used to reach the steady state condition. The steady state
condition criteria were to have steady bed temperature and
steady pressure drop. After steady state was reached, flue gas
composition was measured. The cyclone were emptied and
cleaned after each test, and fly ash samples were collected.
2.3.
Fig. 1 – Schematic diagram of the laboratory-scale fluidizedbed combustor.
gaseous pollutants including CO, SO2, H2S and NOx. Specially,
the influence of CL mass fraction in the mixture and
secondary air is described.
2.
Experimental
2.1.
Experimental set-up
All the experiments were carried out in an atmospheric
bubbling fluidized bed combustor, as illustrated in Fig. 1. The
combustor is a stainless steel pipe of 76 mm I.D. and 1.2 m in
height, placed in an electrically heated oven consisting of
total four silica carbon rods that preheat and make up for the
heat loss from the bed. Fluidization air is introduced into the
bed through a distributor, which is 10 mm thick stainless
steel perforated plate with openings of 1 mm. A secondary
air nozzle, which is a series of small holes arranged on a ring
of 1/4 inch stainless steel tube, is located above the bed
surface in order to promote the mixing of air with fuel. The
fuel mixture is fed into the bed through a screw feeder. A
minor compressed air is introduced into the fuel silo in order
to avoid the flue gas back flow. The bed height can be
controlled by adjusting the inserted height of the discharge
tube from the bottom. Bed materials including the fuel ash
and inertial sand come out of the combustor through the
discharge tube once the bed height exceeds the height of the
discharge tube inside the combustor. The pressure right
before the air distributor is measured with a pressure
transducer to monitor the quality of fluidization. The air is
regulated by mass flow controllers based on the thermal
mass flow sensing technique. The analysis of the flue gas is
Characteristics of fuels
The ultimate and proximate analysis of both CL and coal are
presented in Table 1. CL includes sawdust, wood chip and fecal
matter. As a fuel, it has lower heating value equivalent to low rank
coal (on the order of 5000 BTU/lb) due to high moisture, oxygen and
ash content. High level of volatile matter and very little of fixed
carbon imply that most of combustion for the litter takes place in
the gas phase. The coal used in this experiment is a bituminous
coal with relatively higher sulfur and lower chlorine. Table 2
shows the ash composition of both fuels. Compared with the coal
ash, the ash of CL contains relatively high Ca, K and Na and low Si.
This probably indicates that chicken litter ash has relatively lower
fusion temperature and higher sulfur-retention ability.
3.
Results and discussion
The experimental conditions of the co-combustion tests are
shown in Table 3. In this experiment, baseline data was first
Table 1 – Characteristics of the fuels used
Parameter
Coal
CL
Proximate analysis (%)
Moisture
Ash
Volatile matter
Fixed carbon
2.6
9.4
31.6
56.4
11.3
24.8
57.8
6.1
Ultimate analysis (%,dry)
Carbon
Hydrogen
Oxygen
Nitrogen
Sulfur
Ash
71.3
5.3
8.8
1.4
3.5
9.7
28.2
5.0
35.0
3.4
0.9
27.5
1537
12948
11639
5074
Miscellaneous analysis
Chloride(ppm)
Btu(1b)
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F U EL P RO C ES S IN G TE CH N O L O G Y 8 9 ( 2 0 08 ) 7–1 2
Table 2 – Ash compositions of the fuels used
Sample
name
Coal
CL
Ash compositions (%)
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
P2O5
TiO2
SO3
49.9
35.6
21.2
4.9
19.9
2.1
2.4
13.5
0.8
4.6
0.1
5.8
2.3
12.2
1.0
15.3
0.9
0.2
1.5
5.8
obtained for firing the coal alone and then co-firing tests at CL
mass fractions of 10%, 25% and 50% were performed. The total
calorific input of the fuel was maintained almost constant by
regulating the total feed rate, ensuring that similar combustion conditions existed within the combustor. The coal was
physically mixed with the CL. The furnace temperature and
combustion air stoichiometry were kept at 800 °C and 1.3,
respectively, for all the tests. The ratios of secondary air/total
air were controlled at around 0.3. The test with no secondary
air was also performed in the case of 50% CL to make
comparison.
3.1.
Temperature profile
Fig. 2 shows the axial temperature distributions observed
along the combustor height. It can be seen that the temperature in the bed region is maintained in the range of 860–
880 °C, and the temperature in the freeboard region varies in
the range of 800–860 °C. The temperature drops slightly in the
secondary air injection area owing to the secondary air
consuming energy and increases significantly above the area
because the unburned particles and volatile matter are burned
with the contribution of secondary air.
In addition, in the freeboard region except for the
secondary air injection area, the temperature for the cocombustion is higher than that for the coal combustion alone,
while the reverse is true for the temperature in the bed region.
In fact, the higher the fraction of CL is, the higher is the
freeboard temperature. This temperature variation indicates
Table 3 – Experimental conditions of co-combustion tests
Run
Blend
number fuel
1
2
3
4
5
0%CL/
100%
Coal
10%
CL/
90%
Coal
25%
CL/
75%
Coal
50%
CL/
50%
Coal
50%
CL/
50%
Coal
Feed
rate
(Kg/h)
Furnace
Excess Secondary
temperature air (%) air/total
(°C)
air
0.228
800
30
0.3
0.245
800
30
0.3
0.279
800
30
0.3
0.330
800
30
0.3
0.330
800
30
0
that CL is burned mostly in the freeboard region. This
observation can be explained by the fact that CL has high
volatile matter and low fixed carbon content.
3.2.
CO emissions
The emissions of CO as a function of CL mass fraction are
shown in Fig. 3. It is obvious that CO emissions increase with
an increase of CL mass fraction. Three reasons can be
invoked to account for this: (i) Higher amount of volatile
matter released from CL boosts hydrocarbon concentration
in the combustion atmosphere, which inhibits the further
oxidation of CO. Normally, oxidation of CO occurs through
reactions (1) and (2) with oxygenated free radicals HO and
HO2 [8]. These radicals react easier with hydrocarbons than
with CO.
CO þ OH↔CO2 þ H
ð1Þ
CO þ HO2 ↔CO2 þ HO
ð2Þ
(ii) CL contains high level of chlorine. The formation of HCl
inhibits the oxidation of CO through consuming the radicals
OH and HO2 [9]:
HO2 þ Cl↔HCl þ O2
ð3Þ
H þ Cl þ M↔HCl þ M
ð4Þ
HCl þ OH↔H2 O þ Cl
ð5Þ
(iii) Unburned volatile matter constitutes an additional CO
source.
Fig. 2 – Temperature profile along the combustor height.
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F U EL P RO CE S S IN G TE CH N O L O GY 8 9 ( 2 0 08 ) 7–1 2
Fig. 3 – CO emissions with secondary air.
3.3.
SO2 and H2S emissions
SO2 emissions normally correlate strongly with sulfur content
of the fuel. As mentioned above, CL has low sulfur content in
comparison to the coal. CL addition actually dilutes the sulfur
content of the fuel. Therefore, SO2 emissions decrease with an
increasing CL share as shown in Fig. 4.
Fig. 5 presents the percentage of fuel-S conversion to SO2 as a
function of CL fraction. Obviously, fuel-S conversion decreases
with increasing CL fraction. Two reasons could cause this fact.
Firstly, CL ash has strong retention for sulfur due to relatively
large amount of Ca and Mg present in CL ash. This is evidenced
by the plot of the sulfur content in the collected fly ash with CL
fraction in Fig. 6, where shows that there is an increase in sulfur
content of fly ash as CL fraction increases. Secondly, high
volatiles CL creates strongly reducing atmosphere above the bed
that inhibits the oxidation of H2S to SO2. The variation of H2S/
SO2 ratio with CL fraction (shown in Fig. 7) strongly supports this
point. From Fig. 7, it was found that the ratio of H2S/SO2
increases with a decrease in CL fraction. H2S emissions increase
when a little chicken litter is introduced and decrease with
further increasing CL mass fraction due to low fuel-S input.
3.4.
NO emissions
Under fluidized bed conditions, NO and N2O emissions
originate from the nitrogen of the fuel, so the formation of
Fig. 4 – SO2 emissions with secondary air.
Fig. 5 – Fuel-S conversion rates with secondary air.
thermal NOx is considered insignificant [10,11]. Nitrogen in CL
is mainly released as NH3 [1], which could reduce the NOx
produced from within the bed to N2 according to reaction (6),
or further give NOx according to reaction (7) [12].
2NO þ 4NH3 þ 2O2 ↔3N2 þ 6H2 O
ð6Þ
4NH3 þ 5O2 ↔4NO þ 6H2 O
ð7Þ
Fig. 8 presents NO emissions as a function of CL mass
fraction. It was observed that NO emissions follow an
increasing trend in the case of 10% and 25% CL because of
higher nitrogen contained in CL. However, further increasing
the percentage of CL (in the case of 50% CL), NO emissions
reduce. The probable reason is that in low percentage of
chicken litter, the volatile matter released in the freeboard
could not create strongly reducing atmosphere to suppress the
formation of NOx via reaction (7). With increasing chicken litter
share up to a certain extent, volatiles evolved from the fuel
produce instantaneous fuel-rich condition in the freeboard
region, which leads to the reduction of NOx via reaction (6).
3.5.
Effect of secondary air
In order to elucidate the effect of secondary air on pollutant
emissions, the test without secondary air introduced into
Fig. 6 – Sulfur content in the collected fly ash with secondary
air.
F U EL P RO C ES S IN G TE CH N O L O G Y 8 9 ( 2 0 08 ) 7–1 2
Fig. 7 – H2S emissions and ratio of H2S/SO2 with secondary
air.
the freeboard region was conducted in comparison with
above-mentioned experimental results. This means that all
the air is introduced as fluidizing air. The obtained results
are shown in Fig. 9. As expected, CO emissions significantly
increase owing to the insufficient mixing of fuel with air.
The increment of SO2 emissions is attributed to the less gas–
solid contact in the freeboard region that makes against the
retention of SO2 by ash. There is no reducing zone formed in
the bed region since all the air is introduced as fluidizing air.
Hence, the level of NO with no secondary air introduction
increases. According to these results, it can be concluded
that secondary air should be used and effectively organized
to improve the combustion efficiency and decrease the
pollutant emissions.
4.
Conclusions
Co-combustion tests of coal with CL have been performed in a
laboratory-scale fluidized bed combustor to study the effect of
CL mass fraction and secondary air on the combustion and
emission characteristics of the major gaseous pollutants.
11
Fig. 9 – Effect of secondary air on pollutant emissions in the
case of 50% CL.
Major conclusions from the test results can be summarized
as follows:
• As CL mass fraction increases, the bed temperature
decreases and the temperature in the freeboard region
increases. This is attributed to low fixed carbon and high
volatile matter contained in CL.
• The introduction of high volatile matter CL causes CO
emissions to be increased.
• SO2 emissions are lowered by addition of CL as a result of
fuel-S dilution and CL ash derived natural desulfurization.
Ratio of H2S to SO2 increases with a decrease of CL mass
fraction because high volatile matter released from CL
creates a strong reducing atmosphere that suppresses the
oxidation of H2S.
• Introduction of CL at low concentrations causes higher NO
emissions because more fuel-N is introduced. However,
high levels of CL may reduce NO emissions due to the larger
amount of released volatile matter, which suppresses the
formation of NO.
• Secondary air introduction contributes to lower pollutant
emissions.
Acknowledgement
The authors wish to thank the United State Department of
Energy (No. DE-FC26-03NT41840) and the United State Department of Agriculture (No.6406-12630-002-02S) for their financial
support of this project.
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Fig. 8 – NO emissions as a function of CL mass fraction with
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