2236
Energy & Fuels 2008, 22, 2236–2240
Impact of the Addition of Chicken Litter on Mercury Speciation
and Emissions from Coal Combustion in a Laboratory-Scale
Fluidized Bed Combustor
Songgeng Li,* Shuang Deng, Andy Wu, and Wei-ping Pan
Institute for Combustion Science and EnVironmental Technology, Western Kentucky UniVersity,
Bowling Green, Kentucky 42101
ReceiVed February 26, 2008. ReVised Manuscript ReceiVed May 18, 2008
Co-combustion of chicken litter with coal was performed in a laboratory-scale fluidized bed combustor to
investigate the effect of chicken litter addition on the partitioning behavior of mercury. Gaseous total and
elemental mercury concentrations in the flue gas were measured online, and ash was analyzed for particlebound mercury along with other elemental and surface properties. The mercury mass balance was between 85
and 105%. The experimental results show that co-combustion of chicken litter decreases the amount of elemental
and total mercury in the gas phase. Mercury content in fly ash increases with an increasing chicken litter
share.
1. Introduction
Coal combustion is a significant source of mercury emissions
to the atmosphere, accounting for about one-third of anthropogenic mercury emissions in the United States.1,2 The emission
of mercury into the atmosphere and its transport away from
emission sources impose serious health concerns. Once deposited on soils or water bodies, mercury may accumulate and
concentrate within living organisms from the food chain, causing
various diseases and disorders to animals and humans.3 The
United States Environmental Protection Agency (U.S. EPA)
issued the Clean Air Mercury Rule (CAMR) on March 15, 2005,
which will cut mercury emissions from coal-fired power plants
by as much as 70% from the 1999 level by 2018. This will
affect the U.S. economy and environment.
Poultry litter disposal has received more attention in recent
years because of problems of arranging space large enough to
accommodate ever-increasing quantities of waste in landfills.4,5
It has been estimated that more than 5.6 million tons of litter
are produced annually by the poultry industry in the United
States.6 Poor waste management practices can cause serious
environmental concerns ranging from water and air pollution
to methane emissions, which may contribute to global warming.
Much research4,5 has shown that poultry litter having a caloric
value equivalent to low-rank coals can be combusted to generate
* To whom correspondence should be addressed. Current address:
Department of Chemical and Biomolecular Engineering, The Ohio State
University, 140 West 19th Avenue, Columbus, Ohio 43210. E-mail:
songgengli@hotmail.com.
(1) Electric Power Research Institute (EPRI). An assessment of mercury
emissions from U.S. coal-fired power plants. EPRI report 1000608, Oct.
2000.
(2) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; et al. Fuel Proc.
Technol. 2003, 82, 89–165.
(3) Boldger, P. T.; Szlag, D. C. EnViron. Sci. Technol. 2002, 36, 4430–
4435.
(4) Kelleher, B. P.; Leahy, J. J.; Henihan, A. M.; O’Dwyer, T. F.; Sutton,
D.; Leahy, M. J. Bioresour. Technol. 2002, 83, 27–36.
(5) Abelha, P.; Gulyurtlu, I.; Boavida, D.; Seabra Barros, J.; Cabrita,
I.; Leathy, J.; Kellehr, B.; Leathy, M. Fuel 2003, 82, 687–692.
(6) 29-097.asp″ xmlns:xlink)″http://www.w3.org/1999/xlink″>http://
www.ens-newswire.com/ens/aug2007/2007-08-29-097.asp.
energy. The use of poultry litter together with coal in power
plants seems to be a promising technique for the future in
contributing to fuel cost reduction and waste-disposal solutions.
In comparison to the conventional combustion technology,
fluidized bed combustion is considered an optimal technology
to dispose of animal waste with energy recovery because of 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
Co-combustion of poultry litter with coal has been studied
in a fluidized bed combustor.5,8 It was found that air staging
improved combustion efficiency and lowered NOx emissions.
Co-combustion of poultry litter significantly affected ash
formation, burnout efficiency, and emissions of NOx, SO2, and
chlorine compounds.5 These can affect mercury behavior
considerably.9,10 Unfortunately, few investigations on this aspect
have been reported.
In this work, an investigation on mercury speciation and
emissions during co-firing of chicken litter with coal was
performed in a laboratory-scale atmospheric bubbling fluidized
bed combustor. The influence of chicken litter share is described.
2. Experimental Section
2.1. Experimental Setup. All of the experiments were carried
out in a laboratory-scale atmospheric bubbling fluidized bed
combustor, as illustrated in Figure 1. The combustor is composed
of a stainless-steel pipe of 3 in. inner diameter and 3 feet in height,
placed in an electric-heated oven consisting of four silica carbon
rods, which preheat and make up for the heat loss from the bed.
Fluidization air is introduced into the bed through a distributor,
which is a 10 mm thick stainless-steel perforated plate with openings
(7) Nelson, J. Renewable Energy 1994, 5, 824–828.
(8) Henihan, A. M.; Leahy, M. J.; Leahy, J. J.; Cummins, E.; Kelleher,
B. P. Bioresour. Technol. 2003, 87, 289–294.
(9) Sable, S. P.; Jong, W.; Meij, E.; Spliethoff, H. Energy Fuels 2007,
21, 1883–1890.
(10) Lopes, M. H.; Abelha, P.; Oliveira, J. F. S. EnViron. Eng. Sci. 2005,
22, 205–220.
10.1021/ef8001422 CCC: $40.75  2008 American Chemical Society
Published on Web 06/28/2008
Chicken Litter on Mercury Speciation
Energy & Fuels, Vol. 22, No. 4, 2008 2237
Table 3. Ash Compositions of the Fuels Used
ash compositions (wt %)
sample
name SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O P2O5 TiO2 SO3
coal
49.9
chicken 35.6
litter
21.2
4.9
19.9
2.1
2.4
13.5
0.8
4.6
0.1
5.8
2.3 1.0
12.2 15.3
0.9
0.2
1.5
5.8
by mass flow controllers. A K-type thermocouple is employed to
measure the temperature within the combustor.
Fly ash is collected by the cyclone located at the outlet of the
combustor, while bed ash is discharged from the dense zone of the
combustor directly. The mercury content of the collected fly ash
and bed ash samples are determined by a LECO AMA-254
advanced mercury analyzer, an adsorption spectrophotometer
system. Gaseous mercury samplings are carried out at the outlet of
the cyclone. A PSA analytical semi-continuous emissions monitor-
Figure 1. Schematic diagram of a laboratory-scale fluidized bed
combustor.
Table 1. Experimental Conditions of Co-combustion Tests
chicken litter share on an energy basis (%)
total feed rate (kg/h)
excess air (%)
secondary air/total air
temperature in the bed region (°C)
temperature in the freeboard region (°C)
temperature of the cyclone (°C)
temperature at gaseous mercury sampling point (°C)
0-20
0.23-0.33
30
0.3
855-870
840-800
300-320
234-245
Figure 2. Gaseous mercury emissions and ratio of Hg0/Hg2+.
Table 2. Characteristics of the Fuel Used
parameter
moisture
ash
volatile matter
fixed carbon
carbon
hydrogen
oxygen
nitrogen
sulfur
ash
coal
Proximate Analysis (wt %)
2.6
9.4
31.6
56.4
chicken litter
11.3
24.8
57.8
6.1
Ultimate Analysis (wt %, Dry Basis)
71.3
28.2
5.3
5.0
8.8
35.0
1.4
3.4
3.5
0.9
9.7
27.5
Figure 3. SO2 and HCl concentrations in the flue gas.
Miscellaneous Analysis (Wet Basis)
mercury (ppm, wt)
0.13
<0.01
chloride (ppm, wt)
1537
11 639
Btu (lb)
12 948
5074
of 1 mm. A secondary air nozzle, which is a series of small holes
arranged on a ring of 1/4 in. stainless-steel tube, is located above
the bed surface to promote the mixing of air with fuel. The fuel
mixture is fed into the bed through a screw feeder. 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 windbox pressure is measured with a pressure
transducer to monitor the quality of fluidization. The air is regulated
Figure 4. Sulfur content in the collected fly ash.
2238 Energy & Fuels, Vol. 22, No. 4, 2008
Li et al.
Table 4. Contents of Mercury and Unburnt Carbon in Ash and
Their Surface Area
bed ash
fly ash
sample name
mercury
(ppm, wt)
mercury
(ppm, wt)
unburnt
carbon (%)
surface
area (m2/g)
0% chicken litter
4% chicken litter
10% chicken litter
20% chicken litter
<0.01
<0.01
<0.01
<0.01
0.13
0.27
0.35
0.44
13.3
13.5
13.1
13.3
8.5
9.1
8.9
9.8
ing system is used in this study to analyze the gaseous mercury
and its speciation in the flue gas. A detailed description of this
instrument can be found in the literature.8 Concentrations of major
flue gas components (O2, CO2, SO2, CO, NO, and HCl) are
measured after the cyclone using online flue gas analyzers. All data
are displayed and logged on a PC via a data acquisition unit.
2.2. Experimental Procedure and Conditions. Sand was
chosen as bed material because of its inert nature and availability.
To attain the steady-state combustion and stable bubbling mode, it
is essential to heat 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 a 1-1.5 h period was used to reach
the steady-state condition. The steady-state condition was defined
as maintaining a steady temperature profile and holding a steady
pressure drop. After a steady state was reached, concentrations of
major flue gas components were measured. Mercury speciation
analyses were conducted. The cyclone was emptied and cleaned
after each test, and fly ash and bed ash samples were collected.
The experimental conditions of the co-combustion tests are
shown in Table 1. In this experiment, baseline data was first
obtained for firing the coal alone and then co-firing tests at chicken
litter shares of 4, 10, and 20% on an energy basis 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 fluidizing air was kept
at 70% of the total air supply, and the rest was introduced into the
freeboard as secondary air, aiming to lower CO and NOx emissions.
The combustion air stoichiometry was kept at 1.3 for all of the
tests. During the tests, the temperature in the bed region was
maintained at about 860 °C, and the temperature in the freeboard
region varied from 840 to 800 °C along the height of the freeboard.
The temperatures at the location of the cyclone and gaseous mercury
sampling point were about 300 and 245 °C, respectively.
3. Results and Discussion
3.1. Fuel Characteristics. The proximate, ultimate, and mercury analysis of both chicken litter and coal employed in this
study are shown in Table 2. Table 3 shows the ash compositions
of both fuels. Chicken litter is composed mainly of sawdust,
wood chips, and fecal matter. The main characteristics of
chicken litter are its high level of volatile matter and very little
of fixed carbon. Also worth noting are high chlorine, calcium,
and ash contents in chicken litter because they might affect
mercury speciation. The coal used in this experiment is a
bituminous coal with high sulfur content. Mercury content of
the coal is 0.13 ppm, which is much higher than that of chicken
litter (less than 0.01 ppm). This indicates that mercury emissions
during co-combustion are mainly from the coal.
3.2. Mercury Speciation. Mercury emissions from coal and
waste combustion can be classified into three basic chemical
forms: gaseous elemental, oxidized mercury, and particle-bound
mercury. Oxidized and particle-bound mercury can easily be
removed by the existing air pollution control devices (APCDs)
in power plants.2 Removal of elemental mercury is more
challenging because of its low volatilization temperature, high
equilibrium vapor pressure, and low solubility in water.12
Figure 2 shows the measured average total (Hg0 + Hg2+)
and elemental mercury in the gas phase. It can be seen that the
gaseous total and elemental mercury decrease with an increase
in the share of chicken litter. Looking at the relative amount of
Hg0 and Hg2+ in the gas phase, it is found that the ratio of
Hg0/Hg2+ drops significantly. This means that more elemental
mercury is converted into oxidized mercury as chicken litter is
introduced. Generally, mercury compounds easily vaporize and
convert to elemental mercury at high temperature during
combustion. Once leaving the high temperature furnace environment, the elemental mercury tends to be oxidized under
appropriate conditions. The thermodynamic and kinetic study
indicates that the chlorination is the dominant conversion
pathway for mercury oxidation.13,14 During combustion, chlorine
is initially released primarily as atomic chlorine (Cl), which then
forms HCl and molecular Cl2 by reactions 1–3.15
Cl + H T HCl
(1)
2Cl T Cl2
(2)
HCl + OH T Cl + H2O
(3)
Mercury reacts with the chlorine species by reactions 4–7 to
produce oxidized mercury.13,14
Hg + Cl T HgCl
(4)
HgCl + Cl2 T HgCl2 + Cl
(5)
HgCl + HCl T HgCl2 + Cl
(6)
Figure 5. Mercury content as a function of the chlorine content in fly
ash.
HgCl + Cl T HgCl2
(7)
Table 5. Effect of Secondary Air on Mercury Emissions and
Chlorine Retention by Ash
(11) Cao, Y.; Duan, Y.; Kellie, S.; Li, L.; Xu, W.; Riley, J.; Pan, W.
Energy Fuels 2005, 19, 842–854.
(12) Lopez-Anton, M. A.; Tascon, J. M. D.; Martinez-Tarazona, M. R.
Fuel Proc. Technol. 2002, 77, 353–358.
(13) Sliger, R. N.; Kramlich, J. C.; Marinov, N. M. Fuel Proc. Technol.
2000, 65-66, 423–438.
(14) Laudal, D. L.; Brown, T. D.; Nott, B. R. Fuel Proc. Technol. 2000,
65-66, 157–165.
(15) Vogel., C. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1998,
43, 403–410.
20% chicken litter
without secondary air
with secondary air
particle-bound
gaseous mercury
chlorine in
mercury (ppm, wt)
(ng/Nm3)
fly ash
bed ash fly ash total elemental (ppm, wt)
<0.01
<0.01
0.32
0.44
4500
3800
2100
1721
3921
6554
Chicken Litter on Mercury Speciation
Energy & Fuels, Vol. 22, No. 4, 2008 2239
Figure 6. Mercury distribution and speciation: (a) 0% chicken litter, (b) 4% chicken litter, (c) 10% chicken litter, and (d) 20% chicken litter.
Chicken litter contains 1.16 wt % (Table 2) chlorine, which is
far greater than the chlorine concentration in the coal that was
used in the experiment. The average input flow of chlorine to
the reactor via chicken litter and coal increases with an
increasing chicken litter share. As a result, HCl concentrations
in the flue gas are higher (shown in Figure 3). This may explain
the fact that the ratio of Hg0/Hg2+ decreases with the introduction of chicken litter as indicated in Figure 2.
Although chlorine species dominate the oxidation of elemental
mercury, other flue gas components (such as SO2, NOx, and
H2O) may also affect mercury oxidation.14 Several research
studies16,17 indicate that SO2 inhibits the oxidation of mercury
in flue gas. The reaction mechanism suggested by Qiu et al.17
is that SO2 scavenges the OH radical in flue gas via reactions
8 and 9
HOSO2 + O2 T HO2 + SO3
(8)
HO2 + OH T H2O + O2
(9)
which results in less Cl radical formation according to reaction
3 and therefore less oxidation of mercury in flue gas. Figure 3
presents the SO2 emissions as a function of chicken litter share.
Obviously, SO2 emissions decrease with an increasing chicken
litter share. Two reasons cause this fact. First, chicken litter
has a low sulfur content in comparison to the coal. Chicken
litter addition actually dilutes the sulfur content of the fuel.
Second, chicken litter ash has a strong retention for sulfur
because of the relatively large amount of Ca and Mg present in
chicken litter ash. This is evidenced by the plot of the sulfur
content in the collected fly ash with chicken litter fraction in
Figure 4, which shows that there is an increase in sulfur content
of fly ash as the chicken litter share increases. The combined
effect reduces SO2 emissions as shown in Figure 3. From this
point of view, the addition of chicken litter is also beneficial to
the formation of oxidized mercury. Another factor in the
oxidation of elemental mercury is fly ash compositions. The
oxidation of elemental mercury may occur via a heterogeneous
reaction on fly ash. More evidence is needed to validate this
speculation.
Table 4 presents the content of mercury in the ash. It was
found that the content of mercury in the fly ash collected from
the cyclone is much higher than the samples taken out of the
bed. This is because, at the temperatures used for combustion,
mercury compounds vaporized and passed to the gas phase. The
gaseous mercury was carried away by the combustion gas, and
because the temperature at the cyclone was low (∼300-320
°C), the gaseous mercury was oxidized and adsorbed by the
ash.
(16) Kolker, A.; Senior, C. L.; Quick, J. C. Appl. Geochem. 2006, 21,
1821–1836.
(17) Qiu, J.; Sterling, R. O.; Helble, J. J. Development of an improved
model for determining the effects of SO2 on homogeneous mercury
oxidation. Presented at the 28th International Technical Conference, Coal
Utilization and Fuel Systems, Clearwater, FL, March 10-13, 2003.
Note that the content of mercury in the fly ash for cocombustion is much higher than that for coal alone. According
totheadsorptiontheoryandseveralexperimentalstudies,16,18unburnt
carbon and surface area are important factors for mercury
capture on ash. However, no obvious relation between this fact
and mercury retention was observed in this work because the
amount of unburnt carbon in ash and their surface area are
similar, as shown in Table 4. It implies that other factors play
an important role in mercury capture in the case of chicken litter.
Teller and Quimby19found that activated carbon impregnated
with chloride salts have as much as 300 times greater mercury
removal efficiency than virgin activated carbon. It has also been
shown that a flue gas stream containing HCl vapor can
impregnate activated carbon in situ and improve mercury
removal efficiency. From this perspective, this in situ impregnation is suspected to play a major role because high levels of
HCl in flue gas were observed during co-combustion of chicken
litter and coal. This is confirmed by the plot of the mercury
content with the chlorine content in the collected fly ash in
Figure 5, which shows that there is an increase in mercury
capture onto fly ash as the chlorine content of fly ash increases.
Many research studies5,20 have shown that secondary air
injection improves combustion efficiency and NOx control. More
recent reports21,22indicate secondary air also influences mercury
emissions. It is found that mercury emissions in flue gas
significantly decrease with an increase of the secondary air/
primary air ratio and the extent of the effect of the secondary
air on mercury emissions is strongly correlated with the chlorine
content of the coal and/or the amount of limestone added into
the bed. Because chicken litter has high chlorine and calcium,
secondary air might have a significant role in the mercury
emissions in the case of co-combustion of chicken litter. This
is verified by the results shown in Table 5. It can be seen that,
if all of the air is introduced as fluidizing air, mercury emissions
in flue gas is high, while the correspondent mercury and chlorine
in fly ash is low. Further studies are proceeding in our laboratory
to explore this mechanism.
3.4. Mercury Mass Balance. Mercury mass balance was
estimated on the basis of the total mercury input from the fuels
and the total mercury output including oxidized and elemental
mercury in the gas phase and particle-bound mercury in ash.
Mercury distribution and speciation diagrams for all of the tests
in this study are shown in Figure 6. They are presented as
elemental, oxidized, and particle-bound mercury divided, respectively, by total mercury input. The mass balance ranged
(18) Gibb, W. H.; et al. Fuel Proc. Technol. 2000, 65-66, 365–377.
(19) Teller, A. J.; Quimby, J. M. Presented at 84th Air and Waste
Management Association’s Annual Meeting and Exhibition, Vancouver,
British Columbia, Canada, June 1991.
(20) Okasha, F. Exp. Therm. Fluid Sci. 2007, 32, 52–59.
(21) Sable, S. P.; Jong, W.; Meij, E.; Spliethoof, H. Energy Fuels 2007,
21, 1891–1894.
(22) Liu, K.; Gao, Y.; Riley, J. T.; Pan, W. P.; Mehta, A. K.; Ho, K. K.;
Smith, S. R. Energy Fuels 2001, 15, 1173–1180.
2240 Energy & Fuels, Vol. 22, No. 4, 2008
between 85 and 105%. Note that the proportion of mercury in
ash increases considerably with the introduction of chicken litter.
This means more mercury can be removed by the APCDs in
coal-fired power plants instead of being emitted into the
atmosphere when co-firing chicken litter with coal.
4. Conclusions
Co-combustion tests of chicken litter with coal have been
performed in a laboratory-scale fluidized bed combustor to
study the effect of chicken litter on mercury speciation in
the gas phase and its capture on ash. Major conclusions from
the test results are summarized as follows: (1) Total gaseous
mercury and elemental mercury decreases with an increase
in the share of chicken litter. The ratio of Hg0/Hg2+ in flue
gas sharply decreased from 3.3 to 0.8 as the chicken litter
Li et al.
share increased up to 20% on an energy basis because of a
high chlorine content in chicken litter. (2) Mercury is enriched
in the fly ash and depleted in the bed ash as a result of initial
vaporization at high combustion temperature and subsequent
oxidization and sorption onto ash at low temperature. (3) The
content of mercury in the fly ash increases with an increasing
chicken litter share. This is attributed to high chlorine
contained in the fly ash with the introduction of chicken
litter.
Acknowledgment. The authors thank the United States Department of Energy (DE-FC26-03NT41840) and the United States
Department of Agriculture (6406-12630-002-02S) for their financial
support of this project.
EF8001422