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Chemosphere 240 (2020) 124853
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Chemosphere
journal homepage: www.elsevier .com/locate/chemosphere
Investigation on gaseous pollutants emissions during co-combustion
of coal and wheat straw in a fluidized bed combustor
Zeyu Xue, Zhaoping Zhong*, Xudong Lai
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, 210096, China
h i g h l i g h t s
� Co-combustion decreased SO2, NO and CO emission and carbon content in fly ash.
� Releases of Pb, Zn and Cd are restrained in co-combustion.
� High temperatures promote the transfer of Zn, Pb and Cd in bottom solids.
� Melting in the slag affected the release of Cd and Zn.
� Secondary air promote the release of heavy metals.
a r t i c l e i n f o
Article history:
Received 24 June 2019
Received in revised form
7 September 2019
Accepted 13 September 2019
Available online 19 September 2019
Handling Editor: X. Cao
Keywords:
Co-combustion
Wheat straw
Coal
Pollutant emission
Heavy metals
* Corresponding author.
E-mail address: zzhong@seu.edu.cn (Z. Zhong).
https://doi.org/10.1016/j.chemosphere.2019.124853
0045-6535/© 2019 Elsevier Ltd. All rights reserved.
a b s t r a c t
Co-combustion of coal and wheat straw (WS) was conducted in a lab-scale BFB combustor. Fuel
composition (coal, 70%coalþ30%WS), temperature (750, 800, 850, 900, 950 �C), secondary air ratio (0,
10%, 20%, 30%) were varied to on the release of gaseous pollutant was studied. CO, NOx and SO2 con-
centration in flue gas (FG) were measured on-line by a flue gas analyzer. Fly ash (FA), bottom slag (BS)
and bed material (BM) were collected, digested and analyzed by ICP-OES to determine the distribution of
heavy metals (e.g. Pb, Zn, Cr and Cd). Results indicated that co-combustion could improve the com-
bustion of coal alone by reducing CO, NOx and SO2 emission and carbon content in fly ash effectively. In
co-combustion the increasing secondary air could reduce CO emission and SO2 by enhancing disturbance
and promoting sulfation respectively while the minimum NO emission was reached at the ratio of 20%.
Co-combustion restrained the release of Zn, Cd and Pb compared with coal combustion alone. In co-
combustion, high temperature increased their portion in the flue gas. For Zn, Pb and Cd, their content
in the bottom solids increased while the portion of Cr decreased. Secondary air decreased their content in
fly ash and transferred into flue gas significantly and in bottom solids content of Zn and Pb decreased
while Cd increased.
© 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Biomass is one of the earliest energy resources in human’s so-
ciety for its accessibility and affordability. It has also been ranked as
the forth energy resource after coal, petroleum and nature gas. Due
to its carbon neutrality, biomass plays an essential role in tacking
climate change and energy shortage. However, during the thermal
treatment of biomass, slagging and fouling could occur due to its
high content of alkali and alkaline-earth metals. Co-combustion of
biomass and coal could mitigate these problems caused by biomass
combustion independently. Guo and Zhong (2018) conducted the
co-combustion experiment and proved that biomass could enhance
the ignition property and promote the coal combustion reaction.
Armesto et al. (2013) claimed that an appropriate proportion of
biomass did not affect the combustion efficiency. Co-combustion of
coal and biomass has already been applied in many existing ther-
mal power plants. At present, the global installed capacity of coal-
biomass co-fired power generation was about 100 GW in 2015 and
the maximum value of mass biomass ratio in most power plants
reported was about 20e30% (De and Assadi, 2009). Many re-
searches concerning co-combustion have been carried out so far.
Co-combustion was reported to significantly reduce the release of
both NOx and SOx in existing pulverized-coal fire plants (Sami et al.,
2001). Sung et al. (2016) found that co-combustion obviously
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1
3
primary air inlet
cooling water
inlet
secondary air
inlet
4
5 6
7
9
8
11
13
5
12
emission
10
2
Fig. 1. Schemes of the bubble fluidized bed in this study. (1). hopper (2). primary
feeder (3). secondary feeder (4). air compressor (5). flowmeter (6). air pre-heater (7).
temperature controller (8). electric heater (9). cyclone (10). filter (11). absorption bottle
(12). gas meter (13). gas analyzer.
Z. Xue et al. / Chemosphere 240 (2020) 1248532
reduced SO2 release due to the low contents of S and N of biomass
at high temperatures. Hu et al. (2014) revealed that the decrease of
SO2 emissions during co-combustion could also be owing to the
capture effect by alkali and alkali earth metals from biomass
through sulfation reactions. Basu et al. (2011) contributed the
decrease of NOx emission in co-combustion to the high volatile
matter content of biomass, which could inhibit the formation of
NOx through oxidation-reduction reactions in the dense phase.
Vekemans et al. (2016) confirmed that the adverse effect on the
decrease of SO2/NOx emission was enhanced by the amount of
biomass.
Compared with the pollutants produced by constant elements,
amounts of heavy metals in the coal are relatively low however
these trace elements have been paid special attention due to their
high mobility and bio-toxicity during combustion in recent years.
According to the evaporation characteristics of the element, heavy
metal could be divided into three groups: volatile element (Hg and
Se), semi-volatile element (Pb, Cd, Zn), non-volatile element (Cr, Ni,
Cu, Mn) (Luan et al., 2009). As the result, Cr and Ni transferred into
flue gas mainly by entrainment, Hg and Se by evaporationwhile Pb,
Cd and Zn were affected by the synthetic effects. Li et al. (2018)
indicated that the concentration of several HTEs increased with
the decreasing size in the fly ash.
The evaporation of heavy metal was greatly facilitated by the
rising temperature. In addition, the migration of heavy metals was
affected by other relevant elements. Wang et al. (2016b) conducted
the thermodynamic calculation and found S could impact on Pb and
Cd transformation in presence of Na and Ca. During combustion,
release of Pb could be suppressed by Si through glass transition,
which existed stably in combustion (Sekine et al., 2008;Wang et al.,
2016a). However, Nakada et al. (2008) thought that Pb glass phase
substance was not stable at high temperatures and could be
affected by Cl in the fuels. To be specific, Cl was regarded to
contribute to the release of heavymetals for the lower boiling point
of chlorides than oxides (Zhang et al., 2019). By thermodynamic
calculation, Zn mainly existed in the aluminum-silicates while Cd
may release in the form of Cd(g) and CdCl2(g). Formation of
CdO$SiO2 and CdO$Al2O3 suppressed the migration of Cd. Pb could
be captured by Al, Si to form corresponding aluminates and sili-
cates. Shah et al. (2012) found that Cr mainly existed in Cr(III) in the
bottom slag while Cr(VI) in the fly ash. Belevi and Moench (2000)
claimed that excess air ratio significantly affected the release of
Cu and Zn while had a tiny effect on Cd and Pb during combustion.
Despite the numerous studies concerning gaseous pollutants
during coal combustion before, relative reports on co-combustion
of biomass and coal are rare. In fact, interaction between biomass
and coal during co-combustion is an area in particular need of
study. In order to gain emission behavior of pollutants during co-
combustion, a lab-scale bubbled fluidized bed combustor was set
up to conduct the co-combustion of coal and biomass particles. In
this research, secondary air ratio, combustion temperature,fuel
composition and secondary air ratio were variated to study the
effect on the release characteristics of NOx, SO2 and heavy metals
such as Pb, Zn, Cd and Cr. In this research, biomass mass fraction in
the blends was set as 30% at maximum biomass ratio in present co-
combustion power plants.
2. Experimental work
2.1. Facility and operation process
The experiments were run in a lab-scale bubble fluidized bed
(BFB) combustor (Fig. 1). The BFB system is consisted of two-stage
screw feeder system, two-stage air inlet system, electric heating
furnace, and sample collection system. To prevent pre-pyrolysis/
combustion in the delivering fuels, a water cooling tube was
equipped in the secondary feeder. The main part of the combustor
is 700mm in height and its inner diameter is 32mm. The tem-
peratures are measured by thermocouples located in the dense
phase and dilute phase respectively. Temperature of the dense
phase was selected as combustion temperature, which was
controlled by heating procedure set in advance.
In the dilute zone an intake pipe was introduced to supply
secondary air and the inlet was 150mm below the top part of the
combustor. The flue gas flowed through a cyclone and a filter
respectively to collect fly ash and then analyzed by a gas analyzer
(MGA5, MRU, Germany), which could monitor combustion condi-
tions inside the furnace. Gaseous heavy metal in the flue gas was
captured by two adsorption bottles containing 5% HNO3þ10% H2O2
solution in ice bath. The outlet pipewas enwound by heating belt of
which heating temperature was adjusted at 300 �C to avoid
condensation of flue gas in flow and ensured gaseous fraction of
heavy metals and sulfur could be captured to the greatest extent.
During experiment, air compressor and circulating water pump
started running firstly, then the dense phase was heated at 10 �C/
min from room temperature to the set temperature and kept stable
for 20min, then the fuel was fed into the furnace by screw feeder.
When the temperature exceeded ignition point, fuel could burn
spontaneously in air atmosphere. The fluidized velocity designed
was set as 0.07m/s and the feed quantity of the fuel was about 25 g/
h, which was adjusted by the combustion efficiency monitored by
flue gas analyzer. Each operating condition lasted for 2 h. After each
experiment and the furnace cooled down to room temperature, the
air distributor was detached and the blends of bottom slag and bed
material were collected and weighted.
2.2. Material
The selected fuels for the co-combustion in this study were coal
and wheat straw, collected from Jining City, Shandong Province and
Xingtai City, Hebei Province in China respectively. WS has the ad-
vantages of its high yield in northern China. To avoid agglomeration
brought by alkali metals in WS, the bed material was selected as
bauxite from Shangqiu City, Henan Province (Liu et al., 2007; Zhang
Table 3
Sizes of the materials applied in the research.
WS coal bauxite
True density/gˑcm�3 0.52 1.21 2.64
Size/mm 1.4e1.8 0.8e1.2 0.25e0.4
Z. Xue et al. / Chemosphere 240 (2020) 124853 3
et al., 2018). The materials and their characteristics are listed in
Tables 1e4.
It could be inferred that compared with the average content in
the Chinese and world’s coal, Cd is similar while Pb is between the
two values. Contents of Zn and Cr are both higher than averages
values (Ketris and Yudovich, 2009; Dai et al., 2012).
Table 4
Heavy metal content in the coal, mg/kg.
Pb Zn Cd Cr
Coal in this study 12.91 57.38 0.24 42.4
Average content in China 15.1 41.4 0.25 15.4
Average content in the world 7.8 23.0 0.22 16.0
Table 5
Experimental conditions in the fluidized bed combustor.
Case Fuel composition Temperature/�C Secondary air ratio
1 coal 750 0
2 coal 800 0
3 coal 850 0
4 coal 900 0
5 coal 950 0
6 70%coalþ30%WS 750 0
7 70%coalþ30%WS 800 0
8 70%coalþ30%WS 850 0
9 70%coalþ30%WS 900 0
10 70%coalþ30%WS 950 0
11 70%coalþ30%WS 850 10%
12 70%coalþ30%WS 850 20%
13 70%coalþ30%WS 850 30%
2.3. Experimental conditions and data analysis
Experimental conditions in the fluidized bed reactor are listed in
Table 5. Effects of fuel composition, combustion temperature and
secondary air ratio on the emission characteristics of pollutants
have been investigated.
The concentration of SO2, CO and NO detected in the experiment
should be transformed by the following Eq. (1).
r¼ r’ � 21� 4ðo2Þ
21� 4’ðo2Þ
(1)
where, r and r0 represent the equivalent value and measured value
of gaseous pollutant concentration in the flue gas, respectively, mg/
m3, 4(O2) and 40(O2) are the reference and measured value of O2
content in the flue gas, %. 4(O2) is set as 3.5 in this research as to
make excess air ratio as 1.2, which can be obtained by Eq. (2).
a ¼ 21
21� 4’ðO2Þ
(2)
After combustion and natural cooling, blends of bottom slag (BS)
and bed material (BM), fly ash (FA) in the cyclone were collected
and weighted. The absorption in the two bottles was blended and
diluted in certain proportion. To measure the total content of heavy
metal in the ash residue, EPA 3052 was referred (United States
Environmental Protection Agency (USEPA), 2016). Afterwards, the
concentration of the digestion solution was analyzed by ICP-OES
(Optima 8000, PerkinElmer, USA). The distribution of the element
in each phase was calculated according to Eq. (3) ~ (6).
A¼u� V þmFA �MFA þ mB �MB (3)
Table 1
Composition of fuel ash prepared in muffle furnace at 815 �C in air and bed material
determined by XRF.
Coal ash WS ash Bauxite
SiO2 30.96 53.0 57.31
TiO2 0.829 0.083 1.09
Al2O3 19.040 0.649 28.78
Fe2O3 9.788 0.755 3.39
MnO 0.171 1.051 0.102
MgO 1.114 3.59 0.428
CaO 3.94 7.46 1.25
K2O 0.223 20.9 0.856
Na2O 0.534 0.597 0.282
P2O5 0.290 2.78 0.072
SO3 13.757 1.68 0.004
Cl 0.029 8.25 0.013
Table 2
Proximate and ultimate analysis of fuels, wt%.
Sample Ultimate analysis(wt%, dry and ash-free basis)
N C H O
Coal 1.19 89.73 2.83 2.45
WS 1.44 51.67 6.21 40.45
RFG ¼
u� V
A
� 100% (4)
RFA ¼
mFA �MFA
A
� 100% (5)
RB ¼
mB �MB
A
� 100% (6)
where, A is the total content of the element detected, mg. RFG is the
release ratio in the combustion, u is the concentration of the
absorbing solution, mg/mL, V is the volume of the absorbing solu-
tion, mL, m is the weight of the ash/slag collected, g, M is the trace
metal content in the ash or slag, mg/g, subscript FG, FA and B
represent the flue gas, fly ash and blends of bottom slag and bed
material, respectively.
To measure carbon content in fly ash, it was firstly dried in oven
at 105 �C for 6 h to remove free water then about 5 g of the dried fly
ash was weighted and burned in muffle furnace at 850 �C for
40min. When cooled down, the calcined fly ash was weighted
again. Carbon content (f) was determined by Eq. (7).
f¼G1 � G2
G1
� 100% (7)
Proximate analysis(wt%, dry basis)
S Ash Volatile matter Fixed carbon
3.80 25.96 10.23 63.81
0.23 14.80 64.99 20.21
400
420 coal
co-firing
1000
Z. Xue et al. / Chemosphere 240 (2020) 1248534
G1 and G2 represent themass of dried fly ash and calcined fly ash
respectively.
750 800 850 900 950
240
260
280
300
320
340
360
380
SO
2
co
nc
en
tr
at
io
n/
pp
m
N
O
co
nc
en
tr
at
io
n/
pp
m
T/ C
440
460
480
700
800
900co-firing
coal
Fig. 3. Effect of temperature on NO and SO2 emission in combustion.
3. Results and discussion
3.1. Effect of temperature on CO, SO2 and NOx emission
Results of CO emission in coal combustion and co-combustion
are represented in Fig. 2. As the temperature rose from 750 to
950 �C, CO content in the flue gas decrease from 150 to 89 ppm for
coal combustion alone. In the presence of WS, CO content in the
flue gas decreased significantly due to its improvement in the
combustion efficiency. This promotion was weakened with the
rising temperature. Carbon content in the fly ash resembled that of
CO with the temperature. Carbon content in fly ash could also be
linked to the combustion conditions and higher carbon content
reflected the incomplete combustion. Co-combustion reduced the
carboncontent in fly ash, showing the same varying tendency with
CO emission.
SO2 and NO emissions are the most common gaseous pollutants
derived from coal combustion. S mainly exists in pyrite, thiophene
and thiols in coal and nearly all the released sulfur was converted to
SO2(g) under fuel-lean conditions (Kazanc et al., 2011). As for NOx in
the BFB combustion, it mainly transformed from the oxidation of
fuel-N owing to the relatively low temperature. Fig. 3 shows the
variation of NO and SO2 emission as the function of temperature
from 750 to 950 �C. NO concentration in the flue gas increased from
333 to 408 ppm and 250e366 ppm in coal combustion and co-
combustion respectively. High temperature could promote of oxi-
dization of sulfur compounds by high temperature via Eq. (8). SO2
emission increased monotonically from 790 to 1033 ppm in coal
combustion.
4FeS2 þ11O2ðgÞ/2Fe2O3 þ 8SO2ðgÞ (8)
In co-combustion, SO2 concentration increased from 750 to
850 �C and then fluctuated in the range of 850e950 �C. The
decrease at 900 �C could be owing to the effect of sulfation reaction
in Eq. (9). After 900 �C, the rate of sulfation was exceeded by
oxidation and decomposition of sulfate again and caused the slight
increase of SO2 emission.
4KClþ2SO2ðgÞ þ 2H2OðgÞ þ O2ðgÞ/2K2SO4 þ 4HClðgÞ (9)
750 800 850 900 950
40
60
80
100
120
140
160
C
O
co
nc
en
tr
at
io
n/
pp
m
C
ar
bo
n
co
nt
en
ti
n
fl y
as
h/
%
T/ C
coal
co-firing
2
3
4
5
6
7
8
9
coal
co-firing
Fig. 2. Effect of temperature on CO emission and carbon content in fly ash in
combustion.
3.2. Effect of secondary air on CO, SO2 and NOx emission
As shown in Fig. 4, carbon content in fly ash decreased from
6.26% to 4.27% and 3.39 to 2.72% in coal combustion and co-
combustion respectively as the secondary air ratio increased from
0 to 30%. On one hand, the introduction of secondary air decreased
the fluidized velocity in dense phase, thus increasing the residence
time of the fuel particles in the furnace. On the other hand, the
disturbance brought by secondary air in the dilute phase enhanced
the gas-solid mass transfer, promoting the complete combustion of
the fuel particles. For CO emission, in the co-combustion it
decreased from 75 to 61 ppm during co-combustion. In the pres-
ence of the secondary air, there brought an anoxic environment in
the dense phase where produced large amount of CO, which could
drastically convert into CO2 in the dilute phase with the oxidation
effect of secondary air. However, in coal combustion it decreased
from 119 to 104 ppm at first and then increased to 108 ppm at the
secondary air ratio of 30%. The final twist may be attributed to the
incomplete transformation from CO to CO2 due to short residence
time in the oxidation atmosphere.
Fig. 5 shows the effect of secondary air ratio on NO and SO2
emission. With the increase of secondary air ratio, reducing at-
mosphere in the dense phase prompted the reduction of NO.
Additionally secondary air lowered the temperature in the furnace
and suppressed the formation of NO. When secondary air ratio was
0 10 20 30
60
65
70
75
100
105
110
115
120
C
ar
bo
n
co
nt
en
ti
n
fly
as
h/
%
Secondary air ratio/%
C
O
co
nc
en
tr
at
io
n/
pp
m
coal
co-firing
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
co-firing
coal
Fig. 4. Effect of secondary air ratio on CO emission and carbon content in fly ash in
combustion at 850 �C.
0 10 20 30
240
260
280
300
320
340
360
380
Secondary air ratio/%
N
O
co
nc
en
tr
at
io
n/
pp
m
coal
co-firing
300
375
450
900
975
1050
co-firing
coal
SO
2
co
n c
en
tr
at
io
n/
pp
m
Fig. 5. Effect of secondary air ratio on NO and SO2 emission in combustion at 850 �C.
Z. Xue et al. / Chemosphere 240 (2020) 124853 5
at 30%, N-containing component in the volatiles produced by WS
may react roughly in presence of abundant of oxygen, resulting in a
slight increase of NO during co-combustion. As for SO2 emission, it
was also restrained by the increasing secondary air amount due to
lower temperature. Sulfation reaction in the dilute zone could also
be enhanced by abundant O2 content introduced by secondary air.
3.3. Effect of temperature on trace metal distribution
Zn distribution in the combustion is shown in Fig. 6. Content of
Zn in the bottom solid increased with the temperature from 750 to
950 �C. Higher temperatures promoted the evaporation of Zn spe-
cies, especially chlorides.
It could be seen that Zn was retained in the bottom solids as
temperature increased in both cases. It is also generally believed
that some heavy metals usually existed in the oxides or composite
oxides in bottom solids (bottom slag and bed material). In fact, Zn
could react with Al/Si species and generate stable aluminates and
silicates via Eq. (10) ~ (11) (Diaz-Somoano and Martinez-Tarazona,
2005).
ZnOþAl2O3/ZnAl2O4 (10)
750 800 850 900 950
0
10
20
30
40
50
60
70
80
90
100
R
el
at
iv
e
m
as
sd
i s
tr
ib
ut
io
n
of
Z
n/
%
T/ C
FG
FA
B
BABA BA BA BA
Fig. 6. Relative mass distribution of Zn in the combustion. A: coal combustion; B: co-
combustion.
2ZnOþ SiO2/Zn2SiO4 (11)
However, co-combustion was unfavorable for chemical ad-
sorptions above because of poor contact caused by eutectic com-
pound produced via Eq. (12) (Wang et al., 2016b). Melting and
slagging may slightly inhibit the contact of Zn with bed materials,
decreasing its content in bottom solids.
2KClþnSiO2 þ H2OðgÞ/K2O,nSiO2 þ 2HClðgÞ (12)
During coal combustion and co-combustion, changes of Zn
release were rather minor, from 30% to 35.6% and 26.7%e31.5%
respectively. It may be due to that the release of active and trans-
ferable Zn has almost reached its competition in this temperature
range. Before 800 �C, the incomplete combustion of the coal par-
ticles restrained the release of Zn outwards. In addition, the porous
structure of residual carbon in fly ash could recapture the released
Zn species. As the temperature continued to increase, its content in
flue gas remained stable, which indicated that Zn release almost
reached its completion at 800 �C. It should be noticed that co-
combustion decreased Zn release in the whole temperature
range. A previous study revealed that H2O(g) in flue gas could
restrain the release of Zn by forming oxides with higher boiling
point via Eq. (13) (Durlak et al., 1997). Compared with coal, biomass
combustion produced more steam due to its higher H content and
moisture.
ZnCl2ðgÞþH2OðgÞ/ZnOþ 2HClðgÞ (13)
In the co-combustion, proportion of Zn decreased significantly
in the bottom solids, meanwhile the relative content in the flue gas
also reduced, meaning that co-combustion promoted its transfer
into fly ash.
For Cr in the combustion, it mainly existed in the bottom solids
and the proportion decreased as the temperature increased too as
illustrated in Fig. 7. Similar to Zn, co-combustion could decrease its
proportion in the bottom solids. Furthermore, in the co-
combustion, Cr content in the flue gas remained stable in this
temperature range, indicating that the transfer of Crmainly focused
on fly ash and bottom solids. The reasonmay be that the proportion
of Cr chlorides was relatively lowwhile the oxidewas the dominant
species for Cr, which was stable in solid phase for the high melting/
boiling point of its components. As mentioned before, Cr trans-
ferred into flue gas and fly ash mainly through the entrainment
750 800 850 900 950
0
10
20
30
40
50
60
70
80
90
100
T/ C
FG
FA
B
R
el
at
iv
e
m
as
s d
is
tr
ib
ut
io
n
of
C
r/
%
BABA BA BA BA
Fig. 7. Relative mass distribution of Cr in the combustion. A: coal combustion; B: co-
combustion.
Z. Xue et al. / Chemosphere 240 (2020) 1248536
effect other than evaporation and was enhanced in co-combustion.
Cr was generally perceived to exist in the form of CrO3 in the flue
gas while Cr2O3 in fly ash (Meij andWinkel, 2009). According to our
previous research, CaO was detected in fly ash produced in co-
combustion at 850 �C (Zhang et al., 2018). Reactions in Eq. (14)~(15) occurred in fly ash, favorably affected by increasing tempera-
ture (Stam et al., 2011) (see Fig. 8).
CaCO3/CaOþ CO2ðgÞ (14)
CaOþ Cr2O3/CaCr2O4 (15)
750 800 850 900 950
0
10
20
30
40
50
60
70
80
90
100
R
el
at
iv
e
m
as
sd
is
tr
ib
ut
io
n
of
C
d/
%
T/ C
FG
FA
B
BABA BA BA BA
Fig. 8. Relative mass distribution of Cd in the combustion. A: coal combustion; B: co-
combustion.
750 800 85
0
10
20
30
40
50
60
70
80
90
100
T/
R
el
at
iv
e
m
as
sd
is
tr
ib
ut
io
n
of
Pb
/%
ABA BA
Fig. 9. Relative mass distribution of Pb in the comb
In addition, Cr oxides could also react with other metal oxides
and enriched in fly ash, the related reactions are listed in Eq. (16)~
(18) (Shah et al., 2008; Zhao et al., 2013)
K2Oþ Cr2O3/K2Cr2O4 (16)
FeOþ Cr2O3/FeCr2O4 (17)
MgOþ Cr2O3/MgCr2O4 (18)
In coal combustion, Cd content decreased in fly ash and bottom
solids while increased in flue gas as temperature increased (see
Fig. 8). However in co-combustion, fraction of Cd in the bottom
solids increased from 750 to 900 �C and then decreased at 950 �C.
Co-combustion tended to migrate Cd from flue gas to bottom solids
by forming more CdSO4, CdSiO3 and CdAl2O4. In the beginning the
melt in the slag could capture the released gaseous Cd species by
physical adhesive force (Wang et al., 2014) However, as the tem-
perature continued to rise, the sharp decrease in the specific sur-
face area caused by molten slag inhibited the gas-solid contact.
Similar conclusions are also reported by previous research (Zhong
et al., 2015). Similar to Zn, Cd release was also impaired in co-
combustion due to the effect of vapor in flue gas via Eq. (19)
(Verhulst et al., 1996). Besides chlorides, Cd could also release in
the form of elementary substance.
CdCl2ðgÞþH2OðgÞ/CdOþ 2HClðgÞ (19)
Emission of Pb was enhanced significantly by the rising tem-
perature, as shown in Fig. 9. At high temperatures, Pb mainly
released in the form of Pb(g), PbCl2(g), PbCl(g) and PbO(g) (Aunela-
Tapola et al., 1998). Similar to Cd and Zn, the release of Pb was
impeded in co-combustion. In addition, Ca in the biomass could
0 900 950
FG
FA
B
C
B BA BA
ustion. A: coal combustion; B: co-combustion.
Z. Xue et al. / Chemosphere 240 (2020) 124853 7
also trap Pb and the related reactions could be described by Eq. (20)
~ (22) (Zhao and Lin, 2003). In the bottom solids, Pb content
decreased from 750 to 900 �C and then increased in 950 �C during
coal combustion.
2PbCl2ðgÞþ6CaOþ O2ðgÞ/2Ca2PbO4þ2CaCl2 (20)
2PbOðgÞþ4CaOþ O2ðgÞ/2Ca2PbO4 (21)
PbCl2ðgÞþCaO/PbOþ CaCl2 (22)
with the increasing of temperature, Pb content in bottom solids also
increased significantly, indicating that the high temperature pro-
mote the transfer of Pb from fly ash to the other two phases. Al/Si-
based bed material could function as sorbents to capture Pb and
retained it in the bottom solids. Content of Pb in fly ash varied from
76.5% to 53.6% from 750 to 950 �C in coal combustion and it
decreased from 73.2% to 56.9% in the presence of WS.
3.4. Effect of secondary air on trace metal distribution
It could be inferred from Fig. 10 that the increasing secondary air
ratio basically facilitated the release of heavy metals during co-
combustion meanwhile the content in the fly ash decreased
sharply. It could be explained by the change of carbon content in fly
ash. As mentioned before, there was a positive correlation between
carbon and heavy metal content in fly ash (Sekine et al., 2008).
However, at 30% secondary air ratio, there was a slight decline in
temperature in dilute phase, slowing down the release of the heavy
metals.
Meanwhile, their distribution in the bottom solids appeared
different trends. To be specific, content of Cd in the bottom solids
increased with the increasing secondary air ratio while Pb and Zn
decreased. The content of heavy metals in the bottom solids mainly
depended on the atmosphere in the dense phase zone and inter-
action between fuel and bed materials. It has been discussed that
secondary air brought the anoxic atmosphere in the dense phase
zone, inhibiting the combination of O2 and released metals. In
addition, secondary air could change the local temperature in the
furnace, thus affecting the distribution of the elements.
Pb, Zn and Cr were considered to be thermodynamically stable
in oxides in the bottom slags at high temperatures, as the result the
ratio of these metals in the bottom solids decreased in lack of
0
10
20
30
40
50
60
70
80
90
100 DCB
CrCdPb
R
el
at
iv
e
m
as
sd
is
tr
ib
ut
io
n/
%
FG
FA
B
Zn
A DCBA DCBA DCBA
Fig. 10. Effect of secondary air ratio on heavy metals during co-combustion at 850 �C.
A: secondary air ratio¼ 0; B: secondary air ratio¼ 10%; C: secondary air ratio¼ 20%; D:
secondary air ratio¼ 30%.
abundant oxygen. Slightly different with Pb and Zn, Cr content in
the bottom solids was insensitive to the secondary air ratio in the
co-combustion. This was related to its initial form in the coal, which
was reported to be in the form of FeCr2O4 in coal (Ho et al., 1995).
Effect of the atmosphere was not significant on the distribution of
Cr during the thermal treatment. On the contrary, ratio of Cd in the
bottom solids increased slightly during the increase of secondary
air ratio.
4. Conclusion
Co-combustion of WS and coal was conducted in a lab-scale
bubbled fluidized bed, and temperature, secondary air ratio were
varied to investigate the effect on the emission characteristics of
pollutants respectively. Result indicated that increasing tempera-
ture was basically favorable for the release of NO, SO2 meanwhile it
could reduce the emission of CO and carbon content in the fly ash.
This also facilitated the release of heavy metals by reducing the
physical adsorption of fly ash and migrated from inside of ash
particles to gas phase. Due to the different existing forms at high
temperatures, the relative distribution of the heavymetals behaved
variously. Zn, Cd and Pb content in the bottom solids increasedwith
the increasing temperature while Cr content decreased slightly.
As for the secondary air, it could change the emission and dis-
tribution of the pollutants by affecting the local atmosphere and
temperature in the furnace. The increasing secondary air ratio
decreased carbon content in fly ash, CO and SO2 emissionwhile the
minimum NO emissions was reached at 20% secondary air ratio.
Increasing secondary air could promote the transfer of heavy
metals from fly ash to flue gas. Simultaneously it could decline the
content of Zn and Pb and increase the content of Cd in the bottom
solids.
Acknowledgements
The work described in this paper was financially supported by
the National Key Research and Development Program of China
(2018YFB0605100).
References
Armesto, L., Bahillo, A., Cabanillas, A., Veijonen, K., Otero, J., Plumed, A., Salvador, L.,
2013. Co-combustion of coal and olive oil industry residues in fluidised bed*.
Fuel 82 (8), 993e1000.
Aunela-Tapola, L.A., Aunela-Tapola, L.A., Aunela-Tapola, L.A., 1998. Trace metal
emissions from the Estonian oil shale fired power plant. Fuel Process. Technol.
57 (1), 1e24.
Basu, P., Butler, J., Leon, M.A., 2011. Biomass co-firing options on the emission
reduction and electricity generation costs in coal-fired power plants. Renew.
Energy 36, 282e288.
Belevi, H., Moench, H., 2000. Factors determining the element behavior in munic-
ipal solid waste incinerators. 1. Field studies. Environ. Sci. Technol. 34,
2501e2506.
Dai, S., Ren, D., Chou, C.-L., Finkelman, R.B., Seredin, V.V., Zhou, Y., 2012.
Geochemistry of trace elements in Chinese coals: a review of abundances, ge-
netic types, impacts on human health, and industrial utilization. Int. J. Coal
Geol. 94, 3e21.
De, S., Assadi, M., 2009. Impact of cofiring biomass with coal in power plants e a
techno-economic assessment. Biomass Bioenergy 33, 283e293.
Diaz-Somoano, M., Martinez-Tarazona, A.R., 2005. Retention of zinc compounds in
solid sorbents during hot gas cleaning processes. EnergyFuels 19, 442e446.
Durlak, S.K., Biswas, P., Shi, J., 1997. Equilibrium analysis of the affect of temperature,
moisture and sodium content on heavy metal emissions from municipal solid
waste incinerators. J. Hazard Mater. 56 (1e2), 1e20.
Guo, F., Zhong, Z., 2018. Co-combustion of anthracite coal and wood pellets: ther-
modynamic analysis, combustion efficiency, pollutant emissions and ash slag-
ging. Environ. Pollut. 239, 21e29.
Ho, T.C., Lee, H.T., Shiao, C.C., Hopper, J.R., Bostick, W.D., 1995. Metal behavior during
fluidized bed thermal treatment of soil. Waste Manag. 15 (5e6), 325e334.
Hu, Z., Wang, X., Wang, Z., Wang, Y., Tan, H., 2014. Segmented kinetic investigation
on condensed KCl sulfation in SO2/O2/H2O at 523e1023 K. Energy Fuels 28,
7560e7568.
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref1
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref1
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref1
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref1
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref1
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref2
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref2
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref2
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref2
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref3
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref3
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref3
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref3
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref4
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref4
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref4
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref4
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref5
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref5
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref5
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref5
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref5
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref6
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref6
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref6
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref6
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref7
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref7
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref7
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref8
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref8
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref8
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref8
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref8
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref9
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref9
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref9
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref9
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref10
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref10
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref10
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref10
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref11
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref11
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref11
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref11
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref11
Z. Xue et al. / Chemosphere 240 (2020) 1248538
Kazanc, F., Khatami, R., Manoel Crnkovic, P., Levendis, Y.A., 2011. Emissions of NOx
and SO2 from coals of various ranks, bagasse, and coal-bagasse blends burning
in O2/N2 and O2/CO2 environments. Energy Fuels 25, 2850e2861.
Ketris, M.P., Yudovich, Y.E., 2009. Estimations of Clarkes for Carbonaceous biolithes:
world averages for trace element contents in black shales and coals. Int. J. Coal
Geol. 78, 135e148.
Li, W., Ma, Z., Huang, Q., Jiang, X., 2018. Distribution and leaching characteristics of
heavy metals in a hazardous waste incinerator. Fuel 233, 427e441.
Liu, R.P., Jin, B.S., Zhong, Z.P., 2007. Comparison of two kinds of bed materials during
CFB combustion of cotton stalk. Chem. Eng. Technol. 30, 1434e1439.
Luan, J., Li, A., Su, T., Li, X., 2009. Translocation and toxicity assessment of heavy
metals from circulated fluidized-bed combustion of oil shale in Huadian, China.
J. Hazard Mater. 166, 1109e1114.
Meij, R., Winkel, B.H.T., 2009. Trace elements in world steam coal and their
behaviour in Dutch coal-fired power stations: a review. Int. J. Coal Geol. 77,
289e293.
Nakada, H., Mihara, N., Kawaguchi, Y., Osada, S., Kuchar, D., Matsuda, H., 2008.
Volatilization behavior of lead from molten slag under conditions simulating
municipal solid waste melting. J. Mater. Cycles Waste Manag. 10, 19e23.
Sami, M., Annamalai, K., Wooldridge, M., 2001. Co-firing of coal and biomass fuel
blends. Prog. Energy Combust. Sci. 27 (2), 171e214.
Sekine, Y., Sakajiri, K., Kikuchi, E., Matsukata, M., 2008. Release behavior of trace
elements from coal during high-temperature processing. Powder Technol. 180,
210e215.
Shah, P., Strezov, V., Prince, K., Nelson, P.F., 2008. Speciation of As, Cr, Se and Hg
under coal fired power station conditions. Fuel 87, 1859e1869.
Shah, P., Strezov, V., Nelson, P.F., 2012. Speciation of chromium in Australian coals
and combustion products. Fuel 102, 1e8.
Stam, A.F., Meij, R., Te Winkel, H., Eijk, R.J., Huggins, F.E., Brem, G., 2011. Chromium
speciation in coal and biomass co-combustion products. Environ. Sci. Technol.
45, 2450e2456.
Sung, Y., Lee, S., Kim, C., Jun, D., Moon, C., Choi, G., Kim, D., 2016. Synergistic effect of
co-firing woody biomass with coal on NOx reduction and burnout during air-
staged combustion. Exp. Therm. Fluid Sci. 71, 114e125.
United States Environmental Protection Agency (USEPA), 2016. Microwave Assisted
Acid Digestion of Siliceous and Organically Based Materials.
Vekemans, O., Laviolette, J.-P., Chaouki, J., 2016. Co-combustion of coal and waste in
pulverized coal boiler. Energy 94, 742e754.
Verhulst, D., Buekens, A., Spencer, P.J., Eriksson, G., 1996. Thermodynamic behavior
of metal chlorides and sulfates under the conditions of incineration furnaces.
Environ. Sci. Technol. 30 (1), 50e56.
Wang, S.J., He, P.J., Shao, L.M., Zhang, H., 2016a. Multifunctional effect of Al2O3, SiO2
and CaO on the volatilization of PbO and PbCl2 during waste thermal treatment.
Chemosphere 161, 242e250.
Wang, X., Huang, Y., Zhong, Z., Yan, Y., Niu, M., Wang, Y., 2014. Control of inhalable
particulate lead emission from incinerator using kaolin in two addition modes.
Fuel Process. Technol. 119, 228e235.
Wang, X.Y., Huang, Y.J., Niu, M.M., Wang, Y.X., Liu, C.Q., 2016b. Effect of multi-factors
interaction on trace lead equilibrium during municipal solid waste incineration.
J. Mater. Cycles Waste Manag. 18, 287e295.
Zhang, B., Zhong, Z., Xue, Z., Xue, J., Xu, Y., 2018. Release and transformation of
potassium in co-combustion of coal and wheat straw in a BFB reactor. Appl.
Therm. Eng. 144, 1010e1016.
Zhang, S., Jiang, X., Lv, G., Nixiang, A., Jin, Y., Yan, J., Lin, X., Song, H., Cao, J., 2019.
Effect of chlorine, sulfur, moisture and ash content on the partitioning of As, Cr,
Cu, Mn, Ni and Pb during bituminous coal and pickling sludge co-combustion.
Fuel 239, 601e610.
Zhao, Y., Lin, W.-C., 2003. Multi-functional sorbents for the simultaneous removal of
sulfur and lead compounds from hot flue gases. J. Hazard Mater. 103, 43e63.
Zhao, Y., Zhang, J., Zheng, C., 2013. Release and removal using sorbents of chromium
from a high-Cr lignite in Shenbei coalfield, China. Fuel 109, 86e93.
Zhong, D., Zhong, Z., Wu, L., Xue, H., Song, Z., Luo, Y., 2015. Thermal characteristics of
hyperaccumulator and fate of heavy metals during thermal treatment of sedum
plumbizincicola. Int. J. Phytoremediation 17, 766e776.
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref12
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref13
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref13
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref13
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref13
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref14
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref14
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref14
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref15
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref15
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref15
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref16
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref16
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref16
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref16
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref17
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref17
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref17
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref17
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref18
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http://refhub.elsevier.com/S0045-6535(19)32092-2/sref26
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http://refhub.elsevier.com/S0045-6535(19)32092-2/sref26
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref27
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref27
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref27
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref27
http://refhub.elsevier.com/S0045-6535(19)32092-2/sref28
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	Investigation on gaseous pollutants emissions during co-combustion of coal and wheat straw in a fluidized bed combustor
	1. Introduction
	2. Experimental work
	2.1. Facility and operation process
	2.2. Material
	2.3. Experimental conditions and data analysis
	3. Results and discussion
	3.1. Effect of temperature on CO, SO2 and NOx emission
	3.2. Effect of secondary air on CO, SO2 and NOx emission
	3.3. Effect of temperature on trace metal distribution
	3.4. Effect of secondary air on trace metal distribution
	4. Conclusion
	Acknowledgements
	References