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Full Length Article
Simultaneous NO/CO2 removal performance of biochar/limestone in
calcium looping process
Wan Zhanga
, Yingjie Lia,⁎
, Xiaotong Maa
, Yuqi Qiana
, Zeyan Wangb
a
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
b
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
A R T I C L E I N F O
Keywords:
Limestone
Biochar
Calcium looping
Simultaneous removal
CO2 capture
NO removal
A B S T R A C T
A novel simultaneous NO/CO2 removal system using biochar and calcined limestone in the calcium looping
process was proposed. Coconut shell char and calcined limestone were added into a carbonator in the calcium
looping process as the NO reductant and CO2 sorbent, respectively. The simultaneous NO/CO2 removal per-
formance of coconut shell char/calcined limestone in the calcium looping process was investigated in a bubbling
fluidized bed reactor. NO and CO2 in flue gases are effectively and simultaneously removed by coconut shell
char/calcined limestone in the presence of O2. O2 plays an important role in NO removal by coconut shell char.
The calcined limestone shows a strong catalytic effect on NO reduction by CO generated by the reaction of
coconut shell char and O2. The calcined limestone supports active sites for NO reduction by CO. High CO
concentrations and high carbonation temperatures have positive effects on NO reduction by CO with calcined
limestone catalysis. However, the catalytic effect of calcined limestone is weakened by its carbonation, which is
promoted by the high temperature and additional CO2 produced by the oxidation of char. The simultaneous NO
removal and CO2 capture efficiencies can reach above 97% and 80%, respectively. The porous structure of
coconut shell char is an important factor in enhancing NO reduction with calcined limestone catalysis in the
presence of O2.
1. Introduction
At present, the greenhouse effect caused by the excessive CO2
emissions is a global environmental problem that requires urgent so-
lutions [1,2]. Coal-fired power plants have been regarded as the main
anthropogenic CO2 resources [2]. Among the technologies for miti-
gating CO2 emissions, calcium looping based on the carbonation/cal-
cination cycles of CaO is regarded as one of the most promising tech-
nologies for large-scale CO2 capture from flue gases in coal-fired power
plants [3,4]. The calcium looping system mainly includes a carbonator
and a calciner. The flue gas from a coal-fired power plant first flows into
the carbonator (600–700 °C), where the CO2 in the flue gas is captured
by CaO derived from calcium-based sorbents to generate CaCO3 via the
carbonation reaction. Then, CaCO3 is transported to the calciner (above
800 °C), in which CaCO3 is decomposed into CaO and CO2 under O2/
CO2 or O2/steam combustion of fuel. The enrichment of CO2 and the
regeneration of CaO are realized in the calciner. Then the regenerated
CaO returns to the carbonator for the next CO2 capture cycle [5–7].
Fluidized-bed reactors are usually utilized as carbonators and calciners.
NOx is a major pollutant in flue gases from coal-fired power plants.
NO reduction by char has been proposed as a reasonable way to realize
NO removal because of its low cost and abundant reductant resources
[8,9]. Yan et al. [10] compared NO removal efficiencies of eleven coal
chars in a fixed-bed reactor at 450 °C in the presence of 1% O2 and
found that NO removal efficiencies of different coal chars varied from
20% to 100% due to their different microstructures. Guo et al. [11]
noted that lignite coal char achieved above 50% NO removal at
600–700 °C in a packed-bed reactor. Wang et al. [12] discovered coal-
ash-catalyzed NO reduction by coal char, and the catalytic activity
followed the order Fe2O3 > CaO > MgO > Al2O3 > SiO2. Zhao
et al. [13] added 1 wt% CaO into lignite coal char by the wet impreg-
nation method and found that the NO removal efficiency of the coal
char reached above 60% at above 600 °C in the presence of 0.2% O2.
Illan-Gomez et al. [14] reported CaO on the surface of char enhanced
oxygen transportation in the NO-char reaction. Wang et al. [15] noted
that the NO removal efficiency of coal char mixed with CaO by me-
chanical mixing in a gas-solid suspension reactor reached about 30% in
the absence of O2 at 950 °C. Therefore, CaO is a feasible catalyst for NO
reduction by coal char above 600 °C.
Jensen et al. [16] found that CaO promoted NO reduction during
https://doi.org/10.1016/j.fuel.2019.116428
Received 16 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019
⁎
Corresponding author.
E-mail address: liyj@sdu.edu.cn (Y. Li).
Fuel 262 (2020) 116428
Available online 31 October 2019
0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
T

2. the combustion of char in a fluidized bed. They also mentioned that a
low temperature was better for NO removal in the presence of CaO due
to the high CO concentration generated by char oxidation in fluidized
bed combustion (FBC) [17]. In the calcium looping process, the car-
bonator contains a large amount of CaO. The carbonation temperature
is lower than that in FBC. This difference indicates that CaO in the
carbonator has a good catalytic performance for NO removal by char in
the presence of O2. Shimizu et al. [18,19] found that unburnt coal char
flowing from the calciner into the carbonator decreased the NO con-
centration in the presence of CaO and the NO removal efficiency was
below 25% due to a small amount of coal char. Therefore, NO reduction
by coal char and CO2 capture by CaO may be simultaneously realized in
the carbonator, especially considering the catalytic effect of CaO on NO
reduction by char in the presence of O2.
Sørensen et al. [20] and Li et al. [21] found that biochars (wheat
straw char, sawdust char, rice husk char and corn straw char) showed
higher NO reduction abilities than other tested coal chars at 700–900 °C
due to the larger specific surface areas of biochars, and NO removal
efficiencies of biochars were usually above 70% in the presence of O2 or
CO in the reaction atmosphere. In addition, biochar is friendly to the
environment due to its carbon-neutral nature. Thus, it may be a good
idea to add biochar as a NO reductant in the carbonator. However, NO
removal by biochar in the calcium looping process has seldom been
reported. The effect of CO2 capture by CaO on NO reduction by biochar
has also been not discussed.
In this work, a novel simultaneous NO/CO2 removal system using
biochar and calcium-based sorbent in the calcium looping process is
proposed, as shown in Fig. 1. The simultaneous NO/CO2 removal
system includes a biomass pyrolysis reactor, a carbonator and a cal-
ciner. The biomass is added into the pyrolysis reactor to generate
pyrolysis gases and biochar. The biochar is transported into the car-
bonator, where the NO and CO2 in flue gases from coal-fired power
plants are simultaneously removed by the biochar and CaO in the cal-
cium-based sorbents. CaO may have a catalytic effect on NO reduction
by biochar in the carbonator in the presence of O2 [17,22]. After the
simultaneous removal of NO/CO2, the CaCO3 generated by the carbo-
nation of CaO and the unreacted biochar are sent to the calciner, where
CaCO3 is decomposed into CaO and CO2 and the biochar is burned.
Then the regenerated CaO from the calciner and the biochar generated
by the pyrolysis reactor are transported to the carbonator for the next
simultaneous CO2/NO removal cycle.
There are many types of biochars that can be easily obtained in situ,
such as wheat straw char, sawdust char, rice husk char and corn straw
char. Compared with the mentioned biochars, coconut shell char is well
known for its excellent natural structure and low ash content [23]. In
addition, approximately 60 million-ton coconuts are produced annually
around the world and the producing areas are mainly in Asia [24].
Thus, the production of coconut shell char is inexpensive and can utilize
abundant resources [25,26]. In this work, coconut shell char was
chosen as a typical biochar. The simultaneous NO and CO2 removal
performance of coconut shell char and calcined limestone in the cal-
cium looping process was investigated in a bubbling fluidized bed re-
actor.
2. Experimental
2.1. Materials
The limestone was sampled from Henan Province, China. The
limestone was crushed and screened to a particle size between 0.125
and 0.180 mm. The limestone was calcined in a bubbling fluidized bed
reactor under a pure N2 at 850 °C or under 70% CO2/30% N2 at 950 °C.
The chemical components of the calcined limestone (denoted by CaO)
were analyzed by X-ray fluorescence (XRF, ZSX Primus II), as shown in
Table 1. The coconut shell was sampled from Guangdong Province,
China. The coconut shell char (denoted by CSC) was obtained by the
pyrolysis of coconut shell under a pure N2 atmosphere at 900 °C for
90 min. CSC after acid elution was crushed and screened to a particle
size between 0.125 and 0.180 mm. A coal was sampled from Xinjiang
Province, China. Coal was crushed and screened to a particle size be-
tween 0.125 and 0.180 mm. The coal char (denoted by CC) was gen-
erated by the pyrolysis of coal under a pure N2 atmosphere at 900 °C for
90 min. The elemental analysis of CSC and CC was performed with an
elemental analyzer (EA, Elementar Vario EL), as shown in Table 2.
2.2. Simultaneous NO/CO2 removal tests
The simultaneous NO/CO2 removal by char/CaO in the calcium
looping process was investigated in a bubbling fluidized bed reactor
(BFBR), as shown in Fig. 2. The furnace of the BFBR was a quartz tube
with a perforated distributor plate. The inner diameter and the height of
the tube were 32 and 900 mm, respectively. There was an isothermal
region in the furnace, and the length of this region was 200 mm. The
temperature of the isothermal region represented the reaction tem-
perature of the BFBR. The bottom of the isothermal region was a per-
forated distributor plate. The reaction gases were controlled by mass
flowmeters (Flowmethod FL-802). The mixed gases were sent to the
BFBR by pipelines, which were heated to approximately 300 °C by
electric heating bands. The total flow rate of the reaction gas was 2.2 L/
min, and the particles of the samples were all under bubbling fluidi-
zation. Although CSC was lighter than CaO, CSC did not separate from
CSC/CaO due to the drastic fluidization of CSC/CaO in the BFBR. CSC
and CaO were evenly mixed in the BFBR during the experiments.
Fig. 1. Schematic of simultaneous NO/CO2 removal system using biochar/CaO based on calcium looping process.
Table 1
Chemical components of calcined limestone (wt%).
CaO MgO Al2O3 SiO2 K2O Fe2O3 SrO
95.28 1.53 0.38 2.25 0.25 0.28 0.03
W. Zhang, et al. Fuel 262 (2020) 116428
2

3. To ensure the stable fluidization of the samples in the BFBR, the
thickness of the samples in the BFBR was approximately 2–3 cm under a
steady state. Thus, approximately 16 g of CaO was needed to meet the
above requirements. Char (CSC or CC) and 16 g CaO (fresh CaO or re-
cycled CaO) were mixed according to the specified mass ratios of char
to CaO in a hopper of the BFBR. The furnace of the BFBR was heated by
electrical heating and pure N2 was introduced into the BFBR. When the
temperature of the furnace reached the specified temperature (600 °C,
650 °C and 700 °C), the atmosphere was switched to the reaction at-
mosphere of 15% CO2, 500 ppm NO, 0–5% O2 and N2 balance. At the
same time, a mixture of char and CaO (denoted by CSC/CaO or CC/
CaO) was rapidly added into the furnace from the hopper. The reaction
duration was 15 min. The concentrations of CO, NO, O2 and CO2 in the
exhaust gases from the BFBR were tested and recorded by a Testo 350
flue gas analyzer. The empty bed experiments were performed ac-
cording to the abovementioned procedure, except for the addition of
char and CaO.
The recycled CaO experiencing the different carbonation/calcina-
tion cycles was prepared in the BFBR as follows: at first, CaO was added
into the furnace of the BFBR for 15 min at 650 °C under 15% CO2/85%
N2. The exhaust gas from the BFBR was recorded by the flue gas ana-
lyzer. After carbonation, the atmosphere was switched into pure N2,
and the furnace was heated to a specified calcination temperature (e.g.
850 °C). When the CO2 concentration in the exhaust was 0%, the cal-
cination was finished, and CaO was obtained after participating in one
carbonation/calcination cycle. CaO recycled for use in additional were
prepared according to the same procedure.
During the simultaneous NO/CO2 removal process of char/CaO, the
presence of O2 in the reaction gas resulted in the formation of CO due to
the reaction between the char and O2. To examine the effect of CO on
the simultaneous NO/CO2 removal performance, CaO was added into
the furnace of the BFBR (without the addition of char) for 15 min at
650 °C in the reaction atmosphere including 15% CO2, 500 ppm NO,
1.5% CO and N2 balance.
The NO removal and the CO2 capture efficiencies were defined by
Eqs. (1) and (2), respectively, as follows:
=
−
η t
φ t V φ t V t
φ t V
( )
( )· ( )· ( )
( )·NO
NO,0 0 NO
NO,0 0 (1)
=
−
η t
φ t V φ t V t
φ t V
( )
( )· ( )· ( )
( )·CO
CO ·0 0 CO
CO ·0 0
2
2 2
2 (2)
=
− − −
V t
V
φ t φ t φ t
( )
100% ( ) ( ) ( )/10000
N ·0
CO CO NO
2
2 (3)
where t denotes the carbonation time, s; ηNO(t) represents the NO re-
moval efficiency in the BFBR at t, %; φNO, 0 (t) is the NO concentration
in the exhaust gas before the addition of the sample at t, ppm; V0 de-
notes the total volume of the reaction gas before the addition of the
sample in the BFBR, L/min; φNO (t) denotes the NO concentration in the
exhaust gas after the addition of the sample at t, ppm; V (t) is the total
volume of the reaction gas after the addition of the sample in the BFBR
at t, L/min; ηCO2(t) represents the CO2 capture efficiency in the BFBR at
t, %; φCO2, 0 (t) denotes the CO2 concentration in the exhaust gas before
the addition of the sample at t, %; φCO2 (t) denotes the CO2 con-
centration in the exhaust gas after the addition of the sample at t, %;
VN2,0 represents the N2 volume of the reaction gas before the addition of
the sample in the BFBR, L/min; φCO (t) denotes the CO concentration in
the exhaust gas after the addition of the sample at t, %.
2.3. Characterization
The microstructures of the samples were observed by a scanning
electron microscopy (SEM, JEOL JSM-7600F). The microstructure
parameters, such as the surface area, pore volume and pore size dis-
tribution, of the samples were measured by a nitrogen adsorption
analyzer (Micromeritics ASAP 2020-M). The surface areas were calcu-
lated according to the Brunauer-Emmett-Teller (BET) method. The pore
volumes and pore size distributions were computed using the by
Barrett-Joyner-Halenda (BJH) model.
Table 2
Elementary analysis of CSC after acid elution and CC on air dry basis (wt%).
Sample C H O N S Ash
CSC 96.36 0.40 2.50 0.62 0.09 0.03
CC 77.58 0.64 9.86 0.88 0.93 10.11
Fig. 2. A bubbling fluidized bed reactor system.
W. Zhang, et al. Fuel 262 (2020) 116428
3

4. 3. Results and discussions
3.1. Effect of biochar addition on simultaneous NO/CO2 removal by CaO
Fig. 3 shows the effect of CSC addition on the simultaneous NO/CO2
removal by CaO in the presence and absence of O2. CO2 absorption by
CaO is divided into two reaction stages: a chemical reaction-controlled
stage (rapid absorption of CO2 in approximately 400 s) and a diffusion-
controlled stage (slow CO2 diffusion through the CaCO3 product layer
after approximately 400 s), as shown in Fig. 3(a). Research on CO2
absorption by CaO in the chemical reaction-controlled stage is mean-
ingful for industrial applications due to the short residence time of CaO
in the fluidization. In the absence of O2, the addition of CSC almost does
not affect CO2 absorption by CaO for approximately 400 s, and the CO2
capture efficiency is higher than 90%. However, after the addition of
CSC, the NO concentration in the exhaust gas decreases from 500 to
360 ppm at 400 s due to NO reduction by CSC, and the NO removal
efficiency reaches approximately 34%, as illustrated in Fig. 3(b). The
addition of CSC shows an apparent effect on NO removal in the absence
of O2. In addition, the comparison of NO reduction by CSC and CSC/
CaO (the mass ratio of CSC to CaO = 3:16) for 400 s shows that CaO has
almost no effect on NO reduction by CSC in the BFBR in the absence of
O2. However, some references [12–15] reported that CaO impregnated
into char by the wet impregnation method had a positive effect on NO
reduction by char in a fixed-bed reactor. The contact between CaO and
char particles in the BFBR is significantly less and weaker than that in a
fixed-bed reactor. This difference may be the reason why CaO does not
show an apparent effect on NO reduction by CSC in the BFBR.
In the presence of 4% O2 in the reaction gas, the addition of CSC
(without the addition of CaO) leads to an increase in the CO2 con-
centration from 15% to 18% due to the oxidation of CSC, as presented
in Fig. 3(a). Thus, it is easy to understand why the CO2 capture effi-
ciency of CSC/CaO (above 80% in 400 s) is slightly lower than that of
CaO. However, it should be noted that CSC is carbon-neutral, so the
addition of CSC does not increase the CO2 emissions to the environ-
ment. When only CSC is added to the BFBR in the presence of O2, the
NO concentration in the exhaust gas decreases from 500 to approxi-
mately 100 ppm, and the NO removal efficiency reaches approximately
80% at 400 s, as shown in Fig. 3(b). The presence of O2 is beneficial for
NO removal by char because the oxidation of char continuously renews
the surface intermediates such as carbon-oxygen complexes C (O) and
free sites C* [27,28]. More active sites are generated due to the deso-
rption of CO from the surface of CSC, and the active sites promote NO
reduction by char [29]. After the addition of CSC, the CO concentration
in the exhaust gas increases rapidly in the presence of O2 due to the
reaction of CSC and O2 and is significantly higher than that in the ab-
sence of O2, as shown in Fig. 3(c). Dong et al. [21] found that CO could
reduce NO on the surface of char. Thus, it is inferred that CSC and CO
simultaneously reduce NO in the presence of O2 in the reaction gas.
When CSC and CaO (the mass ratio of CSC to CaO = 3:16) are si-
multaneously added in the BFBR in the presence of O2, the NO con-
centration in the exhaust gas dramatically decreases from 500 to ap-
proximately 20 ppm at 400 s, which means that the NO removal
efficiency reaches approximately 97%, as presented in Fig. 3(b). The
NO concentration in the exhaust gas after the addition of CSC/CaO is
80% lower than that after the addition of CSC at 400 s. The NO removal
Fig. 3. Effect of CSC addition on simultaneous NO/CO2 removal by CaO: (a) CO2 concentration, (b) NO concentration, (c) CO concentration (carbonation:15 min,
650 °C, 15% CO2/500 ppm NO/0 or 4% O2/N2 balance; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
4

5. efficiency of CSC/CaO is 16% higher than that of CSC at 400 s. In ad-
dition, the CO concentration in the exhaust gas after the addition of
CSC/CaO is lower than that after the addition of CSC, as illustrated in
Fig. 3(c). It indicates that the presence of CaO increases the consump-
tion of CO for NO reduction. Therefore, it suggests that the NO reduc-
tion ability of CO is improved by CaO.
3.2. Effect of CaO on NO reduction by CO
O2 in the reaction atmosphere reacts with CSC to form CO and CO2,
so it is necessary to examine the effect of CO on the simultaneous NO/
CO2 removal performance in the calcium looping process. Thus, only
CaO was added into the BFBR with a reaction atmosphere of 15% CO2,
500 ppm NO, 1.5% CO and N2 balance. Fig. 4 depicts the simultaneous
NO/CO2 removal performance of CO and CaO in the absence of CSC. As
shown in Fig. 4(a), NO reduction by CO does not occur in the absence of
CaO at 650 °C. In the presence of CaO and CO, the NO concentration in
the exhaust gas decreases from 500 to 250 ppm at approximately 40 s,
and the NO removal efficiency of CO is approximately 50%. It proves
that CaO strongly catalyzes NO reduction by CO at 650 °C. Kadossov
0 100 200 300 400 500 600 700 800 900
0
50
100
150
200
250
300
350
400
450
500
ηNO
(t < 40s) > 50%
COconcnetrationinexhaustgas(%)
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
NO concentration:
In presence of CaO (16g)
Empty bed
0.000
0.005
1.2
1.3
1.4
1.5
1.6
1.7
CO concentration:
In presence of CaO (16g)
Empty bed
(a)
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
ηCO2
(t < 550s) > 90%
Reaction time (s)
CO2
concnetrationinexhaustgas(%)
In presence of CaO (16g)
Empty bed
(b)
Fig. 4. Effect of CaO on NO removal by CO: (a) NO and CO concentrations, (b) CO2 concentration (carbonation: 15 min, 650 °C, 500 ppm NO/1.5% CO/15% CO2/N2
balance; calciantion: 850 °C, 100% N2).
0 100 200 300 400 500 600 700 800 900
0
50
100
150
200
250
300
350
400
450
500
4%
5%
Empty bed
ηNO
(t ≤ 900s) ≥ 88%
ηNO
(t ≤ 900s) ≥ 94%
ηNO
(t ≤ 900s) ≥ 85%
ηNO
(t ≤ 900s) ≥ 70%
ηNO
(t ≤ 900s) ≥ 25%
ηNO
(t ≤ 420s) ≥ 86%
ηNO
(t ≤ 450s) ≥ 75%
ηNO
(t ≤ 550s) ≥ 33%
ηNO
(t ≤ 215s) ≥ 93%
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
0%
2%
3%
ηNO
(t ≤ 400s) ≥ 97%
(a)
0 100 200 300 400 500 600 700 800 900
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Reaction time (s)
COconcentrationinexhaustgas(%)
O2
concentration:
0%
2%
3%
4%
5%
(b)
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
ηCO2
(t ≤ 550s) ≥ 90%
ηCO2
(t ≤ 400s) ≥ 80%
ηCO2
(t ≤ 420s) ≥ 83%
ηCO2
(t ≤ 450s) ≥ 87%
ηCO2
(t ≤ 215s) ≥ 77%
Reaction time (s)
CO2
concentrationinexhaustgas(%)
O2
concentration:
0%
2%
3%
4%
5%
Empty bed
(c)
Fig. 5. Effect of O2 concentration on simultaneous NO/CO2 removal by CSC/CaO: (a) NO concentration, (b) CO concentration, (c) CO2 concentration (carbonation:
15 min, 650 °C, 15% CO2/500 ppm NO/0–5% O2/N2 balance, mass ratio of CSC to CaO = 3:16; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
5

6. et al. [30] reported that CaO can weakly adsorb CO according to density
functional theory calculations. Valentin et al. [31] found that NO was
adsorbed by the oxygen sites on CaO (1 0 0) with the N atom towards
the surface. Thus, CaO supports active sites for the NO-CO reaction and
promotes the NO-CO reaction. CaO is largely consumed by CO2 to form
CaCO3, as shown in Fig. 4(b). Therefore, the catalytic effect of CaO on
NO reduction by CO is weakened, as CO2 capture by CaO proceeds
because the generated CaCO3 is a less effective catalyst for NO removal
than CaO [22]. The result proves that NO removal by CO generated by
CSC oxidation is catalyzed by CaO. Thanks to the presence of CSC in
CSC/CaO, the carbonation of CaO only slightly decreases NO removal
efficiency of CSC/CaO.
3.3. Effect of O2 concentration on simultaneous NO/CO2 removal by CSC/
CaO
The effect of the O2 concentration in the reaction gas on the si-
multaneous NO/CO2 removal by CSC/CaO is presented in Fig. 5. The
NO concentration in the exhaust gas decreases from 380 to 20 ppm
when the O2 concentration increases from 0% to 4%, as plotted in
Fig. 5(a). O2 is beneficial for NO reduction. As shown in Fig. 5(b), the
CO concentration in the exhaust gas increases as the O2 concentration
increases. A higher concentration of O2 means a more consumption of
CSC by O2. As the O2 concentration increases from 0% to 5%, the CO2
concentration in the exhaust gas during the chemical reaction-con-
trolled stage increases from about 1.7% to 3.9%, and the CO2 capture
efficiency decreases from 90% to 77%, as shown in Fig. 5(c). CO2
generated by CSC oxidation accelerates the consumption of CaO. Thus,
the catalytic effect of CaO on NO reduction weakens with the increasing
O2 concentration. Considering the simultaneous NO and CO2 removal,
the feasible O2 concentration in the reaction gas is 4%.
3.4. Effect of the mass ratio of CSC to CaO on simultaneous NO/CO2
removal by CSC/CaO
The effect of the mass ratio of CSC to CaO on simultaneous NO/CO2
removal by CSC/CaO is shown in Fig. 6. As plotted in Fig. 6(a), the CO2
concentration in the exhaust gas shows a slight increase with the mass
ratio of CSC to CaO increasing from 2:16 to 4:16. The CO2 capture ef-
ficiency declines from 82% to 78% at 400 s. The NO concentration in
the exhaust gas initially decreases from 55 to 20 ppm and then increases
to 31 ppm at 400 s with increasing the mass ratio of CSC to CaO, and the
NO removal efficiency increases from 90% to 97% and then decreases
to 94%, as shown in Fig. 6(b). As the mass ratio of CSC to CaO increases,
the total active sites on CSC increases and NO reduction is promoted.
This is the reason why the NO concentration in the exhaust gas after the
addition of CSC/CaO with a mass ratio of 3:16 is almost one-third of
that after the addition of CSC/CaO with a mass ratio of 2:16. However,
the CO concentration in the exhaust gas decreases with the increasing
mass ratio of CSC to CaO, as shown in Fig. 6(c). A low concentration of
CO has an adverse effect on NO reduction. Thus, the NO concentration
in the exhaust gas shows a slight increase when the mass ratio of CSC to
CaO increases from 3:16 to 4:16. The bubbling fluidization of CSC/CaO
is less effective upon increasing the mass ratio of CSC to CaO from 3:16
to 4:16 due to the size limitation of the BFBR and the total gas flow. The
poor gas-solid mixing results in many O2 molecules in the bubble phase,
which is not beneficial for the reaction between CSC and O2. Therefore,
the CO and CO2 concentrations in the exhaust gas decrease with the
increasing mass ratio of CSC to CaO from 3:16 to 4:16. Therefore, the
mass ratio of CSC to CaO = 3:16 is optimal for simultaneous NO/CO2
Fig. 6. Effect of mass ratio of CSC to CaO on simultaneous NO/CO2 removal by CSC/CaO: (a) CO2 concentration, (b) NO concentration, (c) CO concentration
(carbonation: 15 min, 650 °C, 15% CO2/500 ppm NO/4% O2/N2 balance; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
6

7. removal by CSC/CaO.
3.5. Effect of reaction temperature and calcination condition on
simultaneous NO/CO2 removal by CSC/CaO
The feasible carbonation temperature for CO2 capture by calcium-
based sorbents in the calcium looping process is in the range of
600–700 °C [32–34]. The effect of the reaction temperature on si-
multaneous NO/CO2 removal by CSC/CaO in the BFBR is demonstrated
in Fig. 7. As the reaction temperature increases from 600 to 700 °C, the
CO2 concentration in the exhaust gas in the chemical reaction-con-
trolled stage increases from approximately 1.5% to 9.3%, as depicted in
Fig. 7(a). The CO2 capture efficiency drastically decreases from 90% to
40% because the higher temperature results in more CO2 generated by
the reaction between CSC and O2. As shown in Fig. 7(c), the CO con-
centration in the exhaust gas at 700 °C is higher than that at 600 and
650 °C in 400 s. This result suggests that high temperature accelerates
the release of CO. However, it should be noted that high temperature
also improves the reaction between CO and O2, so the CO concentra-
tions in the exhaust gas at different temperatures are almost the same
above 400 s. Therefore, to achieve a CO2 capture efficiency greater than
80%, the feasible reaction temperature for CO2 capture by CSC/CaO
should not exceed 650 °C. As exhibited in Fig. 7(b), when the tem-
perature increases from 600 to 650 °C, the NO concentration in the
exhaust gas in the chemical reaction-controlled stage decreases from
approximately 70 to 20 ppm, and the NO removal efficiency of CSC/
CaO increases from 89% to 97%. This result is because a higher tem-
perature leads to a better NO reduction ability of char/CO [29]. How-
ever, as the temperature further increases from 650 to 700 °C, the NO
concentration in the exhaust gas decreases slightly from 97% to 94%.
High temperature accelerates the carbonation of CaO [35,36], so more
CaO is converted to CaCO3. Thus, the catalytic effect of CaO on NO
removal by CO is weakened upon increasing the temperature from 650
to 700 °C. To achieve efficient NO removal and CO2 capture by CSC/
CaO, a reaction temperature of 650 °C seems optimal.
In industrial applications, the calcination of CaCO3 occurs under
high CO2 concentrations and high temperatures, which are called se-
vere calcination conditions. Fig. 8 shows the simultaneous NO/CO2
removal performance of CSC/CaO under different calcination condi-
tions. Compared with the mild calcination conditions (850 °C, 100%
N2), the CO2 capture efficiency and the NO removal efficiency decrease
by 2.5% and 3.1% under the severe calcination conditions (950 °C, 70%
CO2/30% N2), respectively, as presented in Fig. 8(a) and (b). The CO
concentration is minimally affected by the calcination conditions, as
plotted in Fig. 8(c). It indicates that the severe calcination conditions
have a slight adverse effect on simultaneous NO/CO2 removal. This
adverse effect is because the severe calcination conditions aggravate the
sintering of CaO, which decreases the surface area and the pore volume
of CaO [37,38]. Thus, the diffusion of CO, NO and CO2 is hindered,
which slows down the simultaneous NO/CO2 removal.
3.6. Effect of the number of cycles on simultaneous NO/CO2 removal by
CSC/CaO
Fig. 9 shows the simultaneous NO/CO2 removal performance of
CSC/recycled CaO for different carbonation/calcination cycles. As the
cycle number increases from 1 to 9, the CO2 concentration in the ex-
haust gas at 400 s increases rapidly and the CO2 capture efficiency of
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
Empty bed
ηCO2
(t ≤ 360s) ≥ 90%
ηCO2
(t ≤ 400s) ≥ 80%
Reaction time (s)
CO2
concentrationinexhaustgas(%)
Temperature:
600
o
C
650
o
C
700
o
C
(a)
ηCO2
(t ≤ 250s) ≥ 40%
0 100 200 300 400 500 600 700 800 900
0
30
60
90
120
150
180
400
450
500
Empty bed
ηNO
(t ≤ 900s) ≥ 94%
ηNO
(t ≤ 900s) ≥ 94%
ηNO
(t ≤ 900s) ≥ 67%
ηNO
(t ≤ 360s) ≥ 89%
ηNO
(t ≤ 250s) ≥ 94%
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
Temperature:
600
o
C
650
o
C
700
o
C
(b)
ηNO
(t ≤ 400s) ≥ 97%
0 100 200 300 400 500 600 700 800 900
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Reaction time (s)
COconcentrationinexhaustgas(%)
Temperature:
600
o
C
650
o
C
700
o
C
(c)
Fig. 7. Effect of reaction temperature on simultaneous NO/CO2 removal of CSC/CaO: (a) CO2 concentration, (b) NO concentration, (c) CO concentration (carbo-
nation: 15 min, 15% CO2/500 ppm NO/4% O2/N2 balance, mass ratio of CSC to CaO = 3:16; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
7

8. CSC/CaO drops due to the sintering of CaO, as shown in Fig. 9(a). As
the cycle number increases from 1 to 9, NO concentration in the exhaust
gas at 400 s rises from 15 to 117 ppm, and the NO removal efficiency of
CSC/CaO decreases from 97% to 77%, as plotted in Fig. 9(b). It means
that the catalytic effect of CaO on NO reduction by CO decreases with
the number of carbonation/calcination cycles. The NO reduction reac-
tion by CO may occur on the surface and in the inner pores of CaO.
Thus, the large surface area and pore volume of CaO are beneficial for
the catalytic effect of CaO on NO reduction by CO. However, the surface
area and the pore volume of CaO both decrease with the number of
cycles due to sintering. Thus, the catalytic effect of CaO on NO reduc-
tion by CO weakens with the number of cycles. To maintain a high NO
removal efficiency in the carbonator, the amount of fresh CaO needs to
be appropriately increased for the industrial application.
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
ηCO2
(t ≤ 400s) ≥ 78%
CaO calcined in:
850
o
C,100 % N2
950
o
C, 70% CO2
/30% N2
Empty bed
Reaction time (s)
CO2
concentrationinexhaustgas(%)
ηCO2
(t ≤ 400s) ≥ 80%
(a)
0 100 200 300 400 500 600 700 800 900
0
10
20
30
40
400
450
500
CaO calcined in:
850
o
C,100 % N2
950
o
C, 70% CO2
/30% N2
Empty bed
ηNO
(t ≤ 400s) ≥ 94%
ηNO
(t ≤ 400s) ≥ 97%
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
(b)
0 100 200 300 400 500 600 700 800 900
0.0
0.3
0.6
0.9
1.2
1.5
1.8
CaO calcined in:
850
o
C,100 % N2
950
o
C, 70% CO2
/30% N2
Reaction time (s)
COconcentration(%)
(c)
Fig. 8. Effect of calcination condition on simultaneous NO/CO2 removal by CSC/CaO: (a) CO2 concentration, (b) NO concentration, (c) CO concentration (carbo-
nation:15 min, 650 °C, 15% CO2/500 ppm NO/4% O2/N2 balance, mass ratio of CSC to CaO = 3:16; calcination: 850 °C, 100% N2 or 950 °C, 70% CO2/30% N2).
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
Empty bed
ηCO2
(t ≤ 180s) ≥ 80%
ηCO2
(t ≤ 200s) ≥ 80% ηCO2
(t ≤ 250s) ≥ 80%
ηCO2
(t ≤ 340s) ≥ 80%
CSC/CaO at
1st cycle
3rd cycle
5th cycle
7th cycle
9th cycle
Reaction time (s)
CO
2
concentrationinexhaustgas(%)
ηCO2
(t ≤ 400s) ≥ 80%
(a)
0 100 200 300 400 500 600 700 800 900
0
25
50
75
100
125
400
450
500
ηNO
(t ≤ 900s) ≥ 78%
ηNO
(t ≤ 900s) ≥ 82%
ηNO
(t ≤ 900s) ≥ 84%
ηNO
(t ≤ 900s) ≥ 88%
ηNO
(t ≤ 900s) ≥ 94%
7th cycle
9th cycle
Empty bed
ηNO
(t ≤ 200s) ≥ 86%
ηNO
(t ≤ 180s) ≥ 84%
ηNO
(t ≤ 250s) ≥ 88%
ηNO
(t ≤ 340s) ≥ 91%
ηNO
(t ≤ 400s) ≥ 97%
1st cycle
3rd cycle
5th cycle
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
(b)
Fig. 9. Effect of number of carbonation/calcination cycles on simultaneous NO/CO2 removal by CSC/CaO: (a) CO2 concentration, (b) NO concentration (carbo-
nation:15 min, 650 °C, 15% CO2/500 ppm NO/4% O2/N2 balance, mass ratio of CSC to CaO = 3:16; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
8

9. 3.7. Comparison of simultaneous NO/CO2 removal by CSC/CaO and CC/
CaO
A comparison of the simultaneous NO/CO2 removal performances
of CSC/CaO and CC/CaO in the BFBR is shown in Fig. 10. The CO2
concentrations in the exhaust gas of CSC/CaO and CC/CaO are ap-
proximately 3.5% and 2.5% at 400 s, respectively, as plotted in
Fig. 10(a). It suggests that the CO2 generated by the oxidation of CSC is
more than that generated by CC. However, it should be noted that CSC
is carbon-neutral. As shown in Fig. 10(b), the NO concentration in the
exhaust gas after the addition of CC/CaO increases rapidly from ap-
proximately 30 to 109 ppm at 400 s and further increases to 165 ppm at
900 s. However, after the addition of CSC/CaO in the BFBR, the NO
concentration in the exhaust gas is less than 20 ppm and remains stable
in 400 s and only increases to 27 ppm at 900 s. The NO removal effi-
ciencies of CSC/CaO and CC/CaO at 400 s are 97% and 81%, respec-
tively. CSC/CaO possesses a much higher NO reduction ability than CC/
CaO. The CO concentration in the exhaust gas of CSC/CaO is much
higher than that of CC/CaO, as presented in Fig. 10(c). The higher
concentration of CO generated by CSC promotes the NO removal under
the catalytic effect of CaO, which is the reason why the NO removal
ability of CSC/CaO is higher than that of CC/CaO.
The SEM images of CSC and CC are presented in Fig. 11. The surface
of CSC is looser than that of CC, as presented in Fig. 11(a) and (c). Many
macropores are observed on the surface of CSC. At higher magnifica-
tion, the pores of CSC seem more abundant than those of CC, as illu-
strated in Fig. 11(b) and (d).
The surface areas and pore volumes of CSC and CC are presented in
Table 3. The surface area and the pore volume of CSC are 2.95 and 2.20
times larger than those of CC, respectively. The porous structure of CSC
favours the reaction between char and O2 to produce CO, which facil-
itates NO removal. The pore size distributions of CSC and CC are shown
in Fig. 12. CSC possesses more pores in the diameter range of
1.7–3.7 nm than CC. Small pores are more beneficial for char oxidation
and NO reduction than large pores [39]. CSC possesses a much higher
porosity than CC, so the NO reduction ability of CSC is higher than that
of CC. In addition, CSC produce more CO than CC in the presence of O2
in the BFBR. Therefore, CSC/CaO shows a higher NO removal ability
than CC/CaO.
4. Conclusion
A novel simultaneous NO/CO2 removal system using biochar/cal-
cined limestone (CaO) in the calcium looping process was proposed.
The simultaneous NO/CO2 removal performance of coconut shell char/
CaO was studied in a bubbling fluidized bed reactor. In the presence of
O2, the addition of coconut shell char (CSC) efficiently reduces NO. CaO
has a positive catalytic effect on NO removal by CO, which is generated
by the oxidation of CSC. CO plays a positive role in NO reduction under
the catalytic effect of CaO. However, CO2 produced by the oxidation of
CO in the presence of O2 accelerates the consumption of CaO due to
carbonation and decreases the catalytic effect of CaO on NO reduction
by CO. The CO2 capture efficiency decreases with increasing O2 con-
centration. High temperature not only promotes the generation of CO2
by oxidation of CSC, which increases the consumption of CaO, but also
promotes NO reduction by CSC and CO. Thus, the NO removal effi-
ciency increases with increasing temperature from 600 to 650 °C and
then decreases upon increasing the temperature to 700 °C. The feasible
0 100 200 300 400 500 600 700 800 900
0
3
6
9
12
15
Empty bed
ηCO2
(t ≤ 400s) ≥ 80%
Reaction time (s)
CO2
concentrationinexhaustgas(%)
CSC/CaO
CC/CaO
(a)
ηCO2
(t ≤ 370s) ≥ 83%
0 100 200 300 400 500 600 700 800 900
0
30
60
90
120
150
180
400
450
500
ηNO
(t ≤ 900s) ≥ 67%
ηNO
(t ≤ 900s) ≥ 94%
ηNO
(t ≤ 400s) ≥ 97%
Reaction time (s)
NOconcnetrationinexhaustgas(ppm)
CSC/CaO
CC/CaO
Empty bed
(b)
ηNO
(t ≤ 400s) ≥ 81%
0 100 200 300 400 500 600 700 800 900
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Reaction time (s)
COconcentrationinexhaustgas(%)
CSC/CaO
CC/CaO
(c)
Fig. 10. Comparison in simultaneous NO/CO2 removal by CSC/CaO and CC/CaO: (a) CO2 concentration, (b) NO concentration, (c) CO concentration (carbonation:
15 min, 650 °C, 15% CO2/500 ppm NO/4% O2/N2 balance, mass ratio of CSC or CC to CaO = 3:16; calciantion: 850 °C, 100% N2).
W. Zhang, et al. Fuel 262 (2020) 116428
9

10. mass ratio of CSC to CaO is 3:16. The simultaneous NO/CO2 removal
efficiencies decline with the number of carbonation/calcination cycles.
The severe sintering of CaO after cycling hinders its catalytic effect on
NO reduction. Due to its porous structure and high surface area, CSC
produces more CO in the presence of O2 than coal char, which is ben-
eficial for NO reduction via CaO catalysis. For CSC/CaO, the CO2 cap-
ture efficiency reaches above 80% and the NO removal efficiency is
97% under the optimal reaction conditions. Simultaneous NO/CO2 re-
moval by biochar and CaO in the calcium looping process seems pro-
mising.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
Financial supports from the National Natural Science Foundation of
China (51876105), the Joint Foundation of National Natural Science
Foundation of China and Shanxi Province for coal-based low carbon
(U1510130) and the Fundamental Research Funds of Shandong
University (2018JC039) are gratefully appreciated.
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