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Nitrogenous Gas Emissions from Coal/Biomass Co-combustion under High Oxygen Concentration in Circulating Fluidized Bed Xin Wang, Qiangqiang Ren, Wei Li, Haoyu Li, Shiyuan Li, and Qinggang Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03141 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017
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Energy & Fuels
1
Nitrogenous Gas Emissions from Coal/Biomass Co-combustion under
2
High Oxygen Concentration in Circulating Fluidized Bed
3
Xin Wang a, b, Qiangqiang Ren*, a, Wei Li a, Haoyu Li a, Shiyuan Li a, Qinggang Lu a
4
a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s
5 6
Republic of China b
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
7
Abstract: Oxy-fuel combustion of coal/biomass is able to realize negative CO2 emission.
8
Increasing the total oxygen concentration in oxy-fuel combustion surely reduces the scale and cost
9
of the flue gas recirculation system. The increase of total oxygen concentration leads to
10
considerable changes in the emission characteristics of nitrogenous gases (nitrogen oxides and
11
their precursors). Here coal/biomass co-combustion tests were conducted in a 0.1MW oxy-fuel
12
circulating fluidized bed combustion apparatus to investigate nitrogenous gas emissions from
13
oxy-fuel combustion of coal/biomass under high total oxygen concentration (50%). HCN was also
14
detected in the flue gas besides NO and N2O especially in the mixed fuel tests. A lower excess
15
oxygen ratio led to less NO and N2O emission, the same as conventional air combustion. The
16
secondary flow ratio affected emissions of nitrogenous gases depending on fuel types and
17
atmospheres. Under the same total oxygen concentration, a higher oxygen concentration in the
18
primary flow led to lower emissions of HCN and N2O. The increase of straw share significantly
19
improved the NO, N2O and HCN emissions. O2/RFG combustion released less nitrogenous gases
20
than O2/CO2 combustion thanks to its longer residence and reaction time. Temperature rise
21
increased NO emission, but reduced N2O and HCN emissions significantly.
22
Keywords: Coal; Biomass: Oxy-fuel co-combustion; CFB; Nitrogenous gases
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1. Introduction
24
The carbon capture and storage (CCS) technology helps with the CO2 emission reduction
25
and low-carbon economy development in China. Among various CCS techniques, the oxy-fuel
26
combustion (O2/CO2) technology shows obvious advantage, feasibility and low cost
27
technology is also suitable for current technological and industrial levels, so it is able to transform
28
the existing combustion systems 1, 2. The biomass utilization technology is considered as zero CO2
29
emission. The biomass-CCS (Bio-CCS) technology, combining the biomass utilization technology
30
and the CCS technology, is used to capture and store the CO2 formed during biomass utilization,
31
which is characteristic of negative CO2 emission 3.
1, 2
. This
32
Biomass fuels are rich in alkali metals (e.g. K and Na), but their direct combustion is limited by
33
agglomeration and corrosion problems 4-6. During co-combustion of biomass fuels and coal, alkali
34
metals react with some elements in coal (e.g. Al and S) to form high-melting-point compounds,
35
thus aggravating agglomeration and corrosion 7, 8. Moreover, retrofitting an existing conventional
36
plant to a co-combustion plant is lower-cost than building a new dedicated biomass-fired plant
37
9
38
technology
39
high combustion stability and efficiency. Furthermore, co-combustion can be operated in a flexible
40
mode, which minimizes the fluctuating supply of biomass and secures the power generation 9. At
41
present, a number of coal/biomass co-combustion power plants have been successfully running for
42
years, and their co-combustion technology is completely mature 10-15. In these plants, the biomass
43
fuels are mainly agricultural straw, waste wood, industrial sludge, and living garbage, while the
44
co-combustion equipment consists of a pulverized coal furnace, a layer gas boiler
. For fossil energy utilization systems, coal/biomass co-combustion is a low-cost and low-risk for
CO2
reduction and
realizes
large-scale
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biomass utilization
16
with
and a
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Energy & Fuels
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circulating fluidized bed (CFB) boiler. These successful power plants provide experience and
46
reference for promotion of the coal/biomass co-combustion technology.
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Compared with the air combustion mode, the oxy-fuel combustion of coal/biomass is still at the
48
laboratory or pilot stage. Canmet Energy Technology Centre in Canada built a 0.8 MW CFB
49
experimental system and conducted many tests of oxy-fuel combustion of coal/biomass
50
found the mixed fuel could be stably burnt in the oxygen-enriched (O2/CO2) atmosphere and the
51
flue gas contained up to 90% CO2 or more. Meanwhile, oxy-fuel combustion of coal/biomass is a
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viable negative CO2 emission technology. Fundación CIUDEN, Spanish Government, built the
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largest and most powerful multi-functional oxy-fuel combustion test system, with the technical
54
support from Foster Wheeler 18, 19. Compared with the air combustion mode, the CIUDEN CFB
55
oxy-fuel combustion system reduced 91% CO2 emission and realized the negative CO2 emission if
56
20% biomass was mixed 19.
17
. They
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Research on oxy-combustion of coal/biomass mixtures and research on the conversion rules of
58
combustion-produced nitrogen oxides are both at the preliminary phase. Combustion tests and
59
numerical simulation both showed the addition of sawdust could reduce the nitrogen content in the
60
mixed fuel, but increased the NO conversion during the co-combustion in O2/N2 and O2/CO2
61
atmospheres
62
chars showed that CO had a more significant influence on NO conversion at 850 °C than at
63
1050-1150 °C 23. A series of coal/biomass co-combustion tests in a CFB furnace showed that NOx
64
and SO2 emissions were reduced in the co-combustion condition thanks to low fuel nitrogen
65
content in the biomass fuel
66
atmosphere did not clearly reflect the real nitrogen content, and the fuel-nitrogen to NOx
20-22
. A study on NO formation during oxy-fuel combustion of coal and biomass
24, 25
. NOx emissions from coal-bagasse burning in an O2/CO2
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conversion was between 20%-50% in all test cases
. Shandong University in China tested the
68
oxy-fuel combustion of coal/biomass in a tube reactor and found the NO release was decreased in
69
the O2/CO2 atmosphere as the mixing ratio of wheat straw and other biomass increased
70
the same O2 concentration, NO release during the coal/biomass co-combustion in the O2/CO2
71
atmosphere was reduced than in the O2/N2 atmosphere. Oxy-fuel combustion tests of coal mixed
72
with Chinese biomass in a 10 kW CFB combustion system showed that the fuel-N to NO
73
conversion depended on the H/N ratio in fuel and that oxygen staging effectively reduced NO
74
emissions under the condition of higher oxygen concentration 28.
27
. Under
75
In a word, research on nitrogen conversion in coal/biomass co-combustion was mainly carried
76
out in the traditional air atmosphere or in an oxy-fuel atmosphere with relatively low total oxygen
77
concentration (about 30%). As for oxy-fuel combustion, there is a lack of systematic tests on
78
coal/biomass co-combustion, and the findings on the release amount of nitrogen oxide during the
79
oxy-fuel combustion of coal/biomass are inconsistent. Moreover, increasing the total oxygen
80
concentration can reduce the scale of the flue gas recirculation system and thus significantly
81
decreases the initial costs. In this study, the oxy-fuel combustion of coal biomass mixtures was
82
tested in a 0.1MW oxy-fuel CFB furnace under high total oxygen concentration (50%).
83
2. Materials and methods
84
2.1 Fuel and material compositions
85
Datong coal (a bituminous coal from Datong, Shanxi Province) and two biomass fuels (corn
86
straw and wheat straw from Hebei Province) were selected. The coals and straws were milled and
87
sieved to particles of 0.355-4 mm. Before tests, coals and straws were mechanically mixed at
88
different proportions.
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The 2-4 mm alumina balls (2.5 kg) and 0.1-2 mm sand (3.5 kg) were mixed and used as the bed
90
material. During the start-up, more sand (0.25-0.355 mm) was added to the bed, if necessary, to
91
build and keep the material circulation.
92
The proximate and ultimate analyses of the feedstock are illustrated in Table 1.
93
Table 1. Proximate and ultimate analyses of feedstock (wt.%)
LHVar Car
Har
Oar*
Nar
Sar
Mar
Aar
Var
FCar MJ kg-1
Datong coal
58.28
3.74
8.61
1.04
0.32
1.87
26.14
27.46
44.53
22.70
Corn straw
37.99
4.64
32.85
1.10
0.16
1.26
22.00
61.34
15.40
14.10
Wheat straw
44.29
5.50
40.81
0.56
0.20
1.80
6.84
71.98
19.38
16.46
94
*: by difference
95
2.2 Experimental methods
96
The experimental system was described in our previous works
29, 30
, and is briefly described
97
here. The furnace has a total height of 6000mm and an inside diameter of 100 mm in the dense
98
phase zone, which expands to 140 mm in the dilute phase zone. The furnace and the cyclone
99
separator are made of refractory material, heating preservation cotton and steel shell, and four
100
water tubes cool the refractory material for a total heat duty of about 70 kW. During tests, the air is
101
supplied by the air compressor, CO2 is provided by CO2 cylinders with a purity of 99.5%, and O2
102
is provided by a liquid cylinder with a purity of 99.6%. The primary flow inlet is located at the
103
bottom of the furnace, and the height of the secondary flow gas holes is 1500 mm above the air
104
distributor. Recycled flue gas (RFG) is supplied by a recirculating fan. O2 and CO2 flow rates are
105
controlled by the mass meters independently. Through the combination of different gas sources,
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O2/CO2 and O2/RFG and other kinds of atmospheres are able to be achieved.
107
Two screw feeders constitute the feeding. One for the fuel feeding is 600 mm above the air
108
distributor, and the other for the sand feeding is 800 mm above the air distributor. Six
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thermocouples (±1oC error) are located at the height of 250 mm, 800 mm, 1520 mm, 2500 mm,
110
4000 mm and 5700 mm, respectively. The oxygen concentration in the flue gas is measured by a
111
zirconia oxygen analyzer. The measurement error for the O2 concentration is ±0.1%. The
112
concentration of CO2, CO, SO2, N2O and NO in the flue gas is monitored on-line by an FTIR
113
analyzer (GASMTE DX4000) before the bag filter. The measurement error for the CO2
114
concentration is ±0.01%, while the errors for other gases (N2O, NO, HCN) concentrations were ±1
115
ppm. The 0.1MW oxy-fuel CFB combustion system is shown schematically in Fig. 1.
116
The coal/biomass oxy-fuel combustion tests were started in an enriched air (O2/N2) firing mode.
117
When the furnace was heated to an expected temperature, the system was switched to the O2/CO2
118
or O2/RFG firing mode by gradually increasing the flow rate of O2/CO2 or O2/RFG and decreasing
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the air flow rate to zero. Coal alone was used to heat up during the start-up, and then mixed fuel
120
was put into the bed.
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Fig. 1 Schematic diagram of the 0.1 MW oxy-fuel CFB combustion system 29, 30
2.3 Experimental conditions
124
Eighteen test cases were designed and the experimental conditions are shown in Table 2. The
125
tests were aimed to investigate the effects of operation parameters on emissions of nitrogenous
126
gases during CFB-based oxy-fuel co-combustion. Given the low ash fusion point of biomass, the
127
biomass share in the tests was no more than 30%. All test data were normalized to mass per unit
128
energy (mg/MJ) for comparison
129
secondary flow, respectively. Excess oxygen ratio is defined as (actual oxygen supply per unit fuel)
130
/ (theoretical oxygen demand per unit fuel) 29. Meanwhile, the error for the O2 concentration in the
131
flue gas was within ±1% as a result of the fluctuation in coal feeding. Each test case would last
132
one hour to get enough data (temperature, gas concentration, et al) in order to compute an average
29, 30
. Here, PF and SF are short for the primary flow and the
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and analyze.
134
Table 2. Experimental conditions during the tests
Biomass Case
Atmosphere
Coal
Setting
Excess O2
Biomass
O2 in PF/SF
share (%)
temperature (oC)
ratio (1)
PF (%)
1
O2/CO2
Datong
/
/
850
1.06
60/40
50
2
O2/CO2
Datong
WS
30
800
1.06
60/40
50
3
O2/CO2
Datong
WS
30
800
1.10
60/40
50
4
O2/CO2
Datong
WS
30
800
1.12
60/40
50
5
O2/RFG
Datong
WS
30
850
1.06
60/40
50
6
O2/RFG
Datong
WS
30
850
1.15
60/40
50
7
O2/RFG
Datong
WS
30
850
1.26
60/40
50
8
O2/CO2
Datong
CS
30
850
1.16
50/50
50
9
O2/CO2
Datong
CS
30
850
1.16
60/40
50
10
O2/CO2
Datong
CS
30
850
1.16
70/30
50
11
O2/RFG
Datong
WS
30
850
1.15
70/30
50
12
O2/CO2
Datong
CS
30
850
1.21
50/50
40
13
O2/CO2
Datong
CS
30
850
1.21
50/50
50
14
O2/CO2
Datong
CS
30
850
1.21
50/50
60
15
O2/CO2
Datong
CS
30
850
1.10
50/50
50
16
O2/CO2
Datong
CS
20
850
1.10
50/50
50
17
O2/CO2
Datong
CS
10
850
1.10
50/50
50
18
O2/CO2
Datong
WS
30
900
1.06
60/40
50
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CS: corn straw; WS: wheat straw
136
2.4 Nitrogenous gases conversion knowledge
137
HCN formation mainly results from the thermal cracking of volatiles and is largely affected by
138
the H radical concentration, and reactions between the H radical and N-containing structures on
139
the char surfaces 31, 32. NO comes from two sources: directly from oxidization of fuel-N and from
140
oxidization of precursors (e.g. HCN, NH3). As reported, NO below 650 oC is mainly formed
141
directly from heterogeneous reactions rather than homogeneous oxidations or from indirect
142
formation during devolatilization
143
devolatilization 35-37. The amine radicals, NH and NH2, are important intermediates in the NH3 to
144
NO oxidation (reactions (1)-(3))
145
main intermediates are HNCO and NCO (reactions (4)-(6)) 35, 38.
33, 34
35, 37, 38
. NH3 and HCN are the main products from the
. HCN is also pivotal in the NO formation, in which the
146
NH3 + O / OH = NH2 + OH / H2O, EO=27.04 kJ/mol, EOH=4.00 kJ/mol (1)
147
NH2 + O / OH = NH + OH / H2O, EO=0.00 kJ/mol, EOH=1.93 kJ/mol (2)
148
NH + O / O2 = NO + H / OH, EO=0.00 kJ/mol, EO2=0.42 kJ/mol (3)
149
HCN + O / OH = NCO / HNCO + H, EO=20.85 kJ/mol, EOH=26.79 kJ/mol (4)
150
HNCO + O / OH = NCO + OH / H2O, EO=47.72 kJ/mol, EOH=15.07 kJ/mol (5)
151
NCO + O / OH = NO + CO / (CO + H), EO=0.00 kJ/mol, EOH=0.00 kJ/mol (6)
152 153
N2O can be considered as the product from the HCN and NH3 incomplete oxidation, as well as from NO reduction 35-37, 38, 40:
154
NCO + NO = N2O + CO, E=3.10 kJ/mol (7)
155
NH + NO = N2O + H, E=0.00 kJ/mol (8)
156
-CN (solid) + NO = N2O + (-C) (9)
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-CNO (solid) + NO = N2O + (-CO) (10)
158
3. Results and discussion
159
3.1 Feasibility and comparison tests
160
Cases 1 and case 2 were carried out to compare mono-combustion and co-combustion as well as
161
to investigate the feasibility of oxy-fuel co-combustion. The fuels were Datong coal in case 1 and
162
Datong coal/wheat straw (coal/straw=70/30) in case 2. Both tests were conducted in the O2/CO2
163
atmosphere. During these cases, both the total oxygen concentration and primary-flow oxygen
164
concentration were 50%, the excess oxygen ratio was kept at about 1.06, and the PF/SF ratio was
165
60/40. Temperature profiles in the furnace and emission factors of nitrogenous gases are shown in
166
Fig. 2.
167
As showed in Fig. 2(b), HCN emission factor was most obvious in case 2. HCN mainly came
168
from the devolatilization stage and then was oxidized to N2 or nitrogen oxides. As reported, no
169
HCN was detected in the flue gas of coal mono-combustion 29, 30. The furnace in our apparatus is
170
only 6 m high and the flow velocity is about 4 m/s, which largely shortens the residence time.
171
Wheat straw is rich in volatile-N, which releases much HCN during the co-combustion, but the
172
residence time is too short for HCN complete oxidization. Moreover, HCN formation mainly
173
results from the thermal cracking of volatiles and is largely affected by the H radical concentration
174
31, 32
175
N sites are consumed rapidly by CO2, thus prolonging the release time of HCN. The prolonged
176
release time and insufficient oxidization time together lead to the formation of HCN in the flue
177
gas.
178
, which is mentioned above in Section 2.4. With the promotion of HCN formation, the active
Compared with the coal mono-combustion mode, the addition of wheat straw led to the
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179
generation of more amine radicals and cyanogen radicals because the high contents of ammonia
180
nitrogen (protein-N and amino-N)
181
critical in NO reduction and N2O generation. As a result, the addition of wheat straw leads to a
182
decrease of NO emission factor and an increase of N2O emission factor.
39
form a reducing atmosphere. Thus, reactions (7)-(10) are
1000 900 800
o
Temperature ( C)
700 600 500
Case 1, Datong coal Case 2, Datong coal/wheat straw=70/30
400 300 200 100 0 0
1000
2000
3000
4000
5000
6000
Height (mm)
183 184
(a)
Datong coal Datong coal/wheat straw=70/30
120
100
Emission (mg/MJ)
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80
60
40
20
0 N2O
NO
HCN
185 186
(b)
187
Fig. 2 Effect of fuel type in O2/CO2: (a) Temperature profiles in the furnace and (b) Emission factors of
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188 189
nitrogenous gases
3.2 Effect of excess oxygen ratio
190
The oxygen concentration in the flue gas is an important parameter during CFBC. Six tests of
191
Datong coal/wheat straw (coal/straw=70/30) were conducted at oxygen concentrations of
192
2.6%-5.04% and 2.6%-10.4% in the flue gas, corresponding to excess oxygen ratios of 1.06-1.12
193
(cases 2, 3, 4 in O2/CO2) and 1.06-1.26 (cases 5, 6, 7 in O2/RFG), respectively. Both the total
194
oxygen concentration and primary-flow oxygen concentration were 50% in all cases, and the
195
PF/SF ratio was 60/40. Temperature profiles in the furnace and emission factors of nitrogenous
196
gases are shown in Fig. 3.
197
The excess oxygen ratio slightly affects the temperature profiles in the O2/CO2 cases. However,
198
in O2/RFG cases which were conducted in semi-closed circulating systems, it was hard to adjust
199
the excess oxygen ratios while keeping the temperature profiles unchanged.
200
Both NO and N2O emission factors were increased with the increasing excess oxygen ratio, but
201
HCN emission factors did not change significantly. It could be predicted that increasing the excess
202
oxygen ratio would promote reactions (1)-(6), thus accelerating NO emission factor. Since air
203
excess ratio and N2O emission factor are positively related 41, 42, reducing the excess oxygen ratio
204
could effectively control NO and N2O emission factors.
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1000 900 800
o
Temperature ( C)
700
O2/CO2, Datong coal/wheat straw=70/30
600
Case 2, excess O2 ratio 1.06
500
Case 3, excess O2 ratio 1.10
400
Case 4, excess O2 ratio 1.12
300
O2/RFG, Datong coal/wheat straw=70/30 Case 5, excess O2 ratio 1.06
200
Case 6, excess O2 ratio 1.15
100
Case 7, excess O2 ratio 1.26
0 0
1000
2000
3000
4000
5000
6000
Height (mm)
205 206
(a)
180
150
Emission (mg/MJ)
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Datong coal/wheat straw=70/30 N2O NO HCN
120
90
60
30
0 1.06
1.08
1.10
Excess oxygen ratio
207 208
(b)
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1.12
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120
Datong coal/wheat straw=70/30 N2O NO HCN
100
Emission (mg/MJ)
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80
60
40
20
0 1.05
1.10
1.15
1.20
1.25
Excess oxygen ratio
209 210
(c)
211
Fig. 3 Effect of excess oxygen ratio: (a) Temperature profiles in the furnace; Emission factors of nitrogenous gases
212
(b) in O2/CO2 and (c) in O2/RFG
213
3.3 Effect of flow staging
214
Flow staging is considered as one major effective method to control NO emission factor in
215
traditional air CFB combustion. For oxy-fuel CFB combustion, especially oxy-fuel CFB
216
co-combustion, whether or not flow staging would reduce emission factors of nitrogenous gases is
217
unclear
218
tests at secondary flow ratios of 30%-50%. Specifically, cases 8-10 in series 1 were conducted
219
using Datong coal/corn straw (coal/straw=70/30) in O2/CO2, while cases 6 and 11 in series 2 were
220
carried out with Datong coal/wheat straw (coal/straw=70/30) in O2/RFG. Both the total oxygen
221
concentration and primary-flow oxygen concentration were 50% in all cases and the excess
222
oxygen ratio in each series kept almost the same. Figure 4 shows the temperature profiles in the
223
furnace and the emission factors of nitrogenous gases.
224
29
. To investigate the effect of flow staging on oxy-fuel combustion, we carried out five
In the O2/CO2 cases, the flow staging has a considerable effect on the temperature profile in the
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furnace, especially in the dense phase zone. Increasing the secondary flow ratio leads to
226
temperature drop near the secondary flow inlet because of its cooling effect, and to the
227
temperature rise near the primary flow inlet due to the drop of the primary flow ratio. The possible
228
reason is that the addition of biomass reduces the ignition temperature of the mixed fuel while the
229
combustion fraction in the dense phase zone increases. Moreover, increasing the secondary flow
230
ratio leads to a reduction in the temperature and oxygen supply in the dense phase zone, which
231
hinders the combustion. In the O2/RFG cases, the flow staging has a similar effect on temperature
232
as in the O2/CO2 cases. However, this effect is not obvious as the RFG temperature is relatively
233
high.
234
In the O2/CO2 cases, NO emission factor first remains constant and then is promoted with the
235
increase of the secondary flow ratio, while N2O and HCN emission factors are first reduced and
236
then intensified. Of the three secondary flow ratios considered, the best secondary flow ratio
237
seems to be 40%. The trend of NO emission factor is similar to that in the coal mono-combustion.
238
As reported, with the same apparatus as in the coal mono-combustion mode, the NO emission
239
factor first declines slightly and then increases with the rise of the secondary flow ratio 29. On the
240
contrary, Czakiert et al. found the flow staging had no effect on N2O emission factor in a 100 kW
241
oxy-CFBC boiler
242
co-combustion mode. A too low secondary flow ratio would weaken the reducibility of the dense
243
phase zone, so the fuel-N is partially converted to N2O and NO instead of N2, but is largely
244
released in the form of HCN and NH3. In the dilute phase zone, HCN and NH3 can be oxidized to
245
NO and N2O. On the contrary, a too high secondary flow ratio would reduce the temperature and
246
oxygen supply in the dense phase zone, which is not conducive to fuel-N release. In the dilute
43
. Nitrogenous gases come from fuel-N conversion in the O2/CO2
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Energy & Fuels
247
phase zone, a large amount of unreleased fuel-N tends to release in the form of NO and N2O in the
248
oxidative environment. Therefore, neither too low nor too high secondary flow ratio is suitable for
249
reduction of NO and N2O emission factors. Unlike the O2/CO2 cases, the NO and N2O emission
250
factors in the O2/RFG cases were promoted with the increasing secondary flow ratio. This proves
251
that flow staging has different effects on nitrogenous gas emission factors depending on the fuel
252
types or the atmospheres.
1000 900 800
o
Temperature ( C)
700 600
Datong coal/corn straw=70/30, O2/CO2
500
Case 8, secondary flow ratio 50% Case 9, secondary flow ratio 40% Case 10, secondary flow ratio 30% Datong coal/wheat straw=70/30, O2/RFG
400 300
Case 6, secondary flow ratio 40% Case 11, secondary flow ratio 30%
200 100 0 0
1000
2000
3000
4000
5000
6000
Height (mm)
253 254
(a)
105
Datong coal/corn straw=70/30 N2O 90
Emission (mg/MJ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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NO HCN
75
60
45
30 30
35
40
45
Secondary flow ratio (%)
255
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256
(b)
Datong coal/wheat straw=70/30 N2O
60
NO HCN
50
Emission (mg/MJ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
40
30
20
10
0 40
30
Secondary flow ratio (%)
257 258
(c)
259
Fig. 4 Effect of flow staging: (a) Temperature profiles in the furnace; Emission factors of nitrogenous gases (b) in
260
O2/CO2 with addition of corn straw and (c) in O2/RFG with addition of wheat straw
261
3.4 Effect of oxygen staging
262
The oxy-fuel CFB combustion is faced with heavy thermal load and fast combustion in the
263
dense phase zone. As a result, slight fluctuations of gas flow supplement and fuel feeding might
264
cause violent temperature variation. Therefore, the temperature in the dense phase zone is very
265
uncontrollable in practice, which limits the total oxygen concentration of the oxy-fuel combustion.
266
The oxygen staging combustion technology, which methodologically changes the oxygen
267
concentrations in the primary and secondary flows, is an effective temperature-controlling method.
268
To investigate the effect of oxygen staging, during Datong coal/corn straw combustion tests (cases
269
12-14) we changed the oxygen concentrations in the primary and secondary flows from 40% to 60%
270
while keeping the total oxygen concentration at about 50% in O2/CO2 atmosphere. The excess
271
oxygen ratio was kept at about 1.21 and the PF/SF ratio was 50/50. Temperature profiles in the
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Page 18 of 27
272
furnace and emission factors of nitrogenous gases are shown in Fig. 5. Reducing the oxygen
273
concentration in the primary flow effectively reduces the temperatures in the dense phase zone and
274
slightly decreases the average temperature in the furnace.
275
The emission factor of each nitrogenous gas is reduced with the rise of oxygen concentration in
276
the primary flow. In particular, the reduction of NO emission factor is slight, indicating the oxygen
277
concentration in the primary flow has almost no significant effect on NO emission factor.
278
Moreover, the HCN and N2O emission factors both decrease obviously owing to the generation
279
and conversion of HCN and N2O. Decreasing the oxygen concentration in the primary flow would
280
reduce the combustion fraction in the dense phase zone, resulting in a temperature drop in this
281
zone and on average (Fig. 5a). As for HCN, a lower temperature and stronger reducibility of the
282
dense phase zone help the conversion of fuel-N to HCN instead of N2 and NO. In the dilute phase
283
zone, a lower temperature also suppresses the HCN to NO/N2O conversion. As for N2O, N2O
284
emission factor is temperature-sensitive and decreases with temperature rising
285
oxygen concentration in the primary flow causes a lower temperature in the furnace and thus a
286
higher N2O emission factor.
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. A lower
Page 19 of 27
1000 900 800
o
Temperature ( C)
700 600 500 400 300
Datong coal/corn straw=70/30 Case 12, O2 concentration in primary flow 40%
200
Case 13, O2 concentration in primary flow 50% Case 14, O2 concentration in primary flow 60%
100 0 0
1000
2000
3000
4000
5000
6000
Height (mm)
287 288
(a)
Datong coal/corn straw=70/30 N2O
120
NO HCN
100
Emission (mg/MJ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
80
60
40
20 40
45
50
55
60
O2 concentration in primary flow (%)
289 290
(b)
291
Fig. 5 Effect of oxygen staging in O2/CO2: (a) Temperature profiles in the furnace and (b) Emission factors of
292
nitrogenous gases
293
3.5 Effect of the mixing ratio
294
The mixing ratio is an important factor for conventional coal/corn-straw co-combustion as well
295
as oxy-fuel co-combustion. Here three tests were conducted at the corn straw share of 10%, 20%
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
296
and 30%, respectively (cases 15-17), with the excess oxygen ratio at 1.10, both the total oxygen
297
concentration and the primary flow oxygen concentration at 50%, and PF/SF ratio at 50/50. Figure
298
6 shows the temperature profiles in the furnace and emission factors of nitrogenous gases. The
299
average furnace temperature drops with the rise of the corn straw share because coal has a larger
300
low heat value (LHV) than corn straw.
301
With the rise of corn straw share, emission factors of NO, N2O and HCN increase by 241.3%
302
(26.4 to 90.1 mg/MJ), 237.5% (17.6-59.4 mg/MJ) and 491.0% (7.8-46.1 mg/MJ), respectively. As
303
showed in Table 1, the nitrogen contents are almost the same between Datong coal and corn straw,
304
indicating the effect of fuel-N contents in different fuels can be ignored. As a result, emission
305
factors of nitrogenous gases owe to different fuel-N forms in coal and corn straw. The forms of N
306
in corn straw are mainly amino-N and protein-N, as well as a small amount of pyridine-N and
307
pyrrole-N 39, which are basically characteristic of volatile-N. However, the main N forms in coal
308
are pyrrole-N, pyridine-N, quaternary-N and N-oxide 44. Volatile-N is prone to be released in
309
forms of nitrogenous gases. This can be interpreted from three aspects: (1) the nitrogen in corn
310
straw is mainly volatile-N which is prone to release and conversion; (2) much nitrogen in coal is
311
char-N which is hard to release and convert; (3), the fixed carbon content in coal is larger than
312
corn straw, so NO is reduced at high temperature
313
2CO+2NO=2CO2+N2 occur easily. As a consequence, the rise of corn straw share results in an
314
increase of volatile-N content and decline of fixed carbon and char nitrogen contents in the mixed
315
fuel. Thus, both the fuel-N to nitrogenous gas conversion and NO/CO/char reactions are easier to
316
happen, which obviously promoting the emission factors and conversions of nitrogenous gases
317
with the rise of corn straw share in the mixed fuel.
38, 45, 46
. Thus, reactions 2C+2NO=2CO+N2 and
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1000 900 800
o
Temperature ( C)
700 600 500 400
Case 15, corn straw share 30% Case 16, corn straw share 20% Case 17, corn straw share 10%
300 200 100 0 0
1000
2000
3000
4000
5000
6000
Height (mm)
318 319
(a)
100 90
N2O
80
NO HCN
70
Emission (mg/MJ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
60 50 40 30 20 10 0
10
20
30
Biomass share (%)
320 321
(b)
322
Fig. 6 Effect of the mixing ratio in O2/CO2: (a) Temperature profiles in the furnace and (b) Emission factors of
323
nitrogenous gases
324
3.6 Effect of temperature in the furnace
325 326
Temperature is an important parameter during the operation of a CFB boiler. Case 2 (about 800 o
C) and case 18 (about 900 oC) with Datong coal/wheat straw (coal/straw=70/30) were conducted
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327
to investigate the effects of temperature on emission factors of nitrogenous gases. Similarly, the
328
excess oxygen ratio, total oxygen concentration, oxygen concentration in the primary flow, and the
329
PF/SF ratio were all kept constant. Figure 7 shows temperature profiles in the furnace and
330
emission factors of nitrogenous gases.
331
The high combustion temperature accelerated the conversion of HCN, which was a product
332
from incomplete combustion. Homogeneous N2O destruction by radicals can be expressed as
333
follows 35:
334
N2O + H = N2 + OH, E=79.03 kJ/mol (11)
335
N2O + OH = N2 + HO2, E=88.16 kJ/mol (12)
336 337
Similarly, under flame conditions or high temperature (>900 °C), the N2O destruction through thermal dissociation in collision with any molecule M is also important 47: N2O + M = N2 + O +M, E=237.01 kJ/mol (13)
338 339
The higher temperature further activates the free radicals (e.g. H and OH), which contribute to
340
reactions (11) and (12). The higher temperature in case 18 also promoted N2O thermal dissociation,
341
leading to the loss of N2O through reaction (13) 48. Thus, temperature rise effectively reduces N2O
342
emission factors.
343
NO emission factor increases slightly with the temperature increasing (from 800 oC to 900 oC).
344
High levels of CO2 will reduce the temperature of the char particles so as to extend fuel-N release
345
time during oxy-fuel combustion. Further, it is difficult for fuel-N from coal char to release
346
completely at the low temperature of 800 oC during coal / biomass co-combustion. At the
347
temperature of 900 oC, the fuel-N releases and converts more completely, while HCN and N2O
348
release lower than those at 800 oC. So NO emission factor at 900 oC is higher than 800 oC.
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1000 900 800
o
Temperature ( C)
700 600 500 400
Datong coal/wheat straw=70/30 o Case 2, 800 C o Case 18, 900 C
300 200 100 0 0
1000
2000
3000
4000
5000
6000
Height (mm)
349 350
(a)
120
Datong coal/wheat straw=70/30 o 800 C o 900 C
100
Emission (mg/MJ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
80
60
40
20
0 N2O
NO
HCN
351 352
(b)
353
Fig. 7 Effect of average temperature in O2/CO2: (a) Temperature profiles in the furnace and (b) Emission factors of
354
nitrogenous gases
355
4. Conclusions
356
Combustion and emission characteristics of coal/straw mixed fuel were investigated in oxy-fuel
357
atmosphere. A series of laboratory-scale CFB tests were conducted at high total oxygen
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358
concentration and some new results are shown:
359
Compared with coal mono-combustion, HCN emission factor is more obvious in coal / biomass
360
co- combustion because of volatile-N rich in straw. At the same total oxygen concentration, a
361
higher oxygen concentration in the primary flow leads to less emission factors of HCN and N2O
362
mainly because of higher average temperature in the furnace. Increasing the straw share
363
significantly promoted the emission factors of NO, N2O and HCN, which was attributed to the low
364
combustion temperature and easy release of fuel-N when straw was added. As a result of the
365
longer residence and reaction time, O2/RFG combustion outperformed O2/CO2 combustion in
366
terms of emission factors of nitrogenous gases. The average temperature rise led to a slight
367
increase of NO emission factor but significant decrease of N2O and HCN emission factors.
368
Author information
369
Corresponding author
370
*Tel: +86-10-82543055, E-mail:
[email protected] 371
Acknowledgements
372
This work is funded by the External Cooperation Program of BIC, Chinese Academy of Sciences
373
(2014DFG61680) and Youth Innovation Promotion Association CAS (No. 2015120).
374
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