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Effect of air distribution on NOx emissions of pulverized coal and char combustion preheated by a circulating fluidized bed shujun zhu, Qinggang Lyu, Jianguo Zhu, Huixing Wu, and Guanglong Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01366 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018
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Energy & Fuels
1
2
Effect of air distribution on NOx emissions of pulverized coal and char
3
combustion preheated by a circulating fluidized bed
4
Shujun Zhua,b, Qinggang Lyu*,a,b, Jianguo Zhu*,a,b, Huixing Wua,b, Guanglong Wua,b
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6
7
a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China b
University of Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT
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This study reports an experimental investigation on the nitrogen oxides (NOx) emissions in pulverized fuel
11
(coal and char) combustion through preheating with a circulating fluidized bed. The high-temperature
12
preheated fuel particles obtained from the circulating fluidized bed would be burned in the down-fired
13
combustor. The focus of research is the trend of NOx emissions with different air distribution through
14
varying secondary air nozzle structures and air ratios as well as tertiary air positions along the down-fired
15
combustor. Under the stable operation, the burning temperature was uniform and the combustion
16
efficiency was high. When the fuel was pulverized coal, the NOx emissions with the secondary air center
17
nozzle structure were almost twice of that with ring nozzle structure. Furthermore, the NOx emissions
18
increased with the secondary air ratio increasing when the nozzle structure was center. However, there was
19
a minimum NOx concentration when the nozzle structure was ring. And the lower NOx emissions were
20
achieved through arranging the tertiary air distribution rationally. In addition, the trend in NO
21
concentration along the down-fired combustor was almost the same irrespective of the fuel (coal or char).
22
But the char combustion efficiency should be paid more attention when the tertiary air position was
23
changed.
24
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Energy & Fuels
1. Introduction
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Nitrogen oxides (NOx) emissions in the utilization of fossil fuel result in air pollution including acid rain
27
and photochemical smog [1]. Owing to the variety and complexity of NOx chemical reactions, much
28
attention has been paid to reduce NOx emissions in fossil fuel combustion [2]. There have already been
29
lots of technologies to control NOx emissions, including gas-staging, fuel-staging, selective catalytic
30
reduction (SCR) as well as selective noncatalytic reduction (SNCR) [3-8]. The technology with air-staging
31
could produce a reductive atmosphere in main combustion zone, and fuel-staging could contribute to
32
reductive reactions in reburning zone. The key theory of SCR and SNCR lies in reduction reactions of
33
nitrogen-containing gas by adding the nitrogen reducing agent. However, the NOx emissions were reduced
34
accompanied by lower combustion efficiency and other chemical catalyst pollution. Moreover, there have
35
been some competitive technologies for years worldwide, which include high-temperature air combustion
36
[9-11] and MILD combustion [12-14]. What they have in common is to maintain a uniform combustion
37
temperature below 1500 ºC in a main combustion region, which could inhibit the formation of thermal NO.
38
There are large reserves of low-rank coal (lignite / sub-bituminous coal) in China, which is more than
39
55%. The traditional combustion methods cannot achieve the clean and efficient use of coal. Therefore, the
40
utilization mode of low-rank coal is gasification or pyrolysis to satisfy the requirements of chemical
41
production. However, with the production of oil gas, there is also a large amount of char, which is a kind
42
of low-volatile, low-calorie inferior fuel compared to raw coal. Hence there are technical difficulties such
43
as difficulty in ignition, poor combustion stability, delayed combustion, and poor burn-out performance,
44
which cause a waste of energy. At present, low-volatile coal is commonly burned in a W-type boiler [15,
45
16], but the NOx emissions reach up to 500 mg/m3 (@6% O2). Also, there are no mature techniques to
46
achieve the efficient combustion of low-volatile char. Therefore, it is meaningful to explore an efficient
47
method to utilize this kind of fuel for energy conservation and emission reduction. 3 ACS Paragon Plus Environment
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A competitive new technology for preheating solid fuel with a circulating fluidized bed (CFB) was
49
proposed to utilize this kind of fuel for energy conservation and emission reduction [17]. First, the solid
50
fuel is preheated in a reducing atmosphere with a CFB. Then the high-temperature preheated fuel (coal gas
51
and solid particles) would be burned in the down-fired combustor (DFC) under air staged combustion.
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Based on the technology, the ignition and burnout process of low-volatile char is smooth. Moreover, the
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high combustion efficiency and low NOx emissions could be implemented simultaneously. The main
54
content of previous study is the feasibility and stability of this technology [18-20]. However, the NOx
55
emissions with different air distribution mode have not been systematically studied. And the adaptability in
56
fuel should be further evaluated.
57
This work investigated the trend of NOx emissions with different air distribution mode when pulverized
58
fuel (coal and char) combustion was preheated through a CFB. The preheating phase was firstly evaluated,
59
which included the variation of temperature in the CFB and the analysis of preheated fuel sampled at the
60
cyclone outlet. Secondly, different secondary air nozzle structures and air ratios as well as tertiary air
61
distribution in DFC were investigated experimentally.
62
2. Experiments
63
2.1. Apparatus and method.
64
The pulverized fuel (coal and char) combustion was conducted in a test system preheated with a CFB,
65
which could be seen in Figure 1. The preheated fuel from the CFB flowed to the DFC through a horizontal
66
pipe with 500 mm in length and 48 mm in diameter. The heating power in test system was approximately
67
30 kW during the stable operation.
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Energy & Fuels 6
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
Secondary Air
4 2
Tertiary Air
3 7 5
Primary Air
8 10
1
11
12
9
1-Air Compressor, 2-Screw Feeder, 3-Riser, 4-Cyclone, 5-U-valve, 6-Sampling Port, 7- Down-fired Combustor, 8-Sampling Port, 9-Water Tank, 10-Water Cooler, 11-Bag Filter, 12-Gas Analyzer
68
Figure 1. Schematic diagram of test system
69
The CFB riser was 78 mm in diameter with a height of 1500 mm. The primary air supplies oxygen for
70
fuel oxidation reactions, which could ensure the preheating temperature above 800 ºC. The gas and the
71
solid particles from the cyclone outlet were defined as coal gas and preheated fuel particles, respectively.
72
The preheated fuel from the CFB flowed to the DFC through a horizontal pipe. The DFC was 260 mm in
73
inner diameter with a height of 3000 mm. Two secondary air nozzle structures were investigated here, as
74
displayed in Fig. 2, which included the center nozzle (nozzle-A) and ring nozzle consisting of four ports
75
(nozzle-B) arranged on the top of the DFC. Furthermore, the tertiary air position was arranged at 600 mm
76
or 1200 mm from the top to guarantee a complete combustion of fuel. A reductive atmosphere was
77
generated before injecting the tertiary air to inhibit NOx generation.
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78 79
Figure 2. Secondary air nozzle structures
80
There are four thermocouples (Ni-Cr/Ni-Si) arranged in the CFB: Three were arranged at 100 mm, 500
81
mm, and 1450 mm above the gas distributor; fourth one was arranged at the return leg. Furthermore, a
82
thermocouple (Ni–Cr/Ni–Si) was arranged in the horizontal pipe connecting the CFB and DFC. Five
83
thermocouples (Pt/Pt-Rh) were also arranged along the DFC, located at 100 mm, 400 mm, 900 mm, 1400
84
mm, and 2400 mm from the top of DFC. The test temperatures were recorded as the average of 30 min of
85
stable operation with variations of less than 10 °C/min. A comparison with standard thermocouples
86
revealed the total uncertainty to be within ± 0.5%. And there were eight sampling locations arranged: One
87
location was arranged at the cyclone outlet to sample the preheated products; one location was arranged at
88
outlet of the bag filter to sample the fly ash; the others were arranged at 100 mm, 400 mm, 900 mm, 1400
89
mm, 2400 mm, and 3000 mm from the top of DFC. The gases at the cyclone outlet were analyzed using a
90
gas chromatographic analyzer as well as a Testo-310 analyzer. And the gases along the DFC were analyzed
91
using a Gasmet FTIR DX-4000 analyzer as well as a KM9106 analyzer. The error in gas concentration is
92
within ±2%.
93
2.2. Fuel characteristics.
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Energy & Fuels
94
Shenmu coal and Shenmu char, which was obtained by the water coke quenching of a two-stage fixed
95
bed pyrolysis furnace, were selected to investigate the trend of NOx emissions during the preheating
96
combustion. The proximate and ultimate analyses of fuel are summarized in Table 1. And the size
97
distribution of fuel particles was between 0.1–0.355 mm. Coal and char had been fed into the CFB
98
separately in different experiments.
99
Table 1 Proximate and ultimate analyses (wt%, air-dried) of fuel items
Shenmu coal
Shenmu char
7.09 32.11 54.56 6.24
3.73 13.90 68.40 13.97
68.70 4.53 12.29 0.90 0.25 24.40
73.77 1.92 5.54 0.78 0.29 23.32
Proximate analysis Moisture Volatile matter Fixed carbon Ash Ultimate analysis Carbon Hydrogen Oxygen Nitrogen Sulfur Low heating value (MJ/kg) 100
2.3. Experimental conditions.
101
The research was carried out by varying the secondary air nozzle structures and air ratios as well as
102
tertiary air distribution. Table 2 lists the experimental parameters. The air stoichiometric ratios of the
103
primary air and the secondary air are λCFB and λSe, respectively. In addition, the tertiary air stoichiometric
104
ratio is defined as λTe. They were summarized here:
105
ುೝ
ߣி =
(1)
ೄ
106
ೄ
ߣௌ =
-
(2)
ೄ
107
ߣ ் =
108
ߣ ்ଵଶ =
109
లబబ ೄ
భమబబ ೄ
(3)
(4)
where AStoic (Nm3/h) is the gas flow rate with stoichiometric complete burnout. APr and ASe are the air flow 7 ACS Paragon Plus Environment
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110
rates of the primary air and the secondary air, while ATe600 or ATe1200 is the tertiary air flow rate at 600 mm or
111
1200 mm from the top of the DFC, respectively.
112 113
In this study, λCFB was set to 0.40, which means that 40% of the stoichiometric oxygen flowed to the CFB, and λStoic (λStoic = λCFB + λSe + ATe600 + ATe1200) was fixed as 1.23. Table 2 Experimental conditions
114 case
air injection arrangement λSe
λTe600
λTe1200
1
nozzle structure
0.480
0.350
0.000
2
0.530
0.000
0.300
0.430
0.000
0.400
0.380
0.000
0.450
5
0.330
0.000
0.500
6
0.480
0.000
0.350
7
0.480
0.175
0.175
3 4
115 116
According to the principle of the ash balance, the conversion ratios of components in fuel, ܥ , during the preheating were calculated here [21]:
ܥ = 1 −
117 118
A /B
భ ×మ మ ×భ
(5)
where A1 and X1 are the ash content and the component X content in the fuel, while A2 and X2 are the ash
119
content and the component X content in the preheated fuel, respectively.
120
3. Results and discussion
121
3.1. Preheating characteristics in CFB.
122
Approximately 40% of the stoichiometric air flowed to the CFB to maintain a preheating temperature of
123
900 ± 10 ºC. Fig. 3 indicates that the recorded temperature distribution remained steady, and the change in
124
these temperature points in the CFB was below 50 ºC. Moreover, the temperature at the cyclone outlet
125
reached above 800 ºC, which was above the ignition temperature of preheated fuel; therefore, the
126
combustion reactions started once the secondary air flowed into the DFC. Fig. 3 also indicates that the
127
measuring pressure drop profile remained steady. The above results have proven that the preheating was
128
stable. 8 ACS Paragon Plus Environment
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Shenmu char
800
100-500mm 500-1450mm cyclone
3.5
Pressure Drop (kPa)
3.0
600 400
2.5 2.0 1.5 1.0 0.5 0.0
200 0 1000
0
15
30
45
60
75
90
Time (min)
Shenmu coal 3.5
800
100-500mm 500-1450mm cyclone
3.0
600 400
100 mm above the air distributor 500 mm above the air distributor 1450 mm above the air distributor return leg outlet of the CFB
200 0
Pressure Drop (kPa)
Temperature (°C)
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
2.5 2.0 1.5 1.0 0.5 0.0 0
15
30
45
60
75
90
Time (min)
0
15
30
45 Time (min)
60
75
90
129 130 131 132
Figure 3. Temperature and pressure drop distribution in the CFB
To evaluate the preheating, the preheated fuel (solid particles and coal gas) were sampled and analyzed, and the results are shown in Tables 3 and 4, respectively. Table 3. Analyses of preheated fuel particles
133
Shenmu coal Preheated particles analysis
Value
Shenmu char
Conversion
Value
ratio (%)
Conversion ratio (%)
Proximate analysis (wt%, air-dried) Moisture
4.75
70.39
1.59
77.57
Volatile matter
9.47
86.97
5.94
77.51
Fixed carbon
71.58
42.02
64.92
49.29
Ash
14.12
0.00
26.55
0.00
Carbon
78.46
49.53
69.05
50.46
Hydrogen
1.25
87.80
1.03
71.77
Ultimate analysis (wt%, air-dried)
Oxygen
0.22
99.21
0.69
96.30
Nitrogen
0.75
63.17
0.62
58.17
Sulfur
0.47
16.92
0.47
32.87
Table 4. Analyses of high-temperature coal gas
134
Shenmu coal
Shenmu char
Value
Value
%
7.76
7.99
CO2
%
14.14
16.03
H2
%
5.87
2.43
CH4
%
1.14
0.45
O2
%
0.00
0.00
Items
Unit
CO
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Page 10 of 21
NO
mg/Nm3
0.00
0.00
NO2
mg/Nm3
0.00
0.00
N2O
3
0.00
0.00
3
2.02
1.43
LHV
mg/Nm
MJ/ Nm
135
According to Eq. (5), the carbon conversion ratio of the preheating was approximately 50% irrespective
136
of the fuel (Shenmu coal or char), i.e., 50% of the carbon was released during the preheating. This shows
137
that almost half the fuel-carbon underwent combustion in the DFC. It can also be concluded that most of
138
the volatile in the fuel was released in the CFB. Despite this, the original volatile content in Shenmu coal
139
was higher than that in char; therefore, the volatile content in preheated Shenmu coal particles was still
140
relatively high.
141
Table 4 shows that the coal gas consisted of H2, CO, and CH4 with an average volume fraction of 5.87%,
142
7.76%, and 1.14%, respectively, when fuel was Shenmu coal. However, the average volume fractions of
143
H2, CO, and CH4 were 2.43%, 7.99%, and 0.45%, respectively, when fuel was Shenmu char. Here, the
144
average volume fraction refers to the ratio of different gas volume to the total gas volume at the cyclone
145
outlet. Similar conclusions with respect to the concentrations of H2, CO, and CH4 have been reported in
146
literature [22, 23]. The low heating value (LHV) of the coal gas was 2.02 and 1.43 MJ/m3, respectively.
147
This can be attributed to the higher hydrogen content in the outlet of the CFB caused by the higher volatile
148
content in the Shenmu coal.
149
During the preheating, the fuel-bound nitrogen in fuel was converted to nitrogen in the coal gas and
150
preheated solid particles. In addition, the elemental nitrogen content of the Shenmu coal and char
151
preheated particles was 0.75% and 0.62%, respectively. According to Table 4 and Eq. (5), the
152
concentrations of NO, NO2, and N2O gases were zero; therefore, 63.17% and 58.17% of the fuel-bound
153
nitrogen was converted into N2, NH3, and HCN. According to our previous study [18, 19], approximately
154
30–45% of the fuel-bound nitrogen in fuel was reduced to N2 in the CFB, contributing to a lower
155
fuel-bound nitrogen flowing into the DFC and lower NOx emissions. 10 ACS Paragon Plus Environment
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Energy & Fuels
3.2. Temperature distributions in DFC.
157
In the experiments, λCFB = 0.40, λSe = 0.48, λTe600 = 0.35, and λStoic = 1.23 were set as the basic
158
experimental parameters. Figure 4 displays the coal combustion temperature distributions along the
159
centerline in the DFC with two kinds of nozzle structures of secondary air (Case 1). The maximum value
160
in temperature of center secondary air nozzle was 1080 ºC at 400 mm from the top of DFC, whereas that
161
of ring secondary air nozzle was 1180 ºC. This shows that nozzle structure-B promoted the mixing of air
162
and high-temperature preheated fuel from the CFB for smoother combustion. Also, the combustion
163
efficiencies of center and ring structures were 97.4% and 98.1%, respectively. The temperature profile
164
with different fuel types were studied under ring secondary air nozzle conditions. Figure 5 indicates that
165
the temperature distribution was almost same before injecting the tertiary air under the conditions of
166
Shenmu coal and char.
167 168
Figure 4. Temperature distribution along the centerline of the DFC when the secondary air nozzle structure was center or
169
ring
170
The maximum value of temperature along the DFC was lower than 1500 ºC. Therefore, thermal NO and
171
prompt NO could be ignored in the fuel combustion [24, 25], which indicated that NOx was mainly from
172
conversion of fuel-bound nitrogen and conversion of NH3 and HCN during the combustion along the DFC.
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173 174
Figure 5. Temperature distribution along the centerline of the DFC when the fuel was pulverized coal or char
175
3.3 Effects of air distribution on NOx emissions.
176
3.3.1 Effects of air distribution on NO concentrations along the centerline of DFC.
177
The NO concentrations in coal combustion along the DFC centerline with two nozzle structures (Case 1)
178
are summarized in Figure 6. Because the gas volume flow rate increased after injecting the tertiary air, the
179
unit of gas concentration was defined as mg/MJ [26]. The NO concentration of ring nozzle was 0 mg/MJ
180
at 100 mm from the top, which indicated that the region from the top to 100 mm of the DFC was in a
181
strong reductive atmosphere. However, the NO concentrations with center nozzle at 100 mm from the top
182
were approximately 130 mg/MJ. This can be explained as follows: Secondary oxygen through nozzle
183
structure-A mixed more early with preheated fuel, which caused a local high-oxidation region and
184
contributed to the increase in NO concentration [20]. Moreover, the combustion reactions closer to the
185
burner nozzle contributed to generate more NOx [27].
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150
center ring
Tertiary air at 600 mm
120
NO (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
90
60
30
0 0
500
1000
1500
2000
2500
3000
Distence from the top( mm)
186 187
Figure 6. NO concentrations along the centerline of the DFC when the secondary air nozzle structure was center or ring
188
When the tertiary gas position was arranged at 600 mm below the top, the NO concentrations slightly
189
decreased below 900 mm from the top with the reduction reactions occurring along the DFC. Furthermore,
190
oxidation and reduction reactions reached a balanced state at last. The main reduction reactions after
191
injecting the tertiary gas were as follows [28, 29]:
192 193
NO + C → CO + 1/2Nଶ C , CO + Oଶ → COଶ
(6) (7)
194
Moreover, the temperature in reductive zone increased with organizing secondary air flow field more
195
rationally, causing intensive heterogeneous reactions [30]. The most important point is that the
196
heterogeneous reactions played the main part in the reduction reactions [31]. Therefore, NO concentrations
197
significantly decreased in the flue gas when the center nozzle structure was varied to ring.
198
The NO2 concentrations with two nozzle structures in the flue gas were almost zero. The NOx emissions
199
for the center and ring structures were 528.73 and 252.07 mg/Nm3 (@6% O2), respectively, less than those
200
in the facilities of fuel combustion with low-NOx techniques [32-34].
201
Similar results have been obtained under the conditions of Shenmu char [20]. The NOx emissions could
202
also be sharply reduced by converting the center to ring nozzle structures. Therefore, the NOx emissions in
203
char combustion with different secondary air nozzle structure are not discussed here.
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Energy & Fuels 204
Figure 7 shows that the NO trend along the centerline in the DFC was the same even when the volatile
205
content in Shenmu coal preheated particles was much higher. The NO concentrations at 400 mm under the
206
Shenmu coal condition were apparently higher than the char condition due to the flow of more amount of
207
nitrogen-containing compounds into the DFC. Furthermore, the NO concentrations decreased more
208
sharply below 900 mm from the top due to more developed pore structure under the Shenmu char
209
conditions. 80
shenmu coal shenmu char 60 Tertiary air at 600 mm
NO (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
Page 14 of 21
40
20
0 0
500
1000
1500
2000
2500
3000
Distance from the top( mm)
210 211
Figure 7. NO concentrations along the centerline of the DFC when the fuel was pulverized coal or char
212
3.3.2 Effects of tertiary air distribution mode on NOx emissions.
213
The tertiary gas distribution included the tertiary gas at 600 mm from the top, at 1200 mm from the top,
214
and uniformity at 600 mm and 1200 mm from the top, which were identified as "at 600 mm", "at 1200
215
mm", and "at 600 and 1200 mm". For other operating parameters, see case 1, case 6, and case 7.
216
As shown in Figure 8, the trend of NOx concentration of pulverized coal combustion in the DFC outlet
217
was the same under center or ring nozzle structures. The secondary gas ratio was kept constant; thus, the
218
released-nitrogen by the oxidation reaction in the reducing zone remained stable. When the tertiary gas
219
was equally injected at 600 mm and 1200 mm from the top, NOx emissions decreased further. This is
220
because that the tertiary gas at 600 mm could not achieve a complete burnout of fuel, only a part in
221
fuel-bound nitrogen could be released. The zone before injecting tertiary gas at 1200 mm from the top was 14 ACS Paragon Plus Environment
Page 15 of 21 222
still in a reductive atmosphere, so the released-nitrogen in the region continued to be reduced, contributing
223
to reduce NOx emissions. When the tertiary air position was varied from 600 mm to 1200 mm from the top,
224
the NO concentrations decreased. The reason for this is the zone before injecting the tertiary gas was in a
225
strong reduction atmosphere with incomplete combustion. Homogeneous reduction and heterogeneous
226
reduction played a leading role in inhibiting NO production. The reductive ratio of NO increased when the
227
length of reductive zone increased, leading to clear decrease in NOx emissions. 600 at 600 mm at 600 and 1200 mm at 1200 mm
500
NOx (mg/Nm3 @6% O2)
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
400
300
200
100 0 center
ring
228 229
Figure 8. NOx emissions with different tertiary air distribution positions when the secondary air nozzle structure was
230
center or ring
231
The combustion efficiencies of ring structure were approximately 98.1% irrespective of the tertiary gas
232
position at 600 mm or 1200 mm. Therefore, the NOx emissions in coal combustion could be reduced by
233
injecting tertiary air at a longer distance from the top in the DFC, which did not reduce combustion
234
efficiency.
235
In Figure 9, the variation in NOx concentrations showed almost the same tendency under the coal and
236
char conditions. The NOx emissions reached the lowest value with the tertiary gas injecting at 1200 mm
237
from the top. The reasons are mentioned above. However, the NOx emissions increased slightly with the
238
tertiary air positions changing from 600 mm to a combination of 600 mm and 1200 mm from the top when
239
fuel was char. This may be due to the oxidation reaction played a leading role compared to the reduction
240
reaction in the zone from 600 mm to 1200 mm from the top. Specific reasons should be further studied. 15 ACS Paragon Plus Environment
Energy & Fuels 280 at 600 mm at 600 and 1200 mm at 1200 mm
240
NOx (mg/Nm3 @6% O2)
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
Page 16 of 21
200
160
120
80 0 shenmu coal
shenmu char
241 242
Figure 9. NOx emissions with different tertiary air distribution positions when the fuel was pulverized coal or char
243
Furthermore, the combustion efficiencies of Shenmu char dropped from 97.8% to 96.9% with tertiary air
244
positions changing from 600 mm to 1200 mm. Compared to Shenmu coal, more attention should be paid
245
to combustion efficiencies under the Shenmu char conditions besides the reduction in NOx emissions by
246
injecting tertiary air at a longer distance from the top in the DFC.
247
3.3.3 Effects of secondary air ratio on NOx emissions.
248
The NOx emissions with different secondary air ratios were studied under two secondary air nozzle
249
structure conditions. The tertiary gas position was arranged at 1200 mm from the top of the DFC. The
250
experimental parameters could be seen in Table 2 (cases 2–6). In this study, the zone before injecting
251
tertiary air was kept in a reductive atmosphere continually by adjusting the total air stoichiometric ratio
252
before injecting tertiary air to less than 1.00.
253
Figure 10 shows that the NOx emissions in coal combustion increased with the secondary air ratio
254
increasing in a lower ratio under the conditions of center nozzle structure. When the secondary and tertiary
255
gas positions were fixed, the ratio variation changed the fuel combustion portion, which led to a variation
256
in the released-nitrogen concentration in the reductive zone. Therefore, reductive reactions were gradually
257
inhibited with the secondary oxygen concentration increasing. In addition, NOx emissions first decreased
258
to a lowest value and then increased with the secondary air ratio increasing under the conditions of ring
16 ACS Paragon Plus Environment
Page 17 of 21 259
nozzle structure. The difference is due to that the variation in NO concentration was an integrated result of
260
various factors including the combustion oxidation reaction and reduction reaction in the reductive
261
atmosphere before the tertiary gas was injected. Furthermore, the local high-oxidation zone was inhibited
262
by changing the nozzle structures. Therefore, the NO reduction reaction became dominant before injecting
263
the tertiary gas. Moreover, it is because that a moderate amount of oxygen would increase the nitrogen
264
reduction reaction rate [35]. The comprehensive result led to the increase in the total reduction amount. As
265
a result, the NOx emissions dropped to the lowest when the secondary air ratio was 0.43. And the NOx
266
emissions also increased with the secondary air ratio continuing to increase. 320 center ring
280
NOx (mg/Nm3 @6% O2)
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Energy & Fuels
240
200
160
120 0 0.33
0.38
0.43
0.48
0.53
Secondary air ratio
267 268
Figure 10. NOx emissions with different secondary oxygen ratios when the secondary air nozzle structure was center or
269
ring
270
In Figure 11, the NOx emissions first decreased to a lowest value and then increased with the secondary
271
air ratio increasing irrespective of fuel types (coal or char). The reasons are mentioned above. Therefore,
272
the NOx emissions dropped to the lowest when the secondary air ratios were 0.43 and 0.48 under the coal
273
and char conditions, respectively. And the NOx emissions also increased with the secondary air ratio
274
continuing to increase.
17 ACS Paragon Plus Environment
Energy & Fuels
shenmu coal shenmu char
210
NOx (mg/Nm3 @6%O2)
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Page 18 of 21
180
150
120
0 0.33
0.38
0.43
0.48
0.53
Secondary air ratio
275 276 277
Figure 11. NOx emissions with different secondary oxygen ratios when the fuel was pulverized coal or char
4. Conclusion
278
Adaptability of fuel is discussed here. Pulverized coal and char achieved stable combustion preheated
279
with a CFB. And the coal gas sampled from the cyclone outlet included N2, CO2, CO, H2, and CH4. The
280
reductive reaction region in the CFB contributed to a lower fuel-bound nitrogen flowing into the DFC and
281
lower NOx emissions.
282
With the secondary air nozzle structures changing from center to ring, NOx emissions reduced from
283
528.73 to 252.07 mg/Nm3 (@6% O2) under the pulverized coal conditions. The NOx formation in coal
284
combustion would be further inhibited by injecting tertiary gas at a longer distance from the top in the
285
DFC. And NOx emissions decreased at a high combustion efficiency. Furthermore, increasing the
286
secondary air ratio could increase NOx emissions under the center nozzle conditions. However, when the
287
nozzle structure was ring, NOx emissions first decreased to a lowest value and then increased.
288
The trend of NO concentration in DFC was almost the same irrespective of fuel (coal or char).
289
Compared to pulverized coal, besides the reduction in NOx emissions by injecting tertiary air at a longer
290
distance from the top in the DFC, more attention should be paid to the combustion efficiencies under the
291
pulverized char conditions.
292 293
AUTHOR INFORMATION 18 ACS Paragon Plus Environment
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Energy & Fuels
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Corresponding Authors
295
*Telephone: +86-010-82543053, Email:
[email protected] /
[email protected] 296 297
ACKNOWLEDGMENT
298
The authors gratefully acknowledge the supports of the National Natural Science Foundation of China
299
(No.51676187).
300 301
REFERENCES
302
(1) You,C.F.; Xu,X.C. Energy 2010, 35 (11), 4467-72.
303
(2) Zhou,H.; Cen,K.; Fan,J. Energy 2004, 29 (1), 167-83.
304
(3) Liu,C.; Hui,S.; Pan,S.; Wang,D.; Shang,T.; Liang,L. Fuel 2015, 139, 206-12.
305
(4) Liu,C.; Hui,S.; Zhang,X.; Wang,D.; Zhuang,H.; Wang,X. Applied Thermal Engineering 2015, 85,
306
278-86.
307
(5) Kuang,M.; Li,Z.; Ling,Z.; Zeng,X. Applied Thermal Engineering 2014, 67(1), 97-105.
308
(6) Kuang,M.; Li,Z.; Liu,C.; Zhu,Q.; Zhang,Y.; Wang,Y. Applied Thermal Engineering 2012, 48, 164-75.
309
(7) Hou,X.; Zhang,H.; Pilawska,M.; Lu,J.; Yue,G. Fuel 2008, 87(15–16) , 3271-7.
310
(8) Daood,S.S.; Javed,M.T.; Gibbs,B.M.; Nimmo,W. Fuel 2013, 105, 283-92.
311
(9) Schaffel-Mancini,N.; Mancini,M.; Szlek,A.; Weber,R. Energy 2010, 35(7) , 2752-60.
312
(10) Zhang,H.; Yue,G.; Lu,J.; Jia,Z.; Mao,J.; Fujimori,T. Proceedings of the Combustion Institute 2007,
313
31(2) , 2779-85.
314
(11) Tamura,M.; Watanabe,S.; Komaba,K.; Okazaki,K. Applied Thermal Engineering 2015, 75, 445-50.
315
(12) Weidmann,M.; Verbaere,V.; Boutin,G.; Honoré,D.; Grathwohl,S.; Goddard,G. Applied Thermal
316
Engineering 2015, 74, 96-101. 19 ACS Paragon Plus Environment
Energy & Fuels 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
317 318
(13) Weidmann,M.; Honoré,D.; Verbaere,V.; Boutin,G.; Grathwohl,S.; Godard,G. Combustion and Flame 2016, 168, 365-77.
319
(14) Stadler,H.; Christ,D.; Habermehl,M.; Heil,P.; Kellermann,A.; Ohliger,A. Fuel 2011, 90(4) , 1604-11.
320
(15) Kuang,M.; Li,Z. Energy 2014, 69(Supplement C) , 144-78.
321
(16) Li,Z.; Kuang,M.; Zhang,J.; Han,Y.; Zhu,Q.; Yang,L. Environmental Science & Technology 2010,
322
44(3) , 1130-6.
323
(17) Zhu,J.; Ouyang,Z.; Lu,Q. Energy & fuels 2013, 27(12) , 7724-9.
324
(18) Ouyang,Z.; Zhu,J.; Lu,Q. Fuel 2013, 113, 122-7.
325
(19) Ouyang,Z.; Zhu,J.; Lu,Q.; Yao,Y.; Liu,J. Fuel 2014, 120, 116-21.
326
(20) Yao,Y.; Zhu,J.; Lu,Q.; Zhou,Z. Journal of Thermal Science 2015, 24(4) , 370-7.
327
(21) Zhu,J.; Yao,Y.; Lu,Q.; Gao,M.; Ouyang,Z. Fuel 2015, 150, 441-7.
328
(22) Chen,G.; Zhang,Y.; Zhu,J.; Cao,Y.; Pan,W. Energy & Fuels 2011, 25(5) , 1964-9.
329
(23) van Eyk,P.J.; Kosminski,A.; Mullinger,P.J.; Ashman,P.J. Energy & Fuels 2016, 30(3) , 1771-82.
330
(24) Fan,W.; Lin,Z.; Li,Y.; Kuang,J.; Zhang,M. Energy & Fuels 2008, 23(1) , 111-20.
331
(25) Fan,W.; Lin,Z.; Li,Y.; Li,Y. Energy & Fuels 2010, 24(3) , 1573-83.
332
(26) Lyu,Q.; Zhu,S.; Zhu,J.; Wu,H.; Fan,Y. Fuel Processing Technology 2018, 176, 43-9.
333
(27) Katsuki,M.; Hasegawa,T. Symposium (International) on Combustion 1998, 27(2) , 3135-46.
334
(28) Furusawa,T.; Tsunoda,M.; Tsujimura,M.; Adschiri,T. Fuel 1985, 64(9) , 1306-9.
335
(29) Mei,L.; Lu,X.; Wang,Q.; Pan,Z.; Ji,Y. Fuel Processing Technology 2014, 118, 192-9.
336
(30) Illan-Gomez,M.; Linares-Solano,A.; Salinas-Martinez de Lecea,C.; Calo,J. Energy & Fuels 1993,
337
7(1) , 146-54.
338
(31) Zhong,B.J.; Shi,W.W.; Fu,W. Fuel processing technology 2002, 79(2) , 93-106.
339
(32) Suda,T.; Takafuji,M.; Hirata,T.; Yoshino,M.; Sato,J. Proceedings of the Combustion Institute 2002, 20 ACS Paragon Plus Environment
Page 20 of 21
Page 21 of 21 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
340
Energy & Fuels
29(1) , 503-9.
341
(33) Liu,H.; Hampartsoumian,E.; Gibbs,BM. Fuel 1997, 76(11) , 985-93.
342
(34) Zhao,Y.; Wang,S.; Nielsen,C.P.; Li,X.; Hao,J. Atmospheric Environment 2010, 44(12) , 1515-23.
343
(35) Wang,J.; Fan,W.; Li,Y.; Xiao,M.; Wang,K.; Ren,P. Energy 2012, 37(1) , 725-36.
344 345
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