Effect of Air Distribution on NOx

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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

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Effect of air distribution on NOx emissions of pulverized coal and char

3

combustion preheated by a circulating fluidized bed

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Shujun Zhua,b, Qinggang Lyu*,a,b, Jianguo Zhu*,a,b, Huixing Wua,b, Guanglong Wua,b

5

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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1 ACS Paragon Plus Environment

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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

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temperature below 1500 ºC in a main combustion region, which could inhibit the formation of thermal NO.

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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

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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

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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

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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.

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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

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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

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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

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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

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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.

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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

ߣ஼ி஻ = ஺

(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

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3.1. Preheating characteristics in CFB.

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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.


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

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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


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)

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

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.


Energy & Fuels

shenmu coal shenmu char

210

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 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

294

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

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Effect of Air Distribution on NOx

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