Biomass Co-combustion under

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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

ACS Paragon Plus Environment

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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


biomass utilization

16

with

and a

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Energy & Fuels

45

circulating fluidized bed (CFB) boiler. These successful power plants provide experience and

46

reference for promotion of the coal/biomass co-combustion technology.

47

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

52

viable negative CO2 emission technology. Fundación CIUDEN, Spanish Government, built the

53

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

57

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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67

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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106

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

109

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

119

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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Energy & Fuels

121 122 123

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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133

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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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Energy & Fuels

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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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157

-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)

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

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


400


300

O2/RFG, Datong coal/wheat straw=70/30 Case 5, excess O2 ratio 1.06

200


100


0 0

1000

2000

3000

4000

5000

6000

Height (mm)

205 206

(a)

180

150

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

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)


1.12

Energy & Fuels

120

Datong coal/wheat straw=70/30 N2O 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

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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


Page 15 of 27

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

225

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


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

Page 16 of 27

NO HCN

75

60

45

30 30

35

40

45

Secondary flow ratio (%)

255


50

Page 17 of 27

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


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

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.


35-37, 38, 40

. 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%


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

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


Page 20 of 27

Page 21 of 27

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


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

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.


Page 22 of 27

Page 23 of 27

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


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

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

References

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