Characteristics of fly ash under oxy-fuel circulating fluidized bed

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Characteristics of fly ash under oxy-fuel circulating fluidized bed combustion Wei Li, Dianbin Liu, and Shiyuan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00934 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Energy & Fuels

1

Characteristics of fly ash under oxy-fuel circulating fluidized

2

bed combustion

3

Wei Li1, Dianbin Liu1,2, Shiyuan Li1,2,*

4 5

1

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China 2

6 7

University of Chinese Academy of Sciences, Beijing, China

Abstract

8

The objective of the present paper is to study the physiochemical properties of fly ash under

9

oxy-fuel circulating fluidized bed (CFB) combustion mode. Tests were conducted in a 50 kW

10

oxy-fuel CFB combustor under both air and oxy-fuel combustion. The analyses of the collected

11

fly ash samples included particle size analysis, N2 adsorption analysis, char carbon, Inductively

12

Coupled Plasma Optical Emission Spectrometer (ICP-OES) and X-ray diffraction (XRD). It was

13

found that the d10, d50 and d90 of fly ash under oxy-fuel combustion mode were larger than that

14

under air combustion mode and increased with the rise of the inlet oxygen concentration. The

15

Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size of fly ash under air

16

combustion are almost the same with that at inlet oxygen concentration of 30%. The element mass

17

concentrations were obviously different between the two combustion modes, but the main mineral

18

phase was insignificantly different. Moreover, the inlet oxygen concentration has no significant

19

effect on the element mass distribution in the fly ash, except for the alkaline earth metal under

20

oxy-fuel combustion.

21

Keywords: Circulating fluidized bed, Oxy-fuel, Fly ash, Physical properties, Chemical

22

composition.

23

*Corresponding Author. Telephone: +86-10-82543055. Fax: +86-10-82543119.

ACS Paragon Plus Environment

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24

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E-mail address: [email protected]

1. Introduction

26

The CO2 capture utilization and storage (CCUS) technology can achieve nearly zero CO2

27

emission, which is of great significance to realizing reduction target of CO2 emission and

28

developing low carbon economy. Oxy-fuel combustion is considered as the most promising CCUS

29

technology [1-3]. It has the advantages of low cost, can be scaled easily and applied to

30

the existed combustion equipment.

31

Compared with conventional air combustion mode, biggish chemical and physical change of

32

the combustion environment might alter the combustion process under oxy-fuel combustion mode.

33

Oxy-fuel combustion related issues including combustion, heat transfer and pollutant formation

34

have been studied in the past decades [2-6]. However, among these researches, there is less

35

research about the characteristics of ashes, including the bottom ash and fly ash, under the

36

oxy-fuel combustion condition. Most literatures regarding oxy-fuel combustion ashes focused on

37

pulverized coal (PC) combustor. Since combustion of low-grade coals can generate abundant fly

38

ash in circulating fluidized bed (CFB) boilers, thus the pollution of fly ash is a more potential

39

problem than in PC boilers [7-9].

40

The effects of oxy-fuel combustion on ash formation mechanisms, chemical composition and

41

particle size distributions (PSD) have been studied in lab-scale and pilot-scale facility. Sheng et al.

42

[10-12] found that O2/CO2 combustion has no significant influence on formation mechanisms of

43

the submicron particles and the fine fragmentation particles using drop tube furnace (DTF).

44

However, there is visible difference of the mass and composition distributions. Suriyawong et al.

45

[13] also found the same conclusions. Yu et al. [14] studied the ash formation in a 100 kW


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Energy & Fuels

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pilot-scale facility under oxy-fuel combustion. The results showed that oxy-fuel atmosphere has

47

no significant impact on the ash PSDs and chemical composition, with the exception of sulfur

48

contents. The ashes under the oxy-fuel combustion has higher sulfur contents comparing air

49

combustion. Li et al. [15] observed that PSDs under two combustion modes are almost the same in

50

a 25 kW quasi one-dimensional down-fired PC combustor. The only difference is the peak of PSD

51

curve moved from 50 µm to 40 µm under oxy-fuel combustion. Zhan et al. [16] also investigated

52

the ash formation under oxy-fuel combustion in a 100 kW down-fired combustor. They concluded

53

that the ash portioning, fouling and slagging under the oxy-fuel combustion have no invisible

54

difference comparing with air combustion. However, the fine particle chemical compositions

55

would be changed.

56

Oxy-fuel CFB combustion outperforms PC combustion owing to several advantages and thus

57

is considered as a more suitable technology for oxy-fuel combustion. In the late years, researcher

58

has done lots of work about oxy-fuel CFB combustion [17-24]. However, literatures about ash

59

issues are rare under oxy-fuel CFB combustion. Wu et al. [25] studied the physiochemical

60

properties of fly ash and bed ash in a 100kW mini-CFB under oxy-fuel combustion, but did not

61

compare air and oxy-fuel combustion modes using the same facility. Furthermore, the influence of

62

the inlet O2 concentration on the physiochemical properties of fly ash was not reported under

63

oxy-fuel combustion. These all are necessary to better understand the characteristic of the oxy-fuel

64

CFB combustion fly ash.

65

The objective of present paper is to investigate the physical and chemical properties of

66

oxy-fuel CFB combustion fly ash and to identify the difference of fly ash under between oxy-fuel

67

and air combustion mode. The influence of inlet O2 concentration was also taken into


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68

consideration. All the tests were done in a 50 kW CFB combustor under O2/CO2 and air

69

combustion mode. And, the collected fly ash samples were analyzed through various methods,

70

including particle size distribution analysis, N2 adsorption analysis, char carbon, Inductively

71

Coupled Plasma Optical Emission Spectrometer (ICP-OES) and X-ray diffraction (XRD).

72

2. Experimental

73

2.1 A 50 kW CFB facility

74

The schematic diagram of the installation is shown in the Figure 2. The inside diameter of

75

furnace is 100 mm. And the height is 3250 mm. Three electrical heaters and a return leg cooler

76

were arranged on the facility. These all help to control of the combustion temperature during the

77

different combustion atmosphere. So, the O2 concentration of inlet gas can be ranged from 21% to

78

50%. More detailed description of the installation has been given elsewhere [24].

79 Figure 1 Schematic diagram of the experimental apparatus

80

81

2.2 Materials


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Energy & Fuels

82

The fuel using in the tests is Datong (DT) coal. The fuel and ash analysis were presented in

83

Tables 1 and Table 2, respectively. The DT coal contained 26.05% ash, which was mainly

84

composed of SiO2 and Al2O3, followed by CaO and Fe2O3. The diameter of the experimental coal

85

is 0.1-1 mm. The diameter of the bed material (silicon sand) is ranged from 0.25 mm to 0.355 mm,

86

with a weight of 2.5 kg. Table 1 Fuel analysis of DT coal

87 LHV (MJ·kg−1)

Proximate analysis (wt%)

Qnet，ar

FCar

Mar

Aar

Var

Car

Har

Oar

Sar

Nar

22.61

44.38

2.2

26.05

27.37

58.08

3.73

8.58

0.32

1.04

Table 2 Ash Composition of DT coal

88

89

Ultimate analysis (wt%)

Component

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

P2O5

K2O

Na2O

Content (wt%)

45.23

37.83

4.02

5.42

0.66

1.62

2.50

0.18

0.32

0.14

2.3 Experimental conditions and analysis methods

90

Experiments were conducted under O2/CO2 and air combustion modes. The O2 concentration

91

in the inlet gas ranged from 30% to 50% under oxy-fuel combustion. Table 3 showed the

92

important operation parameters and the emission dates under the both two combustion mode. All

93

the testes condition were operated at the steady-state for at least 1 h. The temperature profiles of

94

the furnace under both two combustion conditions were showed in Figure 2. The concentrations of

95

gas emissions, such as NO, N2O, SO2, CO and CO2, were monitored via a FTIR gas analyzer

96

(GASMET DX4000, Finland) continuously. All the unit of the emissions were transfer to

97

mass/unit energy input.


Energy & Fuels

3500

Air O2:CO2 30%

m m

2500

O2:CO2 40%

e c a n r u f f o t h g i e H

2000

)

3000

O2:CO2 50%

(

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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1500 1000 500 0 800

820

840

860

880

900

o

Temperature ( C)

98 99

Figure 2 The temperature profiles of the furnace under both two combustion mode Table 3 Operation parameters and the emission dates

100 Conditions

1

2

3

4

Atmosphere

Air

O2/CO2

O2/CO2

O2/CO2

Inlet oxygen concentration, %

21

30

40

50

Gas velocity, m/s

2.5±0.1

2.5±0.2

2.5±0.1

2.5±0.1

Average riser temperature, oC

849±5

846±10

848±7

852±5

Fuel feeding rate, kg/h

2.3±0.5

3.7±0.8

5.1±0.3

6.3±0.5

O2 in flue gas, % d.b

6.0±0.8

5.5±1.1

5.8±0.6

5.6±0.7

CO2 in the flue gas, %

13.5±2.3

91.3±2.5

90.6±1.8

90.9±1.6

CO in the flue gas, mg/MJ

616.8±45.2

502.1±52.3

384.8±46.3

289.9±32.7

SO2 in the flue gas, mg/MJ

279.2±25.5

99.9±12.3

82.3±13.1

72.1±8.5

NO in the flue gas, mg/MJ

67.6±12.6

37.3±6.2

28.0±5.8

18.9±4.8

N2O in the flue gas, mg/MJ

73.6±8.7

64.1±6.3

82.0±5.8

87.7±7.8

101


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102

All the fly ash samples were obtained from the ash-collecting unit, which is arranged on the

103

bottom of the flue gas cooler, after each test. Physicochemical properties of the collected samples

104

under the both two CFB combustion conditions were investigated in terms of PSD, N2 adsorption,

105

char carbon, ICP-OES and XRD.

106

3. Results and discussion

107

3.1 Physical properties

108

3.1.1 Particle size distribution (PSD)

109

Figure 3 showed the PSDs under different combustion conditions. PSDs of fly ashes were not

110

considerably different compared the two combustion modes. The PSDs of curve all samples were

111

showed as S-shaped. However, the center position of the PSD curve moved leftward obviously

112

under the oxy-fuel combustion mode. And as rise of the O2 concentration in the inlet gas, the

113

center position of the particle size distribution curve moved rightward gradually. 100

5

Air O2:CO2 30%

4

80

O2:CO2 40% O2:CO2 50%

3

Volume (%)

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

2 40 1 20 0 0 0.1

1

10

100

1000

Particle Size (µm) 114 115

Figure 3 The particle size distribution under different combustion mode


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116

There are four formation mechanisms of the fly ash particles, including char fragmentation,

117

mineral coalescence, vaporization and subsequent condensation of inorganic matter and excluded

118

mineral char fragmentation [26]. In addition, the collection efficiency of cyclones affects the PSD

119

of fly ash in CFB boilers [27]. The d10, d50 and d90 of different conditions were showed in Table 4.

120

The d10, d50 and d90 are corresponding the particle size with cumulative volume of 10%, 50% and

121

90%, respectively. Table 4 The d10、d50 and d90 under different conditions

122 Condition

d10/µm

d50 /µm

d90 /µm

Air

5.03±1.08

37.17±6.07

166.05±12.87

O2/CO2:30%

2.89±0.32

22.35±5.36

81.09±6.85

O2/CO2:40%

3.56±0.68

26.91±3.25

128.39±25.85

O2/CO2:50%

4.46±0.53

34.80±3.58

162.53±19.53

123 124

The d10, d50 and d90 of oxy-fuel combustion fly ashes all are smaller than those of air

125

combustion fly ash. Zhang et al. found the same results using a DTF. They observed noticeable

126

mineral fragmentation under oxy-fuel combustion [28].

127 128 129

Expect char fragmentation, the char abrasion also considerably affected the PSD of fly ash during the CFB combustion. The rate of char abrasion ܴ௔ can be calculated as follows [29]:

ܴ௔ = −

ௗ௠೟ ௗఛ

=݇

൫௨బ ି௨೘೑ ൯ௐ തതതത ௗ ೛

(1)

130

where ݇ is the constant of Ra; ‫ݑ‬଴ is the fluidized gas velocity and ‫ݑ‬௠௙ is its minimum value,

131

m/s; ܹ is the mass of the char in the furnace, kg; തതത ݀௣ is the average diameter of char, mm.

132

തതത In our experiments, because the same bed material and coal were used, the ‫ݑ‬௠௙ and ݀ ௣


Page 9 of 19

133

were constant. And ‫ݑ‬଴ in the furnace was also kept the same under different combustion modes.

134

However, the coal feeding increased when the combustion mode switched from air to oxy-fuel

135

combustion mode, because the inlet O2 concentration rose from 21% to above 30%. As can be

136

seen in Figure 4, the average pressure drop of the furnace was higher under oxy-fuel combustion,

137

indicating the larger ܹ and ܴ௔ . This was another reason why the diameter of the oxy-fuel

138

combustion fly ash was smaller.

3.0

Air O2/CO2:30%

2.5

Average pressure drop(kPa)

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

O2/CO2:40%

2.0

O2/CO2:50%

1.5

1.0

0.5

0.0 -500

0

500

1000

1500

2000

2500

3000

3500

Height of the furnace (mm)

139 140

Figure 4 The average pressure drop of the furnace

141

It was also indicated oxy-fuel combustion produced finer particles, which may be because the

142

oxygen concentration enhanced the mineral vaporization and nucleation under this mode [15,30].

143

With the increment of the inlet oxygen concentration which would enhance the char

144

fragmentation, the d10, d50 and d90 all increased under oxy-fuel combustion and the char particle

145

temperature noticeably rose. As reported, during single char particle combustion in a fluidized bed,

146

the char particle temperature rose nearly 100 oC as the inlet oxygen concentration increased from

147

21% to 40% under O2/CO2 combustion, although the bed temperature was fixed as 815 oC [31].

148

Thus, the higher temperature promoted the coalescence of minerals, which explained why the fly


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149

ash particles under higher inlet oxygen concentration were bigger-sized.

150

3.1.2 N2 adsorption analysis

151

N2 adsorption analysis of fly ash under different conditions was illustrated in Table 5. The

152

BET surface area, pore volume and pore size at inlet oxygen concentration of 30% were all almost

153

the same between the two combustion modes. The pore structure of the fly ash is highly influences

154

by the char particle temperature. Because of different physiochemical properties between CO2 and

155

N2, inlet oxygen concentration higher than 21% under oxy-fuel combustion mode should be

156

needed to achieve the similar particle temperature as that under air combustion. As reported, the

157

particle temperature of char at inlet oxygen concentration of ~30% under oxy-fuel combustion is

158

almost the same as that under air combustion [31]. However, the particle temperature at higher

159

inlet oxygen concentration under oxy-fuel combustion was higher [31]. The melting of

160

alumino-silicate mineral in char occurs under oxy-fuel condition has been reported [28]. Thus, the

161

BET surface area, pore volume and pore size all decreased with the rise of inlet oxygen

162

concentration (Table 5). Under oxy-fuel combustion, the BET surface area, pore volume and pore

163

size in the fly ash varied within 14.69-23.42 m2/g, 0.024-0.040 cm3/g and 6.36-7.01 nm,

164

respectively. Wu et al. [25] also studied the surface analysis of fly ash in a 100 kWth

165

mini-CFBC facility under oxy-fuel combustion mode using the coal and petroleum coke.

166

They found that the BET surface area, pore volume and pore size in the fly ash are ranged of 5-67

167

m2/g, 0.0083-0.040 cm3/g and 1.20-2.93 nm, respectively. The difference between studies may

168

be caused by the fuel type and operation parameters.

169

Table 5 The N2 adsorption analysis of fly ash under different conditions


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BET Surface Area

Pore Volume

Pore Size

m2/g

cm3/g

nm

Air

23.17±2.18

0.040±0.02

6.72±1.08

O2/CO2:30%

23.42±1.69

0.040±0.03

6.76±0.96

O2/CO2:40%

19.45±1.57

0.034±0.01

7.01±1.35

O2/CO2:50%

14.69±1.52

0.024±0.01

6.36±0.75

Condition

170

3.2 Chemical composition

171

3.2.1 Unburned carbon

172

Figure 5 showed the unburned carbon contents in fly ash under different conditions. The

173

carbon contents ranging from 11.5% to 15.6% were all higher than under the commercial-scale air

174

combustion CFB boiler ranging from 0.5% to 3% [32], but were still within the acceptable range

175

of 5%-20% [27]. The higher unburned carbon content was mainly caused by the relatively short

176

residence time of only ~ 1 sec in such a small-scale CFB combustor as used in the experiments.

20

15

Carbon content (%)

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

10

5

0

Air O2/CO2:30%

O2/CO2:40%

O2/CO2:50%

177 178

Figure 5 The unburn carbon content in fly ash under different conditions


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179

As can be seen in Figure 5, the unburned carbon contents of oxy-fuel combustion fly ash

180

were all lower than those of air combustion fly ash, and decreased as the increase of the O2

181

concentration in the inlet gas, which are consistent with the results in a 1 MW oxy-fuel CFBC [33].

182

It also indicates the higher inlet oxygen concentration is beneficial to improving the coal

183

combustion efficiency.

184

3.2.2 Chemical composition

185

The element mass concentrations in the fly ash under different combustion modes were

186

analyzed by ICP-OES. To eliminate the influence of the unburned carbon, we processed the

187

original ICP-OES data as follows:

188

ா

బ ‫( = ܧ‬ଵ଴଴ି஼

ೠ೙ )

× 100

(2)

189

where ‫ܧ‬଴ and ‫ ܧ‬are the element mass concentration before and after processing, respectively,

190

wt/%; ‫ܥ‬௨௡ is the unburned carbon content, wt/%.

191

Figure 6 showed the processed element mass concentrations in the fly ash under different

192

conditions. The element mass concentrations were obviously different between the two

193

combustion modes. Under oxy-fuel combustion mode, higher mass concentrations of the Ca and

194

Mg, Al and Ti, and lower mass concentrations of alkaline metals (Na and K), Si and S were found.

195

The mass concentrations of Ba and Sr were higher at the inlet oxygen concentration of 30% and

196

40%, but suddenly dropped or were even lower than that under air combustion when the O2

197

concentration in the inlet gas reached 50%. This phenomenon should be reconfirmed and studied

198

in the future. The mass concentrations of Mn, P and Fe were similar between the two modes.


Page 13 of 19

3.0

25

2.5

20

Air O2/CO2:30%

2.0

O2/CO2:40%

15

O2/CO2:50%

1.5

10 1.0 5

0.5

0.0

Mass concentration(wt%)

Mass concentration(wt%)

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

0 Na

K

Ca Mg

S

Ti

Mn

P

Ba

Sr

Si

Al

Fe

199 200

Figure 6 The chemical composition of the fly ash

201

The lower alkaline metal was caused by the higher vaporization degrees under oxy-fuel

202

combustion mode. A fast combustion rate and reduction atmosphere on the surface of char particle

203

may resulted in the higher vaporization degrees under oxy-fuel combustion mode [24, 28]. The

204

vaporization of alkaline metals was harmful for coal combustion, which form the PM2.5 emission

205

and aggravate deposition and slag [34]. In our study, the concentrations of alkaline earth metals in

206

the fly ash were relatively higher. It has been reported that these metals more easily form alkali

207

aluminum silicates under oxy-fuel combustion [35].

208

Compare the air combustion, the S mass concentration in the oxy-fuel combustion fly ash is

209

lower, indicating most of S is released to the flue gas instead of existing in the solid mineral phase

210

under oxy-fuel combustion [35]. Many researches have proved that SO2 emissions under oxy-fuel

211

combustion mode are almost 3-5 times of that under air combustion mode, and the high content of

212

CO2 in the furnace inhibit the self-desulphurization of Ca and Mg [24, 36]. In addition, the SO2 in

213

the furnace would react with vaporized alkaline metals, forming alkali sulfates which deposit on

214

the cool tube surface. Wall et al. [4] and Zeng et al. [29] all found that the higher S content in the


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215

deposition ash under oxy-fuel combustion. Moreover, the intensified carbonation of sulfates in the

216

fly ash under the oxy-fuel combustion due to higher CO2 partial pressure also should be

217

considered [37, 38].

218

The inlet oxygen concentration seemingly has no significant effect on the element mass

219

concentrations in the fly ash, except for the alkaline earth metals (Figure 4). Li et al. [24]

220

concluded the influence of O2 concentration on alkaline earth metals release was not apparent,

221

probably because of the small amounts of Ca and Mg in the their experimental coal. In this study,

222

there are a large number of Ca and Mg in the ash of DT coal, which cannot be ignored. During the

223

oxy-fuel combustion with higher inlet oxygen concentration, the strong combustion reaction

224

would accelerate the heat release, result in the higher char particles temperature. It would promote

225

the formation of Ca(Mg)-Si-Al compounds and thereby fix more Ca and Mg in the fly ash.

226

3.2.3 XRD

227

XRD results of fly ash under different condition were given in Figure 5. The mineral phase of

228

the fly ash did not vary obviously under different combustion modes. As reported similarly, the

229

main mineral phase was not significantly different between two different combustion modes. And

230

the difference of the relative content of mineral phase was due to the different fuel type and

231

combustion temperature [12, 35]. It is also indicated the mineral phase of the fly ash is mainly

232

determined by the coal type, rather than the combustion mode.

233

Regardless of the combustion mode, SiO2 is the main component, followed by Al2O3, and

234

other minor species are KAlSi3O8, CaO and CaCO3 (Figure 7). Because of the high Ca content in

235

the DT coal, Ca-based compounds were detected in the fly ash. It should be noticed that CaSO4

236

was not observed because of the lower content of S in the fly ash. CaO was found under both two


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Page 15 of 19

237

combustion modes, but CaCO3 was only seen under the oxy-fuel combustion mode.

238

Since the O2 concentration in the inlet gas ranged from 30% to 50% during the experiments,

239

the CO2 concentration varied from 50% to above 90% along the height of furnace. According to

240

the equilibrium CO2 pressure over CaCO3 on temperature, as suggested by Baker et al. (Figure 8)

241

[39], CaCO3 seems would react with SO2 directly at temperature of 850 oC, which happened in our

242

experiments. However, the existence of both CaCO3 and CaO in the fly ash indicated sulfur

243

capture under oxy-fuel combustion took place through both direct and indirect sulfation, which

244

was because the higher particle temperature would promote the calcination of CaCO3 under

245

oxy-fuel combustion. This result implied the indirect sulfation reaction of CaCO3 always

246

occurred under oxy-fuel combustion even at lower combustion temperature, especially at high

247

oxygen concentration.

1

Air 1

2

3

41

1

1

1

1

O2/CO2:30% 1 2 3 5 4

Intensity

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

1

1 4

1

O2/CO2:40%

3 2 5 4

4

1

O2/CO2:50%

1 2 3

0

248 249

20

5 4

1 4

40

1

60

80

100

2θ Figure 7 XRD charts fort fly ash under different combustion mode.


Energy & Fuels

1: SiO2, 2: Al2O3, 3: KAlSi3O8, 4: CaO, 5:CaCO3.

250

1.0

Oxy-fuel CFB combustion 0.8

CO2 partial pressure (atm)

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 19

0.6

0.4

Direct sulfation Indirect sulfation

0.2

0.0 700

750

800

850

900

950

1000

o

Temperature ( C)

251

Figure 8 Equilibrium CO2 partial pressure over limestone

252

253

254 255

4. Conclusions

Tests were conducted in a 50 kW CFB combustor under both air and oxy-fuel combustion modes.

256

The d10, d50 and d90 were smaller and increased as the rise of O2 concentration in the inlet gas

257

under oxy-fuel combustion. The BET surface area, pore volume and pore size were all the same

258

between the two combustion modes at inlet oxygen concentration of 30%.

259

The element mass concentrations were obviously different between the two combustion

260

modes. Compared with air combustion, higher mass concentrations of alkaline earth metals (Ca

261

and Mg), Al and Ti, and lower mass concentrations of alkaline metals (Na and K), Si and S were

262

found in the oxy-fuel combustion fly ash. However, the main mineral phase was insignificantly

263

different between the two combustion modes.

264 265

The O2 concentration in the inlet gas seemingly has no significant effect on the element mass concentrations in the fly ash, except for the alkaline earth metals.


Page 17 of 19 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

266

CaO was found under both combustion modes, but CaCO3 was only found under oxy-fuel

267

combustion.

268

Acknowledgments

269

This study is supported by the National Natural Science Foundation of China (Grant

270

51706227).

271

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Characteristics of fly ash under oxy-fuel circulating fluidized bed

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