Further insight into the formation and oxidation of CaCr2O4 during

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Further insight into the formation and oxidation of CaCr2O4 during solid fuel combustion Hongyun Hu, Mengya Shi, Yuhan Yang, Huan Liu, Mian Xu, Junhao Shen, and Hong Yao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05538 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Environmental Science & Technology

1

Further insight into the formation and oxidation of CaCr2O4 during solid fuel

2

combustion

3

Hongyun Hua, Mengya Shia, Yuhan Yanga, Huan Liua,b, Mian Xua, Junhao Shena,

4

Hong Yao a,b,*

5

a

6

Huazhong University of Science and Technology, Wuhan 430074, China

7

b

8

Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,

Department of New Energy Science and Engineering, School of Energy and Power

9

*

Corresponding author. Tel & Fax: +86-27-87545526(O) E-mail: [email protected] (Prof. Hong Yao) 1

ACS Paragon Plus Environment


10

Abstract

11

The control of toxic chromate (Cr6+) formation is still a significant challenge in

12

solid fuel combustion. In particular, the mechanism of chromium transformation from

13

Cr3+ to chromate or other unoxidized forms remains unclear. The present study

14

confirms the formation of a significant unoxidized Cr-containing compound

15

CaCr2O4(Cr3+) during solid fuel combustion. Experiments were conducted, for the

16

first time, to clarify the mechanism of CaCr2O4 oxidation, which is quite different

17

from Cr2O3 oxidation. The findings demonstrate that CaCr2O4 was formed at

18

temperatures above 1200 K, through rapid decomposition of CaCrO4 or slow and

19

direct interaction between CaO and Cr2O3. Compared to Cr2O3, CaCr2O4 could be

20

oxidized at lower temperatures under the influence of free CaO. In the absence of free

21

CaO, the oxidation of CaCr2O4 was minimal; however, in the presence of CaSO4,

22

calcium in the form of CaCr2O4 participated in the oxidation of CaCr2O4. Thus,

23

chromium in the form of CaCr2O4 was more likely to be oxidized when

24

CaCr2O4-containing fly ash was reheated. Fortunately, CaCr2O4 showed slight

25

basicity on the surface, allowing it to react with acidic gases. Accordingly, measures

26

were proposed to suppress the oxidation of CaCr2O4 by stimulating the reactions

27

between CaCr2O4 and acidic substances, like SO2 and Si/Al-compounds. These

28

compounds competed with chromium at high temperatures to react with calcium in

29

the fly ash and in CaCr2O4. As a result, the unoxidized chromium was transformed

30

into highly stable Cr2O3 or Ca3Cr2 (SiO4)3.

2


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TOC/Abstract Art

31

Hated Ash

Cr2O3/Ca3Cr2 (SiO4)3

Cr5+/Cr6+

Reheating

+ Si/Al-compounds CaSO4

+ CaO

Oxidation Suppression (I)

Oxidation (II) Oxidation (I)

SO2

CaSO4 CaCr2O4

Ash Residues

Combustion

+ CaO

Decomposition CaCrO4

Cr2O3

+ CaO + CaO

Solid Fuel

CaSO4 Oxidation Suppression (II)

Organic-Cr / Cr2O3

32

3



33

Introduction

34

Chromium is widely distributed in coal, municipal solid waste, sewage sludge

35

and other solid fuels 1, 2. In China, a large amount of chromium is released within flue

36

gas into atmosphere during solid fuel combustion 3, which is a great threat to human

37

health. Even so, most of the chromium is transferred into ash residues, predominantly

38

as trivalent chromium (Cr3+) 2. More importantly, trivalent chromium can partly

39

oxidize to form more toxic hexavalent chromium (Cr6+), which is classified as a group

40

A inhalation carcinogen 4. Compared with Cr3+, Cr6+ is more easily leached from ash

41

residues, leading to serious water and/or soil pollution 5. Therefore, a series of

42

stringent policies were implemented by the Chinese government for emission control

43

of Cr6+ 6.

44

Generally, the predominant form of chromium in solid fuels is Cr3+, and it is

45

difficult to oxidize during fuel combustion 7, 8. However, a large fraction of Cr6+ was

46

found in the fly ash during the combustion of solid fuels that contained high contents

47

of alkali and/or alkaline earth metal compounds 1, 9. Among these substances, CaO is

48

confirmed to facilitate Cr3+ oxidation over a wide temperature range 10, 11. Conversely,

49

CaO is widely used as a sorbent for the capture of acid gases, such as SO2, and toxic

50

trace elements, such as As2O3, from flue gas

51

conditioner for dewatering of sewage sludge

52

ubiquitous use, CaO is frequently introduced into solid fuels, and it is essential to

53

understand the oxidation of Cr3+ in the presence of CaO.

54

12, 13

. CaO is also commonly used as a

14

. Because of its prevalence and

To date, several studies have been conducted to understand the mechanisms 4


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involved in Cr3+ oxidation in the presence of CaO. The oxidation of Cr3+ was found to

56

be a slow process, and most chromium still existed as Cr3+ in the ash produced in the

57

combustion of coal and waste sludge 8, 11. Nevertheless, the unoxidized Cr3+ may

58

transfer into various species

59

distinguished these unoxidized species or examined their potential oxidizing capacity.

60

When ash residues containing unoxidized Cr3+ are reheated at high temperature, the

61

oxidation of unoxidized Cr3+ compounds likely determines the Cr6+ concentration in

62

the products, such as in the case of fuel combustion in a circulating fluidized bed or in

63

the case of ash thermal treatment for the production of building materials.

5, 15

, and to our knowledge, few studies have

64

Our previous study found that CaCr2O4 was an important intermediate in

65

chromium oxidation in the presence of CaO 16. Unlike Cr2O3, CaCr2O4 is composed

66

of both chromium and calcium. The details regarding the oxidation performance of

67

chromium in CaCr2O4 are still unclear and are surely different from that of Cr2O3,

68

according to our former experiments. On the other hand, it is possible that the calcium

69

in CaCr2O4 participated in chromium oxidation, and its behavior might differ from

70

that of free CaO. In addition, free CaO could react with acid gases and/or acidic

71

inorganic components, such as Si/Al-compounds, in the ash

72

compete with chromium oxides to react with free CaO. As a result, the amount of free

73

CaO available for chromium oxidation decreases. Based on these theories, Mao et al.

74

limited the potential of CaO to promote Cr2O3 oxidation by converting free CaO into

75

phosphate minerals 18. Similarly, Low et al. suppressed the oxidation of Cr2O3 through

76

scavenging of free lime by reaction with kaolinite

19

17

. These substances

. Therefore, the fate of calcium

5



77

likely determined the oxidation of CaCr2O4, and measures could be taken to inhibit

78

CaCr2O4 oxidation based on our understanding of calcium behavior in the process.

79

The aim of this work is to investigate the formation of CaCr2O4 in the Ca-Cr-O

80

system and to evaluate the oxidation of CaCr2O4 in simulated flue gas. In this study,

81

CaCr2O4 was synthesized and well characterized. Then, the oxidation mechanism of

82

CaCr2O4 was determined, considering the effects of free CaO and CaSO4. The fate of

83

calcium in CaCr2O4 was also explored, especially in reactions with SO2 and

84

Si/Al-compounds. Based on our findings, several measures were proposed to suppress

85

the oxidation of CaCr2O4 during reheating of CaCr2O4-containing fly ash.

86

Experimental Procedures

87

Materials. Analytical grade reactants, including CaO, Cr2O3, and CaSO4, were

88

used as raw materials for the investigation of chromium oxidation and for the

89

preparation of test samples: CaCrO4 (Cr6+), Ca3(CrO4)2 (Cr5+) and CaCr2O4 (Cr3+)).

90

To determine the fate of CaCr2O4 during the thermal treatment of fly ash, ash residues

91

were collected from a municipal solid waste incineration plant. Water-washed ash and

92

acid-washed ash samples were obtained after washing the raw fly ash with deionized

93

water and dilute acid solution, respectively. The properties of the ash samples are

94

given in Table S1 and Figure S1 (shown in the Supporting Information). To prepare

95

the mixed samples, all reactants were thoroughly mixed and finely ground prior to the

96

reaction.

97

Oxidation of Chromium Oxides. The oxidation of chromium oxides was

98

conducted in a horizontal tube furnace, and a thermogravimetric analyzer and in situ 6


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high temperature X-ray powder diffraction (XRD) instrument were used to

100

characterize the oxides. The testing temperature, heating rate and atmosphere were

101

well controlled, and the detailed conditions for chromium oxidation in each sample

102

are summarized in Table S2.

103

After the reactions, some Cr-products, like CaCrO4, were directly obtained after

104

cooling. However, for CaCr2O4, some by-products were removed to obtain high purity

105

CaCr2O4. As stated in the following discussion section, CaCr2O4 was formed either

106

through the decomposition of oxidized CaCrO4 (labeled as CaCr2O4-I) or through the

107

interaction between CaO and Cr2O3 (labeled as CaCr2O4-II). Based on the leaching

108

abilities of various Cr-products

109

CaCrO4 by dissolving the formed Ca3(CrO4)2 with deionized water. Similarly,

110

CaCr2O4, formed through the interaction between CaO and Cr2O3, was purified by

111

removing excessive CaO from the products with diluted HCl solution.

20

, CaCr2O4 was separated from the decomposed

112

Characterization of Cr-products. To determine the crystalline structures of the

113

Cr-products, both traditional and in situ high temperature XRD were employed. The

114

microstructure and surface basicity of the samples were determined by scanning

115

electron microscope (SEM) and CO2 temperature-programmed desorption (CO2-TPD)

116

techniques, respectively. To get more details of the Cr-samples, transmission electron

117

microscope (TEM) observations were conducted. Before analysis, the samples were

118

thoroughly dispersed over a standard glass slide and pressed by a copper grid coated

119

with amorphous carbon film.

120

After CaCr2O4 reacted with CaSO4, the formed chromate was leached out of the 7



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products with deionized water, due to the high leaching ability of chromate. The

122

oxidation of CaCr2O4 was evaluated by measuring the concentration of chromium

123

using microwave plasma-atomic emission spectroscopy (MP-AES). However, after

124

the reaction of Cr2O3 with CaO in N2, the distribution of the formed CaCr2O4 and

125

unreacted Cr2O3 in the product could not be distinguished because of their similar

126

leaching abilities. The transformation of Cr2O3 into CaCr2O4 was evaluated according

127

to the contents of Ca and Cr in the products, as detected by X-ray fluorescence (XRF).

128

Specifically, the fraction of chromium in the form of CaCr2O4 was calculated based

129

on the content of Ca in the sample, and the remaining chromium was in the form of

130

Cr2O3.

131

Results and Discussion

132

Formation of CaCr2O4. Thermogravimetric experiments on the decomposition

133

of CaCrO4 were conducted in carrier gas containing various concentrations of oxygen,

134

and the TG/DTG curves are shown in Figure 1. The concentration of oxygen played

135

an important role in the decomposition of CaCrO4. The initial temperature of CaCrO4

136

decomposition decreased with decreasing O2 concentration. In an oxygen-depleted

137

atmosphere (pure N2), the initial decomposition temperature was as low as 1186 K.

138

Figure S2 presents the in situ XRD patterns of the products formed during the

139

decomposition of CaCrO4 in air. It was clear that CaCrO4 decomposed to form

140

Ca3(CrO4)2(Cr5+) and CaCr2O4 (CaCr2O4-I, Eq. (1)). According to Eq. (1), 50% of

141

chromium was transformed into CaCr2O4 in the decomposition of CaCrO4.

142

4CaCrO4 → Ca3(CrO4)2 + CaCr2O4 + 2O2 8


(1)

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Alternatively, the formation of CaCr2O4 might occur through the direct

144

interaction between CaO and Cr2O3. From TG-DSC curves (not shown here), a weak

145

endothermic peak was observed between 1250 K and 1350 K, which was assigned to

146

the formation of CaCr2O4 (CaCr2O4-II, Eq. (2)). To obtain more detailed information,

147

the crystalline structures of the CaO/Cr2O3 mixtures were recorded via in situ high

148

temperature XRD during the heating process under vacuum. The heating program is

149

given in Figure S3. Figure 2 shows the patterns of the products formed at each

150

temperature. The results suggest that the formation of CaCr2O4 through direct

151

interaction between CaO and Cr2O3 was more difficult than via decomposition of

152

CaCrO4. The direct formation of CaCr2O4 began at higher temperature (around 1300

153

K), and the process was quite slow, as evident by the lack of peaks for CaCr2O4 even

154

after heating at 1323 K for 10 min.

155

CaO + Cr2O3 → CaCr2O4

(2)

156

To limit the effect of physical contact between the reactants on CaCr2O4

157

formation, the interactions between CaO and Cr2O3 were conducted with excess CaO.

158

Figure 3 shows the fractions of formed CaCr2O4 as a ratio of total chromium after

159

thermal treatment at temperatures ranging from 1223 K to 1423 K. A similar

160

phenomenon for the slow formation of CaCr2O4 was observed. Additionally, the

161

formation of CaCr2O4 generally took place at higher temperature. At each temperature,

162

limited amounts of CaCr2O4 formed in a short reaction time, and the formation of

163

CaCr2O4 was remarkably stimulated by increasing the reaction time from 1 to 5 min.

164

Further increasing the reaction time from 5 to 10 min resulted in a continuous increase 9



165

in the CaCr2O4 fraction in the products obtained at 1223 and 1323 K. However, few

166

changes were found for CaCr2O4 formation at 1423 K. After heating at 1423 K for 10

167

min, about 10% of chromium still remained unreacted, which was mainly caused by

168

insufficient contact between CaO and Cr2O3 or other unknown factors.

169

According to the results, the formation of CaCr2O4 occurred in the combustion of

170

solid fuel either through the rapid decomposition of CaCrO4 or through the slow and

171

direct interaction between CaO and Cr2O3. Several studies also stated the formation of

172

CaCr2O4 in solid fuel combustion fly ash

173

regarding CaCr2O4 formation, CaCr2O4 was separated from each product by

174

dissolving chromate and excess CaO. After purification, CaCr2O4 was characterized.

175

Although the samples showed similar XRD patterns, the microstructures differed in

176

various CaCr2O4 products (shown in Figure S4). More specifically, CaCr2O4-I was

177

needle/rod-shaped, while CaCr2O4-II was granular. As shown in Figure 4, Cr2O3,

178

CaCr2O4-I and CaCr2O4-II had interplanar distances of 2.52, 2.96 and 2.45 Å,

179

corresponding to the crystal planes (110), (130) and (021), respectively. Different

180

microstructure characteristics of CaCr2O4 might lead to various oxidation

181

performances.

15, 21

. To get more detailed information

182

Oxidation of CaCr2O4. Figure S5 depicts the TG/DTG curves for the oxidation

183

of three Ca-Cr mixtures: CaO with Cr2O3, CaCr2O4-I or CaCr2O4-II. The initial

184

oxidation temperatures progressed in the following order, Cr2O3 (973 K) > CaCr2O4-I

185

(823 K) > CaCr2O4-II (773 K), indicating that the oxidation of CaCr2O4 could take

186

place at lower temperatures than the oxidation of Cr2O3. However, the oxidation of 10


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CaCr2O4 was slower, and this was likely due to the various forms of calcium in the

188

reactants. It is assumed that calcium in free CaO and CaCr2O4 play different roles in

189

the oxidation of chromium. However, the oxidation of CaCr2O4-I and CaCr2O4-II had

190

slight difference in the oxidation capacities. In addition, a sharp mass-loss peak

191

(centered at approximately 1323 K) was recorded in each curve, which was attributed

192

to the decomposition of oxidized chromate.

193

To further clarify the mechanism involved in the oxidation of CaCr2O4, the XRD

194

patterns of the oxidized products, in terms of heating temperature and Ca/Cr molar

195

ratio, were obtained, and the results are displayed in Figure S6. At a molar ratio of 1:1

196

Ca (including calcium from CaCr2O4 and added free CaO) to Cr, most chromium

197

remained unoxidized, especially at 1373 K. The results suggest that calcium in the

198

form of CaCr2O4 could not play the same role as free CaO in the promotion of

199

chromium oxidation. Additionally, the oxidation of CaCr2O4 was mainly determined

200

by free CaO, which was further confirmed by evaluating the oxidation of CaCr2O4

201

mixed with varied free CaO content at 1373 K. From the TG results, shown in Figure

202

5, few changes were found regarding the mass of the sample in the absence of free

203

CaO, demonstrating that calcium in CaCr2O4 did not promote the oxidation of

204

chromium under these experimental conditions. The oxidation of CaCr2O4 was

205

observed upon the addition of free CaO in the sample, which was significantly

206

enhanced by increasing the free CaO content. Apart from the contents of free CaO, the

207

oxidation of CaCr2O4 was strongly affected by temperature. The forms of oxidized

208

chromium differed at various temperatures. As demonstrated by Figure S6, CaCr2O4 11



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209

was oxidized to CaCrO4 (Cr6+) at 1173 K (Eq. (3)) while at 1373 K CaCr2O4 formed

210

thermally stable Ca3(CrO4)2 (Cr5+) (Eq. (4)). Compared to the formation of CaCrO4,

211

higher free CaO content was needed for the formation of Ca3(CrO4)2.

212

CaCr2O4 + CaO + 1.5O2 → 2CaCrO4

(3)

213

CaCr2O4 + 2CaO + O2 → Ca3(CrO4)2

(4)

214

In the fuel combustion process, free CaO probably reacted with acid gases and

215

was rapidly transformed into corresponding Ca-salts 22. CaSO4 was widely distributed

216

in solid fuels and was also the dominant Ca-compound in the fuel combustion.

217

Therefore, chromium oxides were more likely to interact with CaSO4 rather than free

218

CaO, and the oxidation of CaCr2O4 under the influence of CaSO4 should be further

219

investigated. Figure 6 compares the TG/DTG curves for the oxidation of Cr2O3 or

220

CaCr2O4 affected by CaSO4. For the mixture of CaCr2O4 and CaSO4, one mass-gain

221

peak was observed between 1000 K and 1200 K, which was probably related to the

222

oxidation of chromium. In addition, the mass-loss peak around 1350 K was attributed

223

to the decomposition of oxidized products. In contrast, the oxidation of Cr2O3 was

224

minimal, and the mass of the Cr2O3-containing sample slowly decreased with

225

increasing heating temperature. A significant mass-loss peak was observed at 1323 K,

226

and this was likely caused by the release of chromium in the form of Cr2(SO4)3 23, due

227

to the interactions between Cr2O3 and CaSO4 at this high temperature.

228

To further confirm the oxidation of CaCr2O4 in the presence of CaSO4, CaCr2O4

229

was heated under air in a horizontal tube furnace after mixing with various contents of

230

CaSO4 to obtain certain Ca/Cr ratios. The oxidation of CaCr2O4 was evaluated by 12


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measuring highly soluble chromate and the percentage of oxidized chromium at

232

different temperatures, and these results are presented in Figure 7. Without the

233

addition of CaSO4, the oxidation of CaCr2O4 was minimal. After CaSO4 addition, a

234

small fraction of CaCr2O4 was oxidized at 873 K, and the oxidation of CaCr2O4 was

235

strongly stimulated by increasing temperature. About 40% of the chromium in

236

CaCr2O4 was oxidized after heating at 1173 K at a molar ratio of 1:1 Ca to Cr.

237

Interestingly, an increase in the CaSO4 content slightly affected the amount of

238

oxidized CaCr2O4. The XRD patterns of the products (given in Figure S7) show that

239

CaCrO4 was the main oxidized compound, and the unoxidized chromium was

240

transformed into Cr2O3. It is worth noting that CaSO4 remained the predominant

241

crystalline structure in the products. Therefore, CaSO4 was not instrumental in the

242

oxidation of CaCr2O4, but CaSO4 played a significant role in this process, which

243

promoted the transformation of CaCr2O4 into CaCrO4 and Cr2O3 (Eq. (5))

244

2CaCr2O4 + 1.5O2 → 2CaCrO4 + Cr2O3

(5)

245

Inhibition of CaCr2O4 Oxidation. As stated before, CaCr2O4 was more easily

246

oxidized than Cr2O3, even in the absence of free CaO. Therefore, conversion of

247

CaCr2O4 into Cr2O3 is a promising way to lower the oxidation capacity of chromium.

248

However, the problem lies in the removal of calcium from CaCr2O4. It is well known

249

that free CaO is a strong alkaline substance that tends to react with acid gases and

250

other acidic compounds at high temperature

251

might have a similar reaction performance to free CaO, which is mostly determined

252

by the surface basicity of CaCr2O4.

13, 17

. Calcium in the form of CaCr2O4

13



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253

For comparison, the basic sites on the surface of CaCr2O4, Cr2O3 and CaO/Cr2O3

254

mixtures (molar ratio 1:1) were determined by CO2-TPD, and the resulting profiles

255

are given in Figure S8. A broad desorbed peak of CO2 was clearly observed for Cr2O3

256

in the temperature range of 573 to 1073 K, which was mainly assigned to strong basic

257

sites (related to unsaturated O2- ions) 24, 25. The strong basic sites stimulated reactions

258

with acidic compounds at high temperature. After the addition of free CaO, a large

259

amount of CO2 evolution was found at temperatures between 673 and 873 K, which

260

was related to great moderate basic sites (corresponding to M-O pairs)

261

contrast, apart from the strong basic sites, CaCr2O4 contained some moderate basic

262

sites on the surface, as evident by the several weak desorption peaks at low

263

temperatures. As a result, CaCr2O4 might show similar performance to CaO in a

264

reaction with acidic compounds.

25, 26

. In

265

Figure S9 shows the XRD patterns of CaCr2O4 after reaction with SO2. It was

266

found that CaCr2O4 could react with SO2 by forming Cr2O3 and CaSO4 at 1173 K and

267

1373 K (Eq. (6)). However, the reaction was strongly suppressed at 973 K.

268

Furthermore, the oxidation of CaCr2O4 was hardly observed, even in the presence of

269

formed CaSO4, demonstrating that the sulphonation of CaCr2O4 occurred more easily

270

than the oxidation of CaCr2O4.

271

CaCr2O4 + SO2 + 0.5O2 → CaSO4 + Cr2O3

(6)

272

Similarly, the heating temperatures had an obvious effect on the interactions

273

between CaCr2O4 and Si/Al-compounds. Figure 8 displays the XRD patterns of

274

thermally treated mixtures containing CaCr2O4 and Si/Al-rich fly ash. At 973 K and 14


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275

1173 K, Si/Al-compounds hardly reacted with calcium compounds including CaCr2O4.

276

Meanwhile, calcium compounds in ash, such as calcite, stimulated the oxidation of

277

CaCr2O4 by forming CaCrO4, especially in the Ca-rich water-washed fly ash (shown

278

in Table S1). In the acid-washed fly ash, calcite was removed and the remaining

279

calcium was mainly in the form of CaSO4. The formation of CaCrO4 in the

280

acid-washed fly ash further confirmed the promotion of CaCr2O4 oxidation under the

281

influence of CaSO4. Nevertheless, most chromium remained unoxidized in the form

282

of CaCr2O4 after thermal treatment with acid-washed fly ash.

283

In most cases, increasing heating temperature strongly enhanced the interactions 27, 28

284

between Si/Al-compounds and calcium

285

CaCr2O4 was transformed into calcium aluminosilicates (Eq. (7)), and unoxidized

286

chromium was in the form of Cr2O3 in the acid-washed fly ash. In the Ca-rich

287

water-washed fly ash, CaCr2O4 reacted with Si/Al-compounds, and some intermediate

288

products formed in the heating process (Eq. (8)). As a result, a fraction of the

289

chromium

290

Si/Al-compounds in fly ash competed with CaCr2O4 to react with Ca-compounds at

291

1373 K, which effectively suppressed the oxidation of CaCr2O4.

292 293 294

was

transformed

into

. After the reaction, calcium from

Ca3Cr2

(SiO4)3

(Eq.

CaCr2O4 + Si/Al-compounds → CaAl2SiO2 + Cr2O3 Ca-compounds +SiO2 → CaSiO3 CaCr2O4 + 2CaSiO3 +SiO2 → Ca3Cr2(SiO4)3

(9)).

Therefore,

(7) (8) (9)

295

Environmental Implications. Few studies have reported on the formation and

296

oxidation of CaCr2O4 in ash residues during solid fuel combustion. This is largely 15



297

because of the low concentration of chromium in ash samples and limited analytical

298

methods. The findings in this study demonstrate that CaCr2O4 was probably formed

299

through the decomposition of CaCrO4 and/or the direct interaction between CaO and

300

Cr2O3. Compared to Cr2O3, CaCr2O4 was more easily oxidized to form more toxic

301

chromate species (Cr6+ or Cr5+), and this reaction could take place at low temperature,

302

around 773 K. Therefore, measures should be taken to suppress the formation of

303

CaCr2O4 during fuel combustion.

304

In solid fuel samples containing low free CaO content, the reaction between CaO

305

and Cr2O3 predominantly generated CaCr2O4. However, the reaction was very slow,

306

and decreasing the reaction time could effectively inhibit the formation of CaCr2O4.

307

Furthermore, considering the increased CaCr2O4 formation at high temperatures,

308

another promising way for the control of CaCr2O4 formation is to perform fuel

309

combustion at temperatures below 1200 K.

310

In Ca-rich fuel, chromium was probably oxidized to form calcium chromate

311

during combustion 1, 29. An effective method to suppress the oxidation of chromium is

312

to decrease the free CaO content, by separating Ca-rich components from the fuel or

313

enhancing the interactions of Si/Al- or P-compounds with free CaO 18, 19. In addition,

314

increasing the operating temperature could partly decrease the fraction of chromate in

315

ash residues. This effect was attributed to the rapid decomposition of oxidized

316

CaCrO4. As a result, a large fraction of unoxidized chromium was transformed to

317

CaCr2O4. Then, control of CaCr2O4 oxidation was the key issue, especially for ash

318

residue treatment via heating. When ash samples were reheated, CaCr2O4 showed 16


Page 16 of 31

Page 17 of 31


319

great oxidation capacity for the formation of toxic chromate. Even in the absence of

320

free CaO, the oxidation of CaCr2O4 could occur in the presence of CaSO4, which was

321

widely prevalent in the fly ash during fuel combustion and/or desulfurization of flue

322

gas.

323

According to the results in this study, CaCr2O4 contained some basic sites on its

324

surface to support the reactions with acid gases or other acidic compounds. The

325

addition of sulfur-rich materials into the CaCr2O4-containing ash residue enhanced the

326

transformation of CaCr2O4 to CaSO4 and Cr2O3 at high temperature. In this way, SO2

327

produced in the heating process was captured in situ, and the oxidation of chromium

328

was suppressed. Another practical approach was to mix CaCr2O4-containing ash with

329

Si/Al-rich ash before thermal treatment. In this case, calcium from CaCr2O4 was

330

transformed into aluminosilicates, and the chromium remained unoxidized. However,

331

it is worth noting that Ca-compounds in ash might stimulate the oxidation of CaCr2O4,

332

especially at low temperatures like 973 K and 1173 K. Therefore, the operating

333

temperature should be well controlled to suppress CaCr2O4 oxidation by enhancing

334

the interactions between CaCr2O4 and Si/Al-compounds.

335

In summary, in order to control Ca-chromate formation in fuel combustion,

336

attention should be paid to the ash forming process as well as the ash treatment

337

process. The findings from this study broaden our understanding of the formation of

338

CaCr2O4 as well as its thermal behavior. Moreover, the control strategies for

339

chromium oxidation during fuel combustion could be optimized based on these

340

findings. 17



341

Acknowledgments

342

The present work was supported by the National Natural Science Foundation of

343

China (51606075, 51406062, 51506064) and the General Financial Grant from the

344

China Postdoctoral Science Foundation (Grant, 2016M592330). We appreciate the

345

experimental measurements provided by the Analytical and Testing Center of

346

Huazhong University of Science and Technology.

347

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437

Figure captions

438

Figure 1. TG-DSC curves for the decomposition of CaCrO4 in terms of various

439

concentration of oxygen in the carrier gas

440

Figure 2. In situ high temperature XRD patterns of the mixture of CaO and Cr2O3

441

during thermal treatment under vacuum

442

Figure 3. The formation of CaCr2O4 during reactions between CaO and Cr2O3 in N2

443

at a molar ratio of Ca and Cr 4:1

444

Figure 4. TEM images and crystallites size measurements of Cr3+-compounds

445

Figure 5. Thermogravimetric analysis of CaCr2O4 oxidation at 1373 K with the

446

addition of various contents of free CaO

447

Figure 6. TG curves for the oxidation of Cr2O3 and CaCr2O4 under the influence of

448

CaSO4

449

Figure 7. Percentages of oxidized chromium in the heated mixtures of CaCr2O4 and

450

CaSO4 at various mole ratios of Ca and Cr (ND, not detected)

451

Figure 8. XRD patterns of the products from the reactions between CaCr2O4 and (a)

452

water-washed ash, (b) acid-washed ash

453

23



TG / %

100 98

16 1337 K

TG curve

12

96

8

94

4

-1

20

In N2 with 20% O2

DSC / mW mg

102

Page 24 of 31

DSC curve

TG curve

96

6

94

4

92 2

90

DSC curve 0

In pure N2

6 1186 K

TG curve

4

TG / %

88 100 98 96 94 92 90 88 86 84

454

-1

8

DSC / mW mg

TG / %

98

1304 K

2

-1

100

0

In N2 with 5% O2

DSC / mW mg

92

DSC curve 0 373 473 573 673 773 873 973 1073 1173 1273 1373

Temperature / K

455

Figure 1. TG-DSC curves for the decomposition of CaCrO4 in terms of various

456

concentration of oxygen in the carrier gas

24


Page 25 of 31


2 2 1 2

2

Intensity / A.U.

1 3

1.CaO 2.Cr2O3 3.CaCr2O4

4

2

2

4. Experimental carrier

2 1

11

1323 K- 10min 1323 K-1min 1273K-1min 1223 K-1min 1173 K-1min

10 457

20

30

40

50 60 2-Theta / °

70

80

90

458

Figure 2. In situ high temperature XRD patterns of the mixture of CaO and Cr2O3

459

during thermal treatment under vacuum

460

25



Cr(CaCr2O4)/Total Cr (%)

100 1223 K 1323 K 1423 K

80 60 40 20 0 1

461

3 5 Heating time / min

10

462

Figure 3. The formation of CaCr2O4 during reactions between CaO and Cr2O3 in N2

463

at a molar ratio of Ca and Cr 4:1

26


Page 26 of 31

Page 27 of 31


Cr2O3 464

CaCr2O3-I

CaCr2O3-II

Figure 4. TEM images and crystallites size measurements of Cr3+-compounds

465

27



Page 28 of 31

108 CaCr2O4 without CaO

106

CaCr2O4 with CaO n(Ca/Cr) 1:1 CaCr2O4 with CaO n(Ca/Cr) 1.2:1

104 TG / %

CaCr2O4 with CaO n(Ca/Cr) 1.5:1

102 100 98 96

Heating to 1373 K in Argon

Adding 20 % O2

94 0 466

10

20

30 40 50 60 Retention time / min

70

80

90

467

Figure 5. Thermogravimetric analysis of CaCr2O4 oxidation at 1373 K with the

468

addition of various contents of free CaO

469

28


Page 29 of 31


100.5

TG / %

99.0

TG curve for the mixture of CaSO4 and CaCr2O4

97.5 96.0 94.5 93.0 373

470

TG curve for the mixture of CaSO4 and Cr2O3

573

773 973 Temperature / K

1173

1373

471

Figure 6. TG curves for the oxidation of Cr2O3 and CaCr2O4 under the influence of

472

CaSO4

473

29


Percentages of oxidized chromium / %


474

50 Without CaSO4 addition n(Ca):n(Cr) 1:1 n(Ca):n(Cr) 2:1

40 30 20 10 0

ND 873

ND 1023 Temperature / K

ND 1173

475

Figure 7. Percentages of oxidized chromium in the heated mixtures of CaCr2O4 and

476

CaSO4 at various mole ratios of Ca and Cr (ND, not detected)

30


Page 30 of 31

Page 31 of 31


(a)

1.CaCrO4 2.CaCr2O4 3.Ca3Cr2(SiO4)3 4.Quartz SiO2

63 55 4 3 5 4 36

5 56

5.Cr2O3 6.Gehlenite CaAl2SiO7

3

55

1373 K

1 4

1

Intensity / A. U.

1 1

4 5

4

1

4 1 41

41 1

1173 K

1 4 1 4

4 2

10

20

1

1 41 5

30

1

40

(b)

Intensity / A. U.

50 60 2-Theta /°

70

80

90

4.Quartz SiO2 5.Cr2O3

4 5 3 4

24 1 4

4 2 1 20

973 K

1.CaCrO4 2.CaCr2O4 3.Anorthite CaAl2Si2O8

4

10

4

30

32 5

5

4

22

2

22 1

2

40

4

4

1373K

4

4

4

1173K

4

4

4

973K

50 60 2-Theta /°

70

80

90

477

Figure 8. XRD patterns of the products from the reactions between CaCr2O4 and (a)

478

water-washed ash, (b) acid-washed ash

31


Further insight into the formation and oxidation of CaCr2O4 during

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