Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars

Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars

Accepted Manuscript Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars Siim Link, Khanh-Quang Tran, Quang-Vu...

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Accepted Manuscript Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars

Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin PII:

S0360-5442(18)30603-0

DOI:

10.1016/j.energy.2018.04.013

Reference:

EGY 12652

To appear in:

Energy

Received Date:

02 December 2017

Revised Date:

17 March 2018

Accepted Date:

03 April 2018

Please cite this article as: Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin, Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars, Energy (2018), doi: 10.1016/j.energy.2018.04.013

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ACCEPTED MANUSCRIPT 1 2

Catalytic effect of oil shale ash on CO2 gasification of

3

leached wheat straw and reed chars

4 5

Siim Link*

6

Department of Electrical Power Engineering and Mechatronics, School of Engineering,

7

Tallinn University of Technology

8

Ehitajate tee 5, 19086, Tallinn, Estonia

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E-mail: [email protected]

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*corresponding author

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Khanh-Quang Tran

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Department of Energy and Process Engineering, Norwegian University of Science and

14

Technology

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NO-7491 Trondheim, Norway

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Quang-Vu Bach

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Department of Energy and Process Engineering, Norwegian University of Science and

19

Technology

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NO–7491 Trondheim, Norway

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ACCEPTED MANUSCRIPT 22

Patrik Yrjas

23

Johan Gadolin Process Chemistry Centre, Åbo Akademi University

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Piispankatu 8, Turku, FI-20500, Finland

25 26

Daniel Lindberg

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Johan Gadolin Process Chemistry Centre, Åbo Akademi University

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Piispankatu 8, Turku, FI-20500, Finland

29 30

Stelios Arvelakis

31

Bioresource Technology Unit, Laboratory of Organic and Environment Technologies,

32

Department of Chemical Engineering, National Technical University of Athens

33

Zografou Campus, GR-15700, Athens, Greece

34 35

Argo Rosin

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Department of Electrical Power Engineering and Mechatronics, School of Engineering,

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Tallinn University of Technology

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Ehitajate tee 5, 19086, Tallinn, Estonia

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1 Abstract

43

Oil shale ash is a material that could be used as a low cost catalyst for biomass gasification

44

processes. This article analyses the catalytic effect of oil shale ash on CO2 gasification of reed

45

and leached wheat straw chars. A thermogravimetric analyser operated at atmospheric pressure

46

and two different partial pressures of CO2 such as 0.09 atm and 1 atm were applied. The results

47

indicate that oil shale ash has positive effect on char reactivity at the CO2 partial pressure of

48

0.09 atm: the addition of oil shale ash by 30% to reed char lowered the reaction time by 1.3

49

times and to leached wheat straw char by 1.4 times. At the partial pressure of CO2 1 atm, the

50

addition of oil shale ash resulted in the negative effect on the char conversion due to the binding

51

of CO2. The average calculated CO2 binding capacity of oil shale ash was 13.7 mg CO2 per 100

52

mg of oil shale ash.

53 54

Keywords: biomass, oil shale ash, char, gasification, catalyst, reactivity

55 56

2 Introduction

57

Power generation in Estonia is based mainly on oil shale fuel [1], which is characterized by

58

very high mineral matter contents (60-75%) [2]. As a consequence, 5-7 million tonnes of oil

59

shale ash are produced and sent to the landfills in Estonia annually. Only a small percentage of

60

the ash produced finds its way to secondary uses, either as a stabilizing agent of roadbeds in

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construction, or soil conditioner in agriculture [3]. New methods of reusing or recycling of the

62

ash would help reduce the burden to the landfills and contribute to the use of oil shale for

63

sustainable heat and power generation in Estonia.

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Oil shale ashes contain various Ca compounds [4], of which many exhibit their catalytic effect,

65

enhancing the char gasification reactivity [5]. Calcium and several calcium containing

66

compounds (Ca(OH)2, Ca(Ac)2, CaCO3, CaC2O4, Ca(NO3)2) have been studied as catalysts to

67

improve the gasification reactivity of carbon containing materials [6-10]. H. Risnes et al. [6]

68

have reported that the effect of calcium addition as calcium sugar/molasses solutions on straw

69

significantly affects the ash chemistry and the ash sintering tendency but much less the char

70

reactivity. Perander et al. [7] found that the gasification rate of the char increases linearly with

71

an increase in the concentration of Ca and the catalytic activity of Ca is higher than K at the

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beginning of char gasification but the catalytic effect of Ca decreases earlier than the catalytic

73

effect of potassium. Suzuki et al. [8] studied the different loading levels of Ca and found that

74

the gasification rate increases with the loading level. However, the gasification rate decreased

75

rapidly after the char conversion degree of 50%. Struis et al. [9] showed that after char

76

conversion degree of 20%, the reaction rate course of Ca(NO3)2 impregnated charcoal with

77

progressing gasification levels down to that of pure char without additive.

78

A number of low cost materials such as limestone and/or dolomite have been tested as catalysts

79

for biomass gasification. However, most of them have focused on improving the gas

80

composition and tar reduction [11-13] rather than on the gasification reactivity of biomass chars

81

[5]. Moreover, oil shale ash has not been tested as catalyst for biomass gasification. Additives

82

used in the biomass gasification process require detailed knowledge about their impact on the

83

process and the catalytic activity is strongly influenced by the chemistry of mineral matters of

84

the fuel [6]. Therefore, the study reported in this work was carried out, looking at the ability of

85

OSA improving catalytically the gasification of biomass chars. For this purpose, it is reasonable

86

to choose biomass materials with low reactivity as feedstock; otherwise it would be difficult to

87

observe the catalytic effect of OSA. From our previous studies [14-16], both reed and leached

88

wheat straw chars exhibited lower gasification reactivity compared to chars derived from wood

4

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under the conditions investigated. Moreover, reed and wheat straw differ by the content and

90

composition of mineral matter (ash). The choice of biomass materials for the study helps better

91

estimate the use of OSA for feedstocks with diverged ash content and composition. The

92

objective of this study is to determine the effect of OSA on the CO2 gasification reactivity of

93

the selected biomass (common reed and leached wheat straw) chars.

94 95

3 Materials and Methods

96

3.1 Materials

97

The biomass samples selected for the investigation were the following: pelagian reed,

98

originating from the west coast shorelines of Estonia and the islands of Estonia and wheat straw

99

from the area of Thessaly in Greece. The wheat straw sample was pre-treated using the leaching

100

method developed by Arvelakis et al. [17]. The oil shale ash was collected from the heat

101

exchanger of circulating ash (called INTREXTM according to the Foster Wheeler, the supplier

102

of the boiler) of a circulating fluidized bed boiler in the Eesti power plant (one of the Narva

103

power plants of the national energy company Eesti Energia AS).

104

The reed and leached wheat straw (hereafter referred as R and LWS, respectively) were

105

analyzed and characterized regarding to proximate and ultimate analyses, and their gross

106

calorific value, as well as ash chemical analysis in accordance with the American Society for

107

Testing and Materials (ASTM) standard methods, including D 1102-84, D 3175-89a, D 5142-

108

90, D5373-93, D 4208-88, D 2015-95.

109

The proximate analysis of reed char and leached wheat straw char samples (hereafter referred

110

to as RC and LWSC, respectively) was performed by means of a TA Instruments SDT Q600

111

thermogravimetric analyser (TGA). First, the sample was heated at a rate of 20 °C min-1 from

5

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ambient temperature to 105 °C and kept for 10 min in nitrogen gas flow of 90 mL min-1 to

113

determine the moisture content. Then, the temperature was increased, with a heating rate of 50

114

°C min-1, up to 900 °C, at which it was kept for 7 min to determine the volatile content of the

115

sample. Thereafter the temperature was lowered to 600 °C in 20 min, followed by supplying air

116

at the flow rate of 100 mL min-1 to initiate burning for determining the ash content [18].

117

The oil shale ash (hereafter referred to as OSA) fraction with particles smaller than 125 µm in

118

size was selected and characterized regarding the ash chemistry and mineralogy analyses. The

119

ash chemistry analysis was performed in accordance with various standard methods including

120

EVS-EN 196-21, DIN 51729, STI-1-20015, STI-2-20015, and ISO 334. The mineralogy

121

analysis was carried out by means of a Rigaku Ultima IV diffractometer using the X-ray powder

122

diffraction (XRD) method with Cu Kα radiation (λ = 1.5406 Å, 40 kV at 40 mA) and a silicon

123

strip detector D/teX Ultra.

124 125

3.2 Leaching

126

The leaching pre-treatment of the wheat straw was performed using tap water according to the

127

method developed by Arvelakis et al. [17]. During the leaching process, the sample was put in

128

a 200-mesh plastic grid, tied up and submerged into tap water in a plastic 75 l volume barrel.

129

The used mass/water ratio was 66.6 g/L and the retention time was 12 h. At the end of the

130

process, the sample was allowed to dry naturally in air until reaching a constant weight under

131

the sun of the Mediterranean summer within a period of 5 days. During the drying process, the

132

bed thickness of the drying sample was kept below 15 cm to guarantee the fast-drying process

133

without material loss due to possible mould bacterial activity.

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3.3 Char preparation by pyrolysis

135

A cylindrical atmospheric fixed-bed reactor with an electrical heater was used to prepare char

136

from the selected biomass samples. A quartz pipe inside the reactor protects the heating wire

137

against corrosive compounds possibly present in the evolved gas mixture during the pyrolysis.

138

The sample is loaded into the sample holder made of a stainless-steel SS316 bound net with 80

139

μm openings and placed in the quartz pipe. The applied heating procedure was the following:

140

(i) hold the sample at 50 °C for 45 min, (ii) heat the sample to the end temperature of 800 °C at

141

a heating rate of 10 °C min-1, (iii) hold the sample at the end temperature for 15 min, and (iv)

142

cool down. Nitrogen was used as a carrier gas with a flow rate of 1 L min-1 in all pyrolysis

143

experiments. The methodology together with a detailed description of the system is reported in

144

[19].

145

3.4 Test method for CO2 gasification reactivity of char

146

A thermogravimetric analyser (TGA) of TA Instruments SDT Q600 was used for this test. Two

147

partial pressures of CO2, 0.09 and 1.0 atm, were employed for the test at atmospheric pressure,

148

considering possible interferences of CO2 chemistry during the process on the catalytic effect

149

of OSA. The char reactivity was tested at a CO2 partial pressure of 0.09 atm with the following

150

procedure. The sample was heated up from room temperature to 850 °C at a heating rate of 50

151

°C/min in a nitrogen flow rate of 100 mL min-1. When the temperature (850 °C) had been

152

established and kept constant, the gas composition was changed to include CO2 to ensure the

153

flow rate of N2 and CO2 at 91 and 9 mL min-1, respectively. After that, the TGA reactor was

154

held under these conditions until no mass change was observed. Similarly, the char reactivity

155

was tested at a CO2 partial pressure of 1 atm with the following procedure. The sample was

156

heated up from room temperature to 850 °C, at a heating rate of 50 °C/min in a nitrogen flow

157

rate of 100 mL min-1. When the temperature (850 °C) had been established and kept constant,

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the gas was changed to include only CO2 with a flow rate of 100 mL min-1. Then, the TGA

159

reactor was held under these conditions until no mass change was observed.

160

The gasification reactivity is expressed as the char gasification rate over a degree of char

161

conversion. The char gasification rate in units of min-1 at any particular conversion value is

162

defined as shown in Eq. (1).

163

r=dX/dt

164

where r is the reaction rate, and X is expressed as (Eq. (2)):

165

X=(Mo-M(t))/(Mo-Mf).

166

Mo represents the initial mass of the char, M(t) is the mass of the char at time t, and Mf is the

167

mass of the gasification residue.

168

3.5 Thermodynamic calculations

169

In our study, the thermodynamic equilibrium calculations were performed to determine the

170

composition of mineral matter under gasification conditions. For calculations, FactSage 7.1

171

software package was used [20]. The following thermodynamic databases were used for the

172

calculations: FToxid, which includes data for solid oxides and silicates and liquid slag; FTSalt

173

for liquid alkali salts (NaCl-Na2CO3-Na2SO4-KCl-K2CO3-K2SO4) and corresponding alkali salt

174

solid solutions; FactPS for the gas phase and other stoichiometric solid phases. The input for

175

the calculations was the chemical composition of the fuel ashes in Table 2, the temperature was

176

set to 850 °C and the partial pressure of CO2 was set to be 0.09 atm or 1.0 atm.

(1)

(2)

177

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4 Results and discussion

179

4.1 Material characterization

180

Reed (R) and leached wheat straw (LWS) differ by their composition (as seen in Tables 1 and

181

2). The ash content of R and LWS is 3.2% and 5.8% respectively, as dry basis. The main

182

component of reed ash is SiO2. Besides SiO2, the ash of LWS contains also 15% of CaO.

183

Table 1

184

Fuel characterization of the selected biomass samples.

Proximate analysis, dry

Ultimate analysis, dry basis (wt%)

Gross

basis (wt%)

Moisture

calorific

Material (wt%)

value,

Volatile

Fixed

Ash

matter

carbon

N

C

H

S

Cl

O

(MJ/kg)

R

5.4

3.2

80.3

16.5

0.4

47.4

5.7

0.2

n.d.

43.1

20.41

LWS

5.8

5.8

80.7

13.5

0.6

46.3

5.3

0.2

0.1

41.7

20.03

185 186

Table 2

187

Chemical analysis of the feedstock ashes (wt%).

Material

K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 TiO2 SO3

P2O5 Cl

Reed

5.9

8.4

2.9

1.4

73.7

n.d.

1.1

0.1

5.2

1.0

0.6

LWS

4.0

1.9

15.3 2.5

49.9

1.9

0.6

0.1

3.8

n.d.

1.8

OSA

1.0

0.2

43.9 9.6

17.6

5.3

1.5

n.d.

15.3 n.d.

0.4

188 189

On the other hand, the char yield (dry basis) after pyrolysis was 19.4% for reed sample and

190

27.3% for leached wheat straw sample. After pyrolysis, the content of fixed carbon and ash

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increases, but the content of volatile matter decreases compared to parent fuel samples (see also

192

Tables 1 and 3).

193

Table 3

194

Char characterization of the selected biomass samples.

Moisture

Proximate analysis, dry basis (wt%)

Material (wt%)

Ash

Volatile matter

Fixed carbon

RC

1.2

17.7

15.0

67.3

LWSC

2.1

34.6

14.7

50.7

195 196

Results from the XRD analysis of oil shale ash (OSA) are presented in Table 4, which shows

197

that the content of CaO is 2.9% and that of Ca(OH)2 is 18.0%. Kuusik et al. [21] reported 19.9%

198

CaO and 2.1% Ca(OH)2 for the ash collected from the heat exchanger of the circulating ash

199

(INTREX ash) of the fluidized bed boiler. Bityukova et al. [4] have studied also the composition

200

of oil shale ashes. They found that the INTREX ash contains 9.9% CaO and 12.3% Ca(OH)2.

201

The differences between the content of CaO and Ca(OH)2 can be attributed to the chemistry of

202

CaO in the presence of moisture, according to Eq. (3), forming thermally more stable product

203

of calcium hydroxide (portlandite) [4].

204

CaO + H2O = Ca(OH)2 + heat

205

Table 4

206

Mineral composition of the oil shale ash (wt%).

(3)

SiO2 CaO Ca(OH)2 CaSO4 MgO Ca2SiO4 Ca2Mg(Si2O7) CaSiO3 KAlSi3O8 CaCO3 12.0

2.9

18.0

41.0

4.4

5.0

207

10

5.0

5.1

2.6

4.0

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4.2 Thermogravimetric analysis of pure oil shale ash

209

First of all, the behavior of the oil shale ash was thermogravimetrically analyzed under different

210

CO2 partial pressures, as described above. The partial pressure of 0.09 atm characterizes the

211

partial pressure of CO2 in product gas under atmospheric air blown gasification condition [22],

212

and the partial pressure of 1 atm could be considered as the partial pressure of CO2 under

213

pressurized gasification conditions depending on the total pressure of product gas [23-25]. The

214

results of these analyses are shown in Fig. 1.

98

700

96

600

94

500 Phase I: 100% N2

92

400

Phase II: 100% CO2

90

300

Phase II: 9% CO2 + 91% N2

200

Temperature

3000

2800

2600

2400

2200

2000

1600

1400

1200

1000

800

0

600

84

400

100

200

86

0

Temperature, oC

800

88

215

900

100

1800

Mass loss, M/Mo (%)

Phase II

Phase I

102

Time, s

216

Fig. 1. Thermogravimetric analysis of the oil shale ash used for the test.

217

During Phase I, three steps of mass loss could be observed. The first step up to 180 °C is related

218

to the removal of moisture. The second step of mass loss between the temperatures of 375 –

219

450 °C is related to the pyrolysis of unburnt hydrocarbon entrapped in the ash. The third mass

220

loss between 600 – 850 °C is attributed to the decomposition of carbonates [26].

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It is seen that at the CO2 partial pressure of 0.09 atm, there is no increase in mass in Phase II.

222

This indicates that CaO does not react with CO2 under the conditions of our investigation. At

223

the CO2 partial pressure of 1 atm, CaO reacts with CO2 and the mass of ash sample was

224

increased. The average calculated CO2 binding capacity of oil shale ash is 13.7 mg CO2 per 100

225

mg of oil shale ash sample, which is in agreement with previous studies [27].

226

Based on the composition of oil shale ash in Table 2, the equilibrium calculations at 850 °C

227

under different partial pressures of CO2 were performed as well, as shown in Table 5.

228

Table 5

229

Equilibrium calculation of oil shale ash (wt%).

PCO2 (atm) 0.09 1.00

MgO 2.9 2.8

CaO 3.9 0.0

Mineral composition Ca3MgAl4O10 Ca3MgSi2O8 Ca2Fe2O5 11.6 52.0 2.8 11.2 50.5 2.7

CaCO3 0.0 6.7

CaSO4 26.9 26.1

230

The thermodynamic calculations support the experimental data that CO2 is consumed by CaO

231

at the elevated partial pressure of CO2.

232

The observed behavior of oil shale ash under different partial pressures of CO2 is in agreement

233

with the chemical reaction between CaO and CO2 described by Eq. (4), considering the effect

234

of CO2 partial pressure on the equilibrium [28,29]. Indeed, the forward reaction, which reduces

235

the partial pressure of CO2, is favored when the applied partial pressure of the CO2 is increased,

236

considering Le Chatelier’s principle.

237

CaO + CO2 ↔ CaCO3

(4)

238 239

4.3 Char gasification reactivity and the catalytic effect of oil shale ash

240

Figs. 2 and 3 show the data of reaction rates plotted versus the conversion degree for the selected

241

char samples without and with OSA addition, under the CO2 partial pressure condition of 1 atm.

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Fig. 2 shows the reed sample and Fig. 3 the leached wheat straw. The char conversion degree

243

plotted versus the time of the gasification tests is illustrated in Fig. 4 (reed) and Fig. 5 5 (LWS).

244

As can be seen from Figs. 2 and 3, the LWS char is more reactive than the reed char under

245

identical gasification conditions. The reaction rates increase from the start to the points where

246

the char conversion degree is about 20-30%, and then decline. With the addition of OSA, the

247

gasification reactivity of both char samples decreases. The more OSA is added, the lower the

248

reactivity is. In addition, the rate curves for the reed char (Fig. 2) exhibit double peaks, but it is

249

not the case with the LWS char (Fig. 3). 4.5 Reed char

Reaction rate, %/min

4.0

R/10%_OSA

3.5

R/20%_OSA

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

250 251

10

20

30

40

50

60

70

Char conversion degree, % Fig. 2. Reaction rate of reed char, pCO2=1 atm.

252

13

80

90

100

ACCEPTED MANUSCRIPT 4.5

Reaction rate, %/min

4.0 3.5 3.0 2.5 2.0 1.5 LWS char

1.0

LWS/10%_OSA

0.5

LWS/20%_OSA

0.0 0

10

20

40

50

60

70

80

90

100

Char conversion degree, %

253 254

30

Fig. 3. Reaction rate of leached wheat straw char, pCO2=1 atm.

100

Char conversion degree, %

90 80 70 60 50 40 Reed char

30

R/10%_OSA

20

R/20%_OSA

10 0 0

255 256

1500

3000

4500

6000

Time, s Fig. 4. Conversion degree versus time of reed char, pCO2=1 atm.

257

14

7500

9000

ACCEPTED MANUSCRIPT

Char conversion degree, %

100 90 80 70 60 50 40 30

LWS char

20

LWS/10%_OSA

10

LWS/20%_OSA

0 0

258

1000

2000

3000

4000

5000

6000

Time, s

259

Fig. 5. Conversion degree versus time of leached wheat straw char, pCO2=1 atm.

260

The reaction rates versus the char conversion degree of the char samples under the CO2 partial

261

pressure condition of 0.09 atm are shown in Figs. 6 and 7 for the reed and LWS char sample,

262

respectively. The char conversion degrees plotted versus the time of the gasification tests are

263

shown in Figs. 8 and 9. As can see from Figs. 6 and 7, the rate curves for the CO2 gasification

264

of reed and LWS without OSA additions are similar, having one peak only. However, when

265

OSA is added, the peak disappears in all cases. In the case of reed char, OSA additions give

266

rise to increased reaction rate until the conversion degree reaches 15-20% approximately.

267

Within this section of the curves, the more OSA is added, the higher the reaction rate is. In the

268

second section, within 15-35% as char conversion degree, OSA additions seem to have negative

269

effect on the reaction rate. However, the differences between the rate curves are very small. The

270

effects turn to be positive again for the last section from 35% to 100% with pronounced

271

differences and the order of the curves is the same as in the first section.

272

15

ACCEPTED MANUSCRIPT 1.0

Reaction rate, %/min

R/30%_OSA 0.8

R/20%_OSA R/10%_OSA

0.6

Reed char

0.4

0.2

0.0 0

10

30

40

50

60

70

80

90

100

Char conversion degree, %

273 274

20

Fig. 6. Reaction rate of reed char, pCO2=0,09 atm.

275

Reaction rate, %/min

1.4 LWS/30%_OSA

1.2

LWS/20%_OSA LWS/10%_OSA

1.0

LWS char

0.8 0.6 0.4 0.2 0.0 0

276 277

10

20

30

40

50

60

70

Char conversion degree, % Fig. 7. Reaction rate of leached wheat straw char, pCO2=0,09 atm.

278

16

80

90

100

ACCEPTED MANUSCRIPT 100

Char conversion degree, %

90 80 70 60 50

R/30%_OSA

40

R/20%_OSA

30

R/10%_OSA

20

Reed char

10 0 0

10000

30000

Time, s

279 280

20000

Fig. 8. Conversion degree versus time of reed char, pCO2=0,09 atm.

100

Char conversion degree, %

90 80 70 60 50 LWS/30%_OSA

40 30

LWS/20%_OSA

20

LWS/10%_OSA

10

LWS char

0 0

281

5000

10000

15000

20000

25000

Time, s

282

Fig. 9. Conversion degree versus time of leached wheat straw char, pCO2=0,09 atm.

283

In the case of LWS char gasification with OSA additions, the rate curves (Fig. 7) are similar in

284

shape with those of reed char gasification under identical conditions. The curves also exhibit

285

three sections. The main differences between the two cases are that the second section of LWS

17

ACCEPTED MANUSCRIPT 286

curves shows pronounced negative effects of OSA additions on the rate. However, the order of

287

the curves with OSA additions is not consistent.

288

4.4 Discussion

289

Our results show that under the gasification conditions with high CO2 partial pressure, e.g. 1

290

atm, the catalytic effect of oil shale ash is not favored. In fact, as described above, the partial

291

pressure of CO2 is above the equilibrium value of CaCO3 composition. Therefore, CaO is

292

deactivated by reacting with CO2, producing more CaCO3 [30]. This is revealed from Fig. 1

293

under brief discussion in Section 4.2. Consequently, the more OSA is added, the lower the

294

reaction rate. Together with the deactivation of CaO as catalyst, the whole OSA material

295

became non-active. In this case, the non-active OSA blocks also the active sites of char and the

296

contact between CO2 and char is inhibited. Therefore, the lower reactivity (see also Fig. 2 and

297

Fig. 3) under CO2 partial pressure of 1 atm with increasing share of OSA is believed to occur

298

due to the blockage of active sites of char by OSA.

299

Interestingly, in the case of the reed char samples, some fluctuations in the reaction rate were

300

observed after the maximum peaks. It is, however, believed to be caused by the data acquisition,

301

co-considering that previous studies of reed gasification showed no similar behavior [14].

302

On the other hand, both of the pure reed and leached wheat straw char samples (Fig. 2 and 3,

303

Fig. 6 and 7) exhibited lower reactivity when the pCO2 = 0.09 atm, compared to pCO2 = 1 atm.

304

The maximum reaction rates decreased by an order of magnitude. The reaction time for reed

305

char was prolonged 4.4 and for leached wheat straw char 5.1 times (see also Fig. 4 and 5, Fig.

306

8 and 9). The decrease in the reaction rate with the partial pressure of CO2 is in agreement with

307

previous studies [31].

308

Under the gasification conditions of 100% CO2 as a gasification agent, the reed char and leached

309

wheat straw char samples (see also Fig. 2 and Fig. 3) exhibited similar behavior compared to

18

ACCEPTED MANUSCRIPT 310

previous studies [18]. The reactivity of leached wheat straw char is moderately higher than that

311

of reed, i.e. 1.7 times, considering the total reaction times (see also Fig. 4 and Fig. 5).

312

The gasification tests without the addition of OSA (Fig. 2 and 3, Fig. 6 and 7) show that the

313

reaction rate increased at the beginning of the gasification process, achieving a maximum, after

314

which the reaction rate declined. Previous studies suggest that several factors affect the

315

reactivity, which include porous structure, active sites, and mineral matter. According to

316

Radović et al. [32], the reactivity is rather more dependent upon carbon active sites than the

317

total surface area. Van Heek et al. [33] concluded that the extension of total surface area could

318

not be the only dominating factor. It is more likely that the properties of the surface, such as

319

activity and accessibility, give the possibility of blockage areas by minerals. Bar-Ziv et al. [34]

320

suggested that the reactivity evolution during gasification is influenced by changes in the porous

321

structure, and coalescence of microcrystals can be used to represent the change in the

322

concentration of the active sites. Livneh et al. [35] confirmed that the most significant change

323

in reactivity occurs in the range of 0-30% conversion, and the slowdown of reactivity at the

324

conversion rate above 55% can be explained by the consumption of small microcrystals, which

325

are generally more reactive than large ones. The initial increase in the reaction rate observed in

326

our study could be associated with the increase of the surface area as well as active sites in the

327

early phase of gasification. The maximum in the reaction rate is thought to arise from two

328

opposing effects, namely, increase in the reaction surface area as micropores grow and their

329

decline as pores collapse progressively at their intersection (coalescence) [36]. The decrease in

330

the reaction rate is probably due to catalyst deactivation along the process.

331

On the other hand, the gasification tests with OSA additions under the partial pressure of CO2

332

0.09 atm, exhibited high reaction rates in the beginning, up to the conversion degree of 10-20%.

333

After that the rate started to decline. The earlier part of the reaction rate curve is similar to that

334

presented by Struis et al. [9], using Ca(NO3)2 impregnated biomass samples. Many authors have

19

ACCEPTED MANUSCRIPT 335

attributed the loss of activity of CaO to the sintering of CaO [8,9]. Unfortunately, the

336

explanations based on the studies reported and X-ray diffraction (XRD) data are not convincing

337

enough to prove the sintering of CaO. For instance, (i) Struis et al. [9] explain the sintering of

338

CaO based on the XRD of lignite chars performed by Radović et al. [37]. The signal of CaO is

339

not visible in the case of a coal sample, but can be seen in the pyrolysis chars of coal. However,

340

this is not a proof of sintering of CaO; (ii) Suzuki et al. [8] related this phenomenon to gradual

341

sintering of CaO, evidenced by XRD measurement after gasification (gasification temperature

342

was 900 °C). Unfortunately, the results of XRD measurement were not given. Hence, the

343

deactivation of CaO is believed to be unrelated to the sintering of pure CaO because of the

344

interaction with other mineral matter such as Si and K compounds [6]. Additionally, we

345

performed thermodynamic equilibrium modelling with FactSage (seen also in Table 6). As can

346

be seen from Table 6, the Ca is bound in different compounds, mainly with Si, K, S and Mg

347

when OSA is added to biomass chars.

348

Table 6

349

Equilibrium calculation of char mineral matter and oil shale ash (wt%), pCO2=0.09 atm and 850 °C.

R SiO2 Na2Mg2Si6O15 Na2Ca3Si6O16 NaAlSiO4 NaAlSi3O8 KAlSi2O6 KAlSi3O8 CaSiO3 CaMgSi2O6 Ca2MgSi2O7 CaAl2Si2O8 Ca3P2O8 Ca3Fe2Si3O12 CaSO4

79.8 6.4 12.2 1.7 -

R R R 10%OSA 20% OSA 30% OSA 27.2 3.6 9.0 5.3 8.7 2.2 7.4 8.7 27.9 24.2 26.8 39.6 29.2 36.1 0.9 4.8 4.9 4.8 15.5 1.7 11.1

350

20

LWS 32.9 14.1 28.7 18.3 2.6 3.4

LWS LWS LWS 10%OSA 20%OSA 30%OSA 9.9 2.6 2.5 5.5 10.6 4.6 15.6 1.2 29.7 30.3 26.2 27.7 32.9 27.1 13.1 6.0 3.2 3.6 3.9 11.4 15.9 16.5

ACCEPTED MANUSCRIPT 351

Among the studied shares of OSA to the char samples, OSA addition of 30% exhibited the

352

fastest reactions times, i.e. the addition of 30% to the reed char lowered the reaction time by

353

1.3 times and to the leached wheat straw char by 1.4 times.

354

Tuomi et al. [25] observed that the dolomite was more active on tar decomposition in the

355

fluidized-bed gasification tests than in the laboratory scale tests under identical atmospheric

356

pressure conditions and at the same temperature. Similarly, oil shale ash could exhibit enhanced

357

catalytic effects on char conversion in fluidized bed compared to the laboratory scale

358

experiments performed in our study. The importance of even distribution and dispersion, and

359

good contact between catalysts and active sites has been pointed out in other studies as well

360

[7,9,37,38]. Therefore, the impact of OSA as a bed additive in the fluidized bed gasification

361

process needs further investigation.

362 363

5 Conclusions

364

In this study, the catalytic effect of oil shale ash on CO2 gasification of reed and leached wheat

365

straw chars were studied by means of a thermogravimetric analyser, TA Instruments SDT

366

Q600, operated at atmospheric pressure with two different partial pressure levels of CO2: 1.0

367

atm and 0.09 atm. It was observed that the effect was positive when the CO2 partial pressure of

368

0.09 atm was applied. The addition of OSA by 30 wt% to reed char lowered the reaction time

369

by 1.3 times and to leached wheat straw char by 1.4 times, presumably due to the presence of

370

CaO in the OSA. In contrast, when the CO2 partial pressure of 1.0 atm was applied, the effect

371

was negative. This is presumably due to the binding of CaO with CO2 to form CaCO3,

372

considering that CaO but not CaCO3 is directly responsible for the catalytic effect of OSA.

373

Despite of this, the result from this study suggests that OSA can be used as additive to catalyse

374

CO2 gasification of biomass. However, further investigations are needed to optimize the

21

ACCEPTED MANUSCRIPT 375

gasification process with respect to the CO2 partial pressure to be applied. On the other hand, it

376

also suggests that OSA could be used as additive for removing CO2 from hot gas steams such

377

as process or flue gases containing high content of CO2.

378 379

Acknowledgements. Arvo Mere, Peter Backman, Maaris Nuutre, Raaja Aluvee, and Birgit

380

Maaten are acknowledged for their comprehensive assistance in conducting our tests. Rain

381

Veinjärv and Rustam Hasjanov from Enefit Energiatootmine AS are acknowledged for their

382

help in collecting oil shale ash. The work is financially supported by Estonian Ministry of

383

Education and Research (IUT19-4), and by the European Regional Development Fund through

384

the project TK141 “Advanced materials and high-technology devices for energy recuperation

385

systems“.

386 387

6 References

388

[1] Yörük CR, Meriste T, Trikkel A, Kuusik R. Oxy-fuel Combustion of Estonian Oil Shale:

389

Kinetics and Modeling. Energy Procedia 2016;86:124 – 33.

390

[2] Parve T, Ots A, Skrifars BJ, Hupa M. The sintering of Estonian oil shale ashes. Oil Shale

391

1995;12:341-56.

392

[3] Mõtlep R. Composition and diagenesis of oil shale industrial solid wastes. Doctoral thesis.

393

Tartu University Press 2010.

394

[4] Bityukova L, Mõtlep R, Kirsimäe K. Composition of oil shale ashes from pulverized firing

395

and circulating fluidized-bed boiler in Narva thermal power plants, Estonia. Oil shale

396

2010;27:339-53.

22

ACCEPTED MANUSCRIPT 397

[5] Nzihou A, Stanmore B, Sharrock P. A review of catalysts for the gasification of biomass

398

char, with some reference to coal. Energy 2013;58:305-17.

399

[6] Risnes H, Fjellerup J, Henriksen U, Moilanen A, Norby P, Papadakis K, Posselt D, Sørensen

400

L.H. Calcium addition in straw gasification. Fuel 2003;82:641–51.

401

[7] Perander M, DeMartini N, Brink A, Kramb J, Karlström O, Hemming J, Moilanen A,

402

Konttinen J, Hupa M. Catalytic effect of Ca and K on CO2 gasification of spruce wood char.

403

Fuel 2015;150:464–72.

404

[8] Suzuki T, Nakajima H, Ikenaga N, Oda H, Miyake T. Effect of mineral matters in biomass

405

on the gasification rate of their chars. Biomass Conv Bioref 2011;1:17-28.

406

[9] Struis RPWJ, Von Scala C, Stucki S, Prins R. Gasification reactivity of charcoal with CO2.

407

Part II: Metal catalysis as a function of conversion. Chem Eng Sci 2002;57:3593–602.

408

[10] Ganga Devi T, Kannan MP. Calcium catalysis in air gasification of cellulosic chars. Fuel

409

1998;77:1825–30.

410

[11] Kuo JH, Wey MY, Chen WC. Woody waste air gasification in fluidized bed with Ca- and

411

Mg-modified bed materials and additives. Appl Therm Eng 2013;53:42-48.

412

[12] Devi L, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar elimination

413

in biomass gasi cation processes. Biomass Bioenergy 2003;24:125 – 40.

414

[13] Sutton D, Kelleher B, Ross JRH. Review of literature on catalysts for biomass gasification.

415

Fuel Process Technol 2001;73:155–73

416

[14] Link S, Arvelakis S, Hupa M, Yrjas P, Külaots I, Paist A. Reactivity of the Biomass Chars

417

Originating from Reed, Douglas Fir, and Pine. Energy Fuels 2010;24:6533–39.

418

[15] Link S, Kask Ü, Paist A, Siirde A, Arvelakis S, Hupa M, Yrjas P, Külaots I. Reed as a

419

gasification fuel: a comparison with woody fuels. Mires and Peat 2013;13:Article 04,1–12.

23

ACCEPTED MANUSCRIPT 420

[16] Link S, Arvelakis S, Paist A, Hupa M, Yrjas P, Külaots I. Effect of leaching pre-treatment

421

on the char reactivity of pyrolyzed wheat straw. 17th European Biomass Conference and

422

Exhibition, 29 June – 3 July 2009, Hamburg, Germany, 1113-1121.

423

[17] Arvelakis S, Koukios EG. Physicochemical upgrading of agroresidues as feedstocks for

424

energy production via thermochemical conversion methods. Biomass Bioenergy 2002;22:331–

425

48.

426

[18] Link S. Reactivity of Woody and Herbaceous Biomass Chars. PhD Thesis, TUT Press

427

2011.

428

[19] Link S, Arvelakis S, Spliethoff H, De Waard P, Samoson A. Investigation of Biomasses

429

and Chars Obtained from Pyrolysis of Different Biomasses with Solid-State

430

Nuclear Magnetic Resonance Spectroscopy. Energy Fuels 2008;22:3523–30.

431

[20] Bale CW, Bélisle E, Chartrand P, Decterov SA, Eriksson G, Gheribi A, Hack K, Jung I-H,

432

Kang Y-B, Melançon J, Pelton AD, Petersen S, Robelin C, Sangster J, Spencer P, Van Ende

433

M-A. FactSage thermochemical software and databases, 2010–2016. Calphad 2016;54:35-53.

434

[21] Kuusik R, Uibu M, Kirsimäe K. Characterization of oil shale ashes formed at industrial-

435

scale CFBC boilers. Oil Shale 2005;22:407-19.

436

[22] Link S, Arvelakis S, Paist A, Liliedahl T, Rosén C. Effect of leaching pretreatment on the

437

gasification of wine and vine (residue) biomass. Renew Energy 2018;115:1-5.

438

[23] Kitzler H, Pfeifer C, Hofbauer H. Pressurized gasification of woody biomass—Variation

439

of parameter. Fuel Process Technol 2011;92:908–14.

440

[24] Hannula I, Kurkela E. A semi-empirical model for pressurised air-blown fluidised-bed

441

gasification of biomass. Bioresour Technol 2010;101:4608-15.

24

13C

and

23Na

ACCEPTED MANUSCRIPT 442

[25] Tuomi S, Kaisalo N, Simell P, Kurkela E. Effect of pressure on tar decomposition activity

443

of different bed materials in biomass gasification conditions. Fuel 2015;158:293–305.

444

[26] Arvelakis S, Jensen PA, Dam-Johansen K. Simultaneous thermal analysis (STA) on ash

445

from high-alkali biomass. Energy Fuels 2004;18:1066-76.

446

[27] Trikkel A, Keelmann M, Kaljuvee T, Kuusik R. CO2 and SO2 uptake by oil shale ashes.

447

Effect of pre-treatment on kinetics. J Therm Anal Calorim 2010;99:763–69.

448

[28] Ots A. Oil Shale Fuel Combustion. Tallinn Book Printers 2006.

449

[29] Yrjas KP, Zevenhoven CAP, Hupa MM. Hydrogen Sulfide Capture by Limestone and

450

Dolomite at Elevated Pressure. 1. Sorbent Performance. Ind Eng Chem Res 1996;35:176-83.

451

[30] Simell PA, Leppälahti JK, Kurkela EA. Tar-decomposing activity of carbonate rocks under

452

high CO2 partial pressure. Fuel 74;1995:938-45.

453

[31] Samaras P, Diamadopoulos E, Sakellaropoulos GP. The effect of mineral matter and

454

pyrolysis conditions on the gasification of Greek lignite by carbon dioxide. Fuel 75;1996:1108-

455

14.

456

[32] Radović LR, Walker Jr PL, Jenkins RG. Importance of carbon active sites in the

457

gasification of coal chars. Fuel 1983;62:849–56.

458

[33] Van Heek KH, Mühlen HJ. Aspects of coal properties and constitution important for

459

gasification. Fuel 1985;64:1405–14.

460

[34] Bar-Ziv E, Kantorovich I.I. Mutual effects of porosity and reactivity in char oxidation.

461

Progress in Energy and Combustion Science 2001;21:667–97.

462

[35] Livneh T, Bar-Ziv E, Salatino P, Senneca O. Evolution of Reactivity of Highly Porous

463

Chars from Raman Microscopy. Combust Sci Technol 2000;153:65–82.

25

ACCEPTED MANUSCRIPT 464

[36] Struis RPWJ, Von Scala C, Stucki S, Prins R. Gasification reactivity of charcoal with CO2.

465

Part I: Conversion and structural phenomena. Chem Eng Sci 2002;57:3581–92.

466

[37] Radović LR, Walker Jr PL, Jenkins RG. Effect of lignite pyrolysis conditions on calcium

467

oxide dispersion and subsequent char reactivity. Fuel 1983;62:209-12.

468

[38] Radović LR, Walker Jr PL, Jenkins RG. Importance of Catalyst Dispersion in the

469

Gasification of Lignite Chars. Journal of Catalysis 1983;82:382-94.

26

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  

The impact of oil shale ash on gasification is dependent on partial pressure of CO2. Oil shale ash could act as catalyst under atmospheric gasification conditions. Oil shale ash could bind CO2 under pressurized gasification conditions.