Fate of chlorine in rice straw under different pyrolysis temperatures

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Fate of chlorine in rice straw under different pyrolysis temperatures Bingxian Peng, Xinrui Li, Jing Luo, and Xing Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02097 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Energy & Fuels

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Fate of chlorine in rice straw under different pyrolysis temperatures

2

Bingxian Peng*,

3

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, P. R. China

Xinrui Li, Jing Luo, Xing Yu

4 5

ABSTRACT: The fate of Cl in rice straw after pyrolysis in 200-1000 oC was investigated through

6

heating of tubular furnace, sequential extraction, and determination using pyrohydrolysis

7

combined with ion chromatography (PIC) for total Cl, ion chromatography (IC) for inorganic Cl

8

and gas chromatography-mass spectrometer (GC-MS) for CH3Cl. The rice straw and (or) its chars

9

after pyrolysis were characterized with TGA (thermogravimetric analysis), SEM (scanning

10

electron microscope), XRD (X-ray diffraction) and XRF (X-ray fluorescence spectrum). The

11

results showed that, from 200 to 400 oC, the water soluble Cl in rice straw char decreased from

12

60.31% to 40.13%; the released Cl was dominated by Cl-CH3Cl (Cl as CH3Cl) and Cl-HCl (Cl as

13

HCl), with release rate from 10.41 to 16.81% and from 28.76 to 35.34%, respectively. At 400 oC,

14

5.84% of Cl in tar (tar-Cl) was found. In the pyrolysis from 400 to 700 oC, in chars, the water

15

soluble Cl changed a little and organic bound Cl increased from 1.25% to 7.50%; the released

16

Cl-CH3Cl and Cl-HCl respectively decreased from 13.41% to 2.42% and from 29.19% to 12.84%,

17

and the tar-Cl increased from 9.19% to 18.14%. At 700 oC, the Cl as KCl (Cl-KCl) started to

18

release. For the pyrolysis from 700 oC to 1000 oC, the release rate decreased from 40.20% to

19

4.48% for the water soluble Cl and from 7.50% to 0.34% for organic bound Cl, a small amount of

20

residual Cl was found in chars; the released tar-Cl and Cl-KCl increased from 18.74% to 26.01%

21

and from 17.40% to 64.64%, respectively; Cl-HCl decreased from 12.80% to 2.16%. Cl-CH3Cl

ACS Paragon Plus Environment

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disappeared above 800 oC. These results may be caused by the following reasons: the

23

evaporation of water and volatile matter, decomposition and carbonization of organic matter,

24

changes of composition in tar and char, and the reactions of the various Cl forms. During the

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whole pyrolysis stage at 200-1000 oC, the released Cl from rice straw can pose a certain of

26

environmental risks. In order to reduce the release of harmful Cl in rice straw, the measures such

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as fully evaporating water before the pyrolysis, pyrolysis temperature above 700 oC and passing

28

the exhaust gas into NaOH and acetone solution in turn, can be taken.

29 30

1. INTRODUCTION

31

It is an inevitable trend with the social development that non-renewable fossil fuels are

32

replaced by renewable energy sources.1 Biomass is an important and abundant renewable energy

33

source, which contains low N and S compared with fossil fuel, and with net CO2 emission.2 The

34

pollution for biomass is far less than that of fossil fuel during combustion. But biomass contains a

35

lower carbon content which leads to less heat generation. Therefore, biomass usually need to be

36

pyrolyzed to translate into char for replacing coal in combustion, which can increase energy

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utilization of biomass and generate tar and gas.

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Biomass often contains a larger amount of Cl (100-70000 mg/kg),3,4 which is easily released

39

and transferred into gas phase during thermal reaction.5,6 The released Cl can be deposited on the

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downstream equipment, causing corrosion, fouling and slag.7 The chlorine in biomass is mainly

41

released as HCl, KCl, CH3Cl and enter tar through pyrolysis.8-10 HCl is the most important

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atmospheric pollutant except SO2 and NOx. CH3Cl is a carcinogen, with greenhouse effects.9,11,12

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Tar-Cl is toxic and act as environmental hazards.13 For bomass pyrolysis, some studies8-10


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proposed, that the major form of released Cl was HCl in 250-500 oC; some studies 9,11 showed that

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volatilization of Cl-CH3Cl occurred below 350 oC; Wang et al 12 found the released HCl can be

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reacted with char and formed organic Cl; the organic Cl can enter tar at the temperature higher

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than 300 oC. KCl started to sublime at 700-800 oC.14 Above 800 oC, the released Cl mainly stemed

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from KCl sublimation.6,14,15 These studies indicated that Cl released through biomass pyrolysis

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have significant impacts on the environment.

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The transport and transformation of Cl during biomass pyrolysis have attracted considerable

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attention. The release behavior of Cl varies greatly for different biomass at different pyrolysis

52

temperatures. For the pyrolysis of sugarcane trash, switch grass, lucerne and straw rape, the

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release rates of Cl were less than 10% at 200 oC, 30-60% Cl was still present in chars at 900 oC,

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and the released Cl at low temperature might be caused by the reaction of alkali metal Cl with

55

organics.16 During corn straw pyrolysis for 20 min in 500-1150 oC, the chlorine release rate was

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approximately 50% at 500 oC, and then decreased slightly from 500 oC to 700 oC; after that, it

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gradually increased, and complete release for Cl was at 1150 oC.14 As for wheat straw, rice husk

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and sawdust in the process of fixed bed pyrolysis, the chlorine release of was mainly concentrated

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on 200-600 oC, and increased slightly in 600-1000 oC.15 The chlorine release rate of mallee bark in

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pyrolysis was close to 100% when the temperature was higher than 500 oC, and that in gas phase

61

was about 23%, in tar was about 77%.10 These studies indicated that the release behavior of Cl

62

varied largely during different biomass pyrolysis at different temperatures.

63

Rice straw is an important biomass in agriculture, accounting for more than half of total crops

64

in China. The transport and transformation of Cl during rice straw pyrolysis has been concerned

65

by many researchers. Jensen et al17 found that Cl was mainly released in two temperature ranges


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during rice straw pyrolysis, about 60% of Cl released within 200-400 oC, the remainder Cl was

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almost released in 700-900 oC, and the Cl release rate was low in the range of 400-700 oC. The

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chlorine release during pyrolysis at high temperature (1000-1200 oC) can be inhibited by

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torrefaction pretreatment at 250 and 300 oC.18 The literature8 showed that the released Cl

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increased with the rise of temperature and retention time; particle size was one of important

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factors, the Cl release rate of rice straw with 74-124 µm sizes was 60.78% and 27.25% higher than

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that of 250-420 µm sizes at 350 oC. These works indicated that temperature, pretreatment,

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retention time and grain size affect the Cl release during rice straw pyrolysis, and in which

74

temperature was the most important influence factor.

75

It can be seen from above that after biomass pyrolysis, the chlorine would undergo

76

significant migration and transformation, most of Cl enters gas and tar, and a small part of Cl

77

remained in char. The distribution and form of Cl in the products are different under different

78

conditions. Biomass pyrolysis may generate environmentally risky Cl such as CH3Cl, HCl and

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tar-Cl during pyrolysis. However, little research has simultaneously focused on the migration,

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distribution and change of various forms of Cl from rice straw and its environmental effect during

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pyrolysis at different temperatures.

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In this study, the rice straw samples were treated at different pyrolysis temperatures

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(200-1000 oC) to procuce char for burning. TGA, XRD, XRF and SEM were used to characterize

84

the matrix composition and physicochemical properties before and after pyrolysis of rice straw;

85

the total Cl, water souble Cl and organic bound Cl in rice straw and its chars, and the released

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Cl-HCl, Cl-KCl, Cl-CH3Cl and tar-Cl at different temperatures were respectively measured. By

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this way, the fate of Cl in rice straw during pyrolysis and its possible release and transformation


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mechanism were explored, and the environmental risk of Cl in rice straw during pyrolysis at

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different temperatures would be evaluated. The corresponding measures to reduce the release of

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harmful Cl were put forward.

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2. EXPERIMENT

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2.1. Sample collection and preparation

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The experimental samples were rice straw near the Poyang lake in Jiangxi province. The rice

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straw was smashed with a grinder, and the particles with size of 98-125 µm were screened out.

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The particles were air dried at room temperature. The proximate and ultimate analysis of rice

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straw was given in Table 1. From Table 1, it can be seen the rice straw contained more volatile

97

matter, and less fixed carbon, N and S, with 0.65% Cl.

98 99

Table 1 Proximate and ultimate analysis of rice straw. Proximate analysis (w/%, air-dried basis)

100

Ultimate analysis (w/%, air-dried basis)

Moisture

volatiles

Fixed carbon

Ash

C

H

N

O1

S

Cl

4.41

72.22

7.52

15.85

37.09

6.12

0.64

35.89

-2

0.65

1

By difference; 2 Not detected.

101 102

2.2. Rice straw pyrolysis

103

The rice straw pyrolysis apparatus was shown as Figure 1. Before the experiment, a 2.0 g

104

straw powder was put into a porcelain boat which surface was covered with 2.0 g quartz sand

105

(20-50 meshes) to prevent the sample from taking away by N2, then it was placed in water-cooled

106

chamber at the inlet of the silica tube. N2 was injected into the silica tube to exhaust the air, and


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then adjusted to 100 mL/min. Two 50 ml volumetric flasks were respectively put in a solution of

108

30 mL 0.1 M NaOH, and then placed in an ice basin (99.999%)

182

was used as a carrier gas with a flow rate of 100 mL/min. The mass-loss curve (TG) and its

183

derivative (DTG) were recorded as a function of sample temperature. After the experiment, N2

184

flow was kept for 30 min to get rid of the volatile components in gas cell.

185

2.5. Chemical and mineralogical analysis

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Major and minor element analysis (chemical composition) of rice straw was conducted using

187

XRF (S4 PIONEER). The test condition was Rh target, 4 kW, 60 kV, 140 mA, Be-U of detection


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range and 250 µm of minimum analysis of micro zone.

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The mineral compositions of rice straw and its chars at different pyrolysis temperatures were

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analyzed using a D8 ADVANCE X-ray diffractometer (XRD) with Cu K-alpha radiation. The

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pattern was recorded over a 2 h interval of 10-70o, with a step increment of 0.01o.

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The images of the samples to further insight into the morphology changes of the chars from

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rice straw were determined by SEM (S-3400N).

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3. RESULTS AND DISCUSSION

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3.1. Thermal degradation properties of rice straw in N2 atmosphere

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The effects of heating temperature on pyrolysis of rice straw at a heating rate of 10 °C/min were

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shown in Figure 2. The TG and DTG curves suggested that the pyrolysis process can be divided into

198

three stages. The first stage was from room temperature to 200 °C. Because of the removal of moisture

199

and volatile components, the sample weight loss rate was 11.41%. The second stage occurred between

200

200 and 400 °C, the intense decomposition of hemicellulose, cellulose, and lignin in rice straw was

201

observed in the stage.23 With the heating temperature increase from 200 oC to 400 oC, the weight of rice

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straw lost a lot, with a loss rate of 45.40%. The third stage corresponds to the carbonization process

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above 400 °C. Residues in this stage from rice straw were mainly ash and fixed carbon, the weight loss

204

rate was 22.21% from 400 to 1000 °C.


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5000

300

4500

250

TG DTG

4000

200

TG (g)

3500 3000

150

2500 100 2000

DTG (g/min)

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Energy & Fuels

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1500 1000

0

500 -100

0

100

200

300

400

500

600

700

800

900 1000 1100

o

T ( C)

205 206

Figure 2. The TG and DTG curves of rice straw in N2.

207

208

3.2. Change of physicochemical property of rice straw after pyrolysis

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3.2.1. The composition of rice straw ash. The chemical constituents of rice straw ash were

210

analyzed by XRF, which results were shown in Table 3. It can be viewed, in decreasing order of

211

significance, the main chemical components of rice straw ash were SiO2, K2O, Na2O, CaO, MgO,

212

P2O5, Al2O3, MnO and Fe2O3. The content of SiO2 and K2O was respectively 68.72% and 10.32%,

213

indicating a large amount of Si and K in rice straw.

214 215

Table 3. Properties of rice straw ash wd/%. Constituent

SiO2

Al2O3

CaO

Fe2O3

P2O5

K2O

TiO2

MgO

Na2O

MnO

Content (%)

68.72

0.46

1.82

0.26

1.30

10.32

0.05

1.72

1.28

0.37

216


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3.2.2. XRD analysis of rice straw and its chars.

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The XRD analysis of the rice straw and its pyrolysis chars in 200-1000 ℃ for 20 min was shown

219

as Figure 3. It can be seen KCl crystal started to appear in char at 300 oC. With the increase of

220

temperature, the amount of precipitated KCl crystal gradually increased, and achieved the

221

maximum value at 700 oC in the experimental temperatures. To 800 oC, the diffraction peak of

222

KCl decreased, indicating that KCl sublimed to the gas phase. As to 900 oC, only a small amount

223

of KCl crystal existed in char. For 1000 oC, the diffraction peak of KCl crystal disappeared and

224

the stable KAlSi2O6 diffraction peak appeared.

225

226 227

Figure 3.

XRD patterns of rice straw and its pyrolysis chars. a, KCl; b, KAlSi2O6.

228 229

3.2.3. SEM analysis of rice straw and its chars. SEM images of raw rice straw and its pyrolysis

230

chars at different temperatures revealed the change of char morphology. Figure 4 and Figure 5


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respectively showed the structural changes of the epidermis and ground tissue of rice straw and its

232

chars from 200 to 1000 oC of pyrolysis temperature.

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From Figure 4 and studies,24,25 it could be seen that the epidermis of the raw rice straw was

234

highly silicified, with obvious siliceous papillae. The papillaes were arranged in an orderly way,

235

which contained a large amount of amorphous silica (SiO2.nH2O).26,27 At 200 oC, there was no

236

noticeable change on the epidermis; silica mastoids were obviously broken at 300 oC, and

237

completely broken above 400 oC, and the amorphous silica (SiO2·nH2O) was turned into SiO2

238

particles;21 to 800

239

aluminosilicates,19,30 began to appear on the surface of the epidermis, and there were considerable

240

vitreous bodies formed at 900-1000 oC.

oC,

vitreous bodies, which consisted of stable silicates28,29 and

241

In Figure 5, the small coadjacent chambers, which showed rectangular, were come from the

242

plant cells. The sheets forming the chambers were derived from cell walls in the ground tissue.21,31

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Depending on Figure 5, the chambers did not change significantly in 200-300 oC, began to

244

collapse at 400 oC and was obviously broken at 800 oC. To 1000 oC, larger holes appeared in

245

ground tissue.

246

Combined with Figure 2, Figure 4 and Figure 5, it was known during the pyrolysis of rice

247

straw, the epidermal tissue changed markedly at 300 oC, and the ground tissue broke down

248

obviously at 400 oC, which was caused by the decomposition of cellulose, hemicellulose and

249

lignin. After 400 oC, the changes of epidermal and ground tissue were caused by the secondary

250

reactions of char.

251


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252 253

Figure 4. SEM photos of epidermis from rice straw and its pyrolytic chars at temperatures from 200 oC to 1000 oC.

254

REP and EP denote raw epidermis and epidermis of char, respectively.

255

256

257 258

Figure 5. SEM photos of internal surface with ground tissue from rice straw and its pyrolytic chars at temperatures

259

from 200 oC to 1000 oC. RGT and GT denote raw ground tissue and ground tissue of char, respectively.

260

261

3.3. Fate of chlorine in rice straw at different pyrolysis temperatures

262

According to the methods in Table 2, total Cl, water soluble Cl and organic bound Cl in rice

263

straw and its chars at different pyrolysis temperatures were respectively measured. It can be

264

known from the results calculated by Eq. (I), the form of Cl in rice straw was mainly water soluble

265

Cl which accounted for 97.14% of total Cl, and the total Cl and various forms of Cl in raw rice

266

straw and its pyrolysis char were shown in Figure 6.

267 268

On the basis of the results calculated by Eq. (II), (III), (IV) and (V), the changes of Ri with the pyrolysis temperature were shown in Figure 7.


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270 271

Figure 6. Change of Cl in char after rice straw pyrolysis at different temperatures.

272

273 274

Figure 7. Chlorine release after rice straw pyrolysis.


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Compared Figure 6 with Figure 7, the fate of Cl in rice straw and its possible mechanism

276

were illustrated by the following three pyrolysis temperature stages.

277

3.3.1. Pyrolysis in 200-400 oC. It can be seen from Figure 6, there was 60.31% Cl (water soluble

278

Cl) in rice straw char at 200 oC, which indicated that nearly 40% Cl may release with water vapor

279

and volatile components before 200 oC. According to Figure 2, the loss rate of water and volatile

280

matter in this stage was 11.14%, which indicated that water and volatile matter had a stronger

281

capacity for carrying Cl. The Cl in char mainly occurred as water soluble Cl which declined with

282

the rise of temperature, and this form Cl was 40.13% at 400 oC. As shown in Figure 2 and the

283

research,32 the organic matter such as cellulose, hemicellulose and lignin was mainly decomposed

284

during pyrolysis at 200-400 oC, with weight loss rate of 45.40% and Cl release rate of 18.83%,

285

indicating a weaker release of Cl caused by the decomposition of organic matter in rice straw.

286

Figure 7 showed the total Cl release rate of rice straw increased from 39.16% to 57.99% with

287

temperature rise from 200 oC to 400 oC. The tar-Cl began to appear, with a content of 5.84% at

288

400 oC. In this stage, the Cl were mainly released as HCl and CH3Cl. As can be seen from the

289

work,32 tar was generated when biomass was pyrolyzed above 200 oC, which indicated the tar was

290

formed earlier than tar-Cl and both were simultaneously taken out with the decomposition of

291

organic matter in rice straw. At 200 oC, 300 oC and 400 oC, the release rates were 28.76%, 35.34%

292

and 33.81% for Cl-HCl, and 10.41%, 16.81% and 15.58% for Cl-CH3Cl, respectively. There was

293

no KCl released at this stage.

294 295 296

It can be deduced from above, in the pyrolysis temperature of 200-400 oC, most of water soluble Cl in rice straw was released as HCl and CH3Cl, less Cl entered tar. Czégény et al33 thought that during pyrolysis, KCl in biomass can reacted with lignin and


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formed CH3Cl, which showed as Reaction (1). A large amount of lignin in rice straw may be the

298

reason for the generation of CH3Cl during rice straw pyrolysis. Said et al34 thought the one reason

299

of mass loss of rice straw during pyrolysis at 300 oC was the decomposition of lignin. Moreover,

300

the thermogravimetric analysis in Figure 2 showed that the decomposition of rice straw at 300 oC

301

was the fastest, which indicated that the reaction (1) at this stage was strong and the released

302

CH3Cl was more. Du et al15 believed that at the lower pyrolysis temperature, the biomass would

303

also take place Reaction (2) and formed HCl. It can be seen from Table 3 and SEM

304

characterization in Figure 4, the rice straw contains a large amount of SiO2, which may be the

305

reason for the generation of more HCl in this stage.

306 307

2KCl+ nSiO2+ H2O = K2O·(SiO2)n + 2HCl

308

3.3.2. Pyrolysis in 400-700 oC. This was the carbonization stage of rice straw at middle

309

temperature, with the weight loss rate 14.04% (according to Figure 2). From Figure 6, it can be

310

seen the Cl in char mainly comprised of water soluble Cl and organic bound Cl at this stage. From

311

400 oC to 700 oC, the water soluble Cl changed a little, the organic bound Cl increased from

312

1.25% to 7.50%.

313

(2)

On the basis of Figure 7, the total Cl release rate of rice straw decreased slightly, with

314

57.99% at 400 oC and 51.01% at 700 oC. From 500 oC, 600 oC to 700 oC, the release rate of tar-Cl

315

increased from 9.19%, 14.29% to 18.74%; in gas phase, the release rate of Cl-CH3Cl and Cl-HCl

316

decreased from 13.41%, 8.79% to 2.42% and from 29.19%, 27.31 to 12.81%, respectively; KCl

317

began to sublime at 700 oC, with the Cl-KCl release rate of 17.41%.


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318

It was generally believed that the most tar was generated during biomass pyrolysis at 500

319

oC.32

320

stage. It may be mainly caused by the following two reasons: HCl and CH3Cl in quartz tube

321

increased with the rise of temperature, which led to the occurrence of reaction (3) 10 and the

322

increase of organo-Cl; in addition, the composition of tar changed with increasing temperature,32

323

which increased the capture of Cl. The reaction (3) also resulted in the decrease of CH3Cl and

324

HCl.

325

CH3Cl/HCl+OrganicOrganic-Cl

326

3.3.3. Pyrolysis in 700-1000 oC. According to Figure 6, the Cl in rice straw char occurred as water

327

soluble Cl and a small amount of organic bound Cl at this stage. From 700 oC to 1000 oC, water

328

soluble Cl and organic bound Cl decreased sharply from 40.20% to 4.48% and from 7.50% to

329

0.34%, respectively. In addition, the char contained a certain of residual Cl at 900 oC and 1000 oC

330

by the mass balance closure.

331

However, in this study, the tar-Cl and ClCO increased with the increase of temperature at this

(3)

Based on Figure 7, from 700 oC to 1000 oC, the total release rate of Cl from rice straw

332

increased from 50.14% to 92.89%. At this stage, the tar-Cl increased slightly, with release rate

333

from 18.74 to 26.01%; more KCl and less HCl were released, with release rate from 17.40% to

334

64.64% for Cl-KCl and from 12.80% to 2.16% for Cl-HCl. CH3Cl nearly stopped releasing above

335

800 oC.

336

This process was a carbonization stage of rice straw at high temperature, with the weight loss

337

rate of 8.16% (according to Figure 2). The direct sublimation of KCl in char may be the main

338

reason for the release of Cl in rice straw during pyrolysis, which can be inferred by decrease of

339

KCl crystal peak gradually above 700 oC from Figure 3. Based on mass balance closure, part of


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Energy & Fuels

340

Cl-KCl in char was converted to tar-Cl and Cl-HCl. Reactions (3) and (4)11,17 may be the main

341

reasons for the decrease of Cl-HCl and Cl-CH3Cl. Similar to the pyrolysis of 400-700 oC stage, the

342

increase release of organic bound Cl in char and the change of composition in tar (the main

343

components of tar was PAH after 800 oC32)

344

of tar-Cl. In addition, the inclusion of Cl by the vitreous body incluing stable silicate and

345

aluminosilicate generated during the pyrolysis of 900 oC and 1000 oC may be the main reason for

346

the formation of residual Cl in char.7

347

CH3Cl/HCl+Organic-KOrganic+KCl

348

3.4. Environmental effect and prevention of chlorine release during pyrolysis of rice straw

may be the main reason for the continuous increase

(4)

349

From Figure 7, it can be seen that in the pyrolysis of 200-400 oC, relativly larger amounts of

350

Cl-CH3Cl (10.41-16.81%) and Cl-HCl (28.76-35.34%) were released, and the tar-Cl generated at

351

400 oC was 5.84%. Thus, the Cl released by pyrolysis at this stage caused larger pollution to the

352

atmosphere. At 500-700 oC, the released Cl-CH3Cl, tar-Cl and Cl-HCl were respectively

353

2.41-13.41%, 9.19-18.74% and 12.81-29.19%, which indicated the released Cl in this stage had

354

certain impacts on the atmosphere and the surrounding environment. In the pyrolysis of 800-1000

355

oC,

356

without any CH3Cl release, which indicated more pollution to the surrounding environment. From

357

what had been discussed above, the Cl released during the whole stage of rice straw pyrolysis at

358

200-1000 oC can cause a certain of risks to the environment.

the release rates of tar-Cl and Cl-HCl were respectively 16.81-26.01% and 10.24-2.16%,

359

According to the release of Cl in rice straw at different pyrolysis temperatures, the following

360

measures can be taken to reduce the harmful Cl entering the environment. (1) Before pyrolysis of

361

rice straw, fully evaporating water can reduce the release of Cl during pyrolysis at low

362

temperature. (2) The pyrolysis temperature was increased to above 700 oC, so that the Cl in the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

363

straw was mainly released as harmless KCl. (3) The emission of HCl, tar-Cl and CH3Cl which are

364

harmful to the environment can be effectively reduced by passing the exhaust gas into NaOH and

365

acetone solution in turn.

366

4. CONCLUSION

367

(1) At 200-400 oC stage of rice straw pyrolysis, the Cl in char mainly occurred as water soluble Cl;

368

the release rate of Cl in rice straw increased with the rise of temperature. The released Cl-HCl and

369

Cl-CH3Cl were higher. At 400 oC, more than half of the total Cl released, the tar-Cl began to be

370

formed. The fate of Cl in rice straw at this stage may be caused by the following reasons:

371

volatilization of water and volatile matter, decomposition of organic matter, reactions between

372

lignin and KCl and between SiO2 and KCl, H2O.

373

(2) During pyrolysis of rice straw in 400-700 oC, in char, the water soluble Cl changed a little, and

374

the organic bound Cl increased largely. The total Cl release rate decreased slightly; there was not

375

any significant change for tar-Cl; KCl began to sublime at 700 oC; the release rates of Cl-HCl and

376

Cl-CH3Cl decreased obviously. The reasons for Cl fate at this stage were the change of tar

377

composition and the secondary reactions between Cl and char.

378

(3) In the rice straw pyrolysis of 700-1000 oC, the water soluble Cl and organic bound Cl were

379

significantly reduced, a small amount of residual Cl appeared in char. The total Cl release rate

380

increased significantly; the released KCl increased sharply and the release of HCl decreased

381

gradually; the tar-Cl was higher. The principal seasons of Cl fate were the change of composition

382

in tar, the direct sublimation of KCl and the inclusion of Cl by the vitreous body formed by

383

silicate and aluminosilicate.


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Energy & Fuels

384

(4) During all the pyrolysis process in 200-1000 oC, a certain amount of the released tar-Cl,

385

Cl-CH3Cl and Cl-HCl indicated the environmental risks caused by Cl in rice straw cannot be

386

ignored. The measures, such as fully evaporating water before the pyrolysis, pyrolysis temperature

387

above 700 oC and passing the exhaust gas into NaOH and acetone solution in turn, can be taken to

388

reduce the release of harmful Cl.

389 390

■ AUTHOR INFORMATION

391

Corresponding Author

392

*E-mail: [email protected] (B.P.).

393

ORCID

394

Bingxian Peng: 0000-0002-8921-726X

395

Notes

396

The authors declare no competing financial interest

397

398

■ ACKNOWLEDGEMENTS

399

This work was sponsored by National Natural Science Foundation of China (Grant No.

400

21966014) and Science and Technology Research Project from educational department of Jiangxi

401

Province in China (GJJ170173) .

402

403 404

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Fate of chlorine in rice straw under different pyrolysis temperatures

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