Effects of CO2 on sulfur removal and its release behavior during coal pyrolysis

Fuel 165 (2016) 484–489

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Effects of CO2 on sulfur removal and its release behavior during coal pyrolysis Xinlong Wang a, Huiqing Guo a,b, Fenrong Liu a,⇑, Ruisheng Hu a, Meijun Wang c,⇑ a

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China School of Pharmaceutical Science, Inner Mongolia Medical University, Hohhot 010110, China c Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China b

a r t i c l e

i n f o

Article history: Received 11 April 2015 Received in revised form 11 October 2015 Accepted 14 October 2015 Available online 23 October 2015 Keywords: Sulfur removal Sulfur release Coal pyrolysis CO2 atmosphere Py-MS Py-GC

a b s t r a c t In this study, two Chinese coals, Jiexiu (JX) and Yanzhou (YZ) raw coals, their deashed coals and depyrited coals, were used to investigate the effects of CO2 on sulfur removal and its release behavior during coal pyrolysis by pyrolysis with mass spectrometer (Py-MS) and pyrolysis connected with gas chromatogram (Py-GC). It is found that the sulfur removal ratio of YZ and JX coals under CO2 atmosphere is higher than that under Ar atmosphere. Most sulfur removal of JX coal is distributed in tar under Ar atmosphere, while it is distributed in gas phase under CO2 atmosphere. The CO2 atmosphere is very beneficial to H2S, COS and SO2 release into gas phase. That is the maximum evolution peak temperature of H2S and SO2 decreases, and their evolution amount all increase remarkably, especially for COS evolution. The COS evolution of each coal increases with temperature increasing after 850 °C under CO2 atmosphere. This further validates COS formation is related to CO at higher temperatures, while it is unrelated to CO at lower temperatures. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Coal, main energy and chemical raw materials, plays an important role in the energy structure of China. In recent years, there is an increasing demand for clean coals [1–3]. But large-quantity use of coal in China has caused great damage to the environment [4]. The SO2 and CO2 release from coal combustion has led to a series of air pollution related problems, such as acid rain and global warming [5,6]. Therefore, the clean coal utilization has become the urgent topic. Sulfur in coal occurs in both inorganic and organic forms [7–9]. The inorganic sulfur is mostly pyrite and small amounts of sulfates. The organic sulfur is usually categorized as mercaptans, sulfones, aliphatic and aryl sulfides, sulfoxides, disulfides and thiophenes [6,10]. Pyrolysis is a frequent step in various conversion processes, and it is also a simple and effective way for rational utilization of coal and environmental protection [11]. During pyrolysis, both pyrite sulfur and partial organic sulfurs can be removed, a part of sulfurs goes into the gas phase in the form of H2S, COS, SO2, etc., which can be easily recovered [12]. During pyrolysis, sulfurs and minerals or sulfurs and organic matter can interact each other, ⇑ Corresponding authors. Tel./fax: +86 471 4992981. E-mail addresses: [email protected] (F. Liu), [email protected] (M. Wang). http://dx.doi.org/10.1016/j.fuel.2015.10.047 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

the sulfur-containing gases also can react with char and retain in the char [13,14]. The interaction between organic matter and pyrite shows that the behavior of sulfur evolution during pyrolysis may be different in raw coal, deashed coal and pyrite-free coal [15,16]. Therefore, the behavior of sulfur evolution during pyrolysis should be discussed separately. Previous studies have mainly paid attention to desulfurization under inert atmospheres and hydrogen atmosphere, however desulfurization under inert atmospheres during pyrolysis is very low, and hydro-desulfurization still needs higher cost [4,17,18]. Semra’s study [15] shows that pyrolysis under CO2 atmosphere has higher organic sulfur removal ratio at higher temperatures. Pyrolysis under CO2 atmosphere can not only effectively improve desulfurization ratio, but reduce the release of CO2. Due to the very complex presence of sulfur forms in coal, the mechanism of sulfur release and its transformation behavior are still not very clear under CO2 atmosphere, as well the effect of pyrite and mineral matter on them. In this study, the sulfur release and transformation behavior during pyrolysis of two Chinese raw coals, their deashed coals and pyrite-free coals were investigated by Py-MS and with Py-GC under Ar and CO2. This can provide theoretical basis for comprehensive use of coal by pyrolysis connected with pre-desulfurization.

485

X. Wang et al. / Fuel 165 (2016) 484–489 Table 2 Sulfur forms analysis of coal samples.

2. Experimental 2.1. Coal samples

Sample

Jiexiu (JX) and Yanzhou (YZ) raw coals, their deashed coals and pyrite-free coals were used in this study. Organic sulfur content of JX coal is the highest compared with its sulfate sulfur and pyrite, about 92.11%. Pyrite of YZ coal is higher than that in the JX coal, about 34.21%. The deashed coal and pyrite-free coal (thus depyrited dashed coal) were obtained according to the procedures described in literature [14], respectively. Their proximate and element analyses, sulfur forms and ash analyses are shown in Tables 1–3, respectively. The ratio of basic oxides to acid oxide is obtained 2 þK2 OþNa2 O according to the formula: RB=A ¼ Al2 O3 þFe2 O3 þCaOþMgOþTiO . SO3 þP2 O5 þSiO2

2.2. Py-MS equipment Py-MS equipment can be seen elsewhere [19]. About 1.5 g coal sample was placed into a quartz tube fixed-bed reactor (i.d. 30 mm, length 610 mm) and heated from room temperature to 1000 °C at a heating rate of 10 °C/min in a continuous flow of pure Ar or pure CO2 atmosphere at a flow rate of 300 mL/min. A mass spectrometer (QIC-20) was used to measure H2S, COS, SO2, CO, etc. online, about once every 1 min. 2.3. Py-GC equipment

YZ raw coal YZ deashed YZ pyrite-free JX raw coal JX deashed JX pyrite-free

2.4. Calculating methods The char yield (Y) was obtained according to the following forchar mula: Y ¼ W  100%, where Wchar is the weight of char after W coal

pyrolysis and Wcoal is the weight of raw sample. The desulfurization ratio (DR) can be obtained according to the formula: DR% ¼

W s;coal W s;char Y W s;coal

 100%, where Ws,coal and Ws,char is the sulfur

content in the raw coal and char, respectively [20], the desulfurization ratio and char yield are shown in Table 4. The sulfur weight in the form of COS (ms;COS ) obtained according VACOS to the formula: ms;COS ¼ R22:410 6  M s , where R is the heating rate

Sulfur form ratio in total S (%)

St,ad

Ss,ad

Sp,ad

So,ada

Ss

Sp

Soa

3.04 2.48 1.67 2.28 2.27 2.19

0.08 0.06 0.04 0.07 0.04 0.03

1.04 0.69 0.02 0.11 0.21 0.03

1.92 1.73 1.61 2.10 2.02 2.13

2.63 2.42 2.40 3.07 1.76 1.37

34.21 27.82 1.20 4.82 9.25 1.37

63.16 69.76 96.40 92.11 88.99 97.26

Note: St is the total sulfur; Ss is the sulfate sulfur; Sp is the pyrite sulfur; So is the organic sulfur. a By difference.

10 °C/min, V is the flow rate of 180 mL/min, Ms is the atomic weight of sulfur, and ACOS is integrated area of COS. And mS;H2 S and mS;SO2 can also be obtained according to the formula. For example, Fig. 1 shows the COS evolution content (ppm) of JX raw coal obtained by Py-GC under CO2 atmosphere. After integration with temperature, the integrated area can be obtained from this figure as ACOS. AH2 S for H2S and ASO2 for SO2 can be also obtained accordingly. The sulfur weight in different form of sulfur-containing gases is shown in Table 6. The sulfur presence in the gas (Sgas) is the sum of the sulfur weight in the form of H2S, COS and SO2 to the one of sulfur in the raw coal, the formula is: Sgas % ¼

Pyrolysis experiments were carried out in quartz tube fixed bed reactor (i.d. 35 mm, length 700 mm). About 1.0 g coal was pyrolyzed under pure Ar or pure CO2 atmosphere at the temperature range from room temperature to 1000 °C at a flow rate of 180 mL/min at heating rate of 10 °C/min. H2S, COS and SO2 contents (ppm) were analyzed by gas chromatography (GC) with flame photometric detector (FPD) every 50 °C from 50 °C to 1000 °C, offline (SP-7800). The column and detector temperatures were 80 °C and 250 °C, respectively. After pyrolysis, the sample was cooled and collected for further analyzing its weight and sulfur content. And the sulfur content in coal char was measured by automatic sulfur determination analyzer (XK-5000).

Sulfur forms in coal (wt%)

mS;H

2S

þmS;COS þmS;SO W s;coal

2

 100%; the sulfur presence in the char

(Schar) was obtained by the formula: Schar % ¼ 100%  DRð%Þ; the sulfur presence in the tar (Star) was obtained by the following formula: Star % ¼ 100%  Sgas %  Schar %. 3. Results and discussion 3.1. Effects of CO2 on sulfur removal during pyrolysis Table 4 shows the desulfurization ratio of different coals and their char yield during pyrolysis under Ar and CO2 atmospheres. Generally speaking, under Ar atmosphere sulfur removal ratio is related to the volatile content and the stability of organic sulfur in the coal. That is the higher the volatile content is, and the less stable the organic sulfur is, thus the higher sulfur removal ratio is. Under Ar atmosphere, for those two raw coals, the sulfur removal ratio is YZ > JX. This order is very consistent with the volatile content shown in Table 1. This indicates, for JX coal, the organic sulfur is very stable, which can hardly decompose during pyrolysis. For YZ coal with higher pyrite, the desulfurization ratio is deashed coal > depyrited coal > raw coal, this suggests that the deashed treatment and depyrited treatment is helpful for sulfur removal. The reason may be that heat transfer and mass transfer can increase sharply for their deashed and depyrited coals during pyrolysis. This may be also caused by the basic mineral of raw coals

Table 1 Proximate and ultimate analyses of coal samples (wt%). Sample

YZ raw coal YZ deashed YZ pyrite-free JX raw coal JX deashed JX pyrite-free

Proximate analyses

Ultimate analyses

Mad

Aad

Vdaf

FCdaf

Cad

Had

Oada

Nad

Sad

3.19 3.40 3.55 0.55 0.90 0.99

12.09 2.02 0.93 10.25 0.33 0.15

42.85 38.92 39.71 20.75 19.82 20.39

57.15 61.08 60.29 79.25 80.18 9.61

67.03 73.80 75.05 76.52 84.39 84.26

4.63 4.83 4.94 4.04 4.33 4.35

7.98 11.96 12.33 5.04 6.34 6.64

1.32 1.51 1.53 1.32 1.44 1.42

3.76 2.48 1.67 2.28 2.27 2.19

Note: ad is air-dried basis; daf is dried and ash-free basis. a By difference.

486

X. Wang et al. / Fuel 165 (2016) 484–489

Table 3 Ash composition of coal samples (wt%). Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2O

Na2O

P2O5

RB/A

YZ raw JX raw

21.64 55.82

10.72 28.95

24.62 2.67

17.62 4.66

0.94 0.11

1.72 2.95

21.25 2.90

0.42 0.18

0.30 0.28

0.03 0.37

1.31 0.67

Table 4 Char yield and sulfur removal ratio of coal samples during pyrolysis under different atmospheres. Sample

YZ raw coal YZ deashed YZ pyrite free JX raw coal JX deashed JX pyrite free

Ar (%)

CO2 (%)

Char yield

Sulfur removal ratio

Char yield

Sulfur removal ratio

56.81 57.63 54.44 73.25 73.13 70.44

52.27 59.87 59.38 33.05 30.81 35.74

31.80 32.88 31.56 59.10 55.80 55.30

83.00 71.42 74.92 53.76 48.13 51.72

3.2. Effect of CO2 on sulfur release behavior during pyrolysis

absorbing sulfur-containing gases, which could evolve into gas phase from their deashed coals, thus the amount of sulfurcontaining gases increases more than that of raw coals (shown in Table 6). As shown in Table 3, the ratio of basic oxides content to acid oxides content in the ash of YZ coal is higher than that of JX coal, therefore the desulfurization of YZ deashed coal is higher than that of its raw coal. It is really another reason for JX coal with higher organic sulfur, the desulfurization ratio of its deashed coal is the lowest, suggesting that some mineral substance of raw coal can catalyze organic sulfur structure to decompose. There is a little difference for desulfurization ratio of JX raw coal and its depyrited coal. This may be caused by reduction of some organic sulfur by SnCl2 during depyrited process. Under CO2 atmosphere, the desulfurization ratio of all coal samples increases more compared with that under Ar atmosphere. For YZ and JX coals under CO2 atmosphere, the sulfur removal ratio is raw coal > depyrited coal > deashed coal, which is very different from its order above discussed under Ar atmosphere for YZ coal, while it is similar to that under Ar atmosphere for JX coal. This suggests that the mineral matter in coals can catalyze organic sulfur structure and coal structure to decompose for those two coals under CO2 atmosphere. For those two depyrited coals, the organic sulfur removal ratio is YZ > JX, which is further validated that it is consistent with the stability of sulfur in coal, that is the less stable the sulfur is, the higher the sulfur removal is.

300

COS CO2-JX-raw

250

Concentration (ppm)

As shown in Table 5, for most coals, under CO2 atmosphere, the amount of sulfur in gas phase increases more compared with that under Ar. And For YZ and JX raw coal, the sulfur presence in tar also increases more compared with that under Ar. For JX deashed and depyrited coals, most of sulfur removal is distributed in tar under Ar atmosphere, while it is distributed in gas phase under CO2 atmosphere.

200

150

During pyrolysis, sulfur releases with volatile substance evolution in coal. Generally sulfur goes into the gas phase in the form of H2S, COS and SO2 followed by CO. In this study, Py-MS connected with Py-GC was used to investigate the effect of CO2 on H2S, COS and SO2 release behavior during pyrolysis. 3.2.1. Effect of CO2 on H2S release behavior during pyrolysis Fig. 2 shows the H2S evolution profiles of YZ and JX raw coals under Ar and CO2 atmospheres. As shown in Fig. 2, there are two obvious H2S evolution peaks for YZ raw coal under both atmospheres. Under CO2 atmosphere, it can be clearly seen that the maximum evolution peak temperature of each coal decreases more than that under Ar atmosphere. For YZ coal, under Ar atmosphere, the H2S evolution peak temperature is about 501 °C and 618 °C, while it is about 409 °C and 533 °C under CO2 atmosphere, respectively. For JX coal, the maximum peak temperature is about 573 °C under Ar atmosphere, while about 481 °C under CO2 atmosphere. As shown in Figs. 3 and 4, under CO2 atmosphere, for YZ and JX deashed coal and depyrited coal, their maximum evolution peak temperature is also lower than that under Ar atmosphere. This indicates CO2 is beneficial for H2S release at lower temperatures, the reason may be that CO2 could participate in the sulfur decomposition and react with sulfur radicals during pyrolysis. And the first H2S release peak of YZ raw coal and its deashed coal should be mainly attributed to the decomposition of pyrite under these two atmospheres, as the first peak of its depyrited coal disappears; the second one should be attributed to the decomposition of the comparatively stable organic sulfur. As shown in Table 6, H2S evolution amount is the highest compared with SO2 and COS in the gas phase under Ar atmosphere, thus the evolution of H2S would affect the extent of desulfurization. That is the higher the evolution amount of H2S is, the higher desulfurization ratio is (Table 4). By GC analysis, for most coals (shown in Table 6), the evolution amount of H2S under CO2 atmosphere is a little higher than that under Ar atmosphere. For YZ raw coal and JX deashed coal, the amount of H2S evolution increases more in CO2 atmosphere

Table 5 Sulfur presence in different phase (wt%).

100

Sample 50

YZ raw coal YZ deashed YZ pyrite-free JX raw coal JX deashed JX pyrite-free

0 200

400

600

800

1000

Temperature /°C Fig. 1. COS releasing content of JX raw coal obtained by GC under CO2 atmosphere.

a

By difference.

Ar

CO2

Gas

Tara

Char

Gas

Tara

Char

16.43 45.09 44.37 13.11 14.21 12.64

35.84 14.78 15.01 19.94 16.59 23.09

47.73 40.13 40.62 66.95 69.19 64.26

39.57 55.39 56.71 25.92 40.14 29.82

43.43 16.02 18.21 27.84 8.00 21.90

17.00 28.58 25.08 46.24 51.87 48.28

487

X. Wang et al. / Fuel 165 (2016) 484–489 Table 6 The sulfur evolution amount in different form of sulfur-containing gases during pyrolysis under different conditions. Sample

Ar (g/g coal)

YZ raw coal YZ deashed YZ pyrite-free JX raw coal JX deashed JX pyrite-free

CO2 (g/g coal)

H2S

COS

SO2

H2S

COS

SO2

3.62E03 6.49E03 4.58E03 2.89E03 2.49E03 2.44E03

6.87E04 7.84E04 2.95E04 3.39E05 4.50E05 2.45E05

6.87E04 3.91E03 2.53E03 6.35E05 6.92E04 3.01E04

5.05E03 6.72E03 4.72E03 3.01E03 3.51E03 2.74E03

3.98E03 3.51E03 2.35E03 2.50E03 2.92E03 2.55E03

3.00E03 3.51E03 2.41E03 4.06E04 2.68E03 1.23E03

4.00E-011

4.00E-011

CO -JX-raw 2 481°C

H2SCO2/Ar

COSCO2/Ar

SO2CO2/Ar

1.39 1.03 1.03 1.04 1.41 1.13

5.80 4.49 7.97 73.83 65.15 103.57

4.37 0.90 0.95 6.38 3.87 4.10

CO2-JX-pyrite free

2.00E-011

2.00E-011

0.00E+000 9.00E-012

573°C

Ar-JX-raw

6.00E-012

4.00E-011

4.00E-012

2.00E-011

8.00E-011

533°C

CO -YZ-raw

Ar-JX-pyrite free

6.00E-012

Intensity

Intensity

8.00E-012

2

409°C

2.00E-011

CO2-JX-deashed

Ar-JX-deashed

1.00E-011

4.00E-011

4.00E-011

CO2-JX-raw

0.00E+000 4.00E-011

618°c

Ar-YZ-raw

2.00E-011

501°C 8.00E-012

2.00E-011

0.00E+000

4.00E-012 200

400

600

800

1000

200

Temperature /°C

5.00E-011

3.00E-011 0.00E+000 1.00E-011

Ar-YZ-pyrite free

4.00E-011 0.00E+000 1.00E-010

1000

CO2-YZ-pyrite free

Ar-YZ-pyrite free

5.00E-012

CO2-YZ-deashed

0.00E+000

5.00E-011 0.00E+000

800

6.00E-011

CO2-YZ-pyrite free

1.50E-010

CO2-YZ-deashed

Ar-YZ-deashed

Intensity

Intensity

0.00E+000 1.00E-010

600

Fig. 4. H2S evolution profiles of JX raw coal, deashed coal and depyrited coals obtained by MS during pyrolysis under different atmosphere.

4.00E-011 0.00E+000

400

Temperature /°C

Fig. 2. H2S evolution profiles of coal samples obtained by MS during pyrolysis under different atmospheres.

8.00E-011

Ar-JX-raw

CO2-YZ-raw

0.00E+000 1.50E-011

Ar-YZ-deashed

5.00E-011 0.00E+000 3.00E-011

0.00E+000

Ar-YZ-raw

1.50E-010

0.00E+000 200

400

600

800

1000

Temperature /°C

Fig. 3. H2S evolution profiles of YZ raw coal, deashed coal, depyrited coal obtained by MS during pyrolysis under different atmospheres.

CO2-YZ-raw

0.00E+000 3.00E-011

Ar-YZ-raw

0.00E+000 200

400

600

800

1000

Temperature /°C

compared with other coals. The ratio of RH2S under CO2 atmosphere to that under Ar atmosphere is about 1.39 and 1.41 respectively. This further indicates CO2 can promote H2S release, as increasing its evolution amount for the two coals in this condition. 3.2.2. Effect of CO2 on COS release behavior during pyrolysis Figs. 5 and 6 is the COS evolution profiles of YZ and JX coals under the two atmospheres. As shown in Fig. 5, for YZ raw coal, under CO2 atmosphere, it can be also clearly seen that maximum

Fig. 5. COS evolution profiles of YZ raw coal, deashed coal and depyrited coal obtained by MS during pyrolysis under different atmospheres.

evolution peak is at lower temperature than that under Ar atmosphere. This is very similar to its H2S evolution under both atmospheres. However for its deashed and depyrited coals, under CO2 atmosphere, the maximum evolution peak temperature of COS is higher than that under Ar atmosphere. While for JX coal

488

X. Wang et al. / Fuel 165 (2016) 484–489 1.00E-010 5.00E-011 0.00E+000 6.00E-013

CO -JX-pyrite free

1.20E-010

Ar-JX-pyrite free

0.00E+000 3.00E-011

2

Ar-JX-raw

CO -JX-deashed

Intensity

Intensity

2

Ar-JX-deashed

1.50E-012 0.00E+000 6.00E-011 3.00E-011

630°C

2.00E-011

0.00E+000 8.00E-011

0.00E+000 3.00E-012

494°C

6.00E-011

3.00E-013

4.00E-011

CO2-JX-raw

1.00E-011

8.00E-011

CO2-YZ-raw

464°C

4.00E-011 0.00E+000 4.80E-011

CO -JX-raw 2

618°C

Ar-YZ-raw 511°C

3.20E-011 0.00E+000 6.00E-013

Ar-JX-raw

1.60E-011 200

3.00E-013

400

600

800

1000

Temperature /°C 0.00E+000 200

400

600

800

1000

Temperature /°C

Fig. 8. SO2 evolution profiles of coal samples obtained by MS during pyrolysis under different atmospheres.

Fig. 6. COS evolution profiles of JX raw coal, deashed coal, depyrited coal obtained by MS during pyrolysis under different atmospheres.

0.0000006

3.2.3. Effect of CO2 on SO2 release behavior during pyrolysis Fig. 8 shows the SO2 evolution profiles of YZ and JX raw coals under Ar and CO2 atmospheres. Two obvious evolution peaks of SO2 under Ar atmosphere, one obvious peak with shoulder peaks under CO2 atmosphere were detected for YZ raw coal. As shown in Fig. 8, under CO2 atmosphere, it can be also clearly seen that maximum evolution peak temperature of SO2 for each coal is also lower than that under Ar atmosphere. This is very similar to H2S evolution under both atmospheres for YZ and JX raw coals, deashed and depyrited coals. Under Ar atmosphere, for YZ and JX coal, the peak temperature of SO2 maximum evolution is about 618 °C and 630 °C, while it is about 464 °C and 494 °C under CO2 atmosphere, respectively. By GC analysis, the amount of SO2 evolution under CO2 atmosphere is also much higher than that under Ar atmosphere (shown in Table 6) except for a little change of YZ deashed coal and depyrited coal. This indicates that CO2 is also very beneficial to SO2 release, as decreasing the SO2 maximum evolution peak temperature.

JX-CO

0.0000004

0.0000002 4.00E-011

JX-COS

Intensity

2.00E-011 0.00E+000 0.0000015

YZ-CO

0.0000010 0.0000005 0.0000000 1.50E-010

YZ-COS

1.00E-010 5.00E-011 0.00E+000 200

400

600

800

1000

Temperature /°C

Fig. 7. Co and COS evolution profiles of different raw coals obtained by MS during pyrolysis under CO2 atmosphere.

(shown in Fig. 6), under CO2 atmosphere the maximum COS evolution peak temperature of each coal is at higher temperature than that under Ar atmosphere. Under Ar atmosphere, for YZ raw coal, the peak temperature of COS maximum evolution is about 617 °C, it is about 543 °C under CO2 atmosphere. However for JX raw coal, the two peak temperatures are about 393 °C and 596 °C under Ar atmosphere, while about 573 °C, and 830 °C under CO2 atmosphere, respectively. By GC analysis, for each coal, the amount of COS evolution under CO2 atmosphere is much higher than that under Ar atmosphere (shown in Table 6). The maximum ratio of RCOS under CO2 atmosphere to that under Ar atmosphere is 103.57 for JX depyrited coal. This indicates that CO2 is also very beneficial to COS release. In addition, there is a very interested phenomena, that is the COS evolution of each coal increases with the temperature increasing after 850 °C under CO2 atmosphere (shown in Fig. 7), meanwhile the CO evolution of each coal is also increasing. This further validates COS formation is related to CO at higher temperatures [21], while it is unrelated to CO at lower temperatures [22].

4. Conclusions In this study, the following results could been concluded: (1) The sulfur removal ratio of YZ and JX coals under CO2 atmosphere is higher than that under Ar atmosphere. Most sulfur removal of JX coal is distributed in tar under Ar atmosphere, while it is distributed in gas phase under CO2 atmosphere. The effects of ash on sulfur removal are different for different coals under both atmospheres. For YZ coal, the desulfurization ratio is deashed coal > depyrited coal > raw coal under Ar atmosphere, while raw coal > depyrited coal > deashed coal under CO2 atmosphere. This indicates the effect of absorbing sulfur by the alkaline mineral under Ar atmosphere is obvious, while the catalyzed effect of some mineral is evident under CO2 atmosphere. (2) The CO2 atmosphere is very beneficial to H2S, COS and SO2 release into gas phase, the maximum evolution peak temperature of H2S and SO2 decreases sharply, and their evolution amount all increase remarkably, especially for COS evolution. Under both atmospheres, H2S evolution amount is the highest compared with COS and SO2 in the gas phase.

X. Wang et al. / Fuel 165 (2016) 484–489

(3) The COS evolution of each coal increases with temperature increasing after 850 °C under CO2 atmosphere. This further validates COS formation is related to CO at higher temperatures, while it is unrelated to CO at lower temperatures.

Acknowledgements This study was financially supported by the Project of Natural Science Foundation of China (No. 21466025), the Project Natural Science Foundation of Inner Mongolia of China (No. 2013MS0205) and the State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology (2014). References [1] Liao H, Li B, Zhang B. Pyrolysis of coal with hydrogen-rich gases. 2. Desulfurization and denitrogenation in coal pyrolysis under coke-oven gas and synthesis gas. Fuel 1998;77(14):1643–6. [2] Zhang L, Sato A, Ninomiya Y, Sasaoka E. Partitioning of sulfur and calcium during pyrolysis and combustion of high sulfur coals impregnated with calcium acetate as the desulfurization sorbent. Fuel 2004;83(7):1039–53. [3] Duran J, Mahasay S, Stock L. The occurrence of elemental sulphur in coals. Fuel 1986;65(8):1167–8. [4] Cypres R, Furfari S. Fixed-bed pyrolysis of coal under hydrogen pressure at low heating rates. Fuel 1981;60(9):768–78. [5] Liu F, Li W, Guo H, Li B, Bai Z, Hu R. XPS study on the change of carboncontaining group sand sulfur transformation on coal surface. J Fuel Chem Technol 2011;39(2):81–4. [6] Xu L, Yang J, Li Y, Liu Z. Behavior of organic sulfur model compounds in pyrolysis under coal-like environment. Fuel Process Technol 2004;85 (8):101–1024. [7] Bai Y, Wang Y, Zhu S, Li F, Xie K. Structural features and gasification reactivity of coal chars formed in Ar and CO2 atmospheres at elevated pressures. Energy 2014;74:464–70.

489

[8] Rutkowski P, Mullens S, Yperman J, Gryglewicz G. AP-TPR investigation of the effect of pyrite removal on the sulfur characterization of different rank coals. Fuel Process Technol 2002;76(2):121–38. [9] Jorjani E, Yperman J, Carleer R, Rezai B. Reductive pyrolysis study of sulfur compounds in different Tabas coal samples (Iran). Fuel 2006;85(1):114–20. [10] Garcia-Labiano F, Hampartsoumian E, Williams A. Determination of sulfur release and its kinetics in rapid pyrolysis of coal. Fuel 1995;74(7):1072–9. [11] Kawser J, Junichiro H, Li C. Pyrolysis of a Victorian brown coal and gasification of nascent char in CO2 atmosphere in a wire-mesh reactor. Fuel 2004;83 (7):833–43. [12] Zhou Q, Hu H, Liu Q, Zhu S, Zhao R. Effect of atmosphere on evolution of sulfurcontaining gases during coal pyrolysis. Energy Fuels 2005;19(3):892–7. [13] Wang M, Hu Y, Wang J, Chang L, Wang H. Transformation of sulfur during pyrolysis of inertinite-rich coals and correlation with their characteristics. J Anal Appl Pyrol 2013;104:585–92. [14] Liu F, Li B, Li W, Bai Z, Jan Y. Py-MS study of sulfur behavior during pyrolysis of high-sulfur coals under different atmospheres. Fuel Process Technol 2010;91 (11):1486–90. [15] Semra K. Desulfurization of a Turkish lignite at various gas atmospheres by pyrolysis. Effect of mineral matter. Fuel 2003;82(12):1509–16. [16] Oztas N, Yurum Y. Pyrolysis of Turkish Zonguldak bituminous coal. Part 1. Effect of mineral matter. Fuel 2000;79(10):1221–7. [17] Lai Z, Ma X, Tang Y, Lin H. A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA. Energy 2011;36(2):819–24. [18] Gil M, Riaza J, Alvarez L, Pevida C, Pis J, Rubiera F. Oxy-fuel combustion kinetics and morphology of coal chars obtained in N2 and CO2 atmospheres in an entrained flow reactor. Appl Energy 2012;91(1):67–74. [19] Wang M, Tian J, Daniel GR, Chang L, Xie K. Interactions between corncob and lignite during temperature-programmed co-pyrolysis. Fuel 2015;142 (1):102–8. [20] Liu F, Li W, Li B, Chen H. Sulfur removal and its distribution during coal pyrolysis in fluidized bed reactor under oxidative atmospheres. J Fuel Chem Technol 2006;34(4):404–7. [21] Sydorovych Ya, Gaivanovych V, Martynets E. Desulfurization of donetsk basin coals by air–steam mixture. Fuel 1996;75(1):78–80. [22] Liu F, Li W, Chen H, Li B. Sulfur-containing gases analysis during pyrolysis of coal in fluidized bed reactor. J Fuel Chem Technol 2006;34(6):655–9.