Article pubs.acs.org/EF
Cite This: Energy Fuels 2017, 31, 12046-12053
Sulfur Transformation Characteristics and Mechanisms during Hydrogen Production by Coal Gasification in Supercritical Water Shanke Liu, Linhu Li, Liejin Guo,* Hui Jin, Jiajing Kou, and Guoliang Li State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi Province, China ABSTRACT: Coal supercritical water gasification (SCWG) is famous for generating clean gas without SOx pollutant. Study of sulfur transformation characteristics can provide the basis of sulfur removal during hydrogen production by coal gasification in supercritical water (SCW) at the source. In this work, two coals produced from Linfen and Zhangjiamao in China (hereinafter to be referred as L-coal and Z-coal), were chosen as experimental feedstocks to investigate sulfur transformation characteristics during hydrogen production by coal gasification in SCW (550−750 °C, 20 min, 25 MPa). Sulfur transformation pathway and sulfur forms in the products were complex but detected comprehensively. H2S was the only gaseous product instead of SOx, whereas SO42− was the main liquid−sulfur product. Inorganic and organic sulfur compounds were used to investigate sulfur transformation mechanisms. H2S had three sources as follows. First, among inorganic sulfur of raw coal, FeS2 (Pyrite) was chemically stable in SCW lacking of hydrogen. When FeS2 was in hydrogen atmosphere, H2S was generated and FeS2 was converted to Fe1−xS and Fe3O4 under SCW. Second, H2S came from unstable sulfate minerals such as FeSO4 which may decompose and be converted to Fe3O4. Third, organic sulfur, especially thiophene sulfur transformed to H2S. The two sulfur products H2S and SO42− depend on H or OH free radical in SCW. More H free radical provided a reducing environment of SCW to generate H2S at higher temperatures, whereas more OH radical provided an oxidizing environment of SCW to generate SO42− at lower temperatures, but the final trend was generating H2S when coal gasified completely at a higher temperature. The results of this study may provide an experimental basis of solving the SOx emission from coal at the source and demonstrate a promising clean utilization way of coal.
1. INTRODUCTION In recent years, growing coal consumption has resulted in serious environmental problems in China, such as haze and acid rain.1,2 SOx, the byproduct of coal combustion and pyrolysis,3 has played a key role to result in these environment problems. Complex desulfuration technology and equipment were used to clean exhaust gas in coal combustion plants, which result in massive consumption of material and financial resources for desulfuration. Therefore, changing the utilization way of coal is extremely urgent. Supercritical water gasification (SCWG) technology is an innovative technology for energy conversion.4−8 SCWG of coal is a promising clean coal utilization technology,9,10 and is famous of zero SOx pollutant emission.11 Since it was proposed by Modell12 from MIT in 1978, SCWG technology has had a fast development for several years. Studies on sulfur transformation characteristics of coal or model compounds in SCW condition were reported, especially under oxidization condition in SCW. Wang et al. investigated that sulfide, thiosulfate, sulfite, and sulfate were sulfur-containing products of supercritical water oxidization (SCWO) of coal.13 Chen et al. reported that H2S was generated during coal SCWG process while sulfur containing salts were generated during coal SCWO process.14,15 Ma et al. studied kinetics behavior and sulfur transformations of iron sulfide during SCWO and obtained the reaction rates and kinetic parameters.16 Ma et al. also investigated sulfur transformation of methanthiol and thiirane during SCWO and found that no sulfur-containing species existed in the gaseous effluent.17 Fujie et al. studied combustion behavior of brown coal in supercritical water and confirmed that no sulfur-containing gaseous species emission.18 All these works indicated that no SOx gas was generated under © 2017 American Chemical Society
SCWO condition but resulted in problems of salts blocking in the systems. Until now, few works reported sulfur transformation characteristics and mechanisms of coal or biomass under reducing atmosphere of SCW. Yanagida et al. reported that 79% of S moved to the liquid phase while the remainder moved to the solid phase after SCWG treatment of poultry manure.19 This work showed a good utilization way of sulfur containing biomass and presented a referential value for sulfur transformation of coal gasification in supercritical water. Meng et al. studied sulfur transformation in coal during SCW process at temperature 350 to 550 °C and affirmed H2S was the main gaseous sulfur product.20 However, the temperature was relatively lower than 700 °C for coal complete gasification,21 and the sulfur transformation characteristics above 550 °C were not explored. The studies on sulfur transformation characteristics and mechanisms in SCW mainly focused on oxidization environment of SCW or treating homogeneous feedstocks introduced above. Works on reducing environment and heterogeneous feedstocks such as coal were few but important. This paper aimed at studying sulfur transformation characteristics during hydrogen production by coal gasification in SCW by using two coals of different sulfur contents to conduct experiments in a batch reactor with potassium carbonate as catalyst. And then Received: August 28, 2017 Revised: October 10, 2017 Published: October 19, 2017 12046
DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
Article
Energy & Fuels Table 1. Elemental, Proximate Analysis of Raw Coal (Air-Dried Basis) elemental analysis (wt %)
a
proximate analysis (wt %)
coal
C
H
N
S
Oa
Mb
Ac
Vd
FCe
L-coal Z-coal
70.31 68.45
4.18 4.68
1.06 0.93
4.20 0.47
3.14 8.26
1.01 6.63
16.10 10.58
26.60 24.42
56.29 58.37
By difference. bMoisture. cAsh. dVolatiles. eFixed carbon. column C-2000. The carrier gases were high-purity argon. Another gas chromatography (SP-3420A) equipped with a FPD detector and PorapackQ column was used to analyze the sulfur-containing gaseous product quantitatively. And high-purity nitrogen and air were carrier gas and oxidant gas, respectively. The liquid organic products were extracted by ethyl acetate and then analyzed by GC-MS (Agilent6890A-5973N MSD) qualitatively. S2−/HS− and SO32− in liquid products were analyzed by Multiparameter controller (Lovibond-ET99722) quantitatively using spectrophotometric method. SO42− in liquid was analyzed by ion chromatography (IC, Metrohm 930). The sulfur existence forms on the surface of raw coal and residual solid were analyzed semiquantitatively by X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). Characterizations of solid residues of inorganic sulfur were analyzed by X-ray Diffraction (XRD, D/MAX-2400).The total sulfur amounts in residual solid were analyzed quantitatively by sulfur determinator (Sunday SDS720).
typical inorganic and organic compounds were used to investigate H2S generation mechanisms during SCWG process.
2. EXPERIMENTAL SECTION 2.1. Material. The two coals used in this paper were obtained from Linfen, Shanxi Province, and Zhangjiamao, Shaanxi Province, in China. The elemental, proximate and sulfur analysis results of the two coals were shown in Tables 1 and 2. In the tables, the C, H, N, and S
Table 2. Content of sulfur forms in raw coal (Air-Dried Basis)a L-coal
Z-coal
forms
mass/mass of coal·100%
mass/mass of total sulfur·100%
mass/mass of coal·100%
mass/mass of total sulfur·100%
S-o S-p S-s
3.19 0.98 0.03
75.95 23.33 0.72
0.16 0.29 0.02
34.04 61.70 4.26
a
3. RESULTS AND DISCUSSION 3.1. Sulfur Forms in Raw Coal. Sulfur forms in raw coal usually contained organic sulfur, pyrite sulfur and sulfate sulfur.23−25 Different coal may have different sulfur forms such as L-coal and Z-coal. L-coal was a coal with high sulfur content while Z-coal was a coal with low sulfur content. XPS is a classic analysis technology for coal science, especially for investigating functional group of coal.26−29 XPS can be also used in the study of solid products after supercritical water gasification.30 In detail, XPS analysis indicated that sulfur forms31−33 in L-coal were thioalcohol (162.564 eV), thiophene (164.036 eV), marcasite (FeS2, 165.218 eV) and Sulfate (168.978 eV), while sulfur forms in Z-coal were thioalcohol (162.289 eV), thiophene (164.141 eV), pyrite (FeS2, 169.317 eV) and Sulfate (170.749 eV). The relative area fractions of these components by XPS analysis in Figure 1 were near the analysis data in Table 2, which indicated that sulfur forms detected by XPS was relatively reliable. The other distinction between L-coal and Zcoal was that thiophene was the main form in L-coal while FeS2 was the main form in Z-coal. The prominent difference could result in some distinctions during supercritical water gasification. 3.2. Sulfur Transformation Characteristics during Coal SCWG. 3.2.1. Sulfur Transformation Characteristics in Solid Products. Because L-coal has a high sulfur content with a typical sulfur forms component, the solid residues generated from SCWG of L-coal at different temperatures were investigated as shown in Figure 2. We can see that, the peaks of thioalcohol disappeared at 550 °C and above. This result indicated that thioalcohol was unstable and easily gasified. Specially, sulfate sulfur was stable under lower temperature (below 650 °C) but disappeared at 700 °C. The relative areas of FeS2 and thiophene decreased as temperature increased. On the other hand, thiophene decreased relatively more quickly than FeS2 as temperature increased, which indicated that reactivity of thiophene was more sensitive to temperature than FeS2 in SCW. The XPS analysis of solid residual showed that the mechanism of inorganic sulfur transformation was different
S-o, organic sulfur; S-p, pyritic sulfur; S-s, sulfate sulfur.
elemental contents were detected by elemental analyzer and sulfur determinator respectively, the proximate analysis data were detected by proximate analysis instrument and the sulfur forms contents in two coals were analyzed according to Chinese Standard GB/T 215−2003. The coal particle diameter was in the range of 100−150 μm. The sulfur-containing model compound dibenzothiophene (DBT) and pyrite (FeS2) were produced by J&K Chemical Technology Co., Ltd. Analytically pure anhydrous potassium carbonate was produced by Tianjin Honghe Chemical Reagent Factory. Analytically pure absolute ethyl alcohol and FeSO4 were produced by Sinopharm Chemical Reagent Co., Ltd. Chromatographically pure ethyl acetate was produced by Merck. 2.2. Apparatus and Experimental Procedures. Supercritical water gasification of coal were conducted in an autoclave. The details of the autoclave have been described in our previous work.22 Coal (1.32 g), deionized water (25 g), and potassium carbonate (1.32 g, catalyst) were loaded into the bottom of the autoclave. Then high purity argon purged the autoclave after the autoclave was sealed. We adjusted the initial pressure of the autoclave with argon before heating to maintain the pressure of reaction. And then the autoclave was heated to the operating conditions(550 °C-750 °C, 25 MPa) by approximately 11 °C/min and kept for 20 min. After experiments, the autoclave was cooled to 45 °C by water immediately. The gaseous products were collected for analysis. The volumes of gaseous products were measured by a wet gas flowmeter. After the pressure inside decreased to normal pressure, the autoclave was uncovered. The deionized water was used to wash the inner wall of the autoclave and then solid and liquid products were collected in to a beaker, which were separated by filtration for analysis. Inorganic and organic sulfur compounds could suppress interference of complex components in coal to investigate sulfur transformation mechanisms and their SCWG experiments were also conducted in the autoclave. The amounts of inorganic sulfur compounds, ethyl alcohol, water, DBT, and potassium carbonate loaded in the autoclave were introduced in section 3.3 and 3.4, respectively. 2.3. Sample Analysis. The normal compositions of gas sample were analyzed quantitatively by gas chromatography (Agilent7890A) equipped with a thermal conductivity detector (TCD) and a capillary 12047
DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
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3.2.2. Sulfur Transformation Characteristics in Gaseous Products. Figure 3 showed the gas products of coal gasification
Figure 3. Variation in gas production from coal SCWG at different temperatures.
in SCW. H2 and carbonaceous gas (CO2, CH4, CO, C2H4, and C2H6) were the main gaseous products by coal gasification in SCW, whereas H2S was the only sulfur gas product instead of SOx. As we can see in Figure 3, the yields of H2 increased from 9.45 to 55.18 mol/kg for L-coal and 7.02 to 67.38 mol/kg for Z-coal as temperature increased from 550 to 750 °C. Simultaneously, the yields of H2S increased from 0.12 to 0.73 mol/kg for L-coal and 0.024 to 0.108 mol/kg for Z-coal. The results indicated that increasing reaction temperature can enhance reaction rate constant significantly34 which would promote the break of C−S bond and gaseous sulfur molar yields. 3.2.3. Sulfur Transformation Characteristics in Liquid Products. H2S was easily dissolved in water so that S2−/HS− was detected in liquid products after the products were cooled. As temperature increased, the yields of S2−/HS− increased generally as shown in Figure 4. However, the amounts of the S2−/HS− yields were much less than SO42−. SO42− was the main
Figure 1. Sulfur forms in raw coal analyzed by XPS. (a ) L-coal, (b) Zcoal.
Figure 2. Variation in sulfur forms on the surface of solid residues generated by SCWG of L-coal at different temperatures.
from organic sulfur. The detailed discussions about transformation of inorganic sulfur in SCW will be discussed in section 3.3
Figure 4. Variation in liquid sulfur products from coal SCWG at different temperatures. 12048
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Energy & Fuels Table 3. GC-MS Analysis of Sulfur Intermediate Products from SCWG of L-Coal (550°C, 25 MPa, 20 min)
product in liquid. Generally, the yields of L-coal were higher than Z-coal due to a higher sulfur content. In detail, low temperature promoted SO42− generation, while high temperature restrained it. Especially, the maximum yields of SO42− was 0.52 mol/kg of L-coal at 550 °C, which was far more than its amount in raw coal. SO42− may have two sources as follows. First, unstable or soluble sulfate dissolved in water after the product was cooled. Second, OH free radical of SCW or a little amount of residual oxygen in SCW oxidized S(−2 valence state) to S(+6 valence state).20 It is worth mentioning that SO32− was not detected in this work, which was different from the previous work.20 The reason may be explained as H2S and SO32− could not coexist because of a redox reaction at a high temperature condition with rich hydrogen in the reaction system. Organic sulfur intermediate products in liquid from SCWG of coal were detected by GC-MS, and the results of L-coal were shown in Table 3. Dibenzothiophene, benzothiophene and 4methyldibenzothiophene were the three detected organic compounds. However, thioalcohol sulfur was not detected, which indicated that thiophene sulfur was relatively difficult to gasify, whereas thioalcohol sulfur was relatively easy to gasify. And on the other hand, it proved that thiophene sulfur was the main organic sulfur form in coal and indicated the accuracy of XPS analysis of sulfur. For Z-coal with little organic sulfur in raw coal, its organic sulfur intermediate products were too few to be detected by GC-MS. 3.3. Inorganic Sulfur Transformation Mechanisms in SCW. Inorganic sulfur in coal had different structures that may have different mechanisms from organic sulfur in SCW. A series of comparison experiments were conducted to investigate inorganic sulfur transformation in SCW. For simulating atmosphere in SCW, ethyl alcohol was added to generate H2 under particular conditions. The amounts of inorganic compounds, ethyl alcohol and water loaded in the autoclave were 1g, 0.26 mL and 10 g, respectively. Figure 5 showed XRD analysis of inorganic sulfur treated by different conditions listed in Table 4. As we can see in Figure 5b, c, FeS2 was chemically stable at 650 °C in the absence of H2 atmosphere wherever the condition was pyrolysis or SCW. However, FeS2 was partially converted to FeS when it was pyrolyzed at 650 °C in a H2 atmosphere. The reaction can be described as eq 1. FeS2 + H 2 → FeS + H 2S
Figure 5. XRD analysis of inorganic sulfur treated under different conditions listed in Table 4.
Table 4. Treatment Conditions of Inorganic Sulfur at 650°C, 10 min, and 25 MPa (a) (b) (c) (d) (e) (f)
feedstock
state
atmosphere
FeS2 FeS2 FeS2 FeS2+ Ethyl alcohol FeS2+Ethyl alcohol FeSO4+Ethyl alcohol
raw pyrolysis SCW pyrolysis SCW SCW
No H2 No H2 H2 produced by ethyl alcohol H2 produced by ethyl alcohol H2 produced by ethyl alcohol
conversion of FeS2 needed H2 atmosphere. On the other hand, Figure 5d indicated that FeS was much more stable than FeS235,36so that FeS was difficult to be reduced by H2. Differently, as shown in Figure 5e at 650 °C under SCW condition, FeS2 was converted completely, whereas Fe1−xS and Fe3O4 were generated. The distinction was from supercritical water.20 Fe1−xS was generated from FeS2 reduced by H2 in SCW, which can be described in eq 2. The generation of Fe3O4 indicated that Fe1−xS instead of FeS2 can be oxidized by supercritical water as described in eq 3.
(1)
(1 − x)FeS2 + (1 − 2x)H 2 → Fe1 − xS + (1 − 2x)H 2S
This result may explain the reason why FeS2 decreased more slowly than thiophene as temperature rose in Figure 2. The
(2) 12049
DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
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Energy & Fuels Table 5. GC-MS Analysis of Organic Intermediate Products from SCWG of DBT peak area (%) 650 °C retention time (min)
compounds
4.08 7.09 11.45 12.34 14.09 15.04 16.54 17.35 19.63 20.09 20.84 23.32 24.84 27.04 27.36 28.70 28.85 29.71 30.25 30.56 31.36 32.31 32.99 33.52 33.74 35.44 35.79 36.32 37.52 38.92 40.23 40.29 40.49 41.15 42.20 45.48 46.18 46.60 46.87 47.30 49.34 50.25 51.41 53.89 54.34 54.94 55.38 56.49
benzene toluene 3-buten-2-ol styrene 3,3-dimethylhexane 1,2,3,4-tetramethylbenzene propenylbenzene 1-ethenyl-3-ethybenzene acetic acid indene benzofuran 2-methylbenzofuran acetophenone naphthalene cyclopenten-1-ylbenzene, benzothiophene 3,4-dimethylbenzaldehyde 2-methylnaphthalene 2-methylbenzothiophene 1-methylnaphthalene 3-methylbenzothiophene, 7-ethylbenzothiophene biphenyl 1H-inden-1-ome,2,3-dihydro phenol 4-methyl-1′-biphenyl 3-methyl-1′-biphenyl acenaphthene acenaphthylene dibenzofuran 2,4-bis(1,1-dimethylethyl)-phenol fluorene 9H-xanthene 4-methyldibenzofuran benzenecarboxylic acid o-hydroxybiphenyl dibenzothiophene 1-(phenylmethylene)-1H-Indene, phenanthrene 4-methyldibenzothiophene 2-phenyl-1H-Indene 2-phenyinaphthalene triphenylene p-terphenyl fluoranthene p-hydroxybiphenyl m-terphenyl pyrene total
1 min 0.022
750 °C
800 °C
15 min
15 min
15 min
15 min
0.055 0.013
0.129
0.007
0.012
9.666
0.737 0.160 0.009 0.016 0.050 2.001
0.592 0.092
2.192 0.268
0.081 39.231 0.033 1.159 0.778
5 min 0.113 0.063 0.007
0.06
0.014 0.017
0.332
1.901
3.59
0.825 0.409 0.115 0.756 0.228 0.443
0.007 0.018 2.862 0.010 0.634 0.151 0.307 0.167 0.323 0.100
0.096 0.104
0.032 0.103
0.025 0.062
0.057
22.458
37.132
31.009
0.122
0.833 0.153 0.116
0.628 0.135 0.084
5.477
6.588
0.122
0.069
1.424 0.028 0.051 5.931 0.170 4.151 0.286 0.274 0.017
0.315 71.206
0.490 63.036
0.456 0.058
0.216 0.028
0.889 1.061 0.113 0.684 0.369 0.089 18.622
0.048 100
0.057 100
0.010 7.44 0.032 0.111 0.011 0.007 0.028 0.478 47.559 0.027 0.462 0.042 0.013 0.046 0.148 0.057 0.062 0.066 0.079 100
0.038 0.338 40.615 0.025 0.843
0.079
36.952 0.069 1.156 0.034 0.019 4.982 0.142 0.566 0.586 0.024
0.085 8.338 0.024 1.202
0.049 0.304
0.053 0.140
0.223
0.085 100
0.119 100
while FeSO4 was not chemically stable at pyrolysis condition.37 As can be seen in Figure 5f, the residual was converted to Fe3O4 completely in SCW in the presence of H2. When the autoclave was uncovered at the end of this experiment, the smell of H2S was blowing. The special transformation indicated that a redox reaction occurred, which may be described by eq 4
3Fe1 − x S + (4 − 4x)H 2O → (1 − x)Fe3O4 + (1 − 4x)H 2 + 3H 2S
700 °C
(3)
Sulfate in coal was complex and existed as some minerals so that it was difficult to confirm the detail forms in coal until now. Generally, FeSO4 and CaSO4 were mainstream forms of sulfate in coal. Specially, CaSO4 was relatively stable even at 800 °C
3FeSO4 + 11H 2 → Fe3O4 + 3H 2S + 8H 2O 12050
(4)
DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
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Figure 6. A main sulfur transformation pathway of DBT in SCW.
3.4. Organic Sulfur Transformation Mechanisms in SCW. Previous research indicated that supercritical water had prominent reactivity and organic compounds could break the C−C bond easily to generate gas.38,39 C−S bond of organic sulfur such as thioalcohol broke easily in SCW to generate H2S. However, thiophene sulfur was the main organic sulfur form in coal and had a chemical stability by discussed above. So thiophene sulfur transformation mechanisms during SCWG was helpful to study sulfur transformation of coal for us. And sulfur containing organic compounds were usually used to investigate conversion mechanisms during coal or oil thermal conversions.40−43 According to the results of XPS analysis and GC-MS above, we used DBT as a model compound of coal to investigate sulfur transformation mechanisms. The experimental condition were 650−800 °C, 25 MPa, residence time of 1− 15 min. The mass of DBT, water and potassium carbonate (catalyst) were 0.205 g, 10 g, and 0.205 g.The liquid products were extracted by ethyl acetate and then detected by GC-MS. The results were listed in Table 5. As we can see from Table 5, the kinds of organic intermediate products from SCWG of DBT increased first and then decreased as residence time increased and temperature rose. The kinds of products were the most at 650 °C for 15 min residence time and were the least at 800 °C for 15 min residence time. The peak area changes of DBT indicated that DBT almost completely transformed under 800 °C, whereas it was difficult to be gasified under lower temperature such as 650 °C. Among these products, the main sulfur-containing intermediate products were benzothiophene, 2-methylbenzothiophene, 3-methylbenzothiophene, 7-ethylbenzothiophene and 4-methyldibenzothiophene. The phenomenon that peak areas of these sulfur organic intermediate products were very small indicated that the sulfur atoms of DBT were more active in SCW than the nitrogen atoms of indole whose main intermediate product was aniline.22 Among all of the intermediate products, the main products were propenylbenzene, biphenyl, phenol, acenaphthene, dibenzofuran, and naphthalene. Some polycyclic aromatic compounds that were difficult to gasify were generated as the temperature increase rate was slow. It is worth mentioning that when temperature and residence time increased from 650 °C, 1 min to 750 °C, 15 min, the peak area of DBT decreased from 71.206 to 8.338%, whereas dibenzofuran increased from 5.477 to 7.440% and then decreased to 0.566%; propenylbenzene increased from 0 to 39.231% and biphenyl increased from 18.622 to 37.132% first and then kept up to more than 30%. According to these results, we proposed a main transformation pathway of thiophene sulfur of DBT in SCW as shown in Figure 6. In Figure 6, DBT reacted with OH free radical first to generate H2S and dibenzofuran; dibenzofuran reacted with H free radical to generate water and biphenyl; and then biphenyl reacted with H free radical to generate propenylbenzene and CH4. At last the compounds transformed to gas. Though DBT was very chemically stable, the pathway showed the sulfur atom in DBT was more easily attacked than other atoms of DBT, which
also explained why organic sulfur in L-coal solid residues decreased more quickly than inorganic sulfur by XPS analysis. 3.5. H2S Sources and Free Radical Mechanism in SCW. The phenomenon that no SOx generated in gas indicated the mechanism of sulfur transformation in supercritical water gasification (SCWG) under reductive condition was different from the one in SCWO13 or traditional pyrolysis.37 According to previous work, SH free radical was an important intermediate of sulfur transformation during thermal process of coal.44 In SCWO process, enough oxygen promote sulfur transforming to SOx or sulfate. In pyrolysis, active sulfur such as SH free radical attacked functional groups with oxygen to generate SOx or COS. According to the discussions in above sections, H2S was generated from coal gasification in SCW had three sources as follows. First, among inorganic sulfur of raw coal, FeS2 (Pyrite) was chemically stable under pyrolysis and SCW in the absence of hydrogen atmosphere. When FeS2 was in hydrogen atmosphere, H2S was generated and FeS2 was converted to FeS under pyrolysis condition or Fe1‑xS and Fe3O4 under SCW condition. Second, H2S came from unstable sulfate minerals such as FeSO4 which may decompose and be converted to Fe3O4 in the presence of H2 atmosphere. Third, organic sulfur, especially thiophene sulfur transformed to H2S. However, H2S was not the only sulfur-containing product of coal gasification in SCW. The amount of SO42− had some relationship with the amount of H2S as temperature rose. Figure 7 showed sulfur
Figure 7. Sulfur distributions of products in tristate at different temperature.
distributions of products in tristate at different temperatures. At 550 °C, SO42− reached the maximum molar fraction of 43.95% for L-coal and 20.07% for Z-coal. However, at higher temperature such as 750 °C, the fraction of SO42− decreased to 0.85% for L-coal and 3.35% for Z-coal. These results indicated SO42− could be considered as an intermediate product. As the temperature of SCW was high and pressure 12051
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ACKNOWLEDGMENTS This work was financially supported by the China National Key Research and Development Plan Project (Contract 2016YFB0600100), the National Natural Science Foundation of China (Contracts 51527808 and 51323011) and Shaanxi Science & Technology Co-ordination & Innovation Project (Contract 2015TZC-G-1-10)
of SCW was relatively lower, there were many H or OH free radicals in SCW.45,46 Free radical mechanism can make appropriate explanations for these results.47 As temperature was lower, a small amount of H2 generation provided a SCW environment with a large amount of OH free radical with oxidizability, which may oxidize the active SH free radical with stronger reductivity to form SO42−. On the contrary, enough H free radical provided a reductive environment at higher temperatures in the presence of enough H2 generated in SCW. H free radical restrained the generation of high-valence sulfur and promote the generation of H2S from SH free radical. On the other hand, COS or CS2 were usually detected in coal pyrolysis or traditional coal gasification. However, H2S was the only sulfur-containing gas species in SCW because an excessive amount of water molecule may attack them to generate H2S and CO2.
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4. CONCLUSION In this work, two coals produced from Linfen and Zhangjiamao in China were chosen as experimental feedstocks to investigate sulfur transformation characteristics during hydrogen production by coal gasification in SCW. In addition, inorganic and organic sulfur compounds were used to investigate sulfur transformation mechanisms. Sulfur transformation pathway and sulfur forms in the products were detected comprehensively. The main conclusions obtained were as follows: (1) H2S was the only sulfur-containing gaseous product instead of SOx, whereas SO42− was the main liquid-sulfur product. (2) H2S had three sources as follows. First, when FeS2 was in hydrogen atmosphere of SCW, H2S was generated and FeS2 was converted to Fe1−xS and Fe3O4. Second, H2S came from unstable sulfate minerals such as FeSO4 which may decompose and be converted to Fe3O4. Third, organic sulfur such as thiophene sulfur transformed to H2S. (3) The two sulfur products H2S and SO42− depend on H or OH free radical in SCW. When temperature was lower, little amount of H2 was generated and H free radical was not enough which provide a relatively oxidizing environment of SCW to promote SO42− generation. When the temperature was higher, a large amount of H2 was generated and enough H free radical to provide a reductive environment of SCW to promote H2S generation from H and SH free radical. (4) The study showed the process of sulfur transformation from coal to H2S instead of SOx under SCW condition, which may provide an experimental basis of solving the SOx emission from coal at the source and demonstrate a promising clean utilization way of coal.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shanke Liu: 0000-0002-7636-3487 Liejin Guo: 0000-0002-3671-5628 Hui Jin: 0000-0001-9216-7921 Notes
The authors declare no competing financial interest. 12052
DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
Article
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DOI: 10.1021/acs.energyfuels.7b02505 Energy Fuels 2017, 31, 12046−12053
