Sulfur transformation in coal during supercritical water gasification

Fuel 186 (2016) 394–404

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Sulfur transformation in coal during supercritical water gasification Nan Meng a, Dandan Jiang a, Yang Liu b, Zhiyuan Gao b, Yaqin Cao b, Jinli Zhang a, Junjie Gu a,c, You Han a,⇑ a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China ENN Science and Technology Development Co., Ltd., Langfang 065001, PR China c Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa K1S5B6, Canada b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A modified method is used to test the

sulfur forms in char.
FeS2 transforms into Fe1xS, FeS and

Fe3O4 after coal SCW gasification.
Liquid S after coal pyrolysis is S + 4

and S + 6, while S  2 also exists in SCW case.
Supercritical water restricts thiophene and SO2 forming.

a r t i c l e

i n f o

Article history: Received 14 June 2016 Received in revised form 28 July 2016 Accepted 23 August 2016

Keywords: Sulfur transformation Supercritical water gasification Organic sulfur Coal pyrolysis Pyrite

a b s t r a c t The efforts of supercritical water (SCW) on sulfur transformation during coal gasification process were studied in a batch autoclave. Sulfur morphological distribution of two different rank coals on solid, liquid and gas phase at different temperatures were thoroughly investigated during coal SCW gasification compared with coal pyrolysis. Compared with coal pyrolysis, the removal of sulfate-sulfur and pyritic-sulfur in solid state were enhanced during coal SCW gasification, especially with temperature rising. Both in coal pyrolysis and coal SCW gasification, the XRD analysis showed that the pyrite which blended with coal transformed into pyrrhotite, whereas pyrrhotite further converted into magnetite during SCW gasification. Supercritical water had the ability not only restrict thiophene forming, but also oxidize organic-sulfur to produce sulfone, causing that the forming of organic-sulfur was restrained. The sulfate and sulfite were the main liquid-sulfur composition after coal pyrolysis, whereas sulfide also existed in spite of sulfate and sulfite after coal SCW gasification. The sulfide may be produced via H2S absorbing in water. Because the sulfide can be oxidized into sulfate and sulfite by OH radicals in SCW condition, the amount of sulfate and sulfite was much larger than that after coal pyrolysis. The gas sulfur after coal pyrolysis included H2S and SO2, but the abundant H radicals provided by SCW system caused much more H2S produced and inhibited the formation of SO2. The sulfur transformation could be explained by free radical mechanism. Those transformation properties were important for further environmental evaluation and the better using of coal SCW gasification technology with low sulfur pollution. Ó 2016 Elsevier Ltd. All rights reserved. d

d

⇑ Corresponding author. E-mail address: [email protected] (Y. Han). http://dx.doi.org/10.1016/j.fuel.2016.08.097 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

N. Meng et al. / Fuel 186 (2016) 394–404

1. Introduction Coal, the second largest source of energy in the world and taking up to 65% of primary energy consumption in China, has great potential as a substitute of petroleum. However, the utilization of coal is facing with a number of strict environmental challenges. Among various coal utilization technologies, coal gasification in supercritical water (SCW) is developed during the past three decades. By using this technology, low-rank coal without drying can be directly converted to hydrogen, hydrocarbons and oils at relatively low temperatures. Therefore, coal SCW gasification is a promising clean and efficient coal convention technology [1,2]. In recent years, growing environmental concerns have resulted in strict sulfur regulations for coal products. As a main environment-pollution element, sulfur presented in coal can be both in inorganic and organic forms [3–5]. Inorganic sulfur primarily includes sulfate (e.g., CaSO4, BaSO4 and FeSO4) and sulfide (e.g., pyrite, pyrrhotite and marcasite). Organic sulfur mainly classifies as four species: aromatic sulfur (e.g., thiophene), aliphatic sulfur (thiol/thioether), sulfoxide and sulfone. During the traditional conversion or burning process, the sulfur usually turns into SOx or H2S, resulting in air pollution, acid rain and so on [6]. The oxidation of pyritic sulfur in coals is the main cause of the acid mine drainage [7]. Higher concentration of sulfur causes a relatively higher acid dew-point which leads to more serious device corrosion and catalyst poisoning [8,9]. The sulfur contained in coking coal also coats the crystal surface and debases the quality of iron and steel products [10]. Therefore, Understanding the sulfur transformation and clarification of sulfur is very important for coal cleaning utilization. Many experiments showed that the sulfur transformation in coal is complicated, and can be significantly influenced by temperature, coal rank, mineral species, atmosphere, oxygen concentration and etc. [11,12]. Gryglewicz et al. investigated the behavior of sulfur forms during pyrolysis of low-rank coal and their results showed that the organic sulfur was enriched between 330 and 600 °C, while ferrous sulfide began to release sulfur above 1000 °C [13]. Gryglewicz further found that the sulfur removal ratio was decreased with the increasing coal rank [14]. Khan surveyed 32 kinds of bituminous coals during pyrolysis and founded that the yields of sulfur in solid, liquid, and gaseous state were correlated with the contents of total sulfur, pyritic sulfur and organic sulfur in raw coal [15]. Chen et al. reported that the mineral matter in coal could trap sulfur to increase sulfur distribution in char [16]. Bhattacharya’s group investigated the effect of reactive atmosphere on the sulfur emission and their results showed that sulfur distribution in gas phase was various at different atmospheres. For example, Up to 10% of coal sulfur was found to be converted to SO3 under oxy-fuel combustion, whereas SO3 was undetectable during pyrolysis and gasification [17]. Li’s group compared the sulfur release through coal pyrolysis with coal partial oxidation, and they pointed out that small amount of oxygen in inert atmosphere was helpful to improve sulfur removal efficiency without great decrease in char yield [18,19]. In summary, sulfur transformation in coal is affected by many factors. Therefore, new data for sulfur transformation during coal SCW gasification process which is a particular reaction system is needed. The studies related to coal supercritical desulfurization showed that the SCW system had a great effect on the sulfur transformation in coal. For instance, Tao Wang’s group used H2O2 as the oxidant to investigate the oxidation of coal in supercritical water, and they found that the sulfur contained in coal could be gradually oxidized to sulfate in supercritical water medium [20]. Li et al. studied the pyritic and organic sulfur conversion in coal by supercritical ethanol. Their results showed that the Ph2SO was difficult to be removed in supercritical ethanol, but its removal could be

395

improved via adding water into ethanol [21]. Park et al. found the reaction temperature was the most important factor to affect the sulfur removal in sub- and supercritical water, and the sulfur removal in liquid effluents was much greater than that in gas effluents [22]. Chen et al. investigated the influence of pressure, temperature and equivalence ratio of oxygen on sulfur distribution in solid, liquid and gaseous phases [23]. In order to understand the supercritical water desulfurization mechanism, Patwardhan et al. chose a variety of organic sulfurs to measure the kinetics of their decomposition process in the presence of hydrocarbons and supercritical water, and they proposed that the decomposition of sulfurs in presence of SCW was a free-radical process [24]. When Ma et al. investigated the oxidation of iron sulfide in supercritical water in a batch reactor, they revealed that the reaction pathway during iron SCW oxidation was: iron sulfide ? sulfide ? sulfite ? sulfate [25]. The above researches greatly helped us to understand the SCW effect on the sulfur transformation, especially during SCW oxidation process. However, sulfur transformation mechanism during coal SCW gasification process (especially the transformation between the organic sulfur and the inorganic sulfur) is still unclear. Furthermore, owning to the complicated coal composition and uncommon SCW system, many national standard test methods of sulfur forms in coal and char such as American Standard ASTM D-2492 and Chinese Standard GB/T 215 have inevitably introduced errors. In standard methods, ‘sulfate sulfur’ is normally considered as thermal stability sulfate such as CaSO4. Some HCl soluble sulfite or sulfide is neglected and undetected. However, those parts of S could cause serious water pollution for low stability. The ‘pyritic sulfur’ is calculated as pyrite (FeS2), ignoring other HNO3 soluble minerals. All the remaining sulfur is considered as ‘organic sulfur’ for a high cumulative error. Meanwhile, large amounts of water remaining in residue during SCW gasification could influence the mineral solubility. The standard test methods is not suitable for those sample analysis. Hence, a modified method on the basis of Yan et al. [26] (denoted as weight-centrifugal method) is used to calculating the sulfur forms in char after the SCW treated. This method will determine the sulfur forms more accurately compared with other determination methods. In this work, the weight-centrifugal method was used to investigate the sulfur distribution in the solid, liquid and gas phases at different temperatures during coal SCW gasification process. As comparison, the sulfur transformation during coal pyrolysis was also analyzed. Considering the effect of coal rank, two types of coal, a brown coal and a subbituminous coal, were chosen here. In order to direct determine and quantify the sulfur structures, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were also employed. The information could help to understand the mechanism of sulfur removal for cleaner coal utilization and the further design of commercial plants.

2. Material and methods 2.1. Coal sample preparation and reagents The lignite used in this study was Zhaotong coal, from Yunnan province, China, designated as Z-coal. The subbituminous coal was from Wangjiata, Inner Mongolia province, China, designated as W-coal. Those coals were dried under vacuum at 80 °C for 48 h and then crushed to pass through 80 meshes sieve for all experiments. Table 1 showed the proximate and ultimate analyses for those coals. Ultra-pure water (18.2 MXcm) was obtained from a Millipore Milli-Q Advantage A10 system. Hydrochloric acid (36% HCl, BV-III grade) and nitric acid (68% HNO3, BV-III grade) were supplied by Beijing Institute of Chemical Reagents. The purity of

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N. Meng et al. / Fuel 186 (2016) 394–404

Table 1 Proximate and ultimate analysis of raw coal. Proximate analysis (ar,a wt%)

W Z

Sulfur formc (%)

Ultimate analysis (ar, wt%) b

Volatile matter

Ash content

Moisture

C

H

N

S

O

30.60 49.32

22.49 20.41

0.05 0.07

57.76 47.79

3.79 4.48

1.18 1.23

0.74 0.54

13.99 25.48

S-s

S-p

S-o

13.74 16.16

73.32 53.59

12.94 30.25

Ash analysis (wt%)

W Z a b c

SiO2

CaO

Al2O3

Fe2O3

SO3

MgO

K2O

TiO2

P2O5

Na2O

51.45 37.86

10.07 18.22

14.68 14.47

8.48 9.2

8.52 6.95

1.73 4.01

1.87 1.23

0.5 0.98

0.07 0.59

2.29 0.44

ar, as received basis.(samples dry in vacuum at 80 °C). O = 100- ash content-moisture-C-H-N-S. S-s, sulfate sulfur; S-p, pyritic sulfur; S-o, organic sulfur.

N2 and pyrite used in experiment were 99.999% and 98%, respectively. The data in Table 1 clearly showed that Z-coal was a typical lignite coal with low carbon content (47.79%) and high volatile matter (49.32%). Because of the high volatile and water contents, Zhaotong coal, the second largest lignite in China, was a good raw material for coal SCW gasification technology. That’s why it was chosen here for the sulfur transformation investigation during coal SCW gasification. The Z-coal and W-coal had similar ash and total sulfur contents, demonstrating W-coal was a good reference to study the effect of coal rank on the sulfur transformation. Pyrite was the dominant sulfur-bearing mineral (73.32% for Z-coal, 53.59% for W-coal). Meanwhile, Z-coal had comparable organic sulfur contents (30.25%) for its low-rank. Among ash analysis, the silicate and aluminosilicate (SiO2 + Al2O3) accounted for 66.13% in W-coal and 52.03% in Z-coal. The contents of alkalineearth metals in Z-coal was 22.23%, which was twice than that in W-coal, mainly Ca and Mg. 2.2. Experimental instrument and procedure Fig. 1 illustrated the schematic presentation of the experimental apparatus. A batch cylinder autoclave with inner volume about 500 cm3 was used as pressurized reactor, which was heated by a temperature-controlled electric furnace. A compressed gas cylin-

der and an air bag were connected with the reactor through stainless steel tubes. Temperature and pressure in the reactor were measured by a K type thermocouple (with 0.1 °C sensitivity) and a pressure gauge (±0.1 MPa), respectively. In a typical run, the reactor was charged with 14 g coal sample and appropriate water. No extra oxygen was added to avoid the complex changes and interferences by its oxidation. The system was checked for leak with 6 MPa of N2, then was purged with N2 for 3 min to replace residual air. The autoclave was heated up to the reaction temperature at a rate of 5 °Cmin1. During SCW experiments at 450 and 550 °C (designated as 450-S and 550-S), the system pressure changed along with reaction conditions, mainly depending on the temperature and the water content. Hence, the system was maintained in 24.5–25.5 MPa, ±3 °C. If the pressure was beyond the range, recalibration of water content would be required (probably 80 ml for 450-S, 60 ml for 550-S). In subcritical experiment 350-S, the pressure was close to the saturated vapor pressure of water to keep the H2O as vapor steam. In pyrolysis experiments (designated as 350-P, 450-P and 550-P), initial N2 was used to maintain the pressure at 4 MPa on the reaction temperature. After the desired reaction time (15 min) had elapsed, the furnace was removed and the autoclave was cooled down by a fan. The gas were released to the air bag by opening the valve equipped on the autoclave when the temperature dropped to 70–80 °C, avoiding H2S dissolved in water. The sample remained in the autoclave after SCW gasification was separated by vacuum filtration. The char was washed by hot water and dried at 80 °C under vacuum for 24 h. All the liquid retained or in washing process were collected. The tetrahydrofuran was used to clean the system after each experiment.

2.3. Sample analysis and characterization methods

Fig. 1. Schematic diagram of experimental apparatus: (1) N2 high-pressure gas cylinder; (2) heating furnace; (3) valve; (4) high-pressure autoclave; (5) air bag.

The carbon contents in gaseous product such as CO, CO2 and hydrocarbons were assessed by Gas Chromatograph (Agilent 7890A) coupled with flame ionization detector (FID, column: Porapak Q and MS-13X) and thermal conductivity detector (TCD, column: HP-AL/KCL). The concentration of sulfur-containing gases (measured in forms of H2S, COS, SO2, etc., denoted as gas-S) were determined by gas chromatography (Agilent 7890A, column: DB-1) equipped with a flame photometric detector (FPD). The calibration gases and external standard method were employed to quantify the gas concentrations. The total amount of sulfur in the liquid (denoted as liquid-S) was tested by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Thermo Fisher, iCAP 7000). The amount of sulfate (SO2 4 , denoted as S + 6) was detected by ion chromatography (DIONEX ICS-900, column: IonPac AS18) while the amount of sulfide (such as S2, HS, denoted as S  2) was tested by Iodometric

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N. Meng et al. / Fuel 186 (2016) 394–404  2 method. Hence, other sulfite (such as SO2 3 , HSO3 , and S2O3 , denoted as S + 4) was calculated as the liquid-S subtractive S + 6 and S  2. The content of total sulfur and sulfur forms such as sulfate sulfur, pyritic sulfur and organic sulfur (denoted as S-s, S-p and S-o) in coal and the corresponding solid residual (char) were analyzed via a modified test method in Section 2.4. Furthermore, XRD (Bruker D2 PHASER) was employed to provide detailed information about mineralogical characteristics of the char. Diffraction patterns were recorded with Cu Ka (Ka1 = 0.1542 nm) radiation, and the voltage as well as current were 40 kV and 40 mA, individually. Step scanning were performed over a range of 2h from 10° to 90°. Besides, the sulfur species in raw coal and char were investigated by XPS (VG Multilab 2000 X), electron spectrometer from VG Scientific using 300 W Al Ka radiation. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon. The relative concentrations of different sulfur species were calculated according to the corresponding peak values fixed at 163.3 ± 0.4, 164.1 ± 0.2, 166.0 ± 0.5 and 168.2 ± 0.2 eV of binding energy for thiol/thioether sulfur (S1), thiophene (S2), sulfoxide (S3) and sulfones (S4), respectively [27,28].

2.4. The weight-centrifugal test method for sulfur forms in SCW char After SCW treatment, the char mixed with liquid remaining in the reaction kettle. It was hard to gather and quantify all the reaction solution. Washing was necessary for some substances separating out from subcritical water might adhere to the vessel wall or pipeline when temperature cooled down. After filtrating and washing process, some water soluble mineral and organic matter could dissolved in liquid causing an addition of liquid-S. For better comparison, the char in pyrolysis was also washed. Those liquid-S were classified as S  2, S + 4, S + 6 and quantify as above methods, whereas other residue sulfur in char could more precise to reflect the actual situation. According to GB/T 215-2003, the S-o was determined by the difference between the total sulfur and inorganic sulfur, and some iron-sulfur compounds coordinated in the coal structure were likely to be characterized as organic sulfur [29]. Therefore, obvious error was introduced during calculation of the organic sulfur in char. The S-s was calculated by the weight of barium sulfate form HCl solution (gravimetric method). However, HCl could dissolve some sulfite or sulfide. Those forms of S might volatilize as H2S or SO2 during heating in extract HCl solution, and were not calculated for uncombined with Ba. The S-p by the standard method was extract by HNO3, quantified by the concentration of Fe3+ as the stoichiometry of sulfur and iron in FeS2. The pyrite in coal was a complex ore including Zn, Pb and other metal ions which could alternate Fe by isomorphism. During pyrolysis, pyrite might be converted to pyrrhotite (Fe1xS) which partly dissolved in HCl and fully dissolved in HNO3 solution. Hence, the stoichiometry of sulfur and iron considered as 2:1 was inaccurate [26]. Base on the reasons above, a modified calculation method was proposed to identify sulfur forms: (1) Sample A was added to centrifugal tube and treated with 5 M HCl at 80 °C in ultrasonic cleaner for 4 h. The suspension was centrifuged and washed with boiled double-distilled water for several times. The remaining solid was denoted as B. (2) Sample A was treated with 1:7 HNO3 (volume ratio) at 80 °C in ultrasonic cleaner for 4 h. The suspension was centrifuged and washed with boiled double-distilled water for several times. The remaining solid was denoted as C.

(3) Solids B, C were dried at 60 °C for 6 h and then 80 °C for at least 24 h under vacuum. Subsequently, the mass (mA, mB, and mC) and the content of sulfur (SA, SB and SC) were detected by electronic balance and the elemental analyzer, respectively. The S-s sulfur including sulfate, sulfite, thiosulfate and partially HCl-soluble sulfide which may release as H2S, was calculated as:

S-s ¼ ðmA  SA  mB  SB Þ=mA

ð1Þ

The S-p sulfur mainly focused on pyrite was calculated as:

S-p ¼ ðmA  SA  mC  SC Þ=mA  S-s

ð2Þ

The organic sulfur (S-o) was calculated as:

S-o ¼ mC  SC =mA

ð3Þ

Those solid sulfur contents were measured by elemental analyzer (Vario EL cube) and the standard deviation of ±0.1% can be attained. The average value of sulfur content of three parallel experiments was taken as parent value and three effective digits were adopted to denote the value of sulfur content in order to minimizing the random error. The organic sulfur could be detected directly and the accumulative error was avoided. The S-p was calculated based on sulfur content avoiding the interferences from positive ions. All the sulfur was counted without missing, especially sulfite or sulfide. In GB/T 215-2003, the samples need re-washing and filtering until no Cl or Fe3+ were left which was time-consuming. The comparison results of weight-centrifugal method and GB/T 215-2003 were shown in Table 2. The S-p in the modified method was much larger than in GB/T 215 and led the S-o extremely lower. Besides, the modified method had higher precision than gravimetric method, and was more suitable for numerous experiments especially in S-s test. Those experiments without using filter were easier to be repeated. Hence, the modified method was adopted to test the sulfur forms in this study. 3. Results and discussion 3.1. The carbon and sulfur distribution and the conversion ratios Roughly, the material of raw coal converted into gas, liquid and char by SCW gasification. While after the pyrolysis of the dried coal, it mainly converted into gas and char. The carbon contents in three phases were calculated from the results measured by gas chromatography, total organic carbon analyzer and elemental analyzer, respectively. To avoid the errors in the washing process, dry pyrolysis samples without washing were used in carbon calculation. The normalized percentage distribution of carbon was shown in Fig. 2. A large amount of carbon (>63%) remained in solid products after different experiments. Total carbon content in solid decreased associated with the temperature increasing. This high temperature was in favor of carbon volatilization to gas, especially during SCW

Table 2 Comparison of modified and standard test method (mg/g). Total S

Absolute deviation Raw-W 550-SW 550-PW

±0.1 7.42 6.33 9.41

Modified method

GB/T 215-2003

S-s

S-p

S-o

S-s

S-p

S-o

1.02 0.02 0.39

±0.2 5.44 0.52 3.28

±0.2 0.96 5.79 5.74

±0.2 1.08 0.25

±0.4 4.92 0.44 2.66

1.43 5.89 6.51

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Fig. 2. Carbon distribution in three phases: (a) W-coal; (b) Z-coal.

gasification. The carbon content in liquid decreased monotonically as the temperature increasing. Near the critical point, it was beneficial to produce oil which could be collected in liquid. Those abundant oil substances generated at low temperature continued to be gasified. For Z-coal, the liquefaction and gasification conversion were much higher than that of W-coal at the same conditions, due to more volatile matter. Sulfur contents in gas, liquid and solid were calculated and shown in Fig. 3. Most sulfur was remained in char during pyrolysis and was tiny fluctuated with temperatures. During coal SCW gasification, the char-S was decreased from 350 to 450 °C, then increased at 550 °C. At 450 °C where the least sulfur in char during SCW gasification, 37.9% and 19.2% sulfur in W-coal transferred to gas and liquid phase, respectively, whereas 26.6% and 38.8% sulfur in Z-coal migrated to the gas and liquid phase. As shown in Table 1, compared to W-coal, high ash content and the resourceful alkali metals and alkaline earth metals in Z-coal could adsorb more sulfur in liquid [30]. To clearly analyze the carbon and sulfur transformation and compare with coal body, the conversion rate was used to calculate the removal efficiency from solid: the larger the conversion rate, the more easer to release. The conversion rate of coal body, carbon and sulfur were defined as Eqs. (4)–(7) [31]:

Char yield ¼ mchar =mcoal  100%

ð4Þ

Conversion rate of coal body ¼ ð1  char yieldÞ  100% Conversion rate of C ¼ ð1  char yield  C char =C coal Þ  100%

ð5Þ ð6Þ

Conversion rate of S ¼ ð1  char yield  Schar =Scoal Þ  100%

ð7Þ

where mchar and mcoal were the mass of char and raw coal, respectively; Cchar, Ccoal, Schar and Scoal were based on the elemental analyzer results. This calculation method focused on char avoiding the instrumental error in sulfur detection. In Fig. 4a and c, with the temperature increasing in pyrolysis, the conversion rate of carbon and sulfur were gradually increased, although still lower than that of coal body. The conversion rate of coal body and carbon in SCW gasification were higher than that in pyrolysis at the same temperature. Similar tendency was also founded in Xiaolongtan lignite [32]. The higher rank W-coal, possessing the characteristics of subbituminous coal with lower volatile content and more thermally stable macromolecular structure of coal matrix, had lower conversion rate of coal body and carbon than that of Z-coal. Since the main compositions in char were carbon and mineral substance, the conversion rate of carbon had the same tend with coal body with temperature changing. Only about 10% sulfur was removed in pyrolysis. The conversion rates of sulfur in SCW gasification were obviously higher than that

Fig. 3. Sulfur distribution in three phases. (a) W-coal; (b) Z-coal.

N. Meng et al. / Fuel 186 (2016) 394–404

Fig. 4. Conversion rate of coal body, carbon and sulfur: (a) W-coal during pyrolysis; (b) W-coal during SCW; (c) Z-coal during pyrolysis; (d) Z-coal during SCW.

in pyrolysis, especially at 550 and 450 °C. The highest conversion rates of sulfur in two coals were at 450 °C: 57.0% sulfur in Wcoal and 65.4% sulfur in Z-coal were conversed, even above the conversion rates of coal body which were only 30.0% in W-coal and 45.6% in Z-coal, respectively. This meant the SCW tended to promote the release of sulfur from solid compare to other volatiles.

3.2. The distribution of sulfur forms in char The sulfur forms present in raw coal and char, classified as S-s, S-p and S-o, were detected by the weight-centrifugal method and shown in Fig. 5. The S-s and S-p were decreased as temperature rising. The pyrite was slightly decomposed at 350 °C, whereas it decomposed quickly at 550 °C during SCW coal gasification process, causing the convention rate of pyrite in W-coal and Z-coal as high as 88.9% and 93.2%, respectively. On the contrary, only 55.6% and 61.3% pyrite in W-coal and Z-coal conversed during coal pyrolysis. The results showed that the supercritical water could enhance the decomposition of pyrite significantly in both of the two type coals. The detailed mechanism of pyrite decomposition

Fig. 5. The migration of different sulfur forms in char: (a) W-coal during pyrolysis; (b) W-coal during SCW; (c) Z-coal during pyrolysis; (d) Z-coal during SCW. S-s, sulfate sulfur; S-p, pyritic sulfur; S-o, organic sulfur. The unit (mg/g, the same as in the following tables or figures) was sulfur mass content basing on 1 g raw coal after treatments.

399

will be discussed in Section 3.2.2. The S-o in char remarkably increased with the temperature raising from 350 to 550 °C, indicating other forms of sulfur might transfer into organic sulfur. But the amount of S-o in Z-coal after SCW treatment was much lower than that after coal pyrolysis at the same temperature. The organic sulfur transfer mechanism during pyrolysis and SCW gasification will be discussed in detail in Section 3.2.3. The bulk sulfur considered as the sulfur in the whole mass was used to compare with the sulfur on sample surface. There was some differences between them in raw coal or during various processes. The ratio of S/C (molar ratio) on the surface and bulk were calculated from XPS and element analysis, respectively. It represents the sulfur migration path from coal body to surface of coal samples [27,33]. As shown in Table 3, the ratio of S/C on the surface of Z-coal was 3.74  103, which was lower than that of coal body. It suggested that sulfur was rich in the bulk and poor on the coal surface. Whereas during coal pyrolysis process, the content of sulfur was higher at the char surface than in the bulk, indicating the sulfur-containing compounds transferred from inner to the surface. In the case of SCW condition, the mole ratio of S/C on the surface and in the char body were 2.34  103, 2.97  103, respectively, and both lower than that of raw coal. The sulfur-containing compounds were easier to be removed from char into supercritical phase because of the good solubility of SCW. 3.2.1. The sulfate-sulfur transformation In Fig. 5c and d, the S-s of Z-coal in 550 °C, taking up about one third of S-s in raw Z-coal, remained stable during high temperature and SCW treatment, while other two thirds of S-s (about 0.48 mg/ g) was transferred. However, the decomposition degrees of S-s in 350-SZ and 450-SZ were enlarged compared to 350-PZ and 450PZ, respectively. The SCW water benefited those unstable S-s to be removed. The stability of sulfur-bearing minerals were not only influenced by their own decomposition temperature, but also affected by their ambient atmosphere. E.g., the decomposition temperature of CaSO4 and CaS were higher than 1450 and 2000 °C, whereas that of ZnSO4 and FeSO4 were lower at 570 and 470 °C, respectively; in present of coal, ZnSO4 and FeSO4 might decompose as early as 400 and 200 °C, respectively [34]; Huang et al. verified that the Cr7S8 was stable between 400 and 700 °C and it converted with coal into organic sulfur when temperature was higher than 700 °C [35]. Although it was hard to verify the definite types of inorganic salt potentially in S-s, some thermo stabilization minerals which could exist in pyrolysis from 350 to 550 °C might decompose at SCW environment. It was more obvious in Fig. 5a and b that a quarter of S-s retained in 450-PW and 550-PW were thermo stabilization S, whereas those were basically disappeared in 450-SW and 550SW, meaning those parts of S-s were easy to be transformed by SCW gasification. It was apparently that the degree of S-s decomposition in SCW was larger than pyrolysis at the same temperature. Hence, it was a dual effect of thermo and SCW in the migration of minerals. Jin et al. [36] accounted the subcritical water in heating or cooling process had superior solubility for mineral salt transformation which might change S-s form and increase the amount of water-soluble sulfur. 3.2.2. Pyrite transformation in coal environment Atmosphere is one of the important factors influencing sulfur removal, whereas previous studies focus on the sulfur transformation on oxygen environment or only using model compounds environment in SCW [20,24]. However, the coal environment plus supercritical water, containing diversified gas (such as H2, CO, and CO2), organic matter and extreme pressure, is different from single coal pyrolysis environment or coal supercritical water oxidation. Such environment may influence the sulfur removal.

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Table 3 Sulfur/carbon ratio and organic sulfur species. Elemental analysis

Raw-Z 550-PZ 550-SZ a

XPS data a

Organic sulfur species

C wt%

S wt%

S/C 103

C at%

S at%

S/C 103

S1 %

S2 %

S3 %

S4 %

47.79 58.19 60.54

0.54 0.88 0.48

4.23 5.63 2.97

58.77 68.01 76.71

0.22 0.42 0.18

3.74 6.17 2.34

44.44 19.90 14.37

24.39 58.67 52.10

31.17 10.31 8.43

– 11.13 25.10

Atomic ratio.

Among the inorganic constituents of coal, pyrite is the primary sulfur-contributors during coal utilization. The knowledge of its behavior during SCW gasification is very helpful to the cleanly stepping utilization of high-pyrite coals through adjusting and controlling the conversion condition. Bhargava et al. and Huang et al. [37,38] revealed that pyrite evolution was a complicated process and it may have different transformation mechanism under different reaction conditions. Hence, in the first step, pure FeS2 experiments were tested both in pyrolysis and SCW, and the XRD analysis was shown in Fig. 6. The XRD results of the pure FeS2 under pyrolysis or SCW showed no crucial difference compared with the untreated FeS2. The result demonstrated that the pure FeS2 was thermodynamic stable and it did not decompose at 550 °C wherever under pyrolysis or SCW system. The protogenous pyrite in coal was hard to identify by XRD for its low concentration and the interference of other minerals. In order to investigate the transformation behaviors of FeS2 under coal atmosphere, the samples of Z-coal blended with 3 wt% pure FeS2 were tested at 550 °C in pyrolysis and SCW gasification, respectively. Fig. 7 illustrates the changes in the XRD patterns of the corresponding samples. As shown in Fig. 7b, pyrrhotite (Fe1xS) was the dominant mineral phase. The FeS2 completely decomposed into Fe1xS in coal pyrolysis at 550 °C, whereas the complete decomposition of pure pyrite in inert environment was higher than 650 °C [39,40]. This decomposition might be accelerated by H2 as Eq. (8).

FeS2 þ H2 ! FeS=Fe1x S þ H2 S

ð8Þ

After SCW treatment, the Fe1xS was not the only product from FeS2. The XRD in Fig. 7c showed that ferrous sulfide (FeS) and magnetite (Fe3O4) were also produced. Based on the result in the first

Fig. 7. XRD patterns of pyrite transformation in coal atmosphere: (a) raw mixture of Z-coal and 3 wt% FeS2; (b) mixture in 550 °C pyrolysis; (c) mixture in 550 °C SCW; j FeS2, d Fe1xS, N FeS, w Fe3O4.

step that FeS2 did not react with water, the Fe3O4 might derive from further reaction of Fe1xS or FeS. Some articles affirmed that FeS was extremely stable in inert environment under 950 °C [39] and the reaction rate of FeS in the presence of H2 was markedly slow even above 800 °C [41]. Besides, in the presence of CO2, the pyrrhotite converting to Fe3O4 only took place above 750 °C [40]. But under the supercritical water condition, the Fe1xS could be further transformed into Fe3O4 with the interaction of water clusters [42].

Fe1x S=FeS þ ½H2 Oscw ! Fe3 O4 þ H2 S þ H2

Fig. 6. XRD patterns of pure pyrite transformation: (a) raw FeS2; (b) FeS2 in 550 °C pyrolysis; (c) FeS2 in 550 °C SCW; j FeS2.

ð9Þ

3.2.3. The organic sulfur species identify by XPS In order to investigate the mechanism of organic-sulfur transformation during SCW coal gasification, Z-coal and chars were conducted by XPS. As shown in Fig. 8 and summarized in Table 3, sulfur species were classified as thiol/thioether sulfur (S1), thiophene (S2), sulfoxide (S3) and sulfones (S4), respectively. Those samples were pretreated by dilute HNO3, avoiding interference by inorganic substances [21]. In raw coal, sulfur was in species of thiol/thioether, thiophene and sulfoxide. The thiol/thioether, taking up 44.44%, was the dominate sulfur form in the low rank coal. After pyrolysis, thiol/thioether and sulfoxide sulfur decreased from 44.44% and 31.17% to 19.90% and 10.31%, respectively, while thiophene rose from 24.39–58%. In fact, thiol sulfur started to decompose before 300 °C and thioether sulfur decomposed range from 400 to 500 °C, whereas sulfoxide was between 350 and 550 °C [43]. Therefore, as the reaction temperature going up, labile sulfur-containing compounds such as thiol/ thioether sulfur were easier to decompose and form SH/ SR radicals which were cond

d

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Fig. 8. XPS S 2p spectrogram of organic sulfur species: (a) raw Z-coal; (b) 550-PZ; (c) 550-SZ; S1, thiol/thioether sulfur; S2, thiophene; S3, sulfoxide; S4, sulfones.

firmed as the key intermediates in sulfur transformation. Since it was a batch reactor without enough H radicals, those SH radicals tended to react with other volatile materials or organics in char to produce new organic-sulfur compounds such as thiophene and sulfones which were very stable and could not be decomposed even at 900 °C. Hence, sulfur was immobilized in the char as S-o by cyclization or aromatization during pyrolysis: d

d

d

SH þ Volatiles ! S-o

ð10Þ

In SCW, more thiol/thioether and sulfoxide sulfur were removed, meanwhile both of the amount and percentage of thioether sulfur were lower than pyrolysis. Since there were more H radicals in SCW, the interaction between SH radicals and organic compounds was suppressed which inhibited the formation of thiophene [44]. In addition, the OH provided by SCW oxidized more organic sulfur to form sulfone [45], causing the percentage of sulfone was as high as 25.10% in SCW compared with that in pyrolysis (11.13%). d

d

d

OH

OH

RSR0 ƒƒ! RSOR0 ƒƒ! RSO2 R0 d

d

ð11Þ

3.3. Migration of sulfur forms in liquid The quantified classification of sulfur forms in liquid could explain the pathway of sulfur migration. The detected method

for S  2, S + 4 and S + 6 was described in Section 2.4. The S + 4 in W-coal and Z-coal were 0.086 and 0.048 mg/g, while the S + 6 were 0.341 and 0.347 mg/g, respectively. Fig. 9 showed the transformation of sulfur in liquid during various processes. The liquid-S in SCW samples were composed by the initial water and cleaning water in the washing process, while the liquid-S in pyrolysis samples were only the water-soluble S in pyrolysis char after experiment. Hence, S  2 was undetected in raw and pyrolysis char for ignorable content. As the temperature increasing, the liquid-S in pyrolysis was dropped due to the amount of S + 6 decreasing. The S + 6 in pyrolysis derived from the water-soluble sulfate minerals in char. Those minerals were possibly unstable and decomposed at higher temperature as discussed in Section 3.2.1. However, S + 4 and S + 6 in SCW were extremely enlarged compared to that in pyrolysis. With the temperature rising, the S  2 was increased in Fig. 9b, but it had small fluctuation in Fig. 9d. In the case of the abundance of H2S in gas, the alkali and alkaline earth metals in the liquid could absorb them to form S2 or HS. Although the amount of Ca and Mg in Z-coal was twice than that in W-coal (see Table 1), the S  2 was evidently lower than that in W-coal, indicating some absent S  2 in Z-coal might be oxidized, especially at 550 °C. The total amount of S + 4 and S + 6 in liquid phase in 550-SZ was 1.68 mg/g. If we assumed that all the removed S-s transformed into S + 4 and S + 6, which was 0.48 mg/g, the amount of S + 4 and S + 6 in the liquid phase was still larger than this value. Hence, partial S + 4 and S + 6 derived from S  2 oxidation or liquid organic sulfur production other than the S-s dissolution into water during SCW gasification [20]. The reaction pathway could be expressed as: d

OH= OR d

d

OH= OR d

S  2 ƒƒƒƒ! S þ 4 ƒƒƒƒ! S þ 6

ð12Þ

Since there was no additional oxygen in experimental system, the oxygen-containing functional groups in char matrix might be activated by supercritical water and played a positive role in promoting the oxidization of sulfur. The abundant labile oxyorganics in low-rank Z-coal were more active at higher temperature and contributed to the deceasing of S  2 in 550-SZ compared to 550-SW [28,46]. Besides, the OH radicals in SCW also had some capacity to facilitate this oxidation [47]. It was noticeable that although SCW formed more stable sulfur (S + 6), it also produced amount of low valence sulfur in water. The S  2 and S + 4 in liquid were easy to migrate by acid or thermo, and need to be controlled (e.g., alkalization, precipitation, oxidization, etc.) to prevent environmental pollution. Those potential transferability and ecological hazards need to be considered in further effluent disposal. d

Fig. 9. The content of different sulfur forms in liquid: (a) W-coal during pyrolysis; (b) W-coal during SCW gasification; (c) Z-coal during pyrolysis; (d) Z-coal during 3 SCW gasification. S  2, sulfide, such as S2, HS; S + 4, sulfite, such as SO2 , 3 , HSO and S2O2 3 ; S + 6, sulfate.

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Table 4 Sulfur forms in gas (mg/g). PW

a

SW

PZ

SZ

Temperature (°C)

350

450

550

350

450

550

350

450

550

350

450

550

H2S SO2

0.034 0.008

0.213 0.015

0.421 0.030

1.019 nda

2.590 nd

1.979 nd

0.142 0.004

0.172 0.020

0.515 0.042

0.637 0.002

1.377 nd

1.836 nd

Not detected.

3.4. Sulfur in the gas phase During coal pyrolysis, H2S was the main gas-S form and the amount of produced SO2 was about one-tenth of H2S (shown in Table 4), and the amount of both H2S and SO2 increased with temperature rising. Other sulfur forms such as COS, CH4S, were not detected. In the case of coal SCW gasification, SO2 was nearly undetected, and the amount of H2S produced from coal SCW gasification was about 4–10 times higher than that from coal pyrolysis at the same temperature. For instance, the amount of H2S under Z-coal pyrolysis was as low as 0.142 mg/g while it became 0.637 mg/g after SCW gasification at 350 °C. When temperature increased from 350 to 550 °C, the amount of H2S after Z-coal SCW gasification increased gradually. Similar trend was also reported by other research groups when they investigated Datong coal, oil shale and bitumen treated via SCW [22,48,49]. The high H2S content in SCW was not only accelerated via H2 reacted with FeS2 (see Eq. (8)), but also produced from supercritical water reacted with Fe1xS/FeS through Eq. (9). But there was an interesting phenomenon for W-coal. The amount of H2S increased from 350 to 450 °C but it decreased at 550 °C. Correspondingly, the amount of S-o in W-coal after SCW gasification raised obviously at 550 °C (shown in Fig. 5b). The result indicated that more H2S reacted with the char of W-coal to form S-o which caused the amount of H2S decreased in 550-SW.

During coal pyrolysis, the SO2 might derive from organic sulfur. Instable organic sulfur began to release as early as 200 °C before H2 releasing. Without enough H2, active sulfur preferred to reacting with oxygen containing groups and released as SO2 [27,29]. Therefore, the low-rank Z-coal which contents higher oxygenated volatile would produce more SO2 than W-coal. However, in the SCW system, there were enough H radicals to react with sulfur radicals to produce H2S, which inhibited the production of SO2. Furthermore, the SO2 might also dissolve into supercritical water to form SO2 3 . In consequence, SO2 was hardly detected during coal SCW gasification. d

3.5. The pathway of sulfur transformation in coal environment during SCW gasification The transformation of sulfur in coal could been concluded to a two-step mechanism [24,50]. As shown in Fig. 10, in coal pyrolysis, the first step was the dissociation of the sulfur bond from pyrite or unstable organics, and the generation of SH radicals. Then, the second step was the further reaction deriving from those sulfur radicals. In presence of H2 or H radicals, the sulfur radicals could be easily removed as H2S. With OR radicals provide by organic in coal matrix, a small amount of sulfur radicals could combine to generate SO2 or sulfone. In absence of the above radicals, sulfur radicals could combine with carbon atoms in the activated char to form

Fig. 10. Sulfur transformation in coal: SCW gasification vs pyrolysis.

d

d

d

N. Meng et al. / Fuel 186 (2016) 394–404

stable organic sulfur such as complex thiophene. These newformed sulfur species were even more difficult to be removed than the inherent organic sulfur [28,51]. However, supercritical water could offer sufficient H radicals which greatly improved the sulfur removal as H2S. The better supercritical solubility and H radicals also restrained the organic sulfur formation. The SCW system facilitated the decomposition of sulfate and pyritic minerals. Meanwhile, the OH radicals facilitated S2 oxidation both in supercritical fluid (adding anionic valence, such as sulfate and sulfite) and in char (forming some organic sulfur, such as sulfone). d

d

d

4. Conclusions The transformation of sulfur in coal during pyrolysis and SCW gasification were studied from 350 to 550 °C in a batch autoclave. Sulfur distribution of two types of coals in gas, liquid and solid phase were thoroughly investigated in SCW gasification compared with pyrolysis. It was found that the conversion rates of coal body, carbon in SCW were larger than in pyrolysis at the same temperature, and the maximum conversion rates of sulfur in SCW were 57.0% in W-coal and 65.4% in Z-coal at 450 °C, respectively. Because of the good solubility of SCW and the abundant H radicals, the ratio of S/C on the char surface after SCW gasification was not only lower than that on raw coal surface or pyrolysis samples, but also lower than that in char body. SO2 generated in coal pyrolysis experiments was relatively small and mainly derived from organic matter, whereas it was inhibited by H radicals in SCW gasification. H2S was the only gas-sulfur in SCW and its content was higher than that from pyrolysis. The sulfate and sulfite were the main liquid-sulfur contents during SCW gasification and were increased with temperature rising. The sulfide existed after coal SCW gasification and may be produced via H2S absorbing in water. Compared with pyrolysis in solid, the removal of sulfate-sulfur and pyritic-sulfur during SCW gasification were enhanced as temperature rising, whereas the forming of organic-sulfur was suppressed. Pure pyrite was stable under pyrolysis or SCW at 550 °C, while it transformed into pyrrhotite when blended with coal. The pyrrhotite further converted into magnetite during SCW gasification. Organic sulfur species were identified by XPS and thiol/thioether was decomposed after pyrolysis or SCW gasification. And the supercritical water also had the ability to restrict thiophene forming. Free radical mechanism could explain the sulfur transformation in coal pyrolysis and SCW gasification. Those knowledge of transformation properties provided important evidence for further environmental utilization and benefitted the industrial application of the coal SCW gasification process. d

d

Acknowledgment The work was supported by the National High-Tech Research and Development Program of China (2011AA05A201), National Natural Science Foundation of China (21576205), International Science & Technology Cooperation Program of China (2013DFG42680).

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