Inappropriateness of the Standard Method in Sulfur Form Analysis of

Sep 10, 2012 - The solid products of pyrite decomposition during coal pyrolysis under nitrogen atmosphere at different temperatures were investigated ...
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Inappropriateness of the Standard Method in Sulfur Form Analysis of Char from Coal Pyrolysis Jingchong Yan,†,‡ Zonqing Bai,†,* Huiling Zhao,†,‡ Jin Bai,† and Wen Li† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

ABSTRACT: The solid products of pyrite decomposition during coal pyrolysis under nitrogen atmosphere at different temperatures were investigated by X-ray diffraction (XRD), and the solubilities of the corresponding compounds in hydrochloric acid and nitric acid were examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The feasibility of using the standard method for sulfur forms analysis of coal to determine the sulfur forms in char was discussed. It is found that the standard method is unsuitable to analyze the sulfur forms of the char and big error is introduced during calculation of the organic sulfur in char. During pyrolysis, pyrite (FeS2) is converted to pyrrhotite (Fe1−xS) and/or ferrous sulfide (FeS), which dissolve in HCl solution and lead to an elevated value of organic sulfur determined by the standard method. A modified calculation method of sulfur forms in char is recommended. The organic sulfur content in pyrolysis char obtained by this method is much lower and more accurate than that obtained by the standard one. The modified method for the sulfur form analysis of char is superior to the standard one, especially for the chars obtained from coals with high pyrite content and pyrolyzed at high temperatures.

1. INTRODUCTION In recent years, with the widespread use of coal as fuels and chemical materials, the environmental pollution caused by coal combustion has raised more concern for the clean conversion and desulfurization of coal prior to utilization. To this end, researchers worldwide have focused on searching for simple, economic, and efficient desulfurization methods. Unfortunately, owning to the complexity of coal structure and composition, and also the variable structure and distribution of the sulfur compounds in coal, an effective and economically viable desulfurization method remains to be developed. As an intermediate stage in gasification, combustion, and liquefaction, pyrolysis is not only an important method for coal conversion but also an effective method for sulfur removal. Practice shows that the effectiveness of pyrolysis in desulfurization depends largely on the types of coal and the nature of sulfur compounds, as well as the conditions of pyrolysis process, including temperature, heating rate, atmospheres, pressure, residence time, mineral matters, and type of reactor. Over the years, various measures have been taken to reduce the sulfur content of coal pyrolysis char by changing the pyrolysis parameters such as increasing temperature,1 employing various atmospheres,2−5 demineralizing prior to pyrolysis,6,7 extending residence time, and adding various sulfur adsorbents8 or catalysts.9−12 Some of the measures are effective in enhancing the removal of sulfur from coal. A prerequisite for developing effective methods of desulfurization by coal pyrolysis is to accurately determine the sulfur forms and their contents in the feed coal and the produced char. Although much work has been done to reveal the different forms of sulfur in coal and to determine their contents, this is still a difficult problem for scientists and technologists of coal utilization. The wet chemical analysis method to determine sulfur forms in coal is based on the following assumptions.13 © 2012 American Chemical Society

Sulfur in coal comprises organic, pyritic, and sulfate sulfur. Sulfate sulfur and all the nonpyritic iron are dissolved in HCl but not the pyrite itself; pyrite dissolves in HNO3 but not in HCl, and organic sulfur does not dissolve in either acid. Once the sulfate and pyritic sulfur have been determined, all the remaining sulfur is defined as ‘organic sulfur’. This method is superior to other determination methods due to its low cost and briefness, and many national standard methods of sulfur determination of coal such as American Standard14 ASTM D2492 and Chinese Standard15 have been made on the basis of these assumptions. However, the standard method of sulfur determination is not without problems, and it has been criticized by scientists and technologists since the day it was established. The criticisms begin with the assumption that the sulfur in coal exists only in three classes: pyrite (FeS2), sulfate, and organic sulfur. Researchers16 have confirmed the existence of some metal sulfides, such as marcasite (FeS2), sphalerite (ZnS), and galena (PbS) other than pyrite (FeS2), and some iron−sulfur coordination compounds in coal. Obviously, these sulfur compounds are not in accord with the assumptions of the standard method and inevitably introduce errors in sulfur determination. The second criticism17 of the standard method is that the organic content is obtained by the difference between total sulfur and inorganic sulfur (sulfates and pyrite); thus, it is subjected to the accumulative errors in previous determinations of total sulfur, sulfate, and pyrite sulfur. More recent criticisms18 focused on the solubility of various sulfur functionality and sulfur-containing species (or sulfur compounds) during acid treatment. Laban19 raised doubt about the Received: June 21, 2012 Revised: August 30, 2012 Published: September 10, 2012 5837

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The proximate and ultimate analyses of the samples are shown in Table 1.

assumption that all of the ferrous sulfides extracted with nitric acid had the same stoichiometry as pyrite, and sulfur attributable to sulfide minerals other than pyrite would be unaccounted for in the calculation. Furthermore, there might be a contribution of iron from either the partial dissolution of iron silicate or iron oxide minerals, leading to an overestimation of pyritic sulfur. Manovic20 investigated the misleading effects of HCl treatment in the characterization of sulfur in coal by experiment and found that HCl leaching would cause partial dissolution of pyrite and degradation of organic sulfur, thus leading to the overestimation of sulfate sulfur in coal. Despite the validity of the criticisms of the standard methods for sulfur determination in coal, it is generally taken as an accurate method due to the reliability of the determination results in most instances. Moreover, as no universally accepted determination method of sulfur in char has been established, the standard method of sulfur determination of coal has always been used to quantify the different forms of sulfur in char. However, it is well accepted that sulfur compounds in coal undergo a series of changes and conversions during the pyrolysis process, and the sulfur forms are largely different from those in coal. For instance, the pyrite (FeS2) decomposes into various iron−sulfur compounds such as pyrrhotite (Fe1−xS) or ferrous sulfide (FeS) at different temperatures.13 Some sulfates such as ferrous sulfate (FeSO4) and ferric sulfate (Fe2(SO4)3) decompose into iron oxides at 600 °C and even lower temperatures.21,22 In the meantime, the sulfur-containing gases released from coal are captured by the mineral matter and coal matrix and form nascent inorganic and organic sulfur compounds.12 To the best of our knowledge, no work has been done so far to investigate the solubility of the various sulfur compounds of char in HCl and/or HNO3 solutions and to verify the applicability of the standard determination method of sulfur in coal in quantifying the various forms of sulfur in char. The aim of this work is to validate the feasibility of using the standard sulfur determination method of coal to analyze the various sulfur forms of the char from coal pyrolysis. As the content of sulfate and pyrite of coal is determined from the concentration of SO 4 2− in the HCl filtrate and the concentration of Fe3+ in the HNO3 filtrate, respectively, by the standard method, the chars were treated the same as coal by the standard method. The corresponding ion concentration in the filtrate was determined with inductively coupled plasmaatomic emission spectrometry (ICP-AES); then, the content of different forms of sulfur of the char were calculated from the ion concentration in the filtrate. The residue chars were subjected to X-ray diffraction (XRD) analysis to examine the effects of acid treatment on the mineral matters in the char, especially on the Fe−S compounds. Based on ICP and XRD results, as well as the sulfur forms obtained by the standard method, a modified method of sulfur determination of the char was recommended. With the comparison between the results of the modified method and the standard one, we can further evaluate the feasibility of using the standard sulfur determination method for coal in quantifying the sulfur forms in char.

Table 1. Proximate and Ultimate Analyses of YZ Coal (wt %)a proximate analysis (ad)

ultimate analysis (daf)

M

A

V

C

H

Ob

N

S

4.50

12.93

35.32

73.80

5.12

16.08

1.42

3.58

a M, moisture; A, ash; V, volatile; ad, air dry; daf, dry ash free. bby difference.

2.2. Temperature-Programmed Pyrolysis. The pyrolysis experiment was conducted with a horizontal fixed bed reactor. Figure 1 shows the schematic diagram of the pyrolysis equipment. The

Figure 1. Schematic flow diagram of experimental unit for coal pyrolysis. 1, gas cylinder; 2, reducing valve strainer; 3, mass flow controller; 4, temperature controller; 5, quartz tube reactor; 6, electricity furnace ; 7, cooler; 8, oil collector; 9, flue gas absorber. pyrolysis experiment was conducted under N2 atmosphere with flow rate of 120 mL/min. Coal samples (about 10 g)were loaded into the constant temperature zone at room temperature and heated to 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, and 1000 °C, respectively, at the rate of 5 °C/min. The liquid products, tar and water, were condensed and collected in a cold trap. The char was collected after the experiment when the sample was cooled to room temperature. 2.3. Acid Treatment of Coal and Char. All the chars obtained at different temperatures were treated with HCl and HNO3 with the same procedure as the standard method for sulfur forms determination of coal. That is, about 1.0 g of coal or char was boiled mildly with 50 mL 5 M HCl for 30 min; then, the mixture was filtered with slow qualitative filter paper and washed several times with hot distilled water until no Cl− ion was detected by aqueous silver nitrate (AgNO3). The filtrate was collected and concentrated to 200 mL. The residue chars after HCl treatment were denoted as YZ-500-HCl, YZ600-HCl, etc., and the corresponding filtrates from YZ coal and chars were denoted as YZ-HCl, 500-HCl, 600-HCl, etc. The concentrations of various ions in the filtrate were determined by ICP-AES, which is more accurate than the standard method. The residue chars were subjected to XRD analysis for identifying the remaining minerals presented in the samples after HCl treatment. Then, the residue char was boiled with 50 mL 2 M HNO3 for 30 minutes, filtered, and rinsed until no Fe3+ was detected by thiocyanate potassium (KSCN) solution. Similarly, the filtrate was concentrated to 200 mL and analyzed by ICP-AES, and the residue chars were analyzed by XRD. The residue chars were denoted as YZ-500-HCl-HNO3, YZ-600-HCl-HNO3, etc., and the corresponding filtrates were denoted as YZ-HCl-HNO3, 500HCl-HNO3, 600-HCl-HNO3, etc. 2.4. Characterization Methods. The analysis of sulfur forms in the coal and its char was conducted according to the standard method.15 Mineralogical analysis of the chars and the acid treated samples were performed by XRD (Rigaku MiniFlex II DESKTOP, Japan) using Cu Kα radiation (30 kV, 15 mA Kα1 = 0.15408 nm). The samples were ground to less than 74 μm and were scanned with a step size of 4° (2θ) /min over 10−90°.

2. EXPERIMENTAL SECTION 2.1. Coal Sample. The coal sample employed in this work is a high-sulfur subbituminous coal obtained from Yanzhou, Shandong province, China, which was denoted as YZ. The coal was crushed, dried, ground, and sieved. The particle size of the coal is 110−154 μm. 5838

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The concentrations of various ions in the filtrate were also analyzed by ICP-AES (Thermo-Fisher,Thermo Fisher Scientific Inc.).

(10.65 mg/g), whereas the concentration of Fe3+ in the HClHNO3 filtrate is just the opposite. On the other hand, the concentration of SO42− in the HCl-HNO3 filtrate of hightemperature (above 600 °C) chars (1.02−4.46 mg/g char) are much lower than that of YZ-500 char (41.98 mg/g char). The reasons for the discrepancies are that part or all of the FeS2 in the coal has decomposed into Fe1−xS and/or FeS in the char and the decomposition product dissolves in the HCl solution according to the first step of the standard methods for sulfate determination of coal. This can be confirmed by the XRD patterns of the chars and the corresponding acid washed residue chars in Figures 2−4.

3. RESULTS AND DISCUSSION 3.1. Feasibility of the Standard Method in Determining Sulfur Forms in Char. The sulfur form analyses of YZ coal and its chars obtained at different temperatures are conducted according to Chinese standard (GB/T 215-2003). The results are shown in Table 2. Table 2. Sulfur Forms Analyses of YZ Coal and Its Chars Using Standard Method (wt %)a sample

St,ad, %

Sp,ad, %

Ss,ad, %

So,adb, %

raw coal 500 °C char 600 °C char 700 °C char 800 °C char 900 °C char 1000 °C char

3.04 2.96 2.53 2.64 2.45 2.63 2.48

1.56 1.34 n.d. n.d. n.d. n.d. n.d.

0.40 0.35 0.32 0.16 0.12 0.14 0.08

1.08 1.27 2.21 2.48 2.33 2.49 2.40

a

St, total sulfur; Sp, pyrite sulfur; Ss, sulfate sulfur; So, organic sulfur; ad, air dry. bBy difference; n.d., not detected.

According to GB/T-215-2003, the content of sulfate sulfur is determined and calculated by the concentration of SO42− in the HCl filtrate and the content of pyritic sulfur is quantified by the concentration of Fe3+ in the HCl-HNO3 filtrate. This is based on the assumption that all the SO42− in the HCl washed filtrate derive from sulfate of coal/char and all the Fe3+ ions in the HCl-HNO3 filtrate derive from pyrite (FeS2). Thus, the content of FeS2 in the coal or char could be calculated by the Fe3+ concentration in the HCl-HNO3 filtrate and the stoichiometry of sulfur and iron in FeS2. We treated the YZ coal and its chars according to the standard method and determined the concentrations of various ions in the HCl and HCl-HNO3 filtrate, and the results are given in Table 3. The concentration of Fe3+ and SO42− vary among the coal sample and the chars obtained at different temperatures. As the YZ coal is rich in pyrite, which accounts for 51.3% of the total sulfur, the concentration of Fe3+ and SO42− are relatively higher than other ions in the filtrate. Interestingly, the concentration of Fe3+ in the HCl filtrate of chars obtained above 600 °C (23.88− 25.98 mg/g char) are much higher than that of YZ-500 char

Figure 2. XRD patterns for YZ chars.

Figure 2 shows the noticeable presence of quartz (SiO2) and different forms of inorganic sulfur compounds, including FeS2 and decomposition products including Fe1−xS and/or FeS. Obviously, only part of FeS2 is converted into Fe1−xS at 500 °C as there are some peaks of FeS2 remaining in the XRD pattern of the YZ-500 sample. It can be seen clearly that the pyrite completely decomposes into Fe1−xS and FeS above 600 °C and higher temperatures, these results are consistent with the reported literature.23,24 The decomposition of pyrite proceeds according to reactions 1 and 2:

Table 3. Ion Concentrations (mg/g Coal or Char) of Acid Washing Filtrate of YZ Coal and Its Chars by HCl and HNO3 sample

Al3+

Ca2+

Fe3+

K+

Mg2+

Na+

SO42‑

YZ-HCl YZ-HCl-HNO3 500-HCl 500-HCl-HNO3 600-HCl 600-HCl-HNO3 700-HCl 700-HCl-HNO3 800-HCl 800-HCl-HNO3 900-HCl 900-HCl-HNO3 1000-HCl 1000-HCl-HNO3

1.62 0.20 0.72 0.23 6.07 0.23 6.80 0.35 5.68 0.60 5.26 0.49 1.04 0.17

1.37 0.64 2.47 1.14 12.85 0.66 13.20 0.37 12.38 0.94 12.98 0.26 12.60 0.23

8.98 11.20 10.65 13.19 24.14 0.75 25.78 0.24 23.88 0.34 25.98 0.14 23.90 0.15

1.50 0.60 1.06 1.17 1.60 0.90 0.41 0.24 0.34 0.24 0.30 0.06 0.11 0.03

0.72 0.12 0.78 0.22 0.70 0.08 0.67 0.12 0.65 0.16 0.64 0.06 0.46 0.06

1.52 0.86 0.96 0.94 1.00 0.79 0.83 0.89 0.97 1.03 0.66 0.73 0.72 0.61

28.70 39.40 13.64 41.98 12.47 4.46 3.38 2.40 1.55 2.51 1.28 1.17 1.05 1.02

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literature,21,22 sulfates (FeSO4, Fe2(SO4)3, etc.) in the coal decompose into metal oxides and release sulfur-containing gases when heated during the pyrolysis process. This well explains the fact that lower concentration of SO42− is determined in the filtrate of high-temperature char than that of the YZ raw coal and YZ-500 char. By comparing Figure 3 with Figure 4, it is noticed that the peaks of FeS2 remaining in the YZ-500-HCl residue char disappear in the YZ-500-HCl-HNO3 sample, which indicates that the FeS2 remaining in the YZ-500-HCl residue char was dissolved in the HNO3 solution. This agrees with the assumption that FeS2 dissolves in HNO3 but not in HCl solution. The dissolution of FeS2 in HNO3 proceeds according to reaction 5: FeS2 + 4H+ + 5NO−3 → Fe3 + + 2SO4 2 − + 5NO + 2H 2O Figure 3. XRD patterns for HCl washed YZ chars.

This is also the reason that a higher concentration of is detected in the HCl-HNO 3 filtrate than that in the corresponding HCl filtrate of the raw coal and its lowtemperature char (500 °C). No peaks other than that of quartz (SiO2) are observed in Figure 4, indicating that the HNO3 solution has dissolved all the so-called ‘pyrite’ (Fe1−xS or FeS in essence) in the chars. However, a very low concentration of Fe3+ is detected in the HCl-HNO3 filtrate of high-temperature chars, as shown in Table 3, and such a low concentration of Fe3+ is below the detection limit of conventional determination methods such as titration and gravimetry. Therefore, these Fe− S compounds (Fe1−xS and FeS) in the char escape from determination by using the standard method. As a result, the content of organic sulfur obtained by the difference of total sulfur and inorganic sulfur (sum of sulfate and ‘pyrite’) would be overestimated indubitably. In other words, the standard method of sulfur determination of coal is inappropriate to quantify the sulfur forms of the char, especially for the chars obtained from pyrolysis of coals with high pyrite content. 3.2. Modification of the Standard Method in Analyzing Sulfur Forms in Char. Based on this discussion and consideration, we recommend a modified method of sulfur form analysis of char, which was proposed and employed by some researchers recently.25 The procedures are as follows: the char samples are first treated with dilute hot HCl solution and then dried after extraction of sulfates. During the treatment with HCl solution, the ‘pyrite’ decomposes and ‘pyrite sulfur’ is released as H2S. Only organic sulfur remains in the residue char, and then it is determined using the same method as the total sulfur. As the content of ‘pyrite’ sulfur can neither be quantified by the amount of the H2S released in the treatment due to the incomplete capture of H2S released from the system, nor can be calculated from the Fe3+ concentration in the HCl filtrate owing to the interference of other iron-containing species that may dissolve in HCl solution, it is more preferable to be indirectly determined by the difference between the total sulfur and the sum of sulfate and organic sulfur. Equation 6 shows the calculation method of ‘pyrite’ in the char:

Figure 4. XRD patterns for HCl-HNO3 washed YZ chars.

FeS2 → Fe1 − xS → FeS + Sn

(1)

nFeS2 → nFeS + Sn

(2)

These sulfur compounds (Fe1−xS and FeS) dissolve in HCl acid when the chars were treated with hot HCl solution, which was confirmed from the XRD patterns of the residue char after HCl treatment, as shown in Figure 3. The absence of Fe1−xS or FeS in the HCl treated chars indicates that these iron−sulfur compounds have completely dissolved in the HCl solution. The dissolution of these compounds in HCl proceeds according to reaction 3 and 4: FeS + 2H+ → H 2S + Fe 2 +

(3)

Fe1 − xS + 2H+ → H 2S + (1 − x)Fe 2 + 2+

(5)

SO42−

(4)

3+

As the Fe is readily oxidized into Fe by oxygen or HNO3 when boiled in the HCl or HNO3 solution, all the iron ions in the filtrate are in the form of Fe3+. Furthermore, the observation of H2S released during the process of HCl treatment of the char confirms the dissolution of Fe1−xS and FeS in HCl solution. This interprets the fact that very low concentration of Fe3+ is detected in the HCl-HNO3 filtrate of high-temperature char. Beyond that, according to the

Sp = St − (Ss + So)

(6)

where St, Ss, So, and Sp denote total sulfur, sulfate sulfur, organic sulfur, and pyrite sulfur in the char, respectively. Ss could be determined by the amount of SO42− in the HCl filtrate. This is a modified calculation method of sulfur forms determination in the char. The solubility problem of ‘pyrite’ 5840

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supposed to be more accurate than the indirect subtraction method. (c) The content of ‘pyrite’ in the char is not calculated from the Fe3+ in the HNO3 filtrate; thus, the interferences of iron silicates and iron oxide minerals are averted. (d) The content of the sulfate sulfur is calculated from the concentration of SO42− in the HCl filtrate, which is determined by ICP rather than titration with barium chloride (BaCl2), and the accuracy of instrumental analysis is generally considered to be higher than the chemical analysis. Therefore, the sulfate sulfur determined by the modified method is more accurate than the standard one. Due to these reasons, the content of ‘pyrite’ in the char obtained by subtraction of organic and sulfate sulfur from total sulfur would be more close to the real value than the standard method. Despite the superiority of the modified method over the standard method in determining different forms of sulfur in pyrolysis char, one cannot deny that it is still a challenging problem up to now. No method has been established and generally accepted due to the complexity of sulfur forms in char. The standard method of sulfur determination of coal is not suitable for determining different forms of sulfur in char, especially for the coal with high pyrite content. The modified method of sulfur analysis of char improves the accuracy to some extent compared with the standard one, and the results obtained using the modified method are more rational and approximate to the real content of different sulfur forms in char. However, more appropriate sulfur analysis method of char remains to be found so as to determine different sulfur forms with high accuracy and operability.

and accumulative errors in determining the content of organic sulfur could be averted, and thus, the results would be more accurate, especially for high-temperature chars. Manovic25 proposed the relationship of total sulfur content in char after devolatilization and determined the sulfur content of the chars using this modified method, and the results showed fairly good linearity between the experimental and calculated content of sulfur in char. This corroborates the rationality of the modified method in quantifying different sulfur forms in the char. We determined the sulfur forms in the char using the modified method, and the results were shown in Table 4. Table 4. Sulfur Forms Analysis of YZ Coal Pyrolysis Chars Using the Modified Method (wt %)

a

sample

St,ad, %

So,ad, %

Ss,ad, %

Sp,ada, %

500 °C char 600 °C char 700 °C char 800 °C char 900 °C char 1000 °C char

2.96 2.53 2.64 2.45 2.63 2.48

2.59 1.36 1.12 0.92 0.80 0.80

0.45 0.42 0.11 0.05 0.04 0.04

−0.08 0.75 1.41 1.48 1.79 1.64

By difference.

By comparing Table 2 with Table 4, it is noticed that the contents of the organic sulfur obtained by the modified method are much lower than those obtained by the standard method except the YZ-500, which shows the opposite results. In addition, Table 4 shows that the content of ‘pyrite’ in YZ-500 char obtained by the modified method is negative, which is paradoxical to the common sense that sulfur content of coal/ char is always higher than zero. The anomaly of sulfur determination of YZ-500 char is due to the difference of ‘pyrite’ between the YZ-500 char and other high-temperature (above 600 °C) chars. As mentioned, the YZ coal is rich in pyrite, and most of the pyrite remains unconverted at 500 °C, as confirmed from the XRD pattern in Figure 2. The pyrite remaining in the YZ-500 char does not dissolve in HCl solution; thus, it is determined as the organic sulfur of the char using the modified method. As a result, the content of the organic sulfur is highly elevated, and the ‘pyrite’ determined by the difference of total, and the sum of organic and sulfate sulfur is largely underestimated and gives a negative value. However, this is not the case for the chars obtained at relatively higher temperatures (above 600 °C). As shown in the XRD patterns of the high-temperature chars, the ‘pyrite’ in these chars is in the form of Fe1−xS and/or FeS, which could be completely dissolved in dilute HCl solution, and thus avoid being determined as organic sulfur of the char. After the HCl treatment of these high-temperature chars, almost all the remaining sulfur in the residue char is organic sulfur, according to the reasons aforementioned. In other words, the modified method is more appropriate in analyzing high-temperature (above 600 °C) chars rather than low-temperature (below 600 °C) char due to the incomplete decomposition of FeS2 at low temperatures. To sum up, the modified method of sulfur determination of char is superior to the standard method for the following reasons: (a) It takes the difference of sulfur forms between coal and char into account and thus avoids the solubility problems of ‘pyrite’ in the acid treatment. (b) The content of organic sulfur is directly determined by Eschka method, as that of the total sulfur after removal of the inorganic sulfur, which is

4. CONCLUSIONS The chars obtained from YZ coal pyrolyzed under N2 atmosphere at different temperatures were treated with HCl and HNO3 according to the standard method of sulfur determination of coal. The ion concentrations in the acid filtrate were determined by ICP, and the residue chars were analyzed with XRD. The main conclusions could be drawn as follows: The standard method of sulfur determination of coal is not quite suitable for determination of various forms of sulfur in char. The ‘pyrite’ in the char is mainly FeS2, Fe1−xS, and/or FeS, depending largely on the pyrolysis temperature. The content of organic sulfur in the char obtained by the standard method is highly elevated, and thus, the ‘pyrite’ in the char is largely underestimated. The content of organic sulfur in char should be quantified by direct determination rather than by difference. The ‘pyrite sulfur’ in the char, on the other hand, is more appropriate to be determined by the difference of total sulfur and the sum of organic and sulfate one. Taking into account the differences in sulfur forms between coal and char, a new method that is suitable for the sulfur forms analysis in char remains to be found so as to determine their content conveniently and accurately, especially for the char obtained from coals with high pyrite content.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-351-4048967. Fax: +86-351-4050320. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5841

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ACKNOWLEDGMENTS This work was financially supported by National Basic Research Program of China (2011CB201401) and Strategic Guide Project of Coal Utilization of Chinese Academy of Sciences (XDA07060100). The authors also give sincere thanks to Ms. Dongyan Liu for her assistance in ICP analysis and testing.

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