- Email: [email protected]
Thermal and chemical stabilities of arsenic in three Chinese coals
Fuel Processing Technology 85 (2004) 903 – 912 www.elsevier.com/locate/fuproc
Thermal and chemical stabilities of arsenic in three Chinese coals Ruixia Guo, Jianli Yang, Zhenyu Liu * State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China Received 20 May 2003; received in revised form 10 October 2003; accepted 1 November 2003
Abstract The forms and pyrolysis behavior of arsenic in three Chinese coals was studied. The arsenic in the coals and the coal-derived pyrolysis chars were characterized into five forms using a sequential extraction method where a series of solutions with increasing acidity and oxidizability were used. The five arsenic forms are named as ion exchangeable, bound to carbonates, bound to Fe – Mn oxides, bound to organic matter and remained in residue. XRD, SEM-EDX and TOF-SIMS techniques were also used to characterize the coals and the chars. Results show that bleeding ratio of arsenic in the three coals increases with increasing pyrolysis temperature. The type of coal and the arsenic distribution in the coal have significant effect on the pyrolysis behavior of arsenic. The different volatility of arsenic was attributed to its chemical stability and thermal stability. Ion exchangeable arsenic and the arsenic bound to carbonates are not present in the three coals. Other arsenic forms undergo transformation during the pyrolysis, from the solid phase to the vapor phase and from one chemical form to other chemical forms. The main arsenic remained in the residue may include resistant sulfides. D 2004 Elsevier B.V. All rights reserved. Keywords: Arsenic in coal; Pyrolysis; Extraction; Stability
1. Introduction Arsenic is present in coals in ppm order. Its emission from coal combustion, however, results in severe environmental pollution, human health and equipment corrosion problems [1 – 3]. Although there are currently no regulations for arsenic emission from coal combustion, Title III of 1990 Clean Air Act Amendments (CAAA) of USA specifies that * Corresponding author. Tel.: +86-351-4134410; fax: +86-351-4048571. E-mail address: [email protected] (Z. Liu). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.11.032
904
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
arsenic and 10 other metallic species will be controlled to maximum extent technically possible [4]. Pyrolysis has been shown to be promising for desulfurization and denitrification of coal to make clean fuels [5,6]. During the pyrolysis, arsenic in coal will inevitably transform. In this regard, proper control of pyrolysis conditions may result in simultaneous removal of arsenic along with sulfur and nitrogen. Therefore, it is desirable to understand the chemical and thermal stability of arsenic in coals. There have been some reports on the release behavior of arsenic during pyrolysis [7– 9]. The volatility of arsenic was found to vary with coals and pyrolysis conditions. Our early studies on trace elements in Datong coal showed that As, Pb, Cd, Cr and Mn undergo different transformation in pyrolysis [10,11]. The available literatures suggest that the chemical and thermal stabilities of arsenic in coal are related to its forms in coal and the mineral matters and sulfur inherited in coal. The aim of this paper is to explore chemical and thermal stability of arsenic in three Chinese coals. Arsenic was separated into five occurrences by a sequential chemical extraction procedure using a series of solutions of different acidity and oxidizability, from weak to strong. The mineralogical composition of coals and coal-derived pyrolysis chars was characterized by XRD, SEM-EDX and TOF-SIMS. The effects of mineral matters and sulfur in the coals were also discussed.
2. Experimental 2.1. Coal sample Three Chinese coals, Yunnan (YN), Yima (YM) and Datong (DT), were used in this study. The coals were crushed and sieved to 0.16 –0.27 mm and dried prior to use. Proximate and ultimate analyses of the coals are shown in Table 1. The contents of arsenic and major mineral elements in the coals are shown in Table 2. 2.2. Pyrolysis The pyrolysis experiments were conducted in a temperature programmed quartz tube (300 mm id) reactor under a nitrogen stream. The reactor was heated from room temperature to a desired final temperature (300, 500, 700, 800, 900 and 1000 jC) at a
Table 1 Proximate and ultimate analysis of coals Coal sample
DT YM YN
Proximate analysis, ad
Ultimate analysis, ad
V
A
M
C
H
N
S
Oa
26.91 32.71 36.78
10.00 19.93 11.54
0.26 1.80 6.84
79.13 62.76 63.04
4.64 3.38 4.31
0.94 0.67 1.14
1.47 1.84 6.06
3.56 9.62 7.07
ad: air dry. a By difference.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
905
Table 2 Content of arsenic and major mineral elements in coals Coal sample
Asa
Major mineral elementsb Si
Al
Fe
Ca
Mg
DT YM YN
7.9 40.3 350.2
3.09 4.35 2.11
1.04 1.87 1.18
1.20 1.81 2.36
0.14 0.80 0.32
0.04 0.24 0.10
a b
Ag/g coal. g/100 g coal.
rate of 20 jC/min. When the final temperature was reached, the reactor was pulled out from the furnace and cooled down quickly under a nitrogen stream with an electric fan. 2.3. Sample analyses Arsenic in coals and coal-derived chars were extracted and separated into five chemical forms by a sequential extraction procedure as described in the literature [10,12,13]. Through the sequential extraction steps, elements associated with various parts of the coal were removed in the following order: (1) ion exchangeable arsenic compounds determined by dissolution in a sodium acetate solution (pH = 8.5), including water soluble arsenic oxides, arsenic associated with ion exchange sites in organic and inorganic matters; (2) arsenic bound to carbonates determined by dissolution in a sodium acetate – acetic acid solution (pH = 5), including arsenic primarily associated with carbonates; (3) arsenic bound to Fe – Mn oxides determined by dissolution in a hydroxylamine hydrochloride – acetic acid solution, including arsenic associated with Fe/Mn oxide compound, wellcrystallized sulfides or organosulfur compounds; (4) arsenic bound to organic matters determined by dissolution in a hydrogen peroxide/nitric acid – ammonium acetate solution (PH = 2), including arsenic associated with paraffin-like materials and resistant structural (nonhumified) organic matters; and (5) arsenic remained in the residue after the preceding extractions, determined by dissolving the ashed sample in HF –HClO4 solution, including arsenic associated with detrital silicate, aluminosilicate minerals, resistant sulfur compounds, and refractory organic materials. It should be pointed out that the acidity and oxidizability of the extraction solution increases with the extraction order. Arsenic content in the samples was quantified by inductively coupled plasma-atomic spectroscopy (Atomscan16 ICP-AES, TJA company of USA). The detection limit of the instrument is 1.8 Ag/l and the relative standard deviation of the measurement is less than 3%. The thermal stability of the arsenic during pyrolysis was evaluated by bleeding ratio (BR), which is defined as: BR% ¼
As content in coal As content in char char yield 100 As content in coal
In addition, the mineralogical composition in the coals and the coal-derived chars was analyzed by a Rigaku D/max 2500 X-ray diffraction spectrometer (XRD) and a JSM-6301 F scanning electron microscope equipped with an Oxford Link-300 energy dispersive X-
906
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
ray detector (SEM-EDX). In order to detect trace amount of arsenic, Trift II time-of-flight secondary ion mass spectrometer (TOF-SIMS) was also used.
3. Results and discussions 3.1. Thermal stability of arsenic in the coals Release behavior of arsenic during pyrolysis of the three coals is shown in Fig. 1. Generally, bleeding ratio (BR) increases with increasing pyrolysis temperature, especially in the temperature range of 500 –700 jC (Fig. 1a). Temperatures higher than 700 jC show small effect on further release of arsenic. This agrees with the fact that boiling points of evaporable arsenic compounds are usually below 800 jC [14]. It is interesting to note that the increase in BR in the temperature range of 300 – 1000 jC for the three coals are very similar, about 15%, although there is already a BR value of 30% for DT coal at
Fig. 1. BR of As in chars vs. pyrolysis temperature (a) and volatile yield (b).
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
907
temperatures below 300 jC. This behavior seems to suggest that the three coals contain common arsenic compounds in similar distribution. It is important to note that although the BR of arsenic for DT coal is greater than that for YN and YM coals, the amounts of arsenic released into the volatile phase from YN and YM coals are much more than that from DT coal, 60.9 and 7.5 Ag As/g coal from YN and YM coals vs. 3.9 Ag As/g coal from DT coal at 1000 jC due to the much higher arsenic contents of YN and YM coals. Fig. 1b shows the relationship between BR of arsenic and the volatile yields. The points located in the area above the diagonal line represent chars with arsenic content lower than that of the coal, and the points located in the area below the diagonal line represent chars with arsenic content higher than that of the coal. Clearly, the chars derived from YM and YN coals are enriched with arsenic in the temperature range studied. This implies that, unlike DT char, low arsenic YM and YN chars cannot be made through pyrolysis under N2 atmosphere. These differences in thermal stability of arsenic may be related to the occurrence of arsenic, mineral matters and sulfur compounds in the coals. 3.2. Chemical stability and occurrence of arsenic in the coals To evaluate chemical stability and occurrence of arsenic in the coals, sequential extractions were used to separate the arsenic in the coals into five forms as stated in Section 2.3. The results in Fig. 2 show that the ion exchangeable arsenic and arsenic bound to carbonates are not present in the coals, the contents of arsenics bound to Fe– Mn oxides and bound to organic matters are in the range of 10 –30%, and the residue is the most dominant form of arsenic, from 40% to 70%. It can be seen that more arsenic can be extracted from YN coal than from DT and YM coals by the extraction steps 1 –4 (58% vs. 38% and 28%). Since the less acidity and oxidizability solutions in steps 1– 4 were used, the arsenic in YN coal can be classified as chemically less stable compared to that in DT and YM coals. It is important to note that arsenic sulfides, which have been reported to be a main arsenic form in coals, are categorized in the form of bound to Fe –Mn oxides as well as remained in the residue as indicated in Section 2.3. 3.3. Thermal stability of arsenic of different forms To examine thermal stability of arsenic in different chemical occurrences, the chars obtained at 1000 jC were subjected to the sequential extraction, and the results are shown in the right side of Fig. 2. The amounts of arsenic released into volatile phase are also shown. Similar to the coals, ion exchangeable arsenic is not found in all the chars. Small amount of arsenic bound to carbonates is found in DT and YM chars although it is not found in the coals. The decreases of arsenic in the forms of bound to organic matter and bound to Fe –Mn oxides suggest that these arsenic occurrences are usually unstable during the pyrolysis. It is important to note that the most abundant arsenic form in the chars is sill remained in residue. But compared to the arsenic distribution in the coals, the amount of residue-arsenic (arsenic remained in residue) in DT and YM chars decrease, from
908
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
Fig. 2. Distribution of As in DT, YM and YN coals and corresponding chars (1000 jC). Ion exchangeables; Bound to carbonates; Bound to Fe – Mn oxides; Bound to organic matter; Arsenic in the residue; Arsenic released into volatile phase.
61% to 39% for DT coal and from 71% to 57% for YM coal, this indicates that some of the arsenic remained in residue is transformable into the vapor phase during the pyrolysis. However, this behavior is not found for YN coal, which shows an increase in the residue arsenic content, from 41% to 79%. These contradictive phenomena suggest that the transformation of arsenic are very complex, possibly due to effects of coal constituents and the broad classification made by the sequential extraction. It is likely that the difference is partially related to the sulfur contents of the coals, because YN coal contains much more sulfur than DT and YM coals, and the arsenic in residue includes some form of the arsenic sulfides.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
Fig. 3. XRD patterns of DT, YM and YN coals. Q: quartz; C: calcite; A: arsenopyrite; P: pyrite.
909
910
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
3.4. Effect of mineral matters and sulfur in coals on stability of arsenic The mineral contents of the three coals are given in Tables 1 and 2. The difference is significant. Compared to DT and YM coals, YN coal contains much more arsenic and sulfur. To further understand the occurrence of minerals, the coals were characterized using XRD, SEM-EDX and TOF-SIMS techniques. The XRD analyses in Fig. 3 show that the main mineral components in DT and YM coals are quartz and silicates, but in YN coal are pyrite and arsenopyrite, which are consistent with the data in Tables 1 and 2 and may explain the low volatility of arsenic and high residue-arsenic content in YN char. SEM-EDX analyses in Fig. 4 indicate that the YN char after the extraction steps 1– 4, containing only residue-arsenic, contains a large fraction of sulfur on its surface. This may imply that the high residue-arsenic content in the char is due to association of arsenic with
Fig. 4. SEM-EDX results of YN char after the extraction steps 1 – 4.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
911
Fig. 5. TOF-SIMS spectrum of YN char after the extraction steps 1 – 4.
sulfur-bearing compounds. Unfortunately, however, arsenic in this sample was not detected by the technique because of its low concentration. TOF-SIMS was then used for further analysis of the same sample. Fig. 5 shows the presence of As and S as well as Fe, Si, Al, Na and K. These again imply that the residue-arsenic in YN char is possibly associated with resistant sulfur compound.
4. Conclusions Thermal and chemical stabilities of arsenic in three coals are different. Generally, bleeding ratio (BR) of arsenic increases with increasing pyrolysis temperature, especially in the temperature range of 500 – 700 jC. The incremental BRs in the temperature range of
912
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
300– 1000 jC for the three coals are similar, all about 15%. Some of arsenic in coal, 30% in DT coal, is very volatile, which vaporizes below 300 jC. YM and DT coals contain more chemical resistant arsenic than YN coal does. Some arsenic in YN coal is likely associated with pyrite. Ion exchangeable arsenic and arsenic bound to carbonates are not present in the coals. Arsenics bound to organic matters and bound to Fe –Mn oxides are generally thermal unstable. Remained in residue is the main arsenic form in the coals and the chars. Some of the residue-arsenics are transformable in pyrolysis, but some of them are stable due to its association with resistant sulfides.
Acknowledgements The authors gratefully acknowledge the financial support from the Special Funds for Major State Basic Research Project (G19990221), the Chinese Academy of Sciences, and the Natural Science Foundation of China (90210034). Ms. Dongyan Liu and Dr. Baohua Li are also gratefully acknowledged for carrying out ICP-AES and SEM-EDX analyses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
U.S. Environmental Protection Agency, Report to Congress, February 25, 1998. X. Querol, J.L. Fernandea-Turiel, A. Lopez-Soler, Fuel 74 (3) (1995) 331. D.G. Coles, R. Raganil, J. Ondov, G.L. Fisher, Environ. Sci. Technol. 13 (1979) 455. M. Wang, T.C. Keener, S.J. Khang, Fuel Process. Technol. 67 (2000) 147. L. Lin, S.J. Khang, T.C. Keener, Fuel Process. Technol. 53 (1998) 15. T.L. Yeh, T.C. Keener, S.J. Khang, R.G. Jenkins, Fuel Process. Technol. 33 (1993) 33. E. Zajusz-Zubek, J. Konieczynski, Fuel 82 (10) (2003) 1281. Y.H. Wang, H.B. Li, H.T. Huang, Y. Chen, Coal Convers. 25 (3) (2002) 37. J. Wang, A. Takaya, A. Tomita, ACS Div. Fuel Chem. Prepr. 48 (1) (2003) 351. R. Guo, J. Yang, D. Liu and Z. Liu, J. Anal. Appl. Pyrolysis, in press. R. Guo, J. Yang, B. Li, D. Liu, Z. Liu, ACS Div. Fuel Chem. Prepr. 48 (1) (2003) 356. A. Tessler, P.G.C. Campbell, M. Bisson, Anal. Chem. 51 (7) (1979) 844. F. Zhao, PhD thesis, in Chinese with English abstract. Beijing graduate School, China University of Mining and Technology, 1997, p. 63. [14] R. Guo, J. Yang, D. Liu, Z. Liu, ACS Div. Fuel Chem. Prepr. 47 (1) (2002) 201.
Thermal and chemical stabilities of arsenic in three Chinese coals Ruixia Guo, Jianli Yang, Zhenyu Liu * State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China Received 20 May 2003; received in revised form 10 October 2003; accepted 1 November 2003
Abstract The forms and pyrolysis behavior of arsenic in three Chinese coals was studied. The arsenic in the coals and the coal-derived pyrolysis chars were characterized into five forms using a sequential extraction method where a series of solutions with increasing acidity and oxidizability were used. The five arsenic forms are named as ion exchangeable, bound to carbonates, bound to Fe – Mn oxides, bound to organic matter and remained in residue. XRD, SEM-EDX and TOF-SIMS techniques were also used to characterize the coals and the chars. Results show that bleeding ratio of arsenic in the three coals increases with increasing pyrolysis temperature. The type of coal and the arsenic distribution in the coal have significant effect on the pyrolysis behavior of arsenic. The different volatility of arsenic was attributed to its chemical stability and thermal stability. Ion exchangeable arsenic and the arsenic bound to carbonates are not present in the three coals. Other arsenic forms undergo transformation during the pyrolysis, from the solid phase to the vapor phase and from one chemical form to other chemical forms. The main arsenic remained in the residue may include resistant sulfides. D 2004 Elsevier B.V. All rights reserved. Keywords: Arsenic in coal; Pyrolysis; Extraction; Stability
1. Introduction Arsenic is present in coals in ppm order. Its emission from coal combustion, however, results in severe environmental pollution, human health and equipment corrosion problems [1 – 3]. Although there are currently no regulations for arsenic emission from coal combustion, Title III of 1990 Clean Air Act Amendments (CAAA) of USA specifies that * Corresponding author. Tel.: +86-351-4134410; fax: +86-351-4048571. E-mail address: [email protected] (Z. Liu). 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2003.11.032
904
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
arsenic and 10 other metallic species will be controlled to maximum extent technically possible [4]. Pyrolysis has been shown to be promising for desulfurization and denitrification of coal to make clean fuels [5,6]. During the pyrolysis, arsenic in coal will inevitably transform. In this regard, proper control of pyrolysis conditions may result in simultaneous removal of arsenic along with sulfur and nitrogen. Therefore, it is desirable to understand the chemical and thermal stability of arsenic in coals. There have been some reports on the release behavior of arsenic during pyrolysis [7– 9]. The volatility of arsenic was found to vary with coals and pyrolysis conditions. Our early studies on trace elements in Datong coal showed that As, Pb, Cd, Cr and Mn undergo different transformation in pyrolysis [10,11]. The available literatures suggest that the chemical and thermal stabilities of arsenic in coal are related to its forms in coal and the mineral matters and sulfur inherited in coal. The aim of this paper is to explore chemical and thermal stability of arsenic in three Chinese coals. Arsenic was separated into five occurrences by a sequential chemical extraction procedure using a series of solutions of different acidity and oxidizability, from weak to strong. The mineralogical composition of coals and coal-derived pyrolysis chars was characterized by XRD, SEM-EDX and TOF-SIMS. The effects of mineral matters and sulfur in the coals were also discussed.
2. Experimental 2.1. Coal sample Three Chinese coals, Yunnan (YN), Yima (YM) and Datong (DT), were used in this study. The coals were crushed and sieved to 0.16 –0.27 mm and dried prior to use. Proximate and ultimate analyses of the coals are shown in Table 1. The contents of arsenic and major mineral elements in the coals are shown in Table 2. 2.2. Pyrolysis The pyrolysis experiments were conducted in a temperature programmed quartz tube (300 mm id) reactor under a nitrogen stream. The reactor was heated from room temperature to a desired final temperature (300, 500, 700, 800, 900 and 1000 jC) at a
Table 1 Proximate and ultimate analysis of coals Coal sample
DT YM YN
Proximate analysis, ad
Ultimate analysis, ad
V
A
M
C
H
N
S
Oa
26.91 32.71 36.78
10.00 19.93 11.54
0.26 1.80 6.84
79.13 62.76 63.04
4.64 3.38 4.31
0.94 0.67 1.14
1.47 1.84 6.06
3.56 9.62 7.07
ad: air dry. a By difference.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
905
Table 2 Content of arsenic and major mineral elements in coals Coal sample
Asa
Major mineral elementsb Si
Al
Fe
Ca
Mg
DT YM YN
7.9 40.3 350.2
3.09 4.35 2.11
1.04 1.87 1.18
1.20 1.81 2.36
0.14 0.80 0.32
0.04 0.24 0.10
a b
Ag/g coal. g/100 g coal.
rate of 20 jC/min. When the final temperature was reached, the reactor was pulled out from the furnace and cooled down quickly under a nitrogen stream with an electric fan. 2.3. Sample analyses Arsenic in coals and coal-derived chars were extracted and separated into five chemical forms by a sequential extraction procedure as described in the literature [10,12,13]. Through the sequential extraction steps, elements associated with various parts of the coal were removed in the following order: (1) ion exchangeable arsenic compounds determined by dissolution in a sodium acetate solution (pH = 8.5), including water soluble arsenic oxides, arsenic associated with ion exchange sites in organic and inorganic matters; (2) arsenic bound to carbonates determined by dissolution in a sodium acetate – acetic acid solution (pH = 5), including arsenic primarily associated with carbonates; (3) arsenic bound to Fe – Mn oxides determined by dissolution in a hydroxylamine hydrochloride – acetic acid solution, including arsenic associated with Fe/Mn oxide compound, wellcrystallized sulfides or organosulfur compounds; (4) arsenic bound to organic matters determined by dissolution in a hydrogen peroxide/nitric acid – ammonium acetate solution (PH = 2), including arsenic associated with paraffin-like materials and resistant structural (nonhumified) organic matters; and (5) arsenic remained in the residue after the preceding extractions, determined by dissolving the ashed sample in HF –HClO4 solution, including arsenic associated with detrital silicate, aluminosilicate minerals, resistant sulfur compounds, and refractory organic materials. It should be pointed out that the acidity and oxidizability of the extraction solution increases with the extraction order. Arsenic content in the samples was quantified by inductively coupled plasma-atomic spectroscopy (Atomscan16 ICP-AES, TJA company of USA). The detection limit of the instrument is 1.8 Ag/l and the relative standard deviation of the measurement is less than 3%. The thermal stability of the arsenic during pyrolysis was evaluated by bleeding ratio (BR), which is defined as: BR% ¼
As content in coal As content in char char yield 100 As content in coal
In addition, the mineralogical composition in the coals and the coal-derived chars was analyzed by a Rigaku D/max 2500 X-ray diffraction spectrometer (XRD) and a JSM-6301 F scanning electron microscope equipped with an Oxford Link-300 energy dispersive X-
906
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
ray detector (SEM-EDX). In order to detect trace amount of arsenic, Trift II time-of-flight secondary ion mass spectrometer (TOF-SIMS) was also used.
3. Results and discussions 3.1. Thermal stability of arsenic in the coals Release behavior of arsenic during pyrolysis of the three coals is shown in Fig. 1. Generally, bleeding ratio (BR) increases with increasing pyrolysis temperature, especially in the temperature range of 500 –700 jC (Fig. 1a). Temperatures higher than 700 jC show small effect on further release of arsenic. This agrees with the fact that boiling points of evaporable arsenic compounds are usually below 800 jC [14]. It is interesting to note that the increase in BR in the temperature range of 300 – 1000 jC for the three coals are very similar, about 15%, although there is already a BR value of 30% for DT coal at
Fig. 1. BR of As in chars vs. pyrolysis temperature (a) and volatile yield (b).
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
907
temperatures below 300 jC. This behavior seems to suggest that the three coals contain common arsenic compounds in similar distribution. It is important to note that although the BR of arsenic for DT coal is greater than that for YN and YM coals, the amounts of arsenic released into the volatile phase from YN and YM coals are much more than that from DT coal, 60.9 and 7.5 Ag As/g coal from YN and YM coals vs. 3.9 Ag As/g coal from DT coal at 1000 jC due to the much higher arsenic contents of YN and YM coals. Fig. 1b shows the relationship between BR of arsenic and the volatile yields. The points located in the area above the diagonal line represent chars with arsenic content lower than that of the coal, and the points located in the area below the diagonal line represent chars with arsenic content higher than that of the coal. Clearly, the chars derived from YM and YN coals are enriched with arsenic in the temperature range studied. This implies that, unlike DT char, low arsenic YM and YN chars cannot be made through pyrolysis under N2 atmosphere. These differences in thermal stability of arsenic may be related to the occurrence of arsenic, mineral matters and sulfur compounds in the coals. 3.2. Chemical stability and occurrence of arsenic in the coals To evaluate chemical stability and occurrence of arsenic in the coals, sequential extractions were used to separate the arsenic in the coals into five forms as stated in Section 2.3. The results in Fig. 2 show that the ion exchangeable arsenic and arsenic bound to carbonates are not present in the coals, the contents of arsenics bound to Fe– Mn oxides and bound to organic matters are in the range of 10 –30%, and the residue is the most dominant form of arsenic, from 40% to 70%. It can be seen that more arsenic can be extracted from YN coal than from DT and YM coals by the extraction steps 1 –4 (58% vs. 38% and 28%). Since the less acidity and oxidizability solutions in steps 1– 4 were used, the arsenic in YN coal can be classified as chemically less stable compared to that in DT and YM coals. It is important to note that arsenic sulfides, which have been reported to be a main arsenic form in coals, are categorized in the form of bound to Fe –Mn oxides as well as remained in the residue as indicated in Section 2.3. 3.3. Thermal stability of arsenic of different forms To examine thermal stability of arsenic in different chemical occurrences, the chars obtained at 1000 jC were subjected to the sequential extraction, and the results are shown in the right side of Fig. 2. The amounts of arsenic released into volatile phase are also shown. Similar to the coals, ion exchangeable arsenic is not found in all the chars. Small amount of arsenic bound to carbonates is found in DT and YM chars although it is not found in the coals. The decreases of arsenic in the forms of bound to organic matter and bound to Fe –Mn oxides suggest that these arsenic occurrences are usually unstable during the pyrolysis. It is important to note that the most abundant arsenic form in the chars is sill remained in residue. But compared to the arsenic distribution in the coals, the amount of residue-arsenic (arsenic remained in residue) in DT and YM chars decrease, from
908
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
Fig. 2. Distribution of As in DT, YM and YN coals and corresponding chars (1000 jC). Ion exchangeables; Bound to carbonates; Bound to Fe – Mn oxides; Bound to organic matter; Arsenic in the residue; Arsenic released into volatile phase.
61% to 39% for DT coal and from 71% to 57% for YM coal, this indicates that some of the arsenic remained in residue is transformable into the vapor phase during the pyrolysis. However, this behavior is not found for YN coal, which shows an increase in the residue arsenic content, from 41% to 79%. These contradictive phenomena suggest that the transformation of arsenic are very complex, possibly due to effects of coal constituents and the broad classification made by the sequential extraction. It is likely that the difference is partially related to the sulfur contents of the coals, because YN coal contains much more sulfur than DT and YM coals, and the arsenic in residue includes some form of the arsenic sulfides.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
Fig. 3. XRD patterns of DT, YM and YN coals. Q: quartz; C: calcite; A: arsenopyrite; P: pyrite.
909
910
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
3.4. Effect of mineral matters and sulfur in coals on stability of arsenic The mineral contents of the three coals are given in Tables 1 and 2. The difference is significant. Compared to DT and YM coals, YN coal contains much more arsenic and sulfur. To further understand the occurrence of minerals, the coals were characterized using XRD, SEM-EDX and TOF-SIMS techniques. The XRD analyses in Fig. 3 show that the main mineral components in DT and YM coals are quartz and silicates, but in YN coal are pyrite and arsenopyrite, which are consistent with the data in Tables 1 and 2 and may explain the low volatility of arsenic and high residue-arsenic content in YN char. SEM-EDX analyses in Fig. 4 indicate that the YN char after the extraction steps 1– 4, containing only residue-arsenic, contains a large fraction of sulfur on its surface. This may imply that the high residue-arsenic content in the char is due to association of arsenic with
Fig. 4. SEM-EDX results of YN char after the extraction steps 1 – 4.
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
911
Fig. 5. TOF-SIMS spectrum of YN char after the extraction steps 1 – 4.
sulfur-bearing compounds. Unfortunately, however, arsenic in this sample was not detected by the technique because of its low concentration. TOF-SIMS was then used for further analysis of the same sample. Fig. 5 shows the presence of As and S as well as Fe, Si, Al, Na and K. These again imply that the residue-arsenic in YN char is possibly associated with resistant sulfur compound.
4. Conclusions Thermal and chemical stabilities of arsenic in three coals are different. Generally, bleeding ratio (BR) of arsenic increases with increasing pyrolysis temperature, especially in the temperature range of 500 – 700 jC. The incremental BRs in the temperature range of
912
R. Guo et al. / Fuel Processing Technology 85 (2004) 903–912
300– 1000 jC for the three coals are similar, all about 15%. Some of arsenic in coal, 30% in DT coal, is very volatile, which vaporizes below 300 jC. YM and DT coals contain more chemical resistant arsenic than YN coal does. Some arsenic in YN coal is likely associated with pyrite. Ion exchangeable arsenic and arsenic bound to carbonates are not present in the coals. Arsenics bound to organic matters and bound to Fe –Mn oxides are generally thermal unstable. Remained in residue is the main arsenic form in the coals and the chars. Some of the residue-arsenics are transformable in pyrolysis, but some of them are stable due to its association with resistant sulfides.
Acknowledgements The authors gratefully acknowledge the financial support from the Special Funds for Major State Basic Research Project (G19990221), the Chinese Academy of Sciences, and the Natural Science Foundation of China (90210034). Ms. Dongyan Liu and Dr. Baohua Li are also gratefully acknowledged for carrying out ICP-AES and SEM-EDX analyses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
U.S. Environmental Protection Agency, Report to Congress, February 25, 1998. X. Querol, J.L. Fernandea-Turiel, A. Lopez-Soler, Fuel 74 (3) (1995) 331. D.G. Coles, R. Raganil, J. Ondov, G.L. Fisher, Environ. Sci. Technol. 13 (1979) 455. M. Wang, T.C. Keener, S.J. Khang, Fuel Process. Technol. 67 (2000) 147. L. Lin, S.J. Khang, T.C. Keener, Fuel Process. Technol. 53 (1998) 15. T.L. Yeh, T.C. Keener, S.J. Khang, R.G. Jenkins, Fuel Process. Technol. 33 (1993) 33. E. Zajusz-Zubek, J. Konieczynski, Fuel 82 (10) (2003) 1281. Y.H. Wang, H.B. Li, H.T. Huang, Y. Chen, Coal Convers. 25 (3) (2002) 37. J. Wang, A. Takaya, A. Tomita, ACS Div. Fuel Chem. Prepr. 48 (1) (2003) 351. R. Guo, J. Yang, D. Liu and Z. Liu, J. Anal. Appl. Pyrolysis, in press. R. Guo, J. Yang, B. Li, D. Liu, Z. Liu, ACS Div. Fuel Chem. Prepr. 48 (1) (2003) 356. A. Tessler, P.G.C. Campbell, M. Bisson, Anal. Chem. 51 (7) (1979) 844. F. Zhao, PhD thesis, in Chinese with English abstract. Beijing graduate School, China University of Mining and Technology, 1997, p. 63. [14] R. Guo, J. Yang, D. Liu, Z. Liu, ACS Div. Fuel Chem. Prepr. 47 (1) (2002) 201.