- Email: [email protected]
The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor
Fuel 84 (2005) 1874–1878 www.fuelfirst.com
The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor Wen Li*, Hailiang Lu, Haokan Chen, Baoqing Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China Received 15 October 2004; received in revised form 14 March 2005; accepted 15 March 2005 Available online 21 April 2005
Abstract The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO2-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 8C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 8C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 8C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 8C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 8C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 8C high volatility of Cl is obtained in both N2 and CO2 atmospheres. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal; Chlorine; Mode of occurrence; Fluidized-bed reactor; Volatility; Pyrolysis; Gasification
1. Introduction It is well known that there are many kinds of harmful trace elements in coal, which can cause environmental and technological problems during coal utilization. Chlorine is one of the harmful trace elements easily volatilized. A large amount of chloride is present in the gas phase in coal utilization process. Clean air legislation in many countries imposes emission limits on chlorine due to its environmental impact. Meanwhile, high concentration of chlorine in coal can create corrosion and deposition damage in equipment, which leads to a huge cost for solving the technological problems. Information about the modes of occurrence of chlorine in coal and its volatile behavior is important when measures are adopted to control the emission of chlorine during coal conversion. Because of the restriction of * Corresponding author. Tel.: C86 351 4044335; fax: C86 351 4050320. E-mail address: [email protected] (W. Li).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.03.017
determination method and coal complexity, general agreement has not been reached about the modes of occurrence of chlorine in coal. Several forms of chlorine in coal have been reported, such as covalently-bonded Cl, the Cl bound to organic cations in ion exchangeable form, and the Cl ions present in inorganic salts or in water within micropores of coal [1–4]. Literatures show that chlorine is mainly released as hydrogen chloride during heat-treatment of coal in an inert atmosphere or under combustion conditions [5,6]. However, the modes of occurrence of chlorine are much different from coal to coal. Most previous work [5,7–9] about the behavior of chlorine in coal during pyrolysis was performed in a fixed-bed reactor, i.e. in a programmed heating system. The volatilization behavior of chlorine in coal during fluidized bed pyrolysis, moreover, and in gasification is not clear. The purpose of this work is to examine the volatilization behavior of chlorine in coal during fluidized-bed pyrolysis and gasification, and to identify the modes of occurrence of chlorine in coal. In addition, the restraining efficiency of the added CaO on chlorine volatility was studied.
W. Li et al. / Fuel 84 (2005) 1874–1878
2. Experimental 2.1. Coal samples Three Chinese coal samples, Lu-an, Pingshuo and Yanzhou, were used in the experiments. They are selected because the chlorine content in them is high and the volatile matter content is different. The chlorine contents of Lu-an, Pingshou and Yanzhou coal are 369, 347 and 434 mg gK1 on a dry basis, respectively. And the volatile matter content (daf basis) is 15.85, 45.23 and 39.19%, respectively. The coal samples were crushed and sieved to 147–246 mm for use. The coal property is given in Table 1. 2.2. Pyrolysis and gasification tests Pyrolysis tests were performed in a quartz tube fluidizedbed reactor (length of 600 mm and inner diameter of 25 mm) under nitrogen at temperatures ranging from 300 to 900 8C for Lu-an and Pingshuo coal, and from 300 to 600 8C for Yanzhou coal. There is a porous quartz disk located at 360 mm from the top of the reactor to support the coal samples and as a gas distributor. At a predetermined temperature, about 3.0 g coal sample was quickly placed into the quartz tube reactor. The nitrogen flow was 500–1000 ml/min corresponding to the different temperatures. After duration for 30 min, the char sample was cooled down to room temperature by removing the quartz tube reactor from the tube furnace. The chars were collected, weighted and analyzed. To clarify the restraining efficiency of addition of calcium-based mineral matter on chlorine volatility, CaO was added in several runs during Lu-an pyrolysis with the molar ratio of Ca/S equal to 2.8. Gasification tests were carried out under CO2 atmosphere (300 ml/min) at temperature range from 800 to 950 8C. The other conditions are same as those in pyrolysis tests.
1875
forms of chlorine were separated in the following order: (1) water soluble inorganic chlorine: first the samples were dissolved into deionized water and stirred for 3 h, and then further extracted by dimethyl sulphoxide (DMSO) for 1 h; (2) chlorine bound to organic cations: the samples was extracted by 0.1 M KNO3 in 100% DMSO; (3) covalentlybonded organic chlorine: obtained by difference. The amount of chlorine from (2) and (3) is the whole organic chlorine in the sample. 2.4. Determination of chlorine and its volatility Chlorine contents in coal and char were determined by combustion-hydrolysis/potential titration method based on Chinese standard method (GB/T 3558-1996). The basic theory of this method is as follows: the sample (coal or char) is combusted and hydrolysed in a mixed stream of oxygen and steam during which chlorine in the sample is converted into chloride and dissolved quantitatively in water; the chlorine contents in condensate were determined by potential titration with silver nitrate. A standard sample with chlorine content of 570 mg gK1 was used to analyze the precision of the present method using which 565G 10 mg gK1 was obtained. This indicates that the combustion–hydrolysis/potential titration method is effective and exact in determining the chlorine content in the samples. Chlorine volatility during coal pyrolysis and gasification was calculated by the following equation: C m C Y V% Z 1 K 1;d 1;d 100 1 K 1;d 100 (1) C0;d C0;d m0;d where V is the volatility of chlorine, %; C1,d, the content of the chlorine in char, (g gK1; C0,d, the content of chlorine in coal, (g gK1; m1,d, the mass of char, g; m0,d, the mass of coal, g; and Y, the yield of char, %. The difference of chlorine volatility with and without CaO addition is defined as the restraining efficiency.
2.3. Sequential chemical extraction It is difficult to determine the species of chlorine in coal directly, and no reliable analytical methodology has been generally accepted. The modes of occurrence of chlorine in Lu-an and Yanzhou coal and their pyrolysis chars were roughly determined by sequential chemical extraction method suggested by Cox et al [1]. Several associated Table 1 Proximate and ultimate analysis of coal samples Sample
Lu-an Yanzhou Pingshuo a
Proximate analysis (wt%)
Ultimate analysis (wt%, daf)
Mad
Ad
Vdaf
C
0.87 2.31 4.80
12.94 9.99 19.88
15.85 45.23 39.19
88.27 1.23 78.28 1.14 78.76 1.46
By difference.
N
H
S
Oa
3.93 5.04 5.39
0.15 2.66 0.49
6.42 12.88 13.90
3. Results and discussion 3.1. Chlorine volatility during pyrolysis 3.1.1. Effect of temperature Figs. 1 and 2 show the effect of temperature on chlorine contents in char and its volatility during pyrolysis. With increasing temperature, chlorine contents in the chars decrease greatly, which indicates that a large amount of chlorine has volatilized. As shown in Fig. 2, chlorine volatility increases from 4 to 89% for Lu-an coal and from 25 to 82% for Pingshuo coal when raising temperature from 300 to 900 8C; that of Yanzhou coal increases from 14% at 300 8C to 80% at 600 8C. This clearly shows that chlorine volatility of Yanzhou is obviously higher than that of Lu-an and Pingshuo in the temperature range from 300 to 600 8C,
1876
W. Li et al. / Fuel 84 (2005) 1874–1878 450
350
coal
char A
coal
char B
90
Mass percentage / %
Cl content wt / 10–6
100
Lu-an Yanzhou Pingshuo
400
300 250 200 150 100 50
80 70 60 50 40 30 20 10
0 0 25 300
400
500
600
700
800
900
Temperature / °C
0
Lu-an
Yanzhou A;
B;
C;
D
Fig. 1. Effect of pyrolysis temperature on chlorine content in char.
during which more than 68% of chlorine in Yanzhou has already volatilized below 400 8C. Above 700 8C, the Cl volatility is nearly same for Lu-an and Pingshuo coal. This suggests that at high temperatures Cl in coal is mostly evolved, which is independent of the coal type and the form of Cl. It is found that more than 95% of chlorine in Illinois coals was liberated during pyrolysis from 300 to 600 8C in a fixed-bed run [5]; and most chlorine were evolved from the coals studied below 750 8C in a temperature-programmed mode [9]. However, in the fluidized bed runs in this work, the volatility of chlorine is less than that obtained in fixed bed runs at the same temperature. This is believed due to the fast heating rate of the sample in fluidized bed runs. It is well accepted that the chlorine in coal is mainly evolved as HCl during coal pyrolysis. Using IR technique, Tsubouchi et al. [9] have observed that HCl evolved during temperature-programmed pyrolysis of three high rank coals with almost the same carbon contents shows very different rate profiles, which suggests the different Cl functional groups in coals. The different chlorine volatility of Lu-an and Yanzhou coals below 600 8C in this work implies the different Cl forms in them, which will be discussed below in detail.
100
Volatility / %
80
60 Lu-an Yanzhou Pingshuo
40
20
Fig. 3. Modes of occurrence of chlorine in coal and its pyrolysis char (char A-pyrolyzed at 500 8C; char B-pyrolyzed at 400 8C) A- inorganic chlorine; B-chlorine bound to organic cations by ion exchangeable mechanism; Ccovalently-bounded organic chlorine; D-volatility (volatilized into gas and tar).
3.1.2. Modes of occurrence of chlorine and its volatility Fig. 3 shows the results of sequential chemical extraction of Lu-an and Yanzhou raw coals and their chars. The pyrolysis temperature of char A from Lu-an is 500 8C and that of char B from Yanzhou are 400 8C. In the raw coal, the organic Cl is about 41 and 73% for Lu-an and Yanzhou, respectively. And most of the organic Cl is covalently bonded one in both coal samples, which is about 34 and 68% for Lu-an and Yanzhou coal, respectively. After pyrolysis, only a few organic forms of chlorine still remain in char A (about 3%) and char B (about 5%), and they are bound to organic cations by an ion exchangeable mechanism. Most of organic chlorine in Lu-an and Yanzhou coal has already volatilized at low pyrolysis temperatures (below 500 8C). This indicates that covalently bonded organic chlorine shows a high volatile behavior under pyrolysis conditions. About 60% of chlorine in Lu-an coal exists in inorganic forms, which shows a significant decrease in char A (about 43%). The inorganic chlorine in char B is close to that of raw Yanzhou coal (about 27%), which suggests that inorganic forms of chlorine in Yanzhou coal are hardly volatilized at low pyrolysis temperatures (below 400 8C). As shown in previous papers [10,11], the following reactions are main source of volatile Cl into HCl (R and R 0 represent the aliphatic or aromatic function group of coal matrix, M represents alkali or alkaline earth metal): RCl C R 0 H/ RR 0 C HClðgÞ[
(2)
RCl C H2 OðgÞ/ ROH C HClðgÞ[
(3)
Cl2 C H2 OðgÞ/ 1=2O2 C HClðgÞ[
(4)
MClðlÞ C 1=2H2 ðgÞ/ MðgÞ C HClðgÞ[
(5)
0 300
400
500
600
700
800
900
Temperature / °C Fig. 2. Effect of temperature on chlorine volatility during coal pyrolysis.
W. Li et al. / Fuel 84 (2005) 1874–1878
2MClðgÞ C H2 OðgÞ C SO2 C 1=2O2 / MSO4 ðgÞ
1877
100
C 2HClðgÞ[
Lu-an Pingshuo
90
(6)
3.1.3. Effect of added CaO on Cl volatility The effect of added CaO on volatility of chlorine during pyrolysis of Lu-an coal from 300 to 900 8C is compared in Fig. 4. Chlorine volatility with CaO is lower than that without CaO, which clearly shows that evolution of HCl is retarded by the added CaO. Fig. 4 also shows that the restraining efficiency of CaO on chlorine volatility depends on pyrolysis temperature. The high restraining efficiencies are obtained from 450 to 650 8C. It is believed that the capture reaction of HCl by CaO is as following: CaO C 2HCl/ CaCl2 C H2 O
(7)
The thermodynamic equilibrium constant for the reaction is higher than 104 below 900 8C. It is almost irreversible in the conditions used in the present experiments [12]. Restraining efficiency depends on the thermodynamic
Char yield / %
80
Results of sequential chemical extraction has shown that chlorine in Lu-an and Yanzhou coal are mainly in inorganic and organic forms, respectively. According to thermodynamic data of compounds, the vapor pressure of most alkali or alkaline earth metal chlorides is very low below 600 8C. Few inorganic chlorine can be vaporized at low pyrolysis temperatures. It is believed that the organic forms of chlorine are the major source of evolved HCl by reactions (2) and (3) at low pyrolysis temperatures. Some inorganic elemental chlorine can also be vaporized by reaction (4). This can well explain the results of Fig. 2, which shows that the chlorine volatility of Yanzhou coal are significantly higher than that of Lu-an coal below 600 8C, and a large amount of chlorine in Yanzhou coal has already volatilized below 400 8C. With increasing temperature, inorganic forms of chlorine might start to vaporize following the reactions (5) and (6). Thus, chlorine in Lu-an coal also shows high volatility at high pyrolysis temperatures, as shown in Fig. 2.
60 50 40 30 20 10 800
850
900
950
Temperature / °C Fig. 5. Effect of temperature on the char yield during CO2-gasification of coal.
stability of CaCl2. Thermogravimetric curve shows that there is an obvious weight loss started from 740 8C [13], which leads to the decrease of restraining efficiency of CaO above 700 8C. The slow reaction rate should be responsible for the low restraining efficiency of CaO below 450 8C. 3.2. Chlorine volatility during gasification Fig. 5 shows the char yield of Pingshuo and Lu-an coal under CO2-gasification at different temperatures. The char yield decreases from 85% at 800 8C to 55% at 950 8C for Lu-an coal, and that is from 47 to 9% for Pingshuo coal. This suggests the remarkable difference of the CO2gasification reactivity between the two coals. However, the Cl content in the two chars, shown in Fig. 6, exhibits the similar value. Moreover, the release efficiency of Cl, shown in Fig. 7, indicates that nearly 100% of Cl is volatilized at 950 8C from Pingshuo coal, but for Lu-an coal there is a few amount of Cl remained in the char at the same high temperature. This is believed to result from the low gasification reactivity and high char yield of Lu-an coal. The inherent organic Cl in raw coal and the Cl containing in 450
100 without CaO 1% CaO Restraining efficiency
400
Cl content wt / 10–6
80
Cl volatility / %
70
60 40 20
Lu-an Pingshuo
350 300 250 200 150 100 50
0
0 300
400
500
600
700
800
900
Temperature / °C Fig. 4. Effect of CaO additive on chlorine volatility during pyrolysis of Lu-an coal.
0 25
800
850
900
950
Temperature / °C Fig. 6. Effect of gasification temperature on chlorine content in the char.
1878
W. Li et al. / Fuel 84 (2005) 1874–1878
Cl volatility / %
100
95
90 Lu-an Pingshuo 85 800
850
900
950
Temperature / °C Fig. 7. Effect of temperature on chlorine volatility of coal during CO2gasification.
clay might be present in gasification char [4]. In addition, the physic-chemical adsorption of Cl by the char with large surface area is a possible route to form the new stable phase of chlorine [11]. Tsubouchi et al. [9] observed that the organic Cl could be also formed by secondary reaction of HCl evolved at lower temperatures with carbon active sites in the nascent char in pyrolysis. This phenomenon is also expected to exist in the gasification process. Comparing the result in Figs. 3 and 7, we found that Cl volatility of the same coal under different atmospheres is nearly equal at the same temperatures. This indicates that the volatility of Cl, unlike that of F [14], is mainly dependent on the temperature, and not on atmosphere. This might be resulted from the different occurrence modes of Cl and F, which will be the subject of future work.
4. Conclusions The Cl volatility increases with increasing temperature. Below 600 8C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 8C the Cl volatilities for the 3 coals tested are all higher than 80%. Generally, the chlorine volatility of coals in a fixed bed reactor, from the literatures, is greater than that in a fluidized reactor in this work due to the fast heating rate in the later case. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below
500 8C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 8C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 8C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 8C high volatility of Cl is obtained in both N2 and CO2 atmospheres. Up to now, the understanding of chemical forms of chlorine is less than that of other trace elements in coal, especially the organically bonded chlorine which cannot be removed effectively by the simple physical cleaning method. The future work on chlorine in coal should focus on exact determination of organic chlorine form and its thermal behavior in order to design an efficient method before combustion of the coal or char.
Acknowledgements Financial support for this work by the Special Funds for Major State Basic Research Project of China (Contract No. 2004CB217704) are gratefully acknowledged.
References [1] Cox JA, Larson AE, Carlson RH. Fuel 1984;63:1334. [2] Dale LS, Riley KW, Rowland RC. The determination of halogens in coals. 18th annual international Pittsburgh coal conference. Newcastle, NSW, Australia, Dec. 3–7, 2001. CD-ROM. [3] Huggins FE, Huffman GP. Int J Coal Geol 1996;32:31. [4] Vassilev SV, Eskenazy GM, Vassileva CG. Fuel 2000;79:903. [5] Shao D, Hutchinson EJ, Cao H, Pan WP. Energy Fuels 1994;8:399. [6] Bhaskar T, Matsui T, Nitta K, Uddin MA, Muto A, Sakata Y. Energy Fuels 2002;16:1533. [7] Herod AA, Hodges NJ, Pritchard E, Smith CA. Fuel 1983;62:1331. [8] Fynes G, Herod AA, Hodges NJ, Stokes BJ, Ladner WR. Fuel 1988; 67:822. [9] Tsubouchi N, Ohtsuka S, Hashimoto H, Ohtsuka Y. Energy Fuels 2004;18:1605. [10] Wang J, Zhang D, Gao J. Coal Conversion 2003;26:29 (in Chinese). [11] Vassilev SV, Eskenazy GM, Vassileva CG. Fuel 2000;79:923. [12] Li S, Chi Y, Li W, Li R, Qiu K, Li X, et al. Environ Sci 2001;22:95 (in Chinese). [13] Guo X, Yang X, Li H, Chen Y, Li F, Xie K. Acta Scientiae Circum 2000;20:776 (in Chinese). [14] Li W, Lu H, Chen H, Li B. Fuel 2005; 84:353–7.
The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor Wen Li*, Hailiang Lu, Haokan Chen, Baoqing Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China Received 15 October 2004; received in revised form 14 March 2005; accepted 15 March 2005 Available online 21 April 2005
Abstract The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO2-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 8C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 8C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 8C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 8C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 8C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 8C high volatility of Cl is obtained in both N2 and CO2 atmospheres. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal; Chlorine; Mode of occurrence; Fluidized-bed reactor; Volatility; Pyrolysis; Gasification
1. Introduction It is well known that there are many kinds of harmful trace elements in coal, which can cause environmental and technological problems during coal utilization. Chlorine is one of the harmful trace elements easily volatilized. A large amount of chloride is present in the gas phase in coal utilization process. Clean air legislation in many countries imposes emission limits on chlorine due to its environmental impact. Meanwhile, high concentration of chlorine in coal can create corrosion and deposition damage in equipment, which leads to a huge cost for solving the technological problems. Information about the modes of occurrence of chlorine in coal and its volatile behavior is important when measures are adopted to control the emission of chlorine during coal conversion. Because of the restriction of * Corresponding author. Tel.: C86 351 4044335; fax: C86 351 4050320. E-mail address: [email protected] (W. Li).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.03.017
determination method and coal complexity, general agreement has not been reached about the modes of occurrence of chlorine in coal. Several forms of chlorine in coal have been reported, such as covalently-bonded Cl, the Cl bound to organic cations in ion exchangeable form, and the Cl ions present in inorganic salts or in water within micropores of coal [1–4]. Literatures show that chlorine is mainly released as hydrogen chloride during heat-treatment of coal in an inert atmosphere or under combustion conditions [5,6]. However, the modes of occurrence of chlorine are much different from coal to coal. Most previous work [5,7–9] about the behavior of chlorine in coal during pyrolysis was performed in a fixed-bed reactor, i.e. in a programmed heating system. The volatilization behavior of chlorine in coal during fluidized bed pyrolysis, moreover, and in gasification is not clear. The purpose of this work is to examine the volatilization behavior of chlorine in coal during fluidized-bed pyrolysis and gasification, and to identify the modes of occurrence of chlorine in coal. In addition, the restraining efficiency of the added CaO on chlorine volatility was studied.
W. Li et al. / Fuel 84 (2005) 1874–1878
2. Experimental 2.1. Coal samples Three Chinese coal samples, Lu-an, Pingshuo and Yanzhou, were used in the experiments. They are selected because the chlorine content in them is high and the volatile matter content is different. The chlorine contents of Lu-an, Pingshou and Yanzhou coal are 369, 347 and 434 mg gK1 on a dry basis, respectively. And the volatile matter content (daf basis) is 15.85, 45.23 and 39.19%, respectively. The coal samples were crushed and sieved to 147–246 mm for use. The coal property is given in Table 1. 2.2. Pyrolysis and gasification tests Pyrolysis tests were performed in a quartz tube fluidizedbed reactor (length of 600 mm and inner diameter of 25 mm) under nitrogen at temperatures ranging from 300 to 900 8C for Lu-an and Pingshuo coal, and from 300 to 600 8C for Yanzhou coal. There is a porous quartz disk located at 360 mm from the top of the reactor to support the coal samples and as a gas distributor. At a predetermined temperature, about 3.0 g coal sample was quickly placed into the quartz tube reactor. The nitrogen flow was 500–1000 ml/min corresponding to the different temperatures. After duration for 30 min, the char sample was cooled down to room temperature by removing the quartz tube reactor from the tube furnace. The chars were collected, weighted and analyzed. To clarify the restraining efficiency of addition of calcium-based mineral matter on chlorine volatility, CaO was added in several runs during Lu-an pyrolysis with the molar ratio of Ca/S equal to 2.8. Gasification tests were carried out under CO2 atmosphere (300 ml/min) at temperature range from 800 to 950 8C. The other conditions are same as those in pyrolysis tests.
1875
forms of chlorine were separated in the following order: (1) water soluble inorganic chlorine: first the samples were dissolved into deionized water and stirred for 3 h, and then further extracted by dimethyl sulphoxide (DMSO) for 1 h; (2) chlorine bound to organic cations: the samples was extracted by 0.1 M KNO3 in 100% DMSO; (3) covalentlybonded organic chlorine: obtained by difference. The amount of chlorine from (2) and (3) is the whole organic chlorine in the sample. 2.4. Determination of chlorine and its volatility Chlorine contents in coal and char were determined by combustion-hydrolysis/potential titration method based on Chinese standard method (GB/T 3558-1996). The basic theory of this method is as follows: the sample (coal or char) is combusted and hydrolysed in a mixed stream of oxygen and steam during which chlorine in the sample is converted into chloride and dissolved quantitatively in water; the chlorine contents in condensate were determined by potential titration with silver nitrate. A standard sample with chlorine content of 570 mg gK1 was used to analyze the precision of the present method using which 565G 10 mg gK1 was obtained. This indicates that the combustion–hydrolysis/potential titration method is effective and exact in determining the chlorine content in the samples. Chlorine volatility during coal pyrolysis and gasification was calculated by the following equation: C m C Y V% Z 1 K 1;d 1;d 100 1 K 1;d 100 (1) C0;d C0;d m0;d where V is the volatility of chlorine, %; C1,d, the content of the chlorine in char, (g gK1; C0,d, the content of chlorine in coal, (g gK1; m1,d, the mass of char, g; m0,d, the mass of coal, g; and Y, the yield of char, %. The difference of chlorine volatility with and without CaO addition is defined as the restraining efficiency.
2.3. Sequential chemical extraction It is difficult to determine the species of chlorine in coal directly, and no reliable analytical methodology has been generally accepted. The modes of occurrence of chlorine in Lu-an and Yanzhou coal and their pyrolysis chars were roughly determined by sequential chemical extraction method suggested by Cox et al [1]. Several associated Table 1 Proximate and ultimate analysis of coal samples Sample
Lu-an Yanzhou Pingshuo a
Proximate analysis (wt%)
Ultimate analysis (wt%, daf)
Mad
Ad
Vdaf
C
0.87 2.31 4.80
12.94 9.99 19.88
15.85 45.23 39.19
88.27 1.23 78.28 1.14 78.76 1.46
By difference.
N
H
S
Oa
3.93 5.04 5.39
0.15 2.66 0.49
6.42 12.88 13.90
3. Results and discussion 3.1. Chlorine volatility during pyrolysis 3.1.1. Effect of temperature Figs. 1 and 2 show the effect of temperature on chlorine contents in char and its volatility during pyrolysis. With increasing temperature, chlorine contents in the chars decrease greatly, which indicates that a large amount of chlorine has volatilized. As shown in Fig. 2, chlorine volatility increases from 4 to 89% for Lu-an coal and from 25 to 82% for Pingshuo coal when raising temperature from 300 to 900 8C; that of Yanzhou coal increases from 14% at 300 8C to 80% at 600 8C. This clearly shows that chlorine volatility of Yanzhou is obviously higher than that of Lu-an and Pingshuo in the temperature range from 300 to 600 8C,
1876
W. Li et al. / Fuel 84 (2005) 1874–1878 450
350
coal
char A
coal
char B
90
Mass percentage / %
Cl content wt / 10–6
100
Lu-an Yanzhou Pingshuo
400
300 250 200 150 100 50
80 70 60 50 40 30 20 10
0 0 25 300
400
500
600
700
800
900
Temperature / °C
0
Lu-an
Yanzhou A;
B;
C;
D
Fig. 1. Effect of pyrolysis temperature on chlorine content in char.
during which more than 68% of chlorine in Yanzhou has already volatilized below 400 8C. Above 700 8C, the Cl volatility is nearly same for Lu-an and Pingshuo coal. This suggests that at high temperatures Cl in coal is mostly evolved, which is independent of the coal type and the form of Cl. It is found that more than 95% of chlorine in Illinois coals was liberated during pyrolysis from 300 to 600 8C in a fixed-bed run [5]; and most chlorine were evolved from the coals studied below 750 8C in a temperature-programmed mode [9]. However, in the fluidized bed runs in this work, the volatility of chlorine is less than that obtained in fixed bed runs at the same temperature. This is believed due to the fast heating rate of the sample in fluidized bed runs. It is well accepted that the chlorine in coal is mainly evolved as HCl during coal pyrolysis. Using IR technique, Tsubouchi et al. [9] have observed that HCl evolved during temperature-programmed pyrolysis of three high rank coals with almost the same carbon contents shows very different rate profiles, which suggests the different Cl functional groups in coals. The different chlorine volatility of Lu-an and Yanzhou coals below 600 8C in this work implies the different Cl forms in them, which will be discussed below in detail.
100
Volatility / %
80
60 Lu-an Yanzhou Pingshuo
40
20
Fig. 3. Modes of occurrence of chlorine in coal and its pyrolysis char (char A-pyrolyzed at 500 8C; char B-pyrolyzed at 400 8C) A- inorganic chlorine; B-chlorine bound to organic cations by ion exchangeable mechanism; Ccovalently-bounded organic chlorine; D-volatility (volatilized into gas and tar).
3.1.2. Modes of occurrence of chlorine and its volatility Fig. 3 shows the results of sequential chemical extraction of Lu-an and Yanzhou raw coals and their chars. The pyrolysis temperature of char A from Lu-an is 500 8C and that of char B from Yanzhou are 400 8C. In the raw coal, the organic Cl is about 41 and 73% for Lu-an and Yanzhou, respectively. And most of the organic Cl is covalently bonded one in both coal samples, which is about 34 and 68% for Lu-an and Yanzhou coal, respectively. After pyrolysis, only a few organic forms of chlorine still remain in char A (about 3%) and char B (about 5%), and they are bound to organic cations by an ion exchangeable mechanism. Most of organic chlorine in Lu-an and Yanzhou coal has already volatilized at low pyrolysis temperatures (below 500 8C). This indicates that covalently bonded organic chlorine shows a high volatile behavior under pyrolysis conditions. About 60% of chlorine in Lu-an coal exists in inorganic forms, which shows a significant decrease in char A (about 43%). The inorganic chlorine in char B is close to that of raw Yanzhou coal (about 27%), which suggests that inorganic forms of chlorine in Yanzhou coal are hardly volatilized at low pyrolysis temperatures (below 400 8C). As shown in previous papers [10,11], the following reactions are main source of volatile Cl into HCl (R and R 0 represent the aliphatic or aromatic function group of coal matrix, M represents alkali or alkaline earth metal): RCl C R 0 H/ RR 0 C HClðgÞ[
(2)
RCl C H2 OðgÞ/ ROH C HClðgÞ[
(3)
Cl2 C H2 OðgÞ/ 1=2O2 C HClðgÞ[
(4)
MClðlÞ C 1=2H2 ðgÞ/ MðgÞ C HClðgÞ[
(5)
0 300
400
500
600
700
800
900
Temperature / °C Fig. 2. Effect of temperature on chlorine volatility during coal pyrolysis.
W. Li et al. / Fuel 84 (2005) 1874–1878
2MClðgÞ C H2 OðgÞ C SO2 C 1=2O2 / MSO4 ðgÞ
1877
100
C 2HClðgÞ[
Lu-an Pingshuo
90
(6)
3.1.3. Effect of added CaO on Cl volatility The effect of added CaO on volatility of chlorine during pyrolysis of Lu-an coal from 300 to 900 8C is compared in Fig. 4. Chlorine volatility with CaO is lower than that without CaO, which clearly shows that evolution of HCl is retarded by the added CaO. Fig. 4 also shows that the restraining efficiency of CaO on chlorine volatility depends on pyrolysis temperature. The high restraining efficiencies are obtained from 450 to 650 8C. It is believed that the capture reaction of HCl by CaO is as following: CaO C 2HCl/ CaCl2 C H2 O
(7)
The thermodynamic equilibrium constant for the reaction is higher than 104 below 900 8C. It is almost irreversible in the conditions used in the present experiments [12]. Restraining efficiency depends on the thermodynamic
Char yield / %
80
Results of sequential chemical extraction has shown that chlorine in Lu-an and Yanzhou coal are mainly in inorganic and organic forms, respectively. According to thermodynamic data of compounds, the vapor pressure of most alkali or alkaline earth metal chlorides is very low below 600 8C. Few inorganic chlorine can be vaporized at low pyrolysis temperatures. It is believed that the organic forms of chlorine are the major source of evolved HCl by reactions (2) and (3) at low pyrolysis temperatures. Some inorganic elemental chlorine can also be vaporized by reaction (4). This can well explain the results of Fig. 2, which shows that the chlorine volatility of Yanzhou coal are significantly higher than that of Lu-an coal below 600 8C, and a large amount of chlorine in Yanzhou coal has already volatilized below 400 8C. With increasing temperature, inorganic forms of chlorine might start to vaporize following the reactions (5) and (6). Thus, chlorine in Lu-an coal also shows high volatility at high pyrolysis temperatures, as shown in Fig. 2.
60 50 40 30 20 10 800
850
900
950
Temperature / °C Fig. 5. Effect of temperature on the char yield during CO2-gasification of coal.
stability of CaCl2. Thermogravimetric curve shows that there is an obvious weight loss started from 740 8C [13], which leads to the decrease of restraining efficiency of CaO above 700 8C. The slow reaction rate should be responsible for the low restraining efficiency of CaO below 450 8C. 3.2. Chlorine volatility during gasification Fig. 5 shows the char yield of Pingshuo and Lu-an coal under CO2-gasification at different temperatures. The char yield decreases from 85% at 800 8C to 55% at 950 8C for Lu-an coal, and that is from 47 to 9% for Pingshuo coal. This suggests the remarkable difference of the CO2gasification reactivity between the two coals. However, the Cl content in the two chars, shown in Fig. 6, exhibits the similar value. Moreover, the release efficiency of Cl, shown in Fig. 7, indicates that nearly 100% of Cl is volatilized at 950 8C from Pingshuo coal, but for Lu-an coal there is a few amount of Cl remained in the char at the same high temperature. This is believed to result from the low gasification reactivity and high char yield of Lu-an coal. The inherent organic Cl in raw coal and the Cl containing in 450
100 without CaO 1% CaO Restraining efficiency
400
Cl content wt / 10–6
80
Cl volatility / %
70
60 40 20
Lu-an Pingshuo
350 300 250 200 150 100 50
0
0 300
400
500
600
700
800
900
Temperature / °C Fig. 4. Effect of CaO additive on chlorine volatility during pyrolysis of Lu-an coal.
0 25
800
850
900
950
Temperature / °C Fig. 6. Effect of gasification temperature on chlorine content in the char.
1878
W. Li et al. / Fuel 84 (2005) 1874–1878
Cl volatility / %
100
95
90 Lu-an Pingshuo 85 800
850
900
950
Temperature / °C Fig. 7. Effect of temperature on chlorine volatility of coal during CO2gasification.
clay might be present in gasification char [4]. In addition, the physic-chemical adsorption of Cl by the char with large surface area is a possible route to form the new stable phase of chlorine [11]. Tsubouchi et al. [9] observed that the organic Cl could be also formed by secondary reaction of HCl evolved at lower temperatures with carbon active sites in the nascent char in pyrolysis. This phenomenon is also expected to exist in the gasification process. Comparing the result in Figs. 3 and 7, we found that Cl volatility of the same coal under different atmospheres is nearly equal at the same temperatures. This indicates that the volatility of Cl, unlike that of F [14], is mainly dependent on the temperature, and not on atmosphere. This might be resulted from the different occurrence modes of Cl and F, which will be the subject of future work.
4. Conclusions The Cl volatility increases with increasing temperature. Below 600 8C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 8C the Cl volatilities for the 3 coals tested are all higher than 80%. Generally, the chlorine volatility of coals in a fixed bed reactor, from the literatures, is greater than that in a fluidized reactor in this work due to the fast heating rate in the later case. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below
500 8C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 8C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 8C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 8C high volatility of Cl is obtained in both N2 and CO2 atmospheres. Up to now, the understanding of chemical forms of chlorine is less than that of other trace elements in coal, especially the organically bonded chlorine which cannot be removed effectively by the simple physical cleaning method. The future work on chlorine in coal should focus on exact determination of organic chlorine form and its thermal behavior in order to design an efficient method before combustion of the coal or char.
Acknowledgements Financial support for this work by the Special Funds for Major State Basic Research Project of China (Contract No. 2004CB217704) are gratefully acknowledged.
References [1] Cox JA, Larson AE, Carlson RH. Fuel 1984;63:1334. [2] Dale LS, Riley KW, Rowland RC. The determination of halogens in coals. 18th annual international Pittsburgh coal conference. Newcastle, NSW, Australia, Dec. 3–7, 2001. CD-ROM. [3] Huggins FE, Huffman GP. Int J Coal Geol 1996;32:31. [4] Vassilev SV, Eskenazy GM, Vassileva CG. Fuel 2000;79:903. [5] Shao D, Hutchinson EJ, Cao H, Pan WP. Energy Fuels 1994;8:399. [6] Bhaskar T, Matsui T, Nitta K, Uddin MA, Muto A, Sakata Y. Energy Fuels 2002;16:1533. [7] Herod AA, Hodges NJ, Pritchard E, Smith CA. Fuel 1983;62:1331. [8] Fynes G, Herod AA, Hodges NJ, Stokes BJ, Ladner WR. Fuel 1988; 67:822. [9] Tsubouchi N, Ohtsuka S, Hashimoto H, Ohtsuka Y. Energy Fuels 2004;18:1605. [10] Wang J, Zhang D, Gao J. Coal Conversion 2003;26:29 (in Chinese). [11] Vassilev SV, Eskenazy GM, Vassileva CG. Fuel 2000;79:923. [12] Li S, Chi Y, Li W, Li R, Qiu K, Li X, et al. Environ Sci 2001;22:95 (in Chinese). [13] Guo X, Yang X, Li H, Chen Y, Li F, Xie K. Acta Scientiae Circum 2000;20:776 (in Chinese). [14] Li W, Lu H, Chen H, Li B. Fuel 2005; 84:353–7.