Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification

Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification

Fuel 84 (2005) 353–357 www.fuelfirst.com Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification Wen Li*, Hai...

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Fuel 84 (2005) 353–357 www.fuelfirst.com

Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification Wen Li*, Hailiang Lu, Haokan Chen, Baoqing Li State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan South Road No. 27, Taiyuan 030001, People’s Republic of China Received 9 July 2004; received in revised form 13 September 2004; accepted 13 September 2004 Available online 27 October 2004

Abstract The volatilization behavior of fluorine in five Chinese coals was investigated during fluidized-bed pyrolysis and CO2-gasification at a temperature range of 500–900 8C. The effect of co-existed and added calcium on fluorine volatility during pyrolysis was also determined. With increasing pyrolysis temperature, the volatility of fluorine increases. However, the volatility is greatly dependent on the fluorine chemical forms occurred in coal. Except for Datong and Zhungeer coal, more than 65% of fluorine in other three coals occurs as the steady forms. Fluorapatite is not the major carrier of fluorine in the coals studied. Fluorine volatility is retarded by coexisting calcium during coal pyrolysis, indicating that at least part of the stable forms of fluorine in coal might occur as calcium fluoride or calcium fluoride with complex compounds which are stable even at high pyrolysis temperature. The addition of CaO and limestone can suppress the release of fluorine during pyrolysis. The effect of CaO is better than that of limestone. The volatility of fluorine of coal during CO2-gasification depends on not only the occurrence mode of fluorine, but also the gasification reactivity of the coal. Compared with N2 atmosphere, CO2 is more favorable to the release of fluorine from coal. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal; Fluorine; Volatility; Calcium; Pyrolysis; CO2-gasification

1. Introduction Coal is the most abundant fossil fuel in the world and it is and will be the primary energy source in China in less than 50 years to come. In recent years, many toxic trace elements including fluorine in coal attracted much attention [1–4]. Fluorine is one of the harmful trace elements in coal. Though its content is very low, with a mean of 150 mg gK1 in the world coals [1] and 82 mg gK1 in Chinese coals [2], a large amount of toxic compounds of fluorine such as HF, SiF4, and CF4, were released into the atmosphere during coal utilization, leading to harmful effects on the environment and human health. Thus, knowledge about transformation of fluorine during coal conversion is needed. The volatilization behavior of part of trace elements, such as Cl, Hg, Cd, Pb, Mn, As, Be and Ni, during coal pyrolysis has

* Corresponding author. Tel.: C86 351 4044335; fax: C86 351 4050320. E-mail address: [email protected] (W. Li). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.09.008

been investigated [5–8]. And extensive studies have been reported with respect to fluorine emission and control technology during coal combustion [9–11]. However, there is little information about the transformation of fluorine under inert and reduction atmosphere. In this paper, the volatilization of fluorine in five Chinese coals during pyrolysis and CO2-gasification in fluidized-bed reactor was investigated. The effect of calcium in coal and the added calcium on fluorine volatility was also determined.

2. Experimental 2.1. Coal samples Five Chinese lump coal samples were determined in this study Huolinhe and Zhungeer coal are from Inner Mongolia. Datong and Pingshuo coal are from Shanxi Province. Yima coal is from Henan Province. Coal samples were ground

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Table 1 Proximate and ultimate analyses of coal sample Sample

Datong Pingshuo Huolinhe Yima Zhungeer

Proximate analyses/wt%

Ultimate analyses/wt%, daf

Mad

Ad

Vdaf

C

H

S

N

3.6 4.8 16.3 8.8 4.9

14.1 19.9 19.4 17.3 23.4

32.4 39.2 58.5 40.2 37.8

81.62 78.76 73.37 78.10 74.33

5.05 5.39 4.12 3.90 5.65

0.83 0.49 0.52 0.40 0.27

0.87 1.46 1.64 0.86 1.10

and sieved to 60–100 mesh before examination. Their proximate and ultimate analyses are given in Table 1. 2.2. Pyrolysis and gasification tests Pyrolysis tests were carried out in a quartz tube (with inner diameter of 25 mm and length of 600 mm) fluidizedbed reactor with nitrogen flow at temperature ranging from 400 to 900 8C. At a predetermined temperature about 5.0 g coal samples was put into the quartz tube reactor quickly. A mass flow meter controlled the nitrogen velocity ranging from 500 to 900 ml/min for fluidization. Gasification runs were performed at 800–950 8C for Yima and Zhungeer coal with CO2 flow of 300 ml/min. After the desired residence time, the quartz tube reactor was moved quickly to the atmosphere and cooled down to the room temperature. Then the chars were collected for analysis. 2.3. Determination and calculation method Fluorine contents in coal and char were determined by combustion-hydrolysis/fluoride-ion selective electrode method based on Chinese standard method (GB/T 46331997). 0.5 g sample was mixed with 0.5 g quartz sand in a small earthen boat, and then a suitable amount of quartz sand was spread on the mixture. The boat was gradually put into the furnace tube preheated up to 1000 8C. 400 ml/min of oxygen was flowing through the tube, during which the steam was simultaneously introduced to control the volume of condensate at 2.5–3 ml/min and the total volume within 85 ml. During this digestion process the fluorine in coal was totally converted into HF and SiF4, and then dissolved into water. Fluoride-ion selective electrode (pF-1 model, made in Shanghai Leici Company, China) and saturated calomel electrode were used as indicator and reference, respectively, to determine the concentration of fluorine. The PH value of digestion solution was adjusted to be 6, using C3H4OH(COONa)3–KNO3 as a buffer, to avoid the interference of the background matters. The potential analyzer used in this method was G301-A model, made in Institute of Coal Chemistry, Chinese Academy of Sciences. Phosphorus content of raw coal was determined by ICP-AES (inductively coupled plasma-atomic emission spectroscopy, TJA Company of America) using Atomscan16. Mineral matters in coal were calculated from the data of ash analysis result.

Fluorine volatility during coal pyrolysis and CO2gasification was calculated by the following equation     C1;d m1;d C1;d Y V% Z 1 K 100% Z 1 K 100% C0;d C0;d m0;d Where V%: fluorine volatility,%; C1,d: fluorine content of char, mg gK1; C0,d: fluorine content of raw coal, mg gK1; Y: yield of char, %; m1,d: mass of char, g; m0,d: mass of raw coal, g.

3. Results and discussion 3.1. Mode of occurrence of fluorine in coal The electronegativity of fluorine is 4.10 eV which is the highest value in the known elements. Hence, fluorine is the most active non-metallic element and cannot exist in the simple substance in nature. Previous studies showed that the main chemical forms of fluorine in coal were of inorganic association [12,13]. Because of the similar radius of FK and OHK, part of fluorine in coal occurs as independent minerals, such as CaF2 and MgF2; while others replace OHK in other minerals, such as Ca10(PO4)6(OH)F. Due to the complexity of coal composition and strong reaction capacity of fluorine, additive or substitute reactions inevitably take place between fluorine compounds and organic functional groups in coal. Thus, trace amounts of organic fluorine must be existed in coal. As geological environment during coalification and coal ranks are different greatly, the modes of occurrence of fluorine varies widely from coal to coal. Generally it is believed that fluorapatite [Ca10(OH)2KxFx(PO4)6, 0%x%2] is the most important carrier of fluorine [12–14]. Obviously, the mass ratio of phosphorus to fluorine must be higher than 4.9, if fluoraptite is the major form of fluorine in coal. Table 2 gives the content of phosphorus and fluorine and the mass Table 2 Phosphorus and fluorine content in coal samples Sample

Ad/wt%

P/mg gK1

F/mg gK1

Mass ratio (P/F)

Datong Pingshuo Huolinhe Yima Zhungeer

14.1 19.9 19.3 17.3 23.4

68 200 135 234 194

75 135 105 127 609

0.9 1.5 1.3 1.8 0.3

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these coals, which is consistent with the result shown in Table 2. In our previous work [8] the volatility of arsenic in Yima coal was reported. It is found that nearly 70% of arsenic in Yima coal was evaporated and increased slightly when temperature is above 800 8C. But in the case of fluorine in Yima coal less than 30% of fluorine was evaporated even at 900 8C. This strongly suggests that the thermal stability of fluorine-bearing minerals or fluorinecontaining compounds in Yima coal is much higher than that of arsenic-bearing minerals which is mainly pyrite. 3.3. Effect of coexisting calcium on fluorine volatility Fig. 1. Effect of temperature on fluorine volatility during coal pyrolysis.

ratio of the corresponding samples as shown in Table 1. It is seen that the mass ratios of phosphorus to fluorine in five coal samples are all below 4.9, indicating that fluorapatite is not the major form of fluorine in these five coal samples. 3.2. Effect of pyrolysis temperature on fluorine volatility Relationship between pyrolysis temperature and fluorine volatility is given in Fig. 1. It can be seen that fluorine volatility increases with the increasing of pyrolysis temperature. Contrary to expectation, only a small amount of fluorine in coal volatilize during the temperature ranges in this study. Except for Datong and Zhungeer coals, fluorine volatilities are below 35%. It indicates that more than 65% fluorine is thermal stable form in Pingshuo, Yima and Huolinhe coals. The stable forms of fluorine include the fluoride-bearing mineral matters and simple compounds of fluorine. The decomposition temperature, melting and boiling points of these minerals are higher than pyrolysis temperature. The volatile fluorine contains free state of FK adsorbed in coal seams and simple compounds with boiling point below 900 8C. Some organic fluorine might be present in volatile forms, which releases as HF by cracking or condensation reactions during coal pyrolysis. The pyrolysis conditions, especially reaction temperature, greatly influence the volatilization behavior of fluorine in coal. It is reported fluoraptite decomposes at 200 8C to release fluorine which is volatilized up to 50% at 800 8C [15]. The low volatility of fluorine of the five coals at 900 8C indicates that fluoraptite is not the main carrier of fluorine in

The mode of occurrence of fluorine in coal is an important factor that influences its volatile behavior during coal pyrolysis. The vaporization of fluorine might be retarded by its host minerals or coexisting mineral elements. Some fluoride-bearing minerals might be present in coal, for example, fluorophlogopite (KMg3(AlSi3O10)F2, K1 DGF f ;1800 K ZK4171 kJ mol ), has a very high decomposition temperature [15]. Previous studies showed that the decomposition temperature of CaF2 is higher than 1000 8C, and the decomposition temperature of calcium fluoride with complex compounds such as CaF2 CaO, CaF2$CaO$Al2O3, and CaF2$CaO$SiO2, etc, is even higher than 1300 8C [16]. The composition of ash composition in coal is shown in Table 3. Obviously, there are more calcium, silicon and aluminium than that required for formation of fluoridebearing mineral matters or complex compounds as mentioned above, which should be the stable forms of fluorine in coal. The relationship between the calcium content in raw coal and the fluorine volatility during fluidized-bed pyrolysis is shown in Fig. 2. For the coals studied, calcium content increases in the following order: Datong, Pingshuo, Huolinhe, Zhungeer and Yima. Except for Zhungeer coal with a high content of fluorine, fluorine volatility decreases as calcium content of original coal increases during coal pyrolysis. This indicates a strong correlation between fluorine and calcium in original coal at various pyrolysis temperatures. The volatility of fluorine during coal pyrolysis might be retarded by coexisting calcium in coal, indicating that the stable forms of fluorine in coal might exist in a large percentage as calcium fluoride or calcium fluoride based complex compounds.

Table 3 Analyses of ash compositions in coal (product of ash composition and ash content)/wt% Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

K2 O

Na2O

P2O5

Datong Pingshuo Huolinhe Yima Zhungeer

7.67 7.90 12.52 7.65 6.10

3.01 8.78 3.25 3.27 13.46

2.20 1.38 0.93 2.56 0.93

0.42 0.87 1.20 1.81 1.30

0.17 0.30 0.20 0.40 0.26

0.13 0.27 0.16 0.20 0.46

0.22 0.07 0.20 0.24 0.14

0.06 0.02 0.0 0.04 0.0

0.02 0.05 0.03 0.05 0.04

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Fig. 2. Relationship between calcium content in raw coal and fluorine volatility during fluidized-bed pyrolysis.

3.4. The effect of added calcium on volatility of fluorine To further understand the effect of calcium on the fluorine volatility during pyrolysis, CaO and limestone were added into coal with the ratio of 5 and 9%, respectively, to keep the same Ca/S ratio of 5.7. Fig. 3 compares the fluorine volatility of Zhungeer coal with and without calcium additive. For the added CaO the volatility of fluorine greatly decreases in the temperature range tested compared with that of raw coal. For added limestone the fluorine volatility changes little below 700 8C, while it decreases at high temperatures. This suggests that the main fluorine fixation reaction is: CaOC2HF/CaF2CH 2O. Limestone is decomposed little below 700 8C and the competitive reaction of sulfur fixation reaction makes the added limestone has little effect on the volatility of fluorine at low temperature. Fig. 4 shows the effect of calcium amount on the volatility of fluorine at pyrolysis temperature of 900 8C. The restraining efficiency is defined as the difference between fluorine volatility of raw coal and that of calcium added coal. With increasing amount of added calcium the restraining efficiency increases, but more

Fig. 3. Effect of added calcium on fluorine volatility during fluidized-bed pyrolysis of Zhungeer coal.

Fig. 4. Effect of quantity of added calcium on the restraining efficiency of fluorine volatility during Zhungeer coal pyrolysis at 900 8C.

calcium is not favorable to improve fluorine fixation further. The optimum ratio of Ca/S ratio for CaO and limestone to restrain fluorine is about three and four, respectively. 3.5. Fluorine volatility of coal during CO2-gasification The CO2-gasification of Yima and Zhungeer coal was performed at 800–950 8C with residence time of 30 min. Fig. 5 illustrates the yield of residue versus reaction temperature. With increasing temperature the residue yield of Yima coal decreases gradually, but that of Zhungeer coal decreases linearly. The residue yield of Yima coal is much lower than that of Zhungeer coal especially at low temperature, suggesting the higher CO2-gasification reactivity of Yima coal. The content of fluorine in the gasification residue is shown in Fig. 6 in which the value at 25 8C represents the fluorine content in raw coal. The fluorine content in Yima residue is higher than that in raw coal, but it decreases remarkably with increasing temperature. For Zhungeer coal the fluorine content in the residue is about half of that in the raw coal. This implies that the volatility or reactivity of fluorine in Yima coal is lower than that of carbon during CO2-gasification, but the case of Zhungeer coal is just

Fig. 5. Effect of gasification temperature on the yield of residue.

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investigated and the effect of co-existed and added calcium was also examined. The following conclusions are drawn from this work:

Fig. 6. Effect of gasification temperature on the fluorine contents in the residue.

(1) Fluorapatite is not the major form of fluorine in the coals studied. (2) Fluorine volatility increases as reaction temperature increases during coal pyrolysis. Except for Datong and Zhungeer coal, more than 65% of fluorine in other three coals occurs as the stable forms. (3) Fluorine volatility is retarded by co-existing calcium during coal pyrolysis, indicating that at least part of the steady forms of fluorine in coal might occur as calcium fluoride or calcium fluoride with complex compounds. (4) The addition of CaO and limestone can suppress the release of fluorine during pyrolysis. The effect of CaO is better than that of limestone. (5) The volatility of fluorine of coal during CO2-gasification depends on not only the occurrence mode of fluorine, but also the gasification reactivity of the coal. Compared with N2 atmosphere, CO2 is more favorable to the release of fluorine from coal.

Acknowledgements Financial support for this work by the Special Founds for Major State Fundamental Research Project of China (G1999022107) and by Natural Science Foundation of China (29936090) is gratefully acknowledged. Fig. 7. Effect of gasification temperature on the fluorine volatility.

opposite. Fig. 7 shows the fluorine volatility of Zhungeer coal gradually increases with increasing temperature and is higher than that of Yima coal below 900 8C. The fluorine volatility of Yima coal changes little at low temperature and increases remarkably above 850 8C. The different volatility of fluorine in Yima and Zhungeer coal suggests that the release of fluorine depends on not only its occurrence mode, but also the gasification reactivity of the raw coal. Comparing the result in Fig. 1 with that in Fig. 7, one can find that the fluorine volatility of the two coals during CO2-gasification is obviously higher than that during pyrolysis. Especially for Yima coal with high gasification reactivity, the fluorine volatility in gasification is three times higher than that in pyrolysis. It can be concluded that fluorine is more easily released in the reduction atmosphere than in the inert one. 4. Conclusions The volatility of fluorine of five Chinese coals during fluidized-bed pyrolysis and CO2-gasification was

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