1406
Energy & Fuels 2006, 20, 1406-1410
Behavior of Fluorine in the Combustion of Chinese Coal in Small Furnaces Dan Liu,*,† Yuji Sakai,‡ Mitsuo Yamamoto,‡ and Masayoshi Sadakata§ Department of Chemistry Science and Engineering, Ariake National College of Technology, 150 Higashihagio-machi, Omuta, Fukuoka 836-8585, Japan, Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of EnVironmental Chemistry Engineering, The UniVersity of Kougakuinn, 2665-1 Nakanomati, Hatiojisi, Tokyo 192-0015, Japan ReceiVed October 2, 2005. ReVised Manuscript ReceiVed April 5, 2006
Research into fluoride emissions from coal-combustion systems is becoming increasingly important as the issue of fluoride pollution in China becomes more serious. In some rural areas in China, indoor fluoride pollution has particularly been caused by the use of high fluorine content coal in stoves. As a result, many residents suffer from dental fluorosis and bone fluorosis. In this study, X-ray diffraction (XRD) analysis was carried out to confirm the mode by which inorganic fluorides exist in such coals. Analytical results showed that inorganic fluorides in Chinese coals exist mainly as muscovite and apatite. The fluorine concentration in gases emitted from a boat in a quartz tube furnace was measured during combustion of volatile matter and char. The times for volatile matter and char combustions were determined through continuous monitoring of SO2. Experimental results under different combustion conditions showed that fluoride in the emitted gas increased with an increase in oxygen concentration and temperature, while fluoride in the residue decreased with the increase in oxygen concentration and temperature.
1. Introduction In China, domestic coal currently represents 76% of primary energy sources, and this dependency is projected to continue well into the 21st century. It has been forecasted that ∼70% of primary energy consumption in 2010 will be supplied by domestic coal.1 The result from this continued high dependency on coal is increased air pollution from emissions of SOx and dust. Additionally, damage caused to teeth and bones from fluorine compounds has become a serious problem.2,3 While the average fluorine content of Chinese coal is ∼82 mg/kg, which is close to the world average (80 mg/kg),4 in some areas in China, such as Guizhou, Sichuan, the fluorine content of coal is 3 times the world average.5 In South China (Yunnan, Guizhou, Sichuan, Hubei, Hunan, and Guangxi provinces), the fluorine content in coal is about an average of 200 mg/kg.6 Combined with this is the fact that China consumes too much coal, and so the damage from fluorine increases with increased coal combustion. Fluorine damage has been reported in 13 * To whom correspondence should be addressed. E-mail:
[email protected]. † Ariake National College of Technology. ‡ The University of Tokyo. § The University of Kougakuinn. (1) Sadakata, M. Tyuugoku Kannkyou Hanndobooku; Science Forum Press: Tokyo, Japan, 1997; pp 129, 185. (2) Anto, M. Report of Special Research from the National Institute for EnVironmental Studies; SR-33-2000; National Institute for Environmental Studies: Ibaraki, Japan, 2000. (3) Finkelman, R. B.; Orem, W.; Castranova, V.; Tatu, C. A.; Belkin, H. E.; Zhang, B.; Lerch, H. E.; Maharaj, S. V.; Bates, A. L. Int. J. Coal Geol. 2002, 50, 430. (4) Luo, K.; Ren, D.; Xu, L.; Dai, S.; Cao, D.; Feng, F.; Tan, J. Int. J. Coal Geol. 2004, 57, 143-149. (5) Zheng, B. S. Difang Fu Zhongdu ji Gongye Fu Wuran Yanjiu; Chinese Environment Science Press: Beijing, China, 1992; pp 176-185 (6) Zheng, B. S.; Cai, R. G. Chin. J. Control Endemic Dis. 1988, 3 (2), 70-72.
provinces, autonomous regions, and municipalities in China,5 and more than 10 million people in Guizhou Province and surrounding areas suffer from various forms of fluorosis.3,7,8 It is also reported in the Prevention and Treatment of Endemic Diseases of China that fluorine pollution caused by intake of contaminated drinking water and coal-burning pollution has effected 596 million residents, among which 49 million suffer from dental fluorosis and bone fluorosis.9 Fluorine based diseases are acknowledged as local diseases in China.5 Although a daily intake of fluoride by adults that exceeds 8 mg would be harmful,10 a report has shown that the average daily intake of fluoride is 9.7 mg in a fluorine polluted area, because high fluorine content coals have been used.5,7 Because it is difficult to treat fluorine poisoning, it is vital that fluoride emissions be controlled at the sources of coal combustion; in this way, the level of fluoride that people are exposed to could be reduced. Fluorine behavior in coal combustion has been studied by several researchers. Meiji et al. studied the movement of trace elements through a standard pulverized dry-bottom boiler fitted with a high-efficiency electrostatic precipitator. The fuel used was bituminous, and according to the raw data, 120% of the fluorine (inaccuracy due to coal analysis techniques) originally present in the coal was found in the vapor phase in the flue gas (7) Ando, M.; Tadano, M., Yamamoto, S.; Tamaru, K.; Asanuma, S.; Watanabe, T.; Kondo, T.; Sakurai, S.; Ji, R.; Liang, C.; Chen, X.; Zhong, H.; Cao, S. Sci. Total EnViron. 2001, 271, 107-116. (8) Zheng, B.; Huang, R. DeVelopment in Geosciences, Contributions to 28th International Geologic Congress, Washington, DC, 1989; Science Press: Beijing, China, 1990; pp 171-176. (9) Qu, C. The Recent Developments of Health Effect of Water Pollution in China. KEO Discussion Paper (No. G-149); KEO University: Tokyo, Japan, 2001; pp 5-6. (10) IPCS. Fluorine and Fluorides; World Health Organization: Geneva, 1984; pp 25-77.
10.1021/ef050326z CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006
BehaVior of Fluorine in the Combustion of Chinese Coal
Energy & Fuels, Vol. 20, No. 4, 2006 1407
Table 1. Proximate Analysis of Chinese Coal (mass %, dry basis) samples
moisture
ash
VM
FC
Liantang Datang Kaihua Xinji Huaibei
0.5 0.6 1.1 1.7 1.9
86.7 78.6 78.7 56.5 30.9
3.8 5.4 2.4 19.2 26.2
9.0 15.4 17.8 22.6 41.0
Table 2. Ultimate Analysis of Chinese Coal (mass %, daf) samples
C
H
O
N
S
calorific value (kJ/kg)
Liantang Datang Kaihua Xinji Huaibei
65.7 74.2 83.0 72.3 78.9
2.3 1.4 0.5 6.1 5.0
17.7 10.8 3.8 20.1 14.2
1.6 1.3 1.0 1.2 1.4
12.7 12.3 11.7 0.3 0.5
3 100 5 530 6 360 12 260 21 980
downstream of ESP.11 Mu¨nzner reported that, while 10% of the fluorine in coal was released at temperatures under 750 °C, >60% was released at temperatures under 950 °C in fluid-bed combustion.12 Liang et al. noted that, in fluid-bed combustion, fluorine concentration in emitted gases increased as the combustion temperature increased.13 However, research on fluorine behavior in coal-combustion systems using smaller-size furnaces has not been reported, and because 30% of coal used in China is combusted in small-size boilers and stoves,14 it is important to clarify fluorine behavior in these small-size coal-combustion systems. This study aims to clarify the characteristic features of fluorine emissions from high fluorine content coal combustion in small-size furnaces. 2. Experimental Section Five coal samples (Liantang, Datang, Kaihua, Xinji, and Huaibei coals) were used for these experiments. Liantang, Datang, and Kaihua coals came from Zhejiang province, Huaibei coal came from Anhui province, and Xinji coal came from Hangzhou city. Results of proximate and ultimate analyses of these coals are listed in Tables 1 and 2, respectively. X-ray diffraction (XRD) analysis was first carried out to confirm the fluoride forms in the coals. The experimental apparatus used in this study is shown in Figure 1. The length of the pipe to the absorption bulb was intentionally kept short (300 mm) to avoid the potential measurement errors caused by material losses in the pipe. The length of the quartz tube used as the furnace was 760 mm with 25 mm inner diameter and 28 mm outer diameter. The length of the electric heating part was 300 mm and had a 30 mm diameter. Temperature was controlled by thermocouples arranged on the outside of the quartz tube. A combustion boat (80 mm × 13.5 mm × 10 mm) was used in the experiments. To start each experiment, 0.25 g of coal was put in the combustion boat, which was then placed at the end of the quartz tube, where the temperature was below 170 degrees C, for 3 min. At this temperature, volatile matter is not generated. After drying the coal, the combustion boat was moved into the center portion of the quartz tube by pushing it with a quartz bar, and combustion was initiated. (11) IEA Coal Research. Halogen emission from coal combustion; London; p 32; ISBN: 92-90290198-2. (12) Mu¨nzner, H. 8th International Conference on Fluidized-bed Combustion; ASEM: New York, 1985; pp 1219-1226. (13) Liang, D.; Anthony, E. J.; Leowen, B. K.; Yates, D. J. 11th International Conference on Fluidized Bed Combustion; ASME: NewYork, 1991; pp 917-922. (14) Sadakata, M. Tyuugoku de Kannkyo Monntai ni Torikomu (in Japanese); Iwanami Sinsyo Press: Tokyo, Japan, 2000; p 45.
Figure 1. Scheme of the experimental apparatus setup.
The process of coal combustion has two stages. First is volatile matter combustion, which results in a rapid mass decrease. Second is char combustion, which results in a slow mass decrease.15 When coal is heated, volatile matter is released and burns first; after volatile matter combustion is finished, char combustion starts. Fluorine contained in the coal is emitted at both volatile matter and char combustion stages. SO2 emission and production occurs at both the volatile matter combustion step and the char combusting char stages.16 Kim et al. used an SO2 monitor to observe SO2 emitting conditions and to distinguish whether the coal is in the volatile matter combustion stage or if it is combusting char.17 Therefore, fluorine emissions in both stages can be separately measured. The combustion gases emitted in both stages were collected by two absorption bulbs (absorption solution: 0.1M NaOH, 200 mL). One was used to absorb gases emitted during volatile matter combustion and the other was used for char combustion. After adding 50 mL of the buffer solution TISAB (total ion strength adjustment buffer, refer to JIS K0105) to 50 mL of the absorption solution, fluorine concentration was measured using a fluorine ion electrode (Horiba). During coal combustion, much of the fluorine contained in coal is emitted as both gas and particles, while the rest remains in the residue. The following experimental procedures were used to measure the fluorine concentrations in the gases emitted under various experimental conditions. To observe the effects of temperature on fluorine emitted in coal combustion, the temperature of a quartz chamber was controlled between preset temperatures of 400 and 1100 °C until the 0.25 g of coal was completely burnt. The air flow rate during this process was 0.5 l/min. The emitted gas was collected using two absorption bulbs (absorption solution: 0.1 M NaOH, 200 mL), and the fluorine concentration of the solution inside the absorption bulb was measured in units of mg/L. The fluorine weight (mg) inside 0.2 L of NaOH absorption solution could be measured as the fluorine weight (mg) emitted from the coal sample (kg). Therefore, the emitted fluorine concentration (ppm) ) fluorine emitted (mg)/coal sample (kg). Because particles smaller than 10 µm are more likely to remain in the human respiratory system,2 the fluorine concentra(15) Guo, C.-T. Mei Huaxue (in Chinese); Huaxue Gongye Press: Beijing, China, 1999; pp 36-37. (16) Arai, N.; Miura, T.; Miyamae, S. Nennsyouseiseibutu no haxtuseitoyakuseigujyutu; Technosystem Press: Tokyo, Japan, 1997; pp 129, 185. (17) Kim, H.; Lu, G. Q.; Li, T. J.; Sadakata, M. EnViron. Sci. Technol. 2002, 36, 1610-1611.
1408 Energy & Fuels, Vol. 20, No. 4, 2006
Liu et al.
Figure 3. XRD spectra of apatite in Xinji coal. Figure 2. XRD spectra of muscovite and apatite in Liantang coal.
tion in particle form was measured using a filter with a 10 µm mesh diameter, so that particles smaller than 10 µm were absorbed through the filter to a 0.1 M NaOH solution and measured together with the gas form. When coal combustion was completed, the paper filter used for collecting the emitted particles was put into a polycarbonate beaker containing 50 mL of 0.1 M NaOH solution, put on an ultrasonic cleaning machine (US-105), and then washed for 20 min. Finally, 50 mL of buffer solution (TISAB) was added to the polycarbonate beaker, and the fluorine concentration in the liquid was measured.18 Finally, the residue in the combustion boat was completely burnt at 1200 °C for 20 min, and the fluorine concentration in the residue was examined by an absorption method. In Chinese farming districts, people burn coal using combustion systems such as stoves for indoor heating and cooking.2 Since the conditions for coal combustion in these systems are not uniform, it is important to investigate the dependence on oxygen concentration of the fluorine concentration emitted in coal combustion. To test for the effects of oxygen concentration, 0.25 g of coal was burnt for 15 min under different oxygen concentrations (0%, 1%, 5%, 10%, and 20%). This procedure was repeated for each experimental temperature setting. The total flow of gas (N2 + O2) was kept at 0.5 L/min. 3. Results and Discussion Inorganic Fluorides in Chinese Coal. The diffraction spectrums from XRD analyses of five coals were measured. It was confirmed that Liantang, Dantang, and Kaihua coals are the same coal type, while Xinji and Huaibei coals are another. Thus, the experimental results for Liantang and Xinji coals are mainly shown in this paper. XRD analysis showed that inorganic fluorides in the Liantang type coal (Figure 2), existed as muscovite and apatite, while they existed as apatite in the Xinji type (Figure 3). Other inorganic matter may exist but can be disregarded, as they were not detected because they were below XRD detection limits. Emitted Fluorine Concentration during Combustion of Volatile Matter and Char. Combustion times differed according to the type of coal. As shown in Figure 4a, the time for the volatile matter combustion stage of Liantang coal was 100 s, and the time after this to the end of combustion was the time for the char combustion stage. The volatile matter combustion time of Xinji coal was much the same as for Liantang coal, but its char combustion stage time was different (Figure 4b). The (18) Project Administration Bureau of Environment Department. EnVironment Measurement and Analysis References; Project Administration Bureau of Environment Department: Japan, 1978; p 247.
Figure 4. SO2 concentration exhausted during the combustion ((a) Liantang and (b) Xinji coals).
sulfur content in Liantang coal is 10 times the sulfur content in Xinji coal, with the percentage of organic sulfur matter and inorganic sulfur matter in Liantang coal being more than that in Xinji coal. Thus, when the volatile matter and char in Liantang coal combusts (Figure 4a), the SO2 emitted is more than that from Xinji coal (Figure 4b). By comparing the SO2 in Figure 4a coal to that in Figure 4b coal, it can be postulated that, during char combustion, the second peak appeared because emission occurred within a period of 100-500 s, such that emitted SO2 was due to organic sulfur matter combustion and inorganic sulfur matter pyrolysis during this time period. Table 3 presents the emitted fluorine concentrations of the five Chinese coals at the volatile matter and char combustion stages. The Liantang type coals all contain high percentages of fluoride and ash components. Since the fluorine content in coal is in proportion to its ash content, coal with a higher ash content contains a higher fluorine content.19 Because fluorides in (19) Qi, Q. J.; Liu, J. Z.; Zhou, J. H.; Cao, Q. Y.; Jue, K. F. J. Fuel Chem. Technol. 2000, 28, 376-378.
BehaVior of Fluorine in the Combustion of Chinese Coal
Energy & Fuels, Vol. 20, No. 4, 2006 1409
Figure 5. Content of fluorides in flue gas, particles, and residues in Liantang coal combustion. Table 3. Flourine Concentration of Volatile Matter Evolution and Combustion Stage and Char Combustion Stage (900 °C) sample volatile matter evolution and combustion (µg/g) char combustion (µg/g) concentration of flourine (µg/g)
Figure 6. Content of fluorides in flue gas, particles, and residues in Xinji coal combustion.
Liantang Datang Kaihua Xinji Huaibei 95.0
93.1
74.1
290.2
69.2
1656.3 2407
1551.4 1828
580.9 761
249.7 569
346.0 461
Liantang type coals existed mainly as muscovite and apatite, and those in Xinji type coals existed as apatite, fluoride is emitted when apatite and muscovite are pyrolyzed in the char combustion stage.20,21 When the coal combusts, carbon reacts with other elements, for example, O2, and is emitted as CO2 or CO. Organic fluorides are in contact with carbon through chemical bonds in coal, which are broken during combustion, and thus, fluorine is emitted.20 It is thought that the concentration of fluorine emitted is due to the speed of inorganic fluorides pyrolysis and organic fluorides combustion. On the other hand, Xinji type coals are different; they contain higher percentages of volatile components but lower fluorine and ash contents. Therefore, the concentration of the amount of fluorine emitted from them in the volatile matter combustion stage to total fluorine emitted is higher. Relationship between Emitted Fluorine and Temperature during Coal Combustion. The results of measurements of fluorine under different combustion conditions are shown in Figure 5 (Liantang type coal) and Figure 6 (Xinji type coal). Fluorine concentrations in the emitted gases and particles and the combustion residues at each temperature are presented in units of µg/g, that is, the µg of fluorine in the emitted gas and particles and combustion residue obtained from 1 g coal after combustion. In each of the Chinese coals analyzed here, the figures indicate that, as the temperature of coal combustion increases, the amount of fluorine in the emitted gases and particles increases, while fluorine in the combustion residue decreases. It is thought that, when the temperature increases, the combustion rate of organic fluorides and the pyrolysis rates of apatite and muscovite likewise increase.20,21 In thermogravimetricmass spectrometry (TG-MS) tests, pyrolysis of apatite is initiated at 500 °C and that of muscovite is initiated at 800 °C.20,21 Though all five types of Chinese coals contain apatite, it is thought that the fluorine emitted from apatite is low because the maximum content of fluorine in apatite is 3.8%.22 The increase of fluorine emitted is mainly from pyrolysis of muscovite and combustion of organic fluorides, as shown in (20) Liu, D.; Sadakata, M. J. Jpn. Inst. Energy 2003, 82 (9), 679-685. (21) Liu, D.; Nishioka, T; Sadakata, M. J. Ecotechnol. Res. 2002, 8 (2), 120-124.
Figure 7. Relationship between oxygen concentration and released fluorine concentration in Liantang coal.
Figure 5. On the other hand, in the case of Xinji type coal, the increase of fluorine emitted in combustion is caused by the combustion of the organic fluorides in the coal, as shown in Figure 6. The mass balances of fluorine content for each coal at each temperature were between 78% and 108%; the error in the mass balance of trace elements in coal combustion is thought to be an allowable one.23 In this paper, because each coal sample was burned after the quartz tubes were heated to preset temperatures in intervals of 100 °C, the effects of this should influence the samples at each temperature, though the tendency to be influenced would be uniform. Thus, we consider that the emitted gases, particles, and ash fluorine concentration of the burned samples reflected the situation at the preset temperatures. Relationship between Oxygen Concentration and Emitted Fluorine during Coal Combustion. The relationship between oxygen concentration and emitted fluorine in coal combustion is shown in Figure 7 (Liantang type coal) and Figure 8 (Xinji type coal). The results show that, as oxygen concentration increases, the amount of fluorine emitted from all coals increased. These results are due to the fact that, as oxygen concentration increases, the combustion speed of carbon increases, resulting in higher (22) Yokoyama, T.; Asakura, K. Report of BehaVior of Fluorine in Natural EnVironment; T92528; Central Research Institute of Electric Power Industry: Tokyo, Japan, 1994. (23) Yokoyama, T.; Asakura, K.; Seki, T. Report of BehaVior of Trace Elements in Coal Combustion; T88087; Central Research Institute of Electric Power Industry: Tokyo, Japan, 1989.
1410 Energy & Fuels, Vol. 20, No. 4, 2006
Liu et al.
investigation showed that, because of the double effect during combustion, the fluorine emitted increases as oxygen concentration increases. The errors in amounts of fluorine emitted for each coal at each temperature were between 0.3% and 12%. 4. Conclusions
Figure 8. Relationship between oxygen concentration and released fluorine concentration in Xinji coal.
emissions of organic fluorine combined with carbon. Also, as the oxygen concentration increases during coal combustion, the surface temperature of the coal actually rises higher than the preset furnace temperature. This induces rapid decomposition of the inorganic substances, including fluorine, in the coal. The
XRD analyses showed that the inorganic fluorides in Chinese coals are present as muscovite and apatite. We investigated the behavior of fluorine in small-sized coal-combustion facilities and measured the fluorine concentrations emitted during coal volatile matter and char combustion stages. It was observed that the fluorine concentrations in both gas and particle phases increased and that the fluorine concentration in the combustion residue decreased with an increase in combustion temperature. Further, our data showed that the emitted fluorine concentration increased with an increase of oxygen concentration during combustion. Acknowledgment. The authors wish to thank Mrs. Taniguti for experimental work. EF050326Z
