Environ. Sci. Technol. 2000, 34, 2711-2713
Estimation of Mercury Emission from Coal Combustion in China QICHAO WANG,* WENGUO SHEN, AND ZHUANGWEI MA Changchun Institute of Geography, Chinese Academy of Sciences, Changchun 130021, People’s Republic of China
The mercury emissions of coal combustion and the mercury concentration of coals in China were studied. The average mercury concentration of coals is 0.22 mg/ kg. By founding a monitor program, we measured the mercury emission factors of coal-fired boilers, which are from 64.0% to 78.2%. On the basis of other data, we estimated that the total amount of mercury emission from coal combustion was 302.87 t in 1995, including 213.8 t into atmosphere and 89.07 t into ash and cinder. The results show that 2493.8 t of mercury was emitted into the atmosphere during the coal combustion process from 1978 to 1995, and the mercury emission intensity is higher in Beijing, Shanghai, and Tianjin.
Introduction A high Hg content has been found in fish from remote lakes in Scandinavia and North America, which is attributed to long-range transport and subsequent deposition of atmospheric mercury emission (1, 2). In this decade, Hg in the atmosphere over North America is increasing by about 1.5% per year, although industrial Hg consumption has decreased (3). The atmospheric emission of Hg mainly comes from coal combustion and municipal waste incineration (4). To estimate anthropogenic mercury emissions of the whole world accurately and to determine mercury cycling flux, it is important to investigate Hg emission in every country. The mercury emission from coal combustion and municipal waste incineration has been researched in North America and Nordic countries. Coal is the primary energy source, and a great amount of Hg was emitted into the atmosphere in the coal combustion process in China. The Hg emission must directly or potentially be a significant effect on human health (5). China, which is the largest coal consumption country in the world, consumed 1380 Mt of coal in 1995. However, the comprehensive and detailed research on Hg emissions has not been found before now in China. In this paper, we report the mercury content of coals in China and the mercury emission factors in the coal combustion process and study the transport and fate of mercury. In combination with coal consumption, we estimated mercury emission from all kinds of industries and all districts in China, including those emitting into the atmosphere and cinder. Additionally, we also investigated mercury emission from coal combustion from 1978 to 1995.
Materials and Methods Sampling. According to coal resources distribution and coal production, we designed a sampling program. A total of 234 coal samples were collected from the main coal mines that * Corresponding author phone: (086)431-5665374; fax: (086)4315652931; e-mail:
[email protected]. 10.1021/es990774j CCC: $19.00 Published on Web 05/31/2000
2000 American Chemical Society
produced 1040 Mt of coal, 80% of all coal production, in China in 1995. All coal samples were collected from a coal seam and were stored in bottles to prevent contamination. In the laboratory, coal samples were pulverized until the diameter of coal grain was less than 0.18 mm and then were stored in bottles until determination. Some coal, ash, and cinder samples were also collected from power plants and some ordinary industrial or domestic boilers. The method of preparation for ash and cinder was the same as with coal. Determination. The samples were digested with H2SO4HNO3-V2O5. Then we measured Hg concentration using cold vapor atomic absorption (CVAA). At the same time, standard coal and soil samples were used to examine analysis accuracy. The results showed that the digesting and determination methods could meet analysis accuracy. Determination and Calculation of the Hg Emission Factors. The Hg emission factor, Ef (into fly ash), can be calculated following eq 4:
Ef ) Qash/Qtotal
(1)
Qash ) CashMcoalAfFfη
(2)
Qtotal ) CcoalMcoal
(3)
where Ef is the emission factor of Hg into ash(%), Qash is the amount of Hg into ash in the combustion process (mg), Qtotal is the total Hg in coal (mg), Cash is the Hg concentration of ash (mg/kg), Ccoal is the Hg concentration of coal (mg/kg), Mcoal is the mass of coal (kg), Af is ash content of coal (%), Ff is the ratio of the amount of ash into flue gas to the sum of ash, and η is the efficiency of dust remover (%). Substituting eq 2 and eq 3 into eq 1 gives
Ef ) CashAfFfη/Ccoal
(4)
According to the conservation of mass, the relationship within the emission factors is as follows:
Ef + Eb + Ea ) 1
(5)
where 1 refers to the total emission amount of the element, Ef is the ratio of the element in fly ash, Eb refers to the ratio of Hg in bottom ash, and Ea is the ratio of element that goes to the atmosphere. All the parameters in eq 4 were measured directly, Eb was measured using a directly similar method , and Ea is calculated by eq 5. Calculation of Hg Emitted from Coal Combustion. The Hg emission from coal combustion of a department or a province, including Hg into atmosphere and into ashes, is equal to the product of the consumption of coal, the Hg content of coal, and the emission factor.
Results and Discussion Hg Concentration of Coals in China. Table 1 lists the Hg concentration of coals collected from 14 main coal-producing provinces. In most provinces, the standard deviation of Hg concentration is nearly equal or even higher than the mathematical average concentration, which shows that the Hg concentration of coal varies greatly. The content of coal in different mines and different provinces varied greatly, which relates to the geology of coal fields and the type of coal. We found that the distribution of mercury content in coals complies with the following rule: thin coal > lignite > coking coal > anthracite > long flame coal in northeast China. The results indicate that the Hg concentration of coals VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mercury Concentration in Coal of Main Coal Production Provinces in China (mg/kg)
TABLE 3. Mercury Distribution during 1 t of Coal Combustion in Civil and Industry Boilers (mg)
province
range
mathematical average
SD
sample
Hg of coala
Hg into bottom asha
Hg into fly asha
Hg into atmospherea
Heilongjiang Jilin Liaoning Inner Mongolia Beijing Anhui Jiangxi Hebei Shanxi Shaanxi Shandong Henan Sichuan Xinjiang
0.02-0.63 0.08-1.59 0.02-1.15 0.06-1.07 0.23-0.54 0.14-0.33 0.08-0.26 0.05-0.28 0.02-1.95 0.02-0.61 0.07-0.30 0.14-0.81 0.07-0.35 0.02-0.05
0.12 0.33 0.20 0.28 0.34 0.22 0.16 0.13 0.22 0.16 0.17 0.30 0.18 0.03
0.11 0.28 0.24 0.37 0.09 0.06 0.07 0.07 0.32 0.19 0.07 0.22 0.10 0.01
1 2 3 4 5 6 7 8
259 (100) 64 (100) 193 (100) 89 (100) 86 (100) 153 (100) 283 (100) 35 (100)
27 (10.4) 10 (15.6) 49 (25.4) 11 (12.4) 24 (27.9) 40 (26.1) 19 (6.7) 5.9 (8.7)
64 (24.7) 11 (17.2) 27 (14.0) 19 (21.3) 10 (12.0) 22 (14.5) 69 (24.4) 3 (16.9)
168 (64.9) 43 (67.2) 117 (60.6) 59 (66.3) 52 (60.1) 91 (59.4) 195 (68.9) 26 (74.4)
TABLE 2. Mercury Emission Factors of Boiler of Power Plant
sample
Hg concn of coal (mg/kg)
Hg concn of fly ash (mg/kg)
efficiency of dust cleaner (%)
ash content (%)
emission factor into atmosphere (%)
1 2 3 4 5 6 7
0.140 0.105 0.079 0.064 0.095 0.044 0.045
0.049 0.166 0.111 0.100 0.068 0.008 0.011
97 97 97 85 85 95 95
33.5 30.4 34.2 31.5 31.5 37.5 34.9
88.6 53.4 53.4 58.2 80.8 93.5 91.9
collected from Xinjiang, Heilongjiang, Shaanxi, Hebei, Shandong, and Jiangxi provinces is less than 0.20 mg/kg; coals from Beijing, Jilin, and Henan provinces are more than 0.30 mg/kg; coals from Liaoning, Shanxi, Inner Mongolia, and Anhui are between 0.20 and 0.30 mg/kg. Eary (6) reported that the range of Hg concentration of coals was 0.01-1.60 mg/kg. Compared with the data reported by Meij (1), the Hg concentration of coals in China is higher than those in Australia (0.087 mg/kg) and Columbia (0.04 mg/kg), but it is similar to the eastern United States. For other provinces in which the Hg concentration of coal samples was not determined, the data were cited from literature (7-9) or the Hg concentration of coal samples from the same coal fields was used as their Hg concentrations. On the basis of coal production of different provinces, the average Hg concentration of coals in China was calculated. The result is 0.22 mg/kg. Compared with soil, the Hg concentration of coal is much richer. Hg Emission Factors of Different Industries in China. Coal is the most primary energy source in China. Coal-fired power plants consume nearly one-fourth of the total coal consumption. At present, most power plants adopt similar production technology. For example, the coal powder boiler was used widely, and the dust control device mainly includes an electrostatic precipitator and wet scrubber. Two power plants in Changchun were chosen to make experiments for determining Hg emission factor. The results (Table 2) indicate that Hg emission factors into atmosphere range from 53.4% to 93.5%, average 74.3%, while the Hg emission factor into ash is 25.7%. These results are similar to other results reported abroad. Chow et al. (10) have reported that an electrostatic precipitator could remove 30-40% of total Hg of coals in power plants. A similar method was employed to calculate Ef in ordinary industrial and domestic layer-burning boilers. Table 3 lists the monitoring data and results. As for layer-burning boilers, 2712
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The number in parentheses is the ratio of partial Hg to total Hg.
64% of total Hg was sent into the atmosphere with 17.7% and 18.3% sent into bottom ash and fly ash, respectively. As for a pulverized-coal boiler, 74.4% of total Hg was emitted into the atmosphere and 25.6% into cinder. In the smelting and rolling of ferrous metals industry, coal consumption has a distinctive character. Coal is turned into coke, and then coke is used as a raw material in metal smelting and as fuel in the rolling process. Hg is emitted in both coke production and coke utilization. The coking process was monitored in a big coking plant. In the coking process, Hg in coal is transferred to coke and other byproducts such as coke oven gas, coking tar, raw benzene, raw phenols, and ammonium sulfide. On the basis of monitoring data and other data, the Hg emission factor of smelting and rolling of ferrous metals was calculated: 78.2% of total Hg of coals was emitted into atmosphere, 6.8% was emitted into ash, 14.5% was emitted into byproducts, and 0.5% was emitted into wastewater, respectively. In such industries as farming, forestry, mining and quarrying, manufacture, construction, transportation, residential consumption, etc., layer-burning boilers are used widely, and coal is the primary fuel. So the Hg emission factor of layer-burning boilers is adopted as these industries’ emission factor. Hg Emissions of China. In light of the average Hg concentration of coals and coal consumption of different industries in 1995, the amounts of Hg emission from different industries were calculated (Table 4). In 1995, 213.8 and 89.07 t of Hg were emitted into atmosphere and cinder or products, respectively, in the coal consumption process. Main Hg emission industries included power industry, smelting and pressing of ferrous metal, raw chemical material and chemical products, and residential consumption. These industries emitted 63.2% of total Hg that was sent into atmosphere in 1995. Table 5 shows the Hg emission of different provinces in China. The provinces in which Hg emission is more than 20 t/yr are Shanxi, Guizhou, Henan, and Sichuan. Main Hg emission regions are north and southwest China. The Hg emission intensity in Beijing, Shanghai, and Tianjin is higher (>0.2 kg/km2‚yr) in China. The provinces where Hg emission intensity is between 0.2 and 0.1 kg/km2‚yr are Shanxi, Guizhou, and Henan. Environment Risk of Hg Emission in China. Porcella (11) reported that global anthropogenic Hg emission into the atmosphere ranges from 2000 to 4000 t/yr-1, with the high end seemingly most accurate. In 1995, 218.3 t of Hg, which was at least 5% of global Hg emission, was emitted into atmosphere in the process of coal combustion or other utilization in China. Figure 1 shows the change of Hg emission in China. From 1978 to 1995, 2439.8 t of Hg had been emitted into the atmosphere. The average increasing rate was 4.8%/ yr. In some big cities, Hg pollution has become more
TABLE 4. Mercury Emission from All Kinds of Industries in China (1995) sector
coal consumption ( 104 t)
Hg into atmosphere (t)
Hg into cinder (t)
farming, forestry, animal husbandry, fishery industry mining and quarrying manufacturing nonmetal mineral products smelting and pressing of ferrous metal raw chemical materials and products other manufacturing electric power construction transportation, postal, & telecommunications wholesale, retail trade, and catering services others residential consumption total consumption
1856.7 117570.7 9861 63109.5 13424.2 12920.7 10803.5 36764.6 44600.3 439.8 1315.1 977.4 1986.7 13530.1 137676.6
2.61 185.52 13.88 98.78 22.15 22.23 17.83 36.57 72.86 0.62 1.85 1.38 2.8 19.05 213.8
1.47 71.93 7.81 40.10 7.38 6.20 5.94 20.58 25.26 0.35 1.04 0.77 1.57 10.72 89.07
TABLE 5. Mercury Emission of Provinces in China (1995) province
Hg into atmosphere (t)
Hg into cinder (t)
Beijing Hebei Shanxi Inner Mongolia Liaoning Jilin Heilongjiang Shandong Anhui Henan Sichuan Guizhou Yunnan Guangdong Guangxi Jiangshu Shaanxi Xinjiang Ningxia Qinghai Gansu Zhejiang Fujian Jiangxi Hubei Hunan Tianjin Shanghai Hainan
4.50 8.75 29.88 11.20 12.98 7.41 5.80 12.71 6.81 19.99 17.07 22.58 11.80 3.10 2.08 8.47 3.78 0.34 0.33 0.35 2.32 3.02 1.67 3.37 5.51 6.85 2.29 4.36 0.34
1.87 3.64 12.45 4.66 5.40 3.08 2.41 5.29 2.84 8.33 7.11 9.41 4.91 1.29 0.87 3.53 1.57 0.14 0.14 0.15 0.96 1.26 0.69 1.41 2.30 2.85 0.96 1.82 0.14
serious. Junhua (12) reported that Hg from coal combustion was a main pollution source in Beijing. It is very clear that Hg pollution from coal combustion is an important environmental problem in China. Measures that should be made to control Hg emission are as follows: to establish mercury emission environment law to control Hg emitting concentration and total quantity for those main Hg emission industries, to develop mercury remove technology, and to improve amount of washing coal.
Acknowledgments This research was supported by the Ministry of Chinese Science and Technology. We would like to acknowledge Jingyi Liu, An Peng, and Yuhuan Lin for their assistance in sample collection in the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
FIGURE 1. Change of mercury emission in China.
Literature Cited (1) Meij, R. Water Air Soil Pollut. 1991, 56, 21-33. (2) Lindqvist, O.; Jonhansson, K. Water Air Soil Pollut. 1991, 55, 1-32. (3) Lindqvist, O. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Boca Raton, FL, 1994; pp 181-185. (4) Chu, P.; Porcella, D. B. Water Air Soil Pollut. 1995, 80, 135-144. (5) Wang, Q.; Ma, R. China Environ. Sci. 1997, 17 (1), 76-79 (in Chinese). (6) Eary, L. E.; Rai, D.; Mattigod, S. V.; Ainsworth, C. C. J. Eviron. Qual. 1990, 19, 202-214. (7) Tao, S. Chongqing Environ. Sci. 1989, 11 (4), 87-90 (in Chinese). (8) Wang, Q.; Kang, S.; Chen, C.; Wang, Z.; Zou, S. Environ. Chem. 1996, 15, 27-35 (in Chinese). (9) Ni, J.; Feng, X.; Hong, Y. Environ. Chem. 1998, 17, 339-344 (in Chinese). (10) Chow, W.; Miller, M. J.; Forture, J. Power Eng. 1995, 1, 30-34. (11) Porcella, D. B.; Ramel, C.; Jernelov, A. Water Air Soil Pollut. 1997, 97, 205-207. (12) Liu, J.; Wang W.; Peng, A. Environ. Sci. 1998, 18 (3), 135-144 (in Chinese).
Received for review July 12, 1999. Revised manuscript received December 2, 1999. Accepted March 30, 2000. ES990774J
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