A Review of Key Hazardous Trace Elements in Chinese Coals

Review pubs.acs.org/EF

A Review of Key Hazardous Trace Elements in Chinese Coals: Abundance, Occurrence, Behavior during Coal Combustion and Their Environmental Impacts H. Z. Tian,*,† L. Lu,† J. M. Hao,‡ J. J. Gao,† K. Cheng,† K. Y. Liu,† P. P. Qiu,† and C. Y. Zhu† †

State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China ‡ Institue of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Hazardous trace elements (HTEs) can be released into the environment during coal utilization and lead to a high concentration of HTEs in the environment, especially the atmospheric environment, which could be harmful for human health and the eco-environment. This paper summarizes the main characteristics of eight prime environmentally concerning HTEs (Hg, As, Se, Pb, Cr, Cd, Ni, and Sb) in relation to their abundance and distribution, modes of occurrence, behavior during coal combustion, and atmospheric emissions. Particularly, the average content of eight HTEs in Chinese coals is calculated with the available field-test data by using mathematical statistical methods including bootstrap simulation. The bootstrap simulated mean concentration is estimated as Hg 0.20 μg/g, As 5.78 μg/g, Se 3.66 μg/g, Pb 23.04 μg/g, Cd 0.61 μg/g, Cr 30.37 μg/g, Ni 17.44 μg/g, and Sb 2.01 μg/g. Further, the abundance of HTEs in Chinese coals for different coal-forming periods, diverse coalbearing regions and coal ranks are reviewed. The modes of occurrence of these eight HTEs in coals and their behaviors during coal utilization are very complicated due to the diversity of coalification conditions and combustion conditions. Substantial emissions of HTEs from huge amount of coal utilization have been discharged into the atmosphere; atmospheric emissions from coal use in China in the year of 2007 were estimated as Hg 305.9 t, As 2205.5 t, Se 2353.0 t, Pb 12547.0 t, Cr 8217.8 t, Cd 245.4 t, Ni 2308.4 t, and Sb 546.7 t; the provinces with high atmospheric emission intensity are mostly located in highly industrialized and densely populated provinces in China. Thus, advanced HTEs control technologies and management strategies are in great need.

1. INTRODUCTION In recent years, more and more poisoning accidents associated with hazardous trace elements (HTEs) have been reported, and the adverse effects of HTEs on public health and the ecoenvironment are highlighted.1−4 Many national and international organizations, such as the World Health Organization (WHO), the Food and Agriculture Organization (FAO), the European Union (EU), and the United Nations Economic Commission for Europe (UN-ECE), have made efforts to survey the risks of poisoning accidents associated with HTEs and decide on countermeasures to reduce the associated risks on environment and human health. In the 1990 Clean Air Act Amendments (CAAA), 11 elements (including Sb, As, Cr, Pb, Cd, Hg, Ni, Se, Be, Mn, and Co) were listed as key toxic air pollutants, among which Hg, As, Se, Cd, Cr, and Pb were listed as priority elements.5,6 The European Union (EU) and the Canadian Environmental Protection Agency had also listed some HTEs (e.g., As, Cd, Hg, Ni, and Pb) as prime environmental concerns. Recently, with the extensive growth of Chinese economy and industry, more and more poisoning accidents associated with HTEs such as Pb, Cd, and As have happened in China, such as blood Pb excess in children in Fengxiang County of Shaanxi province, Qingyuan City of Guangdong province, Chenzhou City of Hunan Province, and Huaining County of Anhui province, and the Cr contamination incident in Qujing city of Yunnan Province, Cd contamination accident in Longjiang of Guangxi Province and Liuyang of Hunan province; and As © 2013 American Chemical Society

pollution in Guizhou, Hunan, Shandong, and Yunnan provinces.7−13 It is reported that approximately 20% of the total national farmlands have been polluted by HTEs such as Cd, Cr, and Pb.14,15 Airborne HTEs are considered to be one of the main reasons for these incidents and contamination,10,16−18 and indeed, energy consumption, especially coal combustion, and human health problems caused by HTEs have been firmly linked for decades in China.19−26 Until now, some assessments on Chinese and global emissions of Hg, Se, As, Pb, Cd, Cr, Ni, and Sb, and the contribution from various regions and sources have been presented.27−37 These studies indicated that fossil fuel combustion is one of the greatest sources of HTEs into the environment, and therein, coal consumption is regarded as one of the most important HTE emission sources.33−39 Even when presented in only parts per million levels in coal, substantial HTE discharge from huge amounts of coal utilization could lead to dangerous environmental and human health problems during coal mining, processing, and combustion processes.26,40 China is the largest producer and consumer of coal in the world now, accounting for 47% of the world’s total annual coal consumption.41 By the end of 2010, the national total primary energy consumption in China was reported at 3249.4 mtce (million Received: October 24, 2012 Revised: January 14, 2013 Published: January 15, 2013 601

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tonnes of coal equivalent), in which coal accounted for 71.9% of the total primary energy consumption.42 In addition, with the rapid economic growth and urbanization, it is projected that coal will still be the dominate energy type in China in the near future.43 Inevitably, the large quantities of coal consumed nationwide would contribute significantly to the total air burden of HTE discharge. Environmental impacts of trace elements are generally related to the concentration, toxicity, and mobility (modes of occurrence) of these elements in coals.1,23,44,45 From an environmental point of view, the content of HTEs in coals can provide useful information about pollution control during coal combustion and utilization; the modes of occurrence of an element can strongly influence its behavior during coal cleaning, weathering, leaching, combustion, and conversion.46 By now, some publications have reported the concentration, distribution, and origin of selected hazardous elements in Chinese coals sampled from different coal mines, coal fields, and coal-bearing regions.22−24,40,47−57 However, little information about the systemic and integrated assessment of HTEs in Chinese coals has been found, such as the average concentration and regional distribution, the modes of occurrence, as well as their behaviors and the associated emissions to the environment during coal combustion and utilization. In this paper, the abundance of eight key HTEs (Hg, As, Se, Pb, Cr, Cd, Ni, and Sb) in Chinese raw coals are reviewed based on integrated collection and analysis of field test data in published literature and reports; especially, a mathematic statistical methodbootstrap simulationis adopted to evaluate the variations on the average contents of these eight HTEs in Chinese coals by different provinces, different coal-forming periods, and different coal ranks. Moreover, some other important aspects related to geochemistry of these eight HTEs are reviewed, including the modes of occurrence, behavior during combustion, and the associated atmospheric emissions from coal use. However, due to the shortage of field test results of coal samples on Be, Mn, and Co, these three elements, which were also listed as key toxic air pollutants in CAAA-1990, are not reviewed in this article.

(2) Geometric mean: n

X̅g =

X1 + X 2 + ... + X n 1 = n n

i=1

∏ Xi (2)

i=1

n

∑ WX W X + W2X 2 + ··· + WnX n i i Xw = 1 1 = 1n ∑1 Wi W1 + W2 + ··· + Wn (3)

where X̅ w is the weighed mean content of HTE in coal, Xi is the tested content value of HTE in each coal sample, Wi is the replication times of Xi, n is the number of samples. (4) Bootstrap simulation method: The brief mathematical description of bootstrap simulation is as follows.34 A random sample X = (x1, x2, ..., xn) of size n is observed from a completely unspecified probability distribution F. The sampling distribution R (X, F) is the function of X and F. Assume θ = θ(F) is some parameter of F, Fn is the ̂ n) is the empirical distribution function of X, θ̂ = θ(F estimator of θ, and the estimation error can be expressed as R(X , F ) = θ(̂ Fn) − θ(F )

(4)

The basic steps of computing the distribution R(X, F) by bootstrap simulation are summarized as follows: ① The value of observed sample X = (x1, x2, ..., xn) is finite overall sample (called original sample), xi ∼ F(x), i = 1, 2, ..., n. The empirical distribution function of original sample is shown as ⎧0 x < x(1) ⎪ ⎪ Fn = ⎨ k /n x(k) ≤ x < x(k + 1) ⎪ ⎪1 x ≥ x(n) ⎩

(5)

where x(1) ≤ x(2) ≤ ... ≤ x(n) is the statistics of x1, x2, ..., xn sorted in ascending order. ② Monte Carlo simulation is used to randomly simulate N groups of samples x(j) * = (x1*, x2*, ..., xn*, j = 1, 2, ..., N (a very large number) from Fn, and this regeneration sample called bootstrap sample. The generation method of empirical distribution function by Monte Carlo simulation can be expressed as (a) generate random integer η with independence and uniformity between 0 and M (M ≫ n) by computer; (b) let i = η% n, and i is the remainder of n divide η; (c) find the sample xi as the regeneration sample x* in observed samples, and x* is the needed random sample. ③ Calculate the statistics of bootstrap samples: R *(R *, Fn) = θ(̂ Fn*) − θ(̂ Fn) → R n

(6)

where Fn* is the empirical distribution function of bootstrap sample. A small sample cannot derive θ(F), but θ̂(Fn) is used to approximate it. ④ Use the distribution of Rn (under given situation) to simulate the distribution of Tn, say: θ(F) ≈ θ̂ − Rn, which can receive N numbers of θ(F). Then, the distribution and eigenvalue of unknown parameter θ can be obtained.

n

∑ Xi

X1·X 2...·X n= n

where X̅ g is the geometric mean content of HTE in coal, Xi is the tested content value of HTE in each coal sample, and n is the number of samples. (3) Weighted mean:

2. BRIEF INTRODUCTION OF STATISTICAL METHODS INCLUDING BOOTSTRAP SIMULATION When compiling a national HTEs emission inventory, the average content in coal is required for coal combustion sources, especially for developing the regional and provincial emission inventory.32,33 In addition, the average content can also be useful in evaluating the geochemical characteristics of an element in regional coal.8 To obtain systematical assessment results of HTEs in Chinese coals, the average abundance of HTEs has been calculated and assessed by using four kinds of mathematical statistical methods, especially with the introduction of a new method, bootstrap simulation, which is based on a thorough collection of the field measurement results on HTEs concentration in Chinese coals from up-to-date available publications. The mathematical descriptions of these four methods are briefly introduced as follows: (1) Arithmetic mean: X̅a =

n

(1)

where X̅ a is the arithmetic mean content of HTE in coal, Xi is the tested content value of HTE in each coal sample, and n is the number of samples. 602

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Table 1. Comparison of Statistical Mean Content of HTEs in Raw Coals Nationwide in China (μg/g) bootstrap simulationa HTEs

RC

AM

GM

WM

BM

RU

world coals

U.S. coals77

Hg As Se Pb Cd Cr Ni Sb

0−19.10 0−232.00 0−70.60 0−436.33 0−10.20 0−971.00 0−193.00 0−159.05

0.26 6.01 3.89 24.59 0.67 31.98 16.74 1.27

0.15 3.96 1.92 13.98 0.22 15.01 13.31 0.89

0.20 5.77 3.67 23.01 0.61 30.44 17.43 1.94

0.20 5.78 3.66 23.04 0.61 30.37 17.44 2.01

−13.2% to +17.0% −10.0% to +10.2% −12.0% to +11.2% −8.6% to +8.6% −16.4% to +21.3% −10.1% to +11.7% −8.0% to +7.9% −30.0% to +38.1%

0.2078 8.3079 1.3079 25.0078 0.3078 10.0078 20.0080 1.0080

0.18 6.50 2.80 11.00 0.47 15.00 14.00 1.20

The bootstrap samples are generated by simulating N (N = 1000) times, with the fitted distribution of original samples. RC = Range of content, AM = arithmetic mean, GM = geometric mean, WM = weighted mean, BM = bootstrap mean, CI = 95% confidence interval, RU = relative uncertainty, SD = standard deviation. Note: the mean content of U.S. coals and world coals is arithmetic means. a

calculated with simple arithmetic means, and given by different authors and institutions, they show obvious variation, which can be partly explained by the difference in sources and numbers of HTEs contents data samples as well as analytical methods. In addition, the extensive content range of an element reported by different researchers makes it more difficult to the geochemistry characteristics of Chinese coal and developing a reliable atmospheric emission inventory from coal use, and it also increase the uncertainty of emission inventory of HTEs.33,36,37 3.1. Average Content and Its Variability of HTEs in Chinese Coals. To obtain systematic assessment results of HTEs in Chinese coals, we have calculated and assessed the average abundance with the four kinds of mathematical statistical methods introduced above, especially with the introduction of a new methodbootstrap simulation. For comparison, the arithmetic mean, the weighted mean with samples, the geometric mean, and the bootstrap simulation mean content of HTEs are calculated and shown in Table 1; the average contents of American and the world coals are also presented in Table 1 as a reference. The content range of eight HTEs in Chinese coals is also reviewed in Table 1, in which some of the extremely exceptionally high sample values (it means the sample value is far from their adjacent sample values and its frequency is only one time) are excluded. The geometric mean value of all eight HTEs in Table 1 is found to be some lower than the three other statistical means, while the bootstrap mean of these eight HTEs in Chinese coals is basically equal to the weighed mean but lower than the arithmetic mean except for Sb and Ni. Owing to the obvious difference of probability distribution in original data samples, both the bootstrap and the weighted mean could decrease the contribution of very low or high data with low frequency and better reflect the degree of concentration on the original source data by resampling (as introduced in section 2) and weighting with the duplication times of data samples, respectively. In contrast, the arithmetic mean reflects the contribution of very low or high data through equally accounting each original source sample only once. As a result, the bootstrap mean is basically equal to the weighed mean but lower than the arithmetic mean for the six elements (Hg, As, Se, Pb, Cr. and Cd), because there are only a few source data samples with very high content values. In terms of Sb and Ni, the bootstrap mean is basically equal to the weighed mean but higher than the arithmetic mean, owing to a certain number of data samples collected in Guizhou and Guangxi with obviously high content values (see Tables S2−S9 in the Supporting Information). For example, some coals mined in Guizhou are enriched in Sb and Ni, with the maximum content

As a numerical technique originally developed for the purpose of estimating confidence intervals for statistics, bootstrap can provide quantitative information to guide how future emission inventories are improved, and it has been successfully used to estimate the variations and uncertainties of average emission factors.34,58−61 More details of bootstrap simulation can be found in the relevant literature.34,62−64 In this article, bootstrap simulation is applied to evaluate the variability and uncertainty of the contents of eight HTEs (Hg, As, Se, Pb, Cd, Cr, Ni, and Sb) in different classification categories of Chinese coals. Based upon a review of available literature, the tested concentrations of eight HTEs in raw coals are collected and compiled as the original input data for bootstrap simulation (see details in Tables S1−S9 in the Supporting Information). Notably, Beijing, Shanghai, Tianjin, Hainan, and Xizang are not considered, because raw coal output in these areas is very small or even zero. Hong Kong Special Administrative Region, Macau Special Administrative Region, and Taiwan province are also not included because of a lack of data. Also, full particulars of the sources of samples were carefully checked in the cited references to avoid data duplication.

3. ABUNDANCE AND DISTRIBUTION OF HTES IN CHINESE COALS China is a huge country in terms of territory with 34 provinces (autonomous regions and municipalities), in which the coal reserves are very unevenly distributed and allocated. The coal reserves of China are dominantly deposited in the western and northern China, with Shanxi, Shaanxi, and Gansu Province and Inner Mongolia, Xinjiang Uygur, and Ningxia Hui Autonomous Region, possessing more than 80% of Chinese coal resources.65 Further, the concentration of HTEs in coals that are mined from different regions vary substantially and are largely determined by the distinction of coal-forming plants and coal-forming geological environments.24,65,66 There have been some studies reported of the average contents of HTEs in Chinese coals with different domain and different elements coverage.8,24,34,67−72 Here, the arithmetic mean, minimum value, and maximum value of published HTE contents in mined raw coal and the number of samples by provinces are reviewed and classified (Tables S1−S9 in the Supporting Information). In some previous studies,8,24,34,67−72 the national average contents are reported to be 0.71−3.68 μg/ g for Sb, 11.00−56.46 μg/g for Ni, 1.30−12.59 μg/g for As, 0.06−0.48 μg/g for Hg, 0.1−0.99 μg/g for Cd, 11.35−50.40 μg/g for Cr, 0.3−6.22 μg/g for Se, and 8.76−26.95 μg/g for Pb. However, these results on HTEs abundance are generally 603

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Table 2. Bootstrap Mean Concentration of HTEs in Different Coal-Forming Periods and Coal Ranks (μg/g) coal types

Hg

As

Se

Pb

Cd

Cr

Ni

Sb

C−P1 P2 T3 J1−J2 J3−K1 R bituminous anthracite lignite

0.29 0.39 0.50 0.24 0.27 0.09 0.26 0.62 0.24

3.71 6.99 10.37 3.94 6.49 12.46 5.26 4.86 11.79

3.35 4.33 2.76 0.98 0.91 1.19 3.77 5.83 1.98

22.34 24.07 21.81 15.00 15.80 20.24 18.63 22.77 14.90

0.37 0.69 0.82 0.26 0.18 0.44 0.44 0.99 0.49

19.36 31.24 33.43 12.52 22.80 34.46 18.33 22.62 17.94

15.66 26.21 20.30 10.80 13.03 35.84 16.89 15.73 21.86

0.78 1.65 3.90 0.90 1.17 1.77 1.10 0.78 1.14

in anthracite coal. Notably, these findings are only suitable for the nationwide statistical levels and the enrichment of HTEs in different coals may change substantially in special regions. For example, Zhang and coauthors21 indicated that Shanxi anthracite is highly enriched in Hg and Se; bituminous coals are highly enriched in Hg, Cd, Sb, and Se, and the brown coal only contains a higher concentration of Cd. Wang et al.83 had also reported that anthracite coal tended to have a higher concentration of Hg than bituminous coal. The relative uncertainty of HTEs concentrations by different coal forming periods and ranks are also presented (please see Table S10 in the Supporting Information). The uncertainty of HTEs concentrations from T3, J1−J2, J3−K1, and R coals are relatively higher than the other two coal-forming periods. This is mainly because of the collected coal samples that are not evenly distributed either in space or in geological ages. In China, most coal samples are collected from C−P1 coals in Northern China and P2 coals from Southwestern China, while coal samples from T3, J1−J2, J3−K1, and R coal-forming periods are limited in the available published literature. Therefore, in future, detailed field test research and more extensive coal samples from these four coal-forming periods are necessary for improving the representativeness and reducing the uncertainty of trace element abundance assessment in Chinese coals. 3.3. Spatial Distribution of HTEs Content in Coals of Different Provinces and Coal-bearing Regions. The distribution of bootstrap simulated concentrations for the eight HTEs in coals from twenty-six provinces (where raw coal is mined and produced in Chinese official statistics) is listed in Table 3. Here, only the bootstrap mean values are displayed; the other statistical parameters as well as the distribution maps of each element can be seen in Table S11 and Figures S1−S8 in the Supporting Information. It should be noted that the ShenfuDongsheng mining area (Shendong) is included separately, located between the north Shaanxi and south Inner Mongolia as one of the biggest thermal coal production base. For the provinces with very few data samples, such as Guangdong, Gansu, and Zhejiang, in which only one or two averaged content as the original samples are reported, statistical analysis is only conducted on original data, not simulated by bootstrap method. There are mainly five coal-bearing areas (see Figure 1), namely, Northern China, Northeastern China, Northwestern China, Southern China, and the Yunnan-Tibet coal bearing area. In this study, the Southern China and Yunnan-Tibet coal bearing areas are combined together as a single coal bearing area named the Southern China coal bearing area, due to lack of original sample data in the Yunnan-Tibet coal bearing area. The provinces reported with relatively more data sets of HTEs are Guizhou, Shanxi, Anhui, Yunnan, and Inner Mongolia, since many surveys have been done in these areas

of 76.2 μg/g for Sb and 160 μg/g for Ni.20,23 Consequently, the calculated arithmetic mean of Sb and Ni in these Guizhou coals are lower than the bootstrap mean and weighted mean. The national arithmetic averaged contents of Hg, As, Se, Ni, and Sb is somewhat different from the contents reported in the previous publications,33−37 mainly due to the newly updated data sample sources.73−76 It has been demonstrated that coal samples after bootstrap simulation are more centralized, and the bootstrap simulation is a reasonable representation of variability and uncertainty in assessing the average contents of HTEs and their confidential intervals.34,58−61 Therefore, perhaps the bootstrap mean value is much close to the real situation and is more appropriate to evaluate the atmospheric HTEs emissions during coal combustion instead of geometric mean and arithmetic mean values. The ranges of uncertainty (95% confidence intervals) on the average content of total bootstrap samples for nationwide are also calculated (see Table 1). The relative uncertainty ranges of As, Pb, and Ni are nearly symmetrical, and the uncertainty is relatively small, only −10.0% to +10.2% for As, −8.6% to +8.6% for Pb, and −8.0% to +7.9% for Ni. In contrast, even though the uncertainties of the other five HTEs are also relative small, the uncertainty ranges for these five HTEs are distributed positively skewed. 3.2. Distribution Characteristics of HTEs Contents in Coals of Different Coal-Forming Periods and Coal Ranks. The concentrations of trace elements in coals are influenced by various factors such as source material, depositional environment, climate, and hydrologic conditions. Moreover, the tectonic setting, coal rank, and geochemical nature of groundwater and country rocks also have significant impacts during the coal-forming process.1,66,81 Consequently, the abundance of HTEs varies diversely in different coal-forming periods and coal ranks of Chinese coals. All the coal samples cited in this article are sorted into six mainly coal-forming periods, namely, Carboniferous to Early Permian (C−P1), Late Permian (P2), Late Triassic (T3), Early to Middle Jurassic (J1−J2), Late Jurassic to Early Cretaceous (J3−K1), and Tertiary (R).8,82 Further, the input coal samples are classified into three main coal ranks: anthracite, lignite, and bituminous coal. The simulated results by bootstrap for different coal-forming periods and coal ranks are illustrated in Table 2. All the eight HTEs are found to be enriched in Late Permian (P2) and Late Triassic (T3) coals, especially the T3 coals, while they are relatively low enriched in the J1−J2 and J3−K1 coals. This is consistent with previous conclusions by Ren et al. (2006).8 The abundance of HTEs from different coal ranks are also shown in Table 2. It is clear that the concentration of Hg, Se, Pb, and Cr decreases gradually from anthracite through bituminous to lignite. While Ni, As, and Sb are more enriched in lignite than in bituminous, they are minimally concentrated 604

dx.doi.org/10.1021/ef3017305 | Energy Fuels 2013, 27, 601−614

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Table 3. Spatial Distribution of Bootstrap Average Content of HTEs in Coals by Different Provinces and Coal-Bearing Regions (μg/g) province

Hg

As

Se

Pb

Cd

Cr

Ni

Sb

Anhui Chongqing Fujian Gansu Guangdong Guangxi Guizhou Hebei Heilongjiang Henan Hubei Hunan Inner Mongolia Jiangsu Jiangxi Jilin Liaoning Ningxia Qinghai Shaanxi Shandong Shanxi Shendong Sichuan Xinjiang Yunnan Zhejiang Northern China Northwestern China Northeastern China Southern China

0.43 0.31 0.07 0.27 0.07 0.33 0.39 0.15 0.12 0.20 0.20 0.12 0.22 0.69 0.16 0.40 0.17 0.22 0.25 0.21 0.18 0.17 0.41 0.29 0.06 0.36 0.65 0.23 0.18 0.21 0.26

2.89 5.66 9.93 4.14 8.30 16.94 6.68 4.88 3.42 2.20 5.30 10.59 5.77 2.74 7.41 11.57 5.51 3.65 2.68 3.87 5.23 3.84 1.22 5.38 2.97 8.82 12.04 3.92 3.86 6.62 7.38

7.54 3.69 1.22 0.51 0.60 5.03 3.82 2.31 0.90 4.86 8.76 3.72 1.10 6.11 8.39 4.06 0.85 4.27 0.30 3.43 3.66 3.85 2.63 3.31 0.24 1.48 12.02 3.42 3.84 1.61 5.47

13.24 30.44 25.53 8.35 24.40 29.94 23.81 29.30 22.15 16.78 47.39 26.29 26.67 20.98 19.33 29.00 19.68 14.05 10.72 35.17 16.64 26.23 14.84 28.29 2.68 42.54 17.25 20.54 5.55 19.94 19.96

0.11 1.22 0.31 0.08 0.25 0.41 0.79 0.23 0.13 0.54 0.36 0.64 0.10 0.06 0.56 0.15 0.16 1.10 0.03 0.75 0.39 0.75 0.15 1.95 0.12 0.80 0.47 0.67 0.08 0.15 0.78

31.25 28.44 30.48 23.70 74.00 116.41 28.47 32.52 15.48 24.94 40.52 37.03 13.02 19.82 39.75 23.09 26.24 10.63 30.82 32.73 20.62 21.57 27.04 33.00 7.83 73.62 24.20 19.91 12.31 22.68 34.26

19.57 20.90 16.42 19.30 24.90 22.48 22.87 14.61 10.49 11.84 18.61 13.25 6.35 15.48 22.66 15.34 24.13 10.95 12.20 18.86 23.77 15.41 9.82 19.28 8.26 24.32 9.95 13.55 6.34 18.52 23.52

0.25 1.71 0.38 0.7 n.d. 5.55 6.01 0.41 0.79 0.37 1.17 1.54 0.70 0.55 1.83 1.02 0.81 0.27 0.91 2.95 0.47 1.13 0.47 1.70 0.67 0.97 0.73 0.74 0.75 0.65 1.85

and Gansu (0.08 μg/g); lower Cr values are observed in Xinjiang (7.83 μg/g), Ningxia (10.63 μg/g), and Inner Mongolia (13.02 μg/g). This is because most of these regions are located in the Early−Middle Jurassic (J1−J2) coal bearing area in Northwestern China, where the HTEs contents are lower than other coal basins (as mentioned in section 3.2). Pb, Hg, and Ni contents of coals in Northwestern China are also the lowest, compared to the other three coal-bearing areas (see in Table 3). Geographically, Se and Sb contents are lower in Northeastern China coal-bearing region but higher in Southern China coal-bearing area, which is closely in agreement with findings by Ren et al. (2006).8 It is necessary to point out that the relative uncertainty of HTEs content in some provinces is high (see Table S11 in the Supporting Information). For example, the uncertainty of Hg contents in Jilin and Jiangsu provinces demonstrate relatively high uncertainty ranges of more than 100%; Sichuan, Shaanxi, and Shendong also show big uncertainty ranges of Cd contents more than 100%. This is because the source data samples in these areas are more discrete, which might be related to coals of different classes, ages, and seams analyzed by different researchers and institutions. For provinces with few data samples, such as Guangdong and Gansu, where the statistical analysis is only conducted on original data and not simulated by bootstrap, the uncertainties are no doubt the largest. The shortage of the original coal samples contributes to the great uncertainty. Overall, both the number of field test coal samples and their representativeness on regional distribution is still very limited,

Figure 1. Distribution of coal bearing areas and the production of coal by each type.

because of their rich-reserves, intensive coal mining, and typical landscape characteristics. For regional distribution of HTEs content, As (arsenic) is taken as an example in this article. There are nine provinces where the bootstrap mean content of As is higher than the national average level of 5.78 μg/g. The top five high mean values are Guangxi (16.94 μg/g), Zhejiang (12.04 μg/g), Jilin (11.57 μg/g), Hunan (10.59 μg/g), and Fujian (9.93 μg/g). This is closely consistent with the results reported by Wang and his coauthors.84 Thus, coal mined from these provinces should give rise to great environmental concerns during combustion and utilization. The provinces and districts with lower As content are Shendong (1.22 μg/g), Xinjiang (2.97 μg/g), and Jiangsu (2.74 μg/g); lower Cd contents are found in Qinghai (0.03 μg/g), Jiangsu (0.06 μg/g), 605

dx.doi.org/10.1021/ef3017305 | Energy Fuels 2013, 27, 601−614

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Table 4. Summary of the Modes of Occurrence for Eight Key HTEs in Chinese and World Coals HTEs Hg As Se Pb Cd Cr Ni Sb

behavior groups

possible modes of occurrence sulfide-bound (mainly pyrite), clays-bound (correlated with Ca and Al), and organic-bound sulfides affinity (pyrite and chalcopyrite); organic associations; carbonates and silicates organic association; pyrite; accessory selenides aluminosilicate association (low sulfur coal), sulfide affinity (high sulfur coal) galena; organic association; with Fe, sulfide (pyrite-rich coals), associated with aluminosilicate; selenides mainly organic affinity; sulfide; sphalerite organic association and clay (mainly illite), chromite (FeCrO4) (ultramafic deposit origin) multiple associations: sulfides, organics, aluminum-silicate, carbonates affinity; organic and inorganic affinity: organic association, pyrite and accessory sulfides silicates, silicate.

refs

group III

44, 54, 74, 87, 91, 98, 105, 122−124

group II

44, 74, 87, 98, 105, 112, 123, 125−133

group II group III group II

40, 44, 46, 51, 55, 74, 87, 98−105, 123, 126, 133 44, 74, 87, 98, 105, 106, 115, 124−126, 130, 133, 134

group group group group group group

44, 87, 105, 116, 134−136 44, 74, 87, 96, 98, 105, 116, 125, 126, 130, 132−134, 137 44, 74, 87, 98, 105, 123, 125 127−130, 132−134 20, 25, 40, 44, 51, 74, 87, 98, 106, 107, 123, 125, 130, 133, 138−142

II II I II I II

high As content but also closely correlated with the utilization mode of coal as well as the residents’ special living conditions and dietary habit, especially in the western remote areas of China.16,17,73,143 Therein, Guizhou is one of the most typical areas of As contamination, mainly because thousands of the villagers use the pot-bellied coal stove for cooking and heating food and meat indoor without any air pollution control devices, easily leading to As contamination in these areas. It is reported that over 330 000 people in Guizhou were subjected to As poisoning as a result of burning local raw coal indoors, and As poisoning was also found in the Qinling-Bashan mountain area of Shaanxi Province.143 By now, there has been some research using modern techniques and testing methods to study As in Chinese coals. As is mostly correlated with sulfur, especially with pyritic sulfur and organic sulfur, and it could be a substitute for sulfur in pyrite.127−129,144−148 Zhao et al.147 concluded that As concentrations in Chinese coals decreased in the order: sulfide > organic > arsenate > silicate > soluble and exchangeable fractions. Chen et al.127 determined 147 Chinese coal samples from different coal-forming ages and ranks and found that inorganic As mainly occurs in pyrite. However, in organic association, Zhao et al.149 found that As occurred as arsenate (As(V)) and arsenite (As(III)), not in the form of sulfide affinity or As-containing minerals. Ding et al.150 also indicated that As in coals in the organic form mainly existed as As (V). Overall, among the As forms (residence) in Chinese coals, three are dominant: pyritic, organic, and arsenate, as confirmed by Kang et al.73 Locally, some other forms, such as in clay, arsenopyrite, etc., are also possible. Hg contamination in China has also drawn worldwide attention as well as As and other toxic metals. It was indicated that there were no fewer than three forms of Hg in coals, that is, sulfide-bound, clay-bound, and organic-bound.91 The most likely mode of occurrence of Hg in coal is as solid solution in pyrite,54,56,81,87,94,151,152 especially in coals extremely enriched in Hg, such as some coals in Guizhou province.151,153 Hg also has a strong organic affinity1,154−156 and can be claybound.155−157 In low-sulfur coals, Hg has a strong affinity with pyrite and is also organic-bound, while in high-sulfur coals, the most likely mode of occurrence of Hg is as solid solution in pyrite. Silicate-bound Hg may be an important form in some coals because of magmatic influence.54 In addition, there are some studies reporting Hg correlated poorly with sulfur.158 The study of Wang et al.105 also showed that Hg was not generally

especially for several provinces where there are only one or a few data samples. Thus, extensive investigation and much more field tests are required to better understand the abundance and spatial distribution of HTEs in Chinese coals.

4. MODE OF OCCURRENCE OF HTES IN COALS The mode of occurrence of an element refers to how the element is chemically bound and physically distributed throughout the coal. It includes information about the specific mineral and element forms, dispersion within a particular host mineral or maceral, the fraction of the coal the element is associated, the oxidation state that the element occurs, and other factors. The mode of occurrence can also determine the toxicity and behavior of an element during the processes of coal mining, processing, and utilization.85 To discuss the distribution and the associated atmospheric emissions of HTEs from coal use, an understanding of the coal mineralogy of HTEs and its association with inorganic or organic phases is necessary.36,37,86 Therefore, knowledge about the modes of occurrence of HTEs is crucial to the integrated assessment of its recycling and utilization potential and to understand the pathways that is introduced into the environment. To date, a number of studies on the modes of occurrence of HTEs in coals have been carried out throughout the world.73,86−93 The HTE contents and modes of occurrence vary from coal to coal due to the complex coal-forming processes.86,92−96 Finkelman44 found that Cd was mainly associated with sphalerite in coals; Hg and As were found mostly with iron pyrite; Pb occurred in gelenite; and Se was associated in organic matter, pyrite, sulfide, and selenides. Finkelman44 also concluded that the confidence levels of the modes of occurrence of these five elements were relative consistent, whereas for other elements, such as Cr, Ni, and Sb, the confidence levels were low, and little agreement was observed. Davidson97 proved this conclusion by comparing the multiple density separation and sequential leaching results. Chen et al.98 has made a conclusion that As, Pb, Se, and Ni are likely to occur in sulfide minerals;51,87,89,93,99−111 Cr and Pb are associated with clays;53,87,105−107,110,112−116 Ni is related to carbonate minerals;105,112,113,117 while As, Se, Ni, and Cr may be combined with organic matter.99,104,110,118−121 Here, we have reviewed the possible modes of occurrence of these eight key HTEs, as summarized in Table 4. In China, As poisoning associated with coal use is very prominent, it is not only related to the burning of coal with 606

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vapor or gas phase when coal is burned in high temperature. For those elements in group I, when pulverized coal is burned in boiler furnace, part of these elements as purities of coal will evaporated as liquid or solid state particles and suspended in the flue gas, and thus, they will also go through adsorption, condensation, and chemical transformation as the flue gas cooled down in the postcombustion processes,86,175,176 and are easily enriched in fine particles less than 10 μm, particularly in submicrometer particles with mostly sulfate and aluminosilicate affinities. As, Cd, Hg, Sb, Se, and Pb exhibit segregation in the fly ash, while Cr and Ni show no segregation between fly ash and bottom ash. In addition, the enrichment factor of elements increases with decreasing flue-gas temperature and particle size.176 Besides the behavior of HTEs during coal combustion, analytical speciation is necessary to understand their mechanisms of transport and thus their environmental impacts. Among the eight HTEs, Hg, As, and Se belong to the most volatile HTEs, and the total in-stack concentrations of these elements in the vapor phase are reported: up to 98% Hg as Hg, HgO, and CH3Hg; up to 59% Se as Se and SeO2; and 0.7−52% As as As2O3.177 Shah et al.178 studied the speciation of As, Cr, Se, and Hg in coal fired power station conditions in Australia and found that the majority of As in fly ash was present in the less toxic As5+ form and 10% of total As was found to be in the more toxic As3+ form; Se was mainly found as Se4+; Cr6+ was found concentrated in fly ash compared with feed coal; and about 58% total Hg was emitted through flue gas as Hg0. Pb chemical species during coal combustion are predicted to be PbO in molten fly ash, and Se are as gaseous SeO2.167 For Sb, SbO(g) is suggested as the dominant form in high-temperature flue gas,179 and speciation analysis by coal fly ash extracts suggests that the major Sb species are Sb4+ or Sb3+, largely depending on the fly ash samples.180,181 Meng182 reported that, with the temperature below 350 K, the mode of occurrence of Sb is Sb2(SO4)3 (s), then Sb2(SO4)3(s) generated Sb2O5(s) at 350 K; at 750−850 K, Sb2O5(s) begins to decompose to Sb2O4(s); the SbO(g) is the dominant form when the temperature higher than 1100 K. Hg species released from coal combustion process to the environment include three forms: elemental (Hg0), oxidized (Hg2+), and particle bound (HgP). Hg is chemically bound in coal, and during combustion process, it is entirely liberated in its elemental form (Hg0).183 Hg0 could be partly converted to the oxidized form (Hg2+) and partly associated with fly ash particles in the postcombustion zone, and it has been reported that Hg oxidation in the boiler is kinetically controlled and homogeneous oxidation reactions are mainly promoted by chlorine and atomic chlorine.184 Ni is expected to be in the form of oxides, Ni sulfates, and lesser amounts of metallic Ni during high temperature combustion process.185 The prevalent oxidation state of Ni is Ni2+, and other valences (−1, +1, +3, and +4) are also encountered, though less frequently.186,187 During coal combustion, HTEs will be released and redistributed into bottom ash, fly ash, fine fly ash, and even the gaseous phase. These HTEs enriched in fine particles, especially particles less than 2.5 μm, could participate in kinds of chemical reactions in the atmosphere, polluting soil and water environment through transmission and deposition, and even affecting human health through direct intake or food chain.10,16−18 Therefore, the vaporization, nucleation mechanism, the condensation, and the emission in gases, fly and bottom ashes with HTEs deserve to be further investigated.

rich in pyrite-rich coals, differing from many other chalcophile elements. Generally, Pb in coals mostly occurs in gelenite; Ni is likely to occur in sulfide minerals and be related to carbonate minerals; Cd is associated with sphalerite; Se and Sb occur in pyrite; Cr is associated with clays. In addition, all these six elements are possibly combined with organic matter in coals, especially Cd, Sb, Cr, and Se.74−78 However, due to the complexity of coalification conditions, as well as differences among multiple test methods, further investigation about the modes of occurrence of HTEs is greatly needed.

5. BEHAVIOR OF HTES DURING COAL COMBUSTION Some publications have discussed HTEs released from coal combustion with respect to the following aspects: the HTE size-distribution in fly ashes and their enrichment in the submicrometer particles;94,159,160 the formation and transformation of fly ash particles;161,162 the direct gaseous emissions of several volatile HTEs (i.e., Hg, Se and As); and the mobility and leaching behavior of HTEs in coal.86,135,163,164 The behavior of trace elements during coal combustion depends largely on their volatility, with the volatilization trends of trace elements relying on their modes of occurrence, the combustion conditions, combustor structure, as well as the rank of coals.165−170 Trace elements can be categorized into three main groups regarding to their partitioning during coal combustion.85,94,169,171,172 Group I elements are mainly concentrated in the coarse residues or are partitioned equally between coarse residues and suspended particulates (fly ashes in flue gas) (e.g., Mn, Co, V). This group of elements is hardly vaporized and so is equally distributed between bottom ashes and fly ashes and could be normally removed by the conventional particulate control systems. Elements such as As, Cd, Pb, Sb, and Se are in group II and are concentrated more on the fine-grained particles (normally refer to those particles whose aerodynamic diameter is less than 10 μm), which may escape from particulate control systems. These elements show increasing enrichment with decreasing size of fly ash and cooler flue gas and are enriched in fine particles with large specific surface area. Elements such as Hg belong to group III elements, which are readily volatilized during the combustion process and are mainly concentrated in the vapor or gas phase. In addition, Cr and Ni show intermediate partitioning behavior between groups I and II, and Se may display partitioning behavior intermediate between groups II and III. Vaporization behavior is closely related to HTEs’ partitioning in emission streams as well as their different enrichment phenomena. The dynamics of trace elements release during coal pyrolysis process suggested that more than 90% for Cd, 85% for Hg, about 80% for Pb, about 40% for Se, and 30% for Ni were released at the temperature of 1000 °C, and elements such as Ni, Cd and Pb were released to great extent already at the heating temperature of 400 °C.173 Li et al.174 found that, in a stoker-fired combustion unit, elements As, Cr, and Ni were predominantly partitioned in the glassy and refractory bottom ash fractions; Se and Pb were mostly partitioned in fly ash fractions; and volatile elements, including over 90% of Hg and up to 50% of Cd, were mainly partitioned in the flue gas. Vejahati et al.85 indicated that As, Cd, Sb, and Pb performed with a dual behavior during coal combustion: volatilization and condensation on particles of high specific surface. To say that an element is volatile, such as Hg, means that it will become 607

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6. ATMOSPHERIC EMISSIONS AND CONTROL MEASURES OF HTES FROM COAL COMBUSTION Coal is one of the most important sources of energy throughout the world.188 In China, coal combustion in electric power plants and industrial, commercial, and residential sectors has been recognized as one of the major anthropogenic sources of many metals and metalloids into the atmosphere.36−39 It was indicated that the total quantities of trace elements involved in coal combustion were very large, being roughly comparable to the quantities annually mobilized by the natural process of weathering of crustal rocks.189 Recently, atmospheric emissions of different kinds of HTEs from various anthropogenic sources have been reported worldwide.185,190−199 In this article, emissions of typical HTEs from coal combustion as estimated by worldwide and China authorities are summarized in Table 5. According to the global Hg emission assessment, China was the largest single Hg emitter in the world of global emissions.190,199 Studies of Hg emissions in China first focused on anthropogenic sources, especially from coal combustion.206,208,209 Wang et al.206 and Zhang et al.207 were the first to report Hg emissions from coal combustion in China by using coal consumption data and Hg emission factors, citing a value of 213.8 t for the year 1995. Streets and his coauthors then developed comprehensive inventories of Hg emissions of China with high spatial and temporal resolution.195,208−210 In addition, Zheng et al.211 developed a regional Hg emission inventory for the Pearl River Delta (PRD) of China, and they indicated that coal combustion was also the dominant contributors, accounting for 28% of the regional total emissions. Since this century, Hg emission inventories from man-made sources have been developed comprehensively in China.33,195,208,206,212−214 However, the historical trend and current situation of other HTEs emission such as As, Se, Cd, and Pb in China, which have caused more and more environmental problems and poisoning accidents nationwide, are still relatively limited. Tian et al.33−37 have developed relative detailed and completed inventories about anthropogenic atmospheric emissions of Cd, Ni, Sb, Pb, Cr, As, and Se by using the best available emission factors and annual activity levels during the period of 1980−2009, and the temporal and spatial distribution characteristics were also presented. According to the previous studies, industrial coal combustion sector is found to be the biggest single sector for atmospheric HTEs emissions in China, followed by the power generation sector. At the provincial level, atmospheric emissions of HTEs caused by coal combustion are mainly concentrated in more populated and industrialized areas of China where intensive coal use happened, that is, the Yangtze River Delta, the Pearl River Delta, and the Beijing-Tianjin-Hebei region (please see Figures S9−S10 in the Supporting Information).33−37 During the past 11th-Five-Year Plan (2006−2010), the Chinese government has implemented a series of countermeasures to reduce the air pollutants during coal combustion process, for example, shutdown the small utility boilers in thermal power plants, installing advanced air pollution control devices (such as the SNCR and SCR denitrification systems to reduce NOx, high efficiency ESP or fabric filters for capturing particles, and FGD to abate SO2) after power generation and industrial boilers, and these initiatives have achieved significant cobenefit effects for HTEs removal, for example, the emissions of Sb and Ni from power generation sector had declined since 2005 even though the volume of coal consumption kept increasing.35,36

However, with the rapid industrialization in China, the total coal consumption has sharply increased from 2318.51 Mt in 2005 to 3122.36 Mt in 2010.42 As a result, the declined share of HTEs emissions caused by advanced APCDs might be offset by the added large coal combustion (please see Figure S11 in the Supporting Information for more detail). Thus, much more actions should be promulgated and carried out including both technological and managerial measures. Emissions of HTEs from coal burning especially coals with high HTEs content, could elevate the atmospheric HTEs concentration in ambient air, and then, the elevated HTEs concentrations pose a threat to human health and environment directly by human consumption via windblown dust or the hand-to-mouth pathway or by depositing into water and soil environment, thereafter jeopardizing human beings. Recently, control and reduction of atmospheric HTEs from coal combustion have aroused great concern national wide in China, because their adverse impacts of on regional environment and human health risks, as well as the long-distance transport.3,33,208 Lung disease caused by inhaling HTEs contained coal dust is reported in Shenbei Coalfield, Liaoning Provinces.215 In the two notable seleniferous regions in China, the Ankang region in Shaanxi Province and Enshi Prefecture in southwest Hubei Province, many rural inhabitants suffered from Se poisoning owing to the exposure and burning of high Se content hard coal.101,216 Guizhou is classified as the heaviest coal-burning As poisoning district in China.143,217 It is also reported that humans might suffer from various health problems if long-time exposed to airborne contaminants with high concentration HTEs, such as pulmonary dysfunction, cardiovascular dysfunction, and even cancer.13 However, so far in China, the specialized technologies for HTEs removal during coal utilization are quite limited. The existing methods are primarily dependent on the cobenefit of conventional Deduct, De-SO2 and De-NOx technologies, and indeed, these technologies have achieved certain effects for HTEs removal (the removability of different technologies is summarized in Table S12 in the Supporting Information). During the past Five-Year-Plan (2006−2010), different levels of control strategies have been employed in coal utilization in China. Coal cleaning before burning such as coal washing is regarded as an optional effective way to reduce HTEs content in coal, although the main purpose is to wash away part of the ash and sulphide (such as pyrite) and to improve the heating value of coal. The removability of HTEs during physical coal cleaning is mainly determined by the modes of occurrence of HTEs in coals as well as the wash ability of inorganic minerals (the wash ability of different HTEs is summarized in Table S12 in the Supporting Information). It is reported that a large proportion of Hg associated with pyrite can be washed away simultaneously, and almost all the physical coal washing processes tested on Chinese coals demonstrate Hg removal efficiency higher than 50%.218,219 Coal cleaning has the potential to be part of an overall emission control strategy. Overall, clean coal technology development is still a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emissions, not only for conventional air pollutants (PM, SO2, NOx) but also for many kinds of toxic trace elements such as Hg, As, Pb, Sb, etc. The other strategy is to capture trace elements during combustion or postcombustion using different physical and chemical methods, such as capturing the trace elements by using adsorbents and particulate control devices (cyclones, electrostatic 608

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a

609

Hg 660−3512 313 1121 2235 − 19.8 239 2−8 2.6−17.6 190 20.1 1.01−6.32 861 164.7 177.1 284.1 305.9 − − 202.4 213.8 202.4 204.3 256.7 334.0 − − − −

− − − −

Se 900−2755 838 1982 4601 − − − − − − − − − 1494.1 1609.5 2222.7 2353.0 − − −

As 430−3530 607 2416 5011 − − 763 − − − − − − 1412.0 1419.5 2083.4 2205.5 − − − − − − −

1765−14550 28091 51212 119259 − − 13156 − − 1000 − − − 7505.6 6986.0 10950.9 12547.0 12561.8 − −

Pb

− − − −

3920−19630 3353 6234. 14730 2.52 − 2711 − − − − − − 5003.0 4501.9 6933.8 8217.8 8593.4 − −

Cr

atmospheric emissions (t/year) Cd

− − − −

176−892 362 1463 2983 − − 590 − − 550 − − − 131.0 130.5 215.2 245.4 261.5 − −

Ni

− − − −

3275−24150 20417 41228 95287 − − 4797 − − − − − − 1422.6 1338.3 2095.3 2308.4 2393.1 2492.3 −

−: not mentioned in original article. Atmospheric emissions means the HTEs emitted into the air including the fraction both in total particulate matter and in gas phase.

2000 2005 2005 2005 2005 2007 2008 1995 2000 2005 2007 2008 2009 1995 1995 1999 2000 2003 2005

area

worldwide Europe Asia worldwide Finland Mediterranean basin Europe Austrian South Africa Europe Poland Korea worldwide China China China China China China China China China China China China

year

1983 1995

Table 5. Summary of HTEs Emission Estimation from Coal Combustiona Sb

− − − −

353−2255 273 694 1561 − − − − − − − − − 300.1 330.8 530.9 546.7 490.4 505.3 −

refs

195 206, 207 208 195 195 32

38 39 39 39 200 201 29, 202 196 197 203 204 205 199 33−37

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precipitators, fabric filters, and wet scrubbers). Besides, the conventional air pollution control devices that are widely utilized on coal-fired utility boilers or industry boilers for reducing NOx, SO2, and PM, will affect HTEs speciation and are effective in reducing the final HTEs emissions into the atmosphere. In the 12th Five-Year-Plan (2011−2015), more and more combustion devices in power generation and industrial sectors will be mandated to install De-NOx (nitrogen oxides) systems (mainly selective catalytic reduction [SCR] and selective noncatalytic reduction [SNCR]) for minimizing NOx emission; thus the HTEs emission rate per ton of coal will decline further owing to the much higher cobenefit removal efficiency by the combination of SCR+ESPs/FFs+WFGD systems.33−37 Therein, SCR+ESPs/ FFs+WFGD configuration will be the main path to abate Hg discharge from coal-fired power plants in China in the near future.220 However, some advanced multiple-pollutants simultaneous control technologies as well as specific HTEs removal technologies (such as ACI for Hg removal) are necessary for further reduction of elemental HTEs discharge in the long term in China.

fly and bottom ashes with HTEs deserve to be further investigated. (4) In China, coal combustion is one of the dominate sources of atmospheric HTEs emissions. Atmospheric emissions from coal used in China in the year of 2007 were calculated as Hg 305.9 t, As 2205.5 t, Se 2353.0 t, Pb 12547.0 t, Cr 8217.8 t, Cd 245.4 t, Ni 2308.4 t, and Sb 546.7 t, respectively, in 2007. At the regional level, emissions of HTEs are mainly concentrated in the more populated and industrialized areas of China. With the cobenefit of reduction of HTEs by the existing APCDs, HTE removal has achieved certain effects. However, along with the rapid development of Chinese economy, HTE emissions from coal utilization would still be the core of Chinese environmental management and pollution control. Both advanced control technologies and comprehensive managements are greatly in needed for HTE reduction in China.

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ASSOCIATED CONTENT

S Supporting Information *

7. CONCLUSIONS

Twelve tables (Tables S1−S12) and eleven figures (Figures S1−S11) as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org/.

(1) The abundance and distribution of eight typical HTEs in Chinese coals were overviewed on the basis of a thorough investigation of available literature up to date. Especially, the content of eight HTEs (Hg, As, Se, Pb, Cd, Cr, Ni, and Sb) in Chinese coals is assessed by bootstrap simulation method, and the national simulated mean content is concluded to be 0.20 μg/g for Hg, 5.78 μg/g for As, 3.66 μg/g for Se, 23.04 μg/g for Pb, 0.61 μg/g for Cd, 30.37 μg/g for Cr, 17.44 μg/g for Ni, and 2.01 μg/g for Sb. Among different coal-forming periods and diverse coal ranks, the abundance of these eight HTEs differs significantly. Further, the simulated mean content also varies substantially with different provinces and different coal-bearing regions. However, the total number of field test data samples is still very limited to obtain a reliable assessment, especially for several provinces such as Gansu, Zhejiang, and Xinjiang; thus, much more extensive investigation is required for better understandings on these HTEs in Chinese coals. (2) In terms of the modes of occurrence, generally, Hg, As, and Sb are mostly occurred in pyrite; Pb occurs in gelenite; organic and pyritic Se are the dominant forms of Se in coals. Cd is usually associated with sphalerite; Cr is associated with clays; Ni is likely to occur in sulfide minerals and related to carbonate minerals. Nevertheless, other modes of occurrence of these elements are also possible in certain coals, owing to the diversity of coal forming plants and the complex coal-forming processes. (3) Sb, As, Cd, Pb, and Se are partially volatile, and Hg is fully volatile during coal combustion. These elements are concentrated more on the fine-grained particles and enriched in fly ash relative to bottom ash, while Cr and Ni demonstrate no obvious segregation between fly ash and slag ash. The fraction of these HTEs that is enriched in fine particles, especially the fine particles less than 10 μm, may easily go through the conventional APCDs and cause harm to human health and environment. However, due to the complexity of elemental properties and combustion conditions, the vaporization, nucleation mechanism, the condensation, and the emission in gases,

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+86-10) 5880-0176. Fax: (+86-10) 5880-0176. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the National Natural Science Foundation of China (21177012, 40975061), and Beijing Natural Science Foundation (8113032). The authors thank the editors and the anonymous reviewers for their valuable comments and suggestions on our paper.

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REFERENCES

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