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
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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.
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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
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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
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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),
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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
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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
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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.
■
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,
■
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.
■
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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