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Mercury in Chinese Coals: Modes of Occurrence and its Removal Statistical Laws during Coal Separation Jin-he Pan, Changchun Zhou, Longfei Cong, Ningning Zhang, Cheng Liu, Chang-Bin Peng, and Chang-Heng Ou Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01825 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016
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Mercury in Chinese Coals: Modes of Occurrence and its Removal Statistical Laws during Coal Separation Jin-He Pana, Chang-Chun Zhou*,a,b, Long-Fei Conga, Ning-Ning Zhanga, Cheng Liua, Chang-Bin Penga, Chang-Heng Ouyanga a) School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China b) Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education, Xuzhou 221116, China *Corresponding author E-mail:
[email protected] (C. Zhou).
Fax: +86 0516 83591066
Corresponding authors at: Room A505, School of Chemical Engineering and Technology, Nanhu Campus, China University of Mining & Technology, Xuzhou 221116, Jiangsu Province, P.R.CHINA
Abstract As one of the dangerous trace elements in coal, mercury has become an advanced research hotspot in coal purification. The content of ash, sulfur and mercury of raw coals from four high-Hg coal preparation plants located in the main production areas of China was tested. To distinguish the connection between mercury content and the content of ash or sulfur, person correlation analysis and regression correlation analysis were performed by SPSS. The modes of occurrence of mercury in coal samples of these four plants were studied by sequential chemical extraction procedure (SCEP). The laws of mercury removal during coal preparation were researched by analyzing the mercury contents of feedings and products from the four plants. Results showed that mercury in coal was significantly associated with inorganic constituents or sulfur or both of them. Sulfide associated form is the dominant mode of occurrence of mercury in coals. Removal statistical laws of mercury during the process of preparation can be summarized as: (1) mercury in coal tends to concentrate in the fine fraction.; (2) dense medium separation has more obvious effect on mercury removal than flotation; (3) qualified clean coal products with up to standard mercury content is difficult to be obtained by conventional gravity separation or flotation process. The article was to facilitate the understanding of the modes of mercury occurrence in other Chinese coals, and provide a foundation for removal mercury in clean coal technology.
Key words: mercury in coal; correlation between Hg and ash / sulfur; modes of occurrence; removal law; coal
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preparation
1. Introduction Coal is one of the most important energies in the world, especially in China. It plays an important role in the development of social and economic development, but causing environmental pollution1-5. The vast use of coal consumption lead to the high-emission of mercury, which is one of the main sources for mercury pollution. Naturally, mercury pollution has become a worldwide environmental problem and has received unprecedented attention after the Minamata Convention on Mercury6-8. Mercury in coal is almost totally emitted into atmosphere during coal combustion9-11. Consequently, the research of mercury in coal has been a central issue in controlling trace element emissions12. Belkin et al.13 and Kolker et al.14 suggested that means of controlling the emissions during coal combustion process is more complex and more costly than that of reducing mercury content in raw coals before coal combustion. Through flotation method combining with an enhanced gravity separator, Gerald et.al.15 obtained a good mercury removal efficiency with the highest mercury rejection ratio of 80%. Toole-O'Neil et.al.16 reduced the mercury content from 0.23µg/g to 0.16µg/g by coal cleaning experiments. These facts showed that physical separation methods were still convenient and effective method in reducing Hg content. Therefore, the importance of coal preparation cannot be ignored17. To remove mercury from coal efficiently by physical separation or any other preparation, the modes of occurrence of mercury in coal are very necessary to be researched firstly18-19. Due to the different origins and metamorphic grades of coal, the modes of occurrence of mercury in different coal are not quite the same. Mercury in Illinois basin coals is mostly associated with a mixed mineral (pyrite) and organic matter, which has been inferred by Mössbauer and chemical analyses23. Zheng et al.24 found that modes of occurrence of mercury in Permian Huaibei coals included water-leachable,
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ion-exchangeable, organics bounded, carbonate bounded, silicate bounded, and sulfide bounded form. Speciation analysis by using capillary electrophoresis in acid extracts revealed that mercury in South African coals was mainly in the inorganic form, and organic mercury was also detected in very little amount25. Coal cleaning, whose purpose is to remove ash-forming and sulfur-bearing minerals, is the most widely utilization of coal purification26-27. Relying on the existing coal preparation, mercury in coal can be largely removed28. Therefore, the migration and removal of mercury in coal during the current processes of coal preparation plants, especially the reduction of mercury content from raw coal to clean coal, should be studied. This paper is aimed at the comprehensive study on the quality characteristics of typical high mercury coal, the modes of occurrence of mercury in coal, and the statistical laws of the mercury removal during the process of coal preparation. This study will show a theoretical foundation for the removal of mercury in subsequent coal cleaning processes.
2. Material and Methods 2.1 Coal samples Coal samples were collected from four different coal preparation plants (named CP1, CP2, CP3 and CP4 respectively in this paper) that are located in the main producing areas of high-Hg coals which were contained Inner Mongolia, Ningxia, Hebei, and Guizhou, China. These test samples included raw coals and products from each process link of four coal preparation plants. All coal samples were divided into two parts by division. One part was used for experiments, and the other part was split as the stored samples. The representative samples of raw coals were taken as a proximate, total sulfur, and X-ray
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diffraction analysis. As seen from the results in Table 1 and Fig.1, the raw coal from CP1 studied as jet coal with medium sulfur and high ash content, that from CP2 and CP4 researched as 1/3 coking coal with high ash and low sulfur content, and that from CP3 studied as fat coal with middle sulfur and high ash content. In a word, they all belong to soft coal. Table 1 Results of proximate analysis, total sulfur analysis and the mercury content of raw coals. wt%; µg/kg Coal sample
Mad
Aad
Vad
FCad
St,d
Hg
CP1
2.62
31.40
26.38
39.60
1.67
240.47
CP2
0.72
37.22
19.21
42.85
0.59
274.32
CP3
0.67
41.12
12.44
45.76
1.41
177.45
CP4
1.65
45.24
16.58
36.53
0.63
215.71
M: moisture; A: ash; V: volatile; FC: fix carbon; St: total sulfur; ad: air-dried basis. 1600
1000
L
C-calcite P-pyrite
L
1000
O-others
C C
800
K K K K O P
600 400
O
K 600
K K K
400
N
L
P
K K K K
LCC P K
200
L
K PyK
200
K
P
K:kaolinite P:polylithionite Q:quartz Py:pyrite C:calcite O:others
800
Intensity（ cps)
Intensity(CPS)
1200
Q
K
L-serpentine K-kaolinite
1400
0
K K
Py C O
K
K PyQ
Q
0
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
o
2-Theta(o)
2-Theta( )
( )
CP1
CP2 2000
1000
Q K
700
K
600
1800
K:kaolinite Q:quartz P:pyrite C:calcite O:others
800
Q KK K
300
K P
200
O
1200
K 1000
K Q W K
800 600
K O
P C
Q P K Q
100
Q： quartz K： kaolinte W： langite E： iron and manganese oxides O: others
1400
500 400
Q
1600
Intensity(CPS)
900
Intensity(CPS)
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Q
E
K O Q QQ
400
K O
O
200
0 0
10
20
30
40
50
o
60
70
Q
0 0
10
20
2-Theta( )
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30
40
2-Theta(o)
50
60
70
80
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CP3
CP4
Fig.1. XRD results for mineral composition analysis
2.2 Methods Technical route used in this study is shown in Fig.2.
Fig.2. Schematic diagram illustrating the flow of this study The contents of moisture, ash, volatile matter, and fixed carbon in raw coal samples were measured by 5E-MAG6700 automatic proximate analyzer from Changsha Kaiyuan Instruments Co., Ltd. The total sulfur contents were determined by ASTM standard methods D3177-0229. The mercury content in solid samples was detected by DMA-80 mercury analyzer from Milestone. All ash content was combusted from coal samples (except raw coals) in CTM500 muffle furnace, made by Zhang Hong Research Institution, CUMT. Statistics analysis method contained person correlation analysis and regression correlation analysis. Quality parameter correlations of sample quality from the four coal preparation plants were validated by Statistic Package for Social Science (SPSS). From the results of statistics analysis, the correlation between mercury content and the content of ash or sulfur was investigated and modes of
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mercury occurrence in four raw coals were inferred indirectly. The relationship between mercury content with ash or sulfur was determined further through two-dimensional linear regression. For each plant, the mercury concentration (CHg) was mathematically defined by: CHg = ACA+ BCS
(1)
Where A and B are constants, CA and CS are ash content and total sulfur content, respectively. Representative samples of the four raw coals were crushed and ground to less than 100 mesh for the execution of a modificatory sequential chemical extraction procedure (SCEP) as outlined by Zhang et al 30 and Dai et al31-32. SCEP has been applied widely to study the chemical activity and speciation of solid, rock and coal samples33-36. Among various extraction methods, the most broadly used one is BCR-70137-39, which was derived from the research did by Tessier et al.33 and developed by the Community of Bureau of Reference40. Leaching method combined with float-sink step was put forward by Davidson41 and Huggins et al.42, this method can infer the specific elemental forms in different fractions based on their solubility behavior in the various reagents used in different leaching stages43-44. In this study, four-step procedure (as Fig.3.) was used to divide chemistry speciation into five types: ion-exchangeable form, carbonate form, silicate form, sulfide form, and residuum.
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Fig.3. Flow chart of SCEP An evaluation of different separating process for mercury removal was followed with two parameters: concentration coefficient (CC)45-46 and reduction rate15. CC is the ratio of the trace elements content in detected coal samples with average content of trace elements in world coal. The enrichment degree of trace elements in coal is divided into six levels depended on the concentration coefficient (CC). These six grades are defined as follows: CC> 100 as abnormal enrichment, 10
