Abstract
Mercury (Hg) is a toxic trace element emitted from coal conversion and utilization. Samples with different coal ranks and gangue from Ningwu Coalfield are selected and investigated in this study. For understanding dependence of mercury distribution characteristics on coalification degree, Pearson regression analysis coupled with Spearman rank correlation is employed to explore the relationship between mercury and sulfur, mercury and ash in coal, and sequential chemical extraction method is adopted to recognize the Hg speciation in the samples of coal and gangue. The measured results show that Hg is positively related to total sulfur content in coal and the affinity of Hg to different sulfur forms varies with the coalification degree. Organic sulfur has the biggest impact on Hg in peat, which becomes weak with increasing the coalification degree from lignite to bituminous coal. Sulfate sulfur is only related to Hg in peat or lignite as little content in coal. However, the Pearson linear correlation coefficients of Hg and pyritic sulfur are relatively high with 0.479 for lignite, 0.709 for sub-bituminous coal and 0.887 for bituminous coal. Hg is also related to ash content in coal, whose Pearson linear correlation coefficients are 0.504, 0.774 and 0.827 respectively, in lignite, sub-bituminous coal and bituminous coal. Furthermore, Hg distribution is directly depended on own speciation in coal. The total proportion of F2 + F3 + F4 is increased from 41.5% in peat to 87.4% in bituminous coal, but the average proportion of F5 is decreased from 56.8% in peat to 12.4% in bituminous coal. The above findings imply that both Hg and sulfur enrich in coal largely due to the migration from organic state to inorganic state with the increase of coalification degree in Ningwu Coalfield.
Highlights
Introduction published maps and institutional affilMercury (Hg) is considered to be one of the most toxic metal elements which exist in coal
Sub-bituminous coal or bituminous coal, peat has the highest volatile matter with an average value of 58.69% in Table 2 and the highest organic sulfur with an average value of 0.64% in Table 4, which may consider that the dependency of Hg on organic sulfur becomes weak with the increase of coalification degree
The total proportion of F2 + F3 + F4 shown in Figure 14b is gradually increased from 41.5% in peat to 87.4% in bituminous coal with the increase of coalification degree, which is consistent with the above-mentioned correlation between Hg and ash in coal samples and matches with the results in gangue samples
Summary
The analyzed coal samples in this study are conducted from. Ningwu Coalfield (upper Carboniferous Taiyuan Formation, Lower Permian Shanxi Formation) in Shanxi Province, China. The geological structure of Ningwu Coalfield is relatively simple. A total of 71 samples are chosen, including 59 samples with various rank coals from three coal mines and 12 gangue samples from two washing coal plants
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