Selenium in Chinese coal

: distribution, mode of occurrence and environmental geochemistry

Abstract

As is well known, selenium (Se) is one of the essential elements of human beings at low concentrations but toxic at high concentrations. Se exists as a trace element in the Earth’s crust, but coal and the black shale are known as the most Se-enriched carriers: so-called “Se-enriched sedimentary rock”. An environmental lack of Se is prevalent in most of the world and as a result, various forms of extra Se-enriched nutrition are used among people. China not only has a high incidence of endemic Se-deficiency, but also has a high incidence of environmental Se-poisoning. Among the latter, Se-poisoning incidents caused by high-Se coal combustion are the most obvious forms. Selenium is readily released during coal-combustion due to the high volatility of Se. The huge consumption amount of coal has made coal-combustion the main artificial resource of Se in the atmosphere. The content, distribution and mode of occurrence of Se in coals affects the release and migration of Se directly during the use of coal, and finally affects the environment and human health. China has abundant coal reserves produced by the long coal-forming period; thus, the sedimentary environment and coalification are comparatively complex. Systemic studies of Se in Chinese coals are meaningful work for objectively estimating the potential environmental effects of using Chinese coals. This study includes the following aspects: 1. based on the collection of 2,288 coal samples, the average Se content in Chinese coal is 3.91 mg/kg; 2. the Se content in Chinese coal varied remarkably with different provinces or areas, and three groups were defined upon Se content; 3. contents of Se falls gradually over the coal-forming period, and the average content of Se in Paleozoic coals is higher than in the Mesozoic and Cenozoic coals; 4. the content and modes of occurrence of Se in coals of different metamorphic grade were studied and discussed; 5. the formation mechanism of Se in different kinds of coal was systemic discussed; 6. Se in special coal samples (high-sulfur coal and high-chlorine coal ) were also studied. Systemic research on typical coal mining areas are helpful for further understanding of the distribution and enrichment mechanisms of trace elements in coal. Selenium contents in 123 coal and 19 surrounding rock channel samples, which were collected from the fresh working face of main coal seams in seven selected coal mines of the Huaibei coalfield (i.e. Qidong (QD), Taoyuan (TY), Renlou (RL), Haizi (HZ), Liudian (LD), Baishan (BS), and Liuer (LE)), were determined by hydride generation atomic fluorescence spectrometry. Compared with the average China and world coals, Huaibei coals were highly enriched in selenium. The depositional environment of the Permian coal-bearing strata in Huaibei coalfields was inferred by the Boron content in coal. Selenium was distributed unevenly both horizontally and vertically in this area. Pearson correlation coefficients are used to estimate the modes of occurrence of selenium in Huaibei coals. Iron-bearing clay minerals and carbonates were the dominant carriers of selenium in coals of the Lower Shihezi Formation. Selenium in coals from No. 3 and No. 10 coal seams was mainly associated with sulfide minerals. The well-developed faults in the Huaibei coalfield could contribute to the enrichment of selenium in Huaibei coals. The transformation behavior of trace elements in coal during the combustion process has been a focus of interest for a long time, but few researchers have targeted the combustion of coal slime and coal gangue. In fact, the abundant trace elements in coals also exist in coal slime and gangue, which are even more enriched. Low heat value fuels have different physical characteristics, mineral components and modes of occurrence of trace elements. Study of the emission situation of Se from low heat value power plants would be helpful in estimating the environmental risk of the coal-fired power plants more scientifically and accurately. Therefore, Selenium in feed coal, fly ash and bottom ash samples which were collected from a low-heating power plant, were studied. Result showed that Se tends to be enriched by varied degrees in different kinds of fly ash samples. This kind of distribution is shaped by two factors: first, the use of desulfurizing agents produce CaO which may react with gaseous SeO2 which is released by the high temperature; then, the honeycomb-like nature of the material and the tiny size could increase the adsorptive capacity by the enhanced specific surface area of the fly ash particles. Mass balance calculations of feed coal indicate that 80-90% of selenium in coal is distributed in the fly ash, which is captured by an electrostatic precipitator. 6.60-15.23% of the selenium may be disposed into the atmosphere directly or with the flue gas. The environmental impacts of bottom ashes may cause few environmental impacts because of the small proportions involved. The amount of selenium released into air could be very high over time, and further control is necessary for the potential environmental risk. Surface soil samples of Huaibei coal mine area have been contaminated to various degrees, according to the result of the I-geo method. The most polluted soil sample belonged to the moderately polluted category. As far as Se is concerned, the proportion of the polluted sample and the level of Se pollution both have increased from 2009 to 2011. Selenium mainly concentrated at the test site 40 cm below the surface, and then decreased with depth. This distribution pattern indicates the low immigration capacity of Se in local soils, and the high-Se site could be related to external Se sources. The contents of Se in water samples collected in 2009 and 2011 did not show visible differences. Concentrations of Se in most of the water samples are very low without any pollution evident. But there is an obvious evidence of Se enrichment and pollution in sediments and the sediment samples are mainly moderately polluted. However, considering that those water bodies are mainly used for fish-farming, Selenium in the sediment could be secondary sources. Periodic monitoring is necessary for food safety and the health of local residents.

Date of Award	3 Oct 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	T C LAU (Supervisor)