Tendency And Effect Of Environmentally-sensitive Trace Elements In Coal-preparation By-products After Combustion

Coal preparation is an indispensable stage in coal production,during which run-of-mine coal is transformed into products(clean coal)suitable for the market.By-products(coal gangue,coal slime,and middlings)are also produced during this process.Coal-preparation by-products are complex mixtures containing small amounts of coal and many impurities.Coal-preparation by-products are considered as inferior fuels due to their low heat value and high contents of ash,sulphur,and trace elements.Owing to the issues surrounding the disposal of coal-preparation by-products and energy shortages,researchers have strongly advocated harvesting energy from coal-preparation by-products.At present,the main use of coal-preparation by-products is to generate electricity.However,the secondary environmental pollution caused by the comprehensive utilization of coal-preparation by-products has been overlooked.There is little research regarding this in China.The quality(heat value,ash content,and trace element contents.)of different type of coal-preparation by-products(coal gangue,coal slime,and middlings)is quite different.To get a comprehensive understanding of environmental pollution in the surrounding area of enterprises that are engaged in utilizing by-products,and also to compare their environmental pollution degree,an attempt was made to measure contamination of soil and crops around four different coal-preparation by-products fired power plants(a coal gangue-fired power plant in Liupanshui City,Guizhou Province;a coal gangue-fired power plant in Pingxiang City,Jiangxi Province;a coal slime-fired power plant in Jining City,Shandong Province;and a middlings-fired power plant in Datong City,Shanxi Province),by environmentally-sensitive trace elements(ETEs).The contents of 11 ETEs(Be,Cd,Co,Cr,Cu,Mn,Ni,Pb,Sb,V,and Zn)in soil samples and different crops tissues were measured,and their chemical speciation in soil was also determined.The distribution characteristics of ETEs content in the soil-crop system and the speciation characteristics of ETEs in the soil surrounding the study area were analyzed.The associations between the investigated ETEs in soil were analyzed by using multivariate analysis.ETEs absorption and enrichment characteristics of crops were discussed.The migration law of ETEs in the soil-crop system around the study area was studied.The soil and crop pollution due to ETEs was assessed.Health risk from ETEs via consumption of food crops was also estimated.ETEs pollution status in soil and crop around coal-preparation by-products fired power plants were compared to that of coal-fired power plants,and it has also been conducted a comparison between four different coal-preparation by-products fired power plants.In addition,the problems relating to the comprehensive utilization of coal-preparation by-products were summarized and some corresponding suggestions were proposed.The main conclusions were as follows:(1)Based on the descriptive statistics of ETEs contents of soil samples,the distribution characteristics of total content and spciation of ETEs in soil samples,and the associations between ETEs,it may be concluded that the operation of coal-preparation by-products fired power plants had negative impact on the surrounding environment with regard to the content of investigated ETEs.ETEs in soil samples were accumulated to varying degrees in the study area.By assessing S_ace(acid-extractable),the impact of human activities(particularly the recent pollution behaviour)on the environment can be concluded.For most of the investigated ETEs in study area,the changing curve of S_acewith distance was roughly consistent with the regularity of the distribution of the ground concentration of pollutants from elevated point sources.ETEs emission from the coal-preparation by-products fired power plants affected the distribution of ETEs in soil.Therefore,ETEs detached from the original clusters and formed new clusters.These clusters can be interpreted as the different occurrence modes of elements in coal-preparation by-products.(2)The pollution index,Nemero synthesis index,risk assessment code,and potential ecological risk index of ETEs in soil samples of study area had been calculated,taking into account such factors as the total content,chemical speciation,and toxicity of ETEs,and it can be concluded that the soil in the study area was polluted by the above elements to varying degrees.More specifically,the pollution level of soil in different coal-preparation by-products fired power plants was in the following order:coal gangue-fired power plant>coal slime-fired power plant>middlings-fired power plant.With the improvement of coal quality,the adverse effect of ETEs emissions from power plant on environment can be effectively reduced.In addition,the operating time of power plant and its air pollution control facilities were also the important influence factors affecting pollution lever of soil around it.The pollutant emission of power plants differed in different period.However,with the increasingly stringent emission regulating,emission reduction of power plants had been proven fruitful.(3)The Cr and Pb levels in edible parts of partial crop samples exceeded the Chinese food safety standards.Health risk assessment showed that perennial intake of these crops was likely to induce adverse health effects.The ETEs contents of maize tissues largely depend on the bioavailable fraction of the same elements in the soils,rather than their total contents.The ability of different crop tissues to accumulate the same element varied and the ability of the same crop tissue to accumulate different elements also greatly varied.(4)Compared with those reported ETEs pollution in soil around coal-fired power plants,the ETEs pollution in soil around coal gangue-fired power plants located in Liupanshui City and Pingxiang City was serious,whereas the ETEs pollution level in soil around the middlings-fired power plant(Datong City)and the coal slime-fired power plant(Jining City)was general.In view of the fact that the installed capacity of most coal-preparation by-products fired power plant is far less than that of coal fired power plant,the environmental pollution caused by coal-preparation by-products fired power plant is worthy of attention.(5)The edible parts of crop samples around the coal-preparation by-products fired power plants and coal-fired power plants were contaminated to varying degrees.The ability of crop to accumulate ETEs strongly depended on its physiological properties.Local agriculture practices around power plants should be regulated.Planting crops(such as leaf vegetables)that had strong absorption capacity for heavy metal element in the areas around power plants should be avoided.(6)Considering some factors,such as the total content,chemical speciation,and toxicity of ETEs,Co?Cd?Mn?Pb and Sb should be concerned due to their high environmental risk in soil around coal-preparation by-products fired power plants and coal-fired power plants.(7)Comprehensive utilization of such low-grade coal-preparation by-products is a double edged sword,inevitably resulting in secondary environmental pollution.Finding means to utilize by-products in an environmentally friendly way is evidently more important than the pursuit of economic profit.Some relevant measures are therefore suggested,including:the establishment of standardized testing methods and classification indexes for coal-preparation by-products;scientific planning;reasonable site selection;development of specific emission standards and total emission controls for enterprises engaged in utilizing coal-preparation by-products;optimization of management systems;establishment of a whole process tracking management system;and implementation of policies to encourage technical innovation of coal-preparation plants.Harvesting energy from coal-preparation by-products(inferior fuel)is advocated by countries around the world.However,the results of this study indicate that all the investigated coal-preparation by-products fired power plants caused ETEs pollution in their surrounding area,and areas near coal gangue fired power plants suffered more serious ETEs pollution.It deserves significant attention,especially considering the installed capacity of coal-preparation by-products fired power plant and that of coal fired power plant.There are still many problems relating to the comprehensive utilization of coal-preparation by-products.Therefore,enterprises must be more careful when combusting coal-preparation by-products.Coal preparation by-products should not be directly combusted without pre-treatment.It is needed to utilize coal-preparation by-products with more appropriate technology.