A Review of Key Hazardous Trace Elements in Chinese Coals: Abundance, Occurrence, Behavior during Coal Combustion and Their Environmental Impacts

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 mu g/g, As 5.78 mu g/g, Se 3.66 mu g/g, Pb 23.04 mu g/g, Cd 0.61 mu g/g,, Cr 30.37 mu g/g, Ni 17.44 mu g/g, and Sb 2.01 mu g/g. Further, the abundance of HTEs in Chinese coals for different coal-forming periods, diverse coal-bearing 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.