Environmental risk assessment of trace elements in the underground coal gasification residues

Underground coal gasification (UCG) is a new coal utilization technology with safety,economy and environmental protection.However,the stability of underground coal gasification residues has always been the focus of people’s attention,and it is also one of the bottlenecks in the application of the UCG technology.It is of great significance to study the chemical speciation of trace elements (TEs) in the underground coal gasification residues to understand the release and transformation of trace elements and to protect the underground environment.In this paper,Ulanqab lignite was used as the research object,the underground coal gasification sequential transformation products were prepared,and the chemical speciation of trace elements (Hg,F,Se,Zn,Pb,Ba) in the UCG products were studied by using a chemical analysis.The risk assessment code (RAC) was introduced to assess the risk level of trace elements in UCG residues to the underground environment.The results indicate that during the UCG pyrolysis process,Ba exists in the form of BaSO4 and F exists in carbonate minerals.At 900-1 300 ℃ reduction ash,most of Hg has volatilized into the gas and it has no risk to the underground environment.Zn in 900 ℃ and 1 000 ℃ reduction ash has high risk to environment.The risk level of Ba and Pb to underground environment decreases with the increase of reaction temperature.F,Zn,Ba,Se and Pb in the oxidation residue (1 100-1 500 ℃) are mainly in the form of residue.Zn and Se are moderately harmful to the underground environment in the oxidation residue at 1 100 ℃.With the increase of oxidation temperature,the risk level becomes low.Ba and Pb are in low risk level in the residue of underground gasification.When the reaction temperature reaches 1 200 ℃,the risk level of trace elements in the UCG residues is in the low-risk range (RAC<10%),indicating that the UCG residues has reached stability.