Release and transformation mechanisms of hazardous trace elements in the ash and slag during underground coal gasification

[Display omitted] •The chemical speciation of hazardous trace elements was studied.•The existing forms of hazardous trace elements were studied.•The risk assessment code (RAC) was introduced to evaluate the risk level. Underground coal gasification (UCG) is a new coal utilization technology with advantages of high safety and high efficiency. However, the risk of hazardous trace elements (HTEs) in the UCG residue to the underground environment has always been an important issue. In this paper, the UCG sequential transformation products were prepared, and the chemical speciation of hazardous trace elements (Hg, As, Se, Pb, Cr, and Cd) in the UCG products were studied by using a combination of chemical analysis and thermodynamic calculation. Moreover, the risk assessment code (RAC) was introduced to evaluate the risk level of hazardous trace elements to underground environment. The results indicate that during the UCG pyrolysis process, Hg, Pb, Se, and Cd in the Ulanqab coal react with S to form HgS, PbS, SeS, and CdS, respectively. The oxidizable form of Hg (50.00 ± 0.18–66.67 ± 0.23%) and As (78.34 ± 0.37–83.36 ± 0.46%) is the main speciation in 900–1300 °C reduction ash. For the oxidation ash and slag samples, Hg is loaded on the mineral system of Na-Si-O in the form of HgO, and As is combined with the FeAl2O4. Based on the risk assessment code (RAC), Hg, Se and Pb have medium risk level in the coal and semi-coke. Cr and Se in the 900–1200 °C reduction ash can pose high risk on the underground environment. In the oxidation ash and slag, the risk level of HTEs to the environment decreases. The results can provide scientific guidance for the control and treatment of residues in the later-stage of underground coal gasification.