Leaching characteristics of hazardous trace elements in the slag from oxygen-enriched underground coal gasification

Oxygen-enriched gasification is an important method for underground coal gasification to produce high-quality syngas.Underground coal gasification (UCG),as an in-situ coal seam gasification,mainly extracts the energetic components from coal,and leaves the residue in the underground.The hazardous trace elements in coal are enriched in gas,ash with the UCG process.The pollutants in the UCG residue may permeate into the underground water through the long-term leaching,causing the pollution of underground water.Detailed leaching characteristics of hazardous trace elements (HTEs) of underground coal gasification (UCG) slag are essential to understand the migration behavior of HTEs far from the surface and protect the underground water.Ulanqab coal was investigated in this study.UCG slag was prepared through some simulation tests of oxygen-enriched gasification under different atmospheric conditions,and the typical minerals in the coal and slag were identified by XRD and the physical structure of the slag were characterized by the SEM-EDS.The leaching concentration of HTEs including As,Se,Cd,Pb,Ni,Be,Cr,Cu,Zn,Ba,and Ag were identified by ICP-MS and Hg was detected by the direct mercury analyzer DMA 80.Results show that Hg is mostly emitted in the gas and Pb,Be,As,and Se are enriched in the slag.The leaching concentration of gasification slag with different oxygen-enriched level in different pH leaching solution is lower than the relevant limits (GB5085.3—2007),which indicates that the oxygen-enriched underground gasification slag has no risk to the groundwater.Meanwhile,the leaching concentration of HTEs in deionized water is far below the toxicity standard of leaching,and the leaching concentration of slag can fully meet the standard of irrigation water,and will not affect the use of adjacent aquifers.The results can provide a scientific guidance for the control and management of late-stage residues of underground gasification.