Fate of potentially hazardous trace elements during the entrained-flow coal gasification processes in China

With the rapid and wide application of the coal gasification technology in China, the corresponding environmental issues require increasing attention. In this study, the feed coal, coal gasification wastewater samples at key technical links, and the coal gasification residues were collected from three entrained-flow coal gasification plants in Northwest China and were characterized to understand the migration and partition of 21 potentially hazardous trace elements (PHTEs) in coal gasification process and their potential environmental impacts. Compared with Chinese coal, the concentrations of the PHTEs in the feed coals are mainly at the level of "depleted" and "normal". Undergoing the heating process in the gasifier, purification procedure for the syngas, and the residue-wastewater separation, the PHTEs in the feed coals are transferred into gaseous phase, wastewater, and gasification residues. The PHTEs are easier to be released into the gaseous phase during the GSP gasification process compared with the OMB and GE gasification processes. The chalcophile elements Zn, As, Se, Cd, Sb, Tl, Pb, and Hg tend to be enriched in the fine residues, and the retention capability of Hg by the residues is the lowest. With the grey water circulation and the use of additives, the black water samples have higher PHTEs concentrations than other water samples. Except for Sr, Mo, Sb, and Cs, most of the elements in the original water samples can be reduced effectively by removing the particulate matters. In some cases, Cr⁶⁺, Zn, As, Se, Cd, and Pb in the original wastewater samples exceed the limit for industrial use as indicated in the Chinese Environmental Quality Standards for Surface Water and the Emission Standard for Pollutants from Coal Industry. The potential environmental impacts of As and Se in the water cannot be reduced by filtration. The environmental impacts of the coal gasification residues should be further evaluated.