Study on physicochemical properties of “three zone” residues during underground coal gasification

Compared with traditional coal mining and utilization technologies, underground coal gasification (UCG) is known as green coal mining technology. But its potential groundwater pollution is unfavorable for the application and promotion of this technology. The leaching of gasification residues in the gas-fired area based on groundwater is one of the causes of groundwater pollution and the fact that the physicochemical properties of gasification residuals including the composition and pore structure affect the dissolution of harmful substances in the residue and the adsorption of contaminated groundwater.In this paper, by oxygen-enriched /steam two-stage gasification method, model gasification of Jiaozuo anthracite were performed on the self-designed underground coal gasification model test system, and thereafter the corresponding three zone residuals were collected. The physicochemical properties of the surface morphology, pore structure and surface functional groups of the residues were studied by SEM, low temperature nitrogen adsorption, XRD and FTIR. The results show that coal ash, residual cokes of gasification and pyrolysis semi-coke are correspondingly the main components of residuals in the oxidizing zone(OZ), reduction zone(RZ) and dry distillation zone(DDZ). The pores of the residue with reduction are developed, and the specific surface area and pore volume can reach 56.43 m2/g and 0.031 cm3/g respectively;. The specific surface area and pore volume of DDZ residues can be 15.65 m2/g and 0.014 cm3/g respectively. Compared to the residues of RZ and DDZ，the specific surface area and pore volume of OZ residues are relatively smaller, and the OZ residues are mainly composed of quartz and mullite. Graphite-like microcrystalline structures as well as oxygen-containing functional groups, such as phenolic hydroxyl group and ether oxygen are also observed in both samples of RZ and DDZ residues. The physicochemical properties are favorable for both the leaching of hazardous substances and the potential adsorption performance of gasification residuals.This study lays a foundation for understanding the characteristics of the dissolution and migration of pollutants in gasification residues and the adsorption and purification of residual groundwater, enriching the groundwater pollution of coal underground gasification and its prevention and control theory, and also promote the development and application of UCG.