Research progress on influencing factors and evaluation methods of underground coal gasification

Underground coal gasification（UCG） is one of the important potential directions to ensure my country's energy security and clean coal utilization. Analyze relevant domestic and foreign results,and review the principles,influencing factors and evaluation methods of UCG. The coal seam suitable for underground gasification needs to meet the geological conditions:the thickness is greater than 2 m,the inclination angle is less than 70°,the roof and floor with water blocking and heat insulation;avoiding areas with complex geological structures and hydrogeological conditions. The composition of UCG is affected by geological factors such as coal grade,coal quality and water content of coal seams,as well as process factors such as gasification pressure,temperature and gasification agent type. Under air gasification conditions,as the degree of coalification increases,the CO concentration in the gas component increases,the H2 concentration decreases,and the CH4 concentration first increases and then decreases. The calorific value of coal gas and the content of CH4 in coal gas show a trend of first decreasing and then increasing with the increase of fixed carbon content,and a trend of first increasing and then decreasing with the increase of ash yield. The CO2 content in coal gas has a negative correlation with fixed carbon content and ash yield. The gasification temperature and the type of gasification agent not only affect the gas production efficiency,gas quality,and the type and content of pollutants,but also can rupture surrounding rocks,pollute groundwater,and even destroy the stability of the biosphere,atmosphere,hydrosphere,and lithosphere ecosystem.