CO2-air is a potential gasifying-oxidizing medium for two stage mode of underground coal gasification (UCG). The existing literature deficits the UCG studies on the use of CO2-air as a gasifying medium especially for high ash Indian coals. Thus, the present study investigated the viability of utilizing the CO2-air as a gasifying medium for high ash Indian coals using a laboratory scale borehole gasification set-up in a two-stage gasification mode of operation. A typical Indian coal having 42% ash and a North East Indian coal having 4% ash are used for the borehole gasification experiments. The effect of the process parameters such as the molar ratio of O-2/air and the molar ratio of CO2/oxidizing agent on the product gas composition is evaluated. The results show that the gasification of low ash coal and high ash coal produced product gas with a calorific value as high as 260 kJ/mol and 214 kJ/mol, respectively. CO2-air based gasification necessitates the presence of oxygen in the feed gas with O-2/air ratio of 0.1 and 1 for the low and the high ash coal, respectively. Repeatability experiments show 7% and 14% error in the calorific values of syngas for low and high ash coals, respectively. (C) 2017 Elsevier Ltd. All rights reserved.