CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification

Highlights: • Two stage underground coal gasification is carried out using CO₂-air as the gasifying medium. • The average calorific value of 217 and 260 kJ/mol of syngas is obtained using high ash and low ash Indian coal, respectively. • The effect of O₂/air molar ratio and CO₂/oxidant molar ratio on syngas is studied. 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 O₂/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 O₂/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.