Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification

Oxidizing/gasifying agents play an essential role in the economy of underground coal gasification (UCG). The selection of feed gases to UCG depends on the prevailing conditions and inherent properties of a coal seam. Steam based UCG operation would lead to transportation and operational difficulty for efficient gasification. Alternatively, CO2 gas is a potential gasifying medium for coal gasification and this option would ensure the reutilization of waste CO2 gas in a UCG operation. Thus, in the present study, borehole coal combustion and gasification experiments are carried out to simulate CO2 enhanced UCG system using a low ash coal originating from the Northeast region of India. UCG experiments are simulated in a laboratory scale using CO2/O-2 gases and the results are compared with pure oxygen and oxygen enriched air based UCG operation. The composition and calorific value of syngas are analysed for various gasifying agents. The CO2/O-2 borehole experimental studies show that a CO enriched product gas (similar to 40 vol.%) can be generated under dried coal seam conditions. Thermo-gravimetric analysis (TGA) studies show that the rate of pyrolysis is higher under a CO2 atmosphere as compared to N-2 atmosphere. Further, the TGA results show that the generated CO2 pyrolysis products led to substantial char gasification even at low temperatures. The reactivity of tar with CO2 enhanced the calorific value of the product gas due to dry reforming reactions. Also, the presence of suitable inorganic species in the coal progressed the CO2 gasification through ash catalysis.