CO2-O2 dry reforming based underground coal gasification using low and high ash Indian coals

Abstract

CO2-O2 based underground coal gasification (UCG) is a promising technology for exploiting deep coal seams using a greenhouse gasification agent. High molecular weight hydrocarbons, aromatics, tar etc. in the form of volatile matters in a coal seam possess a significant calorific value. During UCG, these components get evolved as vapours and leave the coal seam without undergoing significant cracking and reforming reactions. As a consequence, a substantial amount of tar is produced along with syngas stream. Alternatively, a novel result of the present study shows that tar-CO2 based dry reforming reactions are catalysed by the pyrolysed carbon zones in the borehole at low temperatures and, these reactions enhanced the calorific value of syngas under a CO2 reactive atmosphere. In continuation of our earlier studies, a detailed experimental study is carried out under single and two stage gasification methods using a typical high ash (42% ash) Indian coal and a low ash (4% ash) North East Indian coal. The effect of feed gas flow rate, molar feed ratio and inherent ash content of the coals on the product gas composition is studied. The results show that two stage gasification of high ash coal produces a medium calorific syngas in the order of 189 kJ/mol, which is equivalent to the heating value of a steam-O2 based UCG syngas. Further, the present study shows that a syngas with a high calorific value of 265 kJ/mol can be produced using the low ash coal under two stage gasification mode. Also, the single stage gasification of low ash coal shows the feasibility of producing a syngas with a medium calorific value of 250 kJ/mol.