Comparison between two types of Indian coals for the feasibility of Underground Coal Gasification through laboratory scale experiments

Underground Coal Gasification (UCG) process is studied through systematic laboratory scale experiments. Our earlier published work (Daggupati et al., 2011) demonstrated the various features of the Indian lignite coal in context of its applicability for UCG. In the present work, we study a hard Indian coal, with low volatile matter and moisture content. These results are compared with that of lignite type soft Indian coal, which has relatively high volatile matter and moisture content. The syn-gas produced from hard coal has a calorific value of 69 kJ/mol whereas the syn-gas from lignite coal has a higher calorific value of 170 kJ/mol, under similar conditions, in the laboratory experiments that mimic UCG process. Since UCG is a complex process involving different phenomena like spalling, gas-solid reactions of char and diffusion of gas in the char layer, separate studies on these aspects are required to explain the difference in the behaviors of these two coals in UCG. Spalling tendencies of these two coals are studied qualitatively by performing separate sets of experiments and the findings are used to explain the laboratory scale UCG results. The spalling experiments show that the hard coal has no tendency to spall, but lignite coal spalls, especially under high temperature and reactive atmosphere. The reactivity of the respective chars is studied separately using Thermo Gravimetric Analyzer. It is found that the char produced from lignite coal has higher reactivity of around ten times than the char produced from the hard coal. The paper thus presents a simple laboratory method to evaluate the feasibility of a given coal for UCG with theoretical analysis of the results obtained.