In situ gasification of coal using steam with chemical looping: a technique for isolating CO₂ from burning a solid fuel

In chemical looping combustion (CLC), a metal oxide (generalised as MeO) is used to oxidise a gaseous fuel in: (2n+m) MeO+C n H 2m →(2n+m) Me+mH2O+nCO2. Pure CO2 can then be obtained by cooling the off-gases to condense the steam. The reduced form of the metal oxide, Me, is then transferred to a different reactor, where it is re-oxidised by air in: Me+1/2O2→MeO. The gas from this reactor is N2 containing unused O2. The net effect of these reactions is that the fuel has been burned, with the total heat evolved being the same as for combustion of the fuel in air; however, the resulting CO2 is now pure and not mixed with the nitrogen from the air. This paper shows how it is possible to use CLC with a solid fuel, such as coal char, provided a gasification agent like steam is introduced into the reactor. The gasification agent transfers solid carbon to gaseous CO, which, like the H2 also formed, can be reacted with a solid, e.g. Fe2O3, carrying oxygen, to yield CO2 and H2O. On the basis of a limited series of experiments, the reaction of sintered compacts of Fe2O3 appears to be sufficient to make a semi-batch process feasible when used with steam.