Efficient thermochemical cycle for CO2 gasification of coal using a redox system of reactive iron-based oxide

A two-step thermochemical process using a redox system of metal oxides for converting coal and CO2 to CO was studied on a number of iron-based oxides (ferrites) for the purpose of utilizing solar high-temperature heat below 1173 K. In the first step, ferrite was reacted with coal powder as an oxidant to produce CO, CO2, and the component metals. Pure magnetite (Fe3O4) was readily reduced to FeO but the reduction from FeO to alpha-Fe scarcely proceeded in 30 min of the reaction with coal at 1173 K. It was found that Ni(II)- or In(III)-substitution for Fe(II) or Fe(III) in magnetite strongly enhances the phase transition from FeO to metallic phase to improve the efficiency for coal conversion. The reactive Ni(II)- and In(III)-ferrites were almost reduced to the metallic phases of Ni-Fe alloy and the mixture of alpha-Fe and In, respectively. The Ni(II)- and In(III)-ferrite reactions yielded high coal conversion of 60-70% which was about 3 times as large as that by a conventional single-step CO2 gasification of coal. In the second step, the reduced reactive ferrites were reoxidized with CO2 to generate CO at 1073 K. The metallic phase of alpha-Fe and In was completely converted to oxidized phases with CO2 while the Ni-Fe alloy could not be completely oxidized. The two-step process using In(III)-ferrite could be repeated with high coal conversions of 60-90% by resupplying the consumed coal to the system, although the conversion gradually decreased by the accumulation of ashes. Our findings suggest that the highly efficient net reaction CHn(coal) + CO2 --> 2CO + n/2H(2) may be conducted using concentrated solar radiation as the energy source for high-temperature process heat below 1173 K.