A reactive redox system of metal oxide for a two-step thermochemical cycle for converting coal to upgraded chemical fuels of CO/H₂

Abstract
A two-step thermochemical process using a redox system of metal oxides for converting coal to CO and H₂ with H₂O/CO₂ was studied for the propose of utilizing solar high-temperature heat below 1173 K. In the first step, metal oxide was reacted with coal as an oxidant to produce CO and H₂ : metal oxide + CH_n(coal)→metal + xCO + (1-x)CO₂ + n/2H₂. The metal so formed was reoxidized with H₂O or CO₂ to generate H₂ or CO in the second step. A number of coal-metal oxide systems were thermodynamically examined and demonstrated at 1173 K. The coal-In₂O₃ and coal-SnO systems yielded 82 and 62% respectively. With iron-based oxides, the In(III)-, Ni(II)- and Sn(II)-doped ferrites yielded 60-70% of coal conversions. In these coal-ferrite systems, magnetic separation of recovered ferrite from coal ashes can be expected. The proposed two-step process using In(III)-ferrite could be repeated with highly coal conversion of 60-90%. Our two-step process is superior to the conversions of coal and H₂O/CO₂ to H₂ and CO below 1173 K by the direct single-step reactions, CH_n + H₂O → CO + (n/2+1)H₂ and CH_n + CO₂ → 2CO + n/2H₂.