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

A two-step thermochemical process using a redox system of metal oxides for converting coal to CO and H-2 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 H-2: metal oxide + CHn(coal) --> metal + xCO + (l-x)CO2 + n/2H(2). The metal so formed was reoxidized with H2O or CO2 to generate H-2 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 H-2 and CO below 1173 K by the direct single-step reactions, CHn + H2O --> CO + (n/2+1)H-2 and CHn + CO2 --> 2CO + n/2H(2).