Reactive metal-oxide redox system for a two-step thermochemical conversion of coal and water to CO and H2

A number of metal-oxide redox systems were examined for a two-step thermochemical conversion of coal and water to CO and H2 for the purpose of utilizing solar high-temperature heat below 900°C: metal oxide+CHn (coal)→metal+CO+n/2H2, metal+H2O→metal oxide+H2. In2O3 and SnO2 were found to be the most reactive oxidants for coal under an N2 gas stream below 900°C. These reactive metal oxides were tested for the proposed two-step cyclic process in the pulse feeding mode of H2O, with a temperature swing and under an N2 gas stream. The two-step cyclic process using the In2O3/coal mixture could be repeated with high coal conversions of about 80% in the temperature range of 600–900°C by resupplying the consumed coal to the system. The repeated two-step process via the In2O3/In redox system resulted in efficient performance of the net reaction CHn (coal)+H2O→CO+(n/2+1)H2, while hydrogen was scarcely produced in the continuous feeding mode of H2O/N2 over the In2O3/coal mixture, at a constant temperature of 900°C. The separation of recovered In2O3 from the remaining coal ashes by In2O evaporation was also proposed.