Thermochemical conversion of coal and water to CO and H2 by a two-step redox cycle of ferrite

Redox systems of iron-based oxides (ferrites) mixed with coal powder have been studied to determine the most reactive and selective working materials for thermochemical conversion of coal and water to CO and H2. Reactions were performed in a two-step redox cycle in which the ferrites were reacted with coal powder at 900°C to produce CO, H2, and reduced ferrites (coal-gasification step); these were then reoxidized with water vapor to generate H2 at 700°C in a separate step (water-decomposition step). Magnetite and Mg(II)-, Mn(II)-, Ni(II)-, Zn(II)-, and In(III)-ferrites have been screened for reactivity and selectivity in the coal-gasification step. The In(III)-ferrite showed the greatest reactivity and selectivity for CO formation from coal at 900°C. The CO-production rate in the coal-In(III)-ferrite reaction was 3.5 times as fast as that in the single-step direct coal-H2O reaction. The metallic phase of α-Fe and In, produced by the coal-In(III)-ferrite reaction, was reoxidized to the ferrite phase to generate H2 in the water-decomposition step at 700°C. The amount of H2 evolved using In(III)-ferrite was 5 times larger than that using magnetite. The processes were repeated in the temperature range 700–900°C, with the highly efficient net reaction CHτ(coal) + H2O→CO + (τ/2 + 1)H2.