Thermochemical conversion of coal and water to CO and H 2 by a two-step redox cycle of ferrite

Abstract

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 H 2. Reactions were performed in a two-step redox cycle in which the ferrites were reacted with coal powder at 900°C to produce CO, H 2, and reduced ferrites (coal-gasification step); these were then reoxidized with water vapor to generate H 2 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-H 2O 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 H 2 in the water-decomposition step at 700°C. The amount of H 2 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 CH τ(coal) + H 2O→CO + ( τ 2 + 1)H 2 .