Coal gasification using the ZnO/Zn redox system

A two-step thermochemical process is proposed for converting coal to high-quality synthesis gas. In the first, high temperature, endothermic step, coal is reacted with zinc oxide to form metallic zinc and an H-2-CO gas mixture. In the second, low temperature, exothermic step, zinc is used for splitting water and producing hydrogen and zinc oxide. The hydrogen is employed to enrich and adjust the synthesis gas mixture obtained in the first step, while the zinc oxide is recycled to the first step. Experimental studies have shown a more effective chemical conversion obtained via the proposed two-step scheme as compared to that obtained via the conventional single-step direct steam gasification. CO formation was more favorable with the coal-ZnO redox reaction than with the coal-H2O reaction in the 1173-1373 K temperature range. This highly endothermic reaction could be conducted using concentrated solar radiation as the energy source of high-temperature process heat, allowing for a combined utilization of coal and solar energy, and consequently for a significant reduction of the CO2 emissions derived from the combustion of coal.