Mechanism of iron-catalyzed water vapour gasification of carbon

Abstract

The mechanism of iron-catalyzed water vapour gasification of carbon was studied by temperature programmed desorption measurements of carbon monoxide using a polyvinyl chloride coke (HTT 900°C), which was impregnated with iron sulphate (approx. 2 wt% Fe), pregasified in different hydrogen/water vapour atmospheres and quenched at 300 K · min −1 to temperatures between 500 and 300°C, or at 300 K · s −1 to room temperature. Three carbon monoxide desorption peaks were detected (peak temperatures 550–600°C, 650–700°C, 800°C); they are ascribed to three iron-oxygen surface complexes of different stability. A steady state isothermal water vapour gasification catalyzed by iron is only possible above 700°C with the most stable iron-oxygen surface complexes as intermediates. The results support an oxygen transfer mechanism, where the oxygen transfer from the iron to the carbon surface represents the rate-limiting step. The activation energy of iron-catalyzed water vapour gasification is higher than 360 kJ · mol −1 and therefore higher than that of non-catalyzed gasification (336 kJ · mol −1). The activation energy is attributed to the oxygen transfer step. Acceleration of the gasification by iron at temperatures starting from 700°C may only be affected by a large increase in concentration of active sites for dissociation of water vapour, which represents the rate-limiting step of non-catalyzed gasification.