Char gasification in mixtures of CO2 and H2O: Competition and inhibition

Our understanding of char gasification reaction kinetics at high pressures is generally limited to analyses of data generated using pure reactants, or reactants diluted with inert gases. This paper presents data generated for the reaction of coal chars with mixtures of CO2 and H2O at high pressures, to determine how existing pure-gas rate data can be applied to more realistic gasification systems. The data presented here show that the rate of reaction in a mixture of CO2 and H2O is not the sum of the two pure-gas reaction rates; rather, it is a complex combination of the two that appears to be dependant on the blocking of reaction sites by the relatively slow C–CO2 reaction. Analysis of the results using a Langmuir–Hinshelwood (LH) style of rate equation with the assumption that both reactions compete for the same active surface, produces a model that explains the experimental data. This model allows reaction rates in mixtures of CO2 and H2O to be estimated based on existing, measured pure-gas reactivity data or from ‘first principles’ using knowledge of the rate constants for the LH equation.