Char Gasification Kinetics in Mixtures of CO2 and H2O: The Role of Partial Pressure in Determining the Extent of Competitive Inhibition

There is considerable interest in experimental studies investigating the kinetics of the reactions of carbonaceous chars with mixtures of CO2 and H2O. Several studies report a range of outcomes: some conclude that there is competition between CO2 and H2O for active sites, leading to inhibition of reaction rates of one reactant by another; others conclude that the two reactions occur independently. This work reviews and analyses this recent research activity, highlighting the importance of experimental conditions in generating appropriate gas-solid reaction rate data (many of the published studies have been performed under conditions where it is difficult to gain insight into the mechanisms of gas-solid reaction kinetics in isolation from other chemical and physical processes that may be occurring). This work also uses new data to demonstrate how reactant partial pressures can affect the availability of the reactive surface to a second reactant and, in turn, the extent to which a competitive effect is apparent; relatively low partial pressures of reactants (approximately 0.05 MPa and below) are less likely to show evidence of competition, whereas measurements at higher partial pressures (up to 3.0 MPa) are more likely to reveal competitive behavior. This is consistent with a reaction scheme that has CO2 and H2O competing for reaction sites and inhibitory effects that are only apparent when surface saturation becomes high enough for competition to impact observed kinetics. This result is important and provides further evidence that conventional Langmuir-Hinshelwood reaction schemes for representing char gasification reactions can be applied over a range of pressures and mixtures of reactants.