A kinetic analysis of coal char gasification reactions at high pressures

A Langmuir-Hinshelwood (LH) rate equation is often used for the incorporation of gasification reaction kinetics data into gasification models, as it is applicable over a wider range of conditions than the nth-order rate equation. The use of a LH rate equation at high reactant partial pressures has been questioned, however, with some authors recommending extra terms based on additional reaction steps. Unfortunately, the lack of agreement on the details of these additional steps makes the incorporation of high-pressure gasification reactivity data into gasification models potentially a difficult task. This paper presents further analysis of previously published reactivity data for the reaction of reference chars with 0.1-3.0 MPa of CO2 and, separately, H2O. This analysis is done using LH-style rate equations well-established for use with up to 0.1 MPa of the reactant. It is shown that, in the absence of product gases, these established LH rate formulations can describe the measured high-pressure char-gas reaction kinetics. Furthermore, theoretical predictions of surface coverage phenomena made using these equations agree with experimental measurements of the relative amount of reaction intermediates present on the char surface as the reactant pressure increases. The effects of char surface area, and how reaction at high-pressure develops this surface, are highlighted as an area of investigation requiring more work.