High-Pressure Char Gasification Kinetics: CO Inhibition of the C-CO2 Reaction

Results of high-pressure (up to 3.0 MPa) CO-inhibited char-CO2 reaction rate measurements are presented and analyzed using a Langmuir-Hinshelwood (LH) kinetic model commonly used to describe gasification kinetics at atmospheric pressure. Despite previous success by the authors and others in using this reaction scheme to describe high-pressure kinetics of uninhibited gasification reactions, this work shows that it is unable to describe well CO-inhibited reaction kinetics at (relatively) high partial pressures of CO and CO2. Measurements of the char surface area suggest significant impact of CO and CO2 partial pressures on the development of the char surface area during the reaction (and, therefore, most likely on the term describing the number of active sites on the char surface in the LH rate equation). This term is considered to be a constant in the standard LH formulation; therefore, an alternative LH approach to describe the kinetics is presented, whereby rates are described using the LH equation on a "relative rate" basis (i.e., the CO-inhibited rate normalized to the uninhibited rate at the same P-CO2 and temperature). This technique is able to describe the measured reaction rates well, because the active site term has been removed. This suggests that, at the higher pressures of CO and CO2 than experienced in atmospheric pressure studies, there is a significant effect of the gas concentration on char structural development, which will require more work to be adequately incorporated into rate equations used for describing gasification systems.