The reaction between graphite and CO2 is inhibited by the reaction product, CO, which may be present in the reactant gas or may accumulate within the porosity. In previous studies the importance of inhibition by CO has not always been appreciated. This thesis presents new models for the influence of CO accumulation on the oxidation rate. The models consider mass transport limitations both within the graphite pores and in the bulk gas. They are particularly useful for the design of new experiments and are applied to experiments within the thesis and also to previously reported studies. Oxidation of a highly purified, polycrystalline graphite by CO2 is studied at pressures up to 4.1 MPa, CO concentrations up to 50% and temperatures between 800°C and 1050°C by continuous measurement of weight loss using a microbalance. The experiments consider boch the chemical kinetics and the influence of mass transport on reaction. The results demonstrate the validity of the calculations for CO accumulation and the importance of CO as an inhibitor. For the first time, the kinetics of the approach to the Boudouard equilibrium are reported. At high pressures and high CO concentrations the kinetics are represented by simple equations which are limiting forms derivable from previous kinetic models. These models assume an idealised graphite surface; however, three aspects of the study illustrate the complex and variable nature of the real graphite surface: (i) the manner of the deviation from the simple kinetic equations at low pressures, (ii) the observation of large transient oxidation rates and (iii) the deviation of the estimated equilibrium constant for the Boudouard equilibrium from that predicted by standard thermodynamic data. In addition, a compensation effect is observed in this work and shown to be a statistical artefact resulting from a correlation of errors.
|Date of Award||1984|