Heat and mass transfer limitations in gasification of carbon by carbon dioxide

Rates of gasification of carbon by carbon dioxide were measured in the laboratory using a thermogravimetry apparatus. The experiments were conducted at various temperatures and sample bed depths using powders of graphite and coconut char as sources of carbon, under flowing gas of CO₂. Various CO–CO₂ mixtures were also employed for the study of coconut char. The measured rate data were subjected to a heat and mass transfer analysis to find out the values of isothermal and non-isothermal effectiveness factors. These values were utilized to predict the intrinsic chemical reactivities of graphite and coconut char samples, free from heat and mass-transfer limitations. Extrapolation of experimental data to hypothetical zero bed depth showed good agreements with the above at various temperatures, thus providing valuable cross-checks. The activation energies were found to be 260 and 250 kJ/mol for graphite and coconut char, respectively, under CO₂. Carbon monoxide was found to decrease the rate of gasification but not as drastically as some literature reports predict.