Validity of rate equation based on Langmuir-Hinshelwood mechanism for gasification of carbon - A reappraisal

Literature review on gasification of carbon by carbon dioxide reveals that the most widely accepted mechanism of gasification is based on the adsorption of oxidizing agent, proposed by Langmuir and Hinshelwood. But few investigators have cast doubt on the validity of the mechanism. In this paper, an attempt has been made to recheck the same with the help of critical analysis of literature information as well as rate data generated by the authors on gasification of electrode graphite and coconut char in different gaseous environments, namely, CO2, CO2-Ar and CO2-CO. It has been found that the parameters in the rate equation depend on the type of carbon contrary to the literature postulation that they are universal in nature. Expected linear dependence of rate on gas composition could not be observed under certain experimental conditions. Further, the equilibrium constants of the first elementary step of gasification i.e reversible oxygen exchange step, was found to be widely varying for both, different investigators and for different forms of carbon. Activation energy for the reverse step of oxygen exchange reaction was calculated to be negative for graphite. Thus, the findings of the present study are not in conformity with the behaviour expected if Langmuir-Hinshelwood mechanism is valid for gasification of carbon by carbon dioxide.