Influence of mass transfer on thermogravimetric analysis of combustion and gasification reactivity of coke

Non-isothermal TG/DTG measurements are widely used to determine kinetic parameters of oxidation and gasification of carbons, e.g. by multi-heating rate methods. Thereby it is important to avoid spurious effects due to diffusion limitations. In this work, TG/DTG experiments with activated carbon were conducted under isothermal and non-isothermal conditions. The agreement of the experimental results with results of simulations based on the intrinsic kinetics, pore diffusion and external diffusion in/to the sample (crucible) is very good. The simulations show that mass transfer has an unwanted influence on TG measurements, if the concentration of the gaseous reactant (O2 and CO2) is too low and/or the heating rate is too high. Based on literature data, the simulation was extended to blast furnace coke with a very low reactivity and to highly reactive coke from pyrolysis of miscanthus sinensis. The simulations and theoretical considerations lead to a criterion to exclude the influence of mass transfer on TG measurements a priori. This criterion is valid for combustion as well as for gasification with steam or carbon dioxide.