A predictive multi-step kinetic model of coal devolatilization

Devolatilization is the first step in coal combustion and gasification, thus an accurate kinetic modeling is relevant for the optimal design of these processes. In this work a relatively simple but flexible kinetic model is used to predict the thermal degradation of different coals in a wide range of operating conditions. The main feature of the model lies in its predictive capability: the elemental composition of the starting coal and the operating conditions are the only information required. Three reference coals are used to characterize the devolatilization process. The pyrolysis of each reference coal is described with a multi-step kinetic mechanism effective both at high and low heating rates. The devolatilization of the actual coal is simply obtained as a linear combination of the thermal degradation of the reference coals. The complete kinetic model refers to about 30 reactions and lumped species, which makes this scheme suitable for being adopted in fluidynamic computations. A wide collection of comparisons between model prediction and experimental data validates this model both in terms of residual char and in terms of detailed gas and tar composition. The importance of secondary gas-phase reactions, mainly at high pressure, is also discussed and verified on the basis of an existing detailed kinetic scheme of pyrolysis and oxidation of hydrocarbon fuels.