The role of gasification reactions during pulverized solid fuel combustion: A detailed char combustion model based on measurements of char structure and kinetics for coal and pre-treated biomass

A detailed char combustion model including gasification reactions with emphasis on char structural evolution is developed and validated using combustion experiments from a lab-scale entrained flow reactor burning a bituminous coal and torriefied wood. The char combustion model is a single film model based on nth order Arrhenius type equations for char reactions. Diffusion through the particle boundary layer and inside the particle interior is considered by an effectiveness factor approach. Char reactivity towards O-2, H2O and CO2 is measured for five different char samples from four different parent fuels, being two bituminous coals and two pre-treated biomasses. Char samples are generated at high heating rates in an entrained flow reactor. Reaction rates and intrinsic reactivity values are quantified using a thermo-gravimetric analyzer. Reaction rates for pre-treated biomass samples are one to two orders of magnitude higher compared with the coal samples. The oxygen reaction is around four to five orders of magnitude faster than gasification reactions in the chemically-controlled reaction regime. The char combustion model is able to accurately describe char conversion and char structural changes. Model results reveal that there might be fragmentation effects in early stages of char conversion, presumably for cenospherical char particles. Particle reaction rates and reaction regimes are shown to vary strongly among different particle sizes and trajectories, and even for a single particle along its profile. Therefore, mean values calculated from CFD results have to be interpreted with care and can be misleading. A parameter study on temperature and stoichiometry shows the importance of gasification reactions. Depending on the conditions, gasification reactions are responsible for 2 up to 60 wt.-% of char conversion. They become relevant in regions of high temperatures and oxygen depletion, e.g. inside the flame. For typical furnace conditions of 1300 degrees C and a slightly over-stoichiometric flame, the Boudouard reaction accounted for approximately 10 wt.-% and the heterogeneous water-gas shift reaction for 4 wt.-% of char conversion. Therefore, gasification reactions cannot be neglected when predicting fuel conversion in PF systems, particularly in modern staged combustion systems with reducing furnace conditions. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.