In this work a detailed physical single particle combustion model for black liquor was developed. As a difference to previous models, intra-particle mass transfer during drying and devolatilization was considered, in addition to heat transfer. Relevant chemical reactions and experimentally observed physical combustion phenomena e.g. swelling were implemented. The model was widely tested against experimental data. Char conversion mechanisms were studied in laboratory reactor and furnace conditions addressing the relevant reaction mechanisms needed for developing simplified particle combustion sub-models for use in combination with CFD (Computational Fluid Dynamics).The model succeeded well in predicting experimentally observed combustion behavior. Release rates and yields of mass and carbon were well predicted for the cases studied. The model also succeeded well in predicting particle temperatures during combustion. Values for particle thermal conductivity, devolatilization heat, swelling and shrinkage parameters could be obtained by sensitivity analysis and experimental verification.A novel char conversion mechanism was found by the model, referred to as auto-gasification. Char conversion may take place already during devolatilization as H2O and CO2 flow out from the particle interior and pass through the hot, pyrolyzed particle surface. This auto-gasification mechanism could not be fully validated experimentally. However, an excellent correlatio n with experimental data was obtained when this mechanism was included.It was shown that char conversion mechanisms for black liquor essentially differ from those for other fuels. It was observed that at typical recovery boiler temperatures and gas compositions dominating char conversion mechanisms are H2O gasification, CO2 gasification and carbonate reduction, under non-convective conditions. When convective effects are present, the role of direct char oxidation increases. For high slip velocity conditions, the overlapping of devolatilization and char oxidation is an issue as O2 can reach the particle surface more easily. The concept of envelope flame under furnace conditions should be re-evaluated.The results from this study suggest that the development of a simplified CFD particle combustion model requires the proper understanding of physical and chemical processes taking place in the particle during combustion. In order to transfer the experimental observations to furnace conditions, the relevant mecha nisms that take place need to be understood before the important ones can be selected for CFD-based modeling.
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