Numerical modeling of the drying, devolatilization and char conversion processes of black liquor droplets
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Date
2002-12-13
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Language
en
Pages
74, [121]
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Acta polytechnica Scandinavica. Me, Mechanical engineering series, 163
Abstract
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.Description
Keywords
black liquor combustion, numerical modeling, drying, devolatilization, char conversion
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- Järvinen, M., Zevenhoven, R. and Vakkilainen, E., Auto-gasification of Large Black Liquor Droplets. Apollonia '99 Joint Meeting of 4th Workshop on Transport Phenomena in Two-Phase Flow and EFCE Working Party on Multiphase Fluid Flow. Sozopol, September 11-16 1999, pages 17-22.
- Järvinen, M., Zevenhoven, R., Vakkilainen, E. and Forssén, M., Black Liquor devolatilization and Swelling - Detailed Droplet Model and Experimental Validation. Accepted for publication in Biomass and Bioenergy (2002).
- Järvinen, M. P., Zevenhoven, R. and Vakkilainen, E. K., Importance of Different Char Conversion Mechanisms in Black Liquor Combustion - A Detailed Modeling Approach. Presented at the International Chemical Recovery Conference 2001, Whistler, Canada.
- Forssén, M. and Järvinen, M., Liquor-to-liquor differences in combustion and gasification processes: Simultaneous measurements of swelling and CO<sub>2</sub>, CO, SO<sub>2</sub> and NO formation reveals new data for mathematical models. Presented at the International Chemical Recovery Conference 2001, Whistler, Canada.
- Järvinen, M. P., Zevenhoven, R., Forssén, M. and Vakkilainen, E. K., Effective thermal conductivity and Internal Thermal Radiation in Porous Black Liquor Solids. Accepted for publication in Combustion Science and Technology (2002).
- Järvinen, M. P., Zevenhoven, R. and Vakkilainen, E. K., Auto-Gasification of a Biofuel. Accepted for publication in Combustion and Flame (2002).
- Järvinen, M., Zevenhoven, R. and Vakkilainen, E., 2002. Implementation of a detailed black liquor combustion model for furnace calculations. Published in IFRF Combustion Journal, Article No. 200206, June 2002. http://www.journal.ifrf.net/articles.html