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Modeling countercurrent fixed-bed coal gasification
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Modeling countercurrent fixed-bed coal gasification
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Dissertation
Modeling countercurrent fixed-bed coal gasification
1990
Overview
A one-dimensional model of countercurrent fixed-bed coal gasification has been developed and results have been compared to experimental data obtained from commercial-scale gasifiers. The steady-state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, devolatilization based on chemical functional group composition, coal drying, oxidation and gasification of char, and partial equilibrium in the gas phase. A two-zone, zero-dimensional model is used to estimate effluent gas compositions for use by the one-dimensional model. Predictions and comparisons to experimental data include effluent gas compositions and temperatures, axial temperature profiles, and axial pressure variation. Additional predictions with comparison to limited data include carbon conversion, variable particle size, heat and mass transport properties and species concentration profiles. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction are identified. Furthermore, proper treatment of gas phase chemistry and accounting for variable bed void fraction are necessary to predict realistic axial temperature and concentration profiles. Code evaluation also includes determination of sensitivity of axial temperature, pressure and molar gas concentration profiles to model options, model parameters, and operational parameters. Quantitative agreement with experimental data for gasification of most of the ten coal types ranging from lignite to bituminous in four different fixed-bed gasifiers has been obtained. Various conclusions and recommendations are proposed with regards to fixed-bed modeling and large particle experimentation.
