New Thermodynamic Equilibrium and Computational Fluid Dynamics Approaches for Modelling Biomass Gasifiers

Abstract

Two different routes can be taken to produce valuable energy from biomass: thermochemical and biochemical conversion routes. The focus of this thesis is the subject of the former, namely biomass gasification by addressing the challenges that come with the optimisation and operation of an efficient biomass gasifier using detailed equilibrium and computational fluid dynamics models.

An advanced and comprehensive thermochemical equilibrium model for a downdraft biomass gasifier has been developed to enhance the understanding of gasifier behaviour near equilibrium conditions via prediction of the syngas yield from the reduction zone. The model incorporates the thermodynamic equilibrium of the overall gasification reaction through a stoichiometric approach and includes the prediction of minor gasification byproducts, specifically hydrogen sulphide and ammonia, as sulphur- and nitrogen-based contaminants, respectively. A key feature of the model is the incorporation of a new empirical correlation, derived from relevant existing experimental data, to account for tar mass yield in generic downdraft biomass gasifiers. Additionally, the governing model equations are solved in a fully coupled manner, with the Boudouard reaction applied to predict char yield and the ammonia synthesis reaction used to estimate ammonia production. Notably, the model operates without correction factors and effectively predicts methane concentration, addressing a limitation commonly observed in existing equilibrium models. The outputs of the model include syngas composition, tar and char yields, gasification temperature, cold gas efficiency, and lower heating value for various biomass feedstocks with specified ultimate analyses, across a range of equivalence ratios and moisture contents. Where available, model predictions are compared to corresponding experimental data and demonstrate strong agreement.

Next, computational fluid dynamics is utilised to model a downdraft biomass gasifier through an established solver incorporating the porous media assumption. A detailed and comprehensive chemical reaction scheme is constructed, portraying the chemical reactions occurring in a real unit, efficiently identifying the appropriate range of operating conditions by which the system yields optimum products by tuning the working parameters accordingly to satisfy the production of species from the chemical reactions. The solver, originally designed for modelling biomass gasification in fixed-bed applications, is adapted to include a moving porous bed. Another novel feature of the model is its automatic replenishment functionality, which allows the biomass feedstock to be continually supplied from the top while generating desirable output products at the bottom. The porous bed includes a self-propagating front which facilitates chemical reactions. Transport equations are formulated inside the porous media by applying the spatial averaging methodology and responsible for predicting solid and gas temperature profiles, product gas composition, flow velocity, porosity distribution, and more. The resulting predictions are compared against existing experimental results and those from different CFD approaches for downdraft gasifiers, highlighting the applicability of the solver in addressing fixed-bed biomass gasification challenges.