Modelling of Biomass Combustion and Gasification: from Particle-scale to Reactor-scale

The PhD project is aimed to develop modelling tools to simulate biomass combustion and gasification both in a particle-scale and in a reactor-scale. The work includes the development of comprehensive particle-scale models for biomass devolatilization and char conversion, simplified models for CFD modelling of biomass devolatilization and char conversion, and a meso-scale drag model for CFD modelling of biomass combustion and gasification in fluidized bed reactors. A comprehensive biomass devolatilization model including both external and internal heat transfer has been developed based on the model proposed by Johansen et al. [1]. The model was validated by single wood particle devolatilization experiments under different gas temperatures (1473-1723 K), particle sizes (3-4 mm), particle moisture content (5-52 ar%), and particle dry density (180-1100 kg/m3). Both the modelling and experimental results indicated that the devolatilization time increases linearly with an increase of particle dry density. The gas temperature, particle size, and the particle moisture content influenced significantly on the devolatilization time, while volatile content and the slip velocity have small effects. The kinetics of devolatilization did not influence significantly on the devolatilization time of large wood particles (> 1 mm). Based on the results from the comprehensive devolatilization model, a simple correlation was derived to predict the devolatilization time of wood particles. A comprehensive char conversion model including both external and internal mass and heat transfer, particle shrinkage, the heterogeneous reactions of the char oxidation and gasification, and the homogeneous reactions occurring in the particle boundary layer, has been developed. The model was validated by single particle combustion experiments of pine and beech wood char at high temperatures (1473-1723 K) with different oxygen concentrations (0.0-10.5 vol%) and steam content (25-42 vol%). The modelling results indicated that the CO oxidation in the particle boundary layer has a significant effect on the char conversion process. Both char oxidation and gasification reactions contributed to char conversion, with the latter becoming important at high temperature (e.g. > 1273 K) and large particle size (e.g. > 1 mm) conditions. Based on the comprehensive biomass devolatilization model, a heat transfer corrected isothermal model for biomass devolatilization model has been developed to facilitate CFD modelling of the devolatilization of thermally-thick biomass particles. In the model, two heat transfer corrected coefficients (HT, correction of heat transfer, and HR,i, correction of reaction rates) were introduced based on the difference of external heat transfer and devolatilization rate between the comprehensive biomass devolatilization model and a conventional isothermal model. Compared to the comprehensive devolatilization model, the heat transfer corrected isothermal model predicted a similar devolatilization behaviour with a much lower computational cost. The model was implemented in a Eulerian multiphase flow model to simulate the biomass devolatilization in a batch bubbling fluidized bed reactor. Compared to a conventional isothermal model, the heat transfer corrected isothermal model had similar computational efficiency, but provided more reasonable results for thermally-thick biomass particles. Based on the comprehensive char conversion model, a mass transfer corrected uniform char conversion model was developed to reduce the computational cost of CFD modelling char conversion process. Three coefficients, (HT, correction of heat transfer, and HR,i,, correction of reaction rates, and Hm,i, correction of external mass transfer rate) were introduced by comparing a uniform char conversion model and the comprehensive char conversion model. Compared to the uniform char conversion model, the mass transfer corrected uniform char conversion model gave more reasonable results, and expected to have comparable computation efficiency. A hybrid EMMS drag model has been developed to simulate the hydrodynamics of three dimensional full-loop CFD modelling biomass gasification in a dual fluidized bed system. Compared to the Gidaspow drag model, the pressure distributions predicted by the hybrid EMMS drag model were in a better agreement with the measurement. The effects of solid inventory on the hydrodynamics of the dual fluidized bed system have been evaluated by the hybrid EMMS drag model. It was found that the solid circulation rate increased with an increase of solid inventory. When the solid inventory was larger than 160 kg, an unstable regime was identified for the dual fluidized bed system.