CFD Simulation of Biomass Gasification Fluidized Bed with a Thermal State-Based EMMS Drag Model

As a promising biomass gasification technology, fluidized bed gasification can effectively alleviate the environmental and energy crisis. However, due to the high complexity and coupling relationship between gas-solid flow and chemical reaction in fluidized bed gasifiers, it is a great challenge to design and scale up such fluidized bed gasifiers. Emerging as an efficient studying approach, detailed CFD modeling may help to respond and address this key issue. The EMMS (energy-minimization multi-scale) model, compared with conventional drag models, shows relatively high performance in predicting gas-solid flow characteristics in the fluidized bed. However, most EMMS models are studied under cold model states, hence there are some deviations when they are directly applied to the thermal conditions due to real-time variation of the gas and solids properties. Moreover, most char conversion models are validated with the experimental values without further analysis of the reaction mechanism. In this work, a thermal state-based EMMS drag model coupled with a shrinking core model is proposed to determine the hydrodynamics, heat and mass transfer. With regard to the variations of phases properties due to chemical reactions, the real-time local properties of the two phases are accessed to calculate the heterogeneous drag correlation factor, which can greatly improve the generality and accuracy of EMMS model for the thermal state systems. Meanwhile, the transformation mechanism of control step between chemical reaction and diffusion are fully clarified in char conversion. Eventually, the proposed model is validated with experimental data and theoretical analysis. Predictably, the proposed EMMS model can provide more detailed and reliable information for the fluidized bed gasifier design and scale up.