(44f) An Integrated Mathematical Model of Fluid Dynamics, Heat Transfer and Reaction Kinetics for Fluidized Bed Gasification of Biomass

Heating solid biomass with limited oxygen can gasify it to syngas, which can be used as a fuel or as feedstock for the production of many chemical products. Compared with the traditional fixed bed gasifier, the fluidized bed gasifier has the advantages of more flexibility in feedstock, lower capital and operating cost, and lower oxygen consumption. Fluidized bed gasification is a complex fluid flow, heat transfer and reaction system. During fluidized bed gasification of biomass particles, the fluidizing gases, such as air and steam, induce a multiphase flow pattern of gases and solid particles in a gasifier. As the gasification is an endothermic process, heat has to be provided to the biomass through the hot wall of the gasifier and fluidizing gases. The gasifier typically is filled with an amount of inert heating material to provide a large heat reservoir for maintaining a constant mean bed temperature. The structure of the biomass feedstock meanwhile undergoes thermal cracking as a function of temperature and pressure conditions, ejecting product gases to the fluidizing gases and forming solid char in the matrix of biomass particles. Understanding and quantifying the thermochemical process associated with fluidized bed gasification of biomass is critical to better understanding the conversion mechanisms, optimizing the operating conditions and improving the design of a gasifier. An integrated mathematical model of fluid dynamics, heat transfer and reaction kinetics thus has been developed to describe the fluidized bed gasification of biomass particles. The multiphase fluid flows of gases, biomass particles and inert heating material inside the gasifier are described using Navier-Stokes equations. The drag force between the solid phase and the gas phase is described as part of the body force or resistance in the equations. Turbulence of the flow was taken into account using a k - e model. The Navier-Stokes equations are used to generate the velocity fields of gases and solid particles in the gasifier. Heat transfer between the gas phase and the solid phase, and inside a biomass particle is modeled using energy conservation equations. Energy conservation equations are used to generate the temperature fields of gases and solid particles in the gasifier and the temperature field in a biomass particle. The conversion rate of biomass, as a function of temperature and pressure, is further determined using a reaction kinetics scheme. The model is solved using the finite element method. A laboratory fluidized bed gasifer with a biomass-feeding load of 2.5 kg/hr was constructed. Validation of the model was carried out by comparing predicted temperature and pressure profiles along the gasifier, and the yield and composition of the product gases with experimental values. The validated model was further used to analyze the effects of various operating conditions and gasifier configurations on the yield and composition of syngas.