Multi-fluid CFD modeling of biomass gasification in polydisperse fluidized-bed gasifiers

This work presents an Eulerian computational fluid dynamics (CFD) model of biomass gasification for use in fluidized-bed gasifier (FBG) simulations. The physical and chemical processes of particle gasification and their interaction with the reactive gas flow are modeled within a multi-fluid framework derived from kinetic theory of granular flows. The transport equations of continuous solid phases and species mass fraction of CO, CO2, CH4, H2, H2O, O2, N2, tar, char, and ash are coupled with chemical kinetic models, which describe moisture vaporization, particle devolatilization, homogeneous volatile reaction, heterogeneous char oxidation and char gasification. Continuously variable particle density due to volatilization of lighter components and chemical reactions is implemented to account for the evolution of reacting particles' physical properties. A time-splitting method is employed for decoupling the chemical source from convection. A time-step adaption and restart procedure is implemented to provide the solution stability for strong chemical reaction and to achieve numerical solution efficiency for energy reactor simulations in the time-splitting approach. The chemical source terms based on chemical kinetics are implemented with first-order accuracy in the finite-volume framework. The CFD model is used to simulate wood gasification using air as a fluidization agent in a lab-scale FBG. The simulation results provide detailed information on the dynamic particle processes, char elutriation, and gas composition at the reactor outlet. Operating conditions of different air/biomass mass flow ratio, reactor temperature, and biomass moisture content are simulated and analyzed as to their influence on gas composition and product yields at the gasifier outlet.