(688c) Ddpm-DEM Simulation of Biomass Gasification in a Bubbling Fluidized Bed Reactor

Biomass gasification is a complex chemical process in which biomass is converted into syngas, a combination of carbon dioxide, carbon monoxide, hydrogen, and methane. The syngas could be then used as a fuel in internal combustion engines, gas turbines, or fuel cells for the production of heat, mechanical energy, or power, or as a feedstock for the synthesis of liquid fuels and chemicals. Among the various gasification reactors, the fluidized bed (FB) reactor presents good prospects due to its high rates of heat and mass transfer, good temperature control, and its excellent mixing properties. In a typical FB reactor, fuel feed, together with inert bed material which acts as heat capacitance for the fuel, are fluidized by the gasifying agents, such as air, steam, pure oxygen or their combination. There are many chemical processes within a real biomass FB reactor, such as mixing, segregation, collision, particle heat-up, drying, pyrolysis, volatile matter combustion, and char reaction. Moreover, their scales are greatly separated, which results in detailed study of the entire gasification process being a challenging task. Computational fluid dynamics (CFD) models have become more and more popular in recognizing the dense gas-solid flow dynamics and chemical reactions in FB reactors.

Generally, there are two approaches for modeling gas-particle interaction numerically in dense beds: the Eulerian two fluid model (TFM) approach and the Eulerian–Lagrangian approach. In the TFM approach, solid and gas phases are both treated as interpenetrating continua, and the kinetic theory of granular flows (KTGF) approach is used to calculate particle interaction by solving an additional equation for solids fluctuating energy or granular temperature. The main disadvantage of the TFM approach is the long calculation time required for evaluating the time-averaged solution. The consideration of particle size distribution can significantly increase this time, as an additional dispersed phase for each particle size must be used. In the Eulerian–Lagrangian approach, the gas phase is treated as a continuum while the solid phase is solved by tracking a large number of particles through the flow field calculated from the Navier–Stokes equations for the gas phase. All the forces applied on the particles are calculated to find their new positions and velocities. The solid phase can exchange momentum, mass, and energy with the gas phase. When the particle phase is solved by discrete element method(DEM), the Eulerian-Lagrangian approach is also called DDPM-DEM model. It can offer detailed microscopic information at the particle level, such as particle trajectory, particle-particle and particle-fluid interaction, and transient forces acting on each particle, which is extremely difficult, even impossible to obtain by E-E approach.

A crucial point when using DDPM-DEM is the computing power for simulation comparing with E-E. Therefore, the DDPM-DEM simulations are often restricted to the order of 10⁴ particles and are mostly restricted to 2D or quasi-3D solutions. If chemical reactions are added, computation is more and more complicated and expensive. To date most of the DDPM-DEM studies performed have been focused on the hydrodynamics of the isothermal fluidized bed and there have been few works on the simulation of dense gas-solid flow coupling with chemical reactions.

From all the above mentioned models the E-L approach potentially offers the most accurate description not only of the particle motion, but also of chemical reactions and heat and mass transfer between the dispersed phase and the gas phase at the individual particle scale. Although one can find quite many publications of inert particle simulations using the DEM, only few applications of the DEM for particle systems involving heat and mass transfer and chemical reactions are available in the literature.

The aim of this study is to develop a comprehensive CFD-DEM model capable of describing dense, thermal, and reactive multiphase flows like biomass gasification in a fluidized bed reactor. Based on the model validations against lab-scale bubbling fluidized bed experiments, this study evaluates the effect of different operating parameters on the gasification reactions. In particular, the relationship between gas-solid mixing and chemical reactions is investigated.