The reactor is considered as a one-dimensional plug flow reactor under steady-state conditions [94]. The gas phase is considered as perfectly mixed radially and the solid particles are distributed uniformly in the radial direction. The mass balance for solid and gas components can be described as follows [103]:

10.7.1 CFD Models

CFD modeling solves a set of equations for the conservation of mass, momentum, and energy simultaneously to give the gasifier temperature, the product concentra­tion, and the hydrodynamic parameters at different locations. Due to the complexity of the gasification process, however, there are not many CFD models available for this process and most of them must use fitting parameters and major assumptions for areas where accurate information is not available. Most of the CFD models are for coal gasification and combustion in entrained flow reactors since gas-solid flow is less complex compared to fluidized bed reactors [104] and [105]. A typical CFD model for gasification consists of a set of sub-models for different reactions and phe­nomena, such as drying biomass particles, devolatilization (pyrolysis), secondary pyrolysis, and char oxidation [106]. There are also other sophisticated subroutines for the destruction of solid fuels during gasification and combustion, which could be coupled with transport phenomena of the gasifier [107]. Due, however, to consider­able computational times for CFD models, particularly when chemical reactions are involved, this type of modeling is not very common for fluidized bed gasifiers.