The kinetic modeling of fluidized-bed gasifiers requires several assumptions or submodels. It takes into account how the fluidized-bed hydrodynamics is viewed in terms of heat and mass transfer, and gas flow through the fluidized bed. The bed hydrodynamics defines the transport of the gasification medium through the system, which in turn influences the chemical reaction on the biomass surface. Each of these is subject to some assumptions or involves submodels.

One can use several versions of the fluidization model:

• Two-phase model of bubbling fluidized bed: bubbling and emulsion phases

• Three-phase model of bubbling fluidized bed: bubbling, cloud, and emul­sion phases

• Fluidized bed divided into horizontal sections or slices

• Core-annulus structure

Gas flow through the bed can be modeled as:

• Plug flow in the bubbling phase; ideally mixed gas in the emulsion phase

• Ideally mixed gases in both phases

• Plug flow in both phases (there is exchange between phases)

• Plug flow through the bubble and emulsion phases without mass transfer between phases

• Plug flow of gas upward in the core and solid backflow in the annulus

The following sections present the essentials of a model for a circulating fluidized-bed combustor and one for a bubbling fluidized-bed gasifier (Kaushal et al., 2008). A typical one-dimensional steady-state model of a circulating fluidized-bed combustor, as shown in Figure 5.11, assumes gases as ideal and in the plug-flow regime. The riser is divided into three hydrodynamic zones: lower dense bed zone, intermediate middle zone, and top dilute zone. The solids are assumed uniform in size with no attrition. Char is a homogeneous matrix of carbon, hydrogen, and oxygen.

A bubbling fluidized-bed gasifier is divided into several zones with different hydrodynamic characteristics: dense zone and freeboard zone for bubbling beds; core-annulus for circulating beds. The dense zone additionally deals with the drying and devolatilization of the introduced feed. Superheated steam is introduced at the lower boundary of the dense zone. Each zone is further divided into cells, which individually calculate their local hydrodynamic and thermodynamic state using chosen equations or correlations. The cells are solved sequentially from bottom to top, with the output of each considered the input for the next. The conservation equations for carbon, bed material, and energy are evaluated not in each cell but across the entire zone. Therefore, each

▲

——- Solid

……… Solid + gas

——- Gas/air

FIGURE 5.11 Model of a circulating fluidized-bed gasifier.

zone shows a homogeneous char concentration in the bed material and a uniform temperature. Additional input parameters to the model are geometric data, particle properties, and flow rates.