This section briefly discusses how kinetic models can be applied to the three major gasifier types.

5.6.1 Moving-Bed Gasifiers

A basic moving-bed or fixed-bed gasifier can use the following assumptions:

• The reactor is uniform radially (i. e., no temperature or concentration gradi­ent exists in the radial direction).

• The solids flow downward (in a updraft gasifier) as a plug flow.

• The gas flows upward as a plug flow.

• The interchange between two phases takes place by diffusion.

The mass balance of a gas species, j, can be written (Souza-Santos, 2004, p. 134) as

where ug is the superficial gas velocity, z is the distance, pgJ is the density of the jth gas, and DgJ is the diffusivity of the jth gas. Rm, j, the production or consumption of the jth gas element, is related to Qgasificati0n heat generation or absorption.

Similarly, an energy balance equation can be written for a dz element as

where, Qgasification, Qconv, Qrad, and Qmass are the net heat flow into the element due to gasification, convection, radiation, and mass transfer, respectively. These terms can be positive or negative. pg, Cpg, and Xg are the density, specific heat, and thermal conductivity of the bulk gas, respectively.

Equations (5.79) and (5.80) can be solved simultaneously with appropriate expression for the reaction rate, RmJ.