The kinetic model has two components: hydrodynamics and reaction kinetics. Both affect the overall gasification and are briefly explained below.

Hydrodynamics

A typical fluidized bed gasifier is divided into two zones, the dense zone and the freeboard zone. For the dense zone, the designer needs to compute the size of bubbles, bubble velocity, bubble fraction, and gas exchange between the bubble and the emulsion. Empirical relations for these are available in numerous studies, including Kunii and Levenspiel (1991). The solid distribution profile along the height of the freeboard may be determined using decay factors similar to that given by Kunii and Levenspiel (1991). Gas exchange in this zone is based on being a plug flow riser. More details are given in Nemtsov and Zabaniotou (2008).

Reaction kinetics

Gasification reactions proceed at a finite speed, which is largely governed by the reaction rate of the char, as it is the slowest of all reactions. Pyrolysis and gas — phase reactions are at least an order of magnitude faster than the char conversion. So, the time taken for heating up and devolatilizing of the fuel is much shorter than the time taken for gasification of the char remaining. Thus, the gasification rate of the char is the controlling parameter. The conversion of the porous char particle may be modeled assuming the process to follow either shrinking particle
(diminishing size), shrinking core (diminishing size of the unreacted core), or progressive conversion (diminishing density).