Gas composition exiting a gasifier often varies from the composition predicted by equilibrium models [76]. This is caused by the fact that products exit from the reactor before reaching an equilibrium state and thus demonstrates the need for kinetic models to simulate gasifier behavior. Gasification reactions are divided into three categories: drying, devolatilization, and gasification. The time taken for drying and devolatilization is much faster than gasification of char. Some models assume the first two steps to be instantaneous and that the rate of char gasification controls the overall process [89, 90].

Kinetic models provide information on the progress of the reaction by taking into account the reactor type, size, and its hydrodynamics. In the kinetic model, the reaction kinetic is solved simultaneously with bed hydrodynamics, mass and energy balance to achieve the gas, and tar and char yield at specific operating conditions. Unlike other models, the kinetic model is sensitive to the gas-solid mixing and the flow pattern in the gasifier. Based on the process, this type of model can be divided into three groups: (1) fluidized bed; (2) fixed bed; and (3) entrained flow.