A good understanding of the gasification reaction kinetics and gasifier hydrodynamics is essential for the design, operation, and optimization of gasification processes. This section will discuss the main chemical reactions that govern gasification and the different kinetic models in the scientific literature that are available.

During a gasification process, the biomass particles undergo the following reaction steps:

1. Drying: The biomass particles are heated and dried on entering the reactor (endothermic step).

2. Pyrolysis (thermal cracking): As they reach high temperatures, the biomass particles undergo pyrolysis and decompose to gas and solid char (endothermic step).

3. Gasification:

a. Combustion: The char and gases (condensable and non-condensable) react with oxygen to produce H2O, CO, CO2 (exothermic step).

b. Gasification: Where the produced gas and solid char from previous steps react with the gasifying agent and each other (endothermic step).

c. Tar cracking: The condensable gas decomposes (thermal cracking) to smaller molecular weight components (endothermic).

The level of oxygen in the gas can be set in order for the system to be autothermal. These steps are generally modeled in series, but it is widely accepted that there are no sharp boundaries between them. Table 10.1 lists the important reactions taking place during gasification.