The equilibrium approach is further sub-divided into two models:

1 Stoichiometric model

2 Non-stoichiometric model.

The Stoichiometric model is based on an equilibrium constant. This method requires knowledge of the specific chemical reactions and reaction paths used for the calculation. It means selecting appropriate chemical reactions, and information concerning the value of the equilibrium constant is required. This method, therefore, is not suitable for complex problems where the chemical formulas of the feed or the reaction equations are not well known. This requires the second model, involving minimization of Gibbs free energy (non-stoichiometric model), which is an effective tool to find composition of gases when the reaction paths are unknown (Florin and Harris, 2008). It is a little more complex but advantageous, as a detailed knowledge of the chemical reaction is not needed.

The following section presents a brief discussion on stoichiometric and non­stoichiometric models of computation for the equilibrium concentration of the product gas. It also discusses energy balance, which is essential for an auto­thermal gasification reaction.