Stoichiometric and non-stoichiometric methods for modeling gasification and other reaction equilibria: A review of their foundations and their interconvertibility

The need for chemical reaction equilibrium calculations arises frequently in biomass gasification modeling and in many other fields. The two main formulations are the stoichiometric (S) and non-stoichiometric (NS), each requiring different numerical solution algorithms. The literature typically describes the S formulation as the vanishing of the Gibbs energy changes for a set of chemical reactions, and the NS formulation as the minimization of the system Gibbs energy subject to the element abundance constraints. A recent review (this journal, 131, 109982 (2020)) noted that the literature is contradictory concerning whether S and NS formulations for a given system yield identical solutions, and stated a linear-algebra-based S–NS compatibility criterion for their equality. This review points out three foundational misconceptions in the biomass gasification literature concerning NS and S problem formulations, and shows their clarification by (1) analyzing the different ways in which mass conservation is incorporated in each formulation, and (2) demonstrating how both formulations can be viewed as Gibbs energy minimization strategies. Finally, this review shows how these clarifications lead to (3) extending the S–NS compatibility criterion to an inequality, yielding a straightforward methodology to convert either formulation to a compatible formulation of the other with an identical solution. The explanations are illustrated in the context of a basic biomass gasification problem. Finally, (4) open-source software for chemical equilibrium calculations is briefly reviewed, which obviate the need for researchers to create in-house or to use commercial chemical equilibrium code, allowing them to focus on the modeling aspects of their study.