The equivalence of stoichiometric and non-stoichiometric methods for modeling gasification and other reaction equilibria

The chemical equilibrium of gasification systems is modeled using one of two approaches, the stoichiometric (S) or the non-stoichiometric (NS) method. A complete survey of the gasification literature shows that about half of equilibrium modeling studies use either just the S, or just the NS method, and do not consider whether a different model choice would lead to a different solution. The remaining half of equilibrium modeling studies does motivate their model choice. Among those that discuss their model choice roughly half (15/27) state that the two approaches produce identical solutions. The remaining half propose that the NS method yields superior results (10/27), or the S method may yield superior results (2/27), and thus imply that the methods are not necessarily equivalent. The contradictory conjectures on both sides are made without definitive proof, or reference to definitive proof or evidence. In addition to providing a concise technical review of this question and its role in biomass gasification modeling, a second goal of this technical review article is to provide a tractable, yet rigorous and definitive exposition of the mathematical proof that the stoichiometric and non-stoichiometric approaches are equivalent methods for computing the equilibrium composition of the core gasification problem. This should provide perspective on the existing literature, and helpful guidance for future modeling work. Although the technical review and analysis provided here settles a longstanding contradiction in the gasification literature the relevant proof applies to any reactive system. The systematic template for S vs NS equivalence provided here should therefore be equally useful to researchers modeling and simulating the equilibrium of any other reactive system, since there is always the need to choose between the S or NS approach.