Testing the constrained equilibrium method for the modeling of supercritical water gasification of biomass

Abstract

Most of the modeling approaches for supercritical water gasification (SCWG) of biomass involve the global thermodynamic equilibrium approach which shows the thermodynamic limits of the gasification process. However, the constrained equilibrium method can be a useful tool for the modeling of SCWG of biomass processes for local equilibrium conditions. This study aims to determine the additional constraints for the Gibbs free energy minimization method and test the constrained equilibrium method for the modeling of supercritical water gasification of biomass. Using two different approaches: i) treating the fluid phase as it is composed of one “pseudo” gas phase and one “pseudo” aqueous solution phase (approach I) and ii) treating the fluid phase as one single phase (approach II). Additional constraints including carbon gasification efficiency (CGE), hydrogen gasification efficiency and constrained amounts for specific compounds have been introduced as process dependent values to predict the local equilibrium state compounds. CGE appears to be the most important additional constraint. Setting a constant amount for a specific compound as another constraint improves the accuracy of the model on predicting the composition of the gas products. The model not only predicts the gas formation behavior, but also gives insight into limiting reaction pathways taking place.