Supercritical Water Gasification of Biomass: A Thermodynamic Model for the Prediction of Product Compounds at Equilibrium State

Abstract

This study aims to develop a model to predict the formation of compounds during the supercritical water gasification of biomass with a thermodynamic equilibrium approach. A Gibbs free energy minimization routine has been developed using MatLab software to predict the equilibrium state compounds of the system. Regarding all the calculations, pure condensed phases (both liquids and solids), gases, and aqueous compounds (both neutral and ionic species) have been taken into account in the subcritical region, and in the supercritical region, aqueous hydrate complexes have been taken into account instead of the aqueous compounds. The model has been validated for subcritical and supercritical regions separately. A case study for supercritical water gasification of a microalgae feedstock sample has been performed. The effects of temperature, pressure, and dry matter content on the phase behavior of elements and main product gases have also been investigated. The results of this work show that the model developed on the basis of Gibbs free energy minimization involving multiphase compounds can be used to predict the formation of equilibrium state compounds during supercritical water gasification of biomass.

Supercritical Water Gasification of Biomass: A Literature and Technology Overview

“…The simulations have been performed for a wide range of temperatures (100-580 °C), pressures (25)(26)(27)(28)(29)(30) and dry mass concentrations (5-20 wt%). The same research group [128] has also developed a multi-phase thermodynamic model for the prediction of equilibrium state compounds for supercritical water gasification of biomass systems. The authors validated the model by comparing their predictions with experimental results and model predictions already published.…”

“…However, most of these works concentrate on process modeling which is discussed in detail in the following section. [128] It is a fact that an arbitrary multi-component multi-phase system at a given temperature, pressure and chemical composition will always tend to minimize its total Gibbs free energy till it reaches the global minimum state. However, the predicted compounds at that minimum state are the "unconstrained" equilibrium state compounds.…”

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

“…Numerous research works [5][6][7][8][9][10][11][12] have been published on the kinetic or thermodynamic equilibrium modeling of supercritical water gasification of biomass systems. Most of these works have been carried out with simple or model biomass compounds.…”

“…Letellier et al [25] and Marias et al [26] have developed a thermodynamic equilibrium model for the prediction of compounds based on the reaction equilibrium constant method. Yakaboylu et al [27] and Yanagida et al [28] have used commercial software packages to predict not only the gas phase compounds but also the aqueous and solid phase compounds, and recently Yakaboylu et al [11] have developed a multi-phase thermodynamic model for the prediction of equilibrium state compounds for real biomass systems.…”

“…On the other hand, for the prediction of product compounds, a global thermodynamic equilibrium approach using the Gibbs free energy minimization method has been successfully applied by many authors [10,11,27,28] for different kinds of biomass feedstocks under different process conditions. However, the predicted compounds are the "unconstrained" equilibrium state compounds.…”