Modelling the super-equilibria in thermal biomass conversion: Applications and limitations of the constrained free energy method: Dissertation

In various thermal biomass conversion processes, super-equilibrium concentrations of chemical species are observed. In such cases, the super-equilibrium is consid-ered as a local equilibrium state where an excess amount of certain species or phases exists. Solid biomass conversion in torrefaction and pyrolysis, as well as char conversion in gasification, are kinetically constrained phenomena in which conditions of uperequilibria can be observed. The formation of methane, tar, ammonia and char in biomass gasification can be further described using the super-equilibrium concept. A respective example is the enrichment of sodium, potassium, sulfur and chlorine into flue gas in a kraft pulp recovery boiler. Free radical over-shoot and the related nonequilibrium phenomena during high-temperature combustion result in elevated NO emission levels and may also be contemplated using the super-equilibrium hypothesis. The feasibility of modelling these super-equilibria with a constrained free energy method was evaluated in this study. The applied method is an extension of Gibbs free energy minimisation technique, accompanied by an extension of the thermo-chemical system with additional immaterial constraints. In this study, these additional constraints are defined based on the extent of the reactions. The presented method is an applicable modelling practice in high temperature processes, where local super-equilibrium state is a valid assumption. Suitable processes include biomass gasification, black liquor combustion and NO emission estimation. When low temperature processes are considered, the gas phase composition originates largely from the wood structure and fully kinetic models tend to give a better estimation than the constrained Gibbs'ian method. Kinetic constraints were successfully defined based on empirical constant values, on temperature dependent experimental reaction rate models and on global kinetic models. When models based on elementary kinetic reactions were considered, heavily reduced models were shown to be suitable for incorporation into the constrained free energy method. However, implementing a large number of kinetic reactions in the constrained free energy model was impractical. If a precise estimation of the superequilibrium concentrations of free radicals is needed, detailed kinetic models should be used. Based on the results, this thesis endorses the use of the constrained free energy method for modelling the super-equilibria in thermal biomass conversion. Such a method concurrently provides a unified solution including chemical reactions, and the enthalpic effect and state variables of the system. Thus, it can be considered an alternative to the commonly used global equilibrium models or reduced kinetic models applied when modelling the thermal conversion of biomass.