The char and biomass conversions during gasification, pyrolysis, and torrefaction were studied using the constrained free energy (CFE) method. The Gibbs free energy minimization method is extended by implementing immaterial constraints for describing partial equilibria in the gaseous phase and for kinetically controlling slow reactions associated with the conversion of char and biomass. The connection between immaterial constraints and the affinities of slow chemical reactions are illustrated. The method presented allows for the Arrhenius type of kinetic model to be incorporated into the calculation of local constrained thermodynamic equilibrium. Thus, the kinetically constrained chemical reactions, equilibrium reactions, and reaction enthalpies can be solved simultaneously by applying CFE methodology. A conceivable approach for introducing pseudo-biomass components into the thermochemical system is evaluated. The technique applies to statistical estimates of standard enthalpy and standard entropy based on assumed molecular compositions. When incorporated into a thermodynamic model, the pseudo-components allow for estimating the composition of biomass during the process and the fast volatilization of oxygen- and hydrogen-containing species at the beginning of the processes. The CFE method was successfully used for modeling the char conversion. The high operating temperature of the gasification process justifies the assumption of local equilibrium in the gas phase. The immaterial constraint can be used for controlling the release of carbon to the gas phase as the reaction proceeds. When pyrolysis and torrefaction were studied, the immaterial constraints could be successfully used for describing biomass conversion in solid phases. However, for these processes, the assumption of local equilibrium in the gas phase is not valid, because no equilibrium reactions occur in the low-temperature conditions.