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.
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