Modeling of water evaporation from a shrinking moist biomass slab subject to heating: Arrhenius approach versus equilibrium approach

Abstract

Liquid moisture evaporation and pyrolysis of a biomass fuel slab subject to heating were computationally investigated in a one-dimensional domain where the fuel shrinkage was accounted for. The evaporation dynamics was separately represented by equilibrium and Arrhenius models. The equilibrium model describes evaporation as a thermodynamic process whereas the Arrhenius model treats it as a chemical reaction. For calculations, Gpyro, a pyrolysis computational framework, was used after its source code was revised to include the equilibrium model. By default, Gpyro is only capable of operating with the Arrhenius model. In the present study, also, a continuum description of mass and energy conservation for a pyrolyzating, shrinking porous medium was provided in the form of integral-differential equations. Such a description in the previous Gpyro works was lacking for shrinking objects and the Gpyro documents only provided a numerically discretized equation form. Here, the approximations made to develop this form from the integral-differential equations were clarified. The model was validated against experimental data of cone calorimeter experiments. Two fuel moisture contents (FMC) of 26% and 100% (on dry basis) representing dead and living fuels, respectively, were examined. The living fuel is characterized by a high FMC although the difference between dead and living fuels is not limited to this parameter. The evaporation rate, liquid moisture mass fraction and temperature profiles obtained by using the equilibrium model exhibited abrupt changes at the evaporation front whereas those obtained by using the Arrhenius model showed a smooth behavior throughout the slab. The drying dynamics described by the equilibrium model was more consistent with the underlying physics of evaporation. The equilibrium model demonstrated a distinct evaporation front, did not result in evaporation below the normal boiling point of water and more accurately exhibited the impact of the initial FMC on the drying dynamics. Simulation results showed that the thermochemical evolution of living fuel was appreciably more sensitive to the evaporation model.