A continuum-mechanical derivation of the conservation equations for the pyrolysis and combustion of wood

The equations for conservation of mass, species and energy for both the gas and solid phases are averaged over a characteristic wood volume element during pyrolysis or combustion. The presence of a pure liquid phase in wood is neglected in this study. The wood sample is assumed to obey the continuum hypothesis once phase averaging is completed. Heat conduction, adsorption (and desorption) and chemical reaction occur at the interface between gas and solid. Overall conservation equations are written for mass and energy; the species equations for the two phases are kept separate. Integration of the momentum for the gas phase produces an expression for the velocity field in the sample. Simplified equations are written for two limiting cases of significant practical interest.