Modelling thermal degradation of woody fuel particles

A model for thermal degradation of woody fuel particles is developed. It includes drying, pyrolysis, and char oxidation processes. The model is first applied to assess the validity of the thermally-thin pyrolysis assumption commonly used in wildfire behavior models. For a given external radiant heat flux, the particle size at which transition between thermally-thin and thermally-thick pyrolysis regimes occurs is evaluated by comparing the pyrolysis times computed for both regimes. It is found that, for a given flux, the particle size above which the thermally-thin assumption is questionable, is independent on the moisture content and on the particle surface-area-to-volume ratio. This means that the transition characteristic lengths for spheres, cylinders and slabs are related by: Lcr=Lcr,slab=Lcr,cyl/2=Lcr,sph/3. A Biot number based on the particle surface-area-to-volume ratio, σp can then be defined as Bi=εwQext/λwσpΔT. Results show that the thermally-thin regime can be defined by Bi<0.1 whatever the particle shape. They reveal that the traditional thermally-thin pyrolysis assumption is not suitable to model wildland fire behavior. For thin particles responsible for fire spread pyrolysis is kinetically-controlled while it is controlled by heat diffusion for large particles. Secondly, the model is applied to the combustion of firebrands. Model results are found in good agreement with available experimental data.