Understanding the heat transfer, pyrolysis and ignition of wildland fuels

Abstract

This work provides an understanding of the heat transfer, pyrolysis and ignition of wildland fuels. To do so the wildland fuel morphology, thermal degradation and flammability were assessed through experimentation and literature review.

The burning of wildland fuels is a complex problem. They have a complex elemental structure, chemistry and bulk structure. The processes leading to the ignition depend on these properties and as such the ignition and consequently the flammability is also complex. In this work, pine needle fuel beds are used as typical wildland fuels.

To understand the role of the different processes leading to ignition of natural fuels a systematic experimental campaign was conducted. This covered both assessment of the differences in fuel chemistry and the structure of natural fuels. This involved evaluating the thermal decomposition using TGA and evaluating the flammability of different wildland fuel bed structures under well-defi ned heating conditions using a standardised testing apparatus (Fire Propagation Apparatus).

Different fuel chemistries were evaluated using different species of pine needles. The fuel bed structures were manipulated by changing the fuel element length to form low permeability (high solid fraction) and high permeability (low solid fraction) samples. A wide range of solid fractions are studied here compared to previously work - ranging from 0.03 to 0.51. This allows the flammability of wildland fuel beds to be understood as function of the fuel bed structure. Together this allows an assessment to be made of the relative importance of different fuel bed properties in determining the flammability of different fuel beds.

Initially, classical ignition theory is applied to the problem however, due to the large influence of fuel bed structure it is concluded that this approximation is not t for purpose. Subsequently, a novel framework was employed whereby the ignition processes were interrogated using the experimentally measured sample mass. This involved the superposition of a 1-D heat diffusion equation and the use of thermal degradation chemistry. Using a Genetic Algorithm, the thermo-chemical model was t to the experimental dataset. In this way changes to the effective heat transfer properties of fuel bed were analysed as a function of the fuel bed structure.

It was concluded that fuel bed structure has a dominating effect on the material flammability compared to the fuel chemistry and that the ignition of wildland fuels (and therefore porous fuels) requires a consideration of the heated depth as opposed to a surface temperature at ignition - as is assumed in classical ignition theory. The study also showed that the convective flow through a porous fuel has a large impact on determining the flammability as convective cooling of fine elements by entrained air delays the ignition time and may increase the burning rate. This finding is important for the design of future studies of the burning of wildland fuels as the fuel bed structure is generally not considered beyond the level of fuel loading or bulk density.