A study to investigate pyrolysis of wood particles of various shapes and sizes

Pyrolysis of centimeter-scale wood particles of various sizes and shapes needs to be understood to determine their burning rate and life. Such particles may be thought of as firebrands, which are a major reason for spotting ignition in wildland and wildland-urban interface fires. The burning lifetime of firebrands controls the maximum distance they can travel to cause spotting. To understand and model this, experiments are done in a vertical tube furnace with wood particles of different sizes and shapes. For computations, two classes of shapes, prolate and oblate ellipsoids, were chosen to represent the arbitrary geometry of such particles. Prolate ellipsoids include shapes ranging from thin needles to spheres, whereas, oblate ellipsoids include shapes ranging from thin disks to spheres. The choice of these smooth shapes, while facilitating expedient computations also enables the coverage of wide ranges of particle shapes and surface area to volume ratios (SVR). Model simulations show satisfactory agreement with relevant literature and experimental data. Particle aspect ratio (ϵ, the ratio of minor and major axes), SVR, and equivalent radius (Re) are used to define the particle geometry. Mass loss and center temperature profiles are presented and discussed. It is shown that with the decreasing of aspect ratio, wood particle decomposes faster and the final char fraction becomes smaller. A power-law based correlation between conversion time (tcon) and SVR is derived and verified against experiments. Further, it is shown that an increase in the SVR enhances the production of tar and decreases the yield of char while leaving the yield of gas mostly unaffected.