The effect of size, shape and pyrolysis conditions on the thermal decomposition of wood particles and firebrands

Pyrolysis of moisture-free samples of wood of various sizes (5–20 mm) and shapes (spheres, cylinders, cubes and rectangular blocks) was experimentally and numerically investigated to determine the effect of Shape, Size and Temperature boundary conditions (SST) on the heat and mass transfer processes. These shapes were chosen to enable simplified numerical modeling and to roughly encompass the variety of shapes produced by a wood chipper or firebrands generated by a forest fire. Measured temperatures show two distinct thermal regimes during pyrolysis of all shapes. First, an endothermic reaction that causes a plateau in the particle center temperature. Second, a steep exothermic rise in the center temperature after the endothermic temperature plateau. Simulation results agree well with the experiments. Both show that the temperature at which the pyrolysis occurs has a large influence on the time to completion of pyrolysis and the remaining char mass. Pyrolysis duration increases from sphere to cylinder to cube and follows the mass of the decomposing particle. Both experimental and numerical results show that at higher temperatures, the remaining char mass decreases and the final char mass fraction is lower or smaller particles because of shorter transfer distance for heat and mass transfer. Small, slender particles under high temperature pyrolyze faster, generate less char and produce more volatile compounds. Calculated iso-surfaces for internal pressure generation follow the mass loss measurements. The pressure gradient was found to be the highest for a sphere followed by cylinder and lowest for a cubic particle. Likewise the iso-surfaces for internal heat generation show that pyrolysis is most advanced for sphere followed by cylinder and cube. Two parameters (SSTτ & SSTμ) are defined to correlate the pyrolysis duration (τ) and the final char mass fraction (μ) for various thermal conditions and different particle sizes and shapes. Good correlations were found that may be potentially useful. These correlations may be used to predict the process duration and mass of the volatile compounds for various size and shape particles. They are also useful in determining the lifetime of firebrands.