Measurement of the times for pyrolysis and the thermal diffusivity of a pyrolysing particle of wood and also of the resulting char

Abstract

Cubes and spheres of spruce wood have been prepared, with a fine thermocouple inserted to measure the temperature at their centre. Individual particles were immersed rapidly in a bed of sand (mean size ∼0.2 mm), which was fluidised by nitrogen and held at a fixed temperature up to 700 °C. The rising temperature measured at a particle's centre yielded the effective value of the particle's thermal diffusivity. The temperature response showed evidence of at least two endothermic decomposition reactions, which corresponded to the pyrolysis of fine particles of the wood in a thermogravimetric analyser (TGA). However, the wood undergoing thermal decomposition in a fluidised bed at 500 °C revealed at least one exothermic step at the very end of heating. After being heated this way in a hot bed fluidised by nitrogen, the particles of char formed by spruce wood had very much the same size and shape as the original piece of wood before being heated. These new particles of “char”, formed whilst being heated in a hot fluidised bed, were cooled in a stream of nitrogen and returned to the fluidised bed for re-heating without any complications from pyrolysis. The rise in the char's central temperature with time gave an unambiguous value for the thermal diffusivity of the char. It is clear that volatile matter leaving a particle of wood reduced the rate of heat transfer between a hot fluidised bed and the centre of a devolatilising particle. Also, the time for complete pyrolysis was proportional to the square of the characteristic size (r0) of the spruce being heated. In addition, the time for pyrolysis was proportional to Tbed3±1, so that for a cube of spruce tpyr = 2.9 ± 0.3 × 1015r02/Tbed3 in seconds. Photographic evidence confirmed that devolatilisation of particles of spruce larger than ≈ 2 mm in a fluidised bed follows a shrinking core model and is accordingly controlled by internal heat transfer.