Influence of size and density of fuel on combustion in a packed bed

Controlled experiments on the conversion rate of batches of mono-sized wood particles and wood pellets with similar moisture content, burning under substoichiometric conditions, show no clear influence of particle size. In addition, the measurements show that the fuel density has a very small influence on the conversion rate. Here, the size dependence (3 x 3 x 3, 4 x 16 x 19, 10 x 10 x 10, 30 x 30 x 30, and 20 x 50 x 80 mm for parallelepipedic particles, and diameter 8 x length 25 mm for cylindrical particles) and fuel density (300, 430, 800, and 1100 kg/m(3)) are investigated by a combustion model for a fuel bed. The model calculations confirm that particle density has a very small influence on the conversion rate and that the particle size has an influence on the combustion behaviour of a packed bed: the rate of propagation of the reaction front through the fuel bed is lower for larger particles than for smaller ones. In a bed packed with larger particles, there is a clear temperature difference between the gas and the surface of the solid fuel, whereas the gas and surface temperatures are almost identical for the smaller particles. The processes of drying, devolatilisation, and char combustion of large particles are overlapped, but in a bed of small particles they are sequential. For modelling, this opens up possibilities to simplify the description of the conversion of a bed containing small particles as they can be treated as isothermal. For such particles, the bed can be described as a porous medium where there is a temperature difference between the gaseous and the solid phase. The smallest particle size investigated shows a rather small temperature difference between gas and solid, which indicates that for this particle size the derivation can be further simplified: the temperatures of gas and solid may be given the same value. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.