Summary

Ignition in a fuel bed where the ignition front propagates against the air flow is studied by modelling and through experiments. The model can predict the observed trends when moisture, bed density and air rate change. Geometrical factors, such as particle shape, size and orientation are combined to a single radiation parameter, which can be found experimentally. The main factors affecting ignition speed are the fuel moisture content, fuel volatile content, air flow rate through the bed, air temperature, the bulk density of the fuel bed, particle size, and particle shape. Up to the present most experiments have been carried out with homogenous and rather symmetric particles or with small particle size variation. This is not the case in many actual burners. Some of the measured flame spread rates were high compared to the results from literature, caused by the small bulk density of the bed and the thin particles of the tested fuels. Also wide particle size distribution may increase the ignition velocity. The ignited mass flow rate of dry solids per unit area of the bed (velocity of the ignition front × bed density) is an important parameter when designing combustion equipment, since it is proportional to the combustion rate of the volatiles/grate area.