Explosibility of Coarse Biomass Powders

Pulverised biomass is being used in electric power generation, either co-fired with coal or increasingly as 100% biomass. However, there is minimal information in the literature on the mechanism of flame propagation in pulverised biomass. In the present work the explosion technique was used to obtain fundamental information on the rate of flame propagation, the lean limits of flame propagation and related explosion characteristics of coarse biomass.
A large part of this research involved the modification of the ISO 1m3 method to enable it to be used with coarse fibrous biomass powders. The technique that worked was to follow the Hartmann method and place the dust inside the vessel using a hemispherical bowl and then disperse this dust with a blast of air. This was demonstrated to work with coarse woody biomass and the calibration was established using cornflour and referenced to the standard method.
The MEC and Kst for dusts were shown to have a dependence on the particle size. However, very coarse particles still propagated a flame, with no evidence that this was due to preferentially burning of the finer particles. Biomass particles of 300-500µm were shown to be flammable, i.e. as large as kerosene mist and large than coal particles will propagate a flame. For coarse woody biomass the Kst values were very low <20 bar m/s in many cases, but the peak pressure was high and hence the explosion would destroy biomass handling plant.
This work found that the unburnt material was compressed into a layer against the wall of the vessel ahead of the flame front, thus preventing it from interacting with the flame front. It was postulated that large particles lagged the main flame due to interaction with the explosion induced wind. This led to large particles being pyrolysed behind the flame front and then to arrive last at the wall and so appear as on outer pyrolysed layer on the material compressed against the wall. This explanation also enabled an explanation to be given for the very rich mixtures that could burn with dusts than could not burn if the material was a gas.