Flame development in pulverised biomass for fine and coarse particles

Pulverised biomass is a significant source of carbon neutral renewable electricity and hence plays an important role in carbon reduction strategies. Pulverised biomass is burned either co-fired with coal or increasingly in the UK as 100% pulverised biomass, due to fiscal incentives to do this. The particle size fed to the burners is relatively coarse with a low proportion <75µm and most being in the 75 - 1000 µm size range. There is little information on the flame propagation in clouds of coarsely pulverised biomass and this work was carried out to show coarse biomass could burn in a similar way to fine biomass but with slower flame speeds. There is little in the open literature on flame propagation and explosion risk for fine or coarse biomass particles and this work presents some results using the Hartmann dust explosion tube with high speed photography. The equipment was fitted with a vertical line of three thermocouples (red lines) to detect the flame arrival and determine the mean flame speed as a measure of the mixture reactivity. A second reactivity parameter was the initial rate of pressure rise prior to the vent bursting at the end of the tube. Lycopodium, and oak were investigated. Lycopodium powder is a pollen biomass that is ultra-fine. Oak sawdust was investigated for a fine ≤63µm fraction and for a coarser fraction. The lycopodium flame propagation is shown in Fig. 1. Similar results were obtained for oak milled to <63µm.