Ignition sensitivity of different compositional wood pellets and particle size dependence

Partial or complete substitution of coal with renewable biomass like wood is a sustainable and effective solution to reduce the CO₂ emissions in the atmosphere. Utilization of these woods in the form of compact pellets facilitates in its handling and transportation with higher energy density. However, for electric power generation the pellets are broken up into their constituent milled finer particles for burning as a pulverised biomass flame. There is a dust fire/explosibility hazards in the process handling facilities such as in storage, conveying and milling. In the present work, four commercial pellets were investigated, and each pellet sample was split into three size ranges <63 μm, 63-500 μm and <500 μm. The flame propagation characteristics and their lean flammability limit for each pulverised pellet was determined using modified Hartmann dust explosion tube. It was found that the fine particles, with lower lean flammability limits of 0.3-0.7 equivalence ratio, intensify the explosibility risk (dP/dt of 10-15 bar/s for most reactive concentration) of the dust due to fast volatile release rate. Comparison was also made between particle size distribution (PSD) of the finer fractions (<63 μm) of wood samples in comparison to crop residue samples that showed 70-80% larger size distribution for wood samples due to elongated particles as showed by Scanning Electron Microscope (SEM). Results showed that the ash + moisture content had a stronger effect on wood samples than on agricultural residue's samples. The results showed that the explosibility characteristics of the pellets industry's feed-stocks was variable and dependent on the pellet composition and this needs to be taken into account in explosion protection and in utilising the pellets in the main pulverised biomass combustion.