Burning Properties and Flame Propagation of Varying Size Pulverised Rice Husks

Flame propagation in different size fractions of a rice husk (RH) crop residues were investigated using an ISO 1 m3 dust explosion vessel. This was modified to operate with coarse biomass and for the determination of flame speeds. The flame speed, burning velocity and Kst were found to be greater for the finer fractions compared to the coarser sizes. The MEC were measured at 0.27 equivalence ratio (Ø) for the finest fraction to 1.4Ø for the coarser fraction. The most reactive concentration was measured at lower Ø for fine particles and higher Ø for coarse particles. The maximum Kst for the fine particles was 83 bar m/s and 33 bar m/s for the coarse particles. The size distribution of coarse rice husk particles always has a fine fraction and the flame propagation occurs first in the fine particles, with the coarse particles burning in the hot products of combustion of the fine particles. The fine particle fraction in a coarse mixture has to be flammable and as there is a low proportion of the mixture in the fine fraction, the overall concentration of particles has to increase for the concentration of fines to be flammable. This resulted in the observed lean flammability limit that was richer than stoichiometric for coarse size mixtures.