Explosion and Flame Propagation Properties of Coarse Wood : Raw and Torrefied

A current production torrefaction process was used and the explosion and flame propagation properties were determined at the particle size of the raw (spruce, pine and fir – SPF) and torrefied biomass. The biomass material as received was sieved to <1mm. Size analysis showed that 10% by mass was <100µm and the torrefied sample had 25% <100µm. The CV for the torrefied biomass was 10% greater than that for the raw biomass. The ISO 1 m3 dust explosion vessel was used, with a modified and calibrated biomass dispersion system that could cope with very coarse particles. The explosions did not burn all the dust that was present at the start of the explosion and the residual unburnt dust was shown to be the original dust. The equivalence ratio, O, of the propagating flame was based on the burnt dust concentration, Oburnt. Raw and torrefied samples were found to have minimum explosion concentrations, MEC, of 2.3Oburnt and 1.4Oburnt respectively and this shows that the torrefied sample was more reactive as it had a leaner MEC. The deflagration index, Kst, was higher for the torrefied SPF with a peak at 35 bar m/s compared with 24 for the raw biomass. The peak turbulent flame speeds were similar for torrefied and raw biomass at about 1 m/s. The torrefied biomass was more reactive than the raw biomass mainly due to the smaller particles size and 10% higher CV. The mechanism for coarse particle combustion is considered to be due to the explosion induced wind blowing the finer fractions ahead of the flame which burn first with the coarser fractions gasifying in the rich burnt gases behind the initial flame. The rich MEC was caused by the requirement to have the fine fraction above the MEC when only about 10% of the mixture was fine.