Dust Explosions: Recent Developments Including Pulverised Biomass Plenary Lecture

Dust explosion minimum explosion concentration, MEC, and mixture reactivity measurement are reviewed in terms of the mixture equivalence ratio, O, rather than the concentration units of g/m3, used in all reported dust explosion data. Existing data for MEC implies that HCO dusts are flammable to O ~ 0.2, which is leaner than pure hydrocarbon dusts or gases where the MEC is O = 0.45. Biomass dusts have a HCO elemental composition and they also show very lean MEC at O~0.2. Peak explosion pressure, Pmax, and the explosion reactivity, Kst, are high for a wide range of very rich O for many dusts including biomass and this behaviour is different to that in gaseous explosions. The laminar burning velocity, UL, of dusts was derived from the measured Kst and the ISO 1m3 explosion vessel can be modified to enable spherical flame speeds to be determined for dusts, which gives two techniques to determine UL. It was shown that Kst and KG are linearly related to UL if both are measured in the same 1 m3 explosion vessel. These laminar burning velocities and Kst show that most St1 cellulose type dusts are not very reactive, with Su less than that for gaseous hydrocarbons. The apparent high reactivity of HCO dusts in the MEC data indicates potential problems in the measurement of MEC for dusts. Part of this problem is that in the ISO 1m3 dust explosions around half of the dust is left on the bottom of the vessel at the end of the explosion. Thus the usual nominal concentration of dust in g/m3 is not the concentration that the flame propagated through. This makes the MEC measurement very difficult as the lower the concentration, the lower the proportion of the dust that burns, so that the concentration in the flame front at the MEC is difficult to determine. The Hartmann equipment is potentially a more reliable method of determining the MEC, but this also has its experimental problems, related to whether the injected dust is mixed at ignition. For woody and plant biomass dusts there are additional problems with the fibrous nature of the dusts: the standard dust pot size will not hold sufficient mass and the fibres block in the standard ‘C’ ring dust injection system. Solutions to both of these problems are demonstrated with new calibrated dust injection systems for fibrous biomass. The wide particle size distribution in biomass dusts is a further problem and experiments show that very coarse woody biomass can explode and a mechanism for this is developed involving the propagation of the flame in the fines with the coarse particles gasifying in the burnt gases of the fine flame. This explains the rich MEC for coarse biomass and the lack of a rich limit.