Does the dust explosion risk increase when moving from μm-particle powders to powders of nm-particles?

Based on experience with powders of particle sizes down to the 1–0.1 μm range one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high KSt-values). However, there are two basic physical reasons why this may not be the case. Firstly, complete transformation of bulk powders consisting of nm-particles into dust clouds consisting of well-dispersed primary particles is extremely difficult to accomplish, due to very strong inter-particle cohesion forces. Secondly, should perfect dispersion nevertheless be achieved, the extremely fast coagulation process in clouds of explosive mass concentrations would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal the basic mechanism of flame propagation in dust clouds suggests that increased cloud explosion rates would not be expected as the particle size decreases into the <1 μm range. An overall conclusion is that dust clouds consisting of nm primary particles are not expected to exhibit more severe KSt-values than clouds of μm primary particles, in agreement with recent experimental evidence. In the case of the ignition sensitivity recently published evidence indicates that MIEs of clouds in air of some metal powders are significantly lower for nm particles than for μm particles. A possible reason for this is indicated in the paper.

► Extrapolation of μm powder data predicts very low MIEs and very high KSts for nm powders. (Note that “St” is a subscript to K). ► However, this is not expected in reality for the following reasons. ► Generation of dispersed nm particle clouds of explosive concentrations is practically impossible. ► For organic materials + coals gas phase combustion is rate controlling for particles <0 μm. ► Experiments confirm these expectations, except for very low MIEs of some metal powders.