Olivier Dufaud

Ignition and explosion risks of nanopowders.

Characterization methods with regard to nanopowder flammability and explosivity are presented and illustrated for few nanopowders. Analytical models are developed in order to explain the dependency of the combustion times on the particle diameter. Experimental evidence shows that there exists, for carbonaceous and metallic materials, mainly two combustion regimes that are either kinetically controlled, for small size particles, or diffusion controlled, for large size particles. From the experimentally measured combustion data of those materials, the dependencies of the ignition temperature and the minimal explosive concentration (MEC) with regard to the particle size have been analyzed. We found that the two combustion regimes yield two different tendencies with respect to the particle size. Overall, it is found that as the particle size decreases, minimum ignition temperature (MIT) and minimum ignition energy (MIE) decrease, indicating higher potential inflammation and explosion risks for the use of nanopowders. By contrast, the minimal explosion concentration (MEC) did not show strong variations as the particle size decreases. Rather, a theoretical plateau is observed, which was experimentally confirmed. We also observed that carbon nanopowders exhibit a low propensity to explode while metallic nanopowders can be very reactive, thus delineating high potentials for explosion risks in manufacturing facilities.

Ignition and explosion of nanopowders: something new under the dust

This work deals with the study of ignition and explosion characteristics of nanoparticles. It has been carried out on various powders: zinc, aluminum, carbon blacks... Specific behaviours have been highlighted during the first phase of this project (Nanosafe 2). For instance, it has been demonstrated that there mainly exists two combustion regimes that are either kinetically controlled, for small size particles, or diffusion controlled, for large size particles (generally with diameters greater than 1 or 2 µm). It has been found that as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease (even lower than 1 mJ), indicating higher potential inflammation and explosion risks for metallic nanopowders. Moreover, the presence of agglomerates in the nanopowders could modify their reactivity. Thus, the explosion severity of Al powders tends to increase as the specific surface area decreases, before reaching a peak for 1 µm particle size. These results are essential for industries producing or handling nanopowders in order to propose/design new and proper prevention and protection means. Nevertheless, the validity of the classical characterization tools with regard to nanopowders should be discussed. For example, the experimental laminar flame velocity of Al dusts has been compared to a theoretical one, determined by Huangs model, which assumes that the propagation of the flame is run mainly by conduction. It has shown a good agreement. However, under certain conditions, the Al flame propagation is expected to be mainly conducted by radiation. Two hypotheses can then be made. On the one hand, it can be assumed that the 20 L sphere probably disturbs the flame propagation and thermal mechanisms by absorbing radiation (wall quenching effect). On the other hand, it has been observed, thanks to the use of a high speed camera that the preheating zone is smaller for some nanopowders than for micro-particles (figure below). It could notably be explained by the fact that the flame radiation is absorbed by the cloud of unburnt Al nanopowders. Several other factors may have an impact on the explosion severity. If these points are correctly addressed, it will be possible to get more reliable ignition and explosion characteristics.

Generation, Characterization and Ignition of Lube Oil Mists

In this work, the generation conditions of lube oil mists by spray, their droplet size distributions and their flammability characteristics were studied.At first, tests were carried out on brand new lube oil having a viscosity of 32 mm2.s−1 at 40°C (ISO VG32) and a flashpoint higher than 200°C. Experiments were performed in a vertical semi-open tube with a 0.07 m-square cross section connected to ignition systems supplying energies ranging from 1 mJ to 5 kJ. The average droplet diameter, determined using a laser diffraction sensor, ranged from 4 to 60 μm depending on operating parameters.In the case of brand new mineral oil mists, the probability of ignition by a spark discharge can be considered as low, the minimum ignition energy (MIE) being 2 kJ for 5 μm droplets. The minimum explosive concentration is fairly high, in the order of 250 g.m−3. For mists with droplet diameters around 60 μm, MIE slightly increases to reach 2.5 kJ.The flammability of oil mists generated in case of maloperation (accidental fuel/lube mixing) was also studied. The ignition properties of used lube oil were not notably modified with regard to the brand new oil. However, the addition of organic volatiles compounds leads to a modification of mists flammability, nevertheless it is only perceptible for fuel contents greater than 20%v.© 2015 ASME