Ignition and explosion risks of nanopowders
Introduction
For any powders set in suspension in air, the ease at which they ignite, burn and propagate varies considerably with important factors that have to be characterized so that one can be able to control their explosion propensity. One of them is obviously the particle size, which takes nanopowders now under higher scrutiny for OH&S issues; others are the particle concentration, the degree of agglomeration, and the degree of turbulence [1], [2], [3], [4]. So far, literature studies concerning the evaluation of explosion and flammability hazards of powders were essentially carried out on micro-sized powders [5], albeit there are the recent extensive studies performed by the EU NANOSAFE2 project [6] during the 2005–2009 period and more recently by Wu in Taiwan [7], [8] in 2009–2010. This work proposes to review measured explosion safety parameters of few nanoparticles and nanofibers (carbon blacks, aluminium and multiwalled carbon nanotubes) performed in course of the EU NANOSAFE2 project.
Section snippets
Nanopowders considered from the Geldart's powder classification perspective
Powders can be characterized by their propensity to fluidize in air. Geldart [9] suggested that uniformly sized powders can be classified into four types characterized by the density difference between the particle ρs and the fluid ρg and by their mean particle size (Fig. 1).
Nanopowders are therefore of Geldart C (Cohesive) type, so they naturally tend to easily agglomerate. Group C powders are difficult to fluidize because the interparticle forces are greater than those that the fluid exerts
Dependence of the combustion time with the particle size
Two main categories of particle sizes should be considered depending on micro- or nano-sized particles. Such distinction comes from the two basic combustion mechanisms (diffusion and kinetic controlled) that are at play. Intuitively, as will be demonstrated in the following paragraph, for micro-sized particles, the combustion is mainly controlled by the oxygen diffusion whereas for nano-sized particles, the combustion is kinetically controlled.
The combustion time τb can be estimated through the
Conclusions
In this paper, we have developed analytical models explaining the dependency of the combustion times with the particle diameter. We were able, with experimental validation, to show that there exists for carbonaceous and metallic materials mainly two combustion regimes that are kinetically controlled for small size (nano) particles and diffusion controlled for large size (micro) particles. From the combustion data of these materials, we were able to describe the dependency of the ignition
Acknowledgements
This study was carried out with the financial support of the European Commission through the Sixth Framework program for Research and Technological Development NMP2-CT-2005-515843 contract “NANOSAFE2” and the French Ministry of Ecology, Energy Transport and Sustained Development and the Ministry of Research. We thank Mr. A. Shakesheff from IntrisiqNanomatérials for providing Aluminium nanopowders and Mr. P. Gaillard from Arkema for providing Multiwall Carbon nanotubes used in this study.
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