Nouvelles considérations sur l'explosivité des nanopoudres : un nano-grain de sable dans les rouages des standards

The small size of nanoparticles implies a high specific surface area, which induces original properties when compared to micropowders, such as chemical, mechanical, optical or biological properties. Among these new specificities, nanoparticles are subjected to weak cohesion forces, such as van der Waals, electrostatic or capillary forces, which cause them to agglomerate in a reversible way. However, dust explosions require the dispersion of the dust in the air, which can lead to a breakage of these agglomerates. The aim of this work is then to study the influence of nanoparticles specificities, notably the agglomeration, on their ignition sensitivity and explosion severity, and evaluate the adequacy of the international standard methods to determine these parameters when it comes to nanoparticles. Four types of powders were chosen to highlight the specific behavior of each type of combustible powder: carbon black, nanocellulose, aluminum and carbon-coated silicon. The powders were characterized by Scanning Electron Microscopy (SEM), specific surface measurement and helium pycnometer, and their particle size distribution (PSD) was measured before and after dispersion using different methods. The mean surface diameter, used to consider the high surface area of nanoparticles, considerably decreases after the injection in the 20L sphere, i.e the standard equipment to measure the explosion severity of a dust. This observation highlighted the necessity to characterize the dust after injection in the 20L sphere, to accurately evaluate the explosion risk. Ignition and explosion tests were conducted in standard conditions, but also by varying the powder preparation (aging, drying, sieving, agglomeration), and the operating conditions (dispersion procedure, ignition energy, initial turbulence). Specific behaviors related to the powder nature (carbonaceous, organic or metallic) were then observed, discussed, and alternative measurement methods were proposed. For instance, alternative dispersion nozzles were tested to provide a better cloud homogeneity or to reproduce industrial release conditions. Dispersion procedure and ignition source should be adapted to the minimum ignition energy of the nanopowders to avoid both pre-ignition and overdriving. Variation of the ignition delay time can be helpful to obtain the most conservative results. One of the main proposals consists in the consideration of the laminar burning velocity as a standard characteristic of the explosion severity. Experiments were conducted in a flame propagation tube and a vented 20L sphere to evaluate the unstretched burning velocity of nanocellulose. The results were then compared to an existing correlation based on the pressure-time evolution during standard experiments. Furthermore, a flame propagation model, initially designed for hybrid mixtures, was adapted to the flame propagation in a cloud of organic nanoparticles, showing consistent results with the experiments.La petite taille des nanoparticules induit une grande surface spécifique, qui procure des propriétés inédites, notamment chimiques, mécaniques, optiques ou biologiques, comparées aux particules micrométriques. Parmi ces nouvelles spécificités, les nanoparticules sont soumises à des forces de cohésion plus intenses, telles que des forces de van der Waals, électrostatiques ou capillaires, ce qui les amène à s’agglomérer de manière réversible. Cependant, une explosion de poussières nécessite une dispersion de la poudre dans l’air, ce qui peut mener à une fragmentation de ces agglomérats. L’objectif de ce travail est ainsi d’étudier l’influence des spécificités des nanoparticules, notamment de l’agglomération, sur leur sensibilité à l’inflammation et leur sévérité d’explosion, et d’évaluer l’adéquation des méthodes définies par les standards internationaux pour la détermination de ces paramètres concernant les nanoparticules. Quatre types de poudres ont été considérées pour étudier le comportement spécifique de chaque type de combustible: noirs de carbone, nanocellulose, aluminium et silicium enrobé de carbone. Les poudres ont été caractérisées au repos, et leur distribution de tailles de particules a été mesurée avant et après dispersion à l’aide de méthodes complémentaires. Le diamètre moyen en surface diminue significativement après l’injection dans la sphère de 20L, i.e l’équipement standard utilisé pour mesurer la sévérité d’explosion. Cette observation prouve la nécessité de caractériser la poudre après injection dans la sphère de 20L, pour évaluer le risque de manière fiable. Des essais d’inflammation et d’explosion ont été réalisés dans des conditions standards, mais aussi modifiant la préparation de la poudre (vieillissement, séchage, tamisage, agglomération) ainsi que les conditions opératoires (procédure de dispersion, énergie d’inflammation, turbulence initiale). Des comportements spécifiques liés à la nature de la poudre (carbonée, organique ou métallique) ont été observés, discutés, et des méthodes alternatives de mesure ont été proposées. Par exemple, la procédure de dispersion et la source d'inflammation doivent être adaptées à l'énergie d'allumage minimale des nanopoudres pour éviter à la fois les phénomènes de pré-inflammation et ‘d’overdriving’. Enfin, l’une des principales propositions consiste à considérer la vitesse laminaire de flamme comme un paramètre standard représentant la sévérité d’explosion. Des tests ont été réalisés dans un tube de propagation de flamme et dans une sphère éventée pour évaluer la vitesse non étirée de la nanocellulose. Les résultats obtenus ont alors été comparés à une corrélation existante basée sur les paramètres obtenus lors d’essais standards. De plus, un modèle de propagation de flamme, initialement développée pour des mélanges hybrides, a été adapté pour représenter la propagation de flamme dans un nuage de nanoparticules et a montré des résultats en adéquation avec les résultats expérimentaux

Nanopowders explosion : influence of the dispersion characteristics

International audienceWith the publication of NFPA 68 (2013); a major change is in progress in venting area calculation methods for gas explosions. Old methods referring to the Kg parameter proved to be inappropriate for real applications. The present work provides new data to correlate real gas concentration and initial turbulence conditions to flame propagation and explosion overpressure during a vented gas explosion. Explosion tests were performed in a 4 m3 rectangular chamber equipped with transparent walls and vented (0.49 m2 square vent) on one side. The chamber is filled with a turbulent or quiescent hydrogen-air mixture with a purposely built injection system that allows to vary the turbulence intensity and the length scale. Gas concentration and turbulence parameters are measured with concentration gauges and Pitot probes distributed in the chamber (Duclos, 2017). Then the flame propagation is fully characterized with high speed video and explosion overpressure is measured inside and outside the chamber. The paper presents the parametric study performed by varying the initial turbulence and focuses on its influence on the inside explosion overpressure. Then physics of vented gas explosion is discussed, results are compared to developing phenomenological model

Influence of the particle size distribution on dust explosion: How to choose the right metrics?

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Fast and tiny: A model for the flame propagation of nanopowders

International audienceTo avoid the influence of external parameters, such as the vessel volume or the initial turbulence, the explosion severity should be determined from intrinsic properties of the fuel-air mixture. Therefore, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is both independent of external factors and a useful input for CFD simulation. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion, due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work focuses on a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Assuming the fast devolatilization of organic nanopowders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm/s were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm/s measured experimentally by flame visualization and allows the validation of the model for nanoparticles

Course of explosion behaviour of metallic powders – From micron to nanosize

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‘Knock on nanocellulose’: Approaching the laminar burning velocity of powder-air flames

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Stronger together ? Influence of the agglomeration on nanopowders explosion

Among the factors influencing dust explosion, the particle size distribution (PSD) is both one of the most important and the most complex to consider. For instance, it is commonly accepted that the explosion sensitivity increases when the particle size decreases. Such an assertion may be questionable for nano-objects which easily agglomerate. However, agglomerates can be broken during the dispersion process. Correlating the explosion parameters to the actual PSD of a dust cloud at the moment of the ignition becomes then essential. Powders were characterized before and during their dispersion using in situ laser particle size measurement and a fast mobility particle sizer. The turbulence was determined by particle image velocimetry. The explosion severity and sensitivity of nanocellulose was determined under various operating conditions and for wet and dried powders. The impact of the turbulence on the flame kernel growth was highlighted using a flame propagation tube. It appears that the ignition sensitivity of nanocellulose depends on the PSD and on water activity whereas its explosion severity is less modified by the presence of agglomerates in the raw powder

Some key considerations when evaluating explosion severity of nanopowders

International audienceProtection from explosion events requires the determination of key safety parameters like lower explosion limit, maximum explosion over-pressure, and maximum rate of pressure rise. These parameters are routinely obtained through standard tests performed typically either in a 20 L sphere or a 1 m 3 container. But several aspects are worth a closer investigation. Firstly, the test apparatus must be able to disperse a fairly uniform dust cloud. However, previous investigations showed that actually the current dispersion system can be improved. Indeed, as a function of the sample and the dispersion conditions, the dust concentration in the ignition zone can be significantly different from the nominal dust concentration. Moreover, due to agglomeration/fragmentation phenomena, the particle size distribution can greatly evolve during dust injection. Secondly, the influence of humidity on the explosivity is not considered in current standards. It is just stated that the relative humidity should be checked and noted down, though some provisions exist in American standards. Even when performing the powder injection with synthetic air, the water contained in the residual air of the sphere can impact the chemical reactions occurring during the explosion. Thirdly, the ignition delay time is sometimes modified to study the impact of the dust cloud turbulence on flame propagation but is often misunderstood. For instance, by decreasing the ignition delay time, the injection time of the powder becomes shorter for the same pressure difference. This modifies the dispersion kinetics and can change the particle size distribution. Finally, for very sensitive powders, pre-ignition can occur or the ignition energy sets at 10kJ can lead to an overdriving phenomenon. Maybe these aspects have not been thoroughly considered for micron powders. However, in the case of nanopowders, the importance of these influencing factors was shown in order to duly evaluate explosion parameters. Experimental evidences confirm these aspects and alternative solutions will be presented

A flame propagation model for nanopowders

International audienceThe determination of explosion severity should be made from intrinsic properties of the fuel-air mixture in order to avoid the influence of external parameters, such as the vessel volume or the initial turbulence. To overcome such limitations, the flame propagation of gaseous mixtures is often studied in order to estimate their laminar burning velocity, which is independent of external factors and is a useful input for CFD simulation and for the sizing of protective devices. Experimentally, this parameter is difficult to evaluate when it comes to dust explosion due to the inherent turbulence during the dispersion of the cloud. However, the low inertia of nanoparticles allows performing tests at very low turbulence without sedimentation. Knowledge on flame propagation concerning nanoparticles may then be modelled and, under certain conditions, extrapolated to microparticles, for which an experimental measurement is a delicate task. This work then focused a nanocellulose with primary fiber dimensions of 3 nm width and 70 nm length. A one-dimensional model was developed to estimate the flame velocity of a nanocellulose explosion, based on an existing model already validated for hybrid mixtures of gas and carbonaceous nanopowders similar to soot. Due to the fast devolatilization of organic powders, the chemical reactions considered are limited to the combustion of the pyrolysis gases. The finite volume method was used to solve the mass and energy balances equations and mass reactions rates constituting the numerical system. Finally, the radiative heat transfer was also considered, highlighting the influence of the total surface area of the particles on the thermal radiation. Flame velocities of nanocellulose from 17.5 to 20.8 cm.s-1 were obtained numerically depending on the radiative heat transfer, which proves a good agreement with the values around 21 cm.s-1 measured experimentally by flame visualization and allows the validation of the model for nanoparticles

Nanocellulose explosions: influence of the agglomeration and turbulence on the combustion rate-limiting step and flame propagation

Dust explosion risk assessment is relatively well established for micron-sized particles and requires the determination of key safety parameters representing the ignition sensitivity and explosion severity of the dust. When considering nanoparticles, the particle size distribution (PSD) is more likely to vary during the injection process, due to both the agglomeration phenomenon inherent to strong interactions and the fragmentation phenomenon due to flow shear stresses. As a consequence, safety parameters and their determination methods can differ significantly from micro to nanopowders. A peculiar attention has then to be given to the cloud characteristics (PSD, turbulence), more precisely at the exact moment of ignition. This work focuses on nanocellulose and aims at evaluating the influence of the agglomeration phenomenon and flow turbulence on the dust combustion. Flame propagation tests were performed to evaluate the unstretched burning velocity and explosions tests were carried out to estimate the combustion mechanisms involved