Explosion risks from nanomaterials

International audienceEmerging nanomanufactured products are being incorporated in a variety of consumer products ranging from closer body contact products (i.e. cosmetics, sunscreens, toothpastes, pharmaceuticals, clothing) to more remote body-contact products (electronics, plastics, tires, automotive and aeronautical), hence posing potential health and environmental risks. The new field of nanosafety has emerged and needs to be explored now rather than after problems becomes so ubiquitous and difficult to treat that their trend become irreversible. Such endeavour necessitates a transdisciplinary approach. A commonly forgotten and/or misunderstood risk is that of explosion/detonation of nanopowders, due to their high specific active surface areas. Such risk is emphasized and illustrated with the present development of an appropriate risk analysis. For this particular risk, a review of characterization methods and their limitations with regard to nanopowders is presented and illustrated for a few organic and metallic nanopowders

Ignition and explosion of nanopowders: something new under the dust

International audienceThis 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 Huang's 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

Evolution du niveau d'agglomération de nanopoudres d'aluminium : une approche rhéologique

National audienceFrom micrometric size to nanometric size, the increase of the surface energies allows the agglomeration to be preponderant and then, the risks linked to powders are changed. When the toxicity, the inflammation and explosion risks of nanoparticles are mentioned, the agglomerates must be taken into account. The effect of agglomeration in the behavior of nanopowders has been studied from experimental observation of agglomerated nanopowders of aluminum subjected to shear in a powder rheometer. In order to understand the effect of deagglomeration on the powders, the agglomerate strength has been estimated thanks to the porosity of the powder bed contained in the rheometer cell. This one is ranged between 10 kPa and 1 MPa. The rheological tests show that, compared with the micrometric aluminum, the aluminum nanopowders have a peculiar behavior in the Geldart's classification, being in both class A and C. This fact is due to the facility of nanoparticles to agglomerate and to stay agglomerated.Lorsque l'on passe de la dimension micrométrique à la dimension nanométrique, l'augmentation importante des énergies de surface rend prépondérants les phénomènes d'agglomération. Ceci modifie l'appréhension des risques liés aux poudres. Pour parler de toxicité, de risques d'inflammation et d'explosion des nanoparticules, nous devons prendre en compte la présence d'agglomérats. L'effet de l'agglomération sur la dispersibilité des nanopoudres a été étudié à partir d'observations expérimentales de nanopoudres agglomérées d'aluminium soumises au cisaillement fourni par un rhéomètre à poudres. Pour comprendre l'effet de la désagglomération sur les poudres, nous avons déjà estimé la contrainte d'agglomération mise en jeu à partir de la porosité du lit de poudres contenues dans la cellule du rhéomètre. Cette contrainte est globalement ici comprise entre 10 kPa et 1MPa. Les tests en rhéologie ont montré qu'en comparaison avec une poudre d'aluminium micrométrique, les nanopoudres d'aluminium ont un comportement particulier vis-à-vis de la classification de Geldart, appartenant à la fois aux classes A et C, ceci étant du à la facilité des nanoparticules à s'agglomérer et à le rester

The effect of agglomeration on the emission of particles from nanopowders flow

International audienceThis paper suggests an original method to evaluate the possible emission of particles from a nanopowder submitted to a shear stress in dense phase and the resulting degree of agglomeration of the particles released. The method is based upon the monitoring of the rheological signature of the nanopowders, thanks to a powder rheometer. As a function of the increasing shear rate, the powder flow will evolve from the newtonian state (dense powder) to the coulombian state (dense rheofluidified phase). If the shear rate is high enough, the powder will be set in suspension and the kinetic state (a leaner dense phase submitted to particles collisions) will be reached. The shear stress in this state is dependent on the particle or the agglomerate diameter for cohesive powders, which can be then calculated from rheograms. Carbon black and silica nanopowders have been tested and compared to other experiments carried out on non cohesive glass beads microparticles, chosen as reference. For the different glass beads powders, the average value of their 'agglomerate' diameter is 12% different of the primary diameter, indicating agglomeration of less than two particles. Nanometric agglomerates were found to be of hundred micrometers diameter. That is in line with the high tendency of the nanoparticles to agglomerate. This work can be used to evaluate the current safety tests, such as Hartmann's tube or 20 L sphere apparatuses, to verify whether the standard equipment for microparticles is suitable for the use of nanoparticles. This is linked to research projects like NanoSafe 2

Particle Size Distribution in a Godbert-Greenwald Furnace: Experiments and Modelling

During a dust dispersion, the particle size distribution (PSD) depends on several factors such as the turbulence, the initial particle size and shape as well as the dust concentration. As a consequence, when determining safety parameters using standard procedures, its potential evolution should be considered. Different powders were chosen: glucose, starch, ascorbic acid, glass beads and cellulose. A Godbert-Greenwald furnace was used to disperse the powders and determine their minimum ignition temperature (MIT) according to ISO/IEC 80079-20-2:2016 standard. The PSD of each powder was determined in-situ at different locations using a laser diffraction sensor. Some powders showed clear signs of breakage, as for glucose whose mean diameter decreases from 166 to 76 µm during its dispersion. On the contrary, many samples tended to agglomerate, e.g. starch and cellulose. For instance, the d90 of starch can even be quadrupled under certain conditions. Agglomeration occurs especially for fine dusts due to strong inter-particles forces (e.g. starch) or for elongated fibres due to entanglement phenomenon (e.g. cellulose). During a powder dispersion in the Godbert-Greenwald furnace, the PSD evolves not only as a function of time but also along with its location. The impact of the glass elbow on PSD variation has notably been highlighted by placing the G-G furnace horizontally. For powders showing strong tendency to agglomeration or breakage, the influence of the dispersion pressure has also been studied. The role of such PSD modification on the MIT has been measured and, depending on the dispersion procedure, temperature differences of more than 50°C have been observed. The agglomerate strength was assessed using three models (from Rumpf, Weiler and Kendall works) and compare to the deagglomeration stress exerted on the powders. In the case of cohesive powders, fibres or brittle dusts, attention should be paid to the PSD evolution during MIT determination

Effect of Particle Size Distribution and Inerting Mechanism on Explosion Severity of Organic/mineral Mixtures

The animal feed industry mixes cereals, vitamins, amino acids, and mineral powders to produce a solid mixture called “Premix”. The mitigation of premix explosions is challenging due to the diversity of composition, particle size, and nature of the mixed products. Therefore, determining their explosion safety parameters requires many standardised tests and a time-consuming process. However, it is possible to reduce the extensive use of experimental characterisation by better understanding the physicochemical mechanisms involved. In this context, this project aims to study the influence of Particle Size Distribution (PSD) and the mineral chemical nature on the explosion severity of organic and mineral powder mixtures commonly used for premix manufacturing. Cornflour was mixed with four minerals (sodium chloride, sodium bicarbonate, calcium carbonate, and magnesium oxide) chosen based on their industrial applications and inerting mechanisms (scavenging of radicals, inert gas generation, and heat sink). The powders were sieved to obtain samples with distinct particle size ranges. PSD was analysed ex-situ and in-situ to study the fragmentation behaviour of the products. The explosion tests method was based on the standard ISO/IEC 80079-20-2 using the 20L sphere. The results indicated that due to physical and chemical effects, NaHCO3 is the most efficient inerting agent. Moreover, its initial PSD did not affect the inhibition performance due to its brittleness and the explosibility test pressure gradient, leading to possible inerting overestimation. NaCl reduced the deflagration index (KSt) less efficiently due to the incomplete decomposition into scavenging agents of free radicals, essential for flame propagation. The unsuitable addition of purely thermal inhibitors (CaCO3, MgO) could increase the mixture’s KSt due to a dispersibility improvement, dust cloud PSD reduction and radiation effects. The mineral nature selection during product design could then significantly impact the inherent safety in the premix industry

Self-heating is in the Air': the Role of Oxygen Diffusion on the Thermal Stability of Biomass Piles

This study aims to investigate the thermal stability of biomass piles through a combination of experimental and numerical approaches and explore the impact of particle size and oxygen diffusion. Isothermal basket tests were carried out according to EN-15188:2020 on raw and grinded pellets and sieved dust samples. The progression of the thermal wave inside the baskets was particularly studied by positioning thermocouples at different depths of the pile. The role of oxygen diffusion in the pile was examined by varying the basket size, by modifying the particle size distribution and by partially wrapping the baskets in a protective film. The self-heating behaviour of these piles was also assessed by using the crossing point method. The time evolution of the gases generated was analysed by micro-gas chromatography, especially around the cross-point. In parallel, a three-dimensional model was developed to simulate the thermal behaviour of a quarter cube. The model includes energy balance, considering conductive, convective and radiative heat transfers and a heat-source term. Mass balances for particle size and each species are also considered through consumption and diffusion terms. Shrinking core models were implemented to represent the consumption of reactants. Moreover, thermogravimetric analyses were performed to identify the various reaction stages and determine the activation energy by using Flynn-Wall-Ozawa, Friedman and Kissinger’s methods. This study demonstrates the significant influence of bed permeability, especially related to oxygen accessibility, on the thermal stability of storage facilities. Finally, the predictive model developed could be used to explore the efficiency of safety measures and technological solutions (compaction, storage size reduction, bagging...)

Experimental and Computational Study of the Dispersion and Combustion of Wheat Starch and Carbon-Black Particles During the Standard 20L Sphere Test

PresentationThe 20L sphere is one of the standard devices accepted as an international normativity used for dust explosivity characterization. One concern about the effectiveness and reliability of this test is related to the particle size variation due to particles agglomeration and de-agglomeration. These phenomena are determined by the turbulent regime of the dust cloud during the dispersion. This variable must be considered since it determines the uncertainty level of the ignitability and severity parameters of dust combustion. In this context, this study describes the influence of the cloud turbulence on the dust segregation and fragmentation through an experimental and computational study. The behavior of the gas-solid mixture evidenced with the standard rebound nozzle was compared with that observed with six new nozzle geometries. Thereafter, the variations of the Particle Size Distribution (PSD) that occur during the dispersion within the 20L sphere were analyzed for two different powders: carbon-black and micrometric wheat starch. This description is performed with the implementation of two complementary approaches. On the one hand, an experimental approach characterizes the turbulence levels with Particle Image Velocimetry (PIV) tests that are complemented by the description of the PSD variations with granulometric analyses. On the other hand, a computational approach described the dispersion process with CFD-DEM simulations developed in STAR-CCM+ v11.04.010. The simulation results established that the homogeneity assumption is not satisfied with the nozzles compared in this study. Nonetheless, the particles segregation levels can be reduced using nozzles that generate a better dust distribution in the gas-solid injections. Subsequently, an additional first-approach CFD model was established to study the behavior of the combustion step when a starch/air mixture. This model considers the gas- phase reactions of the combustible gases that are produced from the devolatilization of Wheat starch (CO, CH4, C2H4, C2H6, C2H2 and H2) and allowed to establish the approximate fraction of the particle mass that devolatilizes, as well as to confirm that the modelling of the pyrolysis stage is essential for the correct prediction of the maximum rate of pressure rise

Reuse of medical face masks in domestic and community settings without sacrificing safety: Ecological and economical lessons from the Covid-19 pandemic

The need for personal protective equipment increased exponentially in response to the Covid-19 pandemic. To cope with the mask shortage during springtime 2020, a French consortium was created to find ways to reuse medical and respiratory masks in healthcare departments. The consortium addressed the complex context of the balance between cleaning medical masks in a way that maintains their safety and functionality for reuse, with the environmental advantage to manage medical disposable waste despite the current mask designation as single-use by the regulatory frameworks. We report a Workflow that provides a quantitative basis to determine the safety and efficacy of a medical mask that is decontaminated for reuse. The type IIR polypropylene medical masks can be washed up to 10 times, washed 5 times and autoclaved 5 times, or washed then sterilized with radiations or ethylene oxide, without any degradation of their filtration or breathability properties. There is loss of the antiprojection properties. The Workflow rendered the medical masks to comply to the AFNOR S76-001 standard as “type 1 non-sanitory usage masks”. This qualification gives a legal status to the Workflow-treated masks and allows recommendation for the reuse of washed medical masks by the general population, with the significant public health advantage of providing better protection than cloth-tissue masks. Additionally, such a legal status provides a basis to perform a clinical trial to test the masks in real conditions, with full compliance with EN 14683 norm, for collective reuse. The rational reuse of medical mask and their end-of-life management is critical, particularly in pandemic periods when decisive turns can be taken. The reuse of masks in the general population, in industries, or in hospitals (but not for surgery) has significant advantages for the management of waste without degrading the safety of individuals wearing reused masks