An investigation on the dust explosion of micron and nano scale aluminium particles

A series of dust explosion were conducted to compare the flame structure between nano and micron aluminium dusts. Two-color pyrometer technique is applied to have qualitative observation of flame development. Measurement of temperature indicates that explosion in micron aluminium dust clouds start in a single spot at 3000 K, in contrast, explosion in nano aluminium dust clouds start when hot powder accumulated to a certain amount at lower temperature of 2600 K. For micron aluminium dust clouds, flame at leading edge has the highest temperature and propagates in all directions. On the other hand, flame in nano aluminium dust clouds propagate only upward with the hottest part left behind at the downside. As flame propagates, the temperature at top edge gradually decreases from 2600 K to finally 2000 K, but temperature at bottom edge maintains in 3000 K with no significant displacement. The unevenness of flame structure is considered as the consequence of different particle densities, which suggests that the reaction of nano aluminium particles stays in molten state, meanwhile, the high surface area also leads to unignorable heat loss.     

Dust explosions: How should the influence of humidity be taken into account?

In this work, the influence of humidity on dust explosions of metallic (aluminium) and organic materials (icing sugar, polyethylene and magnesium stearate) has been studied. The impact of pre-humidification of powders on their ignition sensitivity, their volume resistivity and charge decay time has been assessed. The influence of humidity on explosion severity has also been studied by two methods: on the one hand, the dust sample was stored in a controlled workstation at constant relative humidity; on the other hand, the dry dust was dispersed in a humidity controlled atmosphere in the vessel.As expected, the effect of humidity strongly depends on the chemical nature of the particles. Experiments on powders volume resistivity and charge decay time have shown typical trends but have especially pointed out the inadequacy of some standards. Inhibition phenomena have been verified for polyethylene and magnesium stearate, whereas both inhibition and promotion have been observed for icing sugar and could be explained by an evolution of sucrose structure. Dry aluminium dust explosions in humid atmosphere show that water vapour inerts the explosion. However, when aluminium is stored at controlled humidity, the maximum rise of pressure rate increases with the water content, which is probably due to hydrogen generation.     

Explosibility of cork dust in methane/air mixtures

Explosibility studies of hybrid methane/air/cork dust mixtures were carried out in a near-spherical 22.7 L explosibility test chamber, using 2500 J pyrotechnic ignitors. The suspension dust burned as methane/air/dust clouds and the uniformity of the cork dust dispersion inside the chamber was evaluated through optical dust probes and during the explosion the pressure and the temperature evolution inside the reactor were measured. Tested dust particles had mass median diameter of 71.3 μm and the covered dust cloud concentration was up to 550 g/m³. Measured explosions parameters included minimum explosion concentration, maximum explosion pressures and maximum rate of pressure rise. The cork dust explosion behavior in hybrid methane/air mixtures was studied for atmospheres with 1.98 and 3.5% (v/v) of methane. The effect of methane content on the explosions characteristic parameters was evaluated. The conclusion is that the risk and explosion danger rises with the increase of methane concentration characterized by the reduction of the minimum dust explosion concentration, as methane content increases in the atmosphere. The maximum explosion pressure is not very much sensitive to the methane content and only for the system with 3.5% (v/v) of methane it was observed an increase of maximum rate of pressure rise, when compared with the value obtained for the air/dust system.     

Study on explosion risk of aluminum powder under different dispersions

This paper mainly studied the influence of particle size distribution on the explosion risk of aluminum powder under the span of large particle size distribution. The measurement was carried out with the 20 L explosion ball and the Hartmann tube. The statistical analysis was used to analyze the relevance between the parameters of explosion risk and the particle size parameters. Test results showed that with the increase of particle size, the sensitivity parameter increases and the intensity parameter deceleration decreases. The effect of particle size change on MEC and MIE of small particle size aluminum powder is relatively small but greater impact on Pm and (dP/dt)m. The small particle size components greatly increasing the sensitivity of the explosion and accelerating the rate of the explosion reaction; while the large particle size component contributes to the maximum explosion pressure. D_3,2 particle size dust determines the risk of aluminum powder explosion.     

Study of dust cloud behaviour in the modified Hartmann tube using the image subtraction method (ISM)

A dispersion of fine particles in the air is needed for a dust explosion to occur since an explosion is the fast combustion of particles in the air. When particles are poorly dispersed, agglomerated, or their concentration is low, the combustion velocity decreases, and deflagration would not occur. The combustion rate is strictly related to dust concentration. Therefore, the maximum explosion pressure rise occurs at dust concentration close to stoichiometric. Conversely, Minimum Explosion Concentration (MEC) is the lower limit at which self-sustained combustion and a pressure rise are possible. Dust explosion tests are designed to reproduce the dispersion and generation of dust clouds in industrial ambiences by using dispersion devices activated by pressurised air pulses. The resulting dust cloud, which has a marked transient character, is considered representative of real clouds by current standards. Over time, several studies have been carried out to optimise these devices (e.g. to reduce the inhomogeneity of the cloud in the 20 L sphere). The Minimum Ignition Energy (MIE) of dust is measured using the Mike3 modified Hartmann tube, where the ignition attempt is made 60–180 ms after dust dispersion regardless of dust characteristics.This work investigates the dust clouds’ actual behaviour inside the modified Hartmann tube before ignition using high-velocity video movies and a new image post-treatment method called Image Subtraction Method (ISM). Movies are recorded with high-speed cameras at a framerate of 2000 fps and elaborated with an on-purpose developed LabVIEW® code. Concentration (mass per volume) and dispersion pressure are varied to evaluate their effect on dust clouds. Maise starch, iron powder and silica powder are chosen to investigate the effect of particle density and size on the cloud structure and turbulence. This approach will help to investigate the structure of the dust cloud, the shape and size of the particle lumps and the change in dust concentration over time. In addition, information on the actual concentration and cloud turbulence at the ignition location and delay time were obtained, which may help identify the local turbulence scale and widen the characterisation of the cloud generated in the Hartmann tube.     

Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube

In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0–50, 50–96 and 96–180 μm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50–96 μm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m³. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m³, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions.     

Methane/coal dust/air explosions and their suppression by solid particle suppressing agents in a large-scale experimental tube

Methane/coal dust/air explosions under strong ignition conditions have been studied in a 199 mm inner diameter and 30.8 m long horizontal tube. A fuel gas/air manifold assembly was used to introduce methane and air into the experimental tube, and an array of 44 equally spaced dust dispersion units was used to disperse coal dust particles into the tube. The methane/coal dust/air mixture was ignited by a 7 m long epoxypropane mist cloud explosion. A deflagration-to-detonation transition (DDT) was observed, and a self-sustained detonation wave characterized by the existence of a transverse wave was propagated in the methane/coal dust/air mixtures.The suppressing effects on methane/coal dust/air mixture explosions of three solid particle suppressing agents have been studied. Coal dust and the suppressing agent were injected into the experimental tube by the dust dispersion units. The length of the suppression was 14 m. The suppression agents examined in this study comprised ABC powder, SiO₂ powder, and rock dust powder (CaCO₃). Methane/coal dust/air explosions can be efficiently suppressed by the suppression agents characterized by the rapid decrease in overpressure and propagating velocity of the explosion waves.     

超细CaCO3惰化剂对铝合金抛光伴生粉尘爆炸的防控效果*

为探究超细粉体惰化剂对铝合金抛光伴生粉尘爆炸特性的影响规律,利用标准化实验装置及自行搭建的实验平台,在对爆炸基本参数进行测试的基础上,分别研究超细CaCO3粉体对抛光废弃物粉尘点燃敏感度的钝化作用以及对爆炸火焰传播进程的惰化效果,并在相同条件下与同等粒径高纯度铝粉的实验效果进行比对。研究结果表明:铝合金抛光废弃物粉尘最小点火能量为280 mJ,而同等粒径高纯度铝粉最小点火能量为35 mJ;在铝合金抛光废弃物粉尘质量浓度为300 g/m3条件下,发生爆炸的火焰传播速度峰值为7.4 m/s,约为高纯度铝粉的57%,铝合金抛光废弃物粉尘的爆炸敏感度及猛烈度均低于高纯度铝粉;当超细CaCO3粉体的惰化比为30%时,可将铝合金抛光废弃物粉尘的最小点火能量钝化至约1 J,爆炸火焰失去持续传播能力,惰化作用效果充分显现。     

Dust particle sedimentation in the 20 L standard vessel for dust explosion tests

According to the current international standards, to perform the correct evaluation of the explosion and flammability parameters, a uniform distribution of the dust particles should be achieved inside the 20 L and/or 1 m³ standard vessels.CFD simulations have shown that in both standard test vessels (20 L and 1 m³), the dust particles are not uniformly dispersed, being mostly concentrated at the edge of the macro-vortices generated by the injection of the fluid and particle through the nozzle. In addition, only a partial fed of the particles is obtained, and dust particles sedimentation phenomena can occur.As a result, the dust participating to the reactive process may be much lower than the expected nominal concentration in the vessel due to sedimentation and incomplete feeding. Consequently, misleading values of the flammability/explosion parameters could be measured.Particle sedimentation and incomplete feeding depends both on the Stokes number and on the Reynolds number, whereas the concentration distribution depends on the turbulence level, the fluid flow maps, and the number of particles which enter into the vessel through the nozzle.The aim of this work is to evaluate the key parameters (particle size, particle density, and fluid velocity) affecting sedimentation and incomplete feeding in 20 L vessel. To this end, CFD simulations of dust dispersion are performed at varying the particle density and size. Operating maps, in terms of the key parameters and/or their dimensionless combinations, are developed and a correlation for correction of the data is proposed.     

Explosibility of fine graphite and tungsten dusts and their mixtures

To evaluate the explosion hazard of ITER-relevant dusts, a standard method of 20-l-sphere was used to measure the explosion indices of fine graphite and tungsten dusts and their mixtures. The effect of dust particle size was studied on the maximum overpressures, maximum rates of pressure rise, and lower explosive concentrations of graphite dusts in the range 4 μm to 45 μm. The explosion indices of 1 μm tungsten dust and its mixtures with 4 μm graphite dust were measured. The explosibility of these dusts and mixtures were evaluated. The dusts tested were ranked as St1 class. Dust particle size was shown to be very important for explosion properties. The finest graphite dust appeared to have the lowest minimum explosion concentration and be able to explode with 2 kJ ignition energy.     

Experiment-based investigations of magnesium dust explosion characteristics

An experimental investigation was carried out on magnesium dust explosions. Tests of explosion severity, flammability limit and solid inerting were conducted thanks to the Siwek 20 L vessel and influences of dust concentration, particle size, ignition energy, initial pressure and added inertant were taken into account. That magnesium dust is more of an explosion hazard than coal dust is confirmed and quantified by contrastive investigation. The Chinese procedure GB/T 16425 is overly conservative for LEL determination while EN 14034-3 yields realistic LEL data. It is also suggested that 2000-5000 J is the most appropriate ignition energy to use in the LEL determination of magnesium dusts, using the 20 L vessel. It is essential to point out that the overdriving phenomenon usually occurs for carbonaceous and less volatile metal materials is not notable for magnesium dusts. Trends of faster burning velocity and more efficient and adiabatic flame propagation are associated with fuel-rich dust clouds, smaller particles and hyperbaric conditions. Moreover, Inerting effectiveness of CaCO₃ appears to be higher than KCl values on thermodynamics, whereas KCl represents higher effectiveness upon kinetics. Finer inertant shows better inerting effectiveness.     

Suppression effects of powder suppressants on the explosions of oxyhydrogen gas

Suppression tests of oxyhydrogen gas explosions were performed in an explosion tube with five types of dry powder used as the suppressants. The experimental results showed that the powder with large dust cloud density and small radius has better suppression effect, which agrees well with previous correlative results. Moreover, our results also showed that particles with chemical activity and light material density, their suppression effect are more prominent than that of the inert particles with heavy density. To discover the detailed suppression process of dust powder, governing equations were developed based on the homogeneous reactive two-phase flow. The TVD scheme and the Lax–Wendroff–Rubin scheme were adopted to solve the reactive gas phase and particle phase, respectively. The time splitting technique was employed to handle the stiffness of the coupled equations. Our calculated results showed that the dust cloud has the suppression effect on the explosion of oxyhydrogen gas, and with the increase of dust cloud density or the decrease of particle diameter, its suppression effect become more evident, which is in good agreement with our experimental results, in addition, the numerical results showed that with the same particle diameter, the suppression performance is enhanced with the reduction in particle material density.     

Overpressure characteristics of aluminium dust explosion vented through a relief pipe

A parametric experimental study of an aluminium dust explosion, initiated in a vessel and vented through a relief pipe, was performed. The aim is to clarify the overpressure characteristics in a vessel and relief pipe, during aluminium dust explosion venting, especially when a burn-up phenomenon occurs. For a vessel of fixed size, the influence of pipe diameter and pipe length on burn-up was discussed. Results demonstrate that burn-up occurs shortly after flame only enters the initial part of the relief pipe when the original dust concentration in the vessel is at relatively high level, which is usually higher than the optimum concentration obtained from the confined vessel. When burn-up occurs, the maximal overpressure continues to increase rather than to decay along the initial part of the relief pipe. If burn-up is vigorous, a second peak on overpressure-time curve in the vessel could appear. By adding 0.1 g aluminium powders on the membrane, the second overpressure peak may even surpass the first peak. Extending pipe lengths can strengthen the overpressures around the position where burn-up occurs in the relief pipe. Reducing the pipe diameter can increase the burn-up severity in the relief pipe owing to the increased dust concentration and the pressure accumulation.     

Preparation and performance of novel APP/NaY–Fe suppressant for coal dust explosion

Coal dust explosion occurs easily in the coal chemical industry. To ensure safety in industrial production, NaY zeolite was used as carrier modified with Fe ions and combined with ammonium polyphosphate (APP) to prepare a novel composite suppressant for coal dust explosion. The explosion suppression performance of novel APP/NaY–Fe suppressant was investigated by flame propagation inhibition experiments. The results show that Fe ion modification can effectively improve the explosion suppression performance. By increasing content, the explosion suppression performance of the explosion suppressant increases. The maximum explosion pressure P_max of coal dust drops to 0.13 MPa when 50 wt% explosion suppressants were added, and the coal dust explosion cannot continue to expand. Complete suppression of explosion could be achieved by adding 66 wt% explosion suppressants. Combined with XRD, SEM and TG results, the explosion suppression mechanism was proposed. The novel explosion suppressant has high thermal stability, good dispersity and its explosion suppression components distribute uniformly. It shows good explosion suppression performance by the synergistic effect among explosion-suppression components.     

Effect of flame propagation regime on pressure evolution of nano and micron PMMA dust explosions

Experiments using an open space dust explosion apparatus and a standard 20 L explosion apparatus on nano and micron polymethyl methacrylate dust explosions were conducted to reveal the differences in flame and pressure evolutions. Then the effect of combustion and flame propagation regimes on the explosion overpressure characteristics was discussed. The results showed that the flame propagation behavior, flame temperature distribution and ion current distribution all demonstrated the different flame structures for nano and micron dust explosions. The combustion and flame propagation of 100 nm and 30 μm PMMA dust clouds were mainly controlled by the heat transfer efficiency between the particles and external heat sources. Compared with the cluster diffusion dominant combustion of 30 μm dust flame, the premixed-gas dominant combustion of 100 nm dust flame determined a quicker pyrolysis and combustion reaction rate, a faster flame propagation velocity, a stronger combustion reaction intensity, a quicker heat release rate and a higher amount of released reaction heat, which resulted in an earlier pressure rise, a larger maximum overpressure and a higher explosion hazard class. The complex combustion and propagation regime of agglomerated particles strongly influenced the nano flame propagation and explosion pressure evolution characteristics, and limited the maximum overpressure.     

Ignitability assessment of shredder dusts of refrigerator and the prevention of the dust explosion

Explosibility of polyurethane dusts produced in the recycling process of refrigerator and the ways to prevent the dust explosion were studied. In recent years, cyclopentane is often used as the foaming agent and this produces explosive atmosphere in the shredding process. The minimum explosive concentration of polyurethane dust, influence of coexisting cyclopentane gas on the explosibility, effect of relative humidity on the minimum explosive concentration of polyurethane dusts, the minimum ignition energy, influence of cyclopentane mixture on the explosion severity, etc. were investigated.The minimum explosive dust concentration decreased with the increase of cyclopentane concentration and increased with the increase of relative humidity. The minimum ignition energy was about 11 mJ. The ignition energy decreased with the increase of the cyclopentane gas concentration. The cyclopentane gas concentration up to about 5300 ppm did not influence too much on the explosion index (K_st) and maximum explosion pressure. From these, it would be a good way to increase the relative humidity and to regulate the cyclopentane concentration in the shredding process to prevent the dust explosion hazard.     

Quantifying the effect of strong ignition sources on particle preconditioning and distribution in the 20-L chamber

Computational fluid dynamics is used to investigate the preconditioning aspect of overdriving in dust explosion testing. The results show that preconditioning alters both the particle temperature and distribution prior to flame propagation in the 20-L chamber. A parametric study gives the fluid pressure and temperature, and particle temperature and concentration at an assumed flame kernel development time (10 ms) for varying ignitor size and particle diameter. For the 10 kJ ignitor with 50% efficiency, polyethylene particles under 50 μm reach 400 K and may melt prior to flame propagation. Gases from the ignitor detonation displace the dust from the center of the chamber and may increase local particle concentration up to two times the nominal value being tested. These effects have important implications for explosive testing of dusts in the 20-L chamber and comparing to larger 1-m³ testing, where these effects may be negligible.     

Using soybean isoflavone to prevent hydrogen production reaction of aluminium dust and water in wet dust removal systems

Using a dry dust removal system used for aluminium dust collection presents a dust explosion risk, whereas a wet dust removal system presents a risk of hydrogen fire and explosion. Neither system can attain a sufficient level of safety for use at aluminium processing sites. In this paper, soybean isoflavone, a non-toxic and environmentally sustainable flavonoid, was investigated to inhibit hydrogen production from aluminium dust and water. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR) were used to characterize aluminium particles before and after the reaction. Soybean isoflavone was found to inhibit hydrogen production from aluminium dust and water. At a soybean isoflavone solution concentration of 2.1 g L⁻¹, a dense protective film resulting from chemical adsorption on the surfaces of the aluminium particles isolated the aluminium particles from water molecules. This film blocked the reaction pathway between the aluminium particles and water to suppress hydrogen generation. This fundamental study addresses the problems of hydrogen fires and explosions in wet dust removal systems for aluminium dust collection and provides a novel, safe and effective method for aluminium dust removal.     

Effect of particle morphology on metal dust deflagration sensitivity and severity

Combustible dust explosions continue to present a significant threat toward industries processing, storing, or pneumatically conveying metal dust hazards. Through recent years, investigations have observed the influence of particle size, polydispersity, and chemical composition on dust explosion sensitivity and severity. However, studies characterizing the effect of particle shape (or morphology) on metal dust explosibility are limited and merit further consideration. In this work, high-purity aluminum dust samples of three unique particle morphologies were examined (spherical granular, irregular granular, and dry flake). To maintain consistency in results obtained, all samples were procured with similar particle size distribution and polydispersity, as verified by laser diffraction particle size analysis. Scanning electron microscopy (SEM) imaging and Brunauer-Emmett-Teller (BET) experiments were executed to confirm supplier claims on morphology and to quantify the effective surface area associated with each sample, respectively. Investigations performed in a Kühner MIKE3 minimum ignition energy apparatus and a Siwek 20 L sphere combustion chamber resulted in the direct characterization of explosion sensitivity and severity, respectively, as a function of suspended fuel concentration and variable particle morphology. Recommendations to standard risk/hazard analysis procedures and to existing design guidance for the mitigation of deflagrations that originate from ignition of distinctively processed metal dust fuels have been provided.     

Study of the influence of melamine polyphosphate and aluminum hydroxide on the flame propagation and explosion overpressure of aluminum magnesium alloy dust

When aluminum magnesium alloy dust floats in the air, a certain ignition energy can easily cause an accidental explosion. To prevent and control the occurrence of accidental explosions and reduce the severity of accidents, it is necessary to carry out research on the explosion suppression of aluminum magnesium alloy dust. This paper uses a vertical glass tube experimental device and a 20 L spherical explosive experimental device to carry out experimental studies on the suppression of the flame propagation and explosion overpressure of aluminum magnesium alloy dust with melamine polyphosphate (MPP) and Al(OH)₃. With increasing MPP and Al(OH)₃ concentrations, the flame brightness darkened, the flame velocity and propagation distance gradually decreased, and P_max and (dp/dt)_max decreased significantly. When the amount of MPP added reached 60%, the flame propagation distance decreased to 188 mm, which is a decrease of 68%, and the explosion overpressure decreased to 0.014 MPa, effectively suppressing the explosion of aluminum magnesium alloy dust. The experimental results showed that MPP was more effective than Al(OH)₃ in inhibiting the flame propagation and explosion overpressure of the aluminum magnesium alloy dust. Finally, the inhibitory mechanisms of the MPP and Al(OH)₃ were further investigated. The MPP and Al(OH)₃ endothermic decomposition produced an inert gas, diluted the oxygen concentration and trapped active radicals to terminate the combustion chain reaction.