Magnesium Dust Research Articles

Study on the variation regularities of minimum explosion concentration of moist magnesium dust/derived hydrogen hybrids under multiple working conditions

Magnesium is widely used but often accompanied by frequent magnesium powder explosion accidents. If the variation regularity and mechanism of the minimum explosion concentration (cm) of magnesium powder under the multi-factor coupling effect can be understood, the occurrence of magnesium powder explosion accidents can be fundamentally avoided, and intrinsic safety can be achieved. In this paper, experiments were conducted using a self-made 20 L explosion device to explore the effects of particle size, initial temperature, and equilibrium relative humidity (ERH) coupling on the cm of magnesium powder. The study found that an increase in ERH of moist magnesium powder would lead to easier agglomeration of the dust, making it less prone to activation. Additionally, excessive moisture also absorbed more heat, reducing the ignition sensitivity of the dust cloud and resulting in a continuous increase in cm. However, increasing the initial temperature and decreasing the dust particle size both decreased the cm of moist magnesium powder, weakening the effect of ERH on cm. Furthermore, using the analysis methods of orthogonal experimental range and AHP model, it was determined that the influence weights of the factors coupling effects on the cm of magnesium powder, from highest to lowest weight, were: particle size > ERH > initial temperature.Furthermore, in order to deeper understand the mechanism of the action process that affect the cm of moist magnesium powder, additional experiments were conducted to further confirm that the addition of hydrogen derived from the erosion reaction of moist magnesium powder to the explosion system can reduce the cm of magnesium dust clouds. Moreover, a quantitative mathematical model for predicting the cm of magnesium powder/hydrogen two-phase system was also established.

Study on the characteristics parameters of magnesium dust explosion suppression by various inert gases

Alongside the extensive use of magnesium, dust explosion accidents have become increasingly frequent. Conducting research on suppressing magnesium dust explosions is of great significance for the healthy development of the magnesium industry. In this paper, the effects of four inert gases (CO2, N2, Ar, and He) on the explosion characteristics parameters of magnesium dust clouds with different concentrations were investigated using a closed explosion device. It was observed that at low volume fractions of inert gases in the explosion system, there were no significant differences in their effects on the explosion characteristics parameters and the combustion time of magnesium particles. In situations where there was a high level of oxygen, N2 was more effective than Ar in suppressing explosions. However, when the concentration of magnesium dust cloud was higher and the oxygen content was lower within the hybrids, N2 reacted with magnesium dust that was unable to react with oxygen. As a result, the explosion suppression effect gradually shifted from N2 being better than Ar to Ar being better than N2. Moreover, the differences in the effects of four inert gases on particle combustion time became increasingly prominent. However, in all operating conditions, CO2 demonstrated the most effective suppression of magnesium dust explosions, while He exhibited the least effective suppression effect. Furthermore, by employing a chemical reaction kinetics calculation model, it has been discovered that the content of inert gases significantly affected the combustion time and reaction mode of particle combustion, thereby causing variations in the propagation behavior of magnesium dust cloud flames.

Mechanism of influence of water mist in a closed vessel on the explosion overpressure and flame propagation characteristics of magnesium dust

The primary purpose of this paper is to deeply investigate the impact of water mist on the explosive characteristics of magnesium dust and to broaden the inerting and suppression strategies for magnesium dust explosions. The explosion characteristics and flame propagation behaviors of magnesium dust/water mist mixtures were examined in a 16.2-L visualized closed vessel. The explosion products were further analyzed using SEM and XRD. The results indicate that with the addition of water mist, the explosivity of low-concentration magnesium dust (≤600 g/m3) increases. The explosive strength of three higher concentrations of magnesium dust (800 g/m3, 1200 g/m3, 1600 g/m3) notably decreases, with the Pmax dropping by 69.86%, 54.08%, and 46.22% respectively compared to pure magnesium dust. The leading edge of the preheat zone exhibits red radiation, the fluctuation range of the instantaneous flame propagation speed decreases, and cubic particles appear in the explosion products. These changes are mainly attributed to the hydrogen produced in the explosion of the magnesium dust/water mist mixture. Under the combined effects of hydrogen and steam, the preheat zone significantly expands. Moreover, as the concentration of hydrogen increases, its presence weakens the suppressing effect of the water mist, manifested by the evident slowing of the average flame speed, the maximum flame speed, and the rate of decrease in explosive strength. The suppression of magnesium dust explosions by water mist mainly relies on physical actions such as heat absorption, blocking thermal radiation, and diluting oxidizer concentration.

Effect of equilibrium relative humidity on explosion characteristics parameters of humid magnesium dust

In a humid environment, active metal magnesium is susceptible to the hydrogen evolution reaction with water, which significantly increases the risk of explosions in areas where magnesium is produced and used. In this paper, the effects of equilibrium relative humidity (ERH) on the explosion characteristics parameters of magnesium dust under various initial conditions (concentration, particle size and temperature) were studied using a 20 L explosion device. Additionally, the explosion mechanism was thoroughly analyzed by incorporating the oxygen content results of the explosion products. The research revealed that explosion characteristics parameters (Pmax, (dp/dt)max) of magnesium dust cloud initially increased and then decreased with rising ERH when the explosion system was in an “oxygen-rich” state. The maximum increments in Pmax and (dp/dt)max are 130 kPa and 10.73 kPa/ms, respectively. Moreover, a decrease in particle size and an increase in temperature facilitated the generation of more hydrogen from the humid magnesium dust. This phenomenon effectively improved the explosion characteristics parameters of the humid magnesium dust, thereby increasing the sensitivity and risk of the explosion system. However, in the case of a high degree of “oxygen-poor” state, the derived hydrogen instead became burdensome, resulting in an excessive amount of magnesium dust that was unavailable for reaction. This led to a decrease in the explosion characteristics parameters of humid magnesium dust, with the maximum decreases in Pmax and (dp/dt)max being 45.8 kPa and 2.1 kPa/ms respectively.

Comparative analysis of the explosion characteristics of magnesium dust and magnesium-aluminium alloy dust with two concentrations

ABSTRACT Magnesium (Mg) dust and magnesium–aluminum (Mg–Al) alloy dust are commonly produced and form a dust cloud in industrial manufacturing processes, which considerably increases explosion risk. This study examined three metal dusts: Mg, 48 mass% Mg–Al alloy, and 91 mass% Mg–Al alloy. A 20-L spherical explosion device was used to investigate the effect of different Mg concentrations. The results revealed that for all the samples, the explosion pressure (P ex) first increased and then decreased for a dust cloud concentration of 100–1000 g/m3. With the increase of dust particle size, the P ex of samples decreased. With the increase of ignition delay time, the P ex of samples first increased and then decreased, reaching the peak at 60 ms. Under identical conditions, when Mg concentration increased, the alloys became less stable. Thermogravimetry-differential scanning calorimetry findings revealed that the oxidation process of samples could be divided into three stages. The initial oxidation temperature of the alloys was lower than that of the pure Mg; however, in its primary oxidation stage, the mass gain rate of the Mg dust was more rapid than that of the Mg–Al alloys. In the oxidation of the alloys, elemental Mg reacted with oxygen first.

A numerical model for the prediction of the minimum ignition temperature of dust clouds

This paper presents a numerical model for the prediction of the minimum ignition temperature (MIT) of dust clouds. First, a physical model is developed for the dust cloud ignition in the Godbert-Greenwald furnace. A numerical approach is then applied for the MIT prediction based on the physical model. The model considers heat transfer between the air and dust particles, the dust particle reaction kinetics, and the residence times of dust clouds in the furnace. In general, for the 13 dusts studied, the calculated MIT data are in agreement with the experimental values. There is also great accordance between the experimental and numerical MIT variation trends against particle size. Two different ignition modes are discovered. The first one consists in ignition near the furnace wall for bigger particles characterized by rather short residence times. In the second mode, the ignition starts from the center of the furnace by self-heating of the dust cloud for smaller particles with longer residence times. For magnesium, as dust concentration increases, the lowest ignition temperature of the dust cloud IT(conc) decreases first, then transits to increase at a certain point. The transition happens at different dust concentrations for different particle sizes. Moreover, the MIT of the magnesium dust cloud generally increases as particle size increases, but the increasing trend stagnates within a certain medium particle size range.

Explosion overpressure behavior and flame propagation characteristics in hybrid explosions of hydrogen and magnesium dust

The explosion risk of a hybrid mixture of hydrogen and magnesium dust is relatively high during the production of magnesium-based hydrogen storage materials and industrial magnesium products. To explore the explosion characteristics of a hybrid hydrogen/magnesium dust mixture, the overpressure variation and flame propagation characteristics of the hybrid hydrogen/magnesium dust mixture in a 5-L semi-closed explosion duct were experimentally studied. The results show that the explosion pressure (Pex) and the explosion pressure rise rate ((dP/dt)ex) of the hybrid hydrogen/magnesium dust mixture significantly increase only when the hydrogen concentration is greater than 10 %. The flame of a pure magnesium dust explosion mainly consists of the unburned zone, preheating zone, and combustion reaction zone. Compared with pure magnesium dust, the flame structure of the hybrid hydrogen/magnesium dust mixture explosion does not change significantly, but a red emission zone appears at the front of the preheating zone. Moreover, the range of the preheating zone is significantly larger compared to pure magnesium dust, and it increases with the hydrogen concentration. When the hydrogen concentration is low, the flame propagation speed of the hybrid hydrogen/magnesium dust mixture exhibits strong pulsation. The pulsation frequency of the flame propagation speed of the hybrid mixture explosion gradually decreases with the hydrogen concentration, but the average flame propagation speed continually increases. Based on the experimental results, the explosion mechanism of the hybrid hydrogen/magnesium dust mixture was analyzed from the perspective of reaction kinetics by numerical simulation.

Flame propagation and temperature distribution characteristics of magnesium dust clouds in an open space

In order to explore the flame propagation behaviors and temperature characteristics of magnesium dust clouds, the measurement system and the two-color pyrometer technique were used to study the spatial-temporal distribution of flame temperature fields and flame propagation behaviors with different particle sizes. The experiment results showed that the flame propagation characteristics largely depended on the particle size and the flame propagation velocity was higher with decreasing particle sizes. The flame temperatures of the leading edges were kept stable between 3100 and 3300 K, and the average flame temperature was related to the dust concentrations and particle sizes. The average flame temperature of −400 mesh magnesium with 830 g/m3 dust concentration was the highest, and the descent speed of the average flame temperature had a negative correlation with the particle size. The two-color pyrometer technique could evaluate the severity of magnesium dust clouds explosion, which provides a theoretical basis for the prevention.

Numerical Simulation of Magnesium Dust Dispersion and Explosion in 20 L Apparatus via an Euler–Lagrange Method

Computational fluid dynamics (CFD) was used to investigate the explosion characteristics of a Mg/air mixture in a 20 L apparatus via an Euler–Lagrange method. Various fluid properties, namely pressure field, velocity field, turbulence intensity, and the degree of particle dispersion, were obtained and analyzed. The simulation results suggested that the best delayed ignition time was 60 ms after dust dispersion, which was consistent with the optimum delayed ignition time adopted by experimental apparatus. These results indicate that the simulated Mg particles were evenly diffused in the 20 L apparatus under the effect of the turbulence. The simulations also reveal that the pressure development in the explosion system can be divided into the pressure rising stage, the maximum pressure stage, and pressure attenuation stage. The relative error of the maximum explosion pressure between the simulation and the experiments is approximately 1.04%. The explosion model provides reliable and useful information for investigating Mg explosions.

Hydrogen production for fuel cell application via thermo-chemical technique: An analytical evaluation

Fuel cells are considered as the promising candidates for next-generation energy systems for which supplying the required hydrogen is the most important issue. Because of resource scarcity and environmental pollution associated with hydrocarbon reforming, magnesium-water combustion can be applied as an eco-friendly alternative method for hydrogen generation. This study aims to investigate magnesium combustion as a heat and hydrogen source. By proposing an asymptotic model of flame structure, the combustion of single magnesium particle is examined in which the model consists of internal and external zones and a thin reaction layer. Then, the governing equations including mass, energy, and mass fractions of species and their appropriate boundary conditions are derived and solved analytically. Afterwards, with the aid of Taylor approximation, the explicit formulas are extracted for flame location and temperature and magnesium evaporation rate. Subsequently, the effects of various parameters on combustion characteristics and hydrogen production are scrutinized. Eventually, by employing the examined model for single particle, the discrete method is applied to analyze the magnesium dust cloud combustion. The results indicate that the hydrogen production rate is higher than 10-3kg/s for the condition of d=10μm,P=1atm,Le=1 with dust concentration greater than 250mg/liter.

Mechanical Properties and Machinability of Magnesium Alloy AZ31 and AZ91 – A Comparative Review

Throughout the last decade magnesium alloys have slowly overtaken aluminium alloys, especially in the automotive and aerospace industry. Due to its attractive weight-resistance and weight-hardness characteristics, the global production of Magnesium alloys has grown from 360,000 tons in 1999 to 850,000 tons in 2013. Magnesium alloys AZ91 and AZ31 are the most widely used grades. AZ 91 exhibits a Knoop’s hardness (HK) of 76.2 that is higher to the AZ31 with a 51.1 HK. The manufacturing of AZ 91 is through a casting process whereas AZ31 is manufactured by using an extrusion process. The microstructure of AZ31 and AZ91 are highly influenced by the heat generated during the machining process that also affects the surface hardness due to the high thermal conductivity of the material that range from 62 W/m.k to 96W/m.k. Both materials have a very good machinability where milling, drilling, and turning are commonly used. However, extreme care should be taken in terms of machining conditions as Magnesium alloy is highly flammable. However, a safe machining process can be performed by avoiding very fine chips, magnesium dust and also the usage of water-based coolants that react with the raw material. This paper has widely reviewed the studies that have been conducted since the evolution of magnesium alloys in terms of machining parameters, machining consumables and their effect on the mechanical properties, surface conditions and limitations of the various machining processes.

Influential factors on the explosibility of unpremixed hydrogen/magnesium dust

The explosibility of unpremixed hydrogen/magnesium dust in a confined space was simulated and studied using a cubic explosion chamber. Typical influential factors, including hydrogen content in air, depth of magnesium layer, particle size distribution, ambient humidity, initial gas pressure and ignition position, on the explosion pressures of unpremixed hydrogen/magnesium dust in the first and secondary explosions were investigated. Experimental results showed that the biggest peak pressure values in the first and secondary explosions of the unpremixed hydrogen/magnesium dust were both obtained when the hydrogen content was 50%, and nitrogen participated the combustion of magnesium dust in the dust explosion. The effects of magnesium layer depth on the change trend of peak pressure in the first explosion was different from that in the secondary explosion for the unpremixed hydrogen/magnesium dust with a lean hydrogen concentration, and mean particle size of magnesium dust had a significant effect on the explosion pressure. Peak pressures in the first and secondary explosions both presented a trend of increasing at the early stage and then decreased with the increasing humidity or gas pressure, and the combustion mode of the unpremixed hydrogen/magnesium dust varied as the ignition position was changed.

A numerical study on discrete combustion of polydisperse magnesium aero-suspensions

In this numerical study, discrete combustion of polydisperse magnesium dust clouds was investigated. A numerical model accounting for the effects of ignition, thermal conduction, and radiation was formulated to simulate the spatiotemporal distribution of temperature. Three distribution models, i.e., Dagum, log-normal, and Beta prime, were used to describe the magnesium particle-size polydispersity. The numerical model was first validated by comparison against experimental data on discrete combustion of both mono-sized and polydisperse magnesium aero-suspensions. Subsequently, the flame propagation characteristics of mono-sized and log-normally polydisperse cases at two different mean magnesium particle sizes were compared. The comparison shows that polydisperse magnesium dust clouds have higher flame propagation speeds than their mono-sized counterparts. Finally, the differences among the polydisperse cases with different size distributions were compared, revealing that magnesium powders with a higher percentage of small particles give rise to higher flame propagation speeds. Furthermore, results show that in comparison with the Dagum and Beta prime distributions, the log-normal distribution results in a lower flame propagation speed and a higher minimum ignition energy. As either the particle size decreases or the dust-cloud concentration increases, the flame propagation speed increases and the minimum ignition energy decreases.

Predicting Graphite/Metals Explosion Severity: from Experimentation to Modelling

During decommissioning of UNGG (Uranium Natural Graphite Gas) nuclear reactors and wastes reconditioning operations, mixtures of graphite and metal dusts can be encountered. In this perspective, an extensive experimentation has been realized on graphite/magnesium and graphite/iron mixtures to determine their ignition sensitivity and explosivity. The influence of parameters such as the metal content, the ignition energy, the storage humidity, the particle size distribution and the initial turbulence has been considered. It appears that the introduction of such metals can strongly changes the behavior of graphite powders: the maximum rate of pressure rise, for example, increases of roughly 355 % comparing to that of simple graphite when adding 30 % wt. of magnesium dust. By means of a vertical explosion tube, an estimation of the laminar burning velocity has been deduced from the apparent flame velocity through the image processing of explosion videos. This experimental study has provided a solid background to better understand these explosion phenomena, to characterize the mixtures behavior by semi-empirical correlations and also to develop and validate a model predicting the severity of a dust cloud in a confined space. Initially conceived for graphite dusts, this model aspires to provide a description of the flame front propagation, leading to the prediction of the explosivity parameters. The dust cloud combustion has been regarded as the combustion of isolated graphite particles, taking into consideration the oxidation kinetic mechanisms and its rate-limiting steps. A finite volume method has been used for the resolution of the transient conservation equations applied to a one–dimension scheme.

Cut quality characteristics of Al 6061-T6 composites using abrasive water jet machining

This work investigates the cut quality characteristics of composite consisting of Al6061-T6 with magnesium AZ 31 and rock dust as reinforcement material using abrasive water jet machining. To analyse the characteristics of this new composite three different compositions are taken with different proportions of magnesium and rock dust. The composites are produced in the stir casting process, in which aluminium and magnesium are taken as solid metal and rock dust as taken as fine (30 μm) powdered form. The molten metal is converted into a solid workpiece of 100 × 100 × 10 mm. Among various non-traditional machining abrasive water jet machine has minimum thermal deformation. Hence, in this abrasive water jet machine is used to find the cut quality of the new composites. The machining is carried out on Water Jet Germany S 3015 abrasive water jet machine. The experiments for each sample are performed based on Taguchi's L9 orthogonal array. The input parameters are abrasive flow rate, feed rate and standoff distance and output response are material removal rate, surface roughness and kerf deviation. The models developed from ANOVA are significant for the entire three test specimen.

Minimum ignition energies of pure magnesium powders due to electrostatic discharges and nitrogen's effect

This paper experimentally investigated the relation between the minimum ignition energy (MIE) of magnesium powders as well as the effect of inert nitrogen (N2) on the MIE. The modified Hartmann vertical-tube apparatus and four kinds of different-sized pure magnesium powders (median particle size, D50; 28.1 μm–89.8 μm) were used in this study. The MIE of the most sensitive magnesium powder was 4 mJ, which was affected by the powder particle size (D50; 28.1 μm). The MIE of magnesium powder increased with an increase in the N2 concentration for the inerting technique. The magnesium dust explosion with an electrostatic discharge of 1000 mJ was suppressed completely at an N2 concentration range of more than 98%. The experimental data presented in this paper will be useful for preventing magnesium dust explosions generated from electrostatic discharges.

Modelling of combustion of a magnesium dust cloud in heterogeneous media

Combustion of micron-sized magnesium dust particles with spatially discrete sources is studied numerically. The flame propagation speed for various dust concentrations and different particle diameters is estimated. Experimental results for two different burning times are compared. The minimum ignition energy as a function of dust concentration for different particle sizes is studied. The results show good agreement with experimental data.

Comparative study of explosion processes controlled by homogeneous and heterogeneous combustion mechanisms

The dust explosion behaviors induced by two different combustion mechanisms (homogeneous and heterogeneous mechanisms) were comparatively investigated, based on the experiments under different dust concentrations, particle sizes and initial pressures in Siwek 20-L chamber. Based on the thermo-gravimetric analysis (TGA), sweet potato dust and magnesium dust were selected as the representative dusts with homogeneous and heterogeneous combustion mechanisms, respectively. Experiments find that these two dusts have different behaviors in the explosion kinetics due to different combustion mechanisms. For sweet potato dust, the explosion pressure pmax, the pressure rise rate (dp/dt)max and the combustion fraction η exhibit similar variation trends as dust concentration increases and they all reach to the maximum values at the worst-case concentration; while for magnesium dust, the variation of (dp/dt)max is somewhat different from that of pmax, that is, the (dp/dt)max will achieve the maximum at the concentration higher than the worst-case and keep stabilized with further increase of dust concentration. As the particle size decreases, the (dp/dt)max for sweet potato dust will increasingly rise and gradually approach to a stabilized value, but for magnesium dust, the increase of (dp/dt)max becomes pronounced only in the range of smaller particle sizes. To account the effect of initial pressure on pmax under different combustion mechanisms, a dimensionless pressure PR was introduced to denote the relative intensity of explosion. It is found that, for sweet potato dust, the increased initial pressure will promote the explosion process (or with high PR) for the dust cloud with high concentration due to the augmented oxygen concentration, but for the dust cloud with low concentration, the increased initial pressure will suppress the explosion process due to the increased resistance in devolatilization. For magnesium dust, the rise of initial pressure will generally promote the explosion process even for the dust cloud with low concentration; however, in the case of small particle size, the promotion of increased initial pressure to the explosion process is not so pronounced.

마그네슘의 폭발특성에 미치는 평균입경의 영향

본 연구에서는 마그네슘(Mg)분진의 폭발특성에 미치는 평균 입경의 영향을 조사하였다. 이를 위해 20-L분진 폭발시험장치와 10 kJ의 에너지를 갖는 화학 점화기를 사용하여 실험을 수행하였다. 실험에 사용한 Mg분진은 평균입경이 서로 다른 3가지 시료(38, 142, <TEX>$567{\mu}m$</TEX>)를 대상으로 분진농도를 <TEX>$2250g/m^3$</TEX>까지 조사하였다. Mg분진의 폭발하한농도(LEL)는 평균입경 38, <TEX>$142{\mu}m$</TEX>에 있어서 각각 30, <TEX>$40g/m^3$</TEX> 이 얻어졌다. LEL은 입경 증가에 의해 감소 경향을 나타냈는데 Mg분진은 입경이 증가할수록 폭발 확률이 감소함을 의미한다. 최대폭발압력(<TEX>$P_m$</TEX>)과 폭발지수(<TEX>$K_{st}$</TEX>)는 입경이 증가하면 감소하였지만, 평균입경 <TEX>$567{\mu}m$</TEX>의 Mg분진의 경우에는 5 kJ의 착화에너지에서는 폭발 현상이 관찰되지 않았다. A study was carried out on the effect of particle size (mean diameter) on magnesium dust explosion. Experimental investigations were conducted in a 20-L explosion sphere, using 10 kJ chemical ignitors. Explosion tests were performed with three different dusts having mean diameter (38, 142, <TEX>$567{\mu}m$</TEX>) and the dust concentrations were up to <TEX>$2250g/m^3$</TEX>. The lower explosion limits(LEL) of magnesium dusts were about <TEX>$30g/m^3$</TEX> at <TEX>$38{\mu}m$</TEX> and <TEX>$40g/m^3$</TEX> at <TEX>$142{\mu}m$</TEX>. LEL tended to increase with particle size and this means that the explosion probability of magnesium dust decreased with increase of particle size. The maximum explosion presssure (<TEX>$P_m$</TEX>) and <TEX>$K_{st}$</TEX> (Explosion index) decreased with the increase of particle size. For magnesium powder of <TEX>$567{\mu}m$</TEX>, however, the explosive properties were not observed in the 5 kJ ignition energy.

입자크기 변화에 따른 마그네슘 분진의 폭발특성

We conducted experiments to examine the explosion characteristics of different sizes of micro-sized and nano-sized magnesium dust. These explosion characteristics, such as the maximum explosion pressure and maximum rate of pressure rise, depend on the dust concentration. Changes in the particle sizes do not cause clear differences in the explosion characteristics of nano-sized magnesium dust. However, the explosion characteristic values of micro-sized magnesium dust decrease with increasing particle size. Furthermore, nano-sized magnesium dust showed greater explosion characteristics than micro-sized magnesium dust. The calculated flame propagation velocity showed the same trend as the maximum explosion pressure and maximum rate of the pressure rise. The results of classifying the explosion intensity hazards of a magnesium dust explosion showed that the explosion index increased with the decrease of particle size.