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.     

Inhibiting effects by Fe2O3 on combustion and explosion characteristics of ABS resin

The global increase in the use of, and reliance on, plastics has prompted the demand for acrylonitrile-butadiene-styrene (ABS) resin in various fields. With this increased requirement, numerous failures have occurred in the ABS process. Those incidents, resulting from electrostatic discharge, powder accumulation, heat accumulation, construction sparks, and plant fires, have caused dust fire and explosions.In this study, the ABS resin was gleaned from the site and tested for its explosion parameters, including minimum ignition temperature of dust cloud (MIT_C), minimum ignition energy (MIE), and minimum explosion concentration (MEC). To improve loss prevention in the manufacturing process, ferric oxide (Fe₂O₃) as an inert additive was added in the ABS powder. According to the MIE test, Fe₂O₃ has an apparent inhibiting effect on dust explosion for the ABS dust. With the proportion of Fe₂O₃ increased from 25 to 50 mass% in ABS, the MIE increased from 67 to 540 mJ. The explosion tests via 20-L apparatus indicated that Fe₂O₃ mixed with ABS could not increase the MEC significantly. However, the explosion pressure dropped by increasing in the ratio of Fe₂O₃ in ABS. This inerting strategy of ABS was deemed to substantially lessen the probability and severity of fire and explosion.     

Explosion characteristics of micron- and nano-size magnesium powders

Explosion characteristics of micron- and nano-size magnesium powders were determined using CSIR-CBRI 20-L Sphere, Hartmann apparatus and Godbert-Greenwald furnace to study influence of particle size reduction to nano-range on these. The explosion parameters investigated are: maximum explosion pressure (P_max), maximum rate of pressure-rise (dP/dt)_max, dust explosibility index (K_St), minimum explosible concentration (MEC), minimum ignition energy (MIE), minimum ignition temperature (MIT), limiting oxygen concentration (LOC) and effect of reduced oxygen level on explosion severity. Magnesium particle sizes are: 125, 74, 38, 22, 10 and 1 μm; and 400, 200, 150, 100, 50 and 30 nm. Experimental results indicate significant increase in explosion severity (P_max: 7–14 bar, K_St: 98–510 bar·m/s) as particle size decreases from 125 to 1 μm, it is maximum for 400 nm (P_max: 14.6 bar, K_St: 528 bar·m/s) and decreases with further decrease of particle size to nano-range 200–30 nm (P_max: 12.4–9.4 bar, K_St: 460–262 bar·m/s) as it is affected by agglomeration of nano-particles. MEC decreases from 160 to 30 g/m³ on decreasing particle size from 125 to 1 μm, its value is 30 g/m³ for 400 and 200 nm and 20 g/m³ for further decrease in nano-range (150–30 nm). MIE reduces from 120 to 2 mJ on decreasing the particle size from 125 to 1 μm, its value is 1 mJ for 400, 200, 150 nm size and <1 mJ for 50 and 30 nm. Minimum ignition temperature is 600 °C for 125 μm magnesium, it varies between 570 and 450 °C for sizes 38–1 μm and 400–350 °C for size range 400–30 nm. Magnesium powders in nano-range (30–200 nm) explode less violently than micron-range powder. However, likelihood of explosion increases significantly for nano-range magnesium. LOC is 5% for magnesium size range 125–38 μm, 4% for 22–1 μm, 3% for 400 nm, 4% for 200, 150 and 100 nm, and 5% for 50 and 30 nm. Reduction in oxygen levels to 9% results in decrease in P_max and K_St by a factor of 2–3 and 4–5, respectively, for micron as well as nano-sizes. The experimental data presented will be useful for industries producing or handling similar size range micron- and nano-magnesium in order to evaluate explosibility of their magnesium powders and propose/design adequate safety measures.     

Minimum ignition temperatures and explosion characteristics of micron-sized aluminium powder

To forestall, control, and mitigate the detrimental effects of aluminium dust, a 20-L near-spherical dust explosion experimental system and an HY16429 type dust-cloud ignition temperature test device were employed to explore the explosion characteristics of micron-sized aluminium powder under different ignition energies, dust particle sizes, and dust cloud concentration (C_dust) values; the minimum ignition temperature (MIT) values of aluminium powder under different dust particle sizes and C_dust were also examined. Flame images at different times were photographed by a high-speed camera. Results revealed that under similar dust-cloud concentrations and with dust particle size increasing from 42.89 to 141.70 μm, the MIT of aluminium powder increased. Under various C_dust values, the MIT of aluminium dust clouds attained peak value when concentrations enhanced. Furthermore, the increase of ignition energy contributed to the increase of the explosion pressure (P_ex) and the rate of explosion pressure rise [(dP/dt)_ex]. When dust particle size was augmented gradually, the P_ex and (dP/dt)_ex attenuated. Decreasing particle size lowered both the most violent explosion concentration and explosive limits.     

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.     

Experimental study on ignitability of pure aluminum powders due to electrostatic discharges and Nitrogen's effect

In order to prevent dust explosions due to electrostatic discharges (ESD), this paper reports the minimum ignition energy (MIE) of aluminum powders in the air and the effective nitrogen (N₂) concentration for the inert technique. The Hartman vertical-tube apparatus and five kinds of different sized pure aluminum powders (median particle size, D50; 8.53 μm–51.2 μm) were used in this study. The statistic minimum ignition energy (MIE_s) of the most sensitive aluminum powder used in this study was 5 mJ, which was affected by the powder particle size (D50; 8.53 μm). In the case of aluminum powder, the inerting effects of N₂ were quite different from the polymer powders. The MIE of aluminum powder barely changed until the N₂ concentration was 89% in comparison with that of the normal air. When the N₂ concentration was 90%, the MIE of aluminum powders suddenly exceeded 1000 mJ, which does not occur easily with ESD in the industrial process.     

玉米淀粉粉尘爆炸危险性研究

为研究玉米淀粉粉尘爆炸危险性,采用哈特曼管式爆炸测试装置和20 L球爆炸测试装置对200目(<75μm)以下的玉米淀粉粉尘爆炸危险性进行评估,基于静电火花和粉尘质量浓度对粉尘爆炸的影响,对玉米淀粉的静电火花最小点火能量、爆炸下限质量浓度、最大爆炸压力和爆炸指数进行了研究,根据试验结果对玉米淀粉爆炸危险性进行分级。试验结果表明:温度在25℃,喷粉压力为0.80 MPa,粉尘质量浓度在250～750 g/m3范围内,粉尘的最小点火能量随着粉尘质量浓度增加而降低,其最小点火能量在40～80 mJ之间;在点火能量为10 kJ时,粉尘爆炸下限质量浓度在50～60 g/m3之间;在粉尘质量浓度为750 g/m3时,爆炸压力达到最大,为0.66 MPa;在粉尘质量浓度为500 g/m3时,爆炸指数达到最大,为17.21 MPa.m/s,其粉尘爆炸危险性分级为Ⅰ级。     

Inhibiting effect of inhibitors on ignition sensitivity of wood dust

Wood products are easy to produce dust in the production and processing process, and have a serious explosion risk. In order to improve the safety of wood products production, the inhibiting effects of magnesium hydroxide (MTH), SiO₂, melamine polyphosphate (MPP) on the minimum ignition energy (MIE) and minimum ignition temperature (MIT) of wood dust were experimentally studied. The results showed that the inhibiting effects of inhibitors on the MIE of wood dust show the order of MPP > SiO₂>MTH. The order of the inhibiting effects on the MIT of wood dust was MPP > MTH > SiO_2. When 10% MPP was added to wood dust, the time when the flame appears (T_appear) and the time when the flame reaches the top of the glass tube (T_top) obviously rose to 80, 140 ms. Therefore, MPP had the best inhibiting effect on the ignition sensitivity of wood dust.According to thermogravimetry (TG), differential scanning calorimetry (DSC) tests, the introduction of MPP leaded to lower maximum mass loss rate (MMLR), higher temperature corresponding to mass loss of 90% (T_0.1), residual mass and heat absorption. In addition, thermogravimetric analysis/infrared spectrometry (TG-IR) results showed that MPP produced H₂O (g) and NH₃ (g) during the thermal decomposition process, which diluted the oxygen.     

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.     

速生杨木粉尘最小点火能的实验研究

为防止木材加工中木质粉尘燃爆事故的发生，以纤维板生产中常见的原材料速生杨木粉尘作为研究对象，在分析粉尘粒径分布、元素分析、工业分析及形貌特征的基础上，采用1.2 L哈特曼管对3种不同粒径（0~50，＞50~96，＞96~180 μm）速生杨木粉尘进行最小点火能实验，探究点火延迟时间、喷粉压力、质量浓度和粒径分布对速生杨木粉尘最小点火能的影响及变化规律。研究结果表明:在质量浓度为500 g/m3时，分别增加点火延迟时间和喷粉压力，最小点火能都先减小后增大；最佳点火延迟时间和最佳喷粉压力分别为120 ms和120 kPa；粒径对最佳点火延迟时间和最佳喷粉压力无显著影响。在点火延迟和喷粉压力分别为120 ms和120 kPa条件下，最小点火能随质量浓度的增加先减小后增大。粉尘粒径与最小点火能呈正相关性，3种样品的最小点火能分别为1~3，1~3和7~13 mJ，对应的敏感质量浓度分别为500 ，750和1 250 g/m3，属于特别着火敏感性粉尘。     

苯乙烯丙烯酸共聚物/碳黑混合体系粉尘爆炸特性研究

采用MIE-D1.2型最小点火能测试装置及20 L球型粉尘爆炸测试装置,对苯乙烯丙烯酸共聚物/碳黑混合体系粉尘的爆炸特性进行研究。结果表明,过74μm、58μm、47μm孔径筛的粉尘对静电火花敏感,其最小点火能表征值分别为610 mJ、361 mJ、201 mJ。随粉尘质量浓度增加,最小点火能呈现先减小后增加的规律。随粉尘粒径减小,最小点火能与粉尘质量浓度变化关系曲线向低粉尘质量浓度和低点火能量方向偏移,且对应的最敏感爆炸质量浓度从500 g/m~3降至200 g/m~3。随粉尘质量浓度增加,过147μm、74μm、47μm孔径筛的苯乙烯丙烯酸共聚物/碳黑混合体系粉尘爆炸压力及爆炸压力上升速率呈现先增加后减小趋势。在相同粉尘质量浓度下,中位径小于74μm的苯乙烯丙烯酸共聚物/碳黑混合体系粉尘,粉尘的爆炸压力增幅明显减小。苯乙烯丙烯酸共聚物/碳黑混合体系粉尘爆炸下限质量浓度为25 g/m~3,最大爆炸指数为14.636 MPa·m/s,爆炸危险等级划分为St1。     

Applications of dust explosion hazard and disaster prevention technology

Powdered materials are widely used in industrial processes, chemical processing, and nanoscience. Because most flammable powders and chemicals are not pure substances, their flammability and self-heating characteristics cannot be accurately identified using safety data sheets. Therefore, site staff can easily underestimate the risks they pose. Flammable dust accidents are frequent and force industrial process managers to pay attention to the characteristics of flammable powders and create inherently safer designs.This study verified that although the flammable powders used by petrochemical plants have been tested, some powders have different minimum ignition energies (MIEs) before and after drying, whereas some of the powders are released of flammable gases. These hazard characteristics are usually neglected, leading to the neglect of preventive parameters for fires and explosions, such as dust particle size specified by NFPA-654, MIE, the minimum ignition temperature of the dust cloud, the minimum ignition temperature of the dust layer, and limiting oxygen concentration. Unless these parameters are fully integrated into process hazard analysis and process safety management, the risks cannot be fully identified, and the reliability of process hazard analysis cannot be improved to facilitate the development of appropriate countermeasures. Preventing the underestimation of process risk severity due to the fire and explosion parameters of unknown flammable dusts and overestimation of existing safety measures is crucial for effective accident prevention.     

Does the dust explosion risk increase when moving from μm-particle powders to powders of nm-particles?

Based on experience with powders of particle sizes down to the 1–0.1 μm range one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high K_St-values). However, there are two basic physical reasons why this may not be the case. Firstly, complete transformation of bulk powders consisting of nm-particles into dust clouds consisting of well-dispersed primary particles is extremely difficult to accomplish, due to very strong inter-particle cohesion forces. Secondly, should perfect dispersion nevertheless be achieved, the extremely fast coagulation process in clouds of explosive mass concentrations would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal the basic mechanism of flame propagation in dust clouds suggests that increased cloud explosion rates would not be expected as the particle size decreases into the <1 μm range. An overall conclusion is that dust clouds consisting of nm primary particles are not expected to exhibit more severe K_St-values than clouds of μm primary particles, in agreement with recent experimental evidence. In the case of the ignition sensitivity recently published evidence indicates that MIEs of clouds in air of some metal powders are significantly lower for nm particles than for μm particles. A possible reason for this is indicated in the paper.     

Influences of coal dust components on the explosibility of hybrid mixtures of methane and coal dust

In order to study the influences of coal dust components on the explosibility of hybrid mixture of methane and coal dust, four kinds of coal dust with different components were selected in this study. Using the standard 20 L sphere, the maximum explosion pressure, explosion index and lower explosion limits of methane/coal dust mixtures were measured. The results show that the addition of methane to different kinds of coal dust can all clearly increase their maximum explosion pressure and explosion index and decrease their minimum explosion concentration. However, the increase in the maximum explosion pressure and explosion index is more significant for coal dust with lower volatile content, while the decrease in the minimum explosion concentration is more significant for coal dust with higher volatile content. It is concluded that the influence of methane on the explosion severity is more pronounced for coal dust with lower volatile content, but on ignition sensitivity it is more pronounced for coal dust with higher volatile content. Bartknecht model for predicting the lower explosion limits of methane/coal dust mixture has better applicability than Le Chatelier model and Jiang model. Especially, it is more suitable for hybrid mixtures of methane and high volatile coal dust.     

Risk assessment method of polyethylene dust explosion based on explosion parameters

The explosion characteristic parameters of polyethylene dust were systematically investigated. The variations in the maximum explosion pressure (P_max), explosion index (K_st), minimum ignition energy (MIE), minimum ignition temperature (MIT), and minimum explosion concentration (MEC) of dust samples with different particle sizes were obtained. Using experimental data, a two-dimensional matrix analysis method was applied to classify the dust explosion severity based on P_max and K_st. Then, a three-dimensional matrix was used to categorize the dust explosion sensitivity based on three factors: MIE, MIT, and MEC. Finally, a two-dimensional matrix model of dust explosion risk assessment was established considering the severity and sensitivity. The model was used to evaluate the explosion risk of polyethylene dust samples with different particle sizes. It was found that the risk level of dust explosion increased with decreasing particle size, which was consistent with the actual results. The risk assessment method can provide a scientific basis for dust explosion prevention in the production of polyethylene.     

A theoretical model for the prediction of the minimum ignition energy of dust clouds

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIE_cylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ d_p^k with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.     

Prediction of minimum ignition energy of aerosols using flame kernel modeling combined with flame front propagation theory

Flammable aerosols have created many fire and explosion hazards in the process industry, but the flammability of aerosols has not been fully understood. The minimum ignition energy has been widely used as an indicator for flammability of combustible mixtures, but the amount of experimental data on the minimum ignition energy of aerosols is very limited. In this work, the minimum ignition energy of tetralin aerosols is predicted using an integrated model. The model applies the flame front propagation theory in aerosol systems to the growth of the flame kernel, which was created during the spark discharge in the ignition process. The aerosol minimum ignition energy was defined as the minimum level of energy in the initial flame kernel to maintain the kernel temperature above the minimum ignition temperature of 1073 K specific for tetralin aerosols during the kernel growth. The minimum ignition energy obtained in the model is influenced by the fuel-air equivalence ratio and the size of the aerosol droplets. For tetralin aerosols of 40 μm diameter, E_min decreases significantly from 0.32 mJ to 4.3 × 10 e⁻³ mJ when the equivalence ratio rises from 0.57 to 1.0. For tetralin aerosols of 0.57 equivalence ratio, E_min increases from as 0.09 mJ to 0.32 mJ when the droplet diameter rises from 10 μm to 60 μm. The trends are in agreement with previous experimental observations. The method used in current work has the potential to prediction of the minimum ignition energy of aerosol.     

三环唑粉尘爆炸特性研究

利用激光粒度仪对三环唑粉尘的粒径分布进行分析,并用20 L爆炸球测试装置、哈特曼管装置探讨了粉尘质量浓度、点火延迟时间、点火能量、粒径分布对粉尘爆炸的影响并总结了相关规律。实验结果表明:粉尘粒度是影响粉尘最小点火能和爆炸下限的单调因素,粉尘质量浓度是影响粉尘爆炸压力的极值因素,点火延迟时间是影响粉尘最小点火能的极值因素。     

混合金属粉尘爆炸最小点火能量影响因素研究*

为探究混合金属粉尘爆炸危险性及与单一粉体爆炸特性差异，确保车间安全生产，采用粉尘云点火能量测试系统对车间混合金属粉尘及铝粉最小点火能量在不同影响因素下的变化规律及2种粉尘火焰变化特征进行测试。研究结果表明:混合金属粉尘和铝粉最小点火能量在一定范围内（38~96 μm）与粒径呈正相关性，当混合金属粉尘粒径大于75 μm时，所需最小点火能量大于1 000 mJ，其爆炸敏感性迅速降低，此时铝粉仍有较强爆炸敏感性；2种粉尘最小点火能量随质量浓度增加呈先降低后升高的趋势，最小点火能分别为295，15 mJ，对应的敏感质量浓度为600，1 000 g/m3，混合金属粉尘在质量浓度为500~700 g/m3时具有较大爆炸危险性；同铝粉相比，混合金属粉尘点火能量更高、火焰燃烧时间更短、火焰高度更低、爆炸剧烈程度更弱。     

Effect of particle size and dust layer size on ignition characteristics of PMMA dust layer on hot surface

Deposition of combustible dust on a hot surface is a hidden danger of fire. In this work, polymethylmethacrylate (PMMA) dust was selected to analyse the influence of dust layer diameter, dust particle size and dust layer thickness on the ignition characteristics of PMMA dust layer. Critical heating temperatures and ignition time had been measured. The STA-GC/MS-FTIR analysis was used to determine that the main products of PMMA pyrolysis were MMA, CO, CO₂, and C₂H₄, of which CO and C₂H₄ were transported to the ambient to cause gas phase combustion on the surface of the dust layer. For 10 mm thick dust layer, the critical heating temperatures of 5 μm PMMA, 100 nm PMMA, and 30 μm PMMA were 300 °C, 330 °C, and 320 °C. As the thickness of the dust layer increased, the gas transport path became longer, the critical heating temperature and ignition time increased. The characteristic particle size (D [3,2]) was utilized to represent the true particle size, and the ignition time increased with the increase of the characteristic particle size. The increase in the diameter of the dust layer had a slight effect on the temperature history and ignition time of the dust layer.