Niacin,lycopodium and polyethylene powder explosibility in 20-L and 1-m3 test chambers

An experimental program has been undertaken to investigate the explosibility of selected organic dusts. The work is part of a larger research project aimed at examination of a category of combustible dusts known as marginally explosible. These are materials that appear to explode in laboratory-scale test chambers, but which may not produce appreciable overpressures and rates of pressure rise in intermediate-scale chambers. Recent work by other researchers has also demonstrated that for some materials, the reverse occurs – i.e., values of explosion parameters are higher in a 1-m³ chamber than one with a volume of 20 L. Uncertainties can therefore arise in the design of dust explosion risk reduction measures.The following materials were tested in the current work: niacin, lycopodium and polyethylene, all of which are well-known to be combustible and which cover a relatively wide range of explosion consequence severity. The concept of marginal explosibility was incorporated by testing both fine and coarse fractions of polyethylene. Experiments were conducted at Dalhousie University using the following equipment: (i) Siwek 20-L explosion chamber for determination of maximum explosion pressure (P_max), volume-normalized maximum rate of pressure rise (K_St), and minimum explosible concentration (MEC), (ii) MIKE 3 apparatus for determination of minimum ignition energy (MIE), and (iii) BAM oven for determination of minimum ignition temperature (MIT). Testing was also conducted at Fauske & Associates, LLC using a 1-m³ explosion chamber for determination of P_max, K_St and MEC. All equipment were calibrated against reference dusts, and relevant ASTM methodologies were followed in all tests.The explosion data followed known trends in accordance with relevant physical and chemical phenomena. For example, P_max and K_St values for the fine sample of polyethylene were higher than those for the coarse sample because of the decrease in particle size. MEC values for all samples were comparable in both the 20-L and 1-m³ chambers. P_max and K_St values compared favorably in the different size vessels except for the coarse polyethylene sample. In this case, K_St determined in a volume of 1 m³ was significantly higher than the value from 20-L testing. The fact that the 20-L K_St was low (23 bar m/s) does not indicate marginal explosibility of the coarse polyethylene. This sample is clearly explosible as evidenced by the measured values of MEC, MIE, MIT, and 1-m³ K_St (at both 550 and 600 ms ignition delay times).     

Explosibility of micron- and nano-size titanium powders

Explosibility of micron- and nano-titanium was determined and compared according to explosion severity and likelihood using standard dust explosion equipment. ASTM methods were followed using a Siwek 20-L explosion chamber, MIKE 3 apparatus and BAM oven. The explosibility parameters investigated for both size ranges of titanium include explosion severity (maximum explosion pressure (P_max) and size-normalized maximum rate of pressure rise (K_St)) and explosion likelihood (minimum explosible concentration (MEC), minimum ignition energy (MIE) and minimum ignition temperature (MIT)). Titanium particle sizes were ?100 mesh (<150 μm), ?325 mesh (<45 μm), ≤20 μm, 150 nm, 60–80 nm, and 40–60 nm. The results show a significant increase in explosion severity as the particle size decreases from ?100 mesh with an apparent plateau being reached at ?325 mesh and ≤20 μm. Micron-size explosion severity could not be compared with that for nano-titanium due to pre-ignition of the nano-powder in the 20-L chamber. The likelihood of an explosion increases significantly as the particle size decreases into the nano range. Nano-titanium is very sensitive and can self-ignite under the appropriate conditions. The explosive properties of the nano-titanium can be suppressed by adding nano-titanium dioxide to the dust mixture. Safety precautions and procedures for the nano-titanium are also discussed.     

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.     

Explosibility of polyamide and polyester fibers

The current research is aimed at investigating the explosion behavior of hazardous materials in relation to aspects of particulate size. The materials of study are flocculent (fibrous) polyamide 6.6 (nylon) and polyester (polyethylene terephthalate). These materials may be termed nontraditional dusts due to their cylindrical shape which necessitates consideration of both particle diameter and length. The experimental work undertaken is divided into two main parts. The first deals with the determination of deflagration parameters for polyamide 6.6 (dtex 3.3) for different lengths: 0.3 mm, 0.5 mm, 0.75 mm, 0.9 mm and 1 mm; the second involves a study of the deflagration behavior of polyester and polyamide 6.6 samples, each having a length of 0.5 mm and two different values of dtex, namely 1.7 and 3.3. (Dtex or decitex is a unit of measure for the linear density of fibers. It is equivalent to the mass in grams per 10,000 m of a single filament, and can be converted to a particle diameter.) The explosibility parameters investigated for both flocculent materials include maximum explosion pressure (P_max), size-normalized maximum rate of pressure rise (K_St), minimum explosible concentration (MEC), minimum ignition energy (MIE) and minimum ignition temperature (MIT). ASTM protocols were followed using standard dust explosibility test equipment (Siwek 20-L explosion chamber, MIKE 3 apparatus and BAM oven). Both qualitative and quantitative analyses were undertaken as indicated by the following examples. Qualitative observation of the post-explosion residue for polyamide 6.6 indicated a complex interwoven structure, whereas the polyester residue showed a shiny, melt-type appearance. Quantitatively, the highest values of P_max and K_St were obtained at the shortest length and finest dtex for a given material. For a given length, polyester displayed a greater difference in P_max and K_St at different values of dtex than polyamide 6.6. Long ignition delay times were observed in the BAM oven (MIT measurements) for polyester, and video framing of explosions in the MIKE 3 apparatus (MIE measurements) enabled observation of secondary ignitions caused by flame propagation after the initial ignition occurring at the spark electrodes.     

Study on explosion characters of HDPE dust inerted by CaCO3 and NaHCO3

To prevent high density polyethylene (HDPE) dust explosions, this study evaluated HDPE's explosive sensitivity characteristics, and comparatively examined two inert dust types (CaCO₃ and NaHCO₃) to mitigate the explosive sensitivity of HDPE dust. In the serials of experiments, the 1.2 L Hartmann tube and Godbert-Greenwald furnace were used respectively to measure the minimum ignition energy (MIE) and minimum ignition temperature (MIT) of HDPE dust. The findings demonstrated that the MIE and MIT of HDPE are 56.8 mJ and 320 °C under the most sensitive situation. Second, both CaCO₃ and NaHCO₃ can inhibit the explosive sensitivity of HDPE with the variation of several parameters (i.e., quality percentage and particle sizes). Specially, as the quality percentage of 38–48 μm NaHCO₃ come to 70%, the HDPE/NaHCO₃ will not be explosive. Finally, NaHCO₃ had a better inerting effect than CaCO₃ in the reduction of explosive sensitivity of HDPE.     

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.     

Effect of pyrolysis and oxidation characteristics on lauric acid and stearic acid dust explosion hazards

The effect of pyrolysis and oxidation characteristics on the explosion sensitivity and severity parameters, including the minimum ignition energy MIE, minimum ignition temperature MIT, minimum explosion concentration MEC, maximum explosion pressure P_max, maximum rate of pressure rise (dP/dt)_max and deflagration index K_st, of lauric acid and stearic acid dust clouds was experimentally investigated. A synchronous thermal analyser was used to test the particle thermal characteristics. The functional test apparatuses including the 1.2 L Hartmann-tube apparatus, modified Godbert-Greenwald furnace, and 20 L explosion apparatus were used to test the explosion parameters. The results indicated that the rapid and slow weight loss processes of lauric acid dust followed a one-dimensional diffusion model (D1 model) and a 1.5 order chemical reaction model (F1.5 model), respectively. In addition, the rapid and slow weight loss processes of stearic acid followed a 1.5 order chemical reaction model (F1.5 model) and a three-dimensional diffusion model (D3 model), respectively, and the corresponding average apparent activation energy E and pre-exponential factor A were larger than those of lauric acid. The stearic acid dust explosion had higher values of MIE and MIT, which were mainly dependent on the higher pyrolysis and oxidation temperatures and the larger apparent activation energy E determining the slower rate of chemical bond breakage during pyrolysis and oxidation. In contrast, the lauric acid dust explosion had a higher MEC related to a smaller pre-exponential factor A with a lower amount of released reaction heat and a lower heat release rate during pyrolysis and oxidation. Additionally, due to the competition regime of the higher oxidation reaction heat release and greater consumption of oxygen during explosion, the explosion pressure P_m of the stearic acid dust was larger in low concentration ranges and decayed to an even smaller pressure than with lauric acid when the concentration exceeded 500 g/m³. The rate of explosion pressure rise (dP/dt)_m of the stearic acid dust was always larger in the experimental concentration range. The stearic acid dust explosion possessed a higher P_max, (dP/dt)_max and K_st mainly because of a larger pre-exponential factor A related to more active sites participating in the pyrolysis and oxidation reaction. Consequently, the active chemical reaction occurred more violently, and the temperature and overpressure rose faster, indicating a higher explosion hazard class for stearic acid dust.     

Effect of particle size distribution,drying and milling technique on explosibility behavior of olive pomace waste

The Mediterranean area is responsible for about 98% of the olive oil worldwide production, with 900 million olive trees occupying 10 million hectares. However, the processing of 100 kg of olives leads to the production of 40 kg of wastes, mainly constituted by olive pomace, which is potentially recoverable as energetic or material source. In general, in the past 20 years, the exploitation of olive pomace has increased, but along with it, the need for further information about its chemical-physical characterization and the related hazard in industry. Thus, a risk analysis assessment was conducted. When pelletized or in chunks, olive pomace does not pose any greater hazard than a pile of woody material, but when pulverized, it might become dangerous. Two parallel series of experiments were carried out at Dalhousie University (Lab 1) and at Polytechnic of Turin (Lab 2) using the same olive pomace sample, according to slightly different experimental procedures. Olive pomace dust explosibility and flammability parameters were measured: minimum ignition energy (MIE), minimum ignition temperature (MIT), maximum pressure rise rate ((dP/dt)_max and K_St), maximum pressure (P_max), and minimum explosible concentration (MEC). Moreover, the chemical and physical characterization of olive pomace was carried out: moisture content, particle size analysis, Scanning Electronic Microscope (SEM) investigation, thermo-gravimetric analysis (TGA), solid-state Nuclear Magnetic Resonance (NMR), mass spectrometry, calorific value, and bulk density estimation. Different thermal behaviors were observed according to the sieving/grinding pre-treatment. As concern flammability tests, samples seemed not to be sensitive to electric arc ignition (a value of MIE could not be measured), while coarser samples demonstrated higher ignition sensitivity to hot environment sources (MIT furnace) than finer ones. On the other hand, explosion violence parameters were enhanced by decreasing the particle size, while peak pressures were significantly influenced by the heat of combustion and the moisture content. Finally, a new test was developed to quantify the propensity of the raw material to produce fines by abrasion. It is defined “Abrasion by Rolling Test” (ART). The properties of the fines produced were measured as well.     

超细聚苯乙烯微球粉体的燃爆特性研究

为研究超细聚苯乙烯微球粉体的燃爆特性,通过粉尘层最低着火温度测试装置、MIE-D1.2最小点火能测试装置、20 L球形爆炸测试装置,对其最低着火温度、最大爆炸压力、最小点火能量(MIE)等爆炸特性参数进行测定,探讨了加热温度、点火延滞时间、粉尘质量浓度、粉尘粒径对粉体燃爆特性的影响。结果表明:超细聚苯乙烯微球粉尘层在350℃左右时会发生无焰燃烧,且加热温度越高,粉体粒径越小,粉尘层发生着火时所需的时间越短;当粉体质量浓度为250 g/m3时,最大爆炸压力达到0.65 MPa,质量浓度为500 g/m3时,最大爆炸压力的上升速率达90 MPa/s以上;随点火延滞时间增加,最小点火能表现出先缓慢减小再急剧增大的规律;随粉尘质量浓度增加,最小点火能逐渐降低,当粉尘质量浓度超过500g/m3后逐渐趋于稳定。     

不饱和聚酯树脂钮扣粉尘静电爆炸敏感性研究

针对某不饱和聚酯树脂钮扣厂在除尘设备维修过程中发生的粉尘爆炸事故,探究静电引起此次事故的可能性并提出防护措施。通过实验测定不饱和聚酯树脂钮扣粉尘的爆炸特性参数,进而确定其静电爆炸敏感性。结果发现:不饱和聚酯树脂钮扣粉尘云最小点火能MIE为4～10 mJ、最低着火温度MIT为480 ℃、粉尘层最低着火温度LIT＞400 ℃。表明,此粉尘属易燃粉尘,其粉尘爆炸敏感度极高,被静电火花点燃的可能性极大,在生产过程中,应采取静电防护措施。     

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.     

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.     

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.     

Dust explosion risk moderation for flocculent dusts

The research presented in this paper is focused on dust explosions of coarse and fine flocculent (or fibrous) samples of wood and polyethylene. Hybrid mixtures of fibrous polyethylene and admixed ethylene were also studied. Experimentation was conducted by following standardized test procedures and using standardized apparatus for determination of maximum explosion pressure, size-normalized maximum rate of pressure rise, minimum explosible concentration, minimum ignition energy, and minimum ignition temperature. A general trend was observed of enhanced explosion likelihood and consequence severity with a decrease in material diameter, as well as enhanced consequence severity with admixture of a flammable gas to the combustion atmosphere. The same phenomena are well-established for dusts composed of spherical particles; this highlights the importance of inherently safer design and the principle of moderation in avoiding the generation of fine sizes of flocculent dusts and hybrid mixtures of such materials with flammable gases.In addition to presenting experimental findings, the paper describes phenomenological modelling efforts for the flocculent polyethylene using four geometric equivalence models: radial equivalence, volumetric equivalence, surface area equivalence, and specific surface area equivalence. The surface area equivalence model was found to yield the best estimates of maximum rate of pressure rise for the flocculent polyethylene samples investigated experimentally.     

Experimental study on the minimum explosion concentration of anthracite dust: The roles of O2 mole fraction,inert gas and CH4 addition

The explosion characteristics of anthracite coal dust with/without small amount of CH₄ (1.14 vol %) were investigated by using a 20 L spherical explosion apparatus with an emphasis on the roles of oxygen mole fraction and inert gas. Two methods based on overpressure and combustion duration time were used to determine the minimum explosion concentration (MEC) or the lower explosion limit (LEL) of the pure anthracite coal dust and the hybrid coal-methane mixtures, respectively. The experiment results showed that increasing oxygen mole fraction increases the explosion risk of coal dust: with increasing oxygen mole fraction, the explosion pressure (P_ex) and the rate of explosion pressure rise ((dp/dt)_ex)) increase, while MEC decreases. The explosion risk of anthracite dust was found to be lower after replacing N₂ with CO₂, suggesting that CO₂ has a better inhibition effect on explosion mainly due to its higher specific heat. However, the addition of 1.14% CH₄ moderates the inhibition effect of CO₂ and the promotion effect of O₂ on anthracite dust explosion for some extent, increasing explosion severity and reducing the MEC of anthracite dust. For hybrid anthracite/CH₄ mixture explosions, Barknecht's curve was found to be more accurate and conservative than Chatelier's line, but neither are sufficient from the safety considerations. The experimental results provide a certain help for the explosion prevention and suppression in carbonaceous dust industries.     

不同条件下6-APA粉体爆炸最小点火能测试研究

6氨基青霉烷酸（6-APA）是生产阿莫西林的重要中间体,在生产过程的离心机分离及干燥等环节存在粉体燃烧爆炸的危险。利用Hartmann管式粉尘最小点火能测试装置,研究6-APA干粉状态及丙酮存在环境粉体最小点火能变化规律。实验结果表明,6-APA粉体在分散质量为0.6g时,最小点火能为14mJ,参照VDI2263的规定,属于一般着火敏感性粉尘。向粉体中加入丙酮溶剂模拟实际生产环境,实验结果显示粉尘云最小点火能下降明显,且混合物着火能力增强。质量为1g的6-APA粉体与0.5mL丙酮溶剂配比条件下,混合物分散质量为0.6g时,最小点火能为6mJ,在此环境中混合粉体属于特别着火敏感性粉尘。实验结果阐明了6-APA在丙酮存在环境条件下混合粉体燃烧的爆炸危险性,为采取相应的爆炸防护措施提供了实验依据。     

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 p_max, 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 p_max, 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 p_max under different combustion mechanisms, a dimensionless pressure P_R 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 P_R) 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.     

Observations of microscopic characteristics of post-explosion coal dust samples

To reveal the microscopic characteristics of the post-explosion coal dust samples, coal dust explosion tests were performed in a 20 L spherical vessel. The explosion characteristic parameters, such as the maximum pressure (P_max), the maximum rate of pressure rise ((dP/dt)_max), ignition time (t) and the deflagration index (K_St) were recorded. Meanwhile, the post-explosion dust samples were collected and analyzed. The research efforts include particle size distribution analysis, SEM analysis and FTIR analysis of dust samples before and after the explosion. The particle size range of post-explosion dust samples became wider according to the mass percent analysis. The microscopic appearance of samples in same particle size range showed some similarity. The porous structure of dust samples was observed by improving the SEM magnification. The chemical structure of dust samples before and after explosion was analyzed by FTIR.     

Effects of ignition energy on fire and explosion characteristics of dilute hybrid fuel in ventilation air methane

Deflagration explosions of coal dust clouds and flammable gases are a major safety concern in coal mining industry. Accidental fire and explosion caused by coal dust cloud can impose substantial losses and damages to people and properties in underground coal mines. Hybrid mixtures of methane and coal dust have the potential to reduce the minimum activation energy of a combustion reaction. In this study the Minimum Explosion Concentration (MEC), Over Pressure Rise (OPR), deflagration index for gas and dust hybrid mixtures (K_st) and explosive region of hybrid fuel mixtures present in Ventilation Air Methane (VAM) were investigated. Experiments were carried out according to the ASTM E1226-12 guideline utilising a 20 L spherical shape apparatus specifically designed for this purpose.Resultsobtained from this study have shown that the presence of methane significantly affects explosion characteristics of coal dust clouds. Dilute concentrations of methane, 0.75–1.25%, resulted in coal dust clouds OPR increasing from 0.3 bar to 2.2 bar and boosting the K_st value from 10 bar m s⁻¹ to 25 bar m s⁻¹. The explosion characteristics were also affected by the ignitors’ energy; for instance, for a coal dust cloud concentration of 50 g m⁻³ the OPR recorded was 0.09 bar when a 1 kJ chemical ignitor was used, while, 0.75 bar (OPR) was recorded when a 10 kJ chemical ignitor was used.For the first time, new explosion regions were identified for diluted methane-coal dust cloud mixtures when using 1, 5 and 10 kJ ignitors. Finally, the Le-Chatelier mixing rule was modified to predict the lower explosion limit of methane-coal dust cloud hybrid mixtures considering the energy of the ignitors.     

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。