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.     

Flame propagation behaviors of nano- and micro-scale PMMA dust explosions

Flame propagation behaviors of nano- and micro-polymethyl methacrylate (PMMA) dust explosions were experimentally studied in the open-space dust explosion apparatus. High-speed photography with normal and microscopic lenses were used to record the particle combustion behaviors and flame microstructures. Simple physical models were developed to explore the flame propagation mechanisms. High-speed photographs showed two distinct flame propagation behaviors of nano- and micro-PMMA dust explosions. For nano-particles, flame was characterized by a regular spherical shape and spatially continuous combustion structure combined with a number of luminous spot flames. The flame propagation mechanism was similar to that of a premixed gas flame coupled with solid surface combustion of the agglomerates. In comparison, for micro-particles, flame was characterized by clusters of flames and the irregular flame front, which was inferred to be composed of the diffusion flame accompanying the local premixed flame. It was indicated that smaller particles maintained the leading part of the propagating flame and governed the combustion process of PMMA dust clouds. Increasing the mass densities from 105 g/m³ to 217 g/m³ for 100 nm PMMA particles, and from 72 g/m³ to 170 g/m³ for 30 μm PMMA particles, the flame luminous intensity, scale and the average propagation velocity were enhanced. Besides, the flame front became more irregular for 30 μm PMMA dust clouds.     

Analysis of the chosen thermal and flammability parameters of dried sewage dust

The hazardous sludge disposal process in the form of landfills requires the determination inter alia of the flammable and explosion properties of dried sewage sludge dust, which has the ability to ignite and spontaneously combust when stored in silos. At a constant furnace surface temperature, the minimum ignition temperature of the sludge dust layer with a layer thickness of 5 mm is 270 °C, and for a layer thickness of 12.5 mm it is 250 °C. Two selected fire extinguishing powders for Class A, B, C and D fires were used in the study to determine the possibility of reducing the susceptibility of dried wastewater to ignition from heated surface, self-ignition and explosion parameters. The most effective extinguishing powder was ABC Favorit, which increased the value of the minimum ignition temperature of the layer (5 mm thick) to 360 °C and the spontaneous ignition temperature of the sludge with this powder increased by 22 °C at 169.6 cm³ in comparison to the sludge without extinguishing powder, respectively. The lowest self-ignition temperature of 136 °C was recorded for the largest tested volume (169.6 cm³) for dried sewage dust without any fire extinguishing powders. The biggest values of p_max and (dp/dt)_max dried sewage dust were recorded 4.8 bar and 113 bar/s respectively. By analysing the obtained test results, it can be assumed that dried sewage dust is a combustible material with properties similar to biomass.     

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.     

Minimum Ignition Temperature of layer and cloud dust mixtures

The prevention of dust explosions is still a challenge for the process industry. Ignition, in particular, is a phenomenon that is still not completely understood. As a consequence, safety conditions pertaining to ignition suppression are rarely identified to an adequate level. It is well known that, in general, the ignition attitude of a dust depends on several factors, such as the nature of the chemical, the particle size, moisture content, etc., but there is still a lack of knowledge on the effect of the single variables.This paper has the aim of providing data on the Minimum Ignition Temperatures of dust mixtures obtained from a mixing of a combustible dust (flour, lactose, sucrose, sulphur) and an inert dust (limestone, extinguishing powders) as well as from the mixing of two different combustible dusts. Various mixtures with different weight ratios have been tested in a Godbert Greenwald (GG) furnace and on a hot plate in order to measure the effect of mixture composition on the Minimum Ignition Temperature (MIT_L) of the layer and on the Minimum Ignition Temperature (MIT_C) of the cloud. In order to further verify the effects of inert dust particle size, inerts sieved to different size ranges have been tested separately. Generally, both MIT_L and MIT_C increase as the inert content is increased. MIT_C is poorly affected by inert particle size when limestone is used. The MIT_L of pure flour is higher than the MIT_L of mixtures containing up to 40% of 32–75 μm of limestone. This was probably due to the behaviour of pure flour during the test, which demonstrated strong tendency to produce char, cracks in the layer and detachment from the hot plate.     

Ignition of a dust layer by a constant heat flux

In real conditions, the surface temperature of an equipment enclosure covered with a combustible dust layer can significantly rise due to insulating properties of the dust layer. To assess this effect, the measurements of minimum ignition temperature of dust layer at constant temperature of the heated plate t_{t min} (standard method) and the same ignition temperature at constant rate of heat generation t_{h min} for two coal dusts were made. Dust layers of thickness between 5 and 50 mm were tested. For each dust, t_{t min} was higher than t_{h min} for every tested thickness of the layer. The difference was biggest for thin layers and decreased with increase of the layer thickness. The results suggest a deficiency of the standard procedure of measuring minimum ignition temperature of a dust layer.     

4种典型易燃烟煤的着火特性分析*

为了研究典型易燃烟煤的着火特性,预防和控制煤尘爆炸,采用粉尘云最低着火温度实验装置和同步热分析仪,分别研究崔木长焰煤、东荣二矿气煤、察哈素不粘煤和丁集焦煤4种烟煤在不同条件下的煤尘云最低着火温度和煤尘热解过程。研究结果表明:当煤尘云浓度从0.90 kg/m3上升到5.99 kg/m3时,4种煤尘的最低着火温度先降后升,在1.50 kg/m3煤尘云浓度时,4种煤尘的最低着火温度均达到最小,分别为450,580,610,620 ℃。随着升温速率的升高,煤的着火温度、峰值温度、燃尽温度和煤样最大放热量整体呈上升趋势,失重率整体呈下降趋势,在5~20 ℃/min的升温速率范围内,崔木长焰煤、东荣二矿气煤、察哈素不粘煤和丁集焦煤热解过程中的最小着火温度分别为354.17,404.37,443.18,484.13 ℃。4种烟煤的最低着火温度和热解过程中的最小着火温度有相对应关系,研究结论可为以上4个煤矿的具体煤样研究和数据分析提供参考依据。     

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.     

Determination of heterogeneous reaction mechanisms: A key milestone in dust explosion modelling

Reaction kinetics is fundamental for modelling the thermal oxidation of a solid phase, in processes such as dust explosions, combustion or gasification. The methodology followed in this study consists in i) the experimental identification of the reaction mechanisms involved in the explosion of organic powders, ii) the proposal of simplified mechanisms of pyrolysis and oxidation, iii) the implementation of the model to assess the explosion severity of organic dusts. Flash pyrolysis and combustion experiments were carried out on starch (22 μm) and cellulose (53 μm) at temperatures ranging from 973 K to 1173 K. The gases generated were collected and analyzed by gas chromatography. In this paper, a semi-global pyrolysis model was developed for reactive systems with low Damköhler number. It is in good agreement with the experimental data and shows that both carbon monoxide and hydrogen are mainly generated during the pyrolysis of the solid, the generation of the latter compound being greatly promoted at high temperature. A simplified combustion model was also proposed by adding two oxidation reactions of the pyrolysis products. In parallel, flame propagation tests were performed in a semi open tube in order to assess the burning velocity of such compounds. The laminar burning velocity of cellulose was determined to be 21 cm s⁻¹. Finally, this model will be integrated to a predictive model of dust explosions and its validation will be based on experimental data obtained using the 20 L explosion sphere. The explosion severity of cellulose was determined and will be used to develop and adjust the predictive model.     

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.     

Explosibility and thermal decomposition behavior of nitrile rubber dust in industrial processes

The formation of nitrile rubber (NBR) dust clouds during processing can lead to a potential dust explosion under certain conditions. However, the potential explosion hazard posed by NBR dust is usually overlooked by enterprises. In this paper, the explosive properties of NBR dust are investigated using a Hartmann tube, a G-G furnace, and a 20 L explosion chamber. The results showed that NBR dust could cause explosions severe enough to be classified as St-1. In addition, the thermal decomposition behavior of NBR dust under combustion conditions was investigated using a combination of thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA-FTIR). The results indicated that in the early stage, NBR dust mainly undergoes self-thermal decomposition to produce a large amount of combustible gas, which combines with oxygen to form a mixed gas and cause a gas-phase explosion. In addition, the participation of oxygen could lower the initial temperature of NBR dust thermal decomposition. As a result, decomposition occurred more quickly and a large amount of combustible gas was produced, thus expanding the range of dust explosions. Furthermore, these combustible gases exhibit varying degrees of toxicity, seriously affecting the life and health safety of relevant personnel. This work provides theoretical guidance for the development of safe procedures to prevent and address problems during NBR dust processing in enterprises.     

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.     

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.     

Effect of admixture of solid inertant on fire hazard of dust layers oriented at varying degrees of inclination

Unlike metallic dust layers, the layer flammability levels (LFL) of non-metallic dust layers exhibit a wide range from Class 1 (No self-sustained combustion) to Class 6 (explosive combustion). However, determinations of layer flammability have not considered the effect of inclination angle, thereby potentially underestimating fire hazard of combustible dust layers in many industrial situations. In this research, inclined dust layers showed greater fire hazard than did horizontally oriented dust layers. For example, LFL of wood dust jumped from class 3 to class 5 when layers were positioned with an incline. Flame spread rate of PMMA dust layers increased from 1.8 to 3.6 mm/s when the angle of inclination increased from 0 to 40°. Even small amounts of solid inertant significantly decreased surface layer fires. The required amount of inertant to completely inert layer fires was far less than that for smoldering layer fires or dust explosions.     

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.     

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.     

Inerting effect of CaCO3 powder on flame spread of wood dust layer

The effect of CaCO₃ powder, a typical inert dust, on the flame spread characteristics of wood dust layers was studied using an experimental device to understand the ignition characteristics of and develop inert explosion-proof technology for deposited wood dust. The results showed that the flame spread velocity (FSV) of the mixed dust layer was affected by the dispersion effect of CaCO₃ powder and physical heat absorption. As the CaCO₃ powder mass fraction increased, the FSV of the dust layer first increased and then decreased, reaching a peak at a 50% mass fraction. Moreover, the front-end temperature of the flame gradually decreased, and the red spark faded. The combustion reaction of the mixed dust layer could be more completed, and the colour of the combustion residue changed from charcoal black to charcoal grey. The coupling effect of the initial temperature and wind speed can promote an increase in the FSV in the mixed dust layer. The Gauss–Amp model of the FSV of the wood dust layer and mass fraction of CaCO₃ powder showed that the peak of the FSV occurred when the mass fraction of CaCO₃ powder was between 40 and 50%. Thus, a good inerting and explosion-proof effect can be achieved by using CaCO₃ powder with a mass fraction of more than 50%; it can improve the whole inerting process. Inert explosion-proof technology should be considered when assessing fire and explosion risk of dust in real process industry situations.     

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.     

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.     

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。