Ignition temperatures of dust layers: Flaming and non-flaming

Combustible dusts, based on tests conducted in the Setchkin Furnace on five representative dusts, are shown to have two distinct layer minimum ignition temperatures, designated MIT_non-flaming and MIT_flaming. The MIT_non-flaming is the one usually reported in the literature; the MIT_flaming is determined by starting at a much higher temperature and working downwards. MIT_flaming can yield a different ranking of dust flammability than MIT_non-flaming. The MIT_flaming appears to be more closely correlated to the MIT of dust clouds and to the test prescribed for flammable solids in the US Code of Federal Regulations than the MIT_non-flaming.     

Mechanism of dust ignition in incident shock waves

A study is made of dust ignition during transient interaction of an incident shock wave with an extended dust cloud. It has been shown experimentally that in the presence of dispersed phase with volume concentration of 10^–3 the temperature behind a shock wave with Mach number M_s=4.5 can exceed the ambient one by 400 K or more. A physical mechanism is proposed for carrier phase heating, based on the effect of supersonic flow braking behind the shock wave under constraints created by dust particles during velocity relaxation. The gasdynamic functions are obtained as analytic functions of the flow Mach number M. In particular, it is shown that the equation T/T₀=M₀/M is valid for the temperature. The form of M as a function of dispersed-phase parameters and the quantitative nonsteadiness criterion are found. The function M was found to agree well with experiment.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 143–148, May–June, 1993.     

Models for minimum ignition temperature of organic dust clouds

The minimum ignition temperature of dust clouds is one of the important factors required for the design of preventive measures against dust explosion. The mathematical models available to predict this parameter have been analyzed for thier application to organic dust clouds. A solution of the most general model proposed by Mitsui and Tanaka is presented, together with its comparision with experimental data. It has been found to be quite successful in predicting the minimum ignition temperature for metal dusts but not for organic dusts. Recommendations for the development of a new model to predict the minimum ignition temperature of an organic dust, such as polyethylene, have been given.     

Minimum ignition temperature of polyethylene dust: a theoretical model

Dust explosion hazard exists in plants and facilities handling combustible dusts. The minimum ignition temperature of dust clouds is an important parameter requiring special attention to designing the explosion preventive measures. This paper presents a model developed for determining the minimum ignition temperature for an organic dust cloud, polyethylene, simulating the conditions in the Godbert-Greenwald furnace. The model correlates the particle size, as well as the dust concentration with the minimum ignition temperature. It is based on the two-stage oxidation mechanism involving devolatilization/decomposition of the solid particle and homogeneous oxidation of volatile combustible products. In the case of polyethylene, the main combustible gas responsible for ignition and flame propagation has been confirmed to be butylene. The results of the computations were compared with the experimental values and those predicted by Mitsui and Tanaka. The predicted values by the model developed are in close agreement with the experimental data which confirm the proposed ignition mechanism. The model can be used for the prediction of minimum ignition temperature of organic dusts having an autoignition mechanism similar to polyethylene dust. © 1997 John Wiley & Sons, Ltd.     

可燃性气体对PE粉体静电放电点火的影响

运用标准粉尘爆炸测试装置Hartmann管分别测试了聚乙烯(PE)粗料和经过筛分的PE粉体的最小点火能量,比较了PE粉体和可燃性气体共存时(即杂混合物)的最小点火能量与不同形式的静电放电能量.当PE粉体粒径小于2 mm时,随着粒径的增大,可燃性气体对杂混合物最小点火能量的影响也越大.但可燃性气体对PE粉体静电点火的影响变小.当可燃性气体浓度低于10%爆炸下限时,对未经过筛分的原始粉体,各种形式的静电均无法将其点燃;对粒径小于0.5 mm的PE粉体,可以排除电晕放电、刷形放电和堆表面放电作为点火源的可能性:对粒径小于75μm的PE粉体,堆表面放电、火花放电和传播型刷形放电均是可能的点火源.     

Calculation of the ignition sensitivity of dust clouds of varying size distributions

For a cloud of flammable dust in air to ignite, the temperature of the air must be a specific value that depends upon properties of the dust material and of the dust cloud.First, a theoretical treatment is used to explain variations in experimental ignition temperatures in terms of particle size. The theory modifies Cassel and Liebman's method to take account of residence time of dust in experimental furnaces or in hot air. It is shown that it is possible for the ignition temperature of mono-sized coal particles (about 50 μm diameter) to be minimal under a limited residence time.The theory is extended to deal with dust clouds with a distribution in particle size. It is shown that there exists a range of size distributions for which the possibility of ignition is at a maximum. The calculated results are presented in the form of Rosin—Rammler charts indicating the distribution most sensitive to ignition.     

Deposition of gadolinia-doped ceria layers by MOCVD at low temperatures

Electrolyte layers of Gd₂O₃-doped CeO₂ were synthesized by MOCVD with the use of Ce(tmhd)₄ and Gd(tmhd)₃ as reactants in the temperature range of 500–800 °C at reduced pressure. Ar and air were carrier gases. The layers were deposited on the inner surfaces of quartz glass tubes and flat glass substrates. The obtained layers were non-porous and smooth. The layers synthesized at temperature of 500 °C were amorphous. At higher synthesis temperatures, the deposited layers were nanocrystalline. The higher the synthesis temperature, the larger the grains. When the synthesis process was controlled by mass diffusion to the substrate, then a correlation between the chemical composition of reactants and chemical composition of the synthesized layer can be determined. The obtained layers were also examined by scanning electron microscopy, X-ray analysis and UV‐VIS spectroscopy. Additionally, the chemical composition of the synthesized layers was investigated using an EDS analyzer.     

Simulation of the process of coal dust ignition in the presence of metal particles

A mathematical model was developed for the gas-phase ignition of a layer of the dust of typical 2B brown coal by a metal particle heated to high temperatures (above 1100 K) under ideal thermal contact conditions. This model took into account the heating and thermal decomposition of ground coal upon the cooling of a local source, the yield of volatile components, and the formation, heating, and ignition of the gas mixture. The effect of heat source parameters (shape and dimensions) on the fundamental process characteristic—the delay time of ignition—was found. A relationship of the ignition zone position near a hot particle with the heating intensity of a gas mixture of volatile substances and an oxidizing agent was revealed. The results of numerical studies are consistent with well-known experimental data on the conditions and characteristics of ground coal burning on local heating by sources of limited energy capacity.     

Effect of volatiles on ignition delay in coal dust gas suspensions within shock waves

We present the results from an experimental study into the dynamics involved in the formation and the unique features of ignition in shock gas suspensions of coal dust with a dispersion of<40 m and a volatile content of 9, 26, and 55%. We have determined the ignition delays _ig for coal dusts in air and in pure oxygen for a range of Mach numbers from 2.6 to 4.0 in an incident shock wave. We have established that the quantity _ig as well as the nature of the function _ig(1/T) are significantly affected by the kinetics of the volatile yield, in addition to the kinetics of volatile ignition. We have derived an approximation expression for _ig as a function of the content of volatiles, the temperature of the medium, and the partial pressure of the oxygen.Novosibirsk. Translated from Fizika Goreniya i Vzryva, No. 2, pp. 101–111, March–April, 1991.     

Destruction of the surface layer of nitroglycerin powder during ignition at various initial temperatures

Destruction of the surface of nitroglycerin powder during ignition was detected experimentally. It was shown that the degree of destruction depends on the power of the external source, initial temperature, and exposure time. A model of ignition of a solid propellant is proposed which takes into account the occurrence of stresses and deformations in the surface layer during initiation of the chemical reaction. The characteristics of ignition and the degree of destruction are estimated. The results agree qualitatively with experimental data.Tomsk State University, Tomsk 634050. Research Institute of Applied Mathematics and Mechanics, Tomsk 634050. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 4, pp. 10–19, July–August, 1995.     

The effects of coal dust concentrations and particle sizes on the minimum auto‐ignition temperature of a coal dust cloud

Flash fires and explosions in areas containing an enriched combustible dust atmosphere are a major safety concern in industrial processing. An experimental study was conducted to analyse the effects of atmospheric coal dust particle sizes and concentrations on the minimum auto‐ignition temperature (MAIT) of a dust cloud. Two different coal samples from Australian coal mines were used. The coal dust particles were prepared and sized in 3 ranges, of below 74 μm, 74 to 125 μm and 125 to 212 μm, by using a series of sieves and a sieve shaker. A humidifier was used to increase the moisture content of the particles to the required level. All the experiments were conducted in accordance with the ASTM E1491‐06 method in a calibrated Goldbert‐Greenwald furnace. The results from this study indicate that coal dust properties, such as the chemical nature (H/C), concentration, particle size (D₅₀), and moisture content, impact on the MAIT. For coal dust concentrations less than 1000 g.m^?3, the MAIT decreases with increasing coal dust concentrations. On the other hand, for low concentrations of 100 to 15 g.m^?3, the MAIT becomes more reliable for particle size D₅₀ rather than for volatile matters.     

The structure of physically adsorbed layers on graphite at low temperatures

A very brief outline of recent results with references to literature is given.     

Metal-organic chemical vapour deposition of lanthana-doped ceria layers at low temperatures

Non-porous, amorphous and nanocrystalline CeO₂–La₂O₃ layers were synthesized by means of MOCVD using Ce(tmhd)₄ and La(tmhd)₃ as reactants on tubular and planar quartz glass substrates. Different layers were synthesized in the temperature range 500–700 °C using various molar contents of La(tmhd)₃, i.e. 10, 15, 26 and 50%, respectively. A mixture of Ar and air was used as a carrier gas. Air was also an oxygen source necessary for carbon oxidation (a solid by-product of reactant pyrolysis). Therefore, the layers should only contain Ce⁴⁺ ions. When the synthesis process was controlled by the surface reaction rate, there was no correlation between the chemical composition of reactants and that of the layers. Such a dependence was obtained when the synthesis process was controlled by mass diffusion to the substrate. In addition, it was observed that La³⁺ ion segregation can occur during layer synthesis between surface areas with lower and higher amounts of a crystalline phase. The molar percent of La₂O₃ in areas exhibiting less crystallization with smaller grains was higher than in locations with larger grains, because, in this case, intergranular boundary content was lower. X-ray analysis indicates that layers deposited at 500 °C were amorphous, although SEM images show numerous small nuclei of a crystalline phase. It seems that the crystalline phase content was not high enough to be identified by X-ray analysis. Layers deposited at 600 °C consisted of a solid solution of La₂O₃ dissolved in CeO₂. From UV-VIS tests it follows that absorption is very strong up to a wavelength of approximately 320 nm.     

Mathematical simulation of heterogeneous detonation of coal dust in oxygen with allowance for the ignition stage

A consistent physicomathematical model that describes ignition and detonation combustion of a gas suspension of coal-dust particles is developed. The model is based on the concepts of the two-velocity two-temperature continuum of mechanics of heterogeneous media with allowance for reduced reactions of pyrolysis, combustion of volatiles, and combustion of the coke residue. The model is verified with the use of available experimental data on the dependence of the detonation velocity on the initial concentration of the discrete phase and the dependence of the ignition delay on the Mach number of the incident shock wave. An analysis of ignition of the gas suspension of bituminous coal in shock waves shows that the stage of ignition proceeds under conditions of both temperature and velocity nonequilibrium. The influence of particle heating due to stagnation temperature on devolatilization dynamics and ignition delay is established. Examples of computed flow structures behind shock and detonation waves with allowance for the ignition stage are presented.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 89–99, January–February, 2005.     

Using ignition of coal dust produced by different types of mechanical treatment under conditions of rapid heating

Experiments aimed at studying the ignition of coal dust obtained by coal disintegration in high-energy mills are performed in a tubular furnace. Effective kinetic constants of ignition of coal dust ground in a vibrational–centrifugal mill and in a disintegration mill under conditions of rapid heating are determined for the first time. It is shown that the volatile release rate depends on the method of coal grinding.     

Thermal radiation controlled ignition/extinction phenomena in a system of separated catalyst layers with exothermal chemical reaction

In analogy to fixed beds ignition/extinction phenomena were observed in an exothermic and adiabatic chemical tubular reactor containing ten separated layers of catalyst particles. Obviously the thermal feed back by radiative exchange caused the system to perform like a quasihomogeneous medium. The application of the two phase homogeneous model equations revealed that the predicted ignition temperatures, which agreed well with those measured in a packed bed, were on the average 10 K lower than those measured in the multilayer system. This result is attributed to the missing contact points which in packed beds are privileged areas for the progress of ignition.