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.     

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.     

Hot-surface ignition temperatures of dust layers

Minimum dust layer ignition temperatures on a hot surface were determined for several dusts, using a test procedure recommended by the National Academy of Sciences. The dusts included coal, three oil shales, lycopodium spores, corn starch, grain and brass powder. For a few of the dusts the effects of particle size and layer thickness on the minimum ignition temperatures were examined. Test results were repeatable and reliable for the fuels, the lycopodium and the brass powder. The minimum hot-surface ignition temperatures of 12.7-mm thick layers of these dusts ranged from 160°C for brass to 290°C for 20-gal ton⁻¹ oil shale. Flaming combustion was observed only with the brass powder. The minimum ignition temperatures decreased with thicker layers and with smaller particle sizes. Some difficulties were encountered with the corn starch and grain dusts. During heating, the starch charred and expanded; the grain dust swelled and distorted. The test was found acceptable for the purpose of determining the minimum layer ignition temperature of a variety of dusts. To prevent fire hazards due to smoldering or flaming dust layers the temperatures of surfaces on which combustible dusts accumulate should be lower than the minimum hot-surface ignition temperatures of the dusts.     

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.     

A theoretical model of concurrent longitudinal and circumferential superdrawing of hollow polyethylene terephthalate fibers

In a superdrawing process, a polyethylene terephthalate (PET) filament is elongated without developing much orientation and crystallization. Exploiting this phenomenon may bring about lower cost, more flexible and faster response in synthetic fiber production. The concurrent longitudinal and circumferential superdrawing phenomenon of PET hollow fibers is explained using the viscoelastic behavior of a thick walled cylinder under an internal pressure and an axial load in a continuous process. The model defines the stress–strain‐displacement relationship of hollow fibers. The fiber undergoes instantaneous radial superdrawing (increase in thickness) in the process zone followed by concurrent circumferential (increase in void) and longitudinal (increase in length) superdrawing. Based on material viscoelastic properties and processing conditions, the model predicts the threadline tension, internal pressure, and final fiber geometries. Excellent agreement of the model with experimental results is observed over a range of processing conditions. The model is developed from a process engineering viewpoint to enable the analysis of the impact of process parameters during superdrawing on fiber properties. POLYM. ENG. SCI., 50:1773–1779, 2010. © 2010 Society of Plastics Engineers     

中国动力煤的着火温度与着火热的分布规律

为了寻求动力煤成分分布与煤质着火及稳燃特性之间的函数关系,根据燃烧学的热力着火理论,对国内大型电站煤粉锅炉燃用的166个典型的煤质分析数据进行了数学分析。分析了固态碳(C_gt)和当量气态碳(C_qt 3.67H_ar)比值随V_daf的分布规律及其对煤质着火的影响;根据文献中77个煤质的一维沉降炉实验数据,拟合出煤粉的着火温度与V_ad及A_ad的函数关系,计算分析了本文大数据样本煤质的着火温度及着火热的分布规律,提出将着火热和着火温度作为煤质着火特性的图像分辨新方法。据此分析了各类煤质的着火与稳燃特性。结果表明,本方法能明确地定量区分不同类煤质和同类煤质的着火特性差异,且在理论上给出符合宏观规律的合理解释。     

Post-flashover compartment fires: Basis of a theoretical model

A theoretical model of compartment fires in the post-flashover stage is presented. The model incorporates the stirred reactor assumption and window flow approximation of Kawagoe, but treats fuel burning rates and burning regimes in a theoretical manner. The resulting formulation preserves the important features of the fire behavior but is simple enough to permit use for building fire safety design purposes.     

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.     

Ignition properties of polymers evaluated from ignition temperature and ignition limiting oxygen index

An apparatus for measuring ignition temperature and ignition-limiting oxygen index was devised in order to study ignition properties of polymers. Ignition limiting oxygen index, which was suggested by the limiting oxygen index (ASTM D2863-70), was defined as the minimum volume fraction of oxygen required for ignition to occur in a slowly rising gaseous atmosphere. The ignition temperatures and the ignition limiting oxygen indexes of typical polymers were measured by this apparatus. Polymers which produced more ethylene by pyrolysis tend to have lower ignition temperatures. In the case of the same kinds of polymers, plots of ignition temperatures versus ignition limting oxygen indexes gave nearly straight lines (Fig. 3), but the straight line of depolymerization-type polymers differed from that of random degradation-type polymers differed from that of random degradation-type polymers. In most polymers, ignition limiting oxygen indexes were nearly proportional to limiting oxygen indexes, but in the cases of hard poly(vinyl chloride) and soft poly(vinyl chloride), the plots did not follow this relation (Fig. 4). Ignition properties of self-extinguishing polymers were also studied.     

The ignition temperature of lignite char in a fluidized bed combustor

This paper summarises the experimental and modelling work carried out for the variation of bed ignition temperature of a fluidized bed combustor with the char particle diameter and the fluidizing velocity. A lignite char was used and its reactivity was represented using data from Field (1967) and Turnbull and Davidson (1984). The modelling involved solving the steady state heat balance around the fluidized bed combustor at the ignition temperature. A correlation of the total area of char ignited per unit bed mass was determined as a function of the char particle diameter and the fluidizing velocity. This correlation was used to determine the ignition temperature of the fluidized bed combustor operating at different conditions. The fluidized bed combustor heat balance was then solved for the bed ignition temperature which was influenced by both the rate of heat loss from the bed and the reactivity of the char. A sensitivity analysis suggests that the chemical rate reaction coefficient is the most prominent variable when determining the ignition temperature of a fluidized bed combustor.     

Melt-crystallization of n-alkanes and polyethylene in a temperature gradient

Summary When C₃₆H₇₄ was crystallized from a melt-state in a temperature gradient, crystallization proceeded at a crystal-melt boundary plane which is perpendicular to the temperature gradient. The melt-state was studied by striking the incident X-ray near the crystallizing plane.The intensity profile from the melt-state shows weak anisotropy. This fact suggests that the melt-molecules are slightly oriented perpendicular to the crystallizing plane.     

乙烯/聚乙烯两相体系爆炸特性

基于改进的20 L球形爆炸装置,实验测量了乙烯/聚乙烯两相体系爆炸特性参数,系统地分析了两相体系爆炸下限和爆炸强度变化规律,并对比分析了乙烯、聚乙烯和乙烯/聚乙烯3种体系爆炸强度之间的关系。结果表明：乙烯诱导聚乙烯最小爆炸浓度显著降低,低于爆炸下限的乙烯气体与低于最小爆炸浓度的聚乙烯混合后仍具有爆炸危险性。向不同浓度的聚乙烯粉尘中添加乙烯后,爆炸压力p_ex和压力上升速率（dp/dt）_ex均显著提高,但增幅随粉尘浓度的增大而减小。乙烯/聚乙烯两相体系最大爆炸压力p_max和爆炸指数K_st均随乙烯浓度的增大而增大,但不同乙烯浓度下的两相体系最大爆炸压力p_max和爆炸指数K_st均大于单相聚乙烯粉尘,小于单相乙烯气体。     

草酸盐和碳酸氢盐抑制聚乙烯粉尘爆炸特性

为研究草酸盐和碳酸氢盐抑制聚乙烯粉尘爆炸特性,选取NaHCO₃、KHCO₃、Na₂C₂O₄和K₂C₂O₄四种粉体,从火焰结构和火焰传播速度两方面分析其对聚乙烯粉尘爆炸的抑制性能,并结合抑爆粉体的理化性质分析抑爆机理。结果表明,四种抑爆粉体均可抑制聚乙烯粉尘爆炸火焰传播,且抑制效果随抑爆粉体浓度增加而增强。相同条件下,抑爆性能KHCO₃>NaHCO₃>K₂C₂O₄>Na₂C₂O₄,即钾盐粉体抑爆性能优于具有相同酸根离子的钠盐,碳酸氢盐粉体抑爆性能优于具有相同金属离子的草酸盐。另外,结合抑爆粉体热解特性测试及爆炸产物分析,探究了四种抑爆粉体的抑爆机理及离子构成带来的抑爆性能差异性原因。     

文氏管除尘器在氯化聚乙烯除尘中的应用

介绍了氯化聚乙烯生产过程中粉尘的产生以及文氏管除尘器在粉尘治理过程中的应用。并对文氏管除尘器的工艺流程、主要设备系统改进等方面进行了介绍。     

Investigation of ignition of particle-gas flow based on the hot-spot ignition model

Electric-spark ignition of flow of an aluminum particle-air mixture in a channel with a sudden expansion was studied experimentally using the model of hot-spot ignition. The critical radius of the initial site of ignition was determined, the dynamics of the process was studied, and the effect of turbulence intensity on the development of the hot spot was established.     

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.     

Open tubular heterogeneous enzyme reactors: analysis of a theoretical model

A novel reactor, made by coating the inner wall of tubes with an enzyme resin layer, was mathematically modeled. Michaelis-Menten kinetics, laminar flow and isothermal conditions were assumed and substrate diffusion in both the streaming fluid and the catalyst annulus was taken into account. The behavior and effectiveness of the reactor are discussed in terms of characteristic dimensionless groups including the reactor coordinate which is inversely proportional to the average overall rate at a given conversion. An experimental criterion is given for the absence of all diffusional effects which may mask kinetic data. The calculated reactor coordinates can be approximated by the sun of their limiting values pertaining to the kinetically controlled and bulk diffusion controlled reaction thus permitting a simplified treatment of the reactor data and of problems of reactor design. Conditions for extracting kinetic parameters from experimental data are discussed.     

Improved diagnostics for determination of minimum explosive concentration,ignition energy and ignition temperature of dusts

We have designed additional instrumentation for some of the standard tests on dust explosibility and flammability. The new instrumentation defines the experimental conditions of the standard tests more precisely. Additional physical parameters that are measured are relative dust concentrations, energy of spark ignition sources, temperature, and timing of events that occur during the course of the tests.Relative dust concentrations are determined by measuring the attenuation of light through the dust cloud. The light source is a light emitting diode (LED) and the light is detected by a phototransistor (PT). Up to four LED—PT pairs used simultaneously and signals are recorded with a dual-beam oscilloscope with dual-trace vertical amplifiers. Time response of the system is the order of 5 ms. The PT signal is also useful for detecting ignition and the passage of flame fronts.The spark energy in the minimum ignition energy tests is measured by recording the voltage across the spark gap and the spark current with the dual beam oscilloscope. The LED—PT probes have been very useful in the minimum ignition energy tests for measuring the time of the spark relative to the arrival time of the dust at the electrode position.Dynamic measurements of temperature were made in the Godbert—Greenwald furnace, using fast-reponse thermocouples. Ignition delays were also studied using the LED—PT probe.