Parametric study of the explosivity of graphite-metals mixtures

The explosivity of dust clouds is greatly influenced by several parameters which depend on the operating conditions, such as the initial turbulence, temperature or ignition energy, but obviously also on the materials composition. In the peculiar case of a mixture of two combustible powders, the physical and chemical properties of both dusts have an impact on the cloud flammability and on its explosivity. Nevertheless, no satisfactory ‘mixing laws’ predicting the mixture behavior are currently available and the composition variable to be considered for such models greatly depend on the safety parameters which have to be determined: from volume ratios for some thermal exchanges and ignition phenomena, to surface proportions for some heterogeneous reactions and molar contents for chemical reactions. This study is mainly focused on graphite/magnesium mixtures as they are encountered during the decommissioning activities of UNGG reactors (Natural Uranium Graphite Gas). Due to the different nature and reactivity of both powders, these mixtures offer a wide range of interests. Firstly, the rate-limiting steps for the combustion of graphite are distinct from those of metals (oxygen diffusion or metal vaporization). Secondly, the flame can be thickened by the presence of radiation during metal combustion, whereas this phenomenon is negligible for pure graphite. Finally, the turbulence of the initial dust cloud is modified by the addition of a second powder. In order to assess the explosivity of graphite/magnesium clouds, a parametric study of the effects of storage humidity, particle size distribution, ignition energy, and initial turbulence has been carried out. In particular, it was clearly demonstrated that the turbulence significantly influences the explosion severity by speeding up the rate of heat release on the one hand and the oxygen diffusion through the boundary layer surrounding particles on the other hand. Moreover, it modifies the mean particle size and the spatial dust distribution in the test vessel, impacting the uniformity of the dust cloud. Thus, the present work demonstrates that the procedures developed for standard tests are not sufficient to assess the dust explosivity in industrial conditions and that an extensive parametric study is relevant to figure out the explosive behavior of solid/solid mixtures subjected to variations of operating conditions.     

Limiting oxygen concentration for coal dusts for explosion hazard analysis and safety

Investigation of explosion characteristics of coal dust was undertaken as a part of regular research program at CSIR-CBRI, Roorkee, India, for designing explosion safety measures for coal dust handling installations. This paper presents results of detailed experimental work on determination of Limiting Oxygen Concentration (LOC) and influence of reduced oxygen levels on explosion severity data for two types of coals with varying volatile matter as 27.18% (coal A) and 19.69% (coal B) from Jharia coalfield of India determined at ambient conditions with 20-L Spherical Vessel established at CSIR-CBRI. The effects of coal particle size and moisture content were evaluated. Data presented will be used for hazard analysis, designing explosion preventive measures, and explosion severity reduction by involving the use of inert gases for installations handling pulverized coal with similar nature. The importance of ignition source energy in determining LOC data is highlighted. The data collected lead to an extension of the current data for coal dusts as found in the literature. Limiting oxygen concentrations were found as 7% for coal A and 8% for coal B for the size representative to that used in pulverized coal boilers and moisture content ～4%.     

Explosibility parameters for mixtures of pulverized fuel and ash

This paper reports the results of experiments done to examine the explosibility of pulverized fuel dusts (two different coals and petroleum coke) and their waste products (bottom and fly ashes). Tests were conducted for the fuel dusts alone and also blended with other fuels and ashes. The explosion parameters of interest were maximum pressure and rate of pressure rise, and ignition energy and temperature. Petroleum coke was found to present a lesser explosion hazard than the coals. Admixture of either coal or petroleum coke with fly ash resulted in explosible mixtures at volatile contents dependent on the composition of the mixture components and their particle sizes.     

不同挥发分煤尘层最低着火温度变化规律研究

针对煤化工等行业的沉积煤尘热自燃问题,运用煤尘层最低着火温度测试系统,研究了不同挥发分煤尘层的着火状态、不同挥发分及不同厚度煤尘层最低着火温度的变化规律。结果表明:煤尘层厚度为5 mm时,挥发分质量分数大于35%的煤尘在较低温度便出现着火现象,肉眼很容易观察到火星的出现,温度曲线波动剧烈,而对于挥发分质量分数小于15%的煤尘,通过煤尘层内部"温度达到450℃"来判断其着火;在灰分质量分数相当的情况下,煤尘层最低着火温度随挥发分增加呈严格递减的趋势变化;得到了煤尘层厚度和最低着火温度的函数关系式,通过试验得到了挥发分质量分数为37.45%煤尘的重要常数M和N。     

腐朽坑木与煤混合物升温氧化特性研究

为研究煤层及腐朽坑木着火特性,采集巷道内腐朽坑木与煤样并进行混合,对其混合物进行燃点测定和程序升温实验;通过对不同煤木混合物在升温过程中产生气体的规律性进行分析,优选出混合物产生的单一气体指标及复合指标.研究结果表明:随朽木在混合物中的比例上升,混合物燃点呈下降趋势,且CO,C2 H4等指标气体出现更早,CO产生量更大...     

Numerical investigation and theoretical prediction of self-ignition characteristics of coarse coal stockpiles

Spontaneous combustion of coarse coal stockpiles in temporary coal storage yards was investigated numerically using COMSOL Multiphysics software. The main purposes of the numerical investigation were to identify the self-ignition characteristics of coarse coal stockpiles and formulate a theoretical model to predict the self-ignition time and locations of coarse coal piles. A mathematical model for self-ignition of coarse coal piles was developed and the process of spontaneous ignition of coarse coal stockpiles was simulated. The kinetic data of low-temperature oxidation reaction was obtained from the laboratory-scale experiments with bituminous coals taken from Jindi Coal Mine of Shanxi Province in China. The influence of moisture was ignored because the studied coal had low moisture content (mass concentration: 1.87%) and both coal and ambient environment were assumed to be saturated with moisture (or ambient environment was assumed to be dry). The effects of five variables (i.e. wind velocity, oxygen concentration, height, porosity, and side slope) on the spontaneous ignition in coarse coal piles were examined. Simultaneously, a theoretical prediction model was formulated in light of variable analyses and a great number of simulations.Compared to self-ignition characteristics of fine-particle coal piles, several self-ignition characteristics of coarse coal piles were identified by numerical investigation. Wind-driven forced convection plays a predominant role in self-heating of coarse coal piles. As wind velocity increases, the self-ignition location in the pile migrates from the lower part which is close to the surface of the windward side to the upper part near to the surface of the leeward side. Wind velocity increase exerts the positive or the negative effect on self-heating, which depends on a critical wind velocity value to sustain balances of both the heat and the availability of oxygen in the coarse coal pile. The behavior of self-ignition is remarkably sensitive to both oxygen concentration and height, and a coarse coal stockpile will not ignite spontaneously as long as one of two critical variable values is satisfied: oxygen concentration of 5% or height of 3 m. The theoretical prediction model suggests when and where countermeasures should be made to prevent the self-ignition in the coal stockpile with engineering accuracy.     

煤油共生矿区含油煤尘云最低着火温度试验研究

为研究煤油共生矿区含油煤尘最低着火温度的变化规律,选取3种含油浓度不同的煤样,采用粉尘云最低着火温度测定系统,研究含油煤尘云最低着火温度随含油浓度、喷尘压力及煤尘质量的变化规律。研究结果表明:含油煤尘的最低着火温度较不含油煤尘显著降低,且随着煤尘含油浓度的增加,煤尘中挥发分含量增多,煤尘云最低着火温度降低,爆炸危险性增强;低含油浓度煤尘,煤尘受原油挥发分影响较大,在含油浓度为5.7%,4.3%且质量浓度为1364~4550 g/m3时煤尘云最低着火温度随喷尘压力的增大呈先增大后减小的变化趋势,在5.7%,4.3%含油浓度且喷尘压力为0.05 MPa时煤尘MIT随煤尘质量浓度增加先降低后缓慢升高。高含油浓度煤尘,受煤尘团聚现象影响较大,煤尘云最低着火温度随喷尘压力的增加而升高,随煤尘质量的增加呈先减小后增大再缓慢减小的变化规律。     

自适应神经网络在确定落煤残存瓦斯量中的应用

落煤残存瓦斯量的确定是采掘工作面瓦斯涌出量预测的重要环节，它直接影响着采掘工作面瓦斯涌出量预测的精度，并与煤的变质程度、落煤粒度，原始瓦斯含量、暴露时间等影响因素呈非线性关系，人工神经网络具有表示任意非线性关系和学习的能力，是解决复杂非线性，不确定性和时变性问题的新思想和新方法，基于此，作提出自适应神经网络的落煤残丰瓦斯量预测模型，并结合不同矿井落煤残存瓦斯量的实际测定结果进行验证研究，结果表明，自适应调整权值的变步长BP神经网络模型预测精度高，收敛速度快，该预测模型的应用可为采掘工作面瓦斯涌出量的动态预测提供可靠的基础数据，为采掘工作面落煤残存瓦斯量的确定提出了一种全新的方法和思路。     

基于活化能指标的煤自燃倾向性及发火期研究

在煤的自燃及热分析动力学的基础上,结合对神东矿区3种煤样进行了热重实验,运用热重分析手段对煤从常温到燃点之间的氧化热解过程进行了研究。运用不同动力学机制模型函数分别对热重分析数据进行了处理和相关性分析,结果表明煤炭氧化热解过程符合一级化学反应动力学机制,据此求出活化能等动力学参数。对基于活化能指标的煤的自燃倾向性及自然发火期进行了初步研究。经过研究发现,该方法是科学的、客观的。     

煤中掺比伴生硫化物的自燃特性分析

为了深入探究矿井下伴生硫化物对煤自燃及着火燃烧特性的影响,向原煤中添加不同量的含硫物配制4种不同含硫量的煤样,通过TG实验、DSC测试和XRD分析,研究伴生硫化物对煤自燃及着火燃烧特性的影响规律;基于Coats-Redfern法计算煤中掺加不同伴生硫化物时煤燃烧阶段的活化能。研究结果表明:随着煤中掺比伴生硫化物的增多,煤的特征温度相应减小,而吸氧量、可燃和稳燃指数相应增大,原煤中混入伴生硫化物后更易自燃;随着煤中掺比伴生硫化物的增多,煤燃烧阶段的活化能降低,煤更易着火燃烧;伴生硫化物的主要成分为水绿矾、叶绿矾,这些物质在常温下遇水和氧气能够发生化学循环反应,反应放热促使了煤更易自燃;伴生硫化物在温度高于200℃以后整体表现为放热,在温度为565℃时达到放热峰值,这使得煤燃烧阶段的活化能降低,煤更易燃烧。     

基于热爆炸理论的煤燃点确定及动力学分析

为准确判断煤燃点,提高煤自燃灾害防治能力,依据热爆炸理论,结合煤自燃过程放热曲线,将煤自燃升温过程中微分热流曲线上第1处极小值点作为煤的燃点,计算煤着火前后放热过程动力学参数变化.结果表明:随升温速率增加,煤自燃反应放热过程逐渐向高温区域移动,煤燃点逐渐增大,反应的活化能逐渐减小;同一升温速率下燃点之后煤的活化能增大;...     

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。     

When solids meet solids: A glimpse into dust mixture explosions

Mixing an inert solid or a less flammable compound with a combustible dust can be regarded as a direct application of the inherent safety principle of moderation. An experimental investigation was carried out to determine the evolution of the ignition sensitivity and the explosion severity of such various mixtures as a function of their compositions. It demonstrates that the introduction of small amounts of highly combustible powders (such as sulphur or nicotinic acid) to a less flammable dust (such as microcrystalline cellulose or carbon black) can strongly influence the ignition sensitivity as well as the explosion severity.It has notably been shown that the ignition sensitivity of solid/solid mixtures significantly rises up when only 10–5%wt. of highly flammable dust is introduced. Simple models can often be applied to estimate the minimum ignition energy, minimum ignition temperature and minimum explosive concentration of such mixtures. Concerning the dust explosivity, three cases have been studied: mixtures of combustibles dusts without reaction, dusts with reactions between the powders, combustible dusts with inert solid. If the evolution of the maximum explosion pressure can be estimated by using thermodynamic calculations, the maximum rate of pressure rise is more difficult to predict with simple models, and both combustion kinetics and hydrodynamics of the dust clouds should be taken into account. These results were also extended to flammable dust/solid inertant mixture. They clearly show that the concentration of solid inertant at which the ignition is not observed anymore could reach 95%wt. As a consequence, the common recommendation of solid inertant introduction up to 50–80%wt. to prevent dust explosion/ignition should be reconsidered.     

煤低温氧化的实验研究及动力学分析

运用热分析的技术研究阳泉煤的氧化热解反应。将实验煤样进行连续加热氧化到一定温度再恒温和分别加热到一定温度然后恒温这两类实验,对比分析该煤样的低温氧化受热情况并进行动力学分析。得出:两类实验中的热重曲线位置的改变陡缓度不一样,试样量越大,气体的扩散阻力越大,相应的热重曲线位置的改变也越显著;煤低温氧化属于一级化学反应,煤低温氧化反应的活化能随着煤的反应过程的深入而增加,而且煤氧化反应过程是个分阶段的、多步反应以及相互联系促进的过程。     

Influence of coal particles on methane/air mixture ignition in a heated environment

The temperature at which coal dust glows is normally much lower than the auto-ignition temperature (AIT) of methane/air mixtures, and thus a better understanding is needed regarding methane/air ignition in a heated environment in the presence of coal particles. A horizontal tube apparatus was used to test the effect of brown coal and two kinds of bituminous and anthracite on methane/air combustibility. For the four coal samples tested, the presence of coal particles significantly reduced the minimum temperature for ignition of methane/air mixtures in a heated environment. No. 1 bituminous coal with 12 mm diameter decreased the ignition temperature value from 595 to 500 °C. It is thought that pre-ignition of low-AIT volatiles emitted from the heated coal particles ignited the methane/air mixtures. Volatiles, sulfur content, and large porosity of piled coal particles all enhanced ignition of methane/air mixtures in a hot environment, while water content and small particle size reduced ignition. For anthracite, no ignition occurred when temperatures of the heated environment were lower than the AIT of methane (595 °C), except for the 12-mm-diameter sample. Anthracite did not readily ignite methane/air mixtures and the ignition mechanism was somewhat similar to that of a burning cigarette.     

Risk assessment of the ignitability and explosivity of aluminum nanopowders

Previously, an extensive study has been carried out in order to assess the ignition sensitivity and explosivity of aluminum nanopowders. It showed notably that, as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease, indicating higher potential inflammation. However, the explosion severity decreases for diameters lower than 1 μm. As a consequence, this study leads to the conclusions that the ignition sensitivity and explosion severity of aluminum nanopowders may be affected by various phenomena, as pre-ignition, agglomeration/aggregation degree and the intrinsic alumina content. The presence of wall-quenching effects and the predominance of radiation compared to conduction in the flame propagation process have to be discussed to ensure the validity of the 20 L sphere and of the results extrapolation. Based on the peculiar behaviours that had been previously highlighted, a specific risk analysis has been developed in order to assess the fire and explosion risks of such materials. It has been applied to an industrial plant of aluminum nanopowders production. The hazard identification and the consequence modelling steps, especially the quantification of the likelihood and consequences, have been designed specifically. The application of this method has led to the definition of the most adequate safety barriers.     

矿井下着火点位置数值模拟研究

矿井煤炭自燃是煤矿五大自然灾害之一,而煤炭自燃隐蔽着火点位置的确定是解决煤炭自燃问题的关键。本文结合实际情况将火源位置的确定问题归结为热传导方程的寻源反问题。在理想状态下,把煤矿井下热传导的三维模型简化为二维模型,建立热传导方程及初始条件和边界条件,在matlab中编制了有限差分程序对井下着火点的温度场进行了正反演模拟。数值模拟结果表明:该方法能够较准确地反演出火源的特性,并随着离散化程度的提高,离散解逐渐逼近真实解。通过本文的数值模拟我们得知有限差分法求解热传导寻源反演的方法是解决矿井隐蔽火源发火点位置的有效途径之一,对矿井防、灭火研究具有较高的理论和实际应用价值。     

煤粉燃烧性与工业分析、着火温度的相关性研究

喷吹煤粉燃烧性是生产中的一个重要参数,实时掌握原煤燃烧性的波动变化对保证喷煤的安全和稳定十分必要。直接测量煤粉燃烧性的方法成本高,时间长,操作复杂,不利于生产应用;采用测定煤的工业分析和着火温度的方法简单易行,应用范围广泛。该文针对某炼铁厂的15种原煤,进行了工业分析和着火温度的实验研究,引入了统计学中相关系数的概念对数据进行了处理分析,实验证明煤的挥发分和着火温度与煤粉的燃烧性高度线性相关,此方法为利用两参数间接分析煤粉燃烧性提供了理论基础。     

瓦斯对煤尘爆炸特性影响的实验研究

瓦斯的存在对煤尘爆炸特性的理论计算和数值仿真的结果与实际数据有一定差距,因此,通过不同浓度瓦斯与煤尘共存条件下爆炸实验研究,得出了矿井瓦斯对煤尘的最低着火温度、最小点火能量、爆炸下限浓度、最大爆炸压力和最大爆炸压力上升速度等爆炸特性影响的规律即瓦斯对煤尘最低着火温度影响不大;瓦斯可使煤尘的最小点火能量减小,尤其是对难于点燃的煤尘;混合物的爆炸下限浓度随瓦斯浓度的增加而降低;混合物的最大爆炸压力上升速度由于瓦斯的存在而增强,而最大爆炸压力几乎没有变化。同时研究了瓦斯对无爆炸性煤尘的影响。实验研究的结论对于现场防止煤尘爆炸的发生具有指导意义。     

煤岩混合型粉尘云最低着火温度试验研究

为研究半煤岩巷道中岩粉质量分数和煤的挥发分与煤岩混合型粉尘云最低着火温度的关系,选取挥发分差异较大的5种煤样以相同比例配制煤岩混合型粉尘,利用粉尘云最小点火温度测定仪进行煤岩混合型粉尘试验。结果表明,当煤岩混合型粉尘中岩粉质量分数低于40%时,岩粉的混合会导致混合型粉尘云最低着火温度发生小幅度波动;当岩粉质量分数高于40%时,煤岩混合型粉尘最低着火温度会随岩粉质量分数的增加而大幅度升高;挥发分质量分数越小的煤粉,其混合型粉尘云最低着火温度越容易受岩粉质量分数的影响。