Assessment of the thermal hazards and oxidization mechanism of coloured corn starch dust by TG–FTIR

Starch is widely used in industrial production and in every life, and an increasing number of accidents of starch dust burning and explosions are occurring and have caused serious casualties and economic losses. Previous studies on the oxidative properties and microscopic characterization of coloured corn starch dust have been less systematic than the present study. To prevent coloured corn starch dust explosion accidents more effectively, thermogravimetry and Fourier transform infrared spectroscopy were applied to study the oxidation characteristics of coloured corn starch dust. Seven characteristic temperatures were determined from the thermogravimetric curves and derivative thermogravimetric curves of coloured corn starch dust. The entire oxidation process of coloured corn starch dust was divided into five stages, and a 60% mass loss occurred in the rapid oxidation stage. Three iso-conversion methods were used to calculate the apparent activation energy (E_a) and pre-exponential factor (A) at different oxidation stages. The value of E_a was found to be related to the difficulty of the reaction, and it had a positive correlation with lnA. Six kinds of gases were detected during the oxidation process. The oxidation mechanism was further analysed by the macro and micro characterization of the oxidation process. The findings provide a theoretical basis for preventing and controlling explosion accidents that involve coloured corn starch dust.     

Modeling of maize starch explosions in a 12 m3 silo

This paper presents a 2-dimensional numerical model of Eulerian–Lagrangian multi-phase combustion flow to predict maize starch explosions in a 12 m³ silo. The flow field after ignition, flame propagation velocity and pressure development histories etc. during the explosion, are calculated. The data of non-uniform initial conditions including dust concentration, flow velocity and turbulent RMS velocity in the silo for this model are adopted from Hauert, Vogl and Radandt (1994) [Hauert, F., Vogl, A., Radandt, S. (1994). Measurement of turbulence and dust concentration in silos and vessels. 6th international colloquium on dust explosions (pp. 71–80), Shenyang, China, August 28–September 2, 1994.]. A simple concept of dust granule taking into consideration dust dispersion efficiency is proposed and introduced. The Lagrangian method is used to trace trajectories and granules, so it is easier to consider particle size distribution. The k−ε model is used to simulate the turbulence of the gas phase, and the particle's pulsation is modeled by random vector wind generated by the surrounding gas. In the combustion model, vaporization of water, volatilization of volatile, gas phase reaction and the particle's surface reaction are taken into account.     

Detailed analysis of flame propagation during dust explosions by UV band observations

It is important to sufficiently understand the phenomena during the dust explosions in order to take appropriate measures preventing dust explosion accidents. However, at present basic knowledge on flame propagation mechanisms during dust explosions is not enough. In this study, therefore, the flame propagation mechanisms during dust explosions are examined by detailed analyses using a special observation at UV band. Small scale experiments were performed to analyze flame propagating processes in detail. In the experiments, the stearic acid was used as the combustible particle, suspended particles were ignited by an electric spark, and flame propagation through the combustible dust was observed by using a special observation system at UV band. The leading combustion zone is observed to consist of discrete burning blue spot flames by the observation using ordinary photograph system. It is questionable how the leading flame of such discrete structure propagates. In this study, high-speed video images at UV band through a band-pass filter were taken to detect OH emission from combustion reaction zone. Using this method, the propagating flame could be detected clearly and the flame propagation mechanism could be examined in detail. In the conditions performed in this study, discrete flame propagation was not observed and the leading flame was observed to propagate continuously. This result is of importance for understanding the flame propagation phenomena during dust explosion.     

Experimental determination of dust cloud deflagration parameters of selected hydrogen storage materials: Complex metal hydrides,chemical hydrides,and adsorbents

This paper discusses the results of an experimental program carried out to determine dust cloud deflagration parameters of selected solid-state hydrogen storage materials, including complex metal hydrides (sodium alanate and lithium borohydride/magnesium hydride mixture), chemical hydrides (alane and ammonia borane) and activated carbon (Maxsorb, AX-21). The measured parameters include maximum deflagration pressure rise, maximum rate of pressure rise, minimum ignition temperature, minimum ignition energy and minimum explosible concentration. The calculated explosion indexes include volume-normalized maximum rate of pressure rise (K_St), explosion severity (ES) and ignition sensitivity (IS). The deflagration parameters of Pittsburgh seam coal dust and Lycopodium spores (reference materials) are also measured. The results show that activated carbon is the safest hydrogen storage media among the examined materials. Ammonia borane is unsafe to use because of the high explosibility of its dust. The core insights of this contribution are useful for quantifying the risks associated with use of these materials for on-board systems in light-duty fuel cell-powered vehicles and for supporting the development of hydrogen safety codes and standards. These insights are also critical for designing adequate safety features such as explosion relief venting and isolation devices and for supplementing missing data in materials safety data sheets.     

Propagation and extinction of dust flames in narrow channels

This article has investigated the propagation and extinction of aluminum dust cloud flame in a narrow channel. The burned and burning dust particles act as heat sources and the channel walls act as heat sinks. In this method, discrete heat source has been used to analyze dust combustion in a narrow channel. Using the superposition of sources and sinks, the preheat zone temperature is predicted as an indicator of flame propagation or extinction. Dust concentration and channel width are two major parameters which affect the quenching distance and flame propagating speed. Wall temperature affects the heat loss; and by preheating the walls, quenching distance is reduced and flame propagation speed is increased.     

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.     

Initiation of dust explosions by electric spark discharges triggered by the explosive dust cloud itself

An investigation of ignition of dust clouds by the use of electric spark discharges triggered by the explosive dust cloud itself has been conducted. This method of triggering capacitive sparks probably represents a realistic mechanism for initiating accidental dust explosions in industrial practice. Unlike the conventional method for determining the minimum ignition energy (MIE) in the laboratory, the delay between dust dispersion and spark discharge is not a degree of freedom. In stead, the transient dust cloud itself is used to initiate spark breakdown between electrodes set at a high voltage lower than breakdown in pure air. In the present study, different kinds of dusts were tested as ‘spark triggers’, and they exhibited quite different abilities to trigger breakdown. Large particles were found to initiate breakdown at lower voltages than smaller ones. In general, conductive particles were not found to initiate breakdown at lower voltages than dielectric ones when using the same dust concentration.Minimum ignition energies (MIE) of three dusts (Lycopodium clavatum, sulphur and maize starch) were determined using the authors' method of study. The MIEs were somewhat higher than those obtained using conventional methods, but relatively close to the values obtained through conventional methods.     

玉米淀粉粉尘爆炸危险性研究

为研究玉米淀粉粉尘爆炸危险性,采用哈特曼管式爆炸测试装置和20 L球爆炸测试装置对200目(<75μm)以下的玉米淀粉粉尘爆炸危险性进行评估,基于静电火花和粉尘质量浓度对粉尘爆炸的影响,对玉米淀粉的静电火花最小点火能量、爆炸下限质量浓度、最大爆炸压力和爆炸指数进行了研究,根据试验结果对玉米淀粉爆炸危险性进行分级。试验结果表明:温度在25℃,喷粉压力为0.80 MPa,粉尘质量浓度在250～750 g/m3范围内,粉尘的最小点火能量随着粉尘质量浓度增加而降低,其最小点火能量在40～80 mJ之间;在点火能量为10 kJ时,粉尘爆炸下限质量浓度在50～60 g/m3之间;在粉尘质量浓度为750 g/m3时,爆炸压力达到最大,为0.66 MPa;在粉尘质量浓度为500 g/m3时,爆炸指数达到最大,为17.21 MPa.m/s,其粉尘爆炸危险性分级为Ⅰ级。     

隔爆型电机在火炸药危险场所应用的安全性分析

针对普通隔爆型电机应用于火炸药粉尘危险场所的安全性问题进行定量分析,设计壳体粉尘侵入量试验,系统研究不同间隙下的粉尘侵入量,根据试验数据,应用曲线外推法及阿贝尔(Abel)余容状态方程计算爆压,结合材料力学及薄壁理论进行隔爆型电气设备外壳强度及刚度的校核。计算表明:若壳体内粉尘较均匀的悬浮在空中,切向应力与许用应力处于同一数量级;如果火炸药粉尘在轴承室、接线盒等局部堆积成火炸药层,切向应力比许用应力大两个数量级。试验结果强调:用于火炸药粉尘危险场所的电气设备必须有特殊的防爆结构设计,普通的隔爆电机用于火炸药粉尘危险环境时存在一定的安全隐患。     

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.     

外部条件对玉米淀粉粉尘爆炸特性的影响

为了探明外部条件对玉米淀粉粉尘爆炸特性参数的影响,利用20 L球形爆炸装置进行试验测试,探讨了点火能量及粉尘含水量对粉尘爆炸特性的影响,对比研究了CaCO_3和Al(OH)_3两种惰性介质的抑爆效果。结果表明:随点火能量增加,粉尘最大爆炸压力和最大升压速率呈线性上升,在高质量浓度下,粉尘爆炸压力受点火能量的影响更显著;添加CaCO_3和Al(OH)_3能够降低玉米淀粉的爆炸压力,相对于CaCO_3的物理抑爆,Al(OH)_3的物理-化学抑爆效果更佳;玉米淀粉粉尘的最大爆炸压力及爆炸升压速率随粉尘含水量降低而不断增大。     

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.     

Applications of dust explosion hazard and disaster prevention technology

Powdered materials are widely used in industrial processes, chemical processing, and nanoscience. Because most flammable powders and chemicals are not pure substances, their flammability and self-heating characteristics cannot be accurately identified using safety data sheets. Therefore, site staff can easily underestimate the risks they pose. Flammable dust accidents are frequent and force industrial process managers to pay attention to the characteristics of flammable powders and create inherently safer designs.This study verified that although the flammable powders used by petrochemical plants have been tested, some powders have different minimum ignition energies (MIEs) before and after drying, whereas some of the powders are released of flammable gases. These hazard characteristics are usually neglected, leading to the neglect of preventive parameters for fires and explosions, such as dust particle size specified by NFPA-654, MIE, the minimum ignition temperature of the dust cloud, the minimum ignition temperature of the dust layer, and limiting oxygen concentration. Unless these parameters are fully integrated into process hazard analysis and process safety management, the risks cannot be fully identified, and the reliability of process hazard analysis cannot be improved to facilitate the development of appropriate countermeasures. Preventing the underestimation of process risk severity due to the fire and explosion parameters of unknown flammable dusts and overestimation of existing safety measures is crucial for effective accident prevention.     

Effect of ignition delay on explosion parameters of corn dust/air in confined chamber

This paper presents the explosion parameters of corn dust/air mixtures in confined chamber. The measurements were conducted in a setup which comprises a 5 L explosion chamber, a dust dispersion sub-system, and a transient pressure measurement sub-system. The influences of the ignition delay on the pressure and the rate of pressure rise for the dust/air explosion have been discussed based on the experimental data. It is found that at the lower concentrations, the explosion pressure and the rate of pressure rise of corn dust/air mixtures decrease as the ignition delay increases from 60 ms; But at the higher concentrations, the explosion pressure and the rate of pressure rise increase slightly as the ignition delay increases from 60 ms to 80 ms, and decrease beyond 80 ms. The maximum explosion pressure of corn dust/air mixtures reaches its highest value equal to 0.79 MPa at the concentration of 1000 gm⁻³.     

基于正交试验的谷物粉尘燃烧特性影响因素研究

为了研究谷物粉尘燃烧特性影响因素，以可食用玉米淀粉为研究对象，通过ZY6243锥形量热仪（CONE）进行了单因素和正交试验，研究了粉尘粒径、惰性介质CaCO3质量分数、热辐射通量对玉米淀粉燃烧特性的影响效应。应用直观分析法和方差分析法定性地研究了各因素对玉米淀粉热释放速率峰值的影响程度并进行了排序，2种方法得出的结论基本一致。研究结果表明:各因素对玉米淀粉热释放速率峰值的影响程度依次为:CaCO3质量分数＞粉尘粒径＞热辐射通量；其中惰性介质CaCO3对热释放速率峰值和引燃时间的影响最为显著。     

Flame propagation in hybrid mixture of coal dust and methane

To investigate the flame propagation through hybrid mixture of coal dust and methane in a combustion chamber, a high-speed video camera with a microscopic lens and a Schlieren optical system were used to record the flame propagation process and to obtain the direct light emission photographs. Flame temperature was detected by a fine thermocouple. The suspended coal dust in the mixture of methane and air was ignited by an electric spark. The flame propagation speeds and maximum flame temperatures of the mixture were analyzed. The results show that the co-presence of coal dust and methane improves the flame propagation speed and maximum flame temperature notably, which become much higher than that of the single-coal dust flame. The flame front temperature varies with the coal dust concentration.     

基于气体浓度和烟颗粒消光系数的复合火灾预警系统

为解决火灾烟雾颗粒的消光效应对光声火灾检测带来的影响,以近红外激光为光源,结合波长调制技术和谐波检测技术对光声信号和颗粒消光系数进行了测量,并建立了火灾复合探测模型。利用棉绳阴燃、木材热解2组阴燃火实验以及正庚烷明火、木材明火2组明火实验进行了火灾模拟探测,并结合特定算法对结果进行了判定。结果表明系统能较好地对实验产生的CO浓度和烟颗粒消光系数进行探测,且运行稳定,能够满足火灾预警需求。     

碳纤维复合材料粉尘爆炸强度特性研究

碳纤维复合材料是应用于航天、航空领域的高性能材料之一,对于该材料的粉尘爆炸特性还未有相关研究报告。为了研究碳纤维复合材料粉尘的爆炸强度特性,本文采用20L球形粉尘爆炸测试实验系统开展了相关实验研究。实验测得碳纤维复合材料粉尘爆炸下限浓度为50g/m 3,最大爆炸压力为0.48MPa。在测试浓度范围内,最大压力上升速率和爆炸指数均随浓度的增大而变大。另外,在其爆炸强度特性研究的基础上,对产尘车间的环境风险进行了初步辨识,提出了相应的防护措施。本文的研究成果对此类碳纤维复合材料粉尘的工业防护具有实际的指导作用,对于该粉尘的爆炸机理的深入研究也具有一定的参考价值。     

Experimental determination of dust cloud combustion parameters of α-AlH3 powder in its charged and fully discharged states for H2 storage applications

This study discusses results of an experimental program for determination of dust cloud combustion parameters of charged and fully discharged states of metastable alane (aluminum hydride, α-AlH₃ polymorph) powder in air. The measured characterization parameters include: maximum deflagration pressure rise (ΔP_MAX), maximum rate of pressure rise (dP/dt)_MAX, minimum ignition temperature (T_C), minimum explosible concentration (MEC), and minimum ignition energy (MIE). These measured values are used for calculating the associated explosion severity (ES) index, and volume-normalized maximum rate of pressure rise (K_St). The experimental results show values of MEC and T_C of fully discharged alane to be greater than those of the charged alane but measured MIE values are about the same. Moreover, the results show higher reactivity of fully discharged alane dust cloud in air compared to its charged state. For example, ES and K_St of discharged alane dust cloud in air are about 300% and 35% greater, respectively, than ES and K_St of charged alane dust. The higher air reactivity of fully-discharged (primarily Al powder) dust cloud compared to its charged state can be attributed to the higher surface energy (J/m²) of Al compared to that of α-AlH₃. These experimental insights have safety implications in postulated risk scenarios involving light-duty vehicles powered by PEM fuel cells. The core insights and critical data provided by this contribution are useful for supporting development and promulgation of hydrogen safety standards and augmenting property databases of hydrogen storage materials.     

皮带输煤暗道通风排尘改造方案优化研究

为了解决皮带输煤暗道运输过程中粉尘污染严重及冬季供暖不足问题,对暗道通风供热系统进行了优化改造研究。以中煤平朔安家岭264输煤暗道为例,基于气固两相流理论,利用CFD模拟方法,对暗道内由风速对悬浮粉尘浓度分布的影响规律进行数值仿真。模拟结果显示:当输煤暗道排尘风速由0.25 m/s升至0.35 m/s时,粉尘浓度降为3.2 mg/m3,暗道最大通风阻力为245.94 Pa,最低换热量747.68 kW。基于数值模拟结果,从3种优化方案中选定一压一抽联合式通风除尘方案进行改造,现场实测除尘效果良好,表明所提出的输煤暗道通风除尘方案具有实践指导意义。