Bituminous Coal Dust Research Articles

A New Method to Evaluate the Suitability of Ignitor Strength Used in the Tests of Dust Explosions

Based on systematic experiments in the 20-L Siwek sphere, the explosion behaviors of bituminous coal dust were investigated by changing the strength of pyrotechnic ignitor. The effect on the maximum explosion pressure (Pex) and maximum rate of pressure rise ((dp/dt)ex) of the ignitor strength is analyzed. Due to the under-or over-driven phenomena, the overly low or high ignitor strength may result in unrealisticPexand (dp/dt)ex, especially for lower dust concentration. To evaluate the suitability of ignitor strength, the time scales (i.e. the effective action time,teff, and the induction time,ti,) of the ignitor combustion and flame kernel development are introduced by analyzing the ignition process of dust cloud. It can be found that theteffsignificantly heightens and thetigradually decreases with the rise of ignitor strength. Moreover, when theteffis much lower than theti, the under-driven phenomenon will occur, which leads to the underestimatedPexand (dp/dt)ex. Conversely, when theteffis much higher than theti, the explosion tests may be over-driven to cause the overestimatedPexand (dp/dt)ex. For ideal ignitor strength, theteffwould be comparable to theti. Based on these facts, the under-and over-driven phenomena can be effectively verified by comparing theteffand theti. As a result, when the dimensionless time ratioteff/tiis 0.52 or 1.42, the explosion tests in the 20-L sphere would be, respectively, under-driven or over-driven. To obtain the realistic test results in the 20-L sphere, it is suggested that theteff/tivalue should be in the range of 0.8~1.2 as much as possible.

Experimental Investigation on Inerting Mechanism of Dust Explosion

Systematic experiments were carried out on explosions of coal dust-inertant mixtures. Explosion severity was gained by using Siwek 20 L vessel and factors strongly influencing inerting effectiveness, such as deflagration mechanism and inertant composition were taken into account. Tests of thermal-gravimetric analysis were conducted to identify the reaction mechanism of coal dust explosions. As a result, bituminous coal dust deflagrates via the homogeneous mechanism, while anthracite dust mainly features the mechanism of heterogeneous reaction. Results reveal that the inerting functional mechanisms are quite different for bituminous coal and anthracite dusts, and the homogeneous-combustion-dust is much easier to be inhibited than the heterogeneous-reaction-dust. Trends of higher inerting effectiveness are associated with better decomposability, more excellent performance of oxygen blocking. Moreover, the particular efficacy of free ammonia decomposed from monoammonium phosphate in flame extinguishing is identified. Added water shows the best inerting effectives due to particle aggregation induced by moisture, which gives the idea that humidification for coal dust is therefore a cost-effective approach for explosion prevention and mitigation.

The Dust Explosion Characteristics of Coal Dust in an Oxygen Enriched Atmosphere

Abstract The ability to mix pulverised coal with oxygen at concentrations greater than the currently applied 21% may well provide advantages for burner design in oxy/coal fired systems. However the risk of dust explosions increases significantly with increasing oxygen concentration and temperature. In this study the influence of enriched oxygen concentrations is researched on the dust explosion characteristics of Indonesian (Sebuku) high volatile bituminous coal dust and on Pittsburgh Coal ni8. Both ignition sensitivity characteristics (minimum ignition energy and minimum ignition temperatures) and explosion severity characteristics (maximum explosion pressure, Pmax, and maximum rate of pressure rise, Kst) are investigated.

An experimental study of coal dust ignition in wedge shaped hot plate configurations

Ignition behavior of bituminous coal dust deposited between two hot surfaces forming wedges of 60°, and 90° is experimentally studied. Three thermocouples placed along the symmetry plane of the wedge cross-section at various heights from the apex (lowermost, middle and top) are used to record the transient temperature data. Results show that ignition always occurs around the region surrounding the top thermocouple in the case of 60° wedge and both the top and the middle regions in the case of 90° wedge. The trends are explained by investigating three parameters affecting the ignition behavior, namely, the heat transfer from the hot plate to the coal dust, the subsequent chemical heat release, and the heat transfer between different regions within the coal dust. Furthermore, an experimental setup, similar to the standard ASTM E 2021 test, is used to determine the minimum ignition temperatures of coal layers of various thicknesses and to assess the ignition scenario in the wedge configuration using an effective length scale. Measured quantities such as minimum hot plate temperature that causes ignition and the ignition time, as well as the transient heat release rate and heat conducted between various zones, calculated based on the temperature data are used to quantify the three parameters and their effects on ignition behavior.

시멘트 제조공정에서 유연탄 분진의 폭발특성

시멘트제조 공정에 사용하는 유연탄 분진의 폭발특성을 조사하기 위하여 시료의 열적특성과 폭발 실험을 수행하였다. 열적특성은 열중량 분석기(TGA)와 시차주사 열량계(DSC)을 이용하여 온도에 따른 무게 감량과 발열량을 측정하였다. 또 입도별 발화온도와 비표면적을 합께 조사하였다. 하트만식 폭발실험 장치를 직접 제작하여 분진의 농도를 변화시켜가며 분진폭발 실험을 수행하였다. TGA, DSC 및 발화온도 분석결과 실험범위의 입도에 따른 열적 특성은 큰 변화가 없었지만, 비표면적은 입도가 작을수록 큰 것을 확인하였다. 폭발실험에서 폭발확률은 입도가 감소하고 농도가 증가할수록 증가하는 경향이 있었으며, 입도별 최저 폭발하한계 농도를 함께 구하였다. 시멘트 제조공정에 표준적으로 사용하는170/200mesh에서 최저 하한계 농도는 $0.3mg/cm^3$ 이었으며 $0.9mg/cm^3$ 이상에서는 100% 확률로 폭발이 발생하였다. 【We have examined explosion characteristics of bituminous coal dusts in cement manufacturing process. In order to find the thermal properties, we investigated weight loss and ignition temperature of coal materials using TGA and DSC. Also specific surface area of dust was investigated. Dust explosion experiments with Hartman's dust explosion apparatus have been conducted by varying concentration and size of coal dust for explosion probability and lower limit explosion concentration. According to the results for thermal properties, there is a little change by dust size. However, the specific surface area of dust is increased by decreasing dust size. The explosion test results show that small size and increasing concentration of dusts make dust explosion easier. And we find that the lower limit explosion concentration of bituminous coal is $0.3mg/cm^3$ and the probability is 100% on $0.9mg/cm^3$ in 170/200 mesh used in cement manufacturing process.】

Study on mutagenicity of bituminous coal, anthracite and brown coal dust

目的 探讨烟煤尘、无烟煤尘及褐煤尘的致突变作用.方法 分别用烟煤、无烟煤和褐煤粉尘的二甲基亚砜混悬液,作鼠伤寒沙门氏菌回复突变(Ames)和姐妹染色单体交换(SCE)试验.Ames试验采用TA97、TA98、TA100和TA102菌株,并将3种煤尘各分为5000、500、50和5 μg/皿4个剂量组;SCE试验采用健康成人外周血淋巴细胞,煤尘样品分为500、50和5 μg/皿3个剂量组.各剂量组均设3个平行样.试验分为加与不加体外代谢活化系统(S9)两部分,同时设阴性和阳性对照组.Ames试验在细菌培养(37℃)48 h后计算回变菌落数;SCE试验在标本37℃培养72 h后制成染色体标本玻片,油镜下观察计数细胞第二增殖周期分裂相的SCE频率.结果 3种煤尘在加S9与不加S9时,其菌落数均不超过自发回变菌落数的2倍,而且均不能使SCE频率明显升高.结论 在本实验条件下,烟煤尘、无烟煤尘及褐煤尘均未见致突变作用。

High Exposure to Respirable Dust and Quartz in a Labour-intensive Coal Mine in Tanzania

Labour-intensive mines are numerous in several developing countries, but dust exposure in such mines has not been adequately characterized. The aim of this study was to identify and quantify the determinants of respirable dust and quartz exposure among underground coal mine workers in Tanzania. Personal respirable dust samples (n = 134) were collected from 90 underground workers in June-August 2003 and July-August 2004. The development team had higher exposure to respirable dust and quartz (geometric means 1.80 and 0.073 mg m(-3), respectively) than the mining team (0.47 and 0.013 mg m(-3)), the underground transport team (0.14 and 0.006 mg m(-3)) and the underground maintenance team (0.58 and 0.016 mg m(-3)). The percentages of samples above the threshold limit values (TLVs) of 0.9 mg m(-3) for respirable bituminous coal dust and 0.05 mg m(-3) for respirable quartz, respectively, were higher in the development team (55 and 47%) than in the mining team (20 and 9%). No sample for the underground transport team exceeded the TLV. Drilling in the development was the work task associated with the highest exposure to respirable dust and quartz (17.37 and 0.611 mg m(-3), respectively). Exposure models were constructed using multiple regression model analysis, with log-transformed data on either respirable dust or quartz as the dependent variable and tasks performed as the independent variables. The models for the development section showed that blasting and pneumatic drilling times were major determinants of respirable dust and quartz, explaining 45.2 and 40.7% of the variance, respectively. In the mining team, only blasting significantly determined respirable dust. Immediate actions for improvements are suggested to include implementing effective dust control together with improved training and education programmes for the workers. Dust and quartz in this underground mine should be controlled by giving priority to workers performing drilling and blasting in the development sections of the mine.

Continuous wave laser ignition thresholds of coal dust clouds

Laser-based instruments are used in areas where coal dust ignition presents a safety hazard. The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory (NIOSH, PRL) is conducting a study to help determine when an optical beam may be considered a potential ignition source in underground coal mines or coal storage facilities. Researchers conducted experiments to determine threshold igniting powers for Pittsburgh seam bituminous coal dust clouds using an 803 nm continuous wave laser. For fine sized coal dust dispersed in air, concentrations ranging from 600 to 2000 g/m3 were the most easily ignited. A heated layer of coal dust that deposited on the fiber tip during dispersion ignited the dust clouds. Minimum observed igniting powers for laser beams delivered by 200, 400, and 800 μm core fiber optic cables and directed into coal dust clouds were 2.0, 3.0, and 5.0 W, respectively. Threshold igniting power was proportional to beam diameter, and threshold igniting power density decreased with larger fiber diameters. Ignition delay times averaged 0.6 s and did not vary significantly with laser power under initially turbulent test conditions and with flammable targets. Comparisons are made with the results of other researchers.

Flammability limit measurements for dusts in 20-L and 1-m3 vessels

Abstract Two types of flammability limits have been measured for various dusts in the Fike 1-m3 (1000-L) chamber and in the Pittsburgh Research Laboratory (PRL) 20-L chamber. The first limit is the minimum explosible concentration (MEC), which was measured at several ignition energies. In addition to the three dusts studied previously (bituminous coal, anthracite coal, and gilsonite), this work continues the effort by adding three additional dusts: RoRo93, lycopodium, and iron powder. These materials were chosen to extend the testing to non-coal materials as well as to a metallic dust. The new MEC data corroborate the previous observations that very strong ignitors can overdrive the ignition in the smaller 20-L chamber. Recommendations are given in regard to appropriate ignition energies to be used in the two chambers. The study also considered the other limiting component, oxygen. Limiting oxygen concentration (LOC) testing was performed in the same 20-L and 1-m3 vessels for gilsonite, bituminous coal, RoRo93, and aluminum dusts. The objective was to establish the protocol for testing at different volumes. A limited investigation was made into overdriving in the 20-L vessel. The LOC results tended to show slightly lower results for the smaller test volume. The results indicated that overdriving could occur and that ignition energies of 2.5 kJ in the 20-L vessel would yield comparable results to those in the 1-m3 vessel using 10.0 kJ. The studies also illustrate the importance of dust concentration on LOC determinations.

Briquetting of Afşin-Elbistan Lignite

In this study, low-quality Afşin-Elbistan lignite was mixed with a high-quality Siberian bituminous coal and optimum briquetting conditions were investigated for blends with and without binder materials. The blends of lignite and bituminous coal were prepared to contain 30, 40, and 50% bituminous coal and briquetted under pressures of 566 MPa and 708 MPa. The blends of lignite and bituminous coal were also briquetted using sunflower shell, sawdust and molasses as additives. In these experiments, the percentage of the lignite in the blends was fixed at 50%, so that the sum of the percentages of the bituminous coal and binder additive was always 50%. In this way, the low-grade lignite and high-quality bituminous coal dust are converted into a compact and stable fuel which has a higher quality.

Surface composition of respirable silica particles in a set of U.S. anthracite and bituminous coal mine dusts

Respirable particles of high-percentage silica content from anthracite and bituminous coal mine dust samples were analyzed for aluminosilicate clay surface coating, by measuring silicon and aluminum X-ray spectra using scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDS). Silicon and aluminum elemental ratios were determined with incident electron energies of 20 and 5 keV to reveal whether surface occlusion was present. Some 20 respirable-sized, non-agglomerated particles with silicon fraction of signal of 75% or more (for elements above sodium) were analyzed for each of 12 coal mine dust samples. Mine dust samples were from U.S. anthracite and bituminous coal mining regions involved in epidemiological studies of the U.S. National Study of Coal Workers' Pneumoconiosis. Some particles of high-percentage silica content exhibited a decrease in the ratio of silicon-to-aluminum K-alpha line intensities with decreasing electron beam accelerating voltage, consistent with aluminosilicate clay surface contamination or occlusion of a silica particle. Significantly lower frequencies of particles manifesting occluded behavior were found for anthracite dusts versus all bituminous dusts. It is suggested that such occlusion alters the biological availability of the surface of those particles. This may be a factor in the results of attempts to correlate disease prevalence with conventionally measured dust composition, as in the classical failure of coal workers' pneumoconiosis disease prevalence to correlate with silica exposure while being correlated with cumulative total respirable dust exposure and with coal rank.

Design and Evaluation of a Wind Tunnel for Reentrainment Studies of Coal Dust Particles

Abstract A wind tunnel was designed and constructed to study the phenomena of diffusion, deposition, and reentrainment of anthracite and bituminous coal dust particles. Air-velocity profiles and turbulent-intensity profiles were measured in the wind tunnel. A mathematical model of the turbulent flow in the wind tunnel was developed. The solution to the model was derived using a finite difference scheme that was programmed for the computer. This article presents the design and construction of the wind tunnel. The development of the mathematical model, solution procedure, and the computer program are discussed. The characteristics of the wind tunnel were developed by plotting the pressure drop as a function of air velocity. It was observed from the results that the measured and computed velocity profiles agreed well outside the boundary layer. The wind tunnel is suitable for experimental studies on several aspects of particulate phenomena, such as dispersion, diffusion, transport, and reentrainment.

Distinction of Coal Dust Particles from Liquid Droplets by Variations in Azimuthal Light Scattering

Abstract An experimental study was performed to determine the feasibility of optically distinguishing coal particles from water droplets for the purpose of reducing the mass concentration artifact from water sprays and steam reported for optical dust monitors. A single-particle multiangle optical detector was used to measure the scattering of monochromatic, circularly polarized light by mixtures of nonspherical bituminous coal dust particles and 0.5 μm dioctyl sebacate (DOS) droplets, which were used for modeling small, spherical water droplets. A total of 20 different mixtures with known coal number fractions ranging from 6 to 90 percent were measured by the detector using two data acquisition methods known as peak-detection and digital signal evaluation (DSE). Variability in azimuthal scattering by single particles, which is an indicator of the degree of sphericity, was used to discern the fraction of coal particles present in each aerosol mixture. Number concentrations of coal and DOS aerosols were mea...

Hydroxyl radical generation by coal mine dust: Possible implication to coal workers' pneumoconiosis (CWP)

Occupational exposure to coal mine dust causes coal workers' pneumoconiosis (CWP) and other pulmonary diseases by mechanisms that remain unclear. Because the hydroxyl radicals ( ·OH) may play an important role in the pathogenesis of CWP, we studied the potential role of bituminous coal mine dust samples for catalyzing the generation of ·OH from hydrogen peroxide (H 2O 2). These coal mine dusts evaluated represented two geographic areas with diversity in CWP prevalence. Electron spin resonance (ESR), with the aid of spin trapping techniques, was used to measure the ·OH radical generation. Bituminous coal mine dusts representing the Pittsburgh seam in the eastern United States and Blind Canyon seam in the mid-westem United States were used together with a standard coal dust obtained from the National Institute of Standards and Technology, Galthersburg, MD. All the coal mine dust samples generated varying levels of ·OH radicals from H 2O 2 in the presence of a ·OH spin trap 5,5,-dimethyl-l-pyrroline-N-oxide (DMPO). ·OH radical generation by the coal from H 2O 2 was effectively inhibited by deferoxamine and catalase, but only partially inhibited by superoxide dismutase. Metal chelators DETAPAC and EDTA enhanced the radical generation. These results indicated that the Fenton reaction is predominantly involved in the generation of ·OH radicals from H 2O 2. The ·OH-generating potential of all the coal dusts showed a positive correlation with the surface iron content of coal mine dusts. In addition, the potential to induce lipid peroxidation by the coal samples exhibited a good correlation with the available surface iron. Based on the results presented here, we propose that higher concentrations of surface iron in coal mine dust may be involved in the generation of increased levels of ·OH radicals and may play an important role in the development of CWP in different coal mining areas.

Characterization of alveolar macrophage eicosanoid production in a non-human primate model of mineral dust exposure

The relative activation of eicosanoid production which results from the exposure of the alveolar macrophage (AM) to mineral dusts is thought to be a key factor in the pathophysiology of occupational lung disease. We compared in vitro basal and silica-stimulated production of prostaglandin E 2 (PGE 2) and thromboxane A 2 (TXA 2) by AM from normal humans and non-human primates (Macaca nemistrina). In addition, we instilled mineral dusts directly into one lung of the non-human primate and evaluated AM eicosanoid production at two week intervals following dust instillation. Unstimulated AM from humans produce more PGE 2 and TXA 2 than do AM from M. nemistrina. However, in vitro exposure of AM from both species to silica dust produced a qualitatively similar increase in TXA 2 production accompanied by no change in PGE 2 production. Sequential analysis of AM eicosanoid production following a single bolus exposure to bituminous or anthracite coal dusts, titanium dioxide (TiO 2) dust or crystalline silica showed marked variability among individual non-human primates in qualitative and quantitative aspects of dust-induced eicosanoid production. However, the rank order of potency of the different dusts (silica > anthracite > bituminous) correlated with epidemiological evidence relating the type of dust mined to the incidence of pneumoconiosis. These studies suggest that the non-human primate may serve as a model for the study of both the role of eicosanoids in the etiology of dust-induced occupational lung disease and the biochemical basis for individual variability in the response of lung cells to mineral dust exposure.

Minimum Explosible Dust Concentrations Measured in 20-L and 1-M3Chambers

Abstract Minimum explosible concentrations (MEC) of dusts were measured in the Bureau of Mines 20-L chamber and in the Fike L-m3 (1000-L) chamber. The MEC values for gilsonite dust and bituminous coal dust were measured in each chamber at several ignition energies. The explosibility of anthracite coal was also studied in the two chambers. Strong chemical ignitors with energies of 500 to 10 000 J were used in the tests. The uniformity of the dust dispersions in each of the chambers was studied by using optical dust probes. One purpose of the research was to determine if the 20-L chamber was “overdriven” at high ignition energies. The MEC-values measured in the 20-L chamber with 2500-J ignitors were comparable to those measured in the 1-m3 chamber with 10 000 J ignitors. At higher ignition energies in the 20-L chamber, there was evidence of overdriving.

Alveolitis caused by exposure to coal mine dusts: Production of interleukin-1 and immunomodulation by bronchoalveolar leukocytes

Two kinds of coal mine dust, low rank with high quartz (bituminous) and high rank with low quartz (anthracite), were assayed for ability to induce alveolitis and to stimulate interleukin-1 release from normal alveolar macrophages in vitro. Dust-elicited bronchoalveolar leukocytes were also assessed for their effects on macrophage-depleted splenocyte mitogenesis and their ability to produce interleukin-1. Quartz and titanium dioxide were used for comparison as toxic and inert dusts, respectively. Both coal mine dusts caused substantial release of interleukin-1 from normal alveolar macrophages in vitro and the levels were higher than those caused by quartz. Bituminous coal mine dust provoked acute but rapidly subsiding macrophage/neutrophil alveolitis which was greater than that provoked by titanium dioxide; anthracite caused less alveolitis than titanium dioxide and quartz caused large scale, persistent alveolitis. Whole bronchoalveolar leukocytes, including polymorphonuclear neutrophils, elicited by exposure to dust, were less inhibitory to lymphocyte mitogenesis than normal alveolar macrophages and bituminous coal mine dust-induced neutrophils were augmentary to lymphocyte mitogenesis; macrophages from inflamed lungs were, on the whole, inhibitory to lymphocyte mitogenesis. Alveolar macrophages from bituminous coal mine dust- or titanium dioxide-exposed lungs showed increased ability to release interleukin-1 on stimulation in vitro. These findings suggest that bronchoalveolar leukocytes elicited by coal mine dust could modulate immunity by means of interleukin-1 release and enhancement of lymphocyte proliferation.

Effect of in vivo coal dust exposure on arachidonic acid metabolism in the rat alveolar macrophage

Oxygenated metabolites of arachidonic acid (AA) are produced by the alveolar macrophage (AM) and have been shown to mediate inflammatory reactions. We therefore assessed the production of eicosanoids by AM harvested from the lungs of rats exposed to a bituminous coal dust for 2 wk in an inhalation chamber in order to determine if AA metabolism was altered in a manner that may promote an inflammatory response in the lung. Exposure to coal dust resulted in a 66% increase in the number of AM harvested, an increase in thromboxane A2 (TxA2) and leukotriene B4 (LTB4) production to 222% and 181% of control values, respectively, and a decrease in prostaglandin E2 (PGE2) production to 62% of control values. In AM harvested from rats allowed to breath clean air for 2 wk following coal dust exposure, PGE2 production returned to control levels but TxA2 and LTB4 production remained elevated. The TxA2 synthesis inhibitor UK 38,485 reduced TxA2 production in dust-exposed AM both immediately and 2 wk following exposure. Thus, exposure of rats to coal dust significantly alters the metabolism of AA in AM, with potentially important aspects of AA metabolism remaining altered even after a 2-wk recovery period. Based on the established role of eicosanoids in inflammatory and fibrotic processes, these results suggest that the alteration of AM eicosanoid production as a result of the inhalation of coal mine dust may be an important factor in the pathophysiology of coal workers' pneumoconiosis.

Flammability limit measurements for dusts and gases: Ignition energy requirements and pressure dependences

Data are presented for the flammability limits of Pittsburgh Seam bituminous coal dust, polyethylene powder, and methane in air at pressures in the range 0.5 to 3 bar. Explosibility test chambers of 20 and 120 L were used, and ignitability limitations were overcome with efficient pyrotechnic ignitors with nominal energies of 500 to 5,000 J. The propagation criterion used was based on the maximum explosion pressure and the size-normalized maximum rate of pressure rise. The latter is a dynamic criterion that tends to minimize “overdriving” effects as the 20-L data are taken to their asymptotic limits at high ignition energies. The measured lean limits in air at atmospheric pressure are 90 g/m3 for the coal dust, 35 g/m3 for polyethylene, and 4.9 vol pct for methane. The rich limit for methane is 18–19 vol pct, whereas the dusts have no rich limits out to concentrations as high as 4,000 g/m3. A linear, lean-limit pressure dependence was measured for the dusts which was essentially the same as the pressure dependence measured for methane when all are expressed in comparable units: namely, mass concentration of fuel per unit volume of air. This observation further confirms a lean limit dust flame propagation mechanism that is controlled by the gas-phase reaction rate. The limit concentrations are then determined by the dust loading required to generate a lean limit concentration of pyrolysis products in the volatiles-air mixture, and the dust behaves as an equivalent “homogeneous,” premixed gas, regardless of the initial pressure.

Spark ignition energies for dust-air mixtures: Temperature and concentration dependences

Energy requirements for the spark ignition in air of polyethylene powder, lycopodium spores, and Pittsburgh seam bituminous coal dust were measured in a 1.2-L furnace and a 20-L chamber. Dust concentrations and initial temperatures were carefully controlled (at ambient pressure) so that the mixtures were above their lean limits of flammability and below their spontaneous autoignition temperatures. Electrical ignition energy requirements are given in terms of both effective spark gap energies, ε eff , and stored capacitor energies, 1/2 CE 2 . At ambient temperature and pressure, the average minimum ε eff -values for the three dusts were: 40 mJ for lycopodium, 50 mJ for polyethylene, and 160 mJ for the coal dust. The concentrations at which those minima were observed were 300, 500, and 750 g/m 3 , respectively. The measured electrical ignitability ranking is consistent with the data of other researchers but the measured values are systematically higher, probably because of the higher flow and turbulence levels used here. For lycopodium, its ease of ignition allowed its lean limit to be approached readily with spark sources, and measurements of its temperature dependence confirm the validity of the modified Burgess-Wheeler law for a dust. While the concept of a minimum electrical ignition energy for homogeneous gas mixtures is well established, the data reported here, and their analysis, suggest that the concept may not be particularly useful for heterogeneous dust-air mixtures. So many contradictory requirements are involved in experimental conditions, and such extraordinary difficulties and complexities are encountered for dusts, that a reliable determination of a minimum ignition energy that reflects intrinsic flammability behavior is dubious.