Michael J. Pegg

Factors influencing the suppression of coal dust explosions

Abstract Laboratory-scale experiments were conducted to examine the influences of ignition energy, coal dust concentration and particle size of rock dust on suppression of coal dust explosions in a Siwek 20 L spherical explosion chamber. The amount of rock dust required to inert an explosion increased as the ignition energy used to initiate the explosion was increased. Excessively high amounts of rock dust were required when using high ignition energies. This is due to a phenomenon known as overdriving the explosion. Comparison of these results with those of other researchers showed that 5 and 2.5 kJ ignition sources give inerting levels similar to those obtained in mine-scale experiments and in the US Bureau of Mines 20 L chamber, respectively. Results showed that tests with an ignition source > 5 kJ may overestimate the amount of rock dust required to inert an explosion. The amount of rock dust required decreased as coal dust concentration increased beyond that required to produce a stoichiometric volatiles-air mixture. The experimental results also showed that as its particle size decreases, less rock dust is required to inert an explosion. Pocahontas and Pittsburgh coal dusts had similar inerting levels even though Pocahontas has a lower volatile matter than Pittsburgh. This can be attributed to experimental conditions in the 20 L. Siwek chamber, which drive off more volatile matter from the coal than in the standard proximate analysis test.

A comparison of experimental methods to determine the minimum explosible concentration of dusts

An experimental investigation of dust explosions was conducted using a 20 L Siwek explosion chamber, to examine test methods for determination of the minimum explosible concentration (MEC) of combustible dusts. Two methods, one proposed by the American Society for Testing and Materials (ASTM) and the other by the International Electrotechnical Commission (IEC), were compared. The MECs of gilsonite, Pittsburgh coal, oil shale and Pocahontas coal dusts were determined in the 20 L chamber. The test results were interpreted according to the ASTM criterion and the IEC criterion and compared with data available for the four dusts from the US Bureau of Mines 20 L chamber and large-scale testing (1 m3 chamber and mine-scale). MEC values obtained from the IEC test method were generally lower than the values from the ASTM test method. The primary reason for this is the difference in recommended ignition energy between the two methods. Also, an ignition energy of 2500 J was most suitable for determination of minimum explosible concentrations for most of the dusts studied in the 20 L Siwek chamber. This ignition energy gave values that were comparable with data from large-scale experiments.

Laboratory investigation of the dust explosibility characteristics of three Nova Scotia coals

Abstract The explosion characteristics of coal dust/air and methane/coal dust/air mixtures have been determined experimentally. All tests were conducted at initial pressures of nominally 1.0 bar in a 26 / spherical explosion bomb. Run-of-mine coal from the Prince, Lingan and Phalen seams of the Cape Breton Development Corporation was used. Two size fractions of each coal were tested at dust concentrations ranging from the lean flammability limit to 1.0 kg m −3 . The explosion parameters measured for each test were the maximum explosion pressure, P max , and the maximum rate of pressure rise, (d P /d t ) max . Methane addition to the coal dust/air mixtures was found to increase both P max and (d P /d t ) max , the effect being most significant at low dust concentrations. A reduction in mass mean diameter of the coal or an increase in the parent coal volatile content was found to have a similar effect on P max and (d P /d t ) max . These observations are consistent with a description of coal dust flame propagation by gas-phase combustion of devolatilization products.

Inerting of fine metallic powders

Abstract Minimum explosible concentration (MEC) tests were carried out on mixtures of 50:50 AI Mg dust, AI dust, and 70:30 Mg Ca dust. MgO dust was added to these mixtures as an inertant. The results indicate that between 70 and 75% fine MgO dust is required to completely inert the 50:50 AI Mg dust, which is in the same range as the levels of rock dust needed for coal dust inerting. Use of a coarser MgO dust raises the quantity of MgO required to inert. Minimum oxygen concentration tests on the 50:50 AI Mg dust with added MgO correlated with the MEC results. Preparation of inert/metal dust mixtures is a novel method of decreasing the explosion hazard in manufacturing, transportation and for the end-user.

Effects of methane admixture, particle size and volatile content on the dolomite inerting requirements of coal dust

Abstract The dolomite inerting requirements of coal dust/air and methane/coal dust/air mixtures have been determined experimentally. All tests were conducted at

Effect of inerts on layer ignition temperatures of coal dust

An experimental study into the hot surface ignition of coal dust layers has been conducted. Two coals were examined: Prince coal from the Cape Breton Development Corporation and Pittsburgh coal from the United States Bureau of Mines. The effect of admixed inerts (dolomite and limestone) on the dust layer ignition temperature has been analyzed using a steady-state thermal explosion model. The analytical procedure used for evaluating the ignition temperature of a dust layer, heated from below and losing heat from its upper surface by convection, is an extension of the thermal explosion model of Thomas (Ref. [8]); namely, that of Thomas and Bowes (Ref. [15]). To commission the hot plate apparatus and validate the model predictions, a series of experiments were undertaken using sodium dithionite. This material is known to exhibit self-heating and there have been previous layer ignition temperature studies with which to compare results. It was demonstrated that an adequate estimate of the critical ignition temperature may be readily obtained by this analytical method. Furthermore, computed values of the critical ignition parameters for layers of coal dust admixed with inerts, accounting for changes in thermal conductivity, were in reasonable agreement with experimentally determined values.

Explosibility hazard of iron sulphide dusts as a function of particle size

Abstract The explosion characteristics of pyrite and pyrrhotite have been determined as a function of particle size. Explosion tests were conducted in a 201 Siwek chamber using various size fractions of mine samples. It was determined that the critical mass mean diameter for explosibility (i.e. maximum explosible diameter) lies in the range 49–63 μm for the pyrrhotite sample and 85–145 μm for pyrite. A decrease in particle size for each material was found to cause an increase in explosion pressure and rate of pressure rise, and a decrease in the minimum explosible concentration. That pyrite is more of an explosion hazard than pyrrhotite was confirmed and quantified. Testing with FeS and FeS2 chemicals demonstrated the limited applicability of commercial samples to prediction of explosibility behaviour of mine samples. Previous work on coal and metal dusts was analysed to suggest the importance of both homogeneous and heterogeneous reactions in explosion propagation for sulphide dusts.

Lycopodium dust explosions in a Hartmann bomb: effects of turbulence

Abstract An experimental investigation of dust explosions was conducted in a 1.23 I cylindrical pressure vessel known as the Hartmann bomb. A series of explosion runs were carried out using lycopodium as the test dust. The following parameters were varied: ignition delay time; dispersing air pressure; and nominal dust concentration. A companion set of experiments, in which velocity measurements were made during the dust injection process (without ignition), was conducted in a plexiglass version of the Hartmann combustion chamber. These measurements were made using a laser Doppler velocimeter (LDV). The cold-flow velocity measurements indicated that the decay of turbulence in the Hartmann bomb is very rapid and is essentially complete within 200 ms after introduction of the dispersing air. This time frame coincides with the results from the explosion tests, in which the maximum rate of pressure rise displayed a rapid drop in value as the ignition delay time was increased from 40 to 180 ms. The dust constant, K st , was correlated with turbulence intensity expressed as an rms velocity. The correlation obtained was dependent on the nominal dust concentration and the dispersing air pressure. We have demonstrated a methodology whereby initial turbulence may be correlated with the maximum rate of pressure rise during a confined dust explosion. This approach should yield useful results if applied to larger test vessels.

Effectiveness of various rock dusts as agents of coal dust inerting

Abstract An experimental investigation of requirements for coal dust inerting was conducted in a 26 l, spherical chamber. A mine-face sample of coal from the Prince mine of the Cape Breton Development Corporation was tested; rock dusts used were limestone, dolomite (two particle sizes) and magnesite. Limestone, dolomite and magnesite (all having approximately the same particle size) were found to be similar in inerting effectiveness, indicating that the decomposition reaction of the endothermic rock dust did not exert an effect over the short time scales during which the volatiles burn during an explosion. Tests with the two sizes of dolomite showed that a decrease in rock dust particle size could significantly lower the amount required to inert.

Dust explosibility characteristics of azide-based gas generants

Explosibility testing of sodium azide (NaN3), and two new iron oxide-based gas generants, designated as GG1 and GG2, was conducted according to standard procedures. All the dusts were explosible in the range 0–2.0 kg m−3; however, rates of pressure rise were sufficiently low to place them in dust class St. 1. Both the explosion pressures and rates of pressure rise increased without showing a marked maxima for dust loadings up to 2.0 kg m−3. This behaviour is quite different from that observed with most combustible dusts which show maxima in the range 0.50–1.0 kg m−3. The explosibility characteristics depended strongly upon ignition energy. The minimum explosible concentration (MEC) increased, and the rate of pressure rise decreased, on reducing the ignition energy from 5 kJ to 2.5 kJ. Both iron oxide-based gas generant formulations were less explosible as dusts than sodium azide alone. Based on a number of different variables, the following ranking for dust explosibility hazards is proposed: GG2<GG1<sodium azide Of the two formulations, the one which had the higher amount of iron oxide was the least hazardous.