Michael J. Sapko

Experimental Mine And Laboratory Dust Explosion Research At NIOSH

Abstract This paper describes dust explosion research conducted in an experimental mine and in a 20-L laboratory chamber at the Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH). The primary purpose of this research is to improve safety in mining, but the data are also useful to other industries that manufacture, process, or use combustible dusts. Explosion characteristics such as the minimum explosible concentration and the rock dust inerting requirements were measured for various combustible dusts from the mining industries. These dusts included bituminous coals, gilsonite, oil shales, and sulfide ores. The full-scale tests were conducted in the Lake Lynn experimental mine of NIOSH. The mine tests were initiated by a methane–air explosion at the face (closed end) that both entrained and ignited the dust. The laboratory-scale tests were conducted in the 20-L chamber using ignitors of various energies. One purpose of the laboratory and mine comparison is to determine the conditions under which the laboratory tests best simulate the full-scale tests. The results of this research showed relatively good agreement between the laboratory and the large-scale tests in determining explosion limits. Full-scale experiments in the experimental mine were also conducted to evaluate the explosion resistance characteristics of seals that are used to separate non-ventilated, inactive workings from active workings of a mine. Results of these explosion tests show significant increases in explosion overpressure due to added coal dust and indications of pressure piling.

Laboratory and Mine Dust Explosion Research at the Bureau of Mines

This paper describes the results of recent dust explosibility testing in a 20-L laboratory chamber and in the Bruceton and Lake Lynn Experimental Mines in Pennsylvania. Laboratory data on the lean flammability limits for coals and oil shales are compared to mine data for both predispersed tests and nominal loadings that are dispersed by a gas ignition zone. A linear relationship for the lean flammability limits for mixtures of bituminous coal dust and methane gas was measured in both laboratory and mine tests. This paper also compares data on the amount of limestone rock dust necessary to inert coal dusts and coal-methane mixtures. The good agreement between the 20-L chamber data and the mine data means that the laboratory chamber can be used for screening tests before full-scale mine tests.

Particle size and surface area effects on explosibility using a 20-L chamber.

The Mine Safety and Health Administration (MSHA) specification for rock dust used in underground coal mines, as defined by 30 CFR 75.2, requires 70% of the material to pass through a 200 mesh sieve (<75 µm). However, in a collection of rock dusts, 47% were found to not meet the criteria. Upon further investigation, it was determined that some of the samples did meet the specification, but were inadequate to render pulverized Pittsburgh coal inert in the National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) 20-L chamber. This paper will examine the particle size distributions, specific surface areas (SSA), and the explosion suppression effectiveness of these rock dusts. It will also discuss related findings from other studies, including full-scale results from work performed at the Lake Lynn Experimental Mine. Further, a minimum SSA for effective rock dust will be suggested.

Coal dust and gas explosion suppression by barriers

Explosion suppression barriers are devices that contain fire extinguishants that are activated to disperse at some critical point during the propagation of an explosion to suppress it. Suppression of coal dust explosions using barriers (both triggered and passive) has been investigated by the U.S. Bureau of Mines. The work reported in the present study is an update of the continuing study of passive barriers of both rigid and flexible construction. Suppressants tested were water and ABC powder (ammonium phosphate). The coal dust mixtures contained 60 to 65% total incombustible matter and were distributed in the Bureaus single entry experimental mine for a total distance of 111 m (365 ft). Dust explosions were initiated by a 7% methane-air gas zone at the face. The passive barriers were located at distances of 60 to 108 m (200 to 356 ft) from the face. At these distances, the magnitude of the explosion pressure pulse was about 0.70 to 1.14 bar (70 to 114 kPa) at the time the flame front arrived at the barrier. With the exception of the powders, the pressure pulse was sufficiently energetic to fracture the troughs and disperse the suppressants. The powder (approximately 180 kg per test) was almost totally ineffective when used with the rigid barrier because it did not disperse. One to four troughs of water effectively suppressed explosions. It was found that the mounting arrangement for the flexible troughs was most important for successful operation and release of the suppressant. Triggered barrier systems for protection against incipient gas explosions were tested in a simulated longwall panel. Results show that ABC powder was much more effective in suppressing the developing explosion than equal amounts of water released from the same pressurized reservoir. Although water was effective in stopping fully developed dust explosions, it had little effect against an explosion during its incipient stage.

RAPID SAMPLING OF PRODUCTS DURING COAL MINE EXPLOSIONS

This paper describes a U.S. Bureau of Mines investigation of large-scale coal dust explosions in an experimental mine using a high speed clectropneumatic mechanism for the rapid grab-sampling of gases and dusts. This technique enables themonitoring of pyrolysis and charring in fuel dust particles, and the collection of gaseous combustion products, in both large and small-scale explosions. Data obtained from full-scale dust explosion tests at the Bureau of Mines Lake Lynn Test facility show the following: Rapid sampling appears to “freeze” the burned gas compositions at the flame temperature values.Gas samples taken entirely in the flame zone consist of pyrolysis and combustion products with very low residual oxygen.The particles collected in the flame zone show signs of extensive pyrolysis and charring. Measurements of gas concentrations and particle flame temperatures suggest that char burning may have occurred in the flame zone.

Influence of specific surface area on coal dust explosibility using the 20-L chamber

The relationship between the explosion inerting effectiveness of rock dusts on coal dusts, as a function of the specific surface area (cm2/g) of each component is examined through the use of 20-L explosion chamber testing. More specifically, a linear relationship is demonstrated for the rock dust to coal dust (or incombustible to combustible) content of such inerted mixtures with the specific surface area of the coal and the inverse of that area of the rock dust. Hence, the inerting effectiveness, defined as above, is more generally linearly dependent on the ratio of the two surface areas. The focus on specific surface areas, particularly of the rock dust, provide supporting data for minimum surface area requirements in addition to the 70% less than 200 mesh requirement specified in 30 CFR 75.2.