How does limestone rock dust prevent coal dust explosions in coal mines?

Coal dust explosions in underground coal mines are pre­ vented by a generous application of rock dust (usually lime­ stone). If an explosion should occur, the rock dust disperses, mixes with the coal dust and prevents fl ame propagation by acting as a thermal inhibitor or heat sink. To investigate this process in more detail, a number of ex­ plosion experiments using various coal dust and limestone rock dust mixes have been carried out at the Lake Lynn Experimental Mine (LLEM). These were conducted in a single entry of about 488 m (1,600 ft) in length and initiated with a methane gas explosion. A consistent set of postexplosion floor dust samples were taken along the entry after each test. These dust samples have been analyzed in the laboratory using thermogravimetric analysis (TGA) and solubility to determine how the limestone rock dust behaved during the coal dust/rock dust explosions. The preliminary results are reported in this paper and indicate that the chemistry of limestone plays an important role in its capacity to inhibit coal dust explosions. The results also show that it may be possible to estimate the intensity of the explosion using these conventional methods of analysis. Introduction Methane gas is a colorless, odorless, flammable gas that is liberated naturally from coal seams. Methane is particularly dangerous if it reaches concentrations between approximately 5% and 15% in air where the mixture becomes explosive. There­ fore, it is important for mine opera­ tors to monitor the level of methane in coal mines. Current federal safety standards require sufficient ventilation to keep methane levels below 1% (Code of Federal Regulations, Title 30, Part75-323). Explosions in coal mines can be prevented or mitigat­ ed by eliminating ignition sources, by minimizing methane concentrations through the entries with adequate ventila­ tion and by using barriers to suppress propagating ex­ plosions. Methane explosions can also cause subsequent large explosions of coal dust, which can have devastating consequences. To prevent the coal dust from taking part in such an explosion, mine operators routinely cover the fl oor, rib and roof areas of mine entries with a generous applica­ tion of inert rock dust. Pulverized limestone rock dust is commonly used. The percentage of rock dust is frequently checked by mine inspectors. Federal law requires that all areas of a coal mine that can be safely traveled must be kept adequately rock dusted to within 12.2 m (40 ft) of all working faces. In the United States, there are regulations for un­ derground coal mines that require mine operators to dust mine corridors liberally with an inert rock dust, such as pulverized limestone, and maintain a total incombustible content of at least 65% in the non-return (intake) and at least 80% in the return areas (Code of Federal Regulations, Title 30, Part 75-403). The regulations also require additional rock dust where methane is present in any ventilating current and involves adding an extra 1% of incombustible material per 0.1% methane for air in intakes and 0.4% incombustible per 0.1% methane in the returns. Conventionally, it has been accepted that during an explosion, the rock dust disperses, mixes with the coal dust and prevents flame propagation by acting as a thermal inhibitor or heat sink; i.e., the rock dust reduces the fl ame temperature to the point where devolatilization of the coal particles can no longer occur; thus, the explosion is inhibited. The amount of rock dust required to inert such an explosion will depend on the particle size of the inerting agent as well as the particle size and composition of the coal dust. Pulverized limestone (with a low silica content) is commonly used as the rock dust material because it is inexpensive and widely available around the world. How­ ever, other types of minerals such as dolomite and marble dust may also be used. This study investigates how limestone rock dust pre­ vents the propagation of coal dust explosions by assessing its changes in chemical composition. A number of large scale coal dust explosions have been carried out in an ex­ perimental mine. A series of post-explosion fl oor samples were taken after each experiment and analyzed in the laboratory. The post-explosion dust sample studies reported here were part of a larger 2008 LLEM research study on the explosibility of various sizes of coal, which was led by K. L. Cashdollar and E. S. Weiss. Experimental Pittsburgh coal was used in the explosion tests. The coal was mined, ground and pulverized at the National Institute for Occupational Health (NIOSH) Pittsburgh Research Laboratory (PRL), Pittsburgh, PA. Coal dust explosion tests The full-scale, single-entry explosion experiments were carried out at the PRL Lake Lynn Experimental Mine (LLEM). Situated about 80 km (50 miles) southeast of the PRL, the mine consists of an underground limestone mine and surface quarry area. The underground mine section of LLEM is a sophisticated underground testing facility for conducting large-scale gas and coal dust explosions as well as other relevant mine research programs. The entries have been designed to physically match those of commercial coal mines, making them authentic and full-scale. The mine consists of four parallel drifts (A, B, C and D), which are approximately 488 m (1,600 ft) long. In addi­ tion, C and D drifts are connected by another entry, E drift, which is only about 152 m (500 ft) long (Weiss et al. 2006; Cashdollar et. al. 2007; Sapko et. al. 2000). There are seven perpendicular crosscuts connecting A, B and C drifts. The entries are about 6-m (20-ft) wide and about 2-m (6.5-ft) high with cross-sectional areas approximately 12 to 13 m2 (130 to 140 sq ft). All of the explosion experiments described in this pa­ per were done as “single-entry” explosions and were car­ ried out in A drift. Prior to these tests, the crosscuts joining A and B drifts were sealed (PRL, 2008). A list of the explo­ sion program parameters including details of the coal and rock dust mixtures have been summarized in Table 1.