Design & Efficiency of Dust & Water Explosion Barriers Extension to C4030

Current Australian stone dusting regulations have their research origins as far back as the 1920s. Changing mining practices since then have progressively increased the generation of coal dust and reduced particle sizes, therefore the early researched inertisation levels have been rendered obsolete. Stone dust used in Australian mines can be improved by reducing its particle size and/or adding chemical suppressants, including ammonium phosphate and salt.  Neither of these improvements will manifestly increase the probability of passive explosion barriers operating under weak explosion conditions. However, various designs of active barriers appear to solve this problem and their use in Australia should be seriously considered.

Irrespective of the use of active barriers, the incombustibles content of settled dust should be increased to a minimum of 85 per cent for high (>35 per cent) volatile content coals to ensure that the airborne incombustibles is adequate under a credible range of dispersion energies and gas levels.

In the absence of extensive local testing, United States' Bureau of Mines work by Greninger et al. (1990) should be considered as the best linkage between coal volatile content, methane presence and the required inertisation level.

Recommendations

1. Stone dusting is generally considered to be an effective means of ignition prevention and flame suppression when properly distributed and mixed with settled coal dust. There is, however, a body of evidence showing that stone dust can be ineffective under some conditions and that the required levels of stone dust under Australian regulations are assessed as inadequate. 

• Bituminous coals with volatile contents above 14 per cent should be considered hazardous. Overseas research in this area appears comprehensive and applicable to Australia, and suggests that the current minimum incombustibles content in settled dust should be increased to a minimum of 80 per cent, irrespective of the coal volatiles content. South African regulations now require inertisation to 80 per cent incombustibles within 180 m of the face when the volatile content is above this value, and this may be a starting point for discussion. 
• Stone dust is approximately twice as dense as coal dust therefore it is less dispersable than coal dust, and an airborne cloud of coal/stone dust mixture would have a lower incombustibles content than the settled inertised zone from which it was raised. There is theoretical and experimental evidence for this conclusion. 
• The most effective stone dust is one where 70 per cent passes a 75 mm sieve. Further reduction of size does not improve effectiveness because very fine stone dust tends to agglomerate. The optimised distribution compares to 50 per cent being <75 mm, as currently used in Queensland and a review of this regulation is therefore warranted. 
• Size analyses of Australian stone dusts indicate that the materials are generally too coarse for optimal performance. Some products tested from mines do not meet the current standards, and in some cases the manufacturers specification. Quality control testing should be instigated. Overall performance could be enhanced by producing a finer product. 
• It has been shown there is approximately a 5 per cent improvement in stone dust effectiveness between a standard dust (median particle diameter of 80 mm) and the same dust milled to a median particle diameter of 20 mm, as measured in an explosion chamber. When dispersability effects are considered, this difference is expected to be higher. 
• The presence of low concentrations of methane (of the order of 1 to 2 per cent) in the ventilation stream can dramatically increase the explosibility of roadway dust. Thus, at 1.25 per cent methane, the incombustibles should be increased to 80 per cent when the current methane free requirement is 75 per cent. 
• Many other materials apart from stone dust have been studied for their effectiveness as explosion suppressants. Two effective compounds are ammonium phosphate and salt. Both compounds suppress flame development by chemically interacting with the combustion process and, on a mass basis, they are several times more effective than stone dust. Therefore, some more should be initiated to phase in their introduction on a case-by-case basis. Some risk/cost-benefit studies and practical assessments are needed before a final and quantitative recommendation can be offered. 
• Recent research at CSIRO Mining Technology and DMT has resulted in the development of a bagged stone dust barrier. The barrier consists of an array of plastic bags suspended in rows from the roof. Each bag contains between 5 and 6 kg of stone dust that is enclosed with a sealing ring at the top of the bag. Hygroscopic material, for example salt that is an effective chemical flame suppressant, can be sealed from the mine environment in the bagged barrier. Some limited practical testing appears warranted as part of a parcel of measures to improved explosion protection. 
• It appears that explosion safety in high risk areas can be considerably improved by the installation of automatic explosion barriers fitted to coal cutting machines. These barriers are designed to suppress an ignition in the shortest time possible and as close to the point of the ignition as is practicable, so that personnel on the machine and its immediate vicinity are not injured by both the explosion and the means of suppression. Thus, by responding rapidly to a face ignition, only small increases in temperature and pressure occur and there is little generation of toxic gases. No evidence has come to light that suggests these systems are not practical, therefore the authors will proceed with further research into the practicalities of these systems for Australian conditions.