Date of Graduation
Statler College of Engineering and Mineral Resources
Keith A Heasely
Jurgen F Brune
C Aaron Noble
Bleeder systems are an important component for ventilation and the control of methane. The bleeder system of a coal mine contains a mixing zone for methane-laden air from the mined-out portions of the seam to mix with fresh air and the methane concentrations in the bleeders can often be elevated. Bleeders also provides a pathway for coalbed methane-laden ventilation air to quickly flow out of the mine through low resistance airways of a mine, such as supported gateroads, along the un-compacted outer perimeter of the gob, etc. Substantial quantities of coalbed methane are typically removed from underground workings. Although it is a relatively simple task to determine the methane quantities exiting the mine through direct measurements, a clear understanding of the exact manner and associated concentrations in which bleeder entries accumulate and transport methane-air mixtures is not known. The benefit of this improved understanding will decrease the likelihood of an explosion due to unknown accumulations of explosive gases in the bleeder entries, thereby improving worker safety.;In order to provide a better understanding of how a bleeder system works in moving methane through the mine, several field monitoring studies have been designed and completed using a tube bundle system and tracer gas releases. The tube bundle system was installed at a bleederless (progressively sealed) underground coal mine. The tube bundles monitoring points were located at different critical locations surrounding the longwall to specifically monitor gas concentrations and barometric pressures on a 30 minute interval for a period of two years. The tracer gas studies, on the other hand, were conducted at an underground coal operation with a traditional bleeder system. The objectives of these tracer tests were to determine transportation pathways and retention times of tracer gasses to better understand the exact gas movements in longwall gobs. The tracer gas was sampled from different headgate and tailgate entries through sample tubes of different lengths using vacutainers. The gas samples were analyzed using gas chromatography for determining concentration measurements for tracer and other gasses, including methane.;The tube bundle system results showed that falling barometric atmospheric pressure can cause the caved material to outgas higher concentrations of contaminants into the bleeder system. During prolonged atmospheric pressure drops, the gas concentrations leaving the caved material via the bleederless system were measured to increase by over two times the average values. These results strongly suggest that to effectively monitor and detect these outgassing events, the bleeder system requires collecting data more often than the once-a-week regulation stated in the 30 CFR Part §75.364.;The tracer gas testing showed locations of high methane in the bleeders, but the practice in multi panel longwall districts of use premixing of the airflow exiting the longwall panels with cleaner airflow to dilute the methane concentrations to below allowable levels before passing through bleeder evaluation points masked the high methane concentrations. Specifically, samples with methane concentration above 4% were collected from the middle entry of the tailgate, but these airflows were diluted to below 2% just before reaching the bleeder evaluation points, and the mine was unaware of the higher methane levels. This result indicates that premixing of explosive airflow as soon possible, as it exits from the tailgate entries in this case, is beneficial to reducing possible explosions, sampling locations need to be closer to the caved material to better monitor and record the actual conditions existing within the inaccessible bleeder locations.;The explosive mixtures of methane in the bleeder are not theoretical but exist and are measurable with direct and indirect methods within both bleeder and bleederless ventilation system. Obtaining measurements of these mixtures is the first step to be able to better engineer longwall ventilation safety.;The conclusions for this research are: 1) Long duration atmospheric pressure drops of a day or more in length are the controlling factor in increased emission from the caved material. 2) The practice of pre-mixing airflows leaving the middle entries between longwall panels with low methane airflow before reaching the bleeder evaluation points, can mask the existence of explosive mixtures of methane at other locations in the bleeders. 3) Without knowledge of the precise locations of high methane in the bleeder entries, the bleeder system cannot be optimized for minimizing explosive methane concentrations and improve miner safety. To solve the atmospheric pressure drop issue it is recommended that a continuous monitoring system should be installed on surface to record these mine-wide changes in total methane emissions. It is also recommended that bleeder evaluation points should be moved closer to the caved material or the sample tubes should be used to monitor critical locations before mixing occurs. Both of these recommendations will improve the understanding of the nature of gas transportation within the bleeder system and thereby lead to improved worker safety.
Krog, Robert B., "Critical Analysis of Longwall Ventilation Systems and Removal of Methane" (2016). Graduate Theses, Dissertations, and Problem Reports. 6016.