Design and Development of a Multi-Nodal Methane Detection System for Longwall Coal Mining

Methane (CH4) explosions pose significant dangers in longwall mining that may lead to injuries and fatalities. Safety is improved through diligent monitoring of CH4 concentration. Currently, regulations require a CH4 monitor be placed on the shearer, downwind of the cutting head. Portable monitor measurements must be taken at various times and locations. If any CH4 monitor measures a concentration greater than 1%, a warning signal must be given. Based on previous research and the location of the CH4 monitor mounted on the shearer (closest monitor to the face), if 1% methane is measured, the concentration at the face may be already be at the lower explosive limit (5%). If any monitor measures a concentration greater than 2%, production is halted. However, there are spatial and temporal gaps in measurements where a dangerous CH4-air mixture may develop and go undetected. This poses a risk of shearers or other work activity igniting these dangerous mixtures. Through funding provided by The Alpha Foundation for the Improvement of Mine Safety and Health, Inc., a multi-nodal Methane Watchdog System (MWS) was developed to improve CH4 monitoring by decreasing the spatial and temporal measurement gaps. The prototype consisted of 10 sampling nodes distributed along the longwall. Each node had a sampling location near the face and gob. The nodes were connected in series and communicated with a central processing hub. Each node consisted of a sealed box which housed sensors and other components. Two CH4 sensors (metal-oxide and infrared) were mounted in a custom sampling block with climate sensors. Two tubes transported gas samples from relevant locations to the sampling block at the node. The units could sample continuously, alternating between each location. The MWS nodes were powered by low voltage DC power common among shields. In addition, a custom water powered ejector was designed to provide the motive sampling force and represented a critical system component. The ejector was designed to provide sampling for a single unit at flowrate of 2 SLPM. Pressurized water, already powering spray nozzles, would provide an inherently explosion proof motive energy source for active sampling. Ideally, water consumption should be minimized while maintaining enough suction force to draw the sample through the unit at the desired flowrate. An initial ejector design was 3D printed and tested to access its performance. During experimental testing, the ejector demonstrated two distinct operational curves (“High” and “Low” pressure), between which it was believed a flow regime transition from bubble to jet flow occurred. Based on a significant increase in performance post-transition, it was recommended that the ejector operate on the “Low” pressure curve. However, this mode did not meet the flowrate requirement. Thus, a multi-nozzle design was developed and tested, demonstrating the same flow transition. The multi-nozzle ejector was also modelled using a computational fluid dynamics (CFD) software. Experimental points were used to verify the CFD model to predict that a scaled version of the multi-nozzle design met the flowrate and suction force requirements with reduced water consumption