| Longwall mining is a method used in underground coal mining, which is preferred by mine operators due to increased productivity and lower overall injury rates. Coal mines may be considered gassy based on the presence of hazardous gases like, methane, CO and CO2. In the United States, the Mine Safety and Health Administration (MSHA) considers all coal mines to be gassy as a safety measure. In longwall mines, gases from rider coal seams or the floor can migrate into the gob, which has the higher permeability, and then move forward to the working space and areas. The accumulation of methane gas is a safety hazard to the working environment as it can be explosive when mixed with oxygen and exposed to an ignition source. Coal bed degasification, ventilation, nitrogen injection, and gob ventilation boreholes (GVBs) are some of the methods used to control methane hazards.;A well designed mine ventilation system, alone, is limited in its ability to manage methane emissions. When methane emissions are beyond the level that ventilation can handle, additional measures are required, like drilling vertical methane drainage holes, or GVBs, from the surface into the gob area to extract the methane gas; these boreholes are drilled from the surface, above the panel ahead of the mining activity. When the advancing face intercepts the GVBs, the boreholes begin producing methane. It is important to predict the performance of GVBs to ensure a safer working environment in longwall coal mining. The performance of GVBs depends on multiple factors, including; borehole locations, number of boreholes, length of slotted casing, diameter of casing, setting depth of casing above the panel, overburden strata, wellhead vacuum pressure, permeability of caved and fractured zones, and the area of influence of GVBs.;GVBs are widely used in United States (U.S.) underground coal mines for longwall gob degasification. GVBs can recover 30 to 70% of methane emissions from the longwall gob depending on geologic conditions (Mutmansky, 1999). Generally, they are considered useful for reducing methane concentrations in working areas, thereby reducing explosion hazards and creating safer working conditions for a longwall section. The computational fluid dynamics (CFD) modeling work in this dissertation confirms that GVBs are helpful to reduce the methane concentrations at the face. However, they may also draw fresh air from the face into the gob; increasing oxygen ingress into the gob creates explosive gas zones (EGZs) within the gob. It is important to identify the locations for gob ventilation borehole placement to maximize methane extraction and minimize any explosion hazards.;CFD models were developed for this dissertation to analyze the effects of GVB design and operating parameters for methane extraction, the formation of explosive gas zones in the gob and methane concentrations at the longwall face and tailgate. Parameters such as the distance of GVBs from the tailgate and the working face, the borehole diameter, the distance from the top of the coal seam being mined, the wellhead vacuum pressure and the number of GVBs operating on the panel, all have a significant effect on methane extraction, explosive gas mixtures volume and methane concentration in working areas. The CFD studies in this research identified optimal GVB design and operating parameters to maximize benefits and minimize risks. This research used the CFD modeling software package ANSYSRTM FluentRTM along with the output of geomechanical modeling for permeability and porosity input into the CFD models. Earlier research at Colorado School of Mines developed the geomechanical models of the case study mines based on data received from cooperating mines, using the geomechanical software package FLAC3D to determine the permeability of the gob and fractured zone (Marts et al., 2014a; Wachel, 2012; Worrall, 2012).;The main purpose of mine ventilation design is to provide sufficient quantity and quality of air to the workers, and to dilute the concentration of methane and other contaminants. A common perception among mining engineers is that additional air along the longwall face will improve methane dilution on the face and in the tailgate. In this dissertation, a parametric study is presented to discuss the effect of face air quantity on methane concentrations in the tailgate and formation of EGZs in the gob. Counter to conventional wisdom, increased longwall face air quantities may increase the explosion hazard as they result in higher EGZ volumes in the gob and increased methane quantities in the tailgate return. The results have been validated against the data provided by cooperating mines and also compared with published data.;This dissertation provides the industry with a methodology to predict the optimal GVB design and operating parameters and their importance in creating a safe working environment. It will also contribute to the body of knowledge of the effects of face ventilation air quantity changes and how they affect the gob environment and methane dilution in working areas. |
