Computational fluid dynamics modeling of underground coal longwall gob ventilation systems using a developed meshing approach

Abstract

Mining accidents related to ventilation problems that often occur deep within a mine are costly events in terms of production loss, equipment loss, public relations, and ultimately, the cost of human lives. Although the mining process has become safer over the years through mechanical extraction, the operation still requires human control, monitoring, and repair at or near the mining face. Larger operations use the longwall mining process to extract coal by cutting a 3 to 4 foot wide swath out of a long, continuous block of coal. During the operation, the extraction face is ventilated where miners control the cutting process, while the roof is temporarily held up by hydraulic supports called shields. As the shields advance following the cut-out face, the roof behind collapses into a rubblized region called the gob. Air flowing across the face may permeate into the gob, and mine gases (primarily methane) liberated from the overlaying, fractured strata or the mine floor enter the mine ventilation system inside the gob. Continuous monitoring and adjustment of mine ventilation is required to prevent hazardous conditions such as explosive or irrespirable atmospheres. The inaccessibility of the gob prevents any effective monitoring or direct measurements of gas concentration, pressure, velocity or flow characteristics. Research has shown that fresh air from the mine ventilation system can enter the gob where it may create explosive methane-air mixtures. Also, with certain coals, ingress of oxygen into the gob can promote spontaneous combustion of remnant coal. The interaction of mine gas liberation and oxygen ingress into the gob is examined with a Computational Fluid Dynamics (CFD) modeling tool developed during this project to predict hazardous operating conditions and to help design ventilation systems to avoid these hazards. This tool will be used to model the ventilation of a bleeder-ventilated, underground longwall coal panel to assess the development of the explosive methane-air mixtures, in contrast to the current regulatory view that this condition is not recognized as a hazard in a properly working bleeder-ventilated gob system (Wachel, 2012; Worrall, 2012; Gilmore et al., 2013; Marts et al., 2013). This dissertation research employed the CFD modeling software package ANSYS® Fluent® along with the output of a previous model developed using the geomechanical software package FLAC3D, to determine the permeability of the gob. Earlier studies have examined the explosive mixture location and total explosive volume found in the gob and near the face, gob caving characteristics, as well as the ingress of oxygen (Wachel, 2012; Worrall, D. M., 2012; Gilmore et al., 2013; Marts et al., 2013). This research addresses the challenges of applying CFD to model multiple mine ventilation layouts with the development of a modular meshing approach. This modeling approach incorporates the variations in mine lithology through the development of multiple gob flow characteristics that can be applied across a wide range of panel dimensions, and the challenges of modeling large scale mine networks and panel lengths through the application of the modular meshing approach. The modular meshing approach developed in this research project is used to build the CFD models of a longwall gob ventilation system flexible enough to model a variety of different mine layouts, mine ventilation schemes, and gob flow parameters. This is accomplished by the creation of a library of meshed geometry modules that are interfaced together to build the ventilation network surrounding the gob. The CFD mesh is tested by creating several ventilation schemes and various mining conditions. Common operating conditions that meet the mine ventilation regulatory statutes are used as the model boundary conditions. The gob flow characteristics are then validated against a tracer gas study. A mesh module repository is developed to help the mining industry create CFD models that match their mine geometry, allowing access to models with fast compute times, and therefore, removing what once hindered wide spread use of CFD simulations in underground ventilation. This dissertation also presents a methodology used to determine the porosity and permeability parameters, scalable in panel length and width, from the output of a FLAC3D model using a combination of polynomial and exponential functions to fit the data. The modeling tool and methods developed in this research enable mining engineers to design safer longwall mine ventilation systems and to predict results of ventilation changes, thereby preventing conditions that may become hazardous. The resulting work is a significant, progressive step towards prevention of mine fire and explosion disasters related to longwall gob ventilation, which have resulted in the loss of human life. This is accomplished through modeling the commonly used bleeder-ventilated gob ventilation scheme and demonstrating its failure to eliminate the explosive conditions that persist throughout the mine models.