Modeling gas flows in longwall coal mines using computational fluid dynamics

Abstract

Longwall coal mines have been a major and growing source of coal production in the United States for decades. The geological and geochemical nature of coal seams means that methane is frequently present in the coal and surrounding strata of longwall mines. The human coal miners require oxygen to work in these environments, so complex ventilation systems are developed to deliver fresh air to the underground mine. An additional goal of the ventilation system is to prevent explosive mixtures of methane and oxygen from forming in the mine, including within the gob. A computational fluid dynamics (CFD) model was created that accurately simulates the mixing of gases in longwall coal gobs. The longwall panel model simulated the phase leading up to and including shield recovery and therefore examines the ventilation and hazards during that phase of the operation. Special attention was paid to the interaction of gases in the gob, a rubblized zone of rock, potentially methane rich, that is created as the longwall retreats. In order to accurately model behavior, a parallel research used a commercial software package to predict the permeability and porosity of the gob (Wachel, 2012). When coupled with FLUENT, a commercially available CFD software package, a flow model was created that calculated gas concentrations which matched several known monitoring sites in a currently operating longwall mine. Qualitatively, the model's predictions also appeared to conform to observed operating conditions giving further confidence in the model. The validated model framework was used to run many hundreds of simulations. The results of the completed simulations support a number of conclusions. This research suggests that progressively sealed, bleederless longwall panels should strive to maximize nitrogen injection into the headgate side of the panel without creating unsafe low oxygen concentrations in the tailgate return. Nitrogen injected in the tailgate of the panel had a much lower impact on gob safety, but is likely still necessary to sweep the tailgate entries separated from the ventilation system by isolation stoppings as the face advances. The second recommendation of this research is that, while the general shield recovery process used by bleederless longwall panels appears safe, nitrogen should be injected behind the shields in the center of the ventilation circuit to mitigate the hazard created by the build up of high methane environments in the gob immediately behind the shields. Finally, the results suggest that minimizing the concentration of methane in the tailgate return does not always ensure that the optimum methane concentration for the entire longwall gob is being achieved.