Nitrogen injection in progressively sealed longwall gobs and the formation of a complete and dynamic seal
Methane ignition and spontaneous combustion of coal are two common ventilation hazards associated with longwall coal mining. Methane is a coal mine gas emitted from the surrounding strata and the seams themselves as part of the mining process. Methane is diluted by the mine ventilation system used to provide fresh air to the face. Methane air mixtures become explosive between the lower explosive limit of 5.5% methane and the upper explosive limit of 14% methane. Mixtures with higher methane contents will burn as a diffusion flame. Methane ignition has been the cause of several recent mine tragedies including the Upper Big Branch Mine explosion in April 2010 and the Pike River Mine explosion in November 2010. Spontaneous combustion is an exothermic reaction involving coal and oxygen. The initiation of spontaneous combustion is dependent on oxygen concentration and residence time in addition to other factors. If the heat from the reaction is not dissipated the heating can proceed to thermal runaway or a fire that may result in fatalities, equipment losses and or mine closure. Recent spontaneous combustion events have resulted in the temporary closure and loss of longwall equipment and reserves at the Elk Creek Mine from a fire that was discovered in January 2013 and the Soma Mine Disaster that resulted in over 300 fatalities from an explosion that is suspected to have initiated from a spontaneous combustion fire. The investigation is still on going. The focus of this dissertation is to investigate the use of nitrogen to both reduce explosive gas volumes and to reduce the spontaneous combustion potential by diluting oxygen ingress behind the longwall shields. This research will investigate the quantity of nitrogen injection, the injection location and the method of face ventilation to determine the effectiveness of each variable in mitigating the hazards discussed above. The hypothesis is that a back return scheme in conjunction with progressive nitrogen injection creates a safer work environment than a traditional U-Type ventilation scheme in terms of both explosive gas and spontaneous combustion hazards. Knowledge regarding the porous media distribution of the caved gob is required for modeling the gas distributions. Previous findings regarding porous media distribution of the gob and nitrogen injection are presented and discussed in a literature review. A geo-mechanical model was developed to determine the porous media distribution for an active longwall panel. A numerical fluid flow and gas dilution model was developed using the porous media distribution and utilized to study the validity of the hypothesis. Nitrogen injection amount and location was varied for both U-Type and back return face ventilation schemes to determine the effectiveness of each on the desired hazard mitigation. Important conclusions drawn from the research include the following findings. Porous media distributions are noticeably different for static panels compared to active panels simulated by utilizing a method of stepped extraction for the…