Composition Change Model for Sealed Atmosphere in Coal Mines

A sealed atmosphere in a mined-out area of an underground coal mine is simply a volume with homogeneous composition governed by boundary conditions. This paper presents a mathematical model based upon the conservation of mass principle that describes the flow of air (nitrogen and oxygen), methane and carbon dioxide into and out of a sealed atmosphere and the time-dependent changes in gas concentration within the sealed atmosphere. As boundary conditions, the model includes the inflow of methane and carbon dioxide from the surrounding strata, the inflow of nitrogen as an inert gas, the production of carbon dioxide from coal oxidation and the related depletion of oxygen. The model also considers the barometric pressure which can either cause air to flow into the sealed atmosphere or gas to flow out of that volume, depending on the pressure differential with the sealed atmosphere. The mathematical model is implemented into a MATLAB program for calculation and data display. Using a range of realistic input parameters, the model shows that the atmosphere within sealed atmospheres tends to become fuel-rich inert and the pressure tends to increase due to the inflow of methane gas from the surrounding strata. Sealed atmospheres may therefore have a natural tendency to outgas, i.e. to release gas into the mine atmosphere as leakage around and through the seals. The pressure within the sealed atmosphere will depend on the leakage gas flow resistance of the mine seal and the surrounding rock between the sealed atmosphere and the active mine. If this resistance is high enough, it appears possible to maintain the sealed atmosphere in a fuel-rich inert condition and outgassing at all times despite changes in the barometric pressure. Designing the seal and the sealed area to control leakage such that the sealed atmosphere remains inert could result in safer sealed areas with minimal explosion hazard.