Application of numerical gas flows modelling to optimisation of nitrogen injections in the goaf

In Lorraine Collieries (France), the risk of spontaneous combustion in the goaf of longwall coalfaces is relatively high. Nitrogen injections into the goaf are used to control this risk. However, this practice remains relatively empirical in terms of choice of injection locations and flowrates. Carrying out experiments in the goaf is both difficult and expensive. Thus, the CFD modelling of gas flows (air, methane and nitrogen) is an investigation method well adapted to the situation. For 5 years, a research project has been carried out at INERIS with financial funding from Charbonnages de France and European Coal and Steel Community. So a numerical model has been developed and calibrated using a large quantity of field data and then used for various simulations. The results of these simulations indicate that numerical modelling using CFD codes, such as PHOENICS, is a very efficient tool for improving our knowledge of flows in the goaf and for perfecting the various processes and techniques used to control firedamp and spontaneous combustion. For the particular conditions of the modelled faces, optimal nitrogen injection parameters values can now be determined, allowing increased production, diminution of costs and simultaneously safety improvement. This research is still in progress, in order to make the model more realistic. But computer modelling has already become a powerful and practical tool enabling better understanding of the complex phenomena occurring in goaf of coalfaces, thus being a valuable aid for the mining engineer.