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Numerical determination of methane concentration in goaf space |
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Numerical determination of methane concentration in goaf space Authors: J. Szlązak, N. Szlązak
In most Polish mines, there is mining with caving carried out and the seams of mined coal are characte-rized by great fire and methane hazards. As a result of roof rock shift, which takes place after the extrac-tion front of a longwall is shifted, a rubble heap and a fracture zone are formed. Methane liberated from the coal remaining in goaf and through the fissures in the surrounding rocks can be gathered in the goaf space. This process is characterized by unsteadiness in time and space of gas liberation, the dependence of constituent concentration on air flow conditions in adjoining roadways, on filtration rock properties inside the goaf with caving and on atmospheric pressure oscillation. It is very important to know the distribution of methane concentration in goaf when choosing the means of methane hazard prevention. It is necessary to know some selected features of goaf environment, such as goaf porosiły and perme-ability in order to consider the phenomena oceurring in goaf of longwalls with caving. The knowledge of the properties mentioned above is the basis of forming a model of air and gas flow through the goaf center. This article presents the mathematical model for methane transport in goaf space on condition that density distribution of its stream on the boundaries adjoining rock-mass is known. In order to solve equations describing methane transport in goaf, an adequate numerical procedure based on the finite-difference method, which leads to determining fields in question in a discrete form, is suggested. In case of equation (3) calculation methodology, similar to the one used when solving airflow through goaf of longwalls with caving and presented in work (Szlązak, 2000), is used. However, equations (8 and 9) required a different differential approximation as a result of the presence of convection terms. The differential scheme used in this case was suggested by Patankar (1980). On the basis of the presented calculation diagram a computer programme was written. It allows for calculating the distribution of methane concentration in goaf.
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2011