Predicting fugitive gas emissions from gob-to-face in longwall coal mines: Coupled analytical and numerical modeling

Understanding gas emission and transport from the gob to the active working face in longwall coal mines is necessary for mine ventilation and gas control planning and optimization. We document a mine-wide ventilation pressure and flow rate survey (p-Q survey) to establish a ventilation network model – including methane gas concentrations recorded at selected face locations. We propose an analytical pressure gradient model to evaluate gob gas emission and its interaction with the ventilation system. This model combines viscous energy losses along a tortuous gas flow path within the gob materials with kinetic energy losses at irregular cross-sections. A numerical gas emission model was also established to predict gas emission rates at the longwall face and to dynamically determine the gas emission rate from the compacted gob. Field monitoring indicates that steady methane concentrations increase monotonically and almost linearly from headgate to tailgate. The average methane emission rates are estimated as 0.0061 m³/s, 0.0044 m³/s and 0.00215 m³/s for wide, intermediate-width and narrow panels. A numerical network model of the mine was validated then calibrated against the field methane monitoring results at our partner mine. We observe that gob compaction and related porosity reduction significantly affects gas emission rate. An eleven-fold increase in stress (1.70–18.68 MPa) results in a nonlinear decrease in porosity of only ∼75% (from 0.368 to 0.093) but a 56-fold reduction on gas emission rate (compared to the maximum transient gas emission rate). The mine-wide ventilation system is especially sensitive to methane emission rates – a 50% increase in emission rate (from 0.00455 m³/s to 0.00637 m³/s) clearly impacts concentrations in the return branches. Peak methane concentration at related branches increase 39.7%, from 2.24% to 3.13% with the potential to trigger elevated methane alarms. These results can ultimately provide the data for analyzing the interactions between the caved gob and the ventilation system and define mitigation strategies to minimize gas concentrations and hazard.