85 / 2023-06-14 14:21:16

Control practices of longwall face ventilation to mitigate goaf gas emissions onto walkways and tailgate end

Longwall Face Ventilation; Gas Management; Goaf Gas Emissions; CFD Modelling; Tailgate ends

As underground coal extraction progresses into deep and gassy coal seams, the management and mitigation of high-volume goaf gas emissions are presenting significant challenges to mining safety and productivity. Goaf gas migrating onto the longwall (LW) face workplaces may increasingly become an issue leading up to gas exceedance incidents, particularly for longwall panels targeting gassy and multiple seams with high goaf gas emissions (>10000 l/s). High gas emissions could become more challenging for longwalls extracting gassy, low permeability and CO₂-rich seams, where pre-drainage is less effective and too much reliance is placed on the post-drainage system. In combination with literature review, data acquisition and analysis and extensive computational fluid dynamics (CFD) modelling, this study investigates the LW ventilation control practices to mitigate localised high goaf gas emissions onto the LW face, particularly in areas in close proximity to the tailgate end, thus minimising gas-related production delays and potential safety hazards.

A review of historical data and public online resources indicates that as underground coal mines continue to deepen and increase their LW production rates, the effective control of goaf gas (e.g., methane or carbon dioxide) concentration in the LW return airways has become a pressing issue [1-2]. A trip-off of power to LW equipment will occur and underground workers must be withdrawn if the methane/carbon dioxide concentrations exceed the specified standards. In the worse scenario, air mixtures with the methane concentration lying in the explosive range could be present in the LW atmosphere, posing a considerable challenge to mining safety. It is highlighted by reviews of gas fundamental science and LW gas management that the gas distribution “goaf stream” exists, characterised by high levels of methane and reduced oxygen concentrations.

A critical examination of the data collected from participating underground coal mines clearly showed that relying solely on LW ventilation air for gas management would necessitate large and frequently impracticable volume of air, which was attributed to the statutory methane limits in the return airways and at the tailgate (TG) drive. A broad variety of gas exceedance events disclosed in mine surveys have been categorised by location with respect to the LW face (maingate (MG), mid-face and TG), with a special focus on events where ventilation controls are directly related to the exceedance. Themes of gas exceedance events are aligned with the broad principles of LW gas management, and the classification of event themes allows direct alignment with CFD model configuration, boundary conditions and parametric studies.

Three-dimensional (3D) models were developed to conduct steady-state simulations of ventilation behaviour and goaf gas flow dynamics in the LW panels based on site-specific geologic and mining conditions and gas monitoring data collected. Major LW equipment (e.g., hydraulic supports, shearer, armoured face conveyor and TG motor) was incorporated into the CFD models, but only equipment located at the research of interest was included with due consideration of reducing model elements and saving computational time. The simulation results allow for enhanced insight and understanding of gas accumulation mechanisms near the TG ends, and the effectiveness evaluation of different controls/practices for managing elevated gas levels at this localised area, thus identifying best controls/practices and improving mining health and safety. The simulation results indicated that both the TG brattice with appropriate configurations and locations and back-over-bleed ventilation can help considerably reduce the gas concentration to a safe level at the localised TG end, assuming that the airflow quantity through the LW face practically matches with the goaf gas emission rate.

The findings of this study provide recommendations for establishing best practices and designing ventilation and gas control measures, which can serve as general guidance and training resources for effectively managing LW face ventilation and gas emissions.