Underground » Ventilation, Gas Drainage and Monitoring
As longwall mining increasingly targets deeper coal seams, managing high goaf gas emissions has become a significant challenge, particularly in maintaining effective gas management and ensuring uninterrupted mine production. If not properly controlled, goaf gas can migrate to the longwall face, especially at the localised tailgate end, leading to potential safety issues and longwall stoppage. This project reviews various longwall ventilation control practices aimed at mitigating the high localised emissions of goaf gas onto the longwall face and evaluates the effectiveness of these measures using computational modelling, thereby reducing gas-related safety risks and production delays.
The literature review highlights several control measures and engineering designs developed and implemented in Australian mine sites for managing gas on the longwall face. These mainly include pre-drainage and post-drainage techniques, different longwall layouts, ventilation systems, and localised measures, such as curtains and wings, bleeders, and back returns. Essential data and information were collated from 10 participating underground coal mines, and publicly available reports. A critical examination of the data revealed that relying solely on ventilation air for gas management on the longwall face often requires impractically large air volumes due to the statutory methane concentration limits in return airways and at the tailgate drive.
A two-day industry technology transfer workshop was held at the University of Wollongong which provided an opportunity for participants to discuss common issues and failure modes leading to gas exceedance on the longwall face, and share their knowledge and experience with various engineering practices for managing elevated gas levels at the tailgate end. The key outcome of the workshop was a categorised summary of failure modes and control measures for different longwall face locations, detailed in Appendix A.
Three case studies were performed based on site-specific geologic and mining conditions, with a special focus on the longwall layout and ventilation management, and the analysis of gas exceedance events or gas related delays. Interpretation of the gas monitoring data indicated that the shearer position or support operation around the tailgate of the longwall face was the common cause of elevated gas levels at the localised tailgate end, and less effective post-gas-drainage was another factor contributing to high gas concentrations onto the longwall face.
Three dimensional computational models were constructed by incorporating major longwall equipment, and focusing on the tailgate area to optimise computational efficiency. Generic simulations of a U-type ventilation longwall model showed that air quantities provided to the longwall face must match the total gas emission rates. The results indicated that higher airflow rates are required to dilute gas levels near the tailgate drive in CH4-dominant goaf compared to CO2-dominant goaf, given the same goaf gas emission rates and permeability distribution in the goaf area. Site-specific computational modelling results demonstrated that increasing the face ventilation rate had a limited effect on controlling gas concentrations at the tailgate end. However, applying back-overbleed ventilation or tailgate brattice significantly reduced gas levels in that area.
This project enhances the understanding of goaf gas migration onto the longwall faces, particularly at the localised tailgate end, and offers recommendations for improved gas control practices in these areas.