Underground » Ventilation, Gas Drainage and Monitoring
Gas emission prediction plays a critical role in mine ventilation and gas drainage planning and design, to meet coal production targets and safety standards. Gas emission is often expressed in the form of specific gas emission (SGE, unit: m3/t), which refers to the volume of gas emitted per tonne of raw coal produced. The Winter and Flugge models, developed in West Germany in 1950s‐1970s, are the two common methods used in Australian coal mines. However, in practice, the predictions are loosely correlated with actual gas emissions during longwall mining. A comprehensive research program including a literature review, field measurements, numerical modelling, theoretical analysis and case studies was performed to develop a new gas emission prediction method that suits Australian mining conditions.
Investigation into longwall gas emissions at various coal mines suggested that the SGE varies significantly with longwall retreat, but the trend is mostly controlled by the actual gas‐in‐place, interburden thickness between the coal seams, caving mechanics and production rate.
Residual gas content after mining was measured at four different coal mines by drilling into goaf zones (post‐mining goaf holes) and testing coal cores. These measurements identified the key issues associated with the conventional models. Based on the field measurements, a preliminary model was developed in the initial stage of the project. The model is of similar form to the conventional ones, but it adds the mining thickness as a new parameter.
A comprehensive model was consequentially developed. The model incorporates Airey's equation that describes gas emission processes from broken coal, and a three‐zone characterisation of caving mechanics and gas emissions patterns. It takes into account a time‐dependent parameter for different coal seams, which enables prediction of SGE at different production rate.
A spreadsheet tool was developed to facilitate easy uptake of the comprehensive model, as well as to incorporate many other factors that significantly impact longwall SGE but are not considered in the conventional models. These factors include caving parameters, production rate, surrounding gas emissions, longwall face position and gas‐in‐place variations. The spreadsheet tool enables automatic calculation and update as input parameters are varied. It predicts not only SGE, but also its changes with production rate to better support longwall ventilation planning. It can also generate a cumulative gas emission profile in the longitudinal directional of the longwall goaf, which provides an understanding of dynamic gas emissions. Two case studies are presented to demonstrate the use of the spreadsheet tool, along with detailed instructions. The case studies demonstrate that, with appropriate input parameters, the spreadsheet predictions are very close to the actual observations for SGE, as well as its variation with production rate.