Evaluation of coal mine methane inventory methods

Australia uses a blend of IPCC Tier 2 and 3 bottom-up approaches to estimate and report fugitive coal mine methane (CH4) emissions. To date, Tier 3 reporting for underground coal mines, predominantly reliant on direct measurements of ventilated air, lacks systematic assessment against top-down atmospheric measurements. This study employed two aircraft-based approaches to quantify the rate of CH4 emissions from 17 coal mines in the Bowen Basin, Australia. The methods included airborne quantifications compared to operator estimates derived from NGER methods, incorporating data on CH₄ isotope levels and operator reporting across aircraft. The findings revealed discrepancies between the airborne measurements and bottom-up estimates from operators, highlighting the regulatory inadequacies of NGER reporting. Further analysis examined non-coal sources contributing to airborne CH4 emissions and underscored the complexities involved in quantifying these emissions using airborne measurements. The implications of these results for improving inventory methods and regulatory frameworks are discussed.