Estimating greenhouse gas emissions from open-cut coal mining: application to the Sydney Basin

Abstract

A new site-specific (Tier 3) method has been developed to determine greenhouse gas emissions from open coal mining. The Tier 3 method presented here is based on extensive measurement of gas emissions from open-cut coal mines and the physics of gas desorption from coal. It was adopted by Australian National Greenhouse and Energy Reporting in 2009 and since 2012 formed the scientific basis for the Australian Government guidelines on calculating greenhouse gas emissions from open cut mines. The main strength of this method is its site-specific nature and accuracy, as well as its ability to be integrated with routine coal exploration programs. New concepts were produced for the model: a coal mine is regarded as a ‘gas reservoir,’ with coal seam gas being emitted from a ‘gas release zone’ that consists of sedimentary geological units (emission layers) above and below the base of the mine. The primary data required for the method are the in situ gas content and gas composition of the coal and carbonaceous rocks contained within the gas-release zone. These data are obtained through direct measurement of gas desorption from bore cores. To reduce gas drilling, a mine lease is compartmentalised into ‘gas zones’ of similar gas content and reservoir properties. The outputs of the method are emission density (the potential volume of gas emitted from mining site per unit area of the ground surface) and emission factor (the gas volume emitted per tonne of raw coal extracted). Owing to spatial variability and errors of measurement, the estimate of emissions is associated with uncertainty. A simple method of calculating uncertainty of emissions is presented in this work.