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.

为确定露天采煤的温室气体排放量，已经开发出一个新的位点特定的（第三层）方法。这里介绍的第三层法是基于露天煤矿天然气排放量的广泛测量和煤瓦斯解吸的物理性质。它被2009年澳大利亚国家温室和能源报告所采用，并且自2012年以来成为计算露天煤矿的温室气体排放量的澳大利亚政府指南的科学依据。这种方法的主要优点在于它的位点特定的性质和精确性，以及它可以与常规煤炭勘探项目相结合。为该模型形成了一些新的概念：煤矿被视为“气库”，煤层气从“天然气释放带”中排放出来，该带是由煤矿底部之上和之下的沉积地质单元（排放层）组成。该方法所需要的主数据是原位天然气内容和包含在天然气释放区域内的煤和碳质岩石的气体成分。这些数据通过直接测量岩芯的天然气解吸而获得。为了减少天然气钻井，矿井租约按相似的气体内容与储气特性而分成“天然气区”。该方法的结果是排放浓度（采矿点每单位地表面积的潜在天然气排放量）和排放因子（每吨原煤的气体排放量）。由于空间变异和测量误差，排放量的估计具有不确定性。这项研究提出了一种计算排放量不确定性的简单方法。

Keywords:

• greenhouse gas emissions
• coal mining
• open cut
• gas content
• methane (CH₄)
• carbon dioxide (CO₂)

ACKNOWLEDGEMENTS

The author wishes to thank CSIRO and ACARP for providing the necessary funds to undertake this research. The in-kind support provided by numerous coal mines in NSW and Queensland is gratefully acknowledged. The author also acknowledges the contribution of many CSIRO staff who contributed at various stages to mine emissions projects over the past two decades, including J. Carras, C. David, S. Day, the late C. Dokumcu, M. Drummond, R. Fry, P. Hoarau, A. Lange, A. Quintanar, the late D. Roberts and D. Williams.