A Tier 3 method to estimate fugitive gas emissions from surface coal mining

Abstract

Abstract Fugitive coal seam gas emissions from open cut coal mines (surface mining) arise when coal and associated strata are blasted, fractured and disturbed as part of the mining process. This liberates the seam gas that is trapped within coal seams. The Intergovernmental Panel on Climate Change recommends using an emission factor ( EF ) approach as the basis for estimating fugitive coal seam gas emissions. The EF is the volume of gas (m 3 ) released per tonne of coal produced. Three levels of accuracy are associated with estimation of these emissions: Tiers 1, 2 and 3, each having increasing levels of accuracy. Tiers 1 and 2 provide average EF values for the whole country or the coal basin, while Tier 3 provides EF values specific to a coal mine. Until recently, Australian open cuts used nominal EF values of 3.2 m 3 /t and 1.2 m 3 /t for the two main coal producing states of New South Wales and Queensland, respectively. These values were used for all mines in these states, irrespective of the level of ‘gassiness’ of specific coal seams and strata. During the last few years, we have developed a new method for Australian open cut mining that is specific to each mine site. The proposed Tier 3 method, which has been adopted by the National Greenhouse and Energy Reporting, considers the coal seams and clastic rock horizons as gas reservoir units that release their gas during mining. The primary data required are the in situ gas content and gas composition of coal and carbonaceous rocks contained within the column of strata called the ‘gas release zone’. In this methodology, regions of similar gas content and reservoir properties are termed as ‘gas zone’. A small number of drillings are required to characterise the gas zone and to provide inputs to the model. This paper describes this new method of estimating fugitive gas emissions from surface coal mining. To illustrate the procedure of the calculations, the method is applied to an active Australian open cut coal mine.