Evaluating a Tier 3 Method for Estimating Figitive Emissions from Open Cut Coal Mining

Fugitive emissions from open cut coal mines arise when coal and associated strata are broken and disturbed as part of the mining process, liberating the seam gas that might be trapped within coal seams.  The IPCC (Inter-governmental Panel on Climate Change) recommends using an Emissions Factor (EF) approach as the basis for estimating these emissions. The EF is the volume of gas (m3) released per tonne of coal produced.   At present Australia uses a method based on previous CSIRO work from the early 1990's when emission measurements based on plume tracking were made for 17 open cut mines in the Bowen Basin and the Hunter coalfield. The emissions varied widely between mines, eg Emission Factors for Queensland mines ranged from 0.1 - 4.5 m³/t. The values obtained were averaged to produce the current EF values for NSW and Queensland ie 3.2 m³/t and 1.2 m³ /t respectively.

CSIRO, with funding from the Australian Coal Association Research Program (ACARP), has focussed on developing a practical method to estimate mine specific emission rates using gas content and other data gathered from boreholes drilled ahead of mining.  The proposed method is based on the concept of the 'gas release zone' which consists of the geological strata (layers) affected by coal extraction operations. The primary data required is the in situ gas content of coal and rocks contained within the gas release zone, prior to mining.  The basic assumption of the method is that this gas is released on mining, with appropriate allowance for emissions from the mine floor and any residual gas remaining with the coal at the point of use. In this methodology, regions of similar gas content and reservoir properties are termed a 'gas zone'. Emissions Factor values from various locations in a given gas zone is expected to be similar.

The most important parameter of the model is the in-situ gas content. The in-situ gas content of a coal seam can only be determined from the measurement of gas content of  borecores.  Two laboratory methods for determining gas content are presented in this report and one (quick crush) is recommended as preferred.  It is also important to determine gas composition, ie methane (CH₄) and carbon dioxide (CO₂), as CH₄ has a significantly greater greenhouse warming potential than CO₂.

This report details the proposed estimation method and presents a series of case studies illustrating its application to a range of mining scenarios. The case studies were:

• Case 1: A moderate to deep open cut mine with a moderate gas content and data obtained from borecores specifically drilled for this project.    
• Case 2: A moderate to deep open cut mine with a low gas content and data obtained from borecores specifically drilled for this project.
• Case 3: A shallow open cut mine with a low gas content and data obtained from previous exploration drilling as well as from one purpose-drilled borehole.
• Case 4: An open cut mine for which no direct gas content data are available. The case study used the gas content data from an adjacent underground mine. These were extrapolated to shallow depths.

Cases 1 and 2 belonged to two different gas zones (separated by about ~8 km) and measurement of the gas contents from borecores of these two different zones showed significant differences. Within each individual gas zone, however, the measured gas contents were very similar (boreholes drilled at ~0.5 km within the same zone). This indicates the need for an appropriate data collection campaign tailored to each mine.  Case 1 and 2 were also used to estimate the cost of data acquisition.  While this cost will vary between mines an indicative cost per borehole is ~$180,000 if drilled for the sole purpose of gas content determination, and ~$105,000 if the hole is drilled as part of routine exploration. Note that this latter borehole was drilled in a geologically complex area and to a depth of about 200m. For shallower open cuts with similar or less geological complexity this cost should be lower.   

Case 3 illustrates the degree to which drilling costs can be minimised if gas content data is available from prior exploration drilling in the immediate vicinity and at similar depths.  In such cases it may only be necessary to drill additional boreholes to account for known geological anomalies such as faults.

Case 4 illustrates an approach in situations where no gas content data for the target coal seams are available and the data cannot be acquired at the time an estimate is required. In this case the input data for the model were estimated based on the gas content data for an adjacent underground mine. This was done by extrapolating the measured gas content data to shallow depths. This, however, increases significantly the uncertainty of the estimate.

Past studies suggest that residual gas content, ie gas that remains with the coal up to the point of utilisation and would hence form part of the fuel value of coal fired in a power station, is generally insignificant.  However for a few coals tested in this project the residual gas content was ~0.4 m3/t or around 10% of in-situ values.  In such cases the reported emissions should be adjusted to reflect this residual gas.  

In brief the outcomes of this work are as follows:

• A working model of seam gas emissions from open cut mines is now proposed. The model requires the knowledge of the in-situ gas content of coal seams and carbonaceous rock in the overburden, and to some extent, into the underburden. In view of typically large spatial variations in gas content at shallow depths the best way to obtain the gas content data required for the model is to drill and core surface boreholes for the purpose of measuring the in situ gas content. If the boreholes can be drilled as part of the mine routine exploration program the costs would be shared and the emissions estimate portion of the expenditure would be reduced substantially.  
• For planning the drilling program the mine lease may be partitioned into a limited number of gas zones where the coal seams and strata layout and the hydrology would suggest similar gas behaviour. Within the same gas zone no significant variation in gas content should be expected.  
• The gas content measurement method used may influence the gas content value. For the case of mixed gas it is recommended that a quick crush method be used so that the dissolution of CO2 in the measuring water is reduced, and frequent gas composition measurements would be avoided.  

The new model provides a basis for estimating fugitive emissions on a mine by mine basis.  The key input for the model is gas content data for the shallow seams targeted for open cut mining. These data are not, however, routinely measured as part of open cut coal mine exploration drilling. Consequently, application of the model requires that these data be obtained, where necessary.