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Mine Site Greenhouse Gas Mitigation

Towards An Optimal Gas Sampling And Estimation Guideline For GHG Emissions From Open Cut Coal Mines

Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation

Published: July 11Project Number: C19005

Get ReportAuthor: Abouna Saghafi, Peter Hatherly and Kaydy Pinetown | CSIRO

This report describes an approach for estimating fugitive gas emissions from open cut coal mines in line with the Tier 3 model developed by previous ACARP project C15076 (Saghafi, 2008). The model requires gas content, gas composition and the thickness and position of coal seams and carbonaceous layers as the main inputs to calculate the emission factor in terms of volume of gas per tonne of coal produced (m3/t). Emissions can also be defined in terms of the volume of gas which can be emitted per unit area of ground surface (m3/m2). This parameter can be referred to as superficial emission density or emission density.

 

A major concept in the Tier 3 model developed in C15076, is the concept of gas emission layers where each geological layer is treated as an individual emission unit. Another concept is the gas release zone which is the sequence of coal and rock layers contained in a column of strata in the overburden, and to a certain distance in the underburden. The model calculates the potential volume of gas released during mining from each individual emission layer.

 

Some of the input data for the Tier 3 model, such as the density and thickness of layers, are readily available as they are measured through routine geophysical logging of boreholes drilled for exploration purposes. However, gas content and composition data are not routinely measured in open cut mining. As these data form the basis for the estimation of emissions and because of the large spatial variability of gas content and composition in the near surface, it is necessary to undertake sufficient drilling and sampling to accurately determine the distribution of these properties. Actual gas content depends on gas saturation which in turn is a function of groundwater movement and the gas retention capacity of the enclosing strata. To reduce the cost of gas testing, the most straightforward strategy is to collect samples from exploration boreholes when and where it is possible. However, sufficient amounts of core are not always available when they are also needed for quality and geotechnical testing. Therefore, specific gas testing holes may be required.

 

The approach developed in this project (C19005) uses existing geological and geophysical logs to derive the data for the Tier 3 model including gas content (through ash yield and depth), thicknesses and densities for all coal and carbonaceous material. The challenge has been to provide a method for predicting gas content as a function of depth. The gas content model forms the basis of a practical approach to estimating the fugitive emissions from a coal mine. In the course of this project (C19005), a model was developed to predict the potential gas content for various emission layers in the strata above and below the base of a mine. The gas content predictive model is based on the physics of gas storage in coal and carbon rich materials, and predicts potential gas content for the emission layers using, at this stage, the depth and ash data estimated from geophysical logs.

 

One important aspect of the gas content model is that by using this model, the emissions zone of the Tier 3 model can be treated as a continuous medium with emissions layer thicknesses as small as the geophysical log allows.

 

The approach developed in the course of this project is presented in the form of a practical procedure summarised in a flowchart and demonstrated through examples from two mine sites, one in the Bowen Basin and the other in the Hunter Coalfield. The procedure provides a robust method for determining emission factors for open cut mining.

 

In order to use this procedure it is recommended that:

· Basic geological information be available to allow the establishment of a geological context for the gas study. The geological features which have the potential to significantly influence the gas distribution need to be identified.

· Quality geophysical logging data also be available. These data, together with the gas content model, constitute the main input of the approach. Calibrated density logs, in particular, must be available so that coal and carbonaceous layers can be accurately delineated and their ash yields accurately estimated.

· A sufficient number of gas testing holes and core samples be available so that a model for predicting the spatial distribution of the gas content can be built. Accurate gas measurements must be undertaken as the errors in low gas content determination can be high.

· A quality control procedure be followed to check the quality and consistency of all data - gas content and composition, coal quality and geophysical logs.

This approach represents an important first step towards providing a comprehensive model of gas content distribution for emissions calculation. The model as demonstrated in the case studies can be implemented in its current form to provide an emissions estimate. However, further development is encouraged to increase the comprehensiveness and accuracy of the approach. An immediate area of further development of this approach is a method of estimation of the uncertainty of the emissions estimate.

 

The method developed for fugitive gas emissions estimation in this project has been demonstrated at two sites. However, more experience in the use of this method is required. We recommend that further studies at additional sites be undertaken to gain experience with this approach.

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