Toward the Development of an Improved Methodology For Estimating Fugitive Seam Gas Emissions From Open Cut Mining

The project goal was to develop a method for estimating fugitive emissions such that an individual mine could estimate its own emissions based on data related to coal gas properties and mining method. In the proposal for this work it was intended that the method would be based on:

• gas content of the mined coal
• gas content of uneconomic seams and neighbouring strata sent to spoil piles and
• coal production and mining method used.

In the course of project C9063 work was carried out at seven open-cut mines in the Hunter Valley and three in the Bowen Basin where measurements of surface emission and gas content of coal were made. Numerous direct measurements of emissions from uncovered coal seams as well as gas released in coal blast holes and exploration surface holes were made. Fresh coal samples from blasted coal seams were also collected and measured for their gas content and composition. Some samples were allowed to release their gas over periods up to 2 months to investigate the kinetics of gas release.

Measurements of the gas contents in the pit for coal collected from blasted seams in the Hunter Valley mines showed seam gas content of ~0.1 m3/t to more than 1.6 m3/t. Seam gas compositions for these samples varied from almost pure CO2 to 30% CO2, with the remaining gas being CH4. For some coal samples the rates of gas desorption were also measured over a period of a few weeks. For one coal with lump size of 100 mm 50% of the gas was still present after a time period of 6 weeks and 10% still present after ~4.5 months. This suggests that there may be significant amounts of seam gas in the coal leaving the mine. Further work is required to determine the full significance of this observation.

For the Bowen Basin results, measurements of the gas contents of coal seams in pit for the three mines studied varied from almost unmeasurable quantities to 1.5 m3/t. The higher gas contents corresponded to the seams with higher methane composition.

During the course of sample collection and measurement of gas content it was clear that sampling methodology and knowledge of the length of time since the coal seams were uncovered would have significant impacts on the results. Extended time periods from coal being uncovered, to being mined, allow seam gas to desorb. This is an important consideration in the rate of release of seam gas from the uncovered yet unblasted coal seam.

Measurements of the surface emissions from Hunter Valley and Bowen Basin mines showed a wide range of emission rates. For example at one mine the emission rates varied from 0.02 to 0.45 mgs-1m-2 (CO2 equivalents) over essentially similar surfaces.

The surface emissions data generally showed wide variability in the emission rate. This is because the surface emission rates depend on

1. The initial gas content of the coal
2. The elapsed time from when the overburden and coal was disturbed and the measurements made
3. The permeability of the layer over which the measurements were made which is in turn influenced by the mining method and blasting of the coal and overburden

Also the emissions can be expected to decrease with time as the gas desorbs from the target coal seam. Consequently it is not possible, at present, to generalise the above results in a manner so as to arrive at emission factors for the mines studied.

During the final phase of the project, effort concentrated on studying a purposely-drilled surface borehole at a Hunter Valley operation. Gas content of in-situ virgin coal seams up to a depth of 100 m was measured. The gas contents varied from ~0.4 to 3.7 m3/t. As expected, the smaller gas contents corresponded to the shallower seams. The seam gas composition also varied from almost pure CO2 near the surface up to almost 90% CH4 for the deepest seam at 95 m below the surface.

The results demonstrated the significance of water in the boreholes and the necessity to dewater these holes in order to measure gas emission rates. The borehole emissions approximate the emissions likely from a highwall on mine closure. This suggests a method whereby post mining emissions (after mine closure) can be estimated. Further work is required for this methodology to be developed to a stage where it can be used routinely to estimate the fugitive emissions after mine closure.

The fresh borecore samples also provided the opportunity to investigate the kinetics of gas release from fresh coal. For example a sample of the Vaux seam was allowed to desorb for a period over ~2 months. From the desorption history it was concluded that it would take 15 months in order for the coal to release 90% of its gas.

Based on the results of the current study it is clear that sampling coal from open cut operations on an opportunistic basis as attempted during this project is insufficient to allow the appropriate data to be obtained for a Tier 3 methodology to be determined. A dedicated borehole if required so that detailed gas content data can be obtained. These data could then be used, along with data from an extension of the exploration drilling program to include a limited number of gas content measurements, in order to develop a Tier 3 methodology. In addition further work on the emission rates from boreholes could see this approach developed into a method for estimating emissions from final highwalls after mining has ceased.