Estimation of fugitive emissions from open cut coal mining and measurable gas content

To evaluate fugitive emissions from open cut coal mines, emission factor values of 3.2 m3/t and 1.2 m3/t have been used for the two main Australian coal-producing states of New South Wales and Queensland, respectively. CSIRO developed these values in the early 1990s. They were meant for use as average regional values (Tier 2 method), but were subsequently used for all mines, irrespective of the level of ‘gassiness’ of specific coal seams and strata. Over the past decade, A new method has been developed for Australian open cut mining that is specific to each mine site (Tier 3 method). The proposed method has been adopted by National Greenhouse and Energy Reporting and is the basis of Method 2 or 3 for calculation of emissions. The new method is based on an emission model, which considers the coal seams and sedimentary gas-bearing horizons (layers) as individual gas reservoir units. These units release part or all of their gas during mining. The main data required are in situ gas content, gas composition and thickness of the gas-bearing horizons within the column of strata above and below the mine base. In this method, drilling can be reduced by partitioning the mine site into ‘gas zones’ in which similar patterns of gas distribution are expected. Two to three core drillings are required to characterise a gas zone and to provide the main input of the model. Routine geophysical log data can also provide the thickness of gas-bearing layers. Because of the limitations of the standard gas content measuring method, different commercial laboratories claim various limits of detection (i.e. measurability). However, in view of the very different global warming potential values of coal seam gas components, different limits of measurability can lead to significant differences in the estimation of fugitive emissions

Gas Content and Emissions from Coal Mining

Gas content can be considered the most important parameter for assessing emissions from coal seams during and post mining. Traditionally, the purpose of gas content determination was to assess gas outburst potentials and to quantify the magnitude of gas emissions into underground headings and at the coal face. Therefore, it’s a traditional definition; measurement and determination are based on these objectives. However, the calculation of emissions from mining for the purpose of establishing a greenhouse gas inventory would require new definitions for gas content and new measurement methods. Moreover, errors inherent in measuring gas content need to be quantified so that the uncertainty of emissions inventory can be evaluated. Therefore, various definitions of gas content in relation to the purpose of its use are suggested. Anew method of measurement for low gas content coals is discussed. Moreover, various parameters influencing the value of measured gas content are discussed and errors of estimation using the direct method are evaluated

Determination of the Gas Content of Coal

In coal mining the gas content of coal is required primarily to quantify the gassiness of coal for safe mining, but also to quantify potential greenhouse gas emissions from mining. In Australia the gas content of coal is determined using a direct method, whereby the gas desorbed from solid or crushed coal is collected and the volume and composition of the desorbed gas are measured. The determination of gas content is associated with errors of measurement of the volume and composition of the gas. It is undertaken at several stages of gas desorption. Relative errors and resulting uncertainties of determination are more significant for the estimation of lost gas during drilling and gas remaining in coal following the completion of the standard stages of measurements, whence the rate of gas desorption is significantly reduced. This paper discusses the current Australian method and potential errors and uncertainty associated with this method. A new method of measurement for measurement of remaining gas in coal following the completion of standard gas content testing is also suggested. The new method should allow the release of almost all remaining gas in powdered coal following the last stage of standard gas content testing

Gas Wettability of Coal and Implications for Gas Desorption and Drainage

A key parameter affecting the flow of gas in coal is the wetting potential of gas, in comparision to water, to spread over the wall of coal micropores and microfissures. Wettability is quantified in terms of the contact angle of the fluid interface with the solid surface. A fluid with a small angle of contact would spread over the pore walls and eventually displace the non-wetting fluid. Depending on the nature of the coal, gas type and environmental conditions in coal reservoirs, either water or the gas phase could wet coal more strongly. Furthermore, in mixed gas conditions, one gas may be more strongly attached to coal than the other gases. In water-saturated coal, gas desorption in small pores -where most adsorbed gas is stored - can be totally inhibited by water if it is a strong wetting phase. Reducing the hydraulic head (drawdown to achieve the gas desorption pressure) should allow desorption of gas in larger fractures, whereas in small pores, gas desorption could be inhibited by capillary pressure due to the effect of interfacial tension and gas-wetting properties of coal. In this study, we built a new system to quantify the wettability of coal by gas. The contact angle of the water-gas interface with the coal surface inside the gas phase was measured using a captive gas bubble technique. The contact angles of CH4 and CO2 bubbles in water with a coal from the Sydney Basin were measured at different gas-water pressures of up to 15 MPa for CH4 and 6.1 MPa for CO2. The results show that as gas bubbles dissolve in water, the contact angle of the bubble with the coal surface reduces. The contact angle values were smaller for CO2 gas than CH4, and in general, the contact angle value decreases as gas–water pressure increases

Parameters affecting coal seam gas escape through floor and roof strata

Coal seams are compact gas reservoirs and can contain large volumes of methane (CH4) and carbon dioxide (CO2) which are the main constituents of coal seam gas (CSG). CSG is present in various volumes and concentrations across the mining regions in the coalfields of the Sydney and Bowen basins. The variations in actual gas volumes and relative concentration of these gases in coal could be due to different gas generation/accumulation rates and different adsorption capacity of the coals, but also because of the difference in the sealing capacity of the non-coal sediments enclosing the coal seams. It is postulated that the sealing capacity of the main roof and floor rocks at a coal seam could have a major effect on the volume of gas in place (gas content). This paper reports some results of an ongoing investigation on the gas flow parameters which affect the sealing capacity and retention of gas in coal reservoirs. The results discussed here concern, in particular, the matrix permeability (or micro permeability) and the diffusivity of the non-coal horizons in the roof and floor of the coal seams. These properties could be limiting factors on the rate of gas escape from a coal formation to the surrounding strata

A study of potential occurrence of biogenic methane in coal seams

A significant proportion of the total gas emitted from coal mining, particularly for shallow seams (depth), is believed to have been generated from microbial activities within the coal seams and water filling the pores and fractures in coal. To investigate the potential and extent of gas generation in coal due to microbial activities, we developed a method to culture and monitor the production of biogenic methane in coal. We then applied the method to study the process of biogenic methane generation in coals from a mining region in New South Wales. Fresh coal core samples were collected from an exploration borehole drilled into a sequence of coal seams at a greenfield site where five coal seams were located between the depths of 50 to 250m. The formation water was collected from an adjacent borehole drilled into the same sequence of coals. The coal samples were crushed and mixed with formation water and other solutions in glass vials, and then placed in pre-designed incubator at in-situ temperature to allow the production of methane over the life of the project. The results of measurements show that biogenic activities take place and that methane is generated. Methane continued to be produced throughout the life of the project for the studied coals

Developing a New Method to Identify the Source of Gas Emissions into Longwall and Goaf from Surrounding Strata

During coal mining, strata is fractured and gas trapped in the roof and floor of coal seams travels into the workings. Depending on the extent and shape of fractured zones suitable gas drainage patterns are required to maximise the gas capture from strata but also to minimise the cost of operations. In this paper a new method to identify gas emitting zones/seams in the embedding strata and gas migration pathways is presented. The developed method was used in a coal mine in the Southern Coalfield of the Sydney Basin. Geochemical properties of gas trapped in coal seams above and below the mining horizon were analysed and compared with similar properties of gas collected from goaf areas. This study shows that using this method it is possible to identify the source of gas in goaf areas and thus determine the extent of fracturing in the strata around the mined seam

A new method to determine the depth of the de-stressed gas-emitting zone in the underburden of a longwall coal mine

Underground coal mining induces de-stressing and fracturing of strata above and below the targeted seams. This creates a gas-emission zone, which contains gas-bearing coal seams and strata in the roof and floor of the mined (working) seam. In longwall mining, most of the gas released from the emission zone escapes into the coal face and the goaf (caved-in area) behind the coal face, where it presents a safety issue. Depending on the extent and shape of the emission zone, various gas drainage strategies could be applied to maximise capture of the gas from the emitting seams. We developed and trialled a new method to identify the gas emission zone in the underburden of an underground mine in the Sydney Basin, Australia. In this operation, all coal seams are located below the major targeted seam for mining. By measuring the isotopic and molecular composition of gas desorbed from coal cores from exploration drilling and gas collected from the goaf, we identified the source of gas and quantified the limit of the emission zone in the underburden of the working coal seam. This has allowed drainage to be focused and limited to the required depth. Our study will assist others to plan the required depth of gas drainage drilling below the floor of mined seams