Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation
The outcomes of this project are published as two reports. The first summarises the findings of a project aimed at developing a mine ventilation air methane catalytic combustion gas turbine. The second report details the design and testing of new technology that involves the combustion of ventilation air and drainage gas together with coarse reject waste coal from the mine in a rotating kiln.
Report 1: Catalytic Combustion Lean Gas Turbine
This report summarises the findings of a project aimed at developing a mine ventilation air methane catalytic combustion gas turbine. Specifically, the goal of the project is to develop a catalytic combustion gas turbine that can be powered with 1% methane in air. This allows the turbine to utilise a much greater proportion of ventilation air compared to other technologies. Additional methane from drainage gas is required to increase the methane content to a constant concentration of approximately 1%. The efficiency of this power generation system of using methane from ventilation air is estimated to be approximately 20%. The initial stage of this project is documented in this report and encompasses experimental testing of catalysts and the development of a design for a demonstration plant. This report includes a review of mine ventilation air methane utilisation technologies, a review of methane catalytic combustion mechanism and kinetics, a discussion of different types of catalytic combustion reactors, a description of the benchscale rig of CH4 catalytic combustion constructed for the project, a summary of the experimental results and the design of a plant that will demonstrate proof-of-concept.
The experimental rig for the catalytic combustion tests consist of two air heaters, a mixer, a catalytic combustion chamber, a cooler, fuel supplier, power supplier, and sampling system. Bench-scale tests were used to obtain the design parameters of a pilot-scale catalytic combustion gas turbine. Four catalysts were tested at different temperatures, pressures and methane concentrations. Initially, a key operating pressure of 6.2atm was used in tests to establish operating parameters that matched an available turbine, however lower pressures were tested when it was determined that this turbine would not be optimal for the purpose. One of four catalysts was identified as offering the best performance and the highest operating temperature.
Report 2: Rotating Kiln for Solid Rejects and Waste Gas
This report details the design and testing of a new technology that involves the combustion of ventilation air and drainage gas together with coarse reject waste coal from the mine in a rotating kiln. Heat from combustion is converted into electricity by the use of an externally fired gas turbine, although alternative power generation technologies such as a steam turbine with waste heat boiler could be used. Co-firing of waste coal with the ventilation air ensures the oxidation of the low concentration methane. In addition, the waste coal allows the concentration variation to be managed and ensures a constant energy flow for use in power generation. A kiln was chosen, instead of a conventional fluidised bed, in order to study the feasibility of converting the coarse reject material into a lightweight expanded aggregate, a valuable by-product. A life cycle analysis showed the flexibility in the design of this technology provides the maximum greenhouse advantage over other systems because of its ability to burn large quantities of ventilation air.
To provide a pathway for the commercial application of the results of this project and to assist with meeting the costs of the research, a company has been formed, ComEnergy. ComEnergy is a joint venture partnership between the Liquatech Turbine Company, the Sustainable Energy Development Agency of New South Wales (SEDA) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia. During the course of this project, the consortium has built a 200 kW externally fired turbine system which can harvest the heat from the combustion of waste coal and ventilation air. This report details the design and construction of such a facility and its test with waste coal from various NSW mine sites.
Four coals were tested, BHP Billiton's Appin Colliery, South Coal's Metropolitan Colliery, Anglo Coal's Dartbrook Colliery and Xstrata's United Colliery. To reduce the cost and energy consumption of pre-processing the waste coal material, it was decided to test the coals from each mine in the unprocessed state as received from the mine. The only processing was screening out of material with a size greater than 50mm. A small qualitative study was performed on individual aggregate to study the sintering and bubbling processes.
The results showed that although it was possible to burn the waste material from each of the mines in the kiln an additional fuel, in the form of LPG, was required during the tests to maintain combustion and to achieve complete carbon burnout. Further research is required to reduce the need to provide this additional fuel. Other mine site fuels such as drainage gas, tailings or high quality pulverised coal can be substituted for the LPG allowing the technology to be applied to all mines.
One of the primary reasons for choosing the kiln technology to test the coal was the possibility of converting the waste into a valuable lightweight aggregate. However, the high levels of silicon and aluminium within the coals tested, result in a high melting point for the ash. The softening point of the aggregate needs to be exceeded to ensure sintering of the various molten particles. Given the high melting points, and the need to have a high temperature to maintain effective heat transfer, the required flame temperature results in significant levels of oxides of nitrogen (NOx). This, coupled with the already high levels of nitrogen present in the coal, means NOx emissions far exceed the legislated levels. Of the four coals tested, only the waste material from Dartbrook, which has a low ash melting point, was a candidate for aggregate production.
In conclusion it was found that, while the testing of an externally fired turbines produced significant results to allow proceed with the development of a much larger commercial system, a significant amount of work is still needed with regard to the design and operation of the coal burning system and optimisation of the externally fired turbine design. The inability of the system to produce lightweight expanded aggregate on the majority of coals has meant that this burning system need not limit itself to the use of a rotary kiln system.