A power plant in Australia has found an unusual energy source -- methane from the ventilation air produced by its coal mine. At the West Cliff Colliery of BHP Billiton, a mining and resources company, the power plant uses the technology to generate electrical power.
The system, developed by De Pere, Wis.-based Megtec Systems, is based on a patented combination of emission control and steam-cycle technologies. By using a compact and flameless Vocsidizer regenerative thermal oxidizer as an energy source, Megtec made it possible to generate high-grade, superheated steam from a fuel with 0.9 percent methane content. The steam has the same quality as that used by traditional power plants. It is used in this application to drive a conventional steam turbine, which generates electricity that can be delivered to the state grid or directly to the mine, all while reducing harmful emissions that would otherwise be released into the air.
Each hour, the installation is treating 0.9 percent of the methane content in 150,000 scfm of ventilation air. The energy produced from the methane is used to generate steam suitable for running a conventional 6 MW turbine, reportedly resulting in the first power plant of its kind. The volume of ventilation air treated amounts to only 20 percent of the total volume of ventilation air available from the ventilation shaft.
The mine has a ready supply of the gas because when coal is formed, so is methane. Likewise, when the coal is excavated, methane is released. Because methane in air is explosive in concentrations between 5 and 15 percent, ventilation air is used to dilute the methane to levels below 1 percent -- well below the explosion limit. However, this safety measure leads to very large volumes of air with extremely low methane content being released into the atmosphere. Using the gas to power the turbines allows the mine to reduce its emissions.
Methane is a greenhouse gas more than 20 times more potent than CO2. BHP Billiton can now convert the emission reduction to carbon credits corresponding to 250,000 tons of CO2-equivalents (CO2e). A ventilation air methane power plant treating the full volume of ventilation air can generate approximately 1 million carbon credits. In comparison, methane emissions of a cow amount annually to between 1 and 2 tons of CO2e, while a car annually generates emissions corresponding to a typical 2 to 4 tons of CO2e.
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