There are vast, deep coalbeds around the world which contain trapped methane. Coalbed methane (CBM) and low purity mine ventilation air (MVA) are produced as a waste by-product of coal mining. There is also trapped CBM contained in unmineable coalbeds.
For safety reasons, methane is drained from underground coal seams before mining. Due to the explosive nature of methane when mixed with air, liberated CBM in mine workings is a significant safety hazard. It is partially managed by using the mine ventilation system to dilute the CBM concentration to below the lower explosive limit of methane. Other methods include pre-draining the gas from the coal with in-seam or surface boreholes, drilling cross-measure drainage boreholes into adjacent seams which are activated by longwall mining and draining gas.
Vented to atmosphere, this methane is wasted energy and a potent greenhouse gas. Using this gas in a greenhouse gas neutral manner is attracting attention. In such a process, the CO2 produced from a power plant would be injected into the coalbeds to produce more methane, continuing the cycle. In addition, a geological sink is established in the coalbeds, virtually eliminating release of CO2 into the atmosphere.
A number of such projects are being developed around the world. These include:
Appin Tower Power Project, Australia.
Steetley and Bentinck, UK.
Appin is one of five underground mines in the southern coalfields of New South Wales managed by BHP Coal. Two million tonnes a year of coal is mined from the Bulli seam. The Bulli seam has an average in situ gas content of 15 m3/t with a gas composition of 90 per cent methane and 10 per cent CO2. CBM is predrained from the mine by drilling boreholes up to 1 km into the seam in advance of mining. In addition, gas is also collected from abandoned mine workings.
The plant uses CBM and low purity Mine Ventilation Air (MVA) as a fuel. The MVA contains about 0.4 per cent methane. About 20 per cent of the MVA is collected and mixed with CBM. As a result, greenhouse gas emissions have been reduced by two million tonnes per annum of CO2 equivalent by reducing methane emissions, and 0.6 million tonnes per annum of indirect reduction by displacing coal-fired electricity.
The Appin Tower Power Project produces 94 MWe. The plant consists of 94 gas engines from Caterpillar. These engines are 1 MW V16 cylinder twin turbo-charged G3516 gas engines.
The project has been in operation for two years. It has exceeded its design targets for capacity, availability and engine efficiencies. Engine availability has been 90 per cent compared to the original design estimate of 85 per cent. The thermal efficiency of the engines has been improved from 30 per cent to 35 per cent and the net power output per engine has been increased from 926 kWe to 1030 kWe.
The Alberta Research Council is leading a group to exploit CBM from abandoned coalbeds in Alberta by injecting CO2. The novel aspect of this approach is that the injected CO2 is adsorbed in the coal, to be stored in its pore matrix, releasing the trapped methane. This establishes a geological sink in the coalbeds, virtually eliminating any release of CO2. The project had two main objectives:
Reducing greenhouse gas emissions by subsurface injection of CO2.
To enhance coalbead methane recovery factors and production rates as a result of CO2 injection.
The project is divided into three phases:
Proof of concept study.
Implementation of a micro-pilot test.
Implementation of a full-scale pilot project.
Steetley and Bentinck
Independent Energy signed a contract with Deutz UK to design, construct, commission, operate and maintain a 10 MWe gas-fired plant at Bentinck in Nottinghamshire, UK, using methane vented from nearby closed deep coal mines. Deutz will supply three V16-cylinder TBG 632 gas engines, due to start commercial operation in July 2000.
Meanwhile, a 6 MWe plant is operating at Steetley, owned by Independent Energy and developed with Coalgas and Wärtsilä NSD. Power is generated by two 16V25SG Wärtsilä NSD gas engines, each rated at 3 MW. Both units run on CBM, but can also run on natural gas.
To meet methane content levels of 70 per cent, the gas engines were modified. Due to lower energy content (25 MJ/Nm3 for CBM compared to 36 MJ/Nm3 for natural gas), the fuel feed system was rebuilt by:
Increased gas pipe feeding system on the engine.
New main gas valves.
New solenoid driving electronic circuits.
Bigger gas regulating unit.
The volumetric flow for the full output of 3 MW made it necessary to change the main gas valves, applying similar valves to Wärtsilä’s bigger 28SG and 34SG engines. To secure opening and closing of the solenoid-driven main gas valves, the electronically-driven coils need a more powerful current. The gas regulating unit has also increased to a larger size, similar to the Wärtsilä 28SG engine.