Powering Up On Mine Gas

These are testing times for Britain’s domestic coal and gas industries. The recent announcement of a pit closure in Northumberland was yet another indication of the decline in coal mining in the UK. And power generators find it cheaper to import coal from Poland than use domestic sources.

Gas suppliers have also been hit by the UK government’s decision to ban new development in gas-fired power plants unless they show specific environmental benefits. And deregulation has not created the competitive market many hoped for.

Against this background, with power producers reluctant to build new coal-fired plant and energy efficiency the hot topic, it takes innovative engineering to get a generating station off the drawing board. It is with some irony that a recent UK project uses coal mine resources to supply gas for power generation.

The gas supplied at this site is similar to coalbed methane, vented from closed deep coal mines. This gas was, in the past, vented to the atmosphere, thus a waste product from former coal mining operations it is now being used as an energy resource.

Mine gas

Two such projects were brought on stream in 1999 at abandoned collieries in the UK by specialist fuel supplier Coalgas. The first, at Markham in Derbyshire, supplies mine gas via a pipeline to a speciality chemical manufacturer; a second vent at Steetley, Nottinghamshire, fuels a power station owned by Independent Energy.

Both projects have significant environmental benefits, removing methane – a potent greenhouse gas – that would otherwise be vented to the atmosphere. Using the methane reduces the greenhouse impact of the gas by more than 95 per cent.

The Steetley project was developed by Independent Energy, Coalgas and turnkey contractor Wärtsilä NSD, and was funded as a commercial project by both Coalgas and Independent Energy. As a small-scale power station, Steetley is connected directly to the local distribution system, avoiding the transmission charges associated with using the national grid. Coupled with low fuel prices – Independent Energy and Coalgas have contracted the gas supply at a price lower than that for pipeline natural gas – and environmental benefits, Steetley is an attractive scheme.

The scheme is proving such a success that Coalgas is developing two further sites, and has begun investigations to quantify gas emissions for more than 149 other identified vents and boreholes. It has received venture capital backing for its operations and is aiming to have 40 mine gas extraction plants operating by the end of 2004. Coalgas reports that a number of large energy companies are expressing interest in licensing the technology and exporting it to coal-rich overseas markets including China, India and Eastern Europe.

Feasibility

It takes a lot of preparatory work, from feasibility studies and research, shaft surveys and equipment selection, to progress an identified site to one supplying fuel to a power station.

Coalbed methane in working mines is hazardous to operations and is normally removed by mine ventilation systems. In cases of severe methane concentration, it is drained from underground boreholes into pipelines for removal to the surface.

Coalgas’ concept is to view abandoned mine shafts and old mine workings as large underground storage reservoirs. Monitoring of the Steetley and Markham sites enabled information on barometric sensitivity, void space assessment and gas composition to be determined.

Coalgas carried out extensive research using mine plans, survey data and geological reports. Long term monitoring was carried out using sensing and logging equipment. The results were integrated into a database; project economics were weighed up before a decision was taken on project viability.


Figure 1. The Steetley project was funded as a commercial project by Coalgas and Independent Energy
Click here to enlarge image

The consulting group IMC also carried out reserve evaluation studies for the mine gas project on behalf of Independent Energy, including an investigation into the extent of the mine workings, estimates of the gas that might have been produced from the coal seams during the life of the mine, and an estimate of reserves remaining and the rate at which they can be produced.

Coalgas has a licence to extract the gas and has installed gas pumping equipment and associated safety systems. Prior to this project the gas was vented naturally at a rate dependent on atmosphere pressure. To provide the gas at a constant rate for use in power generation it is necessary to use pumps to lower the pressure in the mine.

“Mine gas has a similar origin to coalbed methane but it contains more carbon dioxide and nitrogen as a result of coal seams being exposed to the atmosphere during long term operations,” said David Oldham of Coalgas. “Methane concentrations in the gas vary from 60-80 per cent and it is suitable as a fuel producing very low exhaust emissions. Its calorific value is approximately two-thirds that of North Sea gas.”

Planning permission for the Steetley site was obtained by Coalgas. The power plant is situated on a brownfield site near a major industrial complex. “Noise was a concern because of nearby houses but we had worked with Wärtsilä before on noise sensitive projects and knew we could overcome the problems by housing the engines in pre-cast concrete ‘cells’, and by paying particular attention to the design of the dump radiators,” explained Rob Jones of Independent Energy.

A major hurdle to overcome arose from the fact that Steetley is one of the first projects of its kind in the UK. “There was therefore a need to educate and convince management and funders of the merits of the scheme,” said Jones.

Prime mover

Independent Energy could have considered gas turbines as the power plant’s prime mover. However, there is no heat load and for simple generation, efficiencies in excess of 40 per cent can be achieved with gas engines, compared with around 30 per cent for similar-sized turbines. “The project would have been uneconomic at this level of efficiency and we did not even consider tendering gas turbines for this project,” said Jones. “We also prefer gas engines because they are relatively straightforward to maintain.”


Figure 2. Power at Steetley is generated by two 16V25SG Wartsila NSD gas engines rated at 3 MWe each.
Click here to enlarge image

Power at Steetley is generated by two 16V25SG Wärtsilä gas engines each rated at 3 MWe. Both units are able to run on mine gas but have the capability to run on natural gas. Wärtsilä gas engines each rated at 3 MWe. Both units are able to run on mine gas but have the capability to run on natural gas. W?rtsil? was responsible for the plant design and takes care of the operation and maintenance for the site.

The Wärtsilä W16V25SG is a medium speed (1000 r/mim) lean-burn gas engine for natural gas and coalbed methane. The engine bore is 250 mm and its stroke 300 mm. With 16 cylinders, the maximum electric output is 3.12 MWe and the electrical efficiency 40 per cent.

It is built on a common base frame on which the engine and generator are assembled. For quick installation, some auxiliaries are mounted directly on the common frame such as the lube oil filter and lube oil coolers. The engine is equipped with the Wärtsilä gas engines each rated at 3 MWe. Both units are able to run on mine gas but have the capability to run on natural gas. Wärtsilä was responsible for the plant design and takes care of the operation and maintenance for the site.

In order to meet the methane content levels of around 70 per cent, the gas engines have been modified. Due to the lower energy content (25 MJ/Nm3 for mine gas compared to 36 MJ/Nm3 for natural gas) the fuel feed system has been rebuilt by:

  • Increased gas pipe feeding system on the engine
  • New main gas valves
  • New solenoid driving electronic circuits
  • Bigger gas regulating unit.

The gas pressure is increased in a compressor to about 4 bar. The volumetric flow necessary for the full output of 3 MW has made it necessary to apply valves similar to Wärtsilä’s bigger 28SG and 34SG engines. To secure opening and closing of the solenoid driven main gas valves, the electronically driven coils require a more powerful current. The gas-regulating unit has also been increased to a larger size, similar to the Wärtsilä 28SG engine.

“All these components have been tested in other projects without any problems occurring. The Steetley plant can operate on both natural gas and mine gas. The extraction rate at present is lower than anticipated, so gas is not produced rapidly enough to supply two engines – hence one runs on natural gas and the other on the mine gas. Even when the methane content decreases, the engine can be run in full thanks to the modifications of the fuel system,” said Thomas Stenhede of Wärtsilä.

Projected performance

Although projected to run for many years, it is difficult to be precise about the reserves of mine gas because of the way in which the methane is produced. “Coal has a pore structure – rather like a sponge. The methane is held on the internal surfaces of the pores,” said Oldham.

The methane has to travel through the interconnected pores and then into the fissures within the coal. It then ‘migrates’ into the mine roadways before being pumped to the surface. “The methane therefore follows a very long and tortuous route and it is impossible analytically to establish how much is in connection with the surface and how rapidly it can be extracted.”

The other complication with such a project is that the underground workings can flood with water seeping in from adjacent rock strata; as this water floods the underground roadways it may progressively block some of the gas flow paths.

A small-scale 3 MW power plant using mine gas as a base fuel has strong environmental benefits. The output of methane from a typical mine site in terms of CO2 equivalent would be 307 000 t/year. Using the gas as at Steetley would reduce this to about 4500 t/year CO2 equivalent.

If this saving is projected to 50 abandoned mine gas sites, then by 2004 Coalgas estimates that its operations could save the equivalent of almost 400 000 t of oil. This could translate into as much as 15 per cent of renewable energy production in the UK alone.

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