In recent years, the German Renewable Energy Law (EEG) has provided a strong incentive to develop power generation facilities using coal mine methane (CMM). Although mine gas is not considered renewable, its use in power plants is desirable because of the strong impact of methane on climate change.

The guaranteed payback tariff of 6.6-7.7 r cents per kWh(e) (depending on the plant size) for at least 20 years, helps decrease the investment risk for a mine gas CHP plant.

By the late 1990s only a few pilot plants had been operated in Germany, but adoption of the EEG has provided the basis for the realisation of projects at nearly all available locations.

Due to the favourable payback tariffs for electricity from mine gas, feeding of the electricity to the public grid has now become an attractive alternative. Therefore, CHP projects with a high power-to-heat ratio based on gas engines are currently under development.

The utilisation of CMM from closed-down mines for CHP production started only recently. It is, however, expected that, within the next 2-3 years, almost the total amount of CMM from closed-down mines (estimated at 180 million Nm3 per year – see Table 1) will be used for decentralised CHP production, resulting in a total capacity of more than 50 MWe.

Recently, more than ten CHP projects running on CMM from closed mines have been realised with a total capacity of more than 20 MWe. It is expected that another 50-100 MWe of capacity using CMM from both closed and active coal mines will be installed in the near future.

One of the challenges of using CMM is that the methane concentration is as low as 30 per cent by volume, requiring modifications to the engine and adjustments to the gas-air mixture.

So far CMM has been used in Germany but CBM (coalbed methane) has not yet been employed due to unfavourable geological conditions. However, research on availability and utilisation of CBM is underway.

A brief history of coal mine methane

Until the 1950s, CMM was primarily regarded as a hazard for miners. But then it was realised that as much of the gas as possible needed to be collected and released under controlled conditions.

In the Saarland, a pipeline network was established in the 1950s and continuously extended to today’s length of about 100 km.

Mine gas was (and still is) mainly used in boilers for district heating and co-firing in fossil-fired power stations.

In 1978, the first pipeline for the removal of CMM from a closed-down mine was installed, at Mont Cenis. Initially, the CMM was not used as fuel but released to the atmosphere.

It was not until the late 1980s that public attention was drawn to the emission of greenhouse gases and the resulting danger of climate change. When methane was recognised as a greenhouse gas, 21 times more harmful than CO2, release of CMM into the atmosphere became unacceptable. Instead, use of CMM for power production or CHP became desirable.

In 1998, the first CHP plant running on CMM from a closed-down mine was commissioned, at Mont Cenis. At the same time, G.A.S. Energietechnologie GmbH installed the first containerised CMM power plant (374 kWe), at the Minister Achenbach coal mine.

The necessary permissions for such mine gas power plants mainly involve mining law, building law, and environmental law. The emission limits for gas engines as defined in the German emission law “TA Luft” are given in Table 2.

The need for mobility

Due to uncertainties about the amount and longevity of mine gas supplies at any given site, mobile containerised CHP plants have turned out to be the best concept.

The modular approach allows easy adjustment of the capacity of the plant to the (changing) availability of mine gas. Additionally, use of mobile containers minimises investment risks, lessens space requirements, and reduces lead times.

Recent examples of projects using the containerised mobile CHP concept are: Minister Achenbach Shaft IV; Kurl Shaft III; and Dortmund-Derne.

Minister Achenbach. The Minister Achenbach mine in the Ruhr was closed in 1990. All the shafts were filled and degassing lines installed that terminated under the backfilled material. In the grounds of the Lüntec Technology Park, there is a 250 mm diameter degassing pipe connecting to shaft IV of the abandoned mine. The pipe goes down to a depth of about 500 m and is intended for the release of CMM into the atmosphere. The pipe is provided with a deflagration protection system and a mine flap that opens if the pressure of the shaft is above atmospheric pressure. Penetration of air into the old mine is therefore avoided.

A pipe branch to allow use of some of the coal mine gas was installed between the shaft outlet and the mine flap. Downstream of a rapid action valve and the deflagration protection system the gas is compressed by an rpm-regulated rotary compressor up to 90 mbar(g). The mine gas is then fed into the cogeneration plant module, which initially had a capacity of 374 kWe/538 kWt.

The plant was built by G.A.S. Energietechnologie GmbH under the technical guidance of the Fraunhofer Institute UMSICHT and with financial support from DBU, a German foundation for environmental research.

After more than one year of satisfactory operation, the module was exchanged for a containerised modular plant with a capacity of 941 kWe. This was in the summer of 2000. The footprint for the plant remained unchanged, which demonstrates the advantages of the prefabricated and standardised container approach. The outage period needed to accommodate installation of the new module was less than two weeks.

Kurl Shaft III. In May 2001, Minegas GmbH, a joint venture formed in 2000 between G.A.S. Energietechnologie GmbH, RAG AG, and Lambda GmbH, started up its Kurl Shaft III cogeneration plant. This is at the site of the former Gneisenau mine in Lünen.

The three modules have an output of 1358 kWe each. The electricity is fed to the public grid.

Subsequently, Minegas GmbH has completed another three projects: Werne (2×1000 kWe); Ewald-Fortsetzung (4×1358 kWe); and Consolidation (2×1358 kWe). And by the end of 2003, Minegas GmbH expects to have 50 MWe of CMM-fuelled power plants in operation.

Dortmund-Derne. In November 2001, a 5400 kWe CHP plant in Dortmund-Derne was converted from natural gas to mine gas operation. Due to decreasing electricity prices and increasing gas prices, economic operation of the plant had become more and more difficult. Fortunately, sufficient mine gas for operation of the plant was available nearby from the closed Gneisenau mine. The mine gas is owned by Minegas GmbH. It is compressed on site and then delivered, via a 600 m long pipeline to the CHP plant, which is owned and operated by Grüne Energie Derne GmbH.

Lessons from experience

On the basis of our first two years of operation of CHP plants running on coal mine methane from closed-down coal mines, we can draw the following conclusions:

• Operation of the engines is very dependent on the quality of the mine gas but has generally been satisfactory.

• Availabilities of up to 96 per cent have been achieved.

• Oil change intervals vary between 500 and 2000 hours.

• The high levels of sulphur in the mine gas can cause corrosion in the flue gas heat exchangers.

• Condensation in the gas train must be avoided.

• The long-term availability of mine gas at any particular location is difficult (perhaps even impossible) to assess.

• In Germany each project has to have its own authorisation procedure due to differing local circumstances and this has delayed the implementation of mine gas power plants.

Future projects

In March 2002, G.A.S. Energietechnologie GmbH received a 5 million order for a further six of its CMM-fuelled modular cogeneration plants, including both construction and operation.

To be located at the Lohberg/Osterfeld mine in the Ruhr area, the buyer is Mingas-Power GmbH, a joint venture of RAG Aktiengesellschaft and RWE Power AG, which hopes to construct some 15 coal mine methane installations within the next two years.

The Lohberg/Osterfeld power plant, with a total capacity of 8.2 MWe/9.3 MWt, will be fuelled by coal mine methane from hard coal layers. As well as feeding electricity to the local grid the plant will also produce hot water.

The project lead time will be extremely short, with commissioning planned for August 2002.

At present it is only really in Germany where the law specifically and strongly encourages power and heat production from coal mine gas.

However a similar favourable environment may arise in other countries as they try to achieve their greenhouse gas reduction targets. The potential for putting coal mine gas to good use looks particularly strong in Eastern Europe, North America and China.

Table 1. Approx. annual qantities of coal mine methane in Germany (million Nm3, 1998)
Table 2. TA Luft emissions limits for gas engines