Captured and free

Vattenfall says it is to build the world’s first pilot power plant for ‘carbon dioxide-free coal fired generation’. The 30 MW plant will be built near Berlin and will use carbon dioxide separation with Oxyfuel technology along with underground CO2 storage.

Nigel Blackaby, Features Editor

Back in May, the Swedish energy company Vattenfall announced plans to build what it described as ‘the world’s first pilot plant for a carbon dioxide free coal fired power station’, near Spremberg in the state of Brandenburg to the south of Berlin.

Although the term ‘carbon dioxide-free’ is somewhat of a misnomer, building a plant that captures the CO2 inevitably produced by combusting coal, even on a pilot basis, marks a significant step in the development of clean coal technology. The fact that a major utility believes that commercial operation is a realistic prospect says much about market signals in relation to climate change.

Figure 1. Lignite (brown coal), here being mined, is one of eastern Germany’s primary mineral resources
Click here to enlarge image

Vattenfall’s ambition is to devise secure, feasible technologies for the almost complete elimination of carbon dioxide emissions at cost effective levels. “We believe coal has a future, but this is linked to the carbon dioxide trading system,” says Markus Sauthoff, deputy project leader and head of investment strategy at Vattenfall Europe. “It’s likely that there will be quite ambitious emission reduction targets in the future, and that the price of CO2 trading certificates will be quite high, as a kind of penalty for the use of fossil fuels in power plants,” he says. “So we need to try to reduce our CO2 emissions.”

The International Energy Agency (IEA) predicts that renewable energy sources, such as wind power, will continue to represent less than 15 per cent of world consumption for another 20-30 years. Rising demand for electricity means that use of fossil fuels across the EU is actually forecast to increase. According to the IEA reference scenarios, 50-60 per cent of electricity and heat production will be based on fossil fuels in 2030, compared to 55 per cent in 2002. With the introduction of tougher environmental legislation to limit climate change, the challenge for the power industry will be to find ways of reducing carbon dioxide emissions from fossil fuel plants. The fastest and most efficient way is to replace or upgrade existing older coal fired stations. If today’s installed fleet of coal fired power plants in the EU were to be brought in line with state-of-the-art technology, the associated CO2 emissions would be reduced by almost 20 per cent.

Figure 2. Development of total estimated cost (capture, transport and storage)
Click here to enlarge image

Vattenfall has estimated that its pilot project will cost approximately €40m ($48m) and that the plant can be ready for operation in 2008. It is being developed in collaboration with leading research institutes at German universities. Vattenfall has studied those capture technologies already commercially available in other applications but now needs to optimize the integration of those technologies and to scale-up its equipment significantly.

The pilot plant represents one stage in a plan that will end with the complete design of a 250 MW demonstration plant. The aim is to have a commercial concept available by 2020 at the latest. The planned pilot is therefore an important milestone for the project and much research and development needs to be performed before the demonstration plant can be built.

Long-term project

Engineering of the 30 MW pilot plant started back in 2003 and was stepped up in 2005. The location will be near the existing lignite fired 1600 MW power plant in Schwarze Pumpe, Germany on the site of a previous power station. The plant will be able to utilize the existing infrastructure and feed in energy produced to the power grid. The plan is to operate this plant for at least ten years so that all the necessary tests can be run, after which it would be decommissioned.

Figure 3. O2/CO2 recycle (Oxyfuel) combustion capture process
Click here to enlarge image

Lowering carbon dioxide emissions from fossil fuel fired plants can be addressed through optimizing existing technology, thereby increasing efficiency and reducing emissions per unit of energy. Alongside this is the development of systems that can capture and permanently store carbon dioxide from fossil fuel plants. The Vattenfall project focuses on capture and storage of CO2. In order to be cost competitive within the European Emission Trading Scheme, Vattenfall aims to develop the commercial concept at a total cost of no more than €20 per tonne CO2 avoided.

Carbon capture

The pilot plant will burn lignite (brown coal), one of eastern Germany’s primary mineral resources, in a 30 MWth boiler using oxyfuel technology. “In a conventional power plant we burn lignite in air, which is about 75 per cent nitrogen,” explains Sauthoff. “But in the Oxyfuel process we extract the nitrogen and burn the coal in almost pure oxygen combined with some of the flue gases, which are re-circulated into the furnace. Today we’re testing single components of this at the university level but to stick all the different parts together and to try to operate them in the most effective way will be the task of the pilot plant.”

Carbon dioxide capture and storage (CCS) technology is designed to capture CO2 produced during the combustion process within the power plant and compress it into a liquid form to make transportation easier. CO2 only makes up a small part of the flue gas emitted from a coal fired power plant. Typically, flue gases emitted contain 10-15 per cent CO2, depending on fuel and power plant process. In order to store only the CO2, some method to separate it from the other flue gas constituents is needed.

Here there are two main options. Either an absorption process separates the CO2 after combustion, or the nitrogen in the air is separated before the combustion process (Oxyfuel principle). The result in both cases is a flue gas consisting mainly of CO2 and water vapour. In both cases the process costs energy, thus development work is focusing on reducing this energy consumption in parallel with cost reduction.

Figure 4. Pre-combustion decarbonisation capture process
Click here to enlarge image

Oxyfuel is not the only method of capturing CO2 that’s currently being investigated. There is another so-called ‘pre-combustion’ method, which involves turning the coal into gas before it is burned, and then producing hydrogen that can be used in fuel cells.

Sauthoff admits that it is not yet clear which of the methods will prove to be the most efficient and economically viable. “But Vattenfall does believe that Oxyfuel will be the most favourable and this is why we’ve decided to build the pilot plant,” he says.

Carbon storage

The captured CO2 is then transformed into a liquid and permanently stored deep underground in suitable geological formations. These formations are at a depth of 800 m or more. Here the CO2 is maintained in a liquid state by ambient pressure at that level.

The storage reservoirs are the same kind as those where oil and gas have been stored for millions of years, a porous rock with an impermeable layer on top. At present, several such storage tests are being undertaken around the world. The SACS project in the North Sea is an example that has been used since 1996. Outside Berlin the CO2SINK project is using a redundant underground gas reservoir.

Europe has a more than adequate supply of suitable storage sites. Research is being undertaken into CO2 storage options focusing on on-shore storage close to the power plant, on-shore alternatives some distance from the site or offshore geological storage beneath the sea floor in the North Sea, such as deep saline aquifers or depleted oil and gas fields. The long-term safety and environmental implications of transportation and storage have to be studied. Where CO2 is being stored in oil fields it can also be used in improving production through CO2 injection, a method that has been practiced for many years. CO2 stored underground for a long time will primarily be solved in the reservoir water and then very slowly mineralised.

Research and development of CCS technologies is aimed at making CO2 capture more efficient and less costly. The Vattenfall project will make an important contribution to the field of research.

No posts to display