CO2 scrubbing

Apart from measures for increasing the efficiency of lignite-fired power plants and, hence, to lower CO2 emissions in the long run, we are also working on processes in which the CO2 does not reach the atmosphere in the first place, but is captured before it does. One option here is so-called CO2 scrubbing.

In the chemical industry, the petroleum industry and the gas industry, CO2 scrubbing has been successfully used for many years, but under process conditions that are often considerably different from those which are relevant for application in the field of power plants. So it makes sense to bundle the relevant expertise of strong partners in order to adapt the technology to our needs and to bring energetic and economic efficiency of CO2 scrubbing to a level that meets our requirements. With BASF and Linde we have found two global leaders in their fields as partners for achieving this goal.

The optimised CO2 scrubbing technology is based on the innovative OASE® blue capture technology by BASF as well as optimised plant engineering by Linde. With the jointly developed CO2 scrubbing process, power plants can capture 90% of the CO2 from the flue gas and introduce it into a recycling process or route it into underground storage.

Construction and operation of the CO2 scrubbing pilot plant at the Niederaussem power station

As part of our collaboration a pilot plant was installed at the existing power plant location in Niederaussem. Official commissioning took place on 18 August 2009. During the preparations for this event CO2 had already been captured for the first time in Germany from the flue gas of a conventional coal-fired power station in June of 2009. The pilot plant for CO2 scrubbing was built by Linde and erected at the 1,000 MW lignite unit BoA 1, which is, together with the two new power station units BoA 2&3 at the Neurath site, the most modern and most efficient lignite block in the world with a net efficiency of more than 43%. Therefore the new CO2 capturing technology can be optimally configured for this power station type in Niederaussem.

At approximately 40 metres, the pilot plant for CO2 scrubbing has the same height as future commercial plants. The plant also contains all the individual components of a large plant, just on a smaller scale. The diameter of the absorber column was kept to the minimum size required to produce reliable results. Depending on the trial parameters that have been set, up to 300 kg CO2 per hour (the equivalent of a capture rate of 90%) can be separated from the flue gas stream. During phase 1 of pilot operations of the CO2 scrubbing plant, practical experience was initially gained up until March 2011 and different scrubbing agents were tested under real operating conditions during three six-month test periods in order to identify the optimal scrubbing agent. In the period between the start of trial operations and the end of phase 1, more than 10,000 trial hours were completed.

Breakthrough in capturing carbon dioxide from flue gas of coal-fired power stations

The results of the practical test of the innovative CO2 capture technology are very positive. Compared to the processes which are currently in use, energy consumption can be reduced by approximately 20% with this technology. In addition, the new CO2 solvents are significantly more stable to oxygen so that solvent consumption is considerably reduced. The best solvent was added to BASF’s product portfolio under the brand name of OASE® blue. With a plant availability of over 97%, the pilot plant has exceeded all expectations.

During phase 2 of the test programme further process engineering optimisations were implemented at the plant. The design of the CO2 absorber, for example, where the CO2 is separated from the flue gas, was optimised in such a way by Linde in 2011 that the CO2 can now be removed even more effectively from the flue gas. To this end, newly developed high-tech high-performance packs were installed. With those innovative, honeycomb-like packs made from metal, the exchange surface area and contact time between liquid scrubbing agent and carbon dioxide can be increased further. As a result, the CO2 from the power plant’s flue gas can be bound by the scrubbing liquid more easily. The conversion work commenced in the middle of 2011 and was completed at the end of that year. Tests to date were successful: it was possible to reduce the cross-sectional area of the absorber by more than 35%. Accordingly, CO2 absorbers, for example use in large-scale power plants, can be smaller and therefore cheaper.

In addition, in 2011 a new scrubbing stage for further reducing the emission of solvent traces was retrofitted to the absorber and then tested in order to optimise the emission reduction system. Several very efficient methods for blocking this emission path via the head of the absorber were identified in Niederaussem. The optimised technology is being further explored in a long-term trial using and testing high-resolution online measuring methods during phase 3 of the test programme which will be running until the beginning of 2017. The results from the long-term trial provide valuable insights for the large-scale industrial use of the CCS technology, which the Intergovernmental Panel on Climate Change expect to play a key role in the fight against climate change. In more than 26,000 hours of trial operation clocked at the Niederaussem pilot plant, roughly 7,000 tons of CO2 were captured using the OASE® blue technology.

Additional approaches to optimising the OASE® blue technology are currently being tested in an attempt to reduce the costs of CO2 avoidance – although already low compared with many other options – even further. This includes in particular a variation in the scrubbing agent composition that is to lead to savings in the investment costs of large-scale plants.

RWE Power has invested approximately 20 million euros in total into this development project. The Federal Ministry for Economic Affairs and Energy supported the work by BASF, Linde and RWE in phase 3 with approximately 2.5 million euros. In total the development programme is supported with approximately 10 million euros, a strong indicator for the importance of this undertaking.

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