CO₂ scrubbing process overview

In the medium term, CO₂ scrubbing technology is the only retrofit option available for capturing CO₂

Over the long term, coal will be the cornerstone of a sustainability-focussed energy supply both on a global and on a national scale. For this reason RWE is pressing ahead with the development of technologies that will provide us with a foundation for sustainable coal-based power generation. The focus of our work lies on lignite, one of our domestic resources.
If you want to achieve the CO₂ reduction targets set by policy-makers, over the long term you will need to implement carbon capture and storage (CCS). This holds especially true when taking account of the planned nuclear phase-out.
Solely due to the long investment cycles in the power plant sector – typically 40 years – it will take several decades before the new power plants equipped with fully integrated CO₂ capture technology will be widely realized. Thus, at an early stage RWE Power made a decision to develop advanced CO₂ scrubbing technology; from today’s point of view, CO₂ scrubbing is the only carbon capture technique that allows retrofitting into existing power plants and industrial plants in the medium term.

How does CO₂ scrubbing work?

In principle, the basic CO₂-scrubbing process is very simple. In an absorber about 90% of the CO₂ contained in the flue gas can be bound to a CO₂ solvent at a relatively low temperature and, hence, removed. By raising the temperature, the solvent  “loaded” with CO₂ is rid of it in a desorber and subsequently transported as a lean solvent back to the absorber, where the scrubbing cycle starts again. The captured CO₂ has a high degree of purity and after being compressed is available for transport and underground storage or CO₂ usage.

Cooperation with BASF and Linde to further develop CO₂ scrubbing technology

CO₂ scrubbing has been used successfully in the chemical, petroleum, and gas industries for a long time, though under process conditions that – in some instances – differ considerably from those that are relevant for use in power plants. Hence, what could be more logical than pooling the relevant expertise of strong partners, adapting the technology to our needs, and improving energy and economic efficiency of carbon scrubbing to such an extent that it meets our requirements? In BASF and Linde we have found two partners for this purpose that are global leaders in their respective fields and complement each other perfectly.

A new CO₂ solvent developed by BASF and optimized capture plant technology developed by Linde form the basis of the optimized CO₂ scrubbing technology. The use of the jointly developed CO₂ scrubbing technology will permit power plants to capture 90% of CO₂ of the flue gas and either utilize it to make other products or store it underground.

Erection of a pilot CO₂ scrubbing plant at RWE’s Niederaussem power plant site

Within the scope of a cooperation scheme with the companies BASF and Linde, we erected a pilot plant at the existing Niederaussem power plant site. The pilot plant was commissioned on 18 August 2009. In the course of the preperations, CO₂ could be separated from the flue-gas of a conventional coal power plant in Germany for the first time as early as June 2009. The pilot CO₂ scrubbing plant was built by Linde at the 1,000-MW BoA 1 lignite-fired unit, which, with a net efficiency of over 43%, is the most modern and most efficient lignite-fired unit worldwide. It is equipped with optimized plant technology and is the forerunner to the two power plant units BoA 2&3 that were built at the Neurath site. In Niederaussem, the new carbon capture technology can thus be adapted to this type of power plant in an ideal manner.

The height of the pilot CO₂ scrubbing plant (40 m) corresponds to that of the future commercial plant. The plant also comprises all individual components of large plants, but on a smaller scale. The diameter of the absorber column was limited to the size required to obtain representative results. Depending on the set test parameters, up to 300 kg CO₂ per hour can be separated from a flue gas bypass (corresponds to a capture rate of 90 %).

During phase 1 of the CO₂ scrubbing plant’s pilot operation, initial operating experience was gained until March 2011 and various scrubbing agents tested under real operating conditions in three trials lasting six months each in order to finally identify an optimum solving agent. Between the start of the trial operation and the end of phase 1 of the development programme, more than 10,000 test hours were realized.

Breakthrough in the separation of carbon dioxide from the flue gases of coal-fired power plants

The results of the practical tests conducted to investigate the innovative CO₂ capture technology are encouraging. Compared with the customary processes used today, energy consumption can be reduced by some 20 percent with this innovative technology. In addition, the new solvents feature much higher stability towards oxygen, so that solvent consumption is lowered substantially. With a plant availability of more than 97%, the pilot plant has exceeded all expectations.

Phase 2 of the test programme will start with the implementation of further process optimizations in the plant. The design of the CO₂ absorber, where the CO₂ is separated from the flue gas, will be improved, so that carbon dioxide can be scrubbed even more efficiently from the flue gas. For this purpose, newly developed high-performance packings will be installed. The aim of these honeycombed metal packings is to further increase the exchange surface and hence the contact time between liquid scrubbing agent and carbon dioxide. The CO₂ in the power plant’s flue gas can then be bound even more easily by the scrubbing liquid. If the test is successful, this will allow CO₂ absorbers for major power plants, for example, to be smaller in size and erected at lower costs. The reconstruction work has begun in mid-2011 and will be completed by the end of the year. By late 2013, the optimized technology will then again be trialled in an extended time test. The results obtained in this test are expected to provide valuable insights for a commercial-scale use of the CCS technology that, according to leading members of the Intergovernmental Panel of Climate Change (IPCC), will play a key role in combating climate change.

RWE Power is investing a total of some €15 million in this development project. Germany’s Ministry of Economics and Technology is funding Phase 2 of the joint efforts undertaken by BASF, Linde and RWE with some €4 million. At the start of the project in 2007, the ministry already stressed its importance by awarding funds of €4.5 million.

CO₂ scrubbing for the next generation of lignite fired power plants

The BoA 1 site at Niederaussem is ideally suited for the project for an additional reason. Preparations for commissioning of the prototype lignite-drying plant started in May 2008. Demonstration operation is set to start at the end of 2008/the beginning of 2009. Hence, it is possible to test the pilot CO₂ scrubbing plant at BoA 1 using dry lignite.The coal predrying technique developed by RWE is a key technology for the next generation of lignite-based power plants, allowing the net efficiency of a power plant to be increased by up to 4%-points to more than 47%.

The CO₂ scrubbing technology can be precisely adjusted to the requirements of the next generation of lignite-fired power plants with pre-drying technology. Moreover, a high-performance desulphurization system (REAplus) is being operated in parallel with the CO₂ scrubbing pilot plant at the BoA 1 power plant in collaboration with our partner company Andritz. In an extended time test, flue gas from the REAplus system is conducted to the CO₂ scrubbing pilot plant. The REAplus technology is to help avoid the use of sodium hydroxide in the SO₂ fine scrubbing stage of a future commercial-scale CO₂ scrubbing process, thus lowering operating and investment cost.

CO2 scrubbing process overview 

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