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% - together with the two new power plant units BoA 2&3 at the Neurath site -, is the most modern and most efficient lignite-fired unit worldwide. 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 technology. In addition, the new solvents feature much higher stability towards oxygen, so that solvent consumption is lowered substantially. Under the name OASE® blue, BASF incorporated the best solvent into its product catalogue. With a plant availability of more than 97%, the pilot plant has exceeded all expectations.
In phase 2 of the test programme further process optimizations in the plant were implemented. The design of the CO2 absorber, where the CO2 is separated from the flue gas, was improved by Linde in 2011, so that carbon dioxide can be scrubbed even more efficiently from the flue gas. For this purpose, newly developed high-performance packings were installed. The aim of these honeycombed metal packings is to further increase the exchange surface and enhence 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. The reconstruction work started mid-2011 and was completed by the end of that year.The tests so far were successful: The cross-sectional area of the absorbers could be reduced by more than 35%. This will allow CO2 absorbers for major power plants, for example, to be smaller in size and erected at lower costs.
To optimise the emission reduction system, a new emission mitigation system was also retrofitted into the absorber in 2011 and tested. This system helps to further lower the emission of solvent traces. At the Niederaussem location, several highly effective methods were identified to avoid this emission path via the head of the absorber.
The optimized technology will be trialled in phase 3 of the development project in an extended time test. In this phase, advanced, high-resolution online measuring methods are used and trialed. The results obtained in this test will 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. At the end of 2014, the plant had been in operation for a total of 34,000 hours, more than 26,000 hours of these with theOASE® blue technology.
Future activities of the development programme in Niederaussem will particularly focus on the further increase in the performance of the scrubbing process.
RWE Power is investing a total of some €15 million in this development project. Germany’s Ministry of Economics and Technology is funding Phase 3 of the joint efforts undertaken by BASF, Linde and RWE with some € 2,5 million. The importance of this development project is stressed by the total funding of € 10 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. Linking the PCC pilot plant with BoA 1 provides the only opportunity in the world to test the CO2 scrubbing process using dry lignite thanks to the raw lignite pre-drying prototype (WTA technology) simultaneously operated at this unit. 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 significantly increased. The CO2 scrubbing technology can be precisely adjusted to the requirements of the next generation of highly flexible and highly efficient 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.
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