Cleaning flue gas: Post-combustion CO2 scrubbing
CO2 scrubbing involves a scrubbing agent to separate the carbon dioxide emitted by conventional coal-fired power plants following desulphurisation of the flue gas. This is the only method suitable for retrofitting existing power plants and therefore particularly valuable in the move to spread deployment of CCS technologies.
For many years now, the chemical industry has been using CO2 scrubbing to recover CO2 generated as a by-product of production processes. Scrubbing can also be useful for a coal-fired power plant as it removes CO2 from the flue gas. To reduce carbon emissions, the CO2 scrubbing process is applied downstream of conventional flue gas purification systems.
At the heart of a CO2 scrubbing plant – also known as post-combustion capture (PCC) – is an absorber. This is where the previously desulphurised flue gas comes in counter-flow contact with a scrubbing agent. This watery solution of amines – organic compounds – extracts the CO2 from the flue gas. The low-CO2 flue gas is showered with water before leaving the absorber to remove any traces of the scrubbing agent. It is then released into the atmosphere via a stack or cooling tower. The CO2-saturated scrubbing agent is fed to a desorber where it is heated. This separates the CO2 from the liquid. The liquid is then cooled and fed back to the absorber, where the wash cycle can begin again.
This CO2 scrubbing process is the only method suitable for retrofitting existing power plants. Since summer 2009, energy provider RWE Power has been piloting this process at its lignite power plant in Niederaussem. The company aims to bring CO2 capture to commercial readiness for lignite plants by 2020. With extraction efficiency rates in excess of 90 percent, CO2 scrubbing dramatically reduces combustion gas emissions from power plants. Once captured, the CO2 can be stored underground.
The principle behind the post-combustion method
Linde's Engineering Division is successful worldwide thanks to its core competence in complex, high-tech natural gas liquefaction and air separation plants. This is flanked by extensive engineering expertise in hydrogen, synthesis gas and olefin plants. Drawing on its bundled know-how in chemical plant engineering and over fifty years' experience in gas wash, this division is actively progressing CO2 scrubbing. Although the technology is not new, it needed to be adapted to the specific needs of power plants, which is why RWE Power partnered with BASF and Linde.
Various challenges had to be overcome to build the pilot plant. For example, gas streams in the chemical industry typically contain hardly any oxygen. With a coal-fired power plant, however, the oxygen content in the gas stream is between three and five percent. This meant that the chemical scrubbing process had to be modified. In addition, the previously used plant technology for CO2 scrubbing power industry required greater levels of energy than were desired for use in power plants. And the pilot partners wanted to explore new, improved scrubbing agents that promised lower energy consumption to separate the CO2.
Partnership for a cleaner future
BASF supplies the scrubbing agent for the pilot plant and Linde’s subsidiary Linde Engineering Dresden built the scrubbing facility to the specs of the chemical company. In January 2010, both companies agreed to jointly market the technology aimed at capturing CO2 from flue gases – especially in the Middle East where CO2 is required for enhanced oil recovery. Tests at the pilot plant in Niederaussem will run until 2013. Drawing on the technical and business case findings of the test programme, RWE Power will then decide whether to proceed with a full-scale demo plant with the ability to capture up to one million tonnes of CO2 each year. This plant would provide proof-of-concept for the entire CO2 scrubbing chain – from capture through transport to storage. According to RWE, the pit coal plant currently under construction in Eemshaven in the Netherlands would be an ideal site for the new demo plant.
Linde has also partnered with Mitsubishi Heavy Industries to equip other plants with post-combustion scrubbing technologies. The Japanese company has developed a technology that uses a different scrubbing agent and Linde is to make this innovation available to power plant projects in Europe.
With decades of experience in CO2 management, Linde engineers are also involved in Europe’s largest natural gas liquefaction plant located at Hammerfest in Norway. Here, they built various CO2 scrubbing and compression systems.
Projects
Niederaussem pilot plant: CO2 flue gas scrubbing
The pilot plant in Niederaussem satisfies all the criteria for an industrial-scale carbon capture facility and will thus give valuable research insights. New scrubbing agents aimed at substantially improving carbon capture efficiency are tested. With a net efficiency of over 43 percent, the 1,000-MW lignite-fired block in Niederaussem that houses the pilot facility is the most efficient in the world. An innovative technology based on special chemical solvents cuts energy consumption by 20 percent. RWE, Linde and BASF have been collaborating since 2009 to test this new process.
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