Limits to carbon capture

Carbon capture, removing CO2 from processes instead of releasing it into the atmosphere, is hailed as a means to operate coal-fired electricity plants without CO2 emissions. But, is it really?

At the Paris Climate Conference, the Canadian Premier of Saskatchewan proudly presented the coal-fired plant of the Boundary Dam Project as the showcase for 21-st century technologies for carbon capture and storage or CCS. The prices mentioned for the technology on the web vary from 15 to 150 dollars per tonne of CO2. And that’s not the only problem with CCS.

Professor of engineering thermodynamics Thijs Vlugt at the faculty of Mechanical, Maritime and Materials Engineering has supervised multiple PhD studies into carbon capture. He has doubts about conventional, amine-based CCS technologies, as these are energy-hungry, and they require chemical solvents that may be environmentally harmful.

Roughly speaking, there are two major ways of capturing CO2: pre- or post-combustion The latter covers the attempts of removing CO2 from the flue gasses before these are released into the air. Current CCS projects are of the post-combustion type. Installations can be retrofitted to an existing power plant.

The pre-combustion capture requires a totally different process, and plant, in which the coal is converted into ‘syngas’ (hydrogen (H2) plus carbon monoxide (CO) by gasification of coal. The subsequent shift reaction combines the produced carbon monoxide with steam into carbon dioxide (CO2) and more hydrogen (which can be burnt). This pre-combustion process offers much better perspectives for CO2-capture, said Vlugt.

The trouble with the post-combustion process is that the partial pressure of CO2 in the flue gasses is only 0.1 bar and the gas volumes are enormous. Solvents that bind CO2 from the flue gasses are called amines. Not only do amines emit aerosols, but the whole process also requires enormous amounts of energy (ca. 25% of the primary energy input) to heat the amine solution to more than 100 degrees celcius to release the CO2.

It is possible that the post-combustion capture process can be improved, but it will still require chemical solvents and large amounts of energy.

Vlugt sees better perspectives for pre-combustion capture. Thanks to the higher pressures and smaller volumes, CO2 may be captured from the H2-CO2 mix with organic solvents like methanol, selexol, ionic liquids or with porous materials such as zeolites or metal organic frameworks (MOFs). Heating the solvent a bit, reducing the pressure or cooling the gas mixture are simple and effective means to release the CO2.

The problem is that the precombustion process requires a next-generation coal plant in which the coal is gasified instead of burnt. Nuon operated one such ‘future plant’ in Buggenum, in the south of the Netherlands, but it was divested in 2013.

–> Recent theses on CO2 capture supervised by Professor Vlugt:

M. Ramdin: CO2 Capture with Ionic Liquids: Experiments and Molecular Simulations, December 01, 2015

Balaij, S.P., Absorption of Greenhouse Gases in Liquids, November 23, 2015

P. Khakharia, Aerosol-based Emission, Solvent Degradation, and Corrosion in Post Combustion CO2 Capture, March 02, 2015

M. De Groen, CO2 capture with liquid crystals: A phase equilibrium study, May 26, 2015.

More theses and publications on Prof. Vlugt’s website

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