Professional Engineering

Researchers at the University of Leeds are conducting experiments using the UK’s national synchrotron, the Diamond Light Source, to find ways to improve low-cost methods for carbon capture.

The team, from the university's faculty of engineering, is investigating the efficiency of calcium oxide-based (CaO) materials as carbon dioxide sorbents. Its latest results, published in the journal of Energy & Environmental Science, provide an explanation for one of the key mechanisms involved. Their research results will inform efforts to improve the efficiency of this economically viable method of carbon capture and storage.

Current techniques for post-combustion CCS filter out CO2 from a power plant’s flue gases as it travels up the chimney. The filter is a solvent that absorbs the CO₂, before being heated, releasing water vapour and leaving behind the CO₂. In pre-combustion, the CO₂ is filtered out by use of a catalytic converter before the fossil fuel is burned and the CO₂ is diluted by other flue gases. These methods have the potential to prevent 80% to 90% of a power plant’s carbon emissions from entering the atmosphere.

CaO-based materials have a large range of applications including pre- and post-combustion carbon capture technologies and thermochemical fuel upgrading. They are low-cost, high abundance, have a large sorption capacity and fast reaction rates during the chemical process. They capture CO₂ in the temperature range 400-800°C via the formation of calcium carbonate (CaCO3) which can be regenerated with subsequent release of CO₂, ready for compression and storage. However, after multiple capture and regeneration cycles, the material's capacity for capture decreases due to the loss of surface area through sintering, a process that fuses powders together to create a single solid object. Although the surface area can be restored through hydration, the material suffers a reduction in mechanical strength. If these problems can be overcome, CaO-based materials could provide a low cost answer for carbon capture on a very large scale.

Led by Dr Valerie Dupont and Dr Tim Comyn at Leeds, the team carried out a series of experiments on Diamond’s High resolution powder diffraction beamline, I11, using intense X-rays to study the carbon capture and hydration process in CaO based materials on the nano-scale. Their observations suggest a mechanism for the interaction between CaO and water during hydration.

Tim Comyn explains: “We found that the stresses in the calcium hydroxide phase when bound to CaO were more than 20 times higher than its strength, leading to disintegration and the generation of nano-sized crystallites. Although the generation of a high surface area is a good thing, mechanical friability needs to be kept in check in order to achieve long term reliability for these systems.

“Our analysis provides an explanation of the enhanced capture/disintegration observed in CaO in the presence of steam. Now we understand this, the next step is to develop methods for improving the materials used, and apply the same techniques to other systems.”

Roger Molinder, an Engineering and Physical Sciences Research Council-funded PhD student working on the project, said: “Using the high resolution powder diffraction beamline at the Diamond Synchrotron was key to this discovery; conventional X-ray sources such as those found at most universities in the UK provide data with broad peaks, which do not make this sort of analysis possible. From a rigorous analysis of peak shapes arising from the data, we were able to determine the shape and size of the hydroxide phase, and determine the level of stress. Knowledge of these derived parameters is key to understanding the mechanism of sintering/disintegration.”

Concerns about global warming have prompted both national and international efforts to curb CO₂ emissions. CaO-based materials are a promising candidate for the removal of CO₂ from flue gases at temperatures between 400 and 800°C from processes such as fossil-fuel combustion. They are also being considered as a means to remove the CO₂ that is generated as a result of thermochemical fuel upgrading with biomass sources, which are growing more popular as an alternative to fossil fuels.