Low-cost carbon capture gets X-rayed

Current techniques for post-combustion carbon capture filter out CO₂ from a power plant's flue gases as they travel up a 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 can 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 materials' 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 from the University of Leeds' Faculty of Engineering, 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.

Roger Molinder, an Engineering and Physical Sciences Research Council (EPSRC) funded PhD student on the project, describes, "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 and more popular as an alternative to fossil fuels. Using CaO based materials for carbon capture is just one of the ways to combat global warming. Since CaO based materials are low cost, there is an economic incentive to solve the problem of surface area loss to potentially turn this into a method for large scale CO₂ capture. These recently published results are a promising step towards improving these low cost methods.