CaO readily forms a shell of calcium hydroxide when exposed to water in the air (right). Due to differences in atomic congurations (top left) between the oxide and hydroxides, enormous strains develop due to the interface. These strains of 0.78% lead to stresses 20 times higher than the rupture strength of the hydroxide leading to rupture and the generation of nanoparticles. Deconvolution of the data generated by Diamond (bottom left) allows the Leeds team to determine the size and strain in these layers, from the breadth of the peaks (the peaks from CaOH are far narrower than CaO). Conventional X-ray sources would have considerable peak overlap, making this type of analysis almost impossible.

The Diamond
Light Source is being used to improve low cost methods for carbon
capture. Scientists from the University of Leeds are using the U.K.’s
national synchrotron to investigate the efficiency of calcium oxide
(CaO) based materials as carbon dioxide (CO₂) sorbents. Their results,
published in the journal of Energy & Environmental Science,
provide an explanation for one of the key mechanisms involved. This new
knowledge will inform efforts to improve the efficiency of this
economically viable method of carbon capture and storage.

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 to 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.

In-situ X-ray diffraction of CaO based CO2 sorbents

Source: Diamond Light Source