Calcium looping is a CO2 capture technology that is currently being developed on a 1
MWth pilot-scale. It has advantages including the ability to reclaim high-grade heat,
the use of a relatively inexpensive, abundant and benign sorbent, and the potential
to de-carbonise both power generation and cement manufacture. It makes use of
the reversible carbonation of CaO to remove CO2 from a flue gas and provide pure
CO2 for compression and storage, in a cyclical process. One aspect that is
disadvantageous is the deactivation of CaO-sorbent upon cycling through reactive
sintering; it is enhancement of sorbent that is examined here.
Periodically hydrating sorbent, which can enhance sorbent performance by
increasing reactive porosity, has been investigated:
• Hydration conversion decreases following cycles of carbonation and
calcination and at higher hydration temperatures. The latter has important
consequences for the ability to reclaim high-grade heat from and reduce
thermal cycling during the hydration process;
• Particle breakage can occur upon hydration, which could be problematic for
fluidised processes. This is more significant at lower hydration temperatures
and for more highly sintered sorbents;
• Direct carbonation of hydrated sorbent, rather than following a dehydration
step, results in increased mechanical stability and increased reactivity to CO2;
• Carbonation extent has an approximately linear relationship with prior
hydration extent for equivalent carbonation methods;
• A shrinking core model has been developed to describe the rate of reaction
upon hydration. This successfully describes most data, but deviations are
observed under conditions where pore blockage is likely.
Enhancement of the performance of natural sorbent through surface doping with
potassium compounds has also been investigated. KCl was found to enhance longterm
conversion in the fluidised bed, with two mechanisms proposed:
• Reduced friability of limestone, through KCl melt formation;
• Increased carbonation rates in the slow solid-state diffusion phase.