Research Case Study - The Calcium Looping Cycle
Capturing and storing carbon dioxide (CO2) is the only way which we can continue to use the fossil fuels upon which the energy supply of our current society is based, and one of the only realistic ways to prevent dangerous climate change whilst we move towards a fully sustainable future.
One such technology is the calcium looping cycle, which utilises cheap and abundant calcium oxide (CaO) as a regenerable method of collecting CO2.
Calcium carbonate (CaCO3)is injected into a calciner, operating at around 900oC. CO2 is driven off and captured, with the solid CaO which is formed then being sent to a carbonator, capturing further CO2. The cycle is repeated, with the CaO shuttling CO2 from the carbonator to the calciner. Spent sorbent (the CaO-based substance that absorbs CO2) is then purged from the process to maintain a constant total inventory of CaO. One drawback of the system is that the reactivity of the sorbent decreases quite quickly with time, leading to a large purge requirement.
Research at Imperial focusses on improving this system in a number of ways. Firstly, we have examined the potential for doping the limestone, using a number of different species. By doping CaCO3 with hydrobromic acid (HBr), we have, in the presence of a small quantity of steam, approximately trebled the long-term reactivity of CaO under realistic conditions of calcination and carbonation1.
We have also investigated the potential synergy of CaO looping with cement manufacture, determining whether it is possible to utilise the purged material from the system to produce clinker, which can be ground and mixed with gypsum to make ordinary Portland cement. Our research was the first globally to demonstrate that CaO from this process produced cement clinker with almost identical chemical properties to that produced in the “standard” cement production process. We have also miniaturised the standard tests for strength, setting time etc. of the cement (whilst maintaining reproducibility), allowing us to test the properties of the smaller quantities that we can produce in the laboratory.
[1] - Al-jeboori, Nguyen et al. 2013