Calcium looping reacts calcium oxide (CaO) with the CO2 in flue gas to form calcium carbonate, which is then broken down to capture the CO2 and regenerate the CaO for further use.
Calcium looping (CaL) technology – also known as the regenerative carbon cycle (RCC) – removes carbon dioxide (CO2) from the flue gases of a cement plant (and other power and industrial facilities) using a calcium oxide (CaO) sorbent. The process relies on two reversible chemical reactions: carbonation and calcination.
During carbonation, the sorbent reacts with the CO2 in the flue gas to form calcium carbonate (CaCO3). The CaCO3 is then fed into a calciner, where it is heated to about 850°C-950°C, causing it to decompose into an almost pure CO2 gas stream and solid CaO. The CO2 is removed and further purified, before being sent either for geological storage or onward use. The solid CaO can be sent back for reuse in the process to capture more CO2.
The heat required for calcination can be provided both directly or indirectly.
Direct heating involves the combustion of fuel within the calciner, generally in oxygen-rich (oxyfuel) conditions to prevent the dilution of the CO2 with nitrogen. The provision of the large amounts of oxygen required for oxyfuel combustion is however an energy-intensive process, posing a challenge to the economics and environmental benefit of the process (unless renewable energy is available for oxygen separation). The development of lower-energy separation methods is underway.
Indirect methods are generally less efficient – but do not require large amounts of oxygen. The flue gases from these indirect methods could also be mixed with the flue gases of the cement plant and put through the CaL process, capturing the CO2.
In addition to the need for heat for calcination, the main challenge of CaL technology is the degradation of the CaO sorbent over time. As a result, some of the sorbent must be periodically removed and replaced with fresh sorbent. This adds to the cost of the process, while the spent sorbent must also be disposed of. Potential uses for the spent sorbent that avoid landfilling include cement manufacture and flue gas desulfurisation.