The calcium looping cycle for large-scale CO2 capture

The reversible reaction between CaO and CO2 is an extremely promising method of removing CO2 from the exhaust of a power station, generating a pure stream of CO2 ready for geological sequestration. The technology has attracted a great deal of attention recently, owing to a number of its advantages: the relatively small efficiency penalty which it imposes upon a power station (estimated at 6–8 percentage points, including compression of the CO2); its potential use in large-scale circulating fluidised beds (a mature technology, as opposed to the vastly upscaled solvent scrubbing towers which would be required for amine scrubbing); its excellent opportunity for integration with cement manufacture (potentially decarbonising both industries) and its extremely cheap sorbent (crushed limestone). Unfortunately, sorbent (CaO) derived from natural limestone markedly decreases in its reactivity over a number of cycles of reaction with CO2. Much current and promising research involves the investigation of a number of different methods to either reduce the rate of decay in reactivity, to boost the long-term reactivity of the sorbent or to reactivate the sorbent. Technologies investigated include thermal pretreatment or chemical doping of natural sorbents and the production of artificial sorbents. Attrition of the limestone can be a problem during repeated cycling in, e.g. a circulating fluidised bed, and some of the strategies to enhance the long-term capacity of the limestone to take up CO2 can increase attrition. Strategies to counteract attrition, such as pelletisation of highly reactive materials, have succeeded in reducing, though not eliminating, this problem. Each of these topics is reviewed in detail here, as are potential competing reactions with sulphurous compounds and the large-scale integration of the calcium looping cycle with both a power station and a cement works, including a number of assessments of the economics of the cycle. A number of pilot plants demonstrating the technology have been constructed around the world. No major problems have been encountered thus far, and so calcium looping technology is currently moving to the demonstration scale in a number of locations.