being burned in the calciner to provide energy. Provision of energy to the calciner Heat can be provided for the endothermic calcination step either directly or indirectly. Direct provision of heat involves the combustion of fuel in the calciner itself (fluidised bed combustion). This is generally assumed to be done under oxy-fuel conditions; i.e. oxygen rather than air is used to burn the fuel to prevent dilution of the CO2 with nitrogen. The provision of oxygen for the combustion uses much electricity; other air separation processes are being developed. Indirect provision of heat to the calciner involves either: Combustion of fuel outside the vessel and conduction of energy in to the vessel Combustion of fuel in another vessel and use of a heat transfer medium. Indirect methods are generally less efficient but do not require the provision of oxygen for combustion within the calciner to prevent dilution. The flue gas from the combustion of fuel in the indirect method could be mixed with the flue gas from the process that the CaL plant is attached to and passed through the carbonator to capture the CO2. Recovery of energy from the carbonator Although the heat from the carbonator is not at a high enough temperature to be used in the calciner, the high temperatures involved (>600 °C) mean that a relatively efficient Rankine cycle for generating electricity can be operated. Note that the waste heat from the market-leading amine scrubbing CO2 capture process is emitted at a maximum of 150 °C. The low temperature of this heat means that it contains much less exergy and can generate much less electricity through a Rankine or organic Rankine cycle. This electricity generation is one of the main benefits of CaL over lower-temperature post-combustion capture processes as the electricity is an extra revenue stream (or reduces costs). Sorbent degradation It has been shown that the activity of the sorbent reduces quite markedly in laboratory, bench-scale and pilot plant tests. This degradation has been attributed to three main mechanisms, as shown below. Attrition Calcium oxide is friable, that is, quite brittle. In fluidised beds, the calcium oxide particles can break apart upon collision with the other particles in the fluidised bed or the vessel containing it. The problem seems to be greater in pilot plant tests than at a bench scale. Sulfation Sulfation is a relatively slow reaction (several hours) compared with carbonation (<10 minutes);