the backward, exothermic step is carbonation. A typical Ca-looping process for post-combustion CO2 capture is shown in Figure 1, followed by a more detailed description. Flue gas containing CO2 is fed to the first vessel (the carbonator), where carbonation occurs. The CaCO3 formed is passed to another vessel (the calciner). Calcination occurs at this stage, and the regenerated CaO is quickly passed back to the carbonator, leaving a pure CO2 stream behind. As this cycle continues, CaO sorbent is constantly replaced by fresh (reactive) sorbent. The highly concentrated CO2 from the calciner is suitable for sequestration, and the spent CaO has potential uses elsewhere, most notably in the cement industry. The heat necessary for calcination can be provided by oxy-combustion of coal below. Oxy-combustion of coal: Pure oxygen rather than air is used for combustion, eliminating the large amount of nitrogen in the flue-gas stream. After particulate matter is removed, flue gas consists only of water vapor and CO2, plus smaller amounts of other pollutants. After compression of the flue gas to remove water vapor and additional removal of air pollutants, a nearly pure CO2 stream suitable for storage is produced. The carbonator's operating temperature of 650-700 °C is chosen as a compromise between higher equilibrium (maximum) capture at lower temperatures due to the exothermic nature of the carbonation step, and a decreased reaction rate. Similarly, the temperature of >850 °C in the calcinator strikes a balance between increased rate of calcination at higher temperatures and reduced rate of degradation of CaO sorbent at lower temperatures. Process description CaL is usually designed using a dual fluidised bed system to ensure sufficient contact between the gas streams and the sorbent. The calciner and carbonator are fluidised beds with associated process equipment for separating the gases and solids attached (such as cyclones). Calcination is an endothermic process and as such requires the application of heat to the calciner. The opposite reaction, carbonation, is exothermic and heat must be removed. Since the exothermic reaction happens at about 650 °C and the endothermic reaction at 850-950 °C, the heat from the carbonator cannot be directly used to heat the calciner. The fluidisation of the solid bed in the carbonator is achieved by passing the flue gas through the bed. In the calciner, some of the recovered CO2 is recycled through the system. Some oxygen may also be passed through the reactor if fuel is