text: In the Ca-looping process, a CaO-based sorbent, typically derived from limestone, reacts via the reversible reaction described in Equation () and is repeatedly cycled between two vessels.
  The forward, endothermic step is called calcination while the backward, exothermic step is carbonation.
  A typical Ca-looping process for post-combustion CO  capture is shown in Figure 1, followed by a more detailed description.
  Flue gas containing CO is fed to the first vessel (the carbonator), where carbonation occurs. The CaCO 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 CO  stream behind. As this cycle continues, CaO sorbent is constantly replaced by fresh (reactive) sorbent. The highly concentrated CO 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 CO, 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 CO 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.