2.1 BACKGROUND

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In the Ca-looping process, a CaO-based sorbent, typically derived from limestone, reacts via the reversible reaction described in Equation (1) and is repeatedly cycled between 2 vessels. 

 

CaO (s) + CO2 (g) = CaCO3 (s) ----(1)

 

The forward reaction is known as carbonation, and is an exothermic step. The backwards reaction is the calcination reaction and is endothermic. The figure below describes one potential post-combustion capture process using the calcium looping cycle, as proposed by Shimizu et al. (1999):





























 

 

Figure 2.1 Schematic of post-combustion CO2 capture via the calcium looping process (Fennell, 2010)



As seen in Figure 2.1, flue gas is passed into the carbonator where carbonation of CaO occurs, stripping the flue gas of its CO2. The CaCO3 formed is then passed to the second vessel (calciner) where calcination occurs. The CaO formed is passed back to the carbonator, and a pure stream of CO2 leaves the calciner, suitable for sequestration. This cycle is continued and spent (unreactive) sorbent is continuously replaced by fresh (reactive) sorbent. The highly concentrated CO2 from the calciner can be sent for storage, 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, where coal is burnt in the calciner in an atmosphere of CO2/O2 . Oxy-combustion is necessary in order to maintain a high concentration of CO2 suitable for sequestration and the requisite oxygen for the process is provided by an external air separation unit, estimated to be approximately 1/3 of the size of that required for an oxyfuel-fired power station. 



The carbonation can either occur in situ (i.e. within the gasifier/combustor) or ex situ (i.e. on the product gases), with the former resulting in a reduction in plant complexity at the expense of a higher rate of degradation of sorbent due to contact with ash, sulphur and other impurities in the fuel burned. Heat from the exothermic carbonation of quicklime can be used to run a steam cycle, making up for some of the energy losses elsewhere.





2.2 Thermodynamics of the Process

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​Figure 2.2. Equilibrium vapor pressure of CO2 over CaO as a function of temperature​

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Figure 2.2 is a plot of equilibrium vapor pressure of CO2 over CaO as a function of temperature (McBride et al., 2002). In conditions above the line, carbonation is the dominant reaction whereas in conditions below the line, calcination takes place: partial pressures of CO2 greater than the equilibrium partial pressure at a given temperature will favor carbonation, while those lower than the equilibrium will favor calcination. As a result, if a sorbent is cycled between the two vessels at suitable temperatures, carbonation of

sorbent can be effected in one and calcination in another.



Since a concentrated CO2 stream is aimed at the exit of the calciner, the equilibrium of CO2 on CaO (close to 900 degrees C for pure CO2 at atmospheric pressure) requires calcination to operate at higher temperatures . The temperature of >850 degrees C in the calcinator must also strike a balance between increased rate of calcination at higher temperatures and reduced rate of  degradation of CaO . The carbonator temperature of 650-700 degrees 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 at lower temperatures.

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References:

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​[1] Blamey, J., Anthony, E. J., Wang, J., Fennell, P. S., 2010. The calcium looping cycle for large-scale CO2 capture. Progress in Energy and Combustion Science 36(2): 260-279.​​​

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