The carbonation step is characterized by a fast initial reaction rate abruptly followed by a slow reaction rate (Figure 2).  The carrying capacity of the sorbent is defined as the number of moles of CO reacted in the period of fast reaction rate with respect to that of the reaction stoichiometry for complete conversion of CaO to CaCO. As seen in Figure 2, while mass after calcination remains constant, the mass change upon carbonation- the carrying capacity- reduces with a large number of cycles. In calcination, porous CaO (molar volume = ) is formed in place of CaCO (.). On the other hand, in carbonation, the CaCO formed on the surface of a CaO particle occupies a larger molar volume. As a result, once a layer of carbonate has formed on the surface (including on the large internal surface of porous CaO), it impedes further CO capture. This product layer grows over the pores and seals them off, forcing carbonation to follow a slower, diffusion dependent mechanism.