The use of calcines of natural limestones as CO 2 regenerable sorbents is investigated in this work by studying the decay of the maximum carbonation conversion during many carbonation/ calcination cycles. New experimental information is complemented with a compilation of previously published data on this subject. The observed conversion limits in the reaction of CO 2 with lime are interpreted in terms of a certain loss in the porosity associated with small pores and a certain increase in the porosity associated with large pores. In the carbonation part of every cycle, the CaCO 3 fills up all the available porosity made up of small pores plus a small fraction of the large voids, limited by the thickness of the product layer that marks the onset of the slow carbonation rate. A simple model based on textural changes, observed by scanning electron microscopy, fits equally well all the data from this work and from other authors. The two model parameters are consistent with known mechanism occurring during calcination and carbonation.

Calcium oxide can be an effective CO 2 sorbent at high temperatures. When coupled with a calcination step to produce pure CO 2 , the carbonation reaction is the basis for several hightemperature separation systems of CO 2 . The formation of a product layer of CaCO 3 is known to mark a sudden change in the reaction regime, from a very fast CO 2 uptake to very slow carbonation rates. The critical thickness of this product layer of CaCO 3 has been measured in this work on real sorbent materials, using different limestone precursors and submitting them to many repeated carbonation calcination cycles (up to 100). Mercury porosimetry curves of the calcines and their carbonated counterparts have been obtained and their differences interpreted with a simple pore model, from which the thickness of the product layer is derived. An average value of 49 nm ((19% standard deviation) has been obtained, which is quite insensitive to the type of limestone and to the texture of the calcine as long as the model is fulfilled. The implications of this value on our understanding of the sorbent performance in these CO 2 -capture systems are discussed.

Experiments in a pilot

The use of carbonation/calcination cycles of CaO/CaCO3 is emerging as a viable technique for the capture of CO2 generated in the combustion of coals for power generation. Specifically, the choice of natural limestones as CO2 carriers is an attractive option because they are cheap and abundant materials, although previous studies indicate that the reactivity of the calcines toward CO2 rapidly drops with cycling. This paper reports on the effects of the internal morphology of CaO particles on their capability of absorbing CO2. Calcines of a natural limestone with different initial textures were repeatedly submitted to carbonation/calcination conditions (up to 100 cycles). The textural evolution, as well as the carbonation conversion, of the calcined and recarbonated samples was followed along the experiments. In addition to the known mechanisms of deactivation due to grain growth and limited diffusion of CO2 through the product layer, we have found that pore closure is also taking place in our samples, together with an overall shrinkage of the particle. All of these factors play a role in limiting the maximum carbonation conversions to around 10% after just 100 cycles.