The reversible absorption of CO2 by CaO at high temperature is a promising method for capturing and removing CO2 from a hot gas stream. The main challenge facing the use of this method is the deterioration of CO2 absorption capacity when the method is applied over a large number of CO2 absorption/desorption cycles. Although various techniques have been proposed for improving the cyclic stability and performance of calcium based sorbents, a cost effective method is still needed for industrial applications. Therefore, two promising methods for improving the cyclic stability were selected for further investigation. One method is to optimize the preparation conditions applied to various particle sizes of CaO precursors while the second method involves incorporating an inert material, MgO, in the sorbent. While applying the first method, it was discovered that the absorption capacity and stability of a sorbent derived from limestone is dependent on many factors including the initial calcination atmosphere, temperature and time as well as particle size. It was found that both the absorption capacity and stability were greater for a sorbent derived from 11 ym limestone particles than for one derived from a much coarser material. It was also found that by calcining the 11 ym limestone at 1000oC for 1 hr in 50 vol% CO2, the resulting sorbent had an initial absorption capacity of 7 mmol CO2/g sorbent which only declined to 6.3 mmol CO2/g sorbent over 80 cycles of CO2 absorption/regeneration. A sorbent prepared by calcining calcium acetate at 1000oC for 1 hr in an atmosphere containing from 50 to 100% CO2 exhibited the highest absorption capacity among the materials tested. It was also reasonably stable over 40 cycles tested.

One of the most promising sorbents was prepared from plaster of Paris (calcium sulphate hemihydrate) by treating the material with a cyclic oxidation/reduction process at 1070oC. This sorbent exhibited an increasing trend in absorption capacity throughout a 200 cycle test of CO2 absorption and desorption.

For the second method for improving the cyclic stability of the sorbent, small amounts of MgO were incorporated in a sorbent as an inert diluent and structural stabilizer. It appeared that addition of MgO improved the performance of the sorbent in some cases depending on both the source of MgO and calcination conditions. However, this method did not seem to offer an advantage over the use of dolomite (calcium magnesium carbonate) alone, especially when the particle size of the dolomite was reduced by grinding so that it was more nearly comparable to that of the 11 ym limestone. A sorbent prepared by calcining the ground dolomite at 1000oC for 1 hr in N2 had an absorption capacity in excess of 8 mmol CO2/g sorbent over 80 cycles of CO2 absorption/desorption.