Environ. Sci. Technol. 2010, 44, 9496–9501
Effect of Repeated Steam Hydration Reactivation on CaO-Based Sorbents for CO2 Capture ´ ,* BY VLATKO MATERIC CAROLYN SHEPPARD, AND STUART I. SMEDLEY Industrial Research Limited, 69 Gracefield Road, Seaview, Lower Hutt, New Zealand
Received August 3, 2010. Revised manuscript received October 19, 2010. Accepted November 11, 2010.
Samples of natural limestone and commercial calcium carbonate were subjected to successive calcination and carbonation reactions in a TGA and to repeated activity restoration by steam hydration. Steam hydration of spent lime, followed by heating in CO2, was shown to be an effective method for repeatedly restoring high CO2 capture activity to spent lime during a large number of CO2 capture cycles. Steam hydration was also shown to reduce the decay rate of the CO2 capture activity by increasing the rate of carbonation in the diffusion controlled regime. Repeated hydration-carbonation-calcination cycles led to a considerable expansion of the particles through the formation of large vesicles, likely to lead to high attrition levels when applied in fluidized beds. Based on SEM observation of the particles during hydration-carbonation-calcination cycling, a model was proposed for their progressive weakening. It was concluded that strategies to reduce this weakening must limit the growth of the cracks in the crystal as they are cycled repeatedly.
Introduction In this work, the effect of repeated steam hydration on the CO2 capture properties and the physical structure of a number of calcium oxide sorbents is investigated. The purpose being to investigate the mechanism of reactivation of synthetic and natural CaCO3 sorbents, to observe the physical and chemical effects of repeated hydration-carbonationcalcination cycling on the sorbent and to attempt to understand the processes that give rise to increased attrition as a result of steam hydration. CO2 capture systems based on the reversible carbonationcalcination reaction of lime, referred to as the calcium looping cycle, have been identified as promising systems for economic CO2 capture from stationary sources (1, 2). Such systems could be used in a large number of applications, a survey of which can be found in the literature (3). A major challenge for the practical application of calcium looping is the progressive decrease in the carbonation conversion occurring in the fast, chemically controlled, regime during CO2 capture cycling (3-6). Precipitation of CaCO3 (7, 8), doping of CaO (9), CO2 shocking (10) and reactivation via hydration (11-14) have been suggested as possible methods for increasing the average activity of the sorbent. This work focuses on steam hydration of lime which is an effective reactivation method, restoring the CO2 capture * Corresponding author e-mail:
[email protected]. 9496
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 24, 2010
activity of spent lime even when it was extensively sintered (15). However, steam hydration weakens the lime particles and is likely to lead to increased attrition in a fluidized bed (15, 16). A goal of this work is to discover methods of hydration that reactivate spent sorbents but also produce a sustained higher level of carbonation with repeated carbonationcalcination cycling. A further outcome could be the discovery of hydration methods that do not lead to increased attrition compared to non reactivated materials. These advances would reduce the inventory of lime within the CO2 capture system and lead to reduced capital, operating and material costs with consequent environmental benefits.
Materials and Methods The natural limestones were Taylor’s Agricultural Lime (Aglime), a high purity natural calcite (>97% CaCO3) with a particle size 95%
