Article pubs.acs.org/est
The Mechanism of Vapor Phase Hydration of Calcium Oxide: Implications for CO2 capture Krzysztof Kudłacz† and Carlos Rodriguez-Navarro* Dept. Mineralogy and Petrology, University of Granada, Fuentenueva s/n, 18002 Granada, Spain S Supporting Information *
ABSTRACT: Lime-based sorbents are used for fuel- and flue-gas capture, thereby representing an economic and effective way to reduce CO2 emissions. Their use involves cyclic carbonation/calcination which results in a significant conversion reduction with increasing number of cycles. To reactivate spent CaO, vapor phase hydration is typically performed. However, little is known about the ultimate mechanism of such a hydration process. Here, we show that the vapor phase hydration of CaO formed after calcination of calcite (CaCO3) single crystals is a pseudomorphic, topotactic process, which progresses via an intermediate disordered phase prior to the final formation of oriented Ca(OH)2 nanocrystals. The strong structural control during this solidstate phase transition implies that the microstructural features of the CaO parent phase predetermine the final structural and physicochemical (reactivity and attrition) features of the product hydroxide. The higher molar volume of the product can create an impervious shell around unreacted CaO, thereby limiting the efficiency of the reactivation process. However, in the case of compact, sintered CaO structures, volume expansion cannot be accommodated in the reduced pore volume, and stress generation leads to pervasive cracking. This favors complete hydration but also detrimental attrition. Implications of these results in carbon capture and storage (CCS) are discussed.
■
porous pseudomorphs16 made up of CaO nanoparticles of higher surface area and reactivity than those formed after calcination of CaCO3.17,18 Two modes of hydration, yielding solids with very different properties (crystal size, surface area and reactivity), have been reported:19 (i) wet hydration, which occurs (at low T) via dissolution of the oxide in water and subsequent nucleation and growth of Ca(OH)2 crystals,19−22 and (ii) dry or vapor phase hydration which takes place at a wide range of T (from room T up to ∼450 °C, 1 atm), and has been assumed to take place via a solid-state mechanism.17,23 Although some researchers have proposed wet hydration as a means to reactivate spent CaO,24 vapor phase hydration appears to be the most common and efficient method.2 Vapor phase hydration is also of relevance for heat storage (high-T heat pump),25 and in the production of commercial hydrated lime.19,20 While the mechanism of wet hydration of CaO is now well understood,19−22,26,27 the current mechanistic understanding of the vapor phase hydration of CaO is far from being complete.25 Glasson concluded that the vapor phase hydration of highly reactive (soft-burnt) lime took place via an advancing interface mechanism.23 Interestingly, the initial product of vapor phase
INTRODUCTION Lime-based sorbents are widely used in pre- and postcombustion capture of SO2 and CO2.1,2 Efficient CO2 capture and storage (CCS) using Ca-based sorbents can be achieved by cyclic calcination-carbonation, the so-called Ca-looping cycle.2,3 CO2 is captured following high-T carbonation (typically at ∼650 °C at atmospheric pressure) via the reversible reaction CaO + CO2 = CaCO3 (ΔH298 K = −178 kJ mol−1). Carbonation is typically performed in a fluidized bed reactor.4 The product CaCO3 is calcined (∼900−950 °C) in another reactor and the released (concentrated) CO2 gas is separated for further use or underground mineral sequestration.5,6 One of the most significant limitations of the use of Ca-sorbents for the above-mentioned environmentally relevant process is the huge reduction in carbonation yield (loss-in-capacity) of CaO after a few carbonation-calcination cycles (i.e., less than 20% conversion after ∼10 cycles).7−10 This is reportedly due to sintering of CaO particles and pore (diameter
