Reactivation of a CaO-based sorbent for CO 2 capture from stationary sources

Abstract

A promising method to remove CO 2 from either fuel or flue gases for combustion systems is the calcium looping cycle. Here, solid CaO is reacted in a reversible reaction with CO 2, forming solid CaCO 3 which is transported to another reactor where CaO is regenerated by heating, producing a pure stream of CO 2 for sequestration. The cycle has a number of major advantages over other proposed post-combustion CO 2 capture systems. It imposes a small efficiency penalty, calculated to be as low as 6% including CO 2 compression. It uses the cheapest possible sorbent, crushed rock (limestone). Finally, it can be integrated with cement manufacture, decarbonising this industry in addition to power generation. One problem is that the ability of CaO to take up CO 2 decays rapidly upon cycling between calcined and carbonated forms. Initially, the activity is extremely high (∼0.8 moles of CO 2 taken up per mole of CaO, or 0.45 g CO 2/g CaO). However, after 30 cycles this activity has dropped to ∼0.15 moles/mole, leading to an undesirable heat load in the system caused by heating and cooling unreactive CaO. One solution, investigated here, is reactivation of the sorbent via hydration. Here, the reaction CaO + H 2O = Ca(OH) 2 (which goes to completion) is applied to the spent sorbent by reaction with humid air. The hydroxide is then heated to regenerate CaO with superior porosity and surface area compared with the spent sorbent, effectively doubling its CO 2 uptake. This paper studies the effects of hydration on physical properties of limestone, specifically its propensity to attrit in a fluidised bed. It is shown that the higher the temperature experienced by the particles prior to hydration, the more susceptible they are to attrition. A simple model has been developed to explain this. The results indicated that reactivation would be unsuitable for use with a bubbling fluidised bed CO 2 capture process, though it may be suitable for a moving or fixed bed reactor.