Pelletized CaO-based sorbents treated with organic acids for enhanced CO₂ capture in Ca-looping cycles

Many potential treatments for improving CO₂ capture produce mechanically fragile sorbents therefore pelletization is often a solution to improve the performance of a sorbent for a practical Ca-looping system when a sorbent is first enhanced by some treatment step. However, as pelletization is inherently expensive, the final sorbent should be as reactive as possible and so it may make sense to use a pre-treated sorbent. Here we explore the possible use of acetification, combined with pelletization using a calcium aluminate cement binder to produce pellets. Acetic acid solution (10vol.%) and commercial vinegar were used for limestone treatment both in the form of aqueous solution and vapor and this is as far as we know the first time such vapor phase treatments have been explored. The binder content was 10-14wt%, and both acetified and untreated limestones were used to prepare pellets. This work demonstrated that pellets prepared from acetified limestone with binder are superior to natural limestones. For instance acetified pellets captured 41% more CO₂ than un-acetified pellets after 20 cycles under severe calcination condition at 920°C in CO₂. An important caveat is that the acetification treatment widened pores, resulting in enhanced pellet reactivity with SO₂; therefore, SO₂ must be removed from the flue gas if the aim is to capture CO₂. There are however a number of additional important caveats, first given that acetic acid costs about $900/ton, it is likely that the gains seen here cannot justify such treatments, and second, because Ca acetate is very soluble, it is probably more practical to start with industrial grade Ca acetate (costing about $500/ton) than treating limestone with any grade of acetic acid and then drying and precipitating the resulting product.