(2a) Reactivation of CaO-Based Sorbents for CO2 Capture: Some Aspects of the Carbonation of Ca(OH)2

Calcium-looping is an emerging technology that can be applied to both flue and fuel gas streams for CO₂ capture.  A typical proposed process is described as follows: CaO-based sorbent is repeatedly cycled between two fluidised beds; in the lower temperature carbonator CaO-based sorbent strips CO₂ from a flue or fuel gas via the exothermic formation of CaCO₃; in the calciner, operating at a higher temperature, CaCO­₃ decomposes to provide a pure stream of CO₂ suitable for storage and regenerate CaO.  There are several advantages to this system including the ability to reclaim high-grade heat from the carbonation reaction and the use of cheap non-toxic sorbents, which have the potential to be used in the cement industry after use.

A disadvantage to the system is that the sorbent reduces in reactivity towards CO₂ upon cycling.  This can occur for a number of reasons including loss of reactive porosity through sintering, competing reactions with sulphurous molecules and mass loss through attrition.  One method of overcoming this loss in reactivity is by periodic reactivation of sorbent by hydration, which has the effect of increasing the reactivity of the sorbent for CO₂.  Here, a novel activation strategy proposed by Industrial Research Limited of New Zealand is investigated: they have shown that the temperature of dehydration can be increased in a CO₂ atmosphere, resulting in higher conversions to CaCO₃ and a more mechanically stable sorbent.  The mechanism proposed by Industrial Research Limited is of free water being formed within Ca(OH)₂  prior to dehydration before interacting with CO₂ leading to proton injection into the lattice further delaying dehydration.  In contrast, we speculate that a pore-occlusion mechanism is more likely.

To investigate this, Ca(OH)₂ powder and pellets have been taken and heated separately in various different atmospheres in a TGA with a mass spectrometer performing online gas analysis on the off-gas.  Under a nitrogen atmosphere, both powder and pellets decompose directly to CaO, with decomposition starting at around 400 ºC and resulting in a single peak of H₂O (mass number 18) being observed in the off-gas.  Under a carbon dioxide atmosphere, carbonation of both pellets and powder appears to occur slightly earlier at around 300 ºC and at the same time steam is observed in the off-gas.   However: powder then proceeds to ~ 90% carbonation by ~ 600 ºC with a single peak of H­₂O being observed on the mass spectrometer; pellets reach ~ 80% carbonation by ~ 600 ºC before a second release of H₂O, a small decrease and then increase in mass to ~ 90% carbonation.  The carbonation behaviour of hydrated calcined dolomite (CaCO₃.MgCO₃) has also been investigated.  In this case, under heating in a carbon dioxide atmosphere, a single peak of steam associated with dehydration was observed whilst carbonation was occuring.

These findings are consistent with an occluded pore model, in which an impermeable carbonate layer prevents H₂O diffusion until the occlusion ruptures (molar volume of CaCO₃ and Ca(OH)₂ are 36.9 and 33.7 cm³/mol respectively).  The carbonate layer has a critical thickness that is not reached in the case of the Ca(OH)₂ powder, but is in the case of the Ca(OH)₂ pellets.  The network of MgO formed in the dolomitic particles provides an additional pore network that prevents the formation of an impermeable carbonate layer.