Kinetics studies on wet and dry gas-solid carbonation of MgO and Mg(OH) ₂ for CO ₂ sequestration

Mineral carbonation is a carbon dioxide capture and storage (CCS) route that warrants further investigation. Although most of the CCS research to date has been concerned with underground storage in liquefied form, mineral carbonation is the only method that disposes CO ₂ in a permanent and inherently safe manner. Here, we consider the gas-solid conversion of both MgO and Mg(OH) ₂ with CO ₂ in the presence and absence of steam in an attempt to model and predict the optimum conditions for rapid and complete carbonation. Results from pressurised thermogravimetric analysers (PTGA) and a laboratory scale pressurised fluidised bed (PFB) are presented. The results show that the carbonation of Mg(OH) ₂ is much faster (∼50% in 4 min) in a PFB than the carbonation of comparatively fine MgO (<44 μm) in a PTGA (∼50% in 30 min). Furthermore, the results show that the presence of water vapour is pivotal, giving rise to a clear distinction between MgO and Mg(OH) ₂ carbonation. In the case of MgO, steam (>10%) accelerates the carbonation considerably. However, in the case of Mg(OH) ₂, the addition of steam to the CO ₂ is less important as it is provided intrinsically, as a result of the dehydroxylation of Mg(OH) ₂ at elevated temperatures. Still, humidifying the gas stream can help control dehydroxylation, thereby sustaining carbonation, which typically levels out short of completion. A careful control of the carbonation conditions (temperature, pressure, fluidising velocity, gas composition) and particle properties should allow for close to complete carbonation (>90%) without compromising the carbonation kinetics. Because the PFB carbonation step considered here is part of a larger CCS process (Mg extraction from a natural and abundant mineral followed by production of MgCO ₃), the precipitation stage [Mg(OH) ₂ formation] may be tailored to obtain the necessary particle properties (surface area, porosity).