Using Limestone to Assess the Impacts of Passivation and Ca-Silicate Formation on Lime Carbonation Efficiency for Looping Technology

Abstract

Maximizing process efficiency in the lime looping process for direct air capture (DAC) is essential for reducing costs and energy demands to scale up this carbon dioxide (CO₂) removal (CDR) technology. Here, we investigated two inefficiencies in the lime looping process: (i) surface passivation in relation to the particle size of the initial limestone powder and (ii) the presence of silicate impurities and their thermal reaction with CaCO₃ during calcination. Passivation-limited carbonation to <68% completion for coarser (>25 μm) lime particles compared to ∼88% for finer (<25 μm) particles, emphasizing the need to pulverize limestone to very fine particle sizes to optimize efficiency. Limestones naturally contain unreactive impurities that will reduce CDR efficiency. For instance, the presence of 7.5 wt.% non-carbonates in the limestone used in this study reduced CO₂ capture efficiency by an equal percentage. Additionally, formation of relatively inert Ca-silicates from the reaction of silicates (primarily quartz) with CaO during calcination (900 °C) further reduced efficiency by lowering CaO availability and potentially passivating reactive surfaces. Carbonation of very high-purity limestone (55.6 wt.% CaO) and synthetic CaCO₃ (56.01 wt.% CaO) yielded greater reaction extents of ∼93 and ∼98%, respectively, compared to that of limestones with varying SiO₂ contents (4.4–24.4 wt.%), which achieved ∼86–75% carbonation, respectively. Our findings demonstrate that the use of finely pulverized, very high-purity to impure limestones will be affected by passivation and Ca-silicate formation to varying degrees, incurring process inefficiencies of ∼10–19% due to materials not reacting during carbonation.