Calcium oxide (CaO) looping is a proposed negative emissions technology that involves using CaO powder to passively capture carbon dioxide (CO2) from the atmosphere to form calcium carbonate (CaCO3). The CaCO3 is calcined to regenerate CaO for further reaction while a high-purity stream of CO2 may be utilized or stored. Currently, an understanding of the fundamental reaction processes for CO2 removal using CaO is scarce, inhibiting potential implementation. CaO hydration to calcium hydroxide [Ca(OH)2] and further carbonation to CaCO3 without the addition of liquid water are strongly influenced by humidity. This study aims to provide insight into key reaction pathways and optimal conditions for reaction efficiency to aid the design of direct air capture systems. CaO experiments (0.5, 1, and 1.5 cm thick) demonstrated an optimal thickness of ≤0.5 cm for carbonation (64% CaCO3) with CO2 fluxes ranging from -12 to -117 kg/m2/yr over a 24-day period at 85–95% RH. Controlled relative humidity (20, 40, 60, 80, 95% RH) and humidity swing (60-80% and 60-95% RH) experiments were conducted to quantify reaction rates and extent over 14 days. Although complete hydration was always attained, full carbonation (&gt;90%) was not achieved under most experimental conditions, likely owing to passivation as indicated using electron microscopy. Additionally, CO2 supply may limit carbonation rate for large masses. Therefore, flow-through experiments tested CO2 concentrations (100, 200, 300, 400 ppm), revealing slower reaction yet greater carbonation extent for decreasing CO2 concentrations. Greater CO2 limitations for CaO led to greater hydration extent to Ca(OH)2 prior to carbonation. Carbonation extent was influenced by Ca(OH)2 increasing surface area ~7 fold from CaO. Our findings indicate that complete CaO hydration must occur before most carbonation to minimize passivation. A new understanding of humidity-driven reaction pathways merits high efficiency, aiding in the feasibility of pilot-scale implementation.

Calcium oxide looping: Reaction pathways and optimal conditions for passive direct air capture

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