Carbon dioxide supply and scaling constraints on direct air capture using calcium oxide powder

Calcium oxide (CaO; lime) looping is a carbon dioxide (CO 2 ) removal technology that can mitigate carbon emissions. However, scaling this technology requires a thorough understanding of the rate-limiting effects of CO 2 supply on CO 2 removal rates at and below atmospheric CO 2 concentration levels, as well as with various CaO thicknesses and plot areas. Here, we show that carbonation is readily limited by CO 2 supply even under high flow rates (e.g., 25 mmol CO 2 /h over 100 g CaO). Subsequently, the CO 2 capture efficiencies using CaO and Ca(OH) 2 powders were investigated under ambient CO 2 concentrations (397–490 ppm) to 100 ppm. Ca(OH) 2 carbonation rates increased exponentially with increasing CO 2 concentrations, e.g., 2.1 wt% CaCO 3 /h at 100 ppm compared to 5.2 wt% CaCO 3 /h at ambient CO 2 concentrations, thereby demonstrating the impact of CO 2 concentrations on carbonation efficiency. Column experiments determined CO 2 diffusion limitations for CaO deposits ≥0.5 cm, yielding 64 ± 2 wt% CaCO 3 and an average CO 2 removal rate of 32 kg CO 2 /m 2 /yr (24 days; 85–95% RH). Finally, the CO 2 removal efficiency of CaO plot areas was estimated, revealing the importance of scaling CO 2 supply in concert with increasing CO 2 removal that results from greater thicknesses and masses. • 0.5, 1.0, and 1.5 cm CaO achieved average CO 2 removal rates of 32–35 kg CO 2 /m 2 /yr • CO 2 diffusion was limited through ≥0.5 cm thick CaO powder • Ca(OH) 2 carbonation rates exponentially increased from 100 ppm to 450 ppm CO 2 • The fastest Ca(OH) 2 carbonation rate was 5.2 wt% CaCO 3 /h at 450 ppm CO 2 • CO 2 supply was readily limited for CaO powder, suggesting small plots are efficient