Rates and limitations of CO2 removal by MgO looping

Magnesium oxide (MgO) looping is a proposed carbon dioxide (CO2) removal technology with gigatonne-scale potential [1]. MgO can be spread over large areas, reacting with atmospheric CO2 to form hydrated Mg-carbonates, which can be calcined to release captured CO2 and regenerate MgO. However, no experimental evidence before this study has supported the potential implementation of MgO looping [2]. The carbonation rate of MgO under weathering conditions is highly variable and depends on many factors, including deposit thickness, permeability and water content. In experiments, CO2 removal rates for a 10 cm thick deposit of MgO were 2.2-12.5 kg CO2/m2/yr, varying primarily due to water content. While these rates are significantly faster than those documented for brucite [Mg(OH)2] -bearing mine tailings, only a fraction (<20%) of the initial MgO is predicted to react in 1 yr and areas of 80,000-455,000 km2 are required to capture 1 Gt CO2/yr. Thinner deposits (e.g., 0.5 cm) provide slower CO2 removal rates per area yet more fully react in shorter periods than thicker deposits. For instance, complete carbonation of <2.5 cm thick deposits in 1 yr was predicted. Stable carbon isotopic data demonstrate that carbonation rates are limited by CO2 supply rather than by the dissolution of the reactants, thus the need for greater dispersal over large land areas. Replacing MgO with CaO will improve CO2 supply and carbonation rate given its greater causticity; however, the trade-off of using CaO is that higher temperatures are needed to calcine CaCO3. MgO or CaO looping can be optimized through engineering to enhance carbonation rates and reduce land requirements, which is essential for achieving gigatonne CO2 removal.

[1] McQueen et al., 2020. Nat Com., 2020, 11

[2] Rausis et al., 2022. IJGHGC, 2022, 119, 103701