Atmospheric carbon dioxide removal using layers of lime

Metal oxides such as lime (CaO and Ca(OH)₂) or magnesium oxide (MgO) react spontaneously with CO₂ in the air, under ambient conditions, to form stable carbonate minerals. They are therefore, being used as reactive materials to remove carbon dioxide from the atmosphere to help prevent climate change. In these technologies ‘thin’ layers of calcium or magnesium oxides/hydroxides are spread over an area of land or inside tiered structures to contact the material with CO₂ in the air. The proposed thickness of these layers varies by orders of magnitude between theoretical studies, from 3 to 100 mm, however, there is no published data describing the rates of carbonation as a function of layer thickness for lime. This study monitored the carbonation reaction of 2.5, 5, 10, 25 and 50 mm layers of CaO and Ca(OH)₂ in ambient temperatures and concentrations of CO₂. The results show that repeated spreading of thin layers (<10 mm every 5–10 days) resulted in the largest removal rate per spatial area (>2 t CO₂ ha⁻¹ day⁻¹). However, given that the production costs of zero carbon lime may be substantially greater than the cost of land, it may be more economical to maximise conversion through extended periods between applications.