Reviews and syntheses: Potential and limitations of oceanic carbon dioxide storage via reactor-based accelerated weathering of limestone

Abstract. To achieve climate stabilization, substantial emission reductions are needed. Emissions from industrial point sources can be reduced by applying CO2 emission mitigation methods, which capture carbon dioxide (CO2) before it is released to the atmosphere. Accelerated weathering of limestone (AWL) is such a CO2 emission mitigation approach, in which calcium carbonate (CaCO3) is dissolved and CO2 is stored as dissolved inorganic carbon in the ocean. At present, AWL technology remains at the pilot scale with no industrial implementation. Here, we review the proposed reactor designs for AWL, comparing them in terms of CO2 uptake efficiency, CaCO3 dissolution efficiency, CO2 sequestration efficiency, and water usage. For this, we represent AWL as a four-step process: (i) CO2 uptake, (ii) CaCO3 dissolution, (iii) alkalinization, and lastly (iv) re-equilibration. AWL application is generally characterized by a large water usage and the need for large reactor sizes. Unbuffered AWL approaches show substantial degassing of CO2 back to the atmosphere after the process water is discharged. Buffered AWL approaches compensate the unreacted CO2 by Ca(OH)2 addition, which prevents degassing and hence substantially increases the CO2 sequestration efficiency. Critically however, buffered AWL requires a source of CO2-neutral Ca(OH)2, which is conventionally produced by calcination causing substantial CO2 emissions. The need for process water can be reduced by increasing the CO2 fraction of the gas stream or increasing its pressure. Further optimization of the size distribution of pulverized CaCO3 particles could reduce the amount of Ca(OH)2 needed to buffer the unreacted CO2. The anticipated CO2 sequestration efficiency of buffered AWL is comparable with that projected for large-scale carbon capture and storage (CCS) in geological reservoirs.