Assessment of the potential life-cycle environmental impacts of ocean alkalinity enhancement: from limestone extraction to slaked lime discharge in the sea, including carbon dioxide storage

The global emission pathways which allow keeping the global temperature increase “well below 2°C” as set by the Paris Agreement require a rapid and drastic emission reduction reaching net zero emissions by 2050. Furthermore, hundreds to thousands of gigatonnes of CO₂ need to be removed from the atmosphere cumulatively by 2100 to limit global warming to 1.5°C, which means that a portfolio of different Carbon Dioxide Removal (CDR) terrestrial and marine processes should be developed and upscaled.

Among CDR solutions, Ocean Alkalinity Enhancement (OAE) is the unique one which also counteracts the ongoing ocean acidification. OAE consists of spreading an alkaline material on the sea surface, enhancing the ocean alkalinity and consequently the sea uptake of atmospheric CO_2, which is then stored in the form of dissolved bicarbonate ions (HCO₃^-).

To produce carbon emission-free Ca(OH)₂ and consequently increase the overall process efficiency in CO₂ removal, when slaked lime (Ca(OH)₂) is used as alkaline material for OAE, the CO₂ released from the limestone calcination should be captured and stored. Since the most developed storage alternatives are underground geological formations whose main limitations are the long time required for qualifying their suitability (on the order of years), the geographical uneven distribution and the uncertain injection rate, an alternative storage technology called Buffered Accelerated Weathering of Lime (BAWL) is under study.

BAWL stores CO₂ from calcination in the form of HCO₃^- in seawater using a pipeline where CO₂ dissolves in seawater and reacts with Ca(OH)₂, forming dissolved Ca(HCO₃)₂ mimicking natural weathering but in an accelerated way. The use of Ca(OH)₂ allows to discharge an ionic solution with the seawater pH, to avoid CO₂ degassing and to store permanently CO₂ in the form of bicarbonates. Since the raw materials are CO₂, Ca(OH)₂ and seawater, BAWL fits well with OAE.

To assess the overall process efficiency in CO₂ removal, i.e., the effective CO₂ removal net of life-cycle greenhouse gas (GHG) emissions, the Life Cycle Assessment (LCA) methodology was applied to a process whose system boundaries encompass limestone extraction, other raw materials supply, Ca(OH)₂ discharge in the sea and CO₂ storage through BAWL. In addition to climate change, 15 additional impact categories are considered for the environmental assessment according to the Environmental Footprint method, and the ecoinvent database was used for supporting the life-cycle inventory.

Electricity is considered the energy source for calcination, that requires 83% of the total energy demand, and its production from renewables results as the most impacting phase in most of the impact categories. Thus, the variation of climate change impact was analysed by varying the electricity emission factor. With renewables, the process efficiency is at least 85%, i.e. less than 15% of removed CO₂ compensates the life-cycle GHG emissions removing 1.4 molCO₂/molCa(OH)₂.

Due to the lack of an impact category for assessing ecotoxicity on marine environments, further research is required to include in the LCA methodology the assessment of the benefits from the alkalinity enhancement and the contrast to ocean acidification, as well as the potential risks on the pelagic ecosystem.