Coupling Mineral Carbonation and Ocean Liming

The process by which basic/ultrabasic
silicate minerals (e.g.,
olivine) are reacted with CO2 to produce solid carbonate
minerals (“mineral carbonation”) has been suggested
as a method to sequester carbon dioxide from point sources into stable
carbonate minerals. Alternatively, the addition of lime (produced
from calcining carbonate minerals) to the surface ocean (“ocean
liming”), which results in an increase in ocean pH and a draw-down
of atmospheric CO2 has been proposed as a “geoengineering”
technology, which stores carbon as dissolved alkalinity in the surface
ocean. Combining these approaches, in which the magnesium carbonate
minerals produced from mineral carbonation are used as a feedstock
for ocean liming (mineral carbonation-ocean liming; MC–OL),
may reduce the limitations of individual technologies while maximizing
the benefits. Approximately 1.9 metric tons of magnesium silicate
(producing 0.7 ton of magnesium oxide) are required for every net
ton of CO2 sequestered. A total of 0.7 ton of CO2 is produced from this activity, 70% of which is high-purity (>98%)
from calcining and potentially amenable for geological storage. The
technology can be conceptually viewed as an alternative to direct
air capture and swaps ambient CO2 for high-purity point
source CO2. MC–OL requires approximately 4.9 and
2.2 GJ of thermal and electrical energy ton–1 of
CO2 sequestered. MC–OL has less demand for geological
storage; only 0.5 ton of CO2 needs to be injected for every
ton of CO2 removed from the atmosphere. However, manipulation
of ocean chemistry in this way potentially creates an additional environmental
impact (localized elevated pH or co-dissolution of trace metals) and
requires additional attention.