Alkalinity Generation Constraints on Basalt Carbonation for Carbon Dioxide Removal at the Gigaton-per-Year Scale

The
world adds about 51 Gt of greenhouse gases to the atmosphere
each year, which will yield dire global consequences without aggressive
action in the form of carbon dioxide removal (CDR) and other technologies.
A suggested guideline requires that proposed CDR technologies be capable
of removing at least 1% of current annual emissions, about half a
gigaton, from the atmosphere each year once fully implemented for
them to be worthy of pursuit. Basalt carbonation coupled to direct
air capture (DAC) can exceed this baseline, but it is likely that
implementation at the gigaton-per-year scale will require increasing
per-well CO2 injection rates to a point where CO2 forms a persistent, free-phase CO2 plume in the basaltic
subsurface. Here, we use a series of thermodynamic calculations and
basalt dissolution simulations to show that the development of a persistent
plume will reduce carbonation efficiency (i.e., the amount of CO2 mineralized per kilogram of basalt dissolved) relative to
existing field projects and experimental studies. We show that variations
in carbonation efficiency are directly related to carbonate mineral
solubility, which is a function of solution alkalinity and pH/CO2 fugacity. The simulations demonstrate the sensitivity of
carbonation efficiency to solution alkalinity and caution against
directly extrapolating carbonation efficiencies inferred from laboratory
studies and small-injection-rate field studies conducted under elevated
alkalinity and/or pH conditions to gigaton-per-year scale basalt carbonation.
Nevertheless, all simulations demonstrate significant carbonate mineralization
and thus imply that significant mineral carbonation can be expected
even at the gigaton-per-year scale if basalts are given time to react.