A GEOCHEIMCAL SOLUTION TO THE ATMOSPHERIC CO2 PROBLEM?

If it becomes necessary to purposefully capture and sequester fossil-fuel-generated CO₂, a variety of ocean, land, and underground storage options are being considered. However, many of these approaches are expensive, have potentially serious environmental impacts, and/or may ultimately be ineffective in keeping CO₂ from the atmosphere. As an alternative we propose that CO₂-rich power-plant gases be first hydrated with seawater to produce a carbonic acid solution that in turn is reacted on-site with limestone to form primaily Ca²⁺ and HCO₃^-. This calcium bicarbonate solution is then released and diluted in the ocean where it would add minimally to the existing, large pool of these ions in the sea. Such a process simply speeds up natural, abiotic carbonate weathering and dissolution which will otherwise consume anthropogenic CO₂, but over many millennia.

Using a model of ocean chemistry and transport we show that this process would increase ocean alkalinity, helping to neutralize CO₂ acidity and isolating anthropogenic carbon from the atmosphere. Relative to atmospheric release or direct CO₂ injection into the sea, this method would greatly expand the capacity of the ocean to store anthropogenic carbon while minimizing environmental impacts of this carbon storage on ocean biota. Our calculations indicate that releasing the carbonate-dissolution effluent to the oceans would be less damaging to the marine environment than releasing an equivalent amount of CO₂ directly to the atmosphere.

This sequestration method is also less energy intensive and less expensive than abiotic CO₂ capture and direct ocean or geologic injection schemes. We calculate an energy penalty that may be <5% with a CO₂ capture efficiency which may exceed 50%. Estimated sequestration costs could be as low as $12 per tonne CO₂ sequestered, dependent on reactor configuration and on limestone and water availability and transport. These compare with >$90/tonne CO₂ and >>20% energy penalties estimated for direct, deep ocean or subterranean CO₂ injection. Our initial benchtop-scale experimental simulation of this process indicates that carbonate dissolution could contribute significantly to mitigating adverse impacts of fossil-fuel burning.