The feasibility of enhancing the absorption of CO2 from the atmosphere by adding calcium carbonate (CaCO3) powder to the ocean and of partially reversing the acidification of the ocean and the decrease in calcite supersaturation resulting from the absorption of anthropogenic CO2 is investigated. CaCO3 could be added to the surface layer in regions where the depth of the boundary between supersaturated and unsaturated water is relatively shallow (250–500 m) and where the upwelling velocity is large (30–300 m a 1 ). The CaCO3 would dissolve within a few 100 m depth below the saturation horizon, and the dissolution products would enter the mixed layer within a few years to decades, facilitating further absorption of CO2 from the atmosphere. This absorption of CO2 would largely offset the increase in mixed layer pH and carbonate supersaturation resulting from the upwelling of dissolved limestone powder. However, if done on a large scale, the reduction in atmospheric CO2 due to absorption of CO2 by the ocean would reduce the amount of CO2 that needs to be absorbed by the mixed layer, thereby allowing a larger net increase in pH and in supersaturation in the regions receiving CaCO3. At the same time, the reduction in atmospheric pCO2 would cause outgassing of CO2 from ocean regions not subject to addition of CaCO3, thereby increasing the pH and supersaturation in these regions as well. Geographically optimal application of 4 billion t of CaCO3 a 1 (0.48 Gt C a 1 ) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a 1 after 50 years, 900 Mt CO2 a 1 after 100 years, and 1050 Mt CO2 a 1 after 200 years.
Several low-cost opportunities exist for scrubbing CO2 from waste gas streams, utilizing spontaneous chemical reactions in the presence of water and inexpensive or waste alkaline compounds. These reactions convert CO2 to bicarbonate or carbonate in dissolved or solid form, thus providing CO2 capture and low-risk CO2 storage underground, in the ocean, or in some cases on land. Useful by-products and co-benefits can also be generated by these processes. In certain settings this approach will be significantly less energy intensive, less costly, and less risky than “conventional” molecular CO2 capture and geologic storage.
It has been previously shown that industrial-scale accelerated weathering of limestone, AWL, can effectively convert a significant fraction of US CO2 emissions to long-term storage as bicarbonate in the ocean. Being analogous to the successful, wide-spread use of wet limestone to desulfurize flue gas, AWL reactors could be retrofitted to existing power plants at a cost possibly as low as $3-$4 per tonne CO2 mitigated. Such low costs would especially pertain to coastal power plants where an average of 30,000 tonnes of seawater per GWhe are already pumped through for cooling, and where the majority of coastline (at least in the US) is within 400 km of limestone sources.
Capture and Storage Using Water Co-Produced With Oil
On average 10 barrels of water are brought to the surface with each barrel of oil produced, and the majority of this water is simply pumped back into the reservoir. Our preliminary analysis suggests that most of this water is significantly undersaturated in CO2 relative to industrial waste gas streams that are typically 10% to 20% CO2. Furthermore, such waters can contain significant carbonate ion concentrations, meaning they have an enhanced capacity to react with excess CO2 to form dissolved bicarbonates.
While the US capacity of this CO2 mitigation approach is modest (perhaps 2 million tons/yr) and is best suited to treat CO2 waste streams in the immediate vicinity of the water production, the cost of such CO2 mitigation could be extremely low, perhaps less than $1/tonne CO2.
Co-benefits of CO2 addition to produced water would be the reduction (via lowered pH) of internal pipeline scale formation, a common and expensive problem in the industry. Also, CO2 addition could enhance the oil-water separation process, may reduce downstream microbial fouling, and might enhance oil recovery. Further work is needed to better evaluate the cost/benefit and potential market of this CO2 mitigation approach.
Cement Production can be altered to absorb CO2 instead of releasing CO2.