K.S. Lackner from Columbia University presents a comprehensive review of the progress towards implementing ambient air capture units, and focuses more deeply on capture sorbent development. Lackner discards the notion of using an aqueous sorbent due to the large binding energy required and the corrosiveness of a strong sodium hydroxide solution. Experiments were thus carried out to identify a sorbent with lower binding energy that could still maintain an uptake rate equal or better to that of a sodium hydroxide solution. Ultimately a solid strong-base ion-exchange resin was deemed to be the best sorbent option. Experiments showed that this resin could be loaded with absorbed CO2, and upon exposure to moisture the CO2 would be driven off and the resin would be ready to recommence CO2 uptake after dried. Based on this, Lackner goes on to describe a modular unit that could be easily deployed and could be expected to capture one ton of CO2 per day.
To control corrosion that can potentially occur in the completion equipment, wellhead valve trims and wetted parts of packers should be made of stainless steel, nickel, or Monel. Experiences with injection of supercritical CO2 have demonstrated the need for elastomers and seals resistant to swelling. Additionally, CO2 as a solvent will dissolve any hydrocarbon based material, therefore Teflon, nylon, and hardened rubber are effective materials for use in packing and sealing elements. These refinements made in the design of injection equipment for CO2—EOR can be applied to CCS such that these projects are technically safe and reliable.
Results from the modelling indicate that although depth is an important determinant of storage potential, it is not the most important factor in storage cost. While increased depth can increase the cost by a factor of two, layer thickness and permeability of the storage reservoir can increase cost by a factor of fifty. This hints at the myriad of basin characteristics that need to be assessed before arriving at a viable cost estimate. Additionally, costs within a single basin are likely to differ considerably due to the extreme variability in aquifer characteristics. The most important conclusion that can be drawn from this analysis is that the amount of CO2 storage provided by low-cost regions within saline aquifers in the United States is considerably lower than the estimates reported by previous studies. The study by Eccles et al. suggests that there are only perhaps ten storage reservoirs in the United States that would have an average storage cost of below $10 per ton CO2. If more basins are to become economically viable for CO2 storage, then policymakers will need to devise a regime that imposes a rather significant cost on carbon.—Shanna Hoversten