Conditions for capture

Recent large-scale carbon-capture schemes in the energy sector point to progress, but further development and support are still required to improve viability and widespread deployment.

This year has seen the commencement of operations of two major carbon-capture projects in the US energy sector. In January, the Petra Nova project in Texas started removing CO₂ from the flue-gas streams of a coal-powered generator¹. In April, operations to capture CO₂ emissions from a corn ethanol processing plant began in Decatur, Illinois². The Petra Nova project is a retrofit to an existing 240 MW coal-fired unit, making it the largest post-combustion capture project in the world. Capturing around 90% of the CO₂ from the unit, this translates into an expected sequestration of around 1.4 million tonnes of CO₂ per year. The Decatur facility will store around 1 million tonnes annually and is the world’s first large-scale carbon capture and sequestration (CCS) deployment at a bioethanol facility.

These projects demonstrate that carbon capture from coal power plants and from the biofuels industry can work at scale and help us to understand more about how to safely store the captured CO₂ and ensure it remains in the intended location. But it is sobering to put these amounts of CO₂ sequestration into context alongside the suggested amounts required for significant progress towards meeting climate-change targets. Modelling by the International Energy Agency has suggested that CCS could contribute to 14% of the cumulative emissions reductions required by 2060 in order to limit global temperature increase to 2 °C (ref. 3). This amounts to storing around 400 million tonnes of CO₂ emissions annually by 2025.

Another matter to ponder is the fate of the captured CO₂. At Petra Nova, the CO₂ is piped to an oil field 80 miles away where it is used for enhanced oil recovery, which involves injecting streams of CO₂ into the ground to eke out more oil from the West Ranch oil field, where production was previously declining. In the process, CO₂ is stored in naturally sealed geologic formations. It is estimated that using this technique, production from the oil field will be boosted from around 300 to around 15,000 barrels per day. Therefore, although the process has a favourable effect on CO₂ emissions from the power plant, the captured CO₂ facilitates the production of more fossil fuels. While the CO₂ collected from the bioethanol plant is buried deep underground without further use, and such projects based on bioenergy and CCS could have potential for negative CO₂ emissions, careful whole-systems life-cycle analysis of producing and using bioethanol — even with CCS — is required to understand the true emissions-mitigation potential of such projects, which can be highly case-specific.

Using captured CO₂ to produce more fossil fuels may appear to be somewhat self-defeating in terms of trying to mitigate CO₂ emissions through CCS. However, given that capturing and storing CO₂ represents a cost, naturally industry will seek routes to monetize the captured CO₂ to increase the overall economic viability of such schemes. Therefore, to increase the chances of carbon-capture schemes being more widely deployed, regardless of what is done with the CO₂, the economics will need to be improved and it is likely that targeted policy incentives to motivate further deployment of large-scale CCS will be required. The experience gained by building and operating carbon capture facilities also helps reduce costs in later iterations — the people behind SaskPower’s Boundary Dam Power Station in Canada, which started collecting CO₂ from its coal-fired units in 2014, believe that similar future projects could be delivered with around 30% lower costs⁴. As more ventures are undertaken, further reductions are likely, calling for coordination to ensure that the lessons learned in one project are applied to others.

State-of-the-art technologies for capture of CO₂ are based on amine solvent routes and while the practices are well-developed, continued efforts to improve processes and optimize the solvents may still lead to enhanced energy efficiency of capture and a reduction in costs. Other routes for separating CO₂ from flue gases post-combustion, such as those based on membranes or solid sorbents, are also under investigation. Approaches based on membranes can suffer from low-membrane permeability, which can make dealing with the high flow rates from flue gas stacks challenging. Ways to improve this are being sought; for instance, using nanoparticulate amine-functionalized metal-organic framework fillers has been shown to boost the CO₂ selectivity of polymers without significantly sacrificing their high permeability⁵. New materials for use as solid sorbents with smaller energy penalties associated with carbon capture are also being developed⁶.

Beyond the capture phase of these processes, significant attention is also being paid to the conversion of CO₂ into fuels and chemicals, such as polymers. Utilization of captured carbon in this way may have benefits in terms of emissions mitigation and could provide product streams to improve the overall viability of carbon-capture projects. Moreover, for regions with limited fossil-fuel resources, direct air capture of CO₂ for use as a feedstock could have energy security benefits. Using solar-driven catalytic reactions to generate useful precursors such as CO, which can be used for synthesis of a range of products, might be especially attractive. Indeed, efforts to increase the activity and product selectivity of Earth-abundant catalysts⁷ and to engineer solar-to-chemical systems with the potential for scalability⁸ are key to improving the feasibility of such processes.

Ultimately, whilst the economics of individual CCS technologies and projects will undoubtedly improve — through the lessons learned from large-scale projects, and fundamental advances — capturing CO₂ in order to bury it without further use will remain an additional cost. Therefore, without governmental will and significant policy support, such as adequate carbon pricing or limits on CO₂ emissions, meaningful emissions mitigation through CCS at the scale required to meet climate targets seems unlikely. As with most areas of the energy system, progress in several arenas — fundamental science, technology, engineering and policy — will have to be realised for serious headway to be made.