Photo by D koi on Unsplash
Author: Aloisa Ferrero
The Role of CO2 Capture in the Decarbonization Process
The report published by the International Energy Agency (IEA), “Net Zero by 2050 – A Roadmap for the Global Energy Sector,” identifies CO2 capture, storage, and utilization as a key tool for achieving net-zero emissions by 2050. CCUS (Carbon Capture, Utilization, and Storage) technologies will make it possible to tackle emissions reduction in “hard-to-abate” sectors, increase the production of low-carbon hydrogen, and facilitate the removal of CO2 already present in the atmosphere through BECCS (BioEnergy with CO₂ Capture and Storage) and DAC (Direct Air Capture) systems.
The roadmap sets a global target of capturing 1,670 Mt of CO2 per year by 2030 and 7,600 Mt of CO2 per year by 2050.
Alongside the other key pillars of decarbonization (energy production from renewable sources, energy efficiency, electrification, etc.), the advancement of CO2 capture technologies has the specific goal of facilitating emissions reductions in those sectors where it remains extremely difficult to achieve independence from fossil fuels.
Consider, for example , industrial processes that require both hot water/steam at high temperatures (> 200°C) and electricity . Removing fossil fuels would require replacing the most widely used fuel, natural gas, with renewable equivalents, such as biogas, which is still in short supply today, or by relying on heat pumps powered by photovoltaic panels or wind turbines, which, with such output temperatures, are currently only models in the development phase.
Even with regard to electricity generation, existing gas-fired power plants play an irreplaceable role in balancing the grid. In the event that renewable energy production is lower than expected, consumption is higher, or there are outages at facilities covering the baseload, combined-cycle power plants provide an instantaneous power reserve that allows for responding to peaks and restoring balance with demand. Generating 100% of electricity from renewable sources would require the use of batteries for storage, which are still extremely expensive and unsuitable for compensating for fluctuations over extended periods, such as during the winter. Even seasonal storage systems based on green hydrogen are complex to implement and currently have limited efficiency.
The implementation of CO2 capture systems at fossil fuel-fired power plants could be the solution to accelerate decarbonization in the short term and make it economically sustainable. These systems could be economically competitive even in the short term and integrate with renewable energy sources as a backup system. Meanwhile, by continuing research and development on renewable energy systems and energy storage technologies, it would be possible to increase their efficiency and reduce their costs: these technologies will, in fact, be indispensable for achieving the goals set for 2050 and phasing out the use of fossil fuels entirely.
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Current Status and Future Prospects
CO2 capture is made possible by gas separation processes that have been used in the chemical industry for decades. Once captured, carbon dioxide can be sequestered in permanent storage sites (underground geological formations or depleted gas reservoirs) or reused to produce other carbon-containing materials, such as cement, plastics, and biofuels. In the first case, the term CCS( Carbon Capture and Storage) is used; in the second , CCU (Carbon Capture and Utilization); and when both processes are applied, the term CCUS (Carbon Capture, Utilization, and Storage) is used.
The CCS process consists of three stages: capture, transport, and storage. Each stage has been technically available and used in various industrial sectors for many years, but their combined application to CO₂ emission sources hasso far provedimpractical, both in terms of energy and cost, thus limiting their large-scale implementation. Recently, an alternative related to CCS—namely CCU—has begun to attract attention because, by transforming CO₂ emissions into valuable products, it allows for an economic return at the end of the process that can offset the costs incurred.
CO2 Capture Processes: Three Categories
CO2 capture processes can be grouped into three broad categories:
- Post-combustion capture: The most common technology is based on absorption processes using CO2-affine solvents, such as amines in aqueous solution, with recovery rates of up to 90%. Other technologies also exist, such as polymer membranes, innovative adsorbent materials, innovative solvents (such as amino acids), and molten carbonate fuel cells (MCFCs). The main advantage of this technology lies in its applicability for retrofitting existing plants, while the main drawback is the increased cost associated with the numerous additional components that must be installed and the reduction in plant efficiency due to the heat consumed to regenerate the solvent.
- Pre-combustion capture: This occurs prior to the combustion process, converting the feedstock into hydrogen and carbon dioxide through reforming or gasification processes, and then capturing the CO2 via absorption or adsorption. Although the investment costs are higher than those of other solutions, the simultaneous production of blue hydrogen can be highly competitive, as it can be used to generate electricity, in the chemical industry, or for transportation. It is also possible to produce hydrogen from biomass with CO₂ capture to achieve negative-emission systems.
- Oxy-combustion: This involves an initial process of separating oxygen from nitrogen, which allows only the oxygen stream to be used in the combustion process. Since the combustion products are not diluted by nitrogen from the air, they consist almost exclusively of water and CO₂: the CO₂ is then separated simply by cooling and condensing the water. Although oxy-combustion cycles require a complete redesign of the plant, they are among the most promising technologies due tothe absence of additional pollutant emissions and chemical solvents, as well as separation efficiencies approaching 100%.
In the area of CCU, carbonation—a process that allows CO2 to be stored in solid matrices—enables the production of construction materials, such as aggregates for concrete, bricks, and refractories in general. Research is also being conducted on catalytic processes powered by excess electricity from renewable sources, with the goal of converting CO₂ and H₂O into green, carbon-based fuels.
The world’s first commercial CCS facility was the Sleipner project, which has been operating in the North Sea since 1996. It removes CO2 from natural gas using amine-based technology and then injects it into a depleted reservoir. With regard to post-combustion CCS technology, one of the most significant case studies is located at the Technology Center in Mongstad (Norway), where various types of solvents are being tested. ENEA is involved in several projects in the field of CCU, and at the Polytechnic University of Milan, the GECOS group has been conducting research on CO2 capture systems for over 25 years, focusing in particular on oxy-combustion cycles for power generation (Allam and SCOC-CC) that offer very promising efficiency and cost profiles. The American company Net Power has built a 25 MW industrial-scale demonstration plant based on the Allam cycle.
Finally, CO2 capture and storage enable the creation of negative-emission systems, whether the CO2 is captured directly from the air using Direct Air Capture systems or indirectly from biomass using Bioenergy with CO2 Capture and Storage (BECCS) systems. These technologies will become invaluable in the long term for offsetting distributed and hard-to-avoid emissions—such as those from the agricultural sector—as well as excess emissions produced during the energy transition. The first large-scale DAC facility recently began operations in Iceland. It consists of eight containers, each with an annual capture capacity of 500 metric tons of CO₂. Air is drawn into a collector containing a filter that traps carbon dioxide particles. When the filter is saturated, the collector closes and is heated to approximately 100°C, causing the CO₂ to be released; it is then pumped, along with water, into a deep geological reservoir. The process utilizes waste heat or heat generated from renewable sources.
