At 240 MW, Petra Nova is the world's largest post-combustion carbon capture facility installed on an existing coal-fueled power plant. DOE selected the project to receive up to $190 million as part of the Clean Coal Power Initiative.
Post-combustion Capture refers to capturing carbon dioxide (CO2) from a flue gas generated after combusting a carbon-based fuel, such as coal or natural gas. In conventional fossil fuel power plants, coal or natural gas is burned with air to generate heat energy which is converted to electricity. Of the 4 trillion kilowatt hours of electricity generated in the U.S. in 2019, about 23% was from coal and 38% was from natural gas (EIA, Annual Energy Outlook 2020, Jan 2020). With over 60% of the electricity in the U.S. produced from fossil fuel power plants, deployment of post-combustion capture technologies is vital to reduce CO2 emissions.
One of the biggest challenges in post-combustion capture is separating the relatively low concentration of CO2 from the large amounts of nitrogen in the flue gas. In addition, carbon capture applied to various types of flue gas streams involves unique challenges. In general, natural gas power generation produces an exhaust with essentially zero heavy metal content, lessening flue gas pre-treatment requirements. However, flue gases from natural gas combined cycle (NGCC) plants typically contain ~4% CO2 by volume (compared to a CO2 concentration of 12-15% in flue gases from coal plants) which provides less driving force for CO2 separation, and therefore, requires greater energy input.
R&D efforts are focused on advanced solvent, sorbent, and membrane systems, as well as novel concepts (e.g. hybrid systems that efficiently combine attributes from multiple key technologies) that have the potential to provide step-change reductions in both cost and energy penalties compared to currently available technologies.
Solvent-based CO2 capture involves chemical or physical absorption of CO2 from flue gas into a liquid carrier. The absorption liquid is regenerated by increasing its temperature or reducing its pressure to break the absorbent-CO2 bond. High levels of CO2 capture are possible with commercially-available chemical solvent-based systems; however, these systems require significant amounts of energy for regeneration. R&D objectives include advanced solvents (e.g. water-lean solvents, phase-change solvents, high performance functionalized solvents) that have a lower regeneration energy requirement than existing amine systems, combined with high CO2 absorption capacity and tolerance to flue gas impurities. System advancements include process intensification techniques, methods to mitigate aerosol formation and corrosion, and heat integration approaches.
Sorbent-based CO2 capture involves the chemical or physical adsorption of CO2 using a solid sorbent. Like solvents, solid sorbents are usually regenerated by increasing temperature or reducing pressure to release the captured CO2; however, solid sorbents may have lower regeneration energies compared to solvents due to lower heat capacities. Sorbent technologies are generally less developed than solvents and have heat transfer, stability and attrition challenges. R&D objectives include low-cost durable sorbents that have high selectivity for CO2, high CO2 adsorption capacity, resistance to oxidation, and can withstand multiple regeneration cycles with minimal attrition. System advancements include sorbent process intensification techniques, novel reactor designs, and enhanced process configurations, such as rotating beds for CO2 adsorption and desorption.
Membrane-based CO2 capture uses permeable or semi-permeable materials that allow for the selective transport and separation of CO2 from flue gas. Membrane processes offer potential advantages when applied to post-combustion CO2 capture, including no hazardous chemical storage, handling, disposal or emissions issues, simple passive operation, tolerance to high SOx and NOx content, a reduced plant footprint, efficient partial CO2 capture, and diminished need for modifications to the existing power plant steam cycle. The challenge with membranes is the relatively low partial pressure of CO2 in the fuel gas. R&D objectives include development of low-cost, durable membranes (e.g. polymeric membranes, mixed matrix membranes, sub-ambient temperature membranes) that have improved permeability and selectivity for CO2, thermal and physical stability, and tolerance to contaminants in combustion flue gas. Process enhancements for membrane-based capture systems include low pressure drop membrane modules and utilizing various sweep gases and process configurations.
Novel Concepts for post-combustion capture include hybrid systems that combine attributes from multiple technologies (e.g., solvents and membranes), alternative technologies and processes such as cryogenic separation and electrochemical membranes, and additive manufacturing of novel system components and materials. R&D objectives include development of equipment, materials, and processes which enable intensified thermodynamic operations, improve process performance, and reduce equipment size, lowering capital and operating costs.