- The Chilled Ammonia Process (CAP) is an advanced post-combustion carbon capture technology designed to remove carbon dioxide (CO₂) from flue gas streams, particularly those produced by coal-fired power plants and other high-emission industrial sources. Developed by Alstom (now part of GE Power), CAP uses an aqueous ammonia (NH₃) solution as the solvent to chemically absorb CO₂ at low temperatures—typically between 0°C and 10°C. This chilled environment significantly enhances the absorption capacity and minimizes solvent degradation and ammonia volatilization, making the process more efficient and environmentally manageable compared to traditional amine-based systems.
- In the CAP system, flue gas is first cooled and conditioned to remove particulates, sulfur oxides (SOₓ), and nitrogen oxides (NOₓ), as these impurities can interfere with the absorption chemistry. The clean, cooled flue gas is then brought into contact with the chilled ammonia solution in an absorber column. At these low temperatures, CO₂ reacts with ammonia to form ammonium bicarbonate (NH₄HCO₃) and ammonium carbonate ((NH₄)₂CO₃) in solution. These reactions are reversible, allowing for the easy regeneration of the solvent.
- The CO₂-rich solution is then sent to a regenerator (or stripper), where it is heated to release high-purity CO₂ gas and regenerate the ammonia solution for reuse. The captured CO₂ is typically compressed and either stored (in carbon capture and storage applications) or utilized for other industrial purposes (such as enhanced oil recovery or synthetic fuel production). The regeneration process in CAP is relatively energy efficient due to the favorable thermodynamics of ammonia-carbonate chemistry and the potential for process heat integration.
- One of the most notable advantages of the Chilled Ammonia Process is its low rate of solvent degradation and oxidative breakdown, even in the presence of oxygen-rich flue gases. This contrasts with amine-based systems, which often suffer from chemical degradation, forming corrosive and toxic byproducts. Additionally, ammonia is a widely available and inexpensive chemical, and its emissions can be effectively controlled using ammonia recovery systems and by operating the process under chilled conditions, which reduce ammonia volatility.
- However, the CAP is not without challenges. Operating at low temperatures requires significant refrigeration energy, which can impact overall process efficiency and capital cost. Moreover, ammonia is a toxic and pungent compound, requiring careful handling and control to prevent environmental and occupational exposure. Effective ammonia slip control systems are essential to minimize losses and ensure compliance with environmental regulations. The design and materials used in the system must also resist ammonia-induced corrosion and withstand thermal cycling between the chilled and hot sections of the process.
