Enhancing Carbon Capture by Examining Degradation in Aqueous Amine Solvents and Developing Targeted Mitigation Systems to Reduce Key Environmental Impacts
Global CO2 emissions from industrial, power generation and transportation sources has led to the call for increased implementation of carbon capture strategies. The most developed of these is point source carbon capture, which refers to the process of capturing CO2 directly from large (point) source emitters, before the CO鈧 is released into the atmosphere. The challenge becomes separating CO2 from the other components of the emitted gas, mainly nitrogen. Therefore, these processes typically involve the use of aqueous solutions of amines to absorb (capture) CO鈧 from the gas stream, where the CO2 and the basic amine in water react to form a carbamate and/or bicarbonate, depending on the specific amine used. An advantage when using amine solutions is that this reaction is reversible, as the absorbed CO2 is released when the solution is heated allowing the amine to be reused in multiple cycles of absorption and regeneration.
This type of amine-based carbon capture works well, but it is not without some drawbacks. The temperature swings needed for this desorption process not only requires significant energy input but can also lead to gradual degradation of the amine, commonly referred to as thermal degradation. This can lead to solvent losses, reduced performance, and higher operational costs. In addition, the solvent can degrade due to exposure to oxygen and other contaminants present in the gas (such as SO鈧, NOx). This oxidative degradation can lead to the formation of unwanted byproducts, some of which are regulated volatile organic compounds. To avoid unintended environmental effects, the amine degradation pathways need to be carefully understood and managed. Amine degradation can produce a combination of different species generating a complex matrix that when coupled with the high pH environment, can make degradation remediation challenging. This dissertation focuses on the degradation by-products of amine solvents in carbon capture systems and how the chemical differences between the amine and water impacts the volatility and the removal of these degradation compounds. A better understanding of theses impacts allows for the development of mitigation strategies minimizing any environmental impacts.
Mitigation of the unwanted degradation byproducts is achieved by either removing the contaminants from the solvent or capturing and neutralizing them within the system. First, an assessment was performed to understand the effectiveness of activated carbon adsorption, with implications for treating high pH solutions. While there were some benefits to this methodology, activated carbon adsorption was not completely effective and presented several limitations such as metal leaching from the activated carbon material. Given this, it is necessary to expand into other areas of degradation mitigation. First understanding the potential for emissions of any degradation products, including compounds such as aldehydes, is crucial given their known environmental and human health hazards. These emissions may be impacted by the composition of the amine solvent used, therefore the Henry鈥檚 volatility coefficient of acetaldehyde in relevant amine solutions were determined as a surrogate for other classes of potential degradation compounds. The volatility was determined to be significantly higher from the amine solvent when compared to water, which is critical fundamental information in aiding the development of proper mitigation strategies that can be implemented within capture systems.
Current engineering controls within CO2 capture plants involve the use of water wash systems to reduce amine emissions, however some degradation products are also captured by this system which allows for their targeted neutralization. The composition of the wash-water poses yet another unique challenge as the complex matrix and increased the pH make it difficult to treat via traditional water treatment methods. An electrochemical-mediated treatment method was developed and evaluated to facilitate the decomposition of N-nitrosamines and aldehydes. The experimental results showed that even in the presence of this complex matrix, highly efficient decomposition of these hazardous compounds can be achieved.