https://doi.org/10.1021/acs.iecr.2c02344
“Different amines show varying stability in the CO2 capture process, and every solvent to be used industrially must have high capture efficiency and acceptably high chemical stability under operating conditions. Some solvents are inherently unsuitable for CO2 capture from flue gases because of their degradation and corrosion behavior, like diethylenetriamine (DETA) and ethylenediamine (EDA). There are no general guidelines for when a solvent should be rejected for its chemical stability or corrosive behavior, even though some have developed their own categorization based on testing many amines using the same setup and experimental conditions. (1) However, the limits found there are not necessarily directly transferable to other setups and experimental conditions, as small changes in operating conditions and the presence of different contaminants may significantly impact chemical stability.
The process conditions also impact the chemical stability of all amine-based solvents. For example, exposing the CO
2-loaded solvent to high temperatures (typically around 120 °C) during solvent regeneration and the presence of oxygen and nitrogen oxides (NOx) in the gas to be treated can negatively impact the solvent stability. Furthermore, the presence of sulfur oxides (SOx), metals, or other particulate matter and impurities can either react with the solvent, participate in the degradation reactions, or enhance degradation due to catalytic effects. Independently of the reasons, the result is the formation of unwanted degradation compounds in reactions occurring in parallel with the primary chemical reaction between the absorbent and CO
2. Mitigation technologies can reduce, or ideally eliminate, the formation of these unwanted compounds. Technologies proposed include, for example, using various inhibitors to remove oxygen, radicals, or metals, or remove oxygen from the absorber sump, either using a dissolved oxygen removal apparatus (DORA)
(2) or nitrogen stripping.
How much degradation can be tolerated depends on the solvent, process conditions, presence of impurities, and formed degradation compounds. However, some guidelines are available. Morken et al. presented threshold values for, among others, total heat stable salt (HSS) concentration, metals (Fe
2+/Fe
3+), ammonia emissions, and degradation compounds.
(3) The paper discusses recommended actions when the threshold is reached.
(3) Additionally, Freeman et al. developed a method to estimate the maximum desorber temperature for various amines, typically at rich loadings.
(4) As the method is based on constant loadings and at a certain concentration, it is most suitable for relative comparison between the amines. For MEA, the maximum recommended temperature was around 115 °C, and for piperazine, around 160 °C.
Reducing the formation and presence of degradation compounds is vital. Degradation compounds can be removed, i.e., by replacing solvent inventory (bleed and feed), filtration of particulates (mechanical or active carbon), or using different reclaiming technologies such as thermal reclaiming, ion exchange, and electrodialysis. As given above, Morken et al.
(3) describe actions to be taken when concentrations of degradation compounds and impurities become too high. Further, a recent publication by Moser et al.
(5) describes solvent management strategies during the MEA campaign at the RWE pilot plant.
Degradation compounds influence the CO
2 absorption capacity of the solvent and increase corrosion, foaming, and fouling. For MEA, losses of 0.21–3.65 kg MEA per ton CO
2 captured have been observed for various MEA campaigns at different capture pilot plants.
(5) Other solvents such as Pz and a blend of AMP and Pz have shown less solvent loss than MEA.
(6) Replacing a part of the solvent by feed and bleed or reclaiming, using pretreatment technologies for the flue gas, and mitigation technologies for reducing emission or degradation all come at a cost, and influence which amines may be used on a commercial scale. Solvent degradation may also impact health, safety, and the environment by forming potentially harmful components, such as volatile components and nitrosamines. These will require monitoring to safeguard the health of people surrounding and operating the plant and the environment.
Understanding degradation mechanisms and performing lab-scale experiments are crucial for the use at the industrial scale. Knowledge about degradation mechanisms and how the process conditions impact them helps to develop and design mitigation technologies. Degradation experiments during solvent development help researchers to find optimal solvent composition and operating conditions, i.e., how high temperatures can be used during regeneration. Finally, laboratory data can be used to develop degradation models connected to process simulators allowing detailed studies on how process design impacts degradation. In the future, the models could also be used during industrial operations to predict the need for costly liquid analyses and the operation of mitigation technologies.
Studies that investigated the chemical stability of various solvents started in the 1930s when amines were used for gas treatment offshore and in submarines.
(7−10) Until the end of the 20th century, the experimental focus was on high-temperature degradation in the presence of CO
2, carbamate polymerization reactions being the main degradation pathways. However, the complexity increased when CO
2 removal from oxygen-containing flue gas from power plants was suggested, as oxygen radical reactions could occur. “