https://doi.org/10.1021/acs.iecr.2c02344
“For oxidative degradation, the amount of oxygen present always plays a role. Oxygen is needed to initiate the degradation (30) through the formation of radicals. Radicals have short lifetimes and high reactivity in solutions, and the chemistry around them is complicated to verify. What is known is that when a radical is formed, the proximity of other radicals plays a role in the termination of the reaction (forming a neutral molecule). The identity of the formed radicals is unknown, and no studies exist that follow these radicals toward termination. This is understandable as this would require very complex and expensive experimental work without a guarantee that the efforts will be rewarded with new insight and knowledge. Thus, the research has focused on the termination products like ammonia, alkylamines, aldehydes, and acids.
The general problem with oxidative degradation experiments is that setups and experimental conditions vary greatly, making it difficult to compare data from different research institutions/universities. In our experience, minor differences in the experimental conditions may play significant roles in the outcome of the experiments, as differences in the reactor temperature, gas flow, condenser temperature, and setup (open/closed) impact the amine loss. Typically, a glass or stainless-steel reactor is used for the experiments. The reactor can be pressurized with gas added to the headspace. However, in most cases, the experiments are formed under atmospheric pressures introducing gas continuously into the reactor. The gas introduced contains air or oxygen (6−98%) mixed with N2 with/without CO2 and sometimes other impurities like SOx. The reactor temperature varies between 55−140 °C, and the solution might be preloaded with CO2. The experiments last from 15 days to a month. Additives such as Fe, particulate matter, or fly ash are sometimes present. Variations of the factors described above are ways of accelerating the degradation to reduce the duration of the degradation experiments from months or years to weeks or days. However, no study has investigated how these accelerated conditions influence the formation mechanisms of different degradation compounds or the relative order in which they are formed. Vevelstad et al. performed a principal component analysis (PCA) for MEA and its degradation compounds, comparing samples from two different open degradation setups with varying oxygen and iron concentrations, a cycled setup, and three different samples obtained from pilot campaigns, capturing CO2 from industrial sources. (31) The results showed that the cycled experiment gave a degradation profile similar to pilot samples in terms of degradation compound distribution, and for the open setup, the experiments with oxygen concentration (6% O2) and without the addition of iron were more similar than the experiments at higher oxygen concentration. (31)
Table 1 gives a full overview of the studies conducted with the main goals and findings. The most studied amines are MEA (7,10,13−18,25,27,32−46) and Pz, (21,43,47−50) while AMP, (13,15,18,21,51,52) and MDEA (13,15,18,33,47) are also extensively studied. Some early studies were related to gas conditioning applications and gas purification in submarines. (7,10,32,33) Several studies focus on changes in the molecular structure, (13−16,18,44) showing that some structural features seem to improve oxidative stability. Also, metals’ impact on degradation (10,25,32,40−44,53) and degradation or corrosion inhibitors (10,25,32,36−44) have been investigated. Furthermore, numerous works have proposed pathways for degradation compounds, (14−17,20,21,27,29,33−35,40,43,44,54,55) and extensive work has been performed to identify degradation products of selected solvents. (14,15,19−21,26,27,29,44,48,55,56) Finally, there are studies focusing on the impact of amine concentration, CO2, O2, and temperature variations. (19,20,25,36,37,41,42)“
“Table 1. Overview of Oxidative Degradation Studies, Including Which Amines Were Studied, Their Goals and Main Findings”
”
Introduction
ARTICLE SECTIONS
One of the methods to mitigate global warming is by capturing CO2 from industrial flue gas sources or power plants. Postcombustion CO2 capture using amines is a mature process. The full-scale operation has been employed at Saskatchewan in Canada (Boundary Dam) since 2014 and Petra Nova in Austin/Texas for three years (2017−2020). The solvent technologies in these plants are based on proprietary solvents provided and developed by MHI and Shell Cansolv, while the first full-scale plant in Norway will operate with Aker Carbon Capture’s proprietary solvent technology. The large capture projects mentioned above use proprietary solvent systems, but we expect to see plants using generic solvents and solvent blends with known compositions in the future. Generic solvent systems proposed include, among others, ethanolamine (MEA), piperazine (Pz), and Cesar 1 (a blend of 2-amino-2-methylpropanol/piperazine and AMP/Pz).
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 CO2-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 CO2. 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 (Fe2+/Fe3+), 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 CO2 absorption capacity of the solvent and increase corrosion, foaming, and fouling. For MEA, losses of 0.21–3.65 kg MEA per ton CO2 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 CO2, carbamate polymerization reactions being the main degradation pathways. However, the complexity increased when CO2 removal from oxygen-containing flue gas from power plants was suggested, as oxygen radical reactions could occur. This work gives an overview of the current state of the work on degradation, including degradation mechanisms, known compounds, and experimental approaches for studying degradation. The advantages and disadvantages of different experimental approaches are addressed and discussed. Finally, we summarize some key learnings and recommendations for knowledge gaps to be filled next, helping the research community focus on the knowledge gaps.
2. Degradation Studies
ARTICLE SECTIONS
At first sight, the chemical stability of various solvents looks complicated since many variables influence the formation of unwanted degradation compounds. These include factors like temperature, flue gas composition (O2, NOx, SOx, CO2, particle impurities), plant construction material, hold-up time, and circulation rate. Over the years, simplified lab-scale experiments have focused on specific aspects or factors of degradation chemistry. Combining the understanding from these simplified experiments has resulted in the design of more complex setups that have been capable of capturing the behavior of degradation products in large pilot and demonstration plants such as the SINTEF Tiller CO2 plant and the plant at Technology Centre Mongstad (TCM). (11)
Amine solvents without CO2 are relatively stable even under accelerated conditions in lab-scale experiments. (12,13) But when CO2 is present, the oxidative and thermal stability is reduced. So, ironically, the component we are removing reduces the solvent stability.
In terms of chemical stability, simplified lab-scale systems either focus on oxidative or thermal degradation. Although this division between thermal and oxidative degradation is commonly used, it is not precise as thermal degradation compounds are also formed during oxidative degradation studies. Further, some thermal degradation compounds influence oxidative degradation and vice versa. In reality, in the CO2 capture plant, it is the combination of oxidative and thermal conditions that induce the degradation pathways that take place. Thus, separating oxidative and thermal degradation studies will not reproduce all the pathways taking place in a process.
In the laboratory, the degradation studies typically focus on either amine loss and the effect of structural changes in a molecule (12−18) or investigation of one single amine or amine blend to better understand the solvent degradation pathways. (19−29) Examples of solvent systems investigated in more detail are MEA, Pz, and AMP/Pz.
The motivation in the laboratory scale is, often, identifying stable amines and amine blends, identifying degradation compounds to be followed during piloting and demonstration, and understanding the degradation mechanisms helping to choose operation conditions during piloting and/or demonstration.
An overview of amines that have been part of degradation studies is given in Table S1 in the Supporting Information. The Supporting Information also includes an overview of abbreviations and CAS of other components (Table S2).
Oxidative Degradation Studies
For oxidative degradation, the amount of oxygen present always plays a role. Oxygen is needed to initiate the degradation (30) through the formation of radicals. Radicals have short lifetimes and high reactivity in solutions, and the chemistry around them is complicated to verify. What is known is that when a radical is formed, the proximity of other radicals plays a role in the termination of the reaction (forming a neutral molecule). The identity of the formed radicals is unknown, and no studies exist that follow these radicals toward termination. This is understandable as this would require very complex and expensive experimental work without a guarantee that the efforts will be rewarded with new insight and knowledge. Thus, the research has focused on the termination products like ammonia, alkylamines, aldehydes, and acids.
The general problem with oxidative degradation experiments is that setups and experimental conditions vary greatly, making it difficult to compare data from different research institutions/universities. In our experience, minor differences in the experimental conditions may play significant roles in the outcome of the experiments, as differences in the reactor temperature, gas flow, condenser temperature, and setup (open/closed) impact the amine loss. Typically, a glass or stainless-steel reactor is used for the experiments. The reactor can be pressurized with gas added to the headspace. However, in most cases, the experiments are formed under atmospheric pressures introducing gas continuously into the reactor. The gas introduced contains air or oxygen (6−98%) mixed with N2 with/without CO2 and sometimes other impurities like SOx. The reactor temperature varies between 55−140 °C, and the solution might be preloaded with CO2. The experiments last from 15 days to a month. Additives such as Fe, particulate matter, or fly ash are sometimes present. Variations of the factors described above are ways of accelerating the degradation to reduce the duration of the degradation experiments from months or years to weeks or days. However, no study has investigated how these accelerated conditions influence the formation mechanisms of different degradation compounds or the relative order in which they are formed. Vevelstad et al. performed a principal component analysis (PCA) for MEA and its degradation compounds, comparing samples from two different open degradation setups with varying oxygen and iron concentrations, a cycled setup, and three different samples obtained from pilot campaigns, capturing CO2 from industrial sources. (31) The results showed that the cycled experiment gave a degradation profile similar to pilot samples in terms of degradation compound distribution, and for the open setup, the experiments with oxygen concentration (6% O2) and without the addition of iron were more similar than the experiments at higher oxygen concentration. (31)
Table 1 gives a full overview of the studies conducted with the main goals and findings. The most studied amines are MEA (7,10,13−18,25,27,32−46) and Pz, (21,43,47−50) while AMP, (13,15,18,21,51,52) and MDEA (13,15,18,33,47) are also extensively studied. Some early studies were related to gas conditioning applications and gas purification in submarines. (7,10,32,33) Several studies focus on changes in the molecular structure, (13−16,18,44) showing that some structural features seem to improve oxidative stability. Also, metals’ impact on degradation (10,25,32,40−44,53) and degradation or corrosion inhibitors (10,25,32,36−44) have been investigated. Furthermore, numerous works have proposed pathways for degradation compounds, (14−17,20,21,27,29,33−35,40,43,44,54,55) and extensive work has been performed to identify degradation products of selected solvents. (14,15,19−21,26,27,29,44,48,55,56) Finally, there are studies focusing on the impact of amine concentration, CO2, O2, and temperature variations. (19,20,25,36,37,41,42)”
Table 1. Overview of Oxidative Degradation Studies, Including Which Amines Were Studied, Their Goals and Main Findings
| Amines | Goals | Main findings |
|---|---|---|
| MEA (10,32) | Stabilizing the aq. amine solvents used for CO2 capture in submarines using inhibitors. | Fe and Cu catalyze degradation, while EDTA and sodium salt of N,N-diethanolglycine inhibit oxidative degradation. |
| MEA (34,35) | Studying the formation oxidative degradation products in samples from a CO2 capture plant. | Many degradation mechanisms were proposed. |
| MEA (46,55) | Comparing laboratory and pilot scale degradation. | Significant overlap was found between degradation products from pilot and laboratory scales. Oxidative degradation is the dominant pathway dominant in the pilot scale. |
| MEA (36,37,57,58) | Studying oxidative stability in varying amine concentrations, with and without CO2 or NaVO3 or SOx in pressurized reactors. | Conclusions about the influence of concentration of amine, O2, CO2, SOx, temperature, and corrosion inhibitor were made. Power-law rate model presented. |
| MEA (40) | Studying stability under typical absorber conditions, investigating the effect of iron and inhibitor concentrations on ammonia evolution. | The presence of CO2 hugely increases the rate of degradation. |
| MEA (25,41,42) | Testing oxidative stability under various parameters: pH, CO2 loading, O2/Fe/Cu/MEA concentrations, and inhibitor presence. | Mass transfer of O2 is the limiting factor for the degradation rate of MEA. |
| MEA (39,45) | Studying inhibitors for oxidative degradation of MEA (aq.), and the influence of degradation and corrosion inhibitors on MEA stability. | Inhibitors that successfully worked under simulated absorber conditions were unsuccessful at hindering degradation under cyclic conditions. No inhibitors suitable for both corrosion and degradation inhibition were found. |
| MEA (38) | Studying the effect of stable salts on CO2 solubility, viscosity, thermal and oxidative degradation, and corrosion. | 1–2 wt % KI gave an increase in oxidative stability, without influencing CO2 solubility, viscosity, thermal degradation, and corrosion. |
| MEA (27,54,59) | Studying oxidative stability at different temperatures and pO2 in an open-batch setup. | Monitored MEA loss and 17 different degradation compounds. |
| MEA, TEA, DIPA (7) | Testing resistance of the amine solvents at 85 °C with constant O2 sparging. | MEA was the most resistant amine toward oxidation, followed by TEA and DIPA. |
| MEA, DEA, MDEA (33) | Studying a series of aq., CO2 free amines under oxidative conditions. | Mechanisms for the formation of the primary degradation compounds formic, acetic, oxalic, and glycolic acid were proposed. |
| 16 amines (14,15) | Studying degradation of amine solutions in a pressurized vessel at high temperature (140 °C) in the absence of CO2. | Many oxidative degradation mechanisms were postulated in this work based on results from GC, GC/MS, NMR, and IC. |
| 25 amines (13,18) | Studying structural effects in amines on oxidative stability, and correlations between that and ecotoxicity, biodegradability and thermal stability. | For primary and tertiary alkanolamines, increasing the carbon chain increased oxidative stability. Size/length of alkyl substituents seemed to increase stability. Steric hindrance effect had more impact than electronic effects. A correlation between biodegradation and oxidative degradation was observed, but not between oxidative degradation, ecotoxicity, or thermal degradation. |
| 8 amines (16,17) | Studying oxidative stability in an open and closed batch system. The closed system had gas phase recycling. | Temperature and dissolved metals influence degradation and degradation rate. The open setup generally gave higher amine losses than the closed, with some exceptions. |
| 5 amines (imidazoles) (60) | Studying degradation and toxicity of alkylated imidazoles. | Polyalkylated imidazoles had low oxidative stability. Degradation pathways suggested. |
| Several amines and blends (43,44) | Studying oxidative amine stability and solutions to amine oxidation. Monitoring degradation product formation with and without presence of metals or inhibitors. | Identification of degradation products. |
| Pz (48) | Studying rate of oxidation in the presence of catalysts. | Fe2+, Ni2+, and Cr3+ are only weak oxidation catalysts compared to Cu2+. |
| Pz (49) | Studying oxidation rates and products in a bench-scale cyclic degradation apparatus. Comparing with oxidation in pilot-scale campaigns. | Created a model for degradation and solvent management costs in full-scale. |
| Pz (50) | Studying amine stability under oxidative conditions in an advanced flash stripper. | The stripper configuration seemed to reduce Pz degradation. |
| AMP (51) | Studying oxidative degradation (aq.) in an autoclave type reactor at 100–140 °C. | Degradation rate was found to be mass transfer limited like in Goff and Rochelle. (42) |
| AMP/Pz blends (21) | Studying oxidative stability at temperatures between 80 and 140 °C | The degradation rate of Pz increased in blend with AMP, despite of the same compounds detected in the single amine solutions as in the blends. |
| AMP/KSAR, MMEA, 1-(2HE)PRLD, 2-PPE (29,52,56) | Studying degradation of various amines | 1-(2HE)PRLD and AMP/KSAR more stable than MEA. Volatile compounds were formed during degradation of MMEA. |
| MAPA (24) | Studying MAPA as a solvent for CO2 capture. Degradation properties are included in this evaluation. | MAPA had lower oxidative stability than MEA. |
| EDA (61) | Investigating EDA as a solvent for CO2 capture | Oxidative degradation reduced using inhibitor. |
Lepaumier et al. (14,15) conducted the most comprehensive mechanistic studies of both amine degradation and degradation pathways. The studies investigated six alkanolamines, one alkanolamine/diamine, five ethylenediamines (where a minimum of one nitrogen was a tertiary or a secondary amine), and four additional polyamines. Several of the main conclusions from these works are applicable to other amines, and they can be summarized as follows:
-
Most amines participate in demethylation, methylation, dealkylation reactions, and, to some extent, carboxylic acid formation.
-
Volatile compounds are always formed.
-
Ethanolamines oxidize to amino acids (typically found in small amounts).
-
Ethylenediamines degrade to piperazinone.
-
Tertiary amines are slightly more stable than primary and secondary amines, but an exception is observed when the chain length between two amino groups makes it possible to form five- and six-membered rings.
-
Steric hindrance (AMP) decreases degradation (only AMP investigated). Later, Buvik et al. and Muchan et al. showed that steric effects such as chain length, substituents location in relation to the nitrogen atom, and bond strain positively affect stability. (13,18)
Although the studies mentioned above only studied particular amines, we believe that sterically hindered amines will behave similarly to AMP. Decreasing bond flexibility seems to play a stabilizing role under oxidative conditions. For example, the ring structure of Pz makes the N−H bond more rigid than the N–H bond in MEA, making the initial radical reaction less likely to occur and improving the stability of Pz compared to MEA.”