“Thermal degradation studies, like oxidative degradation studies, have focused on understanding degradation trends due to the molecular structure of the amines or understanding the thermal degradation and its pathways in specific solvent systems. Most of the published thermal degradation experiments are conducted in either stainless steel cylinders or stainless steel autoclaves. In the case of stainless-steel cylinders (the size varies from study to study), they are filled with absorption solutions that contain CO2, which are placed in an oven at the desired temperature (often 110–150 °C). For every sampling, one or two cylinders are taken out of the oven, and the content is analyzed using available analytical schemes. (22,28,29,55,62) The other experiments use a jacketed autoclave in stainless steel, where gas is introduced in the headspace. These experiments are also conducted at a similar temperature as the closed cylinder experiments. (12) The degradation compounds formed in these experiments are mainly described as carbamate polymerization products, meaning that the presence of CO2 is essential for the reaction to occur. Mechanisms and pathways suggested for the identified degradation compounds go back to the 1950s. (8,63) Davis and Eide-Haugmo (62,64) give good overviews of the thermal degradation of different amines, while a comprehensive overview of degradation compounds is given by Gouedard et al. (65) A study has also been conducted to investigate the impact of flue gas contaminants on the thermal degradation of MEA. (66)
The thermal degradation experiments are typically conducted in relatively simple laboratory set-ups, and a large number of studies have been conducted, as shown in
Table 2. Thermal stability in the presence of CO
2 has been more thoroughly studied than stability in the presence of oxygen. This is because, historically, the focus has been on aqueous amine solutions used in natural gas treatment, not for CO
2 capture from oxygen-containing flue gases. Most studies focus on specific blends or single amines, and thoroughly studied amines are MEA,
(8,12,28,62,64,66−77) diethanolamine (DEA),
(12,62,63,73,78−83)N-methyldiethanolamine (MDEA),
(12,47,62,64,73,82,84−88) Pz,
(4,22,47,53,62,64,75,83,86,89−91) AMP,
(12,52,62,64,72,73,75,82,92) and diglycolamine (DGA).
(62,64,71,93,94) Studies on the following amines could also be found: diisopropanolamine (DIPA), 1-(2-hydroxyethyl)pyrrolidine (1-(2HE)PRLD), 2-piperidineethanol (2PPE), 3-amino-1-methylaminopropane (MAPA), various imidazoles, 2-(diethylamino)-ethanol (DEEA), sodium salts of various amino acids,
N-(2-hydroxyethyl)ethylenediamine (AEEA/HEEDA), and ethylenediamine (EDA).
(24,56,60,61,76,95−98) Davies (2009) and Eide-Haugmo (2011)
(62,64) systematically investigated large sets of amines. Most of their work is described in their Ph.D. theses,
(62,64) making the results less available. Thus, in recent years, we have seen publications
(61,98) discussing the potential of amines, like EDA and AEEA/HEEDA, which Davies (2009) and Eide-Haugmo (2011) found to be corrosive and/or unstable. Davies and Eide-Haugmo also give good overviews of thermal degradation pathways suggested from the 1950s to 2009. Furthermore, Lepaumier et al. gave a general overview of degradation mechanisms for alkanolamines and polyamines.
(12,14) In this case, structural variation significantly impacts the chemical stability, where tertiary amines and amines with steric hindrance close to nitrogen often are more stable than others. It is also important to note that the chain length between functional groups in polyamines is essential since longer chains can form stable ring structures. This is often observed for tertiary polyamines such as
N,
N,
N′,
N′-tetramethylbutylenediamine (TMBDA) and
N,
N,
N′,
N′,
N″-pentamethyldiethylenetriamine (PMDETA).
(14)“
Table 2. Overview of Thermal Degradation Studies, Including Which Amines Were Studied, the Goals of the Studies and Their Main Findings
