Mechanism of formation of heat stable salts (HSSs) and their roles in further degradation of monoethanolamine during CO2 capture from flue gas streams

The roles of HSS induced acid products of MEA degradation were evaluated. The results show that formic and acetic acids, formed as a result of MEA oxidation, exist in 2 forms in equilibrium; namely, salts and amides. Specifically, these are formate and acetate, and N-(2-hydroxyethyl)formamide and N-(2-hydroxyethyl)acetamide, respectively. Glycolic acid, also formed due to MEA oxidation, is stable and exists mostly bonded with MEA to produce the glycolate HSS. Its amide is unstable, hydrolyzing back to glycolate and MEA. Oxalic acid was found as a reactive intermediate that mostly decomposed to formic acid, which in turn produced a stable N-(2-hydroxyethyl)formamide. Oxalic acid amide (N-(2-hydroxyethyl)oxamide) could also be formed but its formation was considered a minor route compared with the decomposition route. Succinic acid formed a stable imide (N-(2-hydroxyethyl)succinimide) through an intermediate amide (N-(2-hydroxyethyl)succinamide).