Oxidative Degradation of Diethanolamine Solvent Induced By Nitrogen Dioxide and Dissolved Materials in Post-Combustion Capture of CO2 From Industrial Exhaust Gas Streams

Abstract

Solvent degradation is a serious and well-known operational problem in the postcombustion CO2 capture process using amine-based solvents. It is caused by undesired side reactions between the amine and flue gas impurities such as oxygen, nitrogen oxides (NOx), sulfur oxides (SOx), and fly ash particulate, resulting in a significant decrease in performance and efficiency as well as an increase in operational costs for solvent management. In addition, there are human and environmental concerns regarding the emergence of carcinogenic compounds, N-nitrosamines, which are contributed by the reaction between secondary amines and NOx in the flue gas. Moreover, heavy metalcontaining compounds from fly ash, equipment leaching, and metal-based corrosion inhibitor, once dissolved into the solution, can play a catalytic role by accelerating the degradation reactions. This study aimed to understand the role of degradation-inducing components, namely oxygen, nitrogen dioxide, and dissolved transition metals on the degradation of a secondary amine solvent, diethanolamine (DEA), by measuring the degradation rate of DEA, formation rate of products (i.e. formate and monoethanolamine), and emission rate of ammonia. The degradation experiments were performed at 80 ℃ at atmospheric pressure for 14 days by continuously passing synthetic flue gas through 5 M diethanolamine solution (⍺ = 0.2 mol/mol) associated with dissolved metal. Iron (II), iron (III), and copper (II) at concentrations 0, 1, and 100 μM were studied. The synthetic flue gas contained 6% O2 with 0 – 50 ppm NO2. Degradation products in the liquid phase were identified by GC-MS and CE-DAD. The off-gas was analyzed for ammonia production. Degradation products identified in this study include MEA, MDEA, bicine, ammonia, formate, acetate, glycolate, oxalate, HEOD, BHEP, and N,N-Bis(2- hydroxyethyl)formamide. Formation pathways for all the products are proposed. In terms of the catalytic effect, the ranking was as follows: 1 μM iron (III) > 1 μM iron (II). Copper (II) did not exhibit a catalytic effect towards DEA degradation or ammonia emission. Dissolved metal at concentration of 100 μM, however, considerably reduced the degradation rate of DEA as well as the formation rate of products. This suggests saltingout effect, in which the high concentration of metal salt can lower the solubility of oxygen, resulting in less degradation. NO2 was found to increase the degradation rate of DEA as well as the emission rate of ammonia. The presence of dissolved iron also promoted the DEA degradation rate under the influence of NO2. The synergistic effect of NO2 and dissolved iron caused DEA to degrade more substantially. Therefore, in the real-world operation where NO2 and dissolved iron are encountered, secondary amines like DEA should be avoided.