PNNL

Abstract

Nitrogen oxides, which are present in flue gas, can cause significant negative impacts on amine carbon capture solvents by the formation of heat-stable salts and suspected carcinogens. Thus, to maximize the performance of water-lean solvents, a better understanding of this process in these systems is necessary. Here, a computational study for the fixation of the carbon dioxide (CO2) capture solvent N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) to nitramine/nitrosamine was conducted. Density functional theory calculations and molecular dynamics simulations were used to explore the energetics and reaction pathways. The first step involves the dissociation of the NH of 2-EEMPA, in which the homolytic mechanism is energetically more favorable than the heterolytic mechanism. Interestingly, water that is commonly present in CO2 capture solvents, shows a lower capacity to dissociate the N-H bond compared to NOx or O2. While the first step is highly endothermic, the second step, which involves radical recombination to form N-N bonds, is highly exothermic. Charge analysis indicates that the reaction in the liquid proceeds through a homolytic mechanism, similarly to the gas phase. Our study shows that while NO2 directly reacts with 2-EEMPA, NO has almost no effect. However, in the presence of O2, fixation of 2-EEMPA by NO is enhanced via the formation of N2O4 species which leads to a relatively stable product with HNO2 as intermediate. However, low reaction energies indicate that the formation of nitramine/nitrosamine may be a reversible process, suggesting that 2-EEMPA could be recovered under thermal stripping conditions.