Nitrosamine management in aqueous amines for post-combustion carbon capture
Nitrosamines are one of the largest environmental issues associated with amine scrubbing. Nitrogen dioxide (NO₂) is the primary precursor for nitrosamine formation. In the amine scrubber, NO₂ reacts with amine via a free-radical absorption mechanism, forming an amine radical and nitrite. The overall liquid side mass transfer coefficient (k [subscript g] ’) for NO₂ at absorber conditions is 310⁻⁷, 1010⁻⁷, and >4010⁻⁷ mol/m² s Pa for 9 m monoethanolamine (MEA), 8 m piperazine (PZ) and 7 m methyldiethanolamine (MDEA)/2 m PZ respectively; typical NO₂ uptake in absorbers ranges from 73 % in MEA to over 99 % in MDEA/PZ. The amine radical resulting from NO₂ absorption can react with NO to directly form nitrosamine; in PZ at absorber conditions, approximately 20 % of the amine radical reacts with NO to form n-nitrosopiperazine (MNPZ). Nitrite from NO₂ absorption then cycles to the high temperature stripper where it can nitrosate a secondary amine. Nitrosation at high temperature occurs through nitrite attack on the amine carbamic acid. Primary amines may scavenge nitrite from solution, but typically react slower than secondary amines. For example, N (2 hydroxyethyl)glycine (HeGly), a common secondary amine in degraded MEA, reacts five times faster than MEA. Tertiary amines cannot form carbamates, so they are unable to competitively scavenge nitrite. After nitrosation, the nitrosamine thermally decomposes in the stripper with a pseudo-first order nitrosamine decomposition rate constant. At 163 °C in 8 m PZ, MNPZ decomposes at 6.010⁻⁵ s⁻¹, reaching steady-state in less than a week. Nitrosamines are relatively non-volatile and thermally unstable at reclaiming conditions, allowing for efficient removal through thermal reclaiming; less than 5 % of MNPZ would return to the circulating solvent after thermally reclaiming PZ. Nitrosamines will accumulate to approximately 1 mM, 2 mM, and 21 mM in PZ, MEA, and MDEA/PZ respectively when the stripper is operated at the maximum temperature and thermal reclaiming is used. Gaseous emissions of nitrosamines will be below air quality regulations when scrubber nitrosamine is below 14 mM. If necessary, NO₂ pre scrubbing with sulfite/thiosulfate or triethanolamine can lower nitrosamine by an order of magnitude with no net cost to the carbon capture system.