Absorption of Nitrogen Oxides in Aqueous Amines

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Abstract Nitric oxide (NO) and nitrogen dioxide (NO2) were absorbed into aqueous amine to determine the absorbing species, the absorption kinetics, and the aqueous products at the ppm-level NOx concentrations typical of flue gas from fossil fuel power plants. At flue gas conditions of 0.5-5 ppm of NO2, absorption is dominated by free radical absorption of NO2 as nitrite. NO2 absorption kinetics are first order in NO2 partial pressure, half order in free amine concentration, and fastest in methyldiethanolamine (MDEA). The reaction-enhanced liquid mass transfer coefficient for NO2 absorption in 8 m piperazine (PZ) at absorber conditions is 9.7*10-7 mol/s·m2·Pa, yielding 92% NO2 absorption at a typical A/G of 3.3*106 s·Pa·m2/mol. Similarly, a 9 m monoethanolamine (MEA) solvent will absorb roughly 70% of the inlet NO2 while a 7 m MDEA/2 m PZ solvent will absorb over 99% of the NO2. Nitrite and nitrate are the main NOx absorption products in MDEA with nitrite dominating at low NO2 partial pressures. In PZ, the amine free radical formed during NO2 absorption will react with NO to directly form n-nitrosopiperazine (MNPZ) or react with itself to form 2-piperazinol (2-PZOH). Typical nitrosamine yields in 5 m PZ are around 15% of total absorbed NOx and can be halved with the addition of 200 mM Inhibitor A, a free radical scavenger. Nitrate and nitramine are minor products of NOx absorption, accounting for less than 5% of total absorbed NOx.