Amine scrubbing is the most mature technology for post-combustion CO2 capture. It employs an amine solvent to absorb CO2 from the flue gas of fossil fuel-fired power plants. Amino acids have been proposed as alternatives to amines for CO2 absorption due to their high reactivity with CO2 and low vapor pressure. When selecting amino acids to improve the performance of CO2 capture systems, the potential of these compounds to form carcinogenic N-nitrosamines should be considered to prevent unintended environmental consequences.
In this study, amino acids glycine (Gly), sarcosine (Sar), taurine (Tau), proline (Pro), alanine (Ala), β-alanine, (β-Ala), serine (Ser), phenylalanine (Phe), γ-aminobutyric acid (γ-ABA), glutamic acid (Glu) and aspartic acid (Asp) were selected as model compounds. The amino acids were mixed with strong inorganic bases (e.g., potassium hydroxide) or organic bases (amines) to achieve equimolar concentration of amino acids and bases, and subject to conditions simulating CO2 capture systems. The effects of amino acid structural characteristics and the selection of bases on the formation of N-nitrosamines under absorber conditions were evaluated. The order of amino acid was shown to be a principal factor determining the accumulation of N-nitrosamines. Secondary amino acids exhibited the highest potential to form N-nitrosamines, an order of magnitude higher than that of primary amino acids. This trend is similar to amines. The potential dealkylation of primary amino acid upon nitrosation was examined. Varying alkyl chain length of amino acid (Gly, β-Ala, and γ-ABA) did not significantly impact N-nitrosamine formation. Decarboxylation of amino acid SAR was observed despite the moderate absorber temperature (40 ºC), and a volatile nitrosamine N-nitrosodimethylamine was observed. Considering that the stability of carboxyl group is critical to the ionic nature of amino acids as CO2 capture solvent, the decarboxylation mechanism is investigated. Our systematic investigation of N-nitrosamines formed from amino acids provides a feasible way to benefit environment and human health for amino acid-based CO2 capture.