Controlling Nitrosamines in Absorption-Based Post-Combustion Carbon Dioxide Capture Systems
CO 2 capture is an important strategy to reduce greenhouse gas emissions from fossil fuel combustion. Among the available CO 2 capture methods, amine scrubbing, an absorption-based technology, is the most mature for post-combustion applications and one of the few available options for retrofitting power plants. Amine-based CO 2 absorption has been used at full-scale for capturing CO 2 from coal-fired power plants. A major concern hindering further implementation of the amine-based technology is the formation and release of carcinogenic nitrosamines and nitramines from the CO 2 capture systems. These harmful byproducts with functional groups of nitroso or nitro are formed in the reactions between amines and nitrogen oxides (NO x ) in the flue gas, or nitrite, the hydrolysis product of NO x in the absorption solvent. Several nitrosamines are listed as probable human carcinogens, imposing 10 -6 excess cancer risk when present at low nanograms per liter levels in drinking water. This dissertation aims to mitigate the risks associated with the formation of nitrosamines from absorption-based CO 2 capture systems. First, we critically reviewed the current knowledge of nitrosamine and nitramine formation in CO 2 capture systems. The mechanisms of nitrosamine and nitramine formation in the absorber, desorber, and washwater units were reviewed based on CO 2 capture literature as well as biology and atmospheric studies. This review also shed light on the importance of mitigation strategies, advanced understanding in mechanisms, analytical tool development, and unit integration to improve the quantitative prediction of nitrosamine and nitramine accumulation and emission from CO 2 capture systems. Second, we evaluated the nitrosamine formation potential of two novel solvents, tertiary amines and amino acids, both being considered for their good CO 2 capture performance. A kinetic model was developed for the formation of nitrosamines from tertiary alkanolamines based on experimental results, exhibiting a first-order dependence on nitrite concentration and CO 2 loading, respectively, but a zero-order dependence on amine concentration. We observed the preferential cleavage of 2-hydroxyethyl functional group from tertiary alkanolamines, resulting in nitrosamines of higher volatility than the parent amines. For amino acid solvents, the base added to maintain deprotonation of the amino acids was shown to influence nitrosamine formation. A kinetic model was developed based on experimental data using sarcosine as a model amino acid, which showed that both the fully deprotonated amino acid and amino acid carbamic acid contributed to nitrosamine formation. N-nitrosodimethylamine (NDMA), a highly volatile nitrosamine was detected in sarcosine solvents. It was shown that NDMA formed primarily through decomposition of N-nitrososarcosine under desorber conditions. Finally, a simulation module was developed based on Aspen Plus to predict nitrosamine formation in large-scale CO 2 capture systems. Using diethanolamine (DEA) as the model amine solvent, the absorber module, for which there is relatively high uncertainty in the reaction parameters, was first validated by comparing simulation outputs and experimental results. The simulation showed that the dependence of nitrosamine formation on flue gas composition and desorber temperature was consistent with previous findings from laboratory-scale reactors. Tertiary amine N -methyldiethanolamine (MDEA) was compared with DEA for the formation and emission of nitrosamines. Although MDEA formed less nitrosamines than DEA, it features higher gaseous emissions of nitrosamines than DEA.