Nitrosamine Formation in Absorption-Based Post-Combustion Carbon Dioxide Capture Technologies

PhD Student: Kun Yu

Advisor: Ning Dai

CO₂ capture is an important strategy to reduce greenhouse gas emissions from anthropogenic fossil fuel combustion and other industrial activities. Among the available CO₂ capture methods, including absorption, adsorption, membrane separation, and biofixation, absorption technology is the most mature for post-combustion applications and is one of the few options for retrofitting existing power plants. Amine scrubbing, the absorption of CO₂ by concentrated amine solutions, has been used at full-scale for capturing CO₂ from coal-fired power plants.

A major concern for the implementation of amine-based post-combustion CO₂ capture is the formation of potentially carcinogenic nitrosamines and nitramines. These harmful byproducts form from the reactions between amines and nitrogen oxides (NOₓ) in the flue gas, or nitrite, the hydrolysis product of NOₓ. Several nitrosamines were listed as probable human carcinogens, with threshold levels in drinking water at low nanograms per liter to impose 10-⁶ excess cancer risk.

This dissertation aims to address the risks of absorption-based CO₂ capture technology associated with the formation of nitrosamines and nitramines. First, we investigated nitrosamine formation in the desorber unit from tertiary alkanoamines, which are considered as alternatives to traditional amines for CO₂ capture due to their lower energy requirement. Kinetic modeling based on experimental results indicated that the formation of nitrosamines increased with CO₂ loading, but not amine concentration within the range relevant to CO₂ capture. We identified for the first time the preferential cleavage of 2-hydroxyethyl functional group from tertiary alkanolamines, resulting in nitrosamines with higher volatility that parents amines.

Currently, work is underway to evaluate the formation of nitrosamines from “green solvents” amino acids and ionic liquids under conditions relevant to both the absorber and desorber of CO₂ capture systems. Both of these chemicals exist as ionic species and hence feature low vapor pressure. In addition, the tunable and designable nature of ionic liquids offer more benefits in the selective absorption of CO₂. However, the potential of these novel mixtures in forming nitrosamines and nitramines have not been evaluated.

In addition, we are incorporating nitrosamine formation in Aspen Plus simulator. While Aspen Plus is widely used to predict CO₂ capture performance, its capacity in predicting byproduct formation has not been explored. A customized module will provide a process simulation for nitrosation and nitration, and serve as the basis for evaluating the environmental impacts of absorption-based CO₂ capture systems.