Environmental, health and safety assessment of phase-change solvents for post combustion CO2 capture

Licentiate thesis, 2020

A novel class of solvents exhibiting liquid-liquid phase separation upon reaction with CO2 and/or change in temperature, promises significant reduction of energy requirement of the post combustion capture by chemical absorption. However, proceeding to a large-scale application of novel materials requires holistic evaluation of the aspects related to human health, safety, and environmental impacts currently missing for phase-change solvent alternatives. The current work addresses the gap by performing such an evaluation by help of combined life cycle (LCA) and environmental, health and safety hazard (EHS) assessment. The evaluation is done at the substance level, during the process of design and selection of the solvent alternatives by computer-aided molecular design (CAMD), and the process level, estimating the impact of the capture system deploying phase-change solvents.

The integration of the LCA and EHS impact criteria into the solvent design procedure leads to identification of a much wider set of optimal solvent structures compared to having only thermodynamic properties as objective functions in CAMD. The search enriched the Pareto fronts with the -OH group containing structures beneficial in terms of their lower impact. On one hand, such molecules are highly soluble in water, thus they might not be the best option from the phase-change perspective. On the other hand, there are OH-containing amines proven to exhibit liquid-liquid separation, which have so far received considerably less attention and might require further investigation.

The process level assessment showed that phase-change solvent systems have a potential to be a better alternative to the conventional amine solvent systems due to the reduced reboiler duty and possible lower impact on the environment. Less mobile solvents might be preferable with respect to human safety. With respect to long-term impacts, the process design of the capture systems with the phase-change solvents might promote accumulation of carcinogenic nitrosamines, thus their concentration should be monitored. The life cycle impact was mostly defined by the steam requirement for solvent regeneration and electricity demand for cooling media delivery. The use of renewable electricity and industrial waste heat can decrease the LCA impact of the phase-change capture plant by 70-90%. Then, the remaining impact will be dominated by the degradation behaviour of the solvent molecules, which emphasizes the benefit of the solvents displaying low degradation rates and highlights the importance of experimental studies addressing the degradation behaviour of the solvents.