Discussion of amine volatility and emissions

“The volatility of an amine solvent is an integral parameter when selecting an amine solvent for CO2 capture. Highly volatile amine solvents will increase both the capital and operating costs because the size of the water wash unit both at the top exit of the absorber and desorber will increase while the energy needed for the water wash system and amine losses (increased amine make–up) will both increase. For very expensive amine solvents, the cost associated with increase in their amine make–up could be very high. Researchers [219][220] have previously studied amine volatility and emissions. Results reported by Nguyen et al. revealed that amine volatility is highest at the absorber top at 40 °C while amine volatility decreased as CO2 loading increased (due to reduced free amine present in the solution) [220]. They also reported amine volatility to follow this trend MDEA ∼ PZ < PZ < EDA < MEA < AMP, indicating that blended amine solvents have the potential of reducing amine emissions. According to previous studies [221][222][223], applying MEA for post–combustion CO2 capture emitted between 0.1 and 0.8 kg MEA/tonne CO2 captured with using a water wash. When a water wash unit was installed the MEA emissions can be as low as 0.01–0.03 kg/tonne of CO2 captured [221][224]. Amine volatility and emission can be experimentally studied using a procedure known as ‘iso–kinetic sampling method’. This involves trapping the volatile compounds from the off gas of the absorption unit using a diluted acid placed in series of impingers. This method has been successfully used by many researchers [225][226][227][228][229][230] to evaluate the volatility and emission of amine solvents.

Emitted amine solvents can further undergo reactions with oxidants in the atmosphere which involve oxidized atmosphere to form nitrosamines, nitramines and amides [231]. From the previous sub-section on amine stability, it was seen that nitrosamine and nitramines are more prevalent with secondary amines. Now, the key issue is if secondary amines are avoided for post–combustion CO2 capture, are the tertiary and primary amines eco–friendly and very biodegradable (BOD)?

This is because in the quest to reduce carbon emissions (which is not toxic) care must be taken to avoid secondary pollution through amine emissions which in most cases are toxic to health, environment and aquatic life. Several studies that have reported the improved CO2 capture capability of their amine solvent have not reported their volatility, ecotoxicity and biodegradability limits. Eide-Haugmo et al. studied the biodegradability and ecotoxicity of 43 amine solvents and their results revealed that some of the frequent amine solvents (MDEA, PZ and AMP) commercially applied for CO2 capture has very low biodegradability [232]. Their results also showed that amine solvents with biodegradability below 20% are not acceptable while biodegradability above 60% means that the amine solvent is readily biodegradable. Their study also revealed that toxicity is a problem when the ecotoxicity of an amine is below 10 mg/l [232]. In particular is the volatile trimethylamine with ecotoxicity below 10 mg/l.

Some few questions are yet to be clearly answered;

Should a volatile amine which is very biodegradable and eco–friendly be acceptable for CO2 capture applications?

Should a non–volatile amine that is not readily biodegradable and slightly ecotoxic be acceptable at some applicable concentration?

Can blended amine solutions be a combination of the previous scenarios listed above?

Should blended amine solvents be chosen in terms of being 60–100% BOD and being eco–friendly? Meaning that a 60% BOD amine solvent can be blended with a 25% BOD amine solvent at blend ratio of 70–30% respectively? And should an amine solvent with ecotoxicity below 10 mg/l be blended with another amine solvent with ecotoxicity above 10 mg/l at 30–70% blend ratio respectively?

Therefore, finding a balance between amine ecotoxicity, biodegradability and volatility will help reduce associated costs and risks. This can be achieved by blending amines. In future studies, efforts should be made to incorporate amine solvents that are eco–friendly and readily biodegradable towards CO2 capture.

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