“The loss of organic amines in the absorption process roughly comes from the following scenarios: reactions with residual oxygen in the flue gas (oxidative degradation), thermal degradation at high temperatures (mainly occurs during the operation of the desorption tower) and volatilization of absorbents. Therefore, as an absorbent, amine has good CO2 absorption and desorption capabilities. Still, it also needs to have less oxidative degradation and high-temperature degradation, along with low vaporization. The influence of the organic amine solution’s viscosity cannot be ignored: the higher the viscosity, the slower the mass and heat transfer rate, which leads to an increase in the size of heat exchangers. Similarly, with a lower mass transfer and reaction rate, the need to increase the size of the absorption and desorption tower will also put pressure on the capital. Ideally, high-performance chemical absorbents generally need to have the following properties [6]: The large circulating absorption capacity can reduce the circulating volume of the absorbent, thereby reducing the power of the pump and reducing the energy consumption of the reboiler. The large absorption rate reduces the size of the absorption tower, thereby reducing facility investment. Low degradation and volatility can decrease the solvent replenishment and the environmental pollution, and low viscosity can reduce the size of packing, and of heat exchangers. The low heat of the reaction can reduce the energy consumption required for solvent regeneration.
Different organic amines have different structures, so their absorption characteristics are different due to various absorption mechanisms. Primary and secondary amines have a fast reaction rate, low absorption capacity and high absorption heat [
7]. Tertiary amines and sterically hindered amines have high circulating absorption capacity, slow absorption rate and low heat of absorption [
6,
8,
9]. The mixed amine system refers to a mixed system of amines with different reaction mechanisms, which combines the advantages of two alcohol amines with a high circulating absorption capacity and a rapid absorption rate. For example, the primary amine MEA and cyclic amine PZ with a fast absorption rate are added to a sterically hindered amine with a high capacity [
10,
11,
12]. MEA is the most commonly used absorbent in the process of absorbing CO
2 by amine scrubbing, with the most extended history of use. Due to the accumulated process experience and performance, it is often considered as the reference solvent for new absorbents. Nevertheless, the regeneration energy of the MEA solvent to capture CO
2 is too high and MEA will degrade during desorption operation with high temperature, which will lead to an increase in the cost of overall carbon capture and is unacceptable for long-time, large-scale carbon capture deployment. Due to the fast absorption rate, PZ is usually added to the amine system as a promotor to increase the carbon absorption rate of the system. The disadvantage of piperazine as a promotor is its low boiling point and high melting point. It is easy to crystallize at low temperatures and cannot be configured with higher concentration solutions, therefore reducing the absorption capacity. Its boiling point of 146 °C is within 120–160 °C, the range of maximum working temperature of device, so it is easy to volatilize in the continuous absorption and desorption device and causes solvent loss. Then, PZ derivatives have similar molecular structures as PZ and have also been researched for their activation performance in recent years [
13,
14,
15].”
