Development of non-aqueous amine absorbents for low energy penalty and high efficiency CO₂ capture

Abstract

The escalating carbon dioxide (CO₂) emissions, accounting for 65% of greenhouse gases due to fossil fuel combustion, are a major contributor to global warming. In past decades, there has been a notable and concerning surge in CO₂ emissions into the atmosphere, primarily attributed to the extensive dependency on fossil fuels. The Intergovernmental Panel on Climate Change (IPCC) in 2022 outlined a critical plan to mitigate climate change impacts.

This plan involves a 45% reduction in greenhouse gas emissions by 2030, targeting a cap on global temperature rise at 1.5 °C, and achieving carbon neutrality by 2050. Such ambitious goals necessitate significant cuts in CO₂ emissions on a global scale.

Post-combustion capture, a vital process involving the extraction of CO₂ from flue gas after fuel combustion, is extensively employed in existing power plants. Its popularity stems from its adaptability for retrofitting and the maturity of amine scrubbing technology, which facilitates efficient removal of CO₂. These encompass techniques like adsorption, physical/chemical absorption, membrane separation, bioremediation, and cryogenic separation. Sorption technologies, such as absorption, are the predominant approaches for carbon capture.

However, the high regeneration energy consumption restricted their large-scale deployments.

In recent years, the non-aqueous amine absorbents have emerged as promising low-energy consumption absorbents for post-combustion CO₂ capture. The combination of amines and organic solvents brings a new carbon dioxide absorption mechanism, which provides a theoretical basis for the exploring of novel energy-efficiency and environment-friendly CO₂ absorbents. In this thesis, through the combination of experimental and theoretical research, we have developed a series of non-aqueous amine absorbents which demonstrating reduced energy consumption.

The research work presented in this thesis starts from investigating the CO₂ absorbents screening methods (Chapter 3). In general, an additional activator can promote CO₂ absorption performance. However, it is based on experimental screening. In this study, based on molecular dynamic simulation, we have successfully developed a novel and energy efficient CO₂ absorbent by combining N, N-Dimethylethanolamine (DMEA), and ethylene glycol (EG), with different activators, including ethanolamine (MEA), ethylenediamine (EDA), and piperazine (PZ). The addition of these activators significantly enhances the CO₂ absorption performance of DMEA-based absorbents. Among them, DMEA-PZ-EG exhibits the maximum CO₂ absorption rate and capacity, achieving 6.35 (g-CO₂/ (kg-soln. ·min.)) and 96.5 (g-CO₂/kg-soln.), respectively, representing a 230.1% and 206.35% improvement over 2M DMEA-EG absorbent. Moreover, compared to a 30wt% MEA solution, the energy consumption of DMEA-based absorbents is reduced by 33.93 to 51.56%.

The reaction products were characterized using various spectroscopic techniques, including Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR), and 1D and 2D Nuclear Magnetic Resonance spectroscopy (1H and 13C NMR). The NMR results confirmed that DMEA becomes protonated after CO₂ absorption in the DMEA-EG mixture, indicating that the reaction with CO₂ occurs after the deprotonation of EG, resulting in the formation of organic carbonates, as also observed in the IR spectrum. The strong nucleophilicity of the tertiary amine DMEA promotes proton transfer during the CO₂ absorption process. A molecular dynamics simulation also analysed the effect of activators on CO₂ absorption at molecular level and proposed a new method to predict the CO₂ absorption performance of non-aqueous solvents. Using non-aqueous DMEA absorbents presents a promising approach for improving CO₂ capture efficiency, reducing energy consumption, and facilitating regeneration.

One of the major problems with non-aqueous CO₂ absorbents is their low absorption rate, which makes them ineffective in capturing ultra-low concentrations CO₂ stream. Thus, in Chapter 4, we present the development of an energy-efficient cyclic amine system designed for capturing carbon from both flue gas and ambient air. High energy consumption is a major barrier for the large-scale deployment of carbon capture processes from flue gases or air (direct air capture, DAC). The non-aqueous amines reported in literature possess a high energy efficiency for CO₂ capture from flue gases, they struggle with gas streams containing ultra-dilute CO₂, like air, due to poor absorption kinetics. To address these problems, a novel 2-PE (2-piperidineethanol)/APZ (Aminoethylpiperazine) based CO₂ absorbents were developed in this study.

The experiments and MD simulation results showed that the 2 PE/APZ based absorbents possessed a superior absorption performance both in flue gas and air. Among the developed absorbents, when 2-PE/APZ mixed with DMF (Dimethylformamide), the CO₂ loading reached to 1.004 mole/mole, as theoretical maximum. In DAC tests, 87.31 % CO₂ from air was captured in 24h experiments. The regeneration heat duty of 2-PE/APZ/DMF decreased to 1.694 KJ/g CO₂, a 55.89% reduction compared to the benchmark 30 wt% MEA.

The CO₂ absorption/desorption mechanism was analysed by NMR, In-situ FT-IR, and DFT calculation. It indicated that this significant improvement in CO₂ absorption performance and the reduction in energy consumption are due to the synergistic effect of 2-PE and APZ. During CO₂ absorption, CO₂ react with APZ forming APZ zwitterion rapidly, then deprotonation to the 2-PE. The formation of protonated 2-PEH+ ion pairs with APZCOO- reduces hydrogen bonds and van der Waals forces among the amine-CO₂ complex, facilitating easy regeneration at mild conditions while maintaining high reactivity. The combination of theoretical and experimental results indicates that 2-PE/APZ based absorbents can serve as a promising alternative for carbon capture from flue gas to air with low energy usage.

The two chapters mentioned above have developed a series of low regeneration energy consumption CO₂ absorbents, demonstrating the novel mechanism of CO₂ absorption by non aqueous absorbents. In the Chapter 5 & 6, we are committed to solving the precipitation problem of non-aqueous absorbents during CO₂ absorption process, which produces gelatinous products/precipitation that seriously affects the CO₂ absorption process and damages the absorption equipment. Therefore, combining experimental and theoretical studies, we found that the electrostatic potential of carbamate, instead of van der Waals force, is a major factor controlling the precipitation, and hydrogen bonds can effectively reduce the electrostatic potential of carbamate and prevent precipitation. Single solvent screening experiments have also demonstrated that the absorption rate is closely related to the viscosity of the organic solvent and the affinity of the functional group for CO₂. The polar solvents (Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), and N-Methylformamide (NMF)) exhibit higher absorption rates, but suffer from issues of precipitation. Solvents (Ethylene glycol (EG) and Glycerol) with hydroxyl groups exhibit lower absorption rate, but they don’t have the issue of precipitation. Based on these findings, several novel 2-Amino-2-methyl-1 propanol (AMP)-based non-aqueous absorbents have been developed aiming at reducing the energy penalty, and improving CO2 absorption and desorption performance. Among these absorbents, AMP-EG-DMF (4-3) exhibits maximum CO2 absorption rate and capacity of 9.91 g-CO₂/(kg-soln.·min.) and 122 g-CO₂/(kg-soln.), respectively, which are 64.1% and 28.4% higher than those of 30 wt % AMP aqueous solution, respectively. Additionally, compared to 30 wt% MEA, the energy consumption of AMP-EG-DMF (4-3) shows 46.30% reduction. The addition of EG effectively improves the electrostatic solubility of AMP-carbamate by increasing the number and strength of hydrogen bonds, thus avoiding the generation of precipitation. The final product species and reaction mechanism were analysed by using 13C and 1H NMR, Insitu ATR-FTIR, and quantum chemical calculation. It indicates that bi-solvent AMP-based absorbents can serve as a promising alternative for low-energy CO₂ capture. We have extended the bi-solvent strategy to high CO₂ capture capacity cyclic amine and polyamine based non-aqueous absorbents by adding ethylene glycol (EG). The addition of EG improves the electrostatic solubility of amine-CO₂ products, preventing precipitation. Furthermore, EG positively affects the CO₂ absorption rate and capacity. This bi-solvent strategy has been successfully applied to three single amine absorbents and four blended amine absorbents. It has been demonstrated that this method can be broadly applied to other non-aqueous amine absorbents.

Overall, this thesis has expanded and deepened the understanding of CO₂ capture mechanisms using non-aqueous amine absorbents. On this basis, a series of novel CO₂ absorbents have been developed. The elucidated mechanisms in this thesis can inspire future work on designing and developing more energy-efficient and cost-effective CO₂ absorbents to address the challenge of climate change.