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Energy-efficient electrochemically-driven CO2 capture systems for biogas upgrading

Mohammadpour, Hossein (2022) Energy-efficient electrochemically-driven CO2 capture systems for biogas upgrading. PhD thesis, Murdoch University.

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Biogas encompassing mainly CO2/CH4 mixture has gained increased attention as a renewable energy source over the last years, mainly due to its contributions to greenhouse gas (GHG) abatement. However, this gaseous stream has been undervalued owing to its high CO2 content. Capturing endogenous CO2 from biogas broadens its utilisation as a substitute for natural gas. High energy consumption of traditional CO2 capture technologies has led to an opportunity to develop an alternative technique for large-scale carbon capture. Employing an anion exchange membrane (AEM)-based electrochemical cell for CO2 removal from gas mixtures is a new strategy based on the pH-gradient generated during redox reactions. This technology offers significant practical advantages owing to near-ambient temperature and pressure operating conditions. To open avenues for development research on the electrochemically-driven CO2 capture technique, key principles and features of all major methods for CO2 capture in the literature were summarised as part of this thesis. While to date the (AEM)-based electrochemical cells have received eminent fame for their application in CO2 capture, there are still major enhancement researches required to enhance the technology maturity and reduce costs. In this thesis, the aim was to determine the viability of this CO2 capture system as an alternative to conventional technologies for biogas upgrading. In this context, the main research chapters are summarised as follows:

Firstly, a CO2 absorption column was integrated with an alkaline water electrolyser for biogas upgrading. After scrubbing CO2 in an aqueous absorption column, the resulting bicarbonate solution was fed through the cathode of an anion exchange membrane (AEM)-based electrolyser. With bicarbonate being the dominant anion, it migrates proportionally to the electron flow to the anode from where it is released together with anodic oxygen (O2). The proposed system allows electrochemically-assisted scrubbing and stripping of CO2 without the addition of chemicals. Coulombic efficiency calculations showed that the theoretical electron/carbon ratio of 1 (1 e/ 1 HCO3-) can be achieved by using a pH of 9 while using a traditional pH of around 13 results in more electrons and hence more energy requirement for electrochemical CO2 removal. As predicted from electrochemical stoichiometry, the system optimisation demonstrated that operating the integrated system at pH=9 results in the lowest energy requirement, even though the CO2 absorption rate in the absorbent pH= 13 was about three times higher than that at pH= 9. Results of this study suggest that integrated (AEM)-based alkaline water electrolyser and CO2 absorption column offer a low energy approach for the capture and removal of CO2 from biogas (0.25 to 0.92 kWh/kg CO2). This could potentially reduce the energy demand for CO2 separation from biogas by about 50% compared to the most energy-efficient technologies that are currently available.

Recovery of absorbed CO2 on the anode side of the alkaline membrane diminishes the O2 content in the anodic gas stream. Given this high purity electrolytic O2 has a growing industrial interest in diverse applications such as biological wastewater treatment processes we designed and developed an innovative three-chamber electrochemical cell configuration capable of capturing CO2 and recovering it in an intermediate chamber enabling concomitant anodic high-purity oxygen generation. Our prototype successfully demonstrated that CO2 recovery can be separated from the anodic O2 gas stream by adding a cation exchange membrane (CEM) to the cell. A concise model was also developed to accurately predict the polarisation characteristic of the designed electrochemical cell by considering the numerical coefficient for the Tafel equation and ohmic losses.
Next to the regeneration of spent alkaline solution for CO2 removal from biogas using an alkaline water electrolyser, the generated high-purity electrolytic O2 from the three-chamber electrochemical cell can be used as a valuable by-product in various industrial processes to improve the energy efficiency of the system. Our strategy to cut down the cost of electrochemically-driven CO2 removal was the effective utilisation of high-purity O2 in the aeration process of a wastewater treatment plant. The high-purity electrolytic O2 can be used to substitute the air in the conventional activated sludge process. To determine the benefit of switching from air to oxygen-enriched air or pure oxygen, an analytical model was derived to simulate the dynamic behaviour of a single bubble rising in stagnant water. This study explored the influence of operation parameters such as bubble diameter, and bubble release depth on gas-liquid mass transfer. The predicted results suggested that replacing air with high-purity oxygen in the aeration process of the conventional activated sludge can offset up to 30% of the energy required for water electrolysis. A greater advantage could be obtained by using the generated high-purity oxygen in the aeration system of industries with higher dissolved oxygen requirements such as the aquaculture industry.

Finally, co-mixing electrolytic H2 generated during electrochemical biogas upgrading and CH4 may not be desirable and present some challenges such as the increased probability of ignition and material degradability. In this scenario, electrolytic H2 could be recycled from the cathode to the anode side to replace the kinetically sluggish oxygen evolution reaction (OER). The technical and economic aspect of the H2 recycling cell to generate the pH gradient required for capturing and stripping CO2 was explored. The experimental results showed that the H2 recycling cell enables CO2 capture with a minimum electrochemical work of 0.19 kWh/kg CO2, which is 30% lower than the alkaline water electrolyser system where anodic OER limits the energy efficiency of the process.

The main conclusion drawn from this study is that the electrochemical-driven CO2 capture systems have the potential to reduce energy costs associated with the regeneration of spent alkaline absorbents in biogas upgrading systems.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Mathematics, Statistics, Chemistry and Physics
United Nations SDGs: Goal 7: Affordable and Clean Energy
Supervisor(s): Pivrikas, Almantas and Ho, Goen
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