Biogenic CO2 capture and purification
Biogenic CO2 has been successfully captured from a biogas plant located in Bruck an der Leitha (Austria). After a purification process, gas cylinders containing 99 % (v/v) pure biogenic CO2 was shipped to partners as feedstock for the synthesis of ethylene oxide.
Catalyst development for CO2 reduction to ethylene and water oxidation to H2O2 has been an important research focus in CO2EXIDE. For the CO2 reduction to ethylene, selectivities beyond the state of the art have been achieved at current densities of 200 mA cm-2 and even above. Yet, as one of the most important challenge remains the still limited stability of the catalyst layer during long electrolysis time. For the anodic water oxidation to H2O2, substantial progress beyond the state of the art has been achieved using boron-doped diamond (BDD) electrodes and specific electrolyte conditions, thereby facilitating high H2O2 selectivities and concentrations at high current densities and under stable operating conditions.
Electrocatalytic reactor unit (ERU)
Tailor-made electrolysers with gas diffusion electrodes were developed at different scales, ranging from about 10 cm2 electrodes up to 300 cm2. Electrochemical reactions were typically operated at current densities of 150-200 mA cm-2 and CO2 flow rate of 1500 cm3 min-1. Electrodes with Cu sputter-deposited on carbon-based gas-diffusion layers were used as cathodes, while BDD electrodes served as anodes. Aqueous KHCO3 solutions were used as electrolytes for both half-cell reactions.
Ethylene enrichment unit (EEU)
The gas stream from the ERU contained not only the target product ethylene, but also other components, such as unreacted CO2, methane, carbon monoxide and hydrogen. To increase the ethylene concentration in the gas flow for further chemical conversion, an ethylene enrichment unit (EEU) was developed, based on a membrane technology. The EEU enables ethylene concentrations of more than 30% and ethylene recovery rates of >90%.
Chemical conversion: Epoxidation of ethylene to ethylene oxide
The products of the electrochemical process step, ethylene and H2O2, were converted into ethylene oxide (EO) in a chemical reactor. The process development was conducted in small-scale batch reactor system under mild reaction conditions (50 °C). At the end of the reaction, the temperature was increased to complete the hydrolysis of the generated ethylene oxide to ethylene glycol. The developed process was then transferred into a large-scale (2 L) autoclave system, the ethylene epoxidation unit (EOU), for the CO2EXIDE demonstrator.
In the final phase of CO2EXIDE, the entire process chain was implemented in an integrated demonstrator. The electocatalytic reactor unit (ERU) and ethylene enrichment unit (EEU) were physically connected, and the process chain was investigated with bottled CO2 sourced from a biogas upgrading plant. In combination with the ethylene epoxidation unit (EOU), the CO2EXIDE process chain was successfully demonstrated, enabling the conversion of biogenic CO2 into the platform chemicals ethylene oxide and ethylene glycol.
System-level assessment of environmental and economic performance potentials
Lifecycle and techno-economic assessments of the CO2EXIDE production chain showed advantageous performance potentials. Powered by renewable electricity, the synthesis of ethylene oxide could evolve into a net-zero CO2 emission technology. Ethylene oxide production costs are mainly driven by the overall energy efficiency, electricity prices and investment. A cost-competitive production, compared to established conventional (fossil-based) processes, is only conceivable under favourable boundary conditions: a cost premium is likely to remain for the renewable CO2EXIDE process. Another important issue for the environmental and economic performance is the utilization of side products, such as methane and hydrogen. Nevertheless, regulatory measures, such as carbon pricing, are absolutely necessary to facilitate economic competitiveness.