Feasibility of Flexible CO₂ Conversion Technologies Powered by Renewable Electricity

Abstract

In 2021, the EU set a goal for climate neutrality by 2050, followed by a 2024 recommendation for a 90% reduction in net greenhouse gas (GHG) emissions by 2040 compared to 1990 levels. According to the 2022 IPCC Sixth Assessment Report, achieving the 1.5°C goal is still possible but requires immediate and significant changes across all sectors.

The chemical industry, responsible for 10% of industrial CO2 emissions in 2022, must accelerate its shift away from fossil fuels. This involves replacing fossil-based fuels and feedstocks with alternatives that have lower environmental impacts, such as using CO2 captured from the air or industrial emissions and renewable energy sources like solar, wind, and geothermal. This shift has led to the development of carbon capture and utilization powered by intermittent renewable electricity (IRE).

However, electrochemical processes struggle with fluctuating electricity supplies. Variability in electricity can lower production rates or damage electrolysers. Managing these fluctuations to maintain steady production is difficult with control systems alone, emphasizing the need for flexible operation in electrochemical plants.

Feasibility studies are essential for understanding the market competitiveness of new technologies, identifying potential technical, investment, and environmental challenges.

Electrochemical processes using renewable electricity to produce chemicals are gaining traction. While industrial-scale water electrolysis has been widely studied, less research has focused on electrosynthesis under intermittent electricity supply. This dissertation explores process designs and conditions for scaling up a novel CO2 electrochemical plant using intermittent renewable electricity. The research addresses three sub-questions:

What does flexibility mean in designing novel chemical processes?
How does intermittency affect the techno-economic and environmental performance of a novel CO2 electrochemical plant?
What factors influence the competitiveness of novel CO2 electrochemical technology?

For the first sub-question, a systematic literature review on flexibility was conducted, resulting in a conceptual framework for defining, designing, and evaluating flexibility in novel chemical processes.

For the second sub-question, a microbial electrosynthesis (MES) plant producing hexanoic acid from CO2 was designed and modeled in Aspen Plus. The plant's volume flexibility was explored by varying throughput rates. The plant was coupled with IRE profiles and buffering units using Python scripts, and optimization was performed to enhance economic potential. The study assessed how intermittency impacted the plant’s performance and carbon footprint.

For the third sub-question, the techno-economic and environmental performances of hexanoic acid from MES were compared to its competitors. Two future value chains were considered: one producing hexanoic acid and another upgrading it to n-alkanes for sustainable aviation fuel (SAF). Competitors included plant-based and fermentative hexanoic acid, and certified SAFs.

Technical findings indicate that improving electrolysers’ productivity and product concentration is crucial. Inflexibility in downstream processing (DSP) technologies can affect production and economic outcomes. Designing flexible DSP technologies and sizing equipment to handle fluctuations is essential. Buffering units can mitigate intermittency impacts, and smaller parallel units can maintain production during low electricity supply.

Economic findings suggest that novel electrochemical plants driven by constant grid electricity might struggle in today’s market due to high CAPEX and electricity costs. Intermittency worsens this issue. Enhancing technology, securing cheaper electricity, and optimizing buffering units can improve economic outcomes. Future market conditions and demand shifts could also impact competitiveness.

Environmental findings show that combining electrolysis with renewable electricity has lower impacts than using grid electricity but might not be cleaner than competitors. DSP-related utilities and chemicals can be environmental bottlenecks, linked to selected DSP technologies and CO2 feedstock's carbon intensity.

This dissertation provides an ex-ante assessment of novel technology, acknowledging limitations such as not considering ramping rates. Future research should explore more flexible DSP technologies, modular units, and interactions between different flexibility types. Upgrading hexanoic acid to n-alkanes is currently unattractive environmentally; alternative products like adipic acid could be considered.

Joint efforts between the industrial sector and power suppliers are necessary for significant defossilisation through electricity-based chemical processes.