Combining intermittent renewable electricity (IRE) with carbon capture and utilisation is urgently needed in the chemical sector. In this context, microbial electrosynthesis (MES) has gained attention. It can electrochemically produce hexanoic acid, a value-added chemical, from CO₂. However, there is a lack of understanding regarding how the intermittency of renewable electricity could impact the design of a MES plant. We studied this using Aspen Plus models. A MES plant that was powered by constant grid electricity could operate from 100% down to 70% of its nominal capacity, at which point the heat exchangers and the internal geometrical design of the distillation towers became bottlenecks. The levelised production cost of hexanoic acid (LPC_C6A) was estimated at 4.0 €/kg. Switching to IRE supply increased LPC_C6A to 5.3 €/kg (for wind electricity) and 4.7 €/kg (for hybrid renewable electricity). A battery energy storage system (BESS) was deployed. The lowest LPC_C6A was found at a BESS installation of 29 GJ/h for wind electricity (5.1 €/kg) and at 12 GJ/h for hybrid renewable electricity (4.7 €/kg). In both situations, the volume flexibility of the MES plant was not improved. At the investigated market and operating conditions, coupling IRE to the MES plant was economically infeasible. ... Read more

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. ... Read more

Microbial electrosynthesis (MES) is a novel carbon utilisation technology aiming to contribute to a circular economy by converting CO₂ and renewable electricity into value-added chemicals. This study presents a cradle-to-gate life cycle assessment (LCA) of hexanoic acid (C6A) production using MES, comparing this production with alternative technologies. It also includes a cradle-to-grave LCA for potentially converting C6A into a neat sustainable aviation fuel (SAF). On a cradle-to-gate basis, MES-based C6A exhibits a carbon footprint at 5.5 t CO₂eq/tC6A, similar to fermentation- and plant-based C6A. However, its direct land use is more than one order of magnitude lower than plant-based C6A. On a cradle-to-grave basis, MES-based neat SAF emits 325 g CO₂eq/MJ neat SAF, which is significantly higher than the counterparts from currently certified routes and conventional petroleum-derived jet fuel. However, its negligible indirect land use change emissions might potentially make it competitive against neat SAFs originating from first-generation biomass. ... Read more

CO2 electroreduction driven by renewable energy is a promising technology for defossilizing the chemical industry, but intermittency challenges its operation. This work aims to understand the impacts of intermittency on the design, volume flexibility, and scheduling of a microbial electrosynthesis (MES) plant that converts CO2 to hexanoic acid. A battery and a storage tank were considered to buffer the intermittency. Explorative case studies showed that batteries were economically unfavorable. Restricted by the downstream processing (DSP) flexibility, a storage tank with optimized size combined with optimal scheduling, under the assumed conditions in this work, improved the plant’s volume flexibility only by 10%. The carbon footprint became 3 times lower when switching from grid to renewable electricity, but the levelized production cost of hexanoic acid increased. Hence, coupling with renewable electricity was not economically but environmentally favorable. Developing more flexible DSP technologies or synthesizing higher-purity chemicals are needed to enhance MES’s attractiveness. ... Read more

Incorporating (operational) flexibility into process design has been a key approach to cope with uncertainties. The increasing penetration of renewables and the need for developing new low-carbon technologies will increase the demand for flexibility in chemical processes. This paper presents a state-of-the-art review focusing on the origin, definition, and elements of flexibility in the chemical engineering context. The article points out a significant overlap in terminology and concepts, making it difficult to understand and compare flexibility potential and constraints among studies. Further, the paper identifies a lack of available metrics for assessing specific types of flexibility and the need for developing indicators for exploring the potential flexibility of novel chemical processes. The paper proposes a classification of flexibility types and provides an overview of design strategies that have been adopted so far to enable different types of flexibility. Finally, it offers a conceptual framework that can support designers to evaluate specific types of flexibility in early-stage assessments of novel chemical processes. ... Read more