Ethylene production processes using alternative carbon sources like biomass or CO₂ could have great potential for the olefins industry. Comparing the benefits and pitfalls of different process routes is challenging due to the vastly different feedstocks and key conversion technologies involved. Here, we performed an ex-ante techno-economic and environmental assessment to explore potential trade-offs of three low technology readiness level ethylene production processes. The three routes were: 1) biobased syngas fermentation to ethanol followed by ethanol dehydration, 2) direct electrochemical conversion of CO₂, and 3) indirect CO₂ and H₂O electrolysis to form syngas followed by a Fischer-Tropsch step. This study found three main takeaways. Firstly, the biobased route significantly outperforms the direct and indirect routes in terms of techno-economic and carbon footprint performance. Secondly, the electrolyzer unit is the main factor limiting the techno-economic performance of the direct and indirect cases, reemphasizing the need for continued technological advancements and cost reductions by researchers in this domain. Finally, the indirect plant design, incorporating two electrolyzers and a Fischer-Tropsch step, is not techno-economically feasible for ethylene production, underscoring the need for further research on Fischer-Tropsch plant designs to advance the replacement of traditional fossil-based refineries.

The petrochemical industry needs to reduce the use of fossil fuel as carbon feedstock to reduce its CO2 emissions. Several alternative carbon sources (ACSs), such as biomass, CO2 and plastic waste are being proposed to replace fossil carbon. As each of these ACS process routes has its tradeoffs, it is essential to identify the defossilization pathways that will have the most significant impact. In this work, a superstructure-based optimization approach is presented that can be used to assess defossilization pathways in existing petrochemical clusters. The small case study shows that CO₂ is a promising ACS to replace fossil fuel as the main carbon source but requires a large amount of green hydrogen and significant modifications to the existing cluster.

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.