Performance Assessment of Industrial-scale n-Caproate Production utilizing CO2 as Feedstock

Abstract

The Port of Rotterdam is one of the largest CO2 emitters of the Netherlands. One method to reduce its emissions is by implementing carbon capture and utilization (CCU) technologies. In this research two different CCU routes for industrial-scale n-caproate production are compared. The first route utilizes microbial electrosynthesis (direct route). The second route uses syngas formation via electro-reduction combined with syngas fermentation (indirect route).

Based on the European Union's technology readiness level, the maturity of both routes is ranked as 3 to 4. This indicates that both routes are in the demonstration phase. In this phase scientists play an important role, as the focus is on researching the technology's overall feasibility. It is also the phase in which financial barriers are the most pressing. Early-stage investors can help overcome these barriers, by allocating (financial) resources. Nonetheless, an assessment method taking both scientists' as well as early-stage investors' perspectives into account is missing. This research contributes to filling this knowledge gap.

The methodology applied is bricolage, using a mixture of literature research, simulation data and interviews. Data of the direct route is retrieved from literature research. To obtain data of the indirect route, a simulation is made in Aspen Plus v12. An overview of important assessment parameters are acquired via the conducted interviews.

In total 15 participants are interviewed, of which; 8 scientists working on CCU technologies, 5 early-stage investors, 1 governmental policy executing party and 1 NGO. These interviews, combined with metrics found in literature research, led to an overview of parameters to be assessed for a performance analysis.

Overall, 10 metrics are selected and used for the performance assessment. These include technical, economic, environmental and strategic metrics. The technical assessment showed that the indirect route has a significantly higher energy consumption compared to the direct route (0.35 GJ/kg caproate compared to 0.1 GJ/kg caproate). However, the indirect route has a higher production selectivity towards n-caproate. The economic performance assessment resulted in a lower CAPEX and OPEX for the direct route. Still, for both routes, the minimum n-caproate selling price is below the current n-caproate market price. The net carbon footprint of the indirect route is 7.3 kg CO2 per kg caproate, indicating that a significant amount of CO2 is being emitted during its production.

While an analysis at this maturity stage of the production routes comes with uncertainties, it gives a first sound indication of its performance and bottlenecks. For the indirect route, the energy consumption is dominated by the two electrolyzers used and the downstream processing. This energy consumption is also the main contributor to the OPEX (47%) and the carbon footprint (79%). Another factor largely responsible for the carbon footprint are the emissions during fermentation (19%). The main cost item of the equipment cost is the water alkaline electrolyzer (42% of total cost).

To conclude, the indirect route shows the potential of industrial-scale n-caproate production with a high selectivity. Nonetheless, it is key to reduce its energy consumption and implement more efficient off-gas recycling to improve its techno-economic and environmental performance.