Using exergy to analyse the sustainability of fermentative ethanol and acetate production from syngas via anaerobic bacteria (Clostridium ljungdahlii)

• Exergy-based sustainability assessment of a batch bioreactor for acetate and ethanol fermentation from syngas.
• Application of exergy and eco-exergy approaches for decision-making on syngas pressure.
• 1.8 atm syngas initial pressure as the best condition for acetate and ethanol synthesis.
• Potential application of exergy-based framework for sustainability and productivity of biofuels production.

In this study, exergetic performance assessment of ethanol and acetate fermentation in a batch bioreactor using Clostridium ljungdahlii under various syngas pressures in the range of 0.8–1.8 atm was carried out. The exergetic parameters of the bioreactor were determined using both conventional exergy and eco-exergy concepts in order to select the most sustainable and productive operational conditions. In general, a significant difference was found between the exergetic values of the process using the conventional exergy and eco-exergy approaches. Overall, the eco-exergy concept showed to be a more appropriate approach for analysing and optimizing energy conversion systems including living organisms due to the incorporation of the work of the genetic information embodied in the genomes of microorganisms. The lowest overall normalized exergy destruction was found to be 49.96 kJ/kJ Ac + EtOH and 40.54 kJ/kJ Ac + EtOH at initial pressure of 1.8 atm using the conventional exergy and eco-exergy concepts, respectively. Accordingly, this condition would be recommended to be applied in pilot- or industrial-scale bioreactors. Finally, the employed approach herein could be used to shed light on on-going attempts to improve the performance of the bioreactors for biofuel production considering the sustainability and productivity perspectives.