Carbon Upcycling via Valorization of Syngas Fermentation Process

Abstract

Since the past decades, gaseous waste has been streaming from industrial point sources on a daily basis. Gasification of carbonaceous feedstock has led to the generation of tremendous amount of synthetic gas (syngas; 6 EJ per year) worldwide. The amount of carbon sources in syngas is sufficient for upcycling. However, the technology involved in syngas applications does not meet the economic feasibility of conversion yield and process efficiency. Moreover, the main component of syngas is CO, which microorganisms use as the primary substrate and produce CO2. Consequently, there is an increase in the greenhouse gas (GHG) emission credits of syngas fermentation (kg CO2eq). Thus, the present study aimed to increase the performance of syngas fermentation for efficient conversion in carbon upcycling. Various operating strategies increase carbon upcycling from CO to biochemical products during syngas fermentation. Therefore, a number of optimization processes and operational strategies were tested for specific parameters. Volumetric productivity was found to increase by controlling and turning key controllable parameters such as gas-liquid mass transfer (GLMT), cell density and dilution rate. Cell viability was also a major parameter. Following parameter optimization, biomass boosting and the production stage were operated separately. The acetate concentration reached 34.4 g L-1 with a maximum productivity of 0.72 g L-1 h-1. Further, the types of upcycling carbons (CO2) were expanded to increase carbon conversion to liquid chemicals. Neither H2 nor CO2 were consumed before CO reached a specific concentration, and the low consumption rate of H2 and CO2 was insufficient to cover the CO2 emitted during fermentation. Methanol was used as an additional substrate to improve the consumption rate of CO2 and H2 by Eubacterium limosum KIST612. The total gas consumption and conversion to liquid chemicals (gas to liquid; GTL) in the gas closed system was confirmed. Moreover, the purification of the upcycled carbon (biochemical) by electrodialysis (ED) was cost-efficient and reduced waste. The performance of ED was evaluated (> 99% acetic acid extraction), and the amount of recyclable components was analyzed by simultaneously recycling the medium and performing purification. The cell growth and titer production in recycled medium, which reduced the cost by 16.5%, was similar to growth in carbonate buffered basal medium (CBBM). Assessment was carried out to verify if these processes were environmentally and economically friendly and a cost-effective medium was included for economical operation. Sludge filtrate from a wastewater treatment plant was used instead of the yeast extract of laboratory medium. Acetate production increased by 20%, and the medium cost was reduced by approximately 70%. The processes proposed in this study, were more environmentally effective than using wastewater to upcycle waste gas and more eco-friendly than existing methods due to their ability to increase productivity and recycle and reuse discharged waste gas and wastewater by analyzing GHG emission credits.