Renewable electricity can be leveraged to produce fuels and chemicals from CO₂, offering sustainable routes to reduce the carbon intensity of our energy and products-driven economy.

Ethylene is the most widely produced petrochemical feedstock globally. It is currently produced exclusively from fossil fuels, and its production is the largest CO₂-emitting process in the chemical industry. In this study, we report on a photobiological process for sustained production of ethylene from CO₂. The efe gene encoding an ethylene-forming enzyme from Pseudomonas syringae pv. Phaseolicola was previously expressed in cyanobacterial strains, but was not stable. We modified the gene sequence to enhance its stability, and expressed it in Synechocystis sp. PCC 6803, leading to continuous ethylene production. The same ethylene production rate was sustained across four successive sub-cultures without apparent loss of ethylene-forming ability. Up to 5.5% of the fixed carbon was directed to ethylene synthesis, surpassing the published carbon-partition rate into the TCA cycle. Nitrogen- and phosphorus-enriched seawater can support both growth and ethylene production. Factors limiting ethylene production, including efe expression levels, light intensity and nutrient status, were identified and alleviated, resulting in a peak production rate of 5650 μL L⁻¹ h⁻¹ (7125 μg L⁻¹ h⁻¹, 252 μmol L⁻¹ h⁻¹, or 171 mg L⁻¹ day⁻¹), which is higher than that reported for other algae biofuels and chemicals. This study suggests that Synechocystis, expressing the modified efe gene, has potential to be an efficient biological catalyst for the uptake and conversion of CO₂ to ethylene.

This work presents a detailed analysis of the production design and economics of the cellulosic isobutanol conversion processes and compares cellulosic isobutanol with cellulosic ethanol and n‐butanol in the areas of fuel properties and engine compatibility, fermentation technology, product purification process design and energy consumption, overall process economics, and life cycle assessment. Techno‐economic analysis is used to understand the current stage of isobutanol process development and the impact of key parameters on the overall process economics in a consistent way (i.e. using the same financial assumptions, plant scale, and cost basis). The calculated minimum isobutanol selling price is $3.62/gasoline gallon equivalent ($/ GGE ) – similar to $3.66/ GGE from the n‐butanol process and higher than $3.26/ GGE from the cellulosic ethanol conversion process. At the conversion stage, the n‐butanol process emits the most direct CO ₂ , at 26.42 kg CO ₂ / GGE . Isobutanol and ethanol plants have relatively similar CO ₂ emissions, at 21.91 kg CO ₂ / GGE and 21.01 kg CO ₂ / GGE , respectively. The consumptive water use of the biorefineries increases in the following order: ethanol (8.19 gal/ GGE ) < isobutanol (8.98 gal/ GGE ) < n‐butanol (10.84 gal/ GGE ). Field‐to‐wheel life cycle greenhouse gas ( GHG ) emissions for the ethanol and n‐butanol conversion processes are similar at 4.3 and 4.5 kg CO ₂ ‐eq/ GGE , respectively. The life cycle GHG emissions result for the isobutanol conversion process is 5.0 kg CO ₂ ‐eq/ GGE , approximately 17% higher than that of ethanol. The life cycle fossil fuel consumption is 39 MJ / GGE for n‐butanol, 43 MJ / GGE for ethanol and 51 MJ / GGE for isobutanol. The energy return on investment for each biofuel is also determined and compared: isobutanol (2.2:1) < ethanol (2.7:1) < n‐butanol (2.8:1). © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd

Comprehensive techno-economic analysis across a wide set of technologies and products provides insight into the economic feasibility of electron-mediated CO₂ reduction.

As the demand for utilizing environment-friendly and sustainable energy sources is increasing, the adoption of waste-to-energy technologies has started gaining attention. Producing biogas via anaerobic digestion (AD) is promising and well-established; however, this process in many circumstances is unable to be cost competitive with natural gas. In this research, we provide a technical assessment of current process challenges and compare the cost of biogas production via the AD process from the literature, Aspen Plus process modeling, and CapdetWorks software. We also provide insights on critical factors affecting the AD process and recommendations on optimizing the process. We utilize four types of wet wastes, including wastewater sludge, food waste, swine manure, and fat, oil, and grease, to provide a quantitative assessment of theoretical energy yields of biogas production and its economic potential at different plant scales. Our results show that the cost of biogas production from process and economic models are in line with the literature with a potential to go even lower for small-scale plants with technological advancements. This research illuminates potential cost savings for biogas production using different wastes and guide investors to make informed decisions, while achieving waste management goals.