Syngas fermentation is a biochemical pathway to produce ethanol and has been commercialized successfully. The economic viability of this process could be further improved to become more competitive in the existing ethanol market. Improving gas utilization is the key, and can be done by recycling the unreacted syngas. This work is an early-stage techno-economic assessment of recycling in producing ethanol from Basic Oxygen Furnace (BOF) gas. Economic viability is measured in terms of Relative Competitive Percentage (RCP) and is a measure of closeness to the current market. Two scenarios, firstly a once-through process, and secondly a process with recycling (0.9 split ratio: recycle/purge) of gas is considered. None of them showed a positive RCP as compared to the current ethanol market. Comparing these scenarios, beyond the single pass conversion of 60%, the additional production costs due to recycling become dominating and lead to a lower RCP compared to once-through systems.

Syngas fermentation is an up-and-coming technology that uses acetogenic microorganisms to produce ethanol at the commercial scale. Acetogens can produce many different types of products via their metabolic pathway called the Wood Ljugdahl Pathway (WLP). The WLP can natively produce many different fatty acids and alcohols, and with metabolic engineering, other molecules could be produced through syngas fermentation that are not native to the WLP. In this work isopropanol, 3-hydroxybutyric acid, hexanol, octanol, hexanoic acid, butyric acid and lactic acid were assessed for their feasibility to be produced through syngas fermentation. Two syngas cases were analysed; bio-syngas (H₂:CO of 1:1.907) and basic oxygen furnace gas (H₂:CO of 1:21.667). The feedstock capacity was fixed at 350 ktons/yr based on its availability. Using thermodynamic values, process reactions from the substrate to the product were found. To verify the metabolic feasibility, ATP yields were calculated based on the respective WLPs from the literature. Sensitivity studies of H₂:CO ratios on the carbon yield are carried out to check its effect on the production yields of the product, biomass, and CO₂. Sensitivity analysis showed that a higher H₂:CO ratio in the feedstock will lead to higher production.

The application of membrane assisted fluidized bed reactors for distributed energy production has generated considerable research interest during the past few years. It is widely accepted that, due to better heat and mass transfer characteristics inside fluidized bed reactors, the reactor efficiency can outperform other reactor configurations such as packed bed units. Although many experimental studies have been performed to demonstrate and monitor the long term performance of membrane assisted fluidized bed reactors, the hydrodynamics of membrane-assisted fluidized bed reactors has thus far only been studied in pseudo-2D geometries. In this work the solids concentration inside a real 3D fluidized bed reactor geometry was measured using a fast X-ray analysis technique. Experiments were conducted in absence and presence of two different membrane modules with different configurations and number of membranes (porous Al₂O₃ tubes) for two types of particles, viz. 400–600 μm polystyrene (Geldart B type) and 80–200 μm Al₂O₃ (Geldart A/B type). Results from the experiments with Geldart B type particles revealed that the membrane modules (both the membranes and the spacers) can significantly reduce bubble growth along the fluidized bed resulting in a smaller average bubble diameter, expected to improve the bubble-to-emulsion mass transfer, whereas for the experiments with fine Geldart A/B particles, and at a very high extraction values (40% of the inlet flow), a densified layer with high solids concentration was formed near the membrane, which may impose an additional mass transfer resistance for gas components to reach the surface of the membranes (concentration polarization). The results from this study help designing and optimizing the positioning of the membranes and membrane spacers for optimal performance of fluidized bed membrane reactors.