Liquid biofuels from waste: Modelling thermodynamic limits of the anaerobic biorefinery

• A thermodynamic modelling platform has been developed for liquid biofuel production. • The thermodynamic model indicates upper limits for the production of C2-C5 alcohols. • A thermodynamically consistent model is required for an accurate system description. • The minimum energy quantum alters the product spectrum. • Alcohol recovery is maximized at pH 5 and a hydrogen partial pressure of 0.5 bar. Anaerobic conversions of complex organic waste involve close interactions of different microbial groups and often occur close to thermodynamic equilibrium. A generic mathematical model, the Anaerobic Digestion Model No 1, has been developed to predict biogas production. However, this model does not consider the recovery of value-added products such as alcohols, and neglects thermodynamic constraints. In this study, a novel biorefinery modelling platform for liquid biofuel production has been developed by including thermodynamic boundaries and the formation of C2-C5 alcohols from the respective volatile fatty acids and hydrogen, which are intermediates of the anaerobic digestion process. The thermodynamic model, with a minimum energy quantum of ΔG min = 0 kJ mol −1 , has direct implications for process scale-up by demonstrating upper limits for product titers, productivities and biomass yields. The non-thermodynamic model predicts valerate conversion to propionate, acetate and hydrogen at a positive Gibbs energy change, hence violating the laws of thermodynamics. Additionally, it overestimates ethanol formation by a factor of 8 and propanol formation by a factor of 3 compared to the thermodynamic model with ΔG min = 0 kJ mol −1 . This discrepancy was even more pronounced (factor 137 for propanol and factor 19 for propanol) when assuming a minimum energy quantum of ΔG min = -20 kJ mol −1 . These findings suggest that thermodynamic limitations need to be considered for an accurate mathematical description of liquid biofuel production. Furthermore, it was shown that ΔG min had a pronounced impact on the product spectrum. A two-stage biorefinery process is proposed to recover the remaining effluent fraction. In the first stage, C2-C5 alcohols are produced at an optimum pH of 5, followed by a second stage consisting of biogas production.