Maximizing the sustainability of a macroalgae biorefinery: a superstructure optimization of a volatile fatty acid platform

Abstract

Macroalgae are a valuable energy source that can be transformed into numerous products most notably fuels and chemicals due to their high content of carbohydrates, proteins, and vitamins. In order to use macroalgae on a commercial scale, it is essential to evaluate various potential pathways into value-added products as well as intermediate components and technologies. Furthermore, the prospective processes have to be economically competitive while simultaneously offering minimum environmental impact in terms of carbon and other waste emissions. This in turn presents a large decision-making problem with a significant combinatorial complexity. This study addresses this problem by utilizing a superstructure process design approach for a seaweed biorefinery which produces mixed alcohols and mixed organic acids via an anaerobic digestion/volatile fatty acid route. Seventeen design alternatives have been proposed to determine the optimal design and technical feasibility by maximizing the net present value in the most environmentally beneficial manner. The results indicate that biofuel production from macroalgae is economically viable at a minimum ethanol selling price of $1.17 gal⁻¹. Furthermore, the optimal design enables a 90% reduction in carbon dioxide emissions. Results of the sensitivity analysis show that seaweed price is the most important parameter that can contribute in improving the economics, thereby confirming that the cost-effective and efficient large-scale seaweed cultivation is of utmost importance to the success of macroalgae-based biofuel production.