Multi-objective optimal synthesis of algal biorefineries toward a sustainable circular bioeconomy

Abstract

Production of biodiesel from renewable resources like microalgae biomass presents a potential for reduction of greenhouse gas emissions and fossil fuel energy consumption. The integration of processes from other industries have been implemented in microalgal biorefineries to increase economic sustainability by co-producing several high-value algal-based products. Agro-industrial processes have the potential to be incorporated into the biorefinery because it requires input material flows from other biorefinery process units to cultivate and sell crops for an additional source of revenue and increased carbon sequestration, while generating wastewater that may be used as a cultivation medium for algae or as a resource for other biorefinery processes. Circular bioeconomy, an extension of the circular economy ideology, has the goal of achieving economic and environmental sustainability through maximizing the dedicated recirculation of resource flows, and minimizing waste generation and end-of-life disposal. However, existing modelling studies have not explored this opportunity; previous studies have not considered that resource functionality runs out with repeated recirculation and reuse as it reaches its end of life. In this work, a novel multi-objective optimization model is developed to design and manage closed-loop algal biorefineries integrating agro-industrial processes that captures the effect of recirculation on resource material viability and end-of-life environmental impact. A case study is solved as proof of concept and to illustrate the design methodology, optimal solutions based on economic and environmental performance are analyzed. The results of the case study validate the initial hypothesis that there is a conflict between the economic and environmental objectives since the decision for biofuel production varied for each single objective. With the multi objective model, a balance between the two objectives was found. The results of the optimization model can be applied in the design of an algal biorefinery along with the decisions relating to production quantities incorporating a zero waste outlook.