Fuzzy set scoring methodology for multi-objective sustainability assessment of algal biofuel pathways

Abstract

This work implements a multi-objective scoring methodology based on fuzzy set theory to evaluate and compare the sustainability metrics of microalgal biorefining concepts. Fuzzy membership functions are developed for three key metrics: minimum fuel selling price, global warming potential, and net energy ratio. Membership functions yield satisfaction scores ( λ ) between 0 and 1 for each objective; achieving λ = 1 indicates a sustainability metric has fully met its target, while λ = 0 is unacceptable for the standards of that metric. The individual λ are combined into an overall satisfaction score, λ o , also ranging from 0 to 1, which is used here to simplify the comparison of sustainability metrics. In addition to baseline biorefining conditions, uncertainties across the algal biofuel production system are assessed through a Monte Carlo analysis of λ o . A case study is developed to compare Combined Algal Processing (a biochemical conversion pathway) against hydrothermal liquefaction (a thermochemical conversion pathway). Baseline results show that lipids are always heavily favored for fuel production, but no practical cultivation operations would be able to achieve such lipid-heavy biomass. The Monte Carlo analysis reveals that current algal fuel conversion technologies cannot simultaneously satisfy all key sustainability objectives. If biomass productivity is doubled and the feedstock is high in lipids, hydrothermal liquefaction can achieve a mean λ o = 0.53 ± 0.12 , still well below a desirable λ o of 1. Combined Algal Processing struggles to achieve high satisfaction scores even in optimistic scenarios and requires a high-lipid feedstock to achieve a mean λ o = 0.10 ± 0.03 . Hydrothermal liquefaction can be integrated downstream of Combined Algal Processing to increase fuel yield and optimistically can achieve a mean λ o = 0.48 ± 0.07 . A combination of system improvements, including biomass productivity, feed composition, fuel yield, and implementation of co-product pathways must work together to satisfy all the relevant sustainability metrics for algal biofuels. The scoring methodology developed here can be readily applied to other microalgal biorefinery concepts and, more broadly, to sustainable fuel development and production. • Holistic evaluation of algal based biofuel systems. • Sustainability assessed through economic and environmental metrics. • Fuzzy set scoring system is applied to holistically assess algae fuel pathways. • Overall satisfaction of sustainability targets depends on algae composition. • Thermochemical pathway outscores biochemical pathway when biofuels are sole product.