An assessment of sustainability metrics for waste-to-liquid fuel pathways for a low carbon circular economy

The nexus of solid waste management problems and energy crisis necessitates the utilization of lignocellulosic biomass agro residues and municipal solid wastes for fuel and energy generation. Thermochemical technologies like pyrolysis, hydrothermal liquefaction and gasification are promising options to produce bio-oil, bio-crude and syngas from biomass, which can be further catalytically upgraded to hydrocarbon fuels. While there are technical merits associated with these processes and the products derived from them, the sustainability analysis of the processes from a systems level is imperative to evaluate their commercial viability. This perspective article presents fundamental understanding, importance and analysis of the different sustainability metrics based on mass and energy. The salient properties of biomass and wastes, and that of various fuels derived from them are presented. The three technologies are evaluated based on key metrics such as energy recovery/efficiency, E-factor, process mass intensity and CO₂ footprint. The E-factor for different technologies follows the trend: hydrothermal liquefaction (0.14) ≈ gasification (0.13) < pyrolysis (0.5). However, it is clear from the analysis that the E-factor and GHG emissions of the HTL process, a promising feedstock-agnostic pathway, can be further reduced by valorizing the organic-laden aqueous phase and upgrading the bio-crude to hydrocarbons. The key results of process technoeconomics are presented and assessed from a sustainability viewpoint. This study recommends the use of simple sustainability metrics in research works on thermochemical conversion so that the results from different studies can be compared on a common sustainability platform.