Network-Based Life Cycle Optimization of the Net Atmospheric CO₂-eq Ratio (NACR) of Fuels and Chemicals Production from Biomass

A biofuels and bioproducts conversion network is optimized over unit cost and unit greenhouse gas emissions objectives. We use a multiobjective, functional unit approach based on the life cycle analysis methodology with simultaneous consideration of capital budget constraints. A novel functional unit of mass of input biogenic CO₂-eq is proposed to capture common benefits of bioproducts and biofuels. A novel dimensionless net atmospheric carbon ratio (NACR) for bioconversion processes is defined that captures the life cycle carbon footprint of the process from feedstock cultivation to product end use. The model is formulated as a nonconvex multiobjective mixed integer nonlinear fractional programming problem. We address computational complexity by developing a novel global optimization algorithm that incorporates the parametric algorithm and successive piecewise linear approximations to estimate nonconvex terms, and we introduce nonlinear programming subproblems to ensure global convergence under capital cost budget constraints. We consider large-scale case studies of a bioconversion network of 200 technologies and 142 materials/compounds where fuels and chemicals can be made from biomass. Unit costs range from -$0.27 to -$0.43/kg input CO₂-eq, and the NACRs range from 0.17 to 0.25. An optimal NACR that includes biofuel combustion emissions over a network where only biofuels are produced was found to be 1.90.