The thermochemical-based production of liquid transportation fuels from coal and duckweed biomass is investigated. A complete process superstructure is introduced which includes different alternatives, such as Fischer-Tropsch hydrocarbon production, methanol production, and methanol conversion to gasoline or olefins and subsequently gasoline and distillate. The production of liquid fuels at the lowest cost is determined using a large scale non-convex mixed integer nonlinear (MINLP) model that is solved to global optimality using a branch-and-bound framework. The results suggest that liquid fuels production from coal and duckweed can be economically viable. © 2014 Elsevier B.V.