Biomass is a valuable and currently underutilized feedstock for the energy sector in the United States. Petroleum, a non-renewable resource, accounts for over 35% of the total national energy consumption and is primarily used in the transportation industry . Renewable resources only contribute to 4.7% of liquid fuels production, and thus a significant impact can be made through the use of biomass as a feedstock . Due to absorption of CO2 during photosynthesis, the greenhouse gas reduction resulting from biomass feedstocks is substantial. This, in addition to the domestic feedstock availability of biomass, gives ample motivation for the synthesis and optimization of biomass to liquid transportation fuel (BTL) refineries . Currently, ethanol is the primary fuel produced from biomass, yet it is a poor fuel replacement in the current transportation infrastructure . Technology exists to convert biomass to gasoline, diesel, and kerosene, and process synthesis utilizing global optimization provides a mathematical approach to accurately model and optimize a BTL plant .
Thermochemical methods for converting biomass to fuels are well-known and have been studied using process synthesis for a variety of possible feedstocks [5, 6, 7]. The process synthesis model, which utilize gasification of biomass and fuel production via Fischer-Tropsch or methanol synthesis routes, forms a mixed-integer non-linear program (MINLP) which can be solved to global optimality using a rigorous branch-and-bound framework. These plants can have a significant impact on the liquid fuel supply chain . In the refinery, CO2 can be recycled, vented, or sequestered, and the process incorporates simultaneous heat, power, and water integration . The superstructure yields the breakeven oil price required for the biorefinery to be competitive in today’s energy markets. However, to this point, studies on BTL plants have yet to incorporate biological conversion of biomass into the MINLP superstructure.
The MixAlco process, in which biomass is converted to carboxylic acid salts through fermentation before upgrading to liquid fuels, provides a unique starting point for incorporation of biological routes into the MINLP superstructure [10, 11]. The upgrading of carboxylic acids requires hydrogen, which can be provided through pressure swing adsorption of synthesis gas provided by gasification of lignin residues leaving the fermenter. Gasification of lignin also allows for MixAlco integration with a synthesis gas conversion method. After input-output modeling of the MixAlco process, biological and thermochemical conversion of switchgrass are compared through case studies at capacities ranging from 1000 barrels per day (1 kBD) to 200 kBD. Important financial considerations, including the overall cost of liquid fuels production, the investment cost, greenhouse gas emissions, and product distribution, are analyzed and discussed. Topological decisions for the biorefinery, along with the synergistic impacts of incorporating biological and thermochemical conversion of biomass simultaneously, are also examined.
 U.S. Energy Information Administration, “Monthly energy review," March 2015.
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