Comparison of conversion pathways for lignocellulosic biomass to biofuel in Mid-Norway
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This work investigates one biochemical and one thermochemical biomass-to-liquid biofuel conversion pathway in terms of lignocellulose conversion to liquid Fischer-Tropsch diesel. The focus has been on comparing the two conversion pathways in terms of identifying their energy flows and respective feed to fuel ratios. The conversion pathways investigated comprise two-stage conversion sequences including biomass-to-gas conversion and gas-to-liquid conversion, exerted by anaerobic digestion or gasification followed by Fischer-Tropsch synthesis. A systematic documentation of available technologies regarding the two conversion pathways is performed by literature study. The pathways are modeled in Aspen Plus supplied with FORTRAN declarations. Mass flows and composition for the two pathways are collected from simulations and energy flows are identified by heating value and energy balance calculations. The energy flows are presented graphically and by ESankey-diagrams, and the resulting energy utilities and feed to fuel ratios are presented graphically and in tabular form.The key finding is that for the application to Fischer-Tropsch processes, the biochemical conversion pathway is less energy effective in terms of gas-to-liquid conversion. This result is observed both in terms of energy utility for the pathway and might indicate that biochemical pathways are more energy consuming than conventional thermochemical gas-to-liquid conversion. However, results on feed to fuel ratio indicate that the biochemical conversion of lignocellulose to Fischer-Tropsch diesel is competitive when compared to thermochemical conversion.