Integrating LCA and thermodynamic analysis for sustainability assessment of algal biofuels: Comparison of renewable diesel vs. biodiesel

Advanced biofuels are attracting intense interest from government, industry and researchers as potential substitutes for petroleum gasoline and diesel transportation fuels. Microalgae's advantages as a biofuel feedstock are due particularly to their rapid growth rates and high lipid content. Several life cycle analysis (LCA) studies have been conducted on the production of biodiesel, however less attention has been paid to algae-derived green diesel (renewable diesel II), a promising alternative fuel product. Renewable diesel's advocates suggest that it has superior energy density, shelf stability and can function as a drop-in replacement for petroleum diesel due to their similar chemical composition and fuel properties. Fewer studies have attempted to quantify the sustainability of algae-derived renewable diesel, though renewable diesel options are examined in the current GREET model. This study conducts a well-to-pump LCA focusing on this Renewable Diesel II (RD2) upgrade pathway and comparing it with the corresponding pathway from algal biomass to biodiesel. Particular attention is paid to primary energy use and fossil energy ratio (FER), greenhouse gas emissions, and an initial investigation of thermodynamic metrics. While hydrotreating is less than half as energy intensive a fuel upgrade process as transesterification, the overall life-cycle energy consumption and greenhouse gas emissions are found to be nearly equal for renewable diesel and biodiesel. The complete biofuel production process is only found to be net energy positive for scenarios with reduced burdens from both CO2 sourcing and biomass drying.