Sustainability assessment of second generation bioethanol production: Model-based life cycle assessment of various biochemical conversion paths
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Lignocellulosic biomass is of increasing importance for the bio-economy in providing feedstock for sustainable energy carriers and chemicals. Second generation bioethanol is currently one of the most developed processes based on lignocellulosic biomass as feedstock and the findings from latest research offer valuable insights for the further development of the bio-economy. Main challenge is the recalcitrant behavior of lignocellulosic biomass that requires pretreatment before subsequent hydrolysis and fermentation. Obtaining sugars from lignocellulosic biomass is also important for the production of second generation fuels and chemicals other than bioethanol. Pretreatment can be performed in numerous ways, each especially suitable for certain types of biomass and coming with distinct advantages and disadvantages. In order to determine the environmental performance of second generation bioethanol, the production chain was studied using life cycle assessment (LCA). The bioethanol production process was modeled using a recent NREL model on the biochemical conversion of lignocellulosic biomass. The LCA was especially focused on the pretreatment part of the production chain. The non-renewable energy use (NREU) needed to produce 1 kg of ethanol from corn stover was 8.4 MJ in case of the system expansion approach and 9.5 MJ in case of allocation based on energy content. The corresponding greenhouse gas emissions (GHG) were -1.1 kg CO2 eq. and -1.0 kg CO2 eq. The LCA results were found to be especially sensitive to the pretreatment temperature, acid loading, hydrolysis enzyme loading and conversion efficiencies in the system expansion approach; and toward the pretreatment acid loading, hydrolysis enzyme loading and conversion efficiencies in the energy allocation approach. Further adaptations in the NREL model were made to represent experimental pretreatment results for corn stover and switchgrass, found in literature. These adapted cases show in general lower performance than the calculations based on the original NREL model as result of lower solids loading, lower conversion efficiencies and increased requirements of chemical or higher temperature. However, when increased demand of electricity or decreased electricity production result in the purchase of additional electricity, the environmental impact increases drastically. Comparison of the LCA results based on the NREL model with the available LCA studies on second generation bioethanol showed average environmental performance. Comparison of the results of second generation bioethanol production show better performance compared to usage of first generation ethanol or gasoline as fuel.