Life Cycle Greenhouse Gas Emissions of Biofuels Production from Willow Hot Water Extract

Hot water extraction (HWE), a pretreatment step incorporated in a multi-pathway biorefinery, yields valuable chemicals and sugars that can be used as feedstocks for bioethanol production and other products. A cradle-to-grave life cycle assessment is performed to assess the greenhouse gas (GHG) emissions associated with the production of 1 MJ of ethanol by fermentation of sugars derived from HWE of willow biomass. This study analyzes eight scenarios from the combination of two previous land uses (cropland and grassland), two harvest and storage seasons (summer and winter), and two biorefinery energy sources (biomass and natural gas). Ethanol produced from willow grown on cropland can sequester 0.012 kg CO_2eq MJ^-1 for a supply system incorporating winter harvest and using biomass to meet the energy requirement of the biorefinery. However, for the natural gas-based scenarios, the estimated GHG emissions for willow grown on cropland are 0.078 kg CO_2eq MJ^-1 and 0.118 kg CO_2eq MJ^-1 on grassland. The sensitivity analysis indicates that the life cycle GHG emissions associated with each scenario respond differently to changes in input parameter values; ethanol yield and HWE mass removal have greater influence on the net GHG emissions for natural gas than biomass based scenarios, storage duration is more influential for summer than winter harvests, and the fraction of suitable parcels converted into willow production has a higher impact for grassland than cropland. This study demonstrates that bioethanol produced from willow grown on cropland can be used as a mean of carbon sequestration while producing liquid transportation fuel.