Life cycle assessment of energy and GHG emissions during ethanol production from grass straws using various pretreatment processes

The aim of this study was to perform a well-to-pump life cycle assessment (LCA) to investigate the overall net energy balance and environmental impact of bioethanol production using Tall Fescue grass straw as feedstock. The energy requirements and greenhouse gas (GHG) emissions were compared to those of gasoline to explore the potential of bioethanol as sustainable fuel.

The functional unit used in the study was 10,000 MJ of energy. The data for grass seed production were collected from the farmers in Oregon and published reports. The compositions of straw, pretreatment, and hydrolysis yields were obtained from laboratory experiments. Process models were developed for ethanol production using different pretreatment technologies in SuperPro Designer to calculate the process energy, raw materials, utility use, and emissions related. The Greenhouse Gases Regulated Emissions and Energy use in Transportation model and other literature studies were used to obtain additional data. Systematic boundary identification was performed using relative mass, energy, and economic value method using a 5% cutoff value.

Ethanol yields from grass straw were estimated 256.62, 255.8, 255.3, and 230.2 L/dry metric ton of biomass using dilute acid, dilute alkali, hot water, and steam explosion pretreatments, respectively. Fossil energy required to produce one functional unit was in the range of −1507 to 3940 MJ for different ethanol production techniques. GHG emissions from ethanol LCA models were in the range of −131 to −555.4 kg CO₂ eq. per 10,000 MJ of ethanol. Fossil energy use and GHG emissions during ethanol production were found to be lowest for steam explosion pretreatment among all pretreatment processes evaluated. Change in coproduct allocation from economic to mass basis during agricultural production resulted in 62.4% and 133.1% increase in fossil energy use and GHG emissions respectively.

Technologies used for ethanol production process had major impact on total fossil energy use and GHG emissions. N₂O emissions from the nitrogen fertilizers were major contributor (77%) of total GHG emissions produced during agricultural activities. There was 57.43–112.67% reduction in fossil energy use to produce 10,000 MJ of ethanol compared to gasoline; however, about 0.35 ha of land is also required to produce this energy.