Life cycle assessment of ethanol production from tropical banagrass (Pennisetum purpureum) using green and dry processing technologies in Hawaii

Abstract

This study conducted well-to-pump and well-to wheel life-cycle assessment of fossil energy use and greenhouse gas (GHG) emissions during ethanol production from tropical Banagrass (Pennisetum purpureum) using green-processing (with the use of fresh feedstocks) and dry or conventional processing (with the use of dried feedstocks) in the state of Hawaii. 10 000 MJ of energy was used as a functional unit with a systematic boundary drawn based on relative mass, energy, and economic value method using a 1% cutoff value, and the results were compared to those of conventional gasoline, and ethanol from corn and other ethanol lignocellulosic feedstocks. Detailed techno-economic model was built using the SuperPro designer. Ethanol yields were estimated at 0.27 l/kg (green processing with fungal co-product), 0.27 l/kg (green processing without co-product), and 0.29 l/kg (dry-processing) of feedstock, respectively. The well-to-pump analysis indicate that ethanol production consume 8200 MJ (green processing with co-product), 7600 MJ (green-processing without co-product) and 7200 MJ (dry-processing without co-product) of fossil energy and emit approximately144 kg CO2-eq., 90.6 kg CO2-eq., and 59.1 kg CO2-eq. per 10 000 MJ of ethanol produced, respectively; well-to-wheel analysis showed that 280 g of gCO2-eq., 260 g CO2-eq., and 250 g CO2-eq. of emissions were produced per kilometer by driving Flex Fuel Vehicle. In summary, ethanol produced using the green-processing technology required greater amount of fossil energy and produced more GHG emissions compared to that of dry processing technology, due to additional energy needed for fungal growth and related processes. Process power, enzyme, and chemical production during ethanol processing were identified as emissions hot-spots for both green and dry processing.