Comparative Lifecycle Analysis of BioEnergy Pathways: Cellulosic Ethanol vs Biomass Electricity
This research assesses the comparative environmental profiles of the bioenergy systems - biofuel (ethanol) and biomass electricity - derived from switchgrass. Switchgrass cultivation as a dedicated energy crop is an emerging practice. It has high yield with relatively low nutrient requirements and has a great potential to meet future energy needs.
The contribution of this research is twofold. First, the life cycle energy and GHG emissions of individual bioenergy pathways - producing cellulosic ethanol and biomass electricity - are analyzed in greater detail. In contrast with previous studies, we have not just used a single value of input parameters for different life cycle stages, but instead, have assessed the impact based on a probability distribution by incorporating a Monte Carlo analysis. This has helped to address the variability in the lifecycle impacts of bioenergy systems and establishing a range of energy and greenhouse gas (GHG) impacts, rather than previous single-valued estimates. Second, a framework to compare cellulosic ethanol and biomass electricity lifecycle energy and GHG emissions is provided. We propose the criterion for comparison should not be dictated by absolute emissions along a certain bioenergy pathway, such as producing biofuels or biomass electricity. Instead, we consider the savings in emissions from the displacement of fossil fuel by biomass along each pathway. Based on this criterion, we quantify the lifecycle GHG emissions impacts of each pathway, compare them to the reference fossil energy system and compare their land use efficiency.
The average lifecycle GHG emissions of ethanol are assessed as 35 g CO2-eq/MJ of energy, with a minimum-maximum range of GHG emissions varying from 25 to 50 g CO2-eq/MJ. Switchgrass yield, fertilizer application rates and conversion efficiency are important determinants in the overall variation in the GHG balance. For example, the GHG contribution from the agricultural stage of switchgrass varies over a large range from 500 to 1200 kg CO2-eq/ha/year. A comparison between cellulosic ethanol and a gasoline system shows that on average 55 g CO2-eq/MJ of energy are saved if gasoline use is replaced by cellulosic ethanol derived from switchgrass. The average life cycle GHG emissions from biomass electricity are 100 g CO2-eq/kWh, with a minimum-maximum range of GHG emissions varying from 90 to 110 g CO2 eq/kWh. In comparison to the U.S. grid electricity, 513 g CO2-eq/kWh of energy are saved if biomass electricity from switchgrass displaces the grid electricity. Avoided emissions per unit of energy, however, are also dependent on regional factors, such as regional electricity grid mix.
When comparing these bioenergy systems in terms of land use efficiency, biomass electricity has a better environmental profile in terms of energy sue and GHG emissions than ethanol. 4.5 ton CO2-eq/ha/year emissions are saved if we use all the switchgrass cultivated on a hectare of land to produce ethanol. For biomass electricity, the annual GHG emissions saved are 10 ton CO2-eq/ha/year. The GHG emissions offset from the best case of cellulosic ethanol is comparable to emissions offset from the worst case of biomass electricity.
We have also analyzed another case of the comparative environmental profile of these two bioenergy systems, formulated assuming a special case of biomass electricity use. In this case, all the biomass electricity produced is used to charge electric vehicles and ethanol is used to power Flex Fuel Vehicles (FFVs). 145 g CO2-eq/km will be saved if we use ethanol is used instead of gasoline in FFVs. In the case of electric vehicles, 110 g CO2-eq/km is saved if we use biomass electricity instead of the U.S. average grid power. Thus, biomass electricity is not a very effective alternative if the end goal of bioenergy policies is to use biomass only in the transportation sector. These findings are different from the previous bioenergy comparison studies, which have only estimated the offset of a biomass powered electric vehicle by comparing it to a gasoline-powered car. These studies concluded that biomass electricity has a better environmental profile than cellulosic ethanol.