Environmental Engineering Reference
In-Depth Information
taBle 11.4
Greet analysis of sugarcane ethanol scenarios
scenario
key assumptions
Sugarcane 1 (SC1)
Base case. Sugarcane ethanol produced in Brazil, used in the United States. Farm equipment
manufacture and sugarcane mill construction energy excluded.
Sugarcane 2 (SC2)
Same as 1, except farm equipment manufacture and sugarcane mill construction energy are
included.
Sugarcane 3 (SC3)
Same as 1, except farm equipment manufacture energy is included.
Sugarcane 4 (SC4)
Same as 3, except sugarcane ethanol is consumed in Brazil.
Petroleum
Petroleum produced and consumed in the United States. All infrastructure activities excluded.
Corn
Corn ethanol produced and consumed in the United States. Farm machinery energy included.
Cellulosic
Cellulosic ethanol from switchgrass, produced and consumed in the United States. Farm
equipment manufacture included.
Source:
Wang, M., et al.,
Well-to-Wheels Energy Use and Greenhouse Gas Emissions of Brazilian Sugarcane Ethanol
Production Simulated by Using the GREET Model,
Argonne National Laboratory, Argonne, IL, 2007
scenario was defined as U.S. consumption of sugarcane that was produced and refined into ethanol
in Brazil (SC1). Table 11.4 summarizes the scenarios that were simulated in this study.
Sugarcane is bulky and heavy—70% water by weight—so sugarcane mills are typically close
to the fields. Thermal energy constitutes most energy demand at the processing plant. Wang et al.
(2007) assumed no external thermal energy or electricity input was required to power the conver-
sion process. Electricity and heat co-products can be derived by burning the residue (bagasse) that
remains after the sugarcane is crushed and the juice is extracted.
In this study, over 40% of the electricity generated at the fuel processing plant was assumed to be
surplus, which is marketable for export to the electric grid. In reality, sugarcane ethanol processing
facilities may not be connected to the grid, so electricity export was modeled as an option.
In the first three sugarcane scenarios, ethanol was produced in Brazil and shipped to the United
States for consumption. Wang et al. (2007) modeled transport in Brazil to include pipeline, rail, and
small amounts by truck. The ethanol fuel was then shipped from Brazil to New York or Los Angeles
and consumed near the East and West Coasts (the interior was assumed to consume midwestern
U.S. corn ethanol). A general overview of the life-cycle boundaries is presented in Figure 11.4.
This study also examined the effects of using five different energy sources to power a corn etha-
nol refinery. Figure 11.5 shows the well-to-wheel net energy balance for the base case sugarcane
ethanol scenario (SC1), cellulosic ethanol, and five corn ethanol scenarios: coal, electricity grid
average (Average), natural gas (NG), distiller's grains and solubles (DGS), and biomass.
We can interpret the results as follows: A corn ethanol system that uses a grid average-powered
refinery produces fuel with a 20% net renewable energy gain [Btu content of ethanol - Btu fos-
sil fuel inputs ≈ (200,000 Btu)/(1 million Btu) = 20%]. On a life-cycle energy basis, the scenarios
that used DGS and biomass to power the fuel production facility outperformed the other modeled
systems because DGS and biomass displaced fossil fuel inputs to the production facility. Similarly,
combustion of bagasse can completely power the sugarcane ethanol production facility and result
in export of surplus electricity to the grid. The surplus electricity was assumed to displace natural
gas-generated electricity, which was credited to the sugarcane fuel life-cycle system.
Wang et al. (2007) found little difference amongst the four sugarcane scenarios considered in this
study (see Table 11.4) in terms of fossil energy input requirements. Even when comparing scenarios
SC1, where the ethanol was shipped to the United States for consumption, and SC4, where the etha-
nol was consumed in Brazil, the life-cycle fossil energy requirements were less than 2% different.
This may seem surprising considering the shipping distance from Brazil to America, but the energy
required for shipping, per functional unit, was a relatively small fraction of total life-cycle energy.
