Environmental Engineering Reference
In-Depth Information
1,000,000
800,000
600,000
400,000
200,000
0
-200,000
Corn EtOH
(coal)
Corn EtOH
(average)
Corn EtOH
(NG)
Corn EtOH
(DGS)
Corn EtOH
(biomass)
Sugarcane
(EtOH)
Cellulosic
(EtOH)
Gasoline
-400,000
FIGure 11.5
Net energy balance of gasoline and various bioethanol feedstocks (in Btu). (From 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.)
Wang et al. (2007) found that sugarcane ethanol can reduce life-cycle fossil energy consumption
by 97% relative to gasoline. The savings are largely due to the combustion of bagasse for heat and
electricity in the sugarcane processing facility, which displaces purchased natural gas-generated
electricity. Wang et al. (2007) also found that sugarcane ethanol offers even greater energy benefits
than cellulosic ethanol (see Figure 11.5), although the latter technology is less mature.
11.2.2.2 Biodiesel
In this section we will discuss the results of a comparative LCA of biodiesel and petroleum diesel
(Sheehan et al. 1998). Biodiesel results were calculated for 100% biodiesel (B100), and a 20% bio-
diesel/80% petroleum blend (B20). The functional unit in this study was 1 brake horsepower-hour
(bhp-h) of work delivered by an urban bus engine. In the past, concerns have been raised regarding
biodiesel's cold weather performance. Since the Sheehan study, additives and ASTM blending stan-
dards have been developed to mitigate cold weather performance issues (NBB 2008). In Minnesota,
B2 and B5 (2% and 5% biodiesel, respectively) are currently used, and the mandated blend fraction
is planned to increase to 20% by 2015 (Minnesota 2008). However, legislation dictates that the 20%
blend is only for summer months—a 5% biodiesel blend is allowed for winter months until the cold
weather issues are sufficiently addressed (MDA 2009). Sheehan et al. (1998) used #2 low-sulfur
diesel as a baseline, which also exhibits reduced cold weather performance, although it has a better
tolerance for low temperatures than biodiesel (MDA 2009). The life-cycle model of #2 low-sulfur
petroleum diesel is outlined in Figure 11.6. Sheehan et al.'s (1998) study is slightly dated, but it is
described in this section because of its comprehensiveness, transparency, and role as a foundation
for the numerous biofuel LCAs that followed.
11.2.2.2.1
Diesel Model
Sheehan et al. (1998) focused their comparative biodiesel study on petroleum diesel, but it should
also be noted that 44% of the crude oil refinery's output by mass was gasoline (co-product in Figure
11.6). Sheehan et al. (1998) assumed that foreign and domestic crude petroleum was extracted by
conventional onshore (73%), conventional offshore (20%), and advanced methods (7%). The energy
requirements for producing crude oil using steam injection (an advanced method) are an order of
magnitude higher than those of conventional methods.
Crude oil pumped to the surface is typically mixed with natural gas and water, which must be
separated before the crude can be refined. The natural gas recovered during this production stage
is treated as a co-product; Sheehan et al. (1998) used allocation by mass to assign it 30% of the
extraction process burdens. Crude oil production was modeled as evenly split between foreign and