Environmental Engineering Reference
In-Depth Information
as 25%; both values are close to the assumed values in Sheehan et al. (1998; see Figure 11.7).
However, petroleum refinery energy efficiency could improve in the future. Since the above
Sheehan analysis, a Lawrence Berkeley National Laboratory report (Worrel and Galitsky 2005)
concluded that the typical petroleum refinery could economically increase their energy efficiency
by 10 -20%.
Sheehan et al. (1998) found that using biodiesel in place of petroleum diesel reduces life-cycle
energy consumption and can also significantly reduce life-cycle petroleum consumption; B20 and
B100 were found to reduce life-cycle petroleum consumption by 19 and 95%, respectively, when
compared with petroleum diesel.
11.2.2.3 soybean Biodiesel and renewable Gasoline
A more recent well-to-wheel analysis by Huo et al. (2008) of soybean biodiesel examined six
different fuel production pathways: conventional petroleum gasoline, conventional petroleum
low-sulfur diesel, soybean biodiesel produced by three different methods, and soybean-derived
gasoline. The authors applied the allocation methods listed in Table 11.5 to each of the four soy-
bean fuel systems.
Other studies have discussed the benefits of the displacement allocation method, but there
are reasons for avoiding this approach. For one, it can be difficult to accurately determine
the life-cycle impacts of the displaced conventional products, which must be known to calcu-
late credits for avoided burdens. Second, if a large amount of co-products are generated per unit
of primary product, they will be assigned a large fraction of the burdens. When this occurs and
only the primary product is considered, the cumulative impacts of the production system may
be masked.
Huo et al. (2008) used a hybrid allocation method that considered the energy value of fuel prod-
ucts and market displacement of nonfuel products (i.e., soybean meal) in three of the four soybean
fuel systems (excluding the soybean transesterification system). Life-cycle calculations used the
GREET model to simulate performance in year 2010. Soybean agriculture modeling was updated
with 2007 USDA data, and nitrous oxide (N
2
O) emissions values were updated with 2006 IPCC
data. Potential land-use change was not considered in this study.
After the soybeans were cultivated and crushed to extract the crude soy oil using a hexane sol-
vent, the soy oil was processed using one of four pathways: (1) transesterification with methanol
produced biodiesel along with glycerin; (2) hydrogenation using hydrogen produced supercetane
along with fuel gas and heavy oils; (3) a modified hydrogenation process using hydrogen and differ-
ent inputs than in pathway 2 produced green diesel along with a propane fuel mix; and (4) catalytic
cracking of soy oil produced renewable gasoline along with product gas, light-cycle oil, and clarified
slurry oil. The novelty of the Huo et al. (2008) GREET analysis was modeling of catalytic cracking
and two hydrogenation pathways. The difference between the two hydrogenation processes is that
taBle 11.5
allocation methods for soybean-Based Biofuels
method
description
Displacement
New products with equivalent function replace conventional products. Burdens of
conventional products are avoided and credited to the system.
Energy allocation
Burdens assigned according to energy content of products. Values are constant. Combustion
energy of fuel for primary product, animal digestion energy for co-product.
Market allocation
Burdens assigned according to market value of products. Dynamic values, projected for the
future based on past trends.
Hybrid
Burdens assigned by either displacement or energy allocation depending on whether the
subsystem output is a product or fuel (i.e., glycerin vs. biodiesel)
