Environmental Engineering Reference
In-Depth Information
One problem with using mass or market value is the assumption that process inputs, such as
energy or materials, are concentrated in particular outputs based on one variable (e.g., retail value).
A heavier product does not necessarily require more energy, and a more expensive product will not
necessarily create more impacts, but use of these methods can help relate impacts to products in
socially meaningful ways.
Biofuel life-cycle studies have been moving away from market value and adopting the displace-
ment approach as an allocation method (Kendall and Chang 2009). The displacement method
allows the modeled product to receive credit for burdens that would have been generated had it
not replaced an equivalent conventional product. For example, dried distiller's grain with solubles
(DDGS) is a co-product of corn-grain ethanol production. It is high in protein and can be used
as feed for livestock. When DDGS is used in place of conventional feed, such as corn and soy-
bean meal, less conventional feed production is required [A 2007 U.S. Department of Agriculture
(USDA) survey found that 13.6% of the dairy and hog farmers who responded use DDGS as live-
stock feed, and the single biggest reason it was not used was lack of availability (USDA/NASS
2007)]. An LCA that uses the displacement method for allocation would consider the impacts
of DDGS and the avoided impacts of not using conventional feed. This allocation method was
used in a biofuel life-cycle study to examine the effects of ethanol fuel displacing gasoline (Kim
and Dale 2005). Kim and Dale (2005) used the International Organization for Standardization
(ISO) 14041 standard system expansion approach (ISO 1998) and included alternative products by
expanding the life-cycle system boundaries to allow functionally equivalent product comparisons.
The life-cycle greenhouse gas emissions related to four cropping systems are presented in Table
11.1. The bold avoided system values in Table 11.1 are credits allocated to the biofuel systems.
Ideally, the displacement method would use a general equilibrium model to simulate complex
market interactions and accurately determine the displacement ratio of conventional products by
introduced products (e.g., Does 1 lb of DDGS displace 1 lb of conventional feed? How does broad
introduction of DDGS affect other market prices, and thus consumer choices?). Currently, general
taBle 11.1
example life-cycle GhG emissions results with displacement
method for modeling co-Products
GhG emissions (mg co
2
-eq/ha)
cs
cc
cc50
cwc70
Agricultural Process
23
28.1
55.4
12
Wet milling
79.5
160
158
167
Avoided co-product systems
1.3
-137
-136
-143
Soybean milling
13.7
-
-
-
Biodiesel production
6.2
-
-
-
B20 driving
127
-
-
-
Avoided B20 driving system
-157
-
-
-
Corn stover conversion
-
-
4.2
6.2
Avoided electricity
-
-
-38.1
-56.1
E10 driving
1794
3618
5067
5967
Avoided E10 driving system
-1923
-3877
-5431
-6395
Total
-35.4
-209
-320
-442
Source:
Kim, S. and Dale, B.E.,
Biomass Bioenergy,
29, 426-439, 2005.
Note:
CS, corn-soybean crop rotation (also produced biodiesel in the form of B20
blended fuel); CC, corn only; CC50, corn only with 50% residue removal; and
CwC70, corn only, except wheat in the winter, with 70% residue removal.
