Environmental Engineering Reference
In-Depth Information
1,100,000
1,000,000
900,000
800,000
700,000
600,000
500,000
400,000
300,000
Pump-to-wheel
Well-to-pump, hybrid
Well-to-pump, market
Well-to-pump, energy
Well-to-pump, displacement
Well-to-pump
Green
diesel
Renewable
gasoline
Biodiesel
upercetane
200,000
100,000
0
-100,000
-200,000
-300,000
-400,000
-500,000
FIGure 11.12
Full well-to-wheel petroleum requirements of six fuel production systems. (From Huo, H.,
et al.,
Life-Cycle Assessment of Energy and Greenhouse Gas Effects of Soybean-Derived Biodiesel and
Renewable Fuels.
ANL/ESD 08-2, Argonne National Laboratory, Argonne, IL, 2008.)
approximately 69% of U.S. N
2
O emissions came from croplands in 2007 (EPA 2009c). Nitrogen fer-
tilizer use creates environmental impacts, but it can also increase crop yields; it is a necessary com-
ponent of agriculture system life-cycle models. However, nitrogen fertilizer application rates vary
by more than 200% from state to state (USDA/ERS 2008a), and aside from the variables affecting
soil N content, the conversion rate of soil N to N
2
O is highly uncertain and depends on many factors
(Snyder et al. 2009). It has been suggested that the N
2
O conversion rate should be 3-5 times higher
than the value currently recommended by the IPCC and used in many biofuel LCAs (Crutzen et al.
2007). Modeling of N
2
O field emissions requires further work to reduce the uncertainty (Kendall
and Chang 2009).
On the basis of current models, when indirect land-use change is included within system bound-
aries, life-cycle GHG emissions from corn-grain ethanol can be 93% higher than gasoline GHG
emissions over a 30-year period (Searchinger et al. 2008). Conversion of previously uncultivated
land is likely to increase soil organic carbon losses (Anderson-Teixeira et al. 2009), but indirect
land-use change studies such as Searchinger et al. (2008) have been questioned regarding assump-
tions of free market behavior and geographic selection of land-use change areas (Sylvester-Bradley
2008). A lengthier discussion follows in Section 11.4.
Conservation tillage, also known as “no-till farming,” is an agricultural management practice in
which 30% or more of the soil surface is left covered with crop residue, rather than being plowed
under after harvest. This practice can reduce soil erosion, agricultural production costs, and fossil
fuel consumption. No-till farming is also thought to increase the carbon sequestration potential of
soil, although this claim may be insufficiently supported by evidence (Baker et al. 2007).
11.2.3.2 ethanol
The net greenhouse gas emissions found in the studies assessed by Farrell et al. (2006) with the
EBAMM model are presented in Figure 11.13. Pimentel and Patzek calculated net energy losses and
some of the highest GHG emissions—even higher than those of the EBAMM CO
2
-intensive sce-
nario. These studies have been criticized for using outdated agricultural data as well as inappropri-
ate system boundaries and allocation methods. The ethanol today scenario is an EBAMM specific
case created using the best available data from the six studies.
The largest causes of variation in GHG emissions were limestone application rates and farm
machinery embodied energy assumptions. Also, several parameters affecting N
2
O emissions and
nitrogen displacement were not considered in the six studies: manure application, crop residue left
