Environmental Engineering Reference
In-Depth Information
125
Pimentel
CO
2
intensive
Published values
Commensurate values
Gasoline
EBAMM cases
Graboski
100
Patzek
Gasoline
To day
Wang
Shapouri
de Oliveira
75
50
25
Cellulosic
-
-8
-6
-4
-2
0
246
Net energy (MJ/L)
8 0 2 4 6 8 0 2 4
FIGure 11.13
EBAMM net energy vs. net GHG emissions for corn and cellulosic ethanol. (From Farrell,
A.E., et al.,
Science,
311, 506-508, 2006.)
on the field, growing legumes, wetland conversion to farmland, and use of a corn-soybean rotation
system (soybeans capture nitrogen from the atmosphere, thus reducing fertilizer demand).
Farrell et al.'s (2006) EBAMM analysis of the six studies indicates that agriculture generates
34-44% of life-cycle GHG emissions. On the basis of sensitivity analysis results, Farrell et al.
(2006) concluded that major reductions in net GHG emissions are only likely to be achieved with a
cellulosic ethanol fuel system, as shown in Figure 11.13.
Kim and Dale (2005) evaluated four cropping scenarios, including a corn stover utilization
option. One important feature of their study was its detailed consideration of soil dynamics, particu-
larly nitrogen flux. The peer-reviewed DAYCENT model was used to incorporate organic carbon
and nitrogen soil dynamics. The authors pointed out that 90% of corn stover is left on fields in the
United States, which was assumed to increase soil organic carbon content (sequestration) but also
increases N
2
O emissions.
Table 11.6 shows Kim and Dale's (2005) GHG emissions results. Just as in the nonrenewable
energy results in Table 11.3, life-cycle GHG emissions are also dominated by driving impacts for
each cropping system scenario. Although removing corn stover from the field requires more energy
and results in less soil carbon sequestration, the benefits of increased ethanol production and surplus
electricity generation outweigh these effects. In the CwC70 system, winter wheat crops sequester
more carbon and elevate soil fertility, which increases ethanol production.
The Schmer et al. (2008) analysis of ethanol derived from switchgrass found that this cellulosic
feedstock would reduce life-cycle GHG emissions by 94% compared with gasoline. Combustion of
lignocellulosic plant residue for energy, which displaces fossil fuel electricity, was cited as the main
reason for the GHG savings. This study was significant because agriculture data were obtained from
field-scale plots of marginal cropland rather than small research plots. Modeling was performed
using EBAMM, and the switchgrass was assumed to sequester soil carbon over a 100-year period
on converted croplands.
Sugarcane ethanol, as modeled by Wang et al. (2007), has a different agriculture process than
corn ethanol, including burning of fields before harvest to clear out pests and sharp leaves. The
authors assumed that open-field burning of sugarcane plant remnants was practiced pre- and post-
harvest at a rate of 80%. Field burning emissions include: carbon monoxide (CO), CH
4
, oxides of
nitrogen (NO
x
), N
2
O, particulate matter less than 2.5 μm in aerodynamic diameter (PM
2.5
), par-
ticulate matter less than 10 μm in aerodynamic diameter (PM
10
), volatile organic carbons (VOCs),
and oxides of sulfur (SO
x
). Additionally, N
2
O emissions result from nitrogen fertilizer application.
These emissions are uncertain (see Section 11.2.3.1), but the authors assumed that 1.5% of the
