Environmental Engineering Reference
In-Depth Information
corn ethanol. In the future, cellulosic biofuels will play a greater role as their production costs
are reduced; they should offer significantly greater GHG reduction benefits compared with corn
ethanol and petroleum.
Biomass electricity was found to greatly reduce GHG emissions. Using 100% hybrid poplar in an
IGCC power plant can reduce life-cycle GHG emissions by 94%, and adding CO
2
-seq technology
can reduce life-cycle GHG emissions by 179%, both with respect to coal. The benefits of willow
biomass energy become more apparent when examining direct-fired or gasified willow electricity
compared with co-firing with coal. The NER was found to be 9.9 and 12.9 for direct-firing and gas-
ification, respectively. Hybrid poplar was found to have a NER of 15.6. The high NERs demonstrate
the tremendous fossil energy leveraging of this renewable energy resource. For example, 13 units
of electricity are generated for every 2 units of fossil energy consumed across the full life-cycle of
willow gasification (Keoleian and Volk 2005). The NER is a useful indicator of overall life-cycle
energy performance. When life-cycle boundaries and assumptions are equivalent, the NER also
allows comparison between biofuels and biomass as an energy source.
This chapter focused on liquid fuel production for transportation using internal combustion (IC)
vehicles and biomass production for electricity generation. The NERs for biomass electricity are
generally much higher than those of liquid biofuels. This demonstrates that biomass electricity is
a significantly more efficient method for leveraging fossil energy resources and displacing GHG
emissions. In the future, electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) are
likely to play a role in transportation. Ohlrogge et al. (2009) compared the efficiencies of converting
biomass to electricity with conversion to biofuel for the purpose of powering EVs and IC vehicles,
respectively. Overall, the biomass electricity system was found to be nearly twice as efficient at
delivering the energy originally contained in the biomass to the end user.
Life-cycle energy and GHG emissions were the focal point of this chapter, but other air emissions,
such as criteria pollutants, are often included in life-cycle inventories. As an example, consider bio-
diesel tailpipe emissions, which are important because of their prevalence in highly populated areas
(e.g., biodiesel city buses). Smog is a recurring problem in major urban areas, formed by CO, total
hydrocarbon (THC), and NO
x
interactions with sunlight. Sheehan et al. (1998) found that biodiesel
releases fewer CO and THC emissions but increases NO
x
emissions, each relative to petroleum
diesel. Tailpipe emissions of CO and PM
10
are reduced by 46 and 68%, respectively, when B100
is used in place of diesel fuel. However, the increase in NO
x
is troubling because it is known to
adversely affect the human respiratory system (EPA 2009b). Research on reducing biodiesel NO
x
tailpipe emissions has achieved results through re-engineering of engine timing, use of fuel addi-
tives, and varying fuel properties (McCormick/NREL 2005). Other recent research has concluded
that because of high variance in NO
x
emissions for different engine types and test conditions, no
definitive conclusions on biodiesel NO
x
tailpipe emissions can be drawn (McCormick et al./NREL
2006). There is also concern over gasoline emissions. Preliminary results from a gasoline combus-
tion focused life-cycle study indicate that reducing particulate emissions can significantly reduce
health impacts (LBNL 2009). Hill et al. (2009) found that only cellulosic ethanol derived from corn
stover, switchgrass, mixed prairie vegetation, and miscanthus can reduce the combined costs of
GHG impacts and criteria pollutant PM
2.5
-induced health impacts. Corn ethanol, at best, was found
to cause negative economic impacts equivalent to those of gasoline, and only when produced using
efficient advanced technologies fueled by natural gas (Hill et al. 2009).
The recent trend in LCA of biomass systems has been a move away from attributional and toward
consequential studies (Sheehan 2009). Attributional LCAs evaluate the collective impacts of a sys-
tem and assign a fraction of these impacts to the system's products. Consequential LCAs attempt to
project the net effects of marginal changes within a system, e.g., considering the dynamic effects
increased production would have on a market. Consequential LCA, such as Searchinger et al.'s
(2008) indirect land-use impacts study, seem to offer a more desirable perspective to policy-makers,
but studies of this nature are inherently more complex. Two assumptions often made in biofuel LCAs
are perfect market elasticity (i.e., consumption of one product will lead to an equivalent increase in
