Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 216

ourselves by our cavalier use of fertilizers, the global phosphate supply should be considered a pre-
cious resource that may not be readily available in a couple of decades (DOE, 2010; Vaccari, 2009).
Many of the early sustainability analyses for corn-based ethanol ignored the issue of land use
change altogether, which was a grievous omission. They often did not include the entire system
boundaries, nor did they have a complete inventory (von Blottnitz and Curran, 2007). When
properly accounting for land use change, system boundaries, and a more thoughtful assessment
of the inventory required, corn-based ethanol production shifts from being a good thing for the
environment to a strategy that is ultimately harmful to the environment.
Arguably, the most essential feature of any LCA is to thoroughly document the approach
taken, including purpose of the LCA, system boundaries, critical assumptions, key parameters,
variability in fuel components and uncertainties (Stratton et al. , 2011). In many historical LCAs,
the values for a given input or output is given as a single fixed number (such as the yield of
jatropha oil/ha/y). However, in fact, there are uncertainties in that value associated with real
demographic differences. More uncertainties creep into the analysis fromother variable quantities,
outdated data, or incomplete problem description. To compare different strategies for alternative
fuels, feedstocks need to be compared within an equivalent framework. If the approach is fully
documented, then future analyses can harmonize the assessment by updating or supplementing
the data (e.g., Sun et al. , 2011; von Blottnitz and Curran, 2007; Warner et al. , 2010).
Defining the appropriate criteria by which to measure energy sustainability is still a work in
progress (McBride et al. , 2011). Moreover, the hard data on biofuel production that are needed
for such analyses are full of uncertainty (Murphy et al. , 2011; Slade et al. , 2011), because it is
not available, still emerging, or preliminary. In order to accurately estimate the amount of land
that would be needed globally for producing some specified amount of biofuel, the model must
account for local and regional variations for many variables such as climate, water availability,
crop production techniques, land suitability, and refining capabilities. Wigmosta and co-workers
have developed a comprehensive framework that can account for most of these variables in great
detail for the United States down to a resolution of 30 miles (Wigmosta et al. , 2011). As such
comprehensive tools mature, they will provide even better insight into modeling various scenarios
for biofuel production.
For further reading on LCAs for aviation fuels, see Dray et al. (2010), Jorquera et al. (2010),
Kinsel (2010), Kreutz et al. (2008), Stratton et al. (2010), andYang et al. (2011).
11.7 CONCLUSIONS
Aviation fuel requirements are demanding because of the extreme environment in which aircraft
must operate. Without an alternative to the internal combustion engine in the near term, the indus-
try must rely on drop-in fuels that are compatible with existing engines. Although biodiesel based
on methyl esters are adequate for automotive applications, their poor cold-weather properties
and reduced specific energy make them unsuitable for aviation fuel. Both synthetic paraffinic
kerosene and Hydrotreated Esters and Fatty Acids (HEFA) can meet aviation's needs, but it is
necessary to find sustainable ways of manufacturing these fuels. Some of the environmental and
economic improvements that will bring closure to the problem will include new efficiencies in
strain selection, crop management and production, and more creative approaches to finding uses
for co-products.
ACKNOWLEDGEMENTS
The author would like to gratefully acknowledge the support of NASA Glenn Research Center
and the alternative fuels component of the Subsonic Fixed Wing Program, as well as the wisdom
and guidance of Bob Hendricks and Arnon Chait, and the heroic efforts of the NASA Glenn
Technical Library, especially Marcia Stegenga.