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APPENDIX E
Air Quality and Greenhouse
Gas Benefits
Alternative jet fuels have two principal potential environmental benefits. First, the overall life-
cycle GHG footprint may be lower than that of conventional jet fuel. Second, PM emissions may
be lower. Reductions in NOx have been documented for alternative ground fuels relative to con-
ventional diesel fuel, but there is no current evidence to suggest that the same benefit extends to
alternative jet fuels. The following sections discuss the GHG and PM benefits.
E.1 GHG Life-Cycle Benefits
A key benefit often associated with alternative fuels is the potential to reduce total life-cycle
GHG emissions (specifically carbon reduction) in comparison with conventional petroleum-
based fuels. This potential reduction is quantitatively estimated using the techniques of life-cycle
analysis.
Life-cycle analysis as applied to aviation fuel consists of estimating the amounts of various sub-
stances produced (or consumed) during the complete process of obtaining and using the fuel. The
process is broken down into various stages as the fuel is transformed from its raw form, trans-
ported, and used. Depending on the exact feedstocks, processing technologies, and logistics used,
the life-cycle carbon footprint of the resulting liquid fuel can be more, equal, or less than the con-
ventional petroleum-based jet fuel. Thus, a life-cycle analysis for each particular alternative fuel
is necessary before assigning GHG emissions reduction benefits. Beyond feedstock and process,
the analyses will be project specific with variables such as direct and indirect land use in feedstock
supply brought into consideration (EPA 1999).
While numerous LCA methodologies can be found in the literature, the one that has been
developed and peer reviewed and is being used for United States Air Force (USAF)/Department
of Energy evaluation and is specific to aviation is contained in Framework and Guidance for Esti-
mating Greenhouse Gas Footprints of Aviation Fuels (Allen et al. 2009). That approach was devel-
oped using government funds to ensure that Department of Defense purchases of alternative
fuels conforms to the LCA requirements of Section 526 of the 2007 U.S. Energy Independence
and Security Act (Sissine 2007).
The LCA process accounts for the six stages in the fuel-production life cycle: (1) acquisition
of raw materials, (2) transport of these materials, (3) processing them into aviation fuel,
(4) transport of fuel to the aircraft, (5) combustion of the fuel, and (6) end of life. (In the case of
jet fuel, the end of life stage is not included in the analysis since the fuel is consumed in stage 5.)
In the aggregate, the first four of these stages are often referred to as "well-to-tank" (where the
tank is on the aircraft), and the combustion stage as "tank-to-wake." Analysis of GHG emissions
is performed for each of these stages and includes all inputs and processes associated with a given
88
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Air Quality and Greenhouse Gas Benefits 89
stage. Precise boundaries between each stage are defined so that each element is fully accounted
for but without overlap between stages. Additionally, an overall system boundary is defined.
A recent report (Stratton, Wong, and Hileman 2010) analyzed several feedstocks for the FT
or HEFA processing of aviation fuels using this methodology (see Table 18 and Figure 12). The
nonpetroleum feedstocks coupled with FT processing were coal, natural gas, and switchgrass, as
well as coal and switchgrass combined. The nonpetroleum feedstocks coupled with HEFA pro-
cessing were soybeans, palm, rapeseed, Jatropha, algae, and Salicornia. Depending on assumptions
(particularly those associated with land-use changes associated with growth of the feedstocks),
these pathways were estimated to have life-cycle GHG emissions ranging from less than 1% of
the conventional crude petroleum pathway to over 8 times greater than this pathway. Several
pathways have estimated life-cycle GHG emissions that are less than half of the crude-to-con-
ventional-jet-fuel pathway (switchgrass to FT fuel, Jatropha oil to HEFA, and Salicornia to HEFA
and FT fuel). The variability that can be expected for given processes and feedstocks when land
use and other uncertainties (e.g., yield per acre, energy required for growth, harvesting, and water
extraction) are considered can be observed in the last column in Table 18.
There are other reports exploring the life-cycle GHG footprint of HEFA and FT processes for
the production of alternative jet fuels. Figure 13 shows the results of an analysis of life-cycle GHG
emissions for a variety of alternative fuels, including HEFA/HRJ, alternative (green) diesel, FT
Table 18. Life-cycle GHG emissions expressed as grams CO2 equivalent (g CO2e) per MJ of fuel
energy content (adapted from Stratton, Wong, and Hileman 2010).
Pathway
Combustion
Processing
Feedstock
Transport
Transport
Land-Use
Recovery
WTT CH4
Biomass
WTT N20
Change
Credit
Total
Fuel
Crude to conventional jet fuel 0 4.2 1.5 5.5 0.8 73.2 0.1 2.3 0 87.5
Crude to ULS jet fuel 0 4.2 1.5 7.3 0.8 72.9 0.1 2.4 0 89.1
Oil sands to jet fuel 0 19 1.3 5.5 0.5 73.2 0.1 3.1 0 102.7
Oil shale to jet fuel 0 41.2 0.6 3.3 0.6 73.2 0.2 2.5 0 121.5
Natural gas to FT fuel 0 4.6 0 20.2 1.2 70.4 0 4.6 0 101
Coal to FT fuel with (without) 0 0.8 0.1 19.4 0.6 70.4 0 5.9 0 97.2
carbon capture (117.2) (5.7) (194.8)
Switchgrass to FT fuel -222.7 6.4 0.6 152.1 0.5 70.4 0.2 10.3 -19.8 to 0 -2.0 to
17.7
Coal and switchgrass to FT fuel, -44.3 1.2 0.2 21.9 0.5 70.4 2 4.9 -3.9 to 0 53.0 to
with carbon capture 56.9
Soy oil to HEFA/HRJ -70.5 20.1 1.2 10.3 0.6 70.4 3.6 1.3 0 to 527.2 37.0 to
564.2
Palm oils to HEFA/HRJ -70.5 4.9 3.1 10.3 0.6 70.4 5.1 6.3 0 to 667.9 30.1 to
698.0
Rapeseed oil to HEFA/HRJ -70.5 17.2 3.1 10.3 0.6 70.4 22.4 1.3 0 to 43.0 54.9 to
97.9
Jatropha oil to HEFA/HRJ -70.5 16.7 1.5 10.3 0.6 70.4 9.1 1.2 0 39.4
Algae oil to HEFA/HRJ -70.5 29.6 0.3 10.3 0.6 70.4 8.1 1.8 0 50.7
Salicornia to HEFA/HRJ and FT -105.3 36.8 1.1 38.3 0.5 70.4 4.6 1.3 -41.9 to 0 5.8 to
fuel 47.7
Note: Some totals do not sum due to rounding.
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90 Guidelines for Integrating Alternative Jet Fuel into the Airport Setting
Salicornia to HEFA and FT
Algae oil to HEFA
Jatropha oil to HEFA
Rapeseed oil to HEFA
Palm oils to HEFA
Soy oil to HEFA
Coal and Biomass (w/ CCS)
Switchgrass to FT fuel
Coal to FT fuel (no CCS)
Natural gas to FT fuel
Oil shale to jet fuel
Oil sands to jet fuel
Crude to ULS jet fuel
Crude to conventional jet fuel
0 1 2 3 4 5 6 7 8 9
Figure 12. Relative life-cycle GHG emissions of several pathways for
alternative jet fuels (conventional jet fuel = 1.0; adapted from Stratton,
Wong, and Hileman 2010).
fuels, and conventional fuels (Kalnes, McCall, and Shonnard 2010; Shonnard, Williams, and
Kalnes 2010).
Similar to the observations from Stratton, Wong, and Hileman (2010), feedstock selection
plays a critical role in the contribution of the life-cycle GHG footprint to the process path.
Tallow-based diesel and alternative jet fuel produced from hydroprocessing have the lowest life-
cycle GHG signature since tallow is essentially a waste product and has minimal life-cycle GHG
inputs. Alternative jet fuel made from Jatropha also has a lower life-cycle GHG footprint com-
pared to conventional jet fuel.
Airports are encouraged to conduct or request from potential fuel producers detailed LCA
analysis to determine the life-cycle carbon footprint of the fuels they intend to produce and the
processes they intend to use. The previous estimates are meant to illustrate results from recent
studies and are not intended to be a comprehensive or official representation of life-cycle carbon
Figure 13. Life-cycle GHG emissions for conventional and alternative fuels,
including green diesel (GD), HRJ, FT syndiesel, and biodiesel (BD). Adapted from
Kalnes, McCall, and Shonnard 2010.
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Air Quality and Greenhouse Gas Benefits 91
Figure 14. Particulate emission
index (EI) for immediately behind a
CFM-56 engine using conventional,
Fischer-Tropsch, and blended fuels
as measured by AAFEX. Source:
Beyersdorf and Anderson 2009.
estimates. Given the many uncertainties with respect to feedstocks, production and delivery, and
land-use change impacts, this remains an intensely debated and active field of research.
E.2 Reductions in Criteria Pollutants, Particularly PM 2.5
Another potential benefit of using alternative fuels is the reduction in emissions that affect
local air quality, in particular SOx and PM. These emissions can lead to respiratory diseases, such
as asthma, and are major contributors to acid rain, smog, and reduced visibility (FAA 1997a; EPA
1999). In this section, an overview of the potential of alternative fuels to reduce or mitigate emis-
sions of these pollutants is presented.
Oxides of sulfur present in jet fuel are precursors and indicators of particle and PM2.5 forma-
tion. PM2.5 is known to cause serious health problems and is regulated with separate standards
by the EPA (EPA 2011). Furthermore, as a criteria pollutant, high levels of PM2.5 can lead to areas
in which airports are located to be designated as non-attainment zones, with potential negative
consequences to airport growth and operations.
As alternative fuels are being qualified as blends with petroleum-based jet fuel, the alternative
components of these fuels are essentially sulfur-free. Benefits of alternative fuel regarding PM2.5
emissions have been measured by the USAF and by commercial tests. Figure 14 shows the par-
ticle emission index for conventional fuel, Fischer-Tropsch fuel, and a blend of the two at differ-
ent thrust levels. The decrease associated with the blend becomes larger as the proportion of the
Fischer-Tropsch fuel increases (Corporan et al. 2007). These benefits are substantial when com-
pared to current petroleum-based fuels having typical sulfur contents of approximately 700 parts
per million (Taylor 2009).
In addition to sulfur content, PM formation is also linked to the presence of aromatic com-
pounds in the fuel. Since levels of aromatics in HEFA fuels and fuels produced with Fischer-
Tropsch processes are typically low (Hileman et al. 2009), there would also be a reduction in the
generation of PM from these fuels due to this effect (Morser et al. 2011).
