The five different fuel groups were those derived: from conventional petroleum; from unconventional petroleum; synthetically from natural gas, coal, or combinations of coal and biomass via the FT process; renewable oils; and alcohols.

Of the three types of fuels that show near-term feasibility, the report found that:

• All three are or can easily and inexpensively be made fully compatible with current aircraft and fuel-delivery systems.

• Canadian oil sands and Venezuelan VHOs have the largest potential of several hundred thousand barrels per day of jet fuel, but their use would result in increased GHG emissions.

• The prospects for FT jet fuels depend crucially on construction of a few pioneer commercial plants in the next few years.

• Production of commercial quantities of HRJ depends on the availability of appropriate feedstocks at competitive prices.

Other key findings from the report include:

• Alternative-fuel production benefits commercial aviation regardless of its use in aviation. The expected influx of large amounts of alcohol-based fuels and fuels derived from unconventional petroleum over the next decade may cause long-term world oil prices to be between 5 and 12% lower than they would be in the absence of those fuels. For world crude oil prices in the range of $100 per barrel, this amounts to a price impact of roughly $5 to $13 per barrel. In 2017, jet fuel consumption in the United States (commercial aviation plus military) is projected to be about 1.9 million bpd. Applying the per-barrel savings to this consumption yields net annual jet-fuel cost savings of between $3.2 billion and $8.3 billion, according to the report.

• Reduced GHG impact.Certain HRJ and FT fuels are able to reduce the GHG emissions from aviation. For HRJ to be effective in reducing GHG emissions, the report said, it must be produced from oils that do not incur land-use changes, either directly or indirectly, that cause a large release of other GHGs. This constraint places a severe limit on the amount of climate-friendly HRJ that can be produced within the next decade.

  For FT jet fuels to be effective agents for GHG reduction, they must be produced from biomass or a combination of coal and biomass. In the former case, the fuels will be expensive and demand extensive cultivation of biomass for inputs. In the latter case, capture and sequestration of plant-site carbon emissions would be required, but overall costs would be much less, as would biomass consumption. As with HRJ, the provision of biomass must not incur land-use changes, either directly or indirectly, that cause a large release of GHGs.

  The study found that in the absence of CCS, for oil sands, VHOs, and GTL, life-cycle GHG emissions would increase by 10 to 25% relative to conventional Jet A. For CTL, life-cycle GHG emissions would roughly double.

• Some alternative aviation fuel feedstocks may provide greater benefits in other applications. FT fuels and HRJ are attractive aviation fuels because they have specific energies that are slightly greater than current petroleum-derived jet fuel; however, high-performance diesel fuels can also be made via either FT synthesis or hydroprocessing of renewable oils.

• Alcohols, biodiesel and biokerosene offer no benefit for aviation. Due to their incompatibilities with aircraft fuel systems and their low energy content, alcohol fuels are more appropriate for ground-transportation applications than for use in gas-turbine applications. Biodiesel and biokerosene have poor thermal stability and high freezing points, leading to problems in transportation, storage, and use of these fuels. HRJ may be produced from the same feedstocks used to produce biodiesel and biokerosene, and has none of those issues.

• An ultralow-sulfur (ULS) specification for Jet A would reduce air quality impacts. ULS jet fuel would virtually eliminate secondary particulate matter due to sulfur-oxide emissions while also reducing primary particulate emissions due to sulfur, the report said. The introduction of a ULS jet-fuel specification would act to ease the introduction of FT synthetic fuels and HRJ into commercial aviation, as they pose similar concerns in terms of infrastructure compatibility of lubricity and effect on seals due to their low sulfur and reduced aromatic content.

  Unlike new aircraft and engine technologies, which take some time to diffuse into the fleet, the air-quality benefits of sulfur elimination could be realized as soon as a ULS jet fuel were introduced.

  Adverse effects of ULS jet fuel would include higher fuel prices (by about $0.05 per gallon); an increase (about 1%) in the fuel volume purchased and consumed; a reduction (about 1%) in the aircraft range with full fuel tanks; an increase (by about 2%) in life-cycle GHG emissions; and the elimination of sulfur aerosols, which have a short-term cooling effect. This study did not attempt to assess the balance among these effects to determine whether introduction of a ULS jet-fuel standard is cost beneficial.

• Alternative jet fuels will have a limited impact on fuel price volatility. While a mature alternative-fuel industry could improve the resilience of the fuel-supply chain in the US (e.g., less production in areas vulnerable to operational disruptions from hurricanes), the existence of a commercial alt-fuel industry will not dampen price volatility caused by fluctuations in supply and demand, at least in the next few decades.

The MIT/RAND authors made a number of recommendations, based on their study:

• Measures designed to lower GHG emissions should be broad and place a price on GHG emissions, allowing economically efficient choices to be made across multiple sectors. Aviation should not be treated differently from other sectors.

• Measures designed to promote alternative-fuel use in aviation should consider the potentially large GHG releases associated with land-use changes required for cultivating crops for producing biomass or renewable oils.

• A standard methodology should be developed for assessing life-cycle GHG inventories and impacts of producing and using aviation fuels that takes into account key inputs in producing the fuels and aviation-specific effects associated with high-altitude emissions of gases other than CO₂.

• To improve air quality, the adoption of a reduced-sulfur standard or a ULS jet fuel should be considered, but economic and climate costs and benefits must be weighed carefully.

• Research and testing should be performed using emission measurements from alternative jet fuels to understand the influence of fuel composition on emissions, enabling more effective assessments of the likely effects of deploying alternative aviation fuels.

• Long-term fundamental research should be supported on the creation of alternative middle-distillate fuels for use in ground transportation and aviation.

Although the near-term prospects for alternative jet fuels are limited, more opportunities may be available in the longer term. Multiple alternative jet fuels, biomass-to-liquids and coal-biomass-to-liquids via FT synthesis, and HRJ fuels from renewable oil sources could conceivably reduce aviation’s impact on both global climate change and air quality. The production potential and cost of these fuels depends on their being certified for use in jet engines; on the development of viable, low-cost feedstocks that do not require the use of land that would otherwise be used for food production; and on competition with other potential uses that may be more or less attractive from energy-efficiency, GHG, and economic perspectives. If these criteria are met, then aviation appears to be a ready market for their use. However, the value of using these resources for aviation should be considered in light of potential benefits of use in other parts of the energy sector. Such a comparative analysis was not performed as part of this study.

—Hileman et al.

The work was performed at the request of the Federal Aviation Administration (FAA). It is the result of research and analysis performed under the Partnership for AiR Transportation Noise and Emission Reduction (PARTNER) at MIT and RAND.

Resources

• James I. Hileman, David S. Ortiz, James T. Bartis, Hsin Min Wong, Pearl E. Donohoo, Malcolm A. Weiss, Ian A. Waitz (2009) Near-Term Feasibility of Alternative Jet Fuels (TR-554-FAA)