In-service commercial aircraft emit substantial amounts of aerosol particles that can degrade local air quality and human health near airports, as well as impact Earth’s climate through direct aerosol radiative absorption, contributing cloud condensation nuclei, or modification of the extent and properties of cirrus clouds high in the troposphere. Understanding these impacts is important for assessing the effects of aviation on air quality and climate. In addition, it is important to understand how future changes in the aircraft fleet translate into changes in emissions. Such fleet changes are likely to be brought about by increasing fuel prices, supply security, environmental footprint, and future regulatory pressures.

… biofuels, along with other alternative fuels synthesized via the Fischer−Tropsch process, are characterized by near-zero levels of sulfur and aromatics, which significantly reduce aircraft engine aerosol emissions. Early engine emissions smoke number measurements implicated fuel aromatics as a driver of soot production, and more recent work with a T63 helicopter engine further suggests the naphthalenic subset of aromatic species may be particularly important. Yet, establishing a quantitative link between these fuel property changes and emissions reductions over the full range of fuel aromatic and sulfur contents remains elusive, in part due to a paucity of quantitative particle emission indices spanning the range of low-sulfur fuel contents (<10−50 ppmm S). Additionally, there is an emerging need to expand the current database of aircraft engine emissions14to include alternative fuels and advanced technology engines.

Emissions sampling of real-world engines and airport surveys are important for filling these knowledge gaps but are challenging due to the need to tease out fuel composition effects from a limited number of fuels whose sulfur and aromatic content often vary together. To unravel this complexity, it is necessary to examine the effects of fuel composition changes on the same type of engine, burning a range of both traditional petroleum-based fuels as well as alternative biobased and synthetic aviation fuels. Such a database already exists from NASA ground tests conducted over the past decade but has only now been synthesized to provide a large-N data set for statistical examination.

—Moore et al.

The team combined results from the first Aircraft Particle Emissions Experiment (APEX) (2004); the first Alternative Aviation Fuel Experiment (AAFEX-I) (2009); and the second Alternative Aviation Fuel Experiment (AAFEX-II) (2011) to provide a large-N data set for statistical examination.

• Three different jet fuel types were examined during APEX: (1) a base JP-8 fuel; (2) a blend of the base JP-8 fuel and a large aliquot of tert-butyldisulfide; and (3) a higher aromatic jet A fuel.

• Five different fuels were tested during AAFEX-I including (1) a base JP-8 fuel; (2) an alternative fuel synthesized from natural gas via the Fischer−Tropsch (FT) middle distillate synthesis process by Shell; (3) an alternative fuel synthesized from coal via the FT process by Sasol Corp.; and (4, 5) 50:50 blends of the two FT fuels with the base JP-8.

• Five different fuels were tested during AAFEX-II including (1) a base JP-8; (2) an alternative fuel synthesized from coal via the FT process by Sasol Corp.; (3) the Sasol FT fuel doped with tetrahydrothiophene to produce a high-sulfur, low-aromatic fuel; (4) a hydrotreated esters and fatty acids (HEFA) biojet fuel produced from Cargill beef tallow feedstock by Honeywell UOP; and (5) a 50:50 blend of the HEFA fuel with a JP-8 fuel.

  The JP-8 fuel in the HEFA blend was not the same as the base JP-8 fuel since the fuel was received pre-blended from another aircraft demonstration. Based on the measured fuel properties shown, the other JP-8 fuel possessed similar aromatic content and substantially higher sulfur content than the base JP-8 fuel.

The CFM56-2 was the first high-bypass engine in the 10-ton class and is the foundation for the rest of the CFM engines in service today. It comprises a single-stage, 44-blade turbofan with an annular combustor design and delivers 98 kN of thrust, a bypass ratio of 6.0, and maximum pressure ratio of 31.3. The sturdy, efficient architecture has allowed CFM to become the most popular engine in the air. It flew first on re-engined Boeing 707 aircraft in 1982.

Fuel chemical and physical properties were determined by independent Air Force Research Lab (AFRL) and commercial testing laboratories using standard testing methods.

The researchers used a multiple linear regression model to explain the observed emissions index response variables in terms of the fuel property, engine, and environmental condition predictor variables.

They found that fuel aromatic and sulfur content most affect the volatile aerosol fraction, which in turn dominates the variability (but not necessarily the magnitude) of the number and volume emissions indices (EIs) over all engine powers. The naphthalenic content of the fuel determines the magnitude of the nonvolatile number and volume EI as well as the black carbon mass EI.

Overall, these simple empirical models consisting of less than four variables capture the measured EI variability to within roughly a factor of 2 (i.e., 0.5 ≲ Φ ≲ 2), with adjusted R² values ranging from 0.67 to 0.86. Given the simplicity of the model, this level of agreement is remarkable and establishes a clear, quantitative link between fuel composition and engine emissions indices that is relevant for large-scale, global aviation models.

Direct use of these regression coefficients in such models should be approached with caution, however, as differences across engine manufacturers and types are not considered in the present work. In particular, it is likely that engine differences would alter the relationship between the EIs and engine fuel flow rate and possibly also the regression intercept, and future work is needed both to establish these dependencies and to confirm that the derived relationships between EIs and fuel composition (e.g., sulfur, aromatics, and naphthalenes) uncovered here also apply to other engine systems.

Using the coefficients … for 30−100% engine powers, the present work shows that reducing current JP-8 fuel sulfur from the PQIS average value of 673 ppmm S to zero would reduce the total aerosol number EI by roughly a factor of 4 for the CFM56-2-C1 engines. Reducing the fuel naphthalenes content from 1.27 vol % to zero would produce a similar decrease in the black carbon mass EI.

—Moore et al.

Resources

• Richard H. Moore, Michael Shook, Andreas Beyersdorf, Chelsea Corr, Scott Herndon, W. Berk Knighton, Richard Miake-Lye, K. Lee Thornhill, Edward L. Winstead, Zhenhong Yu, Luke D. Ziemba, and Bruce E. Anderson (2015) “Influence of Jet Fuel Composition on Aircraft Engine Emissions: A Synthesis of Aerosol Emissions Data from the NASA APEX, AAFEX, and ACCESS Missions” Energy & Fuels doi: 10.1021/ef502618w