Reductions in aircraft particulate emissions due to the use of Fischer–Tropsch fuels 1NASA Langley Research Center, Hampton, Virginia, USA
02 Jan 2014
2Aerodyne Research, Inc., Billerica, Massachusetts, USA
3NASA Glenn Research Center, Cleveland, Ohio, USA
4Air Force Research Laboratory, Wright Patterson AFB, Ohio, USA
5Arnold Engineering Development Center, Arnold AFB, Tennessee, USA
6Science Systems and Applications, Inc., Hampton, Virginia, USA
*now at: Worcester Polytechnic Institute, Worcester, Massachusetts, USA
Received: 24 April 2013 – Published in Atmos. Chem. Phys. Discuss.: 10 June 2013 Abstract. The use of alternative fuels for aviation is likely to increase due to
concerns over fuel security, price stability, and the sustainability of fuel
sources. Concurrent reductions in particulate emissions from these
alternative fuels are expected because of changes in fuel composition
including reduced sulfur and aromatic content. The NASA Alternative Aviation
Fuel Experiment (AAFEX) was conducted in January–February 2009 to
investigate the effects of synthetic fuels on gas-phase and particulate
emissions. Standard petroleum JP-8 fuel, pure synthetic fuels produced from
natural gas and coal feedstocks using the Fischer–Tropsch (FT) process, and
50% blends of both fuels were tested in the CFM-56 engines on a DC-8
aircraft. To examine plume chemistry and particle evolution with time,
samples were drawn from inlet probes positioned 1, 30, and 145 m downstream
of the aircraft engines. No significant alteration to engine performance was
measured when burning the alternative fuels. However, leaks in the aircraft
fuel system were detected when operated with the pure FT fuels as a result
of the absence of aromatic compounds in the fuel.
Revised: 21 November 2013 – Accepted: 26 November 2013 – Published: 02 January 2014
Dramatic reductions in soot emissions were measured for both the pure FT
fuels (reductions in mass of 86% averaged over all powers) and blended
fuels (66%) relative to the JP-8 baseline with the largest reductions at
idle conditions. At 7% power, this corresponds to a reduction from 7.6 mg kg−1
for JP-8 to 1.2 mg kg−1 for the natural gas FT fuel. At full
power, soot emissions were reduced from 103 to 24 mg kg−1
(JP-8 and natural gas FT, respectively). The alternative fuels also produced
smaller soot (e.g., at 85% power, volume mean diameters were reduced from
78 nm for JP-8 to 51 nm for the natural gas FT fuel), which may reduce their
ability to act as cloud condensation nuclei (CCN). The reductions in
particulate emissions are expected for all alternative fuels with similar
reductions in fuel sulfur and aromatic content regardless of the feedstock.
As the plume cools downwind of the engine, nucleation-mode aerosols form.
For the pure FT fuels, reductions (94% averaged over all powers) in
downwind particle number emissions were similar to those measured at the
exhaust plane (84%). However, the blended fuels had less of a reduction
(reductions of 30–44%) than initially measured (64%). The likely
explanation is that the reduced soot emissions in the blended fuel exhaust
plume results in promotion of new particle formation microphysics, rather
than coating on pre-existing soot particles, which is dominant in the JP-8
exhaust plume. Downwind particle volume emissions were reduced for both the
pure (79 and 86% reductions) and blended FT fuels (36 and 46%) due to
the large reductions in soot emissions. In addition, the alternative fuels
had reduced particulate sulfate production (near zero for FT fuels) due to
decreased fuel sulfur content.
To study the formation of volatile aerosols (defined as any aerosol formed as
the plume ages) in more detail, tests were performed at varying ambient
temperatures (−4 to 20 °C). At idle, particle number and volume
emissions were reduced linearly with increasing ambient temperature, with
best fit slopes corresponding to −8 × 1014 particles
(kg fuel)−1 °C−1 for particle number emissions and
−10 mm3 (kg fuel)−1 °C−1 for particle volume
emissions. The temperature dependency of aerosol formation can have large
effects on local air quality surrounding airports in cold regions.
Aircraft-produced aerosols in these regions will be much larger than levels
expected based solely on measurements made directly at the engine exit plane.
The majority (90% at idle) of the volatile aerosol mass formed as
nucleation-mode aerosols, with a smaller fraction as a soot coating.
Conversion efficiencies of up to 2.8% were measured for the partitioning
of gas-phase precursors (unburned hydrocarbons and SO2) to form volatile
aerosols. Highest conversion efficiencies were measured at 45% power.
Citation: Beyersdorf, A. J., Timko, M. T., Ziemba, L. D., Bulzan, D., Corporan, E., Herndon, S. C., Howard, R., Miake-Lye, R., Thornhill, K. L., Winstead, E., Wey, C., Yu, Z., and Anderson, B. E.: Reductions in aircraft particulate emissions due to the use of Fischer–Tropsch fuels, Atmos. Chem. Phys., 14, 11-23, doi:10.5194/acp-14-11-2014, 2014.