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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

A Study on the Emissions of Alternative Aviation Fuels

[+] Author and Article Information
Sebastian Riebl, Uwe Riedel

German Aerospace Center (DLR),
Institute of Combustion Technology,
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany

Marina Braun-Unkhoff

German Aerospace Center (DLR),
Institute of Combustion Technology,
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany
e-mail: Marina.Braun-Unkhoff@dlr.de

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 2, 2016; final manuscript received December 30, 2016; published online March 21, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(8), 081503 (Mar 21, 2017) (11 pages) Paper No: GTP-16-1519; doi: 10.1115/1.4035816 History: Received November 02, 2016; Revised December 30, 2016

Currently, the aviation sector is seeking for alternatives to kerosene from crude oil, as part of the efforts combating climate change by reducing greenhouse gas (GHG) emissions, in particular carbon dioxide (CO2), and ensuring security of supply at affordable prices. Several synthetic jet fuels have been developed including sustainable biokerosene, a low-carbon fuel. Over the last years, the technical feasibility as well as the compatibility of alternative jet fuels with today's planes has been proven However, when burning a jet fuel, the exhaust gases are a mixture of many species, going beyond CO2 and water (H2O) emissions, with nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC) including aromatic species and further precursors of particles and soot among them. These emissions have an impact on the local air quality as well as on the climate (particles, soot, contrails). Therefore, a detailed knowledge and understanding of the emission patterns when burning synthetic aviation fuels are inevitable. In the present paper, these issues are addressed by studying numerically the combustion of four synthetic jet fuels (Fischer–Tropsch fuels). For reference, two types of crude-oil-based kerosene (Jet A-1 and Jet A) are considered, too. Plug flow calculations were performed by using a detailed chemical-kinetic model validated previously. The composition of the multicomponent jet fuels was imaged by using the surrogate approach. Calculations were done for relevant temperatures, pressures, residence times, and fuel equivalence ratios φ. Results are discussed for NOx, CO as well as for benzene and acetylene as major soot precursors. According to the predictions, the NOx and CO emissions are within about ±10% for all fuels considered, within the parameter range studied: T = 1800 K, T = 2200 K; 0.25 ≤ φ ≤ 1.8; p = 40 bar; t = 3 ms. The aromatics free GtL (gas to liquid) fuel displayed higher NOx values compared to Jet A-1/A. In addition, synthetic fuels show slightly lower (better) CO emission data than Jet A-1/A. The antagonist role of CO and NOx is apparent. Major differences were predicted for benzene emissions, depending strongly on the aromatics content in the specific fuel, with lower levels predicted for the synthetic aviation fuels. Acetylene levels show a similar, but less pronounced, effect.

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Figures

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Fig. 1

Emission characteristics in a gas turbine, principle power dependency [38]

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Fig. 2

CO emissions: Calculated for four alternative aviation fuels as well as for Jet A-1 and Jet A, at T = 1800 K and at T = 2200 K for p = 40 bar. CO emissions calculated for the GtL fuels are the lowest.

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Fig. 3

CO emission indices, as derived from CO emissions calculated of the six fuels. Note the similarity of the emission levels between the six fuels for all conditions considered.

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Fig. 4

NOx emissions: Simulated at T = 1800 K and at T = 2200 K. Curves follow the depicted trends monotonically.

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Fig. 5

NOx emission indices as derived from NOx emissions calculated of the six fuels, (a) T = 1800 K, 0.25 ≤ φ ≤ 1.4, (b) T = 1800 K, 1.5 ≤ φ ≤ 1.8, (c) T = 2200 K, 0.25 ≤ φ ≤ 1.4, and (d) T = 2200 K, 1.4 ≤ φ ≤ 1.8

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Fig. 6

Benzene emissions simulated at T = 1800 K ((a)(b)) and at T = 2200 K ((c)–(d)). Only minor amounts of benzene remain at t = 0.003 s. For the parameter considered, Jet A-1 shows the highest peak emissions, whereas the GtL fuels show the lowest peak emission values.

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Fig. 7

Acetylene emissions simulated at T = 1800 K ((a)–(b)) and at T = 2200 K ((c)–(d)). Only minor amounts of acetylene remain for fuel-lean mixtures, with significant acetylene levels for fuel rich mixtures.

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