With the characterization of alternative fuels in terms of their chemical composition and thermophysical properties, the Chemical Analytics department is creating a solid foundation for investigating the influence of fuel composition on combustion and emission behavior. The use of alternative liquid fuels in particular represents an analytical challenge since the compounds and compositions often deviate significantly from conventional fuels such as Jet A-1.
For the chemical analysis, advanced two-dimensional GCxGC mass spectrometry is at disposal, which allows the separation of complex fuel matrices due to the modulated series connection of two chromatography columns (polar-non-polar). This analytical method is supported by 1H nuclear magnetic resonance (NMR) spectroscopy, which allows instant determination of the hydrogen content of fuels. Thus, fuel samples can be qualified and quantified with molecular precision and mass fractions of the individual chemical groups (e.g., n-/iso-/cyclo-alkanes, aromatics, oxygenated components), the average molecular formula, and the C/H ratio can be determined.
In addition, thermophysical aviation-related "fit for purpose" parameters of the fuels can be determined. These include for example distillation properties (e.g. initial boiling point, destillation curve, final boiling point), kinematic viscosity and density (including low temperature behavior up to to -47 ° C), surface tension, refraction index and heating value (Qnet and Qgross) of the considered fuels.
Thus, the analytical methods for aviation fuel go well beyond the required ASTM criteria (e.g. cryogenic measurements) and provide individualized validation data for model-based optimization techniques. The accurate characterization of a fuel also provides the basis for the scientific interpretation of its combustion and emission behavior, as well as for the selection of suitable models for the prediction of the reaction kinetics of combustion and pollutant formation.
The applied methods are part of the DLR-VT developed DLR Jet-Fuels Preselection Process®. This process bases on the evaluation of possible aviation fuels through lab-scale experiments, model-based estimations and assessments, as well as comparisons with the broad DLR fuel database. This evaluation allows quick feedback to manufacturers regarding the suitability and/or optimization potential of their fuels as aviation fuels before a time- and cost-intensive approval process is sought.
The DLR Fuels Preselection Process® is currently applied in phase II of the Kopernikus project "Power-to-X" for example, in which the application potential of Fischer Tropsch products (amongst other) in aviation is assessed.