Abstract
Aromatic compounds occurring naturally in jet fuels are precursors for the formation of soot in the exhaust gas of jet engines. Directly emitted in cruising altitude, soot particles lead to the formation of contrails and clouds influencing the radiation balance of the atmosphere. Hence, a detailed knowledge on the effect of aromatics on the sooting behavior is of great importance, especially for the development of alternative synthetic jet fuels. Investigations on the sooting propensity influenced by the molecular structure and concentration of diverse aromatic compounds contained in synthetic and fossil aviation fuels as well as blends of synthetic paraffinic kerosene (SPK) with aromatic compounds (SKA) were carried out experimentally. Using a predefined SPK fuel, five different blends—each containing a single aromatic compound—were prepared in addition to one blend with a typical composition consisting of all these aromatic compounds. In subsequent measurements, the concentration of the aromatics was increased from initially 8.0 vol%, to about 16.5, and 25.0 vol%. The aromatics added were toluene, n-propylbenzene, indane, 1methylnaphthalene, and biphenyl. The studied jet fuels include fossil-based Jet A-1 as well as different synthetic jet fuels (with and without aromatics). Furthermore, the experimental results of the sooting propensity are compared with the results of the hydrogen deficiency model being a measure for the amount of cyclic and unsaturated molecular structures occurring in a hydrocarbon fuel. This study shows the hydrogen deficiency as a useful tool to make predictions about the sooting behavior of different fuels compared to a reference fuel at a specified condition. Additionally, it is observed from the measured sooting propensities as well as from the model predictions of hydrogen deficiency that the structure of aromatic compounds presents greater influence on the soot formation than the aromatic concentration.
Highlights
The combustion of a fuel in a jet engine of an aircraft in cruising altitude is connected with the direct emissions of different pollutants such as carbon dioxide (CO2), nitrogen oxide (NOx), sulfur dioxide (SO2), and soot particles in the upper troposphere and lower stratosphere
Whereas the detail on the volumetric concentration of the aromatics is independent of the synthetic paraffinic kerosene (SPK) surrogate, the amount of hydrogen atoms is calculated for each surrogate mixture consisting of SPK surrogate and aromatics
The various surrogate mixtures were prepared to investigate the influence of aromatic structures and concentration on the sooting behavior of fuels
Summary
The combustion of a fuel in a jet engine of an aircraft in cruising altitude is connected with the direct emissions of different pollutants such as carbon dioxide (CO2), nitrogen oxide (NOx), sulfur dioxide (SO2), and soot particles in the upper troposphere and lower stratosphere. Investigations on the sooting propensity depending on the molecular structure and concentration of different aromatic compounds contained in synthetic and. The soot emissions in terms of particle number concentration were measured continuously in the exhaust gas of a premixed flat flame varying the φ-value, i.e., increasing the fuel fraction in the fuel–air mixture. A useful correlation to compare the sooting propensity of different known jet fuels arises from the definition of the ‘Hydrogen Deficiency’ (HD). The hydrogen deficiency does not provide data to quantify the soot emission, but is beforehand a measure for the amount of cyclic and unsaturated molecular structures occurring in a fuel. Though cyclic and especially aromatic (cyclic and unsaturated) components in a jet fuel account for most of the soot formation in contrast to bulk n-, and iso-alkanes, the hydrogen deficiency correlates with the sooting propensity of jet fuels. The experimental results of the sooting propensity will be compared with the predictions according to the hydrogen deficiency correlation
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