Evaluating Ignition Propensity Of High Cycloparaffinic Content Alternative Jet Fuel By A Chemical Functional Group Approach

The molecular components typically found in today's jet fuels that meet ASTM specification are almost entirely alkanes and (mostly alkyl) aromatics with cycloalkyl contents seldom exceeding 40%. Aromatics and cycloalkyl contents are both important for achieving desirable jet fuel energy density, but it is well recognized that significant reductions in aircraft particulate emissions can be achieved by reducing fuel aromatic content (increasing hydrogen content). New jet fuels derived from non-petroleum and sustainable resources composed of much larger fractions of cycloalkanes spanning the entire molecular weight range are a potential path to maintaining high volumetric energy density even in the absence of aromatics. In this paper, the ignition characteristics of individual cycloalkanes of varying ring carbon number, alkyl side chain structure, and their mixtures with other molecular classes (nand iso-alkanes) are investigated through experimentally determining their Derived Cetane Numbers (DCNs). The measurement reveals cycloheptane to have the highest DCN compared to other saturated cycloalkanes, which can be attributed to the conformal steric structure, previously discussed. A chemical functional group approach is extended to analyze chemical structural effects on the resulting DCN data and prior literature results by separately considering the roles of CH2 groups in cyclo ring, (CH2) cyclo, in comparison to CH2, (CH2)(n), CH3, and benzyl-type groups utilized in the earlier work as chemical functional descriptors. A quantitative structure-property relationship (QSPR) regression model is constructed that well emulates the DCN database and is shown to predict a priori the DCN value of a recently proposed jet fuel composed mostly of cycloalkanes (Shell CPK POSF 13690) and a (previously studied) gas-to-liquid alternative jet fuel (Shell SPK POSF 5729). Feature sensitivity analyses show that the relative significance of (CH2) cyclo on determining DCN is substantially less than that derived from the cycloalkane side-chain structural properties. Finally, the ignition and sooting propensities of these two alternative jet fuels are discussed in a function of their blending ratios with petroleum-derived jet fuel. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.