Understanding the impact of cycloalkane additives on the combustion of HEFA jet fuel

Abstract

Drop-in biofuels, such as Hydroprocessed Esters and Fatty Acids (HEFA), are designed to deliver performance comparable to petroleum-based jet fuels without requiring modifications to existing aircraft engines. These biofuels, which are primarily n- and isoalkanes, have been certified by ASTM for use in blends of up to 50% with conventional Jet A to take advantage of the physical properties of cycloalkanes and aromatics. Cycloalkanes and aromatics are integral components of conventional jet fuels, contributing to desirable physical and combustion properties. However, aromatics are both carcinogenic and major precursors to soot formation, prompting the need for safer and more sustainable alternatives. Bio-derived cycloalkanes have emerged as promising aromatic substitutes, offering comparable fuel properties while mitigating environmental and health risks. HEFA fuels provide an ideal platform for investigating how variations in cycloalkane structures (e.g., monosubstituted, polysubstituted, ring size) uniquely influence fuel reactivity at engine relevant conditions. While the physical properties of cycloalkanes blended with Jet A have been reported in the literature, this study examines the impact of cycloalkane additives on the formation of stable intermediates during HEFA pyrolysis. Combustion studies of jet fuels have shown that larger hydrocarbon molecules undergo pyrolysis to form stable intermediates, such as methane, ethylene, and $>$C2 alkenes. As these intermediates govern the oxidation of the fuel, measuring their time histories and yields provides insight into the fuel reactivity at engine relevant conditions and supports the development of combustion models. Shock tube experiments were conducted to study the pyrolysis of HEFA blends with bio-derived cycloalkanes such as 1,4 dimethylcyclooctane, p-menthane, and n-butylcyclohexane. Multiwavelength laser absorption spectroscopy (LAS) was employed to measure the time-resolved evolution of the stable pyrolysis products. All three cycloalkanes have the same carbon number, allowing for a direct comparison of how structural differences influence the formation of pyrolysis products. Blends containing 30% cycloalkanes by volume in HEFA were analyzed in experiments utilizing 1% fuel/argon test mixtures at a nominal pressure of 2 atm over the temperature range of 1150–1450 K. Additionally, ignition delay times were measured for stoichiometric mixtures for HEFA and cycloalkane blends with oxygen at a nominal pressure of 2 atm over a temperature range of 1200–1400 K. These ignition delay times were used to compare the effect of blending on global combustion behavior. These results suggest that the addition of bio-derived cycloalkanes, which improve the energy density of jet fuels, do not negatively impact the combustion performance of HEFA. Hence, the comparative performance against aromatics should ultimately guide the selection of the most suitable cycloalkane additive. Furthermore, the new measurements reported in this study can also serve as valuable targets to develop accurate chemical kinetic models.