The preignition and autoignition oxidation of alternatives to petroleum derived JP-8 and their surrogate components in a pressurized flow reactor and single cylinder research engine
Authors:   , Kurman Matthew S. , Miller David Leslie , Cernansky Nicholas Peter 
Published: July 2021

The U.S. Department of Defense (DoD) Directive 4140.13 has required the use of JP-8 specification fuels whenever possible. The specifications for JP-8 allow a broad range of thermophysical characteristics, such as chemical composition, distillation characteristics, and heat of combustion, which can be produced from multiple sources and processes. For applications in military power systems operating with compression ignition (CI) engines and some gas turbines, it is necessary to study the effects of fuel source on the preignition and autoignition reactivity that occurs in the 600-1000 K temperature range of the resulting jet fuels and to develop reactivity surrogates for their behavior. The use of alternative sources for JP-8, rather than petroleum sources, has these advantages: improved U.S. energy security, a wider range of feedstocks, and decreased emissions of SO_x and particulate matter. To study the effect of fuel source, the oxidation of three jet fuels from different sources (petroleum, natural gas, and coal) was examined. Preignition experiments were conducted in a pressurized flow reactor (PFR) under lean, dilute conditions at temperatures of 600-800 K and 8 atm pressure. Autoignition experiments were conducted in a single cylinder research engine. Results were compared to a typical sample of petroleum-derived jet fuel. Petroleum, natural gas, and coal derived fuels contain hundreds of components, many with unknown reactivity, and even if known, proper kinetic simulations require computational resources beyond current abilities. To overcome these limitations, the use of surrogates, mixtures of approximately 1-10 components that mimic the properties and behaviors of the real fuel, has been recognized as a feasible approach to chemical kinetic model building. Since the chemical composition varies widely between petroleum and alternative jet fuels, new surrogates need to be developed for alternative fuels as well. Samples of the jet fuels derived from coal, natural gas, and petroleum were oxidized in two complementary experimental facilities to explore their preignition and autoignition behavior. In both facilities, the order of reactivity, based on carbon monoxide production, in descending order was Fischer-Tropsch, petroleum, and coal. The reactivity differences are attributed to composition differences. Results showed that all the fuels exhibit negative temperature coefficient behavior as expected, but Fischer-Tropsch jet fuel produces significantly more carbon monoxide than petroleum and coal derived jet fuel before entering the negative temperature coefficient region. Possible surrogates and their components were also tested in the facilities to elucidate how compositional differences affect preignition and autoignition chemistry. A mixture of n-decane/iso-octane was studied as a surrogate fuel for Fischer-Tropsch jet fuel. The mixture was then tuned to approximate the low and intermediate temperature reactivity of Fischer-Tropsch jet fuel. Fischer-Tropsch jet fuel and the mixture showed similar reactivity based on measurements of carbon monoxide. To further explore the chemistry and impact of the n-decane component of the surrogate for Fischer-Tropsch jet fuel and the decalin component of coal derived jet fuel, PFR experiments were conducted to identify intermediate species profiles using gas chromatography with flame ionization detection and gas chromatography coupled to a mass spectrometer. The major intermediate species that were identified with n-decane oxidation included components of the functional groups aldehydes and alkenes. Cyclic species were the major species observed with decalin oxidation. The research presented will aid in the overall development and use of chemical kinetic models that will be employed for simulating combustion characteristics of gas turbines and CI engines while improving such traits as fuel efficiency, emissions, and power output.