Pyrolysis and Oxidation studies of Real Jet Fuels.

The aim of this study is to provide experimentally determined concentrations of intermediate chemical products of the pyrolysis and oxidation of currently used military jet fuels as a function of temperature and pressure. These values are expected to serve as inputs into emerging models for fuel combustion that have taken advantage of the observation that within a given combustion reaction, there is a separation in time, temperature, and position within a combustion chamber between the pyrolysis process pathways—which can be lumped together for kinetics modeling—and the onset of oxidation. This two‐stage or hybrid approach allows the modeling of both pyrolysis and oxidation to be simplified, which can then lead to greater applicability of these predictive models for actual fuels. Fuels to be considered include JP‐8 (Categories A‐1, A‐2, A‐3), Jet A, Jet A‐1, JP‐7, RP‐2, and JP‐10, available from the Air Force Research Laboratory. Species from the pyrolysis reactions to be detected, identified, and quantified will include the stable intermediates methane (CH4), ethylene (C2H4), propene (C3H6), iso‐ butene (i‐C4H8), 1‐butene (1‐C4H8) and the secondary species acetylene (C2H2), ethane (C2H6), C5, C6, and C7 alkenes, benzene (C6H6), toluene (C7H8), and xylene isomers (C8H10). The chemical kinetics of real military fuels will be studied in the context of the hybrid model of lumped pyrolysis and detailed (foundational) oxidation chemistry over a wider range of propulsion fuels, pressures, and intermediate species than in previous studies. Such new studies are made possible by the well‐established and proven UIC Single Pulse Shock Tubes. Experiments will be conducted over the pressure range of 15‐100 bar, a temperature range of 1100‐1400K, and a nominal reaction time of 2 milliseconds.

A New Approach to the Combustion Chemistry of Low Cetane Number Jet Fuels
The Army’s “Single Fuel Forward” initiative mandates that when logistically necessary, diesel engines must be able to use jet fuel, JP-8, instead of standard diesel fuel. However, JP-8 from conventional feedstocks is not the only type of “JP-8” that the Army may have to accommodate. Up to 50% of a fuel meeting JP-8 specifications may be composed of synthetic alternatives such as Sasol IPK, Shell SPK, Gevo ATJ, and Synthroleum R-8. While such fuels generally have acceptable derived cetane numbers (DCN) to prevent diesel engine operating problems or damage, two of them represent a class of low cetane number JP-8 alternatives that could cause problems: Sasol IPK (DCN 31) and Gevo ATJ (DCN 16). Because conventionally sourced JP-8 itself does not have strictly controlled cetane specifications, a lower DCN JP-8 might be mixed in the field with these low cetane number JP-8 alternatives, compounding the problem.

The work will produce both experimental data and models. The data will be presented as mole fractions or normalized mole fractions plotted with respect to temperature at a given pressure, nominal reaction time, and equivalence ratio, and will have the following features:

a) multiple stable species identified and quantified, including low-temperature chemistry oxygenates revealed by GCxGC analysis and high-temperature reaction products.
b) oxidation (equivalence ratios 0.8-3.0) conditions over the time range of 1-20 msec.
c) pressure range 50-300 bar as needed for a temperature range of 700-1000K.
d) fuels: heptane, isooctane, JP-8 (CN = 45), Sasol IPK, Gevo ATJ and their 50/50 mixtures with JP-8.

Chemical Identifiers of Army Relevant Fuels

Traditional diesel and gasoline fuels used in conventional compression and spark ignition engines can be viewed as chemically opposite fuels. Diesel fuels contain significant amounts of straight-chain hydrocarbon molecules that thermally break down easily and oxidize quickly at low to intermediate temperatures. Gasoline fuels have significant percentages of highly branched saturated hydrocarbon chains that thermally (and oxidatively) break down to branched intermediates that resist subsequent oxidation until temperatures are high. The octane number, associated with gasoline fuels, is often seen as the opposite of the cetane number associated with diesel fuels. It has been suggested that one chemical reason why fuels with high octane numbers have low cetane numbers is that these fuels, because of their structure, produce intermediates that scavenge the radicals that promote combustion at the lower temperature conditions of diesel engines. If the fuel is a highly branched, methyl-substituted fuel, the hypothesized scavenging intermediate is 2-methyl-1-propene (iso-butylene). If the high octane number/ low cetane number fuel contains a high percentage of aromatics, then the hypothesized scavenging intermediate is benzyl radical. These two species, iso-butylene and benzyl radical, are hypothesized to be fuel-related chemical markers associated with the lower cetane number fuels of interest.

The Army test fuels of varying cetane numbers: 30, 35, 40, 45, and 50 will be examined in the high-pressure single pulse shock tube at UIC at conditions spanning pyrolysis to fuel lean oxidation, pressures ranging from 25-100 bar, temperatures ranging from 700 to 1400K and reaction times from 2 – 10 msec. Over this range of conditions, the appearance of isobutylene and di-benzyl (the characteristic species indicating benzyl radical formation) as well as all the other stable pyrolysis and oxidation products will be detected and quantified by GC-MS

High-Pressure Ring Contraction of Cyclic Hydrocarbons

The behaviors of cyclic hydrocarbons in the supercritical fluid phase and gas phase at high pressures and temperatures are of interest because their chemical breakdowns will affect their performance in future combustion engines. These fuels undergo increasing amounts of ring contraction under these conditions instead of fragmentation into smaller molecules leading to increased soot and coke
formation. Ring contraction has been attributed to molecular caging effects on the fuel by the solvent, but more plausible hypotheses for ring-contracted products can also be explored. We propose to examine these hypotheses directly through pyrolysis studies of selected radicals in a high-pressure shock tube,
where the use of an argon bath will rule out any additional solvent effects.
The goal of the proposed work is to test the following two hypotheses: (1) Certain reaction rate coefficient parameters in the literature may be poorly characterized. When accurately determined, these parameters will indicate that the increase with pressure in the rate coefficient value found from the shape and position of the fall-off curves for unimolecular ring contraction is relatively greater than the increase
in the rate coefficient obtained from the shape and position of the fall-off curve for the corresponding unimolecular beta scission reaction. This rate coefficient difference contributes significantly to the increased appearance of ring contraction products at high molecular densities. (2) The negative activation volume of ring contraction reactions leads to an additional, but smaller, increase in their rate coefficients with the pressure that is also partly responsible for the increased appearance of ring contraction products at high molecular densities. The critical radical structure that leads to either contraction or scission is well represented in the 5-hexen-1-yl and 2-ethyl styrene radicals. The proposed work consists of shock tube pyrolysis studies of dilute quantities of these radicals, and of the stable cyclic molecules cyclohexane, methylcyclohexane, decalin, and tetralin at high temperatures, 1000-1300K, pressures from atmospheric to 1000 bar, in argon,
with hydrocarbon pyrolysis product species sampling to identify fuel fragment species. Studies will be performed with the high-pressure single pulse shock tube at the University of Illinois at Chicago (UIC) up to1000 bar and in the low-pressure single pulse shock tube below 15 bar