Thermal Stability of Jet Fuels: The
Kinetics of Forming Deposit Precursors, 03-9020
Printer Friendly Version
David W. Naegeli
Clifford A. Moses
Inclusive Dates: 04/01/97 - 04/01/99
Background - The thermal stability of fuels is
of concern in the maintenance of all types of combustion engines. Thermal stability is
measured in terms of the fuels tendency to form deposits on heat exchangers and
nozzles of jet engines as well as on intake valves, fuel injectors, and combustion chamber
surfaces in gasoline and diesel engines. Several fuel additives have been marketed over
the years to alleviate fuel-deposit problems in engines. Despite extensive research geared
to developing better fuel additives, the mechanism of deposit formation remains an enigma.
An elucidation of the problem could provide a more direct and confident approach to
additive development and fuel formulation.
It is well known that thermal deposits originate from the
autoxidation of fuels. However, beyond that generalization, the mechanism is speculative.
Not long ago, it was assumed that fuels would form deposits rapidly if they autoxidized
fast. Since then, it has been realized that fuels that autoxidize rapidly have a much
lower tendency to form deposits than fuels that show great resistance to autoxidation.
Literature surveys show some variation in the rates at which hydrocarbons autoxidize, for
example, the rate of hydro peroxide formation in hydrocarbons varies as benzylic >
allylic > tertiary > secondary > primary. However, there is no evidence
indicating that some hydrocarbons are vastly more resistant to autoxidation than others.
For example, the difference in the rates of oxidation of alkyl benzenes and normal
paraffins is not that great. Therefore, it has been concluded that naturally occurring
oxidation inhibitors are responsible for the exceptionally slow oxidation of some fossil
Approach - The present investigation shows
that two oxidation-inhibiting mechanisms are in fossil fuels. The first is well known, and
is caused by naturally occurring antioxidants that forestall the oxidation process until
the antioxidant is depleted. The second type of inhibition is caused by antioxidants
created by the oxidation process. These antioxidants reduce the rate of oxidation. This
second form of inhibition seems to be observed only in fuels that contain aromatics and
sulfur. The purpose of this investigation was to understand better the mechanism of
formation of the peroxides, gums, and deposits that form when jet fuels are thermally
stressed. The goal was to create a clearer understanding of the mechanism, which would
then open up new avenues for the development of fuel additives that would inhibit or
prevent the formation of deposits. Test fuels consisting of Jet A, dodecane, and a blend
of dodecane and 15 percent cumene were doped with sulfur compounds and stressed in an
isothermal reactor at 120 to 180°C. Fuel samples were sealed in glass ampoules with an
oxygen-filled headspace. Deposit precursor was determined by laser-induced fluorescence,
hydroperoxides were measured spectrophotometrically, and oxygen consumption was determined
by measuring the pressure change in the ampoule.
Accomplishments - This study shows that
specific sulfur compounds, namely thiols and disulfides, play an important role in the
formation of jet fuel deposits. The results indicate that thiols and disulfides oxidize
faster than other fuel components, including other sulfur compounds. It is proposed that
thiols and disulfides oxidize to sulfones, which liberate sulfur dioxide, SO2,
as they decompose. SO2 reacts with fuel oxidation products to make
gums and deposits. SO2 also oxidizes to SO3
and combines with water to form sulfuric acid. The acid acts as a catalyst that causes
aromatic hydro peroxides to decompose into phenols. The phenols act as antioxidants, which
is the reason fossil fuels are so much more resistant to oxidation than pure hydrocarbons.
A possible explanation of the anomalous relationship between
deposit formation and the oxidation rate is that phenols act not only as oxidation
inhibitors, but they are also involved in the deposit-forming process. Other studies have
shown that phenols increase the formation of gum in thermally stressed fuels.
It was found that adding an organic base such as tridodecyl
amine prevented the formation of phenols. The base neutralizes the acid produced from
sulfur in the fuel, so the aromatic hydro peroxides decompose into oxygen-containing
compounds other than phenols. If phenols are essential to the formation of
high-temperature deposits, the addition of organic bases may improve the thermal stability
of the fuel.
Lubricants, and Vehicle Systems Program
1999 IR&D Home