Carbon deposition from jet fuel on metal surfaces will create problems for the operation of future aircraft. Two jet fuel samples (Jet A and JP-8) were heated in a glass-lined flow reactor in the presence of metal and nonmetal substrates placed in the fuel path. The solid deposits collected on the substrates were examined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and by temperature-programmed oxidation (TPO). The nature and amount of carbonaceous deposits from the thermal decomposition of jet fuel were determined to be dependent on the substrate properties and jet fuel composition. In particular, the catalysis of carbon deposition by active metals was evident in deposits obtained on singlemetal or metal-alloy substrates. Jet A fuel produced much-smaller quantities of carbonaceous solids on active metal substrates than JP-8 fuel did. This variance is attributed to the differences in hydrocarbon and sulfur compound composition of the two fuels.

The deposits formed after short durations of pyrolytic degradation consist of carbonaceous solids growing on metal sulfide particles. Carbonaceous solids contain amorphous films and uniformly sized spheroids. Close association of the carbonaceous film with the sulfide particles suggests that it was produced by a heterogeneous process similar to chemical vapor deposition (CVD), while the morphology of the spheroidal deposits suggests that they were formed by homogeneous nucleation and growth in the fluid phase. Thermal stressing on an alumina-coated SS316 surface and reducing the sulfur content of the jet fuel from 0.10 to 0.01 wt % inhibited metal sulfide formation on the surface. This consequently inhibited the growth of film deposits but not the nature or amount of fluid-phase deposits. These results have shown that the sulfur content of jet fuel and the substrate composition control the heterogeneous carbon deposition. These parameters do not affect the nucleation and growth of the fluid-phase deposits.

Solid deposits from commercially available high-pressure diesel injectors (HPDI) were analyzed to study the solid deposition from diesel fuel during engine operation. The structural and chemical properties of injector deposits were compared to those formed from the thermal oxidative stressing of a diesel fuel range model compound, n-hexadecane at 160°C and 450 psi for 2.5 h in a flow reactor. Both deposits consist of polyaromatic compounds (PAH) with oxygen moieties. The similarities in structure and composition of the injector deposits and n-hexadecane deposits suggest that laboratory experiments can simulate thermal oxidative degradation of diesel in commercial injectors. The formation of PAH from n-hexadecane showed that aromatization of straight chain alkanes and polycondensation of aromatic rings was possible at temperatures as low as 160°C in the presence of oxygen. A mechanism for an oxygen-assisted aromatization of cylcoalkanes is proposed.

Temperature-programmed oxidation (TPO) is used to determine the oxidation reactivity of carbonaceous deposits formed on different substrates via the thermal stressing of jet fuel samples. The multiple CO 2 peaks in TPO profiles are attributed to differences in the oxidation reactivity of the deposits, which is related to their structural characteristics. This study investigates whether the TPO profiles relate to the characteristics of the deposits formed during thermal stressing, or if they result from chemical alterations of the original deposits during the TPO analysis. This question is important for understanding the substrate effects on carbon deposition from heated fuels, as well as the removal of carbonaceous deposits from the aircraft fuel system components. Findings from this study affirm that the results from TPO experiments can be used to characterize the oxidation reactivity of carbonaceous deposits, relative to their molecular and structural characteristics.

Reaction of the organic sulfur compounds in Jet A with Fe-and Ni-based alloy substrates under pyrolytic conditions formed significant amounts of metal sulfides. Pyrrhotite (Fe (1-x) S) and heazlewoodite (Ni 3 S 2 ) were formed on SS316 and Inconel 600 surfaces, respectively, in the short duration experiments. After extended periods of thermal stressing, an additional crystal phase, pentlandite (Fe,Ni) 9 S 8 , was also observed on both surfaces. The lack of FeS 2 (pyrite) formation over extended periods of stressing indicates that the amount of sulfur reacting with the substrates decreased with the increasing thermal stressing time. A focused ion beam (FIB)/SEM analysis showed that the metal sulfide formation can extend up to 2 µm depth from the surface in 2 h of thermal stressing. The formation of metal sulfides on alloy surfaces degrades the alloy surfaces and affects solid carbon deposition from jet fuel.