Ind. Eng. Chem. Res. 2008, 47, 9337–9350
9337
Characterization of Solid Deposits Formed from Short Durations of Jet Fuel Degradation: Carbonaceous Solids Ramya Venkataraman† and Semih Eser* The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
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. Introduction Several researchers have characterized the carbonaceous solids formed from heated jet fuel on metal and nonmetal surfaces.1-6 Several types of deposits have been identified for a given substrate-fuel combination.7-10 The chemical and structural properties of the deposits were shown to be dependent on the nature of the substrate and fuel composition. From a thermodynamic and kinetic point of view, the carbonaceous deposits formed by catalytic mechanisms (filamentous deposits and deposits with pregraphitic order) are believed to grow much faster than the (amorphous) deposits formed via thermal pathways.7,11-14 The heteroatom species, especially the sulfur compounds present in the hydrocarbon fuels, have been shown to affect deposit formation.15-17 In past studies, significant amounts of metal sulfides on the substrates in addition to the carbonaceous solids have also been observed due to the interaction of the organic sulfur compounds with the metals at high temperatures.18,19 The detailed characterization of the metal sulfides formed during jet fuel degradation under pyrolytic conditions is discussed in the accompanying Article. In this study, the carbonaceous deposits formed under an inert atmosphere from short duration thermal stressing experiments have been characterized. This was done in an attempt to understand the formation mechanism of early stage deposits. Knowing the chemical and morphological nature of deposits formed will also aid in developing solutions to inhibit their formation. The use of metal oxide coatings on fuel and oil system components has already been identified as an effective way to inhibit carbonaceous deposit formation from hydrocarbon degradation.20 Characterization of the solid deposits formed on such inert surfaces was also done in this study to assess the effectiveness of selected metal-oxide coatings in reducing solid deposit formation. To study the role of sulfur compounds in deposit formation, an activated carbon adsorbent was used to reduce the sulfur content of the jet fuel sample prior to short duration thermal * To whom correspondence should be addressed. Tel.: (814) 8631392. E-mail:
[email protected]. † Current address: Sinfin-A, Rolls-Royce plc, Derby DE24 8BJ, U.K.
stressing of low-sulfur sample under the same conditions as those used for the as-received jet fuel. Experimental Section Substrates and Fuel Samples. Fe, Ni, and their alloys are the commonly used metals in the construction of jet engine components. For this purpose, SS316, an Fe-rich alloy, and Inconel 600, an Ni-based superalloy, were chosen as the active metal substrates for this study. Alumina-coated SS316 substrates were used as the inert substrates in this study. The similarities and differences in the nature of solids formed on the different types of surfaces have been discussed. SS316 (composition (wt %): Fe 64, Ni 14, Cr 16, C 0.08, Mn 2, P 0.045, S 0.030, Si 1, Cu 2) and Inconel 600 foils (composition (wt %): Ni 74.4, Fe 8.0, Cr 15.5, Cu 0.5, Mn 1.0, Si 0.5, C 0.15, S 0.0015) with dimensions of 13 cm × 0.3 cm × 0.025 cm were placed in a 1/4 in. (OD), 20 cm long, glass-lined, stainless steel tube reactor inserted in a vertical block heater. The fuel used in the study was commercial aviation Jet A with a sulfur content of 0.1 wt %, which is much higher than the sulfur levels in jet fuel samples used in previous studies,21,22 although it lies within the sulfur level specifications for commercial aviation fuel (
