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Ind. Eng. Chem. Res. 2000, 39, 642-645
MATERIALS AND INTERFACES Characterization of Carbon Deposits from Jet Fuel on Inconel 600 and Inconel X Surfaces Orhan Altin† and Semih Eser*,†,‡ Laboratory for Hydrocarbon Process Chemistry, The Energy Institute, 209 Academic Projects Building, and Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
Flow reactor experiments were conducted to study carbon deposit formation from decomposition of a jet fuel (JP-8) at 500 °C and 500 psig for 5 h on the surface of two superalloys, Inconel 600 and Inconel X. The deposits collected on superalloy surfaces were characterized by temperatureprogrammed oxidation, size exclusion microscopy, and energy-dispersive X-ray spectroscopy. Significantly lower deposition on Inconel X compared to that on Inconel 600 was attributed to the presence of minor elemental components, such as Al, Ti, Nb, and Ta in the Inconel X alloy. Introduction
Table 1. Chemical Composition of JP-8 Fuela
The formation of carbon deposits on metallic surfaces from decomposition of jet fuel is a major concern in the development of advanced aircraft in which the fuel is also used as a heat sink.1 It is expected that the fuel would be exposed to temperatures as high as 480 °C (900 °F) and 68 atm (1000 psi).2 A similar problem occurs in the production of olefins during naphtha cracking at higher temperatures (800-1000 °C) but lower pressures (1-4 atm).3 Metal surface composition can strongly affect solid deposition from thermally stressed jet fuel, or model hydrocarbons.4-6 Silica-based surfaces, such as Silcosteel, glass-lined steel, and quartz do not appear to catalyze carbonaceous solid formation during thermal stressing of jet fuel at 500 °C.7 The use of such materials as inert coatings on metal surfaces may, however, suffer from typical limitations of coatings because of the differences in thermal properties of the coating layer and the coated metals. The formation of solid deposition on metal surfaces has been extensively studied in the last three decades.8-12 The solid formation rate and the structure of deposits strongly depend on the composition of the metal surface. Nickel and nickel-containing alloys form solid deposits because of the catalytic activity of nickel in dehydrogenation reactions. Heat-resistant Ni-Fe-Cr-containing alloys are used in many high-temperature applications. Superalloys are based on group VIIIA base elements produced for high-temperature applications demonstrating mechanical strength and surface stability. They have been developed as corrosion-resistant materials for aircraft parts and nuclear reactors.13 Surfaces containing titanium, aluminum, niobium, tantalum, and molybdenum are reported to suppress the solid deposition.5,14,15
C7-C18 C6-C14 C6-C14 C10-C14 C10-C14 C9-C12 alkanes cyclohexanes aromatics decalins tetralins indans
* To whom correspondence should be addressed. Telephone: 814-863-1392. Fax: 814-865-3573. E-mail: sxe2@ psu.edu. † The Energy Institute. ‡ The Pennsylvania State University.
61.3 a
13.5
15.5
0.9
0.4
1.4
In weight percent.
Table 2. Composition of Metal Foilsa,b Ni Fe Cr
Al Ti Mn Cu Nb + Ta
Inconel 600 72 8 15.5 1.0 5000 Inconel X 73 7 15.5 0.7 2.5 0.5 2500
9500
C
Si
S
1500 5000 150 400 2500 50
a
From Goodfellow Co. and expressed in weight percent. b Cu, Nb, Ta, C, Si, and S in ppm.
The objectives of the present study are to evaluate the surface behavior of Inconel 600 and Inconel X superalloys under thermal stressing conditions relevant to the operation of advanced aircraft and to characterize the carbon deposits from thermal decomposition of a JP-8 fuel in a flow reactor. Experimental Section Thermal stressing of a JP-8 fuel was carried out in the presence of Inconel 600 and Inconel X foils. The chemical composition of JP-8 fuel is given in Table 1. Inconel 600 and Inconel X alloys compositions are almost the same when major components such as Ni, Fe, and Cr are compared. The detailed nominal composition of both alloys is given in Table 2. Inconel X also contains some minor components Al, Ti, Nb, and Ta. Addition of Al to alloys gives corrosion resistance and precipitation hardening, Ti exhibits age hardening (γ′ phase), and Nb + Ta gives precipitation hardening (γ′′ phase). For stressing experiments, a 15 × 0.3 cm2 coupon of the superalloy foils was placed at the bottom of a 20 cm, 6.3 mm o.d. (1/4 in.), and 4 mm i.d. glasslined tube reactor. The thickness of the foils was 0.075 mm. Before the introduction of the JP-8 fuel, the reactor was heated to 500 °C (932 °F) wall temperature for 2 h under flowing nitrogen at 34 atm (500 psig). The fuel
10.1021/ie990694o CCC: $19.00 © 2000 American Chemical Society Published on Web 01/29/2000
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Figure 1. TPO profiles of carbon deposition from JP-8 decomposition at 500 °C and 34 atm for 5 h on Inconel 600 and Inconel X alloy.
was preheated to 250 °C (482 °F) before entering the reactor. The reactor wall temperature and the fuel pressure were kept constant throughout the experiments at 500 °C and 34 atm, respectively. Under these experimental conditions, JP-8 is in a supercritical state.16 The flow rate of jet fuel was maintained at 4 cm3/min for 5 h. More details on the flow reactor system can be found elsewhere.17 The stressed foils were analyzed using LECO-RC 412 multiphase carbon determinator to obtain the total carbon deposition on the foils as well as the temperature-programmed oxidation (TPO) profiles of the deposits. The morphology of the deposits was examined with an ISI-DS 130 dual-stage scanning electron microscope (SEM). Results and Discussion The characterization of the carbonaceous deposits samples indicated that metal catalysis of carbon deposition, producing, for example, filamentous carbon, plays a key role in the initiation of deposits. Gas-phase mechanisms for molecular weight growth are not critically important for catalyzed surface reactions, such as filamentous carbon formation, because the precursor species consist of small hydrocarbon radicals (e.g., C1‚‚, C2‚) or small unsaturated hydrocarbons (e.g., olefins, C2), C3)). Gas-phase mechanisms are important, though, for the formation of thermal deposits that involve liquidphase carbonization (e.g., mesophase formation) or particulate growth on the surface. They are also important for subsequent thermally driven deposition on either incipient carbon filaments or other kinds of catalytically formed carbons. Under the same thermal stressing conditions, Inconel 600 collected a much higher quantity of deposit than Inconel X did. As determined by multiphase carbon analysis, the amounts of carbon in the deposits collected on Inconel 600 and Inconel X are 45 and 3 µg/cm2, respectively. In this study, only the deposits collected on the alloy surfaces were measured. Figure 1 shows the TPO profiles of the deposit collected on the alloy foils at 500 °C reactor wall temperature, 34 atm, and for 5 h reaction periods. The profiles plot the amount of carbon gasified during the oxidation of the deposit in the LECO carbon analyzer as a function of temperature measured in the analyzer furnace. A constant heating rate of 30 °C/min was used in the experiments to heat the samples from 100 °C to a maximum temperature of 800 °C with a holding period
of 3 min at 800 °C. The high-temperature peaks in the profiles are attributed to structurally more ordered (less reactive) carbon deposit produced, most probably, by surface catalysis. The low-temperature peaks correspond to less ordered (more reactive and possibly containing significant amount of hydrogen) deposit that may result from secondary deposition processes promoted by the presence of the incipient, catalytically formed, carbon. The secondary deposition refers to thermally driven deposit growth on incipient carbon filaments that leads to the thickening and coating of the filaments by pyrolytic carbon formation or liquidphase carbonization reactions. Alternatively, the lowtemperature peaks may result from gasification of carbon catalyzed by finely dispersed metal particles (e.g., Ni or Fe) in the deposit as found in filamentous carbons.17 Figure 1 shows very different TPO profiles for the deposits on Inconel 600 and Inconel X. The sharp peak at approximately 125 °C in the Inconel 600 profile is attributed to the evaporation of hydrocarbons (from jet fuel decomposition) adsorbed on the deposit surface. The two prominent features of the Inconel 600 TPO profile are the presence of a sharp peak that evolves around 700 °C and two broad peaks that evolve in the temperature range 200-600 °C. The presence of the 700 °C peak can be attributed to the formation of highly ordered carbon structures by surface catalysis, such as crystallites. The formation of crystallites from hydrocarbon decomposition on metal surfaces has been reported for light hydrocarbons (C2 and C3) at higher temperatures (600-800 °C) and atmospheric pressure.18 This is the first observation of the formation of crystallites from jet fuel decomposition at relatively low temperatures and much higher pressures. The broad low-temperature peaks in Inconel 600 TPO profile can be attributed to the presence of filamentous carbon or hydrogen-rich carbonaceous deposits produced by thermal decomposition reactions on incipient deposit surfaces. These observations indicate that Inconel 600 has a high catalytic activity for the deposit formation from jet fuel at 500 °C and 500 psig. In contrast to the behavior of Inconel 600 deposit, the TPO profile of the Inconel X deposit does not show a high-temperature peak but displays a broad lowtemperature peak in the same temperature range as that seen from the Inconel 600 deposit profile. These observations along with much less deposition obtained on the Inconel X surface clearly indicate that this superalloy has an activity for deposit formation much lower than that of Inconel 600. The SEM examination also showed differences in the morphology of the deposits obtained on the two superalloy surfaces. Figure 2a shows filamentous carbon deposits on the Inconel 600 surface. The filaments that are approximately 2 µ in length and 0.3 µ in diameter appear to be coated by a carbonaceous layer from secondary deposition process. An energy-dispersive X-ray spectroscopy (EDS) analysis of these fibrous structures (not shown here) indicated high concentrations of Ni and Fe removed from the surface of Inconel 600. The concentrations of elements Ni, Fe, and Cr are 10 times higher on the fibrous deposits than those of the platelet deposits on the metal surface. On Inconel X, however, the deposits are very few and randomly distributed on the surface as shown in the SEM micrograph in Figure 2b. The morphology of the
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Figure 2. SEM micrographs of deposits collected on Inconel 600 (a) and Inconel X (b) by JP-8 decomposition at 500 °C and 34 atm for 5 h.
deposits on Inconel X has both crystalline and amorphous features. The crystallites with faceted edges that are clearly seen on the surface of stressed Inconel X (Figure 2b) are most likely metal carbides produced by surface reactions. These crystallites were mostly covered by the filaments on the stressed Inconel 600 surface. The deposits have particle size in the range of 0.2-1 µ. By X-ray point analysis, more sulfur was observed on solid deposits than that on metal surfaces. The sulfur may accumulate during deposit formation from JP-8 fuel, which contains 68 ppm S. Inconel X surface was not covered completely by deposits, and the majority of the surface remained undeposited even after a 5 h reaction period. In agreement with TPO analyses, these observations confirm that the Inconel X surface has a much lower activity toward deposit formation than that of Inconel 600. Once the catalytic carbon is formed on metal surfaces, the increased surface area can promote further deposition by free radical reactions leading to the formation of polycyclic aromatic hydrocarbons. These compounds can, in turn, act as precursors for amorphous or globular deposits. There is only one study that we were able to find in the literature related to carbon deposition on Inconel X surface.19 The authors studied solid deposition from heavy oils on Inconel X and observed “cake like” deposits similar to those seen in this study.
The experimental results from this study provide more insight into the effects of metal alloy surfaces on solid deposition considering both hetero- and homogeneous reactions. Both Inconel 600 and Inconel X are used in high-temperature applications, and they contain approximately 72% Ni. The lower activity of Inconel X can be explained by the presence of Al, Ti, Nb, and Ta as minor components that may significantly suppress the carbon deposition.14 It appears that, even at relatively low concentrations, these elements can affect the adsorption of reactive species and their subsequent reactions on alloy surfaces. Thermal decomposition of jet fuels produces reactive gas species that are chemisorbed on metal surfaces.2,19 Ni- and Fe-rich surfaces catalyze decomposition of chemisorbed hydrocarbons to produce carbon and hydrogen. Carbon then diffuses through any dislocations on the metal surface and finally precipitates in these locations.21,22 The increased concentration of precipitated carbon creates a stress at dislocations, and after a certain tensile strength of the metal is reached, the metal crystallite is removed from the surface, producing a filament with a metal particle at the tip. These minor elements Al, Ti, Nb, and Ta in Inconel X may have physically covered the active metals or acted to reduce the carbon solubility at the reaction temperature 500 °C. Presumably, on the Inconel X surface, the catalytic activity of Ni can be suppressed by the formation of Ni3A (A ) Al, Ti, Ta, or Nb) and by the inhibited diffusion of carbon through the bulk due to the high ductility strength of the γ′ structure. In addition, γ′ phase Ni3(Al, Ti) improves the strength of Inconel X against surface cracks.13 The formation of filamentous carbon is controlled by diffusivity and solubility of carbon in metals.23 To some extent, the formation of metal carbides is also controlled by carbon solubility and diffusivity. Data on solubility and diffusivity of carbon in superalloys is very limited. Some studies were reported on carbon diffusion in Ni-Fe-Cr alloys. Grabke et al.,24 for example, observed that increasing Ni content in the ternary alloy reduced carbide formation by decreasing the rate of carbon diffusion in the alloy. It has been found that additions of Ti, Al, Nb, and Ta to various binary alloys reduced carbide formation25-30 because of lower solubility and diffusivity of C in Ti, Al, Nb, and Ta oxides. Further, these oxides have higher thermal stability than the oxides of Ni, Fe, and Cr.31 The presence of Al with Cr could provide more resistance to oxidation and suppress the outward diffusion of metallic elements and the inward diffusion of oxygen, nitrogen, and sulfur. In superalloys, both Ti and Al act as solid solution strengtheners, and in addition, Ti substitutes for aluminum positions in the ordered structure. Nb can combine with Al and Ti to increase the solution temperature of the γ′ structure so that the alloys’ strengthening effect carries to higher temperature,13 because these elements with the higher d-orbital energy level form bonds with nickel atoms stronger than those between Ni and Fe/Cr atoms.32 Conclusions The characterization of the carbonaceous deposits from a JP-8 jet fuel decomposition on superalloy foils indicated that the deposition is initiated by metalcatalyzed surface reactions. Subsequent growth and thickening of the incipient deposit takes place through
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thermally driven pyrolytic carbon formation or liquidphase carbonization. The amount of carbon deposited on superalloys appears to depend on the composition of the metal surface. The Inconel 600 surface collected many more deposits than did that of Inconel X, because of the higher catalytic activity of Inconel 600 toward deposit formation. The TPO profiles and morphology of the deposits on the two superalloys were also very different. The presence of minor components, Al, Ti, Nb, and Ta in Inconel X appears to suppress carbon deposition, because the major component compositions (Ni, Fe, and Cr) of Inconel 600 and Inconel X are practically the same. Acknowledgment This work was funded by the Air Force Research Laboratory/Aero Propulsion and Power Directorate, Wright Patterson AFB. We thank Prof. H. H. Schobert of PSU for his support and Dr. T. Edwards of AFWL/ APPD for helpful comments and discussions. Literature Cited (1) Hazlett, R. N. Physicochemical Aspects of Aviation Fuel Thermal Stability. In Aviation Fuel: Thermal Stability Requirements; Kirklin, P. W., David, P., Eds.; ASTM STP Series, 1138; ASTM: Philadelphia, PA, 1992; Vol. 18. (2) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanisms. Ind. Eng. Chem. Res. 1993, 32, 3117. (3) Reyniers, G. C.; Froment, G. F.; Kopinke, F. D.; Zimmerman, G. Coke Formation in the Thermal Cracking of Hydrocarbons. 4. Modeling of Coke Formation in Naphtha Cracking. Ind. Eng. Chem. Res. 1994, 33, 2584. (4) Atria, J.; Li, J.; Eser, S.; Schobert, H. H.; Cermignani, W. Carbonaceous Deposit Formation from Jet Fuel and Norpar-13. Carbon ’97, 23rd Biennial Conf. Carbon 1997, 2, 312. (5) Albright, L.; F.; McGill, W. A. Aluminized ethylene furnace tubes extend operating life. Oil Gas J. 1987, Aug 31, 46. (6) Zimmermann, G.; Zychlinski, W.; Woerde, H. M.; Oosterkamp, P. van den. Absolute Rates of Coke Formation: A Relative Measure for the Assessment of the Chemical Behavior of HighTemperature Steels of Different Sources. Ind. Eng. Chem. Res. 1998, 37, 4302. (7) Altin, O.; Venkataraman, A.; Eser, S. Analysis of Solid Deposits from Thermal Stressing of a Jet Fuel on Different Surfaces in a Flow Reactor. ACS Symp. Ser., Div. Pet. Chem. Inc. 1998, 43 (3), 404. (8) Albright, L. F.; Marek, J. C. Coke Formation during Pyrolysis: Roles of Residence Time, Reactor Geometry, and Time of Operation. Ind. Eng. Chem. Res. 1988, 27, 743. (9) Graff, J. M.; Albright, I. L. Coke Deposition from Acetylene, Butadiene, and Benzene Decompositions at 500-900 °C on Solid Surfaces. Carbon 1982, 20 (4), 319. (10) Reyniers, M. F. S. G.; Froment, G. F. Influence of Metal Surface and Sulfur Addition on Coke Deposition in the Thermal Cracking of Hydrocarbons. Ind. Eng. Chem. Res. 1995, 34, 773. (11) Trimm, L. D. The Formation and Removal of Coke from Nickel Catalyst. Catal. Rev.-Sci. Eng. 1977, 16, 155. (12) Trimm, D. L. Fundamental Aspects of the Formation and Gasification of Coke. In Pyrolysis-Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (13) Sims, C. T.; Stoloff, N. S.; Hagel, W. C. Superalloys; John Wiley and Sons: New York, 1987.
(14) Baker, R. T. K.; Chludzinski, J. J., Jr. Filamentous Carbon Growth on Nickel-Iron Surfaces: The Effect of Various Oxide Additives. J. Catal. 1980, 64, 464. (15) Altin, O.; Venkataraman, A.; Eser, S. Analysis of Solid Deposits from Thermal Stressing of a JP-8 Fuel on Superalloy Foils in a Flow Reactor. ACS Symp. Ser, Div. Pet. Chem. Inc. 1998, 43 (3), 408. (16) Yu, J.; Eser, S. Determination of Critical Properties (Tc, Pc) of Some Jet Fuels. Ind. Eng. Chem. Res. 1995, 34, 404-409. (17) Li, J.; Eser, S. Carbonaceous Deposit Formation on Metal Surfaces from Thermally Stressed Dodecane. Carbon ‘95, 22nd Biennial Conf. Carbon 1995, 314. (18) Baker, R. T. K.; Harris, P. S. The Formation of Filamentous Carbon. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker Inc.: New York, 1978. (19) Bleekkan, E. A.; Holmen, A. Coke Formation on Metal Foils from Heavy Oils. Carbon 1987, 25 (6), 827. (20) Zabarnick, S. Studies of Jet Fuel Thermal Stability and Oxidation Using Quartz Crystal Microbalance and Pressure Measurements. Ind. Eng. Chem. Res. 1993, 32, 1012. (21) Bianchini, E. C.; Lund, C. R. F. Kinetic Implications of Mechanisms Proposed Carbon Filament Growth. J. Catal. 1989, 117, 455. (22) Snoeck, J.-W.; Froment, G. F.; Fowles, M. Filamentous Carbon Formation and Gasification: Thermodynamics, Driving Force, Nucleation, and Steady-State Growth. J. Catal. 1997, 169, 240. (23) Yang, R. T.; Goethel, P. J.; Schwatz, J. M.; Lund, C. R. F. Solubility and Diffusivity of Carbon in Metals. J. Catal. 1990, 122, 206. (24) Grabke, H. J. Carburization: A High-Temperature Corrosion Phenomenon, MTI Publication No. 52; MTI: St. Louis, MO, 1988. (25) Shatynski, S. R. The Thermochemistry of Transition-Metal Carbides. Oxid. Met. 1979, 13 (2), 105. (26) Lai, G. Y. Resistance to Carburization of Various HeatResistant Alloys. In High-Temperature Corrosion in Energy Systems, Proceedings of the TMS-AIME Symposium; Rothman, M. F., Ed.; The Metalurgical Society of AIME: Warrandale, PA, 1985; p 551. (27) Stahlen, G., von Austenitischen, The Gaseous Carburisation of Austenitic Steels. Werkst. Korros. 1979, 30, 785. (28) Hillert, M.; Qiu, C. A Reassesment of the Fe-Cr-Mo-C System. J. Phase Equil. 1992, 13 (5), 512. (29) Pietzka, M. A.; Schuster, J. C. Summary of Constitutional Data on the Aluminum-Carbon-Titanium System. J. Phase Equilibr. 1994, 15 (4), 392. (30) Lai, G. Y. Carburization and Metal Dusting. High-Temperature Corrosion of Engineering Alloys; ASM International: Materials Park, OH, 1990. (31) Kumar, L.; Venkataramani, R.; Sundararaman, M.; Mukhopadhyay, P.; Garg, P. S. Studies on the Oxidation Behavior of Inconel 625 Between 873 and 1523 K. Oxid. Met. 1996, 45 (1/2), 221. (32) Murata, Y.; Miyazaki, S.; Morinaga, M.; Hashizume, R. Hot Corrosion Resistant and High-Strength Nickel-Based Single Crystal and Directionally-Solidified Superalloys Developed by the d-Electrons Concept. In Superalloys - 1996; Kissinger, R. D., Deye, D. J., Anton, D. L., Cetel, A. D., Nathal, M. V., Pollock, T. M., Woodford, D. A., Eds.; TMS Publications: Warrendale, PA, Sept 1996; p 61.
Received for review September 21, 1999 Revised manuscript received December 6, 1999 Accepted December 11, 1999 IE990694O
