Analysis of Carbonaceous Solid Deposits from Thermal Oxidative

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Ind. Eng. Chem. Res. 2010, 49, 2722–2730

Analysis of Carbonaceous Solid Deposits from Thermal Oxidative Stressing of Jet-A Fuel on Iron- and Nickel-Based Alloy Surfaces Arun Ram Mohan and Semih Eser* The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802

Thermal stressing of Jet-A was conducted in a flow reactor on iron- and nickel-based metal surfaces at a fuel flow rate of 1 mL/min for 5 h at a wall temperature of 350 °C and 3.5 MPa (500 psig) so that both liquidphase autoxidation and thermal decomposition of autoxidation products contribute to the formation of carbonaceous deposits. The deposits produced were characterized by field emission scanning electron microscopy (FESEM) and temperature programmed oxidation (TPO). The effect of metal surface on deposit formation increases in the following order: AISI316 < AISI 321 ≈ AISI 304 < Inconel 600 < AISI 347 < Inconel 718 < FecrAlloy < Inconel-750X. The variation in the activity of the metal substrates is attributed to their reaction with reactive sulfur compounds in the fuel and interaction of oxygenated intermediates formed by autoxidation during thermal stressing. Introduction The formation of carbonaceous deposits from jet fuel on the metal surfaces in the fuel systems before combustion is of major concern for the operation of aircraft engines as they can plug the filters and accumulate on valves, flow lines, and the fuel injector.1,2 Temperature is one of the important parameters affecting the rate of fuel degradation. Autoxidation of jet fuel, which causes the formation of oxygenated products, is predominant at temperatures less than 260 °C.3 These products decompose between 290 and 350 °C beyond which pyrolysis is significant. Fuel composition strongly affects the thermal and oxidative degradation of the fuel and its deposit-forming tendency.4 Below 260 °C, the deposit-forming tendency from jet fuels saturated with air was significantly affected by the nature of exposed metal surfaces.5 It was observed that metals containing copper and vanadium were most active toward carbon deposition. Characterization of deposits formed at 260 °C from Jet-A in the presence of excess air on metal and metal oxide substrates showed similarities in its morphology and chemical composition with soot.6 The morphology of deposits indicated a negligible role of substrates in carbon deposition. The presence of oxygen-containing functional groups and absence of sulfur and nitrogen in the deposits was notable.6 Pyrolytic degradation studies conducted with commercial aviation fuel Jet-A and military jet fuel JP-8 at 500 °C suggested that the amount of deposits formed is strongly influenced by the nature of the metal substrate, the fuel composition, and of any sulfur compounds present.7 Investigation on the effect of substrate on deposit formation from JP-8 at the same temperature has shown the presence of carbon with different levels of structural order ranging from amorphous to crystalline phases.2,8 It was also suggested that the presence of minor components like Ti, Al, Nb, and Ta in the alloy decreases the catalytic activity of iron and nickel by reducing the solubility of carbon in the base metals and suppressing carbon deposition through stabilization of alloy surfaces, making the removal of metal particles from the surface more difficult.2 The interaction of sulfur compounds with metal surfaces is very complex.9 In some cases, it was suggested that they can passivate the metal surface by forming sulfides and blocking * To whom correspondence should be addressed. Tel.: (814) 8631392. E-mail: [email protected].

the active sites.7 On the other hand, it was suggested that sulfur compounds in jet fuel activate the metal surface for carbon deposition by forming metal sulfides under pyrolytic conditions and so increasing the surface area available for carbon deposition.10 Characterization of the deposits obtained during thermaloxidative degradation of n-hexadecane at 160 °C showed the formation of aromatic solids in the fluid phase.11 Experiments primarily focused on the product formation from the decomposition of aerated dodecane at 800 psig in the liquid phase in SS304, SS316, and aluminum tubes suggested that the formation of oxygenated products from hydroperoxide decomposition and hydroperoxide-initiated pyrolysis is predominant in the temperature range between 282 and 400 °C.12 Although there were some differences in the product distribution, the type of metal surfaces did not appear to control the type or amount of product formation. Studies conducted with Jet-A under thermal-oxidative conditions where the fuel exit temperature is less than 350 °C show that surface reactions affect the carbonaceous solid deposit formation.13 The objective of this study is to investigate the effect of various metal alloys AISI304, AISI 316, AISI 347, AISI 321, Fecralloy, Inconel 600, Inconel 718, and Inconel 750-X on carbon deposition from Jet-A at a wall temperature of 350 °C and a reactor pressure of 500 psig. Experimental Section Thermal Stressing Experiments. The elemental composition of eight foil substrates in weight percentage used in this study is given in Table 1. All of the substrates are washed in hexane and dried in argon for an hour before the experiment. The experimental setup for thermal stressing of Jet-A is shown in Figure 1.14 The details of the thermal stressing reactor are described elsewhere.1 The stressing experiment is conducted in a 6.35 mm diameter (1/4 in. o.d.) glass-lined stainless steel reactor that is 20 cm long. The substrate is inserted at the bottom of the isothermal glass-lined stainless steel reactor. The reactor with the foil is heated in the presence of argon at a reactor pressure of 3.5 MPa (500 psig) to 350 °C with the help of a jacketing block heater to maintain isothermal conditions along the length of the reactor and maintained at that temperature for 4 h to obtain thermal equilibrium. Ultra zero air is bubbled into the Jet-A reservoir so that it is saturated with dissolved oxygen during the course of the experiment. The fuel is pumped into

10.1021/ie901283r  2010 American Chemical Society Published on Web 02/19/2010

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

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Table 1. Elemental Composition of Alloys (Goodfellow Ltd.) elemental composition substrate

Fe

Ni

Cr

AISI316 AISI321 AISI304 AISI347 FeCrAl IN600 IN718 IN750-X

64 68 69 67 72.6 8 18.5 7

12 10.5 10 11

18 18 18 18 22 15.5 19 15.5

74.43 52.5 73

Mn 2 2 2 2 1 0.18 0.5

C 0.08 0.08 0.08 0.08 0.03 0.15 0.04 0.04

Ti

Mo

Si

3

1.0 0.5 1.0 1.0 0.3 1.0 0.18 0.25

0.6

0.9 2.5

Figure 1. Flow reactor setup for thermal stressing experiment adapted and modified from ref 14.

the system at 500 psig. It enters the preheating line of 3.175 mm diameter (1/8 in. o.d.) and 2 m in length. The residence time of the fuel in the preheating line is 6.3 min, and it is preheated to 260 °C before entering the reactor. The fuel flow rate, reactor wall temperature, and the pressure are maintained at 1 mL/min, 350 °C, and 500 psig for 5 h. The residence time of the fuel in the reactor is 1.4 min. The fuel is maintained in the fluid phase during the course of experiment. At the end of the experiment, the residual fuel in the reactor was removed by purging it with argon. Characterization of Carbon Deposits. Under pyrolytic conditions, maximum deposition was obtained between 10 and 15 cm from the top of the reactor.1 Preliminary experiments were conducted with AISI 316 and AISI 304 foils to study the variation in deposit formation as a function of reactor length. The length of the foils used in this experiment was 10 cm. With respect to the glass-lined reactor described elsewhere,1 it is located between 7.5 and 17.5 cm from the top of the reactor. TPO conducted on the two sections of the same substrate material, each 5 cm long, showed the same amount of deposits. The nature of the TPO curves corresponding to each substrate is discussed in the next section. The TPO curves from each of the two sections of the same substrate were similar. Therefore, for the TPO of the other substrates, the portion of the foil located between 10 and 15 cm of the top of the reactor was chosen for analysis. The samples were dried under vacuum at 110 °C for 2 h. The morphology of the deposits was examined using a field emission scanning electron microscope (FESEM) JEOL 6700F. X-ray diffraction was performed in the gracing incidence mode to identify the various phases of metal sulfides, if any, in the deposits in the PANalytical X’Pert Pro MPD instrument operated at 45 kV/40 mA and scanned at 0.02 deg/s. The amount of solid carbonaceous deposits formed on each substrate after 5 h of thermal stressing experiment was measured by temperature programmed oxidation (TPO) in a RC412 Multiphase Carbon Analyzer. During TPO, the sample was loaded in a quartz boat and heated from 100 to 900 °C in the presence of

3

Nb

S

1.2

0.03 0.03 0.03 0.03

5.13 0.95

0.0015 0.0008 0.0005

Cu

0.5 0.15 0.25

Al

Y

Zr

4.8

0.1

0.1

0.5 0.7

ultra high purity oxygen at a ramp rate of 30 °C/min and held at 900 °C for 5 min. The carbon dioxide produced was measured in an IR cell. Any CO produced during oxidation was converted to CO2 in the presence of a copper oxide catalyst. The peak positions relate to the oxidation reactivity and thus depend on the structure of solid carbon deposits. The ramp rate during the TPO experiment may influence the position of peaks during the oxidation of carbonaceous deposits.15 However, staged TPO experiments show that the structure of the deposits does not change when the sample is heated in the above-mentioned program sequence in the presence of UHP oxygen.16 The individual peak positions and peak intensities are reproducible. The total amount of solid carbonaceous deposits obtained on each substrate is reproducible to within 10% of the deposit mass. Results and Discussion The analysis of Jet-A by gas chromatograph-mass spectrometry (GC-MS) and GC with pulsed flame photometric detector (GC-PFPD) for the hydrocarbon composition and sulfur compounds are shown in the chromatograms in Figure 2a and b, respectively. The concentration of sulfur compounds in Jet-A was found by elemental analysis to be 1160 ppm by weight. By comparison with standards, some of the peaks in the chromatogram were identified as dimethyl and trimethyl benzothiophenes as shown in Figure 2b. Characterization of sulfur compounds in aviation fuels by atomic emission detector (GCAED) has shown, in the order of increasing retention times, the presence of thiols, sulfides, disulfides (classified as reactive sulfur species), and methyl-substituted thiophenes and benzothiophenes (classified as nonreactive sulfur species). The classification of reactivity of the sulfur compounds is based on their tendency to undergo hydrodesulfurization.17 Therefore, the unidentified peaks observed in the chromatogram with shorter retention times may correspond to reactive sulfur species such as sulfides and disulfides. The presence of more volatile nonreactive species cannot be discounted. Amount of Solid Carbon Deposited on Different Metal Substrates. On the basis of the average amount of deposits obtained from three experiments on each substrate, the variation in the tendency of each substrate for deposit formation from Jet-A is shown in Figure 3. The amount of carbon deposited is lowest for AISI 316 and highest for Inconel 750-X. Among the stainless steel substrates, AISI 347 gave the highest amount of deposits. The elemental composition of AISI 304 and AISI 347 shown in Table 1 is very close to one another except for the presence of Niobium in AISI 347. Thermal stressing experiments with JP-8 on niobium foils under pyrolytic conditions suggest that niobium is catalytically inactive for carbon deposition.8 Similarly, the amount of carbon deposits on AISI 321 is close to that on AISI 304. The composition of major elements and some minor elements is similar to one another except for the presence of titanium in AISI 321. Even though titanium is known to suppress carbon deposition under pyrolytic conditions,

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Figure 2. (a) GC-MS chromatogram of Jet-A showing the composition of the fuel. (b) PFPD chromatogram showing sulfur compounds.

its presence does not make a difference in carbon deposition from Jet-A at 350 °C as compared to other stainless steel foils. It should be pointed out that the iron content of AISI 304 and AISI 321 (69% and 68%, respectively) is higher than that of AISI 316 (64%), which deposits less carbon than those on AISI 304 and AISI 321. Comparison of the elemental composition of Inconel 600 and Inconel 750-X shows the presence of minor elements titanium, niobium, and aluminum in Inconel 750-X in small percentages, which otherwise have a similar composition of major elements.

Inconel 750-X gives greater amount of deposits as compared to Inconel 600. Under pyrolytic conditions, it was observed that the presence of minor elements like Nb, Al, and Ti in the metals, which are added for precipitation strengthening,2 appeared to suppress carbon deposition.2 The ability of these elements to suppress carbon deposition in the metals was attributed to the formation of passivating layers that prevent the access of reactive species, formed during thermal decomposition of hydrocarbons,2 to the base metals iron and nickel, which are known for their catalytic activity toward carbon deposition.18 The difference in


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Figure 3. Carbon deposits on different metal surfaces from Jet-A at 350 °C, 500 psig with a fuel flow rate of 1 mL/min for 5 h. Table 2. Calculated Atomic Percentage of Fe, Ni, and S on the Alloys after 5 h of Thermal Stressing elements considered for sulfide formation substrate

Fe

Ni

S

AISI316 AISI321 AISI304 AISI347 IN600 IN718 IN750-X

54 57 57 56 7 17 6

10 8 8 9 59 45 58

36 35 35 35 35 38 36

the amount of deposits was attributed to the surface composition of the alloy substrates, which influences their catalytic activity for deposit formation at 500 °C. The same explanation does not appear to support the results obtained at 350 °C. As the temperature in these experiments is substantially lower than that of pyrolytic conditions, the metals might exhibit a lower degree of catalytic activity toward deposit formation from pyrolytic decomposition of hydrocarbons. In the intermediate regime, the formation and decomposition of oxygenated products both in the fluid phase and on substrate surface as well as the formation of deposit-forming precursors in the fluid phase may contribute to deposit formation. TPO and FESEM Analysis of Deposits on Various Substrates. The peaks corresponding to the evolution of CO2 at low temperatures are due to the high oxidation reactivity of hydrogen-rich carbon or solid carbon that is structurally less ordered. The CO2 peaks evolving at relatively high temperatures are due to the presence of hydrogen-lean carbon that is structurally more ordered. The sulfur compounds present in Jet-A are expected to react with the metal substrates to produce metal sulfides. An Fe-Ni-S ternary phase diagram at 400 °C19 was used to predict the phases of various metal sulfide structures observed in this study on various substrates. At 350 °C, it is assumed that all of the sulfur in the fuel is converted to metal sulfides during the 5 h duration of the experiment. This assumption is used to calculate the amount of sulfur consumed in the formation of metal sulfides. On the basis of the weight of substrates (1.10 g), amount of sulfur in the jet fuel (1160 ppm), the fuel flow rate (1 mL/min), and the elemental composition of iron and nickel for each substrate from Table 1, the atomic percentages of iron, nickel, and sulfur are calculated and summarized in Table 2. These values are used to predict the sulfide phases observed in FESEM using the ternary phase diagram. The dotted lines in the phase diagram corresponding to 35 at. % sulfur and 55 at. % iron are shown for convenience. They do not have any physical significance. Figure 4 shows the Fe-Ni-S phase diagram used for this purpose. Figures 5 and 6 show the FESEM and TPO of deposits formed on AISI 316 and AISI 321 respectively. As expected,

Figure 4. Fe-Ni-S ternary phase diagram at 400 °C, adapted from Raghavan, 2004. Terminology: γ, continuous solid solution between face centered cubic iron and nickel; py, pyrite FeS2; pn, pentlandite (FeNi)9S8; hz, heazlewoodite Ni3S2; mss, monosulfide solid solution; vio, violarite Ni3S4; vs, vaesite NiS2; R, iron-rich region.

Figure 5a shows the presence of metal sulfides in the form of faceted crystallites denoted as (P) and fibers denoted as (F) on AISI 316. The diameter of the metal sulfide fibers measured in the FESEM is found to be 75 nm. Similar structures were observed during thermal stressing with a different batch of highsulfur containing jet fuel that was exposed to AISI 316 at 470 °C.20 The phase diagram in Figure 4 predicts the formation of pyrrhotites. FESEM shows the presence of faceted prismatic structures and filamentous structures. The presence of this phase in two different morphologies was also observed under pyrolytic conditions.20 The formation of pyrrhotites in the temperature range 250-500 °C on steel surfaces during the processing of crudes is observed in petroleum refining.21 Therefore, it is suggested that these structures belong to the pyrrhotite phase. The amount of carbon deposits on AISI 316 is marginally less than that on AISI 321. The micrographs of Figures 5a and 6a corresponding to AISI 316 and AISI 321 respectively show that the extent of degradation in AISI 316 is less than that on AISI 321 due to the formation of metal sulfides, which increases the surface roughness during the course of experiment. As mentioned above, the iron content of AISI 316 is lower than those of AISI 304 and AISI 321. This may be responsible for the latter showing more extensive metal sulfide formation and roughening of substrate surfaces that creates more area for carbon deposition. The carbonaceous deposits seen as bright regions (A1) in Figure 5a are scattered along the length of the filamentous pyrrhotite structures. The TPO profile for AISI 316 in Figure 5b shows three groups of peaks. The broad spectrum in the low temperature range between 250 and 400 °C can be attributed to relatively more reactive thermal deposits due to the formation of hydrogen-rich carbonaceous solid from higher alkanes in Jet-A.7 These deposits are formed more likely by liquid-phase polymerization reactions and condensation reactions. The micrograph (Figure 5a) also shows the presence of spherulitic carbon (S). These spherulitic carbon structures may have formed by nucleation and growth of precursors in the gas phase followed by their deposition on the surface of sulfides. High-resolution transmission electron microscopy (HRTEM) examination has shown that the spherulitic deposits accumulating on the sulfide particles are amorphous in nature. The intermediate broad peak in the temperature range between 400

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Figure 5. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on AISI 316 from Jet-A at 350 °C and 500 psig for 5 h.


and 500 °C can be attributed to the oxidation of spherulitic deposits.10 The formation of layers of carbonaceous thin films and spherulitic deposits on the surface of metal sulfides has also been observed during pyrolytic degradation of Jet-A at 470 °C.10 The most intense high temperature peak seen in the profile between 500 and 700 °C may be attributed to the carbonaceous film or platelets formed on metal surfaces through dehydrogenative catalysis, producing greater structural order in the solid carbon deposit in Figure 5b. As seen in Figure 6b, the TPO profile for AISI 321 has three peaks that are better resolved and have a more uniform distribution of peak intensities as compared to the TPO profile of the deposits on AISI 316. Similar to the suggested assignments for AISI 316 deposits, the first peak (250 and 400 °C) can be attributed to the hydrogen-rich carbonaceous solid from decomposition of higher alkanes. The intermediate peak (400-500 °C) may be attributed to the oxidation of spherulitic carbon marked as S in Figure 6a and particulate deposits observed as bright white regions on the surface of prismatic metal sulfide crystallites. The high temperature peak between 500 and 700 °C can be attributed to the oxidation of structurally more ordered deposits. The phase diagram in Figure 4 predicts the presence of pyrrhotites. Surface morphology of deposits observed in FESEM shows the presence of prismatic metal sulfides (P) and filaments (F) in Figure 6a. Pyrrhotites are known to have these two morphologies. Therefore, it is suggested that the crystallites on the surface of the AISI 321 are pyrrhotites. The FESEM micrograph of Figure 7a shows the presence of filaments (F), faceted metal sulfides (P), and spherulitic deposits (S) on AISI 304. X-ray diffraction of the sample containing these deposits in Figure 7b shows the presence of hexagonal

pyrrhotites having six-fold symmetry that are observed in the micrograph marked as H1. The ternary phase diagram in Figure 4 predicts the same. It is noteworthy to say that the signal was strong only from this substrate. XRD on other substrates did not produce a good signal to detect the presence of sulfides. The TPO profile in Figure 7c appears to contain two broad peaks. Yet upon closer inspection, one may see that the profile can be deconvoluted to four peaks in the temperature ranges 250-400, 400-600, 550-650, and 550-720 °C. These peaks may be assigned in the order of decreasing reactivity, to hydrogen-rich carbonaceous deposits, spherulitic solid carbon deposits, small particles of ordered carbons formed by metal catalysis, and large platelets or films of ordered carbon structures, respectively. Figure 8a shows the FESEM micrograph of deposits formed on AISI 347. The phase diagram shown in Figure 4 predicted the presence of pyrrhotites, which can be observed in the form of prismatic structure marked as P and filamentous structure marked as F in the micrograph. The amount of carbonaceous deposits on AISI 347 is 1.4 times greater than that on AISI 321. The three broad regions of peaks observed on the TPO profile of AISI 347 in Figure 8b can be similarly assigned to hydrogen-rich carbonaceous deposits (200-450 °C), spherulitic solid carbon deposits (450-600 °C), and structurally ordered deposits formed by metal catalysis respectively. As compared to the TPO of AISI 304 and AISI 321 deposits, the multiple high temperature peaks shifted to higher temperatures, suggesting a stronger catalytic activity of AISI 347 than other stainless steels, and is also evident in the larger amount of carbon deposits formed on AISI 347. Given the only major difference in composition among AISI 347, AISI 304, and AISI 321 is the


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Figure 7. (a) Scanning electron microscopy (SEM), (b) X-ray diffractogram, and (c) temperature programmed oxidation (TPO) of the deposits formed on AISI 304 from Jet-A at 350 °C and 500 psig for 5 h.


presence of minor component Nb (1.2%) in AISI 347, it is not clear as to what causes the catalytic activity. Among all of the iron-rich alloy surfaces selected for thermal stressing at 350 °C, Fecralloy gave the highest amount of carbon deposits. It is well-known that iron and iron oxides catalyze dehydrogenation reaction and carbon deposition.22 The above data suggest that the possibility of sulfide formation should be considered. In the FESEM micrograph shown in Figure 9a, it is interesting to note the absence of metal sulfides and the presence of deposits formed from gas phase denoted as E1, spherulitic deposits (S), and bright regions resembling structurally less ordered carbon aggregates on the surface of the deposits (B). As the metal is an aluminized iron and chromium alloy, passivated by chromium and aluminum, the sulfide formation

is not observed at 350 °C. The TPO profile, Figure 9b, shows the presence of three peaks having similar intensities, resembling the case with AISI 321 (Figure 6b). The presence of aluminum in the metal prevents severe degradation of the surface relative to other metals due to their exposure to sulfur in jet fuel. The behavior of Inconel 600 toward deposit formation is different from other alloys. The amount of metal sulfides formed on Inconel 600 shown in the FESEM micrograph in Figure 10a is much less as compared to iron-rich alloy surfaces that form metal sulfides. Despite the lower amount of metal sulfide formation, the metal shows a relatively higher catalytic activity toward carbon deposition. The composition of the alloy (Table 1) shows the presence of copper as a minor element. Studies conducted to elucidate the effect of metals on deposit formation

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Figure 9. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on FeCrAl alloy from Jet-A at 350 °C and 500 psig for 5 h.

Figure 10. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on Inconel 600 from Jet-A at 350 °C and 500 psig for 5 h.

from jet fuel showed that among the metals copper, nickel, iron, and cobalt, copper showed the highest catalytic activity for deposit formation in the temperature range 200-260 °C.23 Therefore, the presence of copper increases the catalytic activity of metal toward formation and/or decomposition of oxidation products and hence carbon deposition. The oxidation of deposits with different morphologies as observed in FESEM (Figure 10a) gives rise to CO2 peaks in the temperature range between 370 and 600 °C in the TPO profile shown in Figure 10b. The higher intensity of the high temperature peak and greater amount of carbon deposits can be attributed to high catalytic activity of copper. The phase diagram predicts the formation of heazlewoodite during the interaction of metals with sulfur compounds in jet fuel. The formation of filamentous structures was observed on Inconel 600 under pyrolytic conditions.20 TEM investigations of these filamentous structures suggest the presence of heazlewoodite.20 Therefore, it is suggested that the fiber-like structures (F) obtained in this experiment and seen in the micrograph (Figure 10a) are heazlewoodite crystals. The facts that the formation of metal sulfides is less on Inconel 600 and the PFPD data suggest that sulfides and disulfides might dissociate due to weak bonding between sulfur atoms and alkyl carbon atoms and accelerate the chain initiation reactions that contribute to the formation of carbonaceous solids in the intermediate regime. The deposits on the surface of Inconel 718 shown in Figure 11a are structurally disordered carbon seen as bright regions (B), spherulitic deposits (S), and sulfides in the form of fibers (F). The sulfide crystals are suggested to be heazlewoodite crystals based on the phase diagram and the above analysis for Inconel 600. Contrary to the observation under pyrolytic

conditions, the amount of solid carbonaceous deposits on Inconel 718 as shown in Figure 11b is greater than that on Inconel 600. The presence of niobium, titanium, and molybdenum in Inconel 718 appeared to suppress carbon deposition under pyrolytic conditions.7 In the intermediate regime, where both pyrolysis and liquid-phase autoxidation of hydrocarbons contribute to carbon deposition, transition metals exhibit varying degrees of catalytic activity toward the hydrocarbons during dehydrogenation and carbon-carbon bond cleavage in catalytic cracking. The participation of metals with multiple valence states has also been observed in the liquid-phase autoxidation of hydrocarbons. The metals catalyze the reduction-oxidation reaction of hydroperoxides, by forming metal-hydroperoxide complexes,24 and other products formed during the oxidation of hydrocarbons, resulting in the formation of free radicals.25 Iron, cobalt, nickel, copper, chromium, and manganese are generally used as catalysts during oxidation of hydrocarbons to accelerate the reduction-oxidation reaction.25 Sulfur compounds under pyrolytic conditions are known to either promote or inhibit carbon deposition depending upon the type of metal substrate.26 Under pyrolytic conditions, addition of thiophenes, 3-methyl benzothiophenes to n-dodecane inhibited carbon deposition on iron and nickel surfaces by blocking the active sites, whereas benzyl phenyl sulfide promoted the formation of carbonaceous solids on Inconel 718.26 These facts along with the PFPD data could help explain the reason for the formation of more deposits on Inconel 718 relative to Inconel 600. From Figure 3, in the intermediate regime under consideration, it can be seen that among the Inconel alloys used, Inconel 750-X gives the highest amount of deposits. The FESEM


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Figure 11. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on Inconel 718 from Jet-A at 350 °C and 500 psig for 5 h.

Figure 12. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on Inconel 750X from Jet-A at 350 °C and 500 psig for 5 h.

micrograph in Figure 12a shows the presence of disordered carbon seen as bright regions (B) on the surface of prismatic metal sulfides and spherulitic deposits (S). The TPO profile in Figure 12b shows that the structurally less ordered carbon oxidizes to give a broad plateau in the temperature range between 300 and 480 °C, and the relatively ordered deposits oxidize at approximately 600 °C. The formation of metal sulfides in Inconel 750X was not observed under pyrolytic conditions with JP-8 when the concentration of sulfur was 68 ppm.27 It was also observed that metal surfaces exposed to Jet-A had significant degradation at 500 °C as opposed to JP-8. As the concentration of sulfur is 1160 ppm in the Jet-A sample used in these experiments, metal sulfides in the form of fibers (F) and prismatic crystallites (P) formed on the substrate are

observed in the micrograph. The phase diagram predicts the formation of pentlandite and heazlewoodite. Filamentous structures observed in the micrograph are suggested to be heazlewoodite crystallites based on the analysis for Inconel 600. As inconel 750-X is a nickel-rich alloy, heazlewoodite would have formed during the early stages of the reaction. Subsequently, monosulfide solid solution might have formed due to the reaction between organosulfur compounds and the minor element iron. Pentlandites typically form by the reaction between monosulfide species and heazlewoodite. To verify the catalytic activity of metals at 350 °C, thermal stressing was conducted with a silicon substrate. The morphology and amount of the deposits formed on silicon are shown in Figure 13a and b, respectively. The structurally disordered

Figure 13. (a) Scanning electron microscopy (SEM) and (b) temperature programmed oxidation (TPO) of the deposits formed on silicon from Jet-A at 350 °C and 500 psig for 5 h.

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carbonaceous deposits seen as bright white regions (B) and the spherulitic deposits (S) shown in Figure 13a oxidize in the temperature range between 250 and 500 °C as shown in the TPO profile in Figure 13b. The absence of high temperature peaks at temperatures greater than 500 °C suggests that iron- and nickel-rich alloys exhibit catalytic activity for the formation of carbonaceous deposits during thermal stressing at 350 °C. The absence of metal sulfides on the surface of silicon and the lower amount of carbon deposits also suggest that the formation of metal sulfides on iron- and nickel-rich alloys increases the surface area available for carbon deposition. Conclusions The formation of carbonaceous solid deposits on metal substrates in the intermediate regime is influenced by reactive organic sulfides and disulfides in the jet fuel, decomposition of oxidation products from liquid-phase autoxidation, along with metal catalysis and the metal sulfide formation. Characterization of the carbonaceous deposits by FESEM and TPO shows predominantly the presence of spherulitic deposits, which nucleate and grow in the fluid phase. The formation of metal sulfides increases the surface roughness and causes disruption of the surface significantly. On the basis of the characterization and the prediction from phase diagram, it appears that pyrrhotite forms in the iron-rich metals and heazlewoodite forms in the nickel-rich alloy surfaces. Because of the surface disruption, metals with multiple valence states may be exposed to the oxygenated intermediates and participate in the decomposition of hydroperoxides through metal-hydroperoxide complex and other oxidation products formed during liquid-phase autoxidation. Therefore, the amount of carbon deposition on the alloys increased in the following order: AISI316 < AISI 321 ≈ AISI 304 < Inconel 600 < AISI 347 < Inconel 718 < Fecralloy < Inconel 750-X. The presence of molybdenum, titanium, and niobium in smaller amounts does not appear to affect carbon deposition under the experimental conditions. Carbon deposition on Fecralloy, Inconel 600, Inconel 718, and Inconel 750-X shows that the formation of metal sulfides does not necessarily passivate the surface and reduce carbon deposition. Acknowledgment We thank Dr. Cigdem Shalaby and Dr. Dania Alvarez Fonseca at the EMS Energy Institute for assistance in sulfur analysis. The funding for this work was provided by RollsRoyce, IN, and the EMS Energy Institute at Penn State University. Literature Cited (1) Altin, O.; Eser, S. Analysis of solid deposits from thermal stressing of a JP-8 fuel on different tube surfaces in a flow reactor. Ind. Eng. Chem. Res. 2001, 40, 596–603. (2) Altin, O.; Eser, S. Analysis of carboneceous deposits from thermal stressing of a JP-8 fuel on superalloy foils in a flow reactor. Ind. Eng. Chem. Res. 2001, 40, 589–595. (3) Jones, E. G.; Balster, W. J.; Balster, L. M. Aviation fuel recirculation and surface fouling. Energy Fuels 1997, 11, 1303–1308. (4) Jones, E. G.; Balster, L. M.; Balster, W. J. Thermal stability of Jet-A fuel blends. Energy Fuels 1996, 10, 509–515. (5) Taylor, W. F. Kinetics of deposit formation from hydrocarbons. 3. Heterogeneous and homogeneous metal effects. J. Appl. Chem. USSR 1968, 18, 251.

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ReceiVed for reView August 14, 2009 ReVised manuscript receiVed February 7, 2010 Accepted February 10, 2010 IE901283R

Analysis of Carbonaceous Solid Deposits from Thermal Oxidative

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