Deposition of Carbonaceous Solids on Different Substrates from

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Deposition of Carbonaceous Solids on Different Substrates from Thermal Stressing of JP-8 and Jet A Fuels Semih Eser,* Ramya Venkataraman, and Orhan Altin Department of Energy and Geo-EnVironmental Engineering and the Energy Institute, The PennsylVania State UniVersity, 101 Hosler Building, UniVersity Park, PennsylVania 16802

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. Introduction Jet fuel serves as a coolant in current aircraft and may be exposed to higher temperatures, because of increasing thermal loads, before it is burned in the advanced aircraft. High thermal loads can lead to temperatures as high as 500 °C on metal surfaces in the fuel delivery system of advanced aircraft.1-3 Exposure to such high temperatures accelerates the reactions of hydrocarbons and heteroatom species in jet fuel and results in the formation of carbonaceous deposits on metal surfaces. Rapid accumulation of solid deposits on various components of the fuel system, including valves, flow tubes, and nozzles, could cause catastrophic failure of the aircraft engine. An extensive research program at Penn State has been directed to understand the jet fuel degradation and solid deposit formation processes under conditions that are relevant to the operation of advanced aircraft.4-10 Edwards11 recently reviewed the cracking and deposition behavior of hydrocarbon aviation fuels under supercritical conditions. From a thermodynamics point of view, aliphatic hydrocarbons in jet fuel will have a tendency to produce aromatic compounds, and aromatic compounds will form larger aromatic ring systems (polyaromatic hydrocarbons) as precursors to solid deposits through hydrogen redistribution at elevated temperatures. Similarly, heteroatom (S, N, and O) species will have a tendency to produce more-stable compounds when heated to elevated temperatures. The rates at which these reactions occur are dependent on the molecular composition of the fuel, the composition of metal substrate, and catalysis of dehydrogenation reactions on metal surfaces at a given temperature and pressure. This paper further probes the effect of substrate surfaces on the deposition of solids from thermal stressing of a commercial aviation jet fuel (Jet A), and a military jet fuel sample (JP-8). The deposited substrates were examined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and temperature-programmed oxidation (TPO). The companion paper12 discusses the effectiveness of using TPO to relate the oxidation behavior of solid carbons to carbon structure and, thereby, to deposition mechanisms. * To whom correspondence should be addressed. Tel: +1-814-8631392. Fax: +1-814-865-3248. E-mail address: [email protected].

Figure 1. Comparison of gas chromatography-mass spectroscopy (GC/ MS) chromatograms of JP-8 and Jet A fuels.

Materials, Experimental Apparatus and Procedures Substrates and Fuel Samples. Substrates used in this study include two single metals, nickel and iron, a binary Fe/Ni (55/ 45) alloy, two stainless steels (SS 304 and SS 321), two superalloys (Inconel 600 and Inconel 718), and a quartz substrate. Table 1 lists the elemental composition of these substrates. The foils or coupons of these materials, with dimensions of 13 cm × 0.3 cm × 0.015 cm, were placed in a 20-cm-long glass-lined 1/4-in. (outer diameter, od) stainless steel tube reactor that was inserted vertically in a block heater. The two types of jet fuel used in this study were a military jet fuel (JP-8) and a commercial aviation jet fuel (Jet A).13-15 Figure 1 compares the gas chromatography-mass spectroscopy (GC/MS) chromatograms of the JP-8 and Jet A fuel samples used in this study. These chromatograms show that the Jet A fuel sample has relatively high concentrations of higher boiling hydrocarbons (e.g., n-alkanes with higher carbon numbers, compared to JP-8 fuel). Comparing the intensities of the higher alkanes (i.e., n-tetradecane, n-pentadecane, and n-hexadecane, marked by peak numbers 7, 8, and 9, respectively, in Figure 2) with those of the lower alkanes (i.e, n-octane, n-nonane, and

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Figure 2. Total carbon deposit amounts from the decomposition of JP-8 and Jet A fuels on different substrate surfaces at 500 °C and 34 atm (500 psig), with a fuel flow rate of 4 mL/min for 5 h.

n-decane, marked by peak numbers 1, 2, and 3, respectively) clearly shows that Jet A fuel contains higher alkanes (peaks 7, 8, 9) in higher concentrations than JP-8 fuel does. Another difference between the composition of the two fuels used in this study is observed in their sulfur contents: JP-8 fuel has a sulfur content of 500 ppm, whereas Jet A fuel has a sulfur content of 800 ppm. It is important to note that these differences are specific to the jet fuels samples used in this study and cannot be generalized to represent differences between JP-8 and Jet A fuels. Flow Reactor Setup. The flow reactor used consists of a modified Chemical Data Systems (CDS) model 803 bench-scale setup, which has been described elsewhere.16 The major modification to this unit includes using a 20-cm-long, 1/4-in. (od), glass-lined stainless steel tube as the reactor into which 13 cm × 3 mm coupons of metal/alloy foils or 13 cm × 3 mm (od) quartz tubes were inserted through the bottom of the reactor. The reactor was heated electrically in a tightly fitted block heater, and a back-pressure regulator was used to control the pressure in the reactor. The fuel was pumped into the reaction system at a controlled liquid flow rate of 4 mL/min, using a Waters model HPLC 515 pump. Before the introduction of the jet fuel, the reactor was heated to a wall temperature of 500 °C (932 °F) for 2 h under flowing argon at 34 atm (500 psig). The fuel 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. The start time for the reaction was noted from the instant the bulk fuel temperature attained the wall temperature of 500 °C. The total residence time of the fuel in the reactor was 127.5 s. Under these experimental conditions, jet fuels are in a supercritical state.2 During the experiments, the downstream transfer lines were also heated, to prevent condensation within the line. Thermal stressing experiments were conducted for 5 h.

Characterization of Deposits. Carbonaceous deposits produced on the substrates in thermal stressing experiments were analyzed to measure the quantity of carbon in the deposits and to observe their morphology, as well as their reactivity with oxygen. This information could also be useful for understanding the carbon deposition mechanisms. Deposit Morphology. Scanning electron microscopy (SEM) was used to observe the morphology of the solid deposits and substrates. The SEM system used in this study was an ISI ABT SX-40A scanning electron microscope in the Materials Characterization Laboratory at Penn State. Energy-dispersive spectroscopy (EDS) was used to identify the elements that constitute the deposits and to provide an elemental mapping of the surface. The EDS analyses were conducted only on selected sample surfaces, because of the limitations of this technique, in regard to sensitivity, to detect minor elements, particularly at high SEM magnifications. Temperature-Programmed Oxidation (TPO) Using a Carbon Analyzer. After the thermal stressing experiments, the samples were cut into 3-cm segments and analyzed for carbon deposition in a LECO model RC-412 multiphase carbon analyzer.17 Temperature-programmed oxidation (TPO) consists of exposing the sample that contains carbonaceous deposits to a flowing O2 gas /O2-inert gas mixture stream in a furnace while increasing the temperature of the furnace from a minimum of 100 °C to a maximum of 900 °C. A constant heating rate of 30 °C was used in the TPO experiments with a holding period of 3 min at 900 °C. A constant O2 flow rate of 750 mL/min was used in all the analyses. Carbon in the sample, which was placed in a quartz boat, was oxidized by reacting it with ultrahigh-purity (UHP) O2. A downstream CuO catalyst bed ensured that any CO produced during the reaction was converted to CO2. A calibrated IR cell measured the amount of total CO2 produced by the oxidation of the deposit, as a function of furnace temperature. Thus, a profile of CO2 evolution (which is also designated as a TPO profile), normalized by the geometric area of the sample substrate, gives the amount of carbon in the deposit (in units of µg/cm2), as well as information on the oxidation reactivity of the carbonaceous deposit.17,18 Typically, solid deposits that are produced by metal-catalyzed dehydrogenation reactions would have a higher degree of structural order at the molecular level, and a lower H/C ratio, compared to those produced by thermal reactions only. Therefore, solid carbons produced through metal catalysis should have lower oxidation reactivity than the deposits produced by thermal reactions. Therefore, determining the oxidation reactivity of carbonaceous deposits can provide information on the chemical nature of the deposits and possible pathways that lead to the formation of carbonaceous deposits produced from thermal stressing.9,11-13 This assumption is further explored in the companion paper.12

Table 1. Composition of Materials Used in This Study11,14,15 Composition substrate

Ni

Fe

Cr

Mo

Al

Ti

Nb

Cu

Mn

Si

C

1 1 0.3 0.5 0.18

0.08 0.08 0.03 0.15 0.04

S

W

Y

Metal/Alloy nickel iron Fe-Ni SS 304 SS 321 FeCr alloy Inconel 600 Inconel 718 quartz

99 45 10 10.5 74.4 52.5

99 55 69 68 72.6 8 18.5

18 18 22 15.5 19

2 2

0.39 4.8 3.05

0.5

0.9

5.13

0.5 0.15

1.0 0.18

Nonmetallic contains 99.9% pure silica

0.03 0.03 0.3 0.0015 0.0008

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Figure 3. Scanning electron microscopy (SEM) image and temperature-programmed oxidation (TPO) profiles of solid deposits produced from JP-8 and Jet A fuels stressed on Inconel 600 for 5 h at 500 °C and 34 atm.

Results and Discussion Amounts of Carbon Deposition Collected on Different Substrates. Thermal stressing with the jet fuel samples produced solid deposits on all the substrates, to varying extents. Figure 2 shows the amounts of carbon in the deposits on different substrates from the two fuels after 5 h of thermal stressing at 500 °C and 34 atm. The mass of the deposits as measured by the LECO model RC-412 multiphase carbon analyzer were reproducible to within (10 wt % of the deposit mass. The results shown in Figure 2 suggest that both the nature of the substrates and the composition of the fuels affect solid carbon deposition from thermal stressing. Generally, JP-8 fuel produced more carbon deposition than Jet A fuel on a given surface, with the exception of one substrate, quartz. The amounts of deposition on Inconel 718 from the two fuels can be considered to be similar, within experimental error. It should be noted that these data are specific to the two fuel samples used in this study and cannot be generalized to represent expected differences in the thermal stability of Jet A and JP-8 fuels. Figure 2 shows that the largest differences in deposition from JP-8 and Jet A fuels on the metal surfaces occurred with substrates that consisted of single metals (iron and nickel) that are known to be active dehydrogenation (or hydrogenation) catalysts, and their binary alloy Fe/Ni (55/45). It seems that JP-8 fuel is more susceptible to producing carbon deposits than Jet A fuel on these catalytically active substrates, producing 8.3 times (124 versus 15) more deposition on iron, 5.5 times (66 versus 12) more deposition on nickel, and 2.9 times (29 versus 11) more deposition on the Fe/Ni alloy. One could presume that this trend could be explained by either the higher reactivity of JP-8, or passivation of the active metal surfaces by Jet A components, or a combination of both factors. These assumptions are discussed in the next section with reference to TPO results, as well as SEM observations on deposited substrates. Quartz surfaces are usually considered inert surfaces with no activity toward deposition of carbonaceous solids from pyrolysis

of hydrocarbons. As expected, the quartz substrate gave the lowest amount of carbon deposition from JP-8 fuel (3 µg/cm2) among all the substrates; that value is 3 times lower in quantity, compared to the lowest deposit amount observed on the metallic substrates (9 µg/cm2 for SS 321 and FeCr alloy). However, in Figure 2, one can note that Jet A fuel produced substantially more carbon deposition on the quartz substrate (20 µg/cm2) than on all the metallic substrates, except for Inconel 600 and SS 304, which are the most-active metal surfaces. Possible reasons for this anomaly are also discussed in the next section. In absolute quantities, the Inconel 600 and SS304 substrates collected the largest amounts of carbon deposition among the alloys from both JP-8 and Jet A fuels. An earlier study17 also revealed Inconel 600 to be a highly active surface in regard to collecting carbon deposits from a different sample of JP-8 fuel under similar stressing conditions, and identified Inconel 718 as one of the most-stable superalloys. The JP-8 fuel sample used in this study produced ∼4 times more deposit on both the Inconel 600 and Inconel 718 substrates, respectively, than that found in the previous study;17 however, the same proportional relationship in the deposition tendency between the two alloys was maintained. The stability of Inconel 718 was attributed to the presence of the minor elements niobium, titanium, and aluminum, which are added to increase the strength and hardness of the alloy, thus reducing its susceptibility to carbon deposition.11 Compared to SS 304, SS 321 collected much less carbon deposition from either JP-8 or Jet A fuel (see Figure 2). Table 1 shows that SS 304 and SS 321 have similar elemental compositions, except for the presence of titanium in SS 321. Titanium is added to SS 321 to suppress the precipitation of grain-boundary chromium carbides and to reduce its susceptibility to intergranular corrosion.6,17 This stabilization seems to help reduce the carbon deposition, compared to the base alloy SS 304. Similarly, the resistance of FeCr alloy to carbon deposition can be explained by the stabilization of iron with the addition of chromium and aluminum.


Figure 4. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on Inconel 718 for 5 h at 500 °C and 34 atm.

TPO and SEM Analysis of Deposits Collected on Different Substrates. Figures 3-12 show the SEM images and TPO profiles of the solid deposits on different substrates. TPO profiles may give multiple CO2 evolution peaks in the range of 100900 °C (see, for example, the TPO profiles shown in Figure 3). CO2 evolution at relatively low temperatures (typically 500 °C) suggest the presence of a higher degree of structural order in the carbonaceous solids, based on their lower oxidation reactivity. The low reactivity or presence of anisotropic (or pregraphitic) order in the deposits can be related to metal catalysis of the deposit formation, producing solid carbons with moreordered structures. Possible metal catalysis of oxidation during the TPO experiments, or chemical changes that may occur in deposits during the TPO experiments, may complicate a simple interpretation of TPO profiles. However, the companion paper12 confirms that the observed TPO profiles genuinely relate to the structure and properties of the original deposits. The SEM images in Figure 3a and b show that the deposition on Inconel 600 from both JP-8 and Jet A fuels caused extensive roughening of the alloy surface. Faceted particles in JP-8 fuel deposits (Figure 3a), and the faceted and fibrous particles in Jet A fuel deposits (Figure 3b), are metal (iron and nickel) sulfides, as identified using EDS analysis that was performed on these samples, and others (see Figure 6 later in this work). Carbonaceous solids deposited from JP-8 fuel appear as small spherules and films (Figure 3a), which are different from the lacy aggregates that are observed in the deposit from Jet A fuel (Figure 3b). The TPO profiles in Figure 3c and d suggest that the carbon deposits have a range of oxidation reactivities, as illustrated by multiple CO2 evolution peaks. The high-temperature peaks in both profiles, along with substantial surface roughening on the Inconel 600 substrate, indicate the high surface activity of this alloy, as discussed in the previous section.

The Inconel 600 foil that was stressed with Jet A fuel showed more-extensive surface damage, which was most likely due to the higher sulfur content of this fuel, in comparison to JP-8 fuel. It was noted during the thermal stressing experiments that all the metal substrates that were stressed with Jet A fuel corroded more severely than those that were stressed with JP-8 fuel. Earlier studies have shown that the presence of sulfur compounds may inhibit or promote carbon deposition from hydrocarbons on metals, depending on the type of sulfur compounds and the composition of the metal substrate.19-27 More quantitative information on this effect can be obtained in the work of Brandt,21 including the ratio of carbon to sulfur found in deposits on different substrates after thermal stressing with various jet fuels. Thermal stressing of Jet A fuel produced less carbon deposition on all metal substrates, except Inconel 718, than JP-8 fuel did. One may suggest that excessive metal sulfide formation from Jet A fuel passivated some active metal sites that would have participated in carbon deposition. Alternatively, thermal deposits that are formed more readily from higher n-alkanes22 may have blocked access to some active metal sites. As discussed in the Experimental Section, Jet A has greater concentrations of higher n-alkanes that easily decompose through free-radical mechanisms to form carbonaceous solids.22,25 The appearance of apparently amorphous solid carbon aggregates in Figure 3b and the low-temperature TPO peaks in Figure 3d may be related to the formation of such thermal deposits (manifested as a broad peak at ∼200-450 °C) with adsorbed hydrocarbons (which appears as a sharp peak at ∼100 °C). Figure 4 illustrates a sharp contrast in the behavior of Inconel 718, relative to that of Inconel 600. Consistent with the observations discussed in the previous section, Inconel 718 did not show a significant surface activity for carbon deposition from either fuel. Figure 4a and b indicates the formation of metal sulfides on the surface, but in much lesser extents than those observed on Inconel 600 (see Figure 3a and b). A larger quantity of carbonaceous deposition is apparent in Figure 4b, in agreement with the TPO results. There are no significant high-

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Figure 5. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on SS 304 for 5 h at 500 °C and 34 atm.

temperature peaks in the TPO profiles for the deposits from either fuel, as shown in Figure 4c and d. A slightly larger amount of carbon deposition obtained from Jet A fuel, compared to that from JP-8 fuel (22 µg/cm2 versus 18 µg/cm2, respectively), may be attributed, in this case, to some surface activation by the reaction of sulfur compounds in Jet A fuel, as reported by Raymundo-Pinero et al.19 One can note that the CO2 peak in the TPO profile for Jet A fuel deposits (Figure 4d) evolves over a higher temperature range than that observed for JP-8 fuel deposits in Figure 4c, which suggests that the carbonaceous solids obtained the Jet A fuel reaction are more refractory than the JP-8 solids. Figure 5 shows that the SS 304 substrate also behaves differently toward JP-8 and Jet A fuels, as shown by the amount and reactivity of the carbon deposits. Jet A fuel generated a smaller amount of carbon deposition than JP-8 fuel (40 µg/cm2 versus 70 µg/cm2), and the Jet A and JP-8 fuel deposits seem to have a different distribution of oxidation reactivity. As seen for the Inconel 718 surface, the Jet A fuel deposits seem to contain more-refractory solid carbons than the JP-8 fuel deposits (see Figure 5c and d). These differences can be attributed to a more limited surface activity toward Jet A fuel, possibly through the passivation of some active metal sites by the constituent sulfur compounds or the decomposition of higher alkanes, as discussed for the Inconel 600 substrate previously in this section. The TPO of SS 304 deposits does not emit CO2 at temperatures of >600 °C, as observed with the Inconel 600 substrate (see Figure 3c and d). In other words, SS 304 surface does not seem to be as active as the Inconel 600 surface in regard to catalyzing the carbon deposition. The SS 304 substrates used with both JP-8 and Jet A fuels showed some surface roughening (see Figure 5a and b), with more-pronounced damage being observed on the surface that was stressed with Jet A fuel, which was most likely due to more-extensive metal sulfide formation. The EDS spectrum and elemental mapping for the Jet A fuel deposits on SS 304, along with the backscattered electron (BSE) images in Figure 6, confirm that the prominent filamentous structures that are observed in the BSE image consist mainly of iron

sulfides, not carbon filaments, as was also noted by Zhang and Eser6 in a similar study. Elemental maps show a ubiquitous presence of sulfur, along with iron, on the substrate surface. Figures 7 and 8 show the SEM images and TPO profiles of deposits obtained on the SS 321 and FeCr alloy surfaces, respectively. These alloys gave the lowest amount of carbon deposition obtained on metallic substrates in this study, as discussed in the previous section, along with the reasons for the stability of SS 321 and FeCr alloy surfaces. The SEM images and TPO profiles of the deposits on these substrates appear similar, which indicates much less surface degradation, compared to active metal surfaces (e.g., Inconel 600 and SS 304), and relatively flat TPO profiles are observed. For both fuels, the iron substrate showed extensive surface degradation when stressed (see Figure 9a and b), which was due to metal sulfide formation; however, substantially more carbon deposition was produced from JP-8 fuel, as shown in Figure 2 and discussed in the previous section. The TPO profiles (Figure 9c and d) clearly indicate that iron-catalyzed carbon deposition from JP-8 fuel is much more extensive than that from Jet A fuel: high-temperature TPO peaks are much more prominent for the JP-8 fuel deposits than for the Jet A fuel deposits. As discussed previously, lower amounts of carbon deposition obtained from Jet A fuel on iron can be attributed to the different hydrocarbon and sulfur compound composition of Jet A fuel, which may lead to passivation of the active metal sites by metal sulfide formation and/or encapsulation by thermal carbon deposits. Samples of deposits that had been formed on iron-containing surfaces gave, in some cases, a single TPO peak that evolved at ∼400 °C, as shown in Figure 10 for JP-8 fuel deposits on iron and Fe/Ni (55/45). This sharp peak can be ascribed to oxidation catalysis by iron particles that are dispersed in the solid carbons during thermal stressing. Previous studies have shown that iron has maximum catalytic activity, when dispersed in solid deposits, compared to most metals.28 Finding iron in a finely dispersed state in solid deposits also provides evidence for the catalytic effect of iron in carbon deposition.


Figure 6. Energy-dispersive spectroscopy (EDS) spectrum and mapping of the solid deposits on SS 304 after thermal stressing with Jet A fuel for 5 h.

Similar to the behavior of iron foils, nickel foils exhibited extensive surface changes, because of the formation of nickel sulfides from JP-8 and Jet A fuels being stressed (see Figure 11a and b). High-temperature TPO peaks exist in the deposits obtained from both fuels (see Figure 11c and d); however, they are more prominent in the JP-8 fuel deposits, along with a larger amount of carbon deposition obtained from JP-8 fuel than that from Jet A fuel. Figure 12 shows the SEM images and TPO profiles of the deposits formed on the Fe/Ni (55/45) alloy. The SEM images (see Figure 12a and b) show different structures than those

observed in individual iron and nickel substrates. For both fuels, the carbon deposit amounts are much smaller than those produced on the individual metals. This agrees with the results that were reported in a study by Zhang and Eser,6 where smaller carbon deposit amounts were attributed to the presence of a single homogeneous phase (γ-Fe) in the alloy, particularly at an Fe/Ni ratio of 55/45. Figure 13 shows the SEM images and TPO profiles of the deposits formed on a quartz substrate via the thermal stressing of JP-8 and Jet A fuels. The quartz surface eliminates the possibility of any catalytic effect on carbon oxidation during

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Figure 7. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on SS 321 for 5 h at 500 °C and 34 atm.

Figure 8. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on FeCr alloy for 5 h at 500 °C and 34 atm.

TPO experiments. The SEM images of the stressed substrates (see Figure 13a and b) show apparently amorphous particles of carbon deposits. The needlelike deposits observed in Figure 13a are artifacts from cutting the quartz substrate for analysis. As expected, the thermal stressing of JP-8 fuel gave the lowest amount of carbon deposition (3 µg/cm2) obtained on any substrate used in this study and a broad TPO peak centered at ∼420 °C with no high-temperature peaks present (see Figure 13c). However, the thermal stressing of Jet A fuel on quartz produced a much greater amount of carbon deposition (20 µg/

cm2), which gave a multipeak TPO profile (see Figure 13d) that is very different from the TPO profile of the JP-8 fuel deposit. The TPO peaks for Jet A fuel deposits evolved at 103, 320, and 510 °C. The peak at 103 °C can be assigned to adsorbed hydrocarbon species on the deposited surface. The remaining two peaks suggest the presence of two types of deposits with different oxidation reactivities. Particularly interesting is the high-temperature peak that is centered at ∼510 °C, which signals a relatively refractory solid carbon with a lower hydrogen content and/or higher degree of structural order. One should also note that Jet A fuel produced substantially more


Figure 9. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on iron for 5 h at 500 °C and 34 atm.

Figure 11. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on nickel for 5 h at 500 °C and 34 atm. Figure 10. TPO profiles of deposits from JP-8 fuel on iron (top profile) and on Fe/Ni (55/45) alloy (bottom profile).

carbon deposition on the quartz substrate than on all the metallic substrates, except for the most active surfaces (Inconel 600 and SS 304). This is also the first instance in this study where Jet A fuel produced significantly more carbon deposition than JP-8 fuel on any substrate. These results suggest that there may be a different interaction between Jet A fuel and the quartz substrate that would produce more deposition and including some carbonaceous solids with relatively low oxidation reactivity. Morrow and Cody29 reported the formation Lewis acid sites on a silica surface upon dehydroxylation by degassing under vacuum at temperatures of 400-1200 °C. Thermal stressing with jet fuel under the reaction conditions used may lead to dehydroxylation of the silica (quartz), which could produce

Lewis acid sites. Lewis acid sites are known to promote coke formation from the cracking of long-chain alkanes.30 Considering that Jet A fuel contains higher concentrations of longer-chain alkanes, one might suggest that the Lewis acid sites on the silica may interact with these reactive n-alkanes or their cracking products to produce coke. The presence of a relatively refractory solid carbon (coke), along with high concentrations of adsorbed hydrocarbons (peak at 103 °C in the TPO profile), may be explained by cracking/polymerization reactions that are catalyzed by Lewis acid sites. In this context, silica may not be considered as an inert surface. In contrast to Jet A fuel, the JP-8 fuel may not have sufficiently high concentrations of longer-chain (more-reactive) hydrocarbons to interact with the Lewis acid sites produced on the quartz substrate during thermal stressing, as discussed in the Experimental Section. Therefore, JP-8 fuel deposits would not generate

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Figure 12. SEM image and TPO profiles of solid deposits produced from JP-8 and Jet A fuels stressed on Fe/Ni (55/45) alloy for 5 h at 500 °C and 34 atm.

Figure 13. SEM image and TPO profile of carbon deposits produced from JP-8 and Jet A fuels stressed on a quartz substrate for 5 h at 500 °C and 24 atm.

the high-temperature (510 °C) peak and the associated lowtemperature (103 °C) peak that may be attributed to Lewis acid activity. Conclusions The nature and amount of carbonaceous deposition from the thermal decomposition of jet fuel were determined to be dependent on the substrate properties and jet fuel composition, under the experimental conditions used in this study. The catalysis of carbon deposition can occur on active metal

substrates that include single metals and metal alloys. Two most active metal substrates were determined to be Inconel 600 and SS 304, and these collected the largest amount of carbon deposition from both JP-8 and Jet A fuel samples. The JP-8 fuel seemed to be more susceptible to producing carbon deposits than Jet A fuel on the catalytically active substrates, including the single-metal substrates iron and nickel. Substantially lower amounts of carbon deposition obtained from Jet A fuel under the same thermal stressing conditions were ascribed to the higher concentrations of sulfur and heavy (long-chain) alkane concen-


trations in Jet A fuel than those in JP-8 fuel. These characteristics of Jet A fuel can passivate the active sites on metal substrates during thermal stressing through the excessive formation of metal sulfides or encapsulation by thermally produced solid carbons. Higher concentrations of heavier (longer-chain) hydrocarbons in Jet A fuel may also explain the much larger quantity of carbon deposition obtained from Jet A fuel (compared to JP-8 fuel) on the quartz substrate. This may involve the possible formation of Lewis acid sites on the quartz substrate during thermal stressing. This study also confirms the catalytic behavior of iron and iron-based alloys during carbon oxidation, as observed in previous studies. Acknowledgment The authors acknowledge funding from the Air Force Office of Scientific Research (AFOSR) to conduct this study. Literature Cited (1) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanism. Ind. Eng. Chem. Res. 1993, 32, 3117-3122. (2) Yu, J.; Eser, S. Determination of Critical Properties (Tc, Pc) of Some Jet Fuels. Ind. Eng. Chem. Res. 1995, 34, 404-409. (3) Rosenthal, D. J.; Teja, A. S. The Critical Properties of n-Alkanes Using a Low Residence Time Flow Apparatus. AIChE J. 1989, 35, 1829. (4) Li, J. Metal Surface Effects on Deposit Formation from Thermally Stressed Jet Fuel and Model Compounds, Ph.D. Thesis, Pennsylvania State University, University Park, PA, 1998. (5) Yu, J.; Eser, S. Thermal Decomposition of Jet Fuel Model Compounds under Near-Critical and Super-Critical Conditions. 2. Decalin and Tetralin. Ind. Eng. Chem. Res. 1998, 37, 4601-4608. (6) Zhang, F. Carbon Deposition on Heated Alloy Surfaces From Thermal Decomposition of Jet Fuel, M.S. Thesis, Pennsylvania State University, University Park, PA, 2000. (7) Butnark, S. Thermally Stable Coal Based Jet FuelssChemical Composition, Thermal Stability, Physical Properties and Their Relations, Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 2003. (8) Strohm, J. J. Novel Hydrogen donors for the Improved Thermal Stability of Advanced Aviation Jet Fuels, M.S. Thesis, Pennsylvania State University, University Park, PA, 2002. (9) Li, J.; Eser, S. Metal Surface Effects on Deposit Formation in a Flow Reactor and Characterization of Jet Engine Fuel System Deposits. Prepr.sAm. Chem. Soc., DiV. Pet. Chem. 1996, 41 (2), 508-512. (10) Venkataraman, R. Utility of Temperature Programmed Oxidation in Characterizing solid deposition from Heated Jet Fuel, M.S. Thesis, The Pennsylvania State University, University Park, PA, 2004. (11) Edwards, T. Cracking and Deposition Behavior of Supercritical Hydrocarbon Aviation Fuels. Combust. Sci. Technol. 2006, 178, 307-334. (12) Venkataraman, R.; Altin, O.; Eser, S. Utility of Temperature Programmed Oxidation in Characterizing Solid Deposition from Heated Jet Fuel. Ind. Eng. Chem. Res. 2006, 45, 8956-8962. (13) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E. JP8+ 100: The Development of High Stability Jet Fuel. J. Energy Resour. Technol. 1996, 118, 170-179.

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ReceiVed for reView July 25, 2006 ReVised manuscript receiVed October 1, 2006 Accepted October 3, 2006 IE060968P

Deposition of Carbonaceous Solids on Different Substrates from

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