Energy & Fuels 2004, 18, 835-843
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The Effect of Fuel Composition and Dissolved Oxygen on Deposit Formation from Potential JP-900 Basestocks Melissa A. Roan* and Andre´ L. Boehman* The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 4, 2003. Revised Manuscript Received March 4, 2004
Future high-speed aircraft will be challenged to meet their onboard cooling requirements, because of weight limitations and higher heating loads. Therefore, the fuel will be required to serve as both propellant and coolant, and, thus, the fuel will need to be thermally stable. An advanced thermally stable jet-fuel formulation, called “JP-900”, has been under development for such applications. Six formulations of potential JP-900 basestocks were tested in a flow reactor to assess their thermal stability and resultant deposit morphology. Of the six fuels that were tested, three fuels consisted largely of hydroaromatic and aromatic compounds, and the other three fuels consisted mainly of cyclic compounds. The fuels were made from refined chemical oil (RCO), a product of coke manufacture, light cycle oil (LCO), a petroleum product, or blends thereof, and were hydrotreated to different extents. All six fuels showed marked improvements in stability when compared to the conventional military jet fuel JP-8. The fuels that consisted largely of hydroaromatic compounds were stable at high temperatures but produced large amounts of deposits in the autoxidative regime, whereas the fuels that consisted of cycloalkanes were more stable in both the autoxidative and pyrolytic regimes than their less-saturated counterparts. Basestocks derived from RCO were more stable in the pyrolytic regime than those derived from LCO. This observation was attributed to their differences in gross fuel composition. Removal of dissolved oxygen via nitrogen sparging increased both the autoxidative and pyrolytic stability of all six fuels. There were also morphological differences in the high-temperature deposits formed by the sparged and nonsparged fuels, which indicated differences in the deposition mechanism when dissolved oxygen was present in the starting fuel.
Introduction Current jet fuels break down under high temperatures and pressures, forming solid carbonaceous deposits. At best, these deposits detract from the heatexchange potential of the surfaces upon which they form. At worst, they constrict fuel flow, potentially leading to an engine shutdown. Three regimes for deposit formation have been defined1 and are widely accepted. These regimes describe different sets of chemical reactions and are largely defined by the temperatures at which these reactions are expected to occur. Autoxidation occurs at temperatures of decalin > ethylcyclohexane > n-butylcyclohexane > n-decane > n-tetradecane.13 Dramatic differences in pyrolytic stability have been observed between JP-8 that is derived from petroleum and JP-8 that is derived from coal. The coal-derived fuel is significantly more stable and pro(7) Zabarnick, S.; Witacre, S. D.; Zelesnik, P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1996, 41, 438-441. (8) DeWitt, M. J.; Zabarnick, S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2002, 47, 183-186. (9) Song, C.; Peng, Y.; Jiang, H.; Schobert, H. H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1992, 37, 484-492. (10) Altin, O.; Bock, G.; Eser, S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2002, 47, 208-211. (11) Ervin, J. S.; Ward, T. A.; Williams, T. F.; Bento, J. Energy Fuels 2003, 17, 577-586. (12) Yoon, E. M.; Selvaraj, L.; Song, C.; Stallman, J. B.; Coleman, M. M. Energy Fuels 1996, 10, 806-811. (13) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234-243.
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duces far less deposit material under pyrolytic conditions than the petroleum-derived fuel. This difference in stability was attributed to the fact that the coalderived JP-8 consists mainly of naphthenic compounds such as decalin, which have been shown to have a high degree of thermal stability, whereas the petroleumderived JP-8 consists largely of normal alkanes and alkyl aromatics.13 Most studies of pyrolytic reactions are conducted under deoxygenated conditions. The removal of dissolved oxygen simplifies the reaction system and allows examination of the pyrolytic decomposition of a fuel without interference from the autoxidative regime. Although the effect of dissolved oxygen in a fuel has been well-characterized at low temperatures (i.e., autoxidative reactions), little work has been done on the effect of dissolved oxygen in the intermediate and pyrolytic regimes. There is evidence that the removal of dissolved oxygen can increase the pyrolytic deposit formed from certain fuels.14,15 The fuels where this phenomenon is observed have a tendency to be naphthenic in nature, and naphthenic fuels have demonstrated good thermal stability in the pyrolytic regime and hold promise as a candidate JP-900, which is a fuel that would be stable up to 900 °F (482 °C). In addition, few studies have examined pyrolytic reactions in fuels that are not sparged with nitrogen. If the presence or absence of dissolved oxygen in the fuel makes a difference in the deposition behavior of the fuel, that difference should be explored. Seven potential basestocks for JP-900 were examined in this study. The fuels were stressed in a flow reactor, and the deposits were analyzed both quantitatively and qualitatively. Deposit profiles were formed by plotting the amount of carbon measured as a function of axial distance along the reactor (implying a function of temperature). Scanning electron microscopy (SEM) and temperature-programmed oxidation (TPO) were used to compare the morphology of deposits formed from different fuels, to determine if there were any mechanistic differences in deposit formation. Experimental Section Seven fuels were examined in this study. The first was JP8, which is the current military fuel; it was obtained from Wright-Patterson Air Force Base. JP-8 is a petroleum-derived fuel that contains an additive package that consists of a corrosion inhibitor/lubricity enhancer, icing inhibitor, and antistatic additive. Because JP-8 contains an additive package, it is difficult to make mechanistic comparisons between this fuel and the others examined; however, it does serve to provide a benchmark for thermal stability. The other six fuels were produced by PARC Technical Services (Pittsburgh, PA) using their catalyst-screening hydrotreater units. Two aromatic feedstocksslight cycle oil (LCO) and refined chemical oil (RCO)swere heavily hydrotreated. The LCO, which is a product of fluid catalytic cracking, was produced by United Refining Company. The RCO is a distillate from the refining of coal tar that is produced from the coke industry; it has been provided by Koppers Industries, Inc. The feedstockssRCO, (14) Edwards, T.; Liberio, P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43, 353-356. (15) Roan, M. M.S. Thesis, The Pennsylvania State University, University Park, PA, 1999.
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Table 1. Hydrocarbon Composition of Fuels Tested in This Study Component (wt %) fuel
hydroaromatics
othera
21.3
7.8
1.2
25.2 31.6 33.5
22.1 58.1 44.3
0.5 1.6 1.6
0 0 0
0.3 0.1 0.3
6.4 1.6 3.0
n-alkanes
iso-alkanes
cycloalkanes
aromatics
JP-8
28.2
35.1
16.4
hydrotreated light cycle oil (LCO) hydrotreated refined chemical oil (RCO) hydrotreated blend
26.6 0.3 4.7
18.7 0.4 7.4
6.9 8.0 8.5
saturated light cycle oil (LCO) saturated refined chemical oil (RCO) saturated blend
28.5 4.4 15.9
21.5 0.9 10.2
43.4 93.0 70.6
a
“Other” includes heteroatomic compounds, alkenes, and unidentified compounds.
Figure 1. (s) Wetted wall temperature and (- - -) bulk fuel temperature profiles as a function of flow reactor axial distance. LCO, and a 1:1 blend of RCO and LCOswere hydrotreated to reduce the heteroatomic and aromatic content. The initial hydrotreatment of the RCO, the LCO, and the 1:1 blend of the two fuels was accomplished using a Crosfield Ni-Mo catalyst at a pressure of 710 psi and a temperature of 685-725 °C. The products of this process have high concentrations of hydroaromatic and aromatic compounds and are called the hydrotreated fuels in this work. Some of these hydrotreated fuels were then further treated with an Englehard Pt-Pd catalyst at 2100 psi and 400-500 °C. The resultant products have a very high degree of saturation and are called the saturated fuels. They consist mainly of naphthenic compounds. Table 1 shows the hydrocarbon composition for the fuels used in this work. The fuels were stressed in a flow reactor that was constructed at Pennsylvania State University.16 For the experiments with deoxygenated fuel, nitrogen sparging was performed for 2 h before the fuel was stressed. The fuel was then pumped from an airtight reservoir into the reactor by a Waters model 515 HPLC pump at a rate of 10 mL/min. The inlet temperature and pressure were monitored, and the fuel passed through a 2-µm filter to remove any sediment that would have formed during storage. The fuel passed through a Carbolite TVS 12/60/900 three-zone furnace that was set to a temperature of 700 °C. The heated reaction zone was 36 in. long, and the reactor tube was a 304 stainless steel tube (1/4 in. outer diameter, 1/8 in. inner diameter). When the fuel exited the reaction zone, the bulk temperature of the fuel was measured, using a stainless-steel-clad type-K thermocouple. The fuel passed through a 2-µm filter to remove any solids that had formed in the bulk fuel and was then cooled by a counterflow heat exchanger. The pressure was regulated using a Nupro needle valve as a backpressure control valve. (16) Roan, M. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 2003.
The fuel had an inlet temperature of 23 °C and an outlet temperature of ∼480-500 °C. Figure 1 shows the wetted wall temperature and the bulk fuel temperature, as a function of axial distance. The wetted wall temperature was plotted by measuring the wall temperature, using type-K thermocouples that were welded to the reactor tube. The bulk fuel temperature was estimated using a model that was developed by Goel and Boehman.17 The system was maintained at a pressure of 700 psi, and, as a result, the fuel was in a supercritical state in the flow reactor. The run was performed for 7 h, to ensure sufficient deposit buildup for analysis. When the run was completed, the tube was sectioned, washed with hexanes, and then vacuum-dried at 140 °C. The samples were then analyzed using a LECO model R412 carbon determinator to determine the amount of carbon deposit that formed, as a function of axial distance and temperature. Results are presented in units of µg C/cm2 (milligrams of carbon per square centimeter). The LECO model R412 carbon determinator also gives the TPO profiles of each sample. These profiles can be useful in determining the reactivity of the carbon deposits and making inferences about morphology based on these reactivities. SEM analysis was used to examine the deposit morphology. The deposit morphology can provide qualitative information about the deposit formation mechanism.18,19
Results Much variation was observed in the amount of deposit formed in the reaction tube. Figure 2 shows the average carbon deposit profile generated by the blend of hy(17) Goel, P.; Boehman, A. L. Energy Fuels 2000, 14, 953-962. (18) Li, J. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1998. (19) Altin, O.; Eser, S. Ind. Eng. Chem. Res. 2001, 40, 596-603.
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Figure 2. Average carbon deposit profile for hydrotreated light cycle oil (LCO), with 95% confidence limits.
Figure 3. Carbon deposit profiles of fuels run without a nitrogen sparge.
drotreated LCO and RCO. The 95% confidence limits are shown. The greatest variation is observed in the early stages of the reactor tube, where autoxidation is the primary mechanism. In contrast, the deposits in the high-temperature regime were much more repeatable. This phenomenon was observed in all of the hydrotreated fuels. In addition to the variation in the quantity of carbon, there are other issues with the reproducibility of the axial location of the peak carbon deposit, especially in the autoxidative regime. Because of time and equipment restraints, only 15 samples were taken from each reaction tube for carbon analysis. These samples are ∼1 in. long and have been taken from the same axial position in each reaction tube. This is especially important, because not every section of tubing is analyzed, which leads to the concern that peak deposition may be missed. Multiple runs were performed to reduce this possibility. When considering wall temperatures, the autoxidative regime is considered to be the area within 11 in. from the inlet, the intermediate regime is considered to be the area 11-20 in. from the inlet, and the pyrolytic regime is considered to be the area 20-36 in. from the inlet. When considering bulk fuel temperatures, the autoxidative regime is considered to be the area within 16 in. from the inlet, the intermediate regime is considered to be the area 16-25 in. from the inlet, and the pyrolytic regime is considered to be the area 25-36 in. from the inlet. Although there is much variation in these carbon deposit profiles, particularly in the amount of autoxidative carbon measured, the carbon deposit profiles are still useful tools for comparing the relative thermal stabilities of various jet fuels. Figure 3 shows the deposit profiles for the fuels and JP-8 that were run without a nitrogen sparge. The wetted wall temperature and bulk fuel temperature profiles, as a function of axial distance, are those shown in Figure 1. The hydrotreated fuels demonstrate the largest autoxidative instability. The hydrotreated RCO and the hydrotreated blend are the least autoxidatively unstable, and stressing them produces more autoxidative deposit than stressing JP-8 under identical conditions. The carbon deposit formed in the autoxidative regime when the hydrotreated RCO was stressed attained a value of 446 µg C/cm2, whereas the carbon deposit formed under identical conditions from JP-8 was 164
µg C/cm2. The hydrotreated fuels contain a large percentage of hydroaromatics, which oxidize easily; therefore, their instability is expected. However, the hydrotreated LCO produces less autoxidative deposit material than the hydrotreated RCO or the hydrotreated blend, with a peak carbon production of 106 µg C/cm2, as opposed to the peak carbon deposit of 446 µg C/cm2 that was observed with the hydrotreated RCO. The saturated fuels prove to be much more stable in the autoxidative regime, generating a relatively small amount of deposit when stressed. The amount measured ranged from 16 µg C/cm2 for the saturated RCO to 50 µg C/cm2 for the blend of the saturated fuels. The saturated RCO was the most stable in this temperature regime, although the differences in stability were not as pronounced as the differences that were observed with the hydrotreated fuels. All six developmental fuels showed reduced carbon deposit formation in the pyrolytic regime when compared to JP-8. The saturated fuels produced significantly less deposit than the hydrotreated fuels. The lowest amount of deposit was observed with the saturated RCO, measured as 21 µg C/cm2, as compared to 76 µg C/cm2 for the hydrotreated RCO. Another notable trend in the pyrolytic regime is that the fuels that have been derived from LCO produce more pyrolytic deposit material than those that have been derived from RCO. Although the differences between the deposit profiles in the pyrolytic regime are not as pronounced in the autoxidative regime, it is certainly significant. Figure 4 shows the deposit profiles for the six fuels and JP-8 that have been run with a nitrogen sparge. The removal of dissolved oxygen from the system decreases the autoxidative deposit formed, as expected. This is particularly notable in the deposit profiles for the hydrotreated LCO. The peak carbon deposition decreases from 106 µg C/cm2 to 25 µg C/cm2 when the fuel is sparged with nitrogen, which was expected. However, two of the fuelssthe hydrotreated RCO and the blend of hydrotreated fuelssstill produce large amounts of deposit in the autoxidative regime (228 and 1040 µg C/cm2, respectively), despite the absence of dissolved oxygen. The saturated fuels do not experience a large decrease in carbon deposit upon nitrogen sparging; however, relatively little carbon deposit was observed in this regime without a nitrogen sparge.
Deposit Formation from Potential JP-900 Basestocks
Figure 4. Carbon deposit profiles of fuels run with a 2-h nitrogen sparge.
Some decrease in pyrolytic deposition was observed for certain fuels when sparged with nitrogen, especially the hydrotreated LCO. The peak pyrolytic deposit measured at a distance of 35 in. decreases from 39 µg C/cm2 to 11 µg C/cm2 when sparged with nitrogen. Relative to the experimental uncertainty of deposit profiles in the pyrolytic regime, this decrease is statistically significant. A similar decrease in carbon deposit formation is also observed for the hydrotreated RCO, the saturated LCO, and the blend of saturated oils. There is no significant difference in the amount of deposit produced when the fuel is sparged with nitrogen for the blend of hydrotreated fuels or the saturated RCO. Some fuels have been observed to demonstrate increased pyrolytic deposit when dissolved oxygen is removed via nitrogen sparging.14,15 Such fuels have a tendency to be cyclic in nature. The fuels tested in this study do not demonstrate such behavior. However, the original hypothesis was that the fuels with dissolved oxygen present that have undergone autoxidation would demonstrate different pyrolytic behavior from those that had been sparged with nitrogen, because of the presence of oxygenated compounds. The deposits formed by stressing these fuels were subjected to TPO and SEM, to provide information about the mechanisms of deposit formation under the present test conditions. TPO can be used to probe the reactivity of the deposits formed from various fuels. The burn-off temperature of a structure indicates its reactivity with oxygen. The less reactive a carbon structure is, the higher its burn-off temperature will be.19 Although this information cannot be used alone to determine the morphology of various deposits, it can be used to indicate differences in reactivities of various carbonaceous deposits and, thereby, potential differences in structure. Dissimilar morphologies indicate potential differences in the deposit formation mechanisms. Differences were detected in the deposit formed in the intermediate regime in certain fuels. Figure 5 shows SEM photomicrographs of deposits formed from hydrotreated LCO, hydrotreated RCO, and saturated LCO stressed on an “as-received basis” and with the dissolved oxygen removed via nitrogen sparging. The deposits are formed under identical conditions; however, there are clear differences in the morphology of the deposits. The deposits shown in Figure 5 were formed at a wall
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temperature of 480 °C and a bulk fuel temperature of 295 °C. The TPO profiles of the fuels are shown in Figure 6. The TPO profiles for the hydrotreated LCO indicate that the deposits formed under identical temperature conditions and different dissolved oxygen contents have different reactivities in the presence of oxygen. The profile for the deposit produced under nonsparged conditions shows one large carbon burn-off peak at ∼430 °C. The TPO profile for the deposit formed when the fuel is sparged with nitrogen has three distinct peaks: one at 310-410 °C, one at 550 °C, and one at 850 °C. These peaks indicate the presence of carbonaceous formations that have different reactivities toward oxygen and likely have different structures. The deposit formed by the hydrotreated LCO under nonsparged conditions is quite granular and has a relatively uniform structure. The deposit formed by the same fuel under nitrogen-sparged conditions is morphologically different. Several types of structures are visible, including the granular structure that comprises the deposit from the nonsparged fuel, platelets, and filaments. The deposits from the hydrotreated RCO have very similarly shaped TPO profiles. Both profiles have a smaller burn-off peak at 360 °C and a larger burn-off peak at a higher temperature. In the case where the fuel has not been sparged with nitrogen, the larger burn-off peak appears at a lower temperature (430 °C, as compared to 500 °C), thus indicating the presence of a more-reactive species. Despite these similarities, striking differences were observed in the morphologies of the deposits when they were observed using SEM. The deposit from the nonsparged fuel consists of stout, filamentous structures that have diameters of 2-3 µm. In contrast, when the dissolved oxygen is removed via nitrogen sparging, significantly smaller filaments are observed, along with larger hexagonal crystals. Also note that the deposits formed from the hydrotreated RCO have different morphologies from those formed from the hydrotreated LCO. The TPO profiles for the saturated LCO seem to indicate a difference in reactivity of the deposits. Both profiles have a very similar low-temperature peak at ∼310 °C, as well as a high-temperature peak. However, the profile for the deposit from the fuel that has been sparged with nitrogen has a very sharp high-temperature peak at 450 °C, whereas the profile for the deposit from the fuel that has not been sparged with nitrogen has a much broader peak at 500 °C. Both of these deposits have a granular structure that is similar to that of the deposit produced by nonsparged hydrotreated LCO. In addition, both deposits have similar structures to each other, regardless of the presence or absence of dissolved oxygen. However, the deposit produced from the nitrogen-sparged fuel is considerably denser than that produced by the fuel stressed in the as-is condition. Discussion Fuel composition has been shown to have a great effect on thermal stability. Table 1 shows the bulk hydrocarbon composition of the fuels studied. The fuels derived from petroleum sources (JP-8, LCO) have significantly higher n-alkane contents than do the fuels that are derived from coal tar. The hydrotreated fuels
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Figure 5. SEM images (magnification of 10 000×) of fuel deposits formed at a wall temperature of 480 °C and a bulk fuel temperature of 295 °C without a sparge (panels a, c, and e) and with a 2-h nitrogen sparge (panels b, d, and f). Panels a and b are micrographs of hydrotreated refined chemical oil (RCO), panels c and d are micrographs of hydrotreated light cycle oil (LCO), and panels e and f are micrographs of saturated LCO.
consist largely of hydroaromatic and aromatic compounds, whereas the saturated fuels consisted largely of cycloalkanes.
Different components of jet fuel behave differently at high temperatures, both in the autoxidative regime and the pyrolytic regime. Larsen et al.20 showed that dif-
Deposit Formation from Potential JP-900 Basestocks
Energy & Fuels, Vol. 18, No. 3, 2004 841 Table 2. Peroxide Concentration of Fuels before Stressing fuel JP-8 hydrotreated light cycle oil (LCO) hydrotreated refined chemical oil (RCO) hydrotreated blend (opened barrel) hydrotreated blend (fresh barrel) saturated light cycle oil (LCO) saturated refined chemical oil (RCO) saturated blend
peroxide content (ppm) none detected 0.6 212 631 46 6 7 8
ferent compound classes oxidize at different rates. They determined that saturated alkanes (both n-alkanes and cycloalkanes) behaved in a similar manner and were quick to oxidize. Alkyl aromatics and hydroaromatic compounds were determined to be even more reactive, an observation that they attributed to the activating influence of the aromatic ring. On the other hand, PAHs (such as naphthalene) were observed to be quite stable. However, Larsen et al. concluded that the variations of
stability that are due to hydrocarbon structure were less important than the stabilizing effect of natural inhibitors.20 The hydrotreated fuels produce a considerable amount of oxidative deposit upon stressing, particularly the fuels that contain the hydrotreated RCO. Table 1 shows that the hydrotreated RCO consists largely of hydroaromatic compounds (58.1 wt %). The hydrotreated RCO also contains a high percentage of tetrahydronaphthalene (42 wt % of the total fuel), which oxidizes very quickly. The hydrotreated LCO, which produces less oxidative deposit, has a lower weight percentage of hydroaromatic compounds (22 wt %) and an even lower concentration of tetralin (3-4 wt %). The higher percentage of saturated compounds present in the hydrotreated LCO may help to increase its autoxidative stability. The saturated fuels do produce some deposit in the autoxidative regime. However, the amount of deposit produced is much less than that observed from the hydrotreated fuels, which indicates a higher oxidative stability. The saturated fuels are heavily hydrotreated and, therefore, do not likely contain any natural, sulfurcontaining inhibitors. Although saturated compounds can oxidize, the reaction is apparently slow enough that the amount of deposit that is formed is still considerably lower than that formed by the hydrotreated fuels, or by JP-8. Conventionally, sparging with nitrogen to remove dissolved oxygen reduces the formation of autoxidative deposit. This behavior is observed with JP-8, the hydrotreated LCO, and the saturated fuels. However, sparging with nitrogen did not decrease the amount of deposit formed from the hydrotreated blend of fuels or the hydrotreated RCO significantly. To explain this phenomenon, ASTM standard D 370385 was used to determine the peroxide number of the starting fuels. The results are shown in Table 2. The saturated fuels had low peroxide values, which indicates that very little peroxide had formed during storage and, thus, implies that little oxidation had occurred. The hydrotreated LCO had an extremely low peroxide number, which indicated that it was very stable during storage. X-ray analysis21 indicated that there were sulfur compounds present in the fuel, which may have served as antioxidants and prevented peroxide buildup in the hydrotreated LCO. The sulfur content of the hydrotreated LCO may have also contributed to its enhanced autoxidative stability, when compared to the other two hydrotreated fuels. In addition, the hydrotreated LCO, while having a high percentage of
(20) Larsen, R. G.; Thorpe, R. E.; Armfield, F. A. Ind. Eng. Chem. 1942, 34, 183-193.
(21) Badger, M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45, 531-533.
Figure 6. Temperature-programmed oxidation (TPO) profiles for fuel deposits formed at a wall temperature of 480 °C and a bulk fuel temperature of 295 °C (s) without a sparge and (- - -) with a 2-h nitrogen sparge: (a) hydrotreated RCO, (b) hydrotreated LCO, and (c) saturated LCO.
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hydroaromatics, contained only 3-4 wt % of tetralin, which oxidizes very quickly. The hydrotreated RCO and the hydrotreated blend of fuels had much-higher peroxide values than the other fuels that were tested. This may be attributed to the high percentage of easily oxidizable compounds, such as tetralin. The formation of peroxides during storage is a likely explanation for the formation of deposit in the autoxidative regime, even though the oxygen has been removed. The first step in the formation of autoxidative deposits is the formation of peroxides. If the peroxides are already present, they will not be removed by nitrogen sparging, as dissolved oxygen will be; therefore, the autoxidation mechanism will proceed until the peroxides are consumed. Differences in pyrolytic stability were also observed. These differences were not as pronounced as the differences in autoxidative stability, and all six developmental fuels showed significant improvement when compared to JP-8. The differences in pyrolytic stability can also be largely attributed to differences in fuel composition. Alkyl aromatics generally are highly unstable, as are n-alkanes. Cycloalkanes have been shown to be much more stable,9 and, therefore, their presence in a fuel will lend stability to the fuel. Hydroaromatic compounds serve as hydrogen donors and have been shown to increase fuel stability in the pyrolytic regime. The fuels tested were chosen for their high concentrations of hydroaromatic or naphthenic compounds. As previously mentioned, the saturated fuels produce less pyrolytic deposit material than do their hydrotreated counterparts. Although this reduction is not as drastic as that observed when these fuels are compared to JP-8, it is still significant. For example, under nonsparged conditions, at a distance of 33 in. along the reactor tube, hydrotreated LCO produced 35 µg C/cm2. Saturated LCO produced 27 µg C/cm2. Similar behaviors were observed for the other fuels. The developmental fuels have significant differences in composition: the saturated fuels consist mainly of cycloalkanes, and the hydrotreated fuels consist of hydroaromatics and aromatics; however, the differences in composition do not seem to affect their thermal stabilities differently. Although all of the fuels are more stable than JP-8 and produce relatively low amounts of deposit, the fuels that have been derived from the LCO were less stable than those derived from the RCO. The fuels that were derived from the petroleum-based LCO have a higher percentage of n-alkanes (25-27 wt %) and iso-alkanes (19-22 wt %) than do the fuels from the RCO (
