Gas-Phase Hydrodesulfurization of JP-8 Light Fraction Using Steam

Gas-phase hydrodesulfurization of a JP-8 light fraction was investigated over CoMo/Al2O3 and NiMo/Al2O3 catalysts. Use of a light fraction provides a ...
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Gas-Phase Hydrodesulfurization of JP-8 Light Fraction Using Steam Reformate Xiwen Huang and David L. King* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352

Gas-phase hydrodesulfurization of a JP-8 light fraction was investigated over CoMo/Al2O3 and NiMo/Al2O3 catalysts. Use of a light fraction provides a fuel that is more easily desulfurized and allows the process to operate in the vapor phase. This study investigated the utilization of reformate (syngas) from a steam reformer rather than pure H2 as gas feed to HDS unit. This is consistent with what might be available to the military during operation in the field. Dry syngas functions almost as well as pure H2 in the HDS reaction, and sulfur levels below 5 ppmw are readily obtained from a feed initially containing 320 ppmw sulfur. Addition of 40 vol % steam to the syngas has a significant negative impact on HDS performance with CoMo/Al2O3, but only a small effect with NiMo/Al2O3. The impacts of various process conditions on S removal efficiency were examined and will be described. 1. Introduction The U.S. Army is carrying out research and development to provide new technology for field-based power generation. A candidate technology is fuel cells, which can provide a silent, low heat signature, and portable source of power. Fuel cell technology is focusing on several military applications, such as low-power systems (2 kW) for mobile power generation and auxiliary power units.1 Hydrogen is the required fuel for PEM fuel cells, and reforming of JP-8 logistics fuel provides a primary means to supply this hydrogen. One of the major barriers for JP-8 reforming is the presence of significant organic sulfur impurities in JP-8 with contents ranging from a few hundred ppmw to a maximum of 3000 ppmw. Sulfur impurities, even at low levels, are significant poisons for both catalysts in the reformer2-4 and electrode catalysts in the fuel cell.5,6 Thus, a practical method to remove fuel sulfur to ppm or sub-ppm levels is necessary. The only commercially practiced technology for removal of sulfur from liquid hydrocarbon feedstocks is hydrodesulfurization (HDS). Organic sulfur present in the fuel is catalytically converted to H2S over sulfided CoMo/Al2O3 or NiMo/Al2O3 catalysts, typically in three-phase trickle bed reactors under high hydrogen pressure.7 This technology is difficult to adapt to small-scale applications, due to the high temperature and pressure of the operation, the lack of available high pressure hydrogen, and the difficulty in scaling down the trickle bed operation to a compact unit. As a result, alternate sulfur removal technologies, primarily based on adsorption, are also being investigated.8-12 Systems for production of hydrogen or syngas based on integrating an adsorptive desulfurizer with a small reformer have recently been described.13,14 This investigation was initiated in an effort to develop a practical JP-8 desulfurization process based on hydrodesulfurization technology that could be employed by the military in field operations. Unlike an industrial HDS unit that requires a high-pressure hydrogen co-feed, this system was intended to work at reduced pressures using hydrogen-containing reformate. Low-pressure operation obviates the need to employ heavy and expensive equipment. Two themes drove our approach: (i) to * To whom correspondence should be addressed. Phone: (509) 3753908. Fax: (509) 375-2186. E-mail: [email protected].

generate a light boiling fraction of JP-8 that could be vaporized without difficulty (or without carbon formation) so that the HDS reaction could be carried out in the vapor phase (by moving from trickle phase to vapor phase operation, high-pressure requirements might be significantly reduced) and (ii) to utilize syngas in place of hydrogen, since syngas at moderate pressure can be produced in the field by steam reforming of the purified JP-8 fuel. Most of the syngas would be further purified to power the fuel cell, but a fraction of the syngas could be diverted to a compact hydrodesulfurization unit. An added benefit of the approach is that through use of a light cut of JP-8 fuel the catalytic requirements for the HDS process could be reduced, since HDS is known to be more effective if heavy, high boiling sulfur compounds are absent from the fuel. The overall approach consists of the following unit operations: fuel distillation, hydrodesulfurization of the light fraction, and adsorbent polishing of the HDS product as necessary to meet the final fuel specifications. The unused heavier fraction of the fuel would be otherwise utilized, for example, in direct vehicle propulsion. This paper focuses on the hydrodesulfurization portion of the overall approach. Papers describing distillation using microchannel hardware and adsorbent sulfur polishing will be published separately. The entire integrated process is shown schematically in Figure 1. In the envisioned HDS process, syngas reacts with the JP-8 light cut in a single pass. The liquid-phase product is condensed, and the unreacted syngas now containing H2S is separated from the desulfurized JP-8 light cut product and combusted to provide heat to maintain the endothermic steam reforming reaction or the HDS reaction. This paper describes gas-phase hydrodesulfurization studies of JP-8 light cut fuel over commercially available CoMo/Al2O3 and NiMo/Al2O3 catalysts. We found very little available information from the literature on HDS using syngas with these two catalysts, especially for real JP-8 fuel rather than a surrogate mixture. We found even less information with steam also present in the feed. The objective of this study was therefore to identify an optimal process strategy for maximum sulfur removal from JP-8 light cut fuel through an HDS process using syngas (including steam). 2. Experimental Section 2.1. Materials. The catalysts employed were commercial presulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts (Criterion

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Figure 1. Integrated JP-8 desulfurization process.

Catalysts and Technologies). For a typical run, 1 g of catalyst was loaded into the reactor and pretreated for 4 h with a mixture comprising 75 sccm H2 and 4 cm3/h JP-8 light cut at 343 °C and 250 psig. Raw JP-8 fuel with a sulfur content of 1400 ppmw was supplied by BP from a refinery located in Tacoma, WA. The light cut fuel was prepared by glassware distillation with a cutoff temperature of 176 °C. The distillation cut was chosen to eliminate the less reactive, heavy sulfur compounds such as benzothiophene (BT) and alkyl-substituted BTs. Simulated dry reformate (syngas) consisting of 74% H2, 14.1% CO2, and 11.9% CO was provided from gas cylinders. This composition is representative of reformate product that we have obtained from steam reforming of various liquid hydrocarbon fuels. This reformate typically includes about 40 vol % steam based on an initial H2O/carbon feed ratio of 3. 2.2. HDS Reaction System. The HDS reaction was carried out in a fixed bed reactor with a 1-2-g catalyst loading. A thermocouple probe with 10 detection points was inserted along the axis of the reactor to obtain a thermal profile. One of the temperature detection points was located inside the catalyst bed. Simulated dry reformate and JP-8 light cut were introduced to the HDS reactor at 350 °C after being mixed in a microchannel vaporizer. The HDS reaction was carried out near-isothermally, with reaction pressures ranging from 50 psi to 280 psig. The distillation curve of the raw JP-8 was simulated by CHEMCAD, and the results were used to provide an operating window to ensure the fuel remained in the gas phase under all conditions of operation. Liquid product was collected upstream of a backpressure regulator in a pressure vessel held at 4 °C. 2.3. Product Analysis. Sulfur removal efficiency was determined by quantifying the total sulfur concentration in the liquid product. The analytical system utilized a HP5955 gas chromatograph equipped with a sulfur chemiluminescent detector (SCD). A 30-m DB5 column, programmed to operate from 50 to 250 °C, provided separation of the various sulfur components, and 100 ppm sulfur oil was used as calibration standard for calculating the total sulfur concentration, which was obtained by integrating the total area under the sulfur peaks. We assumed a constant response factor for all sulfur species. The hydrocarbon distribution was analyzed using a Varian gas chromatograph equipped with a FID detector and a DB5 column. Semiquantitative analysis of aromatic and olefinic concentrations in the feed was obtained by 1H and 13C NMR.

3. Results and Discussion 3.1. Analysis of JP-8 Fuel Properties. JP-8 fuel composition is similar to Jet A kerosene with additional specifications and additives imposed by the military. The fuel specifications include up to 3000 ppmw sulfur, although in the U.S. sulfur concentrations are generally lower. Most hydrocarbons in JP-8 are paraffins, naphthenes, and aromatics with carbon number between 8 and 16. Figure 2 compares the JP-8 light cut fuel obtained by batch distillation (obtained below a cutoff point of 176 °C, which corresponds to approximately 16 vol % of the fuel) with the raw JP-8. This distillate comprises primarily C8-C11 hydrocarbons with a small fraction above C11. We calculated the thermodynamic properties of a simulated JP-8, using a surrogate mixture involving 46 major components using CHEMCAD. The calculated density and average molecular weight of raw JP-8 were 0.79 g/mL and 160 respectively, which are in good agreement with literature data.15 This simulation was also applied to the JP-8 light cut, which predicts a density of 0.72 g/mL and an average molecular weight of 142. The elimination of high-boiling compounds reduces total sulfur concentration significantly, as shown in Figure 3. The sulfur component distribution of raw JP-8 (Figure 3) shows that the majority of sulfur species comprise dimethyland trimethylbenzothiophenes. Substituted benzothiophenes are present only in very low concentrations. The light cut contains predominantly alkyl-substituted thiophenes, which can be more easily desulfurized by the HDS process.16-20 3.2. Vapor Phase Operating Region for JP-8 Light CutSyngas Mixtures. This proposed HDS process is designed to be operated in the gas phase. VLE data of the reactant mixture (JP-8 light cut and syngas) were used to determine the gasphase operating regime. A phase diagram of the reactant mixture, with a reformate-to-liquid fuel feed (V/L) mole ratio of 2, was generated by CHEMCAD simulation, as shown in Figure 4. With the reaction total pressure maintained below 300 psig, the JP-8 light cut will be fully vaporized at temperatures above 240 °C, and at even lower temperatures if the V/L ratio is increased above 2. The experiments were conservatively designed for operation fully in the vapor phase. 3.3. Comparison of Syngas with H2 for HDS Performance. Presulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts were evaluated. We selected the presulfided versions due to difficulties

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Figure 2. Hydrocarbon distributions in raw JP-8 and its light cut (176 °C).

Figure 3. Sulfur component distributions in raw JP-8 (top) and its light cut (bottom).

Figure 4. Phase diagram of JP-8 light cut-syngas mixture (V/L ) 2).

that could be anticipated in pretreating the catalysts in the field with H2S. The catalyst activation step involved reduction by H2 while co-feeding JP-8 light cut, to maintain sulfur levels in the catalyst bed during pretreatment. The sulfur removal efficiencies from the HDS reaction with CoMo/Al2O3 and NiMo/Al2O3 are compared in Figure 5, with

both H2 and syngas co-feeds. Product sulfur concentrations were reduced from an initial 320 ppm to below 5 ppm with both catalysts. There is only a small difference between syngas and H2-driven HDS in terms of residual product sulfur. To determine whether the slightly poorer performance was simply a result of lower H2 partial pressure in the case of syngas,

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Figure 5. Product sulfur concentration from HDS reaction with syngas and H2 (T ) 350 °C; P ) 250 psi; V/L ) 3; LHSV ) 8).

we performed an additional experiment. We compared HDS performance of NiMo/Al2O3 using 100% H2, a gas mixture comprising 50% He-50% H2, and a syngas mixture of 74% H2, 12% CO, and 14% CO2, as shown in Figure 6. The 50% He-50% H2 gas mixture provided better performance, even though H2 partial pressure is higher in the syngas case. It appears that the presence of CO has a modest negative effect on HDS performance, possibly as a result of competitive adsorption. Careful examination of Figure 6 reveals that all traces show the same residual sulfur-bearing components in approximately the same relative ratios, although the total sulfur concentration is different in each case. This suggests that for the JP-8 light cut used as feedstock, the HDS reaction is nonselective; i.e., there is no observed preferential conversion of the light boilers relative to the heavy ones. We also call attention to the large peak in the chromatogram at 1.5 min elution time that is seen in all three traces. We have confirmed that this is H2S. A second smaller peak seen in the bottom-most trace at ∼1.6 min elution time can be attributed to COS, although we have not made a definitive assignment through the use of a COS standard. This peak is observed whenever syngas is employed rather than H2 and probably arises from the following equilibrium:

CO + H2S ) COS + H2

(1)

Both H2S and COS are present in the liquid product as a result of the method by which we collect HDS product, under pressure at 4 °C. Both of these gases have some solubility under these conditions, but we have found that under a simple gas purge at ambient temperature these components disappear nearly completely from the liquid. 3.4. Syngas-Based HDS Performance of CoMo/Al2O3 and NiMo/Al2O3. Figure 7 compares the sulfur removal efficiency from the JP-8 light cut using syngas with CoMo/Al2O3 and NiMo/Al2O3, both in terms of total sulfur removal as well as removal of the alkylbenzothiophenes. The NiMo/Al2O3 catalyst performs slightly better in both cases. Prevailing theories indicate that two parallel reaction pathways exist in HDS: a hydrogenation pathway involving olefinic and aromatic carbon hydrogenation followed by C-S bond cleavage and a hydrogenolysis pathway involving the direct removal of the sulfur atom by H2.21-23 Both reaction pathways occur simultaneously on different active sites of catalyst surface, and which one dominates is dependent on reaction conditions and the identity of the sulfur compounds. CoMo/Al2O3 catalysts tend to favor hydrogenolysis, while NiMo/Al2O3 favors a hydrogenation pathway.21 On this basis, the NiMo/Al2O3 catalyst should produce a more saturated product than CoMo/Al2O3, which

would be quite acceptable for reforming applications in terms of reducing the tendency for carbon formation caused by the greater difficulty in reforming aromatics. In addition, any hydrogen consumed as a result of the aromatic hydrogenation would be recovered during the subsequent steam reforming step. A proton NMR analysis (not shown) of the JP-8 light fraction indicated that the unsaturated carbon atoms are virtually all aromatic, with negligible olefinic carbon present. Preliminary 13C NMR measurements of sp2 and sp3 carbon in the feed and hydrodesulfurized sample were consistent with a small decrease in total aromatic carbon following treatment with the NiMo/ Al2O3 catalyst and to a lesser extent with CoMo/Al2O3, but reliable quantification requires further and more detailed characterization. We also measured a decrease in unprotonated (increase in protonated) aromatic carbon following HDS processing with NiMo/Al2O3, which we did not see with CoMo/ Al2O3, as shown in Figure 8. This is consistent with greater hydrodealkylation activity being provided by the NiMo/Al2O3 catalyst. 3.5. Effect of Reaction Temperature on Organosulfur Conversion Over NiMo/Al2O3 Catalyst. The HDS reaction of organosulfur compounds is exothermic and essentially irreversible under reaction conditions employed industrially (e.g., 340-425 °C and 55-170 atm).24 The reaction temperature is usually defined by the required reaction rate and thermodynamics, tempered by the rate of catalyst deactivation. The sulfur removal occurs primarily through a hydrogenation pathway on NiMo/Al2O3 catalyst, which can be affected by thermodynamics because hydrogenation of sulfur-containing rings is equilibriumlimited.24 We carried out experiments to identify an optimum reaction temperature of operation. The temperature dependence on the extent of desulfurization for the NiMo/Al2O3 catalyst was investigated over the range 260-380 °C (Figure 9) at a high space velocity (LHSV ) 8). The activity increases substantially with increasing temperature between 260 and 300 °C. Above 300 °C, the sulfur conversion is high and virtually complete by 350 °C. Since further increases in temperature were not required to improve sulfur removal efficiency, and recognizing the potential for carbon formation and/or metal sintering with increasing reaction temperature, 350 °C was selected for the remainder of our studies. 3.6. Effect of Total Reaction Pressure. High-pressure operation is critical for conventional liquid-phase HDS, due to the need to obtain sufficient H2 solubility in the liquid hydrocarbon to allow sufficient access of hydrogen to the catalyst surface. The influence of reaction total pressure on HDS performance with the CoMo/Al2O3 and NiMo/Al2O3 catalysts is shown in Figure 10, The results demonstrate that sulfur conversion is pressure dependent, and an increase in reaction total pressure unsurprisingly leads to higher desulfurization yields. This general trend is observed for both the NiMo/Al2O3 and CoMo/Al2O3 catalysts. However, NiMo/Al2O3 exhibits less sensitivity to pressure variation than does CoMo/Al2O3. With the NiMo/Al2O3 catalyst, low product sulfur (7 ppm) still can be achieved even at a reaction pressures as low as 50 psi. On the other hand, we have observed that increasing pressure above 250 psi provides diminishing returns. 3.7. JP-8 Fuel Space Velocity Effect. HDS activity of the NiMo/Al2O3 catalyst was evaluated to determine the maximum throughput that could be handled by the catalyst under the temperature and pressure conditions typical of our operation (350 °C, 250 psig). The results shown in Figure 11 reveal a modest decrease in desulfurization effectiveness as LHSV is increased from 2 to 8. The slightly better desulfurization perfor-

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Figure 6. Comparison of NiMo/Al2O3 catalyst HDS performance as function of gas co-feed composition (T ) 350 °C; P ) 250 psi; V/L ) 3; LHSV ) 8).

Figure 7. Syngas-based HDS performance on NiMo/Al2O3 and CoMo/ Al2O3 (T ) 350 °C; P ) 250 psi; V/L ) 3; LHSV ) 8).

Figure 8. Unprotonated aromatic carbon percentage in JP-8 light cut and HDS product (syngas co-feed; T ) 350 °C; P ) 250 psi; V/L ) 3; LHSV ) 8).

mance at the lower LHSV value logically correlates with longer residence time. Operationally, a balance can be made between highest sulfur removal efficiency and catalyst and reactor

Figure 9. Effect of temperature on organosulfur conversion over NiMo/ Al2O3 (syngas co-feed; P ) 250 psi; V/L ) 3; LHSV ) 8).

productivity. The LHSV range of 2-8 appears to be appropriate for the catalyst, feedstock, and operating conditions that we have employed. An additional test was carried out in which both the liquid fuel flow rate and catalyst bed volume were doubled while LHSV was kept constant, also shown in Figure 12. Differences in performance were very small, indicating that operation was free of external diffusion limitations. This facilitates higher catalyst throughput and differentiates this process from conventional HDS that operates in the trickle phase. 3.8. Impact of Steam. Direct utilization of reformate as gas feed to the HDS reactor distinguishes this approach from the conventional HDS process. Steam reformate typically contains ∼40 vol % H2O (initial steam/carbon ratio of 3) and could be even greater when processing heavier feeds. The necessity for steam removal from reformate primarily depends on its impact on HDS catalyst activity, although steam may help prevent carbon deposition on the catalyst. We found no information in the literature regarding the effect of steam on a syngas-based HDS process. Figure 12 shows the effect of steam on HDS effectiveness. With CoMo/Al2O3, steam has a modest negative effect on HDS

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Figure 10. Effect of reaction pressure on sulfur conversion (syngas co-feed; T ) 350 °C; V/L ) 3; LHSV ) 8).

Figure 11. Effect of LHSV on sulfur conversion (syngas co-feed; T ) 350 °C; P ) 250 psi; V/L ) 3).

Figure 12. Effect of steam concentration on HDS product sulfur content (T ) 350 °C; P ) 250 psi; V/L (dry syngas) ) 3; LHSV ) 8).

performance at 25% concentration with either hydrogen or syngas. However, at 40% steam content in the feed, performance with syngas is dramatically poorer than with hydrogen. By comparison, the NiMo/Al2O3 catalyst appears to operate well with syngas, even at the higher steam concentration. The NiMo/ Al2O3 catalyst operated better with syngas and steam than did the CoMo/Al2O3 catalyst with hydrogen and steam. With such a high steam tolerance, use of NiMo/Al2O3 can eliminate the

requirement for partial removal of steam from hot reformate before the syngas feed is introduced to the HDS reactor. Competitive absorption of water could help explain the general reduction in HDS performance. Although it has been reported that oxygenated compounds and water have weaker adsorptivity on the sulfide phases of the HDS catalysts than do sulfur-containing compounds,25,26 high water concentrations could still lead to a reduction in available sites through competitive adsorption. In the case of CoMo/Al2O3, it appears to be the combination of syngas in conjunction with high water concentrations that drastically lowers the performance of CoMo/ Al2O3, rather than just H2O alone. Although most likely this effect is due to the presence of CO in the syngas, we can offer no explanation for this result. An important additional observation regarding steam in the feed was that the reduction in catalyst activity following steam addition was irreversible, and removal of steam from the feed did not result in full activity recovery with either catalyst. This could be the result of steam-facilitated sintering or formation of oxide phases that are not readily reconverted to the desired sulfide structure. There is an additional aspect that we have considered in comparing the two catalysts for HDS activity in the presence of steam, that being the role of the water gas shift reaction. The water gas shift activity of the sulfided NiMo/Al2O3 catalyst may be greater than that of sulfided CoMo/Al2O3. This could play a role in the greater water tolerance of NiMo/Al2O3, since as a result the final water concentration would be lower. Ishikuro found that significant WGS reaction takes place during hydrodesulfurization of coal-based model compounds using syngas plus water over NiMo/Al2O3.27 In this work, we did not have available a gas chromatograph to measure differences in CO/ CO2 ratios between feed and product to confirm the water gas shift activity. However, approximately a 7% increase in flow rate of the exiting dry gas product from the HDS reaction was observed with NiMo/Al2O3 when syngas plus steam was employed. By comparison, an increase in dry gas output with the CoMo/Al2O3 catalyst in the presence of steam and syngas was not observed. Approximately 60% of the water gas shift equilibrium would have to occur with NiMo/Al2O3 to be consistent with the increase in product gas flow, assuming no significant contribution from low molecular weight cracking products or hydrodealkylation products from the HDS reaction. Since we saw no increase in dry gas yield with NiMo/Al2O3 when the HDS reaction was carried out without steam in the

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feed, we believe this to be a good assumption. It may well be that the improved performance of the NiMo/Al2O3 catalyst compared with CoMo/Al2O3 may be due to the water gas shift reaction catalyzed by the Ni catalyst component, resulting in lower net steam and higher net hydrogen in the feed. 4. Conclusions This study describes an investigation of gas-phase hydrodesulfurization of a light boiling fraction of JP-8 using syngas as the co-feed. A 176 °C cutoff temperature was selected, yielding 16 vol % of the total feed for subsequent hydrodesulfurization. This light fractional distillate contains a lower total sulfur concentration as well as more easily removed sulfur species compared with the full range JP-8. The sulfur content of the hydrotreated light fraction can be reduced to less than 5 ppmw using dry syngas with conventional CoMo/Al2O3 and NiMo/Al2O3 catalysts. This sulfur level is only slightly higher than is obtained using H2 with these same catalysts. A small amount of COS is produced in the presence of syngas that is not produced when H2 is utilized. We obtained good performance at a temperature and pressure value of 350 °C and 250 psig (H2 partial pressure 185 psig), respectively. Increasing the temperature and pressure above these values provided only marginal improvement in performance. The NiMo/Al2O3 catalyst performs slightly better than CoMo/ Al2O3 in terms of total sulfur removal efficiency and sensitivity to reaction pressure with dry syngas. The presence of steam in the syngas decreases the effectiveness of the HDS reaction. The adverse effect of steam appears to be minor with NiMo/Al2O3 but more severe with CoMo/Al2O3, especially at high steam concentrations. NMR analysis of the hydrotreated product is consistent with a greater hydrogenation capability for the NiMo/ Al2O3 catalyst, resulting in a fuel having a lower concentration of alkylated aromatics. This could be beneficial for a subsequent steam reforming step. Thus the NiMo/Al2O3 catalyst appears for a number of reasons to be superior to CoMo/Al2O3 in this application. Acknowledgment Financial Support by the U.S. Army Tank-Automotive Research, Development and Engineering Center (TARDEC) is gratefully acknowledged. Samples of JP-8 were generously provided by Fairchild Air Force Base (Spokane, WA) and British Petroleum (Tacoma, WA). Complimentary catalyst samples were provided by Criterion Catalysts and Technologies Co. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC05-76RL01830. NMR analysis of the liquid feedstock and hydrodesulfurized products was provided by James Franz. Literature Cited (1) Patil, A., Dubois, T.; Sifer, N.; Bostic, E.; Gardner, K. Portable Fuel Cell Systems for America’s Army: Technology Transition to the Field. J. Power Sources 2004, 136, 220. (2) Cheekatamarla, P.; Lane, A. Catalytic Autothermal Reforming of Diesel Fuel for Hydrogen Generation in Fuel Cells: I. Activity Tests and Sulfur Poisoning. J. Power Sources 2005, 152, 256. (3) Brown, L. A Comparative Study of Fuels for On-Board Hydrogen Production for Fuel-Cell-Powered Automobiles. Int. J. Hydrogen Energy 2001, 26 (4), 381. (4) Darwish, N.; Hilal, N.; Versteeg, G.; Heesink, B. Feasibility of the Direct Generation of Hydrogen for Fuel-Cell-Powered Vehicles by OnBoard Steam Reforming of Naphtha. Fuel 2004, 83 (4-5), 409.

(5) Matsuzaki, Y.; Yasuda, I. The Poisoning Effect of Sulfur-containing Impurity Gas on a SOFC Anode: Part I. Dependence on Temperature, Time, and Impurity Concentration. Solid State Ionics 2000, 132 (3-4), 261. (6) Gorte, R.; Kim, H.; Vohs, J. Novel SOFC Anodes for the Direct Electrochemical Oxidation of Hydrocarbon. J. Power Sources 2002, 106 (1-2), 10. (7) Topsøe H.; Clausen B.; Massoth F. Hydrotreating Catalysis. In Catalysis: Science and Technology; Anderson, J., Boudart, M., Eds.; Springer-Verlag: Berlin, 1996; Vol. 11, pp 1-310. (8) Velu, S.; Ma, X.; Song; C. Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite-Based Adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293. (9) Yang, R. Desulfurization of Transportation Fuels by Adsorption. Catal. ReV. 2004, 46 (2), 111. (10) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T.; Oxidative Desulfurization of Light Gas Oil and Vacuum Gas Oil by Oxidation and Solvent Extraction. Energy Fuels 2000, 14, 1232. (11) Chica, A.; Gatti, G.; Moden, B.; Marchese, L.; Iglesia, E. Selective Catalytic Oxidation of Organosulfur Compounds with tert-Butyl Hydroperoxide. Chem. Eur. J. 2006, 12, 1960 (12) Bosmann, Al; Datsevich, A.; Lauter, J.; Schmitz, C.; Wasserscheid, P. Deep desulfurization of diesel fuel by extraction with ionic liquids. Chem. Commun. 2001, 2494. (13) Velu, S.; Ma, X.; Song, C.; Namazian, M.; Sethuraman, S.; Venkataraman, G. Desulfurization of JP-8 Jet Fuel by Selective Adsorption over a Ni-Based Adsorbent for Micro Solid Oxide Fuel Cells. Energy and Fuels 2005, 19, 1116. (14) Li, Z.; Kabachus, S.; Ye, N.; Fokema, M. Logistic Fuel Processing for Solid Oxide Fuel Cell Applications. Presented at the Fuel Cell Seminar, November 2005, on the Web at www.fuelcellseminar.com/pdf/ 2005/Friday-Nov18/Fokema_Mark_333.PDF (15) Edwards, T.; Maurice, L. Surrogate Mixtures to Represent Complex Aviation and Rocket Fuels. J. Propul. Power 2001, 17(2), 461. (16) Song, C.; Ma, X.; New Design Approaches to Ultra-Clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization. Appl. Catal. B: EnViron. 2003, 41, 207. (17) Link, D.; Baltrus, J.; Rothenberger, K.; Zandhuis, P.; Minus, D.; StriebichLink, R. Class- and Structure-Specific Separation, Analysis, and Identification Techniques for the Characterization of the Sulfur Components of JP-8 Aviation Fuel. Energy Fuels 2003, 17, 1292. (18) Ma, X.; Mochida; I.; Sakanishi K. Hydrodesulfurization Reactivities of Various Sulfur Compounds in Vacuum Gas Oil. Ind. Eng. Chem. Res. 1996, 35, 2487. (19) Ma, X.; Sakanishi, K.; Isoda, T.; Mochida, I. Determination of Sulfur Compounds in Non-Polar Fraction of Vacuum Gas Oil. Fuel 1997, 76, 329. (20) Shafi, R.; Hutchings, G. Hydrodesulfurization of Hindered Dibenzothiophenes: An Overview. Catal. Today 2000, 59, 423. (21) Babich, I.; Moulijn, J. Science and Technology of Novel Processes for Deep Desulfurization of Oil Refinery Streams: A Review. Fuel 2003, 82, 607. (22) Kim, J.; Ma, X.; Song, C.; Lee, Y. Oyama, S. Kinetics of Two Pathways for 4,6-dimethyldibenzothiophene Hydrodesulfurization over NiMo, CoMo Sulfide, and Nickel Phosphide Catalysts. Energy Fuel 2005, 19, 353. (23) Ho, T. Deep HDS of Diesel Fuel: Chemistry and Catalysis. Catal. Today 2004, 98, 3. (24) Girgis, M.; Gates, B. Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991, 30 (9), 2021. (25) Laurent, E.; Delmon, B. Influence of Oxygen-, Nitrogen-, and Sulfur-Containing Compounds on the Hydrodeoxygenation of Phenols over Sulfided Cobalt-Molybdenum/Gamma-alumina and Nickel-Molybdenum/ Gamma-alumina Catalysts. Ind. Eng. Chem. Res. 1993, 32, 2516. (26) Odebunmi, E.; Ollis, D. Catalytic Hydrodeoxygenation: II. Interactions Between Catalytic Hydrodeoxygenation of m-Cresol and Hydrodesulfurization of Benzothiophene and Dibenzothiophene. J. Catal. 1983, 80, 65. (27) Ishikuro, K. Hydrogenation and Hydrodesulfurization of Coal Model Compounds Using Synthesis Gas. Fuel Energy Abstr. 1997, 119.

ReceiVed for reView June 8, 2006 ReVised manuscript receiVed August 3, 2006 Accepted August 10, 2006 IE0607301