Energy Fuels 2009, 23, 6089–6094 Published on Web 10/07/2009
: DOI:10.1021/ef900812w
Hydrogen Production by Noncatalytic Autothermal Reformation of Aviation Fuel Using Supercritical Water Jason W. Picou, Jonathan E. Wenzel, H. Brian Lanterman, and Sunggyu Lee* Department of Chemical & Biological Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409-1230 Received July 29, 2009. Revised Manuscript Received September 15, 2009
The noncatalytic supercritical water reformation of military logistic aviation fuel was studied using a custom-designed 383 mL Haynes Alloy 230 tubular reactor. Experiments were performed at a constant pressure of 24.1 ( 0.1 MPa, a constant temperature of 767 ( 1 °C, and at a constant water-to-fuel ratio of 15 g of water per gram of fuel at various space times and oxygen flow rates. Increasing space time increases the gasification percentage and the resultant hydrogen and carbon dioxide yields; however, the gasification percentage reaches a limit of about 70% after a space time of 79 s when no oxygen was present. The addition of substoichiometric amounts of oxygen does not adversely affect the production of hydrogen gas under certain conditions while increasing carbon gasification and in situ heat generation. Carbon gasification percentages of 86-94 mol %, depending on the space time, were achieved with a molar oxygen-to-fuel ratio of 4.79. A hydrogen yield of 5.3 mol hydrogen per mole of jet fuel, which is 14% of the theoretical maximum, was obtained at a space time of 159 s with no air flow. At the longest space time and a 4.8 oxygen-to-fuel ratio, the hydrogen yield was 5.06, which is 19% of the theoretical maximum. The production of hydrogen from military grade jet fuel would enable armed forces personal to produce electricity on site from a fuel cell, greatly reducing the noise and heat signature compared to internal combustion generators.
supercritical water is almost entirely disrupted, making it more like an organic solvent than ambient water.5 Experiments conducted by this group and others demonstrate the importance of supercritical water in the gasification efficiency, showing that both subcritical pressures and temperatures lead to decreased gasification percentages.6,7 The addition of oxygen in the form of air provides in situ heat generation and leads to autothermal reformation, while increasing gasification and lessening coke formation.8 The absence of a metallic catalyst in the supercritical water reformation process negates sulfur poisoning and coke fouling. Hydrogen gas is a superb energy source due to its cleanliness and efficiency, but since hydrogen gas does not occur naturally on earth in any reasonable quantity, it must be produced from compounds that contain it.9,10 While there are a large number of experimental investigations into the supercritical reformation of alcohols, biomass and waste products to produce hydrogen, very little
Introduction The ability to efficiently produce hydrogen on demand from a small, portable apparatus is imperative to the future viability of the hydrogen economy. There are many difficulties in the reformation of military logistic aviation fuel, due to its hydrocarbon makeup and the high concentration of sulfur. Jet fuel is similar in average carbon number to kerosene and contains branched and cyclic compounds. These higher carbon number hydrocarbons are more difficult to reform, along with the tendency of branched and aromatic compounds to produce coke fouling at high temperatures. For industrial catalytic reformation processes, sulfur must be cleaned from the feedstock fuel since it will poison most reformation catalysts. These difficulties are overcome in a novel, noncatalytic approach using supercritical water partial oxidation for the reformation of jet fuel. In this process, supercritical water functions as a highly energized reforming agent and also as a homogenizing reaction medium. Supercritical water is a nonpolar solvent, allowing straight chained hydrocarbons of up to twenty four carbons to be miscible in supercritical water, as well as oxygen.1-4 Supercritical water is distinct from ambient water in that the hydrogen bonding of
(5) Guo, L. J.; Lu, Y. J.; Zhang, X. M.; Ji, C. M.; Guan, Y.; Pei, A. X. Catal. Today 2007, 129, 275–286. (6) Stever, M. S.; Picou, J. W.; Wenzel, J.; Putta, S.; Lanterman, H. B.; Lee, S. Effects of Supercritical and Subcritical Pressures of Water on Coke Formation During Jet-A Reformation to Hydrogen. Presented at the AIChE Spring National Meeting, New Orleans, LA, April 2008. (7) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, Y.; Aria, K. Ind. Eng. Chem. Res. 2000, 39, 2883–2890. (8) Modell, M. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989; pp 8.153-8.168. (9) Hydrogen Production, Energy Efficiency and Renewable Energy. U.S. Department of Energy. http://www1.eere.energy.gov/hydrogenandfuelcells/production/basics.html (accessed Dec 2007). (10) Hammond, C. R. The Elements. In CRC Handbook of Chemistry and Physics, 57th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1976; pp B-26.
*To whom correspondence should be addressed. E-mail: leesu@ mst.edu. Fax: (573) 202-2361. (1) Holliday, R. L.; Jong, B. Y. M.; Kolis, J. W. J. Supercrit. Fluids 1998, 12, 255–260. (2) Klijn, J. E.; Engberts, J. B. F. N. Nature 2005, 435, 746–747. (3) Levelt Sengers, J. M. H. In Supercritical Fluids: Fundamentals and Applications; Kiran, E., Debenedetti, P. G., Peters, C. J., Eds.; NATO Science Series E: Applied Sciences; Kluwer Academic Publishers: Norwell, MA, 2000; Vol. 366, pp 1-21. (4) Brunner, E. J. Chem. Thermodyn. 1990, 22, 335–353. r 2009 American Chemical Society
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has been targeted to noncatalytic reformation of larger (C5 and above) hydrocarbons.5,11-17 Future technologies may incorporate renewable resources to produce hydrogen, but for a seamless integration into the hydrogen economy, it is imperative that all avenues of production be explored. Also, portable, on-demand reformation of hydrogen from military logistics jet fuel (JP-8), coupled with a fuel cell, would enable armed forces personnel to produce electricity in the field with very little noise or heat signature compared to internal combustion electric generators. The reason for using JP-8 as the hydrogen source are logistical; the armed forces need to have one fuel that all equipment and vehicles can run on to reduce complexity when supplying field units, and currently, that fuel is JP-8. While the hydrogen economy may be years away, a portable hydrogen reformer would be immediately applicable for the armed forces.
Figure 1. Schematic of the supercritical water reformation system at Missouri University of Science and Technology.
gas chromatograph is calibrated to detect hydrogen, nitrogen, carbon monoxide, methane, carbon dioxide, ethyne, ethene, and ethane. The total carbon content of the liquid effluent was analyzed with a Dohrmann DC-190 total organic carbon analyzer.
Apparatus & Chemicals The water used for the reformation experiments was deionized water, and the air used was Airgas Breathing Quality grade D compressed air. The jet fuel used was both civilian jet fuel (Jet-A) and military logistic aviation fuel (JP-8), both of which are an assortment of hydrocarbons including straight chain, branched, and cyclic. An ASTM D2887 boiling range distribution analysis determined that the number of the carbon bonds varied from 7 to 17, with the average being 12 for both fuels.18 The civilian jet fuel contained 0.099 wt % sulfur while the military aviation fuel contained 0.081 wt % sulfur, as determined by Texas Oil Tech Laboratories using ASTM D4294, sulfur content by X-ray.19 Since both Jet-A and JP-8 are similar in boiling point distribution and sulfur content, for the purposes of these experiments and discussion, they are considered identical and are modeled as a single representative molecular species, 1-dodecene, which has the chemical formula C12H24. The reason two different types of jet fuel were used was due to problems acquiring additional military jet fuel. The supercritical water system consists of a liquid feed system, integrated heat exchanger, preheater, air feed system, reactor, zoned reactor heaters, sample collection system, and data acquisition and control system, for which a schematic process flow diagram is illustrated in Figure 1. The supercritical water reactor has a volume of 383 mL and is constructed of Haynes Alloy 230, which is an alloy of mostly nickel, chromium, tungsten, and molybdenum, among other elements.20 This allows the reactor to operate at temperatures up to 800 °C at a pressure of 36 MPa. Analysis of the product gas was performed using a HP 5890 Series A gas chromatograph with a thermal conductivity detector. The
Chemical Reactions There are various chemical reactions that take place during the supercritical water reformation of jet fuel, the most important of which are described below. The overall reformation reaction of jet fuel is ð1Þ C12 H24 þ12H2 O f 12COþ25H2 The endothermic reformation reaction is the most critical reaction, since water participates in the reaction and liberates hydrogen, the desired product. The experiments conducted for this paper were all carried out noncatalytically, in that no catalyst was placed inside the reactor. It has been demonstrated that metallic reactor walls can potentially function as a catalyst in the reformation of various hydrocarbons in supercritical water, but these effects will have to be analyzed in future work.21-24 The reformation reaction is in competition and occurs in parallel with the pyrolysis reaction: ð2Þ C12 H24 f Ca Hb þCx Hy þpH2 ð12 ¼ aþx and 24 ¼ bþyþ2pÞ The pyrolysis reaction is endothermic, but much less so than the reformation reaction, requiring approximately 70 kJ/mol depending on the carbon number of the resultant hydrocarbon fractions. The pyrolysis reaction is thought to be primarily responsible for any gaseous hydrocarbons contained in the product gas, such as methane or ethane. Repeated pyrolysis leaves hydrogen deficient fractions, which eventually become solid coke or function as coke precursors.25,26 In the presence of oxygen, another set of reactions occur. Equation 3 is the partial oxidation of jet
(11) Boukis, N.; Diem, V.; Habicht, W.; Dinjus, E. Ind. Eng. Chem. Res. 2003, 42, 728–735. (12) Bo, Y.; Chao-hai, W.; Cheng-sheng, H.; Cheng, X.; Jun-zhang, W. J. Environ. Sci. 2007, 19, 1424–1429. (13) Lu, Y. J.; Guo, L. J.; Ji, C. M.; Zhang, X. M.; Hao, X. H.; Yan, Q. H. Int. J. Hydrogen Energy 2006, 31, 822–831. (14) Pinkwart, K.; Bayha, T.; Wolfgang, L.; Krausa, M. J. Power Sources 2004, 136, 211–214. (15) Resende, F. L. P.; Fraley, S. A.; Berger, M. J.; Savage, P. E. Energy Fuels 2008, 22, 1328–1334. (16) Wenzel, J. E. The Kinetics of Non-Catalyzed Supercritical Water Reforming of Ethanol. Ph.D. Dissertation, University of MissouriColumbia, Columbia, MO, May 2008. (17) Modell, M. Processing methods for the oxidation of organics in supercritical water. U.S. Patent 4,338,199, 1982. (18) ASTM D2887, Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography; ASTM International: West Conshohochen, PA, 2006. (19) ASTM D4294, Standard Test Method for Sulfur in Petroleum and Petroleum Products by Energy Dispersive X-ray Fluorescence Spectrometry: ASTM International: West Conshohochen, PA, 2006. (20) HaynesÒ 230 Alloy; Haynes High-Temperature Alloys International: Kokomo, IN, 2004; p 2.
(21) Yu, D.; Aihara, M.; Antal, J. M. Energy Fuels 1993, 7, 574–577. (22) Arita, T.; Nakahara, K.; Nagami, K.; Kajimoto, O. Tetrahedron Lett. 2003, 44, 1083–1086. (23) Lachance, R. P. A Fundamental Study of Model Fuel Conversion Reactions in Sub and Supercritical Water. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, June 2005. (24) Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988, 27, 2009– 2014. (25) Nohara, D.; Sakai, T. Ind. Eng. Chem. Res. 1992, 31, 14–19. (26) Lee, S.; Lanterman, H.; Wenzel, J.; Edwards, N.; Adams, A.; Wootton, J.; Garcia, A. Prepr. Pap.;Am. Chem. Soc., Div. Pet. Chem. 2006, 51, 521–523.
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fuel, while eq 4 is the complete oxidation reaction: C12 H24 þ6O2 f 12COþ13H2 C12 H24 þ18O2 f 12CO2 þ13H2 O
Picou et al. Table 1. Theoretical Maximum Hydrogen Gas Production Per Mole of Fuel for Increasing Oxygen-to-Fuel Ratio
ð3Þ ð4Þ
Partial oxidation is the preferred reaction because both hydrogen and carbon monoxide are the products, while total oxidation produces water and carbon dioxide, which are unwanted and wasteful. There is a long history of using both partial and total oxidation in supercritical water.8,17,27,28 In addition to these reactions, the water gas shift reaction may also occur. This would be a highly desirable reaction, since additional hydrogen is produced. In industry, the water gas shift reaction is conducted at temperatures from 150 to 600 °C and is typically carried out over a catalyst of copper and zinc oxide.29,30 The reaction is equilibrium limited and the forward reaction is thermodynamically favored at temperatures of 815 °C or below:31 COþH2 O/CO2 þH2 ð5Þ
molar O2/fuel ratio
mole H2 produced per mole of fuel
0.00 0.80 1.60 4.79
36.0 34.5 33.1 27.2
chromatograph used on these experiments could not differentiate oxygen from nitrogen, it is assumed that all oxygen, fed in substoichiometric amounts, is consumed.27,37 Table 1 shows the relationship between the theoretical maximum amount of hydrogen per mole of jet fuel fed as a function of oxygen to fuel ratio. These values are based on the assumption that all the oxygen is consumed in partial oxidation, the remaining jet fuel is reformed, and all the resultant carbon monoxide reacts with water to produce hydrogen and carbon dioxide The oxygen-to-fuel molar ratio is a measure of how much oxygen was fed as O2 divided by how much fuel was fed. As shown in Table 1, the addition of oxygen reduces the theoretical maximum amount of hydrogen produced, with there being 38% less hydrogen when the oxygen-to-fuel ratio is at 4.79 than when no oxygen is present. The decrease in hydrogen production is necessary for the in situ heat generated by the partial oxidation reaction. The theoretical maximum will allow a comparison between experiments and illustrates the feasibility and challenges of producing hydrogen noncatalytically from jet fuel.
Various other reactions, like methanation or the Boudouard reaction, could also be possible. Previous studies involving the water gas shift reaction in supercritical water resulted in negligible methanation at conditions similar to those for this work.30 The high temperatures used in this paper should limit the exothermic Boudouard reaction, since the equilibrium constant favors carbon monoxide production at temperatures above 680 °C.31-35 The discussion was limited to the aforementioned reactions for simplicity, and since they effectively and accurately describe all of the observed products.36 These reactions listed above plus the other neglected reactions, taken together, could produce a robust kinetic model for the partial oxidation and reformation of jet fuel in supercritical water, but the scope of this paper is in the determination of the effects of water gas shift, reformation, oxidation, and pyrolysis reactions, and a detailed intrinsic kinetic or chemical mechanistic model will require further experimentation. The conclusions of this research warrant further investigation leading to such a kinetic model. The maximum amount of hydrogen gas that could theoretically be produced, assuming all jet fuel is reformed and all the carbon monoxide undergoes the water gas shift reaction is 37 mol of hydrogen gas per mole of jet fuel fed. Any other reactions that take place would result in less hydrogen. When oxygen is added, oxidation reactions occur, decreasing the amount of hydrogen produced per mole of fuel. Since oxygen is a powerful oxidizer in supercritical water and the gas
Experimental Results and Discussion The space time of the fluid in the reactor and the flow rate of air into the reactor were varied to investigate the effects upon the gas composition and fuel gasification. The space time was varied by changing the inlet water and fuel flow rate and calculated as a function of inlet fluid density using the Peng-Robinson equation of state with the van der Waals mixing rule. The temperature was held at 767 ( 1 °C at a constant pressure of 24.1 ( 0.1 MPa with a 15/1 water-to-fuel mass feed ratio. A water-to-fuel ratio of 15/1 corresponds to a 12/1 water-to-carbon (H2O/C) molar ratio, or an aqueous aviation fuel concentration of 6.25 wt %. Three oxygento-fuel (O2/fuel) ratios were tested. Table 2 outlines the temperature, pressure, fuel type, water, fuel, and air flow rates, with the corresponding oxygen-to-fuel ratio and space time for a given experimental run. The experiments were conducted in a randomized order from 1 to 12. Run numbers 6 and 13 are duplicates. The oxygen-to-fuel ratio affects eq 3, the partial oxidation reaction. With the jet fuel modeled as C12H24, the minimum ratio necessary to partially oxidize all the fuel, assuming the reaction continues to completion without any other competing reactions, is an oxygen-to-fuel ratio of 6.0, as determined by eq 3. A substoichiometric O2/fuel ratio of 4.79 would partially oxidize 80% of all incoming fuel given these same assumptions. All the experiments were conducted below a 6.0 oxygen-to-fuel ratio in order to limit the oxidation reactions. As the oxygen-to-fuel ratio increases, the amount of energy liberated by the oxidation reactions increases and the proportion of fuel left
(27) Lilac, W. D. Controlled Depolymerization of Polypropylene via Selective Partial Oxidation in a Supercritical Water Medium. Ph.D. Dissertation, University of Missouri-Columbia, Columbia, MO, Dec 1999. (28) Dickinson, N. L. Pollutant-free low temperature slurry combustion process utilizing the super-critical state. U.S. Patent 4,292,953, 1981. (29) Ahmed, S. Water-Gas Shift Reaction over Cu-Based Mixed Oxide Catalysts. In Proceedings of the 15th Saudi-Japan Joint Symposium, Dhahran, Saudi Arabia, Nov 27-28, 2005. (30) Picou, J. W.; Wenzel, J.; Lanterman, H. B.; Lee, S. Kinetics of Noncatalytic Water Gas Shift Reaction in a Supercritical water Medium. Presented at the AIChE Spring National Meeting, New Orleans, LA, April 2008. (31) Lee, S. Alternative Fuels; Taylor & Francis: Philadelphia, 1996; pp 140-151. (32) McIntosh, S.; Gorte, R. J. Chem. Rev. 2004, 104, 4845–4865. (33) Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649. (34) Garcia-Serna, J.; Garcia-Merino, E.; Cocero, M. J. J. Supercrit. Fluids 2007, 43, 228–235. (35) Mondal, K.; Lorethova, H.; Hippo, E.; Wiltowski, T.; Lalvani, S. B. Fuel Process. Technol. 2004, 86, 33–47. (36) Picou, J. W. Autothermal Non-Catalytic Reformation of Jet Fuel in a Supercritical Water Medium. M.S. Thesis, Missouri University of Science and Technology, Rolla, MO, June 2008.
(37) Hong, G. T.; Spritzer, M. H. Supercritical Water Partial Oxidation. Presentation to the Department of Energy Hydrogen Program Annual Review, Berkeley, CA, May 19-22, 2003.
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Table 2. Experimental Results of Jet Fuel and Air in Supercritical Watera run ID
fuel type
water flow (g/min)
fuel flow (g/min)
air flow (slpm)
oxygen-to-fuel ratio (O2/fuel)
temperature (°C)
space time (s)
3 4 8 7
Jet-A Jet-A JP-8 JP-8
7.6 7.6 7.3 7.5
0.53 0.53 0.48 0.48
0.00 0.25 0.50 1.50
0.00 0.74 1.67 4.99
765 765 763 763
159 153 156 136
12 11 10 9
JP-8 JP-8 JP-8 JP-8
15.1 15.1 15.1 14.3
0.93 0.95 0.95 1.00
0.00 0.50 1.00 3.01
0.00 0.83 1.68 4.80
770 772 772 765
79 77 74 70
1 13 6 2 5
Jet-A JP-8 JP-8 Jet-A JP-8
31.1 29.6 29.4 31.5 29.7
1.97 1.94 1.93 2.00 1.91
0.00 1.00 1.00 1.99 5.99
0.00 0.82 0.83 1.59 5.01
768 773 769 772 765
39 39 40 36 34
a
Temperature was held constant at 767 ( 1 °C, and pressure, at 24.1 ( 0.1 MPa. The space time and the molar oxygen-to-fuel ratio are varied.
was present in all gases divided by the molar flow rate of carbon in the inlet jet fuel. The species gas yield, the right axis of Figure 2, is calculated from the molar flow rate of each gas species divided by the inlet molar flow rate of jet fuel, a dimensionless number, to account for variations in inlet flow rates. The gasification percentage began at 49.7% for the 39 s space time and increased to 71% for the intermediate space time of 79 s, a 43% increase. It then decreased slightly to 70% for the longest space time of 159 s. Even though the space time was doubled from 79 to 159 s, the gasification percentage did not change. This indicates that the gasification of aviation fuel had reached a limit around the space time of 79 s, with longer space times resulting in a constant 70% conversion of the fuel into gas. The remaining 30% of carbon would then be in either the liquid effluent or have become solid due to the pyrolysis reaction. Total organic carbon analysis determined that less than 1% of the carbon that was fed into the system was present in the liquid effluent. In effect, if the carbon does not leave the reactor as gas, it stays behind as solid. Therefore, when there is a carbon gasification percentage of 70%, it is assumed that 30% remains in the reactor as solid The hydrogen gas yield increases linearly with increasing space time, from 1.9 mol of hydrogen per mole of jet fuel fed at a space time of 39 s to a yield of 5.3 at 159 s, a 182% increase. The maximum hydrogen yield of 5.3 is only 14% of the theoretical maximum of 37 mol of hydrogen per mole of fuel. This is due to hydrogen being contained in other species such as methane and ethane and the water gas shift reaction not proceeding to completion, along with the fact that not all of the fuel was gasified. Methane yield follows a pattern similar to the gasification percentage, in that it increases sharply between 39 and 79 s, and then decreases. There is no ethane detectable for the 159 s space time experiment. The decrease in methane and disappearance of ethane indicate that reformation is more active at longer space times. However, since the carbon gasification remains level, then methane and ethane are being reformed preferentially over the coke or coke precursors that remained in the reactor. The carbon dioxide yield increased 294% from 0.57 to 2.25 for increasing space time, while the carbon monoxide yield decreased 48%. This trend indicates that the forward-water gas shift reaction is preferred at longer space times, converting carbon monoxide into carbon dioxide and hydrogen. Other studies of the noncatalytic supercritical water gas shift reaction and the
Figure 2. Gas yield and carbon gasification percentage (CG %) as a function of space time.
to participate in the endothermic reformation reaction decreases. This leads to the autothermal nature of the reactions, in that more energy is liberated through the oxidation reactions than is used in the reformation reaction. The amount of oxygen that is needed so that the energy requirement of the reformation reaction equals the energy liberated by the partial oxidation reaction is equal to a 3.84 oxygen-to-fuel ratio. Experiments 5, 6, and 8 all have oxygen-to-fuel ratios higher than 3.84, meaning more energy is produced by oxidation than used by reformation. Effect of Space Time. First, the effect of space time, without air flow, was investigated. Three experiments, 1, 3, and 12, at three different space times of 39, 79, and 159 s were conducted at an average temperature of 768 ( 1 °C and pressure of 24.09 ( 0.04 MPa. The net effect of the variation in space time on gasification and gas yield is illustrated in Figure 2. The molar carbon gasification percentage (CG %), the left axis of Figure 2, is the molar flow rate of carbon that 6092
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: DOI:10.1021/ef900812w
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Figure 3. Nitrogen-free gas yield and gasification percentage as a function of the oxygen-to-fuel molar feed ratio at a space time of 151 ( 5 s.
Figure 4. Nitrogen-free gas yield and gasification percentage as a function of the oxygen-to-fuel molar feed ratio at a space time of 75 ( 2 s.
reformation of hydrocarbons demonstrate similar responses with increasing space time.13,22,38,39 Effect of Oxygenation. The other variable that was studied was feeding substoichiometric amounts of oxygen into the reactor for each of the three space times discussed previously. Figure 3 depicts the gasification percentage and nitrogen free product gas yield of each species as a function of the oxygento-fuel ratio for experiments conducted at a space time of 151 ( 5 s, experiments 3, 4, 8, and 7. The gasification percentage increased linearly from 70.4% to 94.3%, a 34% increase with the increasing oxygen-to-fuel ratio. The addition of oxygen increased the gasification percentage of the fuel, either through oxidizing the coke formed by pyrolysis or oxidizing the fuel directly and leaving less fuel to be pyrolyzed, or a combination of the two. Hydrogen yield decreased 5.4% as oxygen was added to the system. This indicates that while the addition of oxygen may not encourage hydrogen production at this space time, neither is it greatly hindered. For the 4.8 oxygen-to-fuel ratio, the hydrogen yield was 5.06, which is 19% of the theoretical maximum of 27.4 mol of hydrogen per mole of fuel fed at this oxygen-to-fuel ratio. As the oxygen-to-fuel ratio increased, the methane yield decreased, which may be a result of the oxidation reactions consuming fuel that would have otherwise undergone pyrolysis and become methane and coke, or direct oxidation of the pyrolysis products. The oxidation reactions are responsible for the increase in carbon oxides. The carbon dioxide yield increased from 2.25 to 4.59, a 155% increase from oxygen-free conditions to an oxygento-fuel ratio of 4.8, while the carbon monoxide yield increased by 84%, from 0.32 to 0.91, over the same interval.
The greater increase of carbon dioxide over carbon monoxide could be the result of the water gas shift reaction consuming carbon monoxide and producing carbon dioxide and hydrogen. The hydrogen being produced from the forward water gas shift reaction is masked by the decreasing hydrogen production of the oxidation reactions, since partial oxidation conditions result in less hydrogen than reformation. No ethane was detectable for these experiments. For a space time of 75 ( 2 s, experiments 9-12, Figure 4 shows the effects of adding air to the system. The percent gasification increases 23% with increasing oxygen, which is comparable to the 151 s space time experiments. The gas yield, on the other hand, has changed compared with the previous experiment. The hydrogen gas yield increased 15% with increasing oxygen, while during the 151 s space time experiments, it decreased. On the basis of this result, it may be possible that with even higher oxygen-to-fuel ratios there could be further increases in hydrogen, carbon monoxide, and carbon dioxide yields and the gasification percentage. The maximum hydrogen yield of 3.72 mol of hydrogen per mole of jet fuel was obtained at the highest oxygen-to-fuel ratio of 4.8 and corresponds to 14% of the theoretical maximum of 27.4 mol of hydrogen per mole of jet fuel for that oxygen-to-fuel ratio. The increase in the hydrogen yield could be from partial oxidation competing with pyrolysis, producing more hydrogen and fewer hydrocarbons. Also, oxidation creates more carbon monoxide that could then undergo water gas shift and produce hydrogen. The methane yield decreased 27%, from 4.34 to 3.17, as the oxygen-to-fuel ratio increased. The amount of ethane decreased as the oxygen flow increased. The same trend was seen during the 151 s space time experiments, where increasing oxygen decreased the gaseous hydrocarbon yield due to oxidation competing with
(38) Portela, J. R.; Nebot, E.; Martinez de la Ossa, E. Chem. Eng. J. 2001, 81, 287–299. (39) Sato, T.; Kurosawa, S.; Smith, R. L., Jr.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 2004, 29, 113–119.
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the oxidation reactions. The hydrogen yield increased from 1.9 to 2.4, a 26% increase, which is comparable to the percentage increase for the 75 s space time experiments over the same interval. A hydrogen yield of 2.4, representing 9% of the theoretical maximum, occurred at the highest oxygento-fuel ratio. The increasing carbon monoxide and carbon dioxide yields, 233% and 347%, respectively, again illustrate the increasing oxidation reaction as the O2/fuel ratio increases. Carbon monoxide and carbon dioxide increase at a similar rate, whereas for the previous experiments the carbon monoxide yield increased less than carbon dioxide. This indicates that the forward water gas shift reaction is not as active at these space times than during the longer ones. The ratio of carbon dioxide to carbon monoxide at this space time is about 1/1, while the previously discussed space times had ratios between 2/1 and 5/1. The methane yield remained nearly unchanged at about 3.0, while for the longer space time experiments the methane yield decreased due to the oxygenation and further reformation reactions competing with the pyrolysis reaction. The space time of 38 s might be too brief to allow much reformation to occur, which would explain the higher amounts of methane and ethane. Conclusion The supercritical reformation of jet fuel was studied in a noncatalytic 383 mL Haynes Alloy 230 tubular flow reactor. Without oxygen, carbon gasification increases from a space time of 39 to 79 s, while gasification remains constant for space times greater than 79 s. The forward water gas shift reaction is more active at the longer space times based on the decrease in carbon monoxide and an increase in carbon dioxide. The hydrogen gas yield increased with increasing space time for the interval studied. A maximum hydrogen yield of 5.3 mol of hydrogen per mole of jet fuel, which is 14% of the theoretical maximum, was obtained at a space time of 159 s with no air flow. When air was added to the system, the gasification percentage increased with increasing air flow. The addition of oxygen decreases the amount of hydrogen produced per mole of fuel for the 151 s space time experiments, but the yield of hydrogen increased for the two shorter space time experiments. Except for the 38 s space time experiments, the methane and ethane flow rates decreased for increasing oxygen. Carbon dioxide and carbon monoxide both increase with increasing oxygen due to the oxidation reactions, with carbon dioxide increasing more than carbon monoxide because of the water gas shift reaction. It has been shown that 94% gasification of jet fuel in a noncatalytic partial oxidation supercritical water reformer is achievable, that the forward water gas shift reaction also proceeds noncatalytically in the system, and that the addition of oxygen into the system is not necessarily detrimental to hydrogen gas production.
Figure 5. Nitrogen-free gas yield and gasification percentage of fuel as a function of the oxygen- to-fuel molar feed ratio at a space time of 38 ( 1 s.
pyrolysis or oxidizing the hydrocarbon pyrolysis products. The carbon monoxide yield increases 87%, from 0.7 to 1.31, as the oxygen/fuel ratio increased, while carbon dioxide increased 189%, from 1.2 to 3.46, which is likely the result of the oxidation reaction. This trend, similar to the 151 s space time experiments, where the carbon monoxide yield increases less than carbon dioxide, is again indicative of the water gas shift reaction. In experiments 1, 13, 6, 2, and 5, the oxygen-to-fuel ratio was varied at a space time of 38 ( 1 s, the results of which are shown in Figure 5. Since duplicate runs, experiments 13 and 6, were performed at an oxygen/fuel ratio of 0.822 ( 0.004, the results were averaged and error bars show the standard error. The gasification percentage increased at the oxygento-fuel ratio of 0.82, then decreased. This increase in gasification is unseen in the previous two space times that were studied, which is why duplicate runs were conducted at this condition. The increase in gasification is due to an increase in all gaseous species, most notably methane and carbon oxides. Methane is thought to be a product of pyrolysis, and carbon oxides a product of oxidation, but elucidation of this specific condition as to why it would be more conducive to pyrolysis and oxidation than the two surrounding data points would require more detailed analysis. Excluding the increase at the 0.822 O2/fuel ratio point, there are similar trends compared to the previous two space times examined. The molar carbon gasification percentage increased with an increasing O2/fuel ratio, from 50% to 86%, a 72% increase, which is a greater percentage increase than the previous experiments and is indicative of an increase in
Acknowledgment. The current project was sponsored by the U.S. Army under PE Number 0602705A through a subcontract from DRS Technical Services, Inc. The authors are grateful for insightful technological guidance and supervision provided by Dr. Terry Dubois, CERDEC, U.S. Army, Ft. Belvoir, Virginia.
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