Article Cite This: Energy Fuels 2017, 31, 13802−13814
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Analysis of Catalytic Hydrothermal Conversion Jet Fuel and Surrogate Mixture Formulation: Components, Properties, and Combustion Dianne J. Luning Prak,*,† Mark Romanczyk,‡ Katherine E. Wehde,‡ Sonya Ye,† Margaret McLaughlin,† Peter J. Luning Prak,† Matthew P. Foley,§ Hilkka I. Kenttam ̈ aa,‡ Paul C. Trulove,† Gozdem Kilaz,∥ ‡ ⊥ Lan Xu, and Jim S. Cowart †
Department of Chemistry, U.S. Naval Academy, 572 M Holloway Road, Annapolis, Maryland 21402, United States Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47909, United States § Department of Chemistry, U.S. Naval Academy Preparatory School, 440 Meyerkord Avenue, Newport, Rhode Island 02841, United States ∥ School of Engineering Technology, Purdue University, 401 N. Grant Street, West Lafayette, Indiana 47909, United States ⊥ Department of Mechanical Engineering, U.S. Naval Academy, 590 Holloway Road, Annapolis, Maryland 21402, United States ‡
S Supporting Information *
ABSTRACT: Chemical analysis and property measurements of a catalytic hydrothermal conversion jet (CHCJ) fuel were used to formulate hydrocarbon mixtures for use as fuel surrogates. Using conventional gas chromatography/(electron ionization) quadrupole mass spectrometry (GC/(EI)Q MS) and advanced two-dimensional gas chromatography/(electron ionization) high resolution time-of-flight mass spectrometry (GC×GC/(EI)TOF MS), CHCJ was found to differ from Jet-A fuel and to contain mostly linear alkanes, alkylcyclohexanes, and alkylbenzenes, with small amounts of branched alkanes and multiring aromatic compounds. Various surrogates were prepared containing n-dodecane, n-butylcyclohexane, and n-butylbenzene, and their density, viscosity, speed of sound, surface tension, and derived cetane number (DCN) were measured to determine the compositions that most closely matched that of the CHCJ. The optimal surrogates were (1) n-butylcyclohexane, (2) 0.64 mole fraction of nbutylbenzene in n-dodecane, and (3) three three-component blends of n-butylcyclohexane, n-butylbenzene, and n-dodecane with ratios of n-dodecane to n-butylcyclohexane of 0.25, 0.50, and 0.75 corresponding to a lower, medium, and higher n-butylbenzene concentration. Since fuel logistics in the military could be greatly simplified by use of a single fuel for both jet and diesel engines, this study examined this alternative jet fuel and its potential surrogates with respect to combustion in a diesel engine. Combustion experiments using a Waukesha diesel Cooperative Fuels Research (CFR) engine showed that all surrogate mixtures emulated the combustion engine performance of CHCJ in the areas of thermal efficiency, ignition delay, relative rate of heat release, crank angle degree 50% fuel burned location, and burn duration. All the surrogate mixtures operated in the Waukesha engine all showed statistically similar performance to the CHCJ fuel; however, the midaromatic (n-butylbenzene) threecomponent surrogate was marginally closer than either the higher or lower aromatic blends. These results show that DCN and other physical property measurements of a jet fuel can be used in conjunction with chemical composition to design surrogate fuel mixtures that match jet fuel performance in a diesel engine. These surrogate mixtures can be used in modeling studies to help determine the aspects of jet fuels that would enable them to have acceptable performance in a military diesel engine.
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INTRODUCTION In September 2016, an EA-18G Growler flew successfully on 100% alternative biofuel during a test flight at the Naval Air Station Patuxent River, Maryland.1 This biofuel was produced by Applied Research Associates (ARA) and Chevron Lummus Global using a catalytic hydrothermal conversion process.1,2 Other bio-derived materials must be mixed with petroleumbased fuels for use in jet and diesel engines and, thus, are called “alternative fuel blending components”.3−11 A UH-60 Black Hawk helicopter successfully flew on a 50% mixture of petroleum-based fuel with biofuel blending components derived from isobutanol, but a 70% petroleum mixture was required for startup performance of emergency diesel engines.9−11 The testing of jet fuel in military diesel engines is a response to a Department of Defense (DOD) directive that © 2017 American Chemical Society
challenges fuel producers to make one fuel for use in all military equipment.12 Having a jet fuel that can be used in a diesel engine enables the engine to operate under emergency circumstances when no diesel fuel is present and, in the long term, could greatly simplify fuel supply logistics. Both petroleum-based and bio-based fuels may contain hundreds if not thousands of compounds, and researchers often formulate mixtures containing only a few compounds to act as a model system for the fuels. These formulations, often called surrogate mixtures, reduce the required physical property and chemical reaction data needed to model the combustion Received: October 2, 2017 Revised: November 17, 2017 Published: November 25, 2017 13802
DOI: 10.1021/acs.energyfuels.7b02960 Energy Fuels 2017, 31, 13802−13814
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Energy & Fuels
can incorporate several of these factors and as well as chemical kinetic reaction behavior. Surrogate mixtures designed to match all properties often contain a large number of components, which reduces the utility of the surrogate in simplifying the mixture. In the current study, representative compounds were selected from the categories of compounds found in the fuel, so one-, two-, and three-component mixtures were prepared. These surrogates were combusted without petroleum mixing because previous work had shown that CHCJ could be combusted in a diesel engine without mixing with petroleumbased fuel.62
process. Colket et al. have stated that the combustion of each fuel component can involve hundreds to thousands of reaction steps with hundreds of intermediate product species.13 One model of toluene combustion includes 329 species and 1888 reversible reactions, while n-butylcyclohexane combustion simulations have involved 42 species and 80 reactions.14,15 The goal of this study was to develop a surrogate mixture for a catalytic hydrothermal conversion jet (CHCJ) fuel based on its chemical composition, physical properties, and chemical properties and to test this jet-fuel surrogate in a diesel engine. With a CHCJ surrogate mixture containing only a few components, the combustion behavior of the fuel in a diesel engine can be more easily modeled than a complex fuel mixture. This will help us to understand better what aspects of jet fuel impact diesel fuel combustion. The use of surrogate mixtures to model complex fuel mixtures can be found as far back as in the 1920s where gasoline surrogates (heptane, 2,2,4-trimethylpentane) were used to model knocking behavior, but the use of surrogates for jet fuels is more recent.16 In 2004, three working groups were established by federal agencies to look at the surrogates for gasoline, jet fuel, and diesel fuel with reports being published in 2007.16−18 Various experimental systems have been used to assess the performance of surrogate mixtures, including engines, burners, flame test rigs, rapid compression machines, shock tubes, and variable flow reactors.6,9,19−42 Das et al. tested various jet and diesel fuels and their surrogates containing from two to six compounds to compare their sooting tendencies in methane-air laminar coflow flames.19 To formulate a surrogate, researchers have used chemical composition, combustion-related properties such as cetane number or derived cetane number, research octane number, flash point, lower heating value, total sooting index, and physical properties such as density, viscosity, speed of sound, thermal conductivity, and information from the distillation curve or advanced distillation curve.6,25,43−54 Kim et al.53 considered density, viscosity, surface tension, distillation curve, derived cetane number, hydrogen to carbon ratio, lower heating value, and average molecular weight when developing a surrogate for Jet-A fuel. In our laboratories, we formulated a two-component surrogate for a bio-derived jet fuel based on chemical composition, speed of sound, viscosity, density, and bulk modulus and showed that the diesel engine startup performance of the surrogate mixed with petroleum-based jet fuel was the same as that of the bio-based jet fuel mixed with the petroleum-based jet fuel.9 Molecular modeling has been applied to predict the physical properties of surrogate mixtures. Molecular dynamics simulations have predicted the density, bulk moduli, and heat of vaporization of single components and surrogate fuel mixtures.55 The goal of this study was to design surrogate mixtures that would mimic the diesel engine combustion behavior of CHCJ. An iterative approach was used that included the chemical components, combustion properties (derived cetane number, flash point) and physical properties (density, viscosity, surface tension, bulk modulus, volatility) of the fuel. Many of these properties can impact various aspects of the behavior of fuels in diesel engines, and some of them are included in the fuel specifications.56−59 The start of injection can be influenced by bulk modulus, and the formation of fuel droplets in the engine combustion chamber can be influenced by density, viscosity, and surface tension.60,61 The evaporation of the droplets is influenced by volatility. Derived cetane number and flash point
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EXPERIMENTAL SECTION
Materials. The catalytic hydrothermal conversion jet (CHCJ) fuel was produced by Applied Research Associates, Inc. (ARA) and Chevron Lummus Global. This fuel is produced by using a supercritical water process that converts oils from plants, algae, and animals into a biocrude product, which is then hydrotreated using a proprietary catalyst to saturate the olefins and remove the oxygens.2 Fractionation of the resulting mixture is used to produce jet fuel. The CHCJ (LIMS 9353) was provided by the Naval Air Systems Command. Jet-A was acquired from the Wright-Patterson Air Force Base, Dayton, OH. Various organic compounds were used in the chemical analysis and are listed in the Supporting Information. Physical and Chemical Analysis. The fuel properties quantified in this work were surface tension, viscosity, density, speed of sound, distillation curve, flashpoint, and isentropic bulk modulus, Ev, which can be calculated from the speed of sound (c) and density (ρ):
Ev = c 2ρ
(1)
The measurement procedures have been described in previous publications with instrument tested and calibrated with NIST-traceable and certified standards.6 Briefly, viscosity, speed of sound, density, flash point, and surface tension were measured using Stabinger viscometer (Anton Parr SVM 3000), density and sound analyzer (Anton Parr DSA 5000), Setaflash series 8 flash point tester (Stanhope-Seta model 82000-0, closed-cup), and axisymmetric drop shape analyzer (Kruss DS100), respectively. The flash point tester was operated using flash/no flash mode and temperature ramping conditions, in conformance with ASTM methods E502, D3278, and D7236. Replicate measurements (2−80) were used to determine precision. The distillation curves were measured using a Grabner MINIDIS ADXpert automatic distillation apparatus that measures the distillation information per specifications of ASTM D7344-14 with a correlation to ASTM D86 when using small volumes.63,64 ASTM D7344-14, Standard Test Method for Distillation of Petroleum Products and Liquid Fuels at Atmospheric Pressure (Mini Method), is a method that requires less volume, approximately 20 mL, than the standard ASTM D86, which requires approximately 150 mL. Small volume/rapid analysis reduces the amount of fuel needed for testing. The hydrocarbons present in the CHCJ were determined by using conventional gas chromatography/quadrupole mass spectrometry with electron ionization (EI) [GC/(EI)Q MS] as well as more advanced two-dimensional gas chromatography/high-resolution mass spectrometry, also with EI (GC×GC/(EI)TOF MS). The GC/MS analysis method has been described previously.62 The NIST/EPA/NIH Mass Spectral Library (Version 2.0 g) was used to determine what compounds had EI mass spectra with the closest match to those measured for compounds found in the CHCJ. Then, pure compounds were tested to confirm a match between the mass spectral pattern and the retention time of the pure compound and the component of the CHCJ. The GC×GC/MS analysis was conducted using an Agilent 7890A GC×GC system coupled to a Pegasus-HRT 4D (EI) TOF mass spectrometer (Leco Co., St. Joseph, MI, USA). Approximately 10 μL of CHCJ fuel were added into 1.0 mL of n-hexane. An auto injector (Agilent G4513A) was used to inject 0.5 μL of each prepared sample into a split/splitless injector with a split ratio of 1:20. The injection 13803
DOI: 10.1021/acs.energyfuels.7b02960 Energy Fuels 2017, 31, 13802−13814
Article
Energy & Fuels inlet was held at 260 °C. A constant flow of ultrapure helium carrier gas (1.25 mL/min) was used. The samples were separated using a reversed column configuration consisting of a 60 m polar capillary column (Rxi-17Sil ms, Restek, Bellefonte, PA) in the primary oven followed by a 2 m nonpolar capillary column (Rxi-1 ms, Restek, Bellefonte, PA) in the secondary oven. The Agilent 7890A system was equipped with a quad-jet dual stage thermal modulator between the primary and secondary columns. The thermal modulator was supplied with liquid nitrogen for the cold jet stage during modulation. The modulator time was 10 s that was split between the stages, 5 s each, of which 1.5 s corresponded to hot pulse time and 3.5 s corresponded to cold pulse time. The primary oven was set to 40 °C and the heat was ramped up 1 °C/min until reaching a maximum temperature of 165 °C, which was held for 6 min. The secondary oven temperature was offset by +10 °C from that of the primary oven, and the temperature of the thermal modulator hot jet was offset +70 °C from that of the secondary oven. The transfer line was held at 300 °C throughout the entire experiment. The ion source was kept at 250 °C. The samples were ionized using EI at 70 eV electron energy. The analyte ions were sent into a TOF mass spectrometer (20 m flight length) for high resolution analysis (≥25 000). Mass spectra were collected at an acquisition rate of 200 Hz. The detector voltage was 1750 V. Daily mass calibration and tuning were performed by using perfluorotributylamine (PFTBA). An acquisition delay of 400 s was employed to prevent the ionization of n-hexane solvent. Data were collected over a m/z range of 45−550. Data collection, processing, and analysis were performed by using LECO Visual Basic Scripting (VBS) software, ChromaTOF Version 1.90.60.0.43266. Peak find and Library match Wiley (2011) and NIST (2011) databases were used for mass spectral identifications with a match factor threshold of >800. The mass accuracy of the TOF MS was routinely found to be
