ARTICLE pubs.acs.org/EF
Chemical, Thermal Stability, Seal Swell, and Emissions Studies of Alternative Jet Fuels Edwin Corporan,*,† Tim Edwards,† Linda Shafer,‡ Matthew J. DeWitt,‡ Christopher Klingshirn,‡ Steven Zabarnick,‡ Zachary West,‡ Richard Striebich,‡ John Graham,‡ and Jim Klein§ †
Air Force Research Laboratory, Fuels and Energy Branch, Wright-Patterson AFB, Ohio 45433, United States University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469, United States § Jim Klein LLC, Wright-Patterson AFB, Ohio 45433, United States ‡
ABSTRACT: This effort describes laboratory evaluations of six alternative (nonpetroleum) jet fuel candidates derived from coal, natural gas, camelina, and animal fat. Three of the fuels were produced via Fischer-Tropsch (FT) synthesis, while the other three were produced via extensive hydroprocessing. The thermal stability, elastomer swell capability, and combustion emissions of the alternative jet fuels were assessed. In addition, detailed chemical analysis was performed to provide insight into their performance and to infer potential behavior of these fuels if implemented. The fuels were supplied by Sasol, Shell, Rentech, UOP, and Syntroleum Corporation. Chemical analyses show that the alternative fuels were comprised of mostly paraffinic compounds at varying relative concentrations, contained negligible heteroatom species, and were mostly aromatic-free. The six paraffinic fuels demonstrated superior thermal oxidative stability compared to JP-8, and therefore, have increased resistance to carbon formation when heated and can be exposed to higher temperatures when used to cool aircraft systems. Material compatibility tests show that the alternative fuels possess significant seal swelling capability in conditioned nitrile O-rings; however, elastomer swelling was significantly lower than for JP-8, which may likely result in fuel leaks in aircraft systems. Engine tests with the alternative fuels demonstrated no anomalies in engine operation, production of significantly lower nonvolatile particulate matter (soot), and moderately lower unburned hydrocarbons and carbon monoxide emissions compared to baseline JP-8 fuel. Also, no penalty (i.e., increase) in fuel flow requirement for equal engine power output was observed. In general, this study demonstrates that paraffinic fuels derived from different feedstocks and produced via FT synthesis or hydroprocessing can provide fuels with very similar properties to conventional fuels consisting of excellent physical, chemical, and combustion characteristics for use in turbine engines. These types of fuels may be considered as viable drop-in replacement jet fuels if deficiencies such as seal swell, lubricity, and low density can be properly addressed.
’ INTRODUCTION The growing demand and reduced supply of petroleum products, and instability in petroleum-rich countries, results in high uncertainties and volatility in the cost of energy, particularly transportation fuels. As a result, efficient energy technologies and the development of alternative energy options, such as fuels from domestic alternative sources, have become a national priority. Alternative transportation fuels are desirable both from an energy security and environmental perspective as the preponderance of imported oil is converted to liquid transportation fuels. In the United States, liquid fuel production is roughly 200 billion gal/y, with gasoline, diesel, and jet fuel being produced in approximately a 70/20/10 ratio. Thus, domestically produced alternative fuels could increase energy security. Domestic alternatives for gasoline (ethanol) and diesel (biodiesel) exist, but no operational alternative jet fuels are currently being produced in the United States. Alternative aviation fuels are also of interest for mitigating environmental impacts of fuel use, both on the global (climate change) and local (airport) air quality level. Although aviation contributes only 2% to global CO2 emissions, the U.S. Air Force (USAF) and the aviation industry are committed to contributing to potential solutions.1 Alternative aviation fuels are being sought as “drop-in” replacements for current aircraft, requiring no modification to equipment, aircraft operations, r 2011 American Chemical Society
handling, and transportation. This rules out ethanol (due to safety, performance, handling, and material compatibility issues) and biodiesel (due to low temperature and combustion performance, and storage stability issues). Initial U.S. efforts in developing alternative aviation fuels focused on Fischer-Tropsch (FT) fuels produced from coal, biomass, and/or natural gas. Coal-derived “iso-paraffinic kerosene” (IPK) produced by Sasol in South Africa was approved for aviation use in blends up to 50% by volume in Jet A-1 (on a producer-specific basis) in 1999.2 Generic FT “synthetic paraffinic kerosene” (SPK) was approved for use in blends with JP-8 in MIL-DTL-83133G in 2008 and in blends with commercial Jet A in ASTM D7566 in August 2009 (supported by a Research Report outlining the properties of SPK).3-5 The USAF has been very active in the evaluation, demonstration, and certification of FT fuel blends from natural gas and coal. To date, U.S. military aircraft such as the B-52, C-17, and B-1B have been certified for use of a 50/50 (by volume) JP-8/FT blend. Other aircraft (i.e., F-22, KC-135, F-15, C-5, T-38) have already undergone flight tests and are scheduled to be certified on the FT blend in the near Received: November 10, 2010 Revised: January 27, 2011 Published: March 02, 2011 955
dx.doi.org/10.1021/ef101520v | Energy Fuels 2011, 25, 955–966
Energy & Fuels
ARTICLE
future. The next class of alternative fuel being studied for military/commercial aviation certification is “hydroprocessed renewable jet” (HRJ), which is a hydrocarbon aviation fuel produced from animal fat/vegetable oils (triglycerides) by hydroprocessing. This fuel has also been called bio-SPK and “green jet”. Ground and flight tests were recently (August 2010) completed on the C-17 Globemaster III cargo aircraft operating on blends of a tallow-derived HRJ and JP-8 (50/50) and the tallow-derived HRJ, coal FT and JP-8 (25/25/50). In addition, flight tests on 50/50 HRJ/JP-8 blends have been scheduled for FY11 on F-15 and F-22 aircraft. The ASTM D7566 specification is structured to support various classes of alternative fuels in its Appendices, with HRJ anticipated to be added in the near future as more data become available. It is anticipated that fuel produced from ligno-cellulosic feedstocks will also be added at a future date. In this effort, six alternative jet fuel candidates derived from several feedstocks and processes were evaluated to determine their suitability for use in aircraft. Specifically, the chemical and physical properties, oxidative thermal stability characteristics, elastomer swell compatibility, and combustion emissions were evaluated and compared to specification JP-8 fuels.
Emissions Evaluations. Turbine Engine. A T63-A-700 turboshaft engine was used to study the particulate matter (PM) and gaseous emissions characteristics of the alternative jet fuels. The engine is located in the Engine Environment Research Facility (EERF) in the Propulsion Directorate at Wright-Patterson Air Force Base and is used to evaluate turbine engine lubricants, fuels, and sensors in an actual engine environment. Detailed descriptions of the engine and operating methodology have been provided in previous publications.13,14 Although this engine has a single fuel injector, operates at relatively low pressures (∼250-550 kPa), and represents an older technology, the observed emissions trends of more modern technology engines operated with alternative fuels have been consistent with those observed on this platform.15,16 For this effort, the engine was operated at ground idle (low power) and cruise (high power) conditions. Baseline JP-8 was delivered to the engine fuel pump from a facility underground tank while the neat and blended alternative fuels were supplied from a nitrogen-pressurized (138 kPa) external tank. For all tests, the fuel flow rate was computer controlled to maintain a constant turbine outlet temperature (T5). This was considered the most appropriate approach for emissions comparison between fuels and for best run-to-run engine condition repeatability. For each evaluation, the engine was initially operated on JP-8 at the two power settings, then with the alternative fuel, and finally returned to JP-8 to verify the baseline emissions. Emissions Instrumentation. PM and gaseous emissions were sampled from the engine exit plane using oil-cooled probes maintained at 150 °C and transported to the analytical instruments via heated lines (65 °C). The PM sample stream (mostly nonvolatile) was diluted with nitrogen near the probe tip to prevent water condensation, minimize particle losses in the sample lines, and prevent saturation of the particle counting instruments. The PM emissions were characterized using conventional aerosol instruments and corrected for dilution based on the raw and diluted CO2 measurements. A TSI Model 3022A condensation particle counter (CPC) was used to provide a count of particles per unit volume (particle number (PN)), a TSI Model 3936 scanning mobility particle sizer (SMPS) with a nanodifferential mobility analyzer (nDMA) was used to obtain the particle size distribution from 5 to 150 nm, and a Rupprecht & Patashnick Series 1105 tapered element oscillating microbalance (TEOM) was used to obtain real-time particle mass emissions. For the particle mass emissions, only results for the cruise condition are discussed due to the low particle mass at idle for the SPK fuels. In addition, an in-house designed and built smoke machine was used to collect soot samples for determination of the engine smoke number per SAE ARP 1179.17 Gaseous emissions were sampled with undiluted probes and transported through heated lines kept at 150 °C per the SAE ARP 1256.18 Major and minor gaseous species were quantified using an MKS multi gas 2030 Fourier transform infrared (FTIR) based analyzer and total unburned hydrocarbons were quantified using a CAI 600 heated flame ionization detector. A nondisperse infrared analyzer (NDIR) measured the CO2 for the diluted samples from the particle instruments. Fuel Elastomer Swell Evaluation System. To measure the overall volume swell character of the paraffinic fuels studied here, the volume swell of a nitrile rubber (Parker N0602) was measured in each fuel. Nitrile rubber was selected as the test material as it is a commonly used elastomer in aircraft systems. The plasticizer was extracted from the nitrile rubber before testing to remove the confounding factor of O-ring shrinkage due to the extraction of plasticizer by the fuel (approximately 10% reduction in original O-ring volume). Generally, the volume swell of elastomers by a fuel is dependent on the fuel’s molar volume/ geometry, polarity, and ability of the fuel to serve as the hydrogen donor in a hydrogen bond. Nitrile rubber responds to the three factors identified above as influencing the volume swell of O-ring materials. The volume swell of this material was measured using optical dilatometry.19 Briefly, two O-ring samples were placed in a reservoir with 10 mL of the
’ EXPERIMENTAL SECTION Techniques for Chemical and Physical Analysis. The chemical composition and physical properties of the alternative fuels were evaluated to provide increased insight into their performance and to infer potential behavior during implementation. The analyses included evaluation of JP-8 specification properties, hydrocarbon type, and nonstandard analysis such as gas chromatography/mass spectrometry (GC/MS), gas chromatography with a flame ionization detector (GC/ FID), and high performance liquid chromatography (HPLC). The fuels considered comprise a combination of both commercially available and research fuels. Thermal Oxidative Stability Evaluation Systems. Quartz Crystal Microbalance (QCM). The quartz-crystal microbalance (QCM) has been used extensively to study jet fuel thermal stability and qualify the effects of various jet fuel additives on fuel thermal stability.6-8 The QCM has the capabilities to monitor both headspace oxygen and carbon deposition in situ during fuel thermal stressing. The in situ mass accumulation is determined based on the well-known relationship between the change in surface mass and resonant frequency of the quartz crystal.6 The combined oxygen and mass deposition measurements allow for a greater understanding of the autoxidation process for each fuel. The QCM is a batch experiment that is typically operated by thermally stressing 60 mL of fuel at 140 °C for 15 h. Fuel samples are air saturated prior to heating and the system is closed during operation. Single Tube Flow Reactor System. A single-tube flow reactor system was used to evaluate the relative oxidative stability characteristics of the alternative fuels in a flowing environment. The system has previously been used to evaluate thermal stability characteristics of fuels under both oxidative and pyrolytic conditions.9-12 The reaction zone is comprised of a 91.4 cm actively heated section where the fuel is exposed to sufficient temperature to promote the desired reaction chemistry. The outer wall temperature profile of the reaction tube is monitored using thermocouples (TC) strap-welded at various locations. The bulk fuel outlet temperature is monitored using a TC that is inserted into the outlet fuel flow approximately 17.8 cm downstream of the actively heated zone. After exiting the reaction zone, the fuel is cooled and passed through a 7-μm sintered filter element to remove any solids that are entrained in the fluid. The stability characteristics are determined by quantifying the total carbon deposition on the internal surface of the reaction tube and on the downstream filter. 956
dx.doi.org/10.1021/ef101520v |Energy Fuels 2011, 25, 955–966
Energy & Fuels
ARTICLE
solubilized metals. If not removed, these will deposit in the fixed bed reactors causing excessive pressure drop across the catalyst bed and decrease of catalyst activity. The next step in the process is hydrodeoxygenation (HDO). The HDO product effluent, which contains primarily C15-C18 n-paraffins, is hydrocracked/ isomerized to produce a fuel with a boiling point distribution and freeze point similar to a conventional JP-8. The tested R-8 fuel is very similar to the Fischer-Tropsch (FT) derived fuel produced by Syntroleum, S-8, which was used in the certification of alternative FT fuel blends.23 UOP produced two research aviation HRJ fuels using camelina and beef tallow as feedstocks. In the process, pressurized feedstock is mixed with hydrogen and undergoes catalytic deoxygenation to primarily produce n-paraffins. This product is then hydorcracked/isomerized to satisfy required freeze point characteristics and separated to the desired volatility. The beef tallow HRJ fuel evaluated in this effort is very similar to the HRJ used in the recent C-17 tests. Various physical and chemical properties of the alternative fuels were evaluated and compared to a typical specification JP-8. Selected fuel specification properties are shown in Table 2. In general, all synthetic fuels evaluated had zero to very low aromatic content, negligible sulfur, lower density, and higher hydrogen content compared to conventional JP-8. If used neat, the relative low density of paraffinic fuels is likely to impact aircraft range; however, the impact is highly dependent on whether the aircraft is weight or volume limited.24 Gas chromatograms of the various fuels (with the n-paraffin peaks identified) are shown in Figure 1. As shown in Figure 1 and Table 2, the Rentech FT and all HRJ fuels have a boiling point range similar to a typical JP-8, while the Shell and Sasol FT fuels are much narrower. Results of the hydrocarbon type analyses, performed using ASTM D6379 and D2425, are shown in Tables 3 and 4, and a comparison of the nparaffin content of the fuels is shown in Table 5. As noted previously, the alternative fuels are primarily comprised of normal and branched paraffins with low or negligible aromatic content. On the basis of knowledge of the processing techniques employed for the FT fuels, it was expected that these fuels would be solely comprised of paraffinic compounds and free of aromatics. It appears that aromatic compounds found in the Rentech, Sasol, and R-8 fuels were produced during the upgrading steps of the initial products following FT-synthesis (Rentech and Sasol) or hydrodeoxygenation (R-8). Improved control of the fuel upgrading process parameters will likely eliminate the production of aromatic compounds in these fuels if required. The Shell FT fuel showed a mild degree of branching and was comprised of a much higher n-paraffin concentration than found in typical petroleum-derived jet fuels or the other alternative fuels evaluated. High concentrations of long n-paraffins can result in poor low temperature properties; however, the narrow distillation range produces a fuel with a sufficient freeze point.21 On the contrary, the Sasol fuel had no detectable n-paraffins and a very high degree of branching (e.g., tri- and tetra-methylparaffins). The absence of n-paraffins results in an extremely low freeze point for the Sasol SPK. The Rentech FT and the HRJ fuels have similar n-paraffin distributions, but slightly lower in magnitude than a typical JP-8. These fuels exhibited a mild degree of branching and are very similar in composition to the Syntroleum S-8 fuel used in the FT fuel certification of the B-52.23 As shown in Table 4, significant
test fuel. After 2 min of immersion in the fuel, the samples were digitally photographed every 20 s for the next 3 min. At 6 min total elapsed time, the samples were photographed every 60 min for the next 40 h. After the aging period was completed, the cross-sectional area was determined from the digital images and used as a characteristic dimension proportional to the volume. The final volume swell was calculated as the average value obtained from the two samples.
’ TEST DATA AND DISCUSSION Chemical and Physical Analysis of Fuels. Fischer-Tropsch Fuels Produced by Sasol, Shell, and Rentech. The Sasol, Shell, and
Rentech fuels were produced via indirect liquefaction using FT synthesis. The Sasol and Shell fuels are commercially available. During indirect liquefaction, the feedstock (natural gas for Shell and Rentech; coal for Sasol) is partially oxidized either via gasification or steam reforming to produce synthesis gas (carbon monoxide and hydrogen). The synthesis gas (syngas) is fed to an FT reactor where it is converted into higher molecular weight hydrocarbons. The Sasol SPK (also referred to as isoparaffinic kerosene (IPK)) is produced via oligimerization of C3 and C4 olefins followed by hydrotreating and fractionation to produce a fuel with the desired boiling range.20 This process results in an SPK with a very high degree of branching. The Sasol fuel was the first synthetic fuel to be approved as a blend feedstock with Jet A-1 fuel.2 The Shell SPK was produced via the Shell middle distillate synthesis (SMDS) process.21 The SMDS process produces long-chain paraffins from syngas in multitubular fixed bed reactors, followed by hydrocracking, isomerization and fractionation. The submitted Shell SPK fuel is a very narrow-cut kerosene compared to a typical JP-8. The Rentech fuel hydroprocessing process includes hydrotreating to remove oxygenates, followed by hydrocracking/isomerization, hydrotreating, and fractionation to produce a fuel with a volatility range similar to JP-8. Hydroprocessed Renewable Jet Fuels (HRJ) produced by Syntroleum and UOP. Currently, hydroprocessed renewable jet (HRJ) fuels, also known as Bio-SPKs, are being considered for inclusion as approved blending components for both military and commercial jet fuels.22 As part of the USAF alternate fuels research program, HRJ fuels have been synthesized and evaluated for their potential application in military systems. Syntroleum Corporation produced a research aviation fuel using the Syntroleum Bio-Synfining process. The research fuel is a renewable SPK termed R-8 (i.e., renewable feedstock and JP-8 like). The Bio-Synfining process can produce SPK from waste fats and greases, which do not compete with food crops. The feedstocks used for the research fuel evaluated in this study are shown in Table 1. The Syntroleum process includes a feed pretreatment to reduce the contaminants in the solids. Contaminants include animal solids, rust particles, catalyst components, water, and Table 1. Feedstocks Used to Produce R-8 Fuel component
mass %
poultry fat
46%
yellow grease brown grease
18% 18%
floatation grease
9%
prepared foods
9% 957
dx.doi.org/10.1021/ef101520v |Energy Fuels 2011, 25, 955–966
Energy & Fuels
ARTICLE
Table 2. ASTM Specification Tests of Fuels Evaluated ASTM tests
standard
JP-8
Shell FT
Sasol FT
Rentech FT
R-8 HRJ
tallow HRJ
camelina HRJ
total acid number, mg KOH/g (D3242)
max 0.015
0.005
0.002
0.002
0.004
0.002
0.002
0.002
aromatics, % vol (D1319)
max 25.0
17.2
0.0
0.4
1.7
0.0
0.4
0.0
total sulfur, % wt (D4294 or D2622)
max 0.30
0.064
