Article pubs.acs.org/EF
JP‑5 and HRJ‑5 Autoignition Characteristics and Surrogate Modeling Casey Allen,† Daniel Valco,‡ Elisa Toulson,§ Ji Hyung Yoo,∥ and Tonghun Lee*,⊥ †
Department of Mechanical Engineering, Marquette University, Milwaukee, Wisconsin 53233, United States Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States § Department of Mechanical Engineering, Michigan State University, East Lansing, Michigan 48824, United States ∥ Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, Knoxville, Tennessee 37932, United States ⊥ Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡
S Supporting Information *
ABSTRACT: A heated rapid compression machine has been used to investigate the autoignition behavior of JP-5 and camelinabased hydrotreated renewable jet (HRJ-5) fuels. Testing was conducted at low temperatures (Tc = 627−733 K), low-to-moderate pressures (pc = 5, 10, and 20 bar), and lean (ϕ = 0.25 and 0.50) and stoichiometric mixtures in air. The HRJ-5 fuel, which is 99% paraffinic, exhibited greater reactivity than the JP-5 fuel in the form of shorter ignition delays. The HRJ-5 fuel also exhibited transition into the negative temperature coefficient region at a lower compressed temperature (Tc = 675 K) than the JP-5 fuel (Tc = 700 K). Two surrogate fuel blends and kinetic models intended for Jet-A and kerosene-type fuels are evaluated for their ability to predict JP-5 ignition delay times because JP-5 and Jet-A ignition delay times showed close resemblance. The models reproduced the qualitative trend in the data, including an accurate representation of when the negative temperature coefficient behavior appears. The best agreement between the data and predictions was obtained at pc = 5 bar and ϕ = 1.0, but outside of this region, the disparity was often 2-fold or greater.
1. INTRODUCTION The U.S. Department of Defense (DoD) is known to be the single largest consumer of petroleum-based fuels in the country, accounting for a full 2% of U.S. consumption.1 The 300 000 barrels of liquid petroleum-based fuels consumed each day2 are divided among the military branches, with the U.S. Department of the Navy (DoN) consuming 22% of the DoD total.3 New initiatives are directed at reducing these consumption levels by replacing conventional fuel sources with coal-to-liquid and gasto-liquid alternatives that are domestically produced. This shift is largely motivated by national security concerns, but environmental interests have also led to bio-based liquid fuels being considered as a candidate to replace JP-5 for naval aviation activities. This interest has centered on hydrotreated renewable jet (HRJ) fuels, which are a synthetic jet fuel made through a biomass-to-liquid process. These fuels are also referred to as hydrotreated esters and fatty acids (HEFA). The DoN has already demonstrated the feasibility of operating a F/A-18 Super Hornet using bio-based (camelina seed) HRJ-5 fuel in 50:50 blends with JP-5,4 and in 2012, a “green” strike group comprised of multiple F/A-18 aircraft and seaborne destroyers completed a training mission on fuel blends made with 50% biofuel.2 These accomplishments are important steps toward the DoN goal of obtaining one-half of all of their energy from renewable sources by 2020. Flight tests are the essential step to certify a fuel blend for operational use; however, these tests are costly, and the potential exists to replace or minimize the tests with simulation work. This is certainly true when introducing new fuels, such as HRJ-5, and the simulations can also be an effective cost-containment © 2013 American Chemical Society
mechanism when investigating the performance characteristics of new combustion technology (e.g., lean premixed combustion) with conventional or alternative fuel sources. An important step in conducting these simulations is to identify a simplified fuel surrogate (i.e., a minimum number of fuel components) that can reproduce the combustion characteristics of the actual fuel, whose large number of components makes simulation impractical. The work reported here is aimed at producing the validation data needed to guide the selection of surrogate components for both HRJ-5 and JP-5 and at evaluating the potential to predict JP-5 ignition behavior using currently available kinetic mechanisms and surrogate fuel formulations intended for conventional jet fuels. Characterization of the ignition properties of synthetic fuels is an active area of research to support integration of these fuels into existing infrastructure. Recent studies include that of Hui et al.,5 who investigated the combustion responses of synthetic paraffinic kerosene (SPK) and HRJ fuels for comparison to the conventional jet fuel, Jet-A. Measurements were made of derived cetane numbers (DCNs), ignition delay times, laminar flame speeds, and extinction stretch rates for each of the fuels. With particular relevance to this study, all but one of the SPK and HRJ fuels exhibited higher DCNs than Jet-A (Sasol IPK was the exception), owing to the lack of aromatic content and high n-alkane content in the (bio)synthetic fuels. The Sasol IPK, with its composition overwhelmingly isoparaffinic and Received: August 14, 2013 Revised: November 15, 2013 Published: November 18, 2013 7790
dx.doi.org/10.1021/ef401629d | Energy Fuels 2013, 27, 7790−7799
Energy & Fuels
Article
transducer (Kistler 6125C). The roll-up vortex that can form at the periphery of the test cylinder is captured in a piston crevice that is based on the design of Mittal and Sung.14 Of particular importance for this work, where non-volatile jet fuels are tested, the RCM is heated with six band heaters that are each independently controlled by feedback from six thermocouples placed axially along the test chamber wall. This arrangement ensures that a uniform temperature profile is maintained along the length of the test chamber. Schematics and characterization data describing the RCM are available in prior publications.15,16 Reactive fuel−oxidizer mixtures are prepared in the RCM by filling the test chamber with oxygen and diluent gas (N2 for all tests in this work) before using a fuel injector to spray the fuel into the test chamber. Preparing the mixtures directly in the RCM is beneficial relative to preparation in a mixing vessel because the mixtures are made at the actual test pressure rather than an elevated pressure that would be required to supply the mixture for several tests. This has the effect of reducing the mixture temperature to minimize fuel cracking concerns, and more importantly, it eliminates the concern that fuel may condense in the tubing during transfer to the RCM. The authors have referred to this approach as the direct test chamber (DTC) method. The effectiveness of this approach for preparing reactive mixtures is addressed in a prior work,15 where gas chromatography− mass spectrometry (GC−MS) analysis indicated that the fuel injector can accurately load the desired fuel mass and that the fuel and air become well-mixed in as little as 2 min after injection. 2.2. Test Fuels. The naval jet fuel, JP-5, and its bio-based alternative, HRJ-5, were tested in this work. The fuel samples used for testing were provided by the NAVAIR Naval Fuels and Lubricants Cross-Functional Team. Both of the fuels are complicated hydrocarbon blends but are compositionally distinct in the types of hydrocarbons that they carry. The distinction is illustrated in Figure 1,
cycloparaffinic, exhibited a lower DCN than Jet-A. The ignition delay data reinforced the results of the DCN analysis, specifically with regard to the onset of the first reaction (i.e., first stage of ignition). S-8 synthetic jet fuel and Jet-A ignition delay times under more comprehensive test conditions have been reported by Kumar and Sung,6 and the results clearly establish the greater ignition propensity of S-8 over Jet-A. Allen et al.7 have also investigated the ignition properties of U.S. Air Force HRJ fuels and JP-8, drawing a similar conclusion regarding the relative reactivity of HRJ and conventional jet fuels. The study also revealed that simple single-component (2methylnonane = C10) and two-component (10:90 n-dodecane/ 2-methylundecane = C12) surrogate fuels, when used with detailed kinetic mechanisms could only predict ignition characteristics under a limited set of conditions (stoichiometric conditions and low compressed pressures). Wang and Oehlschlaeger measured ignition delay times for SPKs (S-8, Shell GTL, and Sasol IPK) and conventional jet fuels (Jet-A and JP-8) in a shock tube between temperatures of 651 and 1381 K.8 The data revealed that, in the high-temperature region (>1000 K), these fuels show remarkably similar ignition properties, but the data diverge dramatically in the intermediate- and low-temperature regions. Results of other studies by Gokulakrishnan9 and Kahandawala10 provide additional data showing the greater reactivity of SPKs (S-8, specifically) relative to conventional jet fuels and also provide validation data that can be used in the development of surrogate fuel models. Existing kerosene-based jet fuel surrogate models are evaluated in this work for their ability to predict JP-5 ignition properties.11,12 These surrogates and mechanisms were formulated to best reproduce combustion and physical properties using relatively simple surrogate blend compositions, and they are applied as-is, without modifications to make the surrogates more specific to JP-5. The models were chosen largely because they are the most recent developments in surrogate fuel modeling for jet fuels but partly because surrogate fuel modeling for JP-5 has not been an active topic of research. In fact, the authors are aware of only two other JP-5 surrogates, both of which were proposed by Wood.13 Wood proposed 11- and 12-component surrogate fuels to represent the distillation properties and experimental combustion behavior of JP-5. Experimental validation using a swirl-stabilized burner showed similar combustion response between the surrogates and actual JP-5, except where soot formation was concerned. Unfortunately, many of these surrogate components have no available kinetic model for inclusion in a kinetic model that would predict the measurements reported in this work. Furthermore, because of the lack of experimental data describing the autoignition characteristics of JP-5, comparisons are made to the conventional jet fuel, Jet-A. Although these fuels have unique compositions, as discussed in section 2.2, they are both kerosene-like jet fuels with similar average molecular formulas.
Figure 1. TICs for JP-5, HRJ-5, and Jet-A. where total ion chromatograms (TICs) appear for both of the fuels. The ion current for the data sets has been normalized so that the peak current corresponds to unity. The JP-5 is characterized by prominent peaks that appear for the normal hydrocarbon species (primarily ndecane through n-tetradecane). The balance of the fuel is made up primarily of iso-alkanes, alkylated aromatics, alkenes, and cycloalkanes.17 The TIC data for the current batch of Naval HRJ-5 reveals a composition that is distinctly different. The normal alkane components are not obvious, because virtually the entire fuel is composed of alkanes (99.3 wt %), and the composition is fairly evenly divided between n-alkanes (54 wt %) and iso-alkanes (45 wt %). The TIC data reveal a “bulge” where a high concentration of heavier hydrocarbons exist (C15−C17) for the HRJ-5. The HRJ-5 is also
2. EXPERIMENTAL SECTION 2.1. Heated Rapid Compression Machine and Fuel System. A pneumatically driven, hydraulically stopped rapid compression machine (RCM) has been used to measure the ignition delay times of JP-5 and HRJ-5. The RCM is designed to minimize fluid motion during a rapid compression period that generates the elevated temperature and pressure conditions needed to autoignite the mixture. Pressure is monitored in the test chamber with a piezoelectric pressure 7791
dx.doi.org/10.1021/ef401629d | Energy Fuels 2013, 27, 7790−7799
Energy & Fuels
Article
release. The difference between τ and τ1 is the second-stage ignition delay τ2. Compressed gas temperatures, Tc, are calculated at the end of compression and used as the reference temperature for comparing ignition delay tests. The compressed temperatures are calculated by assuming that the core gas in the RCM undergoes adiabatic compression and that heat losses only occur near the cylinder wall (i.e., heat loss does not affect the core gas temperature). This enables Tc to be calculated using experimentally measured pressure values
characterized by its lack of aromatic content, which is common for synthetic jet fuels and tends to increase the cetane number of the fuel. Using ASTM D4737,18 the cetane index of the JP-5 blend has been calculated as 45.2, but the calculation is not attempted for HRJ-5 because the standard is described as unsuitable for (coal-based) synthetic fuels. The methods of Rao et al.19 have been used to estimate the molecular formulas of the fuels as C12.32H23.31 (JP-5) and C13.93H29.75 (HRJ-5). The larger estimate for HRJ-5 over JP-5 is consistent with the chromatogram data, which exhibits a large amount of HRJ-5 fuel content with a molecular weight higher than C14 hydrocarbons. These formulas are used as part of the compressed temperature calculations. The data used to calculate the cetane index and average molecular formulas appear in Table 1.
∫T
0
JP-5
HRJ-5
0.8150 194 210 235 13.6
0.768 190 240 274 15.2
p γ dT = ln c γ−1 T p0
where T is the temperature, γ is the specific heat ratio (Cp/Cv), p is the pressure, and the subscripts 0 and c correspond to initial and compressed conditions, respectively. To estimate the ideal gas specific heats of the JP-5 blends, a 12-component JP-5 surrogate proposed by Wood has been used. For most of these components, polynomial coefficients for calculating Cp/Ru are available from Goos et al.20 For species that are not described in the Goos et al. thermodynamic database, specific heats have been estimated between 300 and 1000 K using group contribution methods,21 and these calculations are used to determine coefficients a1, a2, a3, a4, and a5 that are required to calculate Cp/Ru using the 7-coefficient NASA polynomial format. One exception to this approach is α-methylnaphthalene, whose polynomial coefficients were obtained from Wang et al.22 The coefficients calculated using group contribution methods appear in Table 2, but the reader is cautioned that these are only valid for Cp calculations and not for other thermodynamic properties. They are also valid only for the temperature range of 300−1000 K. For the HRJ-5 compressed temperature calculations, the thermodynamic properties of ntetradecane were used to represent those of the fuel blend with its estimated average molecular formulas of C13.93H29.75. The use of a single paraffin to represent the heat capacity of a synthetic fuel blend has been previously evaluated,7 and its selection is shown to be arbitrary in determining the compressed temperature. However, it is noted that this evaluation was performed on the basis of the assumption that the gas compressibility of the synthetic fuel and pure component paraffin were similar, which has not been experimentally validated. Ignition delay results are reported with error bars that represent the standard error of the measurements at a particular condition. These data represent no fewer than three tests at each condition. Total ignition delay measurements were found to be repeatable within ±5%, and first-stage ignition delay measurements were repeatable to within ±8%. The main uncertainty in the results is the calculated compressed temperature that has been estimated as approximately ±25−35 K, depending upon the experimental conditions. This uncertainty is represented by error bars in the figures. The calculated uncertainties include contributions from instrument measurements of initial pressure (±0.08%), compressed pressure (±1%), and initial temperature (±3 K), as well as uncertainties in thermophysical property data (δCp/Cp = ±0.5%) and mixture composition (mole fraction uncertainties: δXf/Xf = ±6%, δXO2/XO2 = ±1%, and δXN2/XN2 = ±0.1%). The uncertainty in mixture composition translates to an average uncertainty in the specific heat ratio of δγ/γ = ±3%. Uncertainty in the fuel mole fraction is the main contributor to uncertainty in the compressed temperature. 2.4. Experimental Test Conditions. The ignition characteristics of JP-5 and HRJ-5 have been studied in this work at compressed
Table 1. Fuel Specification Data density at 15 °C (g/mL) T10 (°C) T50 (°C) T90 (°C) hydrogen content (% mass)
Tc
2.3. Data Analysis and Experimental Uncertainty. The reactive pressure tests in this work are characterized by ignition delay periods, which are used for comparison to surrogate kinetic models. The ignition delay periods are illustrated in Figure 2, and the
Figure 2. Sample data for the HRJ-5 ignition test showing pressure history and pressure derivative data (dp/dt). The effect of the filtering operation is shown, and the ignition delay definitions are illustrated for τ1, τ2, and τ. definitions are consistent with that used by many other investigators. The total ignition delay, τ, corresponds to the time period from the end of compression to the main heat release event. The end of compression is approximated by the instant at which the rate of pressure change first becomes negative after the compression stroke. This can be observed in Figure 2, where the derivative of the pressure with respect to time (dp/dt) is plotted alongside the pressure (p) data. The first-stage ignition delay, τ1, which marks the onset of lowtemperature heat release is identified by the time at which the maximum rate of pressure rise occurs because of low-temperature heat
Table 2. Coefficients for Calculating the Molar Constant-Pressure Specific Heat as Cp = Ru(a1 + a2T + a3T2 + a4T3 + a5T4) a1 n-pentylcyclohexane 1,3-diisopropylbenzene 1-phenyl-hexane n-heptylcyclohexane
9.98351320 9.99054061 9.99140951 9.98508861
a2 × × × ×
−1
10 10−1 10−1 10−1
7.17216187 7.86672138 8.08117484 9.26209090
a3 × × × ×
−2
10 10−2 10−2 10−2
9.65431959 5.85806203 4.78187165 8.94833513 7792
a4 × × × ×
−5
10 10−5 10−5 10−5
−1.70588487 −1.38860862 −1.23689844 −1.75240767
a5 × × × ×
−7
10 10−7 10−7 10−7
7.07491893 6.31108931 5.65845755 7.39903534
× × × ×
10−11 10−11 10−11 10−11
dx.doi.org/10.1021/ef401629d | Energy Fuels 2013, 27, 7790−7799
Energy & Fuels
Article
temperatures of 627 ≤ Tc ≤ 733 K, compressed pressures of pc = 5, 10, and 20 bar, and equivalence ratios of ϕ = 0.25, 0.5, and 1.0. The fuel− air mixtures have been made using dry air (21% O2 and 79% N2). Initial temperatures in the RCM varied between 115 and 128 °C, which were chosen to ensure complete fuel vaporization prior to commencing a test. Fuel vaporization has been confirmed using GC− MS in a technique previously described by the authors.15 Variation of the compressed temperatures was achieved by altering the compression ratio, and the highest tested temperature is dictated by the occurrence of low-temperature heat release during the compression stroke. This behavior prevented testing at temperatures above 733 K because top dead center (TDC) cannot be accurately specified nor can the corresponding reference temperature be calculated.
the composition of Jet-A and JP-5 are distinct from one another, the prevailing theory in predicting kinetic behavior of complex fuel blends and surrogates says that global phenomena, such as ignition delay time and flame speed, are primarily influenced by the tendency of a given fuel molecular structure to produce the relevant radical pool that controls the reaction rate.25 For both JP-5 and Jet-A, this important radical pool is expected to be comprised of similar species: alkyl, alkylperoxy, and alkyl hydroperoxy radicals, within a similar molecular weight range. The most significant compositional difference between the fuels is related to aromatic content, where the POSF 4658 is approximately ∼29% aromatic (26% alkylbenzenes and 3% naphthalenes) and the JP-5 contains only 12% aromatics (>93% of these are alkylbenzenes). Nevertheless, our main justification for applying the Jet-A surrogate to JP-5 is that, by our measurements, Jet-A (POSF 4658) and JP-5 exhibit similar ignition delay times in the tested temperature range (at pc = 20 bar and ϕ = 1.0). Results to support this claim are reported in section 4.1. The authors do note that the Dooley Jet-A surrogate was formulated on the basis of a distinct methodology proposed by Dooley et al., whereby global combustion properties (derived cetane number and threshold sooting index), fuel molecular weight, and average molecular formula are used to select for the optimum surrogate blend. This combination of properties will be unique for any particular fuel or fuel blend; thus, we cannot initially expect the Dooley Jet-A surrogate to match a “Dooley JP-5 surrogate” formulated using JP-5 properties. In this sense, application of the Dooley Jet-A surrogate to our data is not a test of the Dooley et al. methodology but a test of whether the surrogate, which rather successfully predicted Jet-A ignition delay times near pc = 20 atm and ϕ = 1.0, can make accurate predictions at a wider set of conditions for another conventional jet fuel (JP-5). The aforementioned JP-5 surrogates of Wood, which were used as a basis for calculating ideal gas heat capacity of the fuel (see section 2.3) is not used for simulating the ignition kinetics of JP-5 because no kinetic mechanism is available for many of the components. All of the kinetic modeling results were obtained using CHEMKIN-PRO. The simulations modeled the full compression stroke of the RCM to account for the influence of the chemical reaction occurring prior to TDC. To facilitate these simulations, effective volume profiles were obtained from the RCM by compressing fuel and nitrogen mixtures meant to mimic the specific heats of the mixtures used in the reactive tests. The pressure data gathered from these tests is used to calculate an effective volume profile that is provided as an input to CHEMKIN.26 The effective volume profiles are available to colleagues by contacting the corresponding author.
3. KINETIC MODELING AND SURROGATE FUEL SELECTION The experimental results obtained in this study have been simulated using two surrogate fuel and kinetic model combinations that are presently available. The Aachen surrogate, with the mechanism developed by Honnet et al.,12 represents one combination, and the surrogate fuel composition is 80% n-decane and 20% 1,2,4-trimethylbenzene, by weight (77.2 and 22.8% molar basis, respectively). The surrogate fuel mechanism is based on the n-decane semidetailed mechanism of Bikas and Peters23 and was validated with non-premixed combustion experiments using JP-8 (POSF 4177) and an unspecified commercial kerosene. The validation experiments showed that the mechanism and surrogate combination could generally predict autoignition temperatures measured in a counterflow burner to within 5%, with increasing accuracy at low strain rates (
