Omid AskariMechanical Engineering Department,

Mississippi State University,

Starkville, MS 39762

Mimmo EliaMechanical and Industrial

Engineering Department,

Northeastern University,

Boston, MA 02115

Matthew FerrariMechanical and Industrial

Engineering Department,

Northeastern University,

Boston, MA 02115

Hameed MetghalchiMechanical and Industrial

Engineering Department,

Northeastern University,

Boston, MA 02115

Auto-Ignition CharacteristicsStudy of Gas-to-Liquid Fuel atHigh Pressures and LowTemperaturesOnset of auto-ignition of premixed gas-to-liquid (GTL)/air mixture has been determinedat high pressures and low temperatures over a wide range of equivalence ratios. TheGTL fuel used in this study was provided by Air Force Research Laboratory (AFRL), des-ignated by Syntroleum S-8, which is derived from natural gas via the Fischer–Tropsch(F–T) process. A blend of 32% iso-octane, 25% n-decane, and 43% n-dodecane isemployed as the surrogates of GTL fuel for chemical kinetics study. A spherical chamber,which can withstand high pressures up to 400 atm and can be heated up to 500 K, wasused to collect pressure rise data, due to combustion, to determine the onset of auto-ignition. A gas chromatograph (GC) system working in conjunction with specializedheated lines was used to verify the filling process. A liquid supply manifold was used toallow the fuel to enter and evaporate in a temperature-controlled portion of the manifoldusing two cartridge heaters. An accurate high-temperature pressure transducer was usedto measure the partial pressure of the vaporized fuel. Pressure rise due to combustionprocess was collected using a high-speed pressure sensor and was stored in a local desk-top via a data acquisition system. Measurements for the onset of auto-ignition were donein the spherical chamber for different equivalence ratios of 0.8–1.2 and different initialpressures of 8.6, 10, and 12 atm at initial temperature of 450 K. Critical pressures andtemperatures of GTL/air mixture at which auto-ignition takes place have been identifiedby detecting aggressive oscillation of pressure data during the spontaneous combustionprocess throughout the unburned gas mixture. To interpret the auto-ignition conditionseffectively, several available chemical kinetics mechanisms were used in modeling auto-ignition of GTL/air mixtures. For low-temperature mixtures, it was shown that auto-ignition of GTL fuel is a strong function of unburned gas temperature, and propensity ofauto-ignition was increased as initial temperature and pressure increased.[DOI: 10.1115/1.4033983]

Keywords: onset of auto-ignition, GTL, syntroleum S-8, spherical chamber, ignitiondelay time, pressure rise, chemical kinetics mechanism

Introduction

The need for finding an alternative to oil-based transportationfuels is greater now than ever before due to environmental impactand supply security. Alternative fuels obtained from feedstockssuch as biomass, natural gas, and coal are called synthetic paraf-finic kerosene (SPK) fuels. Recently, the interest on SPK fuels asa viable alternative fuel for aviation transportation is enormous asthey do not warrant any major modifications to the existing fuelinjection/combustor system. Furthermore, the SPK fuels obtainedthrough F–T synthesis are preferred as the fuel composition canbe appropriately tailored for specific applications. Among SPKfuels, GTL fuel is preferred over conventional jet fuels due tocleaner combustion characteristics as a result of less aromatic con-tent and the near absence of sulfur. [1]. The GTL fuel can also besurrogate for gasoline, diesel, and aviation fuels [2]. Over theyears, the potential for deriving high value products from naturalgas [3–5] and its abundant availability have attracted the pioneersin GTL production technology, such as Syntroleum, Shell, Sasol,and Chevron, to build small- and medium-scale GTL plants acrossthe globe. The availability of the world’s third largest natural gas

reserve in Qatar (with proven reserves of about 890� 1012 cu ft[6]) has led to the construction of world’s largest GTL plant, PearlGTL, jointly by Qatar-Petroleum and Shell at a cost of about $18billion. This has enabled Qatar airways to attempt a commercialflight from London, UK to Doha, Qatar, using a 50–50% blend ofGTL fuel and conventional Jet A-1 fuel [7].

The SPK fuels produced using F–T process are mainly com-posed of normal alkanes, iso-alkanes, and cyclic alkanes, whichare substantially different from those of conventional jet fuelssuch as Jet A, Jet A-1, and JP-8. Synthetic fuels have a very smallconcentration of cycloalkanes in comparing with conventional jetfuels, and this small number varies based on the different produc-tion processes and companies. Moses [8] has done a very goodcomparison between different available synthetic fuels. Heshowed that among SPK fuels, Syntroleum aviation fuel providedby AFRL, designated with S-8, has almost no cycloalkanes. Thedifference in fuel chemical properties will have a significantinfluence on the combustion and emission characteristics incombustors. Therefore, a complete knowledge of fundamentalcombustion parameters for GTL fuel at combustor conditions isessential in order to improve and optimize the combustor designand engine efficiency. Furthermore, these fundamental combus-tion parameters are necessary to develop, validate, and to improvethe prediction capabilities of computational tools and chemicalkinetics models. Among these fundamental combustion

Contributed by the Advanced Energy Systems Division of ASME for publicationin the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received June 1,2016; final manuscript received June 14, 2016; published online July 11, 2016.Assoc. Editor: Arash Dahi Taleghani.

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parameters, the investigation of the onset of auto-ignition [9,10]for GTL fuel is extremely relevant and necessary for the combus-tion community, particularly for gas turbine and internal combus-tion engines in the transportation industry [11,12].

The onset of auto-ignition is the set of thermodynamic condi-tions at which the combustion process occurs spontaneously andsimultaneously throughout the combustible mixture [13]. For anyparticular mixture of fuel, oxidizer, and diluent, the temperatureand the pressure are the key parameters to consider as the driversto auto-ignition [14]. Time is also a factor in driving auto-ignition. A parameter called ignition delay time can beexperimentally determined as the time a mixture may remain at atemperature and pressure before auto-ignition occurs [15]. Typi-cally, rapid compression machines or shock tubes are used todetermine the ignition delay times of a mixture. The results fromsuch experiments, even though widely accepted, do not representthe conditions typically found in practical applications of combus-tion system where the pressure and temperature are constantlychanging due to flame propagation. Using the experimental facil-ity developed in this study, a controlled combustion event inspherical constant volume combustion chamber produces anincrease in the pressure and temperature of the unburned mixture[16]. When auto-ignition conditions are reached, the unburned gasinstantly ignites and produces a series of pressure waves that canbe detected by the pressure transducer. The conditions of theunburned gas that initiate the pressure waves are the conditions atwhich auto-ignition occurs and are more representative of thevarying conditions typically found in a combustion system suchas internal combustion engines.

Kumar and Sung [17] have done an experimental study of auto-ignition characteristics of GTL and conventional fuel/air mixturesand observed two-stage ignition delay response. Their furtherstudy on laminar burning speed indicated that S-8 GTL producesa robust flame compared to Jet-A [18]. Wang and Oehlschlaeger[19] have investigated the ignition behavior of several conven-tional and GTL fuels using a heated shock tube. Their ignitiondelay time measurements were almost the same for GTL and Jet-A at temperatures higher than 1000 K. However, at lower temper-atures significant differences were observed between those ofGTL and Jet-A fuels. In order to simulate the combustion behav-ior of GTL fuel, many groups tried to find a surrogate mixture forGTL fuels. Huber et al. [20] provided surrogates for GTL fuelbased on the thermophysical properties incorporating with volatil-ity data. Naik et al. [21] developed a three-component, iso-octane,n-decane, and n-dodecane, chemical kinetics model as a surrogatefor GTL fuel and compared the predictions with the experimentalresults reported by Ji et al. [22]. The comparison showed that themodel was able to accurately predict the laminar burning speedsof GTL fuels, but can only be used for ignition delay time calcula-tion at high temperatures. Dooley et al. [23] also developed achemical kinetic model based on the mixtures of iso-octane andn-dodecane to mimic the combustion behavior of GTL fuels. Thepredictions of auto-ignition, extinction limits, and species concen-tration profile were reported to agree well with the experimentaldata. The ignition behaviors have been evaluated using differentexperimental configurations such as counterflow burner [24],rapid compression machine [17], and shock tubes [19]. The out-come of these studies highlighted the fact that GTL fuel had amuch larger resistance to flame stretch effects and robust combus-tion behavior when compared to conventional jet fuels. The morenotable combustion behavior was the shorter ignition delay ofGTL fuel than the conventional counterparts. However, it is im-portant to note that all of the above studies have been carried outeither at atmospheric pressure or at discrete intermediate pres-sures. The data for the onset of auto-ignition for GTL fuel havenot comprehensively been measured in the conditions which areidentical to internal combustion engines.

In this study, the onset of auto-ignition for GTL fuel/air mix-tures has been measured for different equivalence ratios of0.8–1.2 and different initial pressures of 8.6, 10, and 12 atm at an

initial temperature of 450 K. The GTL fuel used in this researchwas supplied by AFRL, designated by Syntroleum S-8, which wasderived from natural gas via F–T process. The experiments havebeen done in a high pressure and temperature spherical chamber,and the pressure rise has been collected using high-speed pressuretransducer and data acquisition systems. Critical pressures andtemperatures of GTL/air mixture at which auto-ignition takesplace have been identified by detecting aggressive oscillation ofpressure data during the spontaneous combustion process through-out the unburned gas mixture. Several detailed chemical kineticsmechanisms have been compared and used to study the onset ofauto-ignition conditions. A blend of 32% iso-octane, 25%n-decane, and 43% n-dodecane [21] was employed as the surro-gates of GTL fuel for chemical kinetics study and filling process.

Experimental Setup

The core component of the experimental setup includes a spher-ical combustion chamber that enables the measurement of thepressure rise during flame propagation. The spherical chamber isdesigned to withstand pressures up to 400 atm and is located in anoven which can be heated up to 500 K. It includes three ionizationprobes to check the symmetry of the spherical flame. A Kistler601CA high-temperature pressure transducer in conjunction witha Kistler 5010B charge amplifier is used to record the dynamicpressure rise during the combustion process. The chamber is alsoequipped with two extended spark plugs for ignition at the centerof chamber and a K-type thermocouple to measure the mixturegas temperature. The gas supply manifold, mainly constructed of304 and 316 stainless steel Swagelok fittings and tubing, is predis-posed to accommodate inputs from five simultaneous gas cylin-ders, thus allowing for an inventory of gas mixtures to be alwaysreadily available for immediate use. Two vacuum pumps helpevacuate the system faster and allow for independent testing ofthe two chambers. The temperature controllers are proportionallyprogrammable with redundant safety feature for temperature pro-tection and electric shock.

Another key element of the apparatus is the liquid filling systemwhere liquid fuel is allowed to enter and evaporate in atemperature-controlled portion of the manifold. A high-temperature pressure transducer closely coupled to this section ofthe manifold is used to monitor the partial pressure of the vapor-ized fuel, and hence control the amount of fuel that will be used inthe experiment. A GC system in conjunction with the specializedheated lines to prevent condensation was used to check the com-position of premixed fuel/air mixture inside the chamber to verifythe filling process based on partial pressure method. The configu-ration of experimental facilities and their connections are shownin Fig. 1. All data acquisition and analysis were done by severaldedicated PCs using a high-speed DAQ card and isolated inputand output modules for temperature, voltages, and pressure meas-urements as well as automatic delayed firing via control of thehigh-voltage coil driving the spark plugs. The raw pressure signalproduced by the high-speed pressure transducer contains digitiza-tion noise resulting from data acquisition system process of con-verting a voltage signal to a digital value. The digitization noise isapproximately 0.25 psi/bit, which is certainly negligible for therange of pressure measurements of interest. For any set of data tobe deemed acceptable, three consecutive experimental runs haveto yield the exact pressure curve to ensure that the confidencelevel of the experiments is above 95% [25]. More informationabout the experimental setup can be found in previous publication[3,4,16,26–35].

Results and Discussion

As previously discussed, synthetic fuels produced by the F–Tmethod have become increasingly popular as substitutes to theconventional hydrocarbon fuels such as diesel, gasoline, and jetfuels [36]. The GTL fuel used in this study was provided by the

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AFRL [8,37]. This fuel was produced in the U.S. from natural gasthrough an F–T process called low-temperature cobalt catalyst [8]in a small pilot plant by Syntroleum in Oklahoma. The specifica-tion properties of this fuel are presented in Table 1. It can be seenthat the percentage of aromatic (cyclic) is zero. A blend of 32%iso-octane, 25% n-decane, and 43% n-dodecane [21] wasemployed as the surrogates of GTL fuel for chemical kineticsstudy and filling process. The initial mixture composition forGTL/air mixture is defined as

/ð0:32 C8H18 þ 0:25 C10H22 þ 0:43 C12H26Þþ 15:83ðO2 þ 3:76N2Þ (1)

This surrogate mixture has an empirical formula of C10:22H22:44,H=C ratio of 2.196, an estimated cetane number of 61, and amolar mass of 145.37 g/mol.

In this study, the propensity of GTL/air mixture to auto-igniteand the conditions that produce the onset of auto-ignition weremeasured and analyzed for various equivalence ratios and initialconditions. Measurements for the onset of auto-ignition of GTL/air mixture have been recorded in the spherical chamber for dif-ferent equivalence ratios of 0.8–1.2 and different initial pressuresof 8.6, 10, and 12 atm at initial temperature of 450 K. During thecompression of unburned mixture due to flame propagation, whichacts as a piston, both temperature and pressure of the unburnedgas increase. At a specific condition, auto-ignition of all of theunburned gases occurs. The temperature and pressure that instan-taneously produce auto-ignition are measured and reported as theonset of auto-ignition. To detect the onset of auto-ignition, the

derivative of the measured pressure ðdp=dtÞ was evaluated andused as a nonsubjective method to determine the auto-ignitionpressure. The measured pressure signal begins to oscillate onceauto-ignition occurs due to the shock waves bouncing around inthe combustion chamber. The derivative with respect to time ofthe pressure signal will oscillate very aggressively between posi-tive and negative values once auto-ignition occurs, thus providinga convenient method to detect the onset of auto-ignition. It wasassumed that unburned gas is compressed isentropically up to theonset of auto-ignition, and the temperature at the onset of auto-ignition was determined using isentropic relation of compression.

Figure 2 shows the pressure–time traces of combustion of GTL/air mixture for four initial pressures of 2, 8.6, 10, and 12 atm atinitial temperature of 450 K and equivalence ratio of 0.8. For thecase of initial pressure of 2 atm, no auto-ignition was observed,normal combustion process occurred, and the pressure curve isvery smooth as shown by the solid line in Fig. 2. At the onset ofauto-ignition, a sudden oscillation in the pressure is detected asshown by points A, B, and C in Fig. 2 for initial pressures of 8.6,10, and 12, respectively, along with audible noise as a good

Fig. 1 Overview of experimental facilities

Table 1 Specification properties of Syntroleum S-8 fuel

Flash point (�C) 48Freezing point (�C) �51Density at 15 �C (kg/L) 0.756Viscosity at �20 �C (mm2/s) 4.3Net heat of combustion (MJ/kg) 44.1Conductivity (pS/m) 128Lubricity test (BOCLE) wear scar (mm) 0.59Aromatics (% vol) 0.0Total sulfur (% mass)

indication that unburned mixture was auto-ignited. The auto-ignition pressure traces shown in Fig. 2 are very similar to thosein internal combustion engines reported by Heywood [38]. Theauto-ignition pressures were accurately determined using pressurederivative technique and locating the point at which the pressurederivative with respect to time becomes discontinuous as shownin Fig. 3. It can be observed visually in this figure that the oscilla-tory pressure waves caused by auto-ignition are easily detected bythis technique. As it was mentioned, the corresponding auto-ignition temperatures of unburned gas were measured assumingthat the unburned gas mixture was compressed isentropically.

While development of the chemical kinetics mechanism ofGTL fuel is in its initial stage, there are several mechanisms thatcan predict GTL combustion characteristics such as laminar burn-ing speed and ignition delay time [21,23,39,40]. These mecha-nisms are summarized in Table 2. As shown in the table, in termsof computational time which is proportional to number of speciesand reactions, Dooley et al. mechanism [23] is the most expensivemechanism due to the larger number of species, and Yu et al.mechanism [40] is the least expensive mechanism. All three surro-gates of iso-octane, n-decane, and n-dodecane are included inNaik et al. [21], Dooley et al. [23], and Ranzi et al. [39]mechanisms except Yu et al. mechanism [40] which only has twosurrogates of 2,5-dimethylhexane (one of the octane’s isomers)and n-dodecane. The above proposed composition of 32% iso-octane, 25% n-decane, and 43% n-dodecane is applicable for thefirst three mechanisms in Table 2. For Yu et al. mechanism [40],its own suggested composition of 41.9% 2,5-dimethylhexane and58.1% n-dodecane has been used.

The ignition delay time computations were performed withassumption of constant volume/constant internal energy usingCANTERA solver [41]. The ignition delay time was calculatedwhen the temperature reached a value of 400 K above the initialtemperature. Figure 4 shows a comparison of ignition delay timesbetween available kinetics mechanisms of GTL fuel with the

heated shock tube experimental data of Wang and Oehlschlaeger[19] at equivalence ratio of 1 and pressure of 20 atm for a widerange of temperature from 700 K to 1300 K. As it can be seen,Ranzi et al. [39] and Yu et al. [40] mechanisms have a goodagreement with the experimental results for both low and hightemperatures. Since the agreement of Ranzi et al. [39] mechanismwith experimental data at intermediate temperatures is slightlybetter than those predicted by Yu et al. [40] and also it includesall three surrogates of iso-octane, n-decane, and n-dodecane, itwas used in this study.

Fig. 3 Comparison of pressure rate–time traces of auto-ignition of GTL/air mixture for three different initial pressures of8.6, 10, and 12 atm, at initial temperature of 450 K and equiva-lence ratio of 0.8

Table 2 Comparison of different chemical kinetics mechanisms for GTL fuel

Mechanism name Number of species Number of reactions Available GTL surrogates

Naik et al. [21] 753 7007 Iso-octane, n-decane, and n-dodecaneDooley et al. [23] 3164 21,671 Iso-octane, n-decane, and n-dodecaneRanzi et al. [39] 484 19,341 Iso-octane, n-decane, and n-dodecaneYu et al. [40] 373 2037 2,5-dimethylhexane and n-dodecane

Fig. 4 Comparison of available GTL detailed kinetics mecha-nisms [21,23,39,40] with the experimental data [19] at equiva-lence ratio of 1 and pressure of 20 atm for a wide range oftemperatures

Fig. 5 Theoretical ignition delay time for a wide range of pres-sures and temperatures using Ranzi et al. [39] mechanism forstoichiometric GTL/air mixture

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As shown in Fig. 5, at low-temperature conditions (T< 800 K),both temperature and pressure have a negative dependency onignition delay time. It can be seen that for low-temperature andhigh-pressure conditions indicated by dashed rectangular box, aseither temperature or pressure increases, the ignition delay timedecreases or in other words, the propensity of auto-ignitionincreases. Figure 5 also shows that pressure has a minor effect onignition delay time for low temperatures, and the effect of pres-sure on ignition delay time increases as the temperature increases.It can be concluded from this figure that for low-temperature con-ditions, the ignition delay time is a strong function of temperature.

The experimental critical temperatures and pressures in thiswork at the onset of auto-ignition along with theoretical ignitiondelay time using Ranzi et al. [39] mechanism for GTL/air mixtureat different equivalence ratios are shown in Fig. 6 and also listedin Table 3. In Fig. 6, as mixture pressure increases, the tempera-ture at which auto-ignition occurs decreases for all three equiva-lence ratios. As shown in Fig. 7, increasing the temperature whilethe pressure is decreasing causes the ignition delay time todecrease. The ignition delay time tends to keep its negative de-pendency only with temperature. These experimental runs pro-duced auto-ignition conditions at different pressures and onlyslightly different temperatures, indicating that the auto-ignitioncharacteristics are a strong function of the temperature of theunburned gases.

Summary and Conclusions

In this paper, the onset of auto-ignition of GTL fuel was meas-ured for a wide range of equivalence ratios from 0.8 to 1.2 and

initial pressures of 8.6, 10, and 12 atm at initial temperature of450 K using aggressive oscillation of pressure data during thespontaneous combustion process throughout the unburned mix-ture. The experiments were done in a high-pressure and tempera-ture spherical chamber, and the pressure rise due to combustionwas collected using high-speed pressure transducer and data ac-quisition systems. Several detailed kinetics mechanisms havebeen compared for ignition delay time calculations, and it wasfound that Ranzi et al. mechanism [39] has a better agreementwith the available experimental results in the literature. It wasshown that at low unburned gas temperatures, both temperatureand pressure have a negative dependency on the ignition delaytime, and the auto-ignition characteristics are also a strong func-tion of the unburned gas temperature.

Acknowledgment

This paper was made possible by NPRP award (NPRP 7-1449-2-523) from Qatar National Research Fund (a member of theQatar Foundation).

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