Examination of the Sooting Tendency of Three-Ring Aromatic

May 13, 2010 - Hydrotreating of a 1:1 blend of coal-derived refined chemical oil (RCO), with petroleum-derived light cycle oil (LCO), has been identif...
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Energy Fuels 2010, 24, 3479–3487 Published on Web 05/13/2010

: DOI:10.1021/ef100181s

Examination of the Sooting Tendency of Three-Ring Aromatic Hydrocarbons and Their Saturated Counterparts Eduardo J. Barrientos and Andre L. Boehman* The Earth and Mineral Sciences (EMS) Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802-2308 Received February 14, 2010. Revised Manuscript Received April 19, 2010

Hydrotreating of a 1:1 blend of coal-derived refined chemical oil (RCO), with petroleum-derived light cycle oil (LCO), has been identified as a strategy for the production of an advanced coal-based jet fuel, JP-900. The conversion of the jet fuel section of a conventional refinery for the production of coal-based JP-900 would impact the quantity and quality of the other refinery products, such as diesel. The physical and chemical properties of refinery streams may be perturbed by the introduction of coal components into the refinery, including the sooting tendency. A coal-derived diesel cut was chemically analyzed via gas chromatography-mass spectrometry (GC-MS). The results showed that two- and three-ring aromatics and their hydrogenated derivatives comprised the bulk of the cut. Phenanthrene and its hydrogenated derivatives were chosen as representative compounds of this cut. Existing sooting tendency correlations and soot formation indicators in the literature apply to some two-ring aromatic compounds, but little or no information is available about how to predict the sooting tendency of aromatics with three or more rings. The present study provides an analysis of the effects of three-ring aromatic hydrocarbons on soot formation and evaluated how their saturated counterparts contribute to the reduction of the sooting tendency. This research also generated a consistent set of data for smoke point and threshold sooting index (TSI), a parameter proposed for evaluating the onset of soot formation, for three-ring compounds, which have not been reported previously. It was found that the sooting tendency of aromatics is more accurately estimated by investigating their effect on hydrocarbon mixtures rather than as pure compounds. When adding aromatic compounds with the same number of carbon atoms to a reference mixture of 35% (v/v) toluene and 65% (v/v) n-heptane, the most saturated compounds showed the lowest sooting tendency. At small concentrations, three-ring saturated hydrocarbons in fuel blends did not improve the sooting tendency, in contrast to two-ring saturated compounds, which are good hydrogen donors and have low sooting tendency. Finally, the calculated TSI of the mixtures was used to estimate the TSI and smoke points of the individual three-ring compounds contained in the blend using mixing rules.

and naphthenic content.6,7 Recently, this feature has been under consideration by the aviation industry. Future highperformance aircraft will work under higher temperatures than at present because of their high speed, which will translate into the jet fuels experiencing severe thermal stress. This condition will lead to the buildup of aromatic compounds in the stressed liquid fuel and further formation of pyrolytic solid deposits (i.e., coke) in the fuel line, which is not desirable. Petroleumderived jet fuels have shown a rapid degradation at elevated temperatures, leading to a high degree of solid deposition or coking because of the poor thermal stability of long-chain alkanes.6 The cracking products from long-chain alkanes associated with current jet fuels form C1-C4 gases and cycloalkanes that further develop into different aromatic compounds, leading to the formation of coke. Studies by Andresen et al.7 compared the performance of paraffinic petroleumderived fuels, naphthenic petroleum-derived fuels, and coalderived fuels and showed that coal-derived fuels provide the best suppression of pyrolytic-induced solid deposits and inhibition of gas formation. Despite the considerable advantage of their high thermal stability, coal-derived fuels may increase the soot formation in

1. Introduction The United States of America’s abundant and readily available supplies of domestic coal could serve as the solution to the problems related to the critical need for a domestic, reliable, and affordable supply of liquid fuels. Coal can be converted through proven, existing modern technology into clean, zero-sulfur synthetic hydrocarbons and chemical products. Technology for producing liquid fuels from coal has been available since the 1930s.1 These techniques vary widely but are generally grouped into three basic categories: direct liquefaction, indirect liquefaction, and pyrolysis.1-5 Coal-derived fuels have been found to have high thermal stability at elevated temperatures because of high hydroaromatic *To whom correspondence should be addressed. E-mail: boehman@ ems.psu.edu. (1) Farcasiu, M. PECT Review, U.S. Department of Energy, Washington, D.C., 1991; pp 4-13. (2) Chetina, O. V.; Isagulyants, G. V. Khim. Tverd. Topl. 1986, 20, 51–59. (3) Mochida, I.; Sakanishi, K. Adv. Catal. 1994, 40, 39–85. (4) Davis, B. H.; Occelli, M. L. Fischer-Tropsch Synthesis, Catalysts and Catalysis, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2007; Vol. 163. (5) Schobert, H. The Chemistry of Hydrocarbon Fuels; Elsevier: Amsterdam, The Netherlands, 1990. (6) Song, C.; Eser, S.; Schobert, H. Energy Fuels 1993, 7, 234–243. r 2010 American Chemical Society

(7) Andresen, J. M.; Strohm, J. J.; Sun, L.; Song, C. Energy Fuels 2001, 15, 714–723.

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combustors because of their composition of cycloalkanes and hydroaromatics in contrast with the long-chain paraffins that comprise the bulk of current petroleum-derived fuels. As will be discussed later in this work, the sooting tendency of hydrocarbons in laminar diffusion flames increases in the following order: n-paraffins < iso-paraffins < cycloalkanes < monoaromatics < polycyclic aromatics. The tendency of fuel to produce soot is of relevance, because it affects flame radiation and contributes to exhaust emissions. In diesel engines, the regions of strong soot formation are likely spatially displaced from the peak flame temperature regions; thus, they are at lower temperatures. Studies made by Musculus8 concluded that changes in radiative heat transfer from soot affect in-cylinder temperatures, cooling the diesel spray flame and affecting NOx formation. In jet engines, soot can cause erosion if it impinges on turbine blades and stators and could turn into carbon deposits, which clog filters and plug the holes in the combustor wall that supply dilution air to the combustion section, disrupting the flow pattern of the combustion products.6,9 Soot represents a significant component of the particulate matter (PM) emitted by engines.10 PM emissions from soot contribute to smog and affect local climate. Furthermore, the condensed phase of PM contains polycyclic aromatic hydrocarbons (PAHs), which are classified as known mutagens and human carcinogens by the International Agency for Research on Cancer (IARC).11 The advanced, thermally stable, coal-based jet fuel, JP900, resists deposit formation at temperatures up to 482 °C (900 °F); therefore, it could be used for cooling sensitive parts on high-performance aircraft, as well as providing propulsive energy.12 Hydrotreating of a 1:1 blend of coalderived refined chemical oil (RCO), a byproduct of the coal tar industry, with petroleum-derived light cycle oil (LCO), has been identified as a strategy for the production of JP-900. However, no oil refinery is operated to mainly produce jet fuel, and the conversion of the jet fuel section of a conventional refinery for the production of coal-based JP-900 would impact the quantity and quality of the other refinery products, such as gasoline, fuel oil, petroleum pitch, petroleum coke, and diesel fuel. Much research is required to study the physical and chemical nature of all products that are perturbed by the introduction of coal components into the refinery, including the impact on the sooting tendency of the fuel products. Results of gas chromatography-mass spectrometry (GC-MS) analyses on a coal-derived diesel cut, presented later in section 3.1 of this work, showed that two- and three-ring aromatic compounds and their hydrogenated derivatives comprised the bulk of the cut. Therefore, comprehensive knowledge of the effect of PAHs and their saturated counterparts on the sooting tendency of fuels is a key step in the development of coal-based fuels with high thermal stability.

Previous studies on the sooting tendency have traditionally defined this property in terms of the smoke point, which is the height in millimeters of the highest flame produced without smoking when the fuel is burned in a specified test lamp.13 The smoke point is quantitatively related to the potential radiant heat transfer from the combustion products of the fuel.13 Because radiant heat transfer has a strong influence on the metal temperature of combustor linears and other hot section parts of gas turbines, this test method provides a basis for the correlation of fuel characteristics with the life of these components.13 The United States Air Force (USAF) regulates the smoke point of JP-8, air force current jet fuel, for essentially all aircrafts and ground vehicles, to a minimum value of 25 or 19 mm, with a maximum naphthalene content of 3% (v/v).14 Further studies made by Minchin15 defined the “smoking tendency” as the inverse of the smoke point. These results showed that the tendency to smoke is directly proportional to the aromatic and naphthenic content of a fuel. Clarke et al.16 examined a total of 115 compounds using a new smoke point lamp that allowed them to measure flame heights over a range of 9-450 mm. Their results showed that increased chain length and chain branching increased the sooting tendency in n-paraffins and that aliphatic side chains on the benzene ring of aromatics did not produce a significant reduction in the smoke points, corroborating the results obtained by Minchin. Schalla and McDonald17 measured fuel consumption rates at which hydrocarbons could be burned smoke free. As in previous work, they found that the rates increased with the carbon number, except in the case of some cycloalkanes and aromatics. They attributed these differences to the relative carbon-carbon and carbon-hydrogen bond dissociation energies. They concluded that most unsaturated compounds have the smallest smoke-free fuel consumption rates. Therefore, the sooting tendency of fuels increases with the stability of the carbon chain or skeleton. Despite the improvements made by previous researchers in relating sooting tendency with the molecular structure, they did not take into account the increased height of the flame, which will be required with increasing the molecular weight of the sample. More oxygen is required to diffuse to the flame when the molecular weight is increased to consume a unit volume of the fuel.21 Furthermore, values of smoke point and trends with molecular structures were not satisfactorily consistent in all of the different studies.15-20 Taking this into account, Calcote and Manos21 defined a threshold sooting index (TSI), varying from a value of 0-100, as an indicator of the sooting tendency of pure hydrocarbon compounds. The 0 value, corresponding to the least sooting compound, was assigned to ethane, and the 100 value was assigned to (13) American Society for Testing and Materials (ASTM). ASTM D1322. In Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 2000. (14) Edwards, T.; Harrison, W. E.; Maurice, L. Q. Proceedings of the 39th American Institute of Aeronautics and Astronautics Aerospace Sciences Meeting and Exhibit, Reno, NV, Jan 8-11, 2001; pp 1-11. (15) Minchin, S. T. J. Inst. Pet. Technol. 1931, 17, 102–120. (16) Clarke, A. E.; Hunter, T. G.; Garner, F. H. J. Inst. Pet. 1946, 32, 627–642. (17) Schalla, R. L.; McDonald, G. E. Ind. Eng. Chem. 1953, 45, 1497– 1500. (18) Hunt, R. A. Ind. Eng. Chem. 1953, 45, 602–606. (19) VanTreuren, K. W. Sooting characteristics of liquid pool diffusion flames. M.S. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1978. (20) Schug, K. P.; Manheimer-Timnat, Y.; Yaccarino, P.; Glassman, I. Combust. Sci. Technol. 1980, 22, 235–250. (21) Calcote, H. F.; Manos, D. M. Combust. Flame 1983, 49, 289–304.

(8) Musculus, M. P. B. SAE Tech. Pap. 2005-01-0925, 2005. (9) Chevron. Aviation Fuels Technical Review (FTR-3). Chevron Product Company, Chevron USA, Inc., San Ramon, CA, 2004. (10) Pickett, L. M.; Siebers, D. L. Combust. Flame 2004, 138, 114–135. (11) International Programme on Chemical Safety (IPCS) INCHEM. Soots, International Agency for Research on Cancer (IARC) Summary and Evaluation, 1985; Vol. 35, p 219, http://www.inchem.org/documents/ iarc/vol35/soots.html (accessed on June 2009). (12) Balster, L. M.; Corporan, E.; DeWitt, M. J.; Edwards, J. T.; Ervin, J. S.; Graham, J. L.; Lee, S.; Pal, S.; Phelps, D. K.; Rudnick, L. R.; Santoro, R. J.; Schobert, H. H.; Shafer, L. M.; Striebich, R. C.; West, Z. J.; Wilson, G. R.; Woodward, R.; Zabarnick, S. Fuel Process. Technol. 2008, 89, 364–378.

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naphthalene. The TSI in diffusion flames is given by the equation   MW þb ð1Þ TSI ¼ a SP

aromatics have not been reported in the literature. Furthermore, smoke points for heavy aromatic hydrocarbons are normally small (less than 10 mm), giving imprecise measurements associated with observation errors and providing sooting tendencies that vary widely from experiment to experiment.26 McEnally and Pfefferle27 determined the sooting tendencies for aromatic hydrocarbons using a new parameter defined as the yield sooting index (YSI) given by the equation

where a and b are constants for any given experimental setup, MW is the molecular weight of the compound, and SP is the smoke point of the fuel. The use of TSI represented a great improvement in the cumbersome task of determining the sooting tendency of hydrocarbons because it permits use of all of the literature data in a consistent way to interpret molecular structure effects and create empirical correlations for predicting the sooting tendency of compounds that have not yet been tested. Olson et al.22 used the TSI as defined by Calcote and Manos to convert smoke points measured from 42 pure hydrocarbons and compared to previous data from the literature, providing a combined list of averaged TSI values for 103 fuels. Until this point, the onset of soot formation was measured or calculated only for pure compounds. Gill and Olson23 identified this problem and established mixing rules for calculating TSIs of fuel blends. The mixing rule for diffusion flame TSIs is given by X xi TSIi ð2Þ TSImix ¼

YSI ¼ Cfν, max þ D

ð3Þ

where C and D are apparatus-specific parameters and fν,max is the maximum soot volume fraction measured in a co-flow methane/air non-premixed flame, whose fuel is doped with 400 ppm of the hydrocarbon sample. The C and D parameters were chosen for the YSI of benzene to be 30 and the YSI of 1,2dihydronaphthalene to be 100.27 McEnally and Pfefferle27 calculated YSIs for 6 cycloaliphatics and 62 aromatics and contrasted them with average TSI values from previous studies. Their results show that YSI correlates well with literature values of TSI. The correlation coefficients between YSI and TSI obtained using Hunt’s data18 and TSI obtained by Olson et al.22 for 18 and 15 tested samples, respectively, are 0.8 and 0.84, which leads to the conclusion that, for most hydrocarbons, YSI and TSI are equivalent methods for sooting tendency measurements. It was also found that YSI values have an uncertainty of (3%, which represents an improvement if compared to the approximately (7% uncertainty found in the TSI values reported by Olson and (15% reported using the data of Hunt. According to McEnally and Pfefferle27 the advantages offered by YSI in comparison to TSI correlations are as follows: (1) YSI gives more accurate results than TSI because the maximum soot volume fraction can be measured more precisely than the smoke point for high sooting fuels (i.e., those high in aromatic content). (2) Small samples (approximately 100 μL) are enough for fν,max measurements in comparison to the, at least, 10 mL required for smoke point measurements described in the ASTM D1322 procedure.13 (3) YSI measures the intrinsic sooting tendency of the additives without interference from indirect effects, such as chemical reactions and changes in residence times, because the concentrations of aromatic dopants are less than 400 ppm. (4) YSI indicates the sooting tendencies of aromatics in the chemical environment of an alkane-fueled flame, because methane is the dominant fuel component, which represents a more realistic model for fuels (i.e., current aviation fuels are about 80% alkanes/cycloalkanes and less than 20% aromatics). Further studies on YSI values were made by McEnally and Pfefferle,26 in which 72 nonvolatile aromatic hydrocarbons were tested using the same technique described before, for which only 5 were reported in their previous work.27 The aromatics were dissolved in 2-heptanone before being injected into the methane and air mixture, because most of the compounds studied are solids at room temperature. In this more recent work, the C and D parameters were chosen to hold the YSI of 2-heptanone to 17 and the YSI of phenanthrene to 191. These new YSI values were chosen to produce consistency with the scale based on benzene and 1,2-dihydronaphthalene reported previously27 and to include species with

i

where TSImix is the calculated value for the fuel blend and xi and TSIi are the mole fractions and diffusion flame TSI values for individual fuel compounds. Gill and Olson23 evaluated the mixing rule against data from the literature for six binary fuel blends and two ternary fuel blends, finding results with errors of less than (10%. They attributed the errors to inaccuracies in reading the scale of the smoke point apparatus. They concluded that, for both the binary and ternary blends, the mixing rule provides good estimates of the experimental TSI values. Yang et al.24 found that the TSI model and the mixing rules defined by Gill and Olson23 correlate excellently with hydrocarbon compositions over a wide range of fuel samples, even mixtures as complex as jet fuels, which contain hundreds of compounds. They measured the smoke points of 12 prototype coal-based jet fuels and correlated with the hydrocarbon class compositions using the TSI model. The correlations obtained were compared to data from the literature, which indicated that TSI values have great merit as a predictor of sooting tendency in actual engines. Yan et al.25 identified that limited data are available on smoke point and TSI. They proposed a structural group contribution method to predict TSI values of pure compounds for which no data exist in the literature. They also discussed mixing rules to estimate the sooting tendencies of hydrocarbon mixtures. The results predicted the TSI values of about 70 compounds, with a standard deviation of 1.3 TSI units and an average relative error of 9.08%. However, they found that the methods that they implemented have limitations in predicting the sooting tendency of aromatics, which presented higher relative errors. Despite the advantages provided by the TSI method to predict sooting tendencies, data for three and more ring (22) Olson, D. B.; Pickens, J. C.; Gill, R. J. Combust. Flame 1985, 6, 43–60. (23) Gill, R. J.; Olson, D. B. Combust. Sci. Technol. 1984, 40, 307–315. (24) Yang, Y.; Boehman, A. L.; Santoro, R. J. Combust. Flame 2006, 149, 191–205. (25) Yan, S.; Eddings, E. G.; Palotas, A. B.; Pugmire, R. J.; Sarofim, A. F. Energy Fuels 2005, 19, 2408–2415.

(26) McEnally, C. S.; Pfefferle, L. D. Proc. Combust. Inst. 2009, 32, 637–679. (27) McEnally, C. S.; Pfefferle, L. D. Combust. Flame 2007, 148, 210–222.

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sooting tendencies higher than naphthalene. The results obtained by this work have uncertainties ranging from (3 to (10% depending upon the solubility of each aromatic compound. Another method to measure sooting tendency that has recently been developed by Crossley et al.28 measures the soot formed from pyrolysis of fuels in an oxygen-free environment. This method is referred to as the micropyrolysis index (MPI), which is defined as the amount of carbon deposited from an injection of the fuel sample, normalized to two reference compounds. These compounds are n-octane, which received the value of 5, and decalin, which received the value of 20. MPI experiments were conducted by pyrolyzing 20 μL of vaporized liquid samples, including n-alkanes, iso-alkanes, and cycloalkanes, across a hot bed of 300 mg of alumina (R-Al2O3) kept at 850 °C. After 10 min, the samples were cooled to room temperature and the deposited carbon contained in the alumina bed was placed in a temperature-programmed oxidation (TPO) system to quantify the amount of carbon deposited during the injection. According to Crossley et al.28 this method has the following advantages: (1) The chemistry involved is much simpler because the ability of the molecules to oxidize and the oxygen/ fuel ratio of the flame are not taken into account. (2) It would require smaller sample volumes than the TSI and YSI methods; therefore, it can be used on a laboratory scale with expensive or limited in quantity compounds. (3) The method requires common laboratory equipment; therefore, it can be easily implemented and repeated in existing laboratories. (4) TSI and YSI methods do not make any distinction between the separate effects of partial oxidization and partial pyrolyzation of the fuels on the onset of soot breakthrough. The MPI values were compared to TSI averaged values obtained from the literature. Crossley et al.28 found that, in the case of low sooting non-aromatic hydrocarbons, MPI values correlate well and linearly with TSI values. However, for pure aromatics, there is a lack of correlation between both methods. In the TSI method, oxygen is involved, which partially oxidizes the aromatic compounds, reducing their stability, and which also changes the flame temperature and the point in the flame where soot inception begins. These conditions will not be reflected in the case of MPI values, which explains the lack of correlation. Crossley et al.28 also proposed that the sooting tendency of aromatics is more accurately estimated by investigating their effect on hydrocarbon mixtures rather than as pure compounds. Mixtures with small percentages of aromatics are more representative of actual fuels used in practical applications. Furthermore, pure aromatics are very stable, and their C-C bonds will unlikely break at considerable rates under the pyrolysis conditions of the MPI experiments; rather, they will create bonds with other compounds, forming larger particles. Therefore, small amounts of aromatics when added to fuel mixtures will increase the sooting tendency considerably. When tested as pure compounds, they can only form soot by reacting with themselves, depending upon the stability of their side chains. Aromatic molecules with less stable side chains will produce more carbon deposits, which is the reverse trend from that obtained in fuel mixtures that contain aromatics. In mixtures, the aromatics that have higher sooting tendencies are those that have less side chains, because other

molecules in the mixture can create bonds with the aromatics and generate soot at a faster rate. On the basis of this review of the current information available in the literature, we can conclude that the correlations developed and soot formation indicators studied mostly apply to the extent of diaromatic compounds present in fuels. With the exception of the work by McEnally and Pfefferle,26,27 little or no information is provided about how to predict the sooting tendency of aromatics with three or more rings. Furthermore, the correlations used for predicting the values of unmeasured fuels strongly depend upon the amount of experimental data available, which, in the case of multi-ring PAHs, is very limited. The purpose of this work is to provide an analysis of the effects of three-ring aromatic hydrocarbons on soot formation and to evaluate how their saturated counterparts contribute to the reduction of the sooting tendency. This research will also generate a consistent set of smoke point and TSI data for three-ring aromatic hydrocarbons and their saturated counterparts, never reported in previous work. Additionally, the TSI values obtained in this work will be compared to the YSI values reported by McEnally and Pfefferle.26,27 2. Experimental Section 2.1. Fuel Samples. A coal-derived fuel was produced from hydrotreating a 1:1 blend of LCO and RCO followed by severe hydrogenation and was chemically analyzed via GC-MS. A diesel boiling range cut from this coal-derived fuel (EI-175) served as a surrogate for the diesel cut that would be obtained in an oil refinery producing JP-900 to determine what hydrocarbon structural groups comprise the diesel fraction. The chemical analysis of the EI-175 sample was performed using a Schimadzu GC-17 gas chromatograph (GC) coupled to a Schimadzu QP-5000 mass spectrometer (MS). The GC was outfitted with a Restek XTi-5 column (0.25 mm  30 m  0.25 μm). The column temperature was programmed from 40 to 320 °C with a 4 min initial hold and a heating rate of 4 °C/min with a final 15 min isothermal period. The split ratio was 20:1, and 1 μL was injected. The peak assignments were based on a comparison to National Institute of Standards and Technology (NIST) spectra. Fuel blends of 65% (v/v) n-heptane and 35% (v/v) toluene where doped with two- and three-ring aromatics and their saturated counterparts and tested in a smoke point lamp, as described in section 2.2. The smoke point of n-heptane was found to be too high (147 mm, as reported by Hunt18) to be used as a base fuel for the purposes of this study, because the maximum height of the American Society for Testing and Materials (ASTM) standard smoke point lamp used for these measurements is 50 mm, and we are evaluating the effects of other compounds on the sooting tendency of the selected base fuel. For this reason, n-heptane was blended with toluene, a compound with a higher sooting tendency. The toluene in the base fuel blend also served as a solvent for the solid compounds used in the study. The two- and three-ring aromatics doped into the base fuel were naphthalene and phenanthrene, respectively. The concentration of naphthalene and its saturated counterparts in the blend was 5 wt %. In the case of phenanthrene and its saturated forms, the concentration was changed from 2 to 5 wt % to evaluate their effect on the sooting tendency. The compounds used in the present study are listed in Table 1 with their molecular formula, structure, CAS number, molecular weight, and purity. 2.2. Smoke Points. A standard smoke point lamp for liquid fuels as specified by ASTM D1322-9713 was used for smoke point measurements. A black-painted steel frame surrounding the apparatus was used to control air flow during the measurements.

(28) Crossley, S. P.; Alvarez, W. E.; Resasco, D. E. Energy Fuels 2008, 22, 2455–2464.

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Table 1. List of Compounds Used in This Study with Their Properties name 2,2,4-trimethylpentane 1-methylbenzene methyl-cyclohexane n-heptane naphthalene 1,2,3,4-tetrahydronaphthalene trans-decahydronaphthalne cis-decahydronaphthalene 1-methylnaphthalene phenanthrene 9,10-dihydrophenanthrene 1,2,3,4,5,6,7,8-octahydrophenanthrene perhydrophenanthrene

synonym iso-octane toluene

tetralin trans-decalin cis-decalin

formula

CAS number

MW (g/mol)

purity (%)

C8H18 C7H8 C7H14 C7H16 C10H8 C10H12 C10H18 C10H18 C11H10 C14H10 C14H12 C14H18 C14H24

540-84-1 108-88-3 108-87-2 142-82-5 91-20-3 119-64-2 91-17-8 91-17-8 90-12-0 85-01-8 776-35-2 5325-97-3 5743-97-5

114.22 92.13 98.18 100.20 128.17 132.20 138.24 138.24 142.19 178.22 180.24 186.29 192.34

99.8 99.9 99 99 98 97 99 98 97 98 94 >95 >95

The smoke points for each blend were measured on 3 different days, 5 times per test, and averaged. The results were calibrated in accordance with ASTM D1322-9713 using a standard reference fuel blend of 20% (v/v) toluene and 80% (v/v) 2,2,4-trimethylpentane. The ambient temperature and relative humidity of the test site were monitored to make sure that fluctuations of environmental conditions did not affect the smoke points of the reference blends. The ambient temperature ranged from 20 to 23 °C, and the humidity ranged from 38.5 to 43.7%. The overall experimental uncertainty for the measured smoke points was calculated by taking the root mean square of the zeroth-order and first-order uncertainties.29 The zeroth-order uncertainty was obtained by the resolution of the smoke lamp scale as (0.5 mm. The first-order uncertainties of each compound were calculated on the basis of a 95% confidence level using Student’s t test method, with a two-tailed critical value of 0.05. The obtained overall uncertainty values ranged from (2.6% (5 wt % perhydrophenanthrene) to (3.8% (5 wt % phenanthrene).

3. Results and Discussion 3.1. GC-MS Analysis. Figure 1 shows the resulting chromatograms of the coal-derived diesel cut (EI-175). Total ion chromatograms (TICs) are shown with selected ion chromatograms (SICs) for prominent ions. The major chemical classes present in the cut are two- and three-ring aromatics. Specifically, some components found include phenanthrene and anthracene and their hydrogenated derivatives, as well as fluorene and dimethyl biphenyls. On the basis of the results obtained from the GC-MS analysis, phenanthrene and its hydrogenated derivatives were chosen as representative compounds from the coalderived diesel fraction. The following sections examine and analyze the effects of phenanthrene and its hydrogenated counterparts on the sooting tendency of fuels. 3.2. Sooting Tendency. 3.2.1. Smoke Points. The results of smoke point measurements for the different aromatic dopants and aromatic concentrations are shown in Table 2. As can be observed from Table 2, the overall uncertainties ranged from (0.53 to (0.60 mm. The maximum uncertainty corresponded to 5 wt % phenanthrene. This blend when examined in the smoke lamp presented a flickering flame, which made reading the smoke point on the scale more difficult, which explains the higher uncertainty of the value reported in contrast to the smoke points of the other compounds. This phenomenon can be attributed to a limited solubility of phenanthrene in the fuel blend or the presence of contaminants in the phenanthrene that were not soluble.

Figure 1. GC chromatograms of the coal-derived diesel fraction (EI-175). SICs were selected to describe aliphatics (m/z 71), two-ring aromatics (m/z 128, 142, and 156), three-ring aromatics (m/z 158, 178, 182, and 192), and fluorene and phenanthrene (m/z 166 and 178, respectively).

The smoke points obtained for the different aromatics at 5 wt % concentration are plotted against the smoke point of the base fuel without dopants, in Figure 2, to compare the effects of two- and three-ring aromatics on the sooting tendency of fuels and the more important role of hydrogenation of these compounds. The first conclusion from Figure 2 is that increasing the number of rings in the fuel blend decreases the smoke point of the blend and, therefore, increases the sooting tendency. This result is in accordance with multiple previous studies,15-18,28 where the sooting tendency increases in the order: n-paraffins < iso-paraffins < naphthenics < monoaromatics < two-ring aromatics < three-ring aromatics < more than three-ring aromatics. Another observation from Figure 2 is that, when adding aromatic compounds with the same number of carbon atoms, the most saturated compounds show the highest smoke points. This result is evident when comparing naphthalene with tetralin, in which an improvement of 1.09 mm in the smoke point was achieved. This conclusion was previously reported by Shalla and McDonald.17 From this result, it can also be concluded that, when changing the structure from double- to single-bonded carbons, such as from naphthalene

(29) Moffat, R. J. Exp. Therm. Fluid Sci. 1988, 1, 3–17.

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Table 2. Smoke Points for Different Dopants and Dopant Concentrations concentration of the dopant in the fuel blend 0 wt % dopant naphthalene tetralin trans-decalin cis-decalin phenanthrene 9,10-dihydrophenanthrene 1,2,3,4,5,6,7,8-octahydrophenanthrene perhydrophenanthrene

2 wt %

5 wt %

SP (mm) 20.69 ( 0.54 20.69 ( 0.54 20.69 ( 0.54 20.69 ( 0.54 20.69 ( 0.54 20.69 ( 0.54

17.56 ( 0.58 18.74 ( 0.58

17.29 ( 0.59 18.38 ( 0.58 21.34 ( 0.58 21.30 ( 0.58 15.97 ( 0.60 16.28 ( 0.54

20.69 ( 0.54

19.29 ( 0.59

17.26 ( 0.53

20.69 ( 0.54

20.36 ( 0.56

20.73 ( 0.55

and decalin. They conducted their studies by adding different concentrations of hydrogen donors to a jet fuel (JP-8) that is rich in long-chain paraffins and n-butylbenzene, an aromatic hydrocarbon. They concluded that adding hydrogen donors to the fuels significantly reduced or even eliminated the formation of carbonaceous particles and decreased fuel decomposition and gas formation. This improvement on the stability of the fuels can be attributed to the stabilization of the reactive radicals via hydrogen abstraction from tetralin or decalin, which inhibits the radical decomposition, cyclization, aromatization, and condensation reactions. As reported in previous work by Song et al.,32 decalin is not as active as tetralin at inhibiting solid formation. Hydrogen transfer is higher with tetralin than with decalin, which can be attributed to the fact that the benzylic radical from tetralin is more stable than the tertiary radical formed from hydrogen abstraction of decalin. Therefore, we may ask why decalin tends to increase the smoke point of the blend but tetralin decreases it. This can be attributed first to the more aromatic nature of tetralin in contrast to decalin. Second, this can also be attributed to synergistic effects, such as hydrogen donation. To understand the effect of the later in the sooting tendency, we should study the mechanisms of hydrogen transfer of tetralin and decalin presented in Scheme 1. We can observe that, after hydrogen donation, tetralin is converted to naphthalene and decalin is converted to tetralin. Naphthalene is recognized to have a much higher sooting tendency than tetralin. When the amount of tetralin is small (i.e., 5 wt %), the reactive radicals generated from the pyrolysis of paraffinic compounds, such as n-heptane, and aromatic side chains, such as toluene, accelerate the tetralin dehydrogenation via hydrogen abstraction. Thus, at small concentrations, conversion of tetralin to naphthalene through dehydrogenation becomes a prominent effect, which contributes to the increase of the sooting tendency. In the case of decalin, the effect of the hydrogen transfer is also accompanied by a reduction of the aromatic concentration of the fuel blend, which translates in an increase of the smoke point. The results also show that isomers of decalin, trans and cis, have the same smoke points as reported in previous works.16,18,22 In the case of three-ring aromatics, we observe from Figure 2 that blends with the saturated forms (i.e., perhydrophenanthrene) have almost the same smoke point as the original base fuel. Therefore, in the case of three-ring blends, the effect of reducing the aromatic concentration (increasing the smoke point) is neutralized by the effect of adding three-ring naphthenes (reducing the smoke point). From this result,

Figure 2. Smoke point of base fuel without dopants and with 5 wt % aromatics: (black bars) base fuel without dopant and (gray bars) base fuel with 5 wt % dopant.

to decalin and from phenanthrene to perhydrophenanthrene, the smoke points increase even at concentrations as small as 5 wt %. This change in bond structure is accompanied by a decrease in the amount of energy holding the carbon atoms together, specifically in this case, from bond dissociation energy of 146 (double-bonded) to 83 (single-bonded) kcal/mol.30 Therefore, we conclude that the most thermally stable molecules when added to fuels show the greatest tendency to smoke. Figure 2 also shows the relevance of hydrogenation of PAHs in sooting tendency. When the smoke points of the base fuel without any dopants are compared to the smoke points of the base fuel with 5 wt % of a saturated compound, such as decalin, we can observe an increase of almost 1 mm. This can be attributed to the reduction of the aromatic concentration by adding a saturated compound to the blend and by the role of hydrogen donors during pyrolysis of hydrocarbons. Song et al.31 studied the stability improvement of jet fuels by adding hydrogen donors, such as tetralin (30) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic Press: New York, 1971. (31) Song, C.; Lai, W.-C.; Schobert, H. H. Ind. Eng. Chem. Res. 1994, 33, 548–557.

(32) Song, C.; Nihonmatsu, T.; Nomura, M. Ind. Eng. Chem. Res. 1991, 30, 1726–1734.

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Barrientos and Boehman Table 3. Literature TSI Values of Individual Compounds Used in This Work compound methyl-cyclohexane toluene n-heptane naphthalene tetralin trans-decalin cis-decalin 1-methylnaphthalene

suggested TSI by Olson et al.22 4.9 44 2.6 100 61 15 15 91

differential between the smoke point of the base fuel with dopants and the smoke point of the base fuel without dopants for all of the species and different concentrations studied. 3.2.2. TSI Values. As mentioned in the Introduction of this work, TSI is a better sooting tendency indicator than smoke point because it is independent of measurement conditions and takes into account the effect of the molecular weight on the flame height. Making use of the mixing rule proposed by Gill and Olson23 (eq 2) and the suggested TSI values of Olson et al.,22 we calculated the TSI values of the different fuel blends. The TSI values suggested by Olson et al.22 for the individual compounds used in the different blends are tabulated in Table 3, while calculated TSI values of the blends using the mixing rule are shown in Table 4. The calculated TSI values were validated against the experimental data by plotting them against the MW/SP ratio, as shown in Figure 5. According to eq 1, TSI should be proportional to the ratio of the molecular weight of the fuel to the smoke point. Hydrocarbons of the same class have similar TSI,23 which increases with the carbon number as can be corroborated with the results shown in Table 4. Therefore, to obtain a good curve fit, we expanded the data set by including smoke points of methyl-cyclohexane and 1-methylnaphthalene, which have estimated TSI values of 4.9 and 91, respectively, near both extremes of the TSI scale. 1-Methylnaphthalene was preferred over naphthalene, the upper value of the TSI scale, because it exists in liquid form, which provides an easier and more accurate measurement. From Figure 5, a linear relationship is observed between the calculated TSI and the experimentally determined MW/SP. The equation obtained from the curve fit was   MW þ 2:55 ð4Þ TSI ¼ 3:45 SP

Figure 3. Smoke point of the base fuel with different concentrations of aromatic dopants: (black bars) base fuel without dopant, (gray bars) base fuel with 2 wt % dopant, and (white bars) base fuel with 5 wt % dopant.

Figure 4. ΔSP = base fuel with dopant - without dopant: (black bars) base fuel with 2 wt % dopant, and (gray bars) base fuel with 5 wt % dopant.

Equation 4 has the same form as the TSI definition (eq 1), and the correlation coefficient (R2) is 0.9904, indicating a good linear fit. From this figure, we conclude that (1) calculated TSIs using the mixture rule and based on the suggested values of Olson et al.22 represent a good method for obtaining the TSI of hydrocarbon mixtures that contain aromatics and (2) using eq 4, we can predict the TSI of unknown fuels. The constants obtained in this work, a = 3.45 mm mol g-1 and b = 2.55, are comparable to those obtained by Gill and Olson,23 where a = 3.32 mm mol g-1 and b = -1.47. The a value obtained in this work, which represents the slope that determines how the TSI changes with respect to the fuel property MW/SP, is very similar to the one obtained by Gill and Olson. However, the b value differs by 4 TSI units. This can be attributed to the arbitrary selection of the reference compounds. Using eq 4, we calculated the TSI values of all of the blends, as shown in Table 4. It can be seen that these values

we conclude that, at small concentrations of PAHs in fuel blends, no matter how much we saturate three-ring aromatic hydrocarbons, we will not decrease the sooting tendency as in the case of two-ring saturated compounds (i.e., decalin). The effect of varying the concentration of aromatic dopants in the sooting tendency can be observed in Figure 3. In the case of dopants of phenanthrene, dihydrophenanthrene, and octahydrophenthrene, the smoke point decreased with an increasing dopant concentration. In the case of perhydrophenanthrene, no statistically significant effect was shown. Therefore, we conclude that, at small concentrations of three-ring cycloalkanes (i.e., perhydrophenanthrene), changing the concentration does not affect the smoke point of the fuel blend. For illustrative purposes, Figure 4 shows a comparison of the 3485

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Table 4. Mole Fractions of Compounds in Fuel Blends and Calculated TSI Values of Compounds Considered in This Work mole fractions (xi) of individual compounds blend

toluene

n-heptane

methyl-cyclohexane (pure) base fuel (BF) BF þ 5 wt % naphthalene BF þ 5 wt % tetralin BF þ 5 wt % trans-decalin BF þ 5 wt % cis-decalin BF þ 5 wt % phenanthrene BF þ 5 wt % dihydrophenanthrene BF þ 5 wt % octahydrophenanthrene BF þ 5 wt % perhydrophenanthrene BF þ 2 wt % phenanthrene BF þ 2 wt % dihydrophenanthrene BF þ 2 wt % octahydrophenanthrene BF þ 2 wt % perhydrophenanthrene 1-methylnaphthalene (pure)

0.4260 0.4104 0.4108 0.4116 0.4116 0.4147 0.4147 0.4152 0.4155 0.4214 0.4214 0.4216 0.4218

0.5740 0.5529 0.5535 0.5545 0.5545 0.5588 0.5588 0.5594 0.5599 0.5678 0.5678 0.5681 0.5683

dopant

0.0367 0.0357 0.0339 0.0339 0.0265 0.0264 0.0254 0.0246 0.0107 0.0107 0.0103 0.0100

MW/SP (g mol-1 mm-1)

TSI calculated mixing rule

2.40 4.67 5.66 5.33 4.60 4.60 6.19 6.08 5.73 4.78 5.56 5.21 5.06 4.79 25.85

4.90 20.23 23.16 21.69 20.05 20.05

TSI calculated linear fit 18.67 22.07 20.94 18.41 18.44 23.91 23.51 22.33 19.04 21.72 20.52 20.01 19.10

91.00

Table 5. Calculated TSI and Smoke Points for Individual Three-Ring Compounds compound

calculated TSI

MW (g/mol)

calculated SP (mm)

phenanthrene dihydrophenanthrene octahydrophenanthrene perhydrophenanthrene

158.86 144.50 102.81 93.89

178.22 180.24 186.29 192.34

3.9 4.3 6.4 7.2

we can also evaluate the effect of hydrogenation on the sooting tendency by plotting the smoke points and TSI against the H/C ratio for both the two- and three-ring compounds, as shown in Figures 6 and 7. To obtain a better understanding of how these parameters correlate, we add the smoke point and calculated TSI, 5.8 mm and 80, respectively, of 1,2-dihydronaphthalene from Mensch.33 From Figures 6 and 7, we can observe that the sooting tendency of the individual compounds decreases with increasing the hydrogen/carbon ratio (H/C), which corroborates our previous conclusion that more saturated compounds present lower sooting tendencies. Figure 6 also shows why adding decalin to the base fuel mixture improves the sooting tendency. Decalin has a noticeably higher smoke point with respect the H/C ratio than the other compounds. In the case of three-ring compounds, the relation between the smoke point and H/C is linear and does not vary in a significant way. 3.2.3. Comparison of Calculated TSI for Individual Compounds with YSI. To validate the results obtained in Table 5 for the individual three-ring compounds, we compared the calculated TSI values to the YSIs obtained by McEnally and Pfefferle,26,27 which is the only published data on three-ring aromatic and saturated compounds. However, McEnally and Pfefferle only reported YSI values without providing experimental data on the maximum soot volume fraction ( fν,max). For this reason, the TSI values were directly rescaled into the YSI scale. Figure 8 shows the measured TSI values in Table 5 plotted against the reported YSI values from refs 26 and 27. The equation from the linear correlation converts YSI to the same scale as the TSI values in Table 5. Because only YSI values of phenanthrene and dihydrophenanthrene were reported, we expanded the data by adding the TSI

Figure 5. Relationship of the calculated TSI and experimental determined MW/SP. The linear fit defines calculated TSI at a given MW/SP (y = 2.55 - 3.45x; R2 = 0.9904).

are comparable to the ones obtained using the mixing rule. The root-mean-squared error (rmse) of the TSI calculated by the mixing rule and the one calculated using eq 4 is calculated to be 1.4 using the equation vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u h uP t ðTSIi - TSIÞ2 ð5Þ rmse ¼ n where TSIi is the value calculated using the mixing rule, TSI is the value predicted by eq 4, and n is the number of samples. Therefore, we conclude that the TSI model previously used to characterize the sooting tendency of pure hydrocarbons is also applicable for fuel mixtures. Making use of the mixing rule, we calculated the TSI values of three-ring individual compounds, as shown in Table 5. To make a good estimation, we calculated the values using the data for both concentrations of dopants (2 and 5 wt %) and averaged the results. With the TSI obtained for the individual compounds and eq 4, we calculated the smoke points, which are also shown in Table 5. This procedure for calculating the smoke point can be extended to other three-ring compounds. Now that we obtained the smoke points of the individual compounds,

(33) Mensch, A. A study on the sooting tendency of jet fuel surrogates using the threshold soot index. M.S. Thesis, Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA, 2009.

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Figure 6. Relationship of smoke point and H/C: (;) SP of threering compounds (linear fit: y = 1.52 - 3.47x; R2 = 0.9678) and (- - -) SP of two-ring compounds (three-degree polynomial: y = -33.22 þ 113.35x - 116.13x2 þ 41.79x3; R2 = 1).

Figure 8. Scaling of TSI data. TSI values obtained in this work and suggested by Olson et al.22 plotted against YSI from McEnally and Pfefferle.26 The linear fit defines scaled TSI at a given YSI (y = -2.35 þ 0.80x; R2 = 0.9295). Table 6. Comparison of Calculated TSI of Individual Compounds and Scaled TSI Using the Data of YSI Values from McEnally and Pfefferle26 compound

YSI

scaled TSI

TSI from this work

phenanthrene dihydrophenanthrene

191 193

150.77 152.38

158.86 144.50

More saturated compounds lead to lower sooting tendency, which can be observed by a lower TSI value than its less saturated form phenanthrene. 4. Conclusions The work presented in this paper provides an analysis of the effects of hydrogenation in the sooting tendency of PAHs. On the basis of the results obtained, we conclude the following: (1) By adding aromatic compounds with the same number of carbon atoms, the most saturated compounds will have the lowest sooting tendency. (2) Hydrogenation of PAHs reduces the sooting tendency. This can be attributed to the reduction of the aromatic concentration combined with synergistic effects, such as hydrogen-donation reactions. (3) In blends with three-ring saturated compounds, the effect of reducing the aromatic concentration (reducing the sooting tendency) is neutralized by the increase of the number of rings of the mixture (increasing the sooting tendency). Therefore, at small concentrations of PAHs in fuel blends, no matter how much we saturate three-ring aromatic hydrocarbons, there will not be statistically significant changes on the sooting tendency as in the case of two-ring saturated compounds. (4) The sooting tendency of aromatics is more accurately estimated by investigating their effect on hydrocarbon mixtures rather than as pure compounds. Finally, a consistent set of data of TSI and smoke point for three-ring compounds was reported here for the first time.

Figure 7. Relationship of TSI and H/C: (- - -) TSI of three-ring compounds (linear fit: y = 201.61 - 67.017x; R2 = 0.9205) and (;) TSI of two-ring compounds (linear fit: y = 164.68 - 83.911x; R2 = 0.9956).

values suggested by Olson et al.22 of other compounds used in this study, such as decalin, naphthalene, and 1-methylnaphthalene. From Figure 8, we observe that the correlation coefficient (R2) is 0.9295. Equation 6 scales the YSI values into TSI values to compare both sets of data TSI ¼ 0:8YSI - 2:35 ð6Þ On the basis of eq 6, we obtained the scaled TSI values shown in Table 6. The scaled TSI values were compared to the TSI values, also shown in Table 6. From these results, we observe that both values are very similar, with a rmse value of 7.98. It can be argued that the result obtained for dihydrophenanthrene is more valid that the one reported by McEnally and Pfefferle,26 because it has a lower sooting tendency than phenanthrene, which is in agreement with the literature.

Acknowledgment. The authors thank Dr. Stephen Kirby and Dr. Roberto Santoro for their support on this work.

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