Comparison of Alternative Diesel Fuels with the Composition-Explicit

Oct 19, 2012 - The alternative diesel fuels used in this work were obtained from the National Renewable Energy Laboratory in Golden, Colorado. (AVF an...
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Comparison of Alternative Diesel Fuels with the Composition-Explicit Distillation Curve Method R. V. Gough and T. J. Bruno* S Supporting Information *

ABSTRACT: In recent years, environmental considerations, the potential for supply disruptions, and rising fuel prices have led to the development of alternative diesel fuels produced from nonpetroleum feedstocks. It is important to characterize the properties of these fuels in order to assess the degree of departure of the alternative fuels from the petroleum-derived fuels. One of the most important properties to use for this purpose is the volatility, as expressed by the distillation curve. In this paper, we present advanced distillation curve measurements of three prototype alternative diesel fuels and compare the distillation curve, composition, and combustion enthalpy to those of petroleum-derived diesel fuel. We studied two Fischer−Tropsch diesel fuels, one synthesized from coal-derived gas and one produced from natural gas, and one renewable diesel fuel composed of hydrotreated animal and vegetable fats. We found that the distillation curves of the three alternative diesel fuels are similar to those of petroleum-derived diesel fuel, deviating at most by 30 °C from the distillation curve of petroleum-derived diesel fuel. In general, the most significant deviations from petroleum-derived diesel fuel are found in the “light” region (10−30% distillate volume fraction) and the “heavy” region (70−90% distillate volume fraction) of the distillation curve. The diesel fuel made from natural gas was most similar to petroleum-derived diesel fuel in its volatility and combustion enthalpy.

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little theoretical significance.4 The advanced distillation curve (ADC) method, a technique that has been described in detail previously,5−7 provides (1) a composition-explicit data channel for each distillate fraction (for both qualitative and quantitative analyses), (2) temperature measurements that are true thermodynamic state points that can be modeled with an equation of state, (3) temperature, volume, and pressure measurements of low uncertainty suitable for equation of state development, (4) consistency with a century of historical data, and (5) an assessment of the energy content of each distillate fraction.4,8−11 In this paper, we apply the ADC method to compare samples of prototype alternative diesel fuels produced from different feedstocks. Three diesel fuels were studied: a renewable diesel fuel composed of hydrotreated animal and vegetable fats (AVF) and two Fischer−Tropsch diesel fuels, a gas-to-liquid fuel synthesized from natural gas (GTL) and a coal-to-liquid diesel fuel (CTL). We compare the ADC results of these three alternative diesel fuels to those for a petroleum-derived diesel fuel (PDD). These comparisons are valuable in determining the applicability of alternative diesel fuels as enhancers, extenders, or replacements for petroleum-derived diesel fuel. Additionally, the metrology and measurements discussed in this paper are important for developing a thermodynamic model for alternative fuels with an equation of state.

INTRODUCTION In recent years, environmental considerations, the potential for supply disruptions, and rising fuel prices have led to the development of liquid fuels produced from nonpetroleum sources. Numerous diesel fuels, for example, produced from alternative feedstocks, such as biodiesel fuels and gas-to-liquid fuels, are currently under investigation. Many of these alternative diesel fuels have the advantages of being produced domestically, having lower sulfur content and increased lubricity relative to petroleum-derived fuels, and of decreasing certain emissions.1 For alternative fuels to enhance or replace petroleum-derived diesel fuel, it is desirable that they be fungible and they must meet requirements for fuels, such as ASTM D-975. For this to occur, the alternative diesel fuel and the petroleum-derived fuel should have compatible properties (such as densities, viscosities, cloud point and cold-flow properties, and combustion enthalpies). A particularly important property for fuel replacement design and proper engine performance is fuel volatility.2 The volatility is crucial for engine operation and safety and is very sensitive to compositional variability. Moreover, this property, if measured properly, can be related to fundamental theory and the equation of state. This aspect is distinct from a fit-for-purpose property. Volatility is best measured by the distillation (or boiling) curve, a graphical depiction of the boiling temperature of a fluid or fluid mixture plotted against the volume fraction distilled. The distillation curve is one of the most important and informative properties that can be measured for a complex fluid mixture, because it provides the only practical avenue to assess vapor−liquid equilibrium of a complex fluid. The standard test method, ASTM D-86, provides the usual approach to measurement of the distillation curve, yielding an initial and final boiling point and temperatures at intermediate distillate fractions.3 This method has several major drawbacks, however, including large uncertainties in measured temperatures and © 2012 American Chemical Society

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EXPERIMENTAL SECTION

The n-hexane used as a solvent in this work was obtained from a commercial supplier and was analyzed with gas chromatography (GC) with flame ionization detection (FID) and mass spectrometric (MS) detection.12−14 The n-hexane was injected with a syringe into a split/ Received: August 28, 2012 Revised: October 13, 2012 Published: October 19, 2012 6905

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splitless injector set with a 50:1 split ratio. The injector was operated at a temperature of 325 °C and a constant head pressure of 69 kPa (10 psig). A 30 m capillary column of 5% phenyl 95% dimethyl polysiloxane, having a thickness of 1 μm, was used with a temperature-ramping program from 50 to 170 °C at a heating rate of 5 °C/min. These analyses revealed the purity to be greater than 99%, and the solvent was used without further purification. The alternative diesel fuels used in this work were obtained from the National Renewable Energy Laboratory in Golden, Colorado (AVF and CTL) and from a commercial supplier (GTL). The petroleum-derived diesel (PDD) fuel sample used for comparison with the alternative fuel samples was a typical winter grade, low wax, ultralow sulfur diesel fuel that incorporated a red dye (specifying offroad use) and was refined locally from petroleum of the DenverJulesburg field. This prototype fuel was free of any additives. These samples were stored tightly sealed in plastic bottles, and care was taken to minimize exposure to the air to limit oxidation, evaporation of the more volatile components, or uptake of moisture. The samples were not physically or chemically dried. The AVF and CTL fuels were clear, and the GTL was faintly yellow (the color is from an additive package). The diesel fuel samples were subjected to chemical analysis with GC-MS before the measurement of the distillation curve. Samples were analyzed with a 30 m capillary column of 5% phenyl 95% dimethyl polysiloxane with a thickness of 1 μm. Samples were injected with a syringe into a split/splitless injector set with a 50:1 split ratio. The injector was operated at a temperature of 325 °C and a constant head pressure of 34.5 kPa (5.0 psig). For each fuel sample, a temperature program was used that optimized the separation of components. The following temperature programs were used: for AVF, 40 °C for 4 min followed by temperature ramping at 10 °C/min to 200 °C, and a hold at 200 °C for 10 min; for GTL, 40 °C for 4 min followed by temperature ramping at 10 °C/min to 250 °C, and a hold at 200 °C for 10 min; for CTL and PDD, 70 °C for 2 min followed by temperature ramping at 15 °C/min to 140 °C, 30 °C/min to 240 °C, and 1 °C/min to 250 °C. Mass spectra were collected for each peak from 40 to 550 relative molecular mass (RMM) units. Peaks were identified with guidance from the NIST/EPA/NIH mass spectral database and also on the basis of retention indices.14,15 The method and apparatus for ADC measurements has been reviewed in detail elsewhere4,8,9,11,16−18 and thus only a limited description is provided herein. For each distillation curve measurement, 200 mL of each diesel fuel was placed into the boiling flask with a volumetric pipet. Two thermocouples were inserted into the proper locations to monitor Tk, the temperature of the fluid in the kettle, and Th, the temperature of the vapor at the bottom of the takeoff position in the distillation head. Enclosure heating was then commenced with a three-step program based on a previously measured distillation curve. Volume measurements were made in a level-stabilized receiver, and sample aliquots were collected at the sampling adapter hammock. The uncertainty in the volume measurement that was used to obtain the distillate volume fraction was 0.05 mL in each case. The typical experimental atmospheric pressure for the ADC measurements presented herein was approximately 83.2 kPa. The uncertainty in the individual pressure measurements is 0.003 kPa. Because the measurements of the distillation curves were performed at ambient atmospheric pressure at 1650 m above sea level, for tables, figures, and analysis in this paper, temperature readings were adjusted for what should be obtained at standard atmospheric pressure (1 atm = 101.325 kPa). This adjustment was performed with the modified Sydney Young equation, in which the constant term was assigned a value of 0.00010919−21 corresponding to a n-alkane carbon chain of 12.22 This constant was determined by use of experimental data4 that do not extend to longer chain lengths. The fuels studied here may be better represented by a hydrocarbon chain length longer than 12; however, no data are available to produce an improved correlation. We chose to use the constant term associated with n-dodecane rather than use a predicted value for the constant term (for a larger chain) so that we are consistent with previous measurements of petroleum-derived diesel fuel. We note, however, that, for a carbon chain of 18, the

predicted constant would be 0.000095. Use of this value (in place of that of n-dodecane) would lower the presented temperatures uniformly by 1.2 °C, and the shapes of the distillation curves would be unchanged. For the measurements presented here, a typical temperature adjustment was approximately 8 °C. To provide the composition channel information to accompany the temperature data grid, sample aliquots were withdrawn for selected distillate volume fractions. To accomplish this, aliquots of approximately 7 μL of emergent fluid were withdrawn from the sampling hammock in the receiver adapter with a blunt-tipped chromatographic syringe and added to a crimp-sealed vial containing a known mass (approximately 1 mL) of n-hexane solvent. A sample was withdrawn from the first drop of fluid to emerge from the condenser and then at each of 19 additional volume fractions of distillate, for a total of 20 sample aliquots. Each distillate volume aliquot underwent two analyses. First, each fraction was subjected to chemical analysis and peak identification by GC-MS. The aliquots were also analyzed with GC-FID with external standards of octane and/or dodecane to determine the concentrations of all of the compounds. In both cases, aliquots (1 μL) from the crimp-sealed vials of each sample were injected with an automatic sampler. For both analyses, the temperature program was identical to those described above for the neat diesel fuel analyses. High-purity nitrogen was used as the carrier gas. After determination of the concentrations of major components, an enthalpy of combustion analysis that has been described in detail previously11 was performed on the distillate fractions.

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RESULTS AND DISCUSSION 1. Chemical Composition of Three Alternative Diesel Fuel Samples. In Table S1a, b, and c (see the Supporting Information), we present a listing (in order of RT) of the major species contained in each of the three alternative diesel fuel samples. These tables list compounds whose uncorrected area counts (in GC chromatograms) were in excess of 1.5% of the total area. Use of this area cutoff results in the identification of over half of the total uncorrected area. In the past, we have found that neglecting such minor components provides adequate representation of the fuel mixture, for example, in the calculation of the enthalpy of combustion, discussed in section 5 of the Results and Discussion.11 The chromatographic retention times (in minutes), the CAS registry number, the relative molecular mass (RMM), and the uncorrected raw peak area counts (% of total) of the three alternative diesel fuels as well as the PDD fuel are included in Table S1. These analytical results (composition and relative quantities of components) are consistent with our knowledge of the nature of the feedstock and the processing methods used to obtain each fluid. These analyses are also consistent with the results from the composition-explicit data channel of the ADC, as we will note later in the paper. Many components fall below the 1.5% threshold we have set, and therefore, they are not listed in Table S1. Additionally, in several cases, the exact structure of a branched hydrocarbon could not be determined, particularly in the case of AVF. In these cases, the hydrocarbon is listed without the specific location of the substituent (i.e., xmethyltetradecane) or, even more generally (i.e., “branched C15H32”) if the type of branching was ambiguous and could not be determined. The three alternative fuels consisted entirely of linear and branched alkanes. This is in contrast to typical petroleum-derived diesel fuel that contains aromatics such as substituted benzenes and naphthalenes. 2. Initial Boiling Behavior. Between four and six complete distillation curves were measured for each of the three alternative diesel fuels and the PDD fuel. During the initial heating of the fuels in the distillation flask, the behavior of the 6906

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fluid was observed. Observation of the fluid through a window in the heating enclosure allowed for measurement of the onset of boiling for each of the mixtures (measured with Tk). For the ADC method, we typically record the temperatures at which we visually observe (a) the onset temperature (marked by the first bubble), (b) the sustained bubbling temperature, and (c) vapor rising into the distillation head. The vapor rise temperature is observed with a sharp increase in the temperature of Th. We have shown that the vapor rise temperature is the theoretically significant initial boiling temperature (IBT) of the mixture.4 The IBT is important because it is the only point on the distillation curve at which the liquid composition is explicitly known. It is therefore used in equation of state development. These initial temperature values for all four fuels studied are shown in Table 1. These temperatures have been corrected to

PDD fuel. The GTL fuel has more light components, as seen in Table S1b, Supporting Information , which results in the lower initial boiling temperature. Differences among the alternative diesel fuels are further illustrated with the distillation curves. 3. Distillation Curves. Representative distillation curve data for the three alternative diesel fuels and the PDD fuel are provided in Table 2 and plotted in Figure 1. Listed in Table 2

Table 1. Comparison of the initial Boiling Temperatures of the Three Alternative Diesel Fuels Studied in This Work As Well As PDDa observed temp. onset sustained vapor rise (IBT)

CTL (°C)

GTL (°C)

AVF (°C)

PDD (°C)

207.4 (1.1) 226.3 (1.0) 228.6 (0.7)

179.7 (1.2) 198.4 (1.2) 203.9 (1.1)

201.8 (2.3) 221.2 (1.1) 227.6 (0.5)

186.6 (1.2) 195.8 (3.7) 215.1 (0.6)

a

Figure 1. Average of Tk values for the three alternative diesel fuels studied in this work. PDD is also included. Uncertainty in the temperature measurement is smaller than the symbols for most of the data points. The tick mark at on the y-axis represents the temperature at which vapor first reached the condenser head. This temperature is also the initial boiling temperature (IBT).

standard atmospheric pressure with the modified Sydney Young equation. The expanded uncertainties (k = 2) are presented in parentheses. The vapor rise temperatures for the alternative diesel fuels are fairly similar to that of the PDD fuel; as seen in Table 1, none of the fuels studied boiled at a temperature more than 14 °C higher or lower than that for the

are both the fluid temperature (Tk) and the temperature in the distillation head (Th). The Tk data are true thermodynamic state points, while the Th data are diagnostic and allow for a comparison to historical measurements. These temperatures have been corrected to standard atmospheric pressure with the modified Sydney Young equation; the experimental pressures

The values given are averages of at least four different measurements and the combined uncertainty is shown in parentheses. Temperatures have been adjusted to standard atmospheric pressure with the Sydney Young equation.

Table 2. Representative Distillation Curve Data for the Three Alternative Diesel Fuels Studied in This Work and the PDD Fuel for Comparisona CTL (83.0 kPa)

GTL (83.8 kPa)

AVF (82.2 kPa)

PDD (81.8 kPa)

distillate vol fraction (%)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

236.0 241.6 247.4 253.4 259.3 266.0 272.8 279.4 286.8 294.3 302.0 310.0 318.2 326.3 334.5 344.1 354.0 364.7

207.8 218.0 225.0 231.5 238.0 245.2 251.0 255.9 262.5 269.7 276.5 283.8 290.8 297.3 304.0 315.0 322.7 330.8

212.0 218.6 226.2 234.5 242.7 251.2 259.9 268.3 277.1 285.9 294.5 301.7 308.5 315.3 321.9 328.9 335.8 344.1

182.7 189.3 198.4 206.2 214.9 224.3 232.4 241.3 248.8 258.8 275.1 283.9 290.8 298.5 305.1 313.0 317.6 327.4

244.2 253.6 261.1 267.5 272.5 277.1 280.4 283.2 285.7 288.1 290.0 291.8 293.5 295.4 297.2 299.3 301.7 304.1

191.9 209.5 224.1 238.0 247.5 258.6 265.6 269.7 275.7 278.8 282.3 285.2 286.8 288.7 289.8 292.1 292.7 294.3

225.1 230.4 235.3 240.8 245.8 250.8 256.6 262.0 267.9 274.0 280.1 286.8 293.8 301.2 309.3 318.9 328.4 339.6

207.5 214.8 220.6 228.9 234.2 239.7 246.3 252.1 258.1 265.1 270.4 276.9 283.9 290.7 297.9 305.8 313.9 322.8

a

Temperatures have been adjusted to standard atmospheric pressure with the Sydney Young equation. The pressures at which the measurements were made are provided for each fuel to permit recovery of the actual measured temperatures. The uncertainties are discussed in the text. 6907

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Figure 2. GC-FID chromatograms of distillate volume fractions of the CTL fuel sample.

similar to the PDD fuel; however, in terms of slope, the distillation curve of the CTL fuel is most similar to the PDD fuel. The distillation of the AVF and CTL fuels begin at a temperature slightly (13 °C) above that of the PDD fuel. The GTL fuel begins to distill at a temperature 12 °C lower than the PDD fuel. This lower temperature at which the first drop of GTL distills is due to more light molecular components in this initial distillation fraction and may correlate with longer ignition delay.23,24 Previous research on commercial gas turbine kerosenes (fuels that are similar in many respects to diesel fuels) by use of the ADC method showed significant variability in volatility, composition, and combustion enthalpy.25 It is likely that there is similar variability in diesel fuel, although this has not yet been studied by the ADC method. An initial assessment of the expected variability of commercial diesel fuel indicates that it is likely that both the GTL and CTL fuels are within the current experience base. The volatility of the AVF, with its decreasing slope at higher distillate volume fractions, is atypical. 4. Composition Channel Information. While the gross examination of the distillation curves is instructive and valuable for many design purposes, the composition channel of the ADC approach can provide more information that can aid in developing a better understanding of the thermophysical behavior of the fluid. One can sample and analyze the individual fractions as they emerge from the condenser. Figures 2, 3, and 4 contain a series of chromatograms of selected distillate volume fractions of the CTL, GTL, and AVF fuels,

are provided if one desires to recover the temperatures that were experimentally measured. The relationship between Th and Tk can be seen in Table 2. The convergence of Th and Tk would suggest the presence of a pure fluid or an azeotrope among mixture constituents. The average difference between these two measured temperatures was approximately 19 °C for all distillate volume fractions of all fuels. We did not observe azeotropic convergence with any of the fuels studied here. Figure 1 shows graphically the distillation data of Table 2 in terms of Tk (the temperature in the fluid) vs distillate volume fraction (%). The distillation curves of the CTL, GTL, AVF, and PDD fuels are shown here. In most cases, the uncertainty bars on the fluid temperature measurements are smaller than the symbols used, typically 0.5 °C. The temperature range from the 5 to 90% distillate volume fraction for the AVF fuel spans only 55.6 °C, whereas for the GTL and CTL fuels the temperature range is much larger: 128.7 and 132.1 °C, respectively. The PDD fuel distillation occurs over a range of 114.5 °C. It can be seen that the shapes of the distillation curves are very different as well. The distillation curve of GTL has a constant slope over the duration of the distillation, whereas the distillation curves of CTL and PDD increase in slope slightly at distillate volume fractions higher than 75%. The distillation curve of the AVF fuel exhibits significantly different behavior, largely flattening out above ∼20% distillate fraction. As we will discuss later, the flattening of the AVF fuel’s distillation curve is due to the upper limit chain length (C18) found in the fuel feedstock (animal and vegetable fats). In terms of magnitude, the volatility of the GTL fuel is most 6908

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Figure 3. GC-FID chromatograms of distillate volume fractions of the GTL fuel sample.

respectively. All chromatograms shown in these figures were measured using a GC-FID instrument. The intensity (y-axis) is presented in arbitrary units of area counts. The solvent peaks (n-hexane and minor impurities) have been removed digitally from all chromatograms. Although there are many peaks in each chromatogram, these chromatograms are simpler than those of the neat fluids. The chromatogram labeled “1st drop” is the first drop of distillate that emerges from the condenser and represents an estimated distillate volume fraction of 0.025%. It can be seen in all three figures (Figures 2−4) that the lightest components generally decrease in concentration (peak area) and heavier components increase throughout the distillation, as would be expected. The distribution of peaks is also fairly symmetric at all distillate volume fractions (beyond the first drop) in the case of both of Fischer−Tropsch fuels (CTL and GTL). Each subsequent fraction contains decreasing quantities of lighter compounds and increasing quantities of heavier compounds. A unique property of the AVF diesel fuel, however, is the apparent hydrocarbon length cutoff with noctadecane that can be seen in the chromatograms of Figure 4 (and also Table S1c). As the distillate volume fraction increases, the relative amount of n-octadecane visibly increases; however, no components heavier than n-octadecane (RT = 24.85 min) were observed within the limit of detection of the FID detector, even in the highest distillate volume fraction (90%). This characteristic is due to the composition of the animal and vegetable fat feedstocks, which have no fatty acids with chain lengths longer than C18.26 This feature is responsible for the

decrease in slope (flattening) of the distillation curve of the alternative diesel fuel seen in Figure 1. The fact that the composition approaches these heavy C18 components late in the distillation curve also explains the approach of the Tk and Th values seen in Table 2. 5. Enthalpy of Combustion. The composition-explicit data channel allows for the addition of thermochemical data to the distillation curve.9,11,27,28 The enthalpy of combustion (which we represent as −ΔHc) can be found by multiplying the enthalpy of combustion of each of the pure components29 by the mole fraction of that component and then adding the contributions of the individual components to obtain the enthalpy of combustion, as given by eq 1:9,11,28 −ΔHc = Σxt ( −ΔHi)

(1)

where i refers to the individual components that have been identified or selected and the enthalpy of mixing is ignored. We have discussed the contributions to the overall uncertainty of the enthalpy of combustion calculated from eq 1 elsewhere.9,11,22,27,28 The main sources of uncertainty in the enthalpy of combustion calculation here are due to (1) uncertainty in the values tabulated for the individual enthalpy of combustion values for each component, (2) uncertainty in the measured mole fraction, and (3) uncertainty arising from the absence of data for experimental enthalpy of combustion for some of the constituents. There is also some contribution to the uncertainty due to neglecting the enthalpy of mixing, although this is expected to be small for mixtures of linear and 6909

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Figure 4. GC-FID chromatograms of distillate volume fractions of the AVF fuel sample.

branched alkanes.11 Another source of uncertainty could be the complete misidentification of the compounds on the chromatogram; however, the components of all three alternative diesel fuels were mainly straight-chain alkanes that form a characteristic pattern and are easily identified. Finally, there is some uncertainty due to the neglecting of minor components. In past work we have determined that neglecting peaks with total uncalibrated area percentages of up to 4% increases the uncertainty of the calculated enthalpy by only 1.5%.11 In this work, we include peaks with areas of 1.5% or greater, and so, the contribution of minor species to the overall uncertainty is expected to be even smaller than 1.5%. In view of these sources of uncertainty, the overall combined uncertainty (k = 2) assigned to our enthalpy of combustion values is 5%. The enthalpy of combustion of the three alternative diesel fuels and also the PDD fuel was calculated at ten distillate volume fractions from 0.025% (first drop) to 90%. Figure 5 presents these calculated molar enthalpies of combustion, in −kJ/mol, for the distillate volume fractions of the alternative diesel fuels. For comparison, the enthalpies of combustion for the PDD fuel distillate volume fractions are also presented in Figure 5. These data are also given in tabular form in Table 3. We observed that the enthalpy curves mimic the distillation curves in overall shape and trend. Similar to the distillation curves, the GTL fuel is the most similar at each distillate fraction to the PDD fuel. The calculated molar enthalpy of all but three distillate fractions (50, 60, 70%) of GTL are equal (within uncertainty) to the PDD fuel enthalpy values. The CTL

Figure 5. Enthalpies of combustion (presented in units of −kJ/mol) of the three alternative diesel fuels, presented as a function of distillate volume fraction. The PDD fuel is included for comparison. The uncertainty is discussed in the text.

fuel is offset slightly in the positive direction relative to the PDD fuel. The enthalpy of combustion of the CTL fuel fractions are between −500 and −1400 kJ/mol larger than PDD fractions, with an average difference of 920 kJ/mol. The AVF fuel enthalpy curves contain the same features as the distillation curve, but these features are more pronounced. There is a rapid rise in enthalpy values in the low distillate volume fractions, up to about 30%, at which point there is an 6910

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Table 3. Composite Enthalpy of Combustion, Presented on a Mole Basis (in units of −kJ/mol), of Selected Distillate Volume Fractions of the Three Alternative Diesel Fuels Studied Here As Well As the PDD Fuela

a

distillate vol fraction (%)

CTL

GTL

AVF

PDD

0.025 10 20 30 40 50 60 70 80 90

6028 7084 7479 7852 8323 8993 9637 10366 11359 12369

5522 6183 6543 6968 7585 8357 9163 10134 10897 11663

4825 6946 8728 9927 10150 10297 10365 10469 10541 10719

5498 6481 6718 7025 7272 7637 8246 9403 10746 11985

Figure 6. Enthalpies of combustion on a mass basis (presented in units of −MJ/kg) of the three alternative diesel fuels, presented as a function of distillate volume fraction. The enthalpy of combustion values of the PDD fuel are included for comparison. The uncertainty is discussed in the text.

The uncertainties are discussed in the text.

abrupt flattening of the enthalpy curves at −ΔHc ∼ 10 000 kJ/ mol. We discussed how the AVF fuel contains no hydrocarbons longer than n-octadecane and increasing distillate volume fractions simply contain an increasing mole fraction of C18 compounds. Therefore, it is not surprising that the enthalpy of combustion of the alternative diesel fuel approaches the enthalpy of combustion of pure n-octadecane (−1.12 × 104 kJ/mol) as the distillate volume fraction increases. Although the presentation of enthalpy of combustion terms of kJ/mol is useful for calculations and modeling studies, practical applications may require the calculation of enthalpy on a mass or volume basis. These conversions are simple, requiring only the molar mass or density, respectively, of the major components of the fuel. Table 4 lists the enthalpy of

Table 5. Composite Enthalpy of Combustion, Presented on a Volume Basis (in units of −kJ/L), of Selected Distillate Volume Fractions of the Three Alternative Diesel Fuels Studied Here As Well As the PDD Fuela

Table 4. Composite Enthalpy of Combustion, Presented on a Mass Basis (in units of −MJ/kg), of Selected Distillate Volume Fractions of the Three Alternative Diesel Fuels Studied Here As Well As the PDD Fuela

a

distillate vol fraction (%)

CTL

GTL

AVF

PDD

0.025 10 20 30 40 50 60 70 80 90

44.43 44.16 44.12 44.10 44.06 44.00 43.94 43.89 43.83 43.88

44.33 44.25 44.21 44.17 44.12 44.06 43.99 43.91 43.87 43.86

44.40 44.20 44.00 43.86 43.83 43.81 43.81 43.79 43.79 43.77

43.75 43.27 43.52 43.46 42.96 42.97 43.45 43.64 43.90 43.85

a

distillate vol fraction (%)

CTL

GTL

AVF

PDD

0.025 10 20 30 40 50 60 70 80 90

28179 32962 33241 33418 33886 34348 35143 35701 37247 38193

31407 32295 32640 33084 33111 34026 34665 35022 35319 35828

31085 34009 35628 37551 37983 38085 38072 38078 38067 38109

32419 34691 33826 34249 35699 35665 34981 35311 35936 38105

The uncertainties are discussed in the text.

The uncertainties are discussed in the text.

combustion on a mass basis (in −MJ/kg) of selected distillate volume fractions of the four fuels studied. These results are shown graphically in Figure 6. For each fuel, the enthalpy of combustion on a mass basis varies very little with distillate volume fraction. Within uncertainty, the enthalpy of combustion values of all distillate volume fractions of all fuels studied (alternative and the PDD fuel) are equal. Table 5 lists the enthalpy of combustion of distillate volume fractions of the fuels studied in terms of volume (−kJ/L). These results are shown graphically in Figure 7. The temperature basis for this table was chosen as 25 °C. It can be seen that the volumetric enthalpy of combustion varies with

Figure 7. Enthalpies of combustion on a volume basis (presented in units of −kJ/L) of the three alternative diesel fuels, presented as a function of distillate volume fraction. The enthalpy of combustion values of the PDD fuel are included for comparison. The uncertainty is discussed in the text.

distillate volume fraction in a manner resembling the distillation curves in shape. The AVF fuel exhibits a flattening in volumetric enthalpy of combustion values above a 40% distillate volume fraction. This behavior is similar to the flattening in 6911

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distillation temperature observed in Figure 1 for the AVF fuel; however, the enthalpy of combustion values of the AVF fuel fractions are larger than those of any other fuel. This is most likely due to the higher density of the compounds in the AVF fuel. For most distillate volume fractions, the enthalpy of combustion of the GTL and CTL fuels are slightly lower than that of PDD fuel. The enthalpy results contained in Tables 4 and 5 are subject to the same 5% uncertainty as discussed for the molar enthalpy values. The contribution to uncertainty from molecular mass and density are negligible.

CONCLUSIONS Two Fischer−Tropsch fuels, CTL derived from coal and GTL synthesized from natural gas, and a renewable diesel fuel composed of hydrotreated animal and vegetable fats (AVF) were measured with the ADC metrology. The results were compared to the petroleum-derived diesel (PDD) fuel. The distillation curves of the alternative diesel fuels differed from the PDD fuel by no more than 30 °C, with the GTL fuel deviating the least from the PDD fuel. The temperatures measured in the kettle are true thermodynamic state points that can be used to model each fluid with an equation of state. The ADC metrology allowed for a detailed, fraction-by-fraction chemical analysis of the three alternative diesel fuels, including calculation of the enthalpy of combustion of each distillate volume fraction. The thermochemical data generally follow the same trends as the volatility data. The GTL fuel was also the most similar to PDD on a molar enthalpy basis, deviating by less than 10%. The specific results of these particular fuels should not be ascribed to all fuels produced using a given technology, because the components and properties of a fuel are dependent not only on the catalyst and reaction conditions, but also on how the fluid is upgraded into a finished fuel. There are also batch-to-batch differences within a particular type of fuel, but this variability (compositional and energetic) for these fuels is currently unknown because these are prototype fuels. The thermodynamic consistency of these distillation curve measurements allow the temperature data to be modeled with an equation of state and used for the development of fuel surrogates. ASSOCIATED CONTENT

S Supporting Information *

Listing of the major components of the fuels used in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 303.497.5158. Fax: 303.497.5044. E-mail: bruno@ boulder.nist.gov . Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the assistance of Dr. Randall Shearer at RenTech, Inc. for providing the GTL fuel sample and Dr. Robert McCormick at NREL for providing the AVF and CTL fuel samples. A National Academy of Sciences/National Research Council postdoctoral fellowship is gratefully acknowledged by R.V.G. 6912

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Energy & Fuels

Article

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