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Energy & Fuels 1989,3, 292-298
Low-Temperature Carbon Monoxide Formation as a Means of Assessing the Autoignition Tendency of Hydrocarbons and Hydrocarbon Blends R. D. Wilk,* D. N. Koert, and N. P. Cernansky Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, Pennsylvania 19104 Received October 24, 1988. Revised Manuscript Received February 6, 1989
The autoignition tendencies of several neat hydrocarbon fuels and binary fuel blends were investigated. An atmospheric pressure flow reactor was used to oxidize the fuels at temperatures in the range 570-920 K, which encompassed the low- and intermediate-temperature reaction regimes. The degree of low-temperature CO production was used as a measure of reactivity or autoignition tendency. n-Heptane was used as a base-line fuel and blended in binary mixtures with aromatics, alkenes, isooctane, and MTBE. The aromatic components, isooctane, and MTBE all inhibited the oxidation of n-heptane at low and intermediate temperatures. The alkenes inhibited the low-temperature oxidation but promoted the intermediate-temperature reaction. From the results of the n-heptanelisooctane blends, a correlation was obtained between the maximum low-temperature CO production and octane number. The correlation was used to predict octane numbers of the other neat fuels and binary fuel blends. The technique presented in this paper for determining autoignition tendency offers a promising method of characterizing autoignition behavior and a possible method for identifying complex behavior in fuel blending.
Introduction Addressing some of the major combustion problems such as knock in spark ignition engines and cycle to cycle variations in diesel engines requires a detailed understanding of the autoignition process of hydrocarbon fuels. This is not an easy task since the fuels used in these engines are typically full boiling range distillate fuels and gasolines made up of numerous hydrocarbon components in the Cs-Cle range. The hydrocarbon components that comprise these fuels basically fall into four classes: alkanes, alkenes, naphthenes, and aromatics. The different fuel components can have vastly different autoignition characteristics depending on their oxidation chemistry. It is desirable to be able to predict the autoignition tendency of a multicomponent fuel mixture based on its composition. One of the first steps in accomplishing this goal is understanding the oxidation chemistry of individual fuel components or classes of fuels. This task itself is difficult because the oxidation chemistry of practically all hydrocarbon fuels changes with the physical conditions of the combustion environment such as temperature and pressure. For example, the oxidation of all aliphatic hydrocarbons can be classified into at least three major temperature regimes: low, intermediate, and high.' In general, the temperature boundaries separating each of the reaction regimes are approximately as follows: T < 670 K (low), 670 < T < 900 K (intermediate), and T > 900 K (high). These boundaries only apply to low ( I 1 atm) pressure. The boundaries will shift to higher temperatures with increasing pressure. In each of these temperature regimes, the oxidation chemistry is quite different. This is seen by the different stable intermediates and products formed, the different dominating radical species, and the different rates of oxidation. At temperatures above 670 K, the rate of oxidation decreases with increasing temperature over a range
* Author to whom correspondence should be addressed.
of about 50 K, beyond which the rate increases again. The region where the rate slows is referred to as the region of negative temperature coefficient (NTC) and occurs as a consequence of the transition from the low- to intermediate-temperature reaction mechanism. Characteristic of the low-temperature regime is the occurrence of cool flames. These are exothermicities produced from lowtemperature chain-branching reactions. Cool flames can be either dampened by the NTC region, leading to multiple temperature and pressure oscillations, or followed by a hot ignition (two-stage ignition). Several studies have been undertaken to quantify autoignition tendency by relating it to the chemical processes preceding autoignition. Many of these studies have shown that there is a strong relationship between the autoignition or knocking tendency of a fuel (its octane number) and its low-temperature/cool-flame Fuels with higher octane numbers had lower tendencies to form cool flames. This initial line of work was extended to explore the effects of mixture compwition on the low-temperature/cml-flame behavior.'J*16 These later studies typically involved (1)Wilk, R. D. Ph.D. Thesis 1986,Drexel University. (2)Downs, D.; Street, J. C.; Wheeler, R. W. Fuel 1953,32, 279-309. (3)Sturgis, B.M. SAE Trans. 1965,63,253-264. (4)Walsh, A. D. Ninth Symposium (Znternutionul) on Combustion; The Combustion Institute: Pittsburgh, PA, 1963;pp 1046-1055. (5)Fish, A. Angew. Chem., Znt. Ed. Engl. 1968,7, 45-60. (6)Burgess, A. R.;Laughlin, R. G. W. Combust. Flume 1972, 19, 315-329. (7)Luck, C. J.; Burgess, A. R.; Desty, D. H.; Whitehead, D. M.; Pratley, G. FourteefithSymposium (ZnternutionaZ)on Combustion; The Combustion Institute. Pittsburgh, PA, 1973;pp 501-512. (8)Morley, C. Combust. Sci. Technol. 1987,55,115-123. (9)Croudace, M. C.; Jessup, P. J. heaented a t the 1988 SAE International Fuels and Lubricants Meeting and Exposition, Portland, OR, 1988; SAE Paper No.881604. (10)Ma, A. S. C.; Moore, N. P. Combust. Flame 1966,10, 245-253. (11) Salooja, K.C. Combust. Flame 1968,12, 597-602. (12)Moore, F.; Tipper, C. F. H. Combust. Flame 1972,19, 81-87. (13)Cullis, C.F.; Fish, A,; Gibson, J. F. R o c . R. SOC.London 1969, A31 1, 253-266. (14)Cullis, C. F.;Foster, C. D. Combust. Flame 1974,23, 347-356.
088~-0624/89/2503-0292$01.50/0 0 1989 American Chemical Society
Low-Temperature CO Formation
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t
2
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i
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i
a e
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f
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z, Ti,"
=
=
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Figure 1. Qualitative characteristics of paraffin hydrocarbon
oxidation.
correlating mixture composition with changes in low-temperature reactivity parameters such as induction period preceding a cool flame or the associated pressure rise accompanying a cool flame. A novel approach to quantify autoignition tendency was reported by Brezinsky and Dryer." Their approach is based on the staged chemical processes that occur at high temperatures and lead up to autoignition. This is shown schematically in Figure 1. The point of autoignition occurs when there is rapid energy release and temperature rise that is associated with the conversion of CO to C02. This stage is preceded by preignition processes in which the fuel is completely oxidized to intermediates that are then oxidized, producing CO. The tendency of a particular fuel to autoignite is related to the time at which the maximum temperature gradient occurs. Brezinsky and Dryer used this approach to examine the autoignition behavior of n-octane and isooctane, blends of n-octane and isobutylene, and blends of primary reference fuels n-heptane and isooctane, from which a correlation between the time of maximum temperature gradient and octane number was developed. They also suggested that the degree of CO production can be used as a measure of the extent of reaction, since the conversion of CO to COz is governed by the formation of CO. Livi n g s t ~ n 'also ~ contended that the best indicator of reactivity and significant oxidation is the formation of measurable amounts of CO. This suggestioh regarding the use of degree of CO production as a measure of extent of reaction or as an indicator of the rate of oxidation presented some very interesting possibilities. It can be hypothesized that autoignition (energy release due to the rapid conversion of CO to C02via CO OH C02 H)will occur when a critical concentration of CO is formed in the reaction, relative to
+
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+
(15)Wilk, R. D.;Cohen, R. S.; Cernansky, N. P. Twentieth Symposium (International) on Combustion;The Combustion Institute: Pittsburgh, PA, 1984; pp 187-193. (16)Wilk, R.D.;Cernansky, N. P.; Okada, H. Presented at the Joint Meeting of the Western States and Japanese Sections of the Combustion Institute, Honolulu, HI, 1987;WSS/JS Paper No. 87-24. (17)Brezinsky, K.;Dryer, F. L. Presented at the 1987 SAE International Fuels and Lubricants Meeting and Exposition, Toronto, Canada, 1987;SAE Paper No.872109. (18)Livingston, H.K.Znd. Eng. Chem. 1951,43, 2834-2840.
a particular radical pool, which is temperature dependent. As shown in Figure 1, substantial amounts of CO are formed and subsequently converted to C02 at high temperatures. Significant CO production also occurs when a fuel undergoes oxidation a t low and intermediate temperatures. Wilk et have shown that CO is the major reaction product of propane oxidation at low and intermediate temperatures, even under fuel lean conditions. Results from studies of hydrocarbon oxidation a t low and intermediate indicate that the process at these temperatures can be represented by a staged chemical sequence which is somewhat similar to that shown in Figure 1 for high-temperature oxidation. One difference is that the intermediates produced are different in both chemical character and reactivity. Also, the intermediates and initial fuel may not be completely consumed at the lower temperatures. The major difference though is the lack of significant rapid conversion of CO to C02. Consequently, the chemistry at low and intermediate temperatures is only moderately exothermic (yielding temperature increases on the order of 100 K or less) as compared with the large exothermicity of hightemperature chemistry. However, the chemical processes leading to the production of CO and the exothermicity resulting from low- and intermediate-temperature chemistry occurring during the preignition pbriod can contribute significantly to the critical conditions required for the mixture to autoignite. This is illustrated in the problem of knock in SI engines by end gas autoignition. During the cycle, the end gas can spend appreciable time at low and intermediate temperatdes, where the fuel/oxidizer mixture reacts via low- and intermediate-temperaturechemistry. Green et al.= suggest that this chemistry serves to provide enough heat release to increase the temperature of the end gas enough such that high-temperature chemistry takes over producing autoignition. Thus,understanding the oxidation chemistry as well as the relative effects and interactions of different fuel components at low and intermediate temperatures is important to understanding and controlling autoignition. In this paper, the autoignition tendencies of different classes of single-component pure hydrocarbons and binary blends are examined. The fuel components studied were n-pentane, n-hexane, n-heptane, n-octane, n-decane, isooctane, ethene, propene, 1-octene, toluene, p-xylene and methyl tert-butyl ether (MTBE). These fuels were oxidized in an atmospheric pressure flow reactor a t temperatures ranging from 570 to 920 K, encompassing the lowand intermediate-temperature reaction regimes. The amount of CO formed in the oxidation of these fuels and fuel blends was used as a measure of the autoignition tendency/reactivity.
Experimental Section An atmospheric pressure flow reactoru*26 was used to provide (19)Wilk, R.D.;Cernansky, N. P.; Cohen, R. S. Combust. Sci. Technol. 1986,49,41-78. (20)Wilk, R. D.; Cernansky, N. P.; Cohen, R. S. Combust. Sci. Technol. 1987,52, 39-58. (21)Wilk, R. D.;Cernansky, N. P.; Cohen, R. S. Presented at the Fall Meeting of the Western States SectionlThe Combustion Institute, Davis, CA, 1985;WSSCI Paper No. 85-31. (22)Wilk, R.D.; Miller, D. L.; Cernansky, N. P. Presented at the Fall Meeting of the Western States Section/The Combustion Institute, Tucson,AZ, 1986; WSSCI Paper No. 86-32. (23)Green, R. M.; Cernansky, N. P.; Pitz, W. J.; Westbrook, C. K. Presented at the 1987 SAE International Fuels and Lubricants Meeting and Exposition, Toronto, ON, Canada, 1987;SAE Paper No. 872108. (24)Hsieh, F. T.; Cohen, R. S.; Cernansky, N. P. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982;pp 1331-1336. .
Wilk et al.
294 Energy & Fuels, Vol. 3, No. 3, 1989
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Figure 3. Details of the quartz mixing channel.
a medium in which chemicalphenomena could be studied in near isolation from the effects of physical phenomena such as gradients in temperature and fluid flow. In the flow reactor, prevaporized fuel is mixed into a stream of preheated air diluted with nitrogen and freely flows through an adiabatic quartz reaction tube. Samples of the reacting gas mixture can be drawn off at axial locations referenced to the fuel introduction point. Locations far from the point of fuel injection correspond to relatively long reaction times, while those locations that are close correspond to short reaction times. The range of reaction times is adjustable via changes in the flow rate of the reactants; a high rate of flow allows examination of very short reaction times. A schematic of the atmospheric pressure flow reactor test facility is shown in Figure 2. The flow rates of nitrogen and air are regulated by Porter 200 Series mass flow controllers. The flow rate of the gaseous fuel component is regulated by a Tylan FC 260 mass flow controller. The flow rate of liquid fuel is regulated by two Harvard Apparatus Model 975 syringe pumps. The liquid fuel is atomized in nitrogen by a Delavan SNA 0.02 aspirated nozzle contained in a laboratory oven to assure that the fuel remains vaporized. The oxidizer stream, consisting of a mixture of air and nitrogen, is preheated by three 800-W ceramic bead heaters and an 800-W Lindberg Model 55036 tube furnace. The prevaporized fuel/nitrogen mixture is combined with the preheated oxidizer in a quartz mixing channel, the details of which are shown in Figure 3. The fuel stream is carried in a 7 mm diameter quartz tube that passes through the wall of the 22 mm i.d. quartz flow reactor. The fuel delivery tube has its end sealed and has five 1 mm diameter orifices spaced equally around its circumferencethrough which the fuel flows as transversejets into the oxidizer stream. In order to promote rapid mixing, the 7 mm diameter tube passes through a 10 mesh stainless-steel screen located 5 mm upstream of the orifjces. The reacting mixture remains in plug flow as it continues through the reaction zone of the flow reactor. The flow field in this reactor has been analyzed previously.26 The reaction zone is a 22 mm i.d. X 30 cm long quartz tube contained in a second 800-W Lindberg tube furnace. The reactants, intermediates, and products may be drawn off at any location from 1 to 30 cm from (26) Koert, D. N.; Wilk, R. D.; Partridge, P. A.; Cernansky, N. P. Presented at the 80th Annual Meeting of APCA, New York, 1987;Paper NO.87-1.2.
the injection point via a water-cooledgas samplingprobe, which is described in ref 26. Sampling is not done any closer than 4 cm,however, due to incomplete mixing in this region, which c a m a discontinuity in the species profiles. The extracted sample is quenched in the probe due to the relatively low temperature of the coolant (100 "C). The reactants are then pumped to gas analyzers through a multiport heated samplingsystem described in ref 26 and 27. Measurementsof CO concentration are made using a Thermo Electron Model 48 gas filter correlation CO analyzer, and COz is monitored by using a Horiba Model PIR 2000 general purpose infrared gas analyzer. The accuracy of the CO measurement is M.1ppm, which for the worst case is manifested as an uncertainty of *2% of the measured value. The accuracy of the C02measurement is *8% of the measured value for the worst case. Experiments in this study were conducted in such a way that the concentration of CO was measured at a fiied probe position while the temperature of the flow reactor was varied. The total flow rate through the reactor was maintained at 40.7 slm (standard L/min), and the probe was positioned at 30 cm downstream of the fuel injection point. The temperature was varied by adjusting the electrical heating power on the flow reactor to give an average cooling rate of 15 O C / m i n . The temperatureand CO concentration measurements were simultaneously recorded on a strip chart recorder while the temperature was decreased from 920 to 570 K. The initial oxygen concentrationwas kept constant at 15.8%. The initial fuel concentration was adjusted in each case to maintain an equivalence ratio of 0.5.
Results and Discussion Fuel Structure and Blending Effects. The production of CO was used as a measure of autoignition tendency or rate of oxidation for pure, single-component fuels and fuel blends. In the initial set of experiments, five straight-chain alkanes (pentane, hexane, heptane, octane, and decane) were oxidized in the flow reactor. CO measurements were made at the exit of the reactor as the reaction zone temperature was decreased from 920 to 570 K. The data showing the effects of temperature on CO production for the different straight-chain alkanes are presented in Figure 4. In each case, CO production at the (26) Petrow, E. D.; Savliwala, M. N.; Hsieh, F. T.; Cernansky, N. P.; Cohen, R. S. Presented at the 1978 Passenger Car Meeting, Troy, MI, 1978; SAE Paper No. 780632. (27) Cernansky, N. P.; Savery, C. W.; Suffet, I. H.; Cohen, R. S. Presented at the 1978 SAJ3 Int'l. Congress and Exposition, Detroit, MI, 1978; SAE Paper No. 780223.
Low-Temperature CO Formation high initial temperatures is low (approximately 5 ppm) for each fuel. As the temperature decreases, the CO Ievels decrease to about 1 ppm, rise to a maximum, and then decrease back to very low levels (-1 ppm). The maximum CO yields at these conditions occurred at approximately 675 K for each fuel. The decrease in CO production with temperature above 675 K was not due to the conversion to COP The COz followed the same behavior with temperature, peaking at about 675 K. Actually very little COz was formed at all of the conditions of this study. The relationship between CO and COPformation in these type experiments is discussed in detail in ref 25. The decrease in CO production and the lack of reactivity above 675 K indicates a region of negative temperature coefficient (NTC), where the overall reaction rate decreases with increasing temperature. Previous studies on lower molecular weight fuels such as propane,lg propene,20nbutane,21and isobutane32and heavier fuels such as dodecane%have also indicated NTC regions at approximately the same temperatures. This phenomenon represents and is a consequence of the transition in the controlling reaction channels within the reaction mechanism from lowtemperature to intermediate-temperature chemistry. Thus, the peak CO level observed in the oxidation of each fuel is a result of low temperature chemistry. The peak CO level increased monotonically with increasing fuel chain length. Decane gave a substantial amount of CO while pentme, not shown on Figure 4, resulted in almost no CO production a t these conditions. This observation provides confirmationthat the longer the hydrocarbon chain (for alkanes), the easier the molecule breaks down and oxidizes to intermediates and subsequently CO. The effects of mixture composition of fuel blends on reactivity and autoignition tendency is of great importance. While some mixtures behave linearly with respect to mixture composition, other behave synergistically (less reactive than a linear blend of the two) and still others behave antagonistically (more reactive than a linear blend)? In the next set of experiments, the effects of the addition of aromatic compounds on the reactivity of alkanes were examined. Two typical aromatic blending components, toluene (C8H6CH3)and p-xylene (1,4-(CH3),C6H4),were blended with the baseline alkane, n-hepb e . With the total flow rate in the reactor and the initial oxygen concentration kept constant, the initial fuel concentration was adjusted to give an equivalence ratio of 0.5 for each mixture. The CO concentration at the end of the reactor was followed as temperature was varied. The CO profiles for the blends are presented in Figure 5. Results are presented for two toluene blends (10 and 17.5% toluene by volume) and a p-xylene blend (10%). These profiles show behavior similar to those of the alkanes. Maximum CO levels occur at 668 K for the 10% toluene blend and 658 K for the 17.5% toluene blend. The temperature corresponding to the maximum CO for the p-xylene blend is 693 K. Addition of the aromatic components significantly reduces the reactivity over that of the pure heptane (note the change in scale from Figure 4). One possible cause for this is a dilution effect. Since the aromatics are unreactive under these conditions, perhaps they are acting as a diluent to the n-heptane. If this were the case, the 10%/90% toluene/heptane mixture should decrease the CO yield by 10% over that of the pure heptane, i.e. from 725 to 653 ppm. However, the actual decrease in the peak CO level from the pure heptane to the 10% toluene case was 86%. This suggests that there is definite chemical interaction between the fuel components
Energy & Fuels, Vol. 3, NO.3, 1989 295 100
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resulting in decreases in the production of CO and rate of oxidation. p-Xylene is more inhibiting than toluene as the 10% p-xylene reduces the CO by 90%. In the next set of experiments, straight-chain alkenes and their effects on reactivity were examined. Ethene (CzH4), propene (C3H6),and 1-octene (C8HI8)were each mixed with n-heptane in a 1/ 1ratio by volume. Gaseous ethene and propene were added to the prevaporized heptane, and liquid 1-&ne was blended with liquid heptane prior to vaporization. Initial fuel concentrations were adjusted for each mixture to give an equivalence ratio of 0.5. The CO profiles for the heptane/alkene mixtures are shown in Figure 6. All of the alkenes substantially reduced CO formation over that of the pure heptane. Ethene had the least inhibiting effect of the three alkenes tested, yielding a peak CO concentration of 27.1 ppm at 675 K while propene and 1-&ne produced virtually no CO peak associated with low-temperature oxidation at these conditions.
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Figure 7. Effect of temperature and fuel type on carbon monoxide formation for 10% component blends with n-heptane.
Figure 8. Effect of temperature and mixture composition on carbon monoxide formation for blends of isooctane with n-heptane.
It is interesting to note the behavior of CO with temperature from about 770 to 870 K. With each mixture, the CO levels begin to rise again, indicating that the mixtures are becoming reactive as the temperature is increased. This suggests that intermediate-temperature chemistry is taking over and the reaction rate is increasing by way of intermediate-temperature chain-branching paths. This was not observed to a great extent with the straight-chain alkanes at the same temperatures. Therefore, it appears that the alkenes are having mixed effects on the reactivity. At low temperatures, the alkenes are inhibiting oxidation. A t intermediate temperatures (T > 720 K), they seem to be promoting oxidation as indicated by the increase in CO as compared with the results for the pure alkanes. These results are in agreement with those of Moore and Tipper,12who found that the addition of alkenes retarded low-temperature oxidation but promoted second-stage or intermediate-temperature chemistry. The relative effects of each alkene are different in each temperature regime. For example, when the ethene and 1-octene profiles at low temperatures are compared, the 1-&ne is more inhibiting than the ethene. At intermediate temperatures, the 1octene promotes oxidation more than the ethene. This behavior is probably related to the structure of the two alkenes. 1-Octene has a much longer carbon chain and once initiated (by H removal), “looks like” an alkyl radi~ a l Thus . ~ ~it can decompose rapidly at the higher temperatures, much like alkyl radicals, by @-scission. Ethene, on the other hand, tends to scavenge radicals, such as H02 and OH, as they can add to the ethene a t the site of the carbon-carbon double bond. This makes it less reactive at intermediate temperatures relative to 1-octene. I t is interesting to compare the relative effects of different blending components. Figure 7 shows the relative effects on the CO production of four different blending components, toluene, isooctane, p-xylene, and methyl tert-butyl ether (MTBE). Each component was used in a 10% (by volume) mixture with the base-line fuel nheptane. MTBE, a promising antiknock agent, had the strongest effect, severely inhibiting the low-temperature CO formation. This was followed by the two aromatics toluene and p-xylene. Finally, isooctane had the least inhibiting effect of the four.
The next set of experiments was designed to determine the effects and influence of the presence of isooctane in binary mixtures with n-heptane. These fuels are the primary reference fuels for the octane rating scale, with n-heptane assigned a research octane number (RON) of 0 and isooctane having RON = 100. Isooctane was added to the base fuel heptane in amounts of 10, 17.5, 25, and 50% by volume. Isooctane was also run by itself and showed no observable reaction in the form of CO production, under the flow conditions used, over the entire temperature range examined. The results of these blending studies are shown in Figure 8. The effect of temperature on the CO concentration is shown for the case of pure n-heptane and three of the binary heptane/isooctane mixtures (10, 17.5, and 25% isooctane). Similar to the pure isooctane, the blend with 5090isooctane indicated no CO production over the entire temperature range as well. The other blends all showed CO production that peaked at 675 K as with the pure heptane. The maximum CO production is significantly decreased by the addition of isooctane. Increasing amounts of isooctane in the mixture lead to decreasing levels of low-temperature CO formation. Relation to Octane Number. Different blends of the primary reference fuel components correspond directly to fuels of specific octane numbers depending on the relative amounts of isooctane and n-heptane in the mixture. For example, the blend of 10% isooctane/90% n-heptane has a RON of 10. The octane number is a measure of autoignition tendency or, more precisely, an inverse measure of autoignition tendency since the higher the octane number the less the tendency of the fuel to autoignite. If the degree of CO production of a particular fuel oxidized at a certain condition is also indicative of the autoignition tendency, then there should be a relationship between the level of CO production and the RON. To examine this possibility, the peak CO concentrations obtained for the n-heptanelisooctane mixtures in Figure 8 were plotted as a function of volume percent isooctane in the blend, which is simply the research octane number of the b1end.l’ On the basis of the limited number of blends tested, a linear leasbsquares fit of the data was used as a first approximation to correlate the peak CO concentration with RON. Only the mixtures that showed appreciable CO formation (0, 10,17.5% isooctane) were
(28) Westbrook, C. K. AZAA J. 1986, 24, 2002-2009.
Low-Temperature CO Formation
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Figure 9. Correlation of maximum carbon monoxide concentration with percentage isooctane in blends with n-heptane (re3000
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Figure 10. Comparison of octane number correlation with experimental data for n-alkanes: data points, data obtained from literature values for RONBpWand measured peak CO levels;
straight line, octane number correlation obtained from isooctane-n-heptane blends.
included in the fit. The results, shown in Figure 9, indicate for this limited range of octane numbers that the relationship between low-temperature CO yield and RON can be represented reasonably with a linear fit. We thought it would be interesting to test this correlation by applying it to the other data for the pure fuels and blends. First, the applicability of the correlation was tested with the data for the n-alkanes (from Figure 4). The maximum CO concentrations measured for n-decane, noctane, n-heptane, and n-hexane were plotted against the known research octane number for each of these fuels In Figure 10, these data obtained from the are compared to the correlation equation obtained from the experiments on the primary reference fuels. It can be (29) Obert, E. F. Internal Combustion Engines and Air Pollution; Harper and Row: New York, 1973; pp 234-241. (30) Barnard, J. A.;Bradley, J. N. Flame and Combustion;Chapman Hall: London, 1986; pp 241-250.
seen that the correlation is very successful in predicting experimental results. It is noted that the correlation had to be extrapolated to negative octane numbers in order to compare it to the data for decane and octane, which are both more reactive than heptane and, consequently, have negative octane numbers. Next, the correlation was applied to the CO data for the tolueneln-heptane blends. It is not as easy though to compare the correlation with the results for the binary blends as it was with the pure alkanes. The pure alkanes have standard octane values that have been measured by using the standard engine octane rating protocol. On the other hand, random fuel blends often do not have measured octane numbers. What is usually done to estimate the octane number for a binary blend is to apply a straight line fit between two end points defined by the octane numbers of the two blending components. The octane number for any mixture of the two components can then be estimated by linear interpolation. This value represents a linear composition-averaged octane number for the particular blend. For example, the octane number for a 10% toluene-90% n-heptane mixture is estimated to be 12, by using RON(n-heptane) = 0 and RON(to1uene) = 120. As mentioned earlier, quite often the actual measured octane numbers of blends differ from the compositionaveraged values due to synergistic and antagonistic effects of blending due to the complex chemical interaction of the oxidation chemistry of each individual fuel component. If there is a measured octane number for a certain blend and this number is greater than the composition-averaged value, then the blend is termed synergistic. If the measured octane number is less than the average value, then the blend is an tag on is ti^.^ The correlation was used to calculate research octane numbers for each of the two tolueneln-heptane blends by using the peak CO data. These results were then compared to the straight line defining the composition-averaged octane values. This comparison is shown in Figure 11. The calculated values fall close to the average with the 10% blend indicating synergistic behavior and the 17.5% blend indicating antagonistic behavior. With the limited
Wilk et al.
298 Energy & Fuels, Vol. 3, No. 3, 1989 3000 Linear Blending Lines 25
z
P
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20
2
[CO]= -3246 + 72.03 x (CN) R = 0.99
y,1
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i
MTBEBlend
.
n-HeptaneiMTBE
5t /
1
0
0
0
5
10
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Figure 12. Comparison of predicted octane numbers with linear composition-averaged RON'S for n-heptanelp-xylene and nheptane/MTBE blends: data points, points calculated from correlation; straight line, linear blending lines based on RON(n-heptane)= 0, RON@-xylene)= 118,and RON(MTE5E) = 116.
data for these blends though, the blending behavior over an entire range of mixtures cannot be determined. However, the available data do indicate a couple of possibilities regarding the blending behavior. I t is possible that the behavior of these two fuels will average out to produce linear blending characteristics over the entire range of mixtures. Alternatively, it is possible that their chemical interaction is sufficiently complex to produce variations in the blending behavior from synergistic to linear to antagonistic over different ranges of mixture composition. The data from the n-heptane/MTBE and n-heptane/ p-xylene blends were also used with the correlation to generate calculated octane numbers. These values were then compared to the linear composition-averaged lines for each set of blends (Figure 12). Both the MTBE and the p-xylene blends show distinct synergistic behavior. The preceding analysis, in which a correlation for octane number was obtained, could have been paralleled by a similar approach to look a t the data with respect to an alternative measure of ignition tendency, cetane number. The cetane scale is based on the ignition characteristics of blends of hexadecane and heptamethylnonane and is used for rating the ignition tendency of diesel fuels. Unfortunately, these heavy fuel components are difficult to run in the present experiment. This precluded the development of a comparable cetane correlation. However, on the basis of the data for three of the n-alkanes examined (decane, octane, and heptane), all of which have known cetane values30 a linear fit to the maximum CO data was applied and is shown in Figure 13. I t should be noted that in this experiment the reactor residence time at the constant probe position does vary with reaction temperature. During the experiments where the temperature drops from 920 to 570 K, the reaction time increases by 50%. The preceding correlations were developed for peak CO concentrations that occurred in this temperature range. The peak CO concentrations were found to occur at an average reaction temperature of 675 K based on all the data from the neat fuels and fuel blends. The standard deviation in the temperature for the peak CO concentration was 9.02 K. On the basis of an interval
Cetane Number
Figure 13. Correlation of maximum carbon monoxide concentration with cetane number for n-alkanes.
of three standard deviations about the mean value of the reaction temperature, the variation of the residence time at the peak CO concentration is f4%. Thus, the peak CO concentration measurements that were used for the correlations correspond to the same reaction time within an acceptable range of statistical scatter. Summary and Conclusions The autoignition tendencies of several hydrocarbon fuels and binary fuel blends were investigated. The amount of CO produced in the low-temperatureoxidation regime was used as a measure of reactivity or autoignition tendency. n-Heptane was used as a base-line fuel and blended with various amounts of aromatics, alkenes, a branched alkane, and an ether. All of these blending components, except the alkenes, inhibited the oxidation chemistry of n-heptane at both low temperatures and intermediate temperatures. The alkenes inhibited the low-temperature oxidation but promoted oxidation at intermediate temperatures. A correlation was found between the maximum lowtemperature CO production and octane number. The correlation was used to predict octane numbers of neat fuels and binary fuel blends. Due to the experimental conditions used for this study though, the correlation is limited to low octane numbers (RON< 20) and thus may be limited in its practical application. The potential is there, however, for the extension of this type of analysis to more practical octane numbers and to cetane numbers. The technique presented in this paper for determining autoignition tendency offers a promising method of characterizing autoignition behavior and a possible method for identifying complex behavior in fuel blending. Further, the technique involves relatively simple measurements that could alleviate extensive engine testing. Acknowledgment. This work was supported by the U.S. Army Research Office, Contract No. DAAG 29-85K-0253, and the U.S. Department of Energy, Division of Energy Conversion and Utilization Technologies, Contract NO. DE-FG04-87AL44658. Registry No. MTBE, 1634-04-4;CO, 630-08-0;pentane, 109-66-0; hexane, 110-54-3;heptane, 142-82-5;octane, 11 1-65-9; decane, 124-18-5; isooctane, 540-84-1; ethene, 74-85-1; propene, 115-07-1; 1-octene,111-66-0; toluene, 108-88-3; p-xylene, 106-42-3.
