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Vegetable oils
Most vegetable oils are triglycerides (TGs; triglyceride = TG). Chemically, TGs are the triacylglyceryl esters of various fatty acids with glycerol (Figure 1).
Some physical properties of the most common fatty acids occurring in vegetable oils and animal fats as well as their methyl esters are listed in Table I. Besides these fatty acids, numerous other fatty acids occur in vegetable oils and animal fats, but their abundance usually is considerably lower. Table II lists the fatty acid composition of some vegetable oils and animal fats that have been studied as sources of biodiesel.
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ch2oor 1 |
CH2OH 1 |
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1 CHOOR + I |
3 CH3OH |
— |
1 3 CH3OOCR + CHOH 1 |
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1 ch2oor |
1 CH2OH |
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Triglyceride |
Methanol |
Methyl ester Glycerol |
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Figure 1. Structure of triglycerides and principle of the transesterification reaction (shown for methyl esters; R = (CH2)XCH3 or unsaturated rests according to the fatty acids listed in Table I). |
The most common derivatives of TGs (or fatty acids) for fuels are methyl esters. These are formed by transesterification of the TG with methanol in presence of usually a basic catalyst to give the methyl ester and glycerol (see Figure 1). Other alcohols have been used to generate esters, for example, the ethyl, propyl, and butyl esters.
Selected physical properties of vegetable oils and fats as they relate to their use as DF are listed in Table III. For esters these properties are given in Table IV. Also listed in Table III are the ranges of iodine values (centigrams iodine absorbed per gram of sample) of these oils and fats. The higher the iodine value, the more unsaturation is present in the fat or oil.
That vegetable oils and their derivatives are suited as DF is shown by their CNs (Table III) which generally are in the range suitable for or close to that of DF. The heat
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Table 1. Selected properties of some common fatty acids and esters
octadecenoate); 18:1 |
Continued on next page
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Table I. Continued
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a) Z denotes cis configuration.
b) The numbers denote the number of carbons and double bonds. For example, in oleic acid, 18:1 stands for eighteen carbons and one double bond.
c) Melting points and boiling points given in Ref. 28, pp. C-42 to C-553. Melting points and boiling points of 12:0 -18:0 and 18:3 esters given in Ref. 181.
d) Superscripts in boiling point column denote pressure (mm Hg) at which the boiling point was determined.
e) See Ref. 27.
f) Cetane number from Ref. 21. Number in parentheses indicates purity (%) of the material used for CN determinations as given by the author. Other CNs given in Ref. 21 not tabulated here (purities in parentheses): ethyl caprate (10:0) 51.2 (99.4); ethyl myristate (14:0) 66.9 (99.3); propyl caprate (10:0) 52.9 (98.0); isopropyl caprate (10:0) 46.6 (97.7); butyl caprylate (8:0) 39.6 (98.7); butyl caprate (10:0) 54.6 (98.6); butyl myristate (14:0) 69.4 (99.0).
g) CN from Ref. 17. CNs (lipid combustion quality numbers) deviating from Ref. 21 as given in Ref. 17: Methyl laurate 54, methyl myristate 72, methyl palmitate 91, methyl stearate 159.
contents of various vegetable oils (Table III) are also nearly 90% that of DF2 (11-13). The heats of combustion of fatty esters and triglycerides (14) as well as fatty alcohols (15) have been determined and shown to be within the same range.
The suitability of fats and oils as DF results from their molecular structure and high energy content. Long-chain, saturated, unbranched hydrocarbons are especially suitable for conventional DF as shown by the CN scale. The long, unbranched hydrocarbon chains in fatty acids meet this requirement. Saturated fatty compounds have higher CNs. Other observations (16) are (i) that (a) double bond(s) decrease(s) quality (therefore, the number of double bonds should be small rather than large, (ii) that a double bond, if present, should be positioned near the end of the molecule, and (iii) no aromatic compounds should be present. A correlation to the statement on double bond position is the comparison of the CNs of methyl oleate (Table I), methyl petroselinate (methyl 6(Z)-octadecenoate and methyl cw-vaccenate (methyl ll(Z)-octadecenoate). The CN of methyl petroselinate (petroselinic acid occurs in less common oils such as parsley and celery seed oils) is 55.4 and that of methyl c/s-vaccenate (vaccenic acid occurs in fats such as butter and tallow) is 49.5 (17). In that study the CN of methyl
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Table II. Major fatty acids (in wt.-%) of some oils and fats used or tested as alternative diesel fuels.* All values combined from Refs. 176 and 181._______________________
a) These oils and fats may contain small amounts of other fatty acids not listed here. For example, peanut oil contains 1.2% 20:0, 2.5 22:0, and 1.3% 24:0 fatty acids (181). |
oleate was 47.2, the lowest of these 18:1 methyl esters. The double bond of methyl petroselinate is closer to one end of the molecule. It also has the longest uninterrupted alkyl chain of these compounds, which may play a role because alkanes have higher CNs as discussed above. This complements the observations in Ref. 16. Another possibility is benzene formation by a disproportionation reaction from cyclohexane, which in turn would arise from cleavage of methyl oleate (7 7). The low CN of benzene would account for the lower CN of methyl oleate. The other 18:1 compounds would not form cyclohexane due to the different positions of the double bond.
Table III. Fuel-related properties and iodine values of various fats and oilsa
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Oil or Fat Iodine CN HG Viscosity CP PP FP Value (kJ/kg) (mm2/s) (°С) (°С) (°С)
a) Iodine values combined from Refs. 176 and 181. Fuel properties from Ref. 11. All tallow values from Ref. 177 (No CN given in Ref. 177, calcd. cetane index 40.15).
The combustion of the glyceryl moiety of the TGs could lead to formation of acrolein and this in turn to the formation of aromatics (76), although no acrolein was found in precombustion of TGs (18). This may be one reason why fatty esters of vegetable oils perform better in a diesel engine than the oils containing the TGs (16). On the other hand, as discussed above, benzene may arise from the oleic moiety also.
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Table IV. Fuel-related physical properties of esters of oils and fatsa
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a) CN = cetane number; CP = cloud point, PP = pour point, FP = flash point, b) Some flash points are very low. These may be typographical errors in the references or the materials may have contained residual alcohols, c) Ref. 42. d) Ref. 55. e) Ref. 178. f) Ref. 17. g) Ref. 179. h) Ref. 177. i) Ref. 180. j) Ref. 95. k) Ref. 127. 1) Ref. 123.
However, the high viscosity of the TGs is a major contributing factor to the onset and severity of durability problems when using vegetable oils (19-20).
The above statements on CNs correlate with the values given in Tables I, III and IV. For example, corresponding to components of conventional DF, saturated fatty compounds show higher CNs than the unsaturated compounds. CNs generally increase with increasing chain length (21). The CNs of mixtures are influenced by the nature of their components. Correlation of data from Tables II, III and IV shows that major high — CN components lead to relatively high CNs of vegetable oils or their esters.
In some literature it is emphasized that biodiesel is an oxygenated fuel, thus implying that their oxygen content plays a role in making fatty compounds suitable as DF by “cleaner” burning. However, the responsibility for this suitability rests mainly with the hydrocarbon portion which is similar to conventional DF. Furthermore, the oxygen in fatty compounds may be removed from the combustion process by decarboxylation, which yields incombustible C02, as precombustion (18), pyrolysis and thermal decomposition studies discussed below imply. Also, pure unoxygenated hydrocarbons, like cetane, have CNs higher than biodiesel. Fatty alcohols, whose oxygen content is lower than that of the corresponding esters, also have CNs higher than the corresponding methyl esters as determined with ASTM D613. For example, the CN of 1-tetradecanol is 80.8 (22). The CNs of fatty alcohols also increase with chain length with 1-pentanol having a CN of 18.2 (22). The CNs of 1-hexadecanol and 1-octadecanol were not determined in this work due to their high melting points (22), but ignition delay with the constant volume combustion apparatus (CVCA) vessel discussed below was measured. The CNs of some fatty alcohols were lower when employing the CVCA. Fatty ethers (23) were also shown to have CNs higher than the corresponding fatty esters and were suggested as DF extenders. Their main disadvantage compared to esters is their less straightforward synthesis.
The CNs of esters correlate well with boiling points (27). Quantitative correlations and comparison to numerous other physical properties of fatty esters confirmed that the boiling point gives the best approximation of CN (22).
ASTM D613 is used in determining CNs. For vegetable oil-derived materials, an alternative utilizes a CVCA (24). The amount of material needed for CN determination was reduced significantly with this bomb and it also allows studying materials with high melting points that cannot be measured by ASTM D613. Estimated cetane numbers (ECN) were determined on a revised scale permitting values greater than 100. In this case, the ECN of methyl stearate is 159 and that of methyl arachidate (20:0) is 196 (24). The ECNs of other esters were methyl laurate 54, methyl myristate 72, methyl palmitate 91, and methyl oleate 80. ECNs of fatty alcohols were 1 — tetradecanol 51,1 — hexadecanol 68, 1-octadecanol 81, oleyl alcohol 51, linoleyl alcohol 44, linolenyl alcohol 41, and palmitoleyl alcohol 46. The ECNs of the TGs trilaurin and trimyristin exceeded 100, while the ECN of tripalmitin was 89, tristearin 95, triolein 45, trilinolein 32, and trilinolenin 23. The term “Lipid Combustion Quality Number” with an accompanying scale was suggested instead of CN to provide for values in excess of CN 100.
Often the “cetane index” of a fuel is published and should not be confused with CN. This is an ASTM-approved alternative method for a “non-engine” predictive equation of CN for petroleum distillates (25 and references therein). Equations for predicting CNs are usually not applicable to non-conventional DFs such as biodiesel or other lipid materials (26) . Cetane indices are not given here. A method for estimating the cetane indices of vegetable oil methyl esters has been presented (27).
Besides CN, heat of combustion (HG) is another property of fatty compounds that is essential in proving the suitability of these materials as DF {14). Heats of combustion of fatty compounds, oils and fats as well as their methyl esters are listed in Tables I, III, and IV. For purposes of comparison, the literature values {28) for the heat of combustion of hexadecane (cetane), the high CN standard for conventional DF, is 2559.1 kg-cal (at 20°C). The data in Table I show that the heats of combustion of fatty compounds are similar to those of the compounds of similar CH content (long-chain, unbranched alkanes such as hexadecane) ideally comprising conventional DF. For example, the heat of combustion of methyl palmitate is 2550 kg-cal, that of methyl stearate is 2859 kg-cal, and that of unsaturated methyl oleate is 2828 kg-cal.
Even the combined CN and heat data do not suffice to determine the suitability of a material as DF. This is shown by the data in Tables III, which list the viscosities as well as cloud and pour points of numerous vegetable oils and fats. The viscosity of vegetable oils is approximately one order of magnitude greater than that of conventional DF. The high viscosity with resulting poor atomization in the combustion chamber was identified early as a major cause of engine problems such as nozzle coking, deposits, etc. {14, 29-31). Therefore, neat oils have been largely abandoned as alternative DFs.
Four possible solutions to the viscosity problem have been evaluated (52). The most common applied solution to this problem is the preparation of the methyl esters by transesterification. The three other solutions to the problem of high vegetable oil viscosity are dilution (blending) with conventional DF or other suitable hydrocarbons, microemulsification or (co-solvency), and pyrolysis. These processes are also discussed below. As shown in Table IV, the methyl esters of oils and fats have viscosities approaching that of DF2.
The methyl esters, however, have higher cloud and pour points than their parent oils and fats and conventional DF (Tables III and IV). This is important for engine operation in cold or cooler environments. The cloud point is defined as the temperature at which the fuel becomes cloudy due to formation of crystals which can clog fuel filters and supply lines. The pour point is the lowest temperature at which the fuel will flow. It is recommended by engine manufacturers that the cloud point be below the temperature of use and not more than 6 °С above the pour point.