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VOLUME 10, NUMBER 6
NOVEMBER/DECEMBER 1996
© Copyright 1996 American Chemical Society
Articles Cetane Number Improvement of Diesel Fuel by Autoxidation Kohtaro Hashimoto,* Michiko Ikeda, Mitsuru Arai, and Masamitsu Tamura Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received June 11, 1996. Revised Manuscript Received September 12, 1996X
Diesel fuels having low cetane numbers may have ignition problems such as diesel knock, difficult engine starts in cold weather, and need for addition of cetane number improving agents. As organic peroxides are known to be effective cetane number improving additives, hydroperoxides produced in the autoxidation of hydrocarbons can be expected to improve cetane number. In this study, the cetane numbers of diesel fuels autoxidized in the liquid phase under various conditions have been examined. As a result, autoxidized diesel fuels have been found to have higher cetane numbers. Hydroperoxides are also found to be produced in the autoxidation of diesel fuels. It is suggested that hydroperoxides produced from diesel fuel autoxidation should have an important role in increasing its cetane number.
Introduction The increasing demand for diesel fuels has resulted in the use of greater percentages of cracked distillates. Diesel fuels having poor ignition properties may induce such problems as diesel knock and difficult engine starts in cold weather. Such diesel fuels need improved ignition properties. The ignition properties of diesel fuels can be rated in terms of their cetane numbers. Addition of cetane number improving agents to diesel fuels is one method to improve the ignition properties of diesel fuels. In the 1940s and 1950s, organic peroxides were found to be effective cetane number improving agents as were alkyl nitrates and other compounds.1 Li et al. examined the improvement of some nitrates and organic peroxides in cetane number and suggested that the improvement in cetane number correlated with the * Author to whom correspondence should be addressed (e-mail,
[email protected]). X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Robbins, W. E.; Audette, R. R.; Reynolds, N. E. SAE Q. Trans. 1951, 5, 404-417.
S0887-0624(96)00090-4 CCC: $12.00
number of free radicals produced by the thermal decomposition of the additive during the preignition period.2 Inomata et al. studied the effects of isopropyl nitrate and di-tert-butyl peroxide on the spontaneous ignition of n-butane by the use of a rapid compression machine and concluded that the most important factor is the heat released by the combustion of the additive in the preignition period.3 Al-Rubaie et al. examined the effectiveness of some organic peroxides and nitrates in reducing the ignition delay period and concluded that the primary role of additives was that of heat generation through rapid, exothermic, oxidative degradation following injection to the cylinder.4 Clothier et al. made engine measurements with several additives at lower temperature than in a normally operating diesel en(2) Li, T.; Simmons, R. F. Twenty-First Symposium (International) on Combustion; Combustion Institute: Pittsburgh, PA, 1986; pp 455462. (3) Inomata, T.; Griffiths, J. F.; Pappin, A. J. Twenty-third Symposium (International) on Combustion; Combustion Institute: Pittsburgh, PA, 1990; pp 1759-1766. (4) Al-Rubaie, M. A. R.; Griffiths, J. F.; Sheppard, C. G. W. SAE Tech. Pap. Ser. 1991, No. 91233.
© 1996 American Chemical Society
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1148 Energy & Fuels, Vol. 10, No. 6, 1996
Hashimoto et al.
Figure 1. Cetane number improvement of kerosene by autoxidation: (b) 100 °C; (2) 120 °C; (9) 130 °C; (1) 140 °C. Table 1. Operation Conditions of CFR Engine engine speed (rpm) 900 ( 9 injection timing (°BTDC) 13 intake air temp (°C) 66 ( 0.5 injection rate (mL/min) 13.0 ( 0.2
Figure 2. Mechanism for production of hydroperoxides by autoxidation.
Table 2. Effects of Air Flow Rate on Cetane Number Improvementa
Table 3. Concentrations of Peroxides Produced by Autoxidation
a
air flow rate (mL/min)
cetane no. increase
750 200 30
12.2 11.0 9.1
Autoxidation conditions: temperature, 140 °C; period, 1.5 h.
gine.5 Clothier et al. also reviewed how cetane number improving agents work.6 Our recent study suggests that free radicals in the preignition period should have an important role in improving the ignition properties of diesel fuels7 and that azo compounds, which are known to be radical-generating agents, improve the cetane number.8 However, addition of cetane number improving agents is not always an economically practical solution to improve the ignition properties of diesel fuels. On the other hand, organic hydroperoxides are wellknown to be produced in the autoxidation of hydrocarbons.9,10 Dagaut et al. made extensive study of the products of kerosene.11 Also, the organic hydroperoxides produced from the autoxidation of diesel fuels are expected to improve the cetane number of diesel fuels as well as organic peroxides added. In this study, diesel fuels have been autoxidized in the liquid phase under various conditions and the cetane numbers of autoxidized fuels were investigated. Furthermore, their concentrations of peroxides produced in the autoxidation have been examined. (5) Clothier, P. Q. E.; Moise, A.; Pritchard, H. O. Combust. Flame 1990, 82, 242-250. (6) Clothier, P. Q. E.; Aguda, B. D.; Moise, A.; Pritchard, H. O. Chem. Soc. Rev. 1993, 22, 101-108. (7) Hashimoto, K.; Kawakatsu, Y.; Arai, M.; Tamura, M. J. Jpn. Inst. Energy 1995, 74, 200-204. (8) Hashimoto, K.; Akutsu, Y.; Arai, M.; Tamura, M. Sekiyu Gakkaishi 1996, 39, 166-169. (9) Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. J. Am. Chem. Soc. 1979, 101, 7574-7584. (10) Jensen, R. K.; Korcek, S.; Zinbo, M. Int. J. Chem. Kinet. 1994, 26, 673-680. (11) Dagaut, P.; Reuillon, M.; Cathonet, M.; Voisin, D. J. Chim. Phys. Phys.-Chim. Biol. 1995, 92, 47-76.
temp (°C)
autoxidation period (h)
concn of peroxides (ppm)
∆(CN)
140 140 130 130 120 120
0.5 1.5 1.5 3.0 3.0 6.0
95 649 157 444 75 730
2.9 10.0 2.7 11.4 2.8 9.0
Experimental Section Kerosene (cetane number 43.3) was used as a base fuel because it has a lower cetane number than commercial diesel fuels. One liter of base fuel in a 1 L three-neck flask equipped with a reflux condenser, a gas inlet tube, and a thermometer was heated to a certain temperature in a mantle heater. Air was then introduced into a hot base fuel to initiate autoxidation at 100-140 °C. Autoxidation was carried out for a period of 0.5-6 h. The air flow rate was about 200 mL/min. This rate was changed to about 750 and 30 mL/min at an autoxidation temperature of 140 °C and time of 1.5 h to examine the influence of air flow rate. Nitrogen instead of air was introduced into a hot base fuel under the same condition. The ignition properties of a diesel fuel are rated in terms of its cetane number. Cetane number is defined by reference to the ignition properties of standard mixtures of n-cetane (cetane number 100) and heptamethylnonane (cetane number 15) under standard test conditions. The cetane number of autoxidized fuel was determined by using a CFR engine prescribed by the American Society for Testing and Materials (ASTM D613-84). The engine was operated under the conditions shown in Table 1. Reference fuels having different cetane numbers at 5 cetane number intervals were used so that the cetane number of the test fuel could be determined by interpolation of the compression ratios of the two reference fuels. The concentration of hydroperoxides was measured according to the test prescribed by the American Society for Testing and Materials (ASTM D3703).
Results and Discussion Figure 1 shows the effects of autoxidation on the cetane number. Autoxidized fuels indicate increased cetane number. The higher autoxidation temperature
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Cetane Number Improvement
and longer autoxidation period can give larger cetane number increasing effect. The autoxidation of diesel fuel can be effective for increasing the cetane number. Table 2 shows the effect of air flow rate on cetane number improvement. The effect of air flow rate seems not to be so significant. No cetane number increasing effect was found when nitrogen instead of air was introduced into a base fuel. Table 3 shows the concentrations of hydroperoxides produced by autoxidation. The higher autoxidation temperature and longer autoxidation period can produce higher concentrations of hydroperoxides. No hydroperoxides were detected when nitrogen instead of air was introduced into base fuel. Oxygen dissolved in kerosene may play an important role in autoxidation. Figure 2 shows a mechanism for hydroperoxides formation by autoxidation.10 First, alkyl radials are produced by hydrogen abstraction (reaction 1) from hydrocarbons. Addition reaction of oxygen molecule to alkyl radical (reactions 2 and 5) follows to produce alkylperoxy radical. Intermolecular (reactions 3 and 6) and intramolecular (reaction 4) hydrogen abstraction by alkyl peroxy radicals may produce alkyl hydroperoxide. Thus, hydroperoxides could be produced by autoxidation. Some alkyl hydroperoxides decompose to produce alkoxy radicals and hydroxy radicals (reactions 7 and 8). Radicals produced can also take part in the chain reactions. Radical chain reactions are well-known to occur in the preignition stage of diesel fuel combustion in a diesel
Energy & Fuels, Vol. 10, No. 6, 1996 1149
engine. Alkyl hydroperoxides decompose to produce alkoxy radicals and hydroxy radicals immediately in the diesel engine, which can enhance the chain reactions of diesel fuel. Thus, alkyl hydroperoxides produced by autoxidation can increase the cetane numbers of diesel fuels. There are some problems when hydroperoxides are present in the diesel fuel. For example, peroxides are involved with degradation of elastomers in diesel engines,12 and hydroperoxides are less stable than nitrates and dialkyl peroxides. However, we believe that improving the cetane number by autoxidation is a good method, especially for cold environments, if the autoxidation apparatus and storage conditions are appropriate. Conclusion The cetane numbers of diesel fuels, which were autoxidized in the liquid phase under various conditions, have been investigated. As a result, it is shown that the cetane numbers of autoxidized fuels are increased. It is suggested that hydroperoxides produced in autoxidized fuels should improve cetane numbers by enhancing radical chain reactions during the preignition stage. It can be said that the autoxidation of diesel fuels is effective for cetane number improvement. EF960090M (12) Liotta, F. J. SAE Tech. Pap. Ser. 1993, No. 932767.
