Energy & Fuels 2007, 21, 3415–3419
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Kinetic Study of the Oxidative Degradation of Brazilian Fuel Oils P. M. Crnkovic,* C. R. M. Leiva, A. M. dos Santos, and F. E. Milioli Department of Mechanical Engineering, School of Engineering of São Carlos, UniVersity of São Paulo, São Carlos–SP, 13560-970, Brazil ReceiVed April 27, 2007. ReVised Manuscript ReceiVed July 20, 2007
Complex materials as fuel oils are characterized by physical and chemical parameters. This paper deals with the determination of the activation energy of three kinds of Brazilian fuel oils. As it is possible to establish a direct relation between ignition delay and activation energy, these kinetic parameters can be used to characterize these oils regarding the combustion quality of the fuel. Activation energies were determined as a function of conversion degree (R) and temperature by model-free kinetics. The samples, here named A, B, and C, were supplied by Petrobras-Cenpes, and they were characterized by thermogravimetry (TG) and differential thermal analysis (DTA). Transient experiments were performed from room temperature up to 600 °C at five heating rates (2.5, 5.0, 10.0, 15.0, and 20.0 °C min-1) in an atmosphere of synthetic air and samples of about 20.0 mg were used. In all experiments, three distinct decomposition regions were observed and identified as lowtemperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO). In order to compare these oils, we considered the medium value of the activation energy obtained between conversions of 1% and 90% only for the LTO region. The results are as follows: sample A ) 43.8 kJ mol-1, sample B ) 57.2 kJ mol-1, and sample C ) 61.8 kJ mol-1, showing that the sample A is the best option among the samples studied in this work and that the activation energy is a suitable parameter for this purpose.
1. Introduction Internal combustion engines produce mechanical power from the chemical energy contained in a fuel. They can be classified by application of basic engine design, working cycle (four-stroke or two-stroke cycle), fuel, method of ignition (spark-ignition and compression-ignition), and combustion chamber design among other factors.1 In compression-ignition engines (CI engines), the air is compressed and raised to a high temperature during the compression stroke, just before the beginning of the desired combustion. The fuel is injected by the fuel-injection system into the cylinder engine towards the end of the compression stroke.1,2 However, the combustion process in a CI engine is extremely complex. The details of the process depend on the characteristics of the fuel, the engine’s combustion chamber, and its fuel-injection system and operating conditions. In the combustion process of the CI engine there is a special period, called ignition delay, which is the period between the start of the fuel injection into the combustion chamber and the beginning of combustion. Ignition delay occurs due to the time consumed by both physical and chemical delays. Physical delay is the time between the beginning of injection and the attainment of chemical reaction conditions. In the chemical delay, the reaction starts slowly and then accelerates until inflammation or when ignition has taken place. The chemical components of the ignition delay are controlled by the precombustion reactions of the fuel. Since the ignition characteristics of the fuel affect the ignition delay, the properties of a fuel are very important to * Corresponding author. Phone: (55-16-3373-9390). E-mail: paulam@ sc.usp.br. (1) Heywood, J. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988. (2) Obert, E. F. Internal Combustion Engine, third ed.; International Textbook: Scranton, 1968.
determine the CI engine operating characteristics, such as fuel conversion efficiency, smoothness of operation, misfire, pollution and smoke emission, noise, and ease of starting. The ease of igniting the fuel oil in the engine by autoignition is called ignition quality.2 The ignition quality of a fuel in a CI engine is defined by its cetane number, which is associated with its chemical composition: n-paraffins have a high ignition quality, whereas aromatic and naphthenic compounds have low ignition quality. The hydrocarbon n-hexadecane (C16H34) represents the top of the scale with a cetane number of 100, and the isocetane (2,2,4,4,6,8,8 heptamethylnonane) represents the bottom of the scale with a cetane number of 15. A higher cetane number indicates greater fuel efficiency and obviously decreases the delay.1 As mentioned by Heywood,1 Hardenberg and Hase (1979) developed an empirical formula to predict the duration of the ignition delay period in CI engines and correlated this parameter with activation energy. The authors concluded that the activation energy decreases with increasing fuel cetane number. It is possible to establish a direct relation between activation energy (Ea) and ignition delay by means of an equation expressed by the Arrhenius relation: 1,3–5 τ ) Ap-neEa⁄RT (1) Where τ is the ignition delay, Ea is the activation energy, R is the universal gas constant, and A and n are constants that depend on the fuel. From this principle, the better the ignition process, the shorter the ignition delay and, consequently, the lower the activation energy of the combustion reaction. (3) Lichty, L. C. Combustion Engine Process; Mc Graw-Hill: New York, 1967. (4) Kowalewicz, A. Combustion Systems if High-Speed Piston I.C. Engines; Elsevier: New York, 1984. (5) Jóvaj, M. S. Motores de AutomóVil; Editorial Mir: Moscú, 1982.
10.1021/ef700219u CCC: $37.00 2007 American Chemical Society Published on Web 09/22/2007
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Table 1. Specifications and Properties of Samples A, B, and C classification (Petrobras) kinematic viscosity density 15 °C/4 °C ash sulfur a
sample A
sample B
sample C
MF 380 (A) (50 °C) 300 and 380 cSt 0.98 to 0.99 ∼0.05% ∼1%
fuel oil A2 (60 °C) maximum of 960 cSt 0.98 to 1.02 not specificated by ANPa ∼0.8 a 1%
fuel oil A1 (60 °C) maximum of 620 cSt 0.98 to 1.02 not specificated by ANPa ∼0.8 a 1%
ANP–Agência Nacional de Petróleo (Petroleum National Agency).
The activation energy can be obtained from an experimental attempt, and thermal analysis is an efficient tool for such determination. Traditional kinetic methods to analyze first-order and secondorder reactions are not often applicable to complex reactions of practical interest involving, for instance, thermal degradation of fuel oils. Consequently, a variety of mathematical techniques, called global kinetic analysis, have been developed to characterize the behavior of this type of reaction.6 Previous studies have applied thermal analysis techniques to study the behavior of oxidative degradation of the oil derivative.6–18 In recent years the application of thermal analysis techniques to study the combustion and pyrolysis kinetics of fossil fuels has gained wide acceptance. These experiments are advantageous over those on a larger scale, as they require small samples and are relatively inexpensive and fast.19 According to Kök,9 among the precursors that employed thermal analysis, Tadema (1956) stands out as the first researcher that applied this technique to study the effects of the combustion mixture of several crude oils and clays. The author shows the feasibility of differential thermal analysis (DTA) utilization as a tool in the crude oil combustion study and describes the existence of two main reactions: one at a higher temperature and one at a lower temperature. Verkoczy and Jha8 performed thermogravimetric analysis (TG/DTG) and differential scanning calorimetry (DSC) experiments to determine kinetic parameters for four kinds of heavy oils. Kinetic and thermochemical data are estimated for lowtemperature oxidation, high-temperature oxidation, craking, and coking. Kök9 characterized pyrolysis and thermo-oxidation behavior for two types of crude oils using TGA and DSC in an air and nitrogen atmosphere. In the pyrolysis experiment, they observed two different mechanisms, and in the combustion experiments, three distinct reaction regions were observed: low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO). The author obtained the Ea values between 128.3 and 142.3 kJ mol-1 for both crude oils, respectively. The author correlated Ea with deg API, i.e., higher activation energies
were found as the API gravity of crude oils decreased in the HTO region. According to the American Petroleum Institute, the API gravity provides a way to express the relative density of crude petroleum and petroleum products. The API scale varies inversely to the relative density. Kök et al.10 studied the combustion behavior and kinetics of three crude oils. The authors used a high-pressure thermogravimetric analyzer (HPTGA) to evaluate the effect of total pressure. The Coats and Redfern method was used to obtain the kinetic parameters in the LTO and HTO regions. Within the range of 100–300 psig, the authors observed that no effect of pressure on the kinetic parameters was observed. Kök and Okandan11 also correlated the values of activation energies with the API gravity for six crude oils. They applied the nonisothermal methods using TG/DTG with excess of air, concluding that these parameters are inversely proportional: while the activation energy varies from 67.4 to 131.9 kJ mol-1, the API gravity varies from 26.1 to 11.3. Ali et al.12 focused on TG and DTA as techniques to study the characteristics of four crude oils. They investigated the properties of the volatile components and correlated the characteristics of their thermal-oxidation behavior. Four characteristic regions were observed. The first region indicates the mass loss of free moisture and volatile hydrocarbons (20–280 °C), and the remainder is characterized by the oxidative degradation of different types of hydrocarbons: low molecular weight (280–400 °C), medium molecular weight (400–510 °C), and high molecular weight (510–620 °C), respectively. Although most of the studies on thermal analysis of crude oils have focused on the elucidation of the thermal behavior of the samples and kinetics,20 they have not correlated kinetic studies with the quality of the fuel. This paper presents the determination of the activation energy for one of the combustion regions applying model-free kinetics and thermogravimetric analysis. On the basis of the direct relation between ignition delay and activation energy, the use of the activation energy is proposed for the characterization of different Brazilian oils as a complementary parameter to the cetane number to determine the quality of a fuel.
(6) Burnham, A. K.; Braun, R. L. Energy Fuel 1999, 13, 1–22. (7) Drici, O.; Vossoughi, S. J. Pet. Technol. 1985, 731–735. (8) Verkoczy, B.; Jha, K. N. J. Can. Pet. Technol. 1986, (May–June), 47–54. (9) Kök, M. V. Thermochim. Acta 1993, 21, 315–327. (10) Kök, M. V.; Hughes, R.; Price, D. Thermochim. Acta 1996, 287, 91–99. (11) Kök, M. V.; Okandan, E. J. Therm. Anal. 1997, 48, 343–348. (12) Ali, M. A.; Siddiqui, M. A. B.; Zaidi, S. M. J. J. Therm. Anal. 1998, 51, 307–319. (13) Kök, M. V.; Karacan, O. J. Therm. Anal. 1998, 52, 781–788. (14) Kök, M. V. Thermochim. Acta 2001, 369, 149–155. (15) Kök, M. V.; Keskin, C. Thermochim. Acta 2001, 369, 143–147. (16) Gonçalves, M. L. A.; Teixeira, M. A. G.; Pereira, R. C. L.; Mercury, R. L. P.; Matos, J. R. J. Therm. Anal. Cal. 2001, 64, 697–706. (17) Kök, M.V. J. Therm. Anal. Cal. 2003, 73, 241–246. (18) Kök, M. V.; Sztatisz, J.; Pokol, G. Energy Fuels 1997, 11, 1137– 1142. (19) Millington, A.; Price, D.; Hughes, R. J. Therm. Anal. 1993, 40, 225–238.
2. Experimental In our experiments, three distinct fuel oils supplied by Petrobras–Cenpes were evaluated: MF-380, fuel oil A1, and fuel oil A2, which are here denoted by A, B, and C, respectively. Table 1 presents the specifications and properties of these three samples. The thermogravimetric experiments were performed in a Shimazdu TGA-51H analyzer. The experiment procedure evolved from room temperature to 600 °C, at five heating rates of 2.5, 5.0, 10.0, 15.0, and 20.0 °C min-1. All these experiments were run in duplicate for each sample to obtain the medium curve and calculate the activation energy. Samples of 20.0 ( 0.5 mg and aluminum crucibles were used. The reacting (20) Kök, M. V. J. Therm. Anal.Cal. 2002, 68, 1061–1077.
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atmosphere was synthetic air, which was continuously blown over the samples through the furnace of the analyzer at a flow rate of 100 mL min-1. DTAs were studied qualitatively to identify the low-temperature oxidation and the other two regions. The heating rate was 5 °C min-1, and the other experimental conditions were kept the same as those applied to TG experiments. Kinetic Study. Among the existing kinetic methods, the following can be mentioned: Arrhenius, Coats and Redfern, Michelson and Eirnhorn (ratio model), Ingraham and Marrier, Freeman and Carroll, Flynn and Wall, Ozawa, and model-free kinetics. The model-free kinetics method is based on Vyazovkin′s theory21–24 and applies the isoconversional technique to calculate the effective activation energy (Ea) as a function of the conversion level (R) of a chemical reaction, Ea ) f(R). The experiments performed in nonisothermal conditions are considered more convenient than isothermal runs, as no sudden temperature leap is necessary at the beginning of the event. The approach follows all points of conversion from multiple experiments, avoiding an uncertainty that may result from a single experiment. The theory is based on the assumption that dR ) k(T)f(R) dt
Figure 1. TG and DTG curves of sample A performed at a heating rate of 5 °C min-1.
(2)
where dR/dt is the reaction rate, f(R) is the reaction model, and k(T)is the Arrhenius rate constant. Substituting this eq in eq 2, one has dR ) Ae-Ea⁄RTf(R) dt
(3)
where R is the universal gas constant and A is the Arrhenius parameter, also called the pre-exponential factor. Arrhenius parameters (Ea and A), together with the reaction model, f(R), are called a kinetic triplet.22 Dividing eq 3 by the heating rate β ) dT/dt and rearranging it, one obtains
Figure 2. TG and DTG curves of sample B performed at a heating rate of 5 °C min-1.
A 1 d(R) ) e-Ea⁄RT dT (4) f(R) β Integrating such an equation up to conversion R (at temperature T), one has
∫
∫
R
A T -Ea⁄RT 1 dR ) g(R) ) e dT (5) f(R) β T0 Since E/2RT >> 1, the temperature integral can be approximated by 0
∫
T -E ⁄RT a
T0
e
dT ≈
R 2 -Ea⁄RT Te Ea
(6)
Substituting the temperature integral, rearranging eq 5, and applying the logarithm, one obtains
[
]
ER 1 RA β ln 2 ) ln ERg(R) R TR T R
Figure 3. TG and DTG curves of sample C performed at a heating rate of 5 °C min-1.
(7)
One of the main advantages of this method is the possibility of isolating function g(R) in the linear coefficient. However, the determination of this function in a complex process is very
difficult.22 From this principle, the method is suitable to obtain the activation energy of combustion reactions as these processes are highly complex. 3. Results and Discussion
(21) 53–68. (22) 180. (23) 42–45. (24)
Vyazovkin, S.; Wight, C. A. Thermochim. Acta 1999, 340–341, Vyazovkin, S.; Sbirrazzuoli, N. Anal. Chim. Acta 1997, 55, 175– Vyazovkin, S.; Dollimore, D. J. Chem. Inf. Comput. Sci. 1996, 36, Vyazovkin, S. Int. J. Chem. Kinet. 1996, 28, 95–101.
Figures 1–3 show the TG and DTG curves at a heating rate of 5 °C min-1 for samples A, B, and C, respectively. These curves demonstrate the following regions of oxidative degradation, and based on the literature,9,10,14 three distinct regions were identified. The first region is called low-temperature oxidation (LTO), the
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Figure 5. DTG curves of sample C at heating rates of 2.5 (—), 5.0 (- - -), 10.0 ( · · · ), 15.0 (- · - · -), and 20.0 °C min-1 (- · · - · · -). Table 3. Peak and Burn-Out Temperatures of the Fuel Oils in the LTO Region sample A
Figure 4. DTA curve of samples A, B, and C at a heating rate of 5 °C min-1 .
B
Table 2. Temperature Ranges of the Oxidative Degradation Regions at a Heating Rate of 5 °C min-1 Trange (°C) sample
LTO
FD
HTO
A B C
25–384 25–422 25–395
384–467 422–470 395–456
467–600 470–600 456–600
second transition is designated as fuel deposition (FD), and the final reaction is called high-temperature oxidation (HTO). Under an oxidizing environment, DTA results indicate that the system involves exothermic reactions. Figure 4 represents the DTA curves of the samples A, B, and C, where it is possible to note the exothermic peaks related to the LTO, FD, and HTO regions. Comparing Figures 1–4, it can be observed that the mass loss is accompanied by exothermic peaks due to the oxidative degradation of different classes of hydrocarbons.11 The three regions of thermal degradation and corresponding temperature ranges for each sample at 5 °C min-1 are presented in Table 2. In this work, we opted to study the low-temperature reoxidation region (LTO) in which the mass loss indicates first the vaporization of moisture and volatile hydrocarbons and then the oxidation of low molecular weight compounds by the exothermic reaction.12 According to Jóvaj,5 the burn rate of the fuel is determined by its evaporation followed by the formation of the mixture of the generated vapor with air. Therefore, the activation energy of the first region provides a correlation with the quality of the fuel. According to Yoshiki and Phillips,25 the dominant trend in the LTO region is the formation of higher molecular weight and more complex materials. This step is responsible for the formation of the byproducts that introduce oxygen in the form (25) Yoshiki, K. S.; Phillips, C. R. Fuel 1985, 64, 1591–15.
C
heating rate (°C min-1)
peak temp (°C)
burn-out temp (°C)
2.5 5.0 10.0 15.0 20.0 2.5 5.0 10.0 15.0 20.0 2.5 5.0 10.0 15.0 20.0
336 362 405 434 437 377 391 414 443 442 354 371 382 387 390
359 384 421 458 468 393 422 441 461 454 375 395 432 451 469
of carboxylic acids, aldehydes, ketones, alcohols, and hydroperoxides. The heat released by these primary oxidation processes leads to an increase in the oxidation rate and the promotion of the complete combustion reaction. The heating rate affects the intervals of distinct reaction regions and peak and burn-out temperatures. This effect is shown in Figure 5, where DTG curves of sample C at 2.5, 5.0, 10.0, 15.0, and 20.0 °C min-1 are represented. Peak temperatures indicate the maximum rates of weight loss, and burn-out temperatures represent the point in the TG curve where the reaction is completed. Peak and burn-out temperatures for samples A, B, and C of the LTO region at five heating rates are shown in Table 3. The peak temperature of the LTO region was considered to be the peak related to the oxidation of the low molecular weight compounds. To determine the activation energy by using the model-free kinetic method, it was first necessary to evaluate the conversion degree of the LTO region based on the following relation: R)
m0 - m m0 - m∞
(8)
where m is the sample mass that varies with time, m0 is the initial mass sample, and m∞ is the remaining mass sample. Figure 6 shows the conversion curves as functions of temperature for the LTO region of sample C. The end of the
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Energy & Fuels, Vol. 21, No. 6, 2007 3419
Figure 6. Conversion versus temperature for the first stage of the oxidative degradation (LTO) of sample C. Table 4. Activation Energy for Some Conversions of the Fuel Oils in the LTO Region Ea (kJ mol-1) conversion (%)
sample A
sample B
sample C
10 20 30 40 50 60 80
43.1 43.3 43.8 44.6 44.1 44.6 43.0
45.6 47.6 48.6 47.5 50.0 54.2 75.7
55.6 56.4 58.2 60.0 60.0 63.6 66.2
event (R ) 100%) was considered to be the burn-out temperature, as shown in Table 3. The activation energy was calculated by model-free kinetics and some decomposition levels (10, 20, 30, 40, 50, 60, and 80%) are indicated in Figure 6. The curve ln β/TR2 versus 1/TR was plotted providing a series of straight lines with a slope of –ER/R. Therefore, the activation energy was obtained as a function of conversion. The results are presented in Table 4. Throughout the linear regression, it was possible to establish Ea for all conversion levels. Figure 7 shows a group of these Ea values obtained for samples A, B, and C, where one can observe that Ea varies with the conversion. The activation energy was determined for the LTO region and resulted in 43.8 kJ mol-1 (R ) 0.1–0.9) for oil A. For oil B, the results were on average 47.8 kJ mol-1 (R ) 0.1–0.5) and 66.2 kJ mol-1 (R ) 0.5–0.9). For oil C, the activation energies were 57.8 kJ mol-1 (R ) 0.1–0.5) and 66.0 kJ mol-1 (R ) 0.5–0.9). In order to compare these samples, the medium value was considered for all conversion degrees between 1 and 90%, and the results were
Figure 7. Activation energy along the first stage of oxidative degradation, the low-temperature oxidation region (LTO), of samples A ((), B (×), and C (•).
the following: sample A ) 43.8 kJ mol-1, sample B ) 57.2 kJ mol-1, and sample C ) 61.8 kJ mol-1. 5. Conclusions TG/DTG and DTA curves obtained for three different heavy oils in oxidative atmosphere have demonstrated that there are three groups of reactions occurring in different zones of temperatures. Although the heating rate affects the intervals of distinct reaction regions and peak and burn-out temperatures, the characteristics related to the thermal oxidation behavior of the TG and DTG curves for each sample are maintained. The first region was identified and called low-temperature oxidation (LTO), the second transition was designated as fuel deposition (FD), and the final reaction was called high-temperature oxidation (HTO). This knowledge provides essential data for thermochemical and kinetic studies for these groups of reactions. Besides differentiating the oils, the kinetic parameter Ea, related to thermal oxidation, suggests a peculiar behavior mainly regarding the combustion quality of the fuel. According to eq 1, the shorter the ignition delay, the lower the Ea and, consequently, the better the fuel. Starting from this principle and considering only the low-temperature oxidation region (LTO), sample A (MF 380) is the best option in the combustion process among the samples studied in this work. Acknowledgment. The authors would like to acknowledge Petrobras–Cenpes for supplying the samples and Capes and Fapesp (process no. 04/06894-4) for the financial support given to this research. They are also indebted to Dr. I. Ávila for her collaboration. EF700219U
