Energy & Fuels 2002, 16, 785-790
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Effect of Particle Size and Heating Rate on the Combustion of Silopi Asphaltite N. Emre Altun,*,† Mustafa Versan Kok,‡ and Cahit Hicyilmaz† Department of Mining Engineering, Middle East Technical University, 06531, Ankara, Turkey, and Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received October 22, 2001. Revised Manuscript Received February 12, 2002
In this study, the effects of particle size and heating rate on the combustion properties of Silopi asphaltite were investigated. Nonisothermal thermogravimetry experiments were carried out at three different size fractions and five different heating rates. The TG/DTG experiments were carried out from ambient to 900 °C in air. The data obtained were analyzed for the determination of the combustion characteristics of the sample. Also an Arrhenius type kinetic model complemented with the weighed mean activation energy concept was utilized to find out the kinetics of the reactions.
Introduction Turkey has large sources of asphaltic materials and asphaltite, especially in the Southeastern Region of Turkey, around Siirt, S¸ ırnak, and Hakkari. The amount of asphaltite reserves in this region is approximately 77.5 million tonnes.1 It is known that asphaltite is originated from petroleum via metamorphism. It is a hard, black-colored material with a high melting point of about 200-315 °C, mainly composed of hydrocarbons.2 Generally, it is associated with trace metals such as molybdenum, vanadium, nickel, and uranium. Despite the several ways and alternatives for asphaltite utilization, it has been widely used as a supplement to lignite for domestic heating in the Southeastern Region of Turkey since the 1970s. Several studies have been performed not only on the occurrence, reserves and reserve characteristics of Southeastern Asphaltites of Turkey,2,3 but also on the evaluation of them1,4,5 as a source of molybdenum and nickel, for ammoniac production, liquefaction, etc. However, the studies related to the combustion of asphaltite as a fossil fuel is limited. Therefore, this research aims to investigate the combustion characteristics and reaction kinetics of asphaltite and the effects of particle size and heating rates. * Corresponding author. † Department of Mining Engineering, Middle East Technical University. ‡ Department of Petroleum and Natural Gas Engineering, Middle East Technical University. (1) Hamamci, C.; Kahraman, F.; Du¨z, M. Z. Desulfurization of Southeastern Anatolian Asphaltites by the Meyers Methodology. Fuel Process. Technol. 1997, 50, 171-177. (2) Orhun, F. The Characteristics, Metamorphism Degrees and Classification Problems of Asphaltic Materials in Southeastern Turkey. MTA J. 1969, 72, 146-157. (3) Go¨nenc¸ , O. Asphaltites and Asphaltite Reserves in Turkey. MTA Report, 1990. (4) Erol, M.; Demirel, B.; Togˇrul, T.; C¸ alımlı, A. Separation of Organic Matter from Asphaltite with Supercritical Fluid Mixtures. Fuel Process. Technol. 1995, 41, 199-206.
Table 1. Proximate and Elemental Analysis of Silopi Asphaltitea proximate analysis wt (%) (air-dry) moisture ash volatile matter fixed carbon calorific value a
0.88 35.93 48.86 13.20
elemental analysis wt (%) (air-dry) C H S (N.O)
54.22 5.07 8.23 0.25
5540 kcal/kg
Ref 3.
Experimental Section During the experiments, Silopi asphaltite was used. The sample was crushed into three size fractions of +6, -6+10, and -10+20 mesh to be combusted by thermal analysis methods in a TG/DTG analyzer. Thermogravimetry and differential thermogravimetry (TG/DTG) is one of the most common thermal analysis techniques, that measures the weight loss of a sample as a function of temperature and/or time. Therefore, the TG/DTG curves obtained can be used to determine and measure any reaction involving mass change at different temperature/time intervals for a variety of samples. The method can be successfully utilized for the determination of combustion characteristics, reaction kinetics, and mechanisms for fuels, industrial minerals, etc. Proximate and elemental analyses of the Silopi asphaltite are given in Table 1. The TG experiments were carried out by using a Polymer Laboratories PL-TGA 1500 Analyzer (Figure 1), to obtain the data required to determine the combustion and kinetic characteristics of the sample. Calorific values of the samples were measured by using a Parr oxygen bomb calorimeter according to the ASTM D5865-01ae1 standard test method.6 The procedure of the TG/DTG experiments requires placing the sample in the sample pan, setting the heating rate and the amount of airflow to be supplied, and finally commencing the experiment. During the experiments, representative samples (5) Bolat, E.; Kavlak, C ¸ .; Yalın, G.; Dinc¸ er, S. Liquefaction of Turkish Lignites and Asphaltites with a Turkish Vacuum Residual Oil. Fuel Process. Technol. 1992, 31, 55-67. (6) ASTM D5865-01ae1. Standard Test Method for Gross Calorific Value of Coal and Coke. American Society for Testing and Materials, 2001.
10.1021/ef0102519 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002
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Figure 1. Schematic form of the TG/DTG analyzer. Table 2. Combustion Characteristics of Silopi Asphaltite at Different Heating Rates residue (%) 5 °C/min 10 °C/min 15 °C/min 20 °C/min 25 °C/min 25 °C/min (with 22.5 mL/min air) 5 °C/min 10 °C/min 15 °C/min 20 °C/min 25 °C/min 25 °C/min (with 22.5 mL/min air) 5 °C/min 10 °C/min 15 °C/min 20 °C/min 25 °C/min 25 °C/min (with 22.5 mL/min air)
max peak temp (°C)
burn-out temp (°C)
+6# Fraction 31.6 32.4 33.0 33.7 35.4 35.1
410 438 460 468 479 481
751 783 809 829 847 850
-6+10# Fraction 31.9 32.5 33.6 34.8 35.7 35.5
413 443 467 479 490 491
763 814 830 840 852 854
-10+20# Fraction 32.1 418 32.7 452 33.9 475 35.0 486 36.9 508 36.8 510
780 829 846 860 874 876
of approximately 25 mg from three different fractions of +6, -6+10, and -10+20 mesh sizes were combusted at five different heating rates of 5, 10, 15, 20, and 25 °C/min. All the experiments were carried out with a uniform air flow rate of 15 mL/min from room temperature to 900 °C. Prior to the experiments, the PL TGA 1500 Analyzer was calibrated in order to obtain dependable weight and temperature change data. All the experiments were repeated twice and reproducible results have been successfully obtained.
Results and Discussion Theoretically, the combustion of all fossil fuels is initiated when these contact with oxygen at a certain temperature. Although asphaltite is being utilized as an alternative to coal for domestic heating, the difference in the oxygen contents of coal (∼2-23%) and asphaltite, which involves oxygen or oxygen compounds in very little amounts (Table 1), is one of the important facts showing its petroleum origin.2,7
Table 3. Combustion Characteristics of Silopi Asphaltite According to Different Size fractions heating rate 5 °C/min +6 # -6+10 # -10+20 # 10 °C/min +6 # -6+10 # -10+20 # 15 °C/min +6 # -6+10 # -10+20 # 20 °C/min +6 # -6+10 # -10+20 # 25 °C/min +6 # -6+10 # -10+20 #
residue (%)
max peak temp (°C)
burn-out temp (°C)
31.6 31.9 32.1
410 413 418
751 763 780
32.4 32.5 32.7
438 443 452
783 814 829
33.0 33.6 33.9
460 467 475
809 830 846
33.7 34.8 35.0
468 479 486
829 840 860
35.4 35.7 36.9
479 490 508
847 852 874
Table 4. Calorific Values of Silopi Asphaltite According to Particle Size particle size (mesh)
calorific value (kcal/kg)
+6 -6+10 -10+20
5650 5413 5138
According to the TG/DTG curves of Silopi asphaltite (Figures 2-7), the residue at the end of the reactions increased by the increase in the heating rate at all three fractions (Table 2). At first, it is thought that, airflow of 15 mL/min could have been an insufficient amount for the combustion of asphaltite and might have been responsible for the increase in the residue as the heating rate is increased. To test such a possibility, the air supply was increased 50% from 15 to 22.5 mL/min and the TG/DTG experiments were carried out with this amount for each size fraction at a heating rate of 25 °C/min. However, it is seen that negligible changes occurred and the increase in the air supply proved to (7) Bryers, R. W. Investigation of the Reactivity of Macerals Using Thermal Analysis. Fuel Process. Technol. 1995, 44, 25-54.
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Figure 2. TG curves of +6 mesh fraction at different heating rates.
Figure 3. TG curves of -6+10 mesh fraction at different heating rates.
be ineffective on the amount of residue and other thermal properties (Table 2). It can be said that, due to the petroleum origin of asphaltite, as the heating rate is increased, the melting of the material occurs faster and this melting results in the filling of the pores inside the asphaltite structure. As the pores that act as pipes for the passing of oxygen through the body are filled, the contact of oxygen with the combustible material becomes harder and less in-situ reaction takes place, resulting in a decrease in the efficiency of combustion; consequently, more residue is left. Also, this may be a result of the self-heating of the sample pile in the pan, that might have occurred more slowly at lower heating rates and therefore having resulted in a more effective reaction providing a more efficient combustion. In the TG/DTG curves, maximum peak temperature is the point where the rate of weight loss with respect to time is at a maximum. In the study, it is observed that the maximum peak temperature increased as the heating rate is increased from 5 to 25 °C/min. Another important
point about the maximum peak temperatures is that as the heating rate is increased, the weight losses occurring at the maximum peak temperatures decreased, thus, the slower is the heating rate, the greater is the weight loss at the maximum peak temperatures (Table 2). This may again be a result of the longer reaction time allowed and slower combustion at lower heating rates. Also, this is another evidence showing that the combustion efficiency of asphaltite increases as the heating rate is decreased. Finally, the burn-out temperatures of all fractions increased with the increase in the heating rate (Table 2). When the effect of particle size is considered, the TG/ DTG graphs showed that as the particle size is reduced, the residue at the end of the reaction increases (Table 3). Furthermore, it is observed that the maximum peak temperatures incerased with a decrease in the particle size. As the particle size is decreased from +6 to -10+20 mesh size, the burn-out temperatures increased (Table 3). Finally, the calorific values of the samples showed a
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Figure 4. TG curves of -10+20 mesh fraction at different heating rates.
Figure 5. DTG curves of +6 mesh fraction at different heating rates.
decrease from 5650 to 5138 kcal/kg as the particle size is decreased (Table 4). It is obvious that, unlike the combustion characteristics and TG/DTG data of many Turkish lignites,8-10 in the TG graphs of asphaltite, the regions related to the weight losses due to the evaporation of body moisture and the loss of volatile matter is not clear. Parallel to this, no primary weight loss peaks related to the moisture and volatile matter losses can be observed in the DTG graphs of asphaltite. The main reaction region, in which the major weight loss occurs, begins after approximately 290 °C as soon as the (8) Kok, V. M.; O ¨ zbas¸ , E.; Hic¸ yilmaz, C. Effect of Particle Size on the Thermal and Combustion Properties of Coal. Thermochim. Acta 1997, 302, 125-130. (9) Altun, N. E.; Hic¸ yilmaz, C.; Ko¨k, V. M. Effect of Different Binders on the Combustion Properties of Lignite Part 1: Effect on Thermal Properties. J. Thermal Anal. Calorimetry 2001, 65, 787-795. (10) Altun, N. E.; Ko¨k, V. M.; Hic¸ yilmaz, C. Effect of Different Binders on the Combustion Properties of Lignite Part 2: Effect on Kinetics. J. Thermal Anal. Calorimetry 2001, 65, 797-804.
melting of asphaltite takes place. In this main reaction region, weight losses occur as a result of the combustion of hydrocarbons involved. It can be concluded that, being originated from petroleum, melting of asphaltite is a prerequisite for the initiation of the combustion of it. This fact appears to be one of the most important differences between the combustion of coal and asphaltite. Kinetic Analysis. Since an Arrhenius type kinetic model is one of the most useful concepts reflecting the reaction kinetics of the samples from the TG/DTG experiment data successfully, the kinetic characteristics of the Silopi Asphaltite is analyzed and determined by using this method. According to the model:
dw -E n ) Ar exp w dt RT
( )
(1)
where dw/dt shows the rate of weight change of the reacting material, Ar is the Arrhenius Constant, E is
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Figure 6. DTG curves of -6+10 mesh fraction at different heating rates.
Figure 7. DTG curves of -10+20 mesh fraction at different heating rates.
the activation energy, T is the temperature, R is the gas constant, and n is the reaction order. For analyzing the TG/DTG data, the model assumes that the rate of weight loss of the total sample depends only on the rate constant, the weight of the sample remaining, and the temperature with assumed unity reaction order. Thus, the equation becomes
1 dw -E ) Ar exp w dt RT
( )
( )
(2)
If the logarithm of both sides is taken, the equation takes the following form:
log
E ) log Ar [w1 (dw dt )] 2.303RT
(3)
When log[1/w(dw/dt)] is plotted against1/T, a straight line will be obtained with a slope of -E/2.303R and an intercept of the Arrhenius Constant.8
Similar to the combustion of coal, the individual activation energies for each reaction region can be notionally attributed to different reaction mechanisms. However, they do not give any indication of the contribution of each region to the overall reactivity of the sample, so the Arrhenius type kinetic model is complemented with the concept of weighted mean activation energy, Ewm, and this was applied to determine the overall reactivity of the sample:11
Ewm ) F1E1 + F2E2 + F3E3 + ... + FnEn
(4)
where F1, F2, ..., Fn are the mass fractions of the combustible content of the sample burned during each region of Arrhenius linearity, and E1, E2, ..., En are the individual apparent activation energies obtained over each region of Arrhenius linearity. Figure 8 shows the (11) Cumming, J. W. Reactivity Assessment of Coals via a Weighted Mean Activation Energy. Fuel 1984, 63, 1436-1440.
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Figure 8. log (1/w)(dw/dt) vs 1000(1/T) curves of +6 mesh fraction at a heating rate of 5 °C/min and the main reaction regions. Table 5. Activation Energies of Silopi Asphaltite According to Particle Size and Heating Rate heating rate 5 °C/min +6 # -6+10 # -10+20 # 10 °C/min +6 # -6+10 # -10+20 # 15 °C/min +6 # -6+10 # -10+20 # 20 °C/min +6 # -6+10 # 25 °C/min +6 # -6+10 # -10+20 #
activation energy (kJ/mol) 45.8 46.1 46.5 46.2 46.4 46.8 46.7 47.0 47.4 47.1 47.5 47.9 47.6 48.1 48.6
log(1/w)(dw/dt) vs 1000(1/T) curve for the +6 mesh fraction combusted at 5 °C/min and the application of weighted mean activation energy concept with respect to the reaction regions. The activation energy is the measure of the liability of a sample to begin combusting and complete the reaction. In Table 5, the activation energies of the asphaltite samples are given. As seen from this table, the activation energies of the asphaltite samples are much higher when compared with coal whose activation energy was found to vary between ∼9-29 kJ/mol in a number of studies.8-10,12 Thus, when compared with coal, these results show the difficulty of initiating the
combustion of the asphaltite to be due to its high melting point. It is observed that, as the particle size decreased from +6 to -10+20 mesh, the activation energies and therefore the difficulty of the asphaltite to combust increased. Also, the activation energies of each fraction became greater as the heating rate is increased from 5 to 25 °C/min. Conclusion It is clear that despite its usage as an alternative fuel to coal domestically, it is more difficult to initiate the combustion of asphaltite when compared with it. The combustion characteristics of the Silopi asphaltite improved as the heating rate is decreased, and the most efficient combustion results for all the size fractions were obtained at a heating rate of 5 °C/min. Also, the +6 mesh fraction, which is the largest size among the samples of the study, gave the best combustion results. Another point is that, as the particle size is decreased and heating rate is increased, the kinetic energies of the samples also increased and initiating the combustion was more difficult. As a result, asphaltite seems to have some disadvantages as a heating agent with respect to coal. However, in the case of controlled heating and a successful reaction initiation, it may be an alternative. The comparison of the calorific values of asphaltite and coal may be another approach for the quality of asphaltite as a supply for heating purposes, but the emission of harmful gases and their amounts shall be taken into consideration, too. EF0102519 (12) Ozbas, K. E.; Hicyilmaz, C.; Ko¨k, V. M.; Bilgen, S. Effect of Cleaning Process on Combustion Characteristics of Lignite. Fuel Process. Technol. 2000, 64, 211-220.
