Kinetic Modeling of Scrap Tire Pyrolysis - Energy & Fuels (ACS

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Energy & Fuels 1999, 13, 421-427

421

Kinetic Modeling of Scrap Tire Pyrolysis D. Y. C. Leung* and C. L. Wang Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Received May 14, 1998

In this paper we have studied the kinetics of the thermal decomposition of scrap tire using nonisothermal thermogravimetric methods. Experiments were carried out over the temperature range of 20-600 °C at different heating rates and with various powder sizes. Under the conditions studied, the heating rate has a significant effect on the pyrolysis process while the powder size has little effect on it. The results indicated that the pyrolysis process of tire powder consisted of three stages which are related to the degradation of different tire materials. Two models were developed to simulate this pyrolysis process and to determine the kinetic and process parameters. The predictions by the two models agree well with experimental data.

Introduction Due to the tremendous increase in vehicle usage, the generation rate of scrap tire is ever increasing. About one waste tire is generated per person annually in developed countries.1 Currently, most of the waste tires are disposed of by landfilling or stockpiling. The remains go into a variety of reuse/recycle options, such as being used as a filler in asphalt road pavement, a raw material for the production of secondary products, reclaim rubber, artificial reefs, barriers, and breakwaters etc.2,3 On the other hand, tire is made of rubbery materials in the form CxHy. It has high volatile and carbon contents, and its heating value is higher than that of coal. This makes it a good material for energy recovery if proper technologies are used.3,4 Pyrolysis, incineration/combustion,5-7 co-combustion with coal or other fuels,8,9 and gasification processes are considered to be attractive and practicable methods for recovering energy from scrap tire. Incineration is the direct combustion of chopped or whole tire and has long been adopted in cement kilns.2,3,10 There are numerous studies on the direct combustion of tire. Atal and Levendis5,6 studied the combustion behavior of pulverized waste tires and coal. Several power plants using waste tires as the primary fuel have been commissioned (1) Lee, J. S.; Kim, S. D. Energy 1996, 21, 343-352. (2) Leung, D. Y. C.; Lam, G. C. K. Proceedings of the Third AsianPacific International Symposium on Combustion and Energy Utilization; 1995; pp 11-15. (3) Wu, J. S.; Vallabhapuram, R. Hazardous Waste Management Handbook; Prentice-Hall: Englewood Cliffs, NJ, 1994; pp 170-179. (4) Leung, D. Y. C.; Xu, B. Y.; Wu, C. Z.; Luo, Z. F.; Yin, X. L.; Lui, P. Proceedings of the Asia-pacific Conference on Sustainable Energy and Environmental Technology; 1996; pp 494-500. (5) Atal, A.; Levendis, Y. A. Fuel 1995, 74, 1570-1581. (6) Levendis, Y. A.; Atal, A.; Carlson, J.; Dunayevskiy, Y.; Vouros, P. Environ. Sci. Technol. 1996, 30, 2742-2754. (7) Kim, J. R.; Lee, J. S.; Kim, S. D. Energy 1994, 845-854. (8) Tang, Y.; Curtis, C. W. Fuel Proc. Technol. 1996, 46, 195-215. (9) Tzan, D. Y.; Juch, C. Proceedings of the Third Asia-pacific International Symposium on Combustion and Energy Utilization; 1995; pp 115-121. (10) Teng, H.; Chyang, C. S.; Shang, S. H.; Ho, J. A. J. Air Waste Manage. Assoc. 1997, 47, 49-57.

in Europe and the United States.3 Gasification is the process of converting tire into a mixture of combustible gases with the use of air, oxygen, or steam.1,4 Tire pyrolysis is an incomplete thermal degradation process, generally conducted in the absence of air, which results in liquid and gaseous hydrocarbons and char residual.11-18 These products can be converted for different uses. For instance, the char can be converted to carbon black or activated carbon,19-23 which are widely used chemical products. The liquid hydrocarbons can be converted to some high-value compounds such as oil, limonene, and benzene,24-26 while the gaseous products can be used directly as fuel. Due to the high potential of producing various usable products, tire pyrolysis has received much attention in recent years. The present study is also focused on this issue. Thermogravimetric analysis (TGA), a measurement of weight loss of a sample as a function of time and temperature, has been widely used for many years to (11) Yang, J.; Tanguy, P. A.; Roy, C. AIChE J. 1995, 41, 1500-1512. (12) Yang, J.; Tanguy, P. A.; Roy, C. Chem. Eng. Sci. 1995, 50, 1909-1922. (13) Bouvier, J. M.; Charbel, F.; Gelus, M. Resour. Conserv. 1987, 15, 205-214. (14) Araki, T.; Niikawa, K.; Hosoda, H.; Nishizaki, H.; Mitsui, S.; Endoh, K.; Yoshida, K. Conserv. Recycling 1979, 3, 155-164. (15) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 14741482. (16) Williams, P. T.; Besler, S.; Taylor, D. T.; Bottrill, R. P. J. Inst. Energy 1995, 68, 11-21. (17) Lee, J. M.; Lee, J. S.; Kim, J. R.; Kim, S. D. Energy 1995, 20, 969-976. (18) Kaminsky, W. Resour. Recovery Conserv. 1980, 5, 205-216. (19) Teng, H.; Serio, M. A.; Wojtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Ind. Eng. Chem. Res. 1995, 34, 3102-3111. (20) Merchant, A. A.; Petrich, M. A. Chem. Eng. Commun. 1992, 118, 251-263. (21) Merchant, A. A.; Petrich, M. A. AIChE J. 1993, 39, 1370-1376. (22) Giavarini, C. Fuel 1985, 64, 1331-1332. (23) Ogasawara, S.; Kuroda, M.; Wakao, N. Ind. Eng. Chem. Res. 1987, 26, 2552-2556. (24) Ani, F. N. H.; Zailani, R. Proceedings of the Asia-Pacific conference on sustainable energy and environmental technology; 1996; pp 487-493. (25) Roy, C.; Labrecque, B.; Caumia, B. D. Resour. Conserv. Recycling 1990, 4, 203-213. (26) Bouvier, J. M.; Gelus, M. Resour. Conserv. 1986, 12, 77-93.

10.1021/ef980124l CCC: $18.00 © 1999 American Chemical Society Published on Web 01/16/1999

422 Energy & Fuels, Vol. 13, No. 2, 1999

study the behavior of material pyrolysis.12,24,27-33 Under well-controlled conditions, the heat- and mass-transfer limitations between the sample and the supporting pan and between the sample and the purge gas can be neglected with the use of fine particles (diameter < 2 mm) and a small amount of sample (sample weight < 10 mg).32,33 So the entire pyrolysis reaction rate can be limited to kinetic rate-controlled conditions. This allows the pyrolysis process to be investigated over the entire temperature range. Several studies have been conducted to investigate the pyrolysis of scrap tire in both laboratory and industrial scale. Boukadir et al.34 found that the mechanism of rubber degradation is a two-step reaction with a reaction order of 1-1.5 for the first step and 3 for the second under isothermal conditions. However, they did not find a correct activation energy for the process. Bouvier et al.13 reported that rubber degradation is a one-step mechanism and proposed it to be a first-order reaction. They obtained the apparent activation energy and frequency factor using TGA. Kim et al.28 proposed that three constituents of sidewall and tread rubber of tire might undergo irreversible first-order degradation independently. Chen and Yeh35 investigated the styrene-butadiene rubber using nitrogen as the purge gas with different oxygen contents and obtained its apparent activation energy, frequency factor, and reaction order. Yang et al.31 identified and quantified elastomers and processing oil in tire rubber by derivative thermogravimetric (DTG) curve simulation. The kinetic parameters which have been given for each straight elastomer and processing oil are very useful for TGA curve simulation and numerical modeling of tire pyrolysis. Teng et al.19 used a three-lump model to evaluate the volatile evolution when granulated tires undergo a pyrolysis process. The model assumed that the three reactions are independent and parallel first-order reactions. Other researchers have paid much attention to the weight-loss characteristics and elastomer identification during the pyrolysis of tire,24,27,29,33,36 while the effects of powder size have not been studied in detail. Leung and Wang37 investigated the kinetic behaviors of the pyrolysis and combustion process of scrap tire using TGA and DTG methods. A thermal degradation model was proposed to derive the kinetic parameters, which agreed well with experimental data. However, this model only took into account the reactions of two components occurring at the temperature ranges of (27) Brazier, D. W.; Nickel, G. H. Rubber Chem. Technol. 1975, 48, 661-677. (28) Kim, S. D.; Park, J. K.; Chun, H. D. J. Environ. Eng. 1995, 121, 507-514. (29) Sircar, A. K.; Lamond, T. G. J. Appl. Polym. Sci. 1973, 17, 2569-2577. (30) Sircar, A. K.; Lamond, T. G. Rubber Chem. Technol. 1974, 48, 301-309. (31) Yang, J.; Kaliaguine, S.; Roy, C. Rubber Chem. Technol. 1993, 66, 213-229. (32) Wendlandt, W. W. Thermal Analysis; 1992, Min 8, Behai Hall, Taiwan. (33) Menis, O.; Rook, H. L.; Garn, P. D. The State-of-the-Art of Thermal Analysis; U.S. Government Printing Office: Washington, DC, 1980. (34) Boukadir, D.; David, J. C.; Granger, R.; Vergnaud, J. J. Anal. Appl. Pyrol. 1981, 3, 83-89. (35) Chen, K. S.; Yeh, R. Z. Combust. Flame 1997, 180, 408-418. (36) Tzan, D. Y. L.; Juch, C. I. 3rd Asian-Pacific international symposium on combustion and energy utilization; 1995; pp 115-121. (37) Leung, D. Y. C.; Wang, C. L. J. Anal. Appl. Pyrol. 1998, 45, 153-169.

Leung and Wang

300-420 and 350-500 °C, but the effect of moisture, oil, plasticizers, additives, etc., were neglected. As a result, the plateau peak in the DTG curve of tire pyrolysis at the temperature range of 150-350 °C was not considered. In this paper, we propose two new models, namely, the three-component-simulation model and three-elastomer-simulation model, to simulate the kinetic and process parameters of tire pyrolysis. The former model is an extension of the model developed by the authors in the previous study,37 and the effects of moisture, oil, plasticizers, and additives, etc. were considered. The effects of heating rate and particle size on the pyrolysis kinetics were also studied and discussed. Simulation Theory The pyrolysis reaction can be represented by the following solid pyrolysis equation derived by Vachuska and Voboril:32

dRi ) Ki(1 - Ri)ni dt

(1)

Since the degradation commences at a temperature lower than 800 °C which is within the temperature range of the kinetic reaction, the rate constant Ki can be obtained by Arrhenius’ Law as:

Ki ) Ai exp(-Ei/RT)

(2)

Using eqs 1 and 2, the kinetic parameters could be derived from the TG and DTG curves obtained from experiment. With these parameters and the known heating rate (β), the normalized weight loss ratio (R) for a temperature range 0-T K could be calculated theoretically by integrating eq 1 with the use of eq 2 as follows:

( )

A

dR

E

∫0R (1 - Ri )n ) βi∫0Texp - RTi dT i

i

i

(3)

The right-hand side of the above equation has no exact integral, but by using the relation of Coats and Redfern,38 it can be approximated as:

Ai

( ) Ei

Ai RT2

T exp dT ≈ ∫ 0 β RT β

Ei

( )∑

exp -

Ei

RT

∞

(-2)k

k k)0(E /RT) i

(4) Hence, with the use of eq 4 the solution of eq 3 for a first-order reaction (ni ) 1) is

[

Ri ) 1 - exp -

Ai RT2 β Ei

( )∑

exp -

Ei

RT

∞

(-2)k

]

k k)0(E /RT) i

(5)

while the solution for a second-order reaction (ni ) 2) is (38) Coats, A. W.; Redfern, J. P. Nature 1964, 201, 68-69.

Kinetic Modeling of Scrap Tire Pyrolysis

[

Ri ) 1 - 1 +

Ai RT2 β Ei

( )∑

exp -

Ei

RT

∞

(-2)k

]

k k)0(E /RT) i

Energy & Fuels, Vol. 13, No. 2, 1999 423

-1

(6)

From the above equations, the pyrolysis process parameters, such as R, dR/dt, etc., could be predicted theoretically if the kinetic parameters (E, A, and n) are known in addition to the temperature range and heating rate.

ln

dRT dt

dRi

3

∑ i)1 dt

) ln

[∑ ( ) ] 3

) ln

Ai exp -

i)1

Ei

RT

(1 - Ri)

(8)

Since only component 1 degrades at the lower temperature region (dR2/dt ) 0, dR3/dt ) 0, dR1/dt ) dRT/dt, R2 ) 0, R3 ) 0, and R1 ) RT), eq 8 can be simplified as

ln

[

]

dR1 /(1 - R1) ) ln A1 - E1/RT dt

(9)

Simulation Models Tire rubber is composed of various compounds such as elastomers, carbon black, processing oil, plasticizer, vulcanization agents, and a few additives. The usual types of elastomers used in tire product are natural rubber (NR), butadiene rubber (BR), and styrenebutadiene rubbers (SBR). Different compositions of these straight elastomers in tire rubber show different pyrolysis behaviors since many complex reactions are involved. Therefore, it is impossible to develop a precise kinetic model for determining various kinetic parameters from thermogravimetric data alone. It is generally accepted that the more important parameters in pyrolysis are temperature, heating rate, and sample weight and its loss rate. On the basis of these parameters, two models, namely, the three-component-simulation model and three-elastomer-simulation model, are proposed to predict the weight-loss fraction and weightloss rate profiles of tire powder during the pyrolysis process. These two models are described in the following sections. (i) Three-Component-Simulation Model. This model assumed that tire powder is comprised of only three major degradable components. Component 1 is assumed to be consisting of a mixture of oil, moisture, plasticizer, and additives, while components 2 and 3 consist of NR, BR, SBR, and their combination. Each component decomposes at different temperature regions, producing volatile, gases, and char. The pyrolysis rate is treated as the sum of these three component degradation rates. A schematic diagram of the model is shown below:

It is further assumed that each of these three components undergoes an irreversible first-order degradation independently and follows Arrehenius’ Law for ascertaining the values of the rate constant. In addition, the degradation of component 1 occurs at a lower temperature region while that of components 2 and 3 occur at medium and higher temperature regions, respectively. Experimental results indicated that these three components’ degradations partially overlap in the temperature ranges of 280-350 and 350-440 °C. The kinetics of tire rubber pyrolysis can be presented as

dRT dt

3

)

dRi

3

( ) Ei

Ai exp (1 - Ri) ∑ ∑ RT i)1 dt i)1 )

The plot of the left-hand side of eq 9 versus the reciprocal of temperature would result in a straight line of slope of -E1/R and a Y-intercept of ln A1. With the value of E1 and A1 obtained above, R1 can be calculated using eq 5. Similar procedures can be applied to component 2 and component 3 degradation to obtain E2, A2, R2, E3, A3, and R3. Thus, the whole process parameters of RT and dRT/dt can be simulated after obtaining the kinetic and process parameters of each component. (ii) Three-Elastomer-Simulation Model. In this model, the elastomers NR, BR, and SBR are considered as the three main components of tire rubber, each of which has an oil component. In the tire powder pyrolysis process, the oil component is degraded totally while the carbon black and ash contained in the elastomers are considered as undegradable compounds and remain in the final product. Thus, according to the above simulation theory, the kinetics of tire powder can be described as consisting of the three elastomer components and three processing oil parts. The kinetic equations are shown as

dRT dt

6

)

dRi

∑ i)1 dt 6

)

( ) Ei

Ai exp ∑ RT i)1

(WT0 - WT∞)ni-1(Ri∞ - Ri)ni (10)

i

where ni is either equal to 1 or 2 for each component. By means of eqs 5, 6, and 10, the curve simulation of the weight-loss fraction (TG curve) and weight-loss rate (DTG curve) can be performed by assuming that each component undergoes thermal degradation independently and the degradation processes happen simultaneously or overlap partially. A schematic diagram of the model is shown below

(7)

Taking natural logarithm on both sides of eq 7 yields

In this model, the kinetic parameters (E, A, and n) and the weight fractions of processing oil and char contained in elastomers are either assumed or calcu-

424 Energy & Fuels, Vol. 13, No. 2, 1999

Leung and Wang

Figure 1. TG and DTG curve of tire powder pyrolysis.

lated according to the results obtained by Yang et and the present experiment.

al.31

Figure 2. Normalized weight loss rate at different temperatures for different tire powder sizes.

Experimental Section The characteristics of the pyrolysis process depend heavily on the properties of the tire samples. Hence, their physical, chemical, and thermal characteristics are essential for our understanding of the pyrolysis behavior. The tire powder used in this study, supplied by the Recycling Plant of Guangzhou in China, was produced from granulating scrap tire into four different sizes: 1.18-2.36 (8-16 mesh), 1.0-1.18 (16 mesh), 0.5-0.6 (30 mesh), and 0.355-0.425 mm (40 mesh). Reinforced fibers and steel belts are extracted and removed from scrap tires during the production of the tire powder. The plant produces a large amount of tire powder everyday from both truck tires and car tires, which are well mixed before transport. Since only a small quantity of tire powder was used in the experiment, which was randomly taken from large samples, therefore, their compositions are assumed homogeneous in the present experiment. A proximate analysis of the tire powder indicated that it contains 64.2% volatile, 27.8% fixed carbon, 7% ash, and 1% moisture. The thermogravimetric experiments of tire powder were conducted using a Stanton Redcroft STA1500 Simultaneous Thermal Analyzer. About 8 mg of sample was placed in a platinum pan and heated in an inert atmosphere of nitrogen gas over a temperature range of 20-600 °C at controlled heating rates of 10, 30, 45, and 60 °C min-1. The choice of this sample weight is based on the optimum kinetic rate controlled conditions. The sample weight-loss percentage and sample temperature were recorded continuously as a function of heating time. So the sample weight-loss percentage is a function of both sample temperature and heating time. From the sample weight-loss percentage, the normalized weight-loss ratio (R) of a sample can be determined. The normalized weight-loss rate of a sample can be obtained by differentiating R with respect to time, which is also a function of both sample temperature and heating time. The tests were repeated for each experimental condition with different samples. It was found that the TG and DTG curves obtained from these experiments repeated well. Therefore, only one of the results for each experimental condition is reported here.

Results and Discussion Figure 1 shows the TG (R) and DTG (dR/dt) curves of tire powder pyrolysis for a sample size of 40 mesh and at a constant heating rate of 30 °C min-1. The TG curve shows continuous loss in weight as temperature rises and becomes level at temperatures greater than 510 °C, indicating completion of the pyrolysis process. On the

Figure 3. Normalized weight loss rate at different temperatures and heating rates.

other hand, the DTG curve exhibits three different weight-loss regions over a temperature range of 100560 °C. As mentioned by Leung and Wang,37 these characteristics may be due to the fact that the constituents of tire, i.e., NR, BR, SBR, or their combination, processing oil, carbon black, and other trace materials (such as additives, plasticizers, vulcanization agents), lose their weight at different rates and at different temperature ranges. It is understood that the moisture inside the tire powder would evaporate before the temperature reached 150 °C. At the temperature range of 150-350 °C, the oil, plasticizer, and additives are lost, giving a plateau over this region (region 1). Further increasing the temperature to 600 °C, the degradation of NR, BR, and SBR gives two peaks (identified as peak 1 and peak 2) in the DTG curve at ∼400 and ∼470 °C, respectively. The temperatures corresponding to these two peaks are affected by heating rates but are less dependent on powder sizes, as indicated in Figures 2 and 3. The DTG curves shown in Figure 2 indicated that increasing powder size decreases the peak 1 and peak 2 temperatures by a small temperature range (0-6 °C). On the other hand, increasing the heating rates from 10 to 60 °C min-1 increases the peak 1 and peak 2 temperatures by about 33 °C (Figure 3). From the above graphs and those not shown here, the pyrolysis process parameters of different sample sizes and at different heating rates can be derived, which are shown in Table 1. Scrutinizing these results, it can be found that the normalized weight-loss rate is much

Kinetic Modeling of Scrap Tire Pyrolysis

Energy & Fuels, Vol. 13, No. 2, 1999 425

Table 1. Analyzed Results of Pyrolysis Process of Scrap Tire Powder heating sample starting temp at temp at finishing reaction NWLRa NWLRa weight loss weight loss total rate at peak 1 at peak 2 size temp peak 1 peak 2 temp time at peak 1 at peak 2 weight loss (°C min-1) (mesh no.) (min-1) (min-1) (°C) (°C) (°C) (°C) (min) (%) (%) (%) 10

30

45

60

a

8∼16 16 30 40 8∼16 16 30 40 8∼16 16 30 40 8∼16 16 30 40

185 185 185 185 250 250 250 250 260 260 260 260 260 260 260 260

383 380 379 381 400 404 401 404 410 411 409 414 410 410 416 413

450 458 458 450 480 479 478 471 491 485 484 492 485 490 484

500 500 500 500 520 520 520 520 545 545 545 525 545 545 545 545

29.9 29.9 29.9 29.9 8.5 8.5 8.5 8.5 5.5 5.5 5.5 5.5 4.2 4.2 4.2 4.2

0.11 0.09 0.10 0.11 0.30 0.31 0.31 0.33 0.59 0.49 0.49 0.52 0.82 0.73 0.77 0.78

0.06 0.07 0.07 0.06 0.19 0.19 0.18 0.17 0.30 0.275 0.25 0.41 0.37 0.35 0.33

28.2 22.8 23.4 25.8 20.5 24.2 21.9 22.9 25.2 20.1 19.4 23.10 17.20 19.49 22.25 20.68

56.9 55.4 56.7 53.3 53.9 56.4 53.9 50.2 54.6 52.2 51.58 52.41 51.57 53.32 50.16

68.7 65.8 65.3 63.7 64.4 66.7 63.7 61.4 66.5 65.2 63.6 61.73 64.15 63.13 63.42 60.52

NWLR: normalized weight-loss rate. Table 2. Kinetic Parameters of the Three-Component-Simulation Model and Three-Elastomer-Simulation Model three-elastomer-simulation model

kinetic parameter E (kJ mol-1) A (min-1) n

three-component-simulation model component 1 component 2 component 3 52.5 2.0 × 104 1

164.5 6.3 × 1013 1

136.1 2.3 × 109 1

oil

NR elastomer

oil

BR elastomer

oil

SBR elastomer

43.3 207.0 43.3 215.0 48.0 152.0 4.5 × 103 3.0 × 1014 2.3 × 103 7.1 × 1014 6.9 × 103 3.1 × 1010 1 2 1 1 1 1

Figure 4. Comparison of the simulation of the threecomponent-simulation model with experimental weight loss at a heating rate of 10 °C min-1.

Figure 5. Comparison of the simulation of the threecomponent-simulation model with experimental normalized weight-loss rate at a heating rate of 10 °C min-1.

larger and the reaction time much shorter at higher than at lower heating rates. However, the total weight loss decreases slightly with increasing heating rate. The size of tire powder is found to produce little effect on the pyrolysis process. The DTG curves in Figures 1-3 clearly show the existence of three regions which may be interpreted as being due to loss of the three components arbitrarily defined by the three-component-simulation model. As to these three components’ degradation, the values of the apparent activation energy and frequency factor can be determined by eqs 7-9. The kinetic parameters thus obtained are tabulated in Table 2. The simulated TG and DTG curves obtained from this model match quite well with experimental data as shown in Figures 4 and 5. It is found from the simulations that the total weight loss is ∼65%, among them only ∼7% is due to the degradation of component 1 (moisture, oil, plasticizer, and additives); ∼47% and ∼46% are due to the degradation of component 2 and component 3, respectively. In

addition, component 1 and component 2 are, respectively, the easiest and the most difficult components to be decomposed according to the values of the apparent activation energy shown in Table 2. Kim et al.28 investigated the pyrolysis process of sidewall and tread rubber derived from waste tire using TGA. They also obtained two peaks in their DTG curves. However, their peak 2 is higher than peak 1 while the weight loss under the peak 1 area is much smaller than that under peak 2. These characteristics are different from the present results, which may be due to the different sample compositions used. Nevertheless, they also divided the degradation process into three temperature regions similar to the present identification. For sidewall rubber pyrolysis, the average apparent activation energy and frequency factor were found to be 42, 195, and 204 kJ mol-1 and 1436, 2.1 × 1015, and 2.0 × 1014 min-1 at the lower, medium, and higher temperature regions, respectively. The corresponding values for tread rubber pyrolysis were 39, 209, and 127

426 Energy & Fuels, Vol. 13, No. 2, 1999

Leung and Wang

Figure 6. Comparison of the simulation of the three-elastomer-simulation model with experimental weight loss at a heating rate of 10 °C min-1.

Figure 7. Comparison of the simulation of the three-elastomer-simulation model with experimental normalized weightloss rate at a heating rate of 10 °C min-1.

kJ mol-1 and 934.5, 3.8 × 1016, and 8.8 × 108 min-1. These values are only slightly different from the present results. Yang et al.31 studied the pyrolysis behaviors of NR, BR, SBR, and their mixture using TGA. They did the experiment at heating rates of 1, 10, and 40 °C min-1 and at a temperature range of 30-550 °C. Nitrogen was used as purge gas with a flow rate of 200 mL min-1 which was almost the same as that used in this study. The apparent activation energies and frequency factors were found to be 43.3 kJ mol-1 and 1.92 × 103 min-1 for the oil in NR and BR and 48 kJ mol-1 and 2.54 × 103 min-1 for the oil in SBR. Furthermore, the apparent activation energies and frequency factors were found to be 207 kJ mol-1 and 2.36 × 1016 min-1 for NR, 215 kJ mol-1 and 6.32 × 1014 min-1 for BR, and 152 kJ mol-1 and 4.15 × 1010 min-1 for SBR. All the above reaction orders were assumed to be 1 except the straight elastomer of NR which was assumed to be 2. These kinetic parameters provide a database for the research of tire rubber composition. Table 2 also shows the kinetic parameters of pyrolysis of NR, BR, and SBR as defined in the three-elastomersimulation model. It should be noted that the values of the apparent activation energy and reaction order were based on the results of Yang et al.31 Moreover, the frequency factors are calculated according to the DTG peak temperature obtained from the present experiment, while the weight fractions of processing oil and char contained in the elastomers were calculated based on the results of proximate analysis conducted, experimental TG and DTG curve analysis, and the results of Yang et al.31 As mentioned earlier, the total weight loss is ∼65% of the total sample weight. Among them, NR, BR, and SBR account for ∼59%, ∼27%, and ∼14%, respectively, for this model. The weight loss of processing oil is ∼7% of the total weight loss, which has been included into the weight loss of the three elastomers mentioned above. The char, including carbon black and other undegradable trace materials, accounts for ∼32% of the total sample weight. The simulation curves obtained from this model are also comparable with experimental results as shown in Figures 6 and 7. The frequency factors obtained by Yang et al.31 are only slightly different from those obtained from our model (Table 2) except for a lower frequency factor of

the elastomer in NR for our model. This may be due to the different tire powder samples used for the two experiments. The values of the apparent activation energy of the three elastomers shown in Table 2 indicated that SBR is the easiest to be degraded, followed by NR and then BR. Compared with NR and BR degradation, it was also found that the higher apparent activation energy accords with component degradation in the higher temperature region. This phenomenon agrees with the results reported by other researchers.28,31 It should be noted that the three-component-simulation model and three-elastomer-simulation model have different applications under different conditions. The three-component-simulation model could be used to calculate kinetic parameters such as the apparent activation energy and frequency factor. With the use of these kinetic parameters, the process parameters, such as the normalized weight-loss ratio and normalized weight-loss rate, can be predicted. Thus, this model is applicable when any information about the composition of the tire powder and the kinetic behavior of the tire powder pyrolysis process is not known. On the other hand, the three-elastomer-simulation model could be used to simulate the normalized weight-loss ratio and normalized weight-loss rate of the whole tire powder pyrolysis process. It can also be used to distinguish approximately the elastomer composition and its proportion in the tire powder when the kinetic parameters of the main basic straight elastomer compositions used in the tire product are known. Conclusions Experimental studies on the pyrolysis of tire powder have been conducted. It was found that the pyrolysis of tire powder exhibits three obvious weight-loss regions occurring at temperature ranges of about 150-350, 280-440, and 350-510 °C, respectively. The average total weight loss is ∼65%. The heating rate has a significant effect on the pyrolysis process. There is a shift in the weight-loss regions to a higher temperature range accompanied by an increasing weight-loss rate with increasing heating rate. Also, the pyrolysis time has been shortened sharply for increasing heating rates, while there is no obvious change in the total weight loss. On the other hand, the size of tire powder has no

Kinetic Modeling of Scrap Tire Pyrolysis

significant effect on the pyrolysis process except a slight shift in the peak temperatures. The two models, namely, the three-component-simulation model and three-elastomer-simulation model, are found to be capable of simulating the tire powder pyrolysis process. The simulations, which match well with the experimental TG and DTG measurements, provide mechanistic insights into the tire powder pyrolysis process. The kinetic parameters and results obtained from the present study could be used to simulate the initial process of tire powder combustion and gasification. Acknowledgment. The authors gratefully acknowledge the Hong Kong Research Grant Council and the CRCG of the University of Hong Kong for supporting this project.

Energy & Fuels, Vol. 13, No. 2, 1999 427

Nomenclature A ) frequency factor (min-1) E ) apparent activation energy (kJ mol-1) K ) rate constant (min-1) R ) gas constant ) 8.314 × 10-3 (kJ mol-1 K-1) t ) reaction time (min) T ) sample temperature (K) W0 ) sample weight at start time (mg) W ) sample weight at time t (mg) W∞ ) sample weight at end time (mg) R ) normalized weight-loss ratio ) (W0 - W)/(W0 - W∞) dR/dt ) normalized weight-loss rate (min-1) β ) heating rate ) dT/dt EF980124L