Gas from the Pyrolysis of Scrap Tires in a Fluidized Bed Reactor

May 12, 1977 - the data obtained by pyrolysis of tires in a Pyroprobe 1000, where secondary tar ... The recycling of plastic and rubber wastes is gain...
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Energy & Fuels 1996, 10, 134-140

Gas from the Pyrolysis of Scrap Tires in a Fluidized Bed Reactor Juan A. Conesa,* Rafael Font, and Antonio Marcilla Department of Chemical Engineering, University of Alicante, Apartado 99, Alicante, Spain Received July 27, 1995X

A fluidized sand bed reactor was used to study the production of gases from scrap tires at four nominal temperatures (ranging from 600 to 900 °C). Yields of 17 pyrolysis products (methane, ethane, ethylene, propane, propylene, acetylene, butane, butylene, 1,3-butadiene, pentane, benzene, toluene, xylenes + styrene, hydrogen, carbon monoxide, carbon dioxide, and hydrogen sulfide) were analyzed as a function of the operating conditions. The results are compared with the data obtained by pyrolysis of tires in a Pyroprobe 1000, where secondary tar cracking is small. Correlations between the products analyzed with those of methane are discussed.

Introduction The recycling of plastic and rubber wastes is gaining increasing importance, as landfilling and combustion becomes more expensive, and acceptance of these methods is decreasing.1,2 On the other hand, most plastics are produced from oil and have a high potential as hydrocarbon sources. In addition to direct recycling of homopolymers, the pyrolysis of mixed or contaminated plastics is an interesting way of reutilizing polymers.3 The world production of tires is increasing continuously, and it was estimated to be around 1.975 × 106 t in the European Community in 1990, and 2.8 × 106 t in the USA in 1992.4 As with other organic compounds, tires decompose when heated. Pyrolysis is governed by the following parameters: temperature, retention time of the volatiles at the reaction zone, and pressure and type of gaseous atmosphere. There are two stages in pyrolysis: primary pyrolysis and secondary cracking. Cracking occurs at higher temperatures and enables primary products to be converted into compounds which may have a higher market value. Work on the pyrolysis of natural rubber first began in the 1920s. Initial research was carried out with the object of regenerating the monomer of natural rubber, isoprene. The effect of pressure was investigated by Standinger and Fritschi.5 These two authors showed that distillation at 275-350 °C and at 0.1 mmHg yields 63.5% of liquid phase, approximately half of which consists of isoprene and its dimer dipentene. Standinger and Geiger6 distilled rubber in an inert atmosphere at 1 atm. Using this method, they managed to increase the weight of the liquid phase, composed mainly of isoprene dimers such as limonene and other * To whom correspondence should be addressed. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) Kaminsky, W. Makromol. Chem. Macromol. Symp. 1992, 57, 145-160. (2) Gray, T. A.; Jeon, D. Proc. 32nd Ind. Waste Conf., May 10-12, 1977. (3) Bevis, M. Mater. Eng. 1982, 3, 344-349. (4) Schenecko, Kautschuk Gummi Kunststoffe 47 Jahrgang 1994, No. 12/94, p 885-890. (5) Standinger; Fritschi. Helv. Chim. Acta 1922, 5, 758. (6) Standinger; Geiger. Helv. Chim. Acta 1946, 9, 549.

0887-0624/96/2510-0134$12.00/0

monocyclic terpenes. The authors also reported the presence of cyclohexadiene and methylcyclohexadiene in the liquid phase. Kato and Someshima7 pyrolyzed a solution of rubber in heavy oils (T ) 310-390 °C, P ) 20-500 atm). Reaction products consisted mainly of cracked oil, bitumen, carbon, and gas. Since then, research has started up in a number of new areas, in an attempt to discover ways of converting rubber waste into liquid combustible and fuel. One method is the aromatization of products generated by primary pyrolysis. This has prompted a considerable number of researchers to carry out investigations into the pyrolysis of isoprene and its dimers. Oro, Han, and Zlatkis,8 for instance, pyrolyzed isoprene at temperatures of between 300 and 1000 °C. The influence of temperature on the composition of the reaction products is considerable. While aliphatic components predominate in the liquid phase at low temperatures, at high temperatures decomposition products are entirely aromatic; they consist principally of naphthalene, methylnaphthalenes, xylenes, and trimethylbenzenes, together with toluene. Pyrolysis produces maximum yields of aromatic components at temperatures of between 700 and 800 °C. It was not until 1974 that interest in the solid phase really developed. Most studies concentrated on the recovery of the solid phase with a view to using it in the treatment of waste water once it had been activated. This aspect has been emphasized by Tanaka and Gomyo,9 Jo and Yoda,10 Songa,11 Ishibashi and Noda,12 Kudo13 and Merchant and Petrich.14,15 (7) Kato; Someshima. J. Soc. Chem. Ind. Jpn. 1935, 38, 596. (8) Oro; Han; Zlatkis. Anal. Chem. 1967, 39, 27. (9) Tanata; Gomyo. Japan, Kokai 7438, 895 (Cl. 14 E 331, 13(9) F2) 11 Apr. 1974, Appli. 7281, 419, 16 Aug. 1972. (10) Jo; Yoda (Nippon Zeon Co, Ltd). Japan Kokai 74, 102, S9S (Cl. 14 E 331, I, 12(9) F2, 91 C 91), 17 Sep. 1974, Appli. 73 14, 588, 05 Feb.1973. (11) Songa. Ger. Offen., 2, 328, 400 (Cl. c 01b) 20 Jan. 1974 (12) Ishibashi; Noda; Terada. Japan Kokai 7545, 799 (Cl. COIB, B0ID), 24 Apr. 1975, Appli. 7396, 413, 27 Aug. 1973. (13) Kudo. Japan Kokai 7575, 593 (Cl. COIB, BOID), 20 Jan. 1975, Appli. 73 126, 013, 09 Nov. 1973. (14) Merchant, A. A.; Petrich, M. A. Chem. Eng. Commun. 1992, 118, 251-263. (15) Merchant, A. A.; Petrich, M. A. AIChE J. 1993, 39(8), 1370.

© 1996 American Chemical Society

Gas from the Pyrolysis of Scrap Tires

A number of pyrolysis methods have been, or are being, developed at pilot or industrial scale.16 In many commercial or pilot plant scale tire pyrolysis processes, the gases have been used to heat the reactor; for example, calorific values between 34.6 and 40.0 MJ/m3 have been reported.17,18 Some authors have pointed out the importance of the residence time in the products from tire pyrolysis. For example, Cypres and Bettems16 detected an increase in the production of naphthalene as the residence time increases, pyrolyzing whole tires in a pyrolysis furnace with postcracking, together with a decrease of the liquid fraction and an increase of benzene. Temperature is, without doubt, the most studied variable. The maximum production of the liquid fraction, for example, was found to be at 415 °C (with a 56.6 wt %) by Roy and Unsworth19 using a vacuum reactor, at 600 °C (40.0%) by Kaminsky and Sinn20 in a fluidized bed reactor, and at 850 °C (32.5%) by Williams et al.21 in a static bath reactor. On the other hand, the maximum oil fraction obtained was 58.8% by Williams et al.,22 53.0% by Kawakami et al.,23 and 23% by Collin.24 Kaminsky and Sinn20 reported an increase in methane, hydrogen and other hydrocarbon gases on increasing the reactor temperature from 640 to 840 °C. In accordance with Williams,21 in a whole tires batch pyrolysis unit, the hydrogen amount in the liquids will decrease if T increases, and will increase in the gas phase.25 The amounts of methane and benzene also increase.26 The char fraction was found to decrease as temperature increases from 300 to 720 °C (Williams21). At low temperatures (below 500 °C), fuel oil is produced but not much gas. At high temperatures, light aromatics and larger quantities of gas are produced. No papers have been found with an exhaustive analysis of the influence of the different operating conditions on the yields of the most important compounds formed. In this work, the gas production from the pyrolysis of scrap tires was studied in a fluidized sand bed reactor, where primary and secondary reactions take place. The (16) Cypres, R.; Bettems, B. In Pyrolysis and Gasification Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London, 1989; p 209. (17) Collin, G.; Grigoleit, G.; Bracker, G. P. Chem. Eng. Technol. 1978, 50(11), 836. (18) Kaminsky, W.; Sinn, H.; Janning, J. Chem. Eng. Technol. 1979, 51(5), 419. (19) Roy, C.; Unsworth, J. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London, 1989. (20) Kaminsky, W.; Sinn, H. In Thermal conversion of solid wastes and biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980. (21) Williams, P. T.; Besler, S.; Taylor, D. T. In Waste: Handling, Processing and Recycling. Paper presented at a Seminar organized by the Environmental Engineering Group of the Institution of Mechanical Engineers, and held at the Institution of Mechanical Engineers on 27 April 1993. (22) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 14741482. (23) Kawakami, S.; Inoue, K.; Tanaka, H.; Sakai, T. In Thermal conversion of solid wastes and biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980. (24) Collin, G. In Thermal conversion of solid wastes and biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980. (25) Kaminsky, W. J. Anal. Appl. Pyrol. 1985, 8, 439-448. (26) Kaminsky, W. Resource Recovery Conservation 1980, 5, 205216.

Energy & Fuels, Vol. 10, No. 1, 1996 135 Table 1. Elemental Analysis of the Tire Used in This Work wt % C H N

83.55 7.81 0.39

wt % S O (by diff) ash

1.48 6.77 8.23

results are compared with those obtained in a Pyroprobe 1000, where the residence time of the volatiles in the hot zone of the probe is very short, and consequently, the secondary reactions take place in a short extension (Garcı´a et al.,27 Conesa et al.28,29 ). The following aspects are considered in this paper: (1) influence of pyrolysis temperature on yields of compounds obtained from the pyrolysis of tires; (2) influence of residence time of volatiles in the hot zone of the reactor on yields of compounds obtained from tire pyrolysis; (3) analysis and discussion of the correlation of the results presented with those obtained by using a Pyroprobe 1000 (analytical apparatus). Experimental Section The experimental procedure and the pyrolysis equipment used are described in a previous paper.28 Pyrolysis was carried out in a cylindrical stainless steel reactor. The diameter of the reactor was 6.9 cm. and the length was 43.2 cm. The inert fluidized bed was sand of 0.105-0.210 mm particle size, calcinated at 900 °C, and washed by an acid solution of HCl. The inert gas used was helium of 99.999% purity. Heating was achieved by means of a refractory oven. The bed surface temperature was controlled automatically at four different temperatures: 600, 700, 800, and 900 °C. The reactor was not isothermal, showing a temperature gradient from the sand bed to the top of the reactor. Two chromel-alumel thermocouples were used to control and measure the temperature profile. Some experiments were carried out in a CDS Pyroprobe 1000. This is a pyrolyzer heated by a platinum filament. The length of the coil is around 1 cm. The weight of the sample varies between 0.5 and 2 mg. A scheme of the apparatus can be found elsewhere.30

Materials and Methods The original tire (Dunlop SP Le Mans 165/60R14 75H) was shredded into small pieces with a mean diameter of 5 mm. Elemental analysis was carried out in a Carlo Erba Instrument Model CHNS-O EA110 and the results are shown in Table 1. The temperature of the oven was 1020 °C, and the combustion was carried out in a pure oxygen atmosphere. The standard used was sulfanilamide and the sample weight was 2 mg. Gas Analysis. The compounds obtained were identified and quantified by a Shimadzu GC-14A gas chromatograph using FID and TCD detectors and four different columns: (a) alumina column (2 m × 1/8 in.) for analyzing methane, ethane, ethylene, propane, propylene, acetylene, butane, butylene, 1,3-butadiene, pentane, and benzene (FID detector); (b) Petrocol A column (20 in. × 1/8 in.) for toluene and xylenes + styrene (FID (27) Garcı´a, A. N.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrol. 1992, 23, 99-119. (28) Conesa, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. Energy Fuels 1994, 8, 1238. (29) Conesa, J. A.; Marcilla, A.; Font, R. J. Anal. Appl. Pyrol. 1994, 30, 101. (30) Caballero, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. J. Anal. Appl. Pyrol. 1993, 27, 221-244.

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detector); (c) Porapack R column (2 m × 1/8 in.) for H2S (TCD detector); and (d) Porapack Q column (4 m × 1/8 in.)for H2, CO, and CO2 (TCD detector). Oil Composition. Liquid column chromatography was used to fraction the oils into their component chemical classes. Glass columns packed with silica, RP18 sorbent, were used, preheated for 2 h at 90 °C. The column was then sequentially eluted with pentane, benzene, ethyl acetate, and methanol to produce aliphatic, aromatic, oxygenated, and polar fractions, respectively, using the method developed by Williams et al.31 Each fraction was dried under nitrogen and weighed to determine the chemical class mass composition. The molecular mass range of the oil was determined by size exclusion chromatography (SEC). The system used tetrahydrofuran as the mobile phase, a minicolumn SEC maintained at 0 °C to minimize solvent-solute interactions and improve efficiency, and a dual detection system incorporating a refractive index detection sensitive to all hydrocarbons and an ultraviolet detector at 254 nm sensitive to aromatic compounds. The SEC was calibrated with standard molecular mass polystyrene samples. The range of the system was from a nominal molecular mass of 50-10000 mass units.

Conesa et al.

Fluidization Conditions. Several fluidization runs were performed at room temperature in a glass tube, in order to select the appropriate sample size and the inert gas flow to be used in the fluidized bed reactor, taking into account the following aspects: Particle Size. Small sizes favor the internal heat transfer within the particle, while on the other hand, in fluidized bed reactors, the particles must be greater than a minimum in order to avoid entrainment of fines. This aspect is not very important when the material has a large density, as in the case of tires. Tires shredded to an average diameter of 5 mm were pyrolyzed. In the glass tube it was observed that pieces of tire and the sand particles mixed rapidly in the upper part of the sand bed. Inert Gas Flow. High flow rates improve the sandsolid mixing and the external heat transfer between the hot bed and the cold solid, while on the other hand, an excessive flow rate may cause a considerable entrainment of fines. Furthermore, the gas flow must be limited in order to allow the volatiles to be cracked in the upper part of the reactor. The minimum fluidization velocity of sand (Umf), at the different operating temperatures was theoretically calculated using the Ergun equation.32 The value Umf slightly depends on temperature in the selected range, indicated as follows: 1.35 cm/s at 600 °C, 1.29 cm/s at 700 °C, 1.23 cm/s at 800 °C, and 1.19 cm/s at 900 °C. Consequently, if the helium flow is kept constant in the range 600-900 °C, the fluidization state of the sand bed as well as the heat transfer between the hot bed and the solids will be similar. The selected value for the gas velocity was 3.6 cm/s calculated at the nominal

operating temperature of the sand bed, which is 3 times the Umf at 900 °C (the highest operating temperature) and 2.6 times the Umf at 600 °C (the lowest one). The experimental value for Umf working at 700 °C was 1.26 cm/s, which is in good agreement with that predicted by the Ergun equation. Influence of Temperature and Residence Time on Yields of Products in Tire Pyrolysis. In order to study the maximum yields of gases that can be obtained from tire pyrolysis, 58 experiments were carried out at four sand bed temperatures: from 600 to 900 °C at 100 °C intervals. Yields of methane, ethane, ethylene, propane, propylene, acetylene, butane, butylene, 1,3-butadiene, pentane, benzene, toluene, xylenes + styrene, H2, CO, CO2, and H2S were determined in each experiment. Both the height of the bed (or the mass of sand) and the weight of tire discharged onto the hot bed were modified in each experiment. In the experiments, the values of volume V of the upper part of the reactor (between the top of the fluidized bed and the reactor head) or the sand bed mass and the tire mass m were changed, in order to obtain different yields. The amount of tire discharged was varied from 0.2 to 5 g. Thus, the mean residence time of volatiles is different in each run, depending on the mass of volatiles produced and the volume of the upper part of the reactor. Since the reactor is not isothermal (temperature ranging from that of the sand bed to approximately 300 °C at the top of the reactor), it is difficult to find a parameter that is representative of the tar cracking extension. A magnitude that could be roughly proportional to the residence time is the ratio between the volume of the upper part of the reactor, above the sand bed (V), and the mass of the tires discharged onto the bed (m). When V is constant, the greater the value m, the lower the residence time. When m is constant, the greater the value V, the greater the residence time.28,29,33,34 Nevertheless, note that the extension of the secondary reactions depends also on the temperature profile and the corresponding kinetics. The selection of V/m is only considered for presentation of the experimental results. Figures 1-5 show the variation of the yields vs the ratio V/m for some of the compounds analyzed at the four nominal temperatures. In these graphs, the primary reactions are represented by V/m ) 0 obtained in a Pyroprobe 1000 equipment with the following operating conditions: nominal heating rate 20 000 °C/s; pyrolysis time 20 s; pyrolysis temperature 600, 700, 800, and 900 °C; interphase temperature 200 °C; sample mass, from 0.5 to 1.0 mg. In the Pyroprobe 1000, it can be assumed that the secondary cracking of the tars is small, due to the small volume of the probe that is at the nominal temperature. For nominal heating rates of 20 000 °C/s, heating rates between 250 and 585 °C/s, were estimated (Garcı´a et

(31) Williams, E.; Horne, P.; Williams, P. T.; Bottrill, R. Renewable Energy 1994, 5 (III) 2069-2072. (32) Kunii, D.; Levenspiel, O. Fluidization Engineering, John Wiley and Sons: New York, 1969.

(33) Font, R.; Marcilla, A.; Verdu´, E.; Devesa, J. Ind. Eng. Chem. Process Res. Dev. 1986, 25, 491-496. (34) Font, R.; Marcilla, A.; Verdu´, E.; Devesa, J. Ind. Eng. Chem. Res. 1988, 27, 1143-1149.

Results and Discussion

Gas from the Pyrolysis of Scrap Tires

Figure 1. Variation of the hydrocarbon yields vs. the ratio between the volume V of the upper part of the reactor and the discharged mass m, for total gas.

Energy & Fuels, Vol. 10, No. 1, 1996 137

Figure 3. Same as Figure 1, for ethane.

Figure 4. Same as Figure 1, for carbon monoxide. Figure 2. Same as Figure 1, for methane.

al.27). More detail of the Pyroprobe and its performance can be found elsewhere (Font et al.;35 Caballero et al.30). The extrapolation of the results obtained in the fluidized bed reactor when V/m equals zero shows a good agreement with the results in the Pyroprobe 1000.

Table 2 shows the yields obtained at the maximum V/m for the four temperatures, together with the data obtained in the Pyroprobe (V/m ) 0), of all the compounds analyzed. Table 3 shows the maximum yield of each compound and the corresponding operating condition.

(35) Font, R.; Marcilla, A.; Verdu´, E.; Devesa, J. Ind. Eng. Chem. Res. 1990, 29, 1846-1855.

In spite of the dispersion of the results, due in part to the heterogeneity of the material, the following

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Conesa et al. Table 3. Maximum Yield of Each Compound and Operating Condition temp (°C) methane ethane ethylene propane propylene acetylene butylene butane pentane benzene toluene xylenes + styrene hydrogen CO CO2 H2S butadiene total gas

900 700 800 600 700 600 600 900 700 900 800 700 900 900 800 700 600 700

sand (g)

tire (g)

yield

300 0.2349 950 0.6780 300 0.2440 200 3.2130 950 0.6780 200 0.3620 200 0.3620 300 0.2210 300 0.2280 300 0.2210 1135 0.4160 950 0.6780 300 0.2349 300 0.2705 300 0.3420 300 0.3470 Pyroprobe 300 0.2280

7.64 1.05 6.44 0.36 2.91 0.08 4.89 0.46 1.59 8.73 5.67 8.18 0.74 1.65 3.6 0.81 5.28 37.06

Table 4. Chemical Class Fractioning of the Tire-Derived Pyrolysis Oil (wt % Soluble in Each Disolvent)

Figure 5. Same as Figure 1, for hydrogen sulfide. Table 2. Yield of Each Compound at V/m Maximum and in Pyroprobe V/m (cm3/g) )

T ) 600 °C 0 5895

T ) 700 °C 0 6102

T ) 800 °C 0 5870

T ) 900 °C 0 6295

methane ethane ethylene propane propylene acetylene butylene butane pentane benzene toluene xylenes + styrene hydrogen CO CO2 H2S butadiene total gas

0.05 0.02 0.04 0.05 0.03 0.00 0.33 0.00 0.11 0.00 0.11 0.02

0.15 0.08 0.21 0.05 0.20 0.01 0.89 0.02 0.13 0.07 0.15 0.27

0.17 0.23 0.11 0.04 0.45 0.01 1.41 0.05 0.11 0.23 0.19 0.73

0.19 0.10 0.18 0.02 0.02 0.02 2.80 0.03 0.15 0.56 0.20 0.12

1.61 0.63 2.33 0.16 1.89 0.07 2.41 0.06 1.26 3.42 3.59 4.68

0.00 0.09 0.33 1.02 0.73 2.49 0.02 0.79 5.28 2.56 7.12 29.06

3.96 0.74 4.30 0.13 2.60 0.01 1.93 0.11 1.59 5.68 3.53 7.32

0.01 0.14 0.45 1.02 0.90 2.20 0.05 0.50 3.71 1.30 7.35 37.06

6.15 0.36 6.12 0.02 0.88 0.02 0.18 0.25 0.57 5.36 4.21 3.12

0.02 0.50 0.51 1.08 1.21 3.32 0.01 0.10 1.15 0.03 6.63 32.27

7.57 0.12 4.43 0.00 0.04 0.00 0.00 0.46 0.06 8.73 5.00 0.41

0.05 0.67 0.62 1.21 1.25 3.20 0.00 0.54 1.24 0.03 7.55 32.47

behaviors for gases as a function of the residence time could be deduced: (a) Compounds whose yields clearly increase with the residence time of the volatiles in the reactor: methane, benzene, toluene, xylenes + styrene, and hydrogen. Figure 2 shows the methane data. Similar variations can be observed with the remaining compounds. (b) In the case of CO and CO2 there is a slight increment of the yield between the primary and the secondary reaction. Figure 4 shows the variation for the CO. (c) Compounds whose yields have maximums at a determined V/m: ethane, ethylene, propane, propylene, butane, butylene, acetylene, pentane, 1,3-butadiene, and H2S. Total gas yield has a similar behavior, within the 600-800 °C temperature range, when the bed nominal

pentane

benzene

ethyl acetate

methanol

39.5

19.1

21.3

20.1

temperature is increased and low residence time to achieve the maximum yield is required. Nevertheless, at 900 °C the total gas yield is lower than that for 800 °C at any residence time, showing the presence of cracking of tars to soot. In Figures 1 and 3 the data for total gas and ethane respectively is shown. These behaviors are in agreement with those observed in the high-density polyethylene pyrolysis in the same apparatus.28 Roy and Unsworth,19 Douglas et al.,36 and Kaminsky and Sinn20 have also shown that the gas phase comprises hydrogen, carbon dioxide, carbon monoxide, and hydrocarbons including methane, ethane, ethylene, propane, propylene, butylene, butadiene, and butane. Butadiene is derived from the breakdown of the polymer butadiene-styrene-rubber used in the manufacture of tires. Dodds et al.37 have shown that the rubbers used in tire manufacture are characterized by carbon-carbon double bonds within the rubber molecule. This serves to produce highly reactive free radicals, which tend to be subchains of the original rubber molecule. This explains the presence of styrene, butadiene, and alkenes in the pyrolysis products. The formation of aromatic and polyaromatic compounds such as benzene, toluene, naphthalene, and phenanthrene (Williams et al.22) is to be expected through reactions of styrene. Cypres and Bettens38 have suggested that hydrogen and ethane are derived from secondary aromatization reactions. Liquid and Solid Fractions. The chemical fractioning of the liquid fraction from a 700 °C run (at V/m ) 950 cm3/g) is shown in Table 4 (according to the method developed by Williams et al.31). The calorific value of the sample was 41 600 J/g, and the average (36) Douglas, E.; Webb, M.; Daborn, G. R. Presented at the Symposium on Treatment and Recycling of Solid Wastes, Institute of Solid Wastes Management, Manchester UK, 1974. (37) Dodds, J.; Domenico, W. F.; Evans, D. R.; Fish, L. W.; Lassahn, P. L.; Toth, W. J. “Scrap tires: a resource and technology evaluation of tire pyrolysis and other selected alternative technologies”. US Department of Energy Report EGG-2241, 1983. (38) Cypres, R.; Bettens, B. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Brudgwater, A. V., Eds.; Elsevier Applied Science: London, 1989.

Gas from the Pyrolysis of Scrap Tires

Energy & Fuels, Vol. 10, No. 1, 1996 139

Table 5. Solid Fraction (wt %) Obtained at Each Temperature (Different Residence Times Together) temp (°C)

wt %

600 700 800 900

20.4 21.6 29.1 32.5

Table 6. Properties of the Char Obtained at 700 °C

a

calorific value (J/g)

26973

Elemental analysis (%) C H N Oa S

94.32 1.07 0.97 2.10 1.54

By difference.

Figure 7. Logarithm of the hydrocarbon yield vs logarithm of methane yield: ethylene.

Figure 6. Logarithm of the hydrocarbon yield vs logarithm of methane yield.

molecular weight was 106 (analyzed by size exclusion chromatography). In comparison with Williams et al.39,40 (aproximately 63% pentane fraction, 30% benzene, and 9% ethyl acetate), the oil obtained in this work has a lower aliphatic content. The solid fraction increases at higher temperatures, as is shown in Table 5, in agreement with Williams et al.21 Some properties of this char are shown in Table 6. The calorific value (26 973 J/g) is similar to that of the char obtained by Kim et al.41 (25 330 J/g) and some lower than that obtained by Wolfson42 (30 000 J/g). Correlation of Results. In order to correlate the results obtained by scrap tire pyrolysis in a fluidized bed reactor, logarithms of yields corresponding to each product obtained vs logarithm of methane yields have been plotted for each run (Figures 6-12 shows some cases). This method is widely used in the literature: Funazukuri et al.,43-45 Scott et al.,46 Font et al.,35 Garcı´a (39) Williams, P. T.; Besler, S.; Taylor, D. P. Proc. Inst. Mech. Eng. 1993, 207. (40) Williams, P. T.; Taylor, D. P. J. Anal. Appl. Pyrol. 1994, 29, 111. (41) Kim, J. R.; Lee, J. S.; Kim, S. D. Energy 1994, 19 (8), 845-854. (42) Wolfson, D. E.; Beckman, J. A.; Walters, J. G.; Bennet, D. J. US Department of Interior, Bureau of Mines Report of Investigations 7302,1969. (43) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. Ind. Eng. Chem. Prod. Des. Dev. 1986, 25, 172. (44) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. J. Anal. Appl. Pyrol. 1986, 9, 139.

Figure 8. Logarithm of the hydrocarbon yield vs logarithm of methane yield: benzene.

et al.,27 Caballero et al.,30 and Conesa et al.29 In the pyrolysis of lignocellulosic tars, methane is selected because its formation takes place through a mechanism which is very sensitive to temperature. In this work, methane has also been selected in these plots. If methane yield is considered as an indicator of the extension of pyrolytic reactions, different behaviors can be deduced from these plots: 1. Yields of ethane, propane, propylene, butylene, and pentane show maximums at temperatures near 700 °C, as a consequence of the secondary cracking undergone by these hydrocarbons. At 800 °C, the cracking of these compounds takes place, with an increase in methane production. Figure 6 shows ethane versus methane data. In the case of ethylene, the cracking only takes place at 900 °C (Figure 7). 2. Yields of aromatic compounds, represented by benzene in Figure 8 increase as the methane yield increases. (45) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. J. Anal. Appl. Pyrol. 1988, 13, 103. (46) Scott, D. S.; Piskorz, J.; Radlein, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581.

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Figure 9. Logarithm of the hydrocarbon yield vs logarithm of methane yield: 1,3-butadiene.

Conesa et al.

Figure 12. Logarithm of the hydrocarbon yield vs logarithm of methane yield: hydrogen sulfide.

4. Yields of carbon oxides seem to be approximately constant. Figure 10 shows the CO data. 5. Correlation coefficients for yields of logarithm of hydrogen and hydrogen sulfide versus logarithm of methane yield are 0.746 71 and 0.074 55 respectively (Figures 11 and 12), therefore, hydrogen yield correlates better with methane than hydrogen sulfide. The behavior of these compounds is similar to that observed when pyrolyzing almond shells (Font et al.34), and using municipal solid waste as feeding material (Garcı´a et al.47 ). In both cases, the hydrogen yield increases as the methane yield increases, and the yield of carbon oxides is approximately constant. Conclusions Figure 10. Logarithm of the hydrocarbon yield vs logarithm of methane yield: carbon monoxide.

From the study of tire pyrolysis in a fluidized sand bed reactor, the following conclusions can be deduced: 1. The yield of total gas obtained increases in the range 600-800 °C from 6.3 to 37.1%. At higher temperatures, the yield of total gas decreases slightly. 2. The formation of methane, hydrogen, benzene, and toluene is favored by high residence times, but ethane, ethylene, propane, propylene, butane, butylene, acetylene, 1,3-butadiene, and pentane undergo cracking to different extents at increasing residence times and/or temperature. 3. The maximum yield of total gas obtained at 800 °C from tire pyrolysis is 34.3 % with the following composition: methane 6.11%, ethane 0.39%, ethylene 5.84%, propane 0.01%, propylene 0.74%, acetylene 0.02%, butylene 0.12%, butane 0.17%, pentane 0.38%, benzene 6.76%, toluene 5.04%, xylenes + styrene 3.2%, hydrogen 0.3%, CO 0.95%, CO2 3.6%, H2S 0.64%, and butadiene 0.06%.

Figure 11. Logarithm of the hydrocarbon yield vs logarithm of methane yield: hydrogen.

Acknowledgment. Support for this work was provided by CYCIT-Spain, Research project AMB93-1209.

3. The 1,3-butadiene can be cracked or can suffer reactions of aromatization, and disappears very quickly in comparison with the other hydrocarbons (Figure 9).

EF950152T (47) Garcia, A. N.; Font, R.; Marcilla, A. Energy Fuels, in press.