Tire Pyrolysis: Evolution of Volatile and Semivolatile Compounds

Introduction. John Boyd Dunlop invented the tire in 1888 when the wheels of his son's bicycle were covered and they made the driving much more comfort...
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Energy & Fuels 2000, 14, 409-418

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Tire Pyrolysis: Evolution of Volatile and Semivolatile Compounds Juan A. Conesa,* Andres Fullana, and Rafael Font Department of Chemical Engineering, University of Alicante, Apartads 99, E-03080 Alicante, Spain Received July 20, 1999

The formation/decomposition of several compounds from the pyrolysis of tire wastes is compared in three different types of equipment. The types of laboratory equipment consist of a Pyroprobe, where the secondary reactions are minimized, and a horizontal pyrolysis furnace, where the cracking is severe. The yields obtained in both types of equipment are compared with those previously obtained in a fluidized bed reactor. On the other hand, the kinetic severity function (KSF) has been proposed as a measure of the cracking undergone by the tars produced. This paper focuses on the production of semivolatiles from tire wastes.

Introduction John Boyd Dunlop invented the tire in 1888 when the wheels of his son’s bicycle were covered and they made the driving much more comfortable. From the discovery of this first tire until today, the production of such material has continuously expanded, becoming an industry invoicing, in 1992, more than 53.4 billion dollars.1 As a consequence of the huge increase in tire consumption, the wastes produced from tires have also increased in recent years. Data obtained from the EPA in 19962 show that the weight percentage of tire with respect to the total amount of municipal solid wastes increased from 1.3% in 1960 (representing a total of 1120000 tons) to 1.9% in 1994 (4080000 tons). In the last 30 years, the amount of residues has increased more than a hundred percent. Despite the fact that scrap tires represent slightly more than 12% of all solid waste, scrap tires present a special disposal and reuse challenge because of their size, shape, and physicochemical nature. Scrap tires are not generally collected with household waste by municipal authorities. Thus scrap tires have traditionally been classified as a “special waste” or as a “durable product”. These residues cannot be considered as toxic and harmful residues. In the United States the classification is “municipal solid wastes with special characteristics”. In Spain the classification is “not special residues but different from municipal solid wastes”. The treatment of the residues is possible through two different ways, i.e., the recycling and the controlled landfilling. The controlled landfilling is nowadays the destiny of the major part of the residues, maybe due to the difficulty and high cost of tire recycling. But the landfilling presents several problems. Tires are not * Corresponding author. Telephone: +34-965903400 ext. 2646. Fax: +34-965903826. (1) Sponagel, P. Tires. In Ullman’s Encyclopedia of Industrial Chemistry; VCH Verlagsgesellschaft: New York, 1996. (2) Blummenthal, M. H., Tires. In The McGraw-Hill Recycling Handbook; Lund, H. F., Ed.; McGraw-Hill: New York, 1993.

easily compacted, so scrap tires take up considerable amounts of landfill space. On the other hand, due to the tire’s hollow shape, air and other gases can be trapped, which makes tires buoyant. This opening exposes the landfills to insects, rodents, and birds, and allows landfill gases to escape, all of which are undesirable. The rubber industry has implemented a considerable number of arrangements, year after year, to save raw and other materials to protect the environment. Nevertheless, production wastes continue to remain a serious problem that demands more and more engineering and scientific forces to resolve it. Tire wastes have a high amount of energy, so energy recovery could be a good recycling strategy. Considering the low calorific value of several solid residues (for example, wood 10178 kJ/kg, municipal solid wastes 12374 kJ/kg, lignite 16983 kJ/kg, subbituminous coal 24428 kJ/kg, tires 33035 kJ/kg), it can be seen that, for example, tire is more “energetic” than bituminous coal, a classical energy source. In a previous paper3 a fluidized sand bed reactor was used to study the production of gases by pyrolysis of scrap tires at four nominal temperatures (ranging from 600 to 900 °C). Yields of seventeen pyrolysis products (methane, ethane, ethylene, propane, propylene, acetylene, butane, butylene, 1,3-butadiene, pentane, benzene, toluene, xylenes + stirene, hydrogen, carbon monoxide, carbon dioxide, and hydrogen sulfide) were analyzed as a function of the operating conditions. The results were compared with the data obtained by pyrolysis of tires in a Pyroprobe 1000, where secondary tar cracking is small. In this paper, correlations between the products analyzed with those of methane were discussed. In a recent paper4 the utility of the kinetic severity function (KSF) for the validation of kinetic models was (3) Conesa, J. A.; Font, R.; Marcilla, A. Energy Fuels, 1996, 10 (1), 134-140. (4) Conesa, J. A.; Font, R. Energy Fuels 1999, 13 (03), 678-685.

10.1021/ef990155w CCC: $19.00 © 2000 American Chemical Society Published on Web 01/21/2000

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Figure 1. Schematic of the horizontal pyrolysis furnace.

shown in complex pyrolysis processes where n-pentane is formed. The method requires the calculation of the KSF and the measure of the yields of different compounds in different conditions. The kinetic severity function (KSF) is

KSF )

dt ∫0τ k5 dt ) ∫0τ (1.77 × 1012)exp(- 27989 T(t) )

(1)

where k5 is the kinetic constant of the thermal decomposition of pentane. In another paper4 it was shown that the first condition that the calculation must fulfill is that the slope of the ln(pentane yield) vs KSF graph must be logical, considering the decomposition kinetic law of the n-pentane. The method was applied to polyethylene pyrolysis in order to test the modeling previously carried out5,6 and to analyze the variations of the different yields of each compound vs KSF when the polyethylene waxes are cracked. The main objective of this paper is to show the amount and variety of semivolatile compounds (more than a hundred compounds identified and measured) produced in the pyrolysis of tires, using a laboratory pyrolysis furnace that will be described below. Another important objective of this paper has been the comparison of the new laboratory furnace with the behavior of the fluidized bed reactor. This will be accomplished by (1) measuring the volatiles produced in the new laboratory equipment at different experimental conditions; (2) plotting logarithms of yields corresponding to each product obtained vs the logarithm of methane yields for the three types of equipments: and (3) comparing the results in the sense “equal cracking gives equal composition of gases produced”. In addition, the KSF in the new pyrolysis furnace has been estimated by comparing the results with the KSF calculated in the fluidized bed reactor.

Materials and Equipment 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 83.55% C, 7.81% H, 0.39% N, 1.48% S, and 8.23% ash (oxygen is 6.77% by difference). 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. Fluidized Bed Reactor and Pyroprobe 1000. One type of equipment used was a cylindrical 18/8 stainless steel fluidized sand bed 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. The inert gas used was helium of 99.999% purity. The experimental procedure was as follows. A 0.2-3 g tire sample was placed in the feeder; the flow of the inert gas was fixed, and the oven was switched on. When the reactor reached the selected temperature, the exit flow was shifted to the feeder in order to eliminate the air. After a few minutes, the feed valve was opened and the sample poured onto the hot sand bed (between 100 and 1500 g of sand). After the addition, gases evolved from the reactor were collected, to a volume of 40 L, in a Teflon bag provided with an in-out valve and a septum for sampling and analysis. More details about this equipment can be found in previous papers.3,7 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 approximately 1 cm. The weight of the sample varies between 0.5 and 2 mg. A scheme of the apparatus can be found elsewhere.8 Due to its design, it is supposed the secondary reactions do not take place. Horizontal Pyrolysis Furnace. Two similar horizontal quartz reactors of different sizes were used. Figure 1 shows (5) Conesa, J. A.; Font, R.; Marcilla, A.; Caballero, J. A. J. Anal. Appl. Pyrol. 1997, 40-41, 419. (6) Conesa, J. A.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrol. 1994, 30, 101-120. (7) Conesa, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. Energy Fuels 1994, 8 (6), 1238-1246. (8) Caballero, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. J. Anal. Appl. Pyrol. 1993, 27, 221-244.

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Mathematical Procedures. Simulation of the Pyrolysis in the FBR

Figure 2. Typical temperature profile in the laboratory pyrolysis furnace. the first of them. In both reactors, the sample in a small boat is introduced at constant rate (0.5 mm/s) inside the furnace, and a continuous flow of nitrogen is used to remove the volatiles and semivolatiles evolved. The temperature profile of the furnace is determined previously. Figure 2 shows a typical temperature profile for a run performed at 850 °C as nominal temperature. The volatiles and semivolatiles evolved were collected in a small tube containing a XAD-2 resin and after it in a Tedlar bag. The second reactor (smaller than the first one) is formed by a thin tube placed after a cylinder tube. In this case the volatiles left the reactor through the fine tube, and the residence time is less than in the first reactor. Volatiles Analysis. Volatile compounds evolved from the three types of equipment (fluidized bed reactor (FBR), horizontal pyrolysis furnace, and Pyroprobe) were analyzed by gas chromatography. Three different columns were used: AluminaKCl with a FID detector for light hydrocarbons (CH4, C2H6, C2H4 + C2H2, C3H6, C4 and C5 hydrocarbons), a Carbosieve SII column with a TCD for CO2 analysis, and a Molecular Sieve 5 Plot column to separate nitrogen, oxygen, and CO. Semivolatiles Analysis. In some runs carried out with the furnace, the semivolatiles were retained with a XAD-2 resin (around 0.4 g). The semivolatiles adsorbed in the resin were extracted with 100 mL of dichloromethane, following EPA 3540 C method. Before the extraction 20 µg of a deuterated internal standard (1,4-dichlorobenzene-d4, naphthalene-d3, acenaphthene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12) were added. Then the extract was concentrated to 2 mL using a micro Kuderma-Danish concentrator. The area of the primary ion chromatogram (PIC) was used to differentiate between internal standard and sample compounds. TIC (total ion chromatogram) area of internal standard was used to quantitatively analyze the compounds identified, according to the EPA 8276 C method (section 7.3.3). Ratios PIC/TIC previously determined were used to calculate TIC of internal standards. For the analysis of the extracts, a GC8000 from Fisons Instruments coupled with a mass spectrometer MD8000 was used. The column used in this analysis was a DB-5 column, 30 cm length by 0.25 mm i.d. The compounds were identified by the NIST database, and also using the index from Kovad and Lee.9,10 The accuracy of the identification with the mass spectometer is 85-95%, using the NIST database. When the accuracy is less, the index from Kovad and Lee is used, together with visual identification of the peaks. (9) Lee, M.; Vassilaros, D.; White, C.; Novotny, M. Anal. Chem. 1979, 51 (6), 768-773. (10) Pereira, V.; Kovad, A.; Lee, M. J. High-Resolut. Chromatogr. Chromatogr. Commun. 1986, 9, 328-334.

To calculate the KSF in the FBR, that will be used later, a model for the batch pyrolysis of tire wastes is necessary. The modeling proposed for this material is similar to that presented for other materials such as polyethylene, municipal solid wastes, or almond shells.6,11,12 In the modeling of tire pyrolysis, both primary and secondary processes have been modeled. The kinetics of the primary pyrolysis was investigated in a thermobalance13-15, fitting curves obtained at different heating rates at the same time and using the same set of parameters. In the fluidized bed pyrolysis reactor the secondary kinetics is determined. In the model two parameters concerning the heat transfer are introduced. One of them is related with the heat transfer from the bed to the scrap tire particle, that can consider both external and internal heat transfer. The other is a correcting factor that accounts for the difference in the temperature profile in the upper part of the reactor when pyrolysis volatiles are present. This is due to the fact that actually the temperature profile is measured when helium is passing through the reactor and there can be some differences between the volatiles evolved for a short period of time and the stabilized temperature when helium flows). This model was applied to the pyrolysis of almond shells and polyethylene, obtaining kinetic constants of the tar and waxes similar to those in the literature. In the case of tire wastes, it is proposed that the products of the primary pyrolysis are cracked following two different first-order reactions, one of them producing solid residue and other producing secondary gases: ks1

primary pyrolysis

tire waste 98 aGp + bAp

98Gs ks1

(S1)

98Ss

where the sub-index “p” stands for primary products, and “s” for secondary. In a first intent, the model simplified the secondary reactions to only one, but this model does not permit the decrease of the total amount of gas when increasing temperature from 800 to 900 °C, as it occurs in the reactor.3 In the modeling of the batch pyrolysis of tire wastes in the fluidized bed reactor, it was assumed6 that, as a consequence of the primary reaction undergone by the tire sample discharged onto the hot bed, a small amount of volatiles (∆V1) is generated in a small interval (∆t1). The ∆V1 formed is pushed by the helium (inert gas used) flux through the hot zone above the fluidized sand bed. In this zone, the volatiles are cracked as a consequence of the secondary reactions, and the mixture expands. Over the next time interval (∆t2), another amount of (11) Garcı´a, A. N.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrol. 1992, 23, 99-119. (12) Font, R.; Marcilla, A.; Verdu´, E.; Devesa, J. Ind. Eng. Chem. Res. 1988, 27, 1143-1149. (13) Conesa, J. A.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrol. 1997, 43, 83-96. (14) Conesa, J. A.; Marcilla, A. J. Anal. Appl. Pyrol. 1996, 37, 95110. (15) Conesa, J. A.; Font, R.; Fullana, A.; Caballero, J. A. Fuel 1998, 77 (13), 1469-1475.

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volatiles (∆V2) is generated. This is also pushed by the helium flux to the top of the reactor. Pushed by the helium flux the ∆V1 continues rising and the new volatiles increment ∆V2. The process continues until the total sample decomposition and 99.9% of volatiles evolved have left the reactor. The differential equations used in the model can be found in other papers.4,6 A scheme of the differential equations used in the simulation of the secondary pyrolysis process is as follows. (i) The formation of primary volatiles (gases + tars + waxes) has a kinetic law that could be expressed by

dVVp dTyr ) -VVp,∞ dt dt

(3)

where ∆VV2 is the total volume of gases (primary + secondary) that are within a volume ∆VR considered, ∆VVp0 is the volume of volatiles (gases + tars) generated when time equals zero, and R is an expansion factor that can be calculated for each interval. (iv) The heat transfer between the fluidized bed and the scrap tire particle is modeled using the equation:

dTp U1S (Tb - Tp) ) dt Cp

(4)

where Tb is the temperature of the fluidized bed, Tp is the actual temperature of the sample at time t, and S is the external surface of the sample. U1 is the heat transfer coefficient and Cp the specific heat. (v) The heat transfer between the reactor walls and the volatiles is represented by

dTi ) Hs (TR - Ti) dt

k0 (mass fraction1-n s-1) E (kJ/mol) n Wio

F1

F2

F3

3.68 × 105 70.0 1.294 0.290

4.13 × 1016 212.6 2.400 0.230

5.96 × 1017 249.3 1.497 0.121

(2)

where VVp is the total volatiles volume (primary gases + primary tars and waxes), VVp∞ is its maximum value, and Tyr refers to the tire. The primary tire decomposition (dTyr/dt) follows a 3-reaction model as presented in previous papers.13-15 (ii) The secondary reaction of cracking of the tars, as a function of the extent of the reaction (Xs), is considered to follow a first-order reaction kinetics. (iii) The expansion of the total volatiles (primary and secondary) as a consequence of the cracking of tars in the secondary reaction is calculated using

∆VV2 ) ∆VVp,0 (1 + RXs)

Table 1. Parameters Optimized for the Primary Decomposition of Tire Wastes, with an Inert (wI0 ) 0.357) (from refs 3, 14 and 15)

(5)

where Ti is the real temperature of the volume considered, TR is the reactor temperature according to the temperature profile measured at the position of the volume, and Hs is a fitting parameter. This parameter was introduced because the temperature of the gas mixture (helium + pyrolytic products) can be different from that measured by the thermocouple prior to the discharge of the material. The pyrolytic products are generated very rapidly in a short period of time and the temperature of the thermocouple corresponds to a situation when only helium is circulating in accordance with the heat flux by convention and radiation. The introduction of this factor has been justified in previous

Figure 3. Simulation of fluidized bed reactor runs. Experimental and calculated total yields.

papers6,16 to obtain kinetic constants similar to those found by other researchers and has been also justified by analyzing the decomposition in terms of the kinetic severity function KSF.4 The only difference with respect to the previous papers is the model used for the primary pyrolysis. This reaction is not modeled through a scheme as simple as that presented in (S1), but in the case of the tire wastes there are considered three different fractions:13-15

Fi f (1 - si)Gi + siSi

i ) 1, 2, 3

(S2)

The constants (preexponential factors, activation energies, order of reactions, and initial weight of each fraction) were optimized for this primary decomposition,13,15 and are shown in Table 1. The model was used to correlate 54 runs carried out previously in the FBR.3 The objective function used has been the sum of the squares of the differences between experimental (Yexp) and calculated global yields (Ycalc) of the pyrolysis process:

O.F. )

(Yexp - Ycalc)2 ∑ all the runs

The optimized variables were the following: modulus (U1S/Cp), referring to the heat transfer sample-sand bed. correcting factor Hs of the temperature in the upper part of the reactor. activation energy of the secondary reactions (Es1/R and Es2/R). secondary kinetic constant at 1000 K (ks1,1000 K and ks2,1000 K) (since interrelation between activation energy and kinetic constant at 1000 K is lower than interrelation between the activation energy and the preexponential factor). (16) Conesa, J. A.; Font, R.; Marcilla, A.; Caballero, J. A. J. Anal. Appl. Pyrol. 1997, 40-41, 419-431.

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Energy & Fuels, Vol. 14, No. 2, 2000 413

Figure 4. Comparison between the kinetic constants for secondary cracking obtained by several authors.

The optimization method used was the Flexible Simplex,17 and the fitting is shown in Figure 3. This figure shows the experimental vs the optimized total gas yields. A good agreement could be observed. The values of the parameters for a minimum objective function are

(U1S/Cp) ) 9.31 × 10-2 s-1 Es1/R ) 14335.1 K ks1,1000K ) 6.38 s-1

Hs ) 0.5117 s-1

Once the best parameters were found, the average KSF, considering the thermal treatment in gas phase, was calculated for each differential volume element (KSFi, sub-index ‘i’ represents each increment), using eq 1 and the temperature profile calculated. The average kinetic severity function has been calculated using the following relationship:

Es2/R ) 7168.7 K ks2,1000K ) 5.99 × 10-2 s-1

Figure 4 shows the kinetic constants for the cracking of tars and waxes corresponding to some materials at different temperatures. The values represented correspond to those obtained by Antal18 for tar cracking from cellulose, Diebold19 for wood pyrolysis, Liden et al.20 for biomass, Font et al.21 for almond shells, Conesa et al.6 for the pyrolysis of polyethylene, and present work. As can be seen in Figure 4, the activation energies are similar to those obtained by Antal for the cracking of cellulose tars, assuming also two parallel reactions for the cracking of tars. The ranges of the kinetic constant ks1 and ks2 are very close to those proposed by Diebold,19 Liden,20 or Font21 in the decomposition temperature range. On the other hand, values of parameters related with heat transfer ((U1S/Cp) and Hs) are very close to those obtained previously in the polyethylene pyrolysis.6 (17) Himmelblau, D. M. Process Analysis Statistical Methods; Wiley: New York, 1968. (18) Antal, M. J., Jr Ind. Eng. Chem. Prod. Res. Dev. 1983, 22 (2), 366-375. (19) Diebold, J. The Cracking Kinetics of Depolymerized Biomass Vapors in a Continuous Tubular Reactor. Ph.D. Thesis, School of Mines, Golden, CO, 1985. (20) Liden, A. G.; Berruti, F.; Scott, D. S. Chem. Eng. Comm. 1988, 65, 207-221. (21) Font, R.; Marcilla, A.; Verdu´, E.; Devesa, J. J. Anal. Appl. Pyrol. 1993, 27, 221.

exp(-KSF) )

∑i ∆mi exp(-KSFi) ∑i ∆mi

(6)

In this equation, ∆mi is the weight loss of the tire sample in the primary decomposition in the time interval ∆ti. The reason for using eq 6 is based on the fact that the mean nonreacted pentane is the weighted nonreacted pentane in each volume element. Let us consider with more detail the process of formation and cracking of pentane from tire cracking products: kp

k5

tire cracking products 98 pentane 98 gases or solid residues In addition to the formation of pentane, there is an initial amount of pentane formed by the primary decomposition of tire. kp is the kinetic constant of the tire products cracking, and k5 the kinetic constant for the pentane decomposition. Considering the first reaction as first-order:

( )

CP dCP ) ) -kpCp w ln dt CP0

∫0τ kp dt

(7)

where Cp is the concentration of the species “tire cracking products” in the scheme of reaction. If we

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Figure 5. Behavior of ln(C5) vs KSF for different values of M and C50. Table 2. Example of Runs Performed in the Fluidized Bed Reactor (FBR) and in the Pyroprobea methane ethane ethylene propane propylene acetylene butylene butane pentane benzene toluene xylenes + stirene hydrogen CO CO2 H2S butadiene total gas a

FBR

Pyroprobe

7.64 0.13 4.25 nd 0.03 nd 0.01 0.37 0.05 8.68 4.80 0.37 0.74 1.48 2.85 0.34 0.01 31.75

0.19 0.10 0.18 0.02 0.02 0.02 2.80 0.03 0.15 0.56 0.20 0.12 0.05 0.62 1.25 nd 1.24 8.26

Run

Yields (wt %) of each compound.

assume that kp and k5 have similar activation energy:

∫0τ kpdt ) M ∫0τ k5 dt ) M (KSF)

(8)

where M equals the ratio between the preexponential factor of kp to k5. Then

Cp ) Cp0 exp(-M(KSF))

(9)

On the other hand

d(KSF)

)

kp C exp(-M(KSF)) - C5 k5 p0

(10)

where C5 is the pentane concentration. The differential (eq 10) has the following boundary condition:

when KSF ) 0 then C5 ) C50

PR850 SR650 SR750 SR850 SR950 SR1050

temperature (°C) mass (g) furnace

850 0.10 small

650 0.10 big

CO2 CO methane ethane ethylene propane propylene isobutane acetylene transbutene butene cis-2-butene 1,2-butadiene pentane propyne pentene 2-butyne 1-butyne hexane benzene toluene

nd nd 6.4 0.8 6.2 0.1 2.5 nd 0.4 nd 0.1 1.4 nd 2.4 Nd 0.1 0.2 1.7 0.1 6.9 2.6

Yields 8.4 2.0 1.3 nd 6.0 7.0 1.2 0.5 5.4 6.8 0.2 nd 4.1 1.4 0.2 nd 0.2 0.3 0.5 nd 3.1 0.3 0.2 nd nd nd 1.7 1.2 0.9 nd 0.3 nd 0.8 0.3 2.3 0.1 0.2 nd 6.5 7.0 3.0 2.0

a

750 0.10 big

850 0.10 big

950 0.10 big

1050 0.1000 big

3.7 1.9 8.3 0.3 6.3 nd 0.1 nd 1.1 nd nd nd nd nd 0.2 nd nd nd nd 15.3 0.9

3.8 0.9 7.8 nd 1.8 nd nd nd 1.4 nd nd nd nd nd nd nd nd nd nd 10.6 0.1

6.7 1.1 8.5 nd 0.7 nd nd nd 1.1 nd nd nd nd nd 0.1 nd nd nd nd 6.9 nd

nd ) non detected.

C50 being the concentration of pentane formed by the primary decomposition. Resolving eq 10, we obtain

Cp0 M C5 [exp(-M(KSF)) - exp(-KSF)] + ) C50 C50 1 - M exp(-KSF) (12)

dC5 kp dC5 ) kpCp - k5C5 w ) C - C5 dt k5dt k5 p dC5

Table 3. Conditions of the Runs Performed in the Horizontal Pyrolysis Furnace, and Yields Obtained (wt %)

(12)

Figure 5 shows the curves obtained for different values of M, Cp0 and C50 (parameters that, logically could be different depending on the material). This figure shows that the maximum could vary its position and magnitude, and also that the slope of the curves at low values of KSF could be different at a given value of KSF. The change in the slope could be as high as, for

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Energy & Fuels, Vol. 14, No. 2, 2000 415

Figure 6. Yields of several compounds vs yield of methane for the three types of equipment ([ Pyroprobe, O FBR, and 9 new pyrolysis furnace).

example, to produce an increment from 0.87 to 0.52 when changing M from 1 to 0.6, using the same value of Cp0 and C50. In these graphs, at higher values of KSF, all the curves are converted in straight lines with slope unity.

Results and Discussion Evolution of Volatiles from the Different Types of Laboratory Equipment and Comparison with Those of the Fluidized Bed Reactor. The yields obtained in the

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FBR and in the Pyroprobe were published.3 Table 2 shows the yields of two experiments. Table 3 presents the conditions of the six runs done in the new pyrolysis furnace. The runs SR650 to SR1050 were designed to see the effect of the temperature (from 650 to 1050 °C) in the yield of compounds produced, and one of the runs (PR850) was specifically designed to reduce the secondary reactions in the furnace. This was achieved by using the second pyrolysis reactor, which has a minor length exposed to high temperatures. Table 3 also presents the results of the analysis of the volatiles performed for each run. To compare the results obtained in the different types of equipment, the logarithms of yields corresponding to some of the products obtained (those analyzed in all the equipments) vs logarithm of methane yields have been plotted (Figures 6a-k) for all the runs carried out with the three types of equipment.sPyroprobe, fluidized bed reactor, and the horizontal furnace. This method is widely used in the literature: Funazukuri et al.,22-24 Scott et al.,25 Font et al.,12 Garcı´a et al.,11 Caballero et al.,8 and Conesa et al.3,7 In the pyrolysis of lignocellulosic tars, methane is related because its formation takes place through a mechanism which is very sensitive to temperature. In this work, methane has also been related in these plots. From Figures 6a-k it can be seen that in general the high yield of methane obtained in the horizontal furnace places the yields to the right of the yields corresponding to the FBR. Similarly, the yields of products from Pyroprobe are to the left of the FBR. This would mean that the cracking in the horizontal furnace is higher than in the FBR, since the methane is a typical cracking product.3,7,12 In general, the behavior of the yield of each compound in the Pyroprobe is continued in the FBR, and later in the new furnace, so that the behavior in the FBR at higher levels of cracking can be predicted. In fact, in the figures it can be seen that some of the results obtained in the FBR and in the horizontal furnace can be overstrike. For example, yields of propylene, pentane, and (xylenes + styrene) show a similar trend in both types of equipment. In other compounds the behavior can be predicted. For example, the trend of the yield of compounds such as toluene or ethylene, that in the FBR seems to increase continuously as the cracking increases, is clarified in the new furnace, showing that the yield clearly decreases when cracking exceeds a certain value. When comparing runs PR850 and SC850 in Table 3, the yields follow a similar trend: the methane yield increases as the secondary reaction occurs, while compounds such as propylene are cracked. The data (yield of propylene in the horizontal furnace) ) 2.5% for PR850 could be compared with the maximum yield obtained in the FBR (2.9%). KSF in the FBR. The KSF was calculated in the fluidized bed reactor using the procedure explained above. Figure 7 shows the results for the yield of (22) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. Ind. Eng. Chem., Prod. Des. Dev. 1986, 25, 172. (23) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. J. Anal. Appl. Pyrol. 1986, 9, 139. (24) Funazukuri, T.; Hudgins, R. R.; Silveston, P. L. J. Anal. Appl. Pyrol. 1988, 13, 103. (25) Scott, D. S.; Piskorz, J.; Radlein, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581.

Conesa et al.

Figure 7. KSF in the FBR. Pentane and methane.

pentane and methane. As was pointed out previously,4 the slope of the ln(yield of pentane) vs the KSF must be unity when KSF is very high. Concerning Figure 7, the dispersion of the experimental results is great. Nevertheless, in a similar study that was done with polyethylene waxes,4 the behavior of the yields of several compounds was clear and similar to that presented in this paper for tire decomposition. Considering, moreover, the great amount of experiments done in the fluidized bed pyrolysis reactor (and the difficulty controlling all the experimental variables, such as temperature profile, heterogeneity, etc.), we consider that the behavior is representative of the process. Therefore, it could be concluded from Figure 7 that the cracking of the tars from the tires takes place very quickly, at very low values of the KSF. The great dispersion of the data could be due, on one hand, to the heterogeneity of the sample, and on the other to the extreme experimental conditions of the runs,3 with very small amounts of material being discharged to a hot fluidized sand bed reactor. Figure 7 also shows the slope of the curve shown at KSF > 2. This value (slope ) -0.83) is close to the theoretical value expected (slope ) -1, as has been already shown). This value indicates that the pentane is not formed by secondary cracking and that the disappearance of pentane is in accordance with that predicted by the KSF. If the slope of the plot ln(pentane) vs KSF were very different from unity (i.e., 0.1, 10, or 1000), it would be concluded, with no further calculations, that the modeling is not correct. The kinetic constants for the pentane decomposition used in eq 1 have given good results for many researchers, so the

Volatile and Semivolatile Compounds from Tire Pyrolysis

Energy & Fuels, Vol. 14, No. 2, 2000 417

Table 4. Yields of Semivolatiles in the Pyrolysis Runs (weight ppm) compound

P650

P750

ethylbenzene phenylethyne styrene ethenylmethylbenzene 1-ethyl-3-methylbenzene mesitylene benzonitrile ethenylmethylbenzene 1-ethenyl-3-methylbenzene 1-hexanol, 2-ethyl1-propynyl-benzene 1-ethyl-2,3-dimethylbenzene acetophenone o-isopropenyltoluene 2,5-dimethyl-styrene 1,3-diethenyl-benzene 1H-indene, 1-methylene naphthalene benzo B thiophene 1,2-dihydro-6-methyl-naphthalene isoquinoline 1,2-dihydro-2-methyl-naphthalene benzo[b]thiophene, 6-methyl2-methyl-naphthalene 1-methyl-naphthalene biphenyl acenaphthylene 1,7-dimethyl-naphthalene 2-ethenyl-naphthalene biphenylene acenaphtene 4-methyl-biphenyl 1,4,5-trimethyl-naphthalene phenalene fluorene saturated hydrocarbon