Ind. Eng. Chem. Res. 2004, 43, 5133-5145
5133
Pilot-Scale Pyrolysis of Scrap Tires in a Continuous Rotary Kiln Reactor S.-Q. Li,*,† Q. Yao,† Y. Chi,‡ J.-H. Yan,‡ and K.-F. Cen‡ Department of Thermal Engineering, Tsinghua University, Beijing 100084, China, and Department of Energy Engineering, Zhejiang University, Hangzhou 310027, China
The pilot-scale pyrolysis of scrap tires in a continuous rotary kiln reactor was investigated at temperatures between 450 and 650 °C. As the reactor temperature increased, the char yield remained constant with a mean of 39.8 wt %. The oil yield reached a maximum value of 45.1 wt % at 500 °C. The pyrolytic derived oils can be used as liquid fuels because of their high heating value (40-42 MJ/kg), excellent viscosity (1.6-3.7 cS), and reasonable sulfur content (0.97-1.54 wt %). The true-boiling-point distillation test showed that there was a 39.2-42.3 wt % light naphtha fraction in the pyrolytic oil. The volatile aromatics were quantified in the naphtha fraction using gas chromatography-mass spectrometry. The maximum concentrations of benzene, toluene, xylene, styrene, and limonene in the oil were 2.09 wt %, 7.24 wt %, 2.13 wt %, and 5.44 wt %, respectively. The abundant presence of aromatic groups was also confirmed by functional group Fourier transform infrared analysis. The concentration of polycyclic aromatic hydrocarbons such as fluorine, phenanthrene, and anthracene increased with increasing temperature. The pyrolytic char was composed of mesopores with a Brunauer-Emmett-Teller (BET) surface area of about 89.1 m2/g. The char after carbon dioxide activation had a high BET surface area of 306 m2/g at 51.3% burnoff. The relationship between the surface area and the carbon burnoff was almost linear. Both the original pyrolytic char and the activated char have good potential for use as adsorbents of relatively large molecular species. 1. Introduction The disposal of scrap tires is currently a major environmental and economical issue. Recent estimates of the annual arisings of scrap tires in North America are about 2.5 million tonnes, in European Union about 2.0-2.5 million tonnes, and in Japan about 0.5-1.0 million tonnes.1,2 In China, more than 1.0 million tonnes/year of tires are generated, which results in about 0.22 million tonnes of used tires/year.3 Unfortunately, most of these scrap tires are simply dumped in the open and in landfills in our country. Open dumping may result in accidental fires with highly toxic emissions or may act as breeding grounds for insects. Landfills full of tires are not acceptable to the environment because tires do not easily degrade naturally. In recent years, many attempts have been made to find new ways to recycle tires, i.e., tire grinding and crumbling to recycle rubber powders and tire incineration to supply thermal energy. However, grinding is quite expensive because it is performed at cryogenic temperatures and requires energy-intensive mechanical equipment, while incineration may produce hazardous polycyclic aromatic hydrocarbons (PAHs) and soot during the combustion process.4 Pyrolysis as an attractive method to recycle scrap tires has recently been the subject of renewed interest. Pyrolysis of tires can produce oils, chars, and gases, in addition to the steel cords, all of which have the potential to be recycled. Within the past 2 decades, most experiments have been conducted using laboratory-scale batch units to characterize oil, char, and gas products. * To whom correspondence should be addressed. E-mail:
[email protected]. † Tsinghua University. ‡ Zhejiang University.
Some conclusions from these laboratory-scale studies are as follows: (1) Pyrolytic char has potential as a low-grade carbon black for a reinforcing filler or a printing ink pigment,4-6 as a carbon adsorbent after proper activation,7-9 and as a solid or slurry fuel.10 (2) Pyrolytic oil, a mixture of parafins, olefins, and aromatic compounds, possesses a high calorific value (∼43 MJ/kg) and can be used directly as fuel or can be added to petroleum refinery feedstocks.11-15 Oils can also be properly cut based on their evaporating temperatures to solely produce valuable chemical feedstocks (i.e., benzene, xylene, toluene, and D-limonene), or some of the chemicals can be extracted with residue used as fuel.16-22 (3) Pyrolytic gas contains high concentrations of methane, butadiene, and other hydrocarbons, which results in a high calorific value (35-40 MJ/kg) sufficient to heat the pyrolysis reactor.23-25 The gas is generally not sold as a commercial product but used as a process heat resource because of its low yield (∼10 wt %). However, these laboratory-scale studies differ greatly from the field-scale practical applications. Thus, the pilot-scale studies, which bridge between laboratoryscale data and large-scale applications, are of importance. However, the available literature on the continuous pilot-scale tire pyrolysis test is limited. According to reports, the reactors that can fulfill continuous tire pyrolysis include fluidized beds, vacuum moving beds, two-stage moving beds, ablative beds, and rotary kilns. Representative results for each reactor type are listed below. (1) Vacuum Moving-Bed Process (Laval University). Roy and co-workers have focused on the vacuum pyrolysis process for more than 20 years. The reactor development has gone through a 1 kg batch vacuum
10.1021/ie030115m CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
5134 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
vessel, a 15 kg/h semicontinuous vacuum hearth, and finally a 200 kg/h pilot-scale continuous moving bed.5,25,26 The advantage of the vacuum process is that the evaporating volatiles can be immediately removed from the reactor, which minimizes secondary reactions such as thermal cracking, repolymerization, and recondensation. As a result, the oil yield is dramatically increased at the expense of char and gas. Roy et al. also analyzed end uses of pyrolytic char and oil. For the solid products, they concluded that the Brunauer-Emmett-Teller (BET) surface area and structure of the carbon black after pyrolysis only changed a little. That is, a small amount of carbonaceous deposits formed on the pyrolytic char, which limits its use only as reinforcing filler of low-grade rubbers.5,27 The use of pyrolytic char as an additive for road bitumen was also discussed.28 For the liquids, Roy et al. concluded that the distillation of oils improved its economic value. For instance, the lighter fractions can be used as a source of high-value chemicals such as BTX (benzene, toluene, and xylene) and limonene and as an extender oil in rubber formulation,14-17 while the heavy fractions can be used as additives in road bitumen or as a feedstock for coke production.5 (2) Hamburg Fluidized-Bed Process (University of Hamburg). Kaminsky’s group launched the study of waste polymer pyrolysis in the 1970s. The well-known Hamburg process, using an indirectly heated fluidized bed, was developed with the original aim to yield basic chemicals such as BTX and carbon black for reinforcing at temperatures in excess of 700 °C.19 Laboratory-scale plants have been built with capacities of 0.60-3.0 kg/h of plastic, followed by pilot-scale plants with capacities of 10-40 kg/h of plastic and 120 kg/h of used tires at the University of Hamburg.29,30 In recent years, research efforts have been conducted at lower temperatures in order to improve the oil yield and to reduce the energy input.4 (3) BBC Continuous Ablative Process. Black and Brown reported limited details of the continuous ablation reactor from patent applications.31,32 Ablation is achieved by sliding contact of the rubber particles on a hot metal surface, which resulted in a high yield of liquids because of the fast heating of the tire. BBC Company (Canada) is one representative.33 Their tests in a 50 kg/h pilot-scale reactor demonstrated efficient heat and mass transfer, and a smaller test unit is presently operating at a throughput of 10-25 kg/h. In addition, a 1500-2000 kg/h commercial plant was sold to the Castle Capital Company for use in Halifax, Nova Scotia, Canada. Black and Brown31 found that liquid yields of 54 wt % can be obtained at 470-540 °C with a 0.88 s residence time and a 1.3 mm feeding size. The BTX composition in oils was less than 3.5 wt %. A study by Helleur et al.34 reported that pyrolytic char can be converted to activated char with steam or carbon dioxide. The activated char exhibited excellent quality for the removal of organics and heavy metals from aqueous solutions. (4) Two-Stage Moving-Bed Process. The process developed by the Universite Libre de Bruxelles (ULB) is based on a two-stage pyrolysis mechanism.18 During the first stage, the tire is depolymerized at a relatively low temperature (∼500 °C), while during the second stage, the pyrolytic volatiles are postcracked at temperatures of 750-800 °C. Their objective was just to recycle the BTX and activated carbon. Cypres and Bettens18 had obtained oil yields of 37.0-42.2 wt %,
Table 1. Proximate and Ultimate Analyses of Scrap Tire (Air-Dried Basis) case proximate analysis (wt %) moisture ash volatile matter fixed carbon ultimate analysis (wt %) carbon hydrogen nitrogen sulfur oxygen low heat value (kJ/kg)
this work
Roy et al.26
1.14 4.35 62.24 32.28
0.50 6.10 65.20 28.70
84.08 6.71 0.49 1.51 1.73 34923
81.50 7.10 0.50 1.40 3.40 36800
solid yields of 41.7-45.3 wt %, and gas yields of 6.019.5 wt %. The contents of BTX in oils are quite high, with values of 36.4, 16.8, and 6.95 wt %, respectively. (5) Continuous Rotary Kiln Process. A rotary kiln pyrolyzer offers many unique advantages over other types of reactors. For instance, the slow rotation of the inclined kiln enables well mixing of wastes, and thereby uniform pyrolytic products. Also the residence time of solids can be easily adjusted to provide the optimum conditions of pyrolysis reaction. Solid wastes of various shapes, sizes, and calorific values can be fed into a rotary kiln either in batches or continuously. The typical rotary kiln processes include the Kobe Steel commercial 1 tonnes/h plant, the Italian ENEA Research Center Trisaia pilot-scale plant, and the Kassel University laboratory-scale setup.35-37 These rotary kiln reactors widely exist in the process for pyrolysis or gasification, but the technical- and pilot-scale testing data are quite scarce. Only available data by Kawakami et al. were, in fact, less systemic.38 Within the past years, we have been devoted to the research and development of a rotary kiln pyrolyzer. Pyrolysis of various solid wastes in a laboratory-scale batch kiln39,40 and pilot-scale cold-model test in a continuous rotary kiln41,42 has been successively performed. On the basis of these works, a continuous pilotscale rotary kiln reactor has been self-designed and successfully operated.43 The objective in this paper is to study the pilot-scale pyrolysis characteristics of scrap tires in a continuous rotary kiln. Influences of the reactor temperature on the product distribution as well as the product properties and chemical compositions are intensively discussed. 2. Experimental Section 2.1. Tire Samples. Shredded scrap tires were used with particle sizes of 13-15 mm including the fabric cords but not the steel. The proximate and ultimate analyses on the air-dried basis and the calorific value of scrap tires are listed in Table 1. The data of Roy and Unsworth26 are given for comparison. The properties for two tires were quite similar, except for a small difference in the content of the carbon element. 2.2. Pilot-Scale Pyrolysis Process. The pilot-scale process development unit is shown in Figure 1. It consisted of a pyrolytic rotary kiln main reactor and peripheral systems including a supply system (a storage bin with a screw feeder), a tar condenser and reservoir, a solid residue collection tank, a flue gas cleaner, a demister filter, a gas burner, and an effluent gas sampling system.43 The kiln was designed for continuous operation with tire powder conveyed from a sealed container to the
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5135
Figure 1. Schematic of a pilot-scale continuous rotary kiln pyrolysis reactor: 1, purge-gas tank; 2, PID controller; 3, supply and screw feed; 4, mechanically frictional seal; 5, gear wheel; 6, external electric heater; 7, thermocouples; 8, computer; 9, primary packed condenser; 10, filter; 11, postcombustor; 12, gas sampling; 13, gas flowmeter; 14, induced fan; 15, secondary condenser with ice-water mixture; 16, collector of solid residue. Table 2. Operational Parameters of Each Run in the Pilot-Scale Tire Pyrolysis Test reactor temp/°C
kiln rotating rate/rpm
screw rotating rate/rpm
kiln slope/deg
pressure at the kiln outlet/Pa
450 500 550 600 650
0.45 0.60 0.75 0.90 0.90
5 5 5 5 5
2 2 2 2 2
-10 to -20 -10 to -20 -10 to -20 -10 to -20 -10 to -20
electrically heated rotary kiln by a screw feeder. The feeding rate was regulated from 10 to 30 kg/h. The kiln diameter was 0.3 m, and the overall length was 3.0 m. Three individual proportional-integral-derivative (PID)controlled heaters with a total power of 30 kW were used to heat the kiln. The effective heated length of the kiln was about 1.8 m. The solids were transported in the kiln as a result of inclination and rotation. At the kiln exit, the residue char fell into a sealed 6 L container. The solid residence time was adjusted by changing the kiln rotation to meet the desired condition for the complete pyrolysis. The evolved oil vapors and gases were quickly removed from the reactor by a special induced fan to reduce the residence time. The condenser, with four coils cooled by water and a trap refrigerated by ice, was used to recover pyrolytic oils into the reservoirs. The noncondensable gases passed to the scrubbing unit to remove the acids and finally passed to the burner. Two mechanical friction-type seals, with two wearable sliding rings and graphite between the rings, were used to seal the reaction chamber at both the kiln inlet and outlet. The pressure force on the seal was adjusted by varying the tension in the springs. 2.3. Operating Conditions. Before the experiment, nitrogen was used to purge the reactor for 3-5 min to remove the air. The system was operated at a slightly negative pressure of -20 to -10 Pa to prevent leakage of pyrolytic gases. The operating pyrolysis temperature was varied from 450 to 650 °C. The residence time of the solids must be chosen long enough to complete the pyrolysis. Because the measurement of the solid residence time during pyrolysis in a hot kiln is very difficult, the selection of the solid residence time can be optimized in the following program: conducting the pyrolysis test at a certain rotating rate with the char yield weighted, altering the rotating rate in a step of 0.5 rpm and repeating test, judging whether the difference between char yields in those two tests is less than 1%, and finally determining the rotating rate, i.e., the final residence time. The detailed experimental conditions for each pyrolysis run are presented in Table 2. Each run in Table 2 was reproduced. The feeding rate was 12-15 kg/h, with a total of 50 kg of tire in each run.
2.4. Pyrolytic Oil Characterization. 2.4.1. Determination of the Fuel Properties. The fuel properties of pyrolytic oils were analyzed according to the National Standards of the People’s Republic of China (PRC). The analyses included the relative density or API gravity by the pycnometer method (GB/T 2540-1981), the viscosity by the Ostwald method (GB/T 265-1988), the flash point by the closed-cup method (GB/T 261-1983), the moisture by the Karl Fischer method (GB/T 111331989), the carbon residue by the Conradson method (GB/T 17144-2001), and the calorific value and the sulfur content by the oxygen bomb method (GB/T 3881964). 2.4.2. Functional Group Compositional Analysis. The functional group compositional analysis of the oils was carried out by Nicolet-5DX type Fourier transform infrared (FT-IR) spectroscopy. A thin uniform layer of oils was placed on the sample cell, and peak heights were normalized to the major C-H peak. 2.4.3. True-Boiling-Point (TBP) Distillation of Oils. The FY-II crude oil TBP autoclave, developed by Fu-shun Institute of Petroleum (PRC), was used for the automatic distillation test. The distillation included atmospheric pressure distillation (theoretical plate number, 14-18; reflux ratio, 5:1; ibp, 200 °C) and reduced pressure distillation (1.33 kPa; reflux ratio of 2:1; 200350 °C). In each run, about 5-6 kg of pyrolytic oils was fed into the autoclave. As the temperature was increased from room temperature to 350 °C, the distillable fraction was cut in steps of 25 °C. Then each fraction was weighed to calculate its yield. 2.4.4. Oil Analysis of Aromatic Class. After TBP distillation, the oil was classified as the naphtha fraction (ibp ∼ 200 °C) and the heavy fraction (bp > 200 °C). The naphtha oil can be directly analyzed by Finnigan Voyager gas chromatography-mass spectrometry (GCMS). The analysis was performed on a DB-5 capillary column (30 m × 0.2 mm i.d. × 25 µm), using a 200:1 split ratio and a carrier helium gas at 1.0 mL/min. The distilled naphtha fraction, with an interval of 50 °C, was performed by GC-MS with a different temperature program. As for the oil fraction of bp < 100 °C, the oven temperature was initially set at 35 °C for 10 min, then programmed to 150 °C at 8 K/min, then moved to 250
5136 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 3. Pyrolytic Product Yields of Scrap Tires at Temperatures of 450-650 °C and Their Comparison with Various Typical Processes pilot-scale reactor rotary kiln
vacuum processa,5,16 two-stage processb,18 fluidized-bed process4 ablative process31
hydrocarbon vapor yield/(wt %)
run no.
temp/°C
char yield/ (wt %)
oil yield
gas yield
total volatile
AT01 AT02 AT03 AT04 AT05 H036 H045 H018 Rad-X Rallye V-10
450 500 550 600 650 480 (431)a 534 (510) 520 (500) 450 + 800 450 + 800 450 + 800 550 600 450
43.9 41.3 39.9 39.3 38.8 39.3 38.4 33.4 44.8 44.2 41.7 34.0 40.0 39.1
43.0 45.1 44.6 42.7 42.9 53.7 49.9 56.5 38.5 39.7 41.3 56.8 50.9 52.9
13.1 13.6 15.5 18.0 18.3 7.0 11.7 10.1 16.8 16.1 17.0 9.2 9.1 8.0
56.1 58.7 60.1 60.7 61.2 60.7 61.6 66.6 55.2 55.8 58.3 66.0 60.0 60.9
a
In a vacuum process by Roy et al., 480 (431) represents the heating medium and bed temperatures. b In the two-stage process by Cypres et al., 450 + 800 represents the primary reactor and secondary reactor temperatures; Rad-X, Rallye and V-10 are types of waste tire.
°C at 25 K/min, and finally held for 5 min. The program of the 100-150 °C oil fraction was started at 50 °C for 10 min, moved to 250 °C at 8 K/min, and held for 5 min. The program of the 150-200 °C fraction was started at 70 °C for 10 min, moved to 250 °C at 8 K/min, and held for 10 min. The MS operating conditions were as follows: ion source, 270 °C; electron energy, 70 eV, with a range of m/z 50-500 µm and a scan time every 1.0 ms. The heavy oil (>200 °C) was processed according to the China SY5119-86 standard, the method for the classes of soluble organics in rock and for the major chemical classes in crude oil. The heavy fraction was first solved using n-hexane, and the insoluble material was a bituminous fraction. The glass columns packed with silica gel and Al2O3 were used to separate oil into chemical class fractions followed by GC-MS. The column was then sequentially eluted with sequential elutions of hexane, CH2Cl2/hexane, and ethanol/CHCl3 to produce aliphatic, aromatic, and polar fractions. The aromatic fraction was analyzed by GC-MS to determine identification of PAH. The Wax-10 capillary column (30 m × 0.25 mm i.d.), with a 30:1 split ratio and a 1.0 mL/ min carrier helium, was used. The temperature profile was 75 °C for 3 min, followed by a 3 K/min heating to 180 °C, then a 10 K/min heating to 270 °C, and 20 min of holding. 2.5. Pyrolytic Char Characterization. 2.5.1. Proximate and Ultimate Analyses. The pyrolytic char was characterized using the following PRC standards: the proximate analysis by GB/T 212-2001, the ultimate analysis by GB/T 476-2001, and the size distribution by GB/T 12496.2-1999. 2.5.2. Activation of Tire-Derived Chars. A nearly isothermal, externally heated horizontal quartz-tube reactor (28 mm i.d. × 500 mm) was used to activate the tire-derived char. Prior to activation, the char was sieved to generate a 2.5-7.0 mm size fraction and dried to 105 °C. About 15 g of char was placed in the reactor, with carbon dioxide used as the activated agent. The experiments were conducted between 850 and 950 °C. If not specifically indicated, the activating-agent flow rate was 0.3 L/min and the activation time was 4 h. The tests studied the influences of the activation temperature on the degree of burnoff. The burnoff was defined as
burnoff (wt % daf) ) (w1 - w2)/w1 × 100
(1)
where w1 and w2 are the char mass (dry ash free basis) before and after activation, respectively. Only char obtained at a pyrolytic temperature of 550 °C was selected for activation because temperatures in excess of 500 °C had little effect on the char characterization, as shown in the results. 2.5.3. Surface Area and Pore Structure of Pyrolytic or Activated Char. The surface areas of both the pyrolytic char and the activated carbon were determined by nitrogen adsorption at 77 K using a BET isotherm. The instrument for the nitrogen adsorption experiments was made by Quantachrome Corp. In addition, a mercury intrusion porosimeter by Quantachrome Corp. was used as a complementary tool to study the pore structures of pyrolytic chars. 3. Discussion and Results 3.1. Tire Pyrolysis Reaction Scheme. From the point of view of kinetics, tire pyrolysis is quite complex and consists of more than hundreds of chemical reactions. Although each individual reaction is difficult to determine, these reactions can be generally classified into three groups: primary pyrolysis reaction (250-520 °C), secondary postcracking reaction of pyrolytic volatiles (600-800 °C) that strongly affect BTX yields, and char gasifying reaction with CO2/H2O/O2 in the gases (750-1000 °C). The importance of each reaction group is dependent on two parameters, the temperature and time of residence, as indicated by the rates of reaction in Arrhenius form. These multigroup reaction schemes can be used to understand the pyrolysis results in the following parts. 3.2. Yields of Pyrolytic Products. The yields of pyrolytic char, oil, and gas (in difference) are presented in Table 3 for temperatures of 450-650 °C. Data for other typical pilot-scale continuous pyrolyzers (vacuum, fluidized-bed, ablative, and two-stage bed) are also given in Table 3 as a contrast. First, the solid char yield remains essentially constant with a mean of 39.8 wt %, except for the relatively high value of 43.9 wt % at a temperature of 450 °C. The data suggested that the kinetics of the primary pyrolysis reaction did not complete at low temperature (450 °C).44,45 Compared with other typical processes, Roy et al.5,16 reported that the vacuum moving-bed process with a feedstock of 21-42 kg/h had a char yield of 38.439.3 wt %. The two-stage process by Cypres and
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5137 Table 4. Physical-Chemical Properties of Pyrolytic Oils in a Rotary Kiln and Comparison with the Vacuum Process vacuum process5
rotary kiln pyrolysis run characteristics density (kg/Nm3) gravity (API) viscosity at 50 °C (cS) flash point (°C) LHV (MJ/kg) moisture (wt %) carbon residue (wt %) ash (wt %) ultimate analysis C (wt %) H (wt %) N (wt %) S (wt %) O + others (wt %) H/C atomic ratio
450 °C
500 °C
550 °C
600 °C
650 °C
car tire (no. H20)
truck tire (no. H22)
0.941 18.2 2.87 27.5 41.9 0.52 1.36 traces
0.962 15.0 2.44 17.0 41.7 0.88 1.78 traces
0.987 11.3 3.66 30.0 41.0 1.32 3.09 traces
0.955 16.1 1.63 17.5 41.6 0.80 2.01 traces
0.982 11.9 2.00 13.5 41.0 0.85 3.26 traces
0.950 17.4 9.7 28 43.7 0.3 1.30 traces
0.939 19.3 17.8 22 44.8 1.5 1.20 0.005
84.32 10.92 0.48 0.97 3.31 1.55
84.26 10.39 0.42 1.54 3.39 1.48
84.55 9.59 0.64 1.26 3.96 1.36
86.14 9.54 0.70 1.27 2.35 1.33
86.1 9.06 0.84 1.11 2.89 1.26
85.8 10.7 0.5 0.8 2.2 1.49
87.0 11.1 0.7 1.0 0.2 1.54
Figure 2. TBP distillation test of tire-derived oil at temperatures of 500 and 600 °C.
Bettens18 can produce 41.7-44.8 wt % char with three different brands of tire at a temperature of 450 °C. Kaminsky and Mennerich,4 using a fluidized-bed reactor, showed that the char yield was about 40.0 wt % at 600 °C. In addition, a char yield of about 39.1 wt % was obtained in a continuous ablative process developed by BBC company, Canada.31 Comparison with other processes indicates that the char yield is not sensitive to the reactor types of various heating rates, which differs greatly from the pyrolysis of coal or biomass. This is because that scrap tire is a synthetic chemical feedstock according to a certain formula ratio. Tires are made of a complex blend of elastomers, processing oils, carbon black, and a few additives including mineral fillers, vulcanization agents, palticizer, etc. Because the pyrolytic vapors (elastomers and processing oils) produced during tire pyrolysis by thermogravimetry (TG) are about 62-65 wt %,44,45 the yield of pyrolytic solid residue should theoretically be 35-38 wt %. In the practical process, the char yield is a little higher than this value due to the slight hydrocarbon deposits in the char, for example, about 39.8 wt % in this experiments. As shown in Table 3, the volatiles, consisting of oils and gases, are constant at a value of 60 wt % with various temperatures. However, the oil yields reached a maximum yield of 45.1 wt % at 500 °C and then
decreased, as the temperature increased from 450 to 650 °C. The gas yield correspondingly increased from 13.1 to 18.3 wt % as a result of the serious secondary postcracking of vapors at higher temperature. The maximum yield of oils at 500 °C can be attributed to the balance for the competition between the primary pyrolysis reaction and secondary postcracking reaction. The oil yield in the current kiln process is compared with that of other reactor processes in Table 3. Although the reactor configuration has little influence on the char yield, the oil yield varied greatly for the different reactor types. The vacuum process by Roy and co-workers5,16 had a maximum oil yield of 53.7 wt % at 431 °C. The Hamburg fluidized-bed process by Kaminsky and Mennerich4 produced 56.8 wt % oil at 550 °C. The continuous ablative process had a maximum oil yield of 52.9 wt %.30,31 The maximum oil yield of 41.3 wt % was obtained by Cypres and Bettens18 using the two-stage moving bed, which is lowest among all of the processes. The different oil yields of various reactors due to the different degrees of vapor postcracking can mainly be attributed to the difference of the vapor residence time in the high-temperature zone in each process. It is noted that Kawakami et al.,38 using a similar rotary kiln reactor, obtained a maximum oil yield of 53.0 wt % at 500 °C, which was higher than the data in this experiment. The effective heated length is 1.8 m, and the length of the kiln outlet is 0.6 m. It can be explained that the ratio of length to diameter (L/D ) 8) in this paper might be larger than that from the study of Kawakami et al. The larger the ratio of L/D, the higher the residence time of the vapor. A high vapor residence time in the high-temperature zone caused the reduction of the oil yield in this paper. According to the authors’ experiences, the concentrations of valuable chemicals such as BTX and limonene in the oils exerted more important impacts on the economic viability of the process than the total oil yield. The quantification of the valuable chemicals was essential. It would be discussed in the following part. 3.3. Characterization of Pyrolytic Oils. The pyrolytic oils are derived from (1) processing oils in the original tire, (2) evaporating and decomposing of elastomers such as natural rubber, butadiene rubber, or styrene-butadiene rubber, and (3) other organic additives in tires. The tire-derived oils are mostly unrefined oils with a wide range of boiling fractions. 3.3.1. Fuel Properties. Table 4 presents the fuel properties of the pyrolytic oils carried out in the pilot-
5138 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
scale rotary kiln. The properties of oils from the vacuum process by Roy et al.5 are also given as a contrast. The calorific value of the tire oils is similar for all of the tests, with values of 41.0-41.9 MJ/kg, which is higher than that of scrap tires (34.9 MJ/kg). Moreover, this high calorific value of the derived oil is comparable with that of light fuel oil, indicating the potential for use as liquid fuels for industrial furnaces and power plants. The viscosity of pyrolytic oils was 2.0-3.7 cS (at 50 °C). It is similar to that of diesel fuel (1.8-8.0 cS at 20 °C) but at least 1 order of magnitude lower than that of marine or furnace fuel oils. The fuel viscosity is important because it affects the fuel flow through pipes, the fuel atomization, and the performance and wear of diesel pumps. The viscosity of oils derived from the vacuum process by Roy et al.5 was about 4 times that of the viscosity of oils derived from the current rotary kiln process. The oils derived from a fixed bed by Cunliffe and Williams1 had a viscosity of 2.38 cS at 60 °C, which agrees well with the pyrolytic oils in this paper. Generally, the shorter residence time of vapor caused by the vacuum pump reduced the secondary postcracking reaction and improved the oil yield but also increased the amounts of long-chain molecules in the oils, which resulted in the high oil viscosity and limited its usage. The oil density was 0.94-0.99 kg/m3 over a temperature range of 450-650 °C, while the corresponding API gravity was 11.3-18.2. The density of pyrolytic oils was higher than that of diesel oils (0.78 kg/m3) and approached that of marine fuel oils (0.98 kg/ m3). The effects of temperature on both the oil viscosity and density were indistinct within the error limits of measurement. The flash point of liquid fuel indicates the temperature at which the oil begins to evolve vapors in sufficient quantity to form a flammable mixture with air. As seen in Table 4, the flash point of the tire-derived oils was 13.5-30 °C with no particular trend with increasing temperature. Roy et al.5 reported a value of 22-28 °C, and Cunliffe and Williams1 reported 14-18 °C. The flash point of the tire-derived oil was lower than that of the petroleum-refined fuels. For example, light diesel oil has a required minimum flash point of 45 °C and a heavy gas oil of 65 °C. The low flash point of the tire oil can be attributed to the wide range of unrefined oils in the mixture including some light low-boiling-point hydrocarbons. The low flash point will complicate storage of the oils. The moisture of 0.5-1.3 wt % will also be noticeable during the pyrolytic oil usage. The residue carbon in pyrolytic oil was 1.3-3.3 wt %. The corresponding value reported by Roy et al. was 1.21.3 wt %, and that by Williams was 0.5-2.2 wt %.5-13 A typical diesel fuel would have a carbon residue of approximately 0.2 wt %; however, fuel oils used in a very large diesel engine may have carbon residues of up to 12 wt %. Thus, the tire-derived oil should be used in a large diesel engine or industrial boiler rather a microscale typical diesel. The temperature had only a slight effect on the elemental content of pyrolytic oils. As the temperature increased, carbon increased slowly from 84% to 86% and hydrogen decreased rather appreciably from 10.9 to 9.0. The hydrogen/carbon (H/C) ratio was about 1.26-1.55, which is somewhat lower than that of diesel oil (1.735) and light fuel oil (1.740). With increasing temperature, the H/C ratio decreased; especially, the drop gradient
Table 5. Carbon, Hydrogen, and Nitrogen Profiles in Each Distillate Fraction for Pyrolyic Oils Produced at 600 °C distillant
carbon
hydrogen
nitrogen
H/C ratio
total oils 50-100 °C fraction 100-150 °C fraction 150-200 °C fraction 200-250 °C fraction 250-300 °C fraction 300-350 °C fraction
86.14 87.03 89.83 88.50 86.56 87.22 88.28
9.54 9.93 9.50 9.51 9.06 9.27 9.24
0.70 0.46 0.56 1.13 1.71 1.49 1.12
1.33 1.37 1.27 1.29 1.26 1.28 1.26
between 500 and 550 °C is sharper than that in other zones. This implies that the postcracking of evolved vapor, including chain cracking, dehydrocyclization, and aromatization, becomes serious from this zone onward. Thus, the aromatics in the tire-derived oil increased, as indicated by the reducing H/C ratio. In addition, the nitrogen content increased with increasing temperature because the nitrogen in the char or gas participates in the aromatization process to form polycyclic aromatic nitrogen hydrocarbons (PNAHs). The sulfur content was 0.97-1.54 wt % and similar to that of light fuel oil, which is typically about 1.4-1.5 wt %. Comparison of the pyrolytic oils from the rotary kiln and the vacuum process5 indicated that both the nitrogen and sulfur contents are similar, the hydrogen contents are similar below 500 °C but differ above 500 °C, the carbon content in the rotary kiln is lower than that in the vacuum process by 1-2%, and the oxygen content is higher than that in the vacuum process. Table 5 shows the carbon, hydrogen, and nitrogen contents in different distillate fractions of pyrolyic oil at 600 °C. The H/C ratio was quite constant with a mean of 1.27, except for a value of 1.37 in the 50-100 °C distillate fraction. Therefore, the aromatization degrees of the fractions in excess of 100 °C were similar. The nitrogen content in the low-boiling fractions (150 °C), which suggests that the nitrogen exists mainly as a high-boiling-point PNAH such as caprolectum, quinoline, etc. 3.3.2. TBP Distillation Test of Pyrolytic Oil. The TBP distillation test is one of the best approaches to reflect the true distillation of oil. It plays an important role in developing oil refinery or extraction processes. Although many researchers have emphasized the importance of the TBP distillation test, little literature is found about the TBP test because it needs a great amount of pyrolytic oils (about 5 kg). Roy et al.5 once reported the simple test of the pyrolytic oil distillation. Figure 2 shows the results of the TBP distillation test of the pyrolytic oils at temperatures of 500 and 600 °C. The pyrolytic oils had fractions with a wide range of boiling points, mainly 39.2-42.3 wt % light naphtha (ibp ∼ 200 °C), 32.4-33.2 wt % medium fractions that can be used as diesel oil (200-350 °C), and 25.5-28.5 wt % heavy fractions (>350 °C). Comparison of these results with those of vacuum pyrolysis in Table 6 indicated that (1) the light fraction (39-42.3 wt %) is higher than that in the vacuum pyrolysis oil (26.8 wt %) and (2) the contents of medium fractions of both studies are similar at about one-third of the total oil. Kawakami et al.38 reported a relatively low light fraction of 24.5 wt %. It is concluded that the significant postcracking of the oil resulted in not only a low oil yield but a highly valuable light naphtha fraction as well. Figure 3 shows the distillate fractions at intervals of 25 °C of pyrolytic oil produced at 600 °C. The light
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5139 Table 6. Distillate Fractions for Pyrolytic Oils and Crude Oils oil fraction tire-derived pyrolytic oils (%) crude oils produced in China (%)
oil samples
light fraction (ibp ∼ 200 °C)
medium fraction (200-350 °C)
heavy fraction (>350 °C)
600 °C 500 °C vacuum process5 Daqing Shengli Xinjiang
42.3 39.2 -26.8 10.7 7.6 15.4
33.2 32.4 30.7 17.3 26.3 26.0
24.5 28.4 42.5 69.0 66.0 58.6
Figure 3. Distillate fractions at intervals of 25 °C for tire-derived oil produced at 600 °C.
naphtha fraction (ibp ∼ 200 °C) and the medium fraction (200-350 °C) have normal distribution, with mean values of 150 and 275 °C, respectively. This distribution favors the cut of pyrolytic oil as the naphtha and diesel oil in potential applications. In addition, Table 6 also compares the distillation fractions of the pyrolytic oil and three crude oils in China (Daqing, Shengli, and Xinjiang). The pyrolytic oil is much lighter than the crude oil. The crude oil has a 58.6-69.0 wt % heavy fraction (>350 °C), while the pyrolytic oil only contains a 24.5-28.4 wt % heavy fraction. Therefore, the properties of the pyrolytic oil are superior to those of the crude oil. 3.3.3. Functional Group Compositional Analysis of Pyrolytic Oil. The group composition of the pyrolytic oil was determined by FT-IR spectroscopy. Figure 4 illustrates the spectra of the tire-derived pyrolytic oils at temperatures of 450-650 °C. Using the FT-IR spectra of the oil at 600 °C as an example, the following conclusions can be drawn: (1) The dCH stretching vibrations at 3100-3000 cm-1 indicated the presence of aromatic or alkene groups. The aromatic groups can be distinguished from the alkenes by the breathing vibration, which was defined as the conjugated CdC vibrations occurring in the vicinities of both 1600 and 1500 cm-1. In pyrolytic oil spectra, two conjugated peaks at 1604 and 1495 cm-1 confirmed the presence of the aromatic groups. Furthermore, single, polycyclic, and substituted aromatic groups can be identified by the absorbance peaks of C-H cyclic deforming vibrations at 900-675 cm-1 as well as the resonating peaks of C-H and C-C deforming vibrations at 2000-1600 cm-1. (2) The absorbance peaks at both 1675-1575 and 950-875 cm-1 represented the CdC stretching vibrations, which confirm the presence of alkenes in the pyrolytic oil.
(3) The two distinctly intensive peaks at 2906 and 2869 cm-1 represented the C-H stretching vibrations and were indicative of alkanes. Furthur detail concerning the group compositions can be deduced from the C-H deformations at 1500-1300 cm-1 and the C-H cyclic deformations at 1000-650 cm-1. For instance, the C-H deformations for the tire pyrolytic oil occurred at 1455 cm-1, while the C-H cyclic deformations at 775735 cm-1. Thus, it is deduced that the C-H group in the pyrolytic oil was probably -C-(CH3)n-C- (n < 4) or mC-CH3. (4) The remarkably wide absorbance peaks between 3700 and 3200 cm-1 represented the polar compounds in the pyrolytic oil. These peaks can be possibly attributed to the resonance of the O-H and N-H groups, which indicated an acylamino group. Moreover, the presence of CdO50 nm), meso(2-50 nm), and micropore volumes (50 nm) to the total volume for both materials were negligible. Moreover, excluding the micropores, the mesopores (2-50 nm) in the char were similar to those of the commercial activated carbon. The pore volume profile density reached a maximum value at a pore diameter of 25 nm (rj ≈ 25 nm). Finally, although the lower limit of the mercury intrusion test is 3.6 nm, the data in Figure 6 show that the micropores in pyrolytic char were greatly less developed than those in the commercial activated carbon. Pyrolytic char would be more useful for aqueous adsorption of large molecular
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5143
Figure 6. Pore volume profiles for pyrolytic char and commercial activated carbon measured using the mercury intrusion method.
Figure 7. Burnoff and BET surface area achieved in carbon dioxide for the 2.5-5.1 mm fraction of a 550 °C tire-derived char in relation to the activation temperature.
weight species instead of small molecular weight species, because of the rich mesopores in char. 3.4.4. Activation Characteristics of Pyrolytic Chars. Because the original pyrolytic char can only be used as an adsorbent of large molecular species, activation of the char to produce a relatively high grade activated carbon with both high BET surface area and micropore volume may represent a more economically attractive option. Figure 7 shows the burnoff degree and BET surface area achieved in carbon dioxide using the 2.5-5.1 mm fraction of 550 °C pyrolytic char in relation to the activation temperature. As the activation temperature increased from 850 to 950 °C, the burnoff increased from 21.3% to 51.3% nearly in a linear fashion. The BET surface area also linearly increased from 125 to 306 m2/g. The impact of the activation temperature can be attributed to the Arrhenius law, as discussed in detail by Cunliffe and Williams.47 Figure 8 shows the influence of the burnoff degree on the BET surface area of the activated carbon produced by activation of 550 °C tire-derived char. The maximum BET surface area attained was 306 m2/g at the 51.3% burnoff (950 °C and 4 h), while that of the original pyrolytic char was only 89.1 m2/g. High surface areas for activated tire-derived char similar to these
Figure 8. Influence of burnoff on the BET surface area of activated char produced from the activation of a 550 °C tire-derived char.
values have also been reported in the literature. For example, Teng et al.8 reported that activation of tirederived char with carbon dioxide using TG-DTG resulted in a maximum surface area of 370 m2/g with 50% burnoff. Merchant and Petrich9 demonstrated conversion of the tire-derived char (using a muffle furnace) to activated carbon with a surface area of 500 m2/g at 850 °C in a nitrogen flow containing 40 mol % steam. Mirmiran et al.48 reported that the activation of 550 °C pyrolytic char by carbon dioxide under 900 °C resulted in a maximum surface area of 500 m2/g with 80% burnoff. Cunliffe and Williams47 studied activation of the fixed-bed pyrolytic char and reported that, for 50% burnoff, carbon activated by steam had a surface area of 510 m2/g, while carbon activated by carbon dioxide had a surface area of only 420 m2/g. The activation of the tire-derived char (using an ablative pyrolyzer) with carbon dioxide by Helleur et al.34 obtained a surface area of 240-270 m2/g at about 40% burnoff. The high ash content of the pyrolytic char in the current work partially limited the enlargement of the BET surface area during activation. Therefore, acid demineralization of the char after activation is needed to further improve the surface area of the activated char. In addition, the approximately linear relationship between the surface area and the carbon burnoff in Figure 8 has been observed in many previous studies. Cunliffe and Williams47 found an initial relatively slow increase in the BET surface area with increasing burnoff, followed by a linear increase up to a maximum surface area at 65% burnoff, after which the surface area then decreased. Merchant and Petrich9 not only illustrated the linear relationship between the BET surface area and burnoff but also studied the variation of the micropore volume with burnoff. They concluded that the increasing BET surface area with burnoff below 40% can be attributed to the increasing micropore volume, while the increase with burnoff in excess of 40% was attributed to the increasing meso- and macropore volumes. Furthermore, Teng et al.8 and Mirmiran et al.48 also pointed out the linearity between the surface area and carbon burnoff for various scale reactors. Figure 9 compares the pore volume profiles for activated tire-derived char (pyrolyzed at 550 °C and
5144 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
Figure 9. Pore volume profiles for activated tire-derived char (pyrolyzed at 550 °C and activated at 950 °C) and commercial activated carbon.
activated at 950 °C) and commercial activated carbon. As mentioned earlier, the pore volumes in the original pyrolytic char were much different from those in the commercial activated carbon by at least 1 order of magnitude. As shown in Figure 9, the pore volume profile for the activated tire-derived char is quite similar to that for the commercial activated carbon, especially for the mesopore volume. However, the micropore volume in the activated tire-derived char is still less than that in the commercial activated carbon. This suggests that the pyrolytic or activated tire-derived char should be used as a mesoporous carbon in some special cases, e.g., the aqueous adsorption of large molecular species. Aqueous adsorption using tire-derived char has also been investigated. The adsorption of large-molecule methyl blue by the activated or original tire-derived char was both about 230 mg/g3, which is a little higher than that of the commercial activated carbon. However, the adsorption of the iodine by tire-derived char was only 1/ that of the commercial activated carbon. Further 5 research on the adsorption of specific large-molecule pollutants would be of interest. For example, the molecular sizes of mercury and dioxins, mainly produced from waste incinerators, are 0.45 × 0.45 and 1.0 × 0.3 nm, respectively.49 The effective pore diameter for the adsorption of dioxins is about 2-10 nm, which belongs to the mesopore structure, while that for mercury is 0.5-5 nm, which belongs to the transition zone between micropores and mesopores. Thus, the original inactivated tire-derived char theoretically has the potential for the adsorption of dioxins, and slightly activated tirederived char may be used as an adsorbent for mercury. Further work is needed to develop these applications. 4. Conclusions (1) A rotary kiln is a suitable alternative for a pyrolysis reactor of scrap tires because the solid residence time can be flexibly adjusted to meet the requirement of complete decomposition. Two crucial experiences should be noted in the operation of a pilot-scale rotary kiln pyrolyzer. First, the selection of the solid residence time, measured with difficulty during pyrolysis, can be optimized as follows: conducting similar pyrolysis tests by increasing the rotating rate in a step of 0.5 rpm,
judging whether the difference between char yields in the subsequent tests is less than 1%, and finally determining the solid residence time of complete pyrolysis. Second, the kiln length with respect to diameter plays a crucial role in determining the vapor residence time. The latter directly determines the oil yield as well as its valuable light compositions such as BTX. The optimum time/temperature profiles achieved by the multiple heating zones with different temperatures along the kiln length should been attempted to promote the production of volatile aromatics. (2) As the temperature increased from 450 to 650 °C, the char yield remained essentially constant at a mean of 39.8 wt %. Compared with other typical processes such as vacuum, fluidized-bed, ablative, and two-stage, the char yield is independent of reactor types with different heating rates. The oil yield reached a maximum value of 45.1 wt % at 500 °C and then decreased. The oil yield is less than that of Kawakami et al. using a similar rotary kiln.38 The long vapor residence time in the high-temperature zone caused not only the low yield but also richer single-ring aromatics than other typical processes. The maximum concentrations of benzene, toluene, xylene, styrene, and limonene in the oil were 2.09, 7.24, 2.13, and 5.44 wt %, respectively. The absorbance peaks at 3100-3000, 1604 (1495), and 900-675 cm-1 using FT-IR analysis confirmed the formation of rich aromatics. The increasing temperature resulted in the increment of the aromatics composition, especially the high concentration of PAHs. (3) The TBP distillation test of the tire-derived oil had originally been conducted. It had a 39.2-42.3 wt % naphtha fraction (ibp ∼ 200 °C), a 32.4-33.2 wt % medium fraction that can be used as diesel oil (200350 °C), and a 25.5-28.5 wt % heavy fraction (>350 °C). The content of the naphtha fraction was greatly higher than that by Kawakami et al. (24.5 wt %).38 (4) The pore structure of pyrolytic char was mostly composed of mesopores instead of micropores as measured by both N2 adsorption and mercury intrusion analyses. It is most suitable for aqueous adsorption of large molecular weight species. (5) Pyrolytic char after carbon dioxide activation had a relatively high BET surface area of 306 m2/g at 51.3% burnoff. There is an approximately linear relationship between the surface area and carbon burnoff. Further studies on the adsorption of dioxins and heavy metals are currently underway. Acknowledgment This work was funded by National Natural Science Fundation of China (Grant 50076037). We sincerely thank Prof. Paul Williams, University of Leeds, for the inspiration on the research method by their publications. The authors thank the the staff of Department of Chemistry, Zhejiang University, for their help in the analysis of oil properties. We also acknowledge the help of Dr. R.-D. Li, Dr. J.-T. Huang, Dr. D.-H. Yan, Y.-L. Gao, and S.-E. Wen for their cooperation in the experiments. Literature Cited (1) Cunliffe, A. M.; Walliams, P. T. Composition of Oils Derived from the Batch Pyrolysis of Tyres. J. Anal. Appl. Pyrolysis 1998, 44, 131. (2) De Marco Rodriguez, I.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chmon, M. J.; Caballero, B. Pyrolysis of scrap tyres. Fuel Process Technol. 2001, 72, 9.
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5145 (3) Li, S.-Q. Pyrolysis of solid wastes for clean fuels and chemicals: Lab- and pilot-scale studies. Ph.D. Dissertation, Zhejiang University, Hangzhou, China, 2002. (4) Kaminsky, W.; Mennerich, D. Pyrolysis of synthetic tire rubber in a fluidised-bed reactor to yield 1,3-butadiene, styrene and carbon black. J. Anal. Appl. Pyrolysis 2001, 58-59, 803. (5) Roy, C.; Chaala, A.; Darmstadt, H. The vacuum pyrolysis of used tires End-uses for oil and carbon black products. J. Anal. Appl. Pyrolysis 1999, 51, 201. (6) Bilitewaki, B.; Ha¨rdtle, G.; Marek, K. Usage of carbon black and activated carbon in relation to input and technical aspects of the pyrolysis process. In Pyrolysis and gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Eds.; Elsevier Applied Science: London, U.K., 1989. (7) San Miguel, G.; Fowler, G. D.; Sollars, C. J. Pyrolysis of tire rubber: porosity and adsorption characteristics of pyrolytic chars. Ind. Eng. Chem. Res. 1998, 37, 2430. (8) Teng, H.; Serio, M. A.; Wojtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Reprocessing of used tires into activated carbon and other products. Ind. Eng. Chem. Res. 1995, 34, 3102. (9) Merchant, A. A.; Petrich, M. A. Pyrolysis of scrap tires and conversion of chars to activated carbon. AIChE J. 1993, 39, 1370. (10) Collins, L. W.; Downs, W. R.; Gibson, L. K.; Moore, G. W. An evalation of discarded tires as source of fuel. Thermochim. Acta 1974, 10, 153. (11) De Marco Rodriguez, I.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chmon, M. J.; Caballero, B. Pyrolysis of scrap tyres. Fuel Process Technol. 2001, 72, 9. (12) Williams, P. T.; Bottrill, R. P.; Cunliffe, A. M. Combustion of tyre pyrolysis oil. Trans. Inst. Chem. Eng. 1998, 76B, 291. (13) Williams, P. T.; Besler, S.; Taylor, D. T. The batch pyrolysis of tyre wastesfuel properties of the derived oils and overall plant economics. Proc. Inst. Mech. Eng. 1993, 207, 55. (14) Benallal, B.; Roy, C.; Pakdel, H.; Chabot, S.; Poirier, M. A. Characterisation of pyrolytic light naphtha from vacuum pyrolysis of used tyres. Comparison with petroleum naphtha. Fuel 1995, 74, 1589. (15) Pakdel, H.; Roy, C. Simultaneous gas chromatographicFourier transform infrared spectroscopic-mass spectrometric analysis of synthetic fuel derived from used tire pyrolysis oil, naphtha fraction. J. Chromatogr. A 1994, 683, 203. (16) Pakdel, H.; Pantea, D. M.; Roy, C. Production of dllimonene by vacuum pyrolysis of used tires. J. Anal. Appl. Pyrolysis 2001, 57, 91. (17) Pakdel, H.; Roy, C.; Aubin, H.; Jean, G.; Coulombe, S. Formation of DL-limonene in used tire vacuum pyrolysis oils. Environ. Sci. Technol. 1992, 25, 1646. (18) Cypres, R.; Bettens, B. Production of Benzoles and Active Carbon from Waste Rubber and Plastic Materials by Means of Pyrolysis with Simultaneous Post-cracking. In Pyrolysis and gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Eds.; Elsevier Applied Science: London, U.K., 1989. (19) Kaminsky, W.; Shin, H. Pyrolysis of plastic waste and scrap tyres using a fluidised bed process. In Thermal Conversion of Solid Waste and Biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980; p 423. (20) Kaminsky, W.; Kim, J.-s. Pyrolysis of mixed platics into aromatics. J. Anal. Appl. Pyrolysis 1999, 51, 127. (21) Williams, P. T.; Taylor, D. T. Aromatisation of tyre pyrolysis oil to yield polycylic aromatic hydrocarbons. Fuel 1993, 72, 1469. (22) Williams, P. T.; Bottrill, R. P. Sulfur-polycylic aromatic hydrocarbons in tyre pyrolysis. Fuel 1995, 74, 736. (23) Williams, P. T.; Besler, S.; Taylor, D. T. The pyrolysis of scrap automotive tyres: the influence of temperature and heating rate on product combustion. Fuel 1990, 69, 1474. (24) Williams, P. T.; Besler, S.; Taylor, D. T.; Bottrill, R. P. Pyrolysis of automotive tyre waste. J. Inst. Energy 1995, 68, 11. (25) Roy, C.; Labrecque, B.; de Caumia, B. Recycling of scrap tyres to oil and carbon black by vacuum pyrolysis. Resour., Conserv. Recycl. 1990, 4, 203. (26) Roy, C.; Unsworth, J. Pilot-scale plant demostration of used tyres vacumm pyrolysis. In Pyrolysis and gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Eds.; Elsevier Applied Science: London, U.K., 1989. (27) Roy, C.; Rastegar, A.; Kaliaguine, S.; Darmstadt, H. Physicochemical properties of carbon blacks from vacuum pyrolysis of used tires. Plast. Rubber Compos. Process. Appl. 1995, 23, 21.
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Resubmitted for review January 14, 2004 Accepted April 30, 2004 IE030115M
