pilot-scale pyrolysis of scrap tires in a continuous rotary kiln reactor

Pilot-Scale Pyrolysis of Scrap Tires in a Continuous Rotary KilnReactor

S.-Q. Li,*,† Q. Yao,† Y. Chi,‡ J.-H. Yan,‡ and K.-F. Cen‡

Department of Thermal Engineering, Tsinghua University, Beijing 100084, China, andDepartment of Energy Engineering, Zhejiang University, Hangzhou 310027, China

The pilot-scale pyrolysis of scrap tires in a continuous rotary kiln reactor was investigated attemperatures between 450 and 650 °C. As the reactor temperature increased, the char yieldremained 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 heatingvalue (40-42 MJ/kg), excellent viscosity (1.6-3.7 cS), and reasonable sulfur content (0.97-1.54wt %). The true-boiling-point distillation test showed that there was a 39.2-42.3 wt % lightnaphtha fraction in the pyrolytic oil. The volatile aromatics were quantified in the naphthafraction 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 5.44wt %, respectively. The abundant presence of aromatic groups was also confirmed by functionalgroup Fourier transform infrared analysis. The concentration of polycyclic aromatic hydrocarbonssuch as fluorine, phenanthrene, and anthracene increased with increasing temperature. Thepyrolytic char was composed of mesopores with a Brunauer-Emmett-Teller (BET) surface areaof about 89.1 m2/g. The char after carbon dioxide activation had a high BET surface area of 306m2/g at 51.3% burnoff. The relationship between the surface area and the carbon burnoff wasalmost linear. Both the original pyrolytic char and the activated char have good potential foruse as adsorbents of relatively large molecular species.

1. Introduction

The disposal of scrap tires is currently a majorenvironmental and economical issue. Recent estimatesof the annual arisings of scrap tires in North Americaare about 2.5 million tonnes, in European Union about2.0-2.5 million tonnes, and in Japan about 0.5-1.0million tonnes.1,2 In China, more than 1.0 milliontonnes/year of tires are generated, which results inabout 0.22 million tonnes of used tires/year.3 Unfortu-nately, most of these scrap tires are simply dumped inthe open and in landfills in our country. Open dumpingmay result in accidental fires with highly toxic emis-sions or may act as breeding grounds for insects.Landfills full of tires are not acceptable to the environ-ment because tires do not easily degrade naturally. Inrecent years, many attempts have been made to findnew ways to recycle tires, i.e., tire grinding and crum-bling to recycle rubber powders and tire incineration tosupply thermal energy. However, grinding is quiteexpensive because it is performed at cryogenic temper-atures and requires energy-intensive mechanical equip-ment, while incineration may produce hazardous poly-cyclic aromatic hydrocarbons (PAHs) and soot duringthe combustion process.4

Pyrolysis as an attractive method to recycle scraptires has recently been the subject of renewed interest.Pyrolysis of tires can produce oils, chars, and gases, inaddition to the steel cords, all of which have thepotential to be recycled. Within the past 2 decades, mostexperiments have been conducted using laboratory-scalebatch units to characterize oil, char, and gas products.

Some conclusions from these laboratory-scale studiesare as follows:

(1) Pyrolytic char has potential as a low-grade carbonblack for a reinforcing filler or a printing ink pigment,4-6

as a carbon adsorbent after proper activation,7-9 andas a solid or slurry fuel.10

(2) Pyrolytic oil, a mixture of parafins, olefins, andaromatic compounds, possesses a high calorific value(∼43 MJ/kg) and can be used directly as fuel or can beadded to petroleum refinery feedstocks.11-15 Oils canalso be properly cut based on their evaporating tem-peratures to solely produce valuable chemical feedstocks(i.e., benzene, xylene, toluene, and D-limonene), or someof the chemicals can be extracted with residue used asfuel.16-22

(3) Pyrolytic gas contains high concentrations ofmethane, butadiene, and other hydrocarbons, whichresults in a high calorific value (35-40 MJ/kg) sufficientto heat the pyrolysis reactor.23-25 The gas is generallynot sold as a commercial product but used as a processheat resource because of its low yield (∼10 wt %).

However, these laboratory-scale studies differ greatlyfrom the field-scale practical applications. Thus, thepilot-scale studies, which bridge between laboratory-scale data and large-scale applications, are of impor-tance. However, the available literature on the continu-ous pilot-scale tire pyrolysis test is limited. Accordingto reports, the reactors that can fulfill continuous tirepyrolysis include fluidized beds, vacuum moving beds,two-stage moving beds, ablative beds, and rotary kilns.Representative results for each reactor type are listedbelow.

(1) Vacuum Moving-Bed Process (Laval Univer-sity). Roy and co-workers have focused on the vacuumpyrolysis process for more than 20 years. The reactordevelopment has gone through a 1 kg batch vacuum

* To whom correspondence should be addressed. E-mail:[email protected].

† Tsinghua University.‡ Zhejiang University.

5133Ind. Eng. Chem. Res. 2004, 43, 5133-5145

10.1021/ie030115m CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 06/29/2004

vessel, a 15 kg/h semicontinuous vacuum hearth, andfinally a 200 kg/h pilot-scale continuous moving bed.5,25,26

The advantage of the vacuum process is that theevaporating volatiles can be immediately removed fromthe reactor, which minimizes secondary reactions suchas thermal cracking, repolymerization, and reconden-sation. As a result, the oil yield is dramatically increasedat the expense of char and gas. Roy et al. also analyzedend 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 blackafter pyrolysis only changed a little. That is, a smallamount of carbonaceous deposits formed on the pyrolyticchar, which limits its use only as reinforcing filler oflow-grade rubbers.5,27 The use of pyrolytic char as anadditive for road bitumen was also discussed.28 For theliquids, Roy et al. concluded that the distillation of oilsimproved its economic value. For instance, the lighterfractions can be used as a source of high-value chemicalssuch as BTX (benzene, toluene, and xylene) and li-monene and as an extender oil in rubber formulation,14-17

while the heavy fractions can be used as additives inroad bitumen or as a feedstock for coke production.5

(2) Hamburg Fluidized-Bed Process (Universityof Hamburg). Kaminsky’s group launched the studyof waste polymer pyrolysis in the 1970s. The well-knownHamburg process, using an indirectly heated fluidizedbed, was developed with the original aim to yield basicchemicals such as BTX and carbon black for reinforcingat temperatures in excess of 700 °C.19 Laboratory-scaleplants have been built with capacities of 0.60-3.0 kg/hof plastic, followed by pilot-scale plants with capacitiesof 10-40 kg/h of plastic and 120 kg/h of used tires atthe University of Hamburg.29,30 In recent years, researchefforts have been conducted at lower temperatures inorder to improve the oil yield and to reduce the energyinput.4

(3) BBC Continuous Ablative Process. Black andBrown reported limited details of the continuous abla-tion reactor from patent applications.31,32 Ablation isachieved by sliding contact of the rubber particles on ahot metal surface, which resulted in a high yield ofliquids because of the fast heating of the tire. BBCCompany (Canada) is one representative.33 Their testsin a 50 kg/h pilot-scale reactor demonstrated efficientheat and mass transfer, and a smaller test unit ispresently operating at a throughput of 10-25 kg/h. Inaddition, a 1500-2000 kg/h commercial plant was soldto the Castle Capital Company for use in Halifax, NovaScotia, Canada. Black and Brown31 found that liquidyields of 54 wt % can be obtained at 470-540 °C with a0.88 s residence time and a 1.3 mm feeding size. TheBTX composition in oils was less than 3.5 wt %. A studyby Helleur et al.34 reported that pyrolytic char can beconverted to activated char with steam or carbondioxide. The activated char exhibited excellent qualityfor the removal of organics and heavy metals fromaqueous solutions.

(4) Two-Stage Moving-Bed Process. The processdeveloped by the Universite Libre de Bruxelles (ULB)is based on a two-stage pyrolysis mechanism.18 Duringthe first stage, the tire is depolymerized at a relativelylow temperature (∼500 °C), while during the secondstage, the pyrolytic volatiles are postcracked at tem-peratures of 750-800 °C. Their objective was just torecycle the BTX and activated carbon. Cypres andBettens18 had obtained oil yields of 37.0-42.2 wt %,

solid yields of 41.7-45.3 wt %, and gas yields of 6.0-19.5 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 kilnpyrolyzer offers many unique advantages over othertypes of reactors. For instance, the slow rotation of theinclined kiln enables well mixing of wastes, and therebyuniform pyrolytic products. Also the residence time ofsolids can be easily adjusted to provide the optimumconditions of pyrolysis reaction. Solid wastes of variousshapes, sizes, and calorific values can be fed into arotary kiln either in batches or continuously. The typicalrotary kiln processes include the Kobe Steel commercial1 tonnes/h plant, the Italian ENEA Research CenterTrisaia pilot-scale plant, and the Kassel Universitylaboratory-scale setup.35-37 These rotary kiln reactorswidely exist in the process for pyrolysis or gasification,but the technical- and pilot-scale testing data are quitescarce. Only available data by Kawakami et al. were,in fact, less systemic.38

Within the past years, we have been devoted to theresearch and development of a rotary kiln pyrolyzer.Pyrolysis of various solid wastes in a laboratory-scalebatch kiln39,40 and pilot-scale cold-model test in acontinuous rotary kiln41,42 has been successively per-formed. On the basis of these works, a continuous pilot-scale rotary kiln reactor has been self-designed andsuccessfully operated.43 The objective in this paper isto study the pilot-scale pyrolysis characteristics of scraptires in a continuous rotary kiln. Influences of thereactor temperature on the product distribution as wellas the product properties and chemical compositions areintensively discussed.

2. Experimental Section

2.1. Tire Samples. Shredded scrap tires were usedwith particle sizes of 13-15 mm including the fabriccords but not the steel. The proximate and ultimateanalyses on the air-dried basis and the calorific valueof scrap tires are listed in Table 1. The data of Roy andUnsworth26 are given for comparison. The properties fortwo tires were quite similar, except for a small differencein the content of the carbon element.

2.2. Pilot-Scale Pyrolysis Process. The pilot-scaleprocess development unit is shown in Figure 1. Itconsisted of a pyrolytic rotary kiln main reactor andperipheral systems including a supply system (a storagebin with a screw feeder), a tar condenser and reservoir,a solid residue collection tank, a flue gas cleaner, ademister filter, a gas burner, and an effluent gassampling system.43

The kiln was designed for continuous operation withtire powder conveyed from a sealed container to the

Table 1. Proximate and Ultimate Analyses of Scrap Tire(Air-Dried Basis)

case this work Roy et al.26

proximate analysis (wt %)moisture 1.14 0.50ash 4.35 6.10volatile matter 62.24 65.20fixed carbon 32.28 28.70

ultimate analysis (wt %)carbon 84.08 81.50hydrogen 6.71 7.10nitrogen 0.49 0.50sulfur 1.51 1.40oxygen 1.73 3.40

low heat value (kJ/kg) 34923 36800

5134 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004

electrically heated rotary kiln by a screw feeder. Thefeeding rate was regulated from 10 to 30 kg/h. The kilndiameter 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 wereused to heat the kiln. The effective heated length of thekiln was about 1.8 m.

The solids were transported in the kiln as a result ofinclination and rotation. At the kiln exit, the residuechar fell into a sealed 6 L container. The solid residencetime was adjusted by changing the kiln rotation to meetthe desired condition for the complete pyrolysis. Theevolved oil vapors and gases were quickly removed fromthe reactor by a special induced fan to reduce theresidence time. The condenser, with four coils cooled bywater and a trap refrigerated by ice, was used to recoverpyrolytic oils into the reservoirs. The noncondensablegases passed to the scrubbing unit to remove the acidsand finally passed to the burner. Two mechanicalfriction-type seals, with two wearable sliding rings andgraphite between the rings, were used to seal thereaction chamber at both the kiln inlet and outlet. Thepressure force on the seal was adjusted by varying thetension in the springs.

2.3. Operating Conditions. Before the experiment,nitrogen was used to purge the reactor for 3-5 min toremove the air. The system was operated at a slightlynegative pressure of -20 to -10 Pa to prevent leakageof pyrolytic gases. The operating pyrolysis temperaturewas varied from 450 to 650 °C. The residence time ofthe solids must be chosen long enough to complete thepyrolysis. Because the measurement of the solid resi-dence time during pyrolysis in a hot kiln is very difficult,the selection of the solid residence time can be optimizedin the following program: conducting the pyrolysis testat a certain rotating rate with the char yield weighted,altering the rotating rate in a step of 0.5 rpm andrepeating test, judging whether the difference betweenchar yields in those two tests is less than 1%, and finallydetermining the rotating rate, i.e., the final residencetime. The detailed experimental conditions for eachpyrolysis run are presented in Table 2. Each run inTable 2 was reproduced. The feeding rate was 12-15kg/h, with a total of 50 kg of tire in each run.

2.4. Pyrolytic Oil Characterization. 2.4.1. Deter-mination of the Fuel Properties. The fuel propertiesof pyrolytic oils were analyzed according to the NationalStandards of the People’s Republic of China (PRC). Theanalyses included the relative density or API gravityby the pycnometer method (GB/T 2540-1981), the vis-cosity by the Ostwald method (GB/T 265-1988), the flashpoint by the closed-cup method (GB/T 261-1983), themoisture by the Karl Fischer method (GB/T 11133-1989), the carbon residue by the Conradson method(GB/T 17144-2001), and the calorific value and thesulfur content by the oxygen bomb method (GB/T 388-1964).

2.4.2. Functional Group Compositional Analysis.The functional group compositional analysis of the oilswas carried out by Nicolet-5DX type Fourier transforminfrared (FT-IR) spectroscopy. A thin uniform layer ofoils was placed on the sample cell, and peak heightswere normalized to the major C-H peak.

2.4.3. True-Boiling-Point (TBP) Distillation ofOils. The FY-II crude oil TBP autoclave, developed byFu-shun Institute of Petroleum (PRC), was used for theautomatic distillation test. The distillation includedatmospheric pressure distillation (theoretical plate num-ber, 14-18; reflux ratio, 5:1; ibp, 200 °C) and reducedpressure distillation (1.33 kPa; reflux ratio of 2:1; 200-350 °C). In each run, about 5-6 kg of pyrolytic oils wasfed into the autoclave. As the temperature was in-creased from room temperature to 350 °C, the distillablefraction was cut in steps of 25 °C. Then each fractionwas weighed to calculate its yield.

2.4.4. Oil Analysis of Aromatic Class. After TBPdistillation, 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 FinniganVoyager gas chromatography-mass spectrometry (GC-MS). The analysis was performed on a DB-5 capillarycolumn (30 m × 0.2 mm i.d. × 25 µm), using a 200:1split ratio and a carrier helium gas at 1.0 mL/min. Thedistilled naphtha fraction, with an interval of 50 °C, wasperformed by GC-MS with a different temperatureprogram. As for the oil fraction of bp < 100 °C, the oventemperature was initially set at 35 °C for 10 min, thenprogrammed to 150 °C at 8 K/min, then moved to 250

Figure 1. Schematic of a pilot-scale continuous rotary kiln pyrolysis reactor: 1, purge-gas tank; 2, PID controller; 3, supply and screwfeed; 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

reactortemp/°C

kiln rotatingrate/rpm

screw rotatingrate/rpm

kilnslope/deg

pressure at thekiln outlet/Pa

450 0.45 5 2 -10 to -20500 0.60 5 2 -10 to -20550 0.75 5 2 -10 to -20600 0.90 5 2 -10 to -20650 0.90 5 2 -10 to -20

Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5135

°C at 25 K/min, and finally held for 5 min. The programof the 100-150 °C oil fraction was started at 50 °C for10 min, moved to 250 °C at 8 K/min, and held for 5 min.The program of the 150-200 °C fraction was started at70 °C for 10 min, moved to 250 °C at 8 K/min, and heldfor 10 min. The MS operating conditions were asfollows: ion source, 270 °C; electron energy, 70 eV, witha range of m/z 50-500 µm and a scan time every 1.0ms.

The heavy oil (>200 °C) was processed according tothe China SY5119-86 standard, the method for theclasses of soluble organics in rock and for the majorchemical classes in crude oil. The heavy fraction wasfirst solved using n-hexane, and the insoluble materialwas a bituminous fraction. The glass columns packedwith silica gel and Al2O3 were used to separate oil intochemical class fractions followed by GC-MS. Thecolumn was then sequentially eluted with sequentialelutions of hexane, CH2Cl2/hexane, and ethanol/CHCl3to produce aliphatic, aromatic, and polar fractions. Thearomatic fraction was analyzed by GC-MS to determineidentification of PAH. The Wax-10 capillary column (30m × 0.25 mm i.d.), with a 30:1 split ratio and a 1.0 mL/min carrier helium, was used. The temperature profilewas 75 °C for 3 min, followed by a 3 K/min heating to180 °C, then a 10 K/min heating to 270 °C, and 20 minof holding.

2.5. Pyrolytic Char Characterization. 2.5.1. Proxi-mate and Ultimate Analyses. The pyrolytic char wascharacterized using the following PRC standards: theproximate analysis by GB/T 212-2001, the ultimateanalysis by GB/T 476-2001, and the size distribution byGB/T 12496.2-1999.

2.5.2. Activation of Tire-Derived Chars. A nearlyisothermal, externally heated horizontal quartz-tubereactor (28 mm i.d. × 500 mm) was used to activate thetire-derived char. Prior to activation, the char was sievedto generate a 2.5-7.0 mm size fraction and dried to 105°C. About 15 g of char was placed in the reactor, withcarbon dioxide used as the activated agent. The experi-ments were conducted between 850 and 950 °C. If notspecifically indicated, the activating-agent flow rate was0.3 L/min and the activation time was 4 h. The testsstudied the influences of the activation temperature onthe degree of burnoff. The burnoff was defined as

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 inexcess of 500 °C had little effect on the char character-ization, as shown in the results.

2.5.3. Surface Area and Pore Structure of Pyro-lytic or Activated Char. The surface areas of both thepyrolytic char and the activated carbon were determinedby nitrogen adsorption at 77 K using a BET isotherm.The instrument for the nitrogen adsorption experimentswas made by Quantachrome Corp. In addition, amercury intrusion porosimeter by Quantachrome Corp.was used as a complementary tool to study the porestructures of pyrolytic chars.

3. Discussion and Results3.1. Tire Pyrolysis Reaction Scheme. From the

point of view of kinetics, tire pyrolysis is quite complexand consists of more than hundreds of chemical reac-tions. Although each individual reaction is difficult todetermine, these reactions can be generally classifiedinto three groups: primary pyrolysis reaction (250-520°C), secondary postcracking reaction of pyrolytic vola-tiles (600-800 °C) that strongly affect BTX yields, andchar gasifying reaction with CO2/H2O/O2 in the gases(750-1000 °C). The importance of each reaction groupis dependent on two parameters, the temperature andtime of residence, as indicated by the rates of reactionin Arrhenius form. These multigroup reaction schemescan be used to understand the pyrolysis results in thefollowing parts.

3.2. Yields of Pyrolytic Products. The yields ofpyrolytic char, oil, and gas (in difference) are presentedin Table 3 for temperatures of 450-650 °C. Data forother typical pilot-scale continuous pyrolyzers (vacuum,fluidized-bed, ablative, and two-stage bed) are also givenin Table 3 as a contrast.

First, the solid char yield remains essentially constantwith a mean of 39.8 wt %, except for the relatively highvalue of 43.9 wt % at a temperature of 450 °C. The datasuggested that the kinetics of the primary pyrolysisreaction did not complete at low temperature (450°C).44,45 Compared with other typical processes, Roy etal.5,16 reported that the vacuum moving-bed processwith a feedstock of 21-42 kg/h had a char yield of 38.4-39.3 wt %. The two-stage process by Cypres and

Table 3. Pyrolytic Product Yields of Scrap Tires at Temperatures of 450-650 °C and Their Comparison with VariousTypical Processes

hydrocarbon vapor yield/(wt %)pilot-scalereactor run no. temp/°C

char yield/(wt %) oil yield gas yield total volatile

rotary kiln AT01 450 43.9 43.0 13.1 56.1AT02 500 41.3 45.1 13.6 58.7AT03 550 39.9 44.6 15.5 60.1AT04 600 39.3 42.7 18.0 60.7AT05 650 38.8 42.9 18.3 61.2

vacuum processa,5,16 H036 480 (431)a 39.3 53.7 7.0 60.7H045 534 (510) 38.4 49.9 11.7 61.6H018 520 (500) 33.4 56.5 10.1 66.6

two-stage processb,18 Rad-X 450 + 800 44.8 38.5 16.8 55.2Rallye 450 + 800 44.2 39.7 16.1 55.8V-10 450 + 800 41.7 41.3 17.0 58.3

fluidized-bed process4 550 34.0 56.8 9.2 66.0600 40.0 50.9 9.1 60.0

ablative process31 450 39.1 52.9 8.0 60.9a 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 wastetire.

burnoff (wt % daf) ) (w1 - w2)/w1 × 100 (1)


Bettens18 can produce 41.7-44.8 wt % char with threedifferent brands of tire at a temperature of 450 °C.Kaminsky and Mennerich,4 using a fluidized-bed reac-tor, showed that the char yield was about 40.0 wt % at600 °C. In addition, a char yield of about 39.1 wt % wasobtained in a continuous ablative process developed byBBC company, Canada.31

Comparison with other processes indicates that thechar yield is not sensitive to the reactor types of variousheating rates, which differs greatly from the pyrolysisof coal or biomass. This is because that scrap tire is asynthetic chemical feedstock according to a certainformula ratio. Tires are made of a complex blend ofelastomers, processing oils, carbon black, and a fewadditives including mineral fillers, vulcanization agents,palticizer, etc. Because the pyrolytic vapors (elastomersand processing oils) produced during tire pyrolysis bythermogravimetry (TG) are about 62-65 wt %,44,45 theyield of pyrolytic solid residue should theoretically be35-38 wt %. In the practical process, the char yield isa little higher than this value due to the slight hydro-carbon deposits in the char, for example, about 39.8 wt% in this experiments.

As shown in Table 3, the volatiles, consisting of oilsand gases, are constant at a value of 60 wt % withvarious temperatures. However, the oil yields reacheda 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 13.1to 18.3 wt % as a result of the serious secondarypostcracking of vapors at higher temperature. Themaximum yield of oils at 500 °C can be attributed tothe balance for the competition between the primarypyrolysis reaction and secondary postcracking reaction.

The oil yield in the current kiln process is comparedwith that of other reactor processes in Table 3. Althoughthe reactor configuration has little influence on the charyield, the oil yield varied greatly for the different reactortypes. The vacuum process by Roy and co-workers5,16

had a maximum oil yield of 53.7 wt % at 431 °C. TheHamburg fluidized-bed process by Kaminsky and Men-nerich4 produced 56.8 wt % oil at 550 °C. The continuousablative process had a maximum oil yield of 52.9 wt%.30,31 The maximum oil yield of 41.3 wt % was obtainedby Cypres and Bettens18 using the two-stage movingbed, which is lowest among all of the processes. Thedifferent oil yields of various reactors due to the differ-ent degrees of vapor postcracking can mainly be at-tributed to the difference of the vapor residence timein the high-temperature zone in each process.

It is noted that Kawakami et al.,38 using a similarrotary kiln reactor, obtained a maximum oil yield of 53.0wt % at 500 °C, which was higher than the data in thisexperiment. The effective heated length is 1.8 m, andthe length of the kiln outlet is 0.6 m. It can be explainedthat the ratio of length to diameter (L/D ) 8) in thispaper might be larger than that from the study ofKawakami et al. The larger the ratio of L/D, the higherthe residence time of the vapor. A high vapor residencetime in the high-temperature zone caused the reductionof the oil yield in this paper. According to the authors’experiences, the concentrations of valuable chemicalssuch as BTX and limonene in the oils exerted moreimportant impacts on the economic viability of theprocess than the total oil yield. The quantification ofthe valuable chemicals was essential. It would bediscussed in the following part.

3.3. Characterization of Pyrolytic Oils. The py-rolytic oils are derived from (1) processing oils in theoriginal tire, (2) evaporating and decomposing of elas-tomers such as natural rubber, butadiene rubber, orstyrene-butadiene rubber, and (3) other organic addi-tives in tires. The tire-derived oils are mostly unrefinedoils with a wide range of boiling fractions.

3.3.1. Fuel Properties. Table 4 presents the fuelproperties of the pyrolytic oils carried out in the pilot-

Table 4. Physical-Chemical Properties of Pyrolytic Oils in a Rotary Kiln and Comparison with the Vacuum Process

rotary kiln vacuum process5

pyrolysis run 450 °C 500 °C 550 °C 600 °C 650 °C car tire (no. H20) truck tire (no. H22)

characteristicsdensity (kg/Nm3) 0.941 0.962 0.987 0.955 0.982 0.950 0.939gravity (API) 18.2 15.0 11.3 16.1 11.9 17.4 19.3viscosity at 50 °C (cS) 2.87 2.44 3.66 1.63 2.00 9.7 17.8flash point (°C) 27.5 17.0 30.0 17.5 13.5 28 22LHV (MJ/kg) 41.9 41.7 41.0 41.6 41.0 43.7 44.8moisture (wt %) 0.52 0.88 1.32 0.80 0.85 0.3 1.5carbon residue (wt %) 1.36 1.78 3.09 2.01 3.26 1.30 1.20ash (wt %) traces traces traces traces traces traces 0.005

ultimate analysisC (wt %) 84.32 84.26 84.55 86.14 86.1 85.8 87.0H (wt %) 10.92 10.39 9.59 9.54 9.06 10.7 11.1N (wt %) 0.48 0.42 0.64 0.70 0.84 0.5 0.7S (wt %) 0.97 1.54 1.26 1.27 1.11 0.8 1.0O + others (wt %) 3.31 3.39 3.96 2.35 2.89 2.2 0.2H/C atomic ratio 1.55 1.48 1.36 1.33 1.26 1.49 1.54

Figure 2. TBP distillation test of tire-derived oil at temperaturesof 500 and 600 °C.


scale rotary kiln. The properties of oils from the vacuumprocess by Roy et al.5 are also given as a contrast.

The calorific value of the tire oils is similar for all ofthe tests, with values of 41.0-41.9 MJ/kg, which ishigher than that of scrap tires (34.9 MJ/kg). Moreover,this high calorific value of the derived oil is comparablewith that of light fuel oil, indicating the potential foruse as liquid fuels for industrial furnaces and powerplants.

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 ofmarine or furnace fuel oils. The fuel viscosity is impor-tant because it affects the fuel flow through pipes, thefuel atomization, and the performance and wear ofdiesel pumps. The viscosity of oils derived from thevacuum process by Roy et al.5 was about 4 times thatof the viscosity of oils derived from the current rotarykiln process. The oils derived from a fixed bed byCunliffe and Williams1 had a viscosity of 2.38 cS at 60°C, which agrees well with the pyrolytic oils in thispaper. Generally, the shorter residence time of vaporcaused by the vacuum pump reduced the secondarypostcracking reaction and improved the oil yield but alsoincreased the amounts of long-chain molecules in theoils, which resulted in the high oil viscosity and limitedits usage. The oil density was 0.94-0.99 kg/m3 over atemperature range of 450-650 °C, while the corre-sponding API gravity was 11.3-18.2. The density ofpyrolytic oils was higher than that of diesel oils (0.78kg/m3) and approached that of marine fuel oils (0.98 kg/m3). The effects of temperature on both the oil viscosityand density were indistinct within the error limits ofmeasurement.

The flash point of liquid fuel indicates the tempera-ture at which the oil begins to evolve vapors in sufficientquantity to form a flammable mixture with air. As seenin Table 4, the flash point of the tire-derived oils was13.5-30 °C with no particular trend with increasingtemperature. Roy et al.5 reported a value of 22-28 °C,and Cunliffe and Williams1 reported 14-18 °C. Theflash point of the tire-derived oil was lower than thatof the petroleum-refined fuels. For example, light dieseloil has a required minimum flash point of 45 °C and aheavy gas oil of 65 °C. The low flash point of the tire oilcan be attributed to the wide range of unrefined oils inthe mixture including some light low-boiling-pointhydrocarbons. The low flash point will complicate stor-age of the oils. The moisture of 0.5-1.3 wt % will alsobe 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 1.2-1.3 wt %, and that by Williams was 0.5-2.2 wt %.5-13

A typical diesel fuel would have a carbon residue ofapproximately 0.2 wt %; however, fuel oils used in a verylarge diesel engine may have carbon residues of up to12 wt %. Thus, the tire-derived oil should be used in alarge diesel engine or industrial boiler rather a micros-cale typical diesel.

The temperature had only a slight effect on theelemental content of pyrolytic oils. As the temperatureincreased, carbon increased slowly from 84% to 86% andhydrogen 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

between 500 and 550 °C is sharper than that in otherzones. This implies that the postcracking of evolvedvapor, including chain cracking, dehydrocyclization, andaromatization, becomes serious from this zone onward.Thus, the aromatics in the tire-derived oil increased, asindicated by the reducing H/C ratio. In addition, thenitrogen content increased with increasing temperaturebecause the nitrogen in the char or gas participates inthe aromatization process to form polycyclic aromaticnitrogen hydrocarbons (PNAHs). The sulfur content was0.97-1.54 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 kilnand the vacuum process5 indicated that both the nitro-gen and sulfur contents are similar, the hydrogencontents are similar below 500 °C but differ above 500°C, the carbon content in the rotary kiln is lower thanthat in the vacuum process by 1-2%, and the oxygencontent is higher than that in the vacuum process.

Table 5 shows the carbon, hydrogen, and nitrogencontents in different distillate fractions of pyrolyic oilat 600 °C. The H/C ratio was quite constant with a meanof 1.27, except for a value of 1.37 in the 50-100 °Cdistillate fraction. Therefore, the aromatization degreesof the fractions in excess of 100 °C were similar. Thenitrogen content in the low-boiling fractions (<150 °C)was lower than that in the high-boiling fractions (>150°C), which suggests that the nitrogen exists mainly asa high-boiling-point PNAH such as caprolectum, quino-line, etc.

3.3.2. TBP Distillation Test of Pyrolytic Oil. TheTBP distillation test is one of the best approaches toreflect the true distillation of oil. It plays an importantrole in developing oil refinery or extraction processes.Although many researchers have emphasized the im-portance of the TBP distillation test, little literature isfound about the TBP test because it needs a greatamount of pyrolytic oils (about 5 kg). Roy et al.5 oncereported the simple test of the pyrolytic oil distillation.

Figure 2 shows the results of the TBP distillation testof the pyrolytic oils at temperatures of 500 and 600 °C.The pyrolytic oils had fractions with a wide range ofboiling points, mainly 39.2-42.3 wt % light naphtha (ibp∼ 200 °C), 32.4-33.2 wt % medium fractions that canbe used as diesel oil (200-350 °C), and 25.5-28.5 wt %heavy fractions (>350 °C). Comparison of these resultswith those of vacuum pyrolysis in Table 6 indicated that(1) the light fraction (39-42.3 wt %) is higher than thatin the vacuum pyrolysis oil (26.8 wt %) and (2) thecontents of medium fractions of both studies are similarat about one-third of the total oil. Kawakami et al.38

reported a relatively low light fraction of 24.5 wt %. Itis concluded that the significant postcracking of the oilresulted in not only a low oil yield but a highly valuablelight naphtha fraction as well.

Figure 3 shows the distillate fractions at intervals of25 °C of pyrolytic oil produced at 600 °C. The light

Table 5. Carbon, Hydrogen, and Nitrogen Profiles inEach Distillate Fraction for Pyrolyic Oils Produced at600 °C

distillant carbon hydrogen nitrogen H/C ratio

total oils 86.14 9.54 0.70 1.3350-100 °C fraction 87.03 9.93 0.46 1.37100-150 °C fraction 89.83 9.50 0.56 1.27150-200 °C fraction 88.50 9.51 1.13 1.29200-250 °C fraction 86.56 9.06 1.71 1.26250-300 °C fraction 87.22 9.27 1.49 1.28300-350 °C fraction 88.28 9.24 1.12 1.26


naphtha fraction (ibp ∼ 200 °C) and the mediumfraction (200-350 °C) have normal distribution, withmean values of 150 and 275 °C, respectively. Thisdistribution favors the cut of pyrolytic oil as the naphthaand diesel oil in potential applications.

In addition, Table 6 also compares the distillationfractions of the pyrolytic oil and three crude oils inChina (Daqing, Shengli, and Xinjiang). The pyrolytic oilis much lighter than the crude oil. The crude oil has a58.6-69.0 wt % heavy fraction (>350 °C), while thepyrolytic oil only contains a 24.5-28.4 wt % heavyfraction. Therefore, the properties of the pyrolytic oilare superior to those of the crude oil.

3.3.3. Functional Group Compositional Analysisof Pyrolytic Oil. The group composition of the pyrolyticoil was determined by FT-IR spectroscopy. Figure 4illustrates the spectra of the tire-derived pyrolytic oilsat temperatures of 450-650 °C. Using the FT-IR spectraof the oil at 600 °C as an example, the followingconclusions can be drawn:

(1) The dCH stretching vibrations at 3100-3000 cm-1

indicated the presence of aromatic or alkene groups. Thearomatic groups can be distinguished from the alkenesby the breathing vibration, which was defined as theconjugated CdC vibrations occurring in the vicinitiesof both 1600 and 1500 cm-1. In pyrolytic oil spectra, twoconjugated peaks at 1604 and 1495 cm-1 confirmed thepresence of the aromatic groups. Furthermore, single,polycyclic, and substituted aromatic groups can beidentified by the absorbance peaks of C-H cyclicdeforming vibrations at 900-675 cm-1 as well as theresonating peaks of C-H and C-C deforming vibrationsat 2000-1600 cm-1.

(2) The absorbance peaks at both 1675-1575 and950-875 cm-1 represented the CdC stretching vibra-tions, which confirm the presence of alkenes in thepyrolytic oil.

(3) The two distinctly intensive peaks at 2906 and2869 cm-1 represented the C-H stretching vibrationsand were indicative of alkanes. Furthur detail concern-ing the group compositions can be deduced from theC-H deformations at 1500-1300 cm-1 and the C-Hcyclic deformations at 1000-650 cm-1. For instance, theC-H deformations for the tire pyrolytic oil occurred at1455 cm-1, while the C-H cyclic deformations at 775-735 cm-1. Thus, it is deduced that the C-H group inthe pyrolytic oil was probably -C-(CH3)n-C- (n < 4)or mC-CH3.

(4) The remarkably wide absorbance peaks between3700 and 3200 cm-1 represented the polar compoundsin the pyrolytic oil. These peaks can be possibly at-tributed to the resonance of the O-H and N-H groups,which indicated an acylamino group. Moreover, thepresence of CdO<, indicated by the absorbance peakat 1705 cm-1, enabled the O-H group spectra to becomewide. However, Williams et al.13,24 only found the CdO<group but not the O-H group in the spectra of the tire-derived oil.

Figure 4 also shows the FT-IR spectra for the oils inrelation to the pyrolysis temperature (450-650 °C). TheFT-IR spectra peaks for all of the oils produced atdifferent temperatures were normalized to the mainCH2 peak in each spectrogram, and consequently rela-tive changes of the functional group composition weredetermined. As the pyrolysis temperature increased, theintensities of some peaks changed dramatically, indicat-ing that the corresponding group composition changed.For instance, the aromatic groups, indicated by absor-bance peaks at 3100-3000, 1604 (1495), and 900-675cm-1, increased in intensity with increasing tempera-tures. In addition, the 3000-2800 cm-1 absorbancepeak, indicative of alkanes, and the 1675-1575 and950-875 cm-1 peaks, indicative of alkenes, showed adecrease in intensity as the temperature increased from450 to 650 °C. The FT-IR results agree well with theaforementioned reaction mechanisms that the increas-ing temperature improves the secondary postcrackingof volatile products and further aromatization of thepyrolytic oil.

3.3.4. Volatile Aromatic Hydrocarbon Analysis.Table 7 presents the light volatile aromatics in pyrolyticoils. The high concentration of single-ring aromaticssuch as benzene, toluene, xylene, dimethylbenzene,styrene, and indene was suggested as a potential highvalue feedstock in the plastics/polymer industry, whichcould offset the disposal cost. The high pyrolytic tem-perature promoted the production of volatile aromatics.As the temperature increased from 450 to 600 °C, thebenzene concentration increased from 0.40 to 2.09 wt%, the toluene dramatically from 2.27 to 7.24 wt %, thexylene from 1.54 to 2.13 wt %, the 1,2-dimethylbenzenefrom 2.34 to 6.74 wt %, and the 1,3-dimethylbenzenefrom 0.54 to 1.46 wt %. However, as the temperaturefurther increased from 600 to 650 °C, there was even asmall drop in the BTX concentration. This implies that

Table 6. Distillate Fractions for Pyrolytic Oils and Crude Oils

oil fraction oil sampleslight fraction(ibp ∼ 200 °C)

medium fraction(200-350 °C)

heavy fraction(>350 °C)

tire-derived pyrolytic oils (%) 600 °C 42.3 33.2 24.5500 °C 39.2 32.4 28.4vacuum process5 -26.8 30.7 42.5

crude oils produced in China (%) Daqing 10.7 17.3 69.0Shengli 7.6 26.3 66.0Xinjiang 15.4 26.0 58.6

Figure 3. Distillate fractions at intervals of 25 °C for tire-derivedoil produced at 600 °C.


Diels-Alder aromatization would occur and make single-ring aromatics convert to more PAHs. Within temper-ature ranges of 450-650 °C, the styrene concentrationalways increased. Indene, used to produce indene-coumarone resins in industrial applications, had amaximum concentration of 0.90 wt % at 550 °C.

Limonene, with a high value because of its applica-tions in the formulation of industrial solvents, resins,and adhesives, has attracted the most interest. Table 7indicates that limonene had a high concentration of 5.44wt % at lower temperature and dramatically decreasedto 0.07 wt % as the temperature increased from 450 to650 °C. From the point of view of reaction kinetics, someresearchers found that the degradation of limonene toform a range of BTX and styrene occurred above 500°C.1,17 This greatly agreed well with the limonene datain Table 7. So, it is concluded that the heat transfer inthe rotary kiln was well enough to have a rapid responseto the reaction kinetics.

The rotary kiln process in this paper can producemore volatile aromatics than other one-stage processesaforementioned. Within similar temperature ranges, theconcentrations of benzene, toluene, xylene, styrene, andlimonene in a laboratory-scale fixed bed were up to0.29, 1.77, 1.68, 0.36, and 3.13 wt %, respectively.1 In avacuum process, the concentrations of benzene, toluene,xylene, and limonene were 0.68, 1.86, 1.60, and 4.00 wt%. However, the maximum concentrations of benzene,toluene, xylene, styrene, and limonene in this experi-ment were 2.09, 7.24, 2.13, and 5.44 wt %, respectively.Because the length of the kiln can be varied, the rotarykiln pyrolyzer is flexible with regard to the gas residence

time as well as time/temperature profile. In this paper,the richer single-ring aromatics are caused by the largerkiln length.

3.3.5. PAH Analysis. Table 8 presents the concen-trations of several main PAHs in the tire-derived oilsin relation to the temperature. The increasing temper-ature enhanced the formation of PAHs. For instance,as the temperature increased from 450 to 650 °C, theconcentrations of the fluorene and alkylated fluoreneincreased from 1.17 to 2.57 wt %, those of the phenan-threne homologues from 2.15 to 6.51 wt %, and those ofthe anthracene homologues from 1.13 to 2.15 wt %. Asfar as the two-ring naphthalene and its alkylatedcompounds were concerned, their concentrations firstincreased from 8.61 to 13.14 wt % as the temperatureincreased from 450 to 500 °C. However, there was a dropfrom 13.14 to 10.06 wt % as the temperature furtherincreased to 650 °C. It is deduced that a two-ringalkylated naphthalene might take an active part inDiels-Alder aromatization to form a three-ring PAHsuch phenanthrene and anthracene above 500 °C. Thenaphthalene homologues included naphthalene, meth-ylnaphthalene, ethylnaphthalene, dimethylnaphtha-lene, and trimethylnaphthalene. The recycling of naph-thalene from oils is less possible than BTX. Because theconcentration of alkylated naphthalene was high, twomodifications of the rotary kiln pyrolyzer should beconsidered to maximize the production of single-ringaromatics in the future: (1) multiple heating zones withdifferent temperatures along the kiln length in orderto have the desired time/temperature profiles; (2) asolid-bed catalyst process adjacent to the kiln outlet.

The concentrations of four-ring PAHs were relativelylow, e.g., total alkylated pyrene (0.49-0.83 wt %),fluoranthene (0.07-0.15 wt %), and benzo[b]fluorine(0.06-0.21 wt %). The increment of four-ring PAHs inrelation to the temperature was less clear than that ofthree-ring PAHs.

3.4. Characterization of Pyrolytic Chars. Thepyrolytic char, 38.8-43.9 wt %, is mainly derived from(1) the commercial carbon black filler used in tireproduction (∼30 wt %); (2) the carbonaceous depositsformed by condensation and dealkylation reactions ofhydrocarbons, which is absorbed on the char surface;(3) the residue hydrocarbons from original elastomersor processing oils; (4) other inorganic additives such as

Figure 4. FT-IR spectra of tire-derived pyrolytic oils at various temperatures.

Table 7. Concentrations of Volatile AromaticHydrocarbons in Pyrolytic Oils (wt %)

tire pyrolysis temperature (°C)

volatile hydrocarbons 450 500 550 600 650

benzene 0.401 1.341 1.488 2.107 2.093toluene 2.269 2.789 5.162 7.244 7.056xylene 1.538 1.864 2.051 2.127 2.0151,2-dimethylbenzene 2.398 4.319 5.641 6.774 6.095styrene 1.207 1.302 1.423 1.436 2.6401,3-dimethylbenzene 0.539 1.085 1.406 1.553 1.464trimethylbenzene 4.626 7.325 6.265 6.331 6.099C10-based aromatics 3.815 4.937 5.442 5.014 4.073indene 0.831 0.381 0.900 0.447 0.382limonene 5.440 1.883 0.419 0.122 0.070


zinc and calcium compounds. The properties of pyrolyticchar differ from those of commercial carbon blackbecause of the above complex sources.

3.4.1. Size Distribution of Char. The original ∼15mm tire powder particle underwent a thorough thermalcracking in the kiln. Table 9 presents the size distribu-tion of the tire-derived char in relation to the pyrolysistemperature. The weight- and surface-averaged diam-eters are also given. The data show that 83-84% of thechar particles had fractions with sizes of less than 5.1mm. The fraction above 10 mm was less than 4.5%. Theweight- and surface-averaged diameters were 3.02-3.40and 0.74-1.00 mm, respectively. The pyrolysis temper-ature had little effect on the char size distributionwithin 450-650 °C. It is concluded that the rotary kilnreactor can produce more uniform char than otherprocesses, where the kiln rotation plays a crucial rolein the thorough mixing of char.

3.4.2. Proximate and Ultimate Analyses of Char.(1) Proximate Analysis. Table 10 shows the proximateanalysis (air-dry basis) of pyrolytic char for tempera-tures from 450 to 650 °C. The volatile matter contentdecreased while the fixed carbon content increased withincreasing temperature. The V/FC ratio, defined as theratio of volatile matter to fixed carbon, underwent a slowdrop under 500 °C, then a sharp drop between 500 and550 °C, and finally again a slow drop over 550 °C. It isconcluded that the temperature in excess of 550 °Cfavors the evaporation of volatile matters because the

primary pyrolysis reaction has been completed. Inaddition, Roy et al.27 reported that the volatile mattercontent of pyrolytic char in a vacuum process was 2.1-7.9 wt %, which is similar to that for pyrolytic char over550 °C in this paper.

The calorific value of pyrolytic char was 30.0-31.5MJ/kg but slowly decreased with increasing pyrolysistemperature, among which the drop between 500 and550 °C is relatively large. That is, the calorific value isin relation to the volatile matter content. Collins et al.10

reported a calorific value of 30 MJ/kg and an ignitiontemperature of approximately 510 °C for the tire-derivedchar.

The ash content of pyrolytic char, 12.32-14.58 wt %,is high compared to that of most activated carbons usedfor flue gas or wastewater cleanup. The high ash contentfor the tire-derived char has also been reported previ-ously. For example, Roy et al.27 reported the ash contentof pyrolytic char at 10.6-12.2 wt %, Darmstadt et al.46

at 14.7 wt %, and Merchant and Petrich9 at 11.1-11.9wt %. The major components of the ash of pyrolytic charwere zinc and calcium compounds.47

(2) Ultimate Analysis. Table 11 shows the elementalanalysis of pyrolytic char for various temperatures withadditional contrast data. The chars contained about81.00-82.17 wt % carbon on a dry basis, which contrib-uted more than 90% to the organic matters of the char.The hydrogen content decreased slightly with increasingtemperature, indicating lower formation of solid hydro-

Table 8. Concentrations of PAHs in Pyrolytic Oils (wt %)

tire pyrolysis temperature (°C)

volatile hydrocarbons 450 500 550 600 650

naphthalene homologues 8.618 13.144 11.298 10.143 10.058fluorene homologues 1.169 1.012 1.513 2.270 2.568phenanthrene homologues 2.151 2.356 3.398 3.644 6.510anthracene homologues 1.130 0.736 1.091 1.362 2.152pyrene homologues 0.830 0.756 0.489 0.544 0.685fluoranthene 0.070. 0.086 0.066 0.079 0.148benzo[b]fluorene 0.135 0.058 0.105 0.120 0.214

Table 9. Size Distributions of Tire-Derived Char for Various Pyrolysis Temperatures

pyrolysis reactor temperature (°C)

characteristics 450 500 550 600 650

size distribution<0.074 mm 0.04 0.30 0.55 0.95 0.450.074-0.125 mm 1.08 1.55 2.18 3.20 2.180.125-0.25 mm 3.03 3.15 4.74 3.14 3.880.25-0.335 mm 4.14 6.31 5.40 3.01 4.090.335-1.00 mm 21.48 18.38 21.37 14.18 18.211.00-1.25 mm 3.54 5.17 3.53 4.59 4.501.25-2.0 mm 16.72 14.84 14.35 13.61 14.152.0-2.5 mm 10.75 9.65 8.87 9.58 10.172.5-5.1 mm 22.46 24.65 22.25 27.69 25.795.1-7.mm 1.23 1.11 1.70 2.03 1.197.0-10 mm 11.87 10.63 10.69 14.03 11.82>10 mm 3.67 4.27 4.37 3.99 3.57

average diameterweight-averaged 3.09 3.08 3.02 3.40 3.14surface-averaged 1.00 0.86 0.74 0.75 0.82

Table 10. Proximate Analysis of Pyrolytic Char at Temperatures of 450-650 °C (Air-Dry Basis)

pyrolysis reactor temperature (°C)

proximate analysis (wt %) 450 500 550 600 650

moisture 3.40 2.35 1.28 1.98 1.48ash 12.51 12.32 14.58 14.30 13.94volatile matter 16.61 16.14 6.92 5.86 6.27fixed carbon 67.47 69.19 77.22 77.93 78.32calorific value/(kJ/kg) 31.2 31.5 30.0 30.4 30.1V/FC ratio 0.246 0.232 0.090 0.075 0.080


carbons. The decrease of the volatile matter content withincreasing temperature supports this supposition. Royet al.27 reported a hydrogen content of 0.70-0.97 wt %in a vacuum process, while Cunliffe and Williams47

reported one at 0.7-0.8 wt % in a fixed bed, which areboth about half that in the present experiments. Thehigh hydrogen content or H/C ratio of pyrolytic char(especially below 500 °C) in this paper can be attributedto the carbonaceous deposits formed by the condensationand dealkylation of hydrocarbons due to a relatively longresidence time of vapor.

Within ranges of 500-550 °C, the nitrogen and sulfurcontents of the chars were 0.45-0.61 and 2.28-2.53 wt%, respectively. They both appear to not be influencedby the pyrolysis temperature. Similar sulfur contentdata were reported by Roy et al.27 (2.0-2.5 wt %),Helleur et al.34 (2.0 wt %), and Cunliffe and Williams47

(2.0 wt %). Higher nitrogen contents were also foundby Roy et al.27 (0.70-1.04 wt %) and Cunliffe andWilliams47 (0.6-1.0 wt %).

3.4.3. Pore Characteristics of Pyrolytic Char.Common methods to measure pore distributions includethe nitrogen adsorption method and the mercury intru-sion method. The former is limited to a maximum poresize of 200 nm, while the latter has a measuring rangeof 3.6-104 nm. Both methods were used to give the full-scale analyses of pore sizes of pyrolytic char.

(1) N2 Adsorption Method. The 2.5-5.1 mm sizefraction of the pyrolytic char derived at 550 °C wascrushed to about 200 µm for the N2 adsorption test aswell as the BET determination. Figure 5 illustrates thepore volume and surface profile variation with pore

diameters for pyrolytic char. The total pore volume ofchar or carbons includes the macro- (>50 nm), meso-(2-50 nm), and micropore volumes (<2 nm), amongwhich the micropore volume plays the more importantrole in determining the absorption ability of chars orcarbons.9 The pore sizes should be only slightly largerthan the sizes of the molecules of absorbed materials.Therefore, the effective adsorptive pore sizes of charsor activated carbons are generally less than 2 nm. Asseen in Figure 5, the pore volume of pyrolytic char wasmuch smaller than that of the commercial activatedcarbon by at least 1 order of magnitude, especially inthe micropore volume. The integrated pore volume ofpyrolytic char was 0.053 cm3/g (cubic centimeters pergram), while that of the commercial activated carbonwas 0.5 cm3/g. Therefore, the tire-derived char had lessmicroporosity than the commercial activated carbon.

Figure 5 also shows the specific surfaces of pyrolyticchar as a function of the pore sizes. The mesopores hada relatively large fraction in pyrolytic chars. The surfacearea of chars can also be measured using the BETisotherm. The BET surface area of the 2.5-5.1 mm sizepyrolytic char was 89.1 m2/g. Some investigators havereported similar tire-derived char BET surface areasusing different pyrolysis reactors; for instance, the BETsurface area of the pyrolytic char from the vacuummoving bed by Roy et al.5 was 95 m2/g, that from thefluidized bed by Kaminsky et al.4 was 76-85 m2/g, thatfrom the ablative process by Hellur et al.34 was 51-67m2/g, that from the batch rotary kiln by San Miguel etal.7 was 75-85 m2/g, and that from the batch fixed bedby Cunliffe and Williams47 was 61-68 m2/g. Generally,the BET surface area of pyrolytic char was approxi-mately 1/10 that of the commercial activated carbon.

(2) Mercury Intrusion Method. The meso- andmacropores in the pyrolytic char measured using themercury intrusion porosimeter can be regarded asauxiliary information for the char pore characteristics.Figure 6 compares the pore volume profiles between thepyrolytic char and commercial carbon as a function ofthe pore diameter. The contributions of macroporevolumes (>50 nm) to the total volume for both materialswere negligible. Moreover, excluding the micropores, themesopores (2-50 nm) in the char were similar to thoseof the commercial activated carbon. The pore volumeprofile density reached a maximum value at a porediameter of 25 nm (rj ≈ 25 nm). Finally, although thelower limit of the mercury intrusion test is 3.6 nm, thedata in Figure 6 show that the micropores in pyrolyticchar were greatly less developed than those in thecommercial activated carbon. Pyrolytic char would bemore useful for aqueous adsorption of large molecular

Table 11. Ultimate Analysis of Pyrolytic Chars at Various Temperatures (Dry Basis) and Comparison with VariousTypical Processes

elemental composition (wt %)

reactor pyrolysis temp/°C carbon hydrogen nitrogen sulfur oxygen ash H/C

rotary kiln 450 82.13 2.10 0.54 2.28 nd 12.95 0.31500 82.17 2.28 0.61 2.32 nd 12.62 0.33550 80.82 1.46 0.53 2.41 nd 14.77 0.22600 81.00 1.38 0.51 2.53 nd 14.58 0.20650 81.03 1.98 0.45 2.40 nd 14.15 0.29

vacuum process (Roy et al.27) 500 (10.3 kPa)a 82.56 0.97 1.04 2.06 0.49 12.89 0.14500 (0.3 kPa)b 82.6 0.7 0.7 2.5 2.0 11.5 0.103

ablative process (Helleur et al.34) 550 80.3 1.3 0.3 2.0 0.6 15.2 0.19batch fixed-bed process (Cunliffe and Williams47) 500 83.8 0.8 0.6 2.0 nd 11.9 0.113

600 83.9 0.7 1.0 2.0 nd 12.1 0.100a Data from a 12 kg/h pilot-scale vacuum multihearth reactor. b Data from a laboratory-scale vacuum batch retort reactor.

Figure 5. Pore volume and surface profiles of pyrolytic char andactivated carbon measured using the N2 adsorption method.


weight species instead of small molecular weight spe-cies, because of the rich mesopores in char.

3.4.4. Activation Characteristics of PyrolyticChars. Because the original pyrolytic char can only beused as an adsorbent of large molecular species, activa-tion of the char to produce a relatively high gradeactivated carbon with both high BET surface area andmicropore volume may represent a more economicallyattractive option. Figure 7 shows the burnoff degree andBET surface area achieved in carbon dioxide using the2.5-5.1 mm fraction of 550 °C pyrolytic char in relationto the activation temperature. As the activation tem-perature increased from 850 to 950 °C, the burnoffincreased from 21.3% to 51.3% nearly in a linearfashion. The BET surface area also linearly increasedfrom 125 to 306 m2/g. The impact of the activationtemperature can be attributed to the Arrhenius law, asdiscussed in detail by Cunliffe and Williams.47

Figure 8 shows the influence of the burnoff degreeon the BET surface area of the activated carbonproduced by activation of 550 °C tire-derived char. Themaximum BET surface area attained was 306 m2/g atthe 51.3% burnoff (950 °C and 4 h), while that of theoriginal pyrolytic char was only 89.1 m2/g. High surfaceareas for activated tire-derived char similar to these

values have also been reported in the literature. Forexample, Teng et al.8 reported that activation of tire-derived char with carbon dioxide using TG-DTG re-sulted in a maximum surface area of 370 m2/g with 50%burnoff. Merchant and Petrich9 demonstrated conver-sion of the tire-derived char (using a muffle furnace) toactivated 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 °Cpyrolytic char by carbon dioxide under 900 °C resultedin a maximum surface area of 500 m2/g with 80%burnoff. Cunliffe and Williams47 studied activation ofthe fixed-bed pyrolytic char and reported that, for 50%burnoff, carbon activated by steam had a surface areaof 510 m2/g, while carbon activated by carbon dioxidehad a surface area of only 420 m2/g. The activation ofthe tire-derived char (using an ablative pyrolyzer) withcarbon dioxide by Helleur et al.34 obtained a surface areaof 240-270 m2/g at about 40% burnoff. The high ashcontent of the pyrolytic char in the current workpartially limited the enlargement of the BET surfacearea during activation. Therefore, acid demineralizationof the char after activation is needed to further improvethe surface area of the activated char.

In addition, the approximately linear relationshipbetween the surface area and the carbon burnoff inFigure 8 has been observed in many previous studies.Cunliffe and Williams47 found an initial relatively slowincrease in the BET surface area with increasingburnoff, followed by a linear increase up to a maximumsurface area at 65% burnoff, after which the surfacearea then decreased. Merchant and Petrich9 not onlyillustrated the linear relationship between the BETsurface area and burnoff but also studied the variationof the micropore volume with burnoff. They concludedthat the increasing BET surface area with burnoff below40% can be attributed to the increasing microporevolume, while the increase with burnoff in excess of 40%was attributed to the increasing meso- and macroporevolumes. Furthermore, Teng et al.8 and Mirmiran etal.48 also pointed out the linearity between the surfacearea and carbon burnoff for various scale reactors.

Figure 9 compares the pore volume profiles foractivated tire-derived char (pyrolyzed at 550 °C and

Figure 6. Pore volume profiles for pyrolytic char and commercialactivated carbon measured using the mercury intrusion method.

Figure 7. Burnoff and BET surface area achieved in carbondioxide for the 2.5-5.1 mm fraction of a 550 °C tire-derived charin relation to the activation temperature.

Figure 8. Influence of burnoff on the BET surface area ofactivated char produced from the activation of a 550 °C tire-derivedchar.


activated at 950 °C) and commercial activated carbon.As mentioned earlier, the pore volumes in the originalpyrolytic char were much different from those in thecommercial activated carbon by at least 1 order of mag-nitude. As shown in Figure 9, the pore volume profilefor the activated tire-derived char is quite similar to thatfor the commercial activated carbon, especially for themesopore volume. However, the micropore volume in theactivated tire-derived char is still less than that in thecommercial activated carbon. This suggests that thepyrolytic or activated tire-derived char should be usedas a mesoporous carbon in some special cases, e.g., theaqueous adsorption of large molecular species.

Aqueous adsorption using tire-derived char has alsobeen investigated. The adsorption of large-moleculemethyl blue by the activated or original tire-derived charwas both about 230 mg/g3, which is a little higher thanthat of the commercial activated carbon. However, theadsorption of the iodine by tire-derived char was only1/5 that of the commercial activated carbon. Furtherresearch on the adsorption of specific large-moleculepollutants would be of interest. For example, the mo-lecular sizes of mercury and dioxins, mainly producedfrom waste incinerators, are 0.45 × 0.45 and 1.0 × 0.3nm, respectively.49 The effective pore diameter for theadsorption of dioxins is about 2-10 nm, which belongsto the mesopore structure, while that for mercury is0.5-5 nm, which belongs to the transition zone betweenmicropores and mesopores. Thus, the original inacti-vated tire-derived char theoretically has the potentialfor the adsorption of dioxins, and slightly activated tire-derived 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 apyrolysis reactor of scrap tires because the solid residencetime can be flexibly adjusted to meet the requirementof complete decomposition. Two crucial experiencesshould be noted in the operation of a pilot-scale rotarykiln pyrolyzer. First, the selection of the solid residencetime, measured with difficulty during pyrolysis, can beoptimized as follows: conducting similar pyrolysis testsby increasing the rotating rate in a step of 0.5 rpm,

judging whether the difference between char yields inthe subsequent tests is less than 1%, and finallydetermining the solid residence time of complete py-rolysis. Second, the kiln length with respect to diameterplays a crucial role in determining the vapor residencetime. The latter directly determines the oil yield as wellas its valuable light compositions such as BTX. Theoptimum time/temperature profiles achieved by themultiple heating zones with different temperaturesalong the kiln length should been attempted to promotethe production of volatile aromatics.

(2) As the temperature increased from 450 to 650 °C,the char yield remained essentially constant at a meanof 39.8 wt %. Compared with other typical processessuch as vacuum, fluidized-bed, ablative, and two-stage,the char yield is independent of reactor types withdifferent heating rates. The oil yield reached a maxi-mum value of 45.1 wt % at 500 °C and then decreased.The oil yield is less than that of Kawakami et al. usinga similar rotary kiln.38 The long vapor residence timein the high-temperature zone caused not only the lowyield but also richer single-ring aromatics than othertypical processes. The maximum concentrations ofbenzene, toluene, xylene, styrene, and limonene in theoil were 2.09, 7.24, 2.13, and 5.44 wt %, respectively.The absorbance peaks at 3100-3000, 1604 (1495), and900-675 cm-1 using FT-IR analysis confirmed theformation of rich aromatics. The increasing temperatureresulted in the increment of the aromatics composition,especially the high concentration of PAHs.

(3) The TBP distillation test of the tire-derived oil hadoriginally 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 (200-350 °C), and a 25.5-28.5 wt % heavy fraction (>350 °C).The content of the naphtha fraction was greatly higherthan that by Kawakami et al. (24.5 wt %).38

(4) The pore structure of pyrolytic char was mostlycomposed of mesopores instead of micropores as mea-sured by both N2 adsorption and mercury intrusionanalyses. It is most suitable for aqueous adsorption oflarge molecular weight species.

(5) Pyrolytic char after carbon dioxide activation hada relatively high BET surface area of 306 m2/g at 51.3%burnoff. There is an approximately linear relationshipbetween the surface area and carbon burnoff. Furtherstudies on the adsorption of dioxins and heavy metalsare currently underway.

AcknowledgmentThis work was funded by National Natural Science

Fundation of China (Grant 50076037). We sincerelythank Prof. Paul Williams, University of Leeds, for theinspiration on the research method by their publica-tions. The authors thank the the staff of Departmentof Chemistry, Zhejiang University, for their help in theanalysis of oil properties. We also acknowledge the helpof Dr. R.-D. Li, Dr. J.-T. Huang, Dr. D.-H. Yan, Y.-L.Gao, and S.-E. Wen for their cooperation in the experi-ments.

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Resubmitted for review January 14, 2004Accepted April 30, 2004

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pilot-scale pyrolysis of scrap tires in a continuous rotary kiln reactor

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