TOXIC EMISSIONS FROM PYROLYSIS AND COMBUSTION OF WASTE TIRE CHIPS Jefferson Caponero, Jorge A. S. Tenório, Polytechnic School, University of São Paulo - São Paulo, 05508-900 Brazil [email protected] Yiannis A. Levendis & College of Engineering, Northeastern University - Boston, MA 02115, USA Joel B. Carlson US Army Natick Research, Development and Engineering Center Natick, MA 01760, USA

Abstract This paper reports a laboratory investigation on the emissions from batch pyrolysis and combustion of waste tire chips in fixed beds. The goal is to burn this abundant, and rather renewable, solid waste fuel and identify conditions that minimize toxic emissions. In the experiments waste tire chips (0.5-1 cm) are gasified/burned in a horizontal muffle furnace, at gas temperatures in the range of 500-1000°C. During the experiments the gaseous effluent was mixed with additional air or nitrogen and was channeled in a second furnace where further oxidation or degradation took place. With this arrangement of two furnaces in series, with an additional gas mixing section in between, a marked reduction in the emissions of organic pollutants, PAH, soot and CO was observed.

Introduction The disposal of scrap tires is a serious environmental problem, especially for the major industrial countries. Approximately 264, 164 and 32 millions of tires are disposed each year in USA, Japan and Brazil (Table 1) [1,2,3,4]. In the USA it is about one tire per person and has been increasing in the last years. Nowadays, the major disposal methods have been landfill or build scrap tire stockpiles, either legal or illegal. Scrap tires are a favorable place to the proliferation of rats and insects and therefore pose a potential health hazard. A more serious problem is the fire hazard that this scrap tire pose. It has been reported that there are over 2 billions of tires stockpiled or in landfills just in the USA. Table 1 – Annual production/use of tires in some countries. Production/Use of Tires Country Year Base Ref. (millions of tires/year) EUA 264 1990 2 Japan 164 1992 2 Chorea 38 1992 2 Brazil 32 1999 3 France 27,4 1991 4 Germany 25,3 1991 4 UK 23,2 1991 4 Benelux 7,2 1991 4 The increase use of cars in the industrial countries will exacerbate this problem. Some improvements in the tire technology and the retreating, that have been used extensively for heavy-duty vehicle and airplane tires and in some countries for passengers vehicle tires, have increased the tire life. Those improvements themselves have minimized the tire disposal problem but the actual rate of disposal does not see to reflect this reduction. Presently the major alternatives to disposal in landfills are as a fuel. The tire fuel derived (TDF) has been used successfully in waste-to-energy plants, cement kilns and in paper and pump plants and as asphalt ground substitute, but this last with higher costs. Pyrolysis Though these are good alternatives they are also low-value uses and the value of the scrap tires of these uses are similar to the cost of the tire collection. A growing alternative, which tries to result in a yield of more valuable products, is the pyrolysis of the scrap tires. The process of pyrolysis can be defined as a controlled thermal degradation process and basically it could be of four different types inert, oxidative, reductive or vacuum pyrolysis. The differences among these processes are due to the composition of the atmosphere of the furnace (reactor). In the inert process gases such as nitrogen and argon are used, in the oxidative process: oxygen or steam, and in the reductive process: hydrogen. In the oxidative process a fuel-rich (sub-stechiometric) combustion take place and in the reductive the hydrogenation of the material and subsequent production of sulfidric gas allows a reduction of the sulfur content of the by-products [5]. The vacuum process allows the yielding of higher quantities of oil due to the minimization of the secondary reactions, such as: break of chains due to the temperature, re-polimerization and condensation, gas collision, catalytic break of chains and reductive and oxidative reactions [6]. The pyrolysis has been used in the treatment of tires to its by-products, such as: carbon, gas oil, gas and steel. The typical quantity obtained of it component is: 33–38% of solids; 38–55% of oils and 10–30% of gases. Scrap tires, when compared with others waste components, are a homogeneous form of waste, but depending on the tire grade, age and manufacturer the pyrolyses products may vary, it also can vary second

the process used, temperature and heating rate [6,7]. CYPRES and BETTENS [8] pyrolysed scrap tires from six different manufacturers in a two-stage process comprising initial lowtemperature followed by continuous post-cracking of the volatile materials at higehr temperatures. The process was carried between 450 - 800ºC given small but significant differences in the product yielded and chemical composition. GERSTEN et al. [9] using a batch operated unit with only one stage (retort) at 500ºC have obtained almost 50% of oil (about 75% of the organic basis) and only 4.3% of gas with more than 43% of residues. While Roy and his co-workers [10] with a similar configuration at the same temperature yielded 54% of oil, 11% of gas and 36% of residues. Combustion The best solutions for the waste tire issue have been based in their high energy content. It is due to the energy content of tire (29-39 MJ/kg) that is equal or higher than most typically coal (Figure 1 and Table 2). Table 3 shows the typical tire elementary composition. The combustion behavior of the coal has been extensively studied in the last century and still is. In the last decades a growing literature production has attended to the similar behavior of coal and tire during the combustion process. Tires present comparable carbon, ash, sulfur and nitrogen contents. Even the aromatic structures of tires are similar to those in coal. 36,05 33,03 29,65 24,42 16,98 10,18

Wood

12,37

MSW

Lignite

Coke SubBituminous

Coke Bituminous

Whole Tire

Steel-Free Tire

Figure 1 – Energy content of some fuels (MJ/kg) [11] Table 2 – Contrasting elementary composition of tires and bituminous coke, %wt [12]. Element Coke Tire C 82,4 88 H 5,5 8 O 8 2 N 1,7 0,5 S 2,4 1,5 Table 3 – Elementary composition of tire chunks, %wt [13]. Fixed carbon 33.6 Volatiles 58.7 Ash 7.7 Carbon 80.9 Hydrogen 6.4 Sulfur 1.85 Nitrogen 0.3 Oxygen 3 Heating Value (MJ/kg) 39

The tire volatile mass (~60%) is approximately two times higher the most bituminous coal volatile mass. The major differences are the moisture, markedly low in tires, and zinc content, markedly high in tires [14,15]. The combustion of tires can be divided in two categories: combustion of whole tires and combustion of processed tires. The combustion of whole tires is one of the best ways of recycle tires, but only a few industrial processes can deal with this fuel size. It has been used especially in cement plants. The use of tires instead of the conventional fuel adds some advantages to the process. The high temperature of the rotary cement kiln (2000°C on gases and 1450°C on load) creates a propitious environment to oxidize the tire steel. It gives to the kiln extra energy and substitute part of the iron ore used as a row material. The zinc oxide always present works as a mineralizer in the clinker production lowering the clinkerization temperature [16]. The processed tires can varies from larger pieces (shredder) to ground tire (cryogenic process). The most properties of the combustion of tires are directly related with its initial particle size. LEVENDIS and his co-authors have studied the influence of the particle size in fixed beds and pulverized fuel combustion compared with the coal. The results show that both pulverized and batch combustion of the tire size influence the emissions of PAH and soot. The higher the particle size the higher its emissions. On the other hand, it seen do not influence the nature of the species produced. Similar emissions are reported for both chunks and ground tires at higher temperatures or higher residence times or both in this combustion processes [14,17,18,19]. The carbon accounts for nearly half of the mass of atmospheric fine and ultrafine particles and it is present in basically two forms: organic carbon, which includes hundreds of compounds, and elemental carbon [20]. Knowing that, the challenge in tire combustion/pyrolysis is the development of a fuel/technique that minimizes the emissions of PAH and soot. Materials and Methods Tire chips with dimensions in the order of a centimeter were obtained from a local source and included organic fabrics, such as nylon belts, see Figure 2. No metallic belts were included. Some physical and chemical properties are shown in Table 4. All of the tests conducted in this study involved batch combustion or pyrolysis of tire chips in fixed beds. Pre-weighted amounts of 0.8 g, consisting of a few (typically 3) chips, were placed in porcelain boats and were inserted in the quartz tube of the primary furnace, 4 cm in diameter and 87 cm long. All the combustion experiments were conducted in air. The air flowrate in the first furnace was 4 L/min, and the residence time of the gases between the sample and the Venturi was a fraction of a second. This primary furnace was connected to a secondary muffle furnace (the afterburner), as shown in Figure 3. The dimensions of the secondary furnace were 2 cm in diameter and 38 cm long. The effluent of the first furnace passed through a Venturi (8 mm in diameter) where it was mixed with four radially-positioned perpendicular jets of preheated air, discharging at the periphery of the Venturi, see Figure 3. Mixing of the air from the jets and the gaseous effluent of the combustion occurred in the Venturi. The mixed charge was then sampled at the exit of the primary furnace, and subsequently channeled to the afterburner. The wall temperature of the primary furnace was varied in the range of 500-1000°C, while that of the secondary furnace was kept constant at 1000°C throughout these experiments. The flowrate of additional air supplied at the Venturi, through the four jets, was 2 L/min. For the pyrolysis experiments all the gas flows were kept the same as in the combustion experiments but changing the air by nitrogen. All the other parameters remained the same. The calculated Reynolds number in the Venturi was 425. The residence time of the gases in

the secondary furnace was calculated to be in the range of 0.6 to 0.8 sec, according to the temperature therein, and the Reynolds number was in the range of 75 to 85.

1 cm

Figure 2 - A photograph of the waste tire chips that were burned in these experiments. Table 4 – Some physical and chemical properties of the tire chips used. Fixed carbon

Volatile

Ash

24.88 %

69.73 %

4.03 %

Element

Tire Chips

Carbon Hydrogen Sulfur Others

85.82 7.26 2.33 4.59

Aparent Density 0.4 g/cm3

Heating Residue Value Specific Area 39 74 m2/g MJ/kg Carbon Burned Ash Black Fraction* Residue 87.39 82.20 37.70 10.19 <0.5 <0.5 2.39 2.20 3.00 0.03 15.10 58.80

* calculated

Figure 3 - General schematic showing features of the two laminar-flows, horizontal muffle furnaces in series, separated by an additional air mixing section. Monitoring of Emissions O2, NOx, CO, and CO2 emissions from the combustion of tire chips were monitored at the exits of the two furnaces. After the sampling stage the effluent passed through an ice bath and a mildly-heated Permapure dryer, where moisture was removed. The effluent was then

monitored for NOx using a Beckman 951A chemiluminescent NO/NOx analyzer, for SO2 by means of a Rosemount Analytical 590 UV analyzer, for O2 using a Beckman paramagnetic analyzer and for CO and CO2 with Horiba infrared analyzers. The output of the analyzers was recorded using a Data Translation DT-322 data acquisition board in a microcomputer. The integrated acquisition system used DT VPI to support data acquisition from within a HewlettPackard HP VEE visual programming environment and automatically formed Microsoft Excel data files corresponding to the different experiments. The signals from the analyzers were recorded for the duration of the experiment and subsequently were converted to partial pressures and, eventually, to mass yielded (mg/mass of sample burned/pyrolyzed). Polycyclic aromatic hydrocarbon (PAH) emissions, as well as particulate (mostly soot and, possibly, traces of minerals) from the combustion of tire chips were monitored at the exits of the two furnaces. The PAH’s were sampled by passing half of the effluent through each of the sampling stages consisting of a Graseby sampling head with a filter stage and a glass cartridge containing XAD-4. The sampling stage was placed adjacent to the furnace to minimize losses. Prior to each sampling stage the effluent of the furnaces was mixed with a 2 L/min flow of nitrogen gas, in order to the effluent was cooled and inerted. Subsequently, the particulate emissions were trapped on the upper portion of the sampling stage on a 11 cm diameter Fisher Scientific cellulose filter paper with a nominal particle retention of 1 – 5µm. Gas-phase aromatic hydrocarbon emissions were adsorbed on the bed of Supelco XAD-4 resin. The length of the XAD-4 bed was more than twice its diameter. Following the experiments, the filters and resins were removed and placed in separate glass bottles with Teflon-lined caps, and stored at 4 °C. Prior to extraction with methylene chloride, a 25 µL internal standard containing 50µg each of naphthalene-d8, acenaphthene-d10, anthracene-d10, chrysene-d12 and perylene-d12 was applied to each of the glass bottles containing the samples prior to the analysis by gas chromatography coupled to mass spectrometry (GC-MS). Results and Discussion Upon its introduction to the pre-heated furnace, the fuel bed heated up. Two distinct combustion phases were observed; burning of the volatile followed by burning of the char (which is mostly carbon black). During the devolatilization period a diffusion flame formed over the fuel bed, and a laminar plume of smoke was visible flowing to the exit of the furnace. The flame appeared to be rather steady and lasted for a period of approximately one to two minutes. The rapid mass loss during devolatilization of the tire chips resulted in insufficient mixing of the fuel and the oxidant. This was followed by a lengthy char combustion phase (in the order of 10 min). The two combustion phases were studied in detail in previous work [21,22,23] and it was shown that the vast majority of the organic emissions is released during the combustion of volatile. Combustion of the carbon black is slow and its contributions to the organic pollutant emissions are not significant. In this work the effects of two parameters on the emissions from the volatile combustion of tire chips were monitored, (a) the primary furnace temperature and (b) the effect of the existence of a secondary furnace (afterburner). The primary furnace acted as a gasifier/combustor and its temperature was varied between 500°C and 1000°C. The afterburner' s temperature was kept at 1000°C and the gas residence time therein was a little under a second (0.7-0.8 s, depending on the temperature). All other conditions were kept the same in these experiments. During the pyrolysis experiments only the devolatilization takes place producing a brown smoke that becomes as darker as the temperature of the first furnace increases. At first furnace temperatures around 1000°C the devolatilization period was similar to the one in combustion experiments, at lower

temperatures it takes 3-4 times longer. The reader should keep in mind that batch combustion/pyrolysis of the fuel resulted in transient emission profiles. Emission yields were obtained by integrating each profile and dividing by the mass of the fuel burned/pyrolyzed; in this case only the volatile component of the tire chips. During the combustion, globally neither furnace' s atmosphere was starved for oxygen during the combustion events, as the minimum concentrations in both furnaces never fell below 2%, see Figure 4a. The oxygen concentrations at the exit of the secondary furnace were always lower than the corresponding values at the exit of the primary furnace, Figure 1a-b; both values already including the additional air introduced in the Venturi. This is because further conversion of volatile pyrolyzates and primary combustion products takes place in the secondary furnace, thereby consuming oxygen. The oxygen recorded during the pyrolysis experiments is due to insertion of the tire chips into the furnace. The delay between the insertion and the beginning of the pyrolysis is enough to purge the furnace atmosphere. The maximum partial pressure of oxygen in both furnaces never reaches over 2.8% and the amount inlet is typically 3 orders of magnitude lower than in combustion experiments. Both the primary furnace temperature and the existence of the afterburner affected CO2 emission yield. During the pyrolysis it is more expressive, specially at lower temperatures, see Figure 4c. Maximum CO2 partial pressures were recorded to be in the range of 10% for combustion experiments and two orders of magnitudes lower for pyrolysis experiments, see Figure 4d. Maximum CO partial pressures at the exit of the first furnace were typically below 2.5%, for combustion and 0.25% for pyrolysis; while at the exit of the second furnace they were much lower, Figure 4f. The CO emission yields were drastically reduced by the afterburner, the minimum reduction being by a factor of 3 and the maximum by a factor of 250, see Figure 4d. The reduction of the CO emission present a opposite behavior for pyrolysis and combustion. For combustion, as the first furnace temperature increases the reduction is less expressive. It suggests that as the more the temperature increases the more complete is the combustion in the first furnace and the more the residence time at 1000°C increases the more favorable is the CO reduction in the pyrolysis. Yields of SO2 and NOx from batch combustion of tire chips were only recorded at the exit of the primary furnace, see Figure 4g-h. Both these emissions were found to be generally low. A large fraction of the fuel nitrogen and sulfur remained in the char and the ash, see Table 4 and reference 14. The effect of the afterburner was highly beneficial in reducing the particulate emissions, especially at the lower primary furnace temperatures where a manyfold reduction was observed, see Figure 4i. This is because at the lower gasifier temperatures the particulate matter was significantly composed by oils and tars, forming a light brown cloud, and were readily oxidized (combustion) or degraded (pyrolysis) in the afterburner. At the highest gasifier temperatures the particulate matter was mostly black soot, which is very stable at the temperature of the afterburner (1000°C), and thus was only partially reacted. The cumulative emission yields of all detected semi-volatile polycyclic aromatic hydrocarbons (PAH) covering the mass range from 116 amu (indene) to 278 amu (benzo[b]chrysene and isomers) are shown in Figure 4j. More than 50 PAH compounds were detected by GC-MS, for brevity, only the combined amounts of PAH, both in the condensed phase (cellulose filter paper) and in the gas phase (XAD-4) are plotted in Figure 4j. Generally, only 2-3 ring PAH were present in the gas phase, which means that they were not retained on the filter, adsorbed on particulate. Heavier multi-ring compounds were found in the condensed (solid) phase, i.e., with the collected particulate. The overall trend of the cumulative PAH emissions were to decrease with the gas temperature of the primary furnace,

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during the combustion but it presents a exponential increase during the pyrolysis. Not only the effect of the temperature is the opposite during the pyrolysis but also the afterburner effect.

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(i) (j) Combustion: — — Stage 1, — — Stage 2; Pyrolysis: - - - - Stage 1, - - - - Stage 2 Temperature (°C)

Figure 4 - Partial pressures and emission yields at the exits of both the primary and the secondary furnace, for different primary furnace temperatures (500-1000°C). The afterburner temperature was kept constant at 1000°C.

Conclusions The results observed in these experiments permit to conclude that: The temperature effects of the primary furnace, acting as a gasifier/combustor, was significant for all the pollutants analyzed, as well as the effects of operating an afterburner furnace. The combustion indicated that the existence of the afterburner, operated at 1000°C and 0.7 s residence time, drastically reduced CO emissions. The afterburner propitiate a more effective oxidation in the second furnace, what can be seen by its higher emissions of CO2, and its higher consumption of oxygen, due to the increasing of the residence time of the volatile and particulate matter in the second furnace. NOx and SO2 emissions were found to be generally low. Results also indicated that drastically reduced particulate emission, especially at the lower primary furnace temperatures where a manyfold reduction was observed. The cumulative emission yields of all detected semi-volatile polycyclic aromatic hydrocarbons exhibited an increasing trend with temperature. In contrast, PAH emissions from the secondary furnace were much lower, by factors of 2 to 4, especially at the lower temperatures, due to the effect of the afterburner. The pyrolysis indicated that the oxygen recorded due to insertion of the tire chips into the furnace does not significantly react with the tire chips . The delay between the insertion and the beginning of the pyrolysis is enough to purge the furnace atmosphere; the maximum partial pressure of oxygen in both furnaces never reaches over 2.8% and the amount inlet is typically 3 orders of magnitude lower than in combustion experiments. CO2 partial pressures were recorded to be two orders of magnitudes lower than in combustion. CO partial pressures was at least one order of magnitude lower. SO2 and NOx emissions also were found to be generally low. The effect of the afterburner was also beneficial in reducing the particulate emissions, especially at the lower primary furnace temperatures where a manyfold reduction was observed. The overall trend of the cumulative PAH emissions were to exponential increase with the gas temperature of the primary furnace. Not only the effect of the temperature is the opposite during the pyrolysis but also the afterburner effect. Acknowledgements The authors would like to thank Mr. Eric Wisnaskas for technical assistance with the chromatographic analysis. The authors would also like to acknowledge São Paulo research foundation, FAPESP, for financing Eng. M.Sc. Jefferson Caponero (N. 99/000375-5). References [1]

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[2]

JANG, J. W. et al. Resourc., Conserv. and Recycling. 1-2 (1998) 1..

[3]

CEMPRE - COMPROMISSO EMPRESARIAL PARA RECICLAGEM Pneus. [on line]. 15/jan/1999. Available from World Wide Web: [27/jan/1999 – in Portuguese].

[4]

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[5]

BLUMENTHAL, M. H. Tires. Cap. 18 p. 18.1-18.64 In. LUND, H. F. The McGraw-Hill Recycling Handbook. New York, McGraw-Hill, 1993.

[6]

ROY, C.; LABREQUE, B.; CAUMIA, B. Resourc., Conserv. and Recycling. 3 (1990) 203.

[7]

TENG, H. et al. Ind. & Eng. Chem. Research. 9 (1995) 3102.

[8]

CYPRES, R.; BETTENS, B. In: “Pyrolysis and Gasification” (Eds G. L. Ferrero et al), Elsevier Applied Science, London, 1989

[9]

GERSTEN, J. et al. Fuel 78 (1999) 987.

[10]

CHAALA, A.; ROY, C. Fuel Process. Technol 46 (1996) 227.

[11]

BLUMENTHAL, M. H. Tires. Cap. 18 p. 18.1-18.64 In. LUND, H. F. The McGraw-Hill Recycling Handbook. New York, McGraw-Hill, 1993.

[12]

TENG, H. et al. Reprocessing of used tires into activated carbon and other products. Industrial & Engineering Chemistry Research. v. 34, n. 9, p. 3102-11, sep, 1995.

[13]

LEVENDIS, Y. A.; ATAL, A.; CARLSON, J. B. Combus. Sci. and Tech. 134 (1998) 407

[14]

ATAL, A.; LEVENDIS, Y. A. Fuel 74 (1995) 1570

[15]

CAPONERO, J.; TENORIO, J. A. S. In: “Congresso anual da ABM”, 55°, Associacao Brasileira de Metalurgia e Materiais – ABM. Anais. Sao Paulo, 2000 [in Portuguese]

[16]

CAPONERO, J. “Comportamento da lama de fosfatizacao no processo de producao do clinquer de cimento Portland” Master Thesis, Universidade of Sao Paulo, Sao Paulo, Brazil, 1999 [in Portuguese]

[17]

LEVENDIS, Y. A.; ATAL, A.; CARLSON, J. B. Combust. Sci. and Tech. 134 (1998) 407

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LEVENDIS, Y. A. et al. Combust. Sci. and Tech 131 (1998) 147

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LEVENDIS, Y. A. et al. Environ. Sci. and Tech. 30 (1996) 2742

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VIOLI, A.; D’ANNA, A.; D’ALESSIO, A. Chemical Eng. Sci. 54 (1999) 3433

[21] ATAL, A.; LEVENDIS, Y. A. Fuel. 74 (1995) 1570. [22] LEVENDIS, Y. A. et al. Combust. Sci. and Techn. 131 (1998) 147. [23] LEVENDIS, Y. A; ATAL, A.;.CARLSON, J. Combust. Sci. and Techn. 134 (1998) 407.