Energy & Fuels 2003, 17, 225-239
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Emissions of Batch Combustion of Waste Tire Chips: The Afterburner Effect Jefferson Caponero and Jorge A. S. Teno´rio* Polytechnic School, University of Sa˜ o Paulo, Sa˜ o Paulo, 05508-900 Brazil
Yiannis A. Levendis College of Engineering, Northeastern University, Boston, Massachusetts 02115
Joel B. Carlson United States Army Natick Research, Development and Engineering Center, Natick, Massachusetts 01760 Received June 11, 2002
A laboratory investigation was performed on the emissions from batch combustion of waste tire chips in fixed beds to identify techniques and conditions that minimize toxic emissions. Tire derived fuel (TDF), in the form of waste tire chips (1 cm), was burned in a two-stage combustor. Batches of tire chips were introduced to the primary furnace where gasification and oxidative pyrolysis took place. The gaseous effluent of this furnace was mixed with streams of additional air and, subsequently, it was channeled into the secondary furnace (afterburner) where further oxidation took place. The arrangement of two furnaces in series allows for independent temperature control; varying the temperature in the primary furnace influences the type and the flux of pyrolysates. The additional-air mixing section between the two furnaces allows for mostly heterogeneous and fuel-lean combustion in the afterburner. Results showed that both the operating temperature of the primary furnace, in the range of 500-1000 °C, and the existence of the afterburner had marked influences on the emissions of pollutants. Results showed that for this fuel use of combustion staging, with an additional-air mixing section, had a very beneficial effect. It drastically reduced the emissions of CO (by factors of 3-10), the particulates (by factors of 2-5) and the cumulative PAH (by factors of 2-3). Many health-hazardous PAH components were practically eliminated. Overall oxidizing conditions prevailed and the minimum oxygen mole fraction never fell below 2% in the effluent of either furnace. The operating primary furnace temperature (pyrolysis temperature) also proved to be important, with temperatures at the low side of the 500-1000 °C range producing fewer pollutants, upon treatment in the afterburner.
Introduction The disposal of scrap tires is a challenging environmental problem, especially for the industrialized countries. Approximately 264, 164, and 32 million tires are disposed of each year in the United States, Japan, and Brazil, respectively.1-4 In the United States, this amounts to approximately one used tire discarded per person annually, and the rate has been increasing lately. Nowadays, the major disposal methods have been dumping tires in landfills or in scrap tire stockpiles, either legal or illegal. However, scrap tires are a favorable place for proliferation of rodents and insects; thus, they pose a potential health hazard. A more serious problem is the fire hazard that scrap tires pose. * To whom correspondence should be addressed. Phone: +55 11 3091 5546. Fax: +55 11 3091 5243. E-mail:
[email protected]. (1) Senneca, O.; Salatino, P.; Chirone, R. Fuel 1999, 78, 1575. (2) Jang, J. W.; et al. Resourc., Conserv. Recycl. 1998, 1-2, 1. (3) Cempre-Compromisso Empresarial Para Reciclagem. Pneus. On line 15/Jan/1999. Available from World Wide Web. URL: http:// www.cempre.org.br/ficha8.htm [27/Jan/1999, in Portuguese]. (4) Ferrer, G. Resourc., Conserv. Recycl. 1997, 4, 221.
It has been reported that there are several billion waste tires stockpiled or in landfills, just in the United States. The increased usage of automobiles in the industrial countries is exacerbating this problem. Improvements in the tire construction technology and, also in retreating of tires (in limited applications such as heavy-duty vehicles and airplanes) have increased tire life. Even if such improvements themselves should help minimize the tire disposal problem, the actual rate of tire disposal does not seen to reflect this reduction.5 The composition of tires varies depending on their use. Natural and synthetic rubber, carbon black, steel, aromatic oils, stabilizers, sulfur, and zinc oxide are the major components. Automobile tires are made normally with styrene-butadiene copolymers (SBR) or styrenebutadiene copolymer/polybutadiene mixtures (SBR/BR), while the sidewalls are generally made with addition of natural rubber (NR). Table 1 shows the typical tire compound composition.6 (5) Caponero, J.; Tenorio, J. A. S. In Proceedings of the 55th Congresso anual da ABM; Associac¸ a˜o Brasileira de Metalurgia e Materiais-ABM; Sa˜o Paulo, 2000 [in Portuguese].
10.1021/ef0201331 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/18/2002
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Table 1. Typical Tire Compound Composition (wt %)6 SBR carbon black extender oil zinc oxide stearic acid sulfur accelerator total
62.1 31.0 1.9 1.9 1.2 1.1 0.7 99.9
Presently the major alternative to disposal in landfills is using waste tires as a fuel. The tire-derived fuel (TDF) has been used successfully in cement kilns, lime kilns, paper and pump mills, iron foundries, copper smelters, and waste-to-energy plants. Combustion The reason that waste tires are targeted for combustion is their high-energy content (calorific value), which is in the range of 29-39 MJ/kg; this is equal or higher than that of most typical coals.7 The combustion behavior of the coal has been extensively studied over the last few decades and still is. Recent literature has reported on similarities in the combustion behavior of pulverized coal and tire crumb.8-13 The tires’ content of major elemental species (carbon, minerals, sulfur and nitrogen) is similar to that of coal or coke.14 The tire volatile mass (∼60%) is approximately two times higher than that of most bituminous coals. The major differences are the moisture content, markedly low in tires, and zinc content, markedly high in tires.5,8 The nitrogen content of tires is a fraction of that of coal, while the sulfur content is generally comparable. Test-firing of whole tires or shredded tires, has been conducted in various types of boilers, with or without co-firing with coal, see for example reports by Epri,15 Pirnie,16 Clark et al.,17 Lemieux and co-workers,18,19 Kearney,20 and Harding21 reviewed recent commercial demonstrations of TDF co-firing with coal in various types of boilers. The boilers best suited for using TDF (6) Willians, P. T.; Besler, S. Fuel 1995, 74, 1277. (7) Blumenthal, M. H. Tires. In Recycling Handbook; Inlund, H. F., Ed.; McGraw-Hill: New York, 1993; Chapter 18, pp 18.1-18.64. (8) Atal, A.; Levendis, Y. A. Fuel 1995, 74, 1570. (9) Levendis, Y. A.; Atal, A.; Steciak, J. Combustion and Inorganic Emissions of Ground Waste Tires. In Proceedings of the 20th International Conference on Coal Utilization and Fuel Systems, March 1995, Clearwater, FL. (10) Atal, A.; Steciak, J.; Levendis, Y. A. NOx and SOx Emissions from Pulverized Coal and Waste Tire: The Role of Devolatilization and Char Combustion Phases. In Proceedings of the ASME Heat Transfer Division, HTD, Nov 1995, San Francisco, CA; Vol. 317-2. (11) Levendis, Y. A.; Atal, A.; Carlson, J.; Dunayevskiy, Y.; Vouros, P. Environ. Sci. Technol. 1996, 30, 2742. (12) Levendis, Y. A.; Atal, A.; Courtemanche, B.; Carlson, J. Combust. Sci. Technol. 1998, 131, 147. (13) Courtemanche, B.; Levendis, Y. A. Fuel 1998, 77, 183. (14) Teng, H.; et al. Ind., Eng. Chem. Res. 1995, 9, 3102. (15) EPRI GS-GS-7538. In Proceedings of the Conference on Waste Tires as a Utility Fuel, 1991; Prepared by Electric Power Research Institute: Palo Alto, CA, Sept, 1991. (16) Pirnie, M. Air Emissions Associated with the Combustion of Scrap Tires for Energy Recovery; Prepared for the Ohio Air Quality Development Authority: Columbus, May, 1991. (17) Clarc, C.; Meardon, K.; Russell, D. Burning Tires for Fuel and Tire Pyrolysis: Air Implications. EPA-450/3-91-024 (NTIS PB92-145358); U.S. Environmental Protection Agency: Washington, DC, Dec 1991. (18) Lemieux, P. M.; Ryan, J. V. Air Waste. 1993, 43, 1106. (19) Lemieux, P. M. Pilot-Scale Evaluation of the Potential for Emissions of Hazardous Air Pollutants from Combustion of Tire Derived Fuel. EPA-600-R-94-070; U.S. Environmental Protection Agency: Washington, DC, April 1994. (20) Kearney, A. T. Scrap Tire Use/Disposal Study; Final Report: Prepared for the Scrap Tire Management Council, Sept 1990.
are cyclones, stockers and fluidized beds. If tire-derived fuel (TDF) were to be burned in existing pulverized coalfired utility boilers, it would have to be ground to tire crumb. Previous studies in this laboratory examined the combustion and emissions of pulverized tires, i.e., in the form of tire crumb8-13 and found that extensive grinding is not necessary since tire particles as big as 250 µm burned as fast as much smaller coal particles (≈75 µm, which is the typical size used in pulverized coal boilers). Tire particles also burned somewhat hotter but released 3-5 times less NOx emissions, 10% less CO2 and comparable SO2 (as the sulfur contents of the particular fuels burned were similar). However, they released more PAH’s than coal. Such emissions were reduced by adding air, in excess to that needed for coal.11 Another effective method for minimizing such emissions was found to be co-firing tire crumb with coal in pulverized fuel flames.12 In summary, pulverized-flame type combustion behavior of tire crumb in the laboratory appeared similar to that of pulverized coal and emissions were to some degree comparable, at least under some conditions.8,10-12 Nevertheless, as tire pulverization is currently both challenging and costly, it is economically advantageous to burn tires either as whole units or shredded. Combustion of whole or shredded tires, i.e., chips or chunks with dimensions of 1-3 cm, is currently of technological interest. Combustion of whole tires has found applications in cement kilns and dedicated waste to energy plants. 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. Tires provide the kiln with extra energy and substitute part of the iron ore used as a row material. The zinc oxide, always present, functions as a mineralizer in the clinker production lowering the clinkerization temperature.22 Problems are often encountered with the carbon conversion efficiency. Also, only limited data has been reported in the literature on the emissions from such plants, particularly the organic air toxics. Yet, most people have vivid images of copious amounts of black smoke emitted from burning whole tires, either single or in stockpiles. Thus, past work at this laboratory aimed at investigating whether batch combustion of chunks (chips) of tires (simulating whole tires in a small scale) emits more pollutants than the aforementioned combustion of tire crumb in pulverizedfuel flames. Indeed, the results showed that batch combustion of tire chips resulted in much higher emissions of PAH and particulate matter.23 Generally, combustion of fine fuel particles in pulverized fuel furnaces offers better mixing of the fuel and the oxidant than combustion of chunks of solid fuel. On the other hand, it does not appear to influence the nature of the species produced. Similar species were reported for both tire chip and tire crumb combustion at elevated temperatures and comparable residence times.8,11,12,23 (21) Harding, N. S. Cofiring Tire-Derived Fuel with Coal. In Proceedings of the 27th International Technical Conference on Coal Utilization and Fuel Systems, March 4-7, 2002, Clearwater, FL; pp 477-488. (22) Caponero, J. Comportamento da lama de fosfatizac¸ a˜o no processo de produc¸ a˜o do clı´nquer de cimento Portland. Master Thesis, Universidade of Sa˜o Paulo, Sa˜o Paulo, Brazil, 1999 [in Portuguese]. (23) Levendis, Y. A.; Atal, A.; Carlson, J. B. Combust. Sci. Technol. 1998, 134, 407.
Batch Combustion of Waste Tire Chips
Batch combustion of chunks tire chips has also been contrasted to batch combustion of tire crumb and batch combustion of pulverized coal24 and results may be summarized as follows: (i) Batch combustion of TDF (either crumb or chips) in a muffle furnace generated PAH emissions, which were orders of magnitude higher than those from burning streams of tire crumb in a drop-tube furnace, depending on the bulk equivalence ratio, φ. In contrast, combustion of pulverized coal in batch mode (muffle furnace) and in streams (drop-tube furnace) resulted in comparable amounts of PAH’s. (ii) Batch combustion of pulverized coal or tire crumb generated cumulative CO emissions, which were much higher than those from burning streams of pulverized coal or tire crumb in a drop-tube furnace, at comparable feed rates. (iii) Batch combustion of tire crumb in fixed beds resulted in cumulative PAH emission that were an order of magnitude higher than those from batch combustion of pulverized coal, at comparable mass loadings. (iv) All of the monitored cumulative PAH and CO emissions from both fuels in batch combustion, originated from the volatile flames. Such differences in the emissions of PAH and, eventually, particulate matter can be understood by examining the combustion of tire chips. It begins with the rapid heating of the fuel. When a certain temperature is reached on the particle surface, pyrolysis occurs along with thermal oxidation reactions. When chips are subjected to temperatures in the range of 350-600 °C in an oxygen-lean environment, as the surface temperature increases, decomposition or degradation takes place. The devolatilizates from tires are composed of a wide variety of hydrocarbons, called primary products. These products then undergo secondary reactions, such as, thermal and catalytic cracking, re-polymerization, cyclization of alkyl chains, recondensation oxidation and reduction. Upon ignition, a diffusion flame forms around the tire chip, where a gradient of oxygen partial pressure exists. Fuel pyrolysis generally begins at relatively low temperature as the fuel approaches the flame front. The most accepted mechanism for soot formation from aliphatic fuels is through the formation of acetylene and polyacetylenes at a relatively slow rate. Aromatic fuels may form soot by a similar process, but also through a direct route involving ring condensation or polymerization reactions that build on existing aromatic structures.25 The increase of the pyrolysis rate with temperature and/or with the amount of volatiles in the solid fuel leads to an increasing tendency to form PAH and soot. Particles in the order of 10-20 nm form, which then flocculate and fuse to aggregates of soot. Simplified mechanisms of the soot formation, during the secondary reactions, are presented by WU et al.26 and MASTRAL et al.27 Some PAH components, either in the gas phase or condensed on soot particles, are among the most (24) Levendis, Y. A.; Atal, A. Emissions from Burning Tire-Derived Fuel (TDF): Comparation of Batch Combustion of Tire Chips and Continuous of Tire Crumb. In Proceedings of the 23rd Coal Utilization & Fuel Systems International Conference, March 9-12, 1998, Clearwater, FL. (25) Graham, S. C.; Homer, J. B.; Rosenfeld, J. L. J. Proc. R. Soc. London 1975, 344A, 259-285. (26) Wu, S. Y.; Su, M. F.; Baeyens, J. Powder Technol. 1997, 93, 283. (27) Mastral, A. M.; Collen, M.; Murillo, R. Fuel 1996, 75, 1533.
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problematic emissions of the combustion process. They are considered hazardous due to possible interactions with biological nucleophiles, resulting in inhibition of their regular functions.28 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.29 Therefore, a challenge in TDF combustion/pyrolysis is the development of a controlled combustion technique that minimizes the emissions of PAH and soot. The emissions of NOx in tire combustion were reported to be lower in tire than in coal, burned under identical conditions. Courtemanche and Levendis found emissions NOx from TDF to be 4 times lower than those from coal.13 This was mostly related to the nitrogen content of the fuels. In that work it was also observed that the emissions of SO2 and CO2 were proportional to the sulfur content and the carbon content of both fuels. In summary, previous studies in this laboratory showed that burning chunks of tires can be problematic because of the copious amounts of particulate matter and PAH that are released. Soot, once formed, is difficult to burn in typical post-flame furnace conditions. Thus, this work concentrated on identifying combustion techniques and furnace operating parameters that reduce the emissions from burning waste-tire chips. Chips were again burned in batches in a horizontal muffle furnace. However, this time two furnaces were used. The primary furnace acted as a gasifier/combustor. The gaseous effluent of this furnace was mixed with additional air, in a mixing section, and was channeled to a secondary furnace (afterburner) where additional oxidation took place, see Figure 1, under well-mixed and overall fuellean conditions. It was expected that with the arrangement of two furnaces in series, separated by the mixing section, combustion emissions would be reduced. In these experiments the fuel mass loading in the primary furnace/gasifier was fixed while the temperature was varied in the range of 500-1000 °C; the residence time of the effluent gases in the secondary furnace (afterburner) was under 1 s and the temperature therein was 1000 °C. At the exits of the furnaces CO, CO2, NOx, SO2, polycyclic aromatic hydrocarbon (PAH), and particulate emissions were monitored. Materials and Experimental 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. Physical and chemical properties are shown in Table 2. The elemental analysis resulted in the mass fractions shown in Table 3. All of the tests conducted in this study involved batch combustion of tire chips in fixed beds. Preweighted 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. The tube was placed at the centerline of a horizontal split-cell electric furnace (1 kW max.), see Figure 1. To insert the samples quickly in the furnace the porcelain boats were placed at the end of the inner surface of a half tube (a quartz cylinder longitudinally split along the centerline). The other end of the (28) Mastral, A.; et al. Fuel 1998, 77, 1516. (29) Violi, A.; D’Anna, A.; D’Alessio, A. Chem. Eng. Sci. 1999, 54, 3433.
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Figure 1. Schematic of the bench-scale experimental apparatus.
Figure 2. A photograph of the waste dewired tire chips that were burned in these experiments. Table 2. Some Physical and Chemical Properties of the Tire Chips Used fixed carbon
volatiles
ash
apparent density
heating value
residue specific area
24.9%
69.7%
4.0%
0.4 g/cm3
39 MJ/kg
74 m2/g
Table 3. Composition of Tire Chips Fractions, wt % element
tire chips
burned fractiona
carbon black residue
carbon hydrogen sulfur others
85.8 7.3 2.3 4.6
87.4 10.2 2.4 0.0
82.2
