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Energy & Fuels 2004, 18, 102-115
Emissions of Batch Combustion of Waste Tire Chips: The Hot Flue-Gas Filtering 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 RD&E Center, Natick, Massachusetts 01760 Received February 25, 2003. Revised Manuscript Received October 7, 2003
A laboratory investigation was performed on the emissions from the batch combustion of waste tire chips in fixed beds. Techniques and conditions that minimize toxic emissions were identified. Tire-derived fuel (TDF), in the form of waste tire chips (1 cm in size), was burned in a two-stage combustor. Batches of tire chips were introduced to the primary furnace, where gasification and combustion occurred. The gaseous effluent of this furnace was mixed with streams of additional preheated air in a mixing venturi, and it was then passed through a silicon carbide (SiC) honeycomb wall-flow filter that had been placed inside this furnace. Subsequently, it was channeled into a secondary furnace (afterburner), where further oxidation occurred. The arrangement of the two furnaces in series allowed for independent temperature control; varying the temperature in the primary furnace influenced the type and the flux of pyrolysates. The hot-flue-gas filtering section, ahead of the exit of the primary furnace, allowed the retention and further oxidation of most of the generated particulates and, thus, prevented them from entering the afterburner. Results showed that the combination of the high-temperature ceramic filter with the afterburner treatment was successful in reducing the emissions from the combustion of waste tires. Depending on the temperature of the primary furnace, the final emissions of CO were reduced by factors of 2-6, NOx emissions were reduced by factors of 2-3, particulate emissions were reduced by 2 orders of magnitude (both PM2.5 and PM10), and most individual polycyclic aromatic hydrocarbon (PAH) species emissions were reduced by more than 1 order of magnitude, with the exception of naphthalene, whose reduction was less drastic. The overall combustion effectiveness was enhanced, as evidenced by higher CO2 yields.
Introduction The combustion process of solid fuels, such as waste tires, is often incomplete, and undesirable products of incomplete combustion (PICs) are formed. This can be attributed to the combined effect of local temperatures, inadequate mixing of the fuel purolyzates and air, and local oxygen-starved conditions around the fuel.1 The combustion of tires (single or in stockpiles) emits copious amounts of acrid soot; most people have vivid images of intense black smoke emanating from burning whole tires (single or in stockpiles). Hence, past work at this Northeastern University laboratory was focused on investigating whether the batch combustion of chunks of tires (simulating whole tires in a small scale) does, indeed, emit more pollutants than the continuous combustion of tire crumb injected in a furnace. The results showed that the batch combustion of tire chunks resulted in much higher emissions of PICs, such as * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Fuel Process. Technol. 1996, 47, 1.
polycyclic aromatic hydrocarbons (PAHs) and particulate matter. However, whereas the combustion mode influenced the amounts, it did not influence the nature of the species produced. Similar types of emissions were detected from the combustion of both tire chunks and tire crumbs at comparable temperatures and gas residence times in the furnaces.2-5 This is the second part of an ongoing study that addresses the emission of pollutants from the combustion of waste tire chips and identifies techniques for their minimization. The first part of the study6 investigated the effect of two combustion stages, separated by a section where flue gas was mixed with additional air. Experiments were performed in a laboratory apparatus that was composed of two laminar-flow hori(2) Levendis, Y. A.; Atal, A.; Carlson, J. B.; Dunayevskiy, Y.; Vouros, P. Environ. Sci. Technol. 1996, 30, 2742. (3) Levendis, Y. A.; Atal, A.; Courtemnche, B.; Carlson, J. B. Combust. Sci. Technol. 1998, 131, 147. (4) Atal, A.; Levendis, Y. A. Fuel 1995, 74, 1570. (5) Levendis, Y. A.; Atal, A.; Carlson, J. B. Combust. Sci. Technol. 1998, 134, 407. (6) Caponero, J.; Tenorio, J. A. S.; Levendis, Y. A.; Carlson, J. B. Energy Fuels 2003, 17, 225-239.
10.1021/ef030043b CCC: $27.50 © 2004 American Chemical Society Published on Web 11/22/2003
Batch Combustion Emissions of Waste Tire Chips
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Figure 1. Schematic of the bench-scale experimental apparatus, consisting of two furnaces in series, a mixing venturi, a ceramic filter, and emission sampling stages.
zontal muffle furnaces, simulating two independent combustion stages (see Figure 1). The primary furnace acted as a gasifier/combustor for the tire chips that were placed therein. The combustion effluent (gases laden with particulates) contained PICs. This effluent was blended with additional preheated air in a venturi mixing section, and it was then channeled into a secondary furnace, which acted as an afterburner. Because most pollutants from the combustion of tires emanate during the stage of the homogeneous combustion of devolatilizates, rather than from the heterogeneous char combustion phase,7 this entire investigation is focused on the combustion of the devolatilizates (volatiles are 70 wt % of this tire). The performance of the afterburner was influenced by the operating temperature of the primary furnace, which was varied in the range of 500-1000 °C. It was shown that, in the case of this tire-derived fuel (TDF), it was beneficial to use combustion staging, in conjunction with an additional-air mixing section.6 Emissions of CO were reduced by factors of 3-10, particulate emissions were reduced by factors of 2-5, and cumulative PAH emissions were reduced by factors of 2-3. Operating the primary furnace/gasifier at temperatures in the low side of the 500-1000 °C range resulted in low pollutant yields, upon treatment in the afterburner. That study has been continued in this work, in search of additional techniques that further reduce PICs. Herein, the aformentioned work was complemented by the installation of a high-temperature barrier filter ahead of the exit of the primary furnace (see Figure 1). This ceramic honeycomb filter was used to remove the particulates from the flue gas. Such particulates, being primarily organic in nature (soot, tars, oils, waxes), were retained inside the filter, where they were eventually gasified at the elevated temperatures therein. Under such circumstances, the afterburner was able to treat a practically particle-free effluent. The following two subsections discuss (i) the perils of exposure to soot, to illustrate why filtration is important, and (ii) how soot removal is accomplished. (7) Atal, A.; Steciak, J.; Levendis, Y. A. In Procedings of the ASME Heat Transfer Division; American Society of Mechanical Engineers (ASME) International: New York, 1995; Vol. HTD317-2.
Health-Related Effects of Particulate Matter. The particulate matter emitted from the combustion of waste tires is basically soot and tars, which are byproducts of the incomplete combustion of organic fuels. The soot is composed of carbon and adsorbed organic compounds, such as PAHs; however, it may contain traces of inorganic elements, such as metals, oxides, salts, adsorbed liquids and gases, and nitrogen and sulfur composites.8 Living beings are exposed to soot as they breathe polluted air, as they consume contaminated food, and through contact with the skin. Cigarette smokers and workers of industries that either use soot or generate it as a byproduct, as well as workers of transport companies (such as drivers of diesel-powered vehicles), suffer greater risks from being exposed to high concentrations of soot.9,10 As a component of polluted air, soot is also abundant in cities with large amounts of atmospheric pollution.11 Soot, as a result of its physical nature and chemical composition, has been associated with the increased risk of lung, bladder, and skin cancers.12 Several regulatory and scientific agencies have recognized soot as a carcinogenic substance. The USEPA Carcinogen Assessment Group included soot in their list of potentials carcinogens, the National Toxicology Program classified soot to be “recognized as a human carcinogen”, and the International Agency for Research on Cancer (IARC) also has classified soot to be carcinogenic (Group 1).13 The first case of skin cancer related to soot was identified more than 200 years ago, when an increased risk of cancer was detected in chimney sweepers in England. Since that first report, several epidemiological (8) Health and Environmental Effects of Particulate Matter Fact Sheet. Environmental Protection Agency (US-EPA): Washington, DC, 1999. (http://www.epa.gov/ttn/oarpg/naaqsfin/pmhealth.htm) (9) Boffetta, P.; Jourenkova, N.; Gustavsson, P. Cancer, Causes Control 1997, 8, 444. (10) Stober, W.; Abel, U. Int. Arch. Occup. Environ. Health 1996, 68, (Supplement), S3-S61. (11) Soots, Tars, and Mineral Oils, Eighth Report on Carcinogens. National Toxicology Program: 1998. (http://ntp-server.niehs.nih.gov/ htdocs/8_RoC/KC/SootsTars&Min.html) (12) Mastrangelo, G.; Fadda, E.; Marzia, V. Environ. Health Perspect. 1996, 104, 1166. (13) Soots, Supplement 7; The International Agency for Research on Cancer (IARC): Lyon, France, 1987. (Available online at: http:// 193.51.164.11/htdocs/Monographs/Suppl7/Soots.html.)
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studies have demonstrated the correlation between soot and cancer risk. More recent work on chimney sweepers in Sweden and Denmark showed a significant increase in lung cancer; similar studies in Germany and the United Kingdom reached the same conclusions. One of these studies also reported on the increase of the risk of esophagus cancer, primary liver cancer, and leukemia among the same workers.14 In two independent studies, coke soot was applied in the skin of mice, and tumors were produced in both studies. In another animal study, the implantation of wood-generated soot under the skin produced located tumors in female mice. Extracts of oil from soot produced tumors in mice upon hypodermic applications or upon inhalation of great amounts of these extracts.14 Soot was shown to be mutagenic in several laboratories. Soot extracts were mutagenic to bacteria and cultures of human linfoblasts. Extracts of particulate matter emitted from wood combustion were shown to cause damage to the DNA of ovarian cells of female hamsters.13 Hot Flue-Gas Filtration. High-temperature filtration of the effluents from waste incineration can remove particulate matter such as soot. Suitable barrier filters, such as ceramic honeycomb or foam filters, may allow continuous operation if the retained particles are carbonaceous and can be oxidized therein. High residence times and catalytic effects significantly enhance the soot/tar oxidation efficiency in the filter. Extruded ceramic monolith substrates currently are widely used for automotive and stationary emission control reactors, such as three-way catalysts, selective catalytic reactors (SCRs) for the reduction of nitrogen oxides (NOx), and diesel particulate traps. Monoliths are increasingly under development and evaluation for many new reactor applications, e.g., in chemical and refining processes, catalytic combustion, ozone abatement, etc.15 Cell configurations and properties of monoliths are described in terms of geometric and hydraulic parameters.16,17 These properties can be defined in terms of cell spacing, wall thickness, and cell density, which is the number of cells per unit of cross-sectional area. In designing monolithic catalysts, a balance of geometric parameters such as cell density or wall thickness is necessary to meet the constraints of external processing requirements, such as space velocity, flow rates, and pressure drop. The use of diesel particulate filters (DPFs) constitutes the most effective method for removing soot particles from diesel engines. Filtration occurs when the particleladen exhaust gas is forced through the porous walls that separate adjacent channels of the filter honeycombs. Soot particles are trapped in the entrance channels and filtered exhaust passes outward from the exit channels (see Figure 2). In diesel engine applications, the filter is installed in the tailpipe of a vehicle, which is typically operated below the auto-ignition temperature of soot (ca. 600 °C). Thus, soot accumulates (14) Summary of Data Reported and Evaluation. Cap. 5-Soots; The International Agency for Research on Cancer (IARC): Lyon, France, 1985; Vol. 35, p 219. (Available online at: http://193.51.164.11/htdocs/ Monographs/Vol35/Soots.html.) (15) Williams, J. L. Catal. Today 2001, 69, 3. (16) Cash, T. F.; Williams, J. L.; Zink, U. H. Society of Automotive Engineers (SAE) Brazil, Paper No. 982927, 1998. (17) Day, J. P.; Socha, L. S. Society of Automotive Engineers (SAE), Paper No. 910371, 1991.
Caponero et al.
Figure 2. Schematic showing one channel of a silicon carbide (SiC) honeycomb wall-flow filter.
therein and regeneration of the filter is required for continuous engine operation at acceptable exhaust backpressures. Regeneration techniques include periodic thermal destruction or aerodynamic removal by external means. Cordierite, mullite, and silicon carbide (SiC) are materials that are commercially available for diesel exhaust filters. Development of new materials for DPF-type filters is underway.15 Over the last 15 years, extensive work has been conducted in this Northeastern University laboratory in the field of diesel engine exhaust after-treatment, using ceramic filters. The filtration characteristics for such ceramic monoliths were determined to be excellent, and the pressure drop is acceptably low for unimpeded engine operation. SiC honeycomb wall-flow filters can handle elevated temperature gases (if needed, as hot as 1500 °C) and corrosive gases. Particulate retention efficiencies have been recorded to be in the 97%-99% range, as shown in Figure 6 of the work by Larsen et al.18 Such filters are used in the study herein. Materials and Experimental Methods Tire chips with dimensions on the order of 1 cm were obtained from a local source and included organic fabrics, such as nylon belts (see Figure 3). No metallic belts were included. Some physical and chemical properties are shown in Table 1. The elemental analysis resulted in the mass fractions that are shown in Table 2. All of the tests conducted in this study involved the batch combustion of tire chips in fixed beds. Preweighted sample amounts of 0.8000 ( 0.0005 g, consisting of a few chips (three chips with a volume of ∼ 0.8 cm3 each, to keep the total surface area approximately constant in all samples), were placed in porcelain boats and inserted in the quartz tube of the primary furnace, which was 4 cm in diameter and 87 cm long. The tube was placed at the centerline of a horizontal, split-cell, electric furnace (1 kW maximum power) (see Figure 1). To insert the samples quickly into the furnace, the porcelain boats were placed at the end of the inner surface of a tube with a U-shaped cross section, i.e., a quartz cylinder longitudinally split along the centerline. The other end of this U-shaped tube was (18) Larsen, C.; Levendis, Y. A.; Shimato, K. Filtration Assessment and Thermal Effects on Aerodynamic Regeneration in Silicon Carbide and Cordierite Particulate Filters. Society of Automotive Engineers (SAE), Paper 1999-01-0466, 1999. (Also see Soc. Auto. Eng., [Spec. Publ.] SP 1999, SP-1414.)
Batch Combustion Emissions of Waste Tire Chips
Figure 3. Photograph of the waste de-wired tire chips that were burned in these experiments. Table 1. Some Physical and Chemical Properties of the Tire Chips Used property
value
fixed carbon content volatiles content ash content apparent density heating value residue specific area
24.9% 69.7% 4.0% 0.4 g/cm3 39 MJ/kg 74 m2/g
Table 2. Composition of Tire Chips Fractions Content (wt %) element
tire chips
burned volatile 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
