Energy & Fuels 2000, 14, 275-281
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Fluidized Bed Combustion (FBC) of Fossil and Nonfossil Fuels. A Comparative Study A. M. Mastral,* M. S. Calle´n, and T. Garcı´a Instituto de Carboquı´mica, C/Marı´a de Luna, 12, 50015, Zaragoza, Spain Received April 5, 1999
A comparative study on FBC of coal and waste tire was performed from an environmental point of view. A sub-bituminous coal and a nonspecific waste tire blend were used, respectively, as fossil and nonfossil fuels in a fluidized bed combustion (FBC) experimental laboratory installation. The same FBC conditions were carried out keeping constant the air total flow (860 L/h), the percentage of excess oxygen (20%), and varying the combustion temperature (650, 750, 850, and 950 °C). The main objective was to establish the influence of the fuel used in PAH formation and emission. Regarding the PAH formation and emission in FBC, this is the first time that emissions from coal and waste tire combustion are compared. The PAH were collected in different traps and extracted with dimethylformamide (DMF). PAH analysis and quantification were determined by fluorescence spectroscopy in the synchronous mode (FS). Data on polycyclic aromatic hydrocarbons (PAH) are reported and the results obtained show the following: (1) Independently of the combustion temperature, a drastic increase in PAH emissions is obtained when tire is used instead of coal at the same conditions. In this case, the PAH formation and emission seem to be promoted. (2) The pyrolytic process and the interaction between radicals in the mechanisms implied in PAH formation are highly important. (3) The combustion temperature influence is more remarkable in tire combustion than in coal combustion. In the first one, the lowest PAH emissions are obtained at low combustion temperatures (750 °C) while for coal, the highest PAH emissions appear in the range of 750-850 °C.
Introduction Up to now, coal, has been the main fossil fuel used as energy power1 while other energetic resources are being considered lately as an elimination method. Coal, independently of the structural model chosen to explain its nature and in a very simplified model, consists of condensed aromatic/hydroaromatic structures joined by short aliphatic chains and whose aromaticity increases with the coalification degree. In a typical sub-bituminous coal, the aromaticity is around 0.60, while in a bituminous coal it is around 0.65. Besides C and H, coal contains heteroatoms such as O, N, and S. The heteroatom content varied from one coal to another although O is predominant, this content being higher for the sub-bituminous coal than for the bituminous one and depending, therefore, on the coal rank. The different coal nature, a function of the coalification grade, will affect compound emissions when coal is processed. The coal rank2 will be a decisive factor in choosing the coal type to burn and, not only will the reactivity fuel3 grade be important, but also the material mineral content.4 The mineral content could affect the * Corresponding author. Fax: 976-733318. E-mail: amastral@ carbon.icb.csic.es. (1) Merrick, D. Coal Combustion and Conversion Technology; Macmillan Publishers LTD: London and Basingstoke, 1984; Chapter 2, 25. (2) Paul, J.; Peeler, K.; Lane, G. L. Fuel 1993, 72 (6), 737. (3) Hampartsoumian, E.; Nimmo, W.; Rosenberg, P.; Thomsen, E.; Williams, A. Proc. 11th Annu. Int. Pittsburgh Coal Conf. 1994, 11.
reactivity grade due to the porosity changes, surface area, and active points of concentration. Since 1950, the use of coal has been altered due to the increase of oil and to the disposability of natural gas during 1960. Oil and natural gas have partly the eclipsed coal use, and currently the utilization of new fuels such as biomass,5,6 synthetic waste material,7 and urban wastes,8 as ways of eliminating these wastes, is acquiring a considerable importance in combustion processes. On the other hand, the development of transport in the last years has also generated a great amount of waste tire.9 Although an immediate analysis seems to show that coal and tire are two very different materials, their elemental analysis indicates that their content of C, N, H, O, and S are similar. It is estimated that waste tire production in the last years, is about 2 million tons per year in the European Union10 and 3 million tons per year in America. The nonbiodegradability and the adverse environmental effects of this material are its main disadvan(4) Crelling, J. C.; Hippo, E. J.; Werner, B. A.; Gillespie, E. M. In 49th Ironmaking Conference Proceedings; Detroit, Iron and Steel Society, 1990, 211. (5) Kozinski, J. A.; Saade, R. Fuel 1998, 77 (4), 225. (6) Fahlstedt, I.; Lindman, E. K.; Lindberg, T.; Anderson, J. Proc. 14th Int. Conf. Fluidised Bed Combustion, Vancouver, Canada´, 1997, 295. (7) Durlak, S. K.; Biswas, P.; Shi, J.; Bernhard, M. J. Environ. Sci. Technol. 1998, 32, 2301. (8) Gajdos, R. Resour., Conserv. Recycl. 1998, 23 (1-2), 67. (9) Kujioka, M. The Japan Automobile Tire Manufacturers Association Inc., personal communication, June 1995, Tokyo, Japan. (10) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 1474.
10.1021/ef9900536 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
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tages. The high calorific value is its more important advantage. The majority of the ways to eliminate tires11 are the landfill,12 incineration,13 and combustion 14,15 utilized in power generation. The elimination of these wastes through pyrolysis 16 is also receiving a considerable emphasis because it is possible to obtain oils that can be used as fuels or solvents in coal liquefaction.17 Its high calorific value,18 28-37 MJ/kg, higher than that of most of the coals, in combination with its low content in mineral matter, turn tire into a potential fuel. In fact, it has been classified as “nonfossil fuel,” in some countries, using this material in combustion plants that originally burned only coal or as a constituent of both materials blends.19 Concerning tire composition,20 it depends on the trademark and on the specific use tire is manufactured for. It is mainly formed by a copolymers mixture: natural rubber and synthetic rubber, 2/3 mainly styrenebutadiene (SBR) and polybutadiene (PB), and about 1/3 carbon black (CB).21 Among the principal components of tire, CB is used to strengthen rubber. It is an aggregated of spherical particles linked in a random distribution and characterized by its high surface/weight ratio. There are other additional components such as oils, an aromatic hydrocarbon mixture, sulfur, trace metals, etc., which confer physical characteristics to the rubber and whose ratio varied for different tire kinds. Regarding its elemental analysis, it is worthy to remark that tire and bituminous coals have comparable contents of C, S, and N, with aromatic structures similar to coals in some ways.22 Tire shows a great difference in moisture content and ashes that are generally lower than in coal. Concerning structure, it seems to be clear that whereas coal, a fossil fuel, is mainly an aromatic fuel, tire is a nonfossil fuel basically made up of an aliphatic structure. Power generation has constituted the main way of coal use in most of the industrial countries and this was one of the reasons to develop fluidized bed combustion 23 as an alternative combustion system to the pulverized system majority used. The principal aims of developing a new combustion system were to decrease the economical cost, the use of varied quality coals and to allow an effective and cheap control of inorganic emissions (SOx, NOx, COx).24,25 The control and reduction of inorganic emissions have been studied widely due to the legisla(11) Martin, A. Garbage 1991, May/June 28. (12) Farcasiu, M.; Smith, C. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 472. (13) Proceedings: 1991 Conference on Waste Tires as a Utility Fuel, EPRI GS-7538, Electric Power Research Institute, Palo Alto, CA; 1991. (14) Tesla, M. R. Power Eng. 1994, 43. (15) Mastral, A. M. Final Report CSIC to EU, DG XVII, ECSC, Project ref 7220/ED/026, February 1997. (16) Mastral, A. M.; Murillo, R.; Pe´rez-Surio, M. J.; Calle´n, M. S. Energy Fuels 1996, 10, 941. (17) Liu, Z.; Zondlo, J. W.; Dadybourjor, D. B. Energy Fuels 1994, 8, 607. (18) Ekmann, J. M.; Smouse, S. M.; Winslow, J. C.; Ramezan, M.; Harding, N. S. Cofiring of coal and waste, IEA Coal Research, IEACR/ 90, 1996, 18. (19) Tang, Y.; Curtis, C. W. Fuel Process. Technol. 1996, 46, 195. (20) Mastral, A. M.; Murillo, R. Caucho 1995, 436, 23. (21) Sahouli, B.; Blacher, S.; Brouers, F.; Darmstadt, H.; Roy, C.; Kaliaguine, S. Fuel 1996, 75 (10), 1244. (22) Teng, H.; Serio, M. A.; Bassilakis, R. In Preprints of Papers presented at the 203rd ACS National Meeting, San Francisco, CA, 1993, Vol. 37, No. 2, p 533. (23) Geldart, D. Gas Fluidization Technology; John Wiley & Sons: New York, 1986; Chapter 1, p 1.
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tion to which these emissions are submitted. On the other hand, although there is not still a clear and severe legislation concerning organic emissions, the concern in organic emissions to the environment has grown considerably in the last years due to the negative effects that these emissions show on human health. It seems to be a quantity problem because the inorganic emissions are emitted in higher concentrations than the organic emissions. Despite this, organic emissions can be more dangerous than the inorganic ones but they are emitted in lower amounts. Inside organic emissions, the compounds formed by two or more condensed aromatic rings and formed only by C and H are known as polycyclic aromatic hydrocarbons (PAH).26 In general, the PAH formation and emission mechanisms can be summarized in two mechanisms: pyrolysis and pyrosynthesis.27,28 By heating, the organic compounds are partially fragmented to small and unstable molecules (pyrolysis). These fragments, mainly free radicals, highly reactive and existing only as intermediary compounds for a very short time, through recombination reactions, lead to relatively stable aromatic compounds. (pyrosynthesis). So, PAH formation in pyrolysis oils has been attributed to cyclization DielsAlder reactions of alquenes that react to form cyclic alquenes.29 The dehydrogenation of these cyclic alquenes takes place giving as a result aromatic compounds of just one ring, such as benzene, which through posterior reactions lead to PAH formation.30 Anyway, complex hydrocarbons do not have to break down necessarily in small fragments before undergoing recombination processes. Compounds with several rings present in the fuel molecule can undergo a partial cracking followed by dehydrogenation of the primary radicals. In this way, in the literature, evidence has been found suggesting that lipid acids with a long lineal chain play an important role in the PAH formation mechanism because they allow the alkylation of aromatic units in fossil fuels.31 On the other hand, the transference of inter- and intramolecular hydrogen is also considerable in PAH formation.31 The phenyl radicals also seem to play an important role as intermediary in high-temperature reactions, which lead to polycyclic hydrocarbon formation. Concerning PAH emissions in combustion processes, it is remarkable that the PAH amount emitted is a function of the fuel used. In the case of coal and wood,32 the combustion is normally an incomplete process due to the low combustion temperatures and the deficient air with the subsequent formation of different PAH amounts released to the atmosphere. It has also been (24) Takeshita, M. Environmental performance of coal-fired FBC, IEA Coal Research, IEACR/75, London, 1994; p 18. (25) de Nevers, N. Ingenier’ıa de control de la contaminacio´ n del aire; MCGraw-Hill: Mexico, 1998; capı´tulo 11 y 12, 355, 403. (26) Vo-Dinh, T. Chemical Analysis of Polycyclic Aromatic Compounds; J.Wiley and Sons: New York, 1989; Capı´tulo I, Vol. 101, p 3. (27) Mastral, A. M.; Callen, M. S.; Garcia, T. Environ. Sci. Technol. 1999, 33, 3177. (28) Atal, A.; Levendis, Y. A.; Carlson, J.; Dunayevskiy, Y.; Vouros, P. Combust. Flame 1997, 110, 462. (29) Williams, P. T.; Taylor, D. T. Fuel 1993, 72 (11), 1469. (30) Testaferri, L.; Tiecco, M.; Fiorentino, M.; Troisi, L. J. Chem. Soc., Chem. Commun. 1978, 93. (31) Acevedo, S.; Ranaudo, M. A.; Gutie´rrez, L. B.; Escobar, G. Fuel 1996, 75 (9), 1139. (32) Elomaa, M.; Saharinen, E. J. Appl. Polym. Sci. 1991, 42, 2819.
FBC of Fossil and Non-Fossil Fuels
demonstrated that other materials such as polystyrene and polybutadiene,7 waste tire in fixed bed,33 are a main source of PAH emissions during their combustion. Several studies have also been performed about PAH emissions in power generation burning coal as fuel.34-39 These emissions have been studied in experimental furnaces under more controlled burning conditions.27 All of them concluded that the effect of coal type is less important with respect to the PAH amounts formed than the reactor type. PAH emissions from coal in pressurized fluidized bed reactors, the last generation reactors, have been studied by Szpunar40 finding that these emissions are lower in pressurized systems than in atmospheric systems and lower than the ones obtained in conventional combustion systems. Studies of PAH analyses in tire combustion under controlled conditions in scale laboratory furnaces or in power plants are very scarce. Most of the work is centered in tire pyrolysis under inert atmosphere. Inside pyrolysis, hydropyrolysis and pyrolysis can be distinguished under inert atmosphere. Y. A. Levendis33 has studied PAH emission of waste tire and coal separately, in horizontal and vertical furnaces in fixed bed combustion and pyrolysis conditions finding that PAH emissions are higher during pyrolysis than during the combustion and always higher with tire than with coal. There is a lack of information on devolatilization at fluidized beds in oxidant atmospheres. In this work, coal41 and tire have been burned for the first time in an atmospheric fluidized bed combustion pilot plant42 at the same combustion conditions. The aim has been to compare PAH emissions from both fuels giving more particular information about this type of combustion with experimental results. The possible mechanisms implied in PAH formation and emission are discussed. The PAH studied are those listed by US-EPA as priority pollutants. Experimental Section A low rank coal from Utrillas, NE of Spain, was utilized as fossil fuel (0.5-1 mm particle size). Discarded (33) Levendis, Y. A.; Atla, A.; Carlson, J.; Dunayevskiy, Y.; Vouros, P. Environ. Sci. Technol. 1996, 30, 2742. (34) Truesdale, R. S.; Cleland, J. G. In Proceedings of Residential wood and coal combustion; Air Pollution Control Association, Louisville, KY, 1982; p 115. (35) Calle´n, M. S. Formacio´n y emissio´n de Hidrocarburos Aroma´ticos Policı´clicos en generacio´n de energı´a. Combustio´n en lecho fluidizado a presio´n atmosfe´rica. Ph.D. Dissertation, Febrero 1999, Zaragoza. (36) Mastral, A. M.; Calle´n, M. S.; Mayoral, M. C.; Juan, R.; Galba´n, J. In 8th International Conference on Coal Science; Pajares, J. A., Tasco´n, J. M. D. Eds.; Elsevier Science: Oviedo, 1995: p 1951. (37) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Mayoral, M. C. Proceedings of the International Conference of Coal Science; Ziegler, A., Van Heek, K., DGMK, Alemania, 1997; Vol. 2, p 1155. (38) Tobı´as, A. M.; Garcı´a, R.; Holden, K. M. L.; Mitchell, S. C.; Pis, J. J.; McRae, C.; Snape, C. E.; Moinelo, S. R. Coal Science; Pajares, J. A., Tasco´n, J. M. D., Ed.; Elsevier Science: Oviedo, 1995; p 1947. (39) Bayram, A. Generation of emission factors for PAH due to Turkish Coals burned in different types of combustion units. Ph.D. Dissertation, March 1995, Izmir. (40) Szpunar, C. B. Air toxic emissions from the combustion of coal: identifying and quantifying hazardous air pollutants from US coals, ANL/EAIS/TM-83 Argonne, IL, Argonne National Laboratory, Environmental Assessment and Information Sciences Division, 1992, p 144. (41) Mastral, A. M.; Calle´n, M. S.; Murillo, R. Fuel 1996, 75 (13), 1533. (42) Mastral, A. M.; Calle´n, M. S.; Mayoral, M. C.; Galba´n, J. Fuel 1995, 74, 1762.
Energy & Fuels, Vol. 14, No. 2, 2000 277 Table 1. Tire and Coal Elemental and Immediate Analyses %C (daf) %H (daf) %S (mf) %N (daf) % moisture (af) % ash (mf) % volatiles (ar) % fixed carbon (ar) Calorific value (Kcal/Kg)
tire
coal
88.6 8.3 1.4 0.4 0.9 3.8 67.3 31.1 9220
80.2 6.7 5.7 1.0 22.0 26.9 48.6 28.4 4130
Table 2. Efficiency Values Reached in Coal and Tire Combustion (AFBC, 860 L/h, 20% excess oxygen) as a function of combustion temperature temperature (°C) coal efficiency tire efficiency
650
750
850
950
99.0 90.9
98.8 94.5
99.1 95.1
99.4 91.6
tires, supplied by AMSA (rubber-recycling enterprise), after removing the steel thread and the textile netting, were used as nonfossil fuel (0.6-mm particle size); their analyses are shown in Table 1. The combustion experiments were performed in a fluidized bed pilot plant (2800 W), laboratory scale, consisting of a continuous feeder (40-300 g/h), which permits modification of the burnt coal amount, a fluidized bed reactor, made of Khantal steel (6.7 cm i.d., 76 cm height), a furnace (maximum temperature 1000 °C), and a preheater (up to 800 °C). The pilot plant worked at atmospheric pressure. The entry air was divided into two gas streamflows; one of them was introduced at the bottom of the reactor, enough to fluidize the bed, and the other was introduced at the top of the continuous feeder, facilitating the fuel feed. Both airflows were controlled by the corresponding mass flow controllers. A more specific description of the experimental installation has been previously published.42 Sand was used as a fluidizing agent and the samples were taken for 2-4 h when the plant worked at regimen. Calibration curves with each fuel were experimentally determined in order to establish the combustion conditions at which experiments had to be carried out. The combustion experiments were performed keeping constant the air total flow (860 L/h, double the minimum fluidization velocity), the percentage of excess oxygen (20%), and modifying the combustion temperature (650, 750, 850, and 950 °C). At these conditions, high efficiency values (see Table 2) were reached and the values were calculated in each combustion experience according to the following equation:
% efficiency )
100 × (OMinitial fuel - OMfinal ash) OMinitial fuel
(1)
where OM ) organic matter in fuel fed and in the particulate matter collected in cyclones and ashes of the ash-hopper. The initial and final organic matter content in fuel was calculated following the equations:
OMinitial fuel ) g/h fuel fed × t(1 - (h + Ce)) (2) OMfinal ) (PMcyclones × OMash) + (ashash-hopper (g) × OMash-hopper) (3)
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where t ) sampling time (hours), h ) moisture in elemental analysis (% per unit), Ce ) ash of immediate analysis (% per unit), and PMcyclones) particulate matter collected in cyclones (grams). The combustion gases were passed through a system formed by two cyclones and at the exit of the second cyclone, an aliquot of the combustion gases was taken and forced to pass through the sampling system. Samples were protected from sunlight and kept in a refrigerator until their extraction with DMF for 15 min by ultrasonic bath. After extraction, they were filtered and diluted to determine the PAH content by FS. The analytical protocol and the conditions to PAH quantification were made in previous studies 43 after preparing individual and mixed solutions of model compounds. Formerly, the quantification was performed by addition of a model compound at the optimal conditions of each PAH. The compounds studiedsfluorene, benzo[a]pyrene, pyrene, chrysene, anthracene, acenaphtene, benzo[a]anthracene, dibenzo[a,h]anthracene, coronene, perylene, and benzo[k]fluorantheneswere determined by fluorescence spectroscopy in the synchronous mode (FS). The analytical procedure has been published elsewhere.43 Results and Discussion This work has been performed from a double environmental point of view aimed at (a) eliminating a waste material, taking advantage of its high calorific value and taking care of the possible environmental disturbances; (b) comparing PAH emissions using this waste material with the ones generated by coal. About the two fuels, coal is mostly aromatic, and tire has a major aliphatic structure. In all combustion processes, two main factors must be considered in order to study PAH emissions: (a) the fuel, and (b) the combustion process. Regarding the fuel, coal has been worldwide the main fuel used in combustion aimed at the production of energy. Studying the composition of both fuels (see Table 1), it is observed that coal and tire have similar elemental analysis except for the sulfur content. The main difference is shown in the immediate analysis, proving that tire has a higher content in volatiles and a lower content in mineral matter. From an inorganic point of view, subproducts and emissions will be lower for tire than for coal, but from an organic point of view, it is not possible to guess about these emissions and more concretely about PAH emissions. However, the high calorific value that tire shows, turns it into an interesting fuel, especially in power generation. This tire characteristic is important because waste tire shows great disadvantages of nonbiodegradability and moreover, the fact that it is a nonrecyclable material makes its use as fuel more interesting. In general, about the combustion process, emissions will be a function of the reactor and the reactions implied in the process. The reactor type is determinant and decisive during the formation and emission of specific compounds. Although a lot of studies have been performed concerning inorganic emissions, organic emissions, and (43) Mastral, A. M.; Pardos, C.; Rubio, B.; Galba´n, J. Anal. Lett. 1995, 28 (10), 1883.
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more concretely PAH emissions, they have received little attention. So, in this paper a comparative study has been performed concerning a specific group of organic volatile compounds, PAH. Due to the heating and as a consequence of the thermal breaking experimented in the combustion process, a pyrolytic process takes place that implies the release of part of the organic material. The result is a devolatilization process, independently of the fuel used with the release of volatiles and at the same time the entrainment of unburned material coming from the fuel. Due to the interaction between different particles and their choking with the reactor walls, this unburned material is fragmented in different sizes originating particles of a wide size range which can be elutriated as a consequence of the gas streamflow. The formation of bothsvolatiles and particulate matterswill be a function of the conditions at which the process is carried out. In all good combustion processes, the unburned material must be minimized to get efficiency values close to 100%. In this work, the combustion conditions selected have been chosen to reach high efficiency values and they have been kept constant to compare the emissions obtained with two fuelsscoal and tire. The airflow of 860 L/h, double the minimum fluidization velocity, the percentage of excess oxygen (20%), higher than the percentages used in power generation but chosen to avoid oxygen defect, and the sand, as fluidizing agent to eliminate the possible interference of limestone in PAH formation, have been selected as combustion conditions. Results are shown in Table 2. In coal combustion, it is observed that these values increase when combustion temperature increases. Therefore, the contribution of a bad combustion to PAH emissions will have a minimum influence. In tire combustion (see Table 2), the efficiency values reached are lower than with coal and, in general, they also increase with higher combustion temperatures. With tire as fuel, the influence of the incomplete combustion cannot be reflected and its influence would have to be considered to study PAH emissions. The elutriated material by the gas flow is collected in the two cyclones disposed at the exit of the reactor. The lower efficiency values reached with tire appear reflected in the higher amount of particulate matter collected when tire is used as fuel (in tire combustion the particulate matter collected reached for some runs five times the weight collected in coal combustion). This different behavior of coal and tire can be attributed to carbon black (CB) which, due to its high surface/weight ratio is easily entrained by the gas flow without the chance of being burnt completely. Moreover and as a consequence of the pyrolytic process, the radicals released due to its high reactivity and its short average lifetime undergo different reactions that compete between themselves as a function of the combustion conditions. The reactions experienced by the radicals could include two opposite reactions: condensation reactions, which imply the association between radicals forming compounds of higher molecular weight, and oxidation reactions, which imply the elimination of radicals, leading to the COx and H2O formation and therefore, destroying the chance of forming PAH.
FBC of Fossil and Non-Fossil Fuels
Energy & Fuels, Vol. 14, No. 2, 2000 279
Table 3. Distribution of PAH (µg/kg) Emitted and Collected in Coal and Tire Combustion (AFBC, 860 L/h, 20% excess oxygen) as a Function of Combustion Temperature temperature (°C) 650 cyclones trapping system PAH total
750 coal
850
950
coal
tire
tire
coal
tire
coal
tire
0.2 5.4
1997 0.6 333 2480 15.4 57
0.4 10.2
26032 6166
0.4 5.3
25326 7300
5.6
4477 16.0 390
10.6
32198
5.7
32626
PAH formation can be explained through simple radicals as C2, C3, C4. For example, the association between three simple C2 radicals can lead to the formation of a C6 ring, which by successive addition of other radicals can originate PAH growth in an uncontrolled waterfall mechanism. Once formed, the PAH can be emitted as gas or as particulate matter, supported on the entrained material or generating by themselves the particulate matter (soot).44,45 Some of the generated PAH could be emitted supported on entrained solids. Depending on the facility of the radicals released to be supported on particulate matter collected in the cyclones, it is interesting to know the PAH distribution between the cyclones and the trapping system (see Table 3). In a real situation, the particulate matter collected in the cyclones of a power station is recycled while the gas streamflow is released by a chimney. In coal combustion (see Table 3), the highest PAH concentrations are captured in the trapping system, that is to say, they are emitted to the atmosphere. In tire combustion (see Table 3), the amounts collected in cyclones and in the trapping system are both higher than in coal combustion. As a consequence of the entrainment of particles, most PAH emissions are obtained on the particulate matter of the two cyclones. This can be attributed to carbon black, one of the components of tire which can serve as support of the radicals released and promote the retention and higher emission of PAH. This has also been observed by Masclet46 in PAH emissions in power stations using coal as fuel. Therefore, it is necessary to control the PAH emissions which are going to be released by chimney because these can be transported by air affecting the pollution of compounds evolved to nonspecific areas. Concluding, concerning total PAH emissions in tire combustion, these are much more relevant and are emitted in higher concentrations in tire than in coal combustion. Regarding the distribution of each PAH individual as a function of the combustion temperature (see Table 4), the results obtained corroborate the hypothesis of random interaction between radicals, waterfall mechanism in PAH formation, in such a way that the amount of each PAH is variable but the PAH total amount remains constant. Through this mechanism, it is possible to explain that from a simple PAH of two cycles, as naphthalene, acenaphthylene, phenanthrene, or anthracene can be obtained depending on the addition of (44) Smedley, J. M.; Williams, A.; Hainsworth, D. Fuel 1995, 74 (12), 1753. (45) Mastral, A. M.; Callen, M. S.; Murillo, R. In CARBON’97 Conference Proceedings, University Park, 1997, p 1410. (46) Masclet, P.; Bresson, M. A.; Mouvier, G. Fuel 1987, 66, 5.
C2 or C4 radicals and depending on the place where this addition takes place. So although each individual PAH shows a variation as a consequence of the addition of different radicals, the total of PAH emitted is constant. In coal combustion, when the total PAH emitted is studied as a function of combustion temperature, a distribution showing a maximum between 750 and 800 °C is obtained (see Figure 1a). This could be explained assuming that the different combustion temperatures suppose a change in the velocity at the exit of the reactor. So, an increase in the temperature implies an increase in the exit flow velocity. This could be one of the reasons why at 950 °C, due to the high flow velocity, the possibility of interaction between radicals released diminishes. On the contrary, at 650 °C, the residence time of the radicals in the reactor increases due to a slower exit flow, and so their interaction with oxygen increases. As a result, the total PAH amount formed at both extreme temperatures studied decreases. Anyway, the complexity of the radicals evolved and the difficulty of the process make it very difficult to know the incidence of each factor in PAH emissions. The total PAH distribution emitted in tire combustion (see Figure 1b) as a function of combustion temperature shows a general trend in which a high increase in PAH emissions is produced when the combustion temperature increases, promoting in this way the PAH formation and emission. Williams et al. 29 have also observed this fact in tars from pyrolysis. To explain these variations, considering that the air total flow (860 L/h) and the percentage of excess oxygen are constant (20%), the temperature variation will be determinant. With variable combustion temperatures, the velocity of the combustion gases at the exit of the reactor, the residence time, and the interaction time between the radicals released will vary. At low combustion temperatures, the gas velocity at the exit of the reactor is lower and the residence time in the interior of the reactor is longer, increasing the chance of condensation and oxidation. This seems to indicate that from the two main reactions produced in the interior of the reactor, both oxidation and condensation reactions will be favored. Anyway, and because there is a high percentage of excess oxygen, the elimination by oxidation of the radicals seems to be the predominant reaction, decreasing in this way PAH emissions according to the results obtained. To increase combustion temperature, the gas velocity at the exit of the reactor is higher and the residence, the interaction, and the oxidation time of the radicals are shorter, increasing PAH emissions. Moreover, it is necessary to consider the efficiency values obtained when tire is burned. The reason could be CB. CB seems to be the responsible factor due to its high surface/weight ratio. In fact, the tire composition formed major by a mixture of styrene, polybutadiene, and CB is also going to influence the PAH amount emitted, obtaining higher emissions with tire than with coal. These results corroborate that any combustion process is joined to a pyrolytic process that implies secondary reactions. The formation of aromatic and polycyclic aromatic compounds has been attributed to Diels-Alder cyclization reactions with alquenes, mainly at high combustion temperatures.29,47 Tire pyrolysis leads to ethene, propene, and 1,3-butadiene production that
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Table 4. Total of Each PAH Individual (µg/kg) in Coal and Tire Combustion (AFBC, 860 L/h, 20% excess oxygen) as a Function of Combustion Temperature temperature (°C) 650 fluorene benzo[a]pyrene pyrene chrysene anthracene acenaphthene benzo[a]anthracene Dibenzo[a,h]anthracene coronene perylene benzo[k]fluoranthene a
750
850
950
coal
tire
coal
tire
coal
tire
coal
tire
1.5 traces traces 0.5 1.0 1.3 0.1 0.1 1.0 traces nda
792 nda 309 697 9 145 351 338 1836 nda nda
3.6 0.1 0.4 1.2 1.8 6.8 0.5 0.3 1.2 0.1 nda
90 1 18 16 7 238 7 4 9 nda nda
1.6 0.7 0.2 0.9 1.1 2.5 0.4 0.1 2.8 0.1 0.2
5531 611 8136 3067 2093 8060 3926 65 590 119 and
1.7 traces 0.1 0.1 0.9 1.7 0.2 0.1 0.9 traces and
9893 375 8704 2656 1976 2605 3951 42 2317 107 and
nd) non detected.
Figure 1. Total PAH (µg/kg) emitted in (a) coal and (b) tire combustion (AFBC, 860 L/h, 20% excess oxygen) as a function of combustion temperature.
react to form cyclic alquenes. As a result of association and interaction reactions, PAH are generated. Besides these aromatization reactions, cyclization and alkylation48 reactions are also produced that contribute to the increase in total PAH emitted during tire combustion. The low combustion efficiencies promote these kinds of processes although the inertness of CB must also be considered. This characteristic promotes soot formation (47) Cypre`s, R.; Bettens, B. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridwater, A. V., Eds.; Elsevier Applied Science: London, 1989. (48) Mastral, A. M.; Pe´rez-Surio, M. J. Energy Fuels 1997, 11, 202.
Figure 2. PAH distribution (µg/kg) by ring size in (a) coal and (b) tire combustion (AFBC, 860 L/h, 20% excess oxygen) as a function of combustion temperature.
and aromatic associations as nanotubes, semifullerenes. Several authors16,21,49 have explained that during tire pyrolysis, the long elastomer chains break or fragment, adsorbing on CB surface and forming little aromatic compounds which depend on the pyrolysis conditions and increase with growing temperatures. It is also worth knowing that the PAH distribution by ring size (see Figure 2) is a function of the combustion temperature. In coal combustion (see Figure 2a), independently of the combustion temperature, the main contribution to the total PAH emitted (ng/kg coal burnt) (49) Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Carrasco-Marı´n, F.; Mastral, A. M. Langmuir 1996, 12, 5654.
FBC of Fossil and Non-Fossil Fuels
is due to the most volatile compounds, the PAH of 3 rings. These follow a trend in which the maximum emissions are produced at 750 °C. In tire combustion (see Figure 2b), the PAH (µg/kg tire burnt) of 3 and 4 rings are emitted at high concentrations, especially at the highest combustion temperatures at which the increase in polyaromatic compounds of 3 and 4 rings is drastic. At these high combustion temperatures, the high flows at the exit of the reactor make difficult the interaction between radicals avoiding the formation of higher-molecular-weight compounds. Conclusions According to the combustion results obtained and the corresponding analytical data showed in this paper, it can be concluded that: (1) Rubber from waste tire can be used in FBC as a nonfossil fuel. (2) Fuel nature is determinant in FBC volatile organic emissions and although coal has fundamentally an aromatic structure, the highest polyaromatic emissions
Energy & Fuels, Vol. 14, No. 2, 2000 281
are produced using waste rubber from tire as fuel with mainly aliphatic structure. The influence of the FBC temperature on PAH emissions is different for both fuels. In coal FBC, the temperatures out of the range 750-850 °C, generate low PAH amounts while in tire combustion, 750 °C seems to be the proper temperature to decrease PAH emissions. In all the runs performed, PAH emissions using tire as fuel are much higher than using coal as fuel, independently of the combustion temperature. This shows the importance of the pyrolytic process in PAH formation and emission. One possible explanation for this fact related to solid PAH emissions could be found in tire structure and particularly in one of its components, CB, of high surface/weight ratio and which can serve as support to deposit PAH. Acknowledgment. The authors thank ECSC, project Ref. 7220/ED/089 and CICYT, project Ref. AMB98-1583, for partial financial support of this work. EF9900536
