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Soil, Water, and Air Environmental Impact from Tire Rubber/Coal Fluidized-Bed Cocombustion R. Alvarez,† M. S. Calleń,‡ C. Clemente,† D. Go´mez-Limoń,† J. M. Lo´pez,‡ A. M. Mastral,*,‡ and R. Murillo‡ Instituto de Carboquı´mica, CSIC, Miguel Luesma Casta´ n 4, 50018 Zaragoza, Spain, and ETS Ingenieros de Minas, UPM, Rios Rosas 21, 28003 Madrid, Spain Received March 4, 2004. Revised Manuscript Received July 1, 2004

Tire rubber use in energy generation allows both to get cheaper energy and to eliminate a nonbiodegradable solid residue. Previously, to be established as a common practice in energy generation, its environmental impact must be assessed to ensure a sustainable development. With this aim, rubber from tire and 10/90 and 30/70 tire rubber/coal blends have been burnt in an atmospheric fluidized-bed reactor at different temperatures (750, 800, and 850 °C), keeping the gas speed (0.24 m/s) and the oxygen excess (5%) constant. The environmental impact on soil, water, and air because of the rubber from discarded tire combustion is deeply studied in this work. The disposal and lixiviation of the generated bottom ash as well as the atmospheric emissions in fly ash are analyzed by proximate and ultimate analysis, scanning electron microscopy, X-ray diffraction, inductively coupled plasma optical emission spectroscopy, and gas chromatography/mass spectrometry/mass spectrometry.

Introduction The growing industrial development has driven to a quick waste away society demanding huge energy amounts. However, energy generation is always pollutant and, on the other side, fossil fuels are non-welldistributed, and their conversion into energy has a negative environmental impact.1 These two aspects, high fuel requirements and the contamination caused by fossil fuel use, together with the negative environmental aspects caused by the solid residue generation have led to some residues to be considered as new fuels. In this way, disturbing residues are eliminated by combustion considering them non fossil fuels and taking advantage of their calorific value. That agrees with the two main demanding points by the current European society and government: to disperse and to diversify energy sources. If non fossil fuels are taken into account, the dependence of nonhomogeneously distributed energy sources (coal, petroleum, and natural gas) would diminish and smaller power stations would help to take advantage of the different residues generated through decreasing transport cost. In addition, this policy would manage the dispersion of contaminants. Simultaneously with new fuels introduction, cleaner combustion technologies have been developed in the last years to fulfill legislation. New energy generation systems include fluidized bed combustion (FBC),2 which * To whom correspondence should be addressed. Fax: 34 976 733318. E-mail: [email protected]. † UPM. ‡ CSIC. (1) Finkelman, R. B. In Prospects for coal science in the 21st century; Li, B. Q., Li, Z. Y., Eds.; ICCS: People’s Republic of China, 1999; Vol. II, p 1457.

can be atmospheric (AFBC)3 or pressurized,4 pulverized coal combustion,5 combined cycles,6 or the last developments in integrated coal gasification in combined cycles,7 which minimize emissions however to be more expensive.2 It is speculated that the current global production of waste tires, nonbiodegrable residue, is around 6.5 million t/year,8 with 20% of this amount being used in energy generation. Rubber from waste tires is mainly constituted9 by natural rubber and synthetic rubber, mainly styrene-butadiene and polybutadiene, and carbon black. In addition, certain inorganic components are added as fillers, like SiO2, or catalysts for the vulcanization process, like ZnO. Besides, trace elements could also be found in rubber tire composition as a result of the manufacturing process or because of their external incorporation during the tire life. In general, the nature of tires and coal are compared, structurally both of them are quite different materials, but their elemental analyses show that their contents (2) Beer, J. M.; Massilla, L.; Sarofim, A. F. Fluidized Combustion Systems and Applications; Institute of Energy Symposium Series 4; Institute of Energy: London, 1980. (3) Boyd, T. J.; Divilio, R. J. Proceeding on the 9th Annual Internatinoal Pittsburgh Coal Conference, Pittsburgh, PA, 1992; p 738. (4) Liu, H.; Gibbs, B. M. Fuel 1998, 77 (14), 1579. (5) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of Pulverized Coal; Leatherhead, England, 1976. (6) Quarterly Progress Report for U.S. Department of Energy Contract N° DE-AC22-95PC95144 and United Technologies Research Center; United Technologies Research Center: East Hartford, CT, 1998. (7) Baumann, H. R.; Ulrich, N. Paper presented at Gasification. The Gateway to the Future, Dresden, Germany, 1998. (8) Marco, D.; Laresgoiti, M. F.; Cabrero, M. A.; Torres, A.; Chomon, M. J.; Caballero, B. Pyrolysis of scrap tires. Fuel Process. Technol. 2001, 72 (1), 9-22. (9) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Garcıá, T. Combustion of High Calorific Value Waste Material: Organic Atmospheric Pollution. Environ. Sci. Technol. 1999, 33, 4155-4158.

10.1021/ef0499426 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004

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Table 1. Tire Rubber and Coal Proximate and Ultimate Analyses

a

samplea

tire

coal

% moisture (ar) % ash (ar) % C (ar) % N (ar) % S (ar) % H (ar)

0.94 3.28 86.35 0.18 1.6 7.29

7.49 41.46 39.99 0.93 0.96 3.17

ar: as received.

in C, N, H, O, and S are not very different.10 The main differences between tire rubber and coal can be found in their respective sulfur, moisture, and ash content, usually higher in coal.11 This last fact, together with its high calorific value (28-37 MJ/kg),12 higher than most of the coals, makes tire rubber a potential non fossil fuel to be used to reduce the environmental impact on energy generation. In this framework, this paper is addressed to study the environmental impact of the use of a non fossil fuel, rubber from old tire, to recover its high calorific value. Experimental Section Puertollano low-rank coal and scrap rubber from old tires are the two fuels used in this work. Their proximate and ultimate analyses are shown in Table 1. Their particle sizes ranged between 0.4 and 1 mm. The two fuels characterization, coal and tire, has been carried out using Leco equipment for total sulfur, carbon, hydrogen, and nitrogen analysis. Standard methods have been employed to establish the remaining characteristics, while ASTM 2492-90 standard13 was applied for the determination of organic sulfur. The tire rubber calorific value is 9159 kcal/kg. The AFBC laboratory-scale pilot plant was described in detail in a previous work;14 see Figure 1. This laboratory-scale plant was provided with a continuous feeder, which allowed feeding from 50 up to 300 g/h. The reactor was made of Kanthal (67 mm i.d. and 760 mm height), and it was provided with a bottom ash outlet in such a way that the bed height was constant during the experiments with an approximate value of 350 mm. The feeding system introduces the fuel inside the ash fluidized bed, close to the distributor plate to improve the contact between the solid fuel and the air. The air was blown by a compressor, and the flow was controlled by a mass flow controller. After preheating, the air flow was passed through the distributor plate to fluidize the bed. A temperature controller with a thermocouple situated in the middle of the reactor determined the combustion temperature ((10 °C) by means of a furnace. The combustion gas stream passed through two cyclones situated at the reactor exit, where the fly ashes with the higher particle size were collected. After the cyclones, a 1-µm Teflon filter was used to trap the lower size particles. During the reaction, the bottom ashes were collected in an ash pan connected to the reactor. (10) Teng, H.; Serio, M. A.; Bassilakis, R. Preprints of Papers Presented at the 203rd ACS National Meeting, San Francisco, CA, 1993; American Chemical Society: Washington, DC, 1993; Vol. 37, p 533. (11) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Mayoral, C. In Proceedings of the International Conference of Coal Science; Ziegler, A., Van Heek, A., Eds.; DGMK: Germany, 1997; Vol. II, pp 11551158. (12) Ekmann, J. M.; Smouse, S. M.; Winslow, J. C.; Ramezan, M.; Harding, N. S. Co-firing of coal and waste; IEACR/90; IEA Coal Research: London, 1996; p 18. (13) ASTM. Standard test methods for forms of sulfur in coal (D 2492-90); Instrumental determination of carbon (D5291-92); Sulfur in the analysis samples of coal (D 4239-85). (14) Mastral, A. M.; Calleń, M. S.; Murillo, R.; Garcıá, T. PAH and organic matter associated to particulate matter from atmospheric fluidized bed coal combustion. Environ. Sci. Technol. 1999, 33, 3177.

Figure 1. Scheme of the AFBC laboratory-scale pilot plant. Four different feedings were burnt: 100% tire rubber, 100% coal, 90% coal/10% tire rubber, and 70% coal/30% tire rubber. Different temperatures of 750, 800, and 850 °C were used to carry out the experiments that were performed at a constant gas speed (0.24 m/s) and with excess oxygen (5%). The experimental duration was 2 h after the plant had reached the steady state. From each experiment, representative samples from the ash pan (bottom ash), cyclones (fly ash), the 1-µmpore-size filter, and adsorbent were taken and analyzed. Ash pan and cyclone samples were leached according to a standard procedure15 at atmospheric pressure with different sulfuric acid concentrations, at two temperatures, 30 and 70 °C, and for different times ranging from 10 to 120 min. The corresponding product characterization was carried out by atomic absorption spectrometry (AA) for solutions and by X-ray fluorescence (XRF) and X-ray diffraction (XRD) for solids products. To assess for Zn recovery, the ash pan, cyclone, and filter samples were digested twice with 10 mL of concentrated HNO3 in a Teflon bomb to solubilize the metals in ionic form, heating almost to dryness and rinsing with 1 N HNO3 until a final volume of 50 mL with the aim of analyzing their content in Zn by inductively coupled plasma optical emission spectroscopy (ICP-OES; JY 2000 Ultrace Horiba). Quantification was made at the specific wavelength with corresponding dilutions using Milli-Q water in the quantification range of Zn and using a standard reference material (SRM 1944), which was simultaneously analyzed to real samples. The instrument deviation was checked at the beginning and at the end of each sample set. Blanks were also analyzed and found to be satisfactory. It was not possible to analyze the Zn trapped in the adsorbent samples because their scarce amount was always inferior to the detection limits. In addition, the ash pan and cyclone samples from both fuels and their blends have been analyzed by XRF (Philips PX-1404, equipped with a Sc-Mo tube and using pressed powder pellets) or by AA (Philips PV 9100X/14). The crystalline phases have been determined by XRD (Philips PW-1710 diffractometer, equipped with a graphite monochromator and an automatic divergence slit and operating at 40 kV) using the X-ray line Cu KR. (15) DIN norm ref 38414-S4.

Tire Rubber-Coal Fluidized-Bed Cocombustion The microscopy study has been performed using a Hitachi S-570 scanning electron microscope with a Kevex 3500 microanalyzer (a 10-mm2 detection area) operating at 20 kV. The organic pollutants trapped on the adsorbent, resin XAD2, were extracted by an ultrasonic bath for 15 min three times using 15 mL of dichloromethane (DCM) each time. The solution was filtered through a Millex LCR PTFE Millipore filter (0.45-µm pore size, 25-mm diameter) by a syringe and concentrated, first by a rotary evaporator and finally with a N2 stream almost to dryness, to exchange solvent into hexane prior to gas chromatography (GC) analysis with a mass spectrometer-mass spectrometer detector (GC/MS/MS). Prior to the sample quantification by external standard calibration, standard solutions containing a total of 16 PAH [naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno(1,2,3-cd)pyrene, dibenzo(a,h)anthracene, and benzo(g,h,i)perylene] and obtained from Teknokroma [16 PAH mixture, 2000 µg/mL in DCM/benzene (1:1)] were prepared at different concentrations (75-500 ppb) by appropriate dilution and injected into the GC/MS/MS for determining the linearity. Resin blanks were also prepared for determining detection limits. Sample concentrations were not corrected for blank levels because the average concentrations in blanks were lower than 10% of the concentration in the sample. Samples were analyzed by GC (Varian GC 3800) equipped with a low-bleeding fused-silica capillary column CP-Sil 8 CB (60-m length, 0.25-mm i.d., and 0.25-µm thickness) coupled to a MS/MS detector (Saturn 2200) operating in electron impact mode (70 eV). The temperature-time program at the working conditions of the GC/MS/MS was the following: 60 °C isotherm for 1 min, 10°/min until 300 °C, and isotherm for 15 min. The injector was kept by the following program: 60 °C for 0.5 min, 100 °C/min until 330 °C, and isotherm for 45 min. Helium was used as the carrier gas, and the transfer line was heated at 280 °C. In all cases, 1 µL of sample was injected in splitless mode (1/50, split valve closed for 3.5 min). To check the analytical accuracy and precision, analyses of an appropriate standard reference material (SRM 1944) of the National Institute of Standards and Technology were carried out. Measured values were satisfactorily comparable to certified values with a precision between 0.2% (for benzo[k]fluoranthene) and 22% for all compounds except naphthalene, 38%.

Results and Discussion When coal is total or partially replaced by rubber from tire in FBC, from an environmental point of view, it is necessary to take into account the two main differences between both fuels: first, in general, the mineral matter and sulfur lower contents of tire rubber and, second, its higher carbon black content, a quite inert organic material.16 The influence of these two facts is carefully analyzed in this work in relation to the environmental impact caused by tire rubber combustion on soil, water, and atmosphere. Soil Environmental Impact. Generally, sulfur and mineral matter percentages in coal show variable percentages but are always higher than the ones contained in tire rubber. The rubber sulfur content reaches a maximum 1.7%, and its mineral matter is lower than 4%, percentages not frequently found in coal, (16) Mastral, A. M.; Murillo, R.; Perez-Surio, M. J.; Calleń, M. S. Coal hydro-coprocessing with tires and tire components. Energy Fuels 1996, 10 (4), 941.

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independent of the coal rank, where generally these percentages are higher. To fulfill the present legislation, limestone is added in coal combustion mainly to control SOx emissions, in a proportion of Ca/S that can reach a ratio17 equal to or even higher than 4. That means that huge amounts of the corresponding alkaline solid residue, bottom ash, are generated by coal combustion power stations. This residue could be dramatically reduced by coal/tire rubber cocombustion. In addition, the corresponding mineral matter of the raw fuel will increase this generated solid-alkaline residue. For instance, while the 1 t combustion of a 3.5% sulfur and 5% mineral matter content coal, which can be classified as a nonbad coal, would generate ∼290 kg of bottom ash, the combustion of 1 t of tire rubber would generate ∼140 kg of bottom ash. In the case of Puertollano coal, the one used in this work, which is a low-sulfur, high-ash content coal, the residue generation would reach 527 kg by coal tonnes. However, the reduction of the bottom ash generation is not the only advantage. By tire rubber combustion, a nonbiodegrable residue is eliminated at the same time that the cheaper energy can be obtained. Water Environmental Impact. The disposal by land filling or as artificial hills of solid residues generated at the power stations is a common practice. These solid residues or bottom ash disposal means various tonnes per day, depending on the power station capacity, which can release contaminants, which finally go into the rain and become groundwater. It has been demonstrated that the bottom ash, once disposed of by weathering, can release not only inorganic contaminants, whose nature will depend on the specific mineral components18 of the corresponding fuel, but also organic pollutants19 adsorbed on their porous surface during the combustion process. In addition and because of their high alkalinity (these solids because of their pH could be considered toxic residues according to the EU legislation, Directive 1999/31/EU, April 26, 1999, DOCE 182/L 16-07-99), the pH of the rain and groundwater could be altered, turning them into alkaline waters. The possible water contamination due to the inorganic components of tire rubber bottom ash disposal has been evaluated by lixiviation of the corresponding bottom ash. The exact formula of the tire rubber (see Table 2) components is a very well kept secret manufacturing brand, but the ICP-OES analyses showed that the small percentage of the rubber mineral matter is mainly composed of Si, Zn, Ca, and Al. SiO2 and ZnO are added respectively as filler and catalyst during the tire manufacturing process, and both together are close to 80% by weight of the total rubber mineral matter. The sample is, in addition, an undetermined mixture of recycled tire, and the composition regarding phases determined by LTA ashes is mainly composed of ZnO (17) Mastral, A. M.; Garcia, T.; Navarro, M. V.; Lopez, J. M. The Effect of Limestone on PAH Emissions at Coal AFBC. Energy Fuels 2001, 15, 1469. (18) Alvarez, R.; Mastral, A. M.; Clemente, C.; Go´mez-Limoń, D.; Murillo, R.; Calleń, M. S.; Dıáz, A. Coal-Tyre FBC Products. ICCS preprints, Coalscontributing to sustainable world development; The Australian Institute of Energy: Cairns, Australia, 2003. (19) Mastral, A. M.; Calleń, M. S.; Garcia, T.; Lopez, J. M.; Maranõń, E. Relationship between ecotoxicity and PAH content in coal combustion waste samples. Polycyclic Aromat. Compd. 2002, 22, 571.

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Table 2. Data on the Combustion Efficiency and Mineral Matter Content in Cyclone and Bottom Ash as a Function of the Feed Blend and the FBC Temperature (T ) rubber tire; C ) coal) mineral matter (%) feed blend

temp (°C)

in cyclones

in bottom ash

combustion efficiency (%)

100% T 100% T 100% T 100% C 100% C 70% C/30% T 70% C/30% T 70% C/30% T 90% C/10% T 90% C/10% T 90% C/10% T

850 800 750 850 750 850 800 750 850 800 750

34.6 32.5 18.0 91.4 88.9 62.1 69.0 64.3 79.9 77.5 75.4

64.8 64.7 53.7 98.6 98.3 95.2 95.4 93.8 97.3 97.0 96.5

95.8 96.0 89.3 95.8 96.8 91.3 89.9 88.2 94.0 92.9 91.2

(zincite) and amorphous silica with lower amounts of calcium carbonate, quartz, and some clays. Mineral component transformations at tire rubber FBC have been studied by using XRD and scanning electron microscopy (SEM). It has been observed that the original zincite (ZnO) reacts with the silicate phases, and it is transformed into willemite (Zn2SiO4) and hardystonite (ZnCa2Si2O7). The main bottom ash species detected are willemite, zincite (ZnO), and quartz (SiO2). The diffractogram shows the predominance of the phase willemite (Zn2SiO4) in comparison to the phase zincite. Small signals of quartz and magnetite-like structures (magnetite and franklinite) were also detected. The transformation of zincite into willemite and later into hardystonite is a compromise between the residence time and temperature within the reactor. The attack of silicate phases on zincite is a solid-phase reaction, and it is a slow process. Therefore, for the bottom ash, the transformation of the willemite into hardystonite hardly takes place and no peak is detected, while for the slag formation, with a longer residence time in the reactor, the phase zincite is very small and the willemite and hardystonite phases are comparable. SEM micrographs of bottom ashes show that zincite seems to be the nucleus of several grains covered by willemite, and this, by an intermediate phase between willemite and hardystonite with an excess of silicon, probably originated from zincite by the attack of silica and silicate phases. Figure 2 shows that the idiomorphic grains of willemite having a nucleus of zincite are surrounded by another phase (X) that has silicon, calcium, and zinc but with a lower content in metals than hardystonite. This mineral component evolution during combustion into more inert minerals minimizes the possible bottom ash lixiviation problems. In fact, from the acid leaching of the bottom ash at different temperatures and for variable sulfuric acid concentrations (see Figure 3), it can be deduced that the leaching temperature is the most relevant parameter: the higher the leaching temperature, the higher the bottom ash solubility. The trend for the effect of the combustion temperature was as follows: the higher the combustion temperature, the more stable the corresponding bottom ash. The influence of the leaching time is lower than that of the temperature effect. The acid concentration does not practically affect the rank used (see Figure 3). In addition to this

Figure 2. SEM micrographs of bottom ash from rubber tire combustion at 750 °C (X, intermediate species; W, willemite; Z, zincite).

Figure 3. Percentages of Zn recovery from 90% coal/10% rubber FBC bottom ash by acid lixiviation as a function of the acid concentration, lixiviation time, and temperature obtained by AA.

acid leaching, an alkaline leaching was also performed because of the alkaline character of the bottom ash, showing no effect on zinc silicate mineral species. From the above-commented results, it could be deduced that the rain/groundwater contamination by these bottom ashes is going to be very low because of the lower ambient severity. Concerning the organic adsorbed products, the ecotoxicity value obtained from twelve bottom ash samples was always lower than 3.3, which is the toxic unit (TU)19 limit for a sample to be considered toxic. That means that the corresponding lixiviation of the organic products would not reach the toxic limits. These results on inorganic and organic components of the bottom ash seem to indicate that the negative water environmental impact by tire rubber bottom ash weathering will be lower than the one by coal, which shows higher reactivity.19 Atmospheric Environmental Impact. Zn in tire rubber is present as ZnO. During tire rubber combustion, part of this ZnO can be converted, as is shown in this work, into silicate salts, but, because of its volatility, part of the Zn could be released into the atmosphere. This possibility has been checked by analyzing the smallest particulate matter, lower than 1-µm size, emitted and trapped on the filter at the exit of the cyclones. In addition and in order to know about Zn distribution between byproducts, fly ashes collected in the cyclones and the bottom ashes have also been analyzed by ICP-OES. Data obtained are compiled in

Tire Rubber-Coal Fluidized-Bed Cocombustion


Figure 4. Zn percentages detected in samples from (a) tire rubber FBC, (b) coal FBC, (c) 90% coal/10% rubber blend FBC, and (d) 70% coal/30% rubber blend FBC at different temperatures by ICP-OES.

Figure 5. Zn recovery at each sampling point and total recovered Zn (FBC conditions: feed 70% coal/30% tire; 850 °C; 0.245 m/s).

Figure 4a-d. Independent of the feed blend, Zn is accumulated in the particulate matter trapped in the filter. However, quantitatively this filter particulate matter is not relevant in weight, and the majority of the feeding Zn ends in the cyclone and ash pan. According to delivery problems, 30% tire rubber/70% coal and 10% tire/rubber/90% coal blends seem to be quite possibly fuel blends in power stations and, therefore, the total Zn distribution in these blends among the bottom ash, cyclone, and filter has been calculated and is shown in Figure 3. As can be seen, the amount directly emitted through the atmosphere is not relevant. Figure 5 shows that around 10% of the original Zn is lost in the balance and will correspond to the addition of all analytical errors and to the one adsorbed in the resin that, as commented on before, is below the

detection limit. The fly ashes collected in the cyclones contain the higher Zn amount, and because the material trapped in the cyclones is usually recycled in power generation, according to this Zn distribution, it will finally be present at the ash pan as inert silicates. Concerning other legislated emissions, while NOx and SOx emissions will generally be lower than those in coal combustion because of the lower N and S contents of rubber (see Table 1), the COx emissions will be duplicated. Therefore, because the COx emissions are more relevant in greenhouse effect emissions, tire rubber will have more negative atmospheric impact. Up to 35% of the rubber is carbon black, a quite inert material. The volatile matter component of a fuel is very important during combustion. In FBC of tire,20 it has been shown that the efficiency is determined by the competition between combustion and entrainment of char fines generated by primary fragmentation in the early pyrolysis process and by the propensity of the released volatiles to escape the bed and burn in the freeboard region or to undergo a pyrosynthesis13 process. The carbon black inertness, together with its high surface/weight ratio, means that an important amount of carbon black byproducts is entrained from the FBC reactor, enlarging dramatically the organic emissions. These rubber combustion mechanisms have previously been studied.9 As soon as a rubber particle goes into the reactor, the rubber elastomers are released as (20) Scala, F.; Chirone, R.; Salatino, P. Exp. Therm. Fluid Sci. 2003, 27, 465-471.

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Figure 6. Total PAH trapped and determined by GC/MS/MS in (a) a Teflon filter and (b) BaP, (c) InPy, and (d) B(ghi)Pe contribution to the total PAH amount as a function of the combustion temperature (°C) and feed blend.

radicals and also from the carbon black, but because of its inertness and low density, small unburned fragments from carbon black are also entrained. These fragments stabilize aromatization reactions. The retrogressive interactions between radicals and radical small fragments drive the reaction to a much higher amount of organic polyaromatic emissions21 than that in coal combustion. The organic emissions are partitioned between solid and gas phases. The solid phase that goes through the cyclones and is trapped in the 1-µm Teflon filter shows the highest ecotoxicity values of the solid byproducts from tire rubber FBC.19,22 It is worth commenting that a direct relationship between ecotoxicity and adsorbed polyaromatic compounds was found.18 The gas phase or smallest particulate matter, lower than 1-µm size, released to the atmosphere has been

controlled by using proper adsorbents23 or by catalytic destruction. Concerning these organic emissions released in the gas phase, nothing at present has been done to avoid them because they are not yet under legislation. However, there is a proposal of Directive on BaP24 established in 2003. In this study, organic emissions were adsorbed in the XAD-2 resin and analyzed by GC/MS/MS. Results on (21) Mastral, A. M.; Calleń, M. S.; Garcia, T.; Lopez, J. M. B(a)P, B(a)A and D(a,h)A emissions from energy generation at AFBC. Environ. Sci. Technol. 2001, 35, 2645. (22) Calleń, M. S.; Maranõń, E.; Mastral, A. M.; Murillo, R.; Salgado, P.; Sastre, H. Ecotoxicological assessment of ashes and particulate matter from fluidized bed combustion of coal. Ecotoxicol. Environ. Saf. 1998, 41, 59-61. (23) Mastral, A. M.; Garcıá, T.; Murillo, R.; Calleń, M. S.; Lo´pez, J. M.; Navarro, M. V. Measurements of PAH adsorption on activated carbons at very low concentrations. Ind. Eng. Chem. Res. 2003, 42, 155-161

Tire Rubber-Coal Fluidized-Bed Cocombustion

total polyaromatic emissions from tire rubber/coal FBC are shown in Figure 6a. Total PAH emissions from four feeds are shown in Figure 6a, which also includes the contribution of three PAH: benzo[a]pyrene (BaP; Figure 6b), indene(1,2,3-cd)pyrene (InPy; Figure 6c), and benzo(ghi)perylene [B(ghi)P; Figure 6d], 3 of the 16 PAH listed by USEPA as priority pollutants because of their carcinogenic power. These three PAH are the most abundant detected in tire rubber combustion. Figure 6 shows the corresponding emissions from the 30/70 and 10/90 rubber/coal AFBC blends. In addition, the corresponding emissions from coal combustion at the same conditions are included as reference. As can be seen, the emissions from rubber combustion are, at a minimum, 2 orders of magnitude higher. The huge polyaromatic emissions generated at tire rubber FBC, which can be decreased by introducing higher coal percentages in the combustion blend, seem to point out (24) Commission of the European Communities. Proposal for a Directive of the European Parliament and of the Council relating to arsenic, cadmium, mercury, nickel and PAH in ambient air, Brussels, July 16, 2003; 2003/0164 (COD), COM (2003) 423 final.


that carbon black inertness and density need longer residence times than those used with coal, to oxidize this more stable fuel, and a lower gas speed than 0.24 m/s, to avoid the elutriation of small fragments from carbon black. Therefore, and from all combustion runs and analyses performed, it can be concluded that the atmospheric contamination dramatically increases when tire rubber is used as the fuel. Other different combustion variables compared to the ones used for coal combustion should be used to avoid atmospheric contamination by toxic, mutagenic, and carcinogenic pollutants, as well as hotgas cleaning systems and COx capture systems. Acknowledgment. The authors thank the Spanish Ministry of Environment (Project AMB2000-168) for its partial financial support, the Spanish Science and Technology Ministry, Ramoń y Cajal Program, for R.M. and M.S.C. contracts, and the General Council of Aragoń (DGA, Spain) for a J.M.L. pre-doc grant. EF0499426

Coal

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