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Soil, Water, and Air Environmental Impact from TireRubber/Coal Fluidized-Bed Cocombustion

R. Alvarez,† M. S. Callen,‡ C. Clemente,† D. Gomez-Limon,† J. M. Lopez,‡A. M. Mastral,*,‡ and R. Murillo‡

Instituto de Carboquımica, CSIC, Miguel Luesma Castan 4, 50018 Zaragoza, Spain, andETS 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 anonbiodegradable solid residue. Previously, to be established as a common practice in energygeneration, 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 inan atmospheric fluidized-bed reactor at different temperatures (750, 800, and 850 °C), keepingthe 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 thiswork. The disposal and lixiviation of the generated bottom ash as well as the atmosphericemissions in fly ash are analyzed by proximate and ultimate analysis, scanning electronmicroscopy, X-ray diffraction, inductively coupled plasma optical emission spectroscopy, and gaschromatography/mass spectrometry/mass spectrometry.

Introduction

The growing industrial development has driven to aquick waste away society demanding huge energyamounts. However, energy generation is always pollut-ant and, on the other side, fossil fuels are non-well-distributed, and their conversion into energy has anegative environmental impact.1 These two aspects,high fuel requirements and the contamination causedby fossil fuel use, together with the negative environ-mental aspects caused by the solid residue generationhave led to some residues to be considered as new fuels.In this way, disturbing residues are eliminated bycombustion considering them non fossil fuels and takingadvantage of their calorific value.

That agrees with the two main demanding points bythe current European society and government: todisperse and to diversify energy sources. If non fossilfuels are taken into account, the dependence of non-homogeneously distributed energy sources (coal, petro-leum, and natural gas) would diminish and smallerpower stations would help to take advantage of thedifferent residues generated through decreasing trans-port cost. In addition, this policy would manage thedispersion of contaminants.

Simultaneously with new fuels introduction, cleanercombustion technologies have been developed in the lastyears to fulfill legislation. New energy generationsystems include fluidized bed combustion (FBC),2 which

can be atmospheric (AFBC)3 or pressurized,4 pulverizedcoal combustion,5 combined cycles,6 or the last develop-ments in integrated coal gasification in combined cycles,7which minimize emissions however to be more expen-sive.2

It is speculated that the current global production ofwaste tires, nonbiodegrable residue, is around 6.5million t/year,8 with 20% of this amount being used inenergy generation. Rubber from waste tires is mainlyconstituted9 by natural rubber and synthetic rubber,mainly styrene-butadiene and polybutadiene, and car-bon black. In addition, certain inorganic components areadded as fillers, like SiO2, or catalysts for the vulcaniza-tion process, like ZnO. Besides, trace elements could alsobe found in rubber tire composition as a result of themanufacturing process or because of their externalincorporation 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

* To whom correspondence should be addressed. Fax: 34 976733318. 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.

(2) Beer, J. M.; Massilla, L.; Sarofim, A. F. Fluidized CombustionSystems 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 Interna-tinoal 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 Con-

tract N° DE-AC22-95PC95144 and United Technologies ResearchCenter; United Technologies Research Center: East Hartford, CT,1998.

(7) Baumann, H. R.; Ulrich, N. Paper presented at Gasification. TheGateway 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.; Callen, M. S.; Murillo, R.; Garcıa, T. Combustionof High Calorific Value Waste Material: Organic Atmospheric Pollu-tion. Environ. Sci. Technol. 1999, 33, 4155-4158.

1633Energy & Fuels 2004, 18, 1633-1639

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

in C, N, H, O, and S are not very different.10 The maindifferences between tire rubber and coal can be foundin their respective sulfur, moisture, and ash content,usually higher in coal.11 This last fact, together withits high calorific value (28-37 MJ/kg),12 higher thanmost of the coals, makes tire rubber a potential nonfossil fuel to be used to reduce the environmental impacton energy generation.

In this framework, this paper is addressed to studythe 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 tiresare the two fuels used in this work. Their proximate andultimate analyses are shown in Table 1. Their particle sizesranged between 0.4 and 1 mm. The two fuels characterization,coal and tire, has been carried out using Leco equipment fortotal sulfur, carbon, hydrogen, and nitrogen analysis. Standardmethods have been employed to establish the remainingcharacteristics, while ASTM 2492-90 standard13 was appliedfor the determination of organic sulfur. The tire rubber calorificvalue is 9159 kcal/kg.

The AFBC laboratory-scale pilot plant was described indetail in a previous work;14 see Figure 1. This laboratory-scaleplant was provided with a continuous feeder, which allowedfeeding from 50 up to 300 g/h. The reactor was made ofKanthal (67 mm i.d. and 760 mm height), and it was providedwith a bottom ash outlet in such a way that the bed heightwas constant during the experiments with an approximatevalue of 350 mm. The feeding system introduces the fuel insidethe ash fluidized bed, close to the distributor plate to improvethe contact between the solid fuel and the air. The air wasblown by a compressor, and the flow was controlled by a massflow controller. After preheating, the air flow was passedthrough the distributor plate to fluidize the bed. A temperaturecontroller with a thermocouple situated in the middle of thereactor determined the combustion temperature ((10 °C) bymeans of a furnace. The combustion gas stream passedthrough two cyclones situated at the reactor exit, where thefly ashes with the higher particle size were collected. Afterthe cyclones, a 1-µm Teflon filter was used to trap the lowersize particles. During the reaction, the bottom ashes werecollected in an ash pan connected to the reactor.

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 tocarry out the experiments that were performed at a constantgas speed (0.24 m/s) and with excess oxygen (5%). Theexperimental duration was 2 h after the plant had reachedthe steady state. From each experiment, representative samplesfrom the ash pan (bottom ash), cyclones (fly ash), the 1-µm-pore-size filter, and adsorbent were taken and analyzed.

Ash pan and cyclone samples were leached according to astandard procedure15 at atmospheric pressure with differentsulfuric acid concentrations, at two temperatures, 30 and 70°C, and for different times ranging from 10 to 120 min. Thecorresponding product characterization was carried out byatomic absorption spectrometry (AA) for solutions and by X-rayfluorescence (XRF) and X-ray diffraction (XRD) for solidsproducts.

To assess for Zn recovery, the ash pan, cyclone, and filtersamples were digested twice with 10 mL of concentrated HNO3

in a Teflon bomb to solubilize the metals in ionic form, heatingalmost to dryness and rinsing with 1 N HNO3 until a finalvolume of 50 mL with the aim of analyzing their content inZn by inductively coupled plasma optical emission spectroscopy(ICP-OES; JY 2000 Ultrace Horiba). Quantification was madeat the specific wavelength with corresponding dilutions usingMilli-Q water in the quantification range of Zn and using astandard reference material (SRM 1944), which was simulta-neously analyzed to real samples. The instrument deviationwas checked at the beginning and at the end of each sampleset. Blanks were also analyzed and found to be satisfactory.It was not possible to analyze the Zn trapped in the adsorbentsamples because their scarce amount was always inferior tothe detection limits.

In addition, the ash pan and cyclone samples from both fuelsand 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 havebeen determined by XRD (Philips PW-1710 diffractometer,equipped with a graphite monochromator and an automaticdivergence slit and operating at 40 kV) using the X-ray lineCu KR.

(10) Teng, H.; Serio, M. A.; Bassilakis, R. Preprints of PapersPresented at the 203rd ACS National Meeting, San Francisco, CA, 1993;American Chemical Society: Washington, DC, 1993; Vol. 37, p 533.

(11) Mastral, A. M.; Callen, M. S.; Murillo, R.; Mayoral, C. InProceedings of the International Conference of Coal Science; Ziegler,A., Van Heek, A., Eds.; DGMK: Germany, 1997; Vol. II, pp 1155-1158.

(12) Ekmann, J. M.; Smouse, S. M.; Winslow, J. C.; Ramezan, M.;Harding, N. S. Co-firing of coal and waste; IEACR/90; IEA CoalResearch: London, 1996; p 18.

(13) ASTM. Standard test methods for forms of sulfur in coal (D2492-90); Instrumental determination of carbon (D5291-92); Sulfur inthe analysis samples of coal (D 4239-85).

(14) Mastral, A. M.; Callen, M. S.; Murillo, R.; Garcıa, T. PAH andorganic matter associated to particulate matter from atmosphericfluidized bed coal combustion. Environ. Sci. Technol. 1999, 33, 3177. (15) DIN norm ref 38414-S4.

Table 1. Tire Rubber and Coal Proximate and UltimateAnalyses

samplea tire coal

% moisture (ar) 0.94 7.49% ash (ar) 3.28 41.46% C (ar) 86.35 39.99% N (ar) 0.18 0.93% S (ar) 1.6 0.96% H (ar) 7.29 3.17

a ar: as received.

Figure 1. Scheme of the AFBC laboratory-scale pilot plant.

1634 Energy & Fuels, Vol. 18, No. 6, 2004 Alvarez et al.

The microscopy study has been performed using a HitachiS-570 scanning electron microscope with a Kevex 3500 mi-croanalyzer (a 10-mm2 detection area) operating at 20 kV.

The organic pollutants trapped on the adsorbent, resin XAD-2, were extracted by an ultrasonic bath for 15 min three timesusing 15 mL of dichloromethane (DCM) each time. Thesolution was filtered through a Millex LCR PTFE Milliporefilter (0.45-µm pore size, 25-mm diameter) by a syringe andconcentrated, first by a rotary evaporator and finally with aN2 stream almost to dryness, to exchange solvent into hexaneprior to gas chromatography (GC) analysis with a massspectrometer-mass spectrometer detector (GC/MS/MS).

Prior to the sample quantification by external standardcalibration, standard solutions containing a total of 16 PAH[naphthalene, acenaphthylene, acenaphthene, fluorene, phenan-threne, 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, andbenzo(g,h,i)perylene] and obtained from Teknokroma [16 PAHmixture, 2000 µg/mL in DCM/benzene (1:1)] were prepared atdifferent concentrations (75-500 ppb) by appropriate dilutionand injected into the GC/MS/MS for determining the linearity.

Resin blanks were also prepared for determining detectionlimits. Sample concentrations were not corrected for blanklevels because the average concentrations in blanks were lowerthan 10% of the concentration in the sample.

Samples were analyzed by GC (Varian GC 3800) equippedwith a low-bleeding fused-silica capillary column CP-Sil 8 CB(60-m length, 0.25-mm i.d., and 0.25-µm thickness) coupledto a MS/MS detector (Saturn 2200) operating in electronimpact mode (70 eV).

The temperature-time program at the working conditionsof 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 for0.5 min, 100 °C/min until 330 °C, and isotherm for 45 min.

Helium was used as the carrier gas, and the transfer linewas 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 ofan appropriate standard reference material (SRM 1944) of theNational Institute of Standards and Technology were carriedout. Measured values were satisfactorily comparable to certi-fied 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 fromtire in FBC, from an environmental point of view, it isnecessary to take into account the two main differencesbetween both fuels: first, in general, the mineral matterand sulfur lower contents of tire rubber and, second,its higher carbon black content, a quite inert organicmaterial.16 The influence of these two facts is carefullyanalyzed in this work in relation to the environmentalimpact caused by tire rubber combustion on soil, water,and atmosphere.

Soil Environmental Impact. Generally, sulfur andmineral matter percentages in coal show variablepercentages but are always higher than the onescontained in tire rubber. The rubber sulfur contentreaches a maximum 1.7%, and its mineral matter islower than 4%, percentages not frequently found in coal,

independent of the coal rank, where generally thesepercentages are higher.

To fulfill the present legislation, limestone is addedin coal combustion mainly to control SOx emissions, ina proportion of Ca/S that can reach a ratio17 equal to oreven higher than 4. That means that huge amounts ofthe corresponding alkaline solid residue, bottom ash, aregenerated by coal combustion power stations. Thisresidue could be dramatically reduced by coal/tirerubber cocombustion. In addition, the correspondingmineral matter of the raw fuel will increase thisgenerated solid-alkaline residue. For instance, while the1 t combustion of a 3.5% sulfur and 5% mineral mattercontent coal, which can be classified as a nonbad coal,would generate ∼290 kg of bottom ash, the combustionof 1 t of tire rubber would generate ∼140 kg of bottomash. In the case of Puertollano coal, the one used in thiswork, which is a low-sulfur, high-ash content coal, theresidue generation would reach 527 kg by coal tonnes.

However, the reduction of the bottom ash generationis not the only advantage. By tire rubber combustion, anonbiodegrable residue is eliminated at the same timethat the cheaper energy can be obtained.

Water Environmental Impact. The disposal byland filling or as artificial hills of solid residues gener-ated at the power stations is a common practice. Thesesolid residues or bottom ash disposal means varioustonnes per day, depending on the power station capacity,which can release contaminants, which finally go intothe rain and become groundwater.

It has been demonstrated that the bottom ash, oncedisposed of by weathering, can release not only in-organic contaminants, whose nature will depend on thespecific mineral components18 of the corresponding fuel,but also organic pollutants19 adsorbed on their poroussurface during the combustion process. In addition andbecause of their high alkalinity (these solids because oftheir pH could be considered toxic residues accordingto the EU legislation, Directive 1999/31/EU, April 26,1999, DOCE 182/L 16-07-99), the pH of the rain andgroundwater could be altered, turning them into alka-line waters.

The possible water contamination due to the inorganiccomponents of tire rubber bottom ash disposal has beenevaluated by lixiviation of the corresponding bottom ash.The exact formula of the tire rubber (see Table 2)components is a very well kept secret manufacturingbrand, but the ICP-OES analyses showed that the smallpercentage of the rubber mineral matter is mainlycomposed of Si, Zn, Ca, and Al. SiO2 and ZnO are addedrespectively as filler and catalyst during the tire manu-facturing process, and both together are close to 80%by weight of the total rubber mineral matter. Thesample is, in addition, an undetermined mixture ofrecycled tire, and the composition regarding phasesdetermined by LTA ashes is mainly composed of ZnO

(16) Mastral, A. M.; Murillo, R.; Perez-Surio, M. J.; Callen, M. S.Coal hydro-coprocessing with tires and tire components. Energy Fuels1996, 10 (4), 941.

(17) Mastral, A. M.; Garcia, T.; Navarro, M. V.; Lopez, J. M. TheEffect of Limestone on PAH Emissions at Coal AFBC. Energy Fuels2001, 15, 1469.

(18) Alvarez, R.; Mastral, A. M.; Clemente, C.; Gomez-Limon, D.;Murillo, R.; Callen, M. S.; Dıaz, A. Coal-Tyre FBC Products. ICCSpreprints, Coalscontributing to sustainable world development; TheAustralian Institute of Energy: Cairns, Australia, 2003.

(19) Mastral, A. M.; Callen, M. S.; Garcia, T.; Lopez, J. M.; Maranon,E. Relationship between ecotoxicity and PAH content in coal combus-tion waste samples. Polycyclic Aromat. Compd. 2002, 22, 571.

Tire Rubber-Coal Fluidized-Bed Cocombustion Energy & Fuels, Vol. 18, No. 6, 2004 1635

(zincite) and amorphous silica with lower amounts ofcalcium carbonate, quartz, and some clays.

Mineral component transformations at tire rubberFBC have been studied by using XRD and scanningelectron microscopy (SEM). It has been observed thatthe original zincite (ZnO) reacts with the silicate phases,and it is transformed into willemite (Zn2SiO4) andhardystonite (ZnCa2Si2O7).

The main bottom ash species detected are willemite,zincite (ZnO), and quartz (SiO2). The diffractogramshows the predominance of the phase willemite (Zn2-SiO4) in comparison to the phase zincite. Small signalsof quartz and magnetite-like structures (magnetite andfranklinite) were also detected. The transformation ofzincite into willemite and later into hardystonite is acompromise between the residence time and tempera-ture within the reactor. The attack of silicate phaseson zincite is a solid-phase reaction, and it is a slowprocess. Therefore, for the bottom ash, the transforma-tion of the willemite into hardystonite hardly takes placeand no peak is detected, while for the slag formation,with a longer residence time in the reactor, the phasezincite is very small and the willemite and hardystonitephases are comparable.

SEM micrographs of bottom ashes show that zinciteseems to be the nucleus of several grains covered bywillemite, and this, by an intermediate phase betweenwillemite and hardystonite with an excess of silicon,probably originated from zincite by the attack of silicaand silicate phases. Figure 2 shows that the idiomorphicgrains of willemite having a nucleus of zincite aresurrounded by another phase (X) that has silicon,calcium, and zinc but with a lower content in metalsthan hardystonite.

This mineral component evolution during combustioninto more inert minerals minimizes the possible bottomash lixiviation problems. In fact, from the acid leachingof the bottom ash at different temperatures and forvariable sulfuric acid concentrations (see Figure 3), itcan be deduced that the leaching temperature is themost relevant parameter: the higher the leachingtemperature, the higher the bottom ash solubility. Thetrend for the effect of the combustion temperature wasas follows: the higher the combustion temperature, themore stable the corresponding bottom ash. The influenceof the leaching time is lower than that of the temper-ature effect. The acid concentration does not practicallyaffect the rank used (see Figure 3). In addition to this

acid leaching, an alkaline leaching was also performedbecause of the alkaline character of the bottom ash,showing no effect on zinc silicate mineral species.

From the above-commented results, it could be de-duced that the rain/groundwater contamination by thesebottom ashes is going to be very low because of the lowerambient severity.

Concerning the organic adsorbed products, the ec-otoxicity value obtained from twelve bottom ash sampleswas always lower than 3.3, which is the toxic unit (TU)19

limit for a sample to be considered toxic. That meansthat the corresponding lixiviation of the organic prod-ucts would not reach the toxic limits.

These results on inorganic and organic componentsof the bottom ash seem to indicate that the negativewater environmental impact by tire rubber bottom ashweathering will be lower than the one by coal, whichshows higher reactivity.19

Atmospheric Environmental Impact. Zn in tirerubber is present as ZnO. During tire rubber combus-tion, part of this ZnO can be converted, as is shown inthis 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 thesmallest particulate matter, lower than 1-µm size,emitted and trapped on the filter at the exit of thecyclones. In addition and in order to know about Zndistribution between byproducts, fly ashes collected inthe cyclones and the bottom ashes have also beenanalyzed by ICP-OES. Data obtained are compiled in

Table 2. Data on the Combustion Efficiency and MineralMatter 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

cyclonesin

bottom ashcombustion

efficiency (%)

100% T 850 34.6 64.8 95.8100% T 800 32.5 64.7 96.0100% T 750 18.0 53.7 89.3100% C 850 91.4 98.6 95.8100% C 750 88.9 98.3 96.870% C/30% T 850 62.1 95.2 91.370% C/30% T 800 69.0 95.4 89.970% C/30% T 750 64.3 93.8 88.290% C/10% T 850 79.9 97.3 94.090% C/10% T 800 77.5 97.0 92.990% C/10% T 750 75.4 96.5 91.2

Figure 2. SEM micrographs of bottom ash from rubber tirecombustion 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 theacid concentration, lixiviation time, and temperature obtainedby AA.


Figure 4a-d. Independent of the feed blend, Zn isaccumulated in the particulate matter trapped in thefilter. However, quantitatively this filter particulatematter is not relevant in weight, and the majority ofthe 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 bequite possibly fuel blends in power stations and, there-fore, the total Zn distribution in these blends among thebottom ash, cyclone, and filter has been calculated andis shown in Figure 3. As can be seen, the amountdirectly emitted through the atmosphere is not relevant.Figure 5 shows that around 10% of the original Zn islost in the balance and will correspond to the additionof all analytical errors and to the one adsorbed in theresin that, as commented on before, is below the

detection limit. The fly ashes collected in the cyclonescontain the higher Zn amount, and because the materialtrapped in the cyclones is usually recycled in powergeneration, according to this Zn distribution, it willfinally be present at the ash pan as inert silicates.

Concerning other legislated emissions, while NOx andSOx emissions will generally be lower than those in coalcombustion because of the lower N and S contents ofrubber (see Table 1), the COx emissions will be dupli-cated. Therefore, because the COx emissions are morerelevant in greenhouse effect emissions, tire rubber willhave more negative atmospheric impact.

Up to 35% of the rubber is carbon black, a quite inertmaterial. The volatile matter component of a fuel is veryimportant during combustion. In FBC of tire,20 it hasbeen shown that the efficiency is determined by thecompetition between combustion and entrainment ofchar fines generated by primary fragmentation in theearly pyrolysis process and by the propensity of thereleased volatiles to escape the bed and burn in thefreeboard region or to undergo a pyrosynthesis13 process.The carbon black inertness, together with its highsurface/weight ratio, means that an important amountof carbon black byproducts is entrained from the FBCreactor, enlarging dramatically the organic emissions.These rubber combustion mechanisms have previouslybeen studied.9 As soon as a rubber particle goes intothe reactor, the rubber elastomers are released as

(20) Scala, F.; Chirone, R.; Salatino, P. Exp. Therm. Fluid Sci. 2003,27, 465-471.

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 totalrecovered Zn (FBC conditions: feed 70% coal/30% tire; 850 °C;0.245 m/s).


radicals and also from the carbon black, but because ofits inertness and low density, small unburned fragmentsfrom carbon black are also entrained. These fragmentsstabilize aromatization reactions. The retrogressiveinteractions between radicals and radical small frag-ments drive the reaction to a much higher amount oforganic polyaromatic emissions21 than that in coalcombustion.

The organic emissions are partitioned between solidand gas phases. The solid phase that goes through thecyclones and is trapped in the 1-µm Teflon filter showsthe highest ecotoxicity values of the solid byproductsfrom tire rubber FBC.19,22 It is worth commenting thata direct relationship between ecotoxicity and adsorbedpolyaromatic compounds was found.18

The gas phase or smallest particulate matter, lowerthan 1-µm size, released to the atmosphere has been

controlled by using proper adsorbents23 or by catalyticdestruction. Concerning these organic emissions re-leased in the gas phase, nothing at present has beendone to avoid them because they are not yet underlegislation. However, there is a proposal of Directive onBaP24 established in 2003.

In this study, organic emissions were adsorbed in theXAD-2 resin and analyzed by GC/MS/MS. Results on

(21) Mastral, A. M.; Callen, 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) Callen, M. S.; Maranon, E.; Mastral, A. M.; Murillo, R.; Salgado,P.; Sastre, H. Ecotoxicological assessment of ashes and particulatematter from fluidized bed combustion of coal. Ecotoxicol. Environ. Saf.1998, 41, 59-61.

(23) Mastral, A. M.; Garcıa, T.; Murillo, R.; Callen, M. S.; Lopez, J.M.; Navarro, M. V. Measurements of PAH adsorption on activatedcarbons at very low concentrations. Ind. Eng. Chem. Res. 2003, 42,155-161

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)Pecontribution to the total PAH amount as a function of the combustion temperature (°C) and feed blend.


total polyaromatic emissions from tire rubber/coal FBCare shown in Figure 6a. Total PAH emissions from fourfeeds are shown in Figure 6a, which also includes thecontribution of three PAH: benzo[a]pyrene (BaP; Figure6b), indene(1,2,3-cd)pyrene (InPy; Figure 6c), and benzo-(ghi)perylene [B(ghi)P; Figure 6d], 3 of the 16 PAHlisted by USEPA as priority pollutants because of theircarcinogenic power. These three PAH are the mostabundant detected in tire rubber combustion. Figure 6shows the corresponding emissions from the 30/70 and10/90 rubber/coal AFBC blends. In addition, the corre-sponding emissions from coal combustion at the sameconditions are included as reference.

As can be seen, the emissions from rubber combustionare, at a minimum, 2 orders of magnitude higher. Thehuge polyaromatic emissions generated at tire rubberFBC, which can be decreased by introducing higher coalpercentages in the combustion blend, seem to point out

that carbon black inertness and density need longerresidence times than those used with coal, to oxidizethis more stable fuel, and a lower gas speed than 0.24m/s, to avoid the elutriation of small fragments fromcarbon black.

Therefore, and from all combustion runs and analysesperformed, it can be concluded that the atmosphericcontamination dramatically increases when tire rubberis used as the fuel. Other different combustion variablescompared to the ones used for coal combustion shouldbe used to avoid atmospheric contamination by toxic,mutagenic, and carcinogenic pollutants, as well as hot-gas cleaning systems and COx capture systems.

Acknowledgment. The authors thank the SpanishMinistry of Environment (Project AMB2000-168) for itspartial financial support, the Spanish Science andTechnology Ministry, Ramon y Cajal Program, for R.M.and M.S.C. contracts, and the General Council ofAragon (DGA, Spain) for a J.M.L. pre-doc grant.

EF0499426

(24) Commission of the European Communities. Proposal for aDirective of the European Parliament and of the Council relating toarsenic, cadmium, mercury, nickel and PAH in ambient air, Brussels,July 16, 2003; 2003/0164 (COD), COM (2003) 423 final.


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