Environ. Sci. Technol. 2001, 35, 2645-2649
Benzo(a)pyrene, Benzo(a)anthracene, and Dibenzo(a,h)anthracene Emissions from Coal and Waste Tire Energy Generation at Atmospheric Fluidized Bed Combustion (AFBC) A N A M . M A S T R A L , * M A R IÄ A S . C A L L EÄ N , T O M AÄ S G A R C IÄ A , A N D J O S E M . L O P E Z Instituto de Carboquı´mica, CSIC, C/Marı´a de Luna, 12, 50015, Zaragoza, Spain
The main aim of this work was the analyzing of the release to the atmosphere of benzo[a]pyrene (BaP), dibenzo(a,h)anthracene (D(a,h)A), and benzo[a]anthracene (BaA), three of the most carcinogenic PAHs listed by USEPA as priority pollutants, emitted from combustion at the last generation reactors used nowadays in power generation, fluidized bed reactors, trying to establish their incidence when waste materials are used as “new fuels”. BaP, D(a,h)A, and BaA emissions, once collected in the sampling system, have been analyzed by fluorescence spectroscopy in the synchronous mode (FS) after extraction by ultrasonic bath with dimethylformamide (DMF) as solvent. Concerning to the combustion variables influence, the conclusion reached was that, in coal combustion, the lowest emissions of BaP are generated at percentages of excess oxygen of 20%, at flows corresponding to good fluidization conditions, 860 L/h (double of the minimum fluidization velocity) and temperatures out of 850 °C; the lowest emission of D(a,h)A are emitted at 20% excess oxygen, 900 L/h and out of 750 °C, and the lowest emission of BaA are generated at 20% excess oxygen, 860 L/h and out of the range 750-850 °C. Regarding tire as nonfossil fuel, higher emissions of BaP, BaA, and D(a,h)A are detected in comparison to coal combustion. In coal-tire blend, the BaP, BaA, and D(a,h)A emissions are more similar to the values obtained in coal combustion and not intermediate values as it could be expected by the blend composition.
Introduction The progress and the evolution of the mankind throughout the history have supposed an important technological advance, which implies a high, consume of energy. However, the main advantages of this necessary development could be eclipsed if the subproducts generated are not controlled or used in an environmentally unacceptable manner (1). The subproducts can be classified depending on their utility as recycled or as harmful compounds. A careful control about the last destination of these subproducts must be taken into account, in such a way that, nowadays, no new processes should be undertaken without facing their environmental impact. In power generation, combustion from its beginning and as an exothermic process is being used for energy generation despite its negative environmental impact. Several fuels and * Corresponding author phone: 34-976-733977; fax: 34-976733318; e-mail:
[email protected]. 10.1021/es0015850 CCC: $20.00 Published on Web 05/23/2001
2001 American Chemical Society
innumerable improvements in reactors have been introduced until the establishment of the present state-of-the-art and the use of the last generation reactors of fluidized bed. The main advantages (2) of fluidized bed combustion are the possible control of the legislated emissions (SOx with adsorbent addition (3-5); NOx (4-6) by burning at lower temperatures; and COx by increasing the ratio CO2-CO); the high efficiencies reached; and the tolerance with coal fed properties. The inorganic emissions can be controlled at fluidized bed combustion (FBC) (5), but concerning organic emissions very little is known due to the nonexisting legislation about them. The different development and the existence of different policy for each country make the regulation variable but, the environment, common to the mankind, turns the searching of an only legislation which controls the organic emissions into the prior objective (6). Within these volatile organic emissions (VOC), polycyclic aromatic hydrocarbons (PAHs) have started to acquire increasing concern, mainly BaP, BaA, and D(a,h)A, due to their admitted adverse effects on human health (7). Despite PAHs can have a natural source, the combustion process (8) is the main responsible of PAHs in the environment. One of the most important sources of atmospheric PAHs is the fossil fuels (9-14) utilization. It was thought that the incomplete combustion of fuels was the responsible of the PAHs emissions. Nowadays (13), it is known that the combustion process itself also can generate important amounts of PAHs emissions, regardless if the process efficiency value is close to 100% or not. Therefore, it is necessary to take into account that the PAHs from combustion can have a double origin. Once produced and because PAHs show different vapor pressure, different affinity for the particle organic matrix, and different molecular weight, these compounds can exist in the gas phase or in the particulate matter (13, 14). In the particular case of power stations (15), the gas phase is emitted to the atmosphere by a chimney, releasing the PAHs generated (consequence of the process) and the entrained ones (consequence of the unburned fuel) to the air. Very little is known on PAHs emissions from traditional combustors, like fixed beds (16) or on pulverized coal combustion (17). Spuznar (18) compared PAH emissions from three different coal fired stations detecting the lowest emissions from the largest unit. Studied PAH emissions from new energy systems (FBC) are also scarce. The influence of the coal combustion parameters on PAH emissions have been lately published (19-22): the highest PAH emissions, as a function of the combustion temperature, are produced in the range 750850 °C; a great decrease are observed when the excess oxygen increases from 5 to 10%; and the lowest PAH amounts are emitted at flows of the double of the minimum fluidization velocity. The information on the fuel influence on PAHs emissions is very little. It has been reported that the effect of coal type is much less influencing than the reactor type (23) and that the fuel type, coal or biomass, has less effect than the furnace or than the load size (24). The use of waste materials with high calorific value as new fuels is acquiring considerable importance in combustion processes. Combustion of polymeric materials such as polystyrene (25), polypropylene (26), or waste tires (16, 17, 27-30) generates lower inorganic emissions but higher PAHs emissions than coal, and more severe combustion conditions than those for coal will be VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Tire and Coal Elemental an Immediate Analysisa % C (daf) % H (daf) % S (df) % N (daf) % moisture (af) % ash (df) % volatiles (ar) % fixed carbon (ar) calorific value (Kcal/Kg) particle size (mm) a
tire
coal
88.6 8.3 1.4 0.4 0.9 3.8 67.3 31.1 9220 0.5-1.5
80.2 6.7 5.7 1.0 22.0 26.9 48.6 28.4 4130 1
daf: dry ash free; df: dry free; af: ash free; and ar: as received.
necessary to total carbon black from tires conversion into energy (29). During their migration through the air, PAHs can undergo environmental factors (31) which transform them into products in some cases more dangerous (32) than the originally released. PAHs or their derived compounds can get into water and soil (33) being assimilated by the different organisms (plants, animals, fish, etc.) existing in the corresponding ambient. As result and due to the biological cycle at which living species are submitted, PAHs and derived compounds can get into the human body more or less directly, by ingestion (34), inhalation (35), or contact with the skin (36) forming adducts which could alter the regular behavior of cells. This paper is centered not only in the study of the most carcinogenic PAHs emissions, BaP, D(a,h)A, and BaA, from new combustion technology using “new fuels” but also in the abatement of PAHs emissions searching the right combustion conditions for minimum emissions of each one of them.
Experimental Section The combustion experiences were carried out in an atmospheric fluidized bed combustion installation (76-cm height × 7-cm i.d. reactor), previously described in former works (19, 30). Different combustion variables (temperature: 650950 °C, percentage of excess oxygen: 5-40%, air total flow: 700-1100 L/h) and different fuels (coal, coal-tire blend 1:1 in organic matter and tire) are studied. The coal was a low rank coal from NE of Spain (particle size from 0.5 to 1.0 mm), and the tire was obtained from a nonspecific blend of old tires (1 mm particle size). The analyses of both fuels are shown in Table 1. The trapping system was optimized in order to collect PAHs of different volatility. Two cyclones in battery at the reactor exit captured the particulate matter of the biggest size. The particulate matter of smallest size, not collected into the cyclones, was collected in two successive filters; the first one was a nylon filter (20 µm) followed by a Teflon filter (0.5 µm). An adsorbent material (XAD-2 resin) was placed at the end of the trapping battery to collect the most volatile PAHs. Filters and adsorbent were kept in a freezer and protected of sunlight till their extraction with dymethylformamide (DMF) to avoid photodegradation and decomposition reactions. The extraction was performed by ultrasonic bath for 15 min with DMF and the PAHs content of the samples was analyzed by fluorescence spectroscopy (FS) in synchronous mode with a Spectrofluorimeter LS-50 Perkin-Elmer instrument at the conditions determined in previous work (11).
Results and Discussion In all the combustion process and as a consequence of the thermolysis of the coal structure, the release of radicals are a consequence of the fuel thermal decomposition and a solid nucleus remains as unburned material. While the combustion 2646
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of the radicals is faster, the combustion of the remaining solid needs longer times. As the tendency of the released radicals is to quickly stabilize, if the oxygen is not abundant and they are not eliminated by oxidation reactions, they can undergo retrogressive reactions. So, condensation and association reactions can contribute, together to the entrained unburned material, to the PAHs, soots, and particulate matter formation and emissions (12, 13). Therefore, these reactions and the entrainment of unburned solid particles will influence BaP, BaA, and D(a,h)A emissions and will be depending on the combustion variables and on the fuel nature. Thermal Decomposition. The devolatilization step is one of the most important aspects in the combustion process, being primarily responsible of the partitioning between volatile material and the solid nucleus from the original fuel matrix. This solid, as it happens with the radicals released and as a function of the working conditions, could be totally gasified to COx and H2O, or it could be fragmented as unburned material, which will contribute to the entrainment of particulate matter and to the residues formation (13). This particulate matter can serve as support for deposition of determined pollutants and specially PAHs, which can be associated or adsorbed on its surface depending on their nature, molecular weight, etc. At the power stations, there are two main types of residue, bottom ash from the combustor and fly ash collected in different control devices as electrostatic precipitators or baghouses. Combustion efficiency is determined by the extension of fuel conversion: the lower the carbon content in bottom ash, the higher the efficiency. So, with the aim of producing the maximum energy and in order to avoid unburned material, the combustion conditions have been optimized till work at high efficiency values. In this work, the efficiency values have been calculated with the particulate matter collected in two cyclones and in the bottom ash in an approach to a real situation. The efficiency values have been calculated for the different fuels used, coal, coal-tire blend, and tire, and the results obtained were shown to be dependent on the fuel. In this AFB coal combustion, efficiency reached very close values to 100% (19); therefore, the influence of the unburned material on PAHs and specifically on BaP, D(a,h)A, and BaA emissions would be practically worthless. On the contrary, when tire was introduced as fuel in the combustor (tire and coal-tire blend), the efficiency values reached (20, 28, 30) were lower (for coal-tire blend, maximum value: 96.9%) in comparison to coal combustion, especially when tire was the only fuel fed (maximum efficiency value: 95.1%). In addition, a higher amount of particulate matter, consequence of the incomplete combustion, was collected in the cyclones, and a high increase in the polyaromatic BaA, D(a,h)A, and BaP emissions was produced, particularly in the second cyclone. Particulate matter trapped on the cyclones shows the highest size of the particles emitted, corresponding to the larger unburned particles collected in the first cyclone, while that for those trapped in the second cyclone the surface/ weight increases due to its lower size. This fact seems to corroborate that the particulate matter serves as support to PAHs emissions. In addition, a higher unburned organic matter respect to coal combustion is obtained in tire combustion. As the only variation in the combustion process is the fuel nature, because the same combustion conditions (20% excess oxygen, 860 l/h) were kept constant, it seems to be clear that the fuel nature is responsible for the high increase in BaP, D(a,h)A, and BaA emissions. A possible explanation for the higher emissions in tire combustion could be due to the different chemical structure of the fuels, coal and tire. While coal is mainly aromatic, tire has a not so aromatic structure. Tire is majority constituted by one-third carbon black (CB) and two-thirds of elastomers,
FIGURE 2. Diagram of BaP and D(a,h)A formation from BaA.
FIGURE 1. Total distribution (µg/kg) of (a) BaP, (b) D(a,h)A, and (c) BaA in coal combustion as a function of the percentage of excess oxygen (FS, AFBC, 850 °C, 860 L/h). fundamentally styrene-butadiene (SBR) and polybutadiene (PB) in variable proportions (styrene up to 25% of the 2/3 and extender oils up to 15%) as a function of the trademark of the rubber. From these three main components, its high ratio surface/weight and inertness characterize CB. The light nature of CB and its special characteristics allow its entrainment from the reactor making difficult its total burned and obtaining low efficiencies, showing that other conditions than those used with coal (longer residence time, different combustion temperatures, etc.) would be necessary when tire is introduced as fuel. The CB high surface makes that determined pollutants and concretely PAHs find a support to be transported on. Moreover, other factors must be taken into account. The bond energy between the rubber components is more similar than in coal and as consequence of the heating step, a bulky cleavage of chemical bonds is produced simultaneously, releasing a high amount of radicals and increasing the possibility of condensation reactions. At the point where the bulky devolatilization takes place, the relative oxygen concentration will decrease, the elimination radicals as COx and H2O will be less favored, and as a result, PAHs formation will be promoted. Therefore, it does not matter that tire is more aliphatic than coal because as it has been shown (13, 20) not only the fuel nature plays an important role in the mechanism of PAHs formation but also the combustion process. Radicals Reaction. The radicals released in the devolatilization process, due to their high instability and short average lifetimes, independently of the process that the solid nucleus experiences, tend to stabilize through two different reactions: elimination reactions by oxidation, giving as final products COx and H2O, and condensation reactions by association, which can lead to volatile emissions (PAHs) and/ or soots. The main combustion variables, percentage of excess oxygen, temperature, and flow will influence on these elimination and condensation reactions, and their influence is commented below. (a) Influence of the Excess Oxygen. In the particular case that there is abundant excess oxygen, the radicals will stabilize more easily by oxidation disappearing as COx and H2O and minimizing associations among themselves. The influence of the percentage of excess oxygen has been studied in coal combustion on the particular formation of BaP, D(a,h)A, and BaA (Figure 1). The three compounds follow the same general
trend showed also by the total of PAHs emitted (21). Low percentages of excess oxygen promote the formation of these pollutants, and the minimum emissions appear at the 20% of excess oxygen stoichiometry required for burning the fed fuel. Less favorable conditions of combustion, corresponding to scarce percentages of excess oxygen, generate higher emissions of BaP, BaA, and D(a,h)A. BaA, the least carcinogenic compound compared to BaP and D(a,h)A, is emitted at the highest concentrations. Anyway, a possible abatement could be performed increasing the percentage of excess oxygen when the work is carried out in an experimental pilot plant, but it would imply higher cost. Both factors cost and pollution must be balanced and taken into account in order to reach equilibrium between them. In scarce oxygen atmospheres to eliminate the radicals, the temperature and the air total flow in the interior of the reactor, it is to say, the variables controlling the residence time, will be the variables that will determine the reactions undergone by the radicals. As a result, the formation of different pollutants and PAHs through a pyrosynthesis mechanism will be promoted for short residence times and, in general, for adverse conditions. (b) Influence of the Temperature Combustion. The temperature variation, when the rest of the variables are kept constant (flow and percentage of excess oxygen) will influence on the radicals nature in such way that an increase in the temperature will favor more radicals breaking into smaller ones. The higher the energy, the more bonds breaking. Moreover the chance of forming one or other PAHs is depending on the facility of addition of different radicals to a determined structure, for instance, in our particular case, the BaP and D(a,h)A formation, both compounds constituted by five rings but in different position, can be assumed by the addition of a C2 or a C4 radical to a BaA molecule (see Figure 2). Taking into account these two points, the influence of the combustion temperature has been studied for each of the three fuels. In coal combustion, see Figure 3, it is observed that BaP (Figure 3a) shows a maximum at the combustion temperature of 850 °C, D(a,h)A (Figure 3b) at 750 °C, and finally BaA (Figure 3c) shows two maximum emissions which coincide with the two anterior at 750 °C and 850 °C, respectively. BaP shows higher concentrations at high temperatures, 850 °C. It seems to be reasonable that at high-temperature small radicals as C2 will be more abundant as a consequence of a lower selectivity thermal breaking. D(a,h)A shows the highest emissions at the lowest temperatures, 750 °C, at which the probability of existing C4, larger radicals, is higher. It is also noticeable that BaP reached the highest emissions compared to the other two compounds, being that BaP is the most carcinogenic compound of the three studied (37). According to these results, the temperature that seems to produce the lowest emissions of these three compounds is out of the range of 750-850 °C and, therefore, out of the most common temperature used in coal power stations, 850 °C. VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Total distribution (ng/kg) of (a) BaP, (b) D(a,h)A, and (c) BaA in coal combustion as a function of the combustion temperature (FS, AFBC, 20% excess oxygen, 860 L/h).
FIGURE 4. Total distribution (µg/kg) of (a) BaP, (b) D(a,h)A, and (c) BaA in tire combustion as a function of the combustion temperature (FS, AFBC, 860 L/h, 20% excess oxygen). In tire combustion, and following the scheme shown in Figure 3, the higher the temperature, the higher the radicals crack. This agrees with the increase in BaP and BaA emissions (Figure 4) with the combustion temperature and with the contrary trend in D(a,h)A emissions, which decrease with high temperatures. The higher values of BaP, D(a,h)A, and BaA emissions compared to the ones obtained in coal combustion at the same conditions reflect the contribution of the incomplete combustion and so, the fuel nature, on PAHs emissions. Nevertheless, it is positive that the highest emissions are due to BaA, which is much less harmful than BaP and D(a,h)A. In coal-tire blend, the BaP, BaA, and D(a,h)A emissions are more similar to the values obtained in coal combustion and not intermediate values as it could be expected by the blend composition. For total PAH emissions this fact has been previously reported (16, 17, 28, 30). Regarding BaP and D(a,h)A emissions as a function of the temperature, both of them are emitted at low concentrations, always less than 1 2648
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FIGURE 5. Total distribution (µg/kg) of (a) BaP, (b) D(a,h)A, and (c) BaA in coal combustion as a function of the air total flow (FS, AFBC, 850 °C, 10% excess oxygen). µg/kg. BaA is the compound that shows the highest emission of the three studied. Anyway, for all of them, their distribution as a function of combustion temperature is similar showing the minimum emissions at the combustion temperature of 850 °C, temperature used in power stations. In general, the temperature influence on the radical reactions in the combustion of different fuels, coal, coal-tire blend and tire, is shown to be decisive in order to control dangerous PAHs emissions and the use of tire in atmospheric fluidized bed combustion (AFBC) originates the highest emissions of BaP, D(a,h)A, and BaA. As consequence, tire seems not to be a proper fuel to introduce in AFBC when it is burned at the same combustion conditions than coal. The necessity of establishing a normative concerning organic emissions which controls the environmental emissions is reflected again. At the different combustion temperatures studied and when tire is used as fuel, 750 °C can be considered as the right temperature to abate the emissions of BaP, D(a,h)A, and BaA, although this temperature has been reported not to be the more proper one to abate inorganic emissions (4) for the low combustion efficiency reached. (c) Influence of the Airflow. The other variable influencing on the association between radicals is the air total flow when the remaining variables are kept constant. The airflow will also be directly related to the fluidization velocity, necessary to maintain the bed fluidized, and to the residence time of the radicals in the reactor. At low flows, the residence time of the radicals in the interior of the reactor will be longer and if the working conditions are adequate, the elimination of radicals by oxidation will be favored. At high flows, the radicals will be easily entrained in the outlet stream not favoring the elimination reactions and promoting the radicals’ association (22). In coal combustion, when the air total flow influence is analyzed (Figure 5), BaP and BaA emissions follow the same trend observed for the total of PAHs emissions (22). The increase in the air total flow up to 860 L/h, which corresponds to the double of the minimum fluidization, implies a better mixture between air bubbles and the bed favoring the elimination of radicals by oxidation and decreasing BaP and BaA emissions (Figure 5a,c). Good fluidization conditions seem to be very important to abate the emissions of BaP, BaA, and D(a,h)A. Air total flows higher than 860 L/h imply an increase in BaP and BaA emissions. It can be due to that
TABLE 2. Comparison of the Total of BaP, BaA, and D(a,h)A in AFB Combustion (AFBC, 860 L/h, 20% Excess Oxygen, 850 °C) as a Function of the Fuel Burned BaP (µg/kg) BaA (µg/kg) D(a,h)A (µg/kg) a
coal
coal-tire blend
tire
0.7 0.5 0.1
