Development of Efficient Adsorbent Materials for PAH Cleaning from

One of the main anthropogenic sources of PAH emissions is the combustion of .... PAH concentration in the gas phase at these conditions was around 300...
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Development of Efficient Adsorbent Materials for PAH Cleaning from AFBC Hot Gas A. M. Mastral,* T. Garcı´a, R. Murillo, M. S. Calle´n, J. M. Lo´pez, and M. V. Navarro Instituto de Carboquı´mica, CSIC, P.O. Box 589, M Luesma Castan 4, 50018-Zaragoza, Spain Received March 16, 2003. Revised Manuscript Received July 28, 2003

The aim of this paper is to study the performance of several carbon materials on PAH abatement. For this purpose, hot gas cleaning from an atmospheric fluidized bed combustor pilot plant was analyzed by using five different porous materials. Combustion experiments were carried out at similar conditions to those in power stations: 850 °C, 2% oxygen excess, 0.260 m/s gas speed, and low rank coal as fuel. An aliquot of the combustion gases was flowed through a sampling system consisting of a Teflon filter and two adsorbents: the first was a blend of 0.1 mg of active carbon and 8.0 g of sand and the second was 1.0 g XAD-2 resin used as test adsorbent. The temperature and the space velocity of the adsorption bed were 150 °C and 5 × 104 h-1, respectively. Since there was a considerable decrease in the PAH emission concentration under ppbv levels, it was concluded that active carbons might be a promising technological solution for hazardous emission control. The active carbons that were found more appropriate for this abatement were those with a high porosity development and mean pore size around 1.4 nm, which is approximately twice the PAH molecular size.

Introduction Polycyclic aromatic hydrocarbons (PAH) constitute a group of semivolatile organic compounds released during the combustion of organic material. Although these compounds can also be produced by natural sources, their main origin is anthropogenic. One of the main anthropogenic sources of PAH emissions is the combustion of fossil fuels, a process associated with power generation. PAH originate during these combustion processes and, depending on their volatility, can be either emitted in the gas phase or supported on the particulate matter.1 Moreover, PAH have shown a varied carcinogenicity and mutagenicity.2-3 They are gradually deposited in the respiratory system, particularly affecting the bronchioles and alveoli of the lungs, and therefore they have potentially hazardous health effects. It has been estimated that about 95% of total PAH is associated with breathable particles (size less than 3 µm in diameter4,5). In power generation, fluidized bed combustion (FBC) is mainly used as a system to produce energy while * Corresponding author. Phone: 00 34 976 733977. Fax: 00 34 976 733318. E-mail: [email protected]. (1) Kamens, R.; Odum, J.; Fan, Z.-H. Environ. Sci. Technol. 1995, 29, 43. (2) NRC, National Research Council, 1983. Polycyclic aromatic hydrocarbons: evaluation of sources and effects. Committee on pyrene and selected analogues, Board on Toxicology and Environmental Health Hazard, Commission on Life Sciences, National Academy Press: Washington, DC. (3) EPA 1992, Suspect Chemical Sourcebook, Source List 10. EPA Human Health Assessment Group Substances, 2nd ed.; Clasnsky, K. B., Bathesda, M. D., Eds.; Roytech Publications Inc.: 1992. (4) Venkataraman, C.; Friedlander, S. Environ. Sci. Technol. 1994, 28, 563. (5) Baek, S. O.; Goldstone, M. E.; Kirk, P. W. W.; Lester, J. N.; Perry, R. Chemosphere 1991, 22, 503.

meeting the requirements for legislated emissions. There is a lot of information available regarding fluidized beds6-8 whose physical characteristics affect specific factors such as combustion, efficiency, economical costs, emissions, and wastes generated. The influence of the combustion parameters on the PAH distribution between the gas phase and the particulate matter in FBC has also been widely studied. Bayram9 focused on the influence of the coal particle size on the PAH distribution between both phases, concluding that the smaller the size the higher the PAH emissions, and therefore that the gas phase is the one in which the highest PAH emissions are detected. The influence of temperature, percentage of excess oxygen, superficial speed in the bed, and type of fuel used in an AFB reactor has been studied by Mastral et al., concluding that the highest PAH emissions are produced in the temperature range between 750 and 850 °C.10 Low oxygen excess percentages11 dramatically favored PAH formation and emissions and high gas speed, at which a bed deformation to the slugging regime is produced,12 is also determinant in PAH emission. The influence of the combustion process parameters in an AFBC have also been studied (6) Boyd, T. G.; Divilio, R. J. Proceeding on 9th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 1992; p 738. (7) Liu, H.; Gibbs, B. M. Fuel 1998, 77 (14), 1579. (8) Khan, W. Z.; Gibbs, B. M. Fuel 1995, 74 (6), 800. (9) Bayram, A. PhD, Generation of emission factors for PAH due to Turkish Coals burned in different types of combustion units, Izmir, March 1995. (10) Mastral, A. M.; Calle´n, M. S.; Murillo, R. Fuel 1996, 75 (13), 1533. (11) Mastral, A. M.; Calle´n, M. S.; Mayoral, M. C.; Galba´n, J. Fuel 1995, 74 (12), 1762. (12) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Fuel 1998, 77 (13), 1513.

10.1021/ef030058+ CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003

Adsorbents for PAH Cleaning from AFBC Hot Gas

by Liu et al.,13 finding that the main factors affecting PAH emissions are the oxygen excess percentage and the reactor design. In all the studies performed, the main compounds emitted were those PAH with lower carcinogenic character (PAH with 2, 3, and 4 aromatic rings). This kind of distribution was also observed when other types of fuels, i.e., nonfossil fuels such as tire14-16 or biomass,17 were burned in the fluidized bed. The studies carried out on emissions of organic compounds in combustion processes have shown that, no matter what fuel is used, these compounds are emitted to the atmosphere as gas phase or supported on the particulate matter.18 Taking all these facts into consideration, it seems necessary to develop two different systems to control the total emission of organic compounds: one which is able to trap the emissions supported on the particulate matter and another one for the emissions produced in the gas phase. The control of emissions supported on the particulate matter has focused mainly on systems which are efficient enough to retain particles.19 These mechanisms for AFBC are based on the combination of several control systems such as high efficiency cyclones,20 fabric filters,21,22 and electrostatic precipitators23 which reach efficiencies higher than 99%. When pressurized fluidized bed combustion (PFBC) is used, all these systems are replaced by ceramic filters which show higher efficiency on particle retention and can be operated at high temperature.24 Several technologies for the control of gas-phase emissions have been developed in recent years.25,26 The most promising seem to be biofiltration, absorption in membranes, catalytic destruction, and adsorption by high surface area materials (especially active carbons27,28). So far, the existing literature on PAH emission control at real conditions is scarce and limited to a qualitative study of the process. Weber et al.29 and Liljelind et al.30 studied the total oxidation of PAH by commercial catalysts, REMEDIA D/F Catalytic Filter (13) Liu, K.; Han, W.; Pan, W.-P.; Riley, J. T. J. Hazard. Mater. 2001, 84 (2-3), 29/175-188. (14) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Environ. Sci. Technol. 1999, 33, 3177. (15) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Garcı´a, T. Energy Fuels 2000, 14, 275. (16) Levendis, Y. A.; Atal, A.; Carlson, J. B. Environ. Sci. Technol. 1998, 32, 3767. (17) Sondreal, E. A.; Benson, S. A.; Hurley, J. P.; Mann, M. D.; Pavlish, J. H.; Swanson, M. L.; Weber, G. F.; Zygarlicke, D. J. Fuel Process. Technol. 2001, 71 (1-3), 7. (18) Mastral, A. M.; Calle´n, M. S. Environ. Sci.Technol. 2000, 34, 3051. (19) Lohmann, R.; Ockenden, W. A.; Shears, J.; Jones, K. C. Environ. Sci. Technol. 2001, 35, 4046. (20) Vesilind, P. A. Environmental Pollution and Control; Ann Arbor Science: Michigan, 1975. (21) Yoa, S. J.; Cho, Y. S.; Choi, Y. S.; Baek, J. H. Korean J. Chem. Eng. 2001, 18 (4), 539. (22) Goryachev, I. K. Chem. Pet. Eng. 1998, 34 (11-12), 747. (23) Ritcher, L. A. Staub Reinhalt Luft 1993, 53 (2), 41. (24) Saracco, G.; Specchia, V. Chem. Eng. Sci. 2000, 55, 897. (25) Alvim Ferraz, M. C. M.; Mo¨ser, S.; Tonhaa¨user, M. Fuel 1999, 78, 1567. (26) Alonso, M.; Lorences, M. J.; Pina, M. P.; Patience, G. S. Catal. Today 2001, 67, 151. (27) Lowell, S.; Shields, J. E. Powder, Surface Area and Porosity, 3rd ed.; Chapmand & Hall: London, 1991. (28) Davini, P. Carbon 2001, 39 (14), 2173. (29) Weber, R.; Plinnke, M.; Xu, Z.; Wilken, M. Appl. Catal., Bs Environmental 2001, 31, 195. (30) Liljelind, P.; Unsworth, J.; Maaskant, O.; Marklund, S. Chemosphere 2001, 42, 615.

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System and Shell Dioxin Destruction System, respectively. They found that these catalysts were appropriate to control PAH emissions at low temperatures (150 °C), reaching efficiencies of 98% and over. On the other hand, the PAH hot gas cleaning by active carbons was performed by Grodten et al,31 Cudahy et al.,32 and Chiang et al.33 These authors show that the PAH emissions from a waste incinerator could be abated under detection limits with active carbons by using different technologies such as fixed bed reactor, active carbon injection, or fluidized bed reactor, respectively. Simultaneously, a quantitative study of PAH adsorption on active carbon from model compounds was developed by Mastral et al., who studied the performance of active carbons during the adsorption not only of one compound34,35 but also of binary mixtures,36 finding that the total microporosity is the main factor controlling the adsorption process. In addition, other characteristics of active carbons that favor the adsorption process are (a) a micropore size distribution higher than 0.7 nm, where PAH molecules do not find diffusional problems; (b) high development of the mesoporosity, because mesopores not only drive the adsorbate molecules to the micropores but also promote the multilayer interactions increasing the equilibrium adsorption capacity; and (c) low surface acidity, due to both the hydrophobic nature and the lower humidity adsorption capacity of the PAH. The analytical techniques used to determine PAH have been numerous, depending on the matrix complexity. They can be classified as online methods and discontinuous methods. The online methods37,38 have the advantage of measuring PAH emissions directly from the emission source providing high measurement speed and turning them into powerful tools for online emission control of processes. The analytical techniques must also show high sensitivity, as most compounds to be analyzed are emitted at low concentrations and therefore high sample volumes would be necessary. Despite all these advantages, the main drawback of online methods is related to their cost as a consequence of the prohibitive price of most of these techniques. In these cases, discontinuous methods are widely used. Gas chromatography (GC), capillary GC linked to mass spectrometry analysis39,40 or to Fourier transform infrared spectroscopy,41 high-pressure liquid chromatography with spectrometric detection, either UV-visible absorption42 or the most sensitive fluorescence,43,44 have been used. One of these techniques, which allows the (31) Grodten, T.; Schmidt, D.; Dannecker, W. Gefahrstoffe-Reinhaltung der Luft 1998, 58, 205. (32) Cudahy, J. J.; Helsel, R. W. Waste Management 2000, 20, 339. (33) Chiang, B. C.; Wey, M. Y.; Yang, W. Y. J. Environ. Eng. 2000, 126 (11), 985. (34) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V. Energy Fuels 2002, 16 (1), 205. (35) Mastral, A. M.; Garcia, T.; Callen, M. S.; Navarro, M. V.; Galba´n, J. Environ. Sci. Technol. 2001, 35 (11), 2395. (36) Mastral, A. M.; Garcia, T.; Murillo, R.; Callen, M. S.; Lopez, J. M.; Navarro, M. V.; Galban, J. Energy Fuels 2003, 17 (3), 669-676. (37) Heger, H. J.; Zimmermann, R.; Blumenstock, M.; Kettrup. A. Chemosphere 2001, 42 (5-7), 691. (38) Zimmermann, R.; Heger, H. J.; Blumenstock, M.; Dorfner, R.; Nikolai, U.; Schramm, K. W.; Kettrup, A. Organohalogen Compd. 1999, 40, 321. (39) Pandit, G. G.; Srivastava, P. K.; Mohan Rao, A. M. Sci. Total Environ. 2001, 279 (1-3), 159-165. (40) Pino, V.; Ayala, J. H.; Afonso, A. M.; Gonza´lez, V. J. Chromatogr. A 2000, 869 (1-2), 515. (41) Gurka, D. F. Detectors for Capillary Chromatography; Hill, H. H., McMinn, D. G., Eds.; Eiley: New York, 1992; Chapter 11.

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Figure 2. Scheme of the hot sampling system used to trap PAH from AFBC.

Figure 1. Scheme of the atmospheric fluidized bed combustion (AFBC) laboratory scale pilot plant.

analysis of different PAH simultaneously without separation, is synchronous fluorescence spectrometry.45,46 In this context, this work goes a step forward in the field of hot gas cleaning because, taking advantage of the experience acquired working with model compounds,35,36 five porous materials have been tested to study their performance on PAH retention at real conditions, cleaning up a hot gas stream generated by an atmospheric fluidized bed combustion (AFBC) pilot plant. Experimental Section An AFBC laboratory pilot plant described in a previous work,14 see Figure 1, was used to perform the coal combustion runs. The fuel was a low-rank coal of 0.5-1 mm particle size with an ultimate analysis: %C, 73.8 (daf); %N 0.9 (daf); %S, 6.3 (db); %H, 6.4 (daf); immediate analysis: % moisture, 15.7 (ar); % ashes, 23.9 (ar); % volatiles, 15.0 (ar); % fixed carbon, 45.4 (ar); and calorific value, 17.3 MJ/kg. The combustion experiments were carried out keeping the gas speed (0.260 m/s), the percentage of oxygen excess (2%), and the temperature (850 °C) constant and varying the porous materials located at the exit of the gas stream from the AFBC pilot plant. The hot sampling system shown in Figure 2, was used in order to analyze the behavior of porous materials on PAH retention. An aliquot of the combustion gases was taken at the exit of the second cyclone. The gas stream was forced to pass through the interior of a furnace at a temperature of 150 (42) Lagesson-Andrasko, L.; Lagesson, V.; Andrasko, J. Anal. Chem. 1998, 70, 819. (43) Baudot, Ph.; Viriot, M. L.; Andre´, J. C.; Jezequel, J. Y.; Lafontaine, M. Analusis 1991, 19, 85. (44) Patra, D.; Mishra, A. K. Talanta 2001, 55 (1,3), 143. (45) Mastral, A. M.; Calle´n, M. S.; Garcı´a, T. Fuel Process. Technol. 2000, 67 (1), 1-10. (46) Mastral, A. M.; Callen, M. S.; Galban, J. Fuel 1995, 74, 1762.

°C. In this furnace, the gas stream was led through the sampling system consisting of a Teflon filter, 1 µm pore size and 5 cm diameter, and two adsorbents: the first one was a blend of porous material and sand while the second contained XAD-2 resin as test adsorbent. The porous material was mixed with sand to avoid the axial dispersion in the adsorption bed, which could alter the results. The space velocity in the carbon material bed was 5 × 106 h-1. The combustion gases were flowed through a bubbling system with Na2CO3 and desiccators with silica gel to protect the vacuum pump from acid substances and humidity present in the gas stream. In the same way, the general gas stream was flowed through a bubbling system with Na2CO3 and a condenser to protect the gas counter placed at the end of the line. The aliquot was taken once the system had reached a steady state and the sampling furnace was stabilized at 150 °C. Each combustion experiment consisted of several stages stopping the exhaust gas sampling to replace the XAD-2 test resin while the combustion process was still running. Once the test resin was changed, the gas flow aliquot was again conducted through the sampling system, which still conserved the initial carbonaceous adsorbent. Experiments lasted until the porous material was saturated, which was determined by the presence of PAH emissions in the XAD-2 test resin. The breakthrough volume of each material was determined by a discontinuous method. In this method, the fluorescence spectrum of XAD-2 blank was compared with those spectra obtained in the test adsorbent in each stage in which the sampling process was divided. All the spectra were obtained on a model LS-50 computer-controlled spectrofluorimeter (Perkin-Elmer) equipped with a xenon discharge lamp and composed of two monochromators for excitation and emission. The excitation and emission slits were 2.5 nm, respectively, and fluorescence measurements were performed using standard 1 × 1 quartz cells. All spectra were scanned at a rate of 240 nm cm-1. The XAD-2 resin was extracted in ultrasonic bath three times using dimethylformamide (DMF) as solvent because DMF did not show interference with fluorescence signals and allowed PAH extraction. The final extract volume was 10 mL once the solution was previously filtered with syringe through a Teflon filter of 0.5 µm Millipore Millex LCR 0.5 µm PTFE and analyzed by FS. Five carbon materials of different origins were used to study their behavior in the adsorption of PAH vapor mixtures: CA-3 was a commercial coke from German Rhenish lignite supplied by RWE Rheinbraun. From this German lignite two more samples were produced, GFV1100 and GFV3890. They were treated in a fixed reactor by a carbonization process to reduce

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Table 1. Carbon Material Morphological Properties textural parameters sample

SBET (m2/g)

Vmicrop (cm3/g)

Vt (cm3/g)

Lmicrop (nm)

Vmicrop/Vt

CA-3 CA-8 CA-13 GFV3890 GFV1100

172 547 532 405 202

0.079 0.28 0.28 0.21 0.11

0.21 0.41 0.48 0.33 0.12

1.1 1.1 1.1 1.0 1.0

0.376 0.683 0.583 0.636 0.917

the volatile matter present in the original solid. The processes were carried out in a reactor consisting of a stainless steel vertical cylinder, 54 cm high with an inner diameter of 15 cm. The two cokes were heated at 8 °C/min up to 800 °C and 1000 °C, respectively. The soaking time was 8 h for both samples. CA-8 was an active carbon from apricot stones. It was obtained in a two-step process. In the first step, apricot stones were carbonized in nitrogen at 800 °C for 2 h. In the second step, the carbonization product was activated with steam at 800 °C for 2 h and a steam flow rate of 10 g/min. The heating rate was 20 °C/min for both steps. Finally, CA-13 corresponded to an activated carbon from Spanish lignite. The preparation of this activated carbon was carried out by lignite carbonization, followed by activation in steam. Carbonization and activation were both carried out in a horizontal furnace. The carbonization was conducted in a N2 flow (80 mL/min) at 850 °C with a heating rate of 5 °C/min for 2 h. The activation process was carried out at the same temperature and heating rate. The flow gas used was a steam/N2 mixture (0.2 g/min) for a reaction time of 2 h. The five porous carbons were characterized by N2 adsorption at -196 °C using a Quantachrome Autosorb 1. Prior to the experiment, the samples were heated at 200 °C and then outgassed overnight at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface areas (S), micropore volumes (Vmic), average micropore sizes (Lmic), and micropore size distributions, using the density functional theory (DFT).47 The total pore volume (Vt) was obtained from the N2 volume adsorbed at a relative pressure of 0.95. The experimental error due to sample heterogeneity was around 3%, depending on the sample.

Results and Discussion Combustion experiments were performed at the closest conditions to those which can be found at actual power stations. These conditions were: 850 °C, 2% excess oxygen, and gas speed corresponding to the double of the minimum fluidization speed. Previously published works48,49 have shown that, at these conditions, PAH are mainly emitted in the gas phase where their concentration ranges from 100 to 650 ng/m3 depending on power station, fuel, operating conditions, etc. In the case of the above-mentioned experimental equipment, it was found that the average PAH concentration in the gas phase at these conditions was around 300 ppb. The performance of five carbon materials in the PAH hot gas cleaning from this coal AFBC pilot plant was studied. The morphological characteristics of these carbons calculated from nitrogen adsorption isotherms are shown in Table 1. It can be observed that CA-13 has the highest porosity whereas CFV1100 shows the (47) Lastoskie, C. M.; Gubboms, K. E.; Quirke, N. J. Phys. Chem. 1993, 97, 4786. (48) Masclet, P.; Bresson, M. A.; Mouvier, G. Fuel 1981, 66, 556. (49) Calvo Revuelta, C.; de la Fuente Santiago, E.; Rodrı´guez Vazquez, J. A. Environ. Technol. 1999, 20, 61.

Figure 3. Micropore size distribution. (a) CA-3, gfv3890 and gfv1100, (b) CA-8, and (c) CA-13.

lowest. A different trend is shown for the micropore volumes and surface areas where, for example, CA-8 and CA-13 have similar values and higher than those of the other adsorbents. The ratio of the micropore volumes to the total pore volume reflects the degree of microporosity. The analysis of the values clearly shows that the most homogeneous, from the point of view of the microporosity, is GFV1100 followed by CA-8, GFV3890, and CA-13, whereas CA-3 is the most heterogeneous sample. The five carbon materials show similar values in the micropore mean size, Lmicr. These parameters also show that the carbonization process at 800 °C on the CA-3 coke increases its porosity and its degree of microporosity, whereas a higher carbonization temperature, 1000 °C, decreases the total porosity due

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Figure 4. Fluorescence synchronic spectra with ∆λ ) 5 nm. Blank and XAD-2 testing resin for different volumes taken when the adsorption bed is sand.

to the graphitization process on the adsorbent structure. In fact, the original coal despite being a lignite, it is a coking coal. More details about the differences in micropore sizes can be obtained from the analysis of the pore size distributions (PSDs), see Figure 3. CA-3 pore size distribution is shown in Figure 3A. It is observed that this coke has a unimodal distribution with the majority of pores between 0.7 and 1.4 nm, 1 to 2 times molecular size pores, where the PAH retention could be favored by the overlapping of the adsorption potentials. It can be expected that the carbonization process produces a change in the porosity. In this way, when a closer look to the sizes and micropore volumes of GFV3890 is taken (Figure 3A) both an increase in the pore volume and a change in the PSD is observed, creating a peak corresponding to smaller than 0.7 nm, which may be a critical size for the PAH adsorption. However, the increase produced in the pore volume of pores between 0.7 and 1.4 nm could favor the retention process. As for the hightemperature carbonization adsorbent, GFV1100, it can be observed that the resulting material has a narrower microporosity where the mesopores are destroyed by a graphitization process. Finally, CA-8 and CA-13 have a wide unimodal distribution with a significant volume in pores between 1.4 nm to 2 nm, where the PAH adsorption could take place through cooperative effects. From an environmental point of view, an adsorbent, in this case a porous material, is efficient in combustion gas stream cleaning when it is able to keep the pollutant emissions under a limit value. Taking this into consideration, the measurement of the porous material efficiency was determined from the synchronic fluorescence analysis at ∆λ ) 5 nm of a test adsorbent downstream the active carbon bed. From these spectra, it would be possible to determine in a semiquantitative way the breakthrough point of the adsorbents because the PAH presence produced a variation of the spectrum baseline. Figure 4 shows this variation when sand was used as adsorbent bed and XAD-2 resin as test adsorbent. This Figure 4 corroborates that the analytical method is sensitive enough to determine the breakthrough point, and therefore, to study the performance of the different active carbons used in this work. The process was carried out for the five adsorbents described above. In these active carbons, the analysis of the fluorescence synchronic spectra (∆λ ) 5) of the

Figure 5. Fluorescence synchronic spectra with ∆λ ) 5 nm. Blank and XAD-2 testing resin for different volumes taken when the adsorption beds are the adsorbents: (a) CA-3, (b) C-13, and (c) gfv1100.

test resin, for different volume of the combustion gases flowed through the adsorption bed, allowed the determination of the adsorbent breakthrough point. For each adsorbent, it is considered that the breakthrough point is reached when the baseline value of its fluorescence spectrum increases. The spectra of three of these active carbons are shown in Figure 5, and the breakthrough values of the five studied adsorbents are compiled in Table 2. Data show that the efficiency of these adsorbents in the combustion gas cleaning follows this order: GFV1100 < GFV3890 ≈ CA-3 < CA-8 < CA-13. This behavior is different from the one expected, because no correlation is found between the adsorption efficiency and the Vmicrop or Vt as it was found for the model compound adsorption, single compounds,34,35 and binary mixtures,36 respectively. It is observed that GFV1100 and GFV3890 show breakthrough volumes lower than those corresponding to their Vt values.

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Table 2. Breakthrough Volume Range [L] Obtained for the Active Carbons and Total Pore Volume Higher than 1.4 nm (madsorbent ) 0.1 g, T ) 150 °C, and space velocity ) 5 × 104 h-1) Vb V0 > 1.4 nm

CA-3

CA-8

CA-13

GFV3890

GFV1100

1000-1500 0.14

2000-2500 0.20

2500-3000 0.27

1000-1500 0.14

0-250 0.06

So, other textural characteristics must also influence the adsorption process. After a careful study of the results obtained, a possible adsorption mechanism could be postulated, assuming that only the nonmicroporous surface is taking an active role in PAH abatement (see Table 1). According to breakthrough volume data, Table 2, GFV1100 has hardly any active surface where the PAH retention can take place, CA-3 and GFV3890 have a similar behavior, and finally, the nonmicroporous volume shows the highest activity for CA-13. However, the CA-8 adsorbent does not follow this trend because its breakthrough volume is higher than the one expected from the value of its nonmicroporous surface, similar to the ones observed for CA-3 and GFV3890. A thorough study of the CA-8 PSD shown in Figure 3B indicates that there is a noticeable contribution of micropores higher than 1.4 nm to the total porosity. These supermicropores will favor the PAH retention by cooperative effects. The total pore volumes higher than a pore size of 1.4 nm are compiled in Table 2. It can be observed that there is a high correlation between these values and the breakthrough volumes, r2 ) 0.98. Therefore, the experimental results seem to point out that an important pore size that should be considered for the PAH adsorption process is around 1.4 nm, i.e.,

twice the mean PAH molecular size. As a consequence, the molecular size micropores, which were considered as the most relevant textural characteristic in model compound adsorption,34-36 do not seem to play an important role in the PAH adsorption at real conditions. This fact could be explained taking into account that other smaller volatile organic compounds also emitted in combustion processes and at higher concentrations, together with the water molecules present in the gas stream, could be competing and blocking the narrowest adsorption sites. Moreover, from this study it can be deduced that the carbonization process of a lignite coke does not develop an appropriate porous structure in the active carbons. Even though at low temperatures the total porosity is increased, pores lower than 1.4 nm are mainly created (see Figure 3A) which are not useful for PAH abatement. On the other hand, at high temperatures, not only narrow micropores are created, but also the nonmicroporous surface is destructed (see Table 1). Therefore, the PAH adsorption process is even more impeded. The analysis of the synchronic fluorescence spectra can also be utilized from a qualitative point of view to study the compound types, which are present in the gas combustion and are not retained by the porous adsor-

Table 3. PAH Fluorescence Intervals in the Synchronic Fluorescence Spectra of ∆λ ) 5 nm

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Energy & Fuels, Vol. 18, No. 1, 2004

bent. The determination of these compounds is carried out taking into account the wavelength at which the fluorescence peaks appear on the synchronic spectra of ∆λ ) 5 nm and comparing to the model compound spectra. The possible compounds, which can be found at different wavelengths, are shown in Table 3 and they were obtained from bibliographic data.50,51 Figure 5A shows the spectra obtained for the CA-3 saturation process. It can be observed that the breakthrough volume of CA-3 adsorbent is produced from the 1000 L of combustion gases. According to this, the baseline value of the spectrum is kept practically constant up to this point; in the following 500 L of the combustion gases taken, the breakthrough is produced. The synchronic spectrum of this resin shows two areas with peaks clearly differentiated. In the first one, between 295 and 320 nm, the presence of species mainly with only one conjugated aromatic ring (although more than one ring can be present in the molecule) is observed. According to the data shown in Table 3, the kind of species present will be mainly compounds such as fluorene (Fu), biphenyl (Bf), and/or their derivatives. In this area, the presence of compounds such as naphthalene (,Np) can also be expected. The other area, between 315 and 340 nm corresponds to PAH with two conjugated aromatic rings. Some of the compounds which can be found at these wavelengths are shown in Table 3. Once the breakthrough is produced, a new gas sample of 500 L was taken to check the adsorbent breakthrough. The synchronic spectrum of this new resin shows three zones of different peaks. The first zone, the same as in the previous sample, is located between 295 and 320 nm and so it will mainly correspond to the presence of Fu, Bf, or Np compounds. The peak intensity in this case is higher because the sample gas volume was higher. The second zone is between 320 and 370 nm. The comparison of these values with those obtained for the model compounds indicate that it is mainly composed of 2 and 3 conjugated aromatic rings, although the presence of compounds with a higher number of conjugated aromatic rings, such as pyrene (Py) and chrysene (Cry), could also be obtained. The last zone placed between 370 and 410 nm, corresponds to the presence of compounds with 4 or more aromatic rings. Other compounds with a lower number of rings, such as anthracene (An) and its derivatives, also appear in this area. Finally, the adsorbent saturation is checked repeating the last sample taken and obtaining results similar to the ones obtained before. For this reason, they have not been introduced in Figure 5. It can be noticed that, much in the same way as in the model compounds adsorption for pure components and binary mixtures, the compound breakthrough is produced according to decreasing volatility in the real combustion processes. A similar behavior can be observed in the other active carbons shown in Figure 5B,C. (50) Katoh, T.; Yokoyama, S.; Sanada, Y. Fuel 1980, 59, 845. (51) Vo-Dinh, T. Chemical Ana´ lisis of Polycyclic Aromatic Compounds; Wiley: New York, 1989.

Mastral et al. Table 4. Normalized Efficiency Comparison of Adsorbents between Real Conditions (AFBC) and Model Compound Adsorption single adsorption

binary adsorption

adsorbent

Np

Phe

Phe + H2O

Np/Phe

CA-3 CA-8 CA-13

1.0 3.2 3.2

1.0 2.2 3.0

1.0 2.0 3.0

1.0 2.5 3.3

real conditions 1.0 2.0-2.5 3.0-3.5

Finally, a comparison was carried out between the performance of active carbons working at real conditions and those of the model compound (see Table 4). This comparison was carried out by normalizing the adsorption results to the CA-3 activated carbon efficiency in all the experimental conditions studied. CA-3, CA-8, and CA-13 were chosen to carry out the whole study because they have appropriate porous structures in order to analyze the effect of different morphologies on the adsorption process. GFV1100 was discarded because it has an excessively narrow porosity and, as shown in Table 2, its performance at real conditions was the worst. GFV3890 was also discarded because of its similar morphology to CA-3. This similarity made them show a similar performance in the hot gas cleaning at real conditions. It can be observed that the breakthrough volumes at real conditions found for CA-3, CA8, and CA-13 have the same relationship as the one found when the adsorbent efficiencies of Phe,35 Phe in a gas stream with 10% H2O,34 and Np-Phe36 binary mixtures were studied. In this way, and as was observed for model compounds, it can be deduced that the PAH adsorption is a physical process in which total porosity is the most important textural characteristic of active carbons. However, the critical size pore is different for model compounds and for real condition adsorption because of the presence of other smaller and more volatile organic molecules that could be competing for the micropores. Moreover, the higher concentration of these small molecules in the flue gases, compared to the PAH concentrations, allows them to fill most of the micropores. Due to their higher volatility36 the breakthrough of these molecules occurs before that of the PAH but it is not detected because these small molecules are not fluorescent enough. Therefore, model compound adsorption mainly occurs in the 1 to 2 times molecular size pores (0.7-1.5 nm), whereas PAH adsorption at real conditions seems to take place only in pores more than twice as high as their molecular size. Acknowledgment. The authors thank ECSC and the Spanish Environment Ministry for their partial financial support (Projects 7220/EC/089 and Amb-168), the Regional Government of Arago´n, DGA, Spain, (T. Garcı´a, and J. M. Lopez fellowships) and the Spanish Ministry of Science and Technology, “Ramo´n y Cajal” Program (M. S. Calle´n and R. Murillo contracts). EF030058+