Energy Fuels 2009, 23, 4325–4336 Published on Web 08/11/2009
: DOI:10.1021/ef9003945
Effects of Agglomeration Processes on the Emission Characteristics of Heavy Metals under Different Waste Compositions and the Addition of Al and Ca Inhibitors in Fluidized Bed Incineration Jia-Hong Kuo,† Chiou-Liang Lin,‡ and Ming-Yen Wey*,† †
Department of Environmental Engineering, National Chung Hsing University, Taichung, 402, Taiwan, ROC, and ‡Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, 811, Taiwan, ROC Received May 3, 2009. Revised Manuscript Received July 10, 2009
The agglomeration that occurs in a fluidized bed incineration system results in the unexpected shutdown of the system and causes secondary pollutants such as heavy metals to be generated. Addition of Al- and Ca-based additives is an efficient method to control the particle agglomeration. However, emission behavior of heavy metals in incineration processes could be influenced by the addition of agglomeration inhibitors due to both chemical reaction with metals and the fluidization behavior being changed. Accordingly, the aim of this study is to show the emission characteristics of heavy metals (Pb, Cr, and Cd) under various waste compositions (Cl and S) and the addition of agglomeration inhibitors (Al and Ca) during the defluidization process. A thermodynamic equilibrium simulation was used to predict heavy metal formation and to compare the experimental results. The results showed that the emission concentration of heavy metals increased as a result of the reduction of combustion efficiency caused by Al and Ca during agglomeration inhibition processes. In particular, the heavy metal Cr vaporized easily at the initial stage of the defluidization process, indicating that the chemical reactions changed between Na and Cr by adding Al into the system at high temperature. The different affinities of both Al and Cr reacting with Na in the fluidized bed were considered the main reason for the Cr emission. On the other hand, when the wastes contained Cl, the emission concentration of heavy metals increased because of the formation of low-boiling point metallic chlorides. The addition of Na caused bed agglomeration and decreased the emission of Cd and Cr, compared to when Cl was added. However, Cl was the dominant factor on heavy metal Pb. The emission concentration of heavy metals also decreased evidently when both Na and S were added during defluidization. The formation of high-boiling point sulfates was observed on the surface of the sand bed while heat energy accumulated during defluidization by Na. Moreover, the results of the thermodynamic equilibrium simulation demonstrated that the Cl content easily reacted with the three heavy metals. In addition, Ca influenced the emissions of metals Pb and Cr; however, Cd was affected by Al.
In incineration processes, agglomeration is caused by the combustion of wastes containing adhesion materials such as alkalis, alkaline-earth metals, sulfur, and chlorine, which affect agglomeration and defluidization.7-11 Generally, agglomeration and defluidization occur in the fluidized bed as a result of particles sintering. There are two main types of adhesive mechanisms that cause agglomeration in the fluidized bed: (1) viscous flow sintering and (2) melting and chemical reaction producing liquid-phase materials.12 Yan et al.11 described how the clinker probably formed at a high temperature following the mechanism of viscous flow sintering; likewise, crystalline transformation from quartz to cristobalite and tridymite might be indications of ash agglomeration. Table 1 showed the previous papers that are dealing with the topic of ash/bed agglomeration
Introduction Waste incineration has the advantages of stabilization and reduction. It has become a popular and important waste treatment process in Taiwan, where land is scarce and the population is highly dense. It is imperative for Taiwan to further develop its waste incineration system because of the large amount of waste the country generates every day. Notably, a fluidized bed incinerator has the following advantages over other kinds of incinerators: high heat and mass transfer, good materials mixing, and excellent combustion efficiency.1-4 However, during the process of fluidized bed incineration, an agglomeration of inlet waste with complex composition or impure bed materials may occur. This leads to a reduction in the quality of fluidization and causes defluidization in the fluidized bed.5,6 *To whom correspondence should be addressed. Telephone: þ886-422852455. Fax: þ886-4-22862587. E-mail:
[email protected]. (1) Kuo, J. H.; Lin, C. L.; Wey, M. Y. Energy Fuels 2008, 22, 3789– 3797. (2) Basu, P., Combustion and Gasification in Fluidized Beds; Taylor & Francis Group: 2006; pp 10-11. (3) Baeyens, J.; Van Puyvelde, F. J. Hazard. Mater. 1994, 37, 179–190. (4) Van de Velden, M.; Dewil, R.; Baeyens, J.; Josson, L.; Lanssens, P. J. Hazard. Mater. 2008, 151, 96–102. (5) Tardos, G.; Pfeffer, R. Powder Technol. 1995, 85, 29–35. (6) Steenari, B. M.; Lindqvist, O. Biomass Bioenerg. 1998, 14, 67–76. r 2009 American Chemical Society
(7) Kuo, J. H.; Wey, M. Y.; Lin, C. L. Fuel Process. Technol. 2008, 89, 1227–1236. (8) Lin, C. L.; Wey, M. Y. Fuel 2004, 83, 2335–2343. (9) Lin, C. L.; Wey, M. Y.; Yu, W. J. Combust. Flame 2005, 143, 139– 149. (10) Yan, R.; Liang, D. T.; Laursen, K.; Li, Y.; Tsen, L. T.; Tay, J. H. Fuel 2003, 82, 843–851. (11) Yan, R.; Liang, D. T.; Tsen, L. Energ. Convers. Manage. 2005, 46, 1165–1178. (12) Skrifvars, B. J.; Hupa, M.; Backman, R.; Hiltunen, M. Fuel 1994, 73, 171–176.
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Table 1. Recently Publications Dealing with the Topic of Ash/Bed Agglomeration in Fluidized Bed author Fern andez Llorente et al., 2006, 2008 Davidsson et al., 2007 Lin et al., 2004 Bhattacharya and Harttig, 2003 Vuthaluru and Zhang, 2001 Baeyens and Van Puyvelde, 1994
inlet sources
possible elements for agglomeration
operating temperature
biomass
800-1000 °C
potassium
forestry residues Na added artificial waste high-alkali, highsulfur lignites Bowmans coal
870-900 °C 700-900 °C
potassium and sodium sodium
800-850 °C 800 °C
sodium, chlorine, sulfur, calcium, and magnesium sodium, and potassium
sludge
700-900 °C
alkalis and alkalines
in fluidized bed. Moreover, Ohman and Nortin19 also discussed that melting of the coating was directly responsible for the bed agglomeration, and the melting behavior of the coatings was very sensitive to the relative amount of alkalis present. Thus, bed agglomeration occurred when wastes contained alkalis and thus became a serious problem for fluidized bed operation. To prevent agglomeration in a fluidized bed during incineration, some researchers illustrate that the use of additives is an effective way to avoid agglomeration/defluidization.19-22 Several additives were tested for reducing and controlling the bed agglomeration phenomenon. Elements such as Al and Ca were found to inhibit agglomeration in a fluidized bed. Previous studies have discussed that kaolin has been shown to be effective for deposit control, alkali vapor removal, and bed agglomeration control.19,23,24 Moreover, Ca-related additives have been used as alternative bed materials to control the agglomeration behavior.25 Vuthaluru et al.26-29 studied the control of agglomeration and defluidization during fluidized bed combustion. Their findings showed that sillimanite (Al2SiO5), limestone (Ca-rich), and bauxite (Al-rich) as alternative materials extended fluidized bed combustion operation without defluidization by 7-10 times compared to silica sand as the bed material. Similar results were observed from other research.8,12,13 Consequently, Al- and Ca-related additives were shown to be effective for particle agglomeration control.
agglomeration control strategy
ref
limestone, lime, kaolin, alumina, ophite, dolomite and calcined dolomite addition of kaolin (Al-riched) addition of Ca- and Mg-bearing chemicals
13,14 15 15
acid-washed and calcium-exchanged
8
addition of Ca- and Mg-bearing minerals (calcite and magnetite) addition of coarse solids, clay, and special metal oxides in the feed
17 18
When agglomeration occurs in fluidized bed incineration systems, many fluidization characteristics are altered and the system unexpectedly shuts down, leading to the generation of secondary pollutants. During the waste incineration process, heavy metals can not be destroyed, but they can be highly volatile at a high temperature and thus can be released in the air along with the particulates. These heavy metals emitted from incinerators can leach from ash to contaminate our environment.4 It is a serious topic; hence, the effects of different operating conditions on the formation of metallic compounds must be examined. Heavy metals can react with Cl, S, and O during incineration to produce various compounds. Several studies indicated the emission of heavy metals in municipal solid waste (MSW) incineration.30-36 Accordingly, chlorine has a significant effect on the behavior of metallic species. High HCl concentration in the gas steam maybe increase the volatilization of some specific heavy metals and produce chlorides, especially for heavy metal Pb. In addition, other heavy metals like Cr and Cd existed in wastes not only formed as chlorides but also reacted with oxygen to become metal oxides at high temperature.31 The major sources of organic and inorganic chlorine in MSW are polyvinyl chlorine (PVC) and NaCl, respectively. Many studies have illustrated that there is an increase in the formation of HCl gas during the thermal process when chlorine-related sources exist in the combustion system.34 On the other hand, sulfur is also one of the elements that affect the emission of heavy metals at a high temperature. Some metallic compounds react with sulfur existing in the system to form sulfides at a high boiling point. On the basis of the above discussions, the complex compositions of municipal solid waste may cause the emission characteristics of heavy metals and agglomeration behavior to change in the fluidized bed. The addition of bed additives such as Al- and Ca-based compounds not only controls particle agglomeration/defluidization, but also influences the emission behavior of heavy metals during the control process. The effect of both waste compositions and
(13) Fern andez Llorente, M. J.; Dı´ az Arocas, P.; Gutierrez Nebot, L.; Carrasco Garcı´ a, J. E. Fuel 2008, 87, 2651–2658. (14) Fern andez Llorente, M. J.; Escalada Cuadrado, R.; Murillo Laplaza, J. M.; Carrasco Garcı´ a, J. E. Fuel 2006, 85, 2081–2092. (15) Davidsson, K. O.; Steenari, B.-M.; Eskilsson, D. Energy Fuels 2007, 21, 1959–1966. (16) Bhattacharya, S. P.; Harttig, M. Energy Fuels 2003, 17, 1014– 1021. (17) Vuthaluru, H. B.; Zhang, D.-K. Fuel Process. Technol. 2001, 69, 13–27. (18) Baeyens, J.; Van Puyvelde, F. J. Hazard. Mater. 1994, 37, 179– 190. (19) Ohman, M.; Nordin, A. Energy Fuels 2000, 14, 618–624. (20) Bhattacharya, S. P.; Harttig, M. Energy Fuels 2003, 17, 1014– 1021. (21) Bartels, M.; Lin, W.; Nijenhuis, J.; Kapteijn, F.; van Ommen, J. R. Prog. Energy Combust. Sci. 2008, 34, 633–666. (22) Ohman, M.; Bostrom, D.; Nordin, A. Energy Fuels 2004, 18, 1370–1376. (23) Linjewile, T. M.; Manzoori, A. K. Proceedings of the Engineering Foundation Conference on Impact of Mineral Impurities in Solid Fuel Combustion, Hawaii, November; 1997. (24) Dahlin, R. S.; Dorminey, J. R.; Peng, W.; Leonard, R. F.; Vimalchand, P. Energy Fuels 2009, 23, 785–793. (25) Tangsathitkulchai, C.; Tangsathitkulchai, M. Fuel Process. Technol. 2001, 72, 163–183. (26) Vuthaluru, H. B.; Zhang, D. K. Fuel Process. Technol. 2001, 70, 41–51. (27) Vuthaluru, H. B.; Zhang, D. K. Fuel 2001, 80, 583–598. (28) Vuthaluru, H. B.; Zhang, D. K.; Linjewile, T. M. Fuel Process. Technol. 2000, 67, 165–176. (29) Vuthaluru, H. B.; Linjewile, T. M.; Zhang, D. K.; Manzoori, A. R. Fuel 1999, 78, 419–425.
(30) Wey, M. Y.; Yu, L. J.; Jou, S. I. J. Hazard. Mater. 1998, 60, 259– 270. (31) Wey, M. Y.; Su, J. L.; Yan, M. H.; Wei, M. C. Sci. Total Environ. 1998, 212, 183–193. (32) Chen, J. C.; Wey, M. Y.; Yan, M. H. J. Air Waste Manage. Assoc. 1999, 49, 1116–1120. (33) Chen, J. C.; Wey, M. Y.; Su, J. L.; Heish, S. M. Environ. Inter. 1998, 24, 451–456. (34) Chen, J. C.; Wey, M. Y.; Liu, Z. S. .. J. Environ. Eng 2001, 127, 63–69. (35) Chiang, K. Y.; Wang, K. S.; Lin, F. L.; Chu, W. T. Sci. Total Environ. 1997, 203, 129–140. (36) Zhang, Y.; Chen, Y.; Meng, A.; Li, Q.; Cheng, H. J. Hazard. Mater. 2008, 153, 309–319.
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Figure 1. The bubbling fluidized bed incinerator. (1) auto-feeder controller, (2) PID controller, (3) data collector, (4) autofeeder, (5) blower, (6) flowmeter, (7) thermocouple, (8) pressure detector, (9) preheater chamber, (10) sand bed, (11) electric resistance, (12) U manometer, (13) cyclone, (14) induced fan, (15) bag house.
agglomeration inhibition additives on heavy metals emitted from waste incineration is not clear in agglomeration/defluidization conditions. The emission of pollutants during agglomeration under various waste compositions and bed additives has not been studied yet. Agglomeration occurring in fluidized bed incineration not only affects the fluidization characteristics to change and the system to unexpectedly shut down, but it also leads to the generation of secondary pollutants such as heavy metals. This work estimates the effect of waste composition (Cl and S) and the addition of agglomeration inhibitors (Al- and Ca-related additives) on agglomeration/defluidization and the emission of pollutants during incineration processes. Agglomeration promoter (Na), mixtures of heavy metals (Pb, Cr, and Cd), sulfur, chlorine, and sawdust are used to simulate municipal waste. Different bed additives are used to determine the influence of the emission of heavy metals in defluidization conditions. These data of control agglomeration by additives and emission characteristics of heavy metals influenced by waste compositions can be used as references by those who study the operation of fluidized bed incinerators.
detectors are used to detect the difference between pressure of the sand bed and pressure of the freeboard chamber. These probes are connected to different pressure transmitters (Huba control 692) with the range of measurement from 0 to 9800 Pa. The pressure signals are digitized and recorded by a data acquisition system (ADVANTECH PCLabCard PCI-1711 and ADAMView software). Defluidization is defined as any condition where a well-fluidized bed loses fluidization, whether partial or total.37 Siegell et al.38 reported channeling and a consequent marked reduction in pressure drop under defluidization condition. Atakul et al.39 also studied when the agglomeration condition occurred, it results in a drastic increase in the temperature. However, a simultaneous and rapid decrease in the bed pressure drop was also observed. According to previous studies as above-mentioned, the defluidization time can be determined by detecting the pressure fluctuation during experiment. Therefore, the pressure drop can be used to measure the agglomeration during fluidization. The pressure-versus-time profile and visual observation are used to evaluate defluidization time. Preparing Artificial Feed Wastes. The artificial wastes in this study are simulated as municipal solid waste (MSW), but it is well-known that low concentration of heavy metals is observed in real MSW. Therefore, in order to estimate the emission characteristics of heavy metals and to reduce the experimental errors, increasing the concentration of heavy metals in artificial wastes is required. The artificial solid waste includes sawdust (2.25 g), metal solution (1 mL), and a polyethylene bag (0.5 g). The total mass is 3.75 g. Agglomeration promoter (Na) and heavy metals were added as nitrates. The added metal nitrates were NaNO3, Pb(NO3)2, Cr(NO3)3, and Cd(NO3)2. Sulfur and chlorine contents were added as sulfur powder and PVC particle, respectively. The weight percentages are calculated as atoms of metals not nitrates. The metals to be investigated are dissolved as nitrates in distilled water. The metal solution (1 mL),
Experimental Section Experimental Apparatus. Figure 1 demonstrates the fluidized bed incineration system in the experiments. The reactor is a bubbling fluidized bed incinerator made up of a preheated chamber (50 cm long), a main chamber (105 cm high) with an inner diameter of 10 cm that is made of stainless steel (3 mm thickness, AISI 310). The incineration system is enclosed by electrically resistant material packed with ceramic fibers to thermally insulate the system. The stainless steel porous plate functions as a gas distributor with a 15% open area. Three thermocouples are used to measure the temperatures of the preheated chamber, the sand bed, and the freeboard chamber. The bag filter is used to collect the elutriation particles from the incineration system. The programmed logical control controller is employed to control the temperature of the fluidized bed. Two pressure
(37) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Fuel 2003, 82, 1261–1270. (38) Siegell, J. H. Powder Technol. 1984, 38, 13–22. (39) Atakul, H.; Hilmioglu, B.; Ekinci, E. Fuel Process. Technol. 2005, 86, 1369–1383.
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Table 2. Operating Conditions for the Experiments run (No.) 1 2 3 4 5 6 7 8 9 10
operating conditions blank (no additives) additive
agglomeration test
800-B
temperature (°C)
artificial waste additives
800
800-Ca 800-Al 800-Cl 800-S 800-Na
bed material additives
agglomeration no
sampling heavy metals (Pb, Cr, Cd)
CaO activated Al2O3 PVC sulfur powder 800
800-Na-Ca 800-Na-Al 800-Na-Cl 800-Na-S
yes (addition of Na=1.2 wt % in artificial waste) CaO activated Al2O3 PVC sulfur powder
sulfur powder, and PVC particle are then added to the sawdust and packed in a PE bag. Artificial wastes are stored for a day to make sure that the sawdust can absorb the metal solution completely before the experiment. According to previous studies,42 the original concentration of metals in sawdust is neglected. On the other hand, low concentration of heavy metals has been found generally in real MSW and they are hard to be detected and analyzed by other instrument such as XRD. To estimate the emission characteristics of heavy metals and to reduce the experimental errors, increasing the concentration of heavy metals in artificial wastes is required. The concentrations of Na and other additives (heavy metals, S, and Cl) were 1.2 and 1 wt % of 3.75 g per artificial package, respectively. Silica sand (Sauter mean diameter = 190 μm) contained Al (1.62%), Si (94.88%), K (1.62%), Ca (1.66%), and Fe (0.22%) with an almost constant density (Fp = 2600 kg/m3) was used as the bed material. The minimum fluidization velocity was measured to be approximately 50 L/min. Al- and Ca-based additives are taken as activated Al2O3 and CaO in the experiment. Experimental Procedure. Table 2 listed the various operating conditions in this experiment. The operating temperature was set as 800 °C, and different operating parameters such as waste and bed additives, agglomeration test were included. During the fluidized bed test, approximately 40% excess air (relative the theoretical air) was used under operating temperature. 70 L/min input air at room temperature was determined. According to our previous study,40,41 it was reported how to estimate the quality of fluidization by both pressure fluctuation and gas velocity at various particle size distributions. The results presented that the sand bed displays the best quality of fluidization at 1.3Umf in most cases according to the resulting mean amplitude, dominant frequency, and fluidization index. To evaluate heavy emission of heavy metals, particle agglomeration, and to reduce the experimental errors, it is needed to extend the operating time and sampling time. Consequently, approximately 40% excess air was used and the operating temperature was set as 800 °C. Moreover, the effect of different gas velocities on particle agglomeration has been evaluated in our previous study.8 Applied 1.4U/Umf for fluidized bed tests can be acceptable and suitable for analysis of heavy metals during agglomeration processes. When the temperature of sand (or sand þ additive) bed was in a steady state, then artificial waste was fed into the chamber at the rate of one bag per 20 s, the pressure profiles are then recorded to evaluate the rate of defluidization during addition of Na experiment. The way to add the bed additives is to apply Ca- and Al-based additives (CaO and activated Al2O3
with particle size was 0.59-0.7 mm) mixed with silica sand (particle size was 0.7-0.84 mm, sorbent/silica sand = 1:16) at 800 °C. The total amount of bed materials was 2240 g. After each experiment, the combustion chamber is cooled down to room temperature and the bed materials are collected and analyzed. Sampling and Analysis. Flue gas containing heavy metals was isokinetically sampled during incineration. The U.S. EPA method 5 was employed to sample the heavy metals. Wey et al.42 has described the sampling train method. The metal samples were pretreated by microwave digestion and then were analyzed by flame atomic absorption spectroscopy (FAAS). Field emission scanning electron microscopy/energy dispersive spectrometry (FESEM/EDS) and X-ray diffraction (XRD) studies were then carried out on the identification of compounds and influence of agglomeration under various operating conditions. Thermodynamic Equilibrium Simulation of Heavy Metals during Agglomeration Process at Various Conditions. The aspect of thermodynamic equilibrium is carried out to study the major reaction products of the three metals (Pb, Cr, and Cd) under various conditions. The element potentials method combined with the atom population constraints is used to minimize the Gibbs energy of the system.43 The computer model used for the equilibrium calculation was written by Cruise44 and subsequently modified by Ulrich,45 Mudgett,46 Desrosier,47 and Chen et al.48 For convenience and extensive application, the interface modification and the thermodynamic data of the species of interest were incorporated into this computer program. The thermodynamic data for each species, such as heat capacity, entropy, and enthalpy, were obtained from a reference book and then incorporated into the model. The input parameters, such as bed additives (Al2O3 and CaO), operating temperature (600, 800, and 1000 °C), and waste additives (S and Cl) of the thermodynamic equilibrium simulation are shown in Table 3. Figure 2a indicates the possible chemical routes with Cl and S added into artificial wastes in the combustion system: (1) wastes combusted with Cl and S, (2) presence or absence of Na in the system, and (3) direct combustion or combustion with bed materials (silica sand). On the other hand, controlling the bed agglomeration by adding Al and Ca was also simulated by the model. Figure 2b shows the different reactions during agglomeration inhibition processes: (1) presence or absence of Na in the system, and (2) waste combusted with the (43) Smith, J. M.; Van Ness, H. C. Introduction to Chemical Engineering Thermodynamics, 4th ed.; McGraw-Hill: New York, 1987; pp 538-540. (44) Cruise, D. R. J. Phys. Chem. 1964, 68, 3797. (45) Ulrich, G. A., Technical Report of Cabot Corp; 1967, 67-OX-11. (46) Mudgett, P., Technical Report of Cabot Corp; 1967, 67-OX-11. (47) Desorsiers, R. E., Report of the United State Energy Research and Development Administration No. (E49-18)-2205; US department of Energy: Washington, DC,1977. (48) Chen, J. C.; Wey, M. Y.; Liu, Z. S. J. Environ. Eng. 2001, 63–69.
(40) Lin, C. L.; Wey, M. Y.; Cheng, H. T. Chemosphere 2004a, 56, 911–922. (41) Lin, C. L.; Wey, M. Y. Adv. Powder Technol. 2004b, 15, 79–96. (42) Wey, M. Y.; Ou, W. Y.; Liu, Z. S.; Tseng, H. H.; Yang, W. Y.; Chiang, B. C. J. Hazard. Mater. 2001, 82, 247–262.
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Table 3. Input Data of the Thermodynamic Equilibrium Simulation the composition in feed waste (g) element C H O Pb Cr Cd Cl S Na Ca Al Si Temp.(°C)
no additive
react with Cl, S and SiO2
react with SiO2 þ Al2O3
react with SiO2 þ CaO
167.84 37.57 785.93 1.50 1.50 1.50 0 0 1.80
167.84 37.57 2278.93 1.50 1.50 1.50 1.50 1.50 1.80
167.84 37.57 2269.93 1.50 1.50 1.50 0 0 1.80
167.84 37.57 2255.93 1.50 1.50 1.50 0 0 1.80
0 0 0 600, 800, 1000
The composition in bed materials (g) 0 0 0 56 747 700 600, 800, 1000 600, 800, 1000
70 0 700 600, 800, 1000
The simulation results were carried out separately, and a comparison of blank experiments was included.
Results and Discussions Emission of Heavy Metals (HMs) under Agglomerationinhibiting Processes. In incineration processes, heavy metals can be introduced into an incinerator without waste separation. The emission characteristics of heavy metals in incineration systems depend on their physical and chemical properties such as boiling point and saturation vapor pressure. Hence, the emission concentration of heavy metals during bed agglomeration processes was considered. Agglomeration Inhibited by Calcium Oxide. Figure 3 shows the comparison of emission characteristics of heavy metals with/without Ca inhibition during defluidization processes at 800 °C. The results demonstrated that the addition of CaO into the bed materials significantly influenced the emission of Pb. The emission concentration of Pb decreased when CaO existed in the system. Similar results were shown in the previous study.31 Chen et al.33 estimated that Pb reacted and was adsorbed by Ca-based compounds at a high temperature during the fluidized bed operation. On the other hand, the emission concentrations of Cd and Cr were higher than those in the blank experiments because the fluidization behavior changed with the addition of CaO. The previous study1 also described that the combustion efficiency was influenced by the addition of CaO, which resulted in the bimodal particle distribution system. However, the emission behavior of these three metals changed significantly when Na was added to cause agglomeration in the fluidized bed. The asterisk in Figure 3 points to the defluidization time under various operating conditions. According to Figure 3, the emission concentrations of the added metals increased when the system reached defluidization. Previous studies have discussed this behavior,7,9 that is, as agglomerates accumulate, the fluidized bed system transforms into a fix-bed state, and the temperature increases on the surface. Silica sand and CaO did not mix well during defluidization, and the rising temperature increased the volatility of heavy metals on the bed surface, thus increasing the concentration of heavy metals emitted. Nevertheless, the emission concentration of heavy metals increased gradually with the addition of CaO. The highest concentration peak was likewise observed after system defluidization. The difference in the emission behavior between the Na and NaþCaO system illustrated the relationship between the amounts of heavy
Figure 2. Schematic diagrams of different routes of heavy metals reactions during particle agglomeration in fluidized bed incineration. (a) System with Cl or S in artificial waste. (b) System with Al2O3 or CaO mixed with bed materials.
sand bed or with both additives (Al, Ca) and the sand bed. Different reaction routes may lead to different emission behavior of heavy metals as shown in Figure 2. The topic of the relations between particle agglomeration and emission characteristics of heavy metals was emphasized. Accordingly, it is necessary to understand the reactions of heavy metals on the surfaces of bed materials with various operating condition during defluidization processes. To better understand the topic of heavy metals emitted during agglomeration processes, possible chemical routs are simulated by thermodynamic equilibrium model. Then, the results obtained from simulations also indicate different metal species formed at various operating conditions. 4329
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Figure 4. Comparison of emission characteristics of heavy metals with/without Al inhibition during defluidization processes at 800 °C. ((, Blank; 9, with Al; 2, with Na; b, with Al þ Na; *, defluidization time).
Figure 3. Comparison of emission characteristics of heavy metals with/without Ca inhibition during defluidization processes at 800 °C. ((, Blank; 9, with Ca; 2, with Na; b, with Ca þ Na; *, defluidization time).
observed because of the reaction between Cd and Al2O3. Moreover, Cr emitted at the beginning tended to increase but later decreased. This can be explained by the fact that the emission of Cr was influenced by the different affinities of both Al and Cr reacting with Na in the fluidized bed. At a high temperature, Na reacted with Al initially and then combined with Si to form Na2O-SiO2-Al2O3 (s). At the same time, Cr may have vaporized easily at the initial stage of the fluidized bed operation, and a high emission concentration of Cr was observed because of system agglomeration. When the reaction of Na and Al stopped owing to the limited amount of Al in the bed, Na started to react with Cr to form NaCr2O4; this decreased the emission concentration of Cr. In addition, the increase of Cr emission is considered to have reduced the system combustion efficiency because of particle agglomeration and interaction between the waste and the fluidized bed materials. Effect of Cl Addition on Emission of HMs during Defluidization Processes. Figure 5 shows the emission of heavy metals with the addition of Cl during defluidization processes at 800 °C. During the addition of Na and Cl, the emission concentration of heavy metals increased compared
metals emitted and the agglomeration inhibition tendency. The emission concentration increased slightly with the decrease in agglomeration inhibition tendency. Agglomeration Inhibited by Alumina Oxide. A comparison of the emission characteristics of heavy metals when Al existed during defluidization processes is shown in Figure 4. The emission concentration of heavy metals (Pb and Cr), except for Cd, from the blank experiments (no Al2O3 additive) was the lowest. The emission concentration of Cd compared with other metals was more similar to that of the blank experiment. It can be inferred from the results that Cd easily reacted with Al to form high melting point compounds, such as AlCdO4, and that the amount of Cd emitted from the incinerator decreased. During the particle agglomeration inhibition process, the three heavy metals exhibited different emission trends compared with the noninhibition process before defluidization. The emission concentration of Pb with the addition of Al2O3 showed no difference with the nonaddition condition before defluidization. For Cd, a lower emission concentration was 4330
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Figure 5. Emission of heavy metals with Cl addition during defluidization processes at 800 °C. ((, Blank; 9, with Cl; 2, with Na; b, with Cl þ Na; *, defluidization time).
Figure 6. Emission of heavy metals with S addition during defluidization processes at 800 °C. ((, Blank; 9, with S; 2, with Na; b, with S þ Na; *, defluidization time).
with those of the blank experiments. However, there were some differences among the emission behaviors of the three heavy metals under various conditions. For the emission of Pb, Cl content in wastes was the major factor for the increase in emission concentration. Thus, the addition of Na had fewer effects on heavy metal emission behavior compared to when Cl was added. On the other hand, the effect of Na addition played an important role in the significant increase of Cd emission. The same emission characteristics were observed in the emission of Cr. The emission concentration of Cr increased with the addition of Na, and there was no difference with the addition of Cl compared to that of the blank experiment. The experimental results were in agreement with previous studies. Chen et al.32,49 and Ho et al.50 indicated that the presence of organic chlorine (PVC) increased the distribution of metals in the fly ash and decreased
the adsorption of metals in the silica sand. However, when inorganic chlorine (NaCl) was added, the adsorption of metals in the sand bed increased. Effect of S Addition on Emission of HMs during Defluidization Processes. The results of the emission of heavy metals with the addition of sulfur during defluidization processes are shown in Figure 6. The emission trends of these three volatile metals were similar. Figure 6 shows that the emission concentrations of the added metals increased when S was added into the system. In particular, the emission concentration of heavy metals decreased apparently when both Na and S were added during defluidization. This can be explained by the increased efficiency of silica sand in capturing heavy metals in the presence of sulfur. Moreover, high-boiling point sulfates such as CdS (bp = 1750 °C), PbS (bp= 1118 °C), and Cr2S3 (bp=1350 °C) formed on the bed surface while heat energy accumulated during defluidization. The same results were observed from previous papers.32,33,49 Chen et al.49 discussed that the presence of sulfide (Na2S) increased the adsorption
(49) Chen, J. C.; Wey, M. Y.; Yan, M. H. J. Environ. Eng. 1997, 123, 1100–1106. (50) Ho, T. C.; Chen, C.; Hopper, J. R.; Oberacker, D. A. Combust. Sci. Technol. 1992, 85, 101–116.
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Figure 7. The SEM images (50 and 150) of agglomerates (a) Na with SiO2, (b) Na with SiO2 þ Al2O3, and (c) Na with SiO2 þ CaO.
efficiency of metals in silica sand, particularly for the metals Cd and Pb. Species Analysis of Agglomerates. XRD analysis was applied to identify the dominant species of agglomerates, but only SiO2 was detected in this study. These agglomerates are not crystalline compounds that cannot be found by XRD. Hence, the concentration of major species may be too low to be detected by XRD. The results were in agreement with those of previous papers.7,8 Figure 7 illustrates the liquid bridge formed between two particles at various bed materials. Alkali metal forms eutectics with bed materials to generate low melting point species at high temperatures. Figure 8 demonstrates the results using FESEM/EDS. It was found that Al, Ca, Na, Si, Pb, and Cr were the major elements existing between two agglomerates. The results agreed with those of previous investigations.8,9 Thermodynamic Equilibrium Test. An equilibrium calculation is useful and helpful in understanding the chemical reactions associated with the agglomeration system and provides available information on the emission characteristics of heavy metals at a high temperature. Previous studies also used the thermodynamic equilibrium model to simulate and predict the formation and behavior of metal species in the combustion system.48,51 Tables 4-6 list the results of the major species of the three heavy metals formed in the thermodynamic equilibrium model. Various operating conditions were carried out such as operating temperature (600, 800, and 1000 °C), additives (Na, Cl, and S), and particle agglomeration inhibitors (Al and Ca). The simulation’s parameters were separated into two parts as shown in Figure 2. Table 4 shows the major species of Pb in the results from the thermodynamic model. In the blank test, Pb-related oxides were the major species where silica sand is absent in the reaction. Without particle agglomeration conditions, the
results presented that heavy metal Pb would react with Cl to form PbCl4(g), which was easily vaporized at a high temperature. Then, Pb also reacted with CaO to form Ca2Pb(s), which is the main chemical adsorption mechanism of CaO in the fluidized bed. Nevertheless, there is no difference in the formation of Pb-related compounds in the presence of S and Al compared with the blank experiment. When waste combusted with silica sand, PbSiO3(s) resulted in each case of various conditions. The same results were found from a previous study. Lind et al.52 reported that the gas-to-particle conversion route for Cd, Pb, and Cu was found by chemical surface reaction, probably with silicates. On the other hand, the addition of Na into the system resulted in a reduction of PbCl4 (g); the generation of several Pb-related compounds was likewise observed. Moreover, no significant difference was found on the formation of Pb-related species regardless of whether Na was added or not. The simulation results clearly explained the emission behavior of Pb under various operating conditions in this study. In the present of agglomeration inhibitor, CaO, the emission behavior of heavy metal Pb was reduced due to the formation of Ca2Pb(s) compared with other experiments. However, lowboiling-point species such as PbCl4(g) was found when Cl added that lead to increase the emission concentration of heavy metal Pb. The simulation results of the formation of heavy metal Cr in the fluidized bed are shown in Table 5. The formation of CrCl2, CrCl3, and CrCl4 were observed in the presence of Cl in the system, and CaCr2O4(s) existed when CaO was applied in the system. However, metal Cr preferred to react with Na rather than chromium oxides to form NaCr2O4. Furthermore, CrSi2(s) was the major species in the waste combustion with silica sand. Thus, Cl and Na could affect the emission characteristics of metal Cr during fluidized bed incineration. In the consideration of both results obtained
(51) Ho, T. C.; Chuang, T. C.; Chelluri, S.; Lee, Y.; Hopper, J. R. Waste Manage. 2001, 21, 435–441.
(52) Lind, T.; Valmari, T.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Environ. Sci. Technol. 1999, 33, 496–502.
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Figure 8. SEM/EDS image for analysis of liquid bridge between two particles at 800 °C. (a) Na; (b) Na þ Al2O3; (c) Na þ CaO.
from thermodynamic equilibrium modeling and experiments, some differences can be observed in both results. Although the reaction of Cr with Na, Si, and Ca found in the results of simulation, the emission concentration of Cr increased during agglomeration processes that indicated the effect of fluidization behavior was the dominant factor. However, the simulation results were in agreement with the experimental results to clarify the mechanism for the changes of emission behavior of Cr especially in the present of Al and S. Table 6 shows the formation of Cd-related species in the thermodynamic equilibrium model. From the results, CdO(s) and Cd(g) were the major species in the blank test, and then parts of metal Cd reacted with Cl and S to form CdCl2(g) and CdSO4(s) in the presence of Cl and S in the system, respectively.
Furthermore, when Al2O3 was added into the bed, Al2CdO4(s) was produced. In the particle agglomeration process, there is no difference whether Na was added or not, except in the addition of Cl. Table 6 shows that it was difficult for heavy metal Cd to form CdCl2(g) because Na has a higher affinity with Cl than with Cd. In contrast, if waste combusted with the sand bed in the fluidized bed, then heavy metal Cd would have reacted with silica sand to form CdSiO3(s). On the basis of the results obtained from simulation and experimental works, Al and S could be the effective factors on the emission of heavy metal Cd. Other Cd-related species were found in the model, but it seems there is no difference when Ca and Cl were added in fluidized bed from experimental results because of their high volatility products such as cadmium oxides and cadmium chlorides generated during fluidized bed incineration. 4333
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yes
no
PbCl4 (G) PbO(Yellow) PbO (G) Pb (G) PbO(Red) HPb (G) Pb2 (G) PbS (G) Ca2Pb (S) PbSiO3 (S)
PbCl4 (G) PbO(Yellow) PbO (G) Pb (G) PbO(Red) HPb (G) Pb2(G) PbS (G) Ca2Pb (S) PbSiO3 (S)
no
yes
Pb related species
Si exists?
7.24 10-03
7.24 10-03 7.86 10-08 1.37 10-12
7.24 10-03
7.24 10-03 7.87 10-08 1.35 10-12
600 °C
7.24 10-03
9.61 10-05 4.30 10-08 7.14 10-03 1.39 10-15 7.18 10-18
7.24 10-03
9.62 10 4.23 10-08 7.14 10-03 1.36 10-15 6.96 10-18
-05
800 °C
7.24 10-03
7.21 10-03 2.98 10-05 1.29 10-12 2.42 10-13
7.24 10-03
1.04 10-11 9.87 10-13
-03
1000 °C
7.21 10 2.94 10-05
blank (no additive)
7.24 10-03
5.42 10-03
5.42 10-03 1.82 10-03 1.87 10-07 7.02 10-13
7.24 10-03
2.29 10-04 2.19 10-08 1.59 10-03 2.13 10-16 7.87 10-19
7.24 10-03
7.24 10
-03
800 °C
7.24 10-03
7.24 10
-03
600 °C
chlorine
7.24 10-03
7.24 10-03 1.87 10-07 7.98 10-13
600 °C
7.24 10-03
2.29 10-04 2.50 10-08 7.01 10-03 2.58 10-16 1.02 10-18
800 °C
sulfur
7.24 10-03
1.33 10-12 2.96 10-14 9.60 10-17
7.23 10-03 7.31 10-06
1000 °C
7.24 10-03
7.24 10-03
7.24 10-03
7.24 10-03
Effect of Particle Agglomeration Processes (With Na) 5.42 10-03 7.24 10-03 1.81 10-03 1.87 10-07 2.29 10-04 7.23 10-03 1.61 10-06 7.02 10-13 2.19 10-08 6.43 10-06 7.01 10-03 8.71 10-14 2.13 10-16 2.27 10-13 1.44 10-15 7.87 10-19 2.29 10-14 4.32 10-17
7.24 10-03
7.24 10
-03
1000 °C
effect of waste compositions
7.24 10-03
7.24 10-03 1.83 10-07 6.88 10-13
7.24 10-03
7.24 10-03 1.83 10-07 7.86 10-13
600 °C
7.24 10-03
2.24 10-04 2.15 10-08 7.02 10-03 2.11 10-16 7.74 10-19
7.24 10-03
2.24 10-04 2.46 10-08 7.02 10-03 2.58 10-16 1.01 10-18
800 °C
Al2O3 1000 °C
7.24 10-03
1.12 10-12 2.36 10-14
7.23 10-03 6.45 10-06
7.24 10-03
1.36 10-12 3.07 10-14
7.24 10-03 7.24 10-03
7.24 10-03 7.24 10-03
600 °C
effect of bed additives
7.23 10-03 7.37 10-06
Table 4. The Major Species of Pb in the Thermodynamic Equilibrium Simulation Results (Molar Fraction %)
7.24 10-03 7.24 10-03
7.24 10-03 7.24 10-03
800 °C
CaO
7.24 10-03 7.24 10-03
7.24 10-03 7.24 10-03
1000 °C
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: DOI:10.1021/ef9003945 Kuo et al.
4335
yes
no
Cd (G) CdO (S) CdSO4 (S) Al2CdO4(S) CdS (S) CdSiO3 (S)
CdO (S) CdCl2(G) Cd (G) CdSO4 (S) CdS (G) Al2CdO4(S) CdSiO3 (S)
no
yes
Cd related species
Si exists?
yes
no
NaCr2O4 (S) CaCr2O4 (S) CrSi2 (S) CaCr2O4 (S)
Cr2O3 (S) CrCl2O2 (G) CrO3 (G) CrCl3 (G) CrO2 (G) CrCl2 (G) CrCl4 (G) CrO (G) CaCr2O4 (S) CrSi2 (S) CaCr2O4 (S)
no
yes
Cr related species
Si exists?
1.01 10-10
1.15 10-16
2.88 10-02
2.88 10-02
2.88 10-02
4.59 10-14
1.64 10-20
2.88 10-02
2.88 10-02
2.88 10-02
800 °C
1.33 10-02
1.33 10-05
1.33 10-02
1.35 10-05 1.33 10-02
1.33 10-02
600 °C
1.33 10-02
7.24 10-10
1.33 10-02
1.26 10-09 1.33 10-02
1.33 10-02
1000 °C
600 °C
2.88 10-02
2.88 10-02
2.88 10-02
1.44 10-02 4.90 10-09 8.70 10-14 7.07 10-20 1.65 10-20
2.88 10-02
2.88 10-02
2.88 10-02
1.44 10-02 2.47 10-09 1.02 10-10 4.14 10-18 1.15 10-16 6.91 10-18 1.22 10-20
800 °C
chlorine
1000 °C
600 °C
800 °C
1000 °C
600 °C
7.65 10-20
2.08 10-12
1.44 10-02
2.88 10-02
2.88 10-02
2.88 10-02
2.88 10-02
2.88 10-02
2.88 10-02
9.69 10-18
7.75 10-11
1.98 10-07
1.44 10-02
Effect of Particle Agglomeration Processes (With Na) 2.88 10-02 2.88 10-02 2.88 10-02 2.88 10-02
2.88 10-02
1.77 10-14
1.95 10-09
1.44 10-02
2.88 10-02
2.88 10-02
7.85 10-20
2.15 10-12
1.44 10-02
2.88 10-02
2.88 10-02
1.44 10-02 1.50 10-09 1.03 10-08 6.53 10-17 1.63 10-11 4.46 10-15 2.13 10-20 8.17 10-18
sulfur
2.88 10-02
2.88 10-02
2.88 10-02
1.73 10-14
1.89 10-09
1.44 10-02
800 °C
Al2O3
1000 °C
1.33 10-02
7.35 10-03 6.00 10-03
1.33 10-02
7.24 10-03
600 °C
1.33 10-02
6.46 10-10 1.33 10-02
1.33 10-02
1.07 10-02 2.66 10-03 1.24 10-09
1.33 10-02
6.88 10-06 1.33 10-02
1.33 10-02
1.10 10-02 2.32 10-03 1.33 10-05
800 °C
chlorine
1.33 10-02
1.95 10-13 1.33 10-02
600 °C
1.33 10-02
5.44 10-06 1.33 10-02
800 °C
sulfur
1000 °C
1.33 10-02
1.14 10-19
4.27 10-03
9.07 10-03
1.33 10-02
1.33 10-02
1.33 10-02
1.14 10-19 1.33 10-02
Effect of Particle Agglomeration Processes (With Na) 3.76 10-03 2.45 10-13 6.39 10-06 9.07 10-03 9.59 10-03 1.33 10-02 1.33 10-02 4.27 10-03
1.33 10-02
4.09 10-03 2.03 10-03 7.22 10-03
1000 °C
effect of waste compositions
1.33 10-02
1.33 10-02
1.33 10-02 1.33 10-02
600 °C
1.33 10-02
1.33 10-02
1.33 10-02 1.33 10-02
800 °C
Al2O3
Table 6. The major species of Cd in the thermodynamic equilibrium simulation results (molar fraction %)
2.88 10-02
2.88 10-02
2.88 10-02
8.18 10-18
1.62 10-11
1.03 10-08
1.44 10-02
6.11 10-03
blank (no additive)
800 °C
1.44 10-02
600 °C
1.44 10-02
blank (no additive)
effect of waste compositions
1000 °C
1.44 10-02
1.44 10-02
1.44 10-02
1.44 10-02
1.33 10-02
1.33 10-02
1.33 10-02 1.33 10-02
1000 °C
1.33 10-02
6.35 10-10 1.33 10-02
1.33 10-02
7.24 10-10
1.33 10-02
600 °C
effect of bed additives
2.88 10-02
2.88 10-02
2.88 10-02
9.51 10-18
7.55 10-11
1.91 10-07
600 °C
effect of bed additives
1.44 10-02
Table 5. The Major Species of Cr in the Thermodynamic Equilibrium Simulation Results (Molar Fraction %)
1.33 10-02
6.77 10-06 1.33 10-02
1.33 10-02
7.72 10-06
1.33 10-02
800 °C
CaO
1.44 10-02
1.44 10-02
1.44 10-02
1.44 10-02
800 °C
CaO
1.33 10-02
3.69 10-03 9.65 10-03
1.33 10-02
4.21 10-03
9.13 10-03
1000 °C
1.44 10-02
1.44 10-02
1.44 10-02
1.44 10-02
1000 °C
Energy Fuels 2009, 23, 4325–4336
: DOI:10.1021/ef9003945 Kuo et al.
Energy Fuels 2009, 23, 4325–4336
: DOI:10.1021/ef9003945
Kuo et al.
on the emission of Cd and Cr compared to when Cl was added. However, Cl was the dominant factor for the heavy metal Pb. The emission concentration of heavy metals also decreased evidently when both Na and S were added during defluidization. The formation of high-boiling point sulfates was observed on the surface of the sand bed while heat energy accumulated during defluidization by Na. (3) Simulation works indicated that the emission of heavy metal Pb was influenced mainly by the presence of Cl and Ca. Furthermore, Na, Cl, and Ca were the major factors that affected the emission of heavy metal Cr. Thus, the emission of Cd was affected by both Cl and Al. When waste combusted with the sand bed, the three heavy metals transformed to metal silicates such as PbSiO3(s), CdSiO3(s), and CrSi2(s) in the absence of Ca.
Conclusions The experimental results showed the emission characteristics of heavy metals (Pb, Cr, and Cd) under various waste compositions (Cl and S) and agglomeration inhibitors (Al and Ca) during the defluidization process. The following conclusions were obtained from our study: (1) During agglomeration-inhibition processes caused by Al and Ca, the emission concentration of heavy metals increased because of the reduction in combustion efficiency. In particular, heavy metal Cr vaporized easily at the initial stage of defluidization process, indicating that the chemical reactions changed between Na and Cr through the addition of Al into the system at a high temperature. The different affinities of both Al and Cr reacting with Na in the fluidized bed were considered the main reason for the Cr emission. (2) When wastes contained Cl, the concentration of heavy metals emitted increased because of the formation of lowboiling point metallic chlorides. The addition of Na has effects
Acknowledgment. The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 95-2221-E-005053-MY3.
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