Energy Fuels 2010, 24, 2910–2917 Published on Web 05/05/2010
: DOI:10.1021/ef901595d
Investigation on the Emission of Heavy Metals in Fluidized-Bed Incineration during the Agglomeration/Defluidization Process Chiou-Liang Lin,*,† Shih-Hsien Chang,‡ and Tzu-Huan Peng† † Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China, and ‡Department of Public Health, Chung-Shan Medical University, Taichung 402, Taiwan, Republic of China
Received December 27, 2009. Revised Manuscript Received April 24, 2010
This study investigated the effects of waste components and operating temperatures on heavy metal emission in the agglomeration/defluidization process in a fluidized bed. Artificial waste was used to simulate municipal waste and to induce the formation of agglomerates. Those contain different concentrations of alkali metals, alkaline earth metals, a mixture of metals (Pb, Cr, and Cd), and sawdust. The experimental results indicated that Na increased the risk of agglomeration/defluidization but decreased the emission concentrations of heavy metals. Additionally, the emissions of heavy metals were lower with Ca and Mg addition. Ca and Mg inhibited the generation of agglomerates, thereby maintaining the quality of fluidization and the uniform mixing of the silica sand. Scanning electron microscopy/energydispersive spectrometry (SEM/EDS) analysis showed that Na, Si, Mg, Pb, and Cr were present on particle surfaces. In addition, some compounds with high melting points, such as CaO and MgO, were produced when Ca and Mg were added to the system. These high-melting-point species may inhibit the generation of agglomerates while maintaining the quality of fluidization and the uniform mixing of the silica sand. According to these results, two important mechanisms may decrease the emission of heavy metals: adsorption by silica sand and covering or adherence by Na-containing glasses that form low-melting eutectics. For the three heavy metals studied, their metallic species may react with Na to form eutectic compositions, be covered or adhered to by eutectics, and stay in the sand bed, thereby decreasing the emission concentrations in flue gas. Overall, the emission of heavy metals decreased as Na, Ca, and Mg were added at various temperatures.
particle size, surface area, density, humidity, and momentum of the particles.4-8 Additionally, the chemical reaction mechanism, physical and chemical properties of the particles and the characteristics of the ash may also influence the formation of agglomerates and lead to defluidization.9 The components of municipal wastes used in fluidized-bed incineration processes are complex and contain some materials that have the potential to cause adhesion, such as plastics,10 alkalis, alkaline earth metals, sulfur, chlorine, and iron.11,12 These materials are the major causes of agglomeration and defluidization. Gluckman et al.13 indicated that agglomeration depends upon the stickiness and collision of particles, and Skrifvars et al.14,15 showed the same results. Therefore, two mechanisms of adhesion may be considered: (1) viscous materials cause sintering to occur in glassy materials, which easily flow to other particles, and (2) melting and chemical reactions
1. Introduction The phenomenon of agglomeration/defluidization during fluidized-bed operation frequently causes problems because of the accumulation of agglomerates. This phenomenon leads to changes in fluidization characteristics, such as bubble size, bubble frequency, bubble velocity, and minimum fluidization velocity,1 and influences the operation of the process, even to the extent of shutting down the fluidized bed.2 The quality of fluidization will affect the operation efficiency of the fluidized reactor. Good quality of fluidization is attained when the air is distributed uniformly through the bed and the particles are distributed well in the gas stream.3 Therefore, the characteristics of fluidization will be changed with agglomeration/ defluidization to decrease the quality of fluidization. There are many complex factors that cause agglomeration during the fluidized process, including gas velocity, operating temperature,
(9) Seville, J. P. K.; Silomon-Pflug, H.; Knight, P. C. Powder Technol. 1998, 97, 160–169. (10) Arena, U.; Mastellone, M. L. Powder Technol. 2001, 120, 127–133. (11) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Fuel 2003, 82, 1261–1270. (12) Yan, R.; Liang, D. T.; Tsen, L. Energy Convers. Manage. 2005, 46, 1165–1178. (13) Gluckman, M. J.; Yerushalmi, J.; Squires, A. M. Defluidization characteristics of sticky or agglomerating beds. In Fluidization Technology; Keairns, D. L., Ed.; Hemisphere: Washington, D.C., 1976; Vol. 2, pp 395-422. (14) Skrifvars, B. J.; Hupa, M.; Hiltunen, M. Ind. Eng. Chem. Res. 1992, 31, 1026–1030. (15) Skrifvars, B. J.; Hupa, M.; Backman, R.; Hiltunen, M. Fuel 1994, 73, 171–176.
*To whom correspondence should be addressed. Telephone: þ886-75919722. Fax: þ886-7-5919376. E-mail:
[email protected]. (1) Tardos, G.; Pfeffer, R. Powder Technol. 1995, 85, 29–35. (2) Lin, C. L.; Wey, M. Y. Fuel 2004, 83, 2335–5343. (3) Morse, R. D.; Ballou, C. O. Chem. Eng. Prog. 1951, 47, 199–204. (4) Bartels, M.; Nijenhuis, J.; Lensselink, J.; Siedlecki, M.; de Jong, W.; Kapteijn, F.; van Ommen, J. R. Energy Fuels 2009, 23, 157–169. (5) Bie, R.; Li, S.; Zhao, Y.; Yang, L. Energy Fuels 2009, 23, 4304– 4310. (6) Wank, J. R.; George, S. M.; Weimer, A. W. Powder Technol. 2001, 121, 195–204. (7) Lin, C. L.; Wey, M. Y.; Lu, C. Y. Powder Technol. 2006, 161, 150–157. (8) Lin, C. L.; Kuo, J. H.; Wey, M. Y.; Chang, S. H.; Wang, K. S. Powder Technol. 2009, 189, 57–63. r 2010 American Chemical Society
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generate some liquid-phase materials. These two factors lead to the generation of agglomerates during the operating process. Researchers have analyzed these viscous materials and found that the main components are low-melting-point species, including Al2(SO4)3, Na2SO4, Na2O, Na2SiO3, and V2O5.2,16,17 Some of these elements, particularly alkalis, exist in municipal wastes. During combustion, these compounds form primarily low-melting-point eutectics, which cause agglomeration and defluidization.2,7,18,19 Many parameters affect hydrodynamic fluidization behaviors and the generation of pollutants in fluidized-bed combustion. These factors include the amount of excess air, the operating gas velocity, the operating temperature, the compounds in the waste, and the mixing of the bed materials. Pollutants, such as organics [polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethyl benzene, and xylene (BTEX)], heavy metals, and acid gases, may form during incineration.20 Therefore, the quality of fluidization directly affects pollutant generation. Heavy metal compounds are volatilized at high temperatures to form metallic vapor or sub-micrometer particles. These are released to the environment by adsorption onto the fly ash or emission in the flue gas, resulting in harm to the environment and human health. Accordingly, the operating parameters of the reactor are major factors affecting the distribution of heavy metals. These parameters include the composition of the wastes, the combustion temperature, the gas velocity, the chlorine concentration, the loading of waste, etc. However, the intrinsic characteristic of the metal is also an important factor. Fournier et al.21 pointed out that the distribution of heavy metals is related to the boiling point of the species. Generally, metal compounds with higher saturation vapor pressures are more easily vaporized after combustion, condense on the ash, and are then distributed in fly ash or flue gas. Some heavy metals with higher boiling points are found in the bottom ash. Therefore, the combustion temperature influences the distribution ratio of heavy metals. As the temperature increases, the concentrations of zinc, lead, and cadmium in the bottom ash decrease, while the concentrations of zinc, arsenic, mercury, and lead in the flue gas increase simultaneously.22,23 Besides the combustion temperature and the intrinsic characteristics of the metal, the presence of other elements in the waste also affects the dispersal of heavy metals into the environment. When chlorine exists in the incineration system, the volatility of these metals increases and the concentration of metals in the bottom ash is reduced.24 Hence, the combustion conditions and the
Figure 1. Bubble fluidized-bed incinerator: (1) PID controller, (2) blower, (3) flow meter, (4) thermocouple, (5) pressure transducer, (6) electric resistance, (7) sand bed, (8) feeder, (9) sampling place, (10) cyclone, (11) filter, and (12) induced fan.
amount of other elements (chlorine and sulfur) present in the reactor will also affect the distribution of heavy metals. When adhesive materials accumulate to form agglomerates during fluidized-bed incineration, the characteristics of fluidization, such as bubble size, bubble frequency, bubble velocity, and minimum fluidization velocity, will be changed, which may lead to the formation of secondary pollutants25 during incineration or the unscheduled shutdown of the reactor. Most research in this area has focused on the effects of coal composition on agglomeration. The effects of agglomeration/defluidization on the emission of heavy metals during incineration of wastes have rarely been examined. Therefore, this study focuses on the effects of different waste components and operating parameters on heavy metal emission in the agglomeration/defluidization process. To simplify the factors, artificial waste was used to simulate municipal waste and to produce agglomerates. The artificial waste contains alkali metals (Na), alkaline earth metals (Mg and Ca), a mixture of heavy metals (Pb, Cr, and Cd), and sawdust. The effects of alkali metals, alkaline earth metals, and operating temperature on agglomeration and emission of heavy metals were considered. These results can be used as references for the operation of fluidized-bed incinerators. 2. Experimental Section Figure 1 shows the apparatus used in this experiment. The reactor was a bubbling fluidized bed, and the main chamber was 120 cm high with an inner diameter of 10 cm. The chambers were made of 3 mm thick stainless steel (AISI 310). The apparatus was surrounded by electrically resistant material, which was packed with ceramic fibers for thermal insulation, and equipped with a stainless-steel porous plate with 15% open area for gas distribution. The program logical control (PID) controller was connected with two thermocouples to control the temperature of the chamber. The particles emitted from the combustion chamber were collected by a cyclone connected to a bag filter. To simulate the generation of agglomeration/defluidization during incineration, an alkali metal (Na) was added in artificial wastes to form low-melting-point eutectics. Alkaline earth metals (Mg and Ca) were also added in some test runs to determine their influence on agglomeration and the emission of heavy metals. Silica sand was employed as the bed material in the experiment,
(16) Lin, W.; Dam-Johansen, K.; Frandsen, F. Chem. Eng. J. 2003, 96, 171–185. (17) Atak€ ul, H.; Hilmioglu, B.; Ekinci, E. Fuel Process. Technol. 2005, 86, 1369–1383. (18) Yan, R.; Liang, D. T.; Laursen, K.; Li, Y.; Tsen, L.; Tay, J. H. Fuel 2003, 82, 843–851. (19) Manzoori, A. R.; Agarwal, P. K. Fuel 1994, 73, 563–568. (20) Lin, C. L.; Wey, M. Y.; Cheng, H. T. J. Environ. Eng. 2006, 132, 960–966. (21) Fournier, D. J.; Whitworth, W. E.; Lee, J. W.; Waterland, L. R. The fate of trace metals in a rotary kiln incinerator with a venturi/packed column scrubber. Feb, 1991; EPA/600/S2-90/043. (22) Hiraoka, M.; Takeda, N. Behavior of hazardous substances in stabilization and solidification processes of industrial wastes. In Toxic and Hazardous Waste Disposal; Pojasek, R. B., Ed.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. 3, pp 107-124. (23) Gerstle, R. W.; Albrinck, D. N. J. Air Pollut. Control 1982, 32, 1113–1123. (24) Wey, M. Y.; Huang, J. H.; Chen, J. C. J. Chem. Eng. Jpn. 1996, 29, 743–752.
(25) Lin, C. L.; Wey, M. Y.; Yu, W. J. Combust. Flame 2005, 143, 139–149.
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Table 1. Elemental Analysis of Different Wastes by Weight species
C (%)
H (%)
N (%)
O (%)
sawdust polypropylene (PP) polyethylene (PE)
43.12 86.16 85.71
5.80 12.20 13.04
5.01 1.12 0.86
46.07 0.52 0.39
Table 2. Calculated Results of Stoichiometric Air and Composition of Flue Gas results of stoichiometric air stoichiometric air (L/min)
excess air (40%) (L/min)
43.3
60.6
calculation at 25 °C
composition of flue gas (%) calculation at 800 °C
CO2
O2
N2
H2O
10.70
7.17
67.41
14.72
Figure 2. Effect of different Na concentrations on time to defluidization (the defluidization of the 0.3% Na test was not observed until 14 040 s).
Table 3. Operating Conditions for the Experimentsa concentration (%) run
temperature (°C)
species of heavy metal
Na
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
700 800 900 800 800 800 800 800 800 700 800 900 700 800 900 700 800 900
Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd Pb, Cr, Cd
0.3 0.5 0.7 0.9 1.1 1.3 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7
a
Ca
Mg
0.7 0.7 0.7 0.7 0.7 0.7
Gas velocity, 0.13 m/s; material size, 770 μm; bed height, 18 cm.
which contained SiO2 (97.80%), Al2O3 (2.01%), and Fe2O3 (0.07%). Those were collected by the standard Tyler sieves in the range of 20-25 mesh. The mean particle size was 770 μm, and the particles had an almost constant density across all sizes (Fp = 2600 kg/m3). The minimum fluidization velocity (Umf) of 770 μm is 0.1 m/s. The alkali, alkaline earth, and heavy metals (Pb, Cr, and Cd) were added as nitrates in artificial wastes. The metal nitrates contained NaNO3, Ca(NO3)2, Mg(NO3)2, Pb(NO3)2, Cr(NO3)3, and Cd(NO3)2. The total mass of synthetic solid waste was 3.24 g, which included sawdust (1.6 g), polypropylene (0.35 g), metal solution (1 mL), and a polyethylene (PE) bag (0.29 g). The elemental analysis of sawdust, polypropylene, and polyethylene were detected by an elemental analyzer (EA), and these results are listed in Table 1. These results of elemental analysis can be used to calculate the stoichiometric air (as shown in Table 2). The metal nitrates were dissolved in distilled water, the metal solution (1 mL) was added to sawdust, and then the metal/sawdust was enclosed in a PE bag. The artificial waste was stored for 1 day to ensure that the metal solution was absorbed into the sawdust. The artificial waste was fed into the combustion chamber every 20 s. The temperature was measured in the bed with a thermocouple (as shown in Figure 1). The operating temperatures were controlled at 700, 800, and 900 °C, and the excess air factor (actual air flow/ideal air flow) was maintained at 40% (60 L/ min = 0.13 m/s). Table 3 presents the controlled parameters during the experiment.
Figure 3. Defluidization time at different temperatures.
The minimum fluidization velocity (Umf) was determined from the pressure drop versus gas velocity using the method described in detail by Lin et al.26 When the temperature was in a steady state, air was passed through the combustion chamber and the artificial waste was fed into the chamber at a rate of one bag every 20 s. The pressure versus time profile and visual observation were used to evaluate the defluidization time during incineration. The pressure drop was determined by two pressure detectors located in the sand bed and freeboard. Each detector was associated with a different pressure transmitter, and the range of measurement was 0-1000 mm H2O. Siegell et al.27 and Atak€ ul et al.17 reported channeling and a consequent marked reduction in pressure drop under defluidization conditions. Tardos et al.28 also pointed out that the rapid pressure drop was associated with defluidization. Therefore, the defluidization time can be determined by detecting the pressure fluctuation during the experiment. The pressure fluctuation was measured with experimental time, and the pressure (26) Lin, C. L.; Wey, M. Y.; You, S. D. Powder Technol. 2002, 126, 297–301. (27) Siegell, J. H. Powder Technol. 1984, 38, 13–22. (28) Tardos, G.; Mazzone, D.; Pfeffer, R. Can. J. Chem. Eng. 1985, 63, 384–389.
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Figure 4. Emission of heavy metals at different temperatures, with and without Na added (/ = defluidization time).
3. Results and Discussion
versus time profile was plotted. The pressure fluctuation was maintained stable before defluidization. As the defluidization occurred, a rapid decrease in the bed pressure drop was observed and this time was the defluidization time. The emitted heavy metals were sampled isokinetically (at location 9 in Figure 1) during incineration using the United States Environmental Protection Agency (U.S. EPA) Method 5.29 After the experiment, as the combustion chamber was cooled to room temperature, the agglomerate materials were subsequently collected and analyzed. In addition, the fly ash on the filter was pretreated by microwave digestion. The concentrations of heavy metals were determined using an atomic absorption (AA) spectrophotometer. The agglomerates were also analyzed by scanning electron microscopy/ energy-dispersive spectrometry (SEM/EDS) and X-ray diffraction (XRD) to observe the particle surfaces and metal species present in the agglomerates.
3.1. Effect of the Na Concentration and Temperature on Agglomeration/Defluidization. Figure 2 shows the effects of different Na concentrations on the time to reach agglomeration/ defluidization. As shown in Figure 2, when the Na concentration was greater than 0.5%, the defluidization time rapidly decreased. This result indicates that the Na concentration correlated negatively with the defluidization time, in which the time decreases significantly as the Na concentration increases. According to previous studies,2,7 alkali metals may react with silica sand or impurities in the sand to form lowmelting-point eutectics during combustion. When the operating temperature exceeds the melting points of these eutectics, they transform to a highly viscous liquid phase. These liquid species flow easily to cover the surface of the sand particles via particle collision, which then generates the
(29) Liu, Z. S. J. Hazard. Mater. 2007, 142, 506–511.
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agglomerates. Thus, as the Na concentration increases, more eutectics with low melting points are generated, which then decreases the time to defluidization. Figure 3 depicts the effects of the temperature (700, 800, and 900 °C) on the defluidization time. As shown in Figure 3, an increased operating temperature enhanced the generation of agglomerates. Manzoori and Agarwal19 indicated that the agglomeration rate increased with an increasing temperature, which is in agreement with the experimental data presented here. The amount of liquid-phase eutectics increases when the operating temperature increases. These liquid eutectics are highly viscous and move easily to other particles, which increases agglomeration and defluidization. 3.2. Effect of Mg and Ca on Agglomeration/Defluidization. Figure 3 also displays the effects of Mg and Ca on agglomeration/ defluidization. In general, the addition of Mg and Ca increases the time to defluidization. These alkaline earth metals play an inhibition role in agglomeration. Many researchers have discussed the role of Mg and Ca on agglomeration/defluidization. Arvlakis et al.,11 Manzoori and Agarwal,19 and Skrifvars et al.30 pointed out that Ca and Mg in the system enhance the formation of agglomerates; however, Vuthalrur and Zhang31 showed that Mg and Ca could generate high-melting-point eutectics, which would inhibit agglomeration. Previous studies have analyzed the relationship between the components of coal and metal elements in agglomerates. However, some metals existing in agglomerates are covered by eutectics and do not cause the formation of the agglomerates themselves. This phenomenon has not been observed clearly in previous research. In this study, artificial wastes were used to simulate agglomeration/defluidization. As Ca and Mg were added, the defluidization time increased. According to a previous study,8 Ca and Mg were added to artificial waste to confirm the roles of these alkaline earth metals. The results indicated that the added Ca and Mg inhibit agglomeration and increase the defluidization time. However, the concentration of Na, Ca, and Mg and operating conditions all affected the agglomeration/defluidization. When the mole ratios (Na/Ca and Na/Mg) decreased, the capability of inhibition increased. If the mole ratio was larger than 2, the effect of Ca and Mg on defluidization was insignificant. Additionally, the defluidization time at 700 and 800 °C was 3 or 4 times greater than that at 900 °C. Therefore, the effect of Ca and Mg on agglomeration/defluidization depends upon operating conditions during combustion. However, this experimental result showed that Ca and Mg inhibited agglomeration/defluidization during incineration, because the mole ratios (Na/Ca and Na/Mg) were low. 3.3. Effect of Agglomeration/Defluidization on the Emission of Heavy Metals. Figure 4 shows the emission of heavy metals at different temperatures as artificial wastes with or without Na added. If the artificial wastes did not contain Na, agglomeration/defluidization was not observed during incineration. Under these conditions, the emission of heavy metals was more stable. These experimental runs were operated continuously for 22 min, and the flue gas was sampled to determine the emitted concentrations of heavy metals. As shown in Figure 4, the emission concentrations of the three metals followed the order Cd > Pb > Cr under various conditions. These three metals (Cd, Pb, and Cr) have high,
Figure 5. SEM/EDS analysis of the agglomerates (800 °C, with the addition of Mg).
medium, and low volatility, respectively. The experimental data agreed with the order of the boiling points. When Na was added to the artificial wastes, the emission concentrations of heavy metals were stable before defluidization at 700 and 800 °C. As the system underwent defluidization, the amount of emitted heavy metals increased significantly. According to Chen et al.,32 when silica sand was employed as bed material, the sand adsorbed heavy metals and decreased their emission concentrations. The adsorption efficiency of the three metals followed the order Cr > Pb > Cd and corresponded to the order of their boiling points. Therefore, whether or not Na is added, silica sand plays an important role in the adsorption of heavy metals. Although the agglomerates formed gradually via liquid-phase eutectics, which reduced the quality of fluidization during incineration, most heavy metals were adsorbed by the silica sand, which maintained the stability of the heavy metal emissions. As the bed material reached defluidization, the quality of fluidization decreased significantly and the silica sand did not mix uniformly. Thus, the emission of heavy metals increased when the adsorption efficiency of silica sand decreased. A comparison of Na addition versus no added Na indicates that, at 700 and 800 °C, the emission of heavy metals decreased when Na was added. These results are illustrated in Figure 4. Although the quality of fluidization decreased as the low-melting-point eutectics reduced the adsorption
(30) Skrifvars, B. J.; Backman, R.; Hupa, M.; Sfiris, G.; A˚byhammar, T.; Lyngfelt, A. Fuel 1998, 77, 65–70. (31) Vuthaluru, H. B.; Zhang, D. K. Fuel 2001, 80, 583–598.
(32) Chen, J. C.; Wey, M. Y.; Yan, M. H. J. Environ. Eng. 1997, 123, 1100–1106.
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Figure 6. Effect of Ca and Mg addition on the emission of heavy metals at different temperatures during agglomeration/defluidization (/ = defluidization time).
that the adsorption by silica sand and covering or adherence by eutectics decreases the emission of heavy metals during agglomeration/defluidization. When the agglomerates are generated gradually at 700 and 800 °C, the quality of fluidization decreases slowly and the amount of adsorbed metals decreases insignificantly. In this case, liquid-phase eutectics covering or adhering to heavy metals becomes an important factor in the decreased emission of metals. However, the eutectics are generated in great quantities at 900 °C, which enhances the concentration of heavy metals in the bed. The quality of fluidization decreases quickly, which causes the silica sand to mix non-uniformly. This phenomenon decreases the adsorption efficiency and causes the emission of heavy metals to increase in the entire system.
efficiency of heavy metals, the emission concentrations of the three metals did not increase. These results may be attributed to three causes: (1) the metals were adsorbed by the silica sand, (2) the heavy metals reacted with Na to form eutectics, and (3) the heavy metals were covered by or adhered to the liquid-phase Na eutectics and remained in the eutectics on particle surfaces. Each of these reasons would cause the concentration of heavy metals to decrease in the flue gas during incineration. Figure 5 depicts an SEM/EDS image, showing that eutectics were observed between two particles and on particle surfaces. According to the EDS analysis, these materials contain heavy metal components. However, the emission of heavy metals increased adversely at 900 °C with Na addition. We speculate 2915
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with high melting points, such as CaO and MgO, were formed.34 These high-melting-point species may increase the melting point of the system to inhibit the generation of agglomerates. For the three heavy metals studied (Cd, Pb, and Cr), Cd has the highest volatility. In the combustion system, Cd did not react with other elements to generate eutectics or compounds, because it is volatilized at high temperatures. Therefore, no Cd species were present on the surface of the agglomerate (as shown in Figure 5), which implies that Cd was covered by the eutectics, which may be an important mechanism of decreased emission concentrations in flue gas. Most of the Pb and Cr compounds may form oxides. If these oxides have low melting points, such as some oxides of Pb, they may react with Na to form eutectics. Additionally, these oxides may also be covered by or adhered to eutectics and remain in the sand bed, thereby decreasing emission concentrations in flue gas.
3.4. Effect of Ca and Mg on the Emission of Heavy Metals. The effect of Ca and Mg addition on the emission of heavy metals at different temperatures during agglomeration/ defluidization is illustrated in Figure 6. In this figure, 0.0% Na (average) was used to symbolize the average emission concentration of heavy metals during incineration without Na addition. When Ca and Mg were added, the emission trend was similar to that of 0.0% Na (average); the emission concentrations of heavy metals were stable prior to defluidization. When the system underwent defluidization, the amount of heavy metals emitted increased significantly. The emission concentration of the three metals followed the order of their boiling points. From these experimental results, the emissions of heavy metals in the presence of Ca and Mg are lower than those without Ca and Mg addition. Adding Ca and Mg inhibited the generation of agglomerates, which maintained the quality of fluidization and the uniform mixing of the silica sand. The quality of fluidization decreased slowly and the amount of adsorbed metals decreased insignificantly. A large amount of heavy metals were covered by or adhered to the liquid-phase eutectics and remained in the liquid phase. Therefore, the emission of heavy metals decreased as Ca and Mg were added. The emission concentration of metals at 900 °C without Ca and Mg addition was larger than that of 0.0% Na (average) and unstable. After Ca and Mg were added, the time to defluidization was prolonged. The emission trend was similar to the results at 700 and 800 °C, and the emission concentration decreased at the same time. Therefore, adding Ca and Mg will maintain the quality of fluidization and prolong the defluidization time while simultaneously decreasing the emission concentrations of heavy metals. 3.5. Analysis of Agglomerate Species. In the fluidized bed, the viscous or plastic flow of materials is the most important mechanism to form agglomeration/defluization, because it is the most rapid.9 For glassy, non-crystalline materials, the move by a viscous or plastic mechanism is the major mechanism of agglomeration.9,33 However, a crystalloid component may be formed after incineration. However, only SiO2 was detected by XRD, and this result agreed with studies of Skrifvars et al.30 and Vuthaluru and Zhang.31 On the other hand, the qualitative SEM/EDS results (Figure 5) indicate that Na, Si, Mg, Pb, and Cr are present on particle surfaces. Therefore, these elements might exist in low-melting-point eutectics in agglomerates but may be too thin or not crystallized well enough to be detected by XRD. Additionally, these may also be the noncrystalline eutectics. According to previous studies,16,18 they analyzed the eutectic species on agglomerates. The Na compounds with low melting points, such as Na2O, were found. Additionally, Liu et al.34 also applied the electron spectroscopy for chemical analysis system (ESCA) to analyze the eutectic species, and Na compounds with low melting points, such as Na2CO3, NaNO3, Na2C2O4, NaHCO3, and Na2O, were also found on agglomerates. Therefore, those easily melt to the viscous liquid phase to form agglomerates at high temperatures. When Ca and Mg were added to the system, some compounds
4. Conclusions This study investigated the effects of various components of waste and operating parameters on heavy metal emission in the agglomeration/defluidization process in a fluidized bed. To simplify the factors involved, artificial waste was used to simulate municipal waste and produce agglomerates. The effects of alkali metals, alkaline earth metals, and operating temperature on the agglomeration and emission of heavy metals were considered. The results indicate that the Na concentration and defluidization time were negatively correlated, where the time decreased significantly as the Na concentration increased. Further, the addition of Mg and Ca increased the defluidization time and had an inhibitory role in agglomeration. The experimental results indicate that Na increased the risk of agglomeration/defluidization but decreased the emission concentrations of heavy metals at 700 and 800 °C. We speculate that adsorption by silica sand and covering or adherence by Na-containing eutectics are important mechanisms that decrease the emission of heavy metals during agglomeration/defluidization. However, at 900 °C, the quality of fluidization decreased quickly, causing the silica sand to mix non-uniformly and greatly increasing the generation of eutectics, thereby decreasing adsorption efficiency. Additionally, the emissions of heavy metals in the presence of Ca and Mg were lower than without Ca and Mg addition. The addition of Ca and Mg inhibited the generation of agglomerates, thereby maintaining the quality of fluidization and the uniform mixing of the silica sand. The quality of fluidization decreased slowly and the adsorption amount of metals decreased insignificantly. A large amount of heavy metals was covered by or adhered to the liquid-phase eutectics and remained in the eutectics. Therefore, the emission of heavy metals decreased as Ca and Mg were added at various temperatures. In the analysis of the agglomerates by XRD, only SiO2 was detected and other compounds of Na, Mg, Ca, Pb, Cr, and Cd were not found. In addition to XRD, SEM/EDS analysis was also employed, which indicated that Na, Si, Mg, Pb, and Cr were present on particle surfaces. Therefore, these eutectic species may exist in agglomerates but may be too thin to be detected by XRD. Also, these compounds are not crystalline. When Ca and Mg were added to the system, some compounds with high melting points, such
(33) Mikami, T.; Kamiya, H; Horio, M. Powder Technol. 1996, 89, 231–238. (34) Liu, Z. S.; Lin, C. L.; Chou, J. D. Fuel Process. Technol. 2010, 91, 591–599.
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: DOI:10.1021/ef901595d
Lin et al.
as CaO and MgO were formed, which might inhibit the generation of agglomerates. For the three heavy metals examined, most compounds (Pb and Cr) might be present as oxides but Cd was volatilized at high temperatures. These metallic species may react with Na to form eutectics or be covered by or adhered to eutectics and remain in the
sand bed, thus decreasing the emission concentrations in flue gas. Acknowledgment. The authors thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract NSC 96-2221-E-390-031-MY3.
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