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defluidization on emission of heavy metals for various fluidized parameters in fluidized-bed incineration
Fuel Processing Technology 91 (2010) 52–61
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
The effects of agglomeration/defluidization on emission of heavy metals for various fluidized parameters in fluidized-bed incineration Chiou-Liang Lin ⁎, Ming-Chih Tsai, Chih-Hung Chang Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, 811, Taiwan, ROC
a r t i c l e
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Article history: Received 10 June 2009 Received in revised form 18 August 2009 Accepted 18 August 2009 Keywords: Agglomeration Defluidization Incineration Fluidized bed Heavy metal
a b s t r a c t The agglomeration/defluidization may be produced to generate the secondary pollutant during incineration. However, the effects of agglomeration/defluidization on heavy metal distribution have rarely been examined. Therefore, the effects of the agglomeration/defluidization process on heavy metal emission in flue gas are studied. The artificial waste is employed to simulate municipal waste and to form agglomerates, which contain alkali metals, earth alkali metals, a mixture of metals (Pb, Cr and Cd) and sawdust. The fluidized parameters (including gas velocity, sand particle size and static bed height) are varied to determine their influences on heavy metal emission. The results indicate that addition of Na increases the risk of agglomeration/defluidization, but the emission concentration of heavy metals decreases during agglomeration/defluidization. The heavy metals may react with Na to form the eutectics or are covered and adhered by the liquid-phase eutectics of Na to stay in sand particle and lead to a decrease in the emission of heavy metals. The system was operated at a low gas velocity that not only easily resulted in agglomeration/defluidization but also increased the emission concentration of heavy metals. Large particles (920 μm), which have a poor fluidized quality, had the highest emission concentration. Small particles (645 μm) were uniformly fluidized to enhance the fluidization quality and to decrease the emission concentration. Additionally, adding Ca did not decrease the heavy metal emission concentration, but maintained the fluidization during eutectic accumulation. The Ca prevented the sand bed from quickly achieving defluidization and prolonged the increased emission of heavy metals after defluidization. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Fluidized-bed incinerators have been extensively employed to dispose of waste due to advantages that include good mixing of solids, high heat transfer and large contact surface area [1]. However, agglomeration/defluidization problems that occur during the operation of fluidized beds have frequently caused difficulties. These phenomena are caused by the accumulation of agglomerates. When agglomeration/defluidization occurs, the characteristics of fluidization (such as bubble size, bubble frequency, bubble velocity and minimum fluidization velocity) change [2] and influence the operation of the fluidization, even to the extent of shutting down the fluidized bed [3–6]. Morse and Ballou [7] indicated that good quality of fluidization expressed the air across uniformly through the bed and the particles being distributed well in the fluid stream. So the agglomeration/defluidization will change the characteristics of fluidization to decrease quality of fluidization. For fluidized-bed incineration, the components of the waste are complex and might contain some adhesion materials, such as alkalis, earth alkali metals, sulfur, chlorine and iron [8,9]. These elements will
⁎ Corresponding author. Tel.: +886 7 5919722; fax: +886 7 5919376. E-mail address: [email protected] (C.-L. Lin).
react with silica sand or impurities in sand to generate low melting point eutectics at high temperatures. These eutectics easily flow to other particle surfaces via particle collision to form agglomerates [8,10–12]. Many complex mechanisms lead to agglomeration during the fluidization process; some researchers have indicated that agglomeration depends on the stickiness and the collision of particles [13–15]. According to some studies, agglomerates form by generating low melting point species during combustion. Researchers have analyzed these visco-materials and found some low melting point species, such as Al2(SO4)3, Na2SO4, Na2O, Na2SiO3 and V2O5 [10,16,17]. These species lead to agglomeration/defluidization via the following two mechanisms of adhesion: (1) glassy materials are formed by sintering that easily flow to other particles, and (2) liquid-phase materials are generated by melting and chemical reactions [14,15]. However, some elements, such as alkalis, usually exist in municipal wastes. These elements easily form low melting point eutectics and adhere to the surfaces of particles during incineration to cause agglomeration/ defluidization [10,11,18,19]. During the fluidized-bed incineration process, some conditions affect the hydrodynamic fluidization behavior, such as the amount of excess air, operating gas velocity, operating temperature, particle size, compounds in the waste and mixing of the bed materials. These parameters directly affect the quality of the fluidization and indirectly
0378-3820/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.08.012
C.-L. Lin et al. / Fuel Processing Technology 91 (2010) 52–61
influence the generation of pollutants. Some pollutants, such as organics (PAHs and BTEXs), heavy metals and acid gas, may form during incineration [20]. Of these pollutants, heavy metal compounds are important because they can harm the environment and human health. Heavy metals are volatilized at high temperature to form metallic vapors or submicron particles, which are emitted into the environment by adsorption onto fly ash or mixing with the flue gas. Many operating parameters affect the distribution of heavy metals. These factors include the operating conditions and the intrinsic characteristics of metal, such as the combustion temperature, gas velocity, chlorine, loading of waste, and boiling point of the species. Fournier et al. [21] indicated that the distribution of heavy metals is primarily related to the boiling point of the species. In general, metal compounds are more easily vaporized after combustion which is distributed in fly ash or flue gas because metal compounds have a higher saturation vapor pressure. Some heavy metals with higher boiling points are found in the bottom of the ash. 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 simultaneously increase [22]. Therefore, the operating temperature and intrinsic characteristics influence the distributional ratios of heavy metals. During fluidized-bed incineration, adhesion materials may form agglomerates that affect the characteristics of fluidization (such as bubble size, bubble frequency, bubble velocity and minimum fluidization velocity), leading to the formation of secondary pollutants and the unscheduled shutdown of the reactor. This phenomenon increases the cost of operation and the risk of harming the environment and human health. Previous studies of agglomeration mainly focused on coal or biomass combustion. Most have investigated the effect of various constituent species in coal or biomass on agglomeration behavior [3,23]. However, municipal waste generally contains some alkali and alkali earth metals that most strongly influence the agglomeration. The municipal waste usually contains many heavy metals at the same time. Therefore, an agglomerate may be produced to generate the secondary pollutant, such as emission of heavy metal and organic pollutant during incineration. The effects of agglomeration/defluidization on heavy metal distribution have rarely been examined during incineration. In order to consider the emission of heavy metals during agglomeration/defluidization, this study focuses on the effects of different components of waste and the operating parameters on agglomeration/defluidization. The emission of heavy metals during the agglomeration/defluidization process is discussed. Artificial waste is employed to simulate municipal waste and to form agglomerates to simplify the factor, which contain alkali metals (Na), earth alkali metals (Ca), a mixture of metals (Pb, Cr and Cd) and sawdust. The experimental conditions include the gas velocity, sand particle size and static bed height. While analyzing the results of the experiment, the effects of the alkali metals, earth alkali metals and fluidized parameters on the agglomeration and emission of heavy metals are considered. The results can be used as a reference for the operation of fluidized-bed incinerators. 2. Experimental procedure Fig. 1 presents the bubbling fluidized-bed reactor that was used in this experiment. The main chamber is 120 cm high with an inner diameter of 10 cm. The reactor is made of 3-mm thick stainless steel (AISI 310). The apparatus is surrounded by an electrically resistant material, which is packed with ceramic fibers for thermal insulation. The reactor is equipped with a stainless steel porous plate with a 15% open area to allow for distribution of the gas. The program logical control (PID) controller connects with two thermocouples in order to control the temperature of the chamber. The cyclone is connected to a carbon filter to collect the particles that are emitted from the combustion chamber.
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An alkali metal (Na) was added to the artificial waste to form a low melting point eutectic that simulates the generation of agglomeration/defluidization during incineration. Additionally, in some tests an earth alkali metal (Ca) was added in order to consider the influence on the agglomeration and emission of heavy metals. Silica sand (645, 770 and 920 μm) contained SiO2 (97.80%), Al2O3 (2.01%) and Fe2O3 (0.07%) with a nearly constant density across all sizes (ρp = 2600 kg/ m3), was used as the bed material in the experiment. Those collected by the standard Taylor sieves in the ranges of 25–30, 20–25, and 18– 20 mesh. The total mass of the 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 alkali, alkaline earth and heavy metals (Pb, Cr and Cd) were added as nitrates in the artificial waste. The metal nitrates contained NaNO3, Ca(NO3)2, Pb (NO3)2, Cr(NO3)3 and Cd(NO3)2. The metal nitrates were dissolved in distilled water to form a metal solution (1 mL), which was then added to the sawdust and enclosed in a PE bag. In order to estimate the emission characteristics of heavy metals and to reduce the experimental errors, the 1 wt.% of every artificial waste is employed. Table 1 shows the concentrations (wt.%) of the added metals calculated as atoms of metals and non-nitrates. Before the experiment, the artificial waste was stored for one 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 experimental conditions included the gas velocity (U/Umf = 1.1, 1.3 and 1.5), sand particle size (645, 770 and 920 μm) and static bed height (H/D = 1.5, 1.8 and 2.1). Table 1 shows the parameters that were controlled for during the experiment. When the sand bed temperature was at a steady state, air was passed through the combustion chamber while the artificial waste was fed into the chamber at a rate of one bag every 20 s. The minimum fluidization velocity (Umf) was initially obtained from the pressure drop versus the gas velocity plot at 800 °C. Lin et al. [24] outlined a method for measuring the minimum fluidization velocity. The defluidization time can be determined by detecting the pressure fluctuation during the experiment. Tardos et al. [25,26] pointed out that the channeling and rapid pressure drop was associated with defluidization. Lin and Wey [10] have detailed the method for measuring defluidization time. Therefore, the pressure-versus-time profile and visual observation are used to evaluate the defluidization time. The pressure drop was determined from two pressure detectors that were located in the sand bed and freeboard. The two detectors were associated with different pressure transmitters, and the range of measurement was 0–1000 mm H2O. For a detailed method for measuring defluidization time, refer to Arena and Mastellone [27]. As the artificial waste was fed into the combustion chamber, the emitted heavy metals were isokinetically sampled every 4 min until the defluidization was reached. The U.S. EPA Method 5 was used to sample the heavy metals [28]. After defluidization, the experiment was stopped and the combustion chamber was cooled to room temperature. The agglomerate materials and the sand were subsequently collected and analyzed. In addition, the fly ash on the filter was pretreated by microwave digestion. Then, the concentration of the heavy metals was determined with an atomic absorption spectrophotometer (AA). In order to observe the surface of the particles and the species of the agglomerates, the agglomerates were also analyzed by scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) and X-ray diffraction (XRD). 3. Results and discussion 3.1. The effects of different conditions on agglomeration/defluidization 3.1.1. Effects of fluidized parameters Fig. 2 illustrates the defluidization times for different parameters. The effect of the gas velocity on the defluidization time is illustrated in
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Fig. 1. The 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) cyclone, (10) filter, and (11) induced fan.
Fig. 2(a), which shows that the defluidization time increased with increasing gas velocity. When the gas velocity increased, the momentum of the particles increased. According to previous research [10], Na reacts with silica sand or impurities in the sand to generate low melting point eutectics. In the incineration process, eutectics with low melting points melt to form liquid-phase eutectics at high temperatures. These liquid-phase eutectics easily flow to other particle surfaces via particle collision and have a highly viscous character. If there is not a large enough force to segregate the two particles, both particles adhere. Consequently, the balance of viscous and segregative forces affects the generation of agglomeration/ defluidization. As the gas velocity increases, the momentum of the particles increases while at the same time enhancing both the mixing of the sand bed and the segregative force. Therefore, agglomeration is difficult to generate as the segregative force increases. This conclusion
is similar to that found by Anodrf et al. [29], who concluded that a high velocity prevents agglomeration. Therefore, a high gas velocity can prolong operation by delaying the generation of agglomeration. The effect of particle size on the defluidization time is shown in Fig. 2(b). In this result, the defluidization time of the smaller size particles (645 μm) was larger than that of the larger particles. When the particle size increased, the defluidization time decreased because the momentum of particles and the fluidized characteristics were different. According to the particle classification of Geldart [30], particles can be classified into four types of powder (A, B, C and D) according to size and density. The B powder has a good fluidized behavior and is usually employed as a bed material in fluidized operation. However, the D powder has a larger size and density, poor solid mixing and fluidized quality, and easily spouts during fluidization. Thus, the D powder does not adapt to the bed materials in the
Table 1 Operating conditions for the experiments. Run
Temperature (°C)
Gas velocity (U/Umf)
Material size (μm)
Bed height (H/D)
Species of heavy metal
Concentration (%) Na
Ca
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800
1.1 1.3 1.5 1.3 1.3 1.3 1.3 1.1 1.3 1.5 1.3 1.3 1.3 1.3 1.1 1.3 1.5 1.3 1.3 1.3 1.3
770 770 770 770 770 645 920 770 770 770 770 770 645 920 770 770 770 770 770 645 920
1.8 1.8 1.8 1.5 2.1 1.8 1.8 1.8 1.8 1.8 1.5 2.1 1.8 1.8 1.8 1.8 1.8 1.5 2.1 1.8 1.8
Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb,
– – – – – – – 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7
– – – – – – – – – – – – – – 0.7 0.7 0.7 0.7 0.7 0.7 0.7
Umf = 0.1 m/s, bed height (H) = 18 cm, and inner diameter (D) = 10 cm.
Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd
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Fig. 2. The effects of (a) gas velocity, (b) particle size and (c) static bed height on the time to defluidization.
fluidized operation. In this experiment, the 645- and 770-μm particles belong to the B powder, while the 920-μm particles belong to the D powder. During the experiment, the gas velocity (0.13 m/s) was used as major gas velocity and was determined by a 770-μm particle at 800 °C. This velocity is not large enough to uniformly fluidize for large particle because the coarsest particles (920 μm) need a larger minimum fluidization velocity. Thus, the momentum of the particles is too low and the viscous force is larger than the segregative forces to easily form agglomerates. In contrast, the momentum of the particles increased for small particles at the same velocity. Therefore, the 645μm particles had the longest defluidization time of the three sizes. Fig. 2(c) illustrates the effect of static bed height on the defluidization time. According to the results, the effect of the static bed height on the defluidization time was not significant. Three static bed heights were classified as deep beds for fluidized operation because their H/D ratios were greater than one. These bed heights, with an H/D close to 2, had similar fluidized behaviors, leading to similar defluidization times. 3.1.2. Effect of adding Ca Fig. 2 also shows the effects of the addition of Ca for various fluidized parameters. The defluidization times of the fluid to which Ca was added were greater than those without Ca. The gas velocity, particle size and static bed height had the same effects. According to previous studies, Arvelakis et al. [8], Atakül et al. [17] and Fernández Llorente et al. [31] found that Ca increases the agglomeration/ defluidization; however, Conn [32] and Vuthaluru and Zhang [33] indicate that Ca can generate eutectics with high-melting points that inhibit agglomeration/defluidization. In order to eliminate the interference of other elements, artificial waste was employed to
elucidate the effects of Ca on agglomeration/defluidization with different fluidized parameters. According to our results, Ca can prolong the operating time and inhibit the generation of agglomeration/defluidization. The reason may be that the added Ca reacts with other elements to form high-melting-point materials. This data agrees with that of Lin et al. [6], who also found that the addition of Ca inhibits the generation of agglomeration/defluidization. 3.2. Emission concentrations of heavy metals during agglomeration/ defluidization Figs. 3 to 5 show the emission concentrations of heavy metals at different gas velocities, particle sizes and static bed heights. According to these results, the emission concentrations follow the sequence Cd > Pb > Cr under various conditions. Cd, Pb and Cr have high, medium and low volatilities, respectively, and their melting and boiling points, respectively, are as follows: Cd, 321.18 °C and 765 °C; Pb, 327.6 °C and 1740 °C; and Cr, 1857 °C and 2672 °C. These experimental results agree with the order of the boiling points. The fluidized material adsorbs some heavy metals, which is an important reason to apply this reactor. According to Wei et al. [34], the silica sand adsorbs heavy metals and decreases emission concentrations. Therefore, whether Na or Ca is added or not, the adsorption of heavy metals by silica sand plays an important role in decreasing the emission concentration of heavy metals. During the incineration process, the agglomerate is gradually formed by accumulation of the liquid-phase eutectics in order to reduce the quality of the fluidization. However, most heavy metals are adsorbed by silica sand to maintain stability of the heavy metal emission. As the bed material reaches the
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Fig. 3. The heavy metal emission concentrations for different gas velocities (X: defluidization time).
defluidization, the quality of the fluidization significantly decreases and stops the silica sand from uniformly mixing. The emission of heavy metals increases when the adsorption efficiency of silica sand decreases. As shown in Figs. 3 to 5, most of the emission concentrations were lower when Na was added as compared to the case when no Na is added under the same conditions. In general, eutectics with low melting points accumulated when Na was added in order to decrease the quality of the fluidization. The sand bed did not uniformly mix and may have increased the heavy metal emission concentration. However, the heavy metal emission concentration did not increase when Na was added. We speculate that these results may be attributed to three reasons: (1) adsorption by silica sand; (2) heavy
metals reacting with Na to form eutectics; and (3) heavy metals that are covered by or adhered to the liquid-phase eutectics of Na, causing them to remain in eutectics at the particle's surface. These mechanisms lead the emission of heavy metals to decrease in flue gas during incineration. 3.2.1. Effects of the fluidized parameters Fig. 3 shows the heavy metal emission concentrations at different gas velocities. Of the three gas velocities, the results for 1.1 Umf demonstrate the greatest emission concentration. Although Na and Ca were added, the heavy metal emission was still larger than the case when Na was not added. At 1.1 Umf, the gas velocity was close to the minimum fluidization velocity; thus, the mixture of the sand bed was
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Fig. 4. The heavy metal emission concentrations for different particle sizes (X: defluidization time).
less uniform than that for the high gas velocity. Therefore, the adsorption efficiency of silica sand decreased with decreasing gas velocity. Additionally, the momentum of the particles decreased at low gas velocities, with the result that these particles did not have enough segregative force to form an agglomerate. Thus, when the system is operated at a low gas velocity, it not only leads to agglomeration/defluidization, but also increases the heavy metal emission concentration. Fig. 4 shows the heavy metal emission concentrations for different particle sizes. As shown in the figure, the emission concentration for particles with a 920-μm diameter was the highest. Small particles (645 μm) uniformly fluidized to enhance the adsorption efficiency of the heavy metal. Additionally, the small particles had a higher fluidized quality than the others at the same gas velocity, which led the particles to have a large momentum that increased the segregative
force. Thus, small particles can prolong operation time and decrease heavy metal emission concentrations whether Na and Ca are added or not. The 920-μm particles are classified as a D powder and need a larger minimum fluidization velocity. The 920-μm particles have poor solid mixing and poor fluidized quality during fluidization; therefore, the emission concentration was higher than in the case of no Na addition before defluidization. In this experiment, the addition of Na formed eutectics and decreased the emission of heavy metals, but this phenomenon was not found in the 920-μm test. Fig. 5 illustrates the results for different static bed heights. Three static bed heights had similar emission concentrations. Most of the heavy metal emission concentrations when Na was added were lower than for the cases of not adding Na. In addition, the emission concentration greatly increased with defluidization. Three static bed heights were classified as deep beds for fluidized operation and had
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Fig. 5. The heavy metal emission concentrations for different static bed heights (X: defluidization time).
similar fluidized behaviors that led to similar defluidization times and heavy metal emission concentrations.
3.2.2. Effect of the addition of Ca Figs. 3 to 5 also illustrate the results of adding Ca. After adding Ca, the defluidization time was prolonged and the emission concentration trend was similar to the case where only Na is added. Adding Ca inhibited the generation of agglomerates that maintain the quality of fluidization and the well-mixing of silica sand. A comparison of the case where only Na was added and the case where both Na and Ca were added indicates that the addition of Na greatly decreased the heavy metal emission concentration, but the effect of Ca on the emission concentration was not significant. Therefore, adding Ca does not decrease the emission, but it could maintain the fluidization during eutectic accumulation. The Ca prevents the sand bed from
quickly achieving defluidization and prolongs the large heavy metal emission after defluidization. 3.3. The characteristics of the sand bed after agglomeration/ defluidization 3.3.1. Particle size distribution Fig. 6 illustrates the results of the particle size distribution of the sand bed for different parameters. Comparing with the size distributions of the three operating conditions (no added Na, only added Na and added Na and Ca), the ratios of coarse particles (>770 μm) in the added Na condition and the added Na and Ca condition were larger than that of the no added Na condition. These two conditions had lower ratios of fine particles (<770 μm) than the no added Na condition. According to previous studies [35,36], attrition increases the number of particles and reduces the particle size. Experimental
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Fig. 6. The particle size distributions after agglomeration/defluidization for the following conditions: (a) 1.1 Umf, (b) 1.5 Umf, (c) 1.5 H/D, and (d) 920 μm.
results also indicate that the attrition rate of the sand bed increases with increasing temperature. Therefore, the particle size normally decreases after fluidization at high temperatures. However, Na may react with silica sand or impurities in the sand during the incineration to generate eutectics with low melting points and to form agglomerates. While, the quantity of fine particles increases with collisions between particles, these attrited particles may adhere via melting eutectics to increase particle size. According to these results, the added Na condition and the added Na and Ca condition significantly decreased the proportion of fine particle, thus increasing the relative quantity of coarse particles. Of the three different fluidized parameters, gas velocity had the greatest influence on the particle size distribution. As shown in Fig. 6(a) and (b), the proportion of fine particles increased with increased gas velocity. As the gas velocity increased, the momentum of the particles increased to enhance the collision probability and then the fine particles largely generate. In contrast, the particle and the melting eutectics easily generate agglomerates and rarely separate at low gas velocities. Therefore, the ratio of coarse particles increased at low gas velocities. For results with different static bed heights and sand particle sizes, these parameters were less influenced by particle size distribution.
However, the particle size distribution was still related to the addition of Na and Ca, as can be seen in Fig. 6(c) and (d). 3.3.2. Characteristics of agglomerates After a sieve analysis, the agglomerate particles were collected to analyze the eutectic species by XRD. However, only SiO2 was detected. Thus, the eutectics may be too thin to be detected by XRD, or they are not crystalline. Fig. 7 shows the SEM/EDS image. As shown in the figure, the elements (including Na, Si, Cd, Cr and Pb) were obtained from the agglomerate surface. Therefore, we speculate that the heavy metal species may cover or adhere via eutectics and remain in the sand particles to decrease the emission concentration in flue gas. 4. Conclusions In order to study the effects of the agglomeration/defluidization process on heavy metal emission in flue gas, Na was used to simulate the generation of agglomerates. Additionally, fluidized parameters (including the gas velocity, sand particle size and static bed height) were varied to determine their influences on heavy metal emission. According to our results, a comparison of the case where Na was
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Fig. 7. The FE-SEM/EDS results for the agglomerate (800 °C, 770 μm and 0.7%Na added).
added to the case where no Na was added shows that most of the emission concentrations in the Na added case were lower than those of the no Na added case under the same conditions. Therefore, the addition of Na increases the risk of agglomeration/defluidization, but the heavy metal emission concentration decreases during agglomeration/defluidization. However, the emission concentration greatly increases after defluidization. For heavy metals to remain in the sand particles, we speculate that they react with Na to form eutectics or that they are covered or adhered by the liquid-phase eutectics of Na. These mechanisms decrease the emission of heavy metals in flue gas. At a low gas velocity (1.1 Umf), the sand bed mixture is less uniform than that with a high gas velocity, which decreases the adsorption efficiency of silica sand. Therefore, if the system operates at a low gas velocity, it not only easily generates agglomeration/ defluidization, but also increases the heavy metal emission concentration. Small particles (645 μm) fluidize uniformly to enhance the adsorption efficiency of heavy metals and have a high momentum to prolong the operation time. Larger (920 μm) particles have poor solid mixing and a poor fluidized quality during fluidization; therefore, before defluidization, the emission concentration is higher than that of the no Na added case. However, three static bed heights were classified as deep beds for fluidized operation and had similar fluidized behaviors that lead to similar defluidization times and heavy metal emission concentrations. Additionally, adding Ca did not
decrease the heavy metal emission concentration, but it could maintain the fluidization during eutectic accumulation. The Ca prevents the sand bed from quickly achieving defluidization and prolongs the enhanced emission of heavy metals after defluidization. Acknowledgment The authors thank the National Science Council of the Republic of China, Taiwan for financial support of this research under Contract NSC 96-2221-E-390-031-MY3.
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[21] D.J. Fournier, W.E. Whitworth, J.W. Lee, L.R. Waterland, The Fate of Trace Metals in a Rotary Kiln Incinerator with a Venturi/Packed Column Scrubber, EPA/600/S2-90/ 043 Feb, 1991. [22] R.W. Gerstle, D.N. Albrinck, Atmospheric emissions of metals from sewage sludge incineration, J. Air Polluti. Control Assoc. 32 (1982) 1113–1123. [23] B.M. Jenkins, L.L. Baxter Jr., T.R. Milers, T.R. Miles, Combustion properties of biomass, Fuel Process. Technol. 54 (1998) 17–46. [24] C.L. Lin, M.Y. Wey, S.D. You, The effect of particle size distribution on minimum fluidization velocity at high temperature, Powder Technol. 126 (2002) 297–301. [25] G. Tardos, D. Mazzone, R. Pfeffer, Destabilization of fluidized beds due to agglomeration part I: theoretical model, Can. J. Chem. Eng. 63 (1985) 377–383. [26] G. Tardos, D. Mazzone, R. Pfeffer, Destabilization of fluidized beds due to agglomeration part II: experimental verification, Can. J. Chem. Eng. 63 (1985) 384–389. [27] U. Arena, M.L. Mastellone, The phenomenology of bed defluidization during the pyrolysis of a food-packaging plastic waste, Powder Technol. 120 (2001) 127–133. [28] Z.S. Liu, Control of heavy metals during incineration using activated carbon fibers, J. Hazard. Mater. 142 (2007) 506–511. [29] R. Andorf, L. Mleczko, D. Schweer, M. Baerns, Oxidative coupling of methane in a bubbling fluidized bed reactor, Can. J. Chem. Eng. 69 (1991) 891–897. [30] D. Geldart, Types of gas fluidization, Powder Technol. 7 (1973) 285–292. [31] M.J. Fernández Llorente, J.M. Murillo Laplaza, R. Escalada Cuadrado, J.E. Carrasco García, Ash behaviour of lignocellulosic biomass in bubbling fluidized bed combustion, Fuel 85 (2006) 1157–1165. [32] R.E. Conn, Laboratory techniques for evaluating ash agglomeration potential in petroleum coke fired circulating fluidized bed combustors, Fuel Process. Technol. 44 (1995) 95–103. [33] H.B. Vuthaluru, D.K. Zhang, Remediation of ash problems in fluidised-bed combustors, Fuel 80 (2001) 583–598. [34] M.C. Wei, M.Y. Wey, C.L. Lin, The competitive adsorption of heavy metals under various incineration conditions, J. Chem. Eng. Jpn. 36 (2003) 243–249. [35] C.L. Lin, M.Y. Wey, Effects of high temperature and combustion on fluidized materials attrition in fluidized bed, Korean J. Chem. Eng. 20 (2003) 1123–1130. [36] C.L. Lin, M.Y. Wey, Influence of hydrodynamic parameters on particle attrition during fluidization at high temperature, Korean J. Chem. Eng. 22 (2005) 154–160.
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
The effects of agglomeration/defluidization on emission of heavy metals for various fluidized parameters in fluidized-bed incineration Chiou-Liang Lin ⁎, Ming-Chih Tsai, Chih-Hung Chang Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, 811, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 10 June 2009 Received in revised form 18 August 2009 Accepted 18 August 2009 Keywords: Agglomeration Defluidization Incineration Fluidized bed Heavy metal
a b s t r a c t The agglomeration/defluidization may be produced to generate the secondary pollutant during incineration. However, the effects of agglomeration/defluidization on heavy metal distribution have rarely been examined. Therefore, the effects of the agglomeration/defluidization process on heavy metal emission in flue gas are studied. The artificial waste is employed to simulate municipal waste and to form agglomerates, which contain alkali metals, earth alkali metals, a mixture of metals (Pb, Cr and Cd) and sawdust. The fluidized parameters (including gas velocity, sand particle size and static bed height) are varied to determine their influences on heavy metal emission. The results indicate that addition of Na increases the risk of agglomeration/defluidization, but the emission concentration of heavy metals decreases during agglomeration/defluidization. The heavy metals may react with Na to form the eutectics or are covered and adhered by the liquid-phase eutectics of Na to stay in sand particle and lead to a decrease in the emission of heavy metals. The system was operated at a low gas velocity that not only easily resulted in agglomeration/defluidization but also increased the emission concentration of heavy metals. Large particles (920 μm), which have a poor fluidized quality, had the highest emission concentration. Small particles (645 μm) were uniformly fluidized to enhance the fluidization quality and to decrease the emission concentration. Additionally, adding Ca did not decrease the heavy metal emission concentration, but maintained the fluidization during eutectic accumulation. The Ca prevented the sand bed from quickly achieving defluidization and prolonged the increased emission of heavy metals after defluidization. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Fluidized-bed incinerators have been extensively employed to dispose of waste due to advantages that include good mixing of solids, high heat transfer and large contact surface area [1]. However, agglomeration/defluidization problems that occur during the operation of fluidized beds have frequently caused difficulties. These phenomena are caused by the accumulation of agglomerates. When agglomeration/defluidization occurs, the characteristics of fluidization (such as bubble size, bubble frequency, bubble velocity and minimum fluidization velocity) change [2] and influence the operation of the fluidization, even to the extent of shutting down the fluidized bed [3–6]. Morse and Ballou [7] indicated that good quality of fluidization expressed the air across uniformly through the bed and the particles being distributed well in the fluid stream. So the agglomeration/defluidization will change the characteristics of fluidization to decrease quality of fluidization. For fluidized-bed incineration, the components of the waste are complex and might contain some adhesion materials, such as alkalis, earth alkali metals, sulfur, chlorine and iron [8,9]. These elements will
⁎ Corresponding author. Tel.: +886 7 5919722; fax: +886 7 5919376. E-mail address: [email protected] (C.-L. Lin).
react with silica sand or impurities in sand to generate low melting point eutectics at high temperatures. These eutectics easily flow to other particle surfaces via particle collision to form agglomerates [8,10–12]. Many complex mechanisms lead to agglomeration during the fluidization process; some researchers have indicated that agglomeration depends on the stickiness and the collision of particles [13–15]. According to some studies, agglomerates form by generating low melting point species during combustion. Researchers have analyzed these visco-materials and found some low melting point species, such as Al2(SO4)3, Na2SO4, Na2O, Na2SiO3 and V2O5 [10,16,17]. These species lead to agglomeration/defluidization via the following two mechanisms of adhesion: (1) glassy materials are formed by sintering that easily flow to other particles, and (2) liquid-phase materials are generated by melting and chemical reactions [14,15]. However, some elements, such as alkalis, usually exist in municipal wastes. These elements easily form low melting point eutectics and adhere to the surfaces of particles during incineration to cause agglomeration/ defluidization [10,11,18,19]. During the fluidized-bed incineration process, some conditions affect the hydrodynamic fluidization behavior, such as the amount of excess air, operating gas velocity, operating temperature, particle size, compounds in the waste and mixing of the bed materials. These parameters directly affect the quality of the fluidization and indirectly
0378-3820/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.08.012
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influence the generation of pollutants. Some pollutants, such as organics (PAHs and BTEXs), heavy metals and acid gas, may form during incineration [20]. Of these pollutants, heavy metal compounds are important because they can harm the environment and human health. Heavy metals are volatilized at high temperature to form metallic vapors or submicron particles, which are emitted into the environment by adsorption onto fly ash or mixing with the flue gas. Many operating parameters affect the distribution of heavy metals. These factors include the operating conditions and the intrinsic characteristics of metal, such as the combustion temperature, gas velocity, chlorine, loading of waste, and boiling point of the species. Fournier et al. [21] indicated that the distribution of heavy metals is primarily related to the boiling point of the species. In general, metal compounds are more easily vaporized after combustion which is distributed in fly ash or flue gas because metal compounds have a higher saturation vapor pressure. Some heavy metals with higher boiling points are found in the bottom of the ash. 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 simultaneously increase [22]. Therefore, the operating temperature and intrinsic characteristics influence the distributional ratios of heavy metals. During fluidized-bed incineration, adhesion materials may form agglomerates that affect the characteristics of fluidization (such as bubble size, bubble frequency, bubble velocity and minimum fluidization velocity), leading to the formation of secondary pollutants and the unscheduled shutdown of the reactor. This phenomenon increases the cost of operation and the risk of harming the environment and human health. Previous studies of agglomeration mainly focused on coal or biomass combustion. Most have investigated the effect of various constituent species in coal or biomass on agglomeration behavior [3,23]. However, municipal waste generally contains some alkali and alkali earth metals that most strongly influence the agglomeration. The municipal waste usually contains many heavy metals at the same time. Therefore, an agglomerate may be produced to generate the secondary pollutant, such as emission of heavy metal and organic pollutant during incineration. The effects of agglomeration/defluidization on heavy metal distribution have rarely been examined during incineration. In order to consider the emission of heavy metals during agglomeration/defluidization, this study focuses on the effects of different components of waste and the operating parameters on agglomeration/defluidization. The emission of heavy metals during the agglomeration/defluidization process is discussed. Artificial waste is employed to simulate municipal waste and to form agglomerates to simplify the factor, which contain alkali metals (Na), earth alkali metals (Ca), a mixture of metals (Pb, Cr and Cd) and sawdust. The experimental conditions include the gas velocity, sand particle size and static bed height. While analyzing the results of the experiment, the effects of the alkali metals, earth alkali metals and fluidized parameters on the agglomeration and emission of heavy metals are considered. The results can be used as a reference for the operation of fluidized-bed incinerators. 2. Experimental procedure Fig. 1 presents the bubbling fluidized-bed reactor that was used in this experiment. The main chamber is 120 cm high with an inner diameter of 10 cm. The reactor is made of 3-mm thick stainless steel (AISI 310). The apparatus is surrounded by an electrically resistant material, which is packed with ceramic fibers for thermal insulation. The reactor is equipped with a stainless steel porous plate with a 15% open area to allow for distribution of the gas. The program logical control (PID) controller connects with two thermocouples in order to control the temperature of the chamber. The cyclone is connected to a carbon filter to collect the particles that are emitted from the combustion chamber.
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An alkali metal (Na) was added to the artificial waste to form a low melting point eutectic that simulates the generation of agglomeration/defluidization during incineration. Additionally, in some tests an earth alkali metal (Ca) was added in order to consider the influence on the agglomeration and emission of heavy metals. Silica sand (645, 770 and 920 μm) contained SiO2 (97.80%), Al2O3 (2.01%) and Fe2O3 (0.07%) with a nearly constant density across all sizes (ρp = 2600 kg/ m3), was used as the bed material in the experiment. Those collected by the standard Taylor sieves in the ranges of 25–30, 20–25, and 18– 20 mesh. The total mass of the 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 alkali, alkaline earth and heavy metals (Pb, Cr and Cd) were added as nitrates in the artificial waste. The metal nitrates contained NaNO3, Ca(NO3)2, Pb (NO3)2, Cr(NO3)3 and Cd(NO3)2. The metal nitrates were dissolved in distilled water to form a metal solution (1 mL), which was then added to the sawdust and enclosed in a PE bag. In order to estimate the emission characteristics of heavy metals and to reduce the experimental errors, the 1 wt.% of every artificial waste is employed. Table 1 shows the concentrations (wt.%) of the added metals calculated as atoms of metals and non-nitrates. Before the experiment, the artificial waste was stored for one 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 experimental conditions included the gas velocity (U/Umf = 1.1, 1.3 and 1.5), sand particle size (645, 770 and 920 μm) and static bed height (H/D = 1.5, 1.8 and 2.1). Table 1 shows the parameters that were controlled for during the experiment. When the sand bed temperature was at a steady state, air was passed through the combustion chamber while the artificial waste was fed into the chamber at a rate of one bag every 20 s. The minimum fluidization velocity (Umf) was initially obtained from the pressure drop versus the gas velocity plot at 800 °C. Lin et al. [24] outlined a method for measuring the minimum fluidization velocity. The defluidization time can be determined by detecting the pressure fluctuation during the experiment. Tardos et al. [25,26] pointed out that the channeling and rapid pressure drop was associated with defluidization. Lin and Wey [10] have detailed the method for measuring defluidization time. Therefore, the pressure-versus-time profile and visual observation are used to evaluate the defluidization time. The pressure drop was determined from two pressure detectors that were located in the sand bed and freeboard. The two detectors were associated with different pressure transmitters, and the range of measurement was 0–1000 mm H2O. For a detailed method for measuring defluidization time, refer to Arena and Mastellone [27]. As the artificial waste was fed into the combustion chamber, the emitted heavy metals were isokinetically sampled every 4 min until the defluidization was reached. The U.S. EPA Method 5 was used to sample the heavy metals [28]. After defluidization, the experiment was stopped and the combustion chamber was cooled to room temperature. The agglomerate materials and the sand were subsequently collected and analyzed. In addition, the fly ash on the filter was pretreated by microwave digestion. Then, the concentration of the heavy metals was determined with an atomic absorption spectrophotometer (AA). In order to observe the surface of the particles and the species of the agglomerates, the agglomerates were also analyzed by scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) and X-ray diffraction (XRD). 3. Results and discussion 3.1. The effects of different conditions on agglomeration/defluidization 3.1.1. Effects of fluidized parameters Fig. 2 illustrates the defluidization times for different parameters. The effect of the gas velocity on the defluidization time is illustrated in
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Fig. 1. The 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) cyclone, (10) filter, and (11) induced fan.
Fig. 2(a), which shows that the defluidization time increased with increasing gas velocity. When the gas velocity increased, the momentum of the particles increased. According to previous research [10], Na reacts with silica sand or impurities in the sand to generate low melting point eutectics. In the incineration process, eutectics with low melting points melt to form liquid-phase eutectics at high temperatures. These liquid-phase eutectics easily flow to other particle surfaces via particle collision and have a highly viscous character. If there is not a large enough force to segregate the two particles, both particles adhere. Consequently, the balance of viscous and segregative forces affects the generation of agglomeration/ defluidization. As the gas velocity increases, the momentum of the particles increases while at the same time enhancing both the mixing of the sand bed and the segregative force. Therefore, agglomeration is difficult to generate as the segregative force increases. This conclusion
is similar to that found by Anodrf et al. [29], who concluded that a high velocity prevents agglomeration. Therefore, a high gas velocity can prolong operation by delaying the generation of agglomeration. The effect of particle size on the defluidization time is shown in Fig. 2(b). In this result, the defluidization time of the smaller size particles (645 μm) was larger than that of the larger particles. When the particle size increased, the defluidization time decreased because the momentum of particles and the fluidized characteristics were different. According to the particle classification of Geldart [30], particles can be classified into four types of powder (A, B, C and D) according to size and density. The B powder has a good fluidized behavior and is usually employed as a bed material in fluidized operation. However, the D powder has a larger size and density, poor solid mixing and fluidized quality, and easily spouts during fluidization. Thus, the D powder does not adapt to the bed materials in the
Table 1 Operating conditions for the experiments. Run
Temperature (°C)
Gas velocity (U/Umf)
Material size (μm)
Bed height (H/D)
Species of heavy metal
Concentration (%) Na
Ca
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800
1.1 1.3 1.5 1.3 1.3 1.3 1.3 1.1 1.3 1.5 1.3 1.3 1.3 1.3 1.1 1.3 1.5 1.3 1.3 1.3 1.3
770 770 770 770 770 645 920 770 770 770 770 770 645 920 770 770 770 770 770 645 920
1.8 1.8 1.8 1.5 2.1 1.8 1.8 1.8 1.8 1.8 1.5 2.1 1.8 1.8 1.8 1.8 1.8 1.5 2.1 1.8 1.8
Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb, Pb,
– – – – – – – 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7
– – – – – – – – – – – – – – 0.7 0.7 0.7 0.7 0.7 0.7 0.7
Umf = 0.1 m/s, bed height (H) = 18 cm, and inner diameter (D) = 10 cm.
Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd Cr, Cd
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Fig. 2. The effects of (a) gas velocity, (b) particle size and (c) static bed height on the time to defluidization.
fluidized operation. In this experiment, the 645- and 770-μm particles belong to the B powder, while the 920-μm particles belong to the D powder. During the experiment, the gas velocity (0.13 m/s) was used as major gas velocity and was determined by a 770-μm particle at 800 °C. This velocity is not large enough to uniformly fluidize for large particle because the coarsest particles (920 μm) need a larger minimum fluidization velocity. Thus, the momentum of the particles is too low and the viscous force is larger than the segregative forces to easily form agglomerates. In contrast, the momentum of the particles increased for small particles at the same velocity. Therefore, the 645μm particles had the longest defluidization time of the three sizes. Fig. 2(c) illustrates the effect of static bed height on the defluidization time. According to the results, the effect of the static bed height on the defluidization time was not significant. Three static bed heights were classified as deep beds for fluidized operation because their H/D ratios were greater than one. These bed heights, with an H/D close to 2, had similar fluidized behaviors, leading to similar defluidization times. 3.1.2. Effect of adding Ca Fig. 2 also shows the effects of the addition of Ca for various fluidized parameters. The defluidization times of the fluid to which Ca was added were greater than those without Ca. The gas velocity, particle size and static bed height had the same effects. According to previous studies, Arvelakis et al. [8], Atakül et al. [17] and Fernández Llorente et al. [31] found that Ca increases the agglomeration/ defluidization; however, Conn [32] and Vuthaluru and Zhang [33] indicate that Ca can generate eutectics with high-melting points that inhibit agglomeration/defluidization. In order to eliminate the interference of other elements, artificial waste was employed to
elucidate the effects of Ca on agglomeration/defluidization with different fluidized parameters. According to our results, Ca can prolong the operating time and inhibit the generation of agglomeration/defluidization. The reason may be that the added Ca reacts with other elements to form high-melting-point materials. This data agrees with that of Lin et al. [6], who also found that the addition of Ca inhibits the generation of agglomeration/defluidization. 3.2. Emission concentrations of heavy metals during agglomeration/ defluidization Figs. 3 to 5 show the emission concentrations of heavy metals at different gas velocities, particle sizes and static bed heights. According to these results, the emission concentrations follow the sequence Cd > Pb > Cr under various conditions. Cd, Pb and Cr have high, medium and low volatilities, respectively, and their melting and boiling points, respectively, are as follows: Cd, 321.18 °C and 765 °C; Pb, 327.6 °C and 1740 °C; and Cr, 1857 °C and 2672 °C. These experimental results agree with the order of the boiling points. The fluidized material adsorbs some heavy metals, which is an important reason to apply this reactor. According to Wei et al. [34], the silica sand adsorbs heavy metals and decreases emission concentrations. Therefore, whether Na or Ca is added or not, the adsorption of heavy metals by silica sand plays an important role in decreasing the emission concentration of heavy metals. During the incineration process, the agglomerate is gradually formed by accumulation of the liquid-phase eutectics in order to reduce the quality of the fluidization. However, most heavy metals are adsorbed by silica sand to maintain stability of the heavy metal emission. As the bed material reaches the
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Fig. 3. The heavy metal emission concentrations for different gas velocities (X: defluidization time).
defluidization, the quality of the fluidization significantly decreases and stops the silica sand from uniformly mixing. The emission of heavy metals increases when the adsorption efficiency of silica sand decreases. As shown in Figs. 3 to 5, most of the emission concentrations were lower when Na was added as compared to the case when no Na is added under the same conditions. In general, eutectics with low melting points accumulated when Na was added in order to decrease the quality of the fluidization. The sand bed did not uniformly mix and may have increased the heavy metal emission concentration. However, the heavy metal emission concentration did not increase when Na was added. We speculate that these results may be attributed to three reasons: (1) adsorption by silica sand; (2) heavy
metals reacting with Na to form eutectics; and (3) heavy metals that are covered by or adhered to the liquid-phase eutectics of Na, causing them to remain in eutectics at the particle's surface. These mechanisms lead the emission of heavy metals to decrease in flue gas during incineration. 3.2.1. Effects of the fluidized parameters Fig. 3 shows the heavy metal emission concentrations at different gas velocities. Of the three gas velocities, the results for 1.1 Umf demonstrate the greatest emission concentration. Although Na and Ca were added, the heavy metal emission was still larger than the case when Na was not added. At 1.1 Umf, the gas velocity was close to the minimum fluidization velocity; thus, the mixture of the sand bed was
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Fig. 4. The heavy metal emission concentrations for different particle sizes (X: defluidization time).
less uniform than that for the high gas velocity. Therefore, the adsorption efficiency of silica sand decreased with decreasing gas velocity. Additionally, the momentum of the particles decreased at low gas velocities, with the result that these particles did not have enough segregative force to form an agglomerate. Thus, when the system is operated at a low gas velocity, it not only leads to agglomeration/defluidization, but also increases the heavy metal emission concentration. Fig. 4 shows the heavy metal emission concentrations for different particle sizes. As shown in the figure, the emission concentration for particles with a 920-μm diameter was the highest. Small particles (645 μm) uniformly fluidized to enhance the adsorption efficiency of the heavy metal. Additionally, the small particles had a higher fluidized quality than the others at the same gas velocity, which led the particles to have a large momentum that increased the segregative
force. Thus, small particles can prolong operation time and decrease heavy metal emission concentrations whether Na and Ca are added or not. The 920-μm particles are classified as a D powder and need a larger minimum fluidization velocity. The 920-μm particles have poor solid mixing and poor fluidized quality during fluidization; therefore, the emission concentration was higher than in the case of no Na addition before defluidization. In this experiment, the addition of Na formed eutectics and decreased the emission of heavy metals, but this phenomenon was not found in the 920-μm test. Fig. 5 illustrates the results for different static bed heights. Three static bed heights had similar emission concentrations. Most of the heavy metal emission concentrations when Na was added were lower than for the cases of not adding Na. In addition, the emission concentration greatly increased with defluidization. Three static bed heights were classified as deep beds for fluidized operation and had
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Fig. 5. The heavy metal emission concentrations for different static bed heights (X: defluidization time).
similar fluidized behaviors that led to similar defluidization times and heavy metal emission concentrations.
3.2.2. Effect of the addition of Ca Figs. 3 to 5 also illustrate the results of adding Ca. After adding Ca, the defluidization time was prolonged and the emission concentration trend was similar to the case where only Na is added. Adding Ca inhibited the generation of agglomerates that maintain the quality of fluidization and the well-mixing of silica sand. A comparison of the case where only Na was added and the case where both Na and Ca were added indicates that the addition of Na greatly decreased the heavy metal emission concentration, but the effect of Ca on the emission concentration was not significant. Therefore, adding Ca does not decrease the emission, but it could maintain the fluidization during eutectic accumulation. The Ca prevents the sand bed from
quickly achieving defluidization and prolongs the large heavy metal emission after defluidization. 3.3. The characteristics of the sand bed after agglomeration/ defluidization 3.3.1. Particle size distribution Fig. 6 illustrates the results of the particle size distribution of the sand bed for different parameters. Comparing with the size distributions of the three operating conditions (no added Na, only added Na and added Na and Ca), the ratios of coarse particles (>770 μm) in the added Na condition and the added Na and Ca condition were larger than that of the no added Na condition. These two conditions had lower ratios of fine particles (<770 μm) than the no added Na condition. According to previous studies [35,36], attrition increases the number of particles and reduces the particle size. Experimental
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Fig. 6. The particle size distributions after agglomeration/defluidization for the following conditions: (a) 1.1 Umf, (b) 1.5 Umf, (c) 1.5 H/D, and (d) 920 μm.
results also indicate that the attrition rate of the sand bed increases with increasing temperature. Therefore, the particle size normally decreases after fluidization at high temperatures. However, Na may react with silica sand or impurities in the sand during the incineration to generate eutectics with low melting points and to form agglomerates. While, the quantity of fine particles increases with collisions between particles, these attrited particles may adhere via melting eutectics to increase particle size. According to these results, the added Na condition and the added Na and Ca condition significantly decreased the proportion of fine particle, thus increasing the relative quantity of coarse particles. Of the three different fluidized parameters, gas velocity had the greatest influence on the particle size distribution. As shown in Fig. 6(a) and (b), the proportion of fine particles increased with increased gas velocity. As the gas velocity increased, the momentum of the particles increased to enhance the collision probability and then the fine particles largely generate. In contrast, the particle and the melting eutectics easily generate agglomerates and rarely separate at low gas velocities. Therefore, the ratio of coarse particles increased at low gas velocities. For results with different static bed heights and sand particle sizes, these parameters were less influenced by particle size distribution.
However, the particle size distribution was still related to the addition of Na and Ca, as can be seen in Fig. 6(c) and (d). 3.3.2. Characteristics of agglomerates After a sieve analysis, the agglomerate particles were collected to analyze the eutectic species by XRD. However, only SiO2 was detected. Thus, the eutectics may be too thin to be detected by XRD, or they are not crystalline. Fig. 7 shows the SEM/EDS image. As shown in the figure, the elements (including Na, Si, Cd, Cr and Pb) were obtained from the agglomerate surface. Therefore, we speculate that the heavy metal species may cover or adhere via eutectics and remain in the sand particles to decrease the emission concentration in flue gas. 4. Conclusions In order to study the effects of the agglomeration/defluidization process on heavy metal emission in flue gas, Na was used to simulate the generation of agglomerates. Additionally, fluidized parameters (including the gas velocity, sand particle size and static bed height) were varied to determine their influences on heavy metal emission. According to our results, a comparison of the case where Na was
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Fig. 7. The FE-SEM/EDS results for the agglomerate (800 °C, 770 μm and 0.7%Na added).
added to the case where no Na was added shows that most of the emission concentrations in the Na added case were lower than those of the no Na added case under the same conditions. Therefore, the addition of Na increases the risk of agglomeration/defluidization, but the heavy metal emission concentration decreases during agglomeration/defluidization. However, the emission concentration greatly increases after defluidization. For heavy metals to remain in the sand particles, we speculate that they react with Na to form eutectics or that they are covered or adhered by the liquid-phase eutectics of Na. These mechanisms decrease the emission of heavy metals in flue gas. At a low gas velocity (1.1 Umf), the sand bed mixture is less uniform than that with a high gas velocity, which decreases the adsorption efficiency of silica sand. Therefore, if the system operates at a low gas velocity, it not only easily generates agglomeration/ defluidization, but also increases the heavy metal emission concentration. Small particles (645 μm) fluidize uniformly to enhance the adsorption efficiency of heavy metals and have a high momentum to prolong the operation time. Larger (920 μm) particles have poor solid mixing and a poor fluidized quality during fluidization; therefore, before defluidization, the emission concentration is higher than that of the no Na added case. However, three static bed heights were classified as deep beds for fluidized operation and had similar fluidized behaviors that lead to similar defluidization times and heavy metal emission concentrations. Additionally, adding Ca did not
decrease the heavy metal emission concentration, but it could maintain the fluidization during eutectic accumulation. The Ca prevents the sand bed from quickly achieving defluidization and prolongs the enhanced emission of heavy metals after defluidization. Acknowledgment The authors thank the National Science Council of the Republic of China, Taiwan for financial support of this research under Contract NSC 96-2221-E-390-031-MY3.
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