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Agglomeration characteristics during fluidized bed combustion of salty wastewater
Powder Technology 253 (2014) 537–547
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Agglomeration characteristics during fluidized bed combustion of salty wastewater Jiliang Ma, Daoyin Liu, Zhendong Chen, Xiaoping Chen ⁎ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, #2 Sipailou, District of Xuanwu, Nanjing, 210096, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 25 November 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: Salty wastewater Fluidized bed Incineration Agglomeration Additives
a b s t r a c t Fluidized bed combustion is a well-established and widely used technology for treatment of combustible wastewater. However, agglomeration is always encountered in industrial practice when dealing with wastewater that is rich in alkali metals. The objective of this present paper is to illustrate the agglomeration behaviors during fluidized bed incineration of salty wastewater. Various operational parameters were presented, viz. salt content, bed temperature, fluidizing gas velocity, static bed height, bed materials, and different additives. On the basis of SEM/EDX and XRD analysis of agglomerate samples, the mechanisms of agglomerate formation as well as the inhibition mechanisms of agglomeration for different additives were also discussed. The results show that agglomeration is promoted by the increasing of salt content, bed temperature, bed height, particle size, and by the decline of fluidizing gas velocity. All the additives tested, including CaCO3, Al2O3, Fe2O3 and Kaolin, are effective to different extents in inhibiting agglomeration. Physical effects were found to be the main inhibition mechanisms of agglomeration when the bed temperature is below 850 °C. Moreover, better results were also observed with increasing of additive amounts. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluidized bed combustion (FBC) is a well-established and widely used technology for treatment of combustible wastewater due to good waste flexibility, high incineration efficiency and low emission level [1,2]. However, agglomeration is always encountered in industrial practice when dealing with wastewater that contains high level of alkali elements, which will result in defluidization and unscheduled shutdowns. Agglomeration of bed materials in fluidized bed combustion/ gasification processes has gained attention since 1970s. It is generally believed that bed agglomeration is mostly attributable to the presence of alkali elements, especially Na and K. They can react with SiO2 (when employing sand as bed materials) to form low-melting sodium silicates which will melt under high temperatures then facilitate the occurring of agglomeration [3]. “melt-induced” and “coating-induced” are regarded as two main causes of agglomeration [4,5], although the latter one is more common in the industrial applications [6,7]. Besides the presence of alkali metals, certain operational parameters also affect the agglomeration behaviors [8–14]. It is widely accepted that high level of bed temperature and lower fluidizing gas velocity
⁎ Corresponding author. Tel.: +86 25 8379 3453 (Office), +86 13951898460 (Mobile); fax: +86 25 8379 3453 (Office). E-mail addresses: [email protected] (J. Ma), [email protected] (D. Liu), [email protected] (Z. Chen), [email protected] (X. Chen). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.12.003
can both promote agglomeration [10–12,15]. Some other factors, such as combustion stoichiometry [11,12], reactor scale [13] and fuel blending ratio [16] are also found to affect agglomerating process to some extent. Note that controversy exists as to the effects of bed material size. Lin [15] pointed out that the increasing of particle size accelerates agglomeration. While, Scala [11,12] and Chirone [13] proposed a contrary conclusion based on experimental results. Similarly, controversy also exists with respect to the effects of bed height [10,14]. Meantime, researchers have put forward various methods to effectively inhibit agglomeration, such as fuel pretreatment [17,18], improvements of operational parameters [19–21], replacement of bed materials [22–26] and adding additives [27,28]. Among these methods, adding additives is highlighted thanks to its low cost, high efficiency and easy accessibility. Its principle is to enhance the melting point of particle system by providing alternative reactions with alkalis, which inhibits the formation of low-melting silicates [27,28]. Typical additives include dolomite, CaO, CaCO3, Ca(OH)2, Kaolin, gibbsite, Al2O3, Fe2O3 etc. The reactions between these additives and sodium silicates are as follows [29–31]: Na2 O 3SiO2 þ 2CaO ¼ Na2 O 2CaO 3SiO2
ð1Þ
Na2 O 3SiO2 þ 3SiO2 þ 3CaO ¼ Na2 O 3CaO 6SiO2
ð2Þ
Na2 O 3SiO2 þ 4SiO2 þ Fe2 O3 ¼ Na2 O Fe2 O3 4SiO2
ð3Þ
Na2 O 3SiO2 þ 3SiO2 þ Al2 O3 ¼ Na2 O l2 O3 6SiO2
ð4Þ
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The melting points of above outcomes are 1284 °C, 1030 °C, 955 °C and 1108 °C respectively. Most of previous investigations on agglomeration have focused on the combustion of solid fuels such as coal, sewage sludge, solid waste and biomass fuel. Yet, when incinerating wastewater that is rich in alkali metals in fluidized beds, agglomeration is also a severe problem. Additionally, the mechanism of such agglomeration is unlike that of solid fuels due to different combustion process. Consequently, it requires a detailed understanding on the agglomeration behaviors during incineration of wastewater containing alkali metals. Bie found that the agglomeration and defluidization characteristics in the black liquor incineration processes are very sensitive to bed temperature [8]. He pointed out that agglomeration will not occur until the bed temperature exceeds 973 K when incinerating wastewater containing Na2CO3 or Na2SO4 [32]. Similar behaviors were also observed by Li [9]. Lv [33] indicated the important role of Na in the agglomeration when incinerating wastewater. Moilanen et al. [3] found that chloride is an important factor that affects the agglomerating process. Hupa [34] and Yan [35] also pointed out that the presence of Na and Cl may reduce the melting point of flue ash. Although there have been some investigations on the agglomeration characteristics during incineration of wastewater, the effects of certain operational parameters on agglomeration are still in controversy, especially for the inhibition mechanisms of some widely applied additives. This paper is aimed to illustrate the agglomeration behaviors during incineration of salty wastewater in a bubbling fluidized bed. Various operational parameters were presented, viz. salt content, bed temperature, fluidizing gas velocity, static bed height, bed materials, and different additives. On the basis of SEM/EDX and XRD analysis of agglomerate samples, the mechanisms of agglomerate formation as well as the inhibition mechanisms of agglomeration for four kinds of additives were also discussed.
2. Experimental 2.1. Apparatus The entire experimental facility is shown in Fig. 1. It consists of a fluidized bed reactor, an electrical heater, a temperature controlling system, a wastewater feeding system, a data acquisition system and a gas feeding system. The fluidized bed reactor is a stainless steel column with an internal diameter of 80 mm and a length of 1300 mm. A stainless steel plate fused with an air box was fitted at the bottom of column to act as bed support as well as a gas distributor. Six caps were fused on the plate and arranged on a circle with an external diameter of 56 mm. Four orifices with a diameter of 2.5 mm were perforated on the top of each cap. The reactor was heated by means of an 8 kW electrical heater. To prevent heat loss, a thick layer of ceramic wool sheet enveloped all parts of the reactor. Two K-type thermocouples were inserted into the bed at a distance of 90 mm and 135 mm above the gas distributor. One was connected to a silicon controlled rectifier to regulate heating rate and maintain the bed temperature to a desired value within the accuracy of ±10 °C, and the other one was to monitor the bed temperature. Furthermore, another thermocouple was installed between the reactor and electrical heater to monitor the temperature of reactor wall. A two-fluid spray gun, located at 215 mm above the air distributor, was used for wastewater feeding. The overall bed pressure drop was measured using a differential pressure transducer, which constantly monitored the pressure drop across the bed with two measuring points drilled in the air box and freeboard of the reactor respectively. The pressure profiles collected were logged on a PC via a data acquisition unit. The actual gas flow rate was calculated from a flow meter reading and temperature and pressure corrections. Before entering into the fluidized bed reactor, the fluidizing air was first introduced into a 3 kW preheater.
Fig. 1. Schematic diagram of bubbling fluidized bed test facility.
J. Ma et al. / Powder Technology 253 (2014) 537–547 Table 1 Additive properties.
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Table 2 Experimental conditions.
No.
Additives
Chemical formula
Reagent types
Particle size distribution
1 2 3 4
Calcium carbonate Aluminum oxide Iron oxide Kaolin
CaCO3 Al2O3 Fe2O3 Al2O3 · 2SiO2 · 2H2O
Experimental reagent Experimental reagent Analytical reagent Chemically pure
280–450 μm 280–450 μm / /
2.2. Materials Fresh quartz sand (97.51% SiO2, particle density 2519 kg/m3), sieved in four size ranges, was employed as bed material. Compressed air and high-pressure nitrogen was used as fluidizing gas and wastewater atomization medium respectively. In order to identify the effects of alkali metals, NaCl solutions with different concentrations (2.5, 3.5, 5, 6.5, 7.5 wt.%) were used to simulate the salty wastewater. The solutions were sprayed into the bed with a flow rate of 15 ml/min. CaCO3, Al2O3, Fe2O3 and Kaolin were used as additives whose main properties are plotted in Table 1. Fig. 2 plots the main compositions of Al2O3 and Kaolin through XRD analysis. As can be seen, the intensity peaks of Al2O3 scatters, indicating a low crystallization index. Concerning the Kaolin, its main component is Al2O3 · 2SiO2 · 2H2O.
No. Temperature Concentration Fluidizing gas (°C) of NaCl velocity solution (%) (m/s)
Static bed height (mm)
Sand Bed material type diameter (μm)
Additive type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
200 200 200 200 200 200 200 200 200 200 200 250 230 170 140 200 200 200 200 200 200 200 200
365 365 365 365 365 365 365 365 365 365 365 365 365 365 365 700 525 247.5 365 365 365 365 365
None None None None None None None None None None None None None None None None None None None CaCO3 Al2O3 Fe2O3 Kaolin
800 850 750 700 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800
5 5 5 5 2 3.5 6.5 7.5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.545 0.377 0.293 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461
Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Used Fresh Fresh Fresh Fresh
2.3. Procedures Table 2 summarizes the test conditions in this study. We investigated the effects of different operating parameters, including the NaCl concentration, bed temperature, gas velocity, static bed height, particle size, as well as additive type and amounts on the agglomeration characteristics.
A fixed amount of prepared sand particles was first poured into the furnace from the top. Then the air compressor was switched on, maintaining a slow stream of air through the bed, then the preheater and the electrical heater were turned on simultaneously. When the bed temperature attains a steady pre-set value, the air flow was increased by fluidizing the bed materials vigorously for 3 min to fully mix the particles. Before feeding salty wastewater, a stream of pure water with the same flow rate was first sprayed into the dense-phase by the atomizer, compelling the bed temperature to drop gradually. When the temperature returned to the pre-set value, we then switched to salty wastewater quickly. The salty wastewater feeding was shut down at the onset of defluidization which was evaluated according to the plot of bed temperature and pressure profiles versus time. The time interval between the beginning of feeding wastewater and the onset of bed defluidization was defined as defluidization time (tD). After each test, the bed materials together with the agglomerates were discharged from the reactor and carefully sieved to identify the size distribution. Representative agglomerate samples were analyzed with SEM/EDX to obtain the morphology and chemical composition of the agglomerate surfaces. XRD analysis was also performed to identify the crystalline compounds of agglomerates. 3. Results and discussions 3.1. Determination of tD
Fig. 2. XRD analysis of Al2O3 (a) and Kaolin (b).
In this study, pressure monitoring was employed to identify tD for its rationality in the reflection of fluidization hydrodynamics. Fig. 3 plots the pressure drops and bed temperatures versus time for three representative test conditions. Time zero in the figure represents the commencement of salty wastewater feeding. As shown in Fig. 3 (a), the sharp rise of pressure drop across the bed, which results from the agglomeration around the inner surface of reactor, can be regarded as the onset of bed defluidization. It is a widely accepted method to evaluate the onset of defluidization caused by agglomeration [5,10–12,15]. Besides that, two other different trends in the pressure drops indicating the onset of defluidization have also been observed: i) The pressure drop falls abruptly due to bed channeling (Fig. 3 (b)); ii) The pressure decreases gradually and the bed temperature increases slowly (Fig. 3 (c)).
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Fig. 4. Variation of tD with bed temperature.
Fig. 3. Bed temperature and pressure profile as a function of time.
Besides SEM, EDX analysis was also performed to quantify the distribution of elements on the different parts of agglomerates, the results of which are summarized in Table 3. Obviously, Na and Cl are the dominating elements in coatings. Other elements like Si and O are also frequently detected. The other elements, C, Al and Mg, can be nearly ignored. Below 750 °C, Si is not present on the agglomerate surface, indicating that NaCl does not react with SiO2. Thus, the agglomeration is just induced by the fusion of NaCl within 750 °C. When the temperature exceeds 750 °C, NaCl reacts with SiO2 to form low-melting Na2SiO3 and gas phase HCl. It can be verified by the EDX analysis that the content of Si increases, whereas Cl decreases with the increasing of bed temperature beyond 750 °C. Consequently, molten Na2SiO3 and melting NaCl are the two main binders that cause agglomeration. Fig. 6 plots the XRD analysis of agglomerate samples under different temperatures. As can be seen, only SiO2 and NaCl were found throughout the tested temperature range. No crystalline phases containing Na, Si and O were identified. That conflicts with EDX results presented in Table 3. Previous studies of Bie [8] and Lin [10] also demonstrated that no Na2SiO3 was found in agglomerates. That may be attributable to the fact that the concentration of Na2SiO3 is too low to be detected by XRD. Additionally, Na2SiO3 may also be transferred to amorphous phase in the cooling process, which cannot be detected through XRD either. According to the above analysis, the possible mechanism of bed agglomeration is “melt-induced” below 750 °C and a combination of “coating-induced” and “melt-induced” for higher temperatures (above 750 °C). The following sub processes of bed agglomeration are suggested. First, gaseous alkali species are condensed on the bed material surface and react with SiO2 to produce low-melting eutectics. Then, layers enriched in Na and Si form adhesive coatings around bed material particles and initiate agglomeration.
3.2. Effects of operational parameters 3.2.1. Bed temperature Fig. 4, plotting tD versus bed temperature, indicates a strong dependence of agglomeration on temperature. tD at 850 °C is much less than that at 700 °C or 800 °C. This is consistent with other literature results [8–13]. As bed temperature increases, the amount of liquid eutectics grows. Meanwhile, the viscosity of the eutectics decreases. Consequently, the eutectics tend to transfer to the surfaces of other particles through collisions, thus facilitating the occurrence of agglomeration. Fig. 5 shows the surface morphology of agglomerate samples obtained under different temperatures on the basis of SEM analysis. As can be seen, with the increasing of bed temperature, the amount of flocculent materials, covering the particle surface and filling the gap increases accompanied by the gradual disappearance of particle edges. Note that, at 850 °C, the molten materials that behave as coaters are uniformly distributed on the particle surface. Thus, the substances connecting particles deplete and the particle edges reappear.
3.2.2. NaCl concentration Fig. 7 illustrates the effects of the NaCl concentration on tD. As shown in the figure, tD decreases with the increasing of NaCl concentration, which is consistent with Lin [10] who found that increasing the concentration of Na accelerates defluidization. As has been discussed previously, NaCl in wastewater reacts with SiO2 to form low-melting eutectics at 800 °C. Therefore, the increasing of NaCl concentration raises the amount of low-melting eutectics, thereby promoting agglomeration. Fig. 8 displays SEM images of bed materials and agglomerate samples for different NaCl concentrations. Fresh sand has angular edges and rough surfaces (Fig. 8 (a)). As the concentration of NaCl solution increases, the amount of low-melting eutectics increases and they assemble around the connective bridges between particles and partially cover the particle surfaces. The shape of particles becomes indistinguishable gradually. When the concentration of NaCl exceeds 6.5%, enough eutectics are generated to cover the particle surfaces entirely.
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Fig. 5. The SEM images of agglomerates for various bed temperatures. (a) 700 °C; (b) 750 °C; (c) 800 °C; (d) 850 °C.
3.2.3. Static bed height Fig. 9 presents the effects of static bed height on tD. The results indicate that tD shortened as the static bed height increases. Stable fluidization state lasts for an hour at the height of 140 mm. As the height increases to 170 mm, tD decreases to 35 min, to the same extent as the height of 200 mm. When the static bed height exceeds 230 mm, fast agglomeration occurs in 13 min after wastewater feeding. The variation of tD with static bed height is determined by the position of the wastewater spray gun. Part of wastewater is carried away by flue gas when the bed surface is lower than spray gun. As the static bed height increases, the corresponding distance between bed surface and spray gun is shortened and the amount of salty wastewater sprayed in the bed increases, which generates more low-melting eutectics. Besides that, the large bed height inhibits the particle mixing and weakens the particle mobility. Thus, liquid phase eutectics adhere to the particle easily. Both of these two factors accelerate the defluidization process when the static bed height increases from 140 mm to 230 mm. When the bed height exceeds 230 mm, the spray gun completely submerges the bed materials, which causes all the wastewater to be sprayed into the dense phase and tD to remain nearly unchanged. When concerning the present study, the slight increase of tD with the static bed height increasing from 230 mm to 250 mm may be due to the experimental errors and the uncertainty in the measurement of tD.
3.2.4. Gas velocity tD for different fluidizing velocities is plotted in Fig. 10. The results suggest that tD increases with the growth of gas velocity. When the fluidization number increases from 1.69 to 3.13, tD is prolonged by 53%, which agrees well with those of Lin [10,15]. The higher fluidizing gas velocity leads to a better mixing of particles and larger forces acting on agglomerates. It implies that the already formed agglomerates may be crushed under such conditions and the agglomeration will be delayed. 3.2.5. Particle size Due to the fact that the effects of particle size on agglomeration are still in controversy, this paper recorded tD for the incineration of salty wastewater using different particle sizes. The fluidizing gas velocity
Table 3 EDX analysis of different points on the agglomerates. Zone
C
O
Na
Cl
Si
Al
Mg
1 2 3 4 5 6 7 8 9
0.52 0.49 0 0.53 0.76 0.69 0.75 0.75 1.07
1.8 1.7 0.77 1.47 1.8 4.23 2.04 1.7 5.33
33.88 35.38 35.38 36.74 35.18 29.76 32.93 35.69 25.99
62.5 62.43 63.65 57.95 59.99 57.52 63.35 52.28 39.33
/ / / 2.03 2.27 7.79 0.94 7.36 25.94
/ / / 1.28 / / / 2.22 1.54
1.29 / / / / / / / /
Fig. 6. XRD analyses of agglomerates under various bed temperatures.
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Fig. 7. Variation of tD with NaCl concentration.
Fig. 9. Variation of tD with static bed height.
employed was kept 0.461 m/s. Fig. 11 shows the corresponding results. As can be seen, the increasing of particle size leads to a decline of tD. Due to the constant fluidizing gas velocity, both the momentum and mixing intensity of particles decrease with the increasing of particle diameters. The tendency to agglomerate is inversely proportional to the
momentum of the particles. Thus, tD increases with the decreasing of particle size. In addition, the small specific surface areas of large particles facilitate the formation of thick coatings. That may be another reason for the acceleration of defluidization when fluidizing larger particles.
Fig. 8. SEM images of bed materials for incineration of salty wastewater with different concentrations of NaCl. (a) fresh sand; (b) 3.5 wt.% NaCl; (c) 5 wt.% NaCl; (d) 6.5 wt.% NaCl; (e) 7.5 wt.% NaCl.
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Fig. 10. Variation of tD with gas velocity. Fig. 13. Variation of tD with additive type.
3.2.6. Sand type The variation of tD with bed material type is shown in Fig. 12. As can be seen, using fresh sand as materials can double tD when compared to the case of used sand. It is due to the fact that Na and Cl condensed on the surface of used sand previously accelerate the defluidization process. Therefore, in the industrial applications, it is feasible to gradually update the bed materials by adding fresh sand to prolong the running time of wastewater incinerator. 3.3. Effects of additives 3.3.1. Additive types Fig. 13 plots the values of tD for the cases using different types of additives. As can be seen, all the tested additives can inhibit agglomeration
Fig. 11. Variation of tD with particle size.
effectively. Among them, Al2O3 exhibits the best effect on the controlling of agglomeration. Table 4 shows the size distribution of bed materials discharged from the incinerator after agglomeration for different types of additives. When using Al2O3 powders as additives, the mass fraction of bed material particles with diameters larger than 450 μm (caused by the agglomeration since the mean diameter of bed materials before the test is 365 μm) is the least. It also validates the significant effect of Al2O3 in the prevention of agglomeration from one side. Fig. 14 shows the concentrations of elements in the agglomerates for different additives. With the injection of additives, the concentrations of Na and Cl on the agglomerate surfaces decrease significantly, whereas the concentrations of Ca, Fe, and Al increase dramatically. It indicates that the replacement of low-melting elements by high-melting elements, thereby increasing the melting-point of coating on the particle surface is the main mechanism of additives for the controlling of agglomeration. Note that the concentration of Na on the agglomerate surface for the case of 10% CaCO3 is larger than the case of non-additives, which is different from that of Fe2O3 and Al2O3. That may be due to the uneven distribution of Na on the agglomerates.
3.3.2. Fe2O3 Fig. 15 shows the SEM/EDX analysis results of the agglomerate samples when employing Fe2O3 powders as additives. As can be seen from the SEM image (Fig. 15 (a)), the smooth surface of the bed material particles was covered by a thin layer of additives and the gaps between particles were filled with some floccus. The EDX analysis (Fig. 15 (b)) shows that large amounts of Fe, O and Si, and low quantities of Na and Cl deposit on point 1 (particle surface, see Fig. 15 (a)). An opposite trend was observed when concerning the other points (bonding between particles). Based on these results, we formed a hypothesis: the mechanism for Fe2O3 to inhibit agglomeration is coating the bed material particles with high-melting Fe2O3 powders. In other words, physical effect is the controlling factor. In order to validate the assumption, we carried out a XRD analysis on the agglomerate samples. The results are shown in Fig. 16. It indicates that only SiO2, NaCl and Fe2O3 are found as crystalline substances on the agglomerate. Na2O · Fe2O3 · 4SiO2, theoretically produced by the Table 4 Size distribution of bed materials for different types of additives.
Fig. 12. Variation of tD with bed material type.
Particle size distribution(μm)
Additive types None
Al2O3
CaCO3
Fe2O3
Kaolin
b280 280–450 N450
3.47 91.6 4.93
3.18 94.83 1.99
3.11 94.02 2.87
3.95 93.97 2.08
2.33 94.73 2.94
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Fig. 14. Content of elements on the agglomerate surface for different types of additives.
reaction between Fe2O3 and Na2SiO3, was not observed. It can be inferred that physical effects are indeed the main mechanisms for Fe2O3 to inhibit the agglomeration under 800 °C: the bed material particles are surrounded by Fe2O3, impeding the contact between bed materials and melting NaCl and avoiding the adhesion caused by the collisions between particles, thus postponing the occurring of agglomeration. 3.3.3. CaCO3 Fig. 17 shows the SEM/EDX analysis of agglomerate samples with CaCO3 powders as additives. The SEM image reveals that the low melting eutectics are coated with non-cohesive layers behaving like refractory materials, which effectively reduces the interparticle adherence, thus controlling the agglomeration. The results of EDX analysis (Fig. 17 (b)) indicate that the surface of bed material particles mainly contains Na, Cl and Ca. In the ternary phase system of SiO2–CaO–Na2O,
Fig. 15. SEM and EDX analysis of agglomerate formed during incineration with Fe2O3 as additives. (a) SEM; (b) EDX.
Fig. 16. XRD analysis of agglomerate formed during incineration with Fe2O3 as additives.
the increasing of Ca content can facilitate the formation of highmelting materials and dilute the concentrations of low-melting eutectics (e.g. Na2SiO3), which would decrease the coating cohesion and increase its melting points. The results of XRD analysis of agglomerates are shown in Fig. 18, in which only SiO2, NaCl and CaO are found. The high-melting matter theoretically produced by the reaction between CaO and Na2SiO3 is not observed. Similar to Fe2O3, CaCO3 also inhibits agglomeration through physical effects under 800 °C. CaO powder generated by the decomposition of CaCO3 forms an effectively protective coating around the bed material surface for its high melting point of 2580 °C, which will
Fig. 17. SEM and EDX analysis of agglomerate formed during incineration with CaCO3 as additives. (a) SEM; (b) EDX.
J. Ma et al. / Powder Technology 253 (2014) 537–547
Fig. 18. XRD analysis of agglomerate formed during incineration with CaCO3 as additives.
prevent the bonding of bed materials to each other and control agglomeration. 3.3.4. Al2O3 Fig. 19 shows the SEM and EDX analysis of agglomerates after adding Al2O3 powder into the incinerator. As can be seen from the SEM image (Fig. 19 (a)), the surface of agglomerates is relatively smooth with clear edges and low concentration of molten NaCl. EDX analysis reveals that points 1, 2 and 3 are rich in Al and short of Na and Cl. Fig. 20, plotting the XRD analysis of agglomerates, shows that SiO2 and NaCl rather than high melting Na2O · Al2O3 · 6SiO2 presents in the agglomerates. Higher reaction temperature of Eq. (4) comparing to the bed temperature may be the reason for the absence of
Fig. 19. SEM and EDX analysis of agglomerate formed during incineration with Al2O3 as additives. (a) SEM; (b) EDX.
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Fig. 20. XRD analysis of agglomerate formed during incineration with Al2O3 as additives.
Na2O · Al2O3 · 6SiO2. As neutral oxide, Al2O3 can prevent the formation of low-melting matters and mitigate the sintering of bed materials. In addition, increasing Al2O3 content may raise the melting point of ash and inhibit agglomeration for its “skeleton” effects during ash melting [38]. In a word, the alleviation of agglomeration when using Al2O3 as additives comes from its physical effects. 3.3.5. Kaolin Fig. 21 shows SEM and EDX analysis of agglomerates with Kaolin as additives. Although the agglomerate surface was covered by a thin layer, no flocculent materials between particles are observed. The EDX analysis show that the agglomerate surface contains large amounts of Al and
Fig. 21. SEM and EDX analysis of agglomerate formed during incineration with Kaolin as additives. (a) SEM; (b) EDX.
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J. Ma et al. / Powder Technology 253 (2014) 537–547 Table 5 Size distribution of bed materials for different amounts of CaCO3. Size distribution (μm)
CaCO3 fraction (%) 0
5
10
20
b280 280–450 N450
3.47 91.6 4.93
2.2 94.63 3.17
3.11 94.02 2.87
11.31 86.36 2.33
4. Conclusions
Fig. 22. XRD analysis of agglomerate formed during incineration with Kaolin as additives.
Cabut little Na and Cl, which leads to a growth of melting point of coating and effectively controlling of agglomeration. The high-melting matters (Na2O · Al2O3 · 6SiO2) are not observed in the agglomerates through XRD analysis (shown in Fig. 22). That is because the present bed temperature dissatisfies with the requirement for the reaction between metakaolin and Na2SiO3. This is consistent with the findings of Ohman [36]. Steenari [37] further ascertained the reaction temperature of 900 °C between metakaolin and Na2SiO3. Al2O3 was not observed in Fig. 22 either, owing to the amorphousness of metakaolin after dehydrating reaction.
3.3.6. Additive amount As discussed previously, the addition of CaCO3 can inhibit agglomeration and reduce SO2 emission simultaneously, thus it has been considered as a preferred additive. We carried out a test to investigate the effects of the amount of CaCO3 on the agglomeration characteristics. As can be seen from Fig. 23, tD increases from 35 min to 2 h as the mass fraction of CaCO3 rises from 0 to 20%. Table 5 shows the size distribution of bed materials for different amounts of CaCO3. The mass fraction of bed materials with diameter larger than 450 μm decreases with the increasing of CaCO3 amount, indicating an apparent influence of increasing the CaCO3 amount on the prevention of agglomeration. Note that the mass fraction of bed materials smaller than 280 μm increases sharply for the CaCO3 amount of 20%, which can be attributed to the abrasion between bed materials under longer running time. It reveals that the increasing of CaCO3 may also have adverse effect on the stable running of fluidized bed incinerator. Thus, it is necessary to select an appropriate mass fraction of additives in the industrial application.
Fig. 23. Effects of the amount of CaCO3 on tD.
A thorough study of the agglomerates formed during the incineration of salty wastewater in a lab-scale fluidized bed was performed to evaluate the effects of operational parameters, NaCl content, as well as four kinds of additives on the agglomeration behavior. Based on these investigations, the following conclusions may be drawn: (1) Increasing the concentration of NaCl in wastewater accelerates the agglomerating process. Similar trends can also be observed for the case of higher bed temperature, lower gas velocity, deeper static bed height, and larger particle size. Additionally, compared with fresh sand, agglomeration was accelerated with used sand as bed material. (2) Above 750 °C, agglomeration is attributed to the formation of low-melting eutectics produced by the reaction between NaCl and SiO2. Below 750 °C, agglomeration is just induced by molten NaCl. (3) CaCO3, Al2O3, Fe2O3 and Kaolin can all effectively inhibit the agglomeration. Among them, Al2O3 exhibits the best inhibition results and better effects were observed with the increasing of additive amounts. (4) The inhibition of studied additives on the agglomeration mainly derives from physical effects within 800 °C. The coating formed by additives around the bed material particles is rich in Al, Ca and Fe, leading to a growth of melting points and a decline of adhesion between particles, thus inhibiting the agglomeration. Acknowledgments Financial supports of this work by National Nature Science Foundation of China (51306035), Specialized Research Fund for the Doctoral Program of Higher Education (20130092120010) and Scientific Research Foundation of Graduate School of Southeast University are gratefully acknowledged. References [1] P. Basu, S.A. Fraser, Circulating fluidized bed boilers design and operations, Butterworth-Heinemann-Reed Publishing, London, 1991. [2] R. Yan, D. Liang, K. Laursen, Y. Li, L. Tsen, J.H. Tay, Formation of bed agglomeration in a fluidized multi-waste incinerator, Fuel 82 (2003) 843–851. [3] A. Moilanen, K. Sipilä, M. Nieminen, E. Kurkela, Ash behaviour in thermal fluidized-bed conversion processes of woody and herbaceous biomass, VTT Energy, Espoo, Finland, 1996. [4] J. Shao, D. Lee, Y. Rong, M. Liu, X. Wang, D. Liang, T.J. White, H.P. Chen, Agglomeration characteristics of sludge combustion in a bench-scale fluidized bed combustor, Energy Fuel 21 (2007) 2608–2614. [5] L. Fryda, K. Panopoulos, E. Kakaras, Agglomeration in fluidized bed gasification of biomass, Powder Technol. 181 (2008) 307–320. [6] M. Ohman, L. Pommer, A. Nordin, Bed agglomeration characteristics and mechanisms during gasification and combustion of biomass fuels, Energy Fuel 19 (2005) 1742–1748. [7] E. Brus, M. Öhman, A. Nordin, Mechanisms of bed agglomeration during fluidized-bed combustion of biomass fuels, Energy Fuel 19 (2005) 825–832. [8] R. Bie, Y. Zhao, Z. Chen, J. Lu, L. Yang, Formation mechanisms of agglomeration caused by burning NSSC black liquor in a fluidized bed incinerator, Energy Fuel 23 (2009) 683–689. [9] X. Li, H. Lv, M. Xu, J. Ma, J. Yan, K. Cen, Agglomeration characteristics in fluidized bed incineration of organic-condensed wastewater, J. Chem. Ind. Eng. 56 (2005) 2166–2171.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Agglomeration characteristics during fluidized bed combustion of salty wastewater Jiliang Ma, Daoyin Liu, Zhendong Chen, Xiaoping Chen ⁎ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, #2 Sipailou, District of Xuanwu, Nanjing, 210096, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 25 November 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: Salty wastewater Fluidized bed Incineration Agglomeration Additives
a b s t r a c t Fluidized bed combustion is a well-established and widely used technology for treatment of combustible wastewater. However, agglomeration is always encountered in industrial practice when dealing with wastewater that is rich in alkali metals. The objective of this present paper is to illustrate the agglomeration behaviors during fluidized bed incineration of salty wastewater. Various operational parameters were presented, viz. salt content, bed temperature, fluidizing gas velocity, static bed height, bed materials, and different additives. On the basis of SEM/EDX and XRD analysis of agglomerate samples, the mechanisms of agglomerate formation as well as the inhibition mechanisms of agglomeration for different additives were also discussed. The results show that agglomeration is promoted by the increasing of salt content, bed temperature, bed height, particle size, and by the decline of fluidizing gas velocity. All the additives tested, including CaCO3, Al2O3, Fe2O3 and Kaolin, are effective to different extents in inhibiting agglomeration. Physical effects were found to be the main inhibition mechanisms of agglomeration when the bed temperature is below 850 °C. Moreover, better results were also observed with increasing of additive amounts. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluidized bed combustion (FBC) is a well-established and widely used technology for treatment of combustible wastewater due to good waste flexibility, high incineration efficiency and low emission level [1,2]. However, agglomeration is always encountered in industrial practice when dealing with wastewater that contains high level of alkali elements, which will result in defluidization and unscheduled shutdowns. Agglomeration of bed materials in fluidized bed combustion/ gasification processes has gained attention since 1970s. It is generally believed that bed agglomeration is mostly attributable to the presence of alkali elements, especially Na and K. They can react with SiO2 (when employing sand as bed materials) to form low-melting sodium silicates which will melt under high temperatures then facilitate the occurring of agglomeration [3]. “melt-induced” and “coating-induced” are regarded as two main causes of agglomeration [4,5], although the latter one is more common in the industrial applications [6,7]. Besides the presence of alkali metals, certain operational parameters also affect the agglomeration behaviors [8–14]. It is widely accepted that high level of bed temperature and lower fluidizing gas velocity
⁎ Corresponding author. Tel.: +86 25 8379 3453 (Office), +86 13951898460 (Mobile); fax: +86 25 8379 3453 (Office). E-mail addresses: [email protected] (J. Ma), [email protected] (D. Liu), [email protected] (Z. Chen), [email protected] (X. Chen). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.12.003
can both promote agglomeration [10–12,15]. Some other factors, such as combustion stoichiometry [11,12], reactor scale [13] and fuel blending ratio [16] are also found to affect agglomerating process to some extent. Note that controversy exists as to the effects of bed material size. Lin [15] pointed out that the increasing of particle size accelerates agglomeration. While, Scala [11,12] and Chirone [13] proposed a contrary conclusion based on experimental results. Similarly, controversy also exists with respect to the effects of bed height [10,14]. Meantime, researchers have put forward various methods to effectively inhibit agglomeration, such as fuel pretreatment [17,18], improvements of operational parameters [19–21], replacement of bed materials [22–26] and adding additives [27,28]. Among these methods, adding additives is highlighted thanks to its low cost, high efficiency and easy accessibility. Its principle is to enhance the melting point of particle system by providing alternative reactions with alkalis, which inhibits the formation of low-melting silicates [27,28]. Typical additives include dolomite, CaO, CaCO3, Ca(OH)2, Kaolin, gibbsite, Al2O3, Fe2O3 etc. The reactions between these additives and sodium silicates are as follows [29–31]: Na2 O 3SiO2 þ 2CaO ¼ Na2 O 2CaO 3SiO2
ð1Þ
Na2 O 3SiO2 þ 3SiO2 þ 3CaO ¼ Na2 O 3CaO 6SiO2
ð2Þ
Na2 O 3SiO2 þ 4SiO2 þ Fe2 O3 ¼ Na2 O Fe2 O3 4SiO2
ð3Þ
Na2 O 3SiO2 þ 3SiO2 þ Al2 O3 ¼ Na2 O l2 O3 6SiO2
ð4Þ
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The melting points of above outcomes are 1284 °C, 1030 °C, 955 °C and 1108 °C respectively. Most of previous investigations on agglomeration have focused on the combustion of solid fuels such as coal, sewage sludge, solid waste and biomass fuel. Yet, when incinerating wastewater that is rich in alkali metals in fluidized beds, agglomeration is also a severe problem. Additionally, the mechanism of such agglomeration is unlike that of solid fuels due to different combustion process. Consequently, it requires a detailed understanding on the agglomeration behaviors during incineration of wastewater containing alkali metals. Bie found that the agglomeration and defluidization characteristics in the black liquor incineration processes are very sensitive to bed temperature [8]. He pointed out that agglomeration will not occur until the bed temperature exceeds 973 K when incinerating wastewater containing Na2CO3 or Na2SO4 [32]. Similar behaviors were also observed by Li [9]. Lv [33] indicated the important role of Na in the agglomeration when incinerating wastewater. Moilanen et al. [3] found that chloride is an important factor that affects the agglomerating process. Hupa [34] and Yan [35] also pointed out that the presence of Na and Cl may reduce the melting point of flue ash. Although there have been some investigations on the agglomeration characteristics during incineration of wastewater, the effects of certain operational parameters on agglomeration are still in controversy, especially for the inhibition mechanisms of some widely applied additives. This paper is aimed to illustrate the agglomeration behaviors during incineration of salty wastewater in a bubbling fluidized bed. Various operational parameters were presented, viz. salt content, bed temperature, fluidizing gas velocity, static bed height, bed materials, and different additives. On the basis of SEM/EDX and XRD analysis of agglomerate samples, the mechanisms of agglomerate formation as well as the inhibition mechanisms of agglomeration for four kinds of additives were also discussed.
2. Experimental 2.1. Apparatus The entire experimental facility is shown in Fig. 1. It consists of a fluidized bed reactor, an electrical heater, a temperature controlling system, a wastewater feeding system, a data acquisition system and a gas feeding system. The fluidized bed reactor is a stainless steel column with an internal diameter of 80 mm and a length of 1300 mm. A stainless steel plate fused with an air box was fitted at the bottom of column to act as bed support as well as a gas distributor. Six caps were fused on the plate and arranged on a circle with an external diameter of 56 mm. Four orifices with a diameter of 2.5 mm were perforated on the top of each cap. The reactor was heated by means of an 8 kW electrical heater. To prevent heat loss, a thick layer of ceramic wool sheet enveloped all parts of the reactor. Two K-type thermocouples were inserted into the bed at a distance of 90 mm and 135 mm above the gas distributor. One was connected to a silicon controlled rectifier to regulate heating rate and maintain the bed temperature to a desired value within the accuracy of ±10 °C, and the other one was to monitor the bed temperature. Furthermore, another thermocouple was installed between the reactor and electrical heater to monitor the temperature of reactor wall. A two-fluid spray gun, located at 215 mm above the air distributor, was used for wastewater feeding. The overall bed pressure drop was measured using a differential pressure transducer, which constantly monitored the pressure drop across the bed with two measuring points drilled in the air box and freeboard of the reactor respectively. The pressure profiles collected were logged on a PC via a data acquisition unit. The actual gas flow rate was calculated from a flow meter reading and temperature and pressure corrections. Before entering into the fluidized bed reactor, the fluidizing air was first introduced into a 3 kW preheater.
Fig. 1. Schematic diagram of bubbling fluidized bed test facility.
J. Ma et al. / Powder Technology 253 (2014) 537–547 Table 1 Additive properties.
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Table 2 Experimental conditions.
No.
Additives
Chemical formula
Reagent types
Particle size distribution
1 2 3 4
Calcium carbonate Aluminum oxide Iron oxide Kaolin
CaCO3 Al2O3 Fe2O3 Al2O3 · 2SiO2 · 2H2O
Experimental reagent Experimental reagent Analytical reagent Chemically pure
280–450 μm 280–450 μm / /
2.2. Materials Fresh quartz sand (97.51% SiO2, particle density 2519 kg/m3), sieved in four size ranges, was employed as bed material. Compressed air and high-pressure nitrogen was used as fluidizing gas and wastewater atomization medium respectively. In order to identify the effects of alkali metals, NaCl solutions with different concentrations (2.5, 3.5, 5, 6.5, 7.5 wt.%) were used to simulate the salty wastewater. The solutions were sprayed into the bed with a flow rate of 15 ml/min. CaCO3, Al2O3, Fe2O3 and Kaolin were used as additives whose main properties are plotted in Table 1. Fig. 2 plots the main compositions of Al2O3 and Kaolin through XRD analysis. As can be seen, the intensity peaks of Al2O3 scatters, indicating a low crystallization index. Concerning the Kaolin, its main component is Al2O3 · 2SiO2 · 2H2O.
No. Temperature Concentration Fluidizing gas (°C) of NaCl velocity solution (%) (m/s)
Static bed height (mm)
Sand Bed material type diameter (μm)
Additive type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
200 200 200 200 200 200 200 200 200 200 200 250 230 170 140 200 200 200 200 200 200 200 200
365 365 365 365 365 365 365 365 365 365 365 365 365 365 365 700 525 247.5 365 365 365 365 365
None None None None None None None None None None None None None None None None None None None CaCO3 Al2O3 Fe2O3 Kaolin
800 850 750 700 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800
5 5 5 5 2 3.5 6.5 7.5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.545 0.377 0.293 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461 0.461
Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Fresh Used Fresh Fresh Fresh Fresh
2.3. Procedures Table 2 summarizes the test conditions in this study. We investigated the effects of different operating parameters, including the NaCl concentration, bed temperature, gas velocity, static bed height, particle size, as well as additive type and amounts on the agglomeration characteristics.
A fixed amount of prepared sand particles was first poured into the furnace from the top. Then the air compressor was switched on, maintaining a slow stream of air through the bed, then the preheater and the electrical heater were turned on simultaneously. When the bed temperature attains a steady pre-set value, the air flow was increased by fluidizing the bed materials vigorously for 3 min to fully mix the particles. Before feeding salty wastewater, a stream of pure water with the same flow rate was first sprayed into the dense-phase by the atomizer, compelling the bed temperature to drop gradually. When the temperature returned to the pre-set value, we then switched to salty wastewater quickly. The salty wastewater feeding was shut down at the onset of defluidization which was evaluated according to the plot of bed temperature and pressure profiles versus time. The time interval between the beginning of feeding wastewater and the onset of bed defluidization was defined as defluidization time (tD). After each test, the bed materials together with the agglomerates were discharged from the reactor and carefully sieved to identify the size distribution. Representative agglomerate samples were analyzed with SEM/EDX to obtain the morphology and chemical composition of the agglomerate surfaces. XRD analysis was also performed to identify the crystalline compounds of agglomerates. 3. Results and discussions 3.1. Determination of tD
Fig. 2. XRD analysis of Al2O3 (a) and Kaolin (b).
In this study, pressure monitoring was employed to identify tD for its rationality in the reflection of fluidization hydrodynamics. Fig. 3 plots the pressure drops and bed temperatures versus time for three representative test conditions. Time zero in the figure represents the commencement of salty wastewater feeding. As shown in Fig. 3 (a), the sharp rise of pressure drop across the bed, which results from the agglomeration around the inner surface of reactor, can be regarded as the onset of bed defluidization. It is a widely accepted method to evaluate the onset of defluidization caused by agglomeration [5,10–12,15]. Besides that, two other different trends in the pressure drops indicating the onset of defluidization have also been observed: i) The pressure drop falls abruptly due to bed channeling (Fig. 3 (b)); ii) The pressure decreases gradually and the bed temperature increases slowly (Fig. 3 (c)).
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Fig. 4. Variation of tD with bed temperature.
Fig. 3. Bed temperature and pressure profile as a function of time.
Besides SEM, EDX analysis was also performed to quantify the distribution of elements on the different parts of agglomerates, the results of which are summarized in Table 3. Obviously, Na and Cl are the dominating elements in coatings. Other elements like Si and O are also frequently detected. The other elements, C, Al and Mg, can be nearly ignored. Below 750 °C, Si is not present on the agglomerate surface, indicating that NaCl does not react with SiO2. Thus, the agglomeration is just induced by the fusion of NaCl within 750 °C. When the temperature exceeds 750 °C, NaCl reacts with SiO2 to form low-melting Na2SiO3 and gas phase HCl. It can be verified by the EDX analysis that the content of Si increases, whereas Cl decreases with the increasing of bed temperature beyond 750 °C. Consequently, molten Na2SiO3 and melting NaCl are the two main binders that cause agglomeration. Fig. 6 plots the XRD analysis of agglomerate samples under different temperatures. As can be seen, only SiO2 and NaCl were found throughout the tested temperature range. No crystalline phases containing Na, Si and O were identified. That conflicts with EDX results presented in Table 3. Previous studies of Bie [8] and Lin [10] also demonstrated that no Na2SiO3 was found in agglomerates. That may be attributable to the fact that the concentration of Na2SiO3 is too low to be detected by XRD. Additionally, Na2SiO3 may also be transferred to amorphous phase in the cooling process, which cannot be detected through XRD either. According to the above analysis, the possible mechanism of bed agglomeration is “melt-induced” below 750 °C and a combination of “coating-induced” and “melt-induced” for higher temperatures (above 750 °C). The following sub processes of bed agglomeration are suggested. First, gaseous alkali species are condensed on the bed material surface and react with SiO2 to produce low-melting eutectics. Then, layers enriched in Na and Si form adhesive coatings around bed material particles and initiate agglomeration.
3.2. Effects of operational parameters 3.2.1. Bed temperature Fig. 4, plotting tD versus bed temperature, indicates a strong dependence of agglomeration on temperature. tD at 850 °C is much less than that at 700 °C or 800 °C. This is consistent with other literature results [8–13]. As bed temperature increases, the amount of liquid eutectics grows. Meanwhile, the viscosity of the eutectics decreases. Consequently, the eutectics tend to transfer to the surfaces of other particles through collisions, thus facilitating the occurrence of agglomeration. Fig. 5 shows the surface morphology of agglomerate samples obtained under different temperatures on the basis of SEM analysis. As can be seen, with the increasing of bed temperature, the amount of flocculent materials, covering the particle surface and filling the gap increases accompanied by the gradual disappearance of particle edges. Note that, at 850 °C, the molten materials that behave as coaters are uniformly distributed on the particle surface. Thus, the substances connecting particles deplete and the particle edges reappear.
3.2.2. NaCl concentration Fig. 7 illustrates the effects of the NaCl concentration on tD. As shown in the figure, tD decreases with the increasing of NaCl concentration, which is consistent with Lin [10] who found that increasing the concentration of Na accelerates defluidization. As has been discussed previously, NaCl in wastewater reacts with SiO2 to form low-melting eutectics at 800 °C. Therefore, the increasing of NaCl concentration raises the amount of low-melting eutectics, thereby promoting agglomeration. Fig. 8 displays SEM images of bed materials and agglomerate samples for different NaCl concentrations. Fresh sand has angular edges and rough surfaces (Fig. 8 (a)). As the concentration of NaCl solution increases, the amount of low-melting eutectics increases and they assemble around the connective bridges between particles and partially cover the particle surfaces. The shape of particles becomes indistinguishable gradually. When the concentration of NaCl exceeds 6.5%, enough eutectics are generated to cover the particle surfaces entirely.
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Fig. 5. The SEM images of agglomerates for various bed temperatures. (a) 700 °C; (b) 750 °C; (c) 800 °C; (d) 850 °C.
3.2.3. Static bed height Fig. 9 presents the effects of static bed height on tD. The results indicate that tD shortened as the static bed height increases. Stable fluidization state lasts for an hour at the height of 140 mm. As the height increases to 170 mm, tD decreases to 35 min, to the same extent as the height of 200 mm. When the static bed height exceeds 230 mm, fast agglomeration occurs in 13 min after wastewater feeding. The variation of tD with static bed height is determined by the position of the wastewater spray gun. Part of wastewater is carried away by flue gas when the bed surface is lower than spray gun. As the static bed height increases, the corresponding distance between bed surface and spray gun is shortened and the amount of salty wastewater sprayed in the bed increases, which generates more low-melting eutectics. Besides that, the large bed height inhibits the particle mixing and weakens the particle mobility. Thus, liquid phase eutectics adhere to the particle easily. Both of these two factors accelerate the defluidization process when the static bed height increases from 140 mm to 230 mm. When the bed height exceeds 230 mm, the spray gun completely submerges the bed materials, which causes all the wastewater to be sprayed into the dense phase and tD to remain nearly unchanged. When concerning the present study, the slight increase of tD with the static bed height increasing from 230 mm to 250 mm may be due to the experimental errors and the uncertainty in the measurement of tD.
3.2.4. Gas velocity tD for different fluidizing velocities is plotted in Fig. 10. The results suggest that tD increases with the growth of gas velocity. When the fluidization number increases from 1.69 to 3.13, tD is prolonged by 53%, which agrees well with those of Lin [10,15]. The higher fluidizing gas velocity leads to a better mixing of particles and larger forces acting on agglomerates. It implies that the already formed agglomerates may be crushed under such conditions and the agglomeration will be delayed. 3.2.5. Particle size Due to the fact that the effects of particle size on agglomeration are still in controversy, this paper recorded tD for the incineration of salty wastewater using different particle sizes. The fluidizing gas velocity
Table 3 EDX analysis of different points on the agglomerates. Zone
C
O
Na
Cl
Si
Al
Mg
1 2 3 4 5 6 7 8 9
0.52 0.49 0 0.53 0.76 0.69 0.75 0.75 1.07
1.8 1.7 0.77 1.47 1.8 4.23 2.04 1.7 5.33
33.88 35.38 35.38 36.74 35.18 29.76 32.93 35.69 25.99
62.5 62.43 63.65 57.95 59.99 57.52 63.35 52.28 39.33
/ / / 2.03 2.27 7.79 0.94 7.36 25.94
/ / / 1.28 / / / 2.22 1.54
1.29 / / / / / / / /
Fig. 6. XRD analyses of agglomerates under various bed temperatures.
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Fig. 7. Variation of tD with NaCl concentration.
Fig. 9. Variation of tD with static bed height.
employed was kept 0.461 m/s. Fig. 11 shows the corresponding results. As can be seen, the increasing of particle size leads to a decline of tD. Due to the constant fluidizing gas velocity, both the momentum and mixing intensity of particles decrease with the increasing of particle diameters. The tendency to agglomerate is inversely proportional to the
momentum of the particles. Thus, tD increases with the decreasing of particle size. In addition, the small specific surface areas of large particles facilitate the formation of thick coatings. That may be another reason for the acceleration of defluidization when fluidizing larger particles.
Fig. 8. SEM images of bed materials for incineration of salty wastewater with different concentrations of NaCl. (a) fresh sand; (b) 3.5 wt.% NaCl; (c) 5 wt.% NaCl; (d) 6.5 wt.% NaCl; (e) 7.5 wt.% NaCl.
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Fig. 10. Variation of tD with gas velocity. Fig. 13. Variation of tD with additive type.
3.2.6. Sand type The variation of tD with bed material type is shown in Fig. 12. As can be seen, using fresh sand as materials can double tD when compared to the case of used sand. It is due to the fact that Na and Cl condensed on the surface of used sand previously accelerate the defluidization process. Therefore, in the industrial applications, it is feasible to gradually update the bed materials by adding fresh sand to prolong the running time of wastewater incinerator. 3.3. Effects of additives 3.3.1. Additive types Fig. 13 plots the values of tD for the cases using different types of additives. As can be seen, all the tested additives can inhibit agglomeration
Fig. 11. Variation of tD with particle size.
effectively. Among them, Al2O3 exhibits the best effect on the controlling of agglomeration. Table 4 shows the size distribution of bed materials discharged from the incinerator after agglomeration for different types of additives. When using Al2O3 powders as additives, the mass fraction of bed material particles with diameters larger than 450 μm (caused by the agglomeration since the mean diameter of bed materials before the test is 365 μm) is the least. It also validates the significant effect of Al2O3 in the prevention of agglomeration from one side. Fig. 14 shows the concentrations of elements in the agglomerates for different additives. With the injection of additives, the concentrations of Na and Cl on the agglomerate surfaces decrease significantly, whereas the concentrations of Ca, Fe, and Al increase dramatically. It indicates that the replacement of low-melting elements by high-melting elements, thereby increasing the melting-point of coating on the particle surface is the main mechanism of additives for the controlling of agglomeration. Note that the concentration of Na on the agglomerate surface for the case of 10% CaCO3 is larger than the case of non-additives, which is different from that of Fe2O3 and Al2O3. That may be due to the uneven distribution of Na on the agglomerates.
3.3.2. Fe2O3 Fig. 15 shows the SEM/EDX analysis results of the agglomerate samples when employing Fe2O3 powders as additives. As can be seen from the SEM image (Fig. 15 (a)), the smooth surface of the bed material particles was covered by a thin layer of additives and the gaps between particles were filled with some floccus. The EDX analysis (Fig. 15 (b)) shows that large amounts of Fe, O and Si, and low quantities of Na and Cl deposit on point 1 (particle surface, see Fig. 15 (a)). An opposite trend was observed when concerning the other points (bonding between particles). Based on these results, we formed a hypothesis: the mechanism for Fe2O3 to inhibit agglomeration is coating the bed material particles with high-melting Fe2O3 powders. In other words, physical effect is the controlling factor. In order to validate the assumption, we carried out a XRD analysis on the agglomerate samples. The results are shown in Fig. 16. It indicates that only SiO2, NaCl and Fe2O3 are found as crystalline substances on the agglomerate. Na2O · Fe2O3 · 4SiO2, theoretically produced by the Table 4 Size distribution of bed materials for different types of additives.
Fig. 12. Variation of tD with bed material type.
Particle size distribution(μm)
Additive types None
Al2O3
CaCO3
Fe2O3
Kaolin
b280 280–450 N450
3.47 91.6 4.93
3.18 94.83 1.99
3.11 94.02 2.87
3.95 93.97 2.08
2.33 94.73 2.94
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Fig. 14. Content of elements on the agglomerate surface for different types of additives.
reaction between Fe2O3 and Na2SiO3, was not observed. It can be inferred that physical effects are indeed the main mechanisms for Fe2O3 to inhibit the agglomeration under 800 °C: the bed material particles are surrounded by Fe2O3, impeding the contact between bed materials and melting NaCl and avoiding the adhesion caused by the collisions between particles, thus postponing the occurring of agglomeration. 3.3.3. CaCO3 Fig. 17 shows the SEM/EDX analysis of agglomerate samples with CaCO3 powders as additives. The SEM image reveals that the low melting eutectics are coated with non-cohesive layers behaving like refractory materials, which effectively reduces the interparticle adherence, thus controlling the agglomeration. The results of EDX analysis (Fig. 17 (b)) indicate that the surface of bed material particles mainly contains Na, Cl and Ca. In the ternary phase system of SiO2–CaO–Na2O,
Fig. 15. SEM and EDX analysis of agglomerate formed during incineration with Fe2O3 as additives. (a) SEM; (b) EDX.
Fig. 16. XRD analysis of agglomerate formed during incineration with Fe2O3 as additives.
the increasing of Ca content can facilitate the formation of highmelting materials and dilute the concentrations of low-melting eutectics (e.g. Na2SiO3), which would decrease the coating cohesion and increase its melting points. The results of XRD analysis of agglomerates are shown in Fig. 18, in which only SiO2, NaCl and CaO are found. The high-melting matter theoretically produced by the reaction between CaO and Na2SiO3 is not observed. Similar to Fe2O3, CaCO3 also inhibits agglomeration through physical effects under 800 °C. CaO powder generated by the decomposition of CaCO3 forms an effectively protective coating around the bed material surface for its high melting point of 2580 °C, which will
Fig. 17. SEM and EDX analysis of agglomerate formed during incineration with CaCO3 as additives. (a) SEM; (b) EDX.
J. Ma et al. / Powder Technology 253 (2014) 537–547
Fig. 18. XRD analysis of agglomerate formed during incineration with CaCO3 as additives.
prevent the bonding of bed materials to each other and control agglomeration. 3.3.4. Al2O3 Fig. 19 shows the SEM and EDX analysis of agglomerates after adding Al2O3 powder into the incinerator. As can be seen from the SEM image (Fig. 19 (a)), the surface of agglomerates is relatively smooth with clear edges and low concentration of molten NaCl. EDX analysis reveals that points 1, 2 and 3 are rich in Al and short of Na and Cl. Fig. 20, plotting the XRD analysis of agglomerates, shows that SiO2 and NaCl rather than high melting Na2O · Al2O3 · 6SiO2 presents in the agglomerates. Higher reaction temperature of Eq. (4) comparing to the bed temperature may be the reason for the absence of
Fig. 19. SEM and EDX analysis of agglomerate formed during incineration with Al2O3 as additives. (a) SEM; (b) EDX.
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Fig. 20. XRD analysis of agglomerate formed during incineration with Al2O3 as additives.
Na2O · Al2O3 · 6SiO2. As neutral oxide, Al2O3 can prevent the formation of low-melting matters and mitigate the sintering of bed materials. In addition, increasing Al2O3 content may raise the melting point of ash and inhibit agglomeration for its “skeleton” effects during ash melting [38]. In a word, the alleviation of agglomeration when using Al2O3 as additives comes from its physical effects. 3.3.5. Kaolin Fig. 21 shows SEM and EDX analysis of agglomerates with Kaolin as additives. Although the agglomerate surface was covered by a thin layer, no flocculent materials between particles are observed. The EDX analysis show that the agglomerate surface contains large amounts of Al and
Fig. 21. SEM and EDX analysis of agglomerate formed during incineration with Kaolin as additives. (a) SEM; (b) EDX.
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J. Ma et al. / Powder Technology 253 (2014) 537–547 Table 5 Size distribution of bed materials for different amounts of CaCO3. Size distribution (μm)
CaCO3 fraction (%) 0
5
10
20
b280 280–450 N450
3.47 91.6 4.93
2.2 94.63 3.17
3.11 94.02 2.87
11.31 86.36 2.33
4. Conclusions
Fig. 22. XRD analysis of agglomerate formed during incineration with Kaolin as additives.
Cabut little Na and Cl, which leads to a growth of melting point of coating and effectively controlling of agglomeration. The high-melting matters (Na2O · Al2O3 · 6SiO2) are not observed in the agglomerates through XRD analysis (shown in Fig. 22). That is because the present bed temperature dissatisfies with the requirement for the reaction between metakaolin and Na2SiO3. This is consistent with the findings of Ohman [36]. Steenari [37] further ascertained the reaction temperature of 900 °C between metakaolin and Na2SiO3. Al2O3 was not observed in Fig. 22 either, owing to the amorphousness of metakaolin after dehydrating reaction.
3.3.6. Additive amount As discussed previously, the addition of CaCO3 can inhibit agglomeration and reduce SO2 emission simultaneously, thus it has been considered as a preferred additive. We carried out a test to investigate the effects of the amount of CaCO3 on the agglomeration characteristics. As can be seen from Fig. 23, tD increases from 35 min to 2 h as the mass fraction of CaCO3 rises from 0 to 20%. Table 5 shows the size distribution of bed materials for different amounts of CaCO3. The mass fraction of bed materials with diameter larger than 450 μm decreases with the increasing of CaCO3 amount, indicating an apparent influence of increasing the CaCO3 amount on the prevention of agglomeration. Note that the mass fraction of bed materials smaller than 280 μm increases sharply for the CaCO3 amount of 20%, which can be attributed to the abrasion between bed materials under longer running time. It reveals that the increasing of CaCO3 may also have adverse effect on the stable running of fluidized bed incinerator. Thus, it is necessary to select an appropriate mass fraction of additives in the industrial application.
Fig. 23. Effects of the amount of CaCO3 on tD.
A thorough study of the agglomerates formed during the incineration of salty wastewater in a lab-scale fluidized bed was performed to evaluate the effects of operational parameters, NaCl content, as well as four kinds of additives on the agglomeration behavior. Based on these investigations, the following conclusions may be drawn: (1) Increasing the concentration of NaCl in wastewater accelerates the agglomerating process. Similar trends can also be observed for the case of higher bed temperature, lower gas velocity, deeper static bed height, and larger particle size. Additionally, compared with fresh sand, agglomeration was accelerated with used sand as bed material. (2) Above 750 °C, agglomeration is attributed to the formation of low-melting eutectics produced by the reaction between NaCl and SiO2. Below 750 °C, agglomeration is just induced by molten NaCl. (3) CaCO3, Al2O3, Fe2O3 and Kaolin can all effectively inhibit the agglomeration. Among them, Al2O3 exhibits the best inhibition results and better effects were observed with the increasing of additive amounts. (4) The inhibition of studied additives on the agglomeration mainly derives from physical effects within 800 °C. The coating formed by additives around the bed material particles is rich in Al, Ca and Fe, leading to a growth of melting points and a decline of adhesion between particles, thus inhibiting the agglomeration. Acknowledgments Financial supports of this work by National Nature Science Foundation of China (51306035), Specialized Research Fund for the Doctoral Program of Higher Education (20130092120010) and Scientific Research Foundation of Graduate School of Southeast University are gratefully acknowledged. References [1] P. Basu, S.A. Fraser, Circulating fluidized bed boilers design and operations, Butterworth-Heinemann-Reed Publishing, London, 1991. [2] R. Yan, D. Liang, K. Laursen, Y. Li, L. Tsen, J.H. Tay, Formation of bed agglomeration in a fluidized multi-waste incinerator, Fuel 82 (2003) 843–851. [3] A. Moilanen, K. Sipilä, M. Nieminen, E. Kurkela, Ash behaviour in thermal fluidized-bed conversion processes of woody and herbaceous biomass, VTT Energy, Espoo, Finland, 1996. [4] J. Shao, D. Lee, Y. Rong, M. Liu, X. Wang, D. Liang, T.J. White, H.P. Chen, Agglomeration characteristics of sludge combustion in a bench-scale fluidized bed combustor, Energy Fuel 21 (2007) 2608–2614. [5] L. Fryda, K. Panopoulos, E. Kakaras, Agglomeration in fluidized bed gasification of biomass, Powder Technol. 181 (2008) 307–320. [6] M. Ohman, L. Pommer, A. Nordin, Bed agglomeration characteristics and mechanisms during gasification and combustion of biomass fuels, Energy Fuel 19 (2005) 1742–1748. [7] E. Brus, M. Öhman, A. Nordin, Mechanisms of bed agglomeration during fluidized-bed combustion of biomass fuels, Energy Fuel 19 (2005) 825–832. [8] R. Bie, Y. Zhao, Z. Chen, J. Lu, L. Yang, Formation mechanisms of agglomeration caused by burning NSSC black liquor in a fluidized bed incinerator, Energy Fuel 23 (2009) 683–689. [9] X. Li, H. Lv, M. Xu, J. Ma, J. Yan, K. Cen, Agglomeration characteristics in fluidized bed incineration of organic-condensed wastewater, J. Chem. Ind. Eng. 56 (2005) 2166–2171.
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