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Toward Stable Operation of Sewage Sludge Incineration Plants: The Use of Alumina Nanoparticles to Suppress Adhesion of Fly Ash Juguan Gao, Miki Matsushita, Genki Horiguchi, Ryosuke Fujii, Mayumi Tsukada, Yohei Okada, and Hidehiro Kamiya Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02305 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019
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Toward Stable Operation of Sewage Sludge Incineration Plants: The Use of Alumina Nanoparticles to Suppress Adhesion of Fly Ash Juguan Gao, Miki Matsushita, Genki Horiguchi, Ryosuke Fujii, Mayumi Tsukada, Yohei Okada, Hidehiro Kamiya*
ABSTRACT In sewage sludge incineration plants, fly ash can adhere to the surfaces of both the incinerator and dust filter, and then accumulate over time. Since this grown layer can potentially block gas flow and damage the filter, thus preventing stable and long-term operation of incineration plants, fly ash adhesion must be monitored carefully and suppressed as much as possible. Herein, we used three different fly ash samples to demonstrate that the merger of experimental and theoretical approaches enabled a rational choice of additive that could suppress fly ash adhesion. The tensile strength of ash powder beds provided a benchmark for the adhesive properties, which were further studied by thermodynamic calculations. Experimental and theoretical results suggested that the use of alumina nanoparticles was a promising approach to suppress the adhesion of fly ash containing a relatively high concentration of phosphorus components.
INTRODUCTION Sewage treatment causes a large amount of waste, i.e., sludge. Disposal of sewage sludge involves
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thickening, digestion, and dewatering processes to reduce its volume and to decompose organic materials contained therein. Although effective utilization of thus treated sewage sludge, such as for fertilizer or cement, is of great importance, the final destination is usually the land. In order to further reduce the volume before landfilling, sewage sludge is incinerated into inert ash.1−3 This is especially needed for large disposal stations in urban areas, since suitable sites for landfilling are restricted or not available. In general, incinerated ash is categorized into bottom ash and fly ash. While bottom ash remains in the bottom of the incinerator, fly ash must be collected using an appropriate filter.4−6 During the incineration, fly ash can adhere to the surfaces of both the incinerator and dust filter, and then accumulate over time. Since this grown layer can potentially block gas flow and damage the filter, thus preventing stable and long-term operation of incineration plants, fly ash adhesion must be monitored carefully and suppressed as much as possible. However, fly ash can contain a wide variety of components with varied ratios and therefore, its chemical and physical properties are difficult to predict. Although some additives were found to be effective to suppress fly ash adhesion and have been used in practice with some success, the choice and amount of the additive used are usually determined by trial-and-error approaches. It is generally recognized that fly ash adhesion can be attributed to the formation of a liquid phase. Not only viscous sticky liquids, but also thin free-flowing ones can induce an attractive force between ash particles through liquid bridging, which is caused by capillary negative pressure and
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surface tension. Furthermore, by using the equations proposed by Rumpf9 and Rabinovich et al.,10 we recently demonstrated that even a tiny amount (ppb order) of liquid can potentially create a significant attractive force between particles that promotes fly ash adhesion and its growth.11 This means that fly ash adhesion can take place even in the presence of an undetectable amount of liquid, making it more difficult to understand the mechanisms. If the amount of thus formed liquids is extremely small, analysis and characterization are almost impossible. In the case of sewage sludge incineration plants, fly ash tends to have a relatively high concentration of phosphorus components, which are assumed to induce the adhesion.12, 13 However, the mechanistic aspect remains unclear and therefore, further experimental and theoretical analyses are needed to effectively suppress the ash adhesion. We previously developed a split-type tensile strength tester, equipped with a furnace (see Figure S1 in Supporting Information for the details).14 The tester can directly measure the tensile strength of the powder beds being studied at various temperatures from ambient to around 1000 °C.11, 15−17 We believe that the tensile strength of ash powder beds is closely related to the adhesion properties, leading to better understanding of the mechanistic aspect. The tester is especially important for studying fly ash adhesion at temperatures lower than 1000 °C, since most previous methods in this field generally require temperatures higher than 1000 °C to melt fly ash. Since sewage sludge incineration is usually carried out at temperatures lower than 900 °C, this tester would have a substantial advantage in studying fly ash adhesion. Described herein are experimental and theoretical
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analyses using three different fly ash samples as models, leading to the finding that alumina nanoparticles are promising additives that could suppress the adhesive properties of fly ash.
EXPERIMENTAL SECTION Materials. In this study, three different fly ash samples (FA 1-FA 3) were used as models. All ash samples were collected from commercial sewage sludge incineration plants. The physical and chemical properties of the ash samples are summarized in Table 1 and Table S1. P, Si, and Al were found to be the main components in all the ash samples. The particle size distribution was measured in a dry system using a laser-scattering particle size distribution analyzer (Horiba, LA-950ND). Morphological observations (Figure S2) and chemical composition analyses were carried out using a FE-SEM/EDS system (JEOL, JSM-6335F/JED-2200F). Alumina nanoparticles (AEROXIDE Alu 130) were provided by Nippon Aerosil Co., Ltd (Figure S3 and Table S2).
Table 1. Physical and Chemical Properties of the Ash Samples Used in This Work. Particle Particle Diameter Chemical Components (wt%)b
density (m) (kg/m3)a Laser Ash
FE-SEM
P2O5
SiO2
Al2O3
CaO
MgO
Diffraction
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K2O
Na2O
Fe2O3
SO3
Others
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FA 10.0
4.6
2765
45.7
18.5
12.0
8.4
6.3
4.5
1.3
1.6
0.6
1.1
8.9
4.3
2893
37.0
25.9
14.6
8.8
5.7
2.1
1.6
2.4
0.4
1.6
8.9
3.9
2933
31.7
26.8
23.4
5.6
3.9
3.4
0.6
2.0
0.9
1.8
9.3
4.3
2864
38.1
23.8
16.7
7.6
5.3
3.3
1.2
2.0
0.6
1.5
1
FA
2
FA
3
Ave. a
Measured by pycnometer. bMeasured by EDS analysis.
Characterization. The tensile strength of the ash powder bed was measured by the split-type tester in the temperature range from ambient to 800 °C under air. The respective ash samples (8.0 g) were packed into a cylindrical cell (diameter 50 mm × height 10 mm) and consolidated by uniaxial pressing at 2.66 kPa for 10 min to prepare the powder beds. After consolidation, the volume of the ash powder bed was measured to determine its porosity. The cell containing the ash powder bed was heated to the target temperature at a heating rate of 10 °C/min and held for 10 min at each temperature before measurement of the tensile strength. Thermal properties of each ash sample were also investigated by thermomechanical analysis (TMA, Rigaku, TMA8310) under air at a heating rate of 10 °C/min (Figure S4).
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Theoretical. Thermodynamic calculations were carried out with the FactSage 7.3. GTOX database. Phosphorus system data was used for all calculations.
RESULTS AND DISCUSSION The present work began with the measurement of the tensile strength of the ash powder beds at various temperatures (Figure 1). The tensile strengths were lower than 0.10 kPa at room temperature for all samples, suggesting that the ashes were not adhesive under these conditions. All the tensile strengths remained at relatively low levels until 500 °C, when they gradually increased up to 1.3 kPa at 600 °C. A significant increase was observed for FA 1 between 600 and 700 °C. Although the tensile strengths of FA 2 and FA 3 also increased as the temperature increased, they were lower than that of FA 1, even at 800 °C. Based on the tensile strength measurements, it could be assumed that FA 1 is a representative adhesive ash and FA 3 is a less-adhesive model.
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Figure 1. Tensile Strength of Ash Powder Beds Measured at Various Temperatures (Holding Time 10 min).
To gain mechanistic insight into the adhesive property of the ash samples, thermodynamic calculations were carried out (Figure 2). The behavior of slag (liquid) phase formation usefully explains the results obtained by the tensile strength measurements. The formation of the slag phase was expected to start at 509 °C for FA 1, which can contribute to the increase of tensile strength. In sharp contrast, no slag phase was formed until 703 °C for FA 3, also suggesting that FA 3 is a representative less-adhesive ash in the thermodynamic calculations.
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Figure 2. Behavior of Slag (Liquid) Phase Formation of FA 1-FA 3.
A significant feature of FA 3 is its low phosphorus content and high silicon and aluminum contents. Thermodynamic calculations were carried out using modified conditions based on FA 1 as a representative adhesive ash. When 15 wt% phosphorus was subtracted, the slag formation temperature significantly increased up to 703 °C, which was equal to that of FA 3 (Figure 3). These results suggested that high phosphorus content is the key to the adhesive property. Of course, in practice, a subtraction approach cannot be applied to suppress the adhesion of fly ash.
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Figure 3. Behavior of Slag (Liquid) Phase Formation of FA 1 with Reduced Phosphorus Content.
Unfortunately, when silicon was increased by 10-20 wt%, no changes were observed for the slag formation temperatures, suggesting that high silicon content is not directly related to the adhesive property (Figure 4). This also means that silicon content is not a promising additive to suppress adhesion of fly ash. When alumina content was increased by 10-20 wt%, however, the slag formation temperatures significantly increased up to 614 °C (Figure 5). While this is still lower than that of FA 3 (703 °C), aluminum would be a promising additive to suppress adhesion of fly ash.
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Figure 4. Behavior of Slag (Liquid) Phase Formation of FA 1 with Increased Silicon Content.
Figure 5. Behavior of Slag (Liquid) Phase Formation of FA 1 with Increased Aluminum Content.
To our delight, the tensile strength of FA 1 was usefully decreased when alumina nanoparticles were
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added by dry blending. The adhesive property of FA 1 with just 3 wt% blending of alumina nanoparticles is comparable to that of FA 3 (Figure 6). It should be noted that dry blending is a heterogeneous process and therefore, the local aluminum content could be extremely high at the surface of the ash particles.
Figure 6. Tensile Strength of Ash Powder Beds Measured at 700 °C (Holding Time 10 min) in the Presence of Alumina Nanoparticles.
CONCLUSION In conclusion, the merger of experimental and theoretical approaches has led to the rational finding that alumina nanoparticles are promising additives to suppress the adhesion of fly ash containing a relatively high concentration of phosphorus components. A benchmark for the adhesive properties
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was provided by the tensile strength of ash powder beds measured by a split-type tester. The behavior of slag phase formation determined by thermodynamic calculations usefully explained the results obtained by the tensile strength measurements. We believe that our current approach would be applicable to various fly ash samples to suppress the adhesive properties. Further studies using different ash samples and additives are under investigation in our laboratory.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional figures and tables, including the details of characterization and theoretical calculations (PDF)
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. ORCID Yohei Okada: 0000-0002-4353-1595 Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Nos. 16H02413 and 18H05342 (to H.K.).
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