Toward Stable Operation of Coal Combustion Plants: The Use of

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Toward Stable Operation of Coal Combustion Plants: The Use of Alumina Nanoparticles to Prevent Adhesion of Fly Ash Genki Horiguchi, Ryousuke Fujii, Yusuke Yamauchi, Harumi Okabe, Mayumi Tsukada, Yohei Okada, and Hidehiro Kamiya Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03043 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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Toward Stable Operation of Coal Combustion Plants: The Use of Alumina Nanoparticles to Prevent Adhesion of Fly Ash Genki Horiguchi,1 Ryousuke Fujii,1 Yusuke Yamauchi,2 Harumi Okabe,2 Mayumi Tsukada,1 Yohei Okada,1 Hidehiro Kamiya*,1

1Department

of Chemical Engineering, Tokyo University of Agriculture and Technology,

2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan

2R&D

Department, TEPCO Research Institute, Tokyo Electric Power Company

Holdings, Inc., 4-1 Egasaki-cho, Tsurumi, Yokohama, Kanagawa 230-8510, Japan

ABSTRACT

ACS Paragon Plus Environment

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In pulverized coal combustion plants, low-grade sub-bituminous coal is known to cause adhesion of fly ash and its growth on the surface of the super heater, heat exchanger, and furnace walls, even when co-combusted with high-grade bituminous coal. In this study, 6 different fly ash samples collected from commercial pulverized coal combustion plants were investigated to find that the porosity and the liquid phase would have a significant impact over the tensile strength of the ash powder beds. Herein, we demonstrate that the use of alumina (Al2O3) nanoparticles as an additive is highly effective to prevent such troublesome events, which would be a great aid to facilitate stable and long-term operation of coal combustion plants. The function of Al2O3 nanoparticles is found to be twofold: retaining the porosity and perturbing the chemical composition of the ash samples. This is rationally supported by experimental and theoretical results. The key to our study is a split-type tensile-strength tester that can directly measure the tensile strength of a powder bed being studied at various temperatures.


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INTRODUCTION

Although renewable sources of power, such as sunlight or biomass, are in high demand and thus have been studied extensively world-wide, energy derived from fossil fuels still occupies a central place in our current society. Limited resources, including petroleum and coal, must be utilized effectively to maximize their available energy and therefore, creative methodologies have been devised and proposed to achieve this. From the viewpoint of thermal power generation, coal can roughly be categorized as high-grade or low-grade, generally defined based on the degree of coalification. Since low-grade sub-bituminous coal is well-known to cause adhesion of fly ash and its growth on the surface of super heater, heat exchanger, and furnace walls during the combustion process, it must be co-combusted with high-grade bituminous coal. However, even in such a co-combustion method, these troublesome events tend to occur, inhibiting stable and long-term operation of pulverized coal combustion plants.1–10

Compared with high-grade bituminous coal, low-grade coal contains higher amounts of alkali metals, which are expected to promote adhesion of fly ash and its growth during


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the combustion process.1–6 This can be attributed to the formation of a liquid phase, which bridges the ash particles to other solid surfaces, since alkali metals are known to produce relatively low-melting-point eutectics with some elements derived from ash.7, 11 Such a liquid bridging is expected to induce an attractive adhesion force between the ash particles and other solid surfaces, which is assisted by capillary negative pressure and surface tension to promote the troublesome events.12 To address this issue, additives are generally used in the combustion process.2–4, 13 These additives are expected to perturb the chemical composition of ash that otherwise produces relatively low-melting-point eutectics, preventing the formation of a liquid phase. To this end, for example, Mg2 and Si-Al3, 4 were found to be effective to reduce adhesion of fly ash and its growth. These additives have been in practical use with some success; however, it is perhaps fair to say that they are not general ways to prevent the troublesome events since the chemical composition of low-grade coal is wide-ranging and unpredictable. Both the choice of additives and the amount used must be considered and optimized to the specific coal sample, rendering the entire process cumbersome. A reasonable


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Energy & Fuels

indication for the use of additives would be a great aid to facilitate stable and long-term operation of pulverized coal combustion plants.

We have been investigating the use of a split-type tensile-strength tester that can directly measure the tensile strength of a powder bed being studied at temperatures from ambient to 1000 °C (see Figure S13 in Supporting Information for the details).12 It should be noted that most previous studies in this field have focused on the melting behavior of ash particles at higher than 1000 °C, while adhesion of fly ash and its growth tend to occur on the surface of a super heater in the temperature range of 6001000 °C. By using the tester, we previously demonstrated that a significant increase was induced in the tensile strength of the ash powder bed even in the presence of a tiny amount of alkali metals.7, 10, 12, 13 Furthermore, a field emission scanning electron microscope (FE-SEM) equipped with a heating device enabled the direct observation of morphological changes of fly ash during the heat treatment, which was proven to be a powerful method to analyze the behavior of fly ash during the combustion process.10 Described herein are experimental and theoretical studies using 6 different fly ash


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samples as models, leading to the finding that alumina (Al2O3) nanoparticles are highly effective to prevent adhesion of fly ash and its growth in the practical temperature range.

EXPERIMENTAL SECTION

Materials. In this study, 6 different fly ash samples (FA1-6) were used as models. All ash samples were collected from commercial pulverized coal combustion plants. Physical and chemical properties of the ash samples are summarized in Table 1. Si and Al were found to be the main components in all the ash samples. The number-based particle size distribution was measured by a laser scattering method in a dry system using a laser scattering particle size distribution analyzer (Horiba, LA-950ND). The particle density was measured by a pycnometer (Micromeritics, AccuPyc II 1340). Morphological observations and chemical composition analyses were carried out using a FE-SEM/EDS system (JEOL, JSM-6335F / JED-2200F). See Figures S1 and S2 for further characterization of ash samples. Al2O3 nanoparticles (AEROXIDE Alu 130) were provided by Nippon Aerosil Co., Ltd.


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Table 1. Physical and chemical properties of the ash samples used in this work.

Particle diameter / µm Ash

Laser

FE-SEM

diffraction

observatio n

FA 1 FA 2 FA 3 FA 4 FA 5 FA 6

12.1

9.81

11.3

10.2

11.6

7.87

Chemical componentsb / wt%

Particle densitya / kg m-3

5.0

5.4

5.1

6.5

4.7

5.7

2361

2446

2515

2311

2197

2370

aMeasured

by pycnometer.

bMeasured

by EDS analysis.

Si

Al

66.

24.

1

5

62.

29.

1

9

56.

29.

4

0

64.

22.

1

2

57.

23.

6

5

52.

28.

4

5

other

Na

K

Mg

Ca

Fe

S

1.4

2.3

1.0

0.7

2.6

0.4

1.0

0.1

1.4

0

1.6

0.5

1.8

2.6

1.4

2.3

1.6

2.0

4.1

1.3

1.9

3.1

2.0

1.3

3.7

2.3

0.6

0.7

4.4

1.1

1.6

5.8

2.5

2.3

1.2

2.4

1.1

3.2

5.7

3.8

1.0

1.9

s


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Characterization. The tensile strength of the ash powder bed was measured by the splittype tensile-strength tester in the temperature range from ambient to 900 °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 uni-axial 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 to 60 min at each temperature before measurement of the tensile strength. Thermal properties of each ash sample were also investigated by thermogravimetry-differential thermal analysis (TG-DTA, Rigaku, Thermo plus EVO TG8120) and thermomechanical analysis (TMA, Rigaku, TMA8310) under air at a heating rate of 10 °C/min. Morphological changes of fly ash during the heat treatment were observed by FE-SEM equipped with a heating device.


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Theoretical. Thermodynamic calculations were carried out with FactSage 7.0. FactPS and FTOxid databases were used for all calculations. The volume (V) of the liquid phase formed during the heat treatment was estimated based on the liquid bridging force (Fl) using equation (1), which is a slightly modified version of the equation reported by Rabinovich.11 The surface distance between particles (H) was calculated based on the porosity (ε) using the modified Rumpf equation (2).14 The derivation details are described in the Supporting Information. x: particle diameter; : surface tension (0.5 Nm1);15

: contact angle (10o);15 A: Hamaker constant (5×10-19 J);16 σ: tensile strength of

powder bed.

𝐹l =

𝜋𝑥𝛾cos𝜃

(

4𝑉

)

(1)

―1

1 + ―1 + 1 + 𝜋𝑥𝐻2

𝐻=

𝐴 1 ― 𝜀RT 24𝑥𝜎RT 𝜀RT

(2)

RESULTS AND DISCUSSION


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The present work began with the measurement of the tensile strength of ash powder beds at various temperatures (Figure 1). In general, the tensile strength increased between 800 °C and 900 °C. Only a slight increase was observed for FA3, whereas a huge increase up to 14 kPa in the tensile strength was observed for FA6, suggesting that a significant amount of liquid phase was formed. When the temperature was held at 900 °C for 10-60 min, the tensile strengths of FA4 and 5 also increased to higher than 14 kPa, while those of FA1 and 2 remained at a relatively lower level even after holding for 60 min. It could be assumed that FA4-6 were the ashes that tended to cause the troublesome events during the combustion process.


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Figure 1. The tensile strength of ash powder beds (a) measured at various temperatures (holding time: 10 min) and (b) measured at various holding times (temperature: 900 °C).

To gain further insight into the behavior of the ash samples during heat treatment, the thermal properties were investigated by TG and TMA (Figure 2). In the TG profiles, weight loss was observed at higher than 600 °C in all the ash samples, which was likely caused by gasification of unburned carbon and sulfur components. Although no significant weight loss was observed between 800 °C and 900 °C, where the tensile strength increased in most samples, the TMA profiles suggested that volume loss started to occur in this temperature range. This shrinkage of the ash powder beds was more marked in FA3-6, which further highlighted the events occurring when the temperature was held at 900 °C. As discussed later, the volume of the liquid phase formed during the heat treatment was estimated to be extremely small. Therefore, it is


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not likely that the shrinkage of the ash powder beds was solely responsible for the formation of the liquid phase.


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Figure 2. TG (blue) and TMA (green) profiles of ash samples measured at various temperatures.

When the porosity of the ash samples, calculated from TG and TMA data, was plotted over the tensile strength at 900 °C, a rough correlation was observed (Figure 3). The tensile strength at 900 °C increased as the porosity decreased. Thus, it could be assumed that not only the formation of the liquid phase but also the porosity of the ash samples would be the keys for the increase in tensile strength. Therefore, the use of an additive that can retain the porosity of the ash samples during heat treatment would be effective to decrease the tensile strength. However, the tensile strength increased as


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the heating time was extended, while no change was observed for the porosity. These results clearly suggested that the increase of the tensile strength was not explained simply by the porosity of the ash samples. Taken together, the porosity was an important factor that determined the tensile strength and the increase derived from the formation of the liquid phase would be promoted under extended periods of heat treatment.

Figure 3. Relationship between the porosity and the tensile strength at 900 °C of the ash samples.


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Next, the formation of the liquid phase was studied theoretically. The volume of the liquid phase was estimated based on the liquid bridging force (Fl) and the porosity of the ash powder beds (ε) using equations (1, 2) and the results are summarized in Table 2. In all cases, the volume of the liquid phase formed during heat treatment was estimated to be extremely small, on the order of ppb in V/V over the ash samples. Such tiny volumes of the liquid phase may have a significant impact over the tensile strength of the ash powder beds.

Table 2. Estimated amount of the liquid phase formed during heat treatment.

V/Vp at each holding time at 900 °C / ppba

Ash 10 min

30 min

60 min

FA1

2.21

2.20

4.46

FA2

18.5

28.2

33.9

FA3

10.9

22.1

26.1

FA4

14.7

36.9

84.1


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FA5

22.3

69.8

102

FA6

92.0

138

122

aV

p:

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Volume of one ash particle.

Based on FE-SEM observations, the ash samples were found to show spherical morphology, and were roughly categorized into microparticles and nanoparticles (Figures 4 and S8 in Supporting Information). In general, the microparticles were coated with the nanoparticles. This morphological characteristic could be reasonably understood based on the combustion process. Pulverized coal is initially combusted under high temperature, where most components have the chance to melt. In combustion plants, the temperature is expected to gradually decrease and components with relatively high melting points condense to form microparticles. When the temperature decreases further, the components with relatively low melting points then condense to form nanoparticles on the surface of the microparticles. This mechanism was confirmed by FE-SEM observations with heat treatment. The rough surface of the ash particles became smooth after heat treatment, especially at 800 oC and higher. It


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appears that the nanoparticles melted selectively, while no significant morphological change was observed for the microparticles. This observation also suggested that the liquid phase was mainly derived from the nanoparticles. The volume of the nanoparticles was expected to be extremely small compared to that of the microparticles, which was in good accordance with the estimated values summarized in Table 2.

Figure 4. SEM images of ash samples FA4 (upper) and FA5 (lower) before and after heat treatment.


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With experimental observations and theoretical considerations in hand, we then sought to propose a new additive that could prevent adhesion of fly ash and its growth during the combustion process. Our strategy was twofold: retaining the porosity and perturbing the chemical composition of the ash samples. The main components of the ash samples used in this study were found to be Si and Al. It is well known that Si and Al form low-melting-point eutectics with alkali metals, such as Na or K. In particular, a relatively higher amount of Na was found in FA4-6, the ashes that tended to cause the troublesome events during the combustion process, and therefore, Si, Al, and Na in their oxidized forms were used for the thermodynamic calculations. Based on the phase diagrams of SiO2-Al2O3-Na2O calculated at 800 oC and 900 oC (Figures 5 and S4 in Supporting Information), we found that slag is expected to form in the presence of Na2O, while this is not the case when a certain amount of Al2O3 exists. For example, slag formation is expected to occur in the presence of 10% Na2O in SiO2 (Na2O 10%, SiO2 90%, red spot), which can be prevented when 20% of SiO2 is changed to Al2O3 (Na2O


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10%, SiO2 70%, Al2O3 20%, green spot). The diagram rationally suggests that Al2O3 would be a promising additive to prevent adhesion of fly ash and its growth during the combustion process. Furthermore, among various commercially available Al2O3, we selected nanoparticles to increase the porosity of the ash samples (Figures S9 and S10 in Supporting Information).

Figure 5. Phase diagrams of SiO2, Al2O3 and Na2O at 800 °C (left) and 900 °C (right). Red triangles show the compositions that form the slag phase.


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The results were more effective than expected. When Al2O3 nanoparticles were added to FA4-6 by dry blending, the tensile strength of the ash powder beds dramatically decreased even in the presence of only 0.5-1% of Al2O3 (Figure 6; see Figure S11 in Supporting Information for SEM images and EDS mappings of the samples). Furthermore, the porosity of the ash powder beds was retained at a high level in the presence of Al2O3, however, it did not change significantly when only a small amount of Al2O3 was added. Therefore, the function of Al2O3 would be twofold: retaining the porosity and perturbing the chemical composition of the ash samples. To retain the porosity, a relatively larger amount of Al2O3 was required, whereas the chemical composition was effectively perturbed even by a small amount of Al2O3. It could be reasonably understood that the added Al2O3 would exist at the surface of the ash particles, providing a high local concentration of Al2O3. The bulk components inside the particles were not expected to participate in the chemical reaction at the surface. The added Al2O3 likely formed relatively high-melting-point eutectics at the surface, which contributed to the decrease in the tensile strength of the ash powder beds.


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Figure 6. Relationship between tensile strength (●) and porosity (+) of ash samples at 900 °C in the presence of Al2O3 nanoparticles.

CONCLUSION


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In conclusion, we have successfully demonstrated that the use of Al2O3 nanoparticles is highly effective to decrease the tensile strength of ash powder beds during heat treatment. The function of the Al2O3 nanoparticles is expected to be twofold: retaining the porosity and perturbing the chemical composition of the ash samples. This is rationally supported by experimental and theoretical results described herein. Although the formation of a liquid phase is well-known to cause adhesion of fly ash and its growth during the combustion process, the estimation of the volumes has not been performed previously. We believe that our current findings would be informative for the choice of additives to facilitate the stable and long-term operation of pulverized coal combustion plants. Further studies using different ash samples and additives are under investigation in our laboratory.

ASSOCIATED CONTENT


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Supporting Information. Additional figures and tables including the details of characterization and theoretical calculation.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Number 16H02413 (to H. K.).

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(12) Kamiya, H.; Kimura, A.; Yokoyama, T.; Naito, M.; Jimbo, G. Development of a split– type tensile–strength tester and analysis of mechanism of increase of adhesion behavior of inorganic fine powder bed at high–temperature conditions. Powder Technol. 2002, 127, 239–245.

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Toward Stable Operation of Coal Combustion Plants: The Use of

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