Factors Affecting the Density and Specific Surface Area - American

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Factors Affecting the Density and Specific Surface Area (Blaine Value) of Fly Ash from Pulverized Coal Combustion Hiromi Shirai,* Michitaka Ikeda, and Kenji Tanno Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan ABSTRACT: In the Japanese electric power industry, it is desirable to reduce the cost of treatment and expand the range of use of fly ash, such as in concrete admixtures. Therefore, it is necessary to form high-quality fly ash. To obtain high-quality fly ash, it is important to clarify the factors affecting its properties. In this study, the factors affecting the density and specific surface area (Blaine value) of coal fly ash were investigated on the basis of experimental results obtained using our combustion test facility and the ash data from the boiler of an actual electric power plant. The density was affected by the ash particle size, the true density of the component materials, and the aluminum content, which is closely related to the fusibility. The specific surface area was affected by the particle size distribution and particle shape. The shape was affected by not only the ash particle size but also the unburned carbon concentration and the ash fusibility. It was also found that the specific surface area of the ash generated from our combustion test facility is higher than that of ash from the actual boiler for ash with the same particle size. This result indicates that the shape of particles is affected by their heating and formation histories in the boiler. On the basis of the above findings, correlation equations were obtained for the density and Blaine value.

1. INTRODUCTION The amount of fly ash discharged from existing pulverized coal-fired power plants is increasing and has reached over 10 million tons/year in Japan. The ratio of fly ash in Japan that is effectively used is more than 90%. However, over 60% of fly ash is treated, so that it can be used as a material in cement. In the Japanese electric power industry, it is desirable to reduce the cost of treatment and expand the range of use of fly ash, such as in concrete admixtures. Therefore, it is necessary to form highquality ash. The Japanese Industrial Standard (JIS) for the use of fly ash in cement classifies fly ash into three grades.1 The grade is determined by the properties of the fly ash (particle size, unburned carbon concentration, density, and specific surface area obtained by the Blaine method) and the properties of the mortar (fluidity and strength after solidification) to which the fly ash is added. To manage the ash quality, it is important to develop a method for predicting the ash properties. Various methods2
5 for predicting the particle size and unburned carbon concentration have been proposed. However, the study6 of the prediction of ash density and specific surface area has been insufficient. The purpose of this study is to clarify factors affecting the density and specific surface area (Blaine value) from experimental results obtained using our combustion test facility and the ash data from four boilers of four different electric power plants. 2. EXPERIMENTAL SECTION 2.1. Coal Samples. In the study using our combustion test facility, 18 different brands of bituminous coal with a total of 27 lots were used. The range of properties of the coals is shown in Table 1. Coals with different combustibility and fusibility were selected to cover the range of properties of coals burned in Japanese pulverized coal-fired power r 2011 American Chemical Society

Table 1. Coal Properties property

our test furnace

moisture (wt %)b ash (wt %)c

actual boilera

2
13 7
17

1
9 5
15

volatile matter (wt %)c

31
47

28
45

fixed carbon (wt %)c

41
63

42
58

fuel ratio

0.8
2.3

1.0
2.1

HHV (MJ/kg)c

27
32

27
31

SiO2

45
73

47
75

Al2O3 Fe2O3

18
41 1
13

15
30 2
11

ash composition (wt %)d

a c

CaO

0.5
10

0.8
13

MgO

0.2
1.8

0.2
2.0

TiO2

0.5
3.5

0.6
1.6

P2O5

0.05
2.1

0.07
1.3 0.1
1.7

Na2O

0.05
1.7

K2O

0.7
3.3

0.4
3.3

unburned carbone

1.6
14.5

0.9
7.7

Blended coal properties are included. Dry basis. d JIS M 8815. e JIS A 6201.

b

At equilibrium humidity.

plants, from which fly ash is produced. Furthermore, samples of fly ashes, formed by the combustion of blended coals, were provided from other power plants. The properties of these samples are those of the blended coals. Additionally, in all ash samples from our test furnace and power plants, the content of SiO2 + Al2O3 is more than 75 wt %. Received: July 21, 2011 Revised: October 23, 2011 Published: October 24, 2011 5700

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Figure 1. Schematic diagram of the pulverized coal combustion facility. conjectured that the values were similar to those of the actual boilers. The size of pulverized coal particles and the properties of the coals were confirmed. Before each combustion test, all ash was removed from the facility to prevent contamination of the newly formed ash. After the combustion test, the facility was cooled. Then, fly ash was collected from the bottoms of the ash hopper of the air heater (AH), de-NOx unit, gas cooler, bag filter, electrostatic precipitators (ESPs), and temperature controllers. These ashes, except for the bottom ash in the furnace, were mixed with weight proportional to the collected weight in each unit, and the mixed ash was used as a sample. 2.3. Ash Property Analysis. The ash particle size was measured using a laser diffraction particle size analyzer. The unburned carbon concentration, which is the ignition loss,1 was calculated from the weight difference of the coal ash before and after heating fly ash for 15 min at 975 ( 25 °C. The ash particle density was determined by the displacement of ethanol in a Le Chatelier flask.7 The specific surface area was determined by the Blaine method,7 in which the specific surface area is derived from the resistance to the flow of air through a layer of ash particles using a known standard material. Figure 2. Details of the furnace.

2.2. Pulverized Coal Combustion Facility. A schematic diagram of the pulverized coal combustion facility is shown in Figure 1. The facility consists of a pulverizer, a furnace with three burners, and a gas treatment system. The coal feed rate is determined by the thermal input of the coal. Here, the thermal input is 760 kW per burner. The coal feed rate is approximately 100 kg/h per burner when high-quality bituminous coal is burned. The details of the furnace are shown in Figure 2. The furnace has a width of 0.9 m, a depth of 1.9 m, and a height of 9.5 m. The mean residence time of the combustion gas is approximately 4 s on the assumption that the gas temperature is 1200 °C. This time is similar to that of actual Japanese boilers. The maximum combustion temperature is more than 1400 °C. The wall has a water wall structure, the inert surface of which is covered with a refractory material. In this study, the outlet O2 concentration was set to 4%, which is equivalent to an excess air ratio of 1.24. The standard two-stage combustion ratio was 30%, and the median diameter of the pulverized coal particle size was 35
50 μm. These conditions and the emissions of NOx and unburned carbon are also similar to those of actual Japanese boilers.6 Mixed ash collected from all parts of the facility, except the bottom of the furnace, was used as a sample. On the other hand, it is thought that almost every boiler in large power plants is currently being operated with an outlet O2 concentration of 2
4% and a two-stage combustion ratio of 10
30%. However, the values for fly ash provided from four boilers were unknown; it was

3. RESULTS AND DISCUSSION 3.1. Ash Particle Density. The ash particle density Fash (g/cm3) depends upon the true density without closed voids Ft (g/cm3) and the ratio of the closed void volume/particle volume (void ratio ε). The particle density is given by

Fash ¼ Ft ð1
εÞ

ð1Þ

The factors affecting the true density and void ratio were investigated. 3.1.1. True Density. The true density is the average density of the composed materials. The contents of the materials for five brands of coal were measured by the X-ray diffraction internal standard method using ZnO. As shown in Table 2, the detected crystalline oxides are quartz (SiO2), mullite (3Al2O3 3 2SiO2), hematite (Fe2O3), and magnetite (Fe3O4). The mullite ratio is the proportion of aluminum in coal ash that is converted into mullite. It is calculated from the mullite content measured by the X-ray diffraction internal standard method and the Al2O3 content measured by chemical analysis.8 The glass ratio is the content excluding crystalline oxides and SO3, assuming that all of the materials are oxides. The mullite ratio and glass ratio are more than 50 wt % in these samples, similar to the results in previous studies.9 However, it was difficult to obtain the exact 5701

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Table 2. Materials Comprising Fly Ash measured crystalline (wt %) brand of coala

a

SiO2

3Al2O3 3 2SiO2

Fe2O3

calculated amoruphous (wt %) Fe3O4

Al2O3

SiO2

Fe2O3

mullite ratio (wt %)

glass ratio (wt %)

NL

9.70

33.53

0.34

0.09

11.24

24.57

4.52

68.22

56.34

MO

16.44

20.94

0.12

0.00

10.90

41.07

4.63

57.98

62.50

WB

26.76

12.98

0.56

0.24

8.13

38.88

3.08

53.41

59.46

HV

25.24

18.61

0.35

0.00

9.81

36.46

2.55

57.68

55.80

SK

9.42

36.23

0.00

0.00

12.20

27.64

3.26

68.09

54.35

NL, Newlands; MO, Mount Owen; WB, Wambo; HV, Hunter Valley (Australia); and SK, Senakin (Indonesia).

true density because a large part of the materials was amorphous. Therefore, the true density of ash was estimated on the basis of the following assumption. All aluminum element was assumed to be mullite (3Al2O3 3 2SiO2) or amorphous aluminosilicate because aluminum coexists with silicon in the clay minerals kaolinite and illite.10 Furthermore, a previous study11 indicated that aluminosilicate including mullite is formed at a combustion temperature of 1200
1400 °C. Because the density of amorphous aluminosilicate was unknown, the density of mullite was selected. The silicon element, excluding that contained in mullite, was assumed to be converted into SiO2. Although the density of SiO2 is in the range of 2.2 g/cm3 (amorphous SiO2) to 2.6 g/cm3 (quartz), in this study, the density of amorphous SiO2 was selected because the content of amorphous SiO2 was greater than that of quartz. Various other metal elements were included in the amorphous glass phase, making it impossible to determine the density of the amorphous glass. Therefore, the densities of other metal oxides were set in accordance with those of the oxides defined by chemical analysis8 of the ash. The selected oxides were SiO2 (density Fi = 2.2 g/cm3), 3Al2O3 3 2SiO2 (Fi = 3.1 g/cm3), Fe2O3 (Fi = 5.2 g/cm3), CaO (Fi = 3.4 g/cm3), MgO (Fi = 3.7 g/cm3), P3O4 (Fi = 2.4 g/cm3), TiO2 (Fi = 4.2 g/cm3), Na2O (Fi = 2.3 g/cm3), and K2O (Fi = 2.4 g/cm3). In this study, the content Wi (wt %) of each oxide was recalculated, so that the total content ratio of the oxides was 100 wt %. The true ash particle density Fa (g/cm3) was estimated using the following equation: Fa ¼

∑

100 ðWi =Fi Þ

ð2Þ

The ash was also ground to a mean volume diameter DV‑ash (μm) of less than 6 μm to remove as many of the closed microvoids as possible. DV‑ash is defined as

∑ðWi Di Þ DV-ash ¼ ∑Xi

Figure 3. Comparison between the estimated true density and the density after removing closed voids by grinding ash.

ð3Þ

where Di (μm) is the diameter and Xi is the weight ratio of the particles. A comparison between Fa and the density of pulverized ash particles is shown in Figure 3. It was found that Fa is almost equal to the pulverized ash density. The results indicate that our method can approximately estimate the true density in the case that the content of SiO2 + Al2O3 measured by chemical analysis is more than 75 wt %. 3.1.2. Closed Voids. The void ratio ε was calculated from the measured ash particle density and the estimated true ash particle density Fa using eq 2. The factors affecting the formation of closed voids were clarified using the void ratio. In general,6,12 the

Figure 4. Influence of the ash particle size on the ash particle density.

particle density tends to become low with an increasing particle diameter of the fly ash, as shown in Figure 4. This is due to the increase in the volume of closed voids with the particle diameter. The relationship between ε and DV‑ash is shown in Figure 5. In this study, the ash particle diameter was changed by changing the pulverized coal diameter DV‑coal in the same coal. This confirms that ε is affected by the ash particle size. Small ash particles are formed from small coal particles, which burn rapidly and whose temperature also increases rapidly during combustion. Additionally, the number of coalescent mineral particles is less than that of large coal particles. Small coal particles are advantageous for the coalescence and melting of mineral particles. Therefore, ε is lower in smaller ash particles. On the other hand, the influence of the two-stage combustion ratio on the void ratio was investigated 5702

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Figure 7. Influence of the Al2O3 content on the void ratio. Figure 5. Influence of the ash particle size on the void ratio.

Figure 8. Relationship between the actual density and the estimated density. Figure 6. Influence of the fusibility of ash on the void ratio.

using NL coal. However, the two-stage combustion ratio was changed; its influence was not clear. Furthermore, it was found that ε for NL coal is different from that for WB coal at the same particle size. This indicates that there are other factors strongly affecting ε. As another possible factor, the influence of the fuel ratio, which is an index of combustibility, was investigated. However, ε did not correlate with the fuel ratio. Next, the influence of fusibility was investigated. However, ε did not correlate with the melting point of the ash, as shown in Figure 6. It is considered that the melting point should not be used as an index to evaluate changes in the internal conditions of the ash because the melting point is determined by the shape of a specimen. Therefore, chemical changes in the ash were estimated by FactSage 5.5, which is a thermochemical software and database package. In Figure 6, it is shown that ε correlates with the temperature at which the liquid phase fraction is 90%, which is the weight ratio of the liquid phase of ash to the total of the solid and liquid phases of ash. Furthermore, it was found that the formation of mullite affects the above temperature. The relationship between ε and the content of Al2O3 WAl (wt %) measured by chemical analysis (JIS M 8815) is shown in Figure 7. The high correlation indicates that the content of the Al element in the form of mullite is an important factor affecting ε.

3.1.3. Correlation Equation for the Ash Particle Density. The empirical correlation equation for the ash particle density using the contributory factors was set up as follows: Fash-est ¼ Fa ð1
ðkd1 þ kd2 ðWAl =100Þ þ kd3 ðWAl =100Þ2 þ kd4 Dp ÞÞ

ð4Þ

where kdi is the coefficient calculated by the least-squares method. The relationship between the actual density Fash‑act and the estimated density Fash‑est is shown in Figure 8. It was clarified that the factors affecting density revealed in this study are correct, because the difference between the two densities is within 5% of the actual density. 3.2. Specific Surface Area. In JIS, the specific surface area of the ash SAb (cm2/g) is measured by the Blaine method. However, SAb is not the true specific surface area of the ash, although it is an important value for evaluating the outer surface area, excluding the inner surface area of open microvoids. The specific surface area of a powder is affected by the particle size distribution and the particle shape. 3.2.1. Particle Size. The specific surface area is strongly affected by the surface area of fine particles. Therefore, the influence of the ash particle size on SAb was investigated using the surface 5703

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Figure 9. Influence of the ash particle size on the specific surface area (Blaine value).

Figure 10. Influence of the ash particle size on the shape of ash particles.

mean diameter DS‑ash (μm), which is defined as DS-ash ¼

∑X i

∑ðXi =Di Þ

ð5Þ

The relationship between DS‑ash (μm) and SAb is shown in Figure 9. If the shapes of the particles are the same, SAb increases as DS‑ash decreases. However, a clear correlation between DS‑ash and SAb was not observed. This means that the influence of the particle size on SAb is not strong. 3.2.2. Particle Shape. The specific surface area SAd (cm2/g of ash) was calculated from the ash particle size distribution and ash density on the assumption that ash particles are spherical. The relationship between DS‑ash and the ratio of SAb/SAd, which is the reciprocal of Carman’s shape factor, is shown in Figure 10. SAb/SAd increases with the ash particle diameter. This indicates that the sphericity of the ash particles decreases as the diameter increases. It is thought that the number of coalesced particles in a mineral affects the particle shape similar to its influence on density. Next, SAb/SAd in our furnace was compared to that in an actual boiler. For the same particle size range, SAb/SAd in our furnace was observed to be higher than that in the boiler. This result indicates that the sphericity of particles in our facility is lower than that in the boiler. It is presumed that this is because the residence time of ash particles in the high-temperature zone of our furnace is shorter than that in the boiler.

Figure 11. Influence of the unburned carbon concentration on the shape of ash particles.

Figure 12. Influence of the melting point on the shape of ash particles.

The relationship between the unburned carbon concentration Uc (wt %) and SAb/SAd is shown in Figure 11. Uc, which is affected by the fuel ratio and combustion conditions, such as the coal particle size and two-stage combustion ratio, affects SAb/SAd because the particles containing unburned carbon have a complex shape. In ash with a high unburned carbon concentration, SAb may not be correctly measured because the Blaine method cannot be efficiently applied to particles with greatly different shapes. Also, the figure shows that the difference between SAb/SAd in our furnace and that in an actual boiler at the same unburned carbon concentration is large. This confirms that its difference is not caused by the unburned carbon. On the other hand, SAb/SAd is affected by the fusibility of ash. As shown in Figure 12, it is confirmed that the melting point is correlated with SAb/SAd. In addition, to estimate SAb/SAd from the ash composition without measuring the melting point, an index that correlates with the melting point was investigated. In Figure 13, it is shown that Al2O3/(Fe2O3 + CaO + MgO + Na2O + K2O)13 is useful for this purpose. The relationship between the index and SAb/SAd is shown in Figure 14. In the index range from 1.0 to 3.5, SAb/SAd increases with the index. However, above a value of 3.5, no correlation can be confirmed because there is insufficient data. Therefore, it is thought that the index is useful for values of less than 3.5. 5704

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Figure 13. Relationship between the index of the melting point and the melting point.

Figure 16. Relationship between the actual SAb and estimated SAb.

Findex ¼

Al2 O3 ðFe2 O3 þ CaO þ MgO þ Na2 O þ K 2 OÞ ð7Þ

where ksi is the coefficient calculated by the least-squares method. The correlation between the actual SAb/SAd (SAb/ SAb)act and the estimated SAb/SAd (SAb/SAb)est is shown in Figure 15. This result confirmed that the factors selected in this study are effective for predicting the Blaine value because of the strong correlation between (SAb/SAb)act and (SAb/SAb)est. Furthermore, the correlation between the actual SAb, SAb‑act, and the estimated SAb, SAb‑est, calculated from (SAb/SAb)est by eq 6 and SAd, is shown in Figure 16, which indicates that SAb can be estimated from (SAb/SAb)est and SAd. Figure 14. Relationship between the index of fusibility and the shape of ash particles.

4. CONCLUSION In this study, the influences of combustion conditions and coal properties on the ash particle size, density, and specific surface area were investigated on the basis of experimental results obtained using our combustion test facility and the ash data from the boiler of an actual electric power plant. (1) For the ash particle density, the density was affected by the ash particle size, the true density of the component materials, and the aluminum content, which is closely related to the fusibility. (2) For the specific surface area of ash, the specific surface area was strongly affected by the shape of the ash particles. The shape was affected by not only the ash particle size but also the unburned carbon concentration in the ash and the ash fusibility. From these results, it was found that the ash particle size distribution, ash composition, and unburned carbon concentration are important factors in developing a method for predicting the density and specific surface area of fly ash. ’ AUTHOR INFORMATION

Figure 15. Relationship between the actual SAb/SAd and estimated SAb/SAd.

3.2.3. Correlation Equation for SAb/SAd. The empirical correlation equation for SAb/SAd was set up as follows: ks2

ðSAb =SAd Þest ¼ ks1 ðUc =100Þ Findex ks3 DS-ash ks4

ð6Þ

Corresponding Author

*Telephone: +81-46-856-2121. Fax: +81-46-856-3346. E-mail: [email protected].

’ REFERENCES (1) Japanese Industrial Standard (JIS). JIS A 6201; JIS: Tokyo, Japan, 1999. 5705

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(2) Central Research Institute of Electric Power Industry (CRIEPI). Improvement of Pulverized Coal Combustion Technology for Power Generation; CRIEPI: Tokyo, Japan, 2002; CRIEPI Review Number 46. (3) Yan, L.; Gupta, P. R.; Wall, F. T. Fuel 2002, 81, 337–344. (4) Barta, L. E.; Toqan, M. A.; Beer, J. M.; Sarofim, A. F. Symp. (Int.) Combust., [Proc.] 1992, 1135–1144. (5) Tanno, K.; Shirai, H.; Ikeda, M. Influence Factors on Fly Ash Particle Size Distribution in Pulverized Coal Combustion; Central Research Institute of Electric Power Industry (CRIEPI): Tokyo, Japan, 2011; CRIEPI Report Number M10020. (6) Shirai, H.; Tsuji, H.; Ikeda, M.; Kotsuji, T. Energy Fuels 2009, 23, 3406–3411. (7) Japanese Industrial Standard (JIS) JIS R 5201; JIS: Tokyo, Japan, 1997. (8) Japanese Industrial Standard (JIS) JIS M 8815; JIS: Tokyo, Japan, 1976. (9) Yamamoto, T.; Hironaga, M. Experimental Study on Mechanism of Strength Development of Fly Ash Contained Concrete Derived from Pozzolanic Reaction; Central Research Institute of Electric Power Industry (CRIEPI): Tokyo, Japan, 2007; CRIEPI Report Number N06018. (10) Lauf, R. J. Fuel 1981, 60, 1177–1179. (11) Mayoral, M. C.; Izquierdo, M. T.; Andres, J. M.; Rubio, B. Thermochem. Acta 2001, 373, 173–180. (12) Ghosal, S.; Self, S. A. Fuel 1995, 74, 522–529. (13) New Energy and Industrial Technology Development Organization (NEDO). NEDO Report NEDO-C-9940; NEDO: Kawasaki, Japan, 2000.

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