The 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India

Characterization of Fly Ash from Various Locations of Electrostatic Precipitator S. Shanthakumar Research Scholar, Dept. of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India

D. N. Singh Dept. of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India R. C. Phadke General Manager, CHEMITHON Engineers Pvt. Ltd., Mumbai, India

Keywords: electrostatic precipitator, flue gas conditioning, fly ash, characterization

ABSTRACT: Electrostatic precipitation is a well established technique to control the emission of particulate matter (fly ash or dust particles) from coal-fired thermal power stations. Flue gas conditioning, FGC, is a technique that involves addition of chemical agents to the flue gas in order to increase the collection efficiency of the Electrostatic Precipitators, ESPs. Several efforts have already been made by earlier researchers to characterize the fly ash from hoppers, to ascertain its suitability for various engineering applications. However, how characteristics of the fly ash inside the ESP, from one location to another, change has not yet been investigated in details. Such investigations would help in understanding the influence of flue gas conditioning on functioning of ESPs. With this in view, attempts were made to characterize the fly ash collected at various locations of the ESP unit of a power plant (viz., hopper, bottom portion, collection plate and Induced Draft (ID) fan). The study brings out the influence of flue gas conditioning on overall characteristics of the fly ash.

1 Introduction The population explosion and industrial growth are two traits of present day society, which require more electricity generated form the coal-fired thermal power stations. However, the combustion of coal results in production of an enormous quantity of the ash, which essentially constitutes of bottom- and fly-ash. The fly ash particles that are in the form of suspension in the flue gas, which comes out of the power stations, contribute to an increased suspended particulate matter, SPM. Hence, reduction in emission levels of the SPM becomes essential for safeguarding the environment.

Electrostatic precipitation is a well established technique that employs application of electric field to separate out the suspended particles (fly ash or dust) from the flue gas, which comes out of thermal power stations by collecting it in hoppers (Navarrete et al., 1997; Parker, 1997; Kim and Lee, 1999; Bottner, 2003; Ray, 2004a; Hanne et al., 2006). Previous studies reveal that the collection efficiency of the ash can be enhanced by resorting to the flue gas conditioning, FGC, which deals with addition of different types of chemical additives (viz., sulphur trioxide, ammonia, salts of sodium) and sprinkling of water to the flue gas, which alters the resistivity of the fly ash and hence results in increased collection efficiency (Cheremisinoff, 1977; Brown et al., 1978; Harker and Pimparkar, 1988; ACRL, 1998; Alvarez et al., 2000; Ray, 2004b). With ever growing emphasis on the utilization of the fly ash, mainly in the cement and concrete, ceramic and electronic industries, researchers have conducted several studies on characterization of the fly ash from hoppers (Joshi and Lohtia, 1997; Bayat 1998; Foner et al. 1999; Sear, 2001; Kiattikomal et al. 2001; Singh and Kolay, 2002; Pandian, 2004; Moreno et al. 2005; Das and Yudhbir, 2006; Vassilev and Vassileva, 2007). However, how these characteristics of the fly ash get influenced due to the flue gas conditioning, has not yet been investigated in details.

With this in view, an attempt has been made in this study to characterize the fly ash collected at various locations in the Electrostatic precipitator (ESP) unit of a power plant in India. These locations correspond to hopper, bottom portion, collection plate and Induced Draft (ID) fan of the ESP. Based on extensive investigations, it has been demonstrated that how FGC influences the overall characteristics of the fly ash in the ESP.

2 Materials and methods Fly ash samples were collected from coal-based thermal power station of 210MW capacity, located in India.

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This power station is operated with dual flue gas conditioning system (ammonia and sulfur trioxide as the flue gas conditioning agents). Ash samples from hopper were collected and mix together so as to create a representative sample. Later, samples from different locations of the ESP, as depicted in Fig.1, were also collected during the shutdown of the power station for maintenance work.

Fig. 1 Sampling location in the ESP

Details of these samples are presented in Table 1 and various analyses carried out to characterize these samples are presented in the following.

Table 1 Designation of samples used in the study

Sampling Point Designation Hopper HR Collection plate of ESP ESP Plate Bottom portion of ESP ESP Induced Draft Fan ID

2.1 Physical characterization The sample was analyzed for its specific gravity, G, by using an Ultra-Pycnometer (Quantachrome, USA), which employs Helium gas (ASTM D 5550-00) and the specific surface area, S, by using Blaine’s air permeability apparatus (ASTM C 204-05). Portland cement was used as the standard reference material. The results are presented in Table 2. The particle size distribution characteristics of the sample were obtained by conducting hydrometer tests (ASTM D 422-63, 2002) and the results are depicted in Fig. 2.

2.2 Chemical characterization The pH and electrical conductivity (EC) of the solution of the ash sample was measured by using a water quality analyzer (Model PE 136, Elico Ltd., India). Liquid to solid (L/S) ratio of 10 and 20 was achieved by mixing 10 g and 5 g of the oven dried sample, respectively, with 100 ml of the distilled water. This solution was stirred, continuously, for half an hour and later filtered. This filtrate was used to determine the pH and EC of the solution and the results are presented in Table 2. Table 2 Physical and chemical characteristics of the samples used in the study

pH EC (μS/cm) Sample G S (m2/g) L/S=10 20 10 20

HR 2.05 0.215 5.8 5.7 158 86 ESP Plate 2.27 0.389 4.9 5.0 579 333

ESP 2.36 0.432 4.5 4.8 1982 1004 ID 2.70 0.558 4.5 4.7 1910 1056

From boiler

Stack4

1

2

3

1. Hopper 2. Bottom portion 3. Collection plates 4. Induced Draft Fan

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1E-3 0.01 0.1 10

10

20

30

40

50

60

70

80

90

100

110

% F

iner

Particle Size (mm)

HR ESP Plate ESP ID

Fig. 2 Particle size distribution of fly ash samples

Chemical composition of the fly ash, in the form of major oxides, was determined using an X-Ray Fluorescence (XRF) setup, (Phillips 1410, Holland). Details of the sample preparation are presented in the following. 4 g ash sample, 1 g of microcrystalline cellulose and isopropyl alcohol were mixed thoroughly, and the mixture was kept below an infrared lamp for slow drying. A small aluminum dish (with inner diameter of 33 mm and height of 12 mm) was taken and one third of this dish was filled with methyl-cellulose, followed by filling up the container by the dried sample. The sample was compressed with the help of a hydraulic jack and the chemical composition of the sample was determined by mounting the compressed dish (pellet) in the sample holder of the XRF setup. The results are presented in Table 3.

Table 3 Chemical composition of the samples used in the study

% by weight Sample Al2O3 BaO CaO Fe2O3 K2O MgO Na2O P2O5 SiO2 SO3 TiO2 HR 31.98 0.16 0.70 3.55 1.03 0.44 0.07 0.24 59.90 0.07 1.78

ESP Plate 39.90 0.15 0.69 4.74 1.42 0.73 0.09 0.34 49.87 0.12 1.84 ESP 40.78 0.24 0.73 4.82 1.30 0.67 0.09 0.47 48.57 0.32 1.92 ID 35.87 0.08 0.59 4.42 1.00 0.49 0.05 0.52 54.99 0.24 1.65

2.3 Mineralogical characterization The sample was evaluated for its mineralogical characteristics by employing X-Ray diffraction (XRD) spectrometer (Phillips 2404, Holland) studies, using a graphite monochromator and Cu-Kα radiation. The sample was scanned from 2θ ranging from 5º to 80º. The presence of minerals has been confirmed with the help of the data files presented by the Joint Committee on Powder Diffraction Standards (JCPDS, 1994). The results are depicted in Fig. 3. It can be noted from the figure that the different crystalline phases of minerals present in the fly ash samples are Quartz, mullite and hematite, among which, Quartz is the major one.

2.4. Scanning Electron Microscopy Morphology of the sample was studied by employing a Scanning Electron Microscopy (Hitachi S3400N, USA). The oven dried sample was placed on the sample holder and the images were captured under various magnifications. Prior to this, the sample was applied with the gold coating to avoid the charge effect so that a clear image could be obtained. Micrographs of the fly ash samples are presented in Fig. 4.

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0 10 20 30 40 50 60 70 800

500100015002000

0500

100015002000

0500

100015002000

0500

100015002000

Rel

ativ

e In

tens

ity

Cu-Kα (2θ)

MQQQ

MM

Q

QM

QMQHH

HR

MMQQQ

MM

Q

QM

QMQH

H

ESP plate

M QQQMM

Q

QMQ

MQHH

ESP

M

QQQ

MM

Q

M

Q - Quartz M - Mullite H - Hematite

MH

H

ID

Fig. 3 X-Ray diffractograms of fly ash samples

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(a) (b)

(c) (d)

(e) (f)

Fig. 4 Micro graphs of fly ash samples

3 Results and Discussion The specific gravity, G, and specific surface area, S, of various samples were plotted against various collection points, as depicted in Fig. 5 and Fig. 6, respectively. It can be observed from these figures that both G and S are minimum and maximum for HR and ID, respectively. This indicates that coarsest particles get collected in the hopper while the finest particles get collected in the ID fan, respectively. The particle size distribution, depicted in Fig. 2, clearly indicates the particle size variation for ash samples from different locations and mainly contains silt and clay size fractions.

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HR ESP Plate ESP ID1.50

1.75

2.00

2.25

2.50

2.75

3.00

G

HR ESP Plate ESP ID

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

S (m

2 /g)

Fig. 5 Specific gravity of fly ash samples Fig. 6 Specific surface area of fly ash samples

pH of the solution prepared from the ash samples, decreases slightly as moving towards the farthest portion of the ESP whereas, the electrical conductivity, EC, increases, as depicted in Fig. 7. Further, it can also be noted from Table 3, that the chemical composition of ash samples (in oxide form) by X-Ray fluorescence studies reveals that all the ash samples are having the major oxides (SiO2, Al2O3, Fe2O3) more than 70% and hence they belong to Class F ash (ASTM C 618). The morphology of ash samples obtained by scanning electron microscopy, as depicted in Fig. 4, reveals that the surface of most of the spherical particles has attached microspheres. It can be observed from the figure that the fly ash samples contain mostly hollow spheres i.e., cenospheres (Fig. 4a). Some particles (from hopper) are irregular in shape which indicates the presence of unburned carbon (Fig.4b). It also consists of broken spheres filled with smaller spheres i.e., pleurospheres (Fig. 4c). Also, Figs. 4(d) and 4(e) depict the effect of FGC on fly ash samples collected inside the ESP. It can be observed that the particles get agglomerated due to the increase in cohesion. This results in collection of more particles (mostly small in size) in the ESP and hence, the increase in collection efficiency, which in turns results in less emission from the stack/chimney of the power stations. It can also be observed from Fig 4(f) that the particles from ID fan of the ESP are the finest particles and hence exhibit much more agglomeration.

HR ESP Plate ESP ID1

2

3

4

5

6

7

8

9

10 L/S=10 L/S=20

pH

HR ESP Plate ESP ID

10

100

1000

10000 L/S=10 L/S=20

EC

(μS/

cm)

Fig. 7 pH and EC of fly ash samples

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4 Concluding Remarks This paper presents details of the investigations conducted on fly ash samples, collected at various locations of the ESP unit from a coal based power station in India, where dual flue gas conditioning is being practiced. The study reveals that the ash characteristics vary from one location to another of the ESP unit. In addition, from the SEM micrographs of the fly ash samples, agglomeration of the ash particles can be noted very clearly, which is responsible for an increased collection efficiency of the ESP and hence reduction in SPM level. However, extensive investigations should be conducted to establish relationships between various operational parameters of the ESP units with physico-chemico-mineralogical and morphological characteristics of the conditioned ash.

5 Acknowledgements The first author expresses his sincere gratitude to Mr. S. N. Trivedi, Managing Director, CHEMITHON Engineers Pvt. Ltd., Mumbai, for his encouragement and support during the course of this study. The financial support received from M/s. CHEMITHON Engineers Pvt. Ltd., Mumbai, for conducting this research, under CEPL fellowship, is gratefully acknowledged.

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