A Scanning Electron Microscopy Study on Flue Gas Conditioned Fly

Ashes Collected from Thermal Power Stations

Shanthakumar, S. Singh, D.N.1 Phadke, R.C.2

Associate Professor Professor General Manager

e-mail: shanthakumar.s@vit.ac.in e-mail: dns@civil.iitb.ac.in e-mial: drphadke@chemithon.co.in

School of Mechanical and Building Sciences, VIT University, Vellore1Department of Civil Engineering, IIT Bombay, Mumbai

2Chemithon Engineers Pvt. Ltd, Mumbai

ABSTRACT

Dual flue gas conditioning (DFGC) is a technique in which the chemical additives such as ammonia and sulphur

trioxide are injected together in to the flue gas in order to reduce the suspended particulate matter (SPM) emission

and hence, the improvement in ash collection efficiency of electrostatic precipitator (ESP). Since, the low sulphur

and high ash contents of Indian coals are the major problem for thermal power stations in controlling the SPM

emission, during DFGC process, sulphur trioxide assists in reducing the resistivity of fly ash where as, ammonia

facilitates the agglomeration of ash particles. However, more details on the DFGC in improving ash collection

efficiency of ESP are not yet presented in literature. Hence, in this paper, an effort has been made to investigate

the effect of dual flue gas conditioning on agglomeration of ash particles, by comparing the scanning electron

microscopy (SEM) images of dosed and undosed fly ash samples collected from the thermal power stations located

in India. Based on the investigations, the effect of flue gas conditioning on agglomeration of ash particles have

been highlighted.

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1. INTRODUCTION

The main source of electricity supply in India is coal-fired

thermal power stations, which produces more than two third

of electricity required. During the combustion process, a

huge amount of coal ash (fly ash and bottom ash) is

generated. The fly ash particles that are in the form of

suspension in the flue gas contribute to an increased

suspended particulate matter, SPM. Hence, for safeguarding

the environment, reduction in emission levels of the SPM

becomes essential. In order to achieve this, various devices,

such as cyclone separators, bag filters and/or electrostatic

precipitators (ESPs) are employed (Shanthakumar et al.

2008a). However, electrostatic precipitator is employed in

majority of the thermal power stations due to its main

advantages such as (i) high collection efficiency; (ii) low-

operating costs; (iii) low-pressure drop; and (iv) its

suitability for dealing with particles of different sizes and

variable flue gas volumes (Theodoree & Buonicore

1976).The practice of removing the suspended particles

from the flue gas by applying the electric field is termed as

electrostatic precipitation (White 1963). Moreover, for

reducing the emission of sulphur dioxide from the chimneys

of the coal fired power plants, low sulphur coal is being

used these days (Ray 2004). However, it has been noted

that the resistivity of the fly ash, generally, increases as the

ratio of sulphur to ash content of the coal decreases, which

results in its very low collection efficiency of ESP(Oglesby

& Nichols 1978, EPA 1998). The coal used in various

thermal power stations across the country varies widely in

its properties. The Indian coal contains high ash (35-45%)

and low sulfur (<0.5%) contents (Chandra et al. 2006).

Since the sulfur content in Indian coals is low, the resistivity

increases to the order of 1012 ohm-cm or more and hence,

the ESP collection efficiency is poor (Chandra et al. 2004).

In order to improve the ash collection efficiency of ESPs,

flue gas conditioning, FGC, is employed in thermal power

stations. This technique involves the injection of the

chemical agents (viz., sulphur trioxide, sulphuric acid,

ammonium sulphate, ammonium bisulphate, sulphamic

acid and ammonia for conditioning of flue gas) and water

into the flue gas to modify the properties of the fly ash so

that the efficiency of electrostatic precipitators increases

454 S. Shanthakumar, D.N. Singh and R.C. Phadke

substantially(Dalmon & Tidy 1972, Ditl & Coughlin 1976).

Among all the chemical agents, sulphur trioxide and

ammonia are the commonly and commercially in use at

thermal power stations. The simultaneous and independent

injection of both ammonia and sulphur trioxide is referred

as dual flue gas conditioning (Drbal et al. 1996). At times

thermal power plants require installation of dual flue gas

conditioning, which is found to be more suitable when, (i)

percentage of Al2O

3 and SiO

2 is more than 90% or (ii) fly

ashes exhibit high resistivity (Parker 1997, Porle et al.

1996). It has also been demonstrated by the researchers

that sulphur trioxide conditioning reduces the resistivity

of fly ash particles (Krigmont 1992, Cook 1975, Bayless et

al. 2000), where as ammonia conditioning improves its

surface charge and hence their cohesive properties and

hence minimizes the re-entrainment of particles ( Dismukes

1975, Turner et al. 1994). However, more details on effect

of flue gas conditioning on agglomeration of ash particles

are not yet presented in the literature. Hence, in the present

study, an effort has been made to investigate the effect of

dual flue gas conditioning on agglomeration of ash

particles, by comparing the scanning electron microscopy

(SEM) images of dosed and undosed fly ash samples

collected from the thermal power stations located in India.

Based on the investigations, the influence of flue gas

conditioning on agglomeration of ash particles have been

highlighted.

Materials and Methods

Materials

Fly ash samples were collected from the coal-based 210

MW capacity thermal power station, located in India. These

samples were collected from hoppers of the ESP, when the

power station is operated with out and with different

concentrations of chemical agents (Ammonia and Sulphur

trioxide). The samples of undosed, ammonia dosed, and

dual flue gas conditioning process are denoted as U, A,

and D, respectively. Details of the samples along with their

designation are presented in Table 1. Details of scanning

electron microscopy test conducted on the sample are

presented in the following.

Table 1: Details of the Samples Used in the Present Study

Dosing rate

(kg/hr)

NH3 SO3

Collection

Point

Sample

Designation

- - Hopper U1, U2, U3, U4, U5

25 - Hopper A1, A2, A3, A4, A5, A6

Hopper D1, D2, D3, D4

8 12 ESP Collection

Plate D5, D6, D7

Scanning Electron Microscopy

Scanning electron microscopy (SEM) is one of the most

widely used techniques for the chemical and physical

characterization of fly ash. A focused electron beam is used

to scan the surface of a sample and the SEM provides visual

information based on gray-scale intensity between chemical

phases (Vassilev & Vassileva 2005). SEM images of the

sample were obtained by employing Hitachi make S3400N

and Quanta make 200 ESEM instruments. A representative

portion of fly ash sample was sprinkled on double-sided

carbon tape mounted on a SEM stub and the images of

various magnifications were captured.

2. RESULTS AND DISCUSSION

The morphology obtained at various magnifications by

scanning electron microscopy for the fly ash samples

collected from hoppers of the ESP are depicted in Figures

1-5. It can be observed from the Figure 1 that the fly ash

samples contain hollow spheres, termed as cenospheres

(Sample U3) and some irregular shaped particles (Sample

U1). Also, Figure 2 illustrate that the fly ash samples

consists of broken spheres filled with smaller spheres,

termed as pleurospheres. The micrographs of unburned

carbon particles present in the fly ash samples are depicted

in Figure 3. The presence of unburned carbon particles can

be due to poor combustion of coal in the thermal power

station. It can be observed from the figure that the unburned

carbon particles are highly porous which can be due to the

escape of volatile matter during the combustion process,

which is consistent with the results reported in the literature

(Hwang et al. 2002). Further, in order to exhibit the

influence of flue gas conditioning on agglomeration of ash

particles, scanning electron microscopy images of different

ash samples (undosed, ammonia dosed, dosed with

ammonia and sulphur trioxide) were compared, as depicted

in Figure 4. It can be noted from these micrographs that

for undosed samples (U5), there is almost no agglomeration

of particles. In contrast, the ammonia dosed samples (A3)

reveal agglomeration of particles, which is consistent with

the results reported in the literature (Shanthakumar et al.

2008b). However, samples collected (D3) from the process

in which both ammonia and sulphur trioxide are injected

together, i.e. DFGC exhibit a substantial agglomeration of

fines due to increase in cohesion between the particles.

Incidentally, as depicted in Figure 5, SEM micrographs of

the samples (D5, D6 and D7) from the collection plate of

the ESP, collected immediately after the shutdown, exhibit

very thick agglomerated pockets of ash particles. Hence,

it is reputable that dual flue gas conditioning helps in more

agglomeration of particles, and hence, improves collection

efficiency of ESPs, which in turns results in less emission

from the stack/chimney of the power stations.

A Scanning Electron Microscopy Study on Flue Gas Conditioned Fly Ashes Collected .... 455

U1 U3

Fig. 1: SEM Images of Undosed Samples

A1 A3

Fig. 2: SEM Images of Pleurospheres Present in the Samples

A4 D1

F i g . 3 : SEM Images of Unburned Carbon Particles Present in the Samples

D5 D6 D7

Fig. 5: SEM Images of Samples Collected from Collection Plate of the ESP

U5 D3 A3

Fig. 4: Comparison of SEM Micrographs of Different Samples (Undosed [U], Dosed with Ammonia [A], Dosed with Ammonia and

Sulphur Trioxide [D])

456 S. Shanthakumar, D.N. Singh and R.C. Phadke

3. CONCLUDING REMARKS

The scanning electron microscopy analysis conducted on

the fly ash samples collected from a coal based power station

in India reveal that the ash samples contain cenospheres,

pleurospheres and unburned carbon particles. On

comparison of the SEM micrographs of undosed, ammonia

dosed and dual conditioned samples, the agglomeration of

ash particles have been observed from the ammonia dosed

samples. However, the dual conditioned fly ash samples

exhibit a very thick agglomeration of the ash particles,

which results in increased collection efficiency of the ESP,

which in turn results in less SPM emissions from the stack

of the power stations.

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