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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.
Indian Geotechnical Conference – 2010, GEOtrendz
December 16–18, 2010
IGS Mumbai Chapter & IIT Bombay
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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