Technologies of sulphur dioxide reduction in coal fired thermal power plan

1. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
17 – 19, July 2014, Mysore, Karnataka, India
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 9, September (2014), pp. 206-212
© IAEME:

2. Journal Impact Factor (2014): 7.5377 (Calculated by GISI)
TECHNOLOGIES OF SULPHUR DIOXIDE REDUCTION IN COAL FIRED
THERMAL POWER PLANT
Shreenath1, Ramakrishna Hegde2, Gangadhar Sheri3
1(Department of Mechanical Engineering, Srinivas Institute of Technology, Mangalore-574143, Karnataka, India)
2 (Professor, Department of Mechanical Engineering, Srinivas Institute of Technology Mangalore-574143, Karnataka)
3(AGM Performance, LANCO-UPCL, Nagarjuna Thermal Power Plant, Padubidri Udupi -574113, Karnataka, India)
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ABSTRACT
This paper evaluates various methods to reduce sulphur dioxide from coal fired thermal power plant. Flue Gas
Desulphurization (FGD) is a technology which extracts sulphur dioxides from flue gases produced in coal based thermal
power plants, where sulphur content in coal is more than 0.5%.In FGD maximum sulphur dioxide reduction is possible
by controlling of flow of flue gases into the absorber unit by adjusting the blade pitch of axial flow booster fan using a
hydraulic system and varying the concentration of lime stone slurry. In addition, coal blending is one of the solution for
reduction in sulphur content. The results indicate that maximum SO2-reduction is possible with increase in limestone
purity. 100% SO2-reduction may be possible in FGD but at the higher it will cause the design and installation cost of
FGD Plant.
Keywords: Blade Pitch, Flue Gas Desulphurisation, Limestone Slurry, Sulphur Dioxide.
1. INTRODUCTION
Any thermal power plant produces power using fossil fuels i.e. coal, biomass, agriculture waste etc. When these
fuels are burnt in furnace they produce a lot of harmful chemicals, gases, fumes which are exposed to atmosphere and
contaminate ambient air quality.
We can’t stop installing power plants, because there is a huge gap in demand and supply of electricity all over
the world, especially in our country as we are in developing stage. But to maintain the environment quality, we have to
do some efforts to reduce the exposure of harmful products to as low as possible by developing new technologies and
using already available technologies in market.
Sulphur dioxide (SO2) is a highly toxic air contaminant. When released into the atmosphere [1], SO2 with water,
oxygen, and other power plant pollutants, such as nitrogen oxides, to form acidic compounds. These acidic compounds
return to the earth in the form of acid deposition, which is the major component in acid rain. Sulphate particles contribute
to thousands of premature infant deaths each year.
Sulphur dioxide is one of the main pollutants of the power sector, which makes its reduction important.
Different methods that can be used in coal-fired power plants to reduce pollution are [2]:
1. Fuel switching or blended-fuel use
2. Coal preparation
3. Boiler modernization
4. Technology change
5. Flue gas desulphurisation.
IJMET
© I A E M E

3. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Low sulphur coal blended with coal of inferior quality parameters gives a mixture, which could emit according
to standards. The change to low sulphur coal might need some modifications in power plants.
Boiler modernisation is another possibility of emission reduction. When the boiler’s life cycle concludes, it
might be more efficient to repair it than to replace it with a new one.
Technology change is the radical method of emissions reduction in a coal fired power station. The main
alternative for the power sector is natural gas. It has lots of advantages—emission reduction, better effectiveness, it could
be built in a shorter time than a coal fired power station, and it needs less investment costs.
Flue Gas Desulphurization (FGD) is a technology which extracts sulphur dioxides from flue gases produced in
coal based thermal power plants, where sulphur content in coal is more than 0.5%. The coal produced from Indian mines
contains only 0.4% sulphur contents, so in India this technology was not required much. But, now a lot of companies are
importing coal from other countries like Indonesia, South Africa which contains sulphur contents ranging from 0.6-0.9%.
So it is made mandatory to install the FGD plant to maintain the ambient air quality standards.
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2. FGD PLANT CHEMISTRY
Flue gas desulphurization is a process to extract sulphur contents from flue gases, where there is a chemical
reaction that takes place inside the absorber tank, between limestone slurry, sulphur dioxide, water oxygen given to
system and gypsum is produced inside.
Sulphur dioxide is extracted from flue gases in wet scrubber, slurry of alkaline sorbent limestone react with the
sulphur dioxide [3].The limestone powder used is approximately 85% pure and more than 90% of the limestone particles
have to pass through a 325 mesh screen. This is necessary in order to have the maximum amount of limestone particles
that can possibly contact the sulphur dioxide molecules in the gas stream. For chemical reaction purpose the limestone
powder is mixed with 15-30% slurry introduced into the FGD vessel, re-circulated, and sprayed into the gas stream.
Fig.1: Absorber reaction tank [7]
2.1 ABSORBER TANK REACTION
Flue gases (containing SO2) entered in the absorber (Fig. 1) come in contact with limestone slurry (CaCO3). The
reaction takes place in wet scrubbing process producing CaSO3 (calcium sulphate) and can be expressed as :
CaCO3 (solid) + SO2 (gas) CaSO3 (solid) + CO2 (gas) (1)
When wet scrubbing with Ca(OH)2 (lime slurry), the reaction also produces CaSO3(calcium sulphite) and can be
expressed as [4] :
Ca(OH)2 (solid) + SO2 (gas) CaSO3 (solid)+ H2O (liquid) (2)

4. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
To partially offset the cost of the FGD installation, the CaSO3 (calcium sulphite) is further oxidized to produce
marketable CaSO4·2H2O (gypsum) using oxidation blowers. This technique is also known as forced oxidation :
CaSO3 (solid) + H2O (liquid) + ½O2 (gas) CaSO4.2H2O (3)
CaSO4. 2H2O is called gypsum and has market value [5].
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2.2 FLUE GAS DESULPHURISATION PROCESS
FGD is a process to extract sulphur dioxide from flue gases and to produce gypsum. The process is a
combination of different systems. Main components of FGD Plant are:
1. Flue Gas System
2. Limestone Handling System
3. Process Water System
4. Absorber System
5. Gypsum Handling System
6. Utility Water System
7. DDCMIS
3. METHODOLOGY USED
Following methodologies are used to reduce sulphur dioxide from the flue gases, which reduces the atmospheric
pollution.
1. Control of flow of flue gases into the absorber unit by adjusting the blade pitch of axial flow booster fan using a
hydraulic system.
2. Variation in concentration of lime stone slurry.
3. Blending of coal at various proportions. The different ratio of high and low calorific value coals are fed to the
the boiler.
4. EXPERIMENTAL SET UP
Flue gases entered through inlet damper, and the flow is boosted using an axial flow booster fan. The high flow
flue gases (Fig.2) entered in absorber tank and come out through the outlet damper to chimney. Both inlet and outlet
dampers are hydraulically operated and provided with common hydraulic system consists electrically operated hydraulic
pumps, accumulators, necessary instruments and valves etc. Hydraulic system is operated by the main Distributed
Control System (DCS) as part of FGD operation.
Continuous Emission Monitoring System (CEMS) is provided in Inlet and Outlet flue gas duct of scrubber unit
to monitor the SO2 (Sulphur Dioxide) level of flue gas during FGD operation. By outlet SO2 monitor, the percentage of
scrubbing (SO2 removal) is calculated with reference to inlet SO2 monitor. Both inlet and out let SO2 level will be
displayed in the FGD control room.
Fig.2: Flue gas path [6]

5. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Flue gas desulphurisation module is provided with independent axial type booster fan (Fig.3). The booster fan is
used to control the flow of flue gases into the absorber tank. The blade pitch of the booster fan can be adjusted by means
of hydraulic pumps [7].
Fig.3: Axial Booster Fan [7]
The hydraulic actuator system (Fig.4) comprises the following main elements:
1. An actuating cylinder moving axially along the fan axis and turning with the rotor.
2. A piston within the actuating cylinder which is axially fixed and rotates with the same speed as the cylinder
3. A feedback rod
4. A stationary control valve which receives the command to change the blade angle via an actuating gear unit
outside of the fan housing and converts it into a hydraulic signal.
Pressurized oil will be directed to the appropriate cylinder side, imparting an axial movement to the cylinder.
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This axial displacement causes the impeller blade to turn.
Fig.4: Hydraulic arrangement to operate booster fan [7]

6. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
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5. RESULTS AND DISCUSSION
5.1 SULPHUR REDUCTION BY MEANS OF BLENDING OF COAL
Fig.5: Sulphur content Vs coal blend ratio
Coal imported from South Africa and Indonesia having higher calorific value. It may be noted that the HCV coal
contains maximum amount of sulphur content nearly 0.9%. To reduce sulphur content in coal the low calorific value coal
is blended with HCV coal. Fig.5 shows the ratio of coal blending then reduction in sulphur content.
5.2 COAL CONSUMPTION AT DIFFERENT RATIO OF BLENDING
Fig.6 shows the coal consumption for different blend. If HCV coal is used the coal flow rate is decreased. In
case of HCV coal is blended with LCV coal then coal consumption is increased. Thus depending on type of coal and ratio
of blending coal handling plant needs to be designed.
Fig.6: Coal consumption at various ratio of blending
5.3 SO2 REMOVAL THROUGH FGD IN 100% HCV COAL CONSUMPTION
Fig.7 shows the variation of SO2 removal rate as a function of blade pitch. When high sulphur content coal is
used (100% HCV) the coal consumption rate is 230 tons per hour. The SO2 removal rate varies from 175.95 kg/h to 3519
kg/h. with respect to proportions of flue gases supplied through FGD plant. In 30% booster fan blade pitch opening the
SO2 removal rate is 1056 kg/h. The SO2 removal efficiency is 25.50%. The 85% of efficiency is achieved when
maximum flow is possible.

7. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Fig.7: SO2 removal at 100% HCV coal
5.4 SO2 REMOVAL THROUGH FGD AT VARIOUS PROPORTIONS OF COAL BLENDING
Fig.8: SO2 removal rate in FGD at different proportions of coal blend
A variety of low and high sulphur coals from various sources can be blended in different proportions to meet
normal and optimal limits for SO2 emissions. As sulphur contents are directly proportional in blending. The blend ratio of
component coals can be determined based on sulphur content to meet emission levels. Fig.8 shows the SO2 removal rate
at different proportions of coal blending. From the figure, it is clear that increasing the blade pitch proportionately
increases SO2 reduction across all blends. In LCV coal sulphur removal rate is less because LCV coal contains only 0.1%
of sulphur. The installation of FGD is not required in power plants when used with low calorific value coal.
5.5 SO2 REDUCTION EFFICIENCY WITH CONCENTRATION OF LIMESTONE PURITY
The available limestone consists of 80-95% of reactive calcium. The SO2 removal efficiency is increased with
increase in lime stone purity (Fig.9). In case of 30% of blade pitch opening the SO2 removal efficiency is 24% with 80%
of lime stone purity. The efficiency is increased upto 25.5% and 27% in increase of limestone purity 85% and 90%
respectively.
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8. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
Fig.9: SO2 removal efficiency Vs limestone purity
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6. CONCLUSIONS
The use of high calorific value coal reduces the fuel consumption per unit power but increase in sulphur dioxide
production rate. Fuel blending is one of the best method to reduce sulphur content. Amount of sulphur dioxide is also
reduced by supplying the flue gases through flue gas desulphurisation plant. Maximum efficiency can be obtained by
passing maximum amount of flue gases through FGD but in turn depends on design of FGD plant. With increase in
purity of limestone the sulphur dioxide removal rate can be increased.
7. ACKNOWLEDGMENT
The author would like to thank Engineers, Udupi Power Corporation Limited Padubidri, Karnataka,India for
their technical support to carry out this research.
8. NOMENCLUTURES
CEMS- Continuous Emission Monitoring System
DDCMIS- Distributed Digital Control Monitoring and Information System
FGD- Flue Gas Desulphurisation
HCV- Higher Calorific value
LCV- Lower Calorific Value
SO2- Sulphur Dioxide
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