1. AIR POLLUTION IN THERMAL
POWER PLANTS

2. THERMAL POWER PLANTS
 Utilize fuel to produce steam for
power generation.
 Classified according to type of fuel
used.
Coal power plant
Gas power plant
Oil power plant

3. Combustion of fuel produces
significant amount of air
pollutant.
Depends on the nature of
fuel used
Coal power plant
 Fly ash
 Sulfur dioxides
 Oxides of nitrogen

4.  Oil power plant
 Sulfur dioxides
 Oxides of nitrogen
 Combustion of coal is major
important source for particulate air
pollutant than oil.
 The amount of fly ash and SO2
depend on the sulfur and ash
content of the fuel used.

5. The major pollutants are
 Particulate matter(fly ash and soot)
 Sulfur oxides (SO2 &SO3)
 Oxides of nitrogen(NO&NO2)
 Trace metals Cd, Hg,Pb, Ni,V, As, F
etc. .
 Probability of emission of CO and
unburnt carbon.

6. Power plant pollution control

7. Pollution control devices

9. PARTICULATE MATTER CONTROL
Range: 20 to 40000 mg/m^3
First step: Process control
Second step: Use of collection device

10. General Methods For Control Of
Particulate Emissions
 Five Basic Types of Dust Collectors :
Gravity and Momentum collectors
 Settling chambers, baffle chambers
Centrifugal Collectors
 Cyclones
 Mechanical centrifugal collectors
Fabric Filters
 Baghouses
 Fabric collectors

11. General Methods For Control Of
Particulate Emissions (Contd.)
Electrostatic Precipitators
 Tubular
 Plate
 Wet
 Dry
Wet Collectors
 Spray towers
 Impingement scrubbers
 Wet cyclones
 Peaked towers
 Mobile bed scrubbers

12. Particulate Collection Mechanism
 Gravity Settling
 Centrifugal Impaction
 Inertial Impaction
 Direct Interception
 Diffusion
 Electrostatic Effects

14. Electrostatic precipitation
 Principle
Charging the solid particles suspended in air or
gases by means of gas ions or electrons
produced under a high electric field.
 The ash left behind after combustion of coal is
arrested in high efficiency electrostatic
precipitator.
 The ash collected in ESP is disposed in ash pond
in the form of slurry.

15. Tubular Dust Collector Arrangement for
an ESP

16. Advantages.
 Large gas volume.
 Separation of particles of size s as low as .05
microns.
 Efficiency (average efficiency 98-99%)
 Inexpensive maintenance.

17. Disadvantages
initial cost –expensive
System size
High level of accuracy in manufacturing

18. SOX CONTROL

19. Flue Gas Desulfurization
 SO2 scrubbing, or Flue Gas Desulfurization processes can be
classified as:
 Throwaway or Regenerative, depending upon whether the recovered sulfur
is discarded or recycled.
 Wet or Dry, depending upon whether the scrubber is a liquid or a solid.
 Flue Gas Desulfurization Processes
The major flue gas desulfurization ( FGD ), processes are :
 Limestone Scrubbing
 Lime Scrubbing
 Dual Alkali Processes
 Lime Spray Drying
 Wellman-Lord Process

20. Limestone Scrubbing

21. Limestone Scrubbing
 Limestone slurry is sprayed on the incoming flue
gas. The sulfur dioxide gets absorbed The limestone
and the sulfur dioxide react as follows :
CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3
-+ CO2
CaCO3 + 2HSO3
-+ Ca+2 ----> 2CaSO3 + CO2 + H2O

22. Lime Scrubbing

23. Dual Alkali System
 Lime and Limestone scrubbing lead to deposits inside spray tower.
 The deposits can lead to plugging of the nozzles through which the
scrubbing slurry is sprayed.
 The Dual Alkali system uses two regents to remove the sulfur
dioxide.
 Sodium sulfite / Sodium hydroxide are used for the absorption of
sulfur dioxide inside the spray chamber.
 The resulting sodium salts are soluble in water,so no deposits are
formed.
 The spray water is treated with lime or limestone, along with make-
up sodium hydroxide or sodium carbonate.
 The sulfite / sulfate ions are precipitated, and the sodium hydroxide
is regenerated.

24. Lime – Spray Drying
 Lime Slurry is sprayed into the chamber
 The sulfur dioxide is absorbed by the slurry
 The liquid-to-gas ratio is maintained such that the spray dries
before it reaches the bottom of the chamber
 The dry solids are carried out with the gas, and are collected
in fabric filtration unit
 This system needs lower maintenance, lower capital costs,
and lower energy usage

25. Wellman – Lord Process
Schematic process flow diagram – SO2 scrubbing and recovery
system

26. Wellman – Lord Process
 This process consists of the following subprocesses:
 Flue gas pre-treatment.
 Sulfur dioxide absorption by sodium sulfite
 Purge treatment
 Sodium sulfite regeneration.
 The concentrated sulfur dioxide stream is processed to a
marketable product.
The flue gas is pre - treated to remove the particulate. The sodium
sulfite neutralizes the sulfur dioxide :
Na2SO3 + SO2 + H2O -----> 2NaHSO3

27. Wellman – Lord Process (contd.)
 Some of the Na2SO3 reacts with O2 and the SO3 present in the flue
gas to form Na2SO4 and NaHSO3.
 Sodium sulfate does not help in the removal of sulfur dioxide, and is
removed. Part of the bisulfate stream is chilled to precipitate the
remaining bisulfate. The remaining bisulfate stream is evaporated
to release the sulfur dioxide, and regenerate the bisulfite.

28. NOX CONTROL

29. General Methods For Control Of Nox
Emissions
 NOx control can be achieved by:
 Fuel Denitrogenation
 Combustion Modification
 Modification of operating conditions
 Tail-end control equipment
 Selective Catalytic Reduction
 Selective Non - Catalytic Reduction
 Electron Beam Radiation
 Staged Combustion

30. Fuel Denitrogenation
o One approach of fuel denitrogenation is to remove a large part of the nitrogen
contained in the fuels. Nitrogen is removed from liquid fuels by mixing the fuels
with hydrogen gas, heating the mixture and using a catalyst to cause nitrogen in
the fuel and gaseous hydrogen to unite. This produces ammonia and cleaner
fuel.
 This technology can reduce the nitrogen contained in both naturally
occurring and synthetic fuels.

31. Combustion Modification
 Combustion control uses one of the following
strategies:
 Reduce peak temperatures of the flame zone. The methods are :
 increase the rate of flame cooling
 decrease the adiabatic flame temperature by dilution
 Reduce residence time in the flame zone. For this we change the shape
of the flame zone
 Reduce Oxygen concentration in the flame one. This can be
accomplished by:
 decreasing the excess air
 controlled mixing of fuel and air
 using a fuel rich primary flame zone

32. Catalytic Combustion

33. Catalytic Emission Control

34. Modification Of Operating Conditions
 The operating conditions can be modified to achieve
significant reductions in the rate of thermal NOx
production. the various methods are:
 Low-excess firing
 Off-stoichiometric combustion ( staged combustion )
 Flue gas recirculation
 Reduced air preheat
 Reduced firing rates
 Water Injection

35. Tail-end Control Processes
o Combustion modification and modification of operating
conditions provide significant reductions in NOx, but not
enough to meet regulations.
 For further reduction in emissions, tail-end control equipment is
required.
 Some of the control processes are:
 Selective Catalytic Reduction
 Selective Non-catalytic Reduction
 Electron Beam Radiation
 Staged Combustion

36. Selective Catalytic Reduction (SCR)
Schematic process flow diagram – NOX control

37. Selective Catalytic Reduction (SCR)
 In this process, the nitrogen oxides in the flue gases are reduced to
nitrogen
 During this process, only the NOx species are reduced
 NH3 is used as a reducing gas
 The catalyst is a combination of titanium and vanadium oxides. The
reactions are given below :
4 NO + 4 NH3 + O2 -----> 4N2 + 6H2O
2NO2 + 4 NH3+ O2 -----> 3N2 + 6H2O
 Selective catalytic reduction catalyst is best at around 300 too 400 oC.
 Typical efficiencies are around 80 %

38. Selective Non-catalytic Reduction (SNR)
 At higher temperatures (900-1000oC), NH3 will reduce
NOX to nitrogen without a catalyst.
 At NH3 : NOX molar ratios 1:1 to 2:1, about 40-
60%reduction is obtained.
 SNR is cheaper than SCR in terms of operation cost and
capital cost.
 Tight temperature controls are needed. At lower
temperatures, un-reacted ammonia is emitted. At
higher temperatures ammonia is oxidized to NO.

39. Electron Beam Radiation
 This treatment process is under development, and
is not widely used. Work is underway to determine
the feasibility of electron beam radiation for
neutralizing hazardous wastes and air toxics.
 Irradiation of flue gases containing NOx or SOx produce
nitrate and sulfate ions.
 The addition of water and ammonia produces NH4NO3, and
(NH4)2SO4
 The solids are removed from the gas, and are sold as
fertilizers.

40. Staged Combustion

41. Staged Combustion
 PRINCIPLE
 Initially, less air is supplied to bring about incomplete
combustion
 Nitrogen is not oxidized. Carbon particles and CO are released.
 In the second stage, more air is supplied to complete the
combustion of carbon and carbon monoxide.
30% to 50% reductions in NOx emissions are achieved.

42. REFERENCES
 1. RAO M.N. & RAO H, Air pollution, Tata
McGraw Hill.
 2.Mahajan S.P., pollution control in process
industries, Tata McGraw Hill.

43. THANK YOU….