Air pollution in thermal power plants
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Transcript of Air pollution in thermal power plants
AIR POLLUTION IN THERMAL
POWER PLANTS
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
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
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.
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.
Power plant pollution control
Pollution control devices
PARTICULATE MATTER CONTROL
Range: 20 to 40000 mg/m^3
First step: Process control
Second step: Use of collection device
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
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
Particulate Collection Mechanism
Gravity Settling
Centrifugal Impaction
Inertial Impaction
Direct Interception
Diffusion
Electrostatic Effects
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.
Tubular Dust Collector Arrangement for
an ESP
Advantages. Large gas volume.
Separation of particles of size s as low as .05
microns.
Efficiency (average efficiency 98-99%)
Inexpensive maintenance.
Disadvantagesinitial cost –expensiveSystem sizeHigh level of accuracy in manufacturing
SOX CONTROL
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
Limestone Scrubbing
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
Lime Scrubbing
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.
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
Wellman – Lord Process
Schematic process flow diagram – SO2 scrubbing and recovery
system
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
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.
NOX CONTROL
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
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.
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
Catalytic Combustion
Catalytic Emission Control
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
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
Selective Catalytic Reduction (SCR)
Schematic process flow diagram – NOX control
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 %
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.
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.
Staged Combustion
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.
REFERENCES
1. RAO M.N. & RAO H, Air pollution, Tata
McGraw Hill.
2.Mahajan S.P., pollution control in process
industries, Tata McGraw Hill.
THANK YOU….