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….