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COAL FIRED POWER GENERATION & ASSOCIATED EMISSION CONTROL METHODS

A conventional coal-fired power plant produces electricity by the burning of coal and air in a steam

generator, where it heats water to produce high pressure and high temperature steam. The steam flows

through a series of steam turbines which spin an electrical generator to produce electricity. The exhaust

steam from the turbines is cooled, condensed back into water, and returned to the steam generator to startthe process over. The efficiency of a conventional coal fired power plant varies between 35%-45%.

Fig 1.Schematic Figure of a Conventional Coal fired Power Generation Plant.

1. Cooling tower 11. High pressure steam turbine 20. Fan

2. Cooling water pump 12. Deaerator 21. Reheater

3. Three-phase transmission line 13. Feedwater heater 22.Combustion air intake

4. Step-up transformer 14. Coal conveyor 23. Economiser

5. Electrical generator 15. Coal hopper 24. Air preheater

6. Low pressure steam turbines 16. Coal pulverizer 25. Electrostatic precipitator

7. Condensate and feedwater pumps 17. Boiler steam drum 26. Fan

8. Surface condenser 18. Bottom ash hopper 27. Flue gas desulfurization

9. Intermediate pressure steam 19. Superheater 28. Flue gas stack 

10. Steam control valve

Fig 1.

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Coal transport and delivery: 

Coal is delivered by highway truck, rail, barge or

if the power plant is in the vicinity of a coal

producing facility such as a mine, then it is

delivered to the power production facility bymeans of conveyer belts. The adjacent schematic Fig

2 illustrates the process.

Fuel preparation:

Coal is prepared for use by first crushing the delivered

coal into pieces less than 5 cm in size. The crushed coal is

then transported from the storage yard to in-plant storage

silos by rubberized conveyor belts. Coal from the storage

silos is fed into pulverizers that grind the crushed coal

into the consistency of face powder and mix it with

primary combustion air which transports the pulverized

coal to the steam generator furnace. In power plants that

do not burn pulverized coal, the coal is directly fed to the

burners with specially designed combustors. The powered

coal slurry is fed to the pulverizers, mixed with air for

combustion and then fed to the combustion chamber[1].Atypical coal pulverizer is shown in the adjacent Fig 3.

Boiler Feedwater Heater : 

The feedwater used in the steam generator consists of 

recirculated condensate water and makeup water.The

condensate and feedwater system begins with thewater condensate being pumped out of the low

pressure turbine exhaust steam condenser (commonly

referred to as a surface condenser). The feedwater plus

makeup water flows through feedwater heaters heated

with steam extracted from the steam turbines.

Typically, the total feedwater also flows through a

deaerator that removes dissolved air from the water,

further purifying and reducing its corrosivity[1].A typical heater for a boiler is shown in Fig 4. 

Fig 2.

Fig 3

Fig 4.

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Deaerator : 

A deaerator is a device that is widely used

for the removal of air and other dissolved

gases from the feedwater to steam

generating boilers. In particular, dissolvedoxygen in boiler feedwaters will cause

serious corrosion damage in steam systems

by attaching to the walls of metal piping and

other metallic equipment and forming oxides

(rust). It also combines with any dissolved

carbon dioxide to form carbonic acid that

causes further corrosion[2].A spray type

deaerator is show in Fig 5.

The combination of a feedwater heater and deaerator is depicted in the Fig 6.

Steam Generator : 

The deaerated boiler feedwater enters the economizer where it is preheated by the hot combustion flue

gases and then flows into the boiler steam drum at the top of the furnace. Water from that drum circulates

through the boiler tubes in the furnace walls using the density difference between water in the steam drumand the steam-water mixture in the boiler tubes. Pulverized coal is air-blown into the furnace from fuel

nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation

of the fireball heats the water that circulates through the boiler tubes mounted on the furnace walls. In the

boiler steam drum, the steam is separated from the circulating water. The steam then flows through

superheat tubes that hang in the hottest part of the combustion flue gases path as it exits the furnace[1]. The

entire process is illustrated by the Fig 7. 

Fig 5.

Fig 6.

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Above the critical point for water of 374 °C and 22 MPa, there is no phase transition from water to steam,

but only a gradual decrease in density. Boiling does not occur and it is not possible to remove impurities

via steam separation. Supercritical steam generators operating at or above the critical point of water arereferred to as once-through plants because boiler water does not circulate multiple times as in a

conventional steam generator. Supercritical steam generators require additional water purification steps to

ensure that any impurities picked up during the cycle are removed. This purification takes the form of high

pressure ion exchange units called condensate polishers between the steam condenser and the feed water

heaters. Super-critical coal fired power plants have higher efficiency than conventional power plants.[1]

Steam Turbines and Electrical Generators : 

The staged series of steam turbines includes a high pressure turbine, an intermediate pressure turbine and

two low pressure turbines. A common configuration is that the series of turbines are connected to each

other and on a common shaft, with the electrical generator also being on that common shaft. As steam

moves through the system, it loses pressure and thermal energy and expands in volume, which requires

increasing turbine diameter and longer turbine blades at each succeeding stage.

Fig 7.

Fig 8.

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Superheated steam from the steam generator flows through a control valve into the high pressure turbine.

The control valve regulates the steam flow in accordance with the power output needed from the plant. The

exhaust steam from the high pressure turbine (reduced in pressure and in temperature) returns to the steam

generator's reheating tubes (see the steam generator diagram above) where it is reheated back to 540 °C

before it flows into the intermediate pressure turbine. The exhaust steam from the intermediate pressure

turbine flows directly into the two low pressure turbines and the exhaust steam from the low pressureturbines flows into the surface condenser. A small fraction of steam from the turbines is used to heat the

deaerator and/or the boiler feedwater preheater[1].A combination of a steam turbine and an electrical

generator is shown in Fig 8.

The turbine-driven electrical generator contains a stationary stator and a spinning rotor. The rotor spins in a

sealed chamber cooled with hydrogen gas, selected because it has the highest known thermal conductivity

of any gas and it has a low viscosity which reduces windage losses from friction between the generator

rotor and the cooling gas. The system requires special handling during startup, with air in the chamber first

displaced by carbon dioxide before filling with hydrogen. This ensures that a highly explosive hydrogen-

oxygen environment is not created[1]. 

The power grid frequency is 60 Hz across North America. The electricity flows to a distribution yard where

three-phase transformers step the voltage up to 115, 230, 500 or 765 kV as needed for transmission to its

destination. This is illustrated by the Fig 9.

Fig 9.

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Steam Condensation and Cooling towers:

The exhaust steam from the low pressure turbines is condensed into water in a water-cooled surface

condenser shown in Fig 10. The condensed water is commonly referred to as condensate. The surface

condenser operates at an absolute pressure of about 35 to 40 mmHg which maximizes the overall power

plant efficiency. The surface condenser is usually a shell and tube heat exchanger. Cooling water circulatesthrough the tubes in the condenser's shell and the low pressure exhaust steam is cooled and condensed by

flowing over the tubes. Typically the cooling water causes the steam to condense at a temperature of about

35 °C. A lower condensing temperature results in a higher vacuum (i.e., a lower absolute temperature) at

the exhaust of the low pressure turbine and a higher overall plant efficiency[3].

The condensate from the bottom of the surface condenser is pumped back to the deaerator to be reused as

feedwater. The cooling water used to condense the steam in the condenser returns to its source without

having been changed other than having been warmed. If the water returns to a local water body (rather than

a circulating cooling tower), it is mixed with cool raw water to lower its temperature and prevent thermal

shock to aquatic biota when discharged into that body of water. Another method sometimes utilized forcondensing turbine exhaust steam is the use of an air-cooled condenser. Exhaust steam from the low

pressure steam turbines flows through the air-cooled condensing tubes which usually have metal fins on

their external surface to increase their heat transfer capacity. Ambient air from a large fan is directed over

the fins to cool the tubes and condense the low pressure steam in the tubes. Air-cooled condensers typically

operate at a higher temperature than water-cooled surface condensers. While reducing the amount of water

used in a power plant, the higher condensing temperature results in a higher exhaust pressure for the low

pressure turbines which reduces the overall efficiency of the power plant[1].

Cooling towers are heat rejection systems used primarily to provide

circulating cooling water in large industrial facilities. The circulating

cooling water absorbs heat by cooling and/or condensing the hot

process streams within the industrial facilities. The cooling towers then

reject that absorbed heat by transferring it to the atmosphere. In power

plants, hyperboloid cooling towers(Fig 11) are normally erected

because of their structural strength and minimal usage of materials.[4]

Fig 10.

Fig 1

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Flue Gas stack:

Flue gases are produced when coal is combusted in

the power plant's steam-generating boiler. Flue gas is

usually composed of carbon dioxide (CO2) and water

vapor as well as nitrogen and excess oxygen remainingfrom the intake combustion air. It also contains a small

percentage of pollutants such as particulate

matter, carbon monoxide, nitrogen oxides and sulfur

oxides. The flue gas stacks are often quite tall to

disperse the exhaust pollutants over a greater area and

thereby reduce the concentration of the pollutants[5].

A flue gas stack in a power plant is shown in Fig 12.

The gas exiting the steam generator is laden with particulate matter (PM), referred to as fly ash, which

consists of very small ash particles. The flue gas contains nitrogen along with combustion products carbon

dioxide (CO2), sulfur dioxide (SO2) and nitrogen oxides (NOx).  The major designated air pollutants

emitted by coal-fired power plants are sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter

(PM), and mercury (Hg).

Several techniques and processes are implemented in order to get rid of these harmful contents from the

flue gas mixture released into the atmosphere which constitute the emission avoidance techniques. The

application of a process varies with situation and necessity.

The main processes which are employed to avoid NOx emissions are Selective Catalytic Reduction(SCR)

& Selective Non-Catalytic Reduction(SNCR). Similarly, SO2 gas in the flue gas mixture is eliminated by

Flue Gas Desulfurization(FGD) and structures like Electrostatic Precipitators and Fabric Filters are

employed to remove particulate matter from the flue gas mixture.

Fig