Coal fired electric power plants23

SUMMER TRAINING NTPC Ltd. ,BARH(PATNA)

IIMT INSTITUTE OF ENGINEERING & TECHNOLOGY

ELECTRICITY GENRATION FROM COAL

Presented by: MUNNA KUMAR B.Tech(E.C.-’G1’) 4th yr.Roll no.- 1037131040

Submitted to:- Dr. P. K Singh (seminar co-ordinator)

Electricity Generation From Coal

Electricity Generation From Coal

• Let’s start at the beginning.• Almost everything in our homes and

businesses today run on electricity.• That electricity has to come from somewhere.• Some sources of electricity are nuclear power,

solar power, wind power, and most important for us Coal Power.

The Process

• Coal was used to heat water in boiler room pipes to produce steam

• The steam was used in a reciprocating (piston) steam engine to produce mechanical energy

• The mechanical energy was converted into electricity by a dynamo (generator)

Inside the dynamo room

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Boiler plant can be divided in to three parts. i) water circuit ii) steam circuit iii) The air and fuel gas circuit.  

1) Water circuit

In the water circuit, water is fed from the boiler feed pumps into the boiler through economiser. In the economiser, it receives some heat from the departing flue gas before it goes to the boiler drum. The drum acts as a reservoir for the various water walls of the boiler and also acts as a separation chamber where water is removed from the steam before the steam goes to the superheaters.

From the boiler drum the water passes down through pipes called down comers to headers at the bottom of the boiler water walls.

The tubes which makeup the walls contain a mixture of steam bubbles and water. This mixture being low dense than water in the down comers, rises rapidly and reaches back to the drum and its place is taken by the water flowing through down comers. This produces what we call is natural circulation.

The steam and water mixture which is returned to the drum is separated so that water only (with no steam bubbles) is returned to the down comers, and steam only(with no water droplets) passed to the super heaters.

2) Steam circuit Dry steam from the boiler drum goes to the various superheater sections. Steam from the boiler drum passes through the superheater connecting tubes to the primary superheater, which is positioned in the convection pass. The steam then flows from the primary superheater outlet header to the secondary superheater located in the combustion chamber. Steam then goes to the final superheater which is located in the combustion chamber in the outlet section, it then leaves the final super heater outlet header and passes to the main stem pipe which has a boiler stop valve.

3)Air/gas circuit

To burn the fuel in the combustion chamber air is required. After combustion, the hot gases are to be evacuated from furnace through the heat absorbing surfaces. This air and gas flow is created by the boiler draught system, which may be either natural or mechanised.

The air drawn from the atmosphere is first routed through an air heater where air is heated by the outgoing flue gases. The hot air is then admitted to the furnace through wind box. In coal fired boilers part of this hot air is used for drying the coal in the pulvariser and transporting the pulvarised coal to furnace.

The gases pass through the radiant heat release zone and then through various superheaters and reheaters (in reheat boilers). Normally there will be a primary superheater and secondary superheater.

After passing through the air heater the flue gas goes to the chimney. In between the air heater and chimney it is customary to provide precipitator to remove the flyash from the flue gas (especially in coal fired boilers) and induced draught fans to suck out the flue gases from the furnace (in balanced draught/induced draught boilers). water is the working medium which transfers the heat energy available in the fuel to the turbine in the form of steam.

22° C reduction in flue gas temperature increases boiler efficiency by 1%

Water is chosen as the medium because of the following reasons. a) its easy availability b) its low viscous property c) it has high specific heat d) Its non-reactivity with surfaces with which it comes in to contact.

The following can be termed as boiler pressure parts.

1. Boiler drum 2. Water walls 3. Superheaters 4. Reheaters and 5. Economisers

BOTTOM RNG HDR& Z-PANEL 1ST PASS W.W

1ST PASS W.W O/L HDRS ROOF I/L HEADER

2ND PASS UPPER C-HDR 2nd PASS LOWER C-HDRS

LTSH I/L HEADER LTSH O/L HEADER

D.P.I/L HEADER D.P.O/L HEADER

S.H. HEADER R.H.HEADER

2ND PASS ROOF O/L HDR(REAR ECONOMISER

M.S

H.R.HC.R.H

FROM F.R.S

Steam Generator Bottom Ash Hopper

Bottom Ring Header

Economiser

LTSH

Eco Hopper

Drum

Final SH

Furnace

ReheaterPlaten SH

Front Pass

Rear Pass

Goose Neck

Water Walls

Pent House

Steam cooled Walls

Air Pre Heater

Wind Box

Burners

Platen SH.

375C-425C

Final SH.

500-540C

Economizer

240-310C

LTSH

330-375C

Water Wall

310C

210 MW Boiler: Water and Steam Circuit

BOILER DRUM

The drum acts as reservoir for water & saturated steam and also provides separation and purification of steam.

The feed water to the drum reaches the drum from the boiler feed pump via the economizer.

A stronger material for use in boiler drums is Ducal W30.

Methods of Steam Separation: 1. By Gravity separation This is employed for boilers having low generation rates. 2. By use of Baffles These are in the form of obstacles in the direct path of steam towards outlet.

WATER WALL SYSTEM:

In the boiler the walls of the combustion chamber are formed by tubular wall sections which not only form the enclosure for the furnace but also provides the evaporating surface for the feed water. The water from the boiler drum is admitted in to the water wall tubes through the downcomers and bottom ring headers. As the water circulates through the waterwall tubes, which receive heat from the furnace radiation, water partially evaporates into steam. Water-steam mixture then return back to the boiler drum.

SUPER HEATERS: Super heaters (SH) are meant for raising the steam temperature above the saturation temperature.

The superheated and reheated steam temperature around 540°C and pressure 165 bar.

i) SH (Reheater also) can classified into convection and radiation type as per heat transfer process.

The super heaters and reheaters which are placed above the furnace and can view the flame are called radiant type.

ii) Super heater may be classified also according to the shape of the tube banks and the position of the headers, such as pendant SH, platen SH, horizontal SH, Ceiling SH, wall SH etc.

iii) They may be classified according to their stages of superheating they perform, like primary SH, Secondary SH, Final SH etc.

Reheaters:

Reheaters (RH) are provided to raise the temperature of the steam from which part of energy has already been extracted by HP turbine.

De-superheaters: Though super heaters are designed in such a way that heat absorbed by radiant and convection super heaters always try to maintain the steam temperature constant in practice the necessary control is achieved by using de-super heater.

All modern boiler contact type de-super heaters by which feed water are sprayed directly into the steam for required cooling.

Amount of feed water to be sprayed is controlled by automatic control system which is designed to maintain a set final steam temperature. Provision of manual control is also there for emergency.

ECONOMISERS: The economiser absorbs heat from the flue gas and adds it mainly as sensible heat to the feed water.

The material used in the manufacture of furnace wall tubes for coal fired boiler is ordinary carbon steel but in the 500 MW oil fired units the major proportion of the furnace is constructed from the 1% Cr. ½% Mo Alloy.

In 660 MW units also this material is used for whole of the furnace.

The Boiler Auxiliaries :

• Draft system• Air heaters •Milling systems •Electrostatic precipitators, etc.

DRAFT SYSTEM:

• The combustion process in a furnace can take place only when it receives a steady flow of Air and has the combustion gases continuously removed.

• The Boiler draft system includes Air and Flue gas flow.

• All modern large utility boilers are fired under "balanced draft" condition, i.e. where draft is zero. This condition is created by the combination of "forced draft" and "Induced draft".

SOOT BLOWERS • deposits resulting from the combustion of coal will be deposited on

the boiler tubes at various zones will be cleaned by soot blowing for

effective heat transfer while on-load. AIR HEATERS: • The air heater is required for efficient combustion in the furnace and

also for drying wet coal in the milling plant. to recover "waste" heat from the flue gas to increase boiler efficiency

AIR PREHEATER

MILLING PLANT :• raw coal from the bunker is fed at a regulated rate to the mills through

a feeder. • Air required for drying and transporting the pulverized coal from the

mill is obtained from the FD fan. • Hot air is drawn through air heaters and cold air directly from FD fan

discharge. • The drying and grinding takes place inside the mills. The pulverized

particles are being carried from the mill to the classifier, which is directly mounted on the mill.

• The medium is directed into the burners through various fuel pipelines.

Type of Pulverizers:

Drum/Tube mills: Ball mills: Bowl mills:

ELECTROSTATIC PRECIPITATORS:

Working Principle: • The principles upon which an electrostatic precipitator

operates are that the dust laden gases pass into a chamber where the individual particles of dust are given an electric charge by absorption of free ions from a high voltage D.C. ionising field.

• They are removed by an intermittent blow usually referred to as rapping. This causes the dust particles to drop into dust hoppers situated below the collecting electrodes.

The following fans are used in the boiler houses:

1. Forced Draft fan (F.D. Fan): To take air from atmosphere to supply all the Combustion air. Speeds vary between

600 to 1500 r.p.m.

2. Induced Draft Fan (I.D. Fan): Used only in balanced draft units to suck the gases out of the furnace and throw them

into the stack. Handles flue gases at temperatures of 125 to 200oC. Speed generally does not exceed 1000 rpm. 3. Primary Air Fans (P.A. Fans) or Exhauster Fan: Used for pulverized system Primary air has got two functions viz. Drying the coal and transportation into the

furnace. Usually 1500 r.p.m.

Stator Casing

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Steam Turbine• As mentioned before,

something has to turn the rotor in order to generate electricity.

• In our case the prime mover happens to be a steam turbine.

• Steam comes out of the tubes in the boiler and into a manifold then into the turbine.

• As the steam passes over the turbine blades, torque is produced as a result of the blade shape.

Steam Turbine

Wikipedia

Turbine Generator and Condenser

Steam Turbine• The rate of steam flow controls how fast the turbine

rotates and therefore the frequency of the electricity produced.

• As the steam moves through the turbine energy is extracted which results in a pressure drop.

• Therefore the LP turbine is located at the exit of the HP turbine to extract the maximum amount of energy from the steam before it is sent to the condenser.

• As electricity is generated it leaves the building though a very large circuit breaker and a series of transformers before it enters the power grid.

Path of Electricity

Control Room

Condenser• After the steam leaves the LP turbine it travels to the

condenser, where it is condensed back to liquid water.

• The condenser is a heat exchanger that cools the steam while is passes over tubes that have cold water running through them.

• The cold water removes energy from the heated steam causing it to condense which is necessary for the water to be re-used as feed.

• There is another reason why the condenser is necessary which we will discuss shortly.

Cooling Towers

• The water that runs through the tubes in the condenser must be cooled down in order to condense the steam.

• This is accomplished using very large cooling towers, in which the water is atomized by sprayers and cooled down by atmospheric conditions and fans.

• The substance leaving cooling towers is sometimes mistaken for smoke, but it is in fact just water vapor.

Cooling Towers

Cooling Tower

Main Condenser

Holte International

Feed System

• After the condensate is collected in the hotwell of the condenser it is pumped through the feed system.

• The feed pump increases the pressure of the feed water in order for it to flow back into the boiler to be turned back into steam to start the cycle over again.

• This stage turns out to be the 4th and final stage of something called a heat engine.

Basic Heat Engine

Laws of Thermodynamics

• 1st: “The increase in internal energy of a system is equal to the amount of heat energy added to the system minus the work done by the system on the surroundings.”

Laws of Thermodynamics• 2nd: The temperature differences between systems in

contact with each other tend to even out and that work can be obtained from these non-equilibrium differences, but that loss of heat occurs, in the form of entropy, when work is done.

• 2nd: It is impossible to produce work in the surroundings using a cyclic process connected to a single heat reservoir (Kelvin, 1851).

Laws of Thermodynamics• The second law also states that the maximum

efficiency of a heat engine can be determined by:

η = 1-(Th/Tc)

• Efficiency is also equal to the work output over the heat input.

η = Δ W/Δ QH

Efficiency

• An ordinary power plant operates between the temperatures of 565C and 25C which leads to maximum efficiency of around 64%.

• However, due to the losses mentioned earlier the usual observed efficiency is about 35%.

• This shows how much of the energy stored in the coal is just wasted instead of being converted to electricity.

Steam Cycle

• The previous part of the presentation was to explain the process behind electricity generation that occurs after the coal portion.

• I will now go through some of the components dealing with the coal aspect of the power plant ending at the boiler which is where the steam cycle began.

• Most of what I will discuss is particular to the Co-gen plant that I visited.

Coal Delivery

• After the coal is mined and loaded into trucks it is delivered into chutes that lead to the Bradford Breaker.

• The breaker is a drum with hammers in it that rotates and breaks the coal down into pieces about 4” diameter, which fall through the screen and onto the conveyor belt which leads to the storage facility.

Bradford Breaker

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Bradford Breaker

Coal Storage

• After the coal has been sufficiently reduced in size, it enters a storage facility, in this case, a large dome.

• The coal is then stacked using a machine you will see in the next slide, which rotates and places the coal around the perimeter of the dome.

• This same machine also takes coal from the pile and delivers it to the crusher building.

Coal Storage

Crusher

• The coal is delivered from the storage facility to a device conveniently called a coal crusher.

• This machine takes the 4” pieces of coal and through a series of rollers converts the fuel into a fine powder.

• This powder is necessary for proper combustion in the boilers.

• All newer state of the art power plants are set up to work with pulverized coal, but older plants may operate with lumped coal.

Crusher

Crusher

Fuel Feed System

• The conveyor belt delivers the crushed coal to a series of fuel feeders, which inject coal into the boilers along with a mixture of high pressure air for combustion.

Boiler Base

Boiler Building

Boilers• The final stage for the coal coincides with the

first stage of the steam plant.• The boiler is usually the largest component of

the coal power plant climbing as high as 200 ft.• Inside the boilers the pulverized coal is burned

while it more or less floats with the aid of HP air.

• Lining the entire inside of the boiler are tubes which carry the feed water to be turned into steam.

Boilers

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Boiler

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Inside the Boiler

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After the Boiler• As the coal is burned a large amount of ash is

produced.• Some falls to the bottom and is collected, then

mixed with water and sent to the ash pile.• Very light ash particles also escape with the

exhaust gasses which are captured by a bag filter system.

• This fly ash is collected in a silo until a certain level is reached when it is pumped to the ash pile via HP air.

Fly Ash

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Ash Pile

Ash

• This ash has a basic pH and has some beneficial uses.

• After a truck delivers coal to the plant it is filled with ash to return to the reclamation site.

• The high pH helps treat acid mine drainage.

Ash

• The fly ash is used as a substitute to make Portland cement.

• Another use of fly ash is structural fill for highway embankments and the fill under new highways.

Exhaust Gasses and the Stack• One of the most controversial aspects of a coal

power plant is what comes out of the stack.• During the combustion process dangerous

gasses and particulates are released, such as NOx, SOx, and CO2.

• Controls are in effect for each of these in new plants, however older plants spew thousands of pounds of each of these into the atmosphere every year.

Intermittent Blowdown

• The intermittent blown down is given by manually operating a valve fitted to discharge pipe at the lowest point of boiler shell to reduce parameters (TDS or conductivity, pH, Silica etc) within prescribed limits so that steam quality is not likely to be affected

• TDS level keeps varying• fluctuations of the water level in the boiler.• substantial amount of heat energy is lost with

intermittent blow down.

Continuous Blowdown

• A steady and constant dispatch of small stream of concentrated boiler water, and replacement by steady and constant inflow of feed water.

• This ensures constant TDS and steam purity.

• This type of blow down is common in high-pressure boilers.

Boiler Water Treatment • Internal Water Treatment: It is carried out by adding

chemicals to boiler to prevent the formation of scale by converting the scale-forming compounds to free-flowing sludges, which can be removed by blowdown.

• Limitation: Applicable to boilers, where feed water is low in hardness salts, to low pressures- high TDS content in boiler water is tolerated, and when only small quantity of water is required to be treated.

• Internal treatment alone is not recommended.

External Water Treatment

• Propose: External treatment is used to remove suspended solids, dissolved solids (particularly the calcium and magnesium ions which are a major cause of scale formation) and dissolved gases (oxygen and carbon dioxide).

• Different treatment Process : – ion exchange; – demineralization; – reverse osmosis and– de-aeration.

Demineralization • Demineralization is the complete removal of all salts. • This is achieved by using a “cation” resin, which exchanges the cations

in the raw water with hydrogen ions, producing hydrochloric, sulphuric and carbonic acid.

• Carbonic acid is removed in degassing tower in which air is blown through the acid water.

• Following this, the water passes through an “anion” resin which exchanges anions with the mineral acid (e.g. sulphuric acid) and forms water.

• Regeneration of cations and anions is necessary at intervals using, typically, mineral acid and caustic soda respectively. The complete removal of silica can be achieved by correct choice of anion resin.

De-aeration• When heated in boiler systems, carbon dioxide

(CO2) and oxygen (O2) are released as gases and combine with water (H2O) to form carbonic acid, (H2CO3).

Deaerator

•In de-aeration, dissolved gases, such as oxygen and carbon dioxide, are expelled by preheating the feed water before it enters the boiler.

Exhaust Gasses and the Stack

The Stack

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Reduce Stack Temperature

• Stack temperatures greater than 200°C indicates potential for recovery of waste heat.

• It also indicate the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning.

22o C reduction in flue gas temperature increases boiler efficiency by 1%

THANK YOU

Questions?