The Electric Efficiency of a Conventional Thermal Power Station12

8/8/2019 The Electric Efficiency of a Conventional Thermal Power Station12

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Electrical Efficiency

The electric efficiency of a conventional thermal power station, considered as saleable

energy produced at the plant busbars compared with the heating value of the fuel

consumed, is typically 33 to 48% efficient, limited as all heat engines are by the laws of

thermodynamics. The rest of the energy must leave the plant in the form of heat. This

waste heat can be disposed of with cooling wateror in cooling towers. If the waste heat

is instead utilized for e.g. district heating, it is called cogeneration. An important class of

thermal power station are associated with desalination facilities; these are typically

found in desert countries with large supplies of natural gas and in these plants,

freshwater production and electricity are equally important co-products.

Since the efficiency of the plant is fundamentally limited by the ratio of the absolute

temperatures of the steam at turbine input and output, efficiency improvements require

use of higher temperature, and therefore higher pressure, steam. Historically, other

working fluids such as mercury have been experimentally used in a mercury vapour

turbine power plant, since these can attain higher temperatures than water at lower

working pressures. However, the obvious hazards of toxicity, and poor heat transfer

properties, have ruled out mercury as a working fluid.

Diagram of a typical coal-fired thermal power station

Steam generator

The steam generating boiler has to produce steam at the high purity, pressure and

temperature required for the steam turbine that drives the electrical generator. The
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generator includes the economizer, the steam drum, the chemical dosing equipment,

and the furnace with its steam generating tubes and the superheater coils. Necessary

safety valves are located at suitable points to avoid excessive boiler pressure. The air

and flue gas path equipment include: forced draft (FD) fan, air preheater(APH), boiler

furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitatororbaghouse)

and the flue gas stack.[1][2][3]

For units over about 200 MW capacity, redundancy of key components is provided by

installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating

dampers. On some units of about 60 MW, two boilers per unit may instead be provided.

Boiler furnace and steam drum

Once water inside the boilerorsteam generator, the process of adding the latent heat of

vaporization orenthalpy is underway. The boiler transfers energy to the water by the

chemical reaction of burning some type of fuel.

The water enters the boiler through a section in the convection pass called the

economizer. From the economizer it passes to the steam drum. Once the water enters

the steam drum it goes down the downcomers to the lower inlet waterwall headers.

From the inlet headers the water rises through the waterwalls and is eventually turned

into steam due to the heat being generated by the burners located on the front and rear

waterwalls (typically). As the water is turned into steam/vapor in the waterwalls, the

steam/vapor once again enters the steam drum. The steam/vapor is passed through a

series of steam and water separators and then dryers inside the steam drum. The

steam separators and dryers remove the water droplets from the steam and the cycle

through the waterwalls is repeated. This process is known as natural circulation.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot

blowers, water lancing and observation ports (in the furnace walls) for observation of the

furnace interior. Furnace explosions due to any accumulation of combustible gases after

a trip-out are avoided by flushing out such gases from the combustion zone before

igniting the coal.

The steam drum (as well as the superheater coils and headers) have air vents and

drains needed for initial startup. The steam drum has internal devices that removes

moisture from the wet steam entering the drum from the steam generating tubes. The

dry steam then flows into the superheater coils.
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Geothermal plants need no boiler since they use naturally occurring steam sources.

Heat exchangers may be used where the geothermal steam is very corrosive or

contains excessive suspended solids. Nuclear plants also boil water to raise steam,

either directly passing the working steam through the reactor or else using an

intermediate heat exchanger.

Fuel preparation system

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed

into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is

next pulverized into a very fine powder. The pulverizers may be ball mills, rotating drum

grinders, or other types of grinders.

Some power stations burn fuel oil rather than coal. The oil must kept warm (above its

pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming

unpumpable. The oil is usually heated to about 100C before being pumped through the

furnace fuel oil spray nozzles.

Boilers in some power stations use processed natural gas as their main fuel. Other

power stations may use processed natural gas as auxiliary fuel in the event that their

main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are

provided on the boiler furnaces.

Fuel firing system and igniter system

From the pulverized coal bin, coal is blown by hot air through the furnace coal burnersat an angle which imparts a swirling motion to the powdered coal to enhance mixing of

the coal powder with the incoming preheated combustion air and thus to enhance the

combustion.

To provide sufficient combustion temperature in the furnace before igniting the

powdered coal, the furnace temperature is raised by first burning some light fuel oil or

processed natural gas (by using auxiliary burners and igniters provide for that purpose).

Air path

External fans are provided to give sufficient air for combustion. The forced draft fan

takes air from the atmosphere and, first warming it in the air preheater for better

combustion, injects it via the air nozzles on the furnace wall.

The induced draft fan assists the FD fan by drawing out combustible gases from the

furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring
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through any opening. At the furnace outlet, and before the furnace gases are handled

by the ID fan, fine dust carried by the outlet gases is removed to avoid atmospheric

pollution. This is an environmental limitation prescribed by law, and additionally

minimizes erosion of the ID fan.

Auxiliary systems

Fly ash collection

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric

bag filters (or sometimes both) located at the outlet of the furnace and before the

induced draft fan. The fly ash is periodically removed from the collection hoppers below

the precipitators or bag filters. Generally, the fly ash is pneumatically transported to

storage silos for subsequent transport by trucks or railroad cars.

Bottom ash collection and disposal

At the bottom of every boiler, a hopper has been provided for collection of the bottom

ash from the bottom of the furnace. This hopper is always filled with water to quench the

ash and clinkers falling down from the furnace. Some arrangement is included to crush

the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.

Boiler make-up water treatment plant and storage

Since there is continuous withdrawal of steam and continuous return of condensate to

the boiler, losses due to blow-down and leakages have to be made up for so as to

maintain the desired water level in the boiler steam drum. For this, continuous make-up

water is added to the boiler water system. The impurities in the raw water input to the

plant generally consist of calcium and magnesium salts which impart hardness to the

water. Hardness in the make-up water to the boiler will form deposits on the tube water

surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to

be removed from the water and that is done by a water demineralising treatment plant

(DM). A DM plant generally consists of cation, anion and mixed bed exchangers. The

final water from this process consists essentially of hydrogen ions and hydroxide ions

which is the chemical composition of pure water. The DM water, being very pure,becomes highly corrosive once it absorbs oxygen from the atmosphere because of its

very high affinity for oxygen absorption.

The capacity of the DM plant is dictated by the type and quantity of salts in the raw

water input. However, some storage is essential as the DM plant may be down for

maintenance. For this purpose, a storage tank is installed from which DM water is
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continuously withdrawn for boiler make-up. The storage tank for DM water is made from

materials not affected by corrosive water, such as PVC. The piping and valves are

generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless

steel doughnut float is provided on top of the water in the tank to avoid contact with

atmospheric air. DM water make-up is generally added at the steam space of the

surface condenser(i.e., the vacuum side). This arrangement not only sprays the water

but also DM water gets deaerated, with the dissolved gases being removed by the

ejector of the condenser itself.

Steam turbine-driven electric generator

Rotor of a modern steam turbine, used in a power station

Turbo generator

The steam turbine-driven generators have auxiliary systems enabling them to work

satisfactorily and safely. The steam turbine generator being rotating equipment

generally has a heavy, large diameter shaft. The shaft therefore requires not only

supports but also has to be kept in position while running. To minimise the frictional

resistance to the rotation, the shaft has a number of bearings. The bearing shells, in

which the shaft rotates, are lined with a low friction material like Babbitt metal. Oil

lubrication is provided to further reduce the friction between shaft and bearing surface

and to limit the heat generated.

Barring gear (or Turning gear)

Barring gear is the term used for the mechanism provided for rotation of the turbine

generator shaft at a very low speed (about one revolution per minute) after unit

stoppages for any reason. Once the unit is "tripped" (i.e., the turbine steam inlet valve is
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closed), the turbine starts slowing or "coasting down". When it stops completely, there is

a tendency for the turbine shaft to deflect or bend if allowed to remain in one position

too long. This deflection is because the heat inside the turbine casing tends to

concentrate in the top half of the casing, thus making the top half portion of the shaft

hotter than the bottom half. The shaft therefore warps or bends by millionths of inches,

only detectable by monitoring eccentricity meters.

But this small amount of shaft deflection would be enough to cause vibrations and

damage the entire steam turbine generator unit when it is restarted. Therefore, the shaft

is not permitted to come to a complete stop by a mechanism known as "turning gear" or

"barring gear" that automatically takes over to rotate the unit at a preset low speed.

If the unit is shut down for major maintenance, then the barring gear must be kept in

service until the temperatures of the casings and bearings are sufficiently low.

Condenser

Diagram of a typical water-cooled surface condenser.

The surface condenser is a shell and tube heat exchanger in which cooling water is

circulated through the tubes. The exhaust steam from the low pressure turbine enters

the shell where it is cooled and converted to condensate (water) by flowing over the

tubes as shown in the adjacent diagram. Such condensers use steam ejectorsorrotarymotor-driven exhausters for continuous removal of air and gases from the steam side to

maintain vacuum.

For best efficiency, the temperature in the condenser must be kept as low as practical in

order to achieve the lowest possible pressure in the condensing steam. Since the

condenser temperature can almost always be kept significantly below 100 oC where the
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vapor pressure of water is much less than atmospheric pressure, the condenser

generally works undervacuum. Thus leaks of non-condensible air into the closed loop

must be prevented. Plants operating in hot climates may have to reduce output if their

source of condenser cooling water becomes warmer; unfortunately this usually

coincides with periods of high electrical demand forair conditioning.

The condenser generally uses either circulating cooling water from a cooling tower to

reject waste heat to the atmosphere, or once-through water from a river, lake or ocean.

Feedwater heater

In the case of a conventional steam-electric power plant utilizing a drum boiler, the

surface condenser removes the latent heat of vaporization from the steam as it changes

states from vapour to liquid. The heat content (btu) in the steam is referred to as

Enthalpy. The condensate pump then pumps the condensate water through a feedwater

heater. The feedwater heating equipment then raises the temperature of the water by

utilizing extraction steam from various stages of the turbine. [2][3]

Preheating the feedwater reduces the irreversibilities involved in steam generation and

therefore improves the thermodynamic efficiency of the system.[9] This reduces plant

operating costs and also helps to avoid thermal shock to the boiler metal when the

feedwater is introduced back into the steam cycle.

Superheater

As the steam is conditioned by the drying equipment inside the drum, it is piped fromthe upper drum area into an elaborate set up of tubing in different areas of the boiler.

The areas known assuperheaterand reheater. The steam vapor picks up energy and

its temperature is now superheated above the saturation temperature. The superheated

steam is then piped through the main steam lines to the valves of the high pressure

turbine.

Deaerator

A steam generating boiler requires that the boiler feed water should be devoid of air and

other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the

metal.

Generally, power stations use a deaerator to provide for the removal of air and other

dissolved gases from the boiler feedwater. A deaerator typically includes a vertical,

domed deaeration section mounted on top of a horizontal cylindrical vessel which

serves as the deaerated boiler feedwater storage tank.[2][3][10]
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There are many different designs for a deaerator and the designs will vary from one

manufacturer to another. The adjacent diagram depicts a typical conventional trayed

deaerator.[10][11] If operated properly, most deaerator manufacturers will guarantee that

oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm/L). [10][12]

Auxiliary systems

Oil system

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine

generator. It supplies the hydraulic oil system required for steam turbine's main inlet

steam stop valve, the governing control valves, the bearing and seal oil systems, the

relevant hydraulic relays and other mechanisms.

At a preset speed of the turbine during start-ups, a pump driven by the turbine main

shaft takes over the functions of the auxiliary system.

Generator heat dissipation

The electricity generatorrequires cooling to dissipate the heat that it generates. While

small units may be cooled by air drawn through filters at the inlet, larger units generally

require special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is

used because it has the highest known heat transfer coefficient of any gas and for its

low viscosity which reduces windage losses. This system requires special handling

during start-up, with air in the chamber first displaced by carbon dioxide before filling

with hydrogen. This ensures that the highly flammable hydrogen does not mix withoxygen in the air.

The hydrogen pressure inside the casing is maintained slightly higher than atmospheric

pressure to avoid outside air ingress. The hydrogen must be sealed against outward

leakage where the shaft emerges from the casing. Mechanical seals around the shaft

are installed with a very small annular gap to avoid rubbing between the shaft and the

seals. Seal oil is used to prevent the hydrogen gas leakage to atmosphere.

The generator also uses water cooling. Since the generator coils are at a potential of

about 22 kV and water is conductive, an insulating barrier such as Teflon is used to

interconnect the water line and the generator high voltage windings. Demineralized

water of low conductivity is used.
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Generator high voltage system

The generator voltage ranges from 11 kV in smaller units to 22 kV in larger units. The

generator high voltage leads are normally large aluminum channels because of their

high current as compared to the cables used in smaller machines. They are enclosed in

well-grounded aluminum bus ducts and are supported on suitable insulators. The

generator high voltage channels are connected to step-up transformers for connecting

to a high voltage electrical substation (of the order of 110 kV or 220 kV) for further

transmission by the local power grid.

The necessary protection and metering devices are included for the high voltage leads.

Thus, the steam turbine generator and the transformer form one unit. In smaller units,

generating at 11 kV, a breaker is provided to connect it to a common 11 kV bus system.

Other systems

Monitoring and alarm system

Most of the power plant operational controls are automatic. However, at times, manual

intervention may be required. Thus, the plant is provided with monitors and alarm

systems that alert the plant operators when certain operating parameters are seriously

deviating from their normal range.

Battery supplied emergency lighting and communication

A central battery system consisting of lead acid cell units is provided to supply

emergency electric power, when needed, to essential items such as the power plant's

control systems, communication systems, turbine lube oil pumps, and emergency

lighting. This is essential for a safe, damage-free shutdown of the units in an emergency

situation.

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or

vaporized fluid exits the boiler for use in various processes or heating applications

Fuel

The source of heat for a boiler is combustion of any of several fuels, such as wood,

coal, oil, ornatural gas. Electric steam boilers use resistance orimmersion type heating

elements. Nuclear fission is also used as a heat source for generating steam. Heat

recovery steam generators (HRSGs) use the heat rejected from other processes such

as gas turbines.
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Configurations

Boilers can be classified into the following configurations:

"Pot boiler"or"Haycock boiler": a primitive "kettle" where a fire heats a partially-

filled water container from below. 18th Century Haycock boilers generally

produced and stored large volumes of very low-pressure steam, often hardly

above that of the atmosphere. These could burn wood or most often, coal.

Efficiency was very low.

Fire-tube boiler. Here, water partially fills a boiler barrel with a small volume left

above to accommodate the steam (steam space). The heat source is inside a

furnace or firebox that has to be kept permanently surrounded by the water in

order to maintain the temperature of the heating surface just below boiling point.

The furnace can be situated at one end of a fire-tube which lengthens the path ofthe hot gases, thus augmenting the heating surface which can be further

increased by making the gases reverse direction through a second parallel tube

or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the

gases may be taken along the sides and then beneath the boiler through flues (3-

pass boiler). In the case of a locomotive-type boiler, a boiler barrel extends from

the firebox and the hot gases pass through a bundle of fire tubes inside the barrel

which greatly increase the heating surface compared to a single tube and further

improve heat transfer. Fire-tube boilers usually have a comparatively low rate of

steam production, but high steam storage capacity. Fire-tube boilers mostly burn

solid fuels, but are readily adaptable to those of the liquid or gas variety.

Water-tube boiler. In this type,the water tubes are arranged inside a furnace in a

number of possible configurations: often the water tubes connect large drums,

the lower ones containing water and the upper ones, steam; in other cases, such

as a monotube boiler, water is circulated by a pump through a succession of

coils. This type generally gives high steam production rates, but less storage

capacity than the above. Water tube boilers can be designed to exploit any heat

source including nuclear fission and are generally preferred in high pressureapplications since the high pressure water/steam is contained within narrow

pipes which can withstand the pressure with a thinner wall.

Flash boiler. A specialized type of water-tube boiler.

Fire-tube boiler with Water-tube firebox. Sometimes the two above types have

been combined in the following manner: the firebox contains an assembly of
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water tubes, called thermic syphons. The gases then pass through a

conventional firetube boiler. Water-tube fireboxes were installed in many

Hungarian locomotives, but have met with little success in other countries.

Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork chop

boiler" the water is contained inside cast iron sections. These sections are

assembled on site to create the finished boiler.

Superheated steam boilers

A superheated boiler on a steam locomotive.

Most boilers heat water until it boils, and then the steam is used at saturation

temperature (i.e., saturated steam). Superheated steam boilers boil the water and then

further heat the steam in a superheater. This provides steam at much higher

temperature, and can decrease the overall thermal efficiency of the steam plant due to

the fact that the higher steam temperature requires a higher flue gas exhausttemperature. However, there are advantages to superheated steam. For example,

useful heat can be extracted from the steam without causing condensation, which could

damage piping and turbine blades.

Superheated steam presents unique safety concerns because, if there is a leak in the

steam piping, steam at such high pressure/temperature can cause serious,

instantaneous harm to anyone entering its flow. Since the escaping steam will initially

be completely superheated vapor, it is not easy to see the leak, although the intense

heat and sound from such a leak clearly indicates its presence.

The superheater works like coils on an air conditioning unit, however to a different end.

The steam piping (with steam flowing through it) is directed through the flue gas path in

the boiler furnace. This area typically is between 1300-1600 degrees Celsius (2500-

3000 degrees Fahrenheit). Some superheaters are radiant type (absorb heat by

radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a
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combination of the two. So whether by convection or radiation the extreme heat in the

boiler furnace/flue gas path will also heat the superheater steam piping and the steam

within as well. It is important to note that while the temperature of the steam in the

superheater is raised, the pressure of the steam is not: the turbine or moving pistons

offer a "continuously expanding space" and the pressure remains the same as that of

the boiler.The process of superheating steam is most importantly designed to remove

all droplets entrained in the steam to prevent damage to the turbine blading and/or

associated piping.

Cooling towers

Cooling towers are heat removal devices used to transfer process waste heat to the

atmosphere. Cooling towers may either use the evaporation of water to remove process

heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air

to cool the working fluid to near the dry-bulb air temperature. Common applications

include cooling the circulating water used in oil refineries, chemical plants, power plants

and building cooling. The towers vary in size from small roof-top units to very large

hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres

in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and

80 metres long. Smaller towers are normally factory-built, while larger ones are

constructed on site.

Industrial(used at DCRTPP,Yamunanagar)

Industrial cooling towers can be used to remove heat from various sources such as

machinery or heated process material. The primary use of large, industrial cooling

towers is to remove the heat absorbed in the circulating cooling water systems used in

power plants,petroleum refineries, petrochemical plants, natural gas processing plants,

food processing plants, semi-conductor plants, and other industrial facilities. The

circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling

tower amounts to about 71,600 cubic metres an hour and the circulating water requires

a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).

If that same plant had no cooling tower and used once-through cooling water, it would

require about 100,000 cubic metres an hour [3] and that amount of water would have to

be continuously returned to the ocean, lake or river from which it was obtained and

continuously re-supplied to the plant. Furthermore, discharging large amounts of hot

water may raise the temperature of the receiving river or lake to an unacceptable level

for the local ecosystem. A cooling tower serves to dissipate the heat into the
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atmosphere instead and wind and air diffusion spreads the heat over a much larger

area than hot water can distribute heat in a body of water.

Cooling tower and water discharge of a nuclear power plant

Some coal-fired and nuclear power plants located in coastal areas do make use of

once-through ocean water. But even there, the offshore discharge water outlet requiresvery careful design to avoid environmental problems.

Petroleum refineries also have very large cooling tower systems. A typical large refinery

processing 40,000 metric tonnes of crude oil per day (300,000 barrels per day)

circulates about 80,000 cubic metres of water per hour through its cooling tower system.

Air flow generation methods

With respect to drawing air through the tower, there are three types of cooling towers

the one used at DCRTPP is the natural draught cooling tower:

Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally

rises due to the density differential to the dry, cooler outside air. Warm moist airis

less dense than drier air at the same pressure. This moist air buoyancy produces a

current of air through the tower.

Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow

is assisted by a fan.

Hyperboloid (aka hyperbolic) cooling towers (Image 1) have become the designstandard for all natural-draft cooling towers because of their structural strength and

minimum usage of material. The hyperbolic form is popularly associated with nuclear

power plants. However, this association is misleading, as the same kind of cooling

towers are often used at large coal-fired power plants as well. Similarly, not all nuclear

power plants have cooling towers.
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Categorization by air-to-water flow

Crossflow

Crossflow is a design in which the air flow is directed perpendicular to the water flow

(see diagram below). Air flow enters one or more vertical faces of the cooling tower to

meet the fill material. Water flows (perpendicular to the air) through the fill by gravity.

The air continues through the fill and thus past the water flow into an open plenum area.

A distribution orhot water basin consisting of a deep pan with holes ornozzles in the

bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles

uniformly across the fill material.

Counterflow

In a counterflow design the air flow is directly opposite of the water flow (see diagram

below). Air flow first enters an open area beneath the fill media and is then drawn up

vertically. The water is sprayed through pressurized nozzles and flows downward

through the fill, opposite to the air flow.

Common to both designs:

The interaction of the air and water flow allow a partial equalization and evaporation of

water.

The air, now saturated with water vapor, is discharged from the cooling tower.

A collection orcold water basin is used to contain the water after its interaction with the

air flow.

Both crossflow and counterflow designs can be used in natural draft and mechanical

draft cooling towers.


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Image 1: Natural draft wet cooling hyperboloid towers at Didcot Power Station, U

Economisers

Economizers, or in British English economisers, are mechanical devices intended to

reduce energy consumption, or to perform another useful function like preheating a

fluid. The term economizer is used for other purposes as well. Boiler, powerplant, andheating, ventilating, and air-conditioning (HVAC) uses are discussed in this article. In

simple terms, an economizer is a heat exchanger.

Boilers

In boilers, economizers are heat exchange devices that heat fluids, usually water, up to

but not normally beyond the boiling point of that fluid. Economizers are so named

because they can make use of the enthalpy in fluid streams that are hot, but not hot

enough to be used in a boiler, thereby recovering more useful enthalpy and improving

the boiler's efficiency. They are a device fitted to a boiler which saves energy by using

the exhaust gases from the boiler to preheat the cold water used to fill it (the feed

water).

Powerplants

Modern-day boilers, such as those in coal-fired power stations, are still fitted with

economizers which are descendants of Green's original design. In this context they are

often referred to as feedwater heaters and heat the condensate from turbines before it

is pumped to the boilers.

Economizers are commonly used as part of a HRSG in a combined cycle power plant.

In an HRSG, water passes through an economizer, then a boiler and then a

superheater. The economizer also prevents flooding of the boiler with liquid water that is

too cold to be boiled given the flow rates and design of the boiler.
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A common application of economizers in steam powerplants is to capture the waste

heat from boilerstack gases (flue gas) and transfer it to the boiler feedwater. This raises

the temperature of the boiler feedwater thus lowering the needed energy input, in turn

reducing the firing rates to accomplish the rated boiler output. Economizers lower stack

temperatures which may cause condensation of acidic combustion gases and serious

equipment corrosion damage if care is not taken in their design and material selection.

Superheaters

A superheater is a device in a steam engine that heats the steam generated by the

boiler again, increasing its thermal energy and decreasing the likelihood that it will

condense inside the engine. Superheaters increase the efficiency of the steam engine,

and were widely adopted. Steam which has been superheated is logically known as

superheated steam; non-superheated steam is called saturated steam orwet steam.

Superheaters were applied to steam locomotives in quantity from the early 20th century,

to most steam vehicles, and to stationary steam engines including power stations.

In locomotive use, by far the most common form of superheater is the fire-tube type.

This takes the saturated steam supplied in the dry pipe into a superheater header

mounted against the tube sheet in the smokebox. The steam is then passed through a

number of superheater elementslong pipes which are placed inside special,

widened fire tubes, called flues. Hot combustion gases from the locomotive's fire pass

through these flues just like they do the firetubes, and as well as heating the water they

also heat the steam inside the superheater elements they flow over. The superheater

element doubles back on itself so that the heated steam can return; most do this twice

at the fire end and once at the smokebox end, so that the steam travels a distance of

four times the header's length while being heated. The superheated steam, at the end of

its journey through the elements, passes into a separate compartment of the

superheater header and then to the cylinders as normal.

The benefit of the front end throttle is that superheated steam is immediately available.

With the dome throttle it took quite some time before the super heater actually provided

benefits in efficiency. One can think of it in this way: if one opens saturated steam from

the boiler to the super heater it goes straight through the superheater units and to the

cylinders which doesn't leave much time for the steam to be superheated. With the

front-end throttle, steam is in the superheater units while the engine is sitting at the

station and that steam is being superheated. Then when the throttle is opened,

superheated steam goes to the cylinders immediately.
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Pulverizer

A pulverizer is a mechanical device for the grinding of many different types of

materials. For example, they are used to pulverize coal forcombustion in the steam-

generating furnaces offossil fuel power plants.

Types of Pulverizers

Ball and Tube Mills

A ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three

diameters in length, containing a charge of tumbling or cascading steel balls, pebbles,

or rods.

A tube mill is a revolving cylinder of up to five diameters in length used for fine

pulverization of ore, rock, and other such materials; the material, mixed with water, is

fed into the chamber from one end, and passes out the other end as slime.

Ring and Ball Mill

This type of mill consists of two rings separated by a series of large balls. The lower ring

rotates, while the upper ring presses down on the balls via a set of spring and adjuster

assemblies. The material to be pulverized is introduced into the center or side of the

pulverizer (depending on the design) and is ground as the lower ring rotates causing the

balls to orbit between the upper and lower rings. The pulverized material is carried out

of the mill by the flow of air moving through it. The size of the pulverized particles

released from the grinding section of the mill is determined by a classifer separator.

Electrostatic precipitator

An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulatecollection device that removes particles from a flowing gas (such as air) using the force

of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration

devices that minimally impede the flow of gases through the device, and can easily

remove fine particulate matter such as dust and smoke from the air stream.
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Collection efficiency (R)

The collection efficiency of an electrostatic precipitator is strongly dependent on the

electrical properties of the particles. A widely taught concept to calculate the collection

efficiency is the Deutsch model, which assumes infinite remixing of the particles

perpendicular to the gas stream.

Sulfur Trioxide is sometimes injected into a flue gas stream to lower the resistivity of the

flue gas in order to improve the collection efficiency of the electrostatic precipitator.

Modern industrial electrostatic precipitators

ESPs continue to be excellent devices for control of many industrial particulate

emissions, including smoke from electricity-generating utilities (coal and oil fired), salt

cake collection from black liquor boilers in pulp mills, and catalyst collection from

fluidized bed catalytic cracker units in oil refineries to name a few. These devices treatgas volumes from several hundred thousand ACFM to 2.5 million ACFM (1,180 m/s) in

the largest coal-fired boiler applications.

The original parallel plateweighted wire design (described above) has evolved as more

efficient (and robust) discharge electrode designs were developed, today focusing on

rigid discharge electrodes to which many sharpened spikes are attached, maximizing

corona production. Transformer-rectifier systems apply voltages of 50100 kilovolts at

relatively high current densities. Modern controls minimize sparking and prevent arcing,

avoiding damage to the components. Automatic rapping systems and hopperevacuation systems remove the collected particulate matter while on line, theoretically

allowing ESPs to stay in operation for years at a time.

Electrical generator
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Generator :In electricity generation, an electrical generator is a device that converts

mechanical energy to electrical energy, generally usingelectromagnetic induction. The

reverse conversion of electrical energy into mechanical energy is done by a motor, and

motors and generators have many similarities. A generator forces electric charges to

move through an external electrical circuit, but it does not create electricity or charge,

which is already present in the wire of its windings. It is somewhat analogous to a water

pump, which creates a flow of water but does not create the water inside. The source of

mechanical energy may be a reciprocating or turbine steam engine, water falling

through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand

crank, the sun or solar energy, compressed air or any other source of mechanical

energy.

Other types of generators, such as the asynchronous or induction singly-fed generator,

the doubly-fed generator, or the brushless wound-rotor doubly-fed generator, do not

incorporate permanent magnets or field windings (i.e, electromagnets) that establish aconstant magnetic field, and as a result, are seeing success in variable speed constant

frequency applications, such as wind turbines or otherrenewable energy technologies.

The full output performance of any generator can be optimized with electronic control

but only the doubly-fed generators or the brushless wound-rotor doubly-fed generator

incorporate electronic control with power ratings that are substantially less than the

power output of the generator under control, which by itself offer cost, reliability and

efficiency benefits.
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The Electric Efficiency of a Conventional Thermal Power Station12

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