Steam power plant 2

Steam Power PlantSolar Lounge

Nishkam DhimanAsst Prof : Electrical and Electronics Engineering

Chitkara University, Punjab

Fluidised Bed Combustion

• A fluidised bed combustion may be defined as the bed of solid particles behaving as a fluid.

Principle:• When a gas is passed through a packed bed of finely divided solid particles, it

experiences a pressure drop across the bed. • At low gas velocities, this pressure drop is small and does not disturb the

particles. But if the gas velocity is increased further, a stage is reached, when particles are suspended in the gas stream and packed bed becomes a ‘Fluidised Bed’

• With the further increase in gas velocity, the bed becomes turbulent and rapid mixing of particles occurs.

• The behaviour of this mixture of solid particles and gas is like fluid. Burning of fuel in such a state is known as fluidised bed combustion.

Working

1. On distributor plate are fed fuel and inhert material dolomite(CaMg(CO 3) and from bottom air is suspended. 2. The high velocity of air keeps the solid feed material in suspending condition during burning.3. The generated heat is rapidly transferred to the water passing through the tubes immersed in the bed and generated steam is taken out. 4. During burning sulphur di oxide formed is absorbed by the dolomite and prevents its escape with the exhaust gases. The molten slag is tapped from the top of the bed.5. The primary objective of the inhert material is to control the temp of the bed, it accounts 90% of the bed volume.It should remain in motion with the fuel and at high temp to tune 800°C .

Advantages• 1. As a result of better heat transfer the unit size and capital cost are

reduced.• 2. It can respond rapidly to change in load demand.• 3. Low combustion temperatures 800-950°C inhibits the formation of nitric

oxide and nitrogen oxide.• 4. Since combustion temperature is low the corrosion of tube is reduced.• 5. There is no need to crush the coal to a pulverized form so cost of crushing

is reduced.• 6. Pollution is controlled as combustion of high sulphur content can be used.• 7. FBC can use solid, liquid or gaseous fuel.• 8. Combustion temp can be controlled accurately.• 9. 70% ash containing coal can be burned in FBC, conventional combustion

system becomes unstable even with above 48% ash.

Boilers• A boiler may be defined as a closed vessel in which

steam is produced from water by combustion of fuel. • The performance of a boiler is measured in terms

of its evaporative capacity which is also called as “Boiler Power”. It is defined as the amount of steam produced in kg/hour. It may also be expressed in kg of steam per kg of fuel burnt or kg/hr/m2 of heating surface.

• Boilers are classified according to the following criteria:

1. According to the position of the principle axis• Vertical • Horizontal• Inclined2. According to flow of water and hot gases.• Water tube (Babcock, Wilcox, Striling, Yarrow Boiler)• Fire tube(Cochran, Lancashire, Locomotive)3. According to position of furnace• Internally fired – furnace/fire is inside the boiler shell – (Cochran,

Lancashire)• Externally fired- (Babcock, Wilcox, Striling, Yarrow Boiler)

4. According to application• Stationary • Mobile (Marine, Locomotive…)5. According to circulating water • Natural circulation (circulation of water takes place due to natural convention

currents produced by application of heat) Lancashire, Babcock, Wilcox boiler.• Forced circulation (Water circulation is done by forced pump) Velox, Lamont,

Benson boilers. • 6. According to steam pressure• Low pressure – Produce steam at a pressure below 80bar, Cochran, Cornish,

Lancashire, Locomotive boilers• High pressure - at a pressure 80 bar and above-Babcock, Wilcox, Velox,

Lamont, Benson boilers.

In water tube boilers, water is circulated through tubes andhot products of combustion flow over these tubes

Water Tube Boilers

• Water tube boilers can further be classified as follows:• Horizontal straight tube boilers• Bent tube boilers• Cyclone fired boilers • Water tube boilers have the following advantages:• High pressures (about 140 kg/cm2) can be obtained• Large heating surfaces can be obtained by use of large

number of tubes – therefore steam can be generated easily.• Efficiency is higher because of high velocity of water in

tubes which improves heat transfer

Fire Tube Boilers

• In fire tube boilers, the hot combustion gases pass through the tubes, which are surrounded by water

Fire Tube Boilers

• Fire tube boilers can further be classified as follows:• External furnace• Internal furnace• Fire tube boilers have the following advantages:• Low cost• Fluctuations of steam demand can be easily met• Compact in size • Disadvantage of fire tube boilers is that they contain more water in

the drum and if the flue gas circulation is poor, they can not quickly meet the steam demand. For the same output, the outer shell of fire tube boiler is much larger than the shell of a water tube boiler

Cochran Boiler

• Vertical multitubular boiler, has a number of horizontal fire tubes.

• Shell diameter: 2.75m• Height: 5.79• Working Pressure: 6.5bar(1bar = 100,000 Pa) (max:

15bar)• Steam capacity: 3500kg/h max 4000• Heating surface: 120m2• Efficiency: 70 to 75%

Babcock and Wilcox water-tube boiler

• Horizontal straight tube boiler, stationary or marine purpose.

Diameter of drum: 1.22 to 1.83mLength: 6.096 to 9.144mSize of water tubes : 7.62 to 10.16cmSize of super heater tubes: 3.84 to 5.71cmWorking pressure: 40bar(max)Steaming capacity: 40000kg/h (max)Efficiency: 60 to 80%Angle of inclination of tubes: 15deg.

Locomotive Boiler

• It is mainly employed in locomotives although it may be used as stationary boiler.

• It is compact in size and its capacity for steam production is quite high for its size and can raise the amount of heat quickly.

Dimensions and specifications:

Working

• It consists of cylindrical barrel with rectangular fire box at one end and smoke box at other end. Coal is introduced through the fire hole into the gate which is placed at the bottom of fire box.

• The hot gasses generated are deflected by an arch of fire bricks, so that the walls of the fire box may be heated properly.

• Fire box is entirely surrounded by water.• The hot gasses pass from fire box to smoke box by series of fire tubes,

then they are discharged to atmosphere.• Heat of hot gasses is transmitted to water, the steam generated is

collected over the water surface.• Superheater.• Here natural draught cant be used as it need high chimney height, the

forced draught is created by the exhaust steam.

• Merits1. High steam capacity.2. Low construction cost.3. Low installation cost.4. Compact.• Demerits1. Chances of corrosion.2. Difficult to clean some water spaces.3. Overload cause overheating.4. Practical limitations of pressure and capacity.

Accessories used in steam power plants

• Accessories are the auxiliary plants required for steam boilers for their proper operation and for the increase of their efficiency.

1. Feed Pumps:Feed pump is pump which is used to deliver feed water to the boiler.a) Reciprocatory pumps consists of cylinder and piston, these

are continuously run by steam from the same boiler to which water is to be fed.

b) Rotary Pumps are centrifugal type are either run by small steam turbine or by an electric motor.

Economiser

• An economiser is a device in which the waste heat of the flue gases is utilised for heating the feed water.

1. Independent type: It is installed in the chamber apart from the boiler settings. The camber is situated ate the passage of the flow of the flue gasses from boiler to chimney.

2. Integral type: it is the part of the boiler heating surface and is installed in within the boiler settings

Advantages of Economiser

• The Temperature range between various parts of the boiler is reduced which results in reduction of stresses due to unequal expansion.

• If the boiler is fed with cold water, it may result in chilling the boiler metal.

• Evaporation capacity of boiler is increased.• Overall efficiency of boiler is increased.

• The function of the air pre-heater is to increase the temperature of air before itenters the furnace. It is generally placed after the economizer; so the flue gases pass through the economizer and then to the air preheater.

Super heater

• The function of a super heater is to increase the temperature of the steam above its saturation point. The super heater is very important accessory of a boiler and can be used both on fire-tube and water-tube boilers. The small boilers are not commonly provided with a super heater.

• Super heaters are located in the path of the furnance gases so that the heat is recovered by the superhearter from the hot gases,

• Advantages• Steam consumption of engine or turbine is

reduced.• Erosion of turbine blade is eliminated.• Efficiency of steam plant is increased.

Types of Superheaters

• Convective Superheaters: Makes the use of heat of flue gases.

• Radiant Superheaters: It is placed in the furnace and the wall tubes receives heat from burning fuel through radiant process. It is used where high amount of superheat temperature is required.

• In a reheat turbine the steam first enters high speed turbine so its temp and pressure reduces before entering low speed turbine so a reheater is used to reheat the cooled steam.

Feed Water Heaters

• A feed water heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feed water reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is introduced back into the steam cycle.

Condenser

• The main purposes of the condenser are to condense the exhaust steam from the turbine for reuse in the cycle and to maximize turbine efficiency by maintaining proper vacuum.

• As the operating pressure of the condenser is lowered (vacuum is increased), the enthalpy drop of the expanding steam in the turbine will also increase. This will increase the amount of available work from the turbine (electrical output). By lowering the condenser operating pressure, the following will occur :

• (a) Increased turbine output • (b) Increased plant efficiency • (c) Reduced steam flow (for a given plant output) It is therefore very advantageous to operate the condenser at the lowest possible pressure

• There are two primary types of condensers that can be used in a power plant :

• (a) Direct Contact • (b) Surface • Direct contact condensers condense the

turbine exhaust steam by mixing it directly with cooling water

• Steam surface condensers are the most commonly used condensers in modern power plants. The exhaust steam from the turbine flows on the shell side (under vacuum) of the condenser, while the plant’s circulating water flows in the tube side.

• The source of the circulating water can be either a closed-loop (i.e. cooling tower, spray pond, etc.) or once through (i.e. from a lake, ocean, or river).

• The condensed steam from the turbine, called condensate, is collected in the bottom of the condenser, which is called a hotwell.

• The condensate is then pumped back to the steam generator to repeat the cycle.

Condenser

Evaporators

• These are used to supply of pure water as make up feed for the boilers. Raw water is evaporated by using extracted steam then condensed to give distilled and pure feed water.

• The film type evaporators: In this type water is sprayed on the surface of tubes through which steam is passed. As the water falls on the surface of the heated tubes it evaporates.

• The submerged type evaporators: In this type the bundle of tubes is submerged in water. Vapors formed in the shell pass out of the shell through a moisture separator and enter a feed water condenser.

Cooling Tower

• Cooling Towers have one function : • Remove heat from the water discharged from the condenser

so that the water can be discharged to the river or recirculated and reused.

• The importance of the cooling tower is felt when the cooling water from the condenser has to be cooled.

• The cooling water after condensing the steam, becomes hot and it has to be cooled as it belongs to a closed system.

• The Cooling towers do the job of decreasing the temperature of the cooling water after condensing the steam in the condenser.

• When water is reused in the process, it is pumped to the top of the cooling tower and will

then flow down through plastic or wood shells, much like a honeycomb found in a bee‟s nest. • The water will emit heat as it is downward flowing

which mixes with the above air flow, which in turn cools the water. Part of this water will also evaporate, causing it to lose even more heat.

Natural Draft cooling tower

• Natural draft towers are typically about 120 m high, depending on the differential pressure between the cold outside air and the hot humid air on the inside of the tower as the driving force. No fans are used.

• Mechanical draft towers uses fans (one or more) to move large quantities of air through the tower. They are two different classes :

• (a) Forced draft cooling towers • (b) Induced draft cooling towers

Turbines

• A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion.

• A steam turbine is a prime mover in which the potential energy of the steam is transferred into the kinetic energy, and later in its turn is transformed into mechanical energy of the rotation of the turbine shaft.

Classification of Steam Turbines

Action of steamA. ImpulseB. ReactionC. Combination of impulse and reaction.According to number of pressure stagesA. Single stage turbine : with one or more velocity stages usually of small power capacity, used for driving centrifugal pumps, blowers etcB. Multi Stage turbine: Wide range of capacities from large to small.According to direction of steam flowA. Axial Turbine: Steam flows in a direction parallel to axis of the turbine.B. Radial Turbine: Steam flows in a direction perpendicular to the axis of the turbine

According to method of governingA. Turbines with throttle governing: fresh steam enters through one

or more simultaneously operated throttle valves.B. Turbines with nozzle governing: fresh steam enters through two or

more consecutively operated regulatorsAccording to steam conditions at inlet to the turbineC. Low pressure turbines, 1.2 to 2 ataD. Medium pressure turbines, 40 ataE. High pressure turbines, above 40 ataF. Very high pressure turbines, 170 ata and higher at temp of 550deg

CG. Turbines of supercritical pressure, 225ata and above

According to their usages in IndustryA. Stationary turbines with constant speed of rotation: alternatorsB. Stationary turbines with variable speed of rotation : turbo-blowers, pumpsC. Non stationary turbines with variable speed: steamers, ships, locomotives

• The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft.

• The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.

• The main difference lies in the way the steam expands while it moves through them.

• In Impulse turbine steam expands in the nozzles and its pressures does not alter as it moves over blades.

• In reaction type the steam expands continuously as it passes over the blades and thus there is gradual fall in pressure during expansion.

Nozzles

• A steam nozzle is defined as a passage of varying crossection, through which heat energy of steam is converted to kinetic energy. It produces a steam jet with high velocity to drive steam turbines.

Impulse Turbines

• An impulse turbine has fixed nozzles that orient the steam flow into high speed jets.

• These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction.

• A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

• As the steam flows through the nozzle its pressure falls from inlet pressure to the exit

• Steam Power Plant pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity.

• The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle.

• The loss of energy due to this higher exit velocity is commonly called the “carry over velocity” or “leaving loss”.

Reaction Turbines

• In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor.

• Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor.

• The steam then changes direction and increases its speed relative to the speed of the blades.

• A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

Main Differences of Impulse and Reaction Turbine• 1) In impulse turbine the water flows through the nozzles and impinges on the buckets where as in

reaction turbine the water is guided by the guide blades to flow over the moving vanes.• 2) In impulse turbine the entire water energy is first converted in kinetic energy but there is no

energy conversion in reaction turbine.• 3) ) In impulse turbine the water impinges on the buckets with kinetic energy where as in reaction

turbine the water glides over the moving vanes with pressure energy.• 4) In impulse turbine the work is done only by the change in the kinetic energy of the jet but in

reaction turbine the work is done partly by the change in the velocity head, but almost entirely by the change in pressure head.

• 5) In impulse turbine the pressure of flowing water remains unchanged and is equal to the atmospheric pressure but in reaction turbine the pressure of flowing water is reduced after gliding over the vanes.

• 7) In impulse turbine the water may be admitted over a part of the circumference or over the whole circumference of the wheel but in reaction turbine the water must be admitted over the whole circumference of the wheel.

• 8) It is possible to regulate the flow of water without loss in impulse turbine but in reaction turbine it is not possible to regulate the flow without loss.