Power Plant EnggPower Plant Engg..High Pressure Boilers & FBCHigh Pressure Boilers & FBC

AssiAssi. Professor. ProfessorMechanical Mechanical Engineering Engineering DepartmentDepartment

HIGH PRESSURE BOILERS

� High Pressure Boiler, P > 60 bar

� Critical Pressure Boiler, P = 221.2 bar

� Super Critical Pressure Boiler, P > 221.2 bar

� Sub Critical Boiler P < 221.2 bar generally in

between 130 to 180 barbetween 130 to 180 bar

� Low Capacity Boiler Ms = 4000 to 6000 kg/hr

� Heavy Duty boiler Ms > 100000 kg/hr

DEPENDING ON TYPE OF FIRING ADOPTED

IN BOILERS THEY CAN BE CLASSIFIED AS

� Stoker fired

� Pulverized coal fired

� Down shot fired� Down shot fired

� Fluidized bed boilers

� Cyclone fired

� Chemical recovery boilers

VARIOUS TYPES OF ARRANGEMENT ARE USED BY DESIGNERS IN

DESIGNING THE BOILER FOR MEETING THE END

REQUIREMENT. HENCE BOILERS ARE CLASSIFIED BASED ON THE

ARRANGEMENT AS

� Top supported boilers

� Bottom supported

� Package boilers

Field erected boilers� Field erected boilers

� Drum type boilers

� Single drum

� Bi drum

� Three drums, but these are presently out of use

� Tower type or single pass

� Close coupled

� Two pass boilers

ACCORDING THE "ASME BOILER AND PRESSURE

VESSEL CODE" BOILERS MAY BE CLASSIFIED AS

� Section I Power Boilers - process boilers, power boilers and high pressure boilers

boilers in which steam or other vapor is generated at a pressures exceeding 15 psig

high temperature water boilers intended for operation at pressures exceeding 160 psig and or temperatures exceeding 250 degrees Ftemperatures exceeding 250 degrees F

� Section IV Heating Boilers - commercial boilers, industrial boilers, heating boilers, low pressure boilers

boilers in which steam or other vapor is generated at a pressures not exceeding 15 psig

� high temperature water boilers intended for operation at pressures not exceeding 160 psig and or temperatures exceeding 250 degrees F

CHARACTERISTICS OF HIGH PRESSURE

BOILER

Necessity of forced circulation for water� Necessity of forced circulation for water

� Pressurized combustion

� Increased heat transfer area – water tubes

� Improved medium heating

ADVANTAGES OF HIGH PRESSURE

BOILERS

� The different advantages of high pressure boilers are listed below :

1. The tendency of scale formation is eliminated due to high velocity of

water through the tubes.

2. Light weight tubes with better heating surface arrangement can be

used. The space required is also less. The cost of foundation, the

time of erection and cost are reduced due to less weight of the tubestime of erection and cost are reduced due to less weight of the tubes

used.

3. Due to use of forced circulation, there is more freedom in the

arrangement of furnace, tubes and boiler components.

4. All the parts are uniformly heated, therefore the danger of

overheating is reduced and thermal stress problem is simplified.

5. The differential expansion is reduced due to uniform temperature

and this reduces the possibility of gas and air leakages.

6. The components can be arranged horizontally as high headrequired for natural circulation is eliminated using forcedcirculation. There is a greater flexibility in the componentsarrangement.

7. The steam can be raised quickly to meet the variable loadrequirements without the use of complicated controlrequirements without the use of complicated controldevices.

8. The efficiency of plant is increased up to 40 to 42% byusing high pressure and high temperature steam.

9. A very rapid start from cold is possible if an externalsupply of power is available. Hence the boiler can be usedfor carrying peak loads or standby purposes with hydraulicstation.

La Mont Boiler

�A forced circulation boiler was first introduced in 1925 by La Mont.

Working:

�The feed water from hot well is supplied to a storage and separating drum (boiler) through the economises. The most of the sensible heat is supplied to the feed water passing through the economiser.

�A centrifugal pump circulates the water equal to 8 to 10 times the weight of steam evaporated. This water is circulated through the evaporator tubes and steam evaporated. This water is circulated through the evaporator tubes and the part of the water evaporated is separated in the separator drum.

�The steam separated in the boiler is further passed through the superheater as shown in Fig. and finally supplied to the prime mover.

Capacity:

�These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar. and at a temperature of 500°C.

Limitation

�The main difficulty experienced in the La Mont boiler is the formation andattachment of bubbles' on the inner surfaces of the heating tubes. Theattached bubbles to the tube surfaces reduced the heat flow and steamgeneration as it offers high thermal resistance than water film.

Benson BoilerBenson Boiler

� Benson in 1922 argued that if the boiler pressure was raised to criticalpressure (225 bar), the steam and water have the same density andtherefore the danger of bubble formation can be easily eliminated.

� The technical development at that time did not allow to build turbines forsuch high pressures. The first high pressure Benson boiler was put intooperation in 1927

� During starting, the water is passed through the economiser, evaporator,superheater and back to the feed line via starting valve A.

� During starting the valve B is closed. As the steam generation starts and itbecomes superheated, the valve A is closed and the valve B is opened.

During starting, firstDuring starting, firstcirculating pumps arestarted and then theburners are started toavoid the overheatingof evaporator andsuperheater tubes.

CapacityThe maximum working pressure obtained so far from commercial Benson boiler is500 bar. The Benson boilers of 150 tones/ hr. generating capacity are in use.Boiler having as high as 650°C temperature of steam had been put in service.

Advantages.1. As there are no drums, the total weight of Benson boiler is 20% less than

other boilers. This also reduces the cost of boiler.2. The transfer of Benson's parts is easy as there is no drums and majority of

the parts are carried to the site without pre-assembly.3. The erection of Benson boiler is easier and quicker as all the parts are3. The erection of Benson boiler is easier and quicker as all the parts are

welded at sites and workshop job of tube expansion is altogether avoided.4. The Benson boiler can be erected in a comparatively smaller floor area.

The space problem does not control the size of Benson boiler used.5. The furnace walls of the boiler can be more efficiently protected by using

smaller diameter and closed pitched tubes.6. The super heater in the Benson boiler is an integral part of forced

circulation system, therefore no special starting arrangement forsuperheated is required.

7. The Benson boiler can be started very quickly because of welded joints.

Primaryseparator

Primary Evaporator

Primary Cricket

Schmidt Hartmann Boiler

The arrangement of the boiler components is shown in Fig. The operation of theboiler is similar to an electric transformer. Two pressures are used to effect anintercharge of energy.

�In the primary circuit, the steam at 100 bar is produced from distilled water.The generated steam is passed through a submerged heating coil which islocated in an evaporater drum as shown in figure.

�The high pressure steam in this coil possesses sufficient thermal potentialand steam at 60 bar with a heat transfer rate of 10,000 kJ/m2-hr°C isgenerated in the evaporator drum.

�Natural circulation is used in the primary circuit and this is sufficient to effect�Natural circulation is used in the primary circuit and this is sufficient to effectthe desired rate of heat transfer and to overcome the thermo-siphon head ofabout 2 m to 10 m.

�In normal circumstances, the replenishment of distilled water in the primarycircuit is not required as every care is taken in design and construction toprevent the leakage. But as a safeguard against leakage, a pressure gaugeand safety valve are fitted in the circuit.

�Advantages.

1. There is a rare chance of overheating or burning the highly heatedcomponents of the primary circuit as there is no chance of interruption to thecirculation either by rust or any other material. The highly heated parts runvery safely throughout the life of the boiler.

2. The salt deposited in the evaporator drum due to the circulation of impurewater can be easily brushed off just by removing the submerged coil fromthe drum or by blowing off the water.

3. The wide fluctuations of load are easily taken by this boiler without unduepriming or abnormal increase in the primary pressure due to high thermaland water capacity of the boiler.

4. The absence of water risers in the drum, and moderate temperaturedifference across the heating coil allows evaporation to proceed withoutpriming.

�The major difficulty experienced in La Mont boiler is thedeposition of salt and sediment on the inner surfaces of the watertubes. The deposition reduced the heat transfer and ultimately thegenerating capacity.

�This further increased the danger of overheating the tubes dueto salt deposition as it has high thermal resistance.

�This difficulty was solved in Loeffler Boiler by preventing theflow of water into the boiler tubes.

�Most of the steam is generated outside from the feed water byusing part of the superheated steam coming out from the boiler.

Thermal PP, Ukai

LOEFFLER BOILER

�The pressure feed pump draws the water through the economiser anddelivers it into the evaporator dram.

�About 65 % of the steam coming out of superheater is passed through theevaporator dram in order to evaporate the feed water. The steam circulatingpump draws the saturated steam from the evaporator drum and is passedthrough the radiant superheater and then convective superheater.

� About 35% of the steam coming out from the superheater is supplied to theHP steam turbine. The steam coming out from HP. turbine is passed throughreheater before supplying to LP turbine.reheater before supplying to LP turbine.

� This boiler can carry higher salt concentration than any other type and ismore compact than indirectly heated boilers having natural circulation. Thesequalities fit it for land or sea transport power generation.

Capacity:�Loeffler boilers with generating capacity of 100 tonnes/hr operating at 140bar are already commissioned.

VELOX BOILER

� The velocity of flue gases exceeds the velocity of sound, thereforethe heat transfer from flue gases at a much greater rate than theachieved at low velocity.

�Air is compressed by air compressor which is run by gas turbine. Thefuel and air are injected downwards into a vertical combustion chamber

�The combustion chamber consists of annulus water tubes. Theproduct of combustion (flue gages) are deflected upwards withsupersonic velocity. As a result, the heat is transferred from flue gasesto water at a very high rate.to water at a very high rate.

Capacity:�Steam generating capacity: 100 tones/hoursPressure: 84 bar

Limitation:�The size of the Velox boiler is limited (100 tones/hour) because morepower is required for running the air compressor.� Power produced by gas turbine is not sufficient to run the aircompressor and hence balance power from external source must besupplied to the compressor.

SUPERCRITICAL BOILER

� As pressure of water or steam is raised, the enthalpy of evaporationis reduced. At critical pressure (221.05 bar) the enthalpy of evaporationbecomes zero.

�When water is heated at constant supercritical pressure suddenly it isconverted into steam. the high pressure (above critical point) waterconverted into steam. the high pressure (above critical point) waterenters the tube inlets and leaves at the outlet as the superheatedsteam.

�There is no drum, but there should be a transition section where thewater is likely to flash in order to accommodate the large increase involume.

Guru Nanak Dev Thermal Plant in n

Bathinda City

Supercritical Boiler

Advantages:

� Heat transfer rates are considerably large compared to subcritical boilers.

�There is no drum, less heat capacity of the generator and hence morestable and gives better response.

�There is no two phase mixture and hence the problem of erosion andcorrosion are minimized.corrosion are minimized.

�There is great ease of operation and their comparative simplicity andflexibility made them adaptable to load fluctuations.

� Higher thermal efficiency (about 40 to 42%) of power station can beachieved with use of supercritical boiler.

Limitations

� The high pressure and temperature of supercritical boiler have limits for

use due to availability of material and difficulties experienced in the turbine

and condenser operation because of large volumes.

�The additional problem is created due to the separation of solid impurities

as phase changes. These solids remain in the tubes and block the passage

for the flow of feed water. Therefore it is necessary to treat water thoroughly

before supply to the boiler.

When the combustion is carried out under high pressure bysupplying the compressed air then rate of heat transfer isincreased and heating surfaces required is reduced. This theory isused in supercharged boiler.

�The high pressure compressed air is supplied to combustion chamber.Part of heat of hot gases in the furnace is absorbed by boiler tubes inwhich water evaporated and then it is superheated in superheater.

�The high pressure and temperature exhaust gases from combustionchamber are used to run gas turbine. The work produces by gas turbineis used to run air compressor.

�The exhaust gases coming from gas turbine passes over theeconomizer tubes. Then escape to the atmosphere through chimney.

In economizer usually pressure of gas side is 5 bar andpressure to the steam side of 200 bar are preferred.

Supercharged boiler

Advantages

� Supercharged boiler requires 30 to 25% of heat transfer surface ofconventional boiler due to very high overall heat transfer co-efficient.

� Due to small heat capacity of the boiler, boiler plant gives better response tocontrol.

� Rapid start of the boiler is possible due to less heating surfaces and� Rapid start of the boiler is possible due to less heating surfaces andcompactness.

� The part of the gas turbine output can be used to drive other auxiliaries.

� Comparatively less number of operators are required.

Limitation:It requires tight passage for high pressure gas.

Jaitapur Nuclear PP

Fluidized Bed

Combustion

It is a system in which fluidizedbed which is composed of fuel and inertmaterial is mixed with air/gas in anatmospheric or pressurized vessel andatmospheric or pressurized vessel andcombustion take places in suspendedcondition of particles in gas stream.

� When air or other gas flows upward through bed, the bed solid particles aredisturbed. If velocity increased further a stage is reached and the composed(packed) bed becomes turbulent and rapid mixing of particles occurs. Thebehavior of this mixture of solid particles and air or gas is like a fluid. Burning of afuel in such a stage is known as fluidized bed combustion.

� The mixture of fuel (crushed coal) and inert material (crushed dolomite* or

limestone) are fed on a distribution plate and air is supplied from the bottom of

distribution plate.

� The air is supplied at high velocity so that solid particles of feed material

remains in suspension condition during burning.

�The evaporator tubes are directly immersed in

* A kind of sedimentary rock resembling marble or limestone but rich in magnesium carbonate

�The evaporator tubes are directly immersed in

the fluidized bed and direct contact between the

burning coal particles and tubes produce very high

heat transfer rates.

The world power industry is trying to shift from oil / gas to old faithfulfuel coal and this is possible by Fluidized bed combustion. There are tworeasons for the rapid increase of fluidized bed combustion (FBC) incombustors.

� First, the liberty of choice in respect of fuels. Not only coal, there ispossibility of using fuels which are difficult to burn using other technologies.This is an important advantage of fluidized bed combustion.

�The second reason, a low emissionof nitric oxides and the possibility ofremoving sulfur in a simple manner byusing limestone as bed material.

FBC systems fit into essentially two major groups, atmospheric systems (FBC)and pressurized systems (PFBC), and two minor subgroups, bubbling (BFB)and circulating fluidized bed (CFB).

Conventional FBCAtmospheric fluidized beds use limestone or dolomite to capture sulfur releasedby the combustion of coal. Jets of air suspend the mixture of sorbent andburning coal during combustion, converting the mixture into a suspension ofred-hot particles that flow like a fluid. These boilers operate at atmosphericpressure.

PFBCThe first-generation PFBC system also uses a sorbent and jets of air tosuspend the mixture of sorbent and burning coal during combustion. However,these systems operate at elevated pressures and produce a high-pressure gasstream at temperatures that can drive a gas turbine. Steam generated from theheat in the fluidized bed is sent to a steam turbine, creating a highlyefficient combined cycle system.

Conventional FBC

�The pressure inside the bed is atmospheric.�The bed consisting about 97% limestone or inert material and 3%burning fuel, is suspended by hot primary air entering the bottom of thecombustion chamber.�There are two types of system based on the fuel feeding arrangementas underfeed and overfeed.

�In case of underfeed fuel and limestone are introduced from bottom ofthe fluidized bed.

�The overfeed system is simple in operation and economical in runningbut results in smaller output per m2 area and gives poor desulphurizationperformance.

�Under feed system provides positive load and a compact design butcostly in operation.costly in operation.

Limitation�The main disadvantage using the conventional FBC is thatincombustible particles (ash and metals) of fuel are came downwardsand block the distributor when light materials like wood dust, agriculturalwaste etc. are used as a fuel. There for modifications are required forhigh combustion efficiency.

Bubbling fluidized bed (BFB)

Bubbling caps

For plants with a nominal boiler capacity of over 20 MW is suitable.In BFB furnaces, a bed material is located in the bottom part.

�The primary air is supplied over a nozzle distributor plate and fluidises thebed. The bed material is usually silica sand of about 1.0 mm in diameter; thefluidisation velocity of the air varies between 1.0 and 2.5 m/s.

�The secondary air is introduced through several inlets in the form ofgroups of horizontally arranged nozzles at the beginning of the upper part ofthe furnace (called freeboard) to ensure a staged-air supply to reduce NOx

emissions.

�The fuel amounts only to 1 to 2% of the bed material and the bed has to�The fuel amounts only to 1 to 2% of the bed material and the bed has tobe heated (internally or externally) before the fuel is introduced.

�The advantage of BFB furnaces is their flexibility concerning particle sizeand moisture content of the biomass fuels. Furthermore, it is also possible touse mixtures of different kinds of biomass or to co-fire them with other fuels.

�One big disadvantage of BFB furnaces, the difficulties they have at partialload operation, is solved in modern furnaces by splitting or staging the bed.

Circulation FBC System.

�To increases the efficiency of combustion - provide non uniform fluidizing velocities over the bed,- provide slope to one of the FBC walls- provide sloping distributor plate to give an air slide particles.

�Due to above there is significant improvement and allowed to use light materials as fuels most successfully.

� Light materials were burned within the bed and heavy incombustibles (ash and metals) gathered at the bottom of the sloping distributor.

�The solid fuel enters the furnace from the side of walls. The low velocity (LV), medium velocity (MV) and high velocity (HV) air supplied at different points along the sloping surface of the distributor plate. The secondary air is supplied over the bed. The ash port is provided at lower end of the distributor plate.

�In this system pressurized air (10 bar approximately) is used forfluidization and combustion.

�Boiler exit gas contain enough energy about temperature 850 to 900°Cto drive a gas turbine. The power output of gas turbine is utilized to runthe air compressor and the electric generator.

�Here, the product of combustion have to be sufficient clean for gasturbine to prevent excessive erosion, corrosion or fouling of the turbine.Hence, the flue gases from the combustion chamber are passed througha cyclone separator.a cyclone separator.

Advantages:In PFBC system, high rate of coal loading and burning is achieved.Comparatively less volume of furnace is required, hence plant size isreduced. It has improved desulphurization and low NOx emission.

Disadvantages :(1) The controlling is difficult to control. With compare to conventionalplant, life is low.