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A CASE STUDY ON OPERATION AND

MAINTENANCE OF BOILER

Sibabrata Mohanty1, Ajit Prasad Dash

2, Rajesh Kumar Sahu

3

1,2Asst.Professor,

3Research Scholar, Department of Mechanical Engineering, G.I.E.T, Gunupur,

Rayagada, (Odisha)

ABSTRACT

Coal fired boiler system generates approximately 38% of the electric power worldwide and will continue to be

major contributors in the future. New pulverized coal fired systems routinely installed today generate power at

net thermal cycle efficiency ranging from 34% to 37% (higher heating value) while removing up to 97% of the

combined, uncontrolled air pollution emissions. A new generation of pulverized coal fired boiler technology is

currently under development which will permit generating efficiencies in excess of 42%. This paper highlights

some of today’s design improvements which target reduced emissions and expanded operability, and explores

some of the boiler design implications for the ultra-supercritical conditions needed to achieve the high cycle

efficiencies for the future.

Keywords: Fbc Boiler, Boiler Mountings, Boiler Accessories, Boiler Efficiency

I. INTRODUCTION

1.1 Definition

It is a closed vessel in which steam is produced from water by combustion of fuel.

Boiler is an apparatus in which thermal energy released by combustion of fuel is transferred into water which

vaporizes and gets converted into steam at the desired pressure and temperature.

1.2 History

As far as the early history goes, it is stated that one boiler was recovered from the ruins of Pompoli, made of

bronze. In the early 17th

century, the boilers were manufactured from copper plates and were riveted. The

Lancashire was manufactured in the mid 18th

century at Lancashire, hence the name.

Till the beginning of 19th

century, there were no manufacturing or inspection criteria, which resulted in a large

no of accidents particularly in UK. After a major boiler accident in UK, which claimed ten lives, in 1859 for the

first time in the world, the insurance company was started to examine the boilers. In India, it is learnt that in

1860, 13 people lost their lives in West Bengal in a boiler explosion and some rules were framed in W.B. similar

rules then came to force in Maharashtra. As the rules framed were not uniform in all the provinces, Central

Govt. appointed a special committee to draft a central Act and in 1923, the Indian Boilers Act came to force.

The Act framed in 1923 was a bare Act and in courses of time it was supplemented by Indian Boiler Regulations

1950, which vividly deals about the materials, constructions & inspection procedures. In our country the boilers

are now designed, manufactured & inspected as per the Indian Boiler Act 1923 and IBR 1950, also I.S.O. R831.

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At present the age old riveted Lancashire boiler have become outdated and various new design welded

construction boilers have replaced the same. At present in our country boilers capable of generating steam to

produce 500 MW of power have been manufactured & are in use since long.

1.3 Boiler Mounting

Mountings are the components used for the safety of boiler, the boiler requires the following mountings a feed

check valve to prevent the return of water from the boiler in base the feed pump is to operating, a steam stop

valve to regulate the flow of steam from the boiler, safety valve (at least two) to protect the boiler from

pressures higher than the designed valve, a blow off valve to empty the boiler when needed and to discharge the

mud and sediments that are in the boiler. Water level indicator to show the water level inside the boiler, a

pressure gauge to indicate the pressure of steam in the boiler, fusible plugs to protect the boiler against low

water lever.

1.4 Boiler Accessories

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

their efficiency

According to the principle of operation the steam separator is classified as follows:

Impact or baffle type

Reverse current type

Centrifugal type

1.5 Boiler Efficiency

Boiler efficiency is the ratio of heat actually utilized in generation of steam to the heat supplied by the fuel in the

same period.

Boiler efficiency= ma (h-hf)/c

Where,

ma= mass of water actually evaporated into steam per kg of fuel at the working pressure

C= calorific value of the fuel in kj/kg.

1.6 Heat Losses in a Boiler Plant

The following heat loss occurs in a boiler plant:

Heat loss of flue gases

Heat loss due to incomplete combustion

Heat loss due to un burnt fuel

Convection and radial losses

II. MECHANISM OF FLUIDIZED BED COMBUSTION

When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as

sand supported on fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased,

a stage is reached when the individual particles are suspended in the air steam- the bed is called “fluidized”.

With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation,

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of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes

the appearance of a fluid- “bubbling fluidized bed”.

At higher velocities, bubbles disappear, and particle is blown out of the bed. Therefore, some amounts of

particles have to be re circulated to maintain a stable system- “circulating fluidized bed”. Fluidization depends

largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the

gas velocity. The difference between the mean solid velocity and mean gas velocity is called as slip velocity.

Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact. If

sand particles in a fluidized state is heated to the ignition temperature of coal, and coal is injected continuously

into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidized bed combustion

(FBC) takes place at about 840 to 950 degree Celsius. Since this temperature is much below the ash fusion

temperature, melting of ash and associated problems are avoided. The lower combustion temperature is

achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective

extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is

maintained between minimum fluidization velocity and particle entrainment in the gas stream.

FBC boilers combustion progress requires the three “T”s that is time, temperature and turbulence. In FBC,

turbulence is promoted by fluidization. Improved mixing generates evenly distributed heat at lower temperature.

Residence time is many times greater than conventional grate firing. Thus an FBC system releases heat more

efficiently at lower temperatures.

2.1 Types of Fluidized Bed Combustion Boilers

There are three basic types of fluidized bed combustion boilers:

Atmospheric classic fluidized bed combustion system (AFBC)

Atmospheric circulating (fast) Fluidized bed combustion system (CFBC)

Pressurized fluidized bed combustion system (PFBC)

the dust collectors and the air pre heaters before being exhaust to atmosphere. The main special feature of

atmospheric fluidized bed combustion is the constraint imposed by the relatively narrow temperature range

within which the bed must be operated. With coal, there is risk of clinker formation in the bed if the

temperature exceeds 950 degree Celsius and loss of combustion efficiency if the temperature falls below

800 degree Celsius. For efficient sulphur retention, the temperature should be in the range of 800 to 850

degree Celsius.

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2.2 General Arrangement of Afbc Boiler

AFBC boiler comprises of following systems:

Fuel feeding system

Air distributor

Bed & in-bed heat transfer surface

Ash handling system

Many of these are common to all types of FBC boilers.

2.3 Advantages of Fbc Boilers

High Efficiency

FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. FBC boiler

can operate with overall efficiency of 84% (plus or minus 2%)

Reduction In Boiler Size

High heat transfer rate over a small heat transfer area immersed in bed results in overall size reduction of

the boiler.

Fuel Flexibility

FBC boiler can operate efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects,

and agro wastes can be burnt efficiently.

Ability to Burn Low Grade Fuel

FBC boiler would give the rated output with inferior quality fuel. The boilers can fire coals with ash content

as high as 62% and having calorific value as low as 2,500 kcal/kg.

Pollution Control

SO2 formation can be greatly minimized by addition of limestone or dolomite for high sulphur coals. 3%

limestone is required for every 1% sulphur in the coal feed.

Low Corrosion and Erosion

The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low

particle velocity (of the order 1m/sec)

Less Excess Air- Higher CO2 in Flue Gas

The CO2 in the flue gases will be of the order of 14-15% at full load. Hence the FBC boiler can operate at

low excess air only 20-25%.

Simple Operation, Quick Start-Up

High turbulence of the bed facilitates quick start-up and shut down. Full automation of start up and

operation using reliable equipment is possible.

Provision Of Automatic Coal And Ash Handling System

Automatic system for coal and ash handling can be incorporated making the plant easy to operate

comparable to oil or gas fired installation

Provision Of Automatic Ignition System

Control system using micro-processors and automatic ignition equipment give excellent control with

minimum manual supervision.

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High Reliability

The absence of moving parts in the combustion zone results in a high degree of reliability and low

maintenance cost.

Reduced Maintenance

Routine overhauls are infrequent and high efficiency is maintained for long periods.

III. OPERATION AND MAINTENANCE OF BOILER

3.1 Coal Handling Plant

The requirement of coal handling plant plays an important role

In coal handling plant, raw coal that is brought from outside & bamboo dust is supplied to griggly hoper

whereby help of vibratory machine it is crushed into smaller parts. Then some amount of bamboo dust is also

mixed with charcoal and supplied to primary screen I & II with the conveyor belt I & IA. In primary screen I&II

coal size is checked (i.e. round 6-10 mm). If the required size is obtained then it goes bunker of CF boiler. If

size condition is not satisfied then the coal goes to crusher I & II. When size is maintained if the coal size is not

obtained to the required size, then it goes to the secondary stages of coal sizing. The acceptable coal size is send

to the boiler through the help of conveyor belt IV.

3.2 Demineralization of Water

It means water is free from all minerals. This is done to various defects happens such as boiler corrosion,

formation of sludge and scale which may decrease the volume or capacity of the boiler due to deposition of

minerals on the walls of the boiler.

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It is to remove mineral content of water, in this system raw water enters the hydrogen zeolite exchanger (cation

removal) at the top and flows to the anion exchanger and degasified and finally passes through silicon absorber.

3.3 Starting of The Boiler – Light Up Procedure For Cold Condition

Preparation of furnace when the furnace is empty, check the following:

Air Nozzle – nozzles hole are to be clean. Nozzle top to be intact.

If nozzle are not clean the ore to be cleaned with air hose

Coal feed nozzle – the top refractory is to be there

*if it is spoiled, the refractory is to be put on coal feed nozzle top

Check the following:

All the access doors to be closed

All personnel to be cleared

All foreign material are to be removed from furnace & pressure parts

Starting equipment & interlock are ok

Check the following equipment for adequate lubrication and readiness for services

o ID fan

o FD fan

o PA fan

o Feed pump

o Fuel feeder

o Dynamo drive system

Banker (coal) to be full with 0-6 mm size coal and gates below the banker are in open position and coal is

available at feeder inlet

o Check instrument air above 3 kg/cm2 pressure is available for instrumentation and control.

o Start first ID fan with suction damper in closed position

o Start FD fan with suction dumper in closed position

3.4 Charging Bed Material

Keeping the individual damper of 3 compartments in open position, furnace draft is to be maintained at 5

mm to 7 mm by slightly opening the ID fan damper

Open the furnace man hole and with 2 mazodoors the bed materials is to be charged into the furnace, care to

be taken that no empty bag slips into the furnace. After completely charging the material it should be

leveled by properly operating dampers of ID & FD fans, then close dampers. Open the main holes on both

sides of the furnace.

Take a 24v hand lamp inside and measure the height of the bed in each compartment. The height of bed in

each compartment should be 300 mm -310 mm. Then come out of the furnace and keep the hand lamp on

the bed coils(glowing)

After preparing the bed start PA fan and de choke individual coal feed lines by opening the coal feed line

drain

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Then open the individual mixing air, nozzle plugs and check whether air is being sucked in if it is in suction

it means the line is clear, if not the line is to be cleared till you feel air suction, then plug the nozzle drains

stop the PA fan.

Again enter into the furnace and see for the bed height of starting compartment (it should be 320-325 mm).

Then stop FD fan and ID fan & close the suction damper control room.

3.5 Lighting up of boiler

Before lighting up of boiler fill the boiler with feed water up to half steam drum level, at the time of taking

water ensure that all air vents are in open position and all drains are in closed position. Confirm the level of

water in the drum by seeing gauge glass levels.

Keeping lighting swabs ready

Keep open the furnace door and open the ID fan suction damper

Light up one swab watch stick, light up all swabs one after one throw, then in all directions in the startup

compartment

In each compartment two thermocouples are provided to indicate feed temperature

It is important to increase air flowing radually to increase the top level temp. to 800 to 850 degree

When the bed temperature rises above 600 degree switch on PA fan, the coal transport lines are to be closed

and coal feed drain lines to the cleared

Adjust PA fan suction damper to maintain header pressure in between 750 mm and 950 mm water column.

Open the fuel transport air damper of the startup compartment. The coal feed shall be kept at minimum at

the beginning.

Stabilize the startup compartment at 850 to 875 degree

When the pressure rises slowly and reach 34-35 kg/cm2, the required range is to be opened up slowly.

Before opening range valve, ensure that super heater steam temp. is 380 to 425 ºC.

This is very important. Take boiler on range. Close vent and drain.

3.6 Compartmental Transfer

Compartmental transfer means activating static bed of the furnace adjacent to the activated compartment.

Before activation static compartment the temperature (bed) of the running compartment is to be maintained

at 920 to 900 ºC

The temperature of the operating compartment shall be once again brought up to 900 ºC to 925 ºC before

another attempt is made

This process is to be done a number of times when the temperature of bed in new compartment rises to 600

to 625 ºC the primary air lies in the new compartment shall be opened & fuel feeding can be started. The

bed temp. is to be brought to 800 to 825 ºC with MCR fluidizing air flow condition.

3.7 Boiler Operation

As per the boiler cold start up procedures starts the boiler and stabilize the boiler on coal with adequate bed

temperature.

At steady conditions put the boiler in auto mode

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Depends on the available load number beds in active condition to be determined, either keeping all the beds

in active condition by reducing the bed temperature to acceptable levels or reduce the number of beds and

increase the bed temperature to acceptable levels, (maximum temperature should be around 950 ºC and

minimum should be around 700 ºC)

Based on the available load airflow to be adjusted to required levels.

3.8 Boiler Trip Due To Power Failure

If boiler trips due to power failure, take the following action:

Start stand by feed pump (on second power sources)

Restore drum water level by controlling feed water on manual mode. If instrument air is not available first

close the isolation valves of the inlet of feed control valve and regulate feed water through by pass valve,

keep a constant watch on gauge glass level

Close the inlet damper of ID and FD fan

Close the butterfly damper of wind box

Close all steam outlets from boiler so that boiler is not depressurized immediately

3.9 Boiler Hot Startup Procedures

Open the startup vent valve slightly

Close all the compartment damper

Start the ID fan, FD fan, PA fan

Open slightly the wind box damper of no 1 so bed I fluidizes with minimum air ensure that fluidization is

uniform and satisfactory

Adjust the ID fan damper to maintain the furnace pressure

Adjust the FD fan & PA damper to get the required fan pressure

Start coal feeding without delay otherwise bed may cool down rapidly

Gradually increase the coal feeding observing the rise in bed temperature

Adjust the air flow and coal feeding observing the rise in bed temperature

Take bed no 2 and other beds on line and connect the boiler process

Take feed water control and furnace draft on auto mode

Charge the ESP if the flue gases temperature is ok

Put the boiler into auto mode

Start ID fan, FD, fan PA fan

If the bed level is too high drain the bed and bring down the bed height to around 300 mm by fluidizing the

bed-1, spread some charcoal over the bed no-1, ignite the same

Repeat the cold startup procedures listed in this manual and established the boiler.

3.10 Boiler Normal Shut Down Procedure

Gradually reduce the steam load by reducing to the coal flow to the furnace

Gradually reduce the air flow also to avoid fast cooling the bed

Reduce the firing rate slowly to maintain the temperature drop around 250 per hour

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Cost of coal feeding to the combustor

Do’s connect the boiler from range when steam pressure drop less than the required pressure for process

Open the startup vent slightly and depressurized the boiler gradually (if required) otherwise close all the

vent and drains fully without depressurizing

Switch off the ID fan if required. Keep ID & FD fan dampers is opened to allow the gases to escape.

When the boiler pressure drops to 2 kg/cm2 open the air vent of drum

Maintain normal water level in the boiler

If surface is required to be cooled quickly, fan may be kept running for sometimes, please note that fast

cooling leads for refractory damages, it is suggested to go for natural cooling

3.11 Daily Maintenance Check List During Operation

Check the correctness of water level in the control desk with direct level glass.

Check the level indicator for proper illumination

Check the combustor for proper fluidization through peep holes

Drain the bed do check for unusual combustible in the bottom ash

Check fan unusual noise for steam/pumps

Check vibration in rotary equipment

Check for air/gas leakage from dust & flue

Check for hot spots bulging etc. on casting dusts etc

Check the position of damper & cleanliness

Check the bearing for lubrication & cooling water system

Check all alarm annunciations with respect to set position

Check for water, oil along with instrument air

Check for any unusual smoke conditions

3.12 Do’s & Don’t’s For Fbc Boiler Do’s

Clear the materials before closing the man holes of pressure parts furnace & ducting.

Maintain all instruments in good worker condition

All equipment interlocks should be always in line.

Maintain normal water level in feed water tank, deaerater tab & boiler

Maintain normal water quality as per the remedial limits.

All dampers must be on smooth operation condition.

Pressure rising from cold start must be done as per the cold start up curve.

All the duct joints must be leak proof

Use proper lubricants recommended by the manufacture regularly

State the equipment as per sequence i.e. start compressor, feed water pumps, ID fan, FD fan, rotary air lock

valve, PA fan

Clear all ash present regularly

Regularly clean feed water & transfer pump section strainer

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Operated the boiler within the recommended operation limit

Boiler, piping, duct, feed water & oil tank must be properly emulated.

Servicing of equipment should be done as per the manufacturer’s schedule.

Maintain proper operation log sheets regularly

Maintain the air face from moisture & oily matter of the pressure as recommended.

Come out regular closing of direct water level gauges glass on feed water tank, deaerater tanks & boiler

drum.

3.13 Don’t’s

Don’t by pan any instrument of safety interlock

Don’t throw water as boiler feed water.

Don’t throw big size, wooden logs inside the furnace for slow firing

Don’t start any fan with suction damper in open condition

Don’t start any pump with the delivery valve in open condition

Don’t run the boiler with furnace in pressurized condition

Don’t throttle the feed water pump balancing leak off value while the pump & in operation

Don’t operate the furnace wall header drain valves while the boiler is in operation

Don’t operate the boiler beyond the operation limits.

Don’t leave the furnace door open while the boiler is in operation

Don’t mix up different lubricants

Don’t by pass compressed air dryer, if any problem attend to it at the earliest

Don’t throw water inside the hot furnace.

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IV. CONCLUSION

AFBC boiler generally comes under the category of FBC boiler in which the major portion fuel used in coal,

bamboo dust and some amount of charcoal fines along with main fuel in order to increase the efficiency. Proper

operation and maintenance of AFBC boiler are mostly required to increase its efficiency, longevity, and smooth

running. In order to function properly the water that is used in boiler should be clarified and then it is

demineralized to get free from all types of radicals like anion and cations which are made free from forming of

scale deposition.Similarly the coal which is used should be properly crushed into required size that is about 6

mm to 10 mm. Therefore the fuel and water that comes to the boiler is to be checked properly.

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Operation are carried out in different stages, like coal start up, hot start up and boiler trip due to various reasons

like time consumption and fuel consumption the hot start up is more economic than cold start up. Cold start up

condition requires preparation of furnace when furnace is empty, charging bed material, spreading of charcoal,

feed water. But in hot start up condition does not require such process. In case of boiler trip due to power failure

the feed pump is made stand by and damper are closed and all the outlets for the steam are closed. So that the

boiler is not depressurized immediately. Under any circumstances like pipe leakage or scale deposits the boiler

is needed to shut down. So in order to avoid these problems certain tests like hydraulic and hydrostatic tests are

carried out. These tests happen to be the key note test for every boiler in any plant.

REFERENCES

[1]. Life cycle analysis of small scale pellet boilers characterized by high efficiency and low emissions by B.

Monteleone etal Applied Energy Volume 155, 1 October 2015, Pages 160–170

[2]. Online identification of the lower heating value of the coal entering the furnace based on the boiler-side

whole process models by Liang Xu Fuel Volume 161, 1 December 2015, Pages 68–77

[3]. 9 – CCHP Operation and Maintenance by Masood Ebrahimi Combined Cooling, Heating and Power

Decision-Making, Design and Optimization 2015, Pages 189–196

[4]. Reducing the recycle flue gas rate of an oxy-fuel utility power boiler by Haining Ga Fuel Volume 140,

15 January 2015, Pages 578–589

[5]. Indian standard IS 10391-1982

[6]. IBR- 1950

[7]. R.K. Rajput, Power Plant Engineering, Laxmi Publication, page:154-158; page: 175-185

[8]. G.R.Nagpal, Power Plant Engineering, Khanna Publication, page: 246-259

[9]. S.C.Arora and S.Domkundwar, Power Plant Engineering, DhanpatRai Publication, page:15.1-15.18,

page:19.1-19.30