AA PRESENTATION ON BOILERS PRESENTATION ON BOILERS

Boiler is a closed vessel in which water under pressure is transferred into steam by the application of heat.

In the furnace, the chemical energy in the fuel is converted into heat. It is the function of the boiler to transfer this heat to the water in the most efficient manner.

A boiler should be designed to absorb the maximum amount of heat released in the process of combustion.

Boiler Types and Classifications

Fire in tube or Hot gas through tubes and boiler feed water in shell side

Fire Tubes submerged in waterApplication

Used for small steam capacities

( upto 25T/hr and 17.5kg/cm2

Merits Low Capital Cost and

fuel Efficient (82%) Accepts wide & load

fluctuations Packaged Boiler

Fire Tube Boiler

1.Fire Tube boiler

In boilers of this type most of the work is done by the heat transfer from hot combustion products flowing inside the tubes to the water surrounding the tubes. They may also be called as shell boilers. Fire tube boilers feature simple and rugged construction and are relatively cheaper.

The maximum design working pressure for fire tube boilers is limited to 17-kg/square centimeter (approx.)

Boiler Types and ClassificationsWater Tube Boiler Water flow through tubes

Water Tubes surrounded by hot gasApplication

Used for Power Plants Steam capacities range

from 4.5- 120 t/hr

Characteristics High Capital Cost Used for high pressure

high capacity steam boiler

Demands more controls Calls for very stringent

water quality

2.Water tube Boiler

As the demand for steam is increased and also for a quick method of boiling, in modern boilers water is put into the tubes with the fire and gases are out side the tubes. This is called "water tube boiler". The following are the advantages of water tube boilers.

• Very high capacity can be obtained. •It can also be designed for much higher pressure. •Most parts of the boiler are accessible for cleaning, repairs and inspection. •The general design permits higher operating efficiencies. •The furnace proportions are such that various fuels can be used without making alterations.

Packaged Boiler

Package boilers are generally of shell type with fire tube designMore number of passes-so more heat transferLarge number of small diameter tubes leading to good convective heat transfer.Higher thermal efficiency

Chain Grate or Traveling Grate Stoker Boiler Coal is fed on

one end of a moving steel chain grate

Ash drops off at end

Coal grate controls rate of coal feed into furnace by controlling the thickness of the fuel bed.

Coal must be uniform in size as large lumps will not burn out completely

Spreader Stoker BoilerUses both

suspension and grate burning

Coal fed continuously over burning coal bed

Coal fines burn in suspension and larger coal pieces burn on grate

Good flexibility to

meet changing load requirements

Preferred over other type of stokers in industrial application

Pulverized Fuel Boiler

Tangential firing

Coal is pulverised to a fine powder, so that less than 2% is +300 microns, and 70-75% is below 75 microns.Coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles.

Combustion temperatures from 1300-1700°CParticle residence time in the boiler is 2-5 secondsMost popular system for firing pulverized coal is the tangential firing using four burners corner to corner to create a fire ball at the center of the furnace

Advantages Its ability to burn all ranks of coal from

anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces.

Disadvantages High power demand for pulverizing Requires more maintenance, flyash erosion

and pollution complicate unit operation

Pulverized Fuel Boiler (Contd..)

Fluidized bed Combustion (FBC) boiler

Further, increase in velocity gives rise to bubble formation, vigorous turbulence and rapid mixing and the bed is said to be fluidized.

Coal is fed continuously in to a hot air agitated refractory sand bed, the coal will burn rapidly and the bed attains a uniform temperature

Distributed air is passed upward through a bed of solid particlesThe particles are undisturbed at low velocity.As air velocity is increased, a stage is reached when the particles are suspended in the air

Fluidised Bed Combustion

3. Modern power station boilers.

Modern power station boilers are all water tube boilers consisting essentially of a combustion chamber and an economiser where heat is transferred to boiler water, a drum where separation of steam and water takes place, superheater where superheat is given to saturated steam, gas ducts and air ducts supporting structure, pipe lines with fittings.

Convection Pass

Modern boilers are generally two pass units, the second pass being the convection pass where heat recovery units like primary superheater, economiser and air heater are placed. The convection pass is made of steel duct or in the form of tube wall.

FURNACE: Furnace is the primary part of boiler where the chemical energy available in the fuel is converted to thermal energy by combustion. Furnace is designed for efficient and complete combustion.

Major factors for efficient combustion are time of fuel inside the furnace, temperature inside the furnace mixing between fuel and air.

The Other important feature of the furnace construction is to reduce the air infiltration. This is to ensure sufficient air through the burners for efficient combustion, prevention of heat losses.

Type of furnaces:

•Dry bottom furnace: •Slag type or wet bottom type •Oil fired furnace

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

1) Water circuit

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

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

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

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

2) Steam circuit

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

3)Air/gas circuit

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

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

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

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

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

 

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

The following can be termed as boiler pressure parts.

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

BOILER DRUM

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

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

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

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

WATER WALL SYSTEM:

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

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

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

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

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

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

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

Reheaters:

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

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

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

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

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

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

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

The Boiler Auxiliaries :

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

DRAFT SYSTEM:

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

The Boiler draft system includes Air and Flue gas flow.

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

SOOT BLOWERS deposits resulting from the combustion of coal will be

deposited on the boiler tubes at various zones will be

cleaned by soot blowing for effective heat transfer

while on-load. AIR HEATERS: The air heater is required for efficient combustion in the

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

AIR PREHEATER

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

the mills through a feeder. Air required for drying and transporting the pulverized

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

from FD fan discharge. The drying and grinding takes place inside the mills.

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

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

Type of Pulverizers:

Drum/Tube mills: Ball mills: Bowl mills:

ELECTROSTATIC PRECIPITATORS:

Working Principle: The principles upon which an electrostatic

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

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

The following fans are used in the boiler houses:

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

vary between 600 to 1500 r.p.m.

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

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

transportation into the furnace. Usually 1500 r.p.m.  

4. Seal Air Fan: Used to seal mill bearings, coal feeders and coal pipes

in case of pressure type mill. May take air from atmosphere and supply air to mill at a

pressure higher than mill pressure.

5. Igniter air fan: Used to provide necessary combustion air to the igniter. Two fans are usually provided out of which one will run

and the other will remain as stand by. Typical speed is 1460 RPM.

6. Scanner air fan: Used to provide necessary cooling air to the flame

scanners Two scanner air fans are usually provided, one will run

and other will remain as stand by. Typical speed is 3000 r.p.m.

Why Boiler Blow Down ?

When water evaporates Dissolved solids gets concentrated and Solids precipitates on tubes. Reduces the heat transfer rate

Intermittent Blowdown

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

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

with intermittent blow down.

Continuous Blowdown

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

This ensures constant TDS and steam purity.

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

The quantity of blow down required to control boiler water solids concentration is calculated by using the following formula:(Continuous Blow down)

TDS in feed water

300 ppm

Steam 3 T/hrTDS(T) =0

TDS (C) =3000 ppm Allowable)

Blow down flow rate=300x10%/3000 =1% :=1% of 3,000 = 30 kg/hr

Blow down(B)

Total Dissolved solids

Reduce Stack Temperature

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

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

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

There are two methods of assessing boiler efficiency. 1)      The Direct Method: Where the energy gain of the working

fluid (water and steam) is compared with the energy content of the boiler fuel. 2)      The Indirect Method: Where the efficiency is the difference

between the losses and the energy input.

Boiler Efficiency Evaluation Method

1. Direct Method 2. Indirect Method

 

Performance Evaluation of Boilers

Example:Type of boiler: Coal fired BoilerHeat input dataQty of coal consumed :1.8 TPHGCV of coal :3200K.Cal/kg

Heat output data Qty of steam gen : 8 TPH Steam pr/temp:10 kg/cm2(g)/1800C Enthalpy of steam(sat) at 10 kg/cm2(g) pressure

:665 K.Cal/kg Feed water temperature : 850 C Enthalpy of feed water : 85 K.Cal/kg

Find out the efficiency ?

Efficiency Calculation by Direct Method

Boiler efficiency (): = Q x (H – h) x 100 (q x GCV)  

Where Q = Quantity of steam generated per hour (kg/hr) H = Enthalpy of saturated steam (kcal/kg) h = Enthalpy of feed water (kcal/kg)

q = Quantity of fuel used per hour (kg/hr) GCV = Gross calorific value of the fuel (kcal/kg)

 Boiler efficiency ()= 8 TPH x1000Kg/Tx (665–85) x 100 1.8 TPH x 1000Kg/T x 3200 = 80.0% 

Boiler Flue gas

Steam Output

Efficiency = 100 – (1+2+3+4+5+6+7+8)

(by In Direct Method)

Air

Fuel Input, 100%

1. Dry Flue gas loss2. H2 loss3. Moisture in fuel4. Moisture in air5. CO loss

7. Fly ash loss

6. Surface loss

8. Bottom ash loss

What are the losses that occur in a boiler?

Feed water Blow down

Intermittent Blowdown

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

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

with intermittent blow down.

Continuous Blowdown

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

This ensures constant TDS and steam purity.

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

Boiler Water Treatment Internal Water Treatment: It is carried out

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

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

Internal treatment alone is not recommended.

External Water Treatment

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

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

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

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

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

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

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

De-aeration When heated in boiler systems, carbon

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

Deaerator

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

Boiler Replacement

if the existing boiler is : Old and inefficient, not capable of firing cheaper

substitution fuel,  over or under-sized for present requirements, not designed for ideal loading conditions replacement option should be explored.

Since boiler plants traditionally have a useful life of well over 25 years, replacement must be carefully studied.