36954153 boiler-book

Steam generator Ganesh kumar

A.GANESH KUMAR
DEUTSCHE BABCOCK, INDIA.

For internal circulation only. All rights reserved by author.


DEDICATED TO MY COLLEGE AND MY PROFESSORS.


PREFACE

Dear friends,

This book was prepared in view of giving assistance to design

engineers entering into the boiler field and to plant engineers whom

I have met always in desire to know the ABC of the boiler design

and related calculations. I have made an attempt in bringing close

relation of practical field design and theoretical syllabus of

curriculum. Engineering students, who always wonder how the

theory studying in curriculum will help them in real life of business.

For them this book will give an inspiration.

I have designed this book in two parts. First, the basic theory of

working fluid in the steam plant cycle. This will be the basic

foundation for development of boiler science. Secondly the main

components of steam generator and its design. Also you can find

various useful data for ready reference at the end of this book.

(A.GANESH KUMAR)


CONTENTS

• PREFACE……………………………………………………………………….

1.0 TYPES OF STEAM GENERATORS

1.1 Introduction…………………………………………………………………….
1.2 History of steam generation and use………………………………………
1.3 Shell and tube boiler………………………………………………………….
1.4 Conventional grate type boiler……………………………………………….
1.5 Oil/gas fired boiler…………………………………………………………….
1.6 Pulverized fuel boiler………………………………………………………….
1.7 Fluidized bed boiler……………………………………………………………
1.8 Heat recovery steam generator………………………………………………
1.9 Practical guide lines for selection of boiler………………………………….

2.0 STEAM, GAS and AIR

2.1 Introduction……………………………………………………………………
2.2 Definitions for some commonly used terms………………………………
2.3 Steam………………………………………………………………………….
2.4 Fuel……………………………………………………………………………..
2.5 Gas and air…………………………………………………………………….
2.6 Some commonly used dimensionless numbers and their significance….

3.0 FURNACE

3.1 Introduction……………………………………………………………………
3.2 Effect of fuel on furnace……………………………………………………..
3.3 Forced or Natural Circulation……………………………………………….
3.4 Heat flux to furnace walls…………………………………………………...
3.5 Points to be noted while designing furnace………………………………
3.6 Classification of furnace…………………………………………………….
3.7 Modes of heat transfer in furnace…………………………………………
3.8 Heat transfer in furnace…………………………………………………….
3.9 Furnace construction……………………………………………………….
3.10 Practical guides for designing fluidized bed, conventional
and oil/gas fired furnace…………………………………………………..

4.0 SUPERHEATER

4.1 Introduction…………………………………………………………………..
4.2 Effect of fuel on super heater design………………………………………
4.3 Points to be noted while designing super heater…………………………
4.4 Classification of super heater……………………………………………….
4.5 Designing a super heater……………………………………………………
4.6 Overall heat transfer across bank of tubes……………………………….
4.7 Steam temperature control…………………………………………………
4.8 Pressure drop………………………………………………………………..


5.0 DRUMS

5.1 Intruction……………………………………………………………………. Deleted: od
5.2 Optimal configuration of drums………………………………………………
5.3 Stubs and attachments in the steam drum/shell…………………………..
5.4 Maximum permissible uncompensated opening in drum…………………
5.5 Size of the drum………………………………………………………………
5.6 Drum internals………………………………………………………………..

6.0 EVAPORATOR AND ECONOMISER

6.1 Introduction……………………………………………………………………….
6.2 Difference between evaporator and economiser……………………………..
6.3 Fin efficiency………………………………………………………………………

7.0 AIRHEATER

7.1 Introduction……………………………………………………………………….
7.2 Types of air heater……………………………………………………………….
7.3 Advantages of air heater………………………………………………………..
7.4 Heat transfer in air heater………………………………………………………
7.5 Practical guide lines for designing airheater………………………………….

8.0 DUST COLLECTOR

8.1 Introduction……………………………………………………………………….
8.2 Effects of air pollution……………………………………………………………
8.3 Air quality standards……………………………………………………………..
8.4 Air pollution control devices…………………………………………………….
Centrifugal cyclone dust collector
Bag filter
Electro static precipitator

9.0 WATER CHEMISTRY

9.1 Introduction…………………………………………………………………….
9.2 Names of water flowing in the power plant cycle…………………………..
9.3 Major impurities in water……………………………………………………..
9.4 Effects of various impurities in boiler water………………………………..
9.5 Need for water treatment…………………………………………………….
9.6 External water treatment……………………………………………………..
9.7 Internal water treatment………………………………………………………
9.8 Practical guides for selecting water treatment plant……………………….

10.0 BOILER CONTROLS

10.1 Introduction……………………………………………………………………
10.2 Control philosophy……………………………………………………………
10.3 Drum level control…………………………………………………………….


10.4 Super heater steam temperature control…………………………………..
10.5 Furnace draft control………………………………………………………….
10.6 Combustion control…………………………………………………………...
10.7 Field instruments……………………………………………………………..
10.8 Panel instruments……………………………………………………………

APPENDIX 1 : MOLLIEAR CHART
APPENDIX2 : PSYCHROMETRY CHART
APPENDIX3 : FUEL ANALYSIS
APPENDIX4 : STEAM TABLES
APPENDIX5 : POLLUTION NORMS IN VARIOUS INDIAN STATES
APPENDIX6 : USEFUL DATAS
APPENDIX7 : UNIT CONVERSION TABLE


1.0 TYPES OF STEAM GENERATOR

1.1 INTRODUCTION

Indian power demand is met mainly from thermal, hydro and nuclear power. Non-
conventional energy power production is very much negligible. Out of the main
power producing sources thermal plant produces 48215 MW (69%), hydro plant
produces 19300 MW (28%), nuclear plant produces 2033 MW (3%) as on 31st
March 1992. In the above power plants 72% of the generation is from thermal and
nuclear, where steam generation is one of the main activity. In the years to come,
the demand of electricity is going on increasing and already most of water resources
suitable for power generation is in service. Except from gas turbines power the most
of new electric capacity has to be met by utilizing steam.

Steam boiler today range in size from those to dry the process material 500 kg/hr to
large electric power station utility boilers. In these large units pressure range from
100 kg/cm² to near critical pressures and steam is usually superheated to 550°C. In
India BHARAT HEAVY ELECTRICALS LTD (BHEL) is the pioneer in developing
the technology for combustion of high ash coal efficiently in atmospheric bubbling
fluidized bed. From where lot of industries in boiler manufacturing starts. Only after
the year 1990, India’s foreign policy was changed, various foreign steam generator
manufacture entered into Indian power market bringing various configuration and
competitiveness in the market.

1.2 HISTORY OF STEAM GENERATION AND USE

The most common source of steam at the beginning of the 18th century was the shell
boiler. Little more than a kettle filled with water and heated from the bottom. Olden
day boiler construction were very much thicker shell plate and riveted constructions.
These boilers utilize huge amount of steel for smaller capacity. Followed this shell
and tube type boilers have been used and due to direct heating of the shell by
flames leads severe explosion causing major damages to life and property. For
safety need, after the Indian independence India framed Indian boiler regulations in
1950, similar to various other standards like ASME, BS, DIN, JIS followed world
wide. Till date IBR 1950 is governing the manufacturing and operation of boilers with
amendments then and there. Indian sugar industry uses very low pressure (15
kg/cm²) inefficient boilers during independence now developed to an operating
pressure of 65 kg/cm² and more of combined cycle power plant. If we analysis most
of the boilers erected in pre-independence period were imported boilers only and
now steam generators were manufactured in India to the world standards on budget,
delivery and performance. In power industry India made a break through in the year
1972, India’s first nuclear power plant was commissioned at Tarapore. This plant
was an pilot plant meant for both power and research work. This was made in
collaboration with then soviet republic of Russia. Now India has its own nuclear
technology for designing nuclear power plant. Even though there is a development,
Indian industry has to go a long way in power sectors.


1.3 SHELL AND TUBE BOILER

Steam was originally used to provide heat to the industrial process like drying,
boiling. In small industry the people are not taken care in fuel consumption point,
they have generated steam in crude manner. Shell and tube boilers are old version
of boilers used in industry where a large flue tube was separated by a fixed grate
man power is used to throw husk and shells into the grate and firing was done.

In early days, as individual electric generating stations increased in capacity, the
practice was merely to increase the number of boilers. This procedure eventually
proved to be uneconomical and larger maintenance. Afterwards, individual boilers
were build larger and larger size, however the size became such that furnace floor
area occupation was more. Therefore further research work have been developed in
this area and technologies such as pulverized coal fired furnace, circulated fluidized
bed furnace, pressurized circulated fluidized furnace (still under research stage)
were developed. These modern technologies have higher heat transfer coefficient in
furnace and allow higher volumetric combustion rates.

1.4 CONVENTIONAL GRATE TYPE BOILERS
TECHNOLOGY

This is the oldest method of firing fuel. Fuel will be spread over the grate, where the
fuel is burnt. Fuel feeding will be done manually or mechanically to have a sustained
flame. In this type burning will be done at higher excess air. Incoming air will be
used for cooling the grate.

Types of grate

Common types of grate that are used for fuel are fixed grate, pulsating grate,
dumping grate, travelling grate. Each type of grate differ slightly in their construction
and arrangement. However the combustion phenomenon remains same.

Travelling grate
The travelling type is a continuous grate which slowly convey the burning fuel
through the furnace and discharge the ash to an ash pit. Grate speed is regulated
by the amount of ash discharging to ash pit ( 0 to 7m/hr)

Pulsating grate
The pulsating grate is non- continuous grate. The grate surface extends from the
rear of furnace to ash pit. Here the grate will be given a racking motion at pre
determined frequency depending on the fuel/ash bed depth.

Dumping grate
Dumping grates are also a non-continuous type grate. The grate is split into
longitudinal sections, one for each feeder. Fuel is distributed on the grate and burns.
When ash depth gets to a depth where air can not diffuse it , the grates are tilted or
ash is dumped into the hopper in the following manner.


Alternating fuel feeding is stopped and grate is tilted by lever arrangement, the
actuation can be done either manually or pneumatic cylinder.
In dumping grate the grate sections should be designed in such a way that, while
dumping the ash part of grate surface not available for burning. In poorly designed
dumping grate there may be steam pressure. Therefore while sizing grate sections
care should be taken such that while dumping part of the grate, other fuel feeder and
remaining sections should able to take the full load.

Dumping grate is similar to fixed grates, it is best suitable for bagasse where the fuel
is of low calorific value and having high moisture content. Therefore air alone can
acts as a cooling medium. If we use coal the grate bar may not with stand higher
temperature and additional cooling by water tube is necessary. Travelling grate is
suitable for burning coal and lignite. As the grate rotates, the grate bar gets heated
and cooled by incoming air for the half of the cycle and remaining half of the cycle
grate bar cooled by the incoming air.

Spreader stoker
Mechanical spreader
The spreader stoker feeder takes fuel from the feeder hopper by either a small ram
or a rotating drum and delivers it into a spinning rotor. An adjustable trajectory plate
is located between the feed mechanism and the rotor. Adjusting the trajectory plate
fuel can be feed through out the entire length of the furnace.
Pneumatic spreader
In this rotor is replaced by high pressure air lines from Secondary air fan is used to
spread the fuel into the furnace. The fuel is carried into the furnace by means of
pneumatic system and the air flow adjustment makes the fuel to flow near or farther
of the furnace.

1.5 OIL/GAS FIRED BOILERS
TECHNOLOGY

Flame has a tendency to burn upward only. This forms the basic concept of burner.
Whenever fresh fuel enters into the ignition zone it starts burning upwards and the
flame will not come downwards to the incoming fuel, by this property combustion
can be controlled easily. Hence it is always better to bring the oil or gas train from
bottom of the burner.
A liquid or gas fuel has flowable property by nature and it has a lower ignition
temperature. When the fuel is forced to flow through the nozzle it will spread though
an predetermined length and burn completely from the point of entry to the firing
zone estimated. The fuel flow can be controlled by means of control valves.

CHARACTERISTICS OF OIL

In today’s climate of fluctuating international fuel prices and quality, the emphasis on
the ability of the boiler on low quality fuel oils has become more greater. In the
international market, the quality of the residual fuel oils is constantly getting poorer
due to the development of more sophisticated cracking methods and also our
indigenous crude production falls short of our requirements, about 15 million tons of
crude is imported from outside sources. These outside sources are many, our


refineries handle a variety of crude. Since the inherent properties of the finished
petroleum products are directly dependent on the parent crude, one can imagine the
petroleum involved in producing residual fuel oil within narrow limits of specifications,
especially with respect to specified characteristics like carbon residue, asphaltenes
and metallic constituents is not possible.

Flash point

Flash point is important primarily from a fuel handling stand point. Too low a flash
point will cause fuel to be a fire hazard subject to flashing and possible continued
ignition and explosion. Petroleum products are classified as dangerous or non
dangerous for handling purposes based on flash point as given below.

Classification Flash point Petroleum
Product

Class A Below 23°C Naptha
Petrol
Solvent 1425
Hexane

Class B 23 to 64°C Kerosene
HSD

Class C 65 to 92°C LDO
Furnace oil
LSHS

Excluded Petroleum 93°C and above Tar

Pour Point

The pour point of the fuel gave an indication of the lowest temperature, above which
the fuel can be pumped. Additives may be used to lower the freezing temperature
of fuels. Such additives usually work by modifying the wax crystals so that they are
less likely to form a rigid structure. It is advisable to store and handle fuels around
10°C above the expected pour point.

Viscosity

Viscosity is one of the most important heavy fuel oil characteristics for industrial and
commercial use, it is indicative of the rate at which the oil will flow in fuel systems
and the ease with which it can be atomized in a given type of burner. When the
temperature increases viscosity of fuel will reduce.


The viscosity needed at burner tip for satisfactory atomization for various types of
burners are as follows.

Type of burner Viscosity at burner tip
In centi stokes
Low air pressure 15 to 24
Medium air pressure 21 to 44
High air pressure 29 to 48
Steam jet 29 to 37
Pressure jet less than 15

Metal Content

Sodium, Potassium, Vanadium, Magnesium, Iron, Silica etc. are some of the metallic
constituents present in fuel oil. Of the above metals, sodium and vanadium are the
most troublesome metals causing high temperature corrosion in boiler super heater
tubes and gas turbine blades. Much of the sodium is removed from the crude oil in
the desalting operation, which is normally applied in the refinery and additional
sodium can be removed from the finished fuel oil by water washing and centrifuging.

Vanadium is found in certain crude oils and is largely concentrated in fuel oil
prepared from these crude. No economical means for removal of vanadium from the
residual fuel oil is available. However certain additives like magnesium are available
to minimize the effect of vanadium.

Asphaltene content and Carbon residue

Asphaltenes are high molecular weight asphaltic material and it requires more
residence time for complete combustion. Asphaltenes as finely divided coke may be
discharged from the stack. Residual fuel oils may contain as much as 4%
asphaltenes.

Petroleum fuels have a tendency to form carbonaceous deposits. Carbon residue
figures for residual fuel oils from 1 to 16% by weight. This property is totally
dependent on the type of crude, refining techniques and the blending operations in
refinery.

Fuels with high carbon residue and asphaltenes requires large combustion chamber
and hence while designing the boiler for such fuel the volumetric loading has to be of
the order of 2 lakhs Kcal/m3hr


OIL/GAS FIRING START UP LOGIC
MANUAL TRIP INTERLOCK CHECK
1.CHECK TRIP VALVES IN CLOSED POSITION
2 . CHECK WATER LEVEL IN DRUM
3. EMERGENCY PUSH BUTTON NOT OPERATED
CONTROL SUPPLY LAMP 4. CHECK FAN SUCTION DAMPER IN CLOSED
POSITION
5.CHECK FUEL PUMP/GAS TRAIN DELIVERY
VALVE IN CLOSED CONDITION
6. CHECK MANUAL ISOLATION VALVE IN START FD FAN
CONTROL POWER SUPPLY SELECTOR SWITCH POSITION.
IN GAS/OIL FIRING MODE

FAILED
DEENERGISE TR & PILOTVALVE

DEDUCT PILOT FLAME DEENERGISE TRANSFORMER
ENERGISE GAS/OIL SHUT OFF VALVE TO OPEN
YES AND VENT TO CLOSE

YES DEENERSISE PILOT GAS & RELESASE LOW FIRE POSITION
MAIN FLAME ESTABLISED
NO NO DEENERSISE PILOT GAS

CHECK
1.0PURGE COMPLETED 1.0 OIL/GAS MAIN SHUT OFF VALVE IN CLOSED POSITION
2.0ALL PURGE INTERLOCKS 2.0 RETURN OIL LINE SHUT OFF VALVE CLOSED POSI
ENERGISE IGNITION AGAIN CHECKED 3.0 AIR/ATOMISING STEAM LINE SHUT OFF VALVE CLSOED
TRANSFORMER & 3.0COMPUSTION AIR PR NOT LOW POSITION
PILOT GAS SHUTOFF VALVE 4.0 INSTRUMENT AIR PR NOT LOW 4.0 PILOT GAS/SCAVENGING LINE SHUT OFF VALVE IN CLOSED
5.0 COMBUSTION AIR DAMPER TO POSITION
LOW FIRE POSITION 5.0 FUEL GAS SHUT OFF VALVE I & II IN CLOSED POSITION
PRESS BURNER 6.0OIL/GAS AT REQUIRED PARAMETER PURGE 6.0 NO FLAME INSIDE FURNACE
START BUTTON 7.0 EMERGENCY PUSH BUTTON BUTTON ON 7.0 FUEL PUMP NOT RUNNING
NOT OPERATED 8.0 FURNACE PRESSURE NOT HIGH
8.0SCANNER COOLING AIR PR OK COMBUSTION AIR 9.0 DRUM LEVEL NOT HIGH HIGH & NOT LOW LOW
DAMPER TO LOW 10.0ALL TRIP PARAMETERS OK
AUTO GAS/OIL FIRING INTERLOCKS FIRE POSITION 11.0 FUEL GAS PRESSURE NOT HIGH & NOT LOW
PURGE COMPLETED PURGE IN PROGRESS LAMP ON


1.6 PULVERIZED FUEL BOILERS

TECHNOLOGY

When coal is powdered to micron size it can be conveyed easily by air in pipelines
and the pulverized coal behaves as if that of oil and hence the same can be easily
burnt in pulverized fuel burners. The heat release by the burners in very high and
un-burnt carbon is almost equal to zero. Hence efficiency achieved by pulverized
burners is much more than any type of coal combustion.

MECHANISM OF PULVERIZED FUEL BURNING

There are two systems of pulverized firing 1.0 direct firing 2.0 indirect firing.

In the direct firing system, raw coal from the storage area is loaded on a conveyor
and fed to a coal crusher. A second conveyor system loads coal into the coal
storage bunker located over the coal pulverization system. Coal via gravity feed is
delivered through a down spout pipe to the coal feeder. A coal shutoff gate is
provided prior to the coal feeder inlet to allow emptying the system down stream.
The coal feeder meters the coal to the crusher dryer located directly below the
feeder discharge. A primary air fan delivers a controlled mixture of hot and cold air
to the crusher dryer to drive moisture in the coal facilitating pulverization the primary
air and crushed coal mixture is then fed to the coal pulverizer located below the
crusher dryer discharge. Selection of pulverizer has to be analyzed critically, since it
is one of the important equipment where the wear and tear is more. For the soft
lignite Beter wheel is preferable and for hard lignite, coal like fuels heavy pulveriser
of ball and hammer mill is preferable. The coal is pulverized to a fine powder and
conveyed through coal pipes to the burners. Primary air is the coal pipe
transportation medium.

The indirect firing system utilizes basically the same coal flow path to the pulverizer.
After the classification of pulverized coal, it is delivered to a coal storage bin. When
needed to fire the boiler the pulverized coal is then conveyed to the burners by an
exhaust fan. This method requires very special provisions to minimize risk of fire or
explosion. Of the two systems, the direct firing is more common.
Neyveli lignite power corporation has pulverized boiler of direct firing system.

1.7 FLUIDIZED BED BOILERS
ATMOSPHERIC FLUIDIZED BED COMBUSTION

TECHNOLOGY

When air or gas is passed through an inert bed of solid particles such as sand
supported on a fine mesh or grid. The air initially will seek a path of least resistance
and pass upwards through the sand. With further increase in the velocity, the air
starts bubbling through the bed and particles attain a state of high turbulence. Under
such conditions bed assumes the appearance of a fluid and exhibits the properties
associated with a fluid and hence the name fluidized bed.


MECHANISM OF FLUIDIZED BED COMBUSTION

If the sand, in a fluidized state is heated to the ignition temperature of the fuel and
fuel is injected continuously into the bed, the fuel will burn rapidly and attains a
uniform temperature due to effective mixing. This , in short is fluidized bed
combustion.
While it is essential that the temperature of bed should be equal to the ignition
temperature of fuel and it should never be allowed to approach ash fusion
temperature (1050° to 1150°C ) to avoid melting of ash. This is achieved by
extraction of heat from the bed by conductive and convective heat transfer through
tubes immersed in the bed.

If the velocity is too low fluidization will not occur, and if the gas velocity becomes too
high, the particles will be entrained in the gas stream and lost. Hence to sustain
stable operation of the bed, it must be ensured that gas velocity is maintained
between minimum fluidization and particle entrainment velocity.

Advantages of FBC.

1.0 Considerable reduction in boiler size is possible due to high heat transfer rate
over a small heat transfer area immersed in the bed.

2.0 Low combustion temperature of the order of 800 to 950°C facilitates burning of
fuel with low ash fusion temperature. Prevents Nox formation, reduces high
temperature corrosion and erosion and minimize accumulation of harmful
deposits due to low volatilization of alkali components.

3.0 High sulphur coals can be burnt efficiently without generation of Sox by feeding
lime stone continuously with fuel.

4.0 The units can be designed to burn a variety of fuels including low grade coals
like floatation slimes and washery rejects.

5.0 High turbulence of the bed facilitates quick start up and shut down.

6.0 Full automation of start up and operation using simple reliable equipment is
possible.

7.0 Inherent high thermal storage characteristics can easily absorb fluctuation in fuel
feed rate.


ATMOSPHERIC CIRCULATING FLUIDIZED BED COMBUSTION

TECHNOLOGY

Atmospheric circulating fluidized bed (ACFB) boiler is a devise used to generate
steam by burning solid fuels in a furnace operated under a velocity exceeding the
terminal velocity of bed material. I.e., solid particles are transported through the
furnace and gets collected in the cyclone at the end of furnace and again recycled
into furnace by means of pressure difference between fluidized bed and return
particle.
MECHANISM OF CIRCULATING FLUIDIZED COMBUSTION

The mechanism is similar to AFBC. However in AFBC the fluidization velocity is just
to make the particles in suspended condition. In ACFB boiler, special combination of
velocity by primary air and secondary air, re-circulation rate, size of solids, and
geometry of furnace, give rise a special hydrodynamic condition known as fast bed.

Furnace below secondary air injection is characteristic by bubbling fluidized bed and
furnace above the secondary air injection is characteristic by Fast fluidized bed.
Most of the combustion and sulphur capture reaction takes place in the furnace
above secondary air level. This zone operates under fast fluidization. In CFB boiler
number of important features such as fuel flexibility, low Nox emission, high
combustion efficiency, effective lime stone utilization for sulphur capture and fewer
fuel feed points are mainly due to the result of this fast fluidization.

In fast fluidization heavier particles are drag down known as slip velocity between
gas and solid, formation and disintegration of particles agglomeration, excellent
mixing are major phenomenon of this regime.

CFB is suitable for
1.0 Capacity of the boiler is large to medium.
2.0 The boiler is required to fire a low grade fuel or highly fluctuating fuel quality.
3.0 Sox and Nox control is important.

PRESSURIZED FLUIDIZED BED COMBUSTION

The advantage of operating fluidized combustion at the elevated pressure ( about 20
bar) is, reduction in steam generator size can be achieved and make possible the
development of a coal fired combined cycle power plant. The development of
pressurized fluidized bed combustion is still in research stage only. With help of
pressurized hot gas coming out of the furnace is cleaned primarily by a cyclone like
CFBC boiler and the gas is expanded in a turbine and the exhaust gas from turbine
is further cooled by the heat exchanger. The aim behind the development of
pressurized fluidized bed are:

1.0 To develop steam generator of smaller size for the higher capacity.

2.0 To reduce the cost of generation of power per MW.

3.0 To develop turbines which make use of solid fuels such as coal, lignite etc.,


1.8 HEAT RECOVERY STEAM GENERATOR

In India, coal availability is 97% of the requirement and we are importing coal only for
the process requirement like baking coal for steel plant where high calorific coal is
required. Hence in post independence India coal fired boilers where flourished,
however due to the need of energy conservation and due to process parameter
requirements development of HRSG in recent periods is more. Moreover due to the
development of gas turbines with gaseous and liquid fuels, more GT are being
installed due to their lower gestation period and higher efficiency than Rankine cycle.

As explained earlier HRSG can be classified into two types, one is for maintaining
process parameter such as temperature and other is in the point of economic point
of view.

The process steam generator are generally referred by the term called waste heat
recovery boiler ( WHRB) where the gas contains heat in excess, this excess waste
heat has to be recovered or removed by any means so that the process parameter
can be maintained. ( e.g. Sulphuric acid plant, hydrogen plant, sponge iron plant,
Kiln exhaust etc.,)

The steam generator stands behind the gas turbine are usually referred as Heat
recovery steam generator.

The HRSG or WHRB the design greatly vary with respect to the size of the plant,
the gas flow, gas volumetric analysis, dust concentration and sulphur di oxide
concentration. In HRSG the gas quantity and inlet temperature is fixed and for
different load the variation of heat will not be proportional and hence at part loads the
heat absorbed at different zones will vary widely and hence for different loads the
performance of the HRSG to be done.


2.0 STEAM,GAS and AIR

2.1 INTRODUCTION

In steam generator water, steam, gas and air are the working fluids in this air and
gas have similar properties. Understanding the properties of gas and air are almost
one and the same. I have grouped steam and gas as one unit and water as a
separate unit just because understanding the behavior of steam and gas is more
important in design point of view where as knowledge of water is more important in
operational point of view.

2.2 DEFINITIONS FOR SOME COMMONLY USED TERMS
Heat
Heat is defined as the form of energy that is transferred across a boundary by virtue
of a temperature difference. The temperature difference is the potential and heat
transfer is the flux. In other words heat is the cause and temperature is the effect.

Energy
Energy of a body is its capacity to do work and is measured by the amount of the
work that it can perform.

Potential Energy( mgh = mass x gravitational force x datum level)
Potential energy of a body is the energy it possesses by virtue of its position or state
of strain.

Kinetic energy ( ½ mv² = ½ x mass x velocity²)
Kinetic energy of a body is the energy possessed by it on account of its motion.

Enthalpy
Enthalpy is the quantity of heat that must be added to the fluid at zero degree
centigrade to the desired temperature and pressure. Enthalpy is defined as heat
within or heat content of the fluid.

Entropy
The word entropy is derived from a Greek word called ‘tropee’ which means
transformation. The unit of entropy is Joules/kelvin.

Specific heat
Specific heat of a substance is defined as the amount of heat required to raise the
temperature of one kilogram of substance through one degree kelvin. All liquids and
solids have one specific heat. However gas have number of specific heats depends
on the condition with which it is heated.

Cp = f(T)


Specific heat at constant pressure.
Specific heat of a substance is defined as the amount of heat required at constant
pressure to raise the temperature of one kilogram of substance through one degree
kelvin.

Integral constant pressure specificheat
It is the average heat required to rise the temperature between two temperature
difference t1 and t2 i.e., Cp = ( H2 – H1)/(t2 –t1)

H = f(Cp/T)

Specific heat at constant volume.
Specific heat of a substance is defined as the amount of heat required at constant
volume to raise the temperature of one kilogram of substance through one degree
kelvin.

NTP and STP condition
It is customary to specify the gas or steam properties at NTP or STP condition,
NTP condition is at Normal temperature and pressure, i.e., the properties measured
at 0°C or 273.15 °K and pressure 1.01325 bar or 1.03 atm
STP condition is at Standard temperature and pressure i.e., the properties measured
at 25°C or 298.15°K and pressure 1.01325 bar or 1.03 atm.

Viscosity
Viscosity of a liquid is its property, due to the frictional resistance between the fluid
particles (cohesion between particles) or between fluid and the wall. Viscosity of
fluid controls the rate of flow.

Newton s Law of viscosity
The shear stress on a layer of a fluid is directly proportional to the rate of shear
strain. ( Velocity gradient )

τ α ν/l where τ is shear stress and ν is velocity , l is the distance or gap between
layers.
τ = µ ν/l where µ is the constant of proportionality and is known as absolute
viscosity or dynamic viscosity.

Kinematic viscosity is the ratio of absolute viscosity to density (µ/ρ)

Thermal conductivity
Thermal conductivity is the property of substance, that its ability to conduct heat and
expressed in W/mK.

Kilogram
Kilogram is the mass of one international prototype made of platinum iridium cylinder
preserved at the international bureau of weights and measures at paris.


Meter
Meter is the length between two transverse lines en-grooved in platinum iridium bar
at 0°C. or The meter is the length equal to 1650763.73 vacuum wave length of the
orange light. ( λ = 605.8 mm of the Krypton 86 discharge lamp)

Second
Second is the duration of 9192631770 periods of the radiation corresponding to the
transition between two specified energy level of the Caesium –133 atom. Or
1/86400th part of mean solar day.

Specific volume
Specific volume is the volume occupied per kg of steam or water or fluid.
Specific volume is the inverse of density.

For heat and mass transfer calculations, we have to know the above properties.

The properties where mainly depends on the temperature for gases and temperature
and pressure for steam. The required equation for derivation is given at appropriate
places.

For gaseous fuel,

Cp /R = f(T)

R = Cp – Cv

Cv = Cp - 1
R R

Specific enthalpy wrt NTP,
T
H ‘ = 1/T  Cp dT ( enthalpy with reference to 0°C)
RT  R
Tn

Specific enthalpy wrt STP
T
H* ‘ = 1/T  Cp dT + Hs ( enthalpy with reference to 25°C)
RT  R RT
Ts
Specific entropy,
T
S ‘ = So  Cp dT - ln(P/Pn) ( entropy with reference to 0°C)
R R R
Tn


Specific free enthalpy

G = H -S
RT RT R

The temperature dependent specific heat (Cp) can be represented by an equation of
4 th degree polynomial as shown below

Cp = a1 + a2T + a3T² + a4 T3 + a5T4 (for temperature from 273K to 1000K)
R

Cp = a9 + a10T + a11T² + a12 T3 + a13T4 (for temperature from 1001K to 5000K)
R
Integrating, and adding constant of integration we get

H = a1 + a2T + a3T² + a4T3 + a5 T4 + a8/T (for temperature from 273K to 1000K
RT 2 3 4 5

H* = a1 + a2T + a3T² + a4T3 + a5T4 + a6/T (for temperature from 273K to 1000K
RT 2 3 4 5

S = a1 ln T + a2T + a3T² + a4T3 + a5T4 + a7 – ln(P/Pn)
R 2 3 4

G = a1(1- ln T) - a2T - a3T² - a4T3 - a5T4 + a6 -a7 + ln(P/Pn)
RT 2 6 12 20 T

Dynamic viscosity , thermal conductivity and prandtl number

Dynamic viscosity, thermal conductivity and prandtl number of a flue gas can be fine
easily with help of the properties of nitrogen and following constants.

Var Specific Dynamic Thermal Prandtl number
Heat Viscosity conductivity
Kj/kgK µPa.S W/mK
a1 0.8554535 -0.9124458E 1 -0.1083113E-1 0.492851
b1 0.2036005E-3 0.4564993E-2 0.5596822E-4 -0.1230046E-2
c1 0.4583082E-6 0.2198889E-4 0.7413502E-7 0.1662398E-5
d1 -0.279808E-9 -0.1891235E-7 -0.5901395E-10 -0.1052753E-8
e1 0.5634413E-13 0.5138895E-11 0.1961745E-13 0.2443111E-12

a2 -0.1002311 -0.4267768E1 -0.8035817E-2 -0.8820652E-2
b2 0.7661864E-3 0.4074274E-3 0.110672E-04 0.1855309E-3
c2 -0.9259622E-6 -0.5125357E-5 -0.8397255E-8 -0.3838084E-6
d2 0.5293496E-9 0.738556E-8 0.1130229E-10 0.3256168E-9
e2 -0.109357E-12 -0.343972E-11 -0.5731264E-14 -0.1005757E-12


Dynamic viscosity,

µg = µn + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gas
P1 = a1 + b1T + c 1T² + d1T3 + e1T4
P2 = a2 + b2T + c 2T² + d2T3 + e2T4 where T is temperature in °C

Thermal conductivity,

kg = kn + P1 XH2O + P2 XCO2

P1 = a1 + b1T + c 1T² + d1T3 + e1T4

Prandtl number,

Prg = Prn + P1 XH2O + P2 XCO2

P1 = a1 + b1T + c 1T² + d1T3 + e1T4
Pra = a + bT + cT² + dT3 + eT4

Specific heat,

Cpg = Cpn + P1 XH2O + P2 XCO2

P1 = a1 + b1T + c1T² + d1T3 + e1T4
P2 = a2 + b2T + c2T² + d2T3 + e2T4 where T is temperature in °C

Where 0 ≤XH2O ≤ 0.3 ,0 ≤ XCO2 ≤0.2 , 0 ≤ T ≤ 1200°C

Dynamic viscosity, thermal conductivity and Prandtl number of NITROGEN

Dynamic viscosity Thermal conductivity Prandtl number
µ Pa.s W/mK
a 0.1714237E02 0.2498583E-1 0.6901183
b 0.4636040E-01 0.6535367E-4 0.2417094E-05
c -0.2745836E-4 -0.7690843E-8 0.2771383E-7
d 0.1811235E-7 -0.1924248E-11 -0.3534575E-10
e -0.674497E-11 0.160998E-14 0.1717930E-13
f 0.1027747E-14 -0.2864430E-18 -0.2989654E-17


µn = a + bT + cT² + dT3 + eT4 + fT5
Kn = a + bT + cT² + dT3 + eT4 + fT5
Prn = a + bT + cT² + dT3 + eT4 + fT5
Cpn = a + bT + cT² + dT3 + eT4 + fT5 (for temp.273 K to 1000K)

And Cpn = a1 + b1T + c1T² + d1T3 + e1T4 + f1T5 (for temp. 1001K to 5000K)

273 K to 1000K 1001K to 5000K

a 0.3679321E1 ‘a1 0.2852903E1
b -0.1313559E-2 b1 0.1580411E-2
c 0.2615196E-5 c1 -0.6189378E-6
d -0.9629654E-9 d1 0.1119450E-9
e -0.9928002E-13 e1 -0.7607378E-14
f -0.9723991E3 f1 -0.8019835E3

2.3 STEAM

We can see in day to day life the process of boiling water to make steam. Steam is
water in the vapour or gaseous state. It is in visible, odorless, non-poisonous and
relatively non corrosive to boiler metals. Steam is uniquely adapted by its
advantageous properties for use in industrial process heating and power cycle.
Thermodynamically boiling is the result of heat addition to the water in a constant
pressure and constant temperature process. The heat which must be supplied to
change water into steam without raising its temperature is called the heat of
evaporation or vaporization and the boiling point of a liquid may be defined as the
temperature at which its vapour pressure(pressure exerted due to the vapour of the
liquid) is equal to the total pressure above its free surface. In other words
temperature at which the partial pressure of vapour increases to make total pressure
above the liquid surface. This temperature is also known as the saturation
temperature.

EVAPORATION

Liquid exposed to air evaporate or vapourize. Evaporation is the process takes
place at the surface exposed to atmosphere. If there is any increase in ambient
temperature or increase of the liquid temperature evaporation rate becomes
increased. The reduction in pressure above the liquid surfaces accelerate the
evaporation rate. Evaporation will be there at all temperature and pressure,
unsaturated surrounding environment also one of the factor increases the
evaporation rate.

BOILING

Boiling is the phenomenon takes place at boiling point of the liquid. Boiling takes
place throughout the liquid column. A liquid will boil, when it’s saturated vapour
pressure exceeds the surrounding environment pressure acted upon the liquid.
Hence boiling point of a liquid will change depends on the pressure exerted by the
environment over the surface.


CONDENSATION

Condensation is the change in phase of vapour phase to it’s liquid phase. When
water vapour or steam comes in contact with cooler surfaces, it gives up the heat
and condenses to water. The heat released while changing from vapour phase to
liquid phase is called heat of condensation. In factories the steam released out of
the main steam line or process vents where we can see a remarkable phenomenon
of indication of dryness of steam. If the steam is dry, we can not visualize the steam
coming out of the vent but after some distance we can see a white cloud. This is
due to the condensation of steam which composed of small particles of water formed
when steam cooled in cooler atmosphere. In other case if the steam is wet, the
white smoke cloud is directly released from the vents.

2.4 FUEL

Combustion

Combustion or burning, is a rapid combination of oxygen with a fuel resulting in
release of heat. The oxygen comes from the air, which is about 21% oxygen and
78% nitrogen by volume.

Most fuels contain carbon, hydrogen, and sometimes sulphur as the basic
composition of combustion materials. These three constituents’ reacts with oxygen
to produce carbon-di-oxide, water vapour, suphur di oxides gases respectively and
heat.

Carbon, hydrogen and sulphur are found exists in direct form in most of the solid and
liquid fuels and in gaseous fuels the combustion matter is found as
hydrocarbons(combination of hydrogen and carbon). When these burn, the final
products are carbon di oxide and water vapour unless there is a shortage of oxygen,
in which case the products may contain carbon mono oxide, unburnt hydrocarbons,
and free carbon.

Heat value of fuel

Quantities of heat are measured in BTU, kiloCalories, or joules. A BTU is the
quantity of heat required to raise the temperature of one pound of water one degree
fahrenheit. A kilocalorie is the quantity of heat needed to raise one kilogram of water
one degree celsius.

Experimental measurements have been made to determine the heat released by
perfect combustion of various fuels. The heat value is usually determined by
calorimeters. When a perfect mixture of a fuel and air originally at 15.6°C is ignited
and then cooled to 15.6°C the total heat released is termed the higher heating value
or Gross calorific value. There is also one more term called lower heating value or
the net calorific value it is the quantity of heat equal to gross calorific value minus the
heat absorbed by the latent heat of water moisture( inclusive of moisture generated
due to combustion of hydrogen present in the fuel) at 25°C.


Dulong’s formula is used to find Calorific value of the fuel

HHV(kj/kg) =338.21C% +1442.43(H-O/8)% + 94.18S%

Relation between HHV and LHV
LHV = HHV – (%H2O + %H2x8.94)χ
Where χ is the latent heat of water vapour at reference temperature 25°C
=583.2 kcal/kg

Proximate Analysis

The general procedure for the analysis relating to proximate analysis is describe
below as per IS 1350(partI). For full details, the original standard may be referred to

i) Moisture
The moisture in the coal is determined by drying the known weight of the coal at
108°C±2°C

ii) Volatile matter
The method for the determination of VM consists of heating a weighted quantity of
dried sample of coal at a temperature of 900°±10°C. for a period of seven minutes.
Oxidation has to be avoided as far as possible. VM is the loss in weight less by that
due to moisture. VM is the portion of the coal which, when heated in the absense of
air under prescribed conditions, is liberated as gases and vapour.

iii) Ash
In this determination, the coal sample is heated in air up to to 500°C for minutes from
500 to 815°C for a further 30 to 60 minutes and maintained at this temperature until
the sample weight becomes constant.

iv) Fixed carbon
Fixed carbon is determined by deducting the moisture. VM and ash from 100

Ultimate analysis

The ultimate analysis of fuel gives the constituent elements namely carbon,
hydrogen,nitrogen, sulphur , hydrocarbons, nitrogen etc., For the ultimate analysis
of the coal sample is burnt in a current of oxygen. As a result the carbon, hydrogen,
sulphur oxidized to water, carbon di oxide and sulphur di oxide respectively. These
constituent are absorbed solvents to estimate the percentage of C,H2,S,N etc.,

The classification of Indian coal on the basis of proximate analysis.
S.n Description Grade Specification
1 Non coking coal, produced A GCV exceeding 6200kcal/kg
in all states other than Assam B GCV exceeding 5600Kcal/kg but
Andhrapradesh,Meghalaya, not exceeding 6200Kcal/kg
Arunachalpradesh and Nagland C GCV exceeding 4940kcal/kg
not exceeding 5600Kcal/kg


D GCV exceeding 4200kcal/kg
E GCV exceeding 3360kcal/kg

F GCV exceeding 2400kcal/kg
G GCV exceeding 1300kcal/kg

2 Non coking coal, produced
Assam,Andhrapradesh,Meghalaya, Not graded
Arunachalpradesh and Nagland

3. Coking coal Steel GrI Ash content not exceeding 15%
Steel GrII Ash content 15% to 18%
Washery GrI Ash content 18% to 21%
Washery GrII Ash content 21% to 24%
Washery GrIII Ash content 24% to 28%

2.5 GAS and AIR
IDEAL GAS OR PERFECT GAS

At low pressure and high temperature, all gases have been found to obey three
simple laws. These laws relate the volume of gas to the pressure and temperature.
All gases, which obey these laws, are called ideal gases or perfect gases. These
laws are called ideal gas laws. These laws are applicable to gases, which do not
undergo changes in chemical complexity, when the temperature or pressure is
varied. I.e., in other words laws applicable to gases which do not undergo any
chemical reaction when subject to change in pressure or temperature.

GAS LAWS

Boyle’s law
Boyle’s law states that the pressure is inversely proportional to volume and the
product of pressure and volume is constant
PV =C
Charles law-I
Charles law states that at constant pressure, volume is directly proportional to
temperature.
V/T = C

Charles law-II
Charles law states that at constant volume, pressure is directly proportional to
temperature.
P/T = C


Absolute scale of temperature

This scale of temperature is based on Charles law. According to Charles law at
constant pressure, volume of given mass changes by 1/273 of its volume at 0°C for
every rise or fall in temperature by 1°C. if the volume of the gas at 0°C is Vo and its
volume at t°C,

Vt = V o + Vo x t = Vo (1 + t/273)
273

If t = -273°C, then volume is zero, the hypothetical temperature of –273°C at which
gas will have zero volume is known as absolute temperature or 0°K.

Avagadra s Law

Avagadra’ s law state that the volume occupied by any gas at normal temperature
and pressure is 22.41383 x 10-3 m3 per mol of gas. I.e., volume occupied by a kg mol
of gas is 22.41383 m3/kg mol.

GAS EQUATION

From Boyle’s law PV = nRoT

Where, Ro is UNIVERSAL GAS CONSTANT

n = m/M = Weight of gas in kg at NTP
Molecular weight of the gas in kg

At normal temperature and pressure

Pressure = 1.01325 x 105 N/m²
Temperature = 273 K
Volume = 22.41383 x 10-3 m3
n = 1 mole

Ro= PV/nT = 1.01325 x 105 x22.41383 x10-3/(1 x273) = 8.314 Nm mol-1 K-1

= 8.314 joules /mol K

Gas constant R = Universal gas constant (Ro) / molecular weight (M).

Daltan s law

At a constant temperature, the total pressure exerted by a mixture of non- reacting
gases is equal to the sum of the partial pressure of each component gases of the
mixture. Thus the total pressure P of a mixture of r gases may be represented
mathematically as


r
Pt = Σ pI where pi is the partial pressure of each components gas of the mixture.
i =1

If P and the molar composition (% volume) of the mixture are known pi can be
calculated using the expression pi = xi P

2.6 SOME COMMONLY USED DIMENSIONLESS NUMBERS AND
THEIR SIGNIFICANCE

NUMBER FORMULA SYMBOL DEFINITION & SIGNIFICANCE

Nusselt hd/k Nu Radio of temperature gradients by
conduction and convection at the
surface
-used for convection heat transfer
coefficient determination

Reynolds ρvd/µ Re Inertia force/viscous force
- used for forced convection and
friction factor

Prandtl Cpµ/k Pr Molecular diffusivity of momentum
Molecular diffusivity of heat

Grashof ρ²d3 gß∆T/µ² Gr Buoyancy force x Inertia force
Viscous force x viscous force
- used for natural convection

Biot hd/ks Bi Internal conduction resistance
Surface convection resistance
- used for fin temperature estimation

Peclet vdρCp/k Pe=RePr Heat transfer by convection
Heat transfer by conduction
Stanton h/Cpρv St=Nu/Pe Wall heat transfer rate
Heat transfer by convection

Euler ∆P/ρv² Eu Pressure force/Inertia force
- used to find pressure drop

Froude v²/gl Fr Inertia force/gravity force


Where v is velocity
‘ d is characteristic dimension
Cp is specific heat
ρ is density
g is acceleration due to gravity
h is convection heat transfer coefficient
µ is dynamic viscosity
ß is volumetric expansion coefficient
T is temperature
P is pressure

Ex.01. Estimate the air and flue gas produced per kg of the following coal analysis.
Ultimate analysis: Carbon = 39.9%, Hydrogen = 2.48% , Sulphur = 0.38 %, Nitrogen
= 0.67%, Oxygen = 6.76 %, Moisture =8% and Ash = 42%. The analysis is based
on weight basis. Consider 4% carbon loss in combustion of AFBC system.

AIR REQUIREMENT CALCULATION

Amount of oxygen required for burning coal

C + O2 à CO2 + heat

12 kg of carbon react with 32 kg of oxygen to produce 44 kg of carbon di oxide. I.e.,
one kg of carbon required 32/12 = 2.666 kg of oxygen and produce 44/12 = 3.666kg
of carbon dioxide.

0.399kg of carbon in coal require = 0.39x2.666 = 1.064 kg of oxygen

H2 + 1/2O2 à H2O + heat

2 kg of hydrogen react with 16 kg of oxygen to produce 18 kg of moisture. I.e., one
kg of hydrogen requires 16/2 = 8 kg of oxygen and produce 18/2 = 9 kg of moisture.

0.0248 kg of hydrogen in coal requires = 0.0248x8 = 0.1984 kg of oxygen

S + O2 à SO2 + heat

32 kg of sulphur require 32 kg of oxygen to produce 64 kg of sulphur di oxide. I.e.,
one kg of sulphur require one kg of oxygen and produce 64/32 = 2 kg of sulphur di
oxide.

0.0038 kg of sulphur in coal require =0.0038 x 1 = 0.0038 kg

the other composition like nitrogen, argon(if present) is inert gas and it will not react
with oxygen. Moisture is in saturated form and it does not require oxygen.

The total oxygen required = 1.064 + 0.1984 +0.0038 = 1.2662 kg


The oxygen present in fuel = 0.0676 kg

Net oxygen required = 1.2662 – 0.0676 = 1.1986 kg

Air contains 23.15 % oxygen by weight and hence the air required for 1.1986 kg of
oxygen is = 1.1986/0.2315 = 5.176 kg of dry air.

Amount of wet air required considering 60% Relative humidity = 5.176 x 1.013 =
5.244 kg.

Coal requires 20% excess air for combustion in AFBC system hence wet air required
for burning per kg of fuel = 5.244 x 1.2 = 6.292 kg.

FLUE GAS GENERATION ESTIMATION

Carbon di oxide produced = (0.399 – 0.0188) x 3.666 = 1.3915 kg
Moisture produced = (0.0248 x 9 ) = 0.2232 kg.
Moisture in fuel = 0.08 kg.
Moisture in air = 0.013 x 6.212 = 0.0807 kg.

Total moisture in flue gas = 0.3839 kg

Sulphur di oxide produced = 0.0038 x 2 = 0.0076 kg.

Nitrogen in air = 6.212 x 0.7685 = 4.7739 kg.
Nitrogen in fuel = 0.0067 kg.

Total nitrogen in the fuel = 4.7739 + 0.0067 = 4.7806 kg.

Excess oxygen in gas = (6.212 – 5.176)x0.2315 = 0.2398 kg.

Total Flue gas produced

Per kg of fuel = 1.391 + 0.3839 + 0.0076 + 4.7806 + 0.2398 = 6.803 kg.

Ex.02 Find the weight of water present in atmospheric air at 60% relative humidity
and temperature 40°C.

For 40°C, the saturation pressure of water is = 0.075226 atm (from steam tables)

At 60% RH the partial pressure of water vapour is 0.6 x 0.075226
=0.045135 atm

Weight of moisture present in air = 0.622 x Pw/(1.035 –Pw)

= 0.622 x 0.045135
(1.035 – 0.045135)

= 0.02836 kg/kg.


Ex03. Estimate the efficiency of a boiler firing with coal as a fuel having GCV of
3200 kcal/kg. Furnace is Fluidized bed boiler. Apply ASME PTC 4.1 indirect method
to calculate the efficiency. Flue gas temperature leaving the boiler is140°C and
ambient air temperature is 40°C. Ash content of the fuel is 42.3% and 20% of total
ash is collected in bed and 80% ash is carried in fly ash. As per lab report the loss on
ignition of ash samples collected in bed zone and fly ash zone is 0.1% by weight and
4.4%by weight. The boiler is operating at 20% Excess air and the dry kg/kg of gas
produced =5.91 and dry kg/kg of air required = 5.696. The moisture and hydrogen
present in the fuel is 6% and 2.7% respectively.

Basically following are the losses present in boiler,
1.0 Unburnt carbon loss
2.0 Sensible heat loss through ash
3.0 Moisture loss due to air
4.0 Moisture and combustion of hydrogen in fuel
5.0 Dry flue gas loss
6.0 Radiation loss.

Unburnt Carbon loss =4%

Sensible heat loss in ash,

Flyash = %Flyash x% of ash qty x sp.heat (Tgo – Tamb) x100/GCV

= 0.8 x 0.423 x0.22(140-40) 100/3200
=0.233%

Bed ash

= 0.2x0.423x0.22(900-40)100/3200
=0.5%

Sensible heat loss due to ash = 0.233+ 0.5 =0.733%

Heat loss due to moisture in air

= kg/kg of moist in air x kg/kg of dry air( Enthalpy of steam at Tgo in 0.013ata –
Enthalpy of steam at Tamb in 0.013 ata)

= 0.013 x 5.696 x( 660.33–615.25)100/3200
=0.1043%

Note: The above implies that the water vapour at ambient temperature at partial
pressure exists in steam form and gets superheated at 140°C

Heat loss due to moisture in fuel and combustion of hydrogen,

=(%of moisture in fuel + % of hydrogen x8.94)(Enthalpy of steam –Tamb)100/3200

= (0.06 + 0.027x8.94)(658.37 –40)100/3200


= 5.824%

Note: The above implies that the water moisture present in fuel is in liquid form,
during combustion it will absorb latent heat and superheat from combustion. The
hydrogen present in the fuel react with oxygen to form water. From combustion
equation of hydrogen it is found that 1 kg of hydrogen form 8.94 kg of water.

Dry flue gas loss,

= kg/kg of dry flue gas x (Enthalpy of gas at Tgo –Air enthalpy at Tamb)x100/3200

=Kg/kg of dry flue gas x Spheat (Tgo –Tamb)100/3200

=5.91 x 0.24 x(140 –40)100/3200 = 4.433%

Radiation loss,

From ABMA Chart the loss is estimated as =0.5%

Note: In the indirect method Blow down losses will not be considered into account. It
is assumed the boiler is operated under zero present blow down.

Ex07 Estimate the FD and ID fan flow and power required for a bagasse fired
dumping grate boiler, whose bagasse consumption at 100% MCR capacity is 31000
kg/hr and the boiler is operating at 35% excess air. The fuel air requirement is 3.909
kg/kg of fuel and gas generation is 4.873 kg/kg.

FD fan
Total air requirement = 31000 x 3.909 = 121179 kg/hr.

Fan design flow with 15% margin = 121179 x 1.15/(3600 x1.128)

= 34.31 m3/sec
FD fan head

Pressure head required for air flow sections like airheater, air ducts and grate are to
be calculated. Now in most of the practical applications the pressure drop works out
to be 165 mm WC and the same can be assumed for this calculation.

FD fan head with margin = 165 x 1.2 = 200mmWc

FD fan power required.

= flow x head/102 x efficiency

= 34. 31 x 200 / (102 x 0.8)


= 84.09 KW

Motor selected = 84.09 x 1.1 = 92.5 KW (next nearest motor standard is 110 KW)

ID fan
Total gas produced = 31000 x 4.873 = 151063 kg/hr.

Fan design flow with 25% margin = 151063 x 1.25 x (273 +140)/(3600 x1.295x273)

= 61.27 m3/sec
ID fan head
Pressure head required for gas flow sections like Furnace, Bank, Economiser, air
heater, gas ducts and dust collectors are to be calculated. Now in most of the
practical applications the pressure drop works out to be 230 mm WC and the same
can be assumed for this calculation.

ID fan head with margin = 230 x 1.3 = 300mmWc

ID fan power required.

= flow x head/102 x efficiency

= 61.27 x 300 / (102 x 0.8)

= 225 KW

Motor selected = 225 x 1.1 = 247.7 KW (next nearest motor standard is 250 KW)

Table showing percentage margin on flow and head required for different boiler
application.
S.N Description Grate type AFBC CFBC OIL
fired
1 FD Fan Flow 15% 25% 25% 15%
Head 20% 25% 25% 20%
2 ID Fan Flow 25% 25% 25% 20%
Head 30% 25% 25% 20%
3 SA/PA/OF fan Flow 10% 25% 25% Not
Head 15% 25% 25% applicable

3.0 FURNACE

3.1 INTRODUCTION:

The design of furnace is considered as the vital part in the boiler. The furnace is the
zone experiencing a high temperature in boiler. The performance of the furnace
reflects or has an impact over other parts behind it such as super heater, evaporator,
and air heaters. For instant, how the furnace design affects super heater can be


illustrated with following. If furnace outlet temperature (FOT) is high, then the next
zone is super heater it gets high amount of heat input naturally the metal
temperature is high and the steam temperature also increased, which in turn reflects
in the performance and cost of material. On the other hand if the furnace is over
sized the FOT will be lesser, to get the required steam temperature the super heater
heat transfer area to be increased. If the heat transfer area is increased it calls for
larger space and cost wise it becomes uneconomical.

3.2 EFFECT OF FUEL ON FURNACE DESIGN:

The type of fuel, form of fuel, heat content and the properties of the fuel such as ash
fusion temperature are also form as constraint over the furnace design. The type of
fuel whether solid or liquid or gas and quantity decides how efficiently we can burn.
Whether we can have a burner (for liquid & gases), solids bubbling bed or dumping
or travelling grate. When the fuel is some thing like bagasse (fibrous and long strand
structure) it can be burnt well in dumping or travelling grate.

A gaseous fuel offers fewer problems since it is clean. Fuel oil brings its own
problems like high or low temperature corrosion and additives have to be used. For
coal ash fusion is the problem, since ash slag down deposits on the wall hindering
heat transfer to steam water mixture. Depends on property of coal, whether it can be
crushable to powdered form, pulverized firing or bubbling bed or cyclone furnace can
be decided.

When we go for oil or gas firing, we can have higher heat flux in the furnace because
of the higher emissivity of oil flame and relative cleanliness of walls compared to coal
firing. There by size of furnace will be smaller for oil or gas fired steam generators.
The volume of the furnace for oil fired boilers will be 60 to 65 percentage of
pulverized fuel firing. However, if a furnace designed for both coal and oil it is
normally designed for coal and performance for oil firing in that furnace will be
carried out. When a furnace designed for coal operated with oil, the higher furnace
absorption results in a lower furnace outlet temperature. Lower FOT means super
heater pick up in super heater will be less and steam outlet temperature will be less.
This is avoided by several techniques out of which, when oil is fired FOT will be
increased by gas recirculation, otherwise when coal is fired FOT will be reduced by
some means of bed absorption (This is used in FLUIDISED BED COMBUSTION
techniques). Furnace size also governed by length of flame in gas or oil fired boiler
since the flame should not impinge on the water walls and cause overheating.
Likewise in coal fired boilers flue gas velocity should be optimized to prevent higher
rate of erosion due to carry over particles in flue gas. Normally a flue gas velocity of
6 to 8 meters per sec was allowed for coal fired boilers and 12 to 15 meters per sec
was allowed for bagasse fired boilers.

3.3 FORCED OR NATURAL CIRCULATION:

Water wall is receiving radiation from flames and are exposed to high heat flux and
there is a possibility of over heating. The boiling is the phenomenon, which governs
the rate of heat transfer from combustion to steam water mixture inside the tube. In
boiling when bubbles formed at tube wall hinders the heat transfer which cause


tubes over heating and tube failure. This sort of boiling occurs at nucleate boiling
stage. Therefore proper circulation must be ensured to cool all tube. Circulation
ratio (CR) is the ratio between mass of water circulated inside the boiler to rate of
steam generation. Hence CR is also directly related to dryness fraction of steam by
the expression CR = 1/x. which implies in one circulation 1/CR quantity of dry steam
was produced. Circulation number will be higher when the difference in density
between steam and water is more (i.e.) due to higher difference in density; steam
water mixture velocity will be more thereby overheating will be prevented. If the
proper circulation is not there, circulation in the boiler circuit is effected by means of
external agency (normally a circulation pump will be used). This type of circulation is
called Forced or controlled circulation.

3.4 HEATFLUX TO FURNACE WALLS:

Boiling phenomenon can be represented by a log-log plot of heat flux Vs surface
temp-bulk temperature as shown

Q max.

H
E
A
T

F
L
U
X

A B C D

SURFACE TEMP

The different regimes of boiling indicated by the letters A, B, C, D. Absence of
bubble formation and the influence of natural convection on the heat transfer process
is predominant in the region A (pool boiling). Formation of vapour bubbles at the
nuclei with resulting agitation of liquid by the bubble characteristics at the region B
(nucleate boiling). The most important perhaps the critical region with respect to the
heat flux is C. In this region the unstable film boiling manifests with an eventual
transition to a continuous vapour film. In the final region D film boiling becomes
stabilized. This phenomenon of stable film boiling is referred as “ LEINDENFROST
EFFECT”


In the regime of boiling the maximum wall heat flux is observed in region C. Many
experimentalists refer this state of maximum wall heat flux as “BURN OUT FLUX’.
The reason being when the wall is heated electrically, the heating element frequently
burn out when the wall heat flux reaches Q maximum. Hence the design engineers
should have an idea of average heat flux to the tubes, how they vary around
periphery and fin tip temperature in case of membrane wall construction. Calculation
of fin temperature was discussed in latter part of this chapter.

3.5 POINTS TO BE NOTED WHILE DESIGNING FURNACE

1.0 Optimal heat transfer area to reduce the gas temperature to a temperature
required from the point of super heater.

2.0 Sufficient height to ensure adequate circulation in the water walls

3.0 Fins in the wall to be properly cooled, accordingly the pitch of water wall to be
selected.

4.0 Flames should not impinge on water wall

5.0 Proper provision should be there to remove ash generated.

6.0 Optimal furnace outlet temperature.

7.0 Sufficient residence time inside the furnace for complete combustion

3.6 CLASSIFICATION OF FURNACE

i) According to ash removal

a) Dry bottom: It consists of water walls or refractory walls enclosing the
flame. Ash shall be removed dry from bottom. The fuel used has low heat
flux and high ash fusion temperature.

b) Wet bottom: Ash removed from bottom is of molten form. The fuel having
high heat flux low ash fusion temperature is used. The flue gas generated
here or clean and free from fly ash and hence erosion, fouling problems are
minimized.

ii) According to Type of combustion

a)Conventional firing
1) Travelling grate
2) Dumping grate
3) Pulsating grate
4) Step grate
5) Fixed grate


b)Bubbling Fluidized bed combustion

c)Circulated Fluidized bed combustion

d)Pulverized fuel combustion

e) Cyclone furnace.

iii) According to draft system

a) Balance draft: In balanced draft both Forced draft and Induced draft fans
are used so to maintain vacuum or zero pressure in furnace. There is no
leakage of combustion product in the atmosphere. In the atmospheric
pressure air leaks into furnace. This type of draft system is widely adapted in
industries.

b) Forced draft or pressurized draft: Considering economic aspect in oil or gas
fired boilers Forced draft fan alone used. The furnace pressure will be of the
order of 100 to 150 mm a water column. The furnace has to be designed to
without leakage. Otherwise combustion product will leak into atmosphere.

c) Induced draft: Induced draft fan is used for sucking the flue gas generated.
The furnace pressure will be maintained below atmospheric pressure.

d) Natural draft: There is no draft fan will be provided for this system. Natural
draft generated due to chimney itself used for the boiler draft. Very small
capacity steam generators will be of this type.

3.7 MODES OF HEAT TRANSFER

In general heat transfer from higher temperature to lower temperature is carried out
in three modes.

1.0 Conduction
2.0 Convection
3.0 Radiation

Conduction
Conduction refers to the transfer of heat between two bodies or two parts of the
same body through molecules, which are more or less stationary. Fourier law of
heat conduction states rate of heat flux is linearly proportional to temperature
gradient.

Q = --K dt/dx


Where,
Q rate of heat flux watts per sq.meter
K thermal conductivity (property of material)W/m°k
dt/dx temperature gradient in x –direction
Negative sign indicates heat flows from high temperature to low temperature.

Heat transfer by conduction in plate and cylinder

Plate Q = k.A. (t1 - t2) watts

X

Cylinder Q =k.(A2- A1).(t1- t2)

(r2- r1) ln(A2/A1)
where,
A area of plate
A1 outside cylinder surface
A2 inside cylinder surface
‘r cylinder radius
‘t temperature of surfaces

Convection
Convection is a process involving mass movement of fluids. When a temperature
difference produces a density difference which results in a mass movement.
Newton s law of cooling governs convection. In convection there is always a film
immediately adjacent to wall where temperature varies.

- kf A (tf - tw)
Q =

Where,
is film thickness
kf thermal conductivity of film
h = kf / heat transfer coefficient (kcal/ sq.m hr °C or W/sq.m °C)

Radiation

All bodies radiate heat. This phenomenon is identical to emission of light. Radiation
requires no medium between two bodies, irrespective of temperature the radiation
heat transfer takes place between each other. However the cooler body will receive
more heat then hot body. The rate at which energy is radiated by a black body at
temperature T( °K) is given by Stefan Boltzmann law.

4
Q= AT


Q rate of energy radiation in Watts

A Surface area radiating heat sq.m
–8 4
Stefan boltzmann constant = 5.67 x 10 Watt/sq.m K
–8 4
4.88 x 10 Kcal/sq.m hr K

3.8 HEAT TRANSFER IN FURNACE

Furnace heat transfer is a complex phenomenon, which can not be calculated by a
single formula. It is the combination of above said three modes of heat transfer.
However in a boiler furnace heat transfer is predominantly due to radiation, partly
due to luminous part of the flame and partly due to non-luminous gases. Overall
heat transfer coefficient in furnace is governed by three T’s temperature, turbulence
and time and calculated by two parts.

Hc - heat transfer coefficient by convection
Hr - heat transfer coefficient by radiation.

HEAT TRANSFER COEFFICIENT BY CONVECTION (Hc)

Heat transfer by convection may carry out in turbulent or laminar flow of the fluid. In
forced convection turbulence or laminar flow depends on mean velocity,
characteristic length L, density and viscosity. These variables are grouped together
in a dimensionless parameter called Reynolds number. Reynolds number is the
ratio between inertia force to viscous force.

Reynolds number = (mass x acceleration)/(shear stress x cross sectional area)

Mass = volume x density
Acceleration = velocity / time
Volume = cross sectional area x velocity
Shear stress = dynamic viscosity x velocity gradient(v / l)

Re = density x velocity x characteristic length
Dynamic viscosity.

When Re > 2100 then flow is turbulence
< 2100 then flow is laminar. In practical case the flow is most often
turbulent only.
In free convection turbulence or laminar flow depends on the buoyancy force and
temperature difference, coefficient of volume of expansion. These variables are
grouped to form dimensionless numbers called Grashoff number and Prandl number.
Laminar or turbulence is identified with product of Grashoff number and prandl
number
9
When, Gr.Pr < 10 flow is laminar


9
Gr.Pr > 10 flow is turbulent.

DIMENSIONAL ANALYSIS FOR HEAT TRANSFER COEFFICIENT

The heat transfer coefficient may be evaluated from correlation developed by
dimensional analysis. In this method all the variables related to the phenomenon is
grouped by experience with help of basic fundamental units length, mass, time and
temperature.

The final equation arrived for

FORCED CONVECTION

h = f(L,U, ρ,µ,k,Cp) ,
where,
L characteristic length (meters)
U velocity (meters/second)
ρdensity ( kilogram/ cub.meter)
µ dynamic viscosity(kilogram/meter. Hour)
k thermal conductivity (watts/meter°kelvin)
Cp specific heat(watt/kilogram.°kelvin)
a b c d e f
Let h = B L U ρ µ k Cp , where B,a,b,c,d,e,f are constants
Expressing the variables in terms of their dimensions
-3 -1 a -1 b -3 c -1 -1 d -3 -1 e -2 -1 f
MT = B L .(LT ) .(ML ) .(ML T ) .(MLT ) .(L² T )

a+b-3c-d+e+2f –b-d-3e-2f c+d+e -e-f
= B.L .T .M .

0 = a + b –3c –d +e +2f
-3 = -b –d –3e –2f
1 =c+d+e
-1 = -e - f

The solution of the equation gives,

a = c-1, b =c, d = -c +f, e = 1-f

c-1 c c -c+f -1-f f
h = B. L .U . ρ .µ .k .Cp

by grouping the variables,
-1 c f
h/L k = B.(UL ρ / µ) . (µ. Cp /k)
c f
Nussultes number = B.(Reynolds number) .(Prandl number)

The constants B,c,f are evaluated from experimental data.


For turbulent flow inside tubes and fully developed flow the following equation
attributed to Mr.Dittus and Boelter,
0.8 n
Nu = 0.023 Re Pr where, n = 0.4 when the fluid is heated
n = 0.3 when the fluid is cooled.

For turbulent flow outside tubes
0.8 n
Nu = 0.037 Re Pr where, n = 0.4 when the fluid is heated
n = 0.3 when the fluid is cooled
FREE CONVECTION
Free convection depends on buoyancy force F, which is defined by,
Let a fluid at To with density ρo change to temperature T with density ρ then,

F = (ρo –ρ)g/P = ((ρo/ρ) – 1)g
Now,
ß coefficient of volume expansion

then, 1/ ρ = (1/ρo) + ß(To-T),
ρo = ρ (1 + ß T)

(ρo/ ρ ) – 1 = ß T

F = ßg T

For an ideal gas ß is inversely proportional to temperature,(i.e. dimensional number
for ß is -1 and F is -1 * LT-2 ie LT-2)

By dimensional analysis,
h = B.(Fa.Cpb.Lc. ρ d.µe.k f)

MT-3 -1
= B[ (LT-2)a.(L2 T-2 -1 b
) . Lc.(ML-3)d.(ML-1T-1)e.(MLT-3 -1 f
) ]

1 = d + e+ f
= a + 2b + c –3d –e + f
-3 = -2a –2b-e-3f
-1 = -b-f

solving this equation.
c = 3a – 1,d = 2a , e = b –2a, f = 1- b

h = B[ (gß T)a . Cpb. L 3a-1. ρ2a. µ b-2a. k1-b)]

h = B[ (gß TL3 ρ2/ µ² )a . (µ.Cp/k)b] (k/L)

hL/k = B. Gra. Prb.


a b
Nu = B. Gr . Pr

By large number of experiments made on fluids it has been found that exponents a
a
and b are of the same value. So the expression reduce to Nu = B.(Gr.Pr)

HEAT TRANSFER BY RADIATION Hr

In furnace heat transfer is predominant by luminous and non-luminous radiation. A
general approximate expression may be written for furnace absorption using Stefan
boltzman law.

Q = A w [ g Tg4 – g TS
4
]

g = c c + w w -

emissivity pattern of tri atomic gases such as carbon di oxide and water vapour are
studied by Mr. Hottel and charts are available to predict gas emissivity as a function
of various gas temperature, partial pressure and beam length. I have also furnished
the expression form to find gas emissivity. When c and w are found from graph
c and w can be determined from the following expression or from graph.
Otherwise emissivity of gas can be directly found by the expression given in
equation1.

0.222 1 1
c = EXP _______________ -
P c *L +0.035 ln2.8 ln(p + 1.8)

1/3
0.23 1 2
w = EXP 0.842 -
(0.23 +Pw*L 0.75 0.5+Pw+p

where p is gas pressure in bar(a)
L is beam length meter
w and c are pressure correction factor for gas pressure

absorptive of gasses can be determined at wall temperature.

g = c c + w w -

At wall temperature correction,

Pcw = P c (Tw/Tg) Pww = P w(Tw/Tg)

c = cw (Tg/Tw)0.65 w = ww (Tg/Tw)0.45


cw is a function of Pcw .L and wall temperature for this we have to see the emissivity
in graph
ww is a function of Pww .L and wall temperature for this we have to see the emissivity
in graph

pressure correction is same as gas emissivity factor.
= w = function of P w/(Pc + P w) , Pcw.L + Pww.L, and temperature of wall

The effect of absorptivty is negligible hence the same can be neglected and a
generalized form of Q = A w g [Tg4 –TS4] can be used.

Heat absorption by energy balance method,

Q = [ Wf . lower heat value – W g .gas exit enthalpy]

Where,
A effective projected area of heat transfer including wall opening
w wall emissivity
g gas emissivity
Stefan boltzman constant
Tg Flue gas temperature of mean theoretical flame temperature(adiabatic
temperature)
TS Furnace wall temperature (If calculated for outside heat transfer coefficient or
consider saturation temperature if calculated for over all heat transfer coefficient, the
difference will be of very minor).
W f Fuel burnt
Wg Flue gas produced

Gas emissivity g = 0.9( 1- e –k.L )………………………………………………1

The emissivity of flame is evaluated by

f = ( 1- e –k.L )

where is the characteristic flame filling volume.

= 1.0 for non luminous flame(practical 0.9) of solid fuels.
0.90 for luminous and semi luminous flame of coal .lignite & husk(AFBC )
0.85 for luminous and semi luminous flame of bagasse (conventional firing)
0.72 for luminous and semi luminous sooty flame of liquid fuels
0.62 for luminous and semi luminous flames of refinery gas fuel OR gas/oil
mixture
0.50 for luminous and semi luminous flames of natural gas

L beam length meters = 3.4* volume/surface area.
For cuboid furnace chamber and bundle of tubes.

K attenuation factor, which depends on fuel type and presence of ash and its
concentration. For non-luminous flame


K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + P w)
_______
(Pc +P w)L

For semi luminous flame, the ash particle size and concentration is taken in
calculation

K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + P w)
________ + 7µ(1/dm²TM²)1/2
(Pc +P w)L

dm mean effective diameter of ash particle in micron
dm 13 for coal ground in ball mills
16 for coal ground in medium or high speed mill
20 for coal milled in hammer mill.

µ - ash concentration in gm/Nm^3

TM – furnace mean temperature °k(Some authors will consider this as outlet
temperature, but it is convincing assumption that in furnace zone temperature will be
uniformly spread through out the furnace by radiation effect (spherical). Hence
considering mean temperature for calculating radiation heat transfer coefficient will
be more appropriate. You can appreciate a notable phenomenon of furnace
temperature depends on flame location inside the furnace, in case flame is located at
the center of furnace(like oil fired burners (refer example1)) mean temperature and
outlet temperature will be at the most equal and if flame is located at one end of the
furnace and radiation beam travels a larger distance of furnace(like AFBC boilers
assuming no free board combustion) the furnace temperature near flame will be
higher and it gradually degrees at the furnace exit.

For luminous oil or gas flame

K = (1.6 TM/1000) –0.5

Pw and Pc are partial pressure of water vapour and carbon di oxide

Above equations give only Theoretical values for flame emissivity. In practical cases
a wide variation would be occurred due to:

1.0 Combustion phenomenon itself
2.0 The flame does not fill the furnace fully. Unfilled portion are subject to only gas
radiation


3.0 The emissivity of radiation is far below the flame emissivity. Emissivity of gas
radiation may be in the range 0.15 to 0.3. Therefore overall emissivity of flame
reduces. Hence emissivity changes with respect to location.

Due to the above fact I have tried to give the practical values and graphs for the
emissivity at appropriate places for AFBC, Dumping grate and fired boilers with
working of example.

The heat transfer by radiation is given as Q = A w g [ TM4 – TS4]. But mostly the
heat transfer will be of both convection and radiation occuring simultaneously and so
to put both process on a common basis, we may define a radiation heat transfer
coefficient by symbol Hr.

Qr = Hr. A. (TM – TS)
4 4
Hr = w g[TM -TS ]/(TM-TS )

While considering the total heat transfer by convection and radiation

Q = (Hc + Hr) A (TM –TS) for fired furnace where gas throughout furnace is same.

Q = (Hc + Hr) A Lmtd for AFBC and Radiation chambers.

By this equations we can get theoretical Hr value but in practice these values are
corrected by effectiveness factor. This depends on various manufacturers
experience on their steam generator.(Normally for oil fired boilers the value will be of
0.79 and gas fired boiler 0.67).

3.9 FURNACE CONSTRUCTION :

Basically three types of constructions are used
1.0 Plain tube construction with a refractory lined furnace
2.0 Tangent tube construction
3.0 Membrane wall construction.

Plain tube construction

FURNACE CHAMBER


REFRACTORY

The drawing shown gives complete idea of the above construction. Refractory lined
wall construction is out dated design since it calls for a lot of refractory work and flue
gas leaks are heavy and it can not with stand positive furnace pressure.

Tangent tube construction

FURNACE CHAMBER

REFRACTORY

Tangent tube is a improvement of refractory lined. Here requirement of boiler tubes
is comparatively more and also refractory structure is not eliminated.

Membrane wall construction

In industries widely used boiler furnace construction is of membrane wall
construction type. In this design the tubes are joined by welding a continuous
longitudinal strip forming a solid panel, which can be as large as transportable.
Panels can be welded together on site to form the furnace. The gap between the
tubes(pitch) are maintained in a such a way that the fin can be cooled by either of
the two side tubes and prevent warping of the panel. Water cooled furnaces not only
eliminated problem of rapid deterioration of refractory walls due to slag, but also
reduced fouling of convection heating surfaces to manageable extent, by lowering
the temperatures leaving the furnace. In addition to reducing furnace maintenance
and fouling of convection heating surfaces, water cooling also helped to generate
more steam. Consequently the boiler surface was reduced since additional steam
generating surface was available in water cooled furnace.

Ex.1.0 . Find the furnace outlet temperature for a fluidized bed boiler operating at 15
kg/cm^2(g) having furnace EPRS of 28.43 sq.m and having the following gas
parameters.


Flue gas produced 11016 kg/hr at a temperature of 900°C and partial water vapour
pressure 0.15 ata , partial carbon di oxide pressure 0.14 ata .
The furnace size is 2.424 x 2.828m and height of 1.75meters.

Assume FOT 740°C

Flue gas properties at film temperature. (900+740 +200)/3 = 613.33°C
–5
Dynamic viscosity = 3.7392 x 10 kg/ms
Thermal conductivity = 0.065177 kcal/m hr.°c
Prandl number = 0.7152

Flue gas velocity at outlet = 11016 x (613.33 +273)
3600 x 273 x 1.286 x 2.424 x 2.828

= 1.1269 meter/sec.

Convection heat transfer coefficient at gas side(Hc ) =
(As steam side heat transfer coefficient is very high, in over all heat transfer
coefficient its effect will be negligible)
0.8 n
Nu = 0.037 Re Pr where n= 0.3 for cooling fluid

0.8 n
Hc/kL = 0.037 Re Pr
0.8
Hc = 0.037 x 0.396 x 1.1269 x 1.75 x 0.7152 0.3 x 0.06517/1.75
3.7392 x 10-5

= 3.56 kcal/m^2 hr.°C

Radiation heat transfer coefficient (Hr)

Beam length = 3.4 x(w x d x l)/2(l.w +l.d + w.d )

Substituting w= 2.424,d = 2.828, l =1.75

L = 1.2709 m

For non luminous flame attenuation factor

K = (0.8 + 1.6x 0.15) x(1-0.00038x(820+273)) x (0.14 +0.15)
_________________
(0.14 +0.15)1.2709
= 0.2904


flame emissivity f = 0.9 x (1- e –0.2904 x 1.2709
)
= 0.2778

Wall emissivity w = 0.9 (practically adopted for fluidized bed boilers)

Radiation heat transfer coefficient Hr

= 4.88 x 10-8 x 0.2778 x 0.9 x [(820+273)4 –(200 + 273)4]
[820 –200]

= 27.1 kcal/hr m^2 K

Total heat transfer coefficient Hc + Hr = 3.56 + 27.1 =30.66 kcal/hr m^2 K

Heat transferred Qg = U A (lmtd)

= 30.66 x 28.43 x[(900 - 740)/ln(700/540)]

= 537419 kcal/hr.

Heat lost by gas QL = Wg ( Hi – Ho)

= 11016 (257.3 – 207.45)
= 549147 kcal/hr

Qg not equal to QL try with 745°C.

Ex 02. Evaluate the size of bed for a 10 tph boiler, operating at 14.5 ksc, satuated
steam from and at 100°C. Coal as a fuel. The efficiency of boiler is 80% and GCV of
coal as 3800 kcal/kg , Flue gas produced per kg of fuel is 6.802 kg/kg at 20% excess
air operation.

Heat output = 10000 x 540 = 5400000 kcal/hr.

Heat input = 5400000/0.8 = 6750000 kcal/hr.

Fuel input = 6750000/3800 =1776.3 kg/hr.

Flue gas produced = 1776.3 x 6.802 = 12082.4 kg/hr.

Bed area = (Flue gas qty x bed temp)/(velocity x density of gases)

= 12082.4 x (900 +273)/(3600 x 273 x 1.295 x 2.8)

= 3.977 m^2.

Bed size arrived = 3200 x1250 mm x mm a refractory wall thickness of 370 mm can
be considered and above which water wall is located. Hence a water wall of size
3584 x 1680( 35 @ 112 pitch and 15 @ 112 pitch ) can be obtained.


The sizing of bed area and water wall size is an art rather than a scientific approach
a better configuration has to be arrived on the basis of experience.

Note: From and at 100°C is the term used in boiler industry to specify the heat
capacity of boiler. This is value is assumed that water at 1kg/cm^2 100°C is given as
input and steam drawn at 1kg.cm^2 .(i.e. latent heat at 1kg/cm^2 pressure only
absorbed )

EX 03. Find the furnace outlet temperature of a 55Tph dumping grate bagasse fired
boiler operating at 42 kg/cm^2 and 420°C super heater outlet at furnace exit plane.
The effective projected area of furnace and superheater plane works out to be
212m^2 and 13.6m^2 respectively. Consider convection heat transfer coefficient
negligible and lower heating value of bagasse 1828 kcal/kg, 85% of air required
flows through air heater at a temperature of 170°C and 15% air for fuel distributor
and OFA at 40°C into the furnace. Fuel consumption 24209 kg/hr. 2% of gross heat
input goes as carbon loss and 1% goes as radiation loss.

FURNACE HEAT INPUT

1.0 Fuel heat input = 24209 x 1828 = 44.254 x 10^6 kcal/hr
2.0 Air heat input = 0.85 x 24209 x 3.909 x 0.24 x 170 +
0.15 x 24209 x 3.909 x 0.24 x 40
=3.418 x 10^6 kcal/hr
where,3.909 is air required for burning one kg of bagasse at 35% excess air.
0.24 kcal/kg°c specific heat of air.

3.0 Un burnt carbon loss = 0.02 x 24209 x2272 = 1.1 x 10^6 kcal/hr

4.0 Radiation loss = 0.01 x 24209 x2272 = 0.55 x 10^6 kcal/hr

Where 2272 kcl/kg is GCV of fuel.

NET FURNACE HEAT INPUT = 1+2 –3 –4

= 46.072 X 10^6 KCAL/HR

applying stefan boltzman law,
Q = A w g [ TM4 – TS4]
As it is a bagasse fired boiler volatile combustion is more TM will be equal to
temperature exit and w g is equal to 0.72.

Assuming 890°C as FOT
Saturation temperature 263°c .

Q1 = 212 x 0.72 x 4.88x10^-8 x ( 11634 – 5364)

= 13.01 x 10^6 kcal/hr.
superheater steam outlet 420°c

36954153 boiler-book

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