Boiler Book

A.GANESH KUMAR

DEUTSCHE BABCOCK, INDIA.

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.

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………. 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

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.

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.

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 whereτ is shear stress andν is velocity , l is the distance or gap between layers.

τ =µ ν/l whereµ 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

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 + a5T4 + 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 Heat Kj/kgK Dynamic Viscosity µPa.S Thermal conductivity W/mK Prandtl number a1 b1 c1 d1 e1 0.8554535 0.2036005E-3 0.4583082E-6 -0.279808E-9 0.5634413E-13 -0.9124458E 1 0.4564993E-2 0.2198889E-4 -0.1891235E-7 0.5138895E-11 -0.1083113E-1 0.5596822E-4 0.7413502E-7 -0.5901395E-10 0.1961745E-13 0.492851 -0.1230046E-2 0.1662398E-5 -0.1052753E-8 0.2443111E-12 a2 b2 c2 d2 e2 -0.1002311 0.7661864E-3 -0.9259622E-6 0.5293496E-9 -0.109357E-12 -0.4267768E1 0.4074274E-3 -0.5125357E-5 0.738556E-8 -0.343972E-11 -0.8035817E-2 0.110672E-04 -0.8397255E-8 0.1130229E-10 -0.5731264E-14 -0.8820652E-2 0.1855309E-3 -0.3838084E-6 0.3256168E-9 -0.1005757E-12

Dynamic viscosity,

µg = µn + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gas

P1 = a1 + b1T + c1T² + d1T3 + e1T4

P2 = a2 + b2T + c2T² + d2T3 + e2T4 where T is temperature in °C

Thermal conductivity, kg = kn + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gas

P1 = a1 + b1T + c1T² + d1T3 + e1T4

P2 = a2 + b2T + c2T² + d2T3 + e2T4 where T is temperature in °C

Prandtl number,

Prg = Prn + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gas

P1 = a1 + b1T + c1T² + d1T3 + e1T4

P2 = a2 + b2T + c2T² + d2T3 + e2T4 where T is temperature in °C

Pra = a + bT + cT² + dT3 + eT4 Specific heat,

Cpg = Cpn + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gas

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 µ Pa.s Thermal conductivity W/mK Prandtl number a b c d e f 0.1714237E02 0.4636040E-01 -0.2745836E-4 0.1811235E-7 -0.674497E-11 0.1027747E-14 0.2498583E-1 0.6535367E-4 -0.7690843E-8 -0.1924248E-11 0.160998E-14 -0.2864430E-18 0.6901183 0.2417094E-05 0.2771383E-7 -0.3534575E-10 0.1717930E-13 -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 b c d e f 0.3679321E1 -0.1313559E-2 0.2615196E-5 -0.9629654E-9 -0.9928002E-13 -0.9723991E3 ‘a1 b1 c1 d1 e1 f1 0.2852903E1 0.1580411E-2 -0.6189378E-6 0.1119450E-9 -0.7607378E-14 -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

Whereχ 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

D GCV exceeding 4200kcal/kg

not exceeding 4940Kcal/kg

E GCV exceeding 3360kcal/kg

not exceeding 4200Kcal/kg

F GCV exceeding 2400kcal/kg

not exceeding 3360Kcal/kg

G GCV exceeding 1300kcal/kg

not exceeding 2400Kcal/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.

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= Vo + 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 m3per 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

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 + 1/2O2à 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 + O2à 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 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 Head 15% 20% 25% 25% 25% 25% 15% 20% 2 ID Fan Flow Head 25% 30% 25% 25% 25% 25% 20% 20% 3 SA/PA/OF fan Flow

Head 10% 15% 25% 25% 25% 25% Not 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.

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.

Q rate of energy radiation in Watts A Surface area radiating heat sq.m

Stefan boltzmann constant = 5.67 x 10–8 Watt/sq.m K4 4.88 x 10–8 Kcal/sq.m hr K4

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

Gr.Pr > 109 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)

Let h = B La Ubρc µd ke Cpf , where B,a,b,c,d,e,f are constants Expressing the variables in terms of their dimensions

MT-3 -1 = B La.(LT-1)b.(ML-3)c.(ML-1T-1)d.(MLT-3 -1)e.(L² T-2 -1)f

= B.La+b-3c-d+e+2f. T–b-d-3e-2f. Mc+d+e. -e-f

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

h = B. L

.U

.

.µ

.k

.Cp

by grouping the variables,

h/L-1k = B.(ULρ / µ)c. (µ. Cp /k)f

Nussultes number = B.(Reynolds number)c.(Prandl number)f

For turbulent flow inside tubes and fully developed flow the following equation attributed to Mr.Dittus and Boelter,

Nu = 0.023 Re0.8 Prn where, n = 0.4 when the fluid is heated n = 0.3 when the fluid is cooled. For turbulent flow outside tubes

Nu = 0.037 Re0.8 Prn 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 densityρo change to temperature T with densityρ 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.ρd.µe.kf) 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. L3a-1.ρ2a. µb-2a. k1-b)] h = B[ (gß TL3ρ2/ µ² )a . (µ.Cp/k)b] (k/L) hL/k = B. Gra. Prb.