Airflow in Boiler PDF

AIR FLOW CONTROL IN COAL-FIRED BOILERS
A dissertation submitted in partial fulfillment o f the requirement for the

award o f the degree of
MASTER OF TECHNOLOGY
IN
INSTRUMENTATION
BY
VENKATA RAJESM PA BA LA
(ROLL NO, 207510)
Under the supervision o f
Dr. m m A IAGGI SUBRA W AW A M CV8
Asst. Professor Sr. Superintendent

NIT Kurukshetra NTPC Ramagundam
DEPARTMENT OF PHYSICS
MATRONAL "NSTHTUTE OIF TECHNOLOGY

(Institution o f National Importance)
KURUKSHETRA, HARYANA -136119.
AUGUST - 2009.
/Y ? 7 5 3 0 - 7
P flA ~<>y
ACKNOWLEDGEMENT
I am the student of NATIONAL INSTITUTE OF TECHNOLOGY

KURUKSHETRA is delightful and feeling pride to have undergone Project
work at Ramagundam Super Thermal Power Station of NTPC Limited.
This Project is an embodiment of the effort of several persons to

whom I would like to express my gratitude.
First I would like to express our sincere thanks to Project External

Supervisor C.V.B. Subrahmanyam, Sr. Supdt (O&M), and Meghanathan,
Supdt (O&M), and M. Prasad, Dy. Supdt (O&M), for giving as an
opportunity to undertake a project in Control & Instrumentation Dept.
I wish to express my deep sense of gratitude to my Project Internal

Supervisor Dr. Neena Jaggi, Asst. Professor, Deptt. of Physics, for
excellent guidance and continuous interaction to complete the project
successfully.
I express my sincere thanks to Dr. S.K. Mahna, Professor &

Chairman, Deptt. of Physics, for his excellent co-operation.
Venkata Rajesh Padala

Roll No: 207510
M.Tech (Instrumentation)
CONTENTS
CONTENTS Page N o
CHAPTER I: OBJECTIVE OF THE PROJECT

1.1 Abstract. 1
1.2 Introduction 2
1.3 Overview of NTPC RAMAGUNDAM 2
CHAPTER II: BASICS OF A THERMAL POWER PLANT
2.1 Energy conversion in Power Plant. 7

2.2 Four essential Circuits in thermal power plant 7
2.3 Block diagram representation of Air flow control 10
2.4 Coal feeder and Pulverizers. 11
2.5 Boiler and its auxiliaries. 17
2.6 Air Preheater 23
2.7 Turbine and Generator System. 25
2.8 Induced Draft and Forced Draft Fans. 28
CHAPTER IH: INSTRUMENTATION FOR AIR FLOW

CONTROL
3.1 Role and Path of Air flow in Plant. 31
3.2 Need of Air flow control. 34
3.3 Process inside the thermal plant. 35
3.3.1 Electro Static Precipitators (ESPs) 37
3.3.2 Flue Gas Stack 39
3.3.3 Mechanical System 40
3.4 Sensors used for air flow measurement. 40

3.4.1. Introduction 40
3.4.2. Sensing principle 41
3.4.3 Sensitivity of gas flow sensors 46

3.4.5 Time response of flow sensors 47
3.5 PID Controller 48

3.6 Air flow control - Damper arrangement. 53
CHAPTER IV: LOGIC DIAGRAMS FOR THE CONTROL OF

AIRFLOW 57
4.1 About MaxDNA System in NTPC-Ramagundam 64
4.1.1 MaxDNA System Architecture 65
4.1.2 MaxDNA System with THERMAL STATION 66
4.1.3 MaxSTATION 66
4.1.4 MaxSTORIAN 67
4.1.5 MaxLINK 67
CHAPTER V: RESULTS & CONCLUSION 70
BIBLIOGRAPHY 77
CHAIPTim 1
OBJECTIVE OF THE
PROJECT
Air Flow Control in Coal-fired B oilers-1
1.1 ABSTRACT
Airflow into the Coal-fired boilers finds significant role in

Thermal Power stations, where the generation of electrical voltage takes place. In Coal-
fired Power plants, the air entering the furnace should be optimum for efficient
combustion. For this purpose, we continuously monitor and control the amount of air
entering the furnace from the Atmosphere, with the help of Air Flow Transmitters. In
view of its importance, a study of Air Flow control loop in a small typical 500 MW
generating unit of NTPC Ramagundam, has been taken up.
When the amount of airflow into the furnace lesser than the set
value, then it results in inefficient combustion which considerably pollutes the
atmosphere. But, when the amount of air entering the boiler exceeds the predefined limit,
then the continuous higher airflow into the boilers may cause furnace pressurization
inside the boiler. Also, the amount of air flow into the boiler has to be at optimum value
to ensure a good overall efficiency of the boiler.
In order to provide efficient combustion and to ensure the safety from the
hazardous conditions, the amount of air entering the Boiler needs to be controlled. The
actual air flow at the inlet of the boiler is measured with the help of Air flow
Transmitters. The Set point of air flow is fixed to a particular value (Approximately 5
times the numerical value of the Fuel flow entering the Furnace). The two flow values are
compared and the Controller output is generated by PID Controller according to the error.
This controller output changes the “position of the Control Damper” or “Blade Pitch” of
the Force Draught (FD) Fan with the help of Servo system. Hence, the change in position
of Control damper regulates the Air Flow in to the Boiler.
Air Flow Control in Coal-fired Boilers- 2
1.2 INTRODUCTION
Boiler is the main part of the Coal-fired power plant. Boiler is a

closed vessel in which water or the other fluid is heated. The heated or the vaporized
fluid exits the boiler for use in various processes or heating applications. It incorporates a
fire-box or furnace in order to bum the fuel and generate the heat. This heat is initially
transferred to water to produce steam.
Different types of boilers are available viz. Coal-fired boilers, Fire

tube boilers, Water tube boilers, etc. Coal-fired boilers are generally used in Thermal
power plants. These boilers use Coal as the main fuel along with the Air for the
combustion process. The Air is responsible for 2 types of functions i.e. for the coal to
enter the furnace as well as for the combustion process (by containing O2 in the Air).
So, Air plays a significant role in the combustion process of Coal-

fired boilers in Thermal power stations. Thus, “Air Flow” in to the Coal-fired boilers has
to be controlled to ensure Safety of the equipment and for the Efficient Combustion (to
minimize the Pollution of Air by Flue gases emerging from the plant after the combustion
process)
1.3 OVERVIEW OF NTPC RAMAGUNDAM

NTPC, India's largest power company, was set up in 1975 to accelerate
power development in India. Today, it has emerged as an ‘Integrated Power M ajor’,
with a significant presence in the entire value chain of power generation business.
NTPC has been ranked No. 1 in 'Best Workplaces for Large Organizations' and eighth
overall in 2008 by Great Places to Work in collaboration with the Economic Times.
In the Forbes list of ‘World's 2000 largest companies, 2007’, NTPC

occupies 411th place. With a current generating capacity of 29,894 MW, NTPC has
embarked on plans to become a 75,000 MW company by 2017.
The total installed capacity of the company is 29, 894 MW (including JVs)
with 15 coal based and 7 gas based stations, located across the country. In addition under
JVs, 3 stations are coal based & another station uses LNG as fuel. By 2017, the power
generation portfolio is expected to have a diversified fuel mix with coal based capacity of
around 53000 MW, 10000 MW through gas, 9000 MW through Hydro generation, about
2000 MW from nuclear sources and around 1000 MW from Renewable Energy Sources
(RES). NTPC has adopted a multi-pronged growth strategy which includes capacity
addition through green field projects, expansion of existing stations, joint ventures,
subsidiaries and takeover of stations.
NTPC has been operating its plants at high efficiency levels. Although the company has
19.1% of the total national capacity it contributes 28.5% of total power generation due to
its focus on high efficiency.
GROWTH OF NTPC, INSTALLED CAPACITY & GENERATION:
Growth of NTPC Installed Capacity & Generation

r ■; IN STA LLED C A PA C ITY ■ G EN ERA TIO N
MW
2607-68
Fig 1.1: Installed Capacity and Generation of Power by NTPC.

NTPC Ramagundam is committed to the production and delivery of a

quality and reliable power to the satisfaction of customers and other stake holders,
through systems and processes, in line with our vision, mission and core values.
We will strive through continual improvement:
> To protect our environment, prevent pollution and minimize wastages.

> To provide a safe and healthy working environment to all our employees and
associates.
> To proactively comply with all the statutory and corporate requirements.
> For up gradation of our knowledge, skills and competences.
Fig 1.3: 500MW Unit of NTPC Ramagundam (Unit-7)
The station generates about 2600 MW of power annually. The fuel for
the power generation is taken from the South Godavari Coal Fields and water is taken
from Godavari River. The power generated from the power plant is shared by the south
Indian states of Andhra Pradesh, Karnataka, Tamil Nadu, Kerala and Pondicherry.
Since inception, the station has achieved excellence in all operational

spheres like project implementation, generation, environment management, ash
utilization, etc. The NTPC, Ramagundam, is supplying power to Andhra Pradesh (610
MW), Tamil Nadu (470 MW), Karnataka (345 MW), Kerala (245 MW), Goa (100 MW)
and Pondicherry (50 MW) and remaining 280 MW would be distributed among the states
depending on their requirements. The project is spread over 10,630 acres is utilising
about 30,000 tonnes of coal and 150 cusecs of water every day for generating power.
Control & Instrumentation:
This is the main area, set up to meet the Control & Instrumentation
requirement of NTPC Ramagundam. The company dedicates itself to this task with full
sincerity to ensure rapid economic development of the country based on timely
commissioning of various projects and satisfactory operation/ maintenance of already
commissioned projects at a high level of efficiency, availability and plant utilization
factor as far as the Control & Instrumentation is concerned.
CMAJPTim 2
BASICS OF A THERMAL
POWER PLANT
2.1 ENERGY CONEVRSION IN POWER PLANT
Conversion of energy during the power generation is as shown below.
Chemical Energy M echanical Energy Electricity
Coal is the primary input used as the fuel in thermal plants. This consists of
carbon compounds, which is in the form of Chemical energy. This energy is transformed
in to mechanical form to rotate turbines. The turbines hence rotated and thereby acting as
prime movers to the alternators. These alternators thus produce the electrical voltage.
2.2 FOUR ESSENTIAL CIRCUITS IN THERMAL POWER PLANT:
Basically, the general layout of thermal power plant consists of mainly four
circuits which are,
1. Coal and Ash circuit
2. Air and Gas circuit
3. Feed Water and Steam circuit
4. Cooling Water circuit.
Coal and Ash Circuit:
In this circuit, the coal from the storage is fed to the boiler through coal handling
equipment for the generation of steam. Ash produced due to combustion of coal is
removed to ash storage through ash-handling system.

Air and Gas Circuit:
Air is supplied to the combustion chamber of the boiler either through forced
draught or induced draught fan or by using both. The dust from the air is removed before
supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of
heat and ash are passed through the air-heater where the exhaust heat of the gases is given
to the air and then it is passed through the dust collectors where most of the dust is
removed before exhausting the gases to the atmosphere.
Feed Water and Steam Circuit:
The steam generated in the boiler is fed to the steam prime mover to develop
the power. The steam coming out of the prime mover is condensed in the condenser and
then fed to the boiler with the help of pump. The condensate is heated in the feed-heaters
using the steam tapped from different points of the turbine. The feed heaters may be of
mixed type or indirect heating type. Some of the steam and water are lost passing through
different components of the system; therefore, feed water is supplied from external
source to compensate this loss. The feed water supplied from external source to
compensate the loss. The feed water supplied from external source is passed through the
purifying plant to reduce dissolve salts to an acceptable level. This purification is
necessary to avoid the scaling of the boiler tubes.
Cooling Water Circuit:
The quantity of cooling water required to condense the steam is considerably high
and it is taken from a lake, river or sea. At the Columbia thermal power plant it is taken
from an artificial lake created near the plant. The water is pumped in by means of pumps
and the hot water after condensing the steam is cooled before sending back into the pond
by means of cooling towers. This is done when there is not adequate natural water
available close to the power plant. This is a closed system where the water goes to the
pond and is re circulated back into the power plant. Generally open systems like rivers
are more economical than closed systems.
Site Selection of a Thermal Power Plant:
The important aspect to be borne in mind during site selection for a thermal power
plant are availability of coal, ash disposal facility, space requirement, nature of land,
availability of water, transport facility, availability of labor, public problems, size of the
Total Air Row
The Air flow control can be explained with the help of the block diagram as shown
here. It consists of many important parts during the electricity generation. They are,
1. Pulverizers
2. Boiler
3. Turbine System
4. Generator
5. Electrostatic precipitator(ESP)
6. Induced Draft and Forced Draft Fans.
2.4 COAL FEEDER AND PULVERIZERS
Coal feeders: Mechanical arrangement to transport the coal from remote place in the
plant to the Pulverizers.
mmmu
Fig 2.1: Gravimetric Feeder

The first gravimetric feeders consisted of six major elements, as follows:
1. A cylindrical steel feeder housing (1).

2. A belt conveyor system including drive and tail pulleys, inlet support pan, and a
tension roll to maintain consistent belt tension (31, 32, 37, and 33).
3. A balance-beam weighing system to measure the gravimetric loading on the belt (85).
4. A motor-driven adjustable leveling bar to modulate the loading of material on the belt
(62).
5. A drag-chain cleanout conveyor to eliminate coal accumulation in the bottom of the
feeder housing (53).
6. A variable-speed-belt drive and control system (19).
The feeder was typically located immediately beneath the coal bunker and
immediately over one of the Pulverizers. Coal would pass down into the feeder and onto
the horizontal transfer belt within the feeder body. As the coal proceeded from the inlet
and toward the discharge, it passed over a weighing system comprised of two fixed and
one moveable roller. As the coal density varied, the moveable roller would either rise or
fall and thereby open or close switches controlling a material leveling bar actuator motor.
The leveling bar was located just beyond the coal inlet and, by either raising or lowering
it, exactly 100 pounds of coal could be maintained on the three-roller span which was
equal in length to the head pulley circumference. The feeder, therefore, discharged
exactly 100 pounds of coal for each turn of the head pulley. The head pulley speed was
proportional to the rate of coal fed that could be expressed as pounds of coal per minute
or pounds of coal per hour, as desired. Total turns of the feeder head pulley times 100
equaled the pounds of coat fed during any given period.
By commanding a change in the motor speed, and thus the head pulley
speed, the combustion control system could command instantaneous fuel delivery rate
changes. The simplicity of the system allowed reliable operation in the hostile
environment presented by the coal dust, heat, and pressures common to coal firing
Air Flow Control in Coal-fired Boilers-13
systems. Further refinements were provided to simplify maintenance and to minimize the
possibility of equipment failure.
Pulverizers:
A pulverizer is a mechanical device for the grinding of many different types of

materials. For example, they are used to pulverize coal for combustion in the steam-
generating furnaces of fossil fuel power plants.
Types of Pulverizers:
Ball and Tube Mills
A ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three

diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or
rods.
A tube mill is a revolving cylinder of up to five diameters in length used for fine
pulverization of ore, rock, and other such materials; the material, mixed with water, is fed
into the chamber from one end, and passes out the other end as slime.
Ring and Ball Mill
This type of mill consists of two rings separated by a series of large balls. The lower ring
rotates, while the upper ring presses down on the balls via a set of spring and adjuster
assemblies. The material to be pulverized is introduced into the center or side of the
pulverizer (depending on the design) and is ground as the lower ring rotates causing the
balls to orbit between the upper and lower rings. The pulverized material is carried out of
the mill by the flow of air moving through it. The size of the pulverized particles released
from the grinding section of the mill is determined by a classifier separator.
Vertical Roller Mills
This mill uses hydraulically loaded vertical rollers resembling large tires to pulverize raw
coal fed down onto a rotating table. As the table rotates, the raw coal is pulverized as it
passes underneath the rollers. Hot air forced through the bottom of the pulverizing
chamber removes unwanted moisture and transports the pulverized coal dust up through
the top of the pulverizer and out the exhaust pipes directly to the burner. The more recent
coal pulverizer designs are Vertical Roller.
Dial Conveyer
Fig 2.2: Diagram detailing a direct fired coal burning system.

• Increased thermal efficiency is obtained through pulverization.
• The use of secondary air in the combustion chamber along with the powered
coal helps in creating turbulence and therefore uniform mixing of the coal and the air
during combustion.
• Greater surface area of coal per unit mass of coal allows faster combustion as
more coal is exposed to heat and combustion.
• The combustion process is almost free from clinker and slag formation.
• The boiler can be easily started from cold condition incase of emergency.
• Practically no ash handling problem.
• The furnace volume required is less as the turbulence caused aids in complete
combustion of the coal with minimum travel of the particles.
The pulverized coal is passed from the pulverizer to the boiler by means of the primary
air that is used not only to dry the coal but also to heat it as it goes into the boiler. The
secondary air is used to provide the necessary air required for complete combustion. The
primary air may vary anywhere from 10% to the entire air depending on the design of the
boiler. The coal is sent into the boiler through burners. A very important and widely used
type of burner arrangement is the Tangential Firing arrangement.
Tangential Burners:
The tangential burners are arranged such that they discharge the fuel air mixture
tangentially to an imaginary circle in the center of the furnace. The swirling action
produces sufficient turbulence in the furnace to complete the combustion in a short period
of time and avoid the necessity of producing high turbulence at the burner itself. High
heat release rates are possible with this method of firing.
The burners are placed at the four comers of the furnace. At the Columbia Power
Plant six sets of such burners are placed one above the other to form six firing zones.
These burners are constructed with tips that can be angled through a small vertical arc.
By adjusting the angle of the burners the position of the fire ball can be adjusted so as to
raise or lower the position of the turbulent combustion region. When the burners are tilted
downward the furnace gets filled completely with the flame and the furnace exit gas
temperature gets reduced. When the burners are tiled upward the furnace exit gas
temperature increases. A difference o f 100 degrees can be achieved by tilting the burners.
2.5 BOILER AND ITS AUXILIARIES
Boiler is the main part of the Thermal Power Plant. The function of the Boiler is
to generate steam.
Boiler is a closed vessel in which water or the other fluid is heated. The heated or
the vaporized fluid exits the boiler for use in various processes or heating applications. It
incorporates a fire-box or furnace in order to bum the fuel and generate the heat. This
heat is initially transferred to water to produce steam.
Different types of boilers are available viz. Coal-fired boilers, Fire tube boilers,
Water tube boilers, etc. A water-tube boiler is a type of boiler in which water circulates in
tubes heated externally by the fire. Water-tube boilers are used for high-pressure boilers.
Fuel is burned inside the furnace, creating hot gas which heats up water in the steam-
generating tubes. In smaller boilers, additional generating tubes are separated in the
furnace, while larger utility boilers rely on the water-filled tubes that make up the walls
of the furnace to generate steam.
The heated water then rises into the steam drum. Here, saturated steam is drawn
off the top of the drum. In some services, the steam will re-enter the furnace in through a
superheater in order to become superheated. Superheated steam is used in driving
turbines. Since water droplets can severely damage turbine blades, steam is superheated
to 730°F (390°C) or higher in order to ensure that there is no water entrained in the
steam.
A large amount of fuel is used in thermal power plant and very large amount of
heat is generated and carried by waste gases. The loss would be very high if the waste
gases carry all the heat away. The loss can he halved by installing an economizer and a
preheater in the path of the waste gases. The economizer transfers the heat from the
waste gases to the incoming feed water. This reduces the heat required to convert the feed
water to steam. The air pre heater increases the heat of the air supplied into the boiler for
combustion. This increases the efficiency of the boiler.
Coal-fired boilers are generally used in Thermal power plants. These boilers use coal as
the main fuel along with the Air for the combustion process.
The fire-tube boiler design in which the water surrounds the heat source and the gases
from combustion pass through tubes through the water space is a much weaker structure
and is rarely used for pressures above 350 psi (2.4 MPa).
Fig 2.4: Fire Tube Boiler.

In this type of boilers, the combustion gases from the furnace are made to
pass through the tubes, meanwhile, the feed water into the drum heated by the
temperature of these gases passing over. Two paths, one for the hot gases from furnace to
emerge out and another for the produced steam to exit.
f f» O^nnfl
Fig 2.5: W ater Tube Boilers.
In this type, as the name itself indicates, the water is made to pass through the tubes,
which are surrounded by the very high temperature combustion gases. With the high
temperature of the gases, water gets converted in to steam.
Boiler auxiliaries are:

!
I
1. Super Heater and Reheaters.
2. Economizer.
1 k?
Super Heaters:
1
As the steam is conditioned by the drying equipment inside the drum, it is
piped from the upper drum area into an elaborate set up of tube in different areas of the
boiler. The areas known as superheater and reheater. The steam vapour picks up from
main steam tube when heated with super heaters. The superheated steam is then piped
through the main steam lines to the valves of the high pressure turbine.
Whatever type of boiler is used, steam will leave the water at its surface and pass into the
steam space. Steam formed above the water surface in a shell boiler is always saturated
and cannot become superheated in the boiler shell, as it is constantly in contact with the
watersurface.
If superheated steam is required, the saturated steam must pass through a superheater.
This is simply a heat exchanger where additional heat is added to the saturated steam.
In water-tube boilers, the superheater may be an additional pendant suspended in the

furnace area where the hot gases will provide the degree of superheat required. In other
cases, for example in CHP schemes where the gas turbine exhaust gases are relatively
cool, a separately fired superheater may be needed to provide the additional heat.
Supeitiealed steam
Supefheater pendant
Fig 2.6: A water tube boiler with a superheater.
If accurate control of the degree of superheat is required, as would be the case if the
steam is to be used to drive turbines, then attemperator (Desuperheater) is fitted. This is a
device installed after the superheater, which injects water into the superheated steam to
reduce its temperature.
Economizers
A boiler economizer is a heat exchanger device that captures the "lost or

waste heat" from the boiler's hot stack gas. The economizer typically transfers this waste
heat to the boiler's feed-water or return water circuit, but it can also be used to heat
domestic water or other process fluids. Capturing this normally lost heat reduces the
overall fuel requirements for the boiler. Less fuel equates to money saved as well as
fewer emissions - since the boiler now operates at a higher efficiency. This is possible
because the boiler feed-water or return water is pre-heated by the economizer therefore
the boilers main heating circuit does not need to provide as much heat to produce a given
output quantity of steam or hot water. Again fuel savings are the result. Boiler
economizers improve a boiler's efficiency by extracting heat from the flue gases
discharged.
The flue gases, having passed through the main boiler and the superheater, will still be
hot. The energy in these flue gases can be used to improve the thermal efficiency of the
boiler. To achieve this, the flue gases are passed through an economizer.
Fig 2.7: A boiler consisting of an economizer in a therm al plant.

The economizer is a heat exchanger through which the feedwater is pumped. The
feedwater thus arrives in the boiler at a higher temperature than would be the case if no
economizer was fitted. Less energy is then required to raise the steam. Alternatively, if
the same quantity of energy is supplied, then more steam is raised. This results in a higher
efficiency. In broad terms a 10°C increase in feed water temperature will give 2%
improvement efficiency.
• Because the economizer is on the high-pressure side of the feed pump, feedwater
temperatures in excess of 100°C are possible. The boiler water level controls
should be of the 'modulating' type, (i.e. not 'on-off) to ensure a continuous flow of
feedwater through the heat exchanger.
• The heat exchanger should not be so large that:
o The flue gases are cooled below their dew point, as the resulting liquor
may be acidic and corrosive,
o The feedwater boils in the heat exchanger.
Types of Economizer:
1. Plain Tube Economizer:
These are generally used in case of boilers with natural draught. The tubes are
made of cast iron and their ends are pressed into top and bottom headers. The economizer
is placed in the main flue gas path between the boiler and the chimney. The waste flue
gases flow outside the tubes and heat is transferred to the water flowing inside. High
efficiency can be achieved by maintaining the water walls soot free.
2. Grilled Tube Economizer:
This is the type of economizer used in the power plant. This type of economizer
reduced space considerably. Rectangular grills are cast on the bare tube walls.
Economizer tubes may have finned tubes to increase the heat transfer rate. Thicker fins
offer greater efficiency than thinner ones because of greater surface area.
2.6 AIR PREHEATER
An air preheater or air heater is a general term to describe any device

designed to heat air before another process (for example, combustion in a boiler) with the
primary objective of increasing the thermal efficiency of the process. They may be used
alone or to replace a recuperative heat system or to replace a steam coil.
In particular, this article describes the combustion air preheater used in large
boilers found in thermal power stations producing electric power from e.g. fossil fuels,
biomasses or waste. The purpose of the air preheater is to recover the heat from the boiler
flue gas which increases the thermal efficiency of the boiler by reducing the useful heat
lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or
chimney) at a lower temperature, allowing simplified design of the ducting and the flue
gas stack. It also allows control over the temperature of gases leaving the stack (to meet
emissions regulations, for example).
The flue gases coming out of the economizer is used to preheat the air before
supplying it to the combustion chamber. An increase in air temperature of 20 degrees can
be achieved by this method. The pre heated air is used for combustion and also to dry the
crushed coal before pulverizing.
Steam ' steam Reheated

Drum mam
Reheater
+tQh ftllSIl
twfoirre exhaust
eiowdoi.... steam
Deaerarted
Boiler 6©U*f
‘vV
Hot « r
Note: APH is the air presenter
Fig 2.8: A ir preheater in a therm al plant.

Types of Air Heaters:
Tubular Air Heater:
The flue gas flows outside the tubes in which the air flows heating it. To increase
the time of contact horizontal baffles are provided.
Plate Type Air Heater:
It consists of rectangular flat plates spaced 1.5 to 2 cm apart leaving alternate air
and gas passages. This is not used extensively as it involves high maintenance.
Regenerative Air Heater:
The transfer of heat from hot gas to cold air is done in 2 stages. In the first stage
the heat from the hot gases is passed to the packing of the air heater and the temperature
of the gas is sufficiently reduced before letting it out in the atmosphere. This is called the
heating period. In the second stage the heat from the packing is passed to the cold air.
This is called the cooling period.
The fuel used in thermal power plants cause soot and this is deposited on the
boiler tubes, economizer tubes, air pre heaters etc. This drastically reduces the amount of
heat transfer o f the heat exchangers. Soot blowers control the formation of soot and
reduce its corrosive effects. The types of soot blowers are fixed type, which may be
further classified into lane type and mass type depending upon the type of spray and
nozzle used. The other type of soot blower is the retractable soot blower. The advantages
are that they are placed far away from the high temperature zone, they concentrate the
cleaning through a single large nozzle rather than many small nozzles and there is no
concern of nozzle arrangement with respect to the boiler tubes.

2.7 TURBINE AND GENERATOR SYSTEM
Turbine System:
Turbine is a device consisting of blades mounted on a cylindrical metal

object which is kept on a shaft itself is coupled to the generator. This motion of the
turbine rotor is transmitted to generator in which mechanical energy is transmitted to
electrical energy. The steam produced into boiler expands in the turbine. In the turbine
the thermal energy of the steam is converted into the kinetic energy. Generally turbine
having blades rotating by steam is shown in figure.
Fig 2.9: Typical Turbine structure with series of Blades

Turbine is divided into three categories, they are:
High Pressure Turbine:

The stream from the boiler drum first is sent on to the HPT, where it rotates the
turbine. Here the steam temperature is 540°C and a pressure of 170 Kg/cm2 and most of
the temperature and pressure of is used by the HPT itself.
Intermediate Pressure Turbine:

The steam from the reheater is sent to the IPT, where it is used to rotate the
turbine. This is having temperature of 540°C and pressure of 4.5 kg/cm2.
Low Pressure Turbine:

The expanded steam from the IPT is sent to the LPT but the pressure decreases to
a negative value of -0.8kg/cm2.
The steam after expansion from the turbine goes to the condenser. The use of turbine
increases the efficiency of the plant by decreasing the exhaust power of the steam below
at atmosphere.
Generator:
Generator is a device, which converts mechanical energy of the shaft into

electrical energy by electro magnetic induction. It consists of a stator and rotor and an
excitation system. This electrical power transmitted various load enters through the
transmission lines. NTPC-Ramagundam generators 2600MW power of which 3x200MW
in Stage-1, 3x500MW in Stage-2 and lx500MW in stage-3 capacity.
Operation:
An electrical generator is a machine that converts mechanical energy into
electrical energy. The energy conversion is based on the principle of the production
dynamically induced EMF. Whenever conductor cuts flux dynamically induced EMF is
produced in it according to faraday’s law of electro magnetic induction. This EMF causes
a current to flow if the conductor circuit is closed. Hence, basic essential parts of an
electrical generator are a magnetic field and conductors, which can so more as to cut the
flux.
The basic law or principle of operation of all rotating machine remains the same
that is faraday’s law of electro magnetic induction. It states that whenever there is
relation motion between a conductor and a magnet that is when a moving coil cuts the
magnetic lines then an emf is directly proportional to the rate of change of flux and the
number of turns thus to produce relative motion either the armature rotate on the magnet.
e= N
dt
Thus to produce relative motion either the armature to rotate on the magnet. In a
DC generator, the armature is rotating part and in alternator, it is a stationary part. The
rotation part (rotor) produces the magnetic field and armature winding is the stator.
Specification of 500MW Generator:
Make BHEL
Type THDF115159
Code IEC34-1, VDE-0330
Apparent power 588MW
Active power 500MW
Power factor 0.85 (lag)
Terminal voltage 21KV
Permissible variation in voltage ±5%
Speed / frequency 3000rpm/50Hz
Hydrogen pressure 4Kg/cm2
Field current 4040amp
Field voltage 340V
Class Class F
Type of insulation MICALASTIC
No of terminals brought out 6
2.8 INDUCED DRAFT AND FORCED DRAFT FANS
There are two types of fans are being used in thermal power stations, namely
Forced draft and Induced draft fans. These Fans may be driven by electric motors, steam
turbines, gas or gasoline engines, or hydraulic motors. The overwhelming choice is the
electric motor. Hydraulic motors are sometimes used when power from an electric utility
is unavailable. Hydraulic motors also provide variable speed control, but have low
efficiencies.
AIR FU
Fig 2.10: Forced Draft Fan and path of air flow.
The Forced Draft (FD) Fan, sucks the air from the atmosphere, pressurizes it
and sends in to furnace. Prior sending it to furnace, the pressurized air is heated in
Secondary air pre heaters(SAPHs). The source o f heating in SAPH is the hot flu gas,
.
which are leaving from the boiler. There are 2 FD Fans for each boiler. The pressurized
hot air generally called Secondary Air acts as combustion medium in furnace.
/ \
Fig 2.11: Induced draft fan in power plant.
The path of air flow in Forced Draft (FD) and Induced Draft (ID) is shown above.
ID fans extract ash less flue gases from Electro Static Precipitators and send it to the
chimney. Chimney sends out the gases to atmosphere at a greater height to prevent
pollution.
A ir Flow O a sto i k €©aMfeafi B®flers= 3®
3.1 ROLE AND PATH OF AIR FLOW IN PLANT
The Air is responsible for two types of functions i.e. for the coal to enter
the furnace as well as for the combustion process (by containing O2 in the Air).
Firstly, Air helps the coal powder from the Pulverizers to enter the furnace.
This air doesn’t involve in the combustion process. This air is known as ‘Primary Air’.
The air from the atmosphere at Standard Temperature and Pressure(STP) is drawn in to
the plant by two individual Primary Air Fans (PAF-A, PAF-B).This air enters the milling
systems through Primary Air Heater (PAHs) systems followed by Primary Air Fans
(PAFs), thereby mixes with the coal powder in Millers. This coal with secondary air
enters the furnace and bums there, causing heat energy to build up in the furnace.
Secondly, the air solely responsible for the combustion process along with
the coal in the furnace is called Secondary air. This air is also drawn in to the plant by
two individual Force Draught Fans (FDF-A, FDF-B) from the atmosphere. The FD Fans
supplies the secondary air in to the furnace through the two individual Secondary Air
Heaters (SAHs) followed by FD Fans. The Air Heaters (AHs) heats up the air drawn by
the FD fans or PAFs and admits in to the furnace.
3.2 NEED OF AIR FLOW CONROL
When the amount of airflow in to the furnace lesser than the optimum
value, indicates that fuel (coal containing carbon compounds) amount is more when
compared with the proportionate value of the air flow. Then, it results in inefficient
combustion, during which the flue gases emerging out will considerably consist of
partially burnt fuel with compounds viz., CO2 , SO2 , CO and other dangerous gases. These
gases considerably pollute the Atmosphere.
On the other hand, when the amount of air entering the boiler exceeds
the predefined limit, then fuel will not reach the furnace with high amount of air and
hence the wastage of fuel, which considerably loss of efficiency of power generation. In
addition to this, the continuous higher airflow in to the boilers may cause furnace
pressurization inside the boiler and there may be a chance of severe damage to the
equipment and there by prevailing hazardous situations in the plant.
In order to provide efficient combustion and to ensure the safety from the
hazardous conditions, the amount of air entering the Boiler needs to be continuously
monitored and controlled by effective means.
3.3 PROCESS INSIDE THE THERMAL PLANT
Boiler is the main equipment in the power plant, where the water is
converted in to steam by heating the water, thereby providing sufficient energy to rotate
the turbines. These turbines (act as Prime movers) are connected to the shaft of the
alternators, to produce electricity.
Feeder is a system, which provides the coal with the help of

Conveyor belts. The coal is in solid state with irregular shape and size. This coal is not
convenient enough for the combustion. Instead, this coal is to be converted in to fine
powder with the help of milling systems. This process of changing bulk blocks of coal in
to its fine granular form is known as ‘Pulverization’ and such milling systems(sometimes
called ‘Mills’) doing this function are called as ‘Pulverizers’. Generally there are many
number of pulverizers to pulverize the coal in the plant. Now, the coal powder is fed to
the furnace with the help of Air for the combustion process.
Here, Air is responsible for two functions in the plant. Firstly, it
helps the coal powder from the pulverizers to enter the furnace. This air doesn’t involve
in the combustion process. This air is known as ‘Primary Air’. The air from the
atmosphere at STP is drawn in to the plant by two individual Primary Air Fans (PAF-A,
PAF-B).This air enters the milling systems through Primary Air Heater (PAHs) systems
followed by Primary Air Fans (PAFs), thereby mixes with the coal powder in Millers.
This coal with secondary air enters the furnace and bums there, causing heat energy to
build up in the furnace.
Secondly, the air solely responsible for the combustion process along with
the coal in the furnace is called Secondary air. This air is also drawn in to the plant by
two individual Force Draught Fans (FDF-A, FDF-B) from the atmosphere. The FD Fans
supplies the secondary air in to the furnace through the two individual Secondary Air
Heaters (SAHs) followed by FD Fans. The Air Heaters (AHs) heats up the air drawn by
the FD fans or PAFs and admits in to the furnace.
Aif M®w C©i$wl m CMMSfM Boifera- 36
Furnace is a part of the Boiler. The water fed into the boiler with the help
of Boiler Feed Pump (BFPs) and such water is known as Feed Water. Boiler takes the
water from the Boiler Feed Pumps and first converts it into saturated steam. This
saturated steam is again heated in different stages of Super heaters. The steam from the
super heaters becomes completely dry and the quality of steam becomes suitable to use in
Turbines. The temperature of this steam is around 540° C.
The steam with water droplets is made to pass through Super Heaters
(SHs), to remove the water droplets by further heating up the steam. This steam drives
the Turbine system. The Alternator, whose shaft is connected to the turbine, produces the
electrical voltage.
Unlike the primary air, secondary air is made to enter the furnace directly
as this air is used solely for the combustion of coal in the furnace. During the combustion
of coal and secondary air in the furnace, flue gases will emerge out of the boiler. These
flue gases are passed through heat exchangers, called ‘Economizer’, where the heat
energy is saved by transferring to the water flowing through pipes in Economizer. The
temperature of the flue gases is greatly reduced in Economizer and the gases are passed
through Electro Static precipitators (ESPs) to collect the Ash and other heavy dust
particles. The light gases remaining after the precipitators are pushed out from the plant
to the Flue gas stack or Chimney to the atmosphere. Two Induced Draught Fans (IDF-A,
IDF-B) are used to suck out the flue gases from the plant to the stack
3.3.1 Electro Static Precipitators (ESP’s):

These are used for the dust and ash to be removed, and not to enter the
atmosphere. These ash and heavy dust particles are highly polluting the environment, as
they contain higher amounts of CO2 , CO, SO2 and other Carbon compounds.
Two emission control devices for fly ash are the traditional fabric filters and the more
recent electrostatic precipitators. The fabric filters are large bag house filters having a
high maintenance cost (the cloth bags have a life of 18 to 36 months, but can be
temporarily cleaned by shaking or back flushing with air). These fabric filters are
inherently large structures resulting in a large pressure drop, which reduces the plant
efficiency. Electrostatic precipitators have collection efficiency of 99%, but do not work
well for fly ash with a high electrical resistivity (as commonly results from combustion of
low-sulfur coal). In addition, the designer must avoid allowing unbumed gas to enter the
electrostatic precipitator since the gas could be ignited.
Fig 3.4: Function of Electro Static Precipitators in thermal plants.
The salt & pepper collector/selector, and repelling balloon experiments serve to illustrate
the basis of an electrostatic precipitator. In these experiments a type of electrostatic
collector and electrostatic selector are created. This same principle is used to keep the
environment clean today.
Fig 3.5: Typical Electro Static Precipitators

The flue gas laden with fly ash is sent through pipes having negatively charged
plates which give the particles a negative charge. The particles are then routed past
positively charged plates, or grounded plates, which attract the now negatively-charged
ash particles. The particles stick to the positive plates until they are collected. The air that
leaves the plates is then clean from harmful pollutants.
Electrostatic precipitators are not only used in utility applications but also other
industries (for other exhaust gas particles) such as cement (dust), pulp & paper (salt cake
& lime dust), petrochemicals (sulfuric acid mist), and steel (dust & fiimes).
3.3.2 Flue Gas Stack:
A flue gas stack are a type of chimney, a vertical pipe, channel or similar structure
through which combustion product gases called flue gases are exhausted to the outside
air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is
combusted in an industrial furnace, a power plant's steam-generating boiler, or other large
combustion device. Flue gas is usually composed of carbon dioxide (CO2) and water
vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It
also contains a small percentage of pollutants such as particulate matter, carbon
monoxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall, up
to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a greater
area and thereby reduce the concentration of the pollutants to the levels required by
governmental environmental policy and environmental regulation.
As the need for more power generation increased, each power station had
increased its number of plants, resulting in optimized utilization of space and resources.
One of the key areas for space optimization was chimneys. Chimneys can be defined as a
vertical hollow structure of masonry, steel or reinforced concrete, built to convey gaseous
products of combustion from a building or process facility.
A chimney should be high enough to furnish adequate draft and to discharge the
products of combustion without causing local air pollution. The height and diameter of a
chimney determine the draft. For adequate draft, small industrial boilers and home
heating systems depend entirely upon the enclosed column of hot gas.
In contrast, stacks, which are chimneys for large power plants and process
facilities, usually depend upon force-draft fans and induced-draft fans to produce the
draft necessary for operation, and the chimney is used only for removal of the flue gas.
3.3.3 Mechanical System:
Servo system is also employed to control the Position of control damper or

blade pitch of FD Fans. The servo system consists of a mechanical arrangement and a
high pressured fuel oil. The mechanical arrangement is directly connected to the Control
damper of the FD fans.
3.4 SENSORS USED FOR AIR FLOW MEASUREMENT
An air flow sensor based on a free-standing cantilever structure is as shown in

the figure. A platinum layer is deposited on the silicon nitride layer to form a
piezoresistor, and the resulting structure is then etched to create a freestanding micro
cantilever. When an air flow passes over the surface of the cantilever beam, the beam
deflects in the downward direction, resulting in a small variation in the resistance of the
piezoelectric layer. The air flow velocity is determined by measuring the change in
resistance using an external meter. The experimental results indicate that the flow sensor
has a high sensitivity (0.0284 Q/ms'1), a high velocity measurement limit (45 ms'1) and a
rapid response time (0.53 s).
3.4.1. Introduction:
Flow measurement is a necessary task in such diverse fields as medical

instrumentation, process control, environmental monitoring, and so forth. This is a flow
sensor in which a piezoresistor is deposited on a free-standing micro cantilever structure
and connected to an LCR meter via gold electrodes.
When air flows over the surface of the cantilever structure, the
beam deforms slightly, causing a measurable change in the resistance of the piezoresistor
layer from which the gas flow rate can then be inversely derived. The proposed sensor
can be manufactured with a simplified and cheap fabrication process for measuring high
flow rates.
3.4.2. Sensing principle
The cantilever deflection by air flow can be obtained by combining the effects of the
loads acting separately:
3 _^2
St = — —( 4 £ - a ) + — —(3 £ -6 ) (1)
r 24E l 6EI
Where,
5t is the total deflection of the cantilever, q is the uniform load intensity on the beam
part, E is the Young's modulus of cantilever, I is the moment of inertia, L is the length of
the cantilever, F is the concentrated load on the paddle part, a is the distance from the
fixed end to the uniform load and b is the distance from the fixed end to the concentrated
load. Please note that q (the uniform load intensity) and F (the concentrated load) are
applied by the wind pressure calculated by the Bernoulli’s
Eq.:
Pmnd ~ 0.5 pairVvmd (2)
Where,
Pwind is the wind pressure, p air is the intensity of the air and Vwind is the air flow velocity.
Due to the limit of the available manufacturing equipment, the airflow velocity is
determined by measuring the change in resistance of the platinum pizeoresistor as the
cantilever beam deflects under the influence of a gas flow passing over its surface.
nitridcfSijN*)
platinum
gold
silicon
resistance
meter
Fig 3.1: Schematic illustration of gas flow sensor
As shown in Figure 3.2, when air flows over the cantilever structure, the
beam is deflected in the downward direction causing a change in the cross-sectional area,
and hence the resistance, of the platinum resistor. The corresponding airflow velocity can
then be determined simply by measuring the resistance change using the external LCR
meter.
micro-cantilever
substrate
Fig 3.2: Diagram of gas flow sensor during sensing operation.
Cantilever beam dimensions
To investigate the relationship between the sensitivity of the proposed flow sensor and
the physical dimensions of the cantilever structure, three different cantilever beam widths
(Wbeam) were considered, namely 400 |o.m, 1200 |im and 2000 fim, respectively. As shown
in Figure 3, the platinum piezoresistor had a length of 1500 jam in every case. Note that in
this figure, the black crosshatched areas denote the platinum resistor, while the black
lines indicate the periphery of the cantilever structure.
Fig 3.3: Side View image of Cantilever beam
<*> (t» > <c>
Fig 3.4: Images of Cantilever beams of Different Widths.
(a) Wbeaa = 400 um. (b) = 1.200 um and (c) = 2.000 um

These sensors measure the air flow under ambient temperature conditions
(25°C) in a wind tunnel at airflow velocities ranging from 0 - 4 5 m/sec. The variation in
the sensor resistance was measured using an LCR meter (WK4230, Wayne Kerr
Electronics Ltd.). For reference purposes, the airflow velocity was also measured using a
Pitot tube flow sensor in the wind tunnel.
3.4.3 Sensitivity of gas flow sensors

The resistance signal generated by the flow sensor increases
approximately linearly with an increasing airflow velocity. From inspection, the average
sensitivities of the sensors with cantilevers of width (Wbeam) 400 fim, 1200 nm and
2000 nm are found to be 0.0134, 0.0227 and 0.0284 (Q/ms'1), respectively, with a
maximum error of 2%. In other words, the flow rate sensitivity increases as the width of
the cantilever beam is increased. The experimental results also reveal that the maximum
detectable flow rate is 45 ms"1. The theoretical results are shown and it is found that both
the experimental and theoretical results are o f high correspondence.
_ l .6
Q* 1.4
§ 1.2
1 £
0 5 10 15 20 25 30 35 40 45 50
Fig3.5 Characteristics of flow rate sensitivity for sensors with cantilever tip widths
of: (a) Wbeam = 400 pm [ 0 ], (b) Wbeam = 1,200 pm [ a ] and c) Wbeam = 2,000pm [A].
The flow sensor can be operated under the wind coming from the
opposite direction in Figure 2, but the measurement range is reduced to be below 30m/sec
due to the cantilever structure fracture. Please note that to avoid undergoing torsion
deformation and producing the erratic output problem of the piezoresistor, the flow
sensors can only measure air flow that is directly along the axis o f the cantilever. It can
also be found that due to the limits of the energy conversion between the kinetic energy
of the air flow and the heat and the delay of the heat transfer on the thermal types of flow
sensors, the current non-thermal type of flow sensor can measure higher flow rates
because of its direct piezoresistor deformation caused by the kinetic energy of the air
flow.
3.4.5 Time response of flow sensors
The response time of thermal flow sensors is known to vary from 0.14 ms to
150 ms [1-2]. It is also essential to investigate this performance in non-thermal type flow
sensors. Figures 10, 11 and 12 reveal that the time responses of the sensors with
cantilevers of width 400 |im, 1200 pm and 2000 (j,m are 1.38, 0.99 and 0.53 s (90%),
respectively. In other words, the response time o f the flow sensors decreases as the
cantilever beam width is increased.
4.425
& 4.42
S' 4.415
i 4.41
'5v? 4.405
f* 4.4
4.395
O 300 600 900 1200 1500
Time (m s)
Fig 3.6: Time response of gas flow sensor with cantilever tip widths of
Wbeam = 400 fim for gas flow rate range of 0 to 30 ms'1.
Stability o f flow sensors
The experimental results indicate that at a constant flow rate of 30 m s'1

and a temperature of 25UC, the variation in stability of the three flow sensors is found to
be 0.0275%, 0.0346% and 0.0450% for cantilever tip width (Wbeam) of 400 |im, 1200
p.m and 2000 nm, respectively. As shown the experimental data, the sensors stability
decreases with an increasing cantilever tip width. It is clear to know the increasing
cantilever tip width not only can enhance the flow sensor sensitivity but also decrease the
response time. However, it may cause the flow sensor stability to be reduced. The
resonant frequencies of the cantilevers are estimated to be 17.3 kHz, 8.7 kHz and 5.8
kHz, which are far above the possible environmental activation frequency.
Applications
• Exhaust Stack Flow Monitoring

• Air Control in Drying Processes
• HVAC Air Velocity Measurements
• Fan Supply and Exhaust Tracking.
3.5 PID CONTROLLER
A proportional-integral-derivative controller (PID controller) is a generic

control loop feedback mechanism (controller) widely used in industrial control systems.
A PID controller attempts to correct the error between a measured process variable and a
desired set point by calculating and then outputting a corrective action that can adjust the
process accordingly and rapidly, to keep the error minimal.
The PID controller calculation (algorithm) involves three separate parameters; the
proportional, the integral and derivative values. The proportional value determines the
reaction to the current error, the integral value determines the reaction based on the sum
of recent errors, and the derivative value determines the reaction based on the rate at
which the error has been changing. The weighted sum of these three actions is used to
adjust the process via a control element such as the position of a control valve or the
power supply of a heating element.
By tuning the three constants in the PID controller algorithm, the controller can provide
control action designed for specific process requirements. The response of the controller
can be described in terms of the responsiveness of the controller to an error, the degree to
which the controller overshoots the set point and the degree of system oscillation. Note
that the use of the PID algorithm for control does not guarantee optimal control of the
system or system stability.
Some applications may require using only one or two modes to provide the appropriate
system control. This is achieved by setting the gain of undesired control outputs to zero.
A PID controller will be called a PI, PD, P or I controller in the absence of the respective
control actions. PI controllers are particularly common, since derivative action is very
sensitive to measurement noise, and the absence of an integral value may prevent the
system from reaching its target value due to the control action.
The PID control scheme is named after its three correcting terms, whose sum constitutes
the manipulated variable (MV),where Pout, /out, and Dout are the contributions to the
output from the PID controller from each of the three terms, as defined below.
P k
Fig 3.7: PID Controller with Kp, Kj, Kd

Proportional term
The proportional term (sometimes called gain) makes a change to the output that is
proportional to the current error value. The proportional response can be adjusted by
multiplying the error by a constant Kp, called the proportional gain.
The proportional term is given by:
Where
• Pout: Proportional term of output

• Kp\ Proportional gain, a tuning parameter
• e: Error = S P - PV
• t: Time or instantaneous time (the present) '
A high proportional gain results in a large change in the output for a given change in the
error. If the proportional gain is too high, the system can become unstable (See the
section on loop tuning). In contrast, a small gain results in a small output response to a
large input error, and a less responsive (or sensitive) controller. If the proportional gain is
too low, the control action may be too small when responding to system disturbances.
In the absence of disturbances, pure proportional control will not settle at its target value,
but will retain a steady state error that is a function of the proportional gain and the
process gain. Despite the steady-state offset, both tuning theory and industrial practice
indicate that it is the proportional term that should contribute the bulk of the output
change.
Integral term
The contribution from the integral term (sometimes called reset) is proportional to both
the magnitude o f the error and the duration of the error. Summing the instantaneous error
over time (integrating the error) gives the accumulated offset that should have been
corrected previously. The accumulated error is then multiplied by the integral gain and
added to the controller output. The magnitude of the contribution of the integral term to
the overall control action is determined by the integral gain, Kt.
The integral term is given by:
Where
• /out: Integral term of output

• K,: Integral gain, a tuning parameter
• e: Error = SP - PV
• t: Time or instantaneous time (the present)
• t : A dummy integration variable
The integral term (when added to the proportional term) accelerates the movement of the
process towards set point and eliminates the residual steady-state error that occurs with a
proportional only controller. However, since the integral term is responding to
accumulated errors from the past, it can cause the present value to overshoot the setpoint
value (cross over the set point and then create a deviation in the other direction). For
further notes regarding integral gain tuning and controller stability, see the section on
loop tuning.
Derivative term
The rate of change of the process error is calculated by determining the slope of the error
over time (i.e., its first derivative with respect to time) and multiplying this rate of change
by the derivative gain K<j. The magnitude of the contribution of the derivative term
(sometimes called rate) to the overall control action is termed the derivative gain, Kj.
The derivative term is given by:
Where
• £>out: Derivative term of output

• Kd'. Derivative gain, a tuning parameter
e: Error = S P - PV
t: Time or instantaneous time (the present)
Controller Output is,
u ( t ) = M V ( t ) = K pe { t ) I K i
The derivative term slows the rate of change of the controller output and this effect is
most noticeable close to the controller set point. Hence, derivative control is used to
reduce the magnitude of the overshoot produced by the integral component and improve
the combined controller-process stability. However, differentiation of a signal amplifies
noise and thus this term in the controller is highly sensitive to noise in the error term, and
can cause a process to become unstable if the noise and the derivative gain are
sufficiently large.
The proportional, integral, and derivative terms are summed to calculate the output of the
PID controller. Defining u{t) is the controller output, and the tuning parameters are:
Proportional gain, Kp
larger values typically mean faster response since the larger the error, the larger
the Proportional term compensation. An excessively large proportional gain will
lead to process instability and oscillation.
Integral gain, Kj
larger values imply steady state errors are eliminated more quickly. The trade-off
is larger overshoot: any negative error integrated during transient response must
be integrated away by positive error before we reach steady state.
Derivative gain, KD
larger values decrease overshoot, but slows down transient response and may lead
to instability due to signal noise amplification in the differentiation of the error.
3.6 AIR FLOW CONTROL - DAMPER ARRANGEMENT
Air flow control in coal-fired boilers, in other words, the control of

secondary air flow in to the furnace is needed, as the secondary air is responsible for the
combustion process. In order to control the secondary air flow in to the furnace, we need
to control the ‘Blade pitch’ or Position of control damper of the FD Fan.
Fig 3.8: Diagram which depicts the air flow control in furnace.
FD fans are those, which can be used to draw the secondary air in to the
plant from the environment. FD Fans are equipped with control dampers which controls
the amount of air flow in to the furnace. The control damper is a series of blades
connected in synchronization and based on the position of the control damper; the air
flow entering the furnace will changes.
Damper:
A damper is a valve or plate that stops or regulates the flow of air inside a
duct, chimney, or other air handling equipment. A damper may be used to cut off central
air conditioning (heating or cooling) to an unused room, or to regulate it for room-by-
room temperature and climate control. Its operation can be manual or automatic. Manual
dampers are turned by a handle on the outside of a duct. Automatic dampers are used to
regulate airflow constantly and are operated by electric or pneumatic motors, in turn
controlled by a thermostat or building automation system.
Fig 3.9: Opposed blade dampers in a mixing duct

In a chimney flue, a damper closes off the flue to keep the weather (and birds and other
animals) out and warm or cool air in. This is usually done in the summer, but also
sometimes in the winter between uses. In some cases, the damper may also be partly
closed to help control the rate of combustion. The damper may be accessible only by
reaching up into the fireplace by hand or with a woodpoker, or sometimes by a lever or
knob that sticks down or out. On a woodbuming stove or similar device, it is usually a
handle on the vent duct as in an air conditioning system. Forgetting to open a damper
before beginning a fire can cause serious smoke damage to the interior of a home, if not a
house fire.
The Dampers of Fans being used in thermal power plants are as shown below.
Fig 3.10: Control Damper with Series of Blades
The blades are so arranged in such a way that, the air will not enters in
only when all blades are aligned vertically (90 Deg).When they are aligned in this
position, the damper blades completely block the air to the maximum extent and there is
no probability of air to enter the furnace. And, on the other hand, if the blades are made
to align horizontally i.e. 0 Deg, they allow the air completely to pass through them
thereby the passage of air through the Fan will be maximum.
A ir flow Air flow
F ig i : W hen B la d e s a re alig n ed H orizontally F ig 2 : "When B la d e s are align ed V ertinelly
Air flow
F ig 3: W hen B la d e s are align ed a t an gle b etw een

0 to 9 0 D e j
Fig 3.11: Blades alignment in different angles.
But, apart from these two positions of blades, we can alter the position of
blades continuously from 0% to 100% of opening, so that, air flow can be controlled
accordingly. The position of blade or the position of control damper of the Fan is called
Blade pitch and is the key factor to be controlled for the control of Air flow in to the
Coal-fired Boilers.
The total air flow demand or set point for total air flow can be set to any
particular value from the Operator Work Station OWS .It is set to a value, approximately
5 times to the amount of coal flow in to the furnace. The actual air flow in to the furnace
is measured by using Air flow transmitters and the two values are fed to PID Controller.
The error, i.e. the difference between the actual air flow and the total air flow demand
(Set Point) is derived from the PID controller, it actuates the control damper through a
servo system, which consists of mechanical arrangement and high pressured fuel oil to
change the position of control damper. The position of control damper changes according
to the error signal from the controller.
When the position of control damper changes, accordingly the air flow in
to the furnace changes. Hence, the “regulation of Air flow” in to the Coal-fired Boilers.
CHAPTER 4
LOGIC DIAGRAMS FOR

THE CONTROL OF
AIR FLOW
Air Flow Control in Coal-fired Boilers-
Symbols Used:
LEGEND mrmzNMMon
© PLOW TRANSMITTER
tN T E G E f tM U R
© PRESS TRANSMITTER
BRWE / AUTO MANUAL INTERFACE
LEVEL TRANSMITTER
0
® TEMP ELEMENT CHANGE OVER ELEMENT/SWITCH
OF poTRUE THEN b-c, ELSE a■■»<:]
0 POSfTON TRANSMITTER
© CURRENT TRANSDUCER CHANGE OVER ELEMENT / SWITCH
[IF p-TRUC THEN b»c, ELSE a«*e)
speed transducer
MAXIMUM GATE
E/P CONVERTOR
MINIMUM GATE
PNL MOUNTED PR IND
OPERATOR WORK S T A T I O N /
PNL MOUNTED LVL IND LARGE VIDEO SCREEN
PNL MOUNTED TEMP INO PARAMETER
PNL MOUNTED LVL REG DIGITAL INDICATOR
PNL MOUNTED PR R£C
PNL MOUNTED FLOW REC MOSAIC DUEL SCALE INDICATOR
PNL MOUNTED TEMP REC
c ] MOSAIC SINGLE SCALE INDICATOR
PI PI CONTROLLER
MOSAIC 3P8/3IL CONTROL TILE
PIO P1D CONTROLLER
SET POINT
SELEGTlVe^AVERAGE CKT BIAS STATION
OR GATE
_f«0J function’ generator
NOT GATE
A SUBTRACTOR
AND GATE
LVM LIMIT VALUE MONITOR
PULSER
TX
5gL TX SELECTION CKT SQUARE ROOT
summator MULTIPLIER
ON DELAY
DEAD BAND
ows OPERATOR WORK STATION
NATIONAL THERMAL POWER CORPORATION LTD’
1 RAMACUNDAM STPP, STAGE-III, <1x500 MW).
Flue Gas 0 2:
r G t t j AT AH INLET
NATIONAL THERMAL POWER CORPORATION LTD

ORG NO Sh No: 47
RAMAGUNOAM STPP, STAGE—III, (1x500 MW)
The heat energy developed inside the furnace causes the feed water entering the
boiler to get converted in to steam in the boiler. In order to achieve better combustion in
furnace, the amount of air entering the furnace should be optimum ad should be
controlled if it exceeds the predefined limit.
The analog control scheme for the air flow control is shown in above diagrams.
The symbols used in these schemes are shown.
Four air flow transmitters are used for air flow measurement, among which 2
are placed on the left side and the remaining 2 are placed on the right side of the boiler. In
addition to these transmitters, 4 no. of temperature elements are also arranged at the same
places. The purpose of these temperature elements is for the temperature compensation.
Square root extractors are also provided for linearising the secondary air flow.
The resultant signal is made available at OWS (Operator Work Station) with the help of
MAXDNA System. Among the two air flow transmitters, only one transmitter with low
value is selected from OWS or the average of both transmitter values is taken in to
consideration. The left side air side air flow with temperature
compensations are fed to summer/tfader t^-edctiiaE^TOtal secondary air flow in to the
fomace- t l& G 'l- jf jj

On the other hand, primary, air flow m ^JJche different mills is measured
individually and flows are added total primary air flow in to the
furnace. Later the total primary air flowafirRSSnsecondary air flow are added to obtain
total air flow entering the furnace. Here the amount of air entering is recorded by the
recorder at each stage during the process. The total air flow measured is the toal actual air
flow indicated in the plant. This amount of air flow will be compared with the set point
later.
The Oxygen content in the flue gases (FG O2) at the air heater inlet is measured
with the help of four FG O2 electrodes. These values are brought at OWS so that the
operator can continuously monitor and control the process right before sitting in front of
the panel. Out of four values, only one is taken in to consideration. Set value for FG 0 2
% is given from OWS. The measured value from the electrode is compared and the
generated error is fed to PID controller. When the difference is too high, then alarm is
signaled, indicating the inefficient combustion process in the furnace. The PID controller
output is recorded at A/M (Auto/Manual) panel.
Air flow demand is truncated by a numerical value so as to compare with fuel

flow. Maximum value from the both is considered. On the other hand, the air flow
actually measured is also truncated and compared with the set point. The error signal is
given to PID controller. This controller output generated is interfaced to control damper
of FD Fans through A/M stations and interfaces.
The two control dampers of FDF-A, FDF-B are equally loaded by the controller
output generated by the controller. At each stage, the values of various parameters are
being monitored and recorded too. This controller output operates the control damper of
fans according to the bias given to them.
The air flow according to the damper position is shown with the help of
different angles of blades position. High/low limiters are used to limit-the-value in case
the oxygen analyzer is out of service. Under any circumstances the air flow shouldn’t be
less than 30% MCR (Maximum Continuous Rating) flow. This signal is the developed set
point and the air flow signal will have proportional and integral action in the air flow
controller. This position demand signal will be selected to the corresponding FD fans in
service through auto/manual station. To have equal loading of FD fans, FD fan motor
current is measured. The difference is used for taking corrective action.. The corrected
signal is used to position the FD fan regulating damper. Boiler auxiliaries interlock
system and Maximum deviation limits (MDL) etc are provided.
To ensure air rich furnace at all times, a maximum deviation limit system
(MDL) is used i.e., Whenever the fuel flow is more than the air flow, this will
automatically reduce the fuel flow and increase the air flow to a safe value and both the
air flow and fuel flow control is transferred to manual. Separate auto/manual station and
position indicator for each FD fan regulating device are provided.
The controller output actuates the mechanical system through interface system
and the mechanical system controls the damper position of fans and hence the damper
position of fans regulates the air flow.
4.1 About MaxDNA SYSTEM in NTPC-Ramagundam
Introduction:
METSO AUTOMATION MAX controls is offering the MAXDNA
distributed control system(DCS).This product is the latest in a long evolution os mission
critical systems provided to the power industry worldwide. This system includes a
modem NT native. Human machine interface(maxView,maxTools),a reliable high
performance fully redundant network(maxNet),a high performance Ethernet resident
DPU’s and a field proven, fully tested I/O System which provides solid, reliable service
to his facilities around the world.
The model DPU which runs under Windows real-time, multi tasking OS is
the hardware processing engine of the MAXDNA DCS. The DPU performs primary data
acquisition, control and data processing functions. DPU also known as MAXDPU4E is a
self contained micro processor based rack mounted unit which occupies either a single
scot in a remote- mounted unit carbine using a 4-wide back plane.
4.1.1 MaxDNA System Architecture:
boiler parameters it consists of MaxSTATIONs, maxSTORIAN, Ethernet Network and

remote I/O which connected with maxNET.
As a station on MAXNET, the DPU scans and processes information for use by
other devices in the MAXDNA system. Each DPU perform:
> Comprehensive alarming and calculations.
> Logging of sequence of events (SOE) data at 1 millisecond resolution
> Acquisition of trend information
> Continuous scanning of model I/O processor and I/O modules
> Execution of predefined algorithms called functional blocks for process
control and data acquisition.
> True open system architecture

> Noiseless communication between plant and network
> Measured and control from small process applications to complete plant
control
> Online graphic monitoring
> Online self documentation configuration
> Ultra high speed graphic building and displays updates
> Centralized engineering capability
4.1.4 MaxSTORIAN:
MaxSTORIAN is the serverless systems consist of 40GB memory hard disk
which displays controlling parameters levels and stores the data from Distributed
Processing Unit (DPU).
> Historical data collection and archiving software for maxSTATION

> Data storage maximizes system data capacity
> Relational database to simplify reports and queries for data analysis
> Archived storage to CD-ROM
> Supports interface to plant networks
> Windows 2000 based
4.1.5 MaxLINK:
MaxLINK is connected between the maxSTATION and plant sub systems
through maxNET.
> Interface to external systems: PLC networks (Coal handling plant, ash
handling plant etc.)
> Supports multiple simultaneous communication protocols
> Supports data transfer from other systems and networks
> All data available to maxSTATIONS
> Windows 2000 based
MaxDNA DPU4E:
MaxDNA is a Distributed Control System (DCS). The DPU performs data
acquisition, control and data processing functions.
DPU Features:
- Real-Time multitasking
- Object Oriented database
- One-to-one redundancy capability
- Diagnostics that are accessible by Workstation and PC direct connection
- 8,000 alarm/event queue to insure no-loss of alarms and events
- Flash memory for non-volatile configuration storage and on-line and off
line firmware updates
MaxDNA DPU4E FUNCTIONALITY:

• The DPU4E is the process controller.
• The DPU4E is a multifunction processor that executes
> control algorithms,
> sequence logic,
> SOE time tagged to 1 ms resolution
> Data acquisition.
A DPU consist of a printed circuit board containing the control processor and
Input/Output processor (IOP) attached to a DPU chassis. The DPU’s front panel contains
status LEDs, and takeover and reset buttons, while the DPU’s front chassis panel contains
network, backup and serial port connectors, mode and network address switches, and the
key switch.
maxDNA DPU4E FUNCTIONALITY
MaxDNA THE REMOTE PROCESSING UNIT (RPU):

The Remote Processing Unit (RPU) is the group of equipment that provides
Si The control
© Data acquisition
® And I/O processing functions for the DCS
The equipment includes

£ DPUs
Q Field I/O modules
Q Field terminations
Q Communication networks
© And peripheral equipment such as power supplies, & cabinets.
CHAIPTim §
RESULTS AND
CONCLUSION
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Fig 5. 2: Details of A ir H o . corresponding to the Controller action.
T , . corves shown in the above diagrams are of different colors. These
different colors indicate the different parameters in the power plant viz,
total fuel flow, FDF-A damper position and FDF-B Datnper postUons re p
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f r D F B O am psi Position 50.5468

Air flow Control in Coal-fired Boilers- 73
lHLB02AA101_XQ50.out.S‘[iFDF-B Damper Position
Total S A Flow
Total Aii Flo w Actual
IS t e a m F l o w
O xygen Actual
F O F - A D a m p a i P o s it io n P A H e a tle t P i e s s u i e
D F B D am per P. nation
Fig 5.4: Position of Control dam per of FD Fan-B.
Steam flow : 1764.68 Tph

Total actual fuel flow : 1769.76 Tph.
Primary Air Header Pressure : 1094 Tph
FD Fan-B Damper position : 50.54%
KMBUlte
■U •■'i
!' [Air Flow SP 1623.780® *r> j" 11rital SA1 low 1005.5820fr B<
<* Total Ail Flow Actual 1627.3646 21 loi.il PA Flow 621.8534 f :‘H;
r Oxyijen SP Bad DataJ : JHSteam Flow 1 4 5 5 .8 5 5 2 ^
OxyijHn Adu<il 3.7734 j | -r..
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^F D F B Barapej Position 48.6795 jjiST ACTIIAI. I OAD 470.7579|-'.'V
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f OF A Ddinpei Position
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ST ACTUAL LOAD
Observations:
Total actual Air flow measured : 1627.36 Tph.

Total fuel flow measured : 292.90 Tph.
Flue Gas O2 content : 3% to 5% (Normal)
: 3.77 % (Actual)
FDF-A Control damper : 33.34%
FDF-B Control damper : 48.67%
CONCLUSION
In boilers, there may be flow of air more or lesser than the optimum value
recommended for the plant. If it exceeds the limit, it causes continuous flow of air into
furnace and hence pressurization of boiler, which may lead to hazardous conditions and
wastage of fuel too. If it is present lesser than the limit, it results in inefficient combustion
and hence the atmospheric pollution with dangerous gases like CO2 , SO2 , CO etc.
In order to provide efficient combustion and to ensure safety from the

hazardous conditions, the amount of air entering the furnace needs to be controlled.
AIR FLOW CONTROL is the control based on the continuous monitoring

and regulating the air flow through the damper positions of the two FD fans in the plant.
PID Control is used for this purpose to effectively control the flow of air to enter the
furnace. This Air flow control loop regulates the amount of air entering the boiler in coal-
fired boilers.
BIBLIOGRAPHY
1. www.ntpc.co.in/ramagundam
2. www.thermo.com
3. Methods for Controlling Large Fan of Thermal Power Plant Boiler Using Inverter in
Abnormal State, IEEE Tran. On control parameters; Nobmasa Matsui, Fujio
Kurokawa.
4. Gas Flow Sensor for Flow Rate and Direction Detection, IEEE Tran. On control
Parameters; Yu-Hsiang Wang, Chien-Hsiung T sai.
5. The air monitoring system in production and transmission of Electricity, IEEE Trans.
Franc Jakl, Kresimir Bakic, Leon Vale.
6. Logic diagrams from Unit-7 of NTPC Ramagundam.
7. Boiler combustion air flow measurement by Yokogawa.
8. www.en.wikipedia.org/thermal power stations.htm
9. Article on Emerson Process Management Power & Water Solutions
10. Article on closed loop flow control using instrumented inlet nozzles;
11. http://www.ebmpapst.us
12. Control of Air flow rate with stack voltage measurement by G. VASU and A. K .
TANGIRALA
13. Real time coal-flow and particle size measurement for improved boiler operation by
S. Laux, J. Grusha, K. McCarthy, Foster Wheeler Energy Corporation.
14. Airflow Measurement for HVAC Systems - Technology Comparison by David S.
Dougan.
15. www.labcor.com and article on Air velocity transducers.
16. Electro static precipitator systems, from Wheelabrator Air Pollution Control Inc.

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AIR FLOW CONTROL IN COAL-FIRED BOILERS

A dissertation submitted in partial fulfillment o f the requirement for the

Under the supervision o f

Dr. m m A IAGGI SUBRA W AW A M CV8

Asst. Professor Sr. Superintendent

MATRONAL "NSTHTUTE OIF TECHNOLOGY

I am the student of NATIONAL INSTITUTE OF TECHNOLOGY

This Project is an embodiment of the effort of several persons to

First I would like to express our sincere thanks to Project External

I wish to express my deep sense of gratitude to my Project Internal

I express my sincere thanks to Dr. S.K. Mahna, Professor &

Venkata Rajesh Padala

CHAPTER I: OBJECTIVE OF THE PROJECT

CHAPTER II: BASICS OF A THERMAL POWER PLANT

2.1 Energy conversion in Power Plant. 7

CHAPTER IH: INSTRUMENTATION FOR AIR FLOW

3.3.2 Flue Gas Stack 39

3.3.3 Mechanical System 40

3.4 Sensors used for air flow measurement. 40

3.4.3 Sensitivity of gas flow sensors 46

3.5 PID Controller 48

CHAPTER IV: LOGIC DIAGRAMS FOR THE CONTROL OF

4.1.2 MaxDNA System with THERMAL STATION 66

CHAPTER V: RESULTS & CONCLUSION 70

Airflow into the Coal-fired boilers finds significant role in

Boiler is the main part of the Coal-fired power plant. Boiler is a

Different types of boilers are available viz. Coal-fired boilers, Fire

So, Air plays a significant role in the combustion process of Coal-

1.3 OVERVIEW OF NTPC RAMAGUNDAM

In the Forbes list of ‘World's 2000 largest companies, 2007’, NTPC

GROWTH OF NTPC, INSTALLED CAPACITY & GENERATION:

Growth of NTPC Installed Capacity & Generation

Fig 1.1: Installed Capacity and Generation of Power by NTPC.

NTPC Ramagundam is committed to the production and delivery of a

We will strive through continual improvement:

> To protect our environment, prevent pollution and minimize wastages.

Fig 1.3: 500MW Unit of NTPC Ramagundam (Unit-7)

Since inception, the station has achieved excellence in all operational

Control & Instrumentation:

2.1 ENERGY CONEVRSION IN POWER PLANT

Conversion of energy during the power generation is as shown below.

Chemical Energy M echanical Energy Electricity

2.2 FOUR ESSENTIAL CIRCUITS IN THERMAL POWER PLANT:

circuits which are,

1. Coal and Ash circuit

2. Air and Gas circuit

3. Feed Water and Steam circuit

4. Cooling Water circuit.

Coal and Ash Circuit:

removed to ash storage through ash-handling system.

Air and Gas Circuit:

removed before exhausting the gases to the atmosphere.

Feed Water and Steam Circuit:

purifying plant to reduce dissolve salts to an acceptable level. This purification is

necessary to avoid the scaling of the boiler tubes.

Cooling Water Circuit:

are more economical than closed systems.

Site Selection of a Thermal Power Plant:

2.4 COAL FEEDER AND PULVERIZERS