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 NIT Kurukshetra

Sr. Superintendent 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

CHAPTER I:

Page N o

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 Coalfired 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 Coalfired 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.

Air Flow Control in Coal-fired Boilers- 3

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

2607-68

MW

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

Air Flow Control in Coal-fired Boilers- 5

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

Air Flow Control in Coal-fired Boilers- 6

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

Air Flow Control in Coal-fired Boilers- 7

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 Flow Control in Coal-fired Boilers- 8

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

Air Flow Control in Coal-fired Boilers- 9

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

Air Flow Control in Coal-fired B oilers-11

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

Air Flow Control in Coal-fired B oilers-12

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 steamgenerating 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.

Air Flow Control in Coal-fired B oilers-14

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.

Air Flow Control in Coal-fired Boilers-16 •

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

Air Flow Control in Coal-fired Boilers-17 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 steamgenerating 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.

Air Flow Control in Coal-fired Boilers-18 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 pre- heater 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.

Air Flow Control in Coal-fired Boilers-19 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.

! I

Boiler auxiliaries are: 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.

Air Flow Control in Coal-fired Boilers- 20 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.

Air Flow Control in Coal-fired Boilers- 21

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.

Air Flow Control in Coal-fired Boilers- 22 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.

Air Flow Control in Coal-fired Boilers- 23

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 Drum

Reheated mam Reheater

+tQh ftllSIl

twfoirre exhaust

eiowdoi..

steam

..

Boiler ‘vV

Deaerarted

6©U*f

Hot « r Note: APH is the air presenter

Fig 2.8: A ir preheater in a therm al plant.

Air Flow Control in Coal-fired Boilers- 24

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.

Air Flow Control in Coal-fired Boilers- 25

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

Air Flow Control in Coal-fired Boilers- 26

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

Air Flow Control in Coal-fired Boilers- 27

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

Air Flow Control in Coal-fired Boilers- 28

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, .

Air Flow Control in Coal-fired Boilers- 29 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®

Air Flow Control in Coal-fired Boilers- 31

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.

Air Flow Control in Coal-fired Boilers- 34

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.

Air Flow Control in Coal-fired Boilers- 35

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

Air Flow Control in Coal-fired Boilers- 37

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

Air Flow Control in Coal-fired Boilers- 38 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

Air Flow Control in Coal-fired Boilers- 39

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

Air Flow Control in Coal-fired Boilers- 40

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.

Air Flow Control in Coal-fired Boilers- 41

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 ) r 24E l 6EI

(1)

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.

Air Flow Control in Coal-fired Boilers- 43

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.

Air Flow Control in Coal-fired Boilers- 44

micro-cantilever

substrate I

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.

Air Flow Control in Coal-fired Boilers- 45

Fig 3.3: Side View image of Cantilever beam

<*>

(t» >



Fig 3.4: Images of Cantilever beams of Different Widths.

(a) Wbeaa = 400 um. (b)

= 1.200 um and (c)

= 2.000 um

Air Flow Control in Coal-fired Boilers- 46

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].

Air Flow Control in Coal-fired Boilers- 47

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.

Air Flow Control in Coal-fired Boilers- 48

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

Air Flow Control in Coal-fired Boilers- 49

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

Air Flow Control in Coal-fired Boilers- 50

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

Air Flow Control in Coal-fired Boilers- 51

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

Air Flow Control in Coal-fired Boilers- 52

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.

Air Flow Control in Coal-fired Boilers- 53

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.

Air Flow Control in Coal-fired Boilers- 54

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-byroom 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

Air Flow Control in Coal-fired Boilers- 55

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.

Air Flow Control in Coal-fired Boilers- 56

Air flow

A ir 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

©

PRESS TRANSMITTER

0

LEVEL TRANSMITTER

BRWE / AUTO MANUAL INTERFACE

® 0 ©

TEMP ELEMENT

CHANGE OVER ELEMENT/SWITCH OF poTRUE THEN b-c, ELSE a■■»<:]

tN T E G E f tM U R

POSfTON TRANSMITTER

CHANGE OVER ELEMENT / SWITCH [IF p-TRUC THEN b»c, ELSE a«*e)

CURRENT TRANSDUCER speed transducer E/P CONVERTOR PNL MOUNTED PR IND

MAXIMUM GATE MINIMUM GATE

PNL MOUNTED LVL IND

OPERATOR WORK S T A T I O N / LARGE VIDEO SCREEN

PNL MOUNTED TEMP INO

PARAMETER

PNL MOUNTED LVL REG

DIGITAL INDICATOR

PNL MOUNTED PR R£C PNL

MOSAIC DUEL SCALE INDICATOR

MOUNTED FLOW REC

PNL MOUNTED TEMP REC PI

PI CONTROLLER

PIO

P1D CONTROLLER

c ]

MOSAIC SINGLE SCALE INDICATOR MOSAIC 3P8/3IL CONTROL TILE

SELEGTlVe^AVERAGE CKT

_f«0J A LVM TX

5gL

ows

function’ generator

SUBTRACTOR LIMIT VALUE MONITOR

SET POINT BIAS STATION OR GATE NOT GATE AND GATE PULSER

TX SELECTION CKT

SQUARE ROOT

summator

MULTIPLIER

ON DELAY DEAD BAND OPERATOR WORK STATION NATIONAL THERMAL POWER CORPORATION LTD’ 1 RAMACUNDAM STPP, STAGE-III, <1x500 MW).

Air Flow Control in Coal-fired Boilers- 59

Flue Gas 0 2: r G t t j AT AH INLET

NATIONAL THERMAL POWER CORPORATION LTD ORG NO RAMAGUNOAM STPP, STAGE—III, (1x500 MW)

Sh No: 47

Air Flow Control in Coal-fired Boilers- 62

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

Air Flow Control in Coal-fired Boilers- 63

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 Control in Coal-fired Boilers- 64

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.

Air Flow Control in Coal-fired Boilers- 65

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.

Air Flow Control in Coal-fired Boilers- 67

> 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

Air Flow Control in Coal-fired Boilers- 68

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.

Air Flow Control in Coal-fired Boilers- 69

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

'?May2003 >35:4000 1774 4445(TPH

fOTAlAIR FLOW

Ou^Sampte Oul Sample

600 X '^ f lr O t t lf U E -

QulSaffipie

FAN-AC0N7DAMPER

I ioaoo

.Oii Sample

550294|?

FAN-0 CONT DAMPER

[Thmwl jp7W77 T^mrFfn

rmxtmn

000

[ B S y

542

Im S P R '

173.9

HST«fP

53®

IF U R P R

___ aoo

trrm.y

m h-rrm.V

-0.881

05:30

im a m Silence \1j

•Vk.U! I siSAa

p s& o w . o iu m SSiA -g

SA8C

:

sew SHSKAE3

rosip-A ® S»S

tm j U5B»- asras$ss.v. m » j >k m

P*>

.j

HP H U ,

ID S iE K

Jwwsrj ® 9 3 S 2 |

;

Air Flow Control in Coal-fired Boilers- 71

AH jAohv©

3

PID_B Function Detai for ‘0HHL10DF151 yUNlT7/C0Al/AFL00P/Contral/HHL100Ft5r

|

Alphabetical J Categorized | By Channel) Security j Help Member

f v a l ue

OUTCTL O u tP c t P e rtu rb P ro p S P M u lt PV PVBad

0 .0 4 7 .3 0 4 5 5 8 .0

» > » »

P V In C tl PVQual i t y R a te R a te S P M u lt R eset S hD esc SP SPHighRng SPLowRng SPM odeLocal SPM odeRemote S P O p D o u b leln crm S R O p S in g le ln c rm SPR ateL m t

1.0 1 7 8 6 .9 0 .6 1 7 8 6 .9 0 .0 0 .0 0 .0

»

» “A i r F lo w C o n t r o l l e r " » 1 7 8 4 .9 8 1 1 0 0 0 .0 0 .0 0 .0 1 .8 » 5 .0 » 1 .0 s 1 0 0 .0 0 .0 SPRemAWUDecLmt ■ > 0 .0 SPRemAWUHys SPRemAWUIncLmt t 0 .0 S P R em oteA ctv » T ru e 1 7 8 4 .8 28 S P R e m o te ln C tl S P R e m o te S e le c td » T ru e 0 .0 SPR em oteSusp S P T r g tT r a c k » t r a c k s b o th * 0HHL10DF15T' TagName "" T itle

] C a te g o r y

R e fe re n c e ./ # 5 .O u t C t l

O u tp u t S ta tu s T u n in g T e s t

C o n fig S ta tu s . / # 2 0 7 .O u t O u tp u t 0HHL10DF904. O u tC tl I n p u t ./ # 1 0 7 .I n Q u a l O u tp u t T u n in g C o n fig T u n in g P a r a m e te r ./ # 5 .S P I n C t l R H I ./# S .S P In C tlR L O

i AFSP.Ou t C

S ta tu s O u tp u t O u tp u t In p u t In p u t C o n fig L im it C o n fig L im it C o n fig L im it T uningR em ot T um ngR em ot TuningR em or S ta tu s In p u t S ta tu s In p u t C o n fig P a r a m e te r

tl

S heet

D e s c rip tio n

S e c u r i t y C la s s

C o n tr o l I n c r e m e n t O u tp u t O u tp u t p e r c e n t b a s e d on r a n g e V a lu e o f s t e p c h a n g e f o r t u n i n g P ro p o rtio n a l s e t p n t m u l ti p l i e r P r e v i o u s s c a n s ' PV v a l u e

0 (M odeChange) 5 (C o n fig u ra t1 o 4 (T u n e /A d ju st) 4 (T u n e /A d ju st) 5 (C o n f1 g u r a tio 0(M odeC hange) 0(M odeC hange) 0(M odeC hange)

C o n tr o lle r P ro c e s s V a ria b le PV Q u a l i t y PID R a te Tim e R a te s e t p o i n t m u l t i p l i e r PID R e s e t S h o rt d e s c r ip tio n S e tp o in t

4 (T u n e /A d ju s t> 4 (T u n e /A d ju st) 4 (T u n e /A d ju st) 5 ( C o n fig u ra tio

5 ( C o n fig u ra tio 8(M odeC hange) 0(M odeC hange) S e t L o c a l S e t p o i n t Mode 8(M odeC hange) S e t R em ote S e t p o i n t Mode 0(M odeC hange) In p u t D 5 ( C o n f ig u r a tio In p u t C 5 (C o n f ig u r a tio O u tp u t m c r r a t e o f c h a n g e l i m i t J ( C o n f i g u r a t i o SP W indup d e v i a t i o n t o l e r a n c e 4 (T u n e /A d ju st) H y s t e r e s i s f o r SP a n t i w i n d u p 4 (T u n e /A d ju st) SP W indup d e v i a t i o n t o l e r a n c e 4 (T u n e /A d ju st) O p e r a t i n g i n a u t o when t r u e S (C o n fig u ra tio R em ote S e t p o i n t I n p u t 0(M odeC hange) 0 =» M a n u a l, 1 * A u to 5 ( C o n f ig u r a tio S e t p o i n t R em ote Op S u sp en d 0(M odeC hange) T a r g e t tr a c k in g o p tio n 5 (C o n fig u ra tio TagName S (C o n fig u ra tio C o n fig u ra tio n S h eet T i t l e 5 (C o n fig u ra tio

Ed

- -

23May09 \T P ( M ai* sc

fdtft lu x

Brvk

i: i-s r i234S67«iG12 {m stm p

1 54 3

rM S ~F R ~ Fh r t m p

173.9 539

|

8U

J

d p lv l

51.3

fw fl

" 1582.2

[

1586.0

1 fv v ppT

201.8

fo R P fT

189.6

[f U r

pr

iP4~HDR

8.32 812.8 1788.3

(h w l v l

-28.4

fcSw’

0.883

TAG

'

L ast I BOl H i l

tg

P rin t

Q U IC K TREND

6 9 .0 5 2 M M W C liM s a f iq w p

t CJJB

m m

6 8.656 MMWC1 ■

PJBAIE

K A ti

PAPHAAB FOBS? AH

3 0 t).8 t S A 1 L M P K iu t t i

(G H T JH

?OAPT aH

m

PUELFtDW

on. d e l

bcv;

M LIA E

SADC

ZUSW M

TbBFP-E

m iy -K

OfiCfAN

IHiHEAH

MLBFF-c

------------ —

; h-efp-a

| cond cy

□ ear

!HHLtODF151

j FW&DP.UM i~>4FM7»rt

irw rw a flfi v jm & ch

J i-y<567e33123‘VSe73S0 »234567*30125l<££7& S0123*^.

LOTMW-

“B0K-A1RPL0WC TRLmn"

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

Wtasm t£b®Mfemfi fe mmimomg 8mWawniMiE M®dfe

lufl.00

0 fsjv.

teas

5 S® c

i i : ■:>: >> 22j - 't;d)

’Hli •

22Ju'-JUUtf ;

|A5rFlowSP

H f Total Aii Flow Actual Osqf§|®#i) S P

H T o t^ S S A Flow

1769.7662 F “

I

Tut.il PA Flow

r

Steam flo w

3.7000 f >

j j Ot£^g®Bs A d u s ! t r S E i a m

1764 6809|’ % 4

4.1633 i

3

f r D F B O am psi Position

sm

i 50.5468

| 1094.232lfr « 675.014§fT f* 160 0.5 062TP—

f uni Flow

327.0885

PA Header Piessui®

8 1 8 .1 5 7 2 h t.A t

i

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 Total actual fuel flow

: 1764.68 Tph : 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 Oxyijen SP

1627.3646

21 loi.il PA Flow

621.8534 f :‘H;

r

OxyijHn Adu
Bad DataJ 3.7734

:

JHSteam Flow j

|

Dr A Damper Position

33.3473

I B P A Meadei Pressure

^F D F B Barapej Position

48.6795

jjiST ACTIIAI. I OAD

1 4 5 5 .8 5 5 2 ^ -r.. 820.72OS|"mV..: 470.7579|-'.'V

IFfeSoSs

partis

room m pswiK® 5©ms© ft® « r

ij ittws vara®, a®tom©auur

'laid te a wfa© to asMe^e fa© propsr ©oi it

i-M A il flo w S P lutdlAJr Flow Actual

1251.3358|t p h 1255.S531|i'H 1410.6697 b a d Dat

% g r a Actual f OF A Ddinpei Position

363916

i A Headei Piessiip

f l F R Damper Posiiitm

ST ACTUAL LOAD

Air Flow Control in Coal-fired Boilers- 76

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 %

FDF-A Control damper

: 33.34%

FDF-B Control damper

: 48.67%

(Actual)

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 coalfired boilers.

BIBLIOGRAPHY

Air Flow Control in Coal-fired Boilers- 77

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.