Boiler Feed Water Treatment:

A case Study Of Dr. Mohamod Shareef Thermal Power Station,

Khartoum State, Sudan

Motawakel Sayed Osman Mohammed Ahmed

B.Sc. (Honours) in Textile Engineering Technology

University of Gezira (2006)

A Dissertation

Submitted to the University of Gezira in Partial Fulfillment of the

Requirements for the Award of the Degree of Master of Science

In

Chemical Engineering

Department of Applied Chemistry and Chemical Technology

Faculty of Engineering and Technology

University of Gezira

January,2014

I

Boiler Feed Water Treatment:

A case Study Of Dr. Mohamod Shareef Thermal Power Station,

Khartoum State, Sudan

Motawakel Sayed Osman Mohammed Ahmed

Supervision Committee:

Name Position Signature Dr. Bshir Mohammed Elhassen Main Supervisor …………………. Dr. Mohammed Osman Babiker Co-supervisor …………………..

Date : January , 2014

II

Boiler Feed Water Treatment:

A case Study Of Dr. Mohamod Shareef Thermal Power Station,

Khartoum State, Sudan

Motawakel Sayed Osman Mohammed Ahmed

Examination Committee:

Name Position Signature Dr. Bshir Mohammed Elhassen Chair Person ……………… Dr.Bahaaeldeen Siddig Mohammed External Examiner ………………… Dr. Mustafa Ohag Mohammed Internal Examiner …………………

Date of Examination : 6. January.2014

III

Dedication

This research is affectionately dedicated to the souls of my parents , To my family With gratitude and love To whom ever love knowledge

I

Acknowledgment

I would like to thank Dr. Basher Mohammed Elhassan the main Supervisor for his guidance and help. my thank also extended to Dr. Mohammed Osman Babiker my Co-supervisor for his great help. my thanks also extended to Chemical Engineers in Dr. Mohamod Sharef Thermal Power Station.

II

Boiler Feed Water Treatment:

A case Study Of Dr. Mohamod Sharef Thermal Power Station, Khartoum State, Sudan

Motawakel Sayed Osman Mohammed Ahmed

Abstract A boiler is an enclosed vessel that provides a means for combustion heat to be transferred to water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process. The purpose of this research is to present the detailed findings about boiler feed water treatments, theories and application in power plants field with practical application on pre- treatment plants and anions exchanger unit . The realization of these objectives implies: online data analysis techniques for analytical and experimental work to be applied to raw water and treated water in laboratory, to analyze the water composition and properties through: determination of raw water parameters in reference to specifications. moreover, the work involves analyzing and comparing the water parameters over seasons of the year. The study area is Dr. Mohamod Sharef thermal power station. The samples were taken from the clarifier tank, demin tank, anion exchanger, boiler drum water , condensate water ,dearator water, feed water ,saturated steam and superheated steam and the turbidity ,conductivity and silicate were measured in March and compared with the same samples taken in January and August. In the clarifier tank the samples taken in March had a turbidity =1.88 NTU and the target value <20 NTU that means the obtained value is acceptable and compared with the turbidity reading of samples taken in January (1.72 NTU) and August (4.2 NTU), this means that in August (Autumn) the turbidity was high ,as to the pH the target is 6.5 – 8,its determined value was slightly beyond the standard, this could be due to some colloids that escaped settling in the clarifier. As to the conductivity the seasonal variation as in all readings were below the recommended value (<250µs /cm).The readings (beyond the measured values) showed some deviation from the standard. This means that water had high concentration of solids and impurities after the river station .Most of the values for the samples were within the target range of the turbidity, pH, conductivity and silicate, that means that the chemical dosing is specific and suitable, so it is recommended that the water feed into the boiler must be treated to generate high quality steam. III

معالجة المياه الداخلة للمرجل: منطقة الدراسة في محطة الدكتور محمود شريف للتوليد الحراري، والية الخرطوم، السودان متوكل سيد عثمان محمد احمد

ملخص الدراسة المرجل عبارة عن وعاء مغلق يستخدم حرارة االحتراق لتحويل الماء إلي ماء ساخن أو بخار،الماء الساخن والبخار تحت ضغط يستخدم لنقل الحرارة لعملية ما. الغرض من هذا البحث هو تقديم بالتفصيل متطلبات معالجة المياه الداخلة إلي المراجل نظريا وعمليا في حقول توليد الطاقة مع التطبيق العملي للمعالجة األولية لمحطات المياه ووحدة المبادل األيوني. ويتم تحقيق هذه األهداف بتحليل ودراسة هذه المياه الخام والمياه التي تمت معالجتها ومقارنتها بالمواصفات والمقاييس المطلوبة وكذلك مقارنتها ببقية فصول السنة. تمت الدراسة بمحطة الشهيد الدكتور محمود شريف الحرارية , أخذت العينات من حوض التنقية ومستودع تخزين المياه القادمة من محطة النهر بعد المعالجة األولية ومن المبادل األيوني ومياه اسطوانات المرجل ومنطقة البخار المكثف وماء السخان الطارد للهواء ومياه تغزيه المراجل ومنطقة البخار المشبع والمحمص تم قياس العكورة والموصلية الكهربية واألس الهيدروجيني في شهر مارس وقورنت بعينات أخذت في شهري يناير وأغسطس. في حوض التنقية وجدت العكورة )NTU 1.88( والقيمة المطلوبة )NTU 20>( وهي قيمة مقبولة وعند مقارنتها بالقراءة التي أخذت في شهري يناير أغسطس علي التوالي وجدت )NTU 1.72( و)NTU 4.2( حيث سجلت اعلي قراءة للعكورة في شهر أغسطس )الخريف( ،أما بالنسبة لألس الهيدروجيني فهو مطلوب من )8 - 6.5 ( وكل القيم المقاسة واقعة في نفس المدي بالرغم من وجود بعض الكائنات الحية الموجودة في حوض التنقية. كذلك الموصلية الكهربية كانت قراءتها في كل فصول السنة في المدي المطلوب )250µs /cm>( ,هنالك قراءات أظهرت بعض االنحرافات عن القيمة المطلوبة وهذا يعني أن الماء به مواد صلبة وشوائب بعد محطة النهر. معظم نتائج العينات تقع في المدي المطلوب من العكورة واألس الهيدروجيني والموصلية الكهربية و السيليكا, وهذا يعني أن الجرعات الكيميائية مناسبة ومحددة ،لذلك أوصي بمعالجة المياه الداخلة للمراجل إلنتاج بخار زو جودة عالية.

IV

TABLE OF CONTENTS

Item page Dedication…………………………………………………………….. i. Acknowledgment………………………………………………...... ii. Abstract (English)…………………………………………………….. iii. i Abstract (Arabic)……………………………………………………… iv. Table of Contents……………………………………………………… v. List of Figures…………………………………………………………. i List of Tables…………………………………………………………... Chapter one

Introduction

Introduction…………………………………………………… 1 Research Objectives…………………………………………….. 9 Chapter Two

Literature Review

2.1-Water treatment

2.1.1-Impurities in water………………………………………… 01

2.1.2-Pre-treatment……………………………………………… 14

2.1.3-Demineralization………………………………………….. 22

2.1.4-Additional treatment options ……………………………. 27

2.1.5-Other water purification techniques………………………. 28

2.2- Boiler water treatment:

2.2.1-Boiler feed water…………………………………………. 32

V

2.2.2-Boiler operation………………………………………….. 35

2.2.3-Fly ash collection………………………………………… 41

2.2.4-Boiler make up water treatment plant and storage………. 40

2.2.5- Fuel preparation system………………………………… 42

2.3- Rankine cycle ……………………………………………… 45

2.4- Cooling tower……………………………………………… 47

2.5-Problems in the boiler……………………………………… 49

2.5.1-Corrosion…………………………………………………. 49

2.5.2-Deposits…………………………………………………. . 50

2.5.3-Solubilizing programmed……………………………….. 50

2.6- Chemical treatment……………………………………… . 52

Chapter Three

Materials and Method:

3.1-Area of study …………………………………………… 55

3.2- Chemical treatment (dosing)

3.2.1 River station……………………………………………. 56

3.2.2 Demineralization unit………………………………….. 57

3. 2. 3. Feed water dosing…………………………………… 58

3.2.4-Boiler Internal Treatment Chemicals dosing…………. 61

3.2.5-Cooling tower dosing………………………………… 61

VI

3.3- Laboratory

3.3.1- Turbidity Test……………………………………… 62

3.3.2- Conductivity Test……………………………………. 63

3.3.3-pH Test……………………………………………….. 65

3.3.4 Silica Test…………………………………………….. 68

Chapter Four

Results and Discussion

4.1- Result……………………………………………...... 71

4.2 – Discussion………………………………………….. …. 73

Chapter Five

Conclusions and Recommendation

5.1-Conclusions………………………………………...... 79

5.2-Recommendation……………………………………………. 79

REFERENCES

VII

List of figures:

figure Item page Figure 1 Demineralization unit 23 Figure 2 The chemical dosing of water treatment in power 33 plant. Figure 3 Diagram of boiler feed water deaerator. 34 Figure 4 Diagram of a typical water-cooled surface condenser. 36 Figure 5 The boiler plant overview 43 Figure 6 Rankine cycle. 45 Figure 7 Curve of the four processes in the Rankine cycle. 46 Figure 8 Cooling towers. 48 Figure 9 The drive off oxygen and other dissolved gases. 51 Figure 10 Pre-treatment of water in the river station. 56

Figure 11 Mechanism of demineralization water. 57 Figure 12 Impurities of water in river station. 58 Figure 13 Relationship among concentration, pH, and 59 conductivity for dilute aqueous ammonia solutions. Figure 14 The samples taken. 60 Figure 15 Turbidity meter. 62

Figure 16 Conductivity meter. 64

Figure 17 pH meter. 67 Figure 18 Si –meter (photometer). 69

Figure 19 The seasonal variation of the turbidity in the clarifier 74 tank.

VIII

List of tables:

Tables Item page Table 1 The impurities in water. 14 Table 2 List of problems caused by impurities in water. 54 Table 3 Chemical dosing in the river station. 56 Table 4 Chemical dosing in the demineralization station. 57 Table 5 Feed water dosing. 58 Table 6 Cooling towers dosing. 61 Table 7 The reading parameter of clarifier water tank. 71 Table 8 The reading parameter of demineralization unit. 71 Table 9 The reading parameter of boiler drum water. 71 Table10 The reading parameter of condensate water 70 Table 11 The reading parameter of dearator water. 70 Table 12 The reading parameter of feed water. 17 Table 13 The reading parameter of saturated steam. 72 Table 14 The reading parameter o f superheated steam. 72 Table 15 The reading parameter o f cooling towers. 72 Table 16 The seasonal variation of the turbidity in the 74 clarifier tank Table 17 The seasonal variation of the pH in the clarifier 75 tank Table 18 The seasonal variation of the conductivity in the 75 clarifier tank Table 19 The overall comparative seasonal variation of 75 (turbidity, pH and conductivity) in the clarifier tank Table 20 The seasonal variation of the parameters 76 (turbidity, pH and conductivity) in the cooling tower.

Table 21 The variation of pH, conductivity and silicate of 75 demin tank, anion exchanger , boiler drum water , condensate water ,dearator water, feed water ,saturated steam and superheated steam

IX

Chapter One 1.1- Introduction : A boiler is an enclosed vessel that provides a means for combustion heat to be transferred to water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process. Water is a useful and cheap medium for transferring heat to a process. When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be extremely dangerous equipment that must be treated without most care. The process of heating a liquid until it reaches its gaseous state is called evaporation. Heat is transferred from one body to another by means of (1) radiation, which is the transfer of heat from a hot body to a cold body without a conveying medium, (2) convection, the transfer of heat by a conveying medium, such as air or water and (3) conduction, transfer of heat by actual physical contact, molecule to molecule. i. The heating surface is any part of the boiler metal that has hot gases of combustion on one side and water on the other. Any part of the boiler metal that actually contributes to making steam is heating surface. The amount of heating surface of a boiler is expressed in square meters. The larger the heating surface a boiler has, the more efficient it becomes. The quantity of the steam produced is indicated in tons of water evaporated to steam per hour. Maximum continuous rating is the hourly evaporation that can be maintained for 24 hours. o F & A means the amount of steam generated from water at 100 C to saturated steam at o 100 C . A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definition is somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure (1–300 psi or 6.895–2,068.427 kPa) but, at pressures above this, it is more usual to speak of a steam generator. A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam locomotives, portable engines and steam powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stationary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately-generated steam is transferred to a receiver (tank) on the locomotive. The steam generator or boiler is an integral component of a steam engine when considered as a prime mover. However it needs be treated separately, as to some

1 extent a variety of generator types can be combined with a variety of engine units. A boiler incorporates a firebox or furnace in order to burn the fuel and generate heat. The generated heat is transferred to water to make steam, the process of boiling. This produces saturated steam at a rate which can vary according to the pressure above the boiling water.Higher the furnace temperature, Faster the steam production. The saturated steam thus produced can then either be used immediately to produce power via a turbine and alternator, or else may be further superheated to a higher temperature; this notably reduces suspended water content making a given volume of steam produce more work and creates a greater temperature gradient, which helps reduce the potential to form condensation. Any remaining heat in the combustion gases can then either be evacuated or made to pass through an economiser, the role of which is to warm the feed water before it reaches the boiler. There are many types of boiler: haycock and wagon top boilers For the first newcomen engine of 1712, the boiler was little more than large brewer’s kettle installed beneath the power cylinder. Because the engine’s power was derived from the vacuum produced by condensation of the steam, the requirement was for large volumes of steam at very low pressure hardly more than 1 psi (6.9 kPa). The whole boiler was set into brickwork which retained some heat. A voluminous coal fire was lit on a grate beneath the slightly dished pan which gave a very small heating surface; there was therefore a great deal of heat wasted up the chimney. In later models, notably by John Smeaton, heating surface was considerably increased by making the gases heat the boiler sides, passing through a flue. Smeaton further lengthened the path of the gases by means of a spiral labyrinth flue beneath the boiler. These under-fired boilers were used in various forms throughout the 18th Century. Some were of round section (haycock). A longer version on a rectangular plan was developed around 1775 by Boulton and Watt (wagon top boiler). This is what is today known as a three-pass boiler, the fire heating the underside, the gases then passing through a central square-section tubular flue and finally around the boiler sides, cylindrical fire-tube boiler an early proponent of the cylindrical form, was the American engineer, Oliver Evans who rightly recognised that the cylindrical form was the best from the point of view of mechanical resistance and towards the end of the 18th Century began to incorporate it into his projects. Probably inspired by the writings on Leupold’s “high- pressure” engine scheme that appeared in encyclopaedic works from 1725, Evans favoured “strong steam” i.e. non condensing engines in which the steam pressure alone drove the piston and was then exhausted to atmosphere. The advantage of strong steam as he saw it was that more work could be done by smaller volumes of steam; this enabled all the

2 components to be reduced in size and engines and could be adapted to transport and small installations. To this end he developed a long cylindrical wrought iron horizontal boiler into which was incorporated a single fire tube, at one end of which was placed the fire grate. The gas flow was then reversed into a passage or flue beneath the boiler barrel, then divided to return through side flues to join again at the chimney (Columbian engine boiler). Evans incorporated his cylindrical boiler into several engines, both stationary and mobile. Due to space and weight considerations the latter were one-pass exhausting directly from fire tube to chimney. Another proponent of “strong steam” at that time was the Cornishman, Richard Trevithick. His boilers worked at 40–50 psi (276–345 kPa) and were at first of hemispherical then cylindrical form. From 1804 onwards Trevithick produced a small two-pass or return flue boiler for semi-portable and locomotive engines. The Cornish boiler developed around 1812 by Richard Trevithick was both stronger and more efficient than the simple boilers which preceded it. It consisted of a cylindrical water tank around 27 feet (8.2 m) long and 7 feet (2.1 m) in diameter, and had a coal fire grate placed at one end of a single cylindrical tube about three feet wide which passed longitudinally inside the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside then a third time beneath the boiler barrel before being expelled into a chimney. This was later improved upon by another 3-pass boiler, the Lancashire boiler which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned while the other was operating. Railway locomotive boilers were usually of the 1-pass type, although in early days, 2-pass "return flue" boilers were common, especially with locomotives built by Timothy Hackworth. Multi-tube boilers a significant step forward came in France in 1828 when Marc Seguin devised a two-pass boiler of which the second pass was formed by a bundle of multiple tubes. A similar design with natural induction used for marine purposes was the popular Scotch marine boiler. Prior to the Rainhill trials of 1829 Henry Booth, treasurer of the Liverpool and Manchester Railway suggested to George Stephenson, a scheme for a multi-tube one-pass horizontal boiler made up of two units: a firebox surrounded by water spaces and a boiler barrel consisting of two telescopic rings inside which were mounted 25 copper tubes; the tube bundle occupied much of the water space in the barrel and vastly improved heat transfer. Old George immediately communicated the scheme to his son Robert and this was the boiler used on Stephenson's Rocket, outright winner of the trial. The design formed the basis for all subsequent Stephensonian-built

3 locomotives, being immediately taken up by other constructors; this pattern of fire-tube boiler has been built ever since 1712 boiler was assembled from riveted copper plates with a domed top made of lead in the first examples. Later boilers were made of small wrought iron plates riveted together. The problem was producing big enough plates, so that even pressures of around 50 psi (344.7 kPa) were not absolutely safe, nor was the cast iron hemispherical boiler initially used by Richard Trevithick. This construction with small plates persisted until the 1820s, when larger plates became feasible and could be rolled into a cylindrical form with just one butt-jointed seam reinforced by a gusset; Timothy Hackworth's Sans Pareil 11 of 1849 had a longitudinal welded seam.Welded construction for locomotive boilers was extremely slow to take hold. Once-through monotubular water tube boilers as used by Doble, Lamont and Pritchard are capable of withstanding considerable pressure and of releasing it without danger of explosion. The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines. The solid fuel firing in order to create optimum burning characteristics of the fire, air needs to be supplied both through the grate, and above the fire. Most boilers now depend on mechanical draught equipment rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as chimney height. All these factors make effective draught hard to attain and therefore make mechanical draught equipment much more economical. There are three types of mechanical draught: Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler.The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet or ejector oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draught furnaces have a negative pressure. The forced draught is obtained by forcing air into the furnace by means of a fan (FD fan) and duct-work. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the

4 furnace. Forced draught furnaces usually have a positive pressure. The balanced draught: balanced draught is obtained through use of both induced and forced draught. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draught fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric. The next stage in the process is to boil water and make steam. The goal is to make the heat flow as completely as possible from the heat source to the water. The water is confined in a restricted space heated by the fire. The steam produced has lower density than the water and therefore will accumulate at the highest level in the vessel; its temperature will remain at boiling point and will only increase as pressure increases. Steam in this state (in equilibrium with the liquid water which is being evaporated within the boiler) is named "saturated steam". For example, saturated steam at atmospheric pressure boils at 100 °C (212 °F). Saturated steam taken from the boiler may contain entrained water droplets, however a well designed boiler will supply virtually "dry" saturated steam, with very little entrained water. Continued heating of the saturated steam will bring the steam to a "superheated" state, where the steam is heated to a temperature above the saturation temperature, and no liquid water can exist under this condition. Most reciprocating steam engines of the 19th century used saturated steam, however modern steam power plants universally use superheated steam which allows higher steam cycle efficiency. L.D. Porta gives the following equation determining the efficiency of a steam locomotive, applicable to steam engines of all kinds: power (kW) = steam Production (kg h−1)/Specific steam consumption (kg/kW h). A greater quantity of steam can be generated from a given quantity of water by superheating it. As the fire is burning at a much higher temperature than the saturated steam it produces, far more heat can be transferred to the once-formed steam by superheating it and turning the water droplets suspended therein into more steam and greatly reducing water consumption. The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 1300–1600 degrees Celsius .Some superheaters are radiant type (absorb heat by thermal radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not: the turbine or moving pistons offer a

5

"continuously expanding space" and the pressure remains the same as that of the boiler.[2] The process of superheating steam is most importantly designed to remove all droplets entrained in the steam to prevent damage to the turbine blading and/or associated piping. Superheating the steam expands the volume of steam, which allows a given quantity (by weight) of steam to generate more power. When the totality of the droplets is eliminated, the steam is said to be in a superheated state. In a Stephensonian firetube locomotive boiler, this entails routing the saturated steam through small diameter pipes suspended inside large diameter firetubes putting them in contact with the hot gases exiting the firebox; the saturated steam flows backwards from the wet header towards the firebox, then forwards again to the dry header. Superheating only began to be generally adopted for locomotives around the year 1900 due to problems of overheating of and lubrication of the moving parts in the cylinders and steam chests. Many firetube boilers heat water until it boils, and then the steam is used at saturation temperature in other words the temperature of the boiling point of water at a given pressure (saturated steam); this still contains a large proportion of water in suspension. Saturated steam can and has been directly used by an engine, but as the suspended water cannot expand and do work and work implies temperature drop, much of the working fluid is wasted along with the fuel expended to produce it. Another way to rapidly produce steam is to feed the water under pressure into a tube or tubes surrounded by the combustion gases. The earliest example of this was developed by Goldsworthy Gurney in the late 1820s for use in steam road carriages. This boiler was ultra-compact and light in weight and this arrangement has since become the norm for marine and stationary applications. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall. It can however be susceptible to damage by vibration in surface transport appliances. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are mechanically assembled on site to create the finished boiler. Supercritical steam generators are frequently used for the production of electric power. They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi or 22.06 MPa) that actual boiling ceases to occur, the boiler has no liquid water - steam separation. There is no generation of steam bubbles within the water, because the pressure is above the critical pressure at which steam bubbles can form. It passes below the critical point as it does work

6 in a high pressure turbine and enters the generator's condenser. This results in slightly less fuel use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device. The Feed water for boilers needs to be as pure as possible with a minimum of suspended solids and dissolved impurities which cause corrosion, foaming and water carryover. Various chemical treatments have been employed over the years, the most successful being Porta treatment [citation needed]. This contains a foam modifier that acts as a filtering blanket on the surface of the water that considerably purifies steam quality.When water is converted to steam it expands in volume over 1,000 times and travels a down a steam pipes at over 100 kilometres/hr. Because of this steam is a good way of moving energy and heat around a site from a central boiler house to where it is needed, but without the right boiler feed water treatment, a steam-raising plant will suffer from scale formation and corrosion. At best, this increases energy costs and can lead to poor quality steam, reduced efficiency, shorter plant life and an operation which is unreliable. At worst, it can lead to catastrophic failure and loss of life. While variations in standards may exist in different countries, stringent legal, testing, training and certification is applied to try to minimise or prevent such occurrences. Failure modes include the over pressurization of the boiler ,insufficient water in the boiler causing overheating and vessel failure and pressure vessel failure of the boiler due to inadequate construction or maintenance. The Doble steam car uses a once-through type contra-flow generator, consisting of a continuous tube. The fire here is on top of the coil instead of underneath. Water is pumped into the tube at the bottom and the steam is drawn off at the top. This means that every particle of water and steam must necessarily pass through every part of the generator causing an intense circulation which prevents any sediment or scale from forming on the inside of the tube. Water enters the bottom of this tube at the flow rate of 600 feet (183 m) a second with less than two quarts of water in the tube at any one time. As the hot gases pass down between the coils, they gradually cool, as the heat is being absorbed by the water. The last portion of the generator with which the gases come into contact remains the cold incoming water. The fire is positively cut off when the pressure reaches a pre-determined point, usually set at 750 psi (5.2 MPa), cold water pressure; a safety valve set at 1,200 lb (544 kg) provides added protection. The fire is automatically cut off by temperature as well as pressure, so in case the boiler were completely dry it would be impossible to damage the coil as the fire would be automatically cut off by the

7 temperature. Similar forced circulation generators, such as the Pritchard and Lamont and velox boilers present the same advantages. 13.

8

1.2- Objectives of the study:  The purpose of this research is to present the detailed findings on boiler feed water treatments, theories and application in power plants field with practical application on pre- treatment plants and anions exchanger unit .  The realization of these objectives include: Online data analysis techniques for analytical and experimental work to be applied to raw water and treated water in laboratory to analyze the water composition, and properties. This comprises the following:

- Analyze raw water Parameters with reference to specifications. - Analyze and study the water quality all over seasons of the year. - Specify the type of treatment suitable for the area under study. - Apply the recommended treatment and analyze the obtained water and discuss the results. - Develop relevant recommendation for appropriate treatment processes.

9

Chapter Two Literature Review

2.1- Water Treatment:

- Chemical composition of water:

Pure water, , is a simple combination of hydrogen and oxygen. There are, however, several hybrids forms of water in all supplies. Water often contains about 300 ppm of deuterium oxide, , or heavy water. It has no use as drinking water or in making plants grow, but in pure form has found use in nuclear reactors. For all practical purposes only ordinary water, , is considered for use in boilers.

- Boiling temperature:

The boiling point of water is dependent on pressure. At sea level atmospheric pressure, water boils at about 212oF. With increasing pressure, the boiling point also increases,at a pressure of 200 psig. For example, water boils at a temperature of about 388oF, at the critical pressure of 3200 psig (where water is converted to steam without change in volume), the boiling point is 704oF. As the pressure decreases, the boiling point of water decreases. Under vacuum water will boil at temperatures as low as 35oF.

- Water an ideal medium for carrying heat energy:

It takes one BTU (British ThermalUnit) to raise temperature of one pound of water 1oF. It takes an additional 970 Btu to change one pound of water, at boiling point, to steam. This heat energy is stored in the steam and when it condenses, the energy is given off. Thus much of the heat from burning fuel can be absorbed by boiler water, transported with the steam, and released at the points of use.

2.1.1- Impurities in water:

All natural waters contain various types and amounts of impurities. These impurities cause boiler problems and as such consideration must be given to the quality and treatment required of the water used for generating steam. For any type of treatment, sediment filtration (usually with cartridge filters) is the first step. 10

- Natural water:

Natural waters contain suspended matter, dissolved solids, and dissolved gases. Water being a universal solvent dissolves minerals, rocks and soil that come into contact with it. It dissolves gases from air and gases that are given off from organics in the soil. It picks up suspended matter from the earth. Additionally it may also be contaminated with industrial wastes and process materials.

- Dissolved minerals:

Dissolved minerals picked up by the water consist mainly of calcium carbonate (limestone), calcium sulfate (gypsum), magnesium carbonate (dolomite), magnesium sulfate (epsom salts), silica (sand), sodium chloride (common salt), hydrated sodium sulfate (Glauber salt), and smaller quantities of iron, manganese, fluorides, aluminum, and other substances. The nitrates and phosphates found in water are usually due to sewage contamination.

- Water hardness:

Water containing high amounts of calcium and magnesium minerals is hard water. The amount of hardness in natural water can vary from a few ppm to 500 ppm. Calcium and magnesium compounds are relatively insoluble in water and tend to precipitate out. This causes scale and deposit problems. Such water must be treated to make it suitable for steam generation.

- Dissolved gases in water:

Water contains varying amounts of dissolved air (21% oxygen, 78% nitrogen, 1% other gases including carbon dioxide). Water can contain up to 9 ppm oxygen at room temperature and atmospheric pressure. As the temperature increases, the solubility of oxygen decreases, but water under pressure can hold higher amounts of dissolved oxygen. Nitrogen, being inert, has little effect on water used in boilers. Water can contain 10 ppm of carbon dioxide, sometimes much more than that due to decaying vegetation and organics in soil. Hydrogen sulfide and methane may be dissolved in water but this is rare. These gases can be troublesome when they are present in the feed water.

11

- Other impurities in water:

Natural waters contain varying levels of soil, sand, turbidity, colour, precipitated minerals, oil, industrial wastes and other suspended solid particles. Turbidity is due to very fine organic materials and microorganisms, as well as suspended clay and silt. Colour is due to the decaying vegetable matter.

- Sources of fresh water:

Fresh water can be surface water from rivers, streams, reservoirs or ground water from wells. Generally ground water supplies are more consistent in composition than surface water supplies. Surface water quality is affected by rainfall, soil erosion and industrial wastes, but ground water is usually harder than surface water. The composition of fresh water also varies with the location and type and strata of the earth formations. In limestone areas, for example, water contains large quantities of dissolved calcium. Apart from the geographic variations, the local conditions of a particular area may have a great influence in the composition of the water. i.

The processes below are the ones commonly used in water purification plants. Some or most may not be used depending on the scale of the plant and quality of the raw (source) water. Engineering and technical staff of laboratories have tested natural water and listed common impurities in water, their effects and possible solutions and prepared the table1[1]:

12

Constituent Chemical Difficulties Caused Means of treatment

Formula Turbidity None- expressed in Imparts unsightly Coagulation, settling analysis as units. appearance to water. And filtration. Deposits in water lines, process equipment, etc. Interferes with most process uses. Color None- expressed in May cause foaming in Coagulation and analysis as units. boilers. Can stain filtration. Chlorination, product in process use. Adsorption by activated carbon Hardness Calcium and Chief source of scale Softening. magnesium in Demineralization. salts expressed heat exchange Internal boiler water

as equipment, boilers, treatment. Surface pipes, etc. Forms active agents. curds with soap. Interferes with dyeing, etc.

Alkalinity Bicarbonate ( ). Foaming and carryover Lime and lime-soda

Carbonate ( ), and of solids with steam. softening. Acid hydrate(OH) Embrittlement of treatment. Hydrogen expressed as boiler zeolite softening. steel. Bicarbonate and Demineralization. carbonate produce CO, De-alkalization by anion in steam, a source of exchange. corrosion in condensate lines

Free , HCl, etc. Corrosion Neutralization with expressed as Mineral acid alkalies

Carbon Corrosion Aeration, De-aeration Dioxide and Neutralization

13

Constituent Chemical Difficulties Caused Means of treatment

Formula pH concentration pH depends on acidic or pH can be increased of Ion, alka-line solids in water. by alkalies and pH=log1/ Most natural waters decreased by acids. have a pH of 6.0-8.0.

Sulfate water-- Increase solid Demineralization contents in Chloride Adds to solids content Demineralization and increases corrosive character of water.

Table No (1) the impurities in water iii.

2.1.2- Pre-treatment:

1. Pumping and containment the majority of water must be pumped from its source or directed into pipes or holding tanks. To avoid adding contaminants to the water, this physical infrastructure must be made from appropriate materials and constructed so that accidental contamination does not occur. 2. Screening -The first step in purifying surface water is to remove large debris such as sticks, leaves, rubbish and other large particles which may interfere with subsequent purification steps. Most deep groundwater does not need screening before other purification steps. 3. Storage – Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters. Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during transitory pollution incidents in the source river. 4. Pre-chlorination – In many plants the incoming water was chlorinated to minimize the growth of fouling organisms on the pipe-work and tanks. Because of the potential adverse quality effects (see chlorine below), this has largely been discontinued. 14

Widely varied techniques are available to remove the fine solids, micro-organisms and some dissolved inorganic and organic materials. The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the quality standards expected of the processed water. 1.

- pH adjustment:

Pure water has a pH close to 7 (neither alkaline nor acidic). Sea water can have pH values that range from 7.5 to 8.4 (moderately alkaline). Fresh water can have widely ranging pH values depending on the geology of the drainage basin or aquifer and the influence of contaminant inputs (acid rain).If the water is acidic (lower than 7), lime, soda ash, or sodium hydroxide can be added to raise the pH during water purification processes. Lime addition increases the calcium ion concentration, thus raising the water hardness. For highly acidic waters, forced draft degasifiers can be an effective way to raise the pH, by stripping dissolved carbon dioxide from the water. Making the water alkaline helps coagulation and flocculation processes work effectively and also helps to minimize the risk of lead being dissolved from lead pipes and from lead solder in pipe fittings. Sufficient alkalinity also reduces the corrosiveness of water to iron pipes. Acid (carbonic acid, hydrochloric acid or sulfuric acid) may be added to alkaline waters in some circumstances to lower the pH. Alkaline water (above pH 7.0) does not necessarily mean that lead or copper from the plumbing system will not be dissolved into the water. The ability of water to precipitate calcium carbonate to protect metal surfaces and reduce the likelihood of toxic metals being dissolved in water is a function of pH, mineral content, temperature, alkalinity and calcium concentration.

- Coagulation and flocculation:

One of the first steps in a conventional water purification process is the addition of chemicals to assist in the removal of particles suspended in water. Particles can be inorganic such as clay and silt or organic such as algae, bacteria, viruses, protozoa and natural organic matter. Inorganic and organic particles contribute to the turbidity and colour of water.

The addition of inorganic coagulants such as aluminum sulfate (or alum) or iron (III) salts such as iron (III) chloride cause several simultaneous chemical and physical interactions on and among the particles. Within seconds, negative charges on the particles are neutralized 15 by inorganic coagulants. Also within seconds, metal hydroxide precipitates of the aluminum and iron begin to form. These precipitates combine into larger particles under natural processes such as Brownian motion and through induced mixing which is sometimes referred to as flocculation. The term most often used for the amorphous metal hydroxides is “floc.” Large, amorphous aluminum and iron hydroxides adsorb and enmesh particles in suspension and facilitate the removal of particles by subsequent processes of sedimentation and filtration.

Aluminum hydroxides are formed within a fairly narrow range, typically: 5.5 to about 7.7. Iron (III) hydroxides can form over a larger pH range including pH levels lower than are effective for alum, typically: 5.0 to 8.5. In the literature, there is much debate and confusion over the usage of the terms coagulation and flocculation,where does coagulation end and flocculation begin? In water purification plants, there is usually a high energy, rapid mix unit process (detention time in seconds) where the coagulant chemicals are added followed by flocculation basins (detention times range from 15 to 45 minutes) where low energy inputs turn large paddles or other gentle mixing devices to enhance the formation of floc. In fact, coagulation and flocculation processes are ongoing once the metal salt coagulants are added.

Organic polymers were developed in the 1960s as aids to coagulants and, in some cases, as replacements for the inorganic metal salt coagulants. Synthetic organic polymers are high molecular weight compounds that carry negative, positive or neutral charges. When organic polymers are added to water with particulates, the high molecular weight compounds adsorb onto particle surfaces and through interparticle bridging coalesce with other particles to form floc. PolyDADMAC is a popular cationic (positively charged) organic polymer used in water purification plants.

- Sedimentation:

Waters exiting the flocculation basin may enter the sedimentation basin, also called a clarifier or settling basin. It is a large tank with low water velocities, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between the two processes does not permit settlement or floc break up. Sedimentation basins may be rectangular, where water flows from end to end, or circular where flow is from the centre outward. Sedimentation basin outflow is typically over a weir so only a thin top layer of water that furthest from the sludge. 16

In 1904, Allen Hazen showed that the efficiency of a sedimentation process was a function of the particle settling velocity, the flow through the tank and the surface area of tank. Sedimentation tanks are typically designed within a range of overflow rates of 0.5 to 1.0 gallons per minute per square foot (or 1.25 to 2.5 meters per hour). In general, sedimentation basin efficiency is not a function of detention time or depth of the basin. Although, basin depth must be sufficient so that water currents do not disturb the sludge and settled particle interactions are promoted. As particle concentrations in the settled water increase near the sludge surface on the bottom of the tank, settling velocities can increase due to collisions and agglomeration of particles. Typical detention times for sedimentation vary from 1.5 to 4 hours and basin depths vary from 10 to 15 feet (3 to 4.5 meters). Inclined flat plates or tubes can be added to traditional sedimentation basins to improve particle removal performance. Inclined plates and tubes drastically increase the surface area available for particles to be removed in concert with Hazen’s original theory. The amount of ground surface area occupied by a sedimentation basin with inclined plates or tubes can be far smaller than a conventional sedimentation basin.

- Sludge storage and removal:

As particles settle to the bottom of a sedimentation basin, a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3 to 5 percent of the total volume of water that is treated. The cost of treating and disposing of the sludge can be a significant part of the operating cost of a water treatment plant. The sedimentation tank may be equipped with mechanical cleaning devices that continually clean the bottom of the tank or the tank can be periodically taken out of service and cleaned manually In soil science, the process whereby very small particles aggregate to form crumbs. The term is usually applied to clays. In certain subs oils of arid areas, downward translocation of soluble salts leads to the breakdown of these crumbs in the process of de flocculation.

- Flocculation:

Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no

17 formation of a cake, since all the flocs are in the suspension.A colloid phenomenon in which the disperse phase separates in discrete, usually visible, particles rather than in a continuous mass, as in coagulation.

- Surface chemistry:

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or be readily filtered from the liquid.

- Physical chemistry:

For emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity. Flocculation is thus the initial step leading to further aging of the emulsion (droplet coalescence).

- Floc blanket clarifiers:

A subcategory of sedimentation is the removal of particulates by entrapment in a layer of suspended floc as the water is forced upward. The major advantage of floc blanket clarifiers is that they occupy a smaller footprint than conventional sedimentation. Disadvantages are that particle removal efficiency can be highly variable depending on changes in influent water quality and influent water flow rate.

- Dissolved air flotation:

When particles to be removed do not settle out of solution easily, dissolved air flotation (DAF) is often used. Water supplies that are particularly vulnerable to unicellular algae blooms and supplies with low turbidity and high colour often employ DAF. After coagulation and flocculation processes, water flows to DAF tanks where air diffusers on the tank bottom create fine bubbles that attach to floc resulting in a floating mass of concentrated floc. The floating floc blanket is removed from the surface and clarified water is withdrawn from the bottom of the DAF tank.

18

- Filtration:

After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc.

- Rapid sand filters:

The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odour. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog. To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called back flushing or backwashing) to remove embedded particles. Prior to this step, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or it can be recycled by mixing with the raw water entering the plant although this is often considered poor practice since it re-introduces an elevated concentration of bacteria into the raw water. Some water treatment plants employ pressure filters. These work on the same principle as rapid gravity filters, differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.

Advantages:

 Filters out much smaller particles than paper and sand filters can.  Filters out virtually all particles larger than their specified pore sizes.  They are quite thin and so liquids flow through them fairly rapidly.  They are reasonably strong and so can withstand pressure differences across them of typically 2–5 atmospheres.  They can be cleaned (back flushed) and reused.

19

-Slow sand filters:

May be used where there is sufficient land and space, as the water must be passed very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration. The filters are carefully constructed using graded layers of sand, with the coarsest sand, along with some gravel, at the bottom and finest sand at the top. Drains at the base convey treated water away for disinfection. Filtration depends on the development of a thin biological layer, called the zoogleal layer or Schmutzdecke, on the surface of the filter. An effective slow sand filter may remain in service for many weeks or even months if the pre-treatment is well designed and produces water with a very low available nutrient level which physical methods of treatment rarely achieve. Very low nutrient levels allow water to be safely sent through distribution systems with very low disinfectant levels, thereby reducing consumer irritation over offensive levels of chlorine and chlorine by-products. Slow sand filters are not backwashed; they are maintained by having the top layer of sand scraped off when flow is eventually obstructed by biological growth. A specific "large-scale" form of slow sand filter is the process of bank filtration, in which natural sediments in a riverbank are used to provide a first stage of contaminant filtration. While typically not clean enough to be used directly for drinking water, the water gained from the associated extraction wells is much less problematic than river water taken directly from the major streams where bank filtration is often used.

- Membrane filtration:

Membrane filters are widely used for filtering both drinking water and sewage. For drinking water, membrane filters can remove virtually all particles larger than 0.2 um including giardia and cryptosporidium. Membrane filters are an effective form of tertiary treatment when it is desired to reuse the water for industry, for limited domestic purposes, or before discharging the water into a river that is used by towns further downstream. They are widely used in industry, particularly for beverage preparation (including bottled water). However no filtration can remove substances that are actually dissolved in the water such as phosphorus, nitrates and heavy metal ions.

- Removal of ions and other dissolved substances:

Ultrafiltration membranes use polymer membranes with chemically formed microscopic pores that can be used to filter out dissolved substances avoiding the use of 20 coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes of micro-organisms can be filtered out.

- Ion exchange:

Ion exchange systems use ion exchange resin- or zeolite-packed columns to replace unwanted ions. The most common case is water softening consisting of removal of Ca2+ and Mg2+ ions replacing them with benign (soap friendly) Na+ or K+ ions. Ion exchange resins are also used to remove toxic ions such as nitrate, nitrite, lead, mercury, arsenic and many others. Minerals dissolved in water form electrically charged particles called ions. Calcium carbonate, for example, forms a calcium ion with positive charges (a cation) and a bicarbonate ion with negative charges (an anion). Some synthetic and natural materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all the calcium and magnesium ions are removed and replaced with sodium ions. Ion exchange resins usually are small porous beads that compose a bed several feet deep through which the water is passed. 5.

- Types of ion exchange resins:

Ion exchange resins are two types: cation and anion. Cation exchange resins react only with positively charged ions like and . Anion exchange resins react only with the negatively charged ions like bicarbonate ( ) and sulfate ( ). Although there are many types of cation exchange resins, they usually operate on either a sodium or hydrogen. That is, they are designed to replace all cations in the water with either sodium or hydrogen. The anion resins are of two types: weak base and strong base. Weak base resins will not take out carbon dioxide or silica, but will remove strong acid anions by a process more similar to adsorption than ion exchange. Strong base anion resins, on the other hand, can reduce carbon dioxide and silica as well as strong acid anions to very low values. Strong base anion resins are normally operated on a hydroxide cycle. Chloride anion exchange resin is also used in dealkalization where alkalinity is reduced.

- Ion exchange regeneration:

Ion exchange resins have a certain capacity for removing ions from water and when their capacity is used up they have to be regenerated. The regeneration is essentially reversing the ion exchange process. Cation exchangers operating on the sodium cycle, salt 21

(NaCl) is added to replenish the sodium capacity. Resins operating on the hydrogen cycle are replenished by adding acid ( or HCl). Anion exchangers are normally regenerated with caustic (NaOH) or ammonium hydroxide ( ) to replenish the hydroxide ions. Salt (NaCl) may also be used to regenerate anion resins in the chloride form for dealkalization. Regeneration process involves taking the vessel off line and treating it with concentrated solution of the regenerant. The ion exchange resin then gives up the ions previously removed from water and these ions are rinsed out of the vessel. After the regeneration has been completed, the vessel is ready for further service. 4.

- Split-stream softening:

The effluents from a cation exchanger operating on sodium cycle are blended with effluents from a cation exchanger operating on a hydrogen cycle, to reduce the alkalinity of the water. Since the hydrogen cycle produces acid water while the sodium cycle does not affect alkalinity, the two effluents can be blended together to give the desired reduction in alkalinity. 4.

- Dealkalization:

One of the ion exchange processes for reducing water alkalinity is referred to as dealkalization. In this process the water passes through an ion exchanger operating on the chloride cycle. The exchanger removes alkaline anions such as carbonate, bicarbonate, and sulfates, replacing these ions with chloride. Cation exchange softening precedes dealkalization process.

2.1.3-Demineralization:

When the water is passed through both cation and anion exchange resins it is known as demineralization. In this process the cation exchange is operated on the hydrogen cycle. That is, hydrogen is substituted for all the cations. The anion exchanger operates on the hydroxide cycle, which replaces hydroxide for all of the anions. The final effluent from the process consists essentially of hydrogen ions and hydroxide ions or pure water. The demineralization process can be done by several methods. In the mixed-bed process, the anion and cation exchange resins are intimately mixed in one vessel. Multi-bed arrangements may consist of different combinations of cation exchange beds, weak and strong-based anion exchange beds, and degasifies. 22

23

-Disadvantages of ion exchange:

Sodium cycle ion exchange softening disadvantage is that the total solids, alkalinity, and silica contents of the raw water are not reduced. In the case of cation exchange on the hydrogen cycle, the disadvantage is the corrosion from the acid pH of the effluent. With demineralization, the main difficulty is higher cost, particularly on high solids raw water. Without an excellent pre-filtration arrangement, fouling of the ion exchange material with suspended and colloidal matter in the raw water can produce short runs, ion-exchange degradation, and regeneration difficulties.

- Advantages of ion exchange:

A major advantage of ion exchange softening is the ease of process control. Variations (within reasonable limits) of hardness in raw water or in flow rate do not have an adverse effect on the completeness of softening. The ion exchange system takes up less space than the lime-soda system. Generally the ion exchange demineralization has the ability to produce better quality boiler feedwater at an economical cost than most other methods.

- Deaeration of water:

Dissolved oxygen in water is a major cause of boiler system corrosion. It should be removed before the water is put in the boiler. Feedwater deaeration removes oxygen by heating the water with steam in a deaerating heater. Part of the steam is vented, carrying with it the bulk of the dissolved oxygen.

- Combination of ion exchange and lime process:

As mentioned earlier, water containing suspended solids, organics, or turbidity requires filtration/clarification prior to ion exchange. Because simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation softening. A common combination treatment is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity, silica, and subsequent filtration and a cation exchange softening. This combination accomplishes several functions like softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity. 24

- Reverse osmosis:

Reverse osmosis mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. Reverse osmosis is theoretically the most thorough method of large scale water purification available, although perfect semi- permeable membranes are difficult to create. Unless membranes are well-maintained, algae and other life forms can colonize the membranes. The operating pressures of about 300 to 900 psi are required to achieve this. Reverse osmosis reduces the dissolved solids of the raw water, making the final affluent ready for further treatment. This process is suitable for any type of raw water, but sometimes the installation and operation cost may not be economical.

- Precipitative softening:

Water rich in hardness (calcium and magnesium ions) is treated with lime (calcium oxide) and/or soda-ash (sodium carbonate) to precipitate calcium carbonate out of solution utilizing the common-ion effect.

- Electrodeionization:

Water is passed between a positive electrode and a negative electrode. Ion exchange membranes allow only positive ions to migrate from the treated water toward the negative electrode and only negative ions toward the positive electrode. High purity deionized water is produced with a little worse degree of purification in comparison with ion exchange treatment. Complete removal of ions from water is regarded as electrodialysis. The water is often pre-treated with a reverse osmosis unit to remove non-ionic organic contaminants.

- Disinfection:

Disinfection is accomplished both by filtering out harmful micro-organisms and also by adding disinfectant chemicals. Water is disinfected to kill any pathogens which pass through the filters and to provide a residual dose of disinfectant to kill or inactivate potentially harmful micro-organisms in the storage and distribution systems. Possible pathogens include viruses, bacteria, including Salmonella, Cholera, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. Following the introduction of any chemical disinfecting agent, the water is usually held in temporary 25 storage often called a contact tank or clear well to allow the disinfecting action to complete.

- Chlorine disinfection:

The most common disinfection method involves some form of chlorine or its compounds such as chloramine or chlorine dioxide. Chlorine is a strong oxidant that rapidly kills many harmful micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive solution that releases free chlorine when dissolved in water. Chlorine solutions can be generated on site by electrolyzing common salt solutions. A solid form, calcium hypochlorite, releases chlorine on contact with water. Handling the solid, however, requires greater routine human contact through opening bags and pouring than the use of gas cylinders or bleach which are more easily automated. The generation of liquid sodium hypochlorite is both inexpensive and safer than the use of gas or solid chlorine.

All forms of chlorine are widely used, despite their respective drawbacks. One drawback is that chlorine from any source reacts with natural organic compounds in the water to form potentially harmful chemical by-products. These by-products, tri halomethanes (THMs) and haloacetic acids (HAAs), are both carcinogenic in large quantities and are regulated by the United States Environmental Protection Agency (EPA) and the Drinking Water Inspectorate in the UK. The formation of THMs and haloacetic acids may be minimized by effective removal of as many organics from the water as possible prior to chlorine addition. Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoa that form cysts in water (Giardia lamblia and Cryptosporidium, both of which are pathogenic).

- Chlorine dioxide disinfection:

Chlorine dioxide is a faster-acting disinfectant than elemental chlorine, however it is relatively rarely used, because in some circumstances it may create excessive amounts of chlorite, which is a by-product regulated to low allowable levels in the United States. Chlorine dioxide is supplied as an aqueous solution and added to water to avoid gas handling problems; chlorine dioxide gas accumulations may spontaneously detonate.

26

-Chloramine disinfection:

The use of chloramine is becoming more common as a disinfectant. Although chloramine is not as strong an oxidant, it does provide a longer-lasting residual than free chlorine and it won't form THMs or haloacetic acids. It is possible to convert chlorine to chloramine by adding ammonia to the water after addition of chlorine. The chlorine and ammonia react to form chloramine. Water distribution systems disinfected with chloramines may experience nitrification, as ammonia is a nutrient for bacterial growth, with nitrates being generated as a by-product.

- Solar water disinfection:

One low-cost method of disinfecting water that can often be implemented with locally available materials is solar disinfection (SODIS). Unlike methods that rely on firewood, it has low impact on the environment.

One recent study has found that the wild Salmonella which would reproduce quickly during subsequent dark storage of solar-disinfected water could be controlled by the addition of just 10 parts per million of hydrogen peroxide.

2.1.4- Additional treatment options:

1. Water fluoridation: in many areas fluoride is added to water with the goal of preventing tooth decay. Fluoride is usually added after the disinfection process. In the U.S., fluoridation is usually accomplished by the addition of hexafluorosilicic acid,which decomposes in water, yielding fluoride ions. 2. Water conditioning: This is a method of reducing the effects of hard water. In water systems subject to heating hardness salts can be deposited as the decomposition of bicarbonate ions creates carbonate ions that precipitate out of solution. Water with high concentrations of hardness salts can be treated with soda ash (sodium carbonate) which precipitates out the excess salts, through the common-ion effect, producing calcium carbonate of very high purity. The precipitated calcium carbonate is traditionally sold to the manufacturers of toothpaste. Several other methods of industrial and residential water treatment are claimed (without general scientific acceptance) to include the use of magnetic and/or electrical fields reducing the effects of hard water. 27

3. Plumbo solvency reduction: In areas with naturally acidic waters of low conductivity (i.e. surface rainfall in upland mountains of igneous rocks), the water may be capable of dissolving lead from any lead pipes that it is carried in. The addition of small quantities of phosphate ion and increasing the pH slightly both assist in greatly reducing plumbo-solvency by creating insoluble lead salts on the inner surfaces of the pipes. 4. Radium Removal: Some groundwater sources contain radium, a radioactive chemical element. Typical sources include many groundwater sources north of the Illinois River in Illinois. Radium can be removed by ion exchange, or by water conditioning. The back flush or sludge that is produced is, however, a low-level radioactive waste. 5. Fluoride Removal: Although fluoride is added to water in many areas, some areas of the world have excessive levels of natural fluoride in the source water. Excessive levels can be toxic or cause undesirable cosmetic effects such as staining of teeth. Methods of reducing fluoride levels are through treatment with activated alumina and bone char filter media.

2.1.5- Other water purification techniques:

Other popular methods for purifying water, especially for local private supplies are listed below. In some countries some of these methods are also used for large scale municipal supplies. Particularly important are distillation (de-salination of seawater) and reverse osmosis.

1. Boiling: Water is heated hot enough and long enough to inactivate or kill micro- organisms that normally live in water at room temperature. Near sea level, a vigorous rolling boil for at least one minute is sufficient. At high altitudes (greater than two kilometres or 5000 feet) three minutes is recommended. In areas where the water is "hard" (that is, containing significant dissolved calcium salts), boiling decomposes the bicarbonate ions, resulting in partial precipitation as calcium carbonate. This is the "fur" that builds up on kettle elements, etc., in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water and in fact increases their concentration (due to some water being lost as vapour). Boiling does not leave a residual disinfectant in the water. Therefore, water that is boiled and then stored for any length of time may acquire new pathogens.

28

2. Granular Activated Carbon filtering: a form of activated carbon with a high surface area, adsorbs many compounds including many toxic compounds. Water passing through activated carbon is commonly used in municipal regions with organic contamination, taste or odors. Many household water filters and fish tanks use activated carbon filters to further purify the water. Household filters for drinking water sometimes contain silver as metallic silver nanoparticle. If water is held in the carbon block for longer period, microorganisms can grow inside which results in fouling and contamination. Silver nanoparticles are excellent anti-bacterial material and they can decompose toxic halo-organic compounds such as pesticides into non-toxic organic products. Distillation involves boiling the water to produce water vapor. The vapor contacts a cool surface where it condenses as a liquid. Because the solutes are not normally vaporised, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvapourised liquid carried with the steam. However, 99.9% pure water can be obtained by distillation. 3. Reverse osmosis: Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane. Reverse osmosis is theoretically the most thorough method of large scale water purification available, although perfect semi- permeable membranes are difficult to create. Unless membranes are well-maintained, algae and other life forms can colonize the membranes. 4. The use of iron in removing arsenic from water.. 5. Direct contact membrane distillation (DCMD). Applicable to desalination. Heated seawater is passed along the surface of a hydrophobic polymer membrane. Evaporated water passes from the hot side through pores in the membrane into a stream of cold pure water on the other side. The difference in vapour pressure between the hot and cold side helps to push water molecules through. 6. Desalination - is a process by which saline water (generally sea water) is converted to fresh water. The most common desalination processes are distillation and reverse osmosis. Desalination is currently expensive compared to most alternative sources of water, and only a very small fraction of total human use is satisfied by desalination. It is only economically practical for high-valued uses (such as household and industrial uses) in arid areas. 7. Gas hydrate crystals centrifuge method. If carbon dioxide or other low molecular weight gas is mixed with contaminated water at high pressure and low temperature, gas

29

hydrate crystals will form exothermically. Separation of the crystalline hydrate may be performed by centrifuge or sedimentation and decanting. Water can be released from the hydrate crystals by heating. 8. In Situ Chemical Oxidation, a form of advanced oxidation processes and advanced oxidation technology is an environmental remediation technique used for soil and/or groundwater remediation to reduce the concentrations of targeted environmental contaminants to acceptable levels. ISCO is accomplished by injecting or otherwise introducing strong chemical oxidizers directly into the contaminated medium (soil or groundwater) to destroy chemical contaminants in place. It can be used to remediate a variety of organic compounds, including some that are resistant to natural degradation 9. Water Purification with Moringa Seeds - Crushed Moringa seeds clarify and purify water to suit domestic use and lower the bacterial concentration in the water making it safe for drinking. Moringa seed powder can be used as a quick and simple method for cleaning dirty river water. Studies showed that this simple method of filtering not only diminishes water pollution, but also harmful bacteria. The moringa powder joins with the solids in the water and sinks to the bottom. This treatment also removes 90-99% of bacteria contained in water.excess sodium hydroxide (lye) when its treatment equipment malfunctioned. Many municipalities have moved from free chlorine to chloramine as a disinfection agent. However, chloramine in some water systems, appears to be a corrosive agent. Chloramine can dissolve the "protective" film inside older service line, with the leaching of lead into residential spigots. 10. Demineralized water distillation removes all minerals from water, and the membrane methods of reverse osmosis and nanofiltration remove most to all minerals. This results in demineralized water which is not considered ideal drinking water. The World Health Organization has investigated the health effects of demineralized water since 1980. Experiments in humans found that demineralized water increased diuresis and the elimination of electrolytes, with decreased blood serum potassium concentration. Magnesium, calcium, and other minerals in water can help to protect against nutritional deficiency. Demineralized water may also increase the risk from toxic metals because it more readily leaches materials from piping like lead and cadmium, which is prevented by dissolved minerals such as calcium and magnesium. Low-mineral water has been implicated in specific cases of lead poisoning in infants, when lead from pipes leached at especially high rates into the water. Recommendations for magnesium have been put at a minimum of 10 mg/L with 20–

30

30 mg/L optimum; for calcium a 20 mg/L minimum and a 40–80 mg/L optimum, and a total water hardness (adding magnesium and calcium) of 2 to 4 mmol/L. At water hardness above 5 mmol/L, higher incidence of gallstones, kidney stones, urinary stones, arthrosis, and arthropathies have been observed. Additionally, desalination processes can increase the risk of bacterial contamination. Manufacturers of home water distillers claim the opposite that minerals in water are the cause of many diseases, and that most beneficial minerals come from food, not water. They quote the American Medical Association as saying "The body's need for minerals is largely met through foods, not drinking water." The WHO report agrees that "drinking water, with some rare exceptions, is not the major source of essential elements for humans" and is "not the major source of our calcium and magnesium intake", yet states that demineralized water is harmful anyway. "Additional evidence comes from animal experiments and clinical observations in several countries. Animals given zinc or magnesium dosed in their drinking water had a significantly higher concentration of these elements in the serum than animals given the same elements in much higher amounts with food and provided with low-mineral water to drink." vi.

31

2.2- Boiler water treatment:

2.2.1- Boiler feed water:

Boiler feedwater is the water supplied to the boiler. Often, steam is condensed and returned to the boiler as part of the feedwater. The water needed to supplement the returned condensate is termed. Make-up water is usually filtered and treated before use. Feedwater composition therefore depends on the quality of the make-up water and the amount of condensate returned. Sometimes people think that there is a great deal of similarity between the requirements for potable (drinking) water and the requirements for boiler feedwater. The minerals in drinking water are considered desirable and are absorbed by the body. On the other hand, minerals in water cannot be handled as well by boilers. Although a boiler is a big mass of steel, it is more sensitive to water impurities than the human stomach. For this reason, a lot of care is needed in filtration and treatment of the boiler water supply.

-Purity requirements of feedwater:

Feedwater is a matter both of quantity of impurities and the nature of impurities. Hardness, iron, and silica, for example, are of more concern than sodium salts. The purity requirements of feedwater depend on how much feedwater is used as well as toleration of the particular boiler design (pressure, heat transfer rate etc.). In high-pressure boilers practically all impurities must be removed.The feedwater (make-up water) from outside needs to be treated for the reduction or removal of impurities by first filtration, and then followed by softening, evaporation, deariation, ion exchange etc. Internal treatment is also required for the conditioning of impurities within the boiler system, to control corrosion, as reactions occur in the boiler itself and the steam pipelines.

-Boiler and steam cycle:

In fossil-fueled power plants, steam generator refers to a furnace that burns the fossil fuel to boil water to generate steam. In the nuclear plant field, steam generator refers to a specific type of large heat exchanger used in a pressurized water reactor (PWR) to thermally connect the primary (reactor plant) and secondary (steam plant) systems, which generates steam. In a nuclear reactor called a boiling water reactor (BWR), water is boiled

32 to generate steam directly in the reactor itself and there are no units called steam generators.

Figure No(2) the chemical dosing of water treatment in power plant4.

In some industrial settings, there can also be steam-producing heat exchangers called heat recovery steam generators (HRSG) which utilize heat from some industrial process. The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids.

A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and superheater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, Air Preheater (AP), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or bag house) and the flue gas stack.

- Feed water heating and deaeration:

The feed water used in the steam boiler is a means of transferring heat energy from the burning fuel to the mechanical energy of the spinning steam turbine. The total feed water

33 consists of re circulated condensate water and purified makeup water. Because the metallic materials it contacts are subject to corrosion at high temperatures and pressures, the makeup water is highly purified before use. A system of water softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter.The makeup water in a 500 MW plant amounts to perhaps 120 US gallons per minute (7.6 L/s) to replace water drawn off from the boiler drums for water purity management, and to also off set the small losses from steam leaks in the system.

The feed water cycle begins with condensate water being pumped out of the condenser after traveling through the steam turbines. The condensate flow rate at full load in a 500 MW plant is about 6,000 US gallons per minute (400 L/s).

Figure No (3) Diagram of boiler feed water deaerator (with vertical, domed aeration section and horizontal water storage section).

The water is pressurized in two stages, and flows through a series of six or seven intermediate feed water heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and gaining temperature at each stage. Typically, in the middle of this series of feedwater heaters, and before the second stage of pressurization, the condensate plus the makeup water flows through a deaerator that removes dissolved air from the water, further purifying and reducing its corrosiveness. The water may be dosed following this point with hydrazine, a chemical that removes the remaining oxygen in the 34 water to below 5 parts per billion (ppb). It is also dosed with pH control agents such as ammonia or morpholine to keep the residual acidity low and thus non-corrosive.

2.2.2- Boiler operation:

The boiler is a rectangular furnace about 50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (58 mm) in diameter.Pulverized coal is air-blown into the furnace through burners located at the four corners, or along one wall, or two opposite walls, and it is ignited to rapidly burn, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput. As the water in the boiler circulates it absorbs heat and changes into steam. It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine.Plants that use gas turbines to heat the water for conversion into steam use boilers known as heat recovery steam generators (HRSG). The exhaust heat from the gas turbines is used to make superheated steam that is then used in a conventional water-steam generation cycle. 8.

- Boiler furnace and steam drum:

The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum and from there it goes through downcomers to inlet headers at the bottom of the water walls. From these headers the water rises through the water walls of the furnace where some of it is turned into steam and the mixture of water and steam then re-enters the steam drum. This process may be driven purely by natural circulation (because the water downcomers is denser than the water/steam mixture in the water walls) or assisted by pumps. In the steam drum, the water is returned to the downcomers and the steam is passed through a series of steam separators and dryers that remove water droplets from the steam. The dry steam then flows into the superheater coils. The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing and observation ports (in the furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are avoided by flushing out such gases from the

35 combustion zone before igniting the coal.The steam drum (as well as the super heater coils and headers) have air vents and drains needed for initial start up.

- Super heater:

Fossil fuel power plants often have a superheater section in the steam generating furnace. The steam passes through drying equipment inside the steam drum on to the superheater, a set of tubes in the furnace. Here the steam picks up more energy from hot flue gases outside the tubing and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves before the high pressure turbine. Nuclear-powered steam plants do not have such sections but produce steam at essentially saturated conditions. Experimental nuclear plants were equipped with fossil-fired super heaters in an attempt to improve overall plant operating cost.

- Steam condensing:

The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases.

Figure No (4) typical water-cooled surface condenser.

The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is cooled and converted to condensate (water) by flowing over the tubes as

36 shown in the adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of air and gases from the steam side to maintain vacuum. For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam.Since the condenser temperature can almost always be kept significantly below 100 °C where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensible air into the closed loop must be prevented.

Typically the cooling water causes the steam to condense at a temperature of about 35 °C (95 °F) and that creates an absolute pressure in the condenser of about 2–7 kPa (0.59– 2.1 inHg), i.e. a vacuum of about −95 kPa (−28.1 inHg) relative to atmospheric pressure. The large decrease in volume that occurs when water vapor condenses to liquid creates the low vacuum that helps pull steam through and increase the efficiency of the turbines. The limiting factor is the temperature of the cooling water and that, in turn, is limited by the prevailing average climatic conditions at the power plant's location (it may be possible to lower the temperature beyond the turbine limits during winter, causing excessive condensation in the turbine). Plants operating in hot climates may have to reduce output if their source of condenser cooling water becomes warmer; unfortunately this usually coincides with periods of high electrical demand for air conditioning. The condenser generally uses either circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or once-through water from a river, lake or ocean. A Marley mechanical induced draft cooling tower The heat absorbed by the circulating cooling water in the condenser tubes must also be removed to maintain the ability of the water to cool as it circulates. This is done by pumping the warm water from the condenser through either natural draft, forced draft or induced draft cooling towers (as seen in the image to the right) that reduce the temperature of the water by evaporation, by about 11 to 17 °C (20 to 30 °F) expelling waste heat to the atmosphere. The circulation flow rate of the cooling water in a 500 MW unit is about 14.2 m³/s ( 500 ft³/s or 225,000 US gal/min) at full load.

The condenser tubes are made of brass or stainless steel to resist corrosion from either side. Nevertheless they may become internally fouled during operation by bacteria or algae in the cooling water or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many plants include an automatic cleaning system that circulates sponge rubber balls through the tubes to scrub them clean without the need to

37 take the system off-line. The cooling water used to condense the steam in the condenser returns to its source without having been changed other than having been warmed. If the water returns to a local water body (rather than a circulating cooling tower), it is tempered with cool 'raw' water to prevent thermal shock when discharged into that body of water.

Another form of condensing system is the air-cooled condenser. The process is similar to that of a radiator and fan. Exhaust heat from the low pressure section of a steam turbine runs through the condensing tubes, the tubes are usually finned and ambient air is pushed through the fins with the help of a large fan. The steam condenses to water to be reused in the water-steam cycle. Air-cooled condensers typically operate at a higher temperature than water-cooled versions. While saving water, the efficiency of the cycle is reduced (resulting in more carbon dioxide per megawatt of electricity). From the bottom of the condenser, powerful condensate pumps recycle the condensed steam (water) back to the water/steam cycle.

- Reheater:

Power plant furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high pressure turbine is passed through these heated tubes to collect more energy before driving the intermediate and then low pressure turbines. ii.

- Air path:

External fans are provided to give sufficient air for combustion. The Primary air fan takes air from the atmosphere and, first warming it in the air preheater for better combustion, injects it via the air nozzles on the furnace wall. The induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring through any closing. 8.

- Steam turbine, used in a power station:

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. As steam moves through the system and loses pressure and thermal energy it expands in volume,

38 requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. The entire rotating mass may be over 200 metric tons and 100 feet (30 m) long. It is so heavy that it must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. This is so important that it is one of only five functions of blackout emergency power batteries on site. Other functions are emergency lighting, communication, station alarms and turbo generator lube oil.

Superheated steam from the boiler is delivered through 14–16-inch (360–410 mm) diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4.1 MPa) and to 600 °F (320 °C) in temperature through the stage. It exits via 24–26-inch (610– 660 mm) diameter cold reheat lines and passes back into the boiler where the steam is reheated in special reheat pendant tubes back to 1,000 °F (500 °C). The hot reheat steam is conducted to the intermediate pressure turbine where it falls in both temperature and pressure and exits directly to the long-bladed low pressure turbines and finally exits to the condenser.

The generator, 30 feet (9 m) long and 12 feet (3.7 m) in diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor— no permanent magnets here. In operation it generates up to 21,000 amperes at 24,000 volts AC (504 MWe) as it spins at either 3,000 or 3,600 rpm, synchronized to the power grid. The rotor spins in a sealed chamber cooled with hydrogen gas, selected because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during startup, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly explosive hydrogen–oxygen environment is not created.

The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily and safely. The steam turbine generator being rotating equipment generally has a heavy, large diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. To minimize the frictional resistance to the rotation, the shaft has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between shaft and bearing surface and to limit the heat generated.

39

- Stack gas path and cleanup:

As the combustion flue gas exits the boiler it is routed through a rotating flat basket of metal mesh which picks up heat and returns it to incoming fresh air as the basket rotates, This is called the air preheater. The gas exiting the boiler is laden with fly ash, which are tiny spherical ash particles. The flue gas contains nitrogen along with combustion products carbon dioxide, sulfur dioxide, and nitrogen oxides. The fly ash is removed by fabric bag filters or electrostatic precipitators. Once removed, the fly ash byproduct can sometimes be used in the manufacturing of concrete. This cleaning up of flue gases, however, only occurs in plants that are fitted with the appropriate technology. Still, the majority of coal- fired power plants in the world do not have these facilities.[citation needed] Legislation in Europe has been efficient to reduce flue gas pollution. Japan has been using flue gas cleaning technology for over 30 years and the US has been doing the same for over 25 years. China is now beginning to grapple with the pollution caused by coal-fired power plants.

Where required by law, the sulfur and nitrogen oxide pollutants are removed by stack gas scrubbers which use a pulverized limestone or other alkaline wet slurry to remove those pollutants from the exit stack gas. Other devices use catalysts to remove Nitrous Oxide compounds from the flue gas stream. The gas travelling up the flue gas stack may by this time have dropped to about 50 °C (120 °F). A typical flue gas stack may be 150– 180 meters (490–590 ft) tall to disperse the remaining flue gas components in the atmosphere. The tallest flue gas stack in the world is 419.7 meters (1,377 ft) tall at the GRES-2 power plant in Ekibastuz, Kazakhstan. In the United States and a number of other countries, atmospheric dispersion modeling studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also requires the height of a flue gas stack to comply with what is known as the "Good Engineering Practice (GEP)" stack height. In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.

2.2.3- Fly ash collection:

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the

40 induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.

- Bottom ash collection and disposal:

At the bottom of the furnace, there is a hopper for collection of bottom ash. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site. Ash extractor is used to discharge ash from Municipal solid waste fired boilers.

2.2.4- Boiler make-up water treatment plant and storage:

Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blowdown and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water, and that is done by awater demineralizing treatment plant (DM). A DM plant generally consists of cation , anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.

The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets 41 deaerated, with the dissolved gases being removed by a de-aerator through an ejector attached to the condenser.

2.2.5- Fuel preparation system:

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. The pulverizers may be ball mills, rotating drum grinders, or other types of grinders.Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100 °C before being pumped through the furnace fuel oil spray nozzles.

Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel in the event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.iv.

- Barring gear:

Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches.

This small shaft deflection, only detectable by eccentricity meters, would be enough to cause damaging vibrations to the entire steam turbine generator unit when it is restarted. The shaft is therefore automatically turned at low speed (about one percent rated speed) by the barring gear until it has cooled sufficiently to permit a complete stop.

42

43

- Oil system:

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam stop valve, the governing control valves, the bearing and seal oil systems, the relevant hydraulic relays and other mechanisms.

At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system.

- Generator cooling:

While small generators may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements. Hydrogen gas cooling, in an oil- sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during start-up, with air in the generator enclosure first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air.

The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The hydrogen must be sealed against outward leakage where the shaft emerges from the casing. Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage to atmosphere.

The generator also uses water cooling. Since the generator coils are at a potential of about 22 kV, an insulating barrier such as Teflon is used to interconnect the water line and the generator high voltage windings. Demineralized water of low conductivity is used. iii.

- Generator high voltage system:

The generator voltage for modern utility-connected generators ranges from 11 kV in smaller units to 22 kV in larger units. The generator high voltage leads are normally large aluminium channels because of their high current as compared to the cables used in smaller machines. They are enclosed in well-grounded aluminium bus ducts and are supported on

44 suitable insulators. The generator high voltage leads are connected to step-up transformers for connecting to a high voltage electrical substation (usually in the range of 115 kV to 765 kV) for further transmission by the local power grid. The necessary protection and metering devices are included for the high voltage leads. Thus, the steam turbine generator and the transformer form one unit. Smaller units,may share a common generator step-up transformer with individual circuit breakers to connect the generators .

2.3- Rankine cycle:

Is a mathematical model that is used to predict the performance of steam engines. The Rankine cycle is an idealised thermodynamic cycle of aheat engine that converts heat into mechanical work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. The Rankine cycle, in the form of steam engines, generates about 90% of all electric power used throughout the world,[1]including virtually all biomass, coal, solar thermal andnuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath and Glasgow University professor.

Figure No (6) Rankine cycle

The Rankine cycle closely describes the process by which steam-operated heat engines commonly found in thermal power generation plants generate power. The heat sources used in these power plants are usually nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil. The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid. Also, unless the pressure and temperature reach super critical levels in the steam boiler, the temperature range the cycle can operate over is quite small: steam turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and steam condenser temperatures are around 30°C. This gives a theoretical maximum Carnot efficiency for the steam turbine alone of about 63% compared with an actual overall thermal efficiency of up to 42% for a modern coal-fired 45 power station. This low steam turbine entry temperature (compared to a gas turbine) is why the Rankine (steam) cycle is often used as a bottoming cycle to recover otherwise rejected heat in combined-cycle gas turbine power stations.

Figure No (7) curve of the four processes in the Rankine cycle.

There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the above Ts diagram.

 Process 1-2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage the pump requires little input energy.

 Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated using mollier diagram or h-s chart or enthalpy- entropy chart also known as steam tables.

 Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. The output in this process can be easily calculated using the Enthalpy-entropy chart or the steam tables.

 Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become a saturated liquid.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the T-S diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor 46 ending up in the superheat region after the expansion in the turbine, which reduces the energy removed by the condensers. 14.

- Transport of coal fuel to site and to storage

Most thermal stations use coal as the main fuel. Raw coal is transported from coal mines to a power station site by trucks, barges, bulk cargo ships or railway cars. Generally, when shipped by railways, the coal cars are sent as a full train of cars. The coal received at site may be of different sizes. The railway cars are unloaded at site by rotary dumpers or side tilt dumpers to tip over onto conveyor belts below. The coal is generally conveyed to 3 crushers which crush the coal to about ⁄4 inches (19 mm) size. The crushed coal is then sent by belt conveyors to a storage pile. Normally, the crushed coal is compacted by bulldozers, as compacting of highly volatile coal avoids spontaneous ignition.

The crushed coal is conveyed from the storage pile to silos or hoppers at the boilers by another belt conveyor system. 8.

2.4- Cooling tower:

A condensing turbine uses all the energy from the steam going from high pressure turbine to secondary turbine to condensing turbine then sends the condensate back for reheating. Where a non condensing turbine just uses the high pressure aspect of the steam then returns the low pressure stream back to be reheated. Condensng turbines utilises the entire available drop from high pressure to the vacuum in the condenser; a back pressure turbine only utilises only the top part, whereas an exhaust steam turbine utilises only th bottom part of the pressure drop.

A tower- or building-like device in which atmospheric air (the heat receiver) circulates in direct or indirect contact with warmer water (the heat source) and the water is thereby cooled A cooling tower may serve as the heat sink in a conventional thermodynamic process, such as refrigeration or steam power generation, or it may be used in any process in which water is used as the vehicle for heat removal, and when it is convenient or desirable to make final heat rejection to atmospheric air. Water, acting as the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is recirculated through the system, affording economical operation of the process.

47

Two basic types of cooling towers are commonly used. One transfers the heat from warmer water to cooler air mainly by an evaporation heat-transfer process and is known as the evaporative or wet cooling tower. Evaporative cooling towers are classified according to the means employed for producing air circulation through them: atmospheric, natural draft, and mechanical draft. The other transfers the heat from warmer water to cooler air by a sensible heat-transfer process and is known as the nonevaporative or dry cooling tower. Nonevaporative cooling towers are classified as air-cooled condensers and as air-cooled heat exchangers, and are further classified by the means used for producing air circulation through them. These two basic types are sometimes combined, with the two cooling processes generally used in parallel or separately, and are then known as wet-dry cooling towers.

Evaluation of cooling tower performance is based on cooling of a specified quantity of water through a given range and to a specified temperature approach to the wet-bulb or dry-bulb temperature for which the tower is designed. Because exact design conditions are rarely experienced in operation, estimated performance curves are frequently prepared for a specific installation, and provide a means for comparing the measured performance with design conditions. i.

Figure No (8) cooling towers .

The Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings. Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 48 meters tall and 100 meters in diameter, or rectangular structures (as in Image 3) that can be over 40 meters tall and 80 meters long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.

There are two approaches to removing impurities in steam generating systems: external and internal water treatment. External treatment (also called pretreatment) refers to any process used to improve water quality before it enters the boiler. A sound approach to boiler operation is to use pretreatment in conjunction with a well planned internal chemical treatment program. That's because boiler feedwater, regardless of the type and extent of external treatment, may still contain impurities. Other contaminants may originate from pretreatment system upsets or process contamination in returned condensate. Even a small amount of impurities in boiler feedwater can eventually accumulate in the boiler to dangerously high levels, due to the effect of "cy cling". Therefore, the accepted practice is to use some type of internal chemical treatment. A s in most water-handling systems, the problems associated with deposit formation and corrosion are so closely related that both must be effectively controlled in order to achieve satisfactory results. In deciding on the treatment to be used, the entire system must be considered since all parts of the steam/water system are interrelated. In this unit w e will cover several approaches to chemical treatment, but as treatment of very high pressure boiler systems is substantially different, it is left out. 2.5-Problems in the boiler: 2.5.1- Corrosion: In our effort s to control corrosion in a boiler water system, remember that dissolved oxygen in the boiler water plays a key role in the corrosion process. Oxygen can cause corrosion in the pre boiler, boiler and after boiler sections with metal destruction occurring rapidly . Therefore, it is essential that dissolved oxygen be kept at the lowest possible level throughout the system. This requirement grows more critical in larger systems with high operating pressures because the cost and danger of equipment failure increase proportionately. Primary removal of dissolved oxygen, or deaeration as it is commonly termed, is carried out in the pre boiler sect ion by means of a deaerator preheating the feedwater. This equipment utilize s steam to heat water, and in effect, drive off oxygen and

49 other dissolved gases.The solubility of gases in a liquid decreases as the temperature of the liquid is increased.

Figure No (9) drive off oxygen and other dissolved gases. While modern mechanical deaerator can reduce the dissolved oxy gen level to seven thousandths of appm , any malfunction of the equipment will permit much higher levels of oxy gen to enter the boiler. It is a common practice to add a chemical oxy gen scavenger to remove the last traces of oxy gen in the feedwater prior to its entry into the boiler. Historically, sodium ulphite and hydrazine have been the most commonly used scavengers. Recently, the use of hydrazine has decreased. Substitutes such as Di-Ethyl-Hydroxyl- Amine (DEHA) are now commonly used to scavenge oxy gen. In addition to removing oxy gen, most of the current scavengers with the except ion of sulphite are also metal passivators. The scavengers react with various metal surfaces such as iron oxide and promote the formation of the protective magnetite and copper oxide layers. Oxygen scavengers can be added to either the pre boiler or boiler sections, and as some of them are volatile, and easily vaporize into gases, they can also be carried into the after boiler sect ion along with the steam. Pure steam, as produced by the boiler, should have a neutral pH of 7. However, in an untreated system, it often ha s a pH of around 5 or 6. This is because of the breakdown of alkalinity in the boiler. Alkalinity is made up of dissolved carbon dioxide gas, bicarbonate ions, carbonate ions, and hydroxide ions. A t operating pressures of 10 bar or more, about 80% of all the bicarbonate and carbonate ions w ill break down and form carbon dioxide gas. This gas flashes off; leaving the boiler with the steam and 50 eventually re-dissolves in the condensate to form carbonic acid when the carbonic acid is formed, the pH of the condensate is lowered. To prevent this, neutralizing compounds are added to react with the carbonic acid. Because they must also travel with the steam, they are typically volatile "canine" chemicals such as rnorpholine and cyclo-hexyl-amine. Usually a combination of several amines provides the best protect ion, depending upon the design of the after boiler sect ion, the consumption rate of the amine, and cost. Another type of amine, known as filming amine, is sometimes used to provide protection against corrosion in the after boiler section. These types of amines are much large r molecule s than the volatile amines just discussed. When used correctly, filming amines form a thin w ax-like, non wet table film on the metal surfaces of the steam and condensate lines, and prevent contact between the metal surface and the corrosive condensate. Therefore, attack by both carbon dioxide and oxy gen are minimized. Octadecylamine has been the most widely used filming amine. In recent y ears, soy abased amines have gained popularity. 2.5.2- Deposits: The problem of deposit formation is the next area for discussion. Boiler deposits are controlled and prevented chemically through one of two approaches, a precipitating program or a so lubilizing program. Precipitating programs result in the formation (or precipitation) of sludge instead of scale. The sludge is relatively non-adherent and ca n be removed from the boiler through bottom blowdown. Phosphate-based treatments are the most common precipitating programs. With this approach, desirable hydroxyl apatite and serpentine sludge are formed and removed. Phosphate dosage levels must be carefully monitored and controlled. If not, scales such as calcium silica temagnesium hydroxide and magnesium phosphate may form instead. Dispersants are sometimes added to the program to condition the sludge and prevent excessive sludge buildup.

2.5.3- Solubilizing programmed: The other approach, the solubilizing chemical treatment, is not subject to most of the problems associated with precipitating chemical treatment. In a solubilizing program, hardness ions are kept in soluble form rather than being precipitated to form sludge. Solubilizing programs can be divided into two types. One type utilizes chemicals which react on a one-to-one basis with impurities to keep them solubilized. In other words, one part of chemical is necessary to react with each part of hardness to keep it solubilized. This type of programmed relies on precise proportions and is called stoichiometric treatment.

51

Chelants such add EDTA and NTA are the two most common chemicals used in these types of programs. Chelants are chemicals that tie up dissolved impurities and prevent them from precipitating. F or example, calcium ions in the feedwater are bound by the chelant, preventing their reaction with carbonate, sulphate, or silicate ions to form scale .A major disadvantage in the use of chelants is that they require very careful control to be effectiv e. A low residual of free chelant must be maintained in the boiler for two reasons: 1. Chelants are relatively expensive, and 2. High excess chelant concentrations are corrosive. On the other hand, keeping this low residual chelant level makes such programmers highly sensitive to upsets in the feedwater quality. Excessive hardness levels entering the boiler will consume the free chelant residual resulting in the excess hardness precipitating as scale. The other type of solubilizing approach employs polymers, dispersants, and organic sludge conditioners to prevent scale formation. These chemicals function by absorbing onto the surface of precipitates, effectively blocking their growth into larger particles, and simultaneously keeping the particles relatively soft and non-adherent. Additionally, these chemicals serve to disperse any scale fragments, minimizing their build-up. Since one polymer molecule can react with more than one molecule of scale, these types of programs are referred to as substoichiometric. Because the chemicals function by distorting the scale structure, precise control in these programmers is not as critical as with the chelant programs. However, gross overfeed should be avoided as this can cause particles to bind and settle out on metal surfaces. We have now covered the basic treatment of boiler water systems. It is a complex science, combining knowledge of chemistry, boiler design and equipment operation. Our local representative is the best source for further information of the program recommended for y our system. He is a professional water treatment Technologist -ready to provide his expertise and assistance to help with any problems in y our plant. i. 2.6- Chemical Treatments:

• Lime Softening and Soda Ash Quick or slaked lime (usually calcium hydroxide) is added to hard water to precipitate the calcium, magnesium and, to some extent, the silica in the water. Soda ash is added to precipitate non-bicarbonate hardness. The process typically takes place in a clarifier followed by a hydrogen cycle cation exchange and a hydroxide cycle anion exchange demineralization.

52

- Phosphate Mono:

Di-or trisodium phosphate and sodium polyphosphate can be added to treat boiler feedwater. Phosphate buffers the water to minimize pH fluctuation. It also precipitates calcium or magnesium into a soft deposit rather than a hard scale. Additionally, it helps to promote the protective layer on boiler metal surfaces. However, phosphate forms sludge as it reacts with hardness; blowdown or other procedures should be established to remove the sludge during a routine boiler shutdown.

- Chelates:

Nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (EDTA) are the most commonly used chelates. Chelates combine with hardness in water to form soluble compounds. The compounds can then be eliminated by blowdown. The preferred feed location for chelates is downstream of the feedwater pump. A stainless steel injection quill is required. However, chelates treatment is not recommended for feedwater with high hardness concentration.

- Polymers:

Most polymers used in feedwater treatment are synthetic. They act like chelates but are not as effective. Some polymers are effective in controlling hardness deposits, while others are helpful in controlling iron deposits. Polymers are often combined with chelates for the most effective treatment.

- Oxygen Scavengers:

A deaerator removes most of the oxygen in feedwater; however, trace amounts are still present and can cause corrosion-related problems. Oxygen scavengers are added to the feedwater, preferably in the storage tank of the feedwater, to remove the trace amount of oxygen escaped from the deaerator. The most commonly used oxygen scavenger is sodium sulfite. Sodium sulfite is cheap, effective and can be easily measured in water.

- Neutralizing Amines:

Neutralizing amines are high pH chemicals that can be fed directly to the feedwater or the steam header to neutralize the carbonic acid formed in the condensate (acid attack). The three most commonly used neutralizing amines are morpholine,

53 diethyleminoethanal (DEAE) and cyclohexylamine. Neutralizing amines cannot protect against oxygen attack; however, it helps keep oxygen less reactive by maintaining an alkaline pH.

- Filming Amines: Filming amines are various chemicals that form a protective layer on the condensate piping to protect it from both oxygen and acid attack. The filming amines should be continuously fed into the steam header with an injection quill based on steam flow. The two most common filming amines are octadecylamine (ODA) and ethoxylated soya amine (ESA). Combining neutralizing and filming amine is a successful alternative to protect against both acid and oxygen attack. vi.

Table No (2) list of problems caused by Impurities in water.

54

Chapter Three

Materials and Method

3.1 - Dr.Mohamod Sharef thermal power station:

Dr.Mohamod Sharef thermal power station (former Bahri thermal station) located east of Khartoum North Industrial Zone, the site was chosen for following reasons :

1. Proximity to the areas of electric power consumption. 2. Proximity to rail and road which leads to the easy access of operating materials. 3. Proximity to the distribution and transport stations. 4. Proximity to a water source Blue Nile.  The establishment of the station has been created to meet the shortage in hydro generation, especially in the flood season or in the summer time of low water levels and to meet the growing demand for energy in this period.  Increase the power generated in the national network.  Improve and raise the efficiency of the performance and operation of the national grid.  Meet the demand of the expansion projects in agricultural, industrial and urban.

Sections of the station buildings:

• Turbines and boilers. • The administration building. • Chemistry Lab. • Workshops and the main store. • Garage. • Dump and fuel tanks. • Desalination Station. • Cooling towers and accessories. • Workers housing. • River water station.

55

3.2 Chemical Treatment (Dosing):

3.2.1 River station:

Figure No (10) pretreatment of water in the river station:

unit dosing performance Presettlement 1. Poly Electrolyte 1. To change the bigger suspended solids in sludge. Tank 2. Calcium Hypo Chlorite To kill aquatic organisms (disinfection). 1. Poly Electrolyte 1. To remove suspended solid Clarifier 2. Calcium Hypo Chlorite To kill aquatic organisms(disinfection). Tanks 3. Aluminum sulphate To change the suspended solid in sludge.

Table No (3) chemical dosing in the river station

56

3.2.2 Demineralization unit:

The treatment resins are classified into four basic categories:

 Strong Acid Cation (SAC)  Weak Acid Cation (WAC)  Strong Base Anion (SBA)  Weak Base Anion (WBA).

Figure No (11) the mechanism of demineralization water.

Table No (4) chemical dosing in the demineralization station:

unit dosing performance Cation Exchanger Regeneration of sulpharic Production all cations from acid Resin Anion Exchanger Regeneration of caustic soda Production all anions from Resin Mixed bed Exchanger Regeneration Sulpharic acid Production all cation and and caustic soda anion from Resin

57

Figure No (12) impurities of water in river station.

3. 2. 4. Feed water dosing:

i. Low pressure dosing: Table No (5) Feed water dosing.

unit dosing performance

Ammonia Rise pH (8.8- 9.3)

Feed water Hydrazine Oxygen scavenger

 Ammonia is a weak base. It undergoes the following reaction with water:

+ → + OH

The equation implies, adding ammonia to water changes both the pH and conductivity.

58

Figure No (13) relationship among concentration, pH, and conductivity for dilute aqueous ammonia solutions.

 pH, the measurement of acidity or basicity, is one of the most important factors affecting scale formation or corrosion in a boiler or cooling system. The types of impurities comprising the mineral concentration behave differently at various pHs. Low pH waters have a tendency to cause corrosion, while high pH waters may cause scale formation.  Dissolved oxygen can be eliminated in a boiler system by utilizing a deaerator feed tank, heated feedwater or condensate tank. Introducing an oxygen scavenger into the feedwater system will also help eliminate any dissolved oxygen in the feedwater. The target for dissolved oxygen in the feedwater at the economizer inlet or, in the absence of an economizer, the boiler feedwater inlet is zero dissolved oxygen.

ii. High pressure dosing:

In this system added some chemicals in boiler and to protect tubes from corrosion as well as to prevent the occurrence of deposits on the surface of the pipes. Injection system chemical boiler consists of pumps, pumping given pressure 100-120bar, the chemical materials added is Try Sodium Phosphonate which prepared and injected in the Drum .

59

3.2.5-Boiler Internal Treatment Chemicals dosing:

Boiler internal treatment chemicals maintain clean boiler tube surfaces and minimize corrosion-reducing the chance of overheating or failure. Control of corrosion and deposition helps in increasing plant reliability and availability, protects equipment, and reduces maintenance and fuel costs. In addition to controlling deposition and reducing corrosion, some water treatment chemicals can reduce the tendency for boiler water to carry over to the steam-protecting superheaters and steam turbines from damage. . vi.

3.2.6-Cooling tower dosing:

Table No (6) cooling towers dosing.

unit dosing performance Sulpharic Acid Rise pH (7.6- 7.8) Cooling tower Inhibitors Scale and corrosion Biocide Shock dose

- In general, operation at higher pH requires significant changes in the treatment program. Bleach, for example, may work well at lower pH but it loses its ability to control microbial growth at higher pH and bromine compounds may be required. General corrosion rates tend to become lower as pH increases, but traditional corrosion control chemicals (like phosphate) may increase the risk of scale formation. Finally, the function of scale and deposit inhibitors is extremely pH dependent. A product that works well at lower pH may work poorly or not at all as pH increases. In addition, additional scale inhibitors may be required to stabilize scales that don't form at lower pH. - Biological growth is another extremely important facet to proper cooling water management. Microbes can cause corrosion, fouling, and disease. Oxidizing biocides (chlorine, chlorine dioxide, ozone and bromine) have been employed to keep bacteria under control.

60

61

3.3- Laboratory:

3.3.1- turbidity Test:

The turbidity is a water quality term that refers to the cloudiness of water. It is caused by suspended materials such as clay, silt, organic matter, phytoplankton and other microscopic organisms being picked up by water as it passes through a watershed. The greater the amount of total suspended solids in water, the murkier the water appears, the higher the turbidity measurement will be. Turbidity is an important water quality indicator because contaminants such as bacteria, viruses and parasites can attach themselves to the suspended particles in turbid water. These particles then interfere with disinfection of the water by shielding contaminants from the disinfectant (e.g. chlorine).

Figure No(15 ) turbidity meter.

Method:

The basic instructions for using a turbidity meter are calibrating the meter to a set of turbidity standards by filling a cuvette (small, glass vial) with a known standard and inserting the cuvette into the chamber of the turbidity meter. Normally a meter can be calibrated with two prepared standards. The concentrations of the standards should be chosen from the low and high ends of the range of the meter. Once calibration is complete, individual water samples can then be tested by placing a water sample into a clean cuvette, inserting it into the chamber of the turbidity meter and reading the

62 displayed amount on the meter. When a cuvette is inserted into the meter it should be dry and wiped clean of smears of fingerprints, as anything on the cuvette could lead to an inaccurate reading of turbidity. Always follow the manufacturer’s instructions for calibration and use.

3.3.2- Conductivity Test:

The conductivity monitors the amount of nutrients, salts or impurities in water and is measured in many fields such as the chemical industry, agriculture, public drinking water systems, aquaculture, water conditioning and treatment and environmental monitoring.

Conductivity (EC) is a measure of a material's ability to conduct an electric current. It estimates the amount of total dissolved solids or salts (TDS) or the total amount of dissolved ions in water. The more salt, acid or alkali in a solution, the greater it’s conductivity. Pure water does not conduct electricity. In agricultural systems for instance, the conductivity of the nutrient solution needed for plant growth must be known. If it is too weak, plants will not get the vital elements they need to grow. If it is too strong, plants run the risk of root burn, especially as evaporation occurs within the growing environment. The nutrient solution needs to be measured or monitored regularly so you know when to add more concentrated solution or when to add fresh water. A high electrical conductivity will stress plants and cause productivity losses.

Concerning cooling towers and boilers, companies may be consuming more make- up water than necessary in rinsing their equipment. This contributes to extra water consumption, chemical consumption, generation of wastewater, and in the case of boilers, fuel consumption. In water rinse tanks, contamination is caused by residual surface chemicals being "dragged in" on the parts being rinsed. In HVAC equipment, water impurities occur as a result of minerals and constituents in the feedwater that remain even after pretreatment. Water in rinse tanks, boilers, and cooling tower systems accumulate these impurities over time, requiring addition of fresh water and removal of a portion of the contaminated water to maintain optimal process or system conditions. Conductivity is an easy characteristic to measure, and in these processes, is typically very indicative of the total dissolved solids (TDS) concentration of the water.

63

Method:

There are basically two types of conductivity meters - one is the small "stick" type meter and the other is the larger, more complex and more accurate bench top or portable model. Stick types can fit in use pocket and as such are quite useful. They are not as accurate as the larger models for the larger models often have a number of functions, are often combined with a pH meter and the electrode is separate and plugs into the instrument.

Figure No (16) conductivity meter.

It is very important to calibrate the instrument every time it is used otherwise it will cause inaccurate readings. Calibration means reading a solution of known conductivity and adjusting the meter to read the same. Calibrate the probe using a standard solution in the range of the samples being tested. Place the probe in a standard solution, condition, rinse probe in a second sample of standard solution, use a third sample of standard solution to calibrate, and then adjust the cell constant until the specified value is displayed. Recalibrate when ranges are changed, or if readings seem to be incorrect.

To take aconductivity reading immerse the electrode/probe in the solution, move the electrode up and down a few times to remove any bubbles. Wait thirty seconds for the sample and electrode to come to the same temperature and then read the

64

measurement. The instrument will usually do the temperature correction for you. The electrode should be immersed over the plates and reference electrode. With the stick type meter, immerse the pins to the level indicated in the instructions. It is important that the level of the liquid is not above the waterproofed section of the electrode or meter.

The conductivity of a solution is highly temperature dependent, therefore it is important to either use a temperature compensated instrument or calibrate the instrument at the same temperature as the solution that you want to measure. Electrodes can be cleaned with mild liquid detergent and/or dilute nitric acid (1% wt) by dipping or filling the cell with solution and agitating for 2 to 3 minutes. Dilute HCl (hydrochloric acid) or H2SO4 (sulfuric acid) may also be used. When stronger cleaning is needed, try concentrated HCl mixed into 50% is opropanol (rubbing alcohol). Rinse the cell several times with distilled or deionized water and recalibrate before use. Always follow the manufacturer’s instructions for calibration and use.

With conventional meters, conductivity is obtained by applying a voltage across two probes and measuring the conductance of the solution. Solutions with a high conductivity produce a higher current. The conductivity unit of measurement commonly used is the Siemens/cm (S/cm) or microSiemens/cm (uS/cm).

3.3.3-pH Test:

The pH is the measure of the acidity or alkalinity of a solution. Some of the areas in which pH is monitored are public drinking water systems, industrial and municipal wastewater plants, agriculture, aquaculture, environmental monitoring, pool and water analysis, food and dairy industries, boiler and cooling towers, pulp and paper mills, acid mining and the chemical manufacturing industry.

The pH scale ranges from 0 to 14. A pH of 0-6 is acidic, a pH of 7 is neutral and a pH of 8-14 is alkaline. Pure water has a neutral pH and human saliva is close to neutral, while our blood is slightly alkaline. Seawater is between 7.7 and 8.3 on the pH scale, and products like hand soap, ammonia and bleach are more alkaline in the range from 9.0 – 12.5. Highly alkaline baking soda is often used to raise the pH of acidic water to a more neutral level.

65

In agriculture, pH affects the ability of plant roots to absorb nutrients. Calcium, phosphorus, potassium and magnesium are likely to be unavailable to plants in acidic soils. Plants have difficulty absorbing copper, zinc, boron, manganese and iron in basic soils. By managing soil pH, one can create an ideal environment for plants and often discourage plant pests at the same time.

Aquarists rely heavily on pH measurements for proper fish keeping. Large bodies of water such as lakes and oceans have very little pH fluctuation, making fish intolerant to pH swings. Fish keeping is a delicate pH balancing act, as fish subjected to pH swings are prone to disease and early death.

Due to chemical water treatment and other factors, tap water in many large cities throughout the world tends to be alkaline with a pH near 8.0. Though drinking tap water with a high (alkaline) pH is not harmful, the declining quality of tap water over the years has resulted in many people opting for faucet or pitcher filters to remove chlorine, chloramines, pesticides and other substances. These filters do not alter the pH of the water. Alternately many people choose to buy purified bottled water or fresh spring water, more likely to have a pH closer to neutral.

pH test strips, pocket meters, portable meters or bench top meters are used to measure pH. Meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with data logging capability and computer interfaces. Inexpensive models sometimes require that temperature measurements be entered to adjust for the slight variation in pH caused by temperature fluctuations of the sample. Today most models are equipped with Automatic Temperature Compensation (ATC) where adjustments due to temperature are done internally by the meter. Meters and specialized probes are available for use in applications such as environmental, quality control and laboratories testing.

Method:

A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading. The pH probe measures pH as the activity of hydrogen ions surrounding a thin-walled glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter.

66

Figure No (17) pH meter.

Before testing your samples, calibration of the instrument should be done in accordance with the manufacturer’s instructions daily, though modern instruments will hold their calibration for around a month.

Calibration can be done at one, two or three points on the pH scale depending on the meter. A buffer solution of 7 is used for one point calibration. Meters with two or three point calibration allow you to calibrate at 7 and then calibrate with buffer solutions of 4 and 10 to insure your meter is reading accurately in that range.

After calibration, the probe is rinsed in distilled, deionized water to remove any traces of the buffer solution, blotted with a clean tissue to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in the sample. The pH value is then displayed on the meter. When testing acidic solutions a pH 4.0 buffer should be used as the second calibration solution to insure accurate readings throughout the range. Likewise, when testing alkaline solutions a 10.0 buffer should be used.

Between uses, the probe tip, which must be kept wet at all times, is typically kept immersed in a small volume of storage solution, which is an acidic solution of around 67

pH 3.0. In an emergency, pH 4.0 buffer, pH 7.0 buffer or tap water can be used, but distilled or deionized water must never be used for longer-term probe storage as the relatively ionless water 'sucks' ions out of the probe through diffusion, which degrades it. Always follow the manufacturer’s instructions for calibration and use.

3.3.4 Silica Test:

Determination of silica photometric method by silica molybdenum blue .

Instrument:

Si –meter (photometer).

Reagents:

1. Ammonium molybdate:

50.0 gm of and dissolved into 500 ml of demin water.

1.1- 42.0 ml of (s.gr. 1.84) were toke and diluted into 300ml of demin water and stir. 1.2- Then the solution mixed. 2. Tartaric acid(10% w/v): 2.1- 10.0 gm of tartaric acid weighted and dissolved into 100ml of demin water. 3. Preparation of acid :

3.1- 1.5 gm of acid ( N (OH) H) were weighted. 3.2- 7.0 gm of ( ) were weighted and dissolved into 200ml of demin water. 3.3- 90.0 gm of were weighted and dissolved into 600ml of demin. 3.4- Then were mixed all and completed the volume to 1000ml.

4. (1.5M). 5. Ammonium molybdate(10%).

68

Figure No (18) Si –meter (photometer).

Procedure :

1. 100 ml of sample were added. 2. 3.0ml of ammonium molybdate , were left to stand for 5.0 min. 3. 3.00ml of tartaric acid were added and left to stand for one min. 4. 2.0ml of 1.2.4 acid were added and left to stand for 8.0min. 5. Then the instrument was calibrated. 6. The demin water was used to wash the instrument and drain, then measured for low value (-2.8) and adjust if needed, and measured for high value (21.0) and adjust if needed. 7. This was followed by recording of the reading.

69

Chapter Four

Results and Discussion

4.1- The Results:

4.1.1- River side:

Table No (7) parameters of clarifier water tank:

parameter pH Conductivity Turbidity µs /cm NTU clarifier tank target reading target reading target reading 6.5-8.0 7.09 < 250 217 < 20 1.28

4.1.2 - Demineralization unit:

Table No (8) parameters of demineralization unit:-

parameter pH Conductivity

µs /cm location target reading target reading target reading Demin tank 6.0-7.5 5.95 ≤ 0.2 0.58 < 20 13.0 Anion 6.0-7.5 5.95 ≤ 10 3.93 < 50 12.9

4.1.3- Boiler Drum Water: Table No (9) reading parameters of boiler drum water :

parameter pH Conductivity

µs /cm location target reading target reading target reading

Boiler Drum 9.6-9.9 9.33 ≤ 150 13.7 ≤ 6000 73.1 Water

70

4.1.4- Condensate Water: Table No (10) reading parameters of condensate water: parameter pH Conductivity µs /cm Condensate Water target reading target reading

8.5-9.2 8.03 ≤ 7 4.7

4.1.5- Dearator Water:

Table No (11) reading parameters of dearator water:

parameter pH Conductivity µs /cm Dearator Water target reading target reading 8.5-9.2 9.42 ≤ 7 9.49

4.1.6- Feed Water:

Table No (12) reading parameters of feed water:

parameter pH Conductivity

µs /cm location target reading target reading target reading Feed Water 8.5-9.2 8.51 ≤ 7 5.63 ≤ 60 15.3

71

4.1.7- Saturated Steam:

Table No (13) reading parameter of saturated steam :

parameter pH Conductivity

µs /cm location target reading target reading target reading Saturated Stem 8.5-9.2 7.75 ≤ 10 10.2 ≤ 20 11.5

4.1.8- Superheated Steam:

Table No (14) reading parameters o f superheated steam:

parameter pH Conductivity

µs /cm location target reading target reading target reading Superheated 8.5-9.2 8.12 ≤ 10 10.24 ≤ 20 12.7 Steam

4.1.9- Cooling Towers

Table No (15) reading parameters o f cooling towers: parameter pH Conductivity Turbidity µs /cm NTU Cooling target reading target reading target reading towers 7.5-7.8 7.70 < 1500 823 < 20 6.54

72

4.2 –The Discussion:

4.2.1-Introduction:

The feedwater is composed of makeup water (usually city water from outside boiler room/ process) and condensate (condensed steam returning to the boiler). The feedwater normally contains impurities, which can cause deposits and other related problems inside the boiler. Common impurities in water include alkalinity, silica, iron, dissolved oxygen and calcium and magnesium (hardness). Blowdown, a periodic or continuous water removal process, is used to limit the concentration of impurities in boiler water and to control the buildup of dissolved solid levels in the boiler. Blowdown is essential in addition to chemical treatments. Chemical treatment of water inside the boiler is essential whether or not the water has been pretreated. Internal treatment, therefore, compliments external treatment by taking care of any impurities entering the boiler with the feedwater (hardness, oxygen, silica, iron regardless of whether the quantity is large or small)l. In some cases external treatment of the water supply is not necessary and the water can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low pressure, much of the condensate is returned and the raw water is of good quality. However, in moderate and high pressure boilers, external pretreatment of the make-up water is mandatory for good results. With today's higher heat transfer rates, even a small deposit can cause tube failures or wasted fuel. Clarification is the removal of suspended matter and color from water supplies. The suspended matter may consist of large particles that settle out readily. In these cases, clarification equipment merely involves the use of settling basins or filters. Most often, suspended matter in water consists of particles so small that they do not settle out, but instead pass through filters. The removal of these finely divided or colloidal substances therefore requires the use of coagulants. Chemical treatment is widely used to control dissolved oxygen in a boiler. The cost of operating a chemical treatment program consists of chemical costs and blow down costs. Periodically the water in the boiler must be flushed out to remove non-volatile compounds. They are flushed out of the boiler in a process called blow down. Chemical addition to the water can increase the frequency of blow down, which increases the operating cost of the boiler. There are two components of blow down costs. Water and steam that is purged from the boiler during blow down is sent to drain. This water must be replenished by fresh makeup water and there is a cost associated with it. The second cost is heat or energy cost. The water blow down from the boiler is hot. It is replaced with cold 73 water that must be reheated in order to produce steam. The benefits of Chemical Treatments: Increase boiler efficiency, Reduce fuel, operating and maintenance costs, Minimize maintenance and downtime, Protect equipment from corrosion and extend equipment lifetime. The purposes of an internal treatment program include: To react with incoming feedwater hardness and prevent it from precipitating on the boiler metal as scale, to condition any suspended matter such as hardness sludge in the boiler and make it non adherent to the boiler metal, to control the causes of boiler water carryover, to eliminate oxygen from the feedwater and to provide enough alkalinity to prevent boiler corrosion. vi.

Table No (16) seasonal variation of the turbidity in the clarifier tank:

Parameter Month Turbidity=(<20) January March August NTU 1.72 1.88 4.2

From the clarifier tank the samples taken in March had a turbidity =1.88 NTU and the target value <20 NTU that means the obtained value is acceptable and compared with the turbidity reading for season samples taken in January (1.72 NTU) and august (4.2 NTU) this means that in august the turbidity was high .as shown in the figure No (20) below:

4.5 4 3.5 3 2.5 2 سلسلة1 1.5 1 0.5 0 January March August

Figure No (19) the seasonal variation of the turbidity in the clarifier tank.

74

In January and August were the average of turbidity was 1.72 NTU and 4.2 NTU respectively with relatively noticeable increase in august (flood season) as see in figure No (20).

Table No (17) seasonal variation of the pH in the clarifier tank:-

Parameter month PH=(6.5 - 8) January March August 7.10 8.10 7.69

As to the PH where the target is (6.5 - 8) and the table No (17) shows that .its determined value was slightly beyond the standard this could be due to some colloids that escaped settling in the clarifier . Table No (18) seasonal variation of the conductivity in the clarifier tank:

Parameter Month Conductivity= January March August (<250) µs /cm 220 203 201

As to the conductivity the seasonal variation as in table No (18) shows that all readings were below the recommended value (<250µs /cm). The summing of the seasonal variations of these parameters turbidity, PH and conductivity shows in table No (18).

Table No (19) the overall comparative seasonal variation of

( turbidity, pH and conductivity) in the clarifier tank:

Month parameter January March August Turbidity=(<20) NTU 1.72 1.88 4.2 pH=(6.5 - 8) 7.10 8.10 7.69 Conductivity= (<250) µs /cm 220 203 201

75

Table No (20) seasonal variation of the parameters (turbidity, pH and conductivity). With respect to samples from the cooling towers, where reading for parameters show same deviation due.

Month parameter January March August pH(7.5- 7.8) 6.52 7.70 7.05 Turbidity(<20) NTU 5.99 6.54 7.80 Conductivity(<1500) µs /cm 694 823 910

Table No (20) seasonal variation of the parameters (turbidity, pH and conductivity) in the cooling tower.

Table No (21) the variation of pH, conductivity and silica (SiO2) of demin tank, anion exchanger , boiler drum water , condensate water ,dearator water, feed water ,saturated steam and superheated steam:

pH Conductivity

Sample µs /cm target reading target reading target reading Demin tank 6.0-7.5 5.95 ≤ 0.2 0.58* < 20 13.0 Anion exchanger 6.0-7.5 5.95 ≤ 10 3.93 < 50 12.9 Boiler Drum Water 9.6-9.9 9.33 ≤ 150 13.7 ≤ 6000 73.1 Condensate Water 8.5-9.2 8.03 ≤ 7 4.7 - - Dearator Water 8.5-9.2 9.42* ≤ 7 9.49* - - Feed Water 8.5-9.2 8.51 ≤ 7 5.63 ≤ 60 15.3 Saturated Steam 8.5-9.2 7.75 ≤ 10 10.2* ≤ 20 11.5 Superheated Steam 8.5-9.2 8.12 ≤ 10 10.24* ≤ 20 12.7

* Readings beyond the measured values shows some deviation to the standard. This means that water had high concentration of solids and impurities after the river station.

- Most of the reading values for the sample in the range from the desired value (target value) of the PH , conductivity and silica that means the chemical dosing added is 76

specific and suitable .I made dissection for the chemical adding to show the important of it for any stage (low pressure and high pressure ) bellow :

- Anionic exchanger the second ion exchanger removes all the anionic species (nitrate, chloride, sulphates, silicate) and exchanges them into water molecules are produced ( from the cationic outlet and from the anionic outlet.)Conductivity is about 2 μS/cm (because of some ppb ionic species that are not completely exchanged) and the pH is about 6-7.5. In case of anionic exchanger exhaustion, the first leakage will be chloride because of its lower charge density. Anionic exchangers are regenerated by NaOH, removing all anions trapped by . When generation is completed, the resin is full of sites.

* The types of impurities comprising the mineral concentration behave differently at various pHs. Low pH waters have a tendency to cause corrosion, while high pH waters may cause scale formation. Caustic-based treatments require measuring only pH and conductivity. pH indicates the caustic level and conductivity indicates the total concentration of dissolved solids (treatment chemicals and contaminants).

* If silica is not removed from the boiler feed water, it will concentrate itself on the drum and is carried over in steam to form adherent deposits in the steam passage way distorting the original shape of turbine nozzles and blades. This alters steam velocities and the pressure drops reducing the capacity and efficiency of the turbine.

* Severe conditions can cause excessive rotor thrust while uneven deposition can unbalance the turbine rotor causing vibration problems. Turbine deposits can accumulate in a very short time when steam purity is poor and can only be removed by external service cleaning and blasting aluminium oxide on the surface.

- Chemical treatment of water inside the boiler is essential whether or not the water has been pretreated. Internal treatment, therefore, compliments external treatment by taking care of any impurities entering the boiler with the feedwater (hardness, oxygen, silica, iron regardless of whether the quantity is large or small. In some cases external treatment of the water supply is not necessary and the water can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low pressure, much of the condensate is returned and the raw water is of good quality. However, in moderate and high pressure boilers, external pretreatment of the make-up water is mandatory for good results. With today's higher heat transfer rates, even a small

77 deposit can cause tube failures or wasted fuel. Make up water: demineralization plants failure, resin fines Silica reduces turbine efficiency; sodium (NaOH, NaCl) corrodes boiler tubes and turbine blades. Sulfate comes from residual chemicals used for regenerating ionic exchangers. vi.

78

Chapter Five

Conclusions and Recommendation

5.1- Conclusions :

 Most of the reading values for the samples were in the range of target value for the

turbidity, pH, conductivity and silica ( ), that means the chemical dosing added is suitable.  Common problems to avoid in the preparation of boiler feed water are impurities, carryover, corrosion, and deposits.  Many processes available in the plant, the suitability of each is a function of source quality, operator capacity and financial resources availability.

5.2-Recommendation:

1. Ion exchanger must be regenerated in specific times as recommended by the manufacturer.

2. Technology selection in the preparation of boiler feed water must be made on the basis of the plant features, this necessary for the ensurance of sustainability in ability.

3. To solve operational problems the monitor controller and automatic devices must be updated.

4. Generally to generate high quality steam the boiler feed water must be treated by setting the chemical dosing and control the operations.

79

REFERENCES: Technical Data: Dr. Mohamod Sharef Thermal Power Station. Websites: i. https://en.wikipedia.org/wiki/Boiler_feedwater‎. ii. http://www.centralheatingcentre.com/Central_Heating/boilers. iii. http://www.cleaver-brooks.com/boiler_efficiency_facts.pdf. v. http://www.naturalgas.org/environment/naturalgas.asp. vi. http://www. Ask and Answers.com. vii. http://www.nalco.com.

Books:

1. Edgar Thomas. F & Himmelblau David. M, Optimization of Chemical Engineering Processes 2nd Edition 2001. 2. Fogler H. Scott , Elements of Chemical Reaction Engineering 3rd Edition, 1999. 3. Mc Cabe Warren.L,Smith Julian.C & Peter Harriott., Unit Operations of Chemical Engineering, 17th Edition , 2005. 4. Smith. J.M, Chemical Engineering Kinetics, 3rd Edition, 19 S. Peters 84. 5. Timmerhaus Max, Klaus &West Ronald. E, Plant Design and Economic For Chemical Engineers, 5fth Edition, 2004. 6. Perry Robert. H &Green Don .W, Perry’s Chemical Engineers’ Hand Book 17th Eidition,1999. 7. Power Line, Volume 8, No. 3, December 2003. 8. Parthibhan. K. K,Natural Circulation in Boilers,2002. 9.V.Ganapathy., ABCO Industries, Abilene, Texas, HEAT TRANSFER . 10. E Von Nostrand Reinhold Company – Robert L.Loftness. 11. Energy Hand book, Second edition, 12. Raja.A.K., Amit P. Srivastava, Manish Dwivedi Power Plant Engineering, 2006. 13. Longman Scientific Technical ,Industrial boilers, 1999.

1

14. R.K.,Rajput . Power Plant Engineering,2008. http://www.Ask and Answers.com.

1. (PDF) Combating Waterborne Diseases at the Household Level. World Health Organization. 2007. Part 1.

2. (PDF) Water for Life: Making it Happen. World Health Organization and UNICEF. 2005. 3. Chen, Jimmy, and Regli, Stig. (2002). "Disinfection Practices and Pathogen Inactivation in ICR Surface Water Plants." Information Collection Rule Data Analysis. Denver:American Water Works Association. McGuire, Michael J., McLain, Jennifer L. and Obolensky, Alexa, eds. 4. Aeration and Gas Stripping, Accessed June 4, 2012. 5. CO2 Degasifiers/Drinking Water Corrosion Control, Accessed June 4, 2012. 6. Degassing Towers, Accessed June 4, 2012. 7.American Water Works Association RTW corrosivity index calculator, Accessed June 4, 2012. 8. Edzwald, James K., ed. (2011). Water Quality and Treatment. 6th Edition. New York:McGraw-Hill. ISBN 978-0-07-163011-5 9. Crittenden, John C., et. al., eds. (2005). Water Treatment: Principles and Design. 2nd Edition. Hoboken, NJ:Wiley. 10. Kawamura, Susumu. (2000). Integrated Design and Operation of Water Treatment Facilities. 2nd Edition. New York:Wiley. 11. United States Environmental Protection Agency (EPA)(1990). Cincinnati, OH. "Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities." Document no. 12. Andrei A. Zagorodni (2007). Ion exchange materials: properties and applications. Elsevier. ISBN 978-0-08-044552-6. Retrieved 22 November 2011. 13. Neemann, Jeff; Hulsey, Robert; Rexing, David; Wert, Eric (2004). “Controlling Bromate Formation During Ozonation with Chlorine and Ammonia.” Journal American Water Works Association. 96:2 (February) 26-29.

14. Conroy RM, Meegan ME, Joyce T, McGuigan K, Barnes J (1999 October). "Solar disinfection of water reduces diarrhoeal disease, an update".

2

15. Conroy RM, Meegan ME, Joyce TM, McGuigan KG, Barnes J (2001). "Use of solar disinfection protects children under 6 years from cholera". 16. Rose A. at al. (2006 February). "Solar disinfection of water for diarrhoeal prevention in southern India". 17. Hobbins M. (2003). The SODIS Health Impact Study, Ph.D. Thesis, Swiss Tropical Institute Basel. 18. B. Dawney and J.M. Pearce “Optimizing Solar Water Disinfection (SODIS) Method by Decreasing Turbidity with NaCl”, The Journal of Water, Sanitation, and Hygiene for Development . 19. Sciacca F, Rengifo-Herrera JA, Wéthé J, Pulgarin C (2010-01-08). "Dramatic enhancement of solar disinfection (SODIS) of wild Salmonella sp. in PET bottles by H(2)O(2) addition on natural water of Burkina Faso containing dissolved iron" (epub ahead of print). 20. Centers for Disease Control and Prevention (2001). "Recommendations for using fluoride to prevent and control dental decay caries in the United States". 21. Division of Oral Health, National Center for Prevention Services, CDC (1993) (PDF). Fluoridation census 1992. 22. Reeves TG (1986). "Water fluoridation: a manual for engineers and technicians" (PDF). Centers for Disease Control. 23. US EPA emergency disinfection recommendations. Epa.gov. 24. Savage, Nora; Mamadou S. Diallo (1 October 2005). "Nanomaterials and Water Purification: Opportunities and Challenges". J. Nanopart. Res. May 2011. 25. Hydrates for Gypsum Stack Water Purification. 26. Water Purification with Moringa Seeds. 27. Poulsen, Kevin."Mysterious Glitch Poisons Town Water Supply". Wired. Retrieved 15 November 2012. 28. " Miranda, Kim, Hull, et.a."Changes in Blood Lead Levels Associated with Use of Chloramines in Water Treatment Systems" 03/13/2007.Environmental Health Perspectives. 29. Health risks from drinking demineralised water. (PDF) . Rolling revision of the WHO Guidelines for drinking-water quality. World Health Organization, Geneva, 2004 30. Kozisek F. (2004). Health risks from drinking demineralised water. WHO. 31.Water Distillers – Water Distillation – Myths, Facts, etc. Naturalsolutions1.com. 32. Minerals in Drinking Water. Aquatechnology.net.

3

APPENDIX

4

Date: 4/1/2013

River side:

parameter pH Conductivity Turbidity µs /cm NTU Clarifier tank target reading target reading target reading 6.5-8.0 7.10 < 250 220 < 20 1.72

Deminerlization plant:

parameter pH Conductivity

µs /cm location target reading target reading target reading Demin tank 6.0-7.5 7.46 ≤ 0.2 0.69 < 20 15.40 Anion 6.0-7.5 5.95 ≤ 10 3.93 < 50 13.2

Boiler Drum Water :

parameter pH Conductivity

µs /cm location target reading target reading target reading

Boiler 9.6-9.9 8.11 ≤ 150 60.7 ≤ 6000 2850 Drum Water

5

Condensate Water:

parameter pH Conductivity µs /cm Condensate Water target reading target reading

8.5-9.2 7.36 ≤ 7 9.28

Dearator Water:

parameter pH Conductivity µs /cm Dearator Water target reading target reading

8.5-9.2 9.83 ≤ 7 9.49

Feed Water:

parameter pH Conductivity

µs /cm location target reading target reading target reading Feed Water 8.5-9.2 8.36 ≤ 7 9.74 ≤ 60 17.10

Saturated Steam:

parameter pH Conductivity

µs /cm location target reading target reading target readin g Saturated 8.5-9.2 7.75 ≤ 10 1.20 ≤ 20 11.50 Steam

6

Superheated Steam:

parameter pH Conductivity

µs /cm location target reading target reading target reading Superheated 8.5-9.2 8.12 ≤ 10 10.24 ≤ 20 12.7 Steam

Cooling towers:

parameter pH Conductivity Turbidity µs /cm NTU Cooling target reading target reading target reading towers 7.5-7.8 6.52 < 1500 694 < 20 5.99

7

Date 27/8/2012

River side:

parameter PH Conductivity Turbidity µs /cm NTU Clarifier tank target reading target reading target reading 6.5-8.0 7.69 < 250 201 < 20 4.2

Deminerlization plant:

parameter pH Conductivity

µs /cm location target reading target reading target reading Demin tank 6.0-7.5 7.47 ≤ 0.2 0.60 < 20 16.3 Anion 6.0-7.5 7.40 ≤ 10 0.42 < 50 7.99

Boiler Drum Water :

parameter pH Conductivity

µs /cm location target reading target reading target reading Boiler Drum 9.6-9.9 9.33 ≤ 150 35.6 ≤ 6000 73.1 Water

8

Condensate Water:

parameter pH Conductivity µs /cm Condensate Water target reading target reading

8.5-9.2 8.03 ≤ 7 4.7

Dearator Water:

parameter pH Conductivity µs /cm Dearator Water target reading target reading

8.5-9.2 9.42 ≤ 7 5.8 Feed Water:

parameter pH Conductivity

µs /cm location target reading target reading target reading Feed Water 8.5-9.2 8.51 ≤ 7 5.63 ≤ 60 12.5

Saturated Steam :

parameter pH Conductivity

µs /cm location target readi target reading target reading ng Saturated 8.5-9.2 7.63 ≤ 10 1.47 ≤ 20 11.5 Steam

9

Superheated Steam:

parameter pH Conductivity

µs /cm location target reading target reading target reading Superheated 8.5-9.2 7.76 ≤ 10 1.90 ≤ 20 5.06 Steam

Cooling towers:

parameter pH Conductivity Turbidity µs /cm NTU Cooling target reading target reading target reading towers: 7.5-7.8 7.05 < 1500 910 < 20 7.8

10