Steam Boilers of Therman Power Stations

Citation preview

OF THERMAL POWER STATIONS M.I.Reznikov and Yu.M.Lipov 1. Steam Generation at Electric Power Stations 2. Power-producing Fuels and Their Characteristics 3. Fuel Preparation at Power Stations 4 .Theoretical Principles of Combustion 5 .Combustion Products 6. Efficiency of Fuel Heat Utilization 7.Pulverized Coal-fired Furnaces 8 .Gas and Fuel Oil-fired Fur­ naces 9. Characteristics, Parameters and Motion Equations of Working Fluid 10,Temperature Conditions on Heating Surfaces HHydrodynamicsofOpen Hydraulic Systems 12. Hydrodynamics of Closed Hydraulic Systems 13. Hydro­ dynamics of Bubbling Systems 14.Physico-chemical Principles of Behaviour of Impurities in Working Fluid 15. Water Conditions 16.Processes on the Firesjde of Heating Surfaces 17. Evaporating Heating Surfaces 18. Steam Super­ heaters and Superheat Control 19. Low-temperature Heating Surfaces 20. Heat Exchange in Heating Surfaces of Boilers 21. Layout and Heat Cal­ culation of Steam Boiler 22. Steam Boilers of High-capacity Monobloc Units 23. Steam Boiler Operation 2 4 .Steam Generators of Nuclear Power Sta­ tions 25. Metals for Steam Boilers

MIR PUBLISHERS

M. H. PC3HHK0B, 10. M. JlnnoB IIAPOBblE KOTJIbl TEIlJlOBblX 3 J1EKTPOCTA1II4 MII «3nepron3fl;n» • M ockb «i

OF THERMAL POWER STATIONS M. I. Reznikov, Yu. M. Lipov T ra n sla te d from the Russian

by V adim Afannsyov

Mir Pulilishers ■ Moscow

First published 1985 Revised from the 1981 Russian edition

Ha amxuucKon

so u k s

Q t3ueproiraAaT», 1981 g) English translation, Mir Publishers, 1985

CONTENTS

8 Chapter 6 . Efficiency of Fuel Heat Preface .............................................. ................... Utilization Clmpter I. Steam Generation at Elec­ 6.1. The Ileal Balance and Efficiency tric Power Stations 10 of tho Steam Boiler . . . . 1.1. The Stonm Boiler at a Power 6.2. Analysis of Heat Losses . S t a t i o n .......................................... 10 1.2. Classification of SteamBoilers 15 diaptcr 7. Pulverized Conl-fiied Fur­ 1.3. Flow Diagram of Steam Produc­ naces ............................... tion .............................................. 17 7.1. Principal Characteristics of 1.4. Principal Characteristics of Chamber Furnaces ................ Steam Boilors ............... 21 7.2. Burners and Their Arrangement 7.3. Dry-bottom Furnaces . . . . Chapter 2. Power-producing Fuels and Their Characteristics 22 7.4. Slagging-bottom Furnaces . . 2.1. Kinds and Compositions of Fuels 22 Chapter 8 . Gas and Fuel Oil-fired Fur­ 2.2. Tho Heating Valuo and Resolv­ naces ............................... ed Characteristics of Fuels . . 25 8.1. Fumace D e s ig n ....................... 2.3. Technical Characteristics of So­ ................... lid Fuels ................... 26 8.2. Fuel Oil Burners 8.3. Combustion of Natural Gas 2.4. Technical Characteristics of Fuel Oil and Natural Gnsos . . . . 29 8.4. Combinod Gns-fuol^Oil Bumors 2.5. Main Deposits of Fossil Fuols 31 Qiapter 9. Characteristics, Parame­ ters and Motlon'Equalions Chapter 3. Fuel Preparation at Power of Working Fluid . . . Stations ....................... 32 9.1. Principal Hydrodynamic and 3.1. Methods of Solid Fuel Combus­ Heat-transfer Equations for the tion .......................................... 32 Water-steam Path 3.2. Pulverization Systems . . . . 34 9.2. Characteristics of Motion of a 3.3. Characteristics of Cool Dust. Steam-water Mixture . ■ Optimal Degree of Pulveriza­ tion .......................................... 38 9.3. Regimes of Steam-wnter Mixtu­ re Flow 3.4. Pulverization Equipment . . 41 9.4. Hydraulic Resistances . 3.5. The Preparation of Fuel Oil and Natural Gas ........................... 45 9.5. Thermophysical Properties of Working Fluid in the Path of a Monobloc Unit ....................... Chapter 4. Theoretical Principles of Combustion ................... 47 Chapter 10. Temperature [Conditions 4.1. Tho Kinetics of Combustion Re­ on Heating Surfaces . . actions ...................................... 47 10.1. Classification o. Floating and 4.2. The Mechanisms of Fuel Com­ bustion ...................................... Cooling Modes . 50 10.2. Heat-transfer Crisis in Evapo­ 4.3. Kinetic and Diffusion Regions rating Tubes of Combustion ............... 54 ................... 4.4. The Ignition of Fuel-air Mixtu­ 10.3. Temperature Conditions Along the Length of a Channel . re. Combustion Front . . 56 4.5. The Burn-off Intensity of Fuel 59 10.4. Temperature Conditions Aro­ und tho Periphery of a Channel Chapter 5. Combustion Products . . 60 10.5. Heat Exchango in Steam Ge­ nerators of Nuclear Power Sta­ 5.1. Tho Composition of Combustion tions .......................................... 60 P r o d u c t s ....................... 5.2. Determination of Excess Air Ra­ Uydrodynumics of Open tio for an Operating Boiler . . 64 Chapter 11. Hydraulic Systems . . . 5.3. Toxic Substances in Waste Ga­ 11.1. Classification of Open Hydrau­ ses and Measures of Environmen­ 65 tal C o n t r o l ................... lic Systems ...........................

67 67 70 77 77 80 85 86

90 90 92 95 96

98 98 100 103 105 107 110 110 112 114 118 123 124 124

G 11.2. Hydrodynamic Stability of Flow in Horizontal Evaporat­ ing T u b e s ............................... 11.3. Hydrodynamic Stability of Flow iu Vertical Evaporating Tubes ...................................... 11.4. Maldistribution of Heal . . . 11.5. Effect of Headers on the Di­ stribution of Working Fluid Botween T u b e s ....................... 11.6. Flow P u ls a tio n s ................... Chapter 12. Hydrodynamics of Closed Hydraulic Systems 12.1. Laws of Free Circulation . 12.2. Calculation of Circulation Cir­ cuits ........................... 12.3. General Hydraulic Characte­ ristic of Evaporating Tubes and Its Hole in Estimating tlio Re­ liability of Circulation . . 12.4. Hydrodynamics of Descending Tubes and Its Effect on the Reliability of Circulation . Chapter 13. Hydrodynamics of Bubbl­ ing Systems 13.1. Laws of Bubbling 13.2. Dynamic Layer in Steam Wash­ ers .............................. 13.3. Effect of Non-uniform Heat Release and Impurities on tho Dynamic Two-pnaso Layer Chapter 14. Physico-chemical Prin­ ciples of Behaviour of Im­ purities in Working Fluid 14.1. impurities in Food Water and Their Effect on Equipmont 14.2. Solubility of Impurities in an Aqueous Heat-transfer Agent and Formation of Deposits . . 14.3. Passage of Impurities from Water to Saturated Steam . . Chapter 15. Water Conditions 15.1. Removal of Impurities from the C ir c u it....................... 15.2. Water Conditions of Oncethrough Boilers . . . . 15.3. Non-scaling Water Conditions of Drum-type Boilers . 15.4. Methods for Generating Clean Steam . . . . Chapter 16. Processes on the Fireside of Heating Surfaces 16.1. Mechnnlsm of Scaling . . . 16.2. Abrasion Wear of Convective Heating Surfacos . . . . 16.3. Corrosion of Heating Surfacos Chapter 17. Evaporating Heating Sur­ faces . . .

Contenti

126 130

13G l/il 143 145

145 148

151 155 157

157 162 162

1G5 165 166 172 178

178 180 183 184 192 192 196 198 202

17.1. Heal Absorption by Evnporaling Surfaces and Tiicir Layout 17.2. Reliable Dosigns of Water Walls 17.3. Gas-light Water Walls and Me­ thods for Enhancing Their Re­ liability ................................... 17.4. Refractory-faced Water Walls Chapter 18. Steam Superheaters and Superheat Control . . . 18.1. Classification of Superheaters 18.2. Operation and Reliability of S u p c rlio a te rs........................... 18.3. Positioning of Suporhoators . . 18.4. Superheat Tontperaluro Control Chapter 19. Low-tcinpcralurc Heat­ ing S u rfa c e s .... 232 19.1. Arrangement of Low-tempera­ ture Heating Surfaces . . . . 19.2. Economizors ........................... 19.3. Air Healers ........................... 19.4. Corrosion Control of Air Hea­ ters .......................................... Chapter 20. Heat Exchange in Heat­ ing Surfaces of Boilers . . . . 20.1. Thermal Characteristics of Wator Walls ............................... 20.2. Flamo E m is s iv ity ........ 249 20.3. Calculation of Radiant Meat Transfer in a Furnace . . . . 20.4. Radiant Heat Transfer in Boi­ ler Fluo D u c t s ............ 255 20.5. Couvoclivo Iloal Transfer in Boiler Fluo D u c t s ........ 257 20.6. Volociticsof Gases and Working Fluid inConvectivo Heating Sur­ faces .......................................... Chapter 21. Layout and Heat Calcu­ lation or Steam Boiler . . 21.1. Boiler Layout and Structures 21.2. Thormal Diagram of a Boiler 21.3. Heal Calculation of a Boiler Chapter 22. Steam Boilers of nighcapaclty Monobloc [Units 22.1. Soloction of Boiler Desigp Ac­ cording to tho Type, Capacity and Operating Conditions of Power Station ....................... 22.2. Characteristics of Modern Steam B o ilo rs ...................................... Chapter 23. Steam Boiler Operation 23.1. Operating Conditions and Cha­ racteristics ............................... 23.2. Stoady Regimos of Boilar Ope­ ration ...................................... 23.3. Unsteady Rogimos of Oporation Within Allowable Loads . . . 23.4. Starting-up Circuits of Mono bloc U n i t s ...........................

202

203

210

216 217 217 222 223 225

232 234 236 243 246 240

251

200 262 262 270 273 276

276 281 290 290 292 294 298

Contents 23.5. Shut-down and Load-shedding R e g im e s .................................. 23.6. Regimes of Boiler Firing and Unit Starting ....................... Chapter 24. Slcam Generators of Nuc­ lear I’owcr Stations 24.1. Classification and Characteri­ stics of Steam Generators for Nuclear I’owcr Stations . . . 24.2. Steam Generators with Aqueous Coolant .................................. 24.3. Steam Generators with Liquidmetal and Gaseous Coolants .

303 30G 312 312 214 317

24.4 Nuclear Roactor as a Steam Ge­ nerator ...................................... Chapter 25. Metals for Steam Boilers 25.1. Metal Behaviour at High Tem­ peratures .................................. 25.2. Metals for Steam Boilers . . . 25.3. Strength Calculations . . . . 25.4. Metal Control in Operation . . R eferences.......................................... I n d e x ..................................................

7

321 324 324 327 330 333 335 337

PREFACE

This textbook has been written as a highor-cducntion course in steam boil ers for thermal power stations. It presents the theory of the processes which occur in steam boilers, designs of boilers for thermal power stations and steam generators for nuclear power stations, and the operating principles of boilers and steam generators. The material in the book is based on four fundamental principles which are closely interrelated nnd reflect the current stale of progress in science and technology: (1) the physico-chemi­ cal processes in the fuel, gas-air, and water-steam paLhs of modern highcapacity boilers; (2) the correlation between these physico-chemical pro­ cesses and the design, layout and arrangement of steam boilers and their elements; (3) advanced technological processes and their technical and eco­ nomical substantiation; and (4) the correlation between the processes oc­ curring in boilers and the principles of boiler operation. This mothod of analysis encourages the optimal selec­ tion of technological processes, boiler designs, and operating regimes. At the beginning of the course, we explain the rolo and place of the steam boiler in the general scheme of elec­ tric power production at modorn highcapacity steam-turbine power stations, give the classification of steam boliers, describe the functions of the princi­ pal boiler elements and, in introduc­ tory form, the physico-chemical pro­ cesses which occur in the water-steam, fuel and gas-air paths of boilers. Thus, the students are immediately introduced to the range of topics which are later discussed in more detail. A number of chapters are devoted

to power-producing fuels and their characteristics, fuel preparation for combustion, the theoretical princip­ les of combustion, technology of fuel combustion, nnd efficiency with which heat is utilized in steam boilers. Next the book focuses on the prin­ ciples of hydrodynamics and the tem­ perature and water conditions in steam boilers. This constitutes the range of problems related to the processes of steam generation. Having studied the processes of fuel combustion nnd stonm generation, the reader is acquainted with several par­ ticular designs of steam boilers and steam boiler elements. Special empha­ sis is placed on the processes and plants for high and supercritical steam para­ meters, monobloc units, the utiliza­ tion of non-traditional fuels, and methods for increasing the reliability and efficiency of power plant equip­ ment. Furtheron, the book explains the principles, stages and sequence of heat and hydraulic calculations for steam boilers, including data on the appli­ cation of electronic computers and the development of mathematical mo­ dels of steam boilers. The concluding chapters arc of a generalized nature and describe certain particular de­ signs of modern steam boilers, trends in their development, and principles of boiler operation. In view of recent progress and pers­ pectives in nuclear power engineering nnd the construction of liigh-capacity nuclear power stations, of large theo­ retical and practical interest are data on tho steam generators of nuclear po­ wer stations. For the first time in higher-education textbooks, some pro-

Preface

cesses occurring in tho steam boilers of thermal power stations and steam generators of nuclear power stations arc discussed in pRrallol. In addition, a separate chapter is devoted solely to the steam generators of nuclear power stations. The authors have carefully selected the illustrations for the book. For deeper analysis of the problems being studied, different types of boiler cir­ cuits and designs are compared in illu­ strations. In somo illustrations, boi­ lers or their elements are shown in a simplified form to facilitate tho rea­ der’s understanding of how they func­ tion and the processes which lake placo in thorn. The present book is the rcsull of

9

many years of lecturing a course on steam generators of power stations at the Moscow power engineering insti­ tute, which has been initiated by Acade­ mician M. A. Slyrikovich. The authors would like to express special thanks to their colleagues on the faculty of steam generators of power stations at the Moscow power engineering institute [faculty chair Prof. V. S. Prolopopov, Dr. Sc. (Eng.)]f tho reviowors of the book, the faculty of steam generators at the Saratov polylechnical institute [faculty chair Prof. A. V. Zmachinsky, Dr. Sc. (Eng.)] and B. I. Shmukler, Cand. Sc. (Eng.), for their valuable com­ ments on the manuscript.

STEAM GENERATION AT ELECTRIC POWER STATIONS

1.1. The Steam Boiler at a Power Station An electric power station is an in­ dustrial plant for generation of electric energy. In the USSR and industrially developed countries, the major portion of electric energy is produced at fuelfired (thermal) power stations which utilize the chemical energy of com­ bustion of organic fuels. A certain quantity of electricity is also produced at nuclear power stations, a kind of thermal stations which utilize the energy of nuclear fuels, and at hydraulic power stations which utilize the energy of falling water. Irrespective of the type of station, electric energy is, as a rule, produced on a centralized basis, which means that individual power stations supply energy to a common power grid, and therefore, are combined into po­ wer systems which may cover a large territory with a large number of con­ sumers. This principle increases the reliability of power supply to consu­ mers, decreases the required reserve power, reduces the cost of produced energy due to more rational load on the power stations of u system, and allows the use of power plants of higher unit power. At some power stations, the centralized principle is employed for the supply of heat to consumers in the form of hot water and low-pressure steam, ns well for the supply of elec­ tric energy. Uloclric power stations, eloctric and heat power networks and consumers make up what is called a power system. Individual power sy­ stems may bo interconnected by hightension electric power lines into a power grid. Most of the power grids

in the Soviet Union comprise the supergrid, which is the highest form of organization of energy production. Thermal power stations. Steam-tur­ bine power stations are the^main type of power stations operating on orga­ nic fuels. They are subdivided into condensation plants which produce oloctric energy only and heat-and-power plants which can produce both elec­ tric energy and heat. Steam-turbine power plants are ndvanlegeous over other types in that, they permit concentration of an enor­ mous power in a single unit, have a relatively high economic efficiency and require the lowest capital costs nnd short time of their construction. The main thermal units at a steam-turbine power station are a steam boiler and a steam turbine (Fig. 1.1). A steam boiler is a combination of healing surfaces in which steam is generated from con­ tinuously fed water by utilizing the heat liberated on combustion of orga­ nic fuel which is fed into tho boiler furnace togother with tho air required for combustion. The water supplied into a steam boilor is called feed water. Feed water is prehoated to the saturation temperature and vaporized and the saturated steam thus formed is further superheated. As fuel is burned, it forms combu­ stion products which sorve as u heattransfer agent in tho heating surfacos where it gives up its heat to the water and steam which ure called the wor­ king fluid. On passing the heating surfaces, the combustion products are cooled to a relatively low temperature and ejected from the boiler through a stack into the atmosphere. The slacks of high-power stations have u height

1.1. Steam Boiler at Power Station Superheated steam

11

Superheated steam

I (a)

I

Fig. 1.1. Principal thermal diagram ol (a) condensing station and (6) heat and power station I —attsarn boiler; 2—sleain turbine; j —electric generator; 4—condenser; 1—condensate pum p; a —feed pump; 7—low-pressure heater: t —high-pressure hentor; 9—dcaernlor; JO—m ains w ater heater; 11—Indu­ strial steam extraction; t r —w ater-treatm ent plant

of 200-300 m or even more to mini­ mize local concentrations of conta­ minants in the air. Solid fuels loavo ash and slag on combustion, which are disposed of from the boiler plant. The superheated steam produced in a boiler is supplied into a steam tur­ bine where its thermal energy is con­ verted into mechanical work on the turbine shaft. The latter is connected to an electric generator in which the mechanical energy is transformed into electricity. The waste, or dump, steam is fed from the turbine into a con­ denser, an apparatus in which the steam is cooled and condensed by means of cold water supplied from a natural (river, sea, pond) or artifi­ cial (cooling tower) water source. At modern condensation power plants with a unit power of 150 MW or more, reheat superheating is employed, usually by arranging a single-bank reheat superheater (reheater) (Fig. 1.1a). Double-bank reheat superhenters aro employed at power plants of a very high power; in this scheme, steam is returned to the boiler from two inter­ mediate turbine stages. Reheat super­ heating increases the efficiency of a turbine and accordingly decreases the unit steam consumption for power generation; it also diminishes tho moisture content of the stenm in the low-pressure turbine stages and de­ creases erosion wear of turbine blades.

Tho condensate is pumped by a condensate pump through low-pressure water heaters into a deaerator, whore the condensate is made to boil and is freed from oxygen and carbon dio­ xide that might cause corrosion of tho equipment. Water from the deaerator is fed by means of a feed-water pump through a high-pressure water heater and then into the steam boiler. The condensate in low-pressure water hea­ ters and tho feed water in high-pres­ sure water healers are heated by the steam taken off from the turbine; this is called regenerative water hea­ ting. This method increases the effi­ ciency of a steam-turbine plant and decreases the heal loss in the conden­ ser. Thus, the steam boiler of a conden­ sation power plant (Fig. 1.1a) is sup­ plied with the condonsate formed from the steam produced in the unit. Part of this condensate is lost in the systom ns leakage. At heat and power sta­ tions, another portion of tho steam produced is taken off and supplied ns process steam to industrial consu­ mers and for domestic purposes. At condensation plants, the steam leakage constitutes only a small fraction of the total steam consumption, around 0.5-1%, und is compensated for by make-up water prelrealed in a watertreatment plant. At heat and power stations, the quantity of make-up

12

Ch. 1. Steam Generation at Electric Power Stations

Fig. 1.2. Principal thermal diagrams of (a) single-circuit, (6) two-circuit and (e) throe-circuit nuclear power stations i — reacto r;

stcom turbine; 3—electric generator; 4—condenser; 7—Interm ediate h eat exchanger

water added may attain 30-50% or even more. Make-up water and turbine conden­ sate contain certain impurities, main­ ly dissolved salts, metal oxides and gases. These impurities enter the steam boiler together with the feed water. In the course of vaporization, tho concentration of impurities in the water increases and under certain con­ ditions they can form deposits (scale) on the hcaLing surfaces of the boiler, which impair heat transfer. Further, impuriLies of the wratcr can partially pass to steam on vaporization, which should be avoided where possible, since the steam must be perfectly pure so as to prevent deposition of impurities in the turbine. For the two reasons mentioned, high conta­ mination of feed water is inadmissible; the concentration of impuriLies in feed water is regulated by special standards and must be strictly con­ trolled. The operation of a steam boiler de­ pends on a number of auxiliary devices and mechanisms, which include fuelpreparation devices, feed pumps to supply feed water to the boiler, forceddraft fans to supply air for combu­ stion, induced-draft fans to eject com­ bustion products through the stack into llio atmosphere, etc. A steam boiler and tho whole complex of its auxiliary equipment constitute what is called the boiler plant, or boiler installation. A modern boiler plant is a complicated engineering facility for steam generation in wdiich all working

S—pum p; 6—steam

generator;

processes are mechanized and autom a­ tically controlled; an autom atic pro­ tection system prevents failures of a boiler plant and enhances the relia­ b ility of iLs operation.

The principal trends in Ihe deve­ lopment of steam boilers seem to be as follows: increase of the unit power, increase of the initial pressure and temperature of steam, application of intermediate steam superheating, me­ chanized and automated control, manu­ facture and delivery of boiler equipment in large blocks for easier and quicker assembly. Nuclear power stations. A unit in which a controlled chain reaction of nuclear fission of heavy elements can he effected is called a nuclear reactor. Fuels for nuclear reactors include natural isotopes (such as U23S) and synthetic isotopes (U233, Pu230, etc.). The nuclear energy liberated by the chain reaction of nuclear fission is transformed into heat which is remo­ ved from the reactor by a coolant. A nuclear power station may have one, two or three circuiLs. In a single-circuit nuclear power station (Fig. 1.2a), steam is generated directly in the nuclear reactor which therefore doubles as a steam generator. Single-circuit stations arc simpler and less expensive and contain the leasL number of equipment units. On the other hand, the working fluid (water and steam) is irradiated in the reactor and becomes radioactive, which ne­ cessitates biological shielding of the reactor proper and the equipment of

1.1. Steam Boiler at Power Station

the water-steam circuit of the sta­ tion. Contamination of steam may result in the formation of’deposits on the surfaces of the equipment. Since these deposits are radioactive, repairs of the equipment are more difficult to make. In a two-circuit nuclear power station (Fig. 1.2b), the flow of a liquid, gas or molten metal is heated in the reac­ tor and serves as the heat-transfer agenl which then gives up its heat to the working fluid in a steam genera­ tor. Therefore, a Iwo-circuiL station has an additional unit, a steam gene­ rator which naturally increases its cost. The heat-transfer agenL must have a certain temperature gradient over the working fluid for the heat to be transferred from the former to the latter. For that reason, with water used as the heat-transfer agent, the temperature of steam entering the turbine is lower than that in a single­ circuit sLation. The two-circuit scheme requires that the pressure in the reactor be maintained at a higher level than that of the steam delivered into the turbine. On the other hand, twocircuit nuclear power stations have certain advantages over the single­ circuit type in that radioactivity does not extend beyond the first circuit, and therefore, the turbine and other equipment of the second circuit arc safely accessible for repairs and bio­ logical shielding is indispensable only for the first circuit. In a three-circuit nuclear power sta­ tion (Fig. 1.2c), liquid sodium is used as the heat-transfer agent in the first circuit. Being irradiated in the reac­ tor, sodium is liable to activation with the formation of an isotope pos­ sessing a high energy of y-radiation. For that reason the first circuit is iso­ lated from the working circuit by an intermediate second circuit. In the second circuit, either sodium or an Na-K alloy can be used as the heattransfer agenl. Should the shielding of the second circuit become untight, the radioactive sodium of the firsL circuit is prevented from getling into

13

the second by maintaining the pressure in the second circuit at a higher level than in the first. In the third circuit, water is the working fluid. In threecircuit nuclear power stations, bio­ logical shielding is set up around the first and the second circuit. Combined sleam-and-gas power plants and MHD plants. With intro­ duction of supercritical steam parame­ ters (p = 25.5 MPa, trs = 545°C), se­ condary intermediate steam superhea­ ting (t ,ec = 545°C), heat regeneration, and application of boiler-turbine mo­ nobloc units of high power (1 200 MW and more), the thermal efficiency of thermal power stations has approa­ ched closely its thermodynamic limit (slightly more than 40%). Any fur­ ther increase of the initial steam para­ meters increases substantially the cost of boiler-turbine units, since more expensive high-alloyed steels must be employed. Further, it is difficult to ensure the required reliability of such units. In view of this, combined systems have been designed and tested indu­ strially. They include a steam-turbine plant and a high-temperature gasturbine plant. Of practical interest are the combined steam-gas plants in which a gas turbine operates in the liigh-lemperature portion and a steam turbine, in the low-temperature por­ tion. Two main possible schemes of combined gas-steam power plants are shown in Fig. 1.3. In both, tho gas turbine operates on high-temperature heal. In Fig. 1.3a, this heat is libera­ ted in the combustion chamber into which fuel and compressed atmosphe­ ric air are supplied. The gases of com­ bustion are used to perform work in the gas turbine. The exhaust gases and some of fuel arc fed into the furnace of a steam boiler which generates steam Lo drive a steam turbine. The combu­ stion products as fed into the boiler furnace contain around 1G% oxygen, so that special air supply for com­ bustion in the boiler furnace is not needed, which makes it possible to dispense with an air preheater. The

14

Ch. 1. Steam Generation at Electric Pouter Stations

£PFig. 1.4. Combined stcom-gos power plant on nuclear fuel 1—reactor; 2—compressor; j —ga* turbine; 4— electric generator; 5—steam generator; 6—feed pum p; 7—condenser; A- steam lurblne

ters of steam, the unit fuel consump­ tion of a combined steam-gas plant is 4-6% lower than that of n steamturbine plant. Tho capital expendi­ tures are also lower by 8-12%, Combined steam-gas plants with nuc­ lear reactors have also been developed (Fig. 1.4). In this version, (lie com­ bustion chamber is replaced by a po­ Fig. 1.3. Thermal diagram of n slcam-gas wer reactor with a gaseous heul-lnmspower plant fer agent, such as an inert gas, for t — air: r —compressor; a —fuel; 4- com bustion chnminstance, helium, which allows the ber; S—gas turbine; 8—exhaust gns*\s; 7 —el«*ciric generator; J —steam boiler; 0—slcain turbine; temperature at the reactor oxil to he j o —condenser; J J —condensate pump; J2 —low-pres­ raised up to 1 SOO’C or oven moro. sure beater; i j —deaerator; J4—feed pum p; i s _ bigh-preasurc healer; /d —bent rxclionger; j 7 — High-temperature gas-coolod reactors blgb-preasure steam boiler; JA—em ergency waste gas disposal can be employed efficiently at nuclear power stations with steam turbines. unit fuel consumption of steam-gas In steam-gas power plants operating plants is 3-^i% lower than that of a on nuclear fuels, the steam boiler uti­ steam-turbine plant with the same lizes the heal of exhaust gases of gas initial steam parameters. turbines. Another scheme (Fig. 1.36) com­ Another type oT combined systems prises a high-pressure steam boiler wiLh steam cycle is a magnetohydrodyin which fuol combustion and heal namic (MI1D) plant. Its characteristic transfer lake place at a high pressure feature is that heat is converted into (0.6-0.7 MPa). This makes it possible electricity without tho use of ma­ to intensify theso processes and decrease chines (Fig. 1.5). Atmospheric air is the dimensions of the boiler and compressed in a compressor, preheated thus to save metal substantially. As in in the boiler to 1 000-1 200°C and fed the previous scheme, the gas turbine logelher with fuol into tho combu­ operates on the high-tom porntore heal stion chamber whore tho combustion of combustion products, i.o. the furnace products form at a temperature of gases of the high-pressure steam 2 500°C and are ionized. Intensive gas boiler. The steam generated in the ionization is effected by adding com­ high-pressure boiler is fed into a steam pounds of potassium, caesium and turbine. The combustion products from other alkali metals into the combustion the gas turbino are cooled by a part chamber. of the water flow fed for steam gene­ Hot ionized gases (high-lemperaluro ration. With the same initial paramo- plasma), which possess the proper-

1.2. Clatiijleatlon oj Steam Bolleri

15

steam in the required quantity which can ensure the specified power of the turbine and the specified steam parameters. 1.2. Classification of Steam Boilers

Fig. 1.5. Principal thennnl diagram of MUD power plant I —fu rl; 2—ionizing seeds; 3—hot sir; 4—combu­ stion cham ber; i - M H i ) channel; «—electric mapnets; 7—gaa duct; «—air heater; 9— healing sur­ faces of stcum holler; l o —exit of combustion pro­ ducts: 11—steam boiler; 12— pump; 13— condenser; 14—electric generator; IS —steam turbine; 19— compressor; 17— d .c .-n .r. converter; IS—energy lo line; 19—air

tics of an electric conductor, aro fed through a nozzle into a channel and move in iL at a speed of roughly 700 m/s. Powerful permanent mag­ nets create a magnetic field in the channel. As plasma moves ill the power­ ful magnetic field, ionized gas partic­ les induce a direct current in an elec­ tric circuit which is then converted into an alternating current. The gas flow' leaves the channel at a tempe­ rature of 1 500-2 000°C. This hightempcralure heal of the gases is uti­ lized for preheating of the air lo be supplied lo the combustion chamber and for generation of steam which is fed into a steam turbine. Tho effi­ ciency of MHD plants may be ns high as 50-60%. Roughly 70-80% of tho total electric energy aro producod in the MHD channel and the remainder, in the steam power plant. As may be seen from Lho above prin­ cipal schemes of electric energy produc­ tion at power stations, the steam boi­ ler at a thermal power plant and the steam generator at a nuclear power station arc indispensable units and belong lo Lhe basic units of a power plant of practically any power rating. A steam boiler and steam gene­ rator are intended for production of

According lo the laws of phase trans­ formations, the production of super­ heated steam involves the following sequence of processes: preheating of foed water to the saturation tempe­ rature, steam generation, and super­ heating of saturated steam to the spe­ cified temperature. These processes can occur only within strictly defined li­ mits and can be effected in three types of healing surfaces. Water preheating lo the saturation temperature is done iu an economizer, the formation of steam lakes place in evaporating hea­ ting surfaces, and steam superheating is carried out in a superheater. The working fluid in heating sur­ faces (wraler in the economizer, steamwater mixture in evaporating lubes, and superheated steam in the super­ heater) must move continuously in order lo ensure continuous heat re­ moval and maintain the appropriate temperature conditions for Lhe metal of the healing surfaces. In this process, water in the economizer and steam in the superheater come only once in contact with the heating surface (Fig. 1.6). The economizer offers hyd­ raulic resistance to the motion of wrater, which must be overcome by pro­ vision of a sufficiently high head in the foed pump. The pressure developed by the feed pump must exceed the pres­ sure at the entry to the zone of steam generation by the magnitude of tho hydraulic resistance of the economizer. Similarly, the motion of steam in lho superhoater is due to a pressure gra­ dient between the zone of steam gene­ ration and the steam turbine. The combined motion of water and steam in evaporating lubes, w'hich has to overcome Lhe hydraulic resi­ stance of these Lubes, can he effected in various ways. Accordingly, a di­ stinction is made between natural-

Ch. 1. Steam Generation at Eleetrle Power Station >

16

4 \

S.

-V (a)

B 1 -V

a ^ ? x>

f

T

a (6)

Fig. 1 .6 . Principal schemes of steam generation in boilers 1 , a small con­ centration of fine fractions (below 25 pm) and a small concentration of coarse fractions. Moisture content of pulverized fuel. Moisture content W, %, is an impor­ tant characteristic of pulverized fuel. An incrcaso in moisture content above the recommended level may result in lower boiler productivity and involve difficulties in dust transport: the dust loses fluidity and slumps in bunkers, clogs feeders, chutes, etc. On the other hand, overdried brown coal and coal dust is liable to sclf-ignito in places wdiere it is stored or accumu­ lates and may be explosive when mi­ xed with air. The moisture content of pulverized fuel is usually determined in terms of the hygroscopic moisture content IF'1 (see Sec. 2.3).

Dust explosiveness. When coal dust suspended in air is confined in a closed volume, it will explode more inten­ sively if its unit, surface area is larger (i.c. if it contains more fine fractions) and if it has a higher yield of volatiles. The temperature of Ihe mixture is al­ so of crucial importance. The most dangerous concentrations of coal dust lie within 0.3-0 .6 kg/m3 air. The ignition of nu air-dust mixture in a closed volume results in a sharp rise in temperature nnd pressure. The pressure may rise well above the li­ mits safe for pulverizing equipment. Damage to the boiler plant by explo­ sions is prevented by installing safe­ ty (relief) valves which discharge part of the mixture from the system should the pressure rise excessively. The con­ centration limit of 0 5 in the drying agenL, i.e. the concentration below which fuel dust cannot explode, is equal to 16% for pulverized peat and oil shales, 18% for brown coals, and 19% for coals. The concentration of 0 2 can be decreased by drying pulve­ rized fuel with a mixture of hot air and combustion products. The proba­ bility of explosion is lower in fuels with a lower yield of volatiles. With the yield of volatiles less than 8 %, fuel is explosion-safe. A high tempe­ rature of the dust-air flow promotes the formation of explosive mixtures, and therefore the temperature of the air-dust mixture downstream of the mill must be strictly controlled. For most pulverized fuels, it should not exceed 70-80°C (fuels with a high yield of volatiles) and 130°C in other cases. The optimal degree of grinding. The grinding properties of various fuels are compared in terms of the laborato­ ry coefficient of grindability, lct, which is understood to be the ratio of the unit consumptions of electric energy for grinding, in a standard la­ boratory mill, a reference solid fuel and the fuel being tested, provided that both have the same initial parti­ cle size and the same ground dust cha­ racteristics. Thus k t = ErIE, (3.7)

3.4.

41

Pulverization Equipment

of the boiler plant and the pul­ verization system arc tested at dif­ ferent degrees of fuel grinding. The optimal value of /?9o depends even morn on the yield of volatiles V*, type of mill, and type of dust se­ parator. The effect oT these factors is expressed by the polydispcrsily coef­ ficient n. Tho optimal fineness of fuel grinding can be found by the formula: R%p - 4 + 0.8nVc (3.8) Fig. 3.7. Determination of the optimal grinding degree of fuel 1—fuel with low V* and low fcp 2—fuel with higher values of Vc and Aj

The condilions in intluslrial grin­ ding mills may differ from the stan­ dard laboratory condilions in the ini­ tial moisture content of fuel and its pnrticlesizc, therefore, the grindnbility k of working fuel may differ from the laboratory value /cj. As follows from formula (3.4), grin­ ding of fuel to a coarser size results in energy savings, AEpul, but inevitably increases the time of complete combu­ stion in the furnace and the heat loss with unburned carbon ABf. Therefore, each kind of fuel has a particular fi­ neness range within which the total expenditures on mill grinding Em and the cost of heat loss with unburned carbon in the furnace, 1.5) and very moist brown coals are ground in pulverizing fans. A ball-tube mill (or simply ball mill) has a drum 2-4 m in diameter and 3-10 m long which is partially filled with steel halls 30-60 mm in dia­ meter (Fig. 3.8). The drum is clad with armour plates on the inside and is heat- and sound-insulated on the out­ side. Raw fuel and hot air arc sup­ plied to the drum through the inlet.

Tabic 3.1. Grinding Mills Characteristics Mill type Ball-tube mill Boiler mill Hammer mill 1’addle-type mill Pulverizing fan

UrlndlnK principle

Polallonnl Hpcod, s"i (rpm)

Impact, abrasion Crushing Impact Impact Impact

0-25-0.42 (15 25) 0.85-1.3 (50-80) 12.5-10.3(750-980) 25(1 500) 12-24.5 (735-1470)

Speed characteris­ tic Low-speed Medium-speed High-speed High-speed High-speed

42

Ch. 3. Fuel Preparation at Power Stations

Fuel supply

/////////

7"7 P H I [7777

Pig. 3.8. Ball mill (general view and cross section) 1 —inlul

pipe; 2—supporting bearing; j —hent- and sound-insulated m ill drum ; 4 —outlet pipe, j- -la r g e r gear whi*i;l; 6—reducer gear; 7—electric m otor

pipo. The drum is rolatod by an ele­ ctric motor via a reducor gear and a driven goar wheel attached to the drum. Thu optimal capacity of a mill is obtained at a rotational speed nd = 0.76ncr (3.9) where ncr is the critical rotational speed of the drum, s ' 1, at which balls can ‘slick’ to drum walls due to cen­ trifugal forces:

whore Dd is the drum diameter, m. With the optimal rotational speed of Lho drum, the halls first rise over the drum wall and then detach from it and fall. Fuel is ground by the im­ pact of the falling balls and by the ab­ rasion between them. Final dust is continuously removed from the mill by the ventilating ngcnl—air. The grinding capacity 0 } a mill, B m, depends substantially on the drum length and diuinoler. Yet a larger diam eterap p reciab ly increas­ es the required power of mill motor N m which is roughly proportional to the third powor of D d. Grinding mills are also characteri­ zed by the drying capacity, i.e. the quantity of fuel that can be dried in

the mill from the original moisture content Ww to Lhc desirod value Wd. The two characteristics of a mill should be matched properly, which is done by controlling the flow rnto and temperature of the drying agent at the mill inlet. The power for mill rotation A'm is virtually independent of the mass of charged fuel in view of the large mass of balls and drum. For this rea­ son, as the quantity of charged fuel decreases, the unit energy consumption for grinding, Eg, kW h/kg. increases, since Ee = N J B m (3.11) Thus, it is advisable to run ball mills at full load. The armour plates and halls of a mill are inevitably subject to wenr during mill operation. The extent of wear depends on the abrasive proper­ ties of fuel which can be chnrnctori/.ed by the relative abrasivil y k„br, which is the ratio of the actual wear to that taken as the reforcnco value |0.3 g/(kW h)l. As has been found by experiments, the two relative characteristics, k, and k„br, are closely interrelated (Fig. 3.9). Harder fuols cause greater abrasion wear of the mill elements [291.

3.4. P u l v e r i z a t i o n E q u i p m e n t

l’ig. 3.9. Dependence of fr„/jr on kt real; 3 — Donetsk k"s coal; :i— Chelyabinsk coal pacli' U; 4 — Vorkuta null tr a d e OZli; j Karaganda ri.nl i>rnd