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M. 11. Peammoe, 10. M. Jiunon IIAPOBhiE KOTJlhl TEIIJIOBhiX 8JIEKTPOCTAI-IIJ;MV:I Mo ci\Ba

t}

I

OF THERMAL PO\iVER STATIONS

M. I. Reznikov, Yu. M. Lipov

Trans la ted from the Russinn by

Vadim Afanosycv

Mir Pu lJli shers Mosco'" .

First published 1985 R evised from the 1981 R ussian edit ion

@ c3ueprOII'3AaTt, 1981 @ English translation, Mir Publishe rs , 1985

CONTEl\lTS

Preface Chapter

. . . . . . . . . . . . .

I . Steam Generation at Elec-tric l'owcr Stations

1.1. The Steam Boiler at a Power S tation . . . . . . . . . .

1.2. Classification o f St.oam Bo ile rs 1.3. Flow Diagram of Steam Prod uc-

ll.on 0 0 1.4. Principal. Characteri stics of

S team Boilers . . . . . . .

Chapter 2. Power-producing Fuels and Their Characteristics

2.1. Kinds and Compositions of Fuels 2.2. The H eating Value and Resolv-

ed Charac te ri stics o f Fuels . . 2.3. T echnical Characteristics of So-

lid Fuels . . . . . . . . 2.1o . Technical Characteri s tics of Fue l

Oil and Natural Gnses . . . . 2.5. !\lain Deposits of Foss il Fuels

Chapter 3. Fuel Preparation at Power Stations . . . . . . .

3.1. Methods of Solid Fuel Combus-tion . . . . . . . . . . . .

3.2 . Pulverization Sys tems . . . . 3.3. Charact.oristics of Coal Dus t.

Optimal Degree of Pulveriza-on " 0 0 0 0

3.4. Pulverization Equipment .. 3.5. The Preparation of Fuel Oil and

Natural Gas . . . . . . . .

Chapter 4. Theore tical Principles of Combustion . . .

4.1. The K inotics of Combustion R e-actions . . . . . . . . . . .

4.2. The Mechanisms of Fuel Com-bustion . . . . . . . . . . .

4.3. Kinetic and Diffusion Regions of Combus tion . . . . .

4.4. The Ignition of Fuel-air Mixtu-re. Combustion F ront . .

4.5. The Burn-off Intensity of Fuel

Chopter 5. Com bust ion Producls . . 5.1. Tho Compos ition o f Combustio n

Products . . . . . . . . . . 5.2. Dctcrrninntion of Excess Air Ro-

tio for an Operating Boiler . . 5.3. Toxic Subs tances in Waste Ga-

ses and Measures o f Environm en-t al Co ntrol . . . . . . . . .

8

10

10 15 17

21

22 22

25 26

29 31

Chapter 6. Effic iency of Fuel Heat Utilization . . . . ..

6.1. The IleaL Balance ond Efficiency of t ho S team Boiler . . . .

0.2. Analysis of Heat Losses . . .

Chapter 7. J>ul vcrized Coal-filed Fur-naces . . . . . . . . .

7 .1. P rincipal Characteris tics of Chamber Furnaces . . . . .

7 .2. Burners nnd Their Arrangement 7 .3. Dry-bottom Furnaces . . . . 7 .4. Slagging-bo tt.om Furnaces . .

Chapter 8. Gas ond Fuel Oil-fired Fur-naces . . . . .

8.1. Furnace Design . . . . . 8.2. Fuel Oil Burners . . . . . 8.3. Combus tion of Natural Gas . . 8.4. Combined Gas-fuolt:Oil Burners

Chapter 9. Characteristics, Parame-ters and Molion"'Equations

67

67 70

77

77 80 85 86

90 90 92 95 96

32 o[ Working Fluid . 98 9.1. Principal Hydrodynamic and

32 Heat-transfer Equations for tho 34 Water-steam Path . . . 98

9.2. Characteristics o f Motion of a Steam-water Mixture . . . 100

38 9.3. Regimes o! Steam-water Mixtu-41 re Flow . . . . . . . 103

9.4. Hydraulic Resistances . . 1.05 45 9.5. Thermopbysical Properties of

Working Fluid in the Path of a Monobloc Unit . . . . . . 107

47

47

50

54

56 59 60

60 64

65

Chapter tO. Temperature (Conditions on Be a ling Surfaces .

1 0.1. Classification o~ Heating and Cooling Modes - . . . .

1 0.2. Heat-transfer Cris is in Evapo-rating Tubes . . . . . .

10.3. Temperature Cond itions Along the Length of a Channel .

10.4. Temperature Conditions Aro-und tho Periphery of a Channel

10.5. Heat Exchange in Steam Ge-nerators of Nuclear Po wer S ta-lions . . . . . . .. . .

Chapter I I. Hydrodynamics of Open Hydraulic Systems . . .

11.1. Classification of Open Hydrau-lic S ys tems . . . . . . . .

iiO

110

112

118

1.23

124

124

I

6

11.2. Hydrodynamic Stability of Flow in Horizontal Evaporat-1-ng 'l'ub . cs - . . . . . . . .

11 .3. Hydrodynamic S tabil ity o( Flow in Vertical Evaporating Tubes . . . . . . . . . . .

11 .4. Maldistribution of HeaL . . . 11.5. Effect of Headers on the Di-

stribution of Working fluid Between Tubes .

H .6. Flow Pulsations . . . . . .

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 Tts Hole in Estimating the Ro-liabili ly of Circulation . . .

12.4. Hydrodynamics of Descending Tubes and Its Effect on t he Reliability of Circulation . .

Chapter 13. llydrodynamics of Bubbl-ing Systems

13.1. Laws of Bubbling . . 13.2. Dynamic Layer in S team Wash-

ers . . . .. ... . 13.3. Effect of Non-uniform Heat

Helease and I mpuri ties on the Dynamic Two-phase Layer .

Chapter 14. Physico-chemical Prin-ciples of Behaviour of Im-purities in Working Fluid

14.1. Impurities in Feed Water and Their Effect on Equipment

11o.2. Solubility of Impurities in on Aqueous Heat-transfer Agent and Formation of Deposits . .

14.3. Passage of Impurities from Water to Saturated Steam

Chapter 15. Waler Condi tions 15.1. Removal of I mpurities from

tbe Circuit . . . . . . 15.2. Water Conditions of Once-

through Boilers . . . . 15.3. Non-scaling Water Conditions

. of Drum-type Boilers . 15.4. Methods for Generating Clean

St~am . . .

Cbapter 16. Processes on lbe F ireside of Heating Sudaces

16.1. Mechanism of Scaling . . . 16.2. Abrasion Wear of Convective

Heating Surfaces . . . . . 16.3. Corrosion of Healing Surfaces

Chapter 17. Evaporating Heating S ur-faces . . . . . .

Contents

126

130 136

1ld 143

145 145

148

151

155

157 157

162

162

1.65

165

166

172

178

178

180

1.83

192 192

196 198

202

17 .1.

1. 7. 2. 17 .3.

17 .4.

Heat Absorption by Evaporat-ing Surfaces and Their Layout Reliable Designs of Water Walls Gas-light Water Walls and Me-thods for Enhancing T hei r He-liability .. ....... . Refractory-faced Water Wails

Chapter 18. Steam Supcrhcat.crs and Superheat Control . . .

18.1. Classification of Superheaters 18.2. Operation and Reliability of

Superheaters . . . . . . . . 18.3. Positioning of S uperheaters . . 18.4. Superheat Tom perature Control

Chapter 19. Low-lcmt>erature ing Surfaces

Heat-

19.1. Arrangement of Low-Lempera-luro Heating Surfaces . . . .

19.2. Economizers . . . . . . 19.3. Air Heaters . . . . . . . . 19.4 . Corrosion Control of Air Hea-

to.rs . . . . . . . . . . . .

Chapter 20. Heat Exchange in Heat-ing Surfaces of Boilers . . . .

20.1. Thermal Characteristics or Wa-ter Walls . . . . . . . . .

20.2. Flame Emissivity ..... . 20.3. Calculation of Radiant Heat

Transfer in a Furnace . . . . 20.4. Radiant Hed Trmsfer in Boi-

ler Flue Ducts . . . . . . . 20.5. Couvcctive H eal Transfer in

Boiler Flue Duels . . . . . . 20.6. Velocities of Gases and Working

Fluid in Convective Healing Sur-f aces . . . . . . . . . . . .

Chapter 21. Layout and Heat Calcu-lation o Steam Boiler . .

21.1. Boiler Layout and Structures 21.2. Thermal Diagram of a Boiler 21.3. Heat Calculation of a Boiler

202 203

210 216

217 217

222 223 225

232

232 234 236

2U

246

2lt6 2t,!) 251

255

257

260

262 262 270 273

Chapter 22. Steam Boilers of H igh-capacity Monobloe IUnlts 276

22.1 . Selection of Boiler Desigp. Ac-cording to the Type, Capacity md Operating Conditions of Power Station . . . . . . . 276

22.2. Characteristics of Modern Steam Boilers . . . . . . . . . 281

Chapter 23. Stea m Boiler Operation 2QO 23.1. Operating Conditions and Cha-

racteristics . . . . . . . . 290 23.2. Steady Regimes of Boiler Ope-

ration . . . . . . . . . . . 202 23.3. Unsteady Rogimos of Operation

Witltin Allowable Loads . . . 294 :!3.4. Starling-up Circuits o[ Mono-

bloc Units . . . . . . . . . 298

I

23.5. Shut-down and Load-shedding Regimes . . . . . . : : . . .

23.6. Regimes or Bo1ler fmng and Unit StarLing ...... .

Chapter 24. Steam Generators of Nuc-lear Power Stations

24.1.

24.2.

24.3.

Classification and Characteri-stics of S team Generators fot Nuclear Power Stations . . . Steam Generators \\'i Llt Aqueous Coolant . . . . . . . . . . Steam Generators with Liquid-metal and Gaseous Coola~Ls

I Contents

303

306

312

312

314

317

24 .4 Nuclear Reactor as a Steam Ge-nerator . . . . . . . . . .

Chapter 25. l\lelals or Steam Boilers 25.1. Metal Behaviour at High Tem-

peratures . . . . . . . . 25.2. Metals for Steam Boilers . . 25.3. Stren..:th Calculations . . 25/t. Metal Contro l in Operation llcferences . .. Index . .

7

321

324

324 327 330 333 335 337

PREFACE

This textbook has been written as a higher-education course in steam boilers 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 cl osely interrel ated and reflect the current stale of progress in science and technology: (1) the phys ico-chemi-cal processes in the fuel, gas-air , and water-steam paths of modern high-capacity 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 carrel ation between the processes oc-curring in boilers and the principles of boiler operation. This method of analysis encourages the optimal selec-tion of technological processes, boiler designs, and operating regimes.

At the beginning of the course, we explain the role and place of the steam boiler in the general scheme of elec-tric power production at modern high-capacity steam-turbine power stations, give the classification of steam boliers, describe the functions of the princi-pal boiler elem ents 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 prepnration for combustion , the theoretical princi p-Ies of combustion, techn ology of fuel combustion , and effi ciency with which heat is u tilized in steam boilers. Next the hook focuses on the prin-ciples of hydrodynamics and the tem-perature and water conditions in steam boilers. This constitutes the range of probl ems related to the procc!:ses of steam generation.

Having studied the processes of fu el combustion and steam generation, the reader is acquainted with several par-ticular designs of s team 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 uLiliza-tion of non-traditional fuel s, and methods for increasing the reliability and efficiency of power plant equip-ment.

Furtheron, the book explains tlw principles, stages and sequence of heat and hydraulic calculations for steam boilers, including data on the appli-cation of el ectronic computers and the development of mathematical mo-dels of steam boilers. Tho concluding chapters are 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 and the construction of high-capacity nuclear power stations, of l arge 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 the steam boilers of thermal power stali!)ns and steam generators of nuclear power st~t~ons are discussed in parallel. In addttiOn, a separate chapter is devoted solely to the steam generators of nuclear power stations.

Tho authors have carefully selected the illustrations for the book. For deeper analysis of the proble~s hei~g studied different types of b01ler cu-cuits a~d designs are compared in illu-strations. In some illustrations, boi-lers or their elements are shown in a simplified form to facilitate the rea-der's understanding of how they func-tion and the processes which take place in thoro.

Tho present book is the result of

9

many years of lectwing a course on steam generators of power stations at the l\foscow power engineering insti-tute, which has been initiated by Acade-mician M. A. Styrikovich.

The authors would like to express special tl18nks to their colleagues on tho faculty of steam generators of power stations at tho 1\'loscow pow~r engineering inslitule [faculty char Prof. V. S. Protopopov, Dr. Sc. (Eng.)], the reviewers or the book, the faculty of steam generators at the Saratov polytechnical 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.

I

I 1

I

STEAM GENERATION AT ELECTRIC PO\VER STATIONS

1.1. The Steam Boiler at a Power Station

An electr-ic power station is an in-dustrial plan L for generation of electric energy. In the USSfi and industrially developed countries, the major portion of electric energy is produced at fuel-fired (thermal) power .~lations which utilize tho chemical energy of com-bustion of organic fuels. A certain quantity of electricity is a lso produced at nuclear power stations, a kind of thermal stalions which utilize the energy of nucl ear ru els , and at hydraulic power stations which uti I ize tho energy of fa lling water.

Irrespective of tho typo of station , electric energy is, as a rul e, produced on a centralized basis, which means that individual power stations supply e nergy 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 r eserve power, reduces the cost of produced energy due to more rational load on the power stat ions of a system, and allows tho use of power plants of higher unit powet. At so me power stations , the centralized principle is empl oyed for the supply of heat to consumers in the fo1m of hot water and low-pressure steam, as well fot tho supply of elec-tric energy. Electric power stations, electric and heat power networks and consumers m.ake up wha t is called a power system. Individual power sy-stems may be interconnected by high-tension electric power Jines 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 p1oduction.

Thermal power stations. S team -tur-bine power stations are the:main type of power stations operating on orga-nic fuels . They are subdivided into condensation plants which produce elec-tric energy on ly and heat-and-power p lants which can prod uce both elec-tr ic energy and heaL.

Steam-turbine power p lants nrc ad-vantageous over othe1 types in tha t they permit concentration of an enor-mous power in a single unit, have a relatively high economic effi ciency a nd require the lowest capital costs and 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 stea m boiler is a combination of heating surfaces in which steam is generated from con-tinuously fed water by u t ilizing the heat liberated on combustion of orga-nic fuel which is fed into the boiler furnace together with the air required for combustion. The water supplied into a steam boiler is called feed water. Feed water is preheat ed to the saturation temperature and vaporized and the saturated steam thus formed is further superheated.

As fuel is burned, it forms com bu-stion prod ucts which serve as a heat-transfer agent in the heating surracos where it gives up its heat to tho water and steam which are called th e wor-king fluid. On passing the beating surfaces, the combustion products are cooled to a relatively low tem petature and ejected from the boil er Llnough a stack into the atmosphere. The stacks of high-power stations have a height

1.1. Steam Boller at Power Statton 11

Fuel Air

r --1 t

I

Superheated steam Waste gases

Steam to relleater

Steam from relleater Ash,slo

12 !1

8 G

I (a) f

2_--, J fuel Air f

Superheated steam

Waste gases

t I 10 I I

Ash.stag W 12

9

6

(6) I t

(( ,-----

Fig. 1.1. Principal thermal diagram of (a) condensing station and (b) heat and powe~ station _ team boiler 2-steam turbine; 3-clcc lrlc genera tor; 4- condcnser; S-;cond.ensate P~P G-tee~ ~u,;;p. 7-low-pressure hea ter; 8- hlgh-prcs.ure hetllcr; 9- dcaernlor; 10-mnms \\aler heater, JJ-Indu

s trial stcnm ex lrnctlou; r2- wntcr-treat.rnent plant

of 200-300 m or even more to mi ni-m ize local concentrations of conta-minants i n the air . Solid fuels leave ash and slag on combustion, which are disposed of from the hoi lor pl ant. The superheated steam prod ucod 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, s team 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 ar tifi-cial (cooling tower) water source.

At modern condensation power plantswithaunitpowero150 MW or more, reheat superheating is employed, usually by arranging a single-banlc r eheat superheater (reheater) (Fig. 1.1a). D ouble- banlc reheat superheaters are employed at power plants of a very high power; in this scheme, sleam is returned to the boiler from two inter-m ediate turbine stages. Reheat s uper-heating increases the efficiency of a turbine and accordingly decreases tho unit steam consumption for power generation; it also diminishes t he moisture content of tho steam in tho low-pressure turbine stages and do-creases erosion wear of turbine bl ades.

The condensate is pumped by a condensate pump through low-pressure water heaters into a deaerator, where tho condensate is m ade to boil and is freed from oxygen and carbon dio-x id e that might cause corrosion of the 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 the feed water in high-pres-sure water heaters are heated by the steam taken off from the turbine; this is called regenerative water hea-t ing. Th is method increases the effi-ciency of a steam-turbine plant and decreases the heat loss in the conden-ser.

Thus, the steam boiler of a conden-sation power plant (Fig. 1.1a) is sup-plied with the condensate formed hom the steam produced in the unit. Par t of this condensate is lost in the system as leaka.ge. At heat and power sta-tions, another portion of the steam produced is taken off and supplied as process steam to industrial consu-mers and for domestic purposes. At cond ensation plants, the steam leakage c.onslitutes only a small fraction of l.he total steam consumption, around 0.5-1 %, and is compensated for by mako-u p water pretreated in a water-treatment plant. At heat and power stations , the quantity of make-up

14 Ch. 1. Steam Generation at Electric Power Stations

f8

J 9 3

f

18 7

3 ,..,. t 7

t 14 t (b)

Fig. 1.3. Thermal diagram of a steam-gas power plant

1-alr: t-comprcssor; J - fuel: 4- combuslion chom-ber: S-gas turbine; 0- cxlluust gases: 7-electric generator; a-steam boiler; 9-steam turbine; 10-condcnscr: 11-condcnsntc pump; 12- low-prcs-surc heater; IJ-deacrator; u - rccd pump; JS-blgh-pressure heater; 10- hc.1t exchanger; 17-hlgll-prcssure steam boiler; 18-cmcrgcncy waste

gns dlsposn I

unit fuel consu mption of steam-gas pl ants is 3-4.% l ower than that of a steam-turbine plant with the same ini tial steam parameters .

Another scheme (Fig. 1.36) com-prises a high-pressure steam boiler in which fuel combustion and heal transfer take place at a high pressure (0.6-0. 7 M Pa). This makes it possible to intensify these processes and decrease the dimensions of the boiler and t bus lo save metal substantially. As in the previous scheme, the gas turbin e operates on the high-temperature heat of combustion products, i.e. the fumace gases of the high-pressure steam boiler. The steam generated in the high-pressure boiler is fed into a s team turbine. The combustion products from the gas turbine are cooled by a part of the water flow fed for steam gene-ration . With t he same initial parame-

f 2 J

8

5

Fig. 1.4. Combined steam-gas power plant on nuclear fuel

J - rcactor: 2-comprcssor: J - gns turbine 4-electric generator; 5-stenrn jlenerolor; e.:....rcccl

pump; 7- condenscr: 8- s teom turbine

ters of steam, the unit fuel consuwp-tion of a combined s team-gns plant is 4-6% lower than that o( a stea m-t urbine plant. Tho capital expendi-tures are also lower by 8-12%.

Combined steam-gas plants wHh nuc-lear reactors have a lso boo11 d eve loped (Fig. 1.11). In this version, the co m-bustion chamber is replaced b y n po-wer reactor with n gaseous lleai.-LI' IHIS-fer agent, such as an inert gas, for instance, helium, which allows the temperature at the reactor exit to be raised up Lo 1 500C or even more. High-temperature gas-cooled reactors can be employed efficien tly at nuclear power stations with steam turbines. In steam-gas power plan ts operating on nuclear fuels, the steam boiler uti-lizes t he beat of exhaust gases of gas turbines .

Another t.ype or combi ned systems wilh steam cycle is a rnagnelohydrody-namic (MilD) plant. Its characteristic feature is that heat is converted into electricity without the use of ma-chines (Fig. 1.5). Atmospheric a i1 is compressed in a compressot, preheated in the boiler to 1 000-1 200C and fed together with fuel in to the combu-stion chamber where th e comb ustion products form at a tom perature of 2 500C and are ionized. I ntensive gas ionization is effected by addi ng com-pounds of potassium, caesium and other alkali metals into the combustion chamber.

Hot ionized gases (high-temperature plasma). which possess the proper-

1.2. Clau!flcatton of Steam Bo!lerr 15

2 3 5 5 f

-----

+ 4 ..::18~::J==-_j 7

17 15

19.

8 9

If 0

15 14

13 0 12

Fig. 1.5. Principal thermal diagram o[ M HD power plant

J-fuP.I; 2- ionizing seeds J-hot nlr; 4- combu-stion chamber; o-1\IH]) cionnct; 6-electrlc mns -nets; 7-gas duct; 8-air hea ter; 9- hcu Ung sur-races or steam boiler; JO-cxlt or combustion pro-ducts; 11-steom boiler; JZ-purflll ; H-condcnsc r; 14-electric generator; JS-slco m turbi ne; JG -comprcssor; 17- d.c.-n.c. converter; 18- cncrg}'

lo 1 inc; 79- alr

ties of an electtic conductor, arc fed througl1 a nozzle inLo a channel and move in it at a speed of roughly 700 m/s. Powerful permanent mug-nets create a magnetic field in the channel. .A.s plasma moves in the power-ful magnetic field, ionized gas partic-les induce a direct current in an elec-tric circuit which is then converted into an alter nating current. The gas flow l eaves the channel at a tempe-rature of 1 500-2 000C. This high-temperature heat of the gases is uti-lized for preheating of the air to be supplied to the combustion chamber and for generation of steam which is fed into a steam turbine. The effi -ciency of MHD plants may be as high as 50-60%. Roughly 70-80% of the total electric energy are produced in the MHD ch annel and the remaind er, in the steam power plant.

As may be seen from the above pl'in-ci pal schemes of electric energy prod uc-t ion at power stations, the s team boi-ler at a thermal power plant and the steam generator at a nuclear power station are ind is pensabl e units and belong to the basic units of a power plant of practically any power ra-ting. A steam boiler and steam gene-rator are int.P.nded for productiou of

s team in the required quantity which can ensure the specified power of the turbine and the specified steam parameters.

1.2. Classification of Ste am Boilers According to the l aws of phase trans-

formations, the production of super-heated steam involves the following sequence of processes: preheating of feed water to the sat uration tempe-rature, steam generation, and super-heating of saturated steam to the spe-cified temperature. These processes can occur only within striclly defined li-mits and can be effected in three t ypes of heating surfaces. Water preheating to the saturation temperature is dono in an economizer, the formation of s t eam takes place in evaporating Ilea-ling surfaces , and steam superheating is cat'l'ied out in a superheater.

The working flu id in heating sur-faces (water in the economizer, steam-water mixture i n evaporating tubes, and superheated steam in the super-heater) must move continuously in order to enswe continuous heat re-moval and maintain the appropriate temperature conditions for the metal of the heating surfaces. ln this process, water in the economizer and steam in the su perheater come only once in contact wilh the heating surface (Fig. 1.6). The economizer offers hyd-raulic resistance to the motion of wa-ter, which must be overcome by pro-vision of a sufficiently high head in the feed pump. The presswe developed by tho feed pump must exceed the pres-sure at the entry to the zone of steam generation by the magnitude of the hydraulic resis tance of the economizer. S imilarly, the motion of steam in the superheater is due to a pressure g ra-dien t between the zone of steam gene-r ation and the steam turbine.

The combined motion of water and s team in evaporating tubes, which has to overcome the hydraulic resi-stance of these tubes, can be effected in various ways. Accordingly, a di-stinction is made between n atural-

16 Ch. 1. Steam Generation at Electric Power Statlon1

7 7 7 J 7

9 fO J ' ~

.

4 2 2

4 6 6 6 6 I f 1 8

p ' s,

~ .. "' s.

.

-5 5 5 5 (q) (6) (C) (d) '

Fig. 1.6. Principal schemes of steam generation in boilers (a) nnturul clrculallon; (b) mulllplc forced circulation; (c) once-through scheme; (d) combined circulation 1- tccd pump; .11- cconomlzer; J-drum; 4-downtake tubes; 5-hcndc.r 6- evoporatlng tubes 7-super~

heater; 8- pump lor multiple forced circulation; 8- mlxcr:' 10- bacl

1.8 Cit. 1. Steanl Generation at Electric Power Stations _..;....;.;;_ _____ _

Fig. 1.7. Flow diagram of steam generation 1- coo l pile; 2- bclt conveyer; 3 and 4- bunkcrs; 5-car dumper with a car; G-crushing plant; 7- crusher hunker; 8- coal;:rlndlng rnHI; 9- prhnary air; JO- IucHlir mixture 11-bumcrs; a - boiler rronl; 13 -Aicnm holler; 14- lurnnce space; J5- secondary nlr: JG - Iower rntllntlon section; J 7- mlddle radlat.lon sec-tion: 18- upper rndlnllon section: 19- superhcn lcd steam; 20- convcctivc suprrhc:Aler: 21- alr Intake rrom holler room: 22- air intake rrom the outsido; 2J- cold air duct; 24-rcbcat superheater; 25- horhon-tal gns duct; 2G-eonvective sh!!lt (vertical gas duct); 27- economher; 28-lccd water : 29- al r hea ter;

JO- Iorced-drn rt t on: JJ- ash coll ector; 32-lnduced-drnll ran; .>J-stack; .74-sl:~g-ash channel

the ra.diant hea.ting surfaces. In modern plants, water walls in the furnace arc often made of finned tubes which are welded together to form a continuous gas-tight (gas-impermeable) shell, which is covered on Lhe outside by a shell of a heat-insulating material to mini-mize beat losses to the surroundings; this ensures proper sanitary conditions in the boiler room, and prevents burns o[ the personnel.

The second vertical shaft and the horizontal dueL that connects i t with the furn~ce serve for accommodating beating surfaces which receive heaL by convection (convective sm-faces) and are called respectively the convec-tive shaft and the convective duct.

H aviog given up their heat t.o tho water walls, com bustion products leave the furnace at a tcmpetature of 900-1 200oc (depending on the type of fuel) and enter the horizontal duct.

As water moves through the boiler tubes, it gradually transforms into steam. The heating surfaces in which st.eam is formed are called evaporating, or steam-generating. In once-through boilers, the evaporating beating sur-face is arranged in the lower portion of the furnace and is called the lower radiation section. Vli Lh su percritical steam parameters, it also includ es a radiant economizer. W a tor suppli ed to a boiler is call ed feed water.

Feed water contains certa in impu- ri Ues. During the process of steam gene-ration, the content of steam in the steam-water mixture increases, water evaporates, and the concentration of impuriti es increases. At a certain con-centration at the end of the steam-generating zone, impurities may be de-posited on the internal surfaces of tubes as scale. The cond uctivity of deposits is only a small fraction of that of the tube metal. This impairs heat transfer to the working fluid and , with intensive heating in the boiler furnace, can cause overheating of the metal which then loses strength and can fracture under the pressure of the working ri u id.

The heating sm-face in which steam generation is completed and steam superheating begins is called the tran-sition zone. Dcposil.ion of scale takes place most! y in this zone. In earlier designs of once-through boilers, the conditions of operation of the metal of this zone were made easier by brin-ging the transition zone out of the furnace into the convective duct where the int eusity of heating was roughly one order of magnitude lower (ofiset transition zone). In modern practice, once-through boilers are fed with practically pure water, so that no scale forms und er normal opcraLi ng conditions and the transiti ou zone

--------------------~1~~~- ~F~lo~w~D~'=a~g~ra~r~n~o~f~S~t=e~a~m~P~ro~d~u=c~tw~n~---------------- 19 can be arranged within the furnace; the working fluid passes from the lower radiation section directly into the water walls above it where steam is superheated (radiant superheater). The radiant superheater can inc! ude either two heating surfaces: the me-dium radiation section and the upper radiation section, which are connected in series, or only the upper radiation section immediately downstream of the lower radiation section. Partially superheated stoam flows into tho last heating surface which is arranged in the convective duct; this is the con-vective superheater whoro s t.earo is hea-ted to the specified t emperature. Su-perheated steam of the required para-motel'S (temperature and pressure) is directed into the turbine. Like any heating sur:faco, the convective super-heater is a system of a l arge number of steel lube coils connected in parall el and interconnected by headers at the inl et and outl et ends.

The tempornt.ure of combustion pro-duels downstream of the convective s uperheater is quite high (800-900C). Part of the worked-off steam can be r eturned from the turbine for secon-dary (intermedia.te) superheating, usu-ally to the same temperatm-e as that of steam from the main superheater. This is the intermediate (reheat) super-heater (or, s imply, reheater) .

The combust ion products at the outlet from the intermediate supel-heater are s till rather hot (500-600C) and their heat can be utilized in a convective economizer . Feed water supplied into the convective economi-zer is preheated to a temperature be-low the saturation point and is fed into the lower r adiation section. The temperature of combustion products downstream of the economizer is 300-4500C or sometimes more. F urthe1 heat util iza I. ion is effected in a next convective heating surface, the air heater. It is a system of vertical tubes, with combus tion products flowing in the tubes and air, between them. Tho tcmperatUl'e of air is 30-60C at the inlet to the air healer (cold air) and

250-420C at the outlet (hot air), de-pending on kind of fuel and method of combustion.

With pulverized fuel combustion, t.he preheated air is separated into two flows. T he prirnary air is used for drying of fu el and transport of fuel dust through burners into the boiler furnace. The temperature of this fu el-air mixture is 70-130C. The secondary air is directed immediately through burners into the furnace (by-passing the fuel mills) at the tempe-ra lure it has had after the a it heater.

Downstream of tl1 e air heater, the combustion products have already a rather low temperature (110-160C). Further utilization of their heat is economically inefficient and they are ejected through the s tack into the at-mosphere. They ruc called waste, or chimney, gases.

Upon bm-ning, fu el leaves fly ash which is mostly carried o[f by com-bustion gases. Fly ash is catched (col-lected) in a fly-ash collector which is ar-ranged upstream of the induced-draft fan. This arrangement prevents abra-sion wear of the induced-draft fans and contamination of the atmosphere with fly ash. The collected ash is re moved by means of ash-removal devices. Part of ash falls oot.o the bottom of the boiler furnace and is removed con ti-nuously by the ash-handling system.

Tho flow diagram of steam genera-tion i11 drum-typo boil ers difors frorn that described above only in the de-sign and operation of the boiler pro-per (Fig . 1.8). In this case, the steam-water mixture formed in the fumaco water walls is fed into the boiler drum. The steam separated in the drum is practically dry and is fed first into the superheater and then into thf) turbin e.

As follows from the flow diagram of steam generation (see Fig. 1.7), a boiler plant has the following basic paths :

-the fuel path, i.e. a combinatioq of elements in which solid fuel is trans-ported, crushed, groun d, and delivere

20 Ch. 1. Steam Generation at Elutric Power Stations

4

Fig.

5

I 6 I I

Superheated steam to lur6ine

7 r---- Steam !"rom re!Jealer t o turbine

9 Feed water

10

11

Cold air ...

~.u.w.'t:::'=-; W o s te

1.8. Diagram of free-circulation drum-type boiler

1-Cumace space; t-wnter walls ~-b~trnero; 4-downtnke Lubes; s - drum ; 6-rndlant superhea-ter; 7- convecllvc su perheater; 8- lnLermedlatc superheater (rehcntc r) ; 9-economizer; JO-con-

vectlve gns duct; 11-air heater

The fuel path comprises the crushing equipment, conveyers, crushed-fuel bun-ker, grinding mill, and pulverized-fuel duct leading to the furnace. Up to the crushed-fuel bunkers, fuel is transfer-red by conveyers. Beginning from the grinding mill, the resistance in the fuel path is overcome by the head of the fan

' -the water-steam path, or circuit,

.is a system of series-connected ele-ments for the transport of water, steam-water mixture and superheated steam . The waler-steam circuit includes the

Mill

following elements : an economizer, furnace water walls, and steam super-heaters;

- the air path includes a combina-tion of elements for suction o( atmos-pheric (cold) air, i ts preheating, trans-port and supply into the furnace. The air path comprises a cold air d uct, air heater (ils air s ide), hot-air duct, and burners ;

-the gas path is a complex o[ ele-ments in which the combustion pro-ducts flow from the furnace into the atmosphere ; it begins in the boiler furnace and passes throug h the super-heaters, economizer, air heater (its gas side), ash collect or and stack.

The air and gas paths are connected in series forming what is call ed the gas-air path. The transition from one to the other t akes place in tho Lloiler furnace space. The diagram or a gas-air path is shown in Fig. 1.9a. In this circuiL, a ir is transported by ulowers and the corresponding air path in tho port ion between tho blower and furnace is at a pressure higher than the atmos-pheric. Combus tion products are trans-ported by induced-draft fans arranged downstream of t he boiler, and there-fore, the furnace proper and all gas ducts are at a pressure below the atmospher ic. This is what is called the balanced draft scheme.

The transport of air to the furnace and of combustion products to the atmosphere can be ensured by forced-draft fans only, i.e. without induced-draft fans (Fig. 1.9b). In that case, the furnace and gas ducts are under a certain excess pressure (supercharged). For comparis on, Fig. 1.10 s hows the pressure distribution in the gas-air path

- --

Forcl!llilrort fan t

I D Steam boiler lnduCI!d-drart fan Steam 6oiler (6)'-------....J

Fig . 1.9. Diagrams of gas-air pa ths ~a) pulverized coaHired bulanced-drufL bollrr: (b) fue l oil-fired superchurgcd holler: ---fuel; - a ir:

J- primnry ui r; 11- S

22 Ch. 2. Power-producing Fuels and Their Characlerlsl lcs

PO,:VER-PRODUCING FUELS AND THEIR CHARACTERISTICS

2.1. Kinds and Compositions of Fuels

The development of power engineer-ing is directl y linked to the const-ruction of new thermal and nuclear power stations, i.e. plant s operating on organic or nuclear fu els.

Organic fuels are those which can generate subs tant.ial quantiti es of heat (per unit mass or unit volume) by reacting with oxygen.

Organic fuels suitable for produc-tion of l arge quantit ies of heat with a sufficien t economy are termed power-producing. Their reserves must be enormous and relati vel y easily ex-plorable. Besides, they mus t be of low value as starting materials for other branches of industry. The most popul ar kinds of power-producing fuels employed at thermal power s tations are as follows: solid -coals and l ig-nites and some products prepared from them, anthracite and scmianth-racite, liquid-fuel oil , and gaseous -natural gas. Peat, oil shal es, st abi-lized oil and industrial gaseous fuels (bl ast-furnace and coke-oven gas) are used to a lesser extent, though in some regions of the country t hey con-sti tute an appreciable share of the fuel balance.

Of late, electric energy is produced more and more at nuclear power sta-tions which utilize the energy of nu-clear fission of r adioactive heavy me-tals: uranium (U235) and plutonium (Pu239). The richest uraniu m ore, ura-ninite, contains 65-90% of uranium dioxide UOz in which only 0.72% falls on radioactive U235 and the ba-lance is the common U238 The ori-gin al nuclear fuel is processed in gas-

diffusion plants to ra ise the concen-tration of um to 1.5-3.5%, after which it can be charged into nuclear reactors. One kilogram of U 230 libe-rates on fission roughly 85 mln MJ of heat, which is equivalent to the combustion of 3 500 l of coal with the heating va ltto 24.5 MJ/kg.

In the USSR, thetmal power sta-tions consume roughly 40% of the total organic fuel. Coals, fuel oil and natural gas are tho predomin a nt kinds of organic fu el in their fu el bal ance. The share of coals burned at thermal power stations increases gra-dually due to tho exploitation of new coal fields in Siberia and Northern Kazakhstan . The consumption of fuel oil and natural gas is approximately at the same level as coal. Other k inds of solid fuel , such as peat and oil shales, constitute only 6-7% of the total fuel consumption by thermal power sta-tions. In new coal fi elds, l ess expen-sive opencast mining will be employed increasingly.

All fossil fuels, i.e. solid fuels and petroleum, have formed in the process of long transformation of the original vegetable mass and died-off animals under a layer of earth or water. The process occurred at, a different rate as regards tho carburization of the fuel, i.e. increasing carbon concen-tration and decreasing cortcentrati on of oxygen and hydtogen (Fig . 2.1).

The degree of carburization, which characterizes the dopt.h of chemical transformations in fuel (which is cal-led t he chemical a.ge of fuel) is not de-term ined directly by its geological age, i.e. the duration of carburization pro-cess.

2.1. 1\irzds and Compositions of Fuels 23

Fuel c.cr. 0~ r. H~i' vc.r. 50 ~2.5 6 85

Wootf = 51 39 =s- 70= Peat 058- =:ti;;; ~ =:s~:= Brown coal =

- 75 . /j : -5; 37- -Coal -

.90 - :r: 4: .9 -

Anthracite ~ ~ 9J 2 2 4

Cru.de petroleum is a mixture of -organic compou nds, including minor -quantities of liquid su lphur and nit-rogen compounds, paraffins and re-:sins. Upon dis tillat ion of I ighL It ac-tions and oils (petrol , nnphta, kero-:sene, gas oil, straw oi l), there remain viscous heavy fractions - fuel oil which is used as a liquid power-producing -fuel. The m ineral impurities present in the crude petroleum are mainly -concentr ated in fuel oil.

Natural gases either form together with petroleum or aro produced by

synthesis in the presence of water and metal carbides at Large depths under the action of high pressure and tem-perature. In many cases, natural gas accompanies petroleum. This is what j s called a casing-head gas; it can be used as a power-producing fuel.

Heplacement of solid fuels by li-quid and gaseous improves the opera-ting conditions of power stations and -can decrease sensibly the cost of the equipment and increase its efficiency. For instance, the capi tal expendi tu-res for the construction o[ a power sta-tion to be fired with gas or fuel oil are lower by 20-24% than those for a solid-fuel fired station of the same power. The efficiency of fuel oil-fired plants is 4% higher (in terms of electric energy produced) than that f a solid-fuel fired plant.

The explored reserves of natural gases and petroleu m are however li-mited and constitute only 6% of the total world r eserves of organic fuel. Besides, n atural gases and petroleum a re of the highest value as starting

Fig. 2.1. Comparison or elemental ana-lyses or principal fuels

materials for various branches of na-tional economy. On the other hand, the reserves of coal exceed 7'1% of the total expl ored reserves of fuel in th o world and th erefore coals are the principal organic fuel.

Tile organic mass of solid and liquid fuels consis ts of a large number of compl ex compounds of the pri ncipal five elements: carbon C, hydrogen H, oxygen 0, sulphur S, and nitrogen N. Besides, any fuel contains mineral impurities A, which enter the origi-nal bed mainly from the outside, and moisture W. For that reason, the chemical analysis of solid and liquid fuels is determined not in the terms of their compounds, but as the total mass of chemical elements in 1 kg of fuel, i.e. the elemental analysis of a fuel is determined .

One should d istinguish between the following fivo elemental masses of fuels:

working mass cw+Hw+Ow+Nw+S:,O+Aw+Ww={00%

(2. 1) analytical mass ca+ lJo+Oa+Na+S~+Aa+Wa = 1.00%

(2. 2) dry (moisture-free) mass

C

I

l

l

24 Ch. 2. Power-producing Fuel1 and Their Characteristics

and Coke residue Fig. 2.2. Diagram of elemental ana-lysis of solid fuel

Volatiles Koncom6usli6le

muss

mass

The work ing mass of fuel is the mass in the form it is delivered to the plant. The consumption of fuel and the vol umes of combustion pro-ducts formed are calcul ated in terms of the working mass. The working fuel upon grinding to pulverized state and when dried in laboratory to the air-dry slate, loses the free mois-ture, and its mass is then called analytical. The r emaining moisture in fuel, wa, which is combined in the original substance, is more often cal-led hygroscopic moisture, i. e. wa = = Wh.

Upon heating of fuel to 102-105C, all its moisture evaporates, thus leav-ing the dry mass of fuel. The combus-tible mass of fuel includes the ele-ments of the origina.I organic matter and the su lphur of inorganic combu-stible compounds (for instance, py-rite FeS,); for that reason, it is cal-led the resolved combustible mass.

In equations (2.1) to (2.4), S., is the volatile sulphur, i.e. the sum of pyri-te sulphur and organic sulphur which can be oxidized in the boiler furnace: S., = Sp +So.

The organic mass of fuel is essential-! y its combustible mass plus pyrite sulphur. Apart from the two kinds of sulphur mentioned, there is also the sulphate sulphur S. which enters the composition of higher oxides (such as CaSO,) and cannot be fur ther oxi-dized in combustion. The various masses of solid fuel are shown dia-grammatically in Fig. 2.2. It should be distinguished between the external and internal ball~t in the composition

of a fuel. The former includes moisture and mineral matter and the latter incl udes oxygen and nitrogen which enter the original organic matter.

Combustible elements in fuel are caT-bon, hydrogen and sulphur, with carbon being the principal combustible ele-ment. It has a h igh beating value (34.1 MJ /kg) and constitutes the major por-tion of the working mass of fuel (50-75% in solid fuels and 83-85% in fuel oils). Hydrogen has a very high hea-ting value (120.5 1\0/kg), but its con-tent in solid fuels is not high (Hw = = 2-4%) and is somewhat higher in liquid fuels (10-11%). Sulphur has a low heating value (9.3 MJ/kg} and is present in fuels in minor quan-tities (S10 = 0.3-4%) and for that reason is of low value as a combusti-ble element. The presence of sui phur oxides in combustion products in-creases the risk of corrosion of metallic heating surfaces and in certain con-centr ations may be dangerous to ve-getation and animals; this necessi-tates measures fqr the collection of sul-phur from waste gases. According to their sulphur content, fuel oils are divided into low-sulphur (Sw < < 0.5%), medium-sulphur (Sw = 0.5-2%) and h igh-sulphur (S"' > 2%) gra-des.

In contrast to solid and liquid fuels, gaseous fuels are mechanical mixtures of combustible and non-combustible gases. Nat ural gases consist pref~rabl y of methane CH6 (up to 90-96%) , witl't minor quantities of heavier hydrocar-bons (ethane C1 11 6 , propane C3H8 ,

I

I

I

2.2. Characteristics of Fuels 25

butane ClH10, etc.) which are often written generally as CmHn (1-6%). _Besides, natural gas contains minor quantities of non-combustibles: nit-rogen N 2 (1-4%) and carbon dioxide cot (0.1.-0.2%).

2.2. The Healing Value and Resolved Characteristics of Fuels

The quantity of heal liber ated on combustion of n unit mass or volume of fuel is the principal thermal cha-racteristic of fuels, called the heating, or calorific, value. One should di-st inguish between the upper and lower heating val ue. The upper heating value Qu is the quantity of heat l.iberate~ on combustion of 1 kg of sohd or li-quid or 1 m3 of gaseous fuel under the conditions that water vapours are condensed and the combustion pro-ducts are cooled to 0C. The lower healing value Q1 differs from tho up-per heat.ing value by the heat of ev~poration of the moisture present m the fuel and the moisture formed on combustion of hydrogen. In power plants, moisture of the combustion products remains in a vaporized state and the heat consumed for its evapo-rat ion is lost. With a higher moisture content of fuel, Q1 is lower.

The lower heating value, kJ /kg, can be found by the formula :

Ql = Qu - Qw (2 .. 6} ;

In the general case, the heat of moisture condensation, k J/kg, is:

Qw = 2 500 ( :~ + ~) =22SH+25W (2.7)

where H and W are the concentrations of hydrogen and moisture, %, and 2 500 is the heat of condensation of 1 kg of moisture at atmospheric pres-sure, kJ/kg.

When finding the l ower heating value for other masses of (uel, except for the analytical and working mass, formula (2.7) becomes simpler be-cause the moisture term is excluded:

Qw = 225 H

The higher heaLing value of solid and liquid fuels can be determined experimentally by burning a sample of fuel in a calorimetric apparatus.

The heating value of fuels can be d etermined approximately from their elemental analysis.

In this respect, the most simple and accurate are Mendeleev's formulae which contain empirically found coef-fici ents for various combustible ele-ments. For instance, the formula for determining the lower beating value of the working mass of solid and li-quid fuels is as follows:

Q'{' = 339Cw + 1 030Hw - 109 (OW - SID) - 25WU' (2.8)

where C"', H"' , etc. aro the concentra-tions of elements in the working mass of fu el, %. For gaseous fuels, if !hei r analysis is known exactly, the heattag value of 1 m3 of dry gas can be deter-mined by the formula:

Q1 = 0.01 (Qa2H 2 + QcoCO + Qca,CH4 + Qc,a,C2Ha+ ... ) (2 .9) where H 2 , CO, CH4 , C2H&, etc. are the volume concentrations of com-bustible gases in fuel, %, and Qa, Qco. Qca,, Qc,a etc. are their hea-ling values, kJ/m3 .

Steam boilers of the same steam-generating capacity can consume w_ide-ly different quantities of fuel, snce the heating value of various fuel s may vary within wide limits. For compa-rison o[ the efficiency of various po-wer plants and for simpler calcula-tions of the combustion of various fuel s , the concept of reference fuel has been introduced. It is a cond itio-nal fuel whose heating value is Q, = = 29 .33 MJ /kg (7 000 kcal /kg). The consumption of various kinds of fuel at power plants can be calcu-1 a ted in terms of the reference fuel by the formula:

Q'f B, = BQ, (2.10)

wh ere B r and B aro the consumption of the r eference fuel and natural fuel, respectively.

26 Ch. 2. Power-producing Fuels and Their Characteristics

The method of expressing the ele-mental analysis of fuels and theii external ballast (moisture and ash content) as percentage of the ori-g inal mass of burned fuel is used wide-ly in power engineering. This me-thod, however, is not always conve-nien t for the analysis of operating ~onditions of steam boilers. For in-stance, as the external ballast. of a fuel increases, the heating value of the fuel becomes lower, and therefore, more fuel must be burned to maintain the same steam-generating capacity of I he boiler . As a result, the mass of the ballast en loring t.he furnace increases much more appreciably t,han the fuel consumption, which can form intolerable condilions for boiler ope-ration . Thus, the percentage of moi-sture, ash or su lphur in fuel is not sufficient to characterize the valua-b ilit.y of a fuel.

The mass consumption of fue ls bur-ned in steam boilers can be characte-rized more reliably by relating the concentration of chemical clements and ballas t, per cen t , to the unit of the lower heating value, 1 MJ , which is called the resolved characteristic of fuel.

The resolved moisture content, ash ~ontent and sulphur content (%kg/ /MJ) are determined by the formulae:

W' = ww Qf , S' = sw Q7 (2.11)

For instance, with the same ori-ginal sulphur content (S"' = 3%) of fuel oil (Ql" = 39 MJ /kg) and brown coal (Q't' = 12 MJ/kg), the mass of sulphur oxides car-ried off with com-bus t,ion products is in th e latter case 3.25 t imes h igher, according to the r atio of the resolved sulphur con tents : S' = 0.077 of fuel oil and S' = 0.25 of brown coal.

2.3. Technical Characteristics of Solid F uels

Efficient combustion of fuels in steam boil ers requires knowledge and correct account of a number of deci-

sive characteristics of fuel s whi ch, apart from the heating value, include the ash content, moisture content, the yield of volatiles, etc.

Ash content. Mineral impurit ies are present in all kinds of solid fuol. Their major portion is not associated with the org,anic mass of fuel. As l'e-g ards their origin, mineral impurities can be divided into internal which accumulate in fuel bods in the course of fuel formation and external which pass into the fuel from the surroun-ding gangue during exploitation.

As fuel is burned, its mineral com-ponents are subjected to high -tom-perature transformations. Complex mi-neral compounds of Ute type of clays Al203 2Si0z2H20, felds pars K 20 Al20u 6Si02 , sulphates and carbo-nates CaSO 4 2H20 CaMg(C03 ) 2 , etc. are destroyed and partly afteroxidi-zed by atmospheric oxygen . The resi-due r emaining af ter t he combustion of fuel, i.e. ash, consis ts mainly of a number of oxides: Si0 2 , Al 20 3 , Fe20 3 , CaO, MgO, K 20, Na20 and its mass turns out, on the average, to be 10% smaller than the original mineral mass of fuel. The percentage of the ash re-sidue r elative to unit mass of original fuel is called the ash content.

The properties of ash play an .im-portant part in the operation of a steam boiler. Finest solid particles of ash are entrained by tho flow of furnace gases and carried off from the furnace; this is what is caJled fly ash. Another part of ash is melted in the flame core and drops to the fumace bottom or s Licks to the s urrounding furnace walls and forms slags on solidification, i.e. solid solutions of minerals whose corn-posi tion may differ from that of fly ash.

Of special importance fot the fuol combustion process are the fusibil i l.y characteris tics of ash. Tho mel Ling points of vat"ious minerals and th eir alloys d iffer widely, ranging from 600 to 2 900C. For that r eas on , ash is not melted at a fixed temperature, but is softened gradually and changes from the solid to liquid state with

2.3. Technical Characteristics of Solid Fuels 27

increasing temperatu re. Tho melting temperature of ash is determined by the standard con e t est, i. e. a .cono of s tandard sizo is pressed from ash and placed into a furnace (Fig. 2.3). Du-ring h eaLing , the following characte-ristic temperatures are marked:

t 1 is the beginning of d eformation , when the shape of the cone starts

changing, t , = 1 000-1 200C; t~ is softening, when tho top of the

cone drops to the base or assumes a dropl et-like shape, t~ = 1100-1ll00C; and

t3 is the fluid state, when ash beg ins to spread ove1 the plane base, t 3 = = 1 200-1 500C.

The basic characteris tic of slag is its v iscosi t. y.

Molten slag, if i l is in an actual liquid state, flows freely along a ver-tical or inclir1ed wall when its vis-

cosity is less than 200 P (poise). The temperature of mol ten slag at which i t can now out freely from a hole is

-called the temperature of normal I i-quid slag removal, tsr- The tempe-rature of fus ion of ash and the typical

-coefficients of viscosity can be found from tables of power-producing fuels.

In burning, the major portion of mineral composition of fuol is trans-formed into fine fl y ash which is car-ried off by the gas flow. In furn aces

operating un der different th ermal con-d iti ons of combustion and slag remo-val, the amount o fly ash carry-over, CLc, may range from 0.85-0.95 to

0.2-0.-1. The rema ining pottion drops down onto tho furnace bottom as s lag and is removed therefrom: a3 1 = = 1 - ac. with a higher ash con-tent of fu el and a higher concentra-l ion of fly ash in furnace gases , more intrica te and expens ive ash-collecting devices are needed to prevent poilu-

Fig. 2.3. Cone test of ash J- hcrore heati ng; Z-bcglnning or dcror-

mntlon; J -sortenin((; 4-lluld state

Lion of the air basin . In that case, the speeds of gases in convective ducts of boilers are reduced to avoid abra-sion weat of tubes , while depositi on o ash particles on the h eating surfaces impairs the cond i t ions o heat trans-for. As a resu l t. , the boiJor plant beco-mes too bulky in design.

The yield of volatiles and coke resi-due. If a sa mple of dry solid Iuel is placed into a crucible and heated gradually in an inert medium without air, i ts mass will decrease. At high tern per a tures, oxygon-couta i ning mo-lecules of the fu el dissociate and form gaseous substances which arc called volatiles (CO, 112 , CJI4 , C02 , etc.). The evolution of volatiles from solid fuels takes place in the temperature range 110-1 100C. The hig hest yield (up to 95%) occUIS at a temperature near 800C (Fig. 2 .4). For that rea-son , the yield of volatiles fro m solid fuels is determined conditionally as the decrease of the mass of fuel sam-ple upon holding it in a crucible at t = 850 25C for 7 min utes, rela-ted to the combust ible mass of fu el, vc %.

50

40

JO

20

to

0

yc

I II

. 4/JO

/ /

2 _,........... J

500 800

- 1,--~

~ --- -

,_~

-

I -l

fOOO c

Fig. 2.4. Evolution of vola tiles depending on tern peru ture

/ - brown coal; 2- lean coni; J- ant11racitc; 4 -ultimalc yield or volatiles lor particular tue l

I

I

l I

I

28 Ch. 2. Power-prodrlclng Fuels and Their Characteristics

Since the yield of volatiles is deter-mined in the first place by the con-centrat ion of oxygen in fuel, it is higher in 'younger' fuels (Fig. 2.1). For instance, the yield of volatiles is yc = 45-50% from brown coals, 25-40% from coals, and only 3-4% from anthracites.

The solid combustible residue re-maining upon evolution of volatiles is called coke. It may be either dense (sintered) or loose (powdered). In air, coke ignites at a temperature of 900-12000C. Volatiles evolving from the fu-el ensure earlier ignition of coke, since they can ignite at a lower tempera-ture t han the coke residue (350-600oC) and thus raise quickly the tem-perature of coke particles. Their ef-fect is especially strong at the initi-al stage of fuel burning. Fuels wiLh a higher yield of volatil es ignite more quickly and burn more completely.

Moisture content. It is distinguished between the adv entitious, adsorbed, cellular, and inh eren t moisture. All kinds of moisture, except for inherent moisture, are removed from fuel on heating to 102-105C. The inherent, or hydrate, moisture is firmly bonded with the mineral por tion of fuel and enters the composition of crystals of the substance.

Solid fossil fuels contain mostly ad-sorbed moisture which is determined by the adsorptivity of complex col-loids of the organic mass of fuel . The highest adsor ptivity is exhibited by peat, brown coals and some younger coals. The adsorptivity of a fuel de-termines its hygroscopic moisture con-tent Wh. This moisture characteri-zes indirectly the age of fuel: it is lo-wer in older fuels. For instance, Wh = 10-13% in brown coals and on I y 1.5-2.5% in anthracites. Know-ledge of W1' is essenti al for estimating the admissible moisture content of pulverized coal to avoid sticking of particles (at an excessive moisture content) or explosion of overdried dust.

The adventitious, or mechanically retained moisture appears in fuel upon

contact with water and remains on its surface due to wetting. Its magni-tude depends on the particle size of fuel and the external conditions du-ring transport and storage. Cellular, or capillary, moisture is determined by the porosity of fuel structure. It is most pronoun ced in peat.

A high ' moisture content in t he working mass of fuel may cause se-rious difficulties in fuel combustion. It decreases the heating value of fu -el , increases fuel consumption and the volume of combustion products, and involves higher heat losses with waste gases and greater energy con-sumption for driving the induced-draft fans. An elevated moisture conten t of furnace gases can cause s tronger corrosion of metal in the air heater and increase contamina-tion of the heating surfaces. Wet fu -el is sticky, which involves difficul-ties in its transport and preparation, and besides, it can congeal in winter t ime. The effect of tho sulphur pre-sent in fuel on the boiler operation will be discussed in the section to fol-low.

Table 2.1. Grading or Coals

"" Yield or Chu ractcrls tic " Coal type - volatil es, or coke ., .. V", % residue ~ a::>

Long-flarno D 36 and Powdered, coal more poorly sin to

red Fiery coal G 36 and -

more Fiery fat coal

GZh 31-37 -

Fat coa l Zh 2'-37 -

Fat coking KZb 25-33 Densely coa l sintered Coking coal K 17-33 -Leaned cu- s 14-27 -king coal

ss 17-37 Poorly Low-caking SID-coal tered, pow-

de red Lean con i T 9-17 Poorly sin-

tered, pow-do red

2.4. Characteristics of Fuel Oll and Natural Gases 29

Grading of solid fuels. Solid fuels are grad ed mainly according to their mois-t ure content in the working mass

(brown coals) or the yield of volatiles (coals). For instance, brown coals ~re divided into three groups: B1 w1th moisture content W "' up to 40% ; B2 with ww = 30-40%, and B3 wit.h ww less than 30% . The grading of coals is based on the yield of volatiles and the characteristics of coke (Table 2.1). .-

F ine fractions of fuel (screenings) which remain after screening of the produced fuel are additionally graded by a letter that shows their parl.icle size for instance: Sh - fractions from

6 mm and less; SSh- fract10ns from 13 rom and Jess; R - ord inary (un-screened) fuel, etc.

2.4. Technical Characteristics of Fuel Oil and Natural Gases

The quality of fuel oil has groat ef-fect on its combustion in boiler fur-naces and the scheme of fuel oil pre-paration and supply to a power sta-t ion.

Viscosity. Viscosity is ~ one of the principal technical characteristics of f uel oils and the basis for their gra-ding. Fuel oils are divided into light, medium and heavy grades. Light grades include marine fuel oils (F5 and F ig. 2.5. Effect of temperature on t he viscosity or fuel oil 500

400 JOO

F12), while medium and heavy grades are used as furnace fuel in boilers and other stationary power plants and in process plants. Depending on their viscosity and other physical characte-ristics, furnace fuel oils are subdivi-ded into the following grades: high-quality fuel oils 40V and 100V and furnace fuel oils 40 and 100 (40V and tOOV are heavy grades of fuel oil) [91.

Viscosity of fuel oils is measured in units or kinematic v iscosity (cen-tistokes, eSt) or iu degrees of Engler viscosity (0 E) as measured in an En-gler viscometer by.the time of ~low of a portion of fuel 01l from a calibrated hole at standard tem perature (80C for heavy fuel oils). For normal trans-portation through pipelines and for fino atomization of fuel oil in burners, ils Engler viscosity should be within 2-3.5E. The viscosity of fuel oil hea-vily depends on temperature (Fig. 2.5). Its variations with temperature are due to the presence of paraffinic hydrocarbons in fuel oil. For easy trans-portation in pipes and for norm al operation of fuel oil pumps, the tem-perature of fuel oil should be mainta-ined near 60-70C.

Rheologic properties. At low tem-peratures (10-25C) viscous fuel oils can stick to the surfaces of vessels, pipes, etc. and remain on them in a l ayer whose thickness is greater with

tOO 60 40 ]0 ('... I' I" zo ~ 100 . l5 -.;;.. 80 10 .[ 60 .,., 8 t::; 50-G " 40 ~ ~ 1:; .~ Jo e ,1 "' .....

200 150

I

t'-"'

I'\

"'

t\. r--.. MfOO

I'. )',. I'\ AffOOV

, .,_M~O 1>\.

" M 4 v "'

t\. ...... ......

" " I'\

' "' :;;: ... ~ 20 J ~ 15 2.5 "' E: ~ 2 -"' 10

r--.

'

--

t\. :-.. "' K r-._ ......

r--.. r--..

r-._

" r-._ ~'

r--..

" t'-. 1'\t' 0 50 50 70 80 90 tOO 110 120 fJO 'C

30 Ch. 2. Poroer-producing Fuels and Their Characteristics

a lower temperature. T his effect is due to the rheologic property of fuel oil , i. e. to the rearrangemen t in the structure of hydrocarbon molecu les which takes place on a decrease of tem-perature. Sticking of fuel oil is avoi-ded by heating H to a temperature of 70C or more.

Density. The densi ty of fuel oils is usually measured in relative uni ts, i.e. as tho ratio of the fuel oil density to the density of water at 20C which is p20 = 0.99-1.06. Wi th an increase of temperature, the density of fuel oils decreases and can be found by tho formula:

P:o Pt = 1-H1 (t - 20) (2.12)

where Pt p20 is the relative density of fu el oil a t a given temperature and at 20C and ~ is the coefficient of volume expansion of fuel oil on heati ng by 1C; for fuel oils, ~ = (5.1-5.3) X X 10 - 4

Ash content:. During processing of crude petroleum, the mineral impuri-ties present in it a1c mainJy concen-trated in heavy fractions and especial-ly in fuel oil. The ash residue remained upon com bustion of fuel oil is not large, not more than 0.1 % of the dry mass. Fuel oil ash is ch aracter ized by a certain content of vanadium whose concentration may be as high as 50% or even mo1e.

Moisture content. Fuel oils usu ally contain 1-3% water. T heir moisture content can rise substantially (up to 10-15%) in tho course of fuel oil pre-healing before pouring it from tank cars, owing to the condensation of low-pressure s team which is used for hea-t ing. A small concentration of mois-ture in fuel oil is favourable for ils atomization in burners and improves t he in flamma bil ity ch aracteristics. With an elevated concen tration of moisture, there is a h igh risk of cor-rosion of convective heating su rfaces; this also increases t he loss of heal with combustion products.

Sulphur content. Petroleum and so-lid fuels contain sulphur in the form of complex sulphur compounds.

When petroleum is processed, the ma-jor portion of sulphur compounds (70-90%) passes to h igh-boiling fra -ctions which are the main components of fuel oil. During combustion of fuel oil or solid fuel, su lphur is oxidized to so~ and a minor portion of i t can form the higher oxide 803 (if the1e is en ough oxygen in the combustion zone), which forms a corrosive medium on low-temperature healing surfaces. The content of sulphur in fuel oils is roughly the same as in sol id fuels (Sw = 0.5-3%),' but the corrosive ab i-l ity of the gaseous medium that forms on the combustion of fuel oil is seve-r al Limes higher. Th is is due to t he fact that, unlike fuel oil , sol id fue ls con t ain certain components in t he ash which can neutralize acid med ia.

Congelation temperature . T he con-gelation temperature of petroleum pro-ducts is the temperature at which they become so thick that. remai n in pl ace and do not flow out for 1 mi nute from a test gl ass inclined at 45. High-sulphur fuel oi ls with a high concentra-tion of paraffins (Grades M-100 and M-100V) are characterized by a high congelation tern perature (25-35C). T he congel ation temperature of fuel oil should be considered properly when sel ecting the schemo of its tran-sport and storage.

F lash point. T he flash point is the tem perature at which a rnixttue of fuel oil vapours and air can be igni-ted when it comes in contact with an open fl ame. Fuel oil grades used at power stations have a flash point of 90-140C, while h igh-par affiu ic fue l oil may have a l ower flnsh poin t, (up to 60C); the fl ash point, of crude petr oleum is only 20-40C. In order to av oid fire hazard, prehe:H ing of fuel oil in open systems shou ld be car-r ied out at tem per atures below i ts fl ash point and in all cases, no t. above 95C to p revent boil ing of the mo is tu-re which may be presen t in th e bu lk of fuel oil .

The basic technical characteri!'t ics o[ natural gases are dens ity, ox plos i veness and toxicity.

2.5. Matn Deposits of Fossil Fuels 31

Fig. 2.0. Igni tion ranges of gas-air mix lures at 20C (p = 0.1 MPa) Bas Ff/.rmulo InfLo.,mmabililglimits, % by volume

Hgdrogen Hz CH f:Jtf/r1de

Methane [!IIane Propane ButontJ Elhglene Acetglene

C2 H~ 117.7.'77: '7A C2H2 I P'Z "// '/// '//. '// /"/, '////

Na/urul gas I Olustrurnau gas 1'////. '//, /"//,'1 CokeoYen gus V///////-

Density. Almost a ll kinds of gaseous fuel are l ighter th an air, so that esca-ped gases may coll ect under roofs. Be-fore firing a boiler, i t is essential to check that there are no gases in places of their probable accumulation. Va-rious gases are compared by using the concept of rel ative gas density which is the r atio of the density of a given gas u nder standard conditions (0C, 1 015 Pa) t o the density of air:

(2.13) where Pg and Pa are the densities of gas and air respectively under stan-dard conditions, kg/m3 .

Explosiveness. A mixture of a gas and air in a certain proport ion can ex-pl ode when in contact w ith open fl ame 01 even a spark, i. e. it igni tes and burns at a speed near the velocity of sound propagation. Explosive con-centr ations of combustible gasos in a ir d epend on the composition and pro-perties of a particul ar gas (Fig. 2.6). In concentrations below the l ower li-m it of in.flammability (explosiveness), a gas-air mixture cannot burn. In concentrations above the upper in-

0 20 40 liiJ 80 Jlolvme proportion Of !l_OS

ifl !JOSoir mixture, %

flammab ility Jim i t, a gas-air m ixt.ure burns without explosion .

Toxici ty. Tox icity is t he ability of gases to poison the living organisms. l n that respect, carbon monoxide CO and hydrogen sulphide H 2S are most dangerous.

Since all components in natural gas are intermixed evenly, the concentra-t ion of harmful gases in air can be de-termined by the pres ence of methane whose concen tration is measured by methanomeler. The test determines the explosiveness of the gas mixture. Almost all natural gases are odourless. For easier detection of gas leakages and taking proper safety measures, na-tural gas is odoured before pum ping into a gas pipeline, i.e. a subst ance having a strong smell is added to it .

2.5. Maio Deposits of F ossil Fuels

Fossil solid fuels are distributed nvor t,he USSR terri tory extremely un-even ly. The most developed ind ustri-all y regions in the European part of the country aTe not rich in fuel. Of greatest impor tance here is the Do-

I !

32 Ch. 3. Fuel Preparation at Powcr .. Stations

netsk coal basin which possesses va-rious grades of coal and anthracite, but i ts reserves can no more satisfy the growing demand. Besides, coal seams are thin and deep-lying which makes coal production too expensive.

The main mass of coals is concen-trated in the Central and West Sibe-ria and Kazakhstan. These coals are cheaper than Donetsk coal, notwith-standirlg the costs required to trans-port them to the European regions. Further, there are I arge reserves of brown coals in the Kansko-Achinsky basin (Central Siberia), with power-ful shallow-lying seams which can be produced profitably by open cast te-chnique; this is the cheapest fuel in the USSR. Similar characteristics have Ekibastuz coals (East Kazakh-stan). Kansko-Achinsky brown coals will be processed by a complex techno-logy into valuable chemical products, brown-coal fuel oil and coke breeze, a

fuel of a high .heating value (around 29.3 MJ/kg).

Petroleum fields in Tyumen di-strict are being exploited intensi-vely. The production of petroleum and condensed gas in this'region.amounts to about 50% of the total production in the country.

Natural gas fields have been found in various regions of the country, the most widely known being the Shebe-l inskoe, Dashavskoe and Gazliyskoe. Of late, unique gas fields have been found and are being exploited inten-sively in Turkmenistan, South Urals and Tyumen district (Shatlykskoe, Orenburgskoe, Medvezhye, Urengoi-skoe, Yamburgskoe). The gas reser-ves in these fields constitute almost 50% of the total explored reserves of natural gas in the country. Large re-serves of gas and petroleum have been discovered in Komi autonomous di-strict.

FUEL PREPARATION AT POWER STATIONS

3.1. Methods of Solid Fuel Combustion

Solid fuel combustion in boiler fur-naces can be effected by various me-thods: flame combustion, cyclone com-bustion, or fluidized-bed combustion (Fig. 3.1). Flame combustion is the roost popular in modern power engi-

.

neenng. The classification of combustion

methods is based on the aerodynamic characteristic of the process which de-termines the conditions of contact of the burning fuel with an oxidant [58).

The capacity of a furnace can be ac-tually increased without limit by bur-ning pulverized coal (coal dust) in a

suspended state in the furnace space. This is what is called flame combusti-on (Fig. 3 .1a). In this method, fine particles of fuel are easily moved by the flow of air and combustion pro-ducts through the section of the fur-nace. Combustion takes place in the furnace space within a rather short time of the presence of the particles in the furnace (1-2 s). The rate of fue l burning, and therefore, the amount of heat evolved in time, depend on the combus~ion surface.

In cyclone combustion, fuel particles go through intensive turbulent motion (Fig. 3.1b). In contrast to flame com-bustion, the fuel particles are blown

3.1. Methods of Sollo l'uel Combustion 33

Fuel Air

/ \''' . ... , . . . . . . . . . . : I I 0 0 0 :. : 0 0 ' ' ;I I . . . . . . . . I 1:. 0 0 ': 0 ' J r' . :' /.lJf < _. I I" :r .. ::-i . . . . . . . : . ., ' . ~ . . . . . ,.:. : :./

/ : :. .:. :, .. 0 .,

. .. .. ~ . . . . . ; .. :....:.. ~-....:.~ _;

(a)

0.

Fuel Air

I 0

_:_::.-.::: . . . . .

~ ~ . . . ..

Motten stag-

(6)

----------------

/

l t 1' Air

(c)

Fig. 3.1. Diagrams of combustion of solid fuels (a) !lame combustion; (b) cyclone combustion; (c) fluidized-bed combustion

over by the flow and burn off more quickly. The cyclone method permits the combustion of coarse coal dust and even crushed coal. A cyclone fur-nace develops a higher temperature with the result that the slags are remo-ved in the molten state (slagging-t.ype furnace).

In recent times, a new combustion method has come into use in the field o[ power engineering called fluidized-bed combustion (Fig. 3.1c). Solid fuel , ground to a particle size of 1-6 rom, is placed onto a grate and blown from beneath with an air flow at such speed that the fuel pal'Licles are li fted above the grate and are reciprocated in the vertical plane. In this process, the speed of the gas-air flow within the fluid ized bed is higher than above it. The fine and partially burned parti-cles rise into the upper portion of the fluidized bed where the flow velocity decreases , and there they burn comple-tely. In operation, the fluidized bed increases in volume by 1.5-2 times; its thickness is usually 0.5-1 m.

Heat absorbing surfaces in the form of in-lin e or staggered tube bundles are arranged in and above the volume of the fluidized bed. The unit heat absorption within the fluidized bed increases substantially due to the in-t ensive conductive (contact) heat tran-3-015 2 4

sfer from incandescent fuel particles to heat-absorbing sur[aces, though the temperature of gases in the burning bed remains at a relatively low level (800-'1 000C), which prevents the me-tal from overheating and d iminishes the concentration of harmful nitrogen oxides in combustion products . In addition, this method of combustion makes i t possible to introduce solid additions (say, limestone) into the bed in order to neutralize the" sul-

' phur oxides that form on combus-tion.

Large power stations consume as much as 1 000 t coal per hour or more. Even when delivered in cars of a large load-carrying capacity (60-125 t), 15-30 cars of fuel must be unloaded eve-ry hour, which is only possible with the use of highly efficient car dum-pers.

The process of pulverization, i.e. transformation of lumpy fuel into

dus~. includes two stages (see Sec. 1.3). In the first stage, raw fuel is crushed to a particle size of not more than 15-25 rom. The crushed fuel is then deli-vered into raw coal bunkers and tran-sferred to grinding mills where it is ground to a final particle size of no greater than 500 !LID. During gri nding, the fuel is dried by hot air to ensure it has good dust fluidity.

34 Ch. 3. Fuel Preparation at Power Sta tions

3.2. Pulverization Systems The pul verizalion system is a com-

bination of equ ipment in which solid fuel is ground, dried and La nsf erred to the burnCIs of a boiler furnace.

By the method of delivery of pul-verized fu el to the furnaces; pulveri-zation systems can bo divided in to central and individual. In the former case, the system is .arranged in a se-parate building (ceritral coal pulveri-zing plant) whore coal is pvlverized on a centralized basis and Lhen die s tribu ted through pipelines between the boilers of the s t.alion . In the lat-ter case, each boi ler is provided with i ts own pulverizing eq uipm en t, with certain prov isions being made to tran-sfer th o pul vorized fuel to neighbou-ring boi lers so ns to in crease t he relia-bility of t.h c fu el suppl y.

T he selecti on of a particular pul-,erizalion system for a power station presents a co mplicated technico-economical probl em. Cen tralized pul-vel'izalion sys tems turn ou t to be

more efficien l economically. especiall y when moist brown con I is pulveri zed, but their equi pment is more intri-cate and ex pensive, and. in addition to this, they are not sufficient I y rei iable in operat ion. I nd i\'idu al systems arc simpler and more rei iable and are widely employed by power stat ions [29].

In dividual pul verizat ion systems can be subdivided into the fo llowing types: closed systems whid1 di rectly blow dust into the furnace space, clo-

. sed systems with an inter mediate dust bunker, open system~ which use hot air as a dust carrier . The type of system (either closed or open) is de-termined by the way in whid1 tho dry-ing agent is utilized upon fuel dry iug . In a closed system, it is directed in to the furnace together with dried pul -verized fuel; in an open system, the drying agent is carefully c.lcanod from fu el fines and ejected into tho s ta ck , bypassing the boil er furn ace.

The pulverization system with closed fuel drying and direct dust blowing into the furnace. Crushed fuol is delivered from the coal bunker by a coal feeder into a g l'inding mill (Fig . 3.2). Hot air at a tem perature

to t1, 0 = 250-400C is also fed into the

II

1--..., mill in order to dry the fu el and tran-sfer i t further to the boiler furn ace bur-ners. This is what is called primary air. Coarse fractions of pulverized fu el a re separated in a sepa rator, af-ter which the fuel and ait {which has been moistened by the moisture of

18 fJ f

Fig. 3.2. Individual pulverization system with direct blowing o[ pulverized fuel into furnace for the opera~ion on compressed hot

nJr J - r aw ron I bunker; f - cut-otr gnle va lve; J - coal rccdc: 4- rnw coa l chute; 5-coal mill; G- dust se-parator; 7- dusl duct; 8- burncr; o-steam holler; Jo- rorcQd-drnrt run; JJ- alr hea ter; 12-prlmnry ai r path; 13- sccondnry nlr path; N -secondary nir duct; J5- cold nlr lor mill venltlnllon; 16-exploslon re lic! vnlve; 17- !lnpper ' 'a lve; 18-

nutomatlc cut-orr gate vnlve

fuel) are supplied at a temperature of 80-130C through pulverized fu el pi-pelines to the furn ace burners. T he r emaining h ot air, which is cn ll od secondary air, is fed sepnrntely l.o t;ho burners.

The quantity of prirna1y air I o be used for the drying and I ransport of pulverized fuel depends on tho fuel quality, in particular, tho mois ture content. The amount o[ primary nir {r1) is usually equal to 0.3-0.5 of the total consumption of a ir for c.ornbu-stion and increases with the moisture content. With extreme! y moist fuel,

9.2. Pulverization Sy3rcm. 35-

the use of hot 11 ir only for fuel dryiug becomes economically inefficient and fu el combustion becomes unstable since the greater portion of puhcrizcd fu el enters the combustion zone at a reduced temp~raturc. I n such a case, fuel is dried by a higher-temperature agent, say, by mixing the primary air with a part of the furnace gases.

If the pulverization system is ri-gidly connected with the bo'iler, it should satisfy more stringent requi-rements as regards the reliability of its operation. The number of instal-led grinding mills must bo no less than three and the number of m ills in operation minus one mi ll must en-sure at least 90% of the rated load of the boiler. Hence the productivity of a mill, Bm, should be:

where Bb is the fuel consumption by the boiler at the rated load, kg/s, and zm is the number of ins talled mills belonging to the boiler.

Upon exi ting from the mill sepa-rator, fuel dust is divided between 2-4 pulverized fuel pipelines which are connected with different, not ad-jacent, burners. This is done in order to avoid an uneven temperature d i-stribution in th e furnace space should the mill he stopped for repairs.

In the scheme of Fig . 3.2 the resi-stance in the pulverized fuel path from the mill to the burners is overcome by the. head developed by the forced-draft fan, so that the pulverization system operates under a slightly ex-cessive pressure (stLpercharged); the pressure upstream of the mill is 1-2.5 kPa. An essential condition for the safe operation of the pulverizati-on system and for maintaining the required .cleanliness in the room is that the equipment must be kept per-fectly tight.

The direct blowing system has cer-tain advantages: it is s imple, the pul-verizing equipment is compact, tho consumption of electric energy for pust t ransport is low, and the fuel a

supply can easily be automatically con-troll ed . . .

The puhcrization syslem with closed fu el 'drying an~ intermediate dust bun-ker (Fig. 3.3). ,A characteristic feature of this system is that the prepared pulverized fuel is separated from the transporting air in a eyclone. The dust is directed into an in termediate bun-ker from which it is fed b y special feeders into pulverized fuel pipelines. The mois tened air at the exit from t.he cyclone hap a temperature of 80-100C and contains 10-15% of the fin est coal .dust. This air cannot be disch arged through the stack and for this reason it is blown by the mill ex-hauster into the primary air duct to be distributed among the pulverized fuel pipelines {Fig. 3.3a). The number of pulverized-fuel pipelines and dust feeders is eq ual to the number of bur-ners in Lhe boiler furnace.

Due to the provision of the inter-mediate bunker, there is no need to match the productivity of the mill with that of the boiler, thus each on them can operate at the optimal load_ The mill exhauster forms a negative-pressure in the system, which avoids dust ejection to the surroundings. AUt inleakage through places where the-system is in contact with the surroun-ding atmosphere {raw coal chutes, dust chutes downstream of the cyclone)' is prevented by means of flapper val ves which open only for a short time to allow the mass of fuel that has accumulated on a valve to pass through.

In the combustion of low-active-fuels with a low yield of volatiles~ the temperature of the pulverized fu:.. ol air mixture should be raised to fn:-ci litate the ignition of the dust. This is achieved in a system where hot air is supplied together with pulverized fuel (Fig. 3.3b). In addition to the pri-mary air which is fed into the pulver rization system in an amount of 15-25%, another portion of hot air (20-25% ) is directed into the air duct and then into the pulverized fuel pi-pel iues by an auxiliary hot blast fan:. T n this case, the temperature of th~

I

I

36 Ch. 9. Fuel Preparatton aJ Power Statton

7

2f

4 20 fj

17 to

fl

f9 5 26 12

N

( a) 7 28

f 29 21

20 fj f7 2/l 27 2F 9

!0

23 32 f9

Jf 24 13 f2 30 'f4' if

(b) Fig. 3.3. Individual closed pulverization system with intermediate dust bunker

(a) wltb pulverized rue! carried by drying aecnt; (b) wttb pulverized fuel carried by ho t air and with drying agent discharged Into boiler furnace. Items 1 11 as In Fig. 3.2 additional Items: 19- lueHirying device; 1!0- Cbutc for rctur_n ol Coarse fractions; 21- Cyclone; 2.11- pulvcrlzcd fuel hunker; 1!3- pulveri1.Cd fuel feeder; 1!wl4-11 mlxer1. 25- pnmnry uir duct; 26- mlll ventilator; 27- revers ible screw feeder; 28- molst drying agent ' fuel mcs; 29-!low meter; JO- valvc lor admitting cold air; J1-hol blast ran; J2- dlscbargc burner

pulverized fuel air mixture is close to that of the hot air. The quantity of

~ir supplied to the burners turns out, however, to be insuificient for complete fuel combustion. To correct this, low-temperature moistened primary air with a slight concentration of fine coal dust is fed from the cyclone into either the combustion zone through special discharge burners or into the an-nular channel around the m ain bur-ners.

In the system described above, the load of tho steam boiler is controlled by the dust feeders by using the re-serve of pulverized fuel in the bunker.

Usually two such systems are provi-ded for a boiler . Their productivity in terms of fuel is 15-20% higher than the maximum fuel consumption by the boiler , because of which one of the systems remains inoperative for a cer-tain time. The system can transfer part of the prepared pulverized fuel into the bunkers of other pulverizing systems through a reversible screw feeder. The available reserve of pulve-rized fuel in the bunkers allows short-term stoppage of both mills for inspe-ction or repairs.

A disadvantage of the intermediate bunker system is that its equipment

3.2. Pulverization Systems 37

is too intricate and bulky. Further-more, the system bas an elevated hy-draulic resistance, which increases the consumption of electric energy for dust transport. Tbe storage of a large mass of dry dust increases fire and explosion hazard. Nonetheless, the system can reliably supply steam boi-lers with pulverized fuel and for that reason has found wide application.

The above-mentioned drawbacks of this system become especially prono-

..

unced in the operation of modern high-capacity boilers. In recent ti-mes, a new system of pulverized coal supply has been developed which is characterized by a high concen tration of dust in fuel pipel ines. Jn conventio-nal systems , the concentration of dust in the primary air fl ow is usually 0.4.~0.6 kg per kg air. In the new meth-od, pulverized fuel is transferred by compressed air, wi th a low air flow rate (only 0.1-0.3 % of the total air flow ra te to burners) and with a dus t concent ration as high as 30-60 kg/kg air. Since the quantity of air is not h igh, t.he dust acquires a high fluidity for motion through s mall-diameter pipelines (60-90 mm). In burners, the dust is spread by hot air. The system does not require the bulky, 300-500 mm pipelines for the transport of dust from the dust bun-kers to the furnace burners, sharply decreases the unit energy consumption

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for pneumatic transport and enables one to adjust the fl ow rate of pri