flame safeguard control phần 2 pps

FIG. Sg-SECTIONAL VIEW OF A SINGLE￾RETORT STOKER WITH UNDULATING

GRATES. (Courtesy of Detroit Stoker

Company, Monroe, Michigan.)

FIG.S9-SECTIONAL VIEW OF A SINGLE·RETORT

STOKER WITH STATIONARY GRATES.

(Courtesy of Detroit Stoker Company,

Monroe, Michigan.)

•. -.'

FIG. SO-MULTIPLE·RETORT STOKER (SECTIONAL VIEW OF 1 RETORl).

lively thin sections at fuel baa) over fhe tuyere zones keeps the fuel bed porous. Dumping grates atlhe rear get

where the air is entering. The valleys form aseries at paral· rid at the ash.

leI, active burning lanes down the tength of the stoker. The multiple-retorl slake' was a nalural extension of

The reciprocating grate is built in sections. Adjacent the single-relor! idea. HOWElWJr, its pq:l'Jlarily has waned

sections move in q::lposile directions to cause stroking ac­ 10 lhe poinl where only one or two are sold each year

lion-when one section is moving forward, the other is Ihrou;tloul (he enlire industry. The multlple-relort s1ok.er

moving backward. This reciprocating movement distrib­ was usacl eXlensively tot' burning caking coals, tor which it

utes coal over the grate surface and at the same time is WEIll adapled. Recant successes in this same area by

33 71-97558-1

, -;.••

over1eed stokers, which are much less costly to malnlain,

have just aoout obsoleled multlple-reton stokers.

SUSPENSION FIRING

In susp80Slan firing, pulverized !powdered) coar is

transported to the furnace in an air slream and injected

into the combuslion cl'larrt)er, along with primary air,

(hrou~ 8. nozzle. The nozzle IS usually horizontal, and is

surroune!ed by art.air register Itlrou~ which secondary air

Is admitted. .

Within a fractioo of a IIbcbndafter a fine panicle of pow￾dered coal enters the combustion chamber, the heat pre￾sent raISes 1m lerrperature and distills aI'I' lhe volatile

maUer. The volatiles, moslly hydrocartlOns, Ignile more

easily than the carbon c~1 of lhe COllI. While the

volatiles bum, they heat the remaining carbon particles to

IncandesC8flG8. SecCl'ldary air sweeps past and &Crltls

the hoi carb::ln partiCles, grackJally burning them.

Pulverized coal installatiOnS have lhe high heal effi￾Ciency and quick regulation dJlainable wilh gas and oil,

which are olher examples of suspension firing. They also

represent an efficient method of burning a Cheaper fuel.

The major disadvantage is the expensive pulverizing and

handling equipment required, which results In a relatively

hi~ ~raling cost for mechanical power. They also re￾quire dusl. calchers or precipitalors near urban areas to

keep fly ash from settling oyer the area.

Pulverized coal units are economically feasible only for

plants consuming more lhan a ton 01 coal per hour. These

inslallations handle any type of bituminous coal. They can

handle coke or anthracite in special cases, but it takes

much more power to grind these hard coals. The power re￾quirement also increases rapidly with moisture conlenl, so

the coal is dried as much as possible before pulverizing.

THE DIRECT·FIRING SYSTEM

A variety ol equipment is used to grind and lransp:ln

the coal and inject it into the furnace. Originally, the prepa￾ration pnase was entirely'separate. The central system (or

tin system) consisted of a large pulverizer supplying a

nLR1t:ler 01 furnaces, and had a bin or bunker for sloring

ltie coal 10 await demand. II could operale al optimum ca￾paCity wilhout the need for a back~ pulverizer In case of

an emergency. The coal was a constant.gradeofflneness,

and the burners could be cOntrolled separately with ease.

HOW8ller, the central system had lwo big

disadvantages:

1. The storage bin was a potential fire hazard Sponta￾neCJl.JS iglitlon almost always occurs jf a binful of pow￾dered coal is left undisturbecl for several days.

2. After Sloring the coal for a few hours, caking occurs

due to surface o:.cidation and the coal no 10nQlir flows

fteely.

In the direct·firing system now used, the pulverizer SLP￾piles only one furnace, and has no storagebin. Since there

Is no slorage capaCity, pulverizer operation fluctuates with

load demand ThIs system is simpler, involving less equip￾menl, so It rl!ldJces capltal outlay. II also avoids lhe poten￾lIal fire hazard and caking of lhe bin. The quantity of

pulverized coal in the mill never exceeds a minimal

amount, and piping belween the mill and furnace is short.

The flexibility needed 10 handle a wide range of coal and

load conditions is built into leday's dlrect·firlng systems.

The pufverizing mlJlls the hean of the direct-firing sys.

tem. The funclions of amlll are (1) feeding raw coal at the

proper rate, (2) grinding the COlli to the desired fineness,

and (3) classifying the finished product so oversized parti￾cles are returned to the mill's grincing zone.

In most mills, air performs three functions: (1) it dries

lhe coal. (2) II helps Classify the pulverized coal leaving the

grinding elements, and (3l1t Iransports the finished prc:xj..

uct to the burners. The air is si.Wled by tarceck:lrafl or

negative pressure.

In (he fOlCed-dlalt. arrangement, a fan outside the mill

~lIes air under &nOl..(tl pressure to perform these 3

functions. tr 8 separate air heater is provided, the fan can

be located on the Inlet side 01 the heater and will handle

cold air. When aU combuSlioo air cemes from a single air

heater (general practice in aU but large, central stallons),

the fan moves only heated air. and its size and power re￾QUirements are mUCh greater. In both cases, the fans han￾dle air only, whiCh Is an advanlage, but the pulverizer mUSl

be kepi airtj~1.

In the negative pt&ssure arrangement, an exhauster

fan rl!ldJces lhe air prBSSLl'e on the mill so that internal at￾mospheric pressure can be used. The fan may be com￾bined with the pulverizer proper, or il may be mounled

ellternally. However, il has 10 handle air laden with coal, so

it must be rl.lQQQCl to resist wear.

PULVERIZING MILLS (FIGS. 61 THROUGH 66)

In a typical pulverIzing mill (Fig. 51), a feeder moves the

raw coal from a hopper into a pulverizer at a definite. ad-­

justable rate. The feedet mechanism can be a variable￾stroke plunger, a re1iOlving screw, or a rotaling table. A

crusher-dryer removes surtace moislure from high mois￾ture coals before they get 10 the pulverizer. The pulverizer

ilself crushes Ihe coal into a powder, usually by grinding.

STOR"GE"'N "O'PER~COA~ PIPING

TO BuRNERS

FEEOER

CRuS"ER·

DRYER

C~"SS'F'ERS

Pu~YER'lER

FIG. 61-TYPICAL PULVERIZING MILL.

34

but also by impact and attrition (wearing away by friction). drum, typically a steel barrel with a cast alloy·steelliner.

Air carries the pulverized coal to a cla::isi(ief, which deter­ Steel or special·alloy balls, about 1 fa 2 inches [25 to 50

mines the fineness 01 the coal going to the burners and re­ mm] in diameter, occupy about one-third 01 the drum vol￾turns the ov~rsize particles to the pulverizer. Finally, an air ume. As the drum rotates, the balls are carried part of the

stream carries the classified coal to the burners through way around it and then slide or drop back toward the bot·

pipes. tom. Coal is fed in at both endS at the drum and intermin·

There are several types of pulverizers. Table IV lists gles with the balls. Impact from the 1alllng balls and

typical characteristics 01lhe most common types, which attrition and crushing from the sliding mass pulverize Ihe

will now be·described. -; coal. The pulverized coal exits 1rom txlth endS of the drum,

BALL M1L,L (FIG. 6f) as shown. (In another design, the raw coal enters one enc:t

A baO mill (or tube mill) consists at a horizontal, rotaling and the pulverized coal leaves at the other end.) ..,- . '

TABLE IV -lYPICAl PULVERIZER CHARACTERISTICS

PULVERIZING

MILL TYPE

•

SPEED

(RPM)

CAPACITY

(TONS OF COAL

PER HOUR)

PRINCIPAL

APPLICATION

FIGURE

NO.

Ball Slow

20-25

4.so Abrasive coals. 62

Bowl (SuctionB-) Medium

75-225

4·20 Industrial steam￾generaling systems.

63

Ball-and-Race Medium

75-225

4·20 IndUstrial steam·

generating systems.

64

Roll-and-Race Slow

2().75

55·70 Gentral-station txlilers

lor utilities.

65

Altrition High

above 225

6-32 Nonabrasive coals;

txllh industrial plants

and utility staliOtlS.

66

, ",-•

a Pressurized txlwl mitis are manufactured in larger sizes, up 10 100 tons 01 coal per hour, for the electric utility

industry.

Ball mitis use more power and are noisier than other

types at pulverizers. However, tJ:ley can handle abrasive

coals at less than halt the malnt~ance costs.

l

BOWL MILL (FIG. 63)

A bowl mill is usually a suction machine. An exhauster

keeps the txlwl under slight negallve pressure to draw In

the raw coal Md convey pulverized coal 10 the burners. /4s

the txlwl rolates at a constanl speed, coal Is drawn Into II

and ground between the rollers and grlndlng 00wl.

The exhauster is a ruggedly buill steel-plale tan de￾signed tor handling abrasive materials. A semishrouded

tan wheel wilh so-called ·Whizzerft blades handles lhe

coarser coal particles. This herps !o increase the life ot the

maIn exhauster blades, sInce Ihey only have to corrveythe

finer coal particles.

Bowl mills are also manufaclured for pressurized,

rather than suclion, operation. TIle pressurized mills are

built in larger sIzes (LP to 100 tons ot coal per hour) tor the

electric utility industry. In lhese mills, the rollers are In￾clined more 10 the horizontal than they are in lhe suction

FIG. 62-BALl MILL PULVERIZER. design.

35 71-97558--1