Document 1QdqLxNbB22pqMw98edZ1yRxX

Am ; ;&:.-V :.; x;; ? m y*f1o|.:bwm*fi.dlr*rt"*ol-biidrelp -mis toward fwikica V' TWOT1V7$%gMa .'5P' " '' ' '*' '-IwSjlT'r.t ArSE^o If v':\.i., no , **oW%'flri* "P1*1** alMb.imgM to 1mo!iM*)Mlrtt - .1' CytioA* bumr..whirl* ctvi*id.<al apcii*t'4ailb! v Ii"/'- A. */* lonfroU rap*** h.r.by ramming ohlQh p*rcirrfflfl* ttfndllflt more than one fuel. Many e!fiu bav* ** developed Pig. 19 !ib0* a unit for coal and oil in which primary, secondary ond tertiary air, wider separate control, are used for iiBkdtot firing. Many other combine* .{leas arc available. ' ifaltlBA iqvlpmant. Becouae theae tMJOera are designed to give peak per* (armance at or near rated burning ca* pprjri-- end fall off at low loads, the problem of lighting off is a major one. Usually. *n auxiliary oil or gas burner, I electiieeUy Ignited, la employed for this purpose. The unit of Pig. 21 has a [.ffiechanical-atomiring oil burner. Both burner and igniting electrode* are re* tractable. In addition^ a safety, inter* y^yiwg system prevents oil flow until burner end.electrodes ore extended. AIR RCQUIRCMtNTS Having seen something of the form that pulverized*coal burner* take, we're now reedy to look at some of the rcla* .dons between burner end preparation system, and between burner and fur* nice. Remember that today's unit sys* tern depends on air for drying, classify* log and transporting the pulverised coal. -When that transport air reaches the burner it becomes, willy-nilly, primary sir unless it can be tempered er a portion bled off. Air Supply. If amount of air needed [ffor operations In the mill and that needed at the burner were the tame, -B would he simple. Bach function \'. performed by the air, however, has a v ratio of air to eool that Is best for r'thst function. Hence some compromise b necessary. ?or example, amount of primary air for best Ignition varies [-with the amount of volatile matter In the coal. Rule of thumb puts the per* eentagt of primary air equal to the Percentage of volatiles. With coals haring a low-volatile content, volume t air needed (or other jobs in unit *y*tems may exceed that for ignition. This, for example, is the cate with anthracite. Successful unit-system tuning of anthracite hinges on remov ing excess air from the transport sys tem before It enters the burner to taome primary air. Modem direct* tad tnthraeite designs thus bleed air. Let's assume now that the burner , Oliver* fuel and air In proper proporY hens lor Ignition. Here the furnace en* j`ta tbs picture, since It must supply ' tat sufficient to evaporate ell mois* tae, distill all volatiles, and raise the *nflre mixture to ignition temperature, tad It must do all this in os short a ta* as possible, bo as not to take up ' b 'arle a fraction ol tbe available ^ *taslt time, or furnace volume. ' .**$ lor ignition. With most coals, tat for ignition is no problem. Usuel* ly, the task is to remove effectively the large amounts of heat liberated and to handle the high temperatures result* ing from radiation of incandescent fuel particles. Extensive watercooling is the answer. But with low-volatile fuels, particularly anthracite, the balance be tween heat removal and heat "reflec tion" must be carefully maintained. In furnaces for such fuels, it Is cus tomary to provide some refractory sur face, strategically loeated to insure stable Ignition. Where does the burner figure In all this? In two ways: first in point of ignition and second in speed and com pleteness of mixing. Point of ignition depends on mixture velocity. As a lower limit, this must be greater than speed of flame propagation or, as we learned earlier in gas firing, flash-back may develop. As on upper limit, velocity cannot be so great that incoming mix ture has time to become so lean or diffused with secondary air that fur nace heot Is not enough to Ignite it Control of Mlaiag. The requirement of thorough, complete end rapid mix ing of fuel and air is fundamental. But even beyond this is control of the mixing elements thot affect flame trav el. This control ol flame travel touches on an inherent weakness in pulverisedcoal flring-the difficulty of influencing the "tail" of combustion, as Can be done in fuel-bed firing. It is for this reason that considerable attention is given to the problem of creating tur bulence and aiding this turbulence by burner arrangement Combmtiblo Lot*. Even under tbe best of conditions, as combustion progress es unburned combustibles experience greater difficulty in finding available oxygen. For this reason, a high degree of turbulence becomes of prime lm' portance where air and fuel keep con stantly changing In physical relation ship to each other There Is always a certain amount of combustible loss, the majority of which, in pulverised firing, goes up the stack. Its amount varies with percentage of excess air and ash content of the fuel. As ash percent age increases, percentage of excess air must algo increase to hold combustible lots at a Axed level. Eventually it be comes a question of balancing loss in combustibles against dry-gas loss. Ash and Slag. Ash enter* the furnace picture in another way. Temperatures realised In burning of most coals ere well above those at which ash fuses. Coal burned In suspension, then, has Its ash content at one time or another in a molten state. It Is highly desirable for the ash to be dry before it enters the gas passages of the steam genera* tor Transfer of heot from the suspendcd particles by radiation to tbe furnace surfaces is the only way of cooling tbe ash, and it takes time. Con It be speeded up} Yes, fully water-cooled furnaces ab sorb heat faster. But time for absorb ing heot usually exceeds combustion time. This time, In suspension firing, is roughly equivalent to distance. So the distance between the burner and entrance to the boiler surface U a func tion of the time required for heat ab sorption. The magnitude of this time, or distance, is largely determined by aah content of tbe fuel. DRY-BOTTOM AND SLAO-TAP Ash formed under good combustion conditions contains iron In a highly oxidised form and haa a higher fusingtemperature range than indicated by laboratory conditions. Fineness of pul verization and good combustion reduce carbon content of ash and so tend to ward higher. oxidation of Iron, thus helping to keep ash from slagging walls ond tubes of dry-boltom furnaces. llog*Tap Sumac**. We may, however, elect to keep ash molten by use of a stag-top furnace. Part of ash is re moved from the combustion cone by directing burner flame generally down ward over a pool of molten slag. These fumaees ore suitable for high-heat re leases, and tend to remove the lowfusion-temperature elements of the ash, so less slagging occurs on walls and tubes proper. Slag pool must be kept hot enough to remain molten so slog flows continuously over a watercooled weir or ring into a quenching pit, where it is broken up by water. Cyriom Surnor. In this new furnace design, what might be termed, "delib erate" slagging Is employed. This burn er, Fig. 23, receives crushed, not pul verized, coal in a stream of high-veloclty air tangent to the circular burner housing, which forms a primary watercooled furnace. Coal thrown to the rim of the furnace by centrifugal force and held by a coating of molten ash re ceives a vigorous scrubbing from fast* moving air. Secondary air enters at high velocity also, and parallel to the path of primary coal-air mixture. It might be said that tbe coal in the sticky slag film bums as tl it were in a fuel bed--volatiles are distilled off and carbon is burned out to leave oah. Combustion of volatile matter begins in the burner chamber, and is com pleted in tbe secondary furnace into which the burner chamber discharges. Molten ash, under centrifugal force, clings to the burner-chamber walla. Slight Inclination causes slag to dis charge continuously. Nature of this burning tends to reduce substantially amount of ash .carried in suspension, hence flyash emission is negligible. December 1946 (7611 97