Document RjywVx8yQyx01aeGzGqonZm1E

1 SPREADER-STOKER FURNACES EMP|; PK ~Ci| of"riVu" rn`cool.. Choice ol luolW Coots dm0oPo'"t,d'thoo upoo chortolwcln.rlin.itga ,?tn Want Your Overfeed Stoker to Here are paintert that you can put to work to insure combs lion without smoke. What's more, fundamentals of theory or operation are included for overfeed stokers--chain, Irovolini grate or spreader--that add up to helpful operating know-how By l N HOWirr, Managing Iditof. and J C McCABE, A.ilifonl CdAor Wat rack tw the August 1947 issue of Powsn when we talked of "How Fuel Burns" (p 71), we set up two fuel burning groups for solid fuels: (I) sus pension and (2) fuel bed. An article on the first, suspension or pulverized* coal firing, appeared in January 1948 (P 76). Let's look now at the second, fuelbed firing. It con be done by introduc ing coal mechanically with stokers, or by hand. With stoker firing, two major 6B (1501 methods of supplying coal are em ployed--overfeed and underfeed. By far the majority of overfeed stokers in power-plant service employ the spreader, chain or traveling-grate arrangement. Stoker Firing. In all stokers firing proceeds in four steps: (1) Coal U pushed, dropped or thrown Into a hightemperature region within a furnace. (2) A pan of the coal is distilled off as a combustible gas (mostly hydro carbons and CO) and burned above tbs fuel bed just Uko o gaseous fuel. (M Remaining solid or coke stays red h* and experiences a vigorous scrubM0! by air until completely burned. (*), Ash Is removed from the furnace z*\ Spreader stokers, Pig. 1, take rt* coal (within definite site limitation), whirl it into suspension within tbe furnace by (1) pneumatic or nonies or (2) paddles or wheels ^ throw the cool supplied from so oTtr t POWEft Msreh lf4*- into medium-sized installations; con only. Cut off forced draft and stoker tinuous discharge or traveling grates, speed. Maintain a slight overfire draft into Jorge-sized steam generators. On low-load operation, remember rate Dumping grates require certain con of ignition drops, smoking coo develop. ditions for best results. For instance, How about draft? Air at low pres the grate should be power operated. sure and at temperatures below 300 F Furnace should be designed (n sections, (lo avoid any coking problem) passes that is, giote-iuriaco sections should through the grate. Overfire draft Is have separate forced-draft supply, grate deliberately held low to prevent de- and ashpit. This arrangement permits ' veloptnent of troublesome gas veloci cleaning one section of grate without ties. Air introduced by jets (see be interrupting or disturbing firing on low) at the overfire level is usually held neighboring sections. Continuous discharge units get down to 10-20% of the total supplied for combustion. Its (unction at this around the problem of Interrupting level U one of mixing. Steam jets steaming capacity for ash cleaning by achieve essentially the tame result. removing it continuously and automatic A low overfire draft and low gat ally on a moving grate. velocity improve operation. Here is Class of Fuat. Spreader stokers fire why. The thin fur) bed offers little e wide range of coals (from lignite resistance to incoming air ao cinder through bituminous). Anthracite--that and flue dust are ever present in the is, 100% anthracite--has not yet proved furnace gas, and more so with high successful as fuel. It piles up in front drafts. Somewhere between 30-50% Other coals, however, as well as of the ash content of the coal goes off many byproduct fuels like coke breeze, with the gas stream leaving the furn burn well with spreader-stoker firing. ace. Furthermore, combustible in this Siriog of fuel is highly importont Top same flyash-and-cinder mix constitutes size, for coal, is about 1*4 in. Coarse as much as S0% of its weight. It all sizes wear the feeder, overload and points op the need of (1) low drafts COHT1NUOUS DISCHARGE ol osh on clinker on the grate. Preferred sice is (2) s cinder collection end return. raring grata olds large itoemlng units 44 in. x 0, nut and alack. Site consist mtut be uniform. Segre With some, collectors ere imperative. Cinder Retnjaetten. This system com gation of coals can raise hob with the prises (1) settling chambers somewhere usual thin fuel bed. Avoid it by (1) within the boiler setting (2) a cinder maintaining hopper half full at all times collector usually before the induced- (2) filling hoppers by mechanically draft fan to save wear and tear on (an imokefree? operated spouts, traveling tarries or nonsegregating chutes. So much for fuel and feed, now for blading. All matter collected in the sys tem is fed by mechanics! means to the' , furnace for reignftlon and burning. firing. Fuel-bed thickness, grate speed, Returning cinder usually enters at a Tips lor Smokefree Operation *n Wetcb hol-dio comlit taM.iogregcHoa si fuel . Keep fuel bed level Held overfir* draft low Keep fir* and aibp't clean !faride eneagh fumoco velum* " Eiteblhb turbulence Guard agolott olr loottago : Whb tgrwden, bm centrals forced draft vary with steam demand and the particular coal. For instance, a high-ash low-fusion.teraperaturo clinkering coal requires even distribu tion, uniform size consist and thickness on a thin, undisturbed fuel bed. In general, keeping the fuel bed level and thin gives good performance and low maintenance. Ccod controls help achieve this. A heavy fire clfnkers end smokes. Avoid particularly a heavy fire at front or bridgewall. At the front point abovo the fuel bed (but not too close to the bailer lubes) under the impulse of air or steam jets. These jets give sn added overfire turbulence that promotes good combustion. One operator in a large power plant believes in mixing the collected fines with the incoming coal in the stoker hopper. This procedure bos two oper ating advantages: (!) uniform fire bed (2) reduction of fines in suspension by holding the recirculated dust at fuel- it causes overheating of arch, and at bed level. the bridgewall clinkering develops Recirculation of cinders snd flue dust *d or underfeed hopper source. Fines *d. tobuni in suspension, larger sites rep to the fuel bed. The fuel bed may be one f three f^to--sutiontry, dump or continuous ^ie. Fig. 1. Choice of grate type spends more on steomlng require- "Ws and less on coal characteristics. Stationary grates prove satisfactory tmal]<apaciiy boUen, particularly 7*9 deploying low-ash cools on band? Ignition. Spreader stokers k% a class enjoy the reputation of quick starters. Even with a cold setting, pressure builds up fast So much so that it may exceed, the rale considered safe for expanding parts of the boiler. It follows that the ipreadcr-staker fire comes up quickly on a live-bank. But banking is something of a problem. Be sure to leave e layer of ash on the grate. Cut down air Sow, reduce feeder has strong advantages: (1) reduces stack diocharge and, therefore, "the smoke nuisance" (2) increases over-all efficiency of the individual steam.generator (3) reduces maintenance on induced-draft fan, rotor and casing. These advantages apply to travelingand chain-grate aa well as to spreader- stoker installations. We ere now ready lo score the points where smoke trouble could develop. 1159) W