Document dG46r6JYevNEZDJ1y900w7k5

1 "*ea ln,,oduc* c"l Ifcrouoh o central ocllon, ildawatdt by gravity and aresiure al feeding trough. The caal mavel along the furnace by tom Air lupply panel through grate, and^fuel bed ' 2HlMs-ond-vallBy contour character izes the multiple-retort fuel bed. Heovy blits of cool cover retort sec tion. Thin vollevs He over tuyeres 3WaWraoong of underfeed stoker's fuel bed (s sold to O) extend the fuel flexibility (2) redueo tho mainfenonee <31 reduce the clinker formation i Underfeed Stokers Are Meant CLEAN AIR Do your pari am) they'll do theirs. Underfeed stokers are jrjl herently free of cinder and flue*duit problems, but poor oh oration or overloading upsets this condition. Here are fora you need if operation, fuels, firing are to stay troublefrey l N ROWLEY, Managing tdllar, and J C McCABE, Attltiani Ou^ last article looked into fuelbed firing (Powm, March 1948, p 88). Of the two mechanical forma--over feed and underfeed--it dealt with over feed firing only. Underfeed stoker*, which wi consider here, represent one of the more widely used means for firing bituminous coal. To aet up guides for good, smokefree operation, let's take up (1) the stoker itself and (2) the combustion process. Underfeed stokers come in two broad classifications--the single retort and the multiple. The single -retort works with units up to a steiming capacity of 25,000 lb per hr, and sometimes as high os 40,000. Multiple-retort units are in service for capacities from 30,000 up to 500,000 lb per hr. Single-retort designs follow three basic forms. All employ a central re tort, Fig. 1. Two have stationary fuel grates on each side of the central retort, and the third bs9 mechanically actuated moving grates on either aide. Types one and two differ'principaily in ash removol. The first, now em ployed on small units only, has dead plates at the extreme sides. Ash is re moved by hoe through the furnace doors. The second has uh-dnapfs/ grates at these extreme sides sod ooA-i lecting pita under each grate. i] The third typa with moving teh burning grates employs aih-duoplol or discharging grates al the ext/to* furnace sides and asb-colleetiog pits j One word of eautlon on fuels: Daohkj screening to reduce fines Is adriiaM*4 for single-retort stokers equipped wftk stationary grates. '] Here's how combustion takes pi*. The central retort serves as a feetol' trough or supply channel far totfj'f ducing coal beneath the fuel 84 1284} POWER Mir ') ply to perform some drying of wet coal There is a certain amount of tone- feed for incoming air. It is accom plished by various air-chamber arrange ment* or hollow grates to give an even fuel-bed distribution. But the majority of the air goes up through tbe fuel bed close to the retort. At this point it serves the useful purposes of. promoting (1) rapid Ignition (2) maximum mixing be tween air end rofetile released from the heated raw coal. Some installations put overfire nozxtes in the rear or front walls for added turbulence. The xone of active combustion covers ell the grato area. With narrow stokers (he active cone reaches the area directly above the retort itself. Usually, though. Ignition starts where the coal passes be yond the retort boundaries. Burning . progresses as coal travels towards the Spotted here ora vortou* equipment points to chock In keeping oHher single 'or multiple-retort stoker* operating In the woy thot they should smokofroo yA rtm, plunger or movable-retort bot* l-tao with the aid of secondary recipro- aotlag elements (or in other models a ****-leed system) pushes coal rearp*ini and upward to the fuel-burning fPstex Coal feed is usually varied by |jua|ing Ume of feed stroke and leavRm Icqg\b of stroke always the same. E' coal leaves the retort and lf*ls aver the grates toward the side walls. With stationary grates its move ment depends in pari on gravity and for the rest on tho pressure of addi tional incoming coal. Mechanically moving gratos add to these forces. Air supply passes through the grates and fuel bed under pressure. Since the gntes usually extend into the retorts a short distance, incoming air alto passes through the top section of the fuel sup- furnace aides- > Clinkering difficulties can develop along the sidewalls at the grate level if a cool having s lower ash-fusion tem perature than the design one is substi tuted. Wateroooled sidewalls reduce troubles from this source and thereby extend the range of suitable eoala. Furnace site should be ample for shortest flame travel. Furthermore,' boiler and stoker should be coordi nated. Shortest flame travel runs about 4 ft (or small stokers and up to 8. for large-sited ones. An allowance of an