Document XOGNenboN3g4Nw0MzOgj205zK

FS*1 i Mi i-. GRINDING OF COAL occur* between belle and grinding WHIRLING BOWL AND ROLLCRS mounted on pivoted axee ring*. Adjuttable epringe maintain grinding pressure grind the coal thrown between them by centrifugal force IMPACT FROM HAMMERS of first stags and attrition BALL-MILL PULVERIZER resembles a tumbling barrel with -from pegs of second combine Coreas to pulverize eoal rotating drum filled with eoal and heavy ateel belle 5? '4 4 H II :'Sl ;?: *: ! -:t ' * 1 CLEAN AIR A JOB FOR IK! POWIR (NCIN[(R What Part Does Pulverized Atmospheric pollution from pulverized coal comet mostly from flyath. A 200,000-kw plant burning coal of 15% ash can emit up to 10 tons of ash per hr. Here are basic factors affect ing the problem from coal preparation to final furnace design Let's look at the process of firing pulverised coal so we may (1) appreci 8jr L N ROWLEY, Managing tditer, and J C McCABE. Assistant fd/tor ate the origin and scope of the Qyesh problem (2) recognize the remedies and their costs in dollars sod cents-- plete. First, the coal must bo prepared. the necessary air, placed in the proper ss much as 5-10% added to steam cost Our article, "How Fuels Burn1' temperature environment and given What happens when you elect to fire (Powbh, August 1947, pp 71-72) set time to burn. coal In suspension? Within a fraction of up the premise thot no fuel bums un The two basic systems for these oper a second after o fuel particle in suspen sion enters the furnace, beat raises its temperature and volatile matter boils off. Air enters the furnace through the burner to mix with distilled gss and complete combustion. Heat from this combustion process less, or until. It becomes a gas. Coal, a solid fuel, requires considerable prep aration because it Is a solid and differs in composition Irotn one mine to another. Pulverizing Systems. Pulverizing or breaking up raw eoal to a powderlike ations are (i) the bin-ond-feedar (2) the unit or direct-fired system. The older of the two, tbe bin-ond-feeder system, has its major importance In firing cools like anthracite. Its most serious disadvantage is a heavy capital outlay; usually a sepa lifts the -temperature of the remaining substance is one way of preparing cool rate mill house for drying and pulver fuel particles to incandescence. Sec- . for the combustion process. It can be izing equipment, plus conveyors, bunk ondary air admitted around the burner likened to the "atomization" phase of oil ers and feeders for fuel delivery. opening mixea with the etreom of glow bunting in that the surface ares of the But the bin system has Important ing coal particles. It sweeps past them fuel to be ezposed is greatly increased. advantages: (1) coal preparation Is in to strip or scrub off successive layers of Ones fuel is in this form it must be dependent of furnace operation (2) CO and COt until combustion is coo- transferred to the furnace, mized with pulverizing mills can operate at optl- Coal Firing Play in Air Pollution? oiujd capacity to give uniform fine ness (3) large storage bins net as reser voirs sgainst outages-(4) use of many burners, individually controlled, gives a flexible furnace, permits adjustment to give optimum combustion. Wbst about the second, the unit system? Todoy's unit does four jobs: (l) pulverizes raw coal (2) dries it in the tsme operation (3) sifts or sepa rates the already putverited coal within (he mill (4) transports this powdered coal to the furnace. All at a large cap ital saving. Secret of this saving is a preheated sir sweep admitted to the pulverizing mill. The warm air Is the drying ele ment, sifting agent and transporting medium. This 3-fold function is not without disadvantage. Each of theae op erations has one optimum ratio between sir and powdered coal for best per formance. The same amount of air for II jobs means simply that the choice is the proverbial happy medium. Transporting Cool. The bin-ond-feeder Arrangement for pulverizing gener ally employs a worm-ond-screw feeder to push coal to the burner where il<be comes air-borne. The unit system uses air sweeping through the mill for its transporting medium. Air-borne fuel, actually a dispersion of a solid In a gss a steam jet for the pulverizing force. and not too generally understood, can All strive for uniform fineness si high bo likened for practical purposes to a throughput and low power consump liquid. tion. High moisture and ash contents Burner design follows the liquid con for a given size output; plus a low cept. (ts job is to mix tbe fuel and the grindability index in the raw cool primary or ignition air- (sometimes the undergoing pulverization, conatitute the secondary air, loo) and then introduce limiting factors on mill capacity. The them to the furnace with, often, a degree of fineness achieved in the pul turbulent motion, not too unlike the verizing process directly affects avail function of an oil burner. In addition, able surface and, in turn, ignition and burner design has to do with flame combustion. shape and combustible travel time. Preheated oir dries, classifies and Once the fuel-air mix reaches the. transports powdered coal to the burner, furnace its travel and combustion differs in the direct-fired systems. If it were with furnace design. This* design fol always possible to use this medium as lows one of two principles--the dry the primary or ignition air os well, it bottom or the slag tap. The names would simplify combustion. But this is come from the means by which osb is not always the case. removed. In (he slag tap, employing In'general, as (he volatile eonlenl higher furnace temperatures than dry of a coal decreases, the degree of fine bottom, osh is kept molten so it flows ness (and often work of the pulverizer) from the furnace. To keep it molten increases while the percentage of prl- many designs direct burner flame into - mary air for ignition purposes de or toward the ash. In dry bottom de creases. A rule of thumb places sign the effort is directed toward tem the percentage of primary air equal to peratures below slagging range. that of volatiie<ontent for anthracite. Major Equipment. Pulverisers em Sometimes this percentage of optimum ploy impact, grinding, rolling or crush primary air is so low that any single ing action to powder coal (see photos). volume of sir capable of performing all One recent design, reported capable of the functions of a direct-fired system ratings up to 10,000 lb coal per hr, has far exceeds this optimum figure. Only 76 |M) POWER January 1946 pOWlR January 1946 (151 77