Document p24Bd8EN0N1zxzJXDBjMBo3NE

tTHESf PULVEltlZERS^fUMBiE COAL y.- ---ssssvw' -.v:^ '.v: COAL IS GROUND BETWElN MOViNG SURFACES IN THESE MIltS V \J> . V* ;V-`; / 17, lV*V' i t ' vj-JJ-*'. | *;tatl-mill pvhrwUcr' rsiwnblci a rotating Avm filled villi teal end many fyiv.riiatl.fl Mfc. plan b(w,wvbali,`and grilling Hng*! ,-t,. Wflriiflg ifri r.il~,n^fl.i<, 01. ptTMii lfl.rfrifl . ; Mluuu, maintain grinflln, f..>.at. aod fliinny..'.; to aflow'd^ tooltfcra.a tmtwnmi tli.m ontrit.ol tar.. uninterrupted and uniform aupply of raw coal la a magnetic teparator iomt* where ahead of the pulverizer feeder. It remove* tramp iron, which might block the feeder. Usually a pulverizer does its job best when supplied with row coal having definite lire limits. These vary some what with mill lire. In general, imall mills do best on coals up to about V<* In. top sire. For medium end lorge mills, best top sire goes up to about 1 in. and IV4 in., respectively. For ony spe cific mill the manufacturer's recom mendations should govern. FtoeasM, Quality of mill output, or fineness, is a highly important factor. It U normally expressed as the percent age of pulverised coal passing through a sieve with openings of a specific sise. Commonest sizes for pulverized-coal testing are 50-mesh for determining oversize, end 200-mesh -for powdered dust Number of openings per lineal inch Axes the mesh of a screen. The 50-mesh sieve, for Instance, has 50 openings to the inch, or 2500 to the square inch. Fineness of pulverisation obtained from a mill depends on a number of factors: Orindability, ash content and these. The fineness desired likewise de pends to greet extent on coal char acteristics and the woy they affect behavior In the furnace. For example, coking coals, when exposed to furnace temperature, swell and form light weight, porous coke particles. These can easily float out of the furnace be fore they are burned. As a result,' car bon lots con run high unless pulverise, don is very fine. Free-burning coats do not have the same swelling charac teristic and hence do not require the some fineness. flfart of VoloHlo. Again, high-volatile coats Ignite more readily than lowvolatile. And sinee ease of ignition dictates somewhat the degree to which Aneness is carried, it follows thot highvolatile coals os a cists need less-fine putverisstion than do low-volatile coals. On the other hand, low-volatile coals, with the single exception of an thracite, have higher grindabilities, be cause they ere softer. The balsnee of these compensating factors determines results in any given ease. Before turning to any consideration of fineness specifications, it would pay to see more exactly what the term means. Coal from a pulverizer consists from coarsest to infinitely fine, distrib uted occording to laws of probability. Plotting amount of each size yields e typical frequency curve, with a peakat some particular site. Shape of the 4 curve end sire occurring in greatest ` amount depends on the coal and the pulveriser. In power plants, prime purpose of pulverizing is to eliminate oversize. No matter what the percentage of coal that will pass 200-mesh, os little ss 5% over, the SO-mesh limit may result in slag ging and increased combustible lossThat is why most specifications call for at least 98% through 50-mesh. Itoownicol fUwnati may be defined w the point where total cost per lb of steam, including fuel, operating sod capital costs, becomes s minimum Ir respective of boiler efficiency. Thii recognizes the fact that reduction of s small percentage of coarse particles. obtain complete combustion in furnace, may require on increase Is operating and capital cost that is out of proportion to the value of the I* In efficiency. Mill Capacity. What about factors af- fecting mill capacity? Just as with fin** ness there ere several. Mill capacity grindobility. Ash and moisture content of the coal, and fineness desired, may play important parts. r. A basic premise in setting mill ca, parity la the assumption that enough heated air wilt be available to dry the , '* coal. If this is not the cose, it is the drying operation that limits mill output, not grinding. Thus it may be possible to obtain more capacity with s dry coal of lower grindsbility than t with a wet high grindsbility coal. Nevertheless, a knowledge of grindsbility properties Is helpful Informa tion in rating a pulverizer. In recent years manufacturers hove developed correction factors that take into acV count influence of coal properties. Drying cot. Let's look at one more problem common to ell pulverizing before examining specific mills. That common problem it moisture end tP*c(ficaUy surface moisture. Inherent moisture, thot it, moisture actually within the coal, does not greatly 'fleet pulverizotion. Drying of coal undergoing pulverization, then, need be enough to remove objection. 'We surface moisture. The extent of drying that con be done within a given mill depends tomehot on its design. For example, most mills are arranged so incoming wet coal mixes immediately with the drier coal inside. Without this quick mixing mill feed end may plug with wet eoel Primarily, though, the amount of heat that can be introduced measures the success or failure of the drying step. Where air flow acts as the drying medium (the most usual arrangement) the mill must be able to handle fairly large volumes of high-temperature air. Conditions within the mill should bo held at a relative humidity below that of saturation at mill outlet tempera ture, roughly 150-160 F. Air Supply. With today's steam-gen erating units, a supply of preheated air at about 500 F or higher, depending on coal moisture, can be obtoined readily. The mill should be so constructed that it can handle air at this temperature without need for tempering to avoid preignition with dry coals. We arc now ready to look at typical current pulverizer designs. These em ploy three basic forces, singly or In combination: (1) impact (2) attrition, and (5) crushing. The nature of these forces becomes apparent when their application is seen. Sail Mill*. The ball mills of Fig. 3 and 4 ore essentially horizontal rotat ing cylinders somewhat longer than they are round. This physical shape leads some to describe them as tube mills. Usual design employs cast liners for the interior. Steel or special alloy bolls, ranging from 1 to 2 In. in diam eter, are dumped into the drum and fill it to just under half full. The mill rotates slowly, depending on tube di ameter, but speed is of the order of 20 rptn- The bolls carry part woy around the cylinder circumference, then slide bock toward the bottom. Coal, inter mingled with the bolls, undergoes im pact from falling balls end attrition and crushing from the sliding mass. CtuMlfytflp Action. Hot air passes through the mill to dry coal, classify It after grinding, end transport finished produet to burners. Operation of a typical classifying arrangement can be seen in Fig. 5. Passing through mein vertical body of classifier, coal-air stream undergoes sharp change In di rection, which throws out overslxed particles. These tailing reenter the mill with incoming raw coal. Coal-air stream leaving classifier carries only coal that meets desired fineness limits. Opening the bypass damper In the seroll of the classifier increases coal output end reduces fineness. moisture of the coal moy be some of of s continuous band of particle sizes does not voiy In direct proportion to 1 >0W| December 1944 (755) 91 90 (754) |'r*. ZOWEl , INI