Document gDGZ53mj48ozD0j8d8vbJRKvq

stage of the actual design. Fuel prop erties and plant design tie-in at man/ points, as diagrams on poges 110 end 111 show. Attention to this angle at the drafting-board stage keeps some small limitation from creeping into the plans. Such a restriction might look big when the time comes to operate the plant. SPENDING THE FUEL DOLLAR _So much for fuel selection at a factor in plant design. Let's turn now to the problem of buying fuel for an existing plant--one that was, in all probability, designed without benefit of a fuel turvey. "Needle-in-the-hay-stock" pur chasing leads to high steam cost and Operating headaches. Responsibility for proper selection rests squarely on the shoulders of the power engineer. He is the only man. aside from the inde pendent fuel consultant, who under stands all the facts needed to spend the fuel dollar wisely. The purchasing agent can help with his knowledge of market conditions and buying proce dure, the salesman and fuel-service engineer can supply facts about their fuels, but the power engineer must coordinate these with plant require ments and make the choice. Because coal is a complex substance, selection cannot be reduced to a simple formula. What follows is an outline of a sound procedure which can be followed in most plants. Because it Involves a minimum of testing end a maximum of "horse-sense," It comes closest to meeting small-plant needs. The right coal for any plant b the one that will carry the load, meet all reasonable operating requirements and produce lowest over-all steam cost. While any coal can be burned in any plant, after a fashion, every practical engineer knows that many plants have design and equipment limitations that bar certain-coals, either because they will not carry the load, because relia bility will be seriously impaired, or be cause intolerable operating difficulties will be encountered. To eliminate a lot of unorganised triol-and-error testing, the power engi neer must examine every part of his plant from coal pile to ashpit and stack, with but one thought in mind, to locate ond evaluate every factor affect ing coal selection. Even the engineer familiar with his plant will find such a fuel-engineering analysis reveals limi tations he wasn't aware of or puts the finger on the exact cause for some limi tation only vaguely recognised. Of equal importance, he will And that some limitations thought to exist will disappear on close examination. By removing false restrictions on coal choice, the analysis may actually lead to savings from wider buying latitudes. 112 (77a) POWER * December 1944 The check list on thb page gives some Idea of the questions the power engineer must ask and answer. Experi ence and records will furnish some of the answers definitely and easily; others will have to be determined by trial. For example, a tow boiler setting causes smoke trouble et high burning rates with high-volatile coats. The engineer knows this but has no records that would indicate the maximum amount of volatile matter that will permit operation at high rates without exceeding the smoke requirements. A few tests with sample shipments of coal will give the answer. Limiting factors on coal use will take various shapes in different plants; in many cases the limitation will occur outside the furnace. For example, in adequate ash-handling equipment may require putting a limit on the ash con tent of fuels burned. A plant equipped with considerable air-heater surface, which produces low exit-gas tempera ture, may find it necessary to limit sul phur content to avoid rapid and ex pensive corrosion. In considering furnace and firing equipment, it will be found that each factor of the design must be considered In relation to burning rates encoun tered and nature of load; that it, wheth er it is steady or fluctuating. kM example, minimum ash-fusion perature for clinker-free operational Some properties of coal have Import- ^cc'only as limits, never as preferWrL For example, once the minimum an underfeed stoker depends UrgeK''r on combujUon role. Liktwi,,. luls-Sl Profusion temperature for trouble- of load affects importance of cak^r^l qualities of coals; a fluctuating may bar a strongly caking coal-fug! i&I iSte operation has been determined, liny increase has no actual value. T- ^ completed plant analyais boils use on a stoker with inadequate /down primorily to a set of limitations tion of the fired grate. fuel bed, or from a handi^l $`$1 j^pjcoifd as far 01 possible in jLfepetiies that esn be measured coal (ash The engineer must also reraemhii ',7'| >content, fusion temperature, sulphur that one property of a coal may modify- **1 another. Thus presence of escosts^sl fines may aggravate clinkering tendt$'&| cies, producing trouble with a coaT.'^l having apparently satisfactory oxb'\,*)f fcontent, etc.) with properties that still jjdefy exact expression (caking quality, fathering properties) stoted as clearly pas possible. Armed with this analysis, |tbe power engineer can tackle the list fusion characteristics. >!v'*u| Finally, in making the fuel ana]y&;^| Kof coals available, eliminating immeidlstely those that foil to meet mini- of his plant, the engineer must diitU&^l guish between limitations and preferiVJl ences. For example, in a plant withi^-vl barely enough pulveriser capacity,-UV: <J| grindabillty drops below a drfini^Wif point, the load can't be carried; coals*,'/-* with lower grindabilitlei ore worthleajv.;' However, If pulveriser capacity is :l *mura requirements. ` At this point relative prices enter the ypkture. The list of coals remaining bsfur the Ant elimination should be xict up on the basis of detivered-cost- fper-Btu. This will Immediately permit d.i second elimination, divided into two i-psrts: (1) of the coals meeting all -requirements, eliminate all but the best pie. the engineer may still prefer highjJ /.j grindabillty coals since grinding cab'll will be somewhat lets. Saving in pewe'r v Jtwo or three from the delivered cost Kit#ndpoint, and (2) from the coals ?exhibiting one or more undesirable consumption and cost of rcplscirg't-Ttf wearing ports will Indicate how con be paid for higher grindabillty. ^qualities, eliminate those where low /cost seems insufficient to compensate ifor undesirable properties. Note that up to this point the only testing required hat been of a simple sort and that even such trials as men tioned for setting the upper limit of volatility are only needed where plant records and observations are lacking on a particular point. Elimination can be carried even further without need for fuli-fledgcd evaporation tests. Bach of the coals whose cost per Btu is low should be studied to determine how much weight must be given to the un desirable properties which pull its price down. High ash content may make no difference in a plant where ash can be sold; in a plant where removal must be paid for every Increase in ash con tent involves a definite cash outlay. Thus, more coals may be eliminated. Por the smalt plant without test in struments, this may be enough. What looks like the best value among the remaining cools can be used for a short period. If it behaves satisfactorily and there Is no apparent Increase in oper ating troubles or maintenance It con be used with a fair degree of confidence. If the plant has equipment tor evap oration testing, plant trials will help verify the analysis up to this point and further sort out the remaining coals. By avoiding hit-or-miss plant testing, a large number of coals can be given consideration and the necessary careful testing can be given those worthy of it, all without excessive expense. To be worthwhile, plant tests mutt be carefully made and the results must be analyzed properly. The errors in volved in casual tests will often exceed the differences In coal quality. Every effort must be made to keep load con ditions the same from test to test Boiler surfaces should be equally clean. Finally, each coal should get equal at tention and skill from the operating force. This does not mean each should be fired the same way; it means each should be Ared to get best results, al lowing for its peculiorities on the given equipment. This, in turn, means that each test should be preceded by a "warm-up" period to give the operat ing force o chance to get acquainted with the coal and its behavior. During this-warm-up and the test itself, every thing connected with the coal's per formance should be studied and check ed against the plant analysis. LOWEST-COST COAL If none of the coals display any markedly unsatisfactory operating characteristics, final selection will de pend on over-all steam cost Indicated for each. This will be made up of fuel cost, as calculated from the evapora >-*l tion results, and maintenance and labor costa as estimated from observation. Differences in evaporation results must be taken with a grain of salt since relative coal values ere not the only factors affecting evaporation. Opera tion of other factors can often be deter mined and allowed for by calculating a boiler heat balance. Unless plant testing extends over a fairly long period. It is difficult to esti mate any differences in operating costs due to different coals. Items Uke in creased power cost for handling or pul verizing can be measured but differ ences in maintenance coats will not show up for some time and then must be analysed carefully to be sure they result from some property of the coal. Normally analysis and testing will yield several-coals-having nearly the same value, any one of which will prove economical. Once the job is done, the engineer must stay with it, for fuel values change and so do plant conditions. For example, improved preparation has |b Co ^-'r.Timlrai-'Sileht-'Autofpstic piy,Vs''[ [S^IndustrisrCb-- *v'."i *- - M'Tlmk'enjDei'roitiAile.Co *1 v ' ,t~ &Si'TMich'Cor-v'' Todd^ShipysrdiXorp.-i* *i\ t . c..V."'./'.CCoombbuustion REaqwuiDpmmeenntt'D'Divlv. * ,,< 7 rCTPril,pil Biimer Co t- * Webster :&ngineering< Co*;" JS^^el.'W.Byrning Equij B~^n'(^mbuation gtateWSrTco raised ash-fusion temperature of West Kentucky coals enough to make them "available" to many midwestem plants formerly unable to use them. The vigi lant engineer will keep his eyes open /or such changes. He will also con stantly revise his fuel specifications as changes in equipment and amount and nature of plant load so require. Co' POWER * December 19*0 (777| 112 .v..*' A.L.VUlVjtr'r