N OVER one hundred years of domestic
SYMPOSIUM ON ANTHRACITE Papers presented before the Division of Gas and Fuel Chemistry, and arranged with the help of the Anthracite Institute, at the 89th Meeting of the -4merican Chemical Society, New York, N. Y . , .4pril 22 t o 26, 1935.
A New Principle of Furnace Design for Anthracite
heater manufacturing, surprisingly few changes have been made in the internal arrangement of anthracite-burning furnaces. The original furnaces were thought to be so fundamentally correct and elementary in principle that neither the grate, the furnace, nor the boiler absorbing surfaces could be basically changed or eliminated. An intensive four-year study a t the Anthracite Institute Laboratory, however, has produced convincing and rather startling proof that such is not the case, and it has indicated that radical departures can and should be made in the basic design of furnaces intended for the combustion of anthracite. With a full realization of the potential importance of revolutionary changes in the design of anthracite-burning heaters from standpoints of improved combustion, decreased manual attention, and simplification of construction, theoretical observations in connection with the combustion of anthracite were applied to the construction of actual heaters. The following fundamental changes were included in their design : (1) Increased areas for furnace radiation in order to maintain a temperature equilibrium between a point’ of ignition and of clinker formation. Clinkers were thus eliminated for all types of coal. (2) A total elimination of all space over the fire with complete combustion within the bed of glowing coals. (3) The admission of all air above the outer edge of, rather than through, the grate, with greatly improved conditions of combustion. (4) The use of a solid, unperforated grate as a fuel support only, thus simplifying construction. ( 5 ) Minimizing the importance of flue passes and secondary absorbing surfaces. The weight and size of the heater for any given outDut can thus be greatly reduced without any sacrifice of efficiency or output. (6) The full use of the natural shrinkage of the fuel while burning to replenish the fire automatically-without mechanical. parts. -
I
Description of New Heater ALLEN J. JOHNSON .4nthracite Institute Laboratory, Primos, Delaware County, Pa.
A new type of furnace that eliminates combustion space over the fire and admits all the air for combustion above, rather than through, the grate is described. Various superior features of performance, as compared with conventional units include sustained carbon dioxide above 17 per cent for indefinite periods, absence of appreciable carbon monoxide even in the absence of combustion space, operation at full rating with a minimum of draft, uniform combustion over 2-5 day periods without manual attention, and clinkerless operation even on low-fusion coals at high rates. The latter property is discussed in some detail, and a theory of clinker-proof boiler design is advanced.
This new type of heater (Figures 1 and 2 ) consists essentially of a deep cylinder of coal placed upon a fuel support which is not perforated, since all air for combustion is admitted through minute holes or slots around the periphery of the furnace. Air admitted through these holes passes horizontally through the active fuel bed to a central cylinder which, being water jacketed, acts both as a medium of increased furnace radiation and, to a lesser extent, as a secondary absorbing surface to reduce the stack temperature. Ashes are removed by rotating the fuel support which is so constructed that the ashes will be thrown off a t its edges into a suitable container. Absorption by Radiation Rather than Convection
As an outcome of the popularly accepted theory that in the combustion of any fuel it must first be reduced to gaseous form, and that the function of the boiler is then largely to extract the heat from these gases, previous and present “improvements” in furnace design are invariably predicated upon providing increased absorption areas, or upon providing properly arranged surfaces with which to extract the heat from these hot gases. This is true in spite of the knowledge that over two-thirds of all the heat absorbed by typical cast-iron boilers is by direct radiation within the furnace. In conventional furnaces (Figure 3) it is a common assumption that furnace temperatures are approximately proportionate to the rate of combustion, and conversely that low furnace temperatures can be secured only when burning coal slowly. While this is true in cases where the secondary ab-
oi the boiler arc relicd upon for tlie extnrcI&,perceirt:igcs of tlie heat generated, it is f this 1abor:ktory t.lrat, when buriring a n t l m cite at :ill rates of cornbustinn, an equilibrium can and should lie Iiiaiiitaiiiid Ibet,wrn the Iient geiicr:it.ed by the fuel bed atid tire Bent aiiiorbeil l,y rndiat.ion aitliiir the iurnace. Gaugor et al. ( I ) lrasc rcport,edaii average iaiiitioir t.ciiqn:raturc for antlirarite of 851 I?., with a r~r:txiniiiui ignitiim teiirperatim of zany m(: iiiriijile oi 91 5"; Tiirrier (2) reports :I iriirriruurii aili ii~fteniiigterri!~er:rturcof 2250" F., with an ayerage oC 2831 '. Thus, ercu with extreme therii is n Mfcrerii~ci n ternpcrwture of tit leiist I tween t,lw poiitt at which :mt,hraciti, ignites ai eoinpletc r u n i l ~ u s t i ~arid ~ i ~the t,eiriperutiire a t alricli clinker
dificultie begin. Tlienifwe. x i t h a rmge of over 1:300" lietwcnn cmirplctc cmil~ustionitrid clinker, it is relatively easy to increase tire radiant areal-1,f t,hr fiirl bed and furiurce to such an extent timt tlie funrace triiiperatiircs at all ratings, hoth liiglr and lou~, will always lie lietween tlie ignition t,ornperatilre oi the Imrdest antlrrarite, :and tlie ash-softening temperature of the most Cree-burning arithracite. In actual prrictice, increased raili:mt space was proviiled by iuserting an aiiditional mt,er-eooleil cylinder into the heart of the burning fuel (Figure -I). The depth to ahich this added surf:we peiietrated iirto the fire t.o produce the desired eoriilitimis of Suriiace ternperatore experirrientnlly a i i i l . once ascertained, was foiind to remain coristant svitliir~n m n d rariges of operiition, pr~ivideiithe fuel lied W:IS lrrld at ii fairly uniforiu depth
Elirninai.io,t of Coinhustion Space As :i second step in tlie design of it f u r n a c e f o r burniislr niitliracit,~,a, series {if test.; n a i conducted for detenriiiiiog the exact value ,)f eoiiibuitiiiii suaw above the fire. A ;t,an d a r 1' u n d e r f e d s t o k c r w a s installed in a cylinLlrii~rliroiler in ~ u c l iR rrra~rirerthat, starting with a relatively large f u r n a c e space betwen the top of tlie tuyiw a i d the c m v n sheet, tliis furnace volume could 1,e ilerreased progressivrly u n t,i1 just sufficient room reiuaiiieil f o r t h e
t h : i t absdutely no elfcct u p o n c 0111 b u s t io n w a s noted. While this s t , a t e m e n t , , upon first iinalysis, \vould seem t,o be out OS line with eurrEDt corn b u s t i o n theories, careful examination Enmi :i standpoint of any pus.4,le d clet e r i o u s effects upon combustion indicates that objections to this umcluaiim would have to I J ~lmd i i p o n o n e o r more of tlie fdlowiiig assiuupti~ms:
Smoke might he rirodncrd Furnace tFinperitures rvould be built an ahnornial extent. Unu.-uallv" hieli . oemeutnrrer oi flv mh \Vr.OOld rrsuit. ( 5 ) The pmmwture quenching of the flame noiild result in thr iiroduction of soot mid/ or inmrnplcte oombustiox. (2) (3) up l o (4) .,
.
Refuting these objectims, it WLS iouiid that in actual prtictice tlie question of carbon monoxide production was merely a niatter of maintaining a coristalrt arid proper fuel bed thickness, together a i t h tlie admission of a correct supply of air. The low ternperatores maintained within the furnace simplified the admission of air, because of tho fact that in the abseuce of clinker the fuel bed is always in a miform condition throughout its area. The gerrera.tion of appreciable quantities of carbon riroiroxide in boilers of existing design tlierefore has been reduced to a question of the use of t.he size of fuel for which t,he heater w a s o r i g i n a l l y constructed. The production of smoke or soot, from antiiracite i s practically impossiiiie, hecause insuEeient h y d r o c a r b o n s are present to form tar, soot, or smoke. The sinall percentages of volatile inattor contained in anthracite consist nlmost ent.irely of hydrogen and a few of its simple corrrpounds ( 3 ) . (It is true that this type d condruction would not be suitable for bituminous coal because of the sinoke and incomplete combustion that would 1111doubt.edly eiisue.) ilbnorrna,l filmace telnperabiire haye :already been sirown to he l a r g e l y t h e result of urilialanced furnace conditions;
INDUSTRIAL AND ENGINEERING CHEMISTRY
946
VOL. 21, NO. 8
ff /C/2-NCY
x U w
a a LL 3
RADIATION
'
IN F U R N A C E )
68%
,:
Y
a
g
22
1
CONDUCTION I%
LESS T H A N
1
,
FIGURE 4. ADDITIONAL RADI.4NT FACE IN FUEL BED
FIGURE 3. RELA-
TIVE ABSORPTION IN FURXACE AND FLUESOF TYPICAL
OF
COMPARATIVE S I Z E AND INTERNAL ARR.4SGEYENT COMBUSTION U N I T AND OF CONVENTIONAL
SURFACE-FIRED BOILER The large percentage of volume available for coal storage in t h e Anthraeite I n s t i t u t e unit a n d its small furnace volume are noteworthy.
1
FIGURE7.
I
1NCUES FRON TffYdRE
1
/S TO CROWN S H I E T
I
PO
FURNACE VOLUME ON COhIBUSTION O F P E N N S Y L V l N I A hNTHRACITE
HORIZONT.
