1966 Glennie Fuels and Furnace

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Proceedings of The Soiith African Sugar Techi~ologists'Association-March 1966

FUELS AND FURNACES

By P. R. A. GLENNIE

ScopeThe modern counterpart of the Horseshoe Furnace

The subject matter of this paper, if treated in thegeneral sense, covers an extremely wide field to whichit is impossible to do justice in the time allocated. Itis even difficult to cover the subject adequately if weconfine ourselves only to the furnace design of bagassefired sugar mill boilers.

I therefore intend to give a short summary of pastand present bagasse furnace designs and cover thesubject of auxiliary fuels and how the latter haveaffected basic boiler design considerations.

For the purposes of this paper I have assumed thatthe majority of you have been concerned, one way or

the other, with the practical aspects of steam raisingequipment in sugar mills.

Historical Background

Bagasse has been used as a fuel for the boilingprocesses of sugar production since the 18th century.With the introduction of steam boiler plant in theearly 1 9th century and the growing mechanization ofsugar mills, it became necessary to overcome theproblems of burning this wet and rather unm anageablefuel in large quan tities.

Abo ut 100 years ago the first Dutch ovens appeared,typically the Cook's Hearth furnace, which was the

forerunner of many variations which are still in useup to the present day. However these variations tookdifferent forms in different parts of the world.

The original Cook's Hearth , which consisted of onlyone cell, with an approximately square grate on towhich the fuel was fed in a conical pile, is still thebasic form used in m any parts of the world.

In this country over the years we have seen thedevelopment of this basic design with the introductionof forced draught, p reheated air, flat suspended roofsand arches and the final refinements of seconda ry over-fire air and tertiary air into the secondary combustioncham ber. With the rise in capacity of boiler plant, thenumber of cells was often increased to three, each cell

maintaining the approximately square shape of thegrate.

A parallel development, mainly in the Westernhemisphere, was the Horseshoe Furnace, in which eachcell was built up with refractories with a horseshoeshape in the plan view. The main difference from theCook's Hearth was the elimination of the grate, thefuel burning in a conical pile on the floor. Primary airwas introduced through cast iron tuyeres built intothe first course of brickwork, with secondary airadmitted higher up and all the way round the peri-phery of the ho rseshoe. These furnaces attained veryhigh ratings indeed. With hot air, ratings of up to 1. 5x LOG BTU/hr. per sq. ft. of floor area were fairly

common. This compares with .the Cook's Hearth,where grate ratings seldom exceed 1 .0 x loGBTU/sq.ft./hr. with hot air.

was developed in the i930's in the West Indies andwas known as the Ward Furnace. These furnaceseliminated the Dutch oven construction in front of theboiler and were placed directly below the main boilerfurnace with an intervening refractory throat. Thebasic horseshoe shape was originally retained withlater variations to oval and then rectangular cells.Although these furnaces have been introduced tomany parts of the world, South Africa has been anotable exception.

Another parallel development was the inclined stepgrate originally introduced at the end of the 19thcentury. In its original Dutch oven form it still exists

commonly in the East and West Indies. Particularlyin more primitive areas these furnaces operate onnatural draught without forced draught or airheaters.

In some cases the step grate has been brought inunder the main furnace in a similar way to the WardFurnace. In other cases preheated air and forceddraught have been introduced.

A post war development has been the EisnerFurnace which started as a variation of the step grateDutch oven. In its final form the furnace is also placedbeneath the main combustion chamber, eliminatingthe typical Dutch oven contours, but the method ofcombustion and shape of fuel bed are similar to thatof the step grate.

With the exception of the Eisner Furnace all theforegoing designs with their many variations were inuse throughout the cane sugar milling industry untilthe late 1940's.

In the years immediately prior to the last war coalspreader fired boilers had become extremely popularin the United States both for power station andindustrial use. It was found that this method of firingwas extremely flexible and could be used for allvarieties and sizings of bituminous co al. It could alsobe used for the auxiliary, or even main, firing of refusefuels.

In the vast timber growing area of North America

considerable quantities of electricity are generatedfrom sawmill wood waste and it became commonpractice to fire this waste by spreader methods withcoal or oil as the standby fuel.

It was therefore only logical that the Americanboiler and stoker companies should turn their mindsto the possibility of burning bagasse on spreaderstokers and developments were started in this directionvery shortly,after the second world war. The advan-tages of spreader firing were almost immediatelyapparent and the new system caught on very rapidlyindeed.

In Southern Africa the first spreader fired unitswere installed in 195314 and by 1960 probably 90%

of all new installations were of the spreader type.Many Dutch oven fired boilers were converted to thenew system.



Proceedings of he South African Sugar Technologists' Association-M arch 1966 65

In this country it is indeed remarkable that, in theshor t space of less than 15 years, spreader firing hastaken over almost entirely from the older methods.

Fuels for Sugar Mills

Bagasse always has been, and for the forseeablefuture must be, the primary fuel of a sugar mill.

In the average modern mill, having a cane input ofover 200 tons pe r hou r, the w eight of bagasse producedper day is approximately 1,800 tons and, with its lowbulk density, the volume is prodigious.

Although there have been many schemes to makebetter use of this fibrous substance, particularly inpaper and cardboa rd manufacture, these have not beenof any particular economic significance. It musttherefore be stressed that bagasse must remain theprimary fuel of all cane sugar mills.

With the increase of refining facilities in larger

mills and the necessity to generate electricity forirrigation, the problem often arises of a certain short-age of bagasse and the engineering designer musteither economise in his use of steam or increase hisboiler efficiency to overcom e the shortfall of this fuel.

Only too frequently, and usually to minimise capitalexpenditure, recourse is made to the use of auxiliaryfuels such as wood, coal a nd, in countries where coalis not as cheap a s it is in S outh Africa, oil, rather thanincrease the efficiency of the bagasse firing equipmen t.

Auxiliary fuels, of course, have their advantagesfrom the point or view of shutting down and startingup procedures and in the event of temporary mill

outages. They obviate, to a large extent, the necessityto store very large quantities of bagasse, which is soawkward to handle mechanically.

Of course, with the generation of electricity forirrigation purposes, it is necessary to fire boiler plantwith such auxiliary fuels during mill shutdown andoff-crop periods.

A nice solution from the boiler designers point ofview would be to install the highest possible efficiencyboiler plant so that an excess of bagasse is producedand the fuel stored for periods of mill shutdown.Unfortunately nobody has yet come forward with areally practical solution to the problems of bagassestorage.

In this country we have the factor of probably thecheapest coal supplies in the world and this is themain reason f or the very rapid development of spread-er fired boilers in Southern Africa with their adapta-bility to coal/bagasse dual firing.

In'most other sugar producing countries, particu-larly the West Indies, oil is cheap and hearth typefurnaces, such as the Ward, with oil burners in thesecondary furnace are able, by dint of lower capitalcosts, to hold their own against the spreader stoker.

Due to clinkering difficulties with high furnacetempera tures it is, of course, not practical to fire highash coal in Dutch oven furnaces.

On the other hand wood, with very similar burningqualities to bagasse, can be burnt either in hearthfurnaces, or for that matter, on spreader stokers.

I have often wondered why so much labour isexpended on bringing large quantities of wood logsto the boilerhouse and manhandling them into thefurnaces for lighting up purposes. If this fuel wereput through a simple hogging machine it could soeasily be fed to the boilers through the norm al bagassecarrier system.

In the following table I give the approximatefurnace gas quantities for a boiler steaming at 100,000lb/hr. with the four different fuels that we havediscussed above :

Bagasse Coal Wood Oil

% moisture infuel . . . 50 5 50 -

G.C .V.B TU /lb. 4,150 12,000 4,300 18,500Furnace gas

weight lb/hr. . 205,000 150,000 200,000 120,000

Assuming that u e have a hypothetical boiler thatcan fire all four of these fuels, obvious ly the fu el withthe lowest gas weight will have the m ost hea t removedfrom these gases in the course of their passage throughthe boiler. If we have designed the boiler fo r operatingwith an economiser, or airheater, outlet temperatureof 400" F. on bagasse, the equivalent tempera ture onwood will probably be 390" F, on coal 350" F. andon o il 320" F.

Consequently, with d ual firing and where we desireto install boilers of high efficiency, we came up againstthe problem of corrosion with the higher grade fuel.

Particularly with auxiliary oil firing in boilers

specifically designed for bagasse, it is essential to by-pass heating surfaces and/or provide parallel flowairheaters in ord er to prevent dew point corrosion inthe airheaters and draught plant. This is only one ofthe many criteria for the design of modern highefficiency boilers for sugar mills.

Design Considerations-Hearth Type Furnaces

, With the majority of hearth furnaces, such as theHorseshoe, Ward or designs best known in thiscountry, the Vincent & Pullar and Murray typefurnaces, the fuel bed is ostensibly in the form of acone with the fuel fed from over-head on to either a

grate or the floor of the cell.With very high temperatures maintained in the

hearth cell the moisture in the fuel is driven off andmost of the fuel ignites from the bottom peripheryof the cone upwards. With the fuel falling through theflames a limited proportion representing the smallerparticles of fuel, are flash dried and readily.ignite;sometimes before they reach the cone.

The whole operation necessitates furnace temper-atures of a very high order and this brings with it thefollowing disadvantages :

Firstly, we have the difficulty of slag forma-tion in the primary and secondary combustion

cham bers, although this will vary considerab ly withthe type of cane and the area in which it is grown.Secondly, due to the high temperatures in the fuel





Proceedings of Tlze South African Sugar. Tecltnologists' Association-March 1966 67

required to obtain a satisfactory spread with bagassewhen compared with coal. I t is therefore desirable tohave means of varying over a wide range the speed ofthe roto rs according to th e fuel being burnt.

The second and rather simpler method of distribu-

ting bagasse over the length of the grate is by meansof pneumatic air spreaders. These obviously haveno moving parts and operate on an air supply fromthe boiler secondary air system. However they areunfortunately unsuitable for coal except in a specialapplications.

Consequently, an d particularly in this country wherethe need is to burn coal and bagasse, the tendencyhas been towards the rotor type spreader which canhandle either fuel.

Personally I am very much in favour of air spreadingas it m ust increase th e flash drying of bagasse especiallywhen hot air is used in the spreaders themselves.

The main disadvantages of spreader firing can besummarised as follows :

Firstly, with spreader firing it is essential t o m eterthe fuel in order to obtain a completely steadyrate of feed. This means that we must providesome form of feeder with a variable speed drivean d it is by no means easy to design a feeder whichwill handle such an awkward fuel as bagasse.However this problem has been reasonably wellovercome with the drum type and the slat typeconveyor feeders. The latter has been adapted tometer both the bagasse and coal in the same unit.

Secondly, and partly as a result of the first dis-

advantage, it is necessary to have m ore complicatedcontrols and m ore skilled operators tha n is obtainedwith hearth furnace operation.

Thirdly, due to the relatively thin fuel bed, thereis little reserve of fuel on the grate in the event ofmill stoppage and it is necessary to change over tocoal within a fairly short space of time. Operatorshave to be trained to carry out this operationwithout loss of steam pressure.

Fourth ly, especially where travelling grate stokersare used, the cost of firing equipment is muchhigher with spreader stokers than with the olderhearth designs. This is to a certain extent offset bythe narrower and more highly rated boilers that

can be used with spreader firing.Fifthly, the very nature of spreader firing means

that higher excess air is used in the furnace thanis obtained with hearth furnaces. Th e-basic figuresare probably 40% and 30 % respectively, in otherwords one t hird more excess air is used in the caseof spreader firing and this increases the chimneylosses. Again this feature is offset by the fact thatfurnace temperatures are very much lower withreduced radiation losses.

The advantages of spreader firing far outweigh thedisadvantages and they can be summarised as follows :

Firstly, there is the intimate mixing of fuel and

air in suspension with consequent flash drying.This brings with it the almost complete eliminationof unburnt gases.

Secondly, we have comparatively low furnacetemperatures with a consequent reduction in brick-work maintenance. Also u ~ d e rhis heading shouldbe mentioned the ability to use partially watercooled walls wh ich again have t he effect of reducingfurnace maintenance.

Third ly, the use of auxiliary fuels, such as coal orwood w aste, on the stoker is facilitated. Also in thecase of combined bagasse and oil firing, the heavyslagging which is experienced with Ward Furnacesis not here apparent due to lower furnacetemperatures.

Fourthly, it is possible to mechanise ashingcompletely.

Fifthly, and as mentioned above, it is possibleto obtain a considerable reduction in boiler width.

Sixthly, the spread er fired boiler is highly flexibleand has "quick steaming" characteristics.

One can only come to the conclusion that, especiallywhere larger boilers over 50,000 lb./hr. are concernedand where higher steam pressures and temperaturesare to be used, the only possible economic solutionlies with the spreader stoker. However it is myopinion that, where coal is only used to a limitedextent or where the auxiliary fuel is oil, a compromisesuch as the Medine design described previously maybe a better proposition.

Yet an other variation, which has a rise n. in myexperience, is the case of a boiler which is primarilycoal fired for long offcrop periods for irrigationpurposes and here we used a conventional backward

moving chain grate stoker with air spreader firing ofbagasse. In the actual case this method was adoptedbecause the boiler was secondhand a nd ha d a n existingchain grate stoker. For new plant the backwardmoving travelling grate stoker is slightly moreexpensive than the conventional spreader travellingstoker which operates in the o p p~ si te irection. Itmay therefore not be normally a very economicsolution, but the point is that the combustion ofcoal o n a conventional travelling grate or chain gratestoker is mor e efficient than spreader firing. The m ain .reasons for this are lower excess air and lower gritemission, both giving reduced chimney losses.

Future DevelopmentsThe cane sugar mill of the futu re may well develop

a steam demand in excess of 1,000,000 lb./hr. It willthen be necessary to consider boiler unit sizes of theorder of 300,000 to 400,000 Ib./hr. of stea m.

To the boiler designer this size of unit has noproblems whatsoever as units of ten times this sizeare currently being installed in power stations allover the world.

However the real problem is how we are going tofire these larger units with bagasse and in my opinion,the only possible solution can lie with pulverizing ofbagasse i n milling equipment.

These mills would be swept with very high temper-ature air an d the powdered fuel fed into a bin system.



68 Procee dings of T11e Soutlz Africa n Suga r Te cknolog ists' Association-Marclt 1966

The dry pulverized fuel would be led to specialfeeders which would distribute the fuel to burners setin. completely water cooled boiler furnaces.

We should n ot get too enthusiastic about this systemuntil such time as there is a call for much larger units.

I say this because it is unlikely that such a systemwith its complicated controls and the necessary firingskills, will become econom ic until a boiler sizeof at least 250,000 lb./hr. is reached. I envisage herea minimum of three such boilers which would mean asugar mill handling something over 600 tons of caneper hour.

A second development which I have in mind isthe possibility of somebody solving the problem ofbagasse storage. This would go a long way towardsincreasing the efficiency of the use of bagasse andeffect a saving in the use of auxiliary fuels.

Thirdly and finally I would like to comm ent on thepossibilities of higher efficiency boiler plant being

employed. Q uite a few inills are now using economisersas well as airheaters, particularly where higher boilerpressures have been employed, but in sugar millpractice we are still a long way off the order ofefficiencies employed in power station practice.Especially where large quantities of coal are used,consideration should be given to back end temper-atures of the order of 280" F. on coal, equivalent tosay, 320' F. on bagasse.

With this order of low exit gas temperature it isprobably better to place the Economiser after themain airheater with possibly a small primary cast ironairheater after the Economiser.

With these low outlet gas temperatures it will benecessary to go to concrete, brick or gunite linedmild steel chimneys.

Mr. Hulett: It is mentioned in the paper that thenew large boilers with which the industry may beequipped will require the bagasse to be furtherpulverised. Final bagasse is already being pulverisedby about 5,000 h.p. before it reaches the boilers.How much more horse power does the autho r suggestwould be required ?

Mr. Glennie: It is extremely difficult to assess horsepower requirements for pulverising bagasse at thisstage. However, it should be borne in mind that theuse of dry fuel in firing boiler plant will increase theefficiency to a very considerable extent and that thiswill more than compensate for the extra horse powerrequirements.

Mr. Hulett: The spreader stoker is recommendedbut it is very often the cause of breakdowns. Withregard to furnace designs, I would like to m ention t hatthe Eisner furnace has a simple brick construction

and requ ires very little maintenance.

Mr. Glennie: Returning to the question of futurehigher capacity boilers, it should be pointed out thatif, for example, two 250,000 Ib./hr boilers are in-stalled instead of ten 50,000 lb./hr. units, or five100,000 Ib./hr. units, the capital and amortisationcharges will be considerably less. In the assumptionI have made for mill steam requirements, 1 have nottaken diffusion into account. However, 1understandthat there would be certain savings of steam require-ments.

Mr. Hulett: High velocity secondary a ir in spreader-

fired furnaces boosts the output of a boiler tremen-dously. A twenty per cent increase was obtained in aboiler at Darnall.

ConclusionMr. Glennie: When the bagasse spreader-fired units

have by means covered the of this ,re fir st in tro duced in the ear ly 195O's, it was themost absorbing subject and have probab ly less opinion of a number of people in the boiler industrythan justice to some furnace designs. Fo r any

that these units required adequate provision foromissions in this direction I can only apologise.

secondary air. Subsequently, high pressure secondaryair was introduc ed by most manufacturers.

Mr. Magasiner: What order of boiler capacitydo you recommend for pulverised bagasse units? Mr. Magasiner: There is a lot more horse power

consumption if you use this high pressure, high velo-Mr. Glennie: Definitely not below 250,000 Ib./hr. city air.

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1966 Glennie Fuels and Furnace