SORG Glas_melting Technology

Glass MeltingTechnology

Perfect Solutions for the Glass Industry

2GlasE/10.08/V2.2

Contents

Regenerative Air Preheating page 5

Regenerative End-fired Furnaces page 6

Regenerative Cross-fired Furnaces page 7

Recuperative Air Preheating page 8

Recuperative End-fired Furnaces page 10

Recuperative Side-fired Furnaces page 11

LoNOx® Melter page 12

FlexMelter® page 13

Boro-Oxy-Melter® page 14

VSM® All-electric Furnace page 15

Barrier Wall • SDR – SORG Deep Refiner® • Refining Bank page 16

Booster page 17

Bubbler • CONTI-DRAIN® page 18

Cullet Preheating • NOx Reduction page 19

Individual Burner Control page 20

Oxy-Fuel Heating page 21

Batch Charging Technology page 22

SORG® Furnaces for Special Glasses page 24

3GlasE/10.08/V2.2

Foreword

SORG® customers benefit fromthis success, which is foundedon technical competence, flexi-bility, reliability and, above all, anenormous wealth of experience. SORG® are able to supply manydifferent types and sizes of fur-nace for the widest range ofproduction and site conditions.

Float glass furnaces with a meltingcapacity of 700 t/24 h or moreare an integral part of the pro-gramme, as is the smallest opalglass furnace for lighting ware,with a capacity of 1 t/24 h.

Not only soda-lime glass, butalso lead-free crystal, full leadcrystal, opal and borosilicateglasses of differing compositionsand water glass are melted inSORG® furnaces.The articlesproduced on our installationscover almost the completerange of glassware commonlyproduced – tableware, lightingware, tubing, insulators, fibres,pellets, flat glass, rolled plate,water glass and, of course, alltypes of containers.

4GlasE/10.08/V2.2

SORG® has long been renownedfor furnace development. Thisprogramme has led to significantimprovements in conventionalfurnace technology and to thedevelopment of completely newfurnace concepts.

This brochure contains informationabout conventional furnaces,such as regenerative end-firedfurnaces, and also innovativeconcepts and various supple-mentary systems – all suppliedby SORG®.

“At Home in the World of Glass”– the validity of this statementis demonstrated by over 250SORG® designed furnaces inoperation in more than 60countries throughout theworld.

Regenerative Air Preheating

The regenerators, which forman intermediate storage medi-um, consist of two chambers,each of which is filled with anetwork of refractories, referredto as the packing. The wastegases from the furnace are pas-sed through one of the cham-bers, and the refractories in thechamber are heated up. Thecombustion air enters the furnacethrough the other chamber.

After a certain period of time theflows of air and waste gases arereversed. The combustion air nowflows through the hot chamberand is heated by heat transferfrom the refractories, whilst thewaste gases pass through theother chamber and heat therefractories in this chamber again.

Regenerator chambers are nor-mally vertical constructions inwhich the waste gases pass

downwards, whilst the com-bustion air travels upwards. Onmost furnaces single pass rege-nerators are used where thegases flow in one directionthrough one chamber. However,where there is insufficient cellardepth available for the installationof the necessary packing volumemultiple-pass regenerators canbe used.

There are various forms of rege-nerator packing, but only two arenow widely used. Both designsutilise specially shaped blocks,cross-shapes for the cruciformsystem, and square sectiontube shapes for the chimneyblock system.

Regenerative furnaces can bedivided into two basic types onthe basis of the location of theburner and the flame path:

• end-fired furnaces• cross-fired furnaces

5

Advantages

■ High preheat temperaturesof up to approx. 1350°C possible

■ Excellent energy con-sumption possible

GlasE/10.08/V2.2

In modern fossil-fired furnacesthe heat contained in the wastegases leaving the furnace isused to preheat the combustionair, in order to produce higherflame temperatures and im-prove efficiency. The air pre-heating system most commonlyused is the regenerativesystem.

Typical regenerator chimney block

packing during construction

Energy savings dependency on

preheated air temperature

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Correlation between flame and

preheated air temperature

End-fired Regenerative Furnaces

The raw materials enter the fur-nace through one or two dog-houses installed on the furnacesides, immediately next to therear wall.

Very large furnaces of this typehave a melting area of approxi-mately 150 m2, whilst small unitsof approximately 20 m2 are alsoin operation.

This type of furnace has twoburner ports, located side byside in the furnace rear wall, andthe regenerators are situatedbehind the furnace. Each port isequipped with 2 – 4 burners,depending on the size of thefurnace.

The flame travels forwards fromthe burner port, turns through180° and exits through the secondburner port. This creates a flameand waste gas path in the shapeof a horizontal “U”. As a resultthe combustion gases in thefurnace have a relatively longresidence time, which producesgood energy utilisation.

6

Advantages

■ Very flexible furnace type

■ Lower construction coststhan with cross-fired furnaces

■ Lower energy consumptionthan a cross-fired furnace

GlasE/10.08/V2.2

Typical applications

Glass type:• Soda-lime glass

Typical products:• containers• rolled plate

Melting capacities:• 20 – 450 t/24 h

100 m2 regenerative end-fired furnace

for containers

Cross-fired Regenerative Furnaces

The burner ports are situatedalong the furnace side walls,normally covering almost thecomplete length.

The number of ports depends onthe size of the furnace and usuallylies within the range 3 – 5. Eachport is provided with 2 – 4 burners,according to the furnace size.

The flame travels from one sideof the furnace to the other andthe waste gases are exhaustedexactly opposite the entry burnerport. The maximum flame lengthavailable is therefore determinedby the furnace width.

The two regenerator chambersare located on the sides of thefurnace and, in most cases, theyare almost as long as the tank.With this type of furnace theregenerator chamber suppliesseveral burner ports. Theair/gas ratio of the individualburner ports can only be con-trolled accurately if the regene-rator chambers are split intosub-chambers to accommodatethe number of burner ports.

7

Advantages

■ can also be built for melting capacities > 500 t/24h

GlasE/10.08/V2.2

The single doghouse is situatedon the furnace rear wall and thebatch is usually charged overalmost the complete tank width.

As a result of the greater numberof ports and larger regeneratorchambers, the heat loss area isgreater than with comparableend-fired furnaces. Furnaces ofthis type smaller than approxi-mately 70 m2 are used only rarely.

Typical applications

Glass type:• Soda-lime glass

Typical products:• containers• float glass• rolled plate

Melting capacities:• 200 – 800+ t/24 h

Recuperatively preheated com-bustion air provides stable heatingwithout the flame/waste gas pathreversal that is necessary withregenerative systems. However,there is the disadvantage thatthe air preheat temperatures arelower than with regenerators.

Most recuperative furnaces forglass melting utilise steel recu-perators. These are alwaysinstalled vertically, whereby thewaste gas flows either upwardsor downwards.

Two basic types of steel recuperator are used:

• double shell recuperator• tube cage recuperator

8

Advantages

■ stable flame path without reversal

■ lower investment than regenerators

■ with part load energy consumption increases more slowly than with regenerative furnaces

GlasE/10.08/V2.2

Comparison of energy consumption

of a regenerative and a recuperative

furnace at part load

In the glass industry recup-erators are used to preheatcombustion air. The hot wastegases and the cold combustionair pass through parallel butseparate channels and heattransfer takes place throughthe intermediate wall.

Recuperative Air Preheating

Recuperative Air Preheating

Double shell recuperator

This type of recuperator consistsof two concentric high tempera-ture resistant steel tubes of similardiameter, so that a narrow annularslit is formed between the twotubes. The hot waste gasespass through the inner tube,whilst the combustion air passesthrough the annular slit. The airmay be passed in the samebasic direction as the wastegases (parallel flow) or in theopposite direction (counterflow).Single modules of this type canbe used alone, or they can beplaced one after another to forma complete unit.

9GlasE/10.08/V2.2

The tube cage recuperator

In a tube cage recuperator thecombustion air is led through alarge number of individual smalldiameter steel tubes arranged ina ring around the inner circum-ference of a large diameter outertube, through which the wastegases flow. The outer tube is madeof steel, lined with refractorymaterial.

The small diameter air tubes aresuspended from the top andsealed with refractory materialat the bottom in such a waythat the tubes are free to expand.

This type of recuperator cangive air preheat temperatures ofup to 750 °C. They are usuallyinstalled on larger furnaces.

A double shell recuperator maybe added downstream of thetube cage unit to form a com-plete aggregate.

Conventional recuperative fur-naces can be divided into twotypes on the basis of the locati-on of the burner and the flamepath:

• end-fired furnaces• side-fired furnaces

Tube cage recuperator

Double shell recuperators arecapable of giving a typical airpreheat temperature within therange 450 – 650 °C. The majorityof these units are used for smallfurnaces up to a melting capacityof approximately 50 t/24 h.

End-fired Recuperative Furnaces

There are several installationarrangements for burners andthe recuperator in end-firedrecuperative furnaces, but oneparticular method has proved tobe the most beneficial.

In this version the burners arelocated on the furnace rear wall,with the waste gas port imme-diately above, in the same wall.The flame leaves the burner andtravels along the furnace, turnsupwards and back to exhaustimmediately above the burners.The flame path created is in theshape of a vertical “U”.

10GlasE/10.08/V2.2

The single doghouse is locatedon a side wall, immediately nextto the rear wall.

This furnace concept is primarilyused for small installations, withmelting capacities of up toapproximately 35 t/24 h.

Typical applications

Glass types:• soda-lime glass• lead-free crystal glass• soft borosilicate C glass• water glass

Typical products:• containers• glass bricks• fibres• lighting ware

Melting capacities:• up to approx. 35 t/24 h

Side-fired Recuperative Furnaces

The burners of side-fired recu-perative furnaces are installedalong both side walls and thewaste gases are exhaustedtowards the rear. Either one ortwo exhaust ports are installedin the furnace rear wall or a sidewall.

The furnace can be designedwith either a single or a twindoghouse, located either on therear wall of the tank or on theside, immediately adjacent tothe rear wall.

Temperature control of largerfurnaces can be divided into anumber of control zones alongthe length of the furnace.

11GlasE/10.08/V2.2

The specific energy consumptionof side-fired recuperative furnacesat full load is higher than that ofcomparable regenerative furnaces,but is lower with partial load.

Typical applications

Glass types:• soda-lime glass• soft borosilicate C glass• E glass

Typical products:• containers• glass bricks• rolled plate• fibres

Melting capacities:• 20 – 350 t/24 h

Recuperative side-fired furnace for containers

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LoNOx® Melter

The LoNOx® Melter is a specialkind of recuperative furnaceconcept developed to operatewith unusually low NOx emissionlevels.

The LoNOx® Melter has a long,narrow tank divided into meltingand refining zones. The batch ischarged over the complete widthof the furnace through a dog-house on the rear wall.

Gas or oil burners are installedin the front part of the meltingarea and the waste gases aredrawn off to the back over thetop of the batch and cullet. Anintermediate arch divides thesuperstructure to ensure thatthe raw materials entering thefurnace are not directly heatedby radiation from the hotter partof the furnace. This produces ahigh level of heat transfer bet-ween the waste gases and theraw materials.

12

Advantage

■ NOx values lower than 500 mg/Nm3 possible

GlasE/10.08/V2.2

A refining bank is installed toimprove the refining processand this is followed by a DeepRefiner® (see page 16).

After leaving the furnace throughwaste gas flues installed at therear in the furnace side walls,the waste gases are first passedthrough recuperators to preheatthe combustion air and theythen enter an external culletpreheater where they transfereven more heat.

Although the combustion airpreheat temperature is relativelylow compared with regenerative

furnaces, the extent to whichenergy is retained in the systemis indicated by the low exit tem-perature of approximately 200 °Cat which the waste gases leavethe cullet preheater. The energyconsumption of LoNOx® Meltersis comparable with the bestregenerative furnaces.

Extensive emission measurementshave shown that with this furnaceconcept it is possible to main-tain values less than 500 mg/Nm2,corrected to 8 % O2 in the wastegases, during stable continuousoperation.

NOx measurements on a LoNOx®

Melter at a load of 220 t/24 h

Typical applications

Glass types:• soda-lime flint, amber and

green container glass

Typical products:• containers

Melting capacities:• 150 – 450 t/24 h

FlexMelter®

The FlexMelter® concept is des-igned for continuous or disconti-nuous production of high qualityglass. It is not necessary to drainglass during periods when thefurnace is not being pulled.

The batch is charged over thecomplete width of the furnacethrough a doghouse on theback wall.

A row of electrodes installed atthe end of the melting sectionacts as a barrier between themelting and refining areas andincreases the temperature of theglass flowing into the refining area.

A refining bank is installed toimprove the refining process andprevents any return flow of glass.

FlexMelter® furnaces are desi-gned to operate with a high per-centage of fossil energy. However,the proportions of fossil fuel andelectricity can be determined atthe design stage, depending onthe type of glass being melted.

The flexibility of this type of fur-nace is based on the consequentsplitting of the three main functionsinvolved in the glass meltingprocess – melting, refining andhomogenisation. The furnacedesign prevents any return flowbetween the various areas.

13GlasE/10.08/V2.2

The burners are installed at thefront end of the melting area andin the refining chamber. The wastegases from all burners are passedbackwards along the furnaceand exhausted through portssituated in the superstructureside walls next to the rear wall.Intermediate arches in thesuperstructure prevent directheat transfer by radiation fromthe hottest part of the furnaceto the colder raw materials.This produces a high level ofheat transfer between the wastegases and the raw materials.

Typical applications

Glass types: • soda-lime flint and coloured

glass• potassium and soda crystal

glass• semi and full lead crystal

glass

Typical products:• household and tableware• stemware• high quality flacons• tubing• insulators• automotive headlamp glass

Melting capacities:• 6 – 130 t/24 h

SORG FlexMelter®

14GlasE/10.08/V2.2

Boro-Oxy-Melter®

The melting and refining of boro-silicate glasses (expansion coef-ficient 33 – 36 x 10-7) and TFTglass are difficult and requirespecial furnaces. Relatively hightemperatures are required andthe refining is also made moredifficult by the fact that only alimited number of refining agentscan be used. In addition thevolatilisation of boron-alkalicompounds, that takes placefrom the glass bath surface, leadsto boron-alkali depletion of thesurface glass and, as a result,silica enrichment occurs with atendency to crystallise in theform of cristobalite.

The Boro-Oxy-Melter® is aSORG® development designedto deal with the problems ofmelting such difficult glasses.

The main source of energy is anoxy-fuel heating system with theappropriate burners installed alongboth sides of the superstructure.The energy input is increased bya high-capacity electric boostingsystem in the melting area. ASORG® refining bank, with a muchshallower glass bath depth, isinstalled in the refining area. Therefining process is improved asthe glass is forced upwardstowards the glass bath surface,i.e. into a high temperature area.

Advantages

■ Excellent glass quality

■ Low energy consumptionWith such glasses the use ofcullet is limited and therefore itis possible to use screw chargersthat do not require a doghouse.These chargers are introducedinto the furnace through circularopenings in the rear wall of thesuperstructure.

Only a low waste gas volume isproduced and therefore wasteheat utilisation is not viable.However, the waste gases mustbe cooled, before they are ledto an electro-static filter. Thewaste gases are cooled in aquench chamber operated withair or water, or a combination of both.

Batch charging system

for a Boro-Oxy-Melter®

Typical applications

Glass types:• Pyrex® type borosilicate glass

(α = 32 – 40 x 10-7)• neutral borosilicate glass

(α = 50 – 60 x 10-7)• TFT glass

Products:• tubing• blown ware• pressed ware• flat glass

15GlasE/10.08/V2.2

VSM® All-electric Furnace

Advantages

■ Rotating crown batch charger - simple dust retention

■ Enclosed superstructure –almost no in-factory dusting

■ Top Electrodes – longer campaign

The SORG VSM® all-electric fur-nace is a cold-top, vertical melter,in which the processes of batchcharging, melting, refining andhomogenisation all take place ina vertical direction.

The furnace is normally 6 or 12sided, or round, which permitsa relatively even loading of the 3phase electrical supply. Theelectrodes are divided into twoor more electrode levels to givebetter control of the verticaltemperature gradient throughthe furnace.

Most VSM® furnaces use theSORG® patented rotating crownbatch charging system. Thecomplete furnace crown is sup-ported on roller units fitted withelectric drive motors, and thecrown can be rotated about thevertical axis of the furnace. Anumber of vibratory chutes aremounted on the crown abovesmall openings at varyingdistances from the furnace verticalaxis. As the crown rotates thevibratory chutes lay concentricrings of batch onto the furnacesurface.

This type of batch chargingsystem requires no movingparts within the furnace super-structure. In addition, the com-plete superstructure is easy toseal, which means that a relati-vely small bag filter for theremoval of dust can be easilyattached to the furnace.

Smaller furnaces are providedwith a vibratory or screw feederto transport the raw materialsinto the furnace, and a rotatingdistribution arm within the fur-nace superstructure to spreadthe batch.

Specially developed SORG® TopElectrodes are installed in almost allVSM® furnaces. These electrodes

are inserted through the furnacesuperstructure and enter theglass bath through the surface.They can be swung out of thefurnace for inspection andexchange.

As this type of electrode requiresneither holes nor water-coolingin the furnace side wall blocksthe refractory material is subjectto much less damage causedby temperature changes. TopElectrodes are situated furtheraway from the side walls thanconventional horizontal electrodesand so the convection currentsproduced by the electrodes causeless wear on the refractorymaterial.

Typical applications

Glass types:• fluoride opal glass• lead crystal and lead-free

crystal glasses• soft borosilicate C glass

(insulating fibres)• hard borosilicate Pyrex® glass• soda-lime glass

Products:• lighting ware• tableware• stemware• high quality flacons• fibres• tubing

Melting capacities:• 3 - 180 t/24 h

All-electric VSM® for

60 t/24 h C glass with Top Electrodes

Barrier Wall • SDR – SORG Deep Refiner® •Refining Bank

Barrier wall

In conventional glass melters theconvection currents in the furnacehave a significant influence on themelting capacity and glass quality.

An upward current is producedat the hot spot as a result of thetemperature distribution in thefurnace. This current interruptsthe forward flow of the colderbottom current and diverts it up-wards to areas of higher temper-atures. This prevents lower qualitybottom glass from flowing directlyinto the throat.

Nowadays this effect is intensifiedby a barrier that is anchored inthe bottom and projects upwards.This barrier wall is installed fromone side of the furnace to theother at the location of the hotspot. The height of the barrierdepends on the glass bath depth.

The combination of a well-engineered design (e.g. adouble row of blocks with offsetjoints) and modern refractorymaterials (e.g. chrome blocks)results in an installation that willfunction correctly during the wholeof the campaign.

SDR – SORG Deep Refiner®

The Deep Refiner® is a part ofthe furnace between the barrierwall and the throat where the glassbath is much deeper than in themelting area. The temperatureof the glass in this area is highestat or near the surface as a resultof the normal furnace super-structure heating. This createsan area in the glass bath withslow moving glass currents thatflow down towards the throat.

16

Advantages

■ increases the residence time and improves refining

■ improves the micro-homogeneity of the glass

GlasE/10.08/V2.2

Advantage

■ improved glass quality, especially at higher specific melting rates

The use of the Deep Refiner®

increases the residence time ofthe glass in the furnace, especiallyin the refining area. This givesmore time for the re-adsorptionof the gas from the remainingbubbles and for the establishmentof the micro-homogeneity soimportant in the forming process.The increase in residence timeof the glass in the furnace alsoleads to lower glass temperaturesin the throat and the working end.

The Deep Refiner® concept canbe applied to all types of con-ventional fossil-fired furnaces.

Refining bank

Successful refining of a glass islargely dependent on two factors:time and temperature, wherebytemperature has the most in-fluence. The refining bank is anadditional device in the tank,designed to raise the glass temperature in the refining zone,without increasing the super-structure temperature.

The bottom of the tank is raisedover a certain distance, to givea shallower glass depth. The glassflowing to the throat is forced up-wards into the hotter area near theglass bath surface. This fulfils oneof the prerequisites for improvedrefining – higher temperatures.

Higher glass temperatures in therefining zone result in lower glassviscosity, so the refining gasesreach the glass bath surfacemore easily. The shallower glassbath means that the actualdistance to the surface is shorter– another advantage.

The refining bank is not designedto improve the melting capacityof a furnace. It improves refiningand therefore the achievableglass quality.

Barrier booster

The installation of an electricbooster around the hot spotincreases the convection cur-rents and this raises the bottomtemperature of the glass bath.This has a positive effect on theglass quality.

Furthermore, additional energyis supplied to the glass bathand the temperature of the glassflowing from the hot spot to themelting area is increased. Thiscan lead to a small increase inthe melting capacity.

Booster

17GlasE/10.08/V2.2

Molybdenum electrodes can beinstalled horizontally through thetank side walls, or verticallythrough the furnace bottom. Acombination of these methodsis also possible.

Most SORG® boosting systemsuse induction regulators that pro-vide stepless voltage variation.For smaller installations a thyristorunit is used.

Under some circumstances anelectric booster can be installedin an operating furnace: in somefurnaces provision for a laterinstallation is made during construction.

Boosting systems can be dividedinto three groups according totheir usage:

Melting booster

A boosting system in the meltingarea supplies additional energydirectly to the glass bath andthis leads to a higher meltingcapacity.

Systems with bottom electrodesincrease efficiency, especially inlarger furnaces, and reduceelectrode wear. However, suchsystems are not suitable for furnaces with high cullet ratios.Here there is the risk that toomuch metal is introduced intothe furnace with the cullet.

Whereas small and medium-sizedfurnaces are normally equippedwith a single electrical booster, forlarger installations it is advanta-geous if the installed electricalenergy is split between twoseparate systems. This exertsthe maximum influence on theconvection currents and canproduce high specific meltingcapacities.

0 10 20 30 40 500.4

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Correlation between electric power

of booster and the additional tonnage

in medium-sized and large furnaces

The application of electricalenergy as an additional energysource in conventional fossil-fuel fired furnaces is referredto as electric boosting.

Local booster

Here the electrodes are installedin order to heat a specific areaof the glass bath.

The majority of these boostersare installed in the throat andriser, where the glass can cooldown too much when the melting rate is low. These areasare also at risk during furnacecommissioning before normaltemperatures and flow velocitieshave been reached.

Most installations consist of just2 or 3 electrodes and a smalltransformer with an installedpower between 40 and 100kVA.

CONTI-DRAIN® – cross sectionBubbling principle

Bubbler • CONTI-DRAIN®

Bubbler

Air or other gases are blownthrough special bubbler nozzlesinstalled in the furnace bottom.This produces large bubbles inthe glass and these rise to thesurface and the gas is exhaustedinto the furnace atmosphere.

The upward movement of thebubbles produces strong local-ised convection currents aroundtheir path, and these currentsmove bottom glass upwardscausing an increase in the glasstemperature at the bottom ofthe tank. The bursting bubbleson the glass surface also createan effective barrier that preventsunmelted batch from movingforwards on the glass bath surface.

In most cases the bubbler tubesare installed in a row across thecomplete furnace at the locationof the hot spot to strengthenthe convection currents at thislocation.

Sorg® bubblers normally havebubbler tubes made of molyb-denum disilicide. This material isextremely resistant to high tem-peratures and does not requirecooling.

Ceramic capillary tubes thatbecome blocked more slowly, ifthe air supply fails, may be usedas an alternative. For specialglasses the ceramic may beoverlaid with a platinum alloysheath.

18

Advantages

■ allows controlled drainageof very small amounts (<500 kg/24 h)

■ possible to extract contaminated bottom glass

GlasE/10.08/V2.2

CONTI-DRAIN®

Zircon cords in finished articlesare the result of corrosion of therefractory material that is usedin large quantities in modernglass furnaces.

Zircon has a significantly higherdensity than normal soda-limeglass. As a result, zircon-richmaterial formed as a result ofrefractory corrosion tends tocollect on the furnace bottom.

The Conti-Drain® is designed topermit the continuous draining ofvery small quantities of materialfrom the melter, working end orforehearth bottom. The low drainingvelocity prevents glass from higherregions being pulled down as aresult of the funnel effect andensures that contaminatedmaterial is removed effectivelyfrom the furnace bottom.

The system consists of a high-temperature resistant steel nozzleplate and a small electrical heating

system. The drain nozzle isinstalled on the underside of aspecial refractory drain block inthe bottom. The electric heatingheats the glass in the drain holeand maintains the temperaturenecessary for the required drai-ning rate.

The drain can be started orstopped at any time by switchingthe electrical heating on or off.

Advantages

■ simple method for increasingbottom temperature

■ assists retention of unmelted batch in meltingarea

CONTI-DRAIN® – in operation

Cullet Preheating • NOx Reduction

19GlasE/10.08/V2.2

Cullet Preheating

When the cullet content in thebatch amounts to more than 60%,as is often the case in containerfurnaces, the use of cullet pre-heating to recover additionalheat from the furnace wastegases is an economically viableproposition.

The SORG® cullet preheatingsystem, first introduced in 1987,is based on the direct heatingprinciple, where the hot wastegases and the cullet to be heatedare in direct contact with oneanother.

Cullet enters the tower constructionat the top and slowly makes itsway downwards to the exit. Thewaste gases enter at the bottomon one side and are exhaustedat the top on the other side,and so the cullet and the gasesare in cross counterflow.

The internal arrangement of thepreheater features louvre typevanes, which distribute the wastegases throughout the column ofcullet.

The velocity of the cullet in thepreheater is very low, which givesa residence time of several hours.This leads to very low mechanicalwear of the steel parts of thepreheater.

The waste gases enter the preheater at a temperature ofapproximately 500 °C and leavewith a temperature of approx-imately 200 °C. The culletpreheat temperature is typicallyapproximately 400 °C.

NOx Reduction

SORG® has been confronting theproblem of NOx reduction since1985.

Although prevention of the for-mation of thermal NOx is closelyrelated to better control of thecombustion process, it hasbecome apparent that there isno single system or procedurethat will reduce NOx emissionsto the level required today.

The design of the port necksand the complete furnacesuperstructure are very important,as these features exert a majorinfluence on the mixing of thefuel and the air contained in thecombustion air.

The distribution of the burnersand certain installation details, suchas their position and angle in theport necks, and the sealing in theburner block are also significantfactors as they influence theflame formation.

The heating control system is alsoimportant. On the one hand, astepped control system prevents

unnecessary fluctuations in thefuel supply, especially after thereversing process in regenerativefurnaces. On the other hand, withindividual burner control the fuelsupply to the individual burnerscan be controlled, so each burneris supplied according to itsrequirements.

Finally, the SORG® CascadeHeating System can be installedon regenerative furnaces toreduce NOx emission levels.The system uses a low velocitygas flame in order to createunder-stoichiometric conditionsaround the main flame root. Asa result of this effect the mainflame root is insulated from theoxygen contained in the com-bustion air. NOx production inthis area is significantly reduced.

In most cases it is possible toreduce the emission of thermalNOx to the current limits byimplementing all the measuresmentioned here.

Individual Burner Control

With nearly all types of furnace twoor more burners are groupedtogether and supplied with fuel.The fuel distribution for the indi-vidual burners is controlled locallyby means of control valves.However, as a result of currentefforts to reduce NOx emissionsa much more comprehensiveflame control is now required.

To a great extent NOx generationis dependent on the energy distribution and its stability in theport. In order to achieve constantratios it is absolutely necessaryto meter and control the fuelsupply to the individual burners.

With oxy-fuel heating it is alsopossible to measure and controlthe oxygen quantity on eachindividual burner.

The control equipment, consistingof a control valve and meteringequipment for each individualburner, is installed in a station. Anoverall control valve can beinstalled in the gas supply station,so that normal “all burner” controlcan take place if required.

Typical applications

• can be used for any type of furnace

20

Advantages

■ reduces NOx values

■ compensates for uneven burner conditions

GlasE/10.08/V2.2

Oxygen station for individual

burner control

Oxy-Fuel Heating

Typical applications

• can be used for any type of furnace

21GlasE/10.08/V2.2

Normal combustion air consists ofapproximately 80 % N2 as ballast.The N2 must still be brought tothe normal flame temperatureand this requires energy.

The use of oxygen instead of airsolves this problem and leads tosignificant energy savings.

The lack of N2 ballast leads to agreat reduction in the waste gasvolume, which makes waste gasheat recovery difficult. In certaincases attempts have been madeto use batch preheating to recoverpart of the residual heat.

However it is still usually neces-sary to cool the waste gasesbefore they can be led to anelectric filter. Waste gas coolingtakes place in a quench chamber,that is operated either with airor water, or a combination of both.

The waste gases from oxy-fuelheating contain much morewater (approx. 60 %) than thosefrom conventional heating systemsand this must be taken intoaccount when the waste gassystem is designed.

With oxy-fuel heating specialattention must be paid to refrac-tory selection. The type of material,the manufacturing tolerancesand the quality of the installationwork are important.

As there is very little N2 availablein and around the flame theproduction of thermal NOx isgreatly reduced. However, as thewaste gas volume is also reduced,relatively high volume-related NOxvalues (e.g. mg NOx per Nm3

waste gas) emerge.

Therefore manufactured quantitybased values, such as kg NOxper ton glass must be used toevaluate the emission values.

The main components of aSORG® oxy-fuel heating systemare the supply and control stationsfor oxygen and gas and thegas/oxygen burners.

The supply stations for gas andoxygen contain the pressurecontrol equipment and all nec-essary safety devices and anoverall control valve for regula-ting the total gas or oxygenquantity. The control valves forthe individual burners are installedin the control stations. This givesindividual control of the amountof gas or oxygen on each burner.

In some countries (e.g. USA)oxy-fuel heating systems arenow standard and are used inall sectors of the glass industry.In other parts of the world oxy-fuel heating is mostly used inthe special glass sector.

Advantages

■ extremely low NOx values

■ low energy consumption

Burner installation on a

SORG® oxy-fuel furnace

Batch Charging Technology

The ideal charging pattern in themelter consists of numerous,relatively small batch piles thatalmost cover the glass bath inthe melting zone.

As far as possible the batchpiles should not be in directcontact with the refractorymaterial of the side walls andcorners of the doghouse, or therear and side wall tank blocks,as this causes premature corro-sion of the refractory material.

Heat losses, dusting anduncontrolled infiltration of coldair are three important problemsthat can occur at the doghouse.

Different types of batch chargerwith different characteristics forvarious applications are available.

Pusher

The raw materials taken fromthe feed hopper are placed onthe glass bath by a stationaryvibratory chute. The batch floatson the glass bath and is thenpushed into the furnace by anindependent water-cooled pusher.

As a result, charging and materialmovement in the doghouse arecarried out independently,which produces a good batchcoverage.

The batch cover is optimised bythe operation of a freely pro-grammable swivel mechanismon the charger, allowing thebatch to be charged in severaldirections at slightly differentangles.

A curtain or cover greatly reducesradiation and provides protectionagainst dust and cold air infiltration.

Pusher chargers are used fre-quently, especially for end-firedregenerative furnaces as theyachieve excellent batch coverage.

Blanket charger

The water-cooled chute, that tiltsdown towards the doghouse ismoved backwards and forwardsby an eccentric. During the for-ward movement the batch isdeposited on the back of thechute, whilst the batch floatingon the bath surface in the dog-house is pushed into the furnace.

The basic charging rate can bevaried by setting the stroke lengthand height of a slide baffle on thedischarge outlet of the feed hop-per. The charging rate is varied bystepless adjustment of the chutemovement speed or by switchingthe movement on and off.

This charger is normally installedas a single machine on furnaceswith low melting capacities, e.g.from 40 – 70 t/24 h.

A non-water-cooled version isused for operation on large cross-fired furnaces, where severalchargers are installed next toone another.

Enfolding charger

The raw materials stored in thefurnace bunker are transportedby a vibratory or screw conveyorto a feed hopper installed onthe batch charger. From herethe batch falls by gravity onto awater-cooled tray that pushes itin individual portions into thefurnace. The whole machine isswivelled by a freely programmablemechanism, so that chargingtakes place in several directionsand optimum bath coverage isachieved.

The charger is mounted directlyon the doghouse and seals offthe whole area. This has theadvantage that heat losses arereduced, dusting is low and nocold air can enter the furnace.

22GlasE/10.08/V2.2

The way in which raw materials– normally a mixture of batchand cullet – are charged intothe furnace can have a signifi-cant influence on the meltingprocess.

23GlasE/10.08/V2.2

Batch Charging Technology

Screw charger

The raw materials from a smallbunker are charged into the fur-nace by a rotating screw installedin a water-cooled tube. Thecharging rate is varied by thechanging rotation speed of thescrew.

A conventional doghouse is notrequired for this type of charger,which is installed in a simpleopening in the superstructureside wall.

This simple installation reducesdusting and the infiltration of coldair as it is almost completelysealed.

Screw chargers are normally usedfor small furnaces for specialglasses that use low amounts of cullet.

Rotating crown

The SORG® patented rotatingcrown is used on larger furnaces.

Several small vibratory chutesare installed above the crown,which has small openings forthe introduction of the rawmaterials. The complete crown,with the vibratory chutes, rota-tes around the vertical axis ofthe furnace and the chutesdeposit concentric rings ofbatch on the glass bath surface.

It is easy to completely seal thesystem so there is virtually nodusting in the factory itself. Dustextraction is also simple as no coldair is drawn into the enclosedsuperstructure.

Charging technologyfor electric furnaces

The specific characteristics ofan electric furnace, which normallyhas a batch blanket over thecomplete melting area, require acompletely different chargingtechnology than a conventionalfurnace.

Frequently so-called X-Y chargersare used for electric furnaces.Here a conveyor is introducedthrough an opening in thesuperstructure side wall andmakes programmed movementsto distribute the batch evenlyover the glass bath. As only theconveyor itself is inserted into thefurnace a great deal of space isrequired outside, immediately nextto the furnace. Much more serious,however, is the fact that theopening for the charger causesvery high dusting near the furnace.It is also extremely difficult toextract and clean the batchgases as a great deal of cold airenters through the chargingopening.

Distributor arm

For smaller electric furnacesSORG® uses a distributor arm.The L-shaped, water-cooled armenters the furnace through a slitin the crown and is positionedso that the horizontal part is justabove the glass bath. The batchis charged into the furnace by avibratory chute and spread overthe entire glass surface by therotating distributor arm thatrotates around the vertical axisof the furnace.

SORG® Furnaces for Special Glasses

Lead crystal

Lead-containing glasses aremost successfully melted in all-electric furnaces. The use ofmolybdenum electrodes with lowfrequency electricity producesexcellent results.

Alternatively it is also possible touse a FlexMelter® with fossil-fuelheating, although here the emis-sion levels must be considered.

Fluoride opal glass

Fluoride opal is used for theproduction of bottles (e.g. in thecosmetic sector) and tableware.

Fluoride volatilisation causestwo sorts of problem: fluorideloss can lead to significant pro-blems with the quality (fluorine isresponsible for the opalisation)and fluoride emissions are sub-ject to very low emission limits.

The SORG VSM® all-electric fur-nace (see page 15) provides theideal solution. The cold batchblanket on the glass bath sup-presses the fluoride volatilisation.

SORG VSM® all-electric furnaceswith melting capacities up to 70 t/24 h have been used to meltfluoride opal glass since 1970.

24GlasE/10.08/V2.2

Although the majority of SORG®

furnaces are used to meltsoda-lime glasses, in recentyears a number of interestinginstallations for other glasseshave been made. Below is areview of the various furnacetypes for these glasses.

Borosilicate glass α = 32 - 40 x 10-7

Very high temperatures arerequired to melt and refine hardborosilicate glass. SORG® offertwo different solutions.

The Boro-Oxy-Melter® (see page14) was developed especially forsuch glasses. The combinationof an oxy-fuel heating system in thesuperstructure, a high capacityelectric booster and a refiningbank produces the necessaryconditions. So far this furnacetype has been built for meltingcapacities up to 70 t/24 h.

An alternative for melting this glassis provided by the SORG VSM®

all-electric furnace (see page15). So far VSM® installationswith melting capacities up to 45 t/24 h have been built forthis glass type.

SORG® Furnaces for Special Glasses

E glass

E glass is used for the productionof textile fibres used, for example,to strengthen plastic items suchas printed circuit boards. Theproduction process requiresabsolutely clean glass free ofany gaseous or solid inclusions.

In the main, recuperative side-firedfurnaces are used for this type ofoperation. SORG® have con-structed such installations withdaily melting rates up to 200 t/24 h.

However, the largest SORG®

furnace for this glass type andprobably the largest E glass furnace in the world, is an oxy-fuel melter with a maximummelting capacity of 305 t/24 h.

C glass

C glass is used to produce fibresfor insulating wool. The glass hasa high alkali content combinedwith boron and therefore theproblem of volatilisation fromthe glass bath surface occurs.

C glass has been melted in SORGVSM® all-electric furnaces (seepage 15) since 1975 and excellentresults have been achieved. So far VSM® furnaces for C glasshave melting capacities of up to 160 t/24 h.

However, a recuperative furnaceis most suitable if fossil fuels are tobe used. SORG® installations havemelting capacities up to 120 t/24h.

More recently SORG® oxy-fuelfurnaces have been built for this glass.

25GlasE/10.08/V2.2

Basalt or mineral wool

Mineral wool is an alternativeinsulating material to glass fibre.

The basis material can be meltedin furnaces that are similar to glassmelting furnaces. Such furnaceshave recuperative heating.

The material has a very highiron content and so has a lowradiation transmission. Thereforethe installation of an electricbooster to assist the fossil-fuelheating is very advantageous.

SORG® have built furnaces ofthis type with melting capacitiesup to 120 t/24 h.

For your notes

26GlasE/10.08/V2.2

GlasE/10.08/V2.2

Nikolaus Sorg GmbH & Co KG

Postfach 152097805 Lohr am MainGermany

Tel.: +49 (0) 9352 507 0Fax: +49 (0) 9352 507 196

[email protected]

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