Advanced melting Technology

+ Glass Processing

Continuous glass furnaces •Usual synonyms for a continuous furnace are glass-melting tank or tank furnace.

•These furnaces are applied for

Container glass production

Flat glass (Float & Rolled) production

Most tableware glass production

Fiber & glass wool production

Most specialty glass production (tubes, display glass, glass-ceramics, lighting

bulbs,..)

Float Glass Production

•These furnaces not applied for:

Most hand-made glass

Vitreous silica

Optical glass fibers

Continuous glass furnaces characteristics •Tank of refractory material, continuously charged with mixed batch

•Heat transfer from combustion chamber using fossil fuel (mostly natural gas) firing

with preheated air or oxygen

•All basis process steps in different zones or sections of furnace

•Continuous operation, during campaigns 5-15 years

•Indefinite number of trajectories from batch charger to exit of furnace (throat or canal).

•These furnace types are suitable for the mass production of glass

•The furnace melting capacity (glass pull) usually is expressed in the number of

(metric) tons of glass melted per day (24 hours)

• Depending on the furnace and type of glass produced, the pull can vary from ~ 20

tons per day (TPD) up to > 700 TPD

•Within the melt, currents (glass melt flow patterns) are being generated, both by pull & by free convection •Extra mixing by the application of bubbling or electrodes •Possibility to boost energy input using electrodes •Electric current in melt will release latent energy •Large number of trajectories of material in tank: wide residence time distribution & quality differences depending on route •Temperature gradients in melt: higher levels (close to the surface) are generally hotter than bottom glass melt •Weirs or dams are optionally applied to bring bottom glass to upper glass melt layers •Using air preheating (regenerators/recuperators) or pure oxygen

Continuous glass furnaces characteristics A melting furnace

consists of :

•Melting tank (glass melt bath)

•Superstructure (combustion chamber)

•Throat as connection between the melting end and the riser that brings the

molten glass in the refiner, working end or distributor

•Neck in case of float glass production, between the melting end and working

end

•Working chamber (working end, gathering end, nose, refiner)

•Heat exchangers: regenerators or recuperators

Oxygen-fuel fired furnaces (Oxy-fuel) •The fuel is fired without nitrogen in the applied oxidant (pure oxygen) (lower volumes

of flue gases, less diluted)

•In general, oxy-fuel glass furnaces have the same basic design as recuperative

glass melters, with multiple lateral burners and a limited number of exhaust port(s).

•Most oxygen fired glass furnaces hardly utilise heat recovery systems to pre-heat the

oxygen supply to the burners (there are some developments in oxygen and natural

gas preheating using the heat contents of the flue gases)

•Burners positioned in special burner blocks in the sidewalls

•Typically only 4 to 6 burners per sidewall are installed.

•NB: Burners from opposite sidewalls are preferably not placed in one line. This

would lead to instable flame tips influencing each other.

Advantages

cheaper furnace designs

lower specific NOx emissions (in kg NOx/ton molten glass);

smaller flue gas volumes

smaller footprints for furnace system

reduction in fuel consumption

•Drawbacks

oxygen costs may exceed the reduction in fuel costs

oxygen-firing require higher refractory quality superstructures

Foaming •Due to the release of gas bubbles and certain lifetime of the bubbles at the glass

melt surface, a foam layer may arise

•Foaming is caused by degassing of the glass melt during fusion of the batch

blanket (primary foam) and/or during fining process (secondary foam).

• Foam has a strong insulation effect.

• It prevents the radiative heat penetration from the combustion chamber into the

melt.

• This is unfavorable for the fining process, because especially the primary fining

process needs a high temperature

Emissions from glass furnaces

Important factors: • Temperature of the glass melt surface

• Composition of the glass melt, especially the contents of volatile components

• Sodium sulfate added

• Composition of the furnace combustion atmosphere,

• Gas velocity at the melt surface

• Residence time of the glass melt in the furnace or the specific surface area of

the melt

• Foaming of the glass melt

Refractories and furnace lifetime •The furnace lifetime depends directly on refractory quality

•Lifetimes for melting tanks based on AZS fused cast (Alumina Zircon Silica)

refractories

Container glass furnaces: 12 - 16 years*

Float glass: 14 - 18 years*

Tableware soda-lime-silica glass: 6 - 8 years

•Selection of appropriate refractories is crucial

* Hot repairs included during campaign:

- plates of AZS or Chromium oxide covering soldier blocks

- repairs of open joints & holes with patch

- ceramic welding by metal/metal oxide powder (oxytherm)

Refractory selection – General aspects •Not only the chemical composition, but the microstructure and macrostructure of

the material (grain sizes, binding phases) determine refractory behavior

•Impurities generally decrease the refractory quality

•For combustion chamber applications refractory (crown) temperatures may reach

levels between 1550 and 1700°C (higher temperatures typically observed in furnaces

for borosilicate glasses or glass-ceramics)

•The refractory materials, exposed to molten glass, are operated at about 1100-1550°C

in most cases.

•In regenerators temperatures levels between 1550°C (at the top) and 500°C (at the

bottom) can be observed

•Important factors to consider include:

Temperature resistance and stability at high temperatures

Thermal shock resistance (especially during first heating)

Thermal expansion

Thermal conductivity

Mechanical resistance / Deformation under load

Corrosion resistance (e.g. different behavior in acid or basic environments)

Costs!

Radiant tube technology

The basic idea of a radiant tube burner is to fire the fuel inside a tube. The

released energy is first transferred through a porous material to the tube wall

and then transported to the glass melt by radiation from this wall.

The tube can be placed above or inside the glass melt. Placing the tube inside

the melt results in a major problem. The tube can dissolve in the melt and will

be severely damaged. Dissolving of the tube in the glass melt will lead to lower

glass quality. A possible solution for this problem is to use tubes with an outer

wall made of a material that is more resistant against the glass melt. An

example of such a material is molybdenum.

Porous burner technology

Unlike conventional combustion processes, the porous burner technology does

not operate with free flames. Rather, the combustion takes place in the cavities

of a porous inert medium, resulting in a totally different appearance of the heat

source itself.

Compared to conventional combustion processes with free flames, radiant tube

technology leads to advantages like high power density and low emissions, which

mostly result from the very intense heat transport within the porous structure. The

most important criterion for combustion is the critical pore size inside the porous

structure. Experiments resulted in the following modified Péclet number for flame

propagation in porous media

Pe = modified Péclet number

SL = laminar flame velocity (m/s)

dm = equivalent porous cavity space diameter (m)

cp = specific heat capacity of the gas mixture (J/kg.K)

ρ = density of the gas mixture (kg/m3)

λ = thermal conductivity of the gas mixture (W/m.K)

η = efficiency of the combustion and heat transfer to the

If the modified Péclet number is higher than 65, convective heat transport to the

surroundings dominates over conductive and radiative heat transport to the porous

material. In that case the combustion heat is transported out of the tube and radiation

from the tube to the surroundings is possible.

If the modified Péclet number is lower than 65, conductive and radiative heat

transport to the porous material dominates over convective heat transport to the

surroundings. There is not enough combustion heat that can be transported to the

surroundings.

If the pore size is smaller than the critical

dimension (i.e. when the modified Péclet

number becomes lower than 65), flame

propagation is prohibited and the flame is

quenched. On the other hand, if the pore

size exceeds this critical dimension, flame

propagation inside the porous structure is

possible.

Staged combustion

aluminium oxide fibres or silicon carbide foams

Staged

combustion, compared with unstaged operation, can reduce NOx-emissions by

more than 50%. Two different stages can be distinguished. In the primary zone a

lean combustion takes place and in the second stage methane is added.

high emissivity ceramic coatings

1. high emissivity coatings that will strongly adhere to dense refractories, insulating fire brick, refractory ceramic fiber, and most metals. Coating glass tank refractorieswith emissivity ceramic coatings will provide more even heating, increased productivity, longer refractory life, and fuel savings.

2. It should not be an insulator. It is not a barrier to the conduction of thermal energy through a furnace wall

Insulating refractories are generally placed behind dense refractories at the cold face ofrefractory linings. While this reduces heat loss from a furnace, the amount of heatstored in the refractory is increased and the refractory materials must withstand highermean temperatures. Because the working lining acts as a heat sink, valuable processenergy is absorbed by the refractories and lost by conduction to the cold face of thelining. Additional convective energy held by the furnace combustion gases is lost up theflue (See next Fig)

When coating is applied to the hot face of the furnace refractory in the superstructure and crown, radiant and convective energy from the burners and hot furnace gases are absorbed at the surface of the coating and re-radiated to the cooler glass batch

For effective operation the temperature of the coating surface must be greater than the temperature of the glass, which is always the case whether the glass batch is being melted or whether the molten glass is being refined. The amount of heat re-radiated from coating is predicted by the following equation: Q = Ew x s x (TC

4 – TL4)

Q = re-radiated energy absorbed by the furnace load Ew = emissivity of the coating σ = Stefan-Boltzmann constantTC = coating temperatureTL = load (glass) temperature

Since the temperature of the coating and the temperature of the glass are raised to the fourth power, it is apparent that coating absorbs and re-radiates the most energy when the temperature difference between the coating and the load is the greatest. The application of coating above the melt line increases the radiative component of heating glass at the expense of the convective component. The coating absorbs convective heat from the hot gases and re-radiates this energy to the glass. The result is less energy being lost up the flue and more energy being used to heat the glass. Uncoated refractories have emissivities, Ew, in the range of 0.4-0.6 at glass melting temperatures. The application of coating to the refractory increases the emissivity of the refractory to about 0.9. This means that about 90% of the energy absorbed by the coating is re-radiated to the cooler glass. It is easy to see that by increasing the Ew of the refractory, the heat absorbed by the glass, Q, will increase significantly. This may not be desirable where over-heating can change the viscosity of the glass and alter the entire production process, so something else in the equation must be reduced to compensate for the increase of EBwB, to maintain a constant Q. The factor that must be reduced is the temperature of the coating and the furnace gases, and this is achieved by reducing the total energy input to the furnace. Of course, as total energy is reduced, fuel savings are gained .

Glasses Flat glass (windows)

Container glass (bottles)

Pressed and blown glass (dinnerware)

Glass fibres (home insulation)

Advanced/specialty glass (optical fibres)

The float glass process was developed by Sir Alastair Pilkington and patented by Pilkington in 1959

Functions of a Float Bath

•Need to produce glass as flat as possible–Bottom surface becomes flat since liquid tin (very dense) provides flat surface–Top surface becomes flat by glass flow due to gravity

•Need to produce glass to needed thickness and width–ADS machines are used

•Need to cool glass from 1100 C to 600 C–Coolers are used at appropriate locations

•Tin must not get oxidized–Bath atmosphere kept reducing 95% N2+ 5% H2

Requirements•Denser than glass

•Minimal chemical reactivity with glass

•Glass doesn’t “wet” it

•Boiling point well above 2000 °F

•Melting point below 1100 °F

•Low vapor pressure

•Manageable metal chemistry

•Affordable

Float bath

Glass from the melter flows gently over a refractory spout on to the mirror-like surface of molten tin, starting at 1,100oC and leaving the float bath as a solid ribbon at 600oC. The principle of float glass is unchanged from the 1950s. But the product has changed dramatically: from a single equilibrium thickness of 6.8mm to a range from sub-millimetre to 25mm; from a ribbon frequently marred by inclusions, bubbles and striations to almost optical perfection. Float delivers what is known as firefinish, the lustre of new chinaware.

Production Process The basic science: If molten glass is poured onto a bath of clean molten tin, the glass will spread out in the same way that oil will spread out if poured onto a bath of water. In this situation, gravity and surface tension will result in the top and bottom surfaces of the glass becoming approximately flat and parallel. The molten glass does not spread out indefinitely over the surface of the molten tin. Despite the influence of gravity, it is restrained by surface tension effects between the glass and the tin. The resulting equilibrium between the gravity and the surface tensions defines the equilibrium thickness of the molten glass (T). The resulting pool of molten glass has the shape as the given picture.

When molten glass is poured onto tin, it will spread until the interfacial tensions and gravitational forces are balanced, resulting in a circular glass-tin interfacial shape with an equilibrium thickness of 6.9mm.When forming a continuous ribbon, the glass will always attempt to achieve equilibrium thickness. If the glass thickness is less than 6.9mm, the ribbon will collapse to become thicker.If the glass thickness is greater than 6.9mm, the ribbon will spread to become thinner.

Glass flows onto the tin at a thickness of approximately 5 cm and spreads

to achieve an equilibrium thickness of .6858 cm.

The lehr force is attenuating the glass to a final thickness less than

equilibrium. The internal stress in the glass attempts to narrow the ribbon

back to equilibrium thickness. The outward force imposed by the ADS

machines minimizes the collapse to produce a ribbon of desired thickness

and width.

ADS Machines•Work in conjunction with attenuating lehr force to form ribbon thickness and width•Impart angular force to top surface of ribbon for controlling ribbon width and speed•Affect ribbon thickness contour

•Used for less than equilibrium thickness: ADS “Assisted Direct Stretch”•Used for greater than equilibrium thickness: RADS “ Reverse Assisted Direct Stretch”

CoatingCoatings that make profound changes in optical properties can be applied by advanced high temperature technology to the cooling ribbon of glass. On-line chemical vapour deposition (CVD) of coatings is the most significant advance in the float process since it was invented. CVD can be used to lay down a variety of coatings, less than a micron thick, to reflect visible and infrared wavelengths, for instance. Multiple coatings can be deposited in the few seconds available as the glass ribbon flows beneath the coaters. Further development of the CVD process may well replace changes in composition as the principal way of varying theoptical properties of float glass.

Despite the tranquillity with which float glass is formed, considerablestresses are developed in the ribbon as it cools. Too much stress and theglass will break beneath the cutter. The picture shows stresses throughthe ribbon, revealed by polarised light. To relieve these stresses theribbon undergoes heat-treatment in a long furnace known as a lehr.Temperatures are closely controlled both along and across the ribbon.

InspectionThe float process is renowned for making perfectly flat, flaw-free glass. But to ensurethe highest quality, inspection takes place at every stage. Occasionally a bubble is notremoved during refining, a sand grain refuses to melt, a tremor in the tin puts ripplesinto the glass ribbon. Automated on-line inspection does two things. It reveals processfaults upstream that can be corrected. And it enables computers downstream tosteer cutters round flaws. Inspection technology now allows more than 100 millionmeasurements a second to be made across the ribbon, locating flaws the unaided eyewould be unable to see. The data drives 'intelligent’ cutters, further improving productquality to the customer.

Types of GlassTempered GlassLaminated GlassReflective GlassPattern GlassWired Glass Fireproof Glass

Tempered GlassThe process of creating tempered glass is to heat the original glass to a very high temperature and

cool it down rapidly to form surface compression on the glass.

Tempered glass is 3-4 times as strong as common glass. Broken by an external force, it will become

bean-sized pieces, preventing injury to people.

Laminated Glass:Laminated glass is a combination of two or more glass sheets with one or more interlayers of plastic (PVB)

or resin. In case of breakage, the interlayer holds the fragments together and continues to provide resistance

to the passage of persons or objects. This glass is particularly suitable where it is important to ensure the

resistance of the whole sheet after breakage such as: shop-fronts, balconies, stair-railings, roof glazing.

Bullet resistant glass

Reflective GlassReflective glass refers to coating the surface of the glass with one or many

layers of metal, alloy, or metal compound film to effectively control the

transmission of visible lights and low infrared.

In light of the different performance, the reflective glass can be classified into the

following:

heat-reflecting glass, low-emissivity glass .

The Rolled Glass Process Patterned glass

Pattern GlassPattern glass is produced by coating and rolling clear or body-tinted, translucent clear, flat glass.

The glass is heated to its softening point and passed between two rollers, which emboss the pattern

into the glass.

It not only provides function of visual screen but also creates aesthetic senses of changing

lights and shades.

Patterned glass is made in a single pass process in which glassflows to the rollers at a temperature of about 1050 ºC. The bottomcast iron or stainless steel roller is engraved with the negative of thepattern; the top roller is smooth. Thickness is controlled by adjustment ofthe gap between the rollers. The ribbon leaves the rollers at about 850 ºCand is supported over a series of water-cooled steel rollers to theannealing lehr. After annealing the glass is cut to size.

‘Obscure’ glass is the term used for any glass that distorts the view.

Wire GlassWired glass is a product in which a wire mesh has been inserted during production.

A steel wire mesh is sandwiched between two separate ribbons of semi-molten

glass, and then passed through a pair of metal rollers which squeeze the

"sandwich of glass and wire" together. It has impact resistance similar to

that of normal glass, but in case of breakage, the mesh retains the pieces of glass.

This product is traditionally accepted as low-cost fire glass.

WIRED GLASS

Wired glass is made in adouble pass process. Theprocess uses twoindependently driven pairs ofwater cooled forming rollers eachfed with a separate flow of moltenglass from a common meltingfurnace. The first pair of rollersproduces a continuous ribbon ofglass, half the thickness of the endproduct. This is overlaid with awire mesh. A second feed of glass,to give a ribbon the samethickness as the first, is thenadded and, with the wire mesh"sandwiched", the ribbon passesthrough the second pair of rollers,which form the final ribbon ofwired glass. After annealing, theribbon is cut by special cuttingarrangements.

Glass Containers

Pressed Glass Processing

Softened

Gob

Glass Forming

Casting - molding

Pressing – pressing second mold into molten glass

Core-forming – clay core dipped into molten mass

Fusing – fusing glass rods together around a mold

Blowing – blowing air into a glob

Blow Molding

Softened

glass