Glass Melting Technology

Three basic types of flat glasses are manufactured:

• Plain flat glass Most plain glass used is now float glass. In this process liquid glass is cooled to give a viscosity sufficiently high for forming and it is then drawn across the surface of molten tin. This method may be used to produce very flat glass in large quantities.

• Textured, patterned and wired glass The rolled glass process is used for the manufacture these types. The glass is drawn in a horizontal ribbon on rollers. If flat glass is required from this process it must be ground.

• Laminated glass This is made with two or more sheets of glass which are bonded together with layers of plastic between them

TV glass, glass-ceramics, optical glass, glass tubes, borosilicate, TFT

1. Mechanisms of heat transfer from flames to batch and glass2. Alkali volatilization and particulate matter formation : The mechanisms and pathways

of alkali volatilization, transport, deposition, and emission are examined by optical measurements of sodium species concentrations throughout the combustion space

3. Improve understanding of the mechanisms of bubble formation, growth, and fining :use a laser to illuminate seeds and bubbles in the glass and to follow the growth and rise of the bubbles in images of the region illuminated by the laser.

4. Develop and test sensors for melting parameters (e.g. temperature, viscosity, NOx, colorants, redox state, velocity )

5. Design intelligent model-based control and process optimization systems 6. Observe glass circulation, its dependence on furnace conditions, and its relationship to

defects in glass 7. Improve understanding of combustion dynamics and emissions 8. Mechanism of batch melting 9. Mechanisms of refractory corrosion

Container glass industry1: Oxy fuel furnaces with heat recovery system2: Cross fired furnace (natural gas) with raw material preheater3. Horseshoe furnace (heavy oil) with exhaust gas heat recovery system4.Horseshoe furnace (natural gas)5. Regenerative horseshoe fired furnaces with air preheating.

Flat glass industry1.Flat glass furnace (heavy oil) with exhaust gas heat recovery system2. Float glass furnace (heavy oil / natural gas) with exhaust gas heat recovery

Electric furnace (borosilicate glass)Oxy fuel furnace (borosilicate glass)

Oxy fuel furnaces (TV glass)

NOx reduction measures1.Near stoichometric Combustion2.Sealing measures 3.lambda adjustment4.Furnace geometry5.burner assembly6.adjustable burners

The process of glass melting may be sub-divided into five individual steps.They are distinctly different with respect to• time demand, or dwell time of a volume element of material,• heat demand or, in the case of refining and homogenization, heatrelease• the temperature level at which the process step typically proceeds

In the glass industry, mainly natural gas or fuel oil is used for combustion and consequently energy supply for the batch melting and fining (removal of gases from glass melts) process.

The main constituent of natural gases is methane gas (CH4)Very small amounts of mercaptan is added to give the natural gas an odor for safety reasons. Mercaptan, a harmless, non-toxic substance that has a strong "rotten egg" smell is added as a safety precaution to natural gas (and propane) to make it easier to detect in case of a leak..

Most natural gases hardly contain metals or sulfur. The main emissions from combustion of natural gas are NO, NO2, CO and CO2 and water vapor

Typical temperatures in the core parts of the flames in glass furnaces can be 1950 – 2500 oC. At such high temperatures radical species (e.g. O, OH, H) are important.

Fuel oil

fuel oil on the other hand contains several impurities ,Including sulfur

Fuel oils with higher molar ratio (compared to natural gas) of carbon (C) versus hydrogen (H), of typically about 1:1.6 to 1:1.9, will often show cracking and carbon rich soot formation, especially at air/oxygen lean (fuel rich) conditions. The large number of hot soot particles in the flames behave like very small black bodies emitting light at high intensity (emissioncoefficients for heat radiation approaching the value 1).

1 evaporation of oil from small oil droplets atomized by nozzles at high velocities into the combustion chamber;2 formation of organic vapors of hydrocarbon molecules with many atoms (molecules containing mainly carbon and hydrogen, some oxygen and sulfur);3 attack of these organic molecules by radical species (O, OH, H) and oxidation and radical reactions in many propagating and branching reaction steps;4 in oxygen lean flames: formation of soot, aromatic compounds and CO and prompt NOx (nitrogen – radical hydrocarbon reactions forming HCN or CN that subsequently decompose into NH and N radicals reacting with OH: N + OH -> NO + H) and fuel NO (formation of nitrogen oxides from CN-groups in the fuel releasing HCN, that oxidizes into NO and N2)5 in oxygen rich flames, formation of CO2, H2O and NOx after many intermediate reaction steps and intermediate reaction products. Metals in the fuel are generally oxidized and sulfur is converted into SO2 (at temperatures above 1000 oC, most sulfur is in the SO2 (gas) state, at lower temperatures SO3 becomes thermodynamically more stable in presence of oxygen, but the oxidation of SO2 into SO3 is often a very slow process without a catalyst).

To provide high enough flame temperatures and to improve energy efficiency,preheating of the combustion air is needed. The combustion air is preheated in large heat exchangers by the use of the flue gas heat contents. There are two types of heat exchangers: regenerators (55-65% heat recovery) andrecuperators (25-40%)

Regenerative furnacesRegenerative furnaces usually have two or more regenerators. A regeneratorconsists of a regenerator chamber wherein a checker work of refractory brickshas been stacked. The bricks form a regular construction with channels for theflue gases or combustion air. First the flue gases are transported through oneregenerator heating the checker work. After about 20 minutes the checkerwork is heated to its optimal temperature. Then the combustion air is ledthrough this regenerator and the flue gases through the other. The combustionair is preheated to 1100-1300ºC by the heat of the checker work. Again after20 minutes the checker work is cooled down too much to heat the combustionair and the process is reversed.There are two types of regenerative furnaces: cross-fired and end-firedfurnaces.

Cross-fired furnaceIn this type of furnace, the burners (actually gas or fuel oil injecting lances) aresituated along the sidewalls of the furnace inside or under the burner ports. Adrawback is that the many burner ports in the superstructure may lead toadditional heat losses and leakages of cold ambient air into the furnace or hotcombustion gases out of the furnace.

End-fired furnace: In this type of furnace, the burners and the regenerator chambers are situated at the back wall side. Two burner ports are located in this back wall. The flames or their combustion gases are reversed at the end (against the shadow wall), therefore the name U-flame or horseshoe fired furnace is used.Advantages of this furnace type are that there are only two burner ports andfairly compact regenerators can be built. This is beneficial for the energyconsumption (less heat losses). A disadvantage is that it is difficult to adjust the firing and therefore to control the temperature profile over the length of the furnace

Recuperative furnacesRecuperative furnaces are equipped with one or two recuperators. Arecuperator is a heat exchanger, in which heat is transferred directly from theflue gases to the combustion air in co-current or counter current flow. Thisheat exchange is based upon radiation. These exchangers are therefore calledradiation recuperators. The combustion air is preheated to 600-800°C. Highertemperatures cannot be reached, because the used metallic materials cannotwithstand higher temperature levels.

cross-fired furnace allows to control the position of thehot spot directly by the flame distribution. This makes its operation moreflexible with respect to changing pull rates than the operation of ahorseshoe flame furnace. However, due to the relatively long path of itsflame, the latter furnace type is superior to the above type with respect toenergy utilization.

Emissions

NOx-emissionsNitrogen oxides are especially formed during the combustion process. The oxygen ofthe air reacts with nitrogen of the air or of natural gas. This reaction can only takeplace at high temperatures (above 1350°C) and in the presence of both oxygen andnitrogen at the same spot. Additional NOx-emissions are obtained when nitrates areused. Nitrate is often used to oxidize the batch or melt.SOx-emissionsSulphuric oxides are emitted from almost all glass furnaces. The ratio SO2:SO3 isabout 10:1. The SOx originates from the fining and fluxing agent (sodium sulphate)and from sulphur contamination of the raw materials and fuel oil. Sulphur exists in theflue gases as dust sulphates (e.g. Na2SO4 and K2SO4), SO2, SO3 and as H2SO4 attemperatures below 200°C.Dust emissionsDust is mainly originating from the condensation of the glass melt vaporizationcarry-over products or reaction products of these vaporized compounds during thecooling of the flue gases. Primary dust condensates are for instance sodium sulphates,lead oxides, sodium borates, potassium borates and potassium sulphates. In case fueloil is used also vanadium and nickel oxides may be present. When producingcontainer glass the condensation of sodium compounds accounts for the greater partof the dust emissions.

Continued…….

Chlorides and fluoridesChlorides are mostly present as hydrochloric acid (HCl) in the flue gases. Sources ofchlorides are synthetic sodium carbonate and in smaller amounts dolomite or cullet. Fluorides are primarily present as HF and sometimes as H2SiF6. Mineral raw materials often contain fluorine minerals.Heavy metalsImportant emissions are sometimes lead in container glass furnaces, and vanadiumand nickel in furnaces fired with fuel oil. Selenium, probably as SeO2, and arseniccompounds are sometimes present in the flue gases. These compounds are gaseous at normal flue gas temperatures. This causes problems when removing these compounds from the flue gases. Most of the other heavy metals can be filtered out by dust filtration.

Conventional gas combustionIn this method fuel gas and air are used for the combustion process. To provide thedesired flame temperatures and to achieve affordable energy costs, airwill almost always have to be preheated. Burners are used to inject fuel gasinto the furnace. The fuel gas enters the furnace through a hollow tube (the burner),and is then mixed with the (preheated) air. Due to the high temperatures and thepresence of both fuel gas and oxygen (in air) the mixture will ignite. The energyprovided by this reaction is used for the glass forming reaction and the heating of thefuel gas, air, reaction gases and feed. A great part of this energy will leave the furnacewith the combustion gases. Using regenerators and recuperators respectively 55-65%and 25-40% of this energy can be recycled.

Using a U-turn melting furnace, the flame length needs to be relatively long.Therefore a slow burn out is needed, which can be achieved by adjusting theangular position of the burner. This long flame length reduces the flametemperature (caused by an increased amount of radiation) and the local oxygenconcentration and thus the NOx formation.

Conventional oil combustion

The difference between this method and fuel gas combustion is the usage of fuel oil for the combustion process. Occasionally cold air (primary air) is used for injecting the fuel oil into the furnace. The fuel oil must be heated to 120ºC to reduce its viscosity to the point it is fluid enough for the injection. This preheating of the fuel requires extra energy compared to conventional gas combustion.

The advantage of fuel oil above fuel gas is the higher energy amount per volume and the higher carbon/hydrogen ratio. Due to this higher ratio,the flame will be more luminescent (more carbon soot), which causes lower flame temperatures. This results in lower Nox emissions. Also, less combustion gases are produced. Combined oil combustion is about5% more energy efficient than gas combustion. The disadvantage is the higher amount of polluting compounds (sulfur) and heavy metals(vanadium and nickel) that cause unwanted emissions and corrosion.

Oxy-fuel combustion

In this combustion method oxygen is used instead of air for the combustion of gas. Air contains 79% of inert nitrogen. The inert gases are also heatedto a temperature of about 1500°C. After combustion, these gases leave the furnace at a temperature of about 1450°C. The energy losses by these gases are very high, but nowadays it is possible to generate steam with this energy. This steam can be used inside the factory (for example in the forming process) or to generate electricity. When there is not any inert nitrogen present, theamount of gases that has to be heated is smaller and the amount of flue gases is limited. The energy losses by these inert gases are limited by the use of pure oxygen instead of air for the combustion. The oxygen cannot easily be preheated like air, because of fire and explosion risks. The flame reaches higher temperatures with oxygen combustion. It is possible to insulate the furnace better, because the furnace is more compact and there are no big burner ports present. This insulation is also needed to prevent the alkali containingfumes from attacking the superstructure at the cold areas (<1450°C). The burners used for oxy-fuel combustion are for instance pipe in pipe burners. Thefuel is injected through the inner pipe, the oxygen through the outer.Electricity is needed for the production of oxygen.

The final flame temperature

• the heating value of the fuel, HG or HN (= net calorific heating/combustion value)• the preheat temperature of the combustion air, Ta ,• the average isobaric specific heat values (from 0-Tflame) of the different gases involved: cpi ,• the degree of burn out,• the heat transfer to the melt and sidewalls,• the excess air or oxygen,• the O2-enrichment of air or the use of pure oxygen.

BURNER PRINCIPLE

A burner in a glass furnace, has the function of combining fuel with air or oxygen in a well defined manner in order to get a continuous firing, as this is desired after ignition. In the glass industry burners with pre-mixing of the fuel and combustion air (hot air) are applied, for instance for recuperative furnaces where air is preheated up to 600-750 oC in recuperators by flue gases.

In regenerative furnaces and in oxygen-fuel fired furnaces, the fuel and oxidant (combustion air or oxygen) are often injected separately in the combustion space. Upon mixing (turbulent mixing) of the combustion air or oxygen and the fuel in the combustion chamber of a glass furnace, combustion reactions take place and flames are formed with temperatures between typically 1700-2500 oC. Because of these high combustion temperatures (fast reaction kinetics), the mixing between the reactants (fuel and oxidant) determines the combustion rate.Mixing is often turbulent, we call these flames: turbulent non premixed flames.

A burner has to fulfill boundary conditions or requirements which are:• no attack or contamination (soot) of the burner,• proper positioning of the flame within the combustion chamber, the flames should cover a large surface area of batch blanket and melt but the reactive parts (flame that contains radical species and has high temperature levels) should not touch the refractory walls;• maximum allowable crown temperature of about 1600 °C (in case of silica crowns) ,• controlled heat transfer along the length of the flame,• low production of environmentally hazardous or corrosive components, like NOx and CO.

Burners, as applied in the glass industry, especially in regenerative furnaces, generally consist of a (sometimes water cooled) metal tube, through which the fuel is squirted or injected with a velocity of 50 to more than 100 m/s into the furnace. Lowering the fuel injection velocities generally will cause slower mixing of fuel and combustion air or oxygen and results in longer, wide flames.

The conventional combustion uses no additional oxygen, while oxy-fuel combustion uses pure oxygen. Three intermediate forms are available: oxygen boosting, oxygen lancing and oxygen enrichment.

Oxygen boostingThe boosting concept uses oxy-fuel burnerspositioned within the air-fuel mixture to increaseproduction, quality, efficiency and furnace stability.Oxy-fuel boosting is used to increase the glass pullrate on a furnace. Extra fuel is combusted withoxygen to get higher temperatures. This techniqueuses conventional combustion as main combustiontechnique.

Oxygen lancing

oxygen lancing is the most commonway to use oxygen as a supplement to combustion to raise the production capacity. The injection of oxygen beside, beneath or through air-fuel flames causes glass melting furnaces to reach a higher pull rate, fuel efficiency and glass quality. The oxygen can be injected where it is mostly needed.

Oxygen enrichmentOxygen is injected into the main combustion air header well ahead of the point where the burner enters the furnace. This pre-mix of oxygen is most common on melting furnaces, when it is desirableto use the oxygen to enhance the entirecombustion process in a consistent manner.

Radiant tube technologyThe 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 tubetechnology leads to advantages like high power density and low emissions, whichmostly result from the very intense heat transport within the porous structure. Themost important criterion for combustion is the critical pore size inside the porousstructure. Experiments resulted in the following modified Péclet number for flamepropagation 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 thesurroundings dominates over conductive and radiative heat transport to the porousmaterial. In that case the combustion heat is transported out of the tube and radiationfrom the tube to the surroundings is possible.If the modified Péclet number is lower than 65, conductive and radiative heattransport to the porous material dominates over convective heat transport to thesurroundings. There is not enough combustion heat that can be transported to thesurroundings.

If the pore size is smaller than the critical dimension (i.e. when the modified Pécletnumber becomes lower than 65), flame propagation is prohibited and the flame isquenched. On the other hand, if the pore size exceeds this critical dimension, flamepropagation inside the porous structure is possible.

Staged combustion

aluminium oxide fibres or silicon carbide foams

Heat transfer

• Gas flame with airFor the gas flame we assume a flame temperature of 2023 K and εv of 0.25 .This results in a heat transfer of 162.7 kW/m2. We need to transfer 8.16 MW to theglass, so a flame area of 50 m2 is needed.• Oil flameIn this case we use a flame temperature of 1973 K and εv of 0.5. Then the heattransfer is 180.6 kW/m2. For the required heat flux of 8.16 MW to the glass melt, weobtain a flame area of 45 m2.• Gas flame with oxygenThe flame temperature for a gas/oxygen flame is higher than a gas/air flame.Therefore we assume a value of 2123 K for the flame temperature. An oxygen/gasflame should have a lower emission coefficient than an air/gas flame because of lessformation of soot. We assume that this effect is compensated by the higher CO2 andH2O concentrations. So we take the same εv of 0.25 in both situations. Calculationsfor the heat transfer then result in 252.4 kW/m2. For the flame area we get 32 m2 (ofcourse we need the same amount of heat transfer to the glass melt, 8.16 MW).So the lowest flame area and the highest heat flux can be obtained in the situation ofa gas flame with oxygen.

When the electrodes are placed horizontally the electrodes cause a certain convection pattern that sets up a thermal dam. This dam retards the forward stream of the glass along the tank. Much more favourable is the situation where the electrodes are placed vertically through the bottom of the tank , because in this case the hot glass melt can rise free and unhindered to the surface. This improves the convection current, which results in better mixing, homogenising and good decolourising. Because of the vertical placement of the electrodes, the upward movement of the intensely heated glass near the electrodes is accelerated and the danger of overheating the glass near the electrodes is also limited.

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 refractories with 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 of refractory linings. While this reduces heat loss from a furnace, the amount of heat stored in the refractory is increased and the refractory materials must withstand higher mean temperatures. Because the working lining acts as a heat sink, valuable process energy is absorbed by the refractories and lost by conduction to the cold face of the lining. Additional convective energy held by the furnace combustion gases is lost up the flue (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 x (TC

4 – TL4)

Q = re-radiated energy absorbed by the furnace load Ew = emissivity of the coating σ = Stefan-Boltzmann constant TC = coating temperature TL = load (glass) temperatureSince 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 .