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 glass
2. 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 industry
1: Oxy fuel furnaces with heat recovery system
2: Cross fired furnace (natural gas) with raw material preheater
3. Horseshoe furnace (heavy oil) with exhaust gas heat recovery system
4.Horseshoe furnace (natural gas)
5. Regenerative horseshoe fired furnaces with air preheating.
Flat glass industry
1.Flat glass furnace (heavy oil) with exhaust gas heat recovery system
2. 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 measures
1.Near stoichometric Combustion
2.Sealing measures
3.lambda adjustment
4.Furnace geometry
5.burner assembly
6.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, heat
release
• 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 (emission
coefficients 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) and
recuperators (25-40%)
Regenerative furnaces
Regenerative furnaces usually have two or more regenerators. A regenerator
consists of a regenerator chamber wherein a checker work of refractory bricks
has been stacked. The bricks form a regular construction with channels for the
flue gases or combustion air. First the flue gases are transported through one
regenerator heating the checker work. After about 20 minutes the checker
work is heated to its optimal temperature. Then the combustion air is led
through this regenerator and the flue gases through the other. The combustion
air is preheated to 1100-1300ºC by the heat of the checker work. Again after
20 minutes the checker work is cooled down too much to heat the combustion
air and the process is reversed.
There are two types of regenerative furnaces: cross-fired and end-fired
furnaces.

Cross-fired furnace
In this type of furnace, the burners (actually gas or fuel oil injecting lances) are
situated along the sidewalls of the furnace inside or under the burner ports. A
drawback is that the many burner ports in the superstructure may lead to
additional heat losses and leakages of cold ambient air into the furnace or hot
combustion 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 and
fairly compact regenerators can be built. This is beneficial for the energy
consumption (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 furnaces
Recuperative furnaces are equipped with one or two recuperators. A
recuperator is a heat exchanger, in which heat is transferred directly from the
flue gases to the combustion air in co-current or counter current flow. This
heat exchange is based upon radiation. These exchangers are therefore called
radiation recuperators. The combustion air is preheated to 600-800°C. Higher
temperatures cannot be reached, because the used metallic materials cannot
withstand higher temperature levels.

cross-fired furnace allows to control the position of the
hot spot directly by the flame distribution. This makes its operation more
flexible with respect to changing pull rates than the operation of a
horseshoe flame furnace. However, due to the relatively long path of its
flame, the latter furnace type is superior to the above type with respect to
energy utilization.

Emissions
NOx-emissions
Nitrogen oxides are especially formed during the combustion process. The oxygen of
the air reacts with nitrogen of the air or of natural gas. This reaction can only take
place at high temperatures (above 1350°C) and in the presence of both oxygen and
nitrogen at the same spot. Additional NOx-emissions are obtained when nitrates are
used. Nitrate is often used to oxidize the batch or melt.
SOx-emissions
Sulphuric oxides are emitted from almost all glass furnaces. The ratio SO2:SO3 is
about 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 the
flue gases as dust sulphates (e.g. Na2SO4 and K2SO4), SO2, SO3 and as H2SO4 at
temperatures below 200°C.
Dust emissions
Dust is mainly originating from the condensation of the glass melt vaporization
carry-over products or reaction products of these vaporized compounds during the
cooling of the flue gases. Primary dust condensates are for instance sodium sulphates,
lead oxides, sodium borates, potassium borates and potassium sulphates. In case fuel
oil is used also vanadium and nickel oxides may be present. When producing
container glass the condensation of sodium compounds accounts for the greater part
of the dust emissions.
Continued…….

Chlorides and fluorides
Chlorides are mostly present as hydrochloric acid (HCl) in the flue gases. Sources of
chlorides 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 metals
Important emissions are sometimes lead in container glass furnaces, and vanadium
and nickel in furnaces fired with fuel oil. Selenium, probably as SeO2, and arsenic
compounds 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 combustion
In this method fuel gas and air are used for the combustion process. To provide the
desired flame temperatures and to achieve affordable energy costs, air
will almost always have to be preheated. Burners are used to inject fuel gas
into 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 the
presence of both fuel gas and oxygen (in air) the mixture will ignite. The energy
provided by this reaction is used for the glass forming reaction and the heating of the
fuel gas, air, reaction gases and feed. A great part of this energy will leave the furnace
with 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 the
angular position of the burner. This long flame length reduces the flame
temperature (caused by an increased amount of radiation) and the local oxygen
concentration 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 about
5% 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 heated
to 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, the
amount 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 containing
fumes from attacking the superstructure at the cold areas (<1450°C). The
burners used for oxy-fuel combustion are for instance pipe in pipe burners. The
fuel 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 boosting
The boosting concept uses oxy-fuel burners
positioned within the air-fuel mixture to increase
production, quality, efficiency and furnace stability.
Oxy-fuel boosting is used to increase the glass pull
rate on a furnace. Extra fuel is combusted with
oxygen to get higher temperatures. This technique
uses conventional combustion as main combustion
technique.

Oxygen lancing
oxygen lancing is the most common
way 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 enrichment
Oxygen 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 desirable
to use the oxygen to enhance the entire
combustion process in a consistent
manner.

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

• Gas flame with air
For 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 the
glass, so a flame area of 50 m2 is needed.
• Oil flame
In this case we use a flame temperature of 1973 K and εv of 0.5. Then the heat
transfer is 180.6 kW/m2. For the required heat flux of 8.16 MW to the glass melt, we
obtain a flame area of 45 m2.
• Gas flame with oxygen
The 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/gas
flame should have a lower emission coefficient than an air/gas flame because of less
formation of soot. We assume that this effect is compensated by the higher CO2 and
H2O concentrations. So we take the same εv of 0.25 in both situations. Calculations
for the heat transfer then result in 252.4 kW/m2. For the flame area we get 32 m2 (of
course 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 of
a 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 σ x (TC
4
– TL
4
)
Q = re-radiated energy absorbed by the furnace load
Ew = emissivity of the coating
σ = Stefan-Boltzmann constant
TC = coating temperature
TL = 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 .

Glass melting technology

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Glass melting technology