Field of the Invention
The present invention relates to a combustion process for melting glass, as
well as mainly to a glass melting furnace for implementation of this process, but
the invention can also be applied to other types of high temperature furnaces.
Background of the Invention
Most types of glass, and in particular plate glass and container glass, are manufactured by melting of raw materials in large melting furnaces producing a
few tens to a few hundred metric tons of glass per day and per unit. The fuel used
in such furnaces is generally natural gas or fuel oil, although other fuels can also
be used. Certain furnaces also use electricity to increase production (electric
boosting). High temperature furnaces (typically 1,500°C, but sometimes higher)
are necessary for the melting. Optimal furnace temperature conditions are obtained
by pre-heating the combustion air (typically up to 1,000°C, but sometimes higher).
The heat required for pre-heating the combustion air is transmitted by the waste
gases, which is generally effected by using reversible regenerators. This approach
enables one to obtain a high degree of thermal efficiency combined with high
melting rates. Several types of melting furnaces exist, including:
Cross-fired furnaces: in these furnaces, which have a melting surface area
generally greater than 70 m2 and which operate with a reversal of the direction of
the flame approximately every 20-30 min, the heat contained in the waste gases is
recovered in regenerators made up of stacks of refractory bricks. The cold
combustion air is pre-heated during its passage through the regenerators (rising
air), while the hot waste gases leaving the furnace are used to re-heat other
regenerators (descending waste gases). These furnaces, with an output sometimes.
greater than 600 t/day and which are used for manufacturing plate glass and
container glass, are great energy consumers. The diagram of cross-fired furnace
operation is presented in Figure 1.
End-fired furnaces: in these furnaces, the flame (sometimes called a

horseshoe flame) describes a loop. These furnaces operate with recovery of the
heat of the waste gases by stacks forming regenerators which deliver it to the
combustion air. The diagram of the operation of this type of furnace is presented
in Figure 2.
The fuel is injected into the furnace into or near the air stream leaving the
regenerator. The burners are designed to produce high temperature flames with
good radiative qualities so as to obtain an efficient heat transfer. A certain number
of options exist for producing the comburant/fuel mixture. The names of these
techniques show how the fuel is introduced. The most frequently encountered
configurations are the following:
"Under port": under the air stream,
"Over port": above the air stream,
"Side port": beside the air stream,
"Through port": through the air stream.
The choice among these different injection methods is made so as to obtain
a suitable result for the configuration of the air streams and of the type of melting
furnace used, and as a function of constraints connected with fuel supply
(example: available gas pressure for a furnace supplied with natural gas) or with
the nature of the fuel.
Although such combustion methods are very efficient in terms of furnace
operation, they induce adverse effects such as the production of very high levels of
nitrogen oxides (subsequently called: NOx), one of the most regulated air
pollutants. In the majority of industrialized countries, limits (in terms of
concentration and flow rate) are imposed on large capacity glass making furnaces
in order to reduce NOx emissions. Furthermore, regulation is becoming more
drastic each year.
In high temperature furnaces as in glass melting furnaces, the main avenue

of NOx formation is the "thermal" avenue in which the NOx are produced in zones
of the furnace where flame temperatures are greater than 1,600°C. Beyond this
threshold, the formation of NOx increases exponentially with the flame
temperature. Unfortunately, the combustion techniques generally used in melting
furnaces for creating very radiative flames such as those mentioned in the
preceding induce high flame temperatures (with maxima greater than 2,000°C)
and have the consequence of generating NOx emissions much higher than those
accepted in numerous countries of the world.
Furthermore, one of the consequences of conventional combustion methods
is that there is little heat released by combustion in the major part of the volume of
the furnace, since in effect the combustion products surrounding the flame
gradually cool in giving up their heat to the glass bath.
Over time, the waste gases become less efficient in transferring heat to the
glass bath by radiation. The transfer of heat by radiation from the flame to the
glass bath can be increased to a significant degree if a way is found, to increase the
temperature of the combustion products still present in the melting chamber.
Several techniques exist enabling reduction of regenerative melting furnace
NOx emissions. Among these can be distinguished primary methods (in which
reduction occurs during combustion), secondary methods (in which reduction
occurs by treatment of the combustion products at the furnace outlet) and
intermediate methods in which the treatment occurs at the location of the outlet of
the melting chamber to the regenerators (the Pilkington 3R process or re-burning).
The methods which can be used are the following:
- Primary method - "Low-NOx" burners: There are several types of "low
NOx" burners on the market, that is to say burners which enable reducing the NOx
emissions even when used alone. However, their performances do not always
enable obtaining the necessary reduction levels for compliance with European
regulations or those in force in other countries around the world. More

particularly, the following types of burners are encountered:
Double impulse burners - These burners produce a low gas speed at the
root of the flame so as to reduce the temperature of the flame in the zone where
the majority of the NOx is generated. The burners also increase the luminosity of
the flame, which promotes a lowering of the temperature of the flame front by
increasing the radiative transfer of heat to the glass bath.
Injection of enveloping gas or "Shrouded Gas Injection" - With this
technique, gaseous fuel is injected at low speed above the "underport" burners in
order to block the comburant flows and to delay mixing of the gas at high speed
coming from the "underport" burners with the air streams, thus reducing
temperatures at the root of the flame.
Ultra-low speed injection of the gas - Injections of fuel gas at very low
speeds (less than 30 m/s) are used with special burners cooled by water circulation
in order to minimize the local temperature of the flame and to increase its
luminosity. The efficiency of this type of burner in terms of NOx reduction
depends greatly on the design of the furnace.
Primary method - Staging: This technique uses conventional burners for
injection of the fuel and reduction of the flow of combustion air through the air
stream in order to create conditions of excess fuel and to introduce the rest of the
comburant in another location of the furnace in order to complete oxidation of the
fuel. This method, which can drastically reduce NOx emissions, is nevertheless
difficult to implement and expensive to use since it requires pure oxygen or ducts
for introducing air at temperatures higher than 1,000°C in order to be thermally
efficient (staging of the comburant in cold air would induce a reduction of energy
efficiency). Examples of this staging technique are:
Air staging: Diverting the hot combustion air coming from the regenerators
by using an ejector towards the combustion chamber in the direction of the waste

gases so as to produce complete combustion. This method requires the use of
heat-insulated ducts and cold air for directing the ejector, hence a loss of thermal
efficiency. This technique has only been used on end-fired furnaces, and mainly in
Germany.
Oxygen-enriched air staging or OEAS (for Oxygen Enriched Air Staging):
The combustion air entering the air stream is introduced with an insufficient flow
for complete combustion in order to create sub-stoichiometric conditions, and pure
oxygen or oxygen enriched air is injected at the rear of the furnace towards the
flow of waste gases so as to complete combustion in the recirculation zone of the
furnace. The OEAS injectors are generally installed in underport position
separately from the burners. This technique has been applied successfully in
end-fired furnaces and in cross-fired furnaces, and mainly in the United States.
Among the various staging technologies, the patent WO 97/36134 discloses
a device with line burners. This device makes it possible to stage the fuel within
the air stream. The fuel supplied to the furnace is divided in two, and a portion is
injected upstream of the burner directly into the hot combustion air. This
methodology does not use an injection of fuel directly into the combustion
chamber as in the present invention. The technique uses an injection of fuel but
always coupled with an injection of air.
Primary method - Rich operating conditions:
This technique reduces the NOx emissions by injecting additional fuel into
the combustion chamber so as to create a "reducing atmosphere" in the
combustion chamber. The reducing atmosphere converts the NOx formed in the
flame into nitrogen and oxygen. In this technique, the NOx produced in the high
temperature flame front are reduced in a second step.
In effect, as shown, for example, by the document JP-A-08208240,
additional fuel introduced by injectors situated on the wall supporting the burner,
on the side wall or facing the burner, is added to the original fuel supplying the

burner or burners. However, according to this method, while making possible
considerable NOx reductions in the combustion chamber, it is necessary to provide
additional combustion air after the exit from the combustion chamber. Not only
does this method require additional consumption of fuel, but the additional fuel is
not burned in the combustion chamber and therefore does not participate in the
melting of the glass.
This process uses an over-consumption of fuel in order to reduce the NOx,
and its application can lead to an increase of the temperatures in the regenerators
and in time to degradation of the regenerators.
Secondary method - Treatment of the waste gases: A major portion of the
NOx is treated at the outlet of the furnace by the use of chemical reduction
processes. Such processes require the use of reducing agents such as ammonia or
hydrocarbon-containing combustion residues with or without the presence of
catalysts. Although capable of achieving the NOx level reductions set by
regulations, these processes are very expensive to install and operate, and in the
case of processes based on hydrocarbons such as the 3R process or system
explained hereafter, a 5-15% increase of the fuel consumption is observed.
Examples of this technique are given below:
3R process (Reaction and Reduction in Regenerators; patented process of
the Pilkington company) - In this technique, the gas is injected at the chamber
roof so as to consume any excess air and to produce reducing conditions in the
regenerators situated at the outlet of the furnace, resulting in the conversion of the
NOx into nitrogen and oxygen. Since an excess of gas must be used, it is
consumed in the lower part of the regenerators where the air is infiltrated or
injected. The additional heat generated is often recovered by boilers. In order to
minimize the quantity of gas necessary for the 3R system, it is common to operate
the furnace with the lowest possible excess air. This technology enables achieving
the NOx reduction levels imposed by the current regulations, and even to exceed
them. Generally, 5-15% of the total fuel consumption of the furnace is necessary

for implementation of the 3R process. The reducing atmosphere in the
regenerators is often the cause of problems with the refractory material composing
them.
Selective catalytic reduction or SCR (Selective Catalytic Reduction) - This
method uses a platinum catalyst for reacting the NOx with ammonia or urea so as
to reduce the NOx into N2 and water. The process has to take place at a specific
temperature and requires precise control of the ammonia in order to avoid
accidental pollution-generating discharges. Since this reaction occurs on the
surface, large surface areas of catalyst are necessary, involving relatively large
installations. The chemical process is relatively complex and demanding in terms
of control and maintenance. Very high NOx reduction levels are attained;
however, the contamination of the catalysts by the waste gases loaded with
particles coming from the glass melting furnace poses problems of clogging and
corrosion. After a certain length of time, the catalysts have to be replaced at
considerable cost.
Summary of the Invention
The aim of the invention is thus to propose a process and means making it
possible to remedy all of the above disadvantages.
More particularly, the invention must make it possible to reduce the NOx
emissions while increasing the temperatures of the surrounding waste gases within
the furnace (the NOx emissions produced in these zones are very low). Moreover,
the invention must make it possible to maintain or even increase the transfer of
heat to the glass bath as well as the yield of the furnace.
The aim of the invention is attained with a combustion process for melting
glass according to which two fuels, of the same nature or of different natures, are
introduced into a melting chamber at two locations a distance away from one
another in order to distribute the fuel in the melting chamber for the purpose of

limiting the NOx emissions, with combustion air being supplied at only one of the
two locations.
The aim of the invention is also attained with a glass melting furnace which
has a tank for receiving the glass to be melted and holding the bath of melted
glass, with, above the glass, walls respectively forming a front wall, a rear wall,
side walls and a roof and constituting a melting chamber, also called a combustion
chamber, as well as at least one intake for hot combustion air (the combustion air
intake also being called an "air stream"), for example, at the outlet of a
regenerator, at least one outlet for hot waste gases, and at least one burner for
introducing a first fuel into the melting chamber.
According to the invention, the melting furnace moreover has at least one
injector for injecting a second fuel into a zone of the melting chamber which is a
distance away from the burner and between the roof and a horizontal plane
situated at a level higher than or equal to a horizontal plane passing through a
lower edge of the intake for hot combustion air, the injector being adjustable in
terms of flow in a complementary manner with respect to the burner so that it is
possible to inject up to 100% of the total of the first and second fuels used by the
injector and the burner, regardless of whether the first and second fuels are of the
same nature or of different natures. Advantageously, said horizontal plane
delimiting the zone of injection of the second fuel is situated between the roof and
a horizontal plane whose distance from the glass bath is greater than or equal to
the minimum height of the air stream in the melting chamber.
In no case is the second fuel injected directly into the hot combustion air.
According to the language chosen for the preceding paragraphs, in the
whole of the present text, the term "burner" exclusively designates a means for
injecting and burning the first fuel, while the term "injector" exclusively
designates a means for injecting and burning the second fuel.
Traditionally, and particularly when thinking of existing furnaces which

can be modified in order to implement the invention in them, the burner could also
be called a "burner," and the injector then would have to be called an "auxiliary
burner." However, such language would weigh down the present text and would
be a source of errors.
Likewise, in the description of the furnace according to the invention and
in the description of other furnaces whose burners are situated on a given wall or
which have only one burner, the front wall is that which bears the burner or
burners, the rear wall is the oppolocation wall, and the side walls are the other two
walls. And in the case of a furnace with a non-rectangular base, the present
definition applies in a similar manner to the corresponding wall sections.
Furthermore, any indication of the number of burners or injectors in a
melting furnace according to the invention is given purely as an example and in no
way presumes a particular embodiment of such a furnace. In effect, the principle
of the present invention is just as valid when a melting furnace according to the
invention has a single burner and a single injector as when it has several, and not
necessarily an equal number of burners and injectors.
According to the present invention, the burners present on a traditional
furnace are kept. They are supplemented by one or more injectors, making it
possible to introduce into the melting chamber, in one or more zones a distance
away from the burners, either another fuel or a fraction of the same fuel as that
introduced by the burners. This injection is sometimes called auxiliary-as opposed
to an additional injection, for example, in afterburning-because its purpose is not
to increase the fuel quantity or flow rate but rather to better distribute or spread the
quantity of fuel necessary for the quantity and type of glass to be melted and thus
to obtain a better heat transfer towards the glass to be melted, while at the same
time reducing the NOx emissions.
This arrangement of the invention, which is furthermore just as valid when
the first and the second fuels are of the same nature as when they are of different

natures, is moreover the basis for the so-called "complementary" method of
adjusting the flow rate of the injectors indicated above.
In effect, the flow rate of the second fuel is varied as a function of the flow
rate of the first fuel so that when the burner does not introduce all of the fuel
necessary for melting the glass, the rest is introduced by one (or more) injector(s)
arranged a distance away from the burner and if necessary a distance away from
one another, in regions or zones of the furnace where the second fuel will mix
initially with the re-circulated combustion products, that is to say coming from the
burner or burners and therefore having a low oxygen content, before igniting in
contact with the hot combustion air not consumed by the flame of the burner or
burners.
Let us expressly note on this subject that in the melting furnaces according
to the present invention, there is no secondary air intake for combustion of the
second fuel, since there is no afterburning.
Generally, in order to obtain a reduction of the NOx emission, the burner
operates in an excess of air, that is to say that the burner introduces less first fuel
than the flow rate of combustion air would permit. This lowers the temperature of
the flame of the burner with respect to temperatures that the flame would have
under stoichiometric conditions, and thus reduces the NOx emission.
In the case of our invention, when the first fuel is burned, the combustion
products fill the combustion chamber and are therefore present at the location or at
all the locations where an injector is placed for introducing the second fuel. During
the introduction of the second fuel, it is first diluted by the products of combustion
of the first fuel and then ignites with the arrival of the combustion air not
consumed by the combustion of the first fuel.
With regard to the "distant" arrangement of the injectors, the distance of the
zones (for arrangement of the injectors) away from the burner or burners depends,

for example, on the geometric data of the furnace and therefore on the time that it
takes for the waste gases to arrive at the injector: the injector must be sufficiently
far from the burner to allow the waste gases to arrive at the injector and to mix
with the second fuel before the non-consumed combustion air from combustion of
the first fuel arrives and ignites the second fuel.
The arrangement of one or more injectors with respect to the burner(s) of a
glass melting furnace according to the present invention leads to a gradual
combustion of the fuel introduced in these regions or zones, producing an increase
of the temperature of the waste gases in these fuel rich zones, as well as to an
increase of heat transfer to the glass bath.
The aim of the invention is also attained with a process for operating a
glass melting furnace which has a melting tank for receiving the glass to be melted
and holding the bath of melted glass, with, above the glass, walls forming a
melting chamber, at least one intake for hot combustion air, at least one outlet for
hot waste gases as well as at least one burner and at least one injector for
respectively injecting a first fuel and a second fuel into the chamber.
According to this process, a first fuel and a second fuel, of the same nature
or not, are injected into the furnace by the burner(s) and injector(s), the injector(s)
being arranged on a different wall or on different walls from that on which the
burner(s) is (are) positioned and being a distance away from the burner or burners,
and the burner(s) and the injector(s) are adjusted in a complementary manner so
that the total of the first and second fuels used by injector(s) (4) and burner(s) (1)
corresponds for the most part to the total flow used normally on the furnace,
regardless of whether the first and second fuels are of the same nature or of
different natures.
The fraction of the fuel which is introduced as second fuel, or the quantity
of a second fuel different from the first, is determined for each furnace, and can
range up to the entire quantity of fuel.

With this technique, according to which a first fuel is introduced into the
melting furnace with an excess of air with respect to the stoichiometric flow of
combustion air, since the fuel fraction introduced by the injectors no longer
supplies the burner, the portion of fuel burned with a high temperature flame front
is reduced, thus generating less NOx emission by the thermal avenue.
The combustion air not used by the burner remains available for
combustion of the second fuel introduced by the injector.
It is also likely that the fuel introduced in the zones of the furnace with
high temperatures, but with a reduced oxygen content, will crack in order to
produce soot, thus increasing the heat transfer from these zones to the glass bath.
The potential injection points can be situated on the side and rear walls of
the furnace and on the wall forming the roof. In certain cases, the center of the
roof which, in the case of the traditional rectangular shapes of glass melting
furnaces, is a transverse line of symmetry or a longitudinal line of symmetry of the
roof with respect to a reference direction given by the direction of the burner
flame, can be particularly advantageous for injection of the second fuel, because
by choosing this location it is possible to reduce by two the number of injectors
necessary for execution of the invention.
The selection of the injection points, of the direction of the jets coming out
of the injector and of the speed of these jets is essential for the success of this
combustion technique. The most suitable positions as well as the geometry of the
injectors have to be identified for each melting furnace.
The speed and the direction of introduction of the second fuel have an
influence on the result obtained by implementation of the various arrangements of
the invention. However, these two characteristics are determined during design of
the device. An error in determination of the position of the injectors or of their
geometry can not only compromise the efficiency of the combustion technique but

can also lead to a lowering of the furnace yield as well as to an increase of the
temperature of the refractory regenerators. In extreme cases, premature shutdown
of the furnace can occur.
The most favorable locations for the injectors and the directions and speeds
of fuel injection, but also clear indications as to the injector geometries which risk
being counterproductive, are advantageously determined using models obtained by
computations and tests. Such models are based on a combination of physical and
mathematical modeling techniques and take into consideration the technical
constraints imposed by the construction of each furnace. The adoption of the most
favorable auxiliary combustion configuration suggested by the modeling results in
NOx emissions much lower than those generated by combustion methods different
from those of the invention, and without this being done at the cost of lowering the
furnace yield. The auxiliary fuel ratio is adjusted to obtain a compromise between
furnace efficiency and level of NOx emissions. By predicting the temperature
within the chamber, the model makes it possible to adjust the auxiliary fuel ratio to
avoid any hot spot as well as any cold spot on the internal surfaces of the furnace.
Particular care should be taken to avoid:
condensation of alkaline materials on the roof or walls of the furnace
(wear and tear of the refractory materials),
condensation of hydrocarbons on the internal walls of the furnace,
as well as modification of the nature of the glass by addition of carbon to
its composition.
This is made possible by the modeling which enables one to choose a
sensible location.
Such models make it possible, for a cross-fired furnace, for example, to
determine the injection position situated in the roof and in the center for a burner
as being one of the most favorable for intended reduction of the NOx emissions,

with an injected secondary fuel ratio that can vary as a function of the emission
level limits that need to be achieved for this burner. A great advantage of
symmetrical injection in the roof with respect to lateral injection is the use of the
same injectors for the flame on the left and the flame on the right.
The number of burners to be equipped with an injector can vary as a
function of the overall level of NOx reduction to be achieved for the furnace.
With regard to end-fired furnaces which have two ports at one end of the
melting and refining chamber, and two sealed regenerators, each connecting
respectively with a port, the auxiliary injections in the roof, like the injections on
the walls, should preferably occur in a zone situated between the roof and a
horizontal plane whose distance from the glass bath is greater than or equal to the
minimum height of the air stream.
The injections should occur, symmetrically or not, on both sides of the
furnace. Locating the injection point(s) optimally is done by use of a model, since
end-fired furnaces can differ from one another, mainly because of the width/length
ratio of the furnace.
Consequently, it is proposed to implement the auxiliary combustion
technique developed here while adding to it the combustion technique already
present on the furnace. This is done by adjusting the fuel flows between the
injectors and the burner so as to produce a balance between NOx reduction, the
nature of the glass, and suitable thermal efficiency for each installation under
consideration.
An embodiment of a melting furnace according to the invention enabling
one to obtain NOx reduction while maintaining or even increasing the heat
transfers is described further on.
The approach of the invention also makes possible a gradual
implementation of this novel combustion technique, thus reducing or eliminating

the risks of production loss due to damage to the furnace. Finally, this approach
allows the operator at any time to go back to his initial combustion configuration.
Although developed for use on regenerative glass melting furnaces, the
auxiliary combustion technique of the invention can also be used on other types of
glass melting furnace (for example, Unit-Melter furnaces or recuperator furnaces),
as well as on furnaces other than glass melting furnaces.
Although it is anticipated that the fuel injected by auxiliary routes is natural
gas for furnaces supplied with natural gas or fuel oil, the use of various fuels such
as biogas, hydrogen, LPG and fuel oil is not excluded.
Thus, the present invention relates equally to the following characteristics
considered alone or in any technically possible combination:
the injector or each injector is arranged in a zone situated between the roof
and a horizontal plane whose distance from the glass bath is greater than or equal
to the minimum height of the air stream;
the injector or each injector can be adjusted in terms of flow rate so that it
is possible to inject up to 100% of the total of the fuel used by the injector(s) and
the burner(s);
at least some of the injectors are arranged'on the roof of the furnace;
at least some of the injectors are arranged on the side walls of the furnace;
at least some of the injectors are arranged on the rear wall of the furnace;
at least some of the injectors are arranged on the wall of the furnace on
which the burner is situated;
the injectors are oriented at least approximately in a direction oppolocation
from the main direction of the flames of the furnace burners;
the injectors are oriented at least approximately in the same direction as
the flames of the furnace burners;
the injectors are oriented at least approximately in a direction
perpendicular to the flames of the furnace burners;
the injectors are oriented at least approximately in a direction transverse to

the flames of the furnace burners;
the first fuel and the second fuel are of the same nature;
the first fuel and the second fuel are of different natures.
The injectors can be equipped with a system of rotation (swirler) making it
possible to control the shape of the flame independently of the flow rate of
secondary fuel so that it is possible to inject up to 100% of the total of the fuel
used by the injector(s) and the burner(s) without affecting the glass bath.
The injectors can be equipped with a device making it possible to adjust the
impulse of the fuel (double impulse) independently of the flow rate of secondary
fuel so that it is possible to inject up to 100% of the total of the fuel used by the
injector(s) and the burner(s) without affecting the glass bath.
The injectors can have a non-circular shape or can have multi-jets in order
to adjust the shape of the flame without affecting the glass bath.
In a modified melting furnace according to the invention, reduction of the
nitrogen oxides contained in the combustion products is obtained by using the
combination of the burners already present on the furnace along with auxiliary
injections of fuel in the zones of re-circulation of the waste gases of said furnace.
The injections are made according to one or more jets situated in optimal locations
on the furnace which are defined by using a methodology based on digital
simulation, which can be coupled or not with the representation of the flows by a
mock-up of the furnace. The plane of the injections can be parallel, perpendicular
or transverse to the surface of the glass bath. The invention can be applied in the
domain of reduction of the nitrogen oxides by primary method in glass melting
furnaces. The invention makes it possible:
to reduce the NOx emissions,
to reduce or eliminate the post-treatment costs,
to improve the yield of the furnace, and
to reduce the NOx emissions while improving the yield of the furnace.

Furthermore, the invention
can be applied regardless of the fuel supplying the burner,
can be applied with a fuel supplying the injectors which is of a different
nature from that supplying the burners of the furnace, if necessary the type of
injector being adapted to the chosen fuel,
can be applied with a fuel supplying the injectors which is of the same
nature as that supplying the burners of the furnace, with it then being possible for
the type of injector to correspond to that of the burners with regard to their
adaptation to the fuel,
is implemented directly in the combustion chamber, also called the melting
chamber,
makes it possible to distribute die fuel between the main burners already
equipping the furnace and injectors in such a way as to bring about reduction of
the NOx emissions combined with a suitable yield for each particular furnace,
can be used with under-port burners, side-port burners, through-port
burners or with any other type of burner originally equipping the furnace,
can use the injection, by the injectors, of a fraction of the fuel injected by
the burners but
can just as well use all of die fuel by die injectors.
The auxiliary injection
is not implemented directly in the air stream,
can be done from the roof,
can be done from the walls situated to the front or rear of the furnace,
can be done from the side walls,
uses positions as well as angles and speeds of injections which are
determined by a parametric study using modelings of the furnace that is intended
to be transformed,
can be done widi the same fuel or with a different fuel from that injected
by the burners,
can be done widi natural gas,

can be done with LPG,
can be done with fuel oil,
can be done with coke-oven gas,
can be done with blast furnace gas,
can be done with reforming gas,
can be done with biogas,
can be done with hydrogen,
can be done with any other fuel.
Other characteristics and advantages of the present invention will emerge
from the description hereafter of an embodiment of a furnace according to the
invention.
Brief Description of the Accompanying Drawings
This description is given with reference to the accompanying drawings in which:
Figures 1 and 2 represent two types of melting furnaces used before the
invention;
Figure 3 represents a cross-fired melting furnace according to the
invention in the form of a horizontal section indicating the zone of the auxiliary
injections;
Figure 4 represents, in a diagram, the NOx levels as a function of the
distribution of power between the burners and the associated injectors;
Figure 5 represents a diagrammatic view of a furnace according to the
invention in the form of a vertical section indicating an auxiliary injection zone
example;
Figure 6 represents, in a diagram, a comparison of the levels of NOx and
CO obtained in a furnace with and without use of the invention;

Figure 7 represents, in a diagram, temperature levels obtained in a furnace
with and without use of the invention; and
Figure 8 represents, in a diagram, a comparison of heat transfers obtained
with and without use of the invention.
Detailed Description of the Invention
Figures 1 and 2 each very diagrammatically represent two types of glass
melting furnaces that are traditionally used, namely a cross-fired regenerative
furnace and an end-fired furnace. Both types of furnaces have a rectangular base
bound by four walls, of which the two walls extending in the lengthwise direction
of the furnace are in this case called the side walls and of which the other two
walls are called the transverse walls. At the top, both furnaces are bounded by a
roof.
In a cross-fired regenerative furnace (Figure 1), burners 1 are arranged in
side walls 2 and operate alternately on one side and then the other for
approximately 20-30 min per side. Cold combustion air A is pre-heated in two
heat recuperators R, namely in an alternating manner according to the rhythm of
operation of the burners, in that one of the two recuperators which is near the
burners in operation. The resulting waste gases F then re-heated in that one of the
two recuperators R which is remote from the burners in operation.
In the end-fired glass melting furnace (Figure 2), in which the length of the
furnace does not greatly exceed its width, burners 1 are arranged in transverse wall
3. The range of the flame of each of burners 1 is such that, under the influence of
the oppolocation transverse wall, the end of each of the flames describes a loop.
The cold combustion air is pre-heated in a part of regenerator R with several
chambers before being directed as hot combustion air AC (see fig. 3) towards the
burners. The resulting waste gases are then directed towards the other regenerator
in order to re-heat it.

In both furnaces, the flames are directed approximately parallel to the
surface of glass bath B.
Figure 4 represents, in a diagram indicating the NOx level achieved as a
function of the power distribution between burner 1 and injectors 4, the results
obtained in a semi-industrial test furnace (or a test cell). It should be noted more
particularly that the NOx emission level decreases with the increase of the portion
of fuel injected through the secondary injectors.
Figure 5 once again represents the end-fired furnace of Figure 2, but in this
case with indication of zone IN in which, according to the invention: the
secondary fuel injections must occur in a defined space above the flames, that is to
say between roof V and horizontal plane P whose distance from glass bath B is
greater than or equal to the minimum height of air stream VA, that is to say in a
zone of the melting chamber which is a distance away from the burner and situated
between the roof and horizontal plane P situated at a level higher than or equal to a
horizontal plane passing through the lower edge of the hot combustion air inlet.
The auxiliary injections advantageously but not necessarily take place
symmetrically on both sides of the furnace.
According to an embodiment which is particularly economical in terms of
number of injectors 4, as diagrammatically represented in Figure 3, the injectors
are arranged in a zone corresponding at least approximately to a central zone with
respect to the burners that are arranged in the side walls of the furnace and that
operate in an alternating manner or simultaneously.
In this view, one also sees the introduction of cold combustion air A, its
passage through heat recuperators R in order to be pre-heated before entering
melting tank or chamber L, the exit of the hot waste gases from the melting
chamber, and their passage through the heat recuperators before leaving the
melting furnace. And an example of an injector arrangement is seen more

particularly. Recall that the precise position of each of the injectors is determined
by a combination of computations according to a model and tests with the specific
furnace that is to be equipped with such injectors.
Tests have been done with such a furnace with a unit power of the
under-port burners of 1.03 MW with an angle of injection to the burner of 10°, an
air factor of 1.1, a pre-heated air temperature of 1,000°C and a furnace
temperature of 1,500°C. The results are represented in Figures 4, 6,7 and 8.
Figure 6 represents, in the form of a diagram, the levels of CO and NOx
with 8% oxygen for different distributions of power between a burner and one or
more allotted injectors, the injector or injectors being arranged in the roof of the
furnace.
Figure 7 represents, in the form of a diagram, the temperature levels of the
roof for different methods of operation of the furnace, namely in the case of a
single burner and in the case of a burner with an injector that injects between 30
and 100% of the fuel. It is observed that the process does not bring about any
overheating of the roof.
Figure 8 represents, in the form of a diagram, the heat flows transmitted to
the load without and with secondary injection. In this example, the heat flow is
highest in the case of secondary injections of between 30 and 80% of the fuel.
Figure 6 represents, in the form of a diagram, the levels of NOx and CO of
a furnace without and with auxiliary injection ranging up to 100% of the fuel. It is
observed that the levels of NOx decrease when the auxiliary fuel portion increases.
As for the CO levels, they gradually increase with the auxiliary fuel portion but in
completely tolerable proportions.
A compromise therefore has to be reached between the NOx and CO levels
and the yield. In the example presented, this compromise is reached with a fuel
flow rate of between 50 and 70% of the total flow rate.

WE CLAIM:
1. A process for melting glass in a glass melting furnace comprising a melting
tank for receiving the glass to be melted and holding a bath of melted glass, with
walls forming above the glass a chamber wherein fuel is burned for providing heat
for melting the glass, and having at least one intake for hot combustion air and at
least one outlet F for hot waste gases, wherein a first fuel is introduced into the
chamber at a first location and a second fuel is introduced into the chamber at a
second location separated from the first location and combustion air is supplied
only at the first location for the combustion of the first and second fuel for the
purpose of limiting the NOx emission, said first location being provided on one
wall of the chamber,
characterized by introducing the first fuel at said first location on the said
one wall and by providing the second location on another wall, at a distance from
the first location, and by choosing said second location so that the zones of
combustion of the first and second fuel are separated from one another in order to
improve heat transfer to the glass bath by a more even temperature distributed
over the glass bath.
2. Process as claimed in claim 1, wherein said second location on another
wall is provided at a distance from said first location so that smokes from the said
first fuel can reach said injector and can mix with said second fuel before non used
combustion air from combustion of the first fuel reach said injector.
3. A process as claimed in claim 2, wherein the flux of said second fuel is in a
direction crossing the direction of the flux of the first fuel.
4. A glass melting furnace comprising a melting tank for receiving the glass
to be melted and for accommodating a melted glass bath, with above the glass,
walls respectively forming a front wall (3), a rear wall, side walls (2) and a roof
(V) for constituting a melting chamber with at least one intake (VA) for hot
combustion air, at least one burner (1) for introducing a first fuel into the chamber,

in said front wall, at least one outlet (F) for hot waste gases, and at least one
injector for introducing a second fuel into said chamber, characterized in that the
at least one injector is arranged on a wall other as that on which the burner (1) is
positioned, at a distance away from the burner in a zone of the chamber where the
combustion of the second fuel is separated from the zone of combustion of the first
fuel.
5. A glass melting furnace as claimed in claim 4, wherein said distance
established so that smokes from the said first fuel can reach said injector and can
mix with said second fuel before non used combustion air from combustion of the
first fuel reach said injector.
6. A glass melting furnace, as claimed in any of claim 4 and 5 wherein the
injector (4) is arranged on a wall other as that on which burner (1) is positioned, at
said distance away from the burner in a zone situated between roof (V) and
horizontal plane (P) situated at a level higher than or equal to a horizontal plane
passing through a lower edge of hot air intake (VA), and wherein the injector (4)
can be adjusted in terms of flow rate in a complementary manner with respect to
burner (1) so that it is possible to inject up to 100% of the total of the first and
second fuels used by injector (4) and burner (1), regardless of whether the first and
second fuels are of the same nature or of different natures.
7. A furnace as claimed in any of claims 1 to 6, wherein the at least some of
injectors (4) are arranged on at least one of the axes of symmetry of roof (V) of the
furnace.
8. A furnace as claimed in any of claims 1 to 7, wherein the at least some of
injectors (4) are arranged on at least one of side walls (2) of the furnace.
9. A furnace as claimed in any one of claims 1 to 8, wherein the at least some
of injectors (4) are arranged on the rear wall of the furnace.

10. A furnace as claimed in any one of claims 4 to 9, wherein the injectors (4)
are oriented at least approximately in a direction oppolocation from the direction
of the flame of burner (1) of the furnace.
11. A furnace as claimed in any one of claims 4 to 10, wherein the injectors (4)
are oriented at least approximately in a direction transverse to that of the flame of
burner (1) of the furnace.
12. A furnace as claimed in any one of claims 4 to 11, wherein the
burner(s) (1) and injector(s) (4) are made for first and second fuels of the same
nature.
13. A furnace as claimed in any one of claims 4 to 11, wherein the
burner(s) (l) and injector(s) (4) are made for first and second fuels of different
natures.
14. A furnace as claimed in any one of claims 4 to 13, wherein the burner(s)
(1) and injector(s) (4) are made for first and second fuels belonging to a group of
fuels which includes natural gas, LPG, fuel oil, coke-oven gas, blast furnace gas,
reforming gas, biogas, hydrogen.
15. A furnace as claimed in claims 4 tol4, wherein the at least one of injectors
(4) is equipped with a device for putting the fuel into rotation.
16. A furnace as claimed in claims 4 to 14, wherein the at least one of injectors
(4) is equipped with a device making it possible to adjust the fuel impulse.

Abstract

A Process For Melting Glass And Furnace Therefor
A process for melting glass and a furnace therefor are disclosed. The glass
melting furnace comprises: a melting tank for receiving the glass to be melted and
for accommodating a melted glass bath, with above the glass, walls respectively
forming a front wall (3), a rear wall, side walls (2) and a roof (V) for constituting a
melting chamber with at least one intake (VA) for hot combustion air, at least one
burner (1) for introducing a first fuel into the chamber, in said front wall, at least
one outlet (F) for hot waste gases, and at least one injector for introducing a
second fuel into said chamber, characterized in that the at least one injector is
arranged on a wall other as that on which the burner (1) is positioned, at a distance
away from the burner in a zone of the chamber where the combustion of the
second fuel is separated from the zone of combustion of the first fuel.