EP2257500A1

EP2257500A1 - Glass melting furnace

Info

Publication number: EP2257500A1
Application number: EP09724290A
Authority: EP
Inventors: Olivier Douxchamps; Johan Behen
Original assignee: AGC Glass Europe SA
Current assignee: AGC Glass Europe SA
Priority date: 2008-03-25
Filing date: 2009-03-25
Publication date: 2010-12-08
Also published as: WO2009118337A1; CN101980974A; EA018516B1; JP2011519804A; US8943856B2; KR101600109B1; KR20110015515A; MX2010010555A; EA201001538A1; US20130111956A1; BRPI0909081A2; CN101980974B; US20110017195A1; JP5454810B2

Abstract

The invention relates to a glass melting furnace including a channel-shaped melting end. According to the invention, the raw materials are introduced at an upstream end and the molten glass is recovered at the downstream end, said furnace being heated by burners. At least 65% of the combustion energy is produced by oxycombustion and the burners are distributed on the walls along the length of the furnace. The majority of the flue gases are released close to the upstream end near the raw material inlets, the remainder being released close to the downstream part such as to maintain a dynamic seal in relation to the surrounding atmosphere.

Description

Glass melting furnace

The present invention relates to glass melting furnaces in which the melting energy is produced essentially by burners fed with fuel and oxygen or oxygen-rich gas. These furnaces are usually referred to as "oxy-combustion" furnaces.

The accessory use of oxy-combustion burners is well known in glass melting furnaces. On furnaces operating in a traditional manner with air, it is then necessary to add one or a limited number of oxy-combustion burners. The introduction of these additional burners is generally intended to increase the capacity of existing furnaces, possibly when they see their performance decline because of their age. This situation is for example when the regenerators associated with these furnaces are degraded and no longer allow sufficient heating of the air used for combustion. It may also be to simply increase the capacity of a given furnace by introducing additional sources of energy.

As a general rule, the additional oxy-combustion burners are arranged near the zone of the furnace in which the raw materials are charged. These burners thus activate the fusion of these raw materials. The addition of a few oxy-combustion burners in large-capacity furnaces is usually carried out without any major modification of the general operation of the furnace, in particular, that the regenerators continue to operate and therefore treat both the fumes combustion of burners operating with air and those from burners operating with oxygen. Apart from having an additional source of energy, these systems operating in the so-called "oxy-boosting" mode do not make it possible to benefit from all the known advantages that can result from oxy-combustion. Potential benefits include mainly lower energy consumption and reduced emissions of unwanted smoke.

The oxy-combustion allows energy gain at least for the reason that the energy of the combustion gases is not absorbed in part by the nitrogen of the air. In traditional furnaces, even if part of the energy entrained with nitrogen is recovered in the regenerators, the fumes finally evacuated still cause a significant part. The presence of nitrogen contributes to this loss.

The reduction in energy consumption per unit of production considered has the additional advantage of limiting carbon dioxide emissions and therefore meeting the regulatory requirements in this area.

The presence of nitrogen is also a source of so-called NOx oxides, whose emission is practically prohibited because of the damage related to the presence of these compounds in the atmosphere. In practice users have tried to operate the furnaces under the conditions leading to emissions as limited as possible. In the case of glass furnaces, these practices are not sufficient to meet the very stringent standards in force, and it is necessary to carry out a costly decontamination of fumes by the use of catalysts.

The use of oxygen makes it possible to overcome the problems related to nitrogen in the air, which is not the case in oxy-boosting techniques.

Despite the advantages stated above, the use of oxy-combustion in large glass furnaces has yet to be developed. The reasons are of many natures. First, the use of oxygen is necessarily more expensive than that of air.

The economic balance of the implementation of the oxy-combustion is positive only to the extent that one is able to recover a significant part of the heat of the fumes. Until now the implementation of the recovery of this energy does not seem to have been undertaken satisfactorily. The potential energy gain has not been effectively achieved.

In addition, the implementation of oxy-combustion still poses technical problems that are the counterpart of certain advantages. A recognized difficulty is related to the fact of the corrosion of refractories, which decreases the lifetime of the silica refractories of the vault. Indeed, the high content of H ₂ O in the combustion atmosphere causes two phenomena of deterioration:

the first is related to the diffusion of H ₂ O in the glassy phase of the refractory blocks.

- the second is linked to the condensation of the soda present in the atmosphere on the refractory brick, which implies a high degree of oxidation, six times higher in the case of an oxy-combustion furnace.

To take these conditions into account, it is necessary to use materials that are more resistant to corrosion than those usually chosen. Most commonly, for a variety of reasons, the vault of large glass melting furnaces is made of silica bricks. In the case of an oxy-combustion furnace, it is necessary to resort to materials such as alumina, AZS, spinel, but on the other hand these materials are more expensive and also pose problems. because of a significantly greater weight.

Other new problems have also appeared in the experiment, which require the implementation of specific and new conditions to ensure that this technique actually finds applications that the theory presents as advantageous. The invention relates to methods for implementing the oxy-combustion technique in large glass furnaces which are the subject of the claims appended to this description.

The inventors have been interested in the problem of the economy of this oxy-combustion technique. In particular they propose to ensure that the energy of the furnace fumes is largely recovered and used for the preheating of the oxygen and, where appropriate, the fuels consumed. Some of the heat of the fumes can also be used to preheat the raw materials.

The economic balance, particularly with regard to energy, requires that the heat of the fumes be recovered. The principle is known. The difficulty comes from the implementation of recovery techniques for the operation of the oven itself.

The choice of the inventors is to make sure to use the energy of the fumes, especially to preheat the oxygen. For obvious reasons the use of regenerators for this recovery is excluded. It is necessary to operate in specific exchangers. This operation is difficult to the extent that the hot oxygen is extremely aggressive for all the materials with which it is located. This corrosive nature is all the more marked as the temperature reached by the oxygen is higher.

It is also necessary according to the invention that the furnaces concerned are substantially free of a nitrogen-charged atmosphere. For this reason, unlike some previously proposed solutions, it is preferable to ensure that all furnace burners operate in oxy-combustion. If, nevertheless, it is possible to retain part of the combustion-type combustion, the energy generated by oxy-combustion represents at least 65% of the total energy used in the oven, preferably at least 80%, and particularly preferably at least 90% thereof.

The use of an aero-combustion fraction can come from a limited number of burners operating entirely in aero-combustion, it can also come from the use of an oxygen comprising a certain amount of air. In the latter case, given that the burners used in oxy-combustion have particular characteristics, the oxygen / air mixture must comprise a content of at least 80% oxygen and preferably at least 90% oxygen. .

For the sake of simplification, in the remainder of the description reference is made to F oxy-combustion and oxygen. The developments in this regard, unless otherwise indicated include the implementation of oxy-combustion with oxygen may contain a small proportion of air, or an assembly comprising a limited portion of aero-combustion combined with an oxy-combustion majority.

Independently of the constituents of the furnace atmosphere resulting from the combustion, it is still necessary to prevent as much as possible the penetration of air coming from outside on the one hand to avoid a loss of energy corresponding to the heating of this air, but especially to prevent as much as possible the formation of NOx unwanted due to the passage of this air through the high temperatures of the combustion flame (these temperatures are of the order of 1800 to 2300 ⁰ C depending on the type of oxygen burner chosen).

Whatever the constructions envisaged, the glass furnaces can not be kept perfectly sealed to the outside atmosphere. Efforts reported in this direction have focused mainly on the establishment of physical barriers limiting the flow of gas from the outside to the inside of the oven. These measurements are certainly useful, but appear insufficient if one wants to maintain an atmosphere essentially composed by the combustion gases. According to the invention it is therefore provided to prevent the admission of surrounding atmosphere by arranging the furnace so that it develops a dynamic seal. For this, according to the invention, it is necessary to adjust the flow of fumes in the oven as specified below.

In large melting glass furnaces that use regenerators in particular, the circulation of gases in the furnace is transversal. The burners are distributed on both sides of the basin containing the molten glass. They work alternately. During a period all the burners located on one side of the oven are active. The corresponding fumes are discharged through the ducts located on the wall facing them. The fumes are passed on the regenerators corresponding to the side in question. In the next period, the burners on the other side are activated with the air circulating on the regenerators previously heated, and so on.

For oxy-combustion furnaces, the burners located on either side of the furnace operate continuously. The distribution of the burners on both sides is not controlled by the necessity of this alternation which does not exist, but more by the search for an optimization of the thermal exchange between the flames and the glass bath, or with the supernatant raw materials.

The flames of the oxy-burners are, at equal power, shorter than the flames of the aero-burners. The reason for this is a smaller gas flow due to the absence of nitrogen. So that the energy distribution is as uniform as possible, for a similar oven width, it is therefore desirable to have the burners on both sides to better cover the surface of the bath. An increase in the emission rate of oxy-combustion gases that could lead to lengthening the flame is not desired in particular to not favor the flight of dust.

It is also best to ensure that the flames grow as undisturbed as possible. To avoid the collision of the flames disposed vis-a-vis, they are therefore advantageously arranged staggered.

According to another specificity of the oxy-combustion flames, to achieve the staging of the combustion over the length of the flame, which is preferable, as in the burners in aero-combustion, it is advantageous to ensure that the flame of these burners develops into a sheet located in a plane substantially parallel to the surface of the glass bath. This is obtained for example by means of burners having a plurality of oxygen injection nozzles located on either side of the fuel inlet nozzle, all these nozzles being substantially aligned parallel to the surface of the bath.

The flue gas flow from the flames is not as in aero-combustion transversely. Traffic is organized according to two objectives.

On the one hand it is to ensure that the heat transfer of fumes in the glass bath is as important as possible. In other words, efforts are made to ensure that, at the exit of the furnace, the flue gas temperature is as low as possible, taking into account also the fact that the flames in oxy-combustion are at a higher temperature, and that overall, fumes are also at a higher temperature than in aero-combustion.

To achieve a greater heat exchange it is ensured that the residence time in the oven is extended. Given the fact, for the same dissipated energy, that the volume of fumes is reduced by more than half compared to that of the aeromombustion, for an oven whose volume would be kept the same, all other things being equal, the time of residence of the fumes would necessarily be increased.

On the other hand, the provisions relating to the circulation of fumes still make it possible to improve heat transfer with the bath. In particular this results from the location of the flue gas outlets, the location of the burners and the distribution of the power developed locally by each of these burners.

According to the invention, and to proceed to the best energy transfer with the bath or with the raw materials, it is necessary to circulate the fumes, or at least most of them, in the opposite direction to the flow of the bath. Thus the fumes during their progression in the oven see their temperature decrease until they exit the oven.

For this reason the evacuation of fumes, or at least most of them, is located near the charging of raw materials. One possibility is to ensure that the evacuation of fumes is effected by separate conduits from those through which the raw materials are introduced into the furnace. Another possibility is that this evacuation takes place by the charging channels themselves, and therefore against the flow of raw materials. For the latter possibility it is necessary to avoid the risks of agglomeration related to condensation in contact with these "cold" raw materials, the water vapor contained in the fumes.

To have the best heat transfer, most of the fumes are evacuated near the charging of raw materials. In practice it is at least 65% of the fumes, and preferably at least 75%. The excess smoke that is not discharged as indicated above follows a path designed in particular to maintain the dynamic seal vis-à-vis the external atmosphere. At least part of this excess is advantageously discharged downstream of the furnace. As indicated, this fraction of the fumes is as small as possible. It is advantageously less than 35% of all the fumes, and preferably less than 25%.

The flue gas outlet downstream of the oven is located beyond the last burners. The flue gases must be evacuated before heat transfer is as complete as possible. For this, these gases must stay a little in the oven, hence the need not to put the burners too close to the exhaust ducts.

The presence of outlets downstream of the burner zone makes it possible in particular to prevent air coming from this zone from passing through the zone of the burners. Most of the downstream air comes from the conditioning area. The NOx content is systematically detected at the upstream outlet. If this content proves to be too high, it is possible to correct this content according to the invention by regulating the flow of the outlets. An increase in this downstream discharge causes more air from the downstream furnace and prevents this nitrogen-laden air from passing through the flames and forming NOx.

Advantageously these settings lead to a nitrogen content as low as possible in the fumes discharged into the upstream outlets. The content is preferably maintained below 10% and particularly preferably below 5%.

The temperature of the gases discharged downstream is generally a little higher than that of the fumes discharged upstream for the reason that the latter are in contact with the zones of the oven the warmest, in particular in that near the charging is normally available no burners, and that the coverage of supernatant raw materials absorbs a significant portion of the energy in the melting of these raw materials.

The residence time of the fumes in the furnace proceeds from a set of conditions. Among these, apart from the organization of the flue gas circulation indicated above, it is necessary to add the flow of fumes produced and the volume which these fumes have inside the furnace. For a determined flow of fumes, the average residence time is a function of the available volume. The higher the volume, the higher the residence time, and in principle the more complete is the heat transfer.

In practice the increase in the volume of the furnace influences in a limited way and can lead to a less satisfactory assessment if it is not well controlled for the following reasons. Experience shows firstly that the heat transfer to the mass to melt and with the bath, is carried out mainly radiatively. Convection of fumes contributes only less than 10% of the intake, and is usually less than 8%. Under these conditions, the increase in the residence time of the fumes adds little to this convective intake. Moreover, the increase in the volume of the furnace leads, in addition to the additional investment in refractories, to an additional loss of energy dissipated externally, whatever the quality of the insulation of the furnace, this loss being a function of the wall surface exposed to the surrounding atmosphere.

The residence of the fumes in the furnace advantageously allows to reduce a little their exit temperature. They are usually on an air-combustion furnace at less than 1650 ^° C., preferably at less than 1600 ^° C. and particularly preferably at less than 1550 ^° C. In the case of an oxy-combustion furnace they are are at less than 1500 ⁰ C, preferably less than 1450 ⁰ C and particularly preferably less than 1350 ⁰ C.

Moreover, the volume of the oven also determines the speed of the fumes in it. It is best to make sure that the speed of Flue gas circulation in the oven remains moderate to avoid disturbing the flames. It is still necessary to avoid the flight of dust by passing on the raw materials, dust which should then be eliminated before passing in the heat exchangers.

Experience shows that the average residence time of flue gases in an air-combustion furnace is 1 to 3 seconds. In the case of the invention, an oxy-combustion furnace, the average residence time of the fumes is established between 10 and 40 seconds and most advantageously between 15 and 30 seconds.

The location of the burners, or better the distribution of the energy supply already mentioned, is an important factor both for the energy balance of the furnace but also for the quality of the glass produced.

All major glass furnaces traditionally have two areas corresponding respectively to the merger and refining. Beyond, the flow of the glass continues in a conditioning basin in which the temperature of the glass is gradually lowered to reach its shaping temperature. For the manufacture of flat glass by the "float" technique, this temperature is of the order of 1100 ^° C.

A bottleneck usually separates the refining zone and the conditioning zone. In the jargon glass this neck is called "neck"., It allows in particular to restrict the passage of the atmosphere from one area to another. According to the invention, efforts are made to reduce at best the corresponding opening and consequently the circulation of the atmosphere entering the refining zone from that of conditioning. In all cases, the fumes must not enter the conditioning zone, otherwise dust still in suspension may be formed and deposited on the surface of the glass.

The introduction of non-combustion gases, and in particular, the penetrating gas through the neck is as limited as possible and advantageously does not exceed 15% of the total volume, and preferably is less than 10% of the total volume of gas flowing in the furnace.

The distinction between melting zone and refining zone takes into account what it is traditional to call "convection loops" of glass. These convection loops are generated by two phenomena: natural convection and forced convection. On the one hand the natural convection movements are related to the temperature conditions, related to the power distribution along the furnace ("fire curve"), On the other hand the forced convection movements are related to the modification of flow caused for example by bubblers, mixers or dams. These two convection phenomena lead the glass to progress by developing movements in the melting zone from upstream to downstream on the surface and in the opposite direction near the sole. In the refining zone the direction of circulation is reversed.

In general, the melting zone is that which requires the largest energy input, and therefore the one in which the overall power of the burners is the largest. The distribution is such that this input is not less than 40% of the total, preferably not less than 50%. It can represent up to 80% of the input, but preferably does not represent more than 70% of the energy delivered. The percentages in question refer to the power delivered by the burners that overhang the area in question.

In order for the oven to operate as efficiently as possible, the burners must be properly distributed along the oven. This distribution is not uniform.

It is necessary to avoid the presence of the most powerful burners near the flue gas outlets to minimize the energy losses in the flue gases. However, in the case where the glass temperatures under the molten raw material mat are too low, at the risk of freezing the glass, additional burners may be located in the vicinity of points of charging of raw materials, either on the walls of the furnace or on the vault, An alternative to minimize the energy losses in the fumes can be the use of electro-boosting (the heating of the glass by electrodes through the sole). Heating by means of submerged electrodes has the advantage of a setting adjusted exactly to local needs. In addition, the efficiency of this electric energy input is much greater than in the case of flame heating, which makes it possible to keep it at relatively low levels. Generally speaking, when this energy supplement is used, it does not represent more than 10% of all the energy developed in the furnace, and most often is less than 5%.

The burners are located at a distance from both the charging of the raw materials and the upstream exits of the fumes. The necessary energy input in this zone therefore results, on the one hand, from convection currents within the bath, which are all the more intense as the temperature of the surface coated with the materials to be melted, and that further downstream of the bath. fade, present an important difference. This contribution also comes from fumes that circulate against the current by going to the exits arranged upstream. Overall the temperature in this zone is not the highest in the furnace but remains sufficient to maintain the melting.

If the first burners are located at a certain distance from the flue gas outlet, in order not to delay the melting of the raw materials, it is nevertheless necessary to locate these first burners in an area of the oven in which the bath is still covered with non-combustible materials. melted. This zone preferably does not exceed half the length of the oven, and particularly preferably not one third. It is indeed necessary, beyond the fusion of this "cover", to ensure a perfect melting of the particles of material dispersed in the bath, and to ensure the rise to the highest temperature which allows no only the completion of the melting but also the homogenization of the molten bath. In addition to the location of the burners, the distribution of the delivered power is significant. The power of the burners is highest in the melting zone near the refining zone. In this part the temperature reached is the highest.

In the refining zone, the bath temperature must be maintained overall. The required energy input is therefore more limited. Preferably the burners in this zone are located in the nearest part of the melting zone. The energy supply is preferably decreasing in the direction of progression in the refining zone.

It is necessary to be able to vary the operating parameters of the furnaces, and in particular the total power applied. These variations controlled by the nature of the raw materials, the variations of pulling ... etc, are generally of limited extent. To maintain as much as possible the optimized energy distribution conditions, traditionally the variations relate essentially to the burners located furthest downstream in the direction of progression of the glass. This peculiarity results in variations in the volume of the fumes in this zone. In order consequently to prevent modifications further upstream because of the breaking of the dynamic equilibrium, it is advantageous according to the invention to regulate the flow of the fumes by means of the outlets arranged in the downstream zone of the furnace as indicated previously.

In conventional furnaces operating in air-combustion, the burners are arranged on the side walls of the furnaces so that the flames develop near the surface of the bath. This provision takes into account in part the limited residence time of the fumes in the oven, which are essentially discharged directly to the side of the furnace facing the burner. It appears necessary to promote as much as possible the exchanges, including convection during this short residence time and therefore to ensure that the flames are also in contact with the surface of the bath. In the case of an oxy-combustion operation as in accordance with the invention, the portion of the convection supply, as indicated previously, is limited. It is therefore preferable to arrange the burners on the side walls away from the bath surface, ensuring both adequate distribution of the radiated energy directly from the flame to the bath and to the vault.

Preferably the location of the burners leads to the development of the flames in planes substantially parallel to the surface of the bath, and at least 0.25m above this surface, and preferably at least 0.40m above. This distance can reach 1.0m but preferably is less than 0.80m.

The use of oxy-combustion, as previously indicated, modifies the atmosphere of the oven which is substantially free of nitrogen. In return, it is relatively richer in water vapor. This particularity has a significant influence on the conduct of the merger. In particular, the increase in the water content above the bath is accompanied by an increase in this content in the glass.

The presence of a high water content also favors degassing of the glass and facilitates refining.

A high water content has the possible counterpart the formation of foam on the surface of the bath. The presence of foam is undesirable in particular because it hinders a good heat exchange. Means are known that reduce the foam if it develops, regardless of the measures taken to prevent its occurrence. These means modify the surface tension of the glass, for example using the technique described in publication EP 1 046 618.

Another way to minimize the risk of foam formation in areas where it can be particularly troublesome, especially in the refining zone, it consists in limiting the water vapor content by the choice of fuel used in this part of the furnace.

The oxy-combustion can be conducted with different types of fuel without losing the benefit of the benefits outlined above. The most common fuels are either natural gas or liquid fuels.

With regard to the water vapor content, the use of gas leads to a higher content than with liquid fuels. For this reason, apart from the questions relating to the cost of energy mentioned above, it may be advantageous according to the invention to supply the burners located in the refining zone with liquid fuel. In this way the risk of foaming in the part of the furnace is reduced where it could be the most detrimental.

The economic balance of oxy-combustion is partly based on the cost of oxygen, that of suitable refractories, and secondly on fuel savings and those concerning the partial absence of pollution control of fumes. To have a positive balance it is necessary to recover a significant part of the heat contained in the fumes coming out of the oven. In practice, as in the case of aero-combustion furnaces, the most cost-effective use is to heat the reactants introduced into the furnace: oxygen, fuel and possibly raw materials.

In comparison with the techniques of air-combustion, especially those using regenerators, a difficulty comes from the nature of the necessary facilities. The regenerators can receive the fumes virtually as collected at the outlet of the oven. The materials that compose them, in particular the fittings, generally refractory ceramic materials, easily support the flue gas temperatures and the dust that these fumes can convey. Subsequently the preheating of the air in the hot regenerators does not require any special precautions. The heating of the products used for oxy-combustion, and in particular the heating of oxygen, on the contrary, requires much more restrictive precautions. Installations in which oxygen circulates must be perfectly tight, resistant to high temperatures and oxygen carried at these temperatures.

As for the fumes, we strive to ensure that they cause only a minimum of dust. The removal of flames from the bath surface, particularly in areas where the bath is covered with materials not yet melted, helps to minimize this entrainment. It also contributes to the fact that the burners, when arranged in staggered rows, minimize the turbulence that could result from the impact of the gas flows emanating from the burners facing each other.

The average fume velocity in the longitudinal direction does not generally exceed 3m / s, and is usually less than 2m / s. In the flames this speed is much higher of the order of 30 to 100m / s, this speed being normally lower than that of the flames in aerofuel combustion.

According to the invention, the preheating of oxygen is advantageously carried out in exchangers made of steels showing excellent resistance to hot oxygen. Exchangers and materials suitable for this purpose are described in the unpublished European Patent Application No. 07 107 942 filed May 10, 2007.

The hot oxygen supplied by the exchanger is brought to a temperature which can not exceed 650 ^° C. This value is a function of the resistance that can be obtained with metal alloys that have the best characteristics. This limit makes it possible to guarantee a duration of use related to the type of installation considered.

In practice for better safety, it is preferable to maintain the temperature of the oxygen at less than 600 ^° C. In order for the preheating of oxygen to be sufficient to improve the energy balance significantly, it is preferred to set a temperature of not less than 350 ^° C.

Similarly, the fuel used is advantageously preheated, whether natural gas or liquid fuel. The temperatures reached for the fuel are not dependent on the resistance of the installations. They possibly depend on the possible degradation of these fuels. In particular, it is necessary to avoid "cracking" them, even partially, which would result in fouling of the installations. For natural gases, the preheating temperature is advantageously less than 650 ^° C. and preferably less than 550 ^° C. For heavy fuels, the temperature is generally lower and does not exceed 180 ^° C., and preferably not

150 ° C.

The recovery of the heat of the fumes is largely sufficient to allow heating of the oxygen and the fuel at the temperatures indicated, independently of the efficiency of the heat exchange when it is carried out under the conditions mentioned in the patent application mentioned above. -above. It is still possible with the surplus to preheat the raw materials, or to feed boilers regardless of the use of the steam produced.

The invention is described with certain details in the following with reference to drawing plates in which:

- Figure 1 is a schematic perspective view of an oven according to the invention; - Figure 2 schematically illustrates a top view of the provisions of Figure 1;

FIG. 3 is a general block diagram of the heat exchange circuits used for a furnace according to the invention;

FIG. 4 is a detail of a block diagram relating to the circulation according to a mode of preheating oxygen; - Figure 5 shows a partial view from above the burner arrangements in an oven according to the invention.

The oven shown in Figure 1 is of the type used for large-capacity glass production such as those used to supply the production of flat glass by "float glass" techniques. Furnaces of this type continuously produce quantities of glass that can be up to 1000 tons / day. To achieve these performances the furnaces must have a power of 60 MW.

The oven 1 comprises a basin disposed in a closed enclosure. The assembly consists of refractory materials resistant to temperature, the corrosion of fumes and the aggression of molten materials. The bath level in the pool is represented by a dotted line 2.

The oven is fed with raw materials at one of its ends. The opening through which the charging of these raw materials takes place is schematized in 3. In practice to facilitate the distribution on the surface of the bath several charging points are usually arranged. The outlet of the molten glass, represented by the arrow V, operates at the opposite end by a neck ("neck") 4 of reduced width relative to that of the basin. Most usually the bottom of the neck 4 is at the level of the oven floor.

The neck is not totally immersed in the molten glass. There remains a space between the upper part of the neck and the surface of the sheet of glass. The operating conditions, with regard to the gas flow in the furnace, are adjusted so that the furnace atmosphere does not pass through the neck, to avoid any risk of entrainment of dust in suspension. To ensure this operation it is preferable to keep a small gaseous current, marked by the arrow A ₁ circulating in countercurrent flow melted glass. Being intended only to prevent a gas flow in the opposite direction, this current A is kept as low as possible. It is important to minimize it because it is normally constituted by the air present above the conditioning zone, not shown in Figure 1 (reference 5 in Figure 2), which follows the bottleneck.

Burners whose location is indicated at 6 are arranged along the side walls of the oven, on each side thereof to extend the flames substantially over the entire width of the basin. The burners are spaced from each other so as to distribute the energy supply over a large part of the length of the same melting-refining pool.

The combustion gases F are evacuated for the largest part by the outlets 7 located near the charging zone and at a distance from the nearest burners. In the representation

(Figure 1 and 2) two outlets 7 are arranged symmetrically on the side walls while the charging of raw materials (MP) is in the furnace axis. This is a preferred mode but other arrangements are also possible, such as for example the exit of the gases in the wall 8 closing the furnace in its upstream part. These outlets can also be distributed in a different manner, the important thing being to ensure that the fumes back up against the flow of glass V in the oven. Where appropriate, the exit of the fumes in particular can be carried out, at least partially, by the one or more charging openings.

As indicated above, according to the invention it is ensured that the chamber of the oven is substantially impervious to the penetration of outside air. Flue gas flow upstream prevents penetration of this side of the oven. The passages possibly arranged on the side walls are also essentially impervious to the penetration of the ambient air. To repress the little air that can come from the packaging part 5, a very limited flow of fumes is advantageously arranged downstream of the furnace. These fumes F are evacuated by the outlets 9. In the adjustment of the amount of air flowing from the conditioning zone to the refining zone, in addition to the gas flows generated by the burners, it is also important, as indicated above, to be able to modulate the quantities of fumes extracted in the downstream zone of the reactor. oven and which are evacuated by the exits 9.

The large-capacity glass kiln traditionally comprises two zones, one of which is called fusion, the other of refining. In Figures 1 and 2, these two areas are not delimited.

The boundary between melting and refining is usually not materialized in the kiln structure. In particular if in these furnaces a dam is disposed on the hearth, this dam does not ordinarily coincide with this limit even if it participates in the conditions that lead to its location.

The distinction between melting and refining zone is in all cases functional. It corresponds to the mode of circulation of the glass in the basin. This includes a first convection loop in the melting portion and a loop, the rotation of which is opposite to the first, in the refining portion. In the absence of means directly influencing the circulation, the position of the limit of the melting zone and that of refining is determined by a set of operating parameters which includes the distribution of energy by the burners. In Figure 2 these two areas are represented I and II.

In general, the supply of energy necessary for the melting of the raw materials is greater than that which keeps the glass at a temperature for refining. As a result, the number of burners, and especially the power they deliver, is greater in the melting zone.

If it is useful to bring a maximum of energy for the fusion, and therefore as soon as the raw materials are charged, it is also necessary to avoid placing the first burners too close to the smoke outlets 7, otherwise these very hot fumes would carry with them an excessive amount of energy. The choice of the position of the first burners is therefore the result of a compromise. The first burners are positioned in such a way that they are above the supernatants.

Always to limit the energy losses by the fumes as indicated above, it is also possible to modulate the power of the burners according to their position. The first burners can in particular operate at a lower power than that of burners located further downstream.

According to the modes of operation which prove the most advantageous, the "curve of fire", that is to say the distribution of the temperatures along the furnace, progresses initially from the upstream until towards a neighboring central part from the beginning of the ripening area. The temperature then varies a little while decreasing slightly until the neck 4 preparing the passage in the conditioning zone. For this reason the downstream end of the furnace is normally devoid of burners.

The distribution of the burners is shown in Figure 2 by the axis thereof. They are preferably arranged staggered on either side of the basin, to ensure that the flames emitted in opposite directions do not collide. Laterally they are separated from each other to cover as much as possible the surface of the bath. In this sense also, the burners used are advantageously of the type which develops the flame in the form of a sheet substantially parallel to the surface of the bath. Their individual rating depends on the burners chosen and the number of burners used.

The space available on the side walls of the oven 11 (Figure 5) is limited by the presence of the metal frame 12 supporting the vault of the oven. The beams constituting this frame are closer together as the oven is wider and the refractory ceramic materials are more heavy. For very large furnaces, only two flat burners 13 of the type described in publication WO 2004/094902 may be installed between two successive beams, one on each side of the furnace. These burners organize the staged combustion from a central fuel supply 14, concentric with a first oxygen supply 15, then by means of several secondary oxygen supplies 16, 17, parallel to the first, spaced from it and disposed in a same substantially horizontal plane. These staged combustion burners 13 produce a flame that develops in a plane substantially parallel to the surface of the molten bath. By their construction these burners have a certain width, hence the limited number between two beams.

As shown in Figure 1, the burners open into the furnace chamber at a distance above the surface of the bath. This arrangement, as explained above, makes it possible to distribute the energy radiated from the flame well. It also allows, combined with the height of the vault, a good circulation of the combustion gases including those that go to the main outlets 7 upstream upstream of the furnace. In contrast to air-fired furnaces whose fumes travel in an essentially transverse path, in the case of oxy-combustion furnaces according to the invention these fumes are directed in the length of the furnace and therefore transversely to the direction of the flames which they do not must not disturb. Having both space under the flames and above them, the flow of fumes can be carried out without excessive turbulence detrimental to the good development of the flames.

The fumes at the furnace exit are used in devices intended to recover a portion of the energy entrained by these fumes. If in principle it is possible to heat exchange directly between the fumes and the products to be preheated, the desire to proceed in the best conditions of efficiency and safety leads to more complex exchange facilities. In FIG. 3, however, for the sake of simplicity, the exchange installations are generally represented at 18 and 19. In these installations oxygen, or / and fuel, is heated before being conveyed to the burners by means of pipes 20, 21.

The fumes at their output are initially at temperatures of the order of 1200 to 1400 ^° C. At these temperatures it is preferable to pass them into a "recuperator", that is to say a summary exchanger that reduces the temperature of the fumes for treatment prior to discharge into the atmosphere via a chimney 24. The recuperator is a system in which a fluid circulates against the flow of fumes. In its most basic form there are two concentric pipes. A more elaborate system consists of a bundle of tubes passing through an enclosure where the heat transfer fluid circulates. Both types can be combined.

After this recuperator the fumes are still at high temperature. It is not generally less than 700 ⁰ C except to use very large recuperators. Before being rejected, they undergo a depollution to eliminate, in particular, the oxides of sulfur. This elimination is carried out for example in electro-filters. To prevent the deterioration of these filters it is necessary to lower the temperature further to about 3-400 ^° C. An economical mode consists in diluting the fumes with ambient air.

When brought back to these temperatures, the mixture can still be used as a means for feeding, for example, boilers producing steam. The steam in question is particularly useful for preheating the fuel-liquid. These are advantageously preheated to temperatures between 100 and 150 ⁰ C, and preferably between 120 and 140 ° C.

The exchanger fluid used in the recuperators is for its part used as indicated below with reference to FIG. 4. In the representation of Figure 3, two exchange systems 18, 19, are arranged one on each side of the furnace. The two flue pipes are connected by a pipe 22. The latter, in case of need for maintenance or repair of one of the installations allows to transfer at least a portion of the fumes on the second momentarily, the surplus can also be discharged through the pipes 27 or 28. In the same way a pipe 23 allows as necessary to supply both sides of the furnace heat transfer fluid.

Pipes 27 and 28 make it possible, if necessary, to avoid the passage of the flue gases into the recuperators and direct them directly to the evacuation 24.

In FIG. 3, the fumes F exiting downstream are not shown as conduits to the exchangers. Depending on the configuration of the installation it is also possible to connect these outputs so that all fumes are recovered. To the extent that the "recoverable" energy is surplus to that which can be used this recovery may be omitted where appropriate.

For the reasons indicated above, it is advantageous to carry out heat exchange in two stages. In a first "recuperator" the fumes heat an intermediate fluid, for example air, nitrogen, CO ₂ or any suitable fluid which circulates for example in a loop between this recuperator and one or more exchangers in the (s) which (s) it heats oxygen or fuel. An alternative for the intermediate fluid such as air is to not use the loop and recover the hot air at the outlet of the secondary exchangers by a boiler or other means of energy recovery.

Figure 4 illustrates this principle. The recuperator 25 receives on the one hand the fumes F, and against the current thereof fluid A, for example air. The heated air is directed to a series of exchangers 26 in which it circulates countercurrent oxygen which is preheated before being directed to the burners 13.

In practice, given the difficulty of conveying hot oxygen through long pipelines, because of the cost of piping or heat losses, according to the invention it is advantageously proposed to preheat the oxygen in the vicinity. burners in which this oxygen is consumed. For this reason it is necessary to multiply the exchangers which according to the case each feed one or a small number of burners located in the immediate vicinity.

In FIG. 4, each burner 13 is supplied with a heat exchanger

26.

The air, after preheating the oxygen, is returned to the recuperator 25 or is returned to the furnace flue to be passed through a boiler.

Claims

1. A glass melting furnace comprising a channel-shaped melting basin, the introduction of the raw materials taking place at an upstream end, the molten glass being recovered at the downstream end, furnace heated by means of burners, in which the combustion energy is produced by oxy-combustion for at least 65%, the burners being distributed on the walls in the length of the furnace, in which most of the flue gas discharge is located near the end upstream in the vicinity of the introduction openings of the raw materials, the remainder of the fumes being discharged near the downstream part so as to maintain a dynamic seal with respect to the surrounding atmosphere in which the burners are distributed so that at least 40% and preferably at least 50% of the furnace power is delivered to the melting zone.

2. Oven according to claim 1 wherein the power delivered in the melting zone does not exceed 80% and preferably not 70% of the total power of the oven.

3. Oven according to one of the preceding claims wherein the active burners are arranged at a distance from the charging zone of raw materials and evacuation of fumes.

4. Oven according to one of the preceding claims wherein the first active burners in the direction of progression of the glass are in the length of the oven above the area having supernatants.

5. Oven according to one of the preceding claims wherein the maximum power is delivered in the furnace length at the boundary of the melting and refining zones.

6. Oven according to one of the preceding claims wherein the operating power of the first burners and that of the last burners is less than that of other burners.

7. Oven according to one of the preceding claims wherein the burners open into the furnace chamber at a distance above the molten bath which is not less than 0.25m and preferably not less than 0.25m. 0.40m.

8. Oven according to one of the preceding claims wherein the burners open into the furnace chamber at a distance above the melt which is not greater than 1.0 m and preferably not greater than 0.8 m.

9. Oven according to one of the preceding claims which comprises in the electrode charging zone for the limited supply of energy in this area.

10. Oven according to claim 9 wherein the supply of energy by means of the electrodes does not exceed 10% of all the energy used in the oven and preferably not more than 5%.