Method for operating a melting furnace in the form of a U-shaped flame furnace and melting furnace

Abstract

Method for operating a melting furnace in the form of a U-shaped flame tank or a transverse flame tank, comprising a combustion chamber bounded by a bottom, side walls and a ceiling, which is connected to at least two burner ports, wherein each burner port has at least one burner to which a fuel gas and combustion air are supplied, and wherein several heating electrodes are provided in the U-shaped flame tank or the transverse flame tank, laterally and / or on the bottom side, and the heating takes place simultaneously on the one hand via the at least one burner of a burner port and on the other hand via at least a part of the heating electrodes, wherein the total heating power consists of a combustion power component resulting from the gas combustion and an electrical power component resulting from the electrical heating, characterized in thatthat if the electrical power share is increased and the combustion power share is simultaneously reduced by decreasing the amount of fuel gas and combustion air supplied, an additional proportion of combustion exhaust gas is supplied together with the combustion air, wherein the amount of combustion exhaust gas is dimensioned such that the velocity of the gas mixture of combustion air and combustion exhaust gas is equal to or greater than a minimum velocity value.

Description

Method for operating a melting furnace in the form of a U-shaped flame furnace and melting furnace

[0001] The invention relates to a method for operating a melting furnace in the form of a U-shaped flame furnace or a transverse flame furnace, comprising a combustion chamber bounded by a bottom, side walls and a ceiling, which is connected to at least two burner ports, wherein each burner port has at least one burner to which a fuel gas and combustion air are supplied, and wherein several heating electrodes are provided in the U-shaped flame furnace or the transverse flame furnace, in particular on the bottom side, and the heating takes place simultaneously on the one hand via the at least one burner of a burner port and on the other hand via at least a part of the heating electrodes, wherein the total heating power consists of a combustion power component resulting from the gas combustion and an electrical power component resulting from the electrical heating.

[0002] A melting furnace of this type, in the form of a U-shaped or transverse flame furnace, is used for melting a glass mass. The furnace has a combustion chamber where the actual melting process takes place and into which the raw materials to be melted are fed. In the U-shaped flame furnace, the combustion chamber is connected to two separate burner ports, each with a burner neck. Each burner port contains at least one burner, usually a series of individual burners. The burners are operated with a fossil fuel gas to which combustion air is added in a defined quantity and at a defined combustion air velocity as an oxidizer. The operation of such a U-shaped flame furnace is such that one burner port operates in combustion mode, while the exhaust gases generated by the combustion chamber are drawn off through the other burner port.Since both burner ports are located at the combustion chamber entrance, a U-shaped path results with respect to the flame and exhaust gas direction. This operating configuration remains set for a predetermined time, for example, 20 minutes, after which the operation changes. The burner port that was operating in burner mode switches to exhaust gas extraction mode, through which subsequent exhaust gas is extracted, while the other burner port switches to burner mode.

[0003] Furthermore, a regenerative heat recovery system (regenerator) is installed upstream of the burner ports, in which the combustion air is preheated from the waste heat of the exhaust gas. This system includes, or is associated with, a reversing unit that switches the exhaust gas and combustion air flow when the operation of the burner ports is changed. The basic design of such a regenerative end-fired glass melting furnace is well known.

[0004] While the two burner ports in the U-shaped flame tray are arranged side-by-side at the front, in the transverse flame tray they are located on the side walls. In the latter, one burner port is always positioned directly opposite another. The operating mode also alternates. For example, if firing occurs from the left side, the combustion air, as with the U-shaped flame tray, comes from the regenerator, where it is preheated, and is directed into the combustion chamber. The burners, located laterally in or below the burner port, introduce the fuel gas flow into the combustion chamber. The resulting exhaust gases are drawn off through the opposite burner port. There, they heat the grid in the regenerator. After 20-30 minutes, the sides switch, and the right side now fires while the left side receives the exhaust gas. Typically, several of these burner port pairs are arranged side-by-side along the side wall; larger trays may have up to six or seven pairs.The burner ports and the corresponding regenerators or regenerator chambers can be dimensioned for different air volumes. A single regenerator or regenerator chamber can serve only one burner port, or, as a shared regenerator, several. The design and operation of such a transverse flame tray are also known.

[0005] In addition to such melting furnaces, which enable a hybrid heating operation consisting of a burner heating system powered by fossil fuel gas and an electric heating system powered by electricity, there are also melting furnaces heated purely by fossil fuels known, which only have a fossil fuel burner heating system and which also have two corresponding burner ports with separate burners and a heat recovery system upstream, along with a reversing system.

[0006] Flame stability is crucial for fossil fuel heating systems. To ensure flame stability, a sufficient quantity of oxidizer, typically combustion air, is required. This is mixed with the fossil fuel gas to provide a sufficiently high impulse of the gas mixture at the burner neck. This means that the melting furnace and / or burners must be operated and designed to maintain a sufficiently high gas velocity at the burner neck to generate a sufficiently high impulse, thereby ensuring flame stability and preventing insufficient heating. This also applies to hybrid-heated melting furnaces with additional electric heating.The electric heating power output is limited to such an extent that the gas burners must always operate with sufficient air velocity to ensure flame stability. This means that a specific ratio between combustion power and electric power must be maintained, and the combustion power output cannot be reduced below a certain limit. Reducing the combustion power output reduces the amount of gaseous fuel used and, consequently, the required amount of combustion air. This inevitably leads to a lower air velocity at the burner neck, and if the reduction is too great, to a dangerously low velocity, thus compromising flame stability.Even with purely fossil fuel heating, i.e., when no additional electric heating is provided, it is always necessary to operate with a sufficiently high amount of fuel so that the required amount of combustion air is dimensioned in such a way that sufficient air velocity is achieved at the burner neck and flame stability is ensured.

[0007] Due to operational reasons, this means that in a hybrid melting furnace the ratio of combustion power share to electrical power share is only variable up to a maximum electrical power share, as otherwise the flame-stable burner operation cannot be maintained.

[0008] Since a reduction in carbon dioxide emissions is always sought, especially in energy-intensive production processes such as operating a melting furnace in conjunction with the use of a fossil fuel, but at the same time there is also fluctuating availability of alternatively generated electrical energy from sun and wind, which cannot be used due to the narrow power ratios, there are limits to operation optimized from the perspective of emissions and energy consumption.

[0009] From US Patent 2009 / 0246719A1, a method is known for firing a burner in a process chamber of a furnace, in which the burner is supplied with fuel and combustion air in a ratio that ensures an excess of air. The method includes the steps of reducing the amount of combustion air and maintaining the excess air at the reduced combustion air volume. Further steps include drawing exhaust gas from the process chamber and supplying the burner with exhaust gas at an increasing flow rate at the reduced combustion air volume.

[0010] US 6,126,440 A describes a method and arrangement for converting a regenerative or recuperative furnace system with air-fuel burners for use with synthetic air. A portion of the furnace exhaust gases is recycled and mixed with oxygen to produce synthetic air. This synthetic air is then used for combustion in the furnace. A cassette-based regenerative oxygen-fuel crossfire furnace system utilizes synthetic air, which contains a mixture of recycled exhaust gases and oxygen, in combination with cassette regenerators.

[0011] DE 10 2009 053 755 A1 describes a method and an apparatus for melting glass, wherein fuel and oxidizer are supplied to a melting furnace via a burner arrangement for combustion within the furnace. In addition to the fuel and oxidizer supplied via the burner arrangement, a flow gas is introduced into the furnace at a velocity of at least 50 m / s through one or more flow gas nozzles. Combustion products are entrained within the furnace by the flow gas and, for example, transported back into the combustion zone. This improved recirculation within the furnace slows down combustion, reduces peak temperatures, and suppresses the generation of thermal nitrogen oxides.

[0012] The invention is therefore based on the problem of providing an improved method for operating a melting furnace.

[0013] To solve this problem, a method of the type mentioned at the outset provides that, when the electrical power share is increased and the combustion power share is simultaneously reduced by decreasing the amount of fuel gas supplied and the amount of combustion air supplied, an additional proportion of combustion exhaust gas is supplied together with the combustion air, wherein the amount of combustion exhaust gas is dimensioned such that the velocity of the gas mixture of combustion air and combustion exhaust gas is equal to or greater than a minimum velocity value.

[0014] The method according to the invention makes it possible to significantly increase the electrical power output beyond the existing maximum level. As described, an increase in the electrical power output is associated with a corresponding reduction in the combustion power output. This reduction leads to a decrease in the amount of fossil or gaseous fuel used, but if the reduction is too great, the decreasing combustion air volume and thus the decreasing air velocity at the burner neck can lead to problems with flame stability.According to the invention, this is counteracted in the case of a correspondingly significant reduction in the combustion power share by at least partially compensating for the reduction in the combustion air quantity through the supply of combustion exhaust gas, so that a total gas quantity of combustion air and combustion exhaust gas results, which exhibits a gas mixture velocity at the burner neck opening that corresponds to a minimum velocity value. This means that the gas mixture quantity is specifically increased by this deliberate supply of combustion exhaust gas, so that a sufficient gas velocity results at the cross-sectionally defined burner neck opening from the correspondingly increased gas quantity, and flame stability is ensured.This means that the velocity of the gas mixture at the burner neck is at a sufficiently high level despite a decreasing amount of pure combustion air, and that flame stability is maintained despite a reduction in the amount of fossil fuel.

[0015] The process according to the invention therefore offers the possibility of reducing the use of fossil or other gaseous fuels, combined with a reduction in the carbon dioxide emissions resulting from their combustion. At the same time, the process according to the invention allows flexible access to temporarily available electrical energy from alternative energy sources such as solar and wind power, with a correspondingly high proportion, since the process according to the invention makes it possible to increase the electrical power share significantly beyond the previously known maximum and to reduce the combustion power share significantly below its previously possible minimum. This is because the introduction of a defined quantity of combustion exhaust gas increases the total quantity of the gas mixture of combustion air and combustion exhaust gas, and thus its velocity, thereby generating the required momentum in the direction of glass flow.The power range of a given fuel-based firing system is thus extended downwards, and the flexibility of electricity use is significantly increased.

[0016] These advantages arise with a hybrid regenerative end-fired glass melting furnace, i.e., a melting furnace that is inherently equipped with both burner and electric heating. The method also advantageously allows for the retrofitting of a melting furnace originally designed solely for burner heating with powerful electric heating. This is because the inventive method enables a significantly wider range of variation in the total heating output from combustion and electric power than previously known melting furnaces. Therefore, retrofitting existing, purely burner-fired melting furnaces is justified by the associated flexibility in electricity consumption and the reduction in fossil or other gaseous fuel use.This means that a previously purely fuel-powered melting furnace can be converted into a hybrid melting furnace with additional electric heating, enabling economically viable operation. The same applies to retrofitting an existing hybrid regenerative glass melting furnace, i.e., a melting furnace that is already equipped with both burner and electric heating. Here, too, the process can achieve greater flexibility in terms of electricity consumption and a reduction in the use of fossil or other gaseous fuels.

[0017] As described, the reduction in gas volume resulting from the fuel reduction is compensated for by supplying a corresponding amount of combustion exhaust gas to the combustion air, thus raising the gas mixture velocity back to or above a minimum velocity value that ensures flame stability. This minimum velocity value should be at least 5 m / s, and preferably at least 6 m / s, for a U-shaped flame tray. It has been found that flame stability can be reliably guaranteed above such a minimum velocity value. Depending on the specific operating conditions, the amount of combustion exhaust gas supplied can be adjusted so that the actual gas mixture velocity corresponds to or only slightly exceeds the minimum velocity value. However, the setting can also be such that the minimum velocity value is significantly exceeded.The minimum speed should not exceed 14 m / s, in particular 12 m / s; a target value of 10 m / s is a suitable upper limit.

[0018] In the case of a transverse flame tray, the minimum velocity should be at least 4 m / s, and preferably at least 4.5 m / s. The minimum velocity for this type of tray should not exceed 12 m / s, and preferably 10 m / s; a target value of 8 m / s is a suitable upper limit. Since the flame is fired transversely, it only needs to cover a shorter distance, unlike the longitudinal firing in a U-shaped flame tray. Therefore, a lower minimum velocity is sufficient for adequate flame stability in a transverse flame tray.

[0019] The partial flow of combustion exhaust gas is advantageously controlled by a feed device with adjustable feed rate. This means that the amount of recirculated combustion exhaust gas can be precisely controlled via the feed device. Recirculation can be achieved, for example, using a hot gas fan or a drive nozzle as the feed device. Its controllability allows for sufficiently precise adjustment of the amount of recirculated combustion exhaust gas and thus the amount of gas mixture used for combustion, enabling corresponding adjustment of the flow parameters of the resulting gas mixture in relation to the desired mixture velocity.

[0020] The feeding device is best controlled via control information that can be determined based on various parameters. The following parameters can be used to determine the control information: - air temperature information describing the temperature of the supplied combustion air, - air quantity information describing the amount of combustion air supplied by a combustion air supply device as well as - Gas mixture oxygen information or gas mixture carbon dioxide information describing the oxygen or carbon dioxide content of the gas mixture, - exhaust gas quantity information describing the amount of exhaust gas supplied by a combustion air supply device.

[0021] Air temperature information describing the temperature of the supplied combustion air can be considered as a parameter. This means that a suitable probe is used to measure the combustion air temperature, for example, in the chamber head in front of the respective burner port during burner operation.

[0022] Another parameter to consider is information describing the amount of combustion air supplied by a combustion air supply device. This means that the amount of combustion air supplied, for example, by a combustion air fan, is measured using suitable equipment.

[0023] Finally, a parameter describing the oxygen content of the gas mixture is taken into account. Using a suitable probe, the oxygen content of the gas mixture consisting of combustion air and combustion exhaust gas is measured, for example, in the combustion chamber head. The introduction of combustion exhaust gas, which has a very low residual oxygen content due to incomplete, superstoichiometric combustion of the fuel, inevitably reduces the oxygen content of the gas mixture. If the oxygen content is now determined, for example, in the combustion chamber head of the burning burner port, this parameter can also be used to determine the required amount of recirculated exhaust gas.

[0024] Alternatively, the carbon dioxide concentration in the chamber head / chamber base can also be measured. CO2 is the main component of the combustion exhaust gas, and information about its concentration in the gas mixture can also be used as a parameter in determining the required amount of recirculated exhaust gas.

[0025] Additionally, there are ways to measure the exhaust gas volume directly. However, due to the contamination of the exhaust gas, this should preferably be done without contact.

[0026] In addition to the parameters described above, which are preferably considered together, it is also possible to determine the control information based on exhaust gas oxygen information describing the oxygen content in the combustion gas. By measuring the residual oxygen in the combustion gas using a suitable probe, the intake of false air can be detected and the amount of false air determined. Alternatively or additionally, a measurement error in the air volume measurement can also be detected and, if necessary, compensated for.

[0027] As described, the oxygen content of the gas mixture or the carbon dioxide content of the gas mixture is preferably determined in the head chamber of an exhaust gas heat recovery system located upstream of the combustion chamber. If exhaust gas oxygen content is also determined, this can also be done in the head chamber of the exhaust gas heat recovery system, or in the base chamber of the exhaust gas heat recovery system. Determining the residual oxygen content in the head chamber of the exhaust side enables the previously described determination of false air volume or the identification of any measurement errors in the air volume measurement, as well as the precise determination of the residual oxygen content in the exhaust gas. Determining the oxygen content in the base chamber allows at least the determination of the residual oxygen content and can be used as an optional additional measurement parameter to refine the calculation of the required amount of combustion exhaust gas.

[0028] It is conceivable to measure the relevant parameters in real time. This means that current values ​​can be measured using suitable probes or sensors, some of which are already installed in existing melting furnaces. In the case of retrofitting a melting furnace that has previously only been fuel-fired, appropriate electrodes, usually molybdenum electrodes, must be installed on the furnace side, along with the necessary transformer, wiring, and control technology. The exhaust and air supply system must also include the appropriate supply device for the combustion exhaust gas, such as a fan, as well as the corresponding piping and control components, which must be adapted to the regenerative heating system to extract the exhaust gas flow and mix it with the combustion air flow. Suitable sensors that enable the measurement of the relevant parameters can be easily retrofitted.

[0029] Alternatively, it is also conceivable to determine the combustion exhaust gas volume based on fixed values ​​stored in a corresponding table, which were determined based on previous measurements. The selection of the necessary parameters depends, for example, on the desired power output, for which corresponding parameter sets are stored.

[0030] The combustion exhaust gas is conveniently extracted in a reversing system, which opens or closes the respective exhaust duct of each burner port. Driven by an actuator (e.g., a hot gas fan or nozzle), the exhaust gas is then fed to the combustion air via a pipe. Such a reversing system is part of a heat recovery and / or recirculation system and allows for switching between burner port operations. This reversing system, which directly intervenes in, controls, and defines the operation of each burner port, thus enables the appropriate exhaust gas extraction or supply to the combustion air in a simple manner.

[0031] At exhaust gas recirculation rates exceeding 20%, the increased exhaust gas content can cause the oxygen concentration in the combustion air-gas mixture to drop too low. While there is stoichiometrically enough oxygen in the flow for complete combustion, the lower oxygen concentration can still lead to combustion problems. Further increasing the combustion air would resolve the issue, but this is energetically inefficient. In this case, it can be advantageous to introduce pure oxygen (>90%) into the combustion air-gas mixture. This can be done anywhere within the regenerator, in the burner neck, or even directly in the melting furnace.This means that, according to the invention, if the oxygen content in the gas mixture falls below a predetermined minimum level, pure oxygen is added to the gas mixture so that the oxygen content is above the minimum level but below a maximum level. The actual oxygen content can be continuously monitored, allowing constant control over whether or not oxygen supply is required, and when it can be stopped.

[0032] The oxygen content in the combustion air-gas mixture could also be used as a measure to determine whether additional oxygen supply is necessary. If this falls below a value of, for example, 17%, oxygen can be actively added until an oxygen concentration of, for example, 20.9% is reached, or up to a maximum oxygen concentration of, for example, 30%. According to the invention, the minimum oxygen content should therefore be 20%, preferably 17%, and the maximum oxygen content should be 30%.

[0033] In addition to the method according to the invention, the invention further relates to a melting pan in the form of a U-shaped flame pan or a transverse flame pan, comprising a combustion chamber bounded by a bottom, side walls and a top, which is connected to at least two burner ports, wherein each burner port has at least one burner to which a fuel gas and combustion air are supplied, and wherein several heating electrodes are provided in the U-shaped flame pan or the transverse flame pan, laterally and / or on the bottom side, and the heating takes place simultaneously on the one hand via the at least one burner of a burner port and on the other hand via at least a part of the heating electrodes, wherein the total heating power consists of a combustion power component resulting from the gas combustion and an electrical power component resulting from the electrical heating.comprising a control device for controlling the amount of combustion exhaust gas to be mixed with the combustion air in accordance with the method described above.

[0034] The inventive method and melting furnace offer a number of advantages. The primary advantage is increased flexibility regarding the energy mix used to operate the melting furnace. The inventive method allows, with particular benefit, a significant increase in the electrical power share compared to previous methods, while simultaneously reducing the fossil or gaseous fuel or combustion power share accordingly. This means that operation is possible with a fuel deficit, while flame stability is maintained despite this substantial reduction in fuel consumption due to combustion exhaust gas recirculation.

[0035] In known hybrid melting furnaces, the maximum range within which the electrical power component can be varied is approximately 10-15%. Starting from a design state of the melting furnace with a minimum electrical power component of 0%, this can then be a maximum of 10-15%. If the melting furnace is designed from the outset with a larger minimum electrical power component, for example 20%, the maximum electrical power component is 30-35%. In the inventive method or melting furnace, it is now advantageously possible to increase this maximum electrical power component, or the interval within which the electrical power component can be varied, by several percentage points, in particular by a proportion of 10-20%. That is, the variation interval increases to approximately 25-35%.In the first example, a minimum electrical power share of 0% and a maximum of 25-35% would be possible; in the second example, a minimum of 20% and a maximum of 45-55%. Depending on the design of the melting furnace, a maximum electrical power share is possible due to process limitations. With a U-shaped melting furnace and the recirculation system according to the invention, an increase in the electrical power share to up to approximately 80% is possible; with a transverse flame furnace, an increase to up to approximately 80% is also possible. The remaining power requirements are covered by the fossil fuel combustion.

[0036] Another advantage is the economically viable option of retrofitting an electric auxiliary heater into an existing system designed solely for fuel or gas heating. The ability to operate the system with a relatively high proportion of electric power allows for cost-optimized and flexible operation with regard to the energy source. Depending on the prevailing cost situation, the ratio of fuel to operating electricity can be varied accordingly, with a correspondingly wide range.

[0037] In particular, a stepless change in the ratio of fuel to operating current, i.e., a stepless variation of the power components within the possible range, is possible. This is because the gas mixture can be continuously adjusted or modified as desired via the feed device to achieve the desired gas mixture velocity.

[0038] Furthermore, the air-side heat transfer coefficient can be increased by adding radiatively active gases such as H₂O and CO₂, and the heat from the added exhaust gas stream can be advantageously used to preheat the gas mixture. A reduction in NOx emissions is also possible.

[0039] The figure shows a schematic representation of a melting trough according to the invention to illustrate the method according to the invention.

[0040] An exemplary glass melting plant 1 according to the invention is shown, comprising a melting tank 2 in the form of a U-shaped flame tank, which is bounded by a base 3, side walls 4, and a ceiling (not shown in detail) and has a combustion chamber 5. Two burner ports 6, 7 are provided on an end wall 4, opening into the combustion chamber 5. Fuel-based heating and the extraction of combustion gases are carried out via the burner ports 6, 7. Each burner port 6, 7 has several burners 8, 9, which are supplied with a gaseous fuel that is combusted together with a suitable oxidizer to heat the combustion chamber 5 and melt the material contained in the combustion chamber 5 to form the molten glass. The molten glass is extracted via an outlet 10 and conveyed via a downstream glass conditioning plant to downstream processing equipment.

[0041] Each burner port 6, 7 is connected to a regenerative heat recovery system 11, which extracts heat from the combustion exhaust gas and preheats the oxidizer, usually air. The heat recovery system 11 has two regeneration chambers 12, 13, each equipped with suitable heat storage elements, preferably made of ceramic, for example in the form of grids or similar. The storage elements of the regeneration chambers 12, 13 through which the exhaust gas is drawn heat up, while the previously heated storage elements of the other regeneration chamber 12, 13 transfer their heat to the oxidizer, i.e., the combustion air, which is passed through this chamber for preheating.

[0042] The exhaust gas heat recovery system 11, which, viewed in the direction of the plane of the drawing, is a correspondingly large hollow body divided into chambers and has a corresponding upper chamber head and a lower chamber foot, is associated with a supply and discharge device 14, which serves to supply the ambient or combustion air and to discharge the combustion exhaust gas. A reversing device 15 is associated with the supply and discharge device 14, which controls which burner port 6, 7 operates in combustion mode and which in exhaust mode, whereby the respective direction or distribution of the air supply as well as the exhaust gas discharge must also be controlled accordingly. This reversing is indicated by the double arrow 16.

[0043] Furthermore, a feed device 17 is provided, via which a partial flow of the combustion exhaust gas, which is generally discharged, is extracted before the actual discharge and fed to the combustion air flow, so that a corresponding gas mixture is formed, which is then supplied to the respective active burner port 6, 7.

[0044] Furthermore, in addition to heating via the burners 8, 9, an electric heater 18 is provided, comprising a series of electrodes 19, which are installed on the bottom 3 and side walls 4 of the melting furnace 2 and which are, for example, molybdenum electrodes. These electrodes 19 enable electric heating in addition to the burner heating. This means that simultaneous burner heating and electric heating operation is possible.

[0045] The respective operation is controlled, for example, by a central control unit 20, which controls all controllable elements of the burner heating, the electric heating, the combustion air supply and distribution, and the exhaust gas discharge and distribution. Of course, separate control units 20 can also be provided for the individual groups. The one or more control units 20 also communicate with suitable probes or sensors to acquire relevant parameters, based on which the corresponding control parameters are determined.

[0046] The operation of melting furnace 2 is such that one burner port 6, 7 is always operating in burner mode, meaning that the fuel gas is fed into the combustion chamber via the corresponding burners and combusts there together with the oxidizer, while the other burner port 6, 7 operates in exhaust mode, i.e., the combustion exhaust gas is extracted through it. In the example shown, it is assumed that the burners 8 of burner port 6 are operating in burner mode, while burner port 7 is extracting the exhaust gas.

[0047] For operation, the burners 8 are supplied with a fuel gas B. The oxidizer is a gas mixture G, which is supplied in sufficient quantity to achieve an adequate gas mixture velocity at the burner neck opening, i.e., the area where the burner port 6 opens into the melting chamber 5. The gas mixture velocity must be equal to or greater than a minimum velocity value to ensure flame stability and to introduce sufficient momentum in the flame direction via the gas mixture. This gas mixture is supplied via the supply and discharge device 14 and the respective regeneration chamber, in this case, the regeneration chamber 12. It consists of a portion of combustion air L, which is supplied via the supply and discharge device 14 and the reversing device 15, as the figure illustrates. A portion of the gas mixture G, in addition to the combustion air L, is also a partial flow of the combustion exhaust gas A, which...Partial exhaust gas flow TA is extracted from the main exhaust gas flow via the supply device 17, for example, a suitable fan, a drive nozzle, or similar device. The figure clearly shows that the combustion air flow L and the partial exhaust gas flow TA are combined to form the gas mixture G, which is then supplied to the burner port 6. Prior to this, the gas mixture is heated in the regeneration chamber 12, which was heated in the previous cycle when the burner port 6 operated in extraction mode. This means that the storage elements there release their stored heat to the gas mixture G, preheating it.

[0048] The burner port 7 operates in extraction mode, through which the combustion exhaust gas A is drawn off and fed to the regeneration chamber 13 to heat the storage elements there. The main flow of combustion exhaust gas A is then drawn off via the inlet and outlet device 14 and finally discharged via the reversing device 15. Simultaneously, the partial exhaust gas flow TA is also drawn off here as needed, which occurs as described via the inlet device 17.

[0049] This operating mode is maintained for a specific period, for example, 20-30 minutes. Then, the system switches, meaning the operating modes of burner ports 6 and 7 are reversed. Burner port 6 then operates in exhaust mode, while burner port 7 operates in combustion mode. The reversing device 15 accordingly controls the air and exhaust gas flows, so that the exhaust gas from burner port 6 is discharged and the combustion air L is supplied to burner port 7. The supply device 17 also now controls the partial exhaust gas flow TA in the opposite direction.

[0050] The melting plant shown could be a system originally designed for simultaneous fuel and electric heating operation. However, it is also conceivable that it is a system originally designed for pure fuel operation, to which the electric heating element 18 has been retrofitted with all necessary components, including the corresponding feed system, since this feed system and its control are essential elements of the melting plant 1.

[0051] The melting system 1 according to the invention allows for a wide range of variation in the heating method, meaning that the ratio of the combustion power share resulting from the combustion of the fuel gas to the electrical power share resulting from the electric heating can be varied over a wide range. This enables the recirculation or return of the partial exhaust gas stream TA to the gas mixture for the formation of the gas mixture, as provided for in the invention. This compensates for an insufficient amount of combustion air L resulting from a significant increase in the electrical power share and a reduction in the combustion power share, combined with a reduction in the amount of fuel gas and thus the amount of combustion air. This ensures that a gas mixture velocity at the burner neck is present that corresponds to or slightly exceeds a minimum velocity value, so that flame stability can be maintained.

[0052] A melting plant, for example one that is equipped with electric heating as standard, is designed for a maximum electrical power output, which is usually in the range of a maximum of 10-15% of the total power.

[0053] Typically, such systems are operated with an electrical power output of approximately 5%. Increasing this output to a maximum of 10-15% is only possible at best. As described, a corresponding increase in the electrical power output and a simultaneous decrease in the combustion power output leads to a reduction in the amount of fuel gas and thus also in the required amount of combustion air. This results in a significant reduction in the combustion air velocity at the burner neck, which can fall below a point where flame stability is no longer guaranteed. A combustion air velocity of approximately 7.5 m / s is typically used. To ensure flame stability, this combustion air velocity should not regularly fall below approximately 6 m / s. The 7.5 m / s figure is based on 100% fuel combustion. Due to the different efficiency levels, 10% electrical energy corresponds to approximately...15-20% of fossil energy is used because the electrical heating power is fed directly into the melt, while the burner heating power is fed indirectly. This means that a corresponding increase in the electrical power share leads to a disproportionate reduction in the amount of fuel and thus combustion air, so that, in order to maintain a sufficiently high combustion air velocity, operation can only be achieved with a relatively low electrical power share.

[0054] The invention provides a remedy here by compensating for the combustion air deficit due to the supply of the partial exhaust gas flow TA and raising the gas velocity at the burner neck back to a value that ensures flame stability.

[0055] The following tables contain corresponding operating values ​​in a sample calculation for an 80 m 2U-shaped flame trays for different tonnages (100%, 85%, 70%, 50%) are specified for three different operating scenarios: firstly, for "normal operation," in which the electric heater operates at a power output typical for the corresponding tonnage; secondly, for "operation with maximum electrical energy input," in which the electric heater operates at the maximum permissible power output; and thirdly, for "operation with maximum electrical energy input and recirculation," in which the electrical power output is significantly increased while simultaneously recirculating a portion of the exhaust gas flow and setting the gas mixture velocity to a minimum value. The analysis is based on an 80 m³ / h cylinder. 2 Bathtub with retrofitted electric heating and retrofitted exhaust gas recirculation. Temperature [°C] 1300 A BH [m 2 ] 2,33 Occupancy 100% 85% 70% 50% pull [tpd] 297 253 208 150 Broken glass [%] 50% 50% 50% 50% Normal operation P fossil [KW] 10350 10170 10120 8360 P electric [KW] 2300 1170 0 0 P fossil [%] 81,8% 89,7% 100% 100,0% P electric [%] 18,2% 10,3% 0,0% 0,0% BS[Nm 3 / h] 1001 983 978 808 LS [Nm 3 / h] 10739 10552 10500 8674 LS [Bm 3 / h] 61849 60772 60473 49956 V L BH [m / s] 7,37 7,25 7,21 5,96 Operation with maximum electrical energy consumption P fossil [KW] 9017 8360 8360 8360 P electric [KW] 3300 2527,5 1320 0 P fossil [%] 73,2% 76,8% 86,4% 100,0% P electric [%] 26,8% 23,2% 13,6% 0,0% BS[Nm 3 / h] 872 808 808 808 LS [Nm 3 / h] 9356 8674 8674 8674 LS [Bm 3 / h] 53884 49956 49956 49956 V L BH [m / s] 6,42 5,96 5,96 5,96 Operation with maximum electrical energy input and recirculation P fossil [KW] 9017 7330 5720 5693 P electric [KW] 3300 3300 3300 2000 P fossil [%] 73,2% 69,0% 63,4% 74,0% P electric [%] 26,8% 31,0% 36,6% 26,0% BS[Nm 3 / h] 872 709 553 550 LS [Nm 3 / h] 9356 7605 5935 5907 LS [Bm 3 / h] 53884 43799 34181 34020 V L BH [m / s] 6,42 5,22 4,07 4,06 Recirculation [%] 0,0% 19,0% 52,0% 53,0% TA [Nm 3 / h] 0 1445 3086 3131 GS [Bm 3 / h] 53884 52121 51958 52051 v G BH [m / s] 6,42 6,2 6,2 6,2

[0056] The following applies: Temperature = Temperature of the air or gas mixture at the burner neck A BH = Cross-sectional area of ​​burner neck Utilization = Utilization of the melting furnace according to design conditions Broken pieces = proportion of broken pieces in the glass mass P fossil = necessary fossil fuel power (total or proportional) P electric = required electrical power (total or proportional) BS [Nm 3 / h] = Fuel gas flow rate in standard cubic meters per hour LS [Nm 3 / h] = combustion air flow in standard cubic meters per hour LS [Bm 3 / h] = Combustion air flow in operating cubic meters per hour (at 1300 °C) v L BH [m / s] = Burner neck velocity of the combustion air Recirculation [%] = proportion of the recirculated partial exhaust gas flow, relative to the combustion air flow LS TA [Nm 3 / h] = Partial exhaust gas flow in standard cubic meters per hour GS [Bm 3 / h] = Gas mixture flow rate in operating cubic meters per hour (at 1300 °C) (= Sum of LS in Bm 3 / h and TA in Bm 3 / h) v G BH [m / s] = burner neck velocity of the gas mixture

[0057] Based on empirical data regarding the required melting power of the glass and the wall losses of the furnace, a required fossil fuel power of 10,120 kW can be assumed for the aforementioned example furnace at a rate of 208 tpd (tons per day) with 50% cullet. This means that 50% of the glass is melted from cullet, while the remainder is produced from mixed raw materials.

[0058] For a typical fuel gas with a calorific value of 8900 kcal / Nm³ 3This requires a fuel gas insertion of 978 Nm. 3 The combustion air flow rate LS required for combustion is determined by the air-fuel ratio of the fuel gas (dependent on the gas composition) and the selected excess air (usually around 10%). In this example, a fuel gas to combustion air volume ratio L of approximately 10.7 is assumed. This results in a required combustion air flow rate LS of 10,500 Nm³. 3 / h.

[0059] After the combustion air is heated in the regenerator (regenerative heat recovery system), it flows through the burner neck into the melting chamber where it reacts with the fuel gas B. Typically, the combustion air there has a temperature of 1250 to 1350 °C (1300 °C in this example).

[0060] The following formula applies to the gas laws (Amontons & Gay-Lussac): p1V1T1=p2V2T2(V=Volume; p=Pressure, T=Temperature)

[0061] Thus, the combustion air flow values ​​can be converted from standard conditions (0 °C; 1013.25 hPa) to the prevailing operating conditions. Since the pressure difference is very small (approx. 10 Pa), the pressure is assumed to be constant. If the altitude of the system deviates significantly from the standard conditions at sea level, the pressure can be corrected using an appropriate factor.

[0062] With the combustion air flow under operating conditions and the cross-sectional area of ​​the burner neck (2.33 m²) 2 ) can now determine the velocity at the burner neck opening v L BH[m / s] can be calculated according to the formula vL BH[m / s]=LS[Bm3 / h]:2.33[m2]:3600 [s], or the burner neck velocity of the gas mixture v G BH[m / s] according to the formula vG BH[m / s]=GS[Bm3 / h]:2.33[m2]:3600 [s].

[0063] This analysis assumes a combustion air temperature, and also a gas mixture temperature, of 1300°C. Furthermore, an opening cross-sectional area A at the burner neck BH of A BH = 2.3 m² is assumed. 2 .

[0064] The air speed v L BH[m / s] in the burner neck is calculated as the average of the operating volume flow [Bm 3 / s] of the combustion air and the opening cross-section A of the burner neck [m 2 ] (v=q / A with v= velocity of combustion air; q = operating volume flow [Bm 3 / s]; A = free cross-section). A similar principle applies to the gas mixture velocity v. G BH at the burner neck, the calculation of which also takes into account the partial exhaust gas flow. The combustion air flow LS is given in standard cubic meters per hour [Nm³]. 3 / h] (reference 0°C, 1013.25 hPa) assumed. The operating cubic meters [Bm 3The values ​​[ / h], which are relevant for the velocity, result from the measured standard volumetric flow rate and the ideal gas laws. The differential pressure to the environment can be neglected in this context due to the low values ​​(<<100 Pa). The temperature is of fundamental importance for the operating volumetric flow rate due to the large value of 1200–1300°C (=1473.15 K–1573.15 K). The conversion factor for the assumed 1300°C (=1573.15 K) is 1573.15 / 273.15 = 5.76. For the conversion of standard cubic meters / hour [Nm³ / h] 3 / h] in operating cubic meters / hour [Bm 3 The standard cubic meter value [ / s] must therefore be multiplied by 5.76.

[0065] The values ​​given are for four different tonnages. "Pull" indicates the respective throughput in tons per day. The proportion of cullet fed into the system is 50% in each case. The remainder consists of corresponding mixed raw materials. 1. Normal operation

[0066] Depending on the tonnage, the melting furnace operates with varying combustion and electrical power ratios. At 100% and 85% tonnage, electric heating is also used. The percentage of electrical power is 18.2% at 100% tonnage and 10.3% at 85% tonnage. At 100% tonnage, the electrical power input is 2300 kW, and the fossil power input (resulting from the fuel) is 10350 kW. At 85% tonnage, the electrical power input is 1170 kW and the fossil power input is 10170 kW. Correspondingly low fossil power inputs are reached at 70% and 50% tonnage, with 0 kW of electrical power input in each case.

[0067] Correspondingly staggered to the fossil fuel power consumption, which corresponds to the required amount of fuel (here at a load of 100%, BS = 1001 Nm). 3 / h), the combustion airflow LS, which is measured in Nm, also behaves as follows: 3The unit is given in cubic meters per hour (m³ / h). The combustion airflow at 100% tonnage is 10739 Nm. 3 / h, and decreases accordingly with decreasing tonnage, resulting naturally from the decreasing use of fuel gas.

[0068] The key parameter is the combustion air velocity at the burner neck, which is measured with v L The combustion air velocity is given in m / s. The combustion air velocity is 7.37 m / s at 100% tonnage, 7.25 m / s at 85% tonnage, and 7.21 m / s at 70% tonnage. These values ​​are all significantly above a minimum air velocity value, defined, for example, as 6.2 m / s, which still ensures sufficient flame stability.

[0069] This value is already just barely undershot at 50% tonnage with an air velocity of 5.96 m / s. Operation is barely possible at this point, but no additional electric heating is permitted, as this would cause the value to drop even further. 2. Operation with maximum electrical energy consumption

[0070] Let us assume that the percentage of electrical power is increased to the greatest extent possible. At 100% tonnage, the electrical power share is increased to a maximum of 3300 kW. The percentage share is 26.8%. This represents an increase of 8.6% compared to normal operation. This is accompanied by a reduction in the fossil fuel combustion power share; the utilized power decreases by 1333 kW. Accordingly, the fuel gas flow is reduced to 872 Nm³. 3 / h.

[0071] The decrease in combustion air velocity is clearly evident, now only reaching v due to the reduced amount of fuel gas. L The BH value is 6.42 m / s. This value is still sufficiently high to ensure flame stability, even when drawing the maximum available power input of 3300 kW.

[0072] However, the situation is different at 85% and 70% tonnage. In the case of 85% tonnage, the electrical output can only be increased to 2527.5 kW. While this represents an increase in the electrical power share of 12.9%, it is coupled with a reduction in fossil fuel power by 1810 kW to 8360 kW, and thus a corresponding decrease in the amount of fuel gas by 175 Nm³. 3 / h to 808 Nm 3 / h, each compared to normal operation. However, as a result, the combustion air velocity v is L BH is also calculated here at v L BH = 5.96 m / s, which is just below, for example, a target minimum value of 6.2 m / s. Therefore, a further increase in electrical energy input is not possible.

[0073] The situation is similar at 70% tonnage. Here, normal operation was without auxiliary electric heating; now the electrical power share is 13.6%. This is accompanied by a corresponding decrease in the fossil fuel power used, which drops by 1760 kW to 8360 kW, and the fuel gas volume decreases by 170 Nm³. 3 / h to 808 Nm 3 / h, each compared to normal operation. Here too, the combustion air velocity decreases; it is also v here. L BH = 5.96 m / s. Therefore, a further increase in the electrical power component is no longer possible.

[0074] With a 50% tonnage, an additional electric heater is not possible, as the combustion air velocity is already at the minimum limit during normal operation. 3. Operation with maximum electrical energy input and recirculation

[0075] This table clearly illustrates the inventive effect of exhaust gas recirculation of the partial exhaust gas stream. At 100% tonnage, no recirculation is required, since the combustion air velocity at the burner neck is v L The BH value of 6.42 m / s is sufficiently high, and the system is operated at its maximum electrical output of 3300 kW anyway. Therefore, the supplied combustion exhaust gas volume, i.e., the partial exhaust gas flow TA, is listed as 0 in the table.

[0076] However, the situation is different at 85% and 70% tonnage. At 85% tonnage, the electrical power input can be increased from 2527.5 kW to 3300 kW, accompanied by an increase in the percentage of electrical power from 23.2% to 31%. While the combustion airflow LS decreases from 8674 Nm 3 / h (see 2.) to 7605 Nm 3 / h. This is accompanied by a reduction in the combustion air velocity v. L BH from 5.96 m / s to 5.22 m / s.

[0077] According to the invention, combustion exhaust gas is recirculated as a partial exhaust gas stream TA and fed into the combustion air stream, so that the gas mixture G shown in the figure is formed. The percentage of 19% recirculation refers to the combustion air stream. For 85% tonnage, therefore, in addition to the 7605 Nm 3 / h combustion air still 0.19 x 7605 = 1445 Nm 3 / h of exhaust gas is added as a partial exhaust gas flow TA through recirculation. Consequently, the gas mixture volume flow at the burner neck is increased accordingly, resulting in a gas mixture velocity v. G BH at the burner neck from v G BH = 6.2 m / s. It is evident that the electrical power output can therefore be significantly increased, namely by 7.8%, while simultaneously ensuring flame stability.

[0078] The change is even more significant at 70% tonnage. There, too, the maximum electrical power input is increased to 3300 kW, representing a 23.0% increase. The fossil fuel power output is drastically reduced by 2640 kW, and the corresponding reduction in fuel gas volume is substantial, resulting in a correspondingly significant reduction in the combustion air velocity. L at the Brenner neck it shows 4.07 m / s.

[0079] Due to a recirculation rate of combustion exhaust gas in the gas mixture of 52% (in addition to 5935 Nm) 3 / h combustion air still 0.52 x 5935 = 3086 Nm 3 / h of exhaust gas for the formation of the gas mixture G) can increase the gas mixture velocity v G BH at the burner neck again to the minimum or target limit of v GThe velocity (BH) can be increased to 6.2 m / s. It is evident that the method according to the invention allows for a significant increase in the electrical power component, far exceeding the otherwise possible maximum power component.

[0080] This also applies to a 50% tonnage. Previously, auxiliary electric heating was not possible there. However, as the table shows, it is now possible to operate with an electrical power input of 2000 kW, resulting in an electrical power share of 26% (formerly 0%). Due to the reduction in the proportion of fuel gas, the combustion air velocity is indeed reduced. L BH = 5.96 m / s to 4.06 m / s. By recirculating a partial exhaust gas flow TA of 53% (in addition to 5907 Nm) 3 / h combustion air still 0.53 x 5907 = 3130 Nm 3 / h) to form the gas mixture G, but the gas mixture velocity v can in turn G at the burner neck on v LThe BH = 6.2 m / s is lifted. The method according to the invention thus enables an extremely efficient use of electric heating, with a very significant power share of 26%, which was previously impossible.

[0081] The tables clearly show the decrease in combustion air velocity at the burner neck with increasing use of an electric auxiliary heater, as well as the limits in terms of the possible electrical power share, resulting from a corresponding decrease in combustion air velocity to a value that is below a target velocity value, and at which flame-stable operation is perhaps just possible.

[0082] At the same time, the tables also show the significant effect of the inventive method or the inventive supply of the partial exhaust gas flow with regard to a blatant increase in the electrical power share, up to the point of being able to use an electric auxiliary heater at all.

[0083] It should be noted in this context that the maximum electrical power input is assumed to be limited to 3300 kW. It is obvious that a higher electrical power input is also possible, meaning the electrical power share can be increased even further, while simultaneously increasing the recirculated partial exhaust gas flow (TA) to maintain a sufficiently high gas mixture velocity. As the table shows, at 100% tonnage, no recirculation is required at all, since the gas mixture velocity is inherently sufficiently high at a maximum output of 3300 kW. This means that, if the electric heating system allows it, a significantly higher electrical power share can be achieved, and flame stability can be ensured without problems by recirculating a relatively small partial exhaust gas flow. This also applies at a tonnage of 85%, where only 19% of the partial exhaust gas flow is recirculated.An increase is also possible here, which would allow for an increase in electrical power consumption beyond the 3300 kW specified here.

[0084] The foregoing considerations further demonstrate that existing systems with integrated electric heating can be operated significantly differently according to the invention, in order to dramatically increase the proportion of electric power and simultaneously reduce the proportion of fossil gas, thus enabling the system to operate within a considerably broader energy mix range. This requires only the partial exhaust gas recirculation provided for in the invention to generate the gas mixture, thereby ensuring a sufficiently high gas mixture velocity at the burner neck, at least corresponding to a defined minimum velocity value or slightly above.

[0085] The system is equipped with appropriate sensors or probes that enable the recording of the parameters mentioned above. The corresponding control unit 20 of the glass melting plant 1 processes these parameters to determine the required amount of exhaust gas that must be diverted from the exhaust gas stream and fed to the combustion air to achieve a corresponding increase in electrical power output. The control unit is programmed accordingly to process the relevant parameters using appropriate control algorithms and to determine the control information for the feed unit 17.

[0086] A cross-flame tray can be analyzed in the same way as described above, using the relevant values ​​specific to this type of tray. The results show comparable effects with this type of tray regarding the increase in the electrical power share when the burner neck velocity is raised to or above the minimum velocity of 4 m / s, in particular 4.5 m / s, by the recirculation according to the invention, despite the reduction of the fossil power share.

Claims

[1] Method for operating a melting furnace in the form of a U-shaped flame furnace or a transverse flame furnace, comprising a combustion chamber bounded by a bottom, side walls and a ceiling, which is connected to at least two burner ports, wherein each burner port has at least one burner to which a fuel gas and combustion air are supplied, and wherein several heating electrodes are provided in the U-shaped flame furnace or the transverse flame furnace, laterally and / or on the bottom, and the heating is carried out simultaneously on the one hand via the at least one burner of a burner port and on the other hand via at least a part of the heating electrodes, wherein the total heating power consists of a combustion power component resulting from the gas combustion and an electrical power component resulting from the electrical heating, characterized by, that if the electrical power share is increased and the combustion power share is simultaneously reduced by decreasing the amount of fuel gas supplied and the amount of combustion air supplied, an additional proportion of combustion exhaust gas is supplied together with the combustion air, wherein the amount of combustion exhaust gas is dimensioned such that the velocity of the gas mixture of combustion air and combustion exhaust gas is equal to or greater than a minimum velocity value. [2] Method according to claim 1, characterized by , that the minimum velocity value in the case of a U-shaped flame tray is at least 5 m / s, in particular at least 6 m / s and a maximum of 14 m / s, in particular 12 m / s, and that the minimum velocity value in the case of a transverse flame tray is at least 4 m / s, in particular at least 4.5 m / s and a maximum of 12 m / s, in particular 10 m / s. [3] Method according to claim 1 or 2, characterized by, that the combustion exhaust gas is supplied via a supply device whose supply quantity can be controlled. [4] Method according to claim 3, characterized by , that the control of the feeding device is controlled via control information determined on the basis of the following parameters: - air temperature information describing the temperature of the supplied combustion air, - air quantity information describing the amount of combustion air supplied by a combustion air supply device as well as - a gas mixture oxygen information or gas mixture carbon dioxide information describing the oxygen content or the carbon dioxide content of the gas mixture, - exhaust gas quantity information describing the amount of exhaust gas supplied by a combustion air supply device. [5] Method according to claim 4, characterized by, that the control information is additionally determined on the basis of exhaust gas oxygen information or exhaust gas carbon dioxide information describing the oxygen content or the carbon dioxide content in the combustion exhaust gas. [6] Method according to claim 4 or 5, characterized by , that the gas mixture oxygen information or the gas mixture carbon dioxide information is determined in a chamber head of an exhaust gas heat recovery system upstream of the combustion chamber, and the exhaust gas oxygen information or the exhaust gas carbon dioxide information is determined in the chamber head or chamber foot of the exhaust gas heat recovery system. [7] Method according to any one of claims 4 to 6, characterized by , that if a predetermined minimum oxygen content in the gas mixture is not reached, pure oxygen is added to the gas mixture so that the oxygen content is above the minimum content value, but below a maximum content value. [8] Method according to claim 7, characterized bythat the minimum content value is 20%, preferably 17%, and the maximum content value is 30%. [9] Method according to any one of claims 4 to 8, characterized by that the parameters are measured in real time, or that predefined fixed information is used as parameters. [10] Method according to any one of claims 3 to 9, characterized by that the controllable feed device is a hot gas fan or a drive nozzle. [11] Method according to any of the preceding claims, characterized by , that the combustion exhaust gas is tapped off in a reversing system, by means of which a respective exhaust duct of each burner port is opened or closed, and fed to the combustion air. [12] Melting tank in the form of a U-shaped flame tank or a transverse flame tank, comprising a combustion chamber (5) bounded by a bottom (3), side walls (4) and a top, which is connected to at least two burner ports (6, 7), wherein each burner port has at least one burner (8, 9) to which a fuel gas (B) and combustion air (L) are supplied, and wherein several heating electrodes (19) are provided in the U-shaped flame tank or the transverse flame tank, laterally and / or on the bottom side, and the heating is carried out simultaneously on the one hand via the at least one burner (8, 9) of a burner port (6, 7) and on the other hand via at least a part of the heating electrodes (19), wherein the total heating power consists of a combustion power component resulting from the gas combustion and an electrical power component resulting from the electrical heating,comprising a control device (20) configured for controlling the quantity of combustion exhaust gas to be mixed with the combustion air according to the method of one of the preceding claims.

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