The invention relates to a method for melting glass from charging material in U-flame and cross-fired tank furnaces with burners for fossil fuels and with regenerators for the recovery of waste heat for preheating the combustion air, whereby set amounts of oxygen and primary fuel are introduced into the area around the flame roots under slightly sub-stoichiometric conditions to cover the heat requirement for the melting process, whereby secondary fuel and additional air are introduced beyond the end of the flame in order to further reduce the content of NO.sub.x, and CO in the waste gases by re-burning or post oxidation, in such a way that ultimately at least approximately stoichiometric combustion takes place, and whereby the waste gases are used to preheat the charging material in at least one raw material pre-heater before they enter the waste gas stack.
In glass furnaces with regenerators, whether cross-fired (side port) or U-flame (end port), the waste gas temperatures are normally in the range of 400-500.degree. C. after the reversal of the flow direction of the gases in the furnace. Simultaneously, oxygen concentrations of 4 to 5% are found here, as it is not possible to keep either the regenerator chambers or the subsequent reversal valves completely sealed. Although it is possible to preheat the charging material in raw material pre-heaters with the temperatures mentioned above, it is only possible to achieve a preheating temperature of approximately 300.degree. C. Therefore, such raw material pre-heaters and the corresponding auxiliary equipment can only be amortized over a long time period.
Methods for reducing NO.sub.x are known. One known method referred to as the "gas re-burn" method has the disadvantage that the additional combustion air has to be introduced in a part of the regenerator chamber which is relatively inaccessible, i.e., in the checkerwork, as otherwise there is insufficient time for re-burning or post oxidation, for example in order to complete the oxidation of CO to CO.sub.2. Therefore excessive concentrations of CO must be expected after the filter. Furthermore, the method referred to as "gas re-burn" has the disadvantage that the energy from the supplementary fuel introduced, the so-called secondary fuel quantity, must be considered as lost unless it is used for the production of hot water or steam in a waste heat boiler. However, this method is not as economic as returning the energy directly to the process.
The publications detailed below are all concerned with the problem of reducing the oxides of nitrogen (NO.sub.x) in the waste gases when melting glass in furnaces which have burners for fossil fuels and regenerative heat exchangers. The furnaces can be designed as either cross-fired or U-flame furnaces. A feature common to these furnaces is the fact that the flame direction is reversed at periodic intervals e.g., every 20 minutes. Only a part of the fuel is fed to the flame root (referred to below as the "primary fuel"), a further part of the fuel (referred to below as the "secondary fuel") is introduced beyond the flame end and within the opposite port neck and/or in the regenerative heat exchanger, and if necessary more oxygen is introduced downstream, so that when considered in total, stoichiometric combustion is achieved.
The term "gas re-burn" has become established for this process in scientific fields. The measures described are carried out in the expectation that a large proportion of the thermal energy of the secondary fuel introduced outside the furnace chamber, which can amount to between 5% and 17% of the total fuel requirement, is retained in the heat exchanger, and then transferred to the incoming combustion air, in order to compensate as far as possible for the "energy sacrifice" of maintaining clean air. However this expectation is practically unfulfillable, as the efficiency of modern regenerators cannot be increased any further.
From U.S. Pat. No. 4,372,770 and the corresponding French patent application 2 510 423 it is known that during re-burning, ammonia can be added at gas temperatures between 870.degree. C.-1090.degree. C. in order to reduce the oxides of nitrogen. It is further stated that before the ammonia is added, the secondary fuel should be added at temperatures of at least 1420 .degree. C., in order to destroy part of the oxides of nitrogen in advance. However, this necessitates the introduction of large quantities of secondary fuel and leads to a high thermal load on the materials of the heat exchanger. Furthermore, the heat exchangers are divided into primary and secondary regenerators in the direction of flow, whereby it is advantageous if the secondary fuel is introduced before the primary regenerator and the ammonia is added between the two regenerators. The building and operating costs are therefore considerable. The waste gas temperatures in the direction of the stack are given as being from 700.degree. C.-1090.degree. C., which leads to considerable energy losses. It is also pointed out that the mixing of the reaction components in the regenerator checkerwork is poor.
A similar method with high reaction and waste gas temperatures and two-part regenerators is described in U.S. Pat. No. 4,328,020.
From the publication by Koppang/Moyeda/Donaldson "Glass Furnace NO.sub.x Control with Gas Reburn", published by The American Ceramic Society, .COPYRGT. 1996, pages 19 to 35, ISSN No. 0196-6219, it is known that the ratio of the final content to the initial content of NO.sub.x can be reduced from 42-46% to 15-30% with increasing injection temperature of the secondary fuel. The said injection temperature is thereby raised from 1049.degree. C. to 1621.degree. C. The study comes to the conclusion that approximately 50-60% of the energy from the secondary fuel can be recovered in the regenerators, whereby their temperature rises by approximately 66.degree. C. It is also stated that a minimum time of approximately 0.3 seconds (in practice 0.5 seconds) must be available for the mixing process and the chemical reduction. These authors also come to the conclusion that local gas temperatures of at least 760-871.degree. C. are required for this. The study comes to the further conclusion that the reduction of the nitrogen oxides to nitrogen depends to a large degree on the stoichiometry in the re-burn zone, whereby the optimum level lies in the sub-stoichiometric range between 0.8 and 0.9. It is also mentioned that the disadvantageous cost balance can presumably be partly improved in that the waste gases can be used for preheating the charging material of the glass melting furnace. However this is not possible with the high temperatures given, because the charging material becomes sticky in the raw material pre-heater and does not slide any more. In the European Patents Nos. EP 0 599 547 and EP 0 599 548, it is presumed that sub-stoichiometric combustion, i.e., combustion with an excess of primary fuel or with a deficiency of oxygen, above the glass melt can be expected to produce poor glass quality. In order to correct this it is suggested that the regenerators should be built as gas tight as possible, so that no air can enter in an uncontrolled fashion, and the process should be operated so that the waste gases at the furnace outlet and/or at the regenerator entry contain unburnt primary fuel as a result of sub-stoichiometric combustion in the furnace, which undergoes pyrolysis on its further path into the regenerators, as a result of which radicals can be formed, which lead to a reduction of the nitrogen oxides to nitrogen.
Re-burning as a result of the addition of air occurs behind the regenerator packing, i.e., at the lower exit to the regenerator and shortly before the stack entry. The temperature at this point should be at least 650.degree. C., and preferably higher. It is suggested that a waste heat boiler be installed in the stack to reduce the energy losses. In addition, it is suggested, albeit without a detailed description of solutions, that the waste gases be cooled down before they enter a waste gas treatment plant or electrostatic filter. However, an adequate sealing of the regenerators is practically impossible, at least with an acceptable level of investment.
It is also known from the European Patent Applications EP 0 758 628 and EP 0 759 412 that in glass melting furnaces, waste gases containing combustible materials including CO can be treated by the addition of air and by the burning of the CO beneath the regenerator packing to reduce the contents of CO and NO.sub.x. Sealed regenerator chambers and the addition of suitable excess fuel in the furnace chamber or the sealed regenerators is assumed, so that the reaction temperature is higher than about 650.degree. C. The questions of how the high temperatures may be reduced by a heat exchanger to make them suitable for raw material preheating, and how an acceptable residence time is achieved, remain unanswered.
The not pre-published U.S. Pat. No. 5,795,364 in turn describes the basic principles of the "re-burn process" in a glass melting furnace with regenerative heat exchangers for preheating the combustion air and deals specifically with the problem of improving the mixing of the individual components: primary fuel/primary air, initial combustion gases/secondary fuel and the second combustion gases/secondary air, in order to avoid fuel-rich combustion zones and to reduce the damage to furnace structural materials. This takes place without an increase in the NO.sub.x components by establishing a "coefficient of variation" (COV), which can be determined from a formula, or measured by means of sensors, which determine the gas composition in the individual planes of the combustion zones. The formula mentioned includes a standard deviation (SD) as a variable, the calculation of which leads to the coefficient of variation COV as a difference [1-SD]. The value of the coefficient of variation COV should be approximately 0.4, which results in approximately stoichiometric combustion in the individual combustion zones. The document in question does not deal with the recovery of waste heat at the outlet of the regenerative heat exchanger by means of the introduction of the waste gases into a pre-heater for the charging material, nor with the control of the maximum temperature at the entry to the pre-heater by influencing the volume of secondary fuel for the re-burning.
The not pre-published U.S. Pat. No. 5,823,124 describes a "re-bum process" in which fuel is burned with an oxidation gas in a furnace under approximately stoichiometric conditions, whereby the furnace can also be a glass melting furnace. In an afterburner, which can also be a burner port, the furnace waste gases are enriched with additional fuel and fed with the excess fuel to a burnout unit, which may be a regenerator, recuperator or reactor. Further oxidation gas is supplied to the individual burnout units, in order to achieve approximate stoichiometric combustion. The oxidation gas is hereby added at the inlet of the burnout unit. A cooling medium is added to the burnout unit in order to cool the waste gases, whereby ambient air is suggested for this application. In addition to the preheating of the oxidation gases in the burnout units, they can also be preheated in a heat exchanger in a waste gas stack. However this is the limit of the heat recovery, and the preheating of solid charging material is not mentioned. The cooling air mentioned above is, for example, fed into the waste gas stack together with its total heat content, which therefore prevents the reuse of the energy. The document also deals with the influence on the NO.sub.x content in the waste gases of variation of the oxygen content in the oxidation gas between 20-100%, and comes to the conclusion that this pollutant is at its highest level of over 6% at an oxygen content of approximately 35% and diminishes to almost zero at an oxygen level of 100%, whereby the disadvantages of operating a furnace with almost pure oxygen predominate.
A common factor of all known solutions is that the transport paths and residence times of the gas mixtures are too short for thorough mixing and post oxidation, so that the waste gas temperature is intolerably high, and that the losses through unused and unusable energy from the secondary fuel can only be partly compensated for.
The efficient removal of both NO.sub.x and CO cannot be achieved easily, as this depends on process steps which are diametrically opposed to one another. This is discussed in detail below, based on the familiar Boudouard equilibrium conditions, according to which the balance moves towards CO with increasing temperature at temperatures above 400.degree. C. when carbon or carbon compounds are present. At 650.degree. C., for example, the balance is between approximately 60 vol.-% CO.sub.2 and 40 vol.-% CO.