Patent Application
20090148797
The present invention relates to a method of combustion in a furnace provided with energy recovery means.
Regenerative furnaces are furnaces equipped with stacks of refractories along their sides. These refractories are heat exchangers for recovering the heat from the flue gases leaving the sides of the furnace and for transferring this heat to cold air supplied to the furnace. The refractories of the regenerators are heated to very high temperatures by the flue gases (about 1300 to 1500° C.). In practice, the flue gases leaving via one side of the furnace come into contact with the refractories from their upper part down to their lower part during a cycle, generally lasting about 20 minutes. During the next cycle, the cold combustion air supplied to the burners of the furnace comes into contact the refractories from their lower part to their upper part in order to extract the heat therefrom. The combustion air is then heated to a temperature generally of about 1100 to 1300° C. before being introduced into the furnace combustion chamber. The streams of flue gases and combustion air are reversed at each cycle so that each side of the regenerator can be heated alternately and used to preheat the combustion air. Preheating the combustion air allows for air combustion with a high energy efficiency. For regenerative furnaces, the combustion air is heated continuously by metal heat exchangers fed with flue gases. These regenerative furnaces operate with air-fuel burners (or air burners), that is, burners in which the oxidizer is air. This is also referred to as air-combustion.
The service life of these furnaces is generally about 10 to 15 years. During a run, the basic problem consists in maintaining the production capacity of the furnace despite the increasing wear of the refractories and of the regenerators. This problem may even prove to be more critical in case of a need to increase production beyond the nominal capacity of the furnace, in order to contend with changes in market requirements over time.
Various alternatives are available for maintaining or increasing the production capacity of an existing regenerative furnace. Firstly, it is possible to supplement the heating electrically by installing electrodes through the floor or the side walls of the glass melt tank. This solution has the intrinsic advantage of great flexibility (production capacity can be increased by 10 to 15%) but its implementation is problematic and there is no guarantee of results, because the choice of the power distribution and electrode positioning is empirical (there is no accurate model of the effect of the electrodes on the glass melt). It is also difficult to position hot electrodes (especially if the locations have not been provided for before the construction of the furnace). The investment cost is high (transformers) and the price of electric power significantly higher than that of the fossil fuels conventionally used for glass melting.
Another solution consists in partially converting the air-combustion carried out in the furnace to oxy-combustion (in the context of the present invention, oxy-combustion means combustion using an oxidizer comprising oxygen-containing gas that is richer in oxygen than air). Oxy-combustion can be carried out by using one of the following technologies:
However, these two solutions have drawbacks. The addition of oxy-burners through the furnace roof gives rise to intrinsic difficulties in the positioning of the oxygen burners in the roof: the placement of this type of burner during a run (roof drilling) is problematic. Excessive thermomechanical stresses (in the burner blocks and noses (high pressure and temperature) or of the roof itself (embrittlement)) may cause faster wear of the materials and equipment and, in consequence, more flaws in the glass. An increase is also observed in the dust fly-off from the batch blanket covering the melt in the melting zone (impacting flames) and also the disturbance of the redox state of the glass (greater presence of CO in contact with the glass in the melting zone) which is liable to cause flaws in the glass.
As to the partial or total conversion of air-burners to oxy-combustion, this is mainly carried out by placing oxygen lances under an air duct while adjusting the rate of hot air flow introduced via the duct concerned or by adding oxy-burners on each side of the furnace. The following limitations are observed to these conversions to oxygen currently available on regenerative furnaces:
It is one object of the present invention to provide a method for increasing the capacity (glass output and quality) of the regenerative furnaces using an oxy-combustion technology, a method that is easy to implement.
Another object is to provide a method for increasing the capacity of regenerative furnaces using an oxy-combustion technology suitable for controlling the NOx emissions in the flue gases in comparison with existing technologies.
A further object is to provide a method for adapting the combined air- and oxy-fuel combustion in regenerative furnaces without altering the profile of the air-fuel flame.
For this purpose, the invention relates to a combustion method carried out in a furnace provided with energy recovery means and burners, in which:
The invention also relates to a system for carrying out a combustion in a furnace comprising:
Other features and advantages of the invention will appear from a reading of the following description. Alternative embodiments of the invention are provided as nonlimiting examples, illustrated by the appended drawings in which:
The invention therefore primarily relates to a combustion method carried out in a furnace provided with energy recovery means and burners, in which:
The furnace of the method according to the invention is equipped with energy recovery means used to heat cold air, that is ambient air, by transferring to it the energy recovered from the flue gases. These energy recovery means are placed either on each side of the furnace on the sides thereof, or at the end of the furnace where the batch is charged.
The furnace is also equipped with air-fuel burners. In the context of the present invention, “air-fuel burners” means burners implementing the combustion of air and of a fuel. According to the present method, an air-fuel burner consists of at least one air duct under which or at the center of which at least one fuel injector is placed. The injector is perpendicular to the furnace wall in order to create a flame substantially perpendicular to the furnace wall. However, angle effects may be imparted to the flame. The heat liberated by the flue gases of the air-fuel burners is recovered by the energy recovery means in operation, the operating mode corresponding to the phase in which the energy recovery means recover the heat, while the shutoff mode corresponds to the case in which the energy recovery means give up their heat to cold air. The energy recovery means and the air-fuel burners operate in phase opposition: those air-fuel burners which do not direct their flames to the operating energy recovery means are extinguished, while those air-fuel burners which direct their flame and their flue gases to the operating energy recovery means are in operation.
According to the invention, the method also uses oxy-fuel burners placed under the air duct of at least one air-fuel burner. In the context of the present invention, oxy-fuel burner means a burner using the combustion of a fuel and of an oxygen-rich gas, that is a gas having an oxygen concentration higher than 90% by volume. The oxygen produced by a VSA (vacuum swing adsorption) process is ideal for this purpose. The fuel of the oxy-fuel burners may be identical to or different from that of the oxy-fuel burners. It is understood that the oxy-fuel burners are under an air duct but remain above the melt. The method can use an oxy-fuel burner under an air duct or a plurality of oxy-fuel burners under one or more air ducts. If the furnace is equipped with air ducts placed in the sides, in that case the method preferably uses an even number of oxy-fuel burners placed symmetrically on each side under opposing air ducts. The oxy-fuel burners used according to the invention are of a particular type: they must implement a staged combustion of the fuel that they burn, the oxygen-rich gas being injected in the form of at least three different jets: injection of a first primary jet in contact with the fuel, injection of a second primary jet injected at a distance I1 from the fuel injection point, and injection of a secondary jet injected at a distance I2. Within the context of the present invention, the expression “injection in contact with the fuel” means the fact that one of the primary jets is injected either at the center of the jet of second fuel, or in the form of a sheath around the jet of second fuel, the distance between the first primary jet of oxygen-rich gas and the second fuel therefore being zero. This type of oxy-fuel burner is described in particular in applications WO 02/081967, WO 2004/094902 and WO 2005/059440. The injectors of the oxy-fuel burner are perpendicular to the furnace wall in order to create a flame substantially perpendicular to the furnace wall. However, angle effects may be imparted to the flame. The flame created by the oxy-fuel burner is substantially parallel to that created by the air-fuel burner.
If the furnace is a melting furnace in which the energy recovery means are placed on the sides, then preferably at least one oxy-fuel burner is placed under an air duct located in the furnace melting zone.
When an oxy-fuel burner is placed under an air duct of an air-fuel burner, then the flow rate of oxygen-rich gas injected by the oxy-fuel burner is generally between 20 and 100% of the flow rate of oxygen-rich gas and air injected by said burner and the air duct under which it is placed. The 100% flow rate corresponds to the phase in which the air-fuel burner is extinguished.
Preferably, the combustion carried out by the air-fuel burners is substoichiometric and the combustion carried out by the oxy-fuel burners is superstoichiometric.
According to the invention, it is possible to control the combustion method, and in particular the combustion carried out in the oxy-fuel burners, according to the heat recovery cycles of the energy recovery means. Thus, according to a first alternative, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas represents 70 to 80%, preferably 75%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to provide a broad air and oxygen flame.
According to a second alternative, for each oxy-fuel burner placed under the air duct of an operating air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas represents 45 to 55%, preferably 50%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burner serves to provide a stable air and oxygen flame and to concentrate the heat transfer to the melt placed near the root of the flame.
As to the operation of the oxy-fuel burners, two operating embodiments of the method can be implemented.
According to a first embodiment of the method, only the oxy-fuel burners whose flames are directed at the operating energy recovery means are also in operation. In this embodiment, the oxy-fuel burners operate in parallel to the air-fuel burners: when part of the energy recovery means recover the flue gases from the operating air-burners, the oxy-burners placed under the air duct of these air-fuel burners also operate, and when the latter energy recovery means are extinguished, then these air-burners which supplied heat to them are also extinguished and the oxy-burners placed under the air ducts of these extinguished air-fuel burners are also extinguished.
In a second embodiment of the method, the oxy-fuel burners operate permanently, independently of the operating and shutoff phases of the heat recovery means and the air-fuel burners. During the implementation of this second embodiment, according to a first alternative, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas can represent 70 to 80%, preferably 75%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to reduce the NOx emissions of the flames and to provide a broad flame.
During the implementation of this second embodiment, according to a second alternative, for each oxy-fuel burner placed under the air duct of a stopped air-fuel burner, the flow rate of oxygen-rich gas in the secondary jet of oxygen-rich gas can represent 45 to 55%, preferably 50%, of the total quantity of oxygen-rich gas injected by said oxy-fuel burner. This distribution of oxygen-rich gas in the secondary oxidizing gas injectors of the oxy-burners serves to reduce the NOx emissions of the flames and to increase the temperature of the flue gases.
The invention also relates to a system for carrying out a combustion in a furnace comprising:
The oxy-fuel burner may be selected from those described in applications WO 02/081967, WO 2004/094902 and WO 2005/059440.
In general, in the combustion system according to the invention, the fuel injector of the air-fuel burner is placed under the air duct and said injector and the injectors of the oxy-fuel burner are placed substantially in the same horizontal plane.
Preferably, the oxy-fuel burner is composed of two sets of injectors, said sets being arranged symmetrically about the center of the air duct under which the burner is placed. In this preferred system, it is advantageous to place at least one fuel injector of the air-fuel burner between the two sets of injectors of the oxy-fuel burner. According to a preferred embodiment, the oxy-fuel burner comprises at least two fuel injectors and said fuel injectors are placed on each side of the two sets of injectors of the oxy-fuel burner.
By implementing the method and the combustion system according to the invention as previously described, it is possible to increase the capacity of a furnace equipped with energy recovery means and air-fuel burners and thereby to increase the production capacity of the furnace.
The method and the combustion system according to the invention also serve to carry out combustions of fuels of different types according to the nature of the air- or oxy-fuel burners.
The second embodiment of the method according to the invention (in which oxy-fuel burners operate permanently, independently of the operating and shutoff phases of the heat recovery means and the air-fuel burners) serves to maintain the heating of the melt. Due to the small volume of flue gases created by the oxy-fuel burners, it is in fact unnecessary to extinguish them when the heat recovery means change their operating mode. In this case, the power of the oxy-fuel burners can be adjusted to compensate for the asymmetrical heating of the air-fuel burners.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0553229 | Oct 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/051086 | 10/23/2006 | WO | 00 | 11/3/2008 |
