Patent Publication Number: US-2011061642-A1

Title: Low-nox gas injector

Description:
The invention relates to a combustion process and a combustion device in which fuel is fed by at least one injector. 
     The invention will be more particularly described for a use in melting glass in glass furnaces, especially furnaces for manufacturing flat glass of the float type or furnaces for the manufacture of hollow packaging glass, for example furnaces operating in inversion mode, of the type using regenerators, although it is not in any way limited to such applications. 
     Most combustion processes of the aforementioned type, especially those used in glass furnaces, are faced with problems of undesirable NO x  emission in the combustion flue gas. 
     NO x  has a deleterious effect both on human beings and on the environment. Firstly, NO 2  is an irritant gas causing respiratory disorders. Secondly, in contact with the atmosphere, NO x  can progressively form acid rain. Finally, it causes photochemical pollution since, in combination with volatile organic compounds and solar radiation, NO x  causes the formation of what is called tropospheric ozone, the increase in concentration of which at low altitude becomes harmful to human beings, especially during hot periods. 
     This is why NO x  emissions standards in force are becoming increasingly stringent. Because of the very existence of these standards, manufacturers and operators of furnaces, such as glass furnaces, are constantly preoccupied with minimizing NO x  emissions, preferably down to a level of 800 mg per Nm 3  of flue gas for a side-fired furnace or 600 mg per Nm 3  of flue gas for an end-fired or horseshoe-flame furnace. 
     The parameters that influence NO x  formation have already been analyzed. These are essentially temperature, since above 1300° C. NO x  emission increases exponentially, and excess air, since the NO x  concentration depends on the square root of the oxygen concentration or the N 2  concentration. 
     Many techniques have already been proposed to reduce NO x  emission. 
     A first technique consists in making a reducing agent act on the emitted gas so that the NO x  is converted to nitrogen. This reducing agent may be ammonia, but this results in drawbacks such as the difficulty of storing and handling such a product. It is also possible to use a natural gas as reducing agent, but this is to the detriment of the consumption by the furnace and it increases CO 2  emissions. The presence of reducing gases, such as carbon monoxide, in certain parts of the furnace, such as regenerators, may also cause accelerated corrosion of the refractories in these zones. 
     It is therefore preferable, without this being obligatory, to dispense with this technique, by adopting what are called primary measures. These measures are so called as the aim is not to destroy NO x  already formed, as in the technique described above, but rather to prevent its formation, for example in the flame. These measures are furthermore simpler to implement and, as a consequence, more economic. However, they cannot completely substitute for the aforementioned technique, rather they advantageously supplement it. In any case, these primary measures constitute an indispensable prerequisite for reducing the consumption of reactants for the secondary measures. 
     Without being limited thereto, it is possible to classify the existing measures in several categories:
         a first category consists in reducing NO x  formation using what is called the “reburning” technique, whereby a zone that is short of air is created in the combustion chamber of a furnace. This technique has the drawback of increasing the temperature in the regenerator stacks and, as the case may be, of requiring a specific design of the regenerators and their stacks, most particularly in terms of sealing and corrosion resistance;   a second category consists in acting on the flame by reducing, or even preventing, NO x  formation therein. To do this, the aim may for example be to reduce the excess combustion air. It is also possible to seek to limit temperature peaks while maintaining flame length and to increase the volume of the flame front in order to reduce the average temperature within the flame. Such a solution is for example described in U.S. Pat. No. 6,047,565 and WO 98/02386. It consists of a combustion process for melting glass, in which the fuel feed and the oxidizer feed both take place so as to spread the fuel/oxidizer contact over time and/or to increase the volume of this contact for the purpose of reducing NO x  emission.       

     It will be recalled that an injector is dedicated to propelling fuel, which is to be burnt with an oxidizer. Thus, the injector may form part of a burner, the term “burner” generally denoting the device comprising both the fuel supply and the oxidizer supply. 
     The fuel is a liquid of the fuel oil type or is a gaseous fuel, such as natural gas. Certain injectors, as described in FR 2 834 774 for example, combine at least one liquid fuel supply with a gaseous fuel supply. 
     Moreover, it is known that gaseous fuels produce more NO x  than fuel oil. 
     The object of the invention is to devise a combustion process employing only gaseous fuel but producing only relatively small amounts of NO x . 
     This objective is achieved by the invention, one subject of which is a combustion process, especially for melting glass, in which a flame is created by gaseous fuel, characterized in that several regularly spaced peripheral low-pressure gas jets converge on a central high-pressure gas jet. 
     The central high-pressure gas jet determines the flame length, whereas the overall (low-pressure and high-pressure) gas flow rate determines the power of the flame. The process of the invention makes it possible to maintain a constant flame length, while modifying the power, and vice versa. 
     The peripheral converging low-pressure gas jets delay flame spread. 
     Therefore, the number of adjustment options is increased, especially with shortening of the flame and reduction in NO x  emission. 
     According to preferred features of the process of the invention:
         70 to 90%, preferably 75 to 85%, of the calorific power stems from the low-pressure gas;   the angle of convergence of the peripheral low-pressure gas jets toward the central high-pressure gas jet is between 4° and 10°, preferably between 5° and 8°;   the number of peripheral low-pressure gas jets is between 4 and 16, preferably between 8 and 12;   all the peripheral low-pressure gas jets have the same characteristics: cross section, flow rate and angle of convergence toward the axis of the central high-pressure gas jet.       

     Another subject of the invention is an injector for implementing a process according to the invention, characterized in that it comprises a high-pressure gas feed duct circumscribed in a coaxial low-pressure gas feed duct, the outlet of which is completely obstructed by a flat ring provided with holes of identical cross section, these being regularly spaced around the axis of said feed ducts and all converging at the same angle on said axis. 
     Preferably, the cross sections of the holes—i.e. in planes perpendicular to the axis of the holes—have circular perimeters. 
     Other subjects of the invention are:
         a burner comprising one or more injectors as described above;   a furnace, especially an end-fired furnace or a side-fired furnace, comprising at least one such burner; and   the application of the process, the injector, the burner or the furnace of the invention for limiting NO x  emissions.       

    
    
     
       The invention will now be illustrated by the following example, with reference to the appended drawings in which: 
         FIG. 1  is a front view of a flat ring forming part of an injector of the invention; and 
         FIG. 2  is a sectional view of this flat ring. 
     
    
    
     The flat ring  1  has ten holes  2  regularly spaced around the axis  3 . 
     The circular holes  2  converge at an angle of 6° toward the axis  3 . 
     Moreover, the flat ring  1  has a central hole intended to receive the central high-pressure gas jet, whereas the peripheral low-pressure gas jets pass through the converging holes  2 . 
     Trials were carried out in a 44 m 2  end-fired furnace. 
     The furnace was worked in a first phase with an injector alternately in the right part and left part of the furnace. 
     This was a dual gas momentum injector differing from that of the invention only by the absence of individual converging low-pressure jets. 
     In this example, the injector was in a central position beneath a stream of air and directed upwardly at an angle of 5°, the stream of air being directed downwardly at an angle of 22°. The injector was inclined at 3° of azimuth toward the internal central axis of the furnace. 
     The values are given at 8% O 2  and 5000 ppm CO. 
     The power of the injector was kept constant at 8000 kW. 
     The NO x  emission was 687 mg/Nm 3  for a specific momentum I sp  (defined as the ratio of the total momentum of the fuel jet to the calorific power) of 4 N/MW. 
     The injector was then modified in accordance with the invention, by the use of the flat ring of  FIGS. 1 and 2 . 
     With a specific momentum of 4 N/MW, the NO x  emission dropped to 587 mg/Nm 3 .