Patent Publication Number: US-2010112498-A1

Title: Hollow jet injector for liquid fuel

Description:
The invention relates to a combustion method and device in which the fuel is fed by at least one injector. 
     The invention is described more particularly for an application for melting glass in glass furnaces, in particular furnaces for manufacturing flat glass of the float type or furnaces for manufacturing holloware, for example furnaces operating by inversion like those using checker chambers (energy recuperators), but it is not necessarily limited to these applications. 
     Most combustion methods, particularly those used in glass furnaces, are faced with problems of undesirable NOx emission in the flue gases. 
     NOx gases are harmful to humans and to the environment. In fact, on the one hand, NO 2  is an irritant gas and the cause of respiratory ailments. On the other hand, in contact with the atmosphere, NOx can gradually form acid rain. Finally, they generate photochemical pollution because, in combination with volatile organic compounds and solar radiation, NOx causes the formation of ozone called tropospheric ozone, which, when its concentration increases at low altitude, becomes harmful to humans, especially in very hot periods. 
     This is why the standards in force on NOx emissions are becoming increasingly stringent. Due to the very existence of these standards, furnace manufacturers and operators such as those of glass furnaces are constantly concerned to minimize NOx emissions, preferably to a level lower than 800, or even lower than 600 mg per Sm 3  of flue gases. 
     The parameters that influence NOx formation have already been analyzed. They are essentially the temperature, because above 1300° C. the NOx emissions increase exponentially, due to the excess air, because the NOx concentration varies with the square root of the oxygen concentration or of the N 2  concentration. 
     Numerous techniques have already been proposed for reducing NOx emissions. 
     A first technique consists in using a reducing agent on the emitted gases to convert the NOx to nitrogen. The reducing agent may be ammonia, but this raises 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 of the furnace and increases the CO 2  emissions. The presence of reducing gases in certain parts of the furnace, such as the checker chambers, can also cause accelerated corrosion of the refractories in these zones. 
     It is therefore preferable, but not absolutely necessary, to discard this technique by adopting measures called primary measures. These measures are called primary because, instead of destroying the NOx already formed, as in the technique described above, they attempt to prevent NOx formation, for example in the flame. These measures are also simpler to implement and, in consequence, more economical. However, they cannot completely supplant the abovementioned technique, but can supplement it advantageously. In any case, these primary measures constitute an indispensable prerequisite for decreasing the reactant consumption of the secondary measures. 
     Existing measures can be classed in a nonlimiting manner into several categories:
         a first category consists in reducing the formation of NOx by using the technique called “reburning” whereby an airless zone is created in the combustion chamber of a furnace. This technique has the drawback of increasing the temperature in the stacks of checker chambers, and may necessitate a specific design of the checker chambers and of their stacks, especially in terms of tightness and corrosion resistance;   a second category consists in adjusting the flame to reduce or even prevent the formation of NOx therein. For this purpose, it may be possible to reduce the excess combustion air. It is also possible to try to limit the temperature peaks while maintaining the flame length, and to increase the volume of the flame front to reduce the average temperature in the flame. Such a solution is described for example in U.S. Pat. No. 6,047,565 and WO9802386. It consists of a glass melting combustion method, in which the fuel and oxidizer are both fed in such a way as to spread the fuel/oxidizer contact over time and/or to increase the volume of this contact in order to reduce the NOx emission.       

     It should be recalled that an injector is dedicated to the propulsion of the fuel, which has the function of being burned by an oxidizer. Thus, the injector may form part of a burner, the term burner generally designating the device comprising both the fuel and the oxidizer intake. 
     For the purpose of reducing NOx, EP921349 (or U.S. Pat. No. 6,244,524) has proposed a burner equipped with at least one injector, comprising an intake duct for a liquid fuel, of the fuel oil type, and a spray fluid intake duct placed concentrically around said liquid fuel intake duct, said liquid fuel intake duct comprising an element perforated with oblique channels to shape the liquid fuel into a hollow jet substantially matching the inside wall, the generatrix of each of said channels making an angle of at least 10°, in particular between 15° and 30°, preferably equal to 20°, with the liquid fuel intake direction. 
     It is the object of the invention to further reduce the NOx in comparison with what could be done on the basis of EP921349 (or U.S. Pat. No. 6,244,524). It has in fact been discovered that the reduction of the angle made by the oblique channels to the liquid fuel intake direction made it directly possible to elongate the flame produced, with more uniform flame temperature and less NOx. 
     A further object of the invention is to propose a combustion furnace and method suitable for all mineral glass melt preparation configurations, permitting optimal heat transfer, in particular by generating a flame having an adequate length and a sufficiently large volume to promote the maximum coverage of the glass melt and the melting batch materials. 
     The injector according to the invention is suitable for any type of glass furnace, like end-fired furnaces and cross-fired furnaces, which can be equipped with checker chambers or unit melters. 
     The invention relates to a liquid fuel spray injector comprising a liquid fuel intake duct and a spray fluid intake duct, said liquid fuel intake duct comprising an element perforated with oblique channels for shaping said fuel into a hollow rotating jet before ejection from said injector, the generatrix of each of said channels making an angle of less than 10° with the liquid fuel intake direction. 
     The injector comprises an intake duct for liquid fuel, particularly of the fuel oil type, and a spray fluid intake duct generally placed concentrically around the liquid fuel intake duct, said liquid fuel intake duct comprising an element perforated with oblique channels for shaping said fuel into a hollow rotating jet before ejection from said injector, the generatrix of each of said channels making an angle of less than 10° with the liquid fuel intake direction. 
     The liquid fuel and the spray fluid both discharge onto an outer face of the injector. In general, the spray fluid exits via a concentric orifice about the liquid fuel injection orifice. It is advantageous for the outer face of the liquid fuel intake duct and the outer face of the injector to be in the same plane. 
     The liquid fuel intake duct may also terminate in a nozzle for ejecting the liquid fuel through its outer face. In this case, the outer face of the liquid fuel intake duct is the outer face of the nozzle. The spray fluid intake duct may terminate in a block perforated with an orifice ejecting the spray fluid, at least part of the nozzle being inserted in said block, the outer face (terminal part) of the nozzle being aligned in the plane defined by the outer face of the block (which has no contact with the spray fluid) and onto which the orifice opens. The outer face of the injector therefore corresponds here to the addition of the outer faces of the nozzle and the outer face of the block. The outer face of the liquid fuel intake duct is the outer face of the nozzle here, because the liquid fuel intake duct terminates in a nozzle. 
     The creation of a very specific liquid fuel flow just before it exits from its intake duct permits an effective mechanical spraying of the liquid fuel by the spray fluid when it leaves the duct, suitable for obtaining a heterogeneity of the droplets of the said fuel, and thereby preventing their excessively rapid combustion from taking place, which is a source of NOx formation. In consequence, for a desired flame temperature, it is possible to allow little fuel at the inlet, and hence, in the flame root, thereby further decreasing the risks of NOx formation. 
     The liquid fuel may be ejected at a feed drive pressure of at least 1.2 MPa. 
     Preferably, the liquid fuel is ejected at a temperature of between 100° C. and 150° C., preferably between 120° C. and 140° C. 
     Such a temperature range serves to adjust any type of liquid fuel used in present-day installations, in particular glass furnaces, to the viscosity required immediately before it is ejected from its intake duct. This viscosity may advantageously be at least 5×10 −6  m 2 /s, in particular between 10 −5  and 2×10 −5  m 2 /s. 
     It has been discovered that the angle of the opening cone of liquid fuel ejection is correlated with the angle made by the oblique channels in the element for shaping the liquid fuel into a hollow jet with the liquid fuel intake direction. In consequence, the liquid fuel is ejected in a cone having an apex angle of at least 10°, especially between 3° and 8°. An apex angle of about 5° is particularly suitable. 
     Independently of the geometry of the liquid fuel intake duct and its dimensions, not only do such values serve systematically to obtain an interference between the spray fluid jet and the liquid fuel droplets, a necessary interference in the context of the invention, but also a dispersion of the size of said droplets, so that the resulting flame has a uniform temperature along its whole length. 
     As to the spray fluid, it is ejected very advantageously at a flow rate of not more than 70 Sm 3 /h, generally between 30 and 60 Sm 3 /h. 
     The spray fluid flow rate is correlated with the pressure of said fluid, a pressure that must be reduced to the maximum. By having a maximum flow rate like the rate mentioned above, a sufficient flame length can be obtained for all existing glass furnace configurations. 
     The liquid fuel intake duct may comprise a cylindrical tube and a nozzle. The nozzle may be fixed, in particular screwed, to the end of the cylindrical tube. A particularly suitable nozzle geometry for the injector of the invention is such as to comprise a frustoconical swing chamber prolonged by an end piece having a cylindrical inside wall. During operation, the liquid fuel stream is hollow when rotated, that is when it leaves the element perforated with oblique channels, and until its expulsion from the injector, that is its spraying in droplets. 
     In a particularly preferred manner, the apex angle theta of the swing chamber is at least 30°, preferably between 55° and 65°, in particular 60°, thereby minimizing the pressure drops of the liquid fuel flow. 
     The element used to form the hollow rotating jet of liquid fuel substantially obstructs the liquid fuel intake duct and is perforated with channels, in particular cylindrical, oblique with regard to the liquid fuel intake direction. 
     This element confers on the liquid fuel a rotating flow enabling it to assume the shape of a hollow jet, and gives it a sufficiently high level of mechanical energy for it to be sprayed at the outlet of its intake duct into droplets having an optimal size dispersion. 
     The channels may advantageously be uniformly distributed on the circumference of the element. 
     This element has a suitable shape for being inserted into the liquid fuel intake duct and may, for example, be a cylinder, preferably having two substantially parallel faces (pellet shape). These faces are also preferably oriented in a direction perpendicular to the liquid fuel intake direction. The element comprising the channels may therefore in particular have a cylindrical shape whereof the axis coincides with the liquid fuel intake direction. 
     More advantageously, the orientation of each of the channels is selected so that their generatrix makes an angle alpha of less than 10°, and even of less than 8°, and even of less than 6°, in particular about 5° with the liquid fuel intake direction. Generally, the orientation of each of the channels is selected so that their generatrix makes an angle alpha of more than 2°, or even more than 3°, even more than 4° with the liquid fuel intake direction. 
     This particular orientation serves to obtain a synergy between all the “divided” jets of liquid fuel leaving their corresponding channels, so that, when they exit therefrom, they contribute to the creation, downstream, of a single hollow jet matching the inside wall of any duct following the element comprising the channels (swing chamber followed by end piece for liquid fuel expulsion). 
     The channels pass through the element and each channel is defined in particular by an orifice on each side of the element, that is by two orifices per channel. In general, the center of the orifices of all the channels located on one side of the element are uniformly distributed on a circle of which the center corresponds to the axis of the element and of the injector. It is possible thereby to define two circles each located on either side of the element. In general, the radius R of these two circles may be identical. For example, R may be between 2.5 and 4.5 mm. 
     If S is the surface area of all the channels included in the element, then an S/R ratio of 6 to 13 mm is preferred. 
     According to an additional feature, the element may be mounted in a sealed manner upstream of the nozzle, in the liquid fuel intake duct, preferably against the swing chamber. 
     The terms “downstream” and “upstream” must be understood with reference to the liquid fuel intake direction. 
     As to the spray fluid intake duct, it preferably comprises at least one cylindrical tube at the end of which a block drilled with an orifice is placed, preferably screwed, and in which at least part of the nozzle according to the invention is inserted. 
     Preferably, the orifice of the block and the outer wall of the part of the nozzle inserted therein are positioned concentrically. This preferred arrangement can also be obtained by the abovementioned screwing, for ensuring the self-centering of the elements described above, that is the orifice of the block with regard to the part of the nozzle inserted therein. 
     This concentricity is advantageous insofar as its absence incurs a risk of formation of very large droplets of liquid fuel, of the fuel oil type, at the periphery of the hollow jet which may cause mediocre combustion, particularly with a risk of increasing the threshold of appearance of carbon monoxide. 
     It is preferable for the outer face (terminal part) of the nozzle to be aligned in the plane defined by the outer face of the block, that is the face lacking any contact with the spray fuel, and onto which the orifice opens. In fact, an incorrect alignment implies a modification of the aerodynamics of the liquid fuel and of the spray fluid when they leave their respective intake duct. 
     Advantageously, the injector according to the invention described above is mounted in a sealed manner in a refractory block using a sealing device comprising a plate provided with cooling fins. Such a sealed assembly prevents any undesirable air intake at the downstream end of the injector, an undesirable air that is particularly harmful insofar as it increases the oxygen content in the flame root which constitutes the hottest part thereof. 
     The injector according to the invention may be fixed to an adjustable support, a ventilation nozzle being oriented toward the downstream end of the injector, more particularly toward the abovementioned plate. The support is preferably adjustable for inclination, bearing and translation, in particular to bear against the plate of the sealed device. 
     As to the ventilation nozzle, it blows air, serving to prevent excessive local overheating at the downstream end of the injector. 
     The liquid fuel intake duct may comprise at least one diffuser. 
     The liquid fuel used in the context of the invention is a liquid fossil fuel commonly used in combustion devices to heat glass batch materials in a glass furnace. It may for example be heavy fuel oil. The spray fluid is similarly the one commonly found in conventional installations and which serves to spray the abovementioned liquid fuel. This may for example be air (in this case called primary air as opposed to the secondary air which serves as the main oxidizer). It may also be natural gas, oxygen (in the case of oxycombustion) or steam. The invention applies in particular to fuels of the heavy fuel oil type and it serves to circulate very large throughshapes (500 to 600 kg/h) of this type of fuel on a single injector according to the invention. 
     The liquid fuel flow rate in the injector is to be determined from the type of furnace on which it is to be mounted, of its operating parameters such as its outshape, and the type of liquid fuel used. These values may be determined without difficulty by a person skilled in the art, who can in particular prepare charts by performing tests. A person skilled in the art will also be sure to select a carefully prepared surface, respectively of the swing chamber, the channels, and the end piece of the inside walls, so as to ensure a minimum of pressure drops caused by friction of the liquid fuel flushing said elements at high speed. 
     The injector according to the invention generates little NOx in the combustion chamber, for example a furnace, it operates with a low spray fluid flow rate, allowing a broad and flexible use of the oxidizer, and therefore, ultimately obtaining good results from the energy standpoint. 
     The injector is generally integrated with a burner further comprising an oxidizer inlet. This oxidizer may be air, oxygen-enriched air or pure oxygen. In general, the injector is placed under the oxidizer inlet. For the case in which the oxidizer is air or oxygen-enriched air, the air enters via an opening having a relatively large cross section, which may in particular be between 0.5 and 3 m 2 , a plurality of injectors possibly being combined at each air inlet. 
     The invention is particularly suitable for manufacturing high grade glass, in particular optical, such as flat glass prepared by the float process, or holloware. The furnace equipped with the injector according to the invention emits little NOx, without any risk of reducing combustion that is potentially harmful to the color of the glass. 
     The invention may in particular advantageously supplement the techniques described in U.S. Pat. No. 6,047,565 and WO9802386. 
    
    
     
         FIG. 1  shows a schematic partial section of an injector according to the invention. 
         FIG. 2  shows an element according to the invention, perforated with channels shaping the fuel into a hollow jet in a side view of a section ( FIG. 2   a ) and a plan view ( FIG. 2   b ). 
         FIG. 3  shows a vertical section of a wall of a glass furnace comprising an injector according to  FIG. 1 . 
     
    
    
       FIG. 1  shows a partial section of an injector  1  according to the invention. This injector  1  comprises two fluid feeds, that is respectively the liquid fuel intake duct  2  and the spray fluid intake duct  3 . 
     The abovementioned liquid fuel and spray fluid intake ducts are connected respectively, upstream of the flow of each of the two fluids, to a circuit issuing from a liquid fuel source and a spray fluid source not shown. 
     The liquid fuel intake duct  2  consists essentially of a cylindrical tube  21  at the end whereof a nozzle  22  is screwed. Said nozzle, at its downstream end, comprises a frustoconical swing chamber  23  prolonged by an end piece  24  having a cylindrical inside wall  25 . The apex angle theta of the swing chamber  23  is 60°. 
     Inside the abovementioned nozzle  22  is a cylinder mounted in a sealed manner thrusting against said chamber  23 . Said cylinder  4  is the element perforated with the oblique channels shaping the liquid fuel into a hollow jet. The cylinder  4  comprises channels  41  uniformly distributed on its circumference and has two faces  42 ,  43  parallel to one another and substantially perpendicular to the liquid fuel intake direction symbolized by the arrow f in  FIG. 1 , a direction that is also identical to that of the spray fluid intake. 
     The channels  41  are cylindrical, their generatrix making an angle alpha of 5° with the abovementioned intake direction. 
     As to the spray of fluid intake duct  3 , it essentially comprises a cylindrical tube  31  at the end of which a block  32  is screwed, whereof the inside shoulder  33  thrusts against the downstream end of the tube  31 . 
     The block  32  is perforated with an orifice  34  having a shape suitable for the insertion of part of the nozzle  22 . The block  32 , on the side of the orifice  34 , also has a projecting part  35  suitable for screwing the block  32  to the cylindrical tube  31  to ensure perfect self-centering of the outer wall  26  of the end piece  24  inside the orifice  34 . 
     Due to their matching shapes, the concentricity of the abovementioned two elements  26 ,  34  is perfectly guaranteed, thereby preventing any undesirable modification of the size dispersion of the liquid fuel droplets leaving the duct  2 . 
     The alignment of the terminal part  36  of the nozzle (outer face of the nozzle) in the plane Π is perfectly achieved, said plane Π being the one defined by the outer face  37  of the block, that is the one lacking any contact with the spray fluid, and onto which the orifice  34  opens. 
     Such an arrangement serves to preserve the aerodynamics of the two fluids when they exit from their respective intake ducts. 
       FIG. 2  shows the cylinder  4  in  FIG. 1  in greater detail, in a side section ( FIG. 2   a ) and a plan view ( FIG. 2   b ).  FIG. 2   b  shows that the cylinder comprises 8 channels  20  whereof the centers are uniformly distributed on a circle having a radius R.  FIG. 2   b  only shows the orifice emerging from these channels, that is the orifice opening from the top of the part, except for one of these channels, for which the upper orifice  21  is drawn by a continuous circle and the bottom orifice  22  is drawn by a dotted circle. All the channels are obviously identical.  FIG. 2   a  shows the cylinder in a side view, and only the channel of the orifices  21  and  22  has been shown. The axis of this channel makes an angle alpha with the axis of the cylinder itself which corresponds to the liquid fuel intake direction. In the context of the invention, the angle alpha is lower than 10°. 
       FIG. 3  shows a vertical section of a wall of a glass furnace comprising an injector  5  according to  FIG. 1 . In this particular configuration, it can be seen that the injector  5  comprises a support  6  that is adjustable for inclination, bearing and translation. Fixed to this adjustable support  6  is the injector  5  which bears against the walls of a refractory block  7 , via a plate  8  provided with cooling fins. The refractory block  7  is itself mounted in an opening of the wall of the furnace  9 . 
     The injector  5  also comprises a ventilation nozzle  10  oriented toward the abovementioned plate. 
     Also observable are two flexible intake pipes  11 ,  12  connected respectively to the liquid fuel and spray fluid feed sources, which are not shown. 
     The operation of the injector will now be explained below. 
     When passing through the cylinder  4 , the liquid fuel, conveyed via the cylindrical tube  21 , is divided into as many individual jets as the number of tangential channels  41 . 
     The individual jets then enter the swing chamber  23 , striking its walls, with a minimum of pressure drops due to the fact that the value of the apex angle theta is 60°. 
     The uniform distribution of the tangential channels  41  and the 5° alpha inclination of the generatrix on the whole circumference of the cylinder  4  of each of these channels results in a centrifugation of all the individual jets against the wall of the swing chamber  23  without any interference necessarily occurring between them. 
     This centrifugation in the swing chamber serves, downstream, to enable the fuel to follow a helicoidal trajectory by assuming the shape of a hollow jet matching the inside wall  25  of the end piece  24 . 
     At the outlet of the end piece  24 , the liquid fuel has thereby acquired maximum mechanical energy and, under the influence of the spray fluid, it bursts vertically into very fine droplets having an optimal size dispersion. Such a dispersion gives the flame leaving the injector, once activated by the main oxidizer, a very uniform temperature along its whole length. 
     Such a fuel spray, for a given fuel flow rate, also considerably lengthens the flame in comparison with a spray that would be caused by the same injector  1  without cylinder  4 . 
     The cylinder  4  must be dimensioned in such a way that it is never completely filled, and that according to the invention, a hollow jet substantially matching this inside wall is always obtained. 
     The injector described above is of simple and inexpensive design. It is also completely and easily removable and adaptable to already existing installations. 
     In the figures, the angle alpha is slightly exaggerated to facilitate its understanding. 
     EXAMPLE 1 
     A 144 m 2  (surface area of glass melt) end-fired furnace is equipped with a burner comprising an air inlet stream under which four injectors of liquid fuel oil heated to 130° C. are placed. This burner has a capacity of 15 megawatts. Each injector contains an element for rotating the fuel oil comprising 8 holes 2.3 mm in diameter, whereof the axis makes a 5° angle to the liquid fuel oil intake direction. The axes of these holes are arranged on a circle having a radius of 3.75 mm. The total fuel oil flow rate (sum of flow rates fed to all the injectors) was 2000 kg/h. The air was fed to the burner in stoechiometric conditions with regard to the fuel oil. The NOx measured in the fluid gases was 550 mg per Sm 3 . 
     EXAMPLE 2 
     Comparative 
     The procedure of example 1 was followed, except that the holes had an axis making a 20° angle to the liquid fuel oil intake direction. The NOx measured in the flue gases was 800 mg per Sm 3 .