Abstract:
In at least one of the ducts for feeding a fuel and an oxidant to a burner, a device for flow pulsation is provided to cause in the duct a pulsation of the flow of fluid which is fed at a frequency lower than 3 Hz, typically lower than 1 Hz. Application for example to oxycombustible burners of industrial furnaces having high yield and low emission of nitrogen oxides.

Description:
BACKGROUND OF INVENTION 
     (a) Field of the Invention 
     The present invention concerns techniques of pulsed combustion where a flame of a burner of an industrial furnace which is supplied with fuel and an oxidant is provided with a pulsation, to cause cyclic variations of at least one of the flows of fuel and oxidant which feeds the burner. 
     (b) Description of Prior Art 
     Already known are processes of pulsating combustion, based on the principle of increasing the volume of burned gases. This volume increase acts on devices for controlling the flows of the gases to be burned by causing a decrease of the combustion, and therefore, a decrease of the volume of the burned gases. The upstream pressure of the fluids then reestablishes the flows at their highest value and the combustion increases, thus ensuring a pulsating combustion. 
     Such a process is described in the document EP-A-0.046.898. 
     The document SU-A-857.642 describes a device for pulsating air for the combustion of gaseous or liquid fuels which operates on the principle of a paddle wheel mounted on an axis. 
     It is known that the production of nitrogen oxide, which is harmful to the environment, depends on partial pressures of oxygen, nitrogen, as well as on the temperature of the medium, which is a problem in the case of pure oxygen which is increasingly used in industrial furnaces because of the numerous advantages that it provides. Studies made by the Applicant have shown that, for example, in the case of pure oxygen, the content of nitrogen oxide fumes increases from 180 ppm to 1600 ppm in a so-called adiabatic furnace of 25 KW, where the thermic energy is transmitted to the walls of the furnace and to the fumes produced as a function of a content of nitrogen in the natural gas varying from 2 to 16%. Similarly, the content of nitrogen oxides in the fumes increases from 150 ppm to 500 ppm in a furnace, for example, a furnace for melting glass, where the thermic energy is simultaneously transmitted to the charge, to the walls and to the fumes, as a function of the content of nitrogen in the natural gas used. The Applicant has, on the other hand, observed that, all things being equal, the content of nitrogen oxides in the fumes goes from 700 ppm to 1200 ppm when the content of residual oxygen in the fumes goes from 1% to 4%. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to propose a process of pulsed combustion, enabling more particularly, with a fuel which is highly enriched in oxygen, for example, substantially pure oxygen, to reduce the emission of nitrogen oxides. 
     To do this, according to a characteristic of the invention, the frequency of the cyclic variations of at least one of the flows of fuel and oxidant is lower than 3 Hz, particularly lower than 1 Hz and preferably about 0.2 Hz. 
     According to other characteristics of the invention: 
     the ratio between the duration under maximum flow and the duration under minimum flow of the pulsed flow is between 0.5 and 2, preferably of the order of 1; 
     the volume ratio between a minimum flow and a maximum flow of the pulsed flow is lower than 0.7, typically 0.5, preferably of the order of 0.25; 
     the volume ratio between a minimum flow and a flow which is combined with pulsations is between 0.1 and 2, preferably of the order of 0.25; 
     the cyclic variations are induced at least in the flow of fuel and, in the case where the cyclic variations are induced in the flow of fuel and in the flow of oxidant, these cyclic variations are produced at a frequency which is common to both flows by being preferably dephased by at least π; 2. 
     The fuel contains at least 80% oxygen, and advantageously consists of substantially pure industrial oxygen, the fuel being typically natural gas. 
     The present invention also concerns devices for pulsed combustion to carry out the above processes, comprising a burner of an industrial furnace connected to at least one duct for supplying fuel and at least one duct for supplying oxidant, in which at least one of the supplying ducts is provided with a pulsating means to provide a pulsation of the flow of fluid supplied by the duct at a frequency lower than 3 Hz. 
     The present invention also concerns the use of such a device in a furnace for melting glass. 
     As shown by the experimental results which will be given in more detail later, these characteristics enable to substantially reduce the emission of nitrogen oxides even in the case where pure oxygen is used as oxidant and to maintain the volume content of oxygen in the fumes at a very low value. It can be estimated that the pulsations produced enable to intervene first on the times of residence of the species which are present (co-linear trajectories) and secondly on the meeting times of these species. It is thus possible to act directly on the temperature level due to the fact that, locally, highly over- or under-stoichiometric conditions prevail. The local combustion is therefore carried out in the presence of combustion products from the preceding local combustions and as a result, there is a staging of the combustion mechanism and therefore directly a decrease of the formation of nitrogen oxides. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other characteristics and advantages of the invention will appear from the description which follows of embodiments, given with reference to the annexed drawings, in which: 
     FIG. 1 represents a diagram of the ratio of the nitrogen oxide contents NOx/NOx(ref) with and without pulsation as a function of the frequency of the pulsations made solely on the fuel, such as natural gas; 
     FIG. 2 represents a diagram of the content in percentage of carbon monoxide as a function of the frequency of the pulsations made solely on the fuel, such as natural gas; 
     FIGS. 3 and 4 are diagrams corresponding to those of FIGS. 1 and 2, the pulsations being here made only on the oxidant, such as oxygen; 
     FIGS. 5 and 6 are diagrams corresponding to those of FIGS. 1 and 2, the pulsations being made here simultaneously on the natural gas fuel and on the oxygen oxidant; 
     FIGS. 7 and 8 are diagrams analogue to those of FIGS. 1 and 2 with other operating conditions; 
     FIGS. 9 and 10 represent diagrams of the ratios NOx/NOx(ref) as a function of the ratio C/D, C being the minimum constant flow and D being the flow with addition of pulsations; 
     FIG. 11 is a schematic view of an apparatus according to the invention; 
     FIG. 12 is a schematic view of a variant of the apparatus; 
     FIGS. 13 and 14 are partial schematic views of another embodiment of pulsating means according to the invention; 
     FIG. 15 is a schematic view in cross-section of another embodiment of a pulsating means according to the invention; and 
     FIGS. 16 to 18 are cross-section views of variants of the rotary tubular valve head of the valve of FIG. 15. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 1, the experimentation corresponds to pulsations provided only on the natural gas fuel, the volume ratio of the minimum flow to the maximum flow of natural gas being 0.23. It is noted that there is a ratio of the emission rate of nitrogen oxides to a reference system without pulsation (NOx(ref)) which varies from 0.5 to 1.2 for a pulsation frequency variation of 0.1 at 10.8 Hz. Below 3 Hz, it is observed that there is a substantial reduction of the emissions of nitrogen oxides, which reaches 50% in the pulsation frequency range between 0.1 and 0.3 Hz. 
     Under the same conditions, it is observed, with reference to FIG. 2, that there is nearly no emission of carbon monoxides, except for the very low frequencies, lower than 0.15 Hz. 
     With references to FIGS. 3 and 4, there are similar effects of reduction of the rate of emission of nitrogen oxides and small production of carbon monoxide at very low frequency in the case where there are provided pulsations only on the oxygen oxidant, the optimum range here being between 0.2 and 2 Hz. 
     With reference to FIGS. 5 and 6, which correspond to an experiment where pulsations are provided simultaneously on the natural gas fuel and on the oxygen oxidant, it is noted that there is an important reduction of the emissions of NOx for pulsation frequencies lower than 3 Hz, this reduction reaching more than 90% for frequencies between 0.2 and 0.4 Hz with a dephasing of at least π; 2 between the pulsations of the fuel and of the oxidant. 
     With reference to FIGS. 7 and 8, reduced rates of nitrogen oxides produced in a furnace for melting glass are observed at frequencies lower than 4 Hz with a ratio of the minimum and maximum flows of 0.5 and a duration at minimum flow equal to the duration at maximum flow. The furnace is operated with a minimum flow equal to half the maximum flow, and a dephasing of π between the pulsations and the flows, and a duration at minimum flow substantialy equal to that of the maximum flow: there is a reduction of about 40% of the rate of emission of nitrogen oxides. 
     Referring now to the diagrams of FIGS. 9 and 10, which represent in ordinates, respectively the rates of nitrogen oxides and carbon monoxide and, in abscissae, the ratio between the minimum flow and the maximum flow of the pulsations (expressed by C/P corresponding to a ratio between a constant minimum flow and a supplement of pulsation flow), it is seen that for a substantially reduced emission of nitrogen oxides, the ratio C/P should be between 0.1 and 0.3, i.e. the minimum flow/maximum flow ratio should be between 0.5 and 0.2, preferably of the order of 1. 
     On FIG. 11 there is seen a part 1, defining a combustion chamber, of an industrial furnace, for example, a furnace for melting glass, in which a flame 2 is formed by an oxy-combustible burner 3, consisting of a central tube 4 for supplying natural gas surrounded by a concentric tube 5 for supplying oxygen. The tubes 4 and 5 are connected to gaseous sources not illustrated respectively by means of a duct 11, 21, each incorporating a control electro-valve of low frequency 12, 22 which is by-passed by a deviation including a needle valve 13, 23 supplying a given minimum flow. The gas flows are measured, upstream of the valves 12, 13; 22, 23 by means of by-passed rotameters 14, 24. The electro-valves 12 and 22 enable through their cyclic openings and closings, to provide a pulsation of the natural gas and oxygen flows between a minimum flow supplied by the needle valves 13 and 23 (valves 12 and 22 being closed) and a maximum flow corresponding to the full opening of the electro-valves 12 and 22. The same pulsating means may be applied in simplified cases where a pulsation is carried out only on one of the gases supplied to burner 3, typically on the oxidant. 
     As a variant, as illustrated in FIG. 12, the pulsating means may consist of electronic flow generators 31, 32. 
     FIGS. 13 and 14 represent a variant of the pulsating means in at least one of the ducts supplying the burner by alternate oscillation of a wing 42 pivotally mounted in a duct 41 for feeding the burner with fuel and cyclically operated by means of an electric motor 43, the duct 51 for supplying the other component for the combustion having no pulsating means. 
     In the variant illustrated in FIGS. 15 to 18, the pulsating means comprises a rotary valve including a valve body 61 defining internal cylindrical cavity 62 in which arrive aligned inlet and outlet ducts 63 and 64. A tubular rotary valve head 65 is rotatably mounted in cavity 62, the valve head including a tubular wall provided with angularly distributed openings 68 providing therebetween closing wall portions 66, 67 which, during rotation of the valve head 65, sequentially close the ducts 63, 64. The valve head 65 is mounted at the end of a shaft 69 which pivots in the valve body 61 and is caused to rotate continuously or possibly alternatively, by means of a motor (not illustrated). In the case of a pulsation on the two fluids supplying the burner, a common motor may operate the two rotary valves to ensure the pulsation of the flows of fuel and oxidant. Dephasing of the opening/closing of the valves, and consequently of the pulsations of the flows, is carried out by mere adjustment of the driving pinions between the two valves. 
     In the embodiment of FIG. 15, the valve head 65 includes two diametrically opposed openings 68. Depending on circumstances, it is possible, however, to modify the ratio of durations between opening and closing by providing valve heads having a different arrangement of openings, as illustrated in FIG. 16 (three openings 74-76 with important angular apertures separating three closing zones 71-73 of small angular extension), FIG. 17 (four small openings 85-88 between four closing walls of wide angular extension 81-84) or FIG. 18 (four openings 95-98 et four closing portions 91-94 having substantially the same angular extension). 
     Although the present invention has been described with reference to specific embodiments, it is not limited thereto, but on the contrary is capable of modifications and variants which would appear to one skilled in the art.