Abstract:
The present invention relates to a catalytic combustion device comprising a main combustion zone ( 20, 200 ) including at least one catalytic section ( 5, 103 ) and at least one air/fuel mixing zone ( 11, 117 ), said mixing zone comprising at least one pressurized air inlet ( 1, 101 ) and injection means ( 12, 105 ) for injecting a liquid fuel.  
     According to the invention, injection means ( 12, 105 ) project the liquid fuel onto a hot wall ( 13, 15, 107 ) of said device so as to allow vaporization of said fuel on contact with this wall.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a catalytic combustion device with vaporization of liquid fuel on a hot wall, thus allowing to optimize the preparation of the air/fuel mixture in a combustion zone.  
           [0002]    Conventional combustion, carried out in the presence of a flame and commonly used in combustion methods, is a process that is difficult to control.  
           [0003]    It occurs in a well-determined air/fuel concentration range and leads, besides the formation of carbon dioxide and water, to the production of pollutants such as carbon monoxide and nitrogen oxides.  
           [0004]    Because of the increasingly severe environmental regulations relative to the pollutants emitted by combustion processes (nitrogen oxides, unburnt fuels, carbon monoxide), it has become necessary to find new technologies allowing such emissions to be greatly decreased.  
         BACKGROUND OF THE INVENTION  
         [0005]    Several conventional solutions are known to the man skilled in the art:  
           [0006]    Selective catalytic reduction of nitrogen oxides by ammonia allows to reduce the NO x  concentrations in fumes to about 10 ppm. This solution however requires a particular reactor downstream from the combustion chamber, storage and use of ammonia, and the installation and running costs of such a solution are high.  
           [0007]    Injection of water or steam, which lowers the temperature reached by the combustion gas, thus significantly reducing the NO x  contents to about 50 ppm. The cost of such a device is low, but the running costs are high because of the intensive purification of the water prior to injection and of the overconsumption of fuel due to an energy efficiency decrease. Furthermore, although water injection is sufficient to meet the current standards, it will not meet the future NO x  standards.  
           [0008]    Lean-burn combustion. As it is the case with the present invention, this technology is based on the lowering of the combustion temperatures. It allows the NO x  concentrations to be lowered down to about  20  ppm, but this decrease often occurs to the detriment of the carbon monoxide and unburnt fuel emissions, which are then increased.  
           [0009]    Catalytic combustion is an attractive solution for meeting the increasingly severe standards relative to pollution. In fact, the catalytic combustion chamber advantageously replaces conventional burners because it allows better control of the total oxidation of the fuel in a very wide range of the air/fuel ratio values, thus allowing to work under optimum conditions which greatly reduce nitrogen oxides, unburnt fuel and carbon monoxide emissions. It is well-known that the main characteristic of this particular type of combustion is to provide complete oxidation of the fuels at a relatively low temperature (below 1000° C.) in relation to a conventional combustion.  
           [0010]    It may also be mentioned that catalytic combustion allows a great variety of compounds to be burnt. The applications of catalytic combustion are thus multiple: radiant panels and tubes, catalytic stoves, gas turbines, cogeneration, burners for boilers, catalytic sleeves for tubular reaction systems, hot gas production in the field of direct contact heating and catalytic plate reactors, etc. The possible fields of application of catalytic combustion are described in the literature, for example in: &lt;&lt;Catalytic Combustion: Current Status and Implications for Energy Efficiency in the Process Industries, Heat Recovery System &amp; CHP, 13, No.5, pp. 383-390, 1993 &gt;&gt;.  
           [0011]    Combustion catalysts are generally prepared from a monolithic ceramic or metallic substrate on which a thin support layer consisting of one or more heat-resisting oxides, whose surface and porosity are greater than that of the monolithic substrate, is deposited. The active phase comprising most often essentially metals of the platinum group is dispersed on this support layer.  
           [0012]    Concerning the catalytic combustion processes in the field of energy production and cogeneration, the commonest reactor configuration is a reactor comprising several catalytic zones: the inlet catalyst(s) being more specifically dedicated to the initiation of the combustion reaction, the others being used to stabilize the combustion reaction at high temperature; the number of catalytic stages (or zones) is adjusted according to the conditions imposed by the application considered. It is also possible to replace the first catalytic reaction initiation zone by a pilot burner allowing the reaction to be initiated.  
           [0013]    In the conventional version of the catalytic combustion chamber, i.e. with a mixing zone followed by the catalytic section, preparation of the air/fuel mixture is one of the most critical points.  
           [0014]    Mixing has to be carried out as fast as possible and as homogeneously as possible in order to limit self-ignition risks.  
           [0015]    There are also cases where the temperature of the air at the compressor outlet is too low to allow fast vaporization of the fuel.  
           [0016]    In order to obtain vaporization of a liquid fuel, one of the easiest procedures consists in projecting the fuel at high velocity onto a surface, preferably a plane surface, and perpendicular thereto. Such injection modes are for example used for catalytic cracking, but the grain sizes obtained remain rather coarse (average diameter of the droplets of the order of several hundred microns).  
           [0017]    The work carried out by the applicant has shown that it is possible to substantially improve the homogeneity of the air/fuel mixture and therefore to optimize control of the catalytic oxidation of the fuels, and to limit the discharge of pollutant gases by improving the vaporization of the liquid fuel so as to obtain finer droplets.  
         SUMMARY OF THE INVENTION  
         [0018]    More precisely, the invention relates to a catalytic combustion device comprising a main combustion zone including at least one catalytic stage, at least one air/fuel mixing zone, said mixing zone comprising at least one pressurized air inlet, and injection means for injecting a liquid fuel, characterised in that the injection means project the liquid fuel onto a wall heated by the combustion of the air/fuel mixture in the main combustion zone, so as to allow vaporization of said fuel on contact with this wall.  
           [0019]    The invention allows to substantially reduce the diameter of the liquid droplets by sending a primary liquid jet onto a surface whose temperature is higher than the maximum boiling temperature of said fuel under the pressure conditions of the combustion zone.  
           [0020]    This primary liquid jet can be advantageously sprayed by any injector or spraying system known to the man skilled in the art.  
           [0021]    Injectors allowing primary spraying of the fuel with liquid droplets whose average diameter ranges between 5 and 60 μm (10 −6  metre), preferably between 10 and 40 μm, are generally used.  
           [0022]    It has been found by the applicant that the surface temperature of the wall encountered by the primary jet is advantageously substantially equal to or greater, at the pressure considered, than a first temperature T N  of the wall corresponding to a maximum boiling temperature of the liquid.  
           [0023]    At this temperature T N , the intense thermal exchanges between the wall and the fuel lead to an intense spraying of the liquid fuel (also referred to as Nukiyama temperature). A substantially equal temperature is understood to be a temperature greater or less than said temperature by 100° C., preferably greater or less than said temperature by 50° C., and most preferably greater or less than said temperature by 20° C.  
           [0024]    It has also been found by the applicant that it is possible, according to another embodiment of the invention, to advantageously obtain a great fragmentation of the liquid droplets from the primary jet by applying a temperature substantially ranging between said Nukiyama temperature and a temperature T L  in the neighbourhood of which and beyond which the thermal transfers are reduced by the presence of a vapour film between the droplet and the wall (referred to as Leidenfrost temperature).  
           [0025]    It is also possible, without departing from the scope of the invention, to apply a temperature greater than said Leidenfrost temperature, above which the evaporation time of the liquid droplets decreases as a result of the increase, with the wall, of the heat transfers by conduction, convection and radiation.  
           [0026]    Control of the wall temperature will thus condition the size of the droplets and can be obtained by means of any technique known to the man skilled in the art.  
           [0027]    Such an injection strategy has many advantages during preparation of the air/fuel mixture for catalytic combustion:  
           [0028]    In relation to a conventional configuration of the catalytic combustion device with premixer and catalytic section, a layout with such an injection mode allows to obtain faster vaporization of the liquid fuel, in particular those with rather high final vaporization temperatures. This is the case with certain gas oils for example. Under such conditions, premixing of the air with the fuel can be obtained more rapidly.  
           [0029]    The arrangement which is the object of the present invention can also contribute to cooling the walls of the combustion or postcombustion zones, or of the zone carrying the hot gases to the expander.  
           [0030]    In cases where the temperature of the air at the compressor outlet feeding the catalytic combustion-device is insufficient to obtain complete vaporization of the fuel, the proposed solution allows to overcome this problem thanks to the heat transfer between the combustion or postcombustion zone and the fuel injection zone.  
           [0031]    It allows to envisage a significant reduction in the total volume of the combustion zone since the zone normally reserved for vaporization of the fuel and premixing disappears.  
           [0032]    In general, the hot wall on which the fuel is sprayed is the wall of the combustion or postcombustion zone or of the zone carrying the hot gases resulting from the combustion or the wall of the starting equipment which can be, for example, a flame combustion chamber, an electric heater or any other device known to the man skilled in the art.  
           [0033]    According to an embodiment of the invention, the means intended for injection of the liquid fuel are injectors allowing primary spraying, whose orientation and characteristics are calculated so as to obtain the most homogeneous possible distribution of the fuel in the combustion air, and the size of the droplets sent by said injector ranges between 5 and 60 μm, preferably between 10 and 40 μm, and most preferably between 20 and 30 μm.  
           [0034]    Advantageously, the hot wall of the zone opposite said injection means has a substantially plane shape.  
           [0035]    It is also possible, without departing from the scope of the invention, that the hot wall of the zone opposite the injectors has a curved shape, concave for example.  
           [0036]    It is advantageous that the zone receiving the impact of the fuel jets is equipped with devices allowing to increase the heat transfer from the hot zone to the spraying zone.  
           [0037]    The device according to the present invention finds applications for example in gas turbines equipped with a heat recuperator or in combustion chambers having an annular geometry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    Other features and advantages of the present invention will be clear from reading the description hereafter, given by way of non limitative example, of two different embodiments of the device according to the invention, with reference to the accompanying figures wherein:  
         [0039]    [0039]FIG. 1 shows an example of an embodiment where the fuel is injected onto a hot wall of a combustion initiating device consisting of a pilot burner, and  
         [0040]    [0040]FIG. 2 shows another example of an embodiment where the fuel is injected onto a hot wall of a postcombustion zone. 
     
    
     DETAILED DESCRIPTION  
       [0041]    The combustion device diagrammatically shown in FIG. 1 comprises an inlet  1  for pressurized air coming from a compressor (not shown in the figure). This air circulates in a peripheral annular space  2  prior to reaching a distribution box  3  where it is separated into a stream intended for a combustion initiation device, here a pilot burner  4 , and a stream sent to a catalytic section  5 .  
         [0042]    A device, not shown in the figure, can be provided in the vicinity of this distribution box  3  in order to separate the air in an optimum way whatever the running conditions of the machine.  
         [0043]    The pilot burner shown in FIG. 1 is a conventional flame burner. It comprises a central fuel delivery line  6 , an air box  7 , means  8  such as blades, for example, for adjusting the velocity and the rotation of the combustion air before it enters combustion zone  9  of the pilot burner, an outlet zone  10  for the fumes produced by the pilot burner, said outlet running right through catalytic section  5 .  
         [0044]    This pilot burner can also be an equipment known to the man skilled in the art and reputed to discharge low nitrogen oxides amounts, such as for example systems in which the combustion air is brought into rotation in blades, with injection of the fuel inside the blades, or part thereof, or in the immediate vicinity of these blades.  
         [0045]    Main combustion zone  20  comprises an air/fuel mixing zone  11  arranged downstream from distribution box  3 , liquid fuel mechanical spray injectors  12  equally distributed for example on the periphery of mixing zone  11  and of catalytic section  5 .  
         [0046]    Injectors  12  produce a liquid fuel jet sent onto hot wall  13  of pilot burner  4  and they allow primary spraying of this fuel with liquid droplets whose average diameter ranges between 5 and 60 μm (10 −6  metre), preferably between 10 and 40 μm.  
         [0047]    This jet is preferably substantially perpendicular to the hot wall. Substantially perpendicular means that the angle between the surface of the hot wall in relation to the axis of the jet more preferably ranges between 80° and 100°.  
         [0048]    Of course, this angle can range between 40° and 140°, preferably between 60° and 120°.  
         [0049]    Wall  13  is heated by the combustion of the air/fuel mixture in section  5  and by contact with the hot wall, the liquid fuel is vaporized while dividing into very fine droplets which are some microns in average diameter (10 −6  m) and carried along by the combustion air. The number of injectors, their orientation in relation to the hot surface and the characteristics of the injectors are calculated by the man skilled in the art so as to obtain the most homogeneous possible distribution of the fuel in the gaseous stream, once the fine droplets sprayed. The gaseous air/fuel mixture flows then into catalytic section  5  which often consists of one or more monoliths arranged in parallel or in series, in order to limit pressure drops. When the combustion of the air/fuel mixture is not complete in the catalytic section, it continues in zone  14 , referred to as postcombustion zone, provided therefore.  
         [0050]    Wall  15  which is in contact with postcombustion zone  14  or with catalytic section  5  is also heated by the combustion of the air/fuel mixture in catalytic section  5 , and it is possible to arrange injectors  12  opposite this wall.  
         [0051]    According to a variant, in order to optimize spraying of the droplets, wall  13  of pilot burner  4  opposite the injectors can have a substantially plane shape, or even curved or concave so that all of the liquid fuel droplets sent by the injector impact as perpendicular as possible the hot surface where they are intended to fragment and disintegrate.  
         [0052]    Without departing from the scope of the invention, it is of course possible to use any known device allowing such an effect to be obtained, such as for example the presence of inserts of substantially plane or convex curved shape.  
         [0053]    [0053]FIG. 2 is another possible illustration of the invention.  
         [0054]    It also comprises an inlet  101  for pressurized air coming from the compressor (not shown in the figure), a combustion initiation device  102  (or pilot burner) and main combustion zone  200  with its catalytic section  103  proper.  
         [0055]    The combustion air circulates in a substantially annular peripheral space  104 . The fuel is introduced by means of injectors  105  fastened to and substantially equally distributed on outer wall  106  of annular space  104 . These injectors can be mechanical (without spraying assistance) or air-blast injectors (with the assistance of a spraying fluid) or any other equivalent device. The jets produced by these injectors are sent onto hot wall  107  which separates annular space  104  from zone  108 , which can be a postcombustion zone or simply a connection zone between catalytic section  103  and the expander (not shown in the figure) and, on contact with this hot wall, the liquid fuel is sprayed as very fine droplets.  
         [0056]    As described above, injectors  105  produce a fuel jet with a primary spray containing liquid droplets whose average diameter ranges between 5 and 60 μm (10 −6  metre), preferably between 10 and 40 μm.  
         [0057]    Advantageously, certain parts of wall  107  can be covered with insulating materials in order to prevent hot spots which can lead to an early ignition of the air/fuel mixture.  
         [0058]    Conversely, zone  120  of wall  107 , which receives the impact of the jets, can be equipped with devices such as blades in order to increase the heat transfer from hot zone  108  to spraying zone  104 .  
         [0059]    As in the previous case, the number of injectors, their orientation in relation to the hot wall and their characteristics are calculated by the man skilled in the art so as to obtain the most homogeneous possible distribution of the fuel once the droplets sprayed.  
         [0060]    Annular zone  104  is ended by a distributor  109  which distributes the air/fuel mixture among pilot burner  102  and main catalytic section  103 . This distribution can for example be obtained by means of a mobile shutter  110  which alternately moves in front of inlet  111  of catalytic section  103  or in front of inlet  112  of pilot burner  102 , according to the running conditions of the machine.  
         [0061]    The pilot burner can be a device such as shown in FIG. 1. It can also be a system as shown in FIG. 2, i.e. consisting of an initiating catalytic section  121 , fed by a circuit  113  arranged after distributor  109 . This catalytic section can be a metal monolith preheated by Joule effect, by means of an electric power supply consisting of any electricity source  114 , of two metallic connectors  115  arranged at each end of the monolith and of an electric link  116  connecting said connectors  115  to electricity source  114 .  
         [0062]    Main catalytic section  103  comprises a distribution box  117  for the air/fuel mixture, and this box can be equipped for example with a perforated plate  118  intended to provide homogeneous feeding of all the constituent channels of the monolith.  
         [0063]    This plate  118  can also be a monolith of very limited thickness, intended to stop any flame in case of unwanted self-ignition of the air/fuel mixture, in space  119  between said plate  118  and main catalytic section  103 . The latter can consist of one or more monoliths arranged in series or in parallel.  
         [0064]    As in the previous case, a free space  108  can be provided downstream from catalytic section  103 , before the expander (not shown), which is intended to complete the combustion of the air/fuel mixture if it has not completely burned in the catalytic section.  
         [0065]    Catalytic sections  102  and  103  can use catalysts of different nature. The catalyst of pilot burner  102  can for example have a high precious metal content, precious metals being known for their efficiency for catalytic combustion, and combustion can thus start from 200° C. or 250° C.  
         [0066]    The invention can also be applied to gas turbine configurations with a heat recuperator or to combustion chambers having an annular geometry.