Abstract:
A method for operating a premixing burner is provided. The premixing burner includes a premixing zone. An air mass flow and fuel may be injected into the premixing zone, and a potential hot gas backflow area may form. A fluid containing no fuel is injected downstream from the fuel injection into the premixing zone in order that a hot gas backflow area does not form. A premixing burner including a premixing zone is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/059658, filed Jul. 23, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07014820.0 EP filed Jul. 27, 2007. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a premixing burner, especially a syngas premixing burner, and to a method for operating a premixing burner. 
       BACKGROUND OF INVENTION 
       [0003]    Premixing burners typically comprise a premixing zone in which air and fuel are mixed before the mixture is directed into a combustion chamber. The mixture burns in said chamber, with a hot gas under increased pressure being generated. The hot gas is transferred on to the turbine. The primary consideration when operating premixing burners is to keep nitrous oxide emissions low and to avoid a flame blowback. 
         [0004]    Syngas premixing burners are characterized by syngases being used as a fuel in them. Compared to the classical turbine fuels of natural gas and oil which essentially consist of hydrocarbons, the combustible elements of the syngases are essentially carbon monoxide and hydrogen. Depending on the gasification method and the overall plant concept, the heating value of the syngas is around 5 to 10 times smaller than that of natural gas. 
         [0005]    As well as the stoichiometric combustion temperature of the syngas the mixture quality between syngas and air at the flame front is a significant influencing variable for avoiding temperature peaks and thereby for minimizing thermal nitrous oxide formation. 
         [0006]    The main elements of the syngases, in addition to carbon monoxide and hydrogen, are also inert components. The inert components involved are nitrogen and/or water vapor and where necessary also carbon dioxide. As a consequence of the low heating value, high volume flows of combustion gas must accordingly be introduced into the combustion chamber. The result of this is that far greater injector cross-sections are required for the combustion of low-calorific fuels such as syngases for example than with conventional high-calorific combustion gases. 
         [0007]    The air mass flow introduced into the combustion chamber is typically swirled with the aid of an air swirl generator. The fuel is injected into this swirled air mass flow via one or more rows of circular holes arranged next to one another or behind one another. 
         [0008]    To guarantee an adequate mixing of air and fuel a sufficient penetration depth of the individual jets of fuel into the air mass flow is necessary. Compared to high-calorific burner gases such as natural gas, correspondingly larger, free injection cross sections are required. The result of this is that the fuel jets disturb the sensitive air flow, which in the final analysis leads to a local detachment of the air flow in the feedback region of the fuel jets. The flowback regions forming within the burner are undesired and especially to be avoided at all costs for the combustion of highly-reactive syngas. In the extreme case these local flowback regions lead within the mixture zone of the burner to a flame blowback in the premix zone and thereby to damage to the burner. 
         [0009]    The risk of flame blowback can be largely avoided by highly-reactive syngases being burned in diffusion mode. To realize low nitrous oxide emissions however heavy thinning with inert gases, preferably with steam, is necessary. In the case of a premixing combustion the formation of trail regions or hot gas flowback regions within the burner can be reduced by suitable shaping of the injection holes, but not basically avoided. 
         [0010]    In EP 1 614 963 A1, for reducing the nitrous oxide emissions and for preventing flame blowback in the combustion of low-calorific fuels for the operation of a gas turbine, a method is proposed in which a low-calorific fuel is premixed with the air in stages. 
         [0011]    In EP 1 614 967 A1 a further method for combustion of a low-calorific fuel for the operation of a gas turbine is proposed in which, within the framework of a pre-mixing, the low-calorific fuel is premixed with a low-calorific fuel-air mixture and a conversion of the low-calorific fuel-air mixture is avoided. 
         [0012]    Specifically for preventing flame blowbacks, a gas turbine with an annular combustion chamber is proposed in EP 1 507 120 A1, in which a swirl grid is arranged in a combustion air inlet area around the entire circumference of the annular combustion chamber, whereby a higher flow speed of the incoming combustion air is achieved compared to individual air inlet areas each with a swirl grid. This gives a higher security against flame blowbacks and a lower tendency for the formation of combustion vibrations. 
       SUMMARY OF INVENTION 
       [0013]    To guarantee a secure premixing operation, a flow detachment or a flowback region within the premix zone of the burner is to be avoided at all costs. At the least however potential flowback regions are to be designed such that no damage is caused to the burner. As a rule the flowback regions occur in zones close to the wall in the trail of the fuel gas jets. 
         [0014]    In respect of nitrous oxide minimization the addition of inert mass flows as a thinning medium into the air mass flow or the fuel mass flow (quenching) is usual. The use of the leaner premix technology makes it possible to reduce the quantity of the quenching medium employed, which enhances the economy of the plant. However the lack of inerting then means that the fuel is highly reactive. 
         [0015]    The object of the present invention is to provide an advantageous method for operating a premixing burner in which the formation of hot gas flowback regions is avoided. A further object of the present invention is to provide an advantageous premixing burner. 
         [0016]    These objects are achieved by a method for operating a premixing burner as claimed in the claims and a premixing burner as claimed in the claims. The dependent claims contain further advantageous embodiments of the invention. 
         [0017]    The inventive method relates to a premixing burner which comprises a premixing zone. An air mass flow and fuel are injected into the combustion chamber, in which case a potential hot gas flowback region can form. The inventive method is characterized in that a fluid containing no fuel is injected into the premixing zone downstream of the fuel injection. 
         [0018]    By the local injection of for example cold air into the trail or flowback regions, the formation of said regions within the premixing zone of the burner is largely prevented. And least the fuel in these regions is quenched and cooled off to the extent that no reaction or no ignition of the fuel-air mixture within the premixing zone of the burner can occur. This makes a secure premixing operation of the burner possible. 
         [0019]    In particular the fluid can be injected along the surface located in the potential hot gas flowback region of the premixing zone in the main direction of flow into the premixing zone. The injection of a fluid along the component surface in the main direction of flow prevents the actual formation of the hot gas flowback region and/or quenches and cools the fuel-air mixture at this location such that no ignition conditions obtain. 
         [0020]    The fuel can especially be injected at right angles to the main flow direction of the air mass flow into the premixing zone, which is advantageous in respect of the basic mixing of air and fuel. Basically there is the option of injecting the fuel on the cone side and/or on the hub side into the premixing zone. Furthermore the fuel can be injected via at least one swirler vane into the premixing zone. The fuel concerned can especially be a syngas. 
         [0021]    The fluid injected along the surface located in the potential hot gas flowback region into the premixing zone can for example be air or an inert gas. Gases with very low reaction capabilities, which are only involved in a few chemical reactions for example, are referred to as inert gases. In particular carbon dioxide, water vapor, nitrogen, but also all noble gases can be used as an inert gas. The use of an inert gas is especially suitable if ignition conditions for easily-ignitable fuels are to be avoided. 
         [0022]    When air is used as a fluid injected into the premixing zone it is advantageous to use air of the air mass flow supplied to the premixing zone in any event. For example a proportion of 10% of the overall air fed to the premixing zone can be split off and injected into the latter along the surface of the premixing zone located in the potential hot gas flowback region. The proportion of air injected along the surface of the premixing zone located in the potential hot gas flowback region can be selected as required. The preferred level of the air to be used depends in such cases on the geometry of the premixing zone, on the speed of the air mass flow and on the speed of the injected fuel. 
         [0023]    The inventive premixing burner has a premixing zone, an air swirl generator with an air inlet and one or more fuel nozzles. In such cases the fuel can be injected through the fuel nozzles into an air mass flow swirled by the air swirl generator in the premixing zone, in which case a potential hot gas flowback region can form. The inventive premixing burner is characterized in that the premixing zone surface in the potential hot gas flowback region has at least one opening through which a fluid can be injected into the premixing zone. In particular openings can be present that are arranged so that fluid can be injected in the main direction of flow of the burner along the surface of the premixing zone. 
         [0024]    The local injection of a fluid into the potential hot gas flowback region largely avoids the formation of the hot gas flowback region within the premixing zone of the burner. At the least however the hot gas in the hot gas flowback region is quenched and cooled off such that no reaction in the form of an ignition of the air-gas mixture within the premixing zone of the burner can occur. This prevents flame blowbacks and reduces the nitrous oxide formation, but also allows a secure premixing operation of the burner. 
         [0025]    Preferably the premixing zone surface in the hot gas flowback region features a number of openings. The opening or the openings can advantageously be connected via a fluid channel to the air supply leading to the air swirl generator so that a part of the air can be injected through the opening as a fluid into the combustion chamber. 
         [0026]    The injection nozzles can be located on the cone side and/or on the hub side of the premixing zone. Advantageously the fuel nozzles are arranged in one or more rows lying behind one another downstream of the air swirl generator. This makes a graduated fuel injection possible. Furthermore the fuel nozzles and/or the openings can be located in the air swirl generator, preferably in at least one swirler vane. 
         [0027]    The individual fuel nozzles can be embodied as round holes for example. A further option consists of designing the fuel nozzles such that the fuel can be injected at right angles to the main flow direction of the air mass flow into the combustion chamber, which promotes the premixing. Naturally the fuel can also be injected at any other given angle to the air mass flow. The fuel employed can especially involve a syngas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Further features, characteristics and advantages of the present invention will be explained below on the basis of an exemplary embodiment which refers to the enclosed figures. 
           [0029]      FIG. 1  shows a schematic diagram of a section of a part of the premixing burner. 
           [0030]      FIG. 2  shows a schematic diagram of the flow conditions within the premixing burner depicted in  FIG. 1 . 
           [0031]      FIG. 3  shows a schematic diagram of a section through a part of the inventive premixing burner. 
           [0032]      FIG. 4  shows a schematic diagram of the flow conditions within the premixing burner depicted in  FIG. 3 . 
           [0033]      FIG. 5  shows a schematic diagram of a section through a swirler vane. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0034]    The invention will be explained below in greater detail with reference to  FIGS. 1 to 5 .  FIG. 1  shows a schematic diagram of a section through a part of a conventional premixing burner  1 . The premixing burner  1  includes elements such as a housing  7 , a premixing zone  2 , a swirl generator  10  and/or one or more fuel nozzles  11 . The premixing zone is arranged radial-symmetrically around the central axis  12 . The outer side of the premixing zone  2 , viewed from the central axis  12 , is referred to below as the cone side  3 . The side of the premixing zone  2  facing towards a central axis  12  will be referred to below as the hub side  4 . 
         [0035]    An air mass flow  5  arrives at the swirl generator  10  via an air supply inlet  16 . The air swirl generator  10  swirls the air mass flow  5  and directs this into the premixing zone  2 . From there the air mass flow is forwarded in the main direction of flow  9  to the combustion chamber (not shown). 
         [0036]    On the hub side  4  of the premixing zone  2  are located one or more fuel nozzles  11 . Fuel  6  is directed in the present example through the fuel nozzles  11  at right angles to the main direction of flow  9  of the air mass flow  5  into the premixing zone  2 . A hot gas flowback area  8  is now formed downstream towards the fuel nozzle  11  in the main direction of flow  9 . Instead of a perpendicular injection to the main direction of flow  9  of the air mass flow  5 , the fuel  6  can also be injected at any other given angle to the main direction of flow  9 . 
         [0037]    The flow direction of the injected fuel is indicated by arrow  6 , the flow direction of the supplied air mass flow is indicated by arrow  5 . The main direction of flow inside the premixing zone  2  is marked by arrows  9 . 
         [0038]    The flow conditions inside the premixing zone  2  are depicted in a schematic diagram in  FIG. 2 .  FIG. 2  shows an overhead view of the fuel nozzles  11  from the inside of the premixing zone  2 . The main direction of flow of the air mass flow flowing past the fuel nozzles is indicated by arrows  9 . Hot gas flowback regions  8  now form downstream from the fuel nozzles  11  in the main direction of flow  9 . The flow direction of the hot gas flowing back is indicated by the arrows  13 . 
         [0039]      FIG. 3  shows a schematic diagram of a section through a part of the inventive premixing burner  1 . The basic structure and the principal functioning of the premixing burner  1  depicted in  FIG. 3  essentially correspond to the premixing burner shown in connection with  FIG. 1 . 
         [0040]    In addition to the premixing burner described in connection with  FIG. 1 , the inventive premixing burner features one or more fluid inlet openings  14  which are located downstream of the fuel nozzle or nozzles  11  in the main direction of flow  9 . The fluid inlet openings  14  open out into the premixing zone  2 . Through these fluid inlet openings in the present exemplary embodiment a fluid, for example air or an inert gas, can be injected in the main flow direction  9  into the premixing zone  2 . The flow direction of the injected fluid is indicated by arrows  15 . In this case it runs within the premixing zone  2  essentially in parallel to the main direction of flow  9 . The injected fluid prevents the formation of a hot gas flowback region as occurs with the premixing learner described in conjunction with  FIG. 1 . 
         [0041]      FIG. 4  depicts schematically the flow conditions inside the premixing zone  2  shown in  FIG. 3 . 
         [0042]    An overhead view of the fuel nozzles  11  and the fluid inlet openings  14  viewed from the premixing zone  2  can be seen in  FIG. 4 . The main direction of flow of the air flowing from the swirl generator  10  in the direction of the fuel nozzles  11  and the fluid inlet openings  14  is indicated by arrows  9 . The direction of flow of the fluid injected through the fluid openings  14  is indicated by arrows  15 . The hot gas  13  is carried along in the main direction of flow  9  by the inflowing of fluid. A flowback of the hot gas  13  against the main direction of flow  9  is effectively prevented in this manner. 
         [0043]    In the current exemplary embodiment the fluid injected by the fluid inlet openings  14  involves air which is connected via a fluid channel with the air mass flow  5  and is split off from the latter. It has proved useful in respect of avoiding the flowback of hot gas to introduce around  5 % to 20%, preferably 10%, of the overall air supplied to the premixing zone  2  via the fluid inlet openings  14  into the premixing zone  2 . Instead of air an inert gas, for example carbon dioxide, water vapor or nitrogen can be injected into the premixing zone via the fluid inlet openings  14 . The injection of a noble gas is however basically also possible. 
         [0044]    The fuel can optionally be injected at right angles to the main direction of flow  9  of the air mass flow  5  into the premixing zone  2 , as described in conjunction with  FIG. 1  and  FIG. 3 , or the fuel can be injected at any given angle to the main direction of flow  9  of the air mass flow into the premixing zone  2 . Basically the fuel nozzles and  11  can be located both on the cone side  3  and also on the hub side  4  of the premixing zone  2  or in the swirler vanes  17 . In the event of the fuel nozzles  11  being located on the cone side  3  of the premixing zone  2 , it is advantageous to also place the fluid inlet openings  14  correspondingly on the cone side  3 . The fluid inlet openings  14  should then again be located in the main direction of flow  9  downstream to the fuel nozzles and make it possible to inject the fluid in the main direction of flow  9 . 
         [0045]    The fuel nozzles  11  can be arranged in one or more rows line behind one another downstream of the air swirl generator  10 . They can advantageously be embodied as round holes. The fuel injected through them can especially involve a syngas. 
         [0046]    A further embodiment of variant of the invention is described below in which the fuel  6  and the fluid  15  containing no fuel is injected into the premixing zone via swirler vanes  17 .  FIG. 5  shows a schematic diagram of a section through a swirler vane  17 . The swirler vane  17  has a fuel flow channel  18  within it and a fluid flow channel  19  located downstream to it in the direction of the main flow  9 . 
         [0047]    The fuel  6  is injected via the fuel flow channel  18  through fuel nozzles  11  from the swirler vane  17  into the premixing zone  2 . The fluid  15 , which preferably involves an inert gas, is injected via the fluid flow channel  19  through fluid inlet openings  20 ,  21 ,  22  into the premixing zone  2 . In this case the fluid inlet openings  20 ,  21 ,  22  are located in the main direction of flow  9  downstream from the fuel nozzles  11 . 
         [0048]    In the present embodiment variant a part of the fluid  15  is injected through fluid inlet openings  20  which are arranged downstream next to the fuel nozzles  11 , essentially against the main direction of flow  9  into the premixing zone  2 . By fluid inlet openings  21  arranged further downstream next to the fluid inlet openings  20  a part of the fluid  15  is injected almost at right angles to the main direction of flow  9  into the premixing zone  2 . Further fluid inlet openings  22  are arranged downstream next to the fluid inlet openings  21 , through which a part of the fluid  15  is injected essentially in the main direction of flow  9  into the premixing zone  2 . 
         [0049]    The described arrangement of the fluid inlet openings  20 ,  21 ,  22  avoids any hot gas flowback region arising downstream from the fuel nozzles  11  and thus makes possible a safe premixing operation of the burner.