Abstract:
A system for feeding over-fire air to a combustion chamber ( 10, 24, 42 ) of a steam or hot water generation plant of low-medium capacity, using solid, liquid or gaseous fuel or a combination thereof, comprising a main combustion air duct ( 5 ) supplied to a burner unit ( 11, 25, 43 ) in the combustion chamber and an over-fire air duct ( 12, 27, 44 ), which extends within said combustion chambers and is provided at its end with nozzle means ( 17, 27, 47 ) for said over-fire air.

Description:
DESCRIPTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a simplified system for feeding over-fire air (O.F.A.) designed to be installed on industrial heaters of low-medium capacity, namely lower than 25 MW, apart from the fuel used, with the purpose of keeping the pollutant emissions, in particular nitrogen oxides, as low as possible.  
           [0003]    2. Description of the Prior Art  
           [0004]    A widespread technique to reduce the production of nitrogen oxide (NO x ) in the combustion processes occurring in industrial plants and electric power generation plants is that of the so-called step combustion, which consists in suitably dosing the air and fuel within the combustion system, in such a way to form a fuel rich zone in which the fuel pyrolisis processes occur and NO x  reduction processes are activated. This zone is followed by a fuel lean zone, wherein the combustion reaction is completed by mixing the remaining part of the combustion air with the fuel. In practice, by reducing the oxygen availability in the primary flame zone the speed of formation of nitrogen oxides both from the fixation of the atmospheric nitrogen (thermal NO x ) and from the oxidation of the nitrogen contained in the fuel (chemical NO x ) is inhibited. Due to the reduced presence of the oxygen the nitrogen present in the fuel is forced to recombine with other nitrogen, thus forming molecular nitrogen, N 2 , instead of nitrogen oxide. Similarly, due to the lower temperatures which are reached, lower combustion temperature peaks occur thereby reducing the formation of the thermal NO x .  
           [0005]    A way of putting into practice the step combustion technique is that described in European Patent No. 0452608, according to which combustion air is subdivided into three streams, namely primary air, secondary air and tertiary air, which are supplied to the combustion chamber near the burner outlet coaxially to the fuel inlet. Another way of putting into practice the step combustion is that according to the OFA technique which consists in diverting a portion of the combustion air from the burners to over-fire air ports placed over-fire on the furnace wall downstream of the burner and introducing the portion of combustion air into the furnace through these ports. The amount of diverted combustion air is controlled so that the ignition combustion of the fuel occurs at sub-stoichiometric condition to create a reducing atmosphere which minimizes the formation of nitrous oxides. The over-fire air supply system varies according to the embodiments and often comprises air injectors, swirling vanes, separate blowers and other associated equipment, resulting in installations that are complex and expensive.  
           [0006]    In order to comply with the more and more stringent regulations regarding the reduction of pollutant emissions and in particular nitrous oxide (NO x ) emissions, the retrofitting of the existing plants is necessary. This may involve structural modification to the furnace or additional exhaust gas treatment plants (for example, the so-called DeNO x  plants). In the case of plants for the production of hot water or steam of low-medium capacity, the above mentioned alternatives are both unfeasible both for technical and cost reasons.  
           [0007]    In particular, if a step combustion according to the OFA technique would be used to reduce the NO x  emissions, openings would have to be formed in the part under pressure of the furnace to create the over-fire air supplying ports, which would give rise to many serious problems.  
         SUMMARY OF THE INVENTION  
         [0008]    The object of the present invention is to provide a system for supplying the over-fire air to a furnace of low-medium capacity for hot water or steam generation characterized by a low construction cost, in particular if applied to the retrofitting of existing plants.  
           [0009]    According to the invention the system for supplying the over-fire air to a furnace for the generation of hot water or steam comprises a over-fire air duct extending within the combustion chamber in such a way to provide the over-fire air injection from a rear wall of the furnace with respect to that where the burner is placed, or sidewise with respect to the same wall through an existing opening, for example a manhole, or a dedicated opening, formed in a not pressurized part of the furnace, for example through the refractory material.  
           [0010]    If the over-fire air is injected from the rear wall, the injection is performed through a suitable nozzle which can be coaxial to the burner, if the furnace is equipped with only one burner, or coaxial to the burner assembly, if the furnace is equipped with more than one burner. In the case of a side injection, the nozzles for supplying the over-fire air are arranged near the sidewalls of the furnace, generally near the floor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Features and advantages of the over-fire air supplying system for industrial furnaces according to the present invention will become apparent from the following description of exemplifying, non-limiting embodiments thereof made with reference to the attached drawings wherein:  
         [0012]    [0012]FIG. 1 schematically shows a furnace with relevant supplying system of the main combustion air and of the over-fire air according to the prior art;  
         [0013]    [0013]FIG. 2 shows a system for rear injecting the over-fire air according to the invention;  
         [0014]    [0014]FIG. 3 shows a system for sidewise injecting the over-fire air according to the invention;  
         [0015]    [0015]FIGS. 4 and 5 show two different nozzles for injecting over-fire air in a rear injection system;  
         [0016]    [0016]FIGS. 6 and 7 show two different supplying systems for sidewise injecting the over-fire air according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    The diagram shown in FIG. 1 represents the conventional solution for supplying the combustion air to an industrial furnace, generally indicated at  1 , wherein the combustion air is divided into two streams, one of which is supplied to the burners together with the fuel and the other is directly supplied to the upper part of the furnace as over-fire air. Burners  2 , arranged in one or more arrays, are connected to the relevant conduits  3  and  4  for feeding the fuel and, respectively, for feeding the main combustion air. Ducts  4  branch from a main air header  5  coming from a heat recovery unit  6 , for example a Ljungstrom, in countercurrent to the combustion exhaust gas  7  directed to the stack. The combustion air is supplied by blower means  8  located upstream of the furnace. A certain amount of the combustion air is sucked by a fan  9  from combustion air duct  5  downstream of heat recovery unit  6  to be supplied to the upper part of the furnace, indicated at  1   a,  to complete the combustion. The inlet of the over-fire air in the combustion chamber is achieved through a series of ports formed on the furnace wall.  
         [0018]    The overall flow rate of the combustion air, the flowrate of the air supplied to the single burners, the flowrate of air drawn as over-fire air as well as that of the air admitted through the single supply ports are controllable within large ranges by means of suitable air locks as schematically shown in FIG. 1.  
         [0019]    In FIG. 2 there is shown, in a longitudinal cross-section, a system for the rear injection of the over-fire air according to the present invention. With reference to this figure, there has been indicated at  10  a combustion chamber and generally at  11  a burner or a burner assembly, for example a group of two or four burners. An over-fire air injection device  12  extends within the combustion chamber coaxially to the burner from a nozzle  13  located on the rear wall of the furnace through an adjustment device, comprising a flange  14 , a counterflance  15  and a gasket  16 , through which the protrusion of injection device  12  within the combustion chamber can be regulated so as to allow the axial movement of an injection nozzle  17  mounted at the free end of injection device  12 . Injection device  12  is lined with a layer of insulating material  18 . At the outer side of combustion chamber  10  injection device  12  is connected to an air inlet  19  possibly equipped with a control air lock  20  and a closure flange  21  with a handle  22  extending therefrom for clamping a rod for moving nozzle  17 , in the case of mobile nozzles.  
         [0020]    A system for over-fire air supply with side injection is shown in FIG. 3. In this figure the combustion chamber has been indicated generally at  24  and  25  indicates a generic burner or burner assembly. Located at the front wall of the furnace, in particular below burner  25 , is a nozzle  26  within which a side over-fire air duct  27  extending in combustion chamber  24  is placed. Duct  27  ends with an injection duct  28  transversally arranged to the combustion air flow and equipped with a plurality of tubular nozzles  29  having circular, square or rectangular cross-section. Outside of combustion chamber  24  duct  27  is connected to an air inlet  30  possibly equipped with an adjustable air lock  31  for the over-fire air.  
         [0021]    As a function of the design needs injection duct  28  may extend also along the side walls, taking a U-shaped configuration and the injection nozzles  29  may be replaced by a narrow slit extending along injection duct  28  in such a way to supply the over-fire air to the furnace in the form of a continuous laminar barrier instead of discrete jets.  
         [0022]    Nozzles of the conventional type, which can be used for supplying the over-fire air to the combustion chamber in the case of rear injection of the over-fire air, are shown in FIGS. 4 and 5. The nozzle shown in FIG. 4 provides a continuous radial outlet for the over-fire air and comprises a bottom plate  32  lined with insulating material  33  from which a connection system  34  of rod  23  for moving the nozzle extends. The nozzle is equipped with a plurality of fixing plates  35  for mechanical strengthening and a plurality of blades  36  arranged outside of the nozzle to convey the over-fire air.  
         [0023]    The nozzle shown in FIG. 5 provides a radial outlet in the form of jets for the over-fire air and comprises a bottom plate  37  lined with insulating material  38  from which a connection system  39  of rod  23  for moving the nozzle extends. The nozzle is provided with a plurality of fixing plates  40  for mechanical strengthening and a plurality of diverting blades  41  arranged in the outer side of the nozzle to direct the over-fire air outlet.  
         [0024]    [0024]FIGS. 6 and 7 show other side injection modes for the over-fire air as in the case of FIG. 3, but with an air supply coaxial to the burner and in particular rear coaxial supply (FIG. 6) and front coaxial supply (FIG. 7). In the case of FIG. 6, in which  42  and  43  generally indicate a combustion chamber and, respectively, a burner or burner assembly, an over-fire air supply duct  44  is introduced in the furnace through a manhole  45  and then extends vertically down to the floor and finally runs longitudinally thereon, in particular over the tube bundle. Duct  44  ends with a transverse injection duct  26  bearing tubular injection nozzles  47 . The solution of FIG. 7, on the other hand, can be adopted when a manhole or another opening in a suitable position are unavailable or when the solution of FIG. 3 cannot be adopted. In this case the air supply duct is formed around burner  43  and extends therefrom within the combustion chamber in the same way as shown in FIG. 6.  
         [0025]    Advantageously, in order to further reduce the smoke point of the exhaust gas discharged from the stack, a part of the exhaust gas coming from the furnace, in a percentage not higher than 15%, may be recirculated. It can be injected in the overall combustion air stream, in the air supplied to the burners only, in the over-fire air only or even in one of the burner conduits.  
         [0026]    With respect to the systems of supplying and injecting over-fire air according to the prior art, the system of the invention allows the same typical step combustion to be carried out with the same level of reduction of NO x  (up to about 200 mg/Nm 3 ) by means of a structurally simple solution for low-medium capacity steam and hot water generators and in particular it makes the OFA technique applicable to existing generators of the above mentioned type without requiring modifications of the pressurized parts of the furnace, as existing accesses to the combustion chamber may be used.  
         [0027]    Variations and modifications can be brought to the over-fire air supply system according to the present invention, without departing from the scope of the invention as set forth in the appended claims.