Patent Publication Number: US-2020292170-A1

Title: Device for controlling the combustion process in a power station furnace system

Description:
The invention relates to a device for controlling the combustion process in a power station furnace system with a plurality of burners acting in parallel and arranged in a wall of a combustion chamber and supplied with combustion air via a common wind box, the combustion air being supplied to the individual burner via one or more concentrically annular gaps surrounding the burner. 
     In a power station furnace system, a large number of burners are usually arranged in parallel in a wall of a combustion chamber and are supplied with combustion air via a common wind box. The combustion air is preferably supplied to the individual burner via one or more annular gaps concentrically surrounding the burner. The supply of the combustion air to the annular gap includes means for influencing the quantity of combustion air flowing through the annular gap and subsequently into the combustion chamber. In addition, air guide devices, for example guide vanes whose position can be changed, are arranged in the annular gaps to introduce the combustion air with a spiral motion into the furnace as a swirl flow around a flame forming in front of the burner, wherein the direction of flow of the combustion air flow can be changed by changing the position of the guide vanes. In the case of an arrangement with several concentric annular gaps, both the means for influencing the quantity of combustion air flowing through the annular gap and subsequently into the combustion chamber as well as the air guiding devices, for example guide vanes, can be designed differently in each annular gap and can be controlled separately. By arranging several concentric annular gaps around a burner, the combustion air for the main burner and the afterburning can be introduced separately into the combustion chamber in front of a single burner, i.e. into different combustion zones of the flame in the flow direction and the quantity of combustion air. The guide vanes for generating a swirl flow of the combustion air flow and the means for influencing the quantity of combustion air can be integrated as actuators in a control device for controlling the combustion process, so that the combustion process can be controlled separately for each burner of a power station furnace system. For an optimized control of the combustion process in a power station furnace system, it is necessary to supply each individual burner with an quantity of combustion air for the main burner and the afterburning that is adequate for optimal combustion of the quantity of fuel supplied to the burner, i.e. to control the fuel-air ratio during combustion, which means that with a known quantity of fuel supplied to the burner, the quantity of combustion air flowing through each annular gap surrounding the burner must be determined and, if necessary, subsequently changed. 
     To influence the quantity of combustion air supplied to a burner or a group of burners, it is known to arrange air baffles in the wind box for influencing the combustion air flow within the wind box so as to influence the distribution of the total quantity of combustion air supplied to the wind box among individual burners or groups of burners. The total quantity of combustion air supplied to the wind box can be determined comparatively easily. However, this solution does not enable optimized control of the combustion process in a power station furnace system. 
     To determine the quantity of combustion air supplied to a burner, it is known to measure the velocity of the combustion air flow and to calculate the quantity of combustion air from the geometric dimensions of the cross-sectional area of the duct carrying the combustion air. To measure the velocity of the combustion air flow, dynamic pressure probes which can be introduced into the combustion air flow, also known as Pitot tube or Prandtl&#39;s pitot tube, are known from the prior art. However, dynamic pressure probes of this type cannot be used for measuring the velocity of the combustion air flow in the annular gap of combustion air feed ducts to a burner in a power station furnace system, because the flow of the combustion air in the annular gap is extremely turbulent and has a swirl with strongly curved flow lines, so that only a directional velocity of the combustion air flow can be determined with a dynamic pressure probe when the combustion air flow hits the probe perpendicularly. When the flow is turbulent and the combustion air flow does not strike the dynamic pressure probe perpendicularly, in particular when the direction of the combustion air flow changes, no directional velocity of the combustion air flow can be determined from the differential pressure determined with the dynamic pressure probe. It is therefore not possible to determine the quantity of combustion air flowing through an annular gap by means of dynamic pressure probes arranged in the annular gap. In addition, the combustion air in a coal-fired power station furnace system is heavily loaded with ash particles, which leads to rapid contamination of the dynamic pressure probes. The solution is therefore not applicable for an optimized control of the combustion process in a power station furnace system. 
     The company brochure “Measuring individual burner airflow”, Application Bulletin ICA06 from Air Monitor Corporation, Santa Rosa, Calif. 95406, describes the arrangement of dynamic pressure probes in the flow direction of the combustion air flow upstream of the annular gaps that conduct the combustion air to a burner in a wind box. However, dynamic pressure probes, as described, are considerably susceptible to faults due to contamination. Even when a wind box is arranged upstream of the annular gap, regular, complex maintenance cycles and regular purging of the dynamic pressure probes with cleaned fresh air are therefore necessary for reliable operation. The arrangement described is therefore mostly used only for initial measurements of the burner arrangement, without a real combustion process taking place. It is also not applicable for an optimized control of the combustion process in a power station furnace system. 
     DE 20021 271 U1 describes a sensor device for determining the quantity of combustion air supplied to one burner or to a group of burners of a burner arrangement with a common combustion air supply via a wind box known by using the cross-correlation measurement method, wherein sensor arrangements are arranged within the wind box, each spanning the flow cross section of the wind box, in such a way that the reduced quantity of combustion air supplied to a burner or to a group of burners flows through the sensor arrangements. A sensor arrangement consists of two intersecting individual sensor rods or sensor rod groups which are arranged one behind the other in the flow direction of the combustion air flow and spaced apart from one another, and which span the cross section of the wind box. The velocity of the combustion air flow is determined with correlation methods from the signals generated on the sensor rods as a result of electrical effects, which are caused by electrically charged particles travelling past the sensor rods and transported in the combustion air flow. Based on the velocity of the combustion air flow and the associated geometry of the wind box, the quantity of combustion air flowing through the wind box can be calculated. However, the quantity of combustion air supplied to a single burner can be determined with this device only for special arrangements of burners in connection with a specially designed wind box. Such arrangements of burners and designs of the wind box are rarely significant in practical applications. In addition, this solution has the disadvantage that the interrelated measurements can have a considerable measurement error due to error propagation. This solution is hence also not suitable for an optimized control of the combustion process in a power station furnace system. 
     DE 102012 014260 A1 discloses a device and a method for controlling the fuel-air ratio in the combustion of ground coal in a coal-fired power station furnace system, wherein the measurement of the quantity of combustion air and the measurement of the carrier air volume are obtained with the correlation method by evaluating electrical signals from sensors arranged in the air flow. For this purpose, two sensor rods are arranged one behind the other in the air-guiding channel in the flow direction of the air, in which electrical signals are generated by electrical induction caused by electrically charged particles moving past the sensor rods and guided in the air stream. The signals are supplied to a correlation measuring device. The time required for the electrically charged particles to travel the distance between the two sensor rods is determined using a correlation measurement method. The flow velocity of the air flow is calculated from the time and the distance between the sensor rods, and the air volume is calculated based on the geometry of the air-guiding duct. An electrode and a counter-electrode are arranged upstream of the sensor rods in the direction of flow of the air and are connected to a high-voltage source supplying a voltage between 12 kV and 20 kV. The electrode connected to the high-voltage source is arranged in the air flow in such a way that at least a part of the air flow is exposed to the action of an ion current flowing from the electrode to the counter-electrode and is thus influenced electrically. The device and method described in DE 102012 014 260 A1 cannot be used for optimized control of the combustion process of each individual burner arranged in a power station furnace system. 
     It is common practice to control the combustion process in a power station furnace system based on static characteristic curves, wherein only the quantity of fuel supplied with the combustion air to the burners via a wind box and the overall quantity of combustion air supplied to the burners via the wind box are taken into account as control variables. An optimized control of the combustion process is therefore not possible. 
     It is an object of the invention to provide a device for controlling the combustion process in a power station furnace system which enables an optimized control of the combustion process, i.e. which enables an optimized control of the combustion process of each individual burner arranged in a power station furnace system. 
     This object is attained according to the invention with a device for controlling the combustion process in a power station furnace system unit having the features of the first claim. Claims  2  to  8  describe advantageous embodiments of the invention. 
     A device for controlling the combustion process in a power station furnace system with a plurality of burners arranged in a wall of a combustion chamber, wherein the combustion air is supplied via one or more annular gaps concentrically surrounding the burner and the burner comprises means for influencing the quantity of combustion air flowing through the annular gaps into the combustion chamber, at least means for detecting the quantity of fuel supplied to a burner, and means for determining the quantity of combustion air flowing through the annular gaps. The device for controlling the combustion process is designed in such a way that actuating signals are generated for each means influencing the quantity of the combustion air flowing through the annular gaps surrounding the burner into the combustion chamber. A means for determining the quantity of combustion air flowing through an annular gap includes at least two sensor rods made of an electrically conductive material and forming a corresponding pair, which are arranged in the annular gap transverse to the longitudinal axis of the annular gap or at an angle α to the longitudinal axis of the annular gap with 30°≤α≤90°, one behind the other and in parallel with a mutual spacing a in the flow direction of the combustion air flow, wherein the corresponding sensor rods are arranged such that at least a portion of the combustion air flowing past the first sensor rod in the flow direction of the combustion air also flows past the corresponding second sensor rod of the pair in the flow direction of the combustion air flow. The sensor rods are curved in the longitudinal direction commensurate with the curvature of the annular gap and are electrically insulated from the walls that form the annular gap. The sensor rods are thus arranged in the annular gap in such a way that their longitudinal direction is almost perpendicular or at an angle between 30° and 90° with respect to the flow direction of the combustion air flow, wherein the sensor rods are preferably arranged in the annular gap with a uniform spacing over the length I of the sensor rods with respect to the two walls forming the annular gap. The sensor rods have a length l of l&gt;20 mm, preferably l&gt;200 mm. A means for determining the quantity of combustion air flowing through an annular gap also includes a correlation measuring device to which the sensor rods are electrically connected, with the velocity of the combustion air flow transverse to the longitudinal direction of the sensor rods being measured with the correlation measuring device by evaluating the electrical signals generated by the electrically charged particles transported in the combustion air stream and flowing past the sensor rods. In the event that the sensor rods are not arranged perpendicular to the longitudinal axis of the annular gap, a component of the flow velocity of the combustion air flow in the direction of the longitudinal axis of the annular gap is calculated, and the quantity of combustion air flowing through the annular gap is determined based on the component of the flow velocity of the combustion air flow in the direction of the longitudinal axis of the annular gap and the geometric dimensions of the cross-sectional area of the annular gap. When several annular gaps surround a burner, as described above, sensor rods are arranged in each annular gap and electrically connected to a correlation measuring device, so that the quantity of combustion air flowing through each annular gap surrounding a burner can be determined. Thus, the combustion process of each burner arranged in the wall of a combustion chamber of a power station furnace system can be optimally controlled by supplying to the quantity of fuel supplied to the burner a quantity of combustion air adequate for optimum combustion by determining the quantity of combustion air flowing through the annular gaps surrounding the burner and influencing with the means the quantity of combustion air flowing through the annular gaps into the combustion chamber commensurate with the quantity of combustion air adequate for combustion. 
     The component of the flow velocity of the combustion air flow in the direction of the longitudinal axis of the annular gap refers to the particular component of the flow velocity of the combustion air flow with which the combustion air flow moves in the direction of the longitudinal axis of the annular gap, which hence is the relevant velocity for the transport of a specific quantity of combustion air in a specific unit of time through the annular gap. Due to the high degree of turbulence of the flow of the combustion air in the annular gap, which in a power station furnace system has a width between 20 mm and 200 mm and a circumference between 100 cm and 1500 cm, and in view of any swirl flows of the combustion air flow generated in the annular gap, components of the flow rate of the combustion air flow having different direction and magnitude occur in the annular gap. These various components of the flow velocity of the combustion air flow mentioned above are not relevant for determining the quantity of combustion air supplied to a burner. Important is here only the component of flow velocity of the combustion air flow in the direction of the longitudinal axis of the annular gap, i.e. as described above, only the component of the flow velocity of the combustion air stream with which the combustion air is transported in the longitudinal direction through the annular gap. 
     Surprisingly, it was found that electrical signals are generated on the sensor rods, which are arranged in an annular gap and form a corresponding pair, as a result of the influence caused by electrically charged particles passing by the sensor rods and transported in the combustion air stream, which signals can be evaluated using a correlation measuring device by determining a time offset between the correlating electrical signals which, when divided by the distance a between the corresponding sensor rods, is a measure of the component of the flow velocity of the combustion air flow in the annular gap transverse to the longitudinal direction of the sensor rods. This is surprising because in real measuring arrangements the distance a between the corresponding sensor rods is 2 5 times greater than the width of the annular gap and because although the electrically charged particles move generally in the direction of flow of the combustion air stream, this movement is overlaid, due to the high degree of turbulence of the combustion air flow, by a movement of the electrically charged particles that is predominantly chaotic in terms of magnitude and direction, wherein frequent collisions occur with the walls of the annular gap that is at ground potential and causes an electrical discharge of these particles. 
     It is advantageous for the arrangement of an air guiding device for generating a swirl flow of the combustion air flow to arrange the corresponding sensor rods in the flow direction of the combustion air flow in the annular gap downstream of the air guiding device. 
     It is furthermore advantageous for the arrangement of an air guiding device for generating a swirl flow of the combustion air flow to arrange the sensor rods forming a corresponding pair with a mutual parallel offset, such that at least a portion of the combustion air flowing past the first sensor rod of the corresponding pair also flows in the flow direction of the combustion air past the second sensor rod of the corresponding pair in the flow direction of the combustion air flow. The sensor rods should hereby be sufficiently long, i.e. extend over approximately ¼ of the inner circumference of the annular gap, so that even when the angle of rotation of the swirl flow of the combustion air flow changes, the condition is satisfied that at least a portion of the combustion air flow flowing past the first sensor rod of the corresponding pair in the flow direction of the combustion air flow also flows past the second sensor rod of the corresponding pair in the flow direction of the combustion air flow. 
     Preferably, the sensor rods are constructed as a round rod having a diameter D with 1 mm≤D≤20 mm, or as a square bar having an edge length e in the direction of the width b of the annular gap with 1 mm≤e≤20 mm. Real conditions of practical applications are assumed here, i.e. a width b of the annular gap for supplying the combustion air to a burner in a power station furnace system between 20 mm≤b≤200 mm and a circumference of the annular gap between 100 cm≤circumference of the annular gap≤1500 cm. On the one hand, the sensor rods must be designed so that they do not vibrate in the combustion air flow, but on the other hand they must also not be so large so as to unduly reduce the effective cross section of the annular gap for the passage of the combustion air flow. 
     Advantageously, one or more sensor rods may be electrically and possibly also mechanically segmented in the longitudinal direction of the sensor rod, with the segments forming a sensor rod being arranged in alignment with one another in the longitudinal direction of the segments. The segments of a sensor rod can be electrically connected in series and the electrically segmented sensor rod can be connected as a single electrical unit to an input of the correlation measuring device. However, each segment of an electrically segmented sensor rod may also be electrically connected to a separate input of the correlation measuring device. 
     In a further embodiment, the sensor rods can be designed as film strips constructed of an electrically conductive material which are glued to one of the two walls forming the annular gap and electrically insulated from the wall. 
     In another preferred embodiment of the means for determining the quantity of combustion air flowing through an annular gap, two pairs of corresponding sensor rods are arranged in the annular gap, with each pair being electrically connected to a correlation measuring device, and with the two pairs of corresponding sensor rods being arranged in the longitudinal direction at a different angle α with respect to the longitudinal axis of the annular gap. A pair of corresponding sensor rods is preferably arranged transversely, i.e. at an angle α 1 =90° with respect to the longitudinal axis of the annular gap, while the second pair of corresponding sensor rods is arranged at an angle of α 2 =45° with respect to the longitudinal axis of the annular gap, however under the condition that at least a portion of the combustion air flowing past the first sensor rod of a corresponding pair in the flow direction of the combustion air flow also flows past the second sensor rod of the corresponding pair in the flow direction of the combustion air flow. The velocity of the combustion air flow in the direction of the longitudinal axis of the annular gap is determined by evaluating the signals generated with the first sensor pair, i.e. the sensor pair arranged at an angle α 1 =90° with respect to the longitudinal axis of the annular gap, while a velocity component of the combustion air flowing at an angle α 2 =45° with respect to the longitudinal axis of the annular gap is determined with the second sensor pair, i.e. the sensor pair arranged at an angle α 1 =45° with respect to the longitudinal axis of the annular gap. From both velocities, the swirl angle γ of a combustion air flow having a swirl flow can be calculated by triangulation if the swirl angle γ satisfies the condition (90°−α 1 )&gt;γ&gt;(90°−α 2 ). The angles α 1 =90° of the one pair of corresponding sensor rods and α 2 =45° of the second pair of corresponding sensor rods only represent preferred examples. It will be understood that other angles α 1  and α 2  of the longitudinal directions of the pairs of corresponding sensor rods are also possible if this is necessary to satisfy the condition (90°−α 1 )&gt;γ&gt;(90°−α 2 ). In the event that air guide vanes having variable positions are arranged in the annular gap, the swirl angle can be determined in this manner and intentionally influenced via the position of the air guide vanes, as a result of which the combustion process can additionally be influenced, i.e. controlled. 
     The particular advantage of the invention is that the velocity of the combustion air flow is determined directly in the annular gaps surrounding a burner in a power station furnace system, so that the quantity of combustion air supplied to a burner in a power station furnace system can be determined directly. By influencing the combustion air flow, i.e. the quantity of combustion air flowing through the annular gap, the combustion process in a power station furnace system is optimally controlled according to preselected criteria. 
     Of course, it is also possible in this way to regulate the combustion processes in a power station furnace system. 
     Three exemplary embodiments of the invention will be explained in more detail below. 
    
    
     
       The appended drawings show in: 
         FIG. 1  a partial section of an annular gap surrounding a burner with a corresponding pair of sensor rods arranged in the annular gap, 
         FIG. 2 a    a longitudinal section through a burner with a surrounding annular gap and a corresponding pair of sensor rods arranged in the annular gap, 
         FIGS. 2 b    and c two cross sections through a burner with a surrounding annular gap, each in the plane of the arranged sensor rods, 
         FIG. 3  a partial section of an annular gap surrounding a burner with a corresponding pair of sensor rods arranged in the annular gap at an angle α=45° with respect to the longitudinal axis of the annular gap, 
         FIG. 4 a    a partial section of an annular gap surrounding a burner with two corresponding pairs of sensor rods arranged in the annular gap, wherein the pairs of corresponding sensor rods are in each case arranged at a different angle ααwith respect to the longitudinal axis of the annular gap, and 
         FIG. 4 b    a flat pattern of the annular gap with the corresponding sensor rods arranged on the outer wall of the burner. 
     
    
    
       FIG. 1  shows means for determining the quantity of combustion air flowing through an annular gap  3  with a burner  1  which is coaxially surrounded by a pipe  2  in such a way that an annular gap  3  is formed between the outer wall of the burner  1  and the pipe  2 . The burner  1 , the pipe  2  and the annular gap  3  have a common coaxial longitudinal axis  4 . Combustion air is guided in the annular gap  3 . The pipe  2  has a constriction  5  with a reduction in the annular gap width b to increase the flow velocity v of the combustion air flow. In the region of the constriction  5 , guide vanes  6  are arranged in the annular gap  3 , which cause a swirl flow of the combustion air flow in the annular gap section  3 . 1  downstream of the constriction in the direction of the coaxial longitudinal axis  4 . This annular gap section  3 . 1  has a constant annular gap width b. The direction of flow of the combustion air flow is illustrated by an arrow  7 . The direction of rotation of the swirl flow is illustrated by an arrow  8 . The component of the combustion air flow in the annular gap section  3 . 1  important for determining the quantity of combustion air supplied to the burner  1  is the component of the combustion air flow directed parallel to the coaxial longitudinal axis  4  or orthogonal to the cross-sectional area of the annular gap section  3 . 1  and is illustrated in  FIG. 1  by the arrow  9 . Two sensor rods  10  and  11  are arranged within the annular gap section  3 . 1 . The sensor rods  10  and  11  are each mounted on the outer wall of the burner  1  and electrically insulated by means of two supporting blocks  12 . The sensor rods  10  and  11  are arranged transversely to the longitudinal axis  4  and are adapted in their longitudinal direction to the curvature of the annular gap section  3 . 1  such that they have along their longitudinal extent the same distance c and d to the two walls delimiting the annular gap section  3 . 1 , i.e. the outer wall of the burner  1  and the inside of the pipe  2 . The distance c is the distance between the outer wall of the burner  1  and the sensor rods  10  and  11 , and the distance d is the distance between the inner wall of the pipe  2  and the sensor rods  10  and  11 . The two sensor rods  10  and  11  are equally spaced from the walls delimiting the annular gap section  3 . 1 . They are further arranged so as to be mutually parallel with the spacing a, but rotated radially with respect to one another, wherein the second sensor rod  11  in the flow direction  7  of the combustion air flow is arranged with a parallel displacement in the direction of rotation  8  of the swirl flow of the combustion air flow with respect to the first sensor rod  10  in the flow direction  7  of the combustion air flow.  FIGS. 2 a  to 2 c    illustrate the above-described arrangement of the sensor rods  10  and  11  in the annular gap section  3 . 1 . The sensor rods  10  and  11  are electrically connected to a correlation measuring device  13 . Due to electrical influence caused by electrically charged particles moving past the sensor rods  10  and  11  and transported in the combustion air flow, electrical signals are generated on the sensor rods  10  and  11 , which are evaluated by the correlation measuring device  13  by determining a time offset between the correlating electrical signals, which when divided by the distance a between the sensor rods  10  and  11  is a measure for the component of the flow velocity v of the combustion air flow in the annular gap section  3 . 1  transverse to the longitudinal direction of the sensor rods  10  and  11  in the arrangement of the sensor rods  10  and  11  shown in  FIG. 1 , i.e. in the direction of the longitudinal axis  4  of the annular gap section  3 . 1 . Starting from the component of the flow velocity v of the combustion air flow thus determined in the direction of the longitudinal axis  4  of the annular gap section  3 . 1 , the quantity of combustion air supplied to the burner  1  is determined based on the cross-sectional area of the annular gap section  3 . 1 . At the same time, the quantity of fuel supplied to a burner  1  is measured by using unillustrated means configured to detect the quantity of fuel supplied to the burner  1 , and the combustion process is controlled by changing the quantity of combustion air. 
     In the means shown in  FIG. 3  for determining the quantity of combustion air flowing through an annular gap  3 , the corresponding sensor rods  10  and  11  are arranged at an angle of α=45° with respect to the longitudinal axis  4  of the annular gap. All other features of the annular gap  3  and the arrangement of the sensor rods  10  and  11  in the annular gap section  3 . 1  correspond to those of the means shown in  FIG. 1  for determining the quantity of combustion air flowing through an annular gap  3 . The means shown in  FIG. 3  for determining the quantity of combustion air flowing through an annular gap  3 , as described with reference to  FIGS. 1 and 2 , are used to determine with the correlation measuring device  13  a component of the flow velocity v of the combustion air flow in the annular gap section  3 . 1  directed at an angle of α=45° with respect to the longitudinal axis  4 . The component of the flow velocity v of the combustion air flow in the annular gap section  3 . 1  in the direction of the longitudinal axis  4  of the annular gap section  3 . 1  is calculated by multiplying the component of the flow velocity v determined with the correlation measuring device  13  by sin α, i.e. sin 45°. With the thus calculated component of the flow velocity v of the combustion air flow in the annular gap section  3 . 1  in the direction of the longitudinal axis  4  of the annular gap section  3 . 1 , the combustion air quantity supplied to the burner  1  is then determined using the cross-sectional area of the annular gap section  3 . 1 . 
       FIG. 4 a    shows an arrangement with two pairs of corresponding sensor rods  10 . 1  and  11 . 1 , and  10 . 2  and  11 . 2 , respectively. The corresponding sensor rods  10 . 1  and  11 . 1  are arranged on the outer wall of the burner  1  with their longitudinal direction at an angle α 1 =45° with respect to the longitudinal axis  4 , and the corresponding sensor rods  10 . 2  and  11 . 2  are arranged on the outer wall of the burner  1  with their longitudinal direction at an angle α 2 =90° with respect to the longitudinal axis  4  of the annular gap section  3 . 1 . The two pairs of corresponding sensor rods  10 . 1  and  11 . 1 , and  10 . 2  and  11 . 2 , are each electrically connected to a correlation measuring device  13 . 1  and  13 . 2 , respectively.  FIG. 4 b    shows a flat pattern of this section of the annular gap  3 . 1  with the two pairs of corresponding sensor rods  10 . 1  and  11 . 1 , and  10 . 2  and  11 . 2 , arranged on the outer wall of the burner  1 . This arrangement can be used not only for determining the component of the flow velocity of the combustion air flow v in the direction of the longitudinal axis  4  of the ring gap section  3 . 1  and subsequently the calculation of the quantity of combustion air supplied to the burner, but also for determining the swirl angle γ of a combustion air flow having a swirl flow when the swirl angle γ satisfies the condition (π°−α 1 )&gt;γ&gt;(π°−α 2 ). To this end, the component v 1  of the flow velocity v of the combustion air flow is determined by evaluating the electrical signals generated on the sensor rods  10 . 1  and  11 . 1  using the correlation measuring device  13 . 1 , and the component v 2  of the flow velocity v of the combustion air flow is determined by evaluating the electrical signals generated on the sensor rods  10 . 2  and  11 . 2  using the correlation measuring device  13 . 2 . 
     An exemplary determination of the swirl angle γ of a combustion air flow having a swirl flow will be described below with reference to  FIG. 4 b   . The angle β enclosed between the flow velocity v and the component v 1  of the flow velocity v results from the relationship π°−α 1 +γ, which with α 1 =45° yields β=45°−γ. The angle enclosed between the flow velocity v and the component v 2  of the flow velocity v results from the relationship π°−α 2 +γ, so that with α 2 =90° the angle enclosed between the flow velocity v and the component v 2  of the flow velocity v is equal to the swirl angle γ. The component v 1  of the flow velocity v determined with the corresponding sensor rods  10 . 1  and  11 . 1  and the correlation measuring device  13 . 1  is described by the equation 
         v   1 =cos(45°−γ)· v,  or  v   1 =(cos 45°·cos γ+sin 45°·sin γ)· v.    (1)
 
     The component v 2  of the flow velocity v determined with the corresponding sensor rods  10 . 2  and  11 . 2  and the correlation measuring device  13 . 2  is described by the equation 
         v   2 =cos γ· v,  or cos γ= v   2   /v.    (2)
 
     Substituting equation (2) in equation (1) yields 
         v,= (cos 45°+sin 45°·sin γ/cos γ)· v   2 .   (3)
 
     Transforming equation (3) yields 
         v   1   /v   2 =cos 45°+sin 45°·tan γ, or tan γ=( v   1   /v   2 −cos 45°)/sin 45°.
 
     The swirl angle may thus be calculated from the two determined components v 1  and v 2  of flow velocity v of the combustion air flow according to the equation γ=arctan((v 1 /v 2 −cos 45°)/sin 45°). 
     LIST OF THE REFERENCE SYMBOLS USED 
       1  burner 
       2  pipe 
       3  annular gap 
       3 . 1  annular gap, annular gap section 
       4  longitudinal axis 
       5  constriction 
       6  guide vanes 
       7  arrow, flow direction of the combustion air flow 
       8  arrow, direction of rotation of the swirl flow 
       9  arrow, component of the combustion air flow parallel to longitudinal axis  4   
       10  sensor rod 
       10 . 1  sensor rod 
       10 . 2  sensor rod 
       11  sensor rod 
       11 . 1  sensor rod 
       11 . 2  sensor rod 
       12  supporting block 
       13  correlation measuring device 
       13 . 1  correlation measuring device 
       13 . 2  correlation measuring device