Patent Publication Number: US-6210152-B1

Title: Burner for a heat generator and method for operating the same

Description:
FIELD OF TECHNOLOGY 
     The invention on hand relates to a burner for a heat exchanger according to the preamble of claim  1 . It also relates to a method for operating such a burner. 
     STATE OF THE ART 
     Usually, burners of gas turbines are operated in premix mode. Such premix burners are known from EP-B1-0 321 809 and DE-195 47 913.0. By using upstream fuel injection in such premix burners, the fuel is premixed with the air before the combustion takes place. This provides an explosive mixture for the further combustion inside the burner. In general, it can be noted that such new generation burners offer numerous advantages, for example, a stable flame position, lower pollutant emissions (CO, UHC, NOx), minimal pulsations, complete burnout, a larger operating range, good cross-ignition between the various burners, in particular when creating graduated loads, during which case the burners are operated independently from each other, an adaptation of the flame to the corresponding combustor geometry, a compact design, an improved mixing of the flow media, an improved “pattern factor” of temperature distribution in the combustor, i.e., a balanced temperature profile of the combustor flow. 
     If, however, unforeseen malfunctions occur during operation, this may result in flame instability. Once the flashed-back flame is able to stabilize inside the burner, it burns as a diffusion flame with a very high temperature, at about 1900° C. Within a short time, ranging from 10 to max. 30 seconds, the burner overheats and is destroyed. In any case, the gas turbine must be stopped, inspected, and repaired, resulting in tremendous costs. It was found that, in particular, in prototype gas turbines with new combustion technology or combustion of hydrogen-containing fuels (MBt or LBt gasses) a high risk exists in this regard. 
     DESCRIPTION OF THE INVENTION 
     The invention attempts to solve this problem. The invention, as characterized in the claims, is based on the objective of proposing measures for a burner and a process of the initially mentioned type that would maximize flame stability in the burner. 
     According to the invention it is proposed to provide the burners with a compact, contactless flame monitor in a suitable place. 
     The essential advantages of the invention are that the sensor installed in the burner reports a flashback of the flame. Then the premix fuel mixture is reduced, and the pilot fuel quantity is simultaneously increased, so that the total fuel quantity, and therefore the turbine output, remains constant. Because of the reduction, i.e., of the premix fuel quantity, the flashback flame can no longer stabilize in the burner; it is inevitably flushed out of the burner. This makes it possible to prevent a destruction of the burner. 
     Such a sensor or flame monitor can be realized with high-temperature-resistant glass fibers. These fibers are arranged so that their monitoring field covers the areas at risk, but not the pilot and premix flame burning normally. The UV portion (about 300-330 nm) of the radiation measured by the sensor undergoes a spectral analysis with suitable filters. A flashback in the burner can be detected within a matter of milliseconds via the ratio of the intensity at various wavelengths. If the combustor consists of a number of burners, it is possible to determine with suitable data acquisition in which burner the flame flashback has occurred, and suitable measures for eliminating the causes can be taken. 
     Advantageous and useful further developments of the solution according to the invention are characterized in the remaining claims. 
     The following is a more detailed discussion of the exemplary embodiments of the invention in reference to the drawings. Any characteristics not essential for the direct understanding of the invention have been ignored. Identical elements have been marked in the various figures with the same reference symbols. The flow direction of the media is indicated with arrows. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a schematic view of a burner with integrated sensor; 
     FIG. 2 shows a burner after flashback and with subsequent stabilization of the flame in the burner; 
     FIG. 3 shows a schematic fuel control sequence over time in case of a flame flashback; 
     FIG. 4 shows an integral section through a burner designed as a premix burner with a mixing section downstream from a rotation generator and with pilot burners; 
     FIG. 5 shows a schematic portrayal of the burner according to FIG. 1 with disposition of the additional fuel injectors; 
     FIG. 6 shows a perspective drawing of a rotation generator consisting of several segments, sectioned accordingly; 
     FIG. 7 shows a cross-section through a two-segment rotation generator; 
     FIG. 8 shows a cross-section through a four-segment rotation generator; 
     FIG. 9 shows a view through a rotation generator whose segments are profiled in blade-shape; 
     FIG. 10 shows a variation of the transition geometry between rotation generator and mixing section; and, 
     FIG. 11 shows a tear-off edge for the spatial stabilization of the flowback zone. 
    
    
     METHODS FOR EXECUTING THE INVENTION, COMMERCIAL USABILITY 
     FIG. 1 shows a schematic overview of a premix burner, whereby the design of such a burner has been described in detail in FIGS. 4-11. Principally, this premix burner consists of a rotation generator  100 , of a mixing section  220  following this rotation generator, whereby a system of pilot burners  300  with corresponding pilot flames  70  act in the combustor  30  following the mixing section  220 . In connection with FIG. 2, this FIG. 1 only strives to explain how the flashback  81  of the premix flame  50  which is shown here by means of the flowback bubble, is detected by sensors  400 , and how remedial measures are initiated immediately. In the process, it is always observed that a back-ignition from the combustor  30  to the fuel injectors  116  takes place. A stabilization of this back-ignited flame  80  in the area of the fuel injectors  116  then can no longer be avoided, whereby in this case a diffusion flame with very high temperatures of approximately 1900° C. is created. This flame inevitably results in a destruction of the burner within a matter of a few seconds. At least one sensor  400  is placed immediately downstream from the fuel injectors  116  and is not supposed to monitor either the premix flame  50  nor the pilot flames  70 , but only those areas at risk. Such a sensor  400  preferably consists of high-temperature-resistant glass fibers which are arranged in such a way that their scan angle  402  covers only those areas at risk. The radiation detected by the sensor is further transmitted  401  and undergoes a spectral analysis with suitable filters. A flashback in the burner can be detected within a matter of milliseconds via the ratio of the intensities at various wavelengths. A suitable data acquisition will make it possible to determine in which burner in the system the flame flashback has occurred, whereby specific measures for eliminating the cause then can be taken. 
     FIG. 3 shows which measures are initiated following a flame flashback. When notified that a flashback  81  of the flame has taken place, a control  82  immediately manipulates the fuel quantity for the premix flame  50 , which is immediately reduced according to certain criteria. At the same time, a second control  83  is actuated, which increases the fuel quantity for the pilot burner system  300 , i.e., for the pilot flame  70 . The objective of this counter-acting fuel supply is to keep the turbine output constant. By reducing the fuel quantity for the premix flame  50 , the flashed-back flame is no longer able to stabilize in the burner, it is flushed out of the burner, so that the otherwise inevitable destruction of the burner is in this way safely avoided. FIG. 3 shows the qualitative sequence of the fuel control over time, whereby the flushing out  84  of the flashed-back flame takes place at the extreme points of this control. 
     This process for the direct detection of a flame flashback can be used for all premix burners based on a rotational flow, regardless of how the burner is geometrically constructed, and regardless of which way the rotational flow is created. In particular, this process can be used for the premix burner according to EP-B1-0 321 809, whereby this publication forms an integral part of this specification at hand. 
     FIG. 4 shows the overall construction of a burner that can be operated with a rotational flow. Initially, a rotation generator  100  whose design is shown and explained in more detail in reference to the following FIGS. 5 through 8 is activated. This rotation generator  100  is a conical structure which is impacted repeatedly by a tangentially inflowing combustion air stream  115 . The flow resulting from this is seamlessly fed with the help of a transition geometry located downstream from the rotation generator  100  into a transition piece  200  in such a way that no separation areas can occur there. The configuration of this transition geometry is described in more detail under FIG.  10 . This transition piece  200  is extended on the flow-off side from the transition geometry with a mixing pipe  20 , whereby both parts form the actual mixing section  220 . Naturally, the mixing section  220  may also consist of a single piece, which means that the transition piece  200  and the mixing pipe  20  are then fused to form a single, contiguous structure, whereby the characteristics of each part are preserved. If the transition piece  200  and the mixing pipe  20  are constructed from two parts, these are connected with a bushing ring  10 , whereby the same bushing ring  10  serves on the head side as an anchoring surface for the rotation generator  100 . Such a bushing ring  10  also has the advantage of being able to use different mixing pipes. On the flow-off side of the mixing pipe  20 , the actual combustion chamber  30  of a combustor, which in this case is only symbolized by a flame pipe, is located. The mixing section  220  essentially has the function of providing a defined section downstream from the rotation generator  100 , in which a perfect premixing of fuels of various types can be achieved. This mixing section, i.e., here the mixing pipe  20 , also permits a loss-free guidance of the flow, so that initially no flowback zone or flowback bubble is able to form even in active connection with the transition geometry, so that the mixing quality of all types of fuel can be influenced over the length of the mixing section  220 . However, this mixing section  220  also has another characteristic, namely that the axial speed profile has a distinct maximum on the axis in this mixing section itself, so that a flashback of the flame from the combustor itself should actually be prevented. However, it is correct that with such a configuration this axial speeds decreases towards the wall. In order to prevent a flashback also in this area, the mixing pipe  20  is provided in the flow and peripheral direction with a number of regularly or irregularly distributed bores  21  that have different cross-sections and directions, through which bores a quantity of air flows into the inside of the mixing pipe  20  and induces an increase in the flow speed along the wall in the sense of forming a film. These bores  21  also can be designed so that, in addition, at least an effusion cooling occurs at the inside wall of the mixing pipe  20 . Another possibility for increasing the speed of the mixture within the mixing tube  20  is by constricting the latter&#39;s flow cross-section downstream from the transition channels  201 , which form the already mentioned transition geometry, so that the entire speed level inside the mixing pipe  20  is increased. In the figure, these bores  21  extend at an acute angle to the burner axis  60 . The outlet of the transition channels  201  furthermore corresponds to the narrowest flow cross-section of the mixing pipe  20 . Said transition channels  201  therefore bridge the respective cross-section differential without adversely affecting the formed flow. 
     If the selected measure causes an unacceptable loss of pressure when the pipe flow  40  is guided along the mixing pipe  20 , this can be remedied by providing a diffuser (not shown in the figure) at the end of this mixing pipe. The end of the mixing pipe  20  is therefore followed by a combustor  30  (combustion chamber), whereby a change in cross-section that is a result of a burner front exists between the two flow cross-sections. Only here, a central flame front with a flowback zone that has the characteristics of a bodiless flame retention baffle in relation to the flame front forms. If, during operation, a marginal flow zone forms within this cross-section change in which turbulence separations are created because of the vacuum present there, this results in an increased ring stabilization of the flowback zone. In addition, it must not go unmentioned, that the formation of a stable flowback zone also requires a sufficiently high rotation value in a pipe. If such a rotation value is initially undesired, stable flowback zones can be created by introducing small air flows with strong rotations at the pipe end, for example through tangential openings. In the process it is hereby assumed that the air quantity required for this is about 5 to 20% of the total air quantity. In regard to the design of the burner front at the end of the mixing pipe  20  for stabilizing the flowback zone or flowback bubble, reference is made to the description for FIG.  8 . Regarding the possibility of interfering with a flame flashback, reference is made to FIGS. 1 to  3 . 
     A pilot burner system  300  is provided concentrically to the mixing pipe  20  in the area of the latter&#39;s outlet. This pilot burner system consists of an inner ring chamber  301  into which flows a fuel, preferably a gaseous fuel  303 . Secondary to this inner ring chamber  301 , a second ring chamber  302  is disposed, into which an air quantity  304  flows. Both ring chambers  301 ,  302  have individually designed through-openings in such a way that the individual media  303 ,  304  flow as a result of the function into a mutual, subsequent ring chamber  308 . The passage of the gaseous fuel  303  from the ring chamber  301  into the subsequent ring chamber  308  is achieved by a number of peripherally located openings  309 . The flow-through geometry of these openings  309  is such that the gaseous fuel  303  flows with a high mixing potential into the subsequent ring chamber  308 . The other ring chamber  302  terminates in a perforated plate  305 , whereby the bores  310  provided here are designed so that the air quantity  304  flowing through them results in an impact cooling on the bottom plate  307  of the subsequent ring chamber  308 . This bottom plate has the function of a heat shield in relation to the caloric stress from the combustion chamber  30 , so that this impact cooling must be extremely efficient here. After cooling has taken place, this air mixes inside this ring chamber  308  with the inflowing gaseous fuel  303  from the openings  309  of the upstream ring chamber  301 , before this mixture then flows off into the combustion chamber  30  through a number of bores  306  on the combustion chamber side. The mixture flowing off here burns in the form of a premixed diffusion flame with minimized pollutant emissions and then forms for each bore  306  a pilot burner that acts into the combustion chamber  30  and which ensures a stable operation. 
     An ignition device  311  which in the subsequent ring chamber  308  brings about the ignition of the mixture formed there is conducted through the secondary ring chamber  302  through which an air stream flows. This conduction of the ignition device  311  on the one hand does not require any additional construction measures, and on the other hand this ignition device  311  is continuously cooled by the air  304  which flows there anyway. This is very important, because temperatures of approximately 1000° C. are reached at the tip of a glow igniter  2  pin. But since the operation proposed here requires only a low voltage, but high amps, the susceptibility of the ignition device to condensate water precipitation is eliminated. The arrangement of the glow igniter pin—whereby the use of a spark plug would also be possible—inside the burner results in a low thermal stress on the respective ignition device  311 , so that no additional cooling is necessary and leaks are prevented. 
     FIG. 5 shows a schematic view of the burner according to FIG. 4, whereby here reference is made specifically to the flow around a centrally located fuel nozzle  103  (see FIG. 6) and to the action of fuel injectors  170 . The function of the remaining main components of the burner, i.e., rotation generator  100  and transition piece  200  are described in more detail below in reference to the figures. The fuel nozzle  103  is enclosed at a distance with a ring  190  into which a number of peripherally disposed bores  161  have been integrated, through which an air quantity  160  flows into an annular chamber  180  and there flows around the fuel lance. These bores  161  are placed so as to angle forward in such a way as to create an appropriate axial component on the burner axis  60 . In active connection with these bores  161 , additional fuel injectors  170  which add a certain quantity of a preferably gaseous fuel into the respective air quantity  160  have been provided so that a uniform fuel concentration  150  appears over the flow cross-section in the mixing pipe  20 , as is symbolized in the figure. Exactly this uniform fuel concentration  150 , in particular the strong concentration on the burner axis  60 , ensures that a stabilization of the flame front occurs at the outlet of the burner, especially when using a central injection with liquid fuel, so that any occurrence of combustor pulsations are avoided. 
     In order to better comprehend the construction of the rotation generator  100 , it is advantageous to explain FIG. 6 at least in conjunction with FIG.  7 . If needed, the following text therefore will refer to the other figures when describing FIG.  6 . 
     The first part of the burner according to FIG. 4 is formed by the rotation generator  100  in FIG.  6 . The latter consists of two hollow, conical partial bodies  101 ,  102  which are stacked offset inside each other. The number of conical partial bodies natural may be greater than two, as can be seen in FIGS. 5 and 6. As will also be explained further below, this depends in each case on the operating mode of the burner overall. In certain operating configurations it is possible that a rotation generator consisting of a single spiral is provided. The offset of the respective center axis or longitudinal symmetry axes  101   b ,  102   b  (see FIG. 7) of the conical partial bodies  101 ,  102  relative to each other creates in each case in the adjoining wall, in a mirror-symmetrical arrangement, a tangential channel, i.e., an air inlet slit  119 ,  120  (see FIG. 7) through which the combustion air  115  flows into the interior of the rotation generator  100 , i.e., into the conical cavity  114  of the same. The conical shape of the shown partial bodies  101 ,  102  in the flow direction has a specific fixed angle. Naturally, depending on the specific operating case, the partial bodies  101 ,  102  may have an increasing or decreasing conical angle in the flow direction, similar to a diffuser or confusor. The two last mentioned forms are not shown in the drawing since the expert will be able to understand them easily. The two conical partial bodies  101 ,  102  each have a cylindrical, annular starting part  101   a . The fuel nozzle  103  already mentioned in reference to FIG. 2 which is preferably operated with a liquid fuel  112  is located in the area of this cylindrical starting part. The injection  104  of this fuel  112  coincides approximately with the narrowest cross-section of the conical cavity  114  formed by the conical partial bodies  101 ,  102 . The injection capacity and the type of this fuel nozzle  103  depend on the specified parameters of the respective burner. The conical partial bodies  101 ,  102  also each have a fuel line  108 ,  109  which are located along the tangential air inlet slits  119 ,  120  and are provided with injection openings  117  through which preferably a gaseous fuel  113  is injected into the combustion air  115  flowing there, as is indicated symbolically by arrows  116 . These fuel lines  108 ,  109  are arranged preferably not after the tangential inflow, prior to the entrance into the conical cavity  114 , in order to obtain an optimum air/fuel mixture. The fuel  112  supplied through the fuel nozzle  103  is, as mentioned, usually a liquid fuel, whereby a mixture can be easily formed with another medium also, for example, with recycled flue gas. This fuel  112  is preferably injected at a very acute angle into the conical cavity  114 . This means that after the fuel nozzle  103  a conical fuel spray forms, which is enclosed and reduced by the tangentially inflowing, rotational combustion air  115 . The concentration of the injected fuel  112  is then constantly reduced in axial direction by the inflowing combustion air  115 , resulting in a mixing that approaches an evaporation. If a gaseous fuel  113  is added via the opening nozzles  117 , the fuel/air mixture is formed directly at the end of the air inlet slits  119 ,  120 . If the combustion air  115  is additionally preheated or enriched, for example, with recycled flue gas or exhaust gas, this greatly supports the evaporation of the liquid fuel  112 , before this mixture flows into the next stage, here into the transition piece  200  (see FIGS.  4  and  10 ). The same concepts also apply if liquid fuels are supplied via lines  108 ,  109 . When designing the conical partial bodies  101 ,  102  in regard to the conical angle and the width of the tangential air inlet slits  119 ,  120 , narrow limits must actually be kept, so that the desired flow field of the combustion air  115  is able to form at the outlet of the rotation generator  100 . In general, it can be said that a reduction of the tangential air inlet slits  119 ,  120  promotes the faster formation of a flowback zone already in the area of the rotation generator. The axial speed within the rotation generator  100  can be increased or stabilized with an addition of an air quantity that is described in more detail in reference to FIG. 2 (No.  160 ). A corresponding rotation generation in active connection with the subsequent transition piece  200  (FIGS. 4 and 10) prevents the formation of flow separations within the mixing pipe following the rotation generator  100 . The construction of the rotation generator  100  is also very suitable for changing the size of the tangential air inlet slits  119 ,  120 , so that a relatively large operating bandwidth can be covered without changing the design length of the rotation generator  100 . The partial bodies  101 ,  102  naturally can also be moved relative to each other on a different plane, whereby even an overlapping of them is possible. It is also possible to stack the partial bodies  101 ,  102  spiral-like inside each other by a counter-rotating movement. This makes it possible to change the shape, size, and configuration of the tangential air inlet slits  119 ,  120  as desired, so that the rotation generator  100  can be universally used without changing its design length. 
     FIG. 7, among other things, shows the geometric configuration of optionally provided baffle plates  121   a ,  121   b . They have a flow introduction function and extend, depending on their length, the respective end of the conical partial bodies  101 ,  102  in the flow direction relative to the combustion air  115 . The channeling of the combustion air  115  into the conical cavity  114  can be optimized by opening or closing the baffle plates  121   a ,  121   b  around a pivoting point  123  placed in the area of the entrance of this channel into the conical cavity  114 ; this is, in particular, necessary if the original slit size of the tangential air inlet slits  119 ,  120  should be changed dynamically, for example, in order to change the speed of the combustion air  115 . Naturally, these dynamic measures can also be provided statically, in that baffle plates, as required, form a fixed part with the conical partial bodies  101 ,  102 . 
     Compared to FIG. 4, FIG. 8 shows that the rotation generator  100  is now constructed of four partial bodies  130 ,  131 ,  132 ,  133 . The associated longitudinal symmetry axes for each partial body are designated with the letter “a.” Regarding this configuration, it can be said that as a result of the lower rotation intensity generated with it and in connection with a correspondingly greater slit width, it is ideally suited to prevent the bursting of the turbulence flow on the outlet side of the rotation generator in the mixing pipe, so that the mixing pipe is able to optimally fulfill its intended role. 
     Compared to FIG. 8, the difference in FIG. 9 is that here the partial bodies  140 ,  141 ,  142 ,  143  have a blade profile shape which has been provided to create a certain flow. Other than that, the operating mode of the rotation generator has remained the same. The admixture of the fuel  116  into the combustion air stream  115  is accomplished from the inside of the blade profiles, i.e., the fuel line  108  is now integrated into the individual blades. The longitudinal symmetry axes for the individual partial bodies are also designated with the letter “a” here. 
     FIG. 10 shows a three-dimensional view of the transition piece  200 . The transition geometry is constructed for a rotation generator  100  with four partial bodies, corresponding to FIG. 5 or  6 . Accordingly, the transition geometry has four transition channels  201  as a natural extension of the partial bodies acting upstream, so that the conical quarter surface of said partial bodies is extended until it intersects the wall of the mixing pipe. The same concepts also apply if the rotation generator has been constructed according to a different principle than the one described in reference to FIG.  4 . The surface of the individual transition channels  201  that extends downward in the flow direction has a spiral shape in the flow direction that describes a sickle-shaped progression, corresponding to the fact that the flow cross-section of the transition piece  200  is in this case conically extended in the flow direction. The rotation angle of the transition channels  201  in the flow direction has been chosen so that the pipe flow has then a sufficiently long section available before the change in diameter at the combustor inlet to achieve a perfect premixing with the injected fuel. The above mentioned measures furthermore increase the axial direction at the mixing pipe wall downstream from the rotation generator. The transition geometry and the measures in the area of the mixing pipe bring about a clear increase in the axial speed profile towards the center of the mixing pipe, decisively counteracting the risk of a premature ignition. 
     FIG. 11 shows the already discussed tear-off edge formed at the burner outlet. The flow cross-section of the pipe  20  in this area has the transition radius R whose size depends principally on the flow inside the pipe  20 . This radius R is selected so that the flow closely follows the wall and in this way causes the rotation value to greatly increase. Quantitatively, the size of the radius R can be defined so that it is greater than 10% of the inside diameter d of the pipe  20 . Compared to the flow without a radius, the flowback bubble now increases enormously. This radius R extends up to the outlet plane of the pipe  20 , whereby the angle β between beginning and end of the curvature is less than 90°. The tear-off edge A extends along one leg of the angle β into the interior of the pipe  20  and in this way forms a tear-off stage S relative to the front point of the tear-off edge A whose depth is greater than 3 mm. Naturally, the edge which here extends parallel to the outlet plane of the pipe  20  can now be returned to the stage of the outlet plane with a curved progression. The angle β′ between the tangent of the tear-off edge A and the vertical to the exit plane of the pipe  20  is identical to the angle β. The advantages of this design of the tear-off edge are found in EP-0 780 629 A2 in section “Description of the Invention.” A further design of the tear-off edge for the same purpose can be achieved with torus-like notches on the combustor side. This publication, including its protected scope in regard to the tear-off edge, is an integral part of this specification.