Abstract:
A method and a corresponding device for controlling the temperature of steam in a boiler of a steam generator are provided. The gradual accumulation of dirt on heat exchanger surfaces inside the boiler is incrementally regulated by soot blowers. The targeted influencing of the heat transfer on the heat exchanger surfaces enables the steam temperatures to be controlled and regulated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is the US National Stage of International Application No. PCT/EP2011/056853 filed Apr. 29, 2011 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2010 018 717.8 filed Apr. 29, 2010, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The invention relates to a method for controlling the temperature of steam in a boiler, and to a corresponding device. 
       BACKGROUND OF THE INVENTION  
       [0003]    A fossil-fired steam generator or boiler of a power plant is generally composed of a combustion chamber, an evaporator chamber and a system of heat exchangers which are connected to the evaporator chamber. There are numerous different embodiments of the boiler structures, such as for example drum-type boilers or Benson boilers. In one variant, the evaporator chamber is composed of a pipe arrangement which is in direct thermal contact with the combustion chamber. In the evaporator chamber, the feed water delivered out of a feed water preheater is evaporated to the saturated steam temperature. The steam is subsequently conducted through the system of heat exchangers, which are likewise mostly tubular, in which the steam temperatures are adjusted to the inlet temperatures demanded by the turbines. The system of heat exchangers is conventionally constructed from at least one superheater, reheater, economizer and air preheater. 
         [0004]    During the combustion of solid fossil fuels, flue ash is released which is transported in the flue gas flow to the flue gas outlet and which is then separated or recirculated. Here, some of the ash is deposited on the heat exchanger tubes and other boiler structures, and there, forms in some cases thick deposition layers which can additionally become baked on depending on the coal quality. Said depositions firstly reduce the heat transfer, secondly block the exhaust-gas path, and not least can form conglomerates which are so large that, if they at some time become detached from their support, they can cause considerable mechanical damage as they fall owing to their compact mass and high falling speed. Therefore, by means of steam blowers or water blowers, said lining is removed from time to time. Said process is referred to as “sootblowing”. Thereafter, the heat transfer and thus the steam temperature changes considerably in the cleaned and also in the non-cleaned boiler regions. After all cleaning measures have ended, the boiler gradually becomes fouled again, which in turn correspondingly changes the heat transfer and the steam temperatures. 
         [0005]    The sootblowing is therefore conventionally always performed with the aim of eliminating the fouling of the boiler as globally as possible. Often, sootblowing is performed cyclically, wherein the sequence of the sootblowers is adapted manually according to the thermal state of the boiler, or blowing is performed correspondingly frequently such that no uncontrollable thermal states arise. 
         [0006]    If an automatic system is used for sootblowing, the sootblowing time is calculated on the basis of economic criteria and fouling analyses. The Siemens SPPA-P3000 “cost-optimized sootblowing” system likewise operates on the basis of said criteria. Here, however, the fouling and the resulting thermal losses can be measured only with difficulty. 
         [0007]    A further method for regulating sootblowers is known from U.S. Pat. No. 4,718,376. Here, adjacent sootblowers are combined to form groups of a maximum of four sootblowers. Each group is responsible for a region with a similar deposition characteristic. Furthermore, each sootblower receives a weighting factor which corresponds to a percentage of the total number of sootblowing cycles in which the sootblower is in operation. Each sootblowing cycle begins with the group of sootblowers situated furthest upstream, and progresses in the direction of the flow of the combustion gases. The main criterion on which the execution of the sootblowing is based is that of operating the boiler at or at least close to maximum efficiency. A secondary criterion is that of using as little sootblowing steam as possible. 
         [0008]    The large differences in the fouling before and after the sootblowing of the individual boiler regions and the progressively increasing fouling can disrupt the sensitive balance of the heat distribution, and constitute a significant impediment with regard to the thermal regulability of the boiler. 
         [0009]    Depending on the fouling, a displacement of the heat transfer in the region of the individual evaporator heat exchangers, of the individual superheaters and of the individual reheaters accordingly arises. So-called “thermal imbalances” arise when temperature differences arise in tracts of the heat exchanger which are merged again after the splitting-up of the steam quantities. The temperature differences arise owing to non-uniform splitting-up and non-uniform heat transfers caused by differences in the flue gas flow and temperature. It has also been found that fouling of upstream heat exchangers leads to increased heat absorption in the downstream heat exchangers, and thus represents only a small fraction of an increased waste-gas loss of the boiler. This is defined substantially by the fouling in the eco region. 
         [0010]    Displacements of the heat transfer may be compensated in part by injection regulation in steam coolers provided between the heat exchangers. Here, however, by injection of water into the fresh steam, in principle only cooling can be effected, and only a limited injection quantity can be used. What must be observed here in particular is the negative influence of the reheater injection on the heat demand and the maximum possible performance of the steam-turbine-generator process. The heat demand changes by 0.2% per 1% change in reheater injection rate. 
         [0011]    Owing to fouling, the distribution of the heat transfer between the evaporator and superheater can be displaced to such an extent that firstly the existing injection capacity is no longer sufficient to keep the steam temperature below a desired or safety-related value. Secondly, a situation may arise in which the steam, even in the case of closed injection, no longer reaches the required temperature value. 
         [0012]    Individual heat exchangers without downstream injection-cooling arrangements, such as evaporator regions and final superheaters, however cannot be thermally balanced. 
         [0013]    In addition to the influencing of the steam temperatures by active cooling at individual locations, the heat balance within the boiler can also be influenced by the combustion itself. In the case of a drum-type boiler, the distribution of the heat transfer between the evaporator and superheater is influenced by means of different stratified firing, or by means of a cumbersome pivoting burner device or flue gas recirculation; in the case of a Benson boiler, it is additionally possible to vary the feed water quantity and thus the fresh steam injection quantity. 
         [0014]    Where no pivoting burner device or flue gas recirculation is used, the fresh steam evaporation can be held in the control range only with selective stratified firing, which is not always successful. It is however scarcely possible in this way to adequately control the reheater injection rate. 
         [0015]    Thermal imbalances which arise are compensated for by corresponding safety margins; here, the optimum temperatures are, on average, undershot, which leads in part to an increased heat demand of the process, or reduces the hot steam injection required for controlling the steam temperature to zero. 
         [0016]    In summary, it can be stated that thermal regulability of the boiler, with the aim of stable and optimum thermal conditions of the boiler, based solely on the firing and punctiform injection cooling is highly cumbersome and complex. It is a disadvantage in particular that thermal imbalances can always arise. Additional problems arise owing to the fouling in the boiler region, which always influences the heat transfers at the heat exchanger pipes and is negatively superposed on the regulation process. 
         [0017]    A method and a device for improving the steam temperature control is known from DE 10 2006 006 597 A1. Here, a system is provided for the analysis of the effect of the operation of sootblowers in a heat transfer region of a power plant. Said system determines a steam temperature influencing sequence and calculates a forward control signal which is to be supplied to a steam temperature control system for the heat transfer region. 
       SUMMARY OF THE INVENTION  
       [0018]    It is therefore an object of the present invention to specify an improved method for steam temperature control in a boiler. 
         [0019]    Said object is achieved by the features of independent patent claim. Advantageous embodiments are specified in each case in the dependent patent claims. 
         [0020]    It is the basic concept of the invention for the fouling, which hitherto constituted an imponderable factor in the heat balance and which severely restricted the thermal regulability of the boiler, to now be used in a positive sense by virtue of said fouling being brought about on the heat exchanger surfaces within the boiler in a manner controlled by means of sootblower devices, and the steam temperatures being regulated by means of said setting of the heat transfer at said surfaces. Here, the sootblowing takes place incrementally. With the incremental sootblowing, the thermal characteristics can be controlled through the variation of the operating times of individual sootblowers or individual sootblower groups. Since the sootblower devices are already provided in all power plants, there is accordingly no need for additional measurement instrumentation or machine equipment for steam temperature control. Costs can be saved in this way. 
         [0021]    Here, the fouling is brought about always so as to ensure an equalized overall heat balance within the boiler. The entire plant process is advantageously optimized in this way. This is achieved for example by virtue of evaporator surfaces and superheater surfaces being cleaned such that the heat output is distributed across the evaporator and superheater such that, at all times, taking into consideration the restricted capacity of the steam cooler, firstly the steam setpoint temperatures are always attained and secondly the admissible limit values are not exceeded. Boiler regions of multi-tract form should be cleaned such that, after the steam is split up in the heat exchangers, there are no temperature differences in the steam at the location of subsequent merging. It is basically sought to ensure minimal cleaning of the individual boiler regions at all times, and boiler regions identified as being clean should not be cleaned unnecessarily. Only in this way is it possible to ensure a high efficiency of the overall process. 
         [0022]    The method according to the invention comprises the following steps: 
         [0023]    Subgroups of sootblowers are formed which clean parts of the boiler which are as individually identifiable, and capable of being balanced, as possible. 
         [0024]    Within the technical plant, at least the following parameters are measured: 
         [0025]    Injection rate of the fresh steam and of the reheater steam 
         [0026]    Inlet temperature of steam and flue gas entering the heat exchanger 
         [0027]    Outlet temperature from the heat exchangers 
         [0028]    Fouling factors of individual heat exchangers 
         [0029]    Operating time between one cleaning operation and the next cleaning operation for a sootblower or individual sootblowers of a subgroup. 
         [0030]    From the measured parameters and so as to ensure an equalized overall heat balance within the boiler, the sootblowing time is determined individually for each individual sootblower of the subgroup of sootblowers, and the fouling is thus controlled in the fine range by the regulating system. 
         [0031]    Depending on which region of the boiler the sootblowing is used in, there are different boundary conditions which must be taken into consideration in terms of the regulation technology: In the evaporator region and in the superheater region, it is necessary in particular for the injection rate of the fresh steam and the inlet and outlet temperatures of the superheater to be taken into consideration. In the reheater region, the injection rate of the reheater steam must be taken into consideration, with a view to minimizing said injection rate. In the economizer, in particular the waste gas loss must be taken into consideration. 
         [0032]    If the procedure for a heat exchanger yields a short average operating time across all of the sootblowers of the heat exchanger since the respective last cleaning operation, said heat exchanger is defined as being clean. 
         [0033]    The fouling of individual heat exchangers is determined by virtue of a present heat transfer coefficient at the surfaces under consideration being measured on the basis of a present heat balance. For individual heat exchangers, the degree of fouling is determined by comparison with heat transfer coefficients recorded previously in the clean state, wherein the influence of the relative boiler load is taken into consideration by means of a regression which is linear in regions. The advantage of said design variant lies in the fact that the states “dirty” or “clean” are measured for the first time here. Here, the heat transfer coefficient at a surface under consideration plays a crucial role. The heat transfer coefficient is determined from the heat balance of steam and flue gas. 
         [0034]    The degree of fouling is determined by means of the fouling factor V on the basis of the formula V=1−q/q 0 , wherein q represents the specific heat output of the steam per K temperature difference between the flue gas and steam, and q 0  represents the specific heat output in a state defined as clean. Said specific definition of the fouling advantageously provides a new regulation criterion according to the invention. Here, the fouling of the heat exchanger surfaces is defined quantitatively. 
         [0035]    The advantages of the described invention are numerous and wide-ranging: primarily, the sootblowing is advantageously made part of, and assists, the thermal boiler regulation. The sootblowing takes place fully automatically taking into consideration stable and optimum thermal conditions for the boiler. Even incorrectly dimensioned heat exchangers can be corrected by means of the controllable fouling according to the invention. So-called thermal imbalances at the boiler drawing-in points are automatically compensated. Cleaning-induced temperature fluctuations are minimized. The thermal conditions when relative cleanliness is restored are automatically measured and stored as a measure for the future fouling. For the next onset of a cleaning cycle, one sootblower or individual sootblowers of a subgroup of sootblowers are selected based on the criterion of the maximum operating time between one cleaning operation and the next cleaning operation, whereby a predefinable minimum cycle for each subgroup is ensured. The repeated cleaning of regions which are still clean is prevented through monitoring of the average operating time and taking into consideration the present fouling. The waste gas loss of the boiler can be influenced by means of the modification of the sootblowing cycles. When relative cleanliness of the relevant heat exchanger is restored, the present waste gas loss is automatically measured and stored as a measure for a future increase of the waste gas loss. In summary, it can be stated that the invention minimizes steady-state and dynamic boiler losses without additional outlay in terms of machine technology and personnel. Furthermore, reliable sootblowing with full fouling control is attained, to optimum benefit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0036]    The invention will be explained in more detail below on the basis of an exemplary embodiment illustrated in the drawings, in which: 
           [0037]      FIG. 1  shows a schematic diagram of a steam generator, 
           [0038]      FIG. 2  shows a sketch for explaining the determination of the degree of fouling, 
           [0039]      FIG. 3   a  shows a profile of the steam temperature with a conventional sootblowing algorithm, 
           [0040]      FIG. 3   b  shows a profile of the steam temperature according to an exemplary embodiment of the sootblowing algorithm according to the invention, 
           [0041]      FIG. 4  shows a sketch illustrating a thermal imbalance within the heat exchanger system, and 
           [0042]      FIG. 5  shows a block circuit diagram of an arrangement for implementing the sootblowing algorithm according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]      FIG. 1  illustrates, in highly simplified form, a steam generator. In the combustion chamber BR of the boiler K, a solid fossil fuel, for example carbon dust, is burned. The flue gas RG generated in the process is conducted through the flue gas duct RGK to the flue gas cleaning arrangement RGR. The evaporation of supplied feed water SPW takes place in the pipe systems of the evaporator chamber and of the heat exchanger. The system is conventionally constructed such that feed water is supplied from the feed water tank  1  to the feed water preheater  2  (ECO). From there, the water-steam mixture passes into the drum  3  and is supplied via the downpipes  4 , the distributor collector  5  and the ascending pipes  6  to the superheater ( 7  or Ü) and subsequently to the turbine  8 . The superheater Ü may furthermore also comprise a reheater ZÜ. 
         [0044]    According to the invention on which this application is based, the steam temperature is controlled and regulated by virtue of a certain fouling of the heat exchanger surfaces within the boiler being brought about by means of the sootblower device. 
         [0045]    The fouling on the heat exchanger surfaces is determined as follows: here, fouling is to be regarded as a synonym for losses during the heat transfer between the combustion chamber/flue gas side and the water/steam side of a boiler.  FIG. 2  serves to illustrate the determination of the degrees of fouling or heat exchanger losses. Illustrated in simplified form is a pipe portion, wherein steam D flows with a certain mass flow mD and pressure pD through the interior of the pipe. The temperature TDin is measured at the inlet opening of the pipe, and the temperature TDout is measured at the outlet opening of the tube. Flue gas RG flows with the mass flow mRG and pressure pRG around the pipe. Here, too, temperatures TRGin and TRGout can be determined at the locations of the inlet and outlet openings of the pipe. The heat absorption by the heat exchanger pipe can thus be determined from the water/steam-side measurement variables throughflow, pressure and inlet/outlet temperature. At the flue gas side, the measurement of the mass flow and of the inlet-side and outlet-side temperatures is expedient, wherein missing temperatures and missing flue-gas mass flow can also be calculated in terms of a balance. The heat output of the heat exchanger is newly determined for the clean state after a suitably short average sootblowing cycle, and the boiler model used is adapted correspondingly. Variations of the heat transfer behavior caused by residual lining formation or by a change of coal quality or of the operating conditions are automatically compensated for in this way. 
         [0046]    For every measurable heat exchanger region of the boiler, the heat absorbed during the further operation of the plant is constantly determined on an ongoing basis. Said value is compared with the starting value from the clean state. 
         [0047]    For this purpose, the specific steam output q (or the heat transfer coefficient) is determined from the steam output Q and the difference between the flue gas and steam temperatures ΔT, cf.  FIG. 2 . Said specific steam output q is compared with its starting value in the clean state q_s. This yields the equivalent characteristic values: 
         [0048]    Cleanliness factor CF=q/q_s 
         [0049]    Fouling factor V=1−q/q_s=1−CF 
         [0050]    The invention shall be explained on the basis of  FIG. 3 . By way of an example, the flue gas temperature T is plotted as a function of the time t. The flue gas temperature is inversely proportional to the steam temperature. 
         [0051]      FIG. 3   a  illustrates a conventional sootblowing cycle during a period of uninterrupted operation t R . A period of uninterrupted operation t R  is defined as the operating time between one cleaning operation and the next cleaning operation for a sootblower or a subgroup of sootblowers. After a sootblowing process R, which in this case consists of  6  sootblowers R 1  to R 6 , the flue gas temperature falls sharply, and subsequently rises again continuously with progressive fouling of the pipelines. Finally, sootblowing is performed again, as indicated in  FIG. 3   a  by the sootblowing process Rnext. During every sootblowing process R or Rnext, all of the sootblowers (in this case there are for example six sootblowers R 1  to R 6 ) are in operation simultaneously. 
         [0052]    In  FIG. 3   b , according to the invention, incremental, quasi-continuous operation of the sootblowers is implemented. Instead of one sootblowing process R, a plurality of “smaller” sootblowing processes are now performed after shorter time intervals by means of the individual sootblowers R 1  to R 6 . By contrast, in this exemplary embodiment, the uninterrupted operating time t R  remains constant for each individual sootblower. Within a sootblowing cycle, therefore, the sootblowing process is distributed over time. Sootblowers R 1  begin at the time t 1 , sootblowers R 2  begin at the time t 2 , etc. Associated with said time distribution of the sootblowing there is also a spatial distribution within the technical plant, because the sootblowers are mounted at different locations. 
         [0053]    The effects of the incremental sootblowing on the flue gas temperature are likewise made clear on the basis of  FIG. 3   b . The flue gas temperature T now fluctuates within a significantly smaller interval [Tmax, Tmin]. A further shortening of the time intervals between the operation of the individual sootblowers would thus lead to quasi-continuous operation of the sootblowers and thus also to a quasi-continuous profile of the flue gas or steam temperature. Incremental cleaning of the heat exchanger surfaces thus reduces the extent of the thermal variations in the steam generator. The sootblowing is performed more frequently by means of the individual sootblowers or sootblower groups, and depending on demand, for shorter periods than before. In the case of small steps, quasi-continuous operation of the sootblowers is attained. If always one individual sootblower of the entirety of the sootblowers of the plant is in operation, this can also be referred to as continuous operation. The sootblower regulation can advantageously be integrated into the temperature regulation of the boiler. An automatic activation of individual sootblowers always takes place with consideration being given to plant conditions. Ultimately, the invention permits very fine control of the steam temperatures within the boiler and in the heat exchanger region, both from a time aspect and also from a spatial aspect. 
         [0054]    By means of sootblower optimization, it is possible to compensate thermal imbalances within the heat exchanger system.  FIG. 4  illustrates, in the manner of a sketch, two tracts ST 1  and ST 2  of a heat exchanger, for example of the reheater. As a result of the different soot deposits RA 1  and RA 2  on the pipe systems of the individual tracts, there is a thermal imbalance, that is to say different temperatures T 1  and T 2  prevail at the outlets of the two parallel tracts. The sootblowing should now be carried out where the steam temperature is too low by comparison. 
         [0055]      FIG. 5  illustrates, by way of an example, an embodiment of a controller of a sootblower device in the form of a block circuit diagram. The overall system of the sootblowers RBGS is connected to individual sootblower groups RBG 1  to RBGN and controls these in accordance with the sootblowing algorithm according to the invention. For this purpose, all of the units are connected to a monitoring logic module which in turn is connected to an item of software which comprises an optimization program OP as claimed in one of the claims. 
         [0056]    According to the invention, individual sootblowers or subgroups of sootblowers RBG 1  to RBGN are formed which, altogether, clean individually identifiable heat exchangers and are thus divided such that an individual cleaning operation changes the overall heat transfer of the heat exchanger only slightly. Through measurement of the thermal states and the period of uninterrupted operation of each individual blower or each subgroup, and through automatic cycle control, the fouling of the individual heat exchangers is controlled such that, in steady-state operation of the boiler, the heat absorption by the individual regions can be regulated in the fine range. 
         [0057]    Control variables of the method according to the invention are the times at which the individual sootblowers or subgroups are activated. From these, it is possible to determine both the periods of uninterrupted operation of the individual sootblowers and also the average of the sootblower groups which are assigned to a certain heat exchanger. 
         [0058]    Input variables of the method are the sensor data regarding the temperatures of the water vapor and flue gas (see  FIG. 2 ), the mass flows thereof, and also injection rates of cooling water into fresh steam and reheated steam. Heat balances, heat transfer coefficients and thus the fouling of the individual boiler regions are determined from said variables. 
         [0059]    For the control of the average period of uninterrupted operation of the individual blower groups, firstly the fouling and secondly the steam temperatures, thermal imbalances and likewise injection rates of the fresh steam and of the reheater steam are measured. By measurement of the uninterrupted operating time of the individual sootblowers, subgroups for the next cleaning cycle, and the sootblowing time for these, are selected in a targeted manner. It is the case for all of the heat exchangers that thermal imbalances are always equalized by means of the sootblowing. For the evaporator region, primarily the control of the injection rate of the fresh steam plays a significant role. It must be ensured that, in the case of the superheater, the injection valve position for the fresh steam is in the control range, and the setpoint temperature of the steam is attained. In the reheater region, the injection rate of the fresh steam should tend to zero. In the case of the economizer, it must be taken into consideration that waste gas loss and blowing outlay are balanced. Fouling of the regenerative air preheater will influence the heat balance only insignificantly. What is important here is the avoidance of a deposition between the surfaces, which cannot be reached and eliminated by steam blowers. Therefore, cleaning is performed cyclically in said region and the pressure loss is observed, wherein sootblowing is performed immediately upon the onset of a pressure loss increase. 
         [0060]    In any case, for all of the sootblowers, monitoring is performed to ensure adherence to a minimum cleaning action. This is intended to prevent the formation of conglomerates which are no longer removable or which are dangerously large. On the other hand, if the average cleaning cycle of a heat exchanger becomes very short, the region is defined as being “clean”. Further sootblowing then takes place only when new relevant fouling is identified. Repeated cleaning of clean regions, which causes surface damage, is thus effectively prevented. At the same time, the present heat transfer for the presently clean state can always be newly defined (learned) again, and a corresponding degree of fouling for ongoing operation deter mined from this.