Burner Controller The invention relates to a method for controlling the fuel supply to burners of a burner group, preferably to the burner group of a large-scale industrial plant, in particular a pelletizing plant for example with a traveling grate firing machine, in which several burner groups are disposed, to which the control method according to the invention is to be applied. The invention furthermore relates to a burn- er controller equipped for carrying out this method and to a pelletizing plant with this burner controller. In the method proposed according to the invention the temperature in the burner group is determined as control variable, and in dependence on the control deviation of the temperature determined for the burner group (control variable) to a specified setpoint temperature (setpoint) the fuel supply to the plurality of burners of the burner group is specified as correcting variable.

Such control methods or burner controllers can be used for example in a pelletizing plant, which in WO 96/32510 A1 is described in detail in quite a par- ticular embodiment. The present invention for example relates to the firing zone of the continuous furnace, which includes a plurality of burners arranged in series to the right and left of a traveling grate, which are supplied with fuel via a fuel supply and heat up pellets applied on the traveling grate. The burners provided in a firing zone mostly are controlled via a temperature controller, wherein the flow of the fuel to the respective burners usually is adjusted or controlled via the mean value of all burners present in the firing zone or group. This leads to the fact that all burners in the firing zone are operated with the same fuel quantity and that the burner temperature in the firing zone mostly is not uniformly distributed. Thus, in most cases another temperature exists at the end of a firing zone than at the beginning of the firing zone.

It is the object of the present invention to achieve a better heat distribution within the burner group.

In accordance with the invention, this object is solved with a method according to claim 1 , a burner controller according to claim 9 and a pelletizing plant according to claim 10.

In the method proposed according to the invention it is provided that the controller is formed as temperature-to-flow cascade controller with a temperature master controller for all burners of the burner group and a plurality of fuel supply slave controllers for one individual burner each or for one burner subgroup each of the entire burner group. Preferably, one fuel supply slave controller each is provided for each burner and/or for each burner subgroup of the entire burner group.

The temperature master controller specifies a mean fuel supply as correcting variable for each of the burners of the burner group, i.e. a common mean fuel supply for all burners. Each of the fuel supply slave controllers provided downstream of the temperature master controller in accordance with the invention uses at least one disturbance variable associated to the respective burner and/or the respective burner subgroup, in order to take account of a correction of the mean fuel supply to the individual burner or the burner subgroup. In particular, it is provided that the or a fuel supply slave controller specifies the corrected fuel supply to the respective burner or the respective burner subgroup as setpoint or reference variable. The actual fuel supply, which is measured according to the invention or detected otherwise, is provided as control variable of the fuel supply slave controller, which is adjusted or regulated to the setpoint / the reference variable.

In the cascade control proposed according to the invention, the temperature master controller is a reference controller. The fuel supply slave controllers are follow-up controllers provided downstream of the reference controller. A characteristic of this cascade control consists in that the output or correcting variable of the temperature master controller is the common mean fuel supply for each, i.e. all burners of the burner group. This output or correcting variable of the master temperature controller takes account of the temperature existing in the burner group, in particular a mean temperature or a maximum temperature, and specifies the fuel required on average in the burner group as mean fuel supply, in order to adjust the desired setpoint temperature in the burner group. The fuel supply corrected or influenced by the disturbance variable in connection with the fuel supply slave control for the individual burner or the burner subgroup, which are actuated by the slave controller, then forms the setpoint or the reference variable of this fuel supply slave control, which adjusts or specifies the actual fuel supply to each individual burner or the respectively selected burner subgroup. In accordance with the invention, there are provided several, at least two, fuel supply slave controllers.

Thus, since there are several fuel supply slave controllers for the burners of the burner group, as an individual controller for each burner or as a combined con- trailer for a burner subgroup, the heat distribution within the burner group thus is adapted. This leads to a particularly advantageous equal distribution of the temperature within the burner group and often also helps to save fuel, because to achieve a mean temperature within the burner group an improved efficiency of the burners combined in the burner group is achieved due to the optimized heat distribution. In an ideal burner group, in which each burner would provide the identical heat contribution to the total temperature of the burner group, a suitable control would be achieved already with the master temperature controller, in order to achieve the desired temperature in the burner group with an equal distribution of the heat within the burner group by adjusting the same fuel quantity supplied to all burners. In reality, however, the conditions for the individual burners are not identical. Decisive influences are obtained by the arrangement of the burners in the burner group, because for example burners located at the edge of the burn- er group generally give off more dissipated heat to the outside by radiation than burners located in the interior of the burner group. Further differences can be obtained by constructional conditions of the burner group, for example by a different quality of insulation in the edge region or the flow influence of a wind box shape provided in the region of the burner group. All this leads to the fact that the temperature in the burner group is not maintained exactly and in particular a non-uniform temperature distribution exists in the burner group, when all burners are supplied with the same fuel quantity.

Therefore, it is provided in accordance with the invention to use at least one disturbance variable in the fuel supply slave controller provided downstream of the temperature master controllers, which adjusts or specifies the fuel supply to an individual burner or a burner subgroup which preferably has similar conditions for all burners combined in the burner subgroup. Disturbance variable associated to the burners is understood to be a variable associated to a burner or a selected subgroup of burners, which indicates the deviations in the temperature distribution within the burner group for the respectively selected burner or the respectively selected burner subgroup.

In a particularly preferred embodiment of the present invention it is provided that the mean fuel supply specified by the temperature master controller as starting or correcting variable for an individual burner or a burner subgroup is influenced in dependence on the disturbance variable, in particular by correction factors formed in dependence on the disturbance variable, which are applied to the mean fuel supply, i.e. which for example are multiplied by the value of the mean fuel supply, in order to achieve an individual fuel supply for the and/or each individual burner and/or a/each burner subgroup.

Subsequently, reference in part is only made to one burner or each burner, and in accordance with the invention burner can be understood to be both an indi- vidual burner and a burner subgroup which combines several burners of the entire burner group. This also applies in connection with the temperature measurements explained below.

According to a particularly preferred application, the determined temperature in the burner group used as control variable for the temperature master controller and/or the at least one disturbance variable can be determined from temperature measurements in particular associated to each burner. Such temperature measurements can easily be performed by means of temperature sensors in the range of action of each burner or a burner subgroup. The determined tempera- ture in particular can be formed as maximum value of the temperature values measured for each burner or as maximum value of all temperature values measured at all in the burner group. A basis for the determination of a disturbance variable according to the invention can be the difference of the temperature associated to a burner to a temperature associated to another burner or the determined temperature used as control variable for the temperature master controller. In the case of two burners each arranged as burner pair to the right and left in several rows in direction of movement with respect to a preferred direction, for example a direction of movement of a material to be heated by the burner group, a first disturbance variable each can be determined for a burner pair (i.e. the right and the left burner) and a further disturbance variable each can be determined for all burners arranged to the right and left (i.e. for all burners arranged to the right and left in the several rows in the burner groups).

For the first disturbance variable for example the mean value of the temperature values, which are associated to the respective burners of the burner pair, is determined and the same for example is compared with the temperature determined as control variable of the temperature master controller. From the difference, for example via a functional dependence or a value table, a suitable correction factor Kn is determined for the Nth burner pair.

For the second disturbance variable, the mean value of all temperature values associated to the right and the left burners each can analogously be determined as right and left mean value. These right and left mean values can be compared with the determined temperature serving as control variable of the temperature master controller, the total mean value formed from the right and the left mean value, or the like. From the differences resulting therefrom, for example via a functional dependence or a value table, suitable correction factors KL and KR are determined, which each are applied to the (all) left and right burners, respectively. The above-described concrete method relates to a particularly preferred burner arrangement, to which the invention however is not limited.

To be able to also consider different influences, it is provided in accordance with the invention that a plurality of disturbance variables for each burner or each burner subgroup can act on the mean fuel supply, i.e. the correcting variable of the temperature master controller and the reference variable of the respective fuel supply slave controller. The various disturbance variables can act on the mean fuel supply with equal priority or with a suitable weighting.

In a particularly preferred and easily realizable embodiment, correction factors therefore can be derived from the disturbance variables, which are multiplied by the mean fuel supply. In accordance with the invention, correction factors of individual disturbance variables and/or combined, i.e. in particular multiplied by each other, correction factors of different disturbance variables can be limited to a specified range of values, in order to avoid extreme deflections. A suitable range of values for a correction factor for example can be values from 0.5 to 2.0, which limit a change in the mean fuel supply to half or twice the amount.

For the further protection of the burner system it can be provided that the corrected fuel supply, which is obtained after using the disturbance variable(s) for the fuel supply slave controller, is limited for each burner or each burner subgroup to a maximum fuel supply which can be firmly specified or for example be fixed in a parameterizable manner. It thereby is avoided that the burner system is operated outside the intended design values. According to a preferred embodiment, the burners of the burner group whose fuel supply is to be controlled by the proposed method can be arranged in a matrix form in several rows and/or columns, wherein disturbance variables each are determined for each row and/or each column of the burners. A preferred configuration is obtained with two columns and several rows, so that right and left burners each are arranged as burner pairs in several rows one after the other. This arrangement has already been described in detail. Such arrangement of the burners and formation of the disturbance variables also can be used particularly preferably in pelletizing plants in which material to be heated (pellets on a grate carriage or similar transporting means) is passed through a burner group of a furnace of a traveling grate firing machine in column direction.

A flexible adjustment of the controller can be achieved when the specified set- point temperature of the temperature master controller is specifiable and/or variable, wherein the setpoint rate of change preferably is limited, in order to protect the burner system and achieve longer service lives of the refractory lining in the burner system. An expedient rate of change for example can be set to up to 100 °C per hour, wherein a larger change of the specified setpoint preferably is automatically decreased to this limit value by the controller. Correspondingly, the invention also relates to a burner for controlling the fuel supply to several burners of a burner group, preferably of a large-scale industrial plant, in particular a pelletizing plant for example with a traveling grate firing machine, in which several burner groups are disposed, to which the control method according to the invention is to be applied. The burner controller in- eludes at least one port for a temperature sensor and at least one port for a flow sensor, in particular for measuring the fuel supply, and to a calculating unit.

According to the invention, the above-described method or parts thereof are implemented in the calculating unit in particular by suitable software program means for controlling the fuel supply. According to the invention, the burner controller thus is equipped for carrying out the implemented method. The temperature master controllers and fuel supply slave controllers to be provided can be accommodated in a controller housing or in several different controller housings.

As a particularly preferred application, the invention also relates to a pelletizing plant with a traveling grate firing machine with a plurality of burners, preferably arranged in matrix form, and a burner controller for controlling the fuel supply to the plurality of burners of a burner group, which is formed as described above and equipped for carrying out the above-described method or parts thereof.

Further advantages, features and possible applications of the present invention can also be taken from the following description of an exemplary embodiment and the drawing. All features described and/or illustrated form the subject-matter of the present invention per se or in any combination, also independent of their inclusion in the claims or their back-references.

The only Figure 1 shows a burner group 1 according to the invention, as it is used in burner systems of large-scale industrial pelletizing plants. In the burner group 1 , burners 2 are arranged in two columns R, L and N rows, wherein Figure 1 shows N=3 rows. Each burner 2 is supplied with fuel via a fuel supply conduit 3 and a preferably electromotively or pneumatically operated regulating valve 4 arranged in the fuel supply conduit 3.

In the transport direction indicated by the arrow 5, the material to be heated in the burner group 1 is transported on a traveling grate or similar transporting means over the burners 2, below the burners 2 or more generally past the burners 2, with the transport direction coinciding with the column direction of the burner arrangement. In particular, the material to be heated can be pellets, which in a pelletizing plant are guided through a suitable burner furnace with one or more burner groups 1 .

In the combustion space formed above the burners 2, a certain temperature mostly must be adjusted, in order to achieve the desired effect. This is achieved by means of a temperature controller which adjusts the fuel supply to the individual burners 2 corresponding to the desired temperature. For this purpose, the temperature in the combustion space is repeatedly detected by temperature sensors 6, which each are associated to a burner 2 in the combustion chamber, namely each in exactly one region associated to a burner 2. The temperature values TY determined by the temperature sensors 6 are supplied to a maximum value formation 7, which forms the maximum temperature value of the temperature values TY measured in the burner group and supplies the same as control variable to a temperature master controller 8 (TIC). In the temperature master controller 8, the control difference between the maximum temperature value TY and the temperature setpoint TSP specified for the temperature master controller is formed. To compensate a possible control difference, the temperature master controller 8 specifies a mean fuel supply XAVG as correcting value, with which each burner 2 would have to be supplied, if it would provide the same heat contribution to the total temperature in the combustion chamber corresponding to an ideal case.

In practice, however, this is not the case. Observations have shown that the mean temperature both of the rows and of the columns of the individual burners 2 in the burner arrangement is different. A preferred embodiment of the invention therefore proposes to detect disturbance variables associated to the rows and columns of the burners 2 in the burner arrangement and provide corresponding correction factors, in order to correct the mean fuel supply specified as correcting variable of the temperature master controller.

A first disturbance variable relates to the rows of burners 2 in the burner arrangement, i.e. in the illustrated drawing each of the burner pairs (1 L, 1 R), (2L, 2R) and (3L, 3R). For each of these burner pairs, which for better clarity is not shown in the drawing, the mean temperature each of the temperature sensors associated to the respective burners 2 of a burner pair is formed. From the deviations of these mean temperatures of the various burner pairs (1 L, 1 R), (2L, 2R) and (3L, 3R) from each other, correction factors K1 , K2 and K3 are formed with the objective to adapt the mean temperatures of all N burner pairs in the burner group 1 to each other.

For example, this can be effected such that in addition the mean value of all mean values of the individual burner pairs is formed and each individual mean value of a burner pair is compared with this total mean value. Via a suitable calculation rule or value table, a correction factor KN associated to each burner pair can be determined from this comparison or difference of these values. In Figure 1 , these are the correction factors K1 , K2 and K3, which each are applied to the mean fuel supply XAVG, i.e. multiplied by this value.

A further correction is made for the columns L, R. For this purpose, the meas- ured temperature values of the temperature sensors associated to the right burners 2 (1 R, 2R, 3R) and the left burners 2 (1 L, 2L, 3L) each are determined in an average formation 9. The values present as right and left mean temperature value TYR and TYL are converted into correction factors KL and KR, for example by comparison with their averages (similar to the above-described case), which are applied to the mean fuel supply corrected already by the correction factors K1 , K2 and K3, in order to generate a corrected fuel supply X for each burner 2. Alternatively or in addition, experience values deposited in suitable tables can also be used, for example. In general, disturbance variables for the burners 2 of the burner arrangement thus are considered column by column and line by line, from which column- and line-dependent correction factors K each are obtained, with which the mean fuel supply XAVG supplied by the temperature master controller 8 is corrected, in order to determine a corrected fuel supply X for each burner 2 of the burner group 1 . This corrected fuel supply X is supplied as setpoint for the fuel supply to a fuel supply slave controller 10 associated to each burner 2, which compares the fuel supply setpoint FSP with the currently measured fuel supply to the burner 2 and adjusts or regulates the regulating valves 4 of the burners 2 to the fuel supply setpoint FSP by means of a correcting variable of the fuel supply slave controller 10.

By means of this control, a more uniform temperature profile within each burner group 1 of a burner system can be achieved easily and reliably via the column- and linewise correction. This leads to a more uniform burn-through of the mate- rial to be heated, which in particular can be pellets. As side effect, the individual control of the individual burners also leads to a reduced consumption of fuel.

The fuel supply slave controller 10 thus controls the flow of fuel in the fuel sup- ply conduit 3 and therefore is also referred to as fuel flow slave controller.

To achieve a power limitation for each burner 2, it is furthermore provided that in a maximum value formation 1 1 the corrected fuel supply X is compared with a maximum fuel supply FMAX, which as specified can maximally be supplied to a burner 2. If the corrected fuel supply X exceeds the maximum fuel supply FMAX, the fuel supply setpoint FSP thus is limited to the maximum fuel supply FMAX.

Similarly, for the temperature setpoint TSP which is supplied to the temperature master controller 8, a limitation of the setpoint rate of change to a certain value, for example 100 °C per hour, is provided, which is adjusted by a corresponding limiter 12. In this way, longer service lives of the refractory lining can be achieved, which ages more quickly with rising temperature gradient.

In general, the temperature-to-flow cascade controller proposed according to the invention thus provides for a better heat distribution in the combustion space of a burner group 1 , which also leads to a saving of fuel on the whole. Due to the optional limitation of the maximum fuel supply and the setpoint rate of change, plant-specific parameters can be taken into account and/or the service life of the plant can be prolonged. List of Reference Numerals:

1 burner group

2 burner

3 fuel supply conduit

4 regulating valve

5 transport direction of the material to be heated

6 temperature sensor

7 maximum value formation

8 temperature master controller

9 average formation

10 fuel supply slave controller

1 1 minimum value formation

12 limiter

R, L column of the burner arrangement

n=1 ,2,3 row of the burner arrangement

TT temperature in the region of a burner, disturbance variable

TYL/R temperature of the burners arranged on the left and right, disturbance variable

TY temperature, temperature value of the burner group

TSP setpoint temperature, temperature setpoint

XAVG mean fuel supply (actuating variable of the temperature master controller)

K correction factors

X corrected fuel supply

FSP fuel supply setpoint

FMAX maximum fuel supply