The invention relates to a reactor control unit for controlling a plurality of parallel reactors each being fed with at least one reactant with a time depending feed rate, wherein the control unit is configured to communicate at least one control variable to the plurality of reactors, in particular to communicate a set of control variables to each of the plurality of reactors. The invention further relates to a reactor sys tem comprising a plurality of parallel reactors and a control unit for controlling the plurality of parallel reactors as well as a method for controlling a plurality of par allel reactors.

Reactors, particularly fluidized bed reactors like e.g. roasting reactors, have to be controlled by adjusting a high plurality of parameters. The parameters of the re actors typically include several input material streams like feed material, air, oxy gen, additional gases, water and others. The reactors also have output material streams like the product material and exhaust gases. Additionally the reactors have internal parameters like the process temperature or the composition of the various material streams, especially the output stream of the product. At least some of these parameters cannot be directly controlled and/or depend highly on other parameters of the reactor. Also, there are a number of inner boundary con ditions, like a maximum or a minimum value or a specific ratio to another param eter. Control units for the adjustable parameters of a single reactor are well es tablished

However, often a number of reactors form a combined reactor system, sharing some previous or downward system, like feeding lines. Hence, the available feed has to be shared by the reactors. The same is true for output streams, especially the exhaust gas, which is typically treated in a combined exhaust after-treatment system for all the reactors. As it is very seldom that all reactors are operated at their maximum, it is common to undersize the supply lines and/or the exhaust system. This means, the supply lines cannot supply all reactors running at full capacity at the same time and/or the exhaust after-treatment system cannot handle the exhaust gases of all reac- tors running at full capacity at the same time. Therefore, besides the individual requirements the reactor has to fulfill, also global requirements exist that have to be fulfilled by the combination of the reactors. This renders the control of a reactor system comprising a plurality of reactors highly complicated and thus typically such a reactor system does not use all the available resources and does thus not work at the highest possible efficiency.

It is, therefore, the object of the present invention to improve the efficiency of reactor systems with a plurality of individual reactors. The object is solved by a reactor control unit for controlling a plurality of parallel reactors according to claim 1 .

The reactor control unit is configured to communicate at least one control variable to the reactors, wherein the at least one control variable comprises a feed target, which is a threshold value regarding the specific feed rate of the reactor. In par ticular, the control unit sends one corresponding feed target to every controlled reactor, wherein the reactor can adjust its own actual feed under the condition that it is smaller than the assigned feed target. The feed of the reactor is the amount of input material, put in the reactor, converted therein and output as a product. The feed is therefore more or less proportional to the throughput and the amount of material output of the reactor. The reactor typically tries to maximize its throughput and thus its feed and therefore tries to reach the assigned feed target. The control unit comprises a total feed target, which can be a physical restriction of an apparatus arranged subsequently to the plurality of reactors or can be ad justed by a person due to a specific demand, and indicates the total desired feed, which determines the total throughput of the reactors. The total feed target is stored in the control unit as an upper boundary value. The control unit is config ured to set the respective feed targets of the plurality of reactors such that the sum of the feed targets of the plurality of reactors is lower or equal to the total feed target. Further, the control unit is configured to read the actual feeds from the plurality of reactors, e.g. the feed values as adjusted by the individual reactors, for example by receiving the values from subordinate control units responsible for the individual reactors or access a corresponding memory of such a control unit. Alternatively, the control unit can have direct access to respective sensor units.

Depending on the actual feed rate and the feed target of the reactor, the control unit interprets the reactor as inquiring a higher, the same or a lower feed target. It then decreases the feed target for a reactor that inquires a lower feed target and increases the feed target for a reactor that inquires a higher feed target under the condition that the sum of the feed targets does not exceed the total feed target. Thus, the control unit shifts the several feed targets between the plurality of reac tors, depending on the actual use of the reactors in order to maximize the total throughput of the system of parallel reactors. The control unit preferably con stantly repeats the steps of reading the actual feeds of the reactors and comparing them to the feed targets, determining which reactors inquire a higher and/or lower threshold and increasing and/or decreasing the respective thresholds, when pos sible. The steps can be repeated by the control unit as fast as possible or with a predetermined periodicity.

According to a preferred embodiment, the control unit is configured to calculate a difference between the feed target and the actual feed of each reactor, which is adjusted by the reactor itself. When the calculated difference is larger than a first difference stored in the control unit as a first feed-delta, the reactor seems not to be able to reach the assigned target value. The control unit interprets this as the reactor inquiring for a lower feed target. When the reactor actually reaches its target value or nearly reaches its target value, the difference between the feed target and the feed is smaller than a second feed-delta. The second feed delta can be chosen in the order of magnitude of normal fluctuations of the feed and is thus much smaller than the first feed-delta. The control unit interprets this as the reactor inquiring for higher feed target. Hence, the control unit shifts the target feed from a reactor that was not able to reach its target value to a reactor, which seems to be able to reach an even higher feed. This results in a higher total feed and, thus, in an increased overall throughput.

According to an alternative embodiment, one of the control variables is a lower threshold value regarding the feed of the reactor. The control unit thus sends the feed target as a higher threshold and additionally a lower threshold to the reactor. The reactor has therefore an upper and a lower boundary within which it can ad just its feed. The control unit is configured to calculate a difference between the lower threshold value and the actual feed of each reactor and interpret the reactor as inquiring for lower feed target when a difference between the feed and the lower threshold value regarding the feed of the reactor is smaller than a first feed- delta.

The control unit is further configured to calculate the difference between the feed target and the actual feed. When the feed lies near the feed target, where a dif ference between the feed target and the feed is smaller than a second difference stored in the control unit as a second feed-delta, the reactor is interpreted as inquiring for higher feed target. The sum of the first and second feed-delta can be smaller than the difference between the feed target and the lower threshold value. Thus, the area between the upper and lower threshold is divided in three parts. An upper part, interpreted as inquiring for higher threshold, a middle part as inquiring for the same threshold and a bottom part as inquiring for a lower thresh old. This two-threshold principle is described later regarding the air stream and oxygen stream in more detail and is also applicable with all its details to the feed target and the lower threshold value regarding the feed of the reactor.

According to a preferred embodiment the feed targets of the reactors that inquired a higher feed target are increased such that the sum of all feed targets equals the total feed target. Hence, the free feed target, defined as the difference between the sum of all feed targets and the total feed target, is completely distributed to the reactors that inquired a higher feed target. The control unit can decrease the feed target of the reactors that inquired a lower threshold irrespective of other reactors inquiring for a higher target. This will lead to unassigned or free target. In such a case, the sum of all feed targets is lower than the total feed target. Alternatively, the control unit can be configured to decrease the feed target of the reactors that inquired a lower threshold, only, when at least one other reactor inquires for a higher feed target.

According to a further preferred embodiment the feed targets of the reactors that inquired a higher feed target re increased evenly. Therefore, the free target is distributed evenly to the reactors that inquired a higher feed target. Hence, every reactor that inquired a higher feed target gets the same part of the freed target of a reactor that inquired a lower feed target. The reactors are therefore handled equally, preventing that one reactor has disadvantages due to its position in the control system.

According to a preferred embodiment the feed target is increased and/or de creased by a fixed step and/or rate. The control unit can comprise a predeter mined decrease step for a reactor. The decrease step can be the same or an individual decrease step for every reactor. When the target feed of a reactor that inquired a lower feed target is decreased, the target feed is decreased by this specific step. Likewise, the control unit can comprise a predetermined increase step for a reactor. The increase step can also be the same or an individual in crease step for every reactor. When the target feed of a reactor that inquired a higher feed target is increased, the target feed is increased by this specific step. When the free target is not sufficient to increase the feed target of all reactors that inquired a higher feed target by the predetermined step, the control unit can in crease the feed target of as many reactors as possible. Alternatively, the feed target of all reactors that inquired a higher feed target can be increased by a re duced step, such that the free feed target is distributed completely. It is also pos sible to increase and decrease the feed target by a specific rate, meaning by a predetermined value per unit of time. Thereby the speed with which the feed tar get values change does not depend on the control units calculating power, which affects the repetition rate with which the control unit checks the reactors and sends new feed targets to the reactors. In the same way as above, the control unit can comprise an increase and/or a decrease rate for every individual reactor or a global increase and/or decrease rate for all reactors.

According to a preferred embodiment of the invention, the control unit comprises a maximum feed for at least one of the plurality of reactors, wherein the control unit is configured to set the feed target of the reactor lower than the maximum feed of the reactor. Hence, the control unit knows the maximum feed values of the plurality of the reactors and sets the feed target accordingly such that it does not exceed the maximum feed of the reactor. Additionally the reactor can be con figured to not accept an increase of its feed target that would exceed its maximum feed. Alternatively, the control unit can read the maximum feed from the reactor and set the target feed accordingly, or the control unit can have no information about the maximum feed and set the target feed independently and the reactor itself sets its target value to the maximum feed when a target feed higher than the maximum feed is received. Additionally or alternatively, the control unit can comprise a minimum feed for at least one of the plurality of reactors, wherein the control unit is configured to set the feed target of the reactor higher than the minimum feed of the reactor. Hence, the control unit knows the minimum feed values of the plurality of reactors and sets the feed target accordingly such that it does not fall below the minimum feed target. Additionally, the reactor can be configured to not accept a decrease of its feed target that would fall below its minimum feed. Alternatively, the control unit can read the minimum feed from the reactor and set the target feed accordingly, or the control unit can have no information about the minimum feed and set the target feed independently and the reactor itself sets its target value to the mini mum feed when a target feed lower than the minimum feed is received.

According to a preferred embodiment, the at least one control variable comprises a lower and a higher air stream threshold. Hence, the control unit sends a pair of corresponding air stream thresholds preferably to every controlled reactor, wherein the reactor can adjust its own actual air stream under the condition that it stays within the received thresholds. The control unit comprises a total air stream value, wherein the sum of the higher air stream thresholds of the plurality of reactors is lower or equal to the total air stream value. The control unit is further configured to read an actual air stream from the plurality of reactors by receiving the actual air stream values from a single reactor control unit, accessing a corre sponding memory, having access to respective sensors or receiving the values in another way. The control unit then compares the actual air stream with the lower and higher air stream thresholds and interprets a difference between the higher air stream threshold and the actual air stream, which is smaller than a first air- delta as an inquiry for increase of the air stream thresholds. Furthermore, the control unit interprets a difference between the lower air stream threshold and the actual air stream, which is smaller than a second air-delta as an inquiry for a decrease of the air stream thresholds. The sum of the first and second air-delta can be smaller than the difference be tween the higher air stream threshold and the lower air stream threshold. Thus, the area between the upper and lower threshold is divided in three parts. An upper part, interpreted as inquiring for higher threshold, a middle part as inquiring for the same threshold and a bottom part as inquiring for a lower threshold. Hence, when the reactor adjusts its air stream such that it lies in the upper part of the allowable field, the control unit interprets it as a request for an increase. On the other hand, when the reactor adjusts its air stream such that it lies in the lower part of the allowable field, the control unit interprets it as a request for a decrease.

The control unit then decreases the lower and/or higher air stream thresholds for a reactor that inquired a lower air stream threshold and increases the lower and/or higher air stream thresholds for a reactor that inquired a higher air stream thresh- old under the condition that the total air stream value is not exceeded.

According to a further preferred embodiment, the total air stream value depends on a maximum gas throughput of a joint exhaust gas after-treatment system, which is arranged after the plurality of reactors. The total air stream value can thus be a fixed value calculated once from the maximum gas throughput of the joint exhaust gas after-treatment system, for example the maximum gas through put can be used directly or a specific percentage thereof. Alternatively, the total air stream value is further depending on an oxygen stream value of the plurality of reactors. Since the air stream and the oxygen stream both add to the total exhaust volume, the total air stream value can be constantly adapted to the vary ing oxygen stream. It could further be adapted based on the composition of the feed material, in particular to the percentage of components that result in larger exhaust volumes. According to another preferred embodiment, the control unit comprises a maxi mum air stream for at least one of the plurality of reactors, wherein the control unit is configured to set the lower and/or higher air stream thresholds of the reac tor lower than the maximum air stream of the reactor. Hence, the control unit knows every maximum air stream of the reactors and sets the air stream thresh olds accordingly such that the higher air stream threshold does not exceed the maximum air stream. Additionally the reactor can be configured to not accept an increase of its air stream thresholds that would exceed its maximum air stream. Alternatively, the control unit can read the maximum air stream from the reactor and set the air stream thresholds accordingly, or the control unit can have no information about the maximum air stream and set the air stream thresholds in dependently and the reactor itself sets its air stream thresholds to the highest possible values when a value with a higher air stream threshold that exceeds the maximum threshold is received.

Additionally or alternatively, the control unit can comprise a minimum air stream for at least one of the plurality of reactors, wherein lower and/or higher air stream thresholds of the reactor are set to be higher than the minimum air stream of the reactor. Hence, the control unit knows every minimum air stream of the reactors and sets the air stream thresholds accordingly such that the lower air stream threshold does not fall below the minimum air stream. Additionally, the reactor can be configured to not accept a decrease of its air stream thresholds below its minimum air stream.

Alternatively, the control unit can read the minimum air stream from the reactor and set the air stream thresholds accordingly, or the control unit can have no information about the minimum air stream and set the air stream thresholds inde pendently and the reactor itself sets its air stream thresholds to the lowest values possible, when a lower air stream threshold lower than the minimum air stream is received.

According to a preferred embodiment of the invention, the lower air stream thresh old has a predetermined difference to the higher air stream threshold and the two values are therefore shifted in parallel, when increased or decreased. Alterna tively or additionally higher and lower air stream threshold can be linked by a predetermined factor preferably between 0.6 and 0.95, even more preferred be tween 0.7 and 0.9, in particular 0.8. Both of these alternatives can be used in turns. In particular the control unit can switch from a predetermined difference to a factor and vice versa, depending on operating parameters of the reactor. There fore, only one of the values needs to be handled by the control unit whereas the other value is simply updated accordingly, reducing the workload on the control unit.

According to another preferred embodiment of the invention, the at least one con trol variable comprises a lower and a higher oxygen stream threshold. Hence, the control unit is configured to send a pair of corresponding oxygen stream thresh olds preferably to every controlled reactor, wherein the reactor can adjust its own actual oxygen stream under the condition that it stays within the received thresh olds. The control unit comprises a total oxygen stream value, wherein the sum of the higher oxygen stream thresholds of the plurality of reactors is lower or equal to the total oxygen stream value. The total oxygen stream value can be the max imum possible oxygen supply of an oxygen supply unit common for the plurality of reactors. The control unit is configured to read the actual oxygen streams from the plurality of reactors, compare the actual oxygen streams with the lower and higher oxygen stream thresholds, interpret a difference between the higher oxy gen stream threshold and the actual oxygen stream which is smaller than a first oxygen-delta as an inquiry for increase of the oxygen stream thresholds and in terpret a difference between the lower oxygen stream threshold and the actual oxygen stream which is smaller than a second oxygen-delta as an inquiry for de crease of the oxygen stream thresholds. The sum of first and second oxygen delta can be smaller than the difference between lower and higher oxygen stream threshold. The control unit is further configured to decrease the lower and/or higher oxygen stream thresholds for a reactor that inquired a lower oxygen stream and increase the lower and/or higher oxygen stream thresholds for a reactor that inquired a higher threshold under the condition that the total oxygen stream value is not exceeded.

According to another preferred embodiment, the control unit comprises a maxi mum oxygen stream for at least one of the plurality of reactors, wherein the control unit is configured to set the lower and/or higher oxygen stream thresholds of the reactor lower than the maximum oxygen stream of the reactor. Hence, the control unit knows every maximum oxygen stream of the reactors and sets the oxygen stream thresholds accordingly such that the higher oxygen stream threshold does not exceed the maximum oxygen stream. Additionally the reactor can be config ured to not accept an increase of its oxygen stream thresholds that would exceed its maximum oxygen stream.

Alternatively, the control unit can read the maximum oxygen stream from the re actor and set the oxygen stream thresholds accordingly, or the control unit can have no information about the maximum oxygen stream and set the oxygen stream thresholds independently and the reactor itself sets its oxygen stream thresholds to the highest possible values when a value with a higher oxygen stream threshold that exceeds the maximum oxygen stream is received.

Additionally or alternatively, the control unit can comprise a minimum oxygen stream for at least one of the plurality of reactors, wherein lower and/or higher oxygen stream thresholds of the reactor are set to be higher than the minimum oxygen stream of the reactor. Hence, the control unit knows every minimum oxygen stream of the reactors and sets the oxygen stream thresholds accordingly such that the lower oxygen stream threshold does not fall below the minimum oxygen stream. Additionally, the reactor can be configured to not accept a de crease of its oxygen stream thresholds below its minimum oxygen stream. Alter natively, the control unit can read the minimum oxygen stream from the reactor and set the oxygen stream thresholds accordingly, or the control unit can have no information about the minimum oxygen stream and set the oxygen stream thresh olds independently and the reactor itself sets its oxygen stream thresholds to the lowest values possible, when a lower oxygen stream threshold lower than the minimum oxygen stream is received.

The start-up procedure can be handled such that the reactors are already running, when their control is shifted to the inventive control unit. The control unit can read the parameters like the actual feed, air stream and oxygen stream and accept the control over the reactors only, when the parameters are within required margins like not exceeding the total feed target, the total air stream and/or the total oxygen stream.

The object is also solved by a reactor system comprising a plurality of parallel reactors and a control unit communicating control variables to the reactors, wherein the control unit is a control unit as described above. The reactors are self-controlled and adjust their parameters according to their inner boundary con ditions, like the temperature, the ratios between air, oxygen, feed, sulfur concen tration in the feed or other parameters, and the control variables received from the control unit. Preferably, the reactors are configured to maximize the feed of the reactor and thus the throughput.

According to a preferred embodiment of the inventive reactor system, at least one of the plurality of reactors, in particular all reactors are roasting reactors, in par ticular fluidized bed reactors. According to another preferred embodiment of the inventive reactor system the reactors have a joint exhaust gas after-treatment system with a maximum gas throughput lower than the sum of the maximum exhaust gas outputs of the plural ity of reactors. Hence, when all reactors would run on 100 percent, the exhaust system would be overloaded. Since the reactors are controlled by the inventive control unit, such an overload is prevented.

The object is also solved by a method for controlling a plurality of parallel reactors, wherein a control unit communicates control variables to the reactors, and one of the control variables is a feed target, which is a threshold value regarding the feed of the reactor. The plurality of reactors adjust their parameters according to their inner boundary conditions and the received control variables in order to maximize the target value. The control unit comprises a total feed target and sets the re spective feed targets for the plurality of reactors such that the sum of the feed targets of the plurality of reactors is lower or equal to the total feed target. Further, the control unit reads the actual feeds from the plurality of reactors, compares the actual feeds with the feed targets, and based on the comparison interprets the reactor as inquiring for a higher, the same or a lower feed target. The control unit then decreases the feed target for a reactor that inquired a lower threshold and increases the feed target for a reactor that inquired a higher threshold under the condition that the total feed target is not exceeded. The inventive method can comprises all the steps described above with regard to the control unit and the reactor system.

Further objectives, features, advantages and possible applications of the inven tion can also be taken from the following description of the attached drawings and the example. All features described and/or illustrated form the subject matter of the invention per se or in any combination, independent of their inclusion in the individual claims or their back-references. In the drawings:

Fig. 1 shows a schematic view of the inventive reactor system

Fig. 2 shows a graph of the feed control of three reactors of the inventive reactor system,

Fig. 3 shows a graph of the oxygen stream control of three reactors of the in ventive reactor system;

The inventive reactor system 1 shown in Fig. 1 comprises three reactors 21 , 22, 23. The first reactor 21 has a feed input 41 , which is connected to a total feed supply 4, an oxygen stream input 51 , which is connected to an oxygen supply unit 5 and an air stream input 61 , which is connected to a total air stream supply 6. Likewise, the second reactor 22 has a feed input 42, which is connected to the total feed supply 4, an oxygen stream input 52, which is connected to the oxygen supply unit 5 and an air stream input 62, which is connected to the total air stream supply 6. The third reactor 23 has a feed input 43, which is connected to the total feed supply 4, an oxygen stream input 53, which is connected to the oxygen sup ply unit 5 and an air stream input 63, which is connected to the total air stream supply 6.

First reactor 21 has an output 71 of the produced material, which adds up with the output 72 of the second reactor 22 and the output 73 of the third reactor 23 to the total output 7.

The exhaust gases 81 of the first reactor 21 , the exhaust gases 82 of the second reactor 22 and the exhaust gases 83 of the third reactor 23 are guided to a com mon exhaust after-treatment system 8. A control unit 3 controls the first reactor 21 , the second reactor 22 and the third reactor 23 by sending control variables 31 to the first reactor 21 , control variables 32 to the second reactor 22 and control variables 33 to the third reactor 23. Fur thermore, the control unit 3 reads parameters 34 of the first reactor 21 , parame ters 35 of the second reactor 22 and parameters 36 of the third reactor 23. The control variables 31 , 32 and 33 each comprise a feed target 311 , 321 , 331 , an upper airstream threshold 314, 324, 334 and lower airstream threshold 315, 325, 335 and an upper and lower oxygen threshold.

The reactors 21 , 22 and 23 adjust their feeds 21 1 , 221 , 231 freely without exceed ing the received feed targets 311 , 321 , 331 , their oxygen streams within the mar gins of the upper and lower oxygen stream threshold and their airstreams 214, 224, 234 within the margins of the upper and lower air stream thresholds 314, 315, 324, 325, 334, 335.

The control unit 3 reads an actual feed 211 , air stream 214 and oxygen stream as parameters 34 from the first reactor 21. Likewise, the control unit 3 reads the actual feeds 221 , 231 , air stream 224, 234 and oxygen stream of the second re actor 22 and third reactor as parameters 35 and 36.

The control unit 3 knows the actual demand for the total output 7, which is saved as a total feed target in the control unit 3. Furthermore, the maximum exhaust volume of the exhaust after-treatment system 8 is saved as a total air stream value in the control unit 3 and the maximum possible oxygen supply of the oxygen supply unit 5 is saved in the control unit 3 as a total oxygen stream value.

The control unit periodically performs control steps, for example every 10 sec onds, where it reads the actual feed 21 1 of the first reactor 21 as part of its pa rameters 34. The control unit 3 than checks whether the actual feed 211 lies in an upper part of the allowable feed range in that it calculates the difference of the feed target 311 and the actual feed 211 and compares the difference to a second feed-delta 312. If the difference is smaller than the second feed-delta 312, the actual feed 211 lies in an upper part of the allowable feed range and the control unit 3 interprets this as the reactor 21 inquiring a higher feed target 31 1. The control unit also checks, whether the actual feed 21 1 lies in a lower part of the allowable feed range in that it compares the calculated difference of the feed tar get 31 1 and the actual feed 211 and compares it to a first feed-delta 313. If the difference is larger than the first feed-delta 313, the actual feed 211 lies in a lower part of the allowable feed range and the control unit 3 interprets this as the reactor 21 inquiring a lower feed target 31 1. The control unit 3 performs the same control step for the second reactor 22 and the third reactor 23.

The control unit 3 than decreases the feed target 31 1 , 321 , 331 of all the reactors 21 , 22, 23 that inquired a lower feed target 311 , 321 , 331 by a decrease step, under the condition that this decrease does not result in a feed target 311 , 321 , 331 , which is lower than a minimum feed 213 of the respective reactor 21 , 22, 23. The control unit 3 than increases the feed target 311 , 321 , 331 of all the reactors 21 , 22, 23 that inquired a higher feed target 31 1 , 321 , 331 by an increase step, under the condition that this increase does not result in a feed target 311 , 321 , 331 , which is higher than a maximum feed 212 of the respective reactor 21 , 22, 23 and under the condition that the sum of the feed targets 31 1 , 321 , 331 of the reactors 21 , 22, 23 does not exceed the total feed target.

The control unit also reads the actual air stream 214 of the first reactor 21 as part of its parameters 34. The control unit 3 than checks whether the actual air stream 214 lies in an upper part of the allowable air stream range in that it calculates the difference of the upper air stream threshold 314 and the actual air stream 214 and compares the difference to a second air-delta 316. If the difference is smaller than the second air-delta 316, the actual air stream 214 lies in an upper part of the allowable air stream range and the control unit 3 interprets this as the reactor 21 inquiring higher air stream thresholds 314, 315. The control unit 3 also checks, whether the actual air stream 214 lies in a lower part of the allowable air stream range in that it calculates the difference of the lower air stream threshold 315 and the actual air stream 214 and compares the difference to a first air-delta 317. If the difference is smaller than the first air stream-delta 317, the actual air stream 214 lies in a lower part of the allowable air stream range and the control unit 3 interprets this as the reactor 21 inquiring lower air stream thresholds 314, 315. The control unit 3 performs the same step regarding the air stream for the second reactor 22 and the third reactor 23.

The control unit 3 than decreases the air stream thresholds 314, 315, 324, 325, 334, 335 of all the reactors 21 , 22, 23 that inquired lower air stream thresholds 314, 315, 324, 325, 334, 335 by a decrease step, under the condition that this decrease does not result in air stream thresholds 314, 315, 324, 325, 334, 335, which are lower than a minimum air stream 216 of the respective reactor 21 , 22, 23. The control unit 3 than increases the air stream thresholds 314, 315, 324, 325, 334, 335 of all the reactors 21 , 22, 23 that inquired higher air stream thresholds 314, 315, 324, 325, 334, 335 by an increase step, under the condition that this increase does not result in air stream thresholds 314, 315, 324, 325, 334, 335, which are higher than a maximum air stream 215 of the respective reactor 21 , 22, 23 and under the condition that the sum of the higher air stream thresholds 314, 324, 334 of the reactors 21 , 22, 23 does not exceed the total air stream value.

The control unit also reads the actual oxygen stream of the first reactor 21 as part of its parameters 34. The control unit 3 than checks whether the actual oxygen stream lies in an upper part of the allowable oxygen stream range in that it calcu lates the difference of the upper oxygen stream threshold and the actual oxygen stream and compares the difference to a second oxygen-delta. If the difference is smaller than the second oxygen-delta, the actual oxygen stream lies in an upper part of the allowable oxygen stream range and the control unit 3 interprets this as the reactor 21 inquiring higher oxygen stream thresholds. The control unit 3 also checks, whether the actual oxygen stream lies in a lower part of the allowable oxygen stream range in that it calculates the difference of the lower oxygen stream threshold and the actual oxygen stream and compares the difference to a first oxygen-delta. If the difference is smaller than the first oxygen stream-delta, the actual oxygen stream lies in a lower part of the allowable oxygen stream range and the control unit 3 interprets this as the reactor 21 inquiring lower oxygen stream thresholds. The control unit 3 performs the same steps regarding the ox ygen stream for the second reactor 22 and the third reactor 23.

The control unit 3 than decreases the oxygen stream thresholds of all the reactors 21 , 22, 23 that inquired lower oxygen stream thresholds by a decrease step, under the condition that this decrease does not result in oxygen stream thresholds, which are lower than a minimum oxygen stream of the respective reactor 21 , 22, 23. The control unit 3 than increases the oxygen stream thresholds of all the re actors 21 , 22, 23 that inquired higher oxygen stream thresholds by an increase step, under the condition that this increase does not result in oxygen stream thresholds, which are higher than a maximum oxygen stream of the respective reactor 21 , 22, 23 and under the condition that the sum of the higher oxygen stream thresholds of the reactors 21 , 22, 23 does not exceed the total oxygen stream value.

Figure 2 shows an example for the development over time regarding the feed of the first reactor 21 in the topmost graph, the second reactor 22 in the middle graph and the third reactor 23 in the bottom graph.

The topmost graph regarding the first reactor 21 shows the actual feed 211 , the feed target 31 1 , a first feed-delta 313 and second feed-delta 312 as well as a maximum feed 212 and a minimum feed 213. The first feed-delta 313 and the second feed-delta 312 are differences to the feed target 311 as indicated by the double-ended arrows. The first and second feed-delta 313 and 312 are further shown as dotted lines illustrating the difference to the feed target 31 1 over the shown period of time. Until the time t3 the actual feed 21 1 is varying below the feed target 31 1 and within the two dotted lines. Hence, the difference of the actual feed 211 to the feed target 311 is not larger than the first feed-delta 313 and not smaller than the second feed-delta 312. The control unit 3 thus does not interpret these values as an inquiry for either increase or decrease of the feed target 311 .

The same is true for the third reactor 23 over the complete shown period of time. The actual feed 231 of the third reactor 23 is varying below the feed target 331 and within the two dotted lines illustrating the first and second feed-delta 332, 333. Hence, the difference of the actual feed 231 to the feed target 331 is not larger than the first feed-delta 333 and not smaller than the second feed-delta 332. The control unit 3 thus does not interpret these values as an inquiry for either increase or decrease of the feed target 331 .

The actual feed 221 of the second reactor 22 also starts within the two dotted lines illustrating the first feed-delta 323 and the second feed-delta 322, but crosses the dotted line illustrating the second feed-delta 322 at time ti. Hence, the difference of the actual feed 221 to the feed target 321 is smaller than the second feed-delta 322 at this point and the control unit 3 interprets this as an inquiry for higher feed target 321 . The control unit 3 then calculates the sum of all the feed targets 31 1 , 321 , 331 of the first reactor 21 , the second reactor 22 and the third reactor 23 and subtracts this sum from the total feed target comprised in the control unit 3. In the shown example of figure 2, the sum of the feed targets 31 1 , 321 and 331 equals the total feed target and thus there is no free target. Therefore, even though the second reactor inquired for higher feed target 321 , the feed target 321 of the second reactor 23 is not increased. This is also the case in the time between t2 and t3. At the time t3, the actual feed 211 of the first reactor 21 drops below the lower dotted line and hence the difference of the actual feed 21 1 to the feed target 31 1 is larger than the first feed-delta 313 and the control unit 3 interprets this as an inquiry for a lower feed target 31 1. Since the feed target 311 of the first reactor 21 is still much higher than the minimum feed 213 of the first reactor 21 , the con trol unit 3 decreases the feed target 311 of the first reactor 21 as long as the actual feed 21 1 of the first reactor 21 stays below the dotted line illustrating the first feed- delta 313. This results in free target, meaning that the sum of the feed targets 31 1 , 321 , 331 of the first, the second and the third reactor 21 , 22, 23 is smaller than the total feed target comprised in the control unit 3.

Since the second reactor 22 has an actual feed 221 which is still higher than the upper dotted line illustrating the second feed-delta 322 of the second reactor 22, and hence the difference between the actual feed 221 and the feed target 321 of the second reactor 22 is smaller than the second feed-delta 322 of the second reactor 22, the control unit 3 still interprets this as the second reactor 22 inquiring for higher feed target 321 . Due to the drop of the feed target 31 1 of the first reactor 21 and the resulting free target, the control unit 3 can now increase the feed target 321 of the second reactor 22.

At the time t4, the actual feed 211 of the first reactor 21 raises above the dotted line illustrating the first feed-delta and thus the difference between the actual feed 21 1 and the feed target 31 1 of the first reactor 21 is no longer larger than a first feed-delta 313. The control unit 3 thus does not interpret this as an inquiry for lower feed target 311 anymore. The feet target 31 1 of the first reactor 21 is thus constant again between t4and ts.

From the time ts, the actual feed 211 of the first reactor 21 lies above the dotted line illustrating the second feed-delta 312 and thus the difference between the actual feed 211 and the feed target 31 1 is smaller than the second feed-delta 312. The control unit 3 interprets this as an inquiry for higher feed target 31 1 of the first reactor 21 . Again, the sum of the feed targets 31 1 , 321 and 331 of the first, second and third reactor 21 , 22 and 23 already equals the total feed target and thus no further increase of the feed target 31 1 of the first reactor 21 is possible.

At the time te, the actual feed 221 of the second reactor 22 drops below the dotted line and thus the difference between the actual feed 221 and the feed target 321 of the second reactor 22 is larger than a first feed delta 323 of the second reactor 22 and hence the control unit 3 decreases the feed target 321 of the second re actor 22. Since the first reactor is still inquiring for higher feed target at this time, the feed target 31 1 of the first reactor 21 is increased.

At time 17, the second reactor 22 is still inquiring for lower feed target 321 and hence the feed target 321 of the second reactor 22 is still decreased creating free feed target. Even though the first reactor 21 is still inquiring for a higher feed target 311 , since the difference between the actual feed 211 and the feed target 311 is still smaller than the second feed-delta 312, the feed target 31 1 of the first reactor 21 is not further increased since the feed target 311 has reached the maximum feed 212 of the first reactor 21 .

At the time te the actual feed 211 of the first reactor 21 drops below the first feed- delta 313 again and thus the control unit 3 decreases the feed target 31 1 of the first reactor 21 . Since no other reactor inquired for a higher feed target at that time, the sum of the feed targets 311 , 321 and 331 of the first, the second and the third reactor 21 , 22 and 23 is lower than the total feed target comprised in the control unit 3. Figure 3 shows a corresponding example for the development over time regarding the air stream of the first reactor 21 in the topmost graph, the second reactor 22 in the middle graph and the third reactor 23 in the bottom graph.

The topmost graph regarding the first reactor 21 shows the actual air stream 214, which is adjusted by the reactor between the upper airstream threshold 314 and the lower airstream threshold 315. The topmost graph further shows a first air- delta 317, which is depicted as an double-ended arrow and a dotted line in the distance of the first air-delta to the depicted lower airstream threshold 315. Fur thermore, figure 3 shows the second air-delta as a double-ended arrow and a dotted line with the respective distance to the upper airstream threshold 314. Fur thermore, the upper graph additionally shows a maximum air stream 215 and a minimum air stream 216.

The allowable air stream range between the lower airstream threshold 315 and the upper airstream threshold 314 is divided in three parts by the first air-delta 317 and the second air-delta 316. An upper part, where the difference between the actual airstream and the upper air stream threshold is smaller than the second air-delta, a bottom part, where the difference between the actual airstream and the lower air stream threshold is smaller than the first air-delta and the middle part, where the difference between the actual airstream and the upper air stream threshold is larger than the second air-delta and the difference between the actual airstream and the lower air stream threshold is larger than the first air-delta.

Until the time ts the actual air stream 214 of the first reactor 21 stays within the two dotted lines and thus in the middle part of the allowable range between the lower airstream threshold 315 and the upper airstream threshold 314, in which the control unit 3 interprets the reactor as not inquiring for a higher or a lower threshold. At time ts, the actual air stream falls below the lower dotted line and thus in the bottom part, where the difference between the actual air stream 214 and the lower airstream threshold 315 is smaller than the first air-delta 317 and the control unit 3 interprets the reactor as inquiring for a lower threshold. The control unit 3 de creases upper and lower airstream thresholds until the time 11, when the actual air stream 214 rises again above the lower dotted line, depicting the first air-delta 317.

The middle graph shows the actual air stream 224 of the second reactor 22 with an upper airstream threshold 324 and a lower airstream threshold 325 of the sec ond reactor 22. Again, first and second air-delta 327, 326 of the second reactor

22 are depicted as double-ended arrows and a dotted line. The actual air stream 224 stays within the middle part of the allowable range until time ts. At this time the actual air stream 224 raises above the upper dotted line depicting the second air-delta, 326where the difference between the actual air stream 224 and the up per airstream threshold 324 of the second reactor 22 is smaller than the second air-delta 326 of the second reactor 22. The control unit 3 thus increases the air stream thresholds 324, 325. After the time 17, the actual air stream 224 of the second reactor 22 is still high enough such that the difference between the actual air stream 224 and the upper airstream threshold 324 is smaller than the second air-delta 326. Since the sum of the air stream thresholds 314, 324, 334 already equal the total air stream threshold comprised in the control unit, the air stream thresholds of the second reactor 22 are not further increased.

The bottom graph of figure 3 shows the actual air stream 234 of the third reactor

23 as well as the upper airstream threshold 334, lower airstream threshold 335, the first air-delta 337 and second air-delta 336 of the third reactor 23. Since the actual air stream 234 of the third reactor 23 stays in the middle part of the allow able range the whole time, no decrease or increase is performed by the control unit 3. List of Reference Numerals:

1 reactor system

21 first reactor

22 second reactor

23 third reactor

3 control unit

31 control variables first reactor

32 control variables second reactor

33 control variables third reactor

4 total feed supply

41 feed input first reactor

42 feed input second reactor

43 feed input third reactor

5 oxygen supply unit

51 oxygen stream input first reactor

52 oxygen stream input second reactor

53 oxygen stream input third reactor

6 total air supply

61 air stream input first reactor

62 air stream input second reactor

63 air stream input third reactor

7 total output

71 output first reactor

72 output second reactor

73 output third reactor

8 exhaust gas after-treatment

81 exhaust first reactor

82 exhaust second reactor 83 exhaust third reactor

21 1 actual feed first reactor

212 maximum feed first reactor

213 minimum feed first reactor

214 actual air stream first reactor

215 maximum air stream first reactor

216 minimum air stream first reactor

221 actual feed second reactor

224 actual air stream second reactor

231 actual feed third reactor

234 actual air stream third reactor

31 1 feed target first reactor

312 second feed-delta first reactor

313 first feed-delta first reactor

314 upper air stream threshold first reactor

315 lower air stream threshold first reactor

316 second air-delta first reactor

317 first air-delta first reactor

321 feed target second reactor

322 second feed-delta second reactor

323 first feed-delta second reactor

324 upper air stream threshold second reactor

325 lower air stream threshold second reactor

326 second air-delta second reactor

327 first air-delta second reactor

331 feed target third reactor

332 second feed-delta third reactor

333 first feed-delta third reactor

334 upper air stream threshold third reactor lower air stream threshold third reactor second air-delta third reactor first air-delta third reactor