TECHNICAL FIELD

The present disclosure relates to a method for controlling temperature gradient and average temperature along a rotation axis in a rotary kiln. Further, the disclosure relates to a control system for controlling temperature gradient and average temperature along a rotation axis in a rotary kiln. Yet further, the disclosure relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method. The disclosure also relates to a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method.

BACKGROUND ART

A rotary kiln is a pyro processing device used to raise materials to a high temperature (calcination) in a continuous process. The kiln is a cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its axis. The material to be processed is fed into the upper end of the cylinder. As the kiln rotates, material gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. Hot gases pass along the kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current). The hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln. Such a flame is projected from a burner-pipe (or "firing pipe") which acts like a large Bunsen burner. The fuel for this may be gas, oil, pulverized petroleum coke, pulverized coal, bio-oil, such as tall oil and palm oil, or wood pellets. Materials produced using rotary kilns include: cement, lime, refractories, metakaolin, titanium dioxide, alumina, vermiculite and iron ore pellets.

A rotary kiln may be used in the chemical recovery cycle of chemical pulp industries based on the sulphate process. In one example, dewatered slurry consisting mainly of precipitated calcium carbonate is calcined to calcium oxide. The heating is normally performed by direct combustion. In order to avoid ring formation in the rotary kiln and maintaining even quality of the material, it is important that the temperature profile in the kiln is stable. Usually the temperature is measured at the ends of the kiln. By keeping the end temperatures constant, constant temperature profile throughout the kiln can be achieved.

In order to achieve a constant temperature profile throughout the kiln, the gas flow through the rotary kiln and the fuel input to the burner has to be controlled by actuators. By increasing the gas flow more heat is moved from the burning end zone to the feed end zone. Further, by reducing the gas flow less heat is moved from the burning end zone to the feed end zone. However, the gas flow must be kept above a minimum level so that the kiln receives enough combustion air into the burning end zone. This may be checked by the 0 ₂ content of the gases. Further, the fuel input to the burner influences the temperature profile throughout the kiln.

There have been many attempts to optimise the temperature profile throughout the rotary kiln. Some of them are presented in the following patent applications:

EP2169483A1 relates to a control system for controlling an industrial process. Combining a fuzzy logic indicator (z) with a model based process controller makes it possible to provide robust indicators of the process states (x) for controlling an industrial process in a real plant situation measured process variables (y) possibly contradict each other.

US3437325A relates to a control apparatus for a rotary kiln. The control apparatus comprises first and second heat determining means for providing a first and second control signals in accordance with predetermined heat losses and heat inputs, respectively, associated with the operation of the kiln. The control means is responsive to a predetermined relationship between the first and second control signals for controlling the operation of the heat input source and the drive motor to provide a desired operation of the kiln.

WO0132581A1 relates to a controller for a kiln plant. A thermodynamic controller measures a number of variables including the kiln hood temperature and one or more output gas concentrations, and controls the fuel input to the kiln to maintain the hood temperature within a desired range and a main impeller of the kiln to maintain the measured gas concentrations within a predetermined range.

W02011000430A1 relates to a method and a device for controlling a process for burning lime containing mixture (CaCOs) and converting it to calcined lime (CaO) in a rotary kiln. The method comprises collecting measurement data of the temperature in the wall at a plurality of measuring points along the longitudinal axis of the cavity, predicting the actual temperature gradient along the longitudinal axis of the cavity based at least on the measurement data of the temperature in the wall. Further, the method comprises the steps of, by means of a thermal model describing the temperature along the cavity of the kiln, determining a desired temperature gradient along the cavity based on the predicted temperature gradient along the cavity and a predetermined control strategy controlling the temperature in the kiln so that the area of deposition of lime on the inside of the walls of the kiln is controlled.

Each of those patent applications describes rather complicated methods of temperature control. Hence, there is a need to provide temperature control which is more flexible and easier to implement and maintain.

SUMMARY OF THE INVENTION

The disclosure relates to a method for controlling temperature gradient and average temperature along a rotation axis in a rotary kiln from a burning end zone comprising a burner to a feed end zone comprising material input means, by control of gas flow through the rotary kiln and by control of fuel input to the burner, by means of a control system. The method comprises the steps of:

- receiving an actual burning end zone temperature measured at the burning end zone;

- receiving an actual feed end zone temperature measured at the feed end zone;

- determining an actual average temperature by generating an average of the burning end zone temperature and the feed end zone temperature;

- determining an actual differential temperature by generating a difference between the burning end zone temperature and the feed end zone temperature;

- controlling the average temperature towards an average set point temperature by outputting an average temperature control signal to control the fuel input to the burner; - controlling the differential temperature towards a differential set point temperature by outputting a differential temperature control signal to control the gas flow through the rotary kiln.

An advantage of the method is to reduce the dependency between control of fuel input by burning end zone temperature and gas flow by feed end zone temperature, by introducing new temperature parameters: average temperature to control fuel input to the burner and differential temperature to control the gas flow. Thus, it is possible to control the fuel input and the gas flow with two separate SISO systems. The method provides for improved control of the temperature gradient along the rotation axis in the rotary kiln which increases product quality and process runnability.

In one example, the step of determining the average temperature further comprises that the burning end zone temperature is weighted by a burning end average weight function and the feed end zone temperature is weighted by a feed end average weight function; and that the step of determining the differential temperature further comprises that the burning end zone temperature is weighted by a burning end differential weight function and the feed end zone temperature is weighted by a feed end differential weight function. By introducing average weight functions to the burning end zone temperature and feed end zone temperature for controlling the average temperature, the method can be optimised to further reduce the dependency between fuel input, gas flow, burning end zone temperature and feed end zone temperature. By introducing differential weight functions to the burning end zone temperature and feed end zone temperature for controlling the differential temperature, the method can be optimised to reduce the dependency between fuel input, gas flow, burning end zone temperature and feed end zone temperature.

According to another example, the weight functions are static functions or dynamic systems. Thereby it is possible to reduce the dependency between fuel input, gas flow, burning end zone temperature and feed end zone temperature.

In yet another example, the step of receiving the actual burning end zone temperature further comprises: S

- receiving at least one burning end zone temperature value, wherein each burning end zone temperature value is measured at a predefined position along the rotation axis at the burning end zone; and

wherein the step of receiving the actual feed end zone temperature further comprises:

- receiving at least one actual feed end zone temperature value, wherein each feed end zone temperature value is measured at a predefined position along the rotation axis at the feed end zone;

wherein each of the steps of determining the average temperature unit and the differential temperature unit further comprises:

- determining the weight functions based on the predefined position of each of the at least one burning end zone temperature value and each of the at least one feed end zone temperature value.

By reducing dependency of where, along the rotation axis of the rotary kiln, the burning end zone temperature and the feed end zone temperature respectively is measured. Thus, position based control improves the accuracy of the estimate of the temperature along the rotary kiln. This leads to improved control of the temperature gradient along the rotation axis of the rotary kiln.

According to a further example, the step of receiving the actual burning end zone temperature further comprises the steps of:

- receiving at least two burning end zone temperature values, wherein each burning end zone temperature value is measured at a predefined position along the rotation axis at the burning end zone;

- determining the actual burning end zone temperature by generating an average of the at least two burning end zone temperature values; and

- determining a burning end position by generating an average of the predefined positions where the at least two burning end zone temperature values are measured; and

wherein the step of receiving the actual feed end zone temperature further comprises the steps of:

- receiving at least two actual feed end zone temperature values, wherein each feed end zone temperature value is measured at a predefined position along the rotation axis at the feed end zone; - determining the actual feed end zone temperature by generating an average of the at least two feed end zone temperature values; and

- determining a feed end position by generating an average of the predefined positions where the at least two feed end zone temperature values are measured.

In a another example, the step of determining the average temperature further comprises:

- determining an average position along the rotation axis in the rotary kiln by generating an average of the burning end position and the feed end position;

wherein the step of determining the differential temperature further comprises:

- determining a position difference along the rotation axis in the rotary kiln by generating a difference between the burning end position and the feed end position;

wherein the step of controlling the average temperature further comprises that the average set point temperature is set in dependence of the average position along the rotation axis in the rotary kiln;

wherein the step of controlling the differential temperature further comprises that the differential set point temperature is set in dependence of the position difference along the rotation axis in the rotary kiln.

According to a yet further example, each of the received burning end zone temperature and the received feed end zone temperature is measured at at least one predefined position within the burning end zone and the feed end zone, respectively;

and wherein the method further comprises the step of, prior to the steps of determining an average temperature and determining a temperature gradient:

- determining a temperature profile function along the rotation axis, describing temperature as a function of position along the rotation axis, by adapting the temperature profile function to minimize the difference between the measured temperature measured at the predefined positions along the rotation axis;

- determining the burning end zone temperature and the feed end zone temperature , respectively, based on the adapted temperature profile function.

In a further example, the method further comprises the steps of:

- receiving a fuel input signal measured at the burner; and - controlling fuel input to the burner based on the average temperature control signal and the fuel input signal.

According to another example, the method further comprises the steps of:

- receiving an oxygen level signal measured at the feed end zone; and

- controlling the gas flow by controlling the oxygen level based on the differential temperature control signal and the oxygen level signal. Thus, a specific example of control of oxygen by the differential temperature is disclosed.

Further, the disclosure relates to a control system for controlling temperature gradient and average temperature along a rotation axis in a rotary kiln from a burning end zone comprising a burner to a feed end zone comprising material input means. Fuel input to the burner is controlled by a fuel input unit. A gas flow through the rotary kiln is controlled by a gas flow unit. The control system comprises:

an average temperature unit configured to:

receive an actual burning end zone temperature measured at the burning end zone;

receive an actual feed end zone temperature measured at the feed end zone; determine an actual average temperature by generating an average ofthe burning end zone temperature and the feed end zone temperature;

an average temperature controller configured to:

control the average temperature towards an average set point temperature by outputting an average temperature control signal to the fuel input unit;

a differential temperature unit configured to:

receive the actual burning end zone temperature measured at the burning end zone;

receive the actual feed end zone temperature measured at the feed end zone; determine an actual differential temperature by generating a difference between the burning end zone temperature and the feed end zone temperature;

a differential temperature controller configured to:

control the differential temperature towards a differential set point temperature by outputting a differential temperature control signal to the gas flow unit. Yet further, the disclosure relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the above described method.

The disclosure also relates to a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the above described method.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be further described with reference to the accompanying drawings:

FIG. 1 shows a rotary kiln and the control system for controlling temperature gradient and average temperature according to the disclosure connected thereto.

FIG. 2 illustrates the control system comprising weight functions according to an embodiment of the disclosure connected thereto.

FIG. 3 illustrates a rotary kiln and the control system according to an embodiment.

FIG. 4 shows a method for controlling temperature gradient and average temperature along a rotation axis in a rotary kiln.

DETAILED DESCRIPTION

FIG 1. shows a rotary kiln 100 having a burning end zone BE comprising a burner 120 and a feed end zone FE comprising material input means 130. The term "burning end zone", BE, refers to the half of the rotary kiln 100, along the rotation axis 110 of the rotary kiln, comprising the burner 120. The term "feed end zone", FE, refers to the half of the rotary kiln 100, along the rotation axis 110 of the rotary kiln 100, comprising the material input means 130. The burning end zone BE and the feed end zone FE correspond to different halves of the rotary kiln 100. Thus, together, the burning end zone BE and the feed end zone FE cover the whole length of the rotary kiln 100 along the rotation axis 110.

A control system 1 for controlling temperature gradient and average temperature along a rotation axis 110 in the rotary kiln 100 is connected to the rotary kiln 100. A fuel input to the burner 120 is controlled by a fuel input unit. The fuel may be gas, oil, pulverized petroleum coke, pulverized coal, bio-oil, such as tall oil and palm oil, or wood pellets. A gas flow through the rotary kiln 100 is controlled by a gas flow unit. The gas comprises flue gases.

The control system 1 comprises an average temperature unit 10. The average temperature unit 10 is configured to receive an actual burning end zone temperature TBE-A measured at the burning end zone BE. Further, the average temperature unit 10 is configured to receive an actual feed end zone temperature TFE-A measured at the feed end zone FE. Yet further, the average temperature unit 10 is configured to determine an actual average temperature TA-A by generating an average of the burning end zone temperature TBE-A and the feed end zone temperature TFE-A. Further, the control system 1 comprises an average temperature controller 30. The average temperature controller 30 is configured to control the average temperature TA A towards an average set point temperature TA-SP by outputting an average temperature control signal ST-A to the fuel input unit.

Yet further, the control system 1 comprises a differential temperature unit 20. The differential temperature unit 20 is configured to receive the actual burning end zone temperature TBE-A measured at the burning end zone BE. Further, the differential temperature unit 20 is configured to receive the actual feed end zone temperature TFE-A measured at the feed end zone FE. Yet further, the differential temperature unit 20 is configured determine an actual differential temperature TD-A by generating a difference between the burning end zone temperature TBE-A and the feed end zone temperature TFE-A- Further, the control system 1 comprises a differential temperature controller 40. The differential temperature controller 40 is configured to control the differential temperature TD-A towards a differential set point temperature TD-SP by outputting a differential temperature control signal ST-D to the gas flow unit.

The burning end zone temperature TBE-A may be determined by one burning end zone temperature measurement position 160 or a plurality of burning end zone temperature measurement positions 160 along the rotation axis 110 in the rotary kiln 100. In case of a plurality of burning end zone temperature measurement positions, the temperature measurement positions may be positioned anywhere within the burning end zone. However, in order to increase the possibility to determine the temperature along rotary kiln 100 it is an advantage if the temperature measurement positions are distanced from each other in the direction parallel to the rotation axis 110 in the rotary kiln 100. The feed end zone temperature TFE-A may be determined by one feed end zone temperature measurement position 170 or a plurality of feed end zone temperature measurement positions 170 along the rotation axis 110 in the rotary kiln 100. In case of a plurality of feed end zone temperature measurement positions, the temperature measurement positions may be positioned anywhere within the feed end zone. However, in order to increase the possibility to determine the temperature along rotary kiln 100 it is an advantage if the temperature measurement positions are distanced from each other in the direction parallel to the rotation axis 110 in the rotary kiln 100.

FIG. 2 illustrates the control system 1 comprising weight functions according to an embodiment of the disclosure connected thereto. The average temperature controller 30 is configured to weight the burning end zone temperature TBE-A by a burning end average weight function WBE-A and the feed end zone temperature TFE-A by a feed end average weight function WFE-A. The differential temperature controller 40 is configured to weight the burning end zone temperature TBE-A by a burning end differential weight function WBE-D and the feed end zone temperature T _F E-A by a feed end differential weight function WFE-D-

In one example, the weight functions WFE-A, WBE-A, WFE-D, WBE-D are static functions. The static functions may be static weight factors. In this example, the output, i.e. the value of the function, is dependent only on the input signals.

In another example, the weight functions WFE-A, WBE-A, WFE-D, WBE-D are dynamic systems. The dynamic systems may comprise low pass filters. In this example, the output, i.e. the value of the function, is dependent on current and previous inputs, i.e. a low pass filter.

In one example, each of the average temperature unit 10 and the differential temperature unit 20 is configured to:

receive at least one burning end zone temperature value, wherein each burning end zone temperature value is measured at a predefined position 160 along the rotation axis 110 at the burning end zone BE; and

receive at least one actual feed end zone temperature value, wherein each feed end zone temperature value is measured at a predefined position 170 along the rotation axis 110 at the feed end zone FE;

wherein the average temperature TA-A and the differential temperature TD-A, respectively, is determined by:

determine the weight functions WBE A, WFE-A, WBE-D, WFE-D based on the predefined position 160, 170 of each of the at least one burning end zone temperature value and each of the at least one feed end zone temperature value.

In a further example, each of the average temperature unit 10 and the differential temperature unit 20 is configured to:

receive at least two burning end zone temperature values, wherein each burning end zone temperature value is measured at a predefined position 160 along the rotation axis 110 at the burning end zone BE;

determine the actual burning end zone temperature TBE-A by generating an average of the at least two burning end zone temperature values; and

determine a burning end position by generating an average of the predefined positions 160 where the at least two burning end zone temperature values are measured;

receive at least two actual feed end zone temperature values, wherein each feed end zone temperature value is measured at a predefined position 170 along the rotation axis 110 at the feed end zone FE;

determine the actual feed end zone temperature TFE-A by generating an average of the at least two feed end zone temperature values; and

determine a feed end position by generating an average of the predefined positions 170 where the at least two feed end zone temperature values are measured.

In a yet further example, the average temperature unit 10 further is configured to:

determine an average position along the rotation axis 110 in the rotary kiln 100 by generating an average of the burning end position and the feed end position;

wherein the average set point temperature TA-SP, received by the average temperature controller 30, is set in dependence of the average position along the rotation axis 110 in the rotary kiln 100;

wherein the differential temperature unit 20 further is configured to: determine a position difference along the rotation axis 110 in the rotary kiln 100 by generating a difference between the burning end position and the feed end position;

wherein the differential set point temperature TD-SP, received by the differential temperature controller 40, is set in dependence of the position difference along the rotation axis 110 in the rotary kiln 100.

In another example, each of the received burning end zone temperature TBE-A and the received feed end zone temperature TFE A is measured at at least one predefined position 160, 170 within the burning end zone BE and the feed end zone FE, respectively; and

wherein the average temperature unit 10 and the differential temperature unit 20 further are configured to:

determine a temperature profile function along the rotation axis 110, describing temperature as a function of position along the rotation axis 110, by adapting the temperature profile function to minimize the difference between the measured temperature measured at the predefined positions 160, 170 along the rotation axis 110; and

determine the burning end zone temperature TBE-A and the feed end zone temperature TFE-A, respectively, based on the adapted temperature profile function.

The fuel input unit is configured to control fuel input to the burner 120, based on the average temperature control signal ST- A. In one example, the fuel input unit is a fuel input valve 140, as shown in FIG. 1. However, the fuel input unit may be any other means suitable for supplying fuel input. In another example, the fuel input unit is a fuel input controller 50, as shown in FIG. 3. The fuel input controller 50 is configured to receive a fuel input signal SFUEL measured at the burner 120. Further, the fuel input controller 50 is configured to control fuel input to the burner 120 based on the average temperature control signal ST-A and the fuel input signal SFUEL.

The gas flow unit is configured to control the gas flow through the rotary kiln 100. In one example, the gas flow unit is a fan 150, as shown in FIG. 1. However, the gas flow unit may be any other means for causing a gas flow through the rotary kiln 100. In yet another example, the gas flow unit is an oxygen controller 60. The oxygen controller 60 is configured to receive an oxygen level signal SOXYGEN measured at the feed end zone FE. Further, the oxygen controller 60 is configured to control the gas flow by controlling the oxygen level based on the differential temperature control signal ST -D and the oxygen level signal SOXYGEN.

The average temperature controller 30 may be any of a PID controller, polynomial controller, state feedback controller, fuzzy logic controller or Model predictive controller. The differential temperature controller 40 may be any of a PID controller, polynomial controller, state feedback controller, fuzzy logic controller or Model predictive controller. Further, any other type of controller may be used.

In one example, the fuel input controller 50 may be any of a PID controller, polynomial controller, state feedback controller, fuzzy logic controller or Model predictive controller. In one example, the oxygen controller 60 may be any of a PID controller, polynomial controller, state feedback controller, fuzzy logic controller or Model predictive controller. Further, any other type of controller may be used.

The material input means 130 is arranged to receive any material of the following: lime mud, lime stones or iron ore pellets. However, any other material suitable for the rotary kiln 100 may be used, such as cement, refractories, metakaolin, titanium dioxide, alumina or vermiculite.

FIG. 4 shows a method for controlling temperature gradient and average temperature along a rotation axis 110 in a rotary kiln 100 from a burning end zone BE comprising a burner 120 to a feed end zone FE comprising material input means 130, by control of gas flow through the rotary kiln 100 and by control of fuel input to the burner 120, by means of a control system 1. In detail, the method comprising the steps of:

receiving SI an actual burning end zone temperature TBE-A measured at the burning end zone BE;

receiving S2 an actual feed end zone temperature TFE-A measured at the feed end zone FE;

determining S3 an actual average temperature TA-A by generating an average of the burning end zone temperature TBE-A and the feed end zone temperature TFE-A; > determining 54 an actual differential temperature TD-A by generating a difference between the burning end zone temperature TBE-A and the feed end zone temperature TFE-A;

- controlling 55 the average temperature TA-A towards an average set point temperature TA-SP by outputting an average temperature control signal ST-A to control the fuel input to the burner 120;

- controlling 56 the differential temperature TD-A towards a differential set point temperature TD-SP by outputting a differential temperature control signal ST-D to control the gas flow through the rotary kiln 100.

In one embodiment, the step of determining the average temperature TA A further comprises that the burning end zone temperature TBE-A is weighted by a burning end average weight function WBE-A and the feed end zone temperature TFE-A is weighted by a feed end average weight function WFE-A, and wherein the step of determining the differential temperature TD-A further comprises that the burning end zone temperature TBE-A is weighted by a burning end differential weight function WBE-D and the feed end zone temperature TFE-A is weighted by a feed end differential weight function WFE-D- In one example, the weight functions WFE-A, WBE-A, WFE-D, WBE-D may be static functions or dynamic systems.

The step of receiving the actual burning end zone temperature TBE-A may further comprise receiving at least one burning end zone temperature value, wherein each burning end zone temperature value is measured at a predefined position 160 along the rotation axis 110 at the burning end zone BE. The step of receiving the actual feed end zone temperature TFE-A may further comprise receiving at least one actual feed end zone temperature value, wherein each feed end zone temperature value is measured at a predefined position 170 along the rotation axis 110 at the feed end zone FE. Each of the steps of determining the average temperature TA- A and the differential temperature TD-A further comprises determining the weight functions WBE- A, WFE-A, WBE-D, WFE-D based on the predefined position 160, 170 of each of the at least one burning end zone temperature value and each of the at least one feed end zone temperature value.

The step of receiving the actual burning end zone temperature TBE-A may further comprise the steps of: - receiving at least two burning end zone temperature values, wherein each burning end zone temperature value is measured at a predefined position 160 along the rotation axis 110 at the burning end zone BE;

- determining the actual burning end zone temperature TBE-A by generating an average of the at least two burning end zone temperature values; and

- determining a burning end position by generating an average of the predefined positions 160 where the at least two burning end zone temperature values are measured. The step of receiving the actual feed end zone temperature TFE-A may further comprise the steps of:

- receiving at least two actual feed end zone temperature values, wherein each feed end zone temperature value is measured at a predefined position 170 along the rotation axis 110 at the feed end zone FE;

- determining the actual feed end zone temperature TFE-A by generating an average of the at least two feed end zone temperature values; and

- determining a feed end position by generating an average of the predefined positions 170 where the at least two feed end zone temperature values are measured.

The step of determining the average temperature TA-A may further comprise:

- determining an average position along the rotation axis 110 in the rotary kiln 100 by generating an average of the burning end position and the feed end position;

wherein the step of determining the differential temperature TD A further comprises:

- determining a position difference along the rotation axis 110 in the rotary kiln 100 by generating a difference between the burning end position and the feed end position;

wherein the step of controlling the average temperature TA-A further comprises that the average set point temperature TA-SP is set in dependence of the average position along the rotation axis 110 in the rotary kiln 100;

wherein the step of controlling the differential temperature TD- _A further comprises that the differential set point temperature TD-SP is set in dependence of the position difference along the rotation axis 110 in the rotary kiln 100.

In one embodiment, each of the received burning end zone temperature TBE-A and the received feed end zone temperature TFE-A is measured at at least one predefined position 160, 170 within the burning end zone BE and the feed end zone FE, respectively. In this case, the method may further comprise the step of, prior to the steps of determining an actual average temperature TA A and determining a temperature gradient TG-A:

- determining a temperature profile function along the rotation axis 110, describing temperature as a function of position along the rotation axis 110, by adapting the temperature profile function to minimize the difference between the measured temperature measured at the predefined positions 160, 170 along the rotation axis 110;

- determining the burning end zone temperature TBE-A and the feed end zone temperature TFE- A, respectively, based on the adapted temperature profile function. Yet further, the method may comprise the steps of:

- receiving a fuel input signal SFUEL measured at the burner 120; and

- controlling fuel input to the burner 120 based on the average temperature control signal ST-A and the fuel input signal SFUEL. Further, the method may comprise the steps of:

- receiving an oxygen level signal SOXYGEN measured at the feed end zone FE; and

- controlling the gas flow by controlling the oxygen level based on the differential temperature control signal ST-D and the oxygen level signal SOXYGEN. Further, the disclosure relates to a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method described herein.

Yet further, the computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method described herein.

List of reference numerals

1 control system

10 average temperature unit

20 differential temperature unit

30 average temperature controller

40 differential temperature controller 50 fuel input controller

60 oxygen controller

100 rotary kiln

110 rotation axis of the rotary kiln

120 burner

130 material input means

140 fuel input valve

150 fan

BE burning end zone

FE feed end zone

160 burning end zone temperature measurement position(s)

170 feed end zone temperature measurement position(s)

TBE-A actual burning end zone temperature

TFE-A actual feed end zone temperature

TA-A actual average temperature

TA-SP average set point temperature

TD-A actual differential temperature

TD-SP differential set point temperature

ST-A average temperature control signal

ST-D differential temperature control signal

fuel input signal

oxygen level signal