Method for controlling a thermal combustion system

Abstract

The present invention relates to a method for controlling a thermal combustion system and to a control logic for a combustion system.

Description

[Technical field]

[0001] The present invention relates to a method for controlling a thermal combustion system and to a control logic for a thermal combustion system.

[0002] Combustion can be described as the rapid oxidation of a combustible material, such as a fuel, resulting in the release of usable heat and the production of a visible flame. Combustion remains a challenging subject and is truly interdisciplinary, requiring the fusion of knowledge and skills in multiple fields. Combustion involves the study of chemistry, kinetics, thermodynamics, electromagnetic radiation, multiphase and turbulent flows, aerodynamics and fluid mechanics including heat and mass transfer, and quantum mechanics, to name a few.

[0003] Petroleum refining, chemical processing, and power generation are energy intensive industries involving the use of combustion in burners, process heaters, boilers and cogeneration systems, as well as flares and thermal oxidizers. Combustion for these industries has unique challenges faced in relation to the variety of fuel compositions. Combustion equipment must be flexible to operate in a safe, reliable, efficient, and environmentally responsible manner under a wide range of fuel compositions and conditions. The demands on combustion systems have continually changed and become more demanding over the years. While safety of combustion systems has always been essential, there has been an increased emphasis over time on effective heat transfer, temperature uniformity, scale-up of equipment, efficiency, control, and more recently, environmental emissions and noise generated from combustion. These demands create enormous problems for combustion engineers.

[0004] A combustion system typically includes a fuel supply, a combustion air supply, and an ignition system, all of which together are attached to one or more burners. Typically, these components are operated by a control system. The purpose of the control system is specifically to start, run, and shut down the combustion process and any associated auxiliary processes safely, reliably, and efficiently. The control system consists of various physical and logical components that are selected and assembled according to a control principle, and are arranged to provide the user with an interface that is informative, consistent, and easy to use. At various times during system start-up and normal operation, the state of these systems needs to be verified or changed. During system operation, the control system requires various process information to optimize system efficiency. In addition, the control system constantly monitors all safety parameters and shuts down the combustion system if any safety limit is not met.

[0005] To this end, the combustion control system has a temperature controller and an air-fuel ratio controller as well as an oxygen controller in the combustion oxidizer. The oxygen controller is generally mandatory, but may be considered necessary for efficiency optimization. Additionally, some combustion control systems apply feed-forward principles known in the automation industry. A typical control logic for a combustion oxidizer using multiple sources for fuel and oxygen is described in The John Zink Hamworthy Combustion Handbook - Design and Operations Vol.2, Charles E. Baukal Jr., editor, CRC Press, Boca Raton, Florida 2013 as follows:

[0006] The development of a complete metered control scheme to regulate the fuel and air delivered to the burner starts with an electronically linked parallel positioning scheme. The flow meter is linear with respect to flow rate. Meter output signal scaling provides the required firing speed and air-fuel ratio for use. The transmitter is linear with respect to flow rate, so the combustion air characterizer used for parallel positioning is not required. The temperature indicating controller (TIC) output sets the firing speed by acting as a set point (SP) for each flow controller. The output mode of the controller, reverse acting or direct acting, defines the change in output signal direction with respect to changes in the process variable (PV). For example, if the controller output increases as the PV increases, then the controller mode is direct acting. In a combustion control scheme, a fail-close fuel valve requires a reverse acting flow controller, while a fail-open combustion air valve requires a direct acting flow controller. From the definition of the controller mode, it is clear that the TIC should be reverse acting. That is, with an increase in temperature, which is the PV, the output of the TIC should decrease, decreasing the firing speed. The addition of the high and low signal selectors provides cross limit control for the fully metered control scheme. The low signal selector compares the burn rate demanded by the TIC to the actual combustion air flow ratio and applies the lower of the two signals as the SP to the fuel flow controller. The low signal selector ensures that the fuel SP does not exceed the amount of air available for combustion. The high signal selector compares the burn rate demanded by the TIC to the actual fuel flow and applies the higher of the two signals as the SP to the air flow controller. This ensures that the SP never falls below what is required to burn the actual fuel flow. The high and low signal selectors together ensure that unburned fuel does not develop in the combustion system. The accumulation of unburned fuel can cause detonation. The cross limit control by the signal selectors ensures that the air flow leads the fuel flow at load increases and lags the fuel flow at fuel decreases. This lead / lag action explains why fully metered cross limit control systems are sometimes referred to as "lead-lag" control.Whatever the name, the system functions to maintain the desired air / fuel mixture during load changes. The system may also reduce fuel flow if uniform airflow is lost or reduced. Measurement of O2 content in the flue gas can be used to trim the control scheme. In most systems, the O2 signal should be used to "trim" rather than the primary control. Many O2 analyzers require too much maintenance and / or are too slow to react for use as a primary controller in the combustion process. O2 controllers are utilized for SP injection and provide tuning parameters to aid in customizing the process. High and low signal limiters limit the O2 controller output to the trim function, typically 5-10% of the typical combustion air flow. A multiplicative function (X) on the combustion airflow transmitter signal provides the O2 trim adjustment. The multiplier gives a fixed trim gain. Using an additive function instead of a multiplier would result in a high trim gain at low flows, potentially resulting in combustion air starvation. The O2 trim may be applied to the combustion airflow controller SP rather than the flow transmitter signal. If this technique is used, the airflow signal to the low signal selector must hold the trim change. A cross limit scheme accommodates multiple fuels and O2 sources as shown in Figure 1. If multiple fuels are used, the heating value must be normalized by adjusting the span of the flow transmitter or by adding a heating value multiplier. A similar method is used to normalize the O2 content for multiple air sources.

[0007] However, the typical design of modern combustion control systems does not take into account any variations in the flow rate of combustibles fed to the combustion, such as waste gas streams, which may themselves vary in flow rate. The rapid introduction or sudden loss of a waste stream to a prior art combustion system typically results in a temperature jump, which may cause tripping or disapproval of the waste operation. Furthermore, the final stage of prior art oxidation systems must be operated at above stoichiometric conditions, i.e., excess air or oxygen conditions, for safe operation. This operating principle inevitably leads to the formation of nitrogen oxides, which reach even higher levels if bound nitrogen is present in the waste stream to the combustion system. However, environmental regulations impose strict limits on the emissions of nitrogen oxides from oxidation and combustion processes.

[0008] Therefore, there remains a need for a method of controlling a thermal combustion system that can solve the problems discussed above.

[0009] It has been found that this problem can be solved by first measuring the flow rate of a waste stream from an underlying process of known reactions, and, when a change in the flow rate is measured, predicting whether the measured flow rate will lead to a temperature change in the combustion of the stream in step a). If a temperature change is predicted, the ratio of air to compounds with calorific value and any inert compounds in the waste stream required to keep the temperature constant in the thermal combustion system is calculated and adjusted accordingly by increasing or decreasing the amount of fuel fed to the combustion system and / or by decreasing or increasing the amount of air fed to the combustion system.

[0010] Furthermore, if the waste stream also contains bound nitrogen, it is determined whether the amount of nitrogen oxides formed in the combustion system can be kept to a minimum when said combustion is performed at the adjusted ratio. If not, the ratio of air to the calorific value compounds and any inert compounds in the combustion system is adjusted to a sub-stoichiometric value to keep the amount of nitrogen oxides formed in the combustion system to a minimum. The sub-stoichiometric value is adjusted by an extremum seeking controller.

[0011] An object of the present invention is therefore to provide a method for controlling a thermal combustion system, comprising: a) measuring the flow rate of a waste stream comprising compounds having a calorific value, said waste stream arising from a process underlying a known reaction; b) if a change in flow rate is measured in step a), predicting whether the flow rate measured in step a) will lead to a temperature change in the combustion of the waste stream of step a), said predicting using a chart or equation capable of determining the temperature of the combustion system; c) if a temperature change is expected in step b), c1) calculating the ratio of air, oxygen-enriched air, or oxygen to the compounds having a calorific value in the waste stream of step a) required to maintain a constant temperature in the thermal combustion system; c2) adjusting the ratio calculated in step c1) by increasing or decreasing the amount of fuel supplied to the combustion system and / or by decreasing or increasing the amount of air, oxygen-enriched air, or oxygen supplied to the combustion system; A step of carrying out and d) if the waste stream of step a) also contains bound nitrogen: d1) determining whether the amount of nitrogen oxides formed in the combustion system can be kept to a minimum when said combustion is carried out at the ratio of step c2); d2) if the requirements of step d1) are not met, adjusting the ratio of air, oxygen-enriched air or oxygen to the compounds having a calorific value in the combustion system to a substoichiometric value that keeps the amount of nitrogen oxides formed in the combustion system to a minimum, said substoichiometric value being adjusted by an extremum-seeking controller; A step of carrying out The method includes:

[0012] The advantages of the method according to the invention are: · Have a fast response time to any changes in the mass / volume flow rate of the waste stream; Providing a combustion system that allows for flexible fuel and air, oxygen-enriched air, or oxygen distributions and therefore has a wider operating map; · Significantly reducing temperature spikes in the event of rapid introduction or sudden loss of a waste stream; Allows the combustion oxidizer / combustion system to be constantly operated at the lowest possible load, thereby minimizing the costs of operating the combustion oxidizer / combustion system; Including automatic switchover to sub-stoichiometric operation, especially during the first stage, resulting in safer operation for the operator and reduced nitrogen oxides at the outlet of the combustion oxidizer / combustion system; It is.

[0013] The method according to the invention is not subject to any restrictions regarding the phase of the waste stream, rather the waste stream may be any kind of fluid, i.e. gaseous or liquid waste stream.

[0014] Moreover, the method according to the invention is also not subject to any restrictions with regard to the composition of the waste stream. Thus, said waste stream comprises one or more compounds with a calorific value. Furthermore, said waste stream may also comprise inert compounds, i.e. compounds without a calorific value, such as carbon dioxide. Preferably, the waste stream comprises compounds with a calorific value and inert compounds.

[0015] In the context of the present invention, in relation to an oxidant such as air, air enriched with oxygen, or oxygen, the term substoichiometric is used as known to the skilled artisan to indicate that the amount of said oxidant involved is less than the stoichiometric amount of that oxidant.

[0016] According to the method of the invention, the waste stream originates from a process that is the basis of a known reaction. The compounds with a calorific value contained in the waste stream, as well as any inert compounds, i.e. compounds that do not have a calorific value, are therefore known. Based on the compounds fed to the combustion, the skilled person can formulate a balance equation that is the basis of the combustion reaction in question. The appropriate solution of said balance equation makes it possible to take the necessary measures to keep the operating values ​​constant.

[0017] The composition of the waste stream is substantially constant. It is therefore possible to set up a diagram or equation that gives the temperature of the combustion system as a function of the ratio of air, oxygen-enriched air or oxygen to the compounds with a calorific value and any inert compounds. Needless to say, the inert compounds are not oxidized in the combustion. However, the presence of inert compounds in the waste stream and in the combustion system may affect the temperature in the combustion system due to the heat capacity of the inert compounds. The method according to the invention makes use of the diagram or equation thus obtained. Any change in the flow rate in the waste stream automatically changes the ratio of air, oxygen-enriched air or oxygen to the compounds with a calorific value and any inert compounds. If the change in the flow rate of the waste stream is measured, the change in temperature resulting from said change in flow rate can be predicted. By solving the balance equation of the combustion system (combustion reaction), it is quite possible to take the necessary actions to keep the operating values ​​constant.

[0018] In one embodiment of the method according to the invention, the controller predicts the temperature for the sub-stoichiometric value adjusted in step d2) using a chart or an equation capable of determining the temperature of the combustion system and checks whether the predicted temperature is within an operating window for the combustion system.

[0019] In the context of the present invention, the term operating window or operating window for a combustion system is used to indicate a temperature range within which a combustion system can be operated, with a temperature limit at the lower end of said temperature range and with a temperature limit at the upper end of said temperature range (hereinafter also referred to as maximum temperature). The minimum temperature definition is given by the specific safety and environmental regulations for combustion for the specific combustion system, i.e. for the specific compound with a calorific value and any inert compound that is subjected to combustion, as well as the combustion of the specific compound. Therefore, hereinafter the term minimum temperature is also used as an equivalent to the minimum temperature allowed. The maximum temperature definition is given by the mechanical design conditions of the combustion unit.

[0020] In a preferred embodiment of the method according to the invention, the operating window has a temperature range having a minimum and a maximum temperature, the minimum temperature being given by the minimum temperature permitted for the combustion of the waste stream and the maximum temperature being given by the mechanical design conditions of the combustion unit.

[0021] Therefore, the sub-stoichiometric value adjusted in step d2) needs to be checked whether the corresponding temperature predicted by the controller is within or outside the operating window of the combustion system: if it is within the operating window, the sub-stoichiometric value is accepted, if it is outside the operating window, the sub-stoichiometric value is rejected.

[0022] In a further embodiment of the method according to the invention, the adjusted sub-stoichiometric value in step d2) is accepted if the temperature predicted by the controller is within an operating window of the combustion system.

[0023] In an alternative embodiment of the method according to the invention, if the temperature predicted by the controller is outside the operating window of the combustion system, the sub-stoichiometric value adjusted in step d2) is rejected and the method continues with the value adjusted in step c2).

[0024] In principle, the method according to the present invention is not specifically designed for combustion. Therefore, the method according to the present invention is not subject to any restrictions regarding the number of stages in combustion. Typically, a combustion unit has one or two stages. Nevertheless, it is preferred that the combustion in the method according to the present invention is a two-stage combustion.

[0025] In another embodiment of the method according to the invention, the combustion is a two-stage combustion.

[0026] Preferably, steps a) to d) are carried out in a first stage of combustion, which allows taking appropriate measures for the combustion in the second stage based on the conditions in the first combustion stage, e.g. temperature, to have a combustion outlet that meets safety, legal and environmental regulations regarding the composition of the combustion outlet.

[0027] In a preferred embodiment of the process according to the invention, steps a) to d) are carried out in the first stage in case of a two-stage combustion or in the first and second stages in case of a three-stage combustion.

[0028] For safety reasons, it is preferred that the ratio of air, oxygen-enriched air or oxygen to the compounds with calorific value and any inert compounds in the waste stream of step a) is all adjusted to a value that guarantees an excess of oxygen at the outlet of the combustion unit, in particular that the excess of air, oxygen-enriched air or oxygen is sufficient to guarantee complete combustion.

[0029] In another embodiment of the process according to the invention, the ratio of air, oxygen-enriched air or oxygen to the compounds having a calorific value and any inert compounds in the waste stream of step a) is all adjusted to a value which ensures an excess of oxygen at the outlet of the combustion unit.

[0030] In a further preferred embodiment of the method according to the invention, the excess of air, air enriched with oxygen, or oxygen is sufficient to ensure complete combustion.

[0031] At substoichiometric conditions, situations may arise where the first stage temperature measured or predicted in the first stage may reach or even exceed the maximum temperature. However, this temperature is outside the operating window of the method according to the invention. In such situations, it is preferred that the ratio of air, oxygen-enriched air, or oxygen to the compounds with calorific value and any inert compounds in the combustion system is changed to a value above the stoichiometric value.

[0032] In another preferred embodiment of the method according to the invention, if the temperature measured or predicted in the first stage reaches or exceeds a maximum temperature, the ratio of air, oxygen-enriched air or oxygen to the compounds having a calorific value and to any inert compounds in the combustion system is altered to a value above the stoichiometric value.

[0033] In the context of the present invention, the term sub-stoichiometric is used as known to those skilled in the art to indicate that the amount of a reagent included is greater than the stoichiometric amount of the reagent.

[0034] In detail, step c2) of the method according to the invention comprises measuring the temperature at the outlet of the first stage and the combustion unit, the temperature thus measured being set as a set point for the ratio of air, oxygen-enriched air or oxygen to the compound having a calorific value. The amount of fuel required to keep the temperature constant in the combustion system is then determined. Additionally or alternatively, the amount of air, oxygen-enriched air or oxygen to the compound having a calorific value and any inert compounds required to keep the temperature constant in the combustion system is determined.

[0035] In one embodiment of the process according to the invention, step c2) comprises c2a) measuring the temperature at the first stage and at the outlet of the combustion unit; c2b) setting the temperature measured in step c2a) as a set point for the ratio of air, oxygen-enriched air or oxygen to compound having a calorific value in the first stage or at the outlet; c2c) determining the amount of fuel required to maintain a constant temperature in the combustion system; and / or c2d) determining the amount of air, oxygen-enriched air, or oxygen required to maintain a constant temperature in the combustion system relative to the compound having a calorific value; Includes.

[0036] The technical features of the method according to the invention can be implemented in a control logic for a thermal combustion system.

[0037] Therefore, another object of the present invention is to provide a control logic for a thermal combustion system, comprising: i) a flow meter (1) for measuring the flow rate of a waste stream containing compounds having a calorific value; ii) a temperature indicating controller (2) for predicting whether a change in flow rate measured by the flow meter will result in a temperature change in the combustion of the waste stream; iii) a feedforward block (3) for increasing or decreasing the amount of fuel supplied to the combustion system and / or for increasing or decreasing the amount of air, oxygen-enriched air, or oxygen supplied to the combustion system; and iv) an extremum-seeking controller for adjusting the ratio of air, oxygen-enriched air, or oxygen to a calorific value compound in a combustion system to a substoichiometric value (4); The control logic has the following functions:

[0038] The control logic according to the present invention is not subject to any limitations regarding the phase of the waste stream, rather the waste stream may be any kind of fluid, i.e. gaseous or liquid waste stream.

[0039] Moreover, the control logic according to the invention is also not subject to any limitations regarding the composition of the waste stream. Thus, said waste stream comprises one or more compounds with a calorific value. Furthermore, said waste stream may also comprise inert compounds, i.e. compounds without a calorific value, such as carbon dioxide. Preferably, the waste stream comprises compounds with a calorific value and inert compounds.

[0040] In one embodiment of the control logic according to the present invention, the combustion has two stages, each stage including a flow meter, a temperature indicating controller, and a feed forward block.

[0041] A feedforward block is preferably implemented for each stage of the combustion system. The outlet of each temperature indicating controller (stage 1 and outlet) is a set point of the ratio of air, oxygen enriched air, or oxygen to calorific compounds and inert compounds. At each stage, a feedforward block, also defined as the ratio of air, oxygen enriched air, or oxygen to calorific compounds and inert compounds, is added to the outlet of each temperature indicating controller (1). This ensures that the control logic responds quickly to disturbances in the waste stream flow rate.

[0042] Feedforward is derived from the adiabatic energy balance of the system, assuming that instantaneous combustion of the compounds occurs in the combustion chamber, without considering, among other things, time delays or any heat transfer to the walls. The mathematical problem underlying feedforward is to maintain the temperature in the combustion chamber constant when changes occur in the flow rate of the waste stream. Mathematically, this means that the derivative of the temperature in the combustion chamber over time should be maintained equal to zero. This condition allows the adiabatic energy equation to be rewritten for every change in the flow rate of the waste stream, which in turn allows the adjustment of the fuel flow rate required to maintain a constant temperature in the combustion system (2).

[0043] By a further transformation of each of these equations, the change in the ratio of air, oxygen-enriched air, or oxygen to the calorific and inert compounds can be calculated, which is the feedforward term added to the outlet of the temperature controller.

[0044] Alternatively, the results obtained in (2) can be used in a feedforward block that acts directly on the fuel flow controller, in which case the feedforward block is taken out of the temperature controller outlet (1) and executed on the setpoint of the fuel flow controller.

[0045] In another embodiment of the control logic according to the invention, the feedforward block is adapted to implement steps c2a) to c2c) or c2a), c2b) and c2d) of the method according to the invention.

[0046] In a preferred embodiment of the control logic according to the invention, only the first stage contains an extremum-seeking controller.

[0047] Working Example: Here, the example is carried out using a computational model of the process based on the method shown in FIG. 1. Process modeling is an established and reliable methodology that engineers use to simulate complex chemical processes before building an actual plant. In the context of this example, the combustion system was modeled in a proprietary time-resolved Excel-based simulation. For ease of calculation, the model is adiabatic and contains one zero-dimensional perfectly stirred reactor for each stage. These assumptions make the model (here the resulting combustion temperature) much more sensitive to various flow rate and composition changes, but are sufficient to demonstrate the effectiveness of the controller.

[0048] Using the tools described above, we simulate a change in one of the waste streams (with a low calorific value <5 MJ / kg, and therefore 10 orders of magnitude less than natural gas) as a jump function (0 Nm in one iteration). 3 / h~15000Nm 3 / h). This large flow rate change is what theoretically leads to the high temperature peaks observed in stages 1 and 2 immediately after the waste gas jump. Note that in a real system, this jump cannot occur fast, since the recorded temperatures react much slower than indicated by the theoretical calculations. Therefore, the initial jump can be ignored for this demonstration.

[0049] This large peak inevitably triggers the temperature controller to adjust its output. Without feedforward (feedforward off), the first and second stage combustion temperatures drop more after several iterations than when feedforward is on. The deviation to the temperature setpoint is reduced by more than 50% with the feedforward switch on, and this large gain demonstrates the advantage of the proposed controller method.

[0050] The advantages of the control logic shown in this theoretical simulation are confirmed in practice, since the proposed concept has been retrofitted with two combustion oxidizers, both of which show that the temperature spikes stay well within the operating range when large waste flow rate variations occur (most of the time interruptions of up to 30% of the total combustion chamber volume and heat flow rate).

[0051] This performance can be clearly seen from Figure 4 where the supply of the same waste stream used for the simulation and counting for a heat load of 13 MWth is interrupted (plant trip). Immediately after the waste stream is shut off, the natural gas flow rate increases rapidly, compensating for the loss of the heat load. A temperature drop of only 33°C is recorded.

[0052] Less than 60 minutes after the loss of waste event, the same waste stream was pumped back to the oxidizer at a slightly slower rate than during the waste stream interruption. This time, the temperature of the first stage was barely affected by the waste stream initiation. This indicates that the control logic responded well regardless of the direction of waste stream switch to the oxidizer and the relative amount of energy sent to the oxidizer. [Brief description of the drawings]

[0053] [Figure 1] FIG. 2 is a detailed schematic diagram of a control system according to the present invention; [Diagram 2] FIG. 1 illustrates a John Zink controller. [Diagram 3] FIG. 13 shows a comparison of simulations with and without a feedforward block. [Figure 4] FIG. 13 illustrates the effectiveness of the control scheme in a real combustion chamber.

[0054] Description of the drawings: FIG. 1 is a detailed schematic diagram of a control system according to the present invention having multiple fuels (white circles) and O2 sources (grey circles), with the following reference numerals: (1) a waste fuel flow transmitter; (2) Temperature Indicator Controller (TIC), (3) Feedforward block, (4) Extremum-seeking controller, (5) Differential set point (Δ-SP), (6) Fuel flow transmitter, (7) The sum of (1) and (6), (8) Fuel and minimum air from Δ, (9) Set point (SP), (10) Process variable (PV), (11) Flow Indication Controller (FIC), (12) Fail-close, (13) Fuel gas or fuel oil to the burner; (14) Waste air flow transmitter; (15) Combustion air flow transmitter; (16) The sum of (43) and (44), (17) Process variable (PV), (18) Transmitters for steam load or oxygen control showing analysis (oxygen); (19) Transmitters indicating analysis (oxygen); (20) Set point (SP), (21) High signal selector, (22) Set point (SP), (23) Flow Indication Controller (FIC), (24) Fail open, (25) Combustion air to the burner has.

[0055] FIG. 2 shows a John Zink controller with multiple fuel (white circles) and O2 sources (grey circles), with the following reference numbers: (30) Waste fuel flow transmitter; (31) Fuel flow transmitter, (32) The sum of (31) and (32), (33) Temperature Indicator Controller (TIC), (34) Process variable (PV), (35) low signal selector, (36) Set point (SP), (37) Flow Indication Controller (FIC), (38) Fail-close, (39) Fuel gas or fuel oil to burners; (40) Waste air flow transmitter; (41) Combustion air flow transmitter; (42) The sum of (40) and (41), (43) An oxygen trim transmitter showing analysis (oxygen); (44) Oxygen trim controller showing analysis (oxygen); (45) Oxygen trim low limit selector; (46) Oxygen trim high limit selector; (47) Multiplication function, (48) Process variable (PV), (49) High signal selector, (50) Set point (SP), (51) Flow Indication Controller (FIC), (52) Fail open, (53) Combustion air to the burner has.

[0056] FIG. 3 shows a comparison of simulations with and without the feedforward block.

[0057] Figure 4 shows the effectiveness of the control scheme in a real combustion chamber during a large variation in one of the waste streams. The top graph shows a 18600 Nm 3 The graph in the middle represents a waste stream with an initial flow rate of 750 Nm3 / h and a heating value of 13 MW. The waste stream is abruptly stopped and restarted after 60 minutes. 3This represents the fuel flow to the second burner with an initial flow rate of 22 MW / h and a heating value of 8 MW. When the waste stream flow is stopped, the fuel flow automatically and quickly ramps up to a heating value of 22 MW. When the waste stream resumes, the fuel flow returns to the initial flow rate. The initial combustion temperature is approximately 1150°C. If the waste stream and its 13 MW are suddenly removed from the combustion chamber, the combustion temperature is not significantly affected, as it drops by 33°C to 1150°C - 1117°C.

Claims

1. 1. A method for controlling a thermal combustion system, comprising: a) measuring the flow rate of a waste stream comprising compounds having a calorific value, said waste stream arising from a process underlying a known reaction; b) if a change in flow rate is measured in step a), predicting whether the flow rate measured in step a) will lead to a temperature change in the combustion of the waste stream in step a), said predicting using a chart or equation capable of determining the temperature of the combustion system; c) if a temperature change is expected in step b), c1) calculating the ratio of air, oxygen-enriched air, or oxygen to the heating value compounds in the waste stream of step a) required to maintain a constant temperature in the thermal combustion system; c2) adjusting the ratio calculated in step c1) by increasing or decreasing the amount of fuel supplied to the combustion system and / or by increasing or decreasing the amount of air, oxygen-enriched air, or oxygen supplied to the combustion system; A step of carrying out and d) when the waste stream of step a) also contains bound nitrogen, d1) determining whether the amount of nitrogen oxides formed in the combustion system can be kept to a minimum when the combustion is carried out at the ratio of step c2); d2) if the requirements of step d1) are not met, adjusting the ratio of air, oxygen-enriched air or oxygen to compounds having a calorific value in the combustion system to a substoichiometric value that keeps the amount of nitrogen oxides formed in the combustion system to a minimum, said substoichiometric value being adjusted by an extremum-seeking controller; A step of carrying out A method comprising:

2. 2. The method of claim 1, wherein the controller predicts the temperature for the substoichiometric value adjusted in step d2) using a chart or equation capable of determining the temperature of the combustion system, and checks whether the predicted temperature is within an operating window for the combustion system.

3. 3. The method of claim 2, wherein the operating window comprises a temperature range having a minimum temperature and a maximum temperature, the minimum temperature being given by a minimum temperature permitted for combustion of the waste stream and the maximum temperature being given by mechanical design conditions of a combustion unit.

4. The method of claim 2 or 3, wherein the adjusted sub-stoichiometric value in step d2) is accepted if the temperature predicted by the controller is within the operating window for the combustion system.

5. 4. The method of claim 2 or 3, wherein if the temperature predicted by the controller is outside the operating window for the combustion system, the sub-stoichiometric value adjusted in step d2) is rejected and the method continues with the value adjusted in step c2).

6. The method of any one of claims 1 to 5, wherein the combustion is a two-stage combustion.

7. 7. The process according to claim 1, wherein steps a) to d) are carried out in a first stage in the case of a two-stage combustion or in a first and a second stage in the case of a three-stage combustion.

8. 8. The method according to claim 6 or 7, wherein the ratio of air, oxygen-enriched air or oxygen to compounds having a calorific value in the waste stream of step a) is all adjusted to a value that ensures an excess of oxygen at the outlet of the combustion unit.

9. The method of claim 8, wherein the excess air, oxygen-enriched air, or oxygen is sufficient to ensure complete combustion.

10. 10. The method of any one of claims 6 to 9, wherein if the temperature measured or predicted in the first stage reaches or exceeds a maximum temperature, the ratio of air, oxygen-enriched air, or oxygen to compounds having a calorific value and any inert compounds in the combustion system is changed to a value greater than stoichiometric.

11. The step c2) c2a) measuring the temperature at the first stage and at the outlet of the combustion unit; c2b) setting the temperature measured in step c2a) as a set point for the ratio of air, oxygen-enriched air or oxygen to compound having a calorific value in said first stage or at said outlet; c2c) determining the amount of fuel required to maintain a constant temperature in the combustion system; and / or c2d) determining the amount of air, oxygen-enriched air, or oxygen relative to the compound having a calorific value required to maintain a constant temperature in the combustion system; The method according to any one of claims 1 to 10, comprising:

12. 1. A control logic for a thermal combustion system, comprising: i) a flow meter (1) for measuring the flow rate of a waste stream containing compounds having a calorific value; ii) a temperature indicating controller (2) for predicting whether a change in flow rate measured by said flow meter will result in a temperature change in the combustion of said waste stream; iii) a feedforward block (3) for increasing or decreasing the amount of fuel supplied to the combustion system and / or for increasing or decreasing the amount of air, oxygen-enriched air, or oxygen supplied to the combustion system; and iv) an extremum-seeking controller (4) for adjusting the ratio of air, oxygen-enriched air, or oxygen to the calorific value compounds in the combustion system to a sub-stoichiometric value; The control logic has

13. The control logic of claim 12 , wherein the combustion has two stages, each stage including a flow meter, a temperature indicating controller, and a feed forward block.

14. Control logic according to claim 12 or 13, wherein the feedforward block is adapted to implement the steps c2a) to c2c) or c2a), c2b) and c2d) according to claim 11.

15. 15. A control logic as claimed in claim 13 or 14, wherein only the first stage includes an extremum-seeking controller.

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