EP0697564A1 - Method for controlling and monitoring - Google Patents

Abstract

A self-checking combustion monitor/regulator unit (8) for a heating installation (1) using either solid or liquid fuel, receives a signal from a single sensor (7) which continuously determines the concentrations of oxygen and hydrogen/carbon monoxide in the exhaust gases. The control unit (8) regulates the fuel and air intakes via the respective actuators (4, 5) in accordance with the measured characteristics of the flue gases (6). Predetermined data is used to define acceptable limits of combustion performance and the air supply is increased or reduced to ensure that operation is at all times within these constraints.

Description

The invention relates to a method for regulating and monitoring the combustion of a furnace according to the preamble of claim 1.

To save energy and avoid environmental damage, the monitoring and control of combustion processes in combustion plants is absolutely necessary. The measurement of the oxygen content in exhaust gases alone cannot provide an indication of complete combustion. It is therefore particularly important to record and reduce the proportions of the components that are not burned in the exhaust gas. These unburned components include carbon monoxide and hydrogen. If combustion is incomplete, hydrogen and carbon monoxide emissions always occur together in the exhaust gas. The exact ratio of hydrogen to carbon monoxide, on the other hand, can vary depending on the burner setting, load factor, fuel load, air temperature and air pressure. The occurrence of hydrogen as well as the occurrence of carbon monoxide in the exhaust gas can be used as a guide variable by which it can be recognized whether incomplete combustion is starting.
German patent application P 43 40 534.7 describes a method for regulating and monitoring a combustion system, the operating point of the combustion system being checked cyclically to determine whether its setting ensures the lowest possible excess of oxygen in the exhaust gas. Two sensors are used for the detection of the exhaust gas components, one for determining the oxygen content and the second for detecting of the hydrogen content in the exhaust gas. The signal inputs and signal outputs of the two sensors are connected to the signal inputs and outputs of a processing unit, from which, among other things, all fault messages are output. The output signal of the processing unit is fed to a control device. With its output signal, which is fed to an actuator, this can control the air supply for the combustion system with the aid of an air flap. The oxygen concentration in the exhaust gas can also be determined with the probe, which is provided for detecting the hydrogen, in the state of complete combustion. This makes it possible to use the two probes for mutual monitoring, which increases the safety of the system. A disadvantage of this method, however, is that two probes are required, which doubles the circuit complexity of the control.

The invention has for its object to provide a method for fail-safe monitoring and control of firing systems, for which a minimum of probes and regulating and control devices is required.

This object is achieved by the features of claim 1.

A special feature of the method according to the invention is that only one sensor is required for monitoring the furnace. Its signal is evaluated redundantly with the help of a control and monitoring unit. The control and monitoring unit processes the stationary signal from the sensor, the signals from the actuators for the fuel and air supply and the dynamic signal from the sensor. It also determines the differential change in the sensor signal with the change in the position of the actuator for the air supply. Here, use is made of the fact that the combustion system to be monitored and regulated has at least one adjustable air flap which is driven by a servomotor, and the respective position of the air flap is available to the regulation and monitoring device as a measured value. For regulation and In addition to the stationary sensor signal U, the combustion system is also monitored, the dynamic sensor behavior dU / dt and the differential change in the sensor signal with the change in the actuator position dU / dS. Another advantage of the method is that the functionality of the sensor is continuously checked. This check takes place on the one hand by registering the signal change of the sensor in the event of a brief change in the sensor temperature and on the other hand by registering a short signal increase when the firing system is started. The increase in the sensor signal is caused by a brief increase in hydrogen / carbon monoxide emissions when the furnace is started. The sensor used is described in DE-A-40 21 929. It has two measuring electrodes and a reference electrode. One of the measuring electrodes is oxidation-catalytically inactive and thus enables the hydrogen content in the exhaust gas to be recorded. The second measuring electrode is catalytically active. It can be used to determine the oxygen content in the exhaust gas. In pollutant-free operation, without oxidizable flue gas components, the oxygen concentration of the exhaust gas can be determined from the sensor signal. When combustible gas components such as hydrogen or carbon monoxide appear in the exhaust gas, the sensor signal assumes significantly higher values from which the concentration of the oxidizable gas components can be determined.

Fig. 1

eine Feuerungsanlage mit einer Regelungs- und Überwachungeinheit sowie einem Sensor,

Fig. 2

die Signalverläufe des Sensors,

Fig. 3, 4, und 5

die zeitlichen Verläufe des Sensorsignals,

Fig. 6

die Signalverläufe des Sensors bei Temperaturänderung,

Fig. 7

die Signalverläufe des Sensors nach dem Zünden des Brenners.

Further features essential to the invention are characterized in the subclaims. The invention is explained below with the aid of schematic drawings. Show it:

Fig. 1

a furnace with a control and monitoring unit and a sensor,

Fig. 2

the signal curves of the sensor,

3, 4 and 5

the time profiles of the sensor signal,

Fig. 6

the signal curves of the sensor when the temperature changes,

Fig. 7

the waveforms of the sensor after the burner has been ignited.

Fig. 1 shows a furnace 1, with a burner 2, a furnace 3, an actuator 4 for the fuel supply to the burner, an actuator 5 for the air supply to the burner, one Exhaust gas duct 6, a sensor 7 and a control and monitoring unit 8. The sensor 7 is installed in the exhaust duct 6 at the outlet of the combustion chamber 3. Its signal inputs and outputs 7A and 7B are connected to the signal inputs and outputs 8A and 8B of the control and monitoring unit 8. The signal outputs 8V and 8W of the control and monitoring unit 8 are connected to the signal inputs of the actuators 4 and 5 for the supply of fuel and the supply of air to the burner 2.

der Kurve U größer. Eine unvollständige Verbrennung wird von der Regelungs- und Überwachungseinheit 8 zusätzlich erkannt, wenn die Steigung $\text{α = dU/dS}$

betragsmäßig einen Grenzwert ${\text{α}}_{\text{GM}}{\text{= |dU}}_{\text{M}}{\text{/dS}}_{\text{M}}\text{|}$

bzw. ${\text{α}}_{\text{GÜ}}{\text{= |dU}}_{\text{Ü}}{\text{/dS}}_{\text{Ü}}\text{|}$

überschreitet. Diese Grenzwerte werden in der Regelungs- und Überwachungseinheit 8 ebenfalls einmal gespeichert. Somit kann der Übergang zu einer unvollständigen Verbrennung bei Luftmangel als auch bei Luftüberschuß durch die Regel- und Überwachungseinheit 8 auch auf diese Weise erkannt werden. Gegenmaßnahmen werden von der Regel- und Überwachungseinheit 8 automatisch eingeleitet. Diese können in der Erhöhung der Luftzufuhr bestehen, wenn die Feuerungsanlage in den Bereich des Luftmangels fährt, oder in einer Verminderung der Luftzufuhr, wenn die unvollständige Verbrennung durch Luftüberschuß erfolgt. Bei einem schon gealterten Sensor weist das Sensorsignal U_A beim Einsetzen einer unvollständigen Verbrennung eine geringere Steigung ${\text{α}}_{\text{A}}{\text{=dU}}_{\text{A}}\text{/dS}$

auf, als bei einem neuen Sensor. Die Steigung α_AM bzw. α_AÜ ist jedoch auch jetzt noch größer als ein festgesetzter und gespeicherter Grenzwert α_OM bzw. α_OÜ. Die Grenzwerte α_OM und α_OÜ werden aus dem in Fig.2 aufgetragenen Signalverlauf U_O ermittelt. Dieser ist über der jeweils zugehörigen Position S des Stellgliedes 5 aufgetragen. Der Signalverlauf U_O entspricht dem eines Sensor 7, wenn dieser als reiner Sauerstoffsensor arbeitet oder seine Funktionsfähigkeit verloren hat, Wasserstoff bzw. Kohlenstoff zu erkennen. Jeder Stellgliedposition S ist ein Wert α_O zugeordnet. Diese Werte α_O entsprechen der Steigung ${\text{α}}_{\text{O}}{\text{= dU}}_{\text{O}}\text{/dS}$

des Sensorsignals U_O bei der jeweiligen Stellgliedposition S. Beim Erreichen der Grenzwerte α_OM bzw. α_OÜ erkennt die Regelungs- und Überwachungseinheit 8, daß die Feuerungsanlage in einen Betrieb unvollständigen Verbrennung übergeht. Werden die Werte α_O von dem Sensorsignal U des Sensors 7 erreicht oder unterschritten, so erkennt die Regelungs- und Überwachungseinheit 8, daß der Sensor 7 wegen Überalterung ausgetauscht werden muß.2 shows various states of the furnace 1. Curve U shows the course of the stationary sensor signal, which can pass through areas A, B, C. In area A there is incomplete combustion due to lack of air, in area B there is incomplete combustion due to excess air and in area C there is complete combustion. As shown in Fig. 2, incomplete combustion can occur due to lack of air as well as excess air. If the excess of air is very high, the flame is cooled and the flame is incompletely burned due to the low temperature of the flame. In area C of complete combustion, the residual oxygen in the flue gas can be determined from the stationary sensor signal. In this area, a conventional lambda control can be carried out with the sensor 7. If the burner 2 moves into an area of incomplete combustion, the emission of unburned gas components such as hydrogen and carbon monoxide increases. The consequence of this is that the value of the stationary sensor signal U changes. Incomplete combustion is recognized by the control and monitoring unit 8 when the value of the stationary sensor signal U exceeds a defined limit value U _GM or U _GÜ . As can be seen from the course of the stationary sensor signal U, the limit value U _GM before the transition to area A with lack of air is greater than the limit value U _GÜ before the transition to area B with excess air. These limit values are stored once in the control and monitoring unit 8. If the combustion system moves from the state of complete combustion in the direction of incomplete combustion, the control and monitoring unit 8 recognizes when one of these limit values U _GM or U _{GÜ reaches} whether the combustion system 1 is in the state of incomplete Combustion caused by lack of air or excess air.
In FIG. 2, the signal curve U of the sensor 7 is plotted over the respectively associated position S of the actuator 5. As can be seen from FIG. 2, the slope also increases with the increase in hydrogen and carbon monoxide in the exhaust gas $\text{α = dU / dS}$

the curve U is larger. Incomplete combustion is additionally recognized by the control and monitoring unit 8 when the slope $\text{α = dU / dS}$

a limit in terms of amount ${\text{α}}_{\text{GM}}{\text{= | dU}}_{\text{M}}{\text{/ dS}}_{\text{M}}\text{|}$

respectively. ${\text{α}}_{\text{GÜ}}{\text{= | dU}}_{\text{Ü}}{\text{/ dS}}_{\text{Ü}}\text{|}$

exceeds. These limit values are also stored once in the control and monitoring unit 8. Thus, the transition to incomplete combustion in the event of a lack of air as well as an excess of air can also be recognized by the control and monitoring unit 8 in this way. Countermeasures are initiated automatically by the control and monitoring unit 8. These can consist of an increase in the air supply when the combustion system moves into the area of lack of air, or a reduction in the air supply if the incomplete combustion takes place due to excess air. In the case of an already aged sensor, the sensor signal U _{A has} a smaller gradient when an incomplete combustion is started ${\text{α}}_{\text{A}}{\text{= you}}_{\text{A}}\text{/ dS}$

than with a new sensor. However, the slope α _AM or α _AÜ is now even greater than a fixed and stored limit value α _OM or α _OÜ . The limit values e α _OM and α _OÜ are determined from the signal curve U _O plotted in FIG. This is plotted over the respectively associated position S of the actuator 5. The signal curve U _O corresponds to that of a sensor 7 if it works as a pure oxygen sensor or has lost its ability to detect hydrogen or carbon. A value α _{O is} assigned to each actuator position S. These values α _O correspond to the slope ${\text{α}}_{\text{O}}{\text{= you}}_{\text{O}}\text{/ dS}$

of the sensor signal U _O at the respective actuator position S. When the limit values α _OM or α _{OÜ are reached} , the control and monitoring unit 8 recognizes that the combustion system is incomplete combustion. If the values α _{O are} reached or undershot by the sensor signal U of the sensor 7, the control and monitoring unit 8 recognizes that the sensor 7 must be replaced due to aging.

Another security check for monitoring the furnace can be derived from the dynamic course of the sensor signal dU / dt. This will be explained with reference to FIGS. 3, 4 and 5. In area C, when there is complete combustion, the oxygen content in the exhaust gas changes only slowly. Accordingly, the sensor voltage changes only slowly over time, ie dU / dt is small. If the combustion system 1 is in the state of incomplete combustion irrespective of whether there is a lack of air or excess air, hydrogen or carbon monoxide is emitted. These emissions do not occur uniformly, but more or less pulsating depending on the flame pattern. Sensor 7 follows these rapid changes in hydrogen and carbon monoxide emissions. The sensor signal becomes restless. As shown in FIG. 5, the dynamic signal curve dU / dt exceeds a limit value G _D. Regardless of the stationary sensor signal, the control and monitoring unit 8 can use the course of the dynamic signal to recognize whether the combustion system is in the state of incomplete combustion or not. Countermeasures are then automatically initiated by the control and monitoring unit 8.

All the above-described values α _O and limit values U _GM , U _GÜ , α _GM , α _GÜ , α _OM , α _OÜ , α _AM , α _AÜ , G _D , which are required for monitoring the furnace 1, are preferably used when commissioning the Firing system stored in the control and monitoring unit 8. For this purpose, the combustion system is brought into the states that can occur during its later operation.
The functionality of the sensor 7 itself can also be monitored by registering the sensor signal change when the sensor temperature changes briefly. A brief change in the sensor temperature will cause a brief change in the sensor signal in a functional sensor 7. This test can be performed either when the burner is shut down in air or when the burner is pre-vented, or while it is operating during a complete combustion condition. The temperature change is caused, for example, by a brief change in the heating voltage, as shown in FIG. 6. If no change in the sensor voltage dU is detected, the measuring electrode (not shown here) is faulty and the sensor 7 must be replaced.

The test as to whether the measuring electrode (not shown here) of the sensor 7 is still able to detect hydrogen or carbon monoxide can be carried out during the starting process of the burner. This test is explained with reference to FIG. 7. When the burner is ignited, there is inevitably a brief hydrogen / carbon monoxide emission, which the sensor 7 detects when its sensitive function is in order. If the sensor 7 does not recognize the rise in hydrogen and / or carbon monoxide shortly after the ignition process, it is defective and must be replaced. This is also displayed by the control and monitoring unit 8.

Claims (8)

Method for regulating and monitoring the combustion of a furnace (1) with a burner (2) for solid and flowing fuels, which is followed by a sensor (7), the actuators (4 and 5) for the fuel and air supply for the burner (2) can be controlled by a control and monitoring unit (8), to the signal inputs of which the sensor (7) is connected, characterized in that, for monitoring and regulating the furnace (1), the signal from the sensor (7) is evaluated redundantly and together is processed by the control and monitoring unit (8) with the actual value of the actuator (5) for the supply of air. Method according to Claim 1, characterized in that the limit values (U _GM and U _GÜ ) of the stationary sensor signal (U) are stored in the control and monitoring unit (8) when they are exceeded in the areas of incomplete combustion in the event of excess air or lack of air, and in that the course of the sensor signal is continuously monitored by the control and monitoring unit (8) and the air supply is increased or reduced when one of these limit values (U _GM or U _GÜ ) is reached. Method according to Claim 1, characterized in that the limit values are derived from the course of the signal (U) from the sensor 7, which is plotted over the respectively associated position (S) of the actuator (5) ${\text{α}}_{\text{GM}}{\text{= | dU}}_{\text{M}}{\text{/ dS}}_{\text{M}}\text{|}$

respectively. ${\text{α}}_{\text{GÜ}}{\text{= [you}}_{\text{Ü}}{\text{/ dS}}_{\text{Ü}}\text{|}$

are determined in which a transition to incomplete combustion takes place in the absence of air or excess air, that with these limit values stored in the control and monitoring unit 8 (α _GM and α _GÜ ) the transition to incomplete combustion is recognized, and countermeasures in the form of a Increasing or reducing the air supply can be initiated automatically. Method according to one of claims 1 to 3, characterized in that to detect the malfunction of the sensor (7) the signal curve (U _O ) of the sensor 7 with exclusive oxygen sensitivity is plotted over the respectively associated position (S) of the actuator (5) and a value (α _O ) is determined for each actuator position (S), which is the slope ${\text{α}}_{\text{O}}{\text{= you}}_{\text{O}}\text{/ dS}$

of the sensor signal (U _O ) at the respective actuator position (S) corresponds to the fact that these values (α _O ) as well as the limit values (α _OM or α _OÜ ) at which complete combustion changes to the state of incomplete combustion are detected and once in the control system - And monitoring unit (8) are stored so that reaching or falling below these values (α _O ) or the limit values (α _OM or α _OÜ ) by the signal (U) of the sensor (7) with the help of the control and monitoring unit ( 8) recognized and the transition to incomplete combustion and / or the aging of the sensor (7) is displayed. Method according to one of claims 1 to 4, characterized in that to detect the incomplete combustion, the signal curve dU / dt is monitored by the control and monitoring unit (8) and, when a limit value (G _D ) is exceeded, that in the control and monitoring unit (8) is stored, a control of the combustion system (1) can be carried out for complete combustion. Method according to one of claims 1 to 5, characterized in that for checking the sensor (7) with the aid of the control and monitoring device (8) the heating voltage of the sensor (7) is changed and if the sensor signal (U) remains unchanged, the sensor () 7) is displayed by the control and monitoring unit (8). Method according to one of Claims 1 to 6, characterized in that the short-term emission of hydrogen / carbon monoxide is used to check the sensor (7) when the burner is switched on and, with the sensor signal (U) unchanged, the sensor (7) malfunctions from the control and monitoring unit (8) is displayed. Method according to one of claims 1 to 7, characterized in that all values (α _O ) and limit values (U _GM , U _GÜ , α _GM , α _GÜ , α _OM , α _OÜ , α _AM , α _AÜ , G _D ), which are required for monitoring the combustion system 1, are stored in the control and monitoring unit 8 for the later regulation and monitoring when the combustion system (1) is started up.

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