Device and method for monitoring and controlling a plasma torch

Abstract

Device for monitoring and controlling a plasma torch (10) with a plasma generation unit (20), comprising a current and voltage supply (22) and at least one plasma supply element (25) with which energy can be supplied to a plasma (14) and in whose area of ​​influence the plasma (14) can be generated and maintained, wherein the plasma generation unit (20) is configured to modulate the voltage and / or current supply (22) for the plasma (14) in the acoustic frequency range and wherein a sound detection element (26) is provided with which sound generated by the plasma (14) can be detected and wherein a signal from the sound detection element (26) corresponding to the detected sound serves as a measure of the state of the plasma (14) and this signal is supplied as a control signal to the current and voltage supply (22) for controlling the energy supply to the plasma.

Description

[0001] The invention relates to a device for monitoring a plasma torch according to the preamble of claim 1 and to a method.

[0002] Burners, pilot burners, auxiliary burners, or the like have long been known and are used to ignite burners in industrial furnaces or, depending on their output, in large combustion plants used for heat generation, for example, in process and power plant engineering. These burners are often lance-like in shape and extend into the main burner to provide an ignition flame at the combustion site. The pilot burner has a supply unit that comprises, in a compact unit, an ignition transformer and flame detector or burner control unit with fuel feeds and electrical feeds for igniting and maintaining the pilot flame at the lance tip. Such burners typically operate with a flame whose condition—in particular, whether it is burning or not, or how intensely it burns—must be detected in order to correctly control the combustion process.The use of a plasma torch as a plasma loudspeaker for sound generation is described in DE 29 13 804 C2.

[0003] The invention now relates to ignition burners in which a plasma "burns" instead of a flame. Such burners are also called plasma torches. For example, DE 198 47 848 C1 describes a plasma torch. In contrast to conventional burners, which operate on the basis of a combustion reaction, electrically operated plasmas form considerably hotter flames or arcs, reaching temperatures of 2000 to over 10,000°C. In contrast to the flame of a combustion process, the spatial and thermal expansion of a plasma or arc is very small. The high energy content of the plasma in a relatively small volume shifts the emission of light into the blue spectral range and makes detection with conventional optical flame detectors difficult and expensive, as described, for example, in AT 508 696 A1.Recording the purely electrical variables for plasma generation could ensure that electrical power is being consumed, but not whether it was used to generate a plasma or to generate a plasma in the right place. Furthermore, methods for recording the plasma state via conductivity (ionization) or the plasma temperature via sensors are not suitable due to the high temperatures involved. For this reason, plasma-based torches are difficult and complex to monitor. For example, DE 38 20 858 A1, DD 22 39 56 A1 or DE 40 24 519 A1 each describe a method with which the weld or a weld cut is monitored acoustically. A microphone is used to record an acoustic signal generated by the plasma in the workpiece in question.EP 2 283 511 B1 describes a method by which properties of the electronics generating the plasma, namely the electrical noise and the electro-acoustic emission noise signal, are used to obtain information about the plasma via these noises.

[0004] Based on this, the object of the invention is to enable improved monitoring of a plasma-based burner.

[0005] This object is achieved by a device for monitoring and controlling a plasma torch with a plasma generation unit having the features of claim 1 and a method having the features of claim 11.

[0006] A device according to the invention comprises a plasma generation unit with a current and voltage supply and at least one plasma supply element with which energy can be supplied to a plasma and within whose effective range the plasma can be generated and maintained. According to the invention, the plasma generation unit is designed not only to supply energy to the plasma but also to modulate the voltage and / or power supply in the acoustic frequency range. Furthermore, a sound detection element is provided with which sound generated by the plasma can be detected, and a signal from the sound detection element corresponding to the detected sound serves as a measure of the state of the plasma.

[0007] This is based on the knowledge that sound is generated due to the expansion of the gas heated in the plasma. This is now exploited in the invention, and sound is specifically generated in the plasma in the acoustic frequency range. According to the invention, the sound is generated by simply modulating the energy supply, i.e. the current and / or voltage supply, in the acoustic frequency range. This modulates the plasma in the acoustic frequency range, and it can then be detected as sound using a sound detection element. In the simplest case, the sound can be detected very easily using a variety of conventional microphones as airborne or structure-borne sound. The very inexpensive and easily integrated microphones from mobile communications perform this task very well.

[0008] In its simplest form, this creates a simple plasma detector. When the microphone detects the desired sound (i.e., the modulated tone), a plasma exists; when the microphone detects the desired sound (i.e., the modulated tone), it does not. This can also be used to detect the state of a plasma. Since the sound emission is linked to the plasma itself, it can be used to detect the plasma state. Furthermore, the sound emission is also linked to the shape of the plasma and can therefore also be used to distinguish between desired and undesired plasma shapes.

[0009] The necessary modulation of the current and / or voltage supply is technically known in principle and can be implemented using simple and well-known means, which will be discussed further below.

[0010] In one embodiment, two plasma supply elements are provided and designed as electrodes. The plasma can be generated and maintained at the free ends of the electrodes. The electrodes are connected to the voltage and / or power supply via electrical supply lines.

[0011] Then, for example, the sound generated in the plasma can couple into the electrodes, and the microphone can capture the sound as structure-borne sound via at least one of the electrodes. Capturing structure-borne sound has the advantage of good transmission, thus enabling low-loss transmission of the sound generated in the plasma to the sound-capturing element.

[0012] The plasma itself can be excited capacitively or inductively by radio frequency or microwaves, thus also without the use of electrodes. For example, the plasma supply element can be designed as a resonator or a coil, or the capacitance of flat elements.

[0013] The plasma can be DC (direct current), LF (low frequency), MF (medium frequency), HF (high frequency), or RF (radio frequency) plasma. The invention is applicable to any type of plasma.

[0014] The generated sound can also be transmitted to the sound detection element via the gas in or with which the plasma is operated.

[0015] Advantageously, the acoustic frequency range is between 1 and 300 kHz and thus not only in the audible range, but also in the ultrasonic and infrasonic range.

[0016] At low sound intensities, it is sensible and advantageous to couple the signal of the sound detection element to the modulation signal and to amplify it in a phase-sensitive manner, as is common, for example, with a lock-in method.

[0017] In a further development of the invention, the signal of the sound detection element can be evaluated with regard to its intensity.

[0018] If the burner is designed with a burner tube, which is often the case with pilot burners, for example, and electrodes are provided for energy supply, it is advantageous to provide one electrode as the inner conductor in the burner tube and the other electrode as the outer conductor, which simultaneously forms the burner tube. The sound detection element can then detect the structure-borne sound on the inner conductor and / or outer conductor.

[0019] The task is solved by a method for monitoring a plasma torch with the following steps: - generating a plasma with a plasma generation unit, - Maintaining the plasma by supplying energy via a plasma supply element, - Modulating the energy supply to the plasma with a modulation device in the acoustic frequency range, - detecting a sound generated by the plasma with a sound detection element, - Evaluation of a signal from the sound detection element corresponding to the detected sound in an evaluation unit as a measure of the state of the plasma.

[0020] The evaluation serves to establish a control system for the energy supply to the plasma via sound detection and signal evaluation. The plasma can then be controlled to a desired state in a control loop: energy supply – sound generation – sound detection – evaluation of the sound signal – energy supply.

[0021] The device or method according to the invention can advantageously be used to control the fuel supply to a higher-level combustion unit (e.g., a burner or steam generator). This is due to the approach of, for example, control rooms in power plants, which only allow fuel supply to the burner / steam generator when an ignition plasma is present. This requires a reliable signal coupled to the plasma (e.g., from the evaluation unit), which can be provided by the invention.

[0022] The invention will be explained in detail below using exemplary embodiments with reference to the drawings. The drawings show: Fig. 1 a schematic representation of a pilot burner with a device according to the invention; Fig. 2 to Fig. 4 embodiments of plasma supply elements; Fig. 5 Illustration of an inventive control of the plasma.

[0023] One in Fig. The ignition burner 10 shown in Figure 1 is designed as a plasma torch and generates a plasma 14 in a combustion chamber 12. This plasma 14 is maintained by a power supply and blown out into the combustion chamber 12 with a gas supply. The power and gas are supplied via a lance-like section 16 connected to a power supply unit 18. The power supply unit 18 comprises a plasma generation unit 20, which has a current and voltage supply 22 for the plasma.

[0024] The plasma 14 can be any type of plasma, e.g., DC, LF, MF, HF, or RF plasma. Depending on the type of plasma, corresponding plasma supply elements 25 are provided, in whose effective range the plasma is generated. If the plasma is capacitively excited, the plasma supply elements 25 are designed as electrodes 28, 30 connected to the current and voltage supply 22 ( Fig. 2). If the plasma is excited inductively, the plasma supply element 25 is designed as a coil 32 ( Fig. 3) and when excited by microwaves, the plasma supply element 25 is designed as a resonator, coaxial conductor or waveguide 34 ( Fig. 4).

[0025] In a first embodiment example ( Fig. 1) electrodes are provided, wherein an outer tube 28 of the lance-like section 16 has an outer conductor and an inner conductor 30.

[0026] The current and voltage supply 22 generates a corresponding high voltage for generating and maintaining the plasma 14. In addition, the plasma generation unit 20 has a modulation device 24 that imposes a modulation on the current or voltage for supplying energy to the plasma 14, which modulates the current or voltage within the acoustic frequency range, e.g., between 1 and 300 kHz.

[0027] Modulation is most easily achieved by modulating appropriate driver voltages. This is usually achieved via the regulator circuit of a switching power supply or the base of a power transistor, but in principle, it can be achieved via any electrical or electromagnetic power coupling into the plasma. This can be, for example, the high voltage for an arc or a magnetron, but also the filament voltage of an electron gun or RF generator tube, or even the modulation of a magnetic field via an electromagnet.

[0028] Furthermore, a sound detection element 26 is provided, with which the sound generated by the plasma 14 can be detected. This sound can be detected at suitable locations. For example, the sound can be detected as structure-borne sound on the outer conductor 28 or as structure-borne sound on the inner conductor 30 (in Fig. 1 shown in dashed lines). It is also possible to record the sound directly with a microphone in a burner wall (also shown in dashed lines in Fig. 1). Other possible detection methods are conceivable, e.g., using an ultrasonic transducer, which often operates on a piezoelectric basis. Other detection locations are also conceivable, as long as they can be reached by the sound generated by the plasma. The signals from the sound detection element 26 are evaluated in an evaluation unit 27 and displayed in a suitable manner and / or fed as a control signal to the energy supply unit 18 or the current and voltage supply 22, since the state of the plasma is derived from these signals. This can be used, for example, to set up a control system to keep the plasma in a desired state and to detect malfunctions.

[0029] In one embodiment, microwaves are used for a microwave plasma ( Fig. 4) via a traveling-wave tube (magnetron). The magnetron is operated with a direct voltage of several thousand volts. To generate this high voltage, the mains voltage is first rectified and then chopped using high-speed power switches (switching-mode power supply / inverter). The resulting alternating voltage or pulsating direct voltage is then stepped up to the desired voltage. A subsequent high-voltage rectifier stage then generates the desired HV direct voltage for operating the magnetron. The residual ripple is determined by the switching frequency and can, in the simplest case, be detected as an acoustic signal from the plasma.

[0030] In addition, the switching frequency is usually used as a variable for power control and can therefore also be periodically modulated with the acoustic frequency, which is much lower than the switching frequency. For example, with a switching frequency of 50 kHz, the acoustic frequency of a few kilohertz can be conveniently modulated. This frequency can pass through the HV rectifier without major changes and generates corresponding acoustic signals via the plasma. The detection of the acoustic signal at the switching frequency or modulated frequency serves as clear evidence of the existence of the plasma. Furthermore, the electron generator in the traveling wave tube (filament) can be modulated using both current and voltage in terms of power or temperature, and thus electron emission, which also leads to microwave modulation and sound generation in the plasma at acoustic frequencies.

[0031] In a further embodiment, an arc, such as can be used for a typical arc plasma, is modulated according to the invention. Here, too, switched-mode power supply technology has become established using so-called inverters, whereby in principle the same procedure is applied as in the previous embodiment. However, the alternating voltage or pulsating direct voltage clocked at the switching frequency is transformed to comparatively low voltages and high currents. Subsequent rectification can, but does not have to, take place. The residual ripple or a targeted modulation of the switching stage can be used for modulation with a characteristic acoustic frequency, which can be evaluated via the sound emission by the plasma as evidence of the generated plasma.

[0032] In a further embodiment, a microwave plasma is used again, in which the volume of the structure-borne sound can be used to distinguish between an arc and a plasma flame. The plasma is designed as a high-frequency arc that burns between the inner conductor 30 and the outer conductor 28 and is blown out with the dielectric, thus extending the inner conductor into free space as a flame-like plasma, as shown in Fig. 1 and Fig. 2. The acoustic signal is transmitted via the plasma 14 as structure-borne sound only to the electrically insulated coaxial inner conductor 30 and can therefore only be detected weakly. However, if the plasma 14 burns as a direct arc between the inner and outer conductors of the coaxial lance-like section 16, e.g., due to a failure of the exhaust air, the acoustic signal can also be detected via the non-insulated outer conductor 28 as quite clear structure-borne sound. Thus, a measure of the plasma state can be obtained simply from the difference in the volume of the detected sound.

[0033] In Fig.Figure 5 shows the method according to the invention for monitoring and controlling the plasma 14. In a first step, the plasma 14 is generated using the plasma generation unit 20. In the second step, the plasma is maintained by supplying energy via a plasma supply element, e.g., by supplying electrical power via electrodes. In the third step, the energy supply to the plasma is modulated according to the invention using the modulation device 24 in the acoustic frequency range. In the fourth step, the sound generated by the modulation in the plasma is detected using a sound detection element 26, and in the fifth step, the detected sound is evaluated in the evaluation unit 27.The evaluation result can be displayed in the simplest way, for example by a simple display of whether the plasma is burning or not, or can also be fed back to the energy supply unit, i.e. the current and voltage supply 22, as a control variable, so that a control system is set up to be able to regulate the plasma 14 in a desired state.

Claims

[1] Device for monitoring and controlling a plasma torch (10) with a plasma generation unit (20), comprising a current and voltage supply (22) and at least one plasma supply element (25) with which energy can be supplied to a plasma (14) and in whose effective range the plasma (14) can be generated and maintained, wherein the plasma generation unit (20) is designed to modulate the voltage and / or power supply (22) for the plasma (14) in the acoustic frequency range and wherein a sound detection element (26) is provided with which sound generated by the plasma (14) can be detected and wherein a signal of the sound detection element (26) corresponding to the detected sound serves as a measure of the state of the plasma (14) and this signal is supplied as a control signal to the current and voltage supply (22) for controlling the energy supply for the plasma. [2] Device according to claim 1, characterized bythat two plasma supply elements are designed as electrodes, at whose free ends the plasma can be generated and maintained and which are connected to the voltage and / or power supply via electrical supply lines. [3] Device according to claim 1, characterized by that the plasma supply element is designed as a resonator, antenna or as coil elements or capacitive elements, so that the energy can be coupled into the plasma without electrodes. [4] Device according to one of the preceding claims, characterized by that the acoustic frequency range is between 1 and 300 kHz. [5] Device according to one of the preceding claims, characterized by that the signal of the sound detection element can be evaluated in terms of its intensity. [6] Device according to one of the preceding claims, characterized bythat the sound detection element detects the structure-borne sound coupled into at least one of the electrodes. [7] Device according to one of the preceding claims, characterized by that the sound detection element detects the sound transmitted through a gas surrounding the plasma. [8] Device according to one of the preceding claims, characterized by that one electrode is formed by an inner conductor in a burner tube and the other electrode by the burner tube as an outer conductor and that the sound detection element detects the structure-borne sound on the inner conductor and / or outer conductor. [9] Device according to one of the preceding claims, characterized by that the plasma is excited capacitively, inductively or by microwaves and is formed as DC, LF, MF, HF or RF plasma. [10] Method for monitoring and controlling a plasma torch (10) comprising the steps: - generating a plasma (14) with a plasma generation unit (20), wherein a high voltage for generating and maintaining the plasma (14) is generated with a current and voltage supply (22) of the plasma generation unit (20), - maintaining the plasma (14) by supplying energy via a plasma supply element (25) of the plasma generation unit (20), - modulating the current and voltage supply (22) for the energy supply to the plasma with a modulation device (24) in the acoustic frequency range, - detecting a sound generated by the plasma with a sound detecting element (26), - evaluating a signal of the sound detection element (26) corresponding to the detected sound in an evaluation unit (27) as a measure of the state of the plasma (14), and - Supplying this signal as a control signal to the current and voltage supply (22) for controlling the energy supply for the plasma via the sound detection and evaluation of the signal. [11] Use of a device according to one of the preceding claims 1 to 9 or of the method according to claim 10 for regulating the fuel supply to a higher-level combustion unit.

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