GB2085253A - Monitoring flames by microwaves - Google Patents

Abstract

For monitoring a flame or flames in a furnace, microwave radiation is transmitted, e.g. from an antenna 14 at one side of the furnace, through the flame or flames to a receiver 20 to determine whether or not the radiation has been attenuated by passage through a flame. The radiation is preferably modulated and the modulation detected. Low frequency fluctuations in the signal may be measured and the signal may be processed to determine the fractional fluctuation in flame electron density. <IMAGE>

Description

SPECIFICATION Method and apparatus for flame monitoring This invention relates to a method of and apparatus for monitoring a flame or flames in a furnace.

Particularly in multi-burnerfurnaces, such as in boilers for large electrical power generating stations, the desirability of automatically monitoring flames has long been recognised. If fuel continues to be supplied to a burner, after the flame has been extinguished, the fuel may re-ignite explosively. A human observer can identify a particular flame but continuous human observation is expensive in manpower. Many systems for flame detection have been proposed. In general, such techniques rely on electromagnetic radiation emitted by a flame at optical or near optical frequencies. It is also known however to monitor the pressure fluctuations associated with a flame. While these methods are effective with single flames, they tend to be less reliable in multiple flame furnaces.In a large boiler for example, there may be many burners and it is difficult to have a line of sight through only one flame, particularly bearing in mind that the form of the flame depends on the fuel supply and other factors. Difficulties are aggravated with burners using pulverised fuel, because of the highly luminous and ill-defined nature of the flame.

According to one aspect of the present invention, a method of monitoring a flame in a furnace comprises the steps of radiating electromagnetic energy, in the microwave frequency range, through the location of a flame or flames to be monitored towards a receiving aerial and sensing, from the radiation received at said receiving aerial, the presence or absence of attenuation and/or phase change in the radiated signal path due to passage through the flame. By the microwave frequency range is meant frequency between 300 Mhz and 300 Ghz. Preferably the frequency employed is in the range of 300 Mhz to 3 Ghz.

Microwave radiation is absorbed mainly by ionisation in the flame. This ionisation mainly arises from chemical reactions in the flame, which release heat. A smaller contribution arises from thermal ionisation of easily ionisable impurities. Thus the ionisation is strongly associated with the flame temperature. Hence the presence of a hot flame can be deduced and, under favourable conditions, an indication of its size and temperature can also be deduced from determination of the attenuation in the radiation path. It is preferred to utilise frequencies towards the lower frequency end of the above-defined microwave frequency range.

Low frequencies have the advantage that the receiving aerial can collect more power for a given transmission rating. In a furnace such as that of a large boiler, electromagnetic radiation at these frequencies causes standing wave patterns and other wave interference phenomena. These can greatly affect the received signal level. In general it may be necessary to adjust the position of the aerials empirically so that a maxima in the received signal is obtained. Such adjustments are less critical at lower frequencies. When adjusted for a maximum signal, the system is relatively insensitive to small changes in the antenna position or to frequency variation.

The choice of the frequency is not critical but depends to some extent on anticipated ionisation levels and furnace geometry. It has been found that a frequency of the order of 1 Ghz is suitable for furnaces for large boilers such as are employed in electrical power generating stations.

The transmitting and receiving aerials may be located to transmit the radiation transversely through a flame from a burner. With pulverised coal flames, which are generally cooler than oil and gas flames and have substantially lower levels of ionisation, it may be advantageous to direct the radiation axially or substantially axially along the length of the flame to be monitored. The increased path length through the flame gives increased absorption and hence gives increased sensitivity in detecting the presence or absence of a flame. Particularly with pulverised fuel burners, it is a common practice to have an oil or gas pilot flame; in this case the radiation path may be chosen to pass through both the pilot flame and the main flame so that both these can be monitored.Discrimination between the pilot flame and main flame may be made by determination of the different levels of absorption.

The radiation may be modulated, for example by switching on and off at a frequency, which is low compared with the microwave frequency but which is preferably above 1 KHz, e.g. at a few KHz, so that, by detecting the modulation at the receiver, the received signal can be distinguished readily from electrical interference. Use of such modulation thus facilitates the employment of low transmitter power levels.

It has been found useful, after obtaining a signal representative of the microwave absorption, to determine the fluctuation in that signal arising from fluctuation in the absorbing power of the flame. These are low frequency fluctuations and conveniently the AC component in the range of 0.1 to 10 Hz is separated. From these fluctuations, the fractional fluctuation in flame density may be determined to give an indication of flame stability.

In large furnaces, there may be a regular array of burners. Typically such burners are arranged over a vertical wall of the furnace chamber. Flames from such an array of burners may be monitored by directing radiation in beams along each row and column. Cross talk in the various receivers may be avoided by energising the transmitters for the various rows and columns sequentially with the appropriate receivers being operative in synchronism with the various transmitters. Alternatively the transmitters may be arranged to operate on different frequencies or the various transmitters may be arranged to employ distinguishable modulations.

The invention furthermore includes within its scope a furnace incorporating at least one aerial for radiating microwave radiation disposed inside the furnace, means outside the furnace for energising said radiating aerial and at least one receiving aerial inside the furnace arranged to receive radiation from a transmitting aerial after passing through the location of a flame, said receiving aerial being connected to means outside the furnace responsive to the received signal and arranged for determining whether or not the radiation has been attenuated and/or changed in phase by passage through a flame.

As indicated above, the aerials may be positioned so that the radiation passes transversely through a flame or they may be arranged so that the radiation passes along or substantially along the length of the flame. The aerials may be dipoles e.g. halfwave dipoles mounted a quarter wavelength from the furnace wall and constructed of suitable heat-resisting material, e.g. formed of stainless steel with alumina insulators. If more directionai aerials are required, waveguide fed horns or reflectors or slot aerials may be employed.

In the following description of embodiments of the invention, reference will be made to the accompanying drawings in which: Figure lisa diagram illustrating a monitoring system for monitoring a single flame in a furnace; and Figure 2 is a diagram illustrating a modification of part of the apparatus of Figure 1.

Referring to Figure 1 there is shown diagrammatically a burner 10 for injecting fuel into a furnace to produce a flame 11. This furnace is part of a boiler and parts of the furnace wall are shown at 12, which furnace wall being formed of tubes through which water or steam is passed to be heated. Such a furnace wall will act as a reflector for microwave frequencies. A transmitting aerial 14 in the form of a centre fed halfwave dipole is located within the furnace at a distance a quarter wavelength from the wall, the dipole being connected by a coaxial feeder 15 to a microwave generator 16 operating, in this particular embodiment, at a frequency of 1 GHz. The transmissions are modulated by a moduiator 17 which switches the transmitter on and off to produce a radio frequency signal switched at a frequency of a few KHz.The aerial and feeder system are made of heatproof materials e.g. stainless steel or electrically conductive elements and alumina for insulating elements.

A receiving aerial 20 is located in the furnace in a position such that radiation from the transmitting aerial to the receiving aerial passes through the location of the flame 11. This receiving aerial, in this particular embodiment, is similarto the transmitting aerial, being a halfwave dipole spaced a quarterwavelength in front of the wall of the furnace and is connected by a feeder 22 to a tuned receiver and detector 23. The receiving aerial and feeder are constructed of heat-resistant materials, for example similar to those employed in the transmitting aerial. The output of the receiver and detector 23 is amplified by amplifier 24 and applied to a recorder 25.

The magnitude of the detector output will deopend on the presence or absence of the flame. With radiation at 1 GHz passing through a 16 MW oil fired flame transversely to the length of the flame at a distance of about 1 metre from the burner nozzle, it has been found that the amplitude of the receiver output, using a square law diode detector, was attenuated by about 40% due to the presence of a flame, compared with the signal in the absence of any flame. The amplitude level recorded by the recorder 25 therefore gives an immediate visible indication as to whether a flame is present or absent. Further information however may be obtained from more detailed consideration of the record. The optimum air to fuel ratio corresponds approximately to a maxima in the temperature of a flame and this in turn corresponds to a minimum received microwave signal.Hence the absolute level of the recorded signal gives an indication as to whether the burner is operating satisfactorily. The signal received will fluctuate and hence the recorder will display fluctuations of varying amplitude and frequencies. These are related to variations in the flame and can give additional information on flame stability and quality.

Fouling of the aerials and furnace wall may, particularly because of the presence of standing wave patterns, cause drift in the received signal level, as also may changes in a flame position due to changes in furnace condition or in other flames. Analogue and/or digital data logic processing means indicated at 26 may be provided to interpret such data.

Thermal and pressure effects may cause change in the relative position of the aerials; the transmitter frequency may drift. For these reasons, it may be preferred to provide an automatic frequency control unit 27 to adjust the frequency of the receiver and detector unit in order to maximise the received signals.

Although reference has been made more particularly to determining whether the received signals have been attenuated by ionisation in a flame, such ionisation also causes a phase change and, in some cases, it may be preferred to detect this phase change, e.g. by using phase comparison means comparing the phase of a signal received at the receiving aerial with a signal fed from the transmitter via an alternative transmission path.

To enable the integrity of the aerials to be checked, an auxiliary aerial or aerials may be provided with means for receiving signals from the main transmitting aerial (or aerials) and for transmitting signals to the main receiving aerial (or aerials) without flames in the transmission paths. The auxiliary aerial or aerials enable a check to be made that the main aerials are operated satisfactorily.

In some circumstances, whole furnace monitoring is desired and use may be made of a beam which passes through all or most of the flames in the furnace.

It has been found useful in some cases to separate, from the signal representaive of the microwave absorption, the AC component of that signal which arises from fluctuations in the absorbing power of the flame. By separating a component typically in the rage 0.1 to 10 Hz, an output signal may be obtained which in some circumstances gives a measure of flame stability.

A better stability parameter would be the fractional fluctuation in the flame electron density, f(ne). A simple analysis shows that this is given by:

where a is the rms value of the fluctuation in absorption signal, Vv is the DC value of absorption signal and V0 is the DC value when no flame is present.

Figure 2 illustrates diagrammatically a modification of part of the apparatus of Figure 1 for determining this fractional fluctuation in flame electron density. The output from the amplifier 24 is passed to a filter 30 passing the components in the range 0.1 to 10 Hz to give a signal representing the rms value of the fluctuation in absorption and is also passed to a DC level detector 31. The outputs from units 30,31 are fed to a data processor 32 in which is stored the DC value of the absorption when no flame is present. This processor 32 computes the fractional fluctuation in flame electron density using the above formula and displays this on an indicator 33. The DC component of the signal representing absorption is displayed at 34 also as an analogue signal on a logarithmic scale since the absorption increases logarithmically with increasing electron density in the flames.

Claims (20)

1. A method of monitoring a flame in a furnace comprising the steps of radiating electro-magnetic energy, in the microwave frequency range (as hereinbefore defined), through the location of a flame or flames to be monitored towards a receiving aerial and sensing, from the radiation received at said receiving aerial, the presence or absence of attenuation and/or phase change in the radiated signal path due to passage through the flame.

2. A method as claimed in claim 1 wherein the frequency of the radiation is in the range of 300 Mhz to 3 Ghz.

3. A method as claimed in either claim 1 or claim 2 wherein the transmitting and receiving aerials are located to transmit the radiation transversely through a flame from a burner.

4. A method as claimed in either claim 1 or claim 2 wherein the transmitting and receiving aerials are located to direct the radiation axially or substantially axially along the length of the flame to be monitored.

5. A method as claimed in either claim 1 or claim 2 and for monitoring a flame from a pulverised fuel burner having an oil or gas pilot flame wherein the transmitting and receiving aerials are located so that the radiation path passes through both the pilot flame and the main flame.

6. A method as claimed in any of the preceding claims wherein the radiation is modulated at a frequency which is low compared with the microwave frequency but which is above 1 KHz and wherein the modulation is detected at a receiver.

7. A method as claimed in any of the preceding claims wherein, from a signal representative of microwave absorption, low frequency components in the range of 0.1 to 10 Hz are detected to determine fluctuations in the microwave absorption.

8. A method as claimed in claim 7 wherein means are provided for deriving, from the signal representative of microwave absorption, an output representative of fluctuation in flame electron density.

9. A furnace incorporating at least one aerial for radiating microwave radiation disposed inside the furnace, means outside the furnace for energising said radiating aerial and at least one receiving aerial inside the furnace arranged to receive radiation from a transmitting aerial after passing through the location of a flame, said receiving aerial being connected to means outside the furnace responsive to the received signal and arranged for determining whether or not the radiation has been attenuated and/or changed in phase by passage through a flame.

10. A furnace as claimed in claim 9 wherein the aerials are positioned so that the radiation passes transversely through a flame.

11. A furnace as claimed in claim 9 wherein the aerials are positioned so that the radiation passes along or subsantially along the length of the flame.

12. A furnace as claimed in either claim 9 or claim 10 and having an array of burners arranged in rows and columns wherein the aerials are positioned to direct radiation along each row and column.

13. A furnace as claimed in claim 12 wherein the transmitters for the various rows and columns are energised sequentially.

14. A furnace as claimed in claim 12 wherein the transmitters for the various rows and columns operate on different frequencies.

15. A furnace as claimed in claim 12 wherein the transmitters for the various rows and columns have modulation means arranged for distinguishably modulating the various transmitters.

16. A furnace as claimed in any of claims 9 to 15 wherein the receiver is responsive to the amplitude of the received signals and arranged to determine whether the received radiation has been attenuated by passage through a flame.

17. A furnace as claimed in any of claims 9 to 15 and having means for determining, from the received signal, the low frequency fluctuations in absorption of microwave signals.

18. A furnace as claimed in claim 17 and having signal processing means to determine and to indicate the fractional fluctuation in flame electron density.

19. A furnace with flame monitoring means substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings or with reference to Figure 1 as modified in Figure 2 of the accompanying drawings.

20. A method of monitoring a flame in a furnace substantially as herein before described with reference to Figure 1 of the accompanying drawings or with reference to Figure 1 as modified in Figure 2 of the accompanying drawings.