GB2132342A

GB2132342A - Discrimination between flames of different fuels

Info

Publication number: GB2132342A
Application number: GB08331542A
Authority: GB
Inventors: Richard Booth
Original assignee: Land Combustion Ltd
Current assignee: Land Combustion Ltd
Priority date: 1982-12-14
Filing date: 1983-11-25
Publication date: 1984-07-04
Also published as: GB2132342B; GB8331542D0

Abstract

A method of discriminating between flames of different fuels comprising sensing the light intensity emitted by a flame with sensing means having an output signal determined by said intensity, monitoring the fluctuation in the said intensity, determining the amplitudes of the said fluctuation at a higher and lower frequency and deriving the ratio of the said higher and lower frequency amplitudes, to provide a signal output which by virtue of the numerical value of said ratio signifies that the flame being monitored is of a particular fuel. <IMAGE>

Description

SPECIFICATION Discrimination between the flames of different fuels This invention relates to discrimination between flames produced by the burning of different fuels.

In such areas as single and multi-burner boilers, and particularly those of high output, it is frequently the case that pulverised coal is used as fuel for the burner. However, pulverised coal is not particularly easy to ignite, and accordingly it is frequently the case that in addition to a pulverised coal burner, an oil burner is provided and which is used to ignite the pulverised coal, and occasionally as an additional energy source. It is also not unknown to provide a gas burner for the sole purpose of igniting the oil burner which, in turn, has to ignite the pulverised coal. Once the pulverised coal has been ignited, the oil burner may be shut-down and only re-ignited either at a fresh start-up of the boiler or in an instance where the pulverised coal burner has extinguished, or when further power is required for the boiler.

Similarly, it is usual to shut-down the gas burner and to re-ignite it when required to ignite the oil burner.

In the normal way, after ignition of the oil burner, it is customary to have control means whereby the oil burner is maintained ignited for a pre-determined period of time at the start-up of a boiler, to allow, e.g., a supply mill to feed pulverised coal through the necessary supply lines to the boiler. If for any reason there is a longer delay than expected before pulverised coal arrives at the burner, any non-selective flame-in/flameout monitoring system would be unable to distinguish between the expected stable coal flame and the actual oil flame, and would hence not detect a fault in advance of the shut-off of the oil flame. This would be particularly disadvantageous in a multi-burner boiler, as it would lead to pulverised unignited coal being passed to the boiler atmosphere with the then consequent danger of explosion.

In an attempt to alleviate this difficulty, attempts have been made hitherto to provide a means of discriminating between, particularly, an oil burner flame and a coal burner flame. Thus, it is known to utilise a method relying on the difference between the optical spectra of a coal flame and an oil flame, where an ultra-violet sensitive detector is used to detect the appreciable ultra-violet content of an oil flame spectrum. A flame detected by a second sensor, which shows little or no ultra-violet radiation is assumed to be a coal flame. Such a system has two important disadvantages. In the first place it requires two sensors which of necessity require separate signal processing, and as the two sensors cannot physically be in the same position, great care must be exercised to ensure that both detectors are accurately aligned and focused on to the same point in the flame.Secondly a very reliable flame monitor is required to indicate with certainty the existence of a coal flame.

It is also known to attempt to use the difference in position of the flame front of an oil and a coal burner to determine which flame is currently in being. However it is frequently the case, e.g., when concentric burners are used, that there is no appreciable difference between such flame fronts and hence this technique is not reliable.

The object of the present invention is to provide a means of discriminating between the flames of different fuels which avoids the disadvantages referred to above.

According to the present invention a method of discriminating between flames produced by the burning of different fuels comprises sensing the light intensity emitted by a flame with sensing means having an output signal determined by said intensity, monitoring the fluctuation in the said intensity, determining the amplitudes of the said fluctuation at a higher and lower frequency and deriving the ratio of the said higher and lower frequency amplitudes, to provide a signal output which by virtue of the numerical value of said ratio signifies that the flame being monitored is of a particular fuel.

As an alternative within the same principle, light intensity sensing means can have its output signal applied directly to a discrete fourier transform device to derive directly the spectrum of amplitude fluctuation of light intensity from the flame, the ratio of a higher frequency and lower frequency amplitude of said spectrum again being derived and again the numerical level of the ratio identifying the flame being monitored as the flame of a particular fuel.

Thus, when applied particularly to a boiler having an oil burner for the purpose of igniting a coal burner, on ignition of the oil burner the method of the invention results in an output from, e.g., micro-processor means associated with the sensing means, dictated by the numerical value of the derived ratio. Said signal can then be applied to any suitable means such as a visual display or an audible alarm and at the onset of the coal flame a different numerical level of ratio is provided which when passed to a visual or audible alarm provides accurate advice to the operative that the burner is functioning correctly.If, on the other hand after ignition of the oil flame the known predeterminable time elapses without the coal flame being ignited, the maintenance of a signal stemming from the numerical value of the ratio generated by monitoring the oil flame, can itself be used to provide a visible or audible signal to advise the operative that the coal flame is not ignited.

Quite obviously during the operation of a boiler should a coal flame inadvertently be extinguished then there would be no output which fact can be utilised to provide an immediate visual or audible signal that a flame out condition has occurred.

In practise, it is preferred to monitor a flame within two fixed frequency ranges by suitable filtering, e.g., electronic filtering. Preferably the two fixed filtering ranges are 1 OOHz + 20Hz and 300Hz + 50Hz.

One example of the invention is illustrated in the accompanying drawing, which shows in block diagram form means for putting into effect the method of the invention.

Thus in the drawing incident light from a flame falls on to a photo-electric sensor 1 the output of which is filtered by two band pass filters 2, 3, the filter 2 operating in the range 300Hz and the filter 3 in the range 1 00Hz. The filtered signal from each band pass filter is then passed to a respective rectifier 4 and then to a respective low-pass filter 5. One signal then passes to a multiplier 6 and on to a comparator 7 and the second signal is passed directly to the comparator 7, the output from which signifies the relationship between the ratio of the amplitudes of fluctuation (highest frequency to lowest frequency) and a preset value. If that signal is above the preset value, this signals that an oil flame is present and if lower than that value that a coal flame is present

Claims

1. A method of discriminating between flames produced by the burning of different fuels comprising sensing the light intensity emitted by a flame with sensing means having an output signal determined by said intensity, monitoring the fluctuation in the said intensity, determining the amplitudes of the said fluctuation at a higher and lower frequency and deriving the ratio of the said higher and lower frequency amplitudes, to provide a signal output which by virtue of the numerical value of said ratio signifies that the flame being monitored is of a particular fuel.

2. A method of discriminating between flames produced by the burning of different fuels comprising sensing the light intensity emitted by a flame with sensing means having an output signal determined by said intensity, applying said output directly to a discrete fourier transform device to derive directly the spectrum of amplitude fluctuation of light intensity from the flame, the ratio of a higher frequency and lower frequency amplitude of said spectrum being derived to provide a signal output which by virtue of the numerical value of said ratio signifies that the flame being monitored is of a particularfuel.

3. A method as in Claim 1 or Claim 2, wherein a flame is monitored within two fixed frequency ranges by suitable filtering.

4. A method as in Claim 3, wherein said filtering is electronic filtering.

5. A method as in Claim 3, wherein said two fixed frequency ranges are 100 Hz + 20 Hz and 300 Hz + 50 Hz.

6. A method of discriminating between flames produced by the burning of different fuels substantially as hereinbefore described with reference to the accompanying drawing.