Large boilers, for example, those used in conjunction with steam turbines for electric power generation, are fired by fuels such as coal, oil, gas or liquor. Supporting igniter burners are typically associated with each of the main burners. Because the igniter burners are typically fired with relatively expensive fuels, they are operated only intermittently. More particularly, the igniter burners are preferably fired only upon initial start up of the boiler and thereafter they are only selectably fired for short intervals to light off or support flame at the particular main burner(s) associated with the igniter burner(s).
The prior art has evolved a variety of flame detection techniques for monitoring boiler fires to detect the presence or absence of flame in the boiler regions supported by the various igniter burners. If flames are extinguished in a particular region of the boiler, then the "no flame" condition must be quickly identified or else the main burners continue to supply fuel which may potentially explode if it is not evenly and continuously ignited. Accordingly, highly reliable flame monitoring techniques are required for continuously detecting the presence of flame at regions within the boiler adjacent to each of the burners which fire the boiler.
The apparatus to be described in this application is suitable for use with two types of boiler/burner configurations; namely, "wall" (or "opposed") fired boilers, and "corner" (or "vortex") fired boilers. "Wall" or "opposed" fired boilers incorporate a series of burners mounted on two opposing walls of the four vertical walls of the boiler. Sighting tubes (pipes about 5 cm. in diameter) are positioned across the boiler walls (which are typically about 1.5 meters thick) beside and nearly parallel to each burner head. The sighting tubes are pointed approximately toward the expected location of burner flame. Flame detection apparatus is positioned to "sight" through each tube into the boiler region in which flame is expected.
"Corner" or "vortex" fired boilers incorporate vertically separated stacks of burners in each of the four corners of the boiler. The flames produced by the burners merge in a central vortex within the boiler. The burners may be individually tilted in the vertical plane in order to better control the combustion characteristics and location of the fireball within the boiler. Sighting tubes for corner fired boilers must be flexible so that the flame detection apparatus can continuously track the flame as the burners tilt.
Several prior art flame detectors examine the light emitted by the flame and, from the time variation characteristics of these emissions, determine whether a flame is located near to the burner ("near flame"); or, a fireball is present in the background ("far flame"); or, there is no detectable flame. By monitoring flame flicker (i.e. time variations in the light signal emitted in the frequency band(s) under consideration) such prior art detectors attempt to derive a binary signal representative of "flame" and "no flame" conditions. Pre-determined factors such as the geometry of the detector, the wavelength band it is capable of examining, and the frequency band being monitored affect the characteristics of flame flicker and correspondingly determine the ability of such detectors to accurately detect the presence or absence of flame under varying conditions.
The best prior art flame detectors for use on opposed fired boilers appear to be those which utilize two separate linear arrays of detectors aligned horizontally and vertically to facilitate "X-Y" scanning of selected sub-regions within a region where flame is expected, through electronic selection of an appropriate detector pair. Typically, a zero-crossing waveform shaping analysis is performed on the electronic signals produced by each of the two selected detectors, to generate two bi-level output signals. The output signals are then correlated with one another (prior art detectors of this sort do not however perform true signal correlation, because they work only with binary (i.e. two level) approximations of the detector output signals, rather than with the direct analog outputs of the detectors). If the two signals are highly similar to one another then the correlation result approaches unity. Normally, a result which exceeds some predetermined threshold is accepted as indicating the presence of flame. If the two signals are highly dis-similar to one another then the correlation result approaches zero. A result which does not exceed the aforementioned threshold is normally taken to indicate a "no flame" condition. In some cases, automatic tracking techniques are employed to locate points of maximum correlation in an effort to minimize generation of false "no flame" alarms. It will thus be understood that the prior art is susceptible to error, in that the cumulative approximations inherent in the operation of prior art detectors may result in a "no flame" alarm when flame is in fact present; or, may indicate that flame is present when no flame is in fact present. The prior art tends to overcompensate for these contingencies by allocating flame determination thresholds which minimize generation of false "flame present" signals. However, this necessarily decreases the ability of such prior art devices to respond to flame conditions having light emission characteristics which encompass a large dynamic range.
The inventors believe that superior results may be obtained by concentrating on factors other than flame flicker. More particularly, the inventors believe that superior results may be obtained by analyzing the time.fwdarw.frequency spectral characteristics of light emitted in different visual and infra-red wavebands from the region in which flame is expected, and comparing those characteristics with prestored spectral signatures representative of flame. The present invention accordingly compares short term estimates of the visible and infra-red auto-spectra, the infra-red to visible transfer function, and the infra-red to visible coherence (all of which are hereinafter defined and explained in greater detail), with prestored signatures characteristic of "flame" and "no flame" conditions. The auto-spectra, transfer function and coherence function are used to characterize the relationship between two signals in selected frequency bands. It is this relationship or pattern which is used to identify the flame.