WO1991015715A1 - Burner control - Google Patents

Burner control Download PDF

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Publication number
WO1991015715A1
WO1991015715A1 PCT/GB1991/000468 GB9100468W WO9115715A1 WO 1991015715 A1 WO1991015715 A1 WO 1991015715A1 GB 9100468 W GB9100468 W GB 9100468W WO 9115715 A1 WO9115715 A1 WO 9115715A1
Authority
WO
WIPO (PCT)
Prior art keywords
flame
tne
band
signal
zone
Prior art date
Application number
PCT/GB1991/000468
Other languages
French (fr)
Inventor
Wah H. Wan
Patrick Roy. Mullens
Original Assignee
Credfeld Camtorc Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Credfeld Camtorc Limited filed Critical Credfeld Camtorc Limited
Publication of WO1991015715A1 publication Critical patent/WO1991015715A1/en

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Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23NREGULATING OR CONTROLLING COMBUSTION
    • F23N5/00Systems for controlling combustion
    • F23N5/02Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium
    • F23N5/08Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using light-sensitive elements
    • F23N5/082Systems for controlling combustion using devices responsive to thermal changes or to thermal expansion of a medium using light-sensitive elements using electronic means
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23NREGULATING OR CONTROLLING COMBUSTION
    • F23N2223/00Signal processing; Details thereof
    • F23N2223/08Microprocessor; Microcomputer
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23NREGULATING OR CONTROLLING COMBUSTION
    • F23N2229/00Flame sensors
    • F23N2229/08Flame sensors detecting flame flicker

Definitions

  • This invention relates generally to tne control of fuel burners and in particular to the monitoring of in- dividual fuel burners in an array of such burners includ ⁇ ing, for example, the monitoring of the performance of a start-up burner.
  • the invention will find use in the monitoring of oil. gas and solid fuel burners but is expected to have particular application in a pulverised coal environment.
  • nstallat ons e.g. DOW ⁇ station boilers
  • a plurality of burners which operate in concert to generate the currently r ⁇ uired thermal output.
  • additional burners may need to be brought into operation and conversely, as the required thermal output decreases, burners may need to be taken out of operation.
  • Starting-uo a burner is an operation which requires proper sequencing of events and this is particularly so when a start-up flame using an easily ignitable start-up fuel (e.g. oil) is used to pre-heat the flame zone of a burner to receive a less readily combustible main fuel (e.g. pulverise ⁇ coal or biomass) .
  • start-up fuel e.g. oil
  • the pre-heating stage requires a good control system which will reliably indicate when the start-up fuel is ignited and optionally also when the main fuel is burning eff ciently.
  • This invention therefore seeks to provide a burnei— monitoring system which will obviate at least some cf tne above-noted disadvantages.
  • tnere is provided a method of monitoring a potential flame zone of a burner for the existence of a burner - g ⁇ neratec flarr.e cf a re ⁇ uired type, which method como ⁇ ses ⁇ irecting ra ⁇ iation received from and through at least a Dart of sai ⁇ zone onto a photo-sensitive area of an octo-elect ⁇ c transduce-' device, feeding the outDut cf the transducer ⁇ evice to circuit means adaDted to generate an outout signal wmcn is related to the radiation falling en the sensitive area and determining from the magnitude of a oarameter of tne cutDut signal whether there is or is not a flame of tne recu:red type in the flame zone, wmch method is characterised in that the circuit means induces a band-Dass filter to eliminate the contributions of radiation having freouencies of amolitude variation (
  • the centre fre ⁇ uency of the band-pass filter is tunable and an initial stage of the monitoring method involves setting said centre fre ⁇ uency on the Dasis of a determination of a difference between the outout signal when a flame of the required type is and is net present in said zone.
  • the said centre fre ⁇ uency will be selected to correspond to the maximum amolitude of said difference signal.
  • the pass band of the filter excludes the mains frequency at its low frequency end and exceeds ten times the mains frequency at its high frequency end.
  • the particular pass band selected will depend upon circurnstan- ces, but can be expected to lie somewhere in the range 70 tc SO Hz at the low frequency end and between 1 anc 2 KHz at the high frequency end.
  • a first output signal level ⁇ 'S ' j in the form of a frequency spectrum relating to the background radiation passing througn said zone, to determine a second outout sig -nal level ⁇ SC_____T____r ) in the form of a further frequency spectrum relating to the background radiation passing through said zone anc the radiation created by a flame in said zone and to create a difference signal (S_) which is the difference cetwee.n the signals S g+f _ and S ⁇ (i.e.
  • the range of flicker frequencies being monitored is desirably sub-divided into a substantial number (say 100 in a typical case) of sub-band memory locations and the frequency spectrum is stored as digital numbers (each say between 0 and 99) one in each sub-band memory location.
  • the set of memory locations used to receive the discrete sub-bands of the first output signal level (S ⁇ ) can be used to store the sub-bands of the difference signal (S_), so that the second output signal level (S_ p is used to modify the nurriDer stored in each memory location and is not separately stored as such.
  • a sighting tube leading directly towards tne flame zone is conveniently used to direct radiation to the photo ⁇ sensitive area of the opto-electric transducer an prefer ⁇ ably this tube includes lens means to focus the radiation from the flame zone onto the said area.
  • a single convex lens e.g. of 2.5 cm focal length and 22 mm diameter
  • Detection can be undertaken at any suitable wavelength but operation in the infra-red to near-ultraviolet range is preferred.
  • the method of the invention is expected to find particular utility in the monitoring of the individual burners of a wall-fired pulverised fuel (p.f.) boiler where oil is used for the start-up (or light-up) burner.
  • p.f. wall-fired pulverised fuel
  • the opto-electric transducer is suitably a silicon- based device (e.g. a VACTEC VTB 5050 UV) and the processing of the output signal is conveniently effected using a micro-computer (e.g. an Intel 8032).
  • the band-pass filter ⁇ ing is desirably arranged using a band-pass switched capac ⁇ itor filter circuit. In the case of the monitoring for the presence of gas-fired flame, it is desirable to operate at the ult violet end,of the spectrum and in these circumstances can be useful to use a gas discharge phototube as the opt
  • Apparatus for carrying out the method of the inventi which includes an opto-electric transducer and sign processing equipment to act on the electrical outout the transducer and generate a "flame-on" signal in t
  • the signal processing e ⁇ uipme includes means to determine a flicker frequency which i sensitive to the presence or absence of the looked-f flame, a band-pass filter to receive the outout of t
  • Figure 1 is a schematic view of a modern pluri-burne boiler
  • Figure 2 is the typical layout of a wall-fired boiler
  • Figure 3 is an enlarged cross-section through a flam wall of the boiler of Figure 2
  • Figures 4 and 4A are cross-section and end views respectively, of a p.f. oil start-up burner for use in th flame wall of Figure 3,
  • Figures 5 and ⁇ are power spectral density graons for oil flame flicker and p.f. flame flicker, respectively.
  • FIG. 7 is a schematic view cf flame-flicker cetec- tion equipment in accordance with this invention.
  • Figure S is a block circuit diagram of the electronic equipment for operating the equipment of Figure 7, and
  • Figures 9A to 9C are examples of frequency spectrum signals illustrating how the monitoring metno of the invention is employed in a practical case.
  • FIG. 1 A typical layout of a modern boiler and the associated plants is shown in Figure 1.
  • the main fuel is coal, which is delivered from bunkers 1 to mills 2, where it is ground before being blown into a furnace 3 by a "primary" air stream as pulverised fuel (p.f.).
  • the air required for combustion is blown by a forced draught fan 4 through an air-heater 5, where it is warmed by the flue gases which have passed through the boiler.
  • the 80% or so of air which does not go to the mill 2 is introduced to the furnace through burners 10 as "secondary" air via a windbox 6.
  • a fan 13 provides t suction to exhaust the furnace (which is usually run just sub-atmospheric pressure) and draws the flue g through the boiler, air-heater, precipitator and scrubDe to bring it to the base of the chimney 11 at about at mospheric pressure and 100-200°C. The gas is then drive up the chimney by its own buoyancy.
  • boiler firin configurations There is a wide variety of possible boiler firin configurations. However, the wall- or cornel—fired boiler are the two most commonly used boiler firing con iguration for large industrial boilers although other boiler firin configurations can be found.
  • FIG. 3 A cross-section of the flames produced in a fron wall-fired boiler is shown in Figure 3, the windbox agai being shown at 6 and the burners' flames at 14, Eac burner 10 is associated with a flame-sighting tube 15, onl one of which is shown and this is drawn out of position fo clarity.
  • FIG. 4A A mechanical layout of a typical oil burner is show in Figures 4 and 4A.
  • the primary air and coal enter th combustion chamber through pipe 20 (the p.f. outlet pipe which passes concentrically through a cylindrical secondar air register 21.
  • a coal soreader 24 At the exit 23 of the p.f. outlet pipe 20 is fitted a coal soreader 24 which diverts the p.f. in a conical stream into the surrounding secondary air.
  • the sorea ⁇ e>- 24 is suDDorted on tne end of a tube 25 whien passes ⁇ own tne centre of the p.f. outlet pioe 20 and whicn also carries the barrel of the oil start-uo burner 26.
  • tnus, tre oi anc p.f. burners are concentric.
  • the flame is igmtec i ⁇ .g. PV a prooane/elect ⁇ c torch which is sited alongside tne oil burner barrel within the burner carrier tube).
  • the radiation from a flame varies in amplitude at all times. That is, it is momentarily changing from brighter to dimmer to brighter and so on.
  • the rate of flicker is usua ly too fast to be seen by the eye and its freouency depends to some extent on the type of flame, but it has been found that a substantial amount cf 10-50 Hz flame flickers are present i all flames.
  • Power spectrum analysis is useful in characterizin random phenomena such as identifying sources of mechanica vibration and noise. It is also used in characterizing th energy content of a signal.
  • the idealized power spectra densities for both oil and pulverised coal flame flicke are shown in Figures 5 and 6. In these Figures the or dinate is power per Hertz and the abscissa is frequency i Hertz.
  • FIG. 7 shows a practical emoodiment of flame detec tion equipment in accordance with this invention.
  • Th equipment basically comprises a flame viewing head 30 and control unit 31 linked by a cable 36.
  • the head 30 includes a casing 35 housing a photo sensitive device 34 onto which radiation from a selecte region of a furnace or boiler is directed via a lens 33 i a sighting tube extension sleeve 32.
  • Conveniently th sleeve 32 is made to be an easy but secure fit to a existing sighting tube of a boiler or furnace whose flam performance is to be monitored but the features to achiev this have not been shown.
  • the casing 35 may include venting duct 37 to clear moisture from the optics therei when required.
  • the control unit 31 includes the components snown within the box 50 of the circuit shown in Figure 3 anc these will be discussed more fully hereafter, out tne unit also includes a digital display 38 and a lamp 39.
  • buttons 40-42 are buttons 40-42.
  • Button 40 is a reset button to set the tnresnolc level and buttons 41 and 42 are "initiate search" buttons which are used in setting up the equipment and in par ⁇ ticular putting into the computing memory of the e ⁇ -.oment (by using button 41) the digitized freouency spectrum cf the radiation entering the casing 35 when the flame ⁇ em ⁇ monitored is present in the foreground of the field of view of the head 30 against the background radiation z ⁇ ctner flames in the furnace or boiler and (by using butt r 42; storing the digitized freouency spectrum of only tne back ⁇ ground radiation entering the head 30.
  • the unit 31 also includes a mains supply leac 42 ana a connector 44 for linking the unit to a remote data process ⁇ ing station.
  • the circuit illustrated comprises a signal conditioning circuit 5"! receiving tne input from the cable -35 and an ac amplifier 52 wno ⁇ e gain is adjustable by an adjust control 53.
  • the outout from tne amplifier 52 is fed to a band-pass switched cacacitor filter (BSCF) 54 which receives three further inputs.
  • BSCF band-pass switched cacacitor filter
  • One input is on line 55 from a bank 56 of switches which set the effective "Q" of the BSCF, one input on a line 57 from a central microprocessor 58, and a final input on a line 59 from a clock generator 60 which is controlled by the processor 58.
  • the line 57 forms part of a self-check system whereby the electrical integrity of integers 54, 61, and 62 are periodically checked. Periodically (e.g. every 5 seconds) a self-check signal is issued by the microDrocessor 58. This signal is fed on line 57 anc is ⁇ esigned to turn off the output of the BSCF 54 and hence the output cf the threshold ⁇ etector 62. If the circuit is working properly, removal of the output from the detector 62 should result in a loss of the "flame-on" signal and during tne self-check operation the circuit looks to see that the outout from detector 62 does momentarily disappear. Thus, a self checking facility is implemented by issuing a self-check signal on the line 54 and monitoring the corresDon ⁇ ing response from the output of detector 62.
  • the BSCF acts as a digitally tunable band-pass filter and accordingly is used to generate a series of ac poten ⁇ tial values from the output of the photosensi ive device 34, one for each discrete frequency pass band in turn, which together scan the full range of frequencies cf interest (i.e. in a typical case 80 to 1500 Hz).
  • the ac values are converted to dc potentials in an ac/oc converter
  • the monitoring equipment described herein can also be use ⁇ to indicate the onset of non-optimum combustion con ⁇ itions in the burner being monitored.
  • first digital signal train representing the radiation received in the head 30 when no flame is present at the monitored burner so that it is pure background radiation which is detected (this signal being entered into the memory of processor 58 when button 42 is pressed).
  • This first signal train is represented graphi ⁇ cally in Figure 9A, but in practice would be stored as digital numbers in a hundred or so sub-band memory oca- tions which together cover the entire range of freouencies of interest.
  • the sub-band memory locations are desirably concentrated towards the low end of the spectrum le.g. 70 to 2000 Hz) since practice shows the optimum flicker frequency is likely to be at the low end for the looked-for flame type.
  • this third signal train is scanned to determine the frequency at which the peak amplitude occurs and the tunable bandpass filter is then tuned to target the frequency at which the peak amplitude occurred.
  • the signal obtained from the target flame is compared continuously with a set threshold level in the threshold detector 62 so that whenever the flame signal received, in the frequency range being targeted, is greater than the set threshold level a "flame-on" signal is generated and passed to the microprocessor 58. When the flame signal drops below the set threshold level the "flame-on" signal disap- pears and an appropriate warning is given.
  • the threshold detector 62 ignores signal amplitudes below the set threshold level but accepts signals at or above the set level.
  • the microprocessor 58 By virtue of the "tuning" of the bandpass filter to the flicker fre ⁇ uency of the looked-for type of flame, the appearance of a "flame- on” signal can reliably be used by the microprocessor 58 to transmit initiating signals to field devices (net shown) via a failsafe output unit 64 and the connector 44, and to associated controls such as the display 38 and the in ⁇ icat- ing light 39.
  • Random background frequency and existing flame sources are scanned and an analog signal obtained from these sources is passed through the tunable band pass filter 54 and thence through to the microprocessor 58.
  • the frequency spectrum obtained is accepted by the microprocessor and stored as a first series of digital references.
  • the flame is scanned and new analog signals are obtained from the flame. These signals are then passed through the same tunable band pass filter and thence through to the micro ⁇ processor. The signals obtained from this second source are accepted by the microprocessor and then either stored as a second series of digital references with the first series of references for subsequent differencing or the second series of digital references are directly subtracte ⁇ from the first series and stored as a new series of digital references.
  • the new series of digital references is then scanned to determine the frequency (or frequencies) at whicn tne peak amplitude occurs and the tunable band pass filter is tuned to target a frequency at which the peak amclitu ⁇ e occurred. This sets the unit 31 for future flame momtor- ing operations for the looked-for type of flame.
  • the current output of the opto-electric transducer device (34) is compared with a set threshold level ⁇ : that whenever the flame signal received is greater than the set threshold level a "flame-on" signal is passed to the microprocessor.
  • a common device i.e. the band pass filter
  • Optimum targeted flame detection will be obtained irrespective of (i) the loading of a boiler, i.e. the intensity of background flames and (ii) flame interferences from neighbouring burners.
  • the flame monitoring equipment can be easily and economically implemented using a microprocessor.

Abstract

Monitoring of a burner-generated flame in a flame zone is achieved by watching for changes in amplitude of radiation leaving the flame zone using an opto-electric transducer (34) and a processing circuit (50) which is tuned to a flicker frequency pass band targeted on the looked-for flame. A tunable band-pass switched capacitor filter (54) feeding into a threshold detector (62) via an AC/DC converter (61) is preferred.

Description

BURNER CONTROL
Technical Field
This invention relates generally to tne control of fuel burners and in particular to the monitoring of in- dividual fuel burners in an array of such burners includ¬ ing, for example, the monitoring of the performance of a start-up burner. The invention will find use in the monitoring of oil. gas and solid fuel burners but is expected to have particular application in a pulverised coal environment.
In large fuel-burning nstallat ons (e.g. DOWΘΓ station boilers) a plurality of burners are provided which operate in concert to generate the currently rεαuired thermal output. As the thermal output required increases, additional burners may need to be brought into operation and conversely, as the required thermal output decreases, burners may need to be taken out of operation. Starting-uo a burner is an operation which requires proper sequencing of events and this is particularly so when a start-up flame using an easily ignitable start-up fuel (e.g. oil) is used to pre-heat the flame zone of a burner to receive a less readily combustible main fuel (e.g. pulveriseα coal or biomass) .
The pre-heating stage requires a good control system which will reliably indicate when the start-up fuel is ignited and optionally also when the main fuel is burning eff ciently.
Where there are few burners physically well-spaced- apart conventional techniques can be used for burner monitoring and these include visually observing the flame zone. However, where there are closely packed banks of burners whose flame zones merge, conventional techniques have failed to provide reliable control, leading, inter alia, to the possibilities of inefficient combustion of main fuel, waste of start-uD fuel, ΠSKS of exdosicns and generation of excessive amounts cf environmentally DC! ut¬ ing waste gases.
This invention therefore seeks to provide a burnei— monitoring system which will obviate at least some cf tne above-noted disadvantages.
Summary of the Invention
Accorαing to one aspect of the invention tnere is provided a method of monitoring a potential flame zone of a burner for the existence of a burner - gεneratec flarr.e cf a reαuired type, which method comoπses αirecting raαiation received from and through at least a Dart of saiα zone onto a photo-sensitive area of an octo-electπc transduce-' device, feeding the outDut cf the transducer αevice to circuit means adaDted to generate an outout signal wmcn is related to the radiation falling en the sensitive area and determining from the magnitude of a oarameter of tne cutDut signal whether there is or is not a flame of tne recu:red type in the flame zone, wmch method is characterised in that the circuit means induces a band-Dass filter to eliminate the contributions of radiation having freouencies of amolitude variation (hereafter, for convenience, referred to as "flicker" freouencies) below or above the pass band, in that the Dass band of the filter is set to that part of the frequency SDectrum wnere the flicker frequency of a flame of the required type is detectable against background radiation passing through said zone (i.e. the radiation falling on the sensitive area when there is no burner-generated flame of the required tyDe in the flame zone), and in that the output signal in the pass band is used to determine whether a flame of the reouired type is present in the monitored flame zone.
Preferably the centre freαuency of the band-pass filter is tunable and an initial stage of the monitoring method involves setting said centre freαuency on the Dasis of a determination of a difference between the outout signal when a flame of the required type is and is net present in said zone. Normally the said centre freαuency will be selected to correspond to the maximum amolitude of said difference signal.
Preferably the pass band of the filter excludes the mains frequency at its low frequency end and exceeds ten times the mains frequency at its high frequency end. The particular pass band selected will depend upon circurnstan- ces, but can be expected to lie somewhere in the range 70 tc SO Hz at the low frequency end and between 1 anc 2 KHz at the high frequency end.
In practice we prefer to determine a first output signal level ι'S 'j in the form of a frequency spectrum relating to the background radiation passing througn said zone, to determine a second outout sig -nal level ι SC_____T____r ) in the form of a further frequency spectrum relating to the background radiation passing through said zone anc the radiation created by a flame in said zone and to create a difference signal (S_) which is the difference cetwee.n the signals Sg+f_ and Sβ (i.e. S- = SB+j - Sβ anc to use this difference as indicative of tne flicker freαuency signals actually arising in the flame zone and to tune the band pass filter so that it is centred on the frequency at which the peak amplitude of the difference signal (S_ ) occurs. In this way the band pass filter is set for optimum targeted flame discrimination.
The range of flicker frequencies being monitored is desirably sub-divided into a substantial number (say 100 in a typical case) of sub-band memory locations and the frequency spectrum is stored as digital numbers (each say between 0 and 99) one in each sub-band memory location. For economy of memory requirements, the set of memory locations used to receive the discrete sub-bands of the first output signal level (Sβ) can be used to store the sub-bands of the difference signal (S_), so that the second output signal level (S_ p is used to modify the nurriDer stored in each memory location and is not separately stored as such.
A sighting tube leading directly towards tne flame zone is conveniently used to direct radiation to the photo¬ sensitive area of the opto-electric transducer an prefer¬ ably this tube includes lens means to focus the radiation from the flame zone onto the said area. A single convex lens (e.g. of 2.5 cm focal length and 22 mm diameter) has been found to be a suitable lens means for this aoDlica- tion.
Detection can be undertaken at any suitable wavelength but operation in the infra-red to near-ultraviolet range is preferred.
The method of the invention is expected to find particular utility in the monitoring of the individual burners of a wall-fired pulverised fuel (p.f.) boiler where oil is used for the start-up (or light-up) burner. By means of the invention we have been able to determine whether one burner in a bank of 20 is ignited wnen all other 19 burners are operating and whether that one burner
> is operating satisfactorily on oil, oil and p.f. or just on p.f.. However, the invention is expected to find other applications in the monitoring of the performance of burners and its use with an oil/p.f. burner should be seen to be one example of possible uses and not as a limitation to use.
The opto-electric transducer is suitably a silicon- based device (e.g. a VACTEC VTB 5050 UV) and the processing of the output signal is conveniently effected using a micro-computer (e.g. an Intel 8032). The band-pass filter¬ ing is desirably arranged using a band-pass switched capac¬ itor filter circuit. In the case of the monitoring for the presence of gas-fired flame, it is desirable to operate at the ult violet end,of the spectrum and in these circumstances can be useful to use a gas discharge phototube as the opt
5 electric transducer.
Apparatus for carrying out the method of the inventi which includes an opto-electric transducer and sign processing equipment to act on the electrical outout the transducer and generate a "flame-on" signal in t
10 presence of a looked-for flame in a monitored flame zon is characterised in that the signal processing eαuipme includes means to determine a flicker frequency which i sensitive to the presence or absence of the looked-f flame, a band-pass filter to receive the outout of t
15 transducer, a threshold device to receive the outout of t band-pass filter and generate the "flame-on" signal a means to tune the band-pass filter to the determin flicker frequency. A particularly convenient circuit f processing the output signals from the transducer is
20 tunable band-pass switched capacitor filter feeding into RMS converter.
Brief Description of the Drawings
The invention will now be further described by refer ence to the accompanying drawings, in which:
25 Figure 1 is a schematic view of a modern pluri-burne boiler,
Figure 2 is the typical layout of a wall-fired boiler
Figure 3 is an enlarged cross-section through a flam wall of the boiler of Figure 2,
30 Figures 4 and 4A are cross-section and end views respectively, of a p.f. oil start-up burner for use in th flame wall of Figure 3,
Figures 5 and δ are power spectral density graons for oil flame flicker and p.f. flame flicker, respectively.
Figure 7 is a schematic view cf flame-flicker cetec- tion equipment in accordance with this invention,
Figure S is a block circuit diagram of the electronic equipment for operating the equipment of Figure 7, and
Figures 9A to 9C are examples of frequency spectrum signals illustrating how the monitoring metno of the invention is employed in a practical case.
Description of Preferred Embodiments
A typical layout of a modern boiler and the associated plants is shown in Figure 1.
The main fuel is coal, which is delivered from bunkers 1 to mills 2, where it is ground before being blown into a furnace 3 by a "primary" air stream as pulverised fuel (p.f.). The air required for combustion is blown by a forced draught fan 4 through an air-heater 5, where it is warmed by the flue gases which have passed through the boiler. The 80% or so of air which does not go to the mill 2 is introduced to the furnace through burners 10 as "secondary" air via a windbox 6.
After losing about 30% of the heat of combustion in the furnace by radiation to surrounding evaporator tubes, the flue gas passes over various stages of superheater 7, reheater 8 and economizer 9 surfaces, to which it loses another 50% by a combination of radiation and convection. Finally, a further 10% is transferred to the combustion air in the rotary regenerative air-heater 5 to be returned to the furnace. This small percentage is thus recycled, but about 10% is lost to the atmosphere through a chimney 11. Before reaching the chimney 11, the flue gas i cleaned of particulate matter in an electrostatic precipi tator 12 or a bag filter. Optionally, also, at least so of the sulphur dioxide is removed in a flue-gas desul phurization unit or scrubber. A fan 13 provides t suction to exhaust the furnace (which is usually run just sub-atmospheric pressure) and draws the flue g through the boiler, air-heater, precipitator and scrubDe to bring it to the base of the chimney 11 at about at mospheric pressure and 100-200°C. The gas is then drive up the chimney by its own buoyancy.
There is a wide variety of possible boiler firin configurations. However, the wall- or cornel—fired boiler are the two most commonly used boiler firing con iguration for large industrial boilers although other boiler firin configurations can be found.
A wall-fired p.f. boiler with individual burners 10 i shown in Figure 2. The burners may be arranged on one wal of the furnace, on two opposite walls (opposed-firing) o less commonly, on all four walls.
A cross-section of the flames produced in a fron wall-fired boiler is shown in Figure 3, the windbox agai being shown at 6 and the burners' flames at 14, Eac burner 10 is associated with a flame-sighting tube 15, onl one of which is shown and this is drawn out of position fo clarity.
A mechanical layout of a typical oil burner is show in Figures 4 and 4A. The primary air and coal enter th combustion chamber through pipe 20 (the p.f. outlet pipe which passes concentrically through a cylindrical secondar air register 21. There is a fixed degree of swirl given t the secondary air as it passes through the register 21 an into the burner by a number of peripherally spaced vane a 22. At the exit 23 of the p.f. outlet pipe 20 is fitted a coal soreader 24 which diverts the p.f. in a conical stream into the surrounding secondary air. The soreaσe>- 24 is suDDorted on tne end of a tube 25 whien passes αown tne centre of the p.f. outlet pioe 20 and whicn also carries the barrel of the oil start-uo burner 26. tnus, tre oi anc p.f. burners are concentric.
When the burner is operating on oil ii.e. in its start-uo mode) the flame is igmtec iβ.g. PV a prooane/electπc torch which is sited alongside tne oil burner barrel within the burner carrier tube).
The radiation from a flame varies in amplitude at all times. That is, it is momentarily changing from brighter to dimmer to brighter and so on. Some flames flιcκer at a low frequency. but even those flames that seem to be perfectly steady to the eye, do in fact have amplitude variations (which for convenience herein are being referred to as "flicker") and this flicker car oe detected by a photoelectric detector. The rate of flicker is usua ly too fast to be seen by the eye and its freouency depends to some extent on the type of flame, but it has been found that a substantial amount cf 10-50 Hz flame flickers are present i all flames.
A further characteristic of flames that is of interest to flame failure controls is the light transmission of the combustion products. When a flame is burning without enough air for clean burning, free carbon will be produced which causes a smoky flame and smoke in the combustion products.
The radiation properties of oil and p.f. flames are a result of the complex interaction between aerodynamic, chemical and thermal processes. These interactions αetet— mine the flame ignition patterns and propagation rates and the formation of flame flicker radiation and pollutants. Whereas periodic signals may be completely αescπo by their amplitudes, phases and frequencies, random signal are those where future behaviour cannot be preαicteα an may only be described by quantities such as the powe spectral density, and probability distribution.
Power spectrum analysis is useful in characterizin random phenomena such as identifying sources of mechanica vibration and noise. It is also used in characterizing th energy content of a signal. The idealized power spectra densities for both oil and pulverised coal flame flicke are shown in Figures 5 and 6. In these Figures the or dinate is power per Hertz and the abscissa is frequency i Hertz.
From a comparison between the oil flame spectrum o Figure 5 and the p.f. flame spectrum of Figure 6 it will b noted that at the low frequency end (e.g. below 30 Hz) bot exhibit strong signals but the contribution from the oi flame tails off more slowly at higher frequencies. Simila considerations apply to gas flames.
Figure 7 shows a practical emoodiment of flame detec tion equipment in accordance with this invention. Th equipment basically comprises a flame viewing head 30 and control unit 31 linked by a cable 36.
The head 30 includes a casing 35 housing a photo sensitive device 34 onto which radiation from a selecte region of a furnace or boiler is directed via a lens 33 i a sighting tube extension sleeve 32. Conveniently th sleeve 32 is made to be an easy but secure fit to a existing sighting tube of a boiler or furnace whose flam performance is to be monitored but the features to achiev this have not been shown. The casing 35 may include venting duct 37 to clear moisture from the optics therei when required. The control unit 31 includes the components snown within the box 50 of the circuit shown in Figure 3 anc these will be discussed more fully hereafter, out tne unit also includes a digital display 38 and a lamp 39. to give visual indication of the flame conciticn, and three buttons 40-42. Button 40 is a reset button to set the tnresnolc level and buttons 41 and 42 are "initiate search" buttons which are used in setting up the equipment and in par¬ ticular putting into the computing memory of the eα -.oment (by using button 41) the digitized freouency spectrum cf the radiation entering the casing 35 when the flame εemς monitored is present in the foreground of the field of view of the head 30 against the background radiation z~ ctner flames in the furnace or boiler and (by using butt r 42; storing the digitized freouency spectrum of only tne back¬ ground radiation entering the head 30.
The unit 31 also includes a mains supply leac 42 ana a connector 44 for linking the unit to a remote data process¬ ing station.
Referring now to Figure 8, the circuit illustrated comprises a signal conditioning circuit 5"! receiving tne input from the cable -35 and an ac amplifier 52 wnoεe gain is adjustable by an adjust control 53. The outout from tne amplifier 52 is fed to a band-pass switched cacacitor filter (BSCF) 54 which receives three further inputs. One input is on line 55 from a bank 56 of switches which set the effective "Q" of the BSCF, one input on a line 57 from a central microprocessor 58, and a final input on a line 59 from a clock generator 60 which is controlled by the processor 58.
The line 57 forms part of a self-check system whereby the electrical integrity of integers 54, 61, and 62 are periodically checked. Periodically (e.g. every 5 seconds) a self-check signal is issued by the microDrocessor 58. This signal is fed on line 57 anc is αesigned to turn off the output of the BSCF 54 and hence the output cf the threshold αetector 62. If the circuit is working properly, removal of the output from the detector 62 should result in a loss of the "flame-on" signal and during tne self-check operation the circuit looks to see that the outout from detector 62 does momentarily disappear. Thus, a self checking facility is implemented by issuing a self-check signal on the line 54 and monitoring the corresDonαing response from the output of detector 62.
The BSCF acts as a digitally tunable band-pass filter and accordingly is used to generate a series of ac poten¬ tial values from the output of the photosensi ive device 34, one for each discrete frequency pass band in turn, which together scan the full range of frequencies cf interest (i.e. in a typical case 80 to 1500 Hz). The ac values are converted to dc potentials in an ac/oc converter
61 and these dc values are fed both to a threshold detector
62 and to an analog to digital converter 63.
Since the flicker frequency can change with the efficiency of combustion of a given burner fue , the monitoring equipment described herein can also be useα to indicate the onset of non-optimum combustion conαitions in the burner being monitored.
The microprocessor 58 is the heart of the circuit shown in Figure 8 and performs the following functions.
Firstly, it stores a first digital signal train representing the radiation received in the head 30 when no flame is present at the monitored burner so that it is pure background radiation which is detected (this signal being entered into the memory of processor 58 when button 42 is pressed). This first signal train is represented graphi¬ cally in Figure 9A, but in practice would be stored as digital numbers in a hundred or so sub-band memory oca- tions which together cover the entire range of freouencies of interest. The sub-band memory locations are desirably concentrated towards the low end of the spectrum le.g. 70 to 2000 Hz) since practice shows the optimum flicker frequency is likely to be at the low end for the looked-for flame type.
Secondly, it stores a second digital signal train representing the radiation received in the head 30 when a looked-for fuel flame is present at the monitored burner as seen against the background radiation of the furnace (entered into the memory of processor 58 when button 41 is pressed). This second signal train is represented by Figure 9B, and can again be stored in sub-band memory locations.
Thirdly, it computes the difference between the second and first signal trains and stores this as a third signal train (shown graphically in Figure 9C) whicn basically represents , historically, the contribution to the signal due to the combustion of the looked-for fuel at the mom- tored burner.
Fourthly, this third signal train is scanned to determine the frequency at which the peak amplitude occurs and the tunable bandpass filter is then tuned to target the frequency at which the peak amplitude occurred.
These operations have set the unit 31 for future flame monitoring operations for the looked-for type of flame.
The signal obtained from the target flame is compared continuously with a set threshold level in the threshold detector 62 so that whenever the flame signal received, in the frequency range being targeted, is greater than the set threshold level a "flame-on" signal is generated and passed to the microprocessor 58. When the flame signal drops below the set threshold level the "flame-on" signal disap- pears and an appropriate warning is given.
In essence the threshold detector 62 ignores signal amplitudes below the set threshold level but accepts signals at or above the set level. By virtue of the "tuning" of the bandpass filter to the flicker freαuency of the looked-for type of flame, the appearance of a "flame- on" signal can reliably be used by the microprocessor 58 to transmit initiating signals to field devices (net shown) via a failsafe output unit 64 and the connector 44, and to associated controls such as the display 38 and the inαicat- ing light 39.
Rather than store both the first and second signal trains, it is possible to temporarily store tne first and directly create the difference signal by subtracting the sub-band digital values created by the second train from the temporarily stored first digital values to directly create the required difference signal in the one set of memory locations.
From the above it will be appreciated that the method of the invention resides in the following:-
Random background frequency and existing flame sources are scanned and an analog signal obtained from these sources is passed through the tunable band pass filter 54 and thence through to the microprocessor 58. The frequency spectrum obtained is accepted by the microprocessor and stored as a first series of digital references.
When the flame to be monitored is established, the flame is scanned and new analog signals are obtained from the flame. These signals are then passed through the same tunable band pass filter and thence through to the micro¬ processor. The signals obtained from this second source are accepted by the microprocessor and then either stored as a second series of digital references with the first series of references for subsequent differencing or the second series of digital references are directly subtracteα from the first series and stored as a new series of digital references.
The new series of digital references is then scanned to determine the frequency (or frequencies) at whicn tne peak amplitude occurs and the tunable band pass filter is tuned to target a frequency at which the peak amclituαe occurred. This sets the unit 31 for future flame momtor- ing operations for the looked-for type of flame.
Having set up the unit 31 on the basis of the previous steps, the current output of the opto-electric transducer device (34) is compared with a set threshold level ε: that whenever the flame signal received is greater than the set threshold level a "flame-on" signal is passed to the microprocessor.
Among the advantages of the method of the invention may be mentioned:-
1. A common device (i.e. the band pass filter) can be used for (i) the initial background and targeted flame flicker analyses and (ii) the actual targeted flame detec¬ tion.
2. Optimum targeted flame detection will be obtained irrespective of (i) the loading of a boiler, i.e. the intensity of background flames and (ii) flame interferences from neighbouring burners.
3. The flame monitoring equipment can be easily and economically implemented using a microprocessor.

Claims

1. A, method of monitoring a potential flame zone of burner for the existence of a burner - generated flame of required type, which method comprises directing radiati received from and through at least a Dart of said zone on a photo-sensitive area of an opto-electric transduc device, feeding the output of the transducer device circuit means adapted to generate an output signal wnich related to the radiation falling on the sensitive area a determining from the magnitude cf a parameter of the outp signal whether there is or is not a flame of the reαuir type in the flame zone, characterised in that tne circu means includes a band-pass filter to eliminate the co tributions of radiation having frequencies cf amc itu variation (hereafter, fcr convenience, referred to "flicker" frequencies) below or above tne pass band, that the pass band of the filter is set to that part of t frequency spectrum where the flicker frequency of a fla of the required type is detectable against backgrou radiation passing through saiα zone, and in that the outp signal in the pass band is used to determine whetner flame of the required type is present in tne monitor flame zone.
2. A method according to claim 1, characterised i that the centre frequency of the band-pass filter i tunable and an initial stage of the monitoring metn involves setting said centre frequency on the basis of determination of a difference between the outout sign when a flame of the required type is and is not present i said zone.
3. A method according to claim 2, characterised i that the centre frequency is selected to correspond to tn maximum amolitude of said difference signal.
4. A methoc accorαing to claim 1. cnaracteπsεc in tnat tne pass band of the filter lies in thε range 70 to 2000 Hz.
5. A flame monitoring αevice comprising an ooto- electric transαucer and signal processing eαuioment to act on tne electrical output of tne transαucer ana generate a "flame-on" signal in tne prεsence cf a looκεd-fcr flame in a monitored flame zone, cnaracteπsεo in tnat the signal processing ecuipment includes means tc determine a ~lιcκer frequency which is sensitive to the presence cr aosence of the lookeα-for flame, a band-pass filter to receive tne outout cf the transαucer. a thresnold device to receive tne outout cf the oano-oass filte1- ana generate the "flame-on' signal anc means tc tune tne banα-pass filter tc tne determined flicker freαuency.
6. A flame monitor device accorαing tc claim 5, characterised in that a sighting tube leading directly towards the flame zone is used to direct radiation tc tne pnoto-sensitive area of the opto-electric transcucer.
7. A flame monitor device accorαing to claim 6. character!sεd in that the signtmg tuoe incluαes lens means to focus the radiation from the flame zone ontc tne saic area.
8. A flame monitor device according to claim 5. characterised in that the band-pass filter is a tunaole band-pass switched capacitor filter circuit.
9. A flame monitor device according tc claim 8, characterised in that the tunable band-pass switched capacitor filter circuit feeds its outDUt into an AC/DC converter and then to thε threshold device.
PCT/GB1991/000468 1990-04-03 1991-03-27 Burner control WO1991015715A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB909007448A GB9007448D0 (en) 1990-04-03 1990-04-03 Burner control
GB9007448.5 1990-04-03

Publications (1)

Publication Number Publication Date
WO1991015715A1 true WO1991015715A1 (en) 1991-10-17

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AU (1) AU7660391A (en)
GB (1) GB9007448D0 (en)
WO (1) WO1991015715A1 (en)
ZA (1) ZA912406B (en)

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1960218A1 (en) * 1969-12-01 1971-06-03 Rainer Portscht Temperature radiation detector for automatic fire detection or flame monitoring
US3903418A (en) * 1973-12-14 1975-09-02 Forney International Infrared dynamic flame detector
GB1500771A (en) * 1976-01-20 1978-02-08 Talentum Dev Ltd Flame monitoring apparatus
GB2132342A (en) * 1982-12-14 1984-07-04 Land Combustion Ltd Discrimination between flames of different fuels
US4509041A (en) * 1982-03-22 1985-04-02 The Babcock & Wilcox Company Correlation type flicker flamon
JPS63273726A (en) * 1987-05-06 1988-11-10 Toyota Motor Corp Combustion monitoring method and its device

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1960218A1 (en) * 1969-12-01 1971-06-03 Rainer Portscht Temperature radiation detector for automatic fire detection or flame monitoring
US3903418A (en) * 1973-12-14 1975-09-02 Forney International Infrared dynamic flame detector
GB1500771A (en) * 1976-01-20 1978-02-08 Talentum Dev Ltd Flame monitoring apparatus
US4509041A (en) * 1982-03-22 1985-04-02 The Babcock & Wilcox Company Correlation type flicker flamon
GB2132342A (en) * 1982-12-14 1984-07-04 Land Combustion Ltd Discrimination between flames of different fuels
JPS63273726A (en) * 1987-05-06 1988-11-10 Toyota Motor Corp Combustion monitoring method and its device

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 13, no. 76 (M-800)(3424) 21 February 1989, & JP-A-63 273726 (TOYOTA) 10 November 1988, see the whole document *

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ZA912406B (en) 1992-01-29
AU7660391A (en) 1991-10-30

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