EP2829803B1

EP2829803B1 - Method of stabilizing gas burner combustion using Fast-Fourier-Transformation (FFT)

Info

Publication number: EP2829803B1
Application number: EP13177970.4A
Authority: EP
Inventors: Gerwin Langius
Original assignee: Honeywell Technologies SARL
Current assignee: Garrett Motion SARL
Priority date: 2013-07-25
Filing date: 2013-07-25
Publication date: 2017-05-10
Anticipated expiration: 2033-07-25
Also published as: EP2829803A1

Description

The present patent application relates to a method for operating a gas burner.
EP 1 084 369 B1 and EP 1 179 159 B1 each disclose a method for operating a gas burner. According to this prior art documents, during burner-on phases of the respective gas burner a gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber of the gas burner. The gas/air mixture is provided by mixing an air flow provided by an air duct with a gas flow provided by a gas duct using a mixing device. The quantity of the air flow is adjusted by a fan. The defined mixing ratio of the gas/air mixture is controlled by a controller on basis of a signal provided by an electrical or electronic sensor. According to EP 1 084 369 B1 , the electrical or electronic sensor is coupled to the gas duct and to the air duct. According to EP 1 179 159 B1 , the electrical or electronic sensor is coupled to the gas duct and to a reference point. The electrical or electronic sensor is especially designed as a flow-meter. An actual value corresponding to a pressure ratio between the gas pressure in the gas duct and the air pressure in the air duct or corresponding to a pressure ratio between the gas pressure in the gas duct and the air pressure at the reference point is provided by the electrical or electronic sensor, wherein this actual value is compared with a nominal value. A control variable for a gas valve assigned to the gas duct is generated on basis of the control deviation between the actual value and nominal value, wherein the gas valve is adjusted on basis of this control variable in order to provide the defined mixing ratio of gas and air in the gas/air mixture.
According to EP 1 084 369 B1 and EP 1 179 159 B1 , the defined mixing ratio of gas and air of the gas/air mixture is kept constant over the entire modulation range of the gas burner. In other words, according to the prior art the mixing ratio of the gas/air mixture is kept constant over the entire fan speed range of the fan, either to provide a 1:1 gas-air control having a ratio between the gas pressure and the air pressure of 1:1 over the entire modulation range of the gas burner or to provide a 1:N (N>1) gas-air control having a ratio between the gas pressure and the air pressure of 1:N over the entire modulation range of the gas burner. In both cases a so-called λ-value is usually greater than 1.
During burner-on phases an unstable combustion of the gas/air mixture can occur. Such an unstable combustion can be caused e.g. by cold surfaces of the gas burner, by an incorrect mixing ratio of the gas/air mixture caused by too low calorific gases or by modulation levels being too low.
It is advantage for the gas burner operation to determine if the combustion of the gas/air mixture within the burner chamber is stable or unstable.
WO 2009/89886A2 discloses a method in which stable or unstable combustion of the gas/air mixture within the burner chamber is detected on basis of a signal provided by an ionization sensor. However, the signal provided by an ionization sensor is subject of fluctuations due to ambient noise. For that, slow filtering must be applied to the signal provided by an ionization sensor. However, such a slow filtering of the signal provided by an ionization sensor causes a low ratio of signal fluctuations caused by an unstable combustion versus signal fluctuations caused by ambient noise. This makes it difficult to determine stable or unstable combustion on basis of a signal provided by an ionization sensor.
EP 2 466 202 A2 discloses a method in which the signal provided by the electrical or electronic sensor is used to determine if the combustion of the gas/air mixture within the burner chamber is stable or unstable.
According to EP 2 466 202 A2 the time-based signal provided by the electrical or electronic sensor is sampled at a defined sampling rate, whereby a variance of the samples of the time-based signal is calculated and compared with a reference value in order to determine if the combustion of the gas/air mixture within the burner chamber is stable or unstable. When unstable combustion is determined, the operation of the gas burner will be influenced.
With the method disclosed in EP 2 466 202 A2 to determine if the combustion of the gas/air mixture within the burner chamber is stable or unstable it is impossible to determine reliably if the combustion of the gas/air mixture within the burner chamber is stable or unstable. E.g. noise caused by the fan can be very high and can make it impossible to determine reliably on basis of the variance of samples of the time-based signal if the combustion is stable or unstable. Further on, at high EMC (Electro Magnetic Compatibility) levels the time-signal provided by the electrical or electronic sensor can be influenced which makes it impossible to determine reliably on basis of the variance of samples of the time-based signal if the combustion is stable or unstable.
EP 0 682 210 A1 discloses a combustion control apparatus making use of a flow meter connected to an air duct, wherein it is possible to detect the amount of air flow stability.
EP 2 184 467 A1 relates to a gas turbine method and device. Signals provided by several measurement devices, namely by a process variable measurement device, by pressure variation measurement device and by an acceleration measurement device are used and processes to provide combustion stability.
Against this background, a novel method for operating a gas burner is provided.
The method for operating a gas burner is defined in the claim 1.
The method according to the present application makes it possible to determine reliably if the combustion of the gas/air mixture within the burner chamber is stable or unstable. E.g. noise caused by the fan and/or high EMC (Electro Magnetic Compatibility) levels will not influence the reliability of the determination if the combustion is stable or unstable.
Preferred developments of the invention are provided by the dependent claims and the description which follows. Exemplary embodiments are explained in more detail on the basis of the drawing, in which:

Figure 1: shows a schematic view of a gas burner,
Figure 2: shows a first diagram illustrating a time-based signal provided by the electrical or electronic sensor at a relative low burner load with a stable combustion of the gas/air mixture within the burner chamber;
Figure 3: shows a second diagram illustrating a frequency-based signal obtained through applying a Fourier Transformation on samples of the time-based signal of Figure 2;
Figure 4: shows a third diagram illustrating a time-based signal provided by the electrical or electronic sensor at a relative low burner load with an unstable combustion of the gas/air mixture within the burner chamber;
Figure 5: shows a fourth diagram illustrating a frequency-based signal obtained through applying a Fourier Transformation on samples of the time-based signal of Figure 4;
Figure 6: shows a fifth diagram illustrating a time-based signal provided by the electrical or electronic sensor at a relative high burner load with a stable combustion of the gas/air mixture within the burner chamber;
Figure 7: shows a sixth diagram illustrating a frequency-based signal obtained through applying a Fourier Transformation on samples of the time-based signal of Figure 6;
Figure 8: shows a seventh diagram illustrating a time-based signal provided by the electrical or electronic sensor at a relative high burner load with an unstable combustion of the gas/air mixture within the burner chamber; and
Figure 9: shows an eighth diagram illustrating a frequency-based signal obtained through applying a Fourier Transformation on samples of the time-based signal of Figure 8.

Figure 1 shows a schematic view of a gas burner 10. The gas burner 10 comprises a burner chamber 11 in which combustion of a gas/air mixture takes place during burner-on phases of the gas burner 10. The combustion of the gas/air mixture results into flames 12.
The gas/air mixture is provided to the burner chamber 11 of the gas burner 10 by mixing an air flow with a gas flow.
A fan 14 sucks in air flowing through an air duct 15 and gas flowing though a gas duct 16. A gas valve 17 for adjusting the gas flow through the gas duct 16 and a safety valve 18 are assigned to the gas duct 16. The position of the gas valve 17 is adjusted by a pressure regulator 19.
The gas/air mixture having a defined mixing ratio of gas and air is provided to the burner chamber 11 of the gas burner 10. The gas/air mixture is provided by mixing the air flow provided by an air duct 15 with a gas flow provided by a gas duct 16. The air flow and the gas flow become preferably mixed by a mixing device. Such a mixing device can be designed as a Venturi nozzle (not shown). The quantity of the air flow and thereby the quantity of the gas/air mixture flow is adjusted by the fan 14, namely by the speed of the fan 14. The fan speed can be adjusted by an actuator 22 of the fan 14.
The defined mixing ratio of the gas/air mixture is controlled by a controller 20 on basis of a signal provided by an electrical or electronic sensor 13. According to the invention, the electrical or electronic sensor 13 is coupled to the gas duct 16 and to a reference point 23. The electrical or electronic sensor 13 is preferably designed as a flow-meter.
An actual value corresponding to a pressure ratio between the gas pressure in the gas duct 16 and the air pressure at the reference point 23 is provided by the electrical or electronic sensor 13. This actual value is compared by the controller 20 with a nominal value stored in the controller 20.
The controller 20 generates a control variable for the gas valve 17, namely for an actuator 21 of the gas valve 17, on basis of the control deviation between the actual value provided by the electrical or electronic sensor 23 and the nominal value stored in the controller 20.
The gas valve position of the gas valve 17 is adjusted by the actuator 21 of the same on basis of this control variable in order to provide the defined mixing ratio of gas and air in the gas/air mixture.
The mixing ratio of gas and air of the gas/air mixture provided to the burner chamber 11 is preferably kept constant over the modulation range of the gas burner.
The modulation range of the gas burner 10 is defined by a minimum burner load and by a maximum burner load. At maximum burner load the fan 14 is operated at maximum fan speed. At minimum burner load the fan 14 is operated at minimum fan speed. In other word, the modulation range of the gas burner 10 is defined by the minimum fan speed and by the maximum fan speed.
Alternatively, the mixing ratio of gas and air of the gas/air mixture provided to the burner chamber 11 is changed as a function on the speed of the fan 14. For example, for fan speeds of the fan 14 being smaller than a lower threshold a gas/air mixture is provided having a mixing ratio of gas and air adapted to provide a stable and secure ignition of the gas/air mixture. For fan speeds of the fan 14 being larger than an upper threshold a gas/air mixture is provided having a mixing ratio of gas and air adapted to provide combustion with reduced emissions. For fan speeds being larger than the lower thresholds, especially for fan speeds being larger than the lower thresholds and lower than the upper threshold, the mixing ratio of gas and air of the gas/air mixture can be freely adjustable as a function of the fan speed of the fan 14.
According to the invention, the signal provided by the electrical or electronic sensor 13 is in addition used to determine if the combustion of the gas/air mixture within the burner chamber 11 is stable or unstable.
For determining stable or unstable combustion the time-based signal provided by the electrical or electronic sensor 13 is processed in such a way that the time-based signal provided by the electrical or electronic sensor 13 is sampled at a defined sampling rate providing a defined number of samples of the time-based signal.
Then, a Fourier Transformation is applied to the samples of the time-based signal obtained by the sampling of the time-based signal at the defined sampling rate thereby generating a frequency-based signal.
Then, it is determined if the amplitude of the frequency-based signal within a defined frequency range is larger than a defined threshold, wherein stable combustion is determined when the amplitude of the frequency-based signal within the defined frequency range is lower than the defined threshold, and wherein unstable combustion is determined when the amplitude of the frequency-based signal within the defined frequency range is larger than the defined threshold.
When stable combustion is determined, the operation of the gas burner 10 will not be influenced.
When unstable combustion is determined, the operation of the gas burner will be influenced. It is possible to shut down the gas burner when unstable combustion is detected. Preferably, the modulation range of the gas burner becomes decreased by increasing the minimum burner load or the minimum fan speed when unstable combustion is detected. According to the invention, a Fast Fourier Transformation is applied to the samples of the time based signal obtained by the sampling of the time-based signal of the electrical or electronic sensor 13 at the defined sampling rate thereby generating the frequency-based signal. Such a Fast Fourier is simple and can be executed on simple low performance processors.
Figures 2, 4, 6 and 8 each show the time-based signals 24, 25, 26, 27 provided by the electrical or electronic sensor 13. In Figures 2, 4, 6 and 8 the amplitude a of the signal provided by the electrical or electronic sensor 13 varies or changes over the time t.
Figures 2 and 4 each show time-based signals provided by the electrical or electronic sensor 13 at relative low burner loads, wherein Figures 6 and 8 each show time-based signals provided by the electrical or electronic sensor 13 at relative high burner loads. The time-based signals of Figures 2 and 6 are provided at stable combustion, wherein the time-based signals of Figures 4 and 8 are provided at unstable combustion.
The time-based signals 24, 25, 26, 27 are sampled using a defined sampling rate, e.g. 85.33 Hz. The defined sampling rate is preferably burner load independent. The Fourier Transformation is applied to a defined number of samples, e.g. to 128 samples. The defined number of samples which are subject to the Fourier Transformation is preferably burner load independent.
The Fourier Transformation results in the frequency-based signals 28, 29, 30, 31 shown in Figures 3, 5, 7 and 9.
In Figures 3, 5, 7 and 9 the amplitude A of the signal obtained by the Fourier Transformation varies or changes over the frequency f.
In a defined frequency range 32, e.g. between 20 Hz and 40Hz, it is determined if the amplitude A of the respective frequency-based signal 28, 29, 30, 31 is below or above a defined threshold 33 of e.g. 200 for a relative low burner load or e.g. 1000 for a relative low burner load.
The defined frequency range 32 is preferably burner-load independent. The defined threshold 33 is preferably burner-load dependent.
If in this frequency range 32 the amplitude A of the respective frequency based signal 28, 29, 30, 31 is below the defined amplitude threshold 33, the combustion is stable. Otherwise, the combustion is unstable.
The time-based signals are also often called time-dependent signals and the frequency-based signals are also often called frequency-dependent signals.
The method for operating a gas burner is implemented with the controller 20. The controller 20 determines on basis of the signal provided by the electrical or electronic sensor 13 if the combustion of the gas/air mixture within the burner chamber 11 is stable or unstable. The controller 20 comprises means for performing the method. These means include an interface for receiving the signal from the electrical or electronic sensor 13, a memory for storing data and a processor for performing calculations based on the received signal and stored data.

List of reference signs

10: gas burner
11: burner chamber
12: flame
13: sensor
14: fan
15: air duct
16: gas duct
17: regulating valve
18: safety valve
19: pressure regulator
20: controller
21: actuator
22: actuator
23: reference point
24: time based signal
25: time based signal
26: time based signal
27: time based signal
28: frequency based signal
29: frequency based signal
30: frequency based signal
31: frequency based signal
32: frequency range
33: amplitude threshold

Claims

Method for operating a gas burner (10),
wherein during burner-on phases a gas/air mixture having a defined mixing ratio of gas and air is provided to a burner chamber (11) of the gas burner (10) for combusting the gas/air mixture within the burner chamber (11),
wherein the gas/air mixture is provided by mixing an air flow sucked in by a fan (14) with a gas flow,
wherein the defined mixing ratio of the gas/air mixture is controlled by comparing an actual value of a signal provided by an electrical or electronic sensor (13) with a nominal value for the signal provided by the electrical or electronic sensor (13) and by generating a control variable for a gas valve (17) assigned to a gas duct (16) on basis of the control deviation between the actual value and the nominal value,
wherein the electrical or electronic sensor (13) is coupled to the gas duct (16) and to a reference point (23), wherein the actual value of the signal provided by the electrical or
electronic sensor (13) corresponds to a pressure ratio between the gas pressure in the gas duct and the air pressure at the reference point (23),
wherein the signal provided by the electrical or electronic sensor (13) is in addition used to determine if the combustion of the gas/air mixture within the burner chamber (11) is stable or unstable,
wherein for determining stable or unstable combustion the time-based signal provided by the electrical or electronic sensor (13) is processed in such a way that:
the time-based signal (24, 26, 28, 30) provided by the electrical or electronic sensor (13) is sampled at a defined sampling rate,

a Fourier Transformation is applied to the samples of the time-based signal obtained by the sampling of the time-based signal at the defined sampling rate thereby generating a frequency-based signal (25, 27, 29,31),

it is determined if the amplitude of the frequency-based signal (25, 27, 29, 31) within a defined frequency range (32) is larger than a defined threshold (33), wherein stable combustion is determined when the amplitude of the frequency-based signal within the defined frequency range is lower than the defined threshold, and wherein unstable combustion is determined when the amplitude of the frequency-based signal within the defined frequency range is larger than the defined threshold.
Method as claimed in claim 1, wherein a Fast Fourier Transformation is applied to the samples of the time-based signal obtained by the sampling of the time-based signal at the defined sampling rate thereby generating the frequency-based signal.
Method as claimed in claim 1 or 2, wherein when stable combustion is determined, the operation of the gas burner (10) will not be influenced.
Method as claimed in one of claims 1 to 3, wherein when unstable combustion is determined, the operation of the gas burner (10) will be influenced.
Method as claimed in one of claims 1 to 4, wherein the defined frequency range (32) is burner-load independent.
Method as claimed in one of claims 1 to 5, wherein the defined threshold (33) is burner-load dependent.
Method as claimed in one of claims 1 to 6, wherein the sampling rate is burner-load independent.