JP4274179B2 - Flame detection method and flame detection apparatus - Google Patents

Description

  The present invention relates to a flame detection method and a flame detection apparatus suitable for detecting a flame caused by combustion, particularly a flame caused by lean combustion.

  Low NOx combustion such as high-temperature air combustion is often realized by preheating, premixing, and dilution with an inert gas. When the flame is diluted in this way, it becomes difficult to detect the flame. When highly diluted in air preheated to about the autoignition temperature, generally, determination of whether or not the fuel is combusted is made by detecting the degree of chemical reaction in the gas. For detecting the state of the flame, for example, it is known to analyze the emission spectrum intensity of the combustion flame in the combustion furnace and diagnose the combustion state from the analysis result. Such a system is described, for example, in Japanese Patent Publication (Patent Document 1) having a patent application publication number of JP-A-11-325460.

  The inventors previously proposed that each component such as NO, OH, CH, etc. in the flame is accurately detected using a plurality of types of ultraviolet detectors having different detection wavelength regions. Such a detection method is described in, for example, a Japanese patent application (Patent Document 2) whose patent application publication number is Japanese Patent Application Laid-Open No. 2003-322562.

  However, in the ultraviolet detection device as shown in the above-mentioned Patent Document 2, although the self-luminous spectrum of the flame can be detected well as shown in FIG. 7 (FIG. 4 in the above-mentioned Patent Document 2), for example, its cathode (cathode) There is a problem that not only is it necessary to use a plurality of types of ultraviolet detectors with different detection wavelength regions, but also the configuration becomes large.

  The present invention has been made in view of such circumstances, and its purpose is to provide a flame detection method that can easily detect the state of a flame, focusing on the self-luminous characteristics of the flame in the ultraviolet region. It is in.

  It is another object of the present invention to provide a flame detection method capable of detecting the state of a flame by effectively using an ultraviolet detector having a relatively narrow detection wavelength region.

  Another object of the present invention is to provide a flame detection device having a simple configuration suitable for detecting the state of a flame due to combustion, in particular, the state of a flame due to lean combustion.

In order to achieve the above-described object, the flame detection method according to the present invention includes a combustion of a ratio of two peak intensities and a local equivalence ratio in a self-luminous component of a flame, for example, an ultraviolet region from an OH radical. Focusing on the relationship with the characteristics, each of the self-luminous intensity at different wavelengths with the same radical species peak in the ultraviolet region from the self-luminous component of the flame by combustion, especially the flame by lean combustion, respectively, The emission intensity ratio, which is the ratio of the measured self-emission intensity at each wavelength, is obtained, the relationship between these emission intensity ratios and the flame temperature, and the emission intensity ratio and the air ratio of the mixture used for lean combustion. It is characterized in that flame state detection is performed based on at least one of the above relationships.

Preferably, the self-luminous intensity is measured by measuring the self-luminous spectrum from the specific radical species accompanying the electron transition from the excited state to the ground state due to the lean combustion among the self-luminous components of the flame due to the lean combustion. It is a feature. Further, preferably, an OH band spectrum of the electronic transition A ² Σ ⁺ → X ²計 測 is measured, and in particular, OH (2, 0) near a wavelength of 260 nm, OH (1,0) near a wavelength 280 nm, OH near a wavelength 287 nm It is characterized in that a flame state detection is performed by obtaining a self-luminous intensity ratio of (2, 1) and OH (0, 0) near a wavelength of 306 nm. Preferably, the flame state is detected by paying attention to an OH band spectrum having a wavelength of about 310 nm or less.

  That is, in the potential of the diatomic molecule, the electronic ground state ν ″ and the electronic excited state ν ′ of the diatomic molecule are ν ″ = 0, 1, 2, 3,..., Ν ′ = 0, 1, 2, 3 respectively. , ... and many vibration levels. When a diatomic molecule in the electronically excited state ν ′ = 0, 1, 2, 3,... Returns to the electronic ground state ν ″ = 0, 1, 2, 3,. ν ′, ν ″) indicates the level at that time, and the spectrum at (0, 0) has the strongest light intensity in the OH band.

  In other words, when electrons move from a high energy trajectory to a low energy trajectory, when the energy difference is emitted as light, the spectral components have a wavelength and light intensity that are unique to the energy trajectory. The flame detection method according to the present invention pays attention to the relationship between the wavelength component forming a peak in the self-emission spectrum of such a flame and its emission intensity, and the emission intensity ratio of at least two peaks and the flame temperature or air ratio. From this relationship, the state of the flame in the lean combustion is detected.

Further, the flame detector according to the present invention includes an ultraviolet detector for detecting a plurality of self-luminous intensities at different wavelengths each having a peak of the same radical species in the ultraviolet region from a self-luminous component of a flame caused by combustion, The self-luminous intensity of each wavelength is obtained from the detection signal of the ultraviolet detector, the relationship between the ratio of these self-luminous intensities and the flame temperature, and the relationship between the ratio and the air ratio of the mixture used for the combustion. And a processing device for detecting the state of the flame based on at least one of them.

  Thus, according to the present invention, the self-luminous component of the lean combustion flame is focused on a plurality of self-luminous intensities of the same radical species, specifically the self-luminous spectrum in the ultraviolet region from the OH radical. By simply using an ultraviolet detector that detects a wavelength region of 450 nm, preferably an ultraviolet detector that detects a wavelength region of 250 to 350 nm, it is possible to easily detect a combustion flame state, particularly a lean combustion flame. Moreover, since the intensity of self-luminescence in the above-mentioned wavelength region in the flame is generally higher than the radiation intensity from the wall of the combustion furnace, as described above, by detecting the component of the OH band spectrum having a wavelength of about 310 nm or less, It is possible to reliably detect the presence or absence of a flame, and thus the state of the flame, with little influence from the wall surface of the combustion furnace, which is the background when the flame is detected.

The figure which shows schematic structure of the lean combustion apparatus and flame detection apparatus which are used for the flame detection method which concerns on one Embodiment of this invention. The figure which shows schematic structure of the ultraviolet detector used for a flame detection. The figure which shows the structural example of the drive circuit of an ultraviolet detector. The figure which shows the example of the self-emission spectrum of the OH radical of the flame detected with the flame detection apparatus which concerns on this invention. The figure which shows the relationship between emission intensity ratio _RI , an air ratio, and flame temperature. The figure which shows the relationship between the radiant energy and temperature of a furnace wall. The figure which shows the self-emission spectrum of a flame.

  Hereinafter, a flame detection method and a flame detection apparatus according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a simplified configuration of a lean combustion apparatus in which the method of the present invention is implemented and a flame detection apparatus incorporated in the lean combustion apparatus, and 1 is a combustion furnace. For example, the combustion furnace 1 is a rectangular type that is surrounded by heat-resistant bricks, ceramic fibers, or the like, has a combustion furnace volume of 2.58 × 10 ⁻³ m ^3, and has an exhaust port of 100 × 100 mm on the top. Therefore, the combustion chamber heat load is set to 1.16 × 10 ³ kW / m ³ . The burner 2 provided in the combustion furnace 1 is a wall recess type having an inner diameter of 40 mm and a height of 60 mm. Fuel (for example, propane gas) and air are mixed and supplied to the burner 2 at an air ratio of, for example, 0.8 to 1.4 in a mixer 3 provided immediately before the burner 2. ing. Fuel is supplied to the mixer 3 from the fuel tank F through the metering device V1, the pressure gauge P1, and the flow meter M1, and air is supplied from the blower B through the metering device V2 and the flow meter M2.

  On the other hand, on the side wall surface of the combustion furnace 1, furnace observation windows 4a and 4b in which quartz glass is fitted at heights of 65 mm and 130 mm, respectively, are provided. The light of the flame that emits light by the combustion in the combustion furnace 1, which can be visually recognized from the observation windows 4 a and 4 b in the furnace, is guided to the monochromator 5 (spectrometer) through the optical fiber 6. The monochromator 5 includes a diffraction grating for extracting a desired wavelength component from incident light having various wavelength components, and has a predetermined wavelength range selected according to an angle formed by the diffraction grating and the incident light. The light component is configured to be detected by a light receiving element such as a CCD. By such a monochromator 5, light emission due to the flame of the fuel burned lean by the burner 2 is received and detected, and a voltage (or current) corresponding to the received light intensity is converted. The electric signal (ultraviolet light intensity) detected in this way is taken into the computer (PC) 8 via the A / D converter 7 and the emission intensity ratio between the peak wavelengths is obtained as will be described later. The relationship between the air ratio and the air ratio is examined, and the presence or the state of the flame is detected.

  In this case, a filter (diffraction grating) capable of detecting a wavelength range of 250 to 450 nm is used as the filter (diffraction grating) in the monochromator 5, but for detection of a flame state, the above-mentioned OH band spectrum can be confirmed. Wavelength data is used. Further, since the signal input to the computer 8 includes noise due to flame fluctuation and the dark current of the monochromator 5, the ensemble average and moving average of the input signal (monochromator 5 output signal) are used. This is used as a detection signal.

  Using such a lean combustion apparatus, first, the inside of the combustion furnace 1 is sufficiently heated, and after the furnace temperature is stabilized, the fuel flow rate and the air flow rate are set so that the air ratio becomes 0.8. The self-luminous intensity of the lean burn flame was measured. In addition, when the fuel flow rate was gradually decreased with the air flow rate kept constant, and the measurement was repeated a plurality of times until the air ratio became 1.4 or the blow-off limit was reached, the air flow rate became 80 L / For example, a self-luminescence spectrum as shown in FIG. 4 was obtained at an air ratio of 1.35 min.

The peaks confirmed in the self-emission spectrum shown in FIG. 4 are OH band spectra in the electronic transition A ² Σ ⁺ → X ² 、, and in particular, OH (2, 0) near a wavelength of 260 nm, OH (wavelength near 280 nm) 1, 0), OH (2, 1) near a wavelength of 287 nm, and OH (0, 0) near a wavelength of 306 nm. Therefore, with reference to the emission intensity of OH (0, 0) near the wavelength of 306 nm where the strongest peak appeared, the emission intensity of OH (2, 0) near the wavelength of 260 nm and OH (1,0) near the wavelength of 280 nm. When the ratio of emission intensity (emission intensity ratio) R _I was obtained and the relationship between the emission intensity ratio R _I and the air ratio or flame temperature was investigated for an air flow rate of 90 L / min or less, the relationship shown in FIG. 5 was obtained. It was. In FIG. 5, R260 (expr) represents the ratio of the emission intensity of OH (0, 0) near the wavelength of 306 nm to the emission intensity of OH (2, 0) near the wavelength of 260 nm, and R280 (expr) represents the wavelength of 306 nm. The ratio between the emission intensity of OH (0, 0) near and the emission intensity of OH (1,0) near 280 nm is shown. At this flow rate, the flame was stably present inside the recess burner 2.

On the other hand, although the emissivity of the furnace wall of the combustion furnace 1 varies depending on the material and surface state, it can be regarded as approximately 1.0 in a refractory brick such as alumina. Further, as the temperature of the furnace wall becomes higher, for example, as shown in FIG. 6, the radiation energy of any wavelength increases substantially uniformly with an increase in temperature. Incidentally, the energy of the non-luminous flame is about 10 W / m ^{2 at the} maximum, and at wavelengths corresponding to CH (near 315 nm, 390 nm, 430 nm), it is easily affected by the emissivity of the furnace wall. When the temperature exceeds 1600K, the S / N ratio becomes close to 1, so that the light from the flame (visible wavelength range) is hardly visible. Therefore, it becomes difficult to detect the diluted flame from the wavelength corresponding to CH. Therefore, it is necessary to detect a flame at a wavelength that does not easily depend on the furnace wall.

From this point of view, OH, CH, C ₂ and infrared rays are used in order to detect chemiluminescence generally used for flame detection, and the most suitable chemical species for detection of hydrocarbon flame is CH. It is considered to be. However, this type of chemical species has a relatively long self-emitting wavelength of 431.4 nm, which is suitable for visual confirmation, but is not suitable for detecting a diluted flame.

  In this respect, OH exists not only in the flame but also in the burned hot gas. Therefore, care must be taken in detecting the flame reaction zone. However, the OH self-emission wavelength of 306.4 nm has the highest intensity in the flame reaction zone compared to the emission in the burned gas, and is so strong that the emission from the downstream flame zone can be ignored. Although its intensity is weaker than that of OH, it is also possible to utilize NO emission at a wavelength shorter than 260 nm.

Further, as shown in FIG. 6 described above, when the temperature of the furnace wall becomes higher, the radiant energy becomes stronger according to the Planck equation. Therefore, if the energy distribution in the OH molecule is thermal equilibrium, the emission intensity ratio R _I described above, for example, the ratio I ₂₈₁ / intensity I ₂₈₁ at the wavelength 281.1 nm and the intensity I _{306 at the} wavelength 306.4 nm shown in FIG. I ₃₀₆ depends only on temperature, and has a relationship that the intensity ratio increases as the temperature increases.

  Therefore, paying attention to the above-described lean combustion apparatus shown in FIG. 1, since the measurement is performed in the OH reaction zone, even if the time from the excited state of OH to deactivation is as short as about 400 to 800 nSec, It is unlikely that the distribution of electron energy in the molecule has reached a sufficient thermal equilibrium.

So I tried taking the emission intensity ratio _{R I} of ^{A 2 Σ +} → ^{X 2 Π} in the flame and (0,0) and OH (1, 0) or the like, to obtain a result as shown in Fig. 5 described above It was. In FIG. 5, the curve indicated by a broken line is E (T, λ) where the monochrome emission performance of a Planck black body is E (T, λ).
R _I = E (T, 281 nm) / E (T, 306 nm)
R _I = E (T, 262 nm) / E (T, 306 nm)
Are calculated and written together. R260 (calc) represents the calculated value of the emission intensity ratio for OH (2, 0) near the wavelength of 260 nm, and R280 (calc) represents the calculated value of the emission intensity ratio for OH (1, 0) near the wavelength of 280 nm. However, since the gas temperature measurement by a thermocouple could not obtain a correct value due to the influence of the wall surface, it was calculated using the adiabatic flame temperature by thermochemical equilibrium calculation.

Thus emission intensity ratio determined by R _I and air ratio and the relationship between flame temperature apparently considering, although affected by the noise or the like, close to the change assuming thermal equilibrium shown in the above calculations It was confirmed that the trend was shown. In particular, the emission intensity ratio at R ₂₈₀ with a large signal is almost unchanged under the over-concentration condition with a small air ratio, but when focusing on the self-emission intensity ratio R _I280 at a high flame temperature of about 1500 to 1900 ° C, The value is in the range of 0.20 to 0.32, and is sufficiently strong compared to the radiation intensity of the furnace wall, which is the background of the detection of self-luminescence. Therefore, for example, by paying attention to self-light emission with a wavelength shorter than about 310 nm, and preferably paying attention to self-light emission with a wavelength shorter than 306 nm, it becomes clear that it can be sufficiently utilized for detecting the state of a flame in diluted combustion. It was.

  In place of the expensive monochromator 5 described above as another embodiment of the present invention, for example, a plurality of discharge tube type ultraviolet detectors 9 as disclosed in Japanese Patent Publication No. 44-1039 are combined. It can also be used. As shown in FIG. 2, this ultraviolet ray detector 9 is provided with a net-like anode (anode) 9a and a cathode (cathode) 9b at a predetermined interval in a glass tube that transmits ultraviolet rays, and a Penning mixed gas is supplied. Enclosed. The detectable wavelength in this type of discharge tube type ultraviolet detector 9 is mainly determined by the material of the cathode 9b. That is, ultraviolet light having a wavelength shorter than the wavelength defined by the work function of the material of the cathode 9b is detected. If it is desired to limit the detection wavelength band, the detection light is configured to impinge on the cathode 9b after passing through a predetermined optical bandpass filter. As a drive circuit for the ultraviolet detector 9, for example, the one disclosed in Japanese Patent Publication No. 47-7878 is used.

  That is, the ultraviolet detector 9 is driven by applying an AC voltage of about 300 V through a drive circuit configured as shown in FIG. 3, for example. Then, the ultraviolet ray detector 9 generates a discharge current between the anode (anode) 9a and the cathode (cathode) 9b only when ultraviolet rays having a specific wavelength of a certain intensity or more are irradiated. This discharge current causes a voltage drop in the resistor RL, and generates a voltage or current in cooperation with the capacitor C connected in parallel to the resistor RL.

  Incidentally, this type of discharge tube type ultraviolet detector 9 generally cannot obtain a current output corresponding to the intensity of ultraviolet rays. However, the stronger the ultraviolet rays, the greater the probability that discharge will occur in the ultraviolet detector. For example, by measuring the discharge time, it is possible to obtain a relative output signal corresponding to the intensity of the ultraviolet rays.

  Unlike the monochromator 5 that can change the detection wavelength, the above-described ultraviolet detector 9 can usually detect only a specific wavelength, but has the same detection wavelength as the self-emission spectrum in the ultraviolet region from the OH radical. If two ultraviolet detectors 9 are used, the apparatus can be configured at a lower cost than the monochromator 5. For example, one ultraviolet detector 9 that detects the intensity of OH radical emission near the wavelength of 306 nm may be used, and the other ultraviolet detector 9 that detects the intensity of OH radical emission near the wavelength of 280 nm may be used. . If the emission intensity ratio of the two wavelengths is calculated from the detection results from these two ultraviolet detectors 9, the relationship between the emission intensity ratio and the flame temperature or air ratio can be obtained as described above.

  As yet another embodiment, the detection wavelength of one ultraviolet detector 9 can be changed although it is used in a special way. For example, in the ultraviolet detector 9 that can detect both wavelengths of 306 nm and 280 nm (for example, using silver as the material of the cathode 9b), the sensitivity to the wavelength of 306 nm increases when a high voltage is applied, and the wavelength decreases when the applied voltage is decreased. The phenomenon that the sensitivity with respect to 306 nm falls arises. By utilizing this phenomenon, for example, by switching the applied voltage during measurement, one ultraviolet detector 9 can be used to detect ultraviolet rays having two different wavelengths.

  In the above-described embodiment, the emission intensity is detected for each single radical emission that causes a peak at a specific wavelength using the ultraviolet detector 9 having a narrow detection wavelength band. On the other hand, in another embodiment described below, an ultraviolet detector 9 having a relatively wide detection wavelength band is used. Considering that ultraviolet rays having a wavelength of 200 nm or less are attenuated in the atmosphere and are not detected, for example, the ultraviolet detector 9 having a carbon cathode 9b is an ultraviolet detector having a copper cathode 9b of about 200 to 280 nm. The ultraviolet detector 9 having a silver electrode 9b of about 200 to 300 nm can detect ultraviolet rays in the wavelength band of about 200 to 380 nm, respectively. Therefore, the ultraviolet detector 9 having the carbon cathode 9b detects ultraviolet rays in the dominant wavelength band of the radical emission OH (2, 0) having a peak at a wavelength of 260 nm (detected in the vicinity of the limit wavelength 280 nm of the carbon cathode 9b). Since the sensitivity decreases, radical emission having a peak at 280 nm does not act dominantly). The ultraviolet detector 9 having the copper cathode 9b detects ultraviolet rays in a wavelength band in which radical emission OH (1,0) having a peak at a wavelength of 280 nm is dominant. The ultraviolet detector 9 having a silver cathode 9b detects ultraviolet rays in a wavelength band in which radical emission OH (0, 0) having a peak at a wavelength of 306 nm is dominant. Therefore, as a substitute value for the above-described emission intensity ratio R260, (detection value of the ultraviolet detector 9 having the carbon cathode 9b) / (detection value of the ultraviolet detector 9 having the silver cathode 9b) is adopted, and the above-described emission. As an alternative value of the intensity ratio R280, (detection value of the ultraviolet detector 9 having the copper cathode 9b) / (detection value of the ultraviolet detector 9 having the silver cathode 9b) can be employed. As a result, as in the previous embodiment, it is possible to obtain the relationship between the emission intensity ratio and the flame temperature, or the relationship between the emission intensity and the air ratio. In this embodiment, since the wavelength band to be detected is relatively wide, there is a risk of detecting a plurality of radical emissions. For example, the ultraviolet detector 9 having the copper cathode 9b may detect radical emission OH (2, 1) having a peak at a wavelength of 287 nm as well as radical emission OH (1, 0) having a peak at a wavelength of 280 nm. In such a case, as described above, the voltage applied to the ultraviolet detector 9 is changed to change the sensitivity to the wavelength, and the measurement is performed between the measurement data to reduce the influence on the detected value due to unnecessary radical emission. It is possible.

As described above, in the flame detection method and flame detection apparatus according to the present invention, the OH band spectrum of A ² Σ ⁺ → X ²か ら is measured from the self-luminous component of the flame caused by the lean combustion, and the self-luminous intensity ratio R _I is obtained. By judging, it became clear that the flame temperature or air ratio in dilution combustion can be detected reliably. For this reason, a great number of practical effects are achieved such that various types of lean flame detection can be easily and reliably performed without being affected by thermal radiation on the wall of the combustion furnace.

Claims (5)

Measure multiple self-luminous intensities at different wavelengths with the same radical species peak in the ultraviolet region from the self-luminous component of the flame caused by combustion,
Find the emission intensity ratio, which is the ratio of the measured self-emission intensity of each wavelength,
A flame detection method comprising detecting a flame state based on at least one of a relationship between the emission intensity ratio and a flame temperature and a relationship between the emission intensity ratio and an air ratio of an air-fuel mixture used for the combustion. .   The flame detection method according to claim 1, wherein the self-luminous intensity is measured by measuring a self-luminous spectrum from a specific radical species accompanying an electronic transition from an excited state to a ground state by combustion. . The measurement of the self-luminous intensity is to measure the OH band spectrum of the electronic transition A ² Σ ⁺ → X ² II,
Mutual emission intensity of OH (2,0) near wavelength 260 nm, OH (1,0) near wavelength 280 nm, OH (2,1) near 287 nm, and OH (0,0) near 306 nm The flame detection method according to claim 2, wherein the flame state is detected by obtaining the ratio of the above.   The flame detection method according to claim 1, wherein the state of the flame is detected by paying attention to an OH band spectrum having a wavelength of about 310 nm or less. An ultraviolet detector for detecting a plurality of self-luminous intensities at different wavelengths each having a peak of the same radical species in the ultraviolet region from a self-luminous component of a flame caused by combustion;
The self-luminous intensity of each wavelength is obtained from the detection signal of the ultraviolet detector, the relation between the ratio of the self-luminous intensity and the flame temperature, and the relation between the ratio and the air ratio of the mixture used for the combustion And a processing device for detecting the state of the flame based on at least one of the above.

Images