JPH0220056B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a methane gas concentration measuring method and a measuring device suitable for measuring methane gas concentration in locations where measurement points are far apart, such as in LNG tankers, LNG tanks, and even in coal mine shafts. .

Methane gas is extremely important as a fuel gas, and is contained in large amounts in natural gas and the like.
Particularly in recent years, as city gas has become more caloric, natural gas has been increasingly used as city gas.
Therefore, in order to prevent gas explosions caused by city gas leaks, there is an urgent need to develop a safety system that can reliably and quickly detect methane gas leaks in specific areas such as underground malls and high-rise buildings, and issue warnings. has been done.

Furthermore, methane gas is the main component of coal mine gas generated in coal mines, and a similar system is required to prevent gas explosions caused by coal mine gas or coal dust explosions triggered by these gases.

However, conventionally used methane gas sensors such as catalytic combustion type, thermal conduction type, and semiconductor type have insufficient gas selectivity and response due to their operating principles, and are sensitive to surrounding coexisting gases, temperature, and humidity. There were complaints about reliability.
Therefore, it is unsuitable for mining sites where measurement conditions are strict, and real-time measurement is also difficult. Moreover,
In the case of remote monitoring and telemetry, electrical signals are sent and received, so there are problems that cannot be ignored, such as false alarms caused by electromagnetic induction and accidents caused by cable damage.

This invention was made in view of the above circumstances,
The purpose of the present invention is to provide a method and device for measuring methane gas that is highly reliable even under severe measurement conditions, capable of real-time measurement, capable of extremely remote monitoring and measurement, and without any danger of inducing accidents. It is.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

This invention is based on the fact that optical fibers such as silica-based optical fibers, which have been developed in recent years for optical communications, have wavelengths
The transmission loss is extremely low between 1.0 and 1.8 μm, and within this wavelength range there is a characteristic absorption of methane gas near 1.3 μm and 1.6 μm, and water vapor (H ₂ O)
This was based on the new finding that there is almost no absorption by carbon dioxide (CO ₂ ).

Figure 1 shows the wavelength of silica-based optical fiber from 0.6 μm.
It is a graph showing transmission loss in a wavelength range of 1.8 μm. As is clear from this graph, the wavelength is 1.1~
At 1.7μm, the transmission loss is less than 1dB/Km, especially
It shows an ultra-low loss of 0.2dB/Km near 1.6μm. It can be seen that if such an ultra-low-loss optical fiber is used as an optical transmission line, it is possible to measure the concentration of methane gas in a remote location by spectrophotometry.

Figures 2 and 3 show the characteristic absorption of methane gas, which is the subject of this invention, and the graph in Figure 2 shows the characteristic absorption of methane gas in the 1.33 μm band.
It can be seen that there is a strong absorption band at 1.331 μm. The graph in Figure 3 shows the characteristic absorption of methane gas in the 1.66 μm band, and it can be seen that there is a relatively strong and broad absorption band at 1.666 μm. In the vicinity of these two absorption bands, H ₂ O,
Further measurements confirmed that the characteristic absorption band of CO ₂ was almost absent.

Based on the above findings, for example, if a silica-based optical fiber is used as an optical transmission line and the methane gas characteristic absorption band of wavelength 1.666 μm or 1.331 μm is used, methane gas in a remote location will be almost unaffected by coexisting H ₂ O and CO ₂ . It was found that measurements can be made with high precision without any problems.

Next, a light source that emits light in the near-infrared region with a wavelength of 1.3 μm or 1.6 μm will be described. Light sources in this wavelength range generally include semiconductor laser diodes (LDs) and light emitting diodes (LEDs), but hot wires, discharge tubes, tungsten light bulbs, and the like may also be used. Although LDs can provide high output, their emission wavelength easily fluctuates depending on temperature and power supply voltage, and they are highly monochromatic, so advanced technology is required to use them for such purposes. On the other hand, LEDs have a low output, but their emission spectrum is somewhat broad, so the wavelength stability is good, and it is easy to cover the characteristic absorption wavelength, making it easy to use and sufficient depending on the detection range of the target gas. Available. but,
When using an LED as a light source, the emission spectrum is broad, so a spectrometer is required. There are various types of spectrometers, but here we decided to use an inexpensive band pass filter.

Here, the transmission width of a band-pass filter is generally wide, on the order of 1 to several nm, and if the spectral line of the object to be measured is narrower than this width, it is disadvantageous in terms of efficiency. However, in cases where the width is quite wide, such as 1.331 μm or 1.666 μm for methane gas, we will specifically consider below that even using such a band pass filter will be sufficient to improve the detection efficiency of the entire measurement system. I found it.

Figure 4 shows the intensity distribution of light after passing through the filter using a bandpass filter with a center wavelength of 1.6661 μm and a half-value width of 2 nm and a Gaussian distribution type transmission characteristic.The solid line shows the optical path length of 50 cm for methane gas. The dotted line represents the case where methane gas is not present in the measurement cell at a pressure of 20 Torr. It can be understood that the extinction ratio due to methane gas can be determined by dividing the difference in area between these two curves by the area surrounded by the dotted line.

In Figure 5, the center wavelength is 1.6661μm (A), 1.6666μm
(B) and 1.6656 μm (C) A graph showing the absorption ratio of the 1.666 μm absorption spectrum line of methane gas measured by changing the methane concentration using three types of bandpass filters with a half-width of 2 nm. It is something. The pressure of the mixed gas of methane gas and air was set to 1 atm, and the partial pressure (Torr) of the methane gas therein was varied. As is clear from the graph, if the center wavelength of the filter is different, the extinction ratio will change even if the partial pressure of methane gas is the same, and it can be seen that the filter (A) with a center wavelength of 1.6661 μm gives the highest extinction ratio.

In addition, Figure 6 shows three types of bandpass filters with a center wavelength of 1.6661 μm and half-widths of 1.5 nm (E), 2.0 nm (F), and 2.5 nm (G), which are the same as those shown in Figure 5. The absorbance ratio of methane gas was determined using the following conditions. As a result, for example, 3 Torr of methane gas in the air (corresponding to a concentration of about 6% of the lower explosive limit).
Since it shows the value obtained by taking the logarithm of the number, in the case of 3Torr, the position of log3=0.447 on the horizontal axis scale indicates 3Torr. ) can be detected by using a filter with a half-width of 2.5 nm (G) and measuring an extinction ratio of about 1.5%, that is, a decrease in light intensity. (However, from Figure 6, it can be seen that the filter (E) has the highest sensitivity, but the one with a narrow half-width is somewhat expensive, and the filter (G) can also be used satisfactorily, so (G) (The same filter was selected.) Furthermore, a similar study was conducted for city gas, which contains methane gas. FIG. 7 is a graph when a mixture of city gas and air containing 20% methane gas is used as a sample, and the absorption ratio is measured by varying the amount of city gas in the mixture. For bandpass filters, the center wavelength
The material used is 1.6661 μm and a half width of 2.0 nm.

From the above study results, we found that it is possible to quantify methane gas concentration by using a small LED as a light source and a band pass filter for wavelength selection.

What is shown in FIG. 8 is an example of a methane gas measuring device constructed based on the above knowledge. Reference numeral 1 in the figure is a light source made of a light emitting diode (LED). The 1.3μm band emitted by this light source 1 and
The light in the 1.6 μm band is sent through an optical coupler 2 to an optical fiber with low transmission loss, such as a quartz optical fiber 3, which is an optical transmission path. The silica-based optical fiber 3 has transmission characteristics as shown in Figure 1, and has an extremely low loss of 1.1 to 1.7 μm, so its length can be reduced to several kilometers.
~10km is acceptable. Light from the quartz optical fiber 3 is sent into the measurement cell 4 via an optical coupler 4b. This measurement cell 4 has optical couplers 4b and 4 at both ends of a cylindrical body (outer shell wall) 4a, respectively.
b' is provided to form a closed structure, and the cylinder 4a is formed from a porous material such as porous sintered metal or plastic foam with a continuous pore structure to allow natural flow of the measurement gas. has been done.
When the cylindrical body 4a is made of a porous member in this way, when the atmospheric gas flows in and out of the measuring cell 4, the cylindrical body 4a acts as a filter and removes soot or dust in the atmospheric gas, so that the measuring cell 4 It is possible to prevent soot or dust from entering into the inside of the optical coupler 4b, 4b', and prevent the optical couplers 4b, 4b' from becoming contaminated with soot or dust. Therefore, regardless of the degree of air pollution in the place where the measuring cell 4 is installed, the optical coupler 4
b, 4b' can be ensured for a long period of stable operation. Also, the optical path length of this measurement cell 4 (optical couplers 4b, 4
The distance between b′) was set to 50 to 100 cm as an example. The light emitted from the measurement cell 4 passes through an optical coupler 4b' and is connected to a low transmission loss optical fiber, for example, a quartz optical fiber 5.
sent to. Similarly, this quartz optical fiber 5 is also of low loss. The light that has passed through the optical fiber 5 is sent from an optical coupler 6 to a beam splitter 7 composed of a half mirror, where it is split into two beams. The first beam is sent to a first bandpass filter 8 and the second beam is sent to a second bandpass filter 9 . These filters 8 and 9 are interference filters that utilize the light interference effect of thin films, and multilayer film interference filters are preferably used.
The transmittance at the center wavelength is as high as possible, and the half-value width is
A narrow one of 1.0 to 2.0 nm is desirable. The center wavelength of the first filter 8 is 1.6661 μm or 1.3312 μm, which matches the wavelength of the characteristic absorption of methane gas. Further, the center wavelength of the second filter 9 is set to, for example, 1.62 μm or 1.30 μm, which exhibits no characteristic absorption of moisture or carbon dioxide at wavelengths other than the absorption wavelength of methane gas. By this, the first
The light transmitted through the filter 8 has a wavelength of 1.6661μm or
The transmitted wavelength distribution centered at 1.3312 μm becomes Gaussian-shaped light, and the light transmitted through the second filter 9 has a Gaussian wavelength distribution centered at 1.62 μm or 1.30 μm, which is unrelated to absorption by methane gas. It becomes a form of light. These lights are sent to a first photodetector 10 and a second photodetector 11 each comprising an avalanche photodiode or the like, where they are converted into electrical signals, and amplified by amplifiers 12 and 13. Thereafter, the signal is sent to a signal processing device 14 composed of a microcomputer or the like. here,
The extinction ratio A is determined from the ratio X and (1-X) of the electric signals, and further arithmetic processing is performed using the relationship between the extinction ratio A and the methane gas concentration, which was determined in advance using a standard gas of methane. The concentration of methane gas in the gas present in the chamber 4 is determined, and the result is displayed on the display 15.

FIG. 9 shows another example of the measuring device of the present invention, in which the same components as those shown in FIG. 8 are given the same reference numerals and their explanation will be omitted. In this example, the light leaving the measurement cell 4 passes through a low-loss optical fiber 5, such as a silica-based optical fiber, and is split into two beams by an optical branch 16, which are split into two beams by optical couplers 17 and 18, respectively. The point from which the light is sent to the first filter 8 and the second filter 9 via the chopper 19, and the first and second photodetectors 10, 1
This differs from the previous example in that the electrical signals from 1 are sent to one amplifier 12. In this example, the electrical signals from the photodetectors 10 and 11 are converted into alternating current by the chopper 19, which has the advantage of being easy to amplify.

Note that the example is not limited to the above example, and the light from the light source 1 is split into a plurality of lights by an optical branch path, and these lights are sent to a plurality of measurement cells 4 through separate silica-based optical fibers 3, and the light is transmitted at a plurality of points. It can also be configured to measure methane gas at the same time.

As explained above, according to the methane gas concentration measuring method and measuring device of the present invention, the characteristic absorber of methane gas has a wavelength region with the lowest loss of optical fiber and an absorption band of CO ₂ and H ₂ O. Choose 1.33μm or 1.66μm, which is almost non-existent, and use a light-emitting diode (LED) with good stability as the light source.
The method uses the above-mentioned low-loss silica-based optical fiber for the optical transmission line and a small, inexpensive band-pass filter for wavelength selection to quantify methane gas by near-infrared absorption spectrophotometry, so the measurement cell can be placed in an extremely remote location. It can be installed at any location without electromagnetic induction or short-circuit accidents caused by cable breakage, and can therefore be used to measure methane gas concentration in coal mine tunnel gas and multiple measurement cells in large areas such as underground malls. It is suitable for cases where a system is installed and centrally monitored at one location. Furthermore, it is highly accurate because it is hardly affected by H ₂ O and CO ₂ present in the measurement gas. Moreover, since the outer wall of the measuring cell is made of a porous material, when atmospheric gas flows into and out of the measuring cell, the outer wall acts as a filter to remove soot or dust from the atmospheric gas. It is possible to prevent soot or dust from entering the inside of the measurement cell, and it is possible to prevent the optical coupler from becoming contaminated with soot or dust.
Therefore, even if the measurement cell is installed in a coal mine shaft or an underground mall, the optical coupler can ensure long-term stable operation regardless of the level of air pollution in the installation location, resulting in high measurement accuracy. can be demonstrated over a long period of time. Furthermore, since it is an absorption photometry method, real-time measurement is possible, making it possible to quickly respond to changes in methane concentration.
Furthermore, since a band pass filter with a half width of 1.0 to 2.5 nm is used for wavelength selection, light in the band absorbed by methane gas can be efficiently transmitted and measured accurately, and the device can be made smaller and cheaper. Can be done. Furthermore, high-sensitivity detection of methane below the explosion limit can be achieved even by using a small, low-power, low-output light emitting diode that does not require cooling or the like.

[Brief explanation of drawings]

Figure 1 is a graph showing the transmission loss of the silica optical fiber used in this invention, Figure 2 is the absorption spectrum of methane gas in the 1.33 μm band, Figure 3 is the spectrum of methane gas in the 1.66 μm band, and Figure 4 is the Gaussian spectrum. Figure 5 is a graph showing the intensity distribution of light transmitted through a distributed bandpass filter. Figure 6 is a graph showing the relationship between the concentration of methane gas and the absorption ratio when three types of band transmission filters with different half widths are used, and Figure 7 is a graph showing the relationship between the concentration of city gas in the air and the absorption ratio using a band transmission filter. 8 and 9 are graphs showing the relationship between methane gas concentration in city gas and FIG. 9, which are both schematic configuration diagrams showing examples of the methane gas measuring device of the present invention. DESCRIPTION OF SYMBOLS 1... Light source consisting of a light emitting diode, 3... Quartz-based optical fiber, 4... Measurement cell, 4a... Cylindrical body (outer shell wall), 4b, 4b'... Optical coupler, 5... Quartz-based light Fiber, 7... Beam splitter, 8...
1st band pass filter, 9...2nd band pass filter, 10...1st photodetector, 11...2nd photodetector, 12...amplifier, 14...signal processing device, 15 ...Indicator, 16...Optical branch path, 1
9...Chiyotsupa.

Claims (1)

[Claims] 1. Light in the 1.6 μm band or 1.3 μm band is transmitted through a silica-based optical fiber with low transmission loss in the wavelength range, and the multi-shell wall is made of porous sintered metal or a continuous pore structure. Using a hollow measuring cell made of a porous member made of plastic foam and having optical couplers attached to the outer shell walls facing each other, atmospheric gas flows in and out of the light transmitted by the quartz optical fiber. The light is transmitted to the above measurement cell, and after absorption of methane gas at 1.666μm or 1.331μm in the measurement cell, the light is halved by a silica-based optical fiber with low transmission loss in the wavelength range of 1.6μm band or 1.3μm band. The light is sent to a band transmission filter with a value width of 1.0 to 2.5 nm, and is separated into light of the absorption wavelength of the methane gas and other wavelengths, and these two lights are sent to a photodetector, and the intensity ratio of these lights is determined.
A method for measuring methane gas concentration, characterized in that the methane gas concentration in the measurement cell is measured by this method. 2 A light emitting source that emits light in the 1.6 μm or 1.3 μm band, a quartz-based optical fiber that transmits this light with low transmission loss in the wavelength range, and an outer shell wall made of porous sintered metal or plastic with a continuous pore structure. A hollow measuring cell is constructed of a porous member made of foam to allow atmospheric gas to flow in and out, and an optical coupler is attached to the outer shell wall facing each other. A band-pass filter with a half-value width of 1.0 to 2.5 nm splits the absorbed light into the absorption wavelength of the methane gas and other wavelengths; a photodetector that detects these lights; A methane gas concentration measuring device comprising: an arithmetic processing device that calculates an intensity ratio between a light signal having an absorption wavelength of the methane gas detected by a detector and a light signal having a wavelength other than that.