A portable remote methane detector using an InGaAsP DFB laser
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- Iseki, T. Env Geol (2004) 46: 1064. doi:10.1007/s00254-004-1094-0
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A portable remote methane detector based on infrared-absorption spectroscopy using an InGaAsP distributed-feedback laser is described. This equipment transmits a laser beam and detects a fraction of the backscatter reflected from the target. From this, the detector thereby measures the integrated methane concentration between the detector and the target. The equipment operator can easily detect methane clouds at a distance by hand-scanning the laser beam. To achieve a high sensitivity of detection, wavelength-modulation spectroscopy is applied to the infrared-absorption measurement for methane. The wavelength of the light source is modulated at a frequency of 10 kHz, and the center wavelength is stabilized at the center of the 2ν3 band R(3) line of methane (1.65372 μm). When used with typical targets at distances up to 10 m, the detector has a detection limit of less than 5 ppm-m. To the best of the author’s knowledge, the detector is the only hand-held product capable of remote methane detection. Recently, this novel equipment was commercially introduced into the Japanese market as a product named the Portable Remote Methane Detector (PRMD). Some PRMD units were in research use for detecting methane emissions from garbage landfills, although the PRMD was mainly developed for remote detection of leaks from natural gas pipelines. The author expects that the PRMD could become the standard equipment for field measurements of methane emissions from land.
KeywordsMethaneRemote sensingTunable diode laser absorption spectroscopy (TDLAS)Wavelength modulation spectroscopy (WMS)
Recently, trace gas measurement has become increasingly important due to the widening interest in environmental problems. Laser absorption spectroscopy (LAS) provides one of the best methods for this measurement as it has many advantages over conventional techniques based on gas chromatography or mass spectroscopy. For example, LAS provides real-time measurement, remote and non-destructive sensing, good molecular selectivity, and good isotope sensitivity. Using this method, a tunable diode laser (TDL) is often used as a light source because it is highly monochroic and its wavelength is easily scanned. When using a TDL, LAS is termed as tunable diode laser absorption spectroscopy (TDLAS). As a TDL is relatively inexpensive and does not need a wavelength-selection mechanism such as a spectroscope, it provides a low cost and simple system for a laser-based system. As a result, a number of gas-sensing techniques based on TDLAS have been studied for many years.
Methane (CH4) is, like carbon dioxide (CO2), a greenhouse gas. Although the concentration of methane in natural air (1.7–1.9 ppm) is roughly 200 times lower than that of carbon dioxide (350–370 ppm), the global warming potential (GWP) of methane is estimated to be 23 times larger over a time horizon of 100 years (Watson and others 2001). GWP is defined here as the total impact over time of adding a unit of a greenhouse gas to the atmosphere. Therefore, it is vitally important to measure methane as well as carbon dioxide emissions in studies regarding the greenhouse effect. However, methane-sensing devices need more advanced technology than carbon-dioxide sensing devices due to the low concentration of methane in natural air. The author’s research group recently reported the remote detection (Uehara and Tai 1992) and environmental monitoring (Uehara and others 1997) techniques for methane using a near-infrared TDL. More recently, they reported a portable remote-methane sensor using a near-infrared TDL (Iseki and others 2000). Tokyo Gas Co., Anritsu Corporation, and Tokyo Gas Engineering Co. have improved the portability of this sensor and commercialized it as a novel equipment named the Portable Remote Methane Detector (PRMD; Iseki and Miyaji 2003). To the best of the author’s knowledge, the PRMD is the only hand-held product capable of remote methane detection. This detector is quite suitable for field measurements because of its portability. The PRMD was mainly developed for remote detection of leaks from natural gas pipelines, and more than 40 units are now in actual use in the Japanese gas industry. In addition, some units were in research use for detection of methane emissions from garbage landfills. In the present paper, the author reviews the principle and performance of the PRMD, suggesting its applicability in the detection of geologic methane emissions.
Selection of a light source and an absorption line
To achieve high sensitivity in gas sensing with TDLAS, it is desirable to use as strong an absorption line as possible. Most gas molecules have fundamental bands of vibrational transition in the mid-infrared (2–60 μm) region, whereas overtone and combination bands lie in the near-infrared (0.75–2 μm) region. In general, the fundamental bands have much stronger absorption intensities than the other bands. In the case of methane, there are two strong absorption bands centered at 3.3 (ν3 band) and 7.6 μm (ν4 band). Indeed, some lead-salt based TDLs are available in the mid-infrared region, but they require cryogenic cooling using liquid nitrogen and have shortcomings of maintenance. In contrast, some group III–V semiconductor-based TDLs can operate at room temperature and provide continuous wave operation in the near-infrared region. In addition, these are compact and robust, and can be incorporated within a portable instrument. Of these, the indium–gallium–arsenic–phosphorus (InGaAsP)-distributed-feedback (DFB) laser is particularly suitable for use as a light source of a portable gas-detector. This type of laser was originally developed for telecommunication purposes and is now commercially available in the wavelength region of 1.2–2.1 μm. In this region methane has the strongest absorption band centered at 1.67 μm (2ν3 band), with several lines that do not interfere with absorption of other atmospheric air gases, particularly water vapor. For the present studies, the author has employed an InGaAsP DFB laser whose wavelength is designed around the 2ν3 band R(3) line of methane (1.65372 μm).
Principle of remote detection
Wavelength modulation spectroscopy
The concentration of methane in natural air is 1.7–1.9 ppm, and the peak absorption coefficient of the 2ν3 band R(3) line is 3.8x10−5 ppm−1-m−1 at atmospheric pressure. As a result, methane in natural air with an optical-path length of 1 m gives a peak-absorption coefficient of as little as 6.8x10−5 for the PRMD. Even for methane retention, caused by large amounts of methane emission, the PRMD needs to detect very small absorption levels. For example, methane retention with a methane column density of 100 ppm-m gives the peak absorption coefficient of as little as 7.6x10−3. It should be noted that, in the case of the PRMD, the path-integrated methane concentration is twice as great as the methane-column density because the laser light is received after round-trip propagation between the detector and the target. In addition to this small absorption measurement, the PRMD needs to detect extremely weak power (1 μW at most). To achieve detection of such small absorption levels and weak power, the PRMD employs wavelength modulation spectroscopy (WMS). In WMS, the laser wavelength is modulated at a frequency (f), and the photodetector output is processed by phase-sensitive detection with reference to the fundamental (f), second harmonic (2f), or higher harmonics of modulation. This method is often employed with TDLs because the wavelength of a TDL can be easily modulated by the injection current. In WMS, as the modulation frequency increases, the noise decreases, but the electrical circuitry becomes more complicated and expensive. To balance this, the author set the modulation frequency at f =10 kHz.
In the case of small absorption levels, the 2f signal at the absorption center is proportional to the methane column density and the collection efficiency. In fact, the collection efficiency, defined as the ratio of the received power to the initial laser power (in the absence of methane), changes as a function of target reflectance, distance, and incident angle. On the other hand, the f signal at the absorption center is independent of the methane-column density, but is proportional to the collection efficiency. Therefore, using the ratio between 2f and f signals, the PRMD is able to cancel the change in the collection efficiency (Uehara and Tai 1992) and thereby uses this “ratio detection” technique to obtain the methane-column density.
The specifications of the current version of PRMD
Second harmonic detection of wavelength modulation spectroscopy for infrared absorption using target backscatter return
InGaAsP-distributed feedback laser (wavelength: 1.65372 μm, output: 5 to 10 mW)
Class 2 laser product (by IEC)
1.6 mrad (corresponds to an 8-mm-diameter detection area at 5 m distance)
Fresnel lens (effective diameter: 81 mm)
InGaAs PIN photodiode (packaged with a pre-amplifier)
1.3 kg (including the battery)
W 112xD 250x H 248 mm
Nickel-metal hydrate battery (battery life >90 min @25 °C)
Up to 10 m
The collimator has a full-angle divergence of 1.6x10−3 rad. This corresponds to a detection area of 8-mm diameter at 5-m distance. A Fresnel lens was selected as the collection lens as it is lightweight and inexpensive. In the photodetector, an InGaAs PIN photodiode is packed with a pre-amplifier.
Regarding the response time, the PRMD is much faster than conventional gas detectors. As the time constant for phase-sensitive detection of WMS (equivalent to the response time of the detector) shortens, the detector responds faster, but in doing so the noise level increases. To balance this, the time constant is set at 0.1 s.
Power consumption during operation is on average 5 W and is supplied by a special nickel-metal hydride battery that has a battery life of more than 90 min at 25 °C. The weight of the PRMD is 1.3 kg, including the battery. The PRMD has a width of 112 mm, length of 250 mm, and height of 248 mm as its maximum.
With a decrease in the received light power the noise level increases (Iseki and others 2000) and, therefore, to avoid inaccurate output, the PRMD calculates the methane-column density, but displays a “no measurement” alert if the received light power is below a preset threshold. This threshold is set to 20 nW, at which the noise level is estimated experimentally to be less than 5 ppm-m.
The detection range, or maximum target distance, depends on the target reflectance. Using typical targets, the detection range is up to 10 m with a noise level of less than 5 ppm-m.
Feasibility of environmental application
Although the PRMD was developed for the remote detection of leaks from natural gas pipelines, it can be effectively used for detection of methane emissions from other sources. Actually, the PRMD was used for detection of methane emissions from garbage landfills (Yamada and Ishigaki, personal communication 2002). This implies that the PRMD can detect methane emissions from natural sources such as geologic seeps and microseepage. As the PRMD outputs path-integrated concentration of methane, the methane concentration (in average) is obtained from dividing the detector output by the distance between the detector and target. If vertical wind velocity and methane concentration are measured at the same time, the eddy correlation method (Verma and others 1992) can provide the methane flux from soil to the atmosphere.
Usage in humidity and aerosols
As described earlier, the PRMD uses one of the absorption lines of methane that are free from interference of absorption lines of water. Therefore, the detector is not affected at all by humidity (gaseous H2O). In addition, the detector is almost unaffected by light extinction by most aerosols as it is used with relatively short range. In case of heavy fog with low visibility, however, the PRMD cannot provide the remote sensing functionality.
To improve accuracy
As described earlier, the PRMD has a noise level of less than 5 ppm-m when used with typical targets of up to a distance of 10 m. This means that the noise level is equivalent to an average methane concentration of less than 0.5 ppm when the target distance is set at 10 m. This sensitivity is sufficient for detecting quite large amounts of methane emission. However, higher sensitivity is desired for detecting small amounts of methane emission or monitoring the concentration of methane in the atmospheric air. To achieve this higher sensitivity, one possible method would be to increase the PRMD optical-path length without decreasing the received power. A retroreflector could meet this demand although the line of measurement is fixed between the detector and the retroreflector. Actually, the PRMD was used with a retroreflector for measurement of methane concentrations in a waste-disposal plant (Tanikawa, personal communication 2003). When the PRMD is used with a retroreflector at a distance of 100 m, calculations show that the noise level could equate to an average methane concentration of less than 0.01 ppm. In addition, however, a small correction would be needed to allow for temperature and pressure variations to accurately measure such concentration levels.
Detection of gases other than methane
Absorption bands of typical of gases in (1.2−2.1) μm (only for the principal isotopic species)
Unit in μm
A portable remote methane detector using an InGaAsP DFB laser was developed. This detector is designed as a long-path absorption lidar (laser radar) using a diffused-reflection target. To achieve detection with high sensitivity, the author applies wavelength modulation spectroscopy to the infrared-absorption measurement for methane. The wavelength of the light source is modulated at a frequency of 10 kHz, and the center wavelength is stabilized at the center of the 2ν3 band R(3) line of methane (1.65372 μm). When used with typical targets at distances of up to 10 m, the detector has a detection limit of less than 5 ppm-m. The detection range and sensitivity can, however, be improved using a retroreflector. Although the detector was developed for the remote detection of leaks from natural gas pipelines, it can be effectively used for detection of methane emissions from natural sources, such as geologic seeps and microseepage.
The author expresses thanks to his research group members in Tokyo Gas Co. Ltd., Anritsu Corporation, and Tokyo Gas Engineering Co. Ltd. for their cooperation during the development of the PRMD. The author also wish to thank Dr. Tanikawa, associate professor of the Hokkaido University, Dr. Yamada, Senior researcher of the National Institute for Environmental Studies, and Dr. Ishigaki, researcher of the National Institute for Environmental Studies, for their valuable advise.