New Air Quality Monitoring System Allows Dynamic Monitoring of Methane Gas Leakage

Research and Development of China Instrument Network Instrumentation Researchers at the University of Colorado Boulder conducted field tests on a new type of air quality monitoring system based on frequency comb laser spectroscopy. According to them, the system combined with an atmospheric inversion model can be applied to methane gas. Continuous automatic monitoring of leaks (Optica, doi:10.1364/OPTICA.5.000320). The researchers reported that during the on-site testing they were able to monitor trace methane leaks over a range of a few square kilometres at a rate equivalent to 25% of a person's respiratory rate at rest.

The illustration shows the process of leaked gas monitoring using a mobile dual-frequency comb laser spectrometer. Photo credit: Stephanie Sizemore and Ian Coddington/National Institute of Standards and Technology

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From instantaneous detection to continuous monitoring

The leakage of methane gas generated during human activities not only poses a threat to public safety and causes climate change, but also causes economic losses to oil and gas storage sites and processing plants. Currently, one of the most common methods for detecting methane leaks is to manually use an infrared camera to inspect gas leaks over large areas. This work was laborious and only provided an instantaneous measurement. It is also possible to choose to use aircraft and onboard infrared camera monitoring, but these methods are also expensive and impractical for continuous monitoring.

The team at the University of California, Boulder, claimed that based on the frequency comb technology developed by the Nobel Laureate and Professor John Hall of the University of California, Boulder, they independently developed a continuous monitoring system for the detection of methane. The latest method has been improved. By using laser frequency comb technology as the basis for spectrometers, researchers have created a tool that can quickly and accurately measure high-resolution, broad-band light absorption features with methane gas molecules.

Combining two frequency combs in a dual comb configuration can significantly increase the speed and spectral resolution of the frequency comb spectrometer (see “Double comb spectrum”, OPN, January 2017). Their innovative combination of dual-combined spectroscopy with atmospheric inversion algorithms allows the monitoring unit to pinpoint the location of gas leaks and calculate its leak rate.

Field test of double comb spectrometer

In an outdoor field test, the researchers set up a double comb spectrometer on the Table Mountain research facility at the University of California, Boulder. A 19-inch spectrometer was placed on a 360-degree rotating pedestal. The pedestal was in a circular center about one kilometer in diameter surrounded by a reflector. Gas tanks leaking methane gas were randomly placed in the circle.

When the spectrometer is turned on, it emits a beam with multiple wavelengths in the direction of a mirror. If the light beam passes through the methane gas cloud, the gas molecules absorb a certain wavelength before the light hits the mirror and then return to the detector. By observing the wavelength of absorbed light, researchers can determine whether a specific part of a round “depression” contains methane gas. Applying the atmospheric inversion model to the reflected beam data, the researchers were able to calculate and determine the location of the methane gas and the rate of leakage from the gas tank.

In these field tests, the researchers reported that they were able to detect gas emissions of as low as 1.6 grams per minute in a range of 1 kilometer, and their emission rate was 1000 times lower than the current method. They are also able to detect two simultaneous gas leaks in up to five potential leak sources.

The research team is composed of members of the University of California, Boulder, the National Institute of Standards and Technology, and the National Oceanic and Atmospheric Administration of the United States.

(Original Title: Dynamic Monitoring of Methane Gas Leakage)