Sensitive methane sensing on a budget?

Researchers have developed a new sensor that they suggest could allow practical and low cost detection of low concentrations of methane gas.

The new sensor uses an interband cascade light emitting device (ICLED).

Measuring methane emissions is important because the gas contributes to global warming and air pollution. Relevant sectors include agriculture and the waste industry. Detecting leaks is also critical to the oil and gas industry, since natural gas is mainly composed of methane.

Researchers from Princeton University and the US Naval Research Laboratory have demonstrated a new gas sensor, which uses an interband cascade light emitting device (ICLED) to detect methane concentrations as low as 0.1 ppm. ICLEDs are a new type of higher-power LED that emits light at mid-IR wavelengths, which can be used to measure many chemicals.

The results have been published in Optics Express.

“We hope that this research will eventually open the door to low-cost, accurate and sensitive methane measurements,” said Nathan Li, first author of the paper. “These sensors could be used to better understand methane emissions from livestock and dairy farms and to enable more accurate and pervasive monitoring of the climate crisis.”

Laser-based sensors are currently the gold standard for methane detection, but cost between $10k and $100k each. A sensor network that detects leaks across a landfill, wastewater treatment plant or farm would be prohibitively expensive to implement using laser-based sensors.

Although methane sensing has been demonstrated with mid-IR LEDs, performance has been limited by the low light intensities generated by available devices. To substantially improve the sensitivity and develop a practical system for monitoring methane, the researchers used the new ICLED. “The ICLEDs we developed emit roughly ten times more power than commercially available mid-IR LEDs had generated, and could potentially be mass-produced,” said Jerry Meyer of the US Naval Research Laboratory. “This could enable ICLED-based sensors that cost less than $100 per sensor.”

The new sensor measures IR light transmitted through clean air with no methane and compares that with transmission through air that contains methane. To boost sensitivity, the researchers sent the IR from the high-power ICLED through a 1-meter-long hollow-core fibre containing an air sample. The inside of the fibre is coated with silver, which causes the light to reflect off its surfaces as it travels down the fibre to the photodetector at the other end. This allows the light to interact with additional molecules of methane in the air resulting in higher absorption of the light.

“Mirrors are commonly used to bounce light back and forth multiple times to increase sensor sensitivity but can be bulky and require precise alignment,” said Li. “Hollow core fibres are compact, require low volumes of sample gas and are mechanically flexible.”
To test the new sensor, the researchers flowed known concentrations of methane into the hollow core fibre and compared the IR transmission of the samples with state-of-the-art laser-based sensors. The ICLED sensor was able to detect concentrations as low as 0.1 ppm while showing excellent agreement with both calibrated standards and the laser-based sensor.

“This level of precision is sufficient to monitor emissions near sources of methane pollution,” said Li.