In underground operations, coal mine methane or CMM is released from the coal and surrounding rock strata. (Reference image by TripodStories- AB, Wikimedia Commons).
A research team at Princeton University and the US Naval Research Laboratory developed a new sensor that could allow practical and low-cost detection of low concentrations of methane gas.
In a paper published in the journal Optics Express, the scientists explain that measuring methane emissions and leaks is important to a variety of industries because the gas can be dangerous and, at the same time, it contributes to global warming and air pollution.
In underground operations, for example, coal mine methane or CMM is released from the coal and surrounding rock strata and it can create an explosive hazard. This is why it is normally removed through ventilation systems.
In abandoned and surface mines, on the other hand, methane might escape to the atmosphere through natural fissures or other diffuse sources.
Agricultural and waste industries also emit significant amounts of methane, while natural gas production relies on coalbed methane or CBM, which means that detecting leaks is also critical to the oil and gas industry for both environmental and economic reasons.
ICLED-based sensor to detect methane. (Image by Sameer A. Khan/Fotobuddy, courtesy of The Optical Society).
Given this context, the research team led by Mark Zondlo at Princeton saw an urgency in creating a new kind of sensor. The mechanism uses an interband cascade light emitting device or ICLED to detect methane concentrations as low as 0.1 parts per million. ICLEDs are a new type of higher-power LED that emits light at mid-infrared (IR) wavelengths, which can be used to measure many chemicals.
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 a new ICLED which emits roughly 10 times more power than commercially available mid-IR LEDs had generated.
The way it works is one in which the sensor measures infrared light transmitted through clean air with no methane and compares that with transmission through air that contains methane.
In underground operations, coal mine methane or CMM is released from the coal and surrounding rock strata and it can create an explosive hazard
To boost the system’s sensitivity, the researchers made the infrared light move from the high-power ICLED through a 1-meter-long hollow-core fiber containing an air sample. The inside of the fiber is coated with silver, which causes the light to reflect off its surfaces as it travels down the fiber 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.
To test the new sensor, the researchers flowed known concentrations of methane into the hollow core fiber and compared the infrared 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 parts per million 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,” Nathan Li, first author of the paper, said in the statement. “An array of these sensors could be installed to measure methane emissions at large facilities, allowing operators to affordably and quickly detect leaks and mitigate them.”
Thinking of affordability
By affordable, Li meant that the ICLED-based sensors are designed to be mass-produced.
This means that they would cost less than $100 per sensor, whereas current laser-based sensors – which are the gold standard for methane detection – cost between $10,000 and $100,000 each.
