Mobile ‘dual-comb’ device improves gas detection

26 Mar 2018

NIST-developed detector improves sensitivity and cost model for finding methane, other problem gases.

Accurately detecting, locating and quantifying leaks of methane is critically important for both environmental and economic purposes. However, a group of researchers in the US say that traditional methods are slow, labor-intensive, limited to small coverage areas and expensive to operate over time.

Now, the researchers at National Institute of Standards and Technology), the University of Colorado Boulder and the National Oceanic and Atmospheric Administration have demonstrated a new and better solution: the “mobile dual-comb laser spectrometer”. The work is described in the journal Optica.

This gas detector features a NIST adaptation of a Nobel Prize-winning technology, which can continuously and cost-effectively monitor leaks of methane (CH₄) and other trace gases precisely and over large areas.

Ian Coddington, a NIST physicist and co-author of the Optica paper, potential users of current methane detection and quantification systems – which rely heavily on inspectors using infrared cameras to look laboriously for gas plumes one by one across large sites – often find the costs prohibitive for meeting three needs: coverage of large areas, frequent monitoring and sensitivity.

“The last of these, the problem of sensitivity, really becomes evident when you realize that leaks may have rates as small as 0.2 cubic meters per hour, which is equivalent to just one quarter of the respiration rate of an average human being,” he said.

To address these problems, NIST and its partners have developed a novel observing system that combines the world’s first portable dual-frequency comb laser spectrometer and an array of corner cube retroreflectors. The spectrometer is based on the frequency comb technology for which NIST’s John Hall shared the 2005 Nobel Physics Prize.

The retroreflectors — special mirrors that send light striking them straight back to their source — exhibit an optical phenomenon exploited by NIST’s James Faller in the early 1960s for his design of the laser reflecting arrays, deployed during two Apollo landings to measure the Earth-moon distance.

Combing your flare

An optical frequency comb is a precise tool for measuring different frequencies of light. The new trace gas monitoring system uses a NIST-developed evolution of the frequency comb technology, a dual-comb laser spectrometer where a second comb is added.

“When paired, the combs can act like hundreds of thousands of laser spectrometers working in unison, and yield a device that is 10 to 100 times better than a traditional spectrometer and very sensitive to leaks, even at a great distance,” Coddington added.

“It is comparable to standing at the Lincoln Memorial in Washington, D.C., and detecting one person exhaling among hundreds near the Washington Monument at the other end of the National Mall.”

In the demonstration described in the Optica paper, the researchers ringed a Colorado well field containing multiple methane leak sources with retroreflectors equally spaced along a 1-kilometer-diameter circle. In the center of the ring, they stationed a trailer with the portable dual-comb device inside.

According to Kuldeep Prasad, a NIST mechanical engineer who helped design the computer model, combining local meteorological data with the measurements taken from all of the arcs within the ring of retroreflectors provides a complete accounting of the methane escaping from an entire field of wells.

“The monitoring can be done continuously over time so that every leak in a field is accurately detected and quantified,” he said. “Furthermore, if you set up mobile spectrometers at adjoining fields, you can dramatically expand the total coverage area to several square kilometers.”

“Besides refining the monitoring capabilities of the mobile dual-comb laser spectrometer, our team will next be attempting to bring the operating cost down to a per-well-per-year price tag of around $300,” Prasad said.

“Achieving that goal would make the system affordable for most users.”

Coddington said that the portable dual-comb laser spectrometer system for detecting and quantifying methane leaks already has attracted several industrial partners who are now working with the NIST, CU Boulder and NOAA collaborators on tests to evaluate the system’s performance at operational oil and gas facilities.

‘Greenhouse’ gases

Although its lifetime in the atmosphere is much shorter than the more prevalent greenhouse gas, carbon dioxide (CO₂), methane is significantly more of a problem than CO₂ because by mass it traps more of the sun’s heat — about 25 times more over a 100-year period, according to the Environmental Protection Agency.

Taking that difference into account, the EPA estimates that the methane leaked or intentionally vented from human activities (such as natural gas exploration, storage, coal mining; chemical manufacturing and processing; waste management; and livestock farming accounts for about 10 percent of the warming potential of all greenhouse gas emissions in the United States.

Along with the environmental benefits, more reliable methane monitoring also could reduce the number of costly, disruptive community evacuations by catching leaks from residential or commercial lines before they become threats, and more importantly, save lives and property by helping prevent gas explosions.

The detector developers comment that there is a financial incentive for keeping methane leaks in check: “According to the National Resources Defense Council, capturing currently wasted methane for sale could bring in more than $2 billion of additional revenue each year.”