Mobile ‘dual-comb’ device significantly improves methane leak detection
Accurately detecting, locating and quantifying leaks of methane, the main component of natural gas and a major fuel source worldwide, is critically important for both environmental and economic reasons. Unfortunately, traditional methods are slow, labor-intensive, limited to small coverage areas and expensive to operate over time.
Now, researchers at the National Institute of Standards and Technology (NIST), the University of Colorado Boulder (CU Boulder) and the National Oceanic and Atmospheric Administration (NOAA) have successfully demonstrated a novel solution that features an innovative NIST adaptation of a Nobel Prize-winning technology to continuously and cost-effectively monitor leaks of methane (CH4) and other trace gases with extreme precision and over large areas.
The demonstration is described in a paper published today in the journal Optica.
Although its lifetime in the atmosphere is much shorter than the more prevalent greenhouse gas, carbon dioxide (CO2), methane is significantly more powerful than CO2 because it traps more of the sun's heat pound for pound–about 25 times more over a 100-year period, according to the Environmental Protection Agency (EPA). Taking that difference into account, the EPA estimates that the methane leaked or intentionally vented from human activities such as natural gas exploration, storage and pipelining; 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.
Finally, there's 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.
Current approaches to methane detection rely heavily on inspectors using infrared (IR) cameras to look laboriously for gas plumes one by one across large sites, such as well fields with hundreds of potential leaks. This is time-consuming, requires skilled operators and may only be done once a year or less because of the high cost for monitoring expansive, remote or otherwise difficult-to-survey areas. Aircraft and vehicle-mounted IR cameras or spectrometers offer another option; however, this method also is expensive and may be ill-suited for continuous monitoring.
In fact, said Ian Coddington, a NIST physicist and co-author of the Optica paper, potential users of current methane detection and quantification systems often find the costs prohibitive for meeting three needs: coverage of large areas, frequent monitoring and sensitivity. "The last one really becomes evident when you realize that leaks may have rates as small as 0.2 cubic meters per hour (6 cubic feet per hour), about one-fourth 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 "in-the-field" 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 (see http://www.nist.gov/news-events/news/2005/10/nistjila-fellow-jan-hall-shares-2005-nobel-prize-physics). And 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 (see http://www.nist.gov/nist-time-capsule/any-object-any-need-call-nist/moon-and-back-25-seconds).
As explained in more detail by a NIST fact sheet (see http://www.nist.gov/topics/physics/optical-frequency-combs), an optical frequency comb is a very precise tool for measuring different colors–or frequencies–of light. Scientists start with lasers that emit a continuous train of very brief (femtosecond, or one millionth of one billionth of a second) pulses of light containing millions of different colors, also known as frequency spectra. The evenly spaced, individual lines of the spectra look like the teeth of a comb, giving the tool its name. The laser pulses–with their millions of highly defined teeth–can serve like the marks on a ruler for measuring the spectral signature of any material through which they pass with incredible precision.
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 said. "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.
To detect and quantify methane leaks within a "pie-slice" section of the circle–the area defined by the arc between two consecutive retroreflectors–pulses from the dual-comb laser are first bounced off one reflector back to the spectrometer to quantify residual gas from outside the field. Subtracting that amount from the measurement made by the second reflector yields the level of methane being released within the arc alone. Data from repeated scans of the sector under varying wind conditions (direction and speed) are interpreted by a NIST computer model to define the leak locations and rates.
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."
For gas storage facilities, oil and gas processing plants and other industrial environments where line-of-site leak monitoring isn't possible, Prasad said that the spectrometer can be placed high above the site to detect and quantify methane as it rises.
Development of the NIST-CU Boulder mobile dual-comb laser spectrometer, as well as the associated modeling effort, has been funded by the NIST Greenhouse Gas Measurements Program, the Advanced Research Projects Agency – Energy (ARPA-E), the Department of Energy's Office of Fossil Energy and the Defense Advanced Research Projects Agency (DARPA). Not only does the innovation allow for 24/7, highly reliable methane leak detection and quantification over large areas in the field, but it makes the entire operation more cost-effective than current IR methods.
"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.
S. Coburn, C.B. Alden, R. Wright, K. Cossel, E. Baumann, G-W. Truong, F. Georgetta, C. Sweeney, N.R. Newbury, K. Prasad, I. Coddington and G.B. Ricker. Detection of small and variable trace gas emissions across large regions. Optica. Published 22 March 2018. DOI: 10.1364/OPTICA.5.000320
Michael E. Newman
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