How to determine coal mines’ methane emissions

San Juan generating station and coal mine. (Image by Doc Searls, Wikimedia Commons.)

A research team at Los Alamos National Laboratory found that using multiple methods to measure the ratio of ethane to methane in the ambient air around fossil energy development regions can be used to attribute emissions to specific polluters.

In a paper published in the Journal of Geophysical Research, the scientists explain that while oil, gas and coal production is widely known to emit methane, a potent greenhouse gas, the industry’s geographically vast and variable infrastructure from extraction sites to processing facilities makes attributing these emissions to specific sources challenging.

But by applying their new method in New Mexico’s San Juan Basin, the researchers are convinced that it is possible to identify the sources and, thus, reduce methane emissions.

“We were able to show that individual sources of methane from coal, oil and gas, and fossil fuel infrastructure in New Mexico’s San Juan Basin have different ethane-to-methane ratios that can be detected at various scales, allowing us to differentiate among them,” Aaron Meyer, lead author of the paper, said in a media statement.

Meyer said the results bring an important new capability for attributing natural gas leaks with greater fidelity, which can help nations meet the Global Methane Pledge, signed by the United States, the European Union and about 100 nations at the COP26 climate conference. The pledge aims to reduce methane emissions globally by 30% by 2030, compared to 2020 levels.

The San Juan coal mine

“Natural gas is primarily methane but includes other hydrocarbons, including ethane,” said Manvendra Dubey, co-corresponding author of the article. “The composition of the gas varies with the source. We were able to discriminate among these using unique instruments that span many scales that were deployed by Los Alamos in the Four Corners. Our findings enable emitter accountability to reduce methane emissions, which are 84 times more potent as a warming agent than carbon dioxide, in this decade.”

Focusing on a methane hot spot discovered several years ago above the Four Corners, the study for the first time analyzed observations made at time scales from seconds to hours and length scales from meters to tens of kilometres. The research team took measurements with a mobile ground-based sensing system and studied older data from aircraft campaigns and remote sensing platforms.

San Juan generating station and coal mine
Map showing the San Juan, NM region containing a large coal mine. Known methane emission sources sampled during 2020/2021 mobile surveys and the location of the fourier transform spectrometer instrument are identified. (Image by Journal of Geophysical Research: Atmospheres .)

The analysis determined that the vent shaft of the San Juan coal mine consistently emitted a stable ratio of ethane to methane over eight years of measurements. The ratio held up under a variety of measurement techniques and over a range of distances from the source.

That ethane-to-methane ratio serves as a distinct signature identifying the coal vent and differentiating it from other sources, many of which showed drastically different ratios.

“Despite a diverse and changing emissions environment, we successfully used ethane-to-methane ratios to identify and apportion several sources across scales in space and time,” Meyer said. “Using different measurement techniques, we can leverage the advantages of each to build a systems-level approach to monitoring across an entire basin.”

In the scientist’s view, without the ability to identify, locate and quantify methane emissions, any reduction attempts are thwarted.