FORT COLLINS — A comprehensive study of natural gas transmission and storage facilities led by Colorado State University (CSU) researchers has found that the total amount of methane emitted into the atmosphere from the transmission and storage sector is not statistically different from the emissions from the transmission and storage sector reported in the Environmental Protection Agency’s 2012 Greenhouse Gas Inventory.
The study, published today in the journal Environmental Science & Technology, found overall methane emissions from transmission and storage facilities ranging between 1,220 and 1,950 Gigagrams/year (mean of 1,503 Gg/yr.). The EPA’s Greenhouse Gas Inventory (GHGI), one of the federal agency’s two programs that track methane from the natural gas system, estimated emissions between 1,680 to 2,690 Gg/yr (mean of 2,071 Gg/yr). Because these ranges overlap, the researchers consider the two estimates statistically similar.
The study estimates that total methane emissions from the transmission and storage sector resulted in the loss of 0.28% to 0.45% (mean of 0.35%) of the methane transported in 2012.
The paper is the final analysis of one of the most comprehensive studies of methane emissions from natural gas transmission and storage facilities. It is part of the largest on-site measurement campaign of the U.S. natural gas infrastructure to date.
For the study, researchers from CSU, Carnegie Mellon University and Aerodyne Research, with support from seven natural gas pipeline companies, measured amounts of methane emitted from compressor stations and storage facilities on large natural gas transmission pipelines across the country.
The primary component of natural gas, methane is a greenhouse gas many times more potent than carbon dioxide when released into the atmosphere unburned. The nation’s vast natural gas infrastructure – including wells, pipelines, and storage facilities – is one of many sources of methane emissions in the United States.
The CSU study provides much-needed information on emissions in the U.S. pipeline transmission and storage sector, which includes roughly 1,800 compressor stations and underground storage facilities positioned along approximately 300,000 miles of pressurized natural gas transmission pipelines. The partner company transmission facilities represent approximately 56 percent of the interstate transmission facilities in the U.S.
The research team used 2,292 new on-site measurements and equipment data from 922 facilities to build a computer model to calculate overall emissions.
Based on the results of the study, the authors estimate that approximately one in 25 facilities may be emitting 300 standard cubic feet per minute or more of natural gas at any given time. These emissions account for about one-third of fugitive emissions — unintended emissions from equipment in the pipeline transmission and storage sector — and a quarter of all methane emissions from the sector annually.
“Our data indicate that these releases are both intermittent and unpredictable,” said Daniel Zimmerle, senior research scientist at the CSU Energy Institute, who led the study.
The study also found that fugitive emissions account for 75 percent of all methane emissions in the transmission and storage sectors. Half of these emissions are from major compressor equipment such as seal vents, unit isolation and blow-down valves and rod-packing vents.
Researchers also found that the equipment used in this sector is significantly different than assumed in previous estimates, which can greatly affect the amount of methane being emitted. For example, companies have replaced many smaller engine-driven reciprocating compressors with larger and fewer centrifugal compressors resulting in less unburned methane in exhaust gases.
As part of the study, researchers also compared methane emissions in this sector to the EPA’s Greenhouse Gas Reporting Program (GHGRP). The GHGRP requires reporting of emissions from facilities with annual greenhouse gas emissions greater than 25,000 metric ton carbon dioxide equivalent. The new model indicates facilities that report to the program emit 2.6 times more methane than reported to the EPA.
Zimmerle noted that the difference can be attributed to a combination of factors inherent in the reporting requirements. Factors include emission estimation methods, measurement methods and emissions sources that are not required to be reported under the federal program.
The study was sponsored by the Environmental Defense Fund and supported by Dominion, Dow Chemical, Enable Gas Transmission, Kinder Morgan, Columbia Pipeline Group, TransCanada and The Williams Companies. The Interstate Natural Gas Association of America also participated in the study.
The Colorado State University study is one of 16 organized by the Environmental Defense Fund and industry partners to better quantify the amount of methane released into the atmosphere from the natural gas supply chain. This is the second paper published by researchers at CSU and Carnegie Mellon as part of the project.
The full paper can be found at pubs.acs.org/doi/abs/10.1021/acs.est.5b01669
More information about the study can be found at www.edf.org, ingaa.org, and energyinstitute.colostate.edu/p/transmission-and-storage.html