Greater focus needed on methane leakage from natural gas infrastructure

Ramón A. Alvarez; Stephen W. Pacala; James J. Winebrake; William L. Chameides; Steven P. Hamburg

doi:10.1073/pnas.1202407109

Contributed by Stephen W. Pacala, February 13, 2012 (sent for review December 21, 2011)

Abstract

Natural gas is seen by many as the future of American energy: a fuel that can provide energy independence and reduce greenhouse gas emissions in the process. However, there has also been confusion about the climate implications of increased use of natural gas for electric power and transportation. We propose and illustrate the use of technology warming potentials as a robust and transparent way to compare the cumulative radiative forcing created by alternative technologies fueled by natural gas and oil or coal by using the best available estimates of greenhouse gas emissions from each fuel cycle (i.e., production, transportation and use). We find that a shift to compressed natural gas vehicles from gasoline or diesel vehicles leads to greater radiative forcing of the climate for 80 or 280 yr, respectively, before beginning to produce benefits. Compressed natural gas vehicles could produce climate benefits on all time frames if the well-to-wheels CH₄ leakage were capped at a level 45–70% below current estimates. By contrast, using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, and reducing CH₄ losses from the production and transportation of natural gas would produce even greater benefits. There is a need for the natural gas industry and science community to help obtain better emissions data and for increased efforts to reduce methane leakage in order to minimize the climate footprint of natural gas.

Footnotes

↵¹To whom correspondence may be addressed. E-mail: pacala{at}princeton.edu or ralvarez{at}edf.org.

Author contributions: R.A.A., S.W.P., and S.P.H. designed research; R.A.A. performed research; R.A.A., S.W.P., and S.P.H. analyzed data; and R.A.A., S.W.P., J.J.W., W.L.C., and S.P.H. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1202407109/-/DCSupplemental.
^*Challenges also exist in the quantification of CH₄ emissions from the extraction of coal. We use the term “leakage” for simplicity and define it broadly to include all CH₄ emissions in the natural gas supply, both fugitive leaks as well as vented emissions.
^†This represents an uncertainty range between −19% and +30% of natural gas system emissions. For CH₄ from petroleum systems (35% of which we assign to the natural gas supply) the uncertainty is −24% to +149%; however, this is only a minor effect because the portion of natural gas supply that comes from oil wells is less than 20%.
^‡One-hundred-two times on a mass basis. This value accounts for methane’s direct radiative forcing and a 40% enhancement because of the indirect forcing by ozone and stratospheric water vapor (10).
^§The CH₄ from operation of a CNG automobile was estimated to be 20 times the value for gasoline vehicles (11), which is approximately 20% of the well-to-pump CH₄ leakage on a kg/mmBtu basis. This assumption deserves much further scrutiny.
^¶Our estimate that current well-to-wheels leakage is 3.0% of gas produced assumes that 2.4% of gas produced is lost between the well and the local distribution system (based on EPA’s 2011 GHG emission inventory) and that 0.6% is due to emissions during refueling and from the vehicle itself. For further discussion of the climatic implication of natural gas vehicles see (12).
^||EPA’s GHG inventory suggests leakage from natural gas processing and transmission is 0.6% of gas produced, meaning production leakage must be greater than 2.6% for the total fuel cycle leakage of a power plant receiving fuel from a transmission pipeline to exceed 3.2%.
^**Sites with compressor engines were excluded due to the contractor’s assumption that all engines in the City were uncontrolled, which leads to erroneous emission estimates.
^††Routine emissions do not include such occasional events as well completions and blowdowns. Only 203 of the 254 sites had data for gas production. An Excel spreadsheet containing the Fort Worth data and our calculations is provided in Dataset S1.
^‡‡Emissions query performed on December 5, 2011, using the Data and Maps feature of the U.S. Environmental Protection Agency’s Clean Air Markets Web page (http://camddataandmaps.epa.gov/gdm/).