Washington State University researchers found that methane emissions from local natural gas distribution systems in cities and towns throughout the U.S. have decreased in the past 20 years with significant variation by region.
The researchers found that upgrades in metering and regulating stations, changes in pipeline materials, better instruments for detecting pipeline leaks as well as regulatory changes have led to methane emissions that are from 36% to 70% lower than current Environmental Protection Agency estimates when the data gathered for this study is combined with current pipeline miles and the numbers of facilities. The study also showed significant variations by region, with some areas showing higher than average emissions because of large differences in the age and type of pipe in different parts of the U.S.
- Methane emissions from the distribution system range from approximately 393 to 854 gigagrams (Gg) per year, or between 0.1 and 0.2 percent of the methane delivered nationwide.
- The higher EPA estimates of 1329 Gg are based largely on previous emission measurements from a national study from the early 1990s in which Lamb and his colleagues participated.
- Out of 230 measurements, three large leaks accounted for 50% of the total measured emissions from pipeline leaks. In these types of emission studies, a few leaks accounting for a large fraction of total emissions are not unusual.
Fig 1. Updated U.S. Inventory for Local Distribution Systems
The updated methane emission inventory is summarized in Figure 1. In this inventory the emission factors based on the new measurements from this study are combined with current data for pipeline miles, number of facilities, and other activity factors to estimate the total emissions for each part of the distribution system. The updated inventory also includes estimates of the upper bound estimate based on the variation in emissions from the measurement data.
The updated inventory is compared to the recent EPA 2011 methane emission inventory which largely uses the emission factors from a 1992 national study together with current activity information.
The comparison shows that emissions in the updated inventory are much less than the current inventory, particularly for metering and regulating stations.
Dramatically lower emissions at metering and regulating stations (Figure 2)
- The researchers revisited nine sites from the previous 1992 study.
- They found an average of more than 10 times fewer emissions than measured at these sites 20 years ago.
Fig 2. Re-Visited 1992 GRI/EPA Higher Emitting Sites
Fig 3. Estimates of methane emissions by region for
leaks from underground pipeline mains and services.
Regional differences across the country in local distribution pipeline systems (Figure 3)
- Eastern region accounts for 34% of the total U.S. methane from pipeline leaks.
- Western region contributes 17% of the total U.S. emissions.
- Leaks from cast iron and unprotected steel pipeline mains account for 70% of the eastern emissions and almost half of total U.S. emissions.
- Plastic and unprotected steel account for 92% of U.S. pipeline mains by mileage, but leaks from these pipelines contribute less than 20% of total U.S. emissions.
- Services account for 35% of U.S. emissions.
The study provides the most comprehensive set yet of direct measurements of emissions from the distribution system and, with a series of partner studies, is helping to determine the natural gas industry’s contribution to U.S. greenhouse gas emissions and to global warming. It is published online in Environmental Science & Technology (see paper).
The distribution system includes underground natural gas pipelines as well as metering and regulating facilities and customer meters in cities and towns.