15 February 2014, News Wires – U.S. Environmental Protection Agency (EPA) estimates of methane emissions have been underestimated over the past two decades, according to a study published Friday in the journal Science.
In the new analysis, “Methane Leakage from North American Natural Gas Systems”, researchers compared findings from over 200 studies ranging in scope from local gas processing plants to total emissions from the United States and Canada.
“Actual measurements consistently indicate that methane emission levels are about 50 percent higher than what our national accounts suggest,” said MIT Energy Initiative Director of Research Francis O’Sullivan in a Feb. 13 press release.
O’Sullivan was part of the team of authors who worked on the study, which included representatives from seven universities, national laboratories, federal government bodies and other groups.
“It appears certain that some of this additional methane is from the natural gas system, though likely not all due to the poor state of science surrounding other significant sources,” O’Sullivan commented.
Total methane emissions are typically estimated by multiplying the amount of methane thought to be emitted by a particular type of source, such as natural gas processing plants or belching cattle, by the number of that source type in a region or country. The products are then totaled to estimate all emissions.
Natural gas consists primarily of methane. Methane is emitted intentionally and unintentionally from natural gas infrastructure for safety purposes and through faulty valves and pipeline cracks. In the 1990s, the EPA established emission rates of particular U.S. gas industry components, from wells to burner tips. Since then, many studies have tested gas industry components to determine if the EPA emission rates are accurate; a majority of these studies have found the EPA rates to be too low.
The analysis found that atmospheric studies covering very large areas – where airplanes and towers were used to measure actual methane in the air to test total estimated emission – consistently indicate total U.S. methane emissions of approximately 25 to 75 percent higher than EPA estimates.
One potential reasons gas industry leaks have been underestimated is that emission rates for wells and processing plants were based on operators voluntarily providing information. One EPA study asked 30 gas companies to cooperate, but only six allowed the EPA on site.
“It’s impossible to take direct measurements of emissions from sources without site access,” said Garvin Heath, a senior scientist with the National Renewable Energy Laboratory and study co-author, in a statement. “But self-selection bias may be contributing to why inventories suggest emission levels that are systemically lower than what we sense in the atmosphere.”
The EPA does not include natural methane sources, such as wetlands and geologic seeps. The agency also does not include some emissions caused by human activity, such as abandoned oil and gas wells, because the amounts of associated methane are unknown.
Researchers noted that even small leaks from the natural gas system are important because methane is a potent greenhouse gas – about 30 times more potent than carbon dioxide. The gas industry must address leaks in gas infrastructure to deliver on their promise of less environmental harm, researchers found. However, the analysis found that a few leaks in this systems likely account for much of the problem and could be repaired, citing a previous study that examined 75,000 processing plant components. This study uncovered 1,600 unintentional leaks, but found that 50 faulty components were behind 60 percent of leaked gas at processing plants.
The recent analysis also found that studies showing very high methane emissions in parts of North America with significant gas infrastructure are not representative of the entire gas system.
“If these studies were representative of even 25 percent of the natural gas industry, then that would account for almost all of the excess methane noted in continental-scale studies,” said Eric Kort, an atmospheric professor at the University of Michigan and a study co-author. “Observations have shown this to be unlikely.”
Despite the higher levels of methane emissions than previously estimated, using gas to generate electricity versus coal still reduces the total greenhouse effect over 100 years, as coal burning releases a significant amount of carbon dioxide and mining coal releases methane.
The analysis’ most surprising find was that fueling trucks and buses with natural gas instead of diesel probably raises global temperatures. For natural gas to beat diesel, the gas industry would have to reduce leaks to less than the EPA’s current estimate, which the analysis finds “quite improbable.”
The non-profit organization Novim, formed in 2007 by scientists and engineers with the University of California at Santa Barbara’s Kavli Institute for Theoretical Physics, funded the research through a grant from the Cynthia and George Mitchell Foundation. The foundation asked Novim to examine 20 years of methane studies to explain the wide variation in existing estimates, Marilu Hastings, sustainability program director at the Cynthia and George Mitchell Foundation, said in a Feb. 13 press release.
“Hopefully, this will help resolve the ongoing methane debate.”
The analysis does not attribute percentages of excess emission to gas, oil, coal, agriculture, or landfills as emission rates for most sources are uncertain.
Natural gas flaring has attracted growing attention in the United States as unconventional oil and gas drilling in plays such as the Bakken in North Dakota as resulted in a surge of gas flaring. North Dakota oil production hit a record high in September 2013, Reuters reported.
Producers are flaring gas due to lack of pipeline infrastructure. From May 2012 to May 2013, North Dakota producers flared off approximately 30 percent of gas coming out of the ground, Scientific American reported in September 2013.
– Karen Boman, Rigzone
