The total amount of methane emitted from natural gas transmission and storage facilities in 2012 was 1,220-1,950 gigagrams/year (mean of 1,503 Gg/year), resulting in an estimated loss of 0.35% of the methane transported, according to an industry-backed study released Tuesday.
That amount is not statistically different than was reported in the Environmental Protection Agency's (EPA) 2012 Greenhouse Gas Inventory (GHGI), according to the study, which used new data collected during 2012, including 2,292 onsite measurements, additional emissions data from 677 facilities and activity data from 922 facilities. The largest emission sources were fugitive emissions from certain compressor-related equipment and "super-emitter" facilities, according to the researchers.
The EPA study had estimated methane emissions of 1,680-2,690 Gg/year (mean of 2,071 Gg/year). While the difference of the two studies' conclusions isn't statistically significant, "this is the result of several significant, but offsetting, factors," according to the study's authors, who were led by researchers at Colorado State University (CSU).
"Factors which reduce the study estimate include a lower estimated facility count, a shift away from engines toward lower-emitting turbine and electric compressor drivers, and reductions in the usage of gas-driven pneumatic devices. Factors that increase the study estimate relative to the GHGI include updated emission rates in certain emissions categories and explicit treatment of skewed emissions at both component and facility levels."
But the Interstate Natural Gas Association of America (INGAA), which participated in the study, had a different interpretation of its results. The research indicates that methane emissions from the sector are 27% lower than EPA's estimate, INGAA CEO Don Santa said.
About one in 25 facilities may be emitting 300 standard cubic feet per minute or more of natural gas at any given time, the researchers said.
"Our data indicate that these releases are both intermittent and unpredictable," said Daniel Zimmerle, senior research scientist at the CSU Energy Institute.
The study also found that fugitive emissions account for 75% of all methane emissions in the transmission and storage sectors. Half of the emissions are from major compressor equipment such as seal vents, unit isolation and blow-down valves and rod-packing vents. And the researchers found that the equipment used in the sector is significantly different than assumed in previous estimates, which can greatly affect the amount of methane being emitted. For example, companies have replaced smaller engine-driven reciprocating compressors with larger and fewer centrifugal compressors, resulting in less unburned methane in exhaust gases.
"This finding is important," Santa said. "It underscores why the EPA needs to update the emissions factors it uses to estimate its inventory to reflect more accurately how the transmission and storage sector operates today. EPA largely relies on data from a nearly 20-year-old study to calculate its greenhouse gas inventory. While EPA has appropriately updated emission factors and estimation methods in select cases for other industry sources, including wells in the exploration and production sector, it has not for transmission and storage sector sources.”
The study, which was sponsored by the Environmental Defense Fund, Dominion, Dow Chemical, Enable Gas Transmission Co., Kinder Morgan Inc., Columbia Pipeline Group, TransCanada Corp. and Williams, was published Tuesday in the journal Environmental Science & Technology. It is the final analysis of one of the most comprehensive studies of methane emissions from natural gas transmission and storage facilities, which began two years ago (see Daily GPI, Oct. 15, 2013; June 12, 2013). The previous report from the same researchers concluded that the majority of measured emissions came from only a handful of the sites studied (see Daily GPI, Feb. 10).
An EPA report issued earlier this year found that methane emissions from natural gas production have fallen about 38% since 2005, but emissions from processing increased by about 38% since that year, and rose about 11% from gas transmission and storage sources (see Daily GPI, April 16). Meanwhile, methane emissions from hydraulically fractured gas wells declined about 79% since 2005, EPA said.