Methane to Ethane Ratios of Emissions at Natural Gas Compressor Stations and Storage Facilities
Published on: 2018-09-26
We reported methane emissions of five natural gas transmission and storage facilities as part of the Environmental Defense Fund’s Barnett Coordinated Campaign. Samples included reciprocating engine exhausts, engine crankcases, compressor packing or wet seal vents, turbine exhausts, slop tanks, and leaks. Samples were analyzed with an FTIR spectrometer to determine ethane to methane (C2/C1) and propane to methane (C3/C1) ratios by volume. Hydrocarbon ratios have been used in source apportionment - anthropogenic sources include higher alkanes. Gas composition of shale plays varies by play and within the play. The goal of this study was to analyze the site-to-site and component-to-component variability of C2/C1 ratios and to identify and discuss possible causes of these variations. We determined that C2/C1 ratios varied as much as 7.3 to 61.7% between the different emitters at each site. The main contributor of methane emissions was the reciprocating engine exhaust (46% by mass). Dependent on reciprocating engine configuration, C2/C1 ratios of exhausts ranged from 0.43 ± 0.03 to 2.55 ± 0.18%. At four sites, exhaust C2/C1 ratios were lower than all or most components. The exhaust at Site 2 exhibited higher C2/C1 ratios and included an after treatment catalyst. Average C2/C1 ratios for all sites ranged from 0.17 ± 0.01 to 4.94 ± 0.35%. Local near-field ambient samples were collected within the fence line, near ground level at four sites and yielded C2/C1 ratios between 3.25 ±0.23 - 21.6 ± 1.53%. Only at Site 1 was the calculated C2/C1 signature statistically different to that from pipeline analysis. At Sites 1 and 2, the C2/ C1 ratio was 143 and 0.4% higher, while the remaining sites were all lower than pipeline. In three cases, the C2/C1 ratios of the major emitters – the reciprocating engine exhausts – were statistically different from the pipeline samples.