The E/E pocket reference guide

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37. That review may be out-of-date

Last edited Wed Jan 11, 2012, 12:48 PM - Edit history (1)

http://icp.giss.nasa.gov/education/methane/intro/cycle.html

[font face=Times, Serif][font size=3](Note: This article was originally written in conjunction with the 1997 Global Methane Inventory.[/font][/font]

http://www.epa.gov/outreach/downloads/TechNote_Natural%20gas_4-15-11.pdf

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• According to the latest inventory, CH[font size="1"]₄[/font] emissions from natural gas systems are estimated to be 221.2 MMTCO[font size="1"]₂[/font]e in 2009. Although the primary basis for CH[font size="1"]₄[/font] emissions estimates in the Inventory remains a detailed 1996 study by the Gas Research Institute and EPA (1996 GRI/EPA), significant improvements were made to the emissions estimates in the current inventory, based in part on the large body of work carried out in developing subpart W (Petroleum and Natural Gas Systems) under the GHG Reporting Program. (Please see the Technical Support Document for subpart W at http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD.pdf as well as additional support materials contained in Docket ID No. EPA-HQ-OAR-2009-0923 . )
• The net effect of these changes is a 120% increase from the 2008 emissions estimate. Vented, fugitive and flared emissions from petroleum and natural gas systems are now the largest source of CH[font size="1"]₄[/font] emissions in the United States, replacing enteric fermentation.
• The large increase in emissions was primarily due to methodological improvements to three existing sources and the addition of two new sources.
• The three existing sources contributing to the large increase in emissions were gas well liquids unloading, condensate storage tanks and centrifugal compressors.
• The largest increase in emissions was due to the revised emission factor for gas well cleanups (also referred to as gas well liquids unloading). EPA used well production and well property sample data on well depth, shut-in pressure, and production rates in an engineering equation to re-estimate the average unloading emissions by region for this source (HPDI 2009). This improved data and methodological change increased emissions by more than 22 times while decreasing the substantial uncertainty associated with the previous emission factor that was derived from the EPA/GRI 1996 study. Emissions from liquids unloading in 2009 are estimated to be nearly 96 MMTCO[font size="1"]₂[/font]e.
• For condensate storage tanks, EPA used the same E&P Tank simulation data for hydrocarbon liquids above 45 ̊API flashing emission in tanks as used in previous Inventories to estimate emissions. However, the analysis coupled these flashing emissions simulations with a large sample of condensate production gravities from the HPDI database to improve the factor to account for the average national distribution of condensate gravities. This is different from previous inventories in which a simple average of simulation results for each liquid gravity was used. Additionally, a sample of data from a 2009 Texas Environment Research Consortium (TERC) study was used to account for emissions from separator dump valve malfunctions in the regions represented by the study.
• For centrifugal compressor seals, EPA separated the centrifugal compressors emission source in the natural gas processing, transmission, and underground storage segments into two sources – centrifugal compressors equipped with wet seals and those with dry seals. EPA used data analysis on centrifugal compressor seal oil degassing vent rates (World Gas Conference 2009) to establish an emission factor for centrifugal compressor wet seal degassing and Natural Gas STAR’s Lessons Learned (EPA 2006) document to develop an emission factor for dry seal emissions. The seal oil degassing vent (found with compressors using wet seals) was previously unaccounted for in the Inventory.
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