The attached document presents a compilation of estimation methods for assessing GHGs associated with natural gas transmission and storage facilities: carbon dioxide (CO2), methane, and nitrous oxide emissions from combustion and non-combustion sources. Due to its higher global warming potential (GWP) relative to CO2, methane is an important GHG emission for natural gas systems.
The methodologies, procedures, and examples outlined in the GHG Guidelines address the key GHG emission sources from the transmission and storage sector. The source types for GHGs in gas transmission are typically categorized as:
• Combustion sources (primarily compressor drivers, but also includes on-site auxiliary power, and fleet and construction mobile sources),
• Fugitive emissions from equipment and piping leaks, and
• Process venting and non-routine releases.
It is important to understand that while carbon dioxide from combustion sources can be estimated with a high degree of accuracy based on fuel use and fuel analysis data, other emissions are typically estimated using “emission factors” which are developed from available data and represent “industry-average” operations. Thus, emission estimates for key emissions such as fugitive methane releases include a high degree of uncertainty based on current acceptable practices.