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In our last column, we talked about nitrous oxide as a potent greenhouse gas and how it was often mistakenly associated with power plant emissions. In this column, we will continue with the topic of greenhouse gases (GHGs) but focus on natural gas leakage and unburned hydrocarbons from the U.S. energy infrastructure.
The principal constituent of natural gas is methane which is considered a strong GHG. It is, from a global warming potential perspective, about 21 times more potent than carbon dioxide. While the natural gas industry causes only about 22% of all U.S. methane emissions, reductions are still possible.
Natural gas leaks can occur during the gas production process, its transport and its use. These emissions can be further divided into accidental leakage, incomplete combustion (unburned hydrocarbons), and intentional blowdowns or releases. Most pipeline accidental leakage occurs from valves and seals and can often be reduced or eliminated through proper maintenance and inspection processes.
The combustion process in modern gas turbine combustors is efficient, and the produced amount of unburned hydrocarbons is usually well below 10 ppm. It does not represent a significant contribution to overall GHGs. Gas engines produce significantly higher concentrations of unburned hydrocarbons in the exhaust. Intentional blowdowns or maintenance-related releases of natural gas are caused by scheduled or forced equipment outages and are primarily driven by safety reasons.
During a recent U.S. Department of Energy event that focused on pipeline methane leakage, a presentation by the Pipeline Research Council International (PRCI) included the following numbers for leakage from U.S. compressor stations:
• Reciprocating Compressors 773,000 Mg 31.4%
• Centrifugal Compressors (with wet seals) 232,000 Mg 9.5%
• Gas Engine Exhaust 235,000 Mg 9.5%
• Pneumatic Devices 221,000 Mg 9.0%
• Blowdown & Venting 184,000 Mg 7.5%
These emissions account for about 2% of U.S. GHG emissions on an equivalent CO2 basis. Clearly, the largest contribution comes from leakage through rings and packings of reciprocating compressors. Another substantial percentage comes from incomplete combustion in reciprocating compressor engines.
There was not a lot of detail provided in the PRCI presentation, but one can safely assume that the majority of this leakage and unburned hydrocarbon production is associated with smaller upstream machines in gas-gathering duty. Also, although blowdown and venting may be reduced by better safety technology, reduced maintenance intervention and blowdown-avoidance devices, most of these events are driven by compressor station safety and cannot be completely avoided.
Centrifugal compressor leakage is primarily caused when outdated wet seals are used. Modern dry gas seals on centrifugal compressors almost eliminates this leakage. Because of this, few centrifugal compressor manufacturers still offer machines with wet gas seals.
For the last 15 or 20 years, almost all new models sold have used dry gas seals. Therefore, this methane leakage contribution will continue to decline as older machines are replaced with newer centrifugal compressors or are retrofitted with new seals.
Read more in the March/April 2015 issue of Turbomachinery International magazine