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The discussion on greenhouse gases has to some extent shifted from battling CO2 emissions towards taking other gases that support the greenhouse effect into consideration. Among these are emissions of methane and other hydrocarbons.While the absolute amounts of fugitive emissions due to human action are small compared to CO2 emissions, the potency of methane (and other hydrocarbons) as greenhouse gases is an order of magnitude higher.
Currently, about three quarters of the greenhouse gas emissions are tied to carbon dioxide, while methane is responsible for about one seventh of the emissions. Of course, we have to acknowledge that agriculture contributes about half of the methane gas emissions, while landfills and wastewater contribute another quarter.
So in a big picture view, the fugitive emissions in question are a small contribution to the overall problem. Currently, the methane concentration in the atmosphere is a little below 2 ppm, but it is up from about 0.7 ppm one hundred years ago.
Fugitive greenhouse gas emissions are methane leaks from pipelines and system components such as compressor seals, pump seals, valve packings, flanges and piping connectors. Currently, the emission sources and activity data basis for fugitive emissions are based upon primary equipment that includes subcomponents, such as piping and associated components; compressors; meter stations; interconnects; farm taps; receipt and sales meter stations; border meter stations; gate stations; storage well components; and organic liquids storage tanks.
With regards to compression equipment, we wish to discuss three areas of relevance:
• Unburned hydrocarbon (UHC) emissions from drivers
• Leakage from compressors
• Blowdown requirements
Unburned hydrocarbon emissions from drivers are the result of incomplete combustion. When natural gas is used as fuel, the vast majority of it is converted into water and carbon dioxide in the combustion process. Different types of gas fueled drivers tend to have different levels of unburned hydrocarbon emissions.
Gas turbines have significantly lower levels of unburned hydrocarbon emissions than reciprocating gas engines. Since the starting procedure can create significant amounts of unburned hydrocarbons, starting reliability is an important parameter to consider. Failed starts contribute similar amounts of unburned hydrocarbons as successful starts.
Another source is the leakage from compressors. The natural gas that is compressed can escape from the compressor casing through various paths. These pathsways are different for centrifugal compressors and reciprocating compressors. Modern centrifugal compressors, using dry gas seals, have only one leakage path that exits the compressors through a vent. The amount of leakage is minuscule and the dry gas seals prevent any uncontrolled leakage along the compressor rotor to the atmosphere. The vented gas can be flared (thus converted to CO2 and water) or recompressed to be used as fuel gas.
Reciprocating compressors have various leakage paths mainly through the piston and rod packings, which allow the natural gas to escape through the crank case of the compressor. There are also potential leakage pathways due to valve unloaders or clearance pockets. There are methods to reduce or remove this type of leakage, but they tend to add significantly to the complexity of the system.
Blowdown or system venting events are not directly related to turbomachinery. They can occur during preparation of a pipeline, a compressor station, or equipment for maintenance and inspection. They can also occur in association with an emergency shutdown event, or from accidental releases of pressure relief devices.
These are distinct events, and the frequency is usually directly related to the amount of maintenance events, as well as other planned and unplanned shutdowns. In other words, the more reliable the equipment is, the fewer interventions are necessary and the fewer hydrocarbons are released. In particular, compressor maintenance will require in most cases a blowdown, so the reliability and the maintenance frequency of the compressor are crucial.
Other maintenance events (for example on the driver) usually allow for keeping the equipment in pressurized hold. This means that the compressor, as well as the associated piping, are shut down, but the gas stays at the operating pressure. The same situation can occur if the compressor capacity is not needed temporarily for some time. Centrifugal compressors with dry gas seals, can be kept in pressurized hold indefinitely, assuming accessory systems (like lube oil supply, buffer gas supply and so on) are kept running.
However, keeping units in pressurized hold only makes sense if the equipment can be re-started from pressurized hold, without having to lower the gas pressure. This capability is available for all modern centrifugal compressors driven by gas turbines, and most centrifugal compressor driven by electric motors. Starting reciprocating compressors often requires full or partial blowdown.
It is important that the path-ways of fugitive emissions and their releases are well understood in the industry, and there are significant efforts under way by manufacturers and operators to further reduce their impact.
Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also currently the Chair of the Board of Directors of the ASME International Gas Turbine Institute.
Rainer Kurz is the manager of systems analysis for Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and past chair of the IGTI Oil & Gas applications committee.