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Long duration measurements of pneumatic controller emissions on onshore natural gas gathering stations




Luck, Benjamin Kendell, author
Quinn, Jason, advisor
Zimmerle, Daniel, advisor
Marchese, Anthony, committee member
von Fischer, Joseph, committee member

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Over the last 15 years, advances in hydraulic fracturing have led to a boom of natural gas production the United States and abroad. The combustion of natural gas produces less carbon dioxide (CO2) than the combustion of other fossil fuels per unit of energy released, making it an attractive option for reducing emissions from power generation and transportation industries. Uncombusted methane (CH4) has a global warming potential (GWP) of 86 times that of CO2 on 20 year time scales and a GWP of global warming potential 32 times greater than CO2 on a 100 year time scale. The increase in supply chain throughput has led to concerns regarding the greenhouse gas contributions of CH4 from accidental or operational leaks from natural gas infrastructure. Automated, pneumatic actuated valves are used to control process variables on stations in all sectors of the natural gas industry. Pneumatic valve controllers (PCs) vent natural gas to the atmosphere during their normal operation and are a significant source of fugitive emissions from the natural gas supply chain. This paper outlines the work that was done to improve the characterization of emissions from PCs using long duration measurements. This work was performed as part of the Department of Energy funded Gathering Emission Factor (GEF) study. A thermal mass flow meter based emission measurement system was developed to perform direct measurements of pneumatic controller emissions over multiday periods. This measurement system was developed based on methods used in previous studies, with design modifications made to meet site safety regulations, power supply constraints and measurement duration targets. Emissions were measured from 72 PCs at 16 gathering compressor stations between June, 2017 and May, 2018. The average emission rate of 72 PCs was 10.86 scfh [+4.31/-3.60], which is 91.2% of the EPA's current emission factor for PCs on gathering compressor stations. The mean measurement duration of these 72 samples was 76.8 hours. Due to potential biases associated with flow meter errors, updates to EPA emission factors based on these data are not proposed. However, because all previous studies to quantify PC emissions used short sampling times (typically ≤15 minutes) the long duration measurements provided insight into previously unobserved PC emissions behavior. A panel of industry experts assessed the emissions recordings and found that 30 PCs (42% of measured devices) had emissions patterns or rates that were inconsistent with their design. 73% of emissions measured during this study were attributed to these 30 PCs that were malfunctioning from an emissions perspective. It was also found that PC emission rates are more variable over time than previously thought. Due to this high temporal variability, the short duration observations currently used by leak detection programs to identify malfunctioning equipment have a low probability of providing accurate characterizations of PC emissions. Many natural gas companies are investigating ways to improve the efficiency of their operations and reduce rates of natural gas leakage in their systems. The data presented in this paper improves the characterization of emissions behavior from a significant emission source in the production, processing and transmission sectors of the natural gas supply chain and has implications for organizations with an interest in reducing emissions from PCs.


2019 Spring.
Includes bibliographical references.

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