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Improved characterization of natural gas leak plumes using a Laplace transform approach to correct for instrument response time




O'Brien, Thomas E., author
Marchese, Anthony, advisor
von Fischer, Joe, advisor
Abutayeh, Mohammad, committee member
Weller, Zachary, committee member

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The response time of an instrument is a measure of how quickly it achieves a steady-state value in response to a step change input. The response time directly affects the accuracy of reported values when the instrument is subject to transient conditions. In methane leak surveying, methane gas concentrations are often measured from instruments mounted in moving vehicles. The methane concentrations detected by those instruments are therefore constantly varying. Depending on the physical size of the methane plume, vehicle speed and instrument response time, a vehicle-mounted instrument may or may not reach a steady-state value as the instrument inlet traverses the plume, which can result in an underestimation of the true ambient gas concentration of the plume. For leak detection techniques that rely on downwind concentration measurements to estimate an emission rate and/or detect the presence of a methane emission source, this issue can result in underestimation of emission rates and/or failure to detect an emission source. Further complicating these efforts, different gas analyzers often have different time response characteristics, resulting in an ambiguity in readings achieved by different instruments. In this research, a model was developed to generate improved estimates of the actual gas concentration values and waveform shapes using Laplace transforms, given the time response characteristics of the instruments. Several methane gas analysis instruments used for methane leak detection surveying were experimentally characterized in a laboratory setting to determine their time response characteristics. Controlled release field testing in conjunction with simultaneous methane measurements from multiple instruments mounted on the same moving vehicle were then used to validate this methodology. During field testing, instruments with different response characteristics were plumbed in parallel, thereby sampling the same air stream. The results indicated that the instrument with a faster response time consistently reported significantly higher peak methane concentrations than the slower response instrument. However, by applying the model developed in this study to the raw time-varying gas concentration data, the corrected concentration profiles for the two instruments were found to be nearly identical. The results also indicate that the raw signals from both instruments underrepresented the actual peak methane concentrations for the majority of detected plumes. The results of this study suggest the importance of accurately accounting for instrument time response in downwind, vehicle-mounted methane measurement techniques to ensure that peak methane concentrations are not underreported.


2021 Fall.
Includes bibliographical references.

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climate change
atmospheric methane
greenhouse gasses


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