Abstract
We present field deployment results of a portable optical absorption spectrometer for localization and quantification of fugitive methane (CH4) emissions. Our near-infrared sensor targets the 2ν3 R(4) CH4 transition at 6057.1 cm−1 (1651 nm) via line-scanned tunable diode-laser absorption spectroscopy (TDLAS), with Allan deviation analysis yielding a normalized 2.0 ppmv∙Hz−1/2 sensitivity (4.5 × 10−6 Hz−1/2 noise-equivalent absorption) over 5 cm open-path length. Controlled CH4 leak experiments are performed at the METEC CSU engineering facility, where concurrent deployment of our TDLAS and a customized volatile organic compound (VOC) sensor demonstrates good linear correlation (R2 = 0.74) over high-flow (>60 SCFH) CH4 releases spanning 4.4 h. In conjunction with simultaneous wind velocity measurements, the leak angle-of-arrival (AOA) is ascertained via correlation of CH4 concentration and wind angle, demonstrating the efficacy of single-sensor line-of-sight (LOS) determination of leak sources. Source magnitude estimation based on a Gaussian plume model is demonstrated, with good correspondence (R2 = 0.74) between calculated and measured release rates.
Highlights
Natural gas (NG) harvesting has undergone accelerated growth due to increasing demand for alternative clean energy sources [1,2], with over half-million active wells for oil and gas extraction in the United States alone [3]
Infrared (IR) absorption spectroscopy has emerged in recent years as a promising solution for trace-gas detection which requires high levels of precision and molecular specificity [16,17,18], with applications ranging from health diagnostics [19,20] to environmental [21,22] and industrial process monitoring [23]
Fundamental rovibrational transitions in the mid-IR (3–25 μm) are readily targeted using quantum cascade laser (QCL/ICL) technologies [24,25], while weaker overtone bands are typically measurable in the near-IR (NIR) using conventional tunable diode laser absorption spectroscopy (TDLAS) [17,26,27]
Summary
Natural gas (NG) harvesting has undergone accelerated growth due to increasing demand for alternative clean energy sources [1,2], with over half-million active wells for oil and gas extraction in the United States alone [3]. Point sensors with adequate CH4 sensitivity and molecular specificity should be located on-site for spatially resolved, continuous in situ leak monitoring [10]; the present lack of cost-effective CH4 monitoring solutions prevents large-scale implementation of source attribution technologies for the timely localization and quantification of NG leaks [11,12,13]. Infrared (IR) absorption spectroscopy has emerged in recent years as a promising solution for trace-gas detection which requires high levels of precision and molecular specificity [16,17,18], with applications ranging from health diagnostics [19,20] to environmental [21,22] and industrial process monitoring [23]. We envision our source estimation methods to be widely applicable to a variety of sensing configurations (optical or otherwise), while providing a comparison benchmark for a generation integrated photonic sensors with superior size, weight, power, and cost (SWaP-C) currently under development [18,27,31]
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