Abstract
A tunable diode laser absorption spectroscopy system, employing a 2f wavelength modulation spectroscopy measurement scheme, was developed for remote monitoring of ambient methane fluctuations by way of fiber-optically connected all-optical sensors. Detection of fugitive methane emissions demands a measurement precision less than 2.0 ppmv and a lower detection limit less than ~1.7 ppmv methane in air. To determine the optimum base system configuration, the influence of the laser driving signal frequency and amplitude was characterized to strike a balance between measurement precision and system sensitivity. In addition, relative to the basic system configuration, a 50 and 96 % reduction in measurement deviation was achieved by way of polarization scrambling and thermal stabilization of critical optical components, respectively. For methane concentrations between 2.0 and 50.0 ppmv in air, the laboratory-based system achieved a measured precision of 1.36 ppmv and a lower detection limit of 1.56 ppmv using a 6.0-m single-mode optical fiber and an averaging time of 1 s. The long-term system stability and system performance were analyzed using datasets acquired 4 and 12 months after the initial system calibration, yielding a difference in measured precision within the uncertainties of the calibration gas mixture. Finally, it was determined that fiber length between individual remote optical sensors can lead to a varying measurement bias, which implies that length-specific calibrations for each remote optical sensor may be required for a field implementation.
Highlights
Fugitive emissions, defined as unintended or irregular leaks of gases and vapors, are an important source of air pollution that is difficult to monitor and control [1]
Greenhouse gas data reported by Annex I countries to the United Nations Framework Convention on Climate Change (UNFCCC) suggest that fugitive emissions accounted for ~5 % of global greenhouse gas emissions in 2012 [4]
This paper reports development and quantitative testing of a fiber-coupled, optically networked detection system designed for measuring in situ ambient methane concentrations within an industrial environment, such as an oil and gas processing plant
Summary
Fugitive emissions, defined as unintended or irregular leaks of gases and vapors, are an important source of air pollution that is difficult to monitor and control [1]. Within industrial facilities such as oil and gas processing plants, fugitive methane emissions can be a significant source of greenhouse gas emissions [2, 3]. Detection of fugitive methane is often difficult because the gaseous plumes emanating from individual sources are often intermittent and at low concentrations relative to ambient values of ~1.7 ppmv [5, 6]. Recent work on trajectory and inverse methods for interpreting sensor network data to locate and quantify unknown fugitive sources suggests that a measurement standard deviation of 2 ppmv (equivalent to an absorbance of 8.1 × 10−7 cm−1 for the 2v3 R(3) methane manifold at standard atmospheric pressure and temperatures) is sufficient to locate and potentially quantify fugitive methane sources using statistical methods [9], and possibly as high as 29 ppmv using adjoint-based inverse dispersion modeling [10]
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