Whenever fuel gas (natural gas, coal syngas, biogas, etc.) is generated, transferred or used, its level of contamination, heating value, relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index are typically required. Measurement of various contaminants (e.g. H2S, H2O, O2, CO2) is critical for preventing infrastructure damage due to corrosion or chemical reactivity. Natural gas producers must clean extracted natural gas to remove contaminants and then verify any residual levels before it is introduced into a pipeline. Desulfurizer beds in fuel reformers need periodic replacement or regeneration to prevent H2S breakthrough into the reformed fuel product, and so require frequent contaminant level monitoring. Measurement of key gas parameters, including heating value, relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index, are critical for pricing the fuel, optimizing burner conditions, and determining combustion efficiency.
Fuel producers and their customers typically use up to four separate analyzers (e.g. electrochemical, chilled mirror, lead tape, and gas chromatographs) to analyze fuel gas characteristics, such as amounts of trace contaminants or heating value. For example, a customer site might include a lead-tape system to measure H2S, a chilled mirror instrument to measure H2O and a paramagnetic sensor for O2. Gas chromatographs separate hydrocarbon mixtures into their component species to determine heating value and other gas characteristics (e.g. relative density, compressibility, theoretical hydrocarbon liquid content, and Wobbe index). Each of these analyzers has its own limitations and drawbacks. For example, a lead-tape system requires consumables and frequent servicing, while providing relatively slow readings over a small dynamic range. Likewise, chilled mirror devices are very slow and prone to interference from other condensing components. Finally, gas chromatographs, the current industry standard, are both slow (several minutes per analysis) and require costly consumables and maintenance. The entire suite of instruments is expensive to operate and needs extensive on-site maintenance.
Tunable diode laser absorption spectrometry (TDLAS) has been widely used to measure trace contaminants (e.g. H2S, H2O, O2, CO2 . . . ) in fuel gases and other petrochemicals, but has not been used to determine heating value or other gas characteristics. For example, in a paper by Feng Dong, Christian Junaedi, Subir Roychoudhury, and Manish Gupta, “Rapid, Online Quantification of H2S in JP-8 Fuel Reformate Using Near-Infrared Cavity-Enhanced Laser Absorption Spectroscopy”, Analytical Chemistry 83, pp. 4132-4136 (2011), an off-axis ICOS analyzer operating near 1.59 μm simultaneously quantified H2S, CO2, CH4, C2H4 and H2O in reformed military fuel with rapid, highly precise measurements over a wide dynamic range, with low detection limit and minimal cross-interference with other present species. It was suggested that by including additional near-IR diode lasers at other wavelengths, the instrument could be extended to measure other species, including CO and H2.
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