Development of laser absorption techniques for real-time, in-situ dual-species monitoring (NO/NH3, CO/O2) in combustion exhaust

Abstract Simultaneous dual-species monitoring offers potential for control of large-scale practical combustion systems. The development and demonstration of two dual-species sensors for characterizing NO x abatement (NO/NH 3 ) and combustor performance (CO/O 2 ) are described for potential application in boiler exhaust at coal-fired electric utilities. Tunable laser absorption sensors for simultaneous in-situ detection of these paired species were developed using fundamental-band vibrational transitions in the mid-infrared near 5.2 μm for NO, combination-band transitions near 2.25 μm for NH 3 , overtone-band transitions near 2.3 μm for CO, and electronic transitions in the b-X system near 760 nm for O 2 . Scanned-wavelength, 1 f -normalized wavelength modulation spectroscopy with second harmonic detection (WMS-2 f ) was employed for real-time data processing. Spatial- and time-demultiplexing strategies were used to combine and separate the laser signals. The sensors were tested for simultaneous, continuous monitoring in laboratory combustion exhaust from a premixed ethylene-air flame at atmospheric pressure and varied equivalence ratios with exhaust temperature of ∼620 K. A retro-reflected 3.58 m beam-path was used to mimic a single-ended installation in a boiler exhaust duct. NH 3 mixtures were metered into the flame at different rates to test the response of the NO/NH 3 sensor, and the CO/CO 2 ratio was adjusted by fuel/air equivalence ratio. Trends in the measured concentration ratio of NO to NH 3 were found to agree qualitatively with theoretical expectation, and the CO and O 2 measurements were confirmed by analysis of sampled gases. The laser absorption exhibited the fast time response needed for control sensors. These fast-response, simultaneous dual-species sensors for NO/NH 3 and CO/O 2 show excellent promise for control and optimization of NO x abatement and furnace efficiency in practical combustion systems.

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