Wavelength-modulation-spectroscopy for real-time, in situ NO detection in combustion gases with a 5.2 μm quantum-cascade laser
- 784 Downloads
A mid-infrared absorption strategy with calibration-free wavelength-modulation-spectroscopy (WMS) has been developed and demonstrated for real-time, in situ detection of nitric oxide in particulate-laden combustion-exhaust gases up to temperatures of 700 K. An external-cavity quantum-cascade laser (ECQCL) near 5.2 μm accessed the fundamental absorption band of NO, and a wavelength-scanned, 1f-normalized WMS with second-harmonic detection (WMS-2f/1f) strategy was developed. Due to the external-cavity laser architecture, large nonlinear intensity modulation (IM) was observed when the wavelength was modulated by injection-current modulation, and the IM indices were also found to be strongly wavelength-dependent as the center wavelength was scanned with piezoelectric tuning of the cavity. A quantitative model of the 1f-normalized WMS-2f signal was developed and validated under laboratory conditions. A sensor was subsequently designed, built and demonstrated for real-time, in situ measurements of NO across a 3 m path in the particulate-laden exhaust of a pulverized-coal-fired power plant boiler. The 1f-normalized WMS-2f method proved to have better noise immunity for non-absorption transmission, than wavelength-scanned direct absorption. A 0.3 ppm-m detection limit was estimated using the R15.5 transition near 1927 cm−1 with 1 s averaging. Mid-infrared QCL-based NO absorption with 1f-normalized WMS-2f detection shows excellent promise for practical sensing in the combustion exhaust.
KeywordsNitric Oxide Intensity Modulation Direct Absorption SNCR Combustion Exhaust
Support was provided by the Electric Power Research Institute with Mr. Richard Himes as program manager and the Air Force Office of Scientific Research with Dr. Julian Tishkoff as program manager. We thank Dr. Michael Radunsky and Dr. Sam Crivello of Daylight Solutions for their useful comments on an early version of the manuscript.
- 1.US Environmental Protection Agency, National air quality and emissions trends report, Special Studies ed., Sept, 2003, available at http://www.epa.gov/air/airtrends/aqtrnd03/
- 4.K.C. Smyth, D.R. Crosley, in Applied Combustion Diagnostics, ed. by K. Kohse-Höinghaus, J.B. Jeffries (Taylor and Francis, London 2002), pp. 9–68, Chap. 2 Google Scholar
- 13.A.A. Kosterev, C. Roller, F.K. Tittel, W. Flory, in OSA/CLEO (2003) Google Scholar