Development of laser absorption sensors for combustion gases [electronic resource]
Keywords: Mechanical Engineering

In situ sensors based on laser absorption spectroscopy are developed to monitor key species in combustion exhaust gases. Direct absorption (DA) and wavelength-modulation-spectroscopy (WMS) strategies are investigated for extended near-infrared and mid-infrared laser devices to detect target species in harsh environments at combustion exhaust temperatures in the presence of high moisture level and heavy particulate loading. A real time, in situ sensor for carbon monoxide (CO) was developed using DFB diode lasers near 2.3 µm, with selected transitions (R(10) near 4297.7 cm-1, R(11) near 4300.7 cm-1) in the first overtone band of CO. The sensor was studied in the controlled laboratory environments of a heated cell and a combustion exhaust rig. High temperature water vapor absorption spectra were first measured, and shown to produce < 10 ppm level of interference for CO detection in combustion exhausts at temperatures up to 1200 K. The linestrengths and collisional broadening coefficients for the selected CO lines were then measured to verify the spectroscopic database and provide line shape parameters for calibration-free WMS measurements with second-harmonic detection (2f) and 1f normalization (WMS-2f/1f). This database was then used in a comparative study of DA and WMS methods. CO concentration measurements using scanned-wavelength DA and fixed-wavelength WMS agreed within 5% with those measured by a conventional gas sampling analyzer over the range from < 10 ppm to 2.3%. The CO sensor using the WMS-2f/1f strategy and the selected transitions in the CO overtone band was found to hold good promise for sensitive in situ detection of ppm-level CO in combustion flows, with high resistance to interference absorption from H2O. A novel external-cavity quantum-cascade laser (ECQCL) near 5.2 [micrometers] was investigated to develop a new mid-infrared absorption sensor for in situ detection of nitric oxide (NO) in combustion exhaust gases. The laser has a wide mode-hop-free wavelength tuning range covering the R-branch of the NO fundamental absorption band, with which critical evaluation of the interference absorption by H2O in combustion exhaust gases was enabled. The water vapor absorption spectrum was measured over the range 1880 cm-1 to 1951 cm-1 at combustion exhaust temperatures. Based on the data for water vapor interference and laser performance, three pairs of candidate transitions (R6.5 near 1900 cm-1, R10.5 near 1912 cm-1 and R15.5 near 1927 cm-1) were selected as optimum and subsequently investigated in detail. The tuning characteristics of the ECQCL were studied to understand the nonlinear baseline pattern in its intensity output due to the external-cavity laser architecture, so that both DA and WMS sensing schemes could be developed. Using scanned-wavelength DA, the sensor was validated in a laboratory combustion exhaust rig with a 1.79 m constant-temperature line-of-sight path for NO concentrations between 20 and 95 ppm at 600 K. A wavelength-scanned, WMS-2f/1f strategy was subsequently developed for the ECQCL. Different from the conventional DFB lasers, large nonlinear intensity modulation (IM) was observed when the wavelength was modulated by injection-current modulation, and the IM indices were found to be strongly wavelength-dependent as the center wavelength was scanned with piezoelectric tuning of the cavity. Thus, rather than a single-wavelength laser characterization method applicable for fixed-wavelength WMS schemes or conventional DFB lasers, a new method of measuring the laser modulation characteristics over the entire scan range from the zero-absorption background was proposed. A quantitative model of the WMS-2f/1f signal was then developed and validated. As simultaneous dual-species monitoring offers potential for control of large-scale practical combustion systems, two dual-species sensors for characterizing NOx abatement (NO/NH3) and combustor performance (CO/O2) were developed for potential application in boiler exhaust in coal-fired electric utilities. The fundamental-band vibrational transitions in the mid-infrared near 5.2 µm for NO, combination-band transitions near 2.25 [micrometers] for NH3, overtone-band transitions near 2.3 [micrometers] for CO, and electronic transitions in the b-X system near 760 nm for O2 were used for the sensors. A scanned-wavelength, WMS-2f/1f strategy 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. Trends in the measured concentration ratio of NO to NH3 were found to agree qualitatively with theoretical expectation, and the CO and O2 measurements were confirmed by analysis of sampled gases. These laser absorption techniques exhibited the fast time response needed for control sensors. Prototype sensors for CO and NO were designed and constructed, with refinements from previously demonstrated sensors in laboratory conditions to accommodate the harsh conditions of utility boilers, and were applied in real-time, in situ, continuous monitoring across a 6 m path in the particulate-laden economizer exhaust of a pulverized-coal-fired power plant up to temperatures of 700 K. Smart engineering designs and appropriate detection strategies were employed to ensure the proper functioning of the devices under the harsh conditions in the field. Both the CO and NO sensors successfully captured transient changes in the gas concentration during periods of variation of the intake air level and the SNCR NOx control system. Continuous, unattended monitoring with fast time response and large dynamic range was demonstrated. The field demonstration of the infrared laser absorption sensors showed great potential for real-time combustion exhaust monitoring and control of practical combustion systems.