Grantee Research Project Results
Final Report: Multiplexed Diode-Laser Gas Sensor System for In Situ Multispecies Emissions Measurements
EPA Grant Number: R823933Title: Multiplexed Diode-Laser Gas Sensor System for In Situ Multispecies Emissions Measurements
Investigators: Hanson, Ronald K. , Baer, D. S.
Institution: Stanford University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 30, 1996 (Extended to December 31, 1997)
Project Amount: $194,870
RFA: Exploratory Research - Engineering (1995) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
Increasingly stringent environmental regulations and the drive for more efficient and economical combustion systems motivate the development of improved sensors for real-time process control and emissions monitoring. The objective of the research is to develop novel optical sensors based on tunable diode lasers, fiber-optic components, and absorption spectroscopy techniques for real-time measurements of gas temperature and the concentrations of CO, CO2, NO2, NH3, and H2O at multiple locations in combustion systems for incineration, propulsion, and energy generation applications. The measured values of gas temperature and species concentrations may be incorporated into a closed-loop strategy to demonstrate the utility of diode-laser sensing for on-line control of the combustion process and for emissions monitoring.Summary/Accomplishments (Outputs/Outcomes):
Multiplexed diode-laser sensors based on absorption spectroscopy techniques have been developed to measure gas temperature and the concentrations of CO, CO2, NO2, NH3, and H2O in high- temperature combustion environments. For measurements of CO, CO2, and NH3 concentrations, an external-cavity diode laser (ECDL) operating near 1.55 microns was used to scan at a 10-20 Hz rate over selected CO (3n band), CO2 ((1201)?(0000) band), and NH3 (n1+n3 band) transitions. For measurements of H2O, two distributed feedback (DFB) diode lasers were used to tune at a 10- kHz rate over selected H2O transitions (n1+n3, 2n1 bands) operating near 1. 34 and 1.39 microns. For NO2 measurements, two measurement strategies were pursued. In the first scheme, a commercial diode laser system was used to record absorption measurements near 670 nm. In the second scheme, an ECDL operating near 785 nm was frequency doubled with a quasi-phase-matched nonlinear LiNbO3 waveguide and tuned over absorption features near 395 nm. In the latter stages of the program, a novel ECDL operating near 2.0 microns was tuned over transitions for the simultaneous in situ detection of CO2, H2O, and gas temperature in a combustion system. In general, gas temperature was determined from the ratio of integrated line intensities. Species concentration was determined from the integrated line intensity and the measured temperature.The sensors were applied for simultaneous multi- parameter measurements in laboratory- and industrial-scale combustion facilities. Nonintrusive, line-of-sight measurements of gas temperature and the concentrations of H2O, CO2, and NH3 were recorded in the combustion region (i.e., in situ). Measurements of CO and CO2 concentrations were recorded in the exhaust region using fast extractive-sampling techniques and a multi-pass cell (36-meter folded optical path) for increased sensitivity. The sensors were used to accurately measure (some for the first time) important fundamental spectroscopic parameters (e.g., line strengths, positions, and broadening parameters) of the target species at the probed wavelengths. p>
Multiplexing techniques were developed to combine the outputs of multiple near-IR beams into a single path using commercial fiber-optic splitters and couplers. For in situ combustion measurements, an optical fiber delivered the multi-wavelength beam to the combustor and a GRIN lens collimated the light through the flowfield. The multi-wavelength beam could be split into multiple paths using inexpensive fiber-optic components in order to probe several regions simultaneously and thus significantly reduce the overall cost and maximize the effectiveness of the diode-laser sensor system for remote monitoring. The transmitted multi-wavelength light was de-multiplexed (spectrally separated) into the constituent laser wavelengths by directing the beam at a non-normal incidence angle onto a diffraction grating. The beams were diffracted at angles specific to each wavelength and were subsequently monitored with photodetectors. The detector voltages were digitized by an A/D card installed in a personal computer that was also used for data reduction. Data acquisition and analysis software were developed in- house based on the LabView? platform for visual display of measured parameters in real time.
For CO and CO2 measurements, the diode laser sensor system (using the 1.55-micron ECDL) was applied to monitor gas concentrations in combustion gases using fast-flow sampling and absorption spectroscopy techniques. Recorded survey spectra of the R branch of the CO 3n band and the R-branch of the CO2 2n1+22+n 3 band near 6350 cm-1, and selected lines of the H2On1+2n2 and 2n 2+n3 bands in the spectral region from 6601 cm-1 to 6660 cm-1 were compared with calculated spectra using the HITRAN database. Species-specific measurements of combustion gases in the post-flame region of a premixed laminar flat-flame burner were obtained using a stainless steel, water-cooled sampling probe, thermocouples, and a fast-flow multi-pass absorption cell with a nominal 36-meter long path (and minimum response time of 66 ms; typical response time 1 sec). The measurements of CO and CO2 showed good agreement with calculated equilibrium values of total carbon yield. Measured CO concentrations were below and measured CO2 concentrations were above calculated equilibrium values in fuel rich conditions, indicating some conversion of CO to CO2 in the sampling probe. The minimum detectable absorbence, 8.3'10-5, was recorded in a measurement time of 8.3 ms with a 50-kHz measurement bandwidth (limited by an RC low-pass filter), and corresponded to a CO detectivity of 185 ppm (for a signal-to-noise of unity).
For the NO2 measurements, the absorption coefficients near 395 nm (54'10-20 cm2 molecule-1) exceeded those near 670.2 nm (0.88'10-20 cm2 molecule-1) by a factor of 25 over the measured temperature range 298-500 K and pressure range 0.03-1.0 atm, thereby confirming the merit of near-UV wavelengths for enhanced NO2 detection limits. The NO2 results represent the first high-resolution absorption measurements of NO2 in the near-UV spectral region.
For measurements of NH3, survey spectra were recorded in a 50-cm long test cell (3.27 Torr, 296 K) by tuning the external cavity diode laser over the 1.49-1.56 µm region. The measurements were used to verify (and in some cases, measure for the first time) the line strengths and positions of important transitions and to select the appropriate lines for species detection. The selection of appropriate transitions for in situ measurements was based on the strength and spectral isolation of the measured NH3 lineshapes. The system was applied for in situ NH3 detection in the post-flame gases of a C2H4-air flame by recording high-resolution absorption measurements of the P3(4) doublet. For a minimum detectable absorbence of 10-4 and a 1-meter path, the sensitivity (minimum detectable concentration) for measurements near 1.5 µm is 5 ppm (for a signal to noise ratio of unity).
For simultaneous in situ measurements of CO2, H2O, and gas temperature, a recently developed ECDL (based on strained-InGaAs/InP) operating near 2.0 microns was used to scan over selected CO2 ((1201)?(0000) band) and H2O transitions ((011)?(000), (021)?010) bands) near 1.996 and 1.992 microns, respectively. The system was applied to measure temperature and the concentrations of CO2 and H2O in the post-flame region of a premixed, laminar C2H4-air flat-flame burner in fuel-lean conditions. The laser-based temperature measurements were in agreement with values determined using a (type S) thermocouple to within 3 percent. In addition, the measured CO2 and H2O mole fractions agreed to within 6 percent and 3 percent with calculated equilibrium values at measured temperatures, respectively. The minimum measurable CO2 detectivity in the flame was 200 ppm (1470 K, 1-m path, 200- Hz detection bandwidth). These results represent the first simultaneous in situ measurements of gas temperature and the concentrations of CO2 and H2O in a combustion system.
The project successfully demonstrated that multiplexed diode-laser sensors, comprised of room-temperature tunable diode lasers and fiber-optic components, might be used for real-time measurements of gas temperature and important combustion species in high-temperature flows. The sensors are compact, robust, easy to use and yield fast, continuous, absolute measurements of species concentrations from high resolution absorption lineshapes. The sensors may be used for nonintrusive in situ measurements of gas temperature and major combustion species (e.g., H2O, CO2, NH3), or for sensitive detection of trace species (e.g., CO, NO2) using fast, extractive-sampling techniques for combustion applications, including process control, compliance, and pollution monitoring. Measurements in the flowfield (in situ) are useful to avoid species conversion in probes and adsorption in sampling lines, and offer faster measurement response times that are essentially limited by the tuning rate of the laser (kHz rates typical). Fast, extractive-sampling measurements using multi-pass (folded path) cells offer significantly improved sensitivity owing to the longer optical path length probed, but yield a time response that is limited primarily by the flowthrough time in the cell (1-10 Hz rates typical). p>
The measurements of fundamental spectroscopic parameters (line positions, strengths, broadening parameters) for the probed species, determined from high-resolution absorption lineshapes, were compared with calculated values in the HITRAN96 database. The measurements revealed significant errors in the database at elevated temperatures for H2O, CO2 (near 2.0 microns), and NH3 (near 1.56 and 2.0 microns). These results further emphasize the need for improved spectroscopic measurements at high temperatures of important species, particularly in spectral regions that have not yet been probed extensively with narrow bandwidth tunable lasers (e.g., in the 2-3 micron region).
The results suggest that multiplexed diode-laser absorption sensors may be applied for real-time continuous measurements of gas temperature and the concentrations of the probed species in high-temperature combustion environments for open- and closed-loop control applications. The reliability, accuracy, and flexibility of these compact sensors make them particularly suitable for measurements of a variety of pollutants and combustion efficiency for emissions monitoring and control applications. These sensors may enable improved design and development of large-scale industrial combustion systems. Anticipated availability of diode lasers operating at wavelengths longer than 2 microns will extend the capabilities of these sensors and enable sensitive in situ measurements of important combustion pollutants such as CO, NO, and various hydrocarbons.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 6 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Mihalcea RM, Baer DS, Hanson RK. Tunable diode-laser absorption measurements of NO2 near 670 and 395 nm. Applied Optics 1996;35(21):4059-4064. |
R823933 (Final) |
not available |
Supplemental Keywords:
remote sensing, temperature, waste reduction, pollution prevention, real-time control, atmosphere, combustion, environmental engineering, toxics, pollutants., RFA, Scientific Discipline, Air, Toxics, Waste, Chemistry, HAPS, mobile sources, Environmental Monitoring, tropospheric ozone, Incineration/Combustion, Environmental Engineering, Environmental Law, compliance monitoring, hydrocarbon, emissions measurement, VOCs, chemical contaminants, air pollution, process control, laser gas sensor system, waste incineration, complex combustion effluents, hydrocarbons, incineration, measurement methods , combustion contaminantsRelevant Websites:
http://navier.stanford.edu/hanson ExitProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.