Final Report: Multiplexed Diode-Laser Absorption Sensors for Real-Time Measurements and Control of Combustion Systems

EPA Grant Number: R827123
Title: Multiplexed Diode-Laser Absorption Sensors for Real-Time Measurements and Control of Combustion Systems
Investigators: Hanson, Ronald K. , Baer, D. S. , Jeffries, J. B.
Institution: Stanford University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001 (Extended to September 30, 2002)
Project Amount: $344,605
RFA: Exploratory Research - Environmental Engineering (1998) RFA Text |  Recipients Lists
Research Category: Sustainability , Land and Waste Management , Engineering and Environmental Chemistry


The object of this research project was to exploit emerging semiconductor diode laser technology and demonstrate novel optical absorption-based sensors for a gas-phase combustion effluent. These new lasers, coupled with fiber-optic components, enable simple, robust, and inexpensive devices for measurements of important combustion species. These all-optical sensors are compact and reliable, and allow fast species-specific and sensitive measurements of the target species. This grant supported demonstration measurements for four combustion species: (1) carbon monoxide (CO), a toxic, regulated combustion pollutant; (2) carbon dioxide (CO2), a major-species combustion product and greenhouse gas; (3) ammonia (NH3), an important combustion effluent from thermal de-NOx (also called selective catalytic reduction) and from fuel-bound nitrogen; and (4) H2O, a major-species combustion product. The sensor strategy developed in this program was based on absorption spectroscopy techniques and incorporates new developments in room-temperature continuous-wave semiconductor diode lasers. The range of available diode laser wavelengths recently has been extended into the infrared spectral region near 2.0 and 2.3 microns, thereby enabling sensitive absorption measurements using the strong, first-overtone vibrational bands of CO, and strong combination bands of NH3, CO2, and H2O. These new lasers operate without cryogenic cooling, and thus promise portable and rugged sensors. The diode-laser absorption sensors were used to accurately measure (some for the first time) important spectroscopic parameters of the target species at the probed wavelengths over a range of temperatures and pressures that are relevant to combustion flows.

A field test to demonstrate the sensor strategies for NH3 and CO2 was conducted to monitor the effluent from a bioreactor used to recycle waste water. National Aeronautic and Space Administration (NASA) researchers have worked to develop this waste water bioreactor for use on space missions, and low levels of NH3 effluent eventually can produce hazardous levels in the confined space craft. Therefore, the NH3 must be monitored at sub-parts per million (ppm) levels. In addition, the concentration of CO2 in the effluent provides a measure of bioreactor health and sets regeneration limits. Thus, the CO2 must be quantitatively monitored over a wide range of concentrations. Our sensors were remotely operated and incorporated into a sensor inter-comparison test to validate the bioreactor performance at NASA's Johnson Space Flight Center.

Summary/Accomplishments (Outputs/Outcomes):

In situ combustion diode laser absorption measurements of CO, CO2, and H2O were made in laboratory flames and in combustion exhaust gases. In addition, measurements of NH3 were made in combustion exhaust gases. Measurements of H2O using two different transitions were used to determine gas temperature in both the combustor and exhaust effluent.

For each target species, a significant amount of fundamental spectroscopy research was required for the design of a quantitative absorption-based sensor. First, potential absorption transitions were identified, with model calculations of the spectrum using the high-resolution transmission molecular absorption database (HITRAN). These calculations provided a quantitative assessment of the variation of the absorption signal strength, with gas temperature and the potential interference from optical absorption by other combustion species with absorption transitions that overlap those of the target. After selecting potential absorption transitions, diode lasers at the required wavelengths were fabricated by the telecommunications laser manufacturers. When the diode lasers became available, the wavelength and temperature dependent line strength was determined. An important part of this work was the measurement of the collisional broadening of the selected transition with each major combustion product gas and nitrogen.

Diode-laser absorption sensors have been developed for fast, sensitive, in situ, and combustion exhaust measurements of gas temperature, and the concentrations of the target species CO, NH3, CO2, and H2O. The highlights of this research are summarized below:

Carbon Monoxide (CO). Carbon monoxide is a toxic, regulated pollutant, which can be exhausted in harmful concentrations by fuel rich flames in confined spaces. A diode laser sensor was developed using the newly available distributed feedback lasers near 2.3 µm made by our colleagues at Sarnoff Corporation. In situ measurements of CO concentration were recorded with tunable diode-laser absorption spectroscopy techniques in both the exhaust and immediate post-flame regions of an atmospheric-pressure flat-flame burner operating on ethylene air. Two room-temperature continuous wave (CW) single-mode InGaAsSb/AlGaAsSb diode lasers operating near 2.3 µm were wavelength tuned over individual transitions in the CO first overtone band to record high-resolution absorption line shapes in the exhaust duct 79 cm above the burner (470 K) using the R(15) transition at 4,311.96 cm-1, and the immediate postflame zone 1.5 cm above the burner (1,820-1,975 K) using the R(30) transition at 4,343.81 cm-1. The CO concentration was determined from the measured absorption and the gas temperature. For measurements in the exhaust duct, the noise-equivalent absorbance was 3 x 10-5 (50-kiloHertz (kHz) detection bandwidth, 50-sweep average, 0.1-s measurement time), which corresponds to a CO detection limit of 1.5 ppm-m at 470 K. Wavelength modulation spectroscopy techniques were used to improve the detection limit in the exhaust to 0.1 ppm-m 500-Hz detection bandwidth, 20-sweep average, 0.4-sec measurement time). For measurements in the immediate postflame zone, the measured CO concentrations in the fuel-rich flames were in good agreement with chemical equilibrium predictions. The 0.1 ppm-m sensitivity is sufficient for on road measurements of CO emissions from the cleanest automobiles.

Ammonia (NH3). Ammonia is an important combustion pollutant from coal and other flames with fuel-bound nitrogen. It is used in large-scale industrial combustors for catalytic reduction of NO (here it is important to monitor potential ammonia release or slip in the combustor exhaust). In this work, we examined ammonia spectroscopy near 1.5 m to select transitions appropriate for trace ammonia detection in air quality and combustion emissions-monitoring applications using diode lasers. Six ammonia features were selected for these trace-gas detection applications based on their transition strengths and isolation from interfering species. The strengths, positions, and lower-state energies for the lines in each of these features were measured and compared with values published in the literature. Ammonia slip was measured in the exhaust above an atmospheric pressure premixed ethylene-air burner to demonstrate the feasibility of the in situ diode-laser sensor.

Carbon Dioxide (CO2). High-resolution absorption measurements of CO2 were made in a heated static cell and in the combustion region above a flat-flame burner for the development of an in situ CO2 combustion diagnostic based on a distributed-feedback diode laser operating near 2.0 µm. Calculated absorption spectra of high-temperature H2O and CO2 were used to find candidate transitions for CO2 detection, and the R(50) transition at 1.997 µm (the 1 + 22 + 3 band) was selected on the basis of its line strength and its isolation from interfering high-temperature water absorption. Measurements of spectroscopic parameters are reported, including the line strength, the self-broadening coefficient, the line position made for the R(50) transition, and an improved value for the line strength. The combustion-product populations of CO2 in the combustion region above a flat-flame burner were determined in situ to verify the measured spectroscopic parameters and to demonstrate the feasibility of the diode-laser sensor.

Multi-Species Combustion Monitor (T, H2O, CO2, and CO). A diode-laser sensor system based on absorption spectroscopy techniques was developed to measure CO, CO2, and H2O concentrations and gas temperature non-intrusively in the combustion and exhaust regions above a C2H4-air burner operating at atmospheric pressure. In situ measurements of CO concentration in the combustion region (1.5 cm above the burner) and in the exhaust (79 cm above the burner) were determined from absorption lineshapes of the R(30) and R(15) transitions (2 band) near 2.3 µm, respectively. A minimum sensivity of <10 ppm was demonstrated in the exhaust region. In situ measurements of H2O concentration and gas temperature were determined from absorption lineshapes near 1.343 µm (1 + 3 band), 1.392 µm (21, 1 + 3 bands), and 1.799 µm (22 + 3 2 band) using distributed feedback (DFB) diode lasers. In situ measurements of CO2 were recorded using a DFB laser near 1.997 µm (R [50] transition, 1 + 22 + 3 band). Gas temperature and H2O and CO2 concentrations were monitored simultaneously along a single path in the combustion region (1.5 cm above the burner surface) using a wavelength-multiplexing arrangement. The CO, CO2, and H2O concentration measurements in the combustion region agreed with calculated equilibrium populations within uncertainty (10 percent for CO2 and H2O, 5 percent for CO), and the measured temperatures were in agreement with radiation-corrected type-S thermocouple values to within 4 percent. These results demonstrate the utility of diode laser sensors for fast in situ measurements of multiple important combustion parameters for emissions monitoring and closed-loop control applications.

Field Test of NH3 and CO2 Sensor. Field measurements of NH3 and CO2 concentrations in bioreactor vent gases were recorded at NASA's Johnson Space Center with a portable and automated sensor system over a 45-hour data collection window. Measurements of NH3 and CO2 were made in bioreactor vent gases with DFB diode-laser sensors operating near 2 µm. Calculated spectra of NH3 and CO2 were used to determine the optimum transitions for interrogating with an absorption sensor. For ammonia, a strong and isolated absorption transition at 5,016.977 cm-1 was selected for trace gas monitoring. For CO2, an isolated transition at 5,007.787 cm-1 was selected to measure widely varying concentrations (from a minimum of 500 ppm to 10 percent of the effluent gases) to have sufficient signal for low mole fractions and to avoid being optically thick for high mole fractions. Using direct absorption and a 36-m total path-length multipass flow-through cell, a minimum sensitivity of 0.25 ppm for NH3 and 40 ppm for CO2 was achieved.

All of these results suggest that diode laser absorption techniques may be applied for in situ combustion measurements for infield continuous emission monitoring and combustion control, detailed studies of engine combustion, such as dynamic exhaust gas analysis with single-cycle time resolution, and for development and validation of advanced combustion models.

Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 21 publications 7 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Wang J, Maiorov M, Jeffries JB, Garbuzov DZ, Connolly JC, Hanson RK. A potential remote sensor of CO in vehicle exhausts using 2.3 μm diode lasers. Measurement Science & Technology 2000;11(11):1576-1584. R827123 (2000)
R827123 (2001)
R827123 (Final)
not available
Journal Article Wang J, Maiorov M, Baer DS, Garbuzov DZ, Connolly JC, Hanson RK. In situ combustion measurements of CO using diode-laser absorption near 2.3 μm. Applied Optics 2000;39(30):5579-5589. R827123 (2001)
R827123 (Final)
not available
Journal Article Webber ME, Baer DS, Hanson RK. Ammonia monitoring near 1.5 μm with diode-laser absorption sensors. Applied Optics 2001;40(12):2031-2042. R827123 (2000)
R827123 (2001)
R827123 (Final)
not available
Journal Article Webber ME, Kim S, Sanders ST, Baer DS, Hanson RK, Ikeda Y. In situ combustion measurements of CO2 by use of a distributed-feedback diode-laser sensor near 2.0 μm. Applied Optics 2001;40(6):821-828. R827123 (2001)
R827123 (Final)
not available
Journal Article Webber ME, Claps R, Englich FV, Tittel FK, Jeffries JB, Hanson RK. Measurements of NH3 and CO2 with distributed-feedback diode lasers near 2.0 μm in bioreactor vent gases. Applied Optics 2001;40(24):4395-4403. R827123 (2001)
R827123 (Final)
not available

Supplemental Keywords:

remote sensing, optical absorption, temperature, waste reduction, pollution prevention, control, atmosphere, combustion, environmental engineering, toxics, pollutants., RFA, Scientific Discipline, Air, Waste, Environmental Chemistry, Environmental Monitoring, Incineration/Combustion, Engineering, Chemistry, & Physics, Environmental Engineering, optical sensors, combustion generated radicals, emissions measurement, air pollution, optical sensor, emission controls, process control, carbon dioxide, fiber laser, combustion, real time monitoring, emissions contol engineering, laser absorption sensors, incineration, combustion contaminants

Relevant Websites: Exit

Progress and Final Reports:

Original Abstract
  • 1999 Progress Report
  • 2000 Progress Report
  • 2001 Progress Report