9 Nitric Oxide Detection in Flames by Laser
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Fluorescence DANIEL R. GRIESER and RUSSELL H. BARNES Battelle-Columbus Division, 505 King Avenue, Columbus, OH 43201
Laser-Fluorescence techniques for NO are of interest for studying the mechanisms of NO formation and its influence on chemical processes and pollutant formation in flames. In general, the optical fluorescence techniques provide very high detection sensitivities and good spatial resolution. The method described here for detecting NO in flames is based on the use of a frequency-doubled tunable dye laser to excite transitions in the (0,0) γ-band of NO in the range of 2250 to 2270 Å. Fluorescence is observed at wavelengths associated with the bands involving the (0,0), (0,1), (0,2), and higher ground-state vibrational transitions of the γ-band system. The experimental system used for the NO flame fluorescence measurements is shown in Figure 1. The frequency-doubled beam from the dye laser was focused into the high-temperature reaction zone in a flat flame on a 2-1/4-inch-diameter burner. Fluores cence from the flame was collected in a direction perpendicular to the face of the burner using a cassegrain collection optic and focused through the slits of the spectrometer. The c o l l e c t i o n
o p t i c was l o c a t e d a t a d i s t a n c e o f about 8 inches from the face o f the burner and used with an e f f e c t i v e aperature o f about f/2. An EMI 6256 SA p h o t o m u l t i p l i e r was used as a d e t e c t o r . The output s i g n a l from the d e t e c t o r was processed using an 0RTEC gated photon counting system t h a t was synchronized to the l a s e r p u l s e . E x c i t a t i o n s p e c t r a f o r NO recorded by s e t t i n g the spectrome t e r a t s p e c i f i c wavelengths and c o n t i n u o u s l y scanning the output wavelength o f the dye l a s e r are presented i n Figures 2 and 3. Figure 2 shows a f l u o r e s c e n c e e x c i t a t i o n spectrum f o r a t r a c e o f NO i n n i t r o g e n flowing from the face o f the burner a t room tem perature and no flame present. Figure 2 shows the same e x c i t a t i o n spectrum f o r NO i n the high-temperature zone o f a CH4/O2/N2 flame. Both e x c i t a t i o n s p e c t r a were obtained a t atmospheric pressure a t the same p o s i t i o n above the face o f the burner. In the case o f Figure 2, the spectrometer was s e t a t 2262.0 A f o r the (0,0) y-band, while i n Figure 3 the spectrometer was s e t a t 2368.8 A f o r the (0,1) y-band. The spectrometer s l i t s were s e t a t 3 mm which 0-8412-0570-l/80/47-134-153$05.00/0 © 1980 American Chemical Society
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
154
LASER
PROBES
FOR COMBUSTION
CHEMISTRY
Mirror SPEX Model 1269 1.26-Meter Spectrometer EMI 6256 SA PMT Anagrain Wide Aperture F/1 Collection System for UVA/isible ^
ORTEC 456 High Voltage Power Supply
L ORTEC 9301 Fast Preamplifier
Flat Flame Burner
CH4/O2/N2
ORTEC 930 L Amplifier Discriminator
Figure 1.
Dye laser system for nitric oxide fluorescence measurements in 0 -N flame 2
2
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
CH k
9.
Nitric
GRIESER A N D B A R N E S
Oxide
155
Detection
Prism to Pass Doubled Laser Frequency
Beam Focusing Lens
-6
Laser Beam
ORTEC 467 Time-to-Pulse Height Converter
ORTEC Log-Linear Rate Meter
Molectron Model DL 14 Dye Laser With Frequency Doubling
Molectron Model UV 24 Pulsed Nitrogen Laser
ORTEC 416 A Gate and Delay Generator
Laser SYNC
Strip Chart Recorder
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LASER PROBES FOR COMBUSTION CHEMISTRY
158
i s e q u i v a l e n t to a s p e c t r a l bandpass of about 19.9 ft. In the case o f the flame spectrum, a t r a c e o f NO was added to the n i t r o g e n to enhance the i n t e n s i t y o f the spectrum. I t was, however, p o s s i b l e to d e t e c t the natural l e v e l o f NO i n the flame which was measured p r e v i o u s l y with the same burner by Merryman and Levy (1_) using a quartz probe and chemiluminescence a n a l y z e r and found to be i n the range o f 20-30 ppm. In the case o f Figure 3, i t i s estimated t h a t the l e v e l o f NO i n the doped flame i s about 60 ppm. Scan times o f 60 minutes were r e q u i r e d to record the e x c i t a t i o n s p e c t r a shown i n Figures 2 and 3. The c o n t r i b u t i o n o f Rayleigh s c a t t e r i n g to the spectrum i n Figure 2 would be expected to be small because the cross s e c t i o n s f o r Rayleigh s c a t t e r i n g are s e v e r a l orders-of-magnitude s m a l l e r than f l u o r e s c e n c e cross s e c t i o n s . This was v e r i f i e d e x p e r i m e n t a l l y by comparing s p e c t r a f o r the (0,0) y-band t r a n s i t i o n , with those i n v o l v i n g other lowere l e c t r o n i c v i b r a t i o n a l l e v e l s of the NO y-bands. The s p e c t r a l l i n e s i n Figures 2 and 3 a l l correspond to i d e n t i f i e d r o v i b r o n i c t r a n s i t i o n s (2_ 3). In summary, the r e s u l t s i n t h i s paper demonstrate t h a t l a s e r f l u o r e s c e n c e can be used to d e t e c t NO i n atmospheric-pressure flames. D e t e c t i o n s e n s i t i v i t i e s i n the ppm range were observed with l a s e r pulse energies i n the range o f about 3 y J . T h i s s e n s i t i v i t y can be i n c r e a s e d s i g n i f i c a n t l y by using a higher intensity laser. 9
This research was supported by the D i v i s i o n o f F o s s i l Fuel U t i l i z a t i o n o f the U. S. Department o f Energy under c o n t r a c t no. W-7405-Eng-92, Task 88. Literature Cited 1. 2. 3.
Merryman, E. L., and Levy, A., "Fifteenth Symposium (International) on Combustion", The Combustion Institute, 1974, p. 1073. Zacharias, H., Anders, A., Halpern, J. B., and Welge, K. H., Opt. Commun., 1976, 19, 116. Also see Errata: Opt. Commun., 1977, 20, 449. Deezi, I., Acta Physica, 1958, 9, 125.
RECEIVED
February 1, 1980.
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
