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Probe sampling of oxides of nitrogen from flames
COMBUSTION A N D F L A M E 24, 133-136 (1975)
133
Probe Sampling of Oxides of Nitrogen from Flames J. D. ALLEN British Gas Corporation, Watson House, Peterborough Road, London, SW6 3HN
Predictive models for the rate of formation of nitric oxide in flames are frequently based, at least in part, on empirical data obtained from well-defined combustion systems. The reliability and assessment of these models are, therefore, dependent on the accuracy of experimental measurements. Although recent developments [1 ] in analytical techniques permit reliable routine measurement of low NO x concentrations in combustion products, it is frequently necessary to remove the sample from the different regions of the flame via a probe before the analysis is performed. While the problems associated with the catalytic reduction of NOx by metallic probes are well known [2, 3], we have found that reactions can occur in noncatalytic probes (e.g., silica), which result in anomolously high NO a/NO ratios, without affecting the total NO x content of the sample. NOx formation begins close to the combustion zone of flames and frequently it is assumed, from thermodynamic considerations, that the proportion of NO 2 in this region is insignificant so that only NO need be measured. However, Rosenberg and Larson [4] reported a high proportion of NO 2 in samples withdrawn from the flame front of a premixed methane-air flame via a silica probe and analyzed with a Mast coulometric NO 2 meter. This NO 2 was regarded as being formed in the preheat zone by the upstream diffusion and recombination of NO and O-atoms. NO 2 was observed to disappear in the products following the flame front but to reappear in the postflame zone where excess air was present. We have qualitatively confirmed these observations using premixed methane-air flames of various fuel-air ratios and peak flame temperatures stabi-
lized on a Botha-Spalding burner. NO x concentration profiles through the flame zone were obtained by withdrawing samples via an uncooled fineorifice silica probe and analyzing the products using a Thermo Electron model 12A chemiluminescence analyzer. Although critical flow through the probe orifice was not achieved in this experiment, the rate of cooling of the products was sufficient to quench the high-activation energy reactions forming NO. The NO and NOx concentration profiles through the combustion zone of stoichiometric and fuellean flames were found to differ markedly, as illustrated in Fig. 1. In the visible flame zone, the NOx appears to be predominantly NO 2 . However, because the NO 2 to NO converter of the chemiluminescence monitor, used for the measurement of total NO x, is known to oxidize other nitrogen containing components (such as ammonia) to NO in the presence of oxygen [5], simultaneous measurements of the NO a concentration were made using a galvanic cell detector, similar to that described by Hersch and Deuringer [6]. The two measured profiles (Fig. 1) of the apparent NO 2 in the flame zone are in close agreement. Confirmation of these apparently high NO 2 concentrations has also been obtained by analyzing probed samples using a Griess-Saltzman reagent [7]. Smaller, but still significant, levels of NO 2 have also been observed in samples probed from fuel-rich flames. We believe that the NO 2 observed in these experiments is not present in the combustion zone but arises from reactions in the sampling probe. For efficient quenching of all gas phase reactions, a sampling probe should have a sonic flow orifice downstream of which the sample undergoes rapid adiabatic expansion and cooling. Atoms and
Copyright © 1975 by the Combustion Institute
Published by American ElsevierPublishing Company, Inc.
134
J.D. ALLEN E18 I LU
m o IL
_z 12 ffl W
Z 0 0
e,, 8 Z
Z
~
Z Z
q
ee
0
2
/, 6 8 10 12 I/, 16 PROBE DISTANCE FROM BURNER S U R F A C E - mm
18
20
Fig. 1. NO and NO2 concentrations in samples withdrawn via a fine orifice silica probe traversed along the center-line of a stoichiometric methane/air flat flame on a Botha-Spalding burner. radicals recombine on the probe walls; the concentration of stable molecules thus formed is usually negligible compared with the concentration present in the flame and, consequently, may be ignored. However, NO itself can undergo recombination reactions with radicals and the process of quenching in the sample probe may significantly affect its concentration in the sampled products. Similarly, the products of recombination reactions involving NO cannot be ignored. The major recombination reaction is probably: NO + 0
probe w a ~ r NO 2 .
(1)
Although reaction (1) may be only a minor contributor to the overall destruction o f radicals in the probe, the NO 2 formed can be a significant proportion of the total NO x in the sample. In fact, it is argued that the more efficient the quenching
in the probe, the greater the importance of reaction (1), because, as a result of inefficient quenching, oxygen atoms surviving inside the probe could react with any NO 2 to reform NO, by reaction (2). NO 2 + 0
=NO+O 2.
(2)
However, even in our work where a critical flow probe orifice was not used, the peak NO2]NO x ratio exceeded 90% when the probe was in the visible combustion zone (Fig. 1). The decay of this probe-formed NO2, must be closely related to the decay of the oxygen atom concentration in the post flame zone. It is possible to propose alternative explanations for the observed NO 2 in the probed samples, for example the initial formation of HO 2 or ozone by reaction (3), followed by the oxidation of NO by reaction (4):
BRIEF COMMUNICATION H (or O) + O 2 probe wall HO 2 (or O3), NO+HO2(orO 3)
13 5 (3)
J, NO 2 + O H ( o r O 2). (4)
Whatever the mechanism of NO~ formation in the probe it is evident from our observations that the measurement of total NOx (NO + NO 2) must be performed on samples withdrawn from the combustion zone of flames by fine orifice probes. In those cases where only NO is measured by instruments specific to this gas (e.g., chemiluminescence and nondispersive infrared NO analyzers), without the use of an NO 2 to NO converter, the NO formed in the flame zone may be seriously underestimated. Recently, Pompei and Heywood [8] investigated NO form~ition in fuel-lean kerosene-air flames. Experimental NO profiles, derived from the analysis of samples probed from the flames, were found to be in good agreement with predicted profiles, based on the simple Zeldovich mechanism. It is important to note, however, that an example of the measured NO profiles through the combustion zone exhibits a pronounced minimum similar to those observed in our experiments. If this apparent minimum were associated with a high proportion of NO~ in the samples probed from this region, as in our work, the NO concentration in the flame would have been seriously underestimated. Although the authors regard the agreement between their measured and predicted profiles as somewhat fortuitous in view of the uncertainty in the kinetic rate constants used in the modelling, we feel that, in the absence of confirmatory NOx measurements, the agreement may be even more fortuitous. It is also worth noting that, if only NO is determined, recombination reactions in the sampling probe resulting in the oxidation of NO to NO 2 can mask the detection of "prompt NO" formation [9] in or close to the flame zone of fuel-lean hydrocarbon flames. This effect will be less pronounced in fuel-rich flames due to the lower oxygen atom concentrations, but may explain the apparent insignificance of "prompt NO" in observations such as those of Pompei and Heywood [8]. Recently, Merryman and Levy [10] reported the measurement of NO and NO 2 profile through
low-pressure (0.1 atm) methane-air flames, using a chemiluminescence analyzer. Although the details of the sampling probe were not given, the NO and NO 2 profiles show a remarkable resemblance to those shown in Fig. 1. Confirmation of the presence of NO 2 was made by wet chemical (Saltzman) analyses, although the agreement in the analysis in the postflame zone was poor. Merryman and Levy treat the apparent appearance of NO 2 prior to the abrupt formation of NO (as in Fig. 1) with scepticism, but suggest possible mechanisms for NO 2 formation not involving NO as a precursor. We feel that a more plausible explanation of their "anomolous NO 2" is that NO 2 is formed only in the sampling probe, as described above. This being the case, we feel that the technique of titrating O-atoms with added NO 2 within the sampling probe, employed by Merryman and Levy [10] to measure O-atom profiles in the flame, should be treated with caution since it would appear that the rapid reaction (2) can be readily quenched in the probe. In another paper, Schefer et al. [11 ] described a study of the effect of using cooled and uncooled stainless steel or quartz sampling probes on the analysis for NO and NO 2 in an opposed reacting jet and a turbulent diffusion flame. Analyses for NO and NOx (by a chemiluminescence monitor) in the latter flame were in good agreement for all the probes used. However, although the NOx analyses in an opposed reacting jet flame showed little dependence on the type of sampling probe used, marked differences were observed in the NO (and hence NO 2) analyses. These differences were not related to the probe material [2, 3] (the flame was fuel lean) but to the use of cooled or uncooled probes. The authors considered that all the observed NO 2 was indeed present in the flame zone and that it was catalytically decomposed in the uncooled probes, even those made of quartz. They concluded that only cooled probes can yield reliable NO 2 analyses. We would prefer the explanation of their observations in terms of enhanced NO 2 formation in the cooled probes, as a result of radical recombination reactions, due to the more efficient quenching of the NO 2 + O reaction. There may indeed be conversion of NO 2 to NO in the un-
136 cooled probes and we also believe there to be a significant proportion of NO 2 in the regions surrounding the flame where steep temperature gradients exist (probably arising from the same reactions that we have suggested to occur in a probe but as a result of aerodynamic quenching). However, it seems improbable to us that this NO 2 can be accurately measured in this region by conventional probe sampling techniques. In conclusion, while it is desirable to rapidly quench gas phase reactions in samples withdrawn from flames by probes, the measurement of NO close to the combustion zone should be supplemented b y t h e measurement, in the same probed sample, o f NO 2, which we believe may be formed during radical recombination reactions in the probe. The author is grateful to the Director, Watson House and the British Gas Corporation for permission to publish this note. References 1. Allen, J. D., 3".Inst. Fuel 46, 123 (1973). 2. Halstead, C. J., Nation, G. H., and Turner, L., Analyst 97, 55 (1972).
J.D. ALLEN 3. England, C., Houseman, J., and Teixeira, D. P., Combustion and Flame 20, 439 (1973). 4. Rosenberg, R. B., and Latson, D. H., Formation of nitrogen oxides in aerated methane flames, presented at American Gas Association, Inc., Basic Research Symposium, I.G.T., Chicago, March, 1967. 5. Hodgeson, J. A., et al., A1AA Paper No. 71-1067, Joint Conf. Sensing Environ. Pollutants, California, Nov. 1971. 6, Hersch, P., and Deuringer, R., paper delivered at the 1 lth "Anachem" Conf., Detroit, 1963. 7. Saltzman, B. E.,Anal. Chem. 26, 1949 (1954). 8. Pompei, F., and Heywood, J. B., Combustion and Flame 19, 407, (1972). 9. Fenimore, C. P., 13th Symposium (International) Combustion, The Combustion Institute, Pittsburgh, Pa., 1971, p. 373. 10. Merryman, E. L., and Levy, A., paper presented to the 3rd International Clean Air Congress, Dusseldorf, Oct. 1973. 11. Sehefer, R. W., Mathews, R. D., Cernansky, N. P.~ and Sawyer, R. F., paper presented to the Western States Section, The Combustion Institute, 1973 Fall Meeting, El Segundo, California.
Received 31 January 1974; revised 6 August 1974
133
Probe Sampling of Oxides of Nitrogen from Flames J. D. ALLEN British Gas Corporation, Watson House, Peterborough Road, London, SW6 3HN
Predictive models for the rate of formation of nitric oxide in flames are frequently based, at least in part, on empirical data obtained from well-defined combustion systems. The reliability and assessment of these models are, therefore, dependent on the accuracy of experimental measurements. Although recent developments [1 ] in analytical techniques permit reliable routine measurement of low NO x concentrations in combustion products, it is frequently necessary to remove the sample from the different regions of the flame via a probe before the analysis is performed. While the problems associated with the catalytic reduction of NOx by metallic probes are well known [2, 3], we have found that reactions can occur in noncatalytic probes (e.g., silica), which result in anomolously high NO a/NO ratios, without affecting the total NO x content of the sample. NOx formation begins close to the combustion zone of flames and frequently it is assumed, from thermodynamic considerations, that the proportion of NO 2 in this region is insignificant so that only NO need be measured. However, Rosenberg and Larson [4] reported a high proportion of NO 2 in samples withdrawn from the flame front of a premixed methane-air flame via a silica probe and analyzed with a Mast coulometric NO 2 meter. This NO 2 was regarded as being formed in the preheat zone by the upstream diffusion and recombination of NO and O-atoms. NO 2 was observed to disappear in the products following the flame front but to reappear in the postflame zone where excess air was present. We have qualitatively confirmed these observations using premixed methane-air flames of various fuel-air ratios and peak flame temperatures stabi-
lized on a Botha-Spalding burner. NO x concentration profiles through the flame zone were obtained by withdrawing samples via an uncooled fineorifice silica probe and analyzing the products using a Thermo Electron model 12A chemiluminescence analyzer. Although critical flow through the probe orifice was not achieved in this experiment, the rate of cooling of the products was sufficient to quench the high-activation energy reactions forming NO. The NO and NOx concentration profiles through the combustion zone of stoichiometric and fuellean flames were found to differ markedly, as illustrated in Fig. 1. In the visible flame zone, the NOx appears to be predominantly NO 2 . However, because the NO 2 to NO converter of the chemiluminescence monitor, used for the measurement of total NO x, is known to oxidize other nitrogen containing components (such as ammonia) to NO in the presence of oxygen [5], simultaneous measurements of the NO a concentration were made using a galvanic cell detector, similar to that described by Hersch and Deuringer [6]. The two measured profiles (Fig. 1) of the apparent NO 2 in the flame zone are in close agreement. Confirmation of these apparently high NO 2 concentrations has also been obtained by analyzing probed samples using a Griess-Saltzman reagent [7]. Smaller, but still significant, levels of NO 2 have also been observed in samples probed from fuel-rich flames. We believe that the NO 2 observed in these experiments is not present in the combustion zone but arises from reactions in the sampling probe. For efficient quenching of all gas phase reactions, a sampling probe should have a sonic flow orifice downstream of which the sample undergoes rapid adiabatic expansion and cooling. Atoms and
Copyright © 1975 by the Combustion Institute
Published by American ElsevierPublishing Company, Inc.
134
J.D. ALLEN E18 I LU
m o IL
_z 12 ffl W
Z 0 0
e,, 8 Z
Z
~
Z Z
q
ee
0
2
/, 6 8 10 12 I/, 16 PROBE DISTANCE FROM BURNER S U R F A C E - mm
18
20
Fig. 1. NO and NO2 concentrations in samples withdrawn via a fine orifice silica probe traversed along the center-line of a stoichiometric methane/air flat flame on a Botha-Spalding burner. radicals recombine on the probe walls; the concentration of stable molecules thus formed is usually negligible compared with the concentration present in the flame and, consequently, may be ignored. However, NO itself can undergo recombination reactions with radicals and the process of quenching in the sample probe may significantly affect its concentration in the sampled products. Similarly, the products of recombination reactions involving NO cannot be ignored. The major recombination reaction is probably: NO + 0
probe w a ~ r NO 2 .
(1)
Although reaction (1) may be only a minor contributor to the overall destruction o f radicals in the probe, the NO 2 formed can be a significant proportion of the total NO x in the sample. In fact, it is argued that the more efficient the quenching
in the probe, the greater the importance of reaction (1), because, as a result of inefficient quenching, oxygen atoms surviving inside the probe could react with any NO 2 to reform NO, by reaction (2). NO 2 + 0
=NO+O 2.
(2)
However, even in our work where a critical flow probe orifice was not used, the peak NO2]NO x ratio exceeded 90% when the probe was in the visible combustion zone (Fig. 1). The decay of this probe-formed NO2, must be closely related to the decay of the oxygen atom concentration in the post flame zone. It is possible to propose alternative explanations for the observed NO 2 in the probed samples, for example the initial formation of HO 2 or ozone by reaction (3), followed by the oxidation of NO by reaction (4):
BRIEF COMMUNICATION H (or O) + O 2 probe wall HO 2 (or O3), NO+HO2(orO 3)
13 5 (3)
J, NO 2 + O H ( o r O 2). (4)
Whatever the mechanism of NO~ formation in the probe it is evident from our observations that the measurement of total NOx (NO + NO 2) must be performed on samples withdrawn from the combustion zone of flames by fine orifice probes. In those cases where only NO is measured by instruments specific to this gas (e.g., chemiluminescence and nondispersive infrared NO analyzers), without the use of an NO 2 to NO converter, the NO formed in the flame zone may be seriously underestimated. Recently, Pompei and Heywood [8] investigated NO form~ition in fuel-lean kerosene-air flames. Experimental NO profiles, derived from the analysis of samples probed from the flames, were found to be in good agreement with predicted profiles, based on the simple Zeldovich mechanism. It is important to note, however, that an example of the measured NO profiles through the combustion zone exhibits a pronounced minimum similar to those observed in our experiments. If this apparent minimum were associated with a high proportion of NO~ in the samples probed from this region, as in our work, the NO concentration in the flame would have been seriously underestimated. Although the authors regard the agreement between their measured and predicted profiles as somewhat fortuitous in view of the uncertainty in the kinetic rate constants used in the modelling, we feel that, in the absence of confirmatory NOx measurements, the agreement may be even more fortuitous. It is also worth noting that, if only NO is determined, recombination reactions in the sampling probe resulting in the oxidation of NO to NO 2 can mask the detection of "prompt NO" formation [9] in or close to the flame zone of fuel-lean hydrocarbon flames. This effect will be less pronounced in fuel-rich flames due to the lower oxygen atom concentrations, but may explain the apparent insignificance of "prompt NO" in observations such as those of Pompei and Heywood [8]. Recently, Merryman and Levy [10] reported the measurement of NO and NO 2 profile through
low-pressure (0.1 atm) methane-air flames, using a chemiluminescence analyzer. Although the details of the sampling probe were not given, the NO and NO 2 profiles show a remarkable resemblance to those shown in Fig. 1. Confirmation of the presence of NO 2 was made by wet chemical (Saltzman) analyses, although the agreement in the analysis in the postflame zone was poor. Merryman and Levy treat the apparent appearance of NO 2 prior to the abrupt formation of NO (as in Fig. 1) with scepticism, but suggest possible mechanisms for NO 2 formation not involving NO as a precursor. We feel that a more plausible explanation of their "anomolous NO 2" is that NO 2 is formed only in the sampling probe, as described above. This being the case, we feel that the technique of titrating O-atoms with added NO 2 within the sampling probe, employed by Merryman and Levy [10] to measure O-atom profiles in the flame, should be treated with caution since it would appear that the rapid reaction (2) can be readily quenched in the probe. In another paper, Schefer et al. [11 ] described a study of the effect of using cooled and uncooled stainless steel or quartz sampling probes on the analysis for NO and NO 2 in an opposed reacting jet and a turbulent diffusion flame. Analyses for NO and NOx (by a chemiluminescence monitor) in the latter flame were in good agreement for all the probes used. However, although the NOx analyses in an opposed reacting jet flame showed little dependence on the type of sampling probe used, marked differences were observed in the NO (and hence NO 2) analyses. These differences were not related to the probe material [2, 3] (the flame was fuel lean) but to the use of cooled or uncooled probes. The authors considered that all the observed NO 2 was indeed present in the flame zone and that it was catalytically decomposed in the uncooled probes, even those made of quartz. They concluded that only cooled probes can yield reliable NO 2 analyses. We would prefer the explanation of their observations in terms of enhanced NO 2 formation in the cooled probes, as a result of radical recombination reactions, due to the more efficient quenching of the NO 2 + O reaction. There may indeed be conversion of NO 2 to NO in the un-
136 cooled probes and we also believe there to be a significant proportion of NO 2 in the regions surrounding the flame where steep temperature gradients exist (probably arising from the same reactions that we have suggested to occur in a probe but as a result of aerodynamic quenching). However, it seems improbable to us that this NO 2 can be accurately measured in this region by conventional probe sampling techniques. In conclusion, while it is desirable to rapidly quench gas phase reactions in samples withdrawn from flames by probes, the measurement of NO close to the combustion zone should be supplemented b y t h e measurement, in the same probed sample, o f NO 2, which we believe may be formed during radical recombination reactions in the probe. The author is grateful to the Director, Watson House and the British Gas Corporation for permission to publish this note. References 1. Allen, J. D., 3".Inst. Fuel 46, 123 (1973). 2. Halstead, C. J., Nation, G. H., and Turner, L., Analyst 97, 55 (1972).
J.D. ALLEN 3. England, C., Houseman, J., and Teixeira, D. P., Combustion and Flame 20, 439 (1973). 4. Rosenberg, R. B., and Latson, D. H., Formation of nitrogen oxides in aerated methane flames, presented at American Gas Association, Inc., Basic Research Symposium, I.G.T., Chicago, March, 1967. 5. Hodgeson, J. A., et al., A1AA Paper No. 71-1067, Joint Conf. Sensing Environ. Pollutants, California, Nov. 1971. 6, Hersch, P., and Deuringer, R., paper delivered at the 1 lth "Anachem" Conf., Detroit, 1963. 7. Saltzman, B. E.,Anal. Chem. 26, 1949 (1954). 8. Pompei, F., and Heywood, J. B., Combustion and Flame 19, 407, (1972). 9. Fenimore, C. P., 13th Symposium (International) Combustion, The Combustion Institute, Pittsburgh, Pa., 1971, p. 373. 10. Merryman, E. L., and Levy, A., paper presented to the 3rd International Clean Air Congress, Dusseldorf, Oct. 1973. 11. Sehefer, R. W., Mathews, R. D., Cernansky, N. P.~ and Sawyer, R. F., paper presented to the Western States Section, The Combustion Institute, 1973 Fall Meeting, El Segundo, California.
Received 31 January 1974; revised 6 August 1974