Formantion of NO2 by laminar flames

Eighteenth Symposium (International) on Combustion

FORMATION

OF

The Combustion Institute, 1981

NO 2 BY LAMINAR

FLAMES

K. J. A. HARGREAVES,* R. HARVEY, F. G. ROPER AND D. B. S M I T H British Gas Corporation, London Research Station, Michael Road, London S W 6 2AD

This paper reports findings on the formation of NO~ associated with laminar flames, principally premixed methane-air. A variety of approaches to the problem was used. Preliminary experiments by non-interfering absorption spectroscopy showed some formation of NO 2 from small diffusion flames b u r n i n g within an enclosed box. The main investigation used probe sampling and chemiluminescence analysis to yield more detailed information on the distribution of NOz around a fuel rich premixed methane-air flame. The anomalous formation of NOz in the sampling line was examined and confirmed previous observations that in order to minimise probe-formed NO 2 a low sampling pressure is essential. Under these conditions, NO 2 was found at the edge of the flame and in the post flame gases. Its absence in the hot regions of the flame was in agreement with other recent work. The area immediately surrounding the burner lip was found to be a large source of oxidizing species. However, the oxidizing potential was not fully realised, possibly due to the limited residence time of NO in this region or back reactions destroying the NOz. Evidence of the likely mechanism associated with the NO to NO 2 conversion was obtained from measurements of hydrogen peroxide thought to be involved in the oxidation process. The H20 2 was collected by probe sampling and condensation of flame products, followed by colorimetrie analysis using TiC14 reagent. Probe effects were again taken into consideration. Finally, a mechanism for the oxidation of NO to NO~ is discussed. Reactive species diffuse out of the flame and react with secondary air to produce HO 2 and possibly other oxidizing species. A simple computer model shows that the proposed sequence of reactions can lead to rapid oxidation of NO. 1. I n t r o d u c t i o n

The formation of NOz by flames remains a poorly understood aspect of NO~ chemistry. There is good evidence that NO 2 can be formed in some combustion environments, such as diesel and spark-ignition engine exhausts (1~, gas turbines (2~, and laboratory scale turbulent diffusion flames (31. However, the termolecular oxidation of NO by O~ is very slow for p p m concentrations of NO, and cannot explain these observations. It has been suggested (4'~> that NO2 formation coincides with fast quenching of the flame gases. There are other reports ~6'7~of NO a being formed in primary zones of hot methane-air flames. These results have created some controversy. On the face of it, it seems most unlikely that significant concentrations of NO 2 could exist in such hostile surroundings. * Present Address: British Gas Corporation, Watson House, Peterborough Road, London SW6.

133

It is now clear that considerable problems arise with the use of sampling probes to measure NO and NO 2, Reactions occurring during the sampling process can have a profound effect on the sample, causing the interconversion of NO and NO v Allen (8> drew attention to this and suggested the reaction wall

0 or possibly

+NO

HO 2 + N O

--~ NO2 ~

OH+NO 2

as the culprit, More recent studies by Kramlich and Malte (4~ and by J o h n s o n et al (~ provide a fuller picture, The rapid quenching of the flame gases in the probe appears to produce transient levels of HO e high enough to oxidise NO to NO~. Further evidence of this was provided by Johnson et al, using a neat application of laser fluorescence as a noninterfering method of monitoring NO~ in a premixed flame. They proved that probe measurements vastly over-estimated the real NO~ concentra-

134

COMBUSTION GENERATED P O L L U T I O N

tion, and concluded that significant NO n levels were unlikely to occur in hot flame gases. These studies seem to dispose of the question of NO n in the hot regions of premixed flames. But there still remains the possibility of formation in the cooler regions in the periphery of the flames. Indeed, as Johnson et al state, "the kinetic mechanism developed for the production of NO n in the probe would apply mutatis mutandis to any system wherein flame gases containing NO are cooled suddenly to temperatures below 1100 K. ''~s~ This forms the starting point for this paper.

3 ~2 Ol

2. Initial Experiments

,-.3

Some initial experiments were performed to determine whether NOn formation could be associated with the burning of laminar flames. Absorption spectroscopy was used to monitor NO and NO n to avoid the possibility of probe effects. The methodology was simple. Diffusion flames were burned in an enclosed box. The combustion products were trapped in the box and their change in concentration with time was measured. The box was made of galvanised steel (2.65 m long and 295 mm square cross section), closed with aluminium end-plates containing windows for the spectroscopy. Nitric oxide concentrations were determined at the peak absorption at 226.9 nm. The nitrogen dioxide molecule is far from ideal for quantitative spectroscopy, but there is some structure in the 430 - 450 nm range. The peak at 448 nm was selected for the measurements. The light sources used were a high pressure 450 W xenon lamp (for determining NO) and a 55 W quartz-halogen lamp (for NOn). Spectra were obtained using a 1/2 m Jarrell-Ash monochromator. To obtain the required sensitivity, a multipass system was used. Six traversals of the cell were needed for NO n measurements--a path length of nearly 16 m. For the NO measurements, only two traversals were needed. Small diffusion flames were burned on simple burners made from 1/4 inch o.d. copper tubing, narrowed to a thin slit at the end. Fuels used were methane, ethylene and hydrogen. Flows were adjusted so the flames were non-sooting. Combustion was supported by the air in the box, so flames became progressively vitiated during each run. A typical result is shown in Fig. 1. The significant point is the linear growth rates of NO and NO n. (At later stages of each run, more complex behaviour occurred. But by this time, the level of products in the box was high, and combustion incomplete). The results for all fuels are summarised in Table 1 for the initial phase of each run, when concentrations increased linearly with time. The ratio of the growth rates is the degree of conversion of NO to

m

f

z1 g

V 40

vv

I I 80 TIME (minutes)

I 120

FIG. 1. G r o w t h o f N O and N O n w i t h time f o r

methane diffusion flames. Flow rates: 9 2.83 cm 3 s-l; • 2.50 cm z s-~; 9 1.67 cm z s-m; O 1.42 cm z s -l. (For clarity the origin has been shifted along the time axis). NOn. The main observations were: (i) N O J N O x ratio varied between 44 and 60%; (ii) there was no apparent dependence on fuel type; and (iii) with methane, there does appear to be a correlation with flow rate, conversion varying inversely with flow rate. Tests showed that photolytic effects were abTABLE I Initial rates of formation of NO and NO n by diffusion flame in a closed box Initial rate of formation ppb s -l Fuel Methane

Ethylene Hydrogen

Fuel flowrate, cm ~ s -m NO 1.42 1.67 2.50 2.83 0.68 0.97 10.0 13.5

0.9 1.1 2.2 3.0 0.9 1.7 1.7 0.8

NO n

NOJNO x

1.4 1.4 2.2 2.3 1.2 2.2 1.5 0.8

0.60 0.56 0.50 0.44 0.58 0.56 0.47 0.50

FORMATION OF NO~ BY LAMINAR FLAMES sent, and that the impingementof the flame products on the vessel walls was not important. We do not propose drawing many conclusions from these experiments, as some of the parameters were not well defined. But we do believe the results provide convincing evidence of either a simultaneous formation of NO and NO~ in the flames, or a rapid conversion of NO to NOa on leaving the flame. In the light of the comments in the introduction, the first explanation seems untenable.

3. Experimental 3..1 NO and NO2 Measurements Concentrations of NO and NO 2 were measured at various positions in and around a rich, premixed Bunsen-type flame stabilized on a contoured orifice nozzle burner of 9 mm internal diameter. The burner was housed within a square enclosure surmounted by a cylindrical chimney, secondary air being entrained naturally through window openings at the side of the enclosure. The fuel gas was methane (99% purity) which was mixed with primary 'air' taken from cylinders. The flow rates of gas and air were metered by rotameters which had been calibrated against soap bubble flowmeters. The stoichiometry of the flame investigated was 1.22 (actual fuel/air divided by stoichiometric fuel/air requirement). This flame stoichiometry was used throughout the work. In some of the experiments the fuel-air mixture was seeded with known quantities of cylinder NO, in such a way as to produce minimum changes in flame temperature and conditions. Samples were withdrawn from the flame through a 3 mm o.d. quartz microprobe. The orifice was of 50 lain diameter with a tip designed to minimise interference with the flame and also to rapidly quench the sample. The microprobe was mounted on a three-dimensional micromanipulator for accurate positioning, and was connected to the pyrex sampling system by means of 3 mm i.d. PTFE tubing. NO and NO2 concentrations were determined using a laboratory-built chemiluminescent analyser, similar in construction to instruments described previously.~9'1~ To obtain the total NO, concentration, the NO~ in the sample was converted to NO using a stainless steel thermal converter operating at 650~ In order to reduce the variation in the chemiluminescence quenching efficiencies by the bulk species, a high ratio of ozonized oxygen to sample was used, thus ensuring that the major quenching species was oxygen. The pressure inside the reaction chamber could be adjusted by varying the pumping speed, whilst the pressure in the sampling line could be set by means of a needle

135

valve midway between the probe orifice and the reaction vessel. The latter pressure was monitored by a vacuum gauge positioned immediately after the probe. 3.2 HeO~ in Flames A critical flow quartz microprobe was used to sample flame gases as described in section 3.1, with orifice diameter 80 lam. The probe was connected to a glass sampling system by PTFE tubing 3 mm bore. A glass trap cooled with solid CO~ was used to freeze out water vapour and H202 from the sample gases. Tests showed that the collection efficiency for water was close to 100%; the H~O2 would freeze out with the water with similar efficiency. A twostage rotary pump was used to evacuate the system, and the sampling pressure was controlled by a needle valve. The pressure was measured by a pressure transducer at a T-junctionbetween the sample probe and cold trap. The probe sampling rate from the flame was measured at the start and end of a run, by noting the rate of pressure rise when the system was evacuated and valved off. The total volume of gas sampled during a run was then calculated as the product of sampling time and mean sampling rate. The condensate was analysed for H202 by a colorimetric method using TiCI4 reagent. This gave a yellow-green coloration which is specific to H~O~.<"~ The absorbance at 415 nm was measured by a modified Unicam SP600 spectrophotometer. The colorimetric response was calibrated with H20 ~ solution standardized against acidified KMnO4 solution, and the sensitivity agreed within 2% of the value found by Pilz and Johann. ~1~ Further details are given elsewhere. .

COMBUSTION G E N E R A T E D P O L L U T I O N

136

In the present case the estimated accuracy was +_10%.

4. Results 4.1 NO and NO e Measurements

4.1.1 The Effect of Sample Pressure Variation on Observed NOe Levels In the light of evidence obtained by Kramlich and Malte ~4~ relating to anomalous NO n formation in the sampling probe, an investigation was carried out into the effect of sample probe pressure on the observed NO to NO~ conversion levels. Several sampling positions around the flame edge were selected and the NO and NO n concentrations were measured for a wide range of sample probe pressures. Figure 2 shows the variation of the N O J N O , ratio at different probe pressures obtained on the unseeded CH4-Air flame. A distinct dependence of conversion level on pressure was revealed at particular sampling points, the magnitude of the effect being greatest at the base of the flame (plots 1 and 2). Here, total conversion was approached at pressures > 140 mbar. Moving up the flame edge, the pressure dependence became less dramatic until a position was reached in the exhaust product gases (plot 5)

where the sample probe pressure had no effect on the N O J N O x ratio. At low probe pressures (< 40 mbar) conversion of NO to NO n varied little with pressure, even in the base of the flame. Hence it must be concluded that to minimise errors caused by reactions in the probe, the probe must be operated at a low pressure. In addition, a more thorough set of measurements was carried out at two extremes of pressure within the probe. 4.1.2 Low Pressure Sampling In order to minimise probe-formed NO2 the main investigation on the flame was carried out at pressure of 20 mbar. NO and NO 2 concentrations together with the associated temperatures were measured at points in and around the flame. Particular attention was paid to the region adjacent to the burner lip. The results are shown in Fig. 3. Each sampling point, represented by a circle, contains four pieces of information--the total NO x concentration (ppm), the NO 2 concentration (ppm), the N O J N O x (expressed as a percentage), and the temperature (K) at that particular sampling point. The shape and positions of the primary and secondary flame zones

40-0~

@4

100

25.0

! 6O

~

7-0-

50 NN~(%)

~ 6"0-

40 3O

~5.0z

20 10 0

~ IE4-0-

SAMPLING PRESSURE (mbar) 20

I

40

I

60

I

18p

100

I

120

I

140

I

160

I

Fzc. 2. Effect of sampling pressure on the N O J N O x ratio, for a premixed methane/air flame with 0 = 1.22, methane flowrate 15.2 cm a s -l. (1) 1.0 mm above burner, 1.0 mm outside burner edge ~) 5.0 mm above burner, 3.0 mm outside burner edge ~) 25.0 mm above burner, 4.0 mm outside burner edge (~) 40.0 mm above burner, 3.0 mm outside burner edge (~) 350.0 mm above burner, axially above flame.

3.0~

~

2-O-1.O-

~o ,~,oY/4

,o

,o

f

3.0

"0

RADIAL DISTANCE FROM BURNEREDGE (ram)

FIG. 3. Concentration and temperature measurements for the flame of Fig. 2 at a sampling pressure of 20 mbar.

FORMATION OF NO2 BY LAMINAR FLAMES were estimated from Polaroid photographs taken of the flame prior to sampling. Several important observations may be made: (i) The results support previous conclusions that NO 2 is absent in the hot gases above the primary reaction zone. (ii) NO~ initially appeared at the base of the flame and along the flame periphery, at temperatures below about 1400 K. (iii) Moving radially outwards the N O J NO~ ratio was seen to increase although the actual concentrations were reduced by dilution. (iv) NO2 concentrations increase with height along a given temperature contour. 4.1.3 High Pressure Sampling The pressure in the probe was increased to 200 mbar, and the sampling measurements repeated. Figure 4 shows the results of sampling at this elevated pressure. The outstanding feature evident was that in a localised area immediately above the burner at the flame edge, total conversion of NO to NO2 was observed. The temperature associated with this observation was generally less than 1000 K. At low probe pressure conversions of the order of 10-25% had been observed in this region. Travel-

25"0t

7-0-6-0-P- 5 , 0 -

~ 4.0-

2

IE ~ 3.0-

2-0-

137

ling further away from the base along the flame periphery, the actual ppm NO~ concentrations increased but the conversion steadily dropped. However, high conversion levels (>70%) were still obtained. The results of seeding the flame with cylinder NO are presented in Table II. Again the sample probe pressure was maintained at 200 mbar. The same general features were apparent, in particular the complete conversion of large amounts of added NO to NO2 in the cool base region of the flame. 4.2 H~O~Concentrations in the Flame Gases Concentration profiles of H202 were measured at the edge of the flame for heights above the burner of 1 and 5 mm. The fuel stoichiometry of the flame was 1.22. The results are shown in Fig. 5(a) and (b) for sampling pressures within the probe of 17, 80, and 170 mbar absolute. The results are plotted as mole fractions (XH2o2)against flame gas temperature to allow a direct comparison between different positions within the flame. An indication is also given of the horizontal position to which the flame gas temperature corresponds. The temperature profile measured by the sampling probe differed slightly from the thermocouple measurements, due to the tendency of the probe to "sample forward." In other words, the sample withdrawn by the probe corresponded to a region slightly in advance of the probe orifice. In some cases, the sampling region had a horizontal cross-section of about 2 mm diameter. Thus it is possible that some effects may have been caused by sample gases from regions at different temperatures being mixed within the probe. Although the results in Fig. 5 show some scatter, there appears to be a definite dependence of XH2o2 on sample pressure, suggesting formation within the probe. For a sampling pressure of 17 mbar and a height of 5 mm above the burner, little or no H202 was found below a temperature of 1000 K. For a sampling pressure of 80 mbar at the same height, H202 was observed down to about 500 K. The results at 1 mm above the burner are similar, except that H202 formation is shifted to still lower temperatures, implying a greater degree of disequilibrium in this region. The question of the relative proportions of H202 formed in the flame and in the probe is discussed in section 5.

1.0-

5. Discussion 2'.o ~',4,~4///2~

,.o

2.0

I

3.0

,'.o

RADIALDISTANCE FROM BURNEREDGE(mm) FIG. 4. Concentration and temperature measurements for the flame of Fig. 2 at a sampling pressure of 200 mbar.

The aspects of the results that we wish to discuss are: (i) the probe sampling effects and their implications; (ii) the production of oxidizing species at the edge of the flame; and (iii) the mechanism for NO oxidation.

138

COMBUSTION GENERATED POLLUTION

TABLE II NO and NOa concentrations produced by adding cylinder nitric oxide to a m e t h a n e / a i r flame with 0 = 1.22 Sampling pressure = 200 mbar Sampling position (mm) Concentrations (ppm) Dist. i n / o u t s i d e rim

NO added (ppm)

NOx

1.0

1.0 inside

0 49.5 195.1

11.7 55.6 167.6

2.0

1.0 outside 49.5 98.5 195.1 290.0 473.3

13.0 28.9 41.9 67.2 88.1 128.6

Height

(i)

NO 2

NOz / NOx %

11.7 55.6 167.6

100 100 100

1.1 6.5 18.1 30.3 57.8

13.0 27.8 35.4 49.1 57.8 70.8

100 96 84 73 66 55

NO

4.0

1.0 outside

0 98.5 195.1 290.0 364.3

23.9 66.5 98.2 134.6 152.0

11.0 44.3 66.5 99.0 110.8

12.9 22,2 31,7 35,6 41.2

54 33 32 26 27

15.0

4.0 outside

0 52.0 103.4 195.1 290.0 401.0

36.4 55.4 74.4 101.3 131.4 164.7

23.0 35.6 45.9 69.7 91.8 110.8

13.4 19.8 28.5 31.6 39.6 53.8

37 36 34 31 30 32

25.0

4.0 outside

0 98.5 195.1 318.0

53.6 101.8 38.4 191.1

42.0 84.8 116.1 164.3

11.6 17.0 22.3 26.8

22 17 16 14

Probe Sampling Effects and their Implications

The dependence of NO 2 concentration on sampling pressure as shown in Fig. 2 provides further evidence that sampling anomalies can occur in the measurement of NOx. This supports the conclusions of Kramlich and Malte (4) and of Johnson et al. (5) Our results also indicate the flame regions where such effects are most serious. The probe effects may be minimised by sampling at low pressures. This will both reduce the oxidation rate of NO, and decrease the residence time in the probe. The variation of NO 2 concentration with sampling pressure may be taken to provide a measure of the oxidizing species in the flame. From Fig. 2, such effects are greatest at the base of the flame (curves 1 and 2), but are still significant at the flame edge over half-way up the flame (curve 4). It seems likely

that NO oxidation rates in the flame would vary in similar manner, providing that the temperature is low enough for NO2 to be stable. The data in Fig. 2 can be replotted as in Fig. 6 to fit the curve In (NO/NOx) = A - B (Sampling Pressure) 2 where A and B are constants for each flame position. Although other explanations may be possible, this behaviour agrees with a simple model where: (i) NO is oxidized in the sampling probe by bimolecular reaction; (ii) time available for oxidation is directly proportional to sampling pressure; (iii) the oxidizing species is not significantly depleted (i.e. it is present in large excess or is regenerated after the oxidation). Slight deviations are found from this model for positions 3 and 4, suggesting that for

FORMATION OF NO n BY LAMINAR FLAMES 1C A

8

~n 4 .l

~o o&

9

IIJ

0 .J lak

Z

~n- 10

(a) I I I 500 1000 1500 TEMPERATURE OF SAMPLED GASES (K) J I I I I 2"0 1 "O O RADIAL DISTANCE FROM BURNER EDGE(mm)

8 8 o

o"

os

o o o

z

oz2 o

~0

0 (b) o~ o . d" I 500 1000 150o TEMPERATURE OF SAMPLED GASES (K)

I

I

I

I

I

3-0 2-0 1.0 RADIAL DISTANCE FROM BURNER EDGE (mm)

F1c. 5. HnO n concentrations plotted against flame gas temperature near the edge of the flame from Fig. 2 (a) 1 mm (b) 5 mm above the burner. The symbols represent probe sampling pressures; 9 17 mbar; O 80 mbar; A 170 mbar (absolute).

o,o I

I

L

I

(PROBE SAMPLING PRESSURE)2-BARS2

I

Flc. 6. Data from Figure 2 replotted to show dependence of ln(NO/NO~) on (Sampling Pressure)2. The symbols and numbers on the lines are the same as in Fig. 2.

139

positions high in the flame some depletion of oxidizing species does in fact occur. This behaviour agrees well with the results in Table II, where positions close to the base of the flame showed a high potential for oxidation of NO. There is little doubt that probe effects also arise when sampling for H202. The results in Fig. 5 show a similar pressure dependence to that observed in NO~ measurements. The question then arises: what proportions of the observed H202 are formed in the flame and in the probe? In fact, one may ask whether H20~ would exist at all in the hot regions of the flame (c.f. discussion of NO2). To answer these questions, a comparison with the data of Kramlich and Malte ~'> is useful. Their computer simulation of reactions within the probe showed that about 8 ppm of H202 should be formed in the probe when sampling from a fuel-lean flame at 1660 K. The mechanism was apparently OH recombination. Although our conditions were somewhat different, it is interesting that we found similar concentrations of HnO2 by probe sampling from a flame region at this temperature. At first sight this would imply that our HnOn was probe-formed, and that our OH concentrations were also similar to those of Kramlich and Malte. However in this case, with an OH concentration of ca. 0.3%, a steady state analysis shows that about 2.5 ppm of H202 should be present in the flame itself at 1660 K. (A short extrapolation of the kinetic data was needed beyond 1500 K). While such a calculation cannot be regarded as accurate, it does suggest that a small proportion of the measured HnOn was present in the flame. In the cool regions around the flame, the argument is somewhat different. The recombination of OH to give H20 n is very fast, with equilibration time comparable with diffusion times in the flame. Thus if OH radicals can form H~O2 when cooled in the probe, they can react in similar manner when cooled by diffusion into the cold secondary air. It i s also possible for HnOn to be formed in the flame by disproportionation of HOn, as discussed in section (iii). I n other words, most of the HnOn observed at low pressure when sampling from the cool outer regions of the flame was probably formed in the flame rather than the probe. However, for the present purposes, the origin of the H~O~ is of secondary importance. The important point is that a cool region which yields significant H~On whether formed in the flame or the probe, has the potential for rapid oxidation of NO to NO n. (ii) Production of Oxidizing Species at Flame

Edge The results of spectroscopic and microprobe experiments show that NO~ is produced by laminar flames. The probe measurements further show that it is first observed at the flame periphery, just outside

140

COMBUSTION GENERATED POLLUTION

the secondary reaction zone, where the temperature is less than about 1400 K. The three body reaction NO + NO + 0 2 ~ 2NO2

(1)

is far too slow to explain these results. Previous reports of NO, associated with flames have referred to turbulent or quenched flames. In these cases the mechanism converting NO to NO2 has been assumed to involve the HO, radical NO + HO 2 --->NO~ + OH

(2)

The HO 2is assumed to be formed in regions where high concentration and temperature gradients cause diffusion of radicals into cool boundary layers or eddies where they react. We suggest that the same mechanism applies at the periphery of a simple laminar flame. In particular, the measurements taken in the cool area just above the burner rim point to this region as having great potential for production of oxidizing species. There are three pieces of evidence which demonstrate this point. The large probe pressure dependence of the N O J N O x ratio (shown in Fig. 2) when sampling from the region outside the flame shows that there are above-equilibrium concentrations of reactive species in this relatively cool region, as suggested in section 5(i). The effect is maximised just above the burner rim where almost 100% conversion is observed with a probe pressure of 140 mbar. This is reduced to 25% when the pressure is reduced to 20 mbar. The influence of probe sampling pressure is observed to decrease when sampling from further downstream and finally ceases altogether when sampling from the burnt gases above the flame. These observations are reinforced by the results obtained when NO was artificially added to the flame (Table II). At a position 1 mm above the burner, 170 ppm of NO was fully converted to NO~. At greater distances from the burner, the degree of conversion was less. Although the conversion occurred in the probe, this suggests the presence of comparatively high levels of reactive oxidizing species in this region. The third piece of evidence comes from the measurements of H~O2 at the edge of the flame. The highest concentrations for gas temperatures below 1200 K occur close to the burner rim. Taken together, these observations support the view that the region just above the burner rim is a powerful source of oxidizing species. A mechanism for such formation can be argued as follows: In premixed flames, the highest radical concentrations are found in the primary reaction zone. They then decay towards equilibrium within a few millimetres downstream. Passage of the hot gases through a secondary reaction zone can increase the radical

concentrations, but not to the extent found in the primary reaction zone. The greatest departures from equilibrium will therefore occur at the base of the flame, where primary and secondary reaction zones overlap and are adjacent to cold secondary air. The steep gradients of concentration and temperature in this region will cause rapid diffusion of H and other free radicals into the cold secondary air, to yield conditions necessary for oxidant formation. At greater heights above the burner, such effects (although still present) will be reduced by the sheath of hot products separating the flame from the secondary air. (iii) Mechanism for NO e Formation If we take the base of the flame as a region where oxidative conditions prevail, then a mechanism for NO2 formation can be proposed. This may be extended to other regions of the flame where, although conditions may be less favourable, NO oxidation is still able to occur. The various flame regions will then contribute to NO~ formation in proportions which depend on both their rate of reaction and physical size. H atoms from the primary and secondary reaction zones diffuse out into the cool air surrounding the flame just above the burner lip. At high temperatures the H atoms react via H+O~OH+O

(3)

but at lower temperatures H + O~ + M ~ H O , + M

(4)

is the dominant reaction. HO2 is also destroyed by H atom attack H + HOs --~ OH + OH

(5a)

H + HO2---> H 2 + 02

(5b)

The combination of reactions 4 and 5 sets a ceiling to the possible HO, concentrations, no matter how great the supply of H atoms may be. For a mole fraction of 02 = 0.1, the ceiling is ca. 20 ppm HO 2 at 1500 K, 50 ppm at 1000 K, and 400 ppm at 500 K. Reactions of HO 2 with itself or with other species such as OH will ensure that actual concentrations are less than the above maxima. Once formed in an environment of cool air mixed with combustion products the HO, radical is not very reactive. Reactions with bulk species (e.g. H 2, CO, O~, etc.) will be slow under these conditions. However, it does react rapidly with itself and with NO (reactions 2 and 6). HO 2 + HO 2--* H20~ + O,

(6)

FORMATION OF NO 2 BY LAMINAR FLAMES Reaction 6 forms H202 which is stable at low temperatures, but at higher temperatures may react further via H202 + M ---)OH + OH + M

(7)

H202 + OH --~ HO2 + H20

(8)

thereby regenerating an HO 2 radical that can then react with NO to form NO 2 via reaction 2. The operation of this cycle (reactions 6-8) will remove HOz radicals. The products of NO oxidation by reaction 2 include OH, which may react further to give 1/3(H + 02 + H20 ) by reactions 9 and 10. Alternatively, it may react with any CO or H 2 present to regenerate a H atom and thus form HO~ to continue the cycle. OH + OH--) O + H20

(9)

O + O H ~ H + O2

(10)

OH + CO(H,)---..~ H + C02(H~O)

(11)

or

If any unburnt fuel is present, reaction with OH and subsequently 02 will give alkyl peroxy radicals, which also are efficient at oxidizing NO to NO v The above mechanism (reaction 1-11) was tested by a simple computer simulation which examined the chemical model at fixed temperatures, Other flame processes were ignored. The calculations showed that with reasonable concentrations of H and OH, high rates of conversion of NO to NO~ could occur via the sequence of reactions above. For example at 900 K, with initial H and OH Concentrations of 1 ppm and zero respectively and a CO level of 1%, 25% of the NO was converted to NO 2 in 1 millisecond. In evaluating the above mechanism, it is useful to consider the distribution of NO2 around the flame, as found by low pressure sampling (Fig. 3). (NOJNOx) ratios of up to 25% are found for flame temperatures ca. 1400 K. For this temperature the equilibrium ratio should be 0.5%. But of course, equilibrium conditions do not prevail. The concentrations of NO2 would be governed by formation by HO2 (reaction 2) and destruction by radicals such as H, O, and OH. Thus significant flame-formed NO 2 would be expected in O2-rich conditions up to the temperature where HO 2 dissociates into H + 02, i.e. up to about 1500 K. A second consideration is that maximum concentrations of NO2 increase with height above the burner, implying that conversion occurs all along the flame edge into the burnt gas region. This is illustrated in Fig. 2 where at low sampling pressure the highest (NOJNOx) ratios are found above the flame. Thus although the gases from the base of the flame can oxidize large proportions of

141

NO under appropriate conditions, this potential is not fully realised. The unresolved query is whether the NO oxidation is limited by residence time considerations or by back reactions destroying the NOv

6. Conclusions 1. Under sampling conditions which minimise probe effects, no NO 2 was observed in the hot regions of the flame (>1500 K). 2. NO is oxidized at the edge of the flame, presumably by HO 2 formed by H and other free radicals diffusing out into the secondary air. The oxidation rate is most rapid near the base of the flame, though the maximum degree of conversion in this region appears to be limited by short residence time or back reactions. 3. NO 2 is observed in O2-rich flame gases at temperatures up to 1400 K, in concentrations much greater than expected from equilibration of the reaction 2NO + 02 ~ 2NOv This temperature of 1400 K agrees well with the temperature limit expected for HO 2 formation (1500 K). Acknowledgments We are grateful to K. Bradbury and M. Attwood who performed part of the experimental measurements, and to P. Oliver for his assistance with the computer simulation. We also wish to thank British Gas for permission to publish this paper.

REFERENCES 1. HILLIARD, J. C., AND WHEELER, R. W. " N O 2 in

Engine Exhausts." Ricardo and Co. Rept. DP 18461, 1974. 2. JOHNSON,G. M., AND SMITH, M. Y., Comb. Sci. Technol. 19, 67 (1978). 3. CERNANSKY, N. P., AND SAWYER, R. F., Fifteenth Symposium (International) on Combustion, p1039, The Combustion Institute, 1975. 4. KSAMUCH,J. C., AND MALTE, P. C., Comb. Sci. Technol. 18, 91 (1978). 5. JOHNSON, G. M., SMITH, M. Y., AND MULCAHY,

6.

7.

8. 9.

M. F. R., Seventeenth Symposium (International) on Combustion, p647, The Combustion Institute, 1979. MERRYMAN, E. L., AND LEVY, A., Fifteenth Symposium (International) on Combustion, p1073, The Combustion Institute, 1975. ROSENBERG,R. B., AND LARSON, D. H., AGA Basic Research Symposium, March 1967. ALLEN,J. D., Combust. Flame 24, 133 (1975). FONTIJN,A., SABADELL,A. J., AND RONCO, R. J., Analyt. Chem. 42, 575 (1970).

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10. SlcsBv JR., J. E., BLACK, F. M., BELLAR, T. A., AND KLOSTER~^N, D. L., Environ. Science & Technol. 7, 51 (1973). 11. Pmz, W., ANDJoHANN,I. "'Determining very small amounts of H202 in the air." Int. J. Environ. Analyt. Chem. (1974) 3 (4) p257-270 (In German: SMRE Translation No. 6811).

12. BaADBtrR~, K., ROPEI~, F. G., AND SMITh, D. B. To be published. 13. KASKAN,W. E., Sixth Symposium (International) on Combustion, p134, Reinhold, 1957. 14. FRISTROM,R. M., AND WESrENBERC,A. A., Flame Structure, McGraw-Hill, 1965.

COMMENTS P. C. Malte, University of Washington, USA. You have presented a careful examination of NO 2 formation in flames, which provides additional confirmation of the trends abserved by several persons previously. However, the central question remains unanswered: Does the NO + HOa chemistry occur within the combustion field or within the probe? Could you please comment upon your optical measurements for NO and NO 2 in this respect. Author's Reply. We agree that it is not possible completely to disentangle probe effects from flame processes. There is as yet no complete analysis of the behaviour of aerodynamically quenched p r o b e s - - i n particular to what degree the gases are re-heated and re-compressed by shock waves and boundary layer effects as the supersonic flow returns to sub-sonic (1). Without such informatin, any analysis of reactions in the probe must be incomplete. However, there are three reasons why we believe probe effects do not negate our main conclusions regarding NO 2 in flames: 1) The linear plots in Figure 6 of the paper suggest that the results for hypothetical zero pressure samples would differ little from what was observed at our lowest sampling pressure. There is the possibility that reactions could occur in the initial region of supersonic flow, where conditions are independent of sampling pressure. However, residence times in this region are very short (about 1 Ixsec) and

the pressure drop rapid (thereby reducing reaction rates). So we do not think this can be very significant. 2) Calculations carried out since the paper was submitted show that conditions arising at the flame edge can lead to NO 2 concentrations compatible with our measured levels. The reaction times for NO~ to reach steady state conditions are short compared with times needed for concentration and temperature changes to occur by diffusion and other processes at the flame edge. 3) The clearest evidence that NOa is not all probe-formed comes from the spectroscopic study. These results show unequivocally that rapid NOa formation is associated with laminar flames. Absorption spectroscopy cannot provide detailed information on the spatial distribution of NO and NO 2 in and around the flame. Another diagnostic technique is needed for t h i s - hence our probe study. Taken together, we believe these factors suggest fairly strongly that our measured NOz concentrations are close to those existing in the flame.

REFERENCE 1) SnAeIRO, A. H.: The Dynamics and Thermodynamics of Compressible Fluid Flow, Ronald, 1954.