The Chemistry of Combustion Processes - ACS Publications

6 Formation of NO and N from NH3 in Flames 2

R I C H A R D J . BLINT and C A M E R O N J . D A S C H

Downloaded by MONASH UNIV on June 12, 2016 | http://pubs.acs.org Publication Date: April 16, 1983 | doi: 10.1021/bk-1983-0249.ch006

Physics Department, General Motors Research Laboratories, Warren, MI 48090

Ammonia oxidation at high temperatures is an interesting kinetic system of significant technological importance for NO formation and destruction. Recent work has largely been devoted to the moderate temperature (1300K) region in which NH can quantitatively destroy NO. This report describes flame studies (1800-2800 K) of NH oxi dation in which NH is a model compound for the conver sion of fuel-bound nitrogen to NO and N . A detailed (42 reactions) reaction scheme has been constructed from literature rate constants which predicts our measured flame speeds, major species profiles, and NO levels in ammonia-oxygen-diluent flames. These predictions require that the radical pool size and the N /NO branch ing can be correctly described. This mechanism has been further tested against our measurements of NO emissions from CH -air flames doped with NH andNO.WhileinNH3 flames NH is pivotal, the Fenimore-type "loading" experiments indicate a more prominent role for Ν atoms and the Zeldovitch reactions in hydrocarbon flames. In part, this is a result of the higher radical concentra tions in hydrocarbon flames. Full reaction mechanisms will be necessary to predict the relative conversion of fuel bound nitrogen to NO and N . 3

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The i n t e r a c t i v e k i n e t i c s of NH and NO at high temperatures have r e c e i v e d a great d e a l of a t t e n t i o n i n recent y e a r s . Much of t h i s a t t e n t i o n o r i g i n a t e d from the work of Fenimore who i n v e s t i g a t e d the conversion of f u e l bound n i t r o g e n to NO i n many flames. He found two important p r o p e r t i e s of the NO p r o d u c t i o n : 1) the y i e l d of NO was independent of the f u e l - n i t r o g e n type and 2) the NO tended to be s e l f l i m i t i n g such that a s a t u r a t i o n v a l u e of NO could not be exceeded. 3

Fenimore (1)

formulated a schematic,

two step k i n e t i c

0097-6156/84/0249-0087S06.00/0 © 1984 American Chemical Society Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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r e p r e s e n t a t i o n which d e s c r i b e d t h i s s a t u r a t i o n e f f e c t . The model assumes that a l l the f u e l n i t r o g e n passes through a r e a c t i v e , amine intermediate N * . The f i r s t step i s a general o x i d a t i o n r e a c t i o n which forms NO k* Ν* + Ox -> NO + products and the second step destroys both NO and the NO precursor k*


Ν* + NO

+ N

2

+ products

The NO y i e l d , [ N O ] / [ f u e l - N ] , i s a f u n c t i o n o n l y of [ N O ] s a t s k î [ O x ] / k ; . T h i s formulation f o r the f u e l - n i t r o g e n conver s i o n to NO has been f u r t h e r t e s t e d by Haynes et a l . ( 2 , 3 ) and by Fenimore ( 4 , 5 ) . The e s s e n t i a l feature of t h i s formulation i s the second r e d u c t i o n step. T h i s r e a c t i o n had p r e v i o u s l y been recognized as important from the f a s t r a t e of decay of NO produced i n ammonia flames (6,7). T h i s r e a c t i o n process has been e x p l o i t e d i n the Exxon DeNOx process for NO removal by the c o n t r o l l e d a d d i t i o n of NH to combustion e f f l u e n t s . 3

The simple phenomenology and l a r g e economic importance f o r f u e l n i t r o g e n conversion has encouraged many i n v e s t i g a t o r s to t r y to i d e n t i f y the N* intermediate and i t s r e a c t i o n r a t e s k j and k ? . Neither Kaskan and Hughes (8) or Fenimore (3,4) could c o n c l u s i v e l y demonstrate e i t h e r N H , NH, or Ν as being e x c l u s i v e l y p i v o t a l . D i r e c t measurements of the NH + NO r e a c t i o n r a t e have shown i t to be f a s t (9,10) and important, e s p e c i a l l y at lower temperatures such as 1300K where the DeNOx process i s most e f f i c i e n t . T h i s lower temperature regime i s w e l l understood as a r e s u l t of e x p e r i ments and extensive k i n e t i c modeling (11,12). 2

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Our approach to the f u e l n i t r o g e n conversion to NO problem has been to examine the k i n e t i c s of NH as a model compound u s i n g d e t a i l e d flame c a l c u l a t i o n s with modelable flame experiments. T h i s work has emphasized the major flame p r o p e r t i e s i n c l u d i n g the flame speed, temperature and major species s p a t i a l p r o f i l e s , and the post-flame NO c o n c e n t r a t i o n . These measurements and c a l c u l a t i o n s a r e performed on s t a t i o n a r y f r e e flames. 3

In t h i s paper we f i r s t summarize the a b i l i t y of the NH k i n e t i c mechanism of Dasch and B l i n t (13) to d e s c r i b e the major p r o p e r t i e s of ammonia-oxygen-diluent flames. T h i s mechanism i s constructed s o l e l y from l i t e r a t u r e r a t e constants and i s s p e c i f i c a l l y v a l i d o n l y f o r flames leaner than 0=1.2 Having v a l i d a t e d the ammonia mechanism, we then i n v e s t i g a t e the y i e l d of NO from methane-air flames which have been doped with NH and NO. The NH doped 3

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Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Formation of NO and Ν from 2

NH

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methane flames are a t y p i c a l model system for f u e l - n i t r o g e n NO production. The k i n e t i c modeling(14) of t h i s complete n i t r o g e n and hydrocarbon flame system and i t s success represents a major advance i n the f u l l r e p r e s e n t a t i o n of the f u e l - n i t r o g e n problem. This work r e l i e s on the CH mechanism of Warnatz(15) which has been t e s t e d against many flame p r o p e r t i e s i n c l u d i n g d e t a i l e d temp erature species p r o f i l e s ( 1 6 ) . There are no C-N c o u p l i n g r e a c t i o n s between the C H and NH mechanisms although these r e a c t i o n s are known to be important for the p r o d u c t i o n of "prompt NO" i n r i c h flames (0>1.2). 4


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M i l l e r et a l . (17) have performed s i m i l a r c a l c u l a t i o n s f o r the ammonia flame experiments of Fenimore and Jones (6) and Maclean and Wagner (7). Dean and coworkers ( t h i s Symposium) have a l s o p e r formed experiments and c a l c u l a t i o n s with an emphasis on r a d i c a l intermediates. These c a l c u l a t i o n s show that i n ammonia flames NH i s m a r g i n a l l y more important than NH and N. Furthermore, the important oxidant changes with s t o i c h i o m e t r y . Mixed CH -NH -N0 flames show even a wider v a r i a t i o n i n the r e l a t i v e r o l e of N H , NH, and N. In near s t o i c h i o m e t r i c NO doped methane flames, of course, the Z e l d o v i t c h mechanism i n v o l v i n g Ν i s e x c l u s i v e l y important. S u r p r i s i n g l y , Ν i s a l s o most important i n the NH doped C H flames. This i s a con sequence of the l a r g e r r a d i c a l p o o l i n hydrocarbon flames than NH flames. Under both these extremes most of the doped n i t r o g e n appears as NO. Under combined NH and NO doping the flames are somewhat more s i m i l a r to ammonia flames i n which NH i s important for the r e d u c t i o n of NO to N . These combined NH -N0 doping experiments e x e r c i s e more r i g o r o u s l y the r e d u c t i v e p a r t s of the mechanism than the Fenimore-type NH doping experiments. 2

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It appears that the q u a n t i t a t i v e p r e d i c t i o n of Fenimore's simple [NO]sat parameter w i l l r e q u i r e a l a r g e k i n e t i c mechanism. The combined NH and NO doping experiments t e s t Fenimore*s two step mechanism i n a new way, but i t i s found that the s i n g l e [NO]sat parameter can s t i l l c o r r e l a t e the r e s u l t s ( 1 4 ) . 3

Experiments The experimental methods are e x t e n s i v e l y d e s c r i b e d i n e a r l i e r papers (13,14,16). Temperature and major species p r o f i l e s were determined by spontaneous Raman spectroscopy i n f r e e flames s t a b i l i z e d on a s l o t burner. Flame speeds were determined by p a r t i c l e tracks on the same burner and by the Guoy method on c o n i c a l flames. T o t a l NOx measurements were performed i n the postflame region of a f l a t , water cooled, Meker burner. On the f l a t burner gas flows were adjusted to give a s l i g h t l y wrinkled flame c o r r e sponding to f r e e flame c o n d i t i o n s . Gas samples are extracted with a water c o o l e d , quartz microprobe analyzed with a chemiluminescent


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analyzer. Temperatures measured simultaneously with a c o r r e c t e d thermocouple agree w e l l with c a l c u l a t e d a d i a b a t i c flame tempera tures. The temperatures were constant f o r at l e a s t 5 cm above the burner. The NOx concentrations t y p i c a l l y v a r i e d l e s s than 10% w i t h i n 1.0 cm of the flame f r o n t and then were constant f o r at l e a s t 5 cm. The systematic u n c e r t a i n l y of the NOx measurements i s ±10%.


Computational Method The experimental observables f o r the f r e e flames i n t h i s study were modeled using the GMR flame program(18). T h i s program solves the unsteady s t a t e species and enthalpy conservation equations and allows them to progress i n time u n t i l the steady flame s o l u t i o n i s o b t a i n e d . Boundary c o n d i t i o n s f o r f r e e flames were used. Three types of information are r e q u i r e d f o r these c a l c u l a t i o n s : 1) the r e a c t i o n mechanism, 2) the t r a n s p o r t p r o p e r t i e s and 3) the thermo dynamic p r o p e r t i e s of a l l the s p e c i e s . The r e a c t i o n mechanism (given i n Ref. 16) i s constructed from l i t e r a t u r e v a l u e s . The 10 H-0 r e a c t i o n s are w e l l e s t a b l i s h e d . The 32 a d d i t i o n a l amine and NO r e a c t i o n s are l a r g e l y taken from Ref. 19, 9, and o t h e r s . Although NHi-NHj hydrazine r e a c t i o n s were t e s t e d , they were not important for these near s t o i c h i o m e t r i c flames and were not r e t a i n e d i n the mechanism. Selected values are taken f o r r e a c t i o n s which are r a d i c a l branching or termination r e a c t i o n s and hence e s p e c i a l l y important. The 65 a d d i t i o n a l r e a c t i o n s of the CH mechanism are from Warnatz(20). There are no C-N c o u p l i n g r e a c t i o n s between the NH and C H mechanisms, although i t i s known that these are important i n r i c h e r flames. The d i f f u s i o n and con d u c t i o n terms i n the flame program a r e c a l c u l a t e d from Stockmayer p o t e n t i a l parameters i n the F i c k ' s Law l e v e l of approximation (21). The thermodynamics are taken from the JANAF c o m p i l a t i o n up through the 1978 r e v i s i o n . The heat of formation f o r NH was taken as 84.7 kcal/mole (22). 4

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To t e s t the r e a c t i o n mechanism pure ammonia-oxygen-diluent flames were s t u d i e d under a wide range of s t o i c h i o m e t r i e s and d i l u t i o n s . Peterson and Laurendeau(23) had p r e v i o u s l y studied NH doped H flames which e l u c i d a t e much of the NHi o x i d a t i o n chemistry. The present experiments probe the same r e a c t i o n s and a l s o r e a c t i o n s between nitrogeneous s p e c i e s . Flame speeds were f i r s t considered i n order to e s t a b l i s h a subset of l i t e r a t u r e r e a c t i o n r a t e s which would d e s c r i b e t h i s major flame f e a t u r e . Comparison of the other flame features confirm that the flame speed i s a good t e s t bed for the r e a c t i o n mechanism. 3

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The d i f f i c u l t y and the value of e v a l u a t i n g a r e a c t i o n mechanism based on flame speed over such a l a r g e range of d i l u t i o n s and


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Formation of NO and Ν2 from NH3

BLINT AND DASCH

equivalence r a t i o s i s that r e a c t i o n e f f e c t s at a p a r t i c u l a r temp e r a t u r e range or flame c o n d i t i o n could be inadequate f o r those at another. These flame speed data occur over the a d i a b a t i c flame temperature range of 2270 to 2900 K .


Flame Speeds. O v e r a l l , the c a l c u l a t e d flame speeds agree w e l l with experiment. Flame speed c a l c u l a t i o n s as a f u n c t i o n of d i l uent c o n c e n t r a t i o n with 0=0.8 (the peak of the s t o i c h i o m e t r i c dependence) a r e shown i n F i g u r e 1. The c a l c u l a t e d flame speeds are l e s s temperature dependent than experiment. Considering the s t o i c h i o m e t r i c dependence of the flame speeds, the c a l c u l a t i o n s of flame speed a r e i n greater e r r o r away from the peak (0=0.8) and exceed the experimental e r r o r a t both r i c h and lean l i m i t s . Flame P r o f i l e s Major species and temperature p r o f i l e s p r o v i d e a d e t a i l e d d e s c r i p t i o n of the s t r u c t u r e of these flames. Three flames a t i n i t i a l 0 to N r a t i o s of 0.58 were probed using l a s e r Raman spectroscopy, the equivalence r a t i o s were 0 . 7 , 1.0 and 1.29. The c a l c u l a t e d and measured temperature p r o f i l e s f o r the 0=0.7 flame (given i n F i g u r e 2) shows good agreement. S i m i l a r agreement i s found at 0=1.0 and 1.27. F i g u r e 2 a l s o gives the oxidant decay p r o f i l e which i s another measure of the flame w i d t h . Since the r a t i o of 0 to N i s being measured, d i f f e r e n c e s i n the c a l c u l a t e d and the experimental species p r o f i l e s a r e due t o d i f ference e i t h e r i n the decay r a t e of the oxygen or i n the growth r a t e of the n i t r o g e n . For the flames measured, the 0 / N r a t i o tends t o drop somewhat more q u i c k l y than the c a l c u l a t i o n s . In the leaner two flames the H t o N p r o f i l e reaches a maximum a t about 1800K, w e l l before the temperature maximum. The c a l c u l a t i o n s reproduce the s p a t i a l d i s t r i b u t i o n and s t o i c h i o m e t r i c trends of the H p r o f i l e but a r e a f a c t o r of 3-10 low i n magnitude. While the c a l c u l a t e d flame speeds show d e v i a t i o n s as a f u n c t i o n of equivalence r a t i o , the flame widths seem to be uniformly w e l l described. 2

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Nitrogen Oxide C o n c e n t r a t i o n s . The major o b j e c t i v e of t h i s work i s the understanding and p r e d i c t i o n of the NO c o n c e n t r a t i o n s . The equivalence r a t i o dependence of the NO i s given i n F i g u r e 3. The experimental and c a l c u l a t e d values a r e a l s o compared with the e q u i l i b r i u m NO concentrations f o r the corresponding a d i a b a t i c flame. As has been p r e v i o u s l y observed the NO decreases with i n c r e a s i n g equivalence r a t i o , e x h i b i t i n g a s l i g h t maximum at 0 . 7 . as has been p r e v i o u s l y observed. As seen, the NO i s always "super- e q u i l i b r i u m " and more so i n r i c h flames. G e n e r a l l y the measured and c a l c u l a t e d NO concentrations are the same except f o r very r i c h flames. The c a l c u l a t i o n s a l s o reproduce the steep temp e r a t u r e dependence of the NO c o n c e n t r a t i o n . Not only does the NO c o n c e n t r a t i o n i n c r e a s e with flame temperature, i t i s p r o g r e s s i v e l y d r i v e n more " s u p e r - e q u i l i b r i u m " .


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F i g u r e 1. Experimental and c a l c u l a t e d flame speeds v s . i n i t i a l mole f r a c t i o n o f N i n ammonia-oxygenn i t r o g e n flames with 0= 0 . 8 . 2

™ 40200π 0.1 0.2 0.3 0.4 0.5 υ Ν2 MOLE FRACTION

Figure 2. Experimental and c a l c u l a t e d s p a t i a l p r o f i l e s o f tempera t u r e and 0 / N r a t i o f o r a Φ = 0.7 ammonia-oxygen-nitrogen flame. 9

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Formation of NO and Ν2 from NH3


Major Pathways and Reactions E f f e c t s . As seen above the experimental data (flame speeds, flame widths and NO concentra t i o n s ) a r e w e l l d e s c r i b e d by the k i n e t i c mechanism. A schematic of the whole k i n e t i c c h a i n f o r the combustion of the nitrogeneous species i s shown i n F i g u r e 4. The r e l a t i v e i n f l u e n c e of r e a c t i o n s on a species are determined by a f l u x a n a l y s i s (13,15). T h i s i s a flame averaged a n a l y s i s , but i t t y p i c a l l y gives r e s u l t s s i m i l a r to a comparison of net r e a c t i o n rates i n the r e a c t i o n zone. To f u r ther t e s t the e f f e c t of the s p e c i f i c r e a c t i o n changes we r e c a l c u l a t e s e l e c t e d flames. One important c o n c l u s i o n from such analyses i s that most interme d i a t e species are i n steady s t a t e or " k i n e t i c a l l y l i m i t e d " . This c o n d i t i o n a r i s e s from v e r y f a s t r a d i c a l r e a c t i o n s . These lead to t i g h t k i n e t i c c o u p l i n g and chemical r e d i s t r i b u t i o n among the intermediates much more q u i c k l y than changes due to advection or diffusion. The consumption r e a c t i o n s f o r i n d i v i d u a l species w i l l vary the concentration to match i t s p r o d u c t i o n r a t e . In the flames studied these r a t e s never d i f f e r by more than 20% f o r a l l the r a d i c a l s (H, 0, OH, H 0 , HNO, N H , NH, N) comprising the "rad i c a l p o o l " . The k i n e t i c a l l y l i m i t e d s t a t e allows some s i m p l i f y i n g r e l a t i o n s between the intermediates to be expressed, and these have been discussed by B l i n t and Dasch(13) f o r a number of the nitrogenous i n t e r m e d i a t e s . However, t h i s s t a t e a l s o implies that the s i z e of the intermediate pool w i l l depend on the d i f f e r e n c e between c h a i n branching and terminating r e a c t i o n s . These r e a c t i o n s can be q u i t e slow r e l a t i v e to the c h a i n propagating r e a c t i o n s but have a l a r g e r i n f l u e n c e s i n c e they c o n t r o l the r a d i c a l pool s i z e . These r e a c t i o n s are separately considered f o l l o w i n g the d e s c r i p t i o n of the primary pathways. 2

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In these flames over 95% of the n i t r o g e n atoms are combusted to N . Since N i s predominantly formed by r e a c t i o n s of NO with NHi, approximately h a l f the n i t r o g e n atoms are c y c l e d through NO while the other h a l f are o x i d i z e d no f u r t h e r than N H i . Since the amine r a d i c a l s c o n t r i b u t e to the N formation i n the rough r a t i o s of 3:1:1 f o r NH :NH:N, the primary pathway to N can be w r i t t e n by the two sequences shown i n heavy l i n e . These sequences account for about two t h i r d s of the NO and N f o r m a t i o n . While t h i s i n d i cates a p i v o t a l r o l e f o r N H , n e i t h e r NH nor Ν r e a c t i o n s with NO can be e l i m i n a t e d from the r e a c t i o n scheme. 2

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One of the dominant r e a c t i o n s i s NH with NO branching to the two sets of p r o d u c t s , 2

NH

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+ NO

N N

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+ Η + OH + H 0 2

(RD (R2)

As d i s c u s s e d i n Ref. 16 the branching r a t i o of 70% R l and 30% R2


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Figure 3. Experimental and c a l c u l a t e d maximum NO concentrations as a funct i o n of equivalence r a t i o f o r ammonia-oxygen-nitrogen flames with f i x e d i n i t i a l mole f r a c t i o n of N = 0.4.

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Figure 4. Schematic of the d e t a i l e d k i n e t i c mechanism f o r ammonia combustion.


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was used. To t e s t the e f f e c t of changing the branching r a t i o the flame speeds were r e c a l c u l a t e d u s i n g the branching r a t i o of 40/60 suggested by S i l v e r , e t . a l . ( 9 ) . For flames with i n i t i a l N d i l u t i o n s of 0.4 the flame speeds are slower and the dependence as a f u n c t i o n of equivalence r a t i o i s m o d i f i e d . The maximum i n the flame speed i s s h i f t e d r i c h e r than 0=0.8. The f l u x a n a l y s i s f o r r e a c t i o n s R l and 2 shows a r e d u c t i o n of about 30% f o r the f l u x of the p r o d u c t s . The flame width of the 0=0.5 N ( i n i t i a l ) = 0 . 4 flame increases by almost a f a c t o r of three with the change i n the branching r a t i o . C l e a r l y t h i s branching r a t i o has a l a r g e e f f e c t on the flame p r o p e r t i e s of ammonia flames. 2


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Another r e a c t i o n with a s i g n i f i c a n t e f f e c t on the flame p r o p e r t i e s is NH + 0

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•* HNO + 0

L i t t l e i s known about t h i s r e a c t i o n . We have used the v a l u e sug gested by Peterson and Laurendeau(23). The s e n s i t i v i t y was t e s t e d with the r a t e constant that Branch e t . a l . (12) developed f o r the lower temperatures of the "Thermal Denox" problem. Using t h i s smaller r a t e constant (at flame temperatures) on two flames (0=0.5 N ( i n i t i a l ) = 0 . 4 and 0=0.8 N ( i n i t i a l ) = 0 . 1 ) we f i n d 15-20% decreases i n the flame speed and 40-50% decreases i n the f l u x con t r i b u t i o n from t h i s r e a c t i o n . O v e r a l l the e f f e c t of reducing the NH + 0 r a t e i s to reduce the r a t e of p r o d u c t i o n of HNO i n each of the flames; no s i g n i f i c a n t v a r i a t i o n of t h i s e f f e c t with e q u i v a lence r a t i o was observed. Changing the products of the r e a c t i o n to NO and OH as suggested by B i n k l e y and Melius(24) gives r e s u l t s s i m i l a r to reducing the r a t e . With these products fewer r a d i c a l s are generated; hence the flame speed i s reduced by more than 20% and the flame widths thicken s l i g h t l y . 2

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NO Y i e l d i n Pure and Doped CH Flames 4

Having v a l i d a t e d the mechanism on ammonia-oxygen flames, the y i e l d of NO from n i t r o g e n doped C H - a i r flames was examined. Both NH and NO doping were i n v e s t i g a t e d . Only post-flame NO c o n c e n t r a t i o n s were measured. These are compared with c a l c u l a t i o n s of the f u l l k i n e t i c s and with a d i a b a t i c e q u i l i b r i u m c a l c u l a t i o n s . The c a l c u l a t e d p r o f i l e s i n d i c a t e the complexity of the NO dynamics i n these flames. The temperature and major species p r o f i l e s i n the undoped flames had been studied i n e a r l i e r work(16). Three near s t o i c h i o m e t r i c methane-air flames having i n i t i a l equivalence r a t i o s ( 0 ) of 0.8, 1.0 and 1.2 are d i l u t e d with l e s s than 5 volume percents of NH or NO. In t h i s s e c t i o n NO c o n c e n t r a t i o n is expressed both as a mole f r a c t i o n and as a f r a c t i o n of the t o t a l nitrogen concentration: 4

3

3

NO fraction=[N0]/([N0]+2[N ]+[NH ]+[NH ]+[NH]+[N]+ 2

3

2

[HN0]+2[N 0])


2

96

CHEMISTRY OF COMBUSTION

PROCESSES

T h i s approach removes any e f f e c t s due to changing numbers of molar species w i t h i n the flame. Pure Methane Flames. In a pure methane flames the NO f r a c t i o n undergoes a small jump passing through the flame f r o n t , then increases l i n e a r l y with t i m e - d i s t a n c e . This l i n e a r increase i s due to low NO c o n c e n t r a t i o n a f t e r the flame f r o n t (approximately 30 ppm i n a s t o i c h i o m e t r i c flame) which i s f a r l e s s than the e q u i l i b r i u m c o n c e n t r a t i o n of NO (3100 ppm). The reaction(R3) d r i v i n g the formation of NO i s p a r t of the Z e l d o v i t c h mechanism: 0 + N ·• NO + Ν Ν + 0 NO + 0 Ν + OH + NO + Η

(R3) (R4) (R5)

2


2

The reverse r e a c t i o n s are n e g l i g i b l e . The Z e l d o v i t c h mechanism i s r a t e l i m i t i n g i n these near s t o i c h i o m e t r i c flames having no added n i t r o g e n s p e c i e s . There i s no s i g n i f i c a n t NH or Ν p r o d u c t i o n . NO doped Methane Flames. Most of the NO passes through the flame unreacted, and the NO p r i m a r i l y a c t s as an i n e r t d i l u e n t . Dilut ing the flames with 1.5% NO causes the a d i a b a t i c flame tempera tures to drop by about 100 Κ and the c a l c u l a t e d flame speeds to decrease by about 10%. The NC f r a c t i o n decreases by l e s s than 10% through the flame f r o n t f o r these flames which were doped f a r above the e q u i l i b r i u m NO concentrations (420-3700 ppm depending on stoichiometry). In the experiments there was no post flame decay i n the NO concencentrations ( ± 5 % ) , while the c a l c u l a t i o n s do show some decay (N pathway i s r e s t r i c t e d to the high r a d i c a l region of the flame f r o n t . 2

2

NH Doped Methane Flames. While the a d d i t i o n of NO to a methanea i r flame a c t s p r i m a r i l y as a d i l u e n t , the a d d i t i o n of NH r i c h e n s the mixture l e a d i n g to competition f o r the a v a i l a b l e oxygen. This r i c h e n i n g e f f e c t a l s o reduces the e q u i l i b r i u m NO c o n c e n t r a t i o n of these near s t o i c h i o m e t r i c flames (O.8

The Chemistry of Combustion Processes - ACS Publications

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