The Chemistry of Combustion Processes - American Chemical Society

5 Nitrogen Chemistry in Flames Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 22, 2016 | http://pubs.acs.org Publication Date: April 16, 1983 | doi: 10.1021/bk-1983-0249.ch005

Observations and Detailed Kinetic Modeling A N T H O N Y M . D E A N , M A U - S O N G C H O U , and D A V I D S T E R N Corporate Research—Science Laboratories, Exxon Research and Engineering Company, Linden, NJ 07036

Spatially resolved concentration measurements of NH, NH2, NH3, NO and OH in atmospheric pressure ammonia flames are compared to predictions obtained with a one-dimensional flame algorithm using a detailed reaction mechanism. Several reactions were observed to be equilibrated, and this informa tion was used to obtain estimates of the oscillator strength for NH2 as well as the heat of formation of NH. Use of a conventional mechanism of ammonia oxidation predicted concentration profiles in marked disagreement with the observations. However, it was possible to obtain much more satisfactory fits to the data by including reactions between various NHi (i = 1-2) species to form N-N bonds; these adducts could then decompose to form ultimately N2. These good fits were obtained with rate constants estimated from unimolecular decomposition theory and used with no adjustments. Recent advances i n b o t h l a s e r d i a g n o s t i c i n s t r u m e n t a t i o n and computer m o d e l i n g a l g o r i t h m s have p r o v i d e d k i n e t i c i s t s w i t h e x c i t i n g new o p p o r t u n i t i e s f o r c h a r a c t e r i z a t i o n o f c o m p l e x chemi c a l s y s t e m s . I n t h i s p a p e r we d e s c r i b e o u r u s e o f t h e s e t o o l s t o e l u c i d a t e t h e k i n e t i c s o f n i t r o g e n s p e c i e s i n f l a m e s . Our e f f o r t s h a v e f o c u s e d upon ammonia o x i d a t i o n s i n c e i t c a n b e shown t o p l a y a k e y r o l e i n t e r m s o f NO p r o d u c t i o n i n f u e l - b o u n d n i t r o g e n f l a m e s ( 1 ) a s w e l l a s NO r e d u c t i o n v i a NH3 a d d i t i o n t o f l u e gas i n t h e T h e r m a l DeNO p r o c e s s ( 2 , 3 ) . A l t h o u g h t h e r e h a v e b e e n a h o s t o f p r e v i o u s s t u d i e s o f ammonia o x i d a t i o n i n b o t h s h o c k t u b e s (4) and f l a m e s ( 5 ) , c o n s i d e r a b l e a m b i g u i t y r e m a i n s w i t h r e s p e c t t o t h e d e t a i l s o f t h e mechanism. We h a v e a t t e m p t e d t o remove some o f t h e s e u n c e r t a i n t i e s b y m e a s u r i n g a b s o l u t e c o n c e n t r a t i o n s o f b o t h s t a b l e s p e c i e s and r e a c t i v e i n t e r m e d i a t e s t h r o u g h t h e f l a m e f r o n t r e g i o n and c o m p a r i n g t h e s e c o n c e n t r a t i o n - d i s t a n c e p r o f i l e s t o those o b t a i n e d by n u m e r i c a l l y x

0097-6156/84/0249-0071 $06.00/0 © 1984 American Chemical Society Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

72

CHEMISTRY OF COMBUSTION PROCESSES

s o l v i n g t h e 1-D f l a m e e q u a t i o n s . A d i s c u s s i o n o f t h e measurement t e c h n i q u e s i s f o l l o w e d by t h e m e c h a n i s t i c a n a l y s i s . P o r t i o n s of t h i s m a t e r i a l h a v e b e e n c o v e r e d i n more d e t a i l i n o u r r e c e n t papers (6-8).

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 22, 2016 | http://pubs.acs.org Publication Date: April 16, 1983 | doi: 10.1021/bk-1983-0249.ch005

Experimental

A r r a n g e m e n t and

Observations

F i g u r e 1 shows a s c h e m a t i c o f t h e a p p a r a t u s . T h i s has been d e s c r i b e d i n d e t a i l i n R e f s . 6 and 7; o n l y an o v e r v i e w i s p r e sented here. The 3.8 cm d i a m e t e r b u r n e r i s d e s i g n e d t o p r o d u c e a f l a t h o r i z o n t a l f l a m e f r o n t s o t h a t one c a n map o u t t h e f l a m e c h e m i s t r y by m e a s u r i n g s p e c i e s c o n c e n t r a t i o n s a t v a r i o u s v e r t i c a l d i s t a n c e s above t h e b u r n e r s u r f a c e . T h i s g e o m e t r y a l l o w s one t h e considerable s i m p l i f i c a t i o n of u t i l i z i n g a one-dimensional flame code f o r t h e k i n e t i c a n a l y s i s . The l a s e r beam i s f o c u s e d t o g i v e a c o n s t a n t beam d i a m e t e r o f «"w0.15 mm a c r o s s t h e f l a m e . Again t h i s s i m p l i f i e s the i n t e r p r e t a t i o n of the l i n e - o f - s i g h t absorption measurements. F u r t h e r m o r e , t h i s s m a l l d i a m e t e r p e r m i t s good s p a t i a l r e s o l u t i o n of the flame f r o n t region. I n our atmospheric p r e s s u r e f l a m e s , t h e f l a m e f r o n t was l e s s t h a n 1 mm t h i c k . A v e r a g i n g techniques were used f o r c o l l e c t i o n o f b o t h a b s o r p t i o n and f l u o r e s c e n c e d a t a , w i t h 100 l a s e r p u l s e s p e r d a t a p o i n t b e i n g typical. The l a r g e t e m p e r a t u r e g r a d i e n t s n e a r t h e b u r n e r s u r f a c e c a u s e d some beam s t e e r i n g , b u t d i r e c t measurement showed t h i s d e f l e c t i o n t o be n e g l i g i b l e , i . e . , l e s s t h a n 0.08 mm, a t d i s t a n c e s l a r g e r t h a n 0.3 mm a b o v e t h e s u r f a c e . The gas m i x t u r e s f e d t o t h e b u r n e r w e r e p r e p a r e d i n a s t a i n l e s s s t e e l m a n i f o l d u s i n g e l e c t r o n i c f l o w c o n t r o l l e r s . A range o f r i c h ammonia f l a m e s was s t u d i e d i n w h i c h t h e f u e l e q u i v a l e n c e r a t i o r a n g e d f r o m 1.28 t o 1.81. F l a m e t e m p e r a t u r e s w e r e m e a s u r e d w i t h P t / P t - 1 3 % R h t h e r m o c o u p l e s . The b e a d d i a m e t e r was o n l y 0.12 mm so t h a t t h e r a d i a t i o n c o r r e c t i o n was o n l y 80 K. OH, NH, NH^, and NH3 w e r e measured i n a b s o r p t i o n w h i l e NO was m e a s u r e d i n f l u o r e s c e n c e . The a b s o r p t i o n measurements w e r e reduced to c o n c e n t r a t i o n s v i a curve-of-growth techniques w h i l e t h e NO measurements w e r e c a l i b r a t e d a g a i n s t a b s o r p t i o n i n a l e a n ammonia f l a m e w h e r e t h e NO c o n c e n t r a t i o n was h i g h e r . I t was assumed t h a t t h e e x t e n t o f f l u o r e s c e n c e q u e n c h i n g was t h e same i n t h e r i c h and l e a n f l a m e s ( 7 ) . S u f f i c i e n t s p e c t r o s c o p i c i n f o r m a t i o n was a v a i l a b l e f o r a l l b u t NH2 t o a l l o w a b s o l u t e c o n c e n t r a t i o n a s s i g n m e n t s . N H d a t a c o u l d o n l y be o b t a i n e d a s [NH^] f , w h e r e f ^ was t h e unknown o s c i l l a t o r s t r e n g t h . S i n c e t h e OH a b s o r p t i o n measurements w e r e made o n i n d i v i d u a l r o t a t i o n a l l i n e s , i t was p o s s i b l e t o o b t a i n [ 0 H ] j . A s s u m i n g a B o l t z m a n n d i s t r i b u t i o n f o r t h e r o t a t i o n a l e n e r g y l e v e l s , one c a n o b t a i n the r o t a t i o n a l temperature from a p l o t of ln[0H]j versus Ε . An example o f s u c h d a t a i s shown i n F i g u r e 2. #

2

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


YAG-PUMPED DYE LASER/WEX

REF. DETECTOR

SPECTROMETER

FLAT FLAME

P.M.T.

SAMPLE DETECTOR

BOX-CAR AVERAGER

RATIOMETER/ AVERAGER

F i g u r e 1. S c h e m a t i c d i a g r a m o f t h e e x p e r i m e n t a l a r r a n g e m e n t s f o r l a s e r absorption and l a s e r - i n d u c e d f l u o r e s c e n c e . (Reproduced w i t h p e r m i s s i o n f r o m R e f . 7· C o p y r i g h t 19Ô3, J . Chem. P h y s . )

X

BEAM SPLITTER




74

-4.8 H

ψ=

1.28 ζ = .9 mm T , = 2239 Κ ± 20 Κ rn

-6.4 H

1 0

500

1

1

I

1

1 1—

1000 1500 2000 2500 3000

3500

-1

ROTATIONAL ENERGY (cm ) F i g u r e 2 . Measurement o f OH r o t a t i o n a l t e m p e r a t u r e a t a h e i g h t o f 0 . 9 mm above t h e b u r n e r f o r an ammonia f l a m e w i t h a n e q u i v a l e n c e r a t i o o f 1 . 2 8 . (Reproduced w i t h p e r m i s s i o n from Ref. 6 . C o p y r i g h t 1 9 8 2 , J . Chem. P h y s . )


5.

DEAN ET AL.

75

Nitrogen Chemistry in Flames

R e s u l t s and D i s c u s s i o n


NH2 O s c i l l a t o r S t r e n g t h . As mentioned e a r l i e r , l a c k o f a r e l i a b l e o s c i l l a t o r s t r e n g t h f o r NH2 p r e v e n t e d a s s i g n m e n t o f a b s o l u t e c o n c e n t r a t i o n s . However, we w e r e a b l e t o combine a b s o l u t e measurements o f OH, NH, a n d NH3 w i t h r e l a t i v e measurements o f NH^ to demonstrate t h a t t h e r e a c t i o n s NH

3

+ OH = N H

NH

2

+ OH = NH + H 0

2

+ H 0

(1)

2

(2)

2

were p a r t i a l l y e q u i l i b r a t e d . T h i s f a c t allowed us t o c a l c u l a t e a b s o l u t e NH2 c o n c e n t r a t i o n s a n d t h u s o b t a i n f ^ . The u s e o f two r e a c t i o n s n o t o n l y allowed a c o n s i s t e n c y check, b u t a l s o served t o p r o v i d e an e s t i m a t e f o r AH °(NH). The a p p r o a c h u s e d assumes t h a t H2O r a p i d l y assumes a n e q u i l i brium c o n c e n t r a t i o n i n flames. W i t h ^ 0 f i x e d i n t h i s way, one can e x p l i c i t l y v e r i f y whether o r n o t R e a c t i o n s 1 and 2 a r e e q u i l i b r a t e d by computing t h e r a t i o s : f

[NH ].f /C[NH ].[OH])

(3)

[ΝΗ]/([ΝΗ ].ί..[ΟΗ])

(4)

2

x

3

2

R e p r e s e n t a t i v e v a l u e s o f ( 3 ) a n d (4) a r e t a b u l a t e d i n T a b l e I . Although t h e i n d i v i d u a l c o n c e n t r a t i o n s v a r y w i d e l y , each o f these r a t i o s i s constant w i t h i n experimental e r r o r a t d i f f e r e n t heights above t h e b u r n e r , i n d i c a t i n g t h a t e a c h o f t h e r e a c t i o n s i s e q u i librated. C a l c u l a t i o n o f K e q i s more u n c e r t a i n f o r R e a c t i o n 2 b e c a u s e o f t h e u n c e r t a i n h e a t o f f o r m a t i o n o f NH. G i v e n t h i s u n c e r t a i n t y , one c a n v a r y AHf°(NH) u n t i l f i o b t a i n e d f r o m R e a c t i o n 2 a g r e e s w i t h t h a t o b t a i n e d f r o m R e a c t i o n 1. Such a n a p p r o a c h y i e l d s f i = 5.05 χ 1 0 w i t h A H f ( N H ) = 89 k c a l / m o l e . T h i s v a l u e o f AHf ( N H ) i s c o n s i s t e n t w i t h t h e JANAF v a l u e o f 90+4 k c a l / m o l e ( 9 ) b u t h i g h e r t h a n t h e 84.2±2.3 k c a l / m o l e p r o p o s e d by P i p e r ( 1 0 ) . S i n c e t h e r e a r e a d d i t i o n a l u n c e r t a i n t i e s i n AHf°(NH ) a n d t h e a b s o l u t e c o n c e n t r a t i o n o f NH3, NH, a n d OH, t h e s e v a l u e s o f f ^ a n d AHf°(NH) c a n n o t be t a k e n a s d e f i n i t i v e a s s i g n m e n t s , b u t t h e y do r e p r e s e n t s i g n i f i c a n t i m p r o v e m e n t s o v e r e a r l i e r work. - 5

0

0

2

R o t a t i o n a l E x c i t a t i o n o f OH. One o f t h e most s u r p r i s i n g a s p e c t s o f o u r d a t a was t h e o b s e r v a t i o n o f r o t a t i o n a l l y h o t OH i n t h e f l a m e f r o n t o f φ = 1.28 a n d φ = 1.50 f l a m e s . R o t a t i o n a l t e m p e r a t u r e s ^200 Κ h i g h e r t h a n r a d i a t i o n c o r r e c t e d t h e r m o c o u p l e measurements w e r e o b s e r v e d ; t h e s e w e r e n o t e x p e c t e d s i n c e r o t a t i o n a l energy t r a n s f e r i s so f a s t a t atmospheric p r e s s u r e . Such e x c i t a t i o n was n o t o b s e r v e d b e y o n d t h e f l a m e f r o n t i n a n y o f o u r ammonia f l a m e s a n d n o t e v e n w i t h i n t h e f l a m e f r o n t o f a methane


Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 1.58 8.08

0.79

0.54

0.724

8.46

1935

1929

2.5

3.0

^Reference 6.

d i s t a n c e above burner s u r f a c e .

m

1.55

8.32

0.69

m m in

m

Ο ο

"0 73

c m H δ ζ

00

Ο ηΊ} ο

3

in H

π χ

1.50

8.20

1.11 0.90

0.91

1.37

0.986

+3

9.22

C m

—12

10.0

/10

1938

[NH] [NH,,]»fJOH]

2;0

+3

1.52

C m

22 "

1.40

/10

1.40

^^2^ * ^ i [OH][NH,1

2.56

3

12.0

17

e [NHJ/10 cm~

1941

b

1.5

3

1.45

11

8.33

i

[NIL.]»f /10 cm" 8.71

b

2.04

3

2.54

14

[NH]/10 cm"

5.28

b

= 4.79/2.81/1.00)

14.3

3

2

1938

14

2

1.0

a

Z/mm T/K [OH]/10 cm~

3

φ - 1.28 (NH /0 /N

Table I . P a r t i a l E q u i l i b r i u m i n Ammonia Flames


5.


DEAN ET AL.

11

flame. I t a p p e a r e d t h a t t h e s o u r c e o f t h i s e x c i t e d OH was a n e x o t h e r m i c r e a c t i o n w h i c h was u n i q u e t o t h e f l a m e f r o n t r e g i o n o f t h e ammonia f l a m e . A r a t e a n a l y s i s o f t h e mechanism w h i c h i s d i s c u s s e d l a t e r i n d i c a t e s two r e a c t i o n s w h i c h s a t i s f y t h e s e criteria:


NH

2

+ 0

NH + NO

NH + OH

ΔΗ = -14 k c a l / m o l e

N

ΔΗ = -96 k c a l / m o l e

0

+ OH

Both o f these r e a c t i o n r a t e s peak i n t h e flame f r o n t r e g i o n , and b o t h h a v e t h e p o t e n t i a l t o p r o d u c e v i b r a t i o n a l l y e x c i t e d OH. S i n c e v i b r a t i o n a l r e l a x a t i o n i s much s l o w e r t h a n r o t a t i o n a l r e l a x a t i o n , a p p r e c i a b l e q u a n t i t i e s o f v i b r a t i o n a l l y e x c i t e d OH c o u l d b e f o r m e d . The o b s e r v e d r o t a t i o n a l e x c i t a t i o n c o u l d r e s u l t from cascading o f t h i s e x c i t a t i o n i n t o h i g h r o t a t i o n a l l e v e l s o f the ground v i b r a t i o n a l l e v e l d u r i n g t h e r e l a x a t i o n p r o c e s s . D e t a i l e d K i n e t i c Modeling. Recent advances i n computation t e c h n i q u e s ( 1 1 ) h a v e made i t much e a s i e r t o compute c o n c e n t r a t i o n d i s t a n c e p r o f i l e s f o r f l a m e s p e c i e s . The o n e - d i m e n s i o n a l i s o b a r i c flame equations a r e solved v i a a steady s t a t e s o l u t i o n using f i n i t e difference expressions. A n added s i m p l i f i c a t i o n i s t h a t the energy e q u a t i o n c a n be r e p l a c e d w i t h t h e measured t e m p e r a t u r e profile. I n t h e a d a p t i v e mesh a l g o r i t h m , t h e e q u a t i o n s a r e f i r s t s o l v e d on a r e l a t i v e l y c o a r s e g r i d . Then a d d i t i o n a l g r i d p o i n t s c o u l d be i n c l u d e d i f n e c e s s a r y , and t h e p r e v i o u s s o l u t i o n i n t e r p o l a t e d o n t o t h e new mesh w h e r e i t s e r v e d a s t h e i n i t i a l s o l u t i o n estimate. T h i s p r o c e s s was c o n t i n u e d u n t i l s e v e r a l t e r m i n a t i o n c r i t e r i a were s a t i s f i e d . The s t a r t i n g p o i n t i n d e v e l o p m e n t o f a n ammonia f l a m e mech a n i s m was a mechanism p r e v i o u s l y u s e d t o m o d e l ammonia o x i d a t i o n i n a f l o w t u b e n e a r 1300 Κ ( 3 ) . A d d i t i o n a l r e a c t i o n s w e r e added t h a t were thought t o be important a t t h e h i g h e r flame t e m p e r a t u r e s . C a l c u l a t i o n s w i t h t h i s mechanism p r o d u c e d p r o f i l e s i n marked d i s agreement w i t h o u r d a t a . The p r e d i c t i o n s w e r e s l o w e r t h a n o b s e r v e d ; d e c a y o f NH^ s p e c i e s was much t o o s l o w , a n d OH p e a k e d t o o l a t e b y a b o u t 2.5 mm. To make m a t t e r s w o r s e , f a r t o o much NO was f o r m e d . The NO p r o b l e m was e s p e c i a l l y t r o u b l e s o m e i n t h a t attempts t o i n c r e a s e t h e r a t e o f N H i decay o n l y served t o produce e v e n more NO, s i n c e NO was t h e p r i m a r y d e c a y c h a n n e l f o r t h e N H i s p e c i e s . A p o s s i b l e r e s o l u t i o n o f t h i s dilemma i n v o l v e s r e a c t i o n s o f t h e N H i s p e c i e s w i t h e a c h o t h e r t o f o r m N-N b o n d s . These c o m p l e x e s c o u l d t h e n s p l i t o f f Η atoms t o u l t i m a t e l y f o r m N2I n t h i s way one c o u l d a c h i e v e a n o v e r a l l f a s t e r d e c a y o f NH^ w i t h o u t p r o d u c i n g more NO. I n d e e d , c a l c u l a t i o n s u s i n g s u c h a mechanism showed much b e t t e r a g r e e m e n t w i t h o u r d a t a . Table I I l i s t s t h e " c o n v e n t i o n a l " mechanism a s w e l l a s t h e a d d i t i o n a l NHi + N H i r e a c t i o n s which d r a m a t i c a l l y improve t h e f i t . F i g u r e 3


78


T a b l e I I . M e c h a n i s m f o r R i c h Ammonia F l a m e s k = AT Reaction

n

exp(-E/RT) Α

η

Ε(kcal/mole) Comments


UPDATED FLOW REACTOR MECHANISM 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

NH 3+M=NH 2+H+M NH +H=NH2+H2 NH +0=NH2+OH NH +OH=NH + H 0 NH +0=NH+OH NH +0H=NH+H 0 NH2+H=NH+H2 NH +0 =HNO+OH NH 2+N0=NNH+0H NH +NO=N +H 0 NH +NO=N20+H2 NIT +HNO=NH +NO NH +NNH=N +NH NH+0 =HN0+0 NH+N0=N2+0H NH4OH=N+H20 NH4OH=HN0+H NH+H=N+H2 NH+0=N0+H NH+0=N+0H NH+N=N2+H HN0+M=N0+H+M HN0+0H=N0+H 0 HN0+H=N0+H2 HN0+0=N0+0H HN0+N=N0+NH HN0+N=H+N 0 NNH+M=N 2+H+M NNH+0H=N +H20 NNH+N0=N 2+HNO N+N0=N +0 N+0 =NO+0 N+OH=NO+H N 0+M=N +0+M N 0+H=NH+N0 N 0+H=N +OH N CHO=N +0 N20+0=N0+N0 H +OH=H 0+H H+0 =0H+0 0+H =0H+H H+H0 =0H+0H 0+HO =O +OH OH+H0 =H 0+0 3

3

3

2

2

2

2

2

2

2

2

2

2

2

2

3

2

2

3

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

4.80E+16 2.46E+13 1.50E+12 3.26E+12 2.00E+13 3.00E+10 1.00E+12 5.10E+13 6.10E+19 9.10E+19 5.00E+13 1.75E+14 1.00E+13 6.00E+12 1.20E+13 5.00E+11 5.00E+11 3.00E+13 6.30E+11 6.30E+11 6.30E+11 1.86E+16 3.60E+13 4.80E+12 5.00E+11 1.00E+11 5.00E+10 1.50E+15 3.00E+13 2.50E+12 1.60E+13 6.40E+09 6.30E+11 2.70E+14 3.80E+14 7.60E+13 1.00E+14 1.00E+14 2.20E+13 3.70E+17 1.80E+10 2.50E+14 4.80E+13 5.00E+13

0. 0. 0. 0. 0. 0.68 0.5 0. -2.46 -2.46 0. 0. 0. 0. 0. 0.5 0.5 0. 0.5 0.5 0.5 0. 0. 0. 0.5 0.5 0.5 0. 0. 0. 0. 1.0 0.5 0. 0. 0. 0. 0. 0. -1.0 1.0 0. 0. 0.

93929. 17071. 6040. 2120. 1000. 1290. 2000. 30000. 1866. 1866. 24800. 1000. 0. 3400. 0. 2000. 2000. 0. 0. 8000. 0. 48680. 0. 0. 0. 2000. 3000. 35000. 0. 0. 0. 6300. 0. 54100. 34500. 15100. 28020. 28020. 5150. 17500. 8900. 1900. 1000. 1000.

Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref. Ref.

3 3 3 3 3 3 8 3 3 3 15 3 3 8 8 16 16 1 16 16 16 3 3 16 16 16 16 8 3 3 16 16 16 17 18 19 19 19 3 20 3 3 3 3

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

5.

Table I I .

M e c h a n i s m f o r R i c h Ammonia F l a m e s

A

Reaction


79


DEAN ET A L .

45. 46. 47.

OH+OH=0+H 0 H0 +NO=N02+OH H+N0 =NO+OH

48. 49.

0+N0 =N04O2 H+0 +M=H0 +M H 0/21./*** N02+M=N0+0+M 0+0+M=02+M H+H+M=H2+M H /2.5/ H 0/15./ H+0H+M=H 0+M Η /2.57 H 0/15./

2

2

2

2

2

η

(Continued)

Ε ( k c a l / m o l e ) Comments 3 3 3 3 3

0. 0. 0. 0. 0.

1090. -260. 1500. 600. -995.

1.10E+16 1.38E+18 3.60E+16

0. -1.0 -0.6

66000. 340. 0.

Ref. 3 Ref. 3 R e f . 21

8.80E+21

-2.0

0.

R e f . 21

2

2

50. 51. 52.

Ret. Ref. Ref. Ref. Ref.

6.30E+12 3.43E+12 3.50E+14 1.00E+13 1.50E+15

2

2

53.

2

2

INCLUSION OF NH + NH REACTIONS 16100. 0. 2000.

54. 55. 56.

NH +NH =N H +H NH +NH =N H N H +H=N H +H

1.00E+13 5.00E+12 1.00E+12

0. 0. 0.5

57.

Ν H+OH=N Η +H 0 2 4 2 3 2

3.00E+10

0.68

1290.

58.

N H.+0=N H-+0H 2 4 2 3

2.00E+13

0.

1000

59. 60.

N H =N H +H Ν Η +H=N H +H 2 3 2 2 2

1.20E+13 1.00E+12

0. 0.5

61.

N H^+0H=N H +H 0 2 3 2 2 2

3.00E+10

0.68

1290.

62.

N H +0=N H +OH 2 3 2 2

2.00E+13

0.

1000.

63. 64.

N H =NNH+H Ν H+H=NNH+H 2 2 2

3.40E+12 1.00E+12

0. 0.5

2

2

2

2

2

2

4

2

o

3

2

4

3

2

o

2

3

2

2

0

o

n

0

0

2

0

0

0

0

o

o

0

2

0

58000. 2000.

Ref. 8 Ref. 8 Same a s H+NH2 Same a s OH+NH2 Same a s 0+NH2 Ref. 8 Same a s H+NH2 Same a s 0H+NH2 Same a s 0+NH Ref. 8 Same a s H+NH ?

65000. 2000.

2

65.

N H +OH=NNH+H 0

3.00E+10

0.68

1290.

Same a s 0H+NH2

66.

N H -K)=NNH+OH

2.00E+13

0.

1000.

Same a s 0+NH

67. 68

NH+NH=NNH+H NH+NH =N H +H

5.00E+13 5.00E+13

0. 0.

0. 0.

2

2

2

2

2

2

NOTE :

2

2

2

Réf. S Ref. 8

R a t e c o n s t a n t u n i t s a r e m o l e , cm, s e c ,K.

*** i . e . , r a t e c o n s t a n t i n c r e a s e d b y a f a c t o r o f 21 f o r H 0 a s t h e t h i r d body. 2



80


Distance Above Burner (mm)

Figure 3. Comparison o f ( l e f t ) observed and ( r i g h t ) c a l c u l a t e d p r o f i l e s f o r an ammonia flame with an equivalence r a t i o o f 1.50.


DEAN ET AL.


5.


81

compares t h e s e p r e d i c t i o n s w i t h t h e expanded mechanism t o o u r o b s e r v a t i o n s f o r t h e φ = 1.50 f l a m e . Both c a l c u l a t i o n s and o b s e r v a t i o n s c o n t a i n p o t e n t i a l sources o f e r r o r , a n d d e t a i l e d c o m p a r i s o n s s h o u l d b e made w i t h t h e s e i n m i n d . The c o n c e n t r a t i o n measurements o f NH a n d OH a r e p r o b a b l y a c c u r a t e t o ±20%, e x c e p t t h a t OH i n t h e f l a m e f r o n t r e g i o n c o u l d be l o w i f t h e r e w e r e a p p r e c i a b l e v i b r a t i o n a l e x c i t a t i o n . NO measurements w e r e c a l i b r a t e d v i a a b s o r p t i o n ( 7 ) a n d s h o u l d a l s o be a c c u r a t e t o ±20%. NH3 i s more u n c e r t a i n , ±30%, s i n c e t h e r e i s a l a r g e r u n c e r t a i n t y i n t h e e x t i n c t i o n c o e f f i c i e n t used (12). N H i s p r o b a b l y o n l y a c c u r a t e t o w i t h i n a f a c t o r o f two b e c a u s e of t h e u n c e r t a i n t i e s i n f j _ . Uncertainty i s introduced into the c a l c u l a t i o n s b y t h e u n c e r t a i n h e a t s o f f o r m a t i o n o f NH and NH2. W i t h t h e s e u n c e r t a i n t i e s , major emphasis s h o u l d be p l a c e d upon comparison o f t h e shapes o f t h e c o n c e n t r a t i o n p r o f i l e s . Using these g u i d e l i n e s , t h ecomparisions i n F i g u r e 3 a r e generally quite satisfactory. N o t e t h a t NH i s p r o p e r l y d e s c r i b e d i n terms o f b o t h p r o f i l e shape a s w e l l as a b s o l u t e c o n c e n t r a t i o n . The c a l c u l a t e d NO p r o f i l e n e a r t h e b u r n e r i s t o o h i g h , b u t t h e o v e r a l l d e c a y seems t o b e r e a s o n a b l y w e l l d e s c r i b e d . This d i s crepancy c l o s e t o t h e burner s u r f a c e c a n be r e s o l v e d by u s i n g a l a r g e r r a t e constant f o rthe r e a c t i o n 2

NH + NO + N

2

+ OH

However, c o n s i d e r i n g t h e u n c e r t a i n t y i n a b s o l u t e c o n c e n t r a t i o n s , i t was f e l t t h a t t h i s was i n s u f f i c i e n t j u s t i f i c a t i o n f o r u s e o f a higher value. OH i s a l s o p r o p e r l y p r e d i c t e d , w i t h t h e e x c e p t i o n o f t h e r e g i o n n e a r t h e p e a k , a n d t h i s d i s c r e p a n c y may w e l l b e a m a n i f e s t a t i o n o f v i b r a t i o n a l l y e x c i t e d OH a s d e s c r i b e d e a r l i e r . NH c a n be seen t o have t h e proper shape, and t h e a b s o l u t e concentration p r e d i c t i o n s a r e probably acceptable considering t h e large e r r o r bars here. The p r e d i c t e d ammonia d e c a y r a t e a t l a r g e r d i s t a n c e s a b o v e t h e b u r n e r i s somewhat s l o w e r t h a n o b s e r v e d . However, e v e n i n t h i s c a s e w h e r e t h e f i t l e a v e s some t h i n g t o b e d e s i r e d , i t i s v a s t l y i m p r o v e d o v e r what i t was f o r t h e c a s e where t h e NH. + NH. r e a c t i o n s w e r e e x c l u d e d . In general, theJits i l l u s t r a t e d i n Figure 3 a r e s i m i l a r t o t h o s e o b s e r v e d a t t h e o t h e r two e q u i v a l e n c e r a t i o s ( 8 ) . A p a r t i c u l a r l y encouraging aspect of t h e c a l c u l a t i o n s a t a l l three equivalence r a t i o s i s that they p r o p e r l y p r e d i c t t h e v a r i a t i o n of b o t h peak h e i g h t and peak l o c a t i o n o f t h e r a d i c a l s p e c i e s w i t h r e s p e c t t o changes i n t h e e q u i v a l e n c e r a t i o . Thus, i t a p p e a r s t h a t t h e mechanism g i v e n i n T a b l e I I p r o p e r l y a c c o u n t s f o r most o f o u r e x t e n s i v e d a t a b a s e o n r i c h ammonia f l a m e s . Although i t i s impossible t o prove that t h i s i s t h e c o r r e c t mechanism, t h e p r o p e r p r e d i c t i o n o f s o many s p e c i e s o v e r a r a n g e o f c o n d i t i o n s s t r o n g l y s u g g e s t s t h a t t h e scheme u s e d i s a r e a s o n able approximation t o r e a l i t y . I t i s e v i d e n t t h a t t h e NH-^ + NH^ r e a c t i o n s a r e t h e k e y i n g r e d i e n t t o o b t a i n i n g t h i s good f i t . As 2


82

CHEMISTRY OF COMBUSTION

PROCESSES

mentioned e a r l i e r , omission o f these r e a c t i o n s l e d t o p r e d i c t i o n s w h i c h b o r e no r e s e m b l a n c e t o o u r d a t a ( 8 ) . Figure 4 o u t l i n e s the important r e a c t i o n s of the n i t r o g e n s p e c i e s i n r i c h ammonia f l a m e s . The most i m p o r t a n t r e a c t i o n s p r o d u c i n g N a r e NNH d i s s o c i a t i o n a n d NH + NO. I n t u r n , most o f t h e NNH i s p r o d u c e d f r o m N H2 d i s s o c i a t i o n w h i c h i s p r o d u c e d v i a t h e r e a c t i o n NH + NH2. O t h e r N H i + N H i r e a c t i o n s a r e l e s s i m p o r tant. Thus, t h e N H i ^ i r e a c t i o n s a r e p r i m a r i l y r e s p o n s i b l e f o r Ν 2 p r o d u c t i o n ; i t i s now c l e a r why o m i s s i o n o f t h e s e c h a n n e l s l e d t o s u c h marked changes i n t h e p r e d i c t e d p r o f i l e s . One w o u l d n o t e x p e c t s u c h c h a n g e s i n p r e d i c t e d p r o f i l e s i n l e a n ammonia f l a m e s ; h e r e [NH^] w o u l d b e s u f f i c i e n t l y l o w t h a t t h e N H i + N H i r e a c t i o n s could s a f e l y by omitted. Figure 4 a l s o o u t l i n e s a p o s s i b l e reason f o r the continuing c o n t r o v e r s y a s t o t h e i d e n t i t y o f t h e N H i s p e c i e s w h i c h was r e s p o n s i b l e f o r t h e NH^ + NO r e a c t i o n when e x a m i n i n g NO p r o d u c t i o n i n t h e c o m b u s t i o n o f n i t r o g e n doped f u e l s . An a n a l y s i s o f t h e reactions i n Figure 4 indicates that ther e l a t i v e concentration of N H i s p e c i e s w i l l v a r y w i t h c o n d i t i o n s . I n t h i s work where l a r g e q u a n t i t i e s o f NH a n d NH2 a r e p r e s e n t , N2 p r o d u c t i o n o c c u r s via NHi NHi d i s c u s s e d a b o v e . However, i n t h e more t y p i c a l c a s e o f s m a l l amounts o f n i t r o g e n o u s d i l u e n t , t h e NH^ c o n c e n t r a t i o n s should be t o o l o w f o r these r e a c t i o n s t o be s i g n i f i c a n t . There t h e r e l a t i v e p o p u l a t i o n o f the N H i species w i l l be governed by t h e c o m p e t i t i o n b e t w e e n t h e v a r i o u s NH^ + NO r e a c t i o n s a n d t h e hydrogen a b s t r a c t i o n s : 2

2


+

+

a

s

X + NH. ·> HX + NH. .. ι l - l L a r g e r r a d i c a l c o n c e n t r a t i o n s (X) w o u l d f a v o r h i g h e r concentra t i o n s o f Ν atoms; t h e n Ν + NO w o u l d b e i m p o r t a n t . Lower r a d i c a l c o n c e n t r a t i o n s w o u l d t e n d t o i n c r e a s e t h e i m p o r t a n c e o f NH + NO. F i n a l l y , much l o w e r t e m p e r a t u r e s , i . e . , 1200 Κ f o r t h e T h e r m a l DeNO p r o c e s s , w o u l d make NH^ + NO a k e y r e a c t i o n . x

E s t i m a t i o n o f R a t e C o n s t a n t s f o r N ^ + NH^^ a n d N Q J D i s s o c i a t i o n . The k e y s t o t h e s u c c e s s o f t h e mechanism a r e t h e NH^ + NH^ r e a c t i o n s as w e l l as t h e subsequent u n i m o l e c u l a r d i s s o c i a t i o n o f t h e N^H^ s p e c i e s f o r m e d f r o m t h e s e r e a c t i o n s . A t y p i c a l scheme i s t h e f o l l o w i n g : H

k

NH + N H

f 1T

H H^NNH „NNH

2

— —H N N H

+ Η

r kjM]

H^NNH

lk

d

NNH + Η



DEAN ET AL.


NH

2

NH

3

OH

HNO

NH

2

NH

NH

2

2

NKL

I

NH 2

NO 0

H NO

2

NH NH

2

NH NO

NH

2

NO

2

NH

2

NH, NH

NO

i

2

(

Γ

3

NH

I NO

i Ni 0

4

IH,0

M,H

NNH 1 M,NH

2

Figure k. Important r e a c t i o n s o f n i t r o g e n species i n r i c h amnioni a f1ame s.


2

84


T h i s i s w r i t t e n i n t h e mechanism a s t h e f o l l o w i n g NH + N H N H 2

2

= N H

2

= NNH + H

2

sequence: (68)

+ H

2

(63)


with "68

+

*H

k

+

r

k

s

M

and "63 One t h u s o b t a i n s t h e a p p a r e n t r a t e c o n s t a n t s IC53 a n d k 6 3 b y e v a l u a t i o n o f t h e r a t e constants f o r t h e elementary steps ( k f , k , k g , and k ) a n d u s i n g t a b l e s o f t h e K a s s e l i n t e g r a l ( 1 3 ) t o estimate t h e degree o f f a l l - o f f (I) from t h e l i m i t i n g h i g h p r e s s u r e r a t e c o n s t a n t , k^. k c a n b e t a k e n t o b e t h e h i g h p r e s s u r e recombination r a t e constant; k i s the c o l l i s i o n a l s t a b i l i z a t i o n r a t e c o n s t a n t ; k and k a r e t h e u n i m o l e c u l a r r a t e c o n s t a n t s c o r r e s p o n d i n g t o N-N a n d N-H bond f i s s i o n , r e s p e c t i v e l y , o f t h e c o l l i s i o n complex. These decay r a t e c o n s t a n t s were e s t i m a t e d f r o m t h e RRK e x p r e s s i o n . r

s

f

s

r

H

S-l

w h e r e A i s t h e p r e e x p o n e n t i a l f a c t o r , ε-ε* i s t h e amount o f v i b r a t i o n a l e n e r g y i n e x c e s s o f t h a t r e q u i r e d t o b r e a k t h e bond o f interest, ε i s t h e t o t a l energy o f t h e complex, and S i s t h e e f f e c t i v e number o f o s c i l l a t o r s ( 1 4 ) . S i s computed v i a t h e relation C Q b

"

vib R

F o r t h i s p a r t i c u l a r c a s e o f HNNR^, t h e N-H bond i s o n l y ^ 5 5 k c a l / m o l e w h i l e N-N i s ^ 8 5 k c a l / m o l e . Hence k n » k and k >> k [ M ] a t f l a m e t e m p e r a t u r e s a n d k£g ^ k f = 5 χ 1 0 ^ m^ m o l e " ! s " l , a t y p i c a l recombination r a t e constant. T h u s , t h i s pathway t o N-N bond f o r m a t i o n i s v e r y r a p i d a n d t h i s r e a c t i o n p l a y s a k e y r o l e in the kinetics. r

H

s

C

Summary By c o m b i n i n g l a s e r d i a g n o s t i c s w i t h i m p r o v e d c o m p u t e r a l g o r i t h m s f o r m o d e l i n g l a b o r a t o r y f l a m e s , we h a v e b e e n a b l e t o



5.

DEAN ET AL.


85

develop an improved mechanism f o r ammonia o x i d a t i o n at high temp eratures. The combination of absorption and l a s e r induced f l u o r escence have y i e l d e d absolute concentrations of important flame species. The s u b - m i l l i m e t e r s p a t i a l r e s o l u t i o n allowed us to monitor species w i t h i n the flame f r o n t , and the sub-ppm s e n s i t i v i t y allowed us to measure key r e a c t i v e i n t e r m e d i a t e s . These concentration p r o f i l e s provided us with the necessary data base f o r development of a d e t a i l e d mechanism. Since the a l g o r i t h m used a c c u r a t e l y and e f f i c i e n t l y accounted f o r the e f f e c t of d i f f u s i v e transport w i t h i n the flame, we had the luxury of doing a r e l a t i v e l y simple, steady s t a t e experiment where we could s i g n a l average to o b t a i n adequate s e n s i t i v i t y f o r r a d i c a l s . Since transport was p r o p e r l y handled, we could focus e x c l u s i v e l y upon the k i n e t i c s . In t h i s sense the modeling was s i m i l a r to modeling a shock wave experiment, but here we had the opportunity to monitor r e a c t i v e intermediates. There are a host of h i g h temperature systems which should now be amenable to t h i s combined d i a g n o s t i c / m o d e l i n g approach. It should g i v e k i n e t i c i s t s an e x t r a weapon w i t h which to approach complex systems.

Literature Cited 1. Morley, C., Eighteenth Symposium (International) on Combustion, Combustion Institute, Pittsburgh, PA (1981), p. 23, and references therein. 2. Lyon, R. Κ., U.S. Patent No. 3,900,554 (1975). 3. Dean, A. M.; Hardy, J. E.; Lyon, R. K.; Nineteenth Symposium (International) on Combustion, Combustion Institute, Pittsburgh, PA (182), p. 97. 4. Fujii, N.; Miyama, H.; Koshi, M.; Asaba, T.; Eighteenth Symposium (International) on Combustion, Combustion Institute, Pittsburgh, PA (1981), p. 873, and references therein. 5. Fisher, C. J., Combust. Flame, 30, 143 (1977), and references therein. 6. Chou, M.S.; Dean, A. M.; Stern, D.; J. Chem. Phys., 76, 5334 (1982). 7. Chou, M. S.; Dean, Α. M.; Stern, D.; J. Chem. Phys., 78, 5962 (1983). 8. Dean, A. M.; Chou, M. S.; Stern, D.; Int. J. Chem. Kinetics (submitted). 9. Chase, M. W., Jr.; Curnutt, J. L.; Downey, J. R., Jr.; McDonald, R. Α.; Syverud, A. N.; Valenzuela, Ε. Α.; J. Phys. Chem. Ref. Data, 11. 695 (1982). 10. Piper, L. G.; J. Chem. Phys., 70, 3417 (1979). 11. Smooke, M. D., "Solution of Burner Stabilized Pre-Mixed Laminar Flames by Boundary Value Methods," Sandia National Laboratories Report 81-8040 (1982). 12. Menon, P.G.; Michel, K. W.; J. Phys. Chem., 71, 3280 (1967).



86


13. Emanuel, G., "Table of the Kassel Integral," Aerospace Report No. TR-0200(4240-20)-5 (1969). 14. Benson, S.W., Thermochemical Kinetics, 2nd Edition, Wiley, New York (1976). 15. Roose, T. R., Ph.D. thesis, Stanford University (1981). 16. Westley, F., "Table of Recommended Rate Constants for Chemical Reactions Occurring in Combustion," NSRDS-NSB 67 (1980). 17. Dean, A. M.; Steiner, D. C.; J. Chem. Phys., 66, 598 (1977). 18. Cattolica, R. J.; Dean, A. M.; Smooke, M. D.; "A HydrogenNitrous Oxide Flame Study," Sandia Report SAND 82-8776 (1982). 19. Baulch, D. L . ; Drysdale, D. D.; Home, D. G.; Evaluated Kinetic Data for High Temperature Reactions, Vol. 2, Butterworth, London (1973). 20. Cohen, N.; Westberg, K. R.; "Chemical Kinetic Data Sheets for High Temperature Chemical Reactions," Aerospace Report No. ATR-82(7888)-3. 21. Warnatz, J.; Eighteenth Symposium (International) on Combustion, Combustion Institute, Pittsburgh, PA (1981), p. 369. RECEIVED November

10, 1983


The Chemistry of Combustion Processes - American Chemical Society

Recommend Documents