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Submitted on 1 Jan 1979
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OZONE PRODUCTION IN A HOMOGENEOUS OXYGEN DISCHARGE
B. Eliasson, U. Kogelschatz
To cite this version:
B. Eliasson, U. Kogelschatz. OZONE PRODUCTION IN A HOMOGENEOUS OXY- GEN DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-271-C7-272.
�10.1051/jphyscol:19797133�. �jpa-00219106�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ&ment d( n07, Tome 40, JuiZZet 2979, page C7- 271
OZONE PRODUCTION IN A HOMOGENEOUS OXYGEN DISCHARGE
B. Eliasson and U . Kogelschatz.
B r o w Boveri Research Center, CH-5405 Baden, SwitzerZand.
We r e p o r t on c a l c u l a t i o n s o f t h e e f f i c i e n c y and c o n c e n t r a t i o n o f ozone pro- d u c t i o n i n a s p a t i a l l y homogeneous oxygen d i s c h a r g e . An a p p l i e d e l e c t r i c f i e l d E a c c e l e r a t e s e l e c t r o n s i n a g a s c o n s i s t i n g i n i t i a l l y o f oxygen molecules 0 2 . The e l e c t r o n s have a c e r t a i n e n e r g y c o r r e s - ponding t o t h e e l e c t r o n t e m p e r a t u r e Te which i n t u r n i s a f u n c t i o n o f t h e r a t i o E/p where p i s t h e g a s p r e s s u r e .
The e l e c t r o n s d i s s o c i a t e O 2 m o l e c u l e s i n t o 0 atoms. Through r e c o m b i n a t i o n of atomic oxygen w i t h oxygen m o l e c u l e s ozone i s formed. The p r o d u c t i o n and d e s t r u c t i o n of 0 and O 3 from t h e i n i t i a l m i x t u r e o f e l e c t r o n s and O 2 i s d e s c r i b e d by t h e f o l - lowing r e a c t i o n s :
Along w i t h e , 0 , O 2 a l s o p o s i t i v e and n e g a t i v e i o n s a r e p r e s e n t . We assume t h a t t h e s e c o n s i s t p r e d o m i n a n t l y o f O 2 and 0-
+
i o n s , r e s p e c t i v e l y . These a r e produced a c c o r d i n g t o
The r a t e c o e f f i c i e n t s pi d e s c r i b i n g t h e above r e a c t i o n s a r e e i t h e r known o r can b e c a l c u l a t e d . They a r e f u n c t i o n s o f t h e g a s t e m p e r a t u r e T and t h e e l e c t r o n
g
t e m p e r a t u r e Te. We assume t h a t t h e i o n t e m p e r a t u r e i s e q u a l t o t h e g a s tempera- t u r e .
The time dependent b a l a n c e e q u a t i o n s f o r t h i s system have been s o l v e d f o r two d i f f e r e n t c a s e s : a ) we assume an a r b i t r a - r y i n i t i a l e l e c t r o n c o n c e n t r a t i o n ( 3 x
cmq3) and a p p l y a c o n s t a n t e l e c t r i c f i e l d and b ) we a p p l y a f a s t v o l t a g e p u l s e which i s h i g h enough t o c a u s e i o n i z a t i o n . A t y p i c a l s o l u t i o n i n c a s e a ) i s shown i n
F i g . 1.
F i g . 1: p a r t i c l e c o n c e n t r a t i o n s f o r a c o n s t a n t a p p l i e d e l e c t r i c f i e l d : E/p = 5 V/ (cm T o r r )
,
Tg = 300 K , p = 1 b a r . (The s u b s c r l p . t s e ,1,3,r e f e r t o e l e c t r o n s , 0 , 0 3 , 0
,
02 r e s p e c t i v e l y ) .
There a r e two c h a r a c t e r i s t i c time con- s t a n t s i n v o l v e d i n t h e p r o d u c t i o n o f 0 and 0 3 . A s long a s E/p i s l e s s t h a n t h e break- down v a l u e o f 38 V/(cm T o r r ) a l l e l e c t r o n s
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d i s a p p e a r q u i c k l y t o form n e g a t i v e i o n s t h r o u g h d i s s o c i a t i v e , a t t a c h m e n t . The t i m e c o n s t a n t of t h i s e f f e c t i v e a t t a c h m e n t i s
T, = 1 / ( ( p 2 - p 1 ) n 2 ) . It i s o f t h e o r d e r of some n s e c f o r a p r e s s u r e o f 1 b a r and i s p r o p o r t i o n a l t o l / p .
The Jozone c o n c e n t r a t i o n x 3 i n c r e a s e s w i t h a time c o n s t a n t .r3 >> T,. T~ i s o f t h e o r d e r of v s e c a t a p r e s s u r e o f 1 b a r . I t i s p r o p o r t i o n a l t o l / p 2
.
I n F i g . 2 we show a t y p i c a l s o l u t i o n i n c a s e o f an e l e c t r i c p u l s e whose h e i g h t i s l a r g e r t h a n t h e breakdown v a l u e . The e l e c t r i c f i e l d i n c r e a s e s t o t h e maximum v a l u e o f 4 7 V/(cm T o r r ) w i t h i n 50 n s e c .
F i g . 2 : P a r t i c l e c o n c e n t r a t i o n s f o r a f a s t a p p l i e d v o l t a g e p u l s e o f i n - d i c a t e d shape. (T =300 K , p = l b a r )
9
A t low e l e c t r i c f i e l d s a l l e l e c t r o n s a r e a t t a c h e d t o a t o m i c oxygen a c c o r d i n g t o t h e d i s s o c i a t i v e a t t a c h m e n t d e s c r i b e d by Eq.
(8). A t h i g h e r f i e l d s t h e i o n i z a t i o n ac- c o r d i n g t o E q . ( 7 ) dominates and we have f r e e e l e c t r o n s i n t h e g a s . The f r e e e l e c - t i o n s e x i s t o n l y f o r c a . 30 nsec. The t i m e c o n s t a n t of t h e i n c r e a s e o f t h e ozone con- c e n t r a t i o n i s o f t h e same o r d e r a s b e f o r e . There i s a s u b s t a n t i a l d i f f e r e n c e between t h e r e s u l t s i n F i g . 1 and F i g . 2 though, v i z . whereas i n t h e f i r s t c a s e t h e ozone c o n c e n t r a t i o n i s o f t h e o r d e r of i n t h e second c a s e t h e c o n c e n t r a t i o n r e a c h e s t h e v e r y h i g h v a l u e o f 3 - l o a 3 i n s p i t e o f t h e v e r y low i n i t i a l e l e c t r o n c o n c e n t r a - t i o n (10- 12cm-3). T h i s i s due t o t h e pro-
E/p > 38 V/(cm T o r r ) .
The number o f ozone m o l e c u l e s pro- duced p e r n e g a t i v e c h a r g e a r e shown i n F i g . 3 i n t h e c a s e of an e l e c t r i c p u l s e a s a f u n c t i o n o f E/p. Also shown i s t h e e f f i c i e n c y o f t h e ozone g e n e r a t i o n , i . e . t h e r a t i o mass of ozone produced/energy used. (An e f f i c i e n c y o f 100% c o r r e s p o n d s t o 1220 g/kWh.)
F i g . 3: E f f i c i e n c y Of ozone p r o d u c t i o n and ozon y i e l d p e r n e g a t i v e c h a r g e a s a f u n c t i o n o f E/p.
(T = 300 K , p = 1 b a r ) . 9
There a r e two p e a k s i n t h e e f f i c i e n - cy c u r v e . One around 5 V/ (cm T o r r ) and a n o t h e r around 4 7 V/ (cm T o r r )
.
The peakaround 4 7 V/ (cm T o r r ) i s due t o t h e i n - c r e a s e d p r o d u c t i o n o f e l e c t r o n s .
These c a l c u l a t i o n s show t h a t t o ob- t a i n h i g h e f f i c i e n c y and a h i g h concen- t r a t i o n o f ozone w e have t o g e n e r a t e a f a s t e l e c t r i c p u l s e h i g h e r t h a n t h e break- down v a l u e o f 38 V/ (cm T o r r )
.
The h i g h e s t p u l s e h e i g h t o b t a i n a b l e i s r e l a t e d t o t h e r a t e of i n c r e a s e i n t h e p u l s e h e i g h t . The f a s t e r t h e p u l s e , t h e h i g h e r t h e a p p l i e d E/p v a l u e can be.From t h e s e c a l c u l a t i o n s i t a l s o be- comes a p p a r e n t t h a t a homogeneous p u l s e d d i s c h a r g e c a n r e a c h e f f i c i e n c i e s t h a t a r e c o n s i d e r a b l y h i g h e r t h a n t h o s e r e a c h e d i n a c l a s s i c a l o z o n i z e r w i t h a " s i l e n t " d i s - c h a r g e which i s i n h e r e n t l y inhomogeneous.
d u c t i o n o f an a v a l a n c h e o f e l e c t r o n s once
