Environmental Letters
ISSN: 0013-9300 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/lesa17
Emission Control of Gas Effluents from Geothermal Power Plants Robert C. Axtmann To cite this article: Robert C. Axtmann (1975) Emission Control of Gas Effluents from Geothermal Power Plants, Environmental Letters, 8:2, 135-146, DOI: 10.1080/00139307509437427 To link to this article: http://dx.doi.org/10.1080/00139307509437427
Published online: 02 Sep 2009.
Submit your article to this journal
Article views: 3
View related articles
Citing articles: 8 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=2esa20 Download by: [Universite Laval]
Date: 14 November 2015, At: 08:41
ENVIRONMENTAL LETTERS, 8 (21, 135-146 (1975)
EEIISSION CONTROL OF GAS EFFLUENTS FROM GEOTHERMAL POWER PLANTS
R o b e r t C . Axtmann Department o f Chemical E n g i n e e r i n g and Center f o r Environmental Studies Princeton University P r i n c e t o n , New J e r s e y 0 8 5 4 0 ,
ABSTRACT
Downloaded by [Universite Laval] at 08:41 14 November 2015
Geothermal s t e a m a t t h e w o r l d ' s f i v e l a r g e s t power p l a n t s c o n t a i n s f r o m 0 . 1 5 t o 30% n o n c o n d e n s a b l e g a s e s i n c l u d i n g C O H2S,
H2,
CH4,
N2,
H3B03,
and NH3.
2' A t four of the plants the
gases a r e f i r s t s e p a r a t e d from t h e steam and t h e n d i s c h a r g e d t o the environment; a t t h e f i f t h ,
t h e noncondensables exhaust d i -
r e c t l y t o t h e atmosphere a l o n g w i t h s p e n t steam.
Some C 0 2 a n d
s u l f u r e m i s s i o n r a t e s r i v a l t h o s e f r o m f o s s i l - f u e l e d p l a n t s on a
per m e g a w a t t - d a y b a s i s .
The ammonia a n d b o r o n e f f l u e n t s c a n i n -
t e r f e r e w i t h animal and p l a n t l i f e .
The e f f e c t s o f s u l f u r
( w h i c h e m e r g e s a s H2S b u t may o x i d i z e t o SO2) on e i t h e r a m b i e n t a i r q u a l i t y o r l o n g t e r m human h e a l t h a r e l a r g e l y unknown.
Nost
geothermal t u r b i n e s a r e equipped with d i r e c t contact condensers w h i c h c o m p l i c a t e e m i s s i o n c o n t r o l b e c a u s e t h e y p r o v i d e t w o or more p a t h w a y s f o r t h e e f f l u e n t s t o r e a c h t h e e n v i r o n m e n t .
Use
of d i r e c t c o n t a c t c o n d e n s e r s c o u l d p e r m i t e f f i c i e n t e m i s s i o n control i f coupled t o processes t h a t produce s a l e a b l e q u a n t i t i e s o f p u r i f i e d carbon d i o x i d e and e l e m e n t a l s u l f u r .
135 Copyright 0 1975 by hlarcel Dekker. Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying. microfilming. and recording.01 by any informationstorage and retrievalsystem, without permission in writing from the publisher.
Axnw INTRODUCTION G eo th ermal power i s e n v i r o n m e n t a l l y "cl ean", a c c o r d i n g t o t h e c o n v e n t i o n a l wisdom [ l ] .
at least
That i t enjoys such
a r e p u t a t i o n may r e f l e c t t h e m e a g r e e n v i r o n m e n t a l s c r u t i n y t h a t h a s been a c c o r d e d what i s , a s y e t , a r e l a t i v e l y i n s i g n i f i c a n t technology.
Goldsmith's b r i e f b u t somber a s s e s s m e n t h a s had
l i t t l e c i r c u l a t i o n and, i n any c a s e , a d d r e s s e s p o t e n t i a l geo-
t h e r m a l s o u r c e s i n C a l i f o r n i a [2].
Bowen's r o s i e r a n a l y s i s
draws a l m o s t e x c l u s i v e l y on p u b l i s h e d m a t e r i a l s t h a t d e s c r i b e
Downloaded by [Universite Laval] at 08:41 14 November 2015
a s i n g l e operating p l a n t , a l s o i n C a l i f o r n i a [3]. of the first, on-site,
The r e s u l t s
e n v i r o n m e n t a l s t u d y o f a working geo-
thermal p l a n t have y e t t o appear [4]. T h i s r e p o r t e x p l o r e s one a s p e c t of t h e impact o f geothermal power:
t h e d i s c h a r g e t o t h e environment o f noncondensable g a s e s ,
w i t h p a r t i c u l a r e m p h a s i s o n c a r b o n d i o x i d e an d h y d r o g e n s u l f i d e . The e f f l u e n t s from s i x g e o t h e r m a l f i e l d s a r e co m p ar ed i n m ag n i t u d e w i t h each o t h e r and w i t h t h o s e from f o s s i l - f u e l e d p l a n t s . Some e n v i r o n m e n t a l c o n s e q u e n c e s o f t h e d i s c h a r g e s a r e d i s c u s s e d ; problem a r e a s a r e d e f i n e d ; and a p r o c e s s t h a t c o u l d c o n t r o l t h e e m i s s i o n s and p r o d u c e m a r k e t a b l e b y - p r o d u c t s i s d e l i n e a t e d . CHARACTERISTICS OF GAS FRACTIONS 1.
Concentrations B o t h t h e t o t a l g a s c o n t e n t an d t h e r e l a t i v e c o n c e n t r a t i o n s
o f t h e c o n s t i t u e n t s d e p e n d i n t r i c a t e l y upon t h e g e o c h e m i s t r y o f t h e und erg ro u n d r e s e r v o i r [5-81.
Car b o n d i o x i d e is i n v a r i a b l y
t h e m a i n c o m p o n e n t ( 7 8 - 9 5 % ) w i t h l e s s e r c o n c e n t r a t i o n s o f H2S C.1-17%)>, H2 ( 1 - 1 3 % ) , CH4 ( 0 - 1 2 % ) , N2(0.2-9%),
an d NH3
(0-1.7%);
a n d , o c c a s i o n a l l y , t r a c e a m o u n t s o f a r g o n , e t h a n e , H3B03, HF,
a n d SO2 [9].
HCl,
Reports from a t l e a s t t h r e e f i e l d s i n d i c a t e 136
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS t h a t t h e t o t a l gas f r a c t i o n declines with e x p l o i t a t i o n [lo-121. T h i s , t o o , o c c u r s i n a c o m p l i c a t e d way:
a t W a i r a k e i , New Z e a l a n d
t h e t o t a l g a s c o n c e n t r a t i o n d e c r e a s e d by 5 0 % between 1960 and 1969 w h i l e t h e H2S f r a c t i o n r e m a i n e d v i r t u a l l y c o n s t a n t [ 1 2 ] . T a b l e I g i v e s t h e f i e l d - w i d e a v e r a g e s o f t o t a l g a s (w/o) and mole p e r c e n t a g e s o f C02 and H S a t t h e f i v e l a r g e s t power 2 s t a t i o n s [10,11,13.14];
and a t t h e B r o a d l a n d s , New Z e a l a n d f i e l d
1151, p o r t i o n s o f which h a v e d i s c h a r g e d f o r up t o t h r e e y e a r s b u t f o r which no power s t a t i o n h a s been b u i l t .
The T a b l e demon-
Downloaded by [Universite Laval] at 08:41 14 November 2015
s t r a t e s the s i g n i f i c a n t l y different characteristics of closely a d j a c e n t f i e l d s ( W a i r a k e i and B r o a d l a n d s a r e b a r e l y 10 m i l e s a p a r t ; L a r d a r e l l o and Monte Amiata,
l e s s than 40 miles), but
does n o t r e f l e c t i n t r a - f i e l d d i f f e r e n c e s .
In t h e r e l a t i v e l y
impermeable r e s e r v o i r a t B r o a d l a n d s , H S c o n c e n t r a t i o n s i n s t e a m 2 from a d j a c e n t w e l l s d i f f e r by a s much a s a f a c t o r o f t e n [S].
C02 and H2S C O N T E N T OF GEOTHERMAL STEAM ( 1 9 7 3 )
TABLE I.
Plant
Inrtalled Capacity
(MW[e]) G e y s e r s , USA Lardarello.
Italy
Wairokei, New Z e a l a n d
Average Gas ComC02 p o s i t i o n o f Steam Fraction. (w/o) ( v / o ) 1.0
79
5.0
365
4.9
94
1.6 5.0
192
0.1s
95
75
2.7
858
Monte Amiata, Italy
25 5
200 e s t )
8 Weight p e r c e n t a g e s .
*
(v/o)
396
Cerro P r i e t o , Mexico
Broadlands, New Z e a l a n d
H2S
15#
10-30
95
0.4
3.3
94
1.4
Ref.11 l i s t s no o t h e r c o n s t i t u e n t s .
Broadlands h a s been p a r t i a l l y c h a r a c t e r i z e d , s t a t i o n h a s been b u i l t .
137
b u t no power
AXTMANN ENVIRONMENTAL EFFECTS
The g e o t h e r m a l l i
r a t u r e is permeat 3 w i h references
t h e c o r r o s i v e e f f e c t s o f t h i s omnipresent and maladorous gas
I
--
p a r t i c u l a r l y on power s t a t i o n e q u i p m e n t s u c h a s e l c t r i c a l contacts.
W h i l e t h e e f f e c t s o f a c u t e d o s e s a r e known[16],
t h e geo-
t h e r m a l and i n d u s t r i a l hygiene l i t e r a t u r e s have l i t t l e t o o f f e r on t h e l o n g t e r m e f f e c t s on human h e a l t h o f s u b a c u t e e x p o s u r e .
Downloaded by [Universite Laval] at 08:41 14 November 2015
G o l d s m i t h e q u a t e s H2S e m i s s i o n s t o SO2 e m i s s i o n s [ Z ] ;
but while
H2S i s b e l i e v e d t o o x i d i z e i n t h e a t m o s p h e r e , t h e t i m e s c a l e o f t h e r e a c t i o n i s unknown [17].
S o f a r a s we a r e a w a r e , no
systematic experimental o r a n a l y t i c a l s t u d i e s e x i s t t h a t r e l a t e HzS e m i s s i o n s t o a m b i e n t a i r q u a l i t y a n d human h e a l t h .
2.
Ammonia a n d Boron The h a r m f u l e f f e c t o f d i s s o l v e d NH3 o n f i s h l i f e i n B i g
S u l f u r Creek h a s s t i m u l a t e d t h e o p e r a t o r s of t h e Geysers F i e l d t o r e i n j e c t c o n d e n s a t e f r o m t h e c o o l i n g t o w e r s ( s e e s e c t i o n on Pathways t o t h e Environment below) i n t o t h e h y d r o t h e r m a l r e s e r voir
[lo].
More r e c e n t l y ,
L a r d a r e l l o h a s begun r e i n j e c t i o n
e x p e r i m e n t s aimed a t p r e v e n t i n g d e l e t e r i o u s e f f e c t s o f b o r o n on n e a r b y a g r i c u l t u r a l o p e r a t i o n s [18]. 3.
co2 A s shown i n T a b l e 11, C 0 2 d i s c h a r g e r a t e s p e r u n i t o f e l e c -
t r i c a l p o w e r p r o d u c e d a t some g e o t h e r m a l p l a n t s r i v a l t h o s e f r o m fossil-fueled plants. C02 from man's
A c c o r d i n g t o t h e f a m o u s SMIC R e p o r t [ 1 9 ] ,
e n e r g y - p r o d u c i n g a c t i v i t i e s may t h r e a t e n t h e
s t a b i l i t y of g l o b a l c l i m a t e v i a t h e "green-house
138
e f f e c t " so t h a t
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS TABLE 11. SULFUR A N D C02 DISCHARGE RATES FROM GEOTHERMAL AND COAL PLANTS t
Plant
Sulfur
t onnes (hIW-day)-l
c02 tonnes(&IW-day)-'
t:;cq-
C e r r o P r i e t o , Mexico
0.73
55
4.2
315
Coal (3.5%S)#
0.27
270
25.0
25,000
0.19
5
62.0
1 580
0.15
30
18.0
3,600
Monte A m i a t a ,
Italy*
Broadlands, New Z e a l a n d
,
0.13
47
11.0
4,000
Coal (l.O%S)#
0.077
77
25.0
25,000
Geysers,
0.054
21
1.4
700
0.012
2
Lardarello,
Downloaded by [Universite Laval] at 08:41 14 November 2015
tonnEqday
Italy
USA
Wairakei, New Z e a l a n d
0.32
i
Daily r a t e s o f geothermal p l a n t s a t t h e r a t e d c a p a c i t i e s given i n Table I.
#
1 0 0 0 NW(e) c o a l p l a n t w i t h a t h e r m a l e f f i c i e n c y = 4 0 % j c a l o r i f i c v a l u e o f f u e l = l o 4 BTU/lb; s u l f u r r e t e n t i o n i n a s h = 15%. Assumes 20 w/o g a s f r a c t i o n . A c c o r d i n g t o R e f . 1 1 , t h e l a r g e s t u n i t ( 1 5 bIW[e]) a t Monte A m i a t a u t i l i z e s s t e a m w i t h 30 w/o g a s .
*
62
i n t h i s r e s p e c t , a t l e a s t , g e o t h e r m a l power may o f f e r l i t t l e improvement o v e r c o n v e n t i o n a l t e c h n o l o g y . THE POWER C Y C L E The v a p o r - d o m i n a t e d
r e s e r v o i r s o f n o r t h e r n C a l i f o r n i a and
I t a l y p r o d u c e s u p e r h e a t e d s t e a m which p a s s e s t h r o u g h c e n t r i f u g a l s e p a r a t o r s f o r removal o f e n t r a i n e d rock d u s t and l i q u i d w a t e r . Hot w a t e r r e s e r v o i r s , e . g . ,
i n New Z e a l a n d ,
a t Cerro Prieto,
and
most o t h e r l o c a t i o n s , d e l i v e r a s t e a m - w a t e r m i x t u r e t o t h e s e p a r a t o r s where t h e s t e a m f r a c t i o n i n c r e a s e s a s a r e s u l t o f p r e s s u r e reduction.
A t b o t h t y p e s o f r e s e r v o i r , s u b s t a n t i a l l y a l l t h e non-
condensable gas i n t h e o r i g i n a l f l u i d a r r i v e s a t t h e powerhouse.
139
A
x
m
The s t e a m expands a s i t p a s s e s t h r o u g h t h e t u r b i n e t o a
In o r d e r
c o n d e n s e r where c o o l i n g w a t e r removes t h e w a s t e h e a t .
t o maximize e n e r g y e x t r a c t i o n , a vacuum i s m a i n t a i n e d i n t h e condenser.
Noncondensable g a s e s , i n c l u d i n g i n l e a k a g e a i r ,
l i m i t t h e vacuum and a r e removed w i t h g a s - e j e c t o r s
multi-stage,
s t e a m j e t s or c e n t r i f u g a l e x h a u s t e r s .
--
either
A t a l l the
p l a n t s l i s t e d i n T a b l e I e x c e p t t h a t a t Monte Amiata ( c f . n e x t paragraph), d i r e c t c o n t a c t condensers a r e i n use:
the jet-type
a t L a r d a r e l l o , W a i r a k e i and C e r r o P r i e t o ; t h e p e r f o r a t e d t r a y ,
Downloaded by [Universite Laval] at 08:41 14 November 2015
cascade-type a t Geysers. The n o n c o n d e n s a b l e g a s f r a c t i o n i s much l a r g e r i n geot h e r m a l s t e a m t h a n i n t h e s t e a m o f c o n v e n t i o n a l p l a n t s and ejection costs are correspondingly greater.
About 5 % o f t h e
t o t a l s t e a m a t G e y s e r s i s consumed i n g a s e j e c t i o n
[lo].
Al-
though i t seldom h a s , geothermal s t a t i o n d e s i g n s h o u l d i n v o l v e a c a r e f u l balancing o f t h e degree o f gas e j e c t i o n with t h e improved e f f i c i e n c y t h a t o b t a i n s f r o m low t u r b i n e b a c k - p r e s s u r e
1201.
A t Plonte Amiata t h e g a s l o a d s a r e s o h i g h ( c f . T a b l e 11)
t h a t no a t t e m p t i s made a t s e p a r a t i o n or w a t e r c o o l i n g
... t h e
e x h a u s t e d s t e a m cum g a s i s e x p e l l e d d i r e c t l y t o t h e a t m o s p h e r e
[111. The c h o i c e o f d i r e c t c o n t a c t r a t h e r t h a n i n d i r e c t c o n t a c t ( s u r f a c e ) h e a t e x c h a n g e r s i s a n economic one.
The f o r m e r a r e
s i g n i f i c a n t l y c h e a p e r , r e q u i r e l e s s c o o l i n g w a t e r ( w i t h lower pumping c o s t s ) , and p r e s e n t f e w e r c o r r o s i o n p r o b l e m s .
They
a l s o c a n r e d u c e t h e l o a d on t h e g a s e j e c t o r s w i t h a c o r r e s p o n d i n g r i s e i n t h e n e t e l e c t r i c a l output because t h e water-soluble g a s components t e n d t o d i s s o l v e i n and d i s c h a r g e w i t h t h e
140
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS
cooling water.
A Measurement a t W a i r a k e i i n d i c a t e d t h a t a p p r o x -
i m a t e l y 5 0 % o f t h e C02
+
HZS e x i t s v i a t h i s r o u t e
[21].
Cooling Both C e r r o P r i e t o and G e y s e r s employ m e c h a n i c a l d r a f t cooling towers t o cool t h e t u r b i n e condensate.
No f r e s h w a t e r
s u p p l y i s r e q u i r e d : a t G e y s e r s a p p r o x i m a t e l y 8 0 % o f t h e cond e n s a t e e v a p o r a t e s i n t h e t o w e r ; t h e r e m a i n d e r , which c o n t a i n s a b o u t 2 - 4 0 ppm b o r o n , 2OO.ppm NH3, 450 ppm b i c a r b o n a t e and "some f i n e l y s u s p e n d e d s u l f u r " i s r e i n j e c t e d i n t o t h e r e s e r v o i r
Downloaded by [Universite Laval] at 08:41 14 November 2015
[lo].
Natural d r a f t cooling towers a r e i n u s e a t L a r d a r e l l o ;
a t Monte Amiata, a s m e n t i o n e d a b o v e , s t e a m and h o t g a s e x h a u s t d i r e c t l y t o t h e atmosphere.
Only a t t h e W a i r a k e i p l a n t ,
lo-
c a t e d on t h e b a n k s of t h e Waikato R i v e r , i s t h e r e a w a t e r s u p p l y s u f f i c i e n t f o r straight-through
cooling.
Pathways t o t h e Environment E x c e p t a t Monte Amiata, n o n c o n d e n s a b l e s p a r t i t i o n between t h e g a s e j e c t o r e f f l u e n t s , which emerge from s t a c k s on t h e power s t a t i o n r o o f ,
and t h e c o o l i n g w a t e r s t r e a m .
If the l a t t e r
c i r c u l a t e s t o a c o o l i n g t o w e r , i t c o n t a c t s an a t m o s p h e r e w i t h much l o w e r p a r t i a l p r e s s u r e s of C02 and H2S t h a n e x i s t w i t h i n t h e c o n d e n s e r and 80% o f t h e s t r e a m ' s volume e v a p o r a t e s .
As a
r e s u l t , t h e b u l k o f t h e g a s e s a r e d r i v e n o u t of s o l u t i o n and i n t o t h e t o w e r ' s plume. A t Wairakei,
t h e c o n d e n s a t e e f f l u e n t r a i s e s t h e C 0 2 con-
c e n t r a t i o n i n t h e Waikato R i v e r t o more t h a n f o u r t i m e s t h e normal l e v e l d e t e r m i n e d by e q u i l i b r a t i o n w i t h a t m o s p h e r i c C O z . T r a n s p o r t o f t h e e x c e s s t o t h e a t m o s p h e r e i s r a p i d , however, w i t h a t i m e c o n s t a n t o f a p p r o x i m a t e l y 20 minutes depending
141
A X m
m o s t l y upon t h e e x t e n t o f s u r f a c e wi nds [4]. s u l f u r e f f l u e n t is l e s s c l e a r .
The f a t e o f t h e
W h i l e w a t e r s e v e r a l miles down-
s t r e a m f r o m W a i r a k e i h a s t h e c h a r a c t e r i s t i c H2S o d o r , t h e r e i s some e v i d e n c e t h a t o x i d a t i o n t o s u l f a t e o c c u r s i n t h e c o n d e n s e r O x i d a t i o n t o SO2 i s r e p u t e d t o b e " v e r y f a s t " i n f o g or
[21].
c l o u d d r o p l e t s [ 1 7 ] s u c h as may e x i s t i n t h e j e t c o n d e n s e r s a t Wairakei.
So f a r a s we a r e a w a r e , n o g e o t h e r m a l p l a n t h a s y e t b e e n f i t t e d with emission controls.
P r o c e s s e s under development f o r
Downloaded by [Universite Laval] at 08:41 14 November 2015
t h e G e y s e r s f i e l d i n c l u d e b u r n i n g or s c r u b b i n g t h e g a s e j e c t o r e f f l u e n t s and a d d i n g o x i d a n t s t o t h e c o o l i n g t o w e r s t r e a m s . T h e s e m e a s u r e s c o u l d r e d u c e H2S e m i s s i o n b y 7 0 - 8 0 % [ 2 2 ] . I n summary, H2S a n d C02 i n g e o t h e r m a l s t e a m r e a c h e s t h e a t m o s p h e r e a l m o s t i m m e d i a t e l y f o l l o w i n g p o w e r e x t r a c t i o n or a f t e r s h o r t d e l a y s t h a t a r e a f u n c t i o n o f i n d i v i d u a l p l a n t configurations.
The g a s e j e c t o r e f f l u e n t s h a v e r e a s o n a b l y h i g h
c o n c e n t r a t i o n o f H2S (> 4 , 0 0 0 ppm a t N a i r a k e i [ 4 ] ) a n d C02 (Q 30% [12]).
The u s e o f d i r e c t c o n t a c t c o n d e n s e r s , h o w e v e r ,
insures
t h a t as much a s h a l f o f e a c h g a s i s d i l u t e d t o low a q u e o u s c o n c e n t r a t i o n s w h i c h f a l l t o w a r d t h e ppm r a n g e a s s o o n a s t h e s o l u t i o n s emerge from t h e p l a n t . DISCHARGE RATES T a b l e I1 compares t h e i n s t a n t a n e o u s and d a i l y t o t a l d i s c h a r g e r a t e s o f s u l f u r a n d C02 f r o m t h e p l a n t s l i s t e d i n T a b l e I a n d f r o m two c o a l - f i r e d p l a n t s .
The d a i l y r a t e s o f t h e g e o -
t h e r m a l p l a n t s a r e computed fro m t h e r a t e d c a p a c i t i e s g i v e n i n T a b l e I ; t h o s e f o r t h e c o a l p l a n t s a r e f o r a n o u t p u t o f 1 0 0 0 EllV(e)
142
EMISSION CONTROL FOR GEO'MERMAL POWFX PLANTS
There is no constant factor that relates the gas concentrations given in Table I to instantaneous rates (tons per megawatt-day) o f Table 11: the geothermal fluid from each field has a different enthalpy and the thermal efficiencies of the various plants differ widely. Elsewhere we have argued that effluent rates per unit of electrical energy can be misleading in quantitative comparisons of the environmental impact of geothermal and other power technologies [ 4 ] .
The instantaneous rates ignore the fact that
Downloaded by [Universite Laval] at 08:41 14 November 2015
a hot-water geothermal field continues to discharge at fullrate even when the power station is shut down.
Further, pre-
operational testing of the geothermal reservoir and discharges from "wild bores" (i.e.,
wells that cannot b e capped) contribute
an environmental burden which should be assessed against a plant. In the present case we wish to highlight the potential for producing valuable chemicals; and in the scheme outlined below, such production would be possible only while electrical power is produced. Derpite the caveats of the preceding paragraph, the data of Table I1 are startling from an environmental perspective.
Of
the six fields, only two discharge less sulfur per megawatt-day than does the low-sulfur coal plant; one field emits at a greater rate than the high-sulfur coal plant.
Goldsmith no'tes ominously
that sulfur emissions f r o m plants in California's Imperial Valley are likely to be closer in magnitude to those from Cerro Prieto than from Geysers [ 2 ] . Some of the numbers in Table I1 are in mild disagreement
with those of Bowen [ 3 ] ,
who compared the sulfur emission from a
143
AxRiA"
coal plant with an unrealistically high steam rate to the effluent rate from Geysers; and apparently assumed that the molecular weights of sulfur dioxide and hydrogen sulfide are identical.
CH EM ICA L PRODUCTI0N The industrial exploitation of the geothermal fluids has a long, spotted history.
The first boric acid works at Lardarello
.began operations in 1810 [ll].
Between 1934 and 1954 over
100,000 tons of C02 for dry ice production were mined from geothermal wells near the Salton Sea [23].
Pilot plant studies
Downloaded by [Universite Laval] at 08:41 14 November 2015
of the production of sulfur from hydrogen sulfide at the Namafjall field in Iceland were conducted in the early 1950's [24].
In 1968, Ross pointed out that the Broadlands, New Zealand
field had a sulfur production potential as high as 20,000 tons per year [25].
Several international conferences during the past
fifteen years have discussed the possibility of extracting valuable inorganic salts from geothermal brines. Preliminary design studies indicate that a dual-purpose m
chemical plant, coupled to the gas ejector effluents from indirect contact condensers, could provide emission control for some geothermal fields at minimum cost, and, possibly, at a profit. In one configuration, a low-temperature still cooled by absorption refrigeration would produce liquid C02.
The H2S effluent
from the C02 portion of the plant would then be rich enough to feed a conventional Claus plant for sulfur production.
The
highly exothermic Claus reactions would produce high-grade steam for sale to the power plant or for driving the absorption refrigerator. More detailed studies, now underway, will be the subject of a future communication. 144
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS
ACKNOWLEDGMENTS This work was supported by research grants from the Engineering Foundation and the Council on Environmental Studies of Princeton University; and by a travel grant (GF-41575) from the National Sciences Foundation.
It is a pleasure to acknowledge
useful discussions with Bernard0 Dominguez at Cerro Prieto, C.R. James at Wairakei, Ezio Tongiorgi at Pisa and David Barton at Geysers.
Downloaded by [Universite Laval] at 08:41 14 November 2015
REFERENCES S e e , e.g., John Lear, "Clean Power from Inside the Earth", Saturday Reuiew, Dec. 5 , 1970. bl. Goldsmith, Geothermal Resources in CaZifornia: Poten-
tials a n d Problems, Environmental Quality Laboratory Report No. 5, California Institute of Technology (1971). r31
R.G. Bowen, in Geothermal Energy, P. Kruger and C. Otte. eds., Stanford U. Press, Stanford, California, 1973, Chapter 10.
r41
R.C. Axtmann, Environmental Impact of a Geothermal Power Plant, Science 187 (in press).
[5J
A.J. Ellis, New Zealand J o u r n a l Sci. 5, 434-452 (1962).
r61
A.J. Ellis and N.A.J.
17J
A.J. Ellis and W.A.J. Mahon, Jr., Geochem. et Cosrnochim. A c t a 3, 519-539 (1967).
Mahon, Jr., ibid 9, 440-456 (1966).
W.A.J. Mahon and J.B. Finlayson, An. Journ. Sci. 272, 48-68 (1972). [9 I
O.H. Krikorian, in Geothermal Energy, P. Kruger and C. Otte eds., op. cit., Chapter 17, Table 5. J.P.
Finney, Chem. Eng. Prog. 6 8 171, 83-86 (1972).
Anonymous, Lardare110 and Monte Aniata: Electric Power b y Endogeneous Steam, Ente Nazionale per L'Energia Elettrica, Compartimento di Firenze, Direzione Studi e Ricerche, Roma, 1973.
145
I121
R . B . G l o v e r , I n t e r p r e t a t i o n of Gas C o m p o s i t i o n s f r o m t h e W a i r a k e i F i e l d o v e r Ten Y e a r s , U . N . Symp. D e v e l . U t i l i z a t i o n Geothermal Resources, Pisa, I t a l y , S e p t . 1970, Proc.
i n Press. E l l i s , New Z e a l a n d D e p a r t m e n t o f S c i e n t i f i c a n d I n d u s t r i a l R e s e a r c h R e p o r t CD118/16 RBG 31, May 1 8 , 1 9 7 1 .
131
A.J.
141
Anonymous, P l a n t a G e o t e r m o e Z e c t r i c a C e r r o P r i e t o , C o n i s i o n Federa2 d e E l e c t r i c i d a d , Mexico D.F., J u n e 1 9 7 3 .
151
P.R.L. Browne a n d A . J . (1970).
E l l i s , Am. Journ.
Sci. 2 6 9 , 9 7 - 1 3 1
Downloaded by [Universite Laval] at 08:41 14 November 2015
S. M i n e r , PreZiminary A i r P o l l u t i o n S u r v e y of i y d r o g e n S u l f i d e : A L i t e r a t u r e Review, U.S. D e p t . H e a l t h , E d u c a t i o n , and ,Welfare, N a t i o n a l A i r P o l l u t i o n C o n t r o l A d m i n i s t r a t i o n , R a l e i g h , N.C., October 1969.
Kellogg, R.D. Cadre, E.R. A l l e n , A.L. M a r t e l l , S c i e n c e 175, 587-596 (1972).
W.W. E.A.
P r o f . E. T o n g i o r g i , J u n e 1974.
Lazrus.
and
Pisa, I t a l y , p e r s o n a l communication,
I n a d v e r t e n t C l i m a t e M o d i f i c a t i o n : R e p o r t of t h e S t u d y of Man's Impact on C l i m a t e ( S M I C ) , M.I.T. P r e s s , C a m b r i d g e , Elass. C.R.
(1971),
pp.
84-86,
238-240.
J a m e s , New Zealand E n g i n e e r i n g 2 3 (6),
230-236
(1968).
I . K . W a l k e r , New Z e a l a n d D e p a r t m e n t o f S c i e n t i f i c a n d I n d u s t r i a l R e s e a r c h R e p o r t CD118/16-RBG/1SJ Nov. 3 0 , 1 9 6 6 . Anonymous. Environmental Data S t a t e m e n t G e v 8 e r s U n i t 1 4 . P a c i f i c G a s a n d E l e c t r i c a l Company, S a n F r a n c i s c o , C a l i f . J u l y 1973 1231
K o e n i g . Geothermal E z p l o r a t i o n i n t h e W e s t e r n U n i t e d Symp. D e v e l . U t i l i z a t i o n G e o t h e r m a l R e s o u r c e s , P i s a , I t a l y , 1970, Proc. i n p r e s s .
J.B.
S t a t e s , U.N.
L i n d a l , Timarit 40 ( 5 1 ,
84-87
(1955).
t241
B.
1251
J . B . R o s s , New Zealand Journal S c i e n c e , (1968).
11 ( 2 ) ,
249-257
Received October 22, 1974 Accepted December 13, 1974
146
ISSN: 0013-9300 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/lesa17
Emission Control of Gas Effluents from Geothermal Power Plants Robert C. Axtmann To cite this article: Robert C. Axtmann (1975) Emission Control of Gas Effluents from Geothermal Power Plants, Environmental Letters, 8:2, 135-146, DOI: 10.1080/00139307509437427 To link to this article: http://dx.doi.org/10.1080/00139307509437427
Published online: 02 Sep 2009.
Submit your article to this journal
Article views: 3
View related articles
Citing articles: 8 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=2esa20 Download by: [Universite Laval]
Date: 14 November 2015, At: 08:41
ENVIRONMENTAL LETTERS, 8 (21, 135-146 (1975)
EEIISSION CONTROL OF GAS EFFLUENTS FROM GEOTHERMAL POWER PLANTS
R o b e r t C . Axtmann Department o f Chemical E n g i n e e r i n g and Center f o r Environmental Studies Princeton University P r i n c e t o n , New J e r s e y 0 8 5 4 0 ,
ABSTRACT
Downloaded by [Universite Laval] at 08:41 14 November 2015
Geothermal s t e a m a t t h e w o r l d ' s f i v e l a r g e s t power p l a n t s c o n t a i n s f r o m 0 . 1 5 t o 30% n o n c o n d e n s a b l e g a s e s i n c l u d i n g C O H2S,
H2,
CH4,
N2,
H3B03,
and NH3.
2' A t four of the plants the
gases a r e f i r s t s e p a r a t e d from t h e steam and t h e n d i s c h a r g e d t o the environment; a t t h e f i f t h ,
t h e noncondensables exhaust d i -
r e c t l y t o t h e atmosphere a l o n g w i t h s p e n t steam.
Some C 0 2 a n d
s u l f u r e m i s s i o n r a t e s r i v a l t h o s e f r o m f o s s i l - f u e l e d p l a n t s on a
per m e g a w a t t - d a y b a s i s .
The ammonia a n d b o r o n e f f l u e n t s c a n i n -
t e r f e r e w i t h animal and p l a n t l i f e .
The e f f e c t s o f s u l f u r
( w h i c h e m e r g e s a s H2S b u t may o x i d i z e t o SO2) on e i t h e r a m b i e n t a i r q u a l i t y o r l o n g t e r m human h e a l t h a r e l a r g e l y unknown.
Nost
geothermal t u r b i n e s a r e equipped with d i r e c t contact condensers w h i c h c o m p l i c a t e e m i s s i o n c o n t r o l b e c a u s e t h e y p r o v i d e t w o or more p a t h w a y s f o r t h e e f f l u e n t s t o r e a c h t h e e n v i r o n m e n t .
Use
of d i r e c t c o n t a c t c o n d e n s e r s c o u l d p e r m i t e f f i c i e n t e m i s s i o n control i f coupled t o processes t h a t produce s a l e a b l e q u a n t i t i e s o f p u r i f i e d carbon d i o x i d e and e l e m e n t a l s u l f u r .
135 Copyright 0 1975 by hlarcel Dekker. Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying. microfilming. and recording.01 by any informationstorage and retrievalsystem, without permission in writing from the publisher.
Axnw INTRODUCTION G eo th ermal power i s e n v i r o n m e n t a l l y "cl ean", a c c o r d i n g t o t h e c o n v e n t i o n a l wisdom [ l ] .
at least
That i t enjoys such
a r e p u t a t i o n may r e f l e c t t h e m e a g r e e n v i r o n m e n t a l s c r u t i n y t h a t h a s been a c c o r d e d what i s , a s y e t , a r e l a t i v e l y i n s i g n i f i c a n t technology.
Goldsmith's b r i e f b u t somber a s s e s s m e n t h a s had
l i t t l e c i r c u l a t i o n and, i n any c a s e , a d d r e s s e s p o t e n t i a l geo-
t h e r m a l s o u r c e s i n C a l i f o r n i a [2].
Bowen's r o s i e r a n a l y s i s
draws a l m o s t e x c l u s i v e l y on p u b l i s h e d m a t e r i a l s t h a t d e s c r i b e
Downloaded by [Universite Laval] at 08:41 14 November 2015
a s i n g l e operating p l a n t , a l s o i n C a l i f o r n i a [3]. of the first, on-site,
The r e s u l t s
e n v i r o n m e n t a l s t u d y o f a working geo-
thermal p l a n t have y e t t o appear [4]. T h i s r e p o r t e x p l o r e s one a s p e c t of t h e impact o f geothermal power:
t h e d i s c h a r g e t o t h e environment o f noncondensable g a s e s ,
w i t h p a r t i c u l a r e m p h a s i s o n c a r b o n d i o x i d e an d h y d r o g e n s u l f i d e . The e f f l u e n t s from s i x g e o t h e r m a l f i e l d s a r e co m p ar ed i n m ag n i t u d e w i t h each o t h e r and w i t h t h o s e from f o s s i l - f u e l e d p l a n t s . Some e n v i r o n m e n t a l c o n s e q u e n c e s o f t h e d i s c h a r g e s a r e d i s c u s s e d ; problem a r e a s a r e d e f i n e d ; and a p r o c e s s t h a t c o u l d c o n t r o l t h e e m i s s i o n s and p r o d u c e m a r k e t a b l e b y - p r o d u c t s i s d e l i n e a t e d . CHARACTERISTICS OF GAS FRACTIONS 1.
Concentrations B o t h t h e t o t a l g a s c o n t e n t an d t h e r e l a t i v e c o n c e n t r a t i o n s
o f t h e c o n s t i t u e n t s d e p e n d i n t r i c a t e l y upon t h e g e o c h e m i s t r y o f t h e und erg ro u n d r e s e r v o i r [5-81.
Car b o n d i o x i d e is i n v a r i a b l y
t h e m a i n c o m p o n e n t ( 7 8 - 9 5 % ) w i t h l e s s e r c o n c e n t r a t i o n s o f H2S C.1-17%)>, H2 ( 1 - 1 3 % ) , CH4 ( 0 - 1 2 % ) , N2(0.2-9%),
an d NH3
(0-1.7%);
a n d , o c c a s i o n a l l y , t r a c e a m o u n t s o f a r g o n , e t h a n e , H3B03, HF,
a n d SO2 [9].
HCl,
Reports from a t l e a s t t h r e e f i e l d s i n d i c a t e 136
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS t h a t t h e t o t a l gas f r a c t i o n declines with e x p l o i t a t i o n [lo-121. T h i s , t o o , o c c u r s i n a c o m p l i c a t e d way:
a t W a i r a k e i , New Z e a l a n d
t h e t o t a l g a s c o n c e n t r a t i o n d e c r e a s e d by 5 0 % between 1960 and 1969 w h i l e t h e H2S f r a c t i o n r e m a i n e d v i r t u a l l y c o n s t a n t [ 1 2 ] . T a b l e I g i v e s t h e f i e l d - w i d e a v e r a g e s o f t o t a l g a s (w/o) and mole p e r c e n t a g e s o f C02 and H S a t t h e f i v e l a r g e s t power 2 s t a t i o n s [10,11,13.14];
and a t t h e B r o a d l a n d s , New Z e a l a n d f i e l d
1151, p o r t i o n s o f which h a v e d i s c h a r g e d f o r up t o t h r e e y e a r s b u t f o r which no power s t a t i o n h a s been b u i l t .
The T a b l e demon-
Downloaded by [Universite Laval] at 08:41 14 November 2015
s t r a t e s the s i g n i f i c a n t l y different characteristics of closely a d j a c e n t f i e l d s ( W a i r a k e i and B r o a d l a n d s a r e b a r e l y 10 m i l e s a p a r t ; L a r d a r e l l o and Monte Amiata,
l e s s than 40 miles), but
does n o t r e f l e c t i n t r a - f i e l d d i f f e r e n c e s .
In t h e r e l a t i v e l y
impermeable r e s e r v o i r a t B r o a d l a n d s , H S c o n c e n t r a t i o n s i n s t e a m 2 from a d j a c e n t w e l l s d i f f e r by a s much a s a f a c t o r o f t e n [S].
C02 and H2S C O N T E N T OF GEOTHERMAL STEAM ( 1 9 7 3 )
TABLE I.
Plant
Inrtalled Capacity
(MW[e]) G e y s e r s , USA Lardarello.
Italy
Wairokei, New Z e a l a n d
Average Gas ComC02 p o s i t i o n o f Steam Fraction. (w/o) ( v / o ) 1.0
79
5.0
365
4.9
94
1.6 5.0
192
0.1s
95
75
2.7
858
Monte Amiata, Italy
25 5
200 e s t )
8 Weight p e r c e n t a g e s .
*
(v/o)
396
Cerro P r i e t o , Mexico
Broadlands, New Z e a l a n d
H2S
15#
10-30
95
0.4
3.3
94
1.4
Ref.11 l i s t s no o t h e r c o n s t i t u e n t s .
Broadlands h a s been p a r t i a l l y c h a r a c t e r i z e d , s t a t i o n h a s been b u i l t .
137
b u t no power
AXTMANN ENVIRONMENTAL EFFECTS
The g e o t h e r m a l l i
r a t u r e is permeat 3 w i h references
t h e c o r r o s i v e e f f e c t s o f t h i s omnipresent and maladorous gas
I
--
p a r t i c u l a r l y on power s t a t i o n e q u i p m e n t s u c h a s e l c t r i c a l contacts.
W h i l e t h e e f f e c t s o f a c u t e d o s e s a r e known[16],
t h e geo-
t h e r m a l and i n d u s t r i a l hygiene l i t e r a t u r e s have l i t t l e t o o f f e r on t h e l o n g t e r m e f f e c t s on human h e a l t h o f s u b a c u t e e x p o s u r e .
Downloaded by [Universite Laval] at 08:41 14 November 2015
G o l d s m i t h e q u a t e s H2S e m i s s i o n s t o SO2 e m i s s i o n s [ Z ] ;
but while
H2S i s b e l i e v e d t o o x i d i z e i n t h e a t m o s p h e r e , t h e t i m e s c a l e o f t h e r e a c t i o n i s unknown [17].
S o f a r a s we a r e a w a r e , no
systematic experimental o r a n a l y t i c a l s t u d i e s e x i s t t h a t r e l a t e HzS e m i s s i o n s t o a m b i e n t a i r q u a l i t y a n d human h e a l t h .
2.
Ammonia a n d Boron The h a r m f u l e f f e c t o f d i s s o l v e d NH3 o n f i s h l i f e i n B i g
S u l f u r Creek h a s s t i m u l a t e d t h e o p e r a t o r s of t h e Geysers F i e l d t o r e i n j e c t c o n d e n s a t e f r o m t h e c o o l i n g t o w e r s ( s e e s e c t i o n on Pathways t o t h e Environment below) i n t o t h e h y d r o t h e r m a l r e s e r voir
[lo].
More r e c e n t l y ,
L a r d a r e l l o h a s begun r e i n j e c t i o n
e x p e r i m e n t s aimed a t p r e v e n t i n g d e l e t e r i o u s e f f e c t s o f b o r o n on n e a r b y a g r i c u l t u r a l o p e r a t i o n s [18]. 3.
co2 A s shown i n T a b l e 11, C 0 2 d i s c h a r g e r a t e s p e r u n i t o f e l e c -
t r i c a l p o w e r p r o d u c e d a t some g e o t h e r m a l p l a n t s r i v a l t h o s e f r o m fossil-fueled plants. C02 from man's
A c c o r d i n g t o t h e f a m o u s SMIC R e p o r t [ 1 9 ] ,
e n e r g y - p r o d u c i n g a c t i v i t i e s may t h r e a t e n t h e
s t a b i l i t y of g l o b a l c l i m a t e v i a t h e "green-house
138
e f f e c t " so t h a t
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS TABLE 11. SULFUR A N D C02 DISCHARGE RATES FROM GEOTHERMAL AND COAL PLANTS t
Plant
Sulfur
t onnes (hIW-day)-l
c02 tonnes(&IW-day)-'
t:;cq-
C e r r o P r i e t o , Mexico
0.73
55
4.2
315
Coal (3.5%S)#
0.27
270
25.0
25,000
0.19
5
62.0
1 580
0.15
30
18.0
3,600
Monte A m i a t a ,
Italy*
Broadlands, New Z e a l a n d
,
0.13
47
11.0
4,000
Coal (l.O%S)#
0.077
77
25.0
25,000
Geysers,
0.054
21
1.4
700
0.012
2
Lardarello,
Downloaded by [Universite Laval] at 08:41 14 November 2015
tonnEqday
Italy
USA
Wairakei, New Z e a l a n d
0.32
i
Daily r a t e s o f geothermal p l a n t s a t t h e r a t e d c a p a c i t i e s given i n Table I.
#
1 0 0 0 NW(e) c o a l p l a n t w i t h a t h e r m a l e f f i c i e n c y = 4 0 % j c a l o r i f i c v a l u e o f f u e l = l o 4 BTU/lb; s u l f u r r e t e n t i o n i n a s h = 15%. Assumes 20 w/o g a s f r a c t i o n . A c c o r d i n g t o R e f . 1 1 , t h e l a r g e s t u n i t ( 1 5 bIW[e]) a t Monte A m i a t a u t i l i z e s s t e a m w i t h 30 w/o g a s .
*
62
i n t h i s r e s p e c t , a t l e a s t , g e o t h e r m a l power may o f f e r l i t t l e improvement o v e r c o n v e n t i o n a l t e c h n o l o g y . THE POWER C Y C L E The v a p o r - d o m i n a t e d
r e s e r v o i r s o f n o r t h e r n C a l i f o r n i a and
I t a l y p r o d u c e s u p e r h e a t e d s t e a m which p a s s e s t h r o u g h c e n t r i f u g a l s e p a r a t o r s f o r removal o f e n t r a i n e d rock d u s t and l i q u i d w a t e r . Hot w a t e r r e s e r v o i r s , e . g . ,
i n New Z e a l a n d ,
a t Cerro Prieto,
and
most o t h e r l o c a t i o n s , d e l i v e r a s t e a m - w a t e r m i x t u r e t o t h e s e p a r a t o r s where t h e s t e a m f r a c t i o n i n c r e a s e s a s a r e s u l t o f p r e s s u r e reduction.
A t b o t h t y p e s o f r e s e r v o i r , s u b s t a n t i a l l y a l l t h e non-
condensable gas i n t h e o r i g i n a l f l u i d a r r i v e s a t t h e powerhouse.
139
A
x
m
The s t e a m expands a s i t p a s s e s t h r o u g h t h e t u r b i n e t o a
In o r d e r
c o n d e n s e r where c o o l i n g w a t e r removes t h e w a s t e h e a t .
t o maximize e n e r g y e x t r a c t i o n , a vacuum i s m a i n t a i n e d i n t h e condenser.
Noncondensable g a s e s , i n c l u d i n g i n l e a k a g e a i r ,
l i m i t t h e vacuum and a r e removed w i t h g a s - e j e c t o r s
multi-stage,
s t e a m j e t s or c e n t r i f u g a l e x h a u s t e r s .
--
either
A t a l l the
p l a n t s l i s t e d i n T a b l e I e x c e p t t h a t a t Monte Amiata ( c f . n e x t paragraph), d i r e c t c o n t a c t condensers a r e i n use:
the jet-type
a t L a r d a r e l l o , W a i r a k e i and C e r r o P r i e t o ; t h e p e r f o r a t e d t r a y ,
Downloaded by [Universite Laval] at 08:41 14 November 2015
cascade-type a t Geysers. The n o n c o n d e n s a b l e g a s f r a c t i o n i s much l a r g e r i n geot h e r m a l s t e a m t h a n i n t h e s t e a m o f c o n v e n t i o n a l p l a n t s and ejection costs are correspondingly greater.
About 5 % o f t h e
t o t a l s t e a m a t G e y s e r s i s consumed i n g a s e j e c t i o n
[lo].
Al-
though i t seldom h a s , geothermal s t a t i o n d e s i g n s h o u l d i n v o l v e a c a r e f u l balancing o f t h e degree o f gas e j e c t i o n with t h e improved e f f i c i e n c y t h a t o b t a i n s f r o m low t u r b i n e b a c k - p r e s s u r e
1201.
A t Plonte Amiata t h e g a s l o a d s a r e s o h i g h ( c f . T a b l e 11)
t h a t no a t t e m p t i s made a t s e p a r a t i o n or w a t e r c o o l i n g
... t h e
e x h a u s t e d s t e a m cum g a s i s e x p e l l e d d i r e c t l y t o t h e a t m o s p h e r e
[111. The c h o i c e o f d i r e c t c o n t a c t r a t h e r t h a n i n d i r e c t c o n t a c t ( s u r f a c e ) h e a t e x c h a n g e r s i s a n economic one.
The f o r m e r a r e
s i g n i f i c a n t l y c h e a p e r , r e q u i r e l e s s c o o l i n g w a t e r ( w i t h lower pumping c o s t s ) , and p r e s e n t f e w e r c o r r o s i o n p r o b l e m s .
They
a l s o c a n r e d u c e t h e l o a d on t h e g a s e j e c t o r s w i t h a c o r r e s p o n d i n g r i s e i n t h e n e t e l e c t r i c a l output because t h e water-soluble g a s components t e n d t o d i s s o l v e i n and d i s c h a r g e w i t h t h e
140
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS
cooling water.
A Measurement a t W a i r a k e i i n d i c a t e d t h a t a p p r o x -
i m a t e l y 5 0 % o f t h e C02
+
HZS e x i t s v i a t h i s r o u t e
[21].
Cooling Both C e r r o P r i e t o and G e y s e r s employ m e c h a n i c a l d r a f t cooling towers t o cool t h e t u r b i n e condensate.
No f r e s h w a t e r
s u p p l y i s r e q u i r e d : a t G e y s e r s a p p r o x i m a t e l y 8 0 % o f t h e cond e n s a t e e v a p o r a t e s i n t h e t o w e r ; t h e r e m a i n d e r , which c o n t a i n s a b o u t 2 - 4 0 ppm b o r o n , 2OO.ppm NH3, 450 ppm b i c a r b o n a t e and "some f i n e l y s u s p e n d e d s u l f u r " i s r e i n j e c t e d i n t o t h e r e s e r v o i r
Downloaded by [Universite Laval] at 08:41 14 November 2015
[lo].
Natural d r a f t cooling towers a r e i n u s e a t L a r d a r e l l o ;
a t Monte Amiata, a s m e n t i o n e d a b o v e , s t e a m and h o t g a s e x h a u s t d i r e c t l y t o t h e atmosphere.
Only a t t h e W a i r a k e i p l a n t ,
lo-
c a t e d on t h e b a n k s of t h e Waikato R i v e r , i s t h e r e a w a t e r s u p p l y s u f f i c i e n t f o r straight-through
cooling.
Pathways t o t h e Environment E x c e p t a t Monte Amiata, n o n c o n d e n s a b l e s p a r t i t i o n between t h e g a s e j e c t o r e f f l u e n t s , which emerge from s t a c k s on t h e power s t a t i o n r o o f ,
and t h e c o o l i n g w a t e r s t r e a m .
If the l a t t e r
c i r c u l a t e s t o a c o o l i n g t o w e r , i t c o n t a c t s an a t m o s p h e r e w i t h much l o w e r p a r t i a l p r e s s u r e s of C02 and H2S t h a n e x i s t w i t h i n t h e c o n d e n s e r and 80% o f t h e s t r e a m ' s volume e v a p o r a t e s .
As a
r e s u l t , t h e b u l k o f t h e g a s e s a r e d r i v e n o u t of s o l u t i o n and i n t o t h e t o w e r ' s plume. A t Wairakei,
t h e c o n d e n s a t e e f f l u e n t r a i s e s t h e C 0 2 con-
c e n t r a t i o n i n t h e Waikato R i v e r t o more t h a n f o u r t i m e s t h e normal l e v e l d e t e r m i n e d by e q u i l i b r a t i o n w i t h a t m o s p h e r i c C O z . T r a n s p o r t o f t h e e x c e s s t o t h e a t m o s p h e r e i s r a p i d , however, w i t h a t i m e c o n s t a n t o f a p p r o x i m a t e l y 20 minutes depending
141
A X m
m o s t l y upon t h e e x t e n t o f s u r f a c e wi nds [4]. s u l f u r e f f l u e n t is l e s s c l e a r .
The f a t e o f t h e
W h i l e w a t e r s e v e r a l miles down-
s t r e a m f r o m W a i r a k e i h a s t h e c h a r a c t e r i s t i c H2S o d o r , t h e r e i s some e v i d e n c e t h a t o x i d a t i o n t o s u l f a t e o c c u r s i n t h e c o n d e n s e r O x i d a t i o n t o SO2 i s r e p u t e d t o b e " v e r y f a s t " i n f o g or
[21].
c l o u d d r o p l e t s [ 1 7 ] s u c h as may e x i s t i n t h e j e t c o n d e n s e r s a t Wairakei.
So f a r a s we a r e a w a r e , n o g e o t h e r m a l p l a n t h a s y e t b e e n f i t t e d with emission controls.
P r o c e s s e s under development f o r
Downloaded by [Universite Laval] at 08:41 14 November 2015
t h e G e y s e r s f i e l d i n c l u d e b u r n i n g or s c r u b b i n g t h e g a s e j e c t o r e f f l u e n t s and a d d i n g o x i d a n t s t o t h e c o o l i n g t o w e r s t r e a m s . T h e s e m e a s u r e s c o u l d r e d u c e H2S e m i s s i o n b y 7 0 - 8 0 % [ 2 2 ] . I n summary, H2S a n d C02 i n g e o t h e r m a l s t e a m r e a c h e s t h e a t m o s p h e r e a l m o s t i m m e d i a t e l y f o l l o w i n g p o w e r e x t r a c t i o n or a f t e r s h o r t d e l a y s t h a t a r e a f u n c t i o n o f i n d i v i d u a l p l a n t configurations.
The g a s e j e c t o r e f f l u e n t s h a v e r e a s o n a b l y h i g h
c o n c e n t r a t i o n o f H2S (> 4 , 0 0 0 ppm a t N a i r a k e i [ 4 ] ) a n d C02 (Q 30% [12]).
The u s e o f d i r e c t c o n t a c t c o n d e n s e r s , h o w e v e r ,
insures
t h a t as much a s h a l f o f e a c h g a s i s d i l u t e d t o low a q u e o u s c o n c e n t r a t i o n s w h i c h f a l l t o w a r d t h e ppm r a n g e a s s o o n a s t h e s o l u t i o n s emerge from t h e p l a n t . DISCHARGE RATES T a b l e I1 compares t h e i n s t a n t a n e o u s and d a i l y t o t a l d i s c h a r g e r a t e s o f s u l f u r a n d C02 f r o m t h e p l a n t s l i s t e d i n T a b l e I a n d f r o m two c o a l - f i r e d p l a n t s .
The d a i l y r a t e s o f t h e g e o -
t h e r m a l p l a n t s a r e computed fro m t h e r a t e d c a p a c i t i e s g i v e n i n T a b l e I ; t h o s e f o r t h e c o a l p l a n t s a r e f o r a n o u t p u t o f 1 0 0 0 EllV(e)
142
EMISSION CONTROL FOR GEO'MERMAL POWFX PLANTS
There is no constant factor that relates the gas concentrations given in Table I to instantaneous rates (tons per megawatt-day) o f Table 11: the geothermal fluid from each field has a different enthalpy and the thermal efficiencies of the various plants differ widely. Elsewhere we have argued that effluent rates per unit of electrical energy can be misleading in quantitative comparisons of the environmental impact of geothermal and other power technologies [ 4 ] .
The instantaneous rates ignore the fact that
Downloaded by [Universite Laval] at 08:41 14 November 2015
a hot-water geothermal field continues to discharge at fullrate even when the power station is shut down.
Further, pre-
operational testing of the geothermal reservoir and discharges from "wild bores" (i.e.,
wells that cannot b e capped) contribute
an environmental burden which should be assessed against a plant. In the present case we wish to highlight the potential for producing valuable chemicals; and in the scheme outlined below, such production would be possible only while electrical power is produced. Derpite the caveats of the preceding paragraph, the data of Table I1 are startling from an environmental perspective.
Of
the six fields, only two discharge less sulfur per megawatt-day than does the low-sulfur coal plant; one field emits at a greater rate than the high-sulfur coal plant.
Goldsmith no'tes ominously
that sulfur emissions f r o m plants in California's Imperial Valley are likely to be closer in magnitude to those from Cerro Prieto than from Geysers [ 2 ] . Some of the numbers in Table I1 are in mild disagreement
with those of Bowen [ 3 ] ,
who compared the sulfur emission from a
143
AxRiA"
coal plant with an unrealistically high steam rate to the effluent rate from Geysers; and apparently assumed that the molecular weights of sulfur dioxide and hydrogen sulfide are identical.
CH EM ICA L PRODUCTI0N The industrial exploitation of the geothermal fluids has a long, spotted history.
The first boric acid works at Lardarello
.began operations in 1810 [ll].
Between 1934 and 1954 over
100,000 tons of C02 for dry ice production were mined from geothermal wells near the Salton Sea [23].
Pilot plant studies
Downloaded by [Universite Laval] at 08:41 14 November 2015
of the production of sulfur from hydrogen sulfide at the Namafjall field in Iceland were conducted in the early 1950's [24].
In 1968, Ross pointed out that the Broadlands, New Zealand
field had a sulfur production potential as high as 20,000 tons per year [25].
Several international conferences during the past
fifteen years have discussed the possibility of extracting valuable inorganic salts from geothermal brines. Preliminary design studies indicate that a dual-purpose m
chemical plant, coupled to the gas ejector effluents from indirect contact condensers, could provide emission control for some geothermal fields at minimum cost, and, possibly, at a profit. In one configuration, a low-temperature still cooled by absorption refrigeration would produce liquid C02.
The H2S effluent
from the C02 portion of the plant would then be rich enough to feed a conventional Claus plant for sulfur production.
The
highly exothermic Claus reactions would produce high-grade steam for sale to the power plant or for driving the absorption refrigerator. More detailed studies, now underway, will be the subject of a future communication. 144
EMISSION CONTROL FOR GEOTHERMAL POWER PLANTS
ACKNOWLEDGMENTS This work was supported by research grants from the Engineering Foundation and the Council on Environmental Studies of Princeton University; and by a travel grant (GF-41575) from the National Sciences Foundation.
It is a pleasure to acknowledge
useful discussions with Bernard0 Dominguez at Cerro Prieto, C.R. James at Wairakei, Ezio Tongiorgi at Pisa and David Barton at Geysers.
Downloaded by [Universite Laval] at 08:41 14 November 2015
REFERENCES S e e , e.g., John Lear, "Clean Power from Inside the Earth", Saturday Reuiew, Dec. 5 , 1970. bl. Goldsmith, Geothermal Resources in CaZifornia: Poten-
tials a n d Problems, Environmental Quality Laboratory Report No. 5, California Institute of Technology (1971). r31
R.G. Bowen, in Geothermal Energy, P. Kruger and C. Otte. eds., Stanford U. Press, Stanford, California, 1973, Chapter 10.
r41
R.C. Axtmann, Environmental Impact of a Geothermal Power Plant, Science 187 (in press).
[5J
A.J. Ellis, New Zealand J o u r n a l Sci. 5, 434-452 (1962).
r61
A.J. Ellis and N.A.J.
17J
A.J. Ellis and W.A.J. Mahon, Jr., Geochem. et Cosrnochim. A c t a 3, 519-539 (1967).
Mahon, Jr., ibid 9, 440-456 (1966).
W.A.J. Mahon and J.B. Finlayson, An. Journ. Sci. 272, 48-68 (1972). [9 I
O.H. Krikorian, in Geothermal Energy, P. Kruger and C. Otte eds., op. cit., Chapter 17, Table 5. J.P.
Finney, Chem. Eng. Prog. 6 8 171, 83-86 (1972).
Anonymous, Lardare110 and Monte Aniata: Electric Power b y Endogeneous Steam, Ente Nazionale per L'Energia Elettrica, Compartimento di Firenze, Direzione Studi e Ricerche, Roma, 1973.
145
I121
R . B . G l o v e r , I n t e r p r e t a t i o n of Gas C o m p o s i t i o n s f r o m t h e W a i r a k e i F i e l d o v e r Ten Y e a r s , U . N . Symp. D e v e l . U t i l i z a t i o n Geothermal Resources, Pisa, I t a l y , S e p t . 1970, Proc.
i n Press. E l l i s , New Z e a l a n d D e p a r t m e n t o f S c i e n t i f i c a n d I n d u s t r i a l R e s e a r c h R e p o r t CD118/16 RBG 31, May 1 8 , 1 9 7 1 .
131
A.J.
141
Anonymous, P l a n t a G e o t e r m o e Z e c t r i c a C e r r o P r i e t o , C o n i s i o n Federa2 d e E l e c t r i c i d a d , Mexico D.F., J u n e 1 9 7 3 .
151
P.R.L. Browne a n d A . J . (1970).
E l l i s , Am. Journ.
Sci. 2 6 9 , 9 7 - 1 3 1
Downloaded by [Universite Laval] at 08:41 14 November 2015
S. M i n e r , PreZiminary A i r P o l l u t i o n S u r v e y of i y d r o g e n S u l f i d e : A L i t e r a t u r e Review, U.S. D e p t . H e a l t h , E d u c a t i o n , and ,Welfare, N a t i o n a l A i r P o l l u t i o n C o n t r o l A d m i n i s t r a t i o n , R a l e i g h , N.C., October 1969.
Kellogg, R.D. Cadre, E.R. A l l e n , A.L. M a r t e l l , S c i e n c e 175, 587-596 (1972).
W.W. E.A.
P r o f . E. T o n g i o r g i , J u n e 1974.
Lazrus.
and
Pisa, I t a l y , p e r s o n a l communication,
I n a d v e r t e n t C l i m a t e M o d i f i c a t i o n : R e p o r t of t h e S t u d y of Man's Impact on C l i m a t e ( S M I C ) , M.I.T. P r e s s , C a m b r i d g e , Elass. C.R.
(1971),
pp.
84-86,
238-240.
J a m e s , New Zealand E n g i n e e r i n g 2 3 (6),
230-236
(1968).
I . K . W a l k e r , New Z e a l a n d D e p a r t m e n t o f S c i e n t i f i c a n d I n d u s t r i a l R e s e a r c h R e p o r t CD118/16-RBG/1SJ Nov. 3 0 , 1 9 6 6 . Anonymous. Environmental Data S t a t e m e n t G e v 8 e r s U n i t 1 4 . P a c i f i c G a s a n d E l e c t r i c a l Company, S a n F r a n c i s c o , C a l i f . J u l y 1973 1231
K o e n i g . Geothermal E z p l o r a t i o n i n t h e W e s t e r n U n i t e d Symp. D e v e l . U t i l i z a t i o n G e o t h e r m a l R e s o u r c e s , P i s a , I t a l y , 1970, Proc. i n p r e s s .
J.B.
S t a t e s , U.N.
L i n d a l , Timarit 40 ( 5 1 ,
84-87
(1955).
t241
B.
1251
J . B . R o s s , New Zealand Journal S c i e n c e , (1968).
11 ( 2 ) ,
249-257
Received October 22, 1974 Accepted December 13, 1974
146
