Virginia Electric & Power Co

10. Ohio

Length (km) 440 295 1,048 1,774 1,452 2,032 2,237 2,419 565 174

Annual Capacity (million metric tons)

4.7 11.4 9.8 24.6 9.8 21.6 24.6 22-54

4.9 1.3

F i g u r e 2 . 2 . 1 . E x i s t i n g and proposed c o a l - s l u r r y p i p e l i n e s i n t h e U n i t e d S t a t e s , 1981.

(Source: S l u r r y T r a n s p o r t A s s o c i a t i o n . )

35

Water

and

energy--Demands and effects

P r i n c i p a l w a t e r - u s i n g processes i n t h e energy s e c t o r

Water and energy--Demands and effects

a, U 1 u

38

P r i n c i p a l - t e r - u s i n g processes i n t h e energy s e c t o r

Water and energy--Demands und effects

fabrication

T

Recovered

Reprocessing

I

uranium

PUO,

Mixed oxide fuel fabrication Enrichment

Conversion to UF,

A a nUranium dmills mines

Ore

Sper

V O

uel disposal

Federal waste repository

F i g u r e 2.3.2.2. L i g h t - w a t e r r e a c t o r f u e l c y c l e . (Source: U.S. Department o f Energy, 1981a.)

m e t r i c t o n s o f UF Thus t h e t y p i c e l g l a n t , r a t e d a t 117 m i l l i o n m e t r i c t o n s o i l e q u i v a l e n t p e r y e a r , consumes about 2.3 x 10 m o f w a t e r a n n u a l l y . The e v a p o r a t i v i j Cjonsumption o f c o o l i n g water i s e s t i m a t e d b y t h e U.S. Department o f Energy (1980) a t 20 x 10

The g a s e o u s - d i f f u s i o n p r o c e s s r e q u i r e s a g r e a t e l e c t r i c a l i n p u t t o d r i v e t h e system, and t o m a i n t a i n t h e d e s i r e d o p e r a t i n g temperatures, l a r g e volumes o f c o o l i n g water a r e used.

I n d e e d , n e x t t o t h e w a t e r consumption a t t r i b u t a b l e t o t h e u r a n i u m - m i l l i n g stage, t h i s i s t h e l a r g e s t element o f water consumption i n t h e n u c l e a r - f u e l c y c l e . F o r a modular u n i t o f 12,000 m e t r i c t o n s annual c a p a c i t y , t h e i n p u t o f 9.3 m e t r i c t o n s o f ü,F6 and t h e o u t p u t i s 6.1 t o n s o f en i c e d uranium i n UF form. The w a t e r i n t a k e i s 20 x 10 m

,

o f which 93 p e r c e n t , o r 19 x 10 m

,

i s consumed t k r o u g h e v a p o r a t i o n . The u n i t consumptive use i s e s t i m a t e d b y t h e U.S.

Department o f Energy (1980) t o be about 33 I / t o e . 6 '

m p e r Mtoe.

6 4

⁶

2.4. CONVERSION

"Energy conversion" embraces t h e concept OF changing an energy raw m a t e r i a l o r an i n t e r m e d i a t e p r o d u c t i n t o a more-usable commodity. Examples i n c l u d e t h e d i r e c t b u r n i n g o f c o a l ( F i g u r e 2 . 4 ) , o i l , o r n a t u r a l gas t o c o n v e r t c h e m i c a l energy t o e l e c t r i c a l energy and c o n v e r t i n g t h e t h e r m a l energy produced i n n u c l e a r E i s s i o n t o e l e c t r i c a l energy. "Conversion" can a l s o mean changing t h e f o r m o f t h e raw m a t e r i a l ( f o r example, c o a l ) i n t o a c l e a n , more c o n v e n i e n t f u e l ( f o r example, gas f o r space h e a t i n g ) o r even i n t o a f o r m o f o i l f o r f u r t h e r r e f i n i n g . Much o f t h e c u r r e n t emphasis on c o n v e r s i o n c o n c e n t r a t e s on c o n s e r v i n g c o s t l y o i l t h r o u g h t h e use o f

40

mu

Water

and

snergp.--Demands

and

effects

P r i n c i p a l w a t e r - u s i n g processes i n t h e energy s e c t o r

F i g u r e 2.4.1A.

10

20 30 40 50

CYCLE THERMAL E F F I C I E N C Y , %

Heat r e j e c t i o n by power-plant c o o l i n g systems v s . t h e r m a l e f f i c i e n c y compared w i t h a system h a v i n g 35 p e r c e n t t h e r m a l e f f i c i e n c y . (Source:

Robertson, i n K e s t i n , 1980.)

I n modern s t e a m - e l e c t r i c power p l a n t s

,

maximum t h e r m a l e f f i c i e n c y i s a c h i e v e d b y t h e use o f v e r y high steam temperatures and i n l e t p r e s s u r e s . I n t h e newer f o s s i l - f u e l e d p l a n t s , f o r example, t h e r m a l e f f i c i e n c y o f 40 p e r c e n t i s r e a l i z e d w i t h i n l e t temperatures as high as 538OC and p r e s s u r e s o f 246 kg/cm

.

^{I n} t h e c y c l e employed i n c o n v e n t i o n a l s t e a m - e l e c t r i c g e n e r a t i o n , t h e t h e o r e t i c a l l i m i t o f energy t h a t can be e x t r a c t e d f r o m t h e steam i s about 40 p e r c e n t ; thus, 60 per c e n t o f t h e i n p u t energy a t a minimum i s i n e v i t a b l y l o s t u n l e s s t h i s waste t h e r m a l energy can be p u t t o use i n some o t h e r process r e q u i r i n g low-grade h e a t .

The e v a p o r a t i v e demand o f a s t e a m - e l e c t r i c g e n e r a t o r i s i n v e r s e l y p r o p o r t i o n a l t o i t s t h e r m a l e f f i c i e n c y . About 85 p e r c e n t o f t h e energy i n p u t t o t h e t y p i c a l p l a n t i s used t o d r i v e t h e t u r b i n e s o r i s d i s p o s e d o f as t h e r m a l waste i n t h e f o r m o f warmed water. P r e s e n t n u c l e a r p l a n t s a r e l e s s e f f i c i e n t t h a n f o s s i l - f u e l e d p l a n t s because o f s a f e t y r e s t r i c t i o n s on maximum steam temperatures. A l s o , n u c l e a r p l a n t s d i s s i p a t e h e a t almost e n t i r e l y t o c o o l i n g water because no f l u e gases a r e r e l e a s e d . A c c o r d i n g l y , a t y p i c a l n u c l e a r p l a n t o f 31 p e r c e n t t h e r m a l e f f i c i e n c y r e l e a s e s about 50 p e r c e n t more h e a t t o c o o l i n g w a t e r t h a n a f o s s i l - f u e l e d p l a n t o f comparable c a p a c i t y ( F i g u r e 2.4.1C).

2

Water and energy--Demands

and

effects

44

P r i n c i p a l water-using processes i n t h e energy s e c t o r

Containment structure

Steam line

Turbin e

+

Isolation generator

valves

1 I

Pump

Feed- Condenser

water

i

cooling

water . line

Pressurized water reactor-simplified diagram.

F i g u r e 2.4. I C . Diagram o f flows i n l i g h t - w a t e r r e a c t o r and sketch o f n u c l e a r power-plant l a y o u t . (Source: U.S. Department o f Energy, 1981a.)

Water and energy--Demands

and

effects

PmncipaZ water-using processes in the energy sector

U C a a

$4 al

a

O .rl $4 U O al d al I

al U m

a al rl al 1 u-l

I rl .rl m m O VI rl

50

fj

z

O O

9

4 rl 4 O -4 a h U

u-l O

m $4 M 4 .rl V

O) a m P I U a al m

a 2

^rl

n

?

a 4 c3 al $4

M .rl Er

47

Water and energy--Demands and effects

Principal water-using processes in the energy sector

O) Li 1 M

cr, .d

49

Water and energy--Demands and effects

M e c h a n i c a l - d r a f t c o o l i n g towers use e l e c t r i c fans t o p r o v i d e a i r f l o w t h r o u g h t h e tower f i l l . The u n i t s may be designed f o r t h e fans t o o p e r a t e i n e i t h e r a f o r c e d d r a f t o r an i n d u c e d - d r a f t mode, b u t t h e l a t t e r i s more commonly used. Water t o be c o o l e d i s pumped t o the t o p o f t h e tower, i s d i s t r i b u t e d t h r o u g h headers, t h e n cascades o v e r a s e r i e s o f s p l a s h boards t o t h e base o f t h e tower. The d i r e c t i o n o f a i r f l o w may b e c o u n t e r c u r r e n t o r c r o s s f l o w t o t h e f l o w o f t h e water.

F i g u r e 2.4.1.1B shows a t y p i c a l i n d u c e d - d r a f t ( c r o s s f l o w and c o u n t e r f l o w , wet m e c h a n i c a l - d r a f t ) c o o l i n g tower. The main advantages o f c o u n t e r f l o w towers a r e t h a t t h e process i s more e f f i c i e n t and can be adapted t o more r e s t r i c t e d spaces. The more w i d e l y used c r o s s f l o w t y p e has t h e advantages o f lower a i r - s i d e p r e s s u r e drops and more u n i f o r m d i s t r i b u t i o n o f a i r and w a t e r streams. Each module i s a separage u n i t w i t h i t s own fan, and

t h e l o u v e r e d openings a r e on o n l y two s i d e s , p e r m i t t i n g t h e modules t o be a r r a n g e d end t o end i n l o n g rows. The s l o p e d t r a p e z o i d a l s i d e s o f t h e tower conform t o t h e p a t h o f t h e f a l l . i n g water as i t i s p u l l e d t o t h e c e n t e r b y t h e c r o s s - f l o w i n g a i r s t r e a m . I n t e r n a l s u p p o r t s a r e made o f redwood, t r e a t e d f i r , o r i r o n . The s p l a s h boards a r e made o f p l a s t i c , asbestos cement board, o r p o l y v i n y l c h l o r i d e . Modern m e c h a n i c a l - d r a f t towers have f i b e r g l a s s f a n b l a d e s up t o 8.5 m i n d i a m e t e r and t h e fans a r e d r i v e n b y 150 kW.motors.

A 7

Steam inlet

Water inlei

Shell expansion

Section E-B

F i g u r e 2.4.1.1B. S e c t i o n t h r o u g h m e c h a n i c a l - d r a f t c o o l i n g tower. (Source: Robertson i n K e s t i n , 1980.)

The performance o f n a t u r a l - d r a f t c o o l i n g towers i s more s e n s i t i v e t o l o c a l weather c o n d i t i o n s t h a n i s t h a t o f f o r c e d - d r a f t towers. The n a t u r a l - d r a f t d e s i g n was f i r s t developed i n n o r t h e r n Europe, where wet-bulb temperatures a r e low and h u m i d i t i e s a r e h i g h , thus f a v o r i n g t h a t design. Moreover, peak l o a d s tend t o o c c u r i n w i n t e r , when temperatures a r e l o w e s t , p r o v i d i n g t h e most f a v o r a b l e o p e r a t i n g c o n d i t i o n s f o r t h e n a t u r a l - d r a f t design. I n c o n t r a s t

,

n a t u r a l - d r a f t towers a r e p o o r l y s u i t e d t o h o t , d r y areas, such as t h e s o u t h w e s t e r n U n i t e d S t a t e s , where wet-bulb temperatures tend t o be high and h u m i d i t y low, and peak l o a d s occur i n t h e summer owing t o a i r c o n d i t i o n i n g .

The s e l e c t i o n o f n a t u r a l - d r a f t r a t h e r t h a n f o r c e d - d r a f t c o o l i n g towers a t many s i t e s t h r o u g h o u t the w o r l d t e s t i f i e s t o t h e i r advantages where circumstances a r e f a v o r a b l e . F a v o r a b l e c o n d i t i o n s i n c l u d e (1) low avarage wet-bulb temperature and h i g h r e l a t i v e h u m i d i t i e s , ( 2 ) a b r o a d c o o l i n g range and a h i g h v a l u e f o r approach temperature, ( 3 ) l a r g e w i n t e r l o a d s , ( 4 ) l o n g a m o r t i z a t i o n , ( 5 ) l a r g e s t a t i o n s i z e , ( 6 ) need t o r e s t r i c t g r o u n d - l e v e l

f o g g i n g , and ( 7 ) s i t e s where t h e h e i g h t i s n o t a problem. As i n m e c h a n i c a l - d r a f t towers, t h e r e a r e two b a s i c t y p e s o f n a t u r a l - d r a f t towers, c r o s s f l o w and c o u n t e r f l o w . I n t h e c r o s s f l o w , t h e f i l l o v e r which t h e water cascades i s l o c a t e d i n a r i n g o u s i d e t h e base o f t h e tower and t h e i n s i d e serves m a i n l y as a chimney. I n t h e c o u n t e r f l o w type, t h e f i l l i s i n s i d e t h e base and i s e l e v a t e d so t h a t t h e a i r e n t e r i n g t h e p e r i p h e r y moves upward t h r o u g h t h e

50

P r i n c i p a i i water-using processes i n t h e energy s e c t o r

Water and energy--Demands

and

effects

P r i n c i p a l . w a t e r - u s i n g processes i n t h e energy sector D i r e c t o r i n d i r e c t h y d r o g e n a t i o n and p y r o l y s i s , e i t h e r alone o r i n c o m b i n a t i o n , a r e t h e main processes employed i n s y n f u e l p r o d u c t i o n . D i r e c t h y d r o g e n a t i o n i n v o l v e s e x p o s i n g t h e c o a l t o hydrogen a t h i g h p r e s s u r e . G e n e r a l l y , t h e hydrogen i s produced b y r e a c t i n g steam w i t h carbon char. I n d i r e c t h y d r o g e n a t i o n i s accomplished b y r e a c t i n g c o a l d i r e c t l y w i t h steam o r d i s s o l v i n g the c o a l i n a hydrogen-donor s o l v e n t . i n p y r o l y s i s , c o a l i s h e a t e d i n an i n e r t atmosphere and t h e n i s decomposed t o y i e l d s o l i d carbon and gases and l i q u i d s w i t h h i g h e r hydrogen c o n t e n t s than the o r i g i n a l c o a l . P y r o l y s i s g e n e r a l l y i s a f i r s t s t e p i n most processes. Water vapor and o i l f r a c t i o n s d r i v e n o f f d u r i n g p y r o l y s i s a r e s e p a r a t e d o u t . The water, termed "process condensate ^,'I can be t r e a t e d and reused.

To produce c l e a n - b u r n i n g f u e l s , t h e s u l f u r and n i t r o g e n compounds i n t h e c o a l m u s t be removed. Sulfur, m a i n l y i n t h e form o f hydrogen s u l f i d e , and n i t r o g e n , i n t h e form o f ammonia, a r e p r e s e n t i n t h e gas made f r o m t h e c o a l and i n t h e gas produced i n h y d r o t r e a t i n g p y r o l y s i s o i l s and s y n t h e t i c crudes. Removal o f t h e hydrogen s u l f i d e and ammonia may r e q u i r e a l i q u i d wash u s i n g water, and steam may b e needed f o r h e a t i n g o r f o r gas c l e a n i n g . The demand f o r process water c o n s i s t s m a i n l y o f water f o r h y d r o g e n a t i o n and f o r removal o f contaminants.

C o o l i n g , o r t h e d i s s i p a t i o n o f h e a t , i s an e s s e n t i a l element o f a l l s y n f u e l processes.

The h e a t i n g v a l u e i n t h e i n p u t c o a l t h a t i s n o t r e c o v e r e d i n t h e s y n f u e l p r o d u c t o r b y p r o d u c t s m u s t be t r a n s f e r r e d t o t h e environment. Some o f t h i s waste h e a t i s l o s t d i r e c t l y t o t h e atmosphere, as f l u e gases, as water vapor f r o m c o a l d r y i n g , and i n o t h e r d i r e c t ways ( s u c h as c o n v e c t i o n and r a d i a t i o n from machinery and c o n t a i n e r s ) . However

,

t h e l a r g e s t p r o p o r t i o n o f t h e waste h e a t i s d i s s i p a t e d i n d i r e c t l y t o t h e atmosphere t h r o u g h a h e a t - t r a n s f e r s u r f a c e . The h e a t - t r a n s f e r s u r f a c e may be c o o l e d b y water, which i s t h e n c o o l e d i n a c o o l i n g tower b y e v a p o r a t i o n o f p a r t o f t h e w a t e r . A l t e r n a t i v e l y , t h e h e a t may be d i s s i p a t e d t o a s t r e a m o f a i r p a s s i n g o v e r a m e t a l s u r f a c e as i n an a u t o m o b i l e r a d i a t o r . T h i s l a t t e r method, which consumes no w a t e r , i s termed " d r y c o o l i n g , " and t h e e v a p o r a t i v e systems a r e termed "wet c o o l i n g . " Under t h e same c o n d i t i o n s , a d r y - c o o l i n g system d i s s i p a t e s l e s s h e a t p e r u n i t a r e a than does a wet system; t h e r e f o r e , f o r comparable c o o l i n g c a p a c i t y , a l a r g e r s u r f a c e i s r e q u i r e d f o r d r y c o o l i n g . T h i s adds t o t h e i n i t i a l c o s t o f t h e u n i t . I n a d d i t i o n , a d r y - c o o l i n g system cannot reach as low a temperature as a comparable wet system, r e s u l t i n g i n lowered e f f i c i e n c y i n h o t weather. N e v e r t h e l e s s , i n many stages o f s y n f u e l p r o d u c t i o n , d r y c o o l i n g i s p r e f e r a b l e t o wet c o o l i n g .

The q u a n t i t y o f water r e q u i r e d f o r wet c o o l i n g depends m a i n l y on the o v e r a l l t h e r m a l e f f i c i e n c y o f t h e process employed f o r c o n v e r s i o n . The o v e r a l l e f f i c i e n c y determines t h e p r o p o r t i o n o f waste h e a t t h a t must be d i s s i p a t e d . Not a l l t h e waste h e a t must be d i s s i p a t e d b y c o o l i n g systems, however, because much o f i t escapes t o t h e atmosphere b y o t h e r means. I n g e n e r a l , t h e g r e a t e r t h e hydrogen p r o p o r t i o n i n the f i n a l p r o d u c t , t h e more p r o c e s s i n g i s r e q u i r e d , r e s u l t i n g i n lower t h e r m a l e f f i c i e n c y and h i g h e r c o o l i n g r e q u i r e m e n t s . Thus, c o a l g a s i f i c a t i o n r e q u i r e s more c o o l i n g than c o a l l i q u e f a c t i o n , which i n turn r e q u i r e s more c o o l i n g t h a n p r o d u c t i o n o f s o l i d f u e l , as shown i n t h e t a b l e below.

Carbonjhydrogen Process c o n v e r s i o n R e l a t i v e c o o l i n g Process

w e i g h t r a t i o e f f i c i e n c y ( % ) r e q u i r e m e n t

Coal g a s i f i c a t i o n 3 65-70 6

C o a l l i q u e f a c t i o n 9 70- 75 3

S o l i d f u e l p r o d u c t i o n 16 75-80 1

S y n t h e t i c f u e l s can be produced i n s e v e r a l ways, as shown i n F i g u r e 2.4.3. Coal g e n e r a l l y i s t r e a t e d f i r s t b y p y r o l y s i s , and t h e r e s u l t i n g gaseous, l i q u i d , and s o l i d p r o d u c t s a r e f u r t h e r t r e a t e d . The o f f - g a s e s undergo g a s i E i c a t i o n o r h y d r o g a s i f i c a t i o n , and t h e l i q u i d s a r e h y d r o t r e a t e d t o r a i s e t h e i r hydrogen c o n t e n t and reduce t h e i r s u l f u r and n i t r o g e n c o n t e n t s .

I f t h e h e a v i e r f r a c t i o n s a r e c o o l e d i n s t e a d o f b e i n g h y d r o t r e a t e d , a c l e a n s o l i d f u e l r e s u l t s . The f o l l o w i n g s i m p l i f i e d r e a c t i o n s i l l u s t r a t e t h e m a j o r processes:

Combust i o n :

C + n0,-(2

-

2n)co

+

( 2 n

- i ) c o 2

G a s i f i c ' a t io n :

C + H20(steam)-C0 + H2

Water and energy--Demands and effects

Hydrogenat i c t i : C + H2-

CH4

Water-gas s h i f t r e a c t i o n : CO + H20 H2 + CO2 Methanation:

CO + 3 H 2 w C H 4 + H20

GASIFICATION

SHIFT CONVERSION A N 0 PURIFICATIOW

MEMUM-BIU

. HiGH-Biu M E I H A N A I l O N

( 3 )

( 4 )

( 5 )

CLEAN CASEOUS FUELS

FISCHER.

TROPSCH HYDROCARBON

SYNTHESIS

HinnocinBoN CLEAN FUELS

) LlOUlO U,, STEAM on

SYNTHESIS GAS

GAS 4 s

nr CHAR nt

N I COAL DERIVED LlOUlO

* HYOROCARBON

PYRITIC SULFUR

OtSULFUR12AT1011 FUELS

B Y PHYSICAL.

CHEMICAL OR I H E R M A L

F i g u r e 2.4.3. C o n v e r s i o n methods f o r p r o d u c i n g c l e a n f u e l s f r o m c o a l and o i l shale. (Source:

P r o b s t e i n and Gold, 1978.)

The g a s i f i c a t i o n r e a c t i o n i s endothermic, t h a t i s , i t r e q u i r e s h e a t i n p u t , u s u a l l y s u p p l i e d b y b u r n i n g c o a l o r char ( t h e combustion r e a c t i o n , above). A l l t h e o t h e r r e a c t i o n s a r e e x o t h e r m i c , i n t h a t t h e y g i v e o f f h e a t .

The s t a n d a r d module p l a n t o u t p u t s commonly used i n p l a n n i n g s y n t h e t i c f u e l developments a r e :

6 3

Coal g a s i f i c a t i o n - - 2 . 2 Mtoe (250 x 10 s t a n d a r d E t p e r day) a n n u a l l y . C o a l l i q u e f a c t i o n - - 2 . 5 Mtoe (50,000 b a r r e l s per day) a n n u a l l y .

S o l i d fuel--2.4 Mtoe (10,000 s h o r t tons p e r day) a n n u a l l y .

2.4.3.1. C o a l g a s i f i c a t i o n . S y n t h e t i c gas i n c l u d e s t h r e e b a s i c p r o d u c t s : l o w , medium, and high h e a t - v a l u e gas. The low h e a t - v a l u e gas, o f t e n termed "power gas," has a h e a t i n g v a l u e o f about 890 t o 2,200k c a l / m 3

.

^{I t} i s an i d e a l t u r b i n e f u e l , but because o f i t s l o w h e a t v a l u e i s

54

P r i n c i p a Z I j a t e r - u s i n g processes i n the energy s e c t o r

Water and energy--Demands and effects

P y r o l y s i s , t h e d e s t r u c t i v e d i s t i l l a t i o n o f c o a l , y i e l d s t a r , o i l , w a t e r , char, and non-condensible gases. G e n e r a l l y , t h e condensates a r e f u r t h e r hydrogenated i n o t h e r processes. P y r o l y s i s g e n e r a l l y produces c o n s i d e r a b l e water, n o t o n l y f r o m d i s t i l l a t i o n o f t h e m o i s t u r e c o n t e n t o f t h e c o a l b u t a l s o because most o f t h e oxygen i n t h e input c o a l i s c o n v e r t e d t o water. Water produced t y p i c a l l y i s on t h e o r d e r o f 30 t o 40 p e r c e n t b y weight o f t h e o i l produced.

2 . 4 . 3 . 3 . S o l i d - f u e l p r o d u c t i o n . I n p r o d u c i n g c l e a n , s o l i d f u e l , t h e i n p u t c o a l i s d r i e d and t h e n p a r t i a l l y d i s s o l v e d i n a s o l v e n t t h a t exchanges hydrogen w i t h t h e c o a l . The s o l v e n t i s t h e n mixed w i t h a d d i t i o n a l hydrogen and h e a t e d under p r e s s u r e t o d i s s o l v e t h e r e m a i n i n g carbonaceous m a t e r i a l o f t h e c o a l . The s o l u t i o n i s t h e n f i l t e r e d and t h e s o l v e n t d i s t i l l e d o f f f o r reuse. The r e m a i n i n g s o l i d i f i e d m a t e r i a l c o n t a i n s l e s s than 0 . 2 p e r c e n t ash and l e g s t h a n 1 per c e n t s u l f u r b y w e i g h t ; t h e h e a t i n g v a l u e i s about 1.8 k c a l / k g . The p r o d u c t can be burned d i r e c t l y as f u e l .

2 . 4 . 3 . 4 . Water use. Because the o v e r a l l t h e r m a l . e f f i c i e n c y o f c o a l - s y n f u e l processes i s r e l a t i v e l y h i g h , t h e water consumption b y these processes can be expected t o be moderate. Few o f t h e processes have advanced beyond t h e p i l o t stage, so the demand f o r water, even on a w o r l d s c a l e , i s n e g l i g i b l e t o date. As n o t e d e a r l i e r , g e n e r a l l y t h e lower t h e C/H r a t i o , t h e h i g h e r t h e water consumption. Thus

,

g a s i f i c a t i o n r e q u i r e s more water t h a n l i q u e f a c t i o n , and l i q u e f a c t i o n r e q u i r e s more water t h a n p r o d u c t i o n o f s o l i d f u e l s . T h i s i s i l l u s t r a t e d i n F i g u r e 2 . 4 . 3 . 4 , m o d i f i e d a f t e r P r o b s t e i n and Gold ( 1 9 7 8 ) , which shows t y p i c a l ranges o f water consumption n o r m a l i z e d t o u n i t h e a t i n g v a l u e o f t h e p r o d u c t f o r c o a l - s y n t h e t i c - f u e l processes.

I n each g r o u p i n g , v a l u e s a r e g i v e n f o r consumptive use b y h e a t - d i s s i p a t i o n systems o p e r a t i n g a t maximum, i n t e r m e d i a t e , and m i n i m u m w a t e r - c o o l i n g l e v e l s . Coal g a s i f i c a t i o n ranges from 2,300 t o 3 , 6 0 0 l / t o e , l i q u e f a c t i o n from 1,900 t o 3,000 l / t o e , and s o l i d f u e l p r o d u c t i o n from 1,000 t o 1,600 l / t o e . The d i f f e r e n c e between maximum and m i n i m u m water use i n each c a t e g o r y r e p r e s e n t s a t r a d e o f f between water c o s t s and process c o s t s . A t t h e m i n i m u m l e v e l , c o n s i d e r a b l e e x t r a c a p i t a l and o p e r a t i n g c o s t i s e n t a i l e d i n u s i n g a i r c o o l i n g t o t h e maximum e x t e n t f e a s i b l e because a i r c o o l i n g i n most processes i s i n h e r e n t l y l e s s e f f i c i e n t t h a n water c o o l i n g . Most p l a n t s can be expected t o o p e r a t e i n t h e i n t e r m e d i a t e t o maximum range except where severe w a t e r shortages e x i s t . P r o b a b l y a more s i g n i F i c a n t r e s t r a i n t on development i s

t h e high c a p i t a l c o s t o f p l a n t c o n s t r u c t i o n combined w i t h a l i m i t e d market f o r p r o d u c t s a t t h e p r i c e l e v e l needed t o r e c o v e r i n v e s t m e n t c o s t s .

w 6.000

e

.-

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U

4,000 F

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3,000 I

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2,000

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COAL COAL CLEAN COAL

GASIFICATION LIQUEFACTION (SOLVENT REFINED

(PIPELINE GAS) (FUEL OIL) COAL)

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INTERMEDIATE MINIMUM PRACTICAL

n

F i g u r e 2 . 4 . 3 . 4 . U n i t consumptive use o f water i n c o a l c o n v e r s i o n processes. ( M o d i f i e d from P r o b s t e i n and Gold, 1978.)

56

P r i n c i p a Z w a t e r - u s i n g processes i n t h e energy s e c t o r As o f 1983, t h e most s e r i o u s commercial i n t e r e s t i n t h e e n t i r e spectrum o f s y n f u e l processes i s f o r p r o d u c t i o n o f l o w h e a t - v a l u e gas f o r d i r e c t input t o combined-cycle e l e c t r i c g e n e r a t i n g p l a n t s i n areas t h a t cannot t o l e r a t e a i r p o l l u t i o n from a s t a n d a r d c o a l - f i r e d p l a n t . I n t h i s mode, t h e o u t p u t o f a c o a l - g a s i f i c a t i o n p l a n t i s f e d d i r e c t l y i n t o a gas t u r b i n e t o generate power and the h o t exhaust gases produced a r e used t o r a i s e steam t o generate e l e c t r i c i t y i n a c o n v e n t i o n a l s t e a m - e l e c t r i c c y c l e . By combining c y c l e s i n t h i s way, i t i s p o s s i b l e t o c i r c u m v e n t t h e upper l i m i t o f about 40 p e r c e n t t h e r m a l e f f i c i e n c y o f t h e c o n v e n t i o n a l steam c y c l e and t o achieve an o v e r a l l t h e r m a l e f f i c i e n c y o f 50 p e r c e n t o r b e t t e r through t h e use o f h e a t t h a t o t h e r w i s e would be wasted. A p l a n t employing t h e Texaco oxygen-blown g a s i f i c a t i o n process i s under c o n s t r u c t i o n f o r t h e Southern C a l i f o r n i a E d i s o n Company n e a r Barstow, C a l i f o r n i a , USA. T h i s p l a n t i s now i n o p e r a t i o n (1983) w i t h a 1,000-ton-per-day c o a l g a s i f i e r f e e d i n g gas t o an e x i s t i n g 65 MW b o i l e r . L a t e i n 1985 an' i n t e g r a t e d combined-cycle g e n e r a t o r i s scheduled t o b e g i n o p e r a t i o n a t 90 MW o u t p u t .

The Sasol P l a n t s I and II now o p e r a t i n g i n South A f r i c a g r e t h e w o r l d ' s l a r g e s t c o a l - c o n v e r s i o n f a c i l i t i e s , w i t h a combined i n p u t o f about 1 5 x 10 tons p e r y e a r o f c o a l . When t h e S a s o l P l a n t I I Z comes on l i n e w i t h an a d d i t i o n a l 14 x

l o 6

annual c o a l i n p u t i n 1985, t h e R e p u b l i c o f South A f r i c a e x p e c t s t o s u p p l y 50 p e r c e n t oE i t s l i q u i d - f u e l needs f r o m t h i s complex.

Dans le document reports 42 (Page 46-69)