Desulfurization of Fossil Fuels - ACS Publications - American

Desulfurization of Fossil Fuels J. B. HYNE

Downloaded by EAST CAROLINA UNIV on September 22, 2015 | http://pubs.acs.org Publication Date: January 26, 1979 | doi: 10.1021/bk-1979-0090.ch005

Alberta Sulphur Research L t d . and Department of Chemistry, University of Calgary, Calgary, Alberta T 2 N 1N4

Since the industrial revolution mankind has been dependent on fossil hydrocarbon fuel for a very large proportion of total energy needs. Coal was the dominant form until the first half of the twentieth century when fluid hydrocarbons began to make significant inroads due to easier recovery, handling and transportation. Many of the fluid forms also had lower sulphur content than coal and were therefore more attractive from the environmental impact standpoint. The low sulphur fuels, however, are limited in supply and future energy needs indicate that, until alternate non-hydrocarbon energy sources can be fully developed, there w i l l be an increasing reliance on heavier hydrocarbons, including coal, heavy o i l s , bitumens, etc., all of which tend to be relatively high in sulphur. Coupled with ever stricter environmental control regulations the importance of developing efficient technologies for fuel desulphurisation is clear. This paper presents an examination of the overall question of fossil fuel desulphurisation as i t currently exists including solid, liquid and gas phase hydrocarbon fuels and technologies for their desulphurisation, in situ, in refining, during combustion and post combustion (see Figure 3). Sulphur i n F o s s i l

Fuels

S u l p h u r o c c u r s i n most f o s s i l h y d r o c a r b o n f u e l s i n e i t h e r o r b o t h o f t h e o r g a n i c o r i n o r g a n i c f o r m s . P y r i t e s (FeS2) i s t h e most common o f t h e i n o r g a n i c f o r m s w h i l e o r g a n o s u l p h u r compounds r a n g e w i d e l y o v e r t h e s t r u c t u r a l s p e c t r u m f r o m t h i o e t h e r s and mercaptans i n t h e a l i p h a t i c s e r i e s t o t h i o p h e n e s and complex a r o m a t i c s t r u c t u r e s w h e r e t h e s u l p h u r atoms a r e a n i n t e g r a l p a r t o f the r i n g systems. I n many d e p o s i t s , t h e amount o f s u l p h u r p r e s e n t i n t h e h y d r o carbon f a r exceeds t h e c o r r e s p o n d i n g percentage o f sulphur l i k e l y to have been i n c o r p o r a t e d i n t h e o r i g i n a l b i o l o g i c a l m a t e r i a l f r o m w h i c h t h e f o s s i l f u e l was f o r m e d . Thus t h e s u l p h u r now p r e -

This chapter not subject to U.S. Copyright. Published 1979 American C h e m i c a l Society.

In Chemistry for Energy; Tomlinson, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

46

CHEMISTRY FOR ENERGY

s e n t i n t h e h y d r o c a r b o n d e p o s i t s was e i t h e r c o n c e n t r a t e d d u r i n g the a c c u m u l a t i o n i n t h e f o r m a t i o n o r has been i n c o r p o r a t e d from adjacent inorganic sources. Recent work by W i l s o n O r r (1) and o t h e r s u s i n g s t a b l e i s o tope techniques has p r o v i d e d s t r o n g evidence t h a t i n c o r p o r a t i o n f r o m a n i n o r g a n i c s o u r c e i s t h e most p r o b a b l e e x p l a n a t i o n . I n o r g a n i c s u l p h u r ( s u l p h a t e ) tends t o be r i c h e r i n t h e heavy s u l p h u r i s o t o p e , S , compared w i t h r e f e r e n c e m e t e o r i t i c s u l p h u r whereas o r g a n i c s u l p h u r i s d e p l e t e d i n t h e heavy i s o t o p e . Using t y p i c a l v a l u e s f o r OS **, t h e r e l a t i v e e n r i c h m e n t o r d e p l e t i o n w i t h r e s p e c t t o S , O r r showed t h a t a s much a s 7 8 % o f t h e H2S a s s o c i a t e d w i t h a h y d r o c a r b o n d e p o s i t and 34% o f t h e o r g a n i c a l l y bound s u l p h u r was d e r i v e d f r o m i n o r g a n i c ( f o r m a t i o n r o c k ) s u l phate. A mechanism f o r t h i s i n c o r p o r a t i o n o f s u l p h a t e s u l p h u r i n t o o r g a n i c s t r u c t u r e s i s shown i n F i g u r e 1. A l t h o u g h h y d r o g e n s u l p h i d e a c t s as a c r u c i a l r e a c t i o n i n t e r m e d i a t e undergoing redox r e a c t i o n w i t h sulphate, the o v e r a l l r e a c t i o n i n v o l v e s the reduct i o n o f s u l p h a t e b y h y d r o c a r b o n y i e l d i n g s u l p h u r ( o r H2S) and c a r b o n d i o x i d e . The e l e m e n t a l s u l p h u r p r o d u c e d r e a c t s r e a d i l y w i t h h y d r o c a r b o n s t o y i e l d o r g a n o s u l p h u r compounds t y p i c a l o f those found i n f u e l s . A l l o f t h e s e r e a c t i o n s a r e promoted by deep, h i g h p r e s s u r e , e l e v a t e d temperature b u r i a l o f t h e f o s s i l hydrocarbon. T h i s e f f e c t i s most d r a m a t i c a l l y s e e n i n t h e h i g h c o n c e n t r a t i o n s o f H2S f o u n d i n deep ( 1 0 , 0 0 0 + ) n a t u r a l g a s w e l l s i n many l o c a t i o n s a r o u n d t h e w o r l d . 3 4

3


3 4

T

Formation SO4

+ 3H S — • 2

-> 3S° + ' ( C H ) ' S° + 2S°

2

y

+ 2H 0

2

—>

4S° + 2 H 0 + 20H

2

RH

>

RSH

+ 2RH

•

RSR + H S

3H S + C 0 2

2

2

( A f t e r W i l s o n O r r , 1974) Figure

1.

Organosulfur

compounds

from inorganic

sulfate

T y p i c a l sulphur content values f o r v a r i o u s f o s s i l hydroc a r b o n f u e l s a r e shown i n T a b l e I . The v e r y h i g h a s s o c i a t e d s u l p h u r c o n t e n t o f many o f t h e deep g a s d e p o s i t s i s r e a d i l y s e e n . F o r t u n a t e l y , however, t h i s s u l p h u r c o n t e n t i s p r e s e n t a s r e l a t i v e l y e a s i l y removed H2S a l t h o u g h t h e p r e s e n c e o f s u c h l a r g e q u a n t i t i e s a t e l e v a t e d t e m p e r a t u r e s and p r e s s u r e s c a n p o s e


5.

Desulfurization of Fossil Fuels

HYNE

47

s e r i o u s c o r r o s i o n and m a t e r i a l f a i l u r e problems. Both Canada and France have b u i l t an extensive elemental sulphur production i n dustry based on the recovered H2S from sour n a t u r a l gas. Table I Sulphur Content of F o s s i l Fuels


Gas Phase Natural

L i q u i d Phase Crude O i l :

Bitumen: S o l i d Phase Coal: Lignite: Oil

%H S 54 65 17 ?

Gas:

Shales:

Alberta F o o t h i l l s Mississippi Pyrenees

(production) (production) (production)

Venezuela, Buscan Mexico, Cretaceous Saudi A r a b i a , Manifa Saskatchewan, Weyburn Arkansas, Smackover A l b e r t a , Swan H i l l s Louisiana, Delhi A l b e r t a , Kaybob Athabasca

O i l Sands

°API 10 12 28 24 19 37 44 47 < 10

% S 5.6 5.4 3.0 2.1 2.1 0.8 0.08 0.04 4.7

Eastern North America Western North America

3 -0.5

B.C.,

2.0

Sask., Ont.

Colorado

0.75

The sulphur content of crude o i l s v a r i e s markedly but i s g e n e r a l l y r e l a t e d to the API g r a v i t y of the crude. The very l i g h t crudes (API > 35) g e n e r a l l y tend to be low i n sulphur and are t h e r e f o r e i n c o n s i d e r a b l e demand and command premium p r i c e s . As discussed l a t e r , however, these low sulphur l i g h t crudes are i n r e l a t i v e l y l i m i t e d supply and the trend i s c l e a r l y toward prod u c t i o n of h e a v i e r , higher sulphur crudes. Venezuelan and West e r n Canadian heavy o i l s and bitumens represent the extreme end of the API g r a v i t y spectrum and have correspondingly high sulphur content. Since these d e p o s i t s represent some of the world's l a r gest reserves of f o s s i l hydrocarbon f u e l and w i l l doubtless be produced i n ever i n c r e a s i n g q u a n t i t i e s the development of methodology f o r d e s u l p h u r i s a t i o n i s a matter of importance. While the sulphur values a s s o c i a t e d with gas, o i l and b i t u mens are e i t h e r i n the form of H2S or organosulphur compounds coals can have s i g n i f i c a n t i n o r g a n i c sulphur v a l u e s . T h i s i n t r o duces the p o s s i b i l i t y of d e s u l p h u r i s a t i o n by chemical methods d i f f e r e n t from that r e q u i r e d f o r removal of organic sulphur v a l u e s . Nonetheless, the d e s u l p h u r i s a t i o n problems a s s o c i a t e d



48

w i t h t h e l e s s t r a c t a b l e o r g a n o s u l p h u r compounds s t i l l e x i s t w i t h c o a l s and o t h e r s o l i d p h a s e f o r m s . To d a t e t h e r e h a s b e e n o n l y v e r y l i m i t e d success i n removing organosulphur v a l u e s w i t h o u t e i t h e r g a s i f i c a t i o n or combustion. F i g u r e 2 c l e a r l y shows t h a t C a n a d a s f o s s i l f u e l r e s e r v e s a r e h e a v i l y o r i e n t e d t o w a r d c o a l , t a r sand and h e a v y o i l s . These huge r e s e r v e s o f some 800 b i l l i o n b a r r e l s e q u i v a l e n t ( c . f . c u r r e n t M i d d l e E a s t o i l r e s e r v e s o f some 400 b i l l i o n b a r r e l s ) l e a v e l i t t l e d o u b t a b o u t Canada's l o n g t e r m f o s s i l f u e l s e l f - s u f f i c i e n c y . C o n v e r t i n g t h e s e r e s e r v e s i n t o e n e r g y , h o w e v e r , w i l l be d e p e n d e n t on t h e d e v e l o p m e n t o f new t e c h n o l o g i e s , one o f w h i c h must be e f f e c t i v e containment of the i n h e r e n t sulphur v a l u e s i n order t o minimise environmental impact. I n 1967 i t was e s t i m a t e d (2) t h a t o f t h e 220,000,000 t o n s o f s u l p h u r e m i t t e d t o t h e e a r t h ' s a t m o s p h e r e a n n u a l l y , 75,000,000 t o n s w e r e man made and o f t h a t amount 50,000,000 t o n s r e s u l t e d f r o m c o a l c o m b u s t i o n . I f the extent to w h i c h t h e i n d u s t r i a l i s e d w o r l d becomes d e p e n d e n t u p o n c o a l a s an energy source i s t o i n c r e a s e a g a i n , i t i s c l e a r t h a t removal of sulphur v a l u e s i s e s s e n t i a l to the p r o t e c t i o n of the environment.


1

In Situ Desulphurisation P a r t i a l removal of the sulphur v a l u e s i n f o s s i l hydrocarbon f u e l s c a n be a c c o m p l i s h e d i n p l a c e d u r i n g t h e a p p l i c a t i o n o f s o c a l l e d " i n s i t u " r e c o v e r y t e c h n i q u e s . Perhaps the e a r l i e s t of t h e s e t e c h n i q u e s t o be e x p l o r e d was i n s i t u c o a l g a s i f i c a t i o n . D u r i n g t h e 1 9 4 0 s and 1 9 5 0 s , b o t h t h e R u s s i a n s and t h e B r i t i s h c a r r i e d out e x t e n s i v e f i e l d t r i a l s w i t h i n s i t u c o a l g a s i f i c a t i o n and d e m o n s t r a t e d t h e p r a c t i c a l f e a s i b i l i t y o f o b t a i n i n g a l o w BTU gas p r o d u c t f r o m t h e p r o c e s s . More r e c e n t l y i n s i t u c o a l g a s i f i c a t i o n t r i a l s h a v e b e e n c o n d u c t e d i n A l b e r t a and s e v e r a l p r o j e c t s i n t h e C o l o r a d o o i l s h a l e s h a v e f o c u s s e d on r u b b l i s i n g , and i n s i t u r e t o r t i n g of the s h a l e to l i b e r a t e the hydrocarbon. In s i t u r e c o v e r y t e c h n i q u e s u s i n g f i r e , steam f l o o d i n g , o r a c o m b i n a t i o n o f b o t h , a l s o a p p e a r t o be one o f t h e most p r a c t i c a l r o u t e s f o r f l u i d i s i n g t h e heavy bitumens o f d e e p l y b u r i e d o i l sands thus e n a b l i n g t h e i r r e c o v e r y by d r i l l e d w e l l s r a t h e r t h a n m i n i n g . P e r h a p s t h e most s i g n i f i c a n t a d v a n t a g e o f s u c h i n s i t u t e c h n i q u e s as f a r as d e s u l p h u r i s a t i o n i s c o n c e r n e d , i s t h a t c h e m i c a l r e a c t i o n t e m p e r a t u r e s and p r e s s u r e s c a n be r e a c h e d t h a t w o u l d r e q u i r e e l a b o r a t e and e x p e n s i v e p r o c e s s e q u i p m e n t i f a t t e m p t e d on the surface. Thus c h e m i c a l r e a c t i o n r e g i m e s c a n be g e n e r a t e d " i n s i t u " t h a t w o u l d be l e s s p r a c t i c a l and e c o n o m i c i f t h e f o s s i l f u e l w e r e r e c o v e r e d and s u b j e c t e d t o t r a d i t i o n a l u p g r a d i n g and refining. T h e s e h i g h e r t e m p e r a t u r e and p r e s s u r e c o n d i t i o n s c a n be o f p a r t i c u l a r i m p o r t a n c e i n d e s u l p h u r i s a t i o n r e a c t i o n s . Coker u n i t s i n s u r f a c e equipment can p r o b a b l y s i m u l a t e t h e hydrogen t r a n s f e r r e a c t i o n s that occur i n " i n s i t u " f i r e f l o o d i n g y i e l d i n g l o w e r m o l e c u l a r w e i g h t h y d r o c a r b o n s and h y d r o g e n s u l p h i d e . Thus i n s i t u r e m o v a l o f t h e s u l p h u r f r o m i t s p a r e n t o r g a n o s u l p h u r f

f



5.

HYNE

Desulfurization

of Fossil

49

Fuels

Figure 2. Canada's potential recoverable reserves of fossil fuels (in barreh-of-oil equivalent). Total: 1000 billion; source: Federal Government Publication "An Energy Policy for Canada."

so; POST COMBUSTION h-S0

FLUE GAS CLEAN-UP REMOVE S 0 2

2

ti COMBUSTION

FLUID1SED BED COMBUSTORS WITH CHEMICAL SULPHUR TRAP

De-H S NATURAL GAS HYDROOESULPHURISE CRUDE RAW COAL UPGRADE - MICROBIOLOGY -CHEMICAL COAL GASIFICATION OIL SAND COKING 2

REFINERY

L-HgS

ti IN PLACE Figure

3.

OIL SAND - FIRE/STEAM FLOOO COAL-IN SITU GASIFICATION SHALE-IN SITU RETORT

Desulfurization bon fuels

of

hydrocar-



50

compound i s a c h i e v e d and t h e more t r a c t a b l e g a s e o u s H2S s u l p h u r c a n be removed and p r o c e s s e d , eg. R - S - CH

2

l

- R

r

e

l

0

0

d

j; ^ , > thermal crack

R - R +

HS * 2

/

J °2


desulphurised hydrocarbon

form of

H0 2

+ S

Jo

2

C0

2

Less e a s i l y accomplished under r e a d i l y a c c e s s i b l e s u r f a c e c o n d i t i o n s , h o w e v e r , i s t h e h i g h t e m p e r a t u r e and p r e s s u r e h y d r o l y s i s o f o r g a n o s u l p h u r compounds, eg. R

-

S -

CH

2

- R + HoO

h

l

g

h

t e m

P-V

2RH

+

HS

+

2

CO

press. steam f l o o d I n r e a c t i o n s o f t h i s k i n d n o t o n l y i s t h e s u l p h u r removed f r o m t h e h y d r o c a r b o n b u t t h e h y d r o g e n o f t h e w a t e r ( s t e a m ) becomes i n t i m a t e l y i n v o l v e d i n the o v e r a l l process. Indeed the r e a c t i o n i s t h e c o m b i n a t i o n o f t h e t h e r m a l c r a c k i n g shown i n t h e f i r e f l o o d e x a m p l e and t h e w a t e r gas r e a c t i o n b e t w e e n t h e p r o d u c e d c o k e and s t e a m . R - S - CH

2

- R

C + H0 2

R - S - CH

2

- R + H0 2

• R - R + HS

+ C

2

• H

+

2

y 2RH

CO + HS 2

+

CO

R e c e n t w o r k (3) w i t h m o d e l o r g a n o s u l p h u r compounds h a s s h o wn t h a t a t t e m p e r a t u r e s a b o v e 350°C and p r e s s u r e s i n e x c e s s o f 100 atm (1500 p s i g ) , t h e h y d r o l y t i c d e s u l p h u r i s a t i o n r e a c t i o n o c c u r s r e a d i l y w i t h t h i o e t h e r s , m e r c a p t a n s and o t h e r n o n - t h i o p h e n i c t y p e s o f o r g a n o s u l p h u r compounds. T h i o p h e n e i t s e l f i s more r e s i s t a n t t o t h i s t y p e o f r e a c t i o n b u t d e s u l p h u r i s a t i o n i s s i g n i f i c a n t i n t h e 450 - 500 C r a n g e . More complex f u s e d r i n g s u l p h u r c o n t a i n i n g a r o m a t i c s t r u c t u r e s , c a n , h o w e v e r , be more reactive. C l e a r l y there are c o n s i d e r a b l e advantages to removing s u l p h u r f r o m i t s o r g a n i c s k e l e t o n " i n s i t u " and r e c o v e r i n g i t a t t h e s u r f a c e i n t h e more r e a d i l y h a n d l e d H S f o r m . A l t h o u g h t h e gene r a t i o n o f H S i n s u c h i n s i t u r e c o v e r y p r o c e s s e s may i n t r o d u c e s p e c i a l r e q u i r e m e n t s i n m a t e r i a l s s e l e c t i o n due t o c o r r o s i o n and embrittlement problems the sulphur converted i n t o H S i s sulphur t h a t d o e s n o t consume h y d r o g e n i n h y d r o d e s u l p h u r i s a t i o n r e f i n e r y s t e p s or appear i n r e s i d u a l coke i n coker u p g r a d i n g of heavy o i l or bitumens. Much r e m a i n s t o be l e a r n e d a b o u t t h e r e a c t i o n s o f o r g a n o s u l p h u r compounds a t t h e e l e v a t e d t e m p e r a t u r e s and p r e s s u r e s t h a t c a n be r e a d i l y a c h i e v e d i n i n s i t u r e c o v e r y p r o c e s s e s . The use of t h e n a t u r a l f o r m a t i o n as the c h e m i c a l r e a c t o r p e r m i t s t h e a t t a i n m e n t of r e a c t i o n c o n d i t i o n s t h a t have p r e v i o u s l y been out 2

2

2


5.

HYNE

Desulfurization

of

Fossil

51

Fuels

of t h e range o f p r a c t i c a l i n t e r e s t of t h e o r g a n i c c h e m i s t . The h y d r o l y s i s o f o r g a n i c compounds has b e e n a common a r e a o f s t u d y i n t h e p a s t b u t few h a v e v e n t u r e d i n t o t h e d o m a i n o f h y d r o l y s i s a b o v e 100 C. S i m i l a r l y , the h i g h temperatures a t t a i n a b l e i n i n s i t u r e t o r t i n g and c o m b u s t i o n ( g a s i f i c a t i o n , f i r e f l o o d i n g ) w i l l r e q u i r e a b e t t e r u n d e r s t a n d i n g of m o l e c u l a r rearrangement, c r a c k i n g , d i s p r o p o r t i o n a t i o n and s i m i l a r h y d r o c a r b o n r e a c t i o n s i n a r e a c t i o n t e m p e r a t u r e and p r e s s u r e r e g i m e t h a t h a s r e c e i v e d r e l a tively l i t t l e attention.


Refinery Desulphurisation D e s u l p h u r i s a t i o n o f h y d r o c a r b o n f u e l s has t r a d i t i o n a l l y b e e n c a r r i e d o u t p r i m a r i l y a s p a r t o f t h e r e f i n i n g and u p g r a d i n g p r o c e s s . A c c o r d i n g l y by f a r t h e most a d v a n c e d and b e s t u n d e r s t o o d c h e m i s t r y and t e c h n o l o g y i s t o be f o u n d i n t h i s a r e a . P r i o r t o the advent of major concern f o r environmental impact of f o s s i l f u e l c o m b u s t i o n p r o d u c t s r e l a t i v e l y l i t t l e was done t o d e s u l p h u r i s e h y d r o c a r b o n f u e l s ( p r i n c i p a l l y c o a l ) p r i o r t o c o m b u s t i o n and p a s t e f f e c t s of l a r g e s c a l e consumption of h i g h s u l p h u r c o a l s can s t i l l be s e e n i n m a j o r i n d u s t r i a l i s e d a r e a s a r o u n d t h e w o r l d . The f i r s t m a j o r a t t e m p t a t p r e c o m b u s t i o n d e s u l p h u r i s a t i o n was i n t h e c o a l gas i n d u s t r y and a number o f e f f i c i e n t and e f f e c t i v e t e c h n i q u e s f o r r e m o v a l o f H S , COS, C S , m e r c a p t a n s and other v o l a t i l e sulphur c o n t a i n i n g products of the g a s i f i c a t i o n p r o c e s s were d e v e l o p e d . Many o f t h e s e t e c h n i q u e s f o u n d a p p l i c a t i o n i n t h e s u b s e q u e n t d e v e l o p m e n t o f s o u r n a t u r a l gas p r o c e s s i n g w h e r e l a r g e v o l u m e s o f h y d r o g e n s u l p h i d e had t o be r e moved f r o m t h e h y d r o c a r b o n component. 2

2

Oil P e r h a p s t h e most i n t e n s i v e c h e m i c a l r e s e a r c h e f f o r t i n h y d r o c a r b o n d e s u l p h u r i s a t i o n , h o w e v e r , has b e e n d i r e c t e d t o w a r d the removal of s u l p h u r v a l u e s from crude o i l s . I t i s not appropr i a t e i n a g e n e r a l d e s u l p h u r i s a t i o n overview of t h i s type t o a t tempt t o do o t h e r t h a n p l a c e t h e w h o l e a r e a o f t r a d i t i o n a l c r u d e o i l d e s u l p h u r i s a t i o n i n p e r s p e c t i v e i n r e l a t i o n s h i p to o t h e r hydrocarbon d e s u l p h u r i s a t i o n . By f a r t h e commonest t e c h n i q u e has been h y d r o d e s u l p h u r i s a t i o n r e q u i r i n g t h e g e n e r a t i o n of l a r g e q u a n t i t i e s of m o l e c u l a r hydrogen f o r the c a t a l y t i c e x t r a c t i o n of o r g a n o s u l p h u r v a l u e s f r o m t h e o i l a s h y d r o g e n s u l p h i d e . Many p r o p r i e t r y h y d r o d e s u l p h u r i s a t i o n t e c h n i q u e s have been developed by t h e m a j o r r e f i n e r s e a c h u t i l i s i n g some p a r t i c u l a r a d v a n t a g e o u s f e a t u r e of a c a t a l y s t , temperature or p r e s s u r e r e a c t i o n c o n d i t i o n s (4). P e r h a p s t h e most e l e g a n t a s p e c t o f much o f t h i s p r i o r r e s e a r c h has b e e n i n t h e a r e a o f c a t a l y s t d e v e l o p m e n t w h e r e n o t o n l y e f f i c i e n c y b u t r e s i s t a n c e t o p o i s o n i n g by t h e o t h e r c o n s t i t u e n t s of t h e c r u d e o i l f e e d h a v e r e p r e s e n t e d m a j o r c h a l l e n g e s . S u l p h u r c o n t e n t o f c r u d e o i l s has a l w a y s b e e n r e g a r d e d a s a d i s c o u n t i n g f a c t o r and a s e n v i r o n m e n t a l r e s t r i c t i o n s on s u l p h u r e m i s s i o n s h a v e b e e n t i g h t e n e d , t h e need t o d e s u l p h u r i s e r e f i n e r y




52

f e e d s t o c k s has i n c r e a s e d . I n a d d i t i o n , t h e e v e r g r o w i n g demand f o r h y d r o c a r b o n f u e l s and t h e l i m i t e d s u p p l i e s i n i n d u s t r i a l i s e d c o u n t r i e s has b r o u g h t i n t o t h e f e e d s t o c k s t r e a m h e a v i e r c r u d e s with higher sulphur content. O n l y 36% o f some 13 MM b b l / d . o f c r u d e r u n t o U.S. r e f i n e r i e s i n 1973 was a b o v e 0.5% s u l p h u r , b u t t h i s had jumped t o 4 6 % o f a l a r g e r 15 MM b b l / d . i n J a n u a r y 1978 (5). The t r e n d i s a l s o i l l u s t r a t e d by a c o m p a r i s o n o f t h e h y d r o processing ( i n c l u d i n g hydrodesulphurisation) c a p a c i t i e s i n both t h e U.S. and W e s t e r n E u r o p e b e t w e e n 1967 and 1977. U.S. hydrop r o c e s s i n g c a p a c i t y i n c r e a s e d f r o m 30.5% t o 43.5% d u r i n g t h i s p e r i o d w h i l e t h e E u r o p e a n f i g u r e s show a n e v e n more d r a m a t i c jump f r o m 1 2 % i n 1967 t o 3 1 % o f c r u d e c a p a c i t y i n 1977 ( 6 ) . T h i s t r e n d t o h e a v i e r and h i g h e r s u l p h u r c o n t e n t f e e d s t o c k s i s l i k e l y to continue. As n o t e d e a r l i e r , t h e h e a v y o i l s and b i t u m e n s o f b o t h Canada and V e n e z u e l a , a r e r i c h i n s u l p h u r and w i l l u n d o u b t e d l y be d e s u l p h u r i s e d i n e v e r i n c r e a s i n g amounts a s s u p p l i e s of the l i g h t e r , lower s u l p h u r c o n t e n t f e e d s t o c k s d w i n d l e . I t i s a l w a y s d a n g e r o u s t o c o n c l u d e t h a t l i t t l e r e m a i n s t o be done i n a p a r t i c u l a r r e s e a r c h f i e l d . M o r e t h a n l i k e l y , h a v i n g so c o n c l u d e d , a s i g n i f i c a n t new b r e a k t h r o u g h i s i m m e d i a t e l y a n n o u n ced! C o n s i d e r i n g t h e r e l a t i v e l y u n d e v e l o p e d n a t u r e o f t h e many o t h e r a s p e c t s of h y d r o c a r b o n d e s u l p h u r i s a t i o n , however, i t would seem t h a t f u r t h e r p r o g r e s s i n t h e a r e a o f c a t a l y t i c h y d r o d e s u l p h u r i s a t i o n o f l i q u i d h y d r o c a r b o n f e e d s t o c k s i s l i k e l y t o be l e s s dramatic than i n the past. T h i s d o e s n o t mean, h o w e v e r , t h a t t h e r e i s no need f o r c o n t i n u e d e f f o r t i n i m p r o v i n g h y d r o d e s u l p h u r i s a t i o n processes p a r t i c u l a r l y w i t h r e s p e c t to the techniques f o r h a n d l i n g t h e h e a v y c r u d e c o m p o s i t i o n s and f e e d s t o c k s w i t h high thiophenic type organosulphur content. Gas D e s u l p h u r i s a t i o n o f n a t u r a l gas c o n s i s t s p r i m a r i l y o f r e m o v a l o f h y d r o g e n s u l p h i d e . Thus t h e s u l p h u r c o n t e n t o f t h e g a s eous f o r m o f f o s s i l h y d r o c a r b o n f u e l i s a l r e a d y i n t h e f o r m t o which i t i s normally converted i n other precombustion d e s u l p h u r i sation techniques. D e s p i t e t h i s advantage, however, the d e s u l p h u r i s a t i o n o f s o u r n a t u r a l gas c a n h a v e p r o b l e m s a s s o c i a t e d w i t h t h e s h e e r v o l u m e o f h y d r o g e n s u l p h i d e i n v o l v e d . H2S c o n t e n t o f 15 - 25% i s commonplace i n s o u r gas f i e l d s a r o u n d t h e w o r l d b u t gas w i t h 50 - 65% i s a l r e a d y b e i n g p r o c e s s e d i n b o t h C a n a d i a n and U.S. f i e l d s . C a n a d i a n s o u r gas p r o c e s s i n g p l a n t s h a n d l e some 20,000 t o n s p e r day o f h y d r o g e n s u l p h i d e e x t r a c t e d f r o m t h e n a t u r a l gas p r o d u c t i o n o f t h e f o o t h i l l s g a s f i e l d s o f A l b e r t a and B r i t i s h C o l u m b i a . As t h e d r i l l p e n e t r a t e s i n t o d e e p e r , h o t t e r (200 C) and h i g h p r e s s u r e (1500 atm) f o r m a t i o n s i n s e a r c h f o r more gas and o i l t h e h y d r o g e n s u l p h i d e c o n t e n t of t h e gas g e n e r ally rises. Deep s o u r gas w e l l s i n t h e S o u t h e r n U.S. h a v e shown 65% H S c o n t e n t and w e l l s w i t h a s h i g h a s 90% H S h a v e b e e n completed i n the Canadian Rockies. Processes f o r handling these h i g h c o n c e n t r a t i o n s o f H S i n s u c h l a r g e v o l u m e s must c l e a r l y d i f f e r from those p r e v i o u s l y a v a i l a b l e f o r h a n d l i n g the o f f - g a s e s 2

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f r o m r e f i n e r y h y d r o d e s u l p h u r i s a t i o n o r c o a l gas m a n u f a c t u r e where H2S l e v e l s a r e s i g n i f i c a n t l y l o w e r . A l k a n o l a m i n e s h a v e b e e n w i d e l y u s e d f o r r e m o v a l o f H2S f r o m s o u r n a t u r a l gas. M o n o e t h a n o l a m i n e (MEA) was t y p i c a l l y u s e d t o r e a c t w i t h t h e a c i d i c H2S t o f o r m t h e a l k a n o l a m m o n i u m s u l p h i d e . T h i s s a l t c o u l d be r e a d i l y removed

f r o m t h e h y d r o c a r b o n gas s t r e a m by t h e a q u e o u s s o l u t i o n and the s e p a r a t e d H2S r e g e n e r a t e d by h e a t i n g w i t h l i v e steam. Diethanola m i n e (DEA) and d i i s o p r o p a n o l a m i n e ( D I P A ) , h o w e v e r , h a v e r e p l a c e d t h e s i m p l e r MEA b e c a u s e o f i r r e v e r s i b l e r e a c t i o n of MEA w i t h t h e s m a l l amounts o f COS and C S t h a t a l s o o c c u r i n s o u r n a t u r a l gas. H y d r o g e n s u l p h i d e , h o w e v e r , i s n o t t h e o n l y a c i d gas p r e s e n t i n s o u r gas. I t i s n o r m a l l y a c c o m p a n i e d by s i g n i f i c a n t q u a n t i t i e s of c a r b o n d i o x i d e w h i c h a l s o r e a c t s w i t h the amine s c r u b b i n g solution. T h i s i s b e n e f i c i a l from the s t a n d p o i n t of r e d u c i n g the n o n - c o m b u s t i b l e c o n t e n t o f t h e n a t u r a l gas t h u s i n c r e a s i n g i t s f u e l v a l u e , b u t i t adds t o t h e l o a d t h a t t h e a m i n e must c a r r y and c o m p e t e s w i t h t h e H2S. D i f f e r e n t i a l a b s o r p t i o n o f H2S and CO2 t h e r e f o r e becomes a f a c t o r of c o n s i d e r a b l e i m p o r t a n c e i n p r o c e s s i n g s o u r n a t u r a l gas and c o m b i n a t i o n s o f p h y s i c a l and c h e m i c a l a b s o r p t i o n have been developed such as i n the S u l f i n o l P r o c e s s where d i i s o p r o p a n o l a m i n e ( c h e m i c a l r e a c t a n t ) and t e t r a m e t h y l e n e sulphone ( p h y s i c a l s o l v e n t ) a r e used t o g e t h e r . The a b i l i t y o f most s y s t e m s t o d i s c r i m i n a t e b e t w e e n H2S and CO2 u n d e r e q u i l i brium c o n d i t i o n s i s u s u a l l y not great but the r a t e a t which these two a c i d gas components r e a c t w i t h b a s i c a g e n t s c a n be s i g n i f i c a n t l y d i f f e r e n t . Thus i t may be p o s s i b l e t o d e s i g n a p r o c e s s where t h e d e s u l p h u r i s a t i o n o f s o u r n a t u r a l gas c a n be a c h i e v e d more e f f i c i e n t l y by t h e s e l e c t i v e r e m o v a l o f H2S u s i n g k i n e t i c r a t h e r than e q u i l i b r i u m d i s c r i m i n a t i o n v i s a v i s carbon d i o x i d e . 2

Coal P r e c o m b u s t i o n d e s u l p h u r i s a t i o n o f c o a l has b e e n l a r g e l y associated with coal gasification. R e m o v a l of i n o r g a n i c s u l p h u r , m a i n l y p y r i t e s , and t o some e x t e n t t h e more f r e q u e n t l y o c c u r r i n g o r g a n i c s u l p h u r has b e e n p o s s i b l e by s o - c a l l e d " c o a l c l e a n i n g " i n v o l v i n g p h y s i c a l o r c h e m i c a l l e a c h i n g methods. P r e s e n t technol o g y has b e e n r e c e n t l y r e v i e w e d by D a v i s ( 7 ) . P h y s i c a l s e p a r a t i o n o f s u l p h u r v a l u e s i s l i m i t e d t o t h e i n o r g a n i c a l l y bound s u l p h u r and has l i t t l e i f any e f f e c t on t h e 40 - 70% o r g a n o sulphur content. The d i f f e r e n t i a l d e n s i t y o f p y r i t e and c o a l i s the primary property u t i l i s e d i n cyclone, c o n c e n t r a t i o n t a b l e s and f r o t h f l o t a t i o n t e c h n i q u e s . Some a t t e n t i o n has b e e n g i v e n to magnetic techniques f o r p y r i t e s e p a r a t i o n . C h e m i c a l l e a c h i n g o f p y r i t e has b e e n a c c o m p l i s h e d w i t h aqueous f e r r i c s u l p h a t e under p r e s s u r e . The o v e r a l l r e a c t i o n i s


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oxidation a redox system whereby the f e r r i c i r o n i s reduced t o f e r r o u s ( s u l p h a t e ) and t h e s u l p h i d e o f t h e p y r i t e i s o x i d i s e d t o e l e m e n t a l sulphur. The l a t t e r i s v a p o r i s e d f r o m t h e c o a l and t h e r e d u ced f e r r o u s s u l p h a t e c a n be r e - o x i d i s e d t o t h e f e r r i c s t a t e by oxygenation. Removal of o r g a n i c s u l p h u r f r o m the s o l i d c o a l phase n o r m a l l y r e q u i r e s o x i d a t i o n . Treatment of p u l v e r i s e d c o a l w i t h a l k a l i n e s o d i u m c a r b o n a t e and a e r a t i o n a t p r e s s u r e s up t o 10 atm h a s b e e n r e p o r t e d (8) t o remove a l m o s t a l l i n o r g a n i c and some 30% o f o r ganic sulphur. The s u l p h u r v a l u e s u n d e r g o o x i d a t i o n i n t h e p r o c e s s and 0 Coal - S + Na C0 (alk) »• C o a l + N a ^ O i * p r e s s . ι Î Τ CaC0 r—t + 2 heat * 2

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appear as water s o l u b l e sodium s u l p h a t e . The s o d i u m c a r b o n a t e c a n be r e g e n e r a t e d by t r e a t m e n t o f t h e s u l p h a t e w i t h l i m e and C0 from a limestone k i l n . The e x t r a c t e d s u l p h u r t h u s a p p e a r s f i n a l l y as c a l c i u m s u l p h a t e waste. I t h a s b e e n s a i d t h a t a "bug" c a n be f o u n d t o do any c h e m i c a l t a s k - and do i t b e t t e r ! W h i l e t h e a l l e m b r a c i n g s c o p e o f s u c h a c l a i m may be somewhat e x a g g e r a t e d , t h e r e i s l i t t l e d o u b t t h a t modern m i c r o b i o l o g y i s p l a y i n g a n e v e r i n c r e a s i n g r o l e i n i n d u s t r i a l process design. C o a l d e s u l p h u r i s a t i o n by b a c t e r i a has r e c e n t l y b e e n i m p r o v e d by t h e c o m b i n e d u s e o f t h i o b a c i l l u s f e r r o o x i d a n s and t h i o o x i d a n s ( 9 ) . The f e r r o o x i d a n s h a v e l o n g b e e n known a s t h e c a u s e o f a c i d i c m i n e d r a i n a g e by o x i d i s i n g p y r i t e t o sulphur a c i d . I n c o m b i n a t i o n w i t h the s u l p h u r consuming t h i o o x i d a n s and a d e q u a t e a e r a t i o n o f t h e c o a l s l u r r y b e i n g t r e a t e d , much f a s t e r d e s u l p h u r i s a t i o n i s p o s s i b l e . S u l p h u r c o n t e n t o f c o a l has b e e n r e d u c e d f r o m 5% t o 2% i n n i n e d a y s w i t h p r o m i s e o f r e d u c t i o n t o 1% l e v e l s . D e s u l p h u r i s a t i o n of t h e h y d r o c a r b o n v a l u e s of c o a l as an i n t e g r a l p a r t o f t h e g a s i f i c a t i o n p r o c e s s r e m a i n s t h e most a c t i v e and a t t r a c t i v e p r e c o m b u s t i o n t e c h n i q u e . D e s p i t e t h e number o f s p e c i f i c g a s i f i c a t i o n p r o c e s s e s (10) ( L u r g i , K o p p e r s - T o t z e k , B i Gas, H y g a s , C o g a s , S y n t h a n e , e t c . ) , t h e e s s e n t i a l f e a t u r e s o f c o a l g a s i f i c a t i o n c a n be summarised i n t h e d i a g r a m m a t i c f o r m shown i n F i g u r e 4. The i n o r g a n i c and o r g a n i c s u l p h u r f o r m s i n t h e c o a l a r e l a r g e l y c o n v e r t e d i n t o t h e more r e a d i l y h a n d l e d h y d r o g e n s u l p h i d e f o r m i n t h e i n i t i a l g a s i f i e r s t e p . The HS c o n t e n t o f t h e l o w t h e r m a l c o n t e n t gas f r o m t h e g a s i f i e r i s n o r m a l l y q u i t e l o w ( 0 . 4 % by v o l u m e ) and h i g h c a p a c i t y t e c h n i q u e s 2

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55

Fuels

s u c h a s employed i n s w e e t e n i n g h i g h H2S c o n t e n t s o u r n a t u r a l g a s a r e n o t r e q u i r e d . B o t h p h y s i c a l and c h e m i c a l a b s o r p t i o n t e c h n i ques a s o u t l i n e d i n T a b l e I I a r e u s e d ( 1 1 ) . The S e l e x o l and R e c t i s o l p r o c e s s e s employ a l c o h o l s o r t h e i r e t h e r d e r i v a t i v e s a s p h y s i c a l s o l v e n t s f o r t h e a c i d g a s e s (CO2 and H2S) w h i l e t h e S u l f i n o l P r o c e s s , a s d e s c r i b e d p r e v i o u s l y combines t h e p h y s i c a l s o l v e n t p r o p e r t i e s of tetramethylene sulphone w i t h t h e chemical r e a c t i v i t y of an alkanolamine. The B e n f i e l d p r o c e s s , o t h e r w i s e known a s t h e " h o t p o t " c h e m i c a l l y a b s o r b s t h e a c i d g a s e s i n a q u e o u s p o t a s s i u m c a r b o n a t e s o l u t i o n and t h e J e f f e r s o n DGA s y s t e m a g a i n u t i l i s e s c h e m i c a l r e a c t i o n o f t h e CO2 and H2S w i t h a m i n e t o y i e l d t h e alkanolammonium s a l t . Regeneration of a l l these scrubb i n g s y s t e m s and r e c o v e r y o f t h e H2S and CO2 r i c h a c i d g a s s t r e a m can be r e a d i l y e f f e c t e d by h e a t i n g .

C0 Oxygen Coal Steam

GASIFIER

ACID GAS REMOVAL

SHIFT

Ash

Coal + 0 CO + C 0

2

Gasifier Product +

2

2

+ H

+ CHi* + Figure

H 0 2

CO +

2

C0

2

H 0 2

+ H

2

H S 2

4.

Generalized

coal gasification

sequence

Table I I A c i d Gas ( H ? S , CO?) A b s o r b e r s Process

Licensor

Selexol

Allied

Rectisol

Lurgi

Benfield

Benfield

Chemical

Absorber Polyethyleneglycol Ether Methanol

3 co 0

> f

DGA

J e f f e r s o n Chem.

Sulfinol

Shell

Aqueous P o t a s s i u m Carbonate Diglycolamine

0 w § 0

Tetramethylenesulphone + Diisopropanolamine


tr

4


56

B e c a u s e o f t h e h i g h c o n c e n t r a t i o n s o f C 0 compared w i t h H S found i n c o a l g a s i f i e r p r o d u c t streams 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 of H S and C 0 and t h e a b s o r p t i o n c a p a c i t y o f t h e v a r i o u s s c r u b b i n g s y s t e m s f o r t h e two components c a n b e a c r u c i a l f a c t o r i n s e l e c t i n g a process. A l s o of importance i s t h e p o i n t i n t h e overa l l g a s i f i c a t i o n sequence a t w h i c h t h e a c i d gases and i n p a r t i c u l a r H S i s removed. S e v e r a l t y p e s o f c a t a l y s t c a n b e employed i n the downstream " s h i f t " r e a c t i o n where t h e l o w t h e r m a l c o n t e n t g a s i f i e r p r o d u c t c a n b e u p g r a d e d b y r e a c t i o n o f some o f t h e CO c o n t e n t w i t h w a t e r t o i n c r e a s e t h e h y d r o g e n c o n c e n t r a t i o n . Some of these s h i f t c a t a l y s t s a r e s e n s i t i v e t o H S and i n such cases d e s u l p h u r i s a t i o n o f t h e g a s i f i e r p r o d u c t must b e a c c o m p l i s h e d ahead o f t h e s h i f t r e a c t o r . I f t h e g a s i f i e r p r o d u c t stream i s i n t e n d e d f o r downstream use a s t h e f e e d s t o c k f o r f u r t h e r u p g r a d i n g such a s m e t h a n a t i o n , methanol or F i s c h e r Tropsch s y n t h e s i s , v e r y thorough d e s u l p h u r i s a t i o n i s e s s e n t i a l s i n c e t h e c a t a l y s t s i n these upgrading proc e s s e s a r e h i g h l y s e n s i t i v e t o s u l p h u r p o i s o n i n g . The m e t h a n a t i o n c a t a l y s t s n o r m a l l y c a n n o t t o l e r a t e more t h a n 0.05 ppm o f sulphur i n the feedstock. I n a d d i t i o n to H S sulphur values i n t h e g a s i f i e r p r o d u c t i t may c o n t a i n COS, C S , m e r c a p t a n s and thiophenes. T h e s e a r e n o r m a l l y removed b y a c t i v a t e d c a r b o n o r z i n c o x i d e f i l t e r s ahead o f t h e s e n s i t i v e s y n t h e s i s c a t a l y s t beds. 2

2

2

2

2


2

2

2

O i l Sand B i t u m e n D e s u l p h u r i s a t i o n o f o i l s a n d s b i t u m e n w i t h 4.5% o r g a n o s u l p h u r c o n t e n t may, i n t h e i n s i t u f i r e o r s t e a m f l o o d p r o c e s s e s , be p a r t i a l l y accomplished underground. But f o r the m i n i n g o p e r a t i o n s c u r r e n t l y i n p l a c e and i n t h e p l a n n i n g s t a g e t h e d e s u l p h u r i s a t i o n p r o c e s s must o c c u r d u r i n g t h e r e f i n i n g o r upgrading stages. Hydroprocessing o f e x t r a c t e d bitumen w i l l r e s u l t i n t h e s u l p h u r v a l u e s a p p e a r i n g a s H S i n t h e o f f - g a s e s and t h i s s u l p h u r f o r m c a n b e removed b y a n y o f t h e s e v e r a l a b s o r p t i o n methods d i s c u s s e d p r e v i o u s l y . However, a s i g n i f i c a n t p r o p o r t i o n o f t h e i n i t i a l 4.5% s u l p h u r i n t h e bitumen i s r e t a i n e d i n t h e coke p r o d u c t i o n from t h e f l u i d or d e l a y e d cokers used t o r e d i s t r i b u t e t h e hydrogen c o n t e n t o f the bitumen. I n t h e p r e s e n t GCOS d e l a y e d c o k i n g p r o c e s s f o r u p g r a d i n g A t h a b a s c a o i l s a n d s b i t u m e n some 2 0 % o f t h e c a r b o n o f t h e o r i g i n a l b i t u m e n i s p r o d u c e d a s c o k e c o n t a i n i n g a l m o s t 6% sulphur. The l a r g e r S y n c r u d e o p e r a t i o n w i l l c o n v e r t some 8% o f i n i t i a l bitumen t o coke c o n t a i n i n g 8 - 9 % sulphur. The f u e l v a l u e s o f t h e s e huge c o k e p r o d u c t i o n s c a n o n l y b e r e a l i s e d i f combustion i s p o s s i b l e w i t h i n t h e l i m i t s of environmental t o l e r ance. Thus t h e r e i s a c o n t i n u i n g s t r o n g i n t e r e s t i n methods o f d e s u l p h u r i s i n g coke t h a t w i l l a v o i d t h e n e c e s s i t y f o r i n s t a l l a t i o n o f f l u e gas d e s u l p h u r i s a t i o n a f t e r combustion. Techniques s i m i l a r t o t h a t discussed e a r l i e r f o r t h e des u l p h u r i s a t i o n o f c o a l have been used. Treatment o f sulphur c o n t a i n i n g coke w i t h lime c a n y i e l d c a l c i u m s u l p h i d e which c a n be c o n v e r t e d b a c k t o l i m e w i t h g e n e r a t i o n o f H S b y h y d r o l y s i s . 2

2


5.

HYNE

Desulfurization

of

Fossil

57

Fuels


H y d r o d e s u l p h u r i s a t i o n o f c o k e , h o w e v e r , may be e c o n o m i c a l l y f e a s i b l e (12). While t h i s technique r e q u i r e s a source of hydrogen t h i s , a s w i l l be d i s c u s s e d s h o r t l y , may be p o s s i b l e w i t h o u t a l a r g e net consumption of the hydrogen. Of p a r t i c u l a r i n t e r e s t i s the o b s e r v a t i o n t h a t pre-treatment of the coke w i t h c a u s t i c (13) and p r e - o x i d a t i o n (14) c a n s i g n i f i c a n t l y i m p r o v e h y d r o d e s u l phurisation yields. H y d r o g e n f r o m HpS Throughout the d i s c u s s i o n of r e f i n e r y d e s u l p h u r i s a t i o n t h e common s u l p h u r c o n t a i n i n g end p r o d u c t h a s b e e n H2S. A f t e r e x t r a c t i o n f r o m t h e m a i n f e e d s t o c k s t r e a m by a n a p p r o p r i a t e p h y s i c a l o r c h e m i c a l a b s o r p t i o n method t h e H S c o n c e n t r a t i o n i s n o r m a l l y h i g h enough t o be f e d t o a C l a u s r e a c t o r w h e r e t h e H S i s c o n v e r t e d t o e l e m e n t a l s u l p h u r and w a t e r . 2

2

2H S + 0 2

>

2

2H 0 + 2

0.25S

8

Thus t h e h y d r o g e n g e n e r a t e d f o r u s e i n t h e u p s t r e a m h y d r o d e s u l p h u r i s a t i o n s t e p emerges f r o m t h e s y s t e m a s w a t e r - e n v i r o n m e n t a l l y a c c e p t a b l e b u t v e r y much a " o n c e t h r o u g h " c h e m i c a l u t i l i s a t i o n of t h i s v a l u a b l e r e a g e n t . C o n s i d e r a b l e a t t e n t i o n has been g i v e n r e c e n t l y t o the d i r e c t r e c o v e r y of the hydrogen from hydrogen s u l p h i d e r a t h e r than con v e r s i o n to water. C a t a l y t i c thermal c r a c k i n g of H S i s p o s s i b l e (15,16) and i m p r o v e d c a t a l y s t s p e r m i t t i n g t h e r m a l d e c o m p o s i t i o n at lower temperatures are being i n v e s t i g a t e d . 2

HS 2

< > catalyst heat

H ψ

2

+

S , ψ removal

An i m p o r t a n t f e a t u r e o f any s u c c e s s f u l p r o c e s s b a s e d on c a t a l y t i c t h e r m a l d e c o m p o s i t i o n i s t h a t t h e s y s t e m must be k e p t i n a n " u p s e t e q u i l i b r i u m " c o n d i t i o n s i n c e the e q u i l i b r i u m c o n c e n t r a t i o n of h y d r o g e n i n t h e p r e s e n c e o f s u l p h u r and H S i s l o w a t r e a d i l y a c c e s s i b l e temperatures. By u s e o f a f l o w s y s t e m and s e p a r a t i o n o f t h e r e a c t i o n p r o d u c t s , h o w e v e r , t h e y i e l d o f h y d r o g e n c a n be m a r k e d l y i m p r o v e d by u t i l i s i n g t h e r e l a t i v e k i n e t i c s o f t h e f o r w a r d and r e v e r s e r e a c t i o n s . R e c o v e r y o f t h e h y d r o g e n by c h e m i c a l r e a c t i o n i s a l s o f e a s i ble (17). Many l o w e r s u l p h i d e s o f m e t a l s w i l l r e a c t r e a d i l y w i t h H S t o y i e l d h y d r o g e n and t h e h i g h e r s u l p h i d e o f t h e m e t a l . The h i g h e r s u l p h i d e s a r e u s u a l l y u n s t a b l e a t e l e v a t e d t e m p e r a t u r e s and r e g e n e r a t i o n o f t h e l o w e r s u l p h i d e c a n be e f f e c t e d w i t h recovery of elemental s u l p h u r . 2

2

MS

X

t

+ HS 2

-s

^=t

MS

X + 1

+

H

2

,

Heat



58

S u c h p r o c e s s e s f o r t h e r e c o v e r y o f h y d r o g e n f r o m H2S a r e e n e r g y c o n s u m i n g compared w i t h t h e C l a u s t r e a t m e n t o f t h e p r o d u c t H2S w h i c h i s h i g h l y e x o t h e r m i c . The o v e r a l l e c o n o m i c s o f a s u c c e s s f u l p r o c e s s may, h o w e v e r , be s u c h a s t o make h y d r o g e n r e c o v e r y f r o m H2S more a t t r a c t i v e t h a n c o n t i n u a l h y d r o g e n g e n e r a t i o n f o r a "once t h r o u g h " h y d r o d e s u l p h u r i s a t i o n r o l e .


Combustion D e s u l p h u r i s a t i o n A s c o s t s o f p r e c o m b u s t i o n h y d r o d e s u l p h u r i s a t i o n and p o s t c o m b u s t i o n f l u e gas c l e a n - u p h a v e e s c a l a t e d and a s e n v i r o n m e n t a l r e g u l a t i o n s have f u r t h e r l i m i t e d the s u l p h u r d i o x i d e e m i s s i o n r a t e s , t h e r e has b e e n a g r o w i n g i n t e r e s t i n t e c h n o l o g y d e s i g n e d to e f f e c t f u e l d e s u l p h u r i s a t i o n d u r i n g t h e c o m b u s t i o n p r o c e s s . D e s u l p h u r i s a t i o n d u r i n g f l u i d i s e d bed c o m b u s t i o n o f c o a l h a s b e e n a l e a d i n g technique i n these developments. The p r i n c i p l e o f f l u i d i s e d bed c o m b u s t i o n w i t h s i m u l t a n e o u s d e s u l p h u r i s a t i o n i s b a s e d on t h e t h e r m a l d e c o m p o s i t i o n o f l i m e stone carbonates to y i e l d oxides which then r e a c t w i t h the s u l p h u r o x i d e p r o d u c t s o f c o m b u s t i o n o f b o t h i n o r g a n i c and o r g a n i c s u l p h u r compounds i n t h e h y d r o c a r b o n f u e l . CaC0

CaO

+ S0

3

— •

2

2

- -*

CaO

+

C0

2

CaSOit ash

C o m b u s t i o n t e m p e r a t u r e s a r e h i g h enough t o e n s u r e c a r b o n a t e d e c o m p o s i t i o n and f u l l o x i d a t i o n o f t h e s u l p h u r v a l u e s t o t h e s u l p h a t e form. The r a t i o o f l i m e s t o n e t o f u e l r e q u i r e d f o r e f f e c t i v e des u l p h u r i s a t i o n d e p e n d s u p o n b o t h t h e m i n e r a l and s u l p h u r c o n t e n t of t h e c o a l . I t may be p o s s i b l e t o b u r n l o w s u l p h u r ( CO? heat

C a C 0

2

> CaSO^

NH ΩΗ

Manganese D i o x i d e (Mitsubishi) (23)

Regeneration 3

A c t i v e Magnesia (Showa Hatsuden) (Chemico)

ς

200 - 3 0 0 ° F

η

SO2

M

ç

1400°

n

(24)

1 C ( y

c

n

— — > 15% SO2

} M gS03

MgO

J as

Contact Process

Regeneration

H S0i, 2

4

M o d i f i e d Contact (Monsanto-Penelec) (Tokyo Tech.)

Air + S0

9

0

0

2

°

)S0

F

S0

2

+ H 0 + N0 2

t+

>H S0

2

2

A c t i v a t e d Carbon (Sulfacid-Lurgi) ( H i t a c h i , Tokyo) ( R e i n l u f t , W. Germany) (Westvaco Corp. U.S.)

' - ~ ; Α

ς η B U

2

2

1+

+ H0 2

+ NO

! 0

1/2

30,

u

t

(27)

29,

l+

(NH ) S0

2 NH^OH

M o d i f i e d Chamber (Tyco Labs. Mass)

(28,

2

25

(26)

5

> H S0

3

Vo0U

A

ΐ

Γ

c

t

H

2

2°

>

n l

v

e

Μ

Σ Η

" 2 ^

C a r b o n

31)

REDUCTION TO SULPHUR 7

C a t a l y t i c Redox ( P r i n c e t o n Chem. Res.

S0

2

+ HS

250 - 3 5 0 F U

2

(32)

-> H 0 2

Catalyst

+ S

CO. •

Desulfurization of Fossil Fuels - ACS Publications - American

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