Coal Conversion Technology - ACS Publications - American Chemical...
0 downloads
90 Views
1MB Size
11 Major Technical Issues Facing L o w and Medium B t u
Downloaded via TUFTS UNIV on July 10, 2018 at 02:28:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Gasification E. L. CLARK 4615 North Park Avenue, Chevy Chase, MD 20015
The title of this paper requires some discussion to picture properly the status of coal gasification. An appreciable number of commercial coal gasification plants are operating in several countries throughout the world using several gasification processes. This status indicates that a reasonable number of technical issues have been solved and that a fundamental technical basis for coal gasification exists. Problems still facing the commercial use of coal gasification include the adaptation of existing processes to our environmental standards and to coals of United States origin. The problems of economics are also serious issues which are partly technical in nature. Process improvement and new process development are the technical issues we face in achieving economically competitive coal gasification. Even though Low Btu gas (LBG) and Medium Btu gas (MBG) have become terms of common use, some specification of these gases is desirable. A brief specification is provided in Table 1 which is intended to cover LBG and MBG. The upper and lower values of Btu content per standard cubic foot should be considered as approximate rather than exact limitations. S i m i l a r l y , the term " e s s e n t i a l l y f r e e " i s an attempt t o avoid p r e d i c t i n g what p u r i t y environmental standards might r e q u i r e i n the f u t u r e . The advantages and d i f f i c u l t i e s of g a s i f y i n g c o a l a t e l e v a t e d pressure are not always a p p r e c i a t e d . While combustion of LBG and MBG may take p l a c e a t e s s e n t i a l l y atmospheric p r e s s u r e , the g e n e r a t i o n of these gases a t e l e v a t e d pressure can provide more economical g a s i f i e r o p e r a t i o n and more convenient t r a n s p o r t t o s e v e r a l u s e r s . F i n a l l y , i f a c l e a n gas could be f u r n i s h e d a t e l e v a t e d temperature, t h e thermal content o f the gas would be a v a i l a b l e t o the user. While the s p e c i f i c a t i o n i n Table 1 covers both low and medium Btu gases, we a r e d i s c u s s i n g two d i f f e r e n t m a t e r i a l s . Low Btu gas (LBG) i s produced by the r e a c t i o n of a i r and steam w i t h c o a l and has a h e a t i n g v a l u e g e n e r a l l y 150 t o 170 Btu/Standard Cubic Foot (SCF). Medium Btu gas (MBF) i s produced by the r e a c t i o n of
0-8412-0516-7/79/47-110-183$05.00/0 © 1979 American Chemical Society Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
COAL CONVERSION TECHNOLOGY
184
oxygen and steam w i t h c o a l and has a heating value of 290 t o 350 Btu/SCF. The d i f f e r e n c e i n h e a t i n g value i s c r i t i c a l . Conversion of an e x i s t i n g o i l or n a t u r a l g a s - f i r e d steam generation o r process heating u n i t becomes q u i t e c o s t l y when the a l t e r n a t i v e gaseous f u e l h e a t i n g value drops below 250 Btu/SCF. Ç1) This i n d i c a t e s t h a t end-uses o f LBG may be l i m i t e d t o new p l a n t s designed s p e c i f i c a l l y f o r gaseous f u e l s o f low h e a t i n g value. TABLE 1 LOW BTU GAS SPECIFICATION ο
Heating value above 120 Btu/SCF and below 500 Btu/SCF
ο
E s s e n t i a l l y f r e e of s u l f u r , ammonia, p a r t i c u l a t e s and hazardous i m p u r i t i e s o r byproducts
ο
P r e f e r a b l y provided a t elevated
ο
P r e f e r a b l y provided a t elevated temperature
pressure
Another important d i f f e r e n c e i s that the complexity and minimum economic s i z e i s c o n s i d e r a b l y greater f o r an MBG p l a n t than an LBG p l a n t . Small LBG p l a n t s are on operation supplying gaseous f u e l to small i n d u s t r i a l plants. Bearing these p o i n t s i n mind, we can consider the p o t e n t i a l markets f o r LBG and MBG, Table 2 provides a l i s t i n g of i n d u s t r i a l f u e l usage and power generation s u p p l i e d by petroleum and n a t u a l gas. The f u e l or energy amounts are i n Quads and we can con v e n i e n t l y p i c t u r e the s i z e o f a Quad by n o t i n g i t s equivalence t o one t r i l l i o n cubic f e e t o f n a t u r a l gas o r 1 0 ^ Btu. The data f o r 1974 are approximately the same f o r the p e r i o d of 1974 t o 1977. The data f o r the year 2000 were taken from a p r o j e c t i o n made some time ago by the E l e c t r i c Power Research I n s t i t u t e (EPRI) and appear a b i t on the h i g h s i d e f o r p r o j e c t e d increases f o r e l e c t r i c power and t o t a l energy f o r the year 2000. I n any case, we can conclude that a s i z a b l e market p o t e n t i a l e x i s t s f o r MBG as an a l t e r n a t i v e f u e l f o r e x i s t i n g u n i t s . S i m i l a r l y , the growth p r o j e c t i o n s f o r the f u t u r e i n d i c a t e an adequate p o t e n t i a l f o r LBG as a f u e l f o r new f a c i l i t i e s e s p e c i a l l y f o r e l e c t r i c power generation. We c a u t i o n that the growth p r o j e c t i o n t o 2000 given i n t h i s Table i s q u i t e t e n t a t i v e and s e v e r a l other p r o j e c t i o n s i n d i c a t e lower t o t a l energy demand by that year. The chemical r e a c t i o n s t a k i n g place during the g a s i f i c a t i o n of c o a l are w e l l known. Some of these are l i s t e d i n Table 3. In the r e a c t i o n s l i s t e d , c o a l i s assumed t o be e s s e n t i a l l y carbon. The oxygen i s e i t h e r pure oxygen as used i n MBG production o r
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
11.
CLARK
185
Low & Medium Btu Gasification
oxygen contained i n a i r f o r LBG g e n e r a t i o n . I n the l a t t e r case, n i t r o g e n w i l l be present as a d i l u e n t . The f i r s t three r e a c t i o n s l i s t e d a r e t r u l y g a s i f i c a t i o n r e a c t i o n s i n that they convert a s o l i d (carbon) t o a gas. I t i s apparent that the r e a c t i o n of carbon w i t h oxygen must supply a l l the heat energy r e q u i r e d . TABLE 2 POTENTIAL LOW BTU GAS MARKETS 1974
2000
QUADS 16.0 ^
Industrial
( 1
(20.4p
}
(30)
( 2 )
(75)
( 2 )
7 . 0 ^
E l e c t r i c Power
(20.0)
73
TOTAL
( 2 )
( 2 )
(1)
Market s u p p l i e d by petroleum and n a t u r a l gas.
(2)
T o t a l demand.
(3)
Estimates prepared by EPRI.
150
( 2 )
TABLE 3 COAL GASIFICATION KEY REACTIONS ο
Gasification C + H 0
•CO + H ; Endothermic
2
C + 0
2
—
2
C + 2H ο
*"C°2
Exothermic
;
» CH^; Exothermic
2
Shift CO + H 0
^ C 0 + H ; Exothermic
2
ο
m
QUADS
2
2
Methanation CO + 3 H
2
^ ΰ Η + H 0 ; Exothermic 4
2
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
186
COAL CONVERSION TECHNOLOGY
While some heat energy may be s u p p l i e d by the hydrogénation of carbon to methane, the hydrogen r e q u i r e d f o r t h i s r e a c t i o n must be s u p p l i e d by the s m a l l amount of hydrogen i n the c o a l or by the endothermic r e a c t i o n of steam w i t h carbon. Heat f o r t h i s r e a c t i o n must be s u p p l i e d by combustion of carbon. The two gas phase r e a c t i o n s a l t e r the composition of the gases produced. The s h i f t r e a c t i o n i s s l i g h t l y exothermic. The methanation r e a c t i o n i s s t r o n g l y exothermic but r e q u i r e s the presence of hydrogen and e i t h e r elevated pressure or an a c t i v e c a t a l y s t . As i n most r e a c t i o n s between s o l i d s and gases, the method of o b t a i n i n g contact between c o a l and r e a c t a n t gases i s a c r i t i c a l f a c t o r . F i g u r e 1 shows three a d d i t i o n a l systems f o r s o l i d and gas c o n t a c t i n g . A l l three are used i n c o a l g a s i f i c a t i o n commerc i a l u n i t s or are the b a s i s f o r processes under commercial d e v e l opment. There are advantages and disadvantages to each system. The moving bed u n i t on the l e f t uses f a i r l y l a r g e s i z e s of c o a l w i t h a minimum s i z e of one-quarter i n c h . I t p r o v i d e s countercurrent flow and good heat t r a n s f e r . The f l u i d i z e d bed shown i n the center uses r e l a t i v e l y s m a l l p a r t i c l e s of c o a l which r e s u l t i n a more r a p i d r e a c t i o n r a t e . Both moving bed and f l u i d i z e d bed u n i t s have d i f f i c u l t i e s i n h a n d l i n g c o a l s which agglomerate. Both r e q u i r e p r e c a u t i o n s i n p r e v e n t i n g s o f t e n i n g or m e l t i n g of ash which may cause f o r m a t i o n of c l i n k e r s and may d i s r u p t s o l i d flow. The e n t r a i n e d f l o w u n i t uses f i n e p a r t i c l e s of c o a l ; operates at h i g h e r temperatures to o b t a i n r a p i d r e a c t i o n r a t e s ; and removes ash i n the molten s t a t e or as s l a g . T h i s u n i t can handle any type of c o a l but a t t e n t i o n must be p a i d to the ash components and the m e l t i n g p o i n t and melt v i s c o s i t y to o b t a i n r e l i a b l e o p e r a t i o n . The f i x e d or moving bed has had more usage than any other system. Many s m a l l u n i t s were operated here and abroad. Used w i t h a i r and non-agglomerating c o a l and operated at e s s e n t i a l l y atmospheric p r e s s u r e , such u n i t s were i n e x p e n s i v e , simple to operate and w i d e l y used. The L u r g i u n i t i s the o n l y one which has been designed f o r o p e r a t i o n a t e l e v a t e d p r e s s u r e . I t can be used w i t h a i r or oxygen w i t h the l a t t e r more w i d e l y used. These u n i t s use modest amounts of oxygen (160 to 170 cu. f t . oxygen/1000 c u . f t MBG) but, i n order to p r o t e c t the g r a t e which d i s c h a r g e s the ash, use q u i t e l a r g e q u a n t i t i e s of steam (approx. 75 l b s / 1000 c u . f t . MBG). I n a l l f i x e d bed u n i t s , the hot gases f l o w upward h e a t i n g and d e v o l a t a l i z i n g the c o a l which enters at the top. These v o l a t i l e s condense and r e s u l t i n the p r o d u c t i o n of o i l s , t a r s and v a r i o u s o r g a n i c contaminants. The l a t t e r are found i n the d i s carded water condensate and n e c e s s i t a t e an expensive water c l e a n up system p r i o r to d i s p o s a l . A major problem i s the need t o f i n d a r e l i a b l e use f o r the f i n e c o a l which the f i x e d bed cannot handle. A very l a r g e p i p e l i n e gas p l a n t which p l a n s the product i o n of MBG ( f o r c o n v e r s i o n to methane) from North Dakota l i g n i t e u s i n g L u r g i g a s i f i e r s has arranged to s e l l a l l the f i n e c o a l to a
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Figure 1.
Major gasification systems
Entrained Bed Fluid Bed Fixed Bed
SLAG ASH/CHAR ASH
GAS GAS GAS COAL
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
188
COAL CONVERSION TECHNOLOGY
nearby l i g n i t e - b a s e d power p l a n t c o n c u r r e n t l y under c o n s t r u c t i o n . Whether t h i s p a t t e r n can be continued i n a l l cases where f i x e d bed g a s i f i c a t i o n w i l l be used i s d o u b t f u l . The F l u i d Bed System shown i n F i g u r e 2 i s the b a s i s f o r s e v e r a l developmental processes. The use of f i n e p a r t i c l e s permits t o t a l u t i l i z a t i o n of coal-mine output. One commercial process, the Winkler process, i s used i n A s i a and A f r i c a to provide MBG f o r f u e l and chemical s y n t h e s i s . This commercial process i s operated at atmospheric pressure which i s a disadvantage due to compression c o s t s r e q u i r e d f o r gas t r a n s p o r t a t i o n and most chemic a l end uses. The developmental processes are a l l operated at elevated pressure i n an attempt to remedy t h i s disadvantage. The f l u i d i z e d bed, being a completely mixed system, l i m i t s the carbon conversion which can be obtained. As much as 15% of the c o a l i s not reacted and some use must be made of the high-ash-content char. The use of agglomerating c o a l i s precluded due to the l o s s of f l u i d i z a t i o n i f c o a l p a r t i c l e s s t a r t s t i c k i n g together. The use of f i n e p a r t i c l e s does permit pre-treatment of agglomerating c o a l s p r i o r to f e e d i n g to the g a s i f i e r , but t h i s process a l s o e n t a i l s l o s s e s i n carbon conversion. Another problem area i s the lower p o r t i o n of the f l u i d i z e d bed where a i r or oxygen enters and f i r s t r e a c t s w i t h the c o a l . L o c a l i z e d h i g h temperatures i n areas where adequate turbulence of f l o w may be l a c k i n g can cause s i n t e r i n g together of ash p a r t i c l e s to form c l i n k e r s and d i s r u p t o p e r a t i o n . Reasonable steam and oxygen requirements may make processes based on t h i s system c o m p e t i t i v e i f lower carbon conversion can be tolerated. An important v a r i a n t of the F l u i d Bed system i s under development. This v a r i a n t e l i m i n a t e s use of a i r or oxygen i n the a c t u a l g a s i f i e r . Steam and c o a l are the r e a c t a n t s . Since we know from Table 3 that the r e a c t i o n of steam w i t h c o a l i s endothermic, a heat source must be provided. Hot s o l i d s i n the form of char are heated i n a combustor and are t r a n s f e r r e d to the g a s i f i c a t i o n r e a c t o r as one these processes. In another, hot a l k a l i n e oxides r e a c t w i t h the carbon d i o x i d e i n the gas to form carbonates. The exothermic r e a c t i o n of carbonate formation s u p p l i e s the heat requirements of the steam-carbon r e a c t i o n . Both of these processes depend on a r e a c t i v e c o a l or char to implement the steam-carbon r e a c t i o n . The E n t r a i n e d system i s a h i g h temperature, h i g h r e a c t i o n r a t e process i n which c o a l , oxygen (or a i r ) and steam combine r a p i d l y to produce LBG or MBG. The commercial processes aim p r i m a r i l y at the use of oxygen. S e v e r a l developmental processes use oxygen or air. The most w i d e l y used commercial process (Koppers-Totzek) i s operated at atmospheric pressure. The Texaco p a r t i a l o x i d a t i o n process used w i t h o i l and gas i s under development f o r use w i t h c o a l . S h e l l and Koppers are developing a p r e s s u r i z e d v e r s i o n of the c u r r e n t Koppers-Totzek process. The advantages of the e n t r a i n e d
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
MINEMOUTH COAL
GASIFICATION
FINES REMOVAL
GAS LIQUOR/AIR SEPARATION AND PROCESSING
I GAS COOLING/ | I QUENCH SCRUBBING I
Anatomy of gasification processes, low Btu version
PRETREATMENT (CAKING COALS)
COAL PREPARATION
Figure 2.
I
L.
I
OXYGEN PLANT
SULPHUR REMOVAL
190
COAL CONVERSION TECHNOLOGY
systems a r e : complete conversion of c o a l ; a b i l i t y t o use almost any c o a l , agglomerating or n o t , and of almost any rank; and an apparent l a c k o f adverse environmental impact s i n c e no o i l s , t a r s or contaminants a r e formed. Problem areas i n v o l v e : c o n t r o l of p a r t i c u l a t e emissions; h a n d l i n g of molten s l a g and need f o r s u i t a b l e r e f r a c t o r i e s ; and, due to h i g h e x i t temperatures from the g a s i f i e r , a need t o develop s u i t a b l e heat recovery systems. The entrained systems a r e g e n e r a l l y h i g h oxygen consumers - almost double the requirement f o r the f i x e d bed u n i t s . Energy f o r oxygen p r o d u c t i o n could be recovered from hot g a s i f i e r e f f l u e n t gases i f s u i t a b l e waste heat b o i l e r s and superheaters can be developed. To t h i s p o i n t , we t r u s t that a c l e a r p i c t u r e of the t e c h n i c a l s t a t u s of c o a l g a s i f i c a t i o n i s emerging. We have a reasonable grasp of the chemistry. Three systems f o r h a n d l i n g the mechanics of c o a l r e a c t i o n w i t h steam and air/oxygen have been developed t o the p o i n t where commercial o p e r a t i o n i s p r a c t i c e d . However, t h i s achievement i s not complete enough f o r widespread commercial use i n the United S t a t e s . The technology must be made t o conform w i t h environmental standards, economics and the end-use p a t t e r n s of p o t e n t i a l customers. No meaningful demonstration of c o a l g a s i f i c a t i o n technology has been provided t o e s t a b l i s h t h i s r e quirement and t o prove o p e r a b i l i t y and r e l i a b l e on-stream p e r f o r mance. U n t i l t h i s i s achieved, economic estimates degenerate i n t o i n c o n c l u s i v e paper s t u d i e s and p o t e n t i a l customers cannot accept the r i s k s i n v o l v e d . U n t i l one or more s t a t e - o f - t h e - a r t systems are operated on a commercial s c a l e , the most a t t r a c t i v e advanced systems cannot be moved f u r t h e r toward c o m m e r c i a l i z a t i o n except through massive s u b s i d i e s by the Department of Energy (DOE). These do not seem t o be a v a i l a b l e i n today's s t r i n g e n t budgets which are aimed a t an e l i m i n a t i o n of d e f i c i t s and a r e d u c t i o n i n i n f l a t i o n a r y impact. Some steps forward a r e being made i n e s t a b l i s h i n g r e a l c o s t s , c o l l e c t i n g environmental i n f o r m a t i o n and demonstrating r e l i a b l e o p e r a b i l i t y . S e v e r a l s m a l l f i x e d bed g a s i f i e r s sponsored by DOE and i n d u s t r y a r e under c o n s t r u c t i o n . These w i l l produce LBG f o r i n d u s t r i a l use. A l l of these are a i r blown u n i t s and a r e s t a t e o f - t h e - a r t g a s i f i e r s , A s i z a b l e environmental e v a l u a t i o n and c o n t r o l program i s being implemented. W i t h i n the next 12 t o 18 months we should have o p e r a t i n g and r e l i a b l e economic data on these systems. While the impact on n a t i o n a l energy usage of these r e l a t i v e l y s m a l l u n i t s be n e g l i g i b l e , the data provided w i l l e s t a b l i s h a lower t e c h n i a l r i s k l e v e l f o r l a r g e r f i x e d bed u n i t s . Both e f f l u e n t c o n t r o l and c o n t r o l of i n - p l a n t t o x i c substance l e v e l s w i l l be reduced t o i n d u s t r i a l p r a c t i c e and should make future plants easier to b u i l d . The economic p o s i t i o n of these s m a l l u n i t s i s not very advantageous. I n c e r t a i n end-uses, hot u n p u r i f i e d gas may be u t i l i z e d
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
11.
CLARK
Low ir Medium Btu Gasification
191
d i r e c t l y t o the k i l n burners. P a r t i c u l a t e c o n t r o l i s necessary at the k i l n o u t l e t i n any case and the s m a l l amount of c o a l ash c a r r i e d over from the g a s i f i e r does not seem to a f f e c t b r i c k product q u a l i t y . A s i m i l a r system i s being t e s t e d a t the Bureau of Mines Twin C i t y S t a t i o n i n St. P a u l , Minnesota. Here LBG i s produced and used t o s i n t e r t a c o n i t e pebbles. The process i s c a l l e d endurating o r hardening. Again, the hot gas from the g a s i f i e r i s fed d i r e c t l y t o the s h a f t furnace o r k i l n t o f i r e the "green" t a c o n i t e pebbles and harden them. I n t h i s case, the s m a l l amount o f s u l f u r i n the gas being produced from l i g n i t e i s absorbed by the i r o n oxide pebbles w i t h no s e r i o u s e f f e c t on q u a l i t y . P a r t i c u l a t e c o n t r o l i s maintained a t the o u t l e t of the p r o c e s s i n g k i l n o r furnace. In cases such as the two d e s c r i b e d , we can v i s u a l i z e a com p e t i t i v e p o s i t i o n f o r LBG. Cost estimates f o r these "hot, d i r t y gas" g e n e r a t i o n systems show a f u e l cost of under $ 3 . 0 0 / m i l l i o n Btu i n 1976 d o l l a r s . However, when a p u r i f i c a t i o n system f o r both p a r t i c u l a t e and s u l f u r removal i s added t o these s m a l l - s i z e p r o d u c t i o n u n i t s , the cost i n c r e a s e s d r a s t i c a l l y . The average output o f these s m a l l , a i r - b l o w n g a s i f i e r s o p e r a t i n g a t atmos p h e r i c pressure i s l e s s than 10 tons of coal/hour. S i n g l e t r a i n p u r i f i c a t i o n systems can handle the gas p r o d u c t i o n from as much as 5,000 tons o f coal/day. I t i s obvious that such l a r g e systems are much l e s s c o s t l y per u n i t o f p r o d u c t i o n than a s m a l l u n i t h a n d l i n g the gas produced from 200 t o 250 tons of coal/day. Another disadvantage i n p u r i f y i n g the gas from these s a m l l u n i t s i s t h e i r o p e r a t i o n a t atmospheric p r e s s u r e . The s m a l l e r volume of gas a t e l e v a t e d pressure f u r t h e r reduces the c a p i t a l cost of p u r i f i c a t i o n systems. As a r e s u l t , one f i n d s that p u r i f i e d LBG i n s m a l l u n i t s may double the p r i c e t o over $ 5 . 0 0 / m i l l i o n Btu. Thus, LBG i n s m a l l u n i t s i s o n l y c o m p e t i t i v e i n r a t h e r s p e c i a l cases. For l a r g e r u n i t s u s i n g 10,000 tons of c o a l per day, c o s t s o f p u r i f i e d gas s u i t a b l e f o r combustion under even the most s t r i n g e n t environmental c r i t e r i o n are becoming c o m p e t i t i v e . Costs f o r producing gas by s t a t e - o f - t h e - a r t f i x e d bed systems operated a t 300 p s i g u s i n g c o a l c o s t i n g $ 1 . 0 0 / m i l l i o n Btu have been estimated by EPRI a t $3,00 t o $3.50 f o r LBG and $3.50 t o $4.50 f o r MBG w i t h a l l v a l u e s i n d o l l a r s per m i l l i o n Btu i n mid-1975 d o l l a r s . (_2) The v a r i a t i o n i n cost i s p r i m a r i l y a f u n c t i o n of the o p e r a t i n g f a c t o r which might be assumed. This was v a r i e d from 70% t o 90%, I t should be r e a l i z e d that these are very l a r g e p l a n t s producing s l i g h t l y more than 130 χ 1 0 ( b i l l i o n ) Btu/day. T h i s q u a n t i t y of energy could generate i n excess o f 640 MW of e l e c t r i c i t y (assuming a heat r a t e of 9,000 Btu/KW). F i n d i n g an i n d u s t r i a l p l a n t l a r g e enough i s not e a s i l y achieved even i n today's p o l i c y of very l a r g e i n d u s t r i a l p r o d u c t i o n u n i t s . A survey made f o r the P r e s i d e n t ' s Energy P o l i c y and P l a n n i n g O f f i c e (3) i n 1977 shows 9
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
COAL CONVERSION TECHNOLOGY
192
fewer than 20 p l a n t s l a r g e enough i n n a t u r a l gas consumption t o use the output of an MBG or LBG p l a n t which could process 10,000 tons of coal/day. We a r e forced to conclude that LBG or MBG generated i n f i x e d bed u n i t s could approach commercially c o m p e t i t i v e l e v e l s i n l a r g e p l a n t s . For LBG, which cannot be c o n v e n i e n t l y t r a n s p o r t e d , only very few i n d u s t r i a l p l a n t s could j u s t i f y o n - s i t e generation a t an economic s c a l e . An LBG p l a n t processing 10,000 tons of coal/day could provide energy t r a n s l a t a b l e i n t o 650 t o 800 MW of e l e c t r i c i t y . Remembering that conversion o f e x i s t i n g f a c i l i t i e s t o LBG i s expensive and d i f f i c u l t , we f i n d a r e l a t i v e l y minor r o l e f o r i n d u s t r i a l use except i n new, l a r g e p l a n t s using i n excess of 15 χ 1 0 ^ ( t r i l l i o n ) Btu/year or i n l a r g e power generating p l a n t s of over 500 MW i n s i z e . One must expect that e l e c t r i c power generation o f f e r s the g r e a t e s t p o t e n t i a l f o r LBG and some f u t u r e t h r u s t a t c o m m e r c i a l i z a t i o n might be sponsored or a c t i v a t e d by the public u t i l i t y sector. For MBG, i n d u s t r i a l use i n l a r g e , e x i s t i n g p l a n t s has a meaningful p o t e n t i a l . Of the l a r g e p l a n t s which might support an e c o n o m i c a l l y - s i z e d MBG p l a n t , over h a l f a r e petroleum or p e t r o chemical f a c i l i t i e s . Implementation of MBG to supply f u e l and gaseous feedstocks t o such p l a n t s would almost d i r e c t l y reduce petroleum consumption i n such f a c i l i t i e s . This r e d u c t i o n would be a p p r e c i a b l e s i n c e i t i s estimated that 6% to 10% of the crude petroleum fed t o a r e f i n e r y might be u t i l i z e d t o provide energy for the r e f i n i n g process. A study made f o r DOE has i n d i c a t e d s e v e r a l areas where s u i t a b l e c o n c e n t r a t i o n s of i n d u s t r i a l p l a n t s could be served by a s i n g l e MBG gas producing f a c i l i t y . (40. Due to the need f o r t r a n s p o r t i n g the gas, LBG could not be used. Examples of such areas are: Houston, w i t h a need f o r 149 χ 1 0 Btu/year by 1985; Chicago, w i t h 69 χ 1 0 ; P i t t s b u r g h , w i t h 25 χ 1 0 ; S t . L o u i s , w i t h 20 χ 10 , and P h i l a d e l p h i a , w i t h 37 χ 1 0 . These f i v e areas represent over h a l f the t o t a l United States p o t e n t i a l requirement f o r MBG f u e l . I n d i v i d u a l f a c i l i t i e s i n each of these areas could supply MBG t o many industrial plants. 1
1 2
1 2
1 2
While the e v a l u a t i o n s of cost and p l a n t s i z e discussed i n the preceding paragraphs have been devoted t o f i x e d bed,systems, the conclusions a r e v a l i d f o r a l l c o a l g a s i f i c a t i o n techniques. Estimates of f l u i d bed g a s i f i e r s have a l s o been prepared. (2) U n f o r t u n a t e l y , i n s u f f i c i e n t data a r e a v a i l a b l e t o s u b s t a n t i a t e the o p e r a b i l i t y and a c t u a l p r o d u c t i v i t y which must form the b a s i s for any cost estimate. Using these t e n t a t i v e c o s t s , we f i n d that costs f o r LBG o r MBG might be below $ 3 . 0 0 / m i l l i o n Btu. For entrained flow systems, s t i l l under development, costs i n the below $ 3 . 0 0 / m i l l i o n Btu range are estimated. However, a c t u a l implementation of these advanced systems or even s t a t e - o f - t h e - a r t
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
11.
CLARK
Low ir Medium Btu Gasification
193
systems i s s t i l l a matter of w i l l i n g n e s s t o take a p p r e c i a b l e t e c h n i c a l and economic r i s k s . The c o s t s of MBG and LBG should become c o m p e t i t i v e w i t h energy p r i c e s i n the 1983-85 time span as petroleum p r i c e s i n c r e a s e and n a t u r a l gas p r i c e s a r e d e c o n t r o l l e d . Further development o f improved g a s i f i c a t i o n processes should a l s o have some e f f e c t . The e f f e c t s of new process development a r e , however, l i m i t e d to a c t u a l g a s i f i c a t i o n and these may be s m a l l . The t o t a l f a c i l i t y f o r producing LBG o r MBG i s somewhat more than j u s t g a s i f i c a t i o n . Gas p u r i f i c a t i o n , waste d i s p o s a l and general u t i l i t y requirements are almost a l l standard systems which w i l l only be p a r t i a l l y reduced i n cost by improved c o a l g a s i f i e r technology. F i g u r e 2 diagrams the u n i t s r e q u i r e d t o produce LBG o r MBG. I n the case of LBG, a i r i s u t i l i z e d bypassing the oxygen p l a n t which i s r e q u i r e d f o r MBG p r o d u c t i o n . Gas c o o l i n g , f i n e s removal and s u l f u r removal are s i m i l a r f o r producing both gases. S i m i l a r l y , c o a l p r e p a r a t i o n and pretreatment are performed i n s i m i l a r systems f o r both gases. The a d d i t i o n a l t e c h n i c a l b a r r i e r s which must be overcome have been s t a t e d . P r i m a r i l y , the need i s g r e a t e s t f o r a c t u a l o p e r a t i o n and demonstration of g a s i f i c a t i o n on an i n d u s t r i a l s c a l e . Small g a s i f i e r s are being so demonstrated through the a s s i s t a n c e of the Department of Energy (DOE). A d d i t i o n a l e f f o r t s are underway i n the DOE program. These i n c l u d e an MBG demonstration p l a n t i n which one o f two processes w i l l be t e s t e d : e i t h e r p r o d u c t i o n o f ammonia s y n t h e s i s gas; o r p r o d u c t i o n and d i s t r i b u t i o n of a f u e l gas t o s e v e r a l i n d u s t r i a l and power g e n e r a t i o n customers. The generation of MBG may be a l s o demonstrated i n the p i p e l i n e gas demonstration program. While the MBG produced under t h i s program w i l l be converted t o s y n t h e t i c n a t u r a l gas, the g e n e r a t i o n of MBG demonstrated i n a p i p e l i n e gas p l a n t could be a l s o a p p l i e d t o producing i n d u s t r i a l f u e l o r s y n t h e s i s gas. One s i m i l a r LBG demonstration p l a n t i n the DOE program w i l l use LBG f o r enduration of t a c o n i t e p e l l e t s . While implementation of these demonstration p l a n t p r o j e c t s w i l l depend on the magnitude o f the DOE budget, a very l a r g e share o f the g a s i f i c a t i o n budget i s being committed to t h i s e f f o r t . F o r t u n a t e l y , e f f o r t s i n a d d i t i o n t o those o f DOE a r e being implemented. The Tennessee V a l l e y A u t h o r i t y i s sponsoring the c o n s t r u c t i o n o f an e n t r a i n e d f l o w g a s i f i e r t o operate a t e l e v a t e d pressure and t o provide s y n t h e s i s gas t o t h e i r s m a l l Ammonia P l a n t a t Muscle Shoals, Alabama. I r o n i c a l l y , t h i s ammonia p l a n t was o r i g i n a l l y b u i l t u s i n g coke-fed water gas s e t s f o r s y n t h e s i s gas p r o d u c t i o n . I t was converted t o use n a t u r a l gas steam r e forming when cheap n a t u r a l gas became a v a i l a b l e . The use of c o a l w i l l provide v a l u a b l e data on MBG p r o d u c t i o n and p u r i f i c a t i o n . The C a r t e r O i l Company has reported i t s s t u d i e s on u s i n g
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
194
COAL CONVERSION TECHNOLOGY
Texas l i g n i t e to generate MBG which would be piped to the Houston area f o r f u e l and feedstock use. These s t u d i e s have included the t e s t i n g of Texas l i g n i t e i n commercial g a s i f i c a t i o n u n i t s l o c a t e d abroad. A very l a r g e g a s i f i c a t i o n p r o j e c t f o r c o n v e r t i n g c o a l t o MBG and p i p e l i n e gas i s under c o n s i d e r a t i o n by the F e d e r a l Energy Regulatory Commission w i t h a d e c i s i o n expected by June 1979. Implementation of t h i s p r o j e c t would provide a major demonstration of MBG production and p u r i f i c a t i o n . The design e f f o r t f o r t h i s p r o j e c t has included l a r g e - s c a l e t e s t s of North Dakota l i g n i t e i n commercial c o a l g a s i f i c a t i o n u n i t s . I t i s a n t i c i p a t e d that p r o j e c t s of t h i s magnitude, when s u c c e s s f u l l y operated i n the 1983-85 time p e r i o d , w i l l provide s u f f i c i e n t data so that normal i n d u s t r i a l d e c i s i o n s on use of MBG or LBG can be made. The t e c h n i c a l r i s k should be minimized to permit normal f i n a n c i n g . S e v e r a l important development e f f o r t s could improve the economic s t a t u s of low Btu gas production. Tests performed at W e s t f i e l d , Scotland, j o i n t l y by the Energy Research and Development A d m i n i s t r a t i o n (ERDA) and the American Gas A s s o c i a t i o n (A.G.A.) (5) have demonstrated that f i x e d bed g a s i f i e r s can be used s u c c e s s f u l l y w i t h weakly caking c o a l s (up to a f r e e s w e l l i n g index of 2.5 to 3.0) i f s u i t a b l e s t i r r e r s and d i s t r i b u t o r s are u t i l i z e d . While s m a l l - s c a l e t e s t s at the Morgantown Energy Technology Center have demonstrated on a p i l o t p l a n t s c a l e that even h i g h l y caking c o a l s can be handled, these t e s t s p a r t i a l l y confirmed the Morgantown r e s u l t s on commercial-size g a s i f i e r s . More recent r e s u l t s w i t h f i x e d bed u n i t s at W e s t f i e l d have demons t r a t e d the o p e r a t i o n of a s l a g g i n g bottom i n s t e a d of a g r a t e . This could reduce c o s t s a p p r e c i a b l y f o r f i x e d bed g a s i f i c a t i o n by reducing steam requirements used f o r grate c o o l i n g by over 90%. F u r t h e r , longer-term demonstration of the operating f e a s i b i l i t y of t h i s improved g a s i f i e r appears d e s i r a b l e . A major problem i n f l u i d i z e d bed g a s i f i c a t i o n i s the low carbon conversion. Ash agglomeration could improve carbon conversion and use the f i n e s e f f e c t i v e l y . Test work on t h i s system i s i n progress on a Process Development U n i t s c a l e . F i n a l l y , the use of entrained systems at elevated pressure should improve t h e i r a p p l i c a b i l i t y to a greater v a r i e t y of end uses. The implementation of c o a l g a s i f i c a t i o n w i l l occur as more data are a v a i l a b l e to e l i m i n a t e t e c h n i c a l r i s k . C u r r e n t l y we can v i s u a l i z e a competitive cost or $3.00 to $ 3 . 5 0 / m i l l i o n Btu f o r LBG and MBG i n l a r g e u n i t s u t i l i z i n g 5,000 to 10,000 tons of c o a l per day. These u n i t s could provide a guaranteed supply of gas t o . i n d u s t r y without being d i v e r t e d to use f o r p r i o r i t y consumer needs. MBG p a r t i c u l a r l y could become a d i s t r i b u t e d gas f o r i n d u s t r i a l use. Several areas where s u i t a b l e i n d u s t r i a l p l a n t s are concentrated have been l i s t e d i n a study sponsored by
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
11.
CLARK
Low b- Medium Btu Gasification
195
DOE (4). The implementation of one o r more demonstrations of c o a l g a s i f i c a t i o n by 1983 to 1985 should provide a s o l i d b a s i s f o r commercial use. This represents an unusual o p p o r t u n i t y f o r the gas i n d u s t r y t o extend i t s o p e r a t i n g base and t o ensure f u t u r e s u p p l i e s of c l e a n f u e l f o r consumers and i n d u s t r y . A s i m i l a r o p p o r t u n i t y e x i s t s f o r the p u b l i c u t i l i t y i n d u s t r y i n the p o t e n t i a l of LBG and MBG. The reduced environmental impact of a c o a l g a s i f i c a t i o n p l a n t which produces a p e r f e c t l y c l e a n f u e l e q u i v a l e n t to n a t u r a l gas, compared to d i r e c t combustion of c o a l may a l l o w increased use of c o a l i n areas where increased p o l l u t a n t emission i s barred. As these PSD areas i n c r e a s e i n number, the advantages of c o a l g a s i f i c a t i o n become more apparent. The p o t e n t i a l of more e f f i c i e n t combined c y c l e generation systems which can be used w i t h c o a l - d e r i v e d gases i s an added f a c t o r f o r implementing c o a l g a s i f i c a t i o n .
"LITERATURE CITED" (1)
Low Btu Gas Study, Electric Power Research Institute, Report No. EPRI 265-2, January, 1976.
(2)
Economics of Current and Advanced Gasification Processes for Fuel Gas Production; Electric Power Research Institute Report No. EPRI-AF244, July, 1976.
(3)
Market Potential for Low and Medium Btu Gas, Energy and Environmental Analysis, Inc., November 4, 1977.
(4)
Market Opportunities for Low and Intermediate Btu Gas from Coal in Selected Areas of Industrial Concentration, SRI International, Report No. HCP/T2441-02, June, 1978.
(5)
Trials of American Coals in a Lurgi Gasifier at Westfield, Scotland, Report No. E-105, December, 1972-November, 1974.
RECEIVED
July 11, 1979.
Pelofsky; Coal Conversion Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
