16 Hydrothermal Coal Process E D G E L P. S T A M B A U G H
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Battelle, Columbus Laboratories, 505 King Avenue, Columbus, O H 43201
Coal is the major source of energy for the U.S. and will continue to be so for many years. However, coal is dirty, containing high concentrations of contaminants such as sulfur, nitrogen, and mineral matter. These contaminants, if not removed from the coal before or during combustion, will find their way into the environment and thus constitute a serious health hazard. An alternative to insure a healthier environment, as the consumption of coal as the major source of energy increases, is to remove these contaminants by chemical coal cleaning before combustion. One such method based on hydrothermal technology is the hydrothermal coal process in which certain coals can be chemically cleaned to produce solid fuels which meet Federal sulfur emission standards for new sources. Process Description The basic process, as shown schematically in Figure 1, comprises five major processing operations: coal preparation, hydrothermal (desulfurization) treatment, liquid/solid separation, fuel drying, and leachant regeneration. Coal preparation may entail a simple grinding operation to reduce the raw coal to the desired particle size of 70% -200 mesh or 100% -28 mesh. On the other hand, this operation may involve two operations—grinding of the coal to the desired particle size followed by physical beneficiation to remove a portion of the mineral matter including a portion of the pyritic sulfur. Hydrothermal treatment e n t a i l s b a s i c a l l y three p r o c e s s i n g steps : (1) The g r o u n d c o a l i s m i x e d w i t h a n aqueous a l k a l i n e l e a c h a n t , f o r e x a m p l e , an aqueous s o l u t i o n / s l u r r y o f s o d i u m h y d r o x i d e and l i m e , t o produce a raw c o a l s l u r r y . (2) T h i s r a w s l u r r y i s h e a t e d i n an a u t o c l a v e a t a b o u t 250°-350°C ( s t e a m p r e s s u r e o f 600-2500 p s i g ) t o e x t r a c t a s i g n i f i c a n t p o r t i o n o f t h e s u l f u r and t h e m i n e r a l m a t t e r , d e p e n d i n g on t h e l e a c h a n t and p r o c e s s i n g c o n d i t i o n s .
198
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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16.
STAMBAUGH
Hydrothermal
Coal
Process
199
(3) The c o a l p r o d u c t s l u r r y i s c o o l e d and pumped i n t o a r e c e i v i n g tank. The d e s u l f u r i z e d c o a l i s t h e n s e p a r a t e d f r o m t h e s p e n t l e a c h a n t and washed w i t h w a t e r . The f i n a l p r o d u c t i s a s o l i d f u e l c o n t a i n i n g r e d u c e d c o n c e n t r a t i o n s o f s u l f u r a n d , d e p e n d i n g on t h e leaching c o n d i t i o n s , mineral matter. D r y i n g t h e f u e l t o remove t h e r e s i d u a l m o i s t u r e i s o p t i o n a l . F o r some u s e s , i t may be d e s i r a b l e t o b u r n t h e f u e l wet; i n o t h e r s , r e m o v a l o f a p a r t o r a l l o f t h e r e s i d u a l m o i s t u r e may be desirable. I n a n y e v e n t , t h e c o a l c a n be d r i e d i n c o m m e r c i a l l y available dryers. The s p e n t l e a c h a n t c o n t a i n i n g t h e e x t r a c t e d s u l f u r a s s o d i u m s u l f i d e (Na2S) c a n be r e g e n e r a t e d f o r r e c y c l e i n s e v e r a l ways; one a p p r o a c h i n v o l v e s t r e a t m e n t o f t h e s p e n t l e a c h a n t w i t h c a r b o n d i o x i d e t o remove t h e s u l f u r a s H2S w h i c h i s s u b s e q u e n t l y c o n v e r t e d t o e l e m e n t a l s u l f u r by t h e C l a u s o r S t r e t f o r d p r o c e s s . The r e s u l t i n g l i q u o r c o n t a i n i n g s o d i u m c a r b o n a t e s i s t r e a t e d w i t h l i m e t o c o n v e r t t h e c a r b o n a t e s back t o sodium h y d r o x i d e which i s r e c y c l e d t o t h e d e s u l f u r i z a t i o n segment a f t e r a d j u s t i n g t h e concentration. The c a l c i u m c a r b o n a t e i s t h e r m a l l y decomposed t o l i m e and c a r b o n d i o x i d e f o r r e c y c l e t o t h e l e a c h a n t r e g e n e r a t i o n segment. C a p a b i l i t y o f Process Sulfur Extraction. The p r o c e s s i s c a p a b l e o f c o n v e r t i n g , on a l a b o r a t o r y ( b a t c h ) and m i n i p l a n t (1/4 t o n p e r day) s c a l e , a v a r i e t y o f m a j o r seam c o a l s o f d i f f e r e n t r a n k s and l i g n i t e t o clean s o l i d f u e l s having a t o t a l s u l f u r content equivalent t o o r l e s s t h a n 1.2 l b S0 /10° B t u a s shown i n T a b l e I . T h i s i s a c h i e v e d by e x t r a c t i n g g r e a t e r t h a n 90% o f t h e p y r i t i c s u l f u r f r o m a l a r g e number o f c o a l s and e x t r a c t i n g up t o 50% o f t h e o r g a n i c s u l f u r from c e r t a i n c o a l s . Btu recovery as s o l i d c l e a n f u e l i s i n t h e r a n g e o f 90-95%, d e p e n d i n g on t h e c o a l and p r o c e s s i n g c o n d i t i o n s . The r e m a i n d e r , s o l u b i l i z e d b y t h e l e a c h a n t i s recovered during leachant regeneration. This m a t e r i a l could be u s e d a s p r o c e s s h e a t . T h u s , a w i d e v a r i e t y o f m a j o r seam, h i g h - s u l f u r b i t u m i n o u s c o a l s and s u b b i t u m i n o u s c o a l s i n a d d i t i o n t o l i g n i t e c a n be c o n v e r t e d t o 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 s o l i d fuels. These c o a l s c a n be u s e d d i r e c t l y a s a s o u r c e o f e n e r g y w i t h o u t f u r t h e r c l e a n i n g d u r i n g t h e combustion p r o c e s s , assuming a l l o f t h e s u l f u r i s e m i t t e d t o t h e atmosphere. With a l k a l i hydrothermally t r e a t e d c o a l s , a l l o f the s u l f u r i s n o t e m i t t e d t o the atmosphere. D u r i n g the d e s u l f u r i z a t i o n o p e r a t i o n , t h e c o a l s t r u c t u r e i s opened up t o g i v e a p r o d u c t h a v i n g a s p o n g e - l i k e m o r p h o l o g y ( s e e F i g u r e 2 ) . The p o r o u s s t r u c t u r e a l l o w s t h e a l k a l i t o p e n e t r a t e t h e c o a l p a r t i c l e s and subsequently t o r e a c t w i t h t h e f u n c t i o n a l groups, f o r example, c a r b o x y l i c a c i d groups, o f t h e c o a l molecules. A l s o , the a l k a l i 2
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
9
200
COAL
HEAT
DESULFURIZATION
EXCHANGER
GRINDING MIXING H I G H - SSUULLFFUURR COAL
^
^
j
REGENERATED LEACHANT
TANK
FILTER
S U L F U R REMOVAL AUTOCLAVE
HZ
J]H** DRYER
CLEAN
SPENT LEACHANT
SULFUR
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SULFUR RECOVERY PLANT
Figure 1.
COAL
FOR POWER P L A N T S AND INDUSTRIAL BOILERS
LEACHANT REGENERATION
Schematic of hydrothermal coal process
Figure 2. Comparison of morphology of untreated (top) and HTT coal (bottom)
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
16.
STAMBAUGH
Hydrothermal Coal Process
Table I .
S u l f u r Emissions of Low-Sulfur Coals from Hydrothermal Coal Process
C o a l Source Seam
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201
S0 Raw
o
Equivalent (lb/10 HTT Coal
6
Btu) Coal
Laboratory scale Lower K i t t a n n i n g Upper F r e e p o r t Ohio 6 Pittsburgh 8 . Pittsburgh Lignite Western
2.2 2.4 3.9 4.6 3.4 1.5 1.0
0.9 0.9 1.2 0.9 0.7 1.2 0.3
Prepilot plant Lower K i t t a n n i n g Upper F r e e p o r t
4.0 2.4
1.1 0.8
i s p h y s i c a l l y d e p o s i t e d w i t h i n and on t h e c o a l p a r t i c l e s . Conseq u e n t l y , i n a d d i t i o n to h a v i n g a reduced s u l f u r c o n t e n t , the h y d r o t h e r m a l l y t r e a t e d c o a l i s impregnated w i t h a l k a l i which a c t s as a s u l f u r s c a v e n g e r 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 . I n some r e c e n t c o m b u s t i o n s t u d i e s s u p p o r t e d by EPA ( 1 ) , 41.4-75.7% o f t h e s u l f u r r e m a i n i n g i n t h e t r e a t e d c o a l was c a p t u r e d by t h e a l k a l i i n t h e t r e a t e d c o a l s , as shown i n T a b l e I I . ( T h i s c o m b u s t i o n s t u d y was c o n d u c t e d i n a 1 - l b / h r l a b o r a t o r y c o m b u s t i o n f a c i l i t y . ) S t a c k g a s e s c o n t a i n e d 120-290 ppm SO2 e q u i v a l e n t t o a b o u t 0.3-0.6 l b S 0 / 1 0 6 B t u . T h e o r e t i c a l l y , a s s u m i n g no s u l f u r c a p t u r e , t h e s t a c k g a s e s w o u l d have c o n t a i n e d 490-650 ppm SO2, e q u i v a l e n t t o a b o u t 1.0-1.5 l b S 0 / 1 0 B t u . T h i s added f e a t u r e o f t h e h y d r o t h e r m a l p r o c e s s g r e a t l y i n c r e a s e s t h e number o f c o a l s w h i c h can be u s e d as a s o u r c e o f c l e a n e n e r g y , e s p e c i a l l y t h o s e c o a l s c o n t a i n i n g r e l a t i v e l y h i g h c o n c e n t r a t i o n s of o r g a n i c s u l f u r not s u b j e c t t o r e m o v a l by m e c h a n i c a l c l e a n i n g . 2
6
2
Metal Extraction. Hydrothermal treatment r e s u l t s i n the e x t r a c t i o n o f a number o f t h e t r a c e m e t a l s . Examples of these a r e shown i n T a b l e I I I . T h e r e f o r e , t r a c e m e t a l e m i s s i o n s f r o m t h e c o m b u s t i o n o f h y d r o t h e r m a l l y t r e a t e d c o a l s h o u l d be l o w e r t h a n t h o s e f r o m t h e c o m b u s t i o n o f t h e c o r r e s p o n d i n g raw c o a l s . F u r t h e r m e t a l e x t r a c t i o n i s a c h i e v e d by c h e m i c a l l y d e a s h i n g the h y d r o t h e r m a l l y t r e a t e d c o a l w i t h d i l u t e a c i d , f o r example, sulfuric acid. As an i l l u s t r a t i o n , t h e a s h c o n t e n t o f u n t r e a t e d c o a l s f r o m O h i o , K e n t u c k y , West V i r g i n i a , and P e n n s y l v a n i a r a n g e d f r o m 4.6-13.2%; a s h c o n t e n t o f c h e m i c a l l y d e a s h e d h y d r o t h e r m a l l y t r e a t e d c o a l s r a n g e d f r o m 0.7-5.3%. Improved C o m b u s t i o n B e h a v i o r . C e r t a i n general combustion c h a r a c t e r i s t i c s o f b o t h t h e raw and t r e a t e d c o a l s , s u c h as
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
S0
2
(ppm)
(%)
30.7
41.4
120 75.7
1910 —
1115
290
493
1610
Westland Raw C o a l
1877
495
Martinka Coal Mixed Leachant
Coals
220
250
66.2
651
580
56.9
Westland Coal Mixed Leachant
Westland Coal Caustic
M a r t i n k a c o a l r e p r e s e n t s t h e Lower K i t t a n i n g Seam; W e s t l a n d c o a l r e p r e s e n t s t h e P i t t s b u r g h Seam.
Sulfur capture
SO^ i n s t a c k gas (ppm)
Theoretical
Martinka Coal Caustic
S u l f u r C a p t u r e by A l k a l i i n HTT
Martinka Raw C o a l
Table I I .
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16.
STAMBAUGH
Hydrothermal Coal Process
Table I I I .
203
T o x i c M e t a l s E x t r a c t e d by H y d r o t h e r m a l Treatment o f C o a l
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Concentration Metal
Raw C o a l
Arsenic Beryllium Boron Lead Thorium Vanadium
25 10 75 20 3 40
Average v a l u e s from 3 Ohio
a
(ppm)
Treated
Coai
a
2 3 4 5 0.5 2 coals.
i g n i t i o n t e m p e r a t u r e and r e a c t i v i t y , were d e t e r m i n e d q u a n t i t a t i v e l y f r o m t h e d e r i v a t i v e t h e r m o g r a v i m e t r i c (dTGA) and t h e d i f f e r e n t i a l t h e r m a l (DTA) f u e l a n a l y s e s . The r e s u l t s o f t h e dTGA a n d DTA a r e summarized i n T a b l e s I V and V, r e s p e c t i v e l y . From t h e s e a n a l y s e s , t h e c o m b u s t i o n c h a r a c t e r i s t i c s o f t h e s e c o a l s i n terms o f i g n i t i o n , r e a c t i v i t y , and p o s s i b l y f l a m m a b i l i t y may have b e e n i m p r o v e d by t h e h y d r o t h e r m a l t r e a t m e n t . F o r e x a m p l e , t h e i g n i t i o n t e m p e r a t u r e o f W e s t l a n d c o a l was r e d u c e d f r o m 426°-344°C ( T a b l e I V ) , a r e d u c t i o n o f 82°C, by t r e a t i n g t h e c o a l w i t h s o d i u m h y d r o x i d e and t h e m i x e d l e a c h a n t s y s t e m s . A s i m i l a r e f f e c t was n o t e d by h y d r o t h e r m a l t r e a t m e n t o f t h e M a r t i n k a c o a l w i t h these leachant systems. T h i s was e x p e c t e d i n v i e w o f o t h e r h y d r o t h e r m a l work w h i c h has been c o n d u c t e d a t B a t t e l l e - C o l u m b u s . I n t h i s w o r k , h y d r o t h e r m a l t r e a t m e n t o f c o a l s r e s u l t e d i n a l t e r a t i o n and m o d i f i c a t i o n o f t h e c o a l s t r u c t u r e t o a more s i m p l i f i e d s t r u c t u r e . This i s e v i d e n c e d by t h e f a c t t h a t t h e l i q u i d p r o d u c t s f r o m t h e p y r o l y s i s o f HTT c o a l s c o n t a i n e d l e s s a s p h a l t e n e s t h a n t h e l i q u i d p r o d u c t s f r o m t h e c o r r e s p o n d i n g r a w c o a l s ( 2 ) . The l o w e r m o l e c u l a r w e i g h t o r g a n i c l i q u i d s f r o m t h e HTT c o a l s s h o u l d have a l o w e r i g n i t i o n t e m p e r a t u r e and a h i g h e r d e g r e e o f f l a m m a b i l i t y t h a n t h e h i g h e r m o l e c u l a r w e i g h t l i q u i d s from t h e raw c o a l s . The i n c r e a s e d r e a c t i v i t y i s r e f l e c t e d i n T a b l e V. F o r e x a m p l e , t r e a t m e n t o f t h e M a r t i n k a and W e s t l a n d c o a l s w i t h t h e m i x e d l e a c h a n t s y s t e m r e s u l t e d i n HTT c o a l s w h i c h b u r n e d o u t a t a maximum o f a b o u t 470°C w h e r e a s t h e r a w c o a l s b u r n e d o u t a t a b o u t 585°-600°C. A s i m i l a r e f f e c t , b u t n o t t o t h i s d e g r e e , was observed w i t h t h e sodium h y d r o x i d e - t r e a t e d c o a l s . W h i l e t h e r e may n o t be a d i r e c t c o r r e l a t i o n between c o m b u s t i o n and g a s i f i c a t i o n , h y d r o t h e r m a l t r e a t m e n t o f c o a l w i t h t h e m i x e d l e a c h a n t s y s t e m i n c r e a s e s s t e a m g a s i f i c a t i o n and h y d r o g a s i f i c a t i o n r a t e s by a s much as 40-50 f o l d ( 3 ) . T h i s h a s been a t t r i b u t e d t o a l t e r a t i o n and m o d i f i c a t i o n o f t h e c o a l s t r u c t u r e and t o i m p r e g n a t i o n o f t h e c o a l p a r t i c l e w i t h a c a t a l y s t , i n t h i s case c a l c i u m and/or sodium. T h i s work h a s a l s o
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
range^
622
-
243 432
615
233 426
252 344 488 564 263 360 508 578
Martinka Coal Sodium Treated
275
19.0
250-600
Martinka Raw C o a l
305
21.5
230-570
275
27.5
240-510
Martinka Coal NaOH Leachant
285
23.0
240-465
310
27.0
270-470
Martinka Coal Mixed Leachant
252 376 493 553
Martinka Coal Mixed Leachant
Westland Coal Mixed Leachant
Coals
268 344 494 555
Westland Coal Mixed Leachant
TGA p e r f o r m e d w i t h Cahn E l e c t r o b a l a n c e a t 15°C/min and a i r f l o w o f 800 m l / m i n . T e m p e r a t u r e r a n g e o v e r w h i c h most o f t h e sample i s l o s t .
320
17.5
220-585
Westland Raw C o a l
Westland Coal NaOH Leachant
T h e r m o g r a v i m e t i c A n a l y s e s o f Raw and HTT
T e m p e r a t u r e a t maximum r a t e o f w e i g h t l o s s (°C)
Maximum r a t e o f w e i g h t l o s s (mg/min)
Temperature
TABLE V.
S t a r t i n g e x o t h e r m (°C) I g n i t i o n p o i n t (°C) S e c o n d a r y e x o t h e r m (°C) End o f e x o t h e r m (°C)
Martinka Raw C o a l
Westland Coal Sodium Treated
D i f f e r e n t i a l Thermal A n a l y s e s o f C o a l Samples i n an A t m o s p h e r e o f A i r
We s t l a n d Raw C o a l Low A s h
Table IV.
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>
16.
STAMBAUGH
Hydrothermal Coal Process
shown t h a t t h e m i x e d l e a c h a n t - t r e a t e d c o a l i s more r e a c t i v e the sodium h y d r o x i d e - t r e a t e d c o a l .
205 than
Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch016
Summary Hydrothermal p r o c e s s i n g i s a p o t e n t i a l technology f o r c o n v e r t i n g a v a r i e t y o f m a j o r seam, h i g h - s u l f u r c o a l s and l i g n i t e to c l e a n s o l i d f u e l s having a t o t a l s u l f u r content e q u i v a l e n t t o o r l e s s t h a n 1.2 l b SO2/IO" B t u . D u r i n g t h e t r e a t m e n t , t h e c o a l s are impregnated w i t h a l k a l i which a c t s as a s u l f u r scavenger, thereby reducing s u l f u r emissions s t i l l f u r t h e r d u r i n g combustion. The p r o c e s s i s a l s o e f f e c t i v e i n r e m o v i n g t r a c e m e t a l s s u c h a s b e r y l l i u m , b o r o n , v a n a d i u m , and a r s e n i c ; t h e r e f o r e , t h e g a s e o u s e m i s s i o n s f r o m c o m b u s t i o n o f HTT c o a l s s h o u l d be l e s s p o l l u t i n g i n terms o f s u l f u r and t r a c e m e t a l e m i s s i o n s t h a n f r o m r a w c o a l s . Hydrothermal treatment a l s o appears t o improve t h e combustion c h a r a c t e r i s t i c s o f c o a l i n terms o f i g n i t i o n , r e a c t i v i t y , and possibly flammability. Thus, h y d r o t h e r m a l treatment c o n v e r t s a t l e a s t c e r t a i n h i g h sulfur coals to environmentally acceptable s o l i d fuels with p o t e n t i a l l y improved combustion c h a r a c t e r i s t i c s . These c o a l s a r e p o t e n t i a l s o u r c e s o f e n e r g y f o r p u l v e r i z e d , s t o k e r , and c o a l s l u r r y combustion. Acknowledgement R e s u l t s a s r e p o r t e d were s u p p o r t e d i n p a r t by t h e B a t t e l l e E n e r g y P r o g r a m and i n p a r t by t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n A g e n c y , R e s e a r c h - T r i a n g l e P a r k , NC, u n d e r C o n t r a c t No. 68-02-2119.
Literature Cited
1. Stambaugh, E. P., Levy, Α., Giammar, R. D., Sekhar, K. C., "Hydrothermal Coal Desulfurization with Combustion Results," Proceedings of the Fourth National Conference (October 3-7, 1976) 386-394. 2. Stambaugh, E. P., Feldmann, H. F. , Liu, K. T., Sekhar, K. C., "Novel Concept for Improved Pyrolysis Feedstock Production," 173rd National Meeting American Chemical Society, New Orleans, Louisiana (March 21-25, 1977). 3. Stambaugh E. P., Miller, J. F., Tam, S. S., Chauhan, S. P., Feldmann, H. F., Carlton, H. E., Nack, H., Oxley, J. H., "Battelle Hydrothermal Coal Process," 12th Air Pollution and Industrial Hygiene Conference on Air Quality Management in EPI, University of Austin, Austin, Texas (January 1976).
In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.