Hydrothermal Coal Process - American Chemical Society

16 Hydrothermal Coal Process E D G E L P. S T A M B A U G H

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch016

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.


16.

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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


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


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)


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


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



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 .


16.

STAMBAUGH


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


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



range^

622

-

243 432

615

233 426

252 344 488 564 263 360 508 578

Martinka Coal Sodium Treated

275

19.0

250-600


305

21.5

230-570

275

27.5

240-510

Martinka Coal NaOH Leachant

285

23.0

240-465

310

27.0

270-470


252 376 493 553



Coals

268 344 494 555


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)


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.


>

16.

STAMBAUGH


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


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).


Hydrothermal Coal Process - American Chemical Society

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