Improved Hydrodesulfurization of Coal Char by Acid Leach

21 Improved Hydrodesulfurization of Coal Char by Acid Leach ANN B. TIPTON

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch021

Occidental Research Corporation, La Verne, CA 91750

Occidental Research Corporation's (ORC) Flash Pyrolysis coal liquefaction process (1) is designed to handle caking coals in a single-stage reactor without pretreatment. The process produces a maximum liquid yield and char co-product. When high-sulfur, bituminous coals are used as feed for the process, the char, like the coal, is also high in sulfur. The value of this char as a feed for utility boilers would be increased greatly i f its sulfur content were lowered to meet the EPA sulfur emission standard (1.2 lb SO/MM Btu). Hence, concurrent with the development of the Flash Pyrolysis coal liquefaction process, char desulfurization research has been performed. From this research, a two-step acid leach/hydrodesulfurization (AL/HDS) process has been developed (2). Acid leach, the first step of the process, removes p r i marily iron and calcium sulfides from the char. The absence of these minerals improves significantly the hydrodesulfurization step by allowing operation at shorter residence times and at higher H S concentrations. 2

2

The original research f o rthe acid leach/hydrodesulfurization process, reported by R o b i n s o n (2), studied the effects of temperature, pressure, residence times, s u p e r f i c i a l v e l o c i t y , p a r t i c l e s i z e , char preparation method, c o a l c l a s s (bituminous and subbituminous), a c i d t y p e , a n d H2S c o n c e n t r a t i o n i n t h e treatment g a s (H2). I n most o f t h i s work, chars prepared by p y r o l y s i s o f c o a l s i n a s t a t i c bed u n d e r an inert gas b l a n k e t were used. However, Robinson established that acid-leached Flash Pyrolysis chars have faster h y d r o d e s u l f u r i z a t i o n rates than acidleached static bed chars. Therefore, t h e work r e p o r t e d h e r e was p e r f o r m e d t o e s t a b l i s h t h e d a t a base

280

In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

21.

Hydrodesulfurization

TIPTON

of Coal

Char

281

required f o rthe design o f a n AL/HDS p i l o t p l a n t f o r Flash P y r o l y s i s char from h i g h - s u l f u r , bituminous coal.


Experimental T h e c h a r u s e d i n t h i s s t u d y w a s made i n ORC's Flash Pyrolysis coal liquefaction pilot plant by a s i n g l e p a s s o f -200 m e s h W e s t e r n K e n t u c k y N o . 9 c o a l t h r o u g h t h e p y r o l y s i s r e a c t o r a t 1200°F f o r 2.5 s e c . The f o l l o w i n g a d d i t i o n a l p r o c e s s s t e p s were p e r f o r m e d in laboratory batch reactors. The s i n g l e - p a s s Flash P y r o l y s i s c h a r w a s o x i d i z e d a t 1600°F f o r 0.1 s e c a n d t h e n h e l d i n a f l u i d i z e d b e d u n d e r n i t r o g e n a t 1600°F for 1 hr. The o x i d a t i o n represents a char burning step i n the coal l i q u e f a c t i o n process and t h e a d d i tional heating simulates continuous char recycle. The F l a s h P y r o l y s i s c h a r was a c i d - l e a c h e d five times with 6N h y d r o c h l o r i c a c i d a t 8 0 ° C f o r 5 m i n with a 2:1 a c i d - t o - c h a r ratio. Residual a c i d was removed by water r i n s i n g . The h y d r o d e s u l f u r i z a t i o n reactor and chemical and physical property analyses were described i n detail by Robinson (2). The h y d r o d e s u l f u r i z a t i o n r e a c t o r w a s b u i l t a r o u n d a 1.2-m χ 19-mm i . d . q u a r t z c y l i n d e r i n w h i c h 10-g s a m p l e s w e r e t r e a t e d a t 1600°F, 65 p s i a a n d 0.15 f t / s e c s u p e r f i c i a l v e l o c i t y . Before the t r e a t m e n t g a s was i n t r o d u c e d , t h e reactor and s a m p l e w e r e h e a t e d t o t e m p e r a t u r e a n d h e l d f o r 15 m i n w i t h n i t r o g e n a s t h e f l u i d i z a t i o n medium. The system was also purged with nitrogen during cool down. f

Results

and

Discussion

The c o m p o s i t i o n s of the feed coal, the Flash P y r o l y s i s char and t h e acid-leached char a r e given i n Table I . A comparison o f t h e s u l f u r forms i n feed coal and Flash P y r o l y s i s c h a r shows that the total sulfur was a p p r o x i m a t e l y constant and t h a t pyritic sulfur a n d some organic s u l f u r were c o n v e r t e d t o sulfide sulfur. The conversion of the coal pyritic s u l f u r t o s u l f i d e s u l f u r i n the char i s b e n e f i c i a l t o the AL/HDS process because t h e s u l f i d e form w i l l readily dissolve i n theprocess acids. The absence o f iron s u l f i d e s i s necessary f o r t h e improved hydro desulfurization rate.


COAL

282 Table I. Coal and Char Compos i t i o n


Feed Coal

DESULFURIZATION

a

Flash Pyrolysis Char

Acid-Leached Char

Moisture Ash (dry) Volatile matter (dry) Fixed carbon (dry)

3.43 9.28 38.15 52.57

0.66 24.77 5.17 70.06

2.26 17.97 12.34 69.69

Carbon (dry) Hydrogen (dry) Sulfur (dry) Oxygen and nitrogen (dry)

72.35 4.70 2.96 10.71

72.98 0.83 2.33 0

76.03 1.19 1.06 0

Total sulfur (MAF) Sulfide sulfur (MAF) P y r i t i c sulfur (MAF) Sulfate sulfur (MAF) Organic sulfur (MAF)

3.26 0.13 1.44 0.09 1.60

3.09 1.98 0.17 0 0.95

1.30 0.19 0.07 0 1.04

Chloride (dry)

-

0.05

0.33

Iron (dry) Calcium (dry) Silicon (dry) Aluminum (dry) Potassium (dry) Sodium (dry) Magnesium (dry)

1.32 0.31 2.36 0.86 0.17 0.08

4.01 0.98 5.35 2.03 0.43 0.18 0.12

0.36 0.14 5.66 2.03 0.43 0.18 0.08

Heating value (BTU/lb, dry)

12,780

11,280

EPA standard in % sulfur (MAF)

a

11,540

0.90

Values are wt % except for heating value.


0.84


21.

TIPTON


of Coal

283

Char

A comparison of the compositions i n Table I of t h e F l a s h P y r o l y s i s (FP)char and t h e acid-leached (AL) c h a r shows t h a t t h e a c i d l e a c h removed a p p r o x i mately 90% each o f t h e s u l f i d e sulfur, iron, and c a l c i u m a l o n g w i t h 30% o f t h e a s h . More moles o f i r o n and c a l c i u m were removed than moles o f s u l f u r ; thus, p o t e n t i a l s u l f u r scavengers i n t h e mineral matter such as o x i d e s w e r e a l s o r e m o v e d . Limited h y d r o d e s u l f u r i z a t i o n experiments were p e r f o r m e d w i t h FP c h a r t o c o n f i r m t h e i m p r o v e m e n t s i n hydrodesulfurization after acid leach. The d a t a f o r hydrodesulfurization rates o f FP char with pure h y d r o g e n a t 1600°F a n d 65 p s i a a r e s h o w n i n F i g u r e 1 with curves given f o r total sulfur, sulfide sulfur, and organic sulfur. A l lthree curves gradually decrease at similar rates thus showing t h a t sulfide s u l f u r and o r g a n i c s u l f u r a r e removed s i m u l t a n e o u s l y . F o r t h i s F P c h a r , t h e E P A e m i s s i o n s t a n d a r d o f 1.2 l b S0 /MM B t u i s e q u i v a l e n t t o 0 . 9 0 % t o t a l s u l f u r (MAF) w h i c h was n o t a c h i e v e d a f t e r 150 m i n i n p u r e h y d r o g e n . Hence, f o r FP chars residence time g r e a t e r than 150 min i n p u r e h y d r o g e n w o u l d be r e q u i r e d t o meet t h e EPA s t a n d a r d by h y d r o d e s u l f u r i z a t i o n a l o n e . The data f o rh y d r o d e s u l f u r i z a t i o n r a t e s o f FP c h a r w i t h 0 . 5 % H2S i n h y d r o g e n a r e s h o w n i n F i g u r e 2 with curves given f o r total sulfur, sulfide sulfur, and organic sulfur. The t o t a l s u l f u r and s u l f i d e s u l f u r both i n c r e a s e d , and t h e o r g a n i c s u l f u r remained approximately constant. In these experiments the H S/H r a t i o was 5 χ 1 0 " . A t 1600°F t h e a c c e p t e d experimental e q u i l i b r i u m constant (H2S/H2 ratio) i s 3 χ 1 0 ~ (_3) f o r t h e r e a c t i o n o f F e S w i t h h y d r o g e n : 2

3

2

2

3

FeS

+ H

2

^

Fe +

H S 2

Therefore, t h e 0.5% H S i n H w i l l force the equili brium r e a c t i o n t o t h e l e f t and prevent t h e removal o f sulfide sulfur. The i n c r e a s e i n sulfide sulfur confirms the presence of sulfur scavengers i n the mineral matter. The r e a c t i o n mechanisms o f o r g a n i c s u l f u r removal a r e t o o complex t o p r e d i c t t h e l e v e l o f H S that inhibits hydrodesulfurization. I t c a n be seen i n Figure 2 that a t a 5 χ 1 0 " H S/H ratio, the lowest used i n this study, the removal of organic sulfur i s also inhibited i n FP c h a r . After 150-min residence time, i tappears, i naddition, that there i s a conversion of sulfide sulfur t o organic sulfur as has been observed i n the coking of coals (4,5,6). 2

2

2

3

2

2


COAL DESULFURIZATION


284

Figure 1.


of flash pyrolysis char at 1600°F hydrogen

and 65 psia in



of Coal

Char


TIPTON

Ο-TOTAL SULFUR • - SULFIDE SULFUR Δ-ORGANIC SULFUR

_1_

ΙΟ

20

30

40

50

60

70

RESIDENCE Figure 2.

80

90

.Χ

100

110

120

130 140 150

TIME (MINUTES)

Hydrodesulfurization of flash pyrolysis char at 1600°F in 0.5% HjS/BAL H

and 65 psia

2


COAL

286

Since long residence times are usually required for the h y d r o d e s u l f u r i z a t i o n of coals and chars, the possibility of sulfide sulfur conversion to organic sulfur can i m p o s e a d d e d r e s t r i c t i o n s on allowable H S i n the gas i f operative at lower H S/H ratios. The data f o r h y d r o d e s u l f u r i z a t i o n r a t e s of AL c h a r a t 1600°F a n d 65 p s i a w i t h p u r e h y d r o g e n a n d five levels of hydrogen s u l f i d e i n hydrogen are given in Figure 3. Only the t o t a l s u l f u r curves are shown since they are essentially the same a s the organic s u l f u r c u r v e s f o r AL c h a r s . The s u l f u r l e v e l s reached a s t e a d y s t a t e a f t e r 10 m i n i n a l l c u r v e s e x c e p t pure h y d r o g e n w h i c h r e q u i r e d a b o u t 30 m i n . As the h y d r o g e n sulfide c o n c e n t r a t i o n i n the hydrogen i n c r e a s e s , the hydrogen s u l f i d e i n h i b i t i o n of o r g a n i c s u l f u r removal is e x h i b i t e d i n the g r a d u a l l y i n c r e a s i n g s t e a d y - s t a t e l e v e l of s u l f u r i n the char. From the steady-state levels drawn i n these curves, t h e EPA standard of 0.84% total sulfur (MAF) f o r A L c h a r c a n be met with H S c o n c e n t r a t i o n s as h i g h as 4%. In both the 4.0% and 8.5% H S curves, there i s the i n d i c a t i o n that sulfur i n c r e a s e d a t 150 min r e s i d e n c e time as was observed w i t h t h e FP c h a r and 0.5% H S. This again supports the h y p o t h e s i s that the char's r e a c t i v i t y to t h e a d d i t i o n o f s u l f u r i s i n c r e a s e d by l o n g residence times. The i n h i b i t i o n isotherms are shown i n F i g u r e 4 f o r t o t a l s u l f u r , o r g a n i c s u l f u r , and s u l f i d e sulfur in FP char and AL char. The organic sulfur and sulfide sulfur curves f o r FP c h a r readily show t h a t b o t h f o r m s i n c r e a s e d when t h e a c i d - l e a c h a b l e m i n e r a l s and hydrogen sulfide are present. However, the inhibition isotherms f o r AL char show that organic s u l f u r i s r e m o v e d a n d t h a t no s u l f i d e s u l f u r i s f o r m e d through back r e a c t i o n f o r any of the H S/H ratios (0.005 - 0.093) t e s t e d . Hence, removing the acidl e a c h a b l e m i n e r a l s not only e l i m i n a t e s the scavenging o f h y d r o g e n s u l f i d e by t h e m i n e r a l s b u t a l s o makes i t possible to achieve effective organic sulfur removal in the presence of high H S/H ratios. 2


DESULFURIZATION

2

2

2

2

2

2

2

2

2

Conclusions E f f e c t i v e h y d r o d e s u l f u r i z a t i o n of F l a s h Pyrolysis chars requires essentially H S-free hydrogen and g r e a t e r t h a n 150 m i n r e s i d e n c e t i m e a t 1600°F a n d 65 psia. Effective h y d r o d e s u l f u r i z a t i o n of acid-leached chars c a n be a c h i e v e d w i t h h y d r o g e n m i x t u r e s containing up 2



( MAf )

SULm*

TOTAL

%

(MAF)

SULFUW

TOTAL

%

S U L F U * ( MAF )

TOTAL

%


'\z NOXdLL

fo uoiiDzimJinsdpoipfiH poj {3 xm


288

COAL

DESULFURIZATION

Figure 4. Inhibition isotherms for flash pyrolysis char and acid-leached char at 1600° F, 65 psia, and 150 min residence time.OB) flash pyrolysis char, (O) acid-leached char, ( · " , Q ) starting value.


289

21. TIPTON Hydrodesulfurization of Coal Char

t o 4% H2S i n r e s i d e n c e 1600°F a n d 65 p s i a .

times

o f 10 m i n o r l o n g e r

at


Literature Cited

1. Sass, Α., Chem. Eng. Prog. (1974) 70, 72. 2. Robinson, Leon, Fuel (1976) 55, 193. 3. McKinley, Joseph B . , "Catalysis", P.H. Emmett, Ed., Vol. V, p. 453, Reinhold, New York, 1957. 4. Powell, Alfred R., J. Ind. Eng. Chem. (1920) 12, 1069. 5. Eaton, S.E., Hyde, R.W., Old, B.S., Am. Inst. Mine Met. Eng., Iron Steel Div. Metal Tech. (1948) 19, 343. Tech. Bull. 2453. 6. Cernic-Simic, S., Fuel (1962) 41, 141.


Improved Hydrodesulfurization of Coal Char by Acid Leach

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