Direct Determination of Organic Sulfur in Raw Coals

3 Direct Determination of Organic Sulfur in Raw Coals

Downloaded by UNIV OF ARIZONA on June 27, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch003

B. PARIS Battelle, Columbus Laboratories, 505 King Ave., Columbus, OH 43201

The sulfur content among coals varies to some degree in total quantity and in the forms present. Generally, the forms of sulfur in coals are the inorganic (pyrite and sulfate) and the organic (a complex mixture of organo types). The sulfate content is usually present in low amounts except in some instances where weathering has converted some of the pyrite to sulfate. The pyrite form can occur in a rough size range from 0.25 to 200 μm. The organic sulfur in coals is intimately bound to the coal molecule and is difficult to determine directly when other forms are present. The organic sulfur content in most American coals ranges from about 0.5 to 3%. A total sulfur analysis will determine whether present SO2 emission standards will be met upon combustion of the candidate coal. If the total sulfur is too high and stack gas cleaning is not available, some form of desulfurization is required before this energy source can be used. Cleaning processes can remove varying amounts of the inorganic forms, and some organic sulfur may also be extracted, but for all intents, the content of this latter form may be regarded as the lower level to which coal can be physically beneficiated. Therefore, the baseline level of organic sulfur must be known so that economic and time effective inorganic sulfur extractions can be applied which in turn can comply with combustion standards. C u r r e n t ASTM methods p r o v i d e f o r t h e d i r e c t a n a l y s e s o f t h e t o t a l , s u l f a t e , and p y r i t i c s u l f u r s . The a c c e p t e d t e c h n i q u e s f o r d e t e r m i n i n g t h e s u l f u r f o r m s i n c o a l s a r e t h e ASTM CO s t a n d a r d wet methods. The t o t a l s u l f u r i s d e t e r m i n e d b y t h e E s c h k a method (D3177-75) and t h e s u l f a t e and p y r i t e b y s e l e c t i v e a c i d l e a c h e s (D2492-68). The o r g a n i c s u l f u r i s t h e n o b t a i n e d f r o m t h e d i f f e r e n c e o f t h e t o t a l and t h e two i n o r g a n i c forms s i n c e no r e l i a b l e d i r e c t method i s a v a i l a b l e . The p o s s i b l e a c c u m u l a t i o n o f e r r o r s a n d t h e t i m e r e q u i r e d (^1 1/2 d a y s ) t o o b t a i n t h e o r g a n i c s u l f u r b y d i f f e r e n c e h a s made t h i s a p p r o a c h undesirable but necessary.

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


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Organic Sulfur in Coals

The p r e c i s i o n o f t h e o r g a n i c s u l f u r v a l u e o b t a i n a b l e f r o m t h i s a p p r o a c h was c a l c u l a t e d f r o m numerous a n a l y s e s a t B a t t e l l e Columbus L a b o r a t o r i e s (BCL) i n c o n j u n c t i o n w i t h p r e c i s i o n e s t i m a t e s o f t h e r e s p e c t i v e ASTM methods. A t two s t a n d a r d d e v i a t i o n s ( 9 5 % c o n f i d e n c e l e v e l ) t h e p r e c i s i o n was f o u n d t o be ±25%. I n s t r u m e n t a l methods a l s o c a n be u s e d t o d e t e r m i n e t h e i n o r g a n i c f o r m s and t o t a l s u l f u r f o r a n e s t i m a t i o n o f t h e o r g a n i c s u l f u r content. The two methods u s e X - r a y a n a l y s e s o n p r e s s e d pellets of pulverized coal. I n o n e , H u r l e y and W h i t e ( 2 ) , u s e d X-ray f l u o r e s c e n c e t o c h a r a c t e r i z e a l l o f t h e s u l f u r forms by w a v e l e n g t h s h i f t o f t h e SK^ peak c a u s e d b y v a r i a t i o n i n t h e s u l f u r bonding. The s e c o n d method (3) i s d i s c u s s e d i n an u n p u b l i s h e d r e p o r t on s u l f u r d e t e r m i n a t i o n i n c o a l s and u t i l i z e s X - r a y f l u o r e s c e n c e f o r c o a l and s u l f a t e s u l f u r s and X - r a y d i f f r a c t i o n for p y r i t i c sulfide sulfur. O r g a n i c s u l f u r i s o b t a i n e d by t h e d i f f e r e n c e i n the l a t t e r technique. B o t h o f t h e X - r a y methods a g r e e w i t h c o r r e s p o n d i n g wet c h e m i c a l d a t a on t h e same s a m p l e s . The p r e c i s i o n o f t h e s e i n s t r u m e n t a l methods t h e r e f o r e i s a t l e a s t a s good a s t h e ASTM procedure, but the time o f analyses i s reduced t o s e v e r a l hours. A d i r e c t method f o r d e t e r m i n i n g o r g a n i c s u l f u r i n c o a l s was r e p o r t e d by S u t h e r l a n d ( 4 ) u s i n g t h e e l e c t r o n m i c r o p r o b e o n pressed p e l l e t s of coal. A BCL s u p p o r t e d p r o g r a m , t h e r e f o r e , was i n i t i a t e d t o d e v e l o p a d i r e c t and s p e c i f i c method t o d e t e r m i n e t h e o r g a n i c s u l f u r c o n t e n t i n c o a l s and i n t h e p r e s e n c e o f t h e o t h e r s u l f u r forms. The a p p r o a c h t a k e n e n t a i l e d t h e l o w - t e m p e r a t u r e reaction o f i o n i z e d o x y g e n s p e c i e s w i t h raw c o a l s t o y i e l d v o l a t i l e S 0 components w h i c h a r e t h e r e a c t i o n p r o d u c t s o f t h e o r g a n i c s u l f u r content of the coals. The S 0 p r o d u c t s c a n be c o l l e c t e d i n a s u i t a b l e t r a p m a i n t a i n e d a t l o w t e m p e r a t u r e s and s u b s e q u e n t l y analyzed. X

X

Experimental

Conditions

Equipment. F o r t h i s purpose, t h e i n s t r u m e n t used f o r t h e o x i d a t i o n o f c o a l s d u r i n g t h i s p r o g r a m was t h e L F E LTA 600L l o w temperature asher. The g e n e r a t o r o p e r a t e s a t a c r y s t a l c o n t r o l l e d f r e q u e n c y o f 13.56 MHz and h a s a n o u t p u t c a p a b l e o f d e l i v e r i n g 300 W o f c o n t i n u o u s l y v a r i a b l e power d i s t r i b u t e d among f i v e s e p a r a t e r e a c t i o n c h a m b e r s . N o r m a l a s h i n g i s c a r r i e d o u t u n d e r c o n t i n u o u s r o u g h pumping (300 L/min c a p a c i t y ) so a s t o m a i n t a i n a p r e s s u r e o f 1 t o r r a t an oxygen i n p u t f l o w o f about 50 c c / m i n . The l o w t e m p e r a t u r e o x i d a t i o n t e c h n i q u e h a s b e e n used t o study the m i n e r a l matter content o f c o a l s . As i n d i c a t e d p r e v i o u s l y , the technique u t i l i z e s the o x i d a t i v e p r o p e r t i e s o f t h e r e a c t i v e i o n i c and a t o m i c s p e c i e s o f o x y g e n w h i c h a r e p r o d u c e d when m o l e c u l a r o x y g e n i s p a s s e d t h r o u g h a h i g h f r e q u e n c y field. The i o n i z e d o x y g e n s p e c i e s a r e d i r e c t e d t o t h e s u r f a c e


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COAL DESULFURIZATION

o f t h e raw c o a l where s e l e c t i v e o x i d a t i o n o f t h e o r g a n i c c o n t e n t l e a v e s the m i n e r a l matter r e l a t i v e l y u n a l t e r e d f o r study. The r e a c t i o n produces v o l a t i l e o x i d e s of the t o t a l o r g a n i c content. T h e s e p r o d u c t s a r e CO, CO2, H2O, S0 , and N0 , and t h e y a r e a l l o w e d n o r m a l l y t o be d i s c h a r g e d t h r o u g h the vacuum pump. The t e m p e r a t u r e o f the c o a l s u r f a c e r e a c t i o n under t h e s e c o n d i t i o n s c a n v a r y f r o m 50° t o 300° C d e p e n d i n g p r i m a r i l y upon t h e power a p p l i e d t o t h e RF c o i l s . F r a z e r e t a l . (5) and M i t c h e l l e t a l . (6) used low temperature a s h i n g o f c o a l s to s t u d y m i n e r a l s t a b i l i t y under a v a r i e t y o f c o n d i t i o n s . Although better s t a b i l i t y of m i n e r a l s was o b s e r v e d t h a n t h a t shown by t h e a i r o x i d a t i o n method a t 400°C, i t was n o t e d t h a t p u r e p y r i t e c o u l d be o x i d i z e d a t t e m p e r a t u r e s o f 200 t o 300°C. To p r o v i d e f o r t h e c o l l e c t i o n o f t h e d e s i r e d v a p o r s p e c i e s , t h e e x i t o f one o f t h e r e a c t i o n chambers was r e r o u t e d t o accommodate a g l a s s t r a p p i n g s y s t e m . T h i s m o d i f i c a t i o n i s shown s c h e m a t i c a l l y i n F i g u r e 1. The a l l g l a s s a s s e m b l y , w i t h b a l l j o i n t c o n n e c t o r s i n c l u d e s a s t o p c o c k (D), a low t e m p e r a t u r e t r a p (C) t o c o l l e c t t h e o x i d i z e d s p e c i e s , and a c o n n e c t o r a t (E) t o e i t h e r e v a c u a t e d u r i n g a r u n o r t o a t t a c h a gas b u b b l i n g t r a p f o r c o l l e c t i n g t h e SO2 a t t h e end o f a r u n . The r e m a i n i n g chambers were l e f t i n t a c t so as n o t t o i n t e r f e r e w i t h t h e o p e r a t i o n of the instrument. F i g u r e 2 shows t h e s a m p l e h o l d e r u s e d f o r l o a d i n g t h e s a m p l e f o r the o x i d a t i o n run. Sample p l a t e (A) i s p l a c e d i n t h e h o l d e r as shown a f t e r l o a d i n g w i t h c o a l . The s a m p l e h o l d e r c o n t a i n i n g t h e s a m p l e i s i n s e r t e d i n t h e m o d i f i e d sample chamber. This o v e r a l l arrangement p r o v i d e s ease of h a n d l i n g of the sample.


X

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R e a c t i o n and C o l l e c t i o n P r o c e d u r e . A w e i g h e d sample i s l o a d e d i n t o t h e s a m p l e h o l d e r o n t o a g l a s s p l a t e . The s a m p l e h o l d e r i s i n s e r t e d i n t o t h e r e a c t i o n chamber (A) t h r o u g h p o r t (B). The s y s t e m i s c a r e f u l l y e v a c u a t e d t o a b o u t 0.2 t o r r . As t h e o x i d a t i o n o f t h e c o a l s p r o g r e s s , t h e t e m p e r a t u r e (-196°C) and p r e s s u r e (1 t o r r ) c o n d i t i o n s p e r m i t t r a p p i n g o f t h e SO2 and SO3 g a s e s as s o l i d s . The g e n e r a t o r i s t u r n e d o f f a f t e r , upon v i s u a l e x a m i n a t i o n , t h e s a m p l e s a p p e a r t o be c o m p l e t e l y o x i d i z e d . The o x i d a t i o n time can v a r y from 1 to 3 h o u r s . Helium i s slowly admitted t h r o u g h s t o p c o c k (D) u n t i l t h e s y s t e m c a n be opened t o t h e a i r . A g l a s s i m p i n g e r b o t t l e c o n t a i n i n g a b o u t 50 c c o f 3% H2O2 s o l u t i o n i s a t t a c h e d t o ( E ) . W i t h a s l o w p u r g e o f He t h r o u g h t h e t r a p and i m p i n g e r , t h e Dewar i s removed t o p e r m i t t h e t r a n s f e r o f t h e c o n d e n s e d SO2 i n t o t h e p e r o x i d e s c r u b b e r . The t r a p i s then r i n s e d w i t h a p e r o x i d e s o l u t i o n to c o l l e c t the SO3 component o f t h e p r o d u c t . The two s o l u t i o n s a r e c o m b i n e d f o r s u l f a t e a n a l y s e s u s i n g t h e D i o n e x M o d e l 10 i o n c h r o m a t o g r a p h . Samples. The c o a l s w h i c h w e r e s e l e c t e d f o r t h i s p r o g r a m w e r e u s e d as r e c e i v e d e x c e p t t h a t t h e y w e r e g r o u n d t o p a s s


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-200 mesh s i e v e (74 ym). Wet c h e m i c a l s u l f u r a n a l y s e s were o b t a i n e d on t h e s e c o a l s . I n a d d i t i o n to a r e p r o d u c i b i l i t y study on one c o a l , s e v e r a l o t h e r c o a l s o f v a r y i n g s u l f u r and a s h c o n t e n t s w e r e r u n t o e s t a b l i s h t h e s u i t a b i l i t y o f t h i s method t o c o a l s p o s s e s s i n g v a r y i n g c h e m i c a l c o n t e n t s . The c a n d i d a t e c o a l s a r e l i s t e d i n T a b l e I a l o n g w i t h t h e i r p e r t i n e n t makeup. The s u l f u r v a l u e s w e r e d e t e r m i n e d by ASTM methods. The amount o f c o a l s a m p l e s t a k e n f o r t h e s e r u n s was i n t h e 20-30 mg r a n g e . The r e s u l t i n g s u l f u r c o l l e c t i o n s w e r e e x p e c t e d t o be o f s u f f i c i e n t q u a n t i t y t o be d e t e r m i n e d by t h e a n a l y t i c a l method t o be u s e d . S m a l l c h a r g e s w e r e u s e d i n an e f f o r t t o reduce the o x i d a t i o n time to s e v e r a l hours. A f t e r the i n i t i a l s u r f a c e o f c o a l i s removed, t h e p e n e t r a t i o n o f the i o n i z e d oxygen t o the c o a l u n d e r l y i n g the ash i s time dependent. The p o s s i b l e e r r o r s introduced i n the o v e r a l l d e t e r m i n a t i o n of o r g a n i c s u l f u r f r o m m a c r o s c o p i c i n h o m o g e n i e t i e s a c c e n t u a t e d by the s m a l l s a m p l e c h a r g e s c o u l d be e v a l u a t e d i n t h e p r e c i s i o n series.

Table I .

Samples U s e d i n Low

Sample 719-2 ( O h i o ) 719-3 ( O h i o H a z a r d #4 ( K e n t u c k y ) C o l s t r i p #2 ( W e s t e r n ) Beach B o t t o m #1 (W.Va.)

Total

Temperature

Oxidation Studies

S u l f u r (%) Sulfate Pyritic

Organic

Ash(%) 14.2 17.1 12.8 9.0 25.1

1.58 3.6 0.66 0.18 1.35

2.74 0.07 0.04 0.14 0.03

17.7

0

0

17.7

0

24.7

0

0

24.7

0

6.3 5.2 1.52 0.68 1.97

1.9 1.4 0.82 0.36 0.59

O r g a n i c Compounds Dithiopropionic acid 2-Thiophene Carboxylic Acid

The p u r e o r g a n o s u l f u r compounds shown were a l s o r u n s i n c e the r e s u l t s w o u l d r e p r e s e n t i d e a l r e c o v e r y e x p e r i m e n t s i n t h e absence o f i n t e r f e r i n g m i n e r a l forms. The s a m p l e c h a r g e s u s e d f o r t h e s e r u n s w e r e i n t h e 5-10 mg r a n g e . A s t u d y was a l s o made o f t h e s t a b i l i t y o f FeS2 under t h e ashing conditions f o r coal. O x i d a t i o n o f t h i s compound y i e l d s v o l a t i l e SO2 w h i c h w o u l d be i n d i s t i n g u i s h a b l e f r o m t h a t d e r i v e d from the o r g a n i c s u l f u r i n c o a l . I n a d d i t i o n , t h e p u r e FeS2 was a l s o m i x e d w i t h o t h e r components t o a s s e s s t h e i r e f f e c t upon i t s c o n v e r s i o n . T h e s e m a t e r i a l s i n c l u d e d p u r e S i 0 graphite, a s h , and c o a l . 2


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Experimental

Results

O x i d a t i o n S t u d y o f FeS2I t was f i r s t n e c e s s a r y t o e s t a b l i s h c o n d i t i o n s which prevented or reduced c o n s i d e r a b l y the oxidation of p y r i t e to y i e l d S0 . The e x p e r i m e n t a l c o n d i t i o n s w h i c h w e r e v a r i e d f o r t h e FeS2 s t u d y i n c l u d e d a p p l i e d power t o the p l a s m a , o x y g e n f l o w , p o s i t i o n o f s a m p l e s i n r e l a t i o n t o t h e p l a s m a , p a r t i c l e s i z e o f F e S 2 , and m i x t u r e s o f v a r i o u s m a t e r i a l s w i t h p u r e FeS2» F o r the runs w i t h o n l y FeS2, i t appears t h a t the c o n v e r s i o n t o Fe203 and v o l a t i l e SO2 o c c u r s u n d e r any c o n dition. However, t h e s e v e r i t y o f c o n v e r s i o n i n c r e a s e s d i r e c t l y w i t h t h e a p p l i e d power and i n v e r s e l y w i t h p a r t i c l e s i z e . The p o s i t i o n o f t h e sample i n t h e chamber and t h e r a t e o f o x y g e n f l o w u s e d h a v e m i n i m a l , i f a n y , e f f e c t upon t h e o x i d a t i o n o f FeS . The r u n s made u s i n g m i x t u r e s o f FeS2 w i t h t h e l i s t e d a d d i t i v e s produced v a r i e d r e s u l t s . I t was f o u n d t h a t a d d i n g g r a p h i t e , S1O2, o r an o r g a n i c s u l f u r compound t o p u r e FeS2 d i d n o t p r e v e n t t h e FeS2 r e a c t i o n w i t h o x y g e n . However, a d m i x t u r e s w i t h c o a l o r f l y a s h a p p e a r t o p r e v e n t FeS2 o x i d a t i o n . The r e a s o n f o r t h e a p p a r e n t i n h i b i t i o n o f t h e r e a c t i o n i s n o t known b u t seems t o be a s s o c i a t e d w i t h c h a r a c t e r i s t i c s o f t h e a s h o t h e r t h a n S1O2. The p r e s e n c e o f t h e n a t u r a l a s h i n c o a l s a c c e l e r a t e s the r e a c t i o n o f the o r g a n i c phase w i t h the plasma. I n s a m p l e s c o n t a i n i n g no a s h , s u c h a s g r a p h i t e o r an o r g a n o s u l f u r compound, t h e r e a c t i o n w i t h i o n i z e d o x y g e n i s s l o w e r .


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Coal Studies. C o a l s w e r e o x i d i z e d u s i n g t h e optimum i n s t r u m e n t a l c o n d i t i o n s a s d e t e r m i n e d t o be s u i t a b l e by t h e pyrite studies. T h e s e w e r e f o u n d t o be (a) a p p l i e d power o f 150 W o v e r a l l (20 W/chamber), (b) o x y g e n f l o w a t 30 c c / m i n * and ( c ) a p r e s s u r e o f 1 t o r r . A sample c h a r g e o f a b o u t 25 mg was u s e d . The c o l l e c t e d s u l f a t e s w e r e d e t e r m i n e d on a D i o n e x M o d e l 10 i o n c h r o m a t o g r a p h . B o t h c h e m i c a l and s y s t e m b l a n k s w e r e d e t e r m i n e d t h r o u g h o u t the sample r u n s . The c h e m i c a l b l a n k s w e r e o b t a i n e d on e q u i v a l e n t volumes o f p e r o x i d e - w a t e r s o l u t i o n s as used i n the S 0 r e coveries. The s y s t e m b l a n k s w e r e d e t e r m i n e d f r o m a c t u a l o x i d a t i o n - t r a n s f e r techniques w i t h o u t samples. Both blank l e v e l s w e r e f o u n d t o v a r y f r o m 15 t o 25 y g o f s u l f u r . A t t e m p t s were made t o measure t h e t e m p e r a t u r e o f c o a l s u r f a c e s u n d e r o x i d i z i n g c o n d i t i o n s a t 3 d i f f e r e n t RF power s e t t i n g s u s i n g a r a d i a t i o n thermometer. The s u r f a c e t e m p e r a t u r e s c o r r e s p o n d i n g t o a p p l i e d powers o f 20 W, 30 W, and 40 W w e r e 130°C, 210°C, and 230°C, r e s p e c t i v e l y . T h e r e may be some doubt a b o u t the a b s o l u t e a c c u r a c y o f t h e s e v a l u e s , b u t t h e r e l a t i v e d i f f e r e n c e s s h o u l d be s a t i s f a c t o r y . U s i n g t h e above c o n d i t i o n s f o r c o a l 719-2, a s e r i e s o f 10 r u n s s u p p l i e d a measure o f t h e r e p e a t a b i l i t y o f t h i s t e c h n i q u e f o r t h e o r g a n i c s u l f u r r e c o v e r y i n one c o a l . These r e c o v e r y X


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d a t a and p r e c i s i o n a r e shown i n T a b l e I I . The r a t i o o f t h e obtained/expected values allows f o r the c a l c u l a t i o n of the r e l a t i v e s t a n d a r d d e v i a t i o n o f 19% a t t h e 95% c o n f i d e n c e l e v e l . This i s s l i g h t l y b e t t e r t h a n t h a t a c h i e v a b l e b y t h e i n d i r e c t ASTM method. The r e p e a t a b i l i t y d a t a shown i n T a b l e I I i n c l u d e a w i d e range o f v a r i a b l e s . These i n c l u d e : p o s s i b l e inhomogeneity of s m a l l c o a l c h a r g e s , a b i l i t y t o d u p l i c a t e r e a c t i o n and c o l l e c t i o n t e c h n i q u e s , a n a l y s e s o f s u l f a t e s o l u t i o n s , and r u n s e x t e n d e d over a p e r i o d o f s e v e r a l weeks.


Table

II.

Net O r g a n i c Expected 440 405 420 420 380 430 440 415 430

R e p e a t a b i l i t y Runs o f t h e A n a l y s e s o f O r g a n i c S u l f u r From O x i d a t i o n o f 719-2 C o a l S u l f u r (yg) Obtained

Ratio

450 410 475 360 360 450 440 380 350

Obtained/Expected 1.023 1.012 1.131 0.861 0.949 1.047 1.00 0.916 0.814

Mean % SD ( 9 5 % c o n f i d e n c e l e v e l )

0.971 19.0

I t i s noteworthy to p o i n t out t h e low r e c o v e r y runs which a r e r e p r e s e n t e d i n T a b l e I I by t h e r a t i o v a l u e s 0.861 and 0.814. Although the inhomogeniety o f the s u l f u r d i s t r i b u t i o n i n the c o a l could e x p l a i n the o v e r a l l v a r i a t i o n of the r e s u l t s , another p o s s i b i l i t y f o r t h e l o w r e c o v e r i e s c o u l d be t h e l o s s o f SO2 during transfer. More s p e c i f i c a l l y , t h e l o s s c o u l d have o c c u r r e d d u r i n g the h e l i u m purge i n t o the evacuated system. An i n i t i a l h i g h f l o w o f h e l i u m c o u l d warm t h e d e p o s i t e d S 0 s u f f i c i e n t l y t o cause a s l i g h t v a p o r i z a t i o n l o s s to the system. At a temperature o f -196°C and a p r e s s u r e o f 1 t o r r , t h e c a l c u l a t e d l o s s o f SO2 t h r o u g h v a p o r i z a t i o n c o u l d be 6 n g / h r . However, w i t h t h e same p r e s s u r e c o n d i t i o n s b u t a t t h e h i g h e r t e m p e r a t u r e o f -160°C, t h e v a p o r i z a t i o n l o s s c o u l d be 30 mg/hr. P a s t e x p e r i e n c e h a s shown t h a t SO2 c a n n o t be c o n d e n s e d q u a n t i t a t i v e l y a t t h i s p r e s s u r e and the h i g h e r temperature. Therefore, i t i s p o s s i b l e that f o r the two l o w r e c o v e r y v a l u e s , t h e t e c h n i q u e s o f sample t r a n s f e r w e r e inadequate. 2

6


3.

PARIS

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The s e c o n d s t u d y i n v o l v i n g c o a l 719-2 was t o o x i d i z e f o r S 0 r e c o v e r y a f t e r s p i k i n g w i t h q u a n t i t i e s o f FeS2 h a v i n g a p a r t i c l e s i z e r a n g e o f

Direct Determination of Organic Sulfur in Raw Coals

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