Coal and Coal Products Analytical Characterization Techniques

8 Determination of Chlorine in Organic Combination in the Coal Substance

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J. N. CHAKRABARTI I. S. M., Fuel and Mineral Engineering, Department of Chemistry, Dhanbad 826004, India

Opinions vary regarding the node of combination of chlorine in coals. Early investigators reported that only part of the chlorine could be removed by extraction with water and suggested that some of i t is chemically combined with the coal substance, either as organic chlorine or as adsorbed ions. Later workers, notably Crossley (1952), concluded that virtually the whole of the chlorine is present as alkali chlorides, possibly with small quantities of calcium and magnesium chlorides as well. Edgcombe (1956) found that the alkali in the extracts was equivalent to only part of the total chlorine and concluded that only half of the chlorine could be present as alkali chlorides. He held the view that the volatile chlorine represented that part of the total chlorine which was not present as alkali chlorides and that i t existed in the coal as chloride ions held on the coal substance by an ion-exchange linkage. The nature of chlorine in coal is of major interest in its industrial applications. For example, combustion of high-chlorine coal in boilers results in serious fouling by the formation of alkali-bonded deposits. A correct measure of the chlorine as inorganic chlorides is required in the case of coals that are to be carbonized, since at the temperature of carbonization these salts may react with the refractory walls, forming a glaze. This glazed coating reduces the effective l i f e of a coke oven or of a gas retort. This is precisely the reason why the presence of an appreciable quantity of chlorine in a sample of coal is ordinarily considered detrimental to the brickwork of coke ovens and gas retorts. However, high chlorine contents in coals may not always be prejudicial to the stability of the refractories, as an appreciable percentage of i t may be in organic conbination. In the latter case, there may be either volatilization or decomposition or both. Halogenated organic compounds have hardly any effect on refractories. If there is any liberation of hydrogen chloride gas

O097-6156/82/0205-O185$O6.0O/O © 1982 American Chemical Society

In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


186

COAL AND

COAL

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or simple c h l o r i n e gas due to decomposition, the r e f r a c t o r i e s may be simply bleached. I t i s thus evident that a c o r r e c t e s t i m a t i o n of c h l o r i n e i n organic combination i s of great s i g n i f i c a n c e . In h i s i n v e s t i g a t i o n s on Indian c o a l s , the author proved f o r the f i r s t time the e x i s t e n c e of c h l o r i n e i n organic combination i n the coal substance. I t was found that i n a l l the ten coals examined, the p r o p o r t i o n of c h l o r i n e i n organic combination was a p p r e c i a b l e and i n some cases i t s p r o p o r t i o n rose to 30-36 percent of the t o t a l c h l o r i n e . I t has been suggested that on an average 50% of the t o t a l c h l o r i n e present i n coals of East Midlands C o a l f i e l d i n the U n i t e d Kingdom (which may have t o t a l c h l o r i n e contents as high as 1%) must be assumed to be a s s o c i a t e d w i t h organic matter, probably as h y d r o c h l o r i d e s of p y r i d i n e bases. (Given and Yarzab, 1978.) Work on c h l o r i n e contents of coals from I l l i n o i s Basin has shown that a l l the c h l o r i n e present could not be accounted f o r i n minerals present as h a l i t e (NaCl) or s y l v i t e (KC1) and t h e r e f o r e a s u b s t a n t i a l p r o p o r t i o n i s a s s o c i a t e d w i t h the organic matter. O u t l i n e of the Method The c h l o r i n e i n organic combination i n c o a l i s allowed to combine w i t h hydrogen i n the presence of n i c k e l c a t a l y s t i n a s i l i c a combustion tube. The hydrogen c h l o r i d e gas so formed i s combined w i t h ammonia c i r c u l a t i n g through the r e a c t i o n tube, forming ammonium c h l o r i d e as a sublimate, which i s d i s s o l v e d out by washing w i t h water. Any unreacted hydrogen c h l o r i d e i s a r r e s t e d by hot barium carbonate kept at the e x i t end. The c h l o r i d e formed i s then estimated v o l u m e t r i c a l l y . Y& + CI"

NH 2 HC1 + BaC0

3

•

+ HC1

3

+

HC1 •

NH^Cl

B a C l + H 0 + C0 2

(i)

2

(ii) 2

(ill)

Apparatus and Reagents A.

Apparatus. (a) A s i l i c a tube 55-60 cm long and 15 cm i n diameter (b) Two s i l i c a or p o r c e l a i n boats, one f o r c o a l and the other f o r h o l d i n g barium carobnate (c) Three Meker burners (d) Kipp's apparatus f o r generating hydrogen (e) C o n i c a l f l a s k provided w i t h d e l i v e r y tube and c o n t a i n i n g aqueous ammonia ( f ) Vacuum d e s i c c a t o r


8.


B.

CHAKRABARTi

Chlorine in Organic Combination in Coal

187

Reagents (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (1)

Granualted Zinc Dilute s u l f u r i c acid S o l u t i o n of lead acetate S o l u t i o n of potassium hydroxide Aqueous ammonia Barium carbonate (pure) Acetic acid Concentrated n i t r i c a c i d S i l v e r n i t r a t e s o l u t i o n , 0.05 Ν Nitrobenzene Standard potassium thiocyanate s o l u t i o n , 0.05 Ν F e r r i c alum i n d i c a t o r , saturated s o l u t i o n i n water, suf f i c i e n t n i t r i c a c i d being added t o remove the brown c o l o r , i f any (m) 2 , 4 , 6 - T r i c h l o r o p h e n o l , A.R. (n) Sodium c h l o r i d e , A.R. (o) Sugar c h a r c o a l , s p e c i a l l y prepared.

Procedure and S t a n d a r d i z a t i o n A·

Procedure The s i l i c a tube i s mounted h o r i z o n t a l l y by two clamps, the v e r t i c a l d i s t a n c e being adjusted so as to u t i l i z e f u l l y the h e a t i n g e f f e c t of three Meker burners. One end of the combustion tube i s provided w i t h a cork and d e l i v e r y tube and connected to a constant supply of pure hydrogen (e.g. Kipp's apparatus)· The other end of the combustion tube i s kept open v i a a c o n s t r i c t e d tube (see F i g . 1 ) .

B. S t a n d a r d i z a t i o n F i r s t , an a r t i f i c i a l mixture of pure sodium c h l o r i d e , sugar c h a r c o a l and 2 , 4 , 6 - t r i c h l o r o p h e n o l i s put i n the boat. T h i s i s introduced i n t o the tube near the end connected t o the source of hydrogen and i s placed at a d i s t a n c e of 8 cm (see F i g . 1 ) . A n i c k e l f o i l r o l l e d i n the form of a s p i r a l i s placed a t a d i s t a n c e of 20 cm from the same end. Another boat c o n t a i n i n g pure barium carbonate i s placed j u s t near the open end of the tube. At the o u t s e t , a steady stream of pure hydrogen, s a t u r a t e d w i t h ammonia, i s maintained through the tube. For t h i s purpose, the gas i s bubbled s u c c e s s i v e l y through water, s o l u t i o n s of lead a c e t a t e , potassium hydroxide, and aqueous ammonia (not shown i n F i g . 1). The burner imme d i a t e l y below the n i c k e l f o i l i s s t a r t e d f i r s t . When the e x t e r i o r s u r f a c e of the tube i s red hot (temperature 700-800°C) h e a t i n g of the boat c o n t a i n i n g barium carbonate i s s t a r t e d . The temperature of the boat c o n t a i n i n g the mixture



188

COAL

AND

COAL

.1 S .ο s

! s:

I •5

II

•2.

i 'S

s s .00


PRODUCTS


8.

CHAKRABARTI

189

Chlorine in Organic Combination in Coal

of 2 , 4 , 6 - t r i c h l o r o p h e n o l , sodium c h l o r i d e and sugar charcoal i s r a i s e d g r a d u a l l y and the f u l l temperature e f f e c t i s allowed o n l y a f t e r 15 minutes. Hydrogen combines w i t h c h l o r i n e l i n k e d t o o r g a n i c compounds i n the presence of n i c k e l . This then combines w i t h ammonia, forming ammonium c h l o r i d e , and i s deposited on the i n n e r w a l l s of the tube between the n i c k e l c a t a l y s t and the open end. The l e a s t traces of unreacted hydrogen c h l o r i d e are r e t a i n e d by hot barium carbonate. The r e a c t i o n r e q u i r e s a period of 45 minutes. A f t e r the r e a c t i o n i s over, the tube i s allowed t o c o o l . This i s then washed w i t h water to d i s s o l v e the deposited ammonium c h l o r i d e , a f t e r withdrawing the boats and the s p i r a l . Barium c a r bonate i s a l s o washed i n t o the s o l u t i o n c o n t a i n i n g ammonium c h l o r i d e . This i s then rendered a c i d i c by a c e t i c a c i d , and b o i l e d t o expel hydrocyanic a c i d , i f any. C h l o r i n e i s estimated v o l u m e t r i c a l l y by Volhard's method. T h i s method has been found to be very s u i t a b l e f o r o b t a i n i n g a true measure of the c h l o r i n e l i n k e d to organic compounds. The i n o r g a n i c c h l o r i d e s , however, are not a f f e c t e d by t h i s treatment. T h i s i s v e r i f i e d a t the very outset by t a k i n g a mixture of pure sodium c h l o r i d e , sugar charcoal and 2 , 4 , 6 - t r i c h l o r o p h e n o l i n the boat and e s t i m a t i n g the c h l o r i n e l i b e r a t e d . As the organic compound, i . e . 2 , 4 , 5 - t r i c h l o r o p h e n o l , i s l i k e l y to c o n t a i n some moisture, i t i s d r i e d i n a vacuum d e s i c c a t o r p r i o r to being weighed. The r e s u l t s (Table I ) show that the organic c h l o r i n e alone i s l i b e r a t e d i n t h i s way. The marginal d i f f e r e n c e s between the observed and c a l c u l a t e d values of c h l o r i n e are probably due to the f a c t that the substance was not 100% pure. However, the d i f ferences are w i t h i n the l i m i t s of experimental e r r o r . Having thus standardized the method f o r e s t i m a t i n g organic c h l o r i n e , c o a l samples crushed t o pass through a 240 mes BS, or 6 mesh (64 ym) 15 s i e v e , are put i n the boat and t h e i r organic c h l o r i n e contents determined as described f o r the s t a n d a r d i z a t i o n . For accurate determinations of c h l o r i n e , i t i s a d v i s a b l e t o c a r r y out a blank determination using sugar charcoal i n s t e a d of c o a l and t o determine the c h l o r i n e c o l l e c t e d , i f any, as ammonium c h l o r i d e and a l s o i n the barium carbonate. TABLE I S t a n d a r d i z a t i o n w i t h a Pure Organic Substance Experiment No. 1 2 3 4

Substance 2,4,6-trichlorophenol Do Do (0.144g) + NaCl(0.7917g) Do(0.1509g)+NaCl(1.023g)

Chlorine % Calculated Found 53.49 53.56 54.10 54.09


53.93 53.93 53.93 53.93

COAL AND COAL PRODUCTS

190

The r e s u l t s of experiments w i t h ten coals from d i f f e r e n t l o c a l i t i e s are shown i n Table I I . For a comparative study, the t o t a l c h l o r i n e content of each c o a l sample was a l s o determined by the Eechka method.

TABLE I I


C h l o r i n e Contents of Indian Coals

imp l e No.

Organic C h l o r i n e on d.a.f. Coal (%)

1 2 3 4 5 6 7 8 9 10

0.097 0.124 0.127 0.043 0.032 0.124 0.045 0.082 0.068 0.127

T o t a l Chlorine on d.a.f. Coal (%)

0.395 0.495 0.418 0.426 0.322 0.538 0.551 0.340 0.537 0.350

R a t i o of Total/Organic Chlorine

Organic Chlorine as % of Total Chlorine

4.07:1 4.0:1 3.29:1 9.9:1 10.06:1 4.33:1 12.2:1 4.1:1 7.9:1 2.75:1

24.50 25.05 30.38 10.09 9.93 23.04 8.16 24.10 12.66 36.20

Literature Cited 1. Given, P.H.; Harzab, R.F. In "Methods for Coal and Coal Products", Anal. Vol. II, 1978, 19-20. RECEIVED

July 14, 1982


Coal and Coal Products Analytical Characterization Techniques

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