PLASMA PYROLYSIS OF BROWN COAL

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PLASMA PYROLYSIS OF BROWN COAL

W. Plotczyk, A. Resztak, A. Szymanski

To cite this version:

W. Plotczyk, A. Resztak, A. Szymanski. PLASMA PYROLYSIS OF BROWN COAL. Journal de

Physique Colloques, 1990, 51 (C5), pp.C5-43-C5-48. �10.1051/jphyscol:1990506�. �jpa-00230804�

COLLOQUE DE PHYSIQUE

Colloque C5, supplement au no18, Tome 51, 15 septembre 1990

PLASMA PYROLYSIS OF BROWN COAL

W.W. PLOTCZYK, A. RESZTAK

and

A. SZYMANSKI(')

Laboratory of Plasma Chemistry, Department of Chemistry, University of Warsaw,

02-093

Warsaw, Pasteur

1,

Poland

Abstract

-

The s p e c i f i c energy o f t h e s u b s t r a t e i s d e f i n e d as t h e r a t i o o f t h e plasma j e t energy t o t h e mass of t h e c o a l . The i n f l u e n c e o f t h e epeci-fic e n e r g y o+ t h e b r o w n coal ( 1 0

-

^{35 NJ/kg)}

on t h e y i e l d and s e l e c t i v i t y of t h e g a s w u e prcrducts f o r m a t i o n was determined. The p y r o l y s i s was performed i n d.c. a r c hydrogen plasma j e t w i t h t h e 25 k W power d e l i v e r e d t o i t . The h i g h e r s p e c i f i c energies o f c o a l c o r r e l a t e d t o t h e h i g h e r conversion degrees o f t h e s u b s t r a t e s t o CZHZ and CO as w e l l a s t o t h e h i g h e r s e l e c t i v i t y o f CO formation.

It

has been found t h e o v e r a l l conversion degree o f t h e c o a l t o t h e sum o f gaseous p r o d u c t s was up t o 62 w t . %.

The brown c o a l plasma p y r o l y s i s appears as an i n t e r e s t k n g methad o f unsatrtra- t e d hydrocarbons and CO p r o d u c t i o n . The p r o d u c t s a r e obtained w i t h a h i g h e f f i c i e n c y i n a s i m p l e one s t a g e process. The process i s o n l y a l i t t l e s e n s i - t i v e t o t h e chemical composition o f t h e s u b s t r a t e . The i n f l u e n c e o f process parameters on t h e e f f e c t i v e n e s s o f plasma decomposition o f c o a l s has been examined i n argon / l - S / , hydrogen /S-10/ and n i t r o g e n /11/ plasma j e t s . The process e f f i c i e n c y o f t h e c o a l p y r o l y s i s i n t h e plasma j e t , generated b y a r c discharge, depends on s e v e r a l f a c t o r s /1,2,12,13/. A couple o f them have t o be mentioned:

1. A k i n d o f gas used f o r t h e s t a b i l i z a t i o n o f t h e a r c discharge.

2. The r e a c t o r c o n s t r u c t i o n .

3. A s p e c i f i c energy o f t h e s u b s t r a t e , which means an average energy o f a plasma j e t f a l l i n g on a kg o f t h e s u b s t r a t e .

For a p a r t i c u l a r k i n d of t h e gas f o r m i n g a plasma j e t and a c h w a e n r e a c t o r c o n s t r u c t i o n , i s t h e s p e c i f i c ' e n e r g y o f t h e s u b s t r a t e t h e most s i g n i f i c a n t

( 1 )

Supported by t h e CPBP-0.16 Problems..

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990506

C5-44

COLLOQUE DE PHYSIQUE

parameter i n f l u e n c i n g on t h e d i f - e c t i o n and y i e l d o f t h e r e a c t i o n .

The aim o f t h e p r e s e n t work -was t o determine t h e i n f l u e n c e o f t h e s p e c i f i c energy 04 t h e brown c o a l on t h e y i e l d and s e l e c t i v i t y o f t h e gaseous p r o - d u c t s formation.

2

-

EXPERIMENTAL

The process was c a r r i e d o u t i n a hydrogen plasma j e t generated by d.c. magne- t i c a l l y s t a b i l i z e d a r c discharge /14/ i n t h e plasma r e a c t o r system presented i n Fig. 1. The plasma t o r c h was connected t o a copper made c y l i n d r i c a l r e - a c t i o n chamber. The c o a l was i n t r o d u c e d through a h o l e p e r p e n d i c u l a r t o t h e d i r e c t i o n o f t h e plasma j e t f l o w . The p r o d u c t s were cooled i n a double p i p e c o o l e r o f t h e heat exchanger. The a r c discharge power was set-up a t 25 kW.

The brown c o a l was grinded, then d r i e d i n t h e vacuum a t 378 K and sieved.

A c o a l powder ( t h e f r a c t i o n 0.1

-

0.2 mm) was i n t r o d u c e d i n t o t h e plasma j e t u s i n g a v i b r a t i o n feeder. The s o l i d p r o d u c t s o f t h e r e a c t i o n were separated i n t h e c y c l o n e and sack f l l t e r s .

The elementary a n a l y s i s showed t h e f 01 l o w i ng average s u b s t r a t e composition^

C

-

^{59.7; H}

-

^{5 . 8 ; N}

-

^{0.6; S}

-

1.3; ash 10.2 and 0

-

22.4 w t . % (by d i f f e r - ence).

The amount o f v o l a t i l e matter, b o t h i n a raw m a t e r i a l and t h e s o l i d p r o d u c t s ( c a r b o n i z a t e e ) were determined by measuring t h e mass l o s s a f t e r h e a t i n g t h e sample i n an argon atmosphere i n a speci-a1 q u a r t z f u r n a c e a t 1200 K. I t made p o s s i b l e t o f i n d a mass decrease o f t h e c a l c i n a t e d sample as w e l l as a compo- s i t i o n o f t h e gaseous products. The averaged r e s u l t s o f t h e s u b s t r a t e a n a l y s i s were as f o l l o w s : mass l o s s (corresponded t o v o l a t i l e m a t t e r )

-

^{50.2 w t . %; a}

composition o f gaseous mixture: A r

-

^{39.93 CH4}

-

9.93 CO

-

^{6.8; CaX4}

-

0.18; CzHb

-

^{0.37; Ha}

-

17.72 and Na

-

^1.75 v o l . %.

The c o a l s o l u b i l i t y i n CC14 has been measured i n a Soxhlet apparatus and was found t o be equal t o 3.8 w t . %. The s o l u b i l i t y i n p y r i d i n e as equal t o 9 w t . X The composition o f t h e gaseous prodrtcts o f t h e p y r o l y s i q ^-5 determined by t h e gas chromatography method.

The a n a l y s i s o+ t h e instantaneous gas samples and simultaneous d e t e r m i n a t i o n of t h e average values o f t h e c o a l feed r a t e made i t p o s s i b l e t o d e f i n e t h e elementary c o a l conversion degrees i n t o t h e i n d i v i d u a l p r o d u c t s (U, i as w e l l as i n t o t h e o v e r a l l gaseous p r o d u c t s (U,g)r

-

t h e converei on degree i n t o i n d i v i d u a l product:

where: m* -+mass o f elementary carbon i n t h e i - t h product,

m,

-

mass o f elementary carbon i n t r o d u c e d i n t o t h e r e a c t o r , A,

-

c o n t e n t s of elementary carbon i n t h e s u b s t r a t e ;

-

mass o f t h e elementary carbon i n t h e i - t h p r o d u c t was determined from t h e f ormctl a

t

where: Z A

-

carbon c o n t e n t s i n t h e i - t h product ( w t . %),

-

instantaneous mass stream o f t h e i - t h product, t

-

d u r a t i o n t i m e o f t h e experiment.

-

c o a l t o o v e r a l l gaseous products

The f l u c t u a t i o n s o f t h e c o a l feed r a t e and t h e a r c power have caused t h a t t h e s p e c i f i c energy o f t h e brown c o a l c o u l d be e s t i m a t e d as a parameter averaged i n time. I t was c a l c u l a t e d from t h e formula:

C5-45

where: M

-

d.c. discharge power, t7

-

thermal e f f i c i e n c y o f plasma t o r c h .

W e l d i n g H . f . I n i t i a t o r Mach1 ne

Cathode

Anode

' ~ e a c i i o n c h a m b e r

A n a l y s ~ s A A A

C

- - - -

a

External')

¹

cool e r -

^I I

/ - - - l

to 6 r

/

I n t e r n a l S a c k f i l t e r c

OOI

er Cy c l o n e

F i g . i

-

Schematic diagram o f plasma r e a c t o r .

Taking i n t o c o n s i d e r a t i o n t h e e s t a b l i s h e d c o a l conversion degrees, i t was p o s s i b l e t o e s t i m a t e t h e s e l e c t i v i t y of t h e p y r a l y s i s pracess i n r e s p e c t t o t h e i - t h p r o d u c t . T h i s s e l e c t i v i t y i s d e f i n e s as r e l a t i v e degree o f t h e pre- ferenced d i r e c t i o n o f t h e p y r o l y s i s process l e a d i n g t o t h e i - t h produch f o r - mation, independent +rom t h e t o t a l r e a c t i o n t o t h e carbon. The s e l e c t i v i t y o f t h e process i n r e s p e c t t o t h e i - t h p r o d u c t i s expressed by t h e f o l l o w i n g equa- t i on :

A s o l i d r e a c t i o n product

-

c a r b o n i z a t e , c o n s i s t i n g o f o r g a n i c and i n o r g a n i c compounds was o b t a i n e d i n a d d i t i o n t o gaseous p r o d u c t s as a r e s u l t o f brawn c o a l p y r o l y s i s . The choosen p r o p e r t i e s o f t h e s u b s t r a t e s and c a r b n n i z a t e s were determined by i n f r a r e d s p e c t r o f o t o m e t r i c a n a l y s i s .

The i n f r a r e d s p e c t r a w e r e recorded on a Car1 Zeiss Jena UR-20 sperrtrometer i n t h e L a b o r a t o r y of Molecular Spectrascopy, W n i v e r s i t y of Warsaw. The KBr p e l l e t te?c;hnique o f d i s i n t e g r a t e d samples was used.

3

-

^{RESULTS AND} DXSCUSSlDN

The i n f luence o f s p e c i f i c energy o f the brown c o a l (l0

-

35 HJ/kg> on t h e y i e l d and s e l e c t i v i t y o f gaseaus p r o d u c t s f o r m a t i o n was i n v e s t i g a t e d . I n

COLLOQUE DE PHYSIQUE

t h e experiment, conducted a t a constant a r c power (25 kW), t h e f l o w mass r a t e o f t h e c o a l was v a r i e d i n t h e range 1.08

-

^2.2 kg/h. The r e s u l t s o f t h e ex- periments a r e presented i n F i g . 2. I t was found t h a t t h e main p r o d u c t s o f t h e p y r o l y s i s werer C&=, CzH4, CH4 and CO. The c o a l c o n v e ~ s i o n degree

t o t h e l i q u i d p r o d u c t s and t a r d i d n o t exceed 0.5 w t . h.

Fig. 2

-

The i n f l u e n c e of t h e s p e c i f i c energy o f t h e s u b s t r a t e on t h e conver- s i o n degree (a) and t h e s e l e c t i v i t y o f t h e product f o r m a t i o n (b) f o r : t h e ga- seous p r o d u c t s

-

U,g; t h e acetylene

-

^{U,CsH=; S,CsH23} t h e carbon o x i d e

U,CO; S,CO; t h e methane

-

^{U,CH4( S,CH-} and t h e e t h y l e n e

-

^U,CsH4;

SsCzH4.

From Fig. 2a i t can be seen t h a t t h e i n c r e a s i n g of t h e s p e c i f i c energy caused a h i g h e r conversion o f s u b s t r a t e e.g. f o r t h e sum o f gaseous p r o d u c t s U from 20 t o 62; f o r the CzHa. from 4 t o 14 and f o r t h e CO from 10 t o 42 w t . %, r e s p e c t i v e l y .

For t h e C2H4 and CH4 a d i f f e r e n t p i c t u r e was observed. I t was found t h a t c o n v e r r i o n degree fur those compounds d i d n o t depend s t r i c t l y from t h e speci- f i c energy o f c o a l and were n o t h i g h e r t h a n 4 w t . %.

A s i t can be seen i n Fig, Zb, t h e s e l e c t i v i t y o f t h e p y r o l y s i s process i n r e s p e c t t o t h e CO f ~ r m a t i o n (S,CO) i s much h i g h e r than t h a t f o r t h e a c e t y l e n e

(S,CnH=). An i n c r e a s e o f t h e s p e c i f i c energy o f t h e subsitrate, caused t h e changes of t h e s e l e c t i v i t y of t h e f o r m a t i o n

-

an i n c r e a s e f o r t h e CO (from 52 t o 66%) and decrease f o r CaHS (from 32 t o 24%)

.

The s s l e c t i v i t i e s o f t h e p y r o l y s i s i n r e s p e c t t o t h e CH4 and C&. f o r - mation (S,CH4 and SICPH~, r e s p e c t i v e l y ) , were d i s t i n c t l y lower t h a n those

f o r t h e CO and CZHZ f o r m a t i o n (S,CO and S,C=H=, r e s p e c t i v e l y ) . They

d i d n o t exceed 10% and were independent on t h e s p e c i f i c energy o f t h e c a a l . From t h e F i g . 2b f o l l o w s t h a t t h e CO p a r t i c i p a t i o n i n t h e r e a c t i o n p r o d u c t s i n c r e a s e when t h e s p e c i f i c energy o f t h e c o a l and, i n a consequence, t h e sub- s t r a t e conversion degree becomes h i g h e r .

I t i s worth t o n o t i c e t h a t f o r t h e h i g h s p e c i f i c energy o f t h e s u b s t r a t e a s u m of t h e gaseous p r o d u c t s o f a p y r o l y s i s exceeded t h e c o n t e n t s o f t h e v o l a t i l e p a r t s i n t h e brown c o a l (50 w t . % ) .

Examplary i n f r a r e d s p e c t r a o f s u b s t r a t e and plasma c a r b o n i z a t e s a r e presented on Fig. S.

Fig. 3

-

I n f r a r e d s p e c t r a o f t h e c o a l samples: a) brown c o a l b) c a r b o n i z a t e

-

s p e c i f i c energy 11 MJ/kg, c ) c a r b o n i z a t e

-

s p e c i f i c energy 35 MJ/kg.

As i t f o l l o w from t h e i n f r a r e d spectrophotometric a n a l y s i s t h e a c t i o n o+ a hydrogen plasma j e t on t h e brown c o a l caused disappearance o f bands i n t h e i n f r a r e d spectrum which were c h a r a c t e r i s t i c f o r a m i n i n g coal.

4

-

CONCLUSIONS

The performed experiments have l e a d u s t o t h e c o n c l u s i o n t h a t t h e i n t r o d u c t i o n of t h e brown c o a l i n t o t h e plasma caused i t s e f f e c t i v e decomposition t o t h e simple gaseous compounds: CaH=, CH*, CkH4 and CO. The carbon o x i d e

and t h e a c e t y l e n e were t h e main p r o d u c t s o f t h e p y r o l y s i s i n t h e hydrogen plasma j e t . I t has been e s t a b l i s h e d t h a t an i n c r e a s i n g of t h e c o a l s p e c i f i c energy (10-35 MJ/kgS caused t h e h i g h e r conversion o f t h e s u b s t r a t e e.g. f o r t h e sum o f t h e gaseous products from 20 t o 62, f o r CO from 10 t o 42 and f o r CaHn from 4 t o 14 w t . %. r e s p e c t i v e l y .

A change o f t h e s p e c i f i c energy o f t h e c o a l caused an i n c r e a s e o f t h e s e l e c t - i v i t y o f t h e CO f o r m a t i o n i n a range 52

-

66%. In t h e same t i m e t h e s e l e c t i v i - t y of t h e C ~ H Z f o r m a t i o n changed i n a range 2ii

-

^24%.

The a c t i o n o f a hydrogen plasma j e t on t h e brown coaI caused disappearance o f bands i n t h e i n f r a r e d spectrum which were c h e r a c t e r i s t i c f o r a mining c o a l .

COLLOQUE DE PHYSIQUE

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