Catalytic effect of ZnCl2 during coal pyrolysis

Catalytic effect of ZnCl2 during coal pyrolysis

Fuel Processing Technology, 20 (1988) 51-60 51 Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s ...

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Fuel Processing Technology, 20 (1988) 51-60


Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s




1- I n s t i t u t de Recherches sur la Catalyse, Laboratoire Propre du CNRS conventionn~ A l ' U n i v e r s i t ~ Claude Bernard Lyon I, 2, avenue Albert Einstein, 69626 Villeurbanne C~dex, France. 2- Laboratoire de G~ochimie et M~tallog~nie, CNRS UA 196, 4 place Jussieu, 75252 Paris C~dex 05, France. 3 - Laboratoire de Cristallographie, Universit~ d'Orl~ans, B.P. 6759, 45067 Orleans C~dex 2, France. SUMMARY The slow pyrolysis under helium of a high v o l a t i l e bituminous coal has been studied in the presence or not of ZnCl~ (1-25 % w/w). The hydrogen evolution is enhanced below 600°C but is not modified at higher temperature. The hydrogen y i e l d increases with the ZnCl2 loading and the methane production is suppressed. Py-GC experiments show that ZnCl^ is involved in a low temperature cracking mechanism of the s o l i d . The formation of ZnO and ZnS at 550°C was detected by XRD analysis of pretreated samples ( i n i t i a l loading 10 % w/w). INTRODUCTION From an organic chemistry point of view, coal conversion into char, tar and gas could be imagined as a great number of successive and competitive reactions. The use of catalysts of these reactions could be p o t e n t i a l l y interesting in order to increase the t o t a l conversion and to enhance the production of the most desirable products. Pyrolysis of coal in the presence of inorganic materials has been studied previously under slow heating




A large



additives were tested, selected mainly for t h e i r acido-basic properties. The results showed an increase



production. Georgiadis and Gaillard

char y i e l d (Ref.


and a decrease of



studied more s p e c i f i c a l l y the

effect of ZnCl2 and AICl 3 (3 % w/w) on the pyrolysis of 3 French coals (% V.M. : 24.90, 32.45, 38.20). The same effects on char and t a r yields were observed as well as an increase of

the volumic production of

gas. With the

addition of ZnCl2 and to a lesser degree of AlCl3, hydrogen evolution increases at the expense of

the production of methane and of

other l i g h t


gases. Similar results were obtained by Bodily, et. a l . , ( R e f . 4) when they pyrolysed an Utah coal,


impregnated with

12 % ZnCl2 during

slow heating

experiments under nitrogen. In the presence of the additive, they observed a 0378-3820/88/$03.50


1988 Elsevier Science Publishers B.V.

52 decrease of the weight-loss and an increase of the hydrogen production below 400°C. Due to the presence of ZnCl2, the hydrogen evolution began below 200°C and a maximum appears at 350°C. They also studied the interaction between the coal and ZnCl2. They showed that when the sample (23.1% w/w of ZnCl2) is heated, the a c i d i t y of the resulting char decreases and has almost disappeared at 400°C. The mechanism of this interaction is not well understood. Kandiyoti, et. a l . , ( R e f . 6) studied 3 d i f f e r e n t preparations (ZnCl2 5 % w/w) of Linby coal in a Gray King r e t o r t (5°C min-1). No difference was observed between - a sample impregnated from an aqueous solution of ZnCl2 and dried in a i r (60°C), - the same sample but dried in vacuum (50°C) and - a sample prepared by physically mixing the coal powder and ZnCl2. The effect of the additive was small, within

experimental error, i f

we compare the yields of the i n i t i a l

coal and those obtained for the blank coals prepared as above but without ZnCl2. However, the tar y i e l d of the i n i t i a l coal (untreated) was the highest obtained in these experiments. The present study investigates the c a t a l y t i c effect of ZnCl2 on the release of v o l a t i l e matter during coal slow pyrolysis in an atmospheric pressure of Helium . The transformations of the organic structure of the coal and of the catalyst during the f i r s t stages of the pyrolysis were also studied. EXPERIMENTAL The m a t e r i a l (Merlebach, C, 79.82

used in these

France). The

; H, 4.83

; O, 8.40

r e c e i v e d , the coal p a r t i c l e made by g r i n d i n g ball




under n i t r o g e n

experiments is

a high

c o m p o s i t i o n is as f o l l o w s ; S, 0.79


N, 1.00


; CI,


coal :

; Ash, 4 . 8 0 .


s i z e was m a i n l y lower than 2 mm. The p r e p a r a t i o n was and the r e q u i s i t e for

5 minutes.


The p a r t i c l e


The d r y m i x t u r e

because no d i f f e r e n c e was observed on the y i e l d s added b e f o r e g r i n d i n g .


(% w/w, m o i s t u r e f r e e )

ZnCI 2 in

a stainless

procedure was chosen

o f the p y r o l y s i s


w a t e r was

s i z e was m a i n l y lower than 20 m i c r o m e t e r s .

The samples were s t o r e d under n i t r o g e n . The e f f e c t

o f ZnCI 2 on coal p y r o l y s i s

was s t u d i e d w i t h

a slow h e a t i n g r a t e

(3°C min -1) from ambient t e m p e r a t u r e up t o 950°C b e f o r e c o o l i n g . first

d r i e d a t 50°C in a vacuum oven f o r

a microbalance.

The sample was

12 h then placed in the r e a c t o r pan o f

About 250 mg were used f o r

each run.

The v o l a t i l e

m a t t e r was

c o n t i n u o u s l y removed from the h o t - z o n e by a d o w n - f l o w o f h e l i u m (50 cm3 m i n - l ) . The



analysed on l i n e




a glass



and t h e


S u p e l c o ) . The c o n c e n t r a t i o n s o f H2 and CH4 were measured e v e r y 5 m i n u t e s . sample


by gas chromatography o v e r a c a r b o s i e v e SII column ( s u p p l i e d by

was p y r o l y s e d a t l e a s t 2 t i m e s . The r e p r o d u c i b i l i t y

was w i t h i n


5 %. The





a gas was calculated from the curve of production versus

temperature for this gas. The weight-loss measured by the microbalance represents both the v o l a t i l e matter












decomposition of ZnCl2. At the end of the experiment (950°C), the zinc and the chlorine of the i n i t i a l

sample had almost completely disappeared. I t

is then

d i f f i c u l t to compare by this way samples with d i f f e r e n t ZnCl2 loadings. However, i t can be concluded from the thermogravimetric experiments that no s h i f t of the temperature of the v o l a t i l e matter evolution did take place. A few samples were prepared by preheating the coal without or with 10 % ZnCl 2 up to 425, 450 or 550°C under the same conditions as above. They were studied by PyGc at 750°C and by XRD analysis. RESULTS AND DISCUSSIONS Gas evolution Figure 1 shows the hydrogen evolution during

coal pyrolysis with d i f f e r e n t

ZnCl 2 loadings. The addition of ZnCl2 led to an increase of the release of




5% 10% 25 %




375 E ,Nu



_= 0: ) @ c

125 Z


0 350







Temperature (~C) Figure 1.

Hydrogen evolution during slow pyrolysis (3 °C/min) of Merlebach coal with different ZnCI 2 loadings.

54 hydrogen at low temperature, but no difference was observed above 650°C. These results are similar to those of Bodily et. a l . ,

(Ref. 4). However, i t is shown

in our case that ZnCl2 is active even at low concentration. When only 1% of ZnCl 2 is added, the i n i t i a l

hydrogen evolution is shifted to lower temperature

by 30-40°C, and a shoulder appears on the hydrogen peak. The beginning and the maximum



hydrogen production

concentration of ZnCl2. The variation of

depends strongly the t o t a l




hydrogen y i e l d with the

amount of ZnCl2 added is reported in Figure 2. This production increases with the f i r s t weight percent

of ZnCl2, then a saturation effect is observed.




] 0 I-
















ZnCI 2 Ioedfno (%w/w) ~Kf•

2. Influence of the ZnCle loading on the total hydrogen and methane ( A ) yields

(• )

The methane evolution during pyrolysis is shown in Figure 3. The addition of ZnCl 2 induces a decrease of the methane release but no temperature s h i f t of the initial


and of

the maximum was observed.

In this

case also,


saturation effect on the t o t a l methane y i e l d was obtained when the ZnCl2 loading

was raised (Figure 2).


0% 1% 5% 10% 25 %

100 A

o o1



E '0



\.\ 0 350












Temperature PC) Flgure 3.

Methane evolution during slow pyrolysis (3 °C/min) of Merlebach coal with different ZnCI 2 loadings

These results

are in

good agreement with those


previously by

Georgiadis and Gaillard (Ref. 3) but not with those obtained recently by Neuburg et. al.

(Ref. 7). For the Linby coal, the impregnation with

5 % ZnCl2 induced

an increase of the hydrogen release and also of the methane, ethylene and ethane yields in the temperature range 250-400°C with the same heat treatment. The higher hydrogen y i e l d , the lower yield of methane,and we may assume of the other hydrocarbon products,

indicates that ZnCl2 is

active in


reactions below 600°C. The molar ratios between the increase of the hydrogen production and the zinc chloride loading are l i s t e d in Table 1. These values (>10) show that ZnCl2 is not involved in molecule

a stoichiometric reaction where 1

of ZnCl2 would react with coal and would produce 1 molecule of H2.

This suggests that ZnCl2 is involved in a c a t a l y t i c mechanism of cracking. All these results show that the c a t a l y t i c action of ZnCl2 takes place at low temperature (below 600°C) and that whatever the extent of the cracking is, no modification occurs on the release of hydrogen at higher temperature.

56 TABLE 1.

: Comparison of the molar r a t i o between the increase of hydrogen yield

and the ZnCl2 loading Molar r a t i o hydrogen increase/ZnCl 2 loading

Sample % ZnCl 2 1 5 10 25

17 ii 8 4

Solid evolution ( i ) Rapid pyrolysis-GC of slowly pretreated samples. The transformations of the organic structure of the coal in the temperature range 300-600°C during slow heating (3°C/min) were studied by rapid PyGC at 750°C performed on the i n i t i a l and on slowly preheated samples without and with 10 % ZnCl2. Thus, we obtained + the GC traces of the C6 compounds, from benzene to naphthalene, the response of the heavier compounds being too small. This curve is a kind of "finger p r i n t " of the sample analysed. Without any pretreatment and with these conditions of rapid pyrolysis, the + addition of ZnCl2 has only a l i t t l e effect on the formation of the C6 compounds. No difference can be seen between the coal and the sample with 10 % ZnCl 2 (Figure 4, al, b l ) . A thermal pretreatment of the coal, without or with + 10% ZnCl2, induces a decrease of the C6 compoundsreleased. For the preheated coal, the decrease of the C6+ compounds is low i f we compare with the i n i t i a l coal (Figure 4, al, a2, a3). For the samples, without and with

10 % ZnCl2,

preheated at

425°C, the

reduction is small and the same compounds are obtained (Figure 4, a2, b2). No effect of ZnCl2 can be seen. I f these samples are preheated at 450 °C, a strong effect of

ZnCl2 is


10 % ZnCl2, the production of

observed (Figure 4,



For the sample containing

the C6+ compounds is


reduced. These results

suggest that



differences observed come from the

modification of the chemical nature of the pretreated coal-ZnCl 2 system. The structural changes of the solid during slow heating are d i f f e r e n t without or with ZnCl2. The effect of ZnCl2 in the production of H2 and CH4 would proceed from a catalysis of

coal cracking.

However, a c r i t i c a l

temperature must be

reached since the c a t a l y t i c effect is only clearly shown when the samples are preheated above 425 °C.


8. COAL I-


al : initial



: 425 °c

a3 : 450 °c


/iLi/ , 'it i




b. bl : initial

COAL + 10% ZnCI 2 b2 : 425 °C




Q. C






F i g u r e 4. GC traces of the C~ compounds released by rapid pyrolysis of the Merlebach Coal without and wit~ ]0% ZnCI2, and of the same samples preheated at 425 °C and 450 °C. ( B: Benzene T: Toluene m + p - X : m+p-Xylene o - X : o-Xylene Ph: Phenol o - M e P h : O-Methyl Phenol m s + p - M e P h : m+p-Methyl Phenol D M e P h : di Methyl Phenol N : Naphthalene


[email protected][email protected]


30000 N



[email protected]

c c -

[email protected]

[email protected]@~








s (~-1) Figure 5.

XRD patterns

of Merlebach

coal slowly preheated




with 10% ZnCI 2 initially

ZnCI 2

at 550 °C

59 This is in good agreement with the hydrogen evolution shown in Figure 1. A temperature of 425°C corresponds to the beginning of the hydrogen evolution for a sample with 10 % ZnCl 2" i i ) XRD measurements of slowly/ pretreated samples. Figure 5 shows the XRD pattern of samples pretreated up to 550°C with (10 % w/w) and without ZnCl2. This


corresponds approximatively to






evolution that appears in the presence of ZnCl2. I t is thus possible to observe the transformations of the char and of ZnCl2 that occured during pyrolysis. Fe S represents the ferrous sulphides with d i f f e r e n t stoichiometries. xy The study of the carbon structure shows a decrease of the intensity of the (002) r e f l e c t i o n for the sample i n i t i a l l y containing the

ZnCl2. A low broadening of

(002) r e f l e c t i o n suggests that the presence of the additive during the

pyrolysis induces some d i s t o r t i o n in the aromatic structure. The layers are not as well organized as in the char from the coal alone. The comparison of the 2 curves shows the presence of ZnO and ZnS, and no evidence of the existence of ZnCl2 particles. ZnO was expected as a product of the hydrolysis of ZnCl2. The presence of ZnS is more surprising because the sulfur concentration of the coal is

low. These reactions of transformation of

ZnCl 2 must be taken into account i f we t r y to explain the c a t a l y t i c cracking of coal. We can conclude also that the weight loss of the coal-ZnCl 2 samples during pyrolysis includes a contribution of the catalyst transformation. A correction must be done i f we want to compare samples with d i f f e r e n t loadings. The mean c r y s t a l l i t e size (Ref.


from the

(24 nm), calculated with the Scherrer equation

half-height width



XRD peak indicates that


dispersion of ZnO is rather high. The dispersion of ZnCl2 was certainly better because a sintering probablyoccurredwhen the particles of ZnO are formed. CONCLUSIONS This study confirms the effect of the addition of ZnCl2 (1-25 % w/w) during coal slow pyrolysis under helium. Increase of the hydrogen y i e l d and decrease of

the production of methane suggest that ZnCl2 is

involved in


c a t a l y t i c cracking of the coal at low temperature (below 600°C). The catalyst reduces the gaseous hydrocarbon yields i f the methaneis supposed to be a measure of

the heavier hydrocarbons. However, with our




increase of the coke y i e l d could not be measured accurately. Py-GC experiments show that the catalyst is active during the formation of tar and gas, and not only in the post-cracking of v o l a t i l e matters thermally produced. At 550°C, ZnCl2 is completely transformed into ZnO and ZnS.These kinds a reactions must be studied in order to determine the catalyst

phase and to have

60 a better understanding of the catalytic mechanism. All

data show that

ZnCl2 is

involved in a catalytic mechanism of

cracking. However, i t is not yet possible to state which is the catalytic active phase. Someexperiments are being carried out in order to specify this phase. ACKNOWLEDGEMENTS The authors would l i k e to thank the "Centre de Pyrolyse de Mari~nau" (CPM) for his financial support and for helpful discussions. REFERENCES 1 2 3 4

R. Lessing and M.A.L. Banks, J. Chem. Soc. 125 (1925) 2344-2355. A.W. Gauger and D.J. Salley, Fuel 8 (1929) 79-85. C. Georgiadis and G. Gaillard, Chaleur et Industrie 374 (1956) 247-258. D.M. Bodily, S.H.D. Lee and W.H. Wiser, ACS Div. Fuel Chem. Preprints 19 (I) (1974) 163-166. 5 K. Matsuura, D.M. Bodily and W.H. Wiser, ACS Div. Fuel Chem. Preprints 19 (I) (1974) 157-162. 6 R. Kandiyoti, J . I . Lazaridis, B. Dyrvold and C.R. Weerasinghe, Fuel 63 (1984) 1583-1587. 7 H.J. Neuburg, R. Kandiyoti, R.J. O'Brien, T.G. Fowler and K.J. Bartle, Fuel 66 (1987) 486-492. 8 H.P. Klug and L.E. Alexander, "X-Ray d i f f r a c t i o n procedures" Wiley J. and Sons New-York 1974.