Structural changes in coal chars during gasification

Thermochlmlca Acta, 82 (19841 103--110

103

Elsevmr S c m n c e P u b h s h e r s 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

STRUCTURAL CHANGES IN COAL CHARS DURING GASIFICATION A. de KORANYI and S.G. WILLIAMS British

Gas C o r p o r a t i o n , London Research S t a t i o n , Michael Road, London SW6 2AD,

England

ABSTRACT Two bltuminous coal chars of the same rank were g a s i f i e d a t atmospheric pressure w i t h CO? a t 980 C. P o r o s l t i e s and r e a c t i o n r a t e s were measured a t d m f f e r e n t degree~ of carbon b u r n - o f f . Comparison of changes in p o r o s i t y and r e a c t i v i t y w i t h per cent b u r n - o f f i n d i c a t e s t h a t t o t a l surface area is a s i g n l f m c a n t parameter f o r r e a c t i v m t y but cannot be regarded as the major controlling factor. IHTRODUCTION AS reserves of n a t u r a l gas and o i l

began to d e c l i n e , I n t e r e s t in coal

g a s m f l c a t i o n to produce s u b s t i t u t e n a t u r a l gas, and in coal c h a r a c t e r i s a t i o n has increased.

In p a r t i c u l a r ,

the B r i t i s h

Gas C o r p o r a t i o n has developed and

proven the BGC-Lurgi Slagging G a s i f i e r f o r a wide range of c o a l s . used mn the g a s i f i e r

ms the steam-oxygen g a s i f i c a t l o n

The process

o f coal where the f o l l o w -

ing r e a c t i o n s take place : C +

02

~

CO2

(I)

C +

H20

~

CO +

C -:- CO2

~

2C0

H2

(2) (3)

The r a t e d e t e r m l n i n g step o f the steam-oxygen r e a c t i o n ( I ) as t h a t in the carbon-carbon d i o x i d e r e a c t i o n ( 2 ' 3 ) .

is considered the same

Therefore, as the carbon-

carbon d i o x i d e r e a c t i o n is a l s o the s i m p l e s t o f the heterogeneous g a s l f i c a t i o n r e a c t i o n s , ] t was chosen as the r e a c t i o n to be s t u d i e d here. The r e a c t i v i t y

o f a coal char w i l l

depend, among o t h e r t h i n g s , on the number

of a c t i v e s l t e s ( 4 ' 5 ) , and the pore s t r u c t u r e of the char, as well as on char p r e t r e a t m e n t and the nature and c o n c e n t r a t i o n of mineral m a t t e r .

Although the

number of a c t i v e s i t e s is the most i m p o r t a n t parameter where the r e a c t i o n is chemically controlled,

the development o f pore s t r u c t u r e w i t h carbon b u r n - o f f

may well determine the a c c e s s i b i l i t y

o f r e a c t a n t s to a c t i v e s i t e s and, hence,

determine r e a c t i v i t y . Measurement o f surface areas and m i c r o o o r o s i t y ~4'5'6'7)" of coal chars is complex.

A commonly used equation (D-R) is t h a t of Dubinin and Radushkevich (8)

0040-6031/84/$03.00

© 1984 Elsevier Science Publishers B.V.

104

based on the e a r l i e r pores ~9'I0)'' w

=

Dubinin Polanyi Theory of Volume F i l l i n g

of Micro-

I t has the form :

wo exp {-B(T/B) 2 log 2 ( p U p ) )

(4)

where w is the amount of a d s o r p t i v e taken up a t pressure, p, and wo is the t o t a l micropore volume. The o b j e c t o f t h i s paper is to show how micropore volume and s p e c i f i c surface area of two bituminous coal chars o f the same rank change as the chars are burned away and to see i f these changes c o r r e l a t e w l t h changes in r e a c t i v ity. EXPERIMENTAL R e a c t i v i t y Measurements Two B r i t i s h bituminous coals were used in t h i s work : Markham Main and Manvers Barnborough.

Both belong to the NCB 702 c l a s s i f i c a t i o n ;

the most abundant in the U.K.

They were f i r s t

t h i s rank being

converted to chars by p y r o l y s i s ,

under n i t r o g e n , a t temperatures o f 860~C and 910 ° r e s p e c t i v e l y . U l t i m a t e a n a l y s i s of the chars showed the Markham char to have a carbon c o n t e n t of 87.2% (as received basis) and the Manvers to c o n t a i n 89.4% carbon. Thermogravimetric a n a l y s i s then revealed a v o l a t i l e

carbon content of 7S and 2L~,

respectively. Ten gram samples of the chars were then placed in a d i f f e r e n t i a l r e a c t o r a t atmospheric pressure.

f i x e d bed

They were brought up to r e a c t i o n temperature

(980°C) in a stream o f N2 p r i o r to g a s i f i c a t i o n

w i t h CO2.

The c r o s s - s e c t i o n a l gas f l o w across the 3 cm deep char bed was about 450 moles min - I m-2. Traces o f oxygen were removed upstream o f the char bed by an oxygen t r a p and gas products were analysed downstream by gas chromatography. The r e a c t i o n o f carbon d i o x i d e w i t h carbon produces only carbon monoxide (equation 3).

T h e r e f o r e , the g a s i f i c a t i o n

r a t e of the coal char (R) is h a l f the

r a t e o f p r o d u c t i o n o f CO (Rco), which can be c a l c u l a t e d d i r e c t l y

from a n a l y s i s

of the o u t l e t gas composition and f l o w r a t e . The amount o f carbon burned o f f the char surface (B) at any time ( t )

is

found from : B

=

S

Ri + R ( i - 1 ) 2

(t i

- t(i-1)

The s p e c i f i c g a s i f i c a t i o n

)

(5)

r a t e (Rs), which is defined as the g a s i f i c a t i o n

r a t e per gram o f s o l i d carbon a v a i l a b l e f o r r e a c t i o n a t any time ( t ) , Rs

=

is

:

C/(C F - B)

where CF = t o t a l

(6) amount o f f i x e d carbon in the o r i g i n a l

char.

W h i l s t the carbon l o s t from the char, measured as the percentage o f the

105

original %B =

carbon content burned o f f the char (%B) is : IOOB

(7)

Reaction r a t e s were found to be r e p r o d u c i b l e to w i t h i n +10% f o r the same r e a c t i o n c o n d i t i o n s in a l l cases. Surface Area Measurements S e v e n t y - f i v e m i l l i g r a m samples o f the same chars w i t h p a r t i c l e ~m were placed in a p l a t i n u m mesh bucket w i t h i n a s i l i c a Cahn 2000 E l e c t r o b a l a n c e .

They were g a s i f i e d

d i t i o n s as in the d i f f e r e n t i a l different

s i z e 500 - 850

hangdown f l o w t u b e on a

w i t h CO2 under the same con-

r e a c t o r except t h a t the r e a c t i o n was stopped a t

percentages of b u r n - o f f in o r d e r to measure the changes in surface

porosity. A f t e r samples were g a s i f i e d on the E l e c t r o b a l a n c e , they were outgassed overn i g h t a t 730°C to about 10 -5 bar.

This was to remove any oxygen complex formed

on exposure o f the char to a i r which would block the micropores.

A carbon

d i o x i d e isotherm was then c a r r i e d out a t 195 K. A f t e r completion o f the CO2 i s o t h e r m , samples were outgassed o v e r n i g h t a t 730°C.

They were then s a t u r a t e d a t room temperature w i t h n-nonane according to

the method of Gregg and co-workers t11'12)'"

S a t u r a t i o n was judged to be com-

p l e t e when uptake o f nonane ceased, g e n e r a l l y a f t e r 5 - 16 hours.

This was

f o l l o w e d by degassing to 10-4 b a r , a l s o a t room temperature, to remove excess nonane.

It

is assumed t h a t the long nonane molecules not only f i l l

all

the

micropores in the system but also s t r o n g l y r e s i s t d e s o r p t i o n from the micropores a t room temperature by reason o f t h e i r

shape.

A n i t r o g e n isotherm was then

determined a t 77 K and the BET surface area measured was considered to be the surface area o f mesopores o n l y . RESULTS AND DISCUSSION R e a c t i v i t y Curves One p o t e n t i a l problem w i t h heterogeneous r e a c t i o n s a t high temperature is that of rate limitation

by bulk d i f f u s i o n .

However, Bradshaw et a l (13) showed

t h a t a t atmospheric pressure and up to I000°C bulk d i f f u s i o n e f f e c t s d i d not restrict

the r e a c t i o n o f carbon w i t h CO2 in a thermobalance f l o w system.

In

t h e i r system the Manvers char used was from the same batch as used in t h i s work. I t was of p a r t i c l e

size 15 - 180 Nm and had a maximum r e a c t i o n r a t e , a t 980°C,

o f 9.5 x 10 -4 moles min - I g - l , system and a d i f f e r e n t

particle

w h i l s t in t h i s work, using a d i f f e r e n t

reaction

s i z e , the maximum r e a c t i o n r a t e under the same

c o n d i t l o n s was 8.5 x 10 -4 moles min - I g-1. Plots of r e a c t i v i t y both coal chars.

vs. carbon b u r n - o f f a t 980°C are shown in Figure I f o r

As can be seen, the Markham is by f a r the more r e a c t i v e a t a l l

106

values of measured b u r n - o f f . distinctly

different.

The general shape o f the two p l o t s are a l s o

Both show an i n i t i a l

r i s e in r e a c t l v i t y

which may p o s s i -

b l y be a t t r l b u t e d to carbon b u r n - o f f opening up closed pores, or removal of carbon behaving l i k e d e b r i s i n ] t i a l l y

b l o c k i n g the s t r u c t u r e (14).

However,

above about 20% b u r n - o f f the ;~anvers shows a more or less constant r e a c t i v i t y up to 8Or, w h i l s t the Markham shows a steady increase in r e a c t i v i t y 50%, f o l l o w e d by a sharp f a l l

up to about

above 60%.

4-i3

416

~

. 24

Markham Main

c

E

~20 0

E 16 0 ,t.x

~

..

8

:i'

..

.

. , ,.

+

" ÷ -, - .,,

-

° Ma'~" vers

u

X 4

Nos.=Surface

0

I

I

I

10

20

30

III

40

Areas

f

I

t

I

I

50

60

70

80

90

.~

100

90 burn - off ( d r y - ash - f r e e ) FIG.1

REACTIVITY

At 100% b u r n - o f f the r e a c t i o n w i l l carbon to react.

c l e a r l y cease since there w i l l

However, the d e f i n i t i o n

means t h a t i t is a d i f f e r e n t i a l

VS % B U R N - O F F

of r e a c t i v i t y

be no more

adopted in t h i s work

q u a n t i t y , w i t h the w e i g h t o f carbon remaining as

the denominator, so t h e r e is no fundamental reason why the r a t e should f a l l before 100% b u r n - o f f i s reached.

From these r e s u l t s ,

way or the o t h e r but the f a l l - o f f

in r e a c t i v i t y

there is no evidence one

o f Markham may be due to e f f e c t s

of residual mineral m a t t e r or t o a genuine decrease in a c t i v i t y burn-off.

a t high carbon

However, a p o i n t to be borne in mind is t h a t the carbon b u r n - o f f s are

calculated indirectly

during r e a c t i v i t y

measurements by mass balance and t h a t

e r r o r s in f l o w r a t e s of the product gas are cumulative so t h a t e r r o r s a r i s e in the higher values o f carbon b u r n - o f f .

This in turn a f f e c t s the s p e c i f i c r a t e

107 values which depend on the amount o f s o l i d carbon remaining. able cause o f the s l i g h t

fall

in the r e a c t i v i t y

This is the prob-

of Manvers above 80% b u r n - o f f .

From t h i s p o i n t of view, the g r a v i m e t r i c method o f d e t e r m i n i n g r e a c t i v i t y

is

s u p e r i o r because the amount of carbon g a s i f i e d and the amount of carbon remaining are obtained d i r e c t l y

by the same s i n g l e measurement.

Surface Area and ~licropore Volumes Values f o r the mlcropore volume, Wo, determined from the s i m o l i f i e d D-R e q u a t i o n , and values of "surface area" determined from them are shown in Table I f o r both chars. TABLE I

~ Burn-off

Spec. Rate. (moles min - I g - ] x 10 -4

Meso~orous N2 Surface Aream~,Ig

D-R wo cm3/g

Total D-R Surface Area m~/g

S~ec. Rate/D-R Surface Area (mol.min-lm -2 x 106)

Manvers Barnborough Char (Carbon Content 89.4c,) 12.01 22.43 23.35 39.12 53.19 59.36 67.76 77.45 80.00

6.6 7.4 7.45 7.95 8.25 8.35 8.50 8.50 8.50

29.34 45.68 76.62 166.00 445.00 257.00

0.0569 0.094 0.097S 0.1123 0.1148 0.1328 0.2566 0.1104 0.0638

132 219 228 261 267 309 597 257 148

5.0 3.4 3.3 3.1 3.1 2.7 1.4 3.3 5.7

304 416 401 413 815

4.4 4.3 4.5 5.7 1.6

Markham Main Char (Carbon Content 87.2°) 12.89 23.64 23.55 48.93 78.78

13.5 18.0 18.05 23.4 13.05

40.64 47.50 191.50 384.00

0.1308 0.1785 0.1724 0.1775 0.3506

The problems of measuring surface areas o f microporous carbons have been d l s cussed b e f o r e ( 4 ' 5 ' 6 ' 7 )

Our choice of CO2 adsorbed a t 195 K was based on the

need to use s u f f i c l e n t l y

high a temperature to a l l o w d i f f u s i o n

very small mlcropores a t an acceptable r a t e .

o f CO2 along the

At the same time we wished to work

as near P/Po = I as was p o s s i b l e , w i t h a non-pressurised a p p a r a t u s , to see whether there was any tendency to m u l t i l a y e r f o r m a t i o n a t these h i g h e r p a r t l a l pressures.

One problem t h a t a r i s e s is the value~ o f Po to adopt.

We have

f o l l o w e d the use o f Sing and his co-workers t15s in using t h a t o f the l i q u i d e x t r a p o l a t e d down to 195 K.

The value is 1.86 bar and t h i s means t h a t e f f e c t -

i v e l y values of P/Po > 0.6 cannot be a t t a i n e d . was taken as 0.17 nm.

The c r o s s - s e c t i o n a l area of CO2

108

The isotherms (shown in Figure 2) are e s s e n t i a l l y of Type I, although values f o r P/Po > 0.5 could not be reached, as discussed above, so that i t is d i f f i c u l t to say whether they have any Type I I character.

However, most of the D-R plots

were l i n e a r over the range of P/Po = 0.25 only and showed upward deviations f or higher values of P/Po"

This suggests that the pore f i l l i n g

model accounts for

most of the adsorption observed but that there is a s i g n i f i c a n t contribution from a BET-like m u l t i l a y e r formation.

ManversBarnburgh

M a r k h a m Main

%Burn-off

o % Burn-off • 79

6;B

0.38

~0.16

0"32

0')

=o.14 F

E ~D.12

,/

~' O-2 8

3 0.24 49

EO-IO u ._c 0.08

8

0.20 0.18

J: ¢3

~ 0.06

~ 0-12

0,04

o-o81

0.02

0"04 0

0.1

0"2

0,3

0.4

0-5

P/Po

FIG. 2

0

0~1

0~2

0~3

014

015

P/Po

COAL CHAR C02 ISOTHERMSAT DIFFERENT CONVESION LEVELS

A feature of the adsorption process was i t s slowness at 195 K suggesting thBt d i f f i c u l t i e s a r i s i n g from activated adsorption and slow d i f f u s i o n in narrow pores are not completely overcome even at r e l a t i v e l y high temperature.

Similar

slow adsorption at r e l a t i v e l y high temperatures on microporous carbons have been found by Koresh and Soffer (14) and a t t r i b u t e d by them to carbon 'debris' in the pores slowing down d i f f u s i o n of the adsorbed phase along the pores.

However,

Nandi et al (16) also found that a longer period of time is usually required to establish e q u i l i b r i u m when adsorbing CO2 onto bituminous coals.

Nevertheless,

the problem was p a r t i c u l a r l y troublesome at low values of carbon burn-off and was the reason why no adsorption was carried out on reacted chars.

I t was

a l l e v i a t e d by increasing burn-off which suggests that carbon debris is increasingly removed as reaction proceeds.

109

Combined n-nonane and N2 A d s o r p t i o n I f l a r g e r pores were p r e s e n t , i t would, in p r i n c i p l e ,

be p o s s i b l e to use the

method o f Gregg and hls co-workers to block o f f the genuine micropores (d < 2nm) by p r e - a d s o r b i n g nonane.

The surface area associated w i t h the l a r g e r pores

could be determined by a d s o r p t i o n o f N2 a t 77 K in the usual way).

Results f o r

BET areas obtained by t h i s procedure are given in Table I , column 3.

They show

t h a t at ]ow values f o r carbon b u r n - o f f the mesopores c o n t r i b u t e r e l a t i v e l y little

to the t o t a l

surface area.

However, as r e a c t i o n proceeds and the D-R

surface area i n c r e a s e s , so too does the mesopore surface area. However, some c a u t i o n is needed in i n t e r p r e t i n g

these r e s u l t s .

The uptake o f

N2 in some instances was very slow, i n d i c a t i n g the problem o f a c t i v a t e d d i f f u sion.

Moreover, t - p l o t s (17) of N2 adsorbed on the n o n a n e - f i l l e d char surfaces

were q u i t e anomalous g i v i n g p l o t s in some cases w i t h negative i n t e r c e p t s .

In

the r e s u l t s o f Gregg and c o - w o r k e r s , the removal of the Type I component by filllng

micropores w i t h nonane gives a simple Type I I N2 isotherm which in

t h e o r y gives a l i n e a r t - p l o t

passing through the o r i g i n .

We surmise t h a t the method may break down in the present case because the coal char has a surface t h a t is h i g h l y heterogeneous and in which t h e r e is a broad spread o f pore s i z e ranging c o n t i n u o u s l y from m o l e c u l a r s i z e d pores on one extreme to the mesopore region and up to the macropore region on the o t h e r . C o r r e l a t i o n o f R e a c t i v i t y and Surface Area Markham c h a r , besides having the g r e a t e r r e a c t i v i t y h i g h e r D-R surface area than Manvers. u n i t area is s t i l l

widely different

does g e n e r a l l y have a

On the o t h e r hand, the r e a c t i v i t y

per

f o r the two chars (Table I , column 6).

e v e r , there is a reasonable c o r r e l a t i o n o f surface area w i t h r e a c t i v i t y

How-

up to

b u r n - o f f va]ues of about 50%. T h e r e a f t e r , the two parameters diverge s h a r p l y . These phenomena are i l l u s t r a t e d

in the r e a c t i v i t y

shows values f o r surface area a t d i f f e r e n t

p l o t o f Figure I which a l s o

values o f carbon b u r n - o f f .

(column 2) shows values f o r s p e c i f i c r a t e using the D-R surface area. notable anomaly is seen f o r the Markham char where r e a c t i v i t y sharply a t high values of b u r n - o f f w h i l e surface area is s t i l l

is f a l l i n g

Table I The most off

increasing.

I t is p o s s i b l e t h a t the concept of a c t i v e surface area (ASA) (18) w i t h accessibility

t o a c t i v e s l t e s c o n t r o l l e d by mesopores may be more rewarding and i t

one t h a t we s h a l l pursue f o r f u t u r e work. Thus, w i t h the l i m i t e d evidence presented here, i t definitive

is not p o s s i b l e to be

about c o r r e l a t i o n between micropore surface area and r e a c t i v i t y .

Some l i m i t e d c o r r e l a t i o n

does occur f o r the e a r l i e r

does not hold over complete b u r n - o f f .

p a r t o f the r e a c t i o n but

is

110 SUMMARY AND CONCLUSIONS • Two coal chars of the same rank have remarkably d i f f e r e n t r e a c t i w t i e s . Markham Main is almost twice as reactive as Manvers Barnborough. • D i f f e r e n t shapes of the r e a c t i v l t y plots indicate differences in structure. Markham with a more closed structure than r4anvers, reaches it s maximum l a t e r at about 4 5 burn-off. • Changes in r e a c t i v i t y plots are only p a r t i a l l y p a r a l l e l e d by surface area changes f o r both chars•

I t seems that active surface areas (ASA) may be more

important in c o n t r o l l i n g r e a c t i v i t y • • The constant plateau of r e a c t i v i t y of Manvers a f t e r the maximum is reached indicates a constant dispersion of active sites. The decrease in r e a c t i v i t y of Markham a f t e r the maximum is reached indicates e i t h e r : I) a decrease in active s i t e dispersion; or 2) a decline in a c t i v i t y of active sites with burn-off. • F i n a l l y , ambiguity in porosity r e s u l t s , e s p e c i a l l y towards the end of gasif i c a t i o n indicates a complexity of i n t e r a c t i o n between structure and r e a c t i v i t y and other effects such as the c a t a l y t i c e f f e c t of mineral matter. Also, there appears to be a continuum of a l l types of pores throughout the chars up to nearly f u l l conversion•

Ambiguity also r e f l e c t s the d i f f i c u l t y

in characterising a complex material such as coal which has a f l e x i b l e ~19)'' structure•

Nevertheless, the results are i n t e r e s t i n g and show that more

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