Study of the pore structure and reactivity of Canadian coal-derived chars

Study of the pore structure and reactivity Canadian coal-derived chars Siauw

H. Ng, David

of

P. C. Fung and Sang D. Kim*

Research Scientist, Synthetic Fuels Research Laboratory, Energy CANMET, Energy, Mines and Resources Canada, Ottawa * Department of Chemical Engineering, Korea Advanced Institute Technology, Seoul, Korea (Received 19 June 1987)

Research of Science

Laboratories, and

The pore volume, surface area, compressibility and reactivity of 12 chars derived from Canadian coals ranking from lignite to anthracite, have been determined by mercury porosimetry, gas adsorptiondesorption with N, and CO, adsorbates, helium and mercury densities and thermogravimetric technique. A comparison of the pore structure between the chars and their parent coals was made based on the results

obtained from these techniques. The reactivity of chars was correlated with their physical and chemical properties. It has been found that a single relationship exists between the helium density and the carbon content of the coals and their chars. The chars are more porous but less compressible when compared with their parent (Keywords:

coals.

char;

pore structure: reactivity)

The depletion of oil reserves is a future worldwide problem and alternative energy sources are being sought. One potential alternative receiving increased interest is the direct or indirect conversion of coal or biomass into liquid fuels and the commercial production of gasoline from coal in South Africa proves that this is technologically practical. Recently, the integrated gasification combined cycle process has emerged as one of the most promising new technologies for coal-based electric power generation. The efficiency of any of these coal conversion processes depends largely on coal reactivity, which is greatly influenced by its chemical and physical properties. In a previous publication’, it was shown that the reactivity of a range of Canadian coals varied with their pore structure and coal rank. Chars derived from eight Canadian coals have also been studied with respect to the gasification kinetics and the impact on char reactivity by the parent coal rank, and by the CaO and MgO contents of the chars2. However, the pore system of these chars was not examined and so the object of this study was to determine the surface area and pore structure of a series of chars and examine the effect of these parameters on the char reactivity.

EXPERIMENTAL Muterials

The chars used consisted of the eight chars used previously’ and four additional chars prepared from Canadian coals; Onakawana (lignite) from Ontario, Bienfait (lignite) from Saskatchewan, Tulameen (subbituminous) from Alberta and Mt. Klappan (anthracite) from British Columbia. The devolatilization procedure was similar to that used to prepare the first batch of 8 chars2 but in this instance the coal samples (particle size 3-6 mm) were heated at a rate of 5°C min ’ 0016-2361/88/050700-07$3.00 0 1988 Butterworth &

700

Co. (Publishers) Ltd.

FUEL, 1988, Vol 67, May

under a flow of N, at 2 dm3 min-’ until a temperature of 9OOC was reached. Tables 1 and 2 show the chemical analyses of the four coals and their derived chars. The analyses of the other eight coals and their chars were reported previously2. For physical measurements, the particle size of the char samples was controlled at 425212 Ltrn (4-70 US standard mesh). Reacticity

measurements

The reactivity of char samples was determined thermogravimetrically using an electrobalance at 500°C with air as the gasification medium. A detailed procedure and the apparatus used have been described in an earlier publication2. Apparatus

The mercury densities and pore volumes in the (2.98-0.06 pm) and mesopore (0.06macropore 0.0036pm) ranges were determined using a commercial mercury porosimeter. The surface tension of mercury and contact angle were assumed to be 0.485 Nm-’ and 130”. The helium density measurement was obtained using a helium pycnometer. The N, and CO, sorption studies were conducted using a Carlo Erba Sorptomatic 1822. For N, sorption (77 K), the equilibration time was automatically controlled such that negligible change in pressure was observed over a 4 min period for each pressure point. For CO, adsorption (195 K) the equilibration time was constantly set for 1 h. Equilibrium pressure (P) was gradually increased until it reached 30 % or more of the saturation pressure (P,). The N, surface area and pore size distribution were determined using the BET equation and a procedure developed by Barrett, Joyner and Halenda3 in which the apparent pore radius was calculated by the Kelvin equation4. An effective cross-sectional area of 0.162 nm2 was assumed for the N, molecule.

Pore structure Table 1

Chemical

analyses

and reactivity

of Canadian

chars: S. H. Ng et al.

coal-derived

of coal and char samples Proximate

analysis

(wt 7;)

Ultimate

analysis

(wt Y;)

Ash

Volatile matter

Fixed carbon”

C

H

S

N

0

26.3 30.7

39.6 6.9

34.0 62.4

51.7 67.8

3.5 0.6

1.8 1.2

0.7 1.0

15.9

Bienfait (lignite) coal char

10.9 15.5

44.1 6.0

45.0 78.5

62.8 81.6

4.3 0.6

I.8 0.7

1.2 1.5

19.1 0.1

Tulameen coal char

15.2 23.9

32.4 2.5

52.4 13.6

64.2 73.4

4.2 0.7

4.2 0.5

1.5 1.4

10.7 0.2

21.6 24.5

9.1 1.6

69.4 74.0

69.4 73.4

2.1 0.7

0.4 0.4

1.0 1.1

5.5

Sample

(rank)

Onakawana coal char

(lignite)

(subbituminous)

Mt. Klappan coal char

(anthracite)

” Determined

by difference

Table 2

Chemical

analyses

of ashes in char samples

(wt “;)

Char sample

SiO,

Al@,

Fe@,

TiO,

p205

CaO

M&t

SO,

Na,O

K,O

Onakawana Bienfait Tulameen Mt. Klappan

11.8 3.9 16.1 16.0

3.7 2.4 3.3 4.8

2.2 1.3 2.1 1.3

0.3 0.2 0.2 0.3

0.1 0.2 0.1 0.1

6.2 2.8 0.2 0.3

1.7 0.6 0.2 0.4

3.0 1.8
0.4 1.7
0.2
The CO, surface area was calculated using both the BET equation and the Dubinin-Radushkevich-Kaganer sometimes referred to as equation 5,6 (DRK equation, Dubinin-Polanyi equation), in which the concept of micropore filling in the Dubinin-Radushkevich (DR) equation’ was replaced by surface coverage. Naturally, the DR equation could be used to calculate the micropore volume. The CO, effective cross-sectional area8 and the saturation vapour pressure’ of CO, at 195 K were assumed to be 0.195 nm’ and 186 kPa respectively. RESULTS

AND DISCUSSION

Char is a major product derived from pyrolysis of coal and thus its properties are greatly influenced by the constituents of its parent coal. The extent of change in a property depends on the nature of the coal and the devolatilization procedure used”. Under the fixed condition described previously2, the eight Canadian coalderived chars exhibited some gasification behaviour, which can be correlated with the chemical properties of their parent coals. This has been investigated further by examining the effect of the physical properties of 12 chars originating from coals of different rank. The char samples were first characterized by the mercury porosimetry technique. Figure I shows typical intrusion curves of two chars, one high and one low in porosity. The shape of each curve indicates that at low pressure, the interparticle voids of the char are rapidly filled up by mercury and at high pressure the char is compressed. The combined macro- and mesopore volume was obtained from the difference between the intrusion volume at 413 kPa, and the point at the intercept line (at 413 kPa) extrapolated from the high pressure curve. It should be noted that both the combined volume and the

0.8

0.6

0.4

0.3

0.2

0.1

I

0

0.5

100

I

I

I

200

Hg PRESSURE,

Figure 1 Mercury intrusion Onakawana char; 0, Canmore

curves char

I

I

300

MPb chars:

0,

gradient of the intercept line, which reflect compressibility of the char, are much greater for Onakawana char (low rank lignite char) than those the Canmore char (high rank semi-anthracite char). gradual decrease in the combined volume and

the the for The the

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1988,

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Vol 67, May

701

Pore structure Table 3

Carbon

and reactivity content

of Canadian

and physical

properties

coal-derived

Helium accessible pore volume (ml g-r)

Onakawana Bienfait Coronach Tulameen Sundance Prince Coalspur Devco Mt. Klappan Balmer McIntyre Canmore

70.2 70.5 70.6 75.7 75.7 78.1 19.5 87.1 88.5 89.5 90.5 92.0

Surface area (m* g-‘)

_

(wt%) Parent coal

S. H. Ng et al.

of chars

Cdaf

Char sample

chars:

Char

Helium density (g ml-‘)

Mercury density (gml-‘)

Open porosity (~01%)

97.9 96.6 97.8 96.4 99.4 94.2 98.3 97.0 97.1 97.6 98.1 97.8

2.130 2.020 2.089 1.937 2.177 2.060 2.057 1.913 2.096 1.936 2.023 1.761

0.993 1.293 1.354 1.048 1.491 1.493 1.510 1.749 1.420 1.626 1.562 1.632

53.4 36.0 35.2 45.9 31.5 27.6 26.6 8.6 32.3 16.0 22.8 7.3

Total

Macropore and Mesopore Micropore

Gradient x 105 (mlg-’ MPa-‘)

FiT

co, BET

co, DRK

0.537 0.278 0.260 0.438 0.211 0.185 0.176 .0.049 0.227 0.099 0.146 0.045

0.274 0.093 0.066 0.267 0.05 1 0.112 0.062 0.040 0.088 0.07 1 0.099 0.030

16.6 16.8 10.0 13.9 11.6 10.5 5.1 4.7 11.5 4.3 3.9 6.3

134 38.2 79.9 41.1 81.5 4.01 1.89 1.16 6.73 1.12 2.17 0.95

219 246 241 212 184 20.6 46.5 5.91 I21 5.35 10.5 5.58

399 326 348 287 230 23.7 67.1 6.42 176 6.68 15.6 5.71

0.263 0.185 0.194 0.171 0.160 0.073 0.114 0.009 0.139 0.027 0.047 0.015

60

(B) 0

0

I

I

I

I

.2

.4

.6

.8

RELATIVE

PRESSURE,

&

Figure 3 Gas sorption isotherms of Sundance char: 0, N, adsorption and 0, N, desorption at 77 K; n , COz adsorption at 195 K

I”

o-

60

70

Cd“

80

OF

PARENT

90

COAL,

wt%

Figure 2 Variation of open porosity (A) and pore volume (B) with carbon content of coal-derived char. In (B): 0, total and 0, micropore volume

gradient of the intercept line with increasing coal rank can be seen in Table 3. The mercury density of the chars was measured at 413 kPa assuming that the intrusion volume at this pressure excluded the interparticle voids but included the intraparticle pores I1 . The combination of this apparent density and the ‘true’ density determined by the helium displacement method yielded the total pore volume from which the micropore volume could be calculated by difference. Open porosity defined as the percentage of

702

FUEL, 1988, Vol67,

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total volume of open pores (cavities or channels communicating with the surface of the solids) relative to the char volume could also be calculated. The results are shown in Table 3 and represented graphically in Figure 2. It is seen that as the rank of the parent coals increases, both the open porosity and the ‘He accessible’ pore volume of the chars decrease. It is interesting to note that chars and their parent coals show the same trends in this respect’, except that chars are much more porous but less compressible. Evidently, more open pores were generated during the devolatilization process with the low rank coals showing the most pronounced effect. Figure 3 depicts sorption curves for the Sundance char using N, and CO2 as adsorbates. The N, sorption curves have a characteristic feature of hysteresis loop which can be classified as Type IV isotherm for mesoporous solids’ 2 In contrast, N, isotherms of coals appear to be of Type 11, which is characteristic of less porous solids’. The existence of a hysteresis loop in the sorption isotherm is

I

Pore structure

and reactivity

300

Y 250

\i8 \

7 “Em 200

is !G

150

Y iz 5 VI

loo

50

0 60

70

Cdaf OF PARENT

80 COAL,

90

wt %

Figure 4 Relationship between surface area and carbon coal-derived char: 0, N,; 0, CO, surface area

content

of

explained by one theory as having ‘ink bottle’ pores13~l 6 where capillary condensation and evaporation of the adsorbate take place at different relative pressures. Both N, and CO, surface areas of char samples were calculated from adsorption curves. The results are shown in Tuble 3. It appears that a linear correlation exists between the CO, surface areas of the chars calculated from BET and DRK methods. Consequently, only BET surface areas will be considered in this paper. The relationship between surface area, and the rank of the parent coals is depicted in Figure 4. In general, the surface area decreases sharply as the coal rank increases until a certain value of Cdal is reached. In this study, there seems to be a ‘parabolic’ type correlation between the N, and the CO, surface area (Figure 5) with the former always smaller than the latter. This phenomenon has been explained’ ’ on the grounds that 1. 2.

3.

CO, is a smaller molecule (3.3 A) than a N, molecule (3.64 A); a CO, molecule has higher kinetic energy and thus higher diffusion rate at 195 K than the N, molecule at 77 K: this will permit the CO, to be more accessible than the N, to the micropores; some pores at 77 K may shrink to such an extent that they are no longer accessible to N, molecules.

Other interpretations can also be found in the literature that CO, adsorption may be influenced by the quadrupole moment of CO, molecule interacting with the oxygen functionalities present on the carbon surface18, and that higher surface area may be caused by a CO,-induced swelling effect’ 9. The observation that CO, surface area is always larger than that of N, for chars applies also to the 12 parent coals of this study (unpublished data). Comparison of a char with its parent coal shows that the char has much larger N, surface area until the wt % Cdaf of parent coal reaches z 78 % (Figure 4). Beyond that, the N, surface

of Canadian

coal-derived

chars:

S. H. Ng et al.

areas of both char and coal are small and comparable’. The CO, surface area of a low rank coal is also smaller than that of its corresponding char. Both the surface areas of coal and char decrease with increasing Cdaf. However, the CO2 surface area of the coal rises again after a minimum is reached, but that of the char continues to decline. Thus at high Cdal, the char has a lower CO, surface area than its parent coal. At present one may speculate that the polar interaction of CO, with 0, groups and/or CO,-induced swelling of coals may play an important role in causing the rise of the CO, curve. Also, CO, seems to diffuse at different rates into the pores of coals of different ranks. Nandi and Walker” estimated that for an arbitrary equilibration time of 30 min for each sorption point, anthracite coal would have a much higher percentage of the pore volume filled with CO, at 195 K than a bituminous coal. The fact that considerably good correlation exists between N, and CO, surface areas for chars (Figure 5) but not for coals’ ’ suggests that coals may contain some interfering components which are removed during the devolatilization period. Pore volume can also be derived from CO, adsorption (using DR equation’) and N, desorption (using the BJH’ and the Kelvin4 equations). The resulting ‘CO, accessible’ micropore volume and ‘N, accessible’ pore volume are shown in Table 4 along with the ‘He accessible’ pore volume. It is apparent that for the same char the value obtained from the He technique is the greatest followed by those from the CO, and N, sorption methods. In this study the pore volume obtained from the N, desorption branch covers the widest diameter range (20 to 600 A) and yet its value is, in general, the smallest of the three for the same char sample. This suggests that many of the pores in chars have openings less than 20 A, which are inaccessible to N, at 77 K but accessible to He (the smallest molecule among the three) at room temperature. Some of these small pores ( ~20 A) may have resulted from bigger pores due to pore shrinkage at very low temperature.

140

120 -

CO, SURFACE Figure 5 Correlation coal-derived char

AREA,

rn2g-’

between N, surface area and CO, surface area of

FUEL, 1988, Vol 67, May

703

Pore structure Table 4 methods

and reactivity

Comparison

of micropore

Micropore

of Canadian volumes

volume

coal-derived

obtained

by different

(ml g- i)

Char sample

He accessible

co*

accessible

N, accessible pore volume (ml g-‘)

Onakawana Bienfait Coronach Tulameen Sundance Prince Coalspur Devco Mt. Klappan Balmer McIntyre Canmore

0.263 0.185 0.194 0.171 0.160 0.073 0.114 0.009 0.139 0.027 0.047 0.015

0.136 0.111 0.119 0.098 0.078 0.008 0.023 0.002 0.060 0.002 0.005 0.002

0.108 0.043 0.059 0.045 0.077 0.015 0.008 0.009 0.026 0.008 0.014 0.009

chars:

S. H. PJg et al.

expected. This anthracite char also shows abnormally high surface area and pore volume with respect to the parent coal carbon content. The mechanism of char gasification under the conditions of this study has been explained previously using the shrinking core mode12. The reaction occurs on the outside surface of the char particle but the reaction surface will move inwards as the gasification proceeds, leaving an ash layer behind without changing the external

0 I

I

CO2

ACCESSll3LE

0

Figure 6 accessible

.04

I

1

I

.08 MICROPORE

I

I

Correlation of He accessible micropore micropore volume of coal-derived char

d_g-’

volume

FUEL, 1988, Vol 67, May

ACCESSIBLE

.2 MICROFORE

Figure 7 Effect of He micropore volume derived char: 0, N,; 0, CO, surface area

.3

VOLUME,

on surface

r&g-

area

of coal-

with CO,

In spite of the discrepancy in magnitude, a good correlation is obtained between He and CO, micropore volumes. This is shown in Figure 6, which indicates the existence of two linear regions. The first, with steeper slope, shows the greater difference between He and CO, micropore volumes of chars derived from the high rank coals. Figure 7 depicts the effect of He micropore volume on N, and CO, surface areas of the chars. As expected, surface area increases with increasing micropore volume. However below 0.13 ml g-’ there is little or no change in the N, surface area. This is probably due to the temperature effect which is important in the gas sorption technique but not in the He pore volume measurement. The relationship of reactivity of coal-derived chars with their parent coal ranks has been reported previously2. The char reactivity is defined as the ratio of the reaction rate observed in the initial linear region in a mass conversion-time plot over the initial mass of the char on dry and ash-free basis. It is apparent from Figure 8 that three of the additional chars fit the linear semilog plot between the reactivity and the carbon content of parent coal but the Mt. Klappan char has higher reactivity than

704

He

.I6

.I2 VOLUME,

.I

I5

1.0

F

i

60

70 Cdaf OF

Figure 8 Dependence parent coal

80 PARENT

of char reactivity

90

COAL,wt% on the carbon

content

of the

Pore structure and reactivity

of Canadian

be attributed 1. 2. 3.

v

SURFACE Figure 9 Correlation derived chars: 0, N,;

200

loo

AREA,

300

m2g“

between reactivity and 0. CO, surface area

surface

area

coal-derived

chars:

S. H. Ng et al.

to three main factors:

concentration of active sites (which usually exist in micropores); accessibility of the reactant gas to active sites (in this case it will require more feeder pores); catalysis.

It is noted that all these factors prevail for lignite chars. In an earlier study’ on physical properties of Canadian coals, a correlation was established between helium density and the carbon content of the coals of different origin (Canadian, Japanese, American, British and Czechoslovakian coals). An attempt was made to determine whether the same correlation applies to the chars of this study. The helium densities of the coalderived chars examined were corrected to the mineralmatter-free basis using a density value of 2.7 g ml-’ for the mineral matter”. After correction, the data points of chars were plotted along with those of their parent coals (Figure 11). As expected, the helium density of the char, or coal, decreases initially with increasing carbon content, passes through a minimum and increases to reach a value of 2.25 for pure graphite22.

of coal-

dimension of the char particle. Previous experimental data further confirmed that the initial stage of reaction is kinetically controlled by chemical reaction at the surface of the core and the later stage by the gas diffusion through the ash. It is thus expected that the pore structure of the char will have great impact on the char reactivity. Figure 9 indicates the relationship between the char reactivity and the surface areas measured by two gas adsorption techniques. In both cases, the char reactivity increases rapidly, then slows down gradually and finally rises sharply as the surface area increases. The fast increase at the final stage is attributed to the fact that the three lignite chars (Onakawana, Bienfait and Coronach) with high surface areas are comparatively rich in calcium (6.17, 2.84 and 4.24 wt % CaO respectively), which may induce catalytic gasification2,‘0*21. WalkerlO reported that the carboxyl groups in lignites decompose upon pyrolysis, liberating CO, and resulting in well dispersed metals or metal oxides which act as catalysts for char gasification. The most significant in-situ catalyst in chars produced from low rank coals is CaO because of its abundance and better dispersion during pyrolysis. In contrast, Ca in the higher rank coals is present primarily as larger particle of calcite. Other metals such as Mg, Na, K and Fe have also been reported as good catalysts, but with some uncertainty2,’ ‘v21. The effect of He pore volume on char reactivity is shown in Figure 10. It can be seen that the char reactivity increases with both total and micropore volumes. Its relationship with the total pore volume is linear, but that with the micropore volume deviates from linearity when the three lignite chars are included in the plot. The pronounced effect of the micropore volume of the lignite chars may be attributed to the presence of more feeder pores (macro- and meso-pores) as indicated in Figure 2B. Walker” suggested that the differences in reactivity can

CONCLUSION It has been found that the open porosity, total pore volume, micropore volume, N, and CO, surface areas, gasification reactivity and the apparent compressibility of chars derived from 12 Canadian coals decrease with increase in the carbon content of the coals. Compared with their parent coals, the chars are more porous (especially lignite chars) but less compressible. Lignite chars are characterized by high concentrations of feeder pores, active sites and Ca, which may act as a catalyst in gasification. The combination of these factors has greatly enhanced the reactivity of lignite chars. The char

.

0

.2

.4

He ACCESSIBLE

Figure 10 coal-derived

PORE VOLUME,

Relationship between reactivity and char: 0. total volume; 0, micropore

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1988,

.6 mi_g-’

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volume

Vol 67, May

705

of

Pore structure

79

2.0

coal

Canadian

char

h Japanese q

19 -

of Canadian

coal-derived

.

o Canadian l

-

and reactivity

chars:

chars when the helium density iscorrected content of the char.

REFERENCES

8 American cool, Gan etol

1

2 3 4

5 6 I 8 9 10

15-

1.4 -

11 12 13

1.3 -

1.2 ’

70

1

I

,o

80

,

I

I

90

Cdaf OF COAL / CHAR, wt % Figure 11 and carbon

Relationship between helium density content of coal/coal-derived char

(mineral

matter

free)

reactivity can be correlated with surface areas and pore volumes determined by different methods. The previously found relationship between the helium density and the carbon content of the coals can be further extended to

706

for the mineral

cool, Todo

Foreign cool, Todo

I

S. H. Ng et al

FUEL, 1988, Vol 67, May

14 IS 16 17 18 19 20 21 22

Ng, S. H., Fung, D. P. C. and Kim, S. D. Fuel 1984,63, 1564 Fung, D. P. C. and Kim, S. D. Fuel 1984,63, 1197 Barrett, E. P., Joyner, L. G. and Halenda, P. H. J. Am. Chem. sot. 1951,13, 373 Defay, R., Prigogine, I., Bellemans, A. and Everett, D. H. in ‘Surface Tension and Adsorption’, Longmans, London, 1966, p. 218 Kaganer, M. G. Zhur. Fiz. Khim. 1959, 33, 2202 Gregg, S. J. and Sing, K. S. W. in ‘Adsorption, Surface Area and Porosity’, Academic Press, London, 1982, p. 228 Gregg, S. J. and Sing, K. S. W. in ‘Adsorption, Surface Area and Porosity’, Academic Press, London, 1982, p. 218 Medek, J. Fuel 1977, 56, 131 Debelak, K. A. and Schrodt, J. T. Fuel 1979,5& 734 Walker Jr., P. L. in ‘Fundamentals of Thermochemical Biomass Conversion’ (Eds. R. P. Overend, T. A. Milne and L. K. Mudge) Elsevier, Amsterdam, 1985, p. 485 Gan, H., Nandi, S. P. and Walker Jr., P. L. Fuel 1972,51, 272 Brunauer, S., Emmett, P. H. and Teller, E. J. Am. Chem. Sot. 1938,60, 309 Kraemer, E. 0. in ‘A Treatise of Physical Chemistry’ (Ed. H. S. Taylor), MacMillan, New York, 1931, p, 1661 McBain, J. W. J. Am. Chem. Sot. 1935, 57, 699 Rao, K. S. J. Phys. Chem. 1941,45, 506, 5 17 Katz, S. M. J. Phys. Chem. 1949, 53, 1166 Mahajan, 0. P. in ‘Coal Structure’ (Ed. Robert A. Meyer), Academic Press, New York and London, 1982, p. 51 Deitz, V. R., Carpenter, F. G. and Arnold, R. B. Carbon 1964,1, 245 Reucroft, P. J. and Patel, K. B. Fuel 1983, 62, 279 Nandi, S. P. and Walker Jr., P. L. Fuel 1964,43, 385 Walker Jr., P. L., Matsumoto, S., Hanzawa, T., Muira, T. and Ismail, I. M. K. Fuel 1983, 62, 140 Handbook of Chemistry and Physics, published by the Chemical Rubber Co., 52nd edition, 1971