- Email: [email protected]
Microporous structure of chars produced by pyrolysis of preoxidized coals
Journal
of Analytical and Applied 34 (1995) 13-28
ELSEVIER
JOURNALOI ANALYTICALand APPLIED PYROLYSIS
Pyrolysis
structure of chars produced by pyrolysis of preoxidized coals
Microporous
*, J.J. Pis, J.A. Pajares,
T.A. Centeno lns~itu~o
National
Received
de1 Carhdn.
30 September
Apartado
A.B. Fuertes
73. 33080-Orirk),
1994, accepted
10 October
Spain
1994
Abstract In this work, the effect of coal oxidation and pyrolysis conditions, coal rank and particle size on the microporous structure of chars was analyzed. The study was based on two bituminous coals of different rank: a low-volatile bituminous coal (Figaredo) and a mediumvolatile bituminous coal (Santa BLrbara). Sizes of coal particles between 48 and 855 pm were used. Coals were oxidized in air at several temperatures (150&27O”C) over different time periods (6 h- 15 days). Chars were obtained from coal pyrolysis at temperatures between 600 and 850°C. The carbonized materials were analyzed by adsorption of CO, at O‘C and N, at - 196°C. The experimental results showed that both the duration and temperature of coal oxidation determine the development of the microporous structure of the carbonized materials. Coal oxidation temperature was the variable that most significantly affected the microporous development of chars. The coal oxidation process allowed the size of constrictions at the entrance of the micropores to be altered. In this way, the micropores of chars from oxidized coals were more accessible in spite of being narrower (average micropore width around 0.8 nm). Kqwords:
Chars;
Coals;
Microporosity;
Oxidation;
Pyrolysis
I. Introduction The market
present
interest
trend
for these
* Corresponding Switzerland.
author.
0165-2370/95/$09.50 SSDI
in the
Present
8
manufacture
materials.
In fact,
address:
1995 - Elsevier
0165-2370(94)00863-9
Institut
and there
uses
of active
carbons
is a continuous
de Chimie,
Avenue
increase
Rellevaux.
Science B.V. All rights reserved
reflects
the
in the use
51. 2000 Neuchatel.
14
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (I9951 13-28
of active carbons in water treatment, air pollution and materials recycling, also being used as catalysts and catalyst carriers [ 11. The research into these materials is focused on their fundamental characterization (porosity and surface properties) and on how the properties of these carbons are affected by the type of material precursor and conditions of manufacture [2]. The manufacture of active carbons involves carbonization of the raw material under an inert atmosphere and activation of the carbonized product. Coals and good precursors of active carbons since they can develop a highly porous structure when they are heat treated. However, in the particular case of bituminous coals, heat treatment leads to materials with a poor porous structure [3]. The thermoplastic properties characteristic of these coals favour the alignment of the polycondensed aromatic molecules during the pyrolysis process. This leads to micropore closing and an increase in the graphitic nature of carbonized material. As a result, char particles have relatively inaccessible surface areas. However, chars obtained from coals which do not exhibit plastic properties during the pyrolysis step have less ordered structures and a more accessible microporous structure. Under such circumstances, the gasification of these materials will be carried out in a more uniform way. Evidently, for obtaining an optimum texturai development of bituminous-coals chars it is necessary to control the structural modifications during the pyrolysis, avoiding the plastic stage of coal. As has been reported by different authors [4,5], one method of controlling the thermoplastic properties is by oxidation of coking coal in air at temperatures below the softening range. Although the characteristics of coal play an important role in the textural properties of an active carbon, the pyrolysis and activation conditions significantly influence the porosity of the resulting materials. Knowledge of the relations between the properties and manufacturing methods should help in obtaining active carbons with specific characteristics. Thus, information about the influence of coal oxidation and pyrolysis conditions upon char texture will be useful in controlling the characteristics of active carbons. A systematic analysis of the development of the porous structure in chars obtained from oxidized coals was undertaken. The work presented in this paper describes the effect of coal oxidation and pyrolysis conditions, particle size and coal rank upon the development of porosity in bituminouscoal chars.
2. Experimental 2. I. Raw coal characteristics Two coals from the Central Asturian Basin (North Spain) were used as starting materials: a low-volatile bituminous coal, Figaredo, and a medium-volatile bituminous coal, Santa Barbara. The main characteristics of both coals are given in Table 1 (chemical composition) and Table 2 (fluidity and swelling properties). Figaredo coal has a low mineral matter content (ash content: 3.9%) and a volatile matter content of around 17% (daf). This coal shows very low plasticity values. The Santa
T.A. Centeno et al. /J. Anal. Appl. Pyr+is
Table I Analyses
34 (1995) 13-28
I5
of raw coals
Coal
Figaredo Santa Barbara
Proximate
Figaredo Santa Barbara
((%I)
Ultimate
analysis
(‘Xl, daf)
Moisture
Ash (dry)
V.M. (daf)
C
H
N
S
0 (ditf.)
0.7 0.7
3.9 5.7
17.1 24. I
91.2 90.6
4.2 4.8
I .8
0.6 2.0
2.3
I.6
Table 2 Plastic characteristics Coal
analysis
1.u
of raw coals Arnu
test
Gieseler
test
T, a
T,,”
T,’
ad
b’
T, .’
( “C)
(‘C)
(“(3
(‘%I)
(‘I/;,)
(“C)
T, ’ ( C)
7; g ( C)
(div/min)
462 384
500 419
500 493
-26 -35
-26 133
402
464
503
0 1717
L’Initial softening temperature. ’ Temperature dilatation. ’ Maximum contraction. ’ Maximum temperature. h Maximum fluidity.
of maximum contraction. L Temperature dilatation. f Maximum fluid temperature.
F,nh
of maximum g Solidification
Barbara coal has a lower carbon content, and higher volatile matter and oxygen contents. Moreover, it exhibits marked plastic characteristics, i.e. both swelling (Arnu) and fluidity (Gieseler). Both coals were crushed and sieved, size fractions of 32-63, 125- 180 and 710-1000 pm being used in all subsequent experiments. 2.2. Oxidation Coal samples of approximately 40 g were placed in square trays ( 12 x 12 cm), the bed height being around 3 mm. These were heated at 150, 200, 230 and 270’ C in a laboratory oven with forced air convection. Coal oxidation was carried out for periods of time between 6 h and 15 days. 2.3. Carbonization To pyrolyze the non-oxidized and oxidized coal samples, around 3 g were placed in a quartz reactor inserted in an electric oven. A thermocouple was used to register the temperature inside the reactor. Coal pyrolysis was carried out under a stream of N,. The temperature was raised at a rate of around lOO”C/min up to temperatures between 600 and 850°C. The temperature was kept constant for 1 h. The heat treatment temperature of coal for char preparation is referred to as the HTT.
16
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (I9951 13-28
2.4. Char characterization The porous structure of carbonized materials was determined from the adsorption isotherms of CO, at 0°C and N, at - 196°C. A Carlo Erba volumetric apparatus, Sorptomatic 1900, was used for CO, adsorption, while N, adsorption was carried out in a Micromeritics ASAP 2000. A sample of 1 g was used for each run. The micropore volume, W,, and characteristic energy, E,,, were calculated from COZ adsorption data according to the Dubinin-Radushkevich (D-R) equation: W = W0 exp] - (A l/K)
“I
(1)
where W represents the volume filled at temperature T and relative pressure p/p,,. W, is the total volume of micropores, A = RT In(p,/r)), and E,, and B are specific parameters of the system. W, and E, were obtained from the linear section of a plot of In W vs. (A //I)‘. The accessible micropore width was derived using the following empirical relation [ 61: L(nm)
= 10.8/(E,(kJ/mol)
where L is the average width calculated by the expression s,i(m’/g)
- 11.4) of the micropores.
(2) The
micropore
surface
= 2103 W,(cm3/g)/L(nm)
assuming micropore
slit-shaped and open micropores [6]. In order to obtain the spread distribution, 6, the Dubinin-Stoeckli (D-S) equation [2] was used.
3. Results
and discussion
was
(3) of
3. I. Influence of duration of coal oxidation Tables 3 and 4 compile the data obtained for the Santa Bgrbara and Figaredo chars from the CO2 adsorption. The carbonized materials obtained from the non-oxidized coals show an incipient microporous structure. Thus, the micropore volume of chars from fresh coal are: 0.03 cm3/g for char corresponding to Figaredo coal and 0.10 cm3/g for Santa Barbara char (d, = 125-180 pm). Similar values are observed for the other particle sizes. The low adsorption capacity of the materials resulting from the carbonization of coking coals can be interpreted as a result of pore shrinkage from the reorganization of the internal structure during the heat treatment [ 7,8]. Figs. 1 and 2 show the CO2 adsorption isotherms of chars obtained from Santa Barbara coal and Figaredo coal (d, = 125- 180 pm) oxidized at 200°C for different periods of time. It is evident that coal preoxidation causes an increase in the capacity of adsorption of COZ onto carbonized materials. This indicates that new micropores accessible to CO, are formed in chars after oxidation of the raw coal.
T.A. Centeno et al. 1 J. Anal. Appl. Pyro+i.s
Table 3 Textural characteristics
4 Cm)
32 --63
of chars
from
Figaredo
4,
L
s
(cm’/@
(nm)
(I&,
114 3 l/4 3
19.9 24.5 23.6 24.5 24.6
0.04 0.19 0.21 0.17 0.18
I .4 0.8 0.9 0.8 0.S
43 457 466 405 448
0.22 0.00 0.00 0.00 0.03
l/4 3 114 3
18.8 24.2 23.4 22.2 23.5
0.03 0.15 0.20 0.15 0.19
1.5 0.8 0.9 I .o 0.9
41 360 442 296 416
0.23 0.00 0.00 0.00 0.00
Oxidation time
(days)
(days) _..._~
.-.--.. ____
Non-oxidized 270
Non-oxidized 270
coal
coal
200
coal
Eu (kJ/mol)
Oxidation temperature
200
125-180
obtained
34 (1995) 13-28
6
In relation to this, different authors [9-l l] stated that the increase in oxygen content in the raw coal implies an increase in the char microporosity. This evolution may be explained taking into account that coal oxidation produces the formation of C-C and C-O-C type cross-links between the polycondensed aromatic molecules during the pyrolysis step [7,8]. These bonds, thermally stable at high temperatures, decrease the mobility and arrangement of the carbonaceous macromolecules. Thus, graphitization during carbonization decreases as coal oxidation progresses, contributing to the development of the microporosity in the carbonized materials.
Oxidation time (days)
Oxidation time (days)
G ;;; E A%
0.09
0.06
3
0.00 0.00
0.01
0.02
0.03
0.00 0.00
0.04
0.01
0.03
0.04
P/PO
P/PO Fig. I. CO,-adsorption isotherms (0°C) of chars oxidized at 200°C for different periods of time.
0.02
obtained
Fig. 2. CO,-adsorption isotherms (0°C) of chars obtained oxidized at 200°C for different periods of time.
from
Figaredo
from Santa
Barbara
coal
(d, = 125
180 pm)
coal (d,, = 125
I80 pm)
T.A. Centeno et al. 1 J. Anal. Appl. Pyroiysis
18
Table 4 Textural characteristics
4 (pm)
32-63
of chars
obtained
Oxidation temperature
Oxidation time
(“C)
(days)
Non-oxidized 270
coal 114 112
I 3 200
125Sl80
Non-oxidized 270
114 112
coal l/4 112 3
230
l/4 l/2 3
200
114 112 3
150
710~1000
from Santa
Non-oxidized 270
112 3 15 coal
Bgrbara
34 (1995) 1.7-28
coal
E0 (kJ/mol)
w, (cm%)
s InI (m%)
6
(nm)
18.7 24.4 24.4 24.5 23.9 25.0 24.6 25.0 24.8
0.08 0.20 0.20 0.21 0.21 0.16 0.18 0.18 0.19
1.5 0.8 0.8 0.8 0.9 0.8 0.8 0.8 0.8
107 500 500 525 467 400 450 450 475
0.15 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.03
16.3 24.0 24.6 24.5 23.3 24.5 24.3 24.4 23.3 23.9 24.8 24.7 18.6 20.9 24.7
0.10 0.15 0.17 0.20 0.14 0.17 0.18 0.19 0.13 0.15 0.17 0.18 0.08 0.15 0.17
1.8 0.9 0.8 0.8 0.9 0.8 0.8 0.8 0.9 0.9 0.8 0.8 1.5 1.1 0.8
111 333 425 500 311 425 450 475 289 333 425 450 107 273 425
0.30 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.11 0.00 0.00
17.1 22.2 23.1 24.4 24.0
0.08 0.11 0.12 0.20 0.20
1.6 1.0 1.0 0.8 0.9
100 220 240 500 444
0.16 0.06 0.04 0.00 0.00
L
As shown in Fig. 1, the CO,-adsorption capacity of chars strongly increases during the first stages of coal oxidation. In this way, the oxidation of Figaredo coal (d, = 125- 180 pm) at 200°C for only 6 h implies an increase in the micropore volume of 0.12 cm3/g with respect to the non-oxidized coal char. However, if the coal oxidation is carried out until 3 days, the increase is only 0.04 cm3/g (Table 3). Fig. 2 shows a similar trend in micropore volume for chars obtained from Santa Barbara coal (d, = 125- 180 pm) oxidized at 200°C. No significant change is observed in the micropore volume of chars for periods of coal oxidation longer than 1 day. As has been observed by different authors [ 12,131, coal oxidation does not occur uniformly inside the particles. The extent of oxidation is controlled by oxygen diffusion through the coal microporous matrix. As a consequence, the
T.A. Centeno et al. /J.
Anal. Appl. Pyrolysis 34 (1995) 13-28
19
Fig. 3. Microphotographs of char particles from Santa Barbara coal samples (d, = 125-180 /lrn) at different degrees of oxidation (oxidation temperature = 270 C): (a) Oxidation time = 6 h ( I. Vesicle: 7. anisotropic core; 3, isotropic rim); (b) oxidation time = 3 days.
partially oxidized particles of coal exhibit a characteristic rim, its width depending on time/temperature oxidation conditions. During the pyrolysis stage, the oxidized rim of coal particles does not exhibit plastic properties while the particle non-oxidized core does. Thus, the char particles obtained from oxidized coals present two well-defined regions. This is illustrated by Fig. 3, in which two microphotographs, corresponding to sections of char particles obtained from coal samples oxidized to different extents, are shown. The char particle obtained from partially oxidized coal (at 270°C for 6 h) (Fig. 3(a) exhibits: (1) vesicle formation; (2) an optically
20
T.A. Cenieno et al. / J. Anal. Appl. Pyrolysis
-34 (1995) 13-28
anisotropic core; (3) an optically isotropic rim. In contrast, the char sample corresponding to a highly oxidized coal (at 270°C for 3 days) (Fig. 3(b)) shows an isotropic character without vesicle formation. This is evidence that the hole coal particle is affected by oxidation. The region with isotropic properties corresponds to an accessible structure with well-developed microporosity. The change in char microporosity with extent of coal oxidation will correspond to the growth of the coal oxidation rim. The marked increase in the micropore volume of chars during the first steps of oxidation corresponds to the rapid oxidation of the periphery of coal particles. However, once the external surface of coal is oxidized, the penetration of oxygen into the coal particles occurs by activated diffusion through the micropores, therefore being a very slow process [ 11,121. This fact may explain the subsequent slower increase in the micropore volume of carbonized materials obtained from coals oxidized for long periods of time. Study of the CO2 adsorption data plotted in the D-R coordinates may supply qualitative information on the microporous texture of chars. The D-R plots of chars from Santa Bhrbara coal oxidized at 270°C are represented in Fig. 4. The linearity of D-R (CO,) plots for these materials extends over a large range of relative pressures as is typical for carbons with narrow and uniform microporosity. The parameter E, (Dubinin’s characteristic energy) plays a fundamental role in the characterization of carbons, since it is related to the average width of the micropore system [2]. To estimate the effect of coal oxidation on the microporosity characteristics of carbonized materials, this parameter was calculated for chars obtained
+----7z
7
CO2
%-&A
aA AA
oo A
@'oA
AA
t
0
0
qA
0
0
0
o
0
A
Oxidation time (days) l
0 0 A
0 II4 l/2 3
Fig. 4. Characteristic curves for the adsorption of CO, (0°C) and N, ( - 196°C) on chars Bkbara coal (d, = 125-180 pm) oxidized at 270°C for different periods of time.
from Santa
T.A. Centeno et al. /J. Anal. Appl. Pyrolysis 34 (1995) 13.-28
1.6
K
0.8
9 0.4
0.0
0.0
0.4
0.8 x
Fig. 5. Micropore (s = L/2).
distribution
in chars
from Santa
1.2
1.6
2.0
(nm)
Bkbara
coal (dP = 125- 180 {lrn) oxidized
at ZOO’C
from coals with different degrees of oxidation. As shown in Tables 3 and 4, the value of E, is higher for char particles obtained from coal oxidized samples, suggesting pore narrowing, which is reflected in the derived micropore dimensions (L). This is illustrated by Fig. 5, in which the micropore distribution of chars from Santa Barbara coal (oxidation temperature = 200°C) vs. half-width (X = L/2) is represented. The carbonized materials corresponding to fresh coals show an average micropore width of 1.2 nm for Santa Barbara char and 1.8 nm for Figaredo char In contrast, the chars obtained from oxidized coals present a very homogeneous microporosity (6 z 0) with a micropore size of around 0.8-0.9 nm (Tables 3 and 4). Additionally, the surface area of the micropore walls, Smi, of chars changes with coal oxidation time as micropore volume. Further conclusions concerning the way in which the microporous structure of bituminous-coal chars develops after coal oxidation can be drawn when the results of CO2 and N, adsorption onto these materials are compared. The evolution in the volume of pores of chars accessible to CO, with coal oxidation time might mean that identical microporous systems are developed in these materials, after 1 day of coal oxidation at 270°C. Fig. 4, which presents the CO? and N, adsorption data plotted in the D-R coordinates, shows that this is certainly not true. For low degrees of coal oxidation, particularly for non-oxidized coal chars, the volumes of CO, adsorbed are larger than the corresponding values for N,. Similar results have been reported by other authors who compare N, and CO, adsorption onto very microporous carbons [ 141. This behaviour corresponds to the existence of activated diffusion effects in the N, adsorption. The reason may be that for short periods of coal oxidation there is a high contribution of micropores in char samples, but the entrance of these micropores is very narrow and inaccessible to N,. With an the volume of pores determined by N, increasing degree of coal oxidation, adsorption systematically increases, while micropores accessible to CO2 remain practically constant, the difference between the values of N, and CO? therefore
22
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (1995) 13-28
being smaller. Taking into account, as previously discussed, that coal oxidation does not imply the enlargement of micropores of carbonized materials (Tables 3 and 4) the fact that for long coal oxidation times the volume of pores accessible to N2 is increased suggests the partial elimination of constrictions at the entrance of the micropores as coal oxidation progresses. It is interesting to note that coal oxidation allows the constrictions size at the entrance of the micropores to be controlled. In this way, the microporous texture of chars can be modified by means of coal preoxidation. 3.2. Injhence of coal oxidation temperature It was found that the porous structure of chars is affected by both coal oxidation time and temperature. The latter is the most important factor determining the evolution of char microporosity. The textural characteristics of chars from coals oxidized at different temperatures are indicated in Tables 3 and 4. The development of char microporosity is enhanced by increasing the coal oxidation temperature. A broad variation in micropore volume after short oxidation times is observed as the coal oxidation temperature increases. When Santa Barbara coal (d, = 125- 180 pm), previously oxidized at 270°C for 6 h, is pyrolyzed, a material with a micropore volume of 0.16 cm3/g is obtained (Table 4). This corresponds to an increase of 60% with respect to the carbonized material obtained from fresh coal. In contrast, when oxidation of Santa Barbara coal is carried out at 150°C a similar value is reached after 15 days of coal oxidation (Table 4). The low microporosity of char corresponding to this coal oxidation temperature may be interpreted in terms of the slow diffusion of oxygen inside the coal particles. The increase in oxidation temperature allows the oxygen to diffuse more quickly to the inner part of the coal particle. In this way, coal thermoplasticity is eliminated to a large extent in the particle for relatively short oxidation times. It is also interesting to point out the evolution of char microporosity with coal oxidation time for series corresponding to coal oxidation temperatures higher than 200°C. As shown in Table 4, the modification of the micropore volume of chars corresponding to Santa Barbara coal oxidized at 200, 230 and 270°C presents similar trends. For a given oxidation time, the difference in micropore volume is around 0.01 cm3/g. However, as already reported in a previous paper [ 151,different behaviour is observed in the char microporosity for coal oxidation temperatures lower than 200°C. In Table 4 it is shown that the carbonized material obtained from Santa Barbara coal oxidized at 150°C for 12 h presents a micropore volume slightly lower than for the char from fresh coal. This fact may be explained considering that under these conditions of coal oxidation, the oxygen only affects the surface of the coal particle. In consequence, during the carbonization process, the swelling of the inner part of the coal particle is restricted by the oxidized superficial layer which presents no fluid properties. This would favour the arrangement of carbonaceous macromolecules and imply a decrease in char microporosity. At high oxidation temperatures ( > 200°C) or after long oxidation times at 150°C the oxygen will access deeper layers of coal particles, causing the thermoplastic
T.A. Cenleno ef al. 1 J. Anal. Appl. Pyro1vsi.r 34 (1995) 13-28
23
Coal Oxidation Temperature (” C) 0 270 0 230 A 200 ___
Fig. 6. Micropore
volume
of Santa
Bkbara
chars
vs. O/C atomic
ratlo
of coal (CI, = 115- 180 llni).
properties to disappear over a wider area. A representation of the CO,-micropore volume of chars vs. Santa Barbara coal atomic ratio (O/C) (Fig. 6) shows the grouping of these data, independent of coal oxidation conditions. This suggests that the important factor in coal preoxidation is not the coal oxidation conditions, but the degree of coal oxidation attained (evaluated from the atomic ratio O/C). The chars from coals oxidized at temperatures higher than 200°C show micropore widths around 0.8 nm (Tables 3 and 4). However, when the oxidation of Santa Barbara coal is carried out at 150°C the micropore widths of chars slightly decrease as oxidation progresses, coal oxidation times around of 15 days being necessary to obtain micropores of around 0.8 nm. To illustrate the effect of coal oxidation temperature on pore volume accessible to N,, Table 5 summarizes the percentages of pore volume filled by this adsorptive at relative pressures of 0.01 and 0.1 in the chars from Santa Barbara coal (d,, = 125 180 pm) oxidized at different temperatures for 3 days. It is shown that the percentages of pore volume filled by N, at lower relative pressures increase with coal oxidation temperature, while no significant difference in the microporosity of chars is observed from COz adsorption (Table 4). These results indicate, quantitatively, that the accessibility of pores progressively increase with coal oxidation Table 5 Fractions of pore volume filled by N,( - 196YJ) at different relative pressures for chars from coal Santa BBrbara oxidized at various temperatures (d, = 125- 180 wrn, Oxidation time = 3 days) Coal oxidation temperature (’ CI 0
150 230 270
0.0l/0.90
0. IO/O.90
0.25 0.41 0.43 0.57
0.38 0.54 0.73 0.82
24
T.A. Centeno et al. / J. Anal. Appl. Pyrolysis
34 (1995) 13-28
temperature. Clearly, higher temperatures of coal oxidation lead to the formation of micropores with less constriction. Molecular sieve carbons are known to possess a narrow pore size distribution with pores of bottle-neck shape [ 161. Generally, for manufacturing carbon molecular sieves, the suitable pore structure of the carbon is initially fixed by a controlled heat treatment followed by a change in the entrance to the pores [ 171. Small changes in the size of these constrictions in the micropore structure can impact the rate of diffusion of a gas to a significant extent. The results presented in this work indicate that the size of constrictions at the entrance to micropores can be controlled sufficiently by coal preoxidation. In this way, the oxidation of coal previous to the pyrolysis step may be an interesting way of obtaining carbon molecular sieves. 3.3. Influence of heat treatment temperature (HTT) Different authors have shown that the porous structure of coals can be altered by thermal treatment [ 181. The available surface area of heat treated coals has been shown to reach a maximum level at temperatures of 800-lOOo”C, resulting in a higher adsorption volume, and then to decrease at higher HTTs. With respect to oxidized coals, Siemieniewska et al. [lo] stated that chars from these materials present a more pronounced maximum in the micropore volume, which is displaced to higher pyrolysis temperatures with respect to chars obtained from fresh coals. Oda et al. [9] observed that coal preoxidation produces a marked increase in the micropore volume of chars up to temperatures around 7OO”C, followed by a decrease as a consequence of heteroatoms removal and the rupture of cross-links, resulting in a less porous structure. Parameters from the analysis of CO,-isotherms corresponding to chars obtained by pyrolyzing Santa Barbara coal (d,, = 125 180 pm) at different temperatures are reported in Table 6. The coal was previously subjected to oxidation at 200°C for 3 days. It is observed that pyrolysis temperature has a negligible effect on the COZ accessible pore volume. The micropore volume of chars obtained at different temperatures remains around 0.17 cm3/g. The formation of cross-links resistant to high temperatures, after the oxidation of coal under severe conditions, would explain that no decrease in the microporosity of chars exists between 600 and 850°C [9]. Table 6 Microporosity characteristics of chars obtained by pyrolysis at different coal. Raw coal (d, = 125- 180 pm) oxidized at 200°C for 3 days Heat treatment temperature (“C)
w, (cm3k)
600 700 800 850
0.17 0.17 0.17 0.18
s
temperatures
of Santa
6
G&g) 0.9 0.8 0.8 0.8
378 425 425 450
0.00 0.00 0.00 0.00
BBrbara
T.A. Centeno et al. / J. Anal. Appl. Pyrolysis
I
I
/
2
95
34 (1995) 1% 28
i
I
1
N2
co2
10-I
@
Tf,” C i 0 6. ,:,
ww Fig. 7. Characteristic curves of CO, different heat treatment temperatures.
600 700 800 850
(1O-3)
(0°C) and N,( -196°C) for Santa BBrbara chars obtained Raw coal (d, = 1255 180 pm) oxidized at 200°C for 3 days.
at
With respect to the microporosity characteristics, some authors [IO] have reported that the pore size distribution of chars corresponding to oxidized coals greatly depends on carbonization temperature. However, the results shown in this work indicate that the micropore size of chars from oxidized Santa Barbara coal is around 0.8 nm, indpendent of coal pyrolysis temperature (Table 6). Fig. 7 shows the characteristic curves of Santa Barbara chars obtained at four different temperatures. The differences observed between the volumes of CO, and N, adsorbed confirm the narrow microporosity of these materials. It is noted that the relative uptake of CO, at 0°C and N, at - 196’C by chars could be altered significantly by heat treatment under an inert atmosphere. In this way, a substantial decrease in the adsorption of N, occurs on increasing the carbonization temperature. This fact may be explained as a result of the alignment of aromatic macromolecules, which leads to micropore constrictions on increasingly severe thermal treatment, reducing the accessibility of the N, molecules to the microporous network. 3.4. Influence
of particle
diameter
The influence of particle diameter on char micropore development is determined by the eflect of this variable upon the extent of coal oxidation. As shown in Tables 3 and 4, the particle size does not significantly affect the microporosity of chars
T.A.
26
Centeno
et al. /J.
Anal.
Appl.
Pyrdysis 34 (1995) 13-28
from non-oxidized coals. However, this parameter has a great influence on the microporosity of chars obtained from oxidized coals. When an oxidized Santa Babara coal sample (27OC for 3 days) with an average particle diameter of around 48 pm is carbonized, a material with a micropore volume of 0.20 cm3/g is obtained. In contrast, if an average particle diameter of 855 pm is used, a similar value is reached after 3 days of coal oxidation at 270°C (Table 4). It is interesting to point out that for the manufacture of chars with the same micropore volume, the use of smaller size particles implies a drastic decrease in coal oxidation time. The evolution in the microporosity of chars with coal particle size can be explained, taking into account the lower accessibility of oxygen through the coal particle as the particle size is increased. As a consequence, longer coal oxidation times are required for obtaining equivalent degrees of coal oxidation as a larger particle diameter is utilized. In spite of the increase in micropore volume with coal oxidation time for chars obtained from coals with different particle diameters, the particle size does not modify the micropore characteristics of chars. Thus, the carbonized materials exhibit micropores of around 0.8-0.9 nm and very homogeneous microporosity (6 z 0) (Tables 3 and 4). The influence of particle diameter on the variation in accessibility of micropores to N2 is shown in Fig. 8 in which the characteristic curves (CO, at 0°C and N2 at - 196°C) of chars from Santa Barbara coal oxidized at 270°C for 3 days are represented. The average particle diameters are 45, 153 and 855 pm. It is shown that the elimination of micropore constrictions of chars by means of coal preoxidation takes place more effectively when smaller particles are used.
00
0 0 A
0
30
60
90
120 IS0 180’
400
3243 125480 7104000
800
CRT MpdpYf3)2
1200
1600
2000
(1 O-3)
Fig. 8. Characteristic curves of CO, (0°C) and N, (- 196°C) for chars different particle diameters oxidized at 270°C for 3 days.
from Santa
Bkrbara
coal with
T.A. Cenleno et al. 1 J. Anal. Appl. Pyrolysis 34 (1995) 13-28
31
4. Conclusions The microporous structure of chars obtained from bituminous coals can be modified by means of coal preoxidation. Coal oxidation at temperatures between 150 and 270°C gives rise to chars with well-developed microporosity. An increase in the micropore volume of carbonized materials from oxidized coals is observed. This increment is very marked during the first steps of coal oxidation and the development of char microporosity is enhanced by increasing the coal oxidation temperature. Additionally, coal preoxidation modifies the micropore size distribution: the chars obtained from oxidized coals exhibit very homogeneous microporosity with an average micropore width of around 0.8 nm. In contrast, coal preoxidation allows the size of the constrictions at the entrance of the micropores to be controlled, a progressive elimination of these being observed as the degree of coal oxidation increases. Higher HTTs imply a micropore constriction, reducing the accessibility to the microporous network of chars. The development of microporosity and elimination of micropore constrictions in carbonized materials by means of coal preoxidation take place more effectively when particles with small diameters are used. The effect of coal oxidation conditions, pyrolysis temperature and particle diameter upon the microporous structure of char allows the textural characteristics of carbonized materials to be controlled, with distinct subsequent uses.
Acknowledgements The authors wish to acknowledge Fundacion para el Foment0 en Asturias Tecnologia (FICYT).
the financial support received from the de la Investigation Cientifica Aplicada y la
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
[lo] [ 111 [ 121
H. Janjowska, A. Swiatkowski and J. Choma. in T.J. Kemp (Ed.), Active Carbon, Ellis Howood. London. 1991. R.C. Bansal, J.B. Donnet and H.F. Stoeckli, in Active Carbon, Marcel Dekker, New York. 1988. J.B. Howard, in M.A. Elliot (Ed.), Chemistry of Coal Utilization, Second Suppl. Vol., Wiley. New York. 1981, Chapter 12. D.J. Maloney. R.G. Jenkins and P.L. Walker, Jr., Fuel. 62, (1982) 175. J.J. Pis, A. Cagigas, P. Simon and J.J. Lorenzana, Fuel Process. Technol.. 20 (1988) 307. F. Stoeckli and L. Ballerini, Fuel, 70 ( 1991) 557. Y. Toda, Fuel. 52 (1973) 99. P.K. Singla, S. Miura, R.R. Hudgins and P.L. Silveston, Fuel, 62 (1983) 645. H. Oda, M. Takeuchi and C. Yokokawa, Fuel, 60 (1981) 390. T. Siemieniewska, K. Tomkow, A. Jankowska and M. Jasienko. Fuel, 64 ( 1985) 481. D.J. Maloney and R.G. Jenkins, Fuel, 64 (1985) 1415. H. Sommers and W. Peters, Chem. Ing. Technol., 26 (1954) 441.
28
T.A. Centeno et al. /J.
Anal. Appl. Pyrolysis
34 (1995) 13-28
[ 131 D.L. Carpenter and D.G. Giddings, Fuel, 43 (1964) 247. [14] F. Rodriguez-Reinoso and A. Linares-Solano, in P.A. Thrower (Ed.), Chemistry and Physics Carbon, Vol. 21, 1, Marcel Dekker, New York, 1989. [15] T. Alvarez, A.B. Fuertes, J.J. Pis, J.B. Parra and J.A. Pajares, Fuel, 73 (1994) 1358. [ 161 H. Jtintgen, Carbon, 15 ( 1977) 293. [ 171 S.K. Verma and P.L. Walker, Jr., Carbon, 30 (1992) 829. [18] P. Chiche, P.H. Brunner and P.V. Roberts, Carbon, 18 (1980) 217.
of
of Analytical and Applied 34 (1995) 13-28
ELSEVIER
JOURNALOI ANALYTICALand APPLIED PYROLYSIS
Pyrolysis
structure of chars produced by pyrolysis of preoxidized coals
Microporous
*, J.J. Pis, J.A. Pajares,
T.A. Centeno lns~itu~o
National
Received
de1 Carhdn.
30 September
Apartado
A.B. Fuertes
73. 33080-Orirk),
1994, accepted
10 October
Spain
1994
Abstract In this work, the effect of coal oxidation and pyrolysis conditions, coal rank and particle size on the microporous structure of chars was analyzed. The study was based on two bituminous coals of different rank: a low-volatile bituminous coal (Figaredo) and a mediumvolatile bituminous coal (Santa BLrbara). Sizes of coal particles between 48 and 855 pm were used. Coals were oxidized in air at several temperatures (150&27O”C) over different time periods (6 h- 15 days). Chars were obtained from coal pyrolysis at temperatures between 600 and 850°C. The carbonized materials were analyzed by adsorption of CO, at O‘C and N, at - 196°C. The experimental results showed that both the duration and temperature of coal oxidation determine the development of the microporous structure of the carbonized materials. Coal oxidation temperature was the variable that most significantly affected the microporous development of chars. The coal oxidation process allowed the size of constrictions at the entrance of the micropores to be altered. In this way, the micropores of chars from oxidized coals were more accessible in spite of being narrower (average micropore width around 0.8 nm). Kqwords:
Chars;
Coals;
Microporosity;
Oxidation;
Pyrolysis
I. Introduction The market
present
interest
trend
for these
* Corresponding Switzerland.
author.
0165-2370/95/$09.50 SSDI
in the
Present
8
manufacture
materials.
In fact,
address:
1995 - Elsevier
0165-2370(94)00863-9
Institut
and there
uses
of active
carbons
is a continuous
de Chimie,
Avenue
increase
Rellevaux.
Science B.V. All rights reserved
reflects
the
in the use
51. 2000 Neuchatel.
14
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (I9951 13-28
of active carbons in water treatment, air pollution and materials recycling, also being used as catalysts and catalyst carriers [ 11. The research into these materials is focused on their fundamental characterization (porosity and surface properties) and on how the properties of these carbons are affected by the type of material precursor and conditions of manufacture [2]. The manufacture of active carbons involves carbonization of the raw material under an inert atmosphere and activation of the carbonized product. Coals and good precursors of active carbons since they can develop a highly porous structure when they are heat treated. However, in the particular case of bituminous coals, heat treatment leads to materials with a poor porous structure [3]. The thermoplastic properties characteristic of these coals favour the alignment of the polycondensed aromatic molecules during the pyrolysis process. This leads to micropore closing and an increase in the graphitic nature of carbonized material. As a result, char particles have relatively inaccessible surface areas. However, chars obtained from coals which do not exhibit plastic properties during the pyrolysis step have less ordered structures and a more accessible microporous structure. Under such circumstances, the gasification of these materials will be carried out in a more uniform way. Evidently, for obtaining an optimum texturai development of bituminous-coals chars it is necessary to control the structural modifications during the pyrolysis, avoiding the plastic stage of coal. As has been reported by different authors [4,5], one method of controlling the thermoplastic properties is by oxidation of coking coal in air at temperatures below the softening range. Although the characteristics of coal play an important role in the textural properties of an active carbon, the pyrolysis and activation conditions significantly influence the porosity of the resulting materials. Knowledge of the relations between the properties and manufacturing methods should help in obtaining active carbons with specific characteristics. Thus, information about the influence of coal oxidation and pyrolysis conditions upon char texture will be useful in controlling the characteristics of active carbons. A systematic analysis of the development of the porous structure in chars obtained from oxidized coals was undertaken. The work presented in this paper describes the effect of coal oxidation and pyrolysis conditions, particle size and coal rank upon the development of porosity in bituminouscoal chars.
2. Experimental 2. I. Raw coal characteristics Two coals from the Central Asturian Basin (North Spain) were used as starting materials: a low-volatile bituminous coal, Figaredo, and a medium-volatile bituminous coal, Santa Barbara. The main characteristics of both coals are given in Table 1 (chemical composition) and Table 2 (fluidity and swelling properties). Figaredo coal has a low mineral matter content (ash content: 3.9%) and a volatile matter content of around 17% (daf). This coal shows very low plasticity values. The Santa
T.A. Centeno et al. /J. Anal. Appl. Pyr+is
Table I Analyses
34 (1995) 13-28
I5
of raw coals
Coal
Figaredo Santa Barbara
Proximate
Figaredo Santa Barbara
((%I)
Ultimate
analysis
(‘Xl, daf)
Moisture
Ash (dry)
V.M. (daf)
C
H
N
S
0 (ditf.)
0.7 0.7
3.9 5.7
17.1 24. I
91.2 90.6
4.2 4.8
I .8
0.6 2.0
2.3
I.6
Table 2 Plastic characteristics Coal
analysis
1.u
of raw coals Arnu
test
Gieseler
test
T, a
T,,”
T,’
ad
b’
T, .’
( “C)
(‘C)
(“(3
(‘%I)
(‘I/;,)
(“C)
T, ’ ( C)
7; g ( C)
(div/min)
462 384
500 419
500 493
-26 -35
-26 133
402
464
503
0 1717
L’Initial softening temperature. ’ Temperature dilatation. ’ Maximum contraction. ’ Maximum temperature. h Maximum fluidity.
of maximum contraction. L Temperature dilatation. f Maximum fluid temperature.
F,nh
of maximum g Solidification
Barbara coal has a lower carbon content, and higher volatile matter and oxygen contents. Moreover, it exhibits marked plastic characteristics, i.e. both swelling (Arnu) and fluidity (Gieseler). Both coals were crushed and sieved, size fractions of 32-63, 125- 180 and 710-1000 pm being used in all subsequent experiments. 2.2. Oxidation Coal samples of approximately 40 g were placed in square trays ( 12 x 12 cm), the bed height being around 3 mm. These were heated at 150, 200, 230 and 270’ C in a laboratory oven with forced air convection. Coal oxidation was carried out for periods of time between 6 h and 15 days. 2.3. Carbonization To pyrolyze the non-oxidized and oxidized coal samples, around 3 g were placed in a quartz reactor inserted in an electric oven. A thermocouple was used to register the temperature inside the reactor. Coal pyrolysis was carried out under a stream of N,. The temperature was raised at a rate of around lOO”C/min up to temperatures between 600 and 850°C. The temperature was kept constant for 1 h. The heat treatment temperature of coal for char preparation is referred to as the HTT.
16
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (I9951 13-28
2.4. Char characterization The porous structure of carbonized materials was determined from the adsorption isotherms of CO, at 0°C and N, at - 196°C. A Carlo Erba volumetric apparatus, Sorptomatic 1900, was used for CO, adsorption, while N, adsorption was carried out in a Micromeritics ASAP 2000. A sample of 1 g was used for each run. The micropore volume, W,, and characteristic energy, E,,, were calculated from COZ adsorption data according to the Dubinin-Radushkevich (D-R) equation: W = W0 exp] - (A l/K)
“I
(1)
where W represents the volume filled at temperature T and relative pressure p/p,,. W, is the total volume of micropores, A = RT In(p,/r)), and E,, and B are specific parameters of the system. W, and E, were obtained from the linear section of a plot of In W vs. (A //I)‘. The accessible micropore width was derived using the following empirical relation [ 61: L(nm)
= 10.8/(E,(kJ/mol)
where L is the average width calculated by the expression s,i(m’/g)
- 11.4) of the micropores.
(2) The
micropore
surface
= 2103 W,(cm3/g)/L(nm)
assuming micropore
slit-shaped and open micropores [6]. In order to obtain the spread distribution, 6, the Dubinin-Stoeckli (D-S) equation [2] was used.
3. Results
and discussion
was
(3) of
3. I. Influence of duration of coal oxidation Tables 3 and 4 compile the data obtained for the Santa Bgrbara and Figaredo chars from the CO2 adsorption. The carbonized materials obtained from the non-oxidized coals show an incipient microporous structure. Thus, the micropore volume of chars from fresh coal are: 0.03 cm3/g for char corresponding to Figaredo coal and 0.10 cm3/g for Santa Barbara char (d, = 125-180 pm). Similar values are observed for the other particle sizes. The low adsorption capacity of the materials resulting from the carbonization of coking coals can be interpreted as a result of pore shrinkage from the reorganization of the internal structure during the heat treatment [ 7,8]. Figs. 1 and 2 show the CO2 adsorption isotherms of chars obtained from Santa Barbara coal and Figaredo coal (d, = 125- 180 pm) oxidized at 200°C for different periods of time. It is evident that coal preoxidation causes an increase in the capacity of adsorption of COZ onto carbonized materials. This indicates that new micropores accessible to CO, are formed in chars after oxidation of the raw coal.
T.A. Centeno et al. 1 J. Anal. Appl. Pyro+i.s
Table 3 Textural characteristics
4 Cm)
32 --63
of chars
from
Figaredo
4,
L
s
(cm’/@
(nm)
(I&,
114 3 l/4 3
19.9 24.5 23.6 24.5 24.6
0.04 0.19 0.21 0.17 0.18
I .4 0.8 0.9 0.8 0.S
43 457 466 405 448
0.22 0.00 0.00 0.00 0.03
l/4 3 114 3
18.8 24.2 23.4 22.2 23.5
0.03 0.15 0.20 0.15 0.19
1.5 0.8 0.9 I .o 0.9
41 360 442 296 416
0.23 0.00 0.00 0.00 0.00
Oxidation time
(days)
(days) _..._~
.-.--.. ____
Non-oxidized 270
Non-oxidized 270
coal
coal
200
coal
Eu (kJ/mol)
Oxidation temperature
200
125-180
obtained
34 (1995) 13-28
6
In relation to this, different authors [9-l l] stated that the increase in oxygen content in the raw coal implies an increase in the char microporosity. This evolution may be explained taking into account that coal oxidation produces the formation of C-C and C-O-C type cross-links between the polycondensed aromatic molecules during the pyrolysis step [7,8]. These bonds, thermally stable at high temperatures, decrease the mobility and arrangement of the carbonaceous macromolecules. Thus, graphitization during carbonization decreases as coal oxidation progresses, contributing to the development of the microporosity in the carbonized materials.
Oxidation time (days)
Oxidation time (days)
G ;;; E A%
0.09
0.06
3
0.00 0.00
0.01
0.02
0.03
0.00 0.00
0.04
0.01
0.03
0.04
P/PO
P/PO Fig. I. CO,-adsorption isotherms (0°C) of chars oxidized at 200°C for different periods of time.
0.02
obtained
Fig. 2. CO,-adsorption isotherms (0°C) of chars obtained oxidized at 200°C for different periods of time.
from
Figaredo
from Santa
Barbara
coal
(d, = 125
180 pm)
coal (d,, = 125
I80 pm)
T.A. Centeno et al. 1 J. Anal. Appl. Pyroiysis
18
Table 4 Textural characteristics
4 (pm)
32-63
of chars
obtained
Oxidation temperature
Oxidation time
(“C)
(days)
Non-oxidized 270
coal 114 112
I 3 200
125Sl80
Non-oxidized 270
114 112
coal l/4 112 3
230
l/4 l/2 3
200
114 112 3
150
710~1000
from Santa
Non-oxidized 270
112 3 15 coal
Bgrbara
34 (1995) 1.7-28
coal
E0 (kJ/mol)
w, (cm%)
s InI (m%)
6
(nm)
18.7 24.4 24.4 24.5 23.9 25.0 24.6 25.0 24.8
0.08 0.20 0.20 0.21 0.21 0.16 0.18 0.18 0.19
1.5 0.8 0.8 0.8 0.9 0.8 0.8 0.8 0.8
107 500 500 525 467 400 450 450 475
0.15 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.03
16.3 24.0 24.6 24.5 23.3 24.5 24.3 24.4 23.3 23.9 24.8 24.7 18.6 20.9 24.7
0.10 0.15 0.17 0.20 0.14 0.17 0.18 0.19 0.13 0.15 0.17 0.18 0.08 0.15 0.17
1.8 0.9 0.8 0.8 0.9 0.8 0.8 0.8 0.9 0.9 0.8 0.8 1.5 1.1 0.8
111 333 425 500 311 425 450 475 289 333 425 450 107 273 425
0.30 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.11 0.00 0.00
17.1 22.2 23.1 24.4 24.0
0.08 0.11 0.12 0.20 0.20
1.6 1.0 1.0 0.8 0.9
100 220 240 500 444
0.16 0.06 0.04 0.00 0.00
L
As shown in Fig. 1, the CO,-adsorption capacity of chars strongly increases during the first stages of coal oxidation. In this way, the oxidation of Figaredo coal (d, = 125- 180 pm) at 200°C for only 6 h implies an increase in the micropore volume of 0.12 cm3/g with respect to the non-oxidized coal char. However, if the coal oxidation is carried out until 3 days, the increase is only 0.04 cm3/g (Table 3). Fig. 2 shows a similar trend in micropore volume for chars obtained from Santa Barbara coal (d, = 125- 180 pm) oxidized at 200°C. No significant change is observed in the micropore volume of chars for periods of coal oxidation longer than 1 day. As has been observed by different authors [ 12,131, coal oxidation does not occur uniformly inside the particles. The extent of oxidation is controlled by oxygen diffusion through the coal microporous matrix. As a consequence, the
T.A. Centeno et al. /J.
Anal. Appl. Pyrolysis 34 (1995) 13-28
19
Fig. 3. Microphotographs of char particles from Santa Barbara coal samples (d, = 125-180 /lrn) at different degrees of oxidation (oxidation temperature = 270 C): (a) Oxidation time = 6 h ( I. Vesicle: 7. anisotropic core; 3, isotropic rim); (b) oxidation time = 3 days.
partially oxidized particles of coal exhibit a characteristic rim, its width depending on time/temperature oxidation conditions. During the pyrolysis stage, the oxidized rim of coal particles does not exhibit plastic properties while the particle non-oxidized core does. Thus, the char particles obtained from oxidized coals present two well-defined regions. This is illustrated by Fig. 3, in which two microphotographs, corresponding to sections of char particles obtained from coal samples oxidized to different extents, are shown. The char particle obtained from partially oxidized coal (at 270°C for 6 h) (Fig. 3(a) exhibits: (1) vesicle formation; (2) an optically
20
T.A. Cenieno et al. / J. Anal. Appl. Pyrolysis
-34 (1995) 13-28
anisotropic core; (3) an optically isotropic rim. In contrast, the char sample corresponding to a highly oxidized coal (at 270°C for 3 days) (Fig. 3(b)) shows an isotropic character without vesicle formation. This is evidence that the hole coal particle is affected by oxidation. The region with isotropic properties corresponds to an accessible structure with well-developed microporosity. The change in char microporosity with extent of coal oxidation will correspond to the growth of the coal oxidation rim. The marked increase in the micropore volume of chars during the first steps of oxidation corresponds to the rapid oxidation of the periphery of coal particles. However, once the external surface of coal is oxidized, the penetration of oxygen into the coal particles occurs by activated diffusion through the micropores, therefore being a very slow process [ 11,121. This fact may explain the subsequent slower increase in the micropore volume of carbonized materials obtained from coals oxidized for long periods of time. Study of the CO2 adsorption data plotted in the D-R coordinates may supply qualitative information on the microporous texture of chars. The D-R plots of chars from Santa Bhrbara coal oxidized at 270°C are represented in Fig. 4. The linearity of D-R (CO,) plots for these materials extends over a large range of relative pressures as is typical for carbons with narrow and uniform microporosity. The parameter E, (Dubinin’s characteristic energy) plays a fundamental role in the characterization of carbons, since it is related to the average width of the micropore system [2]. To estimate the effect of coal oxidation on the microporosity characteristics of carbonized materials, this parameter was calculated for chars obtained
+----7z
7
CO2
%-&A
aA AA
oo A
@'oA
AA
t
0
0
qA
0
0
0
o
0
A
Oxidation time (days) l
0 0 A
0 II4 l/2 3
Fig. 4. Characteristic curves for the adsorption of CO, (0°C) and N, ( - 196°C) on chars Bkbara coal (d, = 125-180 pm) oxidized at 270°C for different periods of time.
from Santa
T.A. Centeno et al. /J. Anal. Appl. Pyrolysis 34 (1995) 13.-28
1.6
K
0.8
9 0.4
0.0
0.0
0.4
0.8 x
Fig. 5. Micropore (s = L/2).
distribution
in chars
from Santa
1.2
1.6
2.0
(nm)
Bkbara
coal (dP = 125- 180 {lrn) oxidized
at ZOO’C
from coals with different degrees of oxidation. As shown in Tables 3 and 4, the value of E, is higher for char particles obtained from coal oxidized samples, suggesting pore narrowing, which is reflected in the derived micropore dimensions (L). This is illustrated by Fig. 5, in which the micropore distribution of chars from Santa Barbara coal (oxidation temperature = 200°C) vs. half-width (X = L/2) is represented. The carbonized materials corresponding to fresh coals show an average micropore width of 1.2 nm for Santa Barbara char and 1.8 nm for Figaredo char In contrast, the chars obtained from oxidized coals present a very homogeneous microporosity (6 z 0) with a micropore size of around 0.8-0.9 nm (Tables 3 and 4). Additionally, the surface area of the micropore walls, Smi, of chars changes with coal oxidation time as micropore volume. Further conclusions concerning the way in which the microporous structure of bituminous-coal chars develops after coal oxidation can be drawn when the results of CO2 and N, adsorption onto these materials are compared. The evolution in the volume of pores of chars accessible to CO, with coal oxidation time might mean that identical microporous systems are developed in these materials, after 1 day of coal oxidation at 270°C. Fig. 4, which presents the CO? and N, adsorption data plotted in the D-R coordinates, shows that this is certainly not true. For low degrees of coal oxidation, particularly for non-oxidized coal chars, the volumes of CO, adsorbed are larger than the corresponding values for N,. Similar results have been reported by other authors who compare N, and CO, adsorption onto very microporous carbons [ 141. This behaviour corresponds to the existence of activated diffusion effects in the N, adsorption. The reason may be that for short periods of coal oxidation there is a high contribution of micropores in char samples, but the entrance of these micropores is very narrow and inaccessible to N,. With an the volume of pores determined by N, increasing degree of coal oxidation, adsorption systematically increases, while micropores accessible to CO2 remain practically constant, the difference between the values of N, and CO? therefore
22
T.A. Centeno et al. 1 J. Anal. Appl. Pyrolysis
34 (1995) 13-28
being smaller. Taking into account, as previously discussed, that coal oxidation does not imply the enlargement of micropores of carbonized materials (Tables 3 and 4) the fact that for long coal oxidation times the volume of pores accessible to N2 is increased suggests the partial elimination of constrictions at the entrance of the micropores as coal oxidation progresses. It is interesting to note that coal oxidation allows the constrictions size at the entrance of the micropores to be controlled. In this way, the microporous texture of chars can be modified by means of coal preoxidation. 3.2. Injhence of coal oxidation temperature It was found that the porous structure of chars is affected by both coal oxidation time and temperature. The latter is the most important factor determining the evolution of char microporosity. The textural characteristics of chars from coals oxidized at different temperatures are indicated in Tables 3 and 4. The development of char microporosity is enhanced by increasing the coal oxidation temperature. A broad variation in micropore volume after short oxidation times is observed as the coal oxidation temperature increases. When Santa Barbara coal (d, = 125- 180 pm), previously oxidized at 270°C for 6 h, is pyrolyzed, a material with a micropore volume of 0.16 cm3/g is obtained (Table 4). This corresponds to an increase of 60% with respect to the carbonized material obtained from fresh coal. In contrast, when oxidation of Santa Barbara coal is carried out at 150°C a similar value is reached after 15 days of coal oxidation (Table 4). The low microporosity of char corresponding to this coal oxidation temperature may be interpreted in terms of the slow diffusion of oxygen inside the coal particles. The increase in oxidation temperature allows the oxygen to diffuse more quickly to the inner part of the coal particle. In this way, coal thermoplasticity is eliminated to a large extent in the particle for relatively short oxidation times. It is also interesting to point out the evolution of char microporosity with coal oxidation time for series corresponding to coal oxidation temperatures higher than 200°C. As shown in Table 4, the modification of the micropore volume of chars corresponding to Santa Barbara coal oxidized at 200, 230 and 270°C presents similar trends. For a given oxidation time, the difference in micropore volume is around 0.01 cm3/g. However, as already reported in a previous paper [ 151,different behaviour is observed in the char microporosity for coal oxidation temperatures lower than 200°C. In Table 4 it is shown that the carbonized material obtained from Santa Barbara coal oxidized at 150°C for 12 h presents a micropore volume slightly lower than for the char from fresh coal. This fact may be explained considering that under these conditions of coal oxidation, the oxygen only affects the surface of the coal particle. In consequence, during the carbonization process, the swelling of the inner part of the coal particle is restricted by the oxidized superficial layer which presents no fluid properties. This would favour the arrangement of carbonaceous macromolecules and imply a decrease in char microporosity. At high oxidation temperatures ( > 200°C) or after long oxidation times at 150°C the oxygen will access deeper layers of coal particles, causing the thermoplastic
T.A. Cenleno ef al. 1 J. Anal. Appl. Pyro1vsi.r 34 (1995) 13-28
23
Coal Oxidation Temperature (” C) 0 270 0 230 A 200 ___
Fig. 6. Micropore
volume
of Santa
Bkbara
chars
vs. O/C atomic
ratlo
of coal (CI, = 115- 180 llni).
properties to disappear over a wider area. A representation of the CO,-micropore volume of chars vs. Santa Barbara coal atomic ratio (O/C) (Fig. 6) shows the grouping of these data, independent of coal oxidation conditions. This suggests that the important factor in coal preoxidation is not the coal oxidation conditions, but the degree of coal oxidation attained (evaluated from the atomic ratio O/C). The chars from coals oxidized at temperatures higher than 200°C show micropore widths around 0.8 nm (Tables 3 and 4). However, when the oxidation of Santa Barbara coal is carried out at 150°C the micropore widths of chars slightly decrease as oxidation progresses, coal oxidation times around of 15 days being necessary to obtain micropores of around 0.8 nm. To illustrate the effect of coal oxidation temperature on pore volume accessible to N,, Table 5 summarizes the percentages of pore volume filled by this adsorptive at relative pressures of 0.01 and 0.1 in the chars from Santa Barbara coal (d,, = 125 180 pm) oxidized at different temperatures for 3 days. It is shown that the percentages of pore volume filled by N, at lower relative pressures increase with coal oxidation temperature, while no significant difference in the microporosity of chars is observed from COz adsorption (Table 4). These results indicate, quantitatively, that the accessibility of pores progressively increase with coal oxidation Table 5 Fractions of pore volume filled by N,( - 196YJ) at different relative pressures for chars from coal Santa BBrbara oxidized at various temperatures (d, = 125- 180 wrn, Oxidation time = 3 days) Coal oxidation temperature (’ CI 0
150 230 270
0.0l/0.90
0. IO/O.90
0.25 0.41 0.43 0.57
0.38 0.54 0.73 0.82
24
T.A. Centeno et al. / J. Anal. Appl. Pyrolysis
34 (1995) 13-28
temperature. Clearly, higher temperatures of coal oxidation lead to the formation of micropores with less constriction. Molecular sieve carbons are known to possess a narrow pore size distribution with pores of bottle-neck shape [ 161. Generally, for manufacturing carbon molecular sieves, the suitable pore structure of the carbon is initially fixed by a controlled heat treatment followed by a change in the entrance to the pores [ 171. Small changes in the size of these constrictions in the micropore structure can impact the rate of diffusion of a gas to a significant extent. The results presented in this work indicate that the size of constrictions at the entrance to micropores can be controlled sufficiently by coal preoxidation. In this way, the oxidation of coal previous to the pyrolysis step may be an interesting way of obtaining carbon molecular sieves. 3.3. Influence of heat treatment temperature (HTT) Different authors have shown that the porous structure of coals can be altered by thermal treatment [ 181. The available surface area of heat treated coals has been shown to reach a maximum level at temperatures of 800-lOOo”C, resulting in a higher adsorption volume, and then to decrease at higher HTTs. With respect to oxidized coals, Siemieniewska et al. [lo] stated that chars from these materials present a more pronounced maximum in the micropore volume, which is displaced to higher pyrolysis temperatures with respect to chars obtained from fresh coals. Oda et al. [9] observed that coal preoxidation produces a marked increase in the micropore volume of chars up to temperatures around 7OO”C, followed by a decrease as a consequence of heteroatoms removal and the rupture of cross-links, resulting in a less porous structure. Parameters from the analysis of CO,-isotherms corresponding to chars obtained by pyrolyzing Santa Barbara coal (d,, = 125 180 pm) at different temperatures are reported in Table 6. The coal was previously subjected to oxidation at 200°C for 3 days. It is observed that pyrolysis temperature has a negligible effect on the COZ accessible pore volume. The micropore volume of chars obtained at different temperatures remains around 0.17 cm3/g. The formation of cross-links resistant to high temperatures, after the oxidation of coal under severe conditions, would explain that no decrease in the microporosity of chars exists between 600 and 850°C [9]. Table 6 Microporosity characteristics of chars obtained by pyrolysis at different coal. Raw coal (d, = 125- 180 pm) oxidized at 200°C for 3 days Heat treatment temperature (“C)
w, (cm3k)
600 700 800 850
0.17 0.17 0.17 0.18
s
temperatures
of Santa
6
G&g) 0.9 0.8 0.8 0.8
378 425 425 450
0.00 0.00 0.00 0.00
BBrbara
T.A. Centeno et al. / J. Anal. Appl. Pyrolysis
I
I
/
2
95
34 (1995) 1% 28
i
I
1
N2
co2
10-I
@
Tf,” C i 0 6. ,:,
ww Fig. 7. Characteristic curves of CO, different heat treatment temperatures.
600 700 800 850
(1O-3)
(0°C) and N,( -196°C) for Santa BBrbara chars obtained Raw coal (d, = 1255 180 pm) oxidized at 200°C for 3 days.
at
With respect to the microporosity characteristics, some authors [IO] have reported that the pore size distribution of chars corresponding to oxidized coals greatly depends on carbonization temperature. However, the results shown in this work indicate that the micropore size of chars from oxidized Santa Barbara coal is around 0.8 nm, indpendent of coal pyrolysis temperature (Table 6). Fig. 7 shows the characteristic curves of Santa Barbara chars obtained at four different temperatures. The differences observed between the volumes of CO, and N, adsorbed confirm the narrow microporosity of these materials. It is noted that the relative uptake of CO, at 0°C and N, at - 196’C by chars could be altered significantly by heat treatment under an inert atmosphere. In this way, a substantial decrease in the adsorption of N, occurs on increasing the carbonization temperature. This fact may be explained as a result of the alignment of aromatic macromolecules, which leads to micropore constrictions on increasingly severe thermal treatment, reducing the accessibility of the N, molecules to the microporous network. 3.4. Influence
of particle
diameter
The influence of particle diameter on char micropore development is determined by the eflect of this variable upon the extent of coal oxidation. As shown in Tables 3 and 4, the particle size does not significantly affect the microporosity of chars
T.A.
26
Centeno
et al. /J.
Anal.
Appl.
Pyrdysis 34 (1995) 13-28
from non-oxidized coals. However, this parameter has a great influence on the microporosity of chars obtained from oxidized coals. When an oxidized Santa Babara coal sample (27OC for 3 days) with an average particle diameter of around 48 pm is carbonized, a material with a micropore volume of 0.20 cm3/g is obtained. In contrast, if an average particle diameter of 855 pm is used, a similar value is reached after 3 days of coal oxidation at 270°C (Table 4). It is interesting to point out that for the manufacture of chars with the same micropore volume, the use of smaller size particles implies a drastic decrease in coal oxidation time. The evolution in the microporosity of chars with coal particle size can be explained, taking into account the lower accessibility of oxygen through the coal particle as the particle size is increased. As a consequence, longer coal oxidation times are required for obtaining equivalent degrees of coal oxidation as a larger particle diameter is utilized. In spite of the increase in micropore volume with coal oxidation time for chars obtained from coals with different particle diameters, the particle size does not modify the micropore characteristics of chars. Thus, the carbonized materials exhibit micropores of around 0.8-0.9 nm and very homogeneous microporosity (6 z 0) (Tables 3 and 4). The influence of particle diameter on the variation in accessibility of micropores to N2 is shown in Fig. 8 in which the characteristic curves (CO, at 0°C and N2 at - 196°C) of chars from Santa Barbara coal oxidized at 270°C for 3 days are represented. The average particle diameters are 45, 153 and 855 pm. It is shown that the elimination of micropore constrictions of chars by means of coal preoxidation takes place more effectively when smaller particles are used.
00
0 0 A
0
30
60
90
120 IS0 180’
400
3243 125480 7104000
800
CRT MpdpYf3)2
1200
1600
2000
(1 O-3)
Fig. 8. Characteristic curves of CO, (0°C) and N, (- 196°C) for chars different particle diameters oxidized at 270°C for 3 days.
from Santa
Bkrbara
coal with
T.A. Cenleno et al. 1 J. Anal. Appl. Pyrolysis 34 (1995) 13-28
31
4. Conclusions The microporous structure of chars obtained from bituminous coals can be modified by means of coal preoxidation. Coal oxidation at temperatures between 150 and 270°C gives rise to chars with well-developed microporosity. An increase in the micropore volume of carbonized materials from oxidized coals is observed. This increment is very marked during the first steps of coal oxidation and the development of char microporosity is enhanced by increasing the coal oxidation temperature. Additionally, coal preoxidation modifies the micropore size distribution: the chars obtained from oxidized coals exhibit very homogeneous microporosity with an average micropore width of around 0.8 nm. In contrast, coal preoxidation allows the size of the constrictions at the entrance of the micropores to be controlled, a progressive elimination of these being observed as the degree of coal oxidation increases. Higher HTTs imply a micropore constriction, reducing the accessibility to the microporous network of chars. The development of microporosity and elimination of micropore constrictions in carbonized materials by means of coal preoxidation take place more effectively when particles with small diameters are used. The effect of coal oxidation conditions, pyrolysis temperature and particle diameter upon the microporous structure of char allows the textural characteristics of carbonized materials to be controlled, with distinct subsequent uses.
Acknowledgements The authors wish to acknowledge Fundacion para el Foment0 en Asturias Tecnologia (FICYT).
the financial support received from the de la Investigation Cientifica Aplicada y la
References [I] [2] [3] [4] [5] [6] [7] [8] [9]
[lo] [ 111 [ 121
H. Janjowska, A. Swiatkowski and J. Choma. in T.J. Kemp (Ed.), Active Carbon, Ellis Howood. London. 1991. R.C. Bansal, J.B. Donnet and H.F. Stoeckli, in Active Carbon, Marcel Dekker, New York. 1988. J.B. Howard, in M.A. Elliot (Ed.), Chemistry of Coal Utilization, Second Suppl. Vol., Wiley. New York. 1981, Chapter 12. D.J. Maloney. R.G. Jenkins and P.L. Walker, Jr., Fuel. 62, (1982) 175. J.J. Pis, A. Cagigas, P. Simon and J.J. Lorenzana, Fuel Process. Technol.. 20 (1988) 307. F. Stoeckli and L. Ballerini, Fuel, 70 ( 1991) 557. Y. Toda, Fuel. 52 (1973) 99. P.K. Singla, S. Miura, R.R. Hudgins and P.L. Silveston, Fuel, 62 (1983) 645. H. Oda, M. Takeuchi and C. Yokokawa, Fuel, 60 (1981) 390. T. Siemieniewska, K. Tomkow, A. Jankowska and M. Jasienko. Fuel, 64 ( 1985) 481. D.J. Maloney and R.G. Jenkins, Fuel, 64 (1985) 1415. H. Sommers and W. Peters, Chem. Ing. Technol., 26 (1954) 441.
28
T.A. Centeno et al. /J.
Anal. Appl. Pyrolysis
34 (1995) 13-28
[ 131 D.L. Carpenter and D.G. Giddings, Fuel, 43 (1964) 247. [14] F. Rodriguez-Reinoso and A. Linares-Solano, in P.A. Thrower (Ed.), Chemistry and Physics Carbon, Vol. 21, 1, Marcel Dekker, New York, 1989. [15] T. Alvarez, A.B. Fuertes, J.J. Pis, J.B. Parra and J.A. Pajares, Fuel, 73 (1994) 1358. [ 161 H. Jtintgen, Carbon, 15 ( 1977) 293. [ 171 S.K. Verma and P.L. Walker, Jr., Carbon, 30 (1992) 829. [18] P. Chiche, P.H. Brunner and P.V. Roberts, Carbon, 18 (1980) 217.
of