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Activation of brown-coal chars with oxygen
Activation oxygen
of brown-coal
Kazimierz Tomk6w, Siemieniewska
Anna Jankowska,
chars with
Franciszek Czechowski
and Teresa
Institute of Chemistry and Technology of Petroleum and Coal, Technical University of Wroclaw, Gdariska 7/9, 50-344, Wroclaw, Poland (Received 12 August 1976)
Semicokes and cokes prepared respectively at 773 and 1173 K from brown-coals, xylitic and earthy, from Polish coal seams, were activated with gaseous oxygen (10% oxygen and 90% argon) in a thermogravimetric apparatus to different burn-offs. With increasing temperature of oxygen activation a constant decrease of the sum of micropores and mesopores is observed, but probably as a result of chemisorption of oxygen the micropore volume passesthrough a maximum at 663 K. There is a strong influence of the temperature of carbonization of the char on the formation of porosity in the products of oxygen activation: activated cokes have better adsorptive properties than activated semicokes. The highest value of surface areas (benzene adsorption) are, for semicokes and cokes respectively, 520 and 700 m* g-l. These differences can be attributed to the uniform microporosity in the non-activated coke as distinct from the wide range of the micropore diameters in the non-activated semicoke, and also to the lack of ultramicropores in the former sample. The earthy type of brown coal yields products with a less developed porosity than the corresponding products from the xylitic coal. For the xylitic semicoke as well as for the coke, after continuing the process of activation to burn-offs higher than 50%, a lowering of adsorptive properties is observed.
The action of oxygen on coals, cokes, chars and other carbonaceous materials has been investigated by numerous authors. There are several review articles on this topic written as portions of books, e.g. by van Krevelen’, Dryden’, Puri3, Mattson and Mark4, and numerous publications, among others from research centres in the UK’-‘, France*-‘*, Poland’3-18, Germany19-**, and also in the USSR2324, in the USA2s-27, in India** and Australia29*. However, few of the papers mentioned describe work carried out on chars from brown coals12J920. In a recent publication3’, changes occurring in the porosity of high-temperature brown-coal cokes on activation with carbon dioxide were presented. The present study investigates the action of gaseous oxygen as activating agent for brown-coal chars - for several reasons. First: to study the possible specificity of activation by oxygen as distinct from other activating agents, e.g. carbon dioxide or steam, in developing the porosity of the products of activation. Secondly: gaseous oxygen as an activating agent is readily available and does not require special preparation (as with steam generation). Thirdly: oxygen may be, and frequently is, present in activating mixtures of gases because of the possibility of leakage in equipment, even if activation is supposedly carried out with other vapours. It is therefore useful to know how the presence of oxygen may affect the structure and porosity of activated carbonaceous materials. The aim of this paper is to show some aspects of the influence of the nature of the char taken for oxygen activation on the development of porosity in the products. EXPERIMENTAL
Samples for activation A xylitic brown coal (XBC) and an earthy brown coal
(EBC), both from Polish coal seams, were carbonized under pyrolytic gases in a Fischer-Schrader retort to form chars at 773 K, and maintained under argon at 773 K to constant mass in a thermogravimetric apparatus3* to give semicokes XBC 500 and EBC 500. These semicokes were subsequently heated to 1173 K to constant mass to give cokes XBC 900 and EBC 900. The chemical analyses of the initial coals and the resultant products of carbonization are given in Table 1, which Table
1
Analysesof
Sample
brown coals and products
Ash (%,d.b.)
Volatile matter f%,daf) Xylitic
Coal XBC Char from FischerSchrader retort Semicoke XBC 500 Coke XBC 900
Coal EBC Char from FischerSchrader retort Semicoke EBC 500 Coke EBC 900
Carbon (%,daf)
of carbonization
Hydrogen (%, daf)
Oxygen (diff.) (%, daf)
brcwn-coal
1 .o
67.3
61.9
5.8
32.3
2.4
17.8
82.5
3.4
14.1
2.6
9.6
87.2
2.8
10.0
-
95.8
I.3
2.9
Earthy
brown-coal
2.8
5.4
56’1
67.4
5.7
26.9
9.2
19.8
82.2
3.4
14.4
9.6
14.2
84.8
2.9
12.3
-
97.1
1.0
I.9
11.2
FUEL, 1977, Vol 56, January
101
Activation
of brown-coal chars with oxygen: K. Tomkciw, A. Jankowska, F. Czechowski and J. Siemieniewska
shows that heating the char from the Fischer-Schrader retort in argon to constant mass lowers the volatile matter, decreases the hydrogen and heieroatom content and increases the percentage of carbon. Results (to be published elsewhere) of adsorptive properties of semicokes from brown coals before and after stabilization of their mass indicate that this stabilization opens the microporosity. This could mean that the products of thermal decomposition of the coal evolved on carbonization, which block the pores of the non-stabilized char, are removed during the heat treatment under argon. Activation with oxygen Activation of the semicokes and cokes was carried out in the the~ograv~et~c apparatus, using a mixture of gases composed of 10% oxygen and 90% argon. This percentage of oxygen was taken for practical reasons: at lower concentrations of oxygen the reaction between oxygen and the activated product was very slow and difficult to follow, at higher concentrations the interval between temperatures corresponding to the beginning of the reaction and ignition was very short. Activation with oxygen was preceded by heating the semicoke at 773 K for 1 h in argon, the temperature being subsequently lowered to that of activation, oxygen then being added to the argon and activation continued to a selected burn-off. The sample was cooled under argon. For selected samples, activation with oxygen was followed by heat treatment in argon, also in the same thermogravimetric apparatus. determination of the porous structure The porous structure of the brown-coal semicokes and cokes before and after oxygen activation was characterized by vapour adsorption, using benzene vapour at 298 K. Evaluation of the adsorption and desorption isotherms has been described31. RESULTS AND DISCUSSJO~ Activation of the xylitic semicoke XBC 500 Studies of the action of oxygen, as activating agent for carbonization products of brown coals, were initiated with the xylitic semicoke XBC 500. The xylitic petrograp~c type was taken because of its low mineral-matter content. This low temperature of carbonization was chosen because Rod&, LIEIlo- tOOnm Es,! &Q
5”10nm 3 - 5 nm
I
15-3nm
m
< 1~5lll-n
XBC500 activatedwith oxygen at the temperature. 663K
Figure I Influence of temperature size distribution in xytitic semicoke surface area)
102
of oxygen activation on poreXBC 500 (pore volume and
FUEL, 1977, Vol 56, January
xBC 500 activated to burn-off,
Figure 2 semicoke
Changes in mesopore and micropore volume of xylitic XBC 500 on activation with oxygen at 663 I<
the brown-coal semicoke is produced on a larger industrial scale (production of smokeless fuel, Lurgi furnaces) than is coke, the latter being produced in exceptional cases only. To choose the temperature of activation, experiments were carried out in which the semicoke XBC 500 was activated at different temperatures to the same burn-off of 50%. The results, in the form of pore volume and surface area distributions, are shown in Figure 1. This Figure shows that with increasing temperature of oxygen activation, the sum of the volume of pores directly involved in the adsorption process, i.e. micropores and mesopores (the volume of macropores was not studied) decreases, Evidently, this is so’ because at higher temperatures of activation the oxygen has insufficient time to penetrate into the interior of the particle. Therefore at high temperatures of activation, the burn-off of the sample does not cause much enlargement of the total porosity, but tends to be restricted to pores situated near to the external surface of the particle. This is shown by the fact that the sum of the micropore and mesopore volume of the semicoke activated at the lowest temperature (593 K) is about 0.4 cm3 g-l, while at the highest temperature the corresponding value is only 0.25 cm3 g-l. Analysis of the pore sizes shows another effect. On increasing the activation temperature from 593 to 663 K, there is an increase in the micropore volume. On increasing the activation temperature to 773 K the micropore volume decreases and (since at this heating rate there is no sintering) this could be due to formation of surface oxygen complexes (chemisorbed oxygen) within the microporosity, so blocking access of the benzene adsorbate. An optimum activation temperature was selected as 663 K, and various burn-offs at this temperature are illustrated in Figure 2. With increasing burn-off the sum of micropore and mesopore volumes increases, reaching at the bum-off of about 80-9% almost O-6 cm3 g-l. For the widest mesopores (effective radius from 10 to 100 nm) a very sharp increase in volume is observed above 70% bum-off. Between 30 and 90% burn-off the volume of wide mesopores increases from 0.01 to about 0.3 cm3 g-l and (experimentally) these pores are still present in the ash (O-15 cm3 g-l) as though preserved from the coal structure. On the other hand the narrowest mesopores and micropores diminish in extent when activation is carried out to burn-offs higher than 50%. The maximum volumes are about O-1 cm3 g-l for narrow mesopores and about 0.2 cm3 g-l for the micropores. Pore development in different brown-coal carbonization products For semicokes and cokes obtained from the xylitic and
K. Tomkbw, A. Jankowska,
F. Czechowski and T. Siemieniewska:
earthy brown coals the following procedure was employed: the carbonization products were activated with 10% oxygen in argon (at a temperature close to that which corresponds to the perceptible beginning of reaction between oxygen and the tested semicoke or coke) to a burn-off of S%, and Table 2 Influence of type of brown coal and of the temperature of its carbonization on changes of mass and development of surface area on oxygen activation and subsequent heat treatment
Burn-off
Surface area
(%I
fm2 9-l)
Sem ice kes XBC 500
EBC 500
0
Non-activated Activated with Activated with heated in argon Activated with heated in argon
oxygen oxygen and at 773 K oxygen and at 1073 K
Non-activated Activated with Activated with heated in argon Activated with heated in argon
oxygen oxygen and at 773 K oxygen and at 1073
8.1
155 247
19.8
334
42.8
432
0 8.3
84 30
22.7
41
41.2
53
0 85
70 558
Cokes XBC 900
EBC 900
Non-activated Activated with Activated with heated in argon Activated with heated in argon Non-activated Activated with Activated with heated in argon Activated with heated in argon
Content
oxygen oxygen and at 773 K oxygen and at 1073 K
11.5
oxygen oxygen and at 773 K oxygen and at 1073 K
24.0
674
0 8.0
129 439
11.0 22.2
546
Activation
of brown-coal
chars with oxygen
then the products were heated in argon, first at 773 K and then at 1073 K, always to constancy of mass. The values of burn-offs at the successive steps of this procedure, as well as the respective surface areas, are given in Table 2. It is seen from this Table that semicokes activated in oxygen and then heated in argon at 773 K lose considerably more of their mass than the respective products obtained from cokes. For the xyiitic samples the burn-off of the oxygen-activated semicoke increases from 8.1 to 19.870, while for the oxygen-activated coke this change is only from 8.5 to 115%, although for both carbonization products the temperature of 773 K does not exceed the final temperature of carbonization of the products. Heating in argon at 1073 K brings about a further loss of mass in the cokes (a still more important loss of mass in the semicokes, but here the effect of additional carbonization has now to be considered), and Figure 3 shows that the elemental composition of the oxygen-activated cokes is, after the argon treatment, almost identical with the elemental composition of these cokes before their activation. This would indicate that the temperature of 1073 K is sufficiently high to free the activated brown-coal products from the majority of oxygen chemisorbed during oxygen activation. The pore volumes and pore-size distributions of the products (Figure 4) as well as their surface area values (Table 2) show that, as with activation of brown-coal cokes with carbon dioxide3’, activation with oxygen also leads to a higher development of porosity in the xylitic samples than in the earthy ones. With the earthy semicoke EBC 500 is appeared impossible, when using 10% oxygen, to find a suitable temperature of activation: the semicoke either did not react or, with a very small increase in temperature, ignition occurred. Hence, the initial porosity of this semicoke was not developed but rather destroyed by reaction with oxygen. Further, it is seen in Table 2 and Figure 4 that, for both
of:
ll:‘::::::i oxygen a
hydrogen
m
carbon
XBC
XBC 900
500
EBC 500
Figure 3 Variations of elemental composition heat treatment in neutral atmosphere
EBC 900
of brown-coal
carbonization
products
on activation
with oxygen and subsequent
FUEL,
1977,
Vol 56, January
103
Activation of brown-coal chars with oxygen: K. Tomkciw, A. Jankowska, F. Czechowski and T. Siemieniewska Radius: IIUl
lo-100nm
a
5-1Onm
m
3-5nm
m
<
1,5-3nm
0.L
1.5nm XBC XBC
EBC
Figure 4 Influence tion and subsequent
EBC 900
500
of type of brown coal and final temperature heat treatment
of its carbonization
petrographic types of coal, higher porosity and surface-area values are obtained when the cokes and not the semicokes are activated. The influence of the temperature of carbonization of the xylitic brown-coal char on the formation of porosity on oxygen activation is presented in Figure 5. For each burn-off the surface-area value of the activated coke is higher than the corresponding value for the activated semicoke. The difference is mainly in the development of micropores and to a smaller extent - in the development of the narrow mesopores. The remaining pores contribute little to the total surface area of activated semicokes and cokes. The most striking difference in the mode of formation of microporosity on oxygen activation in the semicoke and in the coke, apart from differences in absolute values of micropore volumes, is the way these pores develop with burn-off: in the semicoke the microporosity is opened gradually, reaching a maximum at the burn-off of 50% and then diminishing at a similar rate. For the coke, micropores are rendered accessible very abruptly and the maximum of their volume is at low burn-offs. For the activated semicoke, the volumes of narrow mesopores are also smaller than the respective volumes in the activated coke, but in both cases the maximum is obtained at burn-offs between 40 and 60%. At still higher burn-offs, oxygen activation of semicokes and cokes leads to a decrease of adsorptive capacity of the activated samples. The differences in the possibilities of development of porosity in brown-coal semicokes and cokes described, dependent upon the nature of the carbonized product (Figures 4 and 5), confirm that on activation the pre-existing porous
104
FUEL,
1977,
Vol
56, January
900
500
on development
of porosity
on oxygen activa-
system of the non-activated product is modified, rather than that new pores are created. The superior development of porosity in the activated cokes, rather than the semicokes, is attributable to the microporosity present in these two products of carbonization before their activation. Studies33 carried out on the development of porosity in a xylitic brown coal on carbonization? comprising adsorption measurements of nitrogen at 77 K, of carbon dioxide at several temperatures, and of other adsorbates of different cross-sectional areas at room temperature, indicate a basic difference in the size of micropores in the semicoke and the coke. The microporous system of the xylitic semicoke consists of pores with a wide range of dimensions: about one third of the micropore volume is formed by.ultra-micropores inaccessible to nitrogen, another one third has dimensions between 0.4 and 0.5 nm, and the remaining micropores are larger than 0.5 nm. The dimensions of micropores in the non-activated xylitic coke are very uniform, 95% of the micropores having a diameter between 0.4 and 0.5 irm. This uniformity and also the complete lack of ultra-micropores - which on activation might easily be inaccessible to the molecules of the activating agent - seem to facilitate the process of oxygen activation of this carbonization product.
CONCLUSIONS The development of porosity in brown-coal semicokes and cokes on activation with gaseous oxygen in the conditions described depends strongly on the origin of the carboniza-
K. Tomkbw,
A. Jankowska,
F. Czechowski
and T. Siemieniewska:
Activation
of brown-coal
XBC 900
s
/ 700
chars with
oxygen
3- 100 nm nm
600
s 3-100nm
-0
20
LO Burn-
60 off
80
600
20
100
( wt % I
Figore 5 Changes in surface area of different off in the process of oxygen activation
LO Burn-off
kind of pores in xylitic
tion product used for activation. Carbonization products from the earthy brown coal develop their porosity to a lesser extent than the respective products from the xylitic brown coal. With the earthy semicoke it was found impossible to carry out oxygen activation with the 10% oxygen in a satisfactory manner and a destruction of the initial porous system of this product was obtained as the result of the action of oxygen. The temperature of oxygen activation of the brown coal carbonization products has a strong influence on the volume of the developed pores and on pore-size distributions. With increasing temperature of activation the volume of the sum of pores responsible for adsorptive properties of the activated product (mesopores and micropores) systematically decreases; however the micropore volume passes through a maximum. There is a pronounced effect of the final temperature of carbonization of the brown-coal char on its further oxygen activation. Coke from the xylitic brown coal gives, on activation with oxygen, products of superior adsorptive properties than corresponding products from the xylitic semicoke. The main difference lies in the development of micropores. In the semicoke the micropores are formed gradually, reaching at the burn-off of 50% a maximum of about 0.2 cm3 g-l, while in the coke the micropores are opened abruptly during the first stages of activation; the volume of 0.2 cm3 g -l is already reached in the xylitic coke at the burn-off of 1% and the maximum volume (O-27 cm3 g-l) corresponds to a burn-off of 25%. This possibility of developing a deeper porosity in the coke than in the semicoke seems to be connected with a highly uniform micropore size-distribution in the initial coke sample and lack of ultra-microporosity, compared with a wider range of pore diameters in the semicoke.
semicoke
60
1 wt %
80
100
I
XBC 500 and coke XBC 900 with increasing burn-
The highest values of surface areas of oxygen-activated semicokes and cokes from the xylitic brown coal are respectively 520 and 700 m2 g-l (benzene adsorption) and correspond in both cases to burn-offs of about 50%. Further continuation of oxygen activation in the applied conditions brings about a decrease of sorptive properties of the activated products.
ACKNOWLEDGEMENTS The authors feel greatly indebted to Dr Harry Marsh of the University of Newcastle upon Tyne, UK, for valuable discussions and for his kindness in reading this article and most useful comments. We also wish to thank sincerely J6zefa Oleszkiewicz and Ahdrzej JankowSki for their help in the preparation and chemical characterization of the samples.
REFERENCES Van Krevelen, D. W. Coal, Elsevier, Amsterdam, 1961, pp 219-262 Dryden, 1. G. C. ‘Chemical Constitution and Reactions of Coal, inchemistry of Coal Wlization, Supplementary Vol. (Ed. H. H. Lowry), New York, 1963, pp 272-289 Puri, B. R. ‘Surface Complexes on Carbon’ in Chemistry and Physics of Carbon. Vol. 6 (Ed. P. L. Walker Jr). Marcel ” Dekker, New York, 1970, pp 193-249 Mattson, J. S. and Mark, H. B. Jr. Acrivated Carbon, Marcel Dekker, New York, 1971, pp 13.-20, 25-86 Marsh, H. and Rand, B. Carbon 1971, 9, 63 Marsh, H. and Foord, A. D. Carbon 1973, 11,421 Marsh, H., Foord, A. D., Mattson, J. S., ‘Thomas, J. M. and Evans, E. L. J. Colloid Interface Sci. 1974,49, 368 Gu&in, H., RebaudiBres, P., Grillet, Y. and Frarqois, M. Bull. Sot. chim. 1964, p 371
FUEL,
1977,
Vol 56, January
105
A~iivat~~~ of brow~-~oa~ chars with oxygen: 9
10 11 12 13 14 15 f6 17 18 19 20
106
K. Tomkow, A. Jankowska,
Gudrin, H., Rebaudibres,‘P. and Francois, M. Bull. Sot. chim. 1965, p 2573 Rebaudj~res, P. Thesis, Orsay, France, 1965 Grillet, Y. Thesis. &say, France, 1969 Guerin, H., Siemieniewska, T., Grillet, Y. and Franqois, M. Carbon 1970,8,727; 1971,9,657 Roga, B. and Ihnatowicz, M. Prace Glbwnego Inst. G~rn~c~wa, Komun. No. 82,195l Olpiriski, W. Archiwum Gbrnicrwa 1962,7, 183 Roga, B. and Pampuch, R. Prace Glbwnego Inst. Gdrnictwa, Komun. No. 189,1956 Szuba, J., Gubrynowicz, L. and Stromich, T. Koks-SmolnGut 1966, 11, 128, 161,209; 1967, 12,45, 116, 146,178, 197 Rozwadowski, M. and Siedlewski, J. ChemiaStosowana 1972,16,123 Siedlewski, J. and Zawadzki, J. Chemia Stosowana 1972,16, 435; 1973; 17,ll Schtitt, E. Brennst.-Chem. 1964,45,144,165,270; 1966, 47.135 W&man, U. Brennst.-Chem. 1966,47, 170; 1967,48,141
FUEL, 1977, Vol56,
January
21 22 23 24 25 26 27 28 29 30 31 32 33
F. Czecbowski and T. ~iemjenje~ka
Langhoff, J. and Peters, W. Erdol u. Kohle. Erdgas. Petrochemie 1964,17,900 Jtintgen, H.and Traencker, K. G. Bren~lst.-Chem. 1964,45, 105 Orieshko, W. F. Dokl. Akad. Nauk SSSR 1952,82,135 Malin. N. W. J. vrikl. chim. 1969.42.1058. 1802 Jenkins, R. G.,Nandi, S. P. and Walker, P. L. Jr Fuel 1973, 52,288 Rodriques-Reinoso, F. and Walker, P. J. Jr Carbon, 1975, 13,7 Rellick, G. S., Thrower, P. A. and Walker, P. L. Jr Carbon 1975, i3,71 Bahl, 0. P. and Manocha, L. M. Carbon 1975,13,297 Breen. J. G. and Evans, D. G. Fuel 1963,42,100 AlJardice, D. J. Carbon 1965,3,215; 1966,4,255 Berger, J., Siemieniew~a, T. and Tomkdw, K. Fuel 1976, 55,9 Tomkdw, K. Prace Naukowe ICHTNW Politechniki Wroclawskiej 1975,25,183 Siemieniewska, T. Koks-Smola-Gaz 1969, 14,134
of brown-coal
Kazimierz Tomk6w, Siemieniewska
Anna Jankowska,
chars with
Franciszek Czechowski
and Teresa
Institute of Chemistry and Technology of Petroleum and Coal, Technical University of Wroclaw, Gdariska 7/9, 50-344, Wroclaw, Poland (Received 12 August 1976)
Semicokes and cokes prepared respectively at 773 and 1173 K from brown-coals, xylitic and earthy, from Polish coal seams, were activated with gaseous oxygen (10% oxygen and 90% argon) in a thermogravimetric apparatus to different burn-offs. With increasing temperature of oxygen activation a constant decrease of the sum of micropores and mesopores is observed, but probably as a result of chemisorption of oxygen the micropore volume passesthrough a maximum at 663 K. There is a strong influence of the temperature of carbonization of the char on the formation of porosity in the products of oxygen activation: activated cokes have better adsorptive properties than activated semicokes. The highest value of surface areas (benzene adsorption) are, for semicokes and cokes respectively, 520 and 700 m* g-l. These differences can be attributed to the uniform microporosity in the non-activated coke as distinct from the wide range of the micropore diameters in the non-activated semicoke, and also to the lack of ultramicropores in the former sample. The earthy type of brown coal yields products with a less developed porosity than the corresponding products from the xylitic coal. For the xylitic semicoke as well as for the coke, after continuing the process of activation to burn-offs higher than 50%, a lowering of adsorptive properties is observed.
The action of oxygen on coals, cokes, chars and other carbonaceous materials has been investigated by numerous authors. There are several review articles on this topic written as portions of books, e.g. by van Krevelen’, Dryden’, Puri3, Mattson and Mark4, and numerous publications, among others from research centres in the UK’-‘, France*-‘*, Poland’3-18, Germany19-**, and also in the USSR2324, in the USA2s-27, in India** and Australia29*. However, few of the papers mentioned describe work carried out on chars from brown coals12J920. In a recent publication3’, changes occurring in the porosity of high-temperature brown-coal cokes on activation with carbon dioxide were presented. The present study investigates the action of gaseous oxygen as activating agent for brown-coal chars - for several reasons. First: to study the possible specificity of activation by oxygen as distinct from other activating agents, e.g. carbon dioxide or steam, in developing the porosity of the products of activation. Secondly: gaseous oxygen as an activating agent is readily available and does not require special preparation (as with steam generation). Thirdly: oxygen may be, and frequently is, present in activating mixtures of gases because of the possibility of leakage in equipment, even if activation is supposedly carried out with other vapours. It is therefore useful to know how the presence of oxygen may affect the structure and porosity of activated carbonaceous materials. The aim of this paper is to show some aspects of the influence of the nature of the char taken for oxygen activation on the development of porosity in the products. EXPERIMENTAL
Samples for activation A xylitic brown coal (XBC) and an earthy brown coal
(EBC), both from Polish coal seams, were carbonized under pyrolytic gases in a Fischer-Schrader retort to form chars at 773 K, and maintained under argon at 773 K to constant mass in a thermogravimetric apparatus3* to give semicokes XBC 500 and EBC 500. These semicokes were subsequently heated to 1173 K to constant mass to give cokes XBC 900 and EBC 900. The chemical analyses of the initial coals and the resultant products of carbonization are given in Table 1, which Table
1
Analysesof
Sample
brown coals and products
Ash (%,d.b.)
Volatile matter f%,daf) Xylitic
Coal XBC Char from FischerSchrader retort Semicoke XBC 500 Coke XBC 900
Coal EBC Char from FischerSchrader retort Semicoke EBC 500 Coke EBC 900
Carbon (%,daf)
of carbonization
Hydrogen (%, daf)
Oxygen (diff.) (%, daf)
brcwn-coal
1 .o
67.3
61.9
5.8
32.3
2.4
17.8
82.5
3.4
14.1
2.6
9.6
87.2
2.8
10.0
-
95.8
I.3
2.9
Earthy
brown-coal
2.8
5.4
56’1
67.4
5.7
26.9
9.2
19.8
82.2
3.4
14.4
9.6
14.2
84.8
2.9
12.3
-
97.1
1.0
I.9
11.2
FUEL, 1977, Vol 56, January
101
Activation
of brown-coal chars with oxygen: K. Tomkciw, A. Jankowska, F. Czechowski and J. Siemieniewska
shows that heating the char from the Fischer-Schrader retort in argon to constant mass lowers the volatile matter, decreases the hydrogen and heieroatom content and increases the percentage of carbon. Results (to be published elsewhere) of adsorptive properties of semicokes from brown coals before and after stabilization of their mass indicate that this stabilization opens the microporosity. This could mean that the products of thermal decomposition of the coal evolved on carbonization, which block the pores of the non-stabilized char, are removed during the heat treatment under argon. Activation with oxygen Activation of the semicokes and cokes was carried out in the the~ograv~et~c apparatus, using a mixture of gases composed of 10% oxygen and 90% argon. This percentage of oxygen was taken for practical reasons: at lower concentrations of oxygen the reaction between oxygen and the activated product was very slow and difficult to follow, at higher concentrations the interval between temperatures corresponding to the beginning of the reaction and ignition was very short. Activation with oxygen was preceded by heating the semicoke at 773 K for 1 h in argon, the temperature being subsequently lowered to that of activation, oxygen then being added to the argon and activation continued to a selected burn-off. The sample was cooled under argon. For selected samples, activation with oxygen was followed by heat treatment in argon, also in the same thermogravimetric apparatus. determination of the porous structure The porous structure of the brown-coal semicokes and cokes before and after oxygen activation was characterized by vapour adsorption, using benzene vapour at 298 K. Evaluation of the adsorption and desorption isotherms has been described31. RESULTS AND DISCUSSJO~ Activation of the xylitic semicoke XBC 500 Studies of the action of oxygen, as activating agent for carbonization products of brown coals, were initiated with the xylitic semicoke XBC 500. The xylitic petrograp~c type was taken because of its low mineral-matter content. This low temperature of carbonization was chosen because Rod&, LIEIlo- tOOnm Es,! &Q
5”10nm 3 - 5 nm
I
15-3nm
m
< 1~5lll-n
XBC500 activatedwith oxygen at the temperature. 663K
Figure I Influence of temperature size distribution in xytitic semicoke surface area)
102
of oxygen activation on poreXBC 500 (pore volume and
FUEL, 1977, Vol 56, January
xBC 500 activated to burn-off,
Figure 2 semicoke
Changes in mesopore and micropore volume of xylitic XBC 500 on activation with oxygen at 663 I<
the brown-coal semicoke is produced on a larger industrial scale (production of smokeless fuel, Lurgi furnaces) than is coke, the latter being produced in exceptional cases only. To choose the temperature of activation, experiments were carried out in which the semicoke XBC 500 was activated at different temperatures to the same burn-off of 50%. The results, in the form of pore volume and surface area distributions, are shown in Figure 1. This Figure shows that with increasing temperature of oxygen activation, the sum of the volume of pores directly involved in the adsorption process, i.e. micropores and mesopores (the volume of macropores was not studied) decreases, Evidently, this is so’ because at higher temperatures of activation the oxygen has insufficient time to penetrate into the interior of the particle. Therefore at high temperatures of activation, the burn-off of the sample does not cause much enlargement of the total porosity, but tends to be restricted to pores situated near to the external surface of the particle. This is shown by the fact that the sum of the micropore and mesopore volume of the semicoke activated at the lowest temperature (593 K) is about 0.4 cm3 g-l, while at the highest temperature the corresponding value is only 0.25 cm3 g-l. Analysis of the pore sizes shows another effect. On increasing the activation temperature from 593 to 663 K, there is an increase in the micropore volume. On increasing the activation temperature to 773 K the micropore volume decreases and (since at this heating rate there is no sintering) this could be due to formation of surface oxygen complexes (chemisorbed oxygen) within the microporosity, so blocking access of the benzene adsorbate. An optimum activation temperature was selected as 663 K, and various burn-offs at this temperature are illustrated in Figure 2. With increasing burn-off the sum of micropore and mesopore volumes increases, reaching at the bum-off of about 80-9% almost O-6 cm3 g-l. For the widest mesopores (effective radius from 10 to 100 nm) a very sharp increase in volume is observed above 70% bum-off. Between 30 and 90% burn-off the volume of wide mesopores increases from 0.01 to about 0.3 cm3 g-l and (experimentally) these pores are still present in the ash (O-15 cm3 g-l) as though preserved from the coal structure. On the other hand the narrowest mesopores and micropores diminish in extent when activation is carried out to burn-offs higher than 50%. The maximum volumes are about O-1 cm3 g-l for narrow mesopores and about 0.2 cm3 g-l for the micropores. Pore development in different brown-coal carbonization products For semicokes and cokes obtained from the xylitic and
K. Tomkbw, A. Jankowska,
F. Czechowski and T. Siemieniewska:
earthy brown coals the following procedure was employed: the carbonization products were activated with 10% oxygen in argon (at a temperature close to that which corresponds to the perceptible beginning of reaction between oxygen and the tested semicoke or coke) to a burn-off of S%, and Table 2 Influence of type of brown coal and of the temperature of its carbonization on changes of mass and development of surface area on oxygen activation and subsequent heat treatment
Burn-off
Surface area
(%I
fm2 9-l)
Sem ice kes XBC 500
EBC 500
0
Non-activated Activated with Activated with heated in argon Activated with heated in argon
oxygen oxygen and at 773 K oxygen and at 1073 K
Non-activated Activated with Activated with heated in argon Activated with heated in argon
oxygen oxygen and at 773 K oxygen and at 1073
8.1
155 247
19.8
334
42.8
432
0 8.3
84 30
22.7
41
41.2
53
0 85
70 558
Cokes XBC 900
EBC 900
Non-activated Activated with Activated with heated in argon Activated with heated in argon Non-activated Activated with Activated with heated in argon Activated with heated in argon
Content
oxygen oxygen and at 773 K oxygen and at 1073 K
11.5
oxygen oxygen and at 773 K oxygen and at 1073 K
24.0
674
0 8.0
129 439
11.0 22.2
546
Activation
of brown-coal
chars with oxygen
then the products were heated in argon, first at 773 K and then at 1073 K, always to constancy of mass. The values of burn-offs at the successive steps of this procedure, as well as the respective surface areas, are given in Table 2. It is seen from this Table that semicokes activated in oxygen and then heated in argon at 773 K lose considerably more of their mass than the respective products obtained from cokes. For the xyiitic samples the burn-off of the oxygen-activated semicoke increases from 8.1 to 19.870, while for the oxygen-activated coke this change is only from 8.5 to 115%, although for both carbonization products the temperature of 773 K does not exceed the final temperature of carbonization of the products. Heating in argon at 1073 K brings about a further loss of mass in the cokes (a still more important loss of mass in the semicokes, but here the effect of additional carbonization has now to be considered), and Figure 3 shows that the elemental composition of the oxygen-activated cokes is, after the argon treatment, almost identical with the elemental composition of these cokes before their activation. This would indicate that the temperature of 1073 K is sufficiently high to free the activated brown-coal products from the majority of oxygen chemisorbed during oxygen activation. The pore volumes and pore-size distributions of the products (Figure 4) as well as their surface area values (Table 2) show that, as with activation of brown-coal cokes with carbon dioxide3’, activation with oxygen also leads to a higher development of porosity in the xylitic samples than in the earthy ones. With the earthy semicoke EBC 500 is appeared impossible, when using 10% oxygen, to find a suitable temperature of activation: the semicoke either did not react or, with a very small increase in temperature, ignition occurred. Hence, the initial porosity of this semicoke was not developed but rather destroyed by reaction with oxygen. Further, it is seen in Table 2 and Figure 4 that, for both
of:
ll:‘::::::i oxygen a
hydrogen
m
carbon
XBC
XBC 900
500
EBC 500
Figure 3 Variations of elemental composition heat treatment in neutral atmosphere
EBC 900
of brown-coal
carbonization
products
on activation
with oxygen and subsequent
FUEL,
1977,
Vol 56, January
103
Activation of brown-coal chars with oxygen: K. Tomkciw, A. Jankowska, F. Czechowski and T. Siemieniewska Radius: IIUl
lo-100nm
a
5-1Onm
m
3-5nm
m
<
1,5-3nm
0.L
1.5nm XBC XBC
EBC
Figure 4 Influence tion and subsequent
EBC 900
500
of type of brown coal and final temperature heat treatment
of its carbonization
petrographic types of coal, higher porosity and surface-area values are obtained when the cokes and not the semicokes are activated. The influence of the temperature of carbonization of the xylitic brown-coal char on the formation of porosity on oxygen activation is presented in Figure 5. For each burn-off the surface-area value of the activated coke is higher than the corresponding value for the activated semicoke. The difference is mainly in the development of micropores and to a smaller extent - in the development of the narrow mesopores. The remaining pores contribute little to the total surface area of activated semicokes and cokes. The most striking difference in the mode of formation of microporosity on oxygen activation in the semicoke and in the coke, apart from differences in absolute values of micropore volumes, is the way these pores develop with burn-off: in the semicoke the microporosity is opened gradually, reaching a maximum at the burn-off of 50% and then diminishing at a similar rate. For the coke, micropores are rendered accessible very abruptly and the maximum of their volume is at low burn-offs. For the activated semicoke, the volumes of narrow mesopores are also smaller than the respective volumes in the activated coke, but in both cases the maximum is obtained at burn-offs between 40 and 60%. At still higher burn-offs, oxygen activation of semicokes and cokes leads to a decrease of adsorptive capacity of the activated samples. The differences in the possibilities of development of porosity in brown-coal semicokes and cokes described, dependent upon the nature of the carbonized product (Figures 4 and 5), confirm that on activation the pre-existing porous
104
FUEL,
1977,
Vol
56, January
900
500
on development
of porosity
on oxygen activa-
system of the non-activated product is modified, rather than that new pores are created. The superior development of porosity in the activated cokes, rather than the semicokes, is attributable to the microporosity present in these two products of carbonization before their activation. Studies33 carried out on the development of porosity in a xylitic brown coal on carbonization? comprising adsorption measurements of nitrogen at 77 K, of carbon dioxide at several temperatures, and of other adsorbates of different cross-sectional areas at room temperature, indicate a basic difference in the size of micropores in the semicoke and the coke. The microporous system of the xylitic semicoke consists of pores with a wide range of dimensions: about one third of the micropore volume is formed by.ultra-micropores inaccessible to nitrogen, another one third has dimensions between 0.4 and 0.5 nm, and the remaining micropores are larger than 0.5 nm. The dimensions of micropores in the non-activated xylitic coke are very uniform, 95% of the micropores having a diameter between 0.4 and 0.5 irm. This uniformity and also the complete lack of ultra-micropores - which on activation might easily be inaccessible to the molecules of the activating agent - seem to facilitate the process of oxygen activation of this carbonization product.
CONCLUSIONS The development of porosity in brown-coal semicokes and cokes on activation with gaseous oxygen in the conditions described depends strongly on the origin of the carboniza-
K. Tomkbw,
A. Jankowska,
F. Czechowski
and T. Siemieniewska:
Activation
of brown-coal
XBC 900
s
/ 700
chars with
oxygen
3- 100 nm nm
600
s 3-100nm
-0
20
LO Burn-
60 off
80
600
20
100
( wt % I
Figore 5 Changes in surface area of different off in the process of oxygen activation
LO Burn-off
kind of pores in xylitic
tion product used for activation. Carbonization products from the earthy brown coal develop their porosity to a lesser extent than the respective products from the xylitic brown coal. With the earthy semicoke it was found impossible to carry out oxygen activation with the 10% oxygen in a satisfactory manner and a destruction of the initial porous system of this product was obtained as the result of the action of oxygen. The temperature of oxygen activation of the brown coal carbonization products has a strong influence on the volume of the developed pores and on pore-size distributions. With increasing temperature of activation the volume of the sum of pores responsible for adsorptive properties of the activated product (mesopores and micropores) systematically decreases; however the micropore volume passes through a maximum. There is a pronounced effect of the final temperature of carbonization of the brown-coal char on its further oxygen activation. Coke from the xylitic brown coal gives, on activation with oxygen, products of superior adsorptive properties than corresponding products from the xylitic semicoke. The main difference lies in the development of micropores. In the semicoke the micropores are formed gradually, reaching at the burn-off of 50% a maximum of about 0.2 cm3 g-l, while in the coke the micropores are opened abruptly during the first stages of activation; the volume of 0.2 cm3 g -l is already reached in the xylitic coke at the burn-off of 1% and the maximum volume (O-27 cm3 g-l) corresponds to a burn-off of 25%. This possibility of developing a deeper porosity in the coke than in the semicoke seems to be connected with a highly uniform micropore size-distribution in the initial coke sample and lack of ultra-microporosity, compared with a wider range of pore diameters in the semicoke.
semicoke
60
1 wt %
80
100
I
XBC 500 and coke XBC 900 with increasing burn-
The highest values of surface areas of oxygen-activated semicokes and cokes from the xylitic brown coal are respectively 520 and 700 m2 g-l (benzene adsorption) and correspond in both cases to burn-offs of about 50%. Further continuation of oxygen activation in the applied conditions brings about a decrease of sorptive properties of the activated products.
ACKNOWLEDGEMENTS The authors feel greatly indebted to Dr Harry Marsh of the University of Newcastle upon Tyne, UK, for valuable discussions and for his kindness in reading this article and most useful comments. We also wish to thank sincerely J6zefa Oleszkiewicz and Ahdrzej JankowSki for their help in the preparation and chemical characterization of the samples.
REFERENCES Van Krevelen, D. W. Coal, Elsevier, Amsterdam, 1961, pp 219-262 Dryden, 1. G. C. ‘Chemical Constitution and Reactions of Coal, inchemistry of Coal Wlization, Supplementary Vol. (Ed. H. H. Lowry), New York, 1963, pp 272-289 Puri, B. R. ‘Surface Complexes on Carbon’ in Chemistry and Physics of Carbon. Vol. 6 (Ed. P. L. Walker Jr). Marcel ” Dekker, New York, 1970, pp 193-249 Mattson, J. S. and Mark, H. B. Jr. Acrivated Carbon, Marcel Dekker, New York, 1971, pp 13.-20, 25-86 Marsh, H. and Rand, B. Carbon 1971, 9, 63 Marsh, H. and Foord, A. D. Carbon 1973, 11,421 Marsh, H., Foord, A. D., Mattson, J. S., ‘Thomas, J. M. and Evans, E. L. J. Colloid Interface Sci. 1974,49, 368 Gu&in, H., RebaudiBres, P., Grillet, Y. and Frarqois, M. Bull. Sot. chim. 1964, p 371
FUEL,
1977,
Vol 56, January
105
A~iivat~~~ of brow~-~oa~ chars with oxygen: 9
10 11 12 13 14 15 f6 17 18 19 20
106
K. Tomkow, A. Jankowska,
Gudrin, H., Rebaudibres,‘P. and Francois, M. Bull. Sot. chim. 1965, p 2573 Rebaudj~res, P. Thesis, Orsay, France, 1965 Grillet, Y. Thesis. &say, France, 1969 Guerin, H., Siemieniewska, T., Grillet, Y. and Franqois, M. Carbon 1970,8,727; 1971,9,657 Roga, B. and Ihnatowicz, M. Prace Glbwnego Inst. G~rn~c~wa, Komun. No. 82,195l Olpiriski, W. Archiwum Gbrnicrwa 1962,7, 183 Roga, B. and Pampuch, R. Prace Glbwnego Inst. Gdrnictwa, Komun. No. 189,1956 Szuba, J., Gubrynowicz, L. and Stromich, T. Koks-SmolnGut 1966, 11, 128, 161,209; 1967, 12,45, 116, 146,178, 197 Rozwadowski, M. and Siedlewski, J. ChemiaStosowana 1972,16,123 Siedlewski, J. and Zawadzki, J. Chemia Stosowana 1972,16, 435; 1973; 17,ll Schtitt, E. Brennst.-Chem. 1964,45,144,165,270; 1966, 47.135 W&man, U. Brennst.-Chem. 1966,47, 170; 1967,48,141
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F. Czecbowski and T. ~iemjenje~ka
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