505 1. Introduction Hard coal belongs to the group of natural

Archives of Mining Sciences 50, Issue 4 (2005) 505–515
505
MIECZYSŁAW ŻYŁA*, JANUSZ CYGANKIEWICZ**, AGNIESZKA DUDZIŃSKA**
SORPTION OF NITROGEN AND CARBON DIOXIDE ON A NUMBER OF SAMPLES OF HARD COAL
HAVING DIFFERENT CARBON CONTENT
SORPCJA AZOTU I DITLENKU WĘGLA NA KILKUNASTU PRÓBKACH WĘGLA KAMIENNEGO
O WZRASTAJĄCEJ ILOŚCI PIERWIASTKA WĘGLA W STRUKTURZE
Specific area values determined for 14 samples of hard coal from Polish mines are clearly different
for samples characterized by different metamorphism degree.
Sorption of carbon dioxide performed at room temperature is almost ten times greater than nitrogen
sorption carried out at temperature of liquid nitrogen (77.5K). Quantities of nitrogen sorbed are relatively
low and still are different for different carbon and volatile matter contents of samples. Similar changes
are observed in the case of sorption of carbon dioxide at room temperature.
Keywords: hard coal, sorption, specific surface, micropores volume, macropores volume, porosity
Wyliczone na badanych czternastu próbkach polskich węgli kamiennych wartości powierzchni
właściwych są wyraźnie zróżnicowane w zależności od stopnia metamorfizmu. Sorpcja ditlenku węgla przeprowadzona w temperaturze pokojowej jest niemal dziesięciokrotnie większa od sorpcji azotu
wyznaczonej w temperaturze ciekłego azotu (77,5K). Ilości sorbowanego azotu są stosunkowo nieduże
a pomimo to wykazują wyraźne różnice w zależności od procentowej zawartości pierwiastka węgla
i części lotnych. Podobne zmiany mają miejsce w przypadku sorpcji ditlenku węgla przeprowadzonej
w temperaturze pokojowej.
Słowa kluczowe: węgiel kamienny, sorpcja, powierzchnia właściwa, objętość mikroporów objętość
makroporów, porowatość
1. Introduction
Hard coal belongs to the group of natural carbonaceous sorbents, such as for example
active carbon whose properties enable to apply it as catalyst, catalyst carrier and sorbent
used for cleaning air and water. Active carbon is used also as a container for fuels and
* WYDZIAŁ PALIW I ENERGII, AKADEMIA GÓRNICZO-HUTNICZA, AL. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND
** ZAKŁAD AEROLOGII, GŁÓWNY INSTYTUT GÓRNICTWA, PL. GWARKÓW 1, 40-166 KATOWICE, POLAND
506
gaseous substances as well as for separation of low- molecular substances (Buczek &
Czepirski, 1999, 2000). These properties are the results of its highly developed structure
of micropores and submicropores. Hard coal has similar, but les developed properties
and its sorption properties are determined by phenomena occurring during carbonization processes of organic material (Buczek & Czepirski, 2000). When one compares
sorption properties of coal and other natural sorbents, such as aluminosilicates it is
easy to say that hard coal’s economical and industrial importance stems from its interesting structure with regard to porosity and multi for chemical and energo-chemical
surface (Jasieńko-Hałat, 1996). The basic framework of hard coal structure consists
of polymer, containing aromatic rings with various condensation degree. On the border
of the polymer some aliphatic and heterocyclic structures can be found, forming the
outer cover, thus hindering the access of vapors and gases into micro-porous polymer.
This structure is unfavorable one for sorption of gases at low temperatures, i.e. liquid
nitrogen 77K (Jasieńko-Hałat, 1996; van Hecek & Hodek, 1994). Nevertheless, hard
coal is favorable for sorption processes occurring at room temperature, responsible for
the natural phenomenon of accumulation of low- molecular compounds such as water,
carbon-dioxide and monoxide, methane and other gaseous substances – that prevents
industrial and ecological disaster from occurring in mines. The important factor occurring within the structure of hard coal is the sorption of water molecules taking place on
oxygen functional groups (Żyła, 1963). The quantity of water sorbed depends on surface
concentration of these groups, that, having polar properties, can bind water molecules
with hydrogen bond (Żyła, 1963).
O.P. Mahajan (Mahajan, 1991) showed in his works, that sorption of water in the first
stage occurs mainly on primary sorption centers. The molecules adsorbed act as secondary sorption centers, and in the final stage it leads to formation of 3D cluster groups. But
hydrophilic groups are separated by hydrophobic parts of surface responsible of sorption
of non-polar compounds (Żyła et al., 1991). On these hydrophobic centers sorption of
non-polar compounds can take place, for example chained-cyclic, aromatic and ring
hydrocarbons. Non-polar units have influence also on sorption of compounds consisting of radicals of aliphatic alcohols, amines and acids. Relation between the quantity
of water and methyl alcohol molecules sorbed is presented in the Figure 1, the values
of am are calculated on the basis of the shapes of sorption isotherm curves determined
for water an methyl alcohol (Żyła et al., 1997; Stachurski & Żyła, 1995). The values of
am are treated as the quantities of molecules of the respective adsorbate sorbed on hard
coal in the first adsorbtion layer, therefore it is the index of primary sorption centers:
curve A represents changes of centers determined from sorption of vapours of CH3OH,
whereas the curve B describes the changes of am values determined for sorption of
water vapor. Generally, the shapes of these curves are similar to each other but in the
case of water vapor the minimum of polar centers of sorption for the samples of coal
having 68 to 92 % of carbon content is clear, whereas the methyl alcohol curve is fairly
descending without distinct minimum. It is so because of dual character of the molecule
507
6
H 2O
5
CH3OH
am [mmol/g]
4
3
Fig. 1. Changes of quantity of polar
sorption centers determined from
water vapor and methyl alcohol
isotherms as a function of carbon
content, %C
2
Rys. 1. Zmiana ilości polarnych centrów
sorpcji wyznaczonych z izoterm
sorpcji par wody i alkoholu metylowego w funkcji procentowej zawartości pierwiastka węgla, %C
1
0
75
80
85
90
95
% C daf
CH3OH, whose radical -CH3 interacts with non-polar parts of coal, and polar hydroxyl
groups interact with polar centers on the surface of coal. This layout is the reason why
molecules of methyl alcohol are not particularly affected by chemical character of coal
(Żyła, 1963). The increase of the value of am for hard coal having above 92% of carbon
content is the result of graphite-like structure of such coal. In the paper by J. Choma
and M. Jaroniec (2001) it was demonstrated that the function of adsorption potential
determined on the basis of water sorption isotherms can be used for calculation of the
quantity of primary centers of adsorption, and it is believed that oxygen functional
groups are such centers. The significant progress on the field of interpretation of polar
substances sorption experiments was made due to determination of theoretical models
of dual sorption (MSD) by J. Milewska-Duda (1991) and al. and multi-sorption (MSW)
(Milewska-Duda, 1993).
The dual sorption model enables to identify in water vapor sorption and methyl alcohol
processes curves describing absorption and adsorption phenomena.
The dual sorption model enables to identify in the water vapor and methyl alcohol
sorption processes the curves describing absorption and adsorption phenomena.
Absorption in this approach boils down to storing of low-molecular substances in
pores of the area between sorbate’s molecules, whereas adsorption has properties of
508
a localized process occurring in micropores (Langmuire’s nature) and macro-pores
(Milewska-Duda, 1991).
Multi-sorption model is an original mathematical approach pertaining to energy and
entropy interaction in hard coals. On the basis of that model several co-relations were
derived occurring between absorption and adsorption processes. This model enables to
better understand swelling processes, to evaluate porous structure parameters and energy distribution on the surface and the molecular structure of coal. This mathematical
approach opened several possibilities to widen the interpretation of fundamental results
of sorption experiments (Milewska-Duda, 1993; Żyła (red.), 2000).
2. Sorption properties of hard coal in respect with gaseous adsorbates
Hard coal is an adsorbent affected by temperature of adsorption process, dimensions
and structure of adsorbate used. In many cases hard coal has properties of molecular
sieves (Jodłowski, 2000; Lasoń (red.), 1988).
3. Low temperature sorption of nitrogen
In this point our attention should be turned into low temperature sorption of nitrogen,
argon or crypton and sorption processes occurring in normal temperature.
Specific surfaces calculated on the basis of low temperature sorption of nitrogen or
argon are many times lower than those calculated from sorption of water, carbon dioxide or methyl alcohol. Decrease in sorption ability of coal in low temperatures is the
result of contraction of thermo-labile structure of coal, hindering access of molecules of
gases into micro and submicropores structure. Besides, at low temperature of sorption
molecules of nitrogen and argon have low kinetic energy and they do not permeate into
micro-pores structure. It was also observed that contraction of coal structure can occur
during progressive process of pores blocking (Jodłowski, 2000).
4. Sorption of carbon dioxide on hard coal
From the papers published in the recent years it emerges that carbon dioxide permeates
well into porous structure of coal (Lasoń (red.), 1988). Measurement of CO2 sorption
isotherms are carried out at temperatures close to ambient temperature and kinetics in
this temperature is sufficient enough to secure sorption equilibrium in short time. It is
the result of small dimensions of CO2 molecule, its dual dipole structure, called quadrupole, and low activation energy. The values of specific surfaces calculated from CO2
adsorption isotherms (at 293 K) are in the range from 150 to 230 m2/g and the majority
509
of the researchers assumes that is the correct estimation (Jodłowski, 2000). From the
publication of M. Wojcik it emerges that the total surface of hard coal can be calculated
from CO2 sorption isotherms. The surfaces of walls of micro and macro-pores are accessible for CO2 molecules at temperature of 293K, enabling CO2 to interact both by the
way of chemical and physical sorption. High values of surface linked to CO2 sorption
are the result of their easy access into the interior of pores and easy propagation that
secures quick transport of CO2 molecule even to the finest pores of coal. It appears that
CO2 sorption, analogically to water vapor sorption, depends on the energetic interaction
with polar groups of coal (Lasoń (red.), 1988). Relatively high values of CO2 sorption
heat values demonstrate that carbon dioxide binding in coal has absorption – adsorption
character (Czapliński (red.), 1994). The values of isosteric sorption heat depend on rank
of coal and real density of a sample (Czapliński (red.), 1994). In CO2 desorption processes temperature of samples decreases, and this temperature depends on desorption rate,
grain dimensions and coal type (Wójcik, 1999; Marsch & Siemieniewska, 1965). The gas
sorbed on coal in certain conditions can exert some work, that is the cause of coal eject
incidents. The CO2 releasing rate depends on its capillary structure, determined in some
way by maceral content of coal matter (Zarębska, 2002; Ceglarska-Stefańska & Zarębska, 2001). After initial ejection burst of CO2 in coal significant amount of CO2 remains
in micro and sub-micro pores (Ciembroniewicz & Marecka, 1992; Nodzeński, 1995).
5. Experimental part
During experiment sorption of carbon dioxide at 293K, and nitrogen at 77,5K was
carried out on more than a dozen samples of coal having clearly different carbon and volatile matter contents and different porosity. The samples were collected in selected Polish
coal mines. Volatile matter content should emphasize the influence of aliphatic acyclic
units located on the borders of aromatic polymer on nitrogen sorption (at 77,5K) and
on carbon dioxide (at 293K). Generally, aliphatic-acyclic hydrocarbons can in elevated
temperatures transform into volatile matter. Changes in coal metamorphism degree have
some effect on changes of surface quantity of oxygen polar group, porosity and volatile
matter content of coal. These parameters should influence also sorption parameters of the
selected coals. From the curves representing CO2 an N2 sorption isotherms the specific
surfaces at 293K (CO2) and 77.5K (N2) were calculated. Basing on the results the graphs
were drawn showing relation between specific surface and carbon content, volatile matter
content and porosity. In the Table 1 the values of the parameters are presented. The coal
samples selected for experiments had carbon content in the range from 69.9 to 87.3%
and their volatile matter content was in the range from 29.1 to 35%. In Table 1 chemical
characteristics of the samples and the results obtained are also presented.
510
TABLE 1
The results of chemical analysis and specific surface calculated
from nitrogen (at 77.5K) and carbon dioxide (at 293K) adsorption
TABLICA 1
Wyniki chemicznej analizy i powierzchni właściwej wyznaczonej
z adsorpcji azotu (77,5K) i ditlenku węgla (w 293K)
No.
Mine
Carbon
content
C, %
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Jaworzno 209
Piast 206
Julian 504
Staszic 402
Staszic 405
Staszic 510
Albert
Jankowice 413/1
Erna 3
Borynia 415/1-2
Borynia 405/1
Marcel 707
Anna 703
Bielszowice 507
64.96
69.10
72.11
72.92
73.59
74.12
76.28
78.43
80.06
81.02
81.20
84.21
84.50
87.34
Volatile
matter
contents
V a, %
29.93
35.00
33.20
34.01
34.18
33.70
33.33
33.11
31.91
29.10
29.30
30.20
30.10
31.17
Surface
Micro-pores
Surface
volume
m2/g BET, m2/g Dubinin,
cm3/g
N2
CO2
13.43
8.96
2.38
3.83
5.54
4.86
3.84
2.89
2.09
2.20
3.02
2.08
2.52
3.83
185.60
181.78
186.16
134.80
147.68
120.67
112.06
126.61
110.80
114.35
118.14
108.84
85.69
92.44
0.074
0.073
0.075
0.054
0.059
0.048
0.050
0.051
0.044
0.046
0.047
0.042
0.034
0.037
The results obtained from the sorption tests are presented in the form of several graphs, showing changes of specific surface in m2/g and porosity as function of carbon and
volatile matter contents. In Figure 2 relation ship between specific surface calculated
from nitrogen sorption and carbon content is presented. This curve has the minimum
being the characteristic feature of coals having carbon content in the range from 77 to
82%, i.e. coking coals with high content of hydrocarbons acyclic rings. The highest values of specific surface showed coal samples of low-carbon content contrasting with the
values for samples of coking coals. Taking into account the observation that sorption of
nitrogen at 77.5K which takes place generally in pores and macro-pores with exclusion
of micro and sub-micro-pores it can be said that aliphatic acyclic hydrocarbons located
on the outer surfaces of aromatic polymer hamper to a great extent access of nitrogen
into this complex. This assumption can be supported by the analysis of values of surface
for coal of low carbon content (13 m2/g) and also coking coals having small surface
(2 m2/g). High carbon content coals (above 85% of C) have twice as high sorption of
nitrogen, that is certainly the result of higher accessibility of macro-pores of those coals
for nitrogen. The curve representing changes of nitrogen surfaces as a function of carbon
content is similar in his shape (Figure 2) to the curve of changes of water vapor sorption
or methyl alcohol (Figure 1). More distinct character has the curve depicting changes of
specific surface calculated from nitrogen sorption as a function of volatile matter content
511
16
14
Surface area, m2/g
12
10
8
Fig. 2. Changes of specific surface values determined from nitrogen sorption
(at 77.5K) as a function carbon
content, %C
6
4
Rys. 2. Zmiana wartości powierzchni
właściwej wyznaczonej z sorpcji
azotu (77,5K) w zależności od
procentowej zawartości pierwiastka
węgla, %C
2
0
60
70
%C
80
90
(Figure 3). In the remaining graphs the changes of specific surfaces calculated from carbon
dioxide sorption as a function of carbon content are represented. The maximal values of
surface can be seen for low-carbon content coals, then go medium carbon –content coals
and in the case of coals of high carbon content this values are slightly increased. From
the Figure 4 it can be seen that the relation is not linear but band-like. The coals having
similar carbon contents can differ in surface values by as high as 25 m2. Probably it is
the effect of clay minerals content in samples, that characterize high absorption power
for CO2. Similarly, high scatter of values can be noted for specific surfaces calculated
from CO2 sorption isotherms, that is depicted in Figure 5 representing changes of specific
surface values in relation to volatile matter content. For samples whose volatile matter
is in the range 29%-33% the specific surface values increase only slightly, and above
33% they increase almost in a straight-line manner. Taking into account the probability
of CO2 penetration into micro-pores system their volumes were calculated using Dubinin-Raduszkiewicz equation, and their values in relation to carbon and volatile matter
contents are presented in figures 6 and 7.
Despite high scatter of results of the calculated micro-pores volume a band-like relation can be determined between pores volume and carbon and volatile matter contents.
Perhaps it is the result of some non-organic compounds, of mineral origin, and their
contribution to overall sorption of CO2 (Krzyżanowski & Żyła, 1988).
512
10
Surface area, m2/g
8
6
4
Fig. 3. Changes of specific surface values
determined from nitrogen sorption
(at 77.5K) as a function of volatile
matter content , %V a
2
Rys. 3. Zmiana wartości powierzchni
właściwej wyznaczonej z sorpcji azotu
(77,5K) w zależności od procentowej
zawartości części lotnych, %V a
0
25
30
% Va
35
40
250
Surface area, m2/g
200
150
Fig. 4. Band-like changes of specific
surface values determined from
carbon dioxide sorption (at 293K)
as a function of carbon content, %C
100
Rys. 4. Pasmowa zmiana wartości powierzchni właściwej wyznaczonej
z sorpcji ditlenku węgla (293K)
w funkcji procentowej zawartości
pierwiastka węgla, %C
50
0
60
65
70
75
%C
80
85
90
513
190
170
Surface area, m2/g
150
130
110
Fig. 5. Band-like changes of specific
surface values determined from carbon
dioxide sorption (at 293K) as a function of volatile matter content, %V a
90
Rys. 5. Pasmowa zmiana wartości powierzchni właściwej wyznaczonej
z sorpcji ditlenku węgla (293K)
w funkcji zawartości części
lotnych, %V a
70
50
25
30
35
40
% Va
0.08
Volume of micropores, cm3/g
0.07
0.06
Fig. 6. Band-like changes of volume
values (cm3/g) determined from
carbon dioxide sorption (calculated
with the use of Dubinin-Raduszkiewicz equation) as a function of carbon
content, %C
0.05
0.04
Rys. 6. Pasmowa zmiana objętości mikroporów (cm3/g) wyznaczonych
z sorpcji ditlenku węgla (wyliczonych
na podstawie równania Dubinina-Raduszkiewicza) w funkcji zawartości
pierwiastka węgla, %C
0.03
0.02
60
70
80
%C
90
514
0.08
Volume of micropores, cm3/g
0.07
0.06
0.05
0.04
Fig. 7. Band-like changes of micro-pores
volume (cm3/g) determined from
carbon dioxide sorption as a function
of volatile matter content, %V a
0.03
Rys. 7. Pasmowa zmiana objętości mikroporów (cm3/g) wyznaczonych
z sorpcji ditlenku w funkcji procentowej zawartości części lotnych, %V a
0.02
27
29
31
% Va
33
35
37
6. Conclusions
1. Values of specific surface calculated from CO2 and nitrogen sorption isotherms
are clearly different. CO2 sorption is almost ten times higher than low-temperature
sorption of nitrogen.
2. Despite relatively low nitrogen sorption the values of specific surface are clearly
related to carbon and volatile matter content.
3. Specific surface values calculated from CO2 sorption isotherms also depend on
carbon and volatile matter contents but this relation is not linear but band-like. The
scatter of the results is probably caused by the presence of some clay minerals of
high CO2 sorption power.
4. Also, the relationship has been observed between micro-pores volume calculated
from CO2 sorption and carbon and volatile matter content.
Research has been undertaken with financial support of Ministry of Science and Information Society
Technologies (Project No. 4 T12A 032 27).
515
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REVIEW BY: PROF. DR HAB. INŻ. BRONISŁAW BUCZEK, KRAKÓW
Received: 03 October 2005