Arch. Min. Sci., Vol. 52 (2007), No 4, p. 573–585

Arch. Min. Sci., Vol. 52 (2007), No 4, p. 573–585
573
JANUSZ CYGANKIEWICZ*, AGNIESZKA DUDZIŃSKA*, MIECZYSŁAW ŻYŁA**
SORPTION AND DESORPTION OF CARBON MONOXIDE IN SEVERAL SAMPLES
OF POLISH HARD COAL
SORPCJA I DESORPCJA TLENKU WEGLA NA KILKU PRÓBKACH POLSKICH
WĘGLI KAMIENNYCH
An experimental study has been carried out on sorption of carbon monoxide in several samples of
hard coal from Polish mines and in active carbon sample as a reference model having well developed
pores structure. It has been demonstrated that sorption of carbon monoxide is influenced by both carbon
and oxygen content in a given coal sample. Then the same samples of coal have been used for carbon
monoxide desorption tests, where it has been demonstrated that this sorption process is irreversible.
Desorption and sorption isotherms do not converge, even under low values of relative pressures. The
quantity of CO remaining within coal matter is diminishing along with increasing carbon content and
with lowering oxygen quantity in coal structure.
Keywords: hard coal, carbon monoxide, sorption, desorption, loop of hysteresis
Badania sorpcji tlenku węgla przeprowadzono na kilku próbkach polskich węgli kamiennych i na
węglu aktywnym jako układzie modelowym o dobrze rozbudowanej strukturze porów. Wykazano, że
sorpcja CO zależna jest zarówno od zawartości pierwiastka węgla jak i tlenu w badanych węglach kamiennych. Przeprowadzono również badania desorpcji tlenku węgla, w których wykazano nieodwracalność
procesu sorpcji na węglach kamiennych. Izotermy desorpcji nie zbiegają się z izotermami sorpcji nawet
przy stosunkowo niskich wartościach ciśnień względnych. Ilość pozostającego w węglu CO zmniejsza
się wraz z wzrastającą zawartością pierwiastka węgla i zmniejszającą się zawartością tlenu w strukturze
węgla kamiennego.
Słowa kluczowe: węgiel kamienny, tlenek węgla, sorpcja, desorpcja, histereza
*
**
CENTRAL MINING INSTITUTE, DEPARTAMENT OF MITING AEROLOGY, PLAC GWARKÓW 1, 40-166 KATOWICE,
POLAND; e-mail: [email protected]
AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF FUELS AND ENERGY, 30-047 KRAKÓW, AL. MICKIEWICZA 30, POLAND
574
1. Introduction
Hard coal is a complicated structure consisting of several aromatic aliphatic and
acyclic compounds formed as a result of geochemical processes underwent by organic
compounds present in plants (Czapliński (red.), 1994). The core framework of hard coal
has the form of organic polymer consisting of aromatic rings bordered by aliphatic hydrocarbons. Complicated chemical structure of hard coal was investigated by numerous
scientists who developed several models of its structure basing on the results of their
experimental studies (Van Krevelen, 1963; Ceglarska-Stefańska, 1975; Milewska-Duda,
1988). The physical structure of coal also has been studied, and especially with regard to
coal porosity (Czapliński (red.), 1994). The results of these studies led to the conclusion
that hard coal is a spatial network-like polymer, whose building elements are aromatic
macro-molecules. In the void spaces of this network molecules are located, built from
of aliphatic and acyclic compounds (Marzec, 1986). In the study by A. Marzec it was
demonstrated that important role is played by electrodonor sites and acceptor sites
rendering the coal surface the double energetic character (Marzec, 1986, 2002), that is
reflected in the results of sorption tests, especially when the sorbate molecules are polar
or dipole ones.
On the other hand the molecules of non-polar compounds and of compounds having
small dimensions, for example nitrogen or argon are very useful for determination of
porosity of adsorbents with regard to their sub-, micro-, mezo- and macropores. Numerous tests and studies were carried out on hard coal sorption of water vapor, methanol,
nitrogen, carbon dioxide and methane (Kross et al., 2002; Li et al., 2003; Webley & Todd,
2003; Żyła et al., 2005). Sorption of carbon monoxide and lower hydrocarbons saturated
and unsaturated was not the subject of research, for the reasons unknown to us, therefore
our research dealing with sorption and desorption of carbon monoxide is an unique one,
as far as we know. Due to low critical temperature of carbon monoxide, equal to 132,8K
determination of its sorption isotherms was carried out at liquid nitrogen temperature
– 77,5K; the tests were carried out on a couple of different types of coal.
The sorption of carbon monoxide molecules, taking place within hard coal structure
can be the root cause of later-on (during mining activity) emissions of CO into mine
atmosphere, where CO concentration in goaf atmosphere can reach some hundredth or
tenth even parts of one percent vol. The most common processes accompanied by CO
emissions are spontaneous heating and underground endogenous fires. Please note, that
that 0,05% concentration of CO in air can be fatal.
In the Polish mining a notion of “CO of natural origin” is usually assumed to describe situation when elevated CO concentrations in mine air are observed even though there are no other signs of self-heating process, such as distinctive evidence from
thermography or presence in air trace amounts of hydrocarbons, hydrogen or other
gases usually accompanying the processes of spontaneous combustion of coal. Also,
in boreholes drilled in coal seams high concentrations of CO are encountered, at times
575
exceeding 1% vol. level (Wacławik et al., 2000). It can be assumed that CO emitted in
such unclear circumstances can be a result of releasing CO due to its desorption from
coal matter.
Because of toxic properties of CO and miners safety we believe it is worth undertaking
a study to determine the conditions favoring significant CO release due to desorption
from within coal structure.
2. Properties of CO
CO in normal conditions is a gas, odorless and without color, is lighter than air and
its water solubility is much lesser than water solubility of CO2.
The structure of CO molecule can be described according to the theory of molecular
orbitals, as consisting of bonding orbital σ and two bonding π orbitals with electrons and
anti-bonding orbitals without electrons. Therefore, the atoms of carbon and oxygen are
connected by triple bonds (one δ bond and two π bonds) (Dzięgielewski, 1985).
The molecule also has the external non-bonding orbital with two electrons, that
influences to the great extent chemical properties of CO. Just because of this pair of
electrons shifted towards the carbon atom and being on the highest energy level the CO
molecule acts as a donor. On the oxygen molecule also a pair of electrons can be found,
but being on much lower energy level (Dzięgielewski, 1985). According to Bielanski
(2000) this nonbonding pair of electrons, responsible for donor property of the molecule
is located on the orbital being the result of so called dygonal hybridization of orbitals
where the 2s orbital of the carbon atom combines with one of the orbitals of oxygen.
CO molecules can create coordinate bonds mainly in complex compounds, where they
act as donors and create carbonyls of metals from lateral groups. The toxic properties
of CO are also related to the structure of CO molecules that combine with ions Fe3+ of
hemoglobin and form ferrous carbonyls (III), thus blocking oxygen transport process
in human body.
Under the influence of electrostatic field electrons of the carbon atom can be shifted
towards positive ions. Also, there are suggestions that sorption of carbon monoxide
can take place on positive ions located on the surface of solids. One can guess that
CO molecules, due to their properties as donors as well as weak acceptors can interact
with π-electrons of solids. This is of some importance during CO sorption in polymeric
structures built from condensed benzene rings, that can be found in the aromatic structure
of hard coal.
The aromatic part of hard coal contains large amount of free π-electrons. Also, it
is highly probable that electrostatic interaction occurs between oxygen and energetic
sites in hard coal, originating from the presence of hydroxyl and carboxyl groups in the
structure of coal.
576
Also, it should be noted that CO molecules show strong ability for de-oxidation,
stronger even than those of hydrogen, therefore a CO molecule transforming easily into
CO2 can reduce in efficient way oxides of many metals.
CO is a weak dipole, its dipole moment is 0,3 .10–30 C.m, the value of critical
temperature of CO is close to the temperature of nitrogen or methane and differs
greatly from the critical temperature of CO2 – namely, their critical temperatures are:
CO = 132,8K, N2 = 126K, CH4 = 190,5K, CO2 = 304,1K.
It should be noted also that kinetic diameter of CO molecules is 0,32 nm and is a little
greater than diameter of nitrogen (0,3) and carbon dioxide (0,28).
The surface area occupied by a CO molecule on surface of a solid is 0,20 nm2
and is larger than surface area taken by a nitrogen or methane molecule (respectively
N2 = 0,162 nm2, CH4 = 0,181 nm2).
The above presented physical properties of the gaseous adsorbates can be of great
importance with regard to their sorption in hard coals having different density of energetic
sites and different porosity.
3. Experimental part of the study
Experiments on sorption and desorption of carbon monoxide were carried out on
six carefully chosen samples of hard coal from Chwałowice hard coal mine (seam No.
404), Zofiówka mine (seam 404/4), Sośnica mine (seam 413), Wesoła mine (seam 501),
Pniówek mine (seam 360), Jaworzno mine (seam 209). The chemical and petrographic
properties of coal samples are presented in the Table 1.
Elementary analysis of the samples was carried out in the Fuel Quality Assessment
Department of the Central Institute of Mining, whereas the sorption and desorption
experiments took place in Mining Ventilation Department of the Institute
For the tests coal samples of 0,125-0,25 mm grain size of were selected from coals having different carbon content and different susceptibility to spontaneous combustion.
Carbon monoxide sorption isotherms were determined at liquid nitrogen temperature
– 77,5K, using volumetric method on the ASAP 2010 apparatus from Micromeritics.
The final vacuum that can be achieved by the pumps set of this apparatus is 5,06 .10–7
Pa. Before each isotherm sorption measurement session the samples were degassed at
318K by stream of helium, the process repeated several times till desired vacuum of
5,063 .10–7 Pa was reached.
At 77,5K diffusion of carbon monoxide into micro and sub-micro pores system is
slow, therefore long is the time to reach sorption equilibrium, and at so low a temperature
there is also possibility of shrinking of coal structure resulting in narrowing of transport
pores, thus diminishing sorption capacity of hard coal.
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TABLE 1
Chemical and petrographic properties of coal samples
TABLICA 1
Charakterystyka chemiczna i apetrograficzna
Contituent content
carbon
sulphur total
sulphur from pyrite
hydrogen
nitrogen
sulphur from ash
sulphur combustible
oxygen (calculated)
moisture
ash
volatile matter
vitynite
reflexivity of vitynite
liptinite (exinite)
inertynite
mineral substance
Samples from the following mines
Symbol Jedn. Pniówek Wesoła Chwałowice Zofiówka Sośnica Jaworzno
p. 360 p. 501
p. 404
p. 404/2 p. 413
p. 209
%
84,24
79,46
79,29
78,62
70,82
57,83
Cta
Sta
%
0,39
0,32
0,35
0,35
3,50
1,10
%
0,01
0,07
0,16
0,01
3,20
0,71
Spa
Hta
%
4,58
4,55
4,97
4,37
3,35
3,37
Na
%
1,52
1,27
1,07
1,15
1,28
0,87
SAa
%
0,07
0,22
0,19
0,18
1,27
0,03
Sca
%
0,32
0,10
0,16
0,17
2,23
1,07
Oda
%
4,58
8,07
8,47
6,17
6,29
11,30
%
1,75
3,69
3,39
0,60
1,85
11,11
Wa
Aa
%
3,01
2,86
2,65
8,92
14,18
14,45
Va
%
27,12
30,40
35,62
27,93
29,88
28,39
Vt
% obj.
73
38
60
91
60
67
R0
%
0,92
0,72
0,70
1,01
0,78
0,51
L
% obj.
7
9
10
1
9
5
I
% obj.
20
53
30
8
31
28
M
% obj.
1
2
1
4
14
11
It should be underlined that at such temperatures (77,7K to 320K) sorption experiments
have been carried out for many years during studies on chemical properties and energy
distribution on coal surface. The results of experiments form the basis for theoretical
studies (Żyła et al., 2005), (Milewska-Duda, 1987) and research pertaining to chemistry
of surface of coal, reactivity of coal and diffusion of gases in pores (Żyła (red), 2000;
Bolt & Innes, 1953; Kawęcka & Lasoń, 1968).
4. The results of sorption experiments
The obtained isotherms of sorption of carbon monoxide (at 77,5K) on six coal samples
from Polish mines are presented in Fig. 1. A distinctive shape of CO sorption isotherm
was obtained from the coal sample from Jaworzno coal mine, having very low carbon
content (11,3%) high moisture content (11,1%) and high overall porosity (15,86%).
High moisture content and high oxygen concentration indicate that there is possibility
that high number of oxygen groups is present in the structure, thus causing high polarity of the surface. One cannot exclude that there is possibility of energetic interaction
between oxygen sorption sites and carbon monoxide molecules. The five remaining
578
70
volume adsorbed, cm3/g
60
50
40
Soœnica p. 413
Pniówek p. 360
Zofiówka p. 404/2
Weso³a p. 501
Chwa³owice p. 404
Jaworzno p. 209
30
20
10
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 1. Isotherms of sorption of carbon monoxide determined at 77.5K for coal samples form coal mines
Rys. 1. Izotermy sorpcji tlenku węgla wyznaczone w temp. 77,5 K dla prób węgli kamiennych
z poszczególnych kopalń
samples of hard coal adsorbed insignificant amounts of CO in comparison with this
sample from Jaworzno mine.
In the Fig. 2 the graph is presented, with its scale adjusted, of the five isotherms of
CO sorption for the samples with low adsorption. The highest sorption capacity had the
sample from the Wesoła coal mine. This sample and the sample from the Chwałowice
volume adsorbed, cm3/g
6
Soœnica p. 413
Pniówek p. 360
Zofiówka p. 404/2
Weso³a p. 501
Chwa³owice p. 404
5
4
3
2
1
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 2. Isotherms of sorption of carbon monoxide determined at 77.5K for hard coal samples
form coal mines
Rys. 2. Izotermy sorpcji tlenku węgla wyznaczone w temp. 77,5 K dla prób węgli kamiennych
z poszczególnych kopalń
579
coal mine had similar oxygen, moisture and carbon contents and despite this similarity
in chemical composition the Chwałowice sample adsorbed significantly less carbon
monoxide the Wesoła sample. This difference in sorption capacity can be attributed to
differences in pertographic composition of the samples – namely the Wesola sample had
the highest content of vitrinite.
The lowest sorption capacity was noted for coal samples from Zofiowka and Pniowek.
It should be stressed that these samples (and especially the Pniowek sample) had the
highest carbon content in their structure and relatively low oxygen content. It is evident
from the presented isotherms the sorption capacity is related to oxygen content and
oxygen sorption centers distribution within the tested samples.
In order to assess the influence of micro and sub-micro porosity on sorption of
carbon monoxide another referential isotherm was determined of CO sorption on active
carbon.
In Fig. 3 isotherms of sorption for well-CO-sorbing coal samples from Jaworzno
and Wesola were confronted with the referential isotherm of sorption determined for the
sample of active carbon. The active carbon showed 5 times greater sorption capacity than
the best sorbing sample form the Jaworzno coal. That difference implicates that despite
hydrophobic surface of active carbon its CO sorption is very high, probably because of
full accessibility of numerous micro and sub-micro pores of active carbon for the tested
adsorbate. For the coals well coalified (Pniowek, Zofiowka) sorption of carbon monoxide
takes place generally on surface of coal grains and at near-surface macro pores.
400
volume adsorbed, cm3/g
350
300
250
200
active carbon
Weso³a p. 501
Jaworzno p. 209
150
100
50
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 3. Isotherms of sorption of carbon monoxide determined at 77.5K for active carbon,
coal from Jaworzno and Wesoła mines
Rys. 3. Izotermy sorpcji tlenku węgla wyznaczone w temp. 77,5 K na węglu aktywnym,
węglu z kopalni Jaworzno i kopalni Wesoła
580
5. Desorption of carbon monoxide
In coal mines it that high CO emissions is frequently observed, having no obvious
connection to any spontaneous heating processes. It is probable that the cause of such
phenomenon is the releasing of CO contained within porous structure of hard coal, i.e.
it can be the effect of CO desorption taking place in favorable conditions.
The experimental tests of CO desorption were carried out on the same coal samples
that had undergone CO sorption, by means of steady lowering of pressure around a given
sample. The tests were performed on the five samples and an active carbon sample treated
as the referential adsorbent having good accessibility of micro and sub-micro porous.
The isotherms obtained of sorption and desorption of CO are presented in figures 4-10.
From the shapes of the isotherms of sorption and desorption on active carbon (Fig. 4)
it is evident that process of sorption is reversible. The isotherm of sorption joins the
isotherm of desorption at as low relative pressure as p/p0 = 0,3. A little, but distinctive,
loop of hysteresis can be observed between the two isotherms. Full reversibility of
sorption indicates that there is dispersional interaction between CO molecules and coal
substance. Durable chemical bonding of CO molecules was not observed.
400
volume adsorbed, cm3/g
350
300
250
200
150
sorption
desorption
100
50
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 4. Isotherms of sorption and desorption of carbon monoxide in active carbon sample
determined at 77.5K
Rys. 4. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K na węglu aktywnym
From the analysis of sorption and desorption isotherms of all hard coal samples it
can be stated that this process is irreversible. Isotherms of sorption did not meet those of
desorption, the hysteresis loop is open. Depending on type of hard coal some different
quantities of carbon monoxide remained inside samples (Fig. 5-9).
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volume adsorbed, cm3/g
2,5
2
1,5
1
sorption
desorption
0,5
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 5. Isotherms of sorption and desorption of carbon monoxide in Sośnica mine
sample determined at 77.5K
Rys. 5. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Sośnica
4,5
volume adsorbed, cm3/g
4
3,5
3
2,5
2
1,5
sorption
desorption
1
0,5
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 6. Isotherms of sorption and desorption of carbon monoxide in Pniówek mine
sample determined at 77.5K
Rys. 6. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Pniówek
The irreversibility of carbon monoxide sorption on all coal samples can be observed.
The quantity of carbon monoxide remaining within coal structure depends on metamorphism degree of a given coal sample. It is clear especially in the case of the sample of
high oxygen content (Jaworzno mine). The hysteresis loop is wide and does not close
582
volume adsorbed, cm3/g
70
60
50
40
30
sorption
desorption
20
10
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 7. Isotherms of sorption and desorption of carbon monoxide in Jaworzno mine
sample determined at 77.5K
Rys. 7. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Jaworzno
4,5
volume adsorbed, cm3/g
4
3,5
3
2,5
2
1,5
sorption
desorption
1
0,5
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 8. Isotherms of sorption and desorption of carbon monoxide in Zofiówka mine
sample determined at 77.5K
Rys. 8. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Zofiówka
even under pressure of p/p0 = 0,2. In the case of highy coalified coals (Pniówek 82,24%
of C) the irreversibility of sorption is less evident, and estimated share of remaining
CO is 2-3%.
583
volume adsorbed, cm3/g
6
5
4
3
sorption
desorption
2
1
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 9. Isotherms of sorption and desorption of carbon monoxide in Wesoła mine
sample determined at 77.5K
Rys. 9. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Wesoła
volume adsorbed, cm3/g
3
2,5
2
1,5
sorption
desorption
1
0,5
0
0
0,2
0,4
0,6
0,8
1
1,2
relative pressure p/p0
Fig. 10. Isotherms of sorption and desorption of carbon monoxide in Chwałowice mine
sample determined at 77.5K
Rys. 10. Izotermy sorpcji i desorpcji tlenku węgla wyznaczone w temp. 77,5K
na węglu z kopalni Chwałowice
A little more of CO remains in the samples of lesser carbon content (79,29 % C and
78,62% Chwałowice i Sośnica). Many times more of adsorbed CO remained in the structure of coal samples from the Jaworzno mine, having 57,83% C and 11,30 % O2. The
estimated amounts reached about 10% even. It was possible in the case of the samples
584
of this coal of accessible micropores structure and numerous energetic sites on surfaces,
because surface energetic sites can combine with CO by non-specific strong bonds. Such
interaction is not observed in the case of CO sorption in active carbon. In this case CO
molecules are engaged by weak bonds of dyspersional interaction.
The results of tests of sorption and desorption of carbon monoxide indicate that within
Polish coal seams of hard coal remain some quantities of adsorbed carbon monoxide
and that the gas can undergo desorption in favorable conditions, thus causing hazard for
miners or triggering false alarms by CO-measuring installations.
6. Conclusions
1. The tests on sorption of carbon monoxide at 77,5K clearly demonstrate that the
quantity of adsorbed carbon monoxide depend on oxygen and carbon content in
coal structure. Probably the process of sorption takes place as the result of specific
interaction between oxygen carbon monoxide molecules with coal surface oxygen
sites. Active carbon, having distinctive hydrophobic surface (non-polar one), sorbs
much larger quantities of carbon monoxide despite the fact that it combines with
carbon monoxide molecules by means of dyspersional interaction only. It is the
result of the presence of much large volume of micro and sub-micro-pores available
for the molecules of this gas. At the liquid nitrogen temperature only a part of the
sub and micro pores system is accessible for carbon monoxide molecules. The
volume of this part depends on carbon content in the polymeric structure of hard
coal. In the case of hard coals that underwent high coalification process the sorption of carbon monoxide takes place mainly on the external surface of coal grains.
2. Another achievement claimed by the authors of this study is the experimental
demonstration of irreversibility of sorption of carbon monoxide. The share of gas
remaining in coal structure depends on oxygen and carbon content in coal structure. Significant volume of non-desorbed carbon monoxide is combined with coal
structure with low carbon content and can reach as high level as 10%. Such volume
of gas can be harmful or even dangerous for miners, when released as a result of
a change of physical conditions around coal seam, especially of temperature.
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Received: 03 July 2007