Arch. Min. Sci., Vol. 55 (2010), No 1, p. 59–67

Arch. Min. Sci., Vol. 55 (2010), No 1, p. 59–67
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Electronic version (in color) of this paper is available: http://mining.archives.pl
PAWEŁ BARAN*, MICHAŁ BROŚ*, ADAM NODZEŃSKI*
STUDIES ON CO2 SORPTION ON HARD COAL IN THE NEAR-CRITICAL AREA WITH REGARD
TO THE ASPECT OF SEQUESTRATION
BADANIA SORPCJI CO2 NA WĘGLU KAMIENNYM W OBSZARZE PRZYKRYTYCZNYM
W ASPEKCIE SEKWESTRACJI
Recently more and more emphasis has been laid on reducing the emission of carbon dioxide. Attempts
are undertaken to reduce the emission of this gas through the works on the low-emission technologies
of fuel combustion. Another way to lower the emission of CO2 is to capture the gas and deposit it in
deep geological layers. One of the options of geological sequestration, next already applied the EOR
method (Nagy & Olajossy, 2008; Núñez-López et al., 2008) is pumping CO2 into deep, fully exploited
or unexploited beds of hard coal.
The issue of carbon dioxide sorption studies is presently handled with regard to the possibility of
sequestrating this gas in deep geological structures. Considering the depth of the storage centre and
assuming that hydrostatic pressure gradient at the level of 10 MPa/km and the geothermal gradient at
the level of 30 K/km (Tarkowski, 2005), it is expected that the stored carbon dioxide will appear in the
supercritical state. The supercritical phase shows the properties of a gas with high density, close to the
density of fluid substances which changes depending on the pressure and temperature in the range from
200 up to 900 kg/m3 (Fig. 1) (Holloway and Savage, 1996).
The definition of supercritical adsorption which is presented in the work is ambiguous in the literature,
due to various approaches regarding the definition of the supercritical phase (fluid). One of the approaches
says that the fluid is supercritical when both the temperature and the pressure reach values above the critical
point. However, adsorption in supercritical conditions (supercritical adsorption) is commonly interpreted
to be a process which takes place in a temperature above the critical temperature (Toth, 2002).
The work presents experimental results of studies on CO2 sorption on high rank coal obtained from
the Lower Silesian Coal Basin. The table 1 provides selected results of the chemical, technological and
density analysis (Nodzeński, 2000). The measurements have been carried out with the use of original
volume apparatus designed to examine the sorption of gases in the supercritical area. The developed
apparatus also makes it possible to measure the sorption of mixtures (e.g. CO2 + CH4). After the state
of balance is established, it is also possible to source the free (unsorbed) phase disturbing the sorption
balance. The principle of sorption measurement at increased pressures by the volume method consists in
decompressing a gas of a known pressure and volume from the batching space to an ampoule containing
a sorbent. Knowing the dead volume of the apparatus and the volume of the batcher, it is possible to
calculate the absorbed quantity by means of the gas laws.
The presented calculation procedure takes into account the CO2 deviations from the properties of the
ideal gas. The gas molar volume for a given pressure and temperature has been calculated by means of
*
AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF FUELS AND ENERGY, AL. MICKIEWICZA 30,
30-059 KAKÓW, POLAND; [email protected]
60
two equations: the Beattie-Bridgeman Equation of State and the Span-Wagner multi-parameter equation
(Michałowski & Wańkowicz, 1993; Span and Wagner, 1996). The figure 3 shows that up to the pressures
of about 2.5 MPa, the molar volumes calculated with the use of both equations of state are convergent.
In case of higher pressures, the calculated values show differences. Following the sorption isotherms
obtained at a subcritical temperature, the thermal equation of sorption (Czepirski & Jagiełło, 1989) was
used to generate an isotherm in 323 K which was then confronted with an isotherm obtained through an
experiment. An unsatisfactory correspondence was found between the isotherm generated by means of the
thermal sorption equation and the isotherm obtain in course of the experiment (Fig. 4). The gas capacity
values obtained as a result of prediction are overestimated by about 5 dm3/kg which testifies to a significant
change of properties of the arrangement after the critical temperature is exceeded.
Keywords: hard coal, carbon dioxide, supercritical adsorption
W ostatnim okresie coraz większy nacisk kładzie się na ograniczenie emisji ditlenku węgla. Zmniejszenie emisji tego gazu próbuje się osiągnąć poprzez prace nad niskoemisyjnymi technologiami spalania
paliw. Innym sposobem obniżenia emisji CO2 jest przechwycenie gazu oraz zdeponowanie go w głębokich
warstwach geologicznych. Jedną z możliwości, geologicznej sekwestracji, obok stosowanej już metody
EOR (Nagy i Olajossy, 2008; Núñez-López i in., 2008) jest wtłoczenie CO2 do głębokich, wyeksploatowanych lub nieeksploatowanych pokładów węgla kamiennego.
Tematyka badań sorpcji ditlenku węgla jest obecnie prowadzona pod kątem możliwości sekwestracji
tego gazu w głębokich strukturach geologicznych. Mając na uwadze głębokość centrum magazynowania
i zakładając gradient ciśnienia hydrostatycznego na poziomie 10 MPa/km oraz gradient geotermiczny
rzędu 30 K/km (Tarkowski, 2005), należy spodziewać się, że zmagazynowany ditlenek węgla będzie
w stanie nadkrytycznym. Faza nadkrytyczna ma właściwości gazu o dużej gęstości, zbliżonej do gęstości substancji ciekłych, która zmienia się w zależności od ciśnienia i temperatury w zakresie od 200 do
900 kg/m3 (rys. 1) (Holloway i Savage, 1996).
Przedstawiono definicję adsorpcji superkrytycznej która w literaturze jest niejednoznaczna, ze względu
na różne podejścia w zdefiniowaniu fazy (płynu) superkrytycznej. Jedno z podejść mówi, że płyn jest
superkrytyczny, gdy zarówno temperatura jak i ciśnienie osiąga wartości powyżej punktu krytycznego.
Jednakże przyjmuje się, że adsorpcja w warunkach superkrytycznych (supercritical adsorption) jest to
proces zachodzący w temperaturze powyżej temperatury krytycznej (Toth, 2002).
Praca prezentuje eksperymentalne wyniki badań sorpcji CO2 na wysokouwęglonym węglu pochodzącym z Dolnośląskiego Zagłębia Węglowego. W tabeli 1 podano wybrane wyniki analizy chemicznej,
technologicznej i densymetrycznej (Nodzeński, 2000). Pomiary wykonane zostały przy użyciu oryginalnej objętościowej aparatury do badania sorpcji gazów w obszarze superkrytycznym. Zbudowany aparat
umożliwia również pomiary sorpcji mieszanin (np. CO2 + CH4). Po ustaleniu się stanu równowagi istnieje
możliwość pobrania fazy wolnej (niezasorbowanej) bez naruszenia równowagi sorpcyjnej. Zasada pomiaru
sorpcji przy podwyższonych ciśnieniach metodą objętościową, polega na rozprężeniu gazu o znanym
ciśnieniu i objętości z przestrzeni dozującej, do ampułki zawierającej sorbent. Znając objętość martwą
aparatu i objętość dozownika, ilość zasorbowaną można obliczyć za pomocą praw gazowych.
Przedstawiono procedurę obliczeniową z uwzględnieniem odchyleń CO2 od właściwości gazu doskonałego. Objętość molową gazu dla danego ciśnienia i temperatury obliczono stosując dwa równania:
równanie stanu Beattie-Bridgemana oraz wieloparametrowe równanie Span’a-Wagner’a (Michałowski
i Wańkowicz, 1993; Span i Wagner, 1996). Rysunek 3 ilustruje, że dla ciśnień do ok. 2,5 MPa, wyliczone
objętości molowe wyznaczone przy użyciu obu równań stanu są zbieżne. W przypadku wyższych ciśnień
obserwuje się już różnice w wyliczonych wartościach. W oparciu o izotermy sorpcji otrzymane w temperaturze podkrytycznej wygenerowano, za pomocą termicznego równania sorpcji (Czepirski i Jagiełło,
1989), izotermę w 323 K a następnie skonfrontowano ją z izotermą otrzymaną w toku eksperymentu.
Stwierdzono niezadowalająca zgodność izotermy wygenerowanej przy użyciu termicznego równania
sorpcji z izotermą otrzymaną z eksperymentu (rys. 4). Wartości gazopojemności otrzymane w wyniku
predykcji są przeszacowane o ok. 5 dm3/kg co świadczy o znaczącej zmianie właściwości układu po
przekroczeniu temperatury krytycznej.
Słowa kluczowe: węgiel kamienny, ditlenek węgla, adsorpcja superkrytyczna
61
1. Introduction
Recently more and more emphasis has been laid on reducing the emission of carbon dioxide.
Attempts are undertaken to reduce the emission of this gas through the works on the low-emission technologies of fuel combustion. Another way to lower the emission of CO2 is to seize the
gas and deposit it in the deep geological layers. One of the options of geological sequestration
is pumping CO2 into deep, fully exploited or unexploited coal beds.
The number of publications concerning the sorption of CO2, CH4 and compounds of these
two gases on hard coals has increased in the recent years (Żyła et al., 2005; Baran et al., 2007;
Jodłowski et al., 2007; Ceglarska-Stefańska et al., 2002, 2008; Czerw & Ceglarska-Stefańska,
2008). These works are carried out also with regard to the possibility of CO2 sequestration
(Zarębska & Dudzińska, 2008; Małopolska & Zarębska, 2008; Ceglarska-Stefańska et al., 2007).
Nevertheless, most publications present the studies on the sorption of these gases in a temperature
below the critical level and at pressures up to 4 MPa. There are very few works which present an
attempt to simulate the real conditions which will occur in the coal bed. It must be considered
that depositing CO2 in coal beds would be performed in very deep beds which are excluded
from exploitation due to high mining costs. Assuming that the hydrostatic pressure gradient is
at the level of 10 MPa/km and the geothermal gradient is at the level of 30 K/km (Tarkowski,
2005), it should be expected that the deposited carbon dioxide will be in the supercritical state.
The supercritical phase has the properties of a gas of high density, close to the density of liquid
substances which changes depending on the pressure and temperature in the range from 200 to
900 kg/m3 (Fig. 1) (Holloway & Savage, 1996).
900
800
density [g/dm 3 ]
700
298 K
600
303 K
500
313 K
400
323 K
300
200
100
0
0
10
20
30
40
50
60
70
80
90
100
110
120
p [bar]
Fig. 1. The CO2 density change diagram in the function of pressure
The definition of supercritical adsorption is ambiguous. This is caused by differences in the
approach taken while defining the supercritical phase (fluid). One of the approaches says that
a fluid is supercritical when both the temperature and the pressure reach values above the critical
point. However, the adsorption in supercritical conditions (supercritical adsorption) is commonly
62
interpreted to be a process which takes place in a temperature above the critical temperature (Toth,
2002). In case of adsorption phenomena, there are three distinct areas:
1. The subcritical area (T < Tc)
2. The near-critical area (Tc < T < Tc+ 10)
3. The supercritical area (T > Tc+ 10).
The decisive factor which has been assumed in this classification is only the temperature in
which the phenomena of adsorption occurs while pressure remains an independent parameter.
This comes from the fact that the phase above the critical temperature cannot undergo condensation regardless of the pressure which occurs in the arrangement.
As the sorption studies are conveyed with the use of devices of various designs, several
research centres conveying such studies undertook attempts to check if the results of works
done by them are comparable (Goodman et al., 2007). They decided to carry out measurements
for the same samples of coals in 6 different laboratories in the world. The procedures of obtaining an adequate moisture of the sample were determined and the deviations from the ideal gas
standard were calculated by means of the same equation of state. All information was gathered
about the apparatus and the measurement procedure which might indicate a factor having the
utmost influence on the comparability of the measurement. It turned out that the concordance of
obtained results was good for pressures up to 8 MPa. In case of pressures above this level, the
results were very divergent, often offering no possibility of capturing the trend. This shows that
the studies on CO2 sorption in the area of near-critical pressure are of significant importance as
far as the formulation of the gas sequestration issue is concerned.
2. Experimental procedures
The measurements have been carried with the use of an original volume apparatus designed
to examine the sorption of gases in the supercritical area (Fig. 2).
The principle of sorption measurement at increased pressures by the volume method consists
in decompressing a gas of a known pressure and volume from the batching space to an ampoule
containing a sorbent. Knowing the dead volume of the apparatus and the volume of the batcher, it
z1
P
z2
Batcher with
adsorbate
Ampoule with
sorbent
Fig. 2. Scheme of apparatus to measure supercritical adsorption
63
is possible to calculate the adsorbed quantity by means of the gas laws. Therefore, the measurement
accuracy depends on the accuracy of the pressure measurement, the device capacity calibration,
the sorbent density and the stability of temperature. The pressure was measured by means of
an S-10 type pressure transmitter by company WIKA, operating in the pressure range between
0 and 100 bar, of the accuracy class of 0.25% BFSL (Best Fit Straight Line). The design of the
apparatus ensures thermostating of both the ampoule and the batcher at the same temperature,
with the accuracy of 0.1 K. The measurement procedure goes as follows:
1) degassing the apparatus at Z1 and Z2 valves open,
2) closing the Z1 valve and filling the batcher,
3) batching the sorbate by opening the Z1 valve,
4) waiting until the state of balance is achieved, closing the Z1 valve and refilling the
batcher.
The criterion of achieving the state of thermodynamic balance was achieving a value of
the gas pressure in the ampoule which was constant in time. Another point of the isotherm was
obtained by batching a subsequent volume of gas.
The adsorbed volume was calculated from the difference of pressures before and after sorption, taking into account the apparatus dead volume (determined by subtracting the coal grain
volume from the total apparatus capacity). The volume of gas (converted into the standard terms:
p = 0.1 MPa, T0 = 298.2 K) contained in the coal mass unit (VpT ), at a given pressure and the
currently measured ampoule temperature, was calculated from the formula:
é
ù 1
Vn
VpT = êVns + mol × Vap ú ×
Vmol
ë
û m
(1)
Vns — the volume of gas contained in the ampoule at the given starting pressure of sorption, converted into standard conditions [dm3],
n
V mol — the gas molar volume in standard conditions [dm3/mol],
Vmol — the gas molar volume, calculated for the given currently measured pressure and
temperature in the ampoule [dm3/mol],
Vap — the dead volume of the ampoule [dm3],
m — mass of coal [kg].
3. Characteristics of the Sample
The measurements have been made on a sample of hard coal obtained from the Lower Silesian Coal Basin. The table 1 provides selected results of the chemical, technological and density
analysis (Nodzeński, 2000). The parameters of the sample transformation correspond to the average values assumed for depth brackets of would-be storage centres (Kotarba et al., 1995).
64
TABLE 1
Selected results of chemical, technological and density analysis of the coal studied
Sample
NR1
C daf [%]
H daf [%]
(O+N)ddaf [%]
V daf [%]
W a [%]
A a [%]
RI
Type of coal*
R0 [%]
ρHe [g/cm3]
ρHg [g/cm3]
.
Vp 102 [cm3/g] **
88,70
3,51
6,97
18,00
0,95
11,20
0
38
1,51
1,46
1,31
7,84
* according to PN-82/C-97002
** total volume of pores
4. Calculation Method
In case of carbon dioxide, it is necessary to take into account imperfections of gas phase.
The gas molar volume for a given pressure and temperature has been calculated using two
equations: the Beattie-Bridgeman equation of state (Michałowski & Wańkowicz, 1993) and the
Span-Wagner multi-parameter equation (Span & Wagner, 1996). The obtained values of molar
volumes have been used to calculate the CO2 gas capacity values which have been presented on
the figure 3. It is noticeable that the differentiation of the gas capacity values occurs at pressures
above 2.5 Mpa.
The thermal equation of sorption in a viral form (Czepirski & Jagiełło, 1989) was used to
describe the obtained experimental data:
ln p =
1
T
n
k
åA v + åB v
i= 0
i
i
i= 0
i
i
+ z ln v
(2)
wherein: Ao .. .An and Bo ... Bk are the best match parameters, (n) and (k) are the orders of approximating polynomials, p — equilibrium pressure, v — amount of absorbed gas.
The thermal equation of sorption enables the prediction of the sorption/desorption isotherm
process in any temperature, following a family of isotherms of points. The isotherm in 323 K has
been generated on the basis of the CO2 gas capacity isotherms (determined in 288 K and 298 K)
on the NR1 coal, sourced from the work (Nodzeński, 2000). The figure 4 presents the comparison
of the generated isotherm and the isotherm obtained in the course of the experiment. It is noticeable that the predicted CO2 gas capacity values are significantly overestimated.
65
35
30
v [NTPdm 3/kg]
25
20
15
10
on the basis of Beattie-Bridgeman
equation
5
on the basis of Span-Wagner
equation
0
0
10
20
p [bar]
30
40
50
Fig. 3. The comparision of CO2 gas capacity isotherms on NR1 coal in 323 K calculated on the basis of
the Beattie-Bridgeman and Span-Wagner equations of state
35
30
v [NTPdm 3/kg]
25
20
15
10
5
Experimental isotherm
Generated isotherm
0
0
10
20
p [bar]
30
40
50
Fig. 4. Comparision of experimental isotherms of CO2 gas capacity on NR1 coal at the temperature 323 K:
the experimental one and the one generated by means of the thermal equation of sorption
66
5. Final Review
1. The work presents the design and the measurement principle of the apparatus for measuring sorption in supercritical conditions (also in the range of overcritical pressures).
2. It has been found that in case of pressures up to about 2.5 MPa, the calculated molar
volumes determined by means of the equation of state both by Beattie-Bridgeman and
Span-Wagner are convergent. In case of higher pressures, differences between the calculated values become noticeable. Presently, most works among the literary resources
refer their calculations to the Span-Wagner multi-parameter equation of state.
3. The CO2 gas capacity isotherm on the examined coal for the temperature of 323 K,
generated by means of the thermal equation of sorption based on the experimental gas
capacity isotherms determined in temperatures below the critical one, shows an unsatisfactory concordance with the isotherm obtained in the course of the experiment. The gas
capacity values obtained as a result of prediction are overestimated by about 5 dm3/kg.
This testifies to a significant change of the arrangement after the critical temperature is
exceeded, and particularly to the dependence of the isosteric enthalpy of sorption on the
temperature.
4. The results of the studies of gas capacity isotherms present an equilibrium state of the
gas–coal arrangement which is of great importance for the stability of the carbon dioxide
sequestration in the coalbed.
Acknowledgment
The work has been carried out of the MNiSW project No N N524 354035
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Received: 17 June 2009