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

Transkrypt

1 Arch. Min. Sci., Vol. 55 (2010), No 1, p Electronic version (in color) of this article is available: 59 PAWEŁ BARAN*, MICHAŁ BROŚ*, ADAM NODZEŃSKI* STUDIES ON CO 2 SORPTION ON HARD COAL IN THE NEAR-CRITICAL AREA WITH REGARD TO THE ASPECT OF SEQUESTRATION BADANIA SORPCJI CO 2 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 CO 2 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 CO 2 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/m 3 (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 CO 2 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. CO 2 + CH 4 ). 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 CO 2 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 RUELS AND ENERGY, AL. MICKIEWICZA 30, KAKÓW, POLAND; pawel.baran@agh.edu.pl

2 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 dm 3 /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 CO 2 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 CO 2 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/m 3 (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 CO 2 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. CO 2 + CH 4 ). 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 CO 2 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 dm 3 /kg co świadczy o znaczącej zmianie właściwości układu po przekroczeniu temperatury krytycznej. Słowa kluczowe: węgiel kamienny, ditlenek węgla, adsorpcja superkrytyczna

3 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 CO 2 is to seize the gas and deposit it in the deep geological layers. One of the options of geological sequestration is pumping CO 2 into deep, fully exploited or unexploited coal beds. The number of publications concerning the sorption of CO 2, CH 4 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 CO 2 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 CO 2 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/m 3 (Fig. 1) (Holloway & Savage, 1996) density [g/dm ] K 303 K 313 K 323 K p [bar] Fig. 1. The CO 2 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

4 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 < T c ) 2. The near-critical area (T c < T < T c + 10) 3. The supercritical area (T > T c + 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 CO 2 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 P z 1 z 2 Ampoule with sorbent Batcher with adsorbate Fig. 2. Scheme of apparatus to measure supercritical adsorption

5 is possible to calculate the absorbed 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, T 0 = K) contained in the coal mass unit (V p T ), at a given pressure and the currently measured ampoule temperature, was calculated from the formula: V T p 63 n V s mol 1 Vn Vap (1) Vmol m V n s the volume of gas contained in the ampoule at the given starting pressure of sorption, converted into standard conditions [dm 3 ], V n mol the gas molar volume in standard conditions [dm 3 /mol], V mol the gas molar volume, calculated for the given currently measured pressure and temperature in the ampoule [dm 3 /mol], V ap the dead volume of the ampoule [dm 3 ], m mass of coal [kg]. 3. Characteristics of the Sample The measurements have been made on a sample of black 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).

6 64 Selected results of chemical, technological and density analysis of the coal studied Sample NR1 C daf [%] 88,70 H daf [%] 3,51 (O+N) daf d [%] 6,97 V daf [%] 18,00 W a [%] 0,95 A a [%] 11,20 RI 0 Typ węgla * 38 R 0 [%] 1,51 ρ He [g/cm 3 ] 1,46 ρ Hg [g/cm 3 ] 1,31 V. p 10 2 [cm 3 /g] ** 7,84 TABLE 1 * according to PN-82/C ** 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 CO 2 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: 1 ln p= T n k i i Ai v + Bi v +zln v (2) i= 0 i= 0 wherein: A o...a n and B o... B k 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 CO 2 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 CO 2 gas capacity values are significantly overestimated.

7 v [NTPdm /kg] on the basis of Beattie-Bridgeman equation 5 on the basis of Span-Wagner equation p [bar] Fig. 3. The comparision of CO 2 gas capacity isotherms on NR1 coal in 323 K calculated on the basis of the Beattie-Bridgeman and Span-Wagner equations of state v [NTPdm /kg] Experimental isotherm Generated isotherm p [bar] Fig. 4. Comparision of experimental isotherms of CO 2 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

8 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 CO 2 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 dm 3 /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 N References Baran P., Hołda S., Macuda J., Nodzeński A., Zawisza L., Investigations of methane content in lignite coals. Mineral Resources Management, t. 23. z. spec. 3, p Ceglarska-Stefańska G., Zarębska K, Aleksandrowicz K., Displacement sorption of CO 2 and CH 4 on low rank hard coal, within a low gas pressure range. Archives of Mining Sciences, vol. 47, iss. 2, p Ceglarska-Stefańska G., Zarębska K., Wolszczak J., Sorption of pure components and mixtures CO 2 and CH 2 on hard coals. Gospodarka Surowcami Mineralnymi, t. 24, z. 4/1, p Ceglarska-Stefańska G., Nodzeński A., Hołda S., Badania układu węgiel-gaz w aspekcie pozyskiwania metanu i sekwestracji CO 2. Gospodarka Surowcami, t. 23, z. spec. 3, s Czepirski L., Jagiełło J., Virial type thermal equation of gas - solid adsorption. Chem. Eng. Sci., 44, 4, 797. Czerw K., Ceglarska-Stefańska G., Dynamika deponowania gazów kopalnianych w strukturze porowatej węgla kamiennego. Gospodarka Surowcami Mineralnymi, t. 24, z. 3/3, s Goodman A.L., Busch A., Bustin M., Chikatamarla L., Day S., Duffy G.J., Fitzgerald J.E., Gasem K.A.M., Gensterblum Y., Hartman C., Jing C., Krooss B.M., Mohammed S., Pratt T., Robinson R.L. Jr., Romanov V., Schroeder K., Inter-laboratory Comparison II: CO 2 Isotherms Measured on Moisture-Equilibrated Argonne Premium Coals at 55 C and Pressures to 15 MPa. International Journal of Coal Geology, 72 (2007), Holloway S., Savage D The potential for aquifer disposal of carbon dioxide in the UK. Energy Conversion and Management, 37,

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