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desorption on bituminous coal—Experiments on cubicoid sample cut from the primal coal lump
International Journal of Coal Geology 85 (2011) 72–77
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Methane and carbon dioxide sorption/desorption on bituminous coal—Experiments on cubicoid sample cut from the primal coal lump Katarzyna Czerw ⁎ Faculty of Energy and Fuels, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 4 October 2010 Accepted 5 October 2010 Available online 13 October 2010 Keywords: Sorption/desorption of mine gases Swelling/shrinkage of coal Expansion/contraction of coal CO2 sequestration ECBM recovery
a b s t r a c t There was performed a research into the sorption of methane and of a mixture of methane and carbon dioxide on low-rank coal (type 32.2, according to PN-82/G-97002). Experiments were carried out by means of the volumetric method on a cubicoid solid sample, sized 20 × 20 × 40 mm, cut out of the primal body. At the same time, the changes occurring in the sample's overall dimensions, which accompanied sorption processes, were monitored. The kinetic curves of methane deposition and evacuation (two measurement cycles) and the deposition kinetic curve for the mixture of carbon dioxide and methane in a coal structure containing methane previously sorbed were determined, as well as the adequate kinetics of coal strains resulting from the sorption/desorption processes. The results show that in the experimental conditions sorption process and the induced strains noticed in the coal–methane system are proportional. When slightly approximated, the correlation between induced strains and the quantity of gases being sorbed ε = f (V) becomes linear. However, the course of sample strain kinetics for the time of experiment in the coal-gaseous mixture indicates the occurrence of sample shrinkage after a dynamic increase in sizes, although the gas was being still collected in the coal structure. Their relation with the quantity of gas under sorption ε = f (V) was linear only in the initial stage of experiment. It was found that in the case of the coal under investigation preferable is the sorption of carbon dioxide, not of methane, and CH4 is displaced from the sorbed phases by CO2. It seems feasible to recover CH4 through capture and storage of CO2 in a coal bed whose structure, parameters and petrographic composition complies with the material under research. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An increase in CO2 in the Earth's atmosphere is considered to be one of the main factors that determine the heating up of the planet's climate. Ecological issues and prospective economic aspects give rise to more interest in the sequestration of carbon dioxide and ECBM process (enhanced coal bed methane recovery), viz. recovery of methane from carboniferous deposits with simultaneous CO2 deposition in them. It constitutes one of the methods aimed at lowering the emission of this gas. The determination of relations occurring between sorption and dilatometric phenomena that occurs simultaneously on hard coal and the understanding of the underlying mechanisms constitute one of the most important issues pertinent to carbon dioxide sequestration and recovery of methane from carboniferous formations. Hence, in the recent few years more and more research into this field has been done. Fundamental issues to be investigated are changes caused by interactions coal–gas (CH4 and CO2) within a coal bed as well as elaboration of analytical and ⁎ Tel.: +48 12 617 21 21; fax: +48 12 617 45 47. E-mail address: [email protected]. 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.10.002
numerical methods as tools for describing the course of such changes. Mine gases are deposited in coal beds in a few forms: (1) adsorbed in micropores and on the surface of larger pores, (2) absorbed in the coal molecular structure, (3) as free gas in fissures and larger pores whose share becomes of significance at higher sorbate pressures, and (4) dissolved in deposit waters (Ceglarska-Stefańska and Zarębska, 2006; Mastalerz et al., 2004; Zarębska and Ceglarska-Stefańska, 2008). From two main components of mine gas, methane has lower diffusivity, which is why its sorption is slower than in the case of carbon dioxide (Busch et al., 2004; Clarkson and Bustin, 1999a,b; Cui et al., 2004). CH4 molecules have a spatial, tetrahedral structure and are apolar. That is why forces of dispersion play the decisive role in interactions of coal–methane. For critical parameters of methane in standard conditions, CH4 is treated as gaseous sorbate. Whereas CO2 molecules have a linear structure, are not polar, but they are endowed with a high quadrupole moment. In standard conditions CO2 is a vapour. It is commonly assumed that the preferable sorption occurring in hard coal is that of carbon dioxide, not methane, which constitutes the foundation for the idea of ECBM and justifies research activities.
K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
However, in the latest dozen or so years, there have been many published papers that advocate the necessity of desisting from that generalization in favor of analyzing respective coal-gas systems. Busch et al. (2006) performed a series of experiments in the field of competitive sorption of gaseous mixtures of CO2 and CH4 on various hard coals of different rank. The most important conclusion of the authors is that the preferential character of sorption does not depend on the gas composition, but on the coals rank, maceral composition and the pressure of sorbate. Generally, a high rank was related to preferential CO2 sorption, however, low-rank coals under analysis exhibited a non-homogeneous selectivity, viz. preferential sorption of CO2 or CH4. The results of papers (Crosdale, 1999; Majewska et al., 2009) also indicate the preferential methane sorption. Instead, the research results described in articles (Ceglarska-Stefańska and Zarębska, 2005; Yu et al., 2008) exhibited the selectivity of sorption of carbon dioxide from gaseous mixtures of CO2 and CH4. In the case of paper (Ceglarska-Stefańska and Zarębska, 2005) an intensification of that tendency was observed with an increased of sorbate pressure. In the case of investigations carried out on hard coal, in which not only the sorption process, but also the phenomena of the sorption induced expansion of sorbents generated by vapors and gases were monitored, essential are shapes and overall dimensions of samples. There is also another very important cause for using solid samples in sorption experiments. It must be clearly underlined that traditional research into the sorption on powders and grain fractions are not representative in relation to the coal bed parameters, since the natural coal porous structure is destroyed and there is no pressure of overburden rocks (Karacan and Mitchell, 2003). Research into coal expansion leads to a general conclusion that coal is a bi-porous, transport-sorption system in which high pressure of gases causes the compression of microporous areas and expansion of macropores, while the expansion occurring in the microporous coal substance causes transport pores to narrow causing the in situ system permeability of the bed to fall (Ceglarska-Stefańska and Zarębska, 2006; Pan and Connell, 2007; Seewald and Klein, 1986). In the references quoted are specified some research results pointing to a linear relation between coal expansion and the volume of gas accumulated in its structure, for example in (Chikatamarla et al., 2004; Levine, 1996, Robertson and Christiansen, 2005, St. George and Barakat, 2001). There are also some available works according to which the course of sorption processes and the range of contingent strains are not subject to a linear dependence (Ceglarska-Stefańska, 1990; Ceglarska-Stefańska et al., 2007, 2008; Pan and Connell, 2007). The purpose of this study is: (1) the determination of the dependence between the range of expansion/contraction of hard coal under investigation and the volume of gases accumulated in its porous structure in the coal–methane system as well as in the system coal–gaseous mixture (CO2 and CH4), (2) experimental verification of the thesis on preferential character of CO2 sorption vs. CH4 and a possibility of displacement of CH4 from the coal structure by means of CO2 in the system under analysis by introducing carbon dioxide into a coal sample already saturated with methane, and (3) gaining some analytical data for predicting the behavior of the coal under investigation in ECMB processes. 2. Experimental The research was done on a cubicoid hard coal sample, grade high volatile bituminous C according to ECE-UN In Seam Coal Classification, (type 32.2, in conformity with the Polish classification as specified in the standard PN-82/G-97002). The coal was conveyed from the mine ‘Brzeszcze-Silesia’, bed 405. The results of the analysis performed on samples are shown in Table 1. The petrographic composition and average reflexivity of vitrinite were carried out at the Department of Deposit and Mining Geology, Faculty of Geology, Geophysics
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and Environmental Protection (AGH University of Science and Technology), by means of a mineralogical microscope AXIOPLAN and a reflectometer Axioplan-MPM-400 made by ZEISS OPTION. The analysis showed that the coal had a striped microstructure with variable thickness (under 0.1 to 1.8 mm) and the average strip thickness was approx. 0.3–0.5 mm. Alternate layers are made of semifusinite, fusinite and collotelinite and trimacerite whose composition is similar to clarodurite and vitrinertoliptite (with microsporinite). Collotelinite contains streaks composed of telinite impregnated with gelinite. Fusinite and semifusinite contain a large number of destroyed fragments (crushed and not impregnated). They belong mainly to degrado form, especially semifusinite. In some places those macerals are resinite impregnated which at such places is accumulated in larger amounts. Trimacerite layers exhibit the predominance of inertinite macerals which, like microsporinite and liptodetrinite are embedded in the filler consisting of collotelinite. A rare component occurring in those films is macrinite and macrosporinite. Minerals are chiefly syngenetic carbonate grains. A coal sample sized 20 × 20 × 40 mm was cut out from the primal body in such a way that the walls sized 20 × 20 mm were parallel to bedding plane. The conducted experiments included the investigation of kinetics of accumulation and evacuation of methane (two measurement cycles) and kinetics of accumulation of mixture of CO2 and CH4 in the structure of coal under analysis that contained methane previously adsorbed. Simultaneously the kinetics of coal sorption induced strains was monitored. Sorption and expansion/ contraction measurements were taken using the apparatus and test procedures followed that described by (Majewska et al., 2008, 2009). The experimental set-up consisted of two individual units working together: (1) the gas sorption apparatus using volumetric method (pressure meter MKS BARATRON 722A with a measuring range of 0– 4.0 MPa and measurement accuracy ±0.001 MPa) and (2) the strain meter for measuring samples external dimensions changes (electrical resistance bridge, type SGM-1C81, constructed in Strata Mechanics Research Institute of The Polish Academy of Science, measuring range of linear strain ~ 4‰, measurement accuracy 0.001‰, with domestic production resistance-type paper strain gauges type RL120). The whole apparatus was thermo-stabilized, which enabled measurements to be taken at constant temperature of 298 K. The experimental procedure included the previous long-term degassing of the sorption apparatus and the sample itself (vacuum 10−5 hPa) and immersions in a helium bath (10 kPa) to secure the removal of possible interfering sorbed gases and vapours, including water molecules. The measurement cycle included a few stages. First, methane was introduced into the degassed measurement cell and the changes in coal storage capacity and sample sizes were monitored until reaching the state close to the equilibrium of linear stresses (sorption 1_CH4). Then, the gas desorption was performed by decreasing the sorbate pressure in the sample cell (desorption 1_CH4). Later on, the measurement ampoule was filled up with an extra portion of sorbate and the observation of advancing changes to storage capacity and sizes of the tested material was continued (sorption 2_CH4). Afterwards, in a similar way another desorption of the gas was performed (desorption 2_CH4). The successive stage was the injection of carbon dioxide into the measurement space containing methane left after the desorption 2_CH4 (sorption 3_ CO2/CH4). Further monitoring of gas storage capacity and sample sizes was carried out. At the end of this part of experiment, a gas sample was collected from the measurement cell to determine its composition through chromatography. A gas chromatography apparatus Hewlett Packard HP 5890 (adsorbent Porapak Q) with a thermal-conductance detector TCD was used. Basing upon the obtained kinetics of coal induced linear strains in the perpendicular (εT) and parallel (εL) directions, the kinetics of volumetric strains (εV) were determined in accordance with the relation: εV = εT + 2 εL.
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K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
Table 1 Characteristic of the coal sample under study. Chemical and technological analysis Wa [%]
Aa [%]
Ca [%]
Cdaf [%]
Ha [%]
1.52
11.23
72.21
82.76
4.31
Petrographic composition Telinite
Collotelinite
Vitrodetrinite
Collodetrinite
Corpogelinite
Gelinite
Total vitrinite
Sporinite
Rezinite
Liptodetrinite
3.0
24.2
1.3
9.2
1.0
2.5
41.2
6.5
2.1
2.3
3. Results and discussion The experimental results are collected in Table 2, and given as plots in Figs. 1–4. They show, respectively, the kinetic curves of accumulation and evacuation of methane and gaseous mixture CO2 and CH4 in the porous structure of the cubicoid hard coal sample and kinetic curves of coal expansion/contraction that accompany sorption processes (Fig. 1), the dependence between coal volumetric strains and the quantity of methane collected in its structure (Fig. 2), the dependence between coal volumetric strains and the amount of mixture of gases accumulated in its structure (Fig. 3) and the value of pressure in the sample cell vs. time throughout all experimental stages (Fig. 4). It needs to be underlined that decisive for the completion of each research stage was reaching the plateau on kinetic curves of linear strains, viz. a state close to dilatometric equilibrium. In no research stage was attained the state close to the sorption equilibrium in the coal–gas system. A low gas storage capacity of coal towards methane and gaseous mixture can be attributed to its low rank (Table 1). It is related to a relatively small share of micro- and ultramicropores in total porosity, even in the macerals of vitrinite group (Chalmers and Bustin, 2007). The share of vitrinite in the coal under analysis is merely 41.2%. Moreover, the presence of functional groups occurring peripherally in appreciable quantities stops, to some extent, the pore clearance and prevents sorbate molecules from free penetration into the coal structure. A relatively high share of macerals of inertinite and liptinite groups in the petrography of the sorbent under investigation, for its meso- and macroporosity, on the one hand decreases the coal sorption capacity, but on the other, this share facilitates the displacement of gas within the coal pore system. Hence, a relatively small extent of strains of the coal under investigation was observed. This is in particular true for fusinite (14.2%) and semifusinite (18.6%), which does not yield to expansion, and which in most cases contain no other components in cellular spaces. Similar conclusions can be found, for example, in Karacan (2003) and Karacan and Mitchell (2003). As can be seen in Fig. 1, the course of both the kinetic curves of methane sorption is different as far as the gas accumulation rate is concerned, which is related to a better availability of coal porous structure in the stage of sorption 2_CH4. Those curves also indicate that coal sorption capacity towards methane is in stage of sorption 1_CH4 slightly smaller than the value obtained at the end of stage sorption 2_CH4. After desorption 1_CH4, in the coal structure is still left approx. 50% of gas stored in the sorption stage. In the case of desorption 2_CH4, a triple reduction of methane pressure in the measurement ampoule caused gas desorption at a level of 50% of gas accumulated during the sorption 2_CH4 and about 70% if referred to the total methane sorbed. According to predictions, linear strains of coal exhibit anisotropy through the entire experiment. The relation between the linear strains εT:εL changes during the experiment from 2.1 for methane sorption stages and 2.8 for methane desorption steps to 1.8 for the sorption 3_CO2/CH4 stage. The sorbent expands/shrinks more in perpendicular
than parallel direction to stratification (Fig. 1). Kinetic curves of CH4 sorption and desorption with corresponding kinetic curves of volumetric strains are featured with a very similar trends. Therefore, with a slight approximation, the dependence of volumetric strain vs. methane sorption/desorption εV = f (V) is more or less linear (Fig. 2). However, those curves show tendency towards a sigmoid shape, because the rate of transformations in coal sizes is slower at the beginning and at the end of each experimental stage. In the first case, it results from pressure increases or decreases in the ampoule due to sorbate dosage or removal, which, respectively, will cause the matrix to contract or expand. In the latter case, this is related to the tendency towards relaxation and re-arrangement of coal structure, when the rate of changes in the quantity of accumulated gas decreases at the end of each stage. The curves that correspond with sorption stages of experiment are inclined to the OX axis at similar angles. The εV = f (V) plots for desorption stages are inclined to OX axis also at similar, but bigger angles that in the case of sorption curves (especially for desorption 1_CH4). It can suggest that the sample shrinkage during desorption occurs faster than the expansion in the course of sorption. It should be also noted that in Fig. 2 each successive curve εV = f (V) is slightly shifted more towards the right side than the curves from earlier experimental stages. The conclusion is that at the same quantities of gas accumulate in the coal structure in the successive experimental stages, the value of corresponding strains diminishes. Such observations corroborate the idea that in those experimental conditions the induced strains are not fully reversible. That could result very likely from the exposure to instantly increased high pressure of gas and also even more likely from sudden decompressions be several MPa than from sorption itself. As reported by Day et al. (2010) exposure to helium and neon can cause pressure induced contraction of coal. Sorption of these gases on coal is almost none hence no swelling occurred in their tests up to 16 MPa. The results presented by Romanov and Soong (2009) showed that the exposure of coal lumps and powders to high pressure helium may change the sorbents helium density and its texture. In the case of coal lumps those changes appeared to be irreversible. Be also remembered that methane was introduced into the sample previously degassed under a high vacuum and in desorption stages the pressure value was not brought down to 1 atm or to vacuum. For its low rank, coal exhibits only a slightly arrange structure. The lack of spatial orientation of high-molecular compounds which constitute the three-dimensional network and high contents of molecular phase caused that the sorbent matrix is relatively flexible within the range of strains resulting from sorption effects. The course of εV = f (V) curves in Fig. 2 can suggest that the availability of coal structure slightly increases in successive experimental methane sorption/desorption cycles, when the sorption capacity towards that gas is relatively constant. Motions of sorbate molecules conveyed in pores to and from CH4 depositing areas and accumulation and evacuation itself takes place each time in a slightly changed coal porous structure. Sorption processes and sorbate pressure cause the coal structure to change gradually. Its constituents are displaced, and the direction of such a transformation is a more stable and more highly associated system. Such
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Chemical and technological analysis Na [%]
Sa [%]
dtrue 103[kg/m3]
dapparent 103[kg/m3]
Porosity [%]
1.19
0.37
1.379
1.344
2.50
Petrographic composition Total liptinite
Fusinite
Semifusinite
Macrinite
Micrinite
Funginite
Inertodetrinite
Total inertinite
Mineral matter
Ro [%]
10.9
14.2
18.6
2.6
1.2
trace
8.4
45.0
2.9
0.78
modifications are called relaxation. Due to them, sorption 2_CH4 was accompanied by smaller strains than in the case of sorption 1_CH4, although the quantity of gas accumulated in the coal structure was slightly bigger. At the Faculty of Energy and Fuels, AGH University of Science and Technology, investigations were made into coal, type 32.2, obtained from the Brzeszcze-Silesia mine. Apart from typical sorption tests performed on grain samples, some experiments were also conducted on coal samples (plates) cut out from the primal lumps, sized 15 × 15 × 5 mm, with simultaneously monitoring of changes in the sizes of such specimens. Sorbates were carbon dioxide, methane and their mixtures. Test results presented in this paper were compared with the data from references (Ceglarska-Stefańska, 1990; CeglarskaStefańska et al., 2008; Milewska-Duda et al., 1994; Zarębska and Ceglarska-Stefańska, 2004, 2008) obtained for the same type of coal from the same mine. It was found that previously measured values of coal CH4 sorption capacity were three times higher, viz. approx. 12 cm3STP/g, for both grain and plate samples. Strain values in the directions perpendicular and parallel to stratification were also higher, viz. 1.8–4.0‰ and 1.7–3.0‰ for similar values of sorbate pressures. It is also necessary to mention that in the coal material described in this paper there were significant amounts of macerals of inertinite group (45%), inc. fusinite (14.25%) and semifusinite (18.6%), whereas the vitrinite share was merely 41.2%. So it must be underlined that in the experiments described in the papers mentioned the research material had a different petrography. It contained more vitrinite (67.5–76.7% and 46.4%), but first of all, definitely less fusinite and semifusinite (in total, under 15%), hence, their higher gas storage capacity and higher linear strains in plate samples. On the other hand, smaller coal sample sizes are related to a smaller share of transport pores and cracks of primal coal. The successive research stage was carried out to check if the displacement of methane molecules in the sorbed phase by carbon dioxide molecules takes place basing upon the change of sorbate composition in the measurement cell. The initially calculated methane content in the mixture was 20.5%. According to chromatography, at the end of stage sorption 3_CO2/CH4, the share of this gas was 34%. Sorbate enriched in methane corroborates that the adsorbed phase was enriched in CO2 whose share was 91.3%. Hence, the conclusion may be drawn that in the case of coal under analysis the preferential sorption of CO2 at the expense of CH4 occurred. The effect of displacement of CH4 from the sorbed phase by CO2 was true for 60%
Taking into account the above it could be speculated that, it is technically possible to enhance the recovery of methane by injecting carbon dioxide in a coal bed whose structure, technical parameters and petrography correspond with those of test material. Strains that accompany sorption processes in the stage sorption 3_CO2/CH4 were twice that high as in the preceding experimental stages (Fig. 1). There occurred an interesting divergence between the course of kinetic curves and linear strains in the directions perpendicular (εT) and parallel (εL) to stratification. In the first 80 h of that experimental stage, the sample exhibited a dynamic elongation perpendicular to bedding plane (εT), while after reaching the maximum value on the kinetic curve of the process, a gradual shrinkage occurred. Changes in coal sizes parallel (εL) to stratification had a definitely less dynamic course. In the kinetic curve related to that process, there was no clear maximum, and therefore, it is impossible to indicate the moment at which the character of (εL) strains changed from expansion to contraction. Still it is safe to say, that it occurred later than in the other direction. The maximum value of volumetric strains was 12.16‰ and was recorded in the 164th hour of that stage. The sample contracted, although the pressure in the sample cell was decreasing (Fig. 4) indicating that the gas was still being accumulated in the coals structure (Fig. 2). Later on, a slight variability in the sample sizes and linear course of sorption kinetics were recorded (Fig. 1). In the experiment carried out for the system coal–gaseous mixture 75% CO2 and 25% CH4 (Ceglarska-Stefańska et al., 2008), the values of linear strains for plate samples were 4.1‰ in both directions to the stratification, while the sorbate pressure being 2.86 MPa. Results of dilatometric measurements for gaseous sorbates (CO2 and CH4) and their mixtures shown in references (Ceglarska-Stefańska, 1990; Milewska-Duda et al., 1994; Zarębska and Ceglarska-Stefańska, 2008), and in this paper indicate a definitely smaller anisotropy of dimensional changes of coal sized 15 × 15 × 5 mm if compared to the course of such transformations for studied samples, sized 20 × 20 × 40 mm. It corroborates the thesis that such tests should be performed on solid samples, larger than a single lithotype, which constitute possibly a representative fragment of primal coal body. It is difficult to indicate clearly the causes of changes in the character of coal strains in the stage of sorption 3_CO2/CH4 (Fig. 3). The author believes that at the initial stage of experiment, carbon dioxide diffusing through the porous coal structure adsorbed in micropores (also in submicropores) and on the surface of larger pores
Table 2 Changes of the amount of accumulated gas and induced strains of the coal samples under study.
Sorption 1_CH4 Desorption 1_CH4 Sorption 2_CH4 Desorption 2_CH4 Sorption 3_CO2/CH4
Time [h]
Initial pressure [MPa]
Final pressure [MPa]
Final perpendicular strain εT [‰]
288.6 143.4 265.7 239.2 579.5
3.68 0.21 3.14 1.01 4.18
3.25 0.44 2.90 1.07 3.36
2.291 0.878 2.213 1.272 5.202
of methane previously accumulated in the coal porous structure.
on
Final parallel strain εL [‰]
1.168 0.315 1.035 0.455 the2.840 outer
portions
Final volumetric strain εV [‰] 4.627 1.508 4.283 2.180 of 10.882 the sample.
At the
Final amount of sorbed gas [cm3STP/g] 3.25 1.71 3.515 2.58 11.65 time, same
desorption of
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K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
Fig. 1. Kinetics of gas accumulation and induced strains.
Fig. 3. Relationship between volumetric strain and volume of accumulated gas in coalmixture of CO2 and CH4 system.
CH4 molecules started, substituted by CO2 molecules. Those processes were accompanied by expansion of microporous areas of coal (vitrinite, collotelinite) and an extra pressure was being put on the interior parts of the sample. Eventually, the clearance of transport pores diminished because of induced swelling of the external section. General compression might occur in areas of low microporosity (inertinite, fusinite and semifusinite). Successive relaxation of coal matrix elements enabled deposition of CO2 molecules inside the matrix also by way of absorption, which probably did not take place in the case of methane. Most probably there was a gradient of gas saturation traveling throughout the sample body and changing the location of its front during the experiment towards the center of the specimen. Assuming the hypothesis that after 164 h of experimental works the degree of saturation of the coal structure with sorbate in the external and middle parts of the sample was relatively close to the maximum, the successive sorption and dilatometric effects, viz. samples shrinkage and change in the sorption kinetics (linear and increasing) might be explained basing in part upon the approach
postulated by the authors of (Karacan and Mitchell, 2003; Larsen, 2004) and taking the aspect of moving saturation peak under consideration. Namely, after an initial increase in coal sizes, its structure yields to re-arrangement and relaxation. Of course, such transformations occur more slowly than gas accumulation in coal. The expanding coal exceeds the state of thermodynamic equilibrium in the coal–gas system, and the consecutive relaxation leads to the restoration of equilibrium. In consequence, CO2 solubility in coal is reduced. The excess of CO2 molecules is shifted back in the system of pores increasing the pore pressure. The gas moves then towards the interior parts of the coal porous system according to the pressure gradient. Further accumulation of gas in the outer parts of coal is likely to occur mainly with free gas, which complies with the opinion of the authors of (Ceglarska-Stefańska et al., 2007). Probably, the sample shrinkage can be attributed to increased pressure of gas in coal pores and the effect of desorption of CO2 molecules previously absorbed. A situation in which the pressure of free gas is sufficient to cause coal compression counterbalanced by the expansion related to
Fig. 2. Relationship between volumetric strain and volume of accumulated gas in coal– methane systems.
Fig. 4. Pressure changes in sample cell.
K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
sorption processes was observed during CO2-ECBM field project performed in the Alison field, located in New Mexico, USA. The injection of CO2 in CH4 containing bed caused an increase in pressure in coal pores, and in consequence, increased bed permeability (Reeves et al., 2003; Siriwardane et al., 2009). Therefore, attention should be also paid to ‘peaks’ occurring in kinetic curves of induced strains at the places corresponding to dosing or releasing of gas to or from the measurement cell, which can imply that the sample reacts very rapidly to the change of pressure in the ampoule and it changes its sizes (Fig. 1). Instead, the effects expansion/contraction caused by sorption/desorption processes becomes noticeable only after about 10 min of contact between coal and sorbate, which is convergent with the opinion of the authors of (Siriwardane et al., 2009) who ascertained that the effect of pressure in pores is more noticeable than coal expansion/contraction. 4. Conclusions The following conclusions can be drawn from this study: • Modeling of deposition, accumulation and emission of mine gases in rock mass entails test on relatively large, solid sample in which natural heterogeneous character of primal coal, dual porosity and contents of respective petrography components is preserved. • Strains observed in the experimental conditions in the system coal– methane could be irreversible; with a certain approximation, the dependence between volumetric stain and the sorption/desorption capacity of methane εV = f (V) can be expressed linearly. • Sorption/desorption processes cause relaxation changes in the coal structure and are aimed at attaining a compact and thermodynamically stable system. • The course of strains observed during the experiment in the system coal–gaseous mixture is characterized by occurring a sample contraction, although the pressure of gas was decreasing and the sorbate was still being accumulated in the coal structure after a dynamic increase in size. • In the case of the coal under investigation, a preferential sorption of carbon dioxide over methane occurred and the displacement of CH4 from the sorbed phase by CO2 molecules had place. Hence, the concept of enhancing methane recovery by injecting carbon dioxide to coal seems, whose structure, technical parameters and petrography comply with the characteristic of the material tested seems possible. Acknowledgments Financial support for this study was provided by AGH/UST framework no. 11.11.210.117. The author wish to thank Prof. Dr. Hab Grażyna Ceglarska-Stefańska (Faculty of Energy and Fuels, AGH University of Science and Technology, Kraków) for the valuable discussions, careful reading of the manuscript and helpful remarks. The author is grateful for the close support of Dr. Eng. Jerzy Ziętek (Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Kraków). The author would also like to express her gratitude to Dr. Hab. Eng. Marian Wagner—AGH Prof. (Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Kraków) for performing the petrographic analysis of coal under study. Special thanks are required for Mr. Jerzy Małyniuk (Faculty of Energy and Fuels, AGH University of Science and Technology, Kraków) for his technical support in experimental work. References Busch, A., Gensterblum, Y., Krooss, B.M., Littke, R., 2004. Methane and carbon dioxide adsorption/diffusion experiments on coal: an upscaling- and modeling approach. International Journal of Coal Geology 60, 151–168.
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Busch, A., Gensterblum, Y., Krooss, B.M., Siemons, N., 2006. Investigation of highpressure selective sorption/desorption behavior of CO2 and CH4 on coals: an experimental study. International Journal of Coal Geology 66, 53–68. Ceglarska-Stefańska, G., 1990. Współzależność procesów sorpcyjnych i dylatometrycznych zachodzących w układach: węgla-para wodna, dwutlenek węgla, metan. Zeszyty Naukowe AGH nr 1371, Chemia z. 16, Wyd. AGH, Kraków. Ceglarska-Stefańska, G., Zarębska, K., 2005. Sorption of carbon dioxide–methane mixtures. International Journal of Coal Geology 62, 211–222. Ceglarska-Stefańska, G., Zarębska, K., 2006. Sorpcja CO2 i CH4 na niskouwęglonych węglach z KWK Brzeszcze. Karbo 1, 31–34. Ceglarska-Stefańska, G., Majewska, Z., Majewski, S., Ziętek, J., Czerw, K., 2007. Rozwój odkształceń węgla kamiennego w procesach sorpcyjno-desorpcyjnych. Gospodarka Surowcami Mineralnymi 23, 41–50. Ceglarska-Stefańska, G., Nodzeński, A., Czerw, K., Hołda, S., 2008. Coal–mine gases systems in the aspects of methane recovery and CO2 sequestration. Proceedings of the 21st World Mining Congress: Prace Naukowe GiG, Górnictwo i Środowisko, 4, pp. 63–71. Chalmers, G.R.L., Bustin, R.M., 2007. On the effect of petrographic composition on coalbed methane sorption. International Journal of Coal Geology 69, 288–304. Chikatamarla, L., Cui, X., Bustin, R.M., 2004. Implications of volumetric swelling/ shrinkage of coal in sequestration of acid gases. paper 0435. Clarkson, C.R., Bustin, R.M., 1999a. The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study: 1, Isotherms and pore volume distributions. Fuel 78, 1333–1344. Clarkson, C.R., Bustin, R.M., 1999b. The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study: 2, Adsorption rate modeling. Fuel 78, 1345–1362. Crosdale, P.J., 1999. Mixed methane/carbon dioxide sorption by coal: new evidence in support of pore-filling models. Proceedings International Coalbed Methane Symposium. Tuscaloosa, Alabama. Cui, X., Bustin, M.C., Dipple, G., 2004. Selective transport of CO2, CH4 and N2 on coals: insight from modeling of experimental gas adsorption data. Fuel 83, 293–303. Day, S., Fry, R., Sakurovs, R., Weir, S., 2010. Swelling of coals by supercritical gases and its relationship to sorption. Energy and Fuels 24, 2777–2783. Karacan, C.Ö., 2003. Heterogeneous sorption and swelling in a confined and stressed coal during CO2 injection. Energy and Fuels 17, 1595–1608. Karacan, C.Ö., Mitchell, G.D., 2003. Behavior and effect of different coal microlithotypes during gas transport for carbon dioxide sequestration into coal seams. International Journal of Coal Geology 53, 201–217. Larsen, J.W., 2004. The effects of dissolved CO2 on coal structure and properties. International Journal of Coal Geology 57, 63–70. Levine, J.R., 1996. Model study of influence of matrix shrinkage on absolute permeability of coal bed reservoirs. In: Gayer, R., Harris, I. (Eds.), Coalbed methane and coal geology: Geological Society Special Publication, London, 109, pp. 197–212. Majewska, Z., Ceglarska-Stefańska, G., Majewski, St., Ziętek, J., Czerw, K., 2008. Differential swelling of coal. 25th Annual International Pittsburgh Coal Conference. Pittsburgh, PA, USA, 29.09.2008–02.10.2008. Majewska, Z., Ceglarska-Stefańska, G., Majewski, St., Ziętek, J., 2009. Binary gas sorption/desorption experiments on bituminous C coal: simultaneous measurements of sorption kinetics, volumetric strain and acoustic emission. International Journal of Coal Geology 77, 90–102. Mastalerz, M., Gluskoter, H., Rupp, J., 2004. Carbon dioxide and methane sorption in high volatile bituminous coals from Indiana, USA. International Journal of Coal Geology 60, 43–55. Milewska-Duda, J., Ceglarska-Stefańska, G., Duda, J., 1994. A comparison of theoretical and empirical expansion of coals in the high pressure sorption of methane. Fuel 73 (6), 975–978. Pan, Z., Connell, L.D., 2007. A theoretical model for gas adsorption-induced coal swelling. International Journal of Coal Geology 69, 243–252. Reeves, S., Taillefert, A., Pekot, L., Clarkson, C., 2003. The Allison Unit CO2—ECBM Pilot: a reservoir modeling study. U.S. Department of Energy. Topical report (DE-FC260NT40924). Robertson, E.P., Christiansen, R.L., 2005. Measurements of sorption-induced strains. Presented at the 2005 International Coalbed Methane Symposium, Tuscaloosa, Alabama. [17–19 May, Paper 0532]. Romanov, V., Soong, Y., 2009. Helium-volume dynamics of Upper Freeport coal powder and lumps. International Journal of Coal Geology 77, 10–15. Seewald, H., Klein, J., 1986. Methansorption an Steinkohle und Kennzeichnung der Porenstruktur. Glückauf Forschungshefte 47 (3), 149–156. Siriwardane, H.J., Gondle, R.K., Smith, D.H., 2009. Shrinkage and swelling of coal induced by desorption and sorption of fluids: theoretical model and interpretation of field project. International Journal of Coal Geology 77, 90–102. St. George, J.D., Barakat, M.A., 2001. The change of effective stress associated with shrinkage from gas desorption in coal. International Journal of Coal Geology 69 (6), 83–115. Yu, H., Zhou, L., Guo, W., Cheng, J., Hu, Q., 2008. Predictions of the adsorption equilibrium of methane/carbon dioxide binary gas on coal using Langmuir and ideal adsorbed solution theory under feed gas conditions. International Journal of Coal Geology 73, 115–129. Zarębska, K., Ceglarska-Stefańska, G., 2004. The expansion and contraction of hard coals during the storage of mine gases as factors inducing stress-changes in the rock strata. Annals of the Polish Chemical Society 3 (3), 1321–1323. Zarębska, K., Ceglarska-Stefańska, G., 2008. The change in effective stress associated with swelling during carbon dioxide sequestration on natural gas recovery. International Journal of Coal Geology 74, 167–174.
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Methane and carbon dioxide sorption/desorption on bituminous coal—Experiments on cubicoid sample cut from the primal coal lump Katarzyna Czerw ⁎ Faculty of Energy and Fuels, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 4 October 2010 Accepted 5 October 2010 Available online 13 October 2010 Keywords: Sorption/desorption of mine gases Swelling/shrinkage of coal Expansion/contraction of coal CO2 sequestration ECBM recovery
a b s t r a c t There was performed a research into the sorption of methane and of a mixture of methane and carbon dioxide on low-rank coal (type 32.2, according to PN-82/G-97002). Experiments were carried out by means of the volumetric method on a cubicoid solid sample, sized 20 × 20 × 40 mm, cut out of the primal body. At the same time, the changes occurring in the sample's overall dimensions, which accompanied sorption processes, were monitored. The kinetic curves of methane deposition and evacuation (two measurement cycles) and the deposition kinetic curve for the mixture of carbon dioxide and methane in a coal structure containing methane previously sorbed were determined, as well as the adequate kinetics of coal strains resulting from the sorption/desorption processes. The results show that in the experimental conditions sorption process and the induced strains noticed in the coal–methane system are proportional. When slightly approximated, the correlation between induced strains and the quantity of gases being sorbed ε = f (V) becomes linear. However, the course of sample strain kinetics for the time of experiment in the coal-gaseous mixture indicates the occurrence of sample shrinkage after a dynamic increase in sizes, although the gas was being still collected in the coal structure. Their relation with the quantity of gas under sorption ε = f (V) was linear only in the initial stage of experiment. It was found that in the case of the coal under investigation preferable is the sorption of carbon dioxide, not of methane, and CH4 is displaced from the sorbed phases by CO2. It seems feasible to recover CH4 through capture and storage of CO2 in a coal bed whose structure, parameters and petrographic composition complies with the material under research. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An increase in CO2 in the Earth's atmosphere is considered to be one of the main factors that determine the heating up of the planet's climate. Ecological issues and prospective economic aspects give rise to more interest in the sequestration of carbon dioxide and ECBM process (enhanced coal bed methane recovery), viz. recovery of methane from carboniferous deposits with simultaneous CO2 deposition in them. It constitutes one of the methods aimed at lowering the emission of this gas. The determination of relations occurring between sorption and dilatometric phenomena that occurs simultaneously on hard coal and the understanding of the underlying mechanisms constitute one of the most important issues pertinent to carbon dioxide sequestration and recovery of methane from carboniferous formations. Hence, in the recent few years more and more research into this field has been done. Fundamental issues to be investigated are changes caused by interactions coal–gas (CH4 and CO2) within a coal bed as well as elaboration of analytical and ⁎ Tel.: +48 12 617 21 21; fax: +48 12 617 45 47. E-mail address: [email protected]. 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.10.002
numerical methods as tools for describing the course of such changes. Mine gases are deposited in coal beds in a few forms: (1) adsorbed in micropores and on the surface of larger pores, (2) absorbed in the coal molecular structure, (3) as free gas in fissures and larger pores whose share becomes of significance at higher sorbate pressures, and (4) dissolved in deposit waters (Ceglarska-Stefańska and Zarębska, 2006; Mastalerz et al., 2004; Zarębska and Ceglarska-Stefańska, 2008). From two main components of mine gas, methane has lower diffusivity, which is why its sorption is slower than in the case of carbon dioxide (Busch et al., 2004; Clarkson and Bustin, 1999a,b; Cui et al., 2004). CH4 molecules have a spatial, tetrahedral structure and are apolar. That is why forces of dispersion play the decisive role in interactions of coal–methane. For critical parameters of methane in standard conditions, CH4 is treated as gaseous sorbate. Whereas CO2 molecules have a linear structure, are not polar, but they are endowed with a high quadrupole moment. In standard conditions CO2 is a vapour. It is commonly assumed that the preferable sorption occurring in hard coal is that of carbon dioxide, not methane, which constitutes the foundation for the idea of ECBM and justifies research activities.
K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
However, in the latest dozen or so years, there have been many published papers that advocate the necessity of desisting from that generalization in favor of analyzing respective coal-gas systems. Busch et al. (2006) performed a series of experiments in the field of competitive sorption of gaseous mixtures of CO2 and CH4 on various hard coals of different rank. The most important conclusion of the authors is that the preferential character of sorption does not depend on the gas composition, but on the coals rank, maceral composition and the pressure of sorbate. Generally, a high rank was related to preferential CO2 sorption, however, low-rank coals under analysis exhibited a non-homogeneous selectivity, viz. preferential sorption of CO2 or CH4. The results of papers (Crosdale, 1999; Majewska et al., 2009) also indicate the preferential methane sorption. Instead, the research results described in articles (Ceglarska-Stefańska and Zarębska, 2005; Yu et al., 2008) exhibited the selectivity of sorption of carbon dioxide from gaseous mixtures of CO2 and CH4. In the case of paper (Ceglarska-Stefańska and Zarębska, 2005) an intensification of that tendency was observed with an increased of sorbate pressure. In the case of investigations carried out on hard coal, in which not only the sorption process, but also the phenomena of the sorption induced expansion of sorbents generated by vapors and gases were monitored, essential are shapes and overall dimensions of samples. There is also another very important cause for using solid samples in sorption experiments. It must be clearly underlined that traditional research into the sorption on powders and grain fractions are not representative in relation to the coal bed parameters, since the natural coal porous structure is destroyed and there is no pressure of overburden rocks (Karacan and Mitchell, 2003). Research into coal expansion leads to a general conclusion that coal is a bi-porous, transport-sorption system in which high pressure of gases causes the compression of microporous areas and expansion of macropores, while the expansion occurring in the microporous coal substance causes transport pores to narrow causing the in situ system permeability of the bed to fall (Ceglarska-Stefańska and Zarębska, 2006; Pan and Connell, 2007; Seewald and Klein, 1986). In the references quoted are specified some research results pointing to a linear relation between coal expansion and the volume of gas accumulated in its structure, for example in (Chikatamarla et al., 2004; Levine, 1996, Robertson and Christiansen, 2005, St. George and Barakat, 2001). There are also some available works according to which the course of sorption processes and the range of contingent strains are not subject to a linear dependence (Ceglarska-Stefańska, 1990; Ceglarska-Stefańska et al., 2007, 2008; Pan and Connell, 2007). The purpose of this study is: (1) the determination of the dependence between the range of expansion/contraction of hard coal under investigation and the volume of gases accumulated in its porous structure in the coal–methane system as well as in the system coal–gaseous mixture (CO2 and CH4), (2) experimental verification of the thesis on preferential character of CO2 sorption vs. CH4 and a possibility of displacement of CH4 from the coal structure by means of CO2 in the system under analysis by introducing carbon dioxide into a coal sample already saturated with methane, and (3) gaining some analytical data for predicting the behavior of the coal under investigation in ECMB processes. 2. Experimental The research was done on a cubicoid hard coal sample, grade high volatile bituminous C according to ECE-UN In Seam Coal Classification, (type 32.2, in conformity with the Polish classification as specified in the standard PN-82/G-97002). The coal was conveyed from the mine ‘Brzeszcze-Silesia’, bed 405. The results of the analysis performed on samples are shown in Table 1. The petrographic composition and average reflexivity of vitrinite were carried out at the Department of Deposit and Mining Geology, Faculty of Geology, Geophysics
73
and Environmental Protection (AGH University of Science and Technology), by means of a mineralogical microscope AXIOPLAN and a reflectometer Axioplan-MPM-400 made by ZEISS OPTION. The analysis showed that the coal had a striped microstructure with variable thickness (under 0.1 to 1.8 mm) and the average strip thickness was approx. 0.3–0.5 mm. Alternate layers are made of semifusinite, fusinite and collotelinite and trimacerite whose composition is similar to clarodurite and vitrinertoliptite (with microsporinite). Collotelinite contains streaks composed of telinite impregnated with gelinite. Fusinite and semifusinite contain a large number of destroyed fragments (crushed and not impregnated). They belong mainly to degrado form, especially semifusinite. In some places those macerals are resinite impregnated which at such places is accumulated in larger amounts. Trimacerite layers exhibit the predominance of inertinite macerals which, like microsporinite and liptodetrinite are embedded in the filler consisting of collotelinite. A rare component occurring in those films is macrinite and macrosporinite. Minerals are chiefly syngenetic carbonate grains. A coal sample sized 20 × 20 × 40 mm was cut out from the primal body in such a way that the walls sized 20 × 20 mm were parallel to bedding plane. The conducted experiments included the investigation of kinetics of accumulation and evacuation of methane (two measurement cycles) and kinetics of accumulation of mixture of CO2 and CH4 in the structure of coal under analysis that contained methane previously adsorbed. Simultaneously the kinetics of coal sorption induced strains was monitored. Sorption and expansion/ contraction measurements were taken using the apparatus and test procedures followed that described by (Majewska et al., 2008, 2009). The experimental set-up consisted of two individual units working together: (1) the gas sorption apparatus using volumetric method (pressure meter MKS BARATRON 722A with a measuring range of 0– 4.0 MPa and measurement accuracy ±0.001 MPa) and (2) the strain meter for measuring samples external dimensions changes (electrical resistance bridge, type SGM-1C81, constructed in Strata Mechanics Research Institute of The Polish Academy of Science, measuring range of linear strain ~ 4‰, measurement accuracy 0.001‰, with domestic production resistance-type paper strain gauges type RL120). The whole apparatus was thermo-stabilized, which enabled measurements to be taken at constant temperature of 298 K. The experimental procedure included the previous long-term degassing of the sorption apparatus and the sample itself (vacuum 10−5 hPa) and immersions in a helium bath (10 kPa) to secure the removal of possible interfering sorbed gases and vapours, including water molecules. The measurement cycle included a few stages. First, methane was introduced into the degassed measurement cell and the changes in coal storage capacity and sample sizes were monitored until reaching the state close to the equilibrium of linear stresses (sorption 1_CH4). Then, the gas desorption was performed by decreasing the sorbate pressure in the sample cell (desorption 1_CH4). Later on, the measurement ampoule was filled up with an extra portion of sorbate and the observation of advancing changes to storage capacity and sizes of the tested material was continued (sorption 2_CH4). Afterwards, in a similar way another desorption of the gas was performed (desorption 2_CH4). The successive stage was the injection of carbon dioxide into the measurement space containing methane left after the desorption 2_CH4 (sorption 3_ CO2/CH4). Further monitoring of gas storage capacity and sample sizes was carried out. At the end of this part of experiment, a gas sample was collected from the measurement cell to determine its composition through chromatography. A gas chromatography apparatus Hewlett Packard HP 5890 (adsorbent Porapak Q) with a thermal-conductance detector TCD was used. Basing upon the obtained kinetics of coal induced linear strains in the perpendicular (εT) and parallel (εL) directions, the kinetics of volumetric strains (εV) were determined in accordance with the relation: εV = εT + 2 εL.
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K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
Table 1 Characteristic of the coal sample under study. Chemical and technological analysis Wa [%]
Aa [%]
Ca [%]
Cdaf [%]
Ha [%]
1.52
11.23
72.21
82.76
4.31
Petrographic composition Telinite
Collotelinite
Vitrodetrinite
Collodetrinite
Corpogelinite
Gelinite
Total vitrinite
Sporinite
Rezinite
Liptodetrinite
3.0
24.2
1.3
9.2
1.0
2.5
41.2
6.5
2.1
2.3
3. Results and discussion The experimental results are collected in Table 2, and given as plots in Figs. 1–4. They show, respectively, the kinetic curves of accumulation and evacuation of methane and gaseous mixture CO2 and CH4 in the porous structure of the cubicoid hard coal sample and kinetic curves of coal expansion/contraction that accompany sorption processes (Fig. 1), the dependence between coal volumetric strains and the quantity of methane collected in its structure (Fig. 2), the dependence between coal volumetric strains and the amount of mixture of gases accumulated in its structure (Fig. 3) and the value of pressure in the sample cell vs. time throughout all experimental stages (Fig. 4). It needs to be underlined that decisive for the completion of each research stage was reaching the plateau on kinetic curves of linear strains, viz. a state close to dilatometric equilibrium. In no research stage was attained the state close to the sorption equilibrium in the coal–gas system. A low gas storage capacity of coal towards methane and gaseous mixture can be attributed to its low rank (Table 1). It is related to a relatively small share of micro- and ultramicropores in total porosity, even in the macerals of vitrinite group (Chalmers and Bustin, 2007). The share of vitrinite in the coal under analysis is merely 41.2%. Moreover, the presence of functional groups occurring peripherally in appreciable quantities stops, to some extent, the pore clearance and prevents sorbate molecules from free penetration into the coal structure. A relatively high share of macerals of inertinite and liptinite groups in the petrography of the sorbent under investigation, for its meso- and macroporosity, on the one hand decreases the coal sorption capacity, but on the other, this share facilitates the displacement of gas within the coal pore system. Hence, a relatively small extent of strains of the coal under investigation was observed. This is in particular true for fusinite (14.2%) and semifusinite (18.6%), which does not yield to expansion, and which in most cases contain no other components in cellular spaces. Similar conclusions can be found, for example, in Karacan (2003) and Karacan and Mitchell (2003). As can be seen in Fig. 1, the course of both the kinetic curves of methane sorption is different as far as the gas accumulation rate is concerned, which is related to a better availability of coal porous structure in the stage of sorption 2_CH4. Those curves also indicate that coal sorption capacity towards methane is in stage of sorption 1_CH4 slightly smaller than the value obtained at the end of stage sorption 2_CH4. After desorption 1_CH4, in the coal structure is still left approx. 50% of gas stored in the sorption stage. In the case of desorption 2_CH4, a triple reduction of methane pressure in the measurement ampoule caused gas desorption at a level of 50% of gas accumulated during the sorption 2_CH4 and about 70% if referred to the total methane sorbed. According to predictions, linear strains of coal exhibit anisotropy through the entire experiment. The relation between the linear strains εT:εL changes during the experiment from 2.1 for methane sorption stages and 2.8 for methane desorption steps to 1.8 for the sorption 3_CO2/CH4 stage. The sorbent expands/shrinks more in perpendicular
than parallel direction to stratification (Fig. 1). Kinetic curves of CH4 sorption and desorption with corresponding kinetic curves of volumetric strains are featured with a very similar trends. Therefore, with a slight approximation, the dependence of volumetric strain vs. methane sorption/desorption εV = f (V) is more or less linear (Fig. 2). However, those curves show tendency towards a sigmoid shape, because the rate of transformations in coal sizes is slower at the beginning and at the end of each experimental stage. In the first case, it results from pressure increases or decreases in the ampoule due to sorbate dosage or removal, which, respectively, will cause the matrix to contract or expand. In the latter case, this is related to the tendency towards relaxation and re-arrangement of coal structure, when the rate of changes in the quantity of accumulated gas decreases at the end of each stage. The curves that correspond with sorption stages of experiment are inclined to the OX axis at similar angles. The εV = f (V) plots for desorption stages are inclined to OX axis also at similar, but bigger angles that in the case of sorption curves (especially for desorption 1_CH4). It can suggest that the sample shrinkage during desorption occurs faster than the expansion in the course of sorption. It should be also noted that in Fig. 2 each successive curve εV = f (V) is slightly shifted more towards the right side than the curves from earlier experimental stages. The conclusion is that at the same quantities of gas accumulate in the coal structure in the successive experimental stages, the value of corresponding strains diminishes. Such observations corroborate the idea that in those experimental conditions the induced strains are not fully reversible. That could result very likely from the exposure to instantly increased high pressure of gas and also even more likely from sudden decompressions be several MPa than from sorption itself. As reported by Day et al. (2010) exposure to helium and neon can cause pressure induced contraction of coal. Sorption of these gases on coal is almost none hence no swelling occurred in their tests up to 16 MPa. The results presented by Romanov and Soong (2009) showed that the exposure of coal lumps and powders to high pressure helium may change the sorbents helium density and its texture. In the case of coal lumps those changes appeared to be irreversible. Be also remembered that methane was introduced into the sample previously degassed under a high vacuum and in desorption stages the pressure value was not brought down to 1 atm or to vacuum. For its low rank, coal exhibits only a slightly arrange structure. The lack of spatial orientation of high-molecular compounds which constitute the three-dimensional network and high contents of molecular phase caused that the sorbent matrix is relatively flexible within the range of strains resulting from sorption effects. The course of εV = f (V) curves in Fig. 2 can suggest that the availability of coal structure slightly increases in successive experimental methane sorption/desorption cycles, when the sorption capacity towards that gas is relatively constant. Motions of sorbate molecules conveyed in pores to and from CH4 depositing areas and accumulation and evacuation itself takes place each time in a slightly changed coal porous structure. Sorption processes and sorbate pressure cause the coal structure to change gradually. Its constituents are displaced, and the direction of such a transformation is a more stable and more highly associated system. Such
K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
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Chemical and technological analysis Na [%]
Sa [%]
dtrue 103[kg/m3]
dapparent 103[kg/m3]
Porosity [%]
1.19
0.37
1.379
1.344
2.50
Petrographic composition Total liptinite
Fusinite
Semifusinite
Macrinite
Micrinite
Funginite
Inertodetrinite
Total inertinite
Mineral matter
Ro [%]
10.9
14.2
18.6
2.6
1.2
trace
8.4
45.0
2.9
0.78
modifications are called relaxation. Due to them, sorption 2_CH4 was accompanied by smaller strains than in the case of sorption 1_CH4, although the quantity of gas accumulated in the coal structure was slightly bigger. At the Faculty of Energy and Fuels, AGH University of Science and Technology, investigations were made into coal, type 32.2, obtained from the Brzeszcze-Silesia mine. Apart from typical sorption tests performed on grain samples, some experiments were also conducted on coal samples (plates) cut out from the primal lumps, sized 15 × 15 × 5 mm, with simultaneously monitoring of changes in the sizes of such specimens. Sorbates were carbon dioxide, methane and their mixtures. Test results presented in this paper were compared with the data from references (Ceglarska-Stefańska, 1990; CeglarskaStefańska et al., 2008; Milewska-Duda et al., 1994; Zarębska and Ceglarska-Stefańska, 2004, 2008) obtained for the same type of coal from the same mine. It was found that previously measured values of coal CH4 sorption capacity were three times higher, viz. approx. 12 cm3STP/g, for both grain and plate samples. Strain values in the directions perpendicular and parallel to stratification were also higher, viz. 1.8–4.0‰ and 1.7–3.0‰ for similar values of sorbate pressures. It is also necessary to mention that in the coal material described in this paper there were significant amounts of macerals of inertinite group (45%), inc. fusinite (14.25%) and semifusinite (18.6%), whereas the vitrinite share was merely 41.2%. So it must be underlined that in the experiments described in the papers mentioned the research material had a different petrography. It contained more vitrinite (67.5–76.7% and 46.4%), but first of all, definitely less fusinite and semifusinite (in total, under 15%), hence, their higher gas storage capacity and higher linear strains in plate samples. On the other hand, smaller coal sample sizes are related to a smaller share of transport pores and cracks of primal coal. The successive research stage was carried out to check if the displacement of methane molecules in the sorbed phase by carbon dioxide molecules takes place basing upon the change of sorbate composition in the measurement cell. The initially calculated methane content in the mixture was 20.5%. According to chromatography, at the end of stage sorption 3_CO2/CH4, the share of this gas was 34%. Sorbate enriched in methane corroborates that the adsorbed phase was enriched in CO2 whose share was 91.3%. Hence, the conclusion may be drawn that in the case of coal under analysis the preferential sorption of CO2 at the expense of CH4 occurred. The effect of displacement of CH4 from the sorbed phase by CO2 was true for 60%
Taking into account the above it could be speculated that, it is technically possible to enhance the recovery of methane by injecting carbon dioxide in a coal bed whose structure, technical parameters and petrography correspond with those of test material. Strains that accompany sorption processes in the stage sorption 3_CO2/CH4 were twice that high as in the preceding experimental stages (Fig. 1). There occurred an interesting divergence between the course of kinetic curves and linear strains in the directions perpendicular (εT) and parallel (εL) to stratification. In the first 80 h of that experimental stage, the sample exhibited a dynamic elongation perpendicular to bedding plane (εT), while after reaching the maximum value on the kinetic curve of the process, a gradual shrinkage occurred. Changes in coal sizes parallel (εL) to stratification had a definitely less dynamic course. In the kinetic curve related to that process, there was no clear maximum, and therefore, it is impossible to indicate the moment at which the character of (εL) strains changed from expansion to contraction. Still it is safe to say, that it occurred later than in the other direction. The maximum value of volumetric strains was 12.16‰ and was recorded in the 164th hour of that stage. The sample contracted, although the pressure in the sample cell was decreasing (Fig. 4) indicating that the gas was still being accumulated in the coals structure (Fig. 2). Later on, a slight variability in the sample sizes and linear course of sorption kinetics were recorded (Fig. 1). In the experiment carried out for the system coal–gaseous mixture 75% CO2 and 25% CH4 (Ceglarska-Stefańska et al., 2008), the values of linear strains for plate samples were 4.1‰ in both directions to the stratification, while the sorbate pressure being 2.86 MPa. Results of dilatometric measurements for gaseous sorbates (CO2 and CH4) and their mixtures shown in references (Ceglarska-Stefańska, 1990; Milewska-Duda et al., 1994; Zarębska and Ceglarska-Stefańska, 2008), and in this paper indicate a definitely smaller anisotropy of dimensional changes of coal sized 15 × 15 × 5 mm if compared to the course of such transformations for studied samples, sized 20 × 20 × 40 mm. It corroborates the thesis that such tests should be performed on solid samples, larger than a single lithotype, which constitute possibly a representative fragment of primal coal body. It is difficult to indicate clearly the causes of changes in the character of coal strains in the stage of sorption 3_CO2/CH4 (Fig. 3). The author believes that at the initial stage of experiment, carbon dioxide diffusing through the porous coal structure adsorbed in micropores (also in submicropores) and on the surface of larger pores
Table 2 Changes of the amount of accumulated gas and induced strains of the coal samples under study.
Sorption 1_CH4 Desorption 1_CH4 Sorption 2_CH4 Desorption 2_CH4 Sorption 3_CO2/CH4
Time [h]
Initial pressure [MPa]
Final pressure [MPa]
Final perpendicular strain εT [‰]
288.6 143.4 265.7 239.2 579.5
3.68 0.21 3.14 1.01 4.18
3.25 0.44 2.90 1.07 3.36
2.291 0.878 2.213 1.272 5.202
of methane previously accumulated in the coal porous structure.
on
Final parallel strain εL [‰]
1.168 0.315 1.035 0.455 the2.840 outer
portions
Final volumetric strain εV [‰] 4.627 1.508 4.283 2.180 of 10.882 the sample.
At the
Final amount of sorbed gas [cm3STP/g] 3.25 1.71 3.515 2.58 11.65 time, same
desorption of
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K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
Fig. 1. Kinetics of gas accumulation and induced strains.
Fig. 3. Relationship between volumetric strain and volume of accumulated gas in coalmixture of CO2 and CH4 system.
CH4 molecules started, substituted by CO2 molecules. Those processes were accompanied by expansion of microporous areas of coal (vitrinite, collotelinite) and an extra pressure was being put on the interior parts of the sample. Eventually, the clearance of transport pores diminished because of induced swelling of the external section. General compression might occur in areas of low microporosity (inertinite, fusinite and semifusinite). Successive relaxation of coal matrix elements enabled deposition of CO2 molecules inside the matrix also by way of absorption, which probably did not take place in the case of methane. Most probably there was a gradient of gas saturation traveling throughout the sample body and changing the location of its front during the experiment towards the center of the specimen. Assuming the hypothesis that after 164 h of experimental works the degree of saturation of the coal structure with sorbate in the external and middle parts of the sample was relatively close to the maximum, the successive sorption and dilatometric effects, viz. samples shrinkage and change in the sorption kinetics (linear and increasing) might be explained basing in part upon the approach
postulated by the authors of (Karacan and Mitchell, 2003; Larsen, 2004) and taking the aspect of moving saturation peak under consideration. Namely, after an initial increase in coal sizes, its structure yields to re-arrangement and relaxation. Of course, such transformations occur more slowly than gas accumulation in coal. The expanding coal exceeds the state of thermodynamic equilibrium in the coal–gas system, and the consecutive relaxation leads to the restoration of equilibrium. In consequence, CO2 solubility in coal is reduced. The excess of CO2 molecules is shifted back in the system of pores increasing the pore pressure. The gas moves then towards the interior parts of the coal porous system according to the pressure gradient. Further accumulation of gas in the outer parts of coal is likely to occur mainly with free gas, which complies with the opinion of the authors of (Ceglarska-Stefańska et al., 2007). Probably, the sample shrinkage can be attributed to increased pressure of gas in coal pores and the effect of desorption of CO2 molecules previously absorbed. A situation in which the pressure of free gas is sufficient to cause coal compression counterbalanced by the expansion related to
Fig. 2. Relationship between volumetric strain and volume of accumulated gas in coal– methane systems.
Fig. 4. Pressure changes in sample cell.
K. Czerw / International Journal of Coal Geology 85 (2011) 72–77
sorption processes was observed during CO2-ECBM field project performed in the Alison field, located in New Mexico, USA. The injection of CO2 in CH4 containing bed caused an increase in pressure in coal pores, and in consequence, increased bed permeability (Reeves et al., 2003; Siriwardane et al., 2009). Therefore, attention should be also paid to ‘peaks’ occurring in kinetic curves of induced strains at the places corresponding to dosing or releasing of gas to or from the measurement cell, which can imply that the sample reacts very rapidly to the change of pressure in the ampoule and it changes its sizes (Fig. 1). Instead, the effects expansion/contraction caused by sorption/desorption processes becomes noticeable only after about 10 min of contact between coal and sorbate, which is convergent with the opinion of the authors of (Siriwardane et al., 2009) who ascertained that the effect of pressure in pores is more noticeable than coal expansion/contraction. 4. Conclusions The following conclusions can be drawn from this study: • Modeling of deposition, accumulation and emission of mine gases in rock mass entails test on relatively large, solid sample in which natural heterogeneous character of primal coal, dual porosity and contents of respective petrography components is preserved. • Strains observed in the experimental conditions in the system coal– methane could be irreversible; with a certain approximation, the dependence between volumetric stain and the sorption/desorption capacity of methane εV = f (V) can be expressed linearly. • Sorption/desorption processes cause relaxation changes in the coal structure and are aimed at attaining a compact and thermodynamically stable system. • The course of strains observed during the experiment in the system coal–gaseous mixture is characterized by occurring a sample contraction, although the pressure of gas was decreasing and the sorbate was still being accumulated in the coal structure after a dynamic increase in size. • In the case of the coal under investigation, a preferential sorption of carbon dioxide over methane occurred and the displacement of CH4 from the sorbed phase by CO2 molecules had place. Hence, the concept of enhancing methane recovery by injecting carbon dioxide to coal seems, whose structure, technical parameters and petrography comply with the characteristic of the material tested seems possible. Acknowledgments Financial support for this study was provided by AGH/UST framework no. 11.11.210.117. The author wish to thank Prof. Dr. Hab Grażyna Ceglarska-Stefańska (Faculty of Energy and Fuels, AGH University of Science and Technology, Kraków) for the valuable discussions, careful reading of the manuscript and helpful remarks. The author is grateful for the close support of Dr. Eng. Jerzy Ziętek (Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Kraków). The author would also like to express her gratitude to Dr. Hab. Eng. Marian Wagner—AGH Prof. (Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Kraków) for performing the petrographic analysis of coal under study. Special thanks are required for Mr. Jerzy Małyniuk (Faculty of Energy and Fuels, AGH University of Science and Technology, Kraków) for his technical support in experimental work. References Busch, A., Gensterblum, Y., Krooss, B.M., Littke, R., 2004. Methane and carbon dioxide adsorption/diffusion experiments on coal: an upscaling- and modeling approach. International Journal of Coal Geology 60, 151–168.
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