Influence of Maximum Pressure on the Path of CO2 Desorption

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Influence of maximum pressure on the path of CO2 desorption isotherm on coal Gongda Wang, Ting Ren, Kai Wang, and Yaqin Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501974a • Publication Date (Web): 16 Oct 2014 Downloaded from http://pubs.acs.org on October 20, 2014

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Influence of maximum pressure on the path of CO2 desorption isotherm on coal Gongda Wanga,b*, Ting Renb, Kai Wanga, Yaqin Wuc a

Faculty of Resources & Safety Engineering, China University of Mining and Technology, Beijing 100083, PR China

b

School of Civil, Mining & Environmental Engineering, University of Wollongong, NSW 2500, Australia

c

School of Mechanical Electronic & Information Engineering, China University of Mining and Technology, Beijing 100083, PR China

Key words: CO2; coal; adsorption; desorption; hysteresis; maximum pressure

Abstract: Coal seams with high CO2 content may have outburst risk and degasification of CO2 has to be conducted before these coal seams can be safely extracted. For geosequestration of CO2 in un-minable coal seams, the injected CO2 may desorb with the reduction of CO2 pressure. Desorption of CO2 dominates these processes, whilst adsorption isotherms are widely used assuming the adsorption-desorption process is fully reversible. In order to understand the difference between CO2 adsorption and desorption isotherms, i.e., sorption hysteresis, as well as the dependence of CO2 sorption hysteresis on maximum pressure, four cycles of CO2 adsorption-desorption experiments are conducted continuously with increasing maximum pressure (1, 2, 3, 4Mpa). The difference of CO2 emission volume 1

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between adsorption and desorption isotherms is compared and a significant deviation (0.059 and 0.032mol/g) has been observed. The adsorption isotherms show a good repeatability, indicating the gas holding capacity does not change during a long-term contact with CO2. However, a distinct difference between the desorption isotherms is observed, the path of desorption isotherm depends on the maximum pressure, and higher maximum pressures can reduce the proneness of CO2 desorption in the pressure range of this study (0-4MPa).

We suggest that desorption isotherms should be used to predict the CO2 emission volume and the long-term storage stability, and the maximum pressure of laboratory sorption test should be decided according to the in-situ coal seam pressure.

1. Introduction CO2 is an important component of coal seam gas as well as methane. Coal seams with high CO2 content have been found in Australia, Czech, France, Poland, Turkey and China, and these are also identified as outburst-prone seams1-3. Degasification of CO2 from coal seams is widely applied to reduce the outburst-proneness prior to coal recovery and during this process, CO2 desorbs from coal matrix, where the adsorption mainly occurs. Adsorption isotherm can reflect the gas holding capacity of coal at any particular pressure and temperature, thus it is widely used to calculate the gas emission volume in an indirect approach4. The sorption isotherm is also an important parameter when using reservoir simulations to predict the actual gas drainage or production process. A number of studies have been reported which showed the discrepancy of the difference between CO2 adsorption and desorption isotherms5-11, however, the mechanism of sorption hysteresis is still an open question. Injection of CO2 into coal seams as a secondary gas to strip methane is an attractive option to enhance coalbed methane (ECBM) production and sequestrate CO2 in un-minable coal 2


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seams12. The injected CO2 will firstly be adsorbed in coal seams but may desorb when the gas pressure decreases due to CO2 diffusion from coal seam to other adjacent geological strata, thus understanding the desorption characteristic of CO2 is also beneficial for accurately predicting long-term stability of the sequestrated CO2. By reviewing the published data on sorption hysteresis on coal, we found the maximum adsorption pressure of CO2 may have impact on hysteresis degree13, however this finding was based on comparison of data from different authors, and the influence of some other factors, such as the coal rank and type, dry method and temperature cannot be ruled out. In order to directly compare the relationship between maximum pressure and CO2 desorption isotherm as well as sorption hysteresis, four cycles of CO2 adsorption-desorption with different maximum pressures are continuously carried out on coals from Sydney Basin, and the applicability of adsorption isotherms and the variability of desorption isotherms are discussed. 2. Experimental procedure The coal samples were collected from Bulli coal seam, Sydney Basin, where high CO2 areas were experienced in Tahmoor, Metropolitan, Appin and West Cliff Colliery

14-15

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Proximate and petrography analyses of Bulli coal seam are sourced from a CSIRO report 16, as shown in Table 1. Fresh coal blocks were collected and crushed, and 2 sizes of fraction (0.50-1.13 mm, 1.13-2.36 mm) were then used to examine the influence of sample size on sorption hysteresis. A gravimetric sorption apparatus was used17-18, and detailed information of the apparatus and calculation method can be found in the supplementary material. It should be noted that the thermodynamic property of CO2 can vary sharply under high pressure, and in this study a relative low pressure range (0-4MPa) was chosen, thus all the results and discussions are based on this pressure range. The experimental procedure is shown in Scheme 1, four cycles of ‘adsorption, desorption and evacuation’ were conducted continuously on the 3


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same coal sample, and the maximum pressure of each sorption cycle was increased gradually to avoid the influence of possible change of coal property due to high pressure. At least 24 hours of waiting time was performed for each pressure step, and only until the change of gas pressure in sample cell was less than 0.001MPa within 6 hours, the sorption equilibrium was assumed to be achieved.

Scheme 1 Schematic flow diagram of the experimental procedure Table 1 Proximate and petrography analyses of Bulli coal16

Moisture Volatile (%) Matter (%) 1.3

21.7

Fixed Carbon (%) 71.4

Vitrinite Reflectance (%) 1.28

Vitrinite (%)

Liptinite (%)

Inertinite (%)

41.6

0.1

55.3

3. Results and discussion Sorption hysteresis, especially the CO2 sorption hysteresis, has been extensively reported in the past, and in our study this phenomenon is also observed in all experimental groups. However, during reviewing the published literature, we found the influence of sorption 4


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hysteresis on CO2 emission or drainage has not been demonstrated well. As an example, the difference of the CO2 emission volume between adsorption and desorption isotherms is compared. Figure 1 shows the sorption isotherms of 0.50-1.13 mm coal sample at a maximum CO2 pressure of 3MPa, dual sorption model is used to represent the sorption isotherms: ܸ = ݇ܲ + ܸ଴ᇱ

௉ ௉ಽᇲା௉

(1)

where V is the CO2 sorption volume in coal; V0 is the maximum CO2 sorption capacity; P is the CO2 pressure; ݇ is Henry’s law dissolution constant; ܸ଴ᇱ and ܲ௅ᇱ are Langmuir-like constants in the dual sorption model. 0.5

Excess sorption volume, mole/g

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V2

V1

0.4

Adsorption point Desorption point Fitted adsorption isotherm Fitted desorption isotherm V1 : Gas emission volume calculated from adsorption isotherm V2 : Gas emission volume calculated from desorption isotherm

0.3

0.2

0.1

0 0

500

1,000

1,500

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Pressure, Kpa

Figure 1 Adsorption and desorption isotherms of Bulli coal (0.50-1.13 mm) at a maximum CO2 pressure of 3MPa As can be seen from Figure 1, when gas pressure drops from 3MPa to 1.5MPa, the volume of CO2 emission using adsorption and desorption isotherms can be correspondingly calculated as V1 =0.059mol/g, and V2 =0.032mol/g. Only approximate half volume of CO2 can desorb comparing with the calculated value using adsorption isotherm. Obviously the ‘weaker’ proneness of CO2 desorption is beneficial for the long-term stability of the sequestration of CO2 in un-minable coal seam. Historically the nature of CO2 sorption, which 5


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has very steep initial sorption capacity compared with methane sorption, is believed to be responsible for the hard-to-drain coal seams with high CO2 composition19. Figure 1 shows that the sorption hysteresis may also contribute to this problem, because the desorption isotherm has a ‘steeper’ initial sorption capacity than the adsorption isotherm. It also indicates that the sorption hysteresis of methane, although less significant than that of CO2, also exists7, 9-11. Adsorption isotherms can be used to evaluate the gas holding capacity of coal seams, whilst desorption isotherms should be used to predict the production of gas drainage and CBM recovery. Figure 2 shows the adsorption isotherms at different maximum pressures. The samples were exposed to pure CO2 for more than 4 months during the whole processes, and a good repeatability of adsorption isotherms can be observed for both samples. It indicates that the coal property does not change in term of adsorption capacity. Goodman et al.20-21 investigated the direct interaction between CO2 and coal using Attenuated Total Reflectance-Fourier Transformation Infrared (ATR-FTIR) spectroscopy, results show the CO2 sorption on coal is a pure physical sorption, which could explain the good repeatability observed in our study. Excess sorption volume, mole/g

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Comparison of adsorption isotherms at different maximum pressure, 0.50~1.33mm coal

0.5

0.4

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0.2

Maximum pressure 1Mpa Maximum pressure 2Mpa Maximum pressure 3Mpa Maximum pressure 4Mpa

0.1

0.0 0

1000

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Pressure, Kpa

(a)

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Comparison of adsorption isotherms at different maximum pressure, 1.33~2.36mm coal

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2000

3000

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(b)

Figure 2 Adsorption isotherms of 0.50-1.13 mm coal (a) and 1.13-2.36mm coal (b) at different maximum CO2 pressure Interestingly, for both samples, desorption isotherms at different maximum pressure do not follow the same path as shown in Figure 3. The desorption isotherm from a higher maximum CO2 pressure lies on the top of the isotherm curves, and the differences between these isotherms appear significant. A strong controlling effect of maximum pressure on the path of desorption isotherm can be observed, and the proneness of desorption decreases with increasing maximum CO2 pressure. Previous laboratory sorption tests were usually conducted at a random selected maximum pressure rather than the in-situ gas pressure in coal seam, and if it was the case, a large deviation of desorption isotherm can be expected. For CO2 sequestration, the result shows that increasing the injection pressure will not only enhance the adsorbed CO2 volume, but also impede desorption process, and thus promote the long-term stability of the sequestration of CO2.

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Comparison of desorption isotherms at different maximum pressure, 0.50~1.33mm coal 0.5

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(a) Excess sorption volume, mole/g

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Comparison of desorption isotherms at different maximum pressure, 1.33~2.36mm coal

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Figure 3 Desorption isotherms of 0.50-1.13 mm coal (a) and 1.13-2.36mm coal(b) at different maximum CO2 pressure Figure 4 shows the relationship between residual gas contents and the corresponding maximum pressures. The residual gas contents were measured after exposing the samples to dry atmosphere for 72 hours, and the change of residual gas contents were less than 0.001mole/g during the last 24 hours, hence the equilibrium is believed to be achieved. It can be seen that with increasing maximum CO2 pressure, aside from one exception, the residual CO2 content increases and a linear relationship can be observed. It should be noted that after 8


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evacuating the samples for 24 hours, the residual gas contents were nearly depleted for all the experimental groups. It indicates that the residual CO2 molecules, although are hard-todesorb in atmosphere, the bonding between them and coal matrix are still physical sorption. It can be seen from Figs. 2, 3 and 4 that the difference of sample sizes used in this study does not have an apparent influence on the excess sorption volumes nor the residual gas contents, the trends of the isotherms and residual gas contents are similar, indicating the test results are by no means of a coincidence. 0.09

Residual gas content, mole/g

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1.13~2.36mm coal 0.50~1.13mm coal

0.08

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0.05

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0.01 500

1,000

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Maximum adsorption pressure, kpa

Figure 4 Dependence of residual gas content on maximum CO2 pressure Previously, several hypotheses have been proposed to explain the sorption hysteresis on coal, such as residual moisture content8-9, chemical interaction22, experimental inaccuracies7, insufficient waiting time5. Based upon the results of our experiment, the influence of residual moisture content and experimental inaccuracies can be excluded, because the good repeatability of adsorption isotherms and the differences of desorption isotherms were observed alternately. As discussed above, neither should the insufficient waiting time nor the chemical interaction be responsible for this phenomenon. For a physical sorption on coal, we suggest a possible explanation of sorption hysteresis: it is related to the kinetic restriction or accessibility of narrow pore throats, which may exist in micropores, submicropores, macromolecular and molecular fractions; enforced by gas pressure, some CO2 molecules 9


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embed into these pores and result in the swelling of coal matrix, which is believed to be a solution process of gas molecules within macromolecules of coal23, and it may causes the deformation of pore configurations, and further narrows the pore throats; the ‘narrowed’ pore throats require more energy for gas molecules to escape from these pores, and thus resulting in the difference between adsorption and desorption isotherms. The role of maximum CO2 pressure can be understood from this explanation: by increasing CO2 pressure, more CO2 molecules will be forced into these restricted pores, leading to a more severe deformation of coal matrix and pore throats, and consequently a lower proneness of desorption. It should also be noted that the coal swelling has great impact on permeability change during gas depletion24-26. Since the coal swelling may be directly proportional to the gas content of coal, the influence of gas sorption hysteresis on gas permeability change appears to be another interesting phenomenon. 4. Conclusion A laboratory study using four cycles of CO2 adsorption-desorption isotherm tests has been performed continuously with increasing maximum pressures. A great difference has been found when comparing the volume of CO2 emission between adsorption and desorption isotherms. A good repeatability of adsorption isotherms and a significant difference of desorption isotherms are observed alternately. The maximum pressure has impact on the path of desorption isotherm, less CO2 desorbs when gas pressure drops from a higher maximum CO2 pressure. The sample size used in this study does not have an apparent influence on the excess sorption volumes nor the residual gas contents, Corresponding Author Gongda Wang, Tel: +61 46687 9391; Email: [email protected]; Address: Room 309, Building 6, University of Wollongong, Wollongong, NSW 2500, Australia

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ACKNOWLEDGMENT The authors acknowledge the financial support provided for this work by the China Scholarship Council (award to Gongda Wang for 1 year’s study at the University of Wollongong) and the Natural Science Foundation of China (51174212).\ REFERENCES 1.

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Influence of Maximum Pressure on the Path of CO2 Desorption

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