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Pore structure characterization of different rank coals using N2 and CO2 adsorption and its effect on CH4 adsorption capacity: A case in Panguan syncline, western Guizhou, China Shida Chen, Shu Tao, Dazhen Tang, Hao Xu, Song Li, Junlong Zhao, Qi Jiang, and Haoxin Yang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017
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Energy & Fuels
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Pore structure characterization of different rank coals using N2 and CO2
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adsorption and its effect on CH4 adsorption capacity: A case in Panguan syncline,
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western Guizhou, China
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Shida Chen a,b, Shu Taoa,b, *, Dazhen Tanga,b, Hao Xua,b, Song Li a,b, Junlong Zhaoa,b, Qi Jiang a,b, Haoxin Yang a,b a
School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, PR China; b
Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &
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Utilization, Beijing 100083, PR China;
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* Corresponding Author-Email:
[email protected] 9
Abstract: To determine the pore structure characteristic of different coal ranks in Panguan
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syncline, both N2 adsorption-desorption (LP-N2GA) and CO2 adsorption (LP-CO2GA) were
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carried out with the goal of revealing the differentiation evolution of total pore volume(TPV),
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specific surface area (SSA), pore size distribution (PSD) and pore shape of 14 coal samples, and
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the influences of SSA to adsorption capability at different sizes (super-microporous2.76nm) and sudden decrease phenomenon when the relative pressure is
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0.5.Thus, it is visible that there are certain pores, whose required relative pressure for
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concentration is higher than the required relative pressure for desorption evaporation, which
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means that the pore type reflected by this curve is the interconnected pore with two ends open,
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such as the parallel plate pore or cylinder pore with two ends open, which is helpful for the
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adsorption, desorption and diffusion of CBM. Represented by the remnant samples, Type B has no
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hysteresis loops or only small hysteresis loops, reflecting that the pore of this type has the same
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relative pressures in adsorption concentration and desorption evaporation. This kind of curves
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mainly consist of semi-open pore with poor connectivity, such as the wedge-shaped, cylindrical
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and slit-shaped pores with one closed side, which has the weakest adsorption and accumulation
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capability, but is helpful for the desorption and diffusion of CBM.
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3.2.2. N2-TPV, N2-SSA and pore size distribution
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The N2-SSA of 14 coal samples is 0.097-0.546 m2/g (mean 0.318 m2/g), the N2-TPV is
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1.01-6.89×10-3×10-3 mL/g (mean 2.76×10-3 mL/g) (Table 2). According to the calculation of the
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occupied percentages of the pore volumes at different pore size, the transition pore (10-100nm,
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73.69-95.21%) is most developed, followed by the micropore (4.79-26.31%). The distribution
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characteristics of the pore volume and N2-SSA at different pore size of 14 coal samples are shown
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in Fig.4. It can be seen that the pore volume distribution of all test samples at different pore size
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appears to be multimodal, and the peak value at 10-100nm is higher than the one at 2-10nm,
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especially for pores with diameter 60-70nm, indicating that the pores of this section has the
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biggest contributions to the pore volume. In addition，YLT-6 and YLT-8 is of poor development
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of pores with diameter 3-8nm. For most of coal samples, the differentials of the pore specific
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surface area to the pore diameter all present as the single-peak model, and the peak value appears
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within 2-3nm, which means that the N2-SSA is mainly contributed by this part of pores. In other
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words, within 2-100nm, the pore volume mainly comes from the contribution of 10-100nm pores,
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and the more the pores in 2-3nm develop, the bigger the N2-SSA is.
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In addition, Fig.5a indicates that a bigger N2-TPV will lead to a higher N2-SSA, and there is a
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positive relationship between the N2-TPV and coal rank. Meanwhile, with the increasing coal rank,
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the N2-SSA rises gradually. However, the increase rate of N2-SSA decrease gradually with the
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increasing vitrinite reflectance, and the increase tendency presents to be a half-reversed “U” shape
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in the whole (Fig.5b). When Ro,ran>1.4%, the variation amplitudes of the micropore proportion and
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N2-SSA are extremely small. When Ro,ran is within 0. 8-1. 4%, the methane in the coal escapes in
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quantity, and then the hydrogenous side chain and key shorten and reduce greatly, so as to decline
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the coal density. In addition, the coal moisture is also decreasing continuously under the pressure,
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and the porosity is also minimizing, with the dramatic declining of the big pores as the primary.
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Meanwhile, the micropore proportion is increasing constantly, so as to result in the fast growth of
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N2-SSA. When Ro,ran reaches 1.4%, the humic gel basically completes the dehydration. Then the
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moisture and porosity in the coal have both reduced to the minimum and the average pore size has
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already been the smallest [18], [20] and [34]. When vitrinite reflectance further increases
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(Ro,ran >1.4%), ketogenic metamorphism is dominant, the micropore content presents to have
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smaller variation tendency.
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3.3. Measurement of super-microporous with LP-CO2GA
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The micropore is an important index to assess the accumulation and adsorption capacity of
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CBM [47] and [48]. However, at present, the super-microporous in diameter less than 2nm has not
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aroused enough attentions, while the characteristics of the super-microporous have vital influences
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to the adsorption performance of the methane [19] and [49]. This study employs the same
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instruments with the N2 adsorption experiment and conducts tests under 273.15K. As to the
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super-microporous, the adsorption mechanism of the carbon dioxide on the coal surface is
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mono-layer adsorption or microporous fill. So, the adsorption curve coincides with the desorption
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curve, and the desorption curve will not be tested again [41] and [44]. Fig.6 indicates the CO2
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adsorption isotherms of the 8 selected samples. It is visible that, under a low pressure, the
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adsorption isotherm of each coal sample （0
