Fuel 183 (2016) 420–431

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Fuel

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Full Length Article

A comparative evaluation of coal specific surface area by CO2 and N2

adsorption and its influence on CH4 adsorption capacity at different poresizes

http://dx.doi.org/10.1016/j.fuel.2016.06.0760016-2361/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: School of Energy Resources, China University ofGeosciences (Beijing), Beijing 100083, PR China.

E-mail address: xuhao600@163.com (H. Xu).

Junlong Zhao a,b, Hao Xu a,b,⇑, Dazhen Tang a,b, Jonathan P. Mathews c, Song Li a,b, Shu Tao a,b

a School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, PR ChinabCoal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development & Utilization, Beijing 100083, PR ChinacDepartment of Energy and Mineral Engineering, The EMS Energy Institute, G3 Center, Pennsylvania State University, University Park, PA 16802, USA

h i g h l i g h t s

� Pore structures of three coal sample series were evaluated.� Coal specific surface areas by CO2 and N2 adsorption were compared.� Specific surface area was correlated with CH4 adsorption capacity.� Theoretical support was provided for CBM refinement efficient development.

a r t i c l e i n f o

Article history:Received 31 March 2016Received in revised form 11 May 2016Accepted 15 June 2016Available online 30 June 2016

Keywords:Coal rankMacrolithotypeSeamAdsorption poreGas adsorption capacityCoal reservoir

a b s t r a c t

Coal has a heterogenous porosity that influences its specific surface area (SSA) and CH4 adsorption anddesorption. However, the pore size distribution obtained with N2 adsorption is only reliable at pore sizes>2 nm omitting the important contribution of micropore (<2 nm). Here, 13 coal samples from three serieswere measured by both the N2 at 77 K and CO2 at 273 K, respectively, to compared the adsorption porestructure characteristics of different coal ranks, seams, and macrolithotypes, which further revealed theinfluences of mesopore (2–50 nm) and micropore on CH4 adsorption capacity at different pore sizes. Thelarger micropore total pore volume (TPV) contributes to the larger micropore SSA. As micropores are com-mon and contribute extensively to most of the SSA (>99%) in these coals, a much better relationship existsbetween the Dubinin-Radushkevich (DR) SSA and CH4 adsorption capacity (Langmuir volume). With theincrease of the coal rank, the CH4 adsorption capacity increases continuously and the DR SSA shows a ten-dency of first decreasing then increasing; at the same coal rank, from the bright to dull coal, the vitrinitecontent as well as the DR SSA and CH4 adsorption capacity decreases; for the three main coal seams inthe Hancheng mine area, the No. 11 coal has the largest DR SSA and CH4 adsorption capacity followedby the No. 3 coal andNo. 5 coal.With CO2 adsorption, it ismore significant than N2 adsorption to accuratelycharacterize the microscopic structure of coal and understand the gas adsorption mechanism.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Coalbedmethane (CBM) has become the focus of exploration anddevelopment in the United States, Canada, Australia, and China[1–5]. However, improved theoretical rationalization is stillnecessary for CBM industry development [6]. Compared with theconventional sandstone and carbonatite reservoirs, the pore sizeof coal reservoir ismuch smaller (nm) [7,8], and the CBM is naturally

adsorbed on the pore surface of the coal matrix [9]. TheInternational Union of Pure and Applied Chemistry (IUPAC) dividedpores into micropore (<2 nm in size), mesopore (2–50 nm), andmacropore (>50 nm) [10], which is most widely applied [11]. Themicro- and mesoporosity are the primary while flow propertiesand beneficially influenced by the macroposity. Usually, themeso- andmicropore are regarded as absorption porewhichmainlycontrols gas adsorption and desorption while the macropore isprimarily considered as seepage pore [12–14]. Therefore, themicro-scopic structure characteristics of coal adsorption are relevant [15].

J. Zhao et al. / Fuel 183 (2016) 420–431 421

The specific surface area (SSA) of a solid (e.g. coal and activatedcarbon) is a property which affects its reactivity as a sorbent and ismeasured commonly [16–18]. The larger the SSA, the more surfacesites are available, resulting in a higher sorption capacity of thesolid [19]. The common direct method for the quantitative deter-mination of SSA in coal is the N2 adsorption at 77 K [9,20]. Theanalysis methods are Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models [21,22]. However, subjecting to theprobing gas molecular kinetic diameter (0.36 nm) and instrumentprecision constraints, the micropore at the size of <2 nm is difficultto be captured completely [23–25]. Therefore, advanced technolo-gies, including low field nuclear magnetic resonance (LFNMR),X-ray computed tomography (X-ray CT), small angle X-ray scatter-ing (SAXS), and small angle neutron scattering (SANS), are alsointroduced to characterize the pore structure heterogeneity[14,26–31].

While many of these cover a wide range of pore sizes, only SAXSand SANS have coverage in this lower range [32]. Unfortunately,these are not commonly available and thus the measurement ofthe smaller pore sizes requires both CO2 and N2 sorption studies.Since the CO2 molecules are more prone to enter small pores dueto a smaller kinetic diameter (0.33 nm) under a higher temperature(273 K) compared with N2 [33–36], the CO2 adsorption has beenused in the activated carbons [37,38], shale gas reservoir [39,40],minerals [19], and silicon carbide-derived carbon SSA evaluation[41]. However, in the CBM industry, although some scholars havealso determined the coal SSA with CO2 adsorption [42–44], mostof them were for the model fitting analysis or simulation. Thus,in this work, both the N2 adsorption at 77 K and CO2 adsorptionat 273 K were conducted and the major objectives are to: (1)determine the SSA with CO2 and N2 adsorption for different coalrank, seam and macrolithotype samples; (2) compare the differentinfluences of SSA on the CH4 adsorption capacity at different poresizes; (3) investigate the difference of CO2 and N2 adsorptionmechanisms; and (4) provide theoretical support for efficientCBM development.

2. Experiments and methods

The fresh bulk coal samples (�15 � 15 � 15 cm3) were obtainedfrom three active mine areas (Jincheng, Hancheng, and Binchang)in China. A total of eight coal mines including Duanshi, Houcun,Sangshuping, Xiangshan, Nangou, Shengjie, Dafosi, and Tingnanwere sampled (Fig. 1). The coals from Jincheng mine area in theQinshui Basin are high-rank, those from Hancheng and Binchangmine areas in the Ordos Basin are middle-rank and low-rank,respectively (Fig. 2). Here, the samples were separated into threeseries (rank, seam, and macrolithotype). Since the coal rank is akey factor determining the gas adsorption capacity [15,24], the coalrank series collected from different areas were first used to discussthe influence of coal rank on the adsorption pore structures; to thesame rank, the Hancheng mine area where the target coal seamsfor CBM development are the Nos. 3, 5, and 11 [4], was furthertaken as an example to investigate the adsorption pore structurecharacteristics in different coal seams; finally, in consideration ofthe heterogeneity in cleats and pore size distribution within coalmacrolithotypes in the same coal seam [45], the coal macrolitho-type series were designed to fully reveal the difference of gasadsorption capacity and pore size distributions. Table 1 showsbasic information of the coal sample collection used for the gasadsorption.

To determine the coal adsorption pore SSA, both N2 adsorptionat 77 K and CO2 adsorption at 273 K were performed with aMicromeritics ASAP 2020 specific surface analyzer [46]. Here, thecoal marcolithotype was identified basing on the overall relative

luster and percentage of bright components (vitrain and clarain,bright (>80%), semi-bright (50–80%), semi-dull (20–50%), and dull(<20%)) according to Zhao et al. and O’Keefe et al. [45,47]. All thesamples were crushed, sieved into a 60–80 mesh (0.18–0.25 mm)cuts, and dried for 48 h at 60 �C. The mesopore Brunauer-Emmett-Teller (BET) and micropore Dubinin-Radushkevich, (DR)SSA as well as the mesopore Barrett-Joyner-Halenda (BJH) andmicropore Dubinin-Astakhov (DA) total pore volume (TPV) weredetermined directly [21,22,48,49]. The SSA calculation models(BET and DR models) are expressed as follows:

BET model :1

W p0p � 1

� � ¼ 1WmC

þ ðC � 1ÞWmC

� pp0

ð1Þ

where W is the weight of gas adsorbed at pressure p; Wm is theweight of gas adsorbed as monolayer; C is a BET constant; p/p0 isthe relative pressure of adsorbate and here, p0 is the N2 saturatedvapor pressure at 77 K.

DR model :VV0

¼ exp � RTbE0

lnp0

P

� �2" #

ð2Þ

where V is the adsorbed volume at relative pressure p0/p, and here,p0 is the CO2 saturated vapor pressure at 273 K; V0 is the maximumgas adsorbed volume; E0 is the adsorption energy can referencefluid; b is the similarity factor; R is the gas constant; T is theequilibrium temperature of gas adsorption.

To compare the influences of BET SSA and DR SSA on the CH4

capacity, the CH4 isothermal adsorption experiments for all thesamples were conducted. The Langmuir volume and Langmuirpressure were obtained [50]. The Langmuir adsorption modelwas given as:

V ¼ pVL

pþ pLð3Þ

where p is the equilibrium gas pressure; V is the volume of gasadsorbed; VL is the Langmuir volume representing the maximumvolume that can be adsorbed at infinite pressure; and pL is theLangmuir pressure at which the adsorbed volume is half theLangmuir volume.

Samples from the blocks were prepared by crushing and sievingto a size of 60–80 mesh, and since moisture is known to reduce theCH4 capacity of coal, samples of 100–125 g were moisturesaturated at 97% humidity for at least four days to obtain in siturelevant values [51]. The experimental temperature and equilib-rium pressure were 30 �C and up to 10 MPa, respectively [6,52].The random vitrinite reflectance measurements and maceralanalyses (500 points) were performed on the same polishedsection of the coal samples using a Leitz MPV-3 photometermicroscope, according to ISO 7404.3-1994 [53] and ISO 7404.5-1994 [54], respectively. A 5E-MAC III infrared fast coal analyzerwas used to determine the moisture, volatile matter, ash yield,and fixed carbon. Tests were performed in accordance withstandard procedures [55–57].

3. Results and discussion

3.1. Proximate analysis, maceral, and random vitrinite reflectance

Table 2 shows the results of proximate analysis (by mass, airdry basis), maceral (by volume, dry but mineral matter includedbasis), and random vitrinite reflectance (Ro, ran, %) for different coalsamples. Of all the samples, the moisture content ranges from 0.14to 11.46%, ash yield is between 7.50 and 29.79%, volatile matter is6.74–24.33%, and fixed carbon range is from 41.45 to 81.86%.Meanwhile, the vitrinite accounts for between 25.4 and 83.7%

Fig. 1. Positions of the Qinshui Basin and Ordos Basin and the distribution of the sampling points.

Fig. 2. Stratigraphic columns of coal-bearing strata and distribution of sampling seams.

422 J. Zhao et al. / Fuel 183 (2016) 420–431

Table 1Basic information of coal sample collection used for the gas adsorption.

Samplenumber

Coalrank

Series Coalseam

Minedistrict

Coal mine

DFS Low Coal rank 4 Binchang DafosiTN Low 4 TingnanNG Middle 11 Hancheng NangouSJ Middle 3 LiaoyuanDS High 3 Jingcheng DuanshiHC High 3 Houcun

XS-B Middle Coal marco-lithotype

3 Hancheng XiangshanXS-SB Middle 3XS-SD Middle 3XS-D Middle 3

SSP-3 Middle Coal seam 3 Hancheng SangshupingSSP-5 Middle 5SSP-11 Middle 11

J. Zhao et al. / Fuel 183 (2016) 420–431 423

while inertinite ranges from 12.6 to 69.1% and that almost noliptinite is present. The proportion of mineral matter (visible)ranges from 1.9 to 34.8%. The random vitrinite reflectance isbetween 0.54 and 3.07%.

3.2. Measurement of SSA with different gases

3.2.1. Measurement of mesopore SSA with N2 adsorptionTable 3 shows that the BET SSA of 13 samples is between 0.19

and 35.01 m2/g, the BJH TPV is 0.001–0.029 mL/g, and the averagepore diameter (APD) is from 1.9 to 10.7 nm by the N2 adsorptionexperiments (samples are dry but mineral matter containingbasis). However, obvious difference exists among three series sam-ples. In the coal rank series, the values of BET SSA and BJH TPV oflow-rank samples are significantly higher than those of middle-and high-rank samples. For the macrolithotype samples at thesame coal rank, from the bright to dull coal, the BET SSA and BJHTPV firstly increase and then decrease whereas their peak valuesare different due to some micropores were filled by minerals inthe semi-dull and dull coals. In addition, among the Nos. 3, 5,and 11 coal seams in the Hancheng mine area, the BET SSA andBJH TPV of No. 11 coal are the highest while those of No. 5 arethe lowest. Overall, except the high rank series, both the middlerank and low rank series show a positive correlation relationshipbetween BJH TPV and BET SSA (Fig. 3).

According to the porous material adsorption/ condensationtheory [58,59], the adsorption/desorption curves could be classifiedinto three types (A, B and C) (Fig. 4). Type A is represented by HC,

Table 2Proximate analysis, maceral, and random vitrinite reflectance for different coal samples.

Sample number Series Ro,ran (%) Proximate analysisa

Mad Aad

DFS Coal rank 0.63 7.81 29.79TN 0.54 11.46 10.13NG 1.80 0.75 10.53SJ 1.79 0.95 7.50DS 3.07 0.14 11.03HC 2.95 0.40 12.87

XS-B Coal marco-lithotype 1.72 0.94 12.03XS-SB 1.72 0.96 15.25XS-SD 1.72 0.84 8.73XS-D 1.72 0.76 12.38

SSP-3 Coal seam 1.69 0.92 13.21SSP-5 1.74 0.97 10.37SSP-11 1.70 1.03 23.21

a Mad = Moisture content (wt%, air dry basis), Aad = Ash yield (wt%, air dry basis), Vad =b The value was computed by volume (vol%), and the samples were dry while the mi

NG, SSP-3, and XS-B, where the adsorption/desorption curves arenearly straight at the start and then increase sharply at the relativepressure approaching to 0.9. An unconspicuous hysteresis loopappears on the curves indicating the pore morphology is dead-end (half-open) with poor connectivity, such as the slit-like andplate-like. The adsorption/desorption curves of type B representedby DS, SJ, XS-SB, XS-SD, and SSP-5 increase slowly with a smallhysteresis loop and similarly to the type A, curves become sharpat the relative pressure of about 0.9 and represent completely-open pores with good connectivity (such as the cylinder-like andplate-like which are open at both ends). Type C is represented byTN, DFS, XS-D, and SSP-11 and the adsorption/desorption curvesincrease steady and an obvious hysteresis loop could be seenwhich corresponds to ink-bottle pores. Among the three types ofadsorption/desorption curves, type A is advantageous to gasdesorption and diffusion in comparison to adsorption, type C isthe opposite and type B is the most favorable for gas adsorption,desorption and diffusion. In addition, the hysteresis loops of thesemi-dull and dull coals don’t coincide at a low relative pressurewhich reveals the average pore size of the adsorbent molecule isof a similar size as that of the adsorptive molecule [60], and onthe basis of the Kelvin equation, the size of the ink-bottle poresis about 3 nm at the relative pressure of 0.5.

3.2.2. Measurement of micropore SSA with CO2 adsorptionIn the CBM industry, the micropore is an important index to

evaluate the CBM adsorption capacity [13], the N2 adsorption ismainly used for the mesopore measurement instead of micropore[61], where the CO2 adsorption could overcome the drawback ofN2 adsorption [38]. As the CO2 adsorption mechanism on the coalsurface is the monolayer adsorption or micropore filling at thescale of <2 nm [48,49], the adsorption and desorption curves arethe same. Table 4 summarizes the micropore SSA and other struc-ture parameters measured by CO2 adsorption. On the basis of DAand DR models, the CO2 DR SSA and DA TPV could be computedand the ranges are 57.4–198.4 m2/g and 0.022–0.096 cm3/g(samples are dry but mineral matter containing basis), respec-tively. Also, Fig. 5 indicates that the larger the DA TPV is, the largerthe DR SSA is, which is consistent with the experimental results ofAmarasekera et al. [62].

Comparing the CO2 adsorption curves (Fig. 6), although theshapes of adsorption isotherms are a slight different at the lowrelative pressure, these shapes are all consistently convex. In thecoal rank series, the gas adsorption capacity of the high-rank sam-ples is the largest followed by the low- and middle-rank samples.In the coal macrolithotype series, from the bright to dull coal, the

(%) Coal compositionb (%)

FCad Vad Vitrinite Inertinite Exinite Mineral

41.45 21.05 25.4 69.1 2.2 3.354.08 24.33 39.3 45.0 5.2 10.571.82 16.90 69.6 20.4 1.0 9.076.65 14.90 83.7 13.7 0.7 1.981.86 6.97 80.7 13.5 0.0 5.879.99 6.74 73.9 21.1 0.0 5.0

67.28 19.75 78.6 12.6 1.2 7.672.19 11.60 69.4 18.3 1.3 11.079.08 11.35 63.8 21.7 0.4 14.176.36 10.50 39.5 25.4 0.3 34.8

65.37 20.50 58.7 27.8 0.7 12.870.21 18.45 57.7 28.5 0.6 13.261.46 14.30 58.7 25.1 0.9 15.3

Volatile matter (wt%, air dry basis), FCad = Fixed carbon (wt%, air dry basis).neral matter was included.

Table 3The SSA and other pore structure parameters by N2 adsorption for different coal samples.a

Sample number Series BET SSA (m2/g) BJH TPV (mL/g) Pore size fraction (%) BJH APD (nm)

<10 (nm) 10–100 (nm) >100 (nm)

DFS Coal rank 0.16 0.019 52.8 35.3 11.9 6.1TN 0.35 0.029 81.9 14.7 3.4 3.9NG 0.04 0.001 87.1 5.6 7.3 2.1SJ 0.30 0.001 83.3 16.7 0.0 2.0DS 0.53 0.002 37.4 57.7 4.9 3.8HC 0.11 0.005 54.8 42.2 3.0 1.9

XS-B Coal marco-lithotype 0.33 0.001 42.5 50.6 6.9 8.2XS-SB 0.66 0.001 46.2 52.5 1.3 9.5XS-SD 0.33 0.006 35.4 48.7 15.9 10.7XS-D 0.28 0.001 49.5 35.6 14.9 6.0

SSP-3 Coal seam 0.25 0.001 40.3 53.6 6.1 8.5SSP-5 0.23 0.001 40.3 54.6 5.1 5.2SSP-11 0.93 0.005 60.8 28.3 10.9 1.9

a Samples were dry but measured at the mineral matter containing basis. SSA = specific surface area; TPV = total pore volume; APD = average pore diameter.

Fig. 3. Relationship of BET total pore volume and BET specific surface area.

424 J. Zhao et al. / Fuel 183 (2016) 420–431

gas adsorption capacity decreases gradually due to the decrease ofthe micropore and vitrinite content and in the coal seam series, theNo. 11 has the largest gas adsorption capacity while No. 5 coalseam has the lowest gas adsorption capacity.

Fig. 7 shows themicropore size distribution, fromwhich the porestructure of 0.4–1.0 nm in size could be characterized by the CO2

adsorption accurately and most of the samples have three peaks(two prominent peaks at the beginning and the end and one sub-sidiary peak in the middle) and the positions of these peaks are at0.52–0.63 nm, 0.77–0.80 nm, 0.86–0.91 nm, respectivily (e.g. DFS,SJ, and XS-B). However, the samples of HC, DS, XS-SB, SSP-3, andSSP-11 shows a four-peak structure and the difference is that thehigh rank samples have the lower the prominent peak positionsrevealing them tighter CBM reservoirs. Meanwhile, the SSP-5 hastwo peaks which just correspond the middle and end peaksrespectively in the three-peak structure. Of all the samples, thetwo prominent peaks at the beginning and the end are higher thanthe subsidiary peak in themiddle-rank and low-rank samples whilethat in the high-rank samples shows an opposite situation.Corresponding to the adsorption curves, the high-rank samples,bright coal samples, and No. 11 coal seam samples have the largerDR SSA and DA TPV than other samples which represent moredeveloped micropore.

3.3. Influence of SSA on CH4 adsorption capacity

3.3.1. Isotherms of the CH4 adsorptionUsually, Langmuir volume is used to determine the CBM

adsorption capacity by coal reservoirs, and the larger Langmuir

volumes have the larger CBM adsorption potential [63]. The CH4

adsorption isotherms indicate that the high-rank samples havethe larger Langmuir volume that is 6–8 times larger than thoseof low-rank. From the bright to dull coal, the Langmuir volumedecreases gradually which is in agreement with Chalmers andBustin [64]. In addition, in the coal seam series, the No. 11 coalhas the largest Langmuir volume (22.6 m3/t), the No. 3 coal comesthe second (21.1 m3/t) and the No. 5 has the smallest value(20.1 m3/t), which demonstrates the No. 11 coal is potentially themost favorable to be developed (Table 5 and Fig. 8).

3.3.2. Relationship between SSA and CH4 adsorption capacityThe adsorption pore SSA is the principal characteristic for the

assessment of gas adsorption capacity of coal due to the CBMstorage mode [9], which was defined as

Sad ¼ SBET þ SDR ð4Þwhere Sad is the adsorption pore SSA, m2/g; SBET is the mesoporeSSA, m2/g; SDR is the micropore SSA, m2/g.

The ash yield and vitrinite content in the maceral analysis havegreat effects on the adsorption pore SSA [65]. For these samples,Fig. 9 shows that the relationship between the ash yield and theadsorption pore SSA is poor whereas a negative correlation existsbetween the mineral content and the adsorption pore SSA consis-tent with Yee et al. [66]. Moreover, with the increase of the coalrank, the adsorption pore SSA show a tendency of ‘‘U” shape (firstdecreasing then increasing, the lowest value at the bituminous coalstage) in agreement with the previous researches [67–71]. The rea-son of this phenomenon is that during the coal coalification, a largequantity of micropores are generated and with the increase of thecoal burial depth, the geothermal temperature and pressure areincreased while the coal permeability tends to be lower (eventhe pore is compressed into elongated shape) due to the blockingof the pore space by bitumen at the catagenetic stage accompaniedwith complex physical and chemical changes generally[29,67,68,72,73]. When the coal evolves to the anthracite coalstage, most of the macropores would convert into microporesbecause of the enhancement of the orientational alignment[2,74]. In addition, the bright coal samples have the greatest gasadsorption capacity due to the highest vitrinite content (vitrinitestend to be rich in microporosity) and lowest mineral mattercontent which is in agreement with Clarkson and Bustin [9] andFaiz et al. [65].

Based on the theory of solid surface physical adsorption, thesolid material adsorption capacity is commonly proportional tothe SSA [72]. However, to the coal, the Langmuir volume has an

Fig. 4. Adsorption and desorption curves of N2 adsorption for different series samples.

Table 4The SSA and other pore structure parameters by CO2 adsorption for different coal samples.a

Sample number Series DR SSA (m2/g) DA TPV (mL/g) Pore diameter of the peak (nm)

DFS Coal rank 148.2 0.067 0.54/0.79/0.86TN 136.7 0.063 0.52/0.79/0.86NG 105.0 0.052 0.58/0.80/0.89SJ 133.9 0.053 0.58/0.77/0.86DS 198.4 0.096 0.45/0.52/0.61/0.87HC 171.8 0.086 0.54/0.66/0.73/0.86

XS-B Coal marco-lithotype 121.3 0.051 0.59/0.80/0.87XS-SB 104.2 0.022 0.75/0.82/0.91XS-SD 98.4 0.040 0.61/0.77/0.87XS-D 93.6 0.038 0.63/0.77/0.86

SSP-3 Coal seam 94.9 0.038 0.56/0.72/0.79/0.86SSP-5 77.4 0.024 0.77/0.86SSP-11 96.3 0.039 0.56/0.72/0.79/0.87

a Samples were dry but measured at the mineral matter containing basis. SSA = specific surface area; TPV = total pore volume.

Fig. 5. Relationship of DA total pore volume and DR specific surface area.

J. Zhao et al. / Fuel 183 (2016) 420–431 425

obvious ‘‘U” shape relationship with the DR SSA instead of the BETSSA (Fig. 10), which demonstrates that the micropore has a largereffect on the gas adsorption than mesopore as expected. Specifi-cally, in the coal rank series, with the increase of the coal rank,the DR SSA first decreases then increases, and to the low rank coal,since the vitrinite content is less than that of the inertinite whichhas no relationship with the Langmuir volume [68,69] and otherrank coals (Table 2), it has a low CH4 adsorption capacity; in themacrolithotype series, from the bright to dull coal, the CH4

adsorption capacity decreases gradually. Here, since the bright coalhas more micropores and higher vitrinite content than dull coal,the CH4 adsorption capacity of the bright coal is larger. Meanwhile,different coal seams also have different gas adsorption capacity(Fig. 11), which is due to the different properties such as the coalrank and composition [75].

Fig. 6. Adsorption curves of CO2 adsorption for different series samples.

Fig. 7. Pore size distribution curves of CO2 adsorption for different series samples.

426 J. Zhao et al. / Fuel 183 (2016) 420–431

Table 5Langmuir volume and pressure by CH4 isothermal adsorption for different coalsamples.a

Samplenumber

Series Langmuir volume(m3/t)

Langmuir pressure(MPa)

DFS Coal rank 13.2 3.4TN 7.4 4.9NG 20.0 0.8SJ 23.4 1.3DS 40.5 2.6HC 38.6 2.8

XS-B Coal marco-lithotype

24.8 0.9XS-SB 22.9 1.3XS-SD 21.1 1.4XS-D 20.1 1.0

SSP-3 Coal seam 21.1 1.4SSP-5 20.1 1.8SSP-11 22.6 1.5

a Samples were measured at the mineral matter containing basis and moisturesaturated at 97% humidity for at least four days to obtain in situ relevant values.

J. Zhao et al. / Fuel 183 (2016) 420–431 427

To further quantify the SSA contribution of the micropore andmesopore to the absorption pore, the adsorption pore SSAcontribution rate was further defined as

g ¼ SBET=DRSad

� 100% ð5Þ

where g is the adsorption pore SSA contribution rate, %; SBET/DR isthe BET or DR SSA, m2/g.

With Eq. (5), Tables 3 and 4, the SSA contribution rate ofmicropore to the adsorption pore, accounting for more than 99%,could be computed (Table 6), indicating that for CH4 adsorption bythe coal, the micropore is dominant and its gas adsorption abilitydepends on the pore size and frequency ofmicro- andmesoporosity.

Fig. 8. Adsorption isotherms fo

Thus, the actual analysis not only can’t ignore the contribution ofmicropore, but also should be given more attention.

3.4. Comparison of the CO2 and N2 gas adsorption mechanisms

From Table 6, the CO2 DR SSA is far greater than the N2 BET SSA.For example, the DR and BET SSA for the DFS sample are 148.2 m2/gand 0.16 m2/g, respectively, indicating that the contribution to theadsorption pore SSA from the micropore is significant and the val-ues of CO2 DR SSA agree with the published data on micropores[45,62,76,77]. This demonstrates that most of the pores of coalare closed to N2 at 77 K but accessible to CO2 at 273 K which alsocould be confirmed by Amarasekera et al. [62]. The explanationfor this phenomenon is that the scale and precision of the twomethods for SSA determination are different (the much largerspecific interactions for CO2 than for N2 are the major source ofthe differences in the coal SSA measured with the two gases)[78], which reflects the CO2 and N2 gas adsorption mechanismsare obviously different.

For the N2 adsorption, the BET theory was established on themultilayer physisorption and mainly used to measure the meso-pore instead of the micropore SSA as the relative pressure rangecalculated by the BET model is between 0.05 and 0.3 [1,21,45],which would be easily influenced by the test conditions and oftenan abnormal phenomenon of the adsorption volume increase withthe decrease of the pressure is found [45,46]. Since theinteractional potentials overlap each other in the micropore walls,the physical adsorption is more intense in the micropore than thatin the mesopore and outside surface [42,79,80], resulting that it israther difficult to measure the micropore SSA accurately at thescale of <2 nm with N2 adsorption and for most coals, it is usuallyknown to yield low values of SSA by N2 adsorption [16,67,81].Additionally, the namopores (0.5–1 nm) mainly are the sphericaland elliptical metamorphic pores or the intermolecular pores

r different series samples.

Fig. 10. Relationship between Langmuir volume and specific surface area for all the samples.

Fig. 9. Relationship of ash yield (air dry basis), mineral and vitrinite contents (dry but mineral matter containing basis), coal rank, and adsorption pore SSA (dry but mineralmatter containing basis).

428 J. Zhao et al. / Fuel 183 (2016) 420–431

[29,82] which results the micropore filling (the relative pressure is<0.01) accompanied with the monolayer adsorption instead of themultilayer adsorption are the adsorption modes. And the specificinteraction would occur between the N2 molecules and the mostof the surface functional groups as well as the exposed ions[83,84], thus only at the certain relative pressure of 10�7–10�5,the N2 micropore filling would happen and the speed of diffusionand adsorption are slow which may limit the ability of N2 at 77 Kto enter the micropores in coal [85]. Meanwhile, the pore mayshrink at the low temperatures (e.g. 77 K used for N2 adsorption)[86].

However, using CO2 as the adsorbent which has smaller molec-ular kinetics diameter and the DR theory basing on the microporefilling, it is easily to obtain the low relative pressure for microporefilling without the vacuum degree of molecular turbine pumpwhen the test temperature is at 273 K (the saturated vapor pres-sure is about 3.48 MPa) [46] although its high saturation pressurerestricts the upper limit of the relative-pressure range which can

be examined due to the higher temperature than N2 adsorption[62], and also the CO2 could induce the pore swell at the 273 K[87]. In addition, it is also suggested that the pores in coal areisolated from each other and that the efficiency of CO2 as an adsor-bate is the result of its ability to diffuse rapidly through the solidcoal by a solubilization mechanism, thereby reaching all the pores,including those regarded as not having surface openings [88]. Thus,the CO2 adsorption could not only avoid the drawback that the N2

molecular can’t be adsorbed completely when the size of themicropore approaches the N2 molecular diameter, but also quicklyreach the adsorption equilibrium without diffusional limitation[40]. And the CO2 adsorption isotherms could yield accurate totalsurface areas for coals, since almost all of their surface area isdue to pores in the microporous region [42,86,88]. This is whythe CO2 adsorption could measure the pore contribution to theSSA in the range 0.33–1.5 nm reasonably, which has also been ver-ified in slit-shaped carbon characterization by Vishnyakov et al.[38].

Fig. 11. Relationship between Langmuir volume and SSA for different series samples.

Table 6The CH4 isothermal adsorption for different coal samples.a

Sample number Series BET SSA (m2/g) DR SSA (m2/g) SSA contribution rate of microporeto adsorption pore (%)

DFS Coal rank 0.16 148.2 99.9TN 0.35 136.7 99.7NG 0.04 105.0 100.0SJ 0.30 133.9 99.8DS 0.53 198.4 99.7HC 0.11 171.8 99.9

XS-B Coal marco-lithotype 0.33 121.3 99.7XS-SB 0.66 104.2 99.4XS-SD 0.33 98.4 99.7XS-D 0.28 93.6 99.7

SSP-3 Coal seam 0.25 94.9 99.7SSP-5 0.23 77.4 99.7SSP -11 0.93 96.3 99.0

a Samples were dry but measured at the mineral matter containing basis. SSA = specific surface area.

J. Zhao et al. / Fuel 183 (2016) 420–431 429

430 J. Zhao et al. / Fuel 183 (2016) 420–431

4. Conclusions

(1) By the N2 and CO2 adsorption, the coal adsorption porestructure could be characterize completely, where the lattercould be used to determine the micropore SSA accurately atthe size of 0.4–1.0 nm. A positive correlation exists betweenthe coal micropore SSA and TPV. The more the mineral con-tent is, the lower the gas adsorption capacity is.

(2) Since the micropore has a much larger contribution rate(>99%) to the adsorption pore SSA than that of the mesopore,a much better relationship exists between the microporeSSA and CH4 adsorption capacity (Langmuir volume).

(3) With the increase of the coal rank, the CH4 adsorption capac-ity increases continuously and the DR SSA shows a tendencyof first decreasing then increasing; at the same coal rank,from the bright to dull coal, the vitrinite content decreasesas well as the micropore SSA and CH4 adsorption capacity;in the coal seams series, the No. 11 coal has the largest DRSSA and CH4 adsorption capacity followed by the No. 3 coalseam and No. 5 coal seam orderly.

Acknowledgments

This work was financially supported by the Key Project of theNational Science & Technology (Grant No. 2016ZX05042-002),the National Natural Science Foundation Project (Grant Nos.41272175, 41530314), and the Fundamental Research Funds forthe Central Universities (Grant 2652015331). The authors aregrateful to anonymous reviewers and the editor for their carefulreviews and detailed comments that helped to substantiallyimprove the manuscript.

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