Pore Structure Characterization of Different Rank Coals Using N2 and

May 2, 2017 - (14-17) The development of a pore-fracture system, in a manner, depends on the metamorphism degree of coal.(18-22) Wang et al.(18) find ...
0 downloads 10 Views 835KB Size
Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Pore structure characterization of different rank coals using N2 and CO2

2

adsorption and its effect on CH4 adsorption capacity: A case in Panguan syncline,

3

western Guizhou, China

4 5 6

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 &

7

Utilization, Beijing 100083, PR China;

8

* Corresponding Author-Email: [email protected]

9

Abstract: To determine the pore structure characteristic of different coal ranks in Panguan

10

syncline, both N2 adsorption-desorption (LP-N2GA) and CO2 adsorption (LP-CO2GA) were

11

carried out with the goal of revealing the differentiation evolution of total pore volume(TPV),

12

specific surface area (SSA), pore size distribution (PSD) and pore shape of 14 coal samples, and

13

the influences of SSA to adsorption capability at different sizes (super-microporous2.76nm) and sudden decrease phenomenon when the relative pressure is

192

0.5.Thus, it is visible that there are certain pores, whose required relative pressure for

193

concentration is higher than the required relative pressure for desorption evaporation, which

194

means that the pore type reflected by this curve is the interconnected pore with two ends open,

195

such as the parallel plate pore or cylinder pore with two ends open, which is helpful for the

196

adsorption, desorption and diffusion of CBM. Represented by the remnant samples, Type B has no

197

hysteresis loops or only small hysteresis loops, reflecting that the pore of this type has the same

198

relative pressures in adsorption concentration and desorption evaporation. This kind of curves

199

mainly consist of semi-open pore with poor connectivity, such as the wedge-shaped, cylindrical

200

and slit-shaped pores with one closed side, which has the weakest adsorption and accumulation

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

201

capability, but is helpful for the desorption and diffusion of CBM.

202

3.2.2. N2-TPV, N2-SSA and pore size distribution

203

The N2-SSA of 14 coal samples is 0.097-0.546 m2/g (mean 0.318 m2/g), the N2-TPV is

204

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

205

occupied percentages of the pore volumes at different pore size, the transition pore (10-100nm,

206

73.69-95.21%) is most developed, followed by the micropore (4.79-26.31%). The distribution

207

characteristics of the pore volume and N2-SSA at different pore size of 14 coal samples are shown

208

in Fig.4. It can be seen that the pore volume distribution of all test samples at different pore size

209

appears to be multimodal, and the peak value at 10-100nm is higher than the one at 2-10nm,

210

especially for pores with diameter 60-70nm, indicating that the pores of this section has the

211

biggest contributions to the pore volume. In addition,YLT-6 and YLT-8 is of poor development

212

of pores with diameter 3-8nm. For most of coal samples, the differentials of the pore specific

213

surface area to the pore diameter all present as the single-peak model, and the peak value appears

214

within 2-3nm, which means that the N2-SSA is mainly contributed by this part of pores. In other

215

words, within 2-100nm, the pore volume mainly comes from the contribution of 10-100nm pores,

216

and the more the pores in 2-3nm develop, the bigger the N2-SSA is.

217

In addition, Fig.5a indicates that a bigger N2-TPV will lead to a higher N2-SSA, and there is a

218

positive relationship between the N2-TPV and coal rank. Meanwhile, with the increasing coal rank,

219

the N2-SSA rises gradually. However, the increase rate of N2-SSA decrease gradually with the

220

increasing vitrinite reflectance, and the increase tendency presents to be a half-reversed “U” shape

221

in the whole (Fig.5b). When Ro,ran>1.4%, the variation amplitudes of the micropore proportion and

222

N2-SSA are extremely small. When Ro,ran is within 0. 8-1. 4%, the methane in the coal escapes in

ACS Paragon Plus Environment

Page 10 of 36

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

223

quantity, and then the hydrogenous side chain and key shorten and reduce greatly, so as to decline

224

the coal density. In addition, the coal moisture is also decreasing continuously under the pressure,

225

and the porosity is also minimizing, with the dramatic declining of the big pores as the primary.

226

Meanwhile, the micropore proportion is increasing constantly, so as to result in the fast growth of

227

N2-SSA. When Ro,ran reaches 1.4%, the humic gel basically completes the dehydration. Then the

228

moisture and porosity in the coal have both reduced to the minimum and the average pore size has

229

already been the smallest [18], [20] and [34]. When vitrinite reflectance further increases

230

(Ro,ran >1.4%), ketogenic metamorphism is dominant, the micropore content presents to have

231

smaller variation tendency.

232

3.3. Measurement of super-microporous with LP-CO2GA

233

The micropore is an important index to assess the accumulation and adsorption capacity of

234

CBM [47] and [48]. However, at present, the super-microporous in diameter less than 2nm has not

235

aroused enough attentions, while the characteristics of the super-microporous have vital influences

236

to the adsorption performance of the methane [19] and [49]. This study employs the same

237

instruments with the N2 adsorption experiment and conducts tests under 273.15K. As to the

238

super-microporous, the adsorption mechanism of the carbon dioxide on the coal surface is

239

mono-layer adsorption or microporous fill. So, the adsorption curve coincides with the desorption

240

curve, and the desorption curve will not be tested again [41] and [44]. Fig.6 indicates the CO2

241

adsorption isotherms of the 8 selected samples. It is visible that, under a low pressure, the

242

adsorption isotherm of each coal sample (0