Full Length Article A comparative evaluation of coal specific surface area by CO 2 and N 2 adsorption and its influence on CH 4 adsorption capacity at different pore sizes 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 China b Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development & Utilization, Beijing 100083, PR China c Department of Energy and Mineral Engineering, The EMS Energy Institute, G3 Center, Pennsylvania State University, University Park, PA 16802, USA highlights  Pore structures of three coal sample series were evaluated.  Coal specific surface areas by CO 2 and N 2 adsorption were compared.  Specific surface area was correlated with CH 4 adsorption capacity.  Theoretical support was provided for CBM refinement efficient development. article info Article history: Received 31 March 2016 Received in revised form 11 May 2016 Accepted 15 June 2016 Available online 30 June 2016 Keywords: Coal rank Macrolithotype Seam Adsorption pore Gas adsorption capacity Coal reservoir abstract Coal has a heterogenous porosity that influences its specific surface area (SSA) and CH 4 adsorption and desorption. However, the pore size distribution obtained with N 2 adsorption is only reliable at pore sizes >2 nm omitting the important contribution of micropore (<2 nm). Here, 13 coal samples from three series were measured by both the N 2 at 77 K and CO 2 at 273 K, respectively, to compared the adsorption pore structure characteristics of different coal ranks, seams, and macrolithotypes, which further revealed the influences of mesopore (2–50 nm) and micropore on CH 4 adsorption capacity at different pore sizes. The larger 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 exists between the Dubinin-Radushkevich (DR) SSA and CH 4 adsorption capacity (Langmuir volume). With the increase of the coal rank, the CH 4 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 vitrinite content as well as the DR SSA and CH 4 adsorption capacity decreases; for the three main coal seams in the Hancheng mine area, the No. 11 coal has the largest DR SSA and CH 4 adsorption capacity followed by the No. 3 coal and No. 5 coal. With CO 2 adsorption, it is more significant than N 2 adsorption to accurately characterize the microscopic structure of coal and understand the gas adsorption mechanism. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Coalbed methane (CBM) has become the focus of exploration and development in the United States, Canada, Australia, and China [1–5]. However, improved theoretical rationalization is still necessary for CBM industry development [6]. Compared with the conventional sandstone and carbonatite reservoirs, the pore size of coal reservoir is much smaller (nm) [7,8], and the CBM is naturally adsorbed on the pore surface of the coal matrix [9]. The International Union of Pure and Applied Chemistry (IUPAC) divided pores into micropore (<2 nm in size), mesopore (2–50 nm), and macropore (>50 nm) [10], which is most widely applied [11]. The micro- and mesoporosity are the primary while flow properties and beneficially influenced by the macroposity. Usually, the meso- and micropore are regarded as absorption pore which mainly controls gas adsorption and desorption while the macropore is primarily considered as seepage pore [12–14]. Therefore, the micro- scopic structure characteristics of coal adsorption are relevant [15]. http://dx.doi.org/10.1016/j.fuel.2016.06.076 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, PR China. E-mail address: xuhao600@163.com (H. Xu). Fuel 183 (2016) 420–431 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel