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Selective adsorption and selective transport diffusion of CO-CH binary mixture in coal ultramicropores 2
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Yongliang Zhao, Yanhui Feng, and Xinxin Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01294 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016
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Environmental Science & Technology
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Selective adsorption and selective transport diffusion
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of CO2-CH4 binary mixture in coal ultramicropores
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Yongliang Zhao,a,b Yanhui Feng, a,c,* and Xinxin Zhang a,c a
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School of Energy and Environmental Engineering, University of Science and Technology
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Beijing, Beijing 100083, China b
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State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University,
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Xi’an 710049, China c
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Beijing key laboratory of Energy Saving and Emission Reduction for Metallurgical Industry, University of Science and Technology Beijing, Beijing 100083, China
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*
Corresponding author
E-mail:
[email protected] (Yanhui Feng) Tel: +86-010-62334971 Fax: +86-010-62329145
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ABSTRACT
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The adsorption and diffusion of the CO2-CH4 mixture in coal and the underlying mechanisms
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significantly affect the design and operation of any CO2-enhanced coal-bed methane recovery
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(CO2-ECBM) project. In this study, bituminous coal was fabricated based on the Wiser
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molecular model and its ultra-microporous parameters were evaluated; molecular simulations
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were established through Grand Canonical Monte Carlo (GCMC) and Molecular Dynamic (MD)
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methods to study the effects of temperature, pressure, and species bulk mole fraction on the
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adsorption isotherms, adsorption selectivity, three distinct diffusion coefficients, and diffusivity
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selectivity of the binary mixture in the coal ultramicropores. It turns out that the absolute
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adsorption amount of each species in the mixture decreases as temperature increasing, but
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increases as its own bulk mole fraction increasing. The self-, corrected, and transport diffusion
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coefficients of pure CO2 and pure CH4 all increase as temperature or/and their own bulk mole
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fractions increasing. Compared to CH4, the adsorption and diffusion of CO2 are preferential in
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the coal ultramicropores. Adsorption selectivity and diffusivity selectivity were simultaneously
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employed to reveal that the optimal injection depth for CO2-ECBM is 800-1000 m at 308-323 K
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temperature and 8.0-10.0 MPa.
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Environmental Science & Technology
Table of Contents
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KEYWORDS:
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Binary mixture; Adsorption selectivity; Diffusivity selectivity; Transport diffusion coefficient;
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CO2-ECBM.
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1. Introduction
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Anthropogenic greenhouse gas (GHG) emissions and their impact on the global climate
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represent one of the defining environmental issues facing modern society. Of particular concern
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is the emission of CO2, which is the largest contributor to climate change effects.1-5 CO2-
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enhanced coal-bed methane recovery (CO2-ECBM) project, which pumps CO2 into deep,
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unmineable coal seams, represents an effective and direct approach to reducing atmospheric CO2
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concentration.6-8 CO2-ECBM is attractive because it can not only store CO2 that has been
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captured but also can recover CH4 as a fuel or a technical gas, which makes CO2 storage an
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interesting economic prospect.9, 10 Prior to any large-scale implementation of this type of project,
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it is necessary to comprehensively establish both the adsorption and diffusion mechanisms of the
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CO2-CH4 mixture in coal.
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Competitive adsorption equilibria in the CO2-CH4 mixture are of key importance for CO2-
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ECBM, so considerable efforts have been made in recent years to determine their adsorption
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properties.11, 12 There are many available methods for measuring the adsorption isotherms of pure
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CO2 and pure CH4 including the volumetric method,13,
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gravimetric method.17 Based on the single component, the literature provides an estimate of the
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maximum capacity for CO2 storage and CH4 recovery; of course, the adsorptions of CO2 and
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CH4 actually happen simultaneously on the coal matrix, i.e., there is adsorption of a CO2-CH4
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mixture. Studies on multi-component adsorption have grown increasingly popular. The
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adsorption of the binary mixture can be measured by a gravimetric-chromatographic method18-20
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or using a high pressure scanning calorimeter.21 According to International Union of Pure and
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Applied Chemistry (IUPAC), the coal pores were classified into four groups: macropores (pore
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size d>50 nm), mesopores (2-50 nm), micropores (d
