Effects of Supercritical CO2 Treatment Temperature on Functional Groups and Pore Structure of Coals

Zhaolong Ge,Mengru Zeng,Yugang Cheng,Xianfeng Liu,Haoming Wang

doi:10.3390/su11247180

Zhaolong Ge, Mengru Zeng + Show 3 more

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https://doi.org/10.3390/su11247180

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Abstract

The buried depth of a coal seam determines the temperature at which CO2 and coal interact. To better understand CO2 sequestration, the pore structure and organic functional groups of coal treated with different ScCO2 temperatures were studied. In this study, three different rank coals were treated with ScCO2 at different temperatures under 8 MPa for 96 h in a geochemical reactor. The changes in pore structure and chemical structure of coal after ScCO2 treatment were analyzed using mercury intrusion porosimetry, attenuated total reflection Fourier transform infra-red spectroscopy, fractal theory, and curve fitting. The results show that the enhancement effect of ScCO2 on pore structure of coal becomes less significant as the increase of buried depth. In most of the treated coal samples, the variation proportion of mesopores decreased and the variation proportion of macropores increased. In the relatively higher rank coals, the degree of condensation (DOC) of aromatic rings decreased after treatment with ScCO2. The DOC values showed a U-shape relationship with temperature, and the aromaticity showed a downward trend with increasing temperature. The chemical structural changes in the relatively lower rank coal sample were complex. These findings will provide an understanding of mechanisms relevant to CO2 sequestration with enhanced coalbed methane recovery under different geothermal gradients and for different ranks of coal.

Highlights

Carbon dioxide capture and sequestration (CCS) is technically and economically feasible, and it will be possible to reduce CO2 greenhouse gas emissions [1,2]
The variation proportion in each pore size class after ScCO2 treatment was calculated by Equation (3)
mercury intrusion porosimetry (MIP) test, fractal theory, ATR-Fourier transform infra-red spectroscopy (FTIR) test, and curve fitting were conducted on three coal samples to understand the effects of supercritical carbon dioxide (ScCO2) treatment on the pore structure and functional groups in the coal at different temperatures

Summary

Introduction

Carbon dioxide capture and sequestration (CCS) is technically and economically feasible, and it will be possible to reduce CO2 greenhouse gas emissions [1,2]. Geological storage as part of CCS is regarded as the most effective method to store CO2 [3,4,5]. A large number of abandoned underground coal mines and deep unmineable coal seams exist worldwide because of resource depletion, low production capacity, and technical limitations. These mines are potential hosts for CO2 storage and filling material consisting of a mixture of fine-fraction waste [6,7,8]. Replacing the CH4 is conducive to coalbed methane extraction and allows the energy resource in the coal seams to be utilized [10,11]. It is estimated that at present, closed mines in China still contain about 42 × 109 tons of coal resources and nearly 500 × 109 cubic meters of unconventional gas [14]

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