Binary gas sorption/desorption experiments on a bituminous coal: Simultaneous measurements on sorption kinetics, volumetric strain and acoustic emission

Abstract

There is still no clear understanding of the specific interactions between coal and gas molecules. In this context sorption–desorption studies of methane and carbon dioxide, both in a single gas environment and gas mixtures, are of fundamental interest. This paper presents the results of unique simultaneous measurements of sorption kinetics, volumetric strain and acoustic emission (AE) on three tetragonal coal samples subjected to sorption of carbon dioxide and methane mixtures. The coal was a high volatile bituminous C coal taken from the Budryk mine in the Upper Silesia Basin, Poland. Three different gas mixtures were used in the sorption tests, with dominant CO₂, with dominant CH₄ and a 50/50 mixture.

The experimental set-up was designed specially for this study. It consisted of three individual units working together: (i) a unit for gas sorption experiments using a volumetric method, (ii) an AE apparatus for detecting, recording and analysing AE, and (iii) a strain meter for measuring strains induced in the coal sample by gas sorption/desorption. All measurements were computer aided.

The experiments indicated that the coal tested showed preferential sorption of CH₄ at 2.6 MPa pressure and exhibited comparable affinities for CH₄ and CO₂ at higher pressures (4.0 MPa). The results of chromatographic analysis of the gas released on desorption suggested that the desorption of methane from the coal was favoured. The relationship between the volumetric strain and the amount of sorbed gas was found to be non-linear. These results were contrary to common opinions on the coal behaviour. Furthermore, it appeared that the swelling/shrinkage of coal was clearly influenced by the network of fractures. Besides, the AE and strain characteristics suggested common sources of sorption induced AE and strain.

The present results may have implications for the sequestration of carbon dioxide in coal seams and enhanced coalbed methane recovery (ECBM).

Introduction

Environmental merits and potential economic advantages are responsible for growing world wide interest in CO₂ sequestration in deep, unmineable coal seams with simultaneous recovery of methane. A comprehensive review on CO₂ sequestration/ECMB recovery by White et al. (2005) brought out many important questions that remain unanswered and showed how much work is still to be done. For numerical modelling of CO₂ sequestration/ECBM recovery clear understanding of specific interactions between coal and gas molecules is essential. Especially, better understanding of CO₂/CH₄ sorption/desorption and induced swelling/shrinkage of coal are of basic importance.

So far, there has been a limited number of experimental studies on sorption of gas mixtures by coal (e.g. Greaves et al., 1993, Ceglarska-Stefańska and Zarębska, 2001, Ceglarska-Stefańska and Zarębska, 2002a, Ceglarska-Stefańska and Zarębska, 2002b, Ceglarska-Stefańska and Zarębska, 2005, Ceglarska Stefańska et al., 2002, Fitzgerald et al., 2005, Harpalani, 2005, Reeves, et al., 2003, Sereshki, 2005, Sudibanriyo et al., 2003, Busch et al., 2006, Majewska and Ziętek, 2008, Zarębska and Ceglarska-Stefańska, 2008).

The applicability of acoustic emission techniques to the study of coal-gas system has been confirmed in a number of studies (Majewska et al., 1994a, Majewska et al., 1994b, Majewska et al., 1996, Majewska et al., 1997, Majewska et al., 1998, Majewska and Mortimer, 1998, Majewska and Mortimer, 2000, Majewska and Mortimer, 2001, Majewska and Mortimer, 2006, Majewska and Ziętek, 1999, Majewska and Ziętek, 2007a, Majewska and Ziętek, 2007b, Majewska and Ziętek, 2008). AE techniques provide good insights into the sorption characteristics of different coals with different sorbats. Acoustic emission phenomena (AE), including the physics background and examples of applications of AE techniques have already been briefly described (Majewska and Ziętek 2008) — see the Appendix A for the salient points.

This study is an extension of our recent work on acoustic emission and swelling/shrinkage induced in coal by displacement sorption/desorption of CO₂ and CH₄ (Majewska and Ziętek 2008). The important observations were: first, AE kinetics and sample swelling/shrinkage bring out the importance of pre-sorption and the sorbate used; second, it matters whether the coal is first exposed to CH₄ or to CO₂; third, injection of CO₂ into coal previously exposed to CH₄ produces considerable swelling of the coal; and fourth, on desorption after CH₄/CO₂ exchange sorption, initial shrinkage is followed by swelling of the coal. It was concluded that future investigations should comprise simultaneous measurements on the kinetics of sorption, on AE and volumetric strain kinetics for coal samples subjected to displacement sorption of CO₂/CH₄ mixture. This could provide direct competition between two kinds of molecules for the available sorption sites. The idea of such combined measurements is in every respect new and requires the use an experimental set-up specially designed for this study.

Combined sorption, strain and AE studies should give better insight into the complex interactions of the coal matrix and gas molecules.