Elsevier

Applied Energy

Volume 110, October 2013, Pages 73-81
Applied Energy

Effects of gaseous and super-critical carbon dioxide saturation on the mechanical properties of bituminous coal from the Southern Sydney Basin

https://doi.org/10.1016/j.apenergy.2013.03.069Get rights and content

Highlights

  • We explore the effect of CO2 saturation on mechanical behavior of bituminous coal.

  • Properties considered: uniaxial compressive strength (UCS) and Young’s modulus (E).

  • Gaseous CO2 saturation was observed to reduce UCS by 53% and E by 36%.

  • Super-critical CO2 saturation was observed to reduce UCS by 79% and E by 74%.

  • CO2 saturation pressure and coal rank each influence mechanical effects observed.

Abstract

A study was initiated to investigate the effects of gaseous and super-critical carbon dioxide (CO2) adsorption on bituminous coal strength. Uniaxial compressive strength (UCS) experiments were conducted on bituminous coal samples from the southern Sydney Basin saturated with gaseous CO2, super-critical CO2 and N2 at various pressures, and a temperature 33 °C. According to the results, gaseous CO2 adsorption causes the UCS and Young’s modulus of the bituminous coal to be reduced by up to 53% and 36%, respectively. Super-critical CO2 adsorption causes more significant modifications to the mechanical properties of the bituminous coal, resulting in 40% greater UCS strength reduction and 100% greater Young’s modulus reduction compared to gaseous CO2 adsorption. The greater influence of super-critical CO2 on the UCS of the bituminous coal is thought to be related to the greater adsorptive potential and coal swelling produced for super-critical CO2. The more significant influence of super-critical CO2 on the Young’s modulus of the bituminous coal is thought to relate to the greater dissolution (and thus coal plasticization) potential of the super-critical CO2. N2 saturation was not observed to have any significant effect on the mechanical properties of the bituminous coal. Acoustic emission data collected during testing support of the notion that the coal mass natural cleat system largely contributes to the susceptibility of coal to mechanical weakening by CO2 adsorption. The results show that the mechanical influence of CO2 adsorption on coal is highly dependent on the phase state of the CO2.

Introduction

Global warming is very likely due to the observed increase in anthropogenic greenhouse gas concentrations [1] Long-term storage of carbon dioxide (CO2) in deep unmineable coal seams is one of the more economically viable of the various methods proposed for CO2 sequestration, as it can be coupled with enhanced coal-bed methane (CH4) production. In coal-seam CO2 sequestration the CO2 is predominantly stored as a relatively stable adsorbed phase, making it a relatively secure CO2 sequestration alternative [2]. However, the adsorption of CO2 onto coal can cause significant reductions in coal seam permeability and strength, which may act to undermine the feasibility of the sequestration process [3]. Although many studies have been conducted on the subject of CO2 adsorption-induced permeability reduction, only minor consideration has been given to the effects of strength reduction. However, in terms of long-term safety, the strength reduction of coal with CO2 adsorption is a great threat, as it may trigger large-scale fracturing under in situ stresses that in turn could allow leakage of the sequestered CO2 into the atmosphere. The process of sorption of CO2 in coal is complex, as CO2 not only adsorbs onto the coal surface, but also dissolves into the coal matrix, which can cause re-arrangement of the coal matrix structure with the release of polycyclic-aromatic-hydrocarbons [4], [5]. Larsen [6] states that the extent of this re-arrangement is dependent on the system temperature and thus that temperature will control the degree of swelling observed with adsorption. The re-arrangement of coal mass structure due to the physico-chemical and thermo-dynamic reactions initiated during CO2 adsorption in the coal mass has been studied by many researchers [7], [8], [9].

The relationship between coal mass strength and the chemical potential of an adsorbate can be expressed using Gibbs’ [10] and Griffith’s [11] theories, which are given in Eqs. (1), (2), respectively,dγ=-(Γidμi)σ=2γEπawhere γ is the surface energy per unit crack length, Γi and dμi are the surface concentration and change in the chemical potential of the ith adsorbate component, respectively, σ is the tensile stress at an existing crack tip required to form a new crack surface, E is the Young’s modulus of the material and a is the crack half length. According to Eq. (1), replacement of the existing adsorbate with one with greater chemical potential causes the surface energy of the rock mass to be reduced. According to Eq. (2), this reduction causes the tensile stress required to form a new crack to be reduced. This implies that material strength can be reduced by replacing the existing adsorbate with a more reactive one with greater chemical potential. The results of the experimental work of Rebinder [12] Rebinder and Venstrem [14] and Likhtman et al. [13] collectively support the CO2 adsorption induced weakening effect and summarized by Eqs. (1), (2).

During the sequestration process, injected CO2 moves along the coal mass natural cleat system and adsorbs onto the coal matrix along the cleat walls, replacing naturally existing CH4, as CO2 has a greater chemical potential. Since CO2 is more reactive than CH4, this process causes the overall strength of the coal seam to be reduced [12], [13]. According to Karacan [4] swelling chiefly occurs in the vitrinitic parts of the coal mass, as these have greater susceptibility to swelling with CO2 adsorption. Lama [15] notes that outburst in coal mining (the sudden and violent failure of the coal mass due to the release of a large amount of gas) is a dangerous manifestation of this adsorption-induced strength reduction.

A number of studies have explored the influence of CO2 adsorption on coal strength. Ettinger and Lamba [16] observed a considerable strength reduction in 6.5–7 mm coal particles due to CO2 saturation. Their results are consistent with the CO2 saturation-induced strength reduction observed by Czaplinski and Holda [17], Hiramatsu et al. [18], Jackson [19] and Holda [20] in their experimental studies. Aziz and Ming-Li [21] showed an increase in CO2 adsorption-induced strength reduction with increasing CO2 pressure. Viete and Ranjith [22] conducted uniaxial tests to examine the effect of CO2 saturation on coal mass strength, using 54 mm diameter by 104 mm long lignite coal samples that had been saturated with air and CO2. The saturations were performed at a gas saturation pressure of 1 MPa, for 3 days. According to their test results, the adsorption of CO2 in lignite caused a reduction in the uniaxial compressive strength (UCS) and Young’s modulus of the coal by around 13% and 26%, respectively. UCS testing carried out by Perera et al. [23] on lignite samples saturated with CO2 at a pressure of 3 MPa, yielded maximum reductions of lignite UCS and Young’s modulus, due to CO2 adsorption, of around 10% and 16%, respectively.

All of the above-described studies considered only the effect of gaseous CO2 adsorption on coal mass strength. However, unmineable coal seams with potential for CO2 sequestration generally occur at great depths, where pressure and temperature may exceed the critical values of CO2 (7.38 MPa and 31.8 °C). Therefore, CO2 sequestration in deep, unmineable coal seams may involve interactions between coal and CO2 in its super-critical state [24]. According to existing findings, super-critical CO2 has quite different properties compared to gaseous CO2 [25], [26], [27], [28]. For example, Huang et al. [27] demonstrated a marked increase in CO2 viscosity with increasing pressure across the CO2 critical point. Zhou et al. [26] observed a significantly enhanced propensity for CO2 adsorption on activated carbon with the phase transition from gaseous to super-critical CO2, which they related to significant changes in the physical properties of CO2 across the critical point. According to Perera et al. [25], [28], drastic changes to this adsorptive potential of CO2 across the gaseous to super-critical phase transition cause significant changes to CO2 permeability in coal.

These permeability modifications are related to increased adsorption induced swelling causing cleat closure and reduced pore space, possibly with a component of the reduction related to the gas-slippage effect and Klinkenberg flow with the decreased compressibility of super-critical CO2 [28]. According to Qu et al. [29], super-critical CO2 has gas-like compressibilities and liquid-like densities. When the results of recent studies carried out on super-critical CO2 behavior and its influence on coal properties are considered, it becomes clear that a completely different coal strength reduction may be expected for super-critical CO2 compared to that expected for gaseous CO2 saturation. Thus, the main objective of this study is to investigate how both gaseous and super-critical CO2 saturation affect the mechanical properties of bituminous coal. The study involved an experimental program of UCS testing on bituminous coal samples saturated with CO2 at various gaseous and super-critical pressures, samples saturated with inert N2 gas at various pressures, and unsaturated samples.

Section snippets

Coal properties

Bituminous coal samples used for the study were obtained from the Bulli coal seam of the Southern Sydney Basin at a depth of around 400 m within the Appin Coal Mine. The physical properties of the bituminous coal are shown in Table 1. Saghafi et al. [30] also used coal from same location and depth. Therefore, the coal physical properties given in Saghafi et al. [30] for the 400 m deep Bulli coal are applicable to this study. The bituminous coal, in the form of large blocks, was transported from

Results and discussion

A total of 18 specimens were tested. For each, the axial stress and strain and AE response were recorded during the entire testing duration. Two unsaturated (‘normal’) coal samples were tested and used as control specimens for the tests carried out under different saturated conditions. Table 3 shows the UCS and Young’s modulus values obtained from all the tested samples. As can be seen from Table 3, the difference in the values obtained for the various mechanical properties from the initial and

Conclusions

The influence of CO2 adsorption on the UCS and Young’s modulus of bituminous coal was examined using a UCS testing program. The results of tests carried out on unsaturated bituminous coal samples from the Southern Sydney Basin were compared with the results of tests performed on coal saturated with N2 and gaseous and super-critical CO2, at various saturation pressures.

Gaseous CO2 adsorption was observed to cause the bituminous coal UCS and Young’s modulus to be reduced by up to 53% and 36%,

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