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Abstract

Expansion/contraction of coal, induced by the sorption of carbon dioxide and methane in isothermal and non-isothermal conditions, was measured. The investigation is of great importance in the context of validating the potential CO₂ sequestration in unmined coal seams. Changes in temperature in underground coal beds can influence the sorption balance, resulting in strains in coal strata, which could lead to the desorption of gas and leaks to the ground surface. The research shows that the strains induced by CO₂ sorption are about twice the size of those resulting from the sorption of CH₄. The linear strains are anisotropic and greater in the direction perpendicular to the bedding plane. The results of the non-isothermal experiments show that a temperature increase gives rise to the sample swelling in the presence of methane, but a different pattern is observed for coal–CO₂ systems, where sample contraction occurs. This behaviour is explained by the different mechanism of CH₄ and CO₂ deposition and by the diversity in the maceral composition of the samples.

Keywords

Coal carbon dioxide methane sorption swelling

Introduction

Global warming is the increase in the temperature of the atmosphere, which leads to changes in global climate patterns and is extremely likely to be caused by the development of industrial production. These changes can have a significant impact on the environment, including a rise in sea levels, the retreat of glaciers and sea ice decline and extreme weather events, such as floods, hot waves, storms and droughts. Lashof and Ahuja (1990) stated that 57–72% of the enhanced greenhouse effect on global warming is attributed to CO₂ emissions from anthropogenic sources. In order to meet the emission targets stated in the Kyoto Protocol (1992), certain solutions may be applied to reduce greenhouse gas emissions and stabilize their concentration in the atmosphere. One of them is CO₂ capture and storage, which includes capturing waste CO₂, then transporting and depositing it by geologic sequestration in deep saline aquifers, oil and gas reservoirs or unmineable coal seams. Coal beds are particularly interesting because they store natural, so-called coal bed methane (CBM). CH₄ can be displaced by CO₂, which is recovered and used to balance a portion of the cost of the sequestration process. Laboratory and field experiments have shown that coal sorbs CO₂ with a higher affinity in comparison with the sorption of CH₄. Coal can sorb approximately twice as much CO₂ by volume than CH₄, meaning that this sequestration method offers potential (Abunowara et al., 2014; Kudasik et al., 2017; Li and Fang, 2014; Marecka, 2007; Masoudian, 2016; Perera et al., 2011; Shi and Durucan, 2005).

Sorption of coal bed gases, methane and carbon dioxide, in the coal matrix changes the mechanical properties of coal. One of the effects is the swelling of coal. In situ, it results in stress and strain variations in a coal bed under overburden pressure (Skoczylas and Wierzbicki, 2014; Wierzbicki, 2017). Studies show that sorption-induced swelling of hard coal is an anisotropic process (Ceglarska-Stefańska and Czapliński, 1993; Ceglarska-Stefanska and Zarebska, 2002; Cui et al., 2007; Czerw, 2011; Espinoza et al., 2013; Karacan, 2007; Perera et al., 2011; Robertson and Christiansen, 2005; St. George and Barakat, 2001; Yang et al., 2011). This behaviour is a consequence of the stress conditions under which the coal was formed, and it can be researched via swelling experiments on coal samples subjected to CO₂ adsorption/desorption cycles using the strain meter technique. The swelling process affects the strength of coal and causes changes in the gas flow mechanism and rate (Karacan, 2007; Perera et al., 2011). It influences speed at which coal can sorb gases and impacts the permeability of the gas reservoir.

The aim of this study is to show the expansion/contraction behaviour of hard coal, subjected to the sorption of carbon dioxide and methane, in the non-isothermal conditions. In order to preserve the original porous structure of the coal, the investigation was performed on cuboid-shaped coal samples. Since the temperature variations affect the sorption equilibrium, they also result in changes in volumetric strains. This investigation is crucial because, in situ, temperature fluctuations could occur, causing changes in strain conditions in coal, which may lead to stress in the rock layers, followed by uncontrolled gas leaks (Baran et al., 2014; Viete and Ranjith, 2007).

Experimental section

Test material

The experiment was performed on two different hard coal samples from a Polish colliery, KWK Pniówek. The mine is located in the sedimentary cyclothems of coal in the Upper Silesian Coal Basin. This cyclothem forms a mudstone series consisting of sedimentary clastic (sandstone and mudstone) and clay (siltstone and coal shale) rocks.

Selected results of the proximate and ultimate analysis and petrography of coal samples P1 and P2 from seams 360/1 and 404/1, respectively, are summarized in Table 1.

Table 1.

Characteristic of the coal material.

Sample	C^daf[%]	S^daf[%]	H^daf[%]	N^daf[%]	O^daf[%]	W^a[%]	A^a[%]	V^daf[%]	Vitrinite[%]	Liptinite[%]	Inertinite[%]
P1	84.24	0.39	4.58	1.52	4.58	1.75	3.01	27.12	73	7	20
P2	84.96	0.58	4.60	1.70	3.76	0.68	3.78	25.50	53	8	39

The elemental analysis was performed in the Central Mining Institute in Katowice. The moisture content was determined in accordance with the procedure set forth in the standard PN-80/G-04511, while ash content was established in accordance with PN-80/G-04512. The oxygen content was computed as the remainder of 100%, taking into account the moisture and ash content.

Test facilities

Specially designed equipment was used for the experiment, enabling both sorption and dylatometric data to be collected simultaneously (Figure 1). The apparatus can be used for measurements of two samples at the same time. Sorption capacity was measured by the manometric method. The device was placed in a water thermostat in order to maintain a constant temperature with an accuracy of 0.1 K. Gas of a known volume and pressure was decompressed and flowed from the reference cell to the selected sample cell, which contains the coal sample. With the knowledge of the dead volume of the apparatus and the volume of the dosing unit, the amount of sorbed gas was calculated on the basis of gas laws. For both carbon dioxide and methane, the deviations in the ideal gas properties were considerable and the difference between ideal and real gas was resolved using the state equations for CO₂ and CH₄. The volume of the dosing gas unit was computed via a measurement of the nitrogen outflow rate. The dead volume of the sample cell and the reference cell was levelled with glass balls and derived using helium. Coal expansion/contraction behaviour was measured with a strain meter engineered at the Strata Mechanics Research Institute of the Polish Academy of Sciences. Linear deformations were picked up by an electric resistance-type strain gauge, combined with a resistance transducer. A more detailed description of the test facilities and apparatus was included in earlier work (Baran et al., 2015).

Figure 1.

Configuration of the measuring apparatus (Baran et al., 2015).

Test procedure

Cuboid-shaped samples, sized 18 × 18 × 40 mm and weighing about 20 g, were cut from the primary coal fragments. On the walls of the samples, resistance-type strain gauges were attached to the coal surface, then arranged longwise and transverse to the bedding plane. Following this, the samples were placed inside the test ampule and connected to the strain meter, after which each sample and sample cell were degassed. Next, the sample cell’s dead volume was determined using helium and the whole system was degassed in order to reach the static vacuum of 10⁻² Pa. In this situation, the state of the sample was most closely related to the state of as-received or the state between dry and as-received, because the temperature of 298 K would not allow for dry sample to be obtained despite a relatively high vacuum. At the beginning of the procedure, all valves were locked and the strain meter was adjusted to zero.

Firstly, methane was admitted to the reference cell and, when the pressure stabilized, the gas was introduced to the sample cell. In the same moment, recording of the experimental data, concerning the pressure changes and the linear strains of the coal, started. During the isothermal step of the experiment, the data on the kinetics of sorption and the kinetics of volumetric changes at the temperature of 298 K were collected for 50 h. The second phase of the experiment included monitoring of the gas pressure changes and linear strains in the system, while the temperature was raised from 298 to 323 K. The rate at which the temperature was increased was 1 K per 5 min. The highest temperature of 323 K was reached after 3 h and for the next 10 h the monitored processes were isothermal. After these stages, the system was degassed and measurements with carbon dioxide were performed according to the same procedure.

The kinetics of volumetric strains (ε_V) was determined according to the equation

ε_{V} = ε_{⊥} + 2 ε_{II}

where ε_⊥ refers to the sorption-induced linear strains in the perpendicular direction, and ε_II refers to strains in the parallel direction.

Results and discussion

As predicted, the results for both coal samples showed a higher sorption capacity for CO₂ when compared to CH₄ (Figure 2). The adsorption process favours the gas with an adequate adsorption affinity and kinetic diameter of the molecule. The physico-chemical properties of carbon dioxide, in the case of a smaller kinetic diameter (CO₂ 0.33 nm vs. CH₄ 0.38 nm) and higher adsorption energy, enable CO₂ to diffuse more easily compared to CH₄ (Zarębska and Ceglarska-Stefańska, 2008). Other aspects of the sorption process that are in favour of CO₂ are the electric properties (the quadrupole moment) and the presence of functional groups in coal. The reactive oxygen groups in the coal polymer structure can restrict access for the tetrahedral molecules of CH₄ to the pore system, while the reaction between CO₂ molecules and the oxygen groups can have a different nature (Gensterblum et al., 2014). Indeed, the coal microporous matrix can sorb larger amounts of carbon dioxide than methane. Another reason for the preferential sorption of carbon dioxide is the difference between the boiling temperatures for CO₂ and CH₄. The boiling temperature of carbon dioxide is higher (194.5 K) than that of methane (111.4 K). Parts of the pore system of coal are inaccessible for the methane molecules because of the necessity of spending substantial energy to expand the pore walls in order to penetrate them. However, with time, the sorption space becomes saturated, leading to vibrations in the coal copolymer network, while the presence of the elastic phase allows CH₄ molecules to penetrate some of those areas. It is possible that methane molecules cause stress relaxation within molecular elements of coal, as well as creeping.

Figure 2.

Kinetics of CO₂ and CH₄ sorption and volumetric strains at 298 K for samples (a) P1 and (b) P2.

When analysing the sorption processes of the hard coal, it is important to take into consideration the complexity of coal behaviour as a specific natural sorbent. At first, gas molecules are adsorbed on the coal surface relatively fast. Later, the adsorbed molecules diffuse into the coal’s internal structure, blurring the distinction between adsorption and absorption. The co-occurrence of these two processes can be jointly referred to as sorption. After a considerable period of time in which dissolved substances are exposed to vapours, coal tends to behave like a deformed glassy polymer (Larsen, 2004) and its structure rearranges. The process is typically slow, because this transformation involves the large-scale motion of molecules. Considering that, coal should not be treated as a rigid solid, as this could lead to the incorrect interpretation of measurement data.

Figure 3 shows that the results of sorption-induced linear expansion at 298 K for samples P1 and P2 are quite similar. As expected, coal tends to expand more in the direction perpendicular to the bedding plane. This is consistent with results reported by other authors (Czerw, 2011; Majewska et al., 2009; Zarębska and Ceglarska-Stefańska, 2008). This anisotropy is more noticeable in the case of sample P1 and stronger for CH₄-induced swelling. The samples expand at a faster rate and twice as much when exposed to carbon dioxide, compared with methane, even though the initial pressure of the dosed gases involved the same or comparable values (P1 12.15 bar, P2 12.56 bar).

Figure 3.

Kinetics of linear strains in coal–CO₂ and coal–CH₄ systems at 298 K for samples (a) P1 and (b) P2.

For both samples, the shapes of kinetic plots presented in Figure 2 for sorption and dilatometric (volumetric) processes are similar. This supports the idea that the deformations in the coal under investigation are attributable to gas deposition.

Mining operations can cause temperature changes of 10° or more. This situation can lead to a disturbance of the equilibrium in the coal–gas system followed by gas pressure change; thus, the amount of sorbed gas will change as well (Baran et al., 2013; Qu et al., 2014; Wang et al., 2015).

Measurements at 298 K became the starting point for further investigation concerning pressure and strain variations occurring during the temperature increase in the coal–gas system. The results are summarized in Figure 4, revealing the gas pressure increase by: 2.1 bar for CO₂ and 1.5 bar for CH₄ for experiments on sample P1; 1.44 bar for CO₂ and 2.28 bar for CH₄ in the case of sample P2.

Figure 4.

Kinetics of the development of linear strains during the temperature increase step for samples (a) P1 and (b) P2.

In coal–methane systems, temperature changes give rise to almost linear swelling of the sample (Figure 4). This behaviour is rather unexpected, while it is a well-established fact that desorption processes give rise to shrinkage of coal. Temperature increases should lead to methane desorption and pressure increases, as well as causing the equilibrium point to shift towards the adsorption range. The amount of methane adsorbed at the final temperature of 323 K was slightly lower than at 298 K (under 5%), even though the dimensions of the coal sample increased. Higher vitrinite content has a positive relation with sorption capacity in relation to methane (Chalmers and Marc Bustin, 2007). For sample P1, a higher value of sorption capacity at 298 K, as well as after the temperature was raised to 323 K, was observed. This does not affect the samples’ volumetric change phenomenon and could be connected to the lack of methane absorption taking place (Baran et al., 2016; Milewska-Duda et al., 2000). The accumulated methane only seems to be present in the form of adsorbed gas and compressed gas in the coal surface area. Under this condition, a slow temperature-induced desorption and potential shrinkage behaviour may be counterbalanced by sorption processes induced by a pressure increase in free gas. This may indicate that the compression of free gas in adsorption areas of meso- and micropores is significant, which in turn allows for the expansion of the coal mass.

The temperature increase of 25 K caused a decrease in CO₂ sorption capacity from 20.7 dm³/kg to about 17.4 dm³/kg for sample P1, and from 11.4 dm³/kg to 9.5 dm³/kg for sample P2. In the case of methane, sorption capacity decreased from 15.4 to 14.6 dm³/kg for Pn1 and from 8.1 to 7.4 dm³/kg for Pn2. The amount of carbon dioxide desorbed from the samples under investigation was relatively small. For the analysed coal–CO₂ systems, the sorption capacity did not correspond directly to the swelling/shrinkage behaviour. In the case of sample P2, the volumetric changes at the beginning and at the end of the experiment (at 298 K and 323 K) remained at the level of about 7‰; but, the amount of sorbed gas was much lower than in case of the initial stage of the experiment for sample P1. Changes in the samples’ overall dimensions in coal–CO₂ systems induced by temperature increase followed a different pattern (Figure 4). For sample P1, the initial temperature change resulted in a slight swelling of coal. Then, after 25 min, a rapid contraction was observed followed by a rapid expansion and another long-term contraction stage. This rapid shrinkage occurred at a temperature close to the critical temperature of carbon dioxide. The observed changes can be explained by a changed mechanism of CO₂ deposition. CO₂ accumulated in pores undergoes a phase transition as a result of capillary condensation, leading to rapid desorption and coal shrinkage. This kind of behaviour was not observed for sample P2, which showed no significant temperature-induced change in its size. These differences can be attributed to the diversity of the samples’ petrographic composition. The amount of vitrinite group macerals was higher and the content of macerals in the inertinite group was adequately lower in the case of sample P1. Research conducted by Karacan and Mitchell (2003) revealed that: (1) at first, vitrinite swells during carbon dioxide accumulation but, after some time, shrinkage occurs; and (2) in the case of inertinite, porosity is shifted towards the mesopores, which makes the desorption process easier to occur. Accumulated carbon dioxide is likely to be present in both adsorbed and absorbed (dissolved) forms, as well as in the form of compressed gas on the coal surface. In view of the above, during the temperature increase, the initial amount of sorbate accumulated in the coal moved into the free gas phase, causing a pressure increase in the system. Desorption of adsorbed gas led to coal shrinkage and increased pressure. Furthermore, lowering the solubility of CO₂ in coal took place, thus reducing the part of the sorption capacity connected to absorption, while also raising the free gas pressure. For sample P1, the amount of the desorbed gas exceeded the extra adsorption capacity at the new equilibrium point, which resulted from increased gas pressure. In the case of sample P2, the temperature-induced desorption was lower than for sample P1. The assumption was that much less of the sorbed CO₂ would be dissolved in the coal matrix. The result only revealed a slight shrinkage in the coal, while a new sorption and dilatometric equilibrium was reached.

Summary and conclusion

In this study, the swelling of hard coal, induced by the sorption of carbon dioxide and methane under isothermal and non-isothermal conditions, was measured. This research is of great importance in validating the potential of CO₂ sequestration in deep unmined coal fields. The investigation shows that the coal strains attributed to CO₂ sorption are about twice the size as those induced by CH₄. The linear strain kinetics show anisotropy, as the expansion is greater in the direction perpendicular to the bedding plane than in the parallel direction. In the context of non-isothermal research data gathered to date, an observation was made that, contrary to coal–CH₄ systems, CO₂ sorption-induced volumetric changes for the samples under investigation took a different course. This variation can be attributed to their petrographic composition. Both samples originated from the same mine, but from two different seams: 360/1 for P1 and 404/1 for P2. While the chemical composition of the samples was similar, the petrographic composition was significantly different. For vitrinite-rich sample P1, CO₂ sorption started with minor swelling, followed by rapid shrinkage after 25 min, continued by gradual contraction and shrinking, due to dominant temperature-induced desorption. The shrinkage in the coal matrix can give rise to the expansion of existing cracks in the coal bed or cause new cracks. For sample P2, a slight shrinkage in the sample for both linear strains was observed. On the other hand, in the coal–CH₄ systems, an increase in the coal’s overall dimensions followed a temperature increase for both samples under investigation. This effect was unexpected and could be explained by the increase in free gas pressure in the pore system.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This study is supported as a part of the Select + Erasmus Mundus Joint Doctorate Program and the AGH University of Science and Technology,Statutory Research Project 11.11.210.374.

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Development of temperature-induced strains in coal–CH 4 and coal–CO 2 systems

Abstract

Keywords

Introduction

Experimental section

Test material

Test facilities

Test procedure

Results and discussion

Summary and conclusion

Footnotes

Declaration of Conflicting Interests

Funding

References