The ability of five kinetic equations to describe the sorption kinetics and expansion rate of solid coal samples was investigated. The conducted experiment included the measurements of sorption of methane and carbon dioxide on cuboidal hard coal samples at high pressure using volumetric method. Simultaneously the kinetics of coal sorption induced swelling was monitored. All of the approaches used to fit the experimental sorption and dilatometric data were literature-based equations. Two equations represent the traditional approach for sorption on powder and grain fractions based on the bidisperse gas transport model, and have been used to interpret and quantify the observed gas uptake rates in coal. The other three kinetic equations are the pseudo-second-order kinetic equation, Elovich equation, and stretched exponential (SE) equation. Four of the five equations are suitable to describe the kinetics of methane and carbon dioxide sorption in the porous structure of solid coal samples and the kinetics of coal expansion that accompany the sorption processes. The SE equation gave the best fit to the experimental data.
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 CO 2 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 CO 2 sorption are about twice the size of those resulting from the sorption of CH 4. 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-CO 2 systems, where sample contraction occurs. This behaviour is explained by the different mechanism of CH 4 and CO 2 deposition and by the diversity in the maceral composition of the samples.
Abstract. The aim of this study was to investigate the ability of kinetic equations to describe the sorption kinetics and expansion rate of solid coal samples. In order to address his issue the sorption kinetics of methane and carbon dioxide on bituminous coals were studied. At the same time, the changes occurring in the sample's overall dimensions, which accompanied sorption processes, were monitored. Experiments were carried out at high pressure by means of the volumetric method on a cubicoid solid samples. Several literature-based modeling approaches and equations are proposed to fit the kinetic curves of gas deposition, as well as the adequate kinetics of coal swelling. First equation represents the traditional approach to interpret experimental data in terms of fast and slow sorption process and consider the combination of two first-order rate functions. The other empirical kinetic equations are: the pseudo-second-order kinetic equation (PSOE), Elovich equation and the stretched exponential equation (SE). Two of the four equations are suitable to describe the kinetics of methane and carbon dioxide sorption and have been successfully used to quantify the observed dilatometric phenomena rates. The stretched exponential equation gave the best fit to the experimental data.
This paper presents experimental results of the sorption-dilatometric kinetics of methane and carbon dioxide on a sample of hard coal from one of the coal mines in the Upper Silesian Coal Basin. The experiment included isothermal and non-isothermal-isobaric stages. For the isothermal stage, it was found that, up to a certain point (about 8 dm3/kg), the sample’s expansion was linearly related to the amount of gas absorbed. Studies on dilatometric kinetics under constant pressure, with a reduction in temperature, indicate that a dominant share of the heat-expanding properties of carbonaceous material influences changes in the size of the sample in the coal-gas system. It was also found that the sample expansion, due to temperature change, was 2.25‰, for the sample in both the vacuum and the non-adsorbing gas atmosphere.
Low-pressure sorption tests were carried out on samples of selected Polish bituminous coal in coal–methane and coal–carbon dioxide systems. The purpose was to determine the relationship between the petrographic composition of low-rank coal and the amount of gas stored in its porous structure and desorbed from it. The influence of the degree of coalification on the amount of sorbed gas was reduced to minimum, because isotherms of deposition and evacuation of gases were determined on the base coal samples and two concentrates of lithotypes, vitrain and durain, isolated from the original coal. It was found that the sorption capacity of carbon dioxide was related to the pertographic composition, but no such correlation was observed in regard to methane. Langmuir and Dubinin–Radushkevich sorption isotherms and the modified desorption model based on Langmuir equation were chosen. The applied equations gave a very good fit to the experimental data. Calculated parameters corresponding to free adsorption energy in the Dubinin–Radushkevich equation allow concluding on the independence of its mechanism from coal petrography and on the preference of carbon dioxide sorption. Calculated adsorption equilibrium constants in Langmuir’s equation show variability with petrographic composition of coal and have lower values for methane than for carbon dioxide. It was shown that the size of the hysteresis loop depends on the petrographic composition of coal and increases with increase in vitrinite content for both sorbates, which was confirmed by values of areal hysteresis index and hysteresis parameter.
The aims of this work were (1) the determination of the efficiency of isolating by hand two lithotypes concentrates, vitrain and durain, from original coal sample of low rank bituminous coal, (2) the evaluation of the relation between macro-and microscopically distinguishable components of samples under investigation and their physicochemical properties and (3) the assessment of the effect of maceral composition of low rank coal on carbon dioxide sorption capacity and desorption process. To address the first issue a petrographic analysis was performed for separated components and the original raw sample. A significantly higher vitrinite content in vitrain concentrate and much lower in durain concentrate in regard to basic coal confirmed that the separation process was satisfactory. The second aspect comprised an investigation into selected physical and chemical properties of original coal and two lithotypes concentrates including helium density, methanol density, FTIR spectra, the amount of oxygen functional groups and swelling properties using pyridine as solvent. The results show that in the case of material under study the coal rank does not preponderate the effect of maceral composition. This third issue was addressed by performing adequate sorption experiments carried out at 298 K at low pressure on basic coal sample and two lithotypes concentrates. The tendency of adsorption capacity of CO 2 to depend on maceral composition was confirmed as it increases with the content of vitrinite. The Dubinin-Radushkevich equation shows a very good agreement with all the experimental data obtained for both lithotypes concentrates and basic coal under investigation.
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