Coal Methane System Research Articles

Properties of Coal of Moderate Metamorphic Development

The physicochemical properties of Kuznetsk Basin coal are studied. The following distinctive features are confirmed for coal of moderate metamorphic development: minimum strength and gas-dynamic potential; and maximum risk of cracking and explosion, as well as maximum instability of the coal–methane system. For bed samples of different rank, we find that the petrographic characteristics of coal of metamorphic stages III–V include critical regions.

The Spatial Distribution of CBM Systems under the Control of Structure and Sedimentation: The Gujiao Block as an Example

A multilayer coal bed methane system with a complex superimposed relationship developed vertically in the Gujiao block. Taking the cumulative thickness of mudstone between neighbouring coal seams and open faults as object of research, the key strata for the division of different CBM systems was defined. The connection of an open fault for the fluid pressure system and the spatial distribution of the CBM system were analysed. The results showed that the porosity of compact mudstone is extremely low. When the mudstone thickness of the roof and floor increases, the gas content increases. Taking the Lijiashe and Wangzhimao faults as the boundary lines, the fluid pressure gradient between the Shanxi and Taiyuan formations is less than 0.08 MPa /100 m in the northern fracture zone, such as the Zhenchengdi, the Xiqu, the Tunlan, and the northern Dongqu well fields. The vertical gas units of different strata may belong to the same CBM system. The structure is simple in the southern Lijiashe fault. The vertical gas units in the southern Lijiashe fault belong to different CBM systems. Combined with the minimum thickness of the mudstone layer (2 m) and coal seam (0.5 m) standards, the spatial distribution of CBM systems was analysed.

A Simple Method for Measuring Basic Parameters of the Coal—Methane System under Mining Conditions

The study is devoted to the methane hazard in hard coal mining. This hazard occurs in almost every coalfield in the world. Ensuring maximum work safety under methane hazard conditions is based, among other things, on reliable, fast and frequent determinations of methane content in a coal seam. The existing methods are time-consuming, and determinations must be performed in laboratories. Indirect methods such as desorbometric methods are burdened with high measurement uncertainties. The study presents a model of methane release from granular coal samples and a device (AMER) developed for measuring methane content in a coal seam under in situ conditions. Measurements are performed in a fully automatic way and preliminary results, based on the approximation of the Crank model, are available within several dozen minutes from the beginning of the measurement. Also, the use of the unipore diffusion equation and a proper software of the device allowed to determine the values of the effective diffusion coefficient. Results of measurements performed with the AMER desorbometer are highly consistent with the results of the measurements performed in the laboratory using a traditional method.

Methods and Models for Analyzing Methane Sorption Capacity of Coal Based on Its Physicochemical Characteristics

The authors study the influence of physicochemical parameters on methane adsorption capacity of coal and offer the analytical method for the methane adsorption capacity for three-phased condition of methane. It is found that in the depth interval to 300 m below surface, methane adsorption capacity measured in lab can exceed natural gas content of coal obtained from geological exploration data by 30%, and the change in the thermodynamic condition of coal–methane system brings irreversible physicochemical consequences in terms of the altered ratios of physical states of the main components. There is no linear connection between natural gas content of a coal bed and its methane adsorption capacity with respect to occurrence depth. The application of Big Data in treatment and interpretation of large data flows is described. The theoretical data predicted using the proposed method and the experimental data on methane content of Kuzbass coal agree.

A Simple Method for Measuring Basic Parameters of the Coal-Methane System under Mining Conditions

A Simple Method for Measuring Basic Parameters of the Coal-Methane System under Mining Conditions

Assessing the Sorption of Methane by Kuznetsk Basin Coking Coal

The physicochemical properties of Kuznetsk Basin coal are studied in the laboratory. The composition and specific surface of the coal samples are compared, as well as their adsorption of CO2 at different pressures and temperatures. In an atmosphere typical of a mineshaft, coal of moderate metamorphic development is characterized by the greatest sorptional potential in dynamic conditions. Current methods permit assessment of the sorptional properties of coking coal, regarded as a coal–methane system.

An Impact of Mechanical Stress in Coal Briquettes on Sorption of Carbon Dioxide

AbstractThe presence of gases (methane or carbon dioxide) in hard coal is connected with numerous threats for miners employed in underground mining facilities. When analyzing the coal-methane system, it is necessary to determine the relationship between pressure and gas sorption. Such a relationship should be determined under conditions similar to the natural ones – when it comes to both temperature and pressure. The present paper discusses the results of research conducted with the use of coal briquettes under the state of mechanical stress. Carbon dioxide sorption isotherms were determined for different values of stress affecting the coal material. For five coal samples collected in different mines of the Upper Silesian Coal Basin, Langmuir’s sorption isotherms were determined. The results point to significant impact that mechanical stress has upon the sorption process. It is about 1 percent of the value obtained for coal not subjected to stress per 1 MPa. The research results can also prove useful when analyzing hard coal seams from the perspective of their carbon dioxide sequestration abilities.

The Repeatability of Sorption Processes Occurring in the Coal-Methane System during Multiple Measurement Series

The aim of this paper was to investigate the repeatability of sorption processes occurring in the coal-methane system during multiple measurement series. For research purposes, three granular samples of coal sorbent were used. The samples were subjected to a series of three measurements, each time performed under the same conditions. During each series of measurements, the following stages took place: outgassing the sample in a vacuum, and then saturating it with methane under the pressure of 1, 3, and 10 bar. Sorption capacities and the values of effective diffusion coefficients were compared. The studies into the repeatability of measurements of sorption capacities, conducted by means of the IGA-001 (Intelligent Gravimetric Analyzer) gravimetric instrument for three measurement cycles, showed that the obtained measurement discrepancies in relation to the mean value did not exceed 1.23%. The discrepancies in question approximated the measurement uncertainty of the IGA-001 device. The evaluation of the recurrence of determining the values of effective diffusion coefficients showed that the obtained discrepancies were no larger than 10%, which might have resulted from problems with fulfilling most of the assumptions of the applied unipore diffusion model. The authors did not observe any changes of the sorption parameters of hard coal during consecutive cycles of methane saturation.

Results of comparative sorption studies of the coal-methane system carried out by means of an original volumetric device and a reference gravimetric instrument

The present paper discusses the idea of an innovative device for sorption measurements, working under isobaric conditions and based on a precise piston pressure regulator. As part of the research, the author compared the results of sorption measurements obtained with the constructed piston sorptomat with the results obtained by means of a reference device. The sorption capacities of 15 granular high-volatile bituminous coal samples, determined on the basis of methane sorption courses registered with both instruments were compared. The author also defined the criterion of compatibility of the obtained results, which stated that the compared results are compatible if their values fall within the range of measurement uncertainties of the instruments with which they had been established. After comparing the results obtained with the original instrument with the results obtained with the reference gravimetric device, it was observed that 30 out of 33 measurements of sorption capacities were classified as compatible (based on the defined measurement compatibility criterion). The mean difference in the readings provided by both instruments was 1.64%, for sorption capacity determined in 33 measurement points and the methane pressure range of 1–10 bar.

Some parameters of coal methane system that cause very slow release of methane from virgin coal beds (CBM)

In some worldwide hard coal basins recovery of methane from virgin coal beds is difficult. In general, mentioned difficulties are related to geo-mechanical, petrographical and physical-chemical properties of coals in question, occurring for example in the Bowen Basin (Australia) or the Upper Silesian Coal Basin (Poland). Among numerous properties and parameters, the following are very essential: susceptibility of coal beds to deformation connected with coal stress state change and contemporary shrinkage of the coal matrix during methane desorption. Those adverse geo-mechanical and physical-chemical effects are accompanied by essential change of the porous coal structure, which under these disadvantageous conditions is very complex. This study aims to show difficulties, which occur in phase of recognition of the methane-reach coal deposit. Volume absorbed methane (not surface adsorbed) in sub-micropores having minimal size comparable with gas molecule diameter must possess energy allowing separation of the nodes and methane release to micropores.

Fast evaluation of the coalbed methane content of coal viewed as an element leading to improvement in exploitation conditions

Abstract Methane, as a co-occurring compound, can be found in the majority of coal basins all over the world. The knowledge of methane content is a basic piece of information used in balancing and describing a given coal seam, in conducting safe exploitation – with methane hazards and methane and rock outburst hazards in mind – as well as in less common procedures, such as coal seam fracturing and methane extraction, or storing carbon dioxide in off-balance coal seams. The author of the present paper outlines a thoroughly modified desorbometric method used to analyze the coal-methane system. The research object, in underground conditions, is the full process of releasing methane from coal, which makes it possible to determine gas losses, the desorbable methane content, and the effective diffusion coefficient. This type of research can be conducted in less than 24 hours due to the reduction of the grain fraction of the analyzed bore dust. The thorough research in underground conditions was preceded by a model analysis of the simultaneous impact of the desorbable methane content in coal and the effective diffusion coefficient upon the value of the desorption indensity index. The influence of a grain fraction on the kinetics of methane release from coal was described, and the 0.20–0.25 [mm] grain fraction was selected. The choice of this particular fraction made it possible to register the full process of methane release from coal, in the assumed time. An original methodology of performing underground measurements was presented, together with an instrument used to perform these measurements. The underground research involved measuring the desorbable methane content according to the presented original methodology, which was done simultaneously with an analysis of the coalbed methane content carried out by means of the bore dust method, an analysis of desorption intensity, and an analysis of the technical parameters of coal. The results point to a substantial discrepancy between the desorbable methane content in coal and the coalbed methane content measured by the bore hole method in compliance with the Polish standard. The mean relative percentage error for 17 measurements was 6.5%. The author provided an example concerning using the effective diffusion coefficient in explaining substantial differences in the desorption intensity index for two samples characterized by the similar coalbed methane content.

New Instruments and Methods for Analysing the Coal-Methane System

Abstract The authors of the present paper designed and constructed a prototype of an instrument which enables fully automated determination of the desorbable methane content and effective diffusion coefficient in underground conditions. Due to microprocessor analysis of the recorded data and the application of the mathematical model of the diffusion process, it is possible to automatically determine the amount of methane whose release from a coal sample occurred before the sample was placed within a measuring instrument. It is also possible to carry out follow-up extrapolation of the recorded data so the time duration needed to determine reliable results can be reduced. The instrument was tested and optimized, and a number of copies sufficient for performing underground tests were constructed. The concept of the instrument represents a totally new approach to the observation of gas release from a coal sample. Instead of short-period measurements, virtually the whole process of methane release from coal is registered and analysed. This is possibly due to the use of a grain fraction lower than one mm which is presently applied for the sake of evaluating the methane- bearing capacity and desorption intensity.

Physical kinetics of coal-methane system: Mass transfer, pre-outburst events

The article gives the review of the present-day development in the physical kinetics of coal-methane system. The Gibbs thermodynamic potential is derived for this system as the function of methane density (persisting order parameter) and coal jointing (non-persisting order parameter). The authors put forward epy diffusion-filtration mechanism of mass transfer in porous material on two time scales and substantiate the concept of “quick” and “slow” methane. Based on the nonequilibrium thermodynamic potential, kinetic equations are derived for gas pressure and jointing in coal (bed). The first equation solution explains the physical effect of temporal gas pressure growth at the maximum of the external rock (bearing) pressure, the second equation enables generalization of the Griffiths failure criterion for a set of gas-filled joints. Mechanism of pre-outburst spalling of gas-saturated coal is analyzed.

Changes in the sorption/diffusion kinetics of a coal-methane system caused by different temperatures and pressures

W artykule przedstawiono wyniki pomiarow kinetyki adsorpcji metanu na weglu wykonane metodą gra­wimetryczną. Rejestracja czasowych zmian sorpcji metanu na weglu oraz wykorzystanie uniporowego modelu dyfuzji pozwolilo na wyznaczenie wspolczynnikow dyfuzji metanu na weglu w temperaturach od 291 K do 331 K i przy ciśnieniach do 1,7 MPa. Wyniki pokazują, ze wartośc wspolczynnika dyfuzji rośnie ze wzrostem tem­peratury. Zaleznośc pomiedzy temperaturą a wspolczynnikiem dyfuzji wydaje sie byc liniowa. Badania desorpcji wykonane w warunkach takich jak pomiar sorpcji pokazaly, ze przy nizszych ciśnieniach sorpcja jest procesem prze- biegającym szybciej. Roznice w kinetykach zanikająprzy ciśnieniu okolo 1,3 MPa. Wartośc wspolczynnika dyfuzji zalezna jest rowniez od ciśnienia rownowagowego, co wskazuje, ze wspolczynnik ten nie jest stalą materialową ukladu wegiel-gaz. W pracy przypomniano rowniez rozwazania Timofeewa na temat wspolczynnika dyfuzji. Autor ten wiąze wspolczynnik dyfuzji z liniową izotermą Henry’ego. Wyniki pokazują, ze wspolczynnik dyfuzji wedlug koncepcji Timofeewa charakteryzuje sie mniejszą zmiennością wywolaną zmianami ciśnienia w porownaniu z koncepcją Crank’a. W temperaturach powyzej 311 K wspolczynnik dyfuzji wedlug Timofeewa wydaje sie byc niezalezny od ciśnienia

Methane and carbon dioxide sorption/desorption on bituminous coal—Experiments on cubicoid sample cut from the primal coal lump

Abstract There was performed a research into the sorption of methane and of a mixture of methane and carbon dioxide on low-rank coal (type 32.2, according to PN-82/G-97002). Experiments were carried out by means of the volumetric method on a cubicoid solid sample, sized 20 × 20 × 40 mm, cut out of the primal body. At the same time, the changes occurring in the sample's overall dimensions, which accompanied sorption processes, were monitored. The kinetic curves of methane deposition and evacuation (two measurement cycles) and the deposition kinetic curve for the mixture of carbon dioxide and methane in a coal structure containing methane previously sorbed were determined, as well as the adequate kinetics of coal strains resulting from the sorption/desorption processes. The results show that in the experimental conditions sorption process and the induced strains noticed in the coal–methane system are proportional. When slightly approximated, the correlation between induced strains and the quantity of gases being sorbed e = f (V) becomes linear. However, the course of sample strain kinetics for the time of experiment in the coal-gaseous mixture indicates the occurrence of sample shrinkage after a dynamic increase in sizes, although the gas was being still collected in the coal structure. Their relation with the quantity of gas under sorption e = f (V) was linear only in the initial stage of experiment. It was found that in the case of the coal under investigation preferable is the sorption of carbon dioxide, not of methane, and CH4 is displaced from the sorbed phases by CO2. It seems feasible to recover CH4 through capture and storage of CO2 in a coal bed whose structure, parameters and petrographic composition complies with the material under research.

Displacement behavior of CH4 adsorbed on coals by injecting pure CO2, N2, and CO2–N2 mixture

Gas adsorption isotherms of Akabira coals were established for pure carbon dioxide (CO2), methane (CH4), and nitrogen (N2). Experimental data fit well into the Langmuir model. The ratio of sorption capacity of CO2, CH4, and N2 is 8.5:3.5:1 at a lower pressure (1.2 MPa) regime and becomes 5.5:2:1 when gas pressure increases to 6.0 MPa. The difference in sorption capacity of these three gases is explained by differences in the density of the three gases with increasing pressure. A coal–methane system partially saturated with CH4 at 2.4 MPa adsorption pressure was experimentally studied. Desorption behavior of CH4 by injecting pure CO2 (at 3.0, 4.0, 5.0, and 6.0 MPa), and by injecting the CO2–N2 mixture and pure N2 (at 3.0 and 6.0 MPa) were evaluated. Results indicate that the preferential sorption property of coal for CO2 is significantly higher than that for CH4 or N2. CO2 injection can displace almost all of the CH4 adsorbed on coal. When modeling the CH4–CO2 binary and CH2–CO2–N2 ternary adsorption system by using the extended Langmuir (EL) equation, the EL model always over-predicted the sorbed CO2 value with a lower error, while under-predicting the sorbed CH4 with a higher error. A part of CO2 may dissolve into the solid organic structure of coal, besides its competitive adsorption with other gases. According to this explanation, the EL coefficients of CO2 in EL equation were revised. The revised EL model proved to be very accurate in predicting sorbed ratio of multi-component gases on coals.