Experimental investigations on effects of gas pressure on mechanical behaviors and failure characteristic of coals

The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction.In this study,triaxial compression and seepage tests were carried out on coal at different gas pressures.The mechanical properties and failure process of coal were studied,as well as the acoustic emission(AE)and strain energy.The influence of gas pressure on the mechanical parameters of this coal was analyzed.Based on the conventional energy calculation formula,the pore pressure was introduced through the effective stress formula,and each energy component of coal containing gas was refined innovatively.The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described.Finally,the influence of gas pressure on coal strength was theo-retically analyzed from the perspectives of MohreCoulomb criterion and fracture mechanics.The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction(positive)and in the confining pressure direction(negative),as well as that induced by the pore pressure(initially negative and then positive).The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal.The quiet period of AE in the initial stage shortens,and AE activity increases during the pre-peak stage under high gas pressure.The fractal characteristics of AE in three stages are studied using the correlation dimension.The AE process has different forms of self-similarity in various deformation stages.

Insight into the key kinetic steps in the pyrolysis of coking and non-coking coals,characterization of the pyrolysis products

The chemical composition,structural and plastometric properties of different-ranked coals from Mongolia deposits were studied.The non-isothermal iso-conversion Ozawa-Flynn-Wall and Friedman model-free methods were used to assess kinetic parameters and to differentiate decomposition steps.Key peculiarities of the pyrolysis kinetics of brown and bituminous coals were revealed and discussed in terms of the composition and plastometric properties.Brown coal was shown to undergo three decomposition steps with ever increasing activation energy as temperature increased because of the decomposition of thermally more and more stable molecular fragments.The pyrolysis of bituminous coals occurred in four steps,the activation energy having extreme mode of temperature dependence.An important new finding was that the temperature range of the second,major pyrolysis step well corresponded to that between the softening and re-solidification temperatures according to Gieseler plastometry.The yield and composition of the pyrolysis products obtained under isothermal conditions were also characterized depending on coal rank and temperature,and the ways for qualified utilizations were offered.

Effects of coals microscale structural features on their mechanical properties,propensity to crushing and fine dust formation

The work is dedicated to revealing the structural features of coals with different ranks,such as anthracites,metaanthracite and graphite,that determine their ability to crush and form fine dust.For this purpose,a combination of various nanoin-dentation techniques and Raman spectroscopy was used.The mechanical behavior of the selected coals was investigated by cyclic nanoindentation with increasing peak load and quasi-static loading.The alteration of the mechanical properties was studied by analysis of elastic moduli and damage indices Rw.Three groups of coals were identified based on their propensity to crushing during cyclic nanoindentation.Coals assigned to the first and second groups are characterized by local destruction in the contact zone with the indenter and the formation of a core of crushed material.Coals assigned to the third group are characterized by bulk destruction(outside the zone of contact with the indenter).In general,the ability of coals to fracture under mechanical loading decreases in the series of metamorphism due to microscale compaction of vitrinite matter.In the series of anthracite,metaanthracite and graphite,it is established that the coal matter compaction takes place for the anthracite and metaanthracite,whereas graphite reveals rather different behavior due to abrupt change of its structure.The ratios between the amorphous and crystalline phases of carbon(S)were determined by deconvolution of coals Raman spectra.The propensity of coals to crushing(a damage index Rw)increases with growth of the proportion of amorphous carbon in the coal matter.For the considered coals and metaanthracite,it is established that the proneness to destruction outside the contact zone with the indenter is determined by the ratio of amorphous and crystalline carbon of 1 and higher.When S parameter is lower than 1,the coals are being crushed only in the zone of contact with the indenter.

Effect of Bio-Based Organic‒Inorganic Hybrid Hydrogels on Fire Prevention of Spontaneous Combustion of Coals

To solve the fire accidents caused by coal combustion,this work prepared four hybrid hydrogel materials using bio-based polymers,flame retardants,and inorganic materials.Compared to pure water and 3.5 wt%MgCl_(2)solution,the as-prepared hydrogel presents good fire prevention performance.In addition,it is found that CO and CO_(2)are not produced by coal when the pyrolysis temperature is lower than 200℃.During low-temperature pyrolysis,CO is more likely to be produced than CO_(2),indicating inadequate pyrolysis behavior.At the same time,the addition of fire-preventing hydrogel can not only decrease the maximum CO_(2)concentration before the critical temperature but also prolong the corresponding time.In addition,based on the cone calorimeter test,the inhibition effects of pure water,magnesium chloride solution,and four hybrid hydrogels on heat release behavior are evaluated.It is demonstrated that different dosages of different hydrogels affected the fire prevention effect.Phosphorous-modified cellulose/silica and carrageenan/DMMP/vermiculite composite hydrogels have the weakest fire prevention effect at 20 g,which is weaker than that of water.However,the fire prevention effect of carrageenan/DMMP/vermiculite composite hydrogels exceeded that of water at 40 and 60 g.Additionally,the fire prevention effect of the sodium alginate/sepiolite/ammonium polyphosphate composite hydrogel is most significant in common tests,attributed to the intumescent flame retardant system.

Micro-structural evolution and their effects on physical properties in different types of tectonically deformed coals

The macromolecular structure of tectonically deformed coals(TDC)may be determined by the deformation mechanisms of coal.Alterations of the macromolecular structure change the pore structure of TDC and thereby impact physical properties such as porosity and permeability.This study focuses on structure and properties of TDC from the Huaibei and Huainan coal mining areas of southern North China.Relationships between the macromolecular structure and the pore structure of TDC were analyzed using techniques such as X-ray diffraction,high-resolution transmission electron microcopy,and the low-temperature nitrogen adsorption.The results indicated that the directional stress condition can cause the arrangement of basic structural units(BSU)more serious and closer.And,the orientation is stronger in ductile deformed coal than in brittle deformed coal.Tectonic deformation directly influences the macromolecular structure of coal and consequently results in dynamic metamorphism.Because the size of BSU in brittle deformed coal increases more slowly than in ductile deformed coal,frictional heating and stress-chemistry of shearing areas might play a more important role,locally altering coal structure under stress,in brittle deformed coal.Strain energy is more significant in increasing the ductile deformation of coal.Furthermore,mesopores account for larger percentage of the nano-scale pore volume in brittle deformed coals,while mesopores volume in ductile deformed coal diminishes rapidly along with an increase in the proportion of micropores and sub-micropores.This research also approved that the deformations of macromolecular structures change nano-scale pore structures,which are very important for gas adsorption and pervasion space for gas.Therefore,the exploration and development potential of coal bed methane is promising for reservoirs that are subjected to a certain degree of brittle deformation(such as schistose structure coal,mortar structure coal and cataclastic structure coal).It also holds promise for TDC resulting from wrinkle structure coal of low ductile deformation and later superimposed by brittle deformation.Other kinds of TDC suffering from strong brittle-ductile and ductile deformation,such as scale structure coal and mylonitic structure coal,are difficult problems to resolve.

Composition and mode of occurrence of minerals in Late Permian coals from Zhenxiong County,northeastern Yunnan,China

Minerals in the Late Permian coals from the Niuchang-Yigu mining area,Zhenxiong County,northeastern Yunnan,China,were investigated using optical microscopy and low temperature ashing plus X-ray diffraction(LTA?XRD).The results showed that minerals in the coal LTAs are mainly quartz,kaolinite,chamosite,mixed-layer illite/smectite(I/S),pyrite,and calcite,with trace amounts of marcasite,dolomite,and bassanite.The authigenic quartz generally occurs in collodetrinite or as a filling in cleats or cell cavities.This silica was mainly derived from aqueous solutions produced by the weathering of basaltic rocks in the Kangdian Upland and from hydrothermal fluids.The presence of b-quartz paramorph grains in collodetrinite probably indicates that these grains were detrital and came from a volcanic ash.Clay minerals are generally embedded in collodetrinite and occur as cell-fillings.Pyrite occurs as framboidal,anhedral,and euhedral grains and a cell-filling.The coals are high in pyrite and the high pyrite content probably results from seawater invading during the stage of peat accumulation.Calcite generally occurs as vein-fillings,indicating an epigenetic origin.

Significant Mercury Isotope Anomalies in Hydrothermal Altered Coals from the Huaibei Coalfield,China

Igneous intrusion into coal-bearing strata can cause an abrupt increase of temperature-pressure and trigger hydrothermal circulation at the igneous rock-coal contact zone. The conductive heat of intruded lnagma and convective migration of secondary hydrothermal fluids have been observed to significantly alter the concentrations and modes of occurrence of mercury （Hg） in thermally-intruded coals.

Insights from principal component analysis applied to Py-GCMS study of Indian coals and their solvent extracted clean coal products

The present work aims at studying five Indian coals and their solvent extracted clean coal products using Py-GCMS analysis and correlating the characterization data using theoretical principal component analysis.The pyrolysis products of the original coals and the super clean coals were classified as mono-,di-and tri-aromatics,while other prominent products that were obtained included cycloalkanes,n-alkanes,and alkenes ranging from C_(10)-C_(29).The principal component analysis is a dimensionality reduction technique that reduced the number of input variables in the characterization dataset and gave inferences on the relative composition of constituent compounds and functional groups and structural insights based on scores and loading plots which were consistent with the experimental observations.ATR-FTIR studies confirmed the reduced concentration of ash in the super clean coals and the presence of aromatics.The Py-GCMS data and the ATR-FTIR spectra led to the conclusion that the super clean coals behaved similarly for both coking and noncoking coals with high aromatic concentrations as compared to the raw coal.Neyveli lignite super clean coal was found to show some structural similarity with the original coals,whereas the other super clean coals showed structural similarity within themselves but not with their original coal samples confirming the selective action of the e,N solvent in solubilizing the polycondensed aromatic structures in the coal samples.

Physicochemical,mineralogy,and thermo-kinetic characterisation of newly discovered Nigerian coals under pyrolysis and combustion conditions

In this study,the physicochemical,microstructural,mineralogical,thermal,and kinetic properties of three newly discovered coals from Akunza(AKZ),Ome(OME),and Shiga(SHG)in Nigeria were examined for potential energy recovery.Physicochemical analysis revealed high combustible but low levels of polluting elements.The higher heating values ranged from 18.65 MJ/kg(AKZ)to 26.59 MJ/kg(SHG).Microstructure and mineralogical analyses revealed particles with a rough texture,surface,and glassy lustre,which could be ascribed to metals,quartz,and kaolinite minerals.The major elements(C,O,Si,and Al),along with minor elements(Ca,Cu,Fe,K,Mg,S,and Ti)detected are associated with clays,salts,or the porphyrin constituents of coal.Thermal analysis showed mass loss(ML)ranges from 30.51%to 87.57%and residual mass(RM)from 12.44%to 69.49%under combustion(oxidative)and pyrolysis(non-oxidative)TGA conditions due to thermal degradation of organic matter and macerals(vitrinite,inertinite and liptinite).Kinetic analysis revealed the coals are highly reactive under the oxidative and non-oxidative conditions based on the Coats-Redfem Model.The activation energy(Ea)ranged from 23.81 to 89.56 kJ/mol,whereas the pre-exponential factor(kQ)was from 6.77×10^(-4)/min to 1.72×10^(3)/min under pyrolysis and combustion conditions.In conclusion,the coals are practical feedstocks for either energy recovery or industrial applications.

Quality of Some Nigerian Coals as Blending Stock in Metallurgical Coke Production

Two Nigerian coals, Lafia-Obi and Chikila were blended in the mass ratio of 70:30 with imported American and Polish coking coals. Proximate analysis, free swelling index, Ruhr dilatometer and Gieseler plastometer were used in assessing the coking qualities of both the single coals and the blends. The results show that the blends are lower in moisture and ash contents;higher in volatile matter and fixed carbon than the single Nigerian coals. The rheological parameters revealed superiority in thermo-plastic properties of the blends over the unblended local coals. Lafia-Obi/Foreign coals blends possess lower ash and better rheological properties compared to Chikila/Foreign coal composites which have high ash and poor rheological properties. These together suggest that amongst the two Nigerian coals, Lafia-Obi is superior for blending with the foreign ones in metallurgical coke production.

Characterization of Tars from the Thermal Processing of Baganuur and Tavan Tolgoi Coals from Mongolia, Using SEC, UV-F, IR and Mass Spectrometry

Tars from two Mongolian coals (Tavan Tolgoi and Baganuur) produced by simple distillation have been characterized using size exclusion chromatography (SEC) with elution in both 1-methyl-2-pyrrolidinone (NMP) and a mixed solvent (NMP and chloroform), UV-fluorescence in chloroform and NMP, gas chromatography (GC), mass spectrometry (GC-MS, probe-MS and LD-MS with thin layer chromatography) and infra-red spectroscopy. The SEC chromatograms using NMP and the solvent mixture NMP: chloroform indicates that similar conclusions can be drawn from using either eluent. The synchronous UV-fluorescence spectra were shifted to longer wavelengths in chloroform solution than in NMP and chloroform may be the better solvent for these tars prepared without extensive secondary thermal treatment. Infra-red spectra indicated differences between the two coal tars that reflected their different ranks, with more oxygenate groups in the lower rank Baganuur coal. Mass spectrometry (GC-MS and probe-MS) of both coal tars confirmed the presence of aliphatic components as well as aromatics and the relatively extensive alkylation of aromatics. Molecular mass ranges indicated for Baganuur tar by SEC compared well with the mass range by LD-MS although the LD-MS extended to higher mass values. The high mass fractions of the tars were revealed by fractionation by thin layer chromatography with the relevant sections of the developed plates inserted directly into the mass spectrometer;laser desorption was directly from the surface of the plate. LD-MS of the unfractionated samples failed to detect the high mass components because of mass discrimination effects. The high mass components were carried over in the distillation by mass transfer of vapours into the condenser.

Compositional and Structural Study of Macerals in Coals and Source Rocks Using SIMS

The authors applied the Secondary Ion Mass Spectrometry (SIMS) technique to the analysis of compositions and structures of vitrinites fusinites, fusinites bitumens and graptolites in the hydrocarbon source rocks with different maturities dscribed their SIMS spectral characteristics and found that different macerals have differnt spectra which, reflected the compositional and structural differences of macerals. Moreover, the change bod of parameter CH2+/CH3+ can be used for the evaluation of thermal evolution regularity of macerals in the hydrocarbon source rocks The study results show that the SIMS technique is a powerful means for microara analysis of macerals in coals and source rocks. It is certain that the study level of macerals can be raised by detailed study of SIMS results of SIMS results of macerals.

Geochemistry of Sulfur and Hazardous Elements in Late Paleozoic Coals and Partings from Taozao Coalfield, Shandong Province, China

The analyses of different sulfur forms, the trace elements in pyrites using electron microprobe and the trace elements in coal using INAA (instrumental neutron activation analysis) of the Late Paleozoic coals from the Taozao coalfield in Shandong Province, China, conclude that most sulfur (>75 %) in high-sulfur coal of Taiyuan Formation occurred as pyrite, in which many hazardous elements co-existed and their concentrations varied with their geological origin. The concentrations of hazardous elements in high-sulfur coals from Taiyuan Formation, composing mainly of Cu, As, U, Pb, Mo and Co, are much higher than those in the low-sulfur coals from Shanxi Formation and Shihezi Formation, because the influence of seawater during and after coal accumulation in Taiyuan Formation is stronger than those in Shanxi and Shihezi formations. Moreover, the element As is related to Fe, and both elements exist mainly in the form of pyrite. The element U is richer in the coal influenced by seawater. In addition, the coal affected by the magmatism contains more U, too. When high-sulfur coals are processed with heavy media washing to remove sulfur and minerals, the majority of hazardous elements will also be removed from the coals.

A New Distinguish Method of Blending Coals Slagging Characteristic

The slagging characteristic of coal effects the safe operation of boiler unit directly, the study on coal slagging is paid more and more attention by scholars of the world. Because of the complexity of coal characteristic, the blending coals slagging characteristic is more complex than single coal. The study method of blending coals characteristic is the same as the single coal method, but the simple method can’t distinguish the blending coals slagging characteristic well and truly. This paper gives the new distinguish method of blending coals slagging characteristic based on experimentation condition, it is the rate of slag and furnace refuse. The rate of slagging on furnace wall and the refuse on furnace bottom has utilized to distinguish the slagging characteristic of coal. The result shows that the rate of slag and furnace refuse method has better veracity.

Rare Earth Element Geochemistry of Late Palaeozoic Coals in North China

Abstract: Instrumental Neutron Activation Analysis (INAA) was done to determine the abundances of rare earth elements (REE) of 58 samples of Late Palaeozoic Carboniferous-Permian coals and related rocks in North China. Detailed study of REE geochemistry shows that the ?REE of most coals studied in this paper is in a normal range between 30×10?6 and 80×10?6 with a mean of 56×10?6. The REE in the Taiyuan Formation in the northern part of North China are much richer than those in the southern part. This is due to the shorter distance to the source area in the north. Moreover, the ?REE is in positive correlation to coal ash, especially closely related to the content of clay minerals <2 μm in size. This reveals that most REE were carried by terrigenous clastic materials, especially fine clay minerals. In the coals the light REE (LREE) are much richer than the heavy REE (HREE), and the LREE/HREE ratio in coals generally varies from 2 to 8. The LREE/HREE ratio of high-ash, low-sulphur coals is higher than that of low-ash, high-sulphur coals, and is even higher in the roof and floor rocks, reaching 12 with the highest (up to 21) in pyrite concretions, which shows that the HREE have a stronger affinity to organic matter in a reducing environment influenced by seawater. Furthermore, Eu is generally depleted in coals. The REE chondrite-normalized distribution patterns are very similar in coals of the whole of North China. From the REE geochemical characteristics it can be concluded that during the formation of Late Palaeozoic Carboniferous-Permian coals in North China, the supply of terrigeneous materials remained quite stable. The ?REE in low-ash, high-sulphur coals is relatively low and the REE are mainly carried by fine-grained minerals and organic matter and a certain percentage of REE are adsorbed by organic matter; while the ?REE in high-ash, low-sulphur coals is higher and the REE are mainly present in detritus. The ?REE of magmatism-influenced coals is the highest, which suggests that the introduction of magmatic substances may increase the ?REE, thus causing the REE distribution patterns to show an abnormal feature. Moreover, some harmful elements such as U, W and As usually increase when the coals are influenced by magmatism.

Paleo-fires and Atmospheric Oxygen Levels in the Latest Permian:Evidence from Maceral Compositions of Coals in Eastern Yunnan,Southern China

Inertinite maceral compositions of the Late Permian coals from three sections in the terrestrial and paralic settings of eastern Yunnan are analyzed in order to reveal the paleo-fire events and the atmospheric oxygen levels in the latest Permian. Although the macerals in the studied sections are generally dominated by vitrinite, the inertinite group makes up a considerable proportion. Its content increases upward from the beginning of the Late Permian to the coal seam near the Permian- Triassic boundary. Based on the microscopic features and the prevailing theory that inertinite is largely a by-product of paleo-fires, we suggest that the increasing upward trend of the inertinite abundance in the latest Permian could imply that the Late Permian peatland had suffered from frequent wildfires. Since ignition and burning depend on sufficient oxygen, a model-based calculation suggests that the 02 levels near the Wuchiapingian/Changhsingian boundary and the Permian-Triassic boundary are 27% and 28% respectively. This output adds supports to other discoveries made in the temporal marine and terrestrial sediments, and challenges the theories advocating hypoxia as a mechanism for the PermianTriassic boundary crisis.

A Carbon Isotopic Stratigraphic Pattern of the Late Palaeozoic Coals in the North China Platform and Its Palaeoclimatic Implications

This paper gives the stable carbon isotopic data in coals from the Late Namurian to Kazanian stages in the Serteng Mt., Xishan and Huainan coalfields of the North China Platform. Its stratigraphic pattern shows that several isotopic shifts are apparent, and the large δ13C negative shifts (approximately 2.5 to 3.0 %%) occurred during the Stephanian, Artinskian and Kazanian are observed in three Permo-Carboniferous coalfields. Those negative shifts are neither related to the coal rank and coal macerals, nor caused by the variety of peat-forming plants. The general decrease in the δ13C values of the Stephanian, Artinskian and Kazanian coals is consistent with an overall decrease in the δ13C values of ambient atmospheric CO2 and/or a relative increase in atmospheric Pco2 during the coal-forming periods. Therefore the authors postulate that the oxidation of peat, and the δ13C-depleted CO2 flux into the atmosphere during the above stages may have contributed to coeval palaeoclimatic warming by way of the greenhouse effect.

Application of thermally stimulated acoustic emission method to assess the thermal resistance and related properties of coals

Traditional methods of coal thermal resistance characterization are informative but considerably timeconsuming and require utilization of a complex and expensive equipment. This limits the effectiveness of their application. In this paper, authors experimentally investigated potential application of thermally stimulated acoustic emission method for developing of relatively simple and rapid coals thermal resistance assessment method. Features of thermally stimulated acoustic emission of anthracite, lignites and bituminous coal samples subject to cyclic thermal loading have been experimentally investigated.For the first time, it has been shown that there exists a relationship of such patterns with structural parameters and properties of the coal samples, as well as their thermal resistance. The results indicate the possibility of applying the method of thermally stimulated acoustic emission to control processes of cryogenic disintegration and thermal resistance of fossil coals. The description of the equipment and methodological support needed for the implementation of this method have been provided.

Exploration of petrographic,elemental,and material properties of dynamic failure-prone coals

The purpose of this study is to explore how the geochemical and petrographic components of coal may impact its physical properties and how these correlate with a history of reportable dynamic failure in coal mines.Dynamic failure events,also termed bumps,bounces,or bursts,are the explosive failures of rock in a mining environment.These events occur suddenly and often with no warning,resulting in worker injury up to and including fatality in greater than 60%of reportable cases through the Mine Safety and Health Administration(MSHA).A database of variables was compiled using publicly available datasets,which includes compositional geographic,strength,and Hardgrove grindability index(HGI)data.Results indicated that bumping coals were less mature,lower in carbon,higher in oxygen,softer,and less well cleated than coals that did not bump.High liptinite content was found to correlate with higher average uniaxial compressive strength(UCS)values.However,no clear and direct correlation between UCS and dynamic failure status was observed.The findings of this study established that differences existed between coals that had versus had not experienced reportable dynamic failure accidents.These differences were inherent to the coal itself and were independent of mining-induced risk factors.Results further illuminated how compositional attribute of coal influenced physical properties and began to clarify potential links between geochemistry and dynamic failure status.Only through the better understanding of risk can more effective mitigating strategies be enacted.

Co-pyrolysis characteristics and interaction route between low-rank coals and Shenhua coal direct liquefaction residue

To reasonably utilize the coal direct liquefaction residue(DLR), contrasting research on the co-pyrolysis between different low-rank coals and DLR was investigated using a TGA coupled with an FT-IR spectrophotometer and a fixed-bed reactor. GC–MS, FTIR, and XRD were used to explore the reaction mechanisms of the various co-pyrolysis processes. Based on the TGA results, it was confirmed that the tetrahydrofuran insoluble fraction of DLR helped to catalyze the conversion reaction of lignite. Also, the addition of DLR improved the yield of tar in the fixed-bed, with altering the composition of the tar. Moreover, a kinetic analysis during the co-pyrolysis was conducted using a distributed activation energy model. The co-pyrolysis reactions showed an approximate double-Gaussian distribution.