Coals Of Different Rank Research Articles

Study on the wetting mechanism of SDBS with coals of different ranks based on surface free energy theory

Surfactants to improve the wetting ability of water sprays are considered an effective method of suppressing coal dust. Sodium dodecyl benzene sulfonate (SDBS) was used to understand the wetting mechanisms of an anionic surfactant on coals of different ranks. Atomic force microscopy (AFM) experiments and molecular dynamics simulations were used to investigate the surface free energy and hydration film. The analysis showed that the hydration film thicknesses obtained by both the experiment and simulation increase and then decrease with increasing SDBS concentration, and the high-rank coal had a thicker hydration film than the low-rank coal. The electrostatic potential distribution, hydrogen bonding, Mulliken charge, and interface formation energy were analyzed to illustrate the wetting mechanism. It was confirmed that coal's wettability is mostly determined by the electrophilicity of hydrogen atoms in oxygen-containing groups, and for high-rank coal, van der Waals forces gradually become the dominant forces in the wetting of the coal by SDBS. In conclusion, surface free energy theory by MD simulation proved to be a reasonable way to reveal the wetting mechanism of surfactants on coal, and the enhancement effect of SDBS on the wettability of high-rank coal is stronger than that of low-rank coal, which contributes to the development of surfactants for practical engineering applications.

Experimental study on inhibition of spontaneous combustion of different-rank coals by high-performance m-Cresol water-based inhibitor solutions

A kind of simple and efficient m-Cresol water-based inhibitor was proposed. The inhibitory effects of its solutions on the spontaneous combustion of different-rank coals were discussed. Furthermore, the micromorphology and wetting characteristics of different-rank coals were analyzed through a scanning electron microscope and a contact angle meter, respectively. Besides, the weight loss rates, oxidation characteristics, pore structure characteristics and free radical changes of different coal samples were investigated by a thermogravimetric analyzer, temperature-programmed experiments, a surface area analyzer and an electron spin resonance spectrometer, respectively. The results indicate that the prepared inhibitor solution at the 0.3 wt% sodium dodecyl sulfate (SDS) not only boasts good emulsification and wetting performance, but also plays a role in permeability enhancement. The m-Cresol/SDS solution can wet the coal surface and permeate the inner pore surface, forming a uniform inhibitor layer on both the coal surface and the inner pore surface to isolate oxygen, retain water and inhibit free radical generation. Moreover, it can not only effectively reduce the weight loss rate, CO production, specific surface areas and free radical concentration in the oxidation process of CoalXLH, CoalBLT and CoalYQ, but also improve the inhibition rate (to 79.1%, 67.4% and 54.3% at 170 °C respectively).

Pyrolysis characteristics and microstructure evolution of different coal types

Coal pyrolysis is the basis of coal gasification, as it is the necessary reaction step in the coal conversion process. To study the pyrolysis characteristics of different coal grades under temperature, thermal simulation tests of three different ranks of coal were conducted, and the pore structure and gas products were analyzed by using liquid nitrogen adsorption and gas chromatographic, and perform fractal fitting for pores of different size. The results show that the pyrolysis gas output of Inner Mongolia lignite is higher than that of Xinjiang long flame coal and Hancheng bituminous coal. With the increase of coal rank, the gas output of each gas component also shows a decreasing trend and the initial temperature and peak temperature of various gases gradually shift to high temperature. With the increase of temperature, the average pore size, volume, specific surface area, and fractal dimension of the three kinds of coal char show an increasing trend. The average pore size and pore volume of coal char increase, and the fractal dimension increase with the increase of coal rank. The comprehensive comparative analysis of the gaseous products, average pore size, pore volume, and fractal dimension of the three coal chars shows that Inner Mongolia lignite has high gas production, large average pore size, good connectivity, and simple pore structure, which is conducive to the escape of pyrolysis gas and the transportation of gasification agent and is the best favorable coal for gasification. The research results are expected to provide experimental guidance for the actual production of underground coal gasification.

Effect of secondary gas-phase reactions (SGR) in pyrolysis of carbon feedstocks for anisotropic carbon materials production – 2: Effects of SGR on tars produced from varying ranks of coal

This work explores the extent to which secondary gas-phase reactions (SGR) in pyrolysis can be used to modify coal tar chemistries from coals of different ranks in favor of improved anisotropy formation in their respective pitches. Pyrolysis was performed on four different coals of varying bituminous rank (Utah Sufco, Wyoming PRB Black Thunder, Illinois #6, and West Virginia Flying Eagle), with SGR temperatures ranging from 800 to 900° C and nominal SGR gas-phase residence times from 1 to 2.5 s. The oxygen content, aliphatic content, and molecular weight distributions of the coal tar samples were measured to indicate the changes with increasing levels of SGR, and microscopy was also used to measure changes in anisotropy formation in the resulting pitch samples. Generally, for all coals tested, increased levels of pyrolysis SGR led to decreased oxygen and aliphatic content, increased molecular weight sizes, and improved anisotropy formation. However, it was clear that the extent of these property changes depended on the chemistry and rank of the starting coal feedstock. Despite the relatively high rank of the Illinois #6 coal, its respective pitch samples performed poorly in improving anisotropy formation, due to its high sulfur content. The PRB, Sufco, and Flying Eagle coals performed better in their anisotropy formation than Illinois #6, depending on their respective coal ranks. Statistical analysis (analysis of variance, ANOVA) performed on the sample characterization data also suggests that SGR temperature is consistently the most dominant and significant effect on the resulting coal products.

Natural Frequency of Coal: Mathematical Model, Test, and Analysis on Influencing Factors

The difficulty in enhancing the low permeability of deep coal seams is the key problem restricting gas extraction. The technology of coal rock resonance and permeability enhancement excited by vibration wave is hailed as a new technology to enhance coal seam permeability. In particular, the effect of resonance and permeability enhancement is remarkable when the excitation frequency is exactly the same as the natural frequency of coal. In order to promote the application of the technology, the first step is to explore the variation characteristics of coal natural frequency and its influencing factors. In this study, two mathematical models of coal natural frequency were established, and the variations and influencing factors of coal natural frequency were discussed through an experiment on the natural frequency of coal. The results show that coal vibration has multiorder natural frequency which grows with the increase of the order. In addition, the natural frequency of coal is closely related to its elastic modulus, density, size, mass, stiffness, and other physical and mechanical parameters. The larger the coal size and mass are, the lower the natural frequency would be. The natural frequency parallel to the bedding plane is higher than that perpendicular to the bedding plane. For the saturated coal sample, moisture changes its density and reduces its elastic modulus. Consequently, its natural frequency is lower than that of the dried coal sample. The difference of organic matter and mineral content coal of different rank affects the physical and mechanical properties of coal, which leads to the difference in natural frequency of different-rank coals. The natural frequencies of different-rank coal show bituminous > anthracite > lignite. The natural frequencies of coal samples under different influencing factors are all tens of Hz. Thus, the vibration excitation of coal under the low-frequency condition is the focus of future research. The study can provide a theoretical basis for the technology of coal resonance and permeability enhancement excited by vibration wave.

Kinetics and oxidation pathways of Fe3+-catalyzed carbon-assisted water electrolysis for hydrogen production

Carbon-assisted water electrolysis can significantly reduce the electricity consumption of electrolytic hydrogen production, but its slow kinetics limit its further development. In-depth investigation of the oxidation pathways and control steps of coal particles during electrolysis can help to improve the reaction rate in a more targeted manner. In this study, the electrochemical activities of several different ranks of coal, activated carbon, and graphite are compared, and the electrooxidation process is disentangled to investigate the kinetic characteristics of the oxidation of solution catalysts, the reaction of coal with solution catalysts and the electrooxidation of coal. Structural characterization is used to investigate the key factors affecting the electrochemical activity. The results show that coal particles mainly undergo indirect oxidation during electrolysis, and direct oxidation only accounts for a small fraction of oxidation; the oxidation of the solution catalyst on the electrode surface is reversible, and the controlling step in the whole process is the redox reaction between the solution catalyst and coal particles. A variety of oxygen-containing functional groups are generated on the surface of coal particles after electrolysis, among which the C-O group plays a more critical role in the oxidation activity.

Vapour Sorption on Coal: Influence of Polarity and Rank

The surface properties of coal, interactions with gaseous and vapour media, and knowledge of the pore structure are important in terms of preparation, use, and utilisation of coal. This publication combines new unpublished data with analyses included in earlier publications by the research team to expand and systematise information on the sorption of water vapour, methanol vapour, and the saturated and unsaturated hydrocarbons hexane, 1-hexene, heptane, 1-heptene, octane on coals of different ranks. The study showed that the affinity of coal for water and methanol is related to the content of oxygen in the coal rather than the rank of the coal. Water sorption is a multilayer phenomenon, while methanol sorption is a monolayer phenomenon. The water monolayer is greater than that of methanol for low-rank coal, but for the higher-rank coals it is the opposite. The sorption capacity of the applied hydrocarbons depends on the presence or absence of a double bond and the size of the molecule. It increases in the order: n-octane < n-heptane/n-hexane < 1-heptene < 1-hexene. The effect of a double bond is dominant over the influence of the length and shape of the molecule.

Effects of CO2 adsorption on molecular structure characteristics of coal: Implications for CO2 geological sequestration

With a series of global problems such as greenhouse effect caused by the gradual increase of atmospheric CO2 concentration, CO2 geological storage has become one of the effective measures. To study the influence of the interaction between CO2 and coal on the molecular structure characteristics of coal, three coals of different ranks were selected for CO2 adsorption and were subjected to X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy experiments. The results indicate that with increasing of CO2 pressure: 1) Due to the solubility differences of different minerals, the contents of quartz in the coal samples gradually increases, while the contents of calcite, pyrite and kaolinite decreases; 2) The average number of the aromatic layers gradually decreased indicates that some aromatic layers are destroyed and order degree is lowered in the coal microcrystalline structure, resulting in a gradual increase of the inter-layer spacing between aromatic layers, the stacking of aromatic layers; 3) The concentration of aromatic rings gradually decreases leading to the intermolecular structural defects are obvious and the gradual evolution of the microcrystalline structure into the more disordered crystal structure; 4) The hydroxyl groups are predominated by oxidation reactions in long-flame coal and lean coal, while they are mainly governed by swelling in anthracite; 5) The ratio between aliphatic and total hydrogen atoms (Hal/H), aliphatic structure (I1) and ‘A’ factor values of the anthracite reveal a slight increase, while the aromaticity 2 (AR2) and degree of condensation 2 (DOC2) values display a decreasing trend. These values of long-flame coal and lean coal only slightly change. The influence of CO2 intrusion on coal properties due to the changes of molecular structure and CO2 trapping mechanisms are discussed. Our research contributes to understand deeply the geological sequestration of CO2 in coal seams and higher CO2 injection pressures should be adopted.

Critical pore size for micropore filling in coal samples with different rank coals.

The objectives of this study were to explore the occurrence and migration of coalbed methane in coals of different ranks and reveal the microscopic reservoir space and the mechanism of coalbed methane. To meet these objectives, this study selected six coal samples of different coal ranks for low-pressure N2 adsorption experiments, explored the critical pore filling characteristics of packed N2 molecules in the coals, and analyzed the low-pressure N2 adsorption/desorption experimental isotherms using the DFT method and DA equation based on the micropore filling theory. Finally, the critical filling pressure and pore size range for micropore filling were determined, and the analysis results were verified by combining the Langmuir, DA, and BET equations. The results showed that, from low to high coal rank, the N2 adsorption/desorption isotherms of the coal samples transition from type Ⅱ to type Ⅰ. The proportion of N2 molecules in low-rank coals in the form of micropore filling and monolayer adsorption was higher than that in high-rank coals. The critical pressure and critical pore size for micropore filling exhibited U-shaped correlations with the coal rank. Low-rank coals (lignite and long flame coal) were gradually filled in the relative pressure range P/P0 ≈ 1E-4–0.03, and medium- and high-rank coals (gas coal, 1/3 coking coal, lean coal, and anthracite) were filled in the relative pressure range P/P0 ≈ 1E-4–0.01; the corresponding critical pore size ranges were 1.7–2.19 and 1.61–2.00 nm, respectively.

Effects of Cofiring Coal and Biomass Fuel on the Pulverized Coal Injection Combustion Zone in Blast Furnaces

CO2 emissions are a major contributor to global warming. Biomass combustion is one approach to tackling this issue. Biomass is used with coal combustion in thermal power plants or with blast furnaces (BFs) because it is a carbon-neutral fuel; therefore, biomass provides the advantage of reduced CO2 emissions. To examine the effect of co-firing on pulverized coal injection (PCI) in BFs, two coals of different ranks were blended with the biomass in different proportions, and then their combustion behaviors were examined using a laminar flow reactor (LFR). The PCI combustion primarily functions as a source of heat and CO to supply the upper part of the BF. To create a similar PCI combustion environment, the LFR burner forms a diffusion flat flame with an oxygen concentration of 26% with a flame temperature of ~2000–2250 K at a heating rate of 105 K/s. The combustion characteristics, such as the flame structure, burning coal particle temperature, unburned carbon (UBC), and CO and CO2 emissions were measured to evaluate their effect on PCI combustion. With the increase in the biomass blending ratio, the brightness of the volatile cloud significantly increased, and the particle temperature tended to decrease. The fragmentation phenomenon, which was observed for certain coal samples, decreased with the increase in the biomass blending ratio. In particular, with an increase in the biomass blending ratio, the optimum combustion point occurred, caused by the fragmentation of coal and volatile gas combustion of biomass.

Structural characterization of high fidelity for bituminous and semi-anthracite: Insights from spectral analysis and modeling

Although molecular structure evolution in response to coalification process has been reported to be prevalent recently, their characterization accuracy for coals of different ranks still needs to be enhanced. In this paper, the occurrence information of the carbon skeleton structure, functional groups, and nitrogen and sulfur heteroatoms in coals of different ranks (Ro, max = 0.99% for XZ-02, 1.39% for YT-09, 1.63% for YT-11, 2.29% for ZC-15) were extracted via ultimate and spectrum analysis. Then four molecular structure models were created using computer-aided molecular design (CAMD). Results show that the oxygen atoms in coal primarily exhibit phenolic hydroxyl groups, ether groups, and carbonyl groups. The nitrogen heteroatoms are present as pyrrole and protonated pyridine, and the sulfur heteroatoms mainly served as thiophene. The content of aliphatic structures gradually decreased as coalification continued. Their length first decreased (Ro, max ≤ 1.9%) and subsequently increased (Ro, max > 1.9%). The content of aromatic structures gradually increased accompanied by the gradually cleaving of oxygen functional groups and alkyl side chain from the aromatic clusters. Hence, the degree of aromatization gradually increased, and condensation reactions dominated in the late stage of coalification. The initial molecular formulas and structural models were determined for the four coals. Also, the bonding modes among the aromatic clusters, aliphatic side chains, and oxygen functional groups within the molecular structure were adjusted based on the comparison between experiment and calculation spectra, finally creating the following molecular representations: C174H128O8N2 (XZ-02), C167H114O5N2S2 (YT-09), C151H96O7N2 (YT-11), and C146H80O4N2 (ZC-15). The fitting carbon spectra of these molecular representations were consistent with the experimental carbon spectra whose error is within the allowable range (relative error < 15%), verifying the applicability of the molecular structure model and the above method. Furthermore, the mechanism of coalification was discussed based on the variations of the side chains and chemical bonds. The above research can provide a theoretical reference for the molecular structural evolution for coals of different ranks and a high fidelity method to create molecular structure representation.

Experimental Study on Inhibition of Spontaneous Combustion of Different-Rank Coals by High-Performance M-Cresol Water-Based Inhibitor Solutions

Experimental Study on Inhibition of Spontaneous Combustion of Different-Rank Coals by High-Performance M-Cresol Water-Based Inhibitor Solutions

Molecule structure and ultrasonic wave velocity of different ranking coals: An experimental perspective

Ultrasonic wave velocity is an important manifestation of coal elastic properties and is extremely useful for coal and coalbed methane exploration. In this study, we focused on analyzing how the molecular structure affects the elastic ultrasonic wave velocity. High-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FT-IR) were used to explain the coal’s molecular structure. Ultrasonic wave velocity measurements were then used to determine the elastic parameters of coals of different ranks. From an elasticity theory point of view, the molecular structure of coal is a simple system consisting of two substructures. The results showed that the correlation between the ultrasonic wave velocity and the molecular structure differs in different coalification stages. In the first stage (Rr = 0.6%–∼1.6%), the ultrasonic wave velocity of coal keeps decreasing during the maturation process due to the aromatization of the coal molecular structure and the cleavage of alkyl side chains. In the second stage (Rr = 1.6%–∼3.0%), the average layer size (La) of the coal molecular structure increases and the average interlayer space decreases, which makes the ultrasonic wave velocity of coal increase continuously. The evolution of the molecular structure of coal is an important influencing factor for its elastic properties. The irregular parts (alkyl side chains and functional groups) play a major role in elastic ultrasonic wave velocity, followed by the average layer size (La) and the average interlayer spacing (d) of aromatic layers. It was determined that the relationship between the coal rank and ultrasonic wave velocity of coal without macro-fractured can be interpreted as the evolution of the molecular structure (two substructures), and the ultrasonic velocity measurement methods were applied to investigate the coal structure. These relations can be useful for geophysical exploration of coal and coalbed methane.

Extraction and Chemical Characterization of Humic Acid from Nitric Acid Treated Lignite and Bituminous Coal Samples

Currently, conversion of coal into alternative fuel and non-fuel valuable products is in demand and growing interest. In the present study, humic acid was extracted from two different ranks of coal, i.e., low rank and high rank (lignite and bituminous), through chemical pretreatment by nitric acid. Samples of lignite and bituminous coal were subjected to nitric acid oxidation followed by extraction using KOH and NaOH gravimetric techniques. The chemical pretreatment of both types of coal led to enhanced yields of humic acid from 21.15% to 57.8% for lignite low-rank coal and 11.6% to 49.6% bituminous high rank coal. The derived humic acid from native coal and nitric acid treated coal was analyzed using elemental analysis, E4/E6 ratio of absorbance at 465 nm and 665 nm using UV-Visible spectrophotometry and Fourier transformed infrared spectroscopy FTIR. The chemical characteristics of coal treated with nitric acid have shown increased molecular weight and improved aromaticity with more oxygen and nitrogen and lower C, H, and sulphur content. The E4/E6 ratio of nitric acid-treated low and high ranks of coal was high. The FTIR spectroscopic data of nitric acid-treated lignite coal indicates an intensive peak of carboxyl group at 2981.84 cm−1, while bituminous coal was shown in cooperation with the N-H group at 2923.04 cm−1. SEM was performed to detect the morphological changes that happen after producing humic acid from HNO3 treatment and native coal. The humic acid produced from HNO3 treated coal had shown clear morphological changes and some deformations on the surface. SEM-EDS detected the major elements, such as nitrogen, in treated humic acid that were absent in raw coal humic acid. Hence, the produced humic acid through HNO3 oxidation showed a more significant number of humic materials with improved efficiency as compared to native coal. This obtained humic acid can be made bioactive for agriculture purposes, i.e., for soil enrichment and improvement in growth conditions of plants and development of green energy solutions.

Effect of Oxygen on Radical Reaction during Oxidation of Different-rank Coals

ABSTRACT The spontaneous combustion disaster caused by low-temperature oxidation of coal often leads to mine fire which poses a serious threat to the safe mining of underground coal resources. Due to the complexity of coal and the variability of gas atmosphere, it is difficult to prevent the spontaneous combustion of residual coal in goaf. In this paper, the radical characteristics during oxidation of lignite, bitumite, and anthracite in anaerobic, anoxic, and normal oxygen atmospheres were measured by preparing mixed gases with different oxygen concentrations. On this basis, the spontaneous combustion characteristics of different-rank coals under different oxygen concentrations were explored. The experimental results show that the primary radical concentrations of different-rank coals are quite different in the initial stage of oxidation. Among the three types of coals, anthracite has the highest primary radical concentration while lignite has the lowest primary radical concentration. As the oxidation proceeds, the radical concentration of lignite grows at a significantly higher rate than those of bitumite and anthracite. In addition, the g factor values of the three types of coal all increase slightly with the rise of temperature, and the g factor value of lignite is always greater than those of bitumite and anthracite. During coal oxidation, the ΔH values of lignite, bitumite and anthracite vary in the range of 0.78–0.70, 0.6–0.48, and 0.70–0.50, respectively. The ΔH value of lignite is larger than those of bitumite and anthracite, but it experiences a smaller decrease than bitumite and anthracite. With the increase of oxygen concentration, the radical concentration and g factor of coal increase while the line width decreases. Through the analysis, the growth rate of radical concentration and g factor, instead of radical concentration, were used to evaluate the oxidation capacity of coal. The results reveal that the radical reaction in lignite is more sensitive to oxygen than those in bituminous and anthracite.

Changes in Microcrystalline Composition, Functional Groups and Combustion Characteristics of Coal in Coalfield Fire Area after Baking by Different Temperature Gradients

ABSTRACT Once fires occur in a coalfield, large quantities of coal in the coalfield will be baked at high temperatures. The changes and differences in the microscopic and spontaneous combustion properties of coals in high-temperature environments (ambient temperature or even after high-temperature baking) in the fire zone have not been studied yet. In this paper, three types of different-rank coals were baked at five different temperatures to investigate their changes in microcrystalline compositions, functional groups and combustion characteristics. The following results were obtained. The temperature treatment causes damage to the microcrystalline structure of different-rank coals. As the baking temperature rises, the aromatic layer spacing decreases, while the ductility and stacking height gradually increase. The aromatic ether content and the aliphatic hydrocarbon content gradually decline, while the -COOH content goes first and then falls. The spontaneous combustion activities of coals around the fire area gradually weaken with the rise of baking temperature. This research is crucial for revealing the effect of temperature, especially fire baking, on the spontaneous combustion characteristics of coal.

Understanding the multiple interactions of inertinite during pyrolysis/carbonisation with vitrinite: A study of two Australian coals of different rank

Macerals (vitrinite and inertinite) were separated from two Australian coals to study interactions during coking. Macerals were blended in various ratios, and their viscoelastic properties and expansion/contraction behaviour measured using rheometry. The minimum viscosity of a vitrinite concentrate (VC) from a high-rank coking coal (vitrinite reflectance Rvo = 1.54%) increased according to an expected exponential trend with increasing amounts of added inertinite concentrate (IC), suggesting no interaction. Volatile release rates (VRR), obtained from thermogravimetric analysis, were the same as that predicted based on the assumption that vitrinite and inertinite behave independently. For a medium-rank coal (Rvo = 1.19%), increasing proportions of its corresponding inertinite also increased viscosity, but the extent that it increased was much less than expected. VRR from these blends were lower than expected from a mass average of the components. When the IC from the high-rank coal blended with the VC of the medium-rank coal, a dramatic increase in viscosity occurred along with retention of volatile matter. These observations have been attributed to inertinite displaying varying degrees of volatile adsorption and softening/deformation. It is evident that inertinite and vitrinite interact in multiple ways during pyrolysis, which could have competing influences on viscosity and, therefore expansion behaviour.

A study on factors influencing CO2 adsorption by coal

The effects of moisture, pH value, and coal rank on adsorption of CO2 by coal were studied using a self-made small sample coal adsorption device. Coal of different ranks was dried, acidified, and alkalized. CO2 gas (16%) was injected into the device at normal temperature and pressure. The results showed that the moisture and pH value in the environment and the coal rank had an influence on the CO2 adsorption capacity of coal. The saturation adsorption capacity of CO2 of acid-treated coal and alkaline-treated coal increased by 3.342%–4.255% and 0.808%–2.22%, respectively, compared to dry coal. The saturated adsorption capacity of CO2 of untreated coal was reduced by 2.176%–2.521% compared to the dry sample. According to experimental data, the existence of moisture inhibited the adsorption of CO2 by coal, and the increase or decrease in the pH value promotes the adsorption of CO2 by coal. The decrease in the pH value had the greatest influence on the adsorption of CO2 by coal. The higher coal rank resulted in the increased saturated adsorption capacity of CO2.

Gas adsorption capacity changes in coals of different ranks after liquid nitrogen freezing

Fracturing coal with liquid nitrogen (LN2) has a positive effect on coalbed methane (CBM) extraction. The purpose of this study is to investigate the gas adsorption capacity of coals of different ranks after the coals are frozen with LN2. For this purpose, gas adsorption tests, contact angle measurements, and Fourier transform infrared spectroscopic analyses were conducted on both non-frozen samples of lignite, bituminous coal, and anthracite and on identical samples after the samples had been frozen by immersion in LN2. The test results show that after LN2 freezing for 180 min, the maximum amounts of gas that can be adsorbed by the three types of coal are reduced, the coals’ contact angles are smaller, and the proportions of several surface functional groups on the coals change. The effects of LN2 freezing on the coals are related to the coal’s rank. With the increase of coal rank, the influence on the gas adsorbed capacity of coal first increases and then decreases. The gas adsorption capacities of the coals are linearly correlated with proportions of aliphatic hydrocarbon, oxygen-containing, and substituted benzene functional groups but in a quadratic polynomial relationship with the contact angle. The results of the research reported here clarify the gas adsorption characteristics for coals of different ranks frozen by LN2 and advance our understanding of LN2 fracturing technology.

Comprehensive index evaluation of the spontaneous combustion capability of different ranks of coal

To investigate the effect of coal metamorphism on the probability of coal spontaneous combustion (CSC), five types of coal sample were subjected to a temperature-programmed testing system. The gas indicators and limiting parameters of CSC were analysed and the characteristics of different ranks of coal were observed during low-temperature oxidation. The CO production rate of each coal sample was positively correlated with temperature and the CO production rate of low-ranked coal was greater than that of high-ranked coal. The CO/O2 and CO productivity trends were the same. Analysis of the change in the alkene ratio revealed that the oxidation reaction became more intense if the coal temperature exceeded the dry cracking temperature. The oxygen consumption rate of each coal sample was positively correlated with temperature, low-ranked coal samples exhibiting higher oxygen consumption rates. Graham’s fire coefficient can be used as an auxiliary gas indicator to judge CSC tendency. Analysis of the limiting parameters of CSC revealed that the low limiting oxygen volume fraction and the minimal thickness of residual coal of high-rank samples were higher, whereas maximal air leakage intensity was lower. This paper further demonstrated differences in CSC tendency depending on the coal rank. These findings clarified certain factors affecting CSC, which may aid in preventing CSC in the future.