Coal Samples Of Different Ranks Research Articles

Experimental Study on Spontaneous Imbibition of Coal Samples of Different Ranks Based on the NMR Relaxation Spectrum.

The comprehension of the mechanisms governing spontaneous water imbibition in gas-water systems plays a significant role in the operation of hydraulic fracturing and the development of coalbed methane (CBM). In this study, nuclear magnetic resonance (NMR) techniques were used to investigate the pore structure and fluid behavior of different rank coal samples during spontaneous imbibition. Analyses were conducted on the 1D NMR T2 spectrum, 2D NMR T1-T2 spectrum, and layer division T2 spectra to achieve accurate and detailed information about the internal pore structure of coal and the characteristics of fluid transport during spontaneous imbibition. Low-rank coal exhibits good pore connectivity and favorable pore sorting characteristics, indicating favorable reservoir conditions. High-rank coal has larger pore spaces and highly developed micropores, which are highly beneficial for gas adsorption. However, its poor pore sorting characteristics and connectivity limit the migration and diffusion of fluids within the reservoir. The imbibition capacity follows a specific order of contribution, with small pores (10-50 nm) having the most significant role, followed by micropores (2-10 nm), ultramicropores (r < 2 nm), mesopores (50-1000 nm), macropores (1000-10,000 nm), and microfractures (r ≥ 10,000 nm). Low-rank coal stands out due to the restricted development of ultramicropores and small pores, leading to a different contribution of imbibition capacity compared to other samples, where macropores and microfractures dominate over all pore types. The coal reservoirs with favorable pore sorting characteristics and pore connectivity tend to exhibit a tendency toward rapid saturation and attainment of a prompt stable state during the hydraulic fracturing process. Finally, the mechanism of the late retreat effect of imbibition and the laws governing different coal ranks, pore structures, and fluid transport were discussed. This study offers comprehensive analyses of the mechanism of coal spontaneous imbibition and the characteristic laws of fluid seepage, providing insights into the optimization of CBM recovery and reservoir management.

Permeability variation and sensitivity of CO2 injection into coals under the control of effective stress and temperature

ABSTRACT The injection of CO2 into coal reservoirs offers a promising approach for reducing CO2 emissions and enhancing coalbed methane production. An inadequate understanding of the CO2 permeability variation in coals under the coupled constraints of temperature and effective stress limits the accurate evaluation of the CO2 injection potential. To address this issue, coal samples of different ranks were collected from the Sihe, Zhangjiamao, and Dahebian coal mines in China (designated as SH-1, ZJM-1, and DHB-1, respectively). These samples were used to investigate the characteristics of permeability variation and sensitivity to coal rank, temperature, and effective stress. The coal samples ZJM-1, DHB-1, and SH-1 had absolute permeability of 0.01355, 0.03953, and 0.00064mD, respectively. The SH-1 sample had the lowest absolute permeability due to its high coal rank, and the DHB-1 sample had the highest absolute permeability due to its tectonically deformed and fractured coal structure. The association of permeability to effective stress can be more accurately described by the permeability model with variable pore compression coefficient than that with constant pore compression coefficient. The CO2 permeability of the coal samples decreased exponentially with increasing effective stress, while the permeability damage rate increased exponentially and the permeability curvature decreased exponentially. Based on the fitted curve of the permeability damage rate to effective stress, three stages of stress sensitivity were identified: rapidly increasing stage, slowly increasing stage, and approximately stable stage. The permeability and temperature sensitivity coefficient exhibited a decreasing trend with increasing temperature, following a power function relationship. Among the undisturbed coal samples, the SH-1 sample showed higher sensitivity to stress and temperature compared to the ZJM-1 sample. The DHB-1 sample, characterized by a fractured coal structure, exhibited the lowest temperature sensitivity and the highest stress sensitivity. The temperature sensitivity of different coal samples tended to be consistent at temperatures above 70°C. Predictive models for CO2 permeability under the dual constraints of temperature and effective stress were constructed for the three samples using the surface fitting method. These models can assist in evaluating the feasibility of CO2 injection into coal reservoirs at various depths in associated areas.

Combined effects of pore structure and surface chemistry on water vapor adsorption characteristics of coal: Equilibrium, thermodynamic and kinetic studies

The combined effects of surface chemistry and pore structure on water vapor adsorption characteristics of coal were studied by evaluating the equilibrium, thermodynamic and kinetic properties. Four coal samples of different rank were fully characterized with gas (N2 and CO2) sorption and Fourier Transform infrared spectroscopy (FTIR) methods. Then measurements of water vapor adsorption equilibrium and kinetics were undertaken at elevated temperatures. Equilibrium and kinetic data were fitted by the modified BET model and the unipore model, respectively. The thermodynamic parameters, as well as diffusion activation energy were estimated based on the adsorption data. The results demonstrate that water vapor adsorption depends on the surface chemistry but unrelated to the pore structure, because the pore space is not completely filled by water molecules even at the saturation pressure. The monolayer adsorption capacity decreases with increasing coal rank. Moreover, water vapor needs lower work to attain equilibrium on high-ranked coal. Also, the binding affinity of water molecule with primary sites is stronger than secondary sites. Furthermore, the diffusion coefficient decreases with coal rank, and the diffusion activation energy for high-volatile bituminous is higher than the other coals, as a result of the complexity and poor connectivity of its pore network.

Diffuse reflectance spectroscopy based rapid coal rank estimation: A machine learning enabled framework

This research aims at studying the ability of using diffuse reflectance spectroscopy (DRS) for discriminating or classifying coal samples into different ranks. Spectral characteristics such as the shape of the spectral profile, slope, absorption intensity of coal samples of different ranks ranging from lignite A to semi-anthracite were studied in the Vis-NIR-SWIR (350–2500 nm) range. A number of classification algorithms (Logistic Regression, Random Forest, and SVM) were trained using the DRS dataset of coal samples. Class imbalances present in the dataset were handled using different approaches (SMOTE and Oversampling of minority classes), which improved the classification accuracy. Coal samples were initially classified into broad classes viz., lignite, sub-bituminous, bituminous, and anthracite with an accuracy of 0.98 and F1 score of 0.75. Later, the same samples were further classified into sub-class levels. The sub-class level classification also obtained good results with an accuracy of 0.77 and F1 score of 0.64. The results demonstrate the effectiveness of rapid coal classification systems based on DRS dataset in combination with different machine learning-based classification algorithms.

Numerical investigation of physicochemical effects of CO2 on coal particle ignition in O2/CO2 ambience

Coal combustion is a complex phenomenon involving multiscale chemical and physical processes. In this paper, the physical and chemical effects of CO2 on the ignition behaviors of dispersed coal particles are investigated based on a self-developed transient ignition model. Attention has been paid on the separate effects of CO2, namely the thermal conductivity, the species diffusion rate, the heat capacity, and the gasification reaction, on the ignition behaviors in oxy-fuel ambience. Differences of coal ignition delay time and ignition modes in air and oxy-fuel ambience are compared. The heterogeneous ignition (HI) is significantly delayed in CO2 ambience, with the most significant contribution from the lower thermal conductivity of CO2. The negative effects of slower species diffusion rates and the endothermic C-CO2 gasification reaction in CO2 are less important; however, they both become prominent with increasing ambient temperature up to 1800 K. By contrast, the gas phase homogeneous ignition (GI) is less sensitive to CO2 enrichment. The positive effects of slower species diffusion rates, together with the negligible effects of higher heat capacity and extra gasification reaction, impair the negative effect from lower thermal conductivity. In summary, the CO2 substitution makes the ignition mode move from the hetero-homogeneous joint regime towards the homogeneous dominant regime. Thus, coal particle ignition under high temperature (1500–1800 K) and low oxygen fraction (~0.1) transits to the homogeneous dominant case, yielding a reduced difference of ignition delay between air and oxy-fuel combustion. This feature has been observed for coal samples of different ranks from several independent macroscopic experiments, and can be well interpreted by the transition mechanism of ignition mode.

Towards valorizing natural coals in sodium-ion batteries: impact of coal rank on energy storage

Coal samples of different ranks were investigated through various compositional, morphological/structural, and textural experiments prior to their electrochemical implementation in Na-ion half-cells. The purity of coals proved insignificant while distinctions in the flake size, pore width, pore distribution, ID/IG ratio, crystallite parameters (La and Lc) along with adjacent parameters, such as the R-empirical parameter, i.e., limited parallel graphene stacking proved more relevant for Na+ storage into the negative host electrodes. Coal powders were identified via a two-step TGA analysis technique displaying the overall carbon content of the coals and the impurities. Coal-based anode materials were prepared from raw and pyrolyzed coals (at 800 °C under argon gas-flow) and cycled in Na-ion half-cells to further investigate the impact of the coal rank on the energetic properties. High volatile bituminous coal with lower graphene stacking and augmented nanoscopic pores delivered higher reversible capacity in comparison with semi-anthracite coal, whether in their raw (67 vs. 54 mAh/g) or pyrolyzed (214 vs. 64 mAh/g) states, respectively vs. Na/Na+. The dominance of HVBC over SAC due to enhanced properties as R-empirical parameter, ID/IG ratio, and internal porosity. This study provides an exhaustive methodology to assess other carbonaceous anode materials further to evaluate their energy storage capabilities.

A method for accurate characterisation of the pore structure of a coal mass based on two-dimensional nuclear magnetic resonance T1-T2

Accurate evaluation of coal pore structure plays a vital role in the technical engineering relating to coalbed methane (CBM) extraction. In view of errors in distinguishing different pore fluid components by using one-dimensional nuclear magnetic resonance (NMR), the two-dimensional NMR T1-T2 was proposed, which finally realised accurate characterisation of the pore structure and fluid components in a coal mass. We conducted T1-T2 sequence testing on coal samples of different ranks: anthracite, bituminous coal, and lignite. The pore characteristics of the coals were analysed by extracting T1-T2 spectral information. The main research results are as follows: anthracite ranked first, followed by lignite and bitumite successively in terms of pore development degree, while lignite outranked anthracite, both outranked bitumite from the perspective of heterogeneity degree. Furthermore, anthracite, lignite, and bitumite are ranked from large to small according to total porosity, residual porosity, effective porosity, and proportion of effective pores in gas extraction. The centroid locations of bitumite and lignite tend to be distributed within long relaxation times, while those of anthracite are inclined to shorter relaxation times. Lignite and anthracite contain more free water and inorganic-pore bound water than bitumite. The ratio of free fluid space to bound fluid space of anthracite is the largest, followed by that of lignite, while that of bitumite is the smallest. Therefore, anthracite and lignite are conducive to CBM extraction, while necessary fracturing measures for increasing the permeability should be taken for bituminous coal.

Nano-mechanical Properties and Pore-Scale Characterization of Different Rank Coals

ABSTRACTCharacterization of coal micro-structure and the associated rock mechanical properties are of key importance for coal seam exploration, coal bed methane development, enhanced coal bed methane production and CO2 storage in deep coal seams. Considerable knowledge exists about coal chemical properties, but less is known about the nanoscale to the micro-scale structure of coals and how they change with coal strength across coal ranks. Thus, in this study, 3D X-ray micro-computed tomography (with a voxel size of 3.43 µm) and nano-indentation tests were conducted on coal samples of different ranks from peat to anthracite. The micro-structure of peats showed a well-developed pore system with meso- and micro-pores. The meso-pores essentially disappear with increasing rank, whereas the micro-pores persist and then increase past the bituminous rank. The micro-fracture system develops past the peat stage and by sub-bituminous ranks and changes into larger and mature fracture systems at higher ranks. The nano-indentation modulus showed the increasing trend from low- to high-rank coal with a perfect linear relationship with vitrinite reflectance and is highly correlated with carbon content as expected.

Effect of Oxygen Functional Groups on the Surface Properties and Flotation Response of Fine Coal, Comparison of Rank with Oxidation

ABSTRACT The surface properties and flotation response of coal samples of different rank correspond to results from similar experiments for coal samples with different degrees of oxidation. Three principal types of oxygen-containing functional groups (C = O, C-O, O = C-O), were detected at the surface of low rank coals and oxidized coals by XPS, complementing results of previous researchers reported in the literature. The hydroxylation of these polar oxygen groups to C-OH groups and O = C-OH groups seems to account for the observed decrease in hydrophobic character and reduced flotation recovery. Hydration of the coal surface is facilitated at these hydroxylated sites by hydrogen bonding with interfacial water molecules.

The effects of CO2 on organic groups in bituminous coal and high-rank coal via Fourier transform infrared spectroscopy

The interactions between supercritical CO2 and coal and their effects on changes in the coal pore structure and organic groups play a critical role in the CO2 geological storage-enhanced coalbed methane recovery. To investigate the effects of supercritical CO2 on organic groups in coals of different ranks and its mechanisms under different temperature and pressure conditions, CO2 sequestration processes in bituminous coals and high-rank coals were replicated using a high-pressure reactor. Four coal samples of different ranks were exposed to supercritical CO2 and water under three temperatures and pressures for 240 h. Fourier transform infrared spectroscopy was used to provide semiquantitative ratios and Fourier transform infrared spectra of coal samples before and after the supercritical CO2–H2O treatment. The results show that interactions between supercritical CO2 and coal were controlled by the coal macromolecular structure, and semianthracite is the inflection point of interaction characteristics for coal samples of different ranks. Bituminous coal, including high- and low-volatility bituminous coal, has a low degree of condensation of its aromatic structure, and its aromatic nuclei can facilitate addition reactions. Swellings primarily break cross-links between aromatic nuclei in the same aromatic layer. These characteristics favor the polymerization addition of aliphatic side chains of aromatic nuclei, causing an increase in the degree of condensation of the aromatic structures in bituminous coal. High-rank coals including semianthracite and anthracite have a high degree of condensation of their aromatic structures, and the aromatic nuclei favor substitution reactions. Swellings primarily break cross-links connecting different aromatic layers, and bond dissociation reactions and sulfuration reactions are more significant for high-rank coal. These characteristics cause a decrease in the degree of condensation of the aromatic structure in high-rank coal. Temperature and pressure have a great impact on interactions between supercritical CO2 and coal and are controlled by the reaction types of the organic groups. With the increase in experimental temperature and pressure, the changes in the organic group content can be classified as the descending type, the rising type, the lower opening parabola type, and the upper opening parabola type. 45.0°C and 10 MPa is the inflection point of the changes in the organic group content. Descending- and rising-type changes favor addition, bond dissociation, and sulfuration reactions, which are endothermic. The reaction rate of supercritical CO2 and the organic groups increases, and the effects caused by temperature and pressure decrease as the temperature and pressure increase. Lower opening parabola- and upper opening parabola-type changes favor substitution, oxidation, and addition polymerization reactions, which are exothermic. These changes were significantly affected by a variety of reactions and were suppressed by high temperature and pressure. When the temperature is ≤45.0°C and the pressure is ≤10 MPa, supercritical CO2 has remarkable effects on alkyl and hydroxy groups and has a stronger effect on bituminous coal. When the temperature is &gt;45.0°C and the pressure is &gt;10 MPa, supercritical CO2 has remarkable effects on oxygen- and sulfur-containing groups and has a greater effect on high-rank coals.

Chemical extraction of rare earth elements from coal ash

The overall goal of this study is to develop a suitable flow sheet to extract rare earth elements (REEs) from coal ash. A total of 14 coal samples of different ranks were examined for REE concentration, and an anthracite coal sample with the highest REE concentration of more than 700 ppm in the coal ash was selected for REE extraction tests. This paper reports on the results of the experimental program completed in the first part of the study, which included high-temperature leaching with nitric acid followed by solvent extraction tests using various organic extractants, namely, tributyl phosphate, Cyanex 572, di-(2-ethylhexyl)phosphoric acid (D2EHPA) and their combinations. A 4×2×2 experimental design was used to conduct a total of 32 high-temperature leaching tests by varying acid molarity at four levels, solids content at two levels and leaching time at two levels. The highest recovery rates of 90 percent for light rare earth elements (LREEs) and 94 percent for heavy rare earth elements (HREEs) were obtained from the optimum leaching test conditions while maintaining impurity recovery to the leachate at less than 40 percent. D2EHPA was found to be the best extractant in this solvent extraction test series, providing an REE recovery rate of nearly 99 percent.

A new method for calculating gas content of coal reservoirs with consideration of a micro-pore overpressure environment

When the gas content of a coal reservoir is calculated, the reservoir pressure measured by well logging and well testing is generally used for inversion calculation instead of gas pressure. However, the calculation result is not accurate because the reservoir pressure is not equal to the gas pressure in overpressure environments. In this paper, coal samples of different ranks in Shanxi and Henan are collected for testing the capillary pressure of coal pores. Based on the formation process of CBM reservoirs and the hydrocarbon generation and expulsion history of coal beds, the forming mechanisms of micro-pore overpressure environments in coal reservoirs were analyzed. Accordingly, a new method for calculating the gas content of coal reservoirs with consideration of a micro-pore overpressure environment was developed. And it was used to calculate the gas content of No. 1 coal bed of the 2nd member of Lower Permian Shanxi Fm in the Zhongmacun Coal Mine in Jiaozuo, Henan. It is indicated that during the formation and evolution of coals, some solid organic matters were converted into gas and water, and gas–water contact is surely formed in pores. In the end, capillary pressure is generated, so the gas pressure in micro-pores is much higher than the hydrostatic column pressure, which results in a micro-pore overpressure environment. Under such an environment, gas pressure is higher than reservoir pressure, so the gas content of coal reservoirs calculated previously based on the conventional reservoir pressure evaluation are usually underestimated. It is also found that the micro-pore overpressure environment exerts a dominating effect on the CBM content calculation of 3–100 nm pores, especially that of 3–10 nm pores, but a little effect on that of pores >100 nm. In conclusion, this new method clarifies the pressure environment of CBM gas reservoirs, thereby ensuring the calculation accuracy of gas content of coal reservoirs.

Coal and solvent properties and their correlation with extraction yield under mild conditions

Coal solvent extraction is a clean coal technology that involves the extraction of organic matter from coal using solvents. In this study, the effects of various coal and solvent properties on extraction yield were studied and their correlations were observed. Solvent extraction was performed for fifteen coal samples of different ranks with eight solvents under mild conditions. Statistical analyses were then conducted to find correlations between the extraction yields and the coal and solvent characteristics. The extraction yield was strongly correlated with the atomic H/C ratio or volatile matter content. Among the solvent properties, the correlation between the electron donor, acceptor number (DN-AN) and yield was confirmed to be high. The results of multiple regression showed that positive correlations were found with the content of volatile matter of coal and polar force, DN-AN of solvent. Whereas negative correlations were found with the Ca/Mg content of coal and dispersion force, hydrogen bonding force of solvent. The regressionequation- calculated value was similar to the experimental value.

Flotation rates and zeta potentials of maceral concentrates of bituminous coals

Maceral separation is one possible technique to expand coking coal resources, either by increasing the blending ratio of non- or slightly caking coal or rationally utilizing the coking coal according to the different behaviors of macerals in coking. The success of maceral enrichment for a given coal usingfiotation is dependent on the differences in flotation kinetic rate among macerals. This paper presents theflotation rates and zeta potentials of the vitrinite-rich and inertinite- rich maceral concentrates obtained from six coal samples of different ranks — from high-volatile B bituminous coal with mean random vitrinite reflectance (Rrandom) of 0.64 percent to low-volatile bituminous coal with Rrandom of 1.73 percent — using a float-sink procedure. The flotation results show that vitrinite-rich concentrates can be produced in tap water using froth flotation only with frother for the high-volatile B and A bituminous coal samples, but there was no good performance for the high-volatile C bituminous and medium- to low-volatile bituminous coal samples. The zeta potential results show that froth flotation could probably separate the high-volatile C bituminous coal samples well by adjusting pH, but there was no good performance for the higher-rank coal samples.

Exploring the potential of petrographics in understanding coal pyrolysis

The effects of heating manner and charring temperature on both the physical and chemical properties of coal were investigated by petrographic techniques. Six coal samples of different rank were subjected to chemical treatment plus heat treatment. The pyrolysis experiments were carried out on acid washed coal in order to avoid ash formation. The chars used in the study were prepared in a tube furnace and the morphology was characterized.The morphology analysis reveals the differences in reactivity among the coals. It was found that a link existed between total reflectance and aromaticity, total reflectance and fuel ratio, total reflectance and surface area, and total reflectance and percentage of carbon dry ash-free basis (daf), which all gave a definite relationship confirming, that reflectance can be used as a measuring index in coal analysis. The mean total reflectance of the lignite coal for the temperature regime 450°C–700 °C is 1.26–4.36 %; sub-bituminous coal is 1.49–4.64 %; bituminous coal is 1.39–4.86 %; semi-anthracite is 2.15–4.86 %; and anthracite is 2.56–4.79 %. Thus, the large difference in the initial total reflectance between the different coal ranks became very narrow by 700 °C.

Comparative Study of the Aromaticity of the Coal Structure during the Char Formation Process under Both Conventional and Advanced Analytical Techniques

Six coal samples of different rank, five southern hemisphere and one northern hemisphere, were studied using both conventional and advanced analytical techniques: scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), carbon nuclear magnetic resonance (C NMR), and X-ray diffraction (XRD). Apart from SEM that was used to study the coal to char morphology, the other analytical techniques were used to determine the molecular structural parameter of coal, specifically, the aromaticity, fa. The application of these techniques to the simulation of coal formation revealed the aromaticity to be between 0.86 and 1.03 for lignite, between 0.86 and 1.03 for sub-bituminous, between 0.87 and 1.03 for bituminous, between 0.88 and 1.03 for semi-anthracite, and between 0.94 and 1.03 for anthracite. This reported value for the aromaticity was obtained from the conventional method of analyses. Similar values showing the same consistencies in coal rank were obtained using the FTIR (between 0.66 and 0.79 for lignite, between 0.58 and 0.90 for sub-bituminous, between 0.84 and 1.00 for bituminous, between 0.94 and 1.00 for semi-anthracite, and between 0.97 and 1.00 for anthracite). The values obtained using both C NMR and XRD were at variance from those obtained using both the conventional and FTIR, which call into question the reliability, authenticity, and dependability of these sophisticated and expensive analytical techniques. It is therefore proposed for researchers and coal scientist to rely on the conventional analysis technique of determining aromaticity, which serves as a predictive index for char reactivity, to understand the data obtained from these advanced analytical techniques.

Effects of Coal Functional Groups on Adsorption Microheat of Coal Bed Methane

This study measured the adsorption heat for five groups of Chinese coal samples of different ranks at 15 °C and 0.101 MPa using a C80 microcalorimeter. The functional groups of the coal samples were determined by infrared spectroscopy according to quantum chemical theory. The effects of coalification and coal functional groups on the adsorption heat of coal for methane were discussed in terms of energy. As a result, this study has further perfected the adsorption theory of coal bed methane. The results show that the adsorption heat of coal for methane first increases, then decreases with increasing coal rank and reaches a minimum at the fat coal stage. This indicates that coalification has a significant effect on the characteristics of the adsorption heat of coal for methane. These effects can be clearly classified into stages. Coalification influences the adsorption heat for methane by changing the type of coal, the content of oxygen-containing functional groups, and the pore structure. Oxygen-containing...

An experimental study on the evolution of aggregate structure in coals of different ranks by in situ X-ray diffractometry

The XRD profiles of raw coal.

Graphene quantum dots, graphene oxide, carbon quantum dots and graphite nanocrystals in coals.

Six coal samples of different ranks have been used to prepare single-layer graphene quantum dots (S-GQDs). After chemical oxidation and a series of centrifugation separation, every coal could be treated into two fractions, namely, CoalA and CoalB. According to the characterization results of TEM, AFM, XRD, Raman and FTIR, CoalA was revealed to be mainly composed of S-GQDs, which have an average height of about 0.5 nm and an average plane dimension of about 10 nm. The obtained S-GQDs showed excitation-dependent fluorescence and excellent electrochemiluminescence. CoalB was found to be some other carbon-based nanomaterials (CNMs), including agglomerated GQDs, graphene oxide, carbon quantum dots and agglomerated carbon nanocrystals. Generally, low-ranked coals might be more suitable for the preparation of S-GQDs. The production yield of S-GQDs from the six investigated coals decreased from 56.30% to 14.66% when the coal rank increased gradually. In contrast, high-ranked coals had high production yield of CoalB and might be more suitable for preparing other CNMs that were contained in CoalB, although those CNMs were difficult to separate from each other in our experiment.

Influences of magmatic intrusion on the macromolecular and pore structures of coal: Evidences from Raman spectroscopy and atomic force microscopy

Magmatic intrusion into coal-bearing sequences can significantly affect the rank of the intruded coal and change the macromolecular and pore structures. Here, we use Raman microscope and Atomic force microscope (AFM) to obtain quantitative information on these transformations. Six coal samples of different ranks taken from a magmatic intrusion zone of Huainan Coalfield, China, were studied. The Raman spectra were fitted with a combination of 8 Lorentzian bands and 1 Gaussian band. We found that the macromolecular structures in coals of different ranks have significant relationships with Raman spectral parameters, particularly the band area ratios (ID1/IG, ID2/IG, ID3/IG, ID4/IG and IG/IAll). With the increase of coal rank, ID1/IG, ID2/IG, ID3/IG and ID4/IG show trends of decreasing intensity, suggesting an enhanced orientation of aromatic hydrocarbons. Observations by Atomic force microscopy indicate that the pore parameters (e.g. amount, size, shape) are quite different for different ranks of coals. The pore size of low-rank bituminous coal is much larger than high-rank anthracite and pore abundance is higher in the former. In addition, there is also good correspondence between the surface topography of coal grains and coal rank.