Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal-based graphene quantum dots(GQDs) were successfully produced via a one-step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur(SHOS) coals were selected as natural S-doped carbon sources for the preparation of S-doped GQDs. The effects of coal properties on coal-based GQDs were analyzed by means of high-resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD), Fourier transform infrared(FTIR) spectroscopy, X-ray photoelectron spectroscopy(XPS), ultraviolet-visible(UV-Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit bluegreen and blue fluorescence under ultraviolet light. Anthracite-based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the highrank bituminous coal-based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low-rank bituminous coal-based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield(QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high-rank SHOS coal(high-rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high-rank bituminous coal with SHOS content can be used as a natural S-doped carbon source to prepare S-doped GQDs, extending the clean utilization of low-grade coal.

Hydrogen Bonds in Coal——The Influence of Coal Rank and the Recognition of a New Hydrogen Bond in Coal

By means of in situ diffuse reflectance FTIR, the IR spectra of 6 coals with different ranks were obtained from room temperature to 230 ℃. A new curve fitting method was used to recognize the different hydrogen bonds in the coals, and the influence of coal ranks on the distribution of hydrogen bonds(HBs) in the coals and their thermal stability were discussed. The results show that there is another new HB(around 2514 cm -1 ) between the -SH in mercaptans or thiophenols and the nitrogen in the pyridine like compounds in the coals, and the evidence for that was provided. The controversial band of the HB between hydroxyl and the nitrogen of the pyridine like compounds was determined in the range of 3028-2984 cm -1 , and the result is consistent with but more specific than that of Painter et al .. It was found that the stability of different HBs in the coals is influenced by both coal rank and temperature. For some HBs, the higher the coal rank, the higher the stability of them. Within the temperature range of our research, the stability of the HB between the hydroxyl and the π bond increases to some extent for some coals at temperatures higher than 110 or 140 ℃.

Effects of coal rank, Fe_3O_4 amounts and activation temperature on the preparation and characteristics of magnetic activated carbon

Coal-based Magnetic Activated Carbons (CMAC’s) were prepared from three representative coal samples of various ranks: Baorigele lignite from Inner Mongolia; Datong bitumite from Shanxi province; and Taixi anthracite from Ningxia Hui Auto-nomous Region. Fe3O4 was used as a magnetic additive. A nitrogen-adsorption analyzer was used to determine the specific surface area and pore structure of the resulting activated carbons. The adsorption capacity was assessed by the adsorption of iodine and methylene blue. X-ray diffraction was used to measure the evolution behavior of Fe3O4 during the preparation process. Magnetic properties were characterized with a vibrating-sample magnetometer. The effect of the activation temperature on the performance of CMAC’s was also studied. The results show that, compared to Baorigele lignite and Taixi anthracite, the Datong bitumite is more appropriate for the preparation of CMAC’s with a high specific surface area, an advanced pore structure and suitable magnetic properties. Fe3O4 can effectively enhance the magnetic properties and control the pore structure by increasing the ratio of meso-pores. An addition of 6.0% Fe3O4 and an activation temperature of 880 °C produced a CMAC having a specific surface area, an iodine adsorption, a methylene blue adsorption and a specific saturation magnetization of 1152.0 m2/g, 1216.7 mg/g, 229.5 mg/g and 4.623 emu/g, respectively. The coal used to prepare this specimen was Datong bitumite.

Multi-scale pore fractal characteristics of differently ranked coal and its impact on gas adsorption

Well-developed pores and cracks in coal reservoirs are the main venues for gas storage and migration.To investigate the multi-scale pore fractal characteristics,six coal samples of different rankings were studied using high-pressure mercury injection(HPMI),low-pressure nitrogen adsorption(LPGA-N2),and scanning electron microscopy(SEM)test methods.Based on the Frankel,Halsey and Hill(FHH)fractal theory,the Menger sponge model,Pores and Cracks Analysis System(PCAS),pore volume complexity(D_(v)),coal surface irregularity(Ds)and pore distribution heterogeneity(D_(p))were studied and evaluated,respectively.The effect of three fractal dimensions on the gas adsorption ability was also analyzed with high-pressure isothermal gas adsorption experiments.Results show that pore structures within these coal samples have obvious fractal characteristics.A noticeable segmentation effect appears in the Dv1and Dv2fitting process,with the boundary size ranging from 36.00 to 182.95 nm,which helps differentiate diffusion pores and seepage fractures.The D values show an asymmetric U-shaped trend as the coal metamorphism increases,demonstrating that coalification greatly affects the pore fractal dimensions.The three fractal dimensions can characterize the difference in coal microstructure and reflect their influence on gas adsorption ability.Langmuir volume(V_(L))has an evident and positive correlation with Dsvalues,whereas Langmuir pressure(P_(L))is mainly affected by the combined action of Dvand Dp.This study will provide valuable knowledge for the appraisal of coal seam gas reservoirs of differently ranked coals.

Physical simulation of hydrodynamic conditions in high rank coalbed methane reservoir formation

In order to select highly productive and enriched areas of high rank coalbed methane reservoirs,based on hydrologic geology as one of the main factors controlling coalbed methane(CBM) reservoir formations,the effect of hydrodynamic forces controlling CBM reservoir formations was studied by a physical simulation experiment in which we used CBM reservoir simulation facilities.The hydrodynamic conditions of high coal rank reservoirs in the Qinshui basin were analyzed.Our experiment shows the following results:under strong hydrodynamic alternating action,δC1 of coalbed methane reservoir changed from the start at -2.95%～-3.66%,and the lightening process occurred in phases;the CH4 volume reduced from 96.35% to 12.42%;the CO2 volume decreased from 0.75% in sample 1 to 0.68% in sample 2,then rose to 1.13% in sample 3;the N2 volume changed from 2.9% in sample 1 to 86.45% in sample 3.On one hand,these changes show the complexity of CBM reservoir formation;on the other hand,they indicate that strong hydrodynamic actions have an unfavorable impact on CBM reservoir formation.It was found that the gas volume and hydrodynamic intensity were negatively correlated and low hydrodynamic flow conditions might result in highly productive and enriched areas of high rank CBM.

Influence of heating rate on reactivity and surface chemistry of chars derived from pyrolysis of two Chinese low rank coals

A series of char samples were derived from pyrolysis of two typical low-rank coals in China(Shengli lignite and Shenmu bituminous coal) at low, medium and fast heating rates, respectively, to the same pyrolysis temperature 750 °C. Then these chars were characterized by means of thermogravimetric analysis and Fourier transform infrared spectrometer with the aim to investigate the influence of heating rate in pyrolysis process on gasification reactivity and surface chemistry of them. Besides, a homogeneous model was used to quantitatively analyze the activation energy of gasification reaction. The results reveal that Shengli lignite and its derived chars behave higher gasification reactivity and have less content of oxygen functional groups than Shenmu coal and chars. Meanwhile, chars derived from Shengli lignite at 50 °C/min and Shenmu coal at 200 °C/min have the greatest gasification reactivity, respectively. The oxygen functional groups in Shengli lignite are easily thermo-decomposed, and they are less affected by the heating rate, while that in Shenmu coal have a significant change with the variation of heating rate.In addition, there is no good correlation between the change of oxygen functional groups and that of the gasification reactivity of the derived chars from pyrolysis at different heating rates.

Resistivity response to the porosity and permeability of low rank coal

Laojunmiao coal samples from the eastern Junggar basin were studied to understand the relationship between coal resistivity and the physical parameters of coal reservoirs under high temperatures and pressures.Specifically,we analysed the relationship of coal resistivity to porosity and permeability via heating and pressurization experiments.The results indicated that coal resistivity decreases exponentially with increasing pressure.Increasing the temperature decreases the resistivity.The sensitivity of coal resistivity to the confining pressure is worse when the temperature is higher.The resistivity of dry coal samples was linearly related to φ~m.Increasing the temperature decreased the cementation exponent(m).Increasing the confining pressure exponentially decreases the porosity.Decreasing the pressure increases the resistivity and porosity for a constant temperature.Increasing the temperature yields a quadratic relationship between the resistivity and permeability for a constant confining pressure.Based on the Archie formula,we obtained the coupling relationship between coal resistivity and permeability for Laojunmiao coal samples at different temperatures and confining pressures.

The mechanism and products for co-thermal extraction of biomass and low-rank coal with NMP

The high-value utilization of low-rank coal would allow for expanding energy sources,improving energy efficiencies,and alleviating environmental issues.In order to use low-rank coal effectively,the hypercoals(HPCs)were co-extracted from two types of low-rank coal and biomass via N-methyl-2-purrolidinone(NMP)under mild conditions.The structures of the HPCs and residues were characterized by proximate and ultimate analysis,Raman spectra,and Fourier transform infrared(FT-IR)spectra.The carbon structure changes within the raw coals and HPCs were discussed.The individual thermal dissolution of Xibu(XB)coal,Guandi(GD)coal,and the biomass demonstrated that the biomass provided the lowest thermal dissolution yield Y1 and the highest thermal soluble yield Y2 at 280℃,and the ash content of three HPCs decreased as the extraction temperature rose.Co-thermal extractions in NMP at various coal/biomass mass ratios were performed,demonstrating a positive synergic effect for Y2 in the whole coal/biomass mass ratios.The maximum value of Y2 was 52.25wt% for XB coal obtained with a XB coal/biomass of 50wt% biomass.The maximum value of Y2 was 50.77wt% for GD coal obtained with a GD coal/biomass of 1:4.The difference for the optimal coal/biomass mass ratios between XB and GD coals could be attributed to the different co-extraction mechanisms for this two type coals.

Structure and production fluid flow pattern of post-fracturing high-rank coal reservoir in Southern Qinshui Basin

Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reservoir in Southern Qinshui Basin. Flow patterns of methane and water in pore-fracture system and hydraulic fracture were discussed by using limit method and average method. Based on the structure model and flow pattern of post-fracturing high-rank coal reservoir, flow patterns of methane and water were established. Results show that seepage pattern of methane in pore-fracture system is linked with pore diameter, fracture width, coal bed pressure and flow velocity. While in hydraulic fracture, it is controlled by fracture height, pressure and flow velocity. Seepage pattern of water in pore-fracture system is linked with pore diameter, fracture width and flow velocity. While in hydraulic fracture, it is controlled by fracture height and flow velocity. Pores and fractures in different sizes are linked up by ultramicroscopic fissures, micro-fissures and hydraulic fracture. In post-fracturing high-rank coal reservoir, methane has level-three flow and gets through triple medium to the wellbore; and water passes mainly through double medium to the wellbore which is level-two flow.

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.

Investigation of Low Rank Coal Gasification in a Two-Stage Downdraft Entrained-Flow Gasifier

Low-rank coal contains more inherent moisture, high alkali metals (Na, K, Ca), high oxygen content, and low sulfur than high-rank coal. Low-rank coal gasification usually has lower efficiency than high-rank coal, since more energy has been used to drive out the moisture and volatile matters and vaporize them. Nevertheless, Low-rank coal comprises about half of both the current utilization and the reserves in the United States and is the largest energy resource in the United States, so it is worthwhile and important to investigate the low-rank coal gasification process. In this study, the two-stage fuel feeding scheme is investigated in a downdraft, entrained-flow, and refractory-lined reactor. Both a high-rank coal (Illinois No.6 bituminous) and a low-rank coal (South Hallsville Texas Lignite) are used for comparison under the following operating conditions: 1) low-rank coal vs. high-rank coal, 2) one-stage injection vs. two-stage injection, 3) low-rank coal with pre-drying vs. without pre-drying, and 4) dry coal feeding without steam injection vs. with steam injection at the second stage. The results show that 1) With predrying to 12% moisture, syngas produced from lignite has 538 K lower exit temperature and 18% greater Higher Heating Value (HHV) than syngas produced from Illinois #6. 2) The two-stage fuel feeding scheme results in a lower wall temperature (around 100 K) in the lower half of the gasifier than the single-stage injection scheme. 3) Without pre-drying, the high inherent moisture content in the lignite causes the syngas HHV to decrease by 27% and the mole fractions of both H2 and CO to decrease by 33%, while the water vapor content increases by 121% (by volume). The low-rank coal, without pre-drying, will take longer to finish the demoisturization and devolatilization processes, resulting in delayed combustion and gasification processes.

Effect of Temperature on the Molecular Weight Distribution in the Different Ranks of Coal during the On-Line Investigation of Coal Pyrolysis Gas Using Direct Photoionization Mass Spectroscopy

Coal pyrolysis gas from different ranks of coal was monitored on real time basis using photoionization mass spectroscopy. The molecular weight distribution of different products as a function of temperature from various coal ranks studied was observed. It was noted that the release of different classes of compounds like phenols, alkenes, alkylated aromatics and aromatic skeletons was temperature dependent. For all the coal ranks at lower temperatures phenols were the main component, with alkenes and alkylated aromatics at slight higher temperatures and aromatic skeletons were released at the highest temperatures studied.

Research on Permeability of Multiphase Medium of Middle to High-Rank Coals

The permeability of coal of middle to high ranks were tested using He,CH 4 and H 2O in single phase medium and using CH 4 and H 2O in double phase medium. The relation between adsorption and permeability of those media was discussed, and the seepage flow characteristics of methane-water medium in coals were analyzed. The result shows that the coalbed methane resource of high-rank coal reservoirs in China is still recoverable.

Carbon monoxide adsorptive capability of low rank coal's maceral

The centrifugal separation with gravity experiment was made for getting every pure macerals like inertinite and vitrinite,and the isothermal adsorption tests of pure mac- eral are carried out at 30,40,50,55,60,65℃,respectively,after analyzing the proximate element and maceral of coal samples,which was aimed to study the CO adsorptive capa- bility of every maceral of low rank coal at difference temperature and pressure.The results show that the adsorption isotherm of CO can be described by Langmuir equation because it belongs to the Type I adsorption isotherm at low temperature(T≤50℃),and the tem- perature effect on coal adsorption is greater than of pressure in lower temperature and pressure area;what's more,the relationship is linear between the coal adsorption quantity of CO and the pressure at high temperature(T>50℃),it can be described by Henry equation(Q=KP),which increases with pressure.Both temperature and pressure has great influence on CO adsorptive capability of low rank coals,especially the temperature's effect is so very complex that the mechanism need to study further.At the same time,the volatile matter,inertinite,oxygen-function groups and negative functional groups are high popu- larly in low rank coal samples,especially,the content of hydroxide(-OH) has great influ- ence on CO adsorption in that the inertinite has stronger effect than vitrinite on adsorptive capability of low rank coal samples,the result is same to the research on CH4 adsorption.

Swelling Measurements of a Low Rank Coal in Supercritical CO₂

Coal swelling in the presence of water as well as CO2 is a well-known phenomenon, and these may affect the permeability of coal. Quantifying swelling effects is becoming an important issue to verify the suitability of particular coal seams for CO2-enhanced coal bed methane recovery projects. In this report, coal swelling experiments using a visualization method in the CO2 supercritical conditions were conducted on crushed coal samples. The measurement apparatus was designed specifically for the present swelling experiment using a visualization method. Crushed coal samples were used instead of block coal samples to shorten equilibrium time and to solve the problem of limited availability of core coal samples. Dry and wet coal samples were used in the experiments because there is relatively limited information about how the swelling of coal by CO2 is affected by water saturation. Moreover, some coal seams are saturated with water in initial reservoir conditions. The maximum volumetric swelling was around 3% at 10 MPa for dry samples and almost half that at the same pressure for wet samples. The wet samples showed lower volumetric swelling than dry ones because the wet coal samples were already swollen by water. Experimental results obtained for swelling were comparable with other reports. Our visualization method using crushed samples has advantages in terms of sample preparation and experimental execution compared with the other methods used to measure coal swelling using block samples.