High Volatile Bituminous Coal Research Articles

Vacuum Pyrolysis of Brazilian coal, peat and biomass – Results on characterization of feedstock, solid residues, pyrolysis liquids and conversion rates

Pyrolysis of solid fuels such as coal, peat and biomass enables conversion into solid and liquid products, with noncondensable gas being a by-product. The present study evaluates conversion characteristics of Brazilian coal, peat and biomass samples using vacuum pyrolysis techniques. Feedstock and their respective solid residues (chars) and pyrolytic liquids obtained under vacuum pyrolysis conditions were characterized by proximate and ultimate analyses, gross calorific value, petrographic analyses, reactivity to CO2, Raman spectroscopy and organic geochemical methods (extraction, liquid chromatography). Chemical and physical properties in the feedstock samples and solid residues are highly variable. In the coals mean vitrinite reflectances ranged from 0.44% to 1.18% Rrandom indicating a rank range from subbituminous to high volatile/medium volatile bituminous coal. Reflectance measurements obtained from vitrinoid particles identified in solid residues from coal, peat and biomass varied from 2.10% to 10.64% Rrandom. Analyses of the liquid products indicate a tendency of the aliphatic fraction to increase in most of the samples during the pyrolysis process, as well as the predominant formation of polar compounds in the condensable liquids. The results of this study suggest that among the coal samples investigated major conversion to liquids and gases (29.7%–33.2%) occurs in the high volatile bituminous coals from Santa Catarina, whereas in the biomass samples Mamona (Castor Beans), wood chips (Eucalyptus), wooden bars (pinus) and signal grass have all conversion rates > 60%. The conversion rates for the peat samples varied between 32.5 and 46.6%. Reflectance values determined on vitrinoid biomass chars indicate a potential use in soil amendment.

Large-scale atomistic model construction of subbituminous and bituminous coals for solvent extraction simulations with reactive molecular dynamics

Large-scale atomistic models for complex polycyclic aromatic hydrocarbon systems help understand the chemical properties and behaviors of complex feedstocks such as coal or petroleum. However, the development and utilization of large-scale models remain limited due to the difficulty in achieving the varied structural characteristics necessary to capture stochastic nature of these feedstocks. We demonstrate a systematic workflow to construct stochastic molecular systems from a broad analytical suite: high-resolution transmission electron microscopy (HRTEM), carbon-13 nuclear magnetic resonance spectroscopy (13C NMR), laser desorption ionization mass spectroscopy (LDI-MS), and elemental analysis. We present a model construction and analysis utility of a new Python-based module. We selected one subbituminous and three high-volatile bituminous coals to construct large-scale models (∼40,000 atoms). The constructed models were utilized to examine the affinity for solvent extraction (naphthalene or tetralin) and the effect of structural properties (e.g., aromatic cluster size, functional groups, and cross-linking) in reactive molecular dynamics simulations. Complex chemical reactions were monitored with bond order transitions, intermediates formation, and mass distributions. Reactive molecular dynamics simulations suggest a plausible chemical extraction process and products for the complex fossil feedstocks. The results indicated that radical formations with bond breaking of bridging oxygens and carbons were required at high temperatures to facilitate hydrogeneration and extraction of gas molecules from radical-free molecules. We observed that aliphatic chains of tetralin were easily decomposed and combined with radicals to form small size of molecules with aryl bonding, mainly increasing molecules in the 500–1000 Da, while naphthalene had little impact on chemical extraction process.

Organic geochemistry and organic petrology of Oligocene and Miocene lignites, subbituminous and high volatile bituminous coals and carbonaceous shales in northern and central Trinidad

The so far unstudied Oligocene and Miocene organic matter–rich deposits of the Cunapo and Nariva Formation of northern and central Trinidad, western tropical Atlantic Ocean have been investigated using organic geochemical and organic petrological techniques to assess the thermal maturity and paleoenvironmental conditions during peat accumulation. Twenty samples were collected along various sections within the Cunapo and Nariva formations. Evaluation based on organic petrological analyses and Rock-Eval pyrolysis reveals that the coals and carbonaceous shales are thermally immature in the lignite and subbituminous rank for Cunapo Formation and mature in the subbituminous to high-volatile bituminous rank for Nariva Formation. The respective maturities correspond to high volatile matter contents.Petrographically, the Cunapo Formation coals are dominated by huminite with minor amounts of inertinite and liptinite, and the Nariva Formation coals contain high amounts of vitrinite, with moderate to low liptinite and low inertinite contents. The detailed petrographic data for Cunapo and Nariva coals indicate predominantely limnic conditions and a herbaceous origin of organic matter. Further conclusions are deduced from petrographic ratios such as gelification-, tissue preservation-, vegetation- and groundwaterAC indices, ash- and sulfur contents, iso- and n-alkane distribution, and 17α(H)-homohopane ratio. In summary, the data supports that both the Cunapo and Nariva coals formed in a tropical peat environment, partly influenced by very high microbial activity and ± in case of Nariva coals - partly grading into ombrogenic mires. The Cunapo coals were influenced by the development of the Orinoco River system during the Miocene and exposure to marine water during transgressive phases in a tidal, coastal environment.

Maceral composition, coal quality and depositional environments of the middle Permian Ukhaakhudag coal deposit, South Mongolia

The Ukhaakhudag coal deposit is located c. 560 km SSE of the city of Ulaanbaatar in the northeastern sector the South Gobi Basin. The coal-bearing strata is part of the middle Permian Tavantolgoi Formation. This study aims to determine the coal petrographic composition and depositional environments of five coal seams in the lower part of the deposit based on petrographic and chemical analyses of 106 composite samples. Vitrinite ranges from 12 to 64 vol.%, and inertinite varies between 9 and 68 vol.%. Liptinite ranges from 1 to 7 vol.%. Microlithotype analyses indicate that coals are primarily vitrinertite, few coals are classified as trimaceralic microlithotypes duroclarite and clarodurite. The inorganic fraction in the studied coals is mainly composed of clay, small amounts of silica, carbonate, and pyrite minerals. The vitrinite random reflectance values of the samples vary between 0.93-1.16 %. Volatile matter content varies from 26.10 to 41.48 wt.% (dry ash free basis). Ash, moisture, and sulfur contents vary between 11.20-44.76 wt.%, 1.63-6.03 wt.% and 0.49-1.67 wt.%, (air dried basis) respectively. Based on random vitrinite reflectance values and volatile matter content, the studied coals are classified as coking (Ch4), fat (Ch5), 1/3 coking (Ch6), gas fat (Ch7) based on the Mongolian system and medium to high volatile bituminous coal when using the ASTM system. The Gelification Index and Tissue Preservation Index of the studied samples suggest most seams accumulated in wet forest swamps with a high tree density. The Middle Permian peats from this location accumulated in mostly alternate oxic and anoxic mire conditions. The climate was drier during the peat accumulation of Seam 0, but it became warmer, and humidity increased starting from Seam 3 accumulation.

Digital rock characterization and CO2 flow simulation of high-volatile bituminous coal: An application to carbon geosequestration

The permeability of CO2 sequestration is influenced by the characteristics of pore–fracture structures in coal, yet the flow simulation via the pore network model (PNM) and the controls of topology and morphology on permeability remain to be understood. First, the pore–fracture structure of high-volatile bituminous coal (HVBC) is characterized using micro-computed tomography, and the flow characteristics of single-phase CO2 in the connected pore–fracture network are simulated by the PNM. Second, the surface roughness of the connected pore–fracture is extracted by fractal characterization, and morphological algorithms are applied to accurately present the pore–fracture network structure. Finally, the implications of structural parameters (pore or throat diameter, coordination number, tortuosity, and sphericity) on CO2 transport are discussed, and the mechanism of pore–fracture structure response due to mineral dissolution in CO2 injection is analyzed. The results shows that the HVBC sample provides significant pore–fracture space (porosity of 10.87%) and flow path (connectivity of 66.50%) for hydrogen and carbon storage. CO2 flow simulation results demonstrate anisotropic flow, with higher CO2 permeability observed in the Z-axis direction (0.061 × 10−3 μm2) compared to the X- and Y-axis directions (0.044 × 10−3 μm2 and 0.059 × 10−3 μm2, respectively). Moreover, the fractures perpendicular to the coal bedding plane in the coal seam with a large tectonic dip strongly influence the flow of CO2 injection. Fractal analysis reveals a positive correlation between fractal dimension and porosity, indicating that structures with higher surface roughness are not convenient for CO2 transport. Significant pore angle characteristics (sphericity average of 0.58), high pore–throat connectivity (coordination number average of 4.31), and low capillary resistance (tortuosity average of 1.19) collectively affect the flow of CO2. Overall, the strongly anisotropic pore–fracture structure contributes to the inhomogeneous flow pattern in CO2 geosequestration. Changes in pore–fracture structure resulting from mineral dissolution during the early stage of CO2–coal matrix interaction in the HVBC reservoir can significantly enhance storage potential. This study contributes to the existing understanding of flow characteristics and provides insights for optimizing CO2 injection efficiency in carbon geosequestration.

Mineralogy and geochemistry of the Middle Jurassic coal from the Hexi Mine, Shenfu Mining Area, Ordos Basin: With an emphasis on genetic indications of siderite

Siderite is a common carbonate mineral in coal and usually contains geochemical indications (elemental associations and isotopic compositions) for sedimentary environments and regional geological history, but the origin and enrichment mechanism of geochemically-pure siderite in coal is not fully understood. A better understanding of siderite origin in coal can help to decipher the depositional environment of peat and related geological processes of rank advances and epigenesis. This paper analyzed the petrological, mineralogical, and geochemical characteristics, particularly the content, occurrence, and origins of siderite in the Middle Jurassic coal from the Hexi mine of the Shenfu mining area, Ordos Basin, using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometry, and isotope ratio mass spectrometry. The results indicate that the No. 4 Coal is rich in inertinite and is defined as an ultra-low to low sulfur, high volatile bituminous coal. Siderite has contents varying from 5.8% to 54.4% (low temperature ash basis) and occurs in radial or concentric circle nodules in the coal and is mainly distributed within the clay matrix. Syngenetic siderite in the Hexi coals is relatively pure in chemical compositions, and was formed in a brackish water environment affected by salt lakes under arid conditions. The high content of inertinite, high Sr/Cu value, and positive Ce anomaly at the uppermost of the coal seam indicate that the escalating intensification of oxidation and evaporation. The Al2O3/TiO2 ratio and the diagrams of Al2O3/TiO2 vs. Zr/ TiO2 and Al2O3/TiO2 vs. Nb/Yb indicate that the sedimentary source rock in the study area is primarily intermediate-felsic. The δ13CPDB-δ18OSMOW diagram shows C and O for the siderite formation in this study were derived from marine carbonate rock alteration of the underlying Ordovician marine carbonate rock basement. The distribution patterns of rare earth elements and Y (Middle-type enrichment and Gd anomalies) in the coal further verify this hypothesis. Furthermore, siderite in the coal exhibits an enrichment in 13C suggest that bacterial fermentation of organic matter as another possible source of CO2. The iron of the siderite in the study coal was probably derived from the leaching of the magnetite-bearing granite.

Nanostructural Characteristic Changes in High-Volatile Bituminous Coal in Response to Differential Tectonic Deformation: Insights Gained from Quantified HRTEM Aromatic Fringes

Nanostructural Characteristic Changes in High-Volatile Bituminous Coal in Response to Differential Tectonic Deformation: Insights Gained from Quantified HRTEM Aromatic Fringes

Mineralogy and enrichment of critical elements (Li and Nb-Ta-Zr-Hf-Ga) in the Pennsylvanian coals from the Antaibao Surface Mine, Shanxi Province, China: derivation of pyroclastics and sediment-source regions

As a critical element, lithium plays an important role in a wide range of modern industries and in particular, battery storage and new energy technologies. Lithium in coal has attracted much attention in recent years because some coals are highly enriched in this element and consequently their corresponding coal combustion products may provide anm alternative source for this element. Previous studies showed that highly enriched Li in coals is generally associated with chlorite group minerals and in most cases attributed to the action of hydrothermal solutions. This paper provides mineralogical and geochemical data on the No. 9 Coal, the major minable coal from the Antaibao Surface Mine, Ningwu Coalfield, Shanxi Province, northern China, and focuses on the modes of occurrence and source of Li in this coal. The No. 9 Coal is a medium ash, medium sulfur, high volatile bituminous coal. The mineralogy of the No. 9 Coal is dominated by kaolinite, calcite, and pyrite, with minor amounts of anatase, rutile, goyazite, quartz, illite, boehmite, and bassanite, and traces of orthoclase, zircon and galena. The studied coal samples are characterized by high contents of Al2O3 (39.80% on average) in comparison with average Chinese coals, and high concentrations of Li, Ga, Se, Zr, Nb, Ta and Th compared with world hard coals. In addition to Li and Al, the critical elements Nb, Ta, Zr, Hf, and Ga in this coal also have a potential economic significance. The geochemical indicators (high Al2O3/TiO2, low Nb/Yb ratios and the specific rare earth element distribution patterns) and the mineral compositions (e.g., very low content of boehmite) indicate that the mineral matter in most coal benches and host rocks is dominantly derived from the Precambrian calc-alkaline granites of the Yinshan and northern Trans North China Orogens, and possibly from the bauxite of the Benxi Formation. Additionally, the geochemical and mineralogical characteristics of the intra-seam partings and some coal benches (quartz-free, kaolinite-dominated mineralogy, high Al2O3/TiO2 and Nb/Yb values) indicate that they were derived from an alkaline intermediate-felsic volcanic source. The highly enriched Li in the studied samples mainly occurs in kaolinite and is attributed to the input of material from sediment-source regions, rather than hydrothermal fluids. In addition, interaction of terrestrial and marine waters may have promoted the Li enrichment process in the partings.

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Correlating Coal Petrology and Methane Adsorption Parameters of High-Volatile Bituminous Coals with P-Wave Velocity.

The parameters of coal petrology and methane adsorption are significant to exploit coal and coalbed methane (CBM). Based on borehole core sampling, a new method using the P-wave velocity to predict coal maceral, coal face index, and Langmuir parameter of high-volatile bituminous coals was proposed. The results showed that the P-wave velocity correlated positively with coal skeletal density, apparent density, and ash yield with fitting coefficients (R 2) of 0.55, 0.57, and 0.57, respectively, but it negatively correlated with coal porosity and moisture content with R 2 of 0.56 and 0.60, respectively. Vitrinite, ranging from 14.8 to 82.7% with an average of 53.8%, positively correlated with coal porosity due to more micropores in vitrinite and thus negatively correlated with the density and P-wave velocity. Inertinite content was in the range of 5.4 to 27.4% with an average of 11.0%, which correlated negatively with the coal porosity and thus positively with the density and P-wave velocity for most of the samples. Furthermore, the P-wave velocity was weakly positively correlated with mineral content, and a negative correlation was found between the P-wave velocity and vitrinite/inertinite ratio (V/I), gelification index (GI), and Langmuir volume (V L). The porosity (Y 1), vitrinite content (Y 2), inertinite content (Y 3), and V L (Y 4) of coals could be predicted based on the equations as follows: Y 1 = 7842.4 e-0.003X , Y 2 = -0.0003X 2 + 1.0731X - 924.09, Y 3 = 0.0003X 2 - 1.2797X + 1405, and Y 4 = -0.04X + 101.24, where X is the P-wave velocity. Generally, P-wave velocity could be largely used to predict the variations of the coal maceral and methane adsorption capacity of high-volatile bituminous coals, providing a new and valuable approach for CBM exploration and gas prevention in coal mines.

Energy efficiency of bio-coal derived from hydrothermal carbonized biomass: Assessment as sustainable solid fuel for municipal biopower plant

The utilization of conventional biomass to generate electricity in the biomass thermal power plant is still inefficient due to high content of moisture, high impurity, and low heating value. To explore this limitation, in this research, hydrothermal carbonization (HTC) was selected as a sustainable process for producing green solid fuel or bio-coal with a rich of carbon content from palm shell and bamboo, remarkably as model biomass raw materials. The effect of biomass to water weight ratio (1:1 to 1:10), reaction temperatures (160 – 250 °C), and reaction times (1 – 6 h) were performed under autogenous pressure of promising aqueous media. The effect of organic and inorganic acid adding in the HTC of palm shell and bamboo was discussed. The obtained bio-coal was further well characterized with proximate and ultimate analysis and fuel properties. From the results, increasing reaction temperature and adding of acids led to increase carbon content, resulting to increase HHV of bio-coal. Effects of feedstocks (palm shell and bamboo), temperature, and types of acids revealed significantly results on the bio-coal yield accompanied with carbon content, the higher heating value (HHV), energy densification, and energy yield. The addition of inorganic acid in the HTC of biomass with high cellulose content provided higher HHV improvement than that of stony biomass. The highest HHV of 27.9 MJ/kg was obtained in bio-coal derived from palm shell feedstock with 1:1 water ratio and hydrothermally at 250 °C for 6 h. Surprisingly, this HHV is comparable to high-volatile bituminous coal. Moreover, an evaluating assessment for the utilization of this bio-coal instead of biomass feedstock to generate electricity in the thermal power plant with size of 9.9 MW capacity could reduce 34.4 % of fuel storage size, save nearly 29,709.36 USD/year in transportation costs and diminish CO2 emission during burning by approximately 56,732 tons/25 years.

The structural evolution characteristics for high volatile bituminous coal by in-situ heating in electronical microscope

In order to visually investigate the aromatic layer arrangement and structural evolution characteristics, high volatile bituminous coal was selected and investigated by in-situ high-resolution transmission electron microscopy (HRTEM) and thermogravimetry coupled with mass spectrometry (TG-MS). HRTEM was applied to quantify the size and arrangement of aromatic layers (such as length, curvature, orientation, d002 and Lc) during in-situ heating from room temperature (RT) to 1000 ℃, thereby revealing aromatic structural evolution characteristics. TG-MS was employed to identify gaseous release characteristics from RT to 900 ℃·H2O, CO2, CH4, C3H6, benzene and H2 were monitored and measured in this study. Based on the HRTEM experimental results, it was found that with temperature increasing, the distributions of aromatic layers changed from disordered to well-aligned and then to disordered again. Among them, the naphthalene structure, the smallest aromatic layer structure identified in the HRTEM image formed by the condensation of two benzene rings, presents the most obvious variation. In addition, the relationship between aromatic structural evolution and gaseous generation was also discussed in this work. The results showed that the structural evolution of high volatile bituminous coal was complex and closely related to the gaseous generation characteristics. At the temperature of RT∼300 ℃, the decomposing of oxygen-containing functional groups caused an increase in the content of naphthalene, the layers became shorter but more disordered, d002 decreased significantly while Lc increased. Up to 400 ℃, the layers were extended and parallel, the d002 decreased and La increased, while Lc remained almost stable, demonstrating that the polymerization of smaller layers formed in the previous stage was the main reaction in this stage. 400–600 ℃ was the main pyrolysis stage with large amounts of volatile matter generated, new active sites were thus formed and reconnected, causing the growth of aromatics and improvement of the order degree. At 600 ℃∼700 ℃, the content of naphthalene increased again, which may possibly due to the decomposition of methylene-bridge bond connected between aromatic rings, accompanied by the release of CH4 and benzene. Then the layers arrangement reached the most order and the perfect crystallite structure began to be formed gradually up to 800 ℃, which is owing to the polycondensation between the aromatics accompanied with the release of H2. As the temperature rise to 1000 ℃, the value of Lc showed a significant increase and the d002 remained a slight decrease, indicating the improving of the stacking degree, however, the formation of new naphthalene through the aromatization of aliphatic rings caused the naphthalene content increase and La reduce significantly.

Particle temperature and composition measurements in the ignition phase of single coal particles and particle groups under conventional and oxy-fuel atmospheres

The transition from single to group particle combustion is experimentally investigated in a laminar flow reactor. Optical techniques including two-colour pyrometry and a sampling probe provide data on surface temperature and composition of high-volatile bituminous coal particles during ignition, volatile combustion, and char burnout. Adjustable particle number densities (PND) permit studying the transition from single to group particle combustion. An optical imaging pyrometer provides simultaneously acquired data on the soot particles during the volatile combustion and the particle temperature, particle size and position in the 3D particle streak. These parameters are investigated as a function of the PND, which is used to differentiate between single and group particle combustion. Additionally, particle sampling provides particle composition data, which together with the particle temperatures extends a previously published data set from the same reactor. The results show a direct relation between the particle temperature during char burnout and the PND. As particles in group combustion absorb a significant amount of thermal energy from the surrounding atmosphere and reduce the oxygen content in the group, the measurements prove a lower temperature for particles that burn in group compared to single particle. Furthermore, sampling at several residence times indicates a notable difference in the burnout process between single particle (SP) and group particle (GP) combustion. While the effect of different atmospheres impacts the results of SP combustion, GP combustion is significantly less affected. The results of the current study are jointly discussed with previously acquired data of the same burner for a thorough interpretation.

Structure and reactivity of chars prepared from low-volatile coal under O2/N2 and O2/CO2 conditions in a flat-flame assisted entrained flow reactor

Accurate assessment of the structural characteristics and reactivity of coal char is crucial for the design of burners. Chars from low-volatile coal were prepared under O2/N2 (ON) and O2/CO2 (OC) conditions in a flat-flame assisted entrained flow reactor, with bituminous coal as a reference. Both the heating rate and residence time were carefully selected to mimic the conditions used in actual industrial boilers. The physical-chemical structures and reactivities of all chars were characterized using Raman and FTIR spectrometers and thermogravimetric analysis. The physical structure results indicated that a high CO2 content inhibited the char yield and specific surface area of chars from low-volatile anthracite coal, while, for chars from high-volatile bituminous coal, CO2 increased the char yield and specific surface area. For chemical structures, the degree of disorder in the carbon skeleton structures and active functional group contents of all chars prepared under OC conditions were greater than those of chars prepared under ON conditions. Moreover, the apparent reactivity results demonstrated that the physical structure characteristics of coal char played a leading role in the apparent reactivity under investigative conditions.

Effects of pore structure changes on the CH4 adsorption capacity of coal during CO2-ECBM

The effects of supercritical CO2 (scCO2) on pores in coal play a critical role in determining the CH4 adsorption and desorption characteristics and CO2 capacity of coal seams during CO2 geological storage-enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of pore structure changes caused by scCO2 on the CH4 adsorption capacity of coals, six coal samples with different ranks are exposed to scCO2 and water for 240 h at 62.5 °C and 15 MPa using a scCO2 geochemical reactor. Then, gravimetric isothermal adsorption tests, mercury intrusion porosimetry, N2 adsorption tests, and CO2 adsorption tests are performed to determine the CH4 adsorption capacity, pore volume, and pore specific surface area (SSA) of the coal samples with and without scCO2-H2O treatment. Finally, the influences of scCO2 on the pore structure and the effects of pore structure changes on the CH4 adsorption capacity of the coal samples are analyzed. The results show that the CH4 adsorption capacity of coal and its changes after scCO2-H2O treatment are strongly affected by density effects and pore selection effects. The CH4 adsorption capacity of coal is determined by the micropore volume and SSA; however, their changes only affect the CH4 adsorption capacity in the micropore filling stage when the pressure is <5 MPa with weak effects on the total CH4 adsorption capacity, which is related to their weak changes after scCO2-H2O treatment compared with macropores and mesopores. The changes in CH4 adsorption capacity mainly depend on the changes in macropore and mesopore SSAs after scCO2-H2O treatment; scCO2 mainly changes the CH4 adsorption capacity in the transitional stage, when the pressure is 5–10 MPa, and the monolayer and multilayer adsorption stage, when the pressure is >10 MPa. Meanwhile, the effect of pore structure changes on the CH4 adsorption capacity of coal with different ranks is complex. The CH4 adsorption capacity of subbituminous coal and medium-volatile bituminous coal increases after scCO2-H2O treatment due to the increases in macropore SSA, that of high-volatile bituminous coal slightly decreases, and that of semianthracite coal and anthracite coal slightly increases because of the changes in mesopore SSA after scCO2-H2O treatment. However, the CH4 adsorption capacity of low-volatile bituminous coal increases, which is the result of changes in the SSA of the mesopores and macropores due to slight changes in the micropore volume and SSA.

Effect of process variables on producing biocoals by hydrothermal carbonisation of pine Kraft lignin at low temperatures

Lignin from pulping is recovered in a wet form, making it ideal for hydrothermal carbonisation (HTC). This study investigates the effects of temperature (200–280 °C), residence time (1–6 h), and water to biomass mass ratio (2:1–6:1) on the composition of the resultant biocoal and the extent of alkaline and alkaline metal removal during HTC of pine Kraft lignin (PKL). Consistent with studies on other biomass materials, temperature exhibited the most significant effect on biocoal yield and properties, followed by residence time showing a marginal effect and varying the water to lignin mass only affecting alkaline and alkaline earth metals removal. Biocoal yields were in the range of 76–95 % (80–97 % on a carbon basis) and, compositionally, the biocoal obtained corresponded to sub-bituminous (220–260 °C) and high volatile bituminous coals (280 °C). Gas yields are low with the gas comprising mainly CO2 (98 v/v). The optimum temperature, time and biomass to water ratio for alkaline and alkaline earth metals removal was 260 °C, 3 h and 5:1 under which 93–95 % for sodium (Na) and potassium (K) and 75–80 % for magnesium (Mg) and calcium (Ca) removal were achieved. Solid-state 13C Nuclear Magnetic Resonance (NMR) revealed that during HTC, the oxygen-containing bonds of esters, ethers, and carboxylic groups were dissociated to produce lower-molecular weight including phenolic compounds dissolved in the process water. These findings have shown that the HTC is a promising alternative thermochemical route for coalification of wet PKL to a low ash coal-like fuel similar to sub-bituminous coal in rank that has potential applications for carbonisation.

Determination of total organic carbon content using Passey's method in coals of the central Kalahari Karoo Basin, Botswana

This paper focuses on determining total organic carbon (TOC) from boreholes in the Kalahari Basin, Botswana, using Passey's method. The Kalahari Karoo basin is one of several basins in southern Africa filled with Late Carboniferous to Jurassic sedimentary strata that host Permian age coal seams. Nine exploration boreholes (wells) drilled in the central Kalahari Karoo basin are used to determine the Total Organic Carbon potential. Vitrinite reflectance (Ro), proximate and ultimate analyses were conducted on cored coal intervals. Passey's ΔLogR method applied in this study employs resistivity and porosity logs to identify and quantify potential source rocks. Results of Passey's method compared with laboratory-measured carbon showed that Passey's method effectively identifies coal intervals. In terms of TOC calculations, the method works poorly in coal metamorphosed by dolerite intrusions. The heat affected coal samples had Ro from 0.77% to 5.53% and increased in maturity from primarily maceral controlled to high volatile bituminous and anthracite coal. Results from proximate analysis showed compositional changes in the coal were controlled by proximity to sill intrusion, with a decrease in Fixed Carbon and an increase in ash yield in the contact metamorphism zone (2–12 m from sill). For the unaltered coal that has undergone burial maturation displaying Ro of 0.44%–0.65%, the method works well. In unintruded boreholes, correlations between Carbon and calculated TOC indicate strong relationships. Passey's ΔLogR method proved to be a suitable method of estimating TOC on coal that has undergone burial maturation. This study has demonstrated that TOC calculated from the sonic log is more reliable in coal not affected by contact metamorphism than TOC calculated from the density log.

Regulation of permeability of plastic layer of coking coal by volatiles from coal pyrolysis

The low permeability of plastic layer of coking coal causes the formation of coking pressure, and the evolution of permeability is closely related to the release behavior of volatiles. In this paper, a low-volatile bituminous coal C1 and a high-volatile bituminous coal C2 were selected as research objectives, and a lignite L1 and two semi-cokes produced from the pyrolysis of C2 at two temperatures were used as the contrast. The permeability tests of single coal, binary coal or semi-coke and separated combination were conducted. The influence of volatiles release behaviors on the evolution of permeability of plastic layer was revealed by thermogravimetric and fluidity analyses. The results showed that there was a low permeability plateau stage of plastic layer of C1, while the permeability of plastic layer of C2 was rapidly improved after reaching the lowest. This was related to the difference in the mass transfer conditions of volatiles from two coals. C2 could enhance the mass transfer driving force of volatiles and bring both inert components (semi-coke after pyrolysis) and transferable hydrogen, thus the permeability of plastic layer could be improved without destroying the stability of plastic layer. The hydrogen-rich volatiles released from C2 before its initial softening temperature helped the low permeability of the plastic layer of C1 be formed and reach the maximum. The volatiles released during the plastic stage of C2 helped maintain the low permeability plateau of plastic layer of C1. It was plausible to further improve the permeability of plastic layer of C1 by consuming part of hydrogen-rich volatiles from C2 with prolonging the volatiles’ reaction.

Structural Model Construction and Optimal Characterization of High-Volatile Bituminous Coal Molecules

The structural characteristics of coal at the molecular level are important for its efficient use. Bituminous coal from the Baozigou Coal Mine is investigated, using elemental analysis, 13C nuclear magnetic resonance, X-ray photoelectron spectroscopy, and Fourier transform infrared. The molecular structure was determined. The aromatic compounds of bituminous coal molecules are primarily two- and three-ring structures, and the aliphatic structures are primarily in the form of methyl, ethyl side chains, and naphthenic hydrocarbons. The ratio of aromatic bridge carbon to peripheral carbon in the molecular structure is 0.279. Oxygen atoms in the form of carbonyl, phenolic hydroxyl and C–O, and nitrogen atoms in pyrroles. Thus, the average structure model of bituminous coal macromolecules was constructed; the molecular formula was C169H128O10N2S, and the molecular weight was 2378. The aromatic structural units in the macromolecular structure of coal include four naphthalenes, three anthracenes, two tetracenes, and heteroatoms in the form of three carbonyl groups, one phenolic hydroxyl group, one pyrrole, and one pyridine. The structure optimization and annealing kinetic simulation of a single macromolecular structure model were performed. Chemical bonds such as bridge bonds and aliphatic bonds were found to be twisted, and π–π interactions between the aromatic sheets in the molecule produced adjacent aromatic sheets. This arrangement tends to be approximately parallel, and the total energy decreases from 6713.401 to 2667.595 kJ/mol, among which the bond stretching energy and van der Waals energy dominate. We used 20 bituminous coal macromolecular models to construct aggregated structural models. After optimization by molecular dynamics simulation, the macromolecules were constrained by the surrounding molecules, and the sheet-like aromatic carbon structures that were originally approximately parallel were distorted. The macromolecular structure model of bituminous coal constructed in this study provides a theoretical model basis for the optimal surfactant.

Effect of water on methane diffusion in coal under temperature and pressure: A LF-NMR experimental study on successive depressurization desorption

Coalbed methane production is a process of desorption, diffusion, and seepage with the successive decrease of reservoir pressure. However, desorption at atmospheric pressure has been mainly used to investigate the gas diffusion of water-bearing coal. The influence of confining pressure and gas pressure on methane diffusion is not considered in this case. So, the question is, how does the variation of the methane diffusion coefficient of water-bearing coal during desorption? How is it different from dry coal? What is the difference between the methane diffusion behavior of anthracite and bituminous coal? To address these problems, we select low-volatile anthracite in the south of Qinshui Basin and high-volatile bituminous coal in the south of Junggar Basin. The methane diffusion of water-bearing and dry coal during the successive depressurization desorption is compared by low field nuclear magnetic resonance. The influence mechanism of water on methane diffusion is explored from the pore structure, wettability, and external stress. The results show that the methane diffusion coefficient of water-bearing anthracite and bituminous coal increases with the decrease of gas pressure. The methane diffusion of water-bearing coal is affected by the combination of the increase of methane desorption volume (positive effect) and the increase of effective stress (negative effect). When the gas pressure decreases from 6 MPa to 2 MPa, the negative effect is dominant, and the effective diffusion coefficient of methane in coal is poor; When the gas pressure is less than 2 MPa, the positive effect is dominant and the effective diffusion coefficient increases significantly. The homogeneity of methane effective diffusion path of water-bearing anthracite is enhanced, resulting in higher methane desorption rate and effective diffusion coefficient than dry coal. The methane effective diffusion pore size of water-bearing bituminous coal decreases, resulting in the effective diffusion coefficient of methane less than that of dry coal. These results are of great significance to reveal the methane diffusion behavior of coalbed methane in the actual production.