Coal Rank Research Articles

Modified network kinetic model for coal pyrolysis with high-value products and low carbon emissions

Pyrolysis is a clean and efficient technology for the conversion of low-rank coal. Its key product is tar with high value. The complex composition of tar limits its further effective utilization. Although most kinetic models can provide the final distribution of three-phase products in coal pyrolysis, an effective prediction method for the specific composition of tar and gas has not yet been developed. Most numerical models serve to improve the total yield of volatile matter and cannot provide theoretical guidance for the improvement of tar quality or the reduction of carbon emission. Based on the basic assumptions of the CPD (Chemical Percolation Devolatilization) model for product molecules structure and generation mechanism, this paper proposed a more sophisticated method for dividing tar composition, which can achieve the division of light, medium, and heavy fractions of tar components according to boiling point. Light gas composition could be obtained with the combination of FG(Functional Group) submodel. The influence of pyrolysis final temperature, heating rate, and environmental pressure was explored for the yield and distribution of products of 18 coals. The simulation results showed that the total yield of tar was affected by both coal structure characteristics and operating parameters. Under conditions favorable for evaporation, such as low pressure and high temperature, the total yield of tar was positively correlated with the average aromatic core size in coal. Under other conditions, tar release was limited by its volatility. Coal with larger average aromatic core size corresponded to less tar yield instead. The pressure in the operating parameters had the most significant influence on the total yield of tar. The heavy fractions of tar are more sensitive to changes in operational parameters, making working conditions that increase the total yield of tar reduce tar quality. The overall carbon emission of pyrolysis is <1 kg CO2/kg tar, which is far lower than the 2–4 kg CO2/kg tar and 3–9 kg CO2/kg tar of direct and indirect liquefaction processes. Under the conditions of sufficient insulation time and no consideration of secondary reactions, the carbon emission of pyrolysis only depended on the content of carboxyl groups in the precursor functional groups. Therefore, the carbon emission intensity decreased with the increase of coal rank. For the optimization for total tar yield and tar quality, 1 atm pressure and 723–923 K temperature was optimal for low rank coals, while low temperature and heating rate with 10–20 atm pressure or high temperature and heating rate with 45–50 atm pressure were optimal for medium rank coals. For the optimization of light tar yield and tar quality, 6–9 atm pressure was optimal for low-rank coals, and 1–3 atm pressure was optimal for medium-rank coals.

Vector characteristics of microscale gas transport in coalbed methane reservoirs

Elucidating the microscale geological control mechanism of gas transport in coalbed methane (CBM) reservoirs is the basis for efficient development of CBM reservoirs. At present, there is a lack of systematic research on the gas transport vector properties under the influence of microscale gravitational field. In this paper, firstly, a coupling model of multi-layer adsorption and surface jump is established, and the gas adsorption/desorption equilibrium mechanism under the condition of multi-layer adsorption is revealed. Secondly, a coupled model of gas transport in bulk gas field and weak adsorption field is established by an area weight method, and the coupling control mechanism of “coal rank-pressure” on gas transport in the direction parallel to wall is clarified. Thirdly, considering the strength distribution characteristics of microscale gravitational field in the direction perpendicular to wall, the “coal rank-pressure” coupling influence mechanism on gas transport mechanism conversion in weak adsorption field is revealed. It is found that (a) microscale gas transport is vectorial, and surface jump is not contradictory to multilayer adsorption; (b) under low pressure, the influence of residual pressure on surface jump rate can reach more than 10 times; (c) coal rank has a significant impact on microscale gas transport by controlling residual pressure, microscale gravitational field thickness and pore radius; (d) under low pressure, the flow perpendicular to wall in weak adsorption field has an obvious transition phenomenon of transport mechanism, and the influence of residual pressure is significant.

Influences of long-duration NO exposure on CO2 adsorption on coals: insights into oxy-coal burning flue gas storage in coal reservoirs

ABSTRACT Introducing oxy-coal burning flue gas into coal reservoirs has the advantages of mitigating emissions of CO2, NO x , and SO2, and producing in-situ coalbed methane (CBM). Given the characteristics of the geologic time scale for fluid sequestration, the long-duration NO exposure was conducted for various ranks of coal matrices to access the potential impacts of NO exposure on CO2 storage. Afterwards, the changes in critical physico-chemical properties and adsorbability of various ranks of coal matrices to CO2 because of long-duration NO exposure were revealed. Finally, the implications to implement oxy-coal burning flue gas sequestration in unmineable coal reservoirs with CBM production were indicated. The results demonstrate that the long-duration interaction with NO reduces the number of micro- and meso-pores of various ranks of coals, especially those with diametres below 4.00 nm. Moreover, the long-duration NO exposure reduces the oxygen-containing functional groups while significantly increasing the amine/amide-N for all the coals. The aforementioned alterations in the surface chemistry property imply that the coal matrix is capable of chemically adsorbing NO, thereby achieving its stable storage in target coal reservoirs. Given the electron donor–acceptor interactions between amine/amide-N and CO2, the NO exposure can elevate the CO2 storage capability of various ranks of coals under typical reservoir temperature and pressure. In summary, introducing oxy-coal burning flue gas into coal reservoirs is capable of stably storing critical gaseous pollutants and simultaneously enhancing the CO2 storage potential of coal reservoirs, thereby updating the existing oxy-coal burning technology and CO2 sequestration in unmineable coal reservoirs with the enhanced CBM recovery technology.

Lithium isotopic composition of two high‑lithium coals and their fractions with different lithium occurrence modes, Shanxi Province, China

Coal and coal-bearing strata have been considered as potential economic sources of lithium, and, as such have attracted significant attention. Significantly high concentration of lithium has been found in the coals from Shanxi Province, North China. This study examined the lithium isotopic composition of two high-Li coals of different ranks (XJ-8 anthracite and DLT-11 bituminous coal) and their fractions (such as organic-, silicate-) obtained through sequential chemical extraction. Lithium was found to be predominantly associated with silicates. Lithium isotopic compositions in DLT-11 are more positive (δ7Li > 0‰) than those in XJ-8 coal (δ7Li < 0‰) irrespective of which are bulk coals or leachates. Specifically, the carbonate fraction in DLT-11 had the most positive lithium isotopic composition among all fractions. Carbonate fluid hosting positive lithium isotopic composition seems to have had an effect on the lithium isotopic composition of DLT-11 coal. Lithium isotopic composition of XJ-8 anthracite may be a result of lithium re-distribution between organic and inorganic components during coalification, and this was also responsible for lithium isotopic homogeneity in the coal seam.

On the interactions between strength and petrology of Pennsylvania anthracite

Six blocks of anthracite, each from a known seam and mine location, were tested for their unconfined compressive strength and elastic modulus. The maceral content and the proximate analysis of the test specimens were examined after the strength tests were completed. The coal rank (2.84 to 5.84%Rmax), the maceral composition (41.9–88.6% vitrinite (mineral-free basis), and as-received ash content (2.76–28.29%) are not good predictors of the strength for these samples. Instead, the highest compressive strength and elastic modulus samples were the anthracites with the highest ash yield. Among the lowest ash specimens, compressive strength decreases with an increase in the ratio of (telovitrinite + fusinite + semifusinite)/detrovitrinite). The cleats and the inorganic (mineral) and organic (maceral) composition of megascopic and microscopic lithologies likely contributed unquantified variables to those determined in this study.

Influence of elevated temperature and gas atmosphere on coke abrasion resistance. Part one: Pilot oven cokes

A novel approach was developed to examine coke abrasion resistance in-situ at temperatures of up to 950 °C and in a controlled gas atmosphere using rotational tribological testing. The originality of this approach lay in the ability to apply tribological testing to the porous coke surface at elevated temperatures under a controlled inert or CO2 reactive atmosphere. Coke wear characteristics were quantified via (i) the application of advanced microscopy and image analysis techniques, and (ii) analysis of the coefficient of friction (COF) during tribological testing.Two pilot oven cokes were examined in this study, which were generated from single coking coals of similar rank but different petrographic composition. The cokes tested showed a deterioration in abrasion resistance even at 400 °C, and this was accentuated at 950 °C.The COF and the severity of the wear as a function of temperature and gas atmosphere were markedly different between the two cokes. Coke C396 from a mid-to-low vol, medium vitrinite coal showed no statistically significant differences between the wear severity results recorded following room temperature testing and high temperature testing in a CO2 environment. Conversely, the more reactive coke C426 with a higher parent coal inertinite content showed clear differences in the extent of abrasive wear at room temperature and at high temperature in both inert and reactive gas atmospheres.This suggests that the reduction in abrasion resistance at elevated temperatures is accentuated in the coke with the highest coke reactivity index (CRI) and the highest content of inertinite maceral derived components (IMDC). Our research has shown that at room temperature, the IMDC show greater resistance to abrasion than the reactive maceral derived components (RMDC). This has implications for understanding the relevance of room temperature testing to the abrasion resistance of coke under practical blast furnace operation conditions. For example, a coke from a coal or blend with a comparatively high inertinite content may show lower resistance to abrasion in the blast furnace than would be expected from tumble drum indices. Larger sample numbers and cokes from a variety of single coals need to be tested in further work to verify these assertions.Finally, a novel method using micro-CT image analysis was developed to examine the wear path generated at the coke surface during tribological testing. The purpose of this was to assess the extent of the abrasive wear by comparing the rendered micro-CT images before and after tribological testing. A colour gradient scale was used to facilitate the quantitative assessment, whereby the colour of each voxel was based on the distance from the nearest voxel that is associated with solid phase.As expected, the specific surface area (SSA) calculated for the upper 1 mm of each sample decreased following tribological testing due to the generation of a wear track which is smoother than the initially rough, porous coke surface. In the C426 coke type examined, the SSA decreased the most for the sample tested in an argon atmosphere at 950 °C, matching the COF and quantified wear severity results, which showed there was a notable increase in abrasive wear to this coke as the temperature was raised from room to elevated temperature.Coupling this method of analysis with 3D visualisation techniques in further work would provide a powerful approach to examine and quantify mechanisms of coke surface breakage and the microstructural and/or microtextural features responsible for the breakage.

Response and Mechanism of Coal Fine Production to Differential Fluid Action in the Baode Block, Ordos Basin

The Baode Block in the Ordos Basin is currently one of the most successfully developed and largest gas field of low–medium rank coal in China. However, the production of coal fine has affected the continuous and stable drainage and efficient development of this area. The special response and mechanism of differential fluid action during the drainage process is one of the scientific issues that must be faced to solve this production problem. In view of this, the evolution laws of a reservoir’s macro–micro physical characteristics under different fluid conditions (fluid pressure, salinity) have been revealed, and the response mechanism of coal fine migration-induced reservoir damage has been elucidated through a nuclear magnetic resonance online displacement system. The results indicated that pores at different scales exhibited varying patterns with increasing displacement pressure. The proportion of the mesopore and transition pore is not affected by salinity and is positively correlated with displacement pressure. When the salinity is between 3000 mg/L and 8000 mg/L, the proportion of macropore and micropore showed parabolic changes with increasing displacement pressure, and there was a lowest point. The evolution law of pore fractal dimension and permeability change rate under the action of different fluids jointly showed that there was an optimal salinity for the strongest reservoir sensitivity enhancement effect. The mechanical and chemical effects of fluid together determined the damage degree of coal reservoir induced by coal fine migration.

Development of Empirical and Artificial Neural Network Model for the Prediction of Sorption Time to Assess the Potential of CO2 Sequestration in Coal.

Geological sequestration of CO2 in a coal seam is considered an attractive option to reduce the carbon footprint. It has an additional advantage of enhancing the recovery of coalbed methane, which has less sorption affinity toward coal in comparison to CO2. Desorption of gases from coal is controlled by various parameters, including reservoir depth and coal rank. A representative factor for desorption and diffusion in coal is the sorption time. It is an indicator which helps in estimation and evaluation of gas movement in the coal seam. Coals exhibiting high sorption time allow greater quantities of CO2 injection and hold potential for CO2 sequestration. Therefore, reliable and cost-effective estimation of sorption time is very important prior to investment in projects related to CO2 sequestration. Generally, proximate and gas content analyses are part of the preliminary analysis of coal for the assessment of its potential as a coal-bed methane reservoir. In this study, data generated using these analyses were found very useful for estimating the sorption time and CO2 sequestration potential of coal. The coal samples were collected from different depths of the Mand Raigarh coalfield for testing, and an empirical equation and artificial neural network (ANN)-based model have been developed to predict the sorption time of coal. The developed empirical equation predicts the sorption time with a coefficient of determination value of 0.88 and a root mean squared error value of ±1.07 days. Furthermore, the developed ANN model has been found to be very efficient in prediction with a correlation coefficient value of 0.97.

Experimental study of coal rank effect on carbon dioxide injection to enhance CBM recovery

Coal seam gas extraction is an important part of green mining. At the same time gas injection for enhanced coalbed methane recovery (ECBM) has been proposed as an environmentally friendly, low-carbon and effective technology to increase methane production. To study the influence of coal rank on carbon dioxide (CO2)-ECBM efficiency, lignite, bituminous coal, and anthracite were tested using the Gas Flow and Displacement Testing Apparatus (GFDTA). Data indicated that: (1) in lignite and anthracite, outflow CO2 and CH4 fractions changed at higher rates as compared with bituminous coal. At the end of the experiments, lignite and anthracite displayed CH4 outflow fractions of 0.19% and 7.73%, respectively; (2) at the initial stage of the experiments, permeability rapidly decreased. Later, a fast increase was followed by a stable phase. In lignite and anthracite, the increment in permeability during the rapid increase stage was significantly larger than that of bituminous coal. (3) The CH4 flow rate of the three coal ranks showed a decreasing trend. Before the breakthrough, CH4 flow rate decreased rapidly and slowed down. After the breakthrough, CO2 flow displayed a linear increasing trend and later reached stability. (4) In general, CH4 production increased with the increase in coal rank. The variation of CH4 production is manifested in two phases with different dominant factors. In the first stage, CH4 production was mainly affected by permeability, and followed the order lignite > anthracite > bituminous. In the second stage, CH4 production was mainly controlled by the displacement effect, following the order anthracite > bituminous coal > lignite.

Understanding the interactions of NaClO oxidant with coal for intensified hydraulic fracturing effectiveness

The efficacy of hydraulic fracturing coalbeds to enhance methane production suffers from a reduction of the effective permeability due to formation damage. To address this practical challenge, this paper explores the feasibility of applying the oxidative treatment to coals in the post-hydraulic fracturing period. Experimental methodologies are utilized to reveal the fundamental interactions of coal with the oxidative in the context of a combined treatment of hydraulic fracturing and NaClO solution injection. Anthracite and bituminous coals are experimented to account for the effect of coal rank. A comprehensive package of techniques, i.e., ultrasonic wave velocity and hardness (for mechanical properties), scanning electron microscopy (for microstructure and morphology), contact angle (for wettability), and FTIR spectroscopy (for functional groups) were employed to comprehensively investigate the impacts of fracturing fluid and NaClO solution on the chemical and physical properties of the coal samples. The results show that the fracturing fluid exerts varying degrees of harm to the permeability of the two coals, while the effects of oxidative treatment are rank specific in dissolving the blockage and smoothening the fracture surface. It is further unveiled that NaClO solution promotes aromatic structure decomposition at the coal surface to form oxygen-containing functional groups, and its effects on coal permeability evolve in multiple stages. The findings of this study shed light upon implementing oxidative treatment to aid hydraulic fracturing performance in CBM for clean coal production in a carbon-constrained world.

Pore structure evolution of tar-rich coal with temperature-pressure controlled simulation experiments

Tar-rich coal, with a tar yield of 7–12%, has significant potential for hydrocarbon resources. In this study, typical tar-rich coal samples from Daliuta mining area were selected as the research object to conduct temperature–pressure controlled simulation experiments that simulated stratigraphic conditions. The pore characteristics of the tar-rich coal during coal metamorphism were studied, and the pore evolution law of the tar-rich coal was revealed. The findings demonstrate that this experiment successfully simulates all coal ranks, from low coal rank to high coal rank. Tar-rich coals contain pores of five different genetic types. Gas pores and fissures exhibit a tendency toward growth, while cellular pores are gradually broken and collapsed by compaction, showing a tendency to reduce pores. Mold pores and intergranular pores remain basically unchanged. According to the quantitative analysis of pore structure, each pore has a different evolutionary pattern. Macropores are the primary source of pore volume, micropores are the most active pore component in evolution, and mesopores exhibit a diminishing and then growing “U” shape characteristic. Three phases of tar-rich coal pore evolution—Ro, max < 0.9%, Ro, max = 0.9–3%, and Ro, max > 3%—can be distinguished, and there is a strong correlation between these stages and the coalification jumps.

Experimental Study on the Influence of Wettability Alteration on Gas–Water Two-Phase Flow and Coalbed Methane Production

The surface wettability is important in the change in the relative permeability of gas and water. Due to the heterogeneous property of coal, it has a mixed wetting state, which makes it difficult to predict the change in permeability. To investigate the influence of different wettabilities on two-phase flow, a total of three different rank coal samples were collected and were treated with different chemicals. The alteration of the coal’s wettability, characteristics of gas–water flow, and relative permeability of the coal after the chemical treatments were analyzed. The research conclusions suggest that (1) the coal samples treated with SiO2 and H2O2 increased the hydrophilicity of the coal surface, while the coal samples treated with DTAB increased the hydrophobicity of the coal surface. Compared to SiO2, both H2O2 and DTAB can form a uniform wetting surface. (2) The wettability alteration mechanism among the three different chemical reagents is different. (3) All the chemicals can change the gas–water interface. The water migrates more easily through the cleats after H2O2 treatment, while it is more difficult for the water to migrate through cleats after the DTAB treatment. (4) There are two types of flow states of gas and water on different wetting surfaces. A slug flow is formed on a hydrophilic surface, while an annular flow is formed on a hydrophobic surface. (5) The crossover point and the residual water saturation of the relative permeability curves were influenced by the surface wettability.

Full-scale pore structure characterization of different rank coals and its impact on gas adsorption capacity: A theoretical model and experimental study

Full-scale pore structure characterization of different rank coals and its impact on gas adsorption capacity: A theoretical model and experimental study

Experimental study on the activation of coal gasification fly ash from industrial CFB gasifiers

Coal gasification fly ash (CGFA) is an industrial solid waste from the coal circulating fluidized bed (CFB) gasification process, and it needs to be effectively disposed to achieve sustainable development of the environment. To realize the application of CGFA as a precursor of porous carbon materials, the physicochemical properties of three kinds of CGFA from industrial CFB gasifiers are analyzed. Then, the activation potential of CGFA is acquired via steam activation experiments in a tube furnace reactor. Finally, the fluidization activation technology of CGFA is practiced in a bench-scale CFB test rig, and its advantages are highlighted. The results show that CGFA is characterized by a high carbon content in the range of 54.06%–74.09%, an ultrafine particle size (d50: 16.3–26.1 μm), and a relatively developed pore structure (specific surface area SSA: 139.29–551.97 m2·g−1). The proportion of micropores in CGFA increases gradually with the coal rank. Steam activation experiments show that the pore development of CGFA mainly includes three stages: initial pore development, dynamic equilibrium between micropores and mesopores and pore collapse. The SSA of lignite fly ash (LFA), subbituminous fly ash (SBFA) and anthracite fly ash (AFA) is maximally increased by 105%, 13% and 72% after steam activation; the order of the largest carbon reaction rate and decomposition ratio of steam among the three kinds of CGFA is SBFA > LFA > AFA. As the ratio of oxygen to carbon during the fluidization activation of LFA is from 0.09 to 0.19, the carbon conversion ratio increases from 14.4% to 26.8% and the cold gas efficiency increases from 6.8% to 10.2%. The SSA of LFA increases by up to 53.9% during the fluidization activation process, which is mainly due to the mesoporous development. Relative to steam activation in a tube furnace reactor, fluidization activation takes an extremely short time (seconds) to achieve the same activation effect. It is expected to further improve the activation effect of LFA by regulating the carbon conversion ratio range of 27%–35% to create pores in the initial development stage.

A micro-macro coupled permeability model for gas transport in coalbed methane reservoirs

It is of great scientific and engineering significances to reveal the coupling mechanism of microscale and macroscale permeabilities in coalbed methane (CBM) reservoirs. In this paper, firstly, the characteristics of the pore-fracture system of coal matrix, such as gas content, permeability, connectivity, geometric size difference, and direction of micro-fracture development, are revealed. Secondly, a coupled permeability model is established, and the coal rank control mechanism on the transformation of microscale and macrosclae permeabilities is clarified. Thirdly, based on the microscale gravitational field model, the coal rank-pressure coupling influence mechanism on slippage/diffusion/jump of adsorbed gas in the surface control field is clarified. Results show that: (a) under a certain temperature, the microscale permeability and macroscale permeability of coal matrix have a statistical correlation, and the degree of coal metamorphism is the only variable controlling the transformation of micro-macro permeabilities. (b) With the increase of coal rank from 0.5% to 3.5%, the conversion coefficient of bulk gas permeability decreases by 15 orders of magnitude. (c) With the increase of coal metamorphism, the porosity and the density of oxygen-containing functional groups decrease, leading to the proportion of the area of adsorbed water film decreases at first and then increases.

Effects of Minerals Type and Content on the Synthetic Graphitization of Coal: Insights from the Mixture of Minerals and Anthracite with Varied Rank

The challenge of how to effectively treat minerals in coal before synthetic graphitization is a practical problem. It is unrealistic to remove minerals completely via physical or chemical methods. So, it is essential to clarify the role of minerals in the synthetic graphitization of coal. Based on the complex mineral composition, the mixture samples consisting of coal and mineral are used to obtain the effect of minerals type and content on the synthetic graphitization of coal. The role of minerals in synthetic graphitization is closely associated with the mineral content and type, as well as the rank. As to the lower-rank anthracite, quartz, kaolinite, and calcite have the role of inhibitor for the yields and defect degrees of corresponding samples after synthetic graphitization derived from the mixtures, but the role of catalyzer for their crystal structure (the degree of graphitization, stacking height, lateral size). The increasing content of quartz, kaolinite, and calcite is harmful for the yield, but useful for the crystal structure and defect degrees; the increasing content of pyrite is harmful for the yield, degree of graphitization, and stacking height, and it is useful for defect degrees. As to the higher-rank anthracite, quartz, kaolinite, and calcite have the role of inhibitor for the yield of corresponding samples after synthetic graphitization, catalyzer for their crystal sizes (stacking height, lateral size), and inertia for their degrees of graphitization. The increasing content of quartz, kaolinite, calcite, and pyrite is harmful for the yield and crystal size. A lower coal rank indicates being more prone to positive mineral effects on synthetic graphitization. The role of minerals in the synthetic graphitization of coal is complex and also represents a coupling relationship with the thermal transformation of anthracite.

In situ synthesis of S‐doped coal‐based carbon quantum dots and its application to Cr6+ detection

AbstractBACKGROUNDCoal‐based carbon quantum dots have better stability compared with other carbon quantum dots prepared from small molecules as carbon sources. Elemental doping can effectively improve the fluorescence performance of carbon quantum dots. However, the electronegativity difference between sulfur and carbon is small and the charge transfer between the two is very weak, so it is very difficult to dope sulfur onto carbon quantum dots by chemical methods. High sulfur content coal can achieve in situ synthesis of sulfur‐containing carbon quantum dots.RESULTSThe results showed that the prepared quantum dots had good stability and which increased with the increase of coal rank. CQDS is well dispersed, with an average particle size between 2.44 and 2.74 nm. It can emit light blue fluorescence under the UV excitation of 365 nm. It was found that trace amount of Cr6+ had a fluorescence quenching effect on CQDS and a good linear relationship was observed in the range of 0–100 μmol·L−1.CONCLUSIONQuantum dots doped with S atoms have better selectivity, interference resistance, and sensitivity in the detection of metal ions. The stability and the particle size of CQDS are increased with the coal rank. The quantum yield of CQDS prepared from high sulfur content coal is higher than low sulfur content coal which has same rank. The addition of Cr6+ to CQDS produces a dynamic fluorescence quenching. This study provides a new idea for the green preparation of coal‐based carbon quantum dots and the ion detection. © 2023 Society of Chemical Industry.

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.

Analysis of char structure and composition from microwave and conventional pyrolysis/gasification of low and middle rank coals

The structure and composition of residual coal char after pyrolysis dictates its reactivity towards gasification. The nature of the char is a result of the parent coal composition and structure as well as the reaction conditions during pyrolysis. Further, due to the unique characteristics of selective dielectric heating, char development during microwave CO2 gasification may differ from conventional thermal gasification, and little is known about the effect of microwave heating on the char structure development. In this work the heating method used is compared (microwave vs conventional thermal) to generate char samples from four different coal types. These coal chars generated by pyrolysis and gasification were characterized by ultimate analysis, surface area analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and dielectric characterization to investigate the chemical and structural differences between microwave- and conventionally-generated chars. The results reveal notable chemical and structural differences between the chars from microwave and conventional pyrolysis and gasification. In general, the conventional chars had greater pore structure development as seen by higher specific surface areas compared to microwave chars, possibly due to rapid thermal collapse of coal pore structure. Microwave pyrolysis chars had greater ordering of the carbon crystal structure for low-ash coals, while graphitization was inhibited for coals with high ash percentage yield. The observations from this study highlight the differences in char characteristics from microwave and conventional pyrolysis and gasification, which can aid microwave gasifier design, implementation, and integration for efficient syngas production and coal ash utilization.

Depositional settings of the Eocene Suhostrel bituminous coal, SW Bulgaria, inferred from organic petrology and molecular proxies

The paper reports results from organic petrological and geochemical characterization of Suhostrel bituminous coal deposit, SW Bulgaria. Three high ash (21–47 wt%) and thin (up to 0.6 m) coal seams, representing splits from a locally compact coal bed (∼1.8 m thick), were formed around the southern and south-western rim of the Padesh Graben during its Eocene evolution. High total organic carbon contents (avg. ∼86 wt%, daf), vitrinite reflectance (avg. Ro = 1.05%) and Tmax (avg. 457 °C) argue for high vol. A/medium vol. bituminous coalification rank. Maceral composition is dominated by vitrinite (avg. ∼ 95 vol%), accompanied by small amounts of liptinite (avg. ∼ 5 vol%). Apart from few occurrences of funginite, inertinite is not present. The organic matter is mostly represented by detrovitrinite and collotelinite, the latter generally occurring in the form of gelified leaf-derived tissues. Liptinite is mainly represented by cutinite and microsporinite and display no evidence for organic matter contribution from algae. Low TPI and VI indices indicate peat formation from vegetation with poor preservation potential. Insignificant contents of gelovitrinite, coupled with high GI indices denote greater importance of the geochemical processes for the gelification of the organic matter. Striking predominance of mid- and long-chain n-alkanes argues for peat formation from a mixture of aquatic macrophytes and terrestrial vegetational communities. Presence of oleanane, albeit in low concentration, provides robust argument for contribution from angiosperm plants, whereas absence of resin-derived sesqui- and diterpenoid hydrocarbons denotes insignificant role of conifers during peat accumulation. Reconstructed depositional settings indicate organic matter accumulation under marginal aquatic, marsh/fen-type peatland with meso- to rheotrophic characteristics (GWI > 0.5). High mineral matter contents, coupled with the abundance of siliciclastic grains, imply hydrologically active environment. Plant remains deposited under oxidizing conditions (Pr/Ph = 1.5–2.5), but because of high water table were likely subjected to frequent (perhaps seasonal) changes in Eh settings. The resultant limited aerobic biodegradation of the plant remains is evidenced by low concentrations of hopanoid biomarkers and their diagenetic degradation products – drimane-type sesquiterpenoids. Post-depositional marine influence and downward infiltrating sulfate-rich waters is considered responsible for the presence of dibenzothiophene derivatives in Suhostrel coal.All molecular maturity-related ratios (e.g. CPI, Ts/(Ts + Tm), βα/(αβ + βα) C30 hopanes, ββ/(ββ + αα) C29 steranes, MPI-1, etc.) are fully compatible with the bulk geochemical parameters (e.g., TOC, Tmax, Ro) and confirm the bituminous coalification rank of the coal. Based on this data, temperatures of ∼130–140 °C were likely reached during burial of the sediments in a zone, characterized by high thermal influx (geothermal gradient ∼43–47 °C/km).