Raw Coal Research Articles

Effect of louver baffle on the stability and separation performance of the gas-solid separation fluidized bed

Bed density stability is often the dominant factor restricting the efficiency of coal beneficiation in gas-solid separation fluidized beds (GSSFB). Here, in order to improve the fluidization quality, a louver baffle was introduced into a conventional GSSFB, converting it into a so-called louver baffle separation fluidized bed (LBSFB). The influence of the louver baffle on the fluidization characteristics and coal separation efficiency of the GSSFB were investigated using a combination of experiments and numerical simulations. The minimum fluidized gas velocity, pressure drop, and cross section density of the bed were measured when baffle was placed at two different positions (H=40 mm and H=80 mm). Meanwhile, the evolution of bubble generation and particle flow were simulated when the baffle was placed at H= 40 mm and the operating gas velocity was 7.52 cm/s. The results showed that the use of a single guide baffle could effectively break up the bubbles in the bed, inhibit bubble growth and particle backmixing, and restore the uniform distribution of the updraft. When the louver baffle was installed at a height of H = 40 mm and the superficial gas velocity was 7.52 cm/s, the standard deviation of radial density fluctuation in the bed was reduced to 0.028 g/cm3, which is more than 35% lower than that of the conventional GSSFB, and that of the axial density fluctuation was reduced to 0.146 g/cm3. Under these conditions, the separation of 6–13 mm raw coal in a LBSFB resulted in a clean coal yield of 69.97% and a probable error, E, value of 0.082 g/cm3

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.

Effect of Secondary Oxidation of Pre-Oxidized Coal on Early Warning Value for Spontaneous Combustion of Coal

The indicative ability of a gas indicator for the spontaneous combustion of coal is affected by the secondary oxidation of oxidized coal, from old goafs, entering a new goaf through air leakages. This phenomenon can affect the accuracy of early warning systems regarding the spontaneous combustion of coal in a goaf. In this research, three kinds of coal were selected to carry out a spontaneous combustion simulation experiment in which a temperature-programmed experimental device was used to analyze the behavior of the index gas towards raw coal and oxidized coal, for which the latter was oxidized at 70 °C, 90 °C, 130 °C, and 150 °C. The results show that the chain alkane ratio in the secondary oxidation process and the trends of oxygen, CO, and C2H4 concentrations are the same as those in the primary oxidation process. On the other hand, the temperature at which C2H4 initially appears, during secondary oxidation, is lower than in primary oxidation. The CO produced in the early stage of secondary oxidation is greater than the CO produced, at the same temperature, in primary oxidation. In this regard, the usage of C2H4 concentration as an indicator with which to judge the occurrence of the spontaneous combustion of coal would allow for an earlier response. In the secondary oxidation process, the temperature of the extreme value of the alkene ratio appears higher than in primary oxidation. The presence of a higher pre-oxidation temperature and a higher proportion of secondary oxidation gas will affect an indicator’s judgement when the primary oxidation enters the severe oxidation stage. The gas produced by secondary oxidation will affect the early warning of the spontaneous combustion of coal in the coal mine goaf, which should be considered in the establishment of an early warning system.

Combustion conditions and feed coals regulating the Fe- and Ti-containing nanoparticles in various coal fly ash.

Quantitative characteristics and sizes of nanoparticles (NPs) in coal fly ash (CFA) produced in coal-fired power plants as a function of coal type and plant design will help reveal the NP emission likelihood and their environmental implications. However, little is known about how combustion conditions and types of coal regulate the NP abundance in CFAs. In this study, based on single particle (SP)-ICP-MS technology, particle number concentrations (PNCs) and sizes of Fe- and Ti-containing NPs in CFAs were determined for samples collected from power plants of different designs and burning different types of coal. The PNCs of Fe- and Ti-containing NPs in all CFAs measured were in the range of 1.3×107 - 3.4×108 and 6.8×106 - 2.2×108 particles/mg, with the average particle sizes of 111nm and 87nm, respectively. The highest Fe-NP PNCs likely relate to the highest contents of Fe and pyrite in the feed coal. In addition, high TOC in CFAs are associated with metal-containing NPs, resulting in elevated abundances of these NPs with relatively large sizes. Moreover, elevated PNCs of NPs were found in CFAs produced by coal-fired power plants burning low-rank coals and with small installed capacity (especially those under 100-MW units). Compared to cyclone filters, ESPs and FFs with higher removal efficiency typically retain more Fe-/Ti- containing NPs with smaller sizes. Based on a structural equation (SE) model, raw coal properties (coal rank and Fe/Ti content), boiler types, and efficiency of particulate emission control devices likely indirectly affect PNCs of Fe- and Ti-containing NPs by influencing TOC contents and their corresponding metal concentrations of CFAs. This study provides the first analytic and comprehensive information concerning the direct and indirect regulating factors on NPs in various CFAs.

Epidemiology of coal miners' pneumoconiosis and its social determinants: An ecological study from 1949 to 2021 in China.

Pneumoconiosis is the most widely distributed occupational disease worldwide. China is currently the largest coal producer and consumer and the country with the most coal miners and cases of coal workers' pneumoconiosis (CWP). Despite more than 70 years of effort, the problem of CWP and silicosis remains serious. There is a lack of analysis of direct data on coal miners' pneumoconiosis from all over the country. This study aimed to describe the epidemiology of coal miners' pneumoconiosis and reveal some important clues regarding its social determinants. The annual incidence rate, 20-year prevalence rate, and incidence rate of coal miners' pneumoconiosis per million tons in China from 1949 to 2021 were calculated by using the data of annual number of coal miners' pneumoconiosis diagnosed and reported from the coal mining and dressing industry, the number of coal miners, and the raw coal production, and the relationship between the incident cases of coal miners' pneumoconiosis and the death toll from coal mine safety accidents was analyzed using Pearson correlation analysis, with the aim of exploring the relationship between the incident cases of coal miners' pneumoconiosis and its social determinants with an ecological study. From 1949 to 2021, there have been more than 462,000 patients with coal miners' pneumoconiosis in China, showing double U-shaped distributions with an increasing trend, accounting for about 50.5% (462,000/915,000) of all diagnosed pneumoconiosis in China, while the incidencet rate of coal miners' pneumoconiosis presents a large W shaped distribution with three peaks over a time span of more than 50 years. From 1949 to 1986, there was a strong correlation between the incident cases of coal miners' pneumoconiosis and raw coal production, the number of coal miners, and the number of deaths from coal mine accidents (r = 0.849, P<0.001; r = 0.817, P<0.001; r = 0.697, P<0.001, respectively), but there was no such correlation found from 1987 to 2006. It was estimated that the annual incidence rate of coal miners' pneumoconiosis in China from 2016 to 2020 was 3.4‰ (95% CI: 2.6-4.3‰), and the prevalence rate across the recent 20-year observation period was 4.8% (95% CI: 4.6-4.9%), both measured at the peak or around the peak over the 70 years. In particular, 1963, 1986, 2006, and 2009 were the four important turning points in time. There was a sustained high level of incident cases of coal miners' pneumoconiosis with double U-shaped curve in China, which may be affected by a variety of social determinants and risk factors.

Experimental study on pressure wave-induced explosion of different types of deposited coal dust

When the gas explosion occurs in a coal mine, the pressure wave will induce coal dust (CD) to participate in the explosion reaction, which will further expand the accident and eventually cause more serious casualties. Based on this hazard, experiments were performed on three representative types of CD with the aid of a self-built experimental pipeline for gas/CD explosion. The explosion characteristics under the condition that pressure waves entrain different CD to participate in the explosion were studied. Furthermore, the influence of volatile content in CD on the compound explosion was analyzed from a microscopic perspective. The following beneficial conclusions were obtained: The overpressure peak value and the flame propagation velocity in the compound explosion are positively correlated with the volatile content in CD. Under those three conditions, the maximum overpressure is obtained at P2 position, and obvious stratification occurs in the process of flame propagation. However, compared with the other two working conditions, the average flame propagation speed of the composite explosion system with anthracite decreased by 51.2% and 52.6%, respectively, and flame surface breaks and becomes discontinuous earlier. After the explosion, the volatile content of three kinds of CD is significantly reduced, and the volatile content of lignite, bituminous coal and anthracite is 75.6%, 75.7% and 44.7% of the industrial analysis value of raw coal, respectively. With reference to the analysis on the microscopic test results, the amount of volatile gas decomposed from volatiles determines the intensity of the homogeneous reaction in the whole combustion process, while the coke formed after the reaction determines the intensity of the non-homogeneous reaction. Therefore, the volatile content in CD is a key factor affecting CD explosion. The research results provide an important scientific and theoretical basis for the prevention, control and reduction of CD explosion disasters.

Thermal Behavior and Kinetics of Shenmu Coal Pyrolyzed under Hydrogen-Rich or Methane-Gas-Rich Atmosphere

The pyrolysis characteristics of Shenmu coal under an atmosphere containing H2 (40%) and another containing CH4 (40%) were studied via a thermogravimetric analysis, and the kinetic parameters of pyrolysis were calculated by using a distributed activation energy model (DAEM). The results showed that H2 promoted the cleavage of CH-like functional groups by providing reactive hydrogen groups to combine with CH groups and –OH groups in the coal. However, the H2 and CH4 atmosphere inhibits the cleavage of oxygen-containing functional groups such as carbonyl groups and C–O groups, and this is unfavorable to the production of CO2 and CO. The pyrolysis weight-loss rate of raw coal decreases with the increase of the heating rate, and the weight-loss curve shifts to the high-temperature region. At the same conversion rate and pyrolysis temperature, the activation energy of pyrolysis under the H2 and CH4 atmospheres is lower than that under the N2 atmosphere, and the activation energy does not conform to the Gaussian distribution. The activation energy of pyrolysis was distributed in a narrow range, in which the activation energy of the H2 and CH4 atmospheres was concentrated in the range of 175–215 kJ/mol and 225–230 kJ/mol, respectively, and the activation energy of the H2 atmosphere was lower than that of the CH4 atmosphere under the same conversion rate.

Sources and succession of microorganisms in industrial coal flotation system

The water system in a coal preparation plant harbors a variety of microorganisms, which may pose a great effect on coal recovery, however, usually be neglected. In this study, high-throughput sequencing and the growth of indigenous microorganisms in coal slurry containing organic reagents were conducted to trace the sources and succession of the microorganisms in a running plant, and the reason for microbial community succession was deeply explained. The microbiological analysis results found that the microorganisms in the coal slurry are assigned to 34 phyla and 98 genera, all source from the coal seam. Proteobacteria, Acidobacteria, Actinobacteria, and Crenachaeota are the dominant microorganisms at the phylum level, and the abundance of Proteobacteria, Bacteroidetes, and Nitrospirae in the flotation feeding decrease, and the abundance of GAL15 increase compared to the raw coal and mine water samples. Environmental factors are important reasons for microbial community succession. Organic reagents such as collectors and frothers commonly used in coal flotation processes can change the growth environment of microorganisms. The strain QX4, isolated from raw coal and identified as Bacillus sp. can accelerate growth in coal slurry containing organic reagents. Compared with raw coal, the content of chemical oxygen demand (COD) and dissolved oxygen (DO) in the flotation feeding increase by 340 mg/l and 5 mg/l respectively, which inhibit the metabolism of Nitrospirae, Bacteroides, Proteobacteria, and Actinobacteria, while improving the metabolism of GAL15, and change the microbial community and promote the microbial succession in the coal flotation system. Therefore, indigenous microorganisms from coal seam can grow and succeed in the coal flotation system, which has a potential effect on coal flotation efficiency.

Fractal Dimension and Nuclear Magnetic Resonance Characteristics of Surfactants for Coal Gas Desorption

In this paper, the fractal dimension of coal surfaces was calculated using the fractal theory, and the influence of different surfactants on the fractal dimension of coal surface was studied. Six kinds of sustainable and traditional surfactants used in coal gas desorption processes were compared and analyzed. We used mineral gas fertilizer coal from Huainan Liuzhuang, China, as the research object and studied sodium dodecyl benzene sulfonate (SDBS), cohol diethanolamide (CDEA), nonylphenol polyoxyethylene ether (NP-10), tea saponin, sucrose ester and rhamnolipid surfactants used to modify coal. The methane adsorption capacity of coal before and after surfactant modification was measured by low field nuclear magnetic resonance methane adsorption, and Langmuir volume and Langmuir pressure were obtained according to the Langmuir fitting equation. The results showed that from the perspective of fractal dimensions, the best surfactant in the context of sustainability (in order) is rhamnolipid, CDEA, tea saponin, sucrose ester. According to the two important parameters VL and PL in the Langmuir equation, the coal sample modified by sucrose ester had the strongest gas adsorption capacity. According to the numerical analysis of the surface fractal dimension DS of the coal modified by surfactants, the soluble organic matter in the raw coal samples dissolved, and the micropore morphology evolved to become mesoporous or macroporous, which is more favorable for desorption. The coal samples modified by rhamnolipid and SDBS had the strongest gas desorption ability.

Kinetic characteristics of desorption and diffusion in raw coal and tectonic coal and their influence on coal and gas outburst

The basic physical parameters and gas desorption characteristics of raw coal and tectonic coal composite seams in northern China were tested, and the gas diffusion kinetic characteristics of raw coal and tectonic coal were investigated. The difference in gas hazard between the raw coal seam and tectonic coal seam was analysed based on this. The results found that: under the same conditions, the adsorption capacity, initial gas discharge rate, accumulated gas desorption volume, initial gas desorption rate and effective diffusion coefficient of tectonic coal are higher than those of raw coal, while Protodyakonov’s coefficient of tectonic coal is much smaller than that of raw coal. This, together with the lower permeability of tectonic coal seam, easily leads to the formation of a gas enrichment zone in tectonic coal seam easily and triggers coal and gas outburst when the external stress environment changes. Under the same conditions, raw coal is more sensitive to changes in initial gas adsorption equilibrium pressure, and the development of cleats and fractures of raw coal is higher than that of tectonic coal, which makes the gas in the area of raw coal easier to extract than that in the area of tectonic coal. When carrying out gas control in the composite coal seam of raw coal and tectonic coal, it is necessary to consider the gas desorption and diffusion characteristics in the raw coal and tectonic coal and to develop precise coal seam gas control measures based on the principles of safety and economy. The results of this study are of great relevance to the prevention and control of coal and gas outburst disasters in raw coal and tectonic coal seams to ensure the safe and efficient mining of coal resources.

Effect of acidification on microscopic properties and pore structure of coal

Acidification is a common method for removing minerals from coal. In this study, four groups of acidification samples were evaluated: raw coal (Raw), hydrofluoric acid (HF), hydrochloric acid (HCl), and mixed HF–HCl acid (Mix) groups. The concentrations of ash and various elements in the sample groups were measured by industrial and elemental analyses. The Mix solution most effectively removed the minerals from coal, with an ash removal fraction of 87.87%. The microscopic characteristics of the four groups were studied using Fourier-transform infrared (FT-IR) and carbon nuclear magnetic resonance spectroscopy (13C NMR). The oxygen-containing functional groups of the coal samples increased after acidification by 76.07%, 14.49%, and 51.06% for the HF, HCl, and Mix groups, respectively. In addition, the content of aliphatic structures decreased during acidification. The adiposity of the Mix, HCl, and HF groups decreased by 16.36%, 8.50%, and 12.73 %, respectively. The results of CO2 and liquid nitrogen adsorption showed that the cumulative pore volume of the HF and Mix groups increased by 24.90% and 21.39%, respectively, while that of the HCl group decreased by 3.23% compared to the Raw group. The changes in the coal microstructures were observed for both micropores and mesopores. The aliphatic content was inversely proportional to the pore volume, whereas some oxygen-containing functional groups enhanced the pore volume. These findings contribute to the body of knowledge related to coal-seam acidification.

Identification of Factors Determining Household PM2.5 Variations at Regional Scale and Their Implications for Pollution Mitigation.

Indoor PM2.5, particulate matter no more than 2.5 μm in aerodynamic equivalent diameter, has very high spatiotemporal variabilities; and exploring the key factors influencing the variabilities is critical for purifying air and protecting human health. Here, we conducted a longer-term field monitoring campaign using low-cost sensors and evaluated inter- and intra-household PM2.5 variations in rural areas where energy or stove stacking is common. Household PM2.5 varied largely across different homes but also within households. Using generalized linear models and dominance analysis, we estimated that outdoor PM2.5 explained 19% of the intrahousehold variation in indoor daily PM2.5, whereas factors like the outdoor temperature and indoor-outdoor temperature difference that was associated with energy use directly or indirectly, explained 26% of the temporal variation. Inter-household variation was lower than intrahousehold variation. The inter-household variation was strongly associated with distinct internal sources, with energy-use-associated factors explaining 35% of the variation. The statistical source apportionment model estimated that solid fuel burning for heating contributed an average of 31%-55% of PM2.5 annually, whereas the contribution of sources originating from the outdoors was ≤10%. By replacing raw biomass or coal with biomass pellets in gasifier burners for heating, indoor PM2.5 could be significantly reduced and indoor temperature substantially increased, providing thermal comforts in addition to improved air quality.

Research on sustainable development evaluation and improvement path of resource-based cities based on coupling of emergy and system dynamics.

The evaluation of the sustainable development of resource-based cities is still one of the hotspots in today's social research. Taking Jining, Shandong Province, as the research object, this work combines a relevant emergy evaluation index system with system dynamics, establishes a resource-based city emergy flow system dynamics model, and studies sustainable development path in the next planning year. In the work, the key factors affecting the sustainable development of Jining are obtained through the coupling of regression and SD sensitivity analysis, and some scenarios are set up by combining them with the local 14th Five-year plan. Besides, the appropriate scenario (M-L-H-H) for Jining's future sustainable development is chosen in accordance with regional circumstances. That is, during the 14th Five-year Plan period, the appropriate development ranges for the growth rate of social fixed assets investment, the growth rate of raw coal emergy, the growth rate of grain emergy and the reduction rate of solid waste emergy are 17.5-18.3%, - 4.0 to - 3.2%, 1.8-2.6% and 4-4.8%. The methodology system constructed in this article can serve as a reference for similar studies, and the research findings can aid the government in formulating pertinent plans for resource-based cities.

A Comprehensive Assessment of Clean Coal Fuels for Residential Use to Replace Bituminous Raw Coal

Residential coal combustion is a major source of air pollution in developing countries, including China. Indeed, precisely measuring the real-time emission of major air pollutants is often challenging and can hardly be repeated at a lab-scale. In this study, for the first time, two clean coals initiated from raw bituminous coal were burned for real-time estimation of air pollution characteristics and their thermal efficiencies in different stoves. Moreover, thermodynamic equilibrium simulations were investigated for slagging parameters using Factsage 7.1 at reaction temperature 800~1600 °C. Results revealed that the firepower of clean coals (Briquetted coal and Semi-coke) was much higher (2.2 kW and 2.1 kW) than raw coal (1.8 kW) in a traditional stove. However, the thermal efficiencies were remarkably increased (13.3% and 13.5%) in an improved stove for briquetted coal and semi-coke, respectively. The emission of major air pollutants including carbon monoxide (CO), sulfur dioxide (SO2), particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), non-methane hydrocarbons (NMHCs) from both coal and semi-coke was significantly reduced. Thermodynamic equilibrium calculations indicate that briquetted coal is not susceptible to slagging under the reaction conditions in the household stove. The current study provides guidance for the selection of alternative and efficient clean coal fuels in rural areas for household purposes coupled with public health and safety.

Change of the Petrographic Characteristics of Semi-Coke in the Iron Ore Sintering Process.

This study focuses on the petrographic analysis method to evaluate semi-coke and its combustion behavior in the sintering process, which has been seldom conducted before. Semi-cokes are different in morphological features, porosity, pore structure, and wall thickness, because of differences in the vitrinite and inertinite of the raw coal. Semi-coke displayed isotropy, and even after the drop tube furnace (DTF) and sintering process, it still retained its optical properties. Eight kinds of sintered ash were observed using reflected light microscopy. Petrographic analyzes for the combustion properties of semi-coke were based on its optical structure, morphological development, and unburned char. The results indicated that microscopic morphology was an important characteristic when trying to understand the behavior and burnout of semi-coke. These characteristics can be used to trace the origin of the unburned char in fly ash. The unburned semi-coke mostly existed in the form of inertoid, mixed dense and mixed porous. Meanwhile, it was found that most of the unburned chars were melted into sinter, resulting in inefficient fuel combustion.

Hybrid nanoarchitectonics of coal-derived carbon with oxidation-induced morphology-selectivity for high-performance supercapacitor

Coal-derived porous carbon with a large specific surface area is a common electrode material for supercapacitors. Its deep and branched micropores, dense bulk morphology and amorphous structure have greatly limited its practical applications. Herein, hybrid carbon materials were obtained from coal through oxidation followed by activation. The method allows tuning the morphology, porosity, structure, and the degree of graphitization. The pre-oxidation with KMnO4 can break raw coal into small hydrocarbon fragments, which deposit and grow on the surface of generated MnO during pyrolysis leading to hybrid carbon with mesoporous and graphitic nanostructures. Meanwhile, homogeneous etching of the carbon skeleton by the reaction intermediate of K2CO3 led to the formation of abundant active sites. Hence, the optimized sample exhibited a high capacitance of 333 F g−1 at 1 A g−1, an excellent rate capability with 58% capacitance retention at 100 A g−1 and superior cycle durability in a three-electrode system. Besides, an assembled symmetric two-electrode device displayed a high energy density of 8.9 Wh·kg−1 at 250 W·kg−1. This work proposed a facile and rational synthesis strategy by balancing the tradeoff between active sites and intrinsic conductivity and thus provided a new avenue for the value-added utilization of coal.

Differentiation of Rare Earth Elements in Coal Combustion Products from the Handan Power Plant, Hebei Province, China

As a potential source of REY (rare earth elements and yttrium), coal and its products have attracted much attention. In this paper, we aimed to study the enrichment and differentiation of rare earth elements in fly ash with different particle sizes and promote the full recovery and utilization of rare earth elements in fly ash. Our objective was to focus on the REY concentration in feed coal and its combustion products from the Handan Power Plant. We particularly focused on the distribution of REY in relation to different particle sizes, as well as on the state of occurrence (affinity), by applying stepwise chemical extraction and performing examinations using inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that the REY content is affected by the mineral composition of coal ash, and REY is more easily enriched in slag and fine ash than coarse ash. In general, the REY content decreased with the decrease in particle size in coarse ash, whereas the REY content increased with the decrease in particle size in fine ash. It was found that the concentration distribution of REY in solid combustion products is as follows: light REY (LREY) &gt; medium REY (MREY) &gt; heavy REY (HREY). This indicates that the enrichment of REY in solid coal products decreases with the increase in atomic number. The results also show that the occurrence state of REY in raw coal and coal ash undergoes no obvious change (residue state &gt; organic/sulfide-bound state &gt; iron–manganese-oxide-bound state &gt; carbonate-bound state &gt; exchangeable state).

Macerals particle characteristics analysis of tar-rich coal in northern Shaanxi based on image segmentation models via the U-Net variants and image feature extraction

To investigate the particle characteristics of tar-rich coal macerals before separation, this study focuses on the tar-rich coal in northern Shaanxi, China, as the research object. The image segmentation models of U-Net variants (Mobile-Unet, VGG-Unet, Res-Unet, and TransUNet) are combined with OpenCV feature extraction to systematically study the particle morphology, particle size, liberation characteristics, and density separation process of coal macerals. The results indicate that the morphology of vitrinite tends to an olive-like shape as the particle size decreases, whereas the morphology of inertinite is complex and changeable. Further, the particle sizes of raw coal, vitrinite, and inertinite range from 0.09 mm to 0.075 mm, and their morphology uniformity is the best in this range. The statistical difference between the grid calculation point method (GCPM) and the pixel area statistics method (PASM) is mainly due to the particle size uniformity. When the particle size is less than 0.03 mm, the error is less than 3.0 %. By narrowing the range of the liberation densities and separating them preferentially, samples within the range of the unliberated densities are returned to the regrinding process to separate and enrich the vitrinite and inertinite groups in coal, which effectively improves their purity and recovery. In summary, Res-UNet, TransUNet, and the PASM not only have considerable potential for the qualitative analysis of maceral compositions and their liberation types but also provide powerful computer-assisted measurement to achieve reasonable separation and utilization of coal macerals.

Pure oxygen condition forced oxidation ignition process of underground coal gasification

AbstractUnderground coal gasification without shaft is a technology that uses directional drilling technology to gasify deeply buried coal. The technology first needs to ignite the coal seam during drilling, but as the depth of the coal seam (drilling) increases, it is very difficult to ignite the underground coal seam. In order to ignite the underground coal seam more safely and effectively, the forced oxidation ignition technology has been developed; that is, the coal seam is heated by hot oxygen, and the coal seam is rapidly oxidized and released to ignite the coal seam. Due to the complex confidential environment, it is challenging to ignite underground coal seams. To ignite the underground coal seams more safely and effectively, this paper used a self‐made reactor to simulate the forced convection heating of the bituminous coal under different oxygen flow rates with hot oxygen and used a Fourier transform infrared spectrometer flue gas analyzer to detect the outlet gas, used ATR‐FTIR spectrometer to analyze the solid residues in different areas, and recorded the time and maximum temperature of coal ignition under other processes. The results showed that low‐flow high‐temperature oxygen could quickly burn coal seams. When the oxygen flow rate was 2.5 m3/h, and the coal seam was ignited when heated to 130°C for 170 min. Through GC‐MS analysis of nine flammable components in the tar components under different processes, the content of short‐chain aliphatic hydrocarbons in the tar decreases, and the content of benzene series tends to increase. Through ATR‐FTIR analysis, the solid sample near the gas injection port contained fewer oxygen functional groups; the middle of the furnace body included the most oxygen functional groups; near the gas outlet contained more oxygen functional groups than the raw coal. CO2 would rise to 3% when coal was about to ignite.

Combustion performance of lignite after efficient upgrading via deep eutectic solvent‐assisted hydrothermal dewatering

AbstractIn order to increase the efficiency of hydrothermal dewatering (HD), a deep eutectic solvent (DES, synthesized by ChCl and ZnCl2) was employed to promote the removal of moisture and oxygen from lignite. The variation of physicochemical structure of upgraded coals greatly affected the combustion performance. The effects of addition amount of DES, upgrading temperature, and the molar ratio by ChCl and ZnCl2 on combustion performance of upgraded coals were analyzed via thermogravimetric analysis. Compared with HD lignite upgraded at 280°C, the combustion curves of DES‐HD lignite were shifted and delayed toward a higher temperature zone as DES additions increased. DES‐HD lignite had lower spontaneous combustion tendency and higher combustion reactivity. When upgrading temperature increased from 250 to 300°C, the combustion curves of lignite upgraded with 3.0‐g DES (ChCl:ZnCl2 = 1:1) first moved to the low‐temperature zone and then shifted to the high‐temperature zone. DES‐HD lignite maintained the advantages of raw coal in terms of combustion characteristics. For lignite upgraded with different molar ratios of ZnCl2 in DES, the difference in combustion behavior was mainly manifested in the burnout stage. The burnout stage was advanced slightly as the molar ratio of ZnCl2 increased.