Small Coal Particles Research Articles

Particle-scale analysis of thermophysical behaviors of coaxial gas swirling jets with binary-size particle mixture

Understanding the multi-physics coupling of coaxial-jet gas-particle swirling flow is crucial for practical applications. In this study, computational fluid dynamics coupled with the discrete element method is utilized to investigate the flow behavior of a coaxial jet composed of powder and coal within an annular pipe. The influence of particle diameter on particle-scale information is examined. When powder diameter is 450 μm, large particles display high Reynolds and Nusselt numbers, whereas small coal particles demonstrate elevated velocities and temperature. The variation in particle diameter affects momentum and energy exchanges at specific locations, but has a negligible impact on the radial velocity distribution and Nusselt number of powder particles. Specifically, increasing the particle diameter from 50 to 450 μm leads to a reduction in swirling velocity at the outlet of the annular pipe from 15.43 m/s to 9.57 m/s for binary mixtures, and from 18.49 m/s to 7.03 m/s for powders.

Study on coupling diffusion of composite dust and cloud-mist dedust technology in fully mechanized driving face of mixed coal-rock roadway

Aiming at addressing the problem of coal dust and rock dust mixed pollution in mixed coal-rock roadways, this study conducted a CFD numerical simulation on the coupling migration and pollution of coal dust and rock dust in the fully mechanized driving face of mixed coal-rock roadway (i.e., roadway section with both coal seam and rock stratum). Research findings: Small coal dust particles mainly migrate backward along the upper space of the roadway, while large coal dust particles mostly diffuse backward along the bottom of the air return side and settle evidently, with the concentration being higher than 1,000 mg/m3. The concentration of rock dust starts to decline significantly to about 350–800 mg/m3 in the middle and rear of the roadway. The concentration of coal dust is relatively high (200–500 mg/m3) in the front of the roadway, and it decreases gradually to around 175 mg/m3 in the middle and rear sections. With the increase of diffusion time T of rock dust and coal dust, the diffusion distance LD shows a linear increasing trend. Both rock dust and coal dust conform to the linear equation LD = 0.82 T + 11. After the application of the cloud-mist dedust technology, all the measuring points in the mixed coal-rock roadway witnessed a dust removal efficiency of above 75 %, which is a mark of effectiveness of the technology in improving the underground working environment.

Cleaning high ash coal waste from coking coal via froth flotation method

In this work, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and laser particle size methods were employed. The surface of the particle obtained from the cyclonic-static microbubble flotation column (FCSMC) concentrate was coated with numerous small particles when compared to the clean coal collected from the Denver flotation cell. Even though the mass recovery of FCMSC concentrate (51.13%) was approximately twice the mass yield of Denver flotation cell concentrate (26.66%), the ash content in FCSMC concentrate (33.07%) was higher than that of the Denver flotation cell concentrate (14.88%). This result can be attributed to the advantages of FCSMC in small coal particle flotation, and this resulted in the misplacement of coarser particles with lower ash content in the tailing. The highest mass recovery of 0.250–0.500 mm in magnesium ion solution was 95.39%, which was slightly high than that (92.84%) in calcium ion solution. However, the concentrate ash content with 0.250–0.500 mm coarse size fraction in magnesium ion solution was 11.41%, which was higher than that (11.14%) in calcium ion solution. Flotation performances of different size fractions in rougher-cleaner flowsheet indicated that flotation responses in calcium ion solution were better than that in magnesium solution.

Determining diffusion coefficients of coal particles by solving the inverse problem based on the data of methane desorption measurements

The diffusion coefficient is the key parameter been used to evaluate the methane diffusion behavior. Generally, the diffusion coefficient is determined by analytical solutions, including the unipore diffusion model and the bidisperse diffusion model. However, the essential assumptions limit the applicability and accuracy of analytical solutions. Inverse modeling has been proved to be useful in a variety of disparate fields. In this work, the detailed process and principles of using inverse modeling in gas diffusion are introduced. The effectiveness and accuracy of MMA, SNOPT and LM algorithms in determining the diffusion coefficient are analyzed. Results show that the inverse problem is a feasible method and the LM algorithm takes the shortest time and is recommended. The analytical solution (diffusion coefficient) of particles whose sizes are 0.074–0.2 mm, 0.25–0.5 mm and 0.5–1 mm are 13.8% times, 5.0% times and 1.6% times larger than that of the numerical solution, respectively. The numerical method has higher computing accuracy of small coal particles. As the current work is the first to present efficient techniques for the solution of diffusion coefficient-related inverse problems in COMSOL, the applicability of inverse modeling in complicated gas diffusion problems has also been prospected.

Investigating the effect of acid erosion on degradation and brittle fractures in coal using etching solutions

ABSTRACT With an increase in the application of deep coal mining, there has been a significant increase in the risk of rock bursts due to increased mine pressures. Thus, research is required on more effective methods of preventing and controlling rock bursts. In this study, three types of solutions were used to study the brittle fracture degradation of coal by acid leaching. The principle of acid leaching and the microscopic mechanical behavior characteristics of brittle fracture degradation were analyzed. The quantitative laws governing different acid leaching times and unloading values were then obtained. The results were as follows. (1) The iron ions in the coal were replaced by a citric acid solution. The scanning electron microscope (SEM) results of the damaged coal sample indicated slight bending of the coal surface, accompanied by the shedding of small coal particles. After immersion, the failure strain increased by 63.6%, and the failure response time increased by 324.1%. (2) After soaking the coal sample in a phosphoric acid solution, numerous bubbles were observed on the coal surface, and the permeability was significantly enhanced. The SEM results of the damaged coal revealed that the coal surface was divided into blocks after soaking, and the minerals in the coal precipitated and accumulated. After immersion, the failure stress decreased by 24.5%. (3) A mixed acid solution, comprising citric and phosphoric acids, resulted in the most significant corrosion effect. The SEM results of the damaged coal revealed that chemical reactions occurred after the soaking process, and minerals were precipitated. Furthermore, uniaxial failure was the most complete. In particular, the failure stress decreased by 49.8%, the failure strain increased by 81.8%, and the failure response time increased by 77.7%. The mixed acid solution enhanced the plastic characteristics of the coal, reduced its strength, and demonstrated favorable characteristics for coal pressure relief. Thus, the results of this study provide significant engineering advancements for chemical pressure reduction technology, which can be applied to deep coal mining.

Gas content of the D6 coal seam

The article deals with the issues of gas content of the most thick and stable D6 coal seam in the Tentek region. This complex structure seam is dangerous in underground mining for gas and dust outbursts, it consists of coal packs separated by interlayers of mudstones, while the lower layer 0.5-1.5 m thick is very soft, has a strong shear disturbance and is most saturated with methane. Extraction of coalbed methane is a necessary process to ensure the safety of mining operations, to reduce its emissions into the atmosphere, and to utilize it as a fuel and a product for obtaining synthetic materials. The regularity of changes in the particle size distribution of the upper thick pack and the lower thin pack indicates the difference in small coal particles in them, while there are much more of them in the lower layer therefore, the specific surface is larger, which is an important factor of the adsorption processes intensity in the accumulation of methane, and during the gas drainage from the seam. The activation energy of methane from carbohydrate has been determined. A quadratic relationship between methane gas evolution and its initial concentration has been shown. In carbohydrate, it depends on the energy of external forces. A complex relationship has been established between the gas pressure in the coal seam and its concentration and characteristics of the «coal-methanenatural moisture» system. Regularities of changing the methane content of the coal seam depending on its fracturing formed due to the effect of the energy of destruction and the energy appearing with increasing the area of a crack in the coal, have been obtained. The effect of the coal mineral composition on the gas content has been shown.

ОСОБЛИВОСТІ ТЕХНОЛОГІЇ ОСВІТЛЕННЯ ПОВЕРХНЕВИХ СТОКІВ ГІРНИЧОВИДОБУВНИХ ПІДПРИЄМСТВ

Introduction. In the process of coal mining and beneficiation, a large amount of wastewater is formed at enterprises. Sewage has a variety of origins, which can be classified into: natural (mine, quarry, drainage); technological (water for carrying out technological process, water for cooling of cars and devices); surface (water from rain, snow and maintenance of the enterprise); household (water is formed from showers, bathrooms, dining rooms). Wastewater from mines and factories is highly polluted and can have toxic effects on the environment. Problem Statement. Natural waters have a high rate of mineralization, salts of heavy metals, iron. Waters are polluted with organic matter, large and small particles of coal and rock. Surface and process effluents are contaminated with coal dust and petroleum products. Domestic waters are contaminated with surfactants, fats, animal and plant food residues, alkali, cadmium, nickel. Purpose. The aim is to study in detail the sources of pollution of mine effluents, to determine their composition and properties. Identify the seasonality of sources. Describe the method of water treatment and suggest possible improvements to existing technology. Materials and methods. The water must undergo some purification before it enters the environment. One of the main processes of wastewater treatment is filtration. This process is the most common and effective, so it should be part of the cleaning technology. The filter element can be a thin partition with pores or three-dimensional elements with a porous filler. This paper presents a technology with a granular filter, also called fast. The fast filter works on the principle of volume filtration, impurities are retained in the entire volume of the filter medium, in the pores and on the surface of the grains. Before filtration, the water is treated with coagulants. Types of granular materials: quartz sand, anthracite, expanded clay, expanded polystyrene, ceramic sand, mesoporous coal. Mesoporous coal is a promising material for filtration. Results. In the result of consideration of the composition and properties of wastewater, the appropriate technological scheme of mine wastewater treatment is selected and described. The technology provides purification from large impurities in open hydrocyclones, water treatment with flocculant, provides a filter and clarifier, enhanced purification from petroleum products in the oil trap and carbon filter. Filling the filter is sent to the coal warehouse, which is a cost-effective solution. Conclusions. The effluents of mining enterprises are saturated with pollutants and have a very aggressive composition, so it is unacceptable to release them into the environment without prior lighting. Should attention be paid to the purification of petroleum products that fall into the water as a result of the operation of machines and devices. As one of the possible methods, the technological scheme, improved by the department with oil trap and filter, is presented. Filter backfill is mesoporous coal, which is a very promising sorbent. Water treated with this technology can be used for recirculating water supply. This is justified not only by the economic aspect, but also by the environmental one. Keywords: mine drains, mining, pollution, petroleum products, filtration, technological scheme.

Indentation of bituminous coals: Fracture, crushing and dust formation

Bituminous coals are still the main source of energy in the world. However, these brittle porous materials are prone to crushing under action of industrial tools. Our early depth-sensing nanoindentation tests of bituminous coals showed that even if the depth of indentation is within the nanoscale, these brittle coals are no longer continuous elastic media within the indentation zone but rather fine powders of crushed particles irrespectively to the coal maceral. Dust formation of the materials is a problem of great practical importance for the mining industry. Indeed, the powders of coal particles formed during crushing may cause not only explosions in mines but they also contaminate the environment around the roads of the coal transportation. In this study, the crushing of coals due to action of rigid conical indenters and formation of small coal particles is investigated. The studies are based on development of the Galanov-Grigoriev (GG) model of crushing of brittle porous materials and the model adjustment to specific features of coal fracture. Using the indentation tests, we estimate the size distribution of the dust particles formed within the region of fully crushed material. Results are presented for coals of three different stages of metamorphism.

Спектральные характеристики свечения поверхности частиц каменных углей во время воздействия лазерных импульсов в режиме свободной генерации

In this work, we investigated the glow spectra of coal fractions with dimensions 63 μm directly during the action of neodymium laser pulses (120 μs). Depending on the radiation energy density H, the emission spectra have a different character. The glow at the minimum values ​​of the laser pulse energy density Hcr (1) is associated with the ignition of small coal particles (~ 1 μm) present in the fractions and the ignition of reactive microprotrusions on the surface of the larger coal particles. The glow spectra at this stage are of a non-thermal nature and are associated with the emission of molecules of volatile substances in the gas phase and the products of their oxidation. With an increase in the laser pulse energy density H, a thermal glow of the surface of larger coal particles is observed, which is described by the Planck formula at T = 3100 K. When H = Hcr (2) is reached, the surface of the coal particles is ignited during the action of the laser pulse. Contributions to the spectra are the glow of the surface of coal particles, emitted carbon particles, and the glow associated with the emission of excited molecules H2, H2O, CO2. With an increase in H> Hcr (2), the processes leading to the glow of coal particles during a laser pulse are similar to those described above for Hcr (2), but the glow intensity increases

Microbubble Modification of Coal Surfaces before Flotation

The flotation of coking coal after treatment of the coal pulp by microbubbles is investigated. Differential granulometric analysis indicates that, when using air microbubbles, the flotation of small coal particles (<50 μm) predominates. The effectiveness of flotation of small coal fractions (<0.1 mm) by microbubbles, without reagent, is comparable with that attained when using traditional apolar and complex reagents.

Effects of particle size on the self-ignition behaviour of a coal dust layer on a hot plate

Knowing the sizes of coal particles is vital for understanding the real ignition behaviour of coal dust deposits on a hot plate. Furthermore, such knowledge facilitates model construction for assessing industrial coal dust explosion risk. Three particle sizes (0.10–0.15, 0.15–0.20, and 0.20–0.30 mm) of coal samples were selected to analyse the influence of particle size on ignition and smouldering behaviour. Characteristic parameters, such as the minimum ignition temperature of a dust layer (MITL), highest temperature in a layer (HTL), ignition delay time, and heating rates were tested and analysed using a dust layer ignition apparatus. The results revealed that the MITL was 200–210 °C and that the MITL tended to decrease with particle size. The aggregation effect improved the MITL containing fine particles. The optimal ignition position was 7.2 mm from the hot plate. As the particle size increased, the ignition delay time of the sample became longer. However, the HTL and the time to reach the maximum temperature reduced accordingly. For corresponding geometrical shapes of a coal dust layer, the results demonstrated that particle size had a limited effect on the void fraction but had a considerable effect on the number of particles. The finer coal dust had a substantial number of particles, thus producing a larger overall specific surface area in which to oxidize and devolatilize. Small coal dust particles pose a severe hazard in industrial sites, and thus, sites should be cleaned regularly to avoid the possibility of unwanted fires or explosions.

Evaluation of coal conversion pathways in fluidized bed chemical looping combustion with oxygen uncoupling (CLOU)

Chemical looping with oxygen uncoupling (CLOU) is a variant of chemical looping combustion that uses oxygen carriers that release gaseous O2 in the fuel reactor to efficiently combust solid char. There are many different pathways by which fuel carbon can be converted in a CLOU system, including reaction of volatiles, oxidation of char and gasification of char followed by conversion of the gaseous products. Therefore, there is competition between volatiles and solid char for both gaseous and metal bound oxygen. This study presents an experimental method of analysis to better understand the conversion pathway and fate of fuel carbon in a fluidized bed reactor system. The analysis is applied to a Cu-based CLOU carrier in a bench-scale reactor, feeding either coal or coal char in small batches. The effect of coal particle size and reactor temperature on the relative contributions of different carbon conversion pathways is evaluated. Under the conditions studied, higher temperatures and smaller char particles were more effectively converted by gaseous O2 released by the CLOU carrier, resulting in less unconverted char. However, there was more unconverted volatiles exiting the reactor from small coal particles and the resulting char had a higher tendency to exit the fuel reactor unconverted. Larger char particles relied more on conversion by gasification and subsequent oxidation of combustible products. Results for other systems will depend on operating conditions and properties of the fuel, but conversion pathways can be systematically evaluated using the method described.

Effects of temperature gradient and particle size on self-ignition temperature of low-rank coal excavated from inner Mongolia, China.

This study investigates the effects of temperature gradient and coal particle size on the critical self-ignition temperature TCSIT of a coal pile packed with low-rank coal using the wire-mesh basket test to estimate TCSIT based on the Frank–Kamenetskii equation. The values of TCSIT, the temperature gradient and the apparent activation energy of different coal pile volumes packed with coal particles of different sizes are measured. The supercriticality or subcriticality of the coal is assessed using a non-dimensional index IHR based on the temperature gradient at the temperature cross-point between coal and ambient temperatures for coal piles with various volumes and particle sizes. The critical value IHRC at the boundary between supercriticality and subcriticality is determined as a function of pile volume. The coal status of supercritical or subcritical can be separated by critical value of IHR as a function of pile volume. Quantitative effects of coal particle size on TCSIT of coal piles are measured for constant pile volume. It can be concluded that a pile packed with smaller coal particles is more likely to undergo spontaneous combustion, while the chemical activation energy is not sensitive to coal particle size. Finally, the effect of coal particle size on TCSIT is represented by the inclusion of an extra term in the equation giving TCSIT for a coal pile.

Diffusion effect and evolution of kinetic parameters during coal char-CO2 gasification

Large coal char particle (LCC, 12 mm and 20 mm cube) and small coal char particle (SCC, less than 50 μm) were gasified under different temperatures in fixed bed reactor and thermogravimetric analyzer, respectively. The effectiveness factor and kinetic parameters (apparent activation energy and apparent pre-exponential factor) were also calculated to investigate the diffusion effect and the evolution of kinetic parameters during coal char-CO2 gasification. It was found that the internal effectiveness factor was not constant during the conversion process of coal char. In the intermediate and later stage of gasification reaction, there was a remarkable increase for the effectiveness factor. The effectiveness factor of LCC and SCC in the higher temperature range were all much less than 1, this indicated that the gasification of LCC and SCC at high temperature was limited by pore diffusion. Apparent activation energy and apparent pre-exponential factor evolved gradually during the whole conversion process, the existence of diffusion effect would lead to a certain change for the evolution law. A kinetic compensation effect was observed between the apparent activation energy and apparent pre-exponential factor during the conversion process of coal char. When the diffusion effect was very significant, it would have a certain weakening effect on the compensation effect.

Influence of Small Coal Particles in Coking Batch on Coal-Tar Quality

Traditional concepts regarding the influence of the entrainment of coal particles in coking on the quality of coal tar may usefully be expanded. In addition to increase in the ash content, the water content of the tar is increased. To eliminate stabilization of the emulsion forming in the condensation department, a limit of 10–13% has been imposed on the content of coal in the ≤0.2 mm class. Analysis of production data shows that the presence of coal particles smaller than 0.071 mm in the batch increases the density of the coal tar and its content of quinoline-insoluble materials. It is found that the water content of the tar is increased more strongly by the aromatic component of the coal than by the mineral component. The emulsifying properties of dusty coal are greatest for Zh coal and least for G coal.

Moisture transport during contact sorption drying of coal fines

ABSTRACTStudies have shown that contact sorption drying is a feasible technique to dewater fine (+0.15mm–0.5mm) and ultra-fine (−0.15 mm) coal. The aim of this article is to further investigate the drying mechanism involved. It could be shown that moisture transfer occurred primarily in the liquid phase, requiring direct contact between the coal and sorbent particles to facilitate the movement of the water. Moisture transfer was enhanced by operating with a higher sorbent to coal mass ratio. When drying smaller coal particle size ranges, the resistance to liquid transport decreased, leading to better desorption results. These improvements were mainly as a result of a greater contact area between the sorbent and coal particles.

On factors affecting coalbed gas content measurement

Coalbed gas content measurement is important for both unconventional gas recovery and mining safety. This study experimentally discussed factors affecting coalbed gas content measurement. It is found that small coal particles demonstrated high desorption rate at the early stage of desorption tests. However, the difference in methane desorption rate reduces when the coal particle size is more than 1.0 mm, indicating that a threshold of coal particle size exists in methane desorption in coal. Also, the particle size distribution tends to be stable when coal sample mass is over 15 g, stabilizing at around 1.75 mm. Therefore, the coal sample over 15 g is recommended to use in desorption tests of coalbed gas content measurement. The temperature and gas pressure can enhance the desorption rate, especially at the early stage of desorption. This temperature effect, however, mainly occurred in the first 15 min of desorption test. After 30 min, the desorption rate tends to be stable, slightly changing from around 0.35 mol/kg/min to 0.75 mol/kg/min. Due to endothermic process, absolute isothermal condition does not exist in desorption test, but this temperature variation mainly occurred in the first 15 min. The results also show incorporating the desorption data from the time of exposure to 13 min of desorption tests is essential to typically reflect methane desorption kinetics in coal. The study also shows that the measurement difference in gas content between the case with sample being air-puffed and -flushed and the case with sample exposed to the air is small. Therefore, the air-puffing and flushing during the sample collection has an insignificant effect the gas contact measurement result. This study can help to advance the understanding of impact factors on coalbed gas content measurement method.

Understand solids loading effects in a dense medium cyclone: Effect of particle size by a CFD-DEM method

Industrial cyclones, such as gas, hydro and dense medium, are widely used in chemical, mineral and process industries to separate particles from fluids or classify particles by size, density or other solid properties. It is well known that the loading of particles can significantly affect fluid flow in such cyclones but the specific effect can be confusing due to a limited fundamental understanding of the working mechanisms involved. In this work, the problem is partially tackled by analysing the influence of particles of different size on the medium flow in a dense medium cyclone (DMC) by using a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM). In the CFD-DEM model, CFD is used to model the motion of slurry medium by numerically solving the local-averaged Navier-Stokes equations at a computational cell scale, facilitated by the Volume of Fluid (VOF) and Mixture multiphase models. DEM is then used to model the motion of discrete particles by applying Newton's laws of motion, facilitated by a coarse-grained (CG) method to model small coal particles. It is found that both the particle and medium flows are sensitive to the particle size. The most notable finding is that spatial distributions of solid flow patterns form due to particles of different size have different trajectories in the cyclone. These lead to significant different distribution of volumetric particle-fluid interaction forces which cause different effects on the fluid flow. The findings are useful for developing a better understanding the working mechanisms of solids loading effects in DMCs and also in other similar swirling multiphase flow systems.

The oxygen evolution during pyrolysis of HunlunBuir lignite under different heating modes

HulunBuir lignite pyrolysis at low temperatures with different coal particle sizes, and different heating modes were studied on a fixed-bed reactor in order to investigate the oxygen evolution characteristics. The results show that pyrolysis temperature heavily influences the oxygen evolution. Almost 70.69wt% of oxygen in lignite transferred into volatile products at 700°C. With an increase of temperature, the proportion of oxygen transferred into pyrolysis water (water-O) and gas (gas-O), while the proportion of oxygen in tar (tar-O) reached a maximum of 2.26wt% at 700°C. The amount of carbonyl groups and ether linkages reached their maxima in char at 600°C and 650°C, respectively. After temperature-programmed pyrolysis and isothermal pyrolysis, there was more oxygen in the chars collected from the smaller coal particles, while after solid heat carrier pyrolysis, the coal particle size had little impact on the oxygen amount in chars (Char-O). At the same pyrolysis temperature, heating rate was determined to be one of the main factors that affecting the oxygen evolution.

Investigation on heating behavior during the preparation of SiC crystals by microwave sintering

Abstractβ‐SiC powders were synthesized by microwave sintering at 1400‐1850°C for 60 minutes with coal minerals. Raw materials were prepared with a sol‐gel method by coating SiO2 on coal mineral particles, and microwave sintering was conducted in a microwave chamber with TE666 resonant mode. Heating rates were controlled by input microwave power and heating behavior during the microwave sintering was also investigated. XRD, SEM, Raman, TEM techniques were carried out to characterize samples. It was found that different particle size of coal mineral particles gave rise to different heating effect, leading to different growth mechanisms of SiC crystals. This might be the unique features for microwave sintering. Particular SiC crystals dominated for small coal mineral particles, which attributed to microwave plasma effect. While SiC whiskers or fibers dominated for large coal mineral particles, which corresponded to microwave coupling effect. Microwave coupling effect prevailed for low heating rate which was apt to form particular SiC crystals. Whereas, higher heating rate enhanced microwave plasma and primarily resulted in SiC whiskers or fibers.