Coal Breakage Research Articles

Grinding Behaviors of Components in Heterogeneous Breakage of Coals of Different Ash Contents in a Ball-and-Race Mill

Coals used for power plants normally have different ash contents, and the breakage of coals by the ball-and-race mill or roller mill is an energy-intensive process. Grinding phenomena in mill of power plants is complex, and it is also not the same with ideal grinding tests in labs. The interaction among various coals would result in changes of grinding behaviors and energy consumption characterization if compared with those of single breakage. In this study, anthracite and bituminous coal of different ash contents were selected to be heterogeneously ground. Quantitation of components in products was realized using the relation between sulfur content of the mixture and mass yield of one component in the mixture. Product fineness t10 of the component was determined, and split energy was calculated on the premise of specific energy balance and energy-size reduction model by a genetic algorithm. Experimental results indicate that breakage rate and product fineness t10 of the mixture decrease with the increase of hard anthracite content in the mixture. Unlike the single breakage, t10 of anthracite in heterogeneous grinding is improved dramatically, and bituminous coal shows the opposite trend. The interaction between components results in the decrease of the specific energy of the mixture if compared with the mass average one of components in single breakage. Breakage resistance of hard anthracite decreases due to the addition of soft bituminous coal, and grinding energy efficiency of anthracite is also improved compared with that of single grinding.

Discussion on the quantification of components in heterogeneous breakage of coals

Heterogeneous breakage is common in industrial production processes, and the quantification of components in the product mixtures is essential for the fundamental research of breakage behavior and interaction among constituents. In this paper, methods to quantify the heterogeneous grinding of minerals are summarized, and some potential ways to determine component yields in ground products of coal mixtures are discussed. The characteristic ratio of diffraction images and organic sulfur content were selected to monitor the breakage. A relation between those two parameters and the yield of components in mixtures was established. This study provides basic information on the heterogeneous grinding of coals of various ash contents, coalification degrees and grindabilities.

Study on the Theory of High Pressure Water Jet Coal Breaking and Its Main Controlling Factors

In order to solve the problems of gas extraction and pressure relief and permeability enhancement under low permeability and high surrounding rock stress conditions in deep coal seams, the ultra-high pressure hydraulic cutting technology is proposed. The mechanical mechanism of coal breaking by high pressure water jet is analyzed. The process of coal breaking by jet is divided into stress wave effect and secondary crack propagation effect. The effects of physical and mechanical properties of coal and rock and jet parameters on jet cutting depth are analyzed theoretically. The results show that the shear strength, tensile strength, fracture and pore development degree of coal and rock correspond to different types of coal and rock failure. With the increase of jet pressure and flow rate (nozzle diameter), the jet cutting depth increases gradually. The influence of jet rotating speed on cutting depth is analyzed. It is concluded that reducing rotating speed should be adopted to improve cutting depth in field practice. The research results can effectively improve the stress state of coal body, play the role of pressure relief and permeability enhancement, and improve coal seam permeability and gas release capacity, which can well meet the control needs of coal mine gas complex dynamic disasters.

Analysis of Similarities and Differences between Acoustic Emission and Charge Signal Based on Fractal Characteristics of Coal Fracture

Rock burst is a catastrophic dynamic disaster caused by sudden failure and instability of coal, which brings threats to deep coal mining; the AE‐charge signals and the fragment distribution are related to both mechanical properties of coal and disaster early warning directly. Hence, the variation of AE and charge induction during coal failure, fractal feature of coal fragments, and their relationship should be studied in depth. In this paper, uniaxial loading test was carried out for coal with bursting tendency samples produced by blocks cored from 800 m depth in Xiaoqing coal mine of the Tiefa Coal Group in northeast China; the fractal characteristics of specimens are obtained by using the statistical fractal method. The mechanics of similarities and differences between acoustic emission and charge signal is investigated by using loading experiments and theoretical analysis. It is found that the fragments of coal have good self‐similarity properties; the fractal dimension of the specimens is in the range 2.085–2.521, the maximum range being 2.300–2.468, which is slightly higher than that of rock. The high‐amplitude pulses of the acoustic emission and charge are concentrated in the macroscopic fissure development and expansion stage but they have asynchronous characteristics between them. The charge generation process is accompanied by the inhomogeneous deformation and sliding friction; the friction slip is the major one and is analysed theoretically. A theoretical model for the force‐electric coupling relationship is established. The statistical results show that both the acoustic emission and the charge signal accumulation have a significantly proportional relationship with the fractal dimension. Both the acoustic emission and charge signal reveal coal breakage evolution process, which will help in obtaining the precursor information on coal failure. Furthermore, the monitoring results can predict the extent of coal mass instability.

Formation of the Aerogas Situation in a Working Face while Mining Callows with Top Coal Caving onto Conveyor

The processes of gas emission into the working face from coal, cut by the mining machine with the simultaneous top coal caving on- to the conveyor located behind the powered support units, are considered. The presence of two methane emission sources in the face requires addi- tional measures to ensure the aerologic safety of mining. Given the high rate of mining operations due to the performance of modern cutting ma- chines, there is a problem of maintaining the methane concentration in the upcast at a level of 1%. In this case, gas released from the coal mass caved onto the rear conveyor is partially added to the volume of gas released from the cut coal into the face, which generally increases the absolute gas content in the stope. Measures are proposed to contain the growth of gas content in the face in the form of alternations of machined coal breaking and top coal caving processes, which do not overlap in time during the en- tire mining cycle. Due to the fact that this technology is experimental, its adaptation to mining and geological conditions of mines is required.

Coal breakage using abrasive liquid nitrogen jet and its implications for coalbed methane recovery

Coalbed methane (CBM) is an important global energy resource. However, reservoir damage and wellbore collapse caused by water and chemical additives invasion are major obstacles that restrict the economic exploitation of CBM. Abrasive liquid nitrogen (LN2) jet assisted-drilling, a non-aqueous technique, is expected to improve drilling efficiency and simultaneously protect formations. To determine its feasibility and the effects of critical parameters, a series of laboratory experiments considering various jetting schemes were conducted. The coal-breakage mechanism was highlighted based on macro and micro analysis. Subsequently, we proposed an innovative CBM stimulation approach referred to as Radial Abrasive LN2 Jet drilling (ALN-RJD) that uses abrasive LN2 jet to drill multiple laterals radiating out from a main borehole. Then, the potential application of this method for CBM recovery enhancement in an economic feasible way was discussed using the numerical simulation and Net Present Value (NPV) method. The results show that the abrasive LN2 jet has superior coal-breakage performance attributed to four coupled effects: high-velocity jet impact, thermal shock, abrasive erosion, and gasification cracking. Moreover, the ALN-RJD can significantly increases the gas production by creating conductive pathways for gas transport in complex lateral-fracture networks. Besides, the economic analysis indicated that ALN-RJD can achieve higher NPV than those of conventional CBM development methods. The key findings of this study are expected to provide a viable alternative for the sustainable development of CBM resources in a profitable and environmentally acceptable manner.

Relationship between fractal meso-structural and mechanical characteristics of lump coal under uniaxial compression at different temperatures

Coal breakage characteristics have significant effects on the clean utilization of coal. The mechanical properties of Shenmu (SM) coal, Hongshaquan (HSQ) coal and Wucaiwan (WCW) coal under uniaxial compression were tested by a self-designed bench at 90 °C, 120 °C, 150 °C and 180 °C. The results show that the mechanical parameters overall decrease with the increase of temperature (T, °C) except that SM coal has an increasing trend from 120 °C to 180 °C due to thermal expansion of coal matrix and moisture inside closed pores. The compressive strength (σm, MPa) of SM coal at 120 °C decreased by 43% compared with it in 90 °C. By means of fractal theory, these mechanical properties can be reflected by the meso-structural characteristics of coal surface. The fractal dimension (D) generally possesses a positive correlation with the stress (σ, MPa). In initial segments, the curves of some coal samples are relatively stable due to compaction and compressive resistance. Two dimensionless parameters, relative fractal dimension (Dr) and relative stress (σr), were introduced to represent the relationship of meso-structural and mechanical properties quantitatively. It is shown that the relative fractal dimension rises quickly when the relative stress is above 0.75, and the relationship of the two dimensionless parameters can be described.

Laboratory experiments on blockage removing and stimulation of CBM reservoirs by composite pulsating hydraulic fracturing of radial horizontal wells

It is a common phenomenon during CBM drilling and production that reservoir damage is not eliminated completely. In view of this, a technical idea of composite pulsating hydraulic fracturing of radial horizontal wells which is conducive to blockage removing and stimulation was put forward in this paper. Speaking of the hydraulic jetting in a multi-branch radial well, it is to conduct pulsating hydraulic fracturing moderately through a high-diversion radial hole, so as to crush and break coal beds near the main hole to the uttermost. Thus, an extensive pressure relief and permeability increase area where high-diversion pathways are combined with fracture networks is formed. Then, to verify its technical principles, laboratory tests on pulsating hydraulic fracturing of radial wells were designed and carried out. Besides, the relationships of the features of acoustic emission (AE) response during the formation of fractures by composite fracturing of radial horizontal wells vs. coal breaking degree and macro fracture morphology were experimentally studied by using a pulse servo fatigue testing machine and an acoustic emission detector. And the following research results were obtained. First, under experimental conditions, fractures initially occur when the pressure of composite pulsating hydraulic fracturing of radial horizontal wells is 1/3–1/4 of the peak pressure of conventional fracturing, and the amount of its AE events is 1.38–7.07 times that of conventional fracturing. Second, when composite pulsating hydraulic fracturing of radial horizontal wells is conducted, AE emission signals respond strongly, the peak pressure for macro fracturing is lower and a larger fracture network can be generated more easily under the same condition. Third, radial laterals amount, borehole length, dynamic loading frequency and amplitude are the important factors affecting the effect of composite pulsating hydraulic fracturing of radial horizontal wells. In conclusion, composite pulsating hydraulic fracturing of radial horizontal wells provides a new idea of removing the blockages in CBM reservoirs and developing CBM efficiently, realizing effective blockage removing and stimulation of CBM wells.

Increasing Spontaneous Combustion Risk While Reducing the Coal Particle Size

Extraction, storage and transportation of coal are complicated by the danger of spontaneous combustion centers due to toxic gases emission. At various technological processes, the formation of clusters consisting of coal particles of various sizes is possible. In the course of the research, the influence of the size of coal particles on the danger of spontaneous combustion of oxidizing material clusters was estimated. The studies of coal chemical activity have shown that for most seams of Kuzbass mines, the amount of absorbed oxygen increases significantly with decreasing particle size. Coal dust can be more active than coal in 3-8 times. For some seams, the coal breakage has almost no effect on its chemical activity in relation to oxygen. It is determined that the duration of incubation period of spontaneous combustion also varies depending on the size of particles forming the coal cluster. For the appearance of spontaneous combustion centers, the size of coal dust clusters may be in 2– 4 times smaller than for coal. In order to increase the safety of work for all the seams it is necessary to study the chemical activity in relation to oxygen, the tendency to spontaneous combustion, and the duration of the incubation period not only for coal samples, but also for coal dust.

Cracking mechanism of coal under high-pressure water jet and its applications for enhanced coalbed methane drainage

This paper is concerned with the mechanism of coal breakage under high-pressure water jet (HPWJ) and its applications. A model of HPWJ impinging on coal target was established to study the cracking mechanism of coal under impact load. The characteristic and pressure distribution of HPWJ, the propagation characteristics of stress wave in coal, the mechanical properties of different coal particles, and the fracture characteristics of coal under HPWJ erosion were investigated theoretically and numerically. The results show that the shock wave and water wedge pressure are the main factors that cause coal breakage and crack propagation. The damage to the far-field coal particles affected by HPWJ is primarily caused by tensile stress, and the damage to the near-field coal particles affected by HPWJ is caused by the coupled effects of tensile stress and compressive stress. An erosion cavity is formed in the coal model with diameters of 1.25 to 2.5 times that of the jet at different depths. Meanwhile, the strong quasi-static pressure at the crack discontinuities further promotes the propagation of radial cracks around the erosion cavity to form a fracture zone, and the diameter of the fracture zone at different depths is 3.5 to 4.0 times that of the jet. In addition, the results of field application show that there is a significant difference between the methane parameters in the hydraulic flushing borehole and the conventional borehole; the average methane volume fraction and the average methane flow rate in hydraulic flushing boreholes are 3.85 and 3.67 times, respectively, that in conventional boreholes. Indicating hydraulic flushing can effectively promote the initiation and propagation of coal cracks. These results are of great significance to improve coalbed methane drainage technology and prevent gas disaster accidents in coal mines.

Analysis of the stress wave effect during coal breakage by a high-pressure abrasive air jet

In view of the defects of borehole collapse, inhibition of gas desorption and migration of gas existing in hydraulic fracturing and other hydraulic permeability–increasing measures for soft coal seams with low-permeability technology is proposed for coal breakage by a high-pressure abrasive gas jet for relieving pressure and increasing permeability. The comparative analysis of gas jet flow field structure between convergent nozzle and Laval nozzle has been given by numerical simulation. For Laval nozzle, the expansion wave and compression wave alternate and move forward steadily in gas jet and vanish when potential core length reaches maximum. So, the Laval nozzle can form more stable flow filed structure of gas jet and avoid shock wave in gas jet. Furthermore, a high-speed camera is adopted to analyze the jet structure and verify the conclusion of numerical simulation. Based on thermodynamic theory, this article calculates and analyzes the critical local sound velocity and pressure generated from the stress wave during the process of coal breakage by the gas jet. Furthermore, experimental coal breakage by a high-pressure abrasive gas jet is carried out. The high-pressure abrasive gas jet impacts the coal body as a quasi-static load and a dynamic load and forms corrosion pits on the surface of the coal body. Penetrating cracks are formed within the coal in the pattern of the loaded stress wave which leads to coal breakage. The effects of porosity and permeability on the propagation of the stress wave in coal are analyzed by establishing the dispersion equation for the spread of the stress wave in coal. The results show that porosity has a significant effect on wave velocity and that the attenuation of the stress wave is intensified with an increase in porosity. Moreover, the stress wave attenuation is more obvious at high frequency. The effect of permeability on the wave velocity is not significant at low frequencies. In contrast, at high frequency and relatively low permeability, the wave velocity increases with the permeability, and the attenuation of the wave velocity initially increases and then decreases. When the permeability is greater than 10−11 m2, the wave velocity is not affected by the permeability. However, the stress wave is not attenuated.

Experiment on coal breaking with cryogenic nitrogen jet

Severe leakage of drilling fluid and reservoir damage due to water and solid invasion during well drilling are the key factors that restrict economic exploitation of coal bed methane (CBM) in China. To solve these issues, this paper presents a method of CBM well drilling with the assistance of cryogenic nitrogen jet, in which no aqueous phase and solid phase are contained. To determine the feasibility of coal breaking by cryogenic nitrogen jet and the effects of different parameters, such as nozzle diameter, dimensionless standoff distance, nozzle pressure difference and exposure time, we conducted corresponding rock breaking experiments on artificial and natural coals with the cryogenic nitrogen jet and the water jet. Rock breaking features and the energy consumption by the two jets were compared. The results indicate that the cryogenic nitrogen jet has better rock breaking performances and consumes less energy. Under the same conditions, the average rock erosion volume and the energy consumption with the cryogenic nitrogen jet were 18.34 times larger and 90.8% lower than those with water jet, respectively. Visible netted fractures could be created on the surface of coal samples blasted with the cryogenic nitrogen jet, and thus rock breaking could be facilitated effectively. The outstanding rock breaking performances of cryogenic nitrogen jet are attributed to three mechanisms: high-velocity jet impact, thermal shock and gasification cracking effect. In addition, under our experimental conditions, the erosion volume and depth were proportional to the pressure difference and inversely proportional to the dimensionless standoff distance. When the nozzle diameter changed from 1.5 mm to 3 mm, the increases of erosion volume were 3% at the pressure difference of 5 MPa and 41.7% at the pressure difference of 10 MPa respectively. This indicates that the effect of nozzle diameter is more significant at relatively higher pressure drops. With the increase of exposure time, both of the rock erosion depth and volume rose, while with a smoother trend. The results of this study are expected to promote the application of cryogenic nitrogen jet in CBM well drilling.

Oxidant stimulation for enhancing coal seam permeability: Swelling and solubilisation behaviour of unconfined coal particles in oxidants

Low permeability renders a significant fraction of coal seam gas (CSG) resources sub-economic. An effective permeability enhancement strategy is thereby crucial in monetising a large proportion of low permeability CSG resources. This paper introduces the concept of using oxidants for permeability enhancement, describes a practical screening method to evaluate potential oxidants and provides knowledge about the coal behaviour in oxidants. A test based on time-lapse photography and image analysis of coal particles immersed in liquid oxidants was used to assess the extent and rate of change of coal particle size. Complementary leaching tests determined the extent of coal solubilisation by quantifying the change in coal mass and leachate organic content. The swelling profiles of coal particles hand-picked from a low permeability CSG coal core (Bowen Basin, Australia) were first examined in solutions of potassium chloride, and then pyridine for the purpose of method development and validation. Finally, the swelling ratio, SR, and rate of swelling Sr (% area change per 6 h), of coal particles immersed in oxidising solutions of sodium hypochlorite (0.1%, 1% and 10% NaClO), potassium permanganate (0.015%, 0.03%, 0.1%, 1%, 3% and 5% KMnO4), hydrogen peroxide (1%, 3%, 10% and 30% H2O2) and potassium persulfate (1% and 3% K2S2O8) were examined.Results provide evidence for coal solubilisation (maximum mass loss = 15%) and the propensity to swell (maximum particle size increase = 15%) in all the candidate oxidant stimulants as well as coal breakage in specific oxidants and at specific concentrations (1% NaClO and 3%, 5% KMnO4). The swelling and solubilisation of the coals used in this study tends to increase with higher oxidant concentrations. Anisotropic swelling was also clearly observed in 1% NaClO. Coal reacted vigorously with NaClO and KMnO4, but only slightly with K2S2O8 and H2O2. Massive coal solubilisation occurs in NaClO and KMnO4, but negligible in K2S2O8 and H2O2. In terms of coal oxidation to enhance permeability, NaClO and KMnO4 seem to be more promising than K2S2O8 and H2O2.For the situation of in situ application, it remains unclear if the net effect of coal swelling and coal solubilisation will increase or decrease permeability. In addition, coal breakage may lead to void space or new cracks in the coal matrix, which could have the potential to increase the coal permeability. Confined core-flooding tests that simulate in situ conditions are required to elucidate this behaviour.

Investigation of the energy-size reduction and mineral liberation of raw coal in the Hardgrove mill

ABSTRACTRaw coal was ground in a Hardgrove mill, and mineral liberation from coal was investigated by float-sink test and electron probe microanalyzer (EPMA). Based on EPMA, associated conditions of mineral were determined. Analyses of energy-size reduction show that energy efficiency decreases with time. For +0.074 mm progenies, yields of particles in −1.5 g.cm−3 decrease with the increase of size and time, and products in +1.8 g.cm−3 show the opposite trend. EPMA of 0.25–0.125 mm, 1.5–1.8 g.cm−3 products ground for 10 min indicates that only part of minerals are liberated. For −0.074 mm products, the yield of particles in −1.5 g.cm−3 decreases by 23.36% in last several minutes, but ash content of +1.8 g.cm−3 also decreases by 13.18%. Since breakage of raw coal in last 9 min does not reduce particle size obviously, and liberation degree cannot be improved dramatically, closed grinding of 10 min in Hardgrove mill is proper to balance the size reduction and mineral liberation.

Experimental and numerical optimization of coal breakage performance parameters through abrasive gas jet

Experimental and numerical optimization of coal breakage performance parameters through abrasive gas jet

Co-Liquefaction of Elbistan Lignite with Manure Biomass; Part 3 - Effect of Reaction Time and Temperature

Most of the liquefaction process were carried out in a batch reactor, in which the residence time of the liquefaction products is long enough to favour the retrogressive reactions. To minimize retrogressive reactions, the liquefaction of coal was carried out in a flowing solvent reactor in which a fixed bed of coal is continuously permeated by hot solvent. Solvent flowing through the coal bed carries the liquefaction products out of the reactor. Unlike experiments carried out under similar conditions in a batch reactor no increase in solid residue is observed during long time high temperature runs in the flowing solvent reactor. There is a greater appreciation of the importance of retrograde, or polymerization, reactions. If the free radicals formed when coal breaks down are not quickly capped with hydrogen, they react with each other to form large molecules that are much harder to break down than the original coal. Reaction time impacts both the co-liquefaction cost and the product yield. So as to study this idea, the experiments of Elbistan Lignite (EL) with manure co-liquefaction carried out by changing the reaction time from 30 to 120 minutes. As a result, the greatest oil products yields obtained at 60 minutes. Therefore, by thinking about the oil products yield values acquired, the optimal reaction time was obtained to be 60 minutes for Elbistan lignite (EL) with manure liquefied with the temperature of 350°C and 400°C. Above 425°C did not examine because solvent (tetraline) loses its function after 425 °C. The obtained optimum temperature found 400°C due to higher total conversion of liquefaction products and also oil+gas yields.

Study on the characteristics of coal rock electromagnetic radiation (EMR) and the main influencing factors

Coal rock would produce electromagnetic radiation (EMR) in the loading process, but study on the influence factors influence on the coal rock EMR characteristics in the mesoscopic level is not insufficient. In the paper, the EMR characteristics of coal and rock samples under uniaxial loading are studied. Several typical microcosmic mechanisms affecting the characteristics of EMR are discussed, such as strength, composition and microstructure of the samples. Results show that the macroscopic structure of the outburst coal is soft, the corresponding EMR signal increases slowly with the loading increase and the EMR peak is smaller. The rockburst coal has a strong brittleness, the EMR signal increases quickly and EMR peak appears while the coal breaks is larger than the outburst coal. The EMR characteristics of rock samples are similar to the rockburst coal, but the EMR peak is the largest. When the coal rock microstructure is complete, the coal rock block is larger and the brittleness is stronger, then the corresponding strength would be larger. And the free charge generated by thermal excitation, field emission and intergranular chemical bond breakage would also be more. In the meantime, the crack propagation rate becomes greater, therefore the EMR is more stronger. The piezoelectric effect is mainly caused by the linear elastic stage of the specimen deformation and rupture, which contributes less to the EMR signals. This study is of great theoretical and practical value for assessing the mechanical state of coal rock through EMR technology, and accurately monitoring and predicting the coal rock dynamic disasters.

A discrete element method-based approach to predict the breakage of coal

Pulverization is an essential pre-combustion technique employed for solid fuels, such as coal, to reduce particle sizes. Smaller particles ensure rapid and complete combustion, leading to low carbon emissions. Traditionally, the resulting particle size distributions from pulverizers have been determined by empirical or semi-empirical approaches that rely on extensive data gathered over several decades during operations or experiments, with limited predictive capabilities for new coals and processes. This work presents a Discrete Element Method (DEM)-based computational approach to model coal particle breakage with experimentally characterized coal physical properties. The effect of select operating parameters on the breakage behavior of coal particles is also examined.

Effect of the particle shape and swirling intensity on the breakage of lump coal particle in pneumatic conveying

This work studied the influence of the particle shape and flow regime on the lump coal breakage in pneumatic conveying using CFD-DEM simulation. A variety of agglomerates with different sphericities were modelled by the parallel bond method to analyse the breakage characteristics of lump coal. The numerical parameters, simulation conditions and CFD-DEM simulation results were separately validated by experimentation. To demonstrate the lump coal breakage process, the mechanism energy variation in the coal agglomerate was analysed. The fragmentation degree of the coal agglomerate was positively correlated with the energy difference in collision. The integrality ratio of the coal agglomerate increased with the particle sphericity and swirling number in pneumatic conveying. A quasi-periodical downgrade in the coal agglomerate integrality ratio was shown in the swirling flow. A remarkable improvement in the coal agglomerate integrality ratio from approximately 0.85 to 0.975 was observed in swirling flow pneumatic conveying. The regression relationship of the particle sphericity and swirling number fit the Exp3p2 exponential function well.

Characterization of Breakage and Washability of ROM Coal using X-ray Computed Tomography

ABSTRACTThe most critical parameters for the efficient design of coal processing operations depend on the assessment of coal breakage and washability characteristics of the coal resource. X-ray computed tomography (XCT) technology has been demonstrated to have the capability of analyzing washability and fragmentation parameters (particle size, energy, coal type). In this regard, progeny size distributions are obtained from sequential modified drop shatter tests (different input energy levels) and from XCT analysis. In this way, the breakage characteristics (selection and breakage functions) of coal particles are established and the prediction of particle size distribution (PSD) for different levels of energy input is demonstrated.