Coal Size Research Articles

Factors affecting co-degradation of coal and straw to enhance biogenic coalbed methane

The formation of biogenic coalbed methane is stoichiometrically limited by relative deficiency of hydrogen in coal. As straw is rich in hydrogen and can be easily degraded by microorganisms, it is suggested that the co-degradation of coal and straw to produce biogenic methane can overcome these limitations. However, the factors affecting methane production in co-degradation of coal and straw were unclear. Here, the type and part of straw and the particle size of coal and straw were evaluated with respect to methane production from Qinshui Basin coal. The results showed that the biomethane yields in co-degradations of coal and different types of straw were all higher than that in degradation of only coal or only straw with the yield from rice and coal 25.36 times greater, the yield from sweet sorghum and coal 22.70 times greater, the yield from wheat and coal 19.90 times greater, and the yield from corn and coal 1.74 times greater. The co-degradation of coal and stems also showed a better enhancing effect than that of coal and roots and coal and leaves based on biogenic methane production. With respect to particle size, the coal had a greater influence on biomethane production than the straw. The highest methane production was obtained in the co-degradation with 0.125–0.150 mm of coal and 5 mm of rice straw. The affects were also shown to not correlate well with the additional of hydrogen. Therefore, it is more likely that the enhanced methane production is a result of stimulating the microbial biodegradation of coal. Further efforts would be made to find the specific compounds in straw which interact with coal directly, in order to better understand the mechanism of co-degradation between coal and straw, and make it simpler to improve biomethane production.

Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS

The pyrolysis behaviors of Huainan coal (HN coal) with three particle sizes, i.e., <40 μm (HN-S), 224–500 μm (HN-M) and 1600–2000 μm (HN-L), were investigated by thermogravimetry (TG) and online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOF MS) using a krypton discharge lamp (10.6 eV) and synchrotron radiation vacuum ultraviolet light (SVUV) (14.2 eV and 15.5 eV) as the ionization sources. The fragment-free mass spectra of coal pyrolysis products were obtained at 600 °C in real time. The released organic volatiles were characterized as alkenes, phenols and aromatics, and their evolved profiles were measured as a function of time in fixed temperature mode and as a function of temperature in programmed-temperature mode. A higher weight loss ratio of the TG curves was obtained when the particle size was increased. The intensities of the individual volatiles in the mass spectra were all enhanced following the increase in the particle size of HN coal. With regard to the time-evolved profiles, by increasing the particle size, the appearance times of volatiles were delayed. Tricyclic aromatics showed multireleasing processes in both time-evolved and temperature-programmed profiles. The release of the guest molecules lagged within the higher temperature region as the coal particle size increased. To explain the evolution trends of most organic volatiles, a mechanism combining intraparticle and interparticle mechanisms is presented.

Effect of Flow-pattern Reconstruction on Combustion Efficiency and Pollutant Emission in CFB Boiler

The effect of flow-pattern reconstruction on combustion efficiency and pollutant emission in CFB boilers was investigated with a 1-D CFB combustion model. The results from the 1-D model agree well with the field test data. It was found that, the mass fraction of fine coal particles should be increased at lower bed pressure drop to achieve flow-pattern optimization. As the bed pressure drop decreases, the carbon content decreases first and then increases, both in fly and bottom ash. The change of bed pressure drop is the main factor in the influence of flow-pattern optimization on SO2 emission. The higher the bed pressure drop is, the lower the SO2 emission concentration is. Bed pressure drop and coal size distribution are the chief factors of NOx emission concentration in flow-pattern optimization. With the decrease in bed pressure drop and increase in mass fraction of fine particles, NOx emission concentration decreases first and then increases. SO2 emission concentration can be decreased below 100 mg/m3 by increasing Ca/S, while the original emission concentration of NOx increases. Therefore, other assistant adjustments are also needed to achieve flow pattern optimization, such as the secondary air proportion and CaO particle size distribution.

Dry coal beneficiation by the semi-industrial Air Dense Medium Fluidized Bed with binary mixtures of magnetite and fine coal particles

Air Dense Medium Fluidized Bed (ADMFB) is deemed to be one of the most efficient methods for dry coal beneficiation. In the present work, a semi-industrial ADMFB system in continuous operation was utilized to study the effects of operating gas velocity, feed coal size, and mixture composition of medium particles on the coal beneficiation in industrial practice. Binary mixtures of magnetite and fine coal particles were used as the medium material, and four different feed coal samples with the size ranges of −50 + 25, −25 + 13, −13 − 6, and −6 + 2 mm were tested individually. The experimental results showed that the influence of excess gas velocity on the dry coal separation is relatively small in the lower flow rates. The separation density and probable error increase with the decreasing of feed coal size, regardless of the type of feed coal. The separation density can be continually reduced by further increasing the fraction of fine coal in the medium material, with the compromise of the increased probable error. Moreover, the ash content and calorific value of −50 + 6 mm coarse coal can be effectively upgraded, but the beneficiation of −6 + 2 mm fine coal was less efficient.

Water-conserving mining influencing factors identification and weight determination in northwest China

Water-conserving mining is an effective way to alleviate the contradiction between fragile ecological environment and high-intensity coal mining in the arid and semi-arid region of northwest China. It needs to consider the engineering and geological conditions, hydrogeological conditions and mining methods of coal seams. From the three aspects, this paper systematically analyzes the influencing factors and establishes an identification model with multi-level structures. The model includes three primary factors (including the engineering and geological conditions, hydrogeological conditions and mining methods), nine secondary factors (including overlying strata thickness, aquiclude, mining parameters and etc.), sixteen third-tier factors (including the faults, aquiclude thickness and effective mining height and etc.) and twelve fourth-tier factors (including the fault throw exponent, aquiclude permeability and coal pillar sizes and etc.). On the basis, the analytic hierarchy process is used to build the judgment matrix and obtain the weight of each influencing factor. The results indicate that the overlying strata thickness, aquiclude and effective mining height are the most important factors among the primary factors of engineering and geological conditions, hydrogeological conditions and mining methods, respectively. The research results could provide theoretical references for the water-conserving mining of coal resources in northwest China.

Blasting Impact Simulation Test and Fragmentation Distribution Characteristics in an Open‐Pit Mine

Based on the split Hopkinson pressure bar (SHPB) test system, dynamic impact tests of coal specimens under different impact pressures were carried out to study the relationship between the impact load and the size of crushed lump coal. Based on the theory of stress wave attenuation, the relationship between the blasting impact load in a single‐hole blasting area of a coal seam and the load applied in an impact failure test of a coal specimen in the laboratory was established. According to the characteristics of the fragmentation distribution of the coal specimens destroyed under a laboratory impact load and the requirement of the minimum cost control of coal blasting in an open‐pit coal mine, the fragmentation size range was divided into three groups: large‐diameter, medium‐diameter, and powder particles. Based on this range, the variation rule of the mass percentage of coal fragments with impact pressure was obtained. Established on the evaluation principle of the blasting effect in an open‐pit coal mine, a good impact fragmentation effect was obtained. The good pressure range is 0.30 MPa ≤ P &lt; 0.90 MPa.

A Study on the Energy Condition and Quantitative Analysis of the Occurrence of a Coal and Gas Outburst

With mining depths increasing, coal and gas outburst disasters are becoming more and more serious and complicated, which directly restricts the production efficiency of coal mines. In order to study the rules of energy dissipation during the occurrence of a coal and gas outburst based on the occurrence mechanisms, a simulation experiment of a coal and gas outburst with a ground stress of 16 MPa and a gas pressure of 0.5 MPa was carried out using a self‐developed large‐scale coal and gas outburst simulation experimental system. A quantitative analysis was given based on the energy model. The results showed the following: (1) In the process of the coal and gas outburst, the main energy source originated from the elastic potential energy of the coal body and the gas internal energy. The main energy loss was used for coal crushing and throwing. (2) The outburst coal sample in this experiment had a mass of 18.094 kg, and the relative outburst intensity was 1.21%. Additionally, the farthest throwing distance of the outburst coal samples was 3.3 m away from the outburst hole wall. The distribution of the outburst coal sample decreased along the roadway, and the proportion of the coal sample grain size in each area first decreased and then increased with the decrease of the grain size. The coal samples with a grain size less than 0.2 mm after the outburst accounted for 6.34% of the mass of the total coal samples. (3) The elastic potential energy of the coal body accounted for 0.34% of the total outburst energy, while the gas internal energy accounted for 99.66%. It was verified that gas internal energy was the key energy source for the coal and gas outburst, and this internal energy was two orders of magnitude more than the elastic potential energy, playing a leading role in the outburst process. After the outburst initiation, most of the energy was consumed in coal crushing, which was in the same order of magnitude as the gas internal energy. Moreover, the energy losses due to friction, vibration, and sound during the outburst process comprised no more than 10% of the total energy. The research results can provide certain guidance for clarifying the mechanism of a coal and gas outburst and the quantitative analysis of outburst energy.

Permeability Characteristics of Broken Coal and Rock Under Cyclic Loading and Unloading

During longwall mining, the permeability evaluation in a caving zone is one of the most challenging risks for mine operators because it controls the behavior of gas flow and it significantly influences gob gas extraction and spontaneous combustion. Based on the stress environment of repeated mining in the Huainan coal mine, this study discusses the results of a laboratory experiment on broken coal and rock in a caving zone of a gob. The stress–permeability relationship of broken coal, rock, and combination of coal and rock samples with various particle sizes under cyclic loading and unloading was experimentally studied. The laboratory test results imply that permeability stress sensitivity and permeability loss of the first loading and unloading were significantly greater than the succeeding cyclic loading and unloading. Re-crushing and re-arrangement were the main two factors that lead to a drastic drop in caving zone porosity causing permeability reduction in the first loading and unloading process. The initial permeability and initial pore compressibility increased logarithmically when particle size increased, while the changing rate of pore compressibility decreased exponentially. The stress sensitivity of the broken coal samples’ permeability increased when particle size increased. Based on the laboratory experiments, the stress–permeability fitting model considering particle size of broken coal and rock in a caving zone during repeated mining was proposed in this study.

Digital holographic sizer for coal powder size distribution measurement: preliminary simulation and experiment

On-line and continuous measurements of coal powder size distribution in coal-fired plants remain challenging. A coal powder size measurement system based on digital holography (DH) is developed. The effects of several factors, i.e. pixel size, recording distance, particle size, and concentration, on particle size measurement by the DH method are quantified by simulation, yielding the suitable pixel size, recording distance and particle concentration. The measurement system is implemented based on the simulation results and applied to measure particle size distribution of pulverized coal samples. Results show that the particles are clearly reconstructed from the hologram, and the measured size distributions agree well with those of the samples measured by laser particle size analyzer. This work shows that the digital holographic sizer has great potential in coal powder size distribution measurement in power plant pipes.

Stepwise recovery of magnesium from low-grade ludwigite ore based on innovative and clean technological route

Abstract A novel and clean technological route for the comprehensive utilization of low-grade ludwigite ore was proposed, in which magnesium was extracted by metallizing reduction-magnetic separation, sulfuric acid leaching and ethanol precipitation operation. Meanwhile, iron-rich product, silicon-rich product and boron-rich product were obtained, respectively. In the process of metallizing reduction-magnetic separation, 94.6% of magnesium was enriched in the non-magnetic substance from the ore reduced at 1250 °C for 60 min with the ore size of 0.50–2.00 mm and coal size of 0.50–1.50 mm. When the non-magnetic substance was leached at 90 °C for 15 min with the liquid-to-solid ratio of 7:1, 87.4% of magnesium was leached into the liquor separated from silicon gathering in leaching residue. The ethanol precipitation was conducted for 30 min with the ethanol-to-original liquid volume ratio of 1.5:1 at room temperature. 97.2% of magnesium was precipitated out with the initial concentration of 0.8 mol/L in the form of MgSO4·7H2O.

Effects of ultrasonic treatment on the particle size, shape and ash content of fine coal

Effects of ultrasonic treatment on the particle size, shape and ash content of fine coal

Investigation on Coal Fragmentation by High-Velocity Water Jet in Drilling: Size Distributions and Fractal Characteristics

Water jet drilling (WJD) technology is a highly efficient method to extract coalbed methane from reservoirs with low permeability. It is crucial to efficiently remove the coal fragments while drilling. In this study, to disclose coal fragmentation features and size distributions under water jet impact in drilling, the image processing method was utilized to obtain the geometric dimensions of coal fragments. The size distributions, morphologies and fractal characteristics of coal fragmentation were studied based on generalized extreme value distribution and fractal theory. The effects of the jet impact velocity and coal strength on the fragmentation features were analyzed. The results show that fine particles dominate the coal fragments in WJD for coal seams with various strengths. In experiments conducted at the Fengchun coal mine, owing to the higher coal strength of the M7 coal seam, the fragmentation degree of coal subjected to water jets during WJD is lower in the M7 coal steam than in the M8 coal seam, which results in a large dominant fragment size and small fractal dimension under the same impact energy. It was found that the higher the jet impact velocity is, the higher the quantity of fragments generated from WJD and the smaller the particle size. The NUM-based cumulative probability distribution curves of coal fragments are more intensive in the range of relatively small particle sizes and then become sparser with the increase in particle size. When the impact velocity increases, (i) the size distribution curves move toward smaller particle sizes, and the dominant fragment size decreases; (ii) the shape (major axis/minor axis) of coal fragments move toward the upper left, and the curve shape for a high impact velocity attains unity more quickly; and (iii) the fractal dimension value increases linearly. In addition, the fractal dimensions are obviously affected by the dominant fragment size; they increase with the decrease in the dominant fragment size. This study can provide a basis for further research on coal fragment transportation in WJD and parameter selection for discharging coal fragments during drilling for CBM development.

A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects

Underground coal gasification (UCG) is a promising option for extracting energy from coal in unworked or hard to access areas of the subsurface. From a geotechnical perspective, UCG involves various complex phenomena resulting from the elevated temperatures induced within the rock surrounding the UCG burn. This paper presents a coupled thermal-mechanical numerical model developed to represent a UCG trial in Wieczorek, Poland. Temperature dependent mechanical properties were assigned according to results obtained from laboratory experiments and data available in the literature. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. This source term was described using a time decay function to reflect the fact that the energy release from coal gradually decreases with time. The mechanical degradation of coal due to burning was simulated by removing the burned zone from the calculation after a specific time, which depended on zone size and type of coal. In this study, it was found that the maximum temperature at the burning zone was always <1000 °C, which agrees with previous research carried out for other UCG trials. The size of the burning zone was predicted to spread about 15 m laterally after 20 days of burning. Ground subsidence was evaluated for single and multiple (parallel) panel simulations; subsidence at the top of the numerical mesh, corresponding to a depth of 395 m below the surface, ranged from 23 mm for a single panel to 85 mm for seven panels. The degradation of mechanical properties of the rock surrounding the burned zone due to heating was found to have a marginal effect on the ground subsidence when parallel burning was carried out. The numerical modelling results obtained from this study may provide guidance for the design and operation of UCG processes.

Aerial and subaquatic oxidation of coal by molecular oxygen

The oxidation behaviour of three coal types towards molecular oxygen was investigated both in aerial and subaquatic conditions. The effects of time, temperature and coal grain size were studied and compared as basic parameters influencing the oxidation process. The obtained results showed that the influence of these factors can be similar both on aerial and subaquatic coal oxidation. Among the samples, subbituminous coal was primarily found to differ with respect to oxidation in aerial and subaquatic environment. A possible explanation for this is a change in its porous system when immersed in water. For aerial oxidation of the studied coal, oxygen consumption rates RO2 (40 °C, grain size 0.06–0.15 mm) in the range of 1·10−6 to 5·10−6 mol O2.(kg.s)−1 were found, which is approximately 2–4 times higher in comparison with RO2 for the subaquatic oxidation of the coal samples.

Deformation, Permeability and Acoustic Emission Characteristics of Coal Masses under Mining-Induced Stress Paths

The geomechanical and seepage evolution characteristics of coal masses during mining are the key factors that affect the drainage of coalbed methane and the safety of coal mining. Nevertheless, the influence of mining paths on coal seam permeability is rarely investigated given the complexity of mining-induced stress experiments. To study the effect of mining-induced stress on coal mining, the mechanical properties, acoustic emission characteristics and energy evolution of coal masses were experimentally evaluated through mining-induced stress experiments. Experimental results indicated that at peak intensity, the deviatoric stress and axial strain of coal samples under the stress path of protective coal-seam mining are lower than those of coal samples under the non-pillar stress path. The unloading ratio of confining pressure is large under a stress path of non-pillar mining, and the elastic energy, the absorbed energy, and the dissipated energy of coal mass are low during destruction. The effect of high confining pressure on AE events is pronounced under the non-pillar mining path. The overall b value under high confining pressure is smaller than that under low confining pressure, and AE events generally have high energy. The fracture structure of coal mass is complex, and the fractal size of coal is large under high unloading rates of confining pressure, which induce the increase of permeability after coal destruction.

Characterizing the Top Coal Cavability with Hard Stone Band(s): Insights from Laboratory Physical Modeling

One of the most challenging problems in longwall top coal caving is the top coal cavability. Stone bands, which are usually found in thick coal seams, have shown adverse effects on top coal fragmentation and cavability. Understanding the characteristics of the stone band caving process is a primary step in evaluating top coal cavability in such conditions. In this study, a series of laboratory tests was implemented to find an appropriate construction material that can truly display the whole caving process of top coal. On this basis, two large-scale physical models comprising sand, gypsum, and mica complying with relevant similarity rules were constructed to investigate top coal cavability with hard stone band(s) according to two real cases. Based on observations from physical models, it is concluded that hard stone band(s) embedded in top coal usually produce poor fragmentation and cavability of the top coal, which is also affected by the strength, thickness, and location of the stone band(s). According to its relation with the caved goaf and the top coal beneath it, the rock beam consisting of the stone band and the above top coal can produce three profiles before it caves, to which suitable fragmenting and caving characteristics were carefully discussed. Summarizing the stone band beam into different profiles enables the characterization of top coal cavability for various configurations of stone bands in top coal. The results also verified that the fragment size of the top coal can be used as an integrated parameter that reflects all potential factors that impact top coal recovery to evaluate its cavability.

CFD modelling of particle size effect on stoker coal­fired boilers combustion

In the previous study, CFD simulation had been developed to predict combustion characteristic on the Fluidized Bed Boiler and Pulverized Boiler. The high demand on coal used for stoker-fired boilers in Indonesia the power plants provide challenge due to low thermal efficiency problem. In this study, CFD simulation is observed to predict the temperature distribution, heat of reaction, CO and CO 2 mass fraction on the stoker-fired boiler. Boiler geometry is modelled as the combustion chamber until the area before economizer. The selection of boundary conditions is set according to the governing equations available in the ANSYS Fluent software. Parameter design of coal which are particle size and properties of coal is determined to investigate the effect of the observed values. Four models are set to provide a combination of particle size and properties of coal. The solution strategy is developed to reduce instability of the simulation process. Coal combustion modelling includes several physical processes that could result in numerical stability issue when all processes are solved at once. The three stages were used to run the solution of the model. Plot of temperature distribution, heat of reaction, CO 2 and CO mass fraction is generated. The maximum temperature in the 1st to 4th model is 1440.95, 1473.85, 1347.72 and 1617.17 [ o K]. The amount of CO produced from each model tends to increase; respectively from the 1 st to 4 th model is 2.314E-07, 5.878E-07, 5.678E-07 and 7.904E-07. Based on the simulation results, it can be seen that the particle size of coal affects the combustion characteristic in the Stoker Coal-Fire Boiler.

Lithotype-based modelling and simulations of coal degradation conditioned by both high and low energy breakage

The control of coal fragmentation and fines generation during mining and processing is important in coal production. A method to characterise, model and simulate coal size degradation and fines generation based on lithotypes has been developed. This method was refined to cover both high energy single impact to mimic blasting and crushing and low energy incremental breakage to mimic coal handling, transiting, stockpiling and processing. The JKRBT was utilised to characterise high energy single impact breakage and drop shatter tests were used to characterise low energy incremental breakage. X-ray Computed Tomography (XCT) scanning was used as an undisruptive technique to estimate size distributions of drill cores in the drop shatter tests. The JK size-dependent breakage model was applied for breakage characterisation, size degradation modelling and fines generation simulation. The results indicate that coal lithotype has a significant influence on coal degradation and fines generation. This paper has demonstrated that the adaption of two distinct breakage characterisation tests and linkage via the one model is a significant advance in quantifying coal degradation and fines generation during coal production.

Investigating char agglomeration in blast furnace coal injection

Blast furnace iron manufacturers aim to reduce expensive coke usage through the injection of coal. This paper investigates contrasting agglomeration behaviour with a view towards optimising blast furnace operations and limiting furnace permeability issues.A drop tube furnace (DTF) was used to investigate the performance of two coal particle size specifications that were representative of injection coal sizes: pulverised (100% < 300 μm, 50% < 75 μm), and granulated (100% < 1 mm, 50% < 250 μm). A range of coals was subjected to DTF testing with issues arising from the injection of caking coals. Results show these coals exhibit signs of agglomeration, a potentially problematic effect concerning blast furnace permeability. Considering gasification reactivity upon leaving the blast furnace raceway, it was found that the agglomerated coal chars do not suffer from poor reactivity and are more reactive than the non-agglomerated chars. Pre-treatment through oxidation was found to be an effective means of eliminating agglomeration in the DTF as a result of the reduction in caking properties.

Development of Simple Techniques for Calculating the Extent of Coal Cleaning-part 1: Estimating Coal Liberation Characteristics

ABSTRACT The extent of coal cleaning is primarily dependent on three core factors: (1) coal liberation, (2) coal washability, and (3) separator performance. The most important factor that determines the extent of coal cleaning is the liberation of organic matter from mineral matter. In the past, various methods for estimating coal liberation characteristics have been used by different researchers such as fractionation method, macroscopic and microscopic techniques, advanced image analysis techniques, and methods based on float-sink data of coal. Some of these methods do not precisely represent coal liberation, some are purely qualitative, and some involve expensive equipment, extensive testing, and cumbersome mathematical analysis. Moreover, other than float-sink analysis, these techniques are not applicable to the original coal size range being treated in a coal preparation plant. This paper presents a critical review of previous work related to the assessment of coal liberation and proposes a simple mathematical expression for quantifying coal liberation characteristics. The proposed method is exclusively based on float-sink data and thus applicable to any coal size range being treated in gravity-based separators. The proposed method was applied to estimate and compare liberation characteristics of coals belonging to different countries of the world by using their published float-sink data.