Large Coal Particles Research Articles

Experimental and molecular dynamics simulation studies on the effect of composite surfactants on the wettability of anthracite coal

In order to improve the wettability of coal bed water injection, the effects of several composite surfactants on the wettability of anthracite coal were investigated. Firstly, the wettability of different composite surfactants on anthracite was analysed by macroscopic experiments (surface tension and settling time) to determine the optimal concentration and optimal ratio of composite surfactants. Secondly, the effects of adsorption of different composite surfactants on the physicochemical properties of the coal dust surface were investigated by mesoscopic experiments (scanning electron microscopy, energy spectrometer analysis, infrared spectroscopy, X-ray electron spectroscopy and zeta potential), and the results showed that the content of hydrophilic functional groups (Si-O-Si, C-O-C, C-O and CO) on the surface of anthracite coal after treatment with the composite surfactant increased significantly, and the surface potential value decreased significantly in absolute value, agglomerated large coal particles would be formed on the coal surface, and the cracks between coal particles were favourable for the penetration of aqueous solution. Finally, the synergistic mechanism of different composite surfactants was analysed from the perspective of microscopic molecular simulation. The results showed that the composite surfactants enhanced the interaction energy of the coal/composite surfactant/water system, improved the relative concentration distribution of the system, and increased the diffusion coefficient of water molecules. The results of the study can provide valuable guidance for the development of new composite surfactants with wetting effect, which has important application value for mine dust control.

Study on particle size and density segregation law in the separating process of poor quality coal with high gangue content under the action of composite force field

ABSTRACT Thin-seam mining frequently captures a significant proportion of gangue and interbedded coal in surface coal mines. Consequently, raw coal has a broad range of particle size distributions and a high percentage of components with intermediate densities, which are complex and challenging to separate. This study aims to determine the method of product division and the segregation law of particle size and density of raw coal during the separation process using a vibration and airflow composite force field. A difference in the particle size segregation of the coal particles was evident. Particle-size segregation primarily occurs in the separation area. Large-particle-size coal has a wide distribution range, spreading over the entire bed and fully utilizing the bed to achieve separation. The material becomes increasingly displaced into the separation area as its particle size decreases, thereby weakening the separation effect. Large coal particles can undergo density segregation in the vertical direction, with low-density material placed in the upper layer. This ensures that low-density material is incorporated into the refined coal product. Regardless of density, the coal tended to tilt more toward the bottom layer as the particle size decreased. This results in a poor density stratification effect with refined coal products mixed with gangue and an increase in the mismatch content. The extent of fine coal discharge was determined to be two-fifths of the entire length of the discharge side. This ensures that the majority of refined coal is recovered and that there is no excessive gangue mixing with the refined coal output. Achieving a 55.54% yield, refined coal lowers its ash content by 16%, increases its calorific value by 1400 kcal/kg, and effectively separates coal with a high gangue concentration.

Influence of coal quality on the tendency for slagging formation on a spreader stoker boiler

ABSTRACT During the combustion of solid fuels in power generation systems, one of the biggest problems related to ash is the surface deposition and accumulation of molten or sintered slag, commonly known as slagging. This research evaluated the influence of coal quality on the tendency for slagging based on the evaluation of practical criteria for industry, such as traditional indices, ash load, and coal granulometry. The indices were evaluated from standardized analyses of the composition and fusibility temperatures of the ash of eight bituminous coals. Ash fusibility tests were performed in both oxidizing and reducing atmospheres. Under oxidizing conditions, iron had a dominant influence on the formation characteristics of slagging, resulting in higher melting temperatures. Coals with low frequencies of hard deposit formation, as determined by their ash composition, contributed significantly to deposition problems due to their high ash load and granulometry. Furthermore, large coal particles require more time to burn, which can lead to caking or slag and increased carbon loss in ash. Therefore, when using empirical indices to assess the tendency for slagging, it is essential to consider ash content and ensure an appropriate distribution of coal sizes for combustion.

Assessing the impact of specific weight of different-sized particles on operational performance of coal preparation plant

In case of firing pulverized coal in power steam boiler certain fuel preparation process has to be taken in order to ensure stable and optimal combustion in boiler furnace. Before entering furnace low caloric coal is introduced from coal bunker to the coal preparation system where milling/pulverization and drying processes of raw large coal particles are conducted. For the purpose of defining fineness of grinding of pulverized coal mill gaseous mixture at mill outlet is introduced to the separator where large coal particles, separated from outgoing mill gaseous mixture, are recirculated to mill for regrinding. In this paper character of two-phase flow in inertial separator at milling plant in TPP Nikola Tesla Unit B under various operating conditions has been analyzed. The CFD approach has been used for calculating two-phase flow in separators flow domain. Measurement data taken on site along with results of performed heat balance calculations of milling plant have been used for validating calculation model as well as for setting appropriate boundary conditions in CFD model. The CFD calculations has been performed for different positions of all regulating flaps, recirculation rates of gaseous phase and values of specific weight of solid phase in two-phase mixture. For evaluating separators operating performance at different operating regimes changes of milling capacity and fineness of grinding of pulverized coal at separators outlet have been observed. Additionally, deviation rate of trajectories of different-sized particles to the streamlines of gaseous phase has been examined.

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.

Experimental Study on Spontaneous Combustion Characteristics of Large Coal Particles after Soaking.

The spontaneous combustion of coal is affected by many factors, among which the influence of water is significant and complicated. To explore the influence of water on the spontaneous combustion characteristics of goaf residual coal, coal samples with similar particle size distributions to those of goaf residual coal were prepared. After the coal samples were immersed in water for 7–21 days and the external flowing water was drained, spontaneous combustion experiments were carried out using a temperature-programmed method. The results showed that soaking in water could promote and inhibit the spontaneous oxidative combustion of large coal particles in different temperature ranges. When the coal temperature was below 50 °C, water immersion had a significant inhibition effect on coal oxidation and spontaneous combustion. When the temperature of coal was 50–110 °C, soaking in water for 7 days could promote the oxidation and spontaneous combustion of coal. However, soaking for 14 and 21 days had a significant inhibition effect in this temperature range. When the coal temperature was higher than 110 °C, water immersion had a significant inhibition effect on the coal. Moreover, a prolonged immersion time significantly enhanced the inhibition effect. When the immersion time was less than 21 days, the spontaneous combustion of large coal particles by short-term soaking was mainly inhibited.

Influence of the contraction angle on the injection of large coal particles

The CFD–DEM coupling numerical calculation and pneumatic conveying experiment were carried out on the injection of large coal particles. The influence of contraction angle on the flow field and coal particles was studied by the distribution of turbulent kinetic energy, instantaneous distribution of particles and trajectories of particles. The performance of injection was studied by injected particles, remaining particles and mass flux of particles. The results show that the center of turbulence is located in the top of the mixing section and constriction section. The particle collision frequency increases with the increase of the contraction angle, and the conveying state of coal particles changes from uniform flow to bottom flow. When the contraction angle is 12.5°, the injector has the maximum efficiency. Since the range of mass flux is less than 1%, 10°–15° can be regarded as the optimal contraction angle.

Experimental study on collision characteristics of large coal particles (7–15 mm) in 90° elbows of pneumatic conveying systems

Pneumatic conveying of coal particles widely used for environmental friendliness and automation. In this study, orthogonal experiments were carried out in 90° elbows of large coal particles (7–15 mm) to study the effect of the elbow shape, particle size, airflow velocity, and gas supply pressure on elbow erosion. Elbows with double-layer wall structures were designed to present the erosion characteristics, particle breakage ratio, and pressure drop. Results indicate that the angle between the erosion center and the tangential direction of the elbow inlet is approximately 25°–30°. The elbow shape and airflow velocity have significant effects on all three evaluation indices, whereas the particle size and gas supply pressure have significant effects on the pressure drop. The optimal parameters of each factor were confirmed: the blind elbow with a plug, particle size of 9–11 mm, airflow velocity of 20 m/s, and gas supply pressure of 0.4 MPa.

Ignition and Combustion of Large Coal Particles in Cold Nitrogen–Oxygen Mixtures

Ignition and Combustion of Large Coal Particles in Cold Nitrogen–Oxygen Mixtures

Insitu gasification – chemical looping combustion of large coal and biomass particles: Char conversion and comminution

Utilization of large solid fuel particles in fluidized bed – Chemical Looping Combustion (CLC) has the benefits of reduced energy penalty related to carbon capture. When large fuel particles (mm-sized) are used, comminution plays a vital role in the fuel conversion rate, characteristics of ash, inventory loading and thus in the effective design of CLC systems. The present work deals with the conversion of char of large fuel particles and char fragmentation phenomenon in the insitu-gasification CLC environment. Three different coals and a woody biomass in the size range of + 8–25 mm are tested at three different bed temperatures (800 to 950 °C) in a hematite-based batch CLC unit, using steam as the fluidizing/gasification agent. The char conversion time is found to increase by 60 to 170% when particle size changes from 8 to 25 mm and reduced by 42 to 86% with the increase in bed temperature. Regardless of fuel type and feed size, the inception of char fragmentation is noticed in the very first quarter of conversion indicating its significant influence on the char burnout time. A minimum critical char size exists below which char weakening does not yield breakage, whose values vary between 4.4 and 14.2 mm depending on fuel type and feed size. Fuel type is found to be the prime influencer of char conversion time and fragmentation phenomena. This study recommends the use of large particles of all fuels up to 25 mm in CLC systems without any prior size reduction, except the high-ash coals.

Experimental and modelling studies on combined pyrolysis and gasification of large coal particle in steam and CO2 ambience

Experimental and modelling studies on combined pyrolysis and gasification of large coal particle in steam and CO2 ambience

Large Eddy Simulation on the Effects of Coal Particles Size on Turbulent Combustion Characteristics and NOx Formation Inside a Corner-Fired Furnace

Abstract The effects of pulverized coal particles’ sizes on the coal combustion characteristics are numerically studied in a laboratory-scale tangentially fired furnace. The turbulent gas flow and the coal particle motion are solved by employing the large eddy simulation (LES) and the discrete phase model (DPM). The mixture fraction probability density function (MF-PDF) is coupled to simulate the non-premixed pulverized coal combustion. It is found that the coal combustion efficiency is positively affected by the dispersion of coal powders. The particle dispersion and the coal combustion are augmented by the intensive impingement caused by the corner-injected flow. Large coal particles, with their greater inertia, enhance particle agglomerations, which limit the combustion of volatile and char. Accordingly, the average flame temperature decreases with the growing particle sizes. Also, the O2 concentration increases slightly because of the incomplete coal combustion, and the CO2 concentration decreases gradually. In contrast, the CO concentration increases markedly in the furnace center due to the presence of a reducing atmosphere. The NO concentration exhibits an exponential decline with the increased particle size. A relatively stable combustion and a relatively low NOx formation are acquired inside such a corner-fired furnace when the particle Stokes number is a little greater than 1.

Prediction improvements of ignition characteristics of isolated coal particles with a one-dimensional transient model

A modified 1-D transient model considering intra-particle thermal conduction is adopted to improve the predictions of the ignition characteristics of isolated coal particles. The study aims at resolving the incorrect prediction on the variation trend of ignition temperature Ti with the change of oxygen concentration XO2, interpreting the contradictory dependencies on coal particle size and furnace temperature and clarifying the conditions when the intra-particle thermal conduction should be considered. The predictions are compared with microgravity data in which the buoyancy effect is minimized. The results reveal that the previous ignition model with transient adiabatic criterion fails to predict the Ti variation with XO2, since it cannot accurately predict Ti and delay time in the low XO2 region. Instead, the ignition model with flammability limit ignition criterion can well predict Ti in a wide range of XO2. Intra-particle thermal conduction causes remarkable temperature differences for large coal particles, and moreover, the variation trends of surface and center temperatures with particle size are very different. The center temperature at ignition drops remarkably with increasing particle size, while the surface temperature barely changes or slightly increases with particle size. At the same particle size, the variation trends of surface and center temperatures with furnace temperature are also opposite. The ignition mode and variation trend of ignition surface temperature with particle size depends on the heating rate and particle size itself. The contradictory experimental results reported by different researchers are attributed to the particle size and temperature measurement location. The conditions necessary to consider the intra-particle thermal conduction are discussed. Lastly, the effect of the intraparticle thermal conduction is shown on an ignition mode diagram.

Soot formation characteristics in a pulverized coal flame formed in a swirling flow

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.

Simulation study on interaction coefficient of DEM in non–spherical large size (5–30 mm) coal particles

The pneumatic conveying system of coal particles can greatly reduce the dust and improve the environmental quality at underground mining workface and the surrounding of coal enterprises. The particle shape and the interaction coefficients between particles and the contact surface play important roles in the pneumatic conveying and CFD–DEM simulation. In order to build the semblable shape models and obtain the accurate interaction coefficients of large coal particles, this article establishes the contact model by the particle overlap method and describes the mathematical model of the shape characteristics for large coal particle. The particle models are simulated by adopting the multi-index mixed orthogonal experiments. The accumulation density, the porosity and the error between simulation and experiment are taken as the indexes, and the particle models and the particle contact coefficients are taken as the orthogonal test factors. As a result, three more accurate particle models and their interaction coefficients are obtained, which provide the model basis for the pneumatic conveying of large coal particles.

Research on the Pressure Dropin Horizontal Pneumatic Conveying for Large Coal Particles

As a type of airtight conveying mode, pneumatic conveying has the advantages of environmental friendliness and conveying without dust overflow. The application of the pneumatic conveying system in the field of coal particle conveying can avoid direct contact between coal particles and the atmosphere, which helps to reduce the concentration of air dust and improve environmental quality in coal production and coal consumption enterprises. In order to predict pressure drop in the pipe during the horizontal pneumatic conveying of large coal particles, the Lagrangian coupling method and DPM (discrete particle model) simulation model was used in this paper. Based on the comparison of the experimental results, the feasibility of the simulation was verified and the pressure drop in the pipe was simulated. The simulation results show that when the flow velocity is small, the simulation results of the DPM model are quite different from that of the experiment. When the flow velocity is large, the large particle horizontal pneumatic conveying behavior predicted by the model is feasible, which can provide a simulation reference for the design of the coal pneumatic conveying system.

Study of devolatilization during chemical looping combustion of large coal and biomass particles

Chemical Looping Combustion (CLC) is one of the emerging technologies for carbon capture, with less energy penalty. The present way of using pulverized coals in a fluidized bed (FB)-CLC have limitations like loss of unconverted char and gaseous combustibles, which could be mitigated by use of coarser fuel particles. Devolatilization time is a critical input for the effective design of FB-CLC systems, primarily when large fuel particles are used. The present study investigates the devolatilization time and the char yield of three coals of two shapes, namely, two high ash Indian coals and a low ash Indonesian coal and a wood (Casuarina equisetifolia) in the size range of +8–25 mm, at different fuel reactor temperatures (800–950 °C) of a hematite based CLC unit. The devolatilization times of single fuel particles during CLC are determined using a visual method called ‘Color Indistinction Method’. Indonesian coal has the longest devolatilization time among the fuels, and biomass has the least. Increasing the bed temperature enhances the rate of volatile release, whereas this effect is less pronounced in larger particles. Devolatilization of Indonesian coal is found to be strongly influenced by the changes in operating conditions. With the decrease in sphericity, a maximum of 56% reduction in devolatilization time is observed for the +20–25 mm slender particles of Indonesian coals when compared to the near-round particles. The maximum average char yields at the end of the devolatilization phase for coal and biomass are about 55–76% and 16% respectively. Char yield in coal particles increases with an increase in particle size, whereas biomass particles show relatively consistent yield across all experimental conditions. Increase in bed temperature reduces the char yields of coal up to 12% and in biomass up to 30%. High volatile Indian coal is the most influenced fuel by the changes in fuels shape. A correlation for determining devolatilization time under CLC environment is presented, and it successfully fits most of the experimental values within ±20% deviation for coals (R2 = 0.95) and within ±15% deviation for biomass (R2 = 0.97).

Prediction of Horizontal Pneumatic Conveying of Large Coal Particles Using Discrete Phase Model

The pneumatic conveying focusing on gas‐solid two‐phase flow plays an important role in a conveying system. Previous work has been conducted in the fields of small particles, where the size was less than 5 mm; however, there are few studies regarding large sizes (>5 mm). In order to predict the horizontal pneumatic conveying of large coal particles, the coupling methods based on the Euler–Lagrange approach and discrete phase model (DPM) have been used for the simulated research. Compared with the experimental results under the same working condition, the particle trajectory obtained by simulation is similar to the particle distribution at the same position in the experiment, and it turns out that the simulation method is feasible for the horizontal pneumatic conveying of large particles. Multifactor simulations are also carried out to analyse the effects of particle size, flow field velocity, solid‐gas rate, and pipe diameter on the wall abrasion during horizontal pneumatic conveying, which provides simulation reference and design guide for pneumatic conveying of large particles.

Effect of swirling flow on large coal particle pneumatic conveying

In this paper, the CFD-DEM coupling method was used for the large coal particles simulation. It was found that the axial velocity of particles was symmetrically distributed along the radial direction, and the tangential velocity was distributed symmetrically along the axis of the pipe. The concentration of coal particles in central region is conducive to maintaining large particle integrity and reducing pipe wear. The static pressure drop reflects the instantaneous energy consumption of the conveying system, and the total pressure drop calculated by the integral represents the total energy consumption. The standard deviation of static pressure reflects the degree of particles dispersion in the pipeline, and the high degree of dispersion is more conducive to convey. Based on all the above factors, when conveying large coal particles with 5 mm ≤ dp ≤ 15 mm, the range of optimum swirl number is 0.3 ≤ So ≤ 0.4.

Experimental study on the injection performance of the gas-solid injector for large coal particles

The gas-solid injector is the power source of the particle pneumatic conveying system. The injector structural parameters have a large impact on the pneumatic conveying system. To obtain the optimal structural parameters of the injector for large particles, the injection ratio in the pure flow field experiment (PFFE), the average static pressure of stable conveying (pressure index) and the particle transport time (time index) in the particle injection experiment (PIE) are used as the performance indexes. The influence of the structural factors on the injection performance in the PFFE and the PIE are obtained by the multi-index orthogonal test scheme. The optimal structural parameters of the injector in the PFFE and the PIE are compared and verified, providing significant guidance for practical applications and an experimental basis for the large particle pneumatic conveying system.