•A modified model “rough stiff particles with soft contacts” was proposed.•Acceptable agreement was found between the predicted and measured tensile strength.•The bulk flow properties of fuel powders were analyzed in view of micro contacts mechanics.This work studied the mechanical properties of a series of industrial fuel powders: bituminite, lignite, and petroleum coke. Sieved cuts of these powders were assessed and the flow properties of each sample were used to calculate tensile strengths as functions of consolidation stress. In addition, BET surface areas and dispersive surface energies were estimated from surface energy analysis. To analyze the bulk flow properties of these fuel powders in terms of micro-contact mechanics, the fundamentals of fuel powder adhesion and consolidation were reconsidered based on the “stiff particles with soft contacts” model proposed by Tomas. In the present work, a multi-contact concept was introduced to account for the irregular shapes of actual particles. This modified model was based on elastic–plastic contact deformation theory and was employed to describe the contact between rough particles and to estimate the associated inter-particle forces. The results were used in conjunction with the Rumpf approach to relate the isostatic tensile strength to the degree of consolidation. Applying average values for the powder compressibility parameters allowed the model to be used for predictive purposes, and an acceptable level of agreement was found between predicted and measured tensile strengths.Download high-res image (122KB)Download full-size image

•The effect of particle size distribution on the bulk flow behavior was investigated.•A model was provided to predict the isostatic tensile strength of powder characterized by wide size distribution.•Bulk flow properties of the industrial powder were investigated.Two groups of lignite powders characterised by different particle size distributions were prepared herein to investigate the effect of particle size distribution (PSD) on the flow properties of fine powders. The first group was composed of samples with a narrow PSD and prepared using an air classifier. The second group was made of two different samples of the same lignite material characterised by industrial-grade particle size distributions, which were much wider in range than the samples of the first group. The experimentally determined flow properties were used to understand the effect of the PSD. The packing properties and the flow behaviour of all the coal powder samples were characterised in terms of compressibility and flow properties using an FT4 powder flow rheometer. Furthermore, the Brunauer, Emmett and Teller surface areas and the dispersive surface energies were determined using a surface energy analyzer. The samples with similar mean particle sizes, but different particle size distributions, provided significantly different results. Accordingly, a micro-scale approach inspired by the Rumpf and Molerus approach was proposed to obtain a theoretical insight of the effect of the particle size distribution on the bulk flow properties of these powders. This approach was modified to account for wide particle size distributions. A procedure to estimate the tensile strength of the industrial lignite powders starting from the data relative to the narrow-size samples was also developed. The approach validity was demonstrated by the acceptable agreement between the values of the isostatic tensile strength for industrial pulverized coals estimated with the model application and the values directly calculated from the measured flow properties. The bulk flow properties of the industrial grade pulverized lignite were investigated based on the model results to highlight the most significant phenomena affecting the powder flow properties.Download high-res image (153KB)Download full-size image

•The effect of fine particles on the bulk and flow behavior of pulverized coal was investigated.•The comparison between the influences of big fines (< 20 μm) and small fines (20–40 μm) was given.•A model was provided to describe the collapse process of pulverized coal in the hopper.In this paper, pulverized coal from industrial entrained-flow pressurized gasifier was selected to study the influence of fine particles on bulk and flow properties of powder. The original material for this study was an industrial grade pulverized coal containing around 50 wt.% of fines with size smaller than 40 μm. The influence of fines (below 40 μm) on the bulk and flow behavior of the pulverized coal was analyzed and a comparison between the influence of two size-cuts (< 20 μm, 20–40 μm) was further given. Experimental results were obtained based on bulk and flow tests, including compressibility, permeability and shear tests etc. In parallel, a transparent Perspex hopper and an outlet uniaxial stress tester apparatus were fabricated to determine the discharge behavior and the collapse phenomena in the hopper, respectively. The stress at the outlet of the hopper was decreased gradually until reached a stable value and the more fine particles a pulverized coal contains, the more time required for it to stabilize. Accordingly, the pulverized coal performed different discharge behaviors at different packing time. Finally, a packing model was proposed to quantitatively describe the packing process of pulverized coal in the hopper and explain the various discharge behaviors at different packing times.

This paper aims at predicting the solid-mass flow rate of pulverized coal in dense-phase pneumatic conveying systems. A one-dimensional model was developed based on the proper modification of a dilute-flow model by inserting a two-phase flow multiplier. The relationships between the model parameters (discharge coefficient and pressure ratio parameter) and the hydrodynamic nondimensional numbers (Reynolds and Stokes numbers) were determined, and the fitting coefficients were obtained from calibration tests by using regression analysis. Two methods attached to the model were proposed to predict the solid-mass flow rate, and both can provide satisfactory predictions with acceptable errors. These results indicated the possibility of applying a venturi device to measure the solid-mass flow rate at high-pressure and dense-phase conditions.

•This work provides experimental data on aerated discharge at a pilot scale.•Powder characterized with shear testing and fluidization.•Internal relations between fluidization and aerated discharge were highlighted.•The relevance of the aggregative behavior of the powder was determined.•Solid discharge rates were estimated by an aggregative discharge model.A pilot-scale loop feeding system was used to investigate the discharge behavior of pulverized coal. Experiments on discharge of pulverized coal from an aerated hopper under several aeration rates were carried out. The flow properties of the pulverized coal were measured with different shear testing methods. A previously developed predictive model was adopted to estimate the discharge of pulverized coal. Powder flow properties extrapolated to zero consolidation were used for model application. A correlation between the voidage between aggregates and the gas superficial velocity was obtained from independent fluidization tests and applied into aerated discharge with proper modifications. Acceptable agreement was found between the predicted and the measured solid discharge rates. These results indicate the applicability of the model also to predict the powder flow from pilot scale units.Download full-size image

This study investigated the applicability of computational particle fluid dynamic (CPFD) numerical scheme for simulating flows in a 3-D hopper. A glass bead with a particle size distribution was used as the experimental material, which is characterized on the limit between group A and group B powder. The discharge of glass bead particles was simulated using the CPFD model. The flow snapshots and the solid discharge rates were successfully captured by the CPFD calculations and compared well with the experimental results. It therefore confirmed a good feasibility of CPFD method to simulate the complex flow in the 3-D hopper. Consequently, the snapshots as well as the concrete values of the solid volume fraction, particle and gas velocities, and pressure in the hopper were given by the CPFD model. Based on the simulation results and the hopper structure, three flow regions were divided and flow characteristics in these regions were analyzed. It shows dense-flow in the hopper region, dilute-flue in the transition region, and the negative pressure gradient in the standpipe.

•The gas–solid flow through venturi is studied under high pressure and concentration.•The flow characteristics of single-phase and gas–solid through venturi are compared.•The influencing factors of pressure drop for the gas–solid flow are researched.•The pressure drop models of gas–solid flow are established adopting Farber's method.The present work details the flow characteristics and pressure drop of the gas–coal mixture through venturi under high pressure and concentration. A series of experiments of both single-phase gas and gas–coal mixture flows through the venturi were carried out, and the distribution of pressure, volumetric loading ratio and superficial gas velocity were obtained and compared. The results show that a sharp decrease in static pressure and volumetric loading ratio was observed inside the venturi. The degree of the decrease of pressure in the diffuser section is the lowest (≤ 20%). When keeping the average throat gas velocity same, the inlet gas velocity of gas–coal mixture flows is lower than that of single-phase gas flow, while the outlet gas velocity is higher. In addition, the variation of throat gas velocity is more remarkable. It may indicate a greater energy transfer with the presence of particles. The pressure drop of the gas–coal mixture increases with the increase of superficial gas velocity, volumetric loading ratio and gas density. Further, the pressure drop models of single-phase gas flow and gas–solid flows through the venturi have been established by adopting the Farbar's approach based on the mathematical regression analysis. The models contribute to predict the pressure drop of venturi with deviations below 25%.The experimental program was undertaken on the facility of dense phase pneumatic conveying of pulverized coal, where the venturi was installed on the vertical pipeline. The schematic diagram was illustrated in Fig. 1. The whole system consisted of a gas supply system, feeding and receiving vessels, pipelines, a venturi, a measuring system, a filter unit, and a data acquisition system.

•The performance of pilot-scale entrained-flow gasifier was investigated.•Effects of coal property, feeding mode and gasifier lining type were discussed.•The performance of industrial-scale gasifier was predicted through simulation.Effect of the coal property, the coal feeding mode (coal-water slurry (CWS) or dry coal powder (dry feeding)) and the gasifier lining type (refractory brick wall or membrane wall) on the performance of pilot-scale entrained-flow coal gasifier with a throughput of 30 tons of coal per day was systematically investigated. Based on the pilot plant results, the impact of coal type, coal feeding mode and the gasifier lining type on the coal gasification process was analysed quantitatively. Moreover, through simulation, raw material consumptions with different combinations of feeding mode and lining type for commercialized gasifiers were predicted, which provides useful information in choosing a suitable gasification technology and the optimization of coal gasification process.

The pneumatic conveying experiments at the high solid–gas ratio (120–300 kg/m3) were carried out to compare their conveying characteristics separately using CO2 and air as carrier gas. The differences between them are found to be related with fluidized state of pulverized coal in the feeding vessel and can be explained with material properties involving particle–gas interactions, permeability. Employing electrical capacitance tomography (ECT), the transition from full pipe flow to annular flow was observed with increasing the superficial gas velocity (3.0–8.5 m/s) in the vertical pipe and the flow patterns with CO2 and air are found to be similar. The analysis shows there is a significant association between sharp fluctuations of the pressure signals and gas slug. When gas slug was observed in the pipe, there was a large pressure pulse. The occurrence of gas slug is also found to be higher in the transporting with CO2 as carrier gas. The pressure signals may therefore be used to identify gas slug appearing in the pipe. The energy analysis is presented to find that the energy consumption with CO2 is about 7.5% higher than that with air at the same gas flow rate, but the required energy with CO2 is increased by about 20% than that with air at the same solid mass flow rate.Pulverized coal mass flow rate increases with the increasing gas flow rate. The solid mass flow rate for CO2 is reduced by 7%, compared with that for air at the same flow rate.Highlights► The different conveying characteristics using CO2 and air are firstly proposed. ► Full pipe flow and annular flow are observed in vertical conveying with CO2 by ECT. ► Sharp fluctuations of pressure signals are found to be related with gas slug. ► Energy consumption with CO2 is higher than that with air at the same gas flow rate.

The influence of gas type (air and CO2) and hopper pressure (0–400 kPa) on the discharge of the pulverized coal was investigated. For aerated discharge, fluidization in the hopper is the initial state of the discharge process; the state of gas–solid fluidization affects the subsequent hopper discharging significantly. Compared to air, CO2 showed a weaker ability to fluidize the pulverized coal, and thus it was more difficult to improve the hopper discharge at atmospheric pressure. On the other hand, increasing the hopper pressure did not affect the basic discharge law but increased the discharge rate to a certain degree. In addition, with the increase of the hopper pressure, the discharge differences between the air and CO2 aeration series reduced, because the discharge rate had a larger promotion from atmospheric pressure to 400 kPa for the CO2 case.

Using CO2 as a carrier gas in pneumatic conveying of pulverized coal instead of N2 has been paid more and more attention with the wide application of dry-feed entrained-flow coal gasification technology due to its attractive effects as a reactant. Accordingly, a pilot-scale performance of entrained-flow gasification using CO2 as the carrier gas of pulverized coal was investigated at gasifier temperatures between 1300 and 1400 °C. The differences in solid mass flow rate and flow stability are insignificant in the dense-phase pneumatic conveying between CO2 and N2 carrier provided that the conveying pressure differences and solid velocities in the pipeline are kept constant. Regarding syngas composition in dry basis, there was a desired drop in inert gas (N2) concentration from about 6% to less than 2%, while the CO2 concentration increased by about 4%age points in optimal operation conditions when CO2 carrier substituted for N2. At the same time, gasification profiles in different operation conditions indicated that CO2 carrier can act as an auxiliary gasification agent. Steam as another important gasification agent, however, cannot be completely substituted by CO2 carrier gas.

The principle aim of this paper is to understand the crystallization of coal ash slags and the effects on the viscosity by means of high temperature viscosity measurements, in combination with FactSage modeling, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Four coal ashes with the fusion temperatures between 1130 and 1470 °C applied in entrained flow gasifiers in China were prepared for this study. The thermodynamic modeling was carried out using FactSage 6.2 software to predict the composition of homogeneous liquid slag systems as well as heterogeneous slag systems. It can be concluded that the viscosity of coal ash samples is closely related to the phases at high temperatures. The viscosity increases significantly until the mass percentage of the solid phases reaches a certain value (15.15–33.82%). For the coal ash samples enriched in Al2O3 and SiO2 with high AFT, mullite is the first solid phase forming in the liquid slag above 1600 °C, followed by quartz, anorthite, and ferro-cordierite, until the molten slag finally transforms to a 100% solid state. For the coal ash samples enriched in CaO and Fe2O3 with relatively lower AFT, anorthite is the first solid phase forming and separating from the liquid slag, followed by the ferrous aluminosilicate. The crystallization temperature of solid phases, as well as the crystallization rate, is determined by the chemical composition of coal ash samples. The XRD findings were further supported with FactSage thermochemical modeling. The Krieger–Dougherty equation combining with the Watt and Fereday model was used to simulate the viscosity results, which provided a good fit for coal ash samples with a low proportion of crystalline phases.

The influence of gas type (air, He, H2 and CO2) on the fluidization and discharge characteristics of the pulverized coal was investigated in this paper. It was concluded that the gas viscosity was dominant over the gas density in influencing the fluidization quality. The increase of gas viscosity improved the fluidization uniformity, delayed the visible bubbling and promoted the discharge stability. Compared with air, a higher proportion of H2 rises through the coal bed while a higher proportion of CO2 escapes through the hopper outlet during the discharge process. Experimental results show that the discharge rate increases with the addition of H2 in the whole experimental range and CO2 always corresponds to the minimum discharge rate. In spite of the very different effects produced by the various gases, the relationship between the discharge rate and the pressure difference does not change a lot. Nedderman's mode was used to predict the discharge rate, which agreed well with the experimental data with the error kept below 20% except individual points at low aeration rate.The hopper, with an outlet diameter of 40 mm, is made from transparent Perspex to allow a visual inspection of the inside. It has a bin with a diameter of 560 mm and a height of 1800 mm. The conical hopper with a half opening angle of 15°, can be used to add the aerating gas.Highlights► Gas viscosity has a greater effect on fluidization properties than gas density. ► H2 can easily penetrate the coal bed and form a bubbling fluidization regime. ► CO2 can hardly fluidize the pulverized coal for its strong gas adsorption. ► The higher solid discharge rate corresponds to the more effective fluidization. ► The solid discharge rates can be predicted well by Nedderman's model.

Research on flow patterns can provide a better understanding of particle dynamics in dense-phase pneumatic conveying of pulverized coal. In this paper, electrical capacitance tomography (ECT) has been employed to study flow patterns in the 20 mm diameter vertical riser. Three flow patterns were identified on the basis of ECT image analysis, and their characteristics and formation mechanisms were discussed. The flow patterns at high solid concentration were more asymmetrical than those at low solid concentration. Solid concentration signals obtained from ECT were analyzed by different signal analysis methods including relative standard deviation (RSD), probability density function (PDF), and power spectral density function (PSD), which were verified to be effective enough to identify the characteristics of the different flow patterns. Additionally, the Bi model (Bi, H. T.; Grace, J. R. Int. J. Multiphase Flow1995, 21, 1229–1239) was modified to effectively predict chocking velocity in the vertical dense-phase pneumatic conveying of pulverized coal.

An aerated discharge system was established in this paper to investigate the discharge of pulverized coal from a pressurized aerated hopper. Two opposite effects of aerated gas on solid were first revealed. “Fluidized pressurization”, which effectively fluidized the solid and improved the subsequent hopper discharge, was then developed. The effect of hopper pressure on the discharge of pulverized coal was studied. Our experimental results showed that the gas volumetric flow rate increased and the gas superficial velocity decreased with the increase of the hopper pressure in the range of 0–1800 kPa. It was confirmed that more energy was needed; the uncertainty and instability was increased to discharge pulverized coal at higher pressures. Gas momentum flux was defined and used to describe the effect of aeration. The optimum gas momentum flux, which was independent of the hopper pressure, was obtained on the basis of the experimental data. The optimum gas volumetric flow rates and the optimum gas superficial velocities corresponding to the maximum solid discharge rates were further predicted, which agreed well with the experimental data. On the other hand, the hopper pressure also showed a positive effect on the solid discharge, as the maximum solid discharge rate increased gradually with the hopper pressure until a limit value of about 8000 kg/h was reached at 800 kPa.

Understanding flow patterns and their variability is important for optimal design and trouble free dense phase pneumatic conveying of pulverized coal in a horizontal tube. Employing the electrical capacitance tomography (ECT), six flow patterns were identified and utilized for quantitative analysis based on the value and distribution of cross-sectional solid concentration. The dense-phase flow patterns in the horizontal tube of the pneumatic conveying system were somehow variable even when the operating conditions were unchanged. The probability calculation results suggest changing multiple flow patterns with one or two dominant flow for each of the seven sets of experimental conveying conditions and that a finite change in the dominant flow pattern would occur with an increasing superficial gas velocity. The power spectral density (PSD) function and the Hurst exponent of the pressure signals of the pulverized coal were well correlated with its flow patterns in a horizontal tube. The PSD functions and probability density functions (PDFs) of the void fraction signals from ECT are found to be related with flow patterns and can be used to quantitatively identify flow regimes. The ECT data may therefore be utilized for monitoring the flow patterns in a horizontal tube employed for pneumatic conveying of pulverized coal.The sensor was composed of eight sensing electrodes and radial electrodes placed outside a PVC pipe with 20 mm ID and 1.5 mm pipe thickness. The eight sensing electrodes 80 mm long were supported by the radial electrodes.Research Highlights► Research flow patterns of pulverized coal dense phase conveying through ECT. ► Six flow patterns are classified and utilized for quantitative analysis. ► Correlate flow patterns and the measurement signals using various methods.

The cohesive characteristics of pulverized coal were revealed firstly by investigating the flow properties and fluidization behaviours of coal B and coal Y. Different from the classic fluidization profiles, the cohesive coals displayed complex behaviours during the fluidization process. Saw toothed pressure curves were found in the process with increasing aeration velocity. The prevailing mechanisms for cohesive coal discharge were analyzed and verified through the experiments. Experimental results of hopper discharge showed that most models had relatively large deviations to predict the discharge rate. Based on Huang's model, an improved model for discharge rate prediction within ± 20% deviation in the experimental range was presented.The hopper, with an outlet diameter 40 mm, is made from transparent Perspex to allow a visual inspection. It has a bin with a diameter of 560 mm and a height of 1800 m. The conical hopper with a half opening angle of 15°, can be used to add the aeration gas.Research Highlights► The pressurized entrained-flow gasification process is promising. Stable and effective discharge of the pulverized coal is an essential part of it. ► The strong cohesive characteristic of the pulverized coal was revealed. ► The prevailing mechanisms for cohesive coal discharge were analyzed and verified.

Pressure drops are measured on different nozzles of various pipe sizes in dense phase pulverized coal pneumatic conveying. From the experimental results, we conclude that the effect of the gas phase nozzle pressure drop is negligible when comparing with the solid phase pressure drop in the experimental range. The main influence factors contributing to the nozzle pressure drop are gas and solid mass flow rate, solids loading ratio, and the diameters of the nozzle inlet and outlet. A new model was developed to predict the nozzle pressure drop in dense phase pneumatic conveying of pulverized coal based on the Barth's pneumatic conveying theory. The pressure drop predictions from the model are in good agreement with the experimental values. The model quantified the important influence factors of the nozzle pressure drop.Graphical abstractDifferent from the flow in pipe, the special geometry of the nozzle makes the gas-solid flow more complex. Based on Barth’s pneumatic conveying theory, a model was developed to predict the nozzle pressure drop in dense phase pneumatic conveying of pulverized coal. The predictions of the pressure drop from the model are in good agreement with the experimental values.

This paper presents some experimental results of discharge characteristic of cohesive fine coal from aerated hopper. A plexiglas hopper was used to visualize the flow pattern of the coal discharge. Arch can be easily formed during the gravity discharge. The addition of gas can improve the stability and the flow rate of the discharge. At low gas flow rate, two kinds of flow pattern existed at the same time, mass flow and swirl flow. As the gas flow rate increased up to a high level, gas balanced arch occurred, and the discharge rate decreased sharply. In addition, the effect of aeration pattern and outlet diameter on the coal discharge has been studied. Different aeration position results in different flow pattern and stability. Controlling the gas flow rate can realize maximum discharge rate. During the aerated discharge, the influence of outlet diameter and bulk status is changed with the gas flow rate compared with the gravity discharge. According to this law, a new model has been developed to predict the mass flow rate.Graphical abstractThis paper presents some experimental results of discharge characteristic of cohesive fine coal from aerated hopper. A plexiglas hopper was used to visualize the flow pattern of the coal discharge. Arch can be easily formed during the gravity discharge. The addition of gas can improve the stability and the flow rate of the discharge. The effect of aeration pattern and outlet diameter on the coal discharge has been studied.

This paper presents the effects of particle size, moisture content, and coal type on the flowability of pulverized coals. A plexiglas hopper for visualizing the coal discharge and a Jenike-type shear apparatus for measuring friction characteristics were used to investigate the flowability. The fine coal displays weaker flowability because of its larger specific surface area and stronger agglomeration in comparison to the relatively large-size pulverized coal. The number of agglomerated particles C0 increases with the decrease in the particle size, which is also confirmed by the scanning electron microscopy (SEM) images. Increasing the moisture content tends to make the pulverized coal more cohesive. Cohesive forces acting among wet coal particles are mainly due to capillary forces associated with liquid bridging, which increases with the moisture content. However, the moisture content to some extent may act as a lubricant and improve flow along the wall surface. The coal type also has a significant effect on the flowability because of the differences in their compositions and physical structures. The measuring results show that the flowability of pulverized coal improves with the increasing coal rank.

Performance of an entrained-flow gasification technology of pulverized coal in pilot-scale plant is introduced. The gasifier was operated for a throughput of 30–45 t coal per day at pressures of 1–3 MPa. Dense-phase pneumatic conveying was employed for coal's feeding to the gasifier using nitrogen and carbon dioxide as carrier gas, respectively. Effects of the operating conditions including oxygen/carbon ratio and steam/carbon ratio on gasification results were investigated, and the concentration of (CO + H2) in gaseous products reached up to about 97% (vol., dry basis) when CO2 was employed as carrier gas. Moreover, performances of some important instruments in the conveying system of pulverized coal, such as the level indicator and the solid mass flow meter, were also investigated. The typical operating results in this plant such as (CO + H2) concentration, oxygen consumption, coal consumption, carbon conversion and cold gas efficiency were almost as good as those of some well-known dry-fed entrained-flow coal gasification plants.

•Powder discharge from hopper-standpipe system was modelled by CPFD method.•Solids and gas flow in hopper-standpipe geometries were obtained and analyzed.•Hopper outlet diameter and half angle were varied to evaluate their effect.•CPFD computation times are in a range between tens and hundreds of minutes.•The CPFD model predicts solid mass flow rates with errors generally below 5%.In this paper, computational particle fluid dynamic (CPFD) modelling approach was used to describe the discharge of a fine glass beads powder from different hopper-standpipe geometries. The comparison between the CPFD predictions and the experimental results in terms of solid discharge rates, surface cone shape during discharge and pressure drops in the standpipe. The comparison allowed to assess on the possibility to use the CPFD modelling approach to simulate the powder flow in the hopper-standpipe system even accounting for the rather complex interactions between the interstitial gas and the particles occurring in the presence of a standpipe. Furthermore, the effect of hopper geometry on powder discharge was investigated with the CPFD model and verified experimentally in some purposely built hoppers. Finally, the relationships between the hopper geometry parameters (hopper outlet diameter and hopper half angle) and the flow parameters (solid discharge rate, height of characteristic surface, particle volume fraction, particle velocity, gas pressure and flow pattern) were obtained.