Particle residence time distribution and axial dispersion coefficient in a pressurized circulating fluidized bed by using multiphase particle-in-cell simulation

The particle residence time distribution(RTD)and axial dispersion coefficient are key parameters for the design and operation of a pressurized circulating fluidized bed(PCFB).In this study,the effects of pressure(0.1-0.6 MPa),fluidizing gas velocity(2-7 m·s^(-1)),and solid circulation rate(10-90 kg·m^(-2)·s^(-1))on particle RTD and axial dispersion coefficient in a PCFB are numerically investigated based on the multiphase particle-in-cell(MP-PIC)method.The details of the gas-solid flow behaviors of PCFB are revealed.Based on the gas-solid flow pattern,the particles tend to move more orderly under elevated pressures.With an increase in either fluidizing gas velocity or solid circulation rate,the mean residence time of particles decreases while the axial dispersion coefficient increases.With an increase in pressure,the core-annulus flow is strengthened,which leads to a wider shape of the particle RTD curve and a larger mean particle residence time.The back-mixing of particles increases with increasing pressure,resulting in an increase in the axial dispersion coefficient.

Numerical Simulation of Pressurized Spouted Fluidized Bed for Coal Semi-Gasification

Numerical simulation study is conducted for a pressurized spouted fluidized bed coal carbonizer, in which hydrodynamics of pressurized spouted fluidized bed, chemical reactions and energy balance are taken into account. The effect of operating conditions such as bed pressure, air and steam mass flow ratio, temperature on product compositions in the bed is investigated. According to the calculated results, bed pressure and bed temperature have the key effects on coal semi gasification.

Computational study of bubble coalescence/break-up behaviors and bubble size distribution in a 3-D pressurized bubbling gas-solid fluidized bed of Geldart A particles

A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles.High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow.A finegrid,which is in the range of 3–4 particle diameters,was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase.A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm(MS3 DATA)and applied to detect the bubble statistics,such as bubble size,location in each time frame and relative position between two adjacent time frames,from numerical simulations.The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing.The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm.The results showed that the bubble size distributed non-uniformly over cross-sections in the bed.The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained,and the bubble rise velocity follows Davidson’s correlation closely.Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work.The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.

A critical review on direct catalytic hydrogasification of coal into CH_(4):catalysis process configurations,evaluations,and prospects

Coal catalytic hydrogasification(CCHG)is a straightforward approach for producing CH_(4),which shows advantages over the mature coal-to-CH_(4) technologies from the perspectives of CH_(4) yield,thermal efficiency,and CO_(2) emission.The core of CCHG is to make carbon in coal convert into CH_(4) efficiently with a catalyst.In the past decades,intensive research has been devoted to catalytic hydrogasification of model carbon(pitch coke,activated carbon,coal char).However,the chemical process of CCHG is still not well understood because the coal structure is more complicated,and CCHG is a combination of coal catalytic hydropyrolysis and coal char catalytic hydrogasification.This review seeks to shed light on the catalytic process of raw coal during CCHG.The configuration of suitable catalysts,operating conditions,and feedstocks for tailoring CH_(4) formation were identified,and the underlying mechanisms were elucidated.Based on these results,the CCHG process was evaluated,emphasizing pollutant emissions,energy efficiency,and reactor design.Furthermore,the opportunities and strategic approaches for CCHG under the restraint of carbon neutrality were highlighted by considering the penetration of“green”H2,biomass,and CO_(2) into CCHG.Preliminary investigations from our laboratories demonstrated that the integrated CCHG and biomass/CO_(2) hydrogenation process could perform as an emerging pathway for boosting CH_(4) production by consuming fewer fossil fuels,fulfilling the context of green manufacturing.This work not only provides systematic knowledge of CCHG but also helps to guide the efficient hydrogenation of other carbonaceous resources such as biomass,CO_(2),and coal-derived wastes.

Effects of pressure on flow characteristics in a pressurized circulating fluidized bed

The effects of operating pressure on the gas-solid flow characteristics in a pressurized circulating fluidized bed(CFB)gasification system were studied by experiment and simulation.Elevated operating pressure was found to increase the particle uniformity in the axial and radial directions.Experiments and simulations both showed that as the operating pressure increased,the pressure drops along the riser height decreased.The experimental pressure drops at the same locations decreased when the operating pressure increased,but the opposite trend was observed in the simulation results.A proper drag model must be developed for the accurate simulation of pressurized CFB operation,and the accuracy of electrical capacitance tomography(ECT)measurement and cross-correlation analysis should be improved for better prediction of the particle circulation flux.

Thermal performance analysis of advanced partial gasification combined cycle

This paper presents the thermodynamic performance analysis and comparison of four kinds of advanced pressurized fluidized bed combustion combined cycle (APFBC-CC) system schemes, two based on pressurized fluidized bed (PFB) combustion and the other two based on atmospheric circulating fluidized bed (CFB) combustion. The results show that the first scheme avoids high temperature gas filter, but has the lower cycle efficiency and syngas heating value. The second scheme can gain the highest cycle efficiency, however it is better to now lower the filter operating temperature. The third and fourth schemes, based on CFB, have lower efficiencies than the second one. But the fourth one, with preheating air/steam for gasification, can obtain the highest heating value of syngas and gain higher efficiency than the third one.

Statistical and frequency analysis of the pressure fluctuation in a fluidized bed of non-spherical particles

In this paper, the pressure fluctuation in a fluidized bed was measured and processed via standard devia- tion and power spectrum analysis to investigate the dynamic behavior of the transition from the bubbling to turbulent regime. Two types （Geldart B and D） of non-spherical particles, screened from real bed materials, and their mixture were used as the bed materials. The experiments were conducted in a semi- industrial testing apparatus. The experimental results indicated that the fluidization characteristics of the non-spherical Geldart D particles differed from that of the spherical particles at gas velocities beyond the transition velocity Uo The standard deviation of the pressure fluctuation measured in the bed increased with the gas velocity, while that measured in the plenum remained constant. Compared to the coarse particles, the fine particles exerted a stronger influence on the dynamic behavior of the fluidized bed and promoted the fluidization regime transition from bubbling toward turbulent. The power spectrum of the pressure fluctuation was calculated using the auto-regressive （AR） model; the hydrodynamics of the flu- idized bed were characterized by the major frequency of the power spectrum of the pressure fluctuation. By combining the standard deviation analysis, a new method was proposed to determine the transition velocity Uk via the analysis of the change in the major frequency. The first major frequency was observed to vary within the range of 1.5 to 3 Hz.

Effect of agitation on the characteristics of air dense medium fluidization

In order to study the effect of agitation on the characteristics of air dense medium fluidization, we designed and constructed an agitation device. Analyses were then conducted on the fluidization characteristics curves, the bed density stability and the average bubble rise velocity Uaunder different agitation conditions. The results indicated that a lower bed pressure drop(without considering lower gas velocity in a fixed bed stage) and higher minimum fluidized velocity are achieved with increasing agitation speed.The height d(distance between the lower blades and air distribution plate) at which the agitation paddle was located had a considerable effect on the stability of the bed density at 9.36 cm/s < U < 10.70 cm/s. The higher the value of d, the better the stability, and the standard deviation of the bed density fluctuation r dropped to 0.0364 g/cm^3 at the ideal condition of d = 40 mm. The agitation speed also had a significant influence on the fluidization performance, and r was only 0.0286 g/cm^3 at an agitation speed of N = 75 r/min. The average bubble rise velocity decreased significantly with increasing agitation speed under the operating condition of 1.50 cm/s < U–U_(mf)< 3.50 cm/s. This shows that appropriate agitation contributes to a significant improvement in the fluidization quality in a fluidized bed, and enhances the separation performance of a fluidized bed.

Flow-regime transitions in fluidized beds of non-spherical particles

Fluidized beds frequently involve non-spherical particles, especially if biomass is present. For spheri- cal particles, numerous experimental investigations have been reported in the literature. In contrast, complex-shaped particles have received much less attention. There is a lack of understanding of how par- ticle shape influences flow-regime transitions. In this study, differently shaped Geldart group D particles are experimentally examined. Bed height, pressure drop, and their respective fluctuations are analyzed. With increasing deviation of particle shape from spheres, differences in flow-regime transitions occur with a tendency for the bed to form channels instead of undergoing smooth fluidization. The correlations available in the literature for spherical particles are limited in their applicability when used to predict regime changes for complex-shaped particles. Hence, based on existing correlations, improvements are derived.

Characterization of gas-liquid-solid fluidized beds by S statistics

The hydrodynamics of a gas-liquid-solid fluidized bed was investigated by applying the S statistics method to pressure fluctuations measured under various operating conditions in a laboratory-scale bed. S statistics tests reveal the existence of three transition velocities, especially at low gas velocities. Four distinct fluidization regimes, namely, the compacted bed, agitated bed and coalesced and discrete bubble regimes were detected. A comparison of reconstructed attractors of pressure fluctuations measured at different axial positions along the riser and with various solid loadings showed significant differences in the signals compared before fluidization, especially at minimum liquid agitation velocity. Close to the minimum liquid fluidization velocity and high liquid velocities, the variation in particle size has an insignificant effect on the bed hydrodynamics. Therefore, S statistics is a reliable method to demar- cate different fluidization regimes and to characterize the influence of various operating conditions on the hydrodynamics of gas-liquid-solid fluidized beds. The method is applicable in large-scale industrial installations to detect dynamic changes within a bed, such as regime transitions or agglomeration.

Hydrodynamics of three-phase fluidization of homogeneous ternary mixture in a conical conduit-Experimental and statistical analysis

Hydrodynamics of conical fluidized bed differ from that of columnar beds by the fact that a velocity gradient exists along the axial direction of the bed.The gas–liquid–solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operations.Such a device is of considerable industrial importance as evident from its wide applications in chemical,refining,petrochemical,biochemical processing,pharmaceutical and food industries.To explore this,a series of experiments have been carried out for homogeneous well-mixed ternary mixtures of dolomite of varying compositions in a three-phase conical fluidized bed.The hydrodynamic characteristics determined included the bed pressure drop,bed fluctuation and bed expansion ratios.The single and combined effects of operating parameters such as superficial gas velocity,superficial liquid velocity,initial static bed height,average particle size and cone angle on the responses have been analyzed using response surface methodology(RSM).A 25 full factorial central composite experimental design has been employed.Analysis of variance(ANOVA) showed a high coefficient of determination value and satisfactory prediction second-order regression models have been derived.Experimental values of bed pressure drop,bed fluctuation and bed expansion ratios have been found to agree well with the developed correlations.

Magnetically assisted gas-solid fluidization in a tapered vessel:Part II Dimensionless bed expansion scaling

The article presents an effort to create dimensionless scaling correlations of the overall bed porosity in the case of magnetically assisted fluidization in a tapered vessel with external transverse magnetic field. This is a stand of portion of new branch in the magnetically assisted fluidization recently created concerning employment of tapered vessels. Dimensional analysis based on ＂pressure transform＂ of the initial set of variables and involving the magnetic granular Bond number has been applied to develop scaling relationships of dimensionless groups representing ratios of pressures created by the fluid flow, gravity and the magnetic field over an elementary volume of the fluidized bed. Special attention has been paid on the existing data correlations developed for non-magnetic beds and the links to the new ones especially developed for tapered magnetic counterparts. A special dimensionless variable Xp = （Ar△Dbt）1/3√RgMQ combining Archimedes and Rosensweig numbers has been conceived for porosity correlation. Data correlations have been performed by power-law, exponential decay and asymptotic functions with analysis of their adequacies and accuracies of approximation.

Research and Development on PFBC-CC in China and Jiawang Pilot Plant Project

Pressurized Fluidized Bed Combustion(PFBC) is recognized as an advanced coal-fired technology which can improve efficiency in combined cycle scheme and reduce environmental pollution. Progressive status on PFBC-CC in China is presented in this paper. Test results on a 1 MWt bench scale experimental PFBC facility is reviewed briefly. Based on retrofitting of an old steam power plant located at Jiawang,a project to construct a PFBC-CC pilot plant is under way. Designed capacity of the pilot plant is about 15 MWe, 3 MWe from gas cycle and 12 MWe from steam cycle. The system configuration, main design parameters, estimated technical performance as well as construction schedule of the pilot plant are described. The bright future for PFBC-CC in China is also indicated.