Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.

Wavelet denoising and nonlinear analysis of solids concentration signal in circulating fluidized bed riser

Particles,particle aggregates,and reactor walls complicate the dynamic microstructures of circulating fluidized beds(CFBs).Using local solids concentration data from a 10-m-high and 76.2-mm-inner-diameter riser with FCC(Fluid Catalytic Cracking)particles(dp=67μm,ρp=1500 kg/m^3),this paper presents an improved denoising process for use before nonlinear chaos analysis.Using the soft-threshold denoising method in the wavelet domain with experimental empty bed signals as base data to estimate the denoising threshold,an efficient denoising algorithm was proposed and used for the dynamic signals in CFBs.Analysis shows that for the local solids concentration time series,high-frequency fluctuations may be one of the system properties,while noise interference can also make a low-frequency contribution.An exact denoising method is needed in such cases.The correlation dimension and Kolmogorov entropy were calculated using denoised data and the results showed that the particle behavior in the CFB is highly complex.Generally,two correlation dimensions coexist in a low-flux CFB.The first correlation dimension is low and corresponds to small-scale fluctuations that reveal a high-frequency pseudo-periodic movement,but the second correlation dimension is high and corresponds to large-scale fluctuations that indicate multi-frequency movements,including particle aggregation and breakage.At the same axial level,the first correlation dimensions change slightly with radial position,and the main tendency is high at the center but slightly lower near the wall.However,the second correlation dimensions show large changes along the radial direction,are again high in the core region,and after r/R≥0.6(r as radial position,R as radius of the riser),the dimensions clearly drop down.This indicates that the particle behavior is more complex and has higher degrees of freedom at the center,but clusters near the wall are restrained to some degree because of wall effects.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

Effect of a pulsating flow on hydrodynamics and fluid catalytic cracking chemical reaction in a circulating fluidized bed riser

The three-dimensional computational fuid dynamics(3D-CFD)of a pulsating flow applied to the fluid catalytic cracking(FCC)reaction was investigated in the riser of a circulating fluidized bed reactor.The kinetic parameters of the FCC and coke burning reactions for predicting the reactant conversion and product yield percentages were applied.To increase the reactant conversion level and product yield.the effect of the pulsating flow operating parameters was considered using a 2k statistical experimental design with four factors(amplitude,frequency,types of the waveform,and amplitude ratio).The 3DCFD simulation was successfully validated from the experimental literature data.The frequency and type of the waveform were found to be the significant operating parameters.The expression of the fitted regression model and response surface contour were derived and revealed that the pulsating flow provides a higher reactant conversion level and product yield percentages compared to a non-pulsating or steady flow.

High-density circulating fluidized bed gasifier for advanced IGCC/IGFC-Advantages and challenges

Coal-fired Integrated Gasification Combined Cycle （IGCC） and Integrated coal Gasification Fuel-cell Com- bined cycle （IGFC） are being developed as high-efficiency electric power generation technology. However, the highest theoretical gross thermal efficiency of the conventional IGCC]IGFC is still below 52~. In order to obtain higher power generation efficiency, an advanced IGCC （A-IGCC） or advanced IGFC （A-IGFC） sys- tem making use of the exergy recuperation concept by recycling waste heat from gas turbine or fuel cells for steam gasification of coal and biomass was proposed in our laboratory, Corresponding to this system, a novel high-density triple-bed combined circulating fluidized bed （TBCFB） gasifier, composed of a downer pyrolyzer, a bubbling fluidized bed char gasifier, and a riser combustor, was proposed to replace traditional gasifiers such as the entrained flow bed gasifier. The new system is expected to more effectively utilize the waste heat from gas turbines or fuel cells and the heat produced by the combustion of the unreacted char in the riser combustor for pyrolysis and gasification of coal and biomass. In this short review, the advantages and future challenges in the development of high-density TBCFB gasifier are presented and discussed.

Particle size distribution in CPFD modeling of gas-solid flows in a CFB riser

A computational particle fluid dynamics （CPFD） numerical method to model gas-solid flows in a circulating fluidized bed （CFB） riser was used to assess the effects of particle size distribution （PSD） on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distri- butions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and wails also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.

Cluster identification using image processing

By closely examining hue, saturation and value （HSV） images of the solids holdup distribution in a riser, it can be seen that a ＂cluster＂ is the combination of a relatively stable core cluster of the highest solids holdups and constantly changing cluster clouds of solids holdups that are higher than the dilute phase. Based on this analysis, a threshold selection method maximizing the inter-class variance between the background and foreground classes is introduced. A systematic cluster identification process is therefore proposed that： （｜） applies the threshold selection method to obtain the critical solids holdup threshold ~c to discriminate dense and dilute phases and （2） applies the method again in the dense phase regions to obtain the cluster solids holdup threshold Ssct that identifies the core clusters. Using this systematic process, clusters of different shapes and sizes and a relatively clear boundary can be visualized clearly and identified accurately. Using ~sct, the core cluster fraction is calculated by dividing the total number of pixels in the core cluster by the total number of image pixels. The variation of the core cluster fraction according to operating conditions is also discussed.