Numerical simulation of solid circulation mechanism and gas flow paths in a chemical looping combustion system

To study the gas-solid flow characteristics in a chemical looping combustion system integrated with a moving bed air reactor,a 3D full-loop numerical model was established using the Eulerian-Eulerian approach integrated with the kinetic theory of granular flow.The solid circulation mechanism and gas leakage performance were studied in detail.The simulation results showed that in the start-up process,the solid circulation rate first increased to approximately 5 kg/s and then dropped to approximately 1.2 kg/s;this observation was related to the dynamic control of the pressure distribution.In this system,the gas leakage between the inertial separator,upper air reactor,and lower air reactor was restrained by adjusting the pressure difference,thus obtaining optimal gas flow paths.When the pressures at the outlets of the inertial separator,upper air reactor,and lower air were 7.4,11.0,and 14.6 kPa,respectively,the gas leakage ratio was less than 1%in the system.

Heat Transfer in a Liquid-Solid Circulating Fluidized Bed Reactor with Low Surface Tension Media

Heat transfer characteristics between the immersed heater and the bed content were studied in the riser of a liquid-solid circulating fluidized bed, whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of liquid velocity, particle size, surface tension of liquid phase and solid circulation rate on the overall heat transfer coefficient were examined. The heat transfer coefficient increased with increasing particle size or solid circulation rate due to the higher potential of particles to contact with the heater surface and promote turbulence near the heater surface. The value of heat transfer coefficient increased gradually with increase in the surface tension of liquid phase, due to the slight increase of solid holdup. The heat transfer coefficient increased with the liquid velocity even in the higher range, due to the solid circulation prevented the decrease in solid holdup, in contrast to that in the conventional liquid-solid fluidized beds. The values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.

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.

Solids flow characteristics and circulation rate in an internally circulating fluidized bed

Internally circulating fluidized beds(ICFBs)enable effective control of the reactions and heat distribution in reactors.The ICFB contains two or more connected fluidized regions with different gas velocities to promote controlled solid circulation.The control of solid circulation rate(G_(0))is a critical factor.We recorded single particle trajectories by tracing a fluorescent particle,based on which particle flow behaviors were analyzed in different regions.G_(0)was obtained for a wide range of operating parameters.An increase in gas velocity in the down-and upflow beds shortened the particle circulation time in both beds and G_(0)increased significantly.As the static bed height increased,the differential pressure on both sides of the circulation port increased,which resulted in an increase in the solid circulation rate.As the orifice area increased,the flow resistance through the orifice decreased and thus the solid circulation rate increased.G_(0)increased with the decrease in particle size.The gas velocity in the upflowing bed and orifice area was the most important parameter to control the solid circulation rate.G_(0)was compared with the experimental measurements in literature and predictions using the correlation based on Bernoulli’s equation,and they agreed well.

Experimental study of the solid circulation rate in a pressurized circulating fluidized bed

The solid circulation rate is essential for design of pressurized circulating fluidized beds(PCFBs).With increasing pressure from atmospheric pressure to a few bars,the gas density linearly increases with the pressure,which affects the gas-solid flow characteristics.In this work,experiments were performed at room temperature in a cold PCFB apparatus with a riser of 3.3 m in height and 0.05 m in diameter.The solid circulation rate was studied from 20 to 80 kg/(m^(2)·s)under various conditions with increasing pressure from 0.1 to 0.6 MPa and fluidizing gas velocity from around 1.5 to 8.0 m/s for different Geldart B group particles.Most of the conditions were in the flow regimes of core-annulus flow(CAF)only and CAF with a turbulent fluidized bed at the bottom.The trend of the apparent slip factor with the dimensionless slip velocity was similar at different pressures and for different average particle sizes,and it converged to an exponential function.An empirical equation was obtained by fitting the solid circulation rate with the operating parameters(particle transport velocity,particle volume fraction,Archimedes number,and Froude number),which is helpful for design and operation of PCFBs.

Gas-solid Flow Characteristics in High-density CFB

The gas-solid flow characteristics in the riser of a high density CFB of square (0.27 m×0.27 m×10.4 m) or circu-lar (? 0.187m×10.4 m) cross section, using Geldart B particles (quartz sand), was investigated experimentally. The influence of riser structure on the hydrodynamic behaviors of a high-density circulating fluidized bed was investigated. The solid circulation rate was up to 321 kg/(m2s) with the circular cross-section under the operating conditions of the main bed air velocity 12.1 m/s and loosen wind and back-feed wind flow 25.1 m3/h. Different operating conditions on realizing high density circulation was analyzed, while both solids circulation rate and particle holdup depended highly on operating conditions. The circulating gas-solid flow was accompanied by an evidently-dense character in the riser's bottom zone and became fully developed in the middle and upper zones.

Hydrodynamic behavior of an internally circulating fluidized bed with tubular gas distributors

To better understand the hydrodynamic behavior of an internally circulating fluidized bed, solids holdup in the down-comer （Eso）, solids circulation rate （Gs） and gas bypassing fraction （from down-comer to riser y^R, and from riser to down-comer yRD） were experimentally studied. The effects of gas velocities in the riser and in the down-comer （UR and UD）, orifice diameter in the draft tube （dor）, and draft tube height （HR） were investigated. Experimental results showed that increase of gas velocities led to increase in Gs and yDR, and slight decrease in yeD. Larger orifice diameter on the draft tube led to higher 8sD, Gs and yDR, but had insignificant influence on YRD. with increasing draft tube height, both Gs and YDR first increased and then decreased, while yRD first decreased and then increased. Proposed correlations for predicting the hydrodynamic parameters agreed reasonably well with experimental values.

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed,named circulating conventional fluidized bed(CCFB)which operates below particle terminal velocity was proposed and experimentally studied.The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D.and 4.5 m in height with five different types of particles.Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity.The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates.The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions.The effect of particle properties and operating conditions on bed behavior was investigated as well.Results show that particles with higher terminal velocity have higher average solids holdup.

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

To study the influence of back feeding particles on gas-solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds （CFB）. The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u0 = 4 m/s and Gs = 21 kg/（m^2 s）, the back feeding particles were found to penetrate the lean gas-solid flow near the entrance （rear） wall before reaching the opposite （front） wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate （u0 =4 m/s, Gs = 33 kg/（m^2 s）） resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity （u0 =3.2 m/s） and Gs =21 kg/（m2 s）, beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the flnidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of uo = 4 m/s and Gs = 21 kg/（m^2 s）, addition of coarse particles （dp= 1145 μm） into the bed made the radial distribution of solids more symmetrical.

Performance evaluation of circulating fluidized bed incineration of municipal solid waste by multivariate outlier detection in China

This first nationwide survey was conducted to evaluate the overall performance of the circulating fluidized bed （CFB） incineration of municipal solid waste （MSW） in 2014-2015 in China. Total 23 CFB incineration power plants were evaluated. The data for monthly average flue gas emission of particles, CO, NOx, SO2 and HCl were collected over 12 consecutive months. The data were analyzed to assess the overall performance of CFB incineration by applying the Mahalanobis distance as a multivariate outlier detection method. Although the flue gas emission parameters had met the Chinese national emission standards, there were 11 total outliers （abnormal behavior） detected in 6 out of 23 CFB incineration oower olants from the oersoective of the MSW incineration performance. The results demonstrate that it is more important for a better perlbrmance of CFBs to reduce the lrequencles ot the MSW load changes, rather than the magnitudes of the MSW load changes, particularly reducing the frequencies in the range of 10% and more of the load changes, under the same and stable conditions. Furthermore, the overloading occurs more often than the underloading during the operation of the CFB incineration power plants in China. The frequent overloading is 0% to 30% ot the designed capacity. To achieve the stable performance of CFBs in practice, an appropriately designed MSW storage capacity is suggested to build in a plant to buffer and reduce the frequency of the load changes.

Simulation of mass balance behavior in a large-scale circulating fluidized bed reactor

We determine using a compound model the influence of the mass of granular matter on the behavior of a supercritical circulating fluidized bed （CFB） reactor. Population balance enables a stationary-regime modeling of the mass flow of granular matter inside a CFB unit in a large-scale. The simulation includes some important dynamic processes of gas-particle flows in fluidized bed such as attrition, fragmenta- tion, elutriation, and fuel combustion. Numerical calculations with full boiler loading were performed of operational parameters such as furnace temperature, furnace pressure, feeding materials mass flows, and excess air ratio. Furthermore, three bed inventory masses were adopted as experimental variables in the simulation model of mass balance. This approach enables a sensitivity study of mass flows of granular matter inside a CFB facility. Some computational results from this population balance model obtained for a supercritical CFB reactor are presented that show consistency with the operational data for large-scale CFB units.