Effects Of Particle Cluster On Gas-solids Contact Characteristics In A Circulating Fluidized Bed

With the wide application of gas-solids circulating fluidized bed?CFB?reactor in the fields of energy,chemical industry and biomass processing,studies on CFB reactors become the hotspot of the fundamental research in gas-solids fluidization.Studies have reported that particle cluster in the CFB reactor results in the non-uniform gas-solids flow,which in turn leads to lower gas-solids contact efficiency,worse selectivity and weaker heat/mass transfer.In order to investigate the influence of hydrodynamics,particle cluster in particular,on the gas-solids contact efficiency,experimental work on gas-solids hydrodynamics and ozone decomposition reaction is carried out under the operating conditions with superficial gas velocity?U_g?of 5?9 m/s and solid circulation rate?G_s?of 100?300 kg/m²s in a 12 m rectangular CFB system.Solids holdup and particle velocity are characterized by non-uniform distributions both axially and laterally.Flow structure of solids holdup in axial direction from the bottom dense phase to the upper dilute phase can be divided into three sections,including developing region,the fully-developed region and dense region at the top.Cross-sectional average solids holdup is directly proportional to the parameter of G_s^0.6/??_pU_g?^1.3.Lateral distributions of solids holdup also show the non-uniform characteristics with a dilute zone in the center and a dense zone near the wall region.For all operation conditions in this study,solids holdup increases with the increase of G_s and/or the decrease of U_g.Similarly,there are also three regions along the vertical axis of the CFB reactor and a nearly parabola profiles at the lateral direction for solids flow according to the distributions of particle velocity.The above non-uniformity of hydrodynamics is mainly caused by the existence of particle cluster.The average solids holdup of particle cluster ranges from 0.002 to 0.28,which is higher than the local solids holdup by 30%.The influence of operating conditions on the average solids holdup of particle cluster is consistent with that on both solids holdup and particle velocity mentioned above.It is found that there is a nearly quantitative relationship between the average solids holdup of particle cluster and the local solids holdup.The particle cluster time fraction is between 30%and 50%,and can reach up to40%near the wall region in the fully-developed region showing a quantitative correlation with the local solids holdup as well.The particle cluster frequency is closely related to the degree of gas turbulence in the CFB system.It is as low as about 100 Hz at the bottom and as high as 1200 Hz at about 1.3 m along the axis of the CFB reactor.Based on the above work on hydrodynamics,ozone decomposition is utilized to study the gas-solids contacting characteristics.Results show that ozone concentration increases slightly at the bottom and decreases gradually along the CFB reactor.Ozone conversion at the bottom accounts for nearly 80%of the total conversion in the CFB reactor.There is a typical parabolic distribution of the ozone concentration in the lateral direction.At lower U_g or higher G_s operating conditions,the lateral profiles of ozone concentration become more uniform,and the ozone conversion is up to 90%near the wall region.According to the above discussion,a quantitative relationship between ozone concentration and hydrodynamics,especially the particle cluster is found.An empirical correlation between ozone concentration,solids holdup and parameters of particle cluster including cluster time fraction,frequency and its average solids holdup is obtained.Gas-solids contact efficiency calculated by the obtained correlation shows good agreement with the experimental data and the calculation error is below 20%.