Flow And Clustering Characteristics Of B Particles In The Riser Of A Circulating Fluidized Bed

Coal is the most important primary energy source in China.At present,coal gasification is a clean,economical coal utilization technology,and the circulating fluidized bed(CFB)gasification is an efficient,non-bubble gas-solid contact technology.In this thesis,the flow and clustering characteristics of Geldart B particles are studied in a CFB riser.The hydrodynamic experiments were carried out on a CFB riser with height of 5.5m and inner diameter of 60 mm.The flow characteristics of the Geldart B particles in the riser were measured using a dynamic pressure detector and a PV-5A particle holdup meter.Based on the local particle solids voltage signal measured,the axial and radial concentration distributions of Geldart B fly ash in the riser were obtained.It is found that the axial solid concentration of the fly ash particles is higher at the bottom part and lower in the upper part.The radial solid concentration is generally lower in the central region and higher in the near-wall region.Increasing solids circulation rate or decreasing superficial gas velocity can increase the solid concentration in the riser.Cluster properties of the fly ash were studied by the threshold method.The concentration,duration and time fraction of clusters in the riser show a similar radial distribution to the solid holdup,that is,the central region is lower and the wall region is higher.In the axial direction,with the increase of axial height,the concentration,duration and time fraction of cluster were decreased.The increase of the circulation rate and the decrease of superficial gas velocity can increase the concentration,duration and time fraction of the clusters.The frequency of clusters in the riser shows a high center and low wall distribution,and the frequency of clusters increases with height.With the increase of solid circulating rate and superficial gas velocity,the frequency of clusters increased.