A MATHEMATICAL-MODEL FOR SEGREGATION OF LIMESTONE COAL MIXTURES IN SLUGGING FLUIDIZED-BEDS

Coal gasification in fluidised beds requires deep beds to give sufficient residence time to the gas. The aspect ratio in pilot-scale equipment commonly implies that the bed will be in slugging regime. A mathematical model has been developed in order to predict the concentration profiles of coal-limestone mixtures in slugging fluidised beds. A new concept of solid circulation, based on the oscillatory movements induced by the gas slugs, is presented. Also, a simple mechanism based on the free falling of isolated particles is proposed to derive an expression for the segregation flow. Finally, the exchange of solids between phases is assumed similar to that present in bubbling beds. The model has been validated with a number of experimental concentration profiles obtained in a column of 0.13 m ID varying the sorbent particle size, the superficial gas velocity, the overall concentration of sorbent and the bed height. The final version of the model only requires experimental conditions and particle properties to calculate a segregation profile. The intervals of applicability of the model and methods for extrapolation to conditions out of the experimental range are discussed.