The so-called differential density fluidized bed means the fluidized bed system composed by different particles with bigger difference in density. In the fluidized bed system composed of particles with different density, the distribution of particles in the bed along the height will be mainly controlled by the density of particles in bed, namely the particles with big density will be distributed at the bottom of the material layer, and the particles with small density will tend to be distributed at upper part of the materials layer, although their particle size sometimes is bigger than the that of the particles at lower part of the materials layer with big density.