3. Results and discussion
3.1. Minimum fluidization velocity of lignite
Minimumfluidization velocity (umf) is an important factor in understanding
the hydrodynamic behavior of fluidized beds. The minimum
fluidization velocity was obtained from the intersecting point of the
defluidization curve with the constant pressure line.
3.1.1. Effect of the sound frequency
To investigate the effects of the sound frequency on the minimum
fluidization velocity of lignite, experiments were carried out with the
sound frequency varying from 1 kHz to 4 kHz and sound pressure
level fixed at 140 dB. Fig. 2 showed that the plot of pressure drops versus
gas velocity for the lignite particles. According to Fig. 2, the minimum
fluidization velocity was determined at different values of sound
frequency. The results are given in Fig. 3. The minimum value of minimum
fluidization velocity was obtained at optimum sound frequency
of 1.5 kHz. It can be seen from Fig. 3 that the minimum fluidization
velocity of lignite decreased with increasing sound frequency, and
further increased.
For the high moisture content lignite particles, the cohesive force
between two particles is predominantly the liquid bridge force and
hydrogen bond force. The cohesive force decreases, reaches a minimum,
and then increases with the increase of sound frequencies [13]. With
sound excitation, the disruption of lignite particle agglomerates depends
on the sound prompting energy. However, low frequency sound waves
cannot affect the fluidized quality of lignite continuously. In addition, a