Investigation of unstable solids circulation behavior in a circulating fluidized bed with sweeping bend return using pressure frequency analysis

• Start-up of circulating fluidized bed shows an oscillation circulation behavior.
• Pressure frequency distribution distinguishes solids exchange and flow structure.
• Oscillation circulation is activated by critical gas flow rate and valve opening.
• Oscillation circulation model changes with the increase of bed inventory.

The unstable solids circulation behavior in a circulating fluidized bed (CFB) with sweeping bend return is investigated by monitoring pressure fluctuations in a bench-scale model. The pressure fluctuations at the top and bottom of the riser and downer are processed using the Fast Fourier Transform (FFT) technique to extract the dominant frequency and its amplitude in order to quantify the effects of riser inlet gas velocity, valve opening and bed inventory on the operation of the unit. The differential pressure drop fluctuations between riser and downer are processed in a similar manner. The study reveals that the unstable solids circulation behavior is cyclical vis-a-vis gas–solids flow pattern and solids exchange between riser and downer, named as solids oscillation circulation behavior. A dominant frequency of 0.25 Hz is observed for all pressure fluctuation signals corresponding to the cyclical change of bed inventory and gas flow rate in riser and downer induced by the oscillation circulation behavior. An additional dominant frequency for the pressure fluctuation signals at the riser and downer bottom is also observed related to the gas–solid flow pattern. The solids exchange frequency is strongly related to the dominant frequency of the differential pressure drop. The work also demonstrates that the oscillation behavior is activated by a critical inlet gas velocity or valve opening and that the oscillation frequency increases with the decrease of inlet gas velocity, while it does not change with the valve opening. The solids exchange amount decreases with the decrease of gas flow and small percentage valve opening. The oscillation behavior changes from sine model to rectangular with the increase of bed inventory and thus slugging solids circulation is established.

Figure optionsDownload as PowerPoint slide