Identification and instability characterization of a high-density particle band in a cyclone pyrolyzer using fiber optic sensing

In the cyclone pyrolyzer, the high-density particle band (HDPB) is considered a critical hydrodynamic feature for promoting rapid mixing and efficient heat transfer between binary particles. However, the HDPB frequently exhibits unstable flow structures due to the inherent instability of the gas flow field, and this instability characterization of HDPB has not yet been reported in the literature. In this work, the instability characterization and identification of HDPB in the cyclone pyrolyzer were systematically investigated using fiber optic sensing. A solids holdup threshold of 0.05 was established to quantitatively distinguish between dispersed particles and HDPB. The results indicate that when the average solids holdup exceeds 0.15, the particle is predominantly concentrated within the HDPB, meaning its basic formation. The mass fraction of HDPB consistently exceeds its time fraction, and complete formation criteria of HDPB are identified as time fraction >0.3 with corresponding mass fraction >0.9. Dynamic analysis reveals that HDPB duration increases with an increase in the average solids holdup, while its occurrence frequency first increases and then decreases. Frequency domain analysis shows that the solids holdup fluctuation signal exhibits only a single dominant frequency peak at the measurement location, indicating that the solid phase primarily is governed by a dominant periodic flow structure. These findings provide fundamental insights into hydrodynamic of the HDPB and offer practical operational guidelines for the stability optimization of cyclone pyrolyzer.