Review of cluster characteristics in circulating fluidized bed (CFB) risers

- Review of experimental data on cluster characteristics in CFB risers.
- Remarkable qualitative consistency of cluster characteristics trends across studies.
- No direct correlation between any cluster characteristic and a single parameter.
- Discrepancies of an order-of-magnitude due to instrument and analysis differences.

Particle clusters are well acknowledged to affect the hydrodynamics and overall performance of gas-solid fluidized beds. Since one of the first reports on the clustering phenomenon in 1948, the understanding of particle clustering has been rigorously attempted via both modeling and experimental efforts, with significant traction gained especially in the last few decades. Accordingly, the current review targets at providing a comprehensive landscape of the experimental cluster trends to summarize the findings, in particular on circulating fluidized bed (CFB) risers, to date. More questions than answers seem to have sprouted from the abundant experimental data available, which impedes model development. The quantitative comparison of cluster characteristics across studies must be treated with caution, because of (i) different riser configurations, instruments and analysis methods, which can lead to discrepancies of an order-of-magnitude; (ii) the impact on cluster characteristics by an interplay of a host of factors, hence the influence of a single parameter is not straightforward, even within the same study; (iii) the irregularity in the form of clusters, hence the definition and/or measurement of the various cluster characteristics differ; and (iv) the general lack in the reporting of the actual particle size distribution. What is remarkable is that the trends of the cluster characteristics are relatively consistent despite different experimentalists and units.