Hydrodynamics of a Few Coarse Particles in a Fluidized Bed of Fine Particles

Fluidized bed reactors have been extensively employed for coal gasification. Typically, in Winkler type gasifier, feed coal (~ 1-3 mm) is fed in a bed of fine sand (~ 0.1- 0.5 mm) particles and fluidized with the aid of steam and air. However, more often than not, unburnt coal particles escape the bed, which is subsequently recovered in cyclone and recirculated back. It can also be lost along with ash recovery from the bottom discharge. Both these aspects lead to loss of feed and eventual lower carbon conversion. This further leads to frequent plant shut down and a drop in reactor performance. Until now, such issues are largely addressed based on variation in dynamic bed pressure drop and heuristic experience of operators. However, to develop deeper insights into the complex hydrodynamic behavior of coal particles in the bed, one needs to employ a particle tracking study, wherein, a typical path of the fresh feed are traced. This should be performed taking into account for various fractions of coarser coal and finer sand particles in the bed.

With this broad motivation, the present study thus employs Radioactive Particle Tracking (RPT) studies in which the coarser glass beads (2 mm) are tracked in a matrix of fine (~0.5 mm) glass beads in a binary fluidized bed (0.10 cm in ID). This in effect mimics an actual bed of fluidization of coal in the sand in a laboratory scale facility. Although several techniques have been implemented in the past, based on the principle of tracing the path of a single radiotracer particle, RPT remains a popular velocimetry technique [1, 2]. In order to effect such a study, it was planned to conduct the measurements in a mixture of particles of known distributions wherein a marker ⁴⁶Sc tracer was used as a solid tracer to track the coarser fraction of glass beads (2 mm). For the range of superficial velocities studied (2.97- 4.24 m/s), and bed composition (10- 50 weight % of 2mm glass beads) it was found that the bed composition did have a strong influence on the occurrences of solids in the bed. Further, the instantaneous tracer locations showed that it ensured an unbiased scanning of the entire test section. Also, the mean velocities revealed two distinctive zones – a high-velocity region around core while relatively lower velocity towards the wall. Detailed analysis of the results will be discussed in the final presentation.

References

[1] Upadhyay, R. K., & Roy, S. (2010). Investigation of hydrodynamics of binary fluidized beds via radioactive particle tracking and dual‐source densitometry. Can. J. Chem. Eng. 88(4), 601-610.

[2] Roy, S. (2017). Radiotracer and particle tracking methods, modeling and scale‐up. AIChE J. 63(1), 314-326.