Wood chips flow in a rotary kiln: Experiments and modeling

• An increase in the kiln slope or rotational speed induces a decrease in the mean residence time.
• Wood chips flow in the rotary kiln can be considered as a plug flow with small extent of dispersion.
• For high solid hold-up, axial segregation phenomena are highlighted.
• There is no significant difference between the flows of raw and torrefied biomass.
• A modified Saeman model predicts accurately the mean residence time and the bed depth profile along the kiln.

Rotary kilns are well suited for processing woody biomass by torrefaction to produce bioenergy. One of the key issues for process design lies in the flow pattern modeling. The Saeman model is classically used to predict the mean residence time (MRT) and the bed depth profile of powder materials in rotary kilns. Its ability to describe wood chips flow arises. In the present study, residence time distribution (RTD) experiments are conducted with raw and torrefied wood chips. Effects of operating parameters – kiln slope, rotational speed and inlet flow-rate – on the average residence time, the variance and the mean solid hold-up are discussed. A plug flow with small extent of dispersion is emphasized, even if some segregation phenomena are highlighted. Torrefaction did not evidence any significant influence on the flow pattern. With a discrepancy of 20% between the measured and computed mean residence time, the predictive capacity of the classical Seaman model proved to be insufficient. The model is adapted to predict accurately the load profile and the mean residence time of particles with parallelepiped form. The discrepancy between experimental and calculated results is so reduced from 20% to 5% for the MRT and from 25% to 5% for the mean solid hold-up.