Clarifying sub-processes in continuous ring die pelletizing through die temperature control

• Process parameters were varied independently in pilot scale fuel pelletizing.
• Die temperature was negatively correlated with bulk density and durability.
• Continuous ring die and single pelletizing differ regarding the effects of temperature.
• Steam conditioning showed a strong negative correlation with energy consumption.
• Steam conditioning had no effect on bulk density and durability.

A pilot scale pelletizer with a custom-made die temperature control system was used for pelletizing of a typical Nordic softwood blend in an experimental design where die temperature, moisture content, and steam conditioning were varied independently. Steam conditioning, expressed as material temperature, showed a strong negative correlation with the pelletizer motor current, but had no significant effect on other responses. Die temperature was negatively correlated to bulk density and durability. This negative correlation is contradictory to results from a pilot scale study where die temperature co-varied with other factors, and to results from single pelletizing studies that do not mimic the friction originated pressure build-up that is required for pellet formation in a continuous process.