New tool for improved control of sub-process interactions in rotating ring die pelletizing of torrefied biomass

•New concept for controlling feed layer formation when pelletizing torrefied biomass.•Softwood forest residues and a hardwood species torrefied at 308 °C for 9 min were pelletized.•Two ring die pelletizers were used. One with a stationary and one with a rotating die.•Lower initial moisture content enabled better flow properties.

A new concept was developed for feed layer formation control and to obtain continuous pellet production when pelletizing torrefied biomass. The materials pelletized were softwood forest residues and a hardwood species which both had been torrefied at 308 °C for 9 min. The torrefied wood chips were milled over a screen size of 6 mm and the torrefied feedstock moisture content was adjusted to about 9% before pelletizing. Two types of pelletizers were used; one with a stationary ring die and one with a rotating ring die. With a traditional, non-cooled die configuration, the die temperature increased to 75-78 °C. During temperature increment, pellet production deteriorated and finally ceased at approximately 80 °C. This phenomenon was caused by a breakdown of the feed-layer formation between the free rolling rollers and the die. However, continuous production could be sustained when the die was cooled. A new tool was developed based on nozzle injection of water directly onto the feed layer. By this course of action pellet production was sustained at temperatures well above 80 °C. This proof-of-concept for a new tool to control sub-process interactions in ring die pelletizing also includes use of low initial moisture content to utilize the flowability of torrefied particulates and, thus, avoid problems connected to feeding, conveying and silo discharging which frequently occurs at higher feedstock moisture contents.