Effects and mechanism of ball milling on torrefaction of pine sawdust

•The structure of pine sawdust was destroyed into irregular granules by ball milling.•Ball milling reduced the thermal stability of hemicellulose, cellulose, and lignin.•Torrefied ball-milled samples had higher heating values than hammer-milled samples.•Biomass pretreated by ball milling and torrefaction can produce good solid fuels.

The effects and mechanism of ball milling on the torrefaction process were studied. Ball- and hammer-milled (screen size 1 mm) pine sawdust samples were torrefied at three temperatures (230, 260, and 290 °C) and two durations (30 and 60 min) to investigate into their torrefaction behavior and physicochemical properties. The results showed that, under identical torrefaction conditions, torrefied ball-milled pine sawdust had a higher carbon content and fixed carbon, and lower hydrogen and oxygen contents than torrefied hammer-milled pine sawdust. Torrefied ball-milled pine sawdust produced lower mass and energy yields, but higher heating values than torrefied hammer-milled pine sawdust. Ball milling destroyed the crystalline structure of cellulose and thus reduced the thermal stability of hemicellulose, cellulose, and lignin, causing them to degrade at relatively lower temperatures. In conclusion, biomass pretreated with a combination of ball milling and torrefaction has the potential to produce an alternative fuel to coal.