Improving Energy Density and Grindability of Wood Pellets by Dry Torrefaction

In dedicated wood pellet combustion and cofiring with coal in large pulverized fuel furnaces, poor grindability and low bulk density of biomass are important issues for lowering the unburned carbon in ash and achieving high cofiring ratios with coal for pulverized fuel combustion furnaces. In this study, the torrefaction of wood pellets was investigated for improvement of energy density and grindability. The torrefaction tests were performed using a fixed bed reactor for a temperature range of 210-310 °C and holding time of 15-60 min. The mass yield varied from 86.18 to 39.46% accompanied by an increase in the carbon content and heating value. The properties of torrefied wood pellets (TWP) were correlated with the mass yield for use with different time-Temperature histories. The bulk density decreased by the mass yield rose to a power of 0.538. The energy density of TWP was higher in the initial torrefaction stage with a peak of 10.41 GJ/m³ but was below that for the original pellets when the mass yield was approximately ≤60%. The grindability of TWP increased almost linearly with the degree of torrefaction, and the mass yield of 80% attained the lower range of the grindability of coal.