Microwave torrefaction of douglas fir sawdust pellets

Microwave torrefaction of Douglas fir pellets was investigated and the effects of process conditions on the yields of products were determined. The reaction temperature and time significantly influenced the yields of torrefied biomass, bio-oil, and noncondensable gases. Three linear models were developed to predict the product yield, as a function of reaction temperature and time. Gas chromatography/mass spectroscopy (GC/MS) analysis for bio-oils showed that the bio-oils were mainly composed of furans, phenolics, sugars, ketones/aldehydes, and organic acids. The amounts of each compound varied with the reaction conditions. Over the reaction temperature of 275 °C, noncondensable gases were mainly composed of CO ₂ and CO and its yields and compositions were significantly influenced by the temperature. Higher heating values (HHVs) of torrefied biomass were 20.90-25.07 MJ/kg, which is an ∼6%-31% increase, compared to the HHV of raw biomass. The energy yields of torrefied biomass, 67.03%-90.06%, implied that most energy was retained in the torrefied biomass. One linear model as a function of reaction temperature and time was developed to predict the energy yield. Mass and energy balance analysis showed that the total energy recovery was ∼79%-88%, implying that the microwave torrefaction of biomass is practical and energy-efficient.