Torrefaction of wheat and barley straw after microwave heating

Microwave irradiation was used in this study for the torrefaction of wheat and barley straw. The torrefaction effect was studied by varying the microwave power level (200–300 W), reaction time (10–20 min) and moisture content of biomass (5–15%). Mass yield and energy yield of the torrefied biomass was determined. Fuel properties like H/C and O/C ratio were assessed from elemental composition. Grinding characteristics and hydrophobicity of the torrefied sample were studied and compared with the raw biomass. Barley straw tended to carbonize more under microwave irradiation with 29.1% increase in the C content against 16.2% in the case of wheat straw when torrefied at 300 W for 20 min. Both H/C and O/C ratio decreased with increase in power and reaction time. The energy density increased by 14–15% in wheat straw and 21–23% in barley straw under suitable reaction condition. Mass and energy yields were 64.0–97.8% and 73.8–98.4%, respectively for wheat straw. In barley straw, mass and energy yields were 42.7–97.4% and 52.5–97.3%, respectively. Moisture content of the biomass did not affect the reaction as much as other parameters and the mass yields were comparable between different moisture contents. Grindability of the biomass improved significantly after torrefaction. The particle size ratio between torrefied and untreated straw after grinding was 0.66 and 0.61 for wheat and barley, respectively. The torrefied biomass was more hydrophobic and the moisture uptake was reduced by 61–68% under suitable torrefaction condition. Microwave irradiation can be used effectively for torrefaction of the two biomass investigated at moderate power and short process time.