Torrefaction of raw and blended corn stover, switchgrass, and prairie grass

Torrefaction is a promising thermal pretreatment process to prepare biomass for use in energy production. Torrefaction of corn stover, switchgrass, and prairie grass at three selected temperatures (250°C, 300°C, or 350°C) for 3 h was carried out using a laboratory-scale batch reactor. Torrefaction of a blend of these three feedstocks at the same conditions was also examined. The effects of temperature and feedstock type on the yields and properties of the products (bio-char, bio-oil, and torrefaction off-gases) were analyzed. The bio-char produced from all of the feedstocks had higher carbon content and higher heating value (HHV) and lower moisture content (MC) when the torrefaction temperature increased. The HHV of bio-chars produced at 350°C were 25.15 MJ kg^-1 for corn stover, 27.94 MJ kg^-1 for switchgrass, 28.75 MJ kg^-1 for prairie grass, and 28.79 MJ kg^-1 for the blend. The carbon contents of the bio-chars increased from 42.36% to 60.31% for corn stover, from 43.94% to 70.95% for switchgrass, from 44.27% to 66.28% for prairie grass, and from 43.52% to 64.47% for the blend with torrefaction at 350°C. The results also indicated that the bio-oil and off-gases produced at lower temperatures had low value for further utilization. The MCs of the bio-oils produced in the torrefaction process were very high: 52.41% to 71.75% for corn stover, 58.01% to 74.22% for switchgrass, 65.44% to 75.07% for prairie grass, and 79.70% to 83.33% for the blend. The concentrations of combustible gases (H2 and CO) in the off-gas were less than 12% for all feedstocks. Compared to the individual feedstocks, the blend exhibited a very limited synergistic effect. Blending different biomass species for torrefaction may be an option to produce a uniform feedstock for biofuel production.