Tars from Fluidized Bed Gasification of Raw and Torrefied Miscanthus x giganteus

The current study investigates the effect of temperature, equivalence ratio, and biomass composition on tar yields and composition. Torrefied and raw Miscanthus x giganteus (M×G) were used as biomass feedstocks in an atmospheric bubbling fluidized bed gasifier for experiments undertaken between 660 and 850 °C and equivalence ratios from 0.18 to 0.32. Tar was sampled according to the solid phase adsorption method and analyzed by gas chromatography. There is an indication that torrefied M×G produces higher amounts of total GC-detectable tar as well as higher yields of 20 individually quantified tar compounds compared with those of raw M×G. Under similar gasification conditions (800 °C and an equivalence ratio of 0.21), the total GC-detectable tar for torrefied M×G is approximately 42% higher than that for raw M×G. Higher tar yields are observed to be related to higher lignin and lower moisture content of torrefied M×G. The effect of temperature on tar yields is in good agreement with the literature. The highest yield of total GC-detectable tar, secondary tars, and tertiary-alkyl tars is observed between 750 and 800 °C, followed by a decrease at higher temperature, whereas tertiary-polycyclic aromatics increase with the temperature over the range tested. The effect of equivalence ratio on total GC-detectable tar is not clear because data points vary significantly (up to 47%) over the range of equivalence ratios tested. Temperature is the main driver for tar production and its chemical composition; however, this study indicates that tar yields depend significantly on biomass composition.