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 (MxG) were used as biomass feedstocks in an atmospheric bubbling fluidized bed gasifier for experiments undertaken between 660 and 850 degrees 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 MxG produces higher amounts of total GC-detectable tar as well as higher yields of 20 individually quantified tar compounds compared with those of raw MxG. Under similar gasification conditions (800 degrees C and an equivalence ratio of 0.21), the total GC-detectable tar for torrefied MxG is approximately 42% higher than that for raw MxG. Higher tar yields are observed to be related to higher lignin and lower moisture content of torrefied MxG. 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 degrees 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.