Bed Agglomeration during the Steam Gasification of a High-Lignin Corn Stover Simultaneous Saccharification and Fermentation (SSF) Digester Residue

This research investigates the bed agglomeration phenomena during the steam gasification of a high-lignin residue produced from the simultaneous saccharification and fermentation (SSF) of corn stover in a bubbling fluidized bed. The studies were conducted at 895 degrees C using alumina as bed material. Biomass was fed at 1.5 kg/h, while steam was fed to give a velocity equal to 2.5 times the minimum fluidization velocity, with a steam/carbon ratio of 0.9. The pelletized feedstock was co-fed with a cooling nitrogen stream to mitigate feed line plugging issues. Tar production was high at 50.3 g/Nm(3), and the fraction of C10+ compounds was greater than that observed in the gasification of traditional lignocellulosic feedstocks. Carbon closures over 94% were achieved for all experiments. Bed agglomeration was found to be problematic, indicated by pressure drop increases observed below the bed and upstream of the feed line. Two size categories of solids were recovered from the reactor: +0.25 mm and -0.25 mm. After 2.75 h of experiment, 61.7 wt % was recovered as -0.25 mm particles and 38.2 wt % of the recovered reactor solids were +0.25 mm. A sizable percentage (31.8 wt %) was +0.841 mm. The -0.25 mm particles were mainly formed by the initial bed material (Al2O3). Almost 50 wt % of the +0.841 mm particles was found to be formed by organics. The unreacted carbon remaining in the reactor resulted in a low conversion rate to product gas. Inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) confirmed that the large agglomerates (+0.841 mm) were not encapsulated bed material but rather ungasified feedstock pellets with sand particles attached to it.