Production of Deoxygenated Biomass Fast Pyrolysis Oils via Product Gas Recycling

A bench scale fluidized bed system was modified to recycle and utilize the gaseous products of biomass fast pyrolysis as fluidization gas and to create a reactive gas atmosphere to replace, in part or in full, the added nitrogen gas. The effect of the presence of the reducing atmosphere on the pyrolysis oils produced was studied for three different biomass feedstocks, white oak, switchgrass, and pennycress presscake, and compared with those produced under an inert N-2 atmosphere as the control. The reductive atmosphere, consisting primarily of CO, CO2, H-2, and light hydrocarbons, had a significant deoxygenation effect on the pyrolysis oil produced from oak and switchgrass. The effect was significantly smaller for the pyrolysis of pennycress presscake, which inherently produces a lower oxygen product. When oak and switchgrass were used as feedstocks, pyrolysis oils with molar C/O ratios of 9.1 and 8.5 were achieved, respectively, which is increased from about 2.1 for the control experiments for each feedstock. The pyrolysis oils produced under the reducing atmosphere were richer in aromatic hydrocarbons and nonmethoxylated phenolics and had lesser concentrations of levoglucosan and acids than were the pyrolysis oils in the control experiments. These pyrolysis oils had lower total acid numbers (TAN) and higher energy content than those produced in the control experiments. These results compare favorably to zeolite catalyzed fast pyrolysis but without the added catalyst cost and concern for catalyst deactivation or need for regeneration.