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Deoxygenation of Biomass Pyrolysis Vapors via in Situ and ex Situ Thermal and Biochar Promoted Upgrading

Raymundo, Lucas M., Mullen, Charles A., Strahan, Gary D., Boateng, Akwasi A., Trierweiler, Jorge O.
Energy & fuels 2019 v.33 no.3 pp. 2197-2207
BTEX (benzene, toluene, ethylbenzene, xylene), Panicum virgatum, biochar, biomass, carbon, catalysts, catalytic activity, fluidized beds, fungibility, nitrogen, oxygen, pyrolysis, pyrolysis oils, refining, rice hulls, temperature, vapors, zeolites
Production of stable, partially deoxygenated biomass pyrolysis bio-oils is needed to increase the fungibility of bio-oil for refining into fuels and chemicals. While zeolite catalyzed upgrading is commonly used to produce such liquids, the associated catalyst deactivation is a significant hurdle to overcome. Our group has previously reported the thermal deoxygenation of pyrolysis vapors that is carried out under an atmosphere partially consisting of recycled tail gas and without the use of externally added catalysts. In this study, thermal deoxygenation was further studied in a new and scaled-down (laboratory scale) pyrolysis system to allow for a systematic study and to better understand the factors affecting vapor deoxygenation. Temperature excursions from the fast pyrolysis temperatures near 500 °C and/or the catalyzing effect of accumulated biochar were hypothesized to be potential key parameters affecting vapor deoxygenation. Therefore, experiments were conducted by utilizing a recycled gas and inert atmosphere (N₂) while varying process temperatures in the 500–750 °C range in both a fluidized bed pyrolysis reactor (in situ) and a static secondary chamber (ex situ) positioned downstream, after removal of bio-char from the vapor stream. Based on this arrangement, the oxygen content of bio-oils produced varied with temperature changes as follows: 31, 30, and 19 wt % for in situ temperatures of 500, 600, and 700 °C, and 31, 27, 23, and 19 wt % for ex situ temperatures of 500, 600, 700, and 750 °C, respectively. Compared with the use of nitrogen as carrier gas, utilization of the recycled gas atmosphere increased the yield of liquids and decreased production of gases. Organic bio-oil carbon yield was 18.5% from biomass under recycled gas at 700 °C and only with 12.8% under N₂; however, the oxygen contents of the bio-oils were similar. Concentrations of BTEX were higher in bio-oils produced under the recycled gas atmosphere. Experiments were also conducted with biochars from switchgrass and rice hulls loaded in the ex situ chamber and maintained at 500 and 600 °C fixed beds. Bio-oils with oxygen contents as low as 19 wt % were produced with rice hull biochar at 600 °C. This suggests that biochar could have a catalytic deoxygenation effect particularly in a fixed or plugged bed, motivating further exploration of biochar as a catalyst for bio-oil deoxygenation.