Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2013
Publication Date: 7/24/2013
Citation: Mullen, C.A., Boateng, A.A., Goldberg, N.M. 2013. Production of deoxygenated biomass fast pyrolysis oils via product gas recycling. Energy and Fuels. 27:3867-3874.
Interpretive Summary: In order to meet the renewable fuels standards set by the US government, 21 billion gallons of advanced bio-fuels will need to be produced by 2022. The largest source of feedstock for production of these advanced biofuels is lignocellulosic biomass, including woody materials, herbaceous grasses and crop residues (e.g. corn stover, straws). One promising process to convert biomass to a liquid is fast pyrolysis which produces a product called pyrolysis oil (bio-oil) which can be refined to “green” gasoline and diesel fuels that are indistinguishable from those produced from petroleum. Pyrolysis involves heating the biomass in the absence of air, usually by using inert nitrogen gas in the reactor. Pyrolysis oil is currently incompatible with petroleum for refining because it is corrosive and unstable, meaning its viscosity greatly increases during storage creating processing problems. Chemically, these unfavorable properties are due to the fact that pyrolysis oil, unlike petroleum, consists of reactive oxygenated molecules. The most popular method to reduce the oxygen content of pyrolysis oil while it is being produced is to incorporate a catalyst within the pyrolysis process; however, this has its own problems of yield losses, expense and deactivation of the catalyst. We have discovered that by performing the pyrolysis in the presence of gases that are the by-product of the reaction by recycling of the tail gas through the process and thereby replacing the nitrogen gas, we can produce a deoxygenated pyrolysis oil without the use of added catalysts. Therefore we can avoid the pitfalls of their use. We were able to decrease the oxygen content of pyrolysis oils of switchgrass from 40 wt% to 12 wt% by using this process. This information will be valuable to anyone considering pyrolysis as a conversion method for producing advanced biofuels from wood, grasses, crop residues or other biomass.
Technical Abstract: 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, 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 N2 atmosphere as the control. The reductive atmosphere, consisting primarily CO, CO2, H2 and light hydrocarbons had an autocatalytic effect where significant deoxygenation of the pyrolysis oil produced from oak and switchgrass was achieved. The effect was significantly smaller for the pyrolysis of pennycress presscake which inherently produces a lower oxygen product. Using oak and switchgrass as feedstocks, pyrolysis oils with molar C/O ratios of 11.6 and 8.5 were achieved respectively, 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 non methoxylated 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.