Pyrolytic Fractionation: A Promising Thermochemical Technique for Processing Oleaginous (Algal) Biomass

We report the development of a two-step pyrolytic fractionation approach that is especially applicable to processing oleaginous algae feed stocks. The first step is a low-temperature pyrolysis (T ∼ 300–320 °C) to produce bio-oils from degradation of protein and carbohydrate fractions. Solid residues left behind can subsequently be subjected to a second higher temperature pyrolysis (T ∼ 420–430 °C) to volatilize and/or degrade triglycerides to produce fatty acids, their derivatives, and long chain hydrocarbons. Thus, pyrolytic fractionation can be used to “fractionate” oleaginous biomass and separately recover triglyceride degradation products. Proof-of-concept micropyrolyzer and subsequent lab-scale fixed-bed experiments were performed using oleaginous Chlorella sp. and Scenedesmus sp. to demonstrate the pyrolytic fractionation technique and determine bio-oil yields. As expected, triglyceride-specific bio-oils were rich in hydrocarbons and free fatty acids, were nearly free of water, short-chain organic acids, and other carbohydrate degradation products, and had low N-content. Due to production of “high quality” triglyceride-specific bio-oil vapors, pyrolytic fractionation would allow product upgrading via in situ gas-phase catalytic processes to generate drop-in fuels (hydrocarbons) or specialty chemicals (e.g., fatty amides), without the need to condense the vapors. A conceptual process design is developed and energy requirements for pyrolytic fractionation are assessed and discussed.