Pyrolysis of a naturally dried Botryococcus braunii residue

Abstract

The extant chlorophyte microalga Botryococcus braunii is a potential source of biofuel. In any future biofuel plant, it may be dried and stockpiled after harvesting and then pyrolyzed to generate oil. To investigate the formation of bio-oil from B. braunii, its naturally occurring residue known as coorongite was pyrolyzed non-isothermally and isothermally under about 1 atm of pure nitrogen carrier gas. The apparent pyrolysis activation energy of coorongite (25 kJ/mol) is much lower than those of most kerogens, which are on the order of 130-250 kJ/mol. However, it approaches that reported for a Moroccan marine oil shale, implying similarities in their responses to pyrolysis. Non-isothermal pyrolysis by thermogravimetry coupled with infrared spectroscopy (TG-IR) revealed coorongite to contain a significant amount of alkanes. Molecular analysis of the isothermal pyrolysates by gas chromatography-mass spectrometry (GC-MS) identified a homologous series of normal alkanes and alkenes (C 9-C21), normal ketones (C8-C12), alkylaromatic compounds, carboxylic acids, and phenols. Structural group quantitation by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy showed that the pyrolysate is the result of several processes, with thermal decarboxylation forming alkanes, dehydrogenation ± cyclization forming alkenes and aromatic hydrocarbons, and some compounds being products of simple physical volatilization. Complementary analysis of the pyrolysis residues using solid-state 13C NMR and IR revealed that their sp3C-H carbon atoms would also be volatilized if treated by hydrocracking. These results suggest that stockpiled B. braunii may benefit from pyrolytic removal of carboxyl groups prior to further upgrading by hydrocracking and hydrogenation. © 2012 American Chemical Society.