Characterization of oil-extracted residue biomass of Botryococcus braunii as a biofuel feedstock and its pyrolytic behavior

Botryococcus braunii (B. braunii), a green colonial microalga, is one of the most potentially significant renewable energy sources, because of its high productivity of hydrocarbon oils. In order to maximize energy utilization from this alga, it should be important to recover energy from not only the extracted oil but also the biomass residue after oil-extraction. In this work, oil-extracted residue biomass of an identified strain of B. braunii (BOT-22) with different original hydrocarbon content (HC%: from 28 to 51wt%) has been characterized as a biofuel feedstock in terms of the elemental composition as well as the pyrolytic behavior by thermogravimetric (TG) analysis and steam pyrolysis test. This paper has revealed that the B. braunii with higher HC%, even biomass residue after oil-extraction, is superior in quality of a biofuel feedstock because of higher content of C, H and volatile matter and lower content of N, O, S, P, ash and fixed carbon as compared to that with lower HC% and other algal biomass. Steam pyrolysis of the oil-extracted residue biomass of the B. braunii at 873–973K can produce highly carbonized char (∼10%-C), gases of C1–C4 (∼30%-C) and H2, and tar (∼60%-C) including mainly heavy component such as polycyclic aromatic hydrocarbons. From the analysis of the pyrolyzed products and its higher heating values from 29 to 36MJ/kg, the residue biomass of B. braunii can be classified as a hydrogen-rich solid fuel comparable to fossil fuel such as a coal rather than other algal and terrestrial biomass. In summary, not only hydrocarbon oils produced from the B. braunii but also its oil-extracted residue biomass should be a superior feedstock for biofuel production.