Oxidative pyrolysis of spirulina: Impacts of oxygen on bio-oil and property of biochar

Externally added oxygen could react with volatiles in oxidative pyrolysis, which might also react with biochar and tailor property of biochar at varied temperatures via distinct ways. This was investigated herein through oxidative pyrolysis of spirulina in a mixture of N2/O2 (11vol%O2) at 300 to 600°C. The results suggested that the added oxygen oxidized light volatiles and heavy tar, increasing production of bio-oil while diminishing formation of heavy π-conjugated organics. Oxidation of biochar occurred above 300°C, forming additional oxygen-containing organics but showing little impact on mass yield until 600°C. The excessive oxidation reactions formed oxygen-rich biochar at 300 or 400°C, while the oxygen-containing organics started to be cracked at 500 or 600°C, producing the biochar of fragmented surface. The additional oxygenated organics formed reduced thermal stability of the biochar but improved the overall combustion performance of the resulting biochar at 400°C via lowering ignition temperature and burnout temperature, due to the presence of abundant oxygenated aliphatic structures. The characterization with in-situ IR technique showed that the additional functionalities generated were mainly in the form of -OH, adsorbed CO2, C=O, and C-O-C via oxidation of the aliphatic structures, which was less thermally stable than the C-N groups formed through dehydrogenation of amino acid in the oxidative pyrolysis.