Microwave-assisted pyrolysis and analytical fast pyrolysis of macroalgae: product analysis and effect of heating mechanism

Abstract

Macroalgae are emerging feedstocks for sustainable biofuel and chemical production. This study investigates microwave-assisted pyrolysis (MAP) and analytical fast pyrolysis of three different macroalgae species, viz., Kappaphycus alvarezii, Sargassum wightii and Turbinaria ornata at 500 °C. With an aim to understand the effect of heating mechanism on the formation of products and their composition, pyrolysis experiments were conducted in a batch microwave reactor and in a Curie point analytical pyrolyzer. The bio-oil, gas and char yields from MAP of the macroalgae were in the range of 26–32, 41–46 and 22–33 wt%, respectively. Moreover, high yields of CO (37–40 vol%), CH₄ (16–25 vol%) and H₂ (29–32 vol%) were recorded. Furan derivatives and anhydrosugars were the major organics from analytical fast pyrolysis, while anhydrosugars were absent in the bio-oil from MAP of the macroalgae. High selectivities to aromatics (20%) and furan derivatives (40%) were observed. A higher degree of deoxygenation and condensation was observed from MAP as compared to analytical fast pyrolysis. Nitrogen in the macroalgae got transformed into ammonia and heterocyclic nitrogen-containing organics in MAP, while amines were the major nitrogen-containing organics from analytical fast pyrolysis. Sulfur was detected in the form of SO₂ gas in both the pyrolysis processes, while it was also captured in the organic phase in MAP. Generally, secondary cracking reactions were more pronounced in MAP owing to the microwave plasma spots in the reaction mixture, while, owing to the short residence time in the analytical Curie point pyrolyzer, the primary pyrolysis vapors did not undergo secondary gas phase transformations or interactions with the bio-char.