Upgrading of oils derived from hydrothermal treatment of microalgae by catalytic cracking over H-ZSM-5: A comparative Py-GC-MS study

Hydrothermal Treatment (HTT) of microalgae produces an organic liquid (HTT oil) with potentiality as feedstock in biofuel development. Through this process, an oil with oxygen content intermediate between pyrolysis oil from lignocellulosic biomass and fossil oil can be obtained even from lipid-poor microalgal species. However, the elevated content of nitrogen-containing organic compounds derived from algal proteins represents one of the main drawbacks. In this study, catalytic pyrolysis of HTT oil with H-ZSM-5 zeolite was investigated as an upgrading process, in order to reduce the nitrogen content and improve fuel quality. Oils obtained at various HTT experimental conditions from Desmodesmus sp. were mixed with H-ZSM-5 and pyrolyzed at 600 C within a Py-GC-MS system for the analysis of the evolved pyrolysis products. The results were compared with those resulting from catalytic pyrolysis of cellulose, n-hexatriacontane (C₃₆H₇₄) and pyrolysis oils from poplar (original and after hydrogenation). The relative comparison showed that hydrocarbon yields from pyrolysis of HTT oils over H-ZSM-5 were intermediate between those of hexatriacontane and cellulose. The high nitrogen content of the HTT oils did not show any marked effect on yields of hydrocarbons and nitrogen removal was remarkably high in all the investigated HTT oils. Although the data should be confirmed on larger scale, and recycling of catalyst has to be proven, obtained results indicate that it would be possible to introduce HTT oil from microalgae in existing refineries.