Bio-fuel production from catalytic microwave-assisted pyrolysis of the microalgae Schizochytrium limacinum in a tandem catalytic bed

The formation of oxygenated and nitrogenated compounds during microalgae pyrolysis interferes with bio-oil usage as a viable competitor against fossil fuels. Aiming to reduce these undesirable substances and improve aromatic hydrocarbon production, the catalytic microwave-assisted pyrolysis of the microalgae Schizochytrium limacinum in a tandem catalytic bed was investigated. This study evaluated the effects of the reaction temperature and the tandem catalytic bed (NiHTC/SiC and NiHZSM 5/SiC: hydrotalcite and zeolite catalysts) on the product distribution and bio-oil composition. Experimental results showed that ex-situ catalytic microwave-assisted pyrolysis was able to produce satisfactory liquid yields, reaching 58 % in the optimal test. The results showed that NiHTC/SiC (hydrotalcite) favored de-acidification and de-nitrification reactions, leading to more intermediate compounds, such as ketones and alcohols. These compounds were then converted into hydrocarbons over the acid sites of the NiHZSM-5/SiC (zeolite) catalyst. Maximum hydrocarbon and benzene yields of 95 % and 78 %, respectively, were reached at 500 °C and a NiHTC/SiC:NiHZSM-5/SiC catalyst ratio of 1:1. In addition, the regeneration and recycling of the catalytic bed were successfully performed and did not affect catalytic activity regarding hydrocarbon generation. This study also provided crucial data on improving energy recovery (which ranged from 70 to 94 %) and bio-oil quality during the catalytic pyrolysis of microalgae. The satisfactory results indicated that the biomass energy was successfully stored as a liquid fuel.