Production of bio-oil by catalytic pyrolysis of microalgae using Li-LSX-zeolite and alkali silicates

Abstract

Microalgae are promising feedstock for bio-fuels due to high productivity, fast growth rate, and for not being in competition with food crops. Also, catalytic pyrolysis couples a cost effective thermo-chemical conversion process to produce fuels and chemicals from biomass. In this work, different species of microalgae were investigated to assess their potential as pyrolysis feedstock. Pyrolysis of Isochrysis sp. produced the largest bio-oil yield (37 wt.%) with a low content in aromatics and N-compounds compared to the other microalgae bio-oils, suiting the requirements for producing bio-oil. Isochrysis microalgae therefore selected for catalysts screening study; Evaluation of Li-LSX-zeolite, microsilica, LiNa-FA and Na-FA for the bio-oil upgrading. The bio-oil yield reduced but its quality improved in the presence of the catalysts. Li-LSX-zeolite showed a good catalytic performance, principally for bio-oil denitrogenation and good activity for olefins and aromatics production. Since the catalyst resulted the most promising material for enhancing the quality of Isochrysis pyrolysis, a detailed parametric study was carried out. The optimum conditions for producing bio-oil include temperatures close to 500 °C, residence time of ~ 8 secs and a catalyst: microalgae ratio of 1:1. However, N content in bio-oil can be reduced from 3.9 wt.% (non-catalytic) to barely 0.8 wt.% by increasing the catalyst ratio to 3:1, which also boosts the aromatics to five-fold those non-catalytically obtained. Due to high ash contents in the microalgae, chemical pre-treatment was carried out before pyrolysis. Pre-treatment of the algae in acid able to increase the bio-oil yield (38 wt.%) while base pre-treatment favoured gas production (61 wt.%). The acid pretreated microalgae increased the formation of aliphatic in bio-oil simultaneously reduce N-compounds. The use of catalyst over several regeneration cycles on Isochrysis sp. microalgae was studied. The pre-treatment greatly affects the catalyst activity over cycles by enhanced the bio-oil yield and tuned the composition of the bio-oil with high aliphatic compounds. However, the denitrogenation capacities wasd low due to the deactivation of the acid sites of the Li-LSX-zeolite. In conclusion, Li-LSX-zeolite showed a very good denitrogenation activity, with mild deoxygenation capacity which can be useful for the production of olefins and aromatics from microalgae.