Fast pyrolysis of microalgae in a falling solids reactor: Effects of process variables and zeolite catalysts

Non-catalytic and catalytic pyrolysis of microalgae were carried out to generate an organic liquid fuel precursor. The impacts of several process variables on the fast pyrolysis in a falling solids reactor are reported, including temperature, particle size, flow rate, and atmosphere (N2, H2O and CO2). Experiments were carried out with duckweed as the biomass to provide some comparison. The speciated organic phase product data were classified according to the different compound types including hydrocarbons, alcohols, oxygenates, and nitrogenates. In-situ catalytic pyrolysis produced an organic phase with an increased fraction of hydrocarbons and decreased fraction of oxygenates, evidence for carbon removal chemistries such as decarboxylation and decarbonylation. The noncatalytic pyrolysis gave the highest total liquid yield while catalytic pyrolysis resulted in the highest yield of the desired hydrocarbon fraction. A comparison of four exchanged ZSM-5 catalysts (H-, Fe-, Cu-, and Ni-) indicates that the protonated zeolite provided the largest enhancement among the catalysts of the liquid product yield and composition: H-ZSM-5 increased the yield of the hydrocarbon fraction in the organic phase from 21% to 43%, a 100% relative increase, and exhibited the least coking. The effects of biomass weight hourly space velocity, and comparisons between H-ZSM5 powder and monolithic catalysts are also reported. The implications of the findings for the conversion of microalgae to liquid fuels are discussed.

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► Pyrolysis of microalgae and duckweed in falling solids reactor.
► Effect of process variables on product yields and compositions determined.
► Catalytic pyrolysis enhances bio-oil composition.
► Shape selective ZSM-5 catalysts enhance hydrocarbon yield.