Microwave-assisted catalytic biomass pyrolysis: Effects of catalyst mixtures

Abstract

This paper studied the catalytic effects of mixtures of different catalysts (i.e., K3PO4 “KP”, Bentonite “Bento” and Clinoptilolite “Clino”) on biomass catalytic pyrolysis compared to the single catalyst under microwave heating and thermogravimetric analyzer (TGA). Since some catalysts can promote microwave absorption while others show promising catalytic effects, it is possible to use mixtures of catalysts to improve high microwave heating rate and simultaneously catalyze biomass pyrolysis reactions. 10K3PO4/10Bentonite (10 wt.%KP+10 wt.%Bento) catalyst mixture increased the mass loss rate by 85% and 45% at 100 and 25 °C/min, respectively, compared to switchgrass without catalyst. The highest microwave heating rates for torrefaction (110–260 °C) and pyrolysis stages (260–400 °C) were found for the catalyst mixtures, with the highest being achieved by 10KP/10Bento. It was also found that the catalyst mixtures affected the type of coke deposited on catalyst surfaces during pyrolysis, which plays an important role on determining the microwave heating rate under microwave catalytic pyrolysis by affecting the dielectric properties of the bed material and then microwave absorption. Oxygenated-coke deposited on catalyst surfaces reduces the microwave heating rate markedly, while graphitic-coke increases the microwave heating rate considerably. Finally, catalyst combinations improved the catalytic performance significantly and reduced the catalyst loads which in turn will reduce the production cost of bio-oil and biochar from microwave-assisted catalytic pyrolysis.