Assessing biomass catalytic pyrolysis in terms of deoxygenation pathways and energy yields for the efficient production of advanced biofuels

Abstract

The present work focuses on the pathways through which catalytic pyrolysis of biomass into bio-oil proceeds and the effect of the operation conditions on parameters like bio-oil oxygen composition and mass yield, and also additional indicators, such as the distribution of both the oxygen and the chemical energy contained in the initial biomass among the different products. Acid washed wheat straw was used as biomass feedstock. The pyrolysis tests were performed in a lab-scale downdraft fixed-bed reactor working at atmospheric pressure, employing a nanocrystalline H-ZSM-5 zeolite as catalyst. A systematic study was carried out by decoupling both the thermal and catalytic reactions in order to evaluate the influence of three key variables: temperature of the thermal zone; temperature of the subsequent catalytic step and catalyst/biomass ratio. Increasing the pyrolysis temperature in the thermal zone resulted in more bio-oil* (bio-oil in water-free basis) production to the detriment of char and water fractions. In contrast, a significant reduction in bio-oil* fraction, due to decarbonylation and decarboxylation, occurred when increasing the catalytic bed temperature from 400 up to 500 °C. A similar effect was observed by varying the catalyst/biomass ratio since it increased the production of CO, CO₂, light olefins and coke at the expense of a decline in the bio-oil* yield. Nevertheless, this bio-oil contains oxygen amounting to as low as 10 wt%, while retaining about 38% of the energy yield. Char, coke and gaseous hydrocarbons contain a great part of the biomass chemical energy, hence their formation should be suppressed or minimized to further improve the bio-oil* energy yield. At high catalyst/biomass ratios, the bio-oil becomes rich in aromatic compounds, both oxygenated and hydrocarbons, while the amount of sugars, furans, carboxylic acids, and other oxygenated products is strongly reduced.