Optimal Conditions of Thermal Treatment Unit for the Steam Reforming of Raw Bio-oil in a Continuous Two-step Reaction System

Abstract

The separation of pyrolytic lignin in a previous thermal treatment step is essential for the viability and efficiency of the continuous hydrogen production by steam reforming (SR) of raw bio-oil. This work aimed to establish the conditions of this thermal step that maximize the bio-oil fraction liable to valorization in the subsequent SR reactor, and that lead to a better behavior of the catalyst. The influence that temperature (400- 800 °C) and steam-to-carbon ratio S/C (1.5-6.0) have on the composition of resulting volatile stream and on the solid fraction (pyrolytic lignin) deposition was analyzed by feeding raw bio-oil, and a mixture of raw bio-oil and with 20 wt% of ethanol. The thermal treatment temperature affects both the yield of pyrolytic lignin (which decreases with temperature, especially in the range 400-500 °C) and its composition (so that the H/C ratio decreases as the temperature is higher). The yield of liquid fraction and its total content of oxygenates decrease notably above 500 °C. Levoglucosan content decreases, while that of phenols (especially above 650 °C), carboxylic acids (mainly acetic acid) and acetaldehyde increase markedly as temperature is raised. Temperature rise enhances formation of gaseous products (mainly CO and CO₂), with H₂, CH₄ and hydrocarbons being promoted above 600 °C. The effect of thermal step temperature on the Ni/La₂O₃-??Al₂O₃ catalyst behavior was studied by analyzing the evolution with time-on-stream of bio-oil conversion and product yields. Consequently, 500 °C is the thermal treatment temperature that leads to a better compromise between H₂ yield and catalyst stability, since the resulting oxygenated composition causes less deactivation of the reforming Ni/La₂O₃-??Al₂O₃ catalyst, thereby attaining complete and stable bio-oil conversion and H₂ yield close to 90 %.