Characteristics of dehydration during rice husk pyrolysis and catalytic mechanism of dehydration reaction with NiO/γ-Al2O3 as catalyst

Abstract

Dehydration is the first chemical reaction in biomass pyrolysis. The process of dehydration has great influence on the subsequent chemical reactions and product distribution. There is little research on dehydration characteristics of real biomass during pyrolysis. Therefore, in this study the dehydration characteristics were investigated using rice husk as the biomass representative in the pyrolysis experiments. On this basis, NiO/γ-Al2O3, was used as the catalyst for rice husk pyrolysis to study its catalytic mechanism for the dehydration reaction of biomass. Results of the pyrolysis experiments indicated that dehydration reaction mainly occurred in the temperature range of 250–350 °C, in which the mass of water produced was around 5.92% of the dry raw material. Through comparing the dehydration curve with thermogravimetric (TG) curve of rice husk, it was found that the dehydration was one of the primary reactions during the pyrolysis, and the accumulated water produced attained to 33.82 wt% of total volatile products of the pyrolysis at 400 °C. When catalyst NiO/γ-Al2O3 was used with NiO loading in the range of 5–20%, the production of water increased by 0.88–1.04 times at 350 °C under the pyrolysis condition in this study. The results of catalytic pyrolysis showed that γ-Al2O3 was the major active component for catalyzing the dehydration reactions. Through characterization of the catalysts by temperature programmed desorption of ammonia (NH3-TPD), X-ray diffraction (XRD), and scanning electron microscopy (SEM), it was found that the weak acid sites on γ-Al2O3 surface played the most important role in the catalytic dehydration reactions.