Catalytic upgrading of pyrolytic vapors from the vacuum pyrolysis of rape straw over nanocrystalline HZSM-5 zeolite in a two-stage fixed-bed reactor

• A 2-in-1 process is proposed to directly catalyze the vacuum pyrolysis vapors.
• Temperature, catalyst mass and Si/Al ratio affect the product yields significantly.
• Oil phase yield is optimized by employing RSM and obtained at a higher value of 9.81%.
• Oil phase has higher qualities with the HHV of 33.80 MJ/kg and pH value of 5.15.
• A large amount of aromatic hydrocarbons and light phenols are detected in oil phase.

This investigation involved the vacuum pyrolysis of biomass (rape straw) and the online catalytic upgrading of pyrolytic vapors in the second stage of the reactor. Based on the previous study on vacuum pyrolysis of biomass, effects of the operating factors including catalyzing temperature, biomass to catalyst mass ratio and catalyst Si/Al ratio on product yields and qualities have been studied using nanocrystalline HZSM-5 zeolite. Optimization of the oil phase yield was further conducted by employing response surface methodology (RSM). The statistical analysis showed that the operating factors significantly affected the product yields. The optimal conditions for the maximum oil phase yield were obtained at catalyzing temperature of 491.2 °C, biomass to HZSM-5 mass ratio of 0.302 and catalyst Si/Al ratio of 53.3. Confirmation runs gave 9.82% and 9.80% of the oil phase yield compared to 9.90% of predicated value. The H/C, O/C molar ratios (dry basis), density, pH value, dynamic viscosity and high heat value of the oil phase obtained at optimal conditions were 1.518, 0.195, 0.96 g/cm3, 5.15, 5.12 mm2/s and 33.80 MJ/kg, respectively. Liquid product compositions were examined using gas chromatography/mass spectrometry (GC/MS). A large amount of aromatic hydrocarbons and light phenols were detected in the oil phase which contributed to a relatively low polydispersity of 1.35 (M¯n=218.14 g/mol and M¯w=293.73 g/mol). The aqueous phase also contained a large number of light aromatics. The use of nanocrystalline HZSM-5 caused a remarkable increase of the desirable compounds including aromatic hydrocarbons, light phenols and alcohols, and a significant decrease of the unstable and corrosive compounds (e.g., acids, aldehydes and ketones). Besides, thermo-gravimetric analysis of the oil phase was conducted within air atmosphere. It was indicated that the oil phase had higher yield and qualities which might be used as an engine fuel.