Optimized catalytic stepwise pyrolysis for converting cotton straw into hydrocarbon-rich bio-oil in N₂ and CO₂

Stepwise pyrolysis is recognized as a promising method in converting biomass into sustainable bio-oils. However, a systematic study on the stepwise parameters (e.g., low- and high-pyrolysis temperatures, duration, atmosphere) is still lacking. Herein, the stepwise catalytic fast pyrolysis (CFP) of cotton stove over ZSM-5 was conducted, aiming to reveal the impact of aforementioned parameters on bio-oils production. Results showed that the low-temperature pyrolysis benefitted the decomposition of biomass, while the high-temperature counterpart favored the upgrading of bio-oil. The combination of a low-temperature pyrolysis (400 ℃ for 2 min) and high-temperature pyrolysis (600 ℃ for 3 min) led to a satisfied hydrocarbons selectivity (35.8 area%) and bio-oil yield (11.4 wt%). This was superior to the one-step pyrolysis conducted at 600 ℃ for 5 min, which decreased the values to 32.2 area% and 11.2 wt%, respectively. The CO₂ introduction into N₂ atmosphere in stepwise pyrolysis further enhanced the hydrocarbon-rich bio-oil production, and a maximum bio-oil yield of 15.9 wt% was obtained in 100 vol% CO₂, while the highest hydrocarbons selectivity of 51.8 area% was reached in 80 vol% CO₂. This work confirmed the advantages of stepwise pyrolysis for producing high-quality bio-oils and provided a reference for pyrolysis in CO₂ atmosphere.