Catalytic Pyrolysis of Biomass: The Effect of Hydrothermal Treatment of FCC Catalyst and ZSM-5 Additive

The process configuration of catalytic pyrolysis of biomass in a continuous circulating fluidized-bed reactor mimics the fluid catalytic cracking (FCC) process used in the petroleum refineries process to upgrade heavy hydrocarbon molecules to more valuable petroleum products. In the FCC process, the spent catalyst is continuously regenerated at high temperatures and pressures, as a result the catalyst undergoes hydrothermal treatment (deactivation). The influence of hydrothermal deactivation of catalyst used in catalytic pyrolysis of biomass is of importance. In this study, fresh FCC catalyst and ZSM-5 additive were deactivated with 100% steam at 1350 ^oF (732 ^oC) and 1450 ^oF (788 ^oC) for 4 hours in a fluidized state. The catalytic pyrolysis of hybrid poplar with both fresh and steam treated catalysts were conducted in a 50 mm bench scale bubbling fluidized bed reactor at 475^oC and a weight hourly space velocity (WHSV) of 2 h^-1.  The catalysts were evaluated with respect to the product yields and the property of the upgraded bio-oil. The surface area measurements of the catalyst showed a reduction in total surface area of the FCC catalyst with about 40% loss of zeolite surface area after steam treatment at 787.8 ^oC for 4 hours. However, the steaming of the ZSM-5 additive at both conditions resulted in a slight increase in the total surface area. The significant loss in the surface area within the micropore of the FCC catalyst is indicative of the loss in catalytic activity.  The hydrothermal treatment of the FCC catalyst positively affected the catalytic pyrolysis product yields. The organic and gas yields increased and the water and char/coke yields decreased after steam treatment. From the study, the properties of the bio-oils evidently suggested that the FCC catalyst was still active following treatment at 788 ^oC.  The viscosity and the density of the bio-oils from the steamed FCC catalyst were lower than those produced with the fresh FCC catalyst. Steaming at 1350 ^oF (732 ^oC) resulted in bio-oil with the highest HHV (28.55 MJ/kg). In the case of the ZSM-5 additive, the extent of deactivation was reflective only in the organic and gas yields. The organic yield increased and the gas yield decreased. Due to the lower tendency of ZSM-5 to form coke, steaming of the catalyst did not show any significant decrease in the char/coke yield as was seen in the FCC catalyst. Additionally, the ZSM-5 additive did not show similar trend in the improvement of the properties of the bio-oils produced with the steam treated FCC catalyst. The bio-oils from the steamed ZSM-5 additives were rather higher in viscosity and density. The GC analysis of the product gases suggested that steam treatment of the catalyst prolonged the catalyst lifetime owing to the fact the concentrations of CO and CO₂ and CH₄ were fairly stable for the steamed catalyst than for the fresh catalyst. The FTIR and ¹³C-NMR spectra of the bio-oil showed generally that steaming of the catalyst increased the selectivity for the production of aromatic hydrocarbons.