Experiences from an extended catalytic fast pyrolysis production run

In this work, a continuous four day catalytic fast pyrolysis experiment was carried out in VTT's Process Development Unit (PDU) using pine sawdust and a spray dried HZSM-5 catalyst. The PDU employs circulating fluidized bed technology with online catalyst regeneration. Approximately 250 kg of pyrolysis oil were produced, along with 600 kg of aqueous side products. On average, the carbon content of the pyrolysis oil was 72 wt. %, and the oxygen content 22 wt. %. The pyrolysis oil contained 24 % of the original biomass carbon, whereas 14 % were present in the aqueous fraction. The catalytically produced pyrolysis oil contained mostly water-insoluble compounds, i.e. lignin-derived material and some aromatic hydrocarbons, while majority of the polar oxygenates from cellulose and hemicellulose concentrated in the aqueous fraction. The elemental composition of the pyrolysis oil remained quite stable during the four day experiment. Clear changes were, however, observed in the properties and behavior of the catalyst. Coke formation decreased over time, as did the surface area and porosity of the catalyst. This was accompanied by a loss of crystallinity and changes in the catalyst structure. Biomass alkali metals (Ca, K, Mg and P) also deposited on the catalyst, and the overall alkali content of the catalyst reached 1.1 wt. % at the end of the experiment. A linear correlation between catalyst alkali metal content and acidity was observed, which suggested possible substitution of HZSM-5 proton sites with biomass alkalis.