Regeneration of pristine HZSM-5 extrudates during the production of deeply deoxygenated bio-oil from ex situ catalytic fast pyrolysis of biomass in a bench-scale fluidised-bed reactor

Abstract

Ex situ catalytic fast pyrolysis (ex-CFP) of biomass applying ZSM-5 catalysts is an effective method for deoxygenating the pyrolysis vapour, thus producing low-oxygen bio-oil in a single step. The catalysts deactivate after reactions but can be regenerated to recover their performance. Most of the previous studies on catalyst regeneration applied modified ZSM-5 catalysts to produce partially deoxygenated bio-oil with an oxygen content of around 10–25 wt%. In the deep deoxygenation region with bio-oil oxygen content below 5 wt%, the regeneration of pristine HZSM-5 catalyst used in ex-CFP where filtered pyrolysis vapour is upgraded has not been elucidated. Therefore, it is the main purpose of the current study to demonstrate the deep-deoxygenation capability of an unmodified HZSM-5 catalyst that has been previously depreciated. In this work, eucalyptus wood was pyrolysed in a bench-scale bubbling fluidised-bed reactor close-coupled with a separate catalytic reactor containing a fixed bed of pristine HZSM-5 extrudates. The produced catalytic bio-oil appears in 3 phases: light bio-oil, medium aqueous phase and heavy bio-oil. The light bio-oil has a very low oxygen content of ∼1 wt%, containing mainly monocyclic aromatic hydrocarbons, especially benzene, toluene and xylene. The heavy liquid had an oxygen content of 5–8 wt%, containing mainly naphthalene derivatives. The average degree of deoxygenation achieved in this work was 91% throughout the 10 experiments using fresh and regenerated catalysts. The unmodified HZSM-5 extrudates can be considered regenerable for up to 9 cycles with minor catalyst deactivation. The yields of main products including total bio-oil, char and gas were unaffected by the catalyst regeneration. However, the yield of light bio-oil appeared to gradually decrease with regeneration cycles. The regenerated catalyst could retain its surface area, morphology and structural framework to a great extent, albeit with small changes occurring on the surface area and volume of the micropores as well as its crystallinity and crystalline size.