Pilot-scale validation of Co-ZSM-5 catalyst performance in the catalytic upgrading of biomass pyrolysis vapours

Abstract

The main objective of the present work was the evaluation of commercial ZSM-5 catalysts (diluted with a silica–alumina matrix) in the in situ upgrading of lignocellulosic biomass pyrolysis vapours and the validation of their bench-scale reactor performance in a pilot scale circulating fluidized bed (CFB) pyrolysis reactor. The ZSM-5 based catalysts were tested both fresh and at the equilibrium state, and were further promoted with cobalt (Co, 5% wt%) using conventional wet impregnation techniques. All the tested catalysts had a significant effect on product yields and bio-oil composition, both at bench-scale and pilot scale experiments, producing less bio-oil but of better quality. Incorporation of Co exhibited no additional effect on water or coke production induced by ZSM-5, compared to non-catalytic fast pyrolysis. On the other hand, Co addition significantly increased the formation of CO₂ compared to the CO increase which was favored by the use of ZSM-5 alone. These changes in CO₂/CO yields are indicative of the different decarbonylation/decarboxylation mechanism that applies for Co₃O₄ compared to ZSM-5 zeolite, due to the differences in their acidic properties (mainly type of acid sites). Co-promoted ZSM-5 catalysts simultaneously enhanced the production of aromatics and phenols with a more pronounced performance in the pilot-scale experiments resulting in the formation of a three phase bio-oil, rather than the usual two phase catalytic pyrolysis oil (aqueous and organic phases). The third phase produced is even lighter than the aqueous phase and consists mainly of aromatic hydrocarbons and phenolic compounds. Addition of Co in ZSM-5 is thus suggested to strongly enhance aromatization reactions that result in selectivity increase towards aromatics in the bio-oil produced. Possible routes of catalyst deactivation in the pilot plant's continuous operation process have been suggested and are related to pore blocking and masking of acid sites by formed coke (reversible deactivation), partial framework dealumination of the fresh zeolitic catalyst, and accumulative ash deposition on the catalyst that depends on the nature of biomass (content of ash).