Characteristics and origin of char and coke from fast and slow, catalytic and thermal pyrolysis of biomass and relevant model compounds
Char and coke from biomass catalytic pyrolysis have different origins. They cannot be lumped as one since they occupy different locations on the catalyst surface and, thus, contribute differently to catalyst deactivation. In this study, catalyst (ZSM-5) deactivation in the perspective of comparison of char and coke from pyrolysis of different biomass types is investigated. Pine sawdust, glucose, and cellulose are used as feedstocks in the pyrolysis experiments. Biomass char and coke samples produced via slow and fast, thermal and catalytic pyrolysis are characterized with respect to their overall content, oxidation reactivity, catalyst surface area, pore size distribution changes, bonding groups and their effect on catalyst performance. In particular, it is shown that char forms as an external layer on the catalyst surface and in its macropores, whereas coke forms inside the zeolite micropores via hydrogen transfer and addition reactions. The catalyst effect on glucose and pine slow catalytic pyrolysis is minor compared with that on cellulose slow catalytic pyrolysis, due to macropore blocking by char formation. In fast catalytic pyrolysis, catalyst deactivation is mainly attributed to micropore blocking by coke formation. Char and coke are shown to coexist on the catalyst surface after fast catalytic experiments, with the char content after glucose fast catalytic pyrolysis being 30 wt% of the total solid residue. The origins of char and coke in the cellulose, hemicellulose and lignin components of pine are identified and mechanisms for their formation are proposed.