Solid Pyrolysis Modelling by a Lagrangian and Dimensionless Approach--Application to Cellulose Fast Pyrolysis

Abstract

Many kinds of solids (e.g., biomass, thermoplastic and coal) thermally decompose according to similar types of kinetic pathways. They usually include a first step giving rise to more or less stable solid or liquid species followed by competitive reactions with formation of the final products (solids, vapours, and/or gases). In order to be extended to several types of solids, the present pyrolysis model relies on an original dimensionless Lagrangian approach. Mathematical equations of mass and heat balances are written in the assumptions that non-volatiles products form distinct layers which are separated by each other by moving interfaces that propagate towards the inner parts of the sample. The conditions required to apply this Lagrangian approach to solid thermal decomposition are discussed. The time evolution of pyrolysis products and sample internal temperature profiles are obtained by numerical solving of equations written in a reduced form. The first results of simulations performed in a large range of dimensionless parameters values (thermal Thiele, Biot and thermicity criteria) are reported. It is pointed out that variations of reduced masses as a function of reduced time significantly depend on operating conditions (particularly thermal Thiele and Biot numbers). The results of the model are finally compared to experimental data reported in the literature for cellulose fast pyrolysis. The agreement is quite good considering the uncertainties with which the physicochemical parameters are known from the literature.