ANALYTICAL MODELS FOR LARGE PARTICLES

For small particles the kinetics are sufficient to predict the reaction rate. However, for large particles, both the physical and the chemical changes are essential for obtaining a global pyrolysis rate. To formulation an analytical pyrolysis model, the known parameters that can influence the pyrolysis process must be considered. These affect the energy and mass flows into and out of a pyrolysing particle given by the following methods:

* heat transfer from the reactor environment to the particle surface by convection, and/or radiation and and/or conduction;

* heat transfer from the outer surface of the particle into the interior of the particle by conduction and in a few situations to a lesser degree by convection;

* convective heat transfer between the volatile reaction products leaving the reaction zone and the solid matrix ;

* primary pyrolysis leads to conversion of the biomass to gas, char and a primary liquid product;

* secondary and tertiary pyrolysis leads to conversion of the primary product to a gas, char and a secondary liquid product which then forms primary and secondary products;

* changes in physical properties, enthalpy and heats of reaction of the biomass

* changes in the enthalpy of the pyrolysis products;

* diffusion of volatiles out of the solid and away from the particle surface. Pressure gradients may also occur due to vapour formation in larger particles.

Process Parameters that can influence pyrolysis are given below in Table 2.3 with effects.

These processes are all temperature dependent and, since temperature changes with time and space, they will also be time and spatially dependent. Furthermore, they will also be dependent on the physical structure of the particle along with its properties such as density, thermal properties, size and the orientation of the particle with respect to grain.