Kinetic analysis and modeling of wood pyrolysis under nonisothermal conditions

The rate of pyrolysis is a major concern when investigating pyrolysis, gasification and combustion of solids such as wood, coal, and plastics. Heating rate, which is an important process variable in pyrolysis, is commonly regarded as affecting not only the rate of pyrolysis, but also the actual mechanism of the reactions. Up to this time, the role of heating rate in pyrolysis has not been sufficiently defined in quantitative terms.

A new approximation equation for the integral analysis of solid pyrolysis kinetics has been developed in this work. Comparison of this equation, which was derived from the technique of integration by parts, and the well known Coats-Redfern equation, which was obtained from an asymptotic expansion, proved that this approximation is superior.

The new approximation was combined with the first order Arrhenius kinetic formula in order to study dynamic thermogravimetric analysis (TGA) data of small sized (<_ 40 mesh) wood particles (spruce and redwood) with the heating rate varying from 10 C/min to 160 C/min.

A compensation effect in nonisothermal pyrolysis kinetics of fine wood particles is presented for the first time and can be used to determine the apparent kinetic parameters by using heating rate as the only process variable. The effect of heating rate on the reaction rate and overall conversion was also studied. These methods predict the apparent kinetic parameters and thus, the decomposition behavior during nonisothermal pyrolysis in TGA with constant heating rate. By using a multi-segment technique incorporated with the conventional differential method, the effect of heating rate inside larger particles, namely a-cellulose cylinders of 1.75 cm radius, was studied and the kinetic parameters were determined. The results of a correlation between heating rate and kinetic parameters at various radial locations within the a-cellulose cylinder are presented.

A mathematical model was developed to describe the thermal behavior of a finite-size wood sample pyrolyzing in a furnace with a linearly increasing temperature, i.e., constant heating rate. The physical processes incorporated in the model include: (1) transient conduction, (2) internal convection of volatiles, (3) Arrhenius decomposition of active material into volatiles and residual char, (4) porous structure of pyrolyzing solid, (5) accumulation of volatile products, (6) temperature-dependent heat of pyrolysis, and (7) variable density, specific heat, and thermal conductivity. The problem formulation led to a system of coupled-nonlinear-parabolic partial differential equations which were solved iteratively by the Crank-Nicolson method.

Comparisons of calculated values and experimental data for a pyrolyzing beech cylinder showed wery good agreement for both the overall weight loss and temperature profiles within the sample.

Further theoretical computations showed that the mathematical model could predict the moving-boundary phenomena during wood pyrolysis at elevated temperatures as experimentally observed by some investigators. Studies on the scale effect in the pyrolysis of larger particles were performed based on the proposed model. The calculated results were comparable to previous works.