ANALYSIS OF CHEMICAL AND PHYSICAL PROCESSES DURING THE PYROLYSIS OF LARGE BIOMASS PELLETS

The detailed chemical and physical processes that occur during the pyrolysis of large biomass pellets have been studied both experimentally and mathematically. The quantitative effects on product distribution of chemical composition and physical variables, such as external heat flux, pellet length, density and wood grain orientation, are determined systematically by using a Box-Behnken experimental design. The yield of each product is reported as a function of these variables in the form of a second order polynomial.; The experimental apparatus consists of a single pellet reactor with one-dimensional radiant heat flux (2-6 cal/cm('2)-sec) applied to a surface of the cylindrical pellet. Volatile products, which are collected by a cold trap and an automatic gas sampling system, are analyzed by gas chromatography. Temperatures along the pellet length are measured by an optical pyrometer and thermocouples, and the pellet density is obtained by an X-ray technique.; The theoretical analysis extends previous mathematical models to include a multi-step reaction mechanism which predicts char yield. Variable properties, heat, and mass transfer effects during the pyrolysis are also treated. The differential equations are solved using a finite difference method. Predictions of product distribution and volatile release rates as well as temporal temperature profiles are in good agreement with the experimental results. Modeling results indicate the temperature and product distribution are sensitive to the values of the rate coefficient for char production and thermal conductivity. Experimental results in large particle pyrolysis show a different maximum release rate for each volatile component which offers a possibility for increased selectivity. Heat flux has the most significant effect on the pyrolysis rate and product distribution. Pellet length and grain orientation are secondary. When the major heat flux is perpendicular to the wood grain, shrinkage can enhance the pyrolysis rate.; The results obtained from this study will be useful in many applications such as improving wood combustion and fire safety. The methodology used in this work may also apply to coal and oil-shale pyrolysis.