A Numerical Study of Fast Pyrolysis of Beetle Killed Pine Trees

Since 1990s, as a result of unprecedented drought and warm winters, mountain pine beetles have devastated mature pine trees in the forests of western North America from Mexico to Canada. Especially, in the State of Wyoming, there are more than 1 million acres of dead forest now. These beetle killed trees are a source of wildfire and if left unharvested will decay and release carbon back to the atmosphere. Fast pyrolysis is a promising method to transfer the beetle killed pine trees into bio-oils. In the present study, an unsteady state mathematical model is developed to simulate the fast pyrolysis process, which converts solid pine wood pellets into char (solid), bio-oils (liquid) and gaseous products in the absence of oxidizer in a temperature range from 500°C to 1000°C within short residence time. The main goal of the study is to advance the understanding of kinetics and convective and radiative heat transfer in biomass fast pyrolysis process. Conservation equations of total mass, species, momentum, and energy, coupled with the chemical kinetics model, have been developed and solved numerically to simulate fast pyrolysis of various cylindrical beetle killed pine pellets (10 mm diameter and 3 mm thickness) in a reactor (30 mm inside diameter and 50 mm height) exposed to various radiative heating flux (0.2 MW/m2 to 0.8 MW/m2). A fast pyrolysis kinetics model for pine wood that includes competitive path ways for the formation of solid, liquid, and gaseous products plus secondary reactions of primary products has been adapted. Several heat transfer correlations and thermo property models available in the literature have been evaluated and adapted in the simulation. Finite element method is used to solve the conservation equations and a 4th order Runge-Kutta method is used to solve the chemical kinetics. Unsteady-state two dimensional temperature and product distributions throughout the entire pyrolysis process were simulated and the simulated product yields were compared to the experimental data available in the literature. This study demonstrates the importance of the secondary reactions and appropriate convective and radiative modeling in the numerical simulation of biomass fast pyrolysis.