A Study of Pyrolysis of Charring Materials and its Application to Fire Safety and Biomass Utilization.

Abstract

A theoretical and experimental study of pyrolysis of charring material applicable to fire safety and biomass utilization is presented in this thesis. This work is divided into three parts. In the first part, thermal decomposition and pressure generation in charring solids undergoing opposed-flow flame spread is numerically studied with a detailed physics-based model. The result indicates that the char density and product yields are functions of depth due to an insulating char layer. The characteristics of various simplifying model assumptions such as global reaction, infinite rate kinetics and no convective gas transport were examined. In the second part, a method of determining the pyrolysis temperature by enforcing mass and energy balance is proposed and validated by comparison with the decomposition kinetics model and the experiments. This pyrolysis temperature has the form of pyrolysis rate weighted average temperature for the entire charring process. Heat flux, sample size, heat of pyrolysis and kinetic parameters are the most important for determining an appropriate pyrolysis temperature. A non-dimensional correlation to determine an appropriate T_p was proposed. Excellent agreement between the pyrolysis front model using the correlation and experimental data of wood cylinder pyrolysis was achieved. Finally, pyrolysis of wood sphere is studied both experimentally and theoretically. Weight loss and temperatures of the sample were measured during the experiments. Center temperature showed two distinct thermal behaviors with endothermic and exothermic reactions. The numerical study revealed the following findings: (i) Contribution of secondary tar decomposition and lignin decomposition to the temperature peak are small. (ii) Exothermic intermediate solid decomposition is responsible for the temperature peak. (iii) The temperature plateau is caused by endothermic cellulose decomposition. Based on the experimental and numerical results, a novel wood pyrolysis model is proposed. The model consists of three endothermic parallel reactions producing tar, gas and intermediate solid, followed by exothermic intermediate solid conversion to char and exothermic tar decomposition to char and gas. A three-dimensional pyrolysis model for arbitrary geometry charring material was developed using front tracking method. The model was applied for wood sphere pyrolysis analysis and validated by comparison with the one-dimensional model.