Pyrolysis and propensity to self Ignition of long-term low- temperature wood chars

Abstract

Cases have shown that when wood was exposed to long-term low temperature heat sources with different oxygen exposure conditions; it turned into reactive chars that carried fire risk. However, there is still little experimental data explaining the pyrolysis process and reactivity of these chars. This thesis provided a comprehensive study with experimental evidences on the pyrolysis and propensity to self ignition of long-term low-temperature wood chars.

Pyrolysis experiments were carried out on Kapur and Nyatoh hardwood species in ovens isothermally at low temperatures 160 °C, 175 °C and 190 °C for extended durations up to 153 days in both anaerobic and aerobic conditions. Empirical pyrolysis kinetic models of Nyatoh and Kapor wood under isothermal conditions in air were developed based on weight loss history of the pyrolysis process. Thermogravimetric analysis (TGA) and Fourier Transform Infrared spectroscopy (FTIR) were employed as analytical methods to determine the oxygen chemisorption and functionality of these chars.

It was shown that except for the initial fast degradation, the wood degraded following a first order reaction process. The activation energies of the two kinds of hardwood used in this study were different although these values were still in the range reported in literature. The difference could be explained by the different chemical composition with different proportion of cellulose, hemicelluloses and lignin presenting in each wood specie.

Chars created at low temperature for long duration in both aerobic and anaerobic conditions were proved to be reactive and susceptible to oxygen chemisorptions. These chars carried the potential of self ignition. Under the same exposure conditions, anaerobic wood char had higher initial rate of oxygen chemisorptions, in other words, it is evidenced that anaerobic chars are more reactive. Through FTIR investigation, anaerobic chars had more reactive aliphatic groups, especially aliphatic alpha-CH2. For less reactive aerobic char, it was believed that benzylic and hydroaromatic groups were the reactive sites responsible for the oxygen chemisorption.

The reactivity of the chars increased with the degree of pyrolysis, the char reactivity would reach the highest at charcoal condition with the final weight around 19-25%. The pyrolysis kinetic model could be used to obtain the rough estimate of the heating period to reach different degrees of degradation under air condition as a function of temperature of Nyatoh and Kapor wood.

The theoretical understanding had practical meaning to the extensive use of wood in related to fire safety and fire protection.