Pyrolysis Kinetics And Flammability Study Of Forest Fuels

Forest fire is a natural disaster that frequently happens worldwide. Under specific conditions, forest fires may cause great losses of properties and casualties. Especially forest fires may also threaten the safety of Wildland-Urban Interface areas. The ignition and spread of forest fires, are not only related to the properties of plant species, but also closely related to the mechanisms of pyrolysis and combustion. Therefore, research on the mechanisms of pyrolysis and combustion of forest fuels is valuable for the development of ignition and fire spread models, the assessment of forest fire risks, and the prevention and control of forest fires.The aim of this paper is to develop a unified kinetic mechanism that combines solid and gas phase reactions for simulating both the pyrolysis and combustion behaviors of lignocellulosic materials. A modified hybrid genetic algorithm is used for the optimization of kinetic parameters. The sensitivity of mass loss characteristics of pyrolysis to the experimental conditions is analyzed. The influence of heating rates, particle size and sample mass on the processes and kinetics of pyrolysis is discussed. The flammability of different parts of a plant is evaluated from the aspects of kinetics.The work and results of the thesis are summarized as follows.A unified kinetic scheme combining the solid and gas phase reactions is developed for simulating the pyrolysis and combustion of forest fuels. Previous pyrolysis and combustion models were inconsistent with each other, and most of them were developed only based on the mass loss in solid phase, for which reactions in gas phase were ignored. Using the TG-FTIR-MS thermal analysis systems, the mass loss rate in solid phase and evolution of products in gas phase of three forest fuels under inert and oxidative atmospheres were recorded. A unified kinetic scheme is proposed for simulating the pyrolysis and combustion reactions. Based on the traditional three parallel reaction model, the pyrolysis of forest fuels is considered to be the sum of devolatilization processes of three pseudo-components, and the forth reaction, oxidation of char, is added for the combustion mechanism. The presented scheme focuses on the consistent compositions for the major pyrolytic reactions of pyrolysis and combustion. The unified scheme not only reproduces the mass loss curves and captures most of pyrolysis and combustion features, but also is reasonable in chemical sense. A modified hybrid genetic algorithm is presented for optimization of kinetic parameter with high convergence speeds.Kissinger equation is used to explain deviation of mass loss rate curves caused bv the increasing of heating rates, and the peak temperatures of mass loss rate is predicted under different heating rates. Previous studies were mainly focused on the apparent temperature deviation, and the peak temperatures of mass loss rate curves were predicted by linear extrapolation. From the intrinsic kinetics aspect, this work explained the effects of heating rate on the reaction processes, and predicted the peak temperatures of mass loss rate under different heating rates.The effects of particle size on the mass loss processes and kinetics of pyrolysis of three forest fuels under regimes of kinetics is discussed. The regime of kinetics is verified for all of the particles (<1500μm) under study. Particles of three forest fuels, Pine Needle (PN), Pine Branch (PBr), and Pine Bark (PB) with different dimensions were pyrolyzed under heating rate of2℃/min to investigate the effect of particle sizes. It is concluded that the difference of mass loss rates and kinetics is mainly due to the variation of chemical composition, which is verified by the industrial analysis results.The flammability of forest fuels is evaluated from the aspect of kinetics. The flammability of PN, PBr and PB of Pinus Sylvestris is compared by the thermal stability and the kinetics of the pyrolysis and combustion processes. It is concluded that PN holds the highest ignitability due to the lowest initial degradation temperatures and activation energies during the initial stage. PB is considered to be most sustainable in fire due to the lowest reaction rates and the highest activation energies during the whole process. PBr holds the highest combustibility due to the two highest peak of mass loss rates.