Thermal Decomposition Kinetics And Smoldering Mechanism Of Forest Peat

Ground fire in the forest is a kind of smoldering fire of humus and is a great threat to the forest ecosystem and atmospheric circulation. Previous research mainly focuses on the empirical model of ignition probability of humus, and there is much lack of the mechanism research of humus smoldering. As the change of global climate, ground fires have occurred over the world and much attention begins to be paid on forest humus smoldering, which has gradually become a frontier and hot research topic in the field of fire science.The humus material is formed by forest litter through biochemical degradation. It contains many different kinds of organic compounds with complicated structure, nonuniform contents of water and mineral matters. The smoldering process is influenced by the duff depth, wind speed and so on, and the research on this process faces great challenges. In this thesis, the peat is selected as the research material because of its similar thermal decomposition behavior with that of forest humus. The uniform structure and compounds of peat will benefit the mechanism research. The organic matter of peat reacts at high temperature with oxygen by pyrolysis and oxidation reactions and releases heat to maintain the smoldering spread. The composition and content of organic matter will affect the energy released. Water in the peat will evaporate by absorbing energy. The porosity of peat influences the energy density, the flow of gas and heat conduction. The influcence of many factors on peat smoldering was investigated experimentally. From the perspective of the chemical reactions and energy converasion, the numerical model is set up to discuss the inherent law of peat smoldering.This thesis firstly measured the elements of forest peat, the coefficient of thermal conductivity, the specific heat coefficient, the true density of particles and the apparent density of ash. The porosity and the standard mass without water and ashes were defined. It provided reliable data for the experiments and simulations of peat smoldering.The micro-scale chemical reaction mechanism of peat was studied by thermal analysis experiments under different oxygen concentrations, and the chemical reaction model was established. It was found that the pyrolysis process of peat in nitrogen atmosphere was composed of three weight loss processes, which was different from those in the aerobic conditions. The three component superposition reaction scheme was established for peat pyrolysis in nitrogen. For peat decomposition under the aerobic with different oxygen concentrations, a two-step consecutive reaction model was established. The methods of Kissinger, FWO, Starink, Friedman and Gyulai were applied to obtain the activation energy of peat decomposition process. Results were in accordance with the two-step reaction model, which verified the rationality of this reaction model. The inconsistent phenomenon of DSC and DTG peaks was discovered and analyzed. The simultaneous simulation method of two kinds of curves was proposed and the method could obtain the chemical composition and heat release at the same time for peat smoldering simulation.The humus and soil samples were collected layer by layer from the forests experiencing surface fire and the forests not subjected to fire, from HuZhong and JiaGeDaJi nature reserves in China where wild fires often occur. The thermal analysis experiments of five kinds of duff samples were conducted in air condition. The DTG cures were standardized by using the loss mass of the pyro lysis stage. The results showed that there was almost the same weight loss rate of five samples in the pyro lysis stage and DTG cures was clearly divided into two groups of mass loss rate curves, corresponding to the samples affected by surface fire and not affected by surface fire. The burned ratio of peat due to forest fire was evaluated which could be used as a severity evaluation of forest fire.The smoldering experiment device was designed to study the natural vertical downward smoldering fire. Series of smoldering experiments of peat with different water content, porosity and particle size were conducted. The particle size, porosity and water content of samples were controlled carefully in the experiments. The porosity was kept same during the series smoldering experiments of peat with different water content and particle size. So the apparent density of organic matter of peat was same. The reasons of the stable speed of pyro lysis frontier and the continuously thickening oxidation zone were illustrated. The dual effect of strengthening and restraining smoldering spread of water and porosity parameters was discovered. Many special phenomenon were also discovered and explained, such as black carbons on the wall of device, the conical shape after extinguish and the continuous high temperature on the furnace bottom. These results will be helpful to the cognition of smoldering.By the comparison of the DTG curves and the character of smoldering zones in the experiments, four zones of smoldering frontier and smoldering spread rate were defined. The temperature-time-position data of smoldering experiments were dealt with the surface interpolation smooth method. And the frontier spread rates and thicknesses of four zones were obtained. This smooth method can decrease the influence of experiment error, such as non-uniform position of thermalcouples inserted in the device.Based on the micro-scale chemical reaction model and the equation of energy, a smoldering spread rate model and a moving boundary smoldering model were established. The moving boundary and the accumulation of ashes problems were solved.