Mechanism Studies On Thermal Decomposition And Ignition Of Charring Materials By Irradiance

The pyrolysis and ignition of solid fuels is the initiation of combustion and very important for the sequent flame spread and fire growth. In this paper, the pyrolysis and ignition of charring materials (especially wood) was investigated both experimentally and theoretically. Through the work of this study, the database of fire characteristics of the combustibles was supplemented, and the results will be reliable for fire alarm and fire prevention.In order to understand the phenomena of pyrolysis of wood in a real fire, the kinetic studies of the pyrolysis of wood under different atmosphere (nitrogen, air and CO₂) were conducted by thermal gravimetric analysis (TGA). It was found that the kinetic mechanisms of pyrolysis of wood were related to the process of pyrolysis of the main components of wood (Hemi-cellulose, cellulose and lignin). Effects of oxygen concentration and heating rate on the kinetic mechanism of pyrolysis of wood were also studied by TGA. As the oxygen concentration increased, the process of pyrolysis of wood was changed from a single stage to double stages, then a fast single stage under the oxygen concentration exceeding 70%. The "shoulder" peak in the low-temperature mass loss stage of pyrolysis of wood was gradually not obvious as the heating rate increased. It was explained that the pyrolysis of hemi-cellulose and cellulose occurred in a similar range of temperature with the high heating rate. A two-pseudo components two-stage kinetic model was proposed to simulate the process of pyrolysis of wood under oxiding atmosphere, and the key kinetic parameters for the pyrolysis reactions of wood were calculated by the model.Then, the bench-scaled experimental studies on pyrolysis and ignition of wood were launched in the apparatus for investigating the initial stage of a fire. Several key characteristics for the pyrolysis and ignition of wood, such as temperature distribution, mass loss rate, ignition time and ignition temperature, were all studied in the experiments. Effects of heat flux, species, moisture content and grain orientation on the process of pyrolysis and ignition were also studied in the experiments. The critical heat flux of woods under different conditions was obtained by using the extrapolation method based on the ignition data from experiments. Two parameters (volume shrinkage coefficient and surface crackle coefficient) were firstly defined to describe the structural changes of wood during the process of pyrolysis and ignition. It was found that the structure of hard wood was more difficult to be changed than that of soft wood and the hard wood was safer too. The conclusions are useful for improving and developing new fire-proof materials.Based on the experimental studies on the pyrolysis and ignition of wood, a thermal balanced model of pyrolysis and ignition of wood was suggested to describe the process by means of the first principle of heat conduction in a solid, considering the effect of moisture content on the thermal physical properties. The key characteristics of pyrolysis and ignition wood was described by an analytical expression and predicted with several given parameters. The predicted data was found to match well with experimental results under relatively high heat flux. The thermal balanced model is easy for predicting the process of pyrolysis and ignition of wood, but the predictions of had a limited precision and application range as the chemical reactions of pyrolysis, thermal resistance of char layer and reactions heat were all not considered in the model. Then, a comprehensive model (PDE model) considering both physical and chemical phenomena was proposed to simulate the process of pyrolysis and ignition of wood, taking account for evaporation of moisture content as a chemical process. The effects of void fraction and char layer on the apparent conductivity of wood were also considered in the model. The PDE model were solved numerically with some selected parameters, and then temperature distribution, the formation process of resultants of pyrolysis, ignition time and thermal conductivity of the solid were predicted and compared with experimental data. The mechanisms of several key factors (such as heat flux, moisture content, species and grain orientation) affecting the process of pyrolysis and ignition of wood were also analyzed by the predictions of the PDE model. The conclusions give better understandings on the initiation of a fire and prevention of the occurrence of fires.