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The sugar industry is confronted with the problem of safe bagasse storage over extended periods. In term of fire safety, there have been many instances that the spontaneous ignition promotes ignition inside a large bagasse stockpile and eventually the spontaneous ignition process develops to a flaming fire. This paper presents a method to determine a safe size for bagasse stockpile from spontaneous ignition. The kinetic parameters for spontaneous ignition of bagasse were estimated based on two methods: the Frank-Kamenetskii method and the crossing point method. The bagasse activation energies experimentally determined were ranged from 89 to 109 kJ/mol. Based on the calculated kinetic parameter values, the bagasse stockpile safe sizes for a sugar factory were estimated in term of graphical solutions. For a fixed surrounding temperature, as the bagasse stockpile radius or length increases, the height of the stockpile that spontaneous ignition does not occur decreases and approaches the asymptotic value as the stockpile radius or length approaches infinity. The graphical solutions showed that a bagasse stockpile with any radius or length stored with height below the asymptotic height for a given surrounding temperature was considered to be safe from spontaneous ignition. Applying the calculated activation energy of 89 kJ/mol, the asymptotic heights for bagasse stockpiles were 10.0 m, 7.8 m, and 6.0 m for surrounding temperatures of 40 o C, 45 o C and 50 o C, respectively.

A theoretical model for autoignition of wood is developed. The model considers the processes occurring in both solid and gas phases. In the solid phase, a one-dimensional heat conduction model is employed. Char surface oxidation, which can lead to glowing ignition, is taken into account at the solid-gas interface surface. By "glowing ignition", it means the onset of surface combustion. Criteria for glowing ignition are developed based on a surface energy balance. In the gas phase, a transient two-dimensional laminar boundary layer approximation for gas phase transport equations is constructed. The gas phase model is coupled with the solid phase model via the solid-gas interface surface. Flaming autoignition occurs when the maximum gas reaction rate exceeds a critical value. A numerical result from the coupled gas phase and solid phase models shows that autoignition of the combustible gases behaves in two fashions as autoignition type I at high heat flux (i q ′ ′ > 40 kW/m 2) and autoignition type II at low heat flux (i q ′ ′ < 40 kW/m 2). In the type I autoignition, the flaming occurs just an instant after glowing ignition is initiated, while in the type II autoignition, the solid undergoes glowing ignition long before the flaming is achieved. Comparisons between the theoretical and experimental results are presented to demonstrate capabilities and limitations of the present model.

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