## 1 Introduction  
Spontaneous combustion, or spontaneous ignition, is the process by which a substance begins its combustion without an external ignition source. During this process, the inner exothermic reactions raise oxidation velocity, which increases temperature and leads to self-heating. Moreover, temperature increases may produce decomposition reactions that enhance the ignition of the substance [1]. Therefore, this process becomes a danger from the point of view of industrial safety, in addition to transportation and storage. Some biomass treatment plants had accidents, due to this combustion process, as has been compiled by [PERSON] et al. [2]. Any material capable of being oxidized by air may reach spontaneous combustion at a certain temperature. In the beginning, process is slow, and corresponds to material's exothermic oxidation, which produces a heat release. If the heat is not dissipated into the surrounding environment, it may generate a temperature increase. When the temperature rises, so does the oxidation reaction velocity,which means a greater heat production, that may rapidly accelerate to high temperatures able to ignite the material. There are some other inner heat sources that may help self-heating processes, such as:  
* Presence of compounds more oxidizable than the material itself, either because of contamination or naturally (as it happens with the presence of pyrite in coals) [3]. * Aerobic or anaerobic bacterial fermentation [4]. * Gaseous substance fixation, as the adsorption or condensation heat is high [5]. For example, water vapor fixation in some solvents on active coal  
The importance of these processes and the consequences of the accidents produced by self-heating, result in extensive scientific research [6; 7; 8]. To determine the beginning of the process, different methods have been developed, most of them based on thermal analysis techniques, as thermogravimetry and differential scanning calorimetry [9; 10], as well as experimental techniques based on thermal decomposition [11], emission of flammable volatiles [12], critical self-ignition temperature [13], and exothermic reaction against peroxide attack [14]. A truly valuable method to quantify self-combustion risk is to determine the self-ignition temperature (SIT; according to EN 15188). The method is based on obtaining the environmental temperature at which the generated heat, due to the exothermic processes, is greater than the heat emitted to the environment, hence the heat build-up accelerates the oxidation reaction until ignition occurs. This temperature depends on the volume of the product submitted to these conditions, so that the higher the volume, the lower the self-ignition temperature. However, higher volumes increase induction time which is the period of time needed to raise the temperature of the product 60 K over the environmental temperature. This means that SIT will depend on time and volume. Experimentally, SIT is determined for different sample volumes under isothermal conditions and extrapolate the obtained results in order to predict at which temperature, the ignition of a certain sample will be produced, and the required time. Hence, this method turns to be useful for its application on great storage processes. This method has been used previously to characterize the self-ignition of agricultural materials stored in silos [15; 16], coals with different ranges and origin [17], biomasses [18], and thermally dried sewage sludge [19], among others. Research regarding numerical simulations has also been carried out [20; 21]. Moisture is one of the most important parameters regarding solid dust flammability, and whose study is one of the most difficult ones because it could help or hinder the material ignition. It has been noticed that small quantities of moisture may help the reaction as the reabsorption of moisture is an exothermic process that might accelerate the increase of temperature in a self-heating process [22].