## 1 Introduction
However, when moisture percentage increases, it is more important its role as a thermal drain and involves water heating, retarding the beginning of the organic material heating. The influence of moisture on the self-ignition process has been widely studied [23]. Particle size also affects compaction, density, or particle aggregation; thus, it will have an effect on sample moisture. Usually, higher density or particle compaction means lower residual moisture. On the other hand, the granulometry effect is simple: Fine particles help dust ignition, simply because the available surface area is increased. But, if those particles are too fine and devolatilization does not control the explosion rate [24], there will not be an increase in combustion rate among smaller sizes. In other words, when the particle size is increased, the devolatilization reaction takes place at higher temperatures. But, on the other hand, if very small particle sizes are considered, the differences between the granulometry will not be the dominant role in the ignition or explosion of the dust. Particle size is an essential parameter for every ignition process, either when considering dust explosions [25] or when the product is dispersed in the air shaping a cloud before the possible ignition of the cloud. Under those conditions, the oxidation reaction that occurs over the particle surface is more rapidly accelerated, the greater the surface area is available. Therefore, granulometry is a key factor for ignition sensibility and explosion severity for dust clouds. Some models of particle size effect have been developed [26; 27], but not considering self-ignition processes. When the product is deposited as a layer or as a great pile, the importance of granulometry is not so obvious [28]. The influence of the compaction level has been studied and related to the thermal susceptibility using thermogravimetry analysis [29]. It has been observed that sample compaction increases its reactivity during pyrolysis processes, probably due to the heat conductivity increase. Differences in self-heating when testing pellets and un-compacted samples have also been studied [30]. When the material is stored in big piles, the accessibility of oxygen to the inner surface of the material may be reduced, and the oxidation reaction may be delayed [30]. When studying self-heating processes, it is very important to define the two mechanisms acting in the process: Conductivity contact with a hot surface and oxygen diffusion, due to air inlet inside the material particles. It is well known that reaction progress might be affected by oxygen availability, hence greater compaction of a pile or layer will difficult the oxidation reaction progress. Besides that, compaction produces an increase in density and thermal conductivity, which means that self-heating tendency will be reduced because the material is able to evacuate the generated heat in an easier way, retarding self-ignition. The influence of the granulometry on the ignition temperature of a dust layer was studied [31], noticing that such temperature decreases as the thickness of the layer increases, the particle size decreases, and the compaction of the product increases. The behavior of a material layer deposited on a warm surface is different from the behavior of a pile [32], such as an unthreshed grain. In the first scenario, the key factor is the thermal reaction propagation, as there is a great heat contribution through the hot surface. In the second scenario, access to oxygen is the essential parameter for the beginning of the reaction [33]. Moreover, when a fire is produced by biomass fuel, it is known that the thickness of the flame reaction zone is related to the particle size: As the particle size increases, so does the flame reaction zone thickness [34]. As it has been noticed, there are multiple parameters influencing self-ignition, and they may affect, to a greater or lesser, the extent the measured variables used to characterize the process.