## 2 Materials and Methods  
The tested samples included different substances, such as wood biomass, agricultural biomass, or coals: Pine shaving (PS), wheat straw (WS), olive pellets (OP), two different brands of charcoal briquettes typically used in barbecues (Cb1 and Cb2) and dog food (DF). Table 1 shows the codes used for the sample's classification. The samples were subjected to six different tests: Granulometric analysis, moisture analysis, Classification test N.4 of Division 4.2 for self-heating substances, thermogravimetry (TGA), and differential scanning calorimetry (DSC) analysis, and gas emission test. The tests were chosen in order to define different parameters related to self-heating processes. While the N4.2 test provides information regarding self-heating at isothermal conditions, it does not take into account the temperatures at which the process begins. On the other hand, TGA, DSC, and gas emission tests provide information about the beginning of the process, the stages of the same, and the temperatures at which they take places, but it does not consider heating mechanisms. For each sample, three different sizes have been studied: Original product on its commercial shape (coarse), the sample milled with a particle size smaller than 1 mm, and fine fraction smaller than 0.075 mm. For the grinding of the samples, two different mills were used: A crusher and a blade mill. Afterward, sieving was carried out in order to obtain two differentiable fractions: Coarse fraction with a particle size just below 1 mm, and fine fraction lower than 75 \(\upmu\)m. After milling and sieving, a granulometric analysis was performed using laser diffraction Malvern Mastersizer 2000 instrument and obtaining a precise granulometric curve. Moisture was determined using Mettler Toledo HB43-Shalogen analyzer. Because of its size, it was not possible to use the laser diffraction for the group of coarse samples, thus, the samples were measured manually and using sieves. Test N.4 Test Method for Self-Heating Substances (United Nations Manual of Test and Criteria for Transport of Goods)  
Once samples were prepared on the sizes described above, and the two mentioned basic characteristics were determined (moisture and precise particle size), the N4 4.2 Division test of the Manual of Tests and Criteria of the United Nations was carried out over all the samples [35]. This method characterizes self-combustion risk for different materials by assessing certain danger levels during transport, related to the volume of the product and the packing used. This method is part of a set of numerous and diverse tests universally used to establish the different risks that substances may have (such as an explosion, flammability, toxicity, corrosion, radioactivity, etc. ), and there are many procedures or tests that assign to the substances a classification linked to an acceptable safety level for transport [36]. The European Agreement concerning the International Transport of Dangerous Goods by Road (ADR) assigns the 4.2 class to those materials that may spontaneously ignite. For solids, the criteria for the classification of substances that experience spontaneous heating is based on the spontaneous ignition temperature of charcoal, which is 50 \({}^{\circ}\)C for a cubic sample of 27 m\({}^{3}\)[37]. Test methodology, named N4 test, is based on applying this data to a 1 L volume, at which a self-ignition temperature of 140 \({}^{\circ}\)C will correspond. This test is also used as criteria for the transport of the International Maritime Dangerous Goods Code (IMDG). When this method was applied to some dried sludge from wastewater, considered as a highly interesting biomass because of its energetic application, positive results were obtained, hence they are only exempt from classification if they are transported in packages smaller than 3 m\({}^{3}\)[19]. The aim of this test is to determine if a substance undergoes self-heating spontaneously when having an air contact without energy supply or without an external ignition source.