When a fuel burns without external heat sources, this is called spontaneous combustion.
The main causes for spontaneous combustion can be two: self-heating, provoked by processes like fermentation or oxidation, or side elements, allowing its evolution. Some substances, containing oxidising chemicals, start the oxidation even without air, facilitating the spontaneous combustion. During the fermentation, on the other hand, enzymes facilitate a biological oxidation. During the fermentation, on the other hand, enzymes facilitate a biological oxidation. Starting from atmospheric oxygen, they lead the fermenting substance to 40-45°C during a first phase and 70-75°C during a second phase, that is to say the maximum temperature reachable by microbes. After that, other exothermic oxidation reactions are needed to reach the right temperature for spontaneous combustion. Many are the processes leading to spontaneous combustion, like decomposition (complex molecules splitting into simpler molecules) and polymerization (simple molecules bonding into more complex molecules). Obviously, the above processes alone are not enough to provoke a spontaneous combustion. Other factors contribute to its evolution, like temperature, the position of materials, the size and porosity of the material, humidity, ventilation, the presence of catalysts and the ignition time.

HOW TO PREVENT SPONTANEOUS COMBUSTION

Materials like coal, oil paint, cloth and textile, leather, hay, are subject to spontaneous combustion.Following these steps is enough to avoid it:
1) Avoid making big heaps of such materials, all stash piles must be no larger than 100-150 m3 and no higher than 3-4 m. Keep aisles between the piles, at least 2 m large;
2) Use fire walls in storages. No area should be larger than 3000 m2;
3) If you are storing these materials in silos, provide the possibility of transferring all the stashed materials to another silo in case of irregular overheating;
4) If possible,  clean/dry up your products;
5) Cool down all products that went through thermal treatment before being stored;
6) Always keep under control the temperature of the stored products;
7) Provide for an intermediate maturation storage, adequately separated from the other.