The steam explosion process is based on the use of saturated or slightly supersaturated steam at pressure rising up to 32 bar with the related equilibrium temperature of 137°C, the biomass to be treated gets in contact with vapor for the process defined timing, resulting from the equation of the selected severity of the process. After this time, the material is immediately expanded into an atmospheric pressure vessel. The sudden reduction of pressure causes a violent vapor expansion, resulting in the breaking up of the chemical linkages in the treated material. The steam explosion reactor consists of two vessels, one for the pressurization of the biomass (R101) and the other for the following expansion (V101) of the biomass at atmospheric pressure. The pressurization vessel has a total volume of 33 liters, can reach 26 bar of pressure and 227°C thanks to a temperature and pressure control system. Instead, the expansion vessel is directly connected to the atmosphere with a funnel and work at 1 bar maximum pressure, it has a hermetic butterfly valve for the recovery of the exploded biomass.