Fires that occur in silo storages are difficult to extinguish and can create dangerous conditions. Fires in storages of wood pellets often occur because of the heat generated by oxidation reaction inside the silo. Indeed conventional extinguishment techniques are not suitable for this kind of problem. The most used and efficient technique is to fill the silo with inert gas like nitrogen, the process is very easy, the aim is to replace all the air containing oxygen by nitrogen and thus quench the fire, and stop the oxidation process. This technique has been used several times with success, the most recent case in Sweden is the extinguishment of a fire silo in Kristinehamn (November 2007). A lot of empirical knowledge has been gathered about the subject but still some questions are not answered, like: What is the behavior of the gas spreading inside the silo? What is the impact of the porous media on the flow and the concentration distribution inside the silo? Is it possible to set a general law to have the number of inlets against the total area that has to be covered? This report tries to find answers to the previous questions by simulating the filling with nitrogen of a silo full of wood pellets using the COMSOL software. Ignition and pyrolysis is not included in the simulation. Here we are investigating the gas distribution in a cold silo. The major observations resulting from the study show that the concentration distribution is in the shape of a parabolic front of 100% concentration that grows faster upwards than laterally, this means that for high and narrow silos only one inlet is enough to fill it completely. Inlet velocities or diameters have no major impact on the gas distribution, the main parameter is the injection rate. This work was made by Mickaël Freindorf during his internship at SP as a part of the second year at ENSIAME (French engineering school specialized in mechanics). Mickael’s supervisor at SP Fire Technology was Per Blomqvist. The work was part of BRANDFORSK project 602-071 “Inertering av Siloanläggningar”.