What does the OBIGGS replace oxygen with in the fuel tanks?

The On-Board Inert Gas Generation System (OBIGGS) is designed to enhance aircraft safety by reducing the flammability of fuel vapors in fuel tanks. It does this by replacing the oxygen present in the tanks with nitrogen, an inert gas that does not support combustion. By displacing oxygen with nitrogen, the OBIGGS system effectively minimizes the risk of an explosion or fire in the fuel tanks during flight operations.

Using nitrogen is advantageous because it is abundant, non-toxic, and does not contribute to combustion, making it ideal for this application in aviation. Other gases like helium and hydrogen are either not suitable due to their flammable characteristics or because they do not provide the same inert qualities as nitrogen. Carbon dioxide could also be considered, but it does not possess the same effectiveness in displacing oxygen for the purpose of preventing fire hazard in closed systems like fuel tanks. Thus, nitrogen is the optimal choice for OBIGGS to maintain safety standards.