It depends what you mean by fuel!
Many fuels give off energy when they react. This energy raises the temperature, which makes the reaction happen more quickly, often starting a chain reaction that continues until all the fuel is gone. This process is commonly called combustion or burning.
Nitrogen gas alone is very un-reactive, so simply trying to burn it for energy doesn't work. Therefore, it's not a fuel in the conventional sense. It can react when brought into contact with certain metals, but these reactions generally don't release enough energy for combustion.
However, if you define fuel as something that is used to do work, such as drive an engine, nitrogen can be used in certain circumstances. When a gas expands, it pushes against the walls of whatever container it is in. This expansion can be used to do useful work, such as push a piston. Furthermore, when a substance goes from a liquid to a gas, it expands greatly. This principle can be used to drive an engine using the expansion of liquid nitrogen into a gas. In this way, the nitrogen is the "fuel" for the engine.
For the fuel to work though, the nitrogen must be cooled to a low temperature so that it turns into a liquid. This process takes energy. So, even though you get energy out of the nitrogen as it drives your engine, you don't get out more than you spent cooling it down in the first place!
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