Can mercury be used as fuel?

Not actually Because, 1. Mercury is not a flamable compound like petrol. 2. Mercury expands on heating hence it is being used in thermometers taking an advantage of its high thermal expansion property. 3. read more

It is difficult to use as a fuel, but it is very useful as a reaction mass for space travel. Ion drive engines can use mercury, it's conductive and easy to ionise and its high weight gives good Isp. There are other better alternatives, depending on the type of drive but mercury is a good one. read more

However, it is bombarded by radioactive particles from the sun and it may be a source of radioactive fuel for a suitably designed engine. Also, any planet can be used to get a gravity slingshot effect, so Mercury could be used to accelerate a space ship or probe, which is what you use fuel for. read more

The role of the mercury in this (ionic motors, I think) is somewhat similar to the role of water in a steam engine. Water is vaporized and the high pressure vapors are used to push the pistons. But the water is not the fuel. read more