A World War I or II flamethrower basically shoots gasoline or some other petroleum product. We know that these types of incendiaries were capable of knocking out tanks because of things like Molotov Cocktails being recorded knocking out WWII era tanks (wiki article has nice quotes).

However, a Flamethrower is not tactically a good choice for attacking a tank. Flamethrowers have a very short range, and are difficult to use from concealment because they can't be operated while lying down and the flame gives away the user's position. This makes them very vulnerable to machine gun fire either from the tank itself or from supporting units. By the end of WWII flamethrowers had become a fairly niche equipment choice only used in urban situations for this reason. Infantry units generally had much more effective means of attacking armored units, either with anti-tank rocket launchers (i.e. bazookas) or supporting fire from heavier weapons.