I ran my Natural Gas Dryer ON Propane?

the previous posters both have points, another to consider is, if the orifice is off, you could get a lot of soot buildup. soot is carbon, and once it gets thick enough, can actually catch fire.

Natural Gas Dryer

The only difference between running natural gas and propane is that propane is a heavier molecule than the methane of the natural gas, and as such it requires a bit more oxygen in the mixture. Also, the heat of the flame is different---natural gas has bluish throughout, and propane has red tipped or something.

The reason you need to NOT do this without a conversion kit is if the air mixture is off, and you use propane in a natural gas appliance, you might form carbon monoxide etc, which obviously you don't want to do.

The orifices are different sizes because Propane is delivered in higher pressure compared to Natural Gas. So it will run a lot hotter and can cause a fire.

Propane burns hotter than natural gas. It the orifice is not replaced with a smaller one, it may run too hot, maybe trip a hi-limit.