I remember seeing a movie when I was a kid. It was a cartoon about a male scientist who, through some means that I have forgotten, was brought into a sort of ‘magical’ realm where dragons existed. I recall that the guy didn’t believe that they could possibly be real, but there one was. In the end he figures out that dragons consume limestone, which produces some gas, which they ignite with a sort of sparking device in their mouths.
That sounds like “Flight of Dragons” an animated film based on the book of the same name: http://en.wikipedia.org/wiki/Flight_of_Dragons
Pretty cool movie. 
I could never figure out whether that movie was called it Flight of Dragons or George and the Dragon because they kept calling it both whenever it hit the airwaves.
Either way it is most likely the movie you seek and also an awesome movie.