I google'ed it and got this info.
"Disney uses a machine called a Scent Emitting System which goes by the nickname "The Smellitzer." The nickname, bestowed by Imagineers, is a play on "Howitzer," which is a powerful, short-barreled artillery gun.
Smells are achieved by using scented pellets and/or liquid in the device, and blowing air through it and into the area of the ride that requires the scent. At the other side of the area, a powerful fan pull the scent back out of the air so that it doesn't drift into other parts of the ride or show.
Some of the rides, locations and attractions which use Scent Emitting Devices:
Main Street USA (Bet you thought that baking and candy scent was real! Only part of it is.)
Soarin'
Ellen's Energy Adventure
Journey into Imagination
Mickey's PhilharMagic
It's Tough to Be a Bug
Kali River Rapids
Spaceship Earth"