In the Pentecostal Church you often see people use the word "fire" during exorcisms. Why? To burn the demon out?
I would like to see some scripture that can back up the doctrine or culture.
There are two basic analogies for fire in the NT: first is the everlasting fire the condemned souls are cast into; the second one is the purifying fire associated with the Holy Spirit. Mt 25:41 is an example of the former:
Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
Also Luke 9:54:
And when his disciples James and John saw this, they said, Lord, wilt thou that we command fire to come down from heaven, and consume them, even as Elias did?
Of the second, there's Mt 3:11:
I indeed baptize you with water unto repentance. but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
And obviously, the famous Pentecost narrative from Acts 2:
1 When the day of Pentecost came, they were all together in one place. 2 Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. 3 They saw what seemed to be tongues of fire that separated and came to rest on each of them.
Now, the thing about Pentecostalism is that there isn't a Catechism or some sort of definitive compendium of doctrine, but in general, a pastor will invoke fire as a combination of the two metaphors: the fire that purifies the possessed person from the unclean spirit, and the fire that punishes the same unclean spirit in the “place prepared for the devil and his angels.”