Remigius of Auxerre
Catena Aurea · Matthäusevangelium 3
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: Whose fan is in His hand, and He will throughly purge His floor, and gather His wheat into the garner; but He will burn up the chaff with unquenchable fire.
The unquenchable fire is the punishment of eternal damnation; either because it never totally destroys or consumes those it has once seized on, but torments them eternally; or to distinguish it from purgatorial fire which is kindled for a time and again extinguished.