The Supplementum explains the punishment of the damned by fire:
Now a spirit is united to a body in two ways. In one way as form to matter, so that from their union there results one thing simply: and the spirit that is thus united to a body both quickens the body and is somewhat burdened by the body: but it is not thus that the spirit of man or demon is united to the corporeal fire. In another way as the mover is united to the things moved, or as a thing placed is united to place, even as incorporeal things are in a place. In this way created incorporeal spirits are confined to a place, being in one place in such a way as not to be in another.
It’s interesting that the union of the separated soul or angel to corporeal fire is a literal dualism – the damned are bound to the fire so as to be punished by it, which punishment consists in being forced about by an alien physical being. Here at last, a role for a Cartesian soul-body unity in Thomistic thought!
This has been clear about dualisms since the beginning: Plato understood that a spirit being bound to a body could only be a punishment, and that release from such a state could only be for the better. Thomas agrees, and even intensifies this fundamental intuition by arguing that being bound to a body is the ultimate punishment- he just distinguishes such a soul-body union from hylomorphic unity.
Presumably, the damned soul cannot be unified hylomorphically to fire since doing so makes it one nature with the fire, and such a nature is either impossible, or, if possible, would be an entity like the human torch. One can’t punish the rational nature, however, either with the impossible or by turning it into another sort of rational nature with a new set of powers and enjoyments.
Just Thomism 