Alma links the place of fire and brimstone to the chains of hell. Both of these terms are metaphors and it is most probable that his audience understood them as metaphors rather than descriptions of a reality. He notes that the torments of the unrepentant "shall be as a lake of fire and brimstone." He has used the chains of hell previously. These are descriptions of the eternal emotional condition, not the physical environs.
Literary: The allusion to a lake of fire and brimstone depends upon language frequently used in both the Old and New Testaments. For the meaning of the allusion, see this commentary, following 2 Nephi 28:23.