pyrolysis (n.)

before vowels pyr-, word-forming element form meaning "fire," from Greek pyr (genitive pyros) "fire, funeral fire," also symbolic of terrible things, rages, "rarely as an image of warmth and comfort" [Liddell & Scott], from PIE root *paewr- "fire." Pyriphlegethon, literally "fire-blazing," was one of the rivers of Hell.

scientific/medical word-forming element meaning "loosening, dissolving, dissolution," from Greek lysis "a loosening, setting free, releasing; dissolution; means of letting loose," from lyein "to unfasten, loose, loosen, untie," from PIE root *leu- "to loosen, divide, cut apart."