A process for treating acid gas comprising hydrogen sulfide comprises introducing a first acid gas into a reducing furnace to produce a first oxidized gas stream, cooling the first oxidized gas stream in a first heat recovery system, introducing the cooled gas stream into an oxidizing furnace to produce a second oxidized gas stream and cooling the second oxidized gas stream in a second heat recovery system. The acid gas is preferably produced in a refinery after treatment of sour gas or sour water. Spent acid may be fed into oxidizing furnace as part of a sulfuric acid recovery process. The spent acid can be spent sulfuric acid from a sulfuric acid alkylation process.