Since its development in 1936, the modified Claus technology has been the preferred process used by refineries and gas processing plants for the recovery of sulfur from acid-gas streams. Modern sulfur plants include a burner and a thermal reactor tied to a waste-heat exchanger as the first and second process units followed by a catalytic reaction section consisting of two or more catalytic converters. If sulfur recovery higher than 98% is required to meet the low-sulfur emission standards set by legislation, a tail-gas cleanup process must be included in the design of the sulfur recovery unit (SRU). The modified Claus process, however, has a number of operating challenges that are associated with the reactions and occur in the burner and thermal reactor. Some of these problems are: sustainable flame stability, operational reliability, minimum acid-gas concentration, strict control of the acid gas to air ratio, soot formation, catalyst fouling, and the formation of undesired byproducts such as carbonyl sulfide (COS) and carbon disulfide (CS2).