The thermal combustion step of the modified Claus process is carried out in a water-wall boiler instead of a refractory-lined reaction furnace. The water-wall boiler replaces the conventional refractory-lined furnace and waste heat boiler when air, oxygen, or oxygen-enriched air is used as the oxidant. The acid gas combustion may be hydrogen sulfide-rich or hydrogen sulfide-lean with or without contaminants such as hydrocarbons and ammonia. Thermal combustion is performed in a single stage, without a recycle, for all levels of oxygen enrichment.