Efficient waste heat recovery process from sulfur containing flue gas

A method of recovering waste heat from flue gas containing particulate solids, CO.sub.2, SO.sub.2, SO.sub.3, and H.sub.2 O, said process comprising:

• (a) passing hot flue gas in indirect heat exchange with combustion air in an air preheater having a heat exchange surface attackable by sulfuric acid, so as to release heat to said heat exchange surface and to partially cool said flue gas to a temperature within the range of about 350.degree. F. to above the condensation temperature of sulfuric acid;
• (b) passing resultant partially cooled flue gas through a waste heat exchanger in indirect heat exchange contact with boiler feed water to preheat the boiler feed water and to further cool the flue gas to another temperature still above the condensation temperature of sulfuric acid;
• (c) passing resultant further partially cooled flue gas through a particulate removal zone to separate the particulate solids from the partially cooled flue gas;
• and
• (d) passing resultant solids-depleted, partially cooled flue gas in heat exchange contact with combustion air in a regenerator system containing heat exchange surfaces resistant to attack by sulfuric acid, to release additional heat and to cool said flue gas to a temperature below the condensation temperature of sulfuric acid, thereby depositing sulfuric acid on said heat exchange surfaces, and to preheat the combustion air, and then passing the thus-preheated combustion air to further heating in step (a) in said air preheater, and wherein during the step of preheating combustion air in the regenerator system, condensed sulfuric acid is removed therewith and passed with the preheated air to further heating in step (a), and then to a furnace generating the flue gas wherein SO.sub.3 entrained in the combustion air fed to the furnace is decomposed to form SO.sub.2 and oxygen, and whereby due to the temperature of the preheated air being above the sulfuric acid dewpoint, the sulfuric acid does not condense on preheater surfaces of step (a) and no special provisions are required to remove condensed H.sub.2 SO.sub.4 from the regenerator system.