Process for burning limestone, dolomite or the like and annular shaft furnace for performing the same

In an annular shaft furnace for burning and sintering material in lump form, which material is selected from a group consisting of limestone, dolomite and the like, said furnace from top to bottom having a preheating zone, an upper burning zone, a lower burning zone and a cooling zone having a supply of cooling air at its base for flowing upward toward the lower burning zone, each of said upper and lower burning zones having cumbustion chambers with burners lying in a plane with the planes being spaced apart along the axis of the furnace, said furnace containing a substantially hollow shaft insert being closed at an upper end and coacting with the walls of the furnace to form an annular shaft at least in the area of the burning zones and cooling zones, said shaft insert being subdivided by a partition into an upper chamber and a lower chamber, said lower end of said shaft insert being located in the cooling zone while the upper end is positioned in the burning zone and terminates above the plane of the burners for said upper burning zone, said upper chamber of the shaft insert having circulating gas openings adjacent the upper end of the cooling zone with suction means for applying a suction to the chamber to draw a mixture of cooling air flowing upward in the cooling zone and fuel gases flowing downward from the lower burning zone into the gas openings to form a mixture of cooling air and fuel air flowing upward in said upper chamber, said furnace including means for introducing said mixture into the combustion chambers of the lower burner plane with the improvements comprising the lower chamber of the shaft insert being sealed at its lower end from the inner area of the furnace, said lower chamber having cooling air openings positioned below the circulating gas openings for receiving cooling air prior to reaching the upper end of the cooling zone, means including a suction blower for drawing cooling air through said cooling air openings downward in said lower chamber and then discharging said withdrawn cooling air into the combustion chambers at the upper burning zone;

• and said suction means including injectors discharging into the combustion chambers at the lower burning zone and means for supplying an expansion agent to said injectors including means for providing a compressed preheated mixture of a controllable composition of furnace flue gases from the upper end of the shaft furnace and primary air, said means for supplying being adjusted to cause a substoichiometrical burning of the fuel in the lower burner zone to create a reducing atmosphere.