The limestone, heated by the high temperature flue gas in the tail of rotary kiln, will be pushed back into the rotary kiln by hydraulic push rod. The limestone is calcined in the rotary kiln under high temperature and then put into vertical cooler. In the cooler, the limestone will be cooled by cold air and discharged in to atmosphere. Preheated secondary air enters into the kiln. Waste gas at the tail of the kiln is discharged after being processed.