a grate - kiln pelletizing furnace includes a grate that conveys pelletized material to a rotary kiln , a cooler that cools pelletized material from the rotary kiln , and a gas flow apparatus that directs a stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln . the gas flow apparatus also directs a stream of gas from the grate to the rotary kiln to vitiate the preheated process air .