Method and apparatus for injecting air into long cement kilns

A method for reducing the concentration of nitrogen oxides in effluent gas emissions during operation of a conventional long rotary cement kiln comprising a rotary vessel having an upper kiln gas exit end with an adjacent mineral drying zone, and opposite fired clinker exit end with an adjacent clinkering zone and an intermediate mineral calcining zone wherein during operation a kiln gas stream flows from said clinkering zone through the calcining zone and out the kiln gas exit end of the rotary vessel and wherein in-process mineral flows from the drying zone through the clinkering zone, said method comprising the steps of:

• delivering combustible material into the rotary vessel to burn it in contact with the in-process mineral at a point downstream relative to kiln gas flow of the clinkering zone to create a reducing zone extending downstream from the burning combustible material;
• and
• injecting air through at least one port in the wall of the rotary vessel and directing said injected air into the kiln gas stream upstream relative to kiln gas flow of the drying zone at multiple angles so that at least a portion of the injected air is directed toward the in-process mineral continuously during kiln operation.