Question

why is there a high percentage of nitrogen in the mixture of gases coming out of a kiln

Answer 1

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Answer 2

High process temperatures are required to convert the raw material mix to Portland cement clinker. Kiln charge temperatures in the sintering zone of rotary kilns range at around 1450 °C. To reach these, flame temperatures of about 2000 °C are necessary. For reasons of clinker quality the burning process takes place under oxidising conditions, under which the partial oxidation of the molecular nitrogen in the combustion air resulting in the formation of nitrogen monoxide (NO) dominates.
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Answer 3

air is roughly 70% Nitrogen