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Numerical simulation to grasp the flame structure and NO emissions in a H 2 -air counterflow diffusion flame diluted with CO 2 is conducted for a wide range of atmospheric air temperatures and highly preheated air temperatures. Special concern is given to the important role of the chemical effects of added CO 2 , especially in highly preheated air temperature flames diluted with CO 2 . There exists a limit of the oxidizer-side temperature below which flame cannot be sustained. It is observed in highly preheated air temperature flames that intensely diluted cases with CO 2 show extremely low NO emission levels. The chemical effects of added CO 2 reduce flame strength. It is also seen that the difference between the maximum flame temperature and the preheated air temperature becomes smaller and smaller with increasing preheated air temperature and mole fraction of added CO 2 , and this thus implies the acquisition of evenly distributed gas temperatures in industrial furnaces. The NO emission index increases as the oxidizer-side temperature increases and decreases as the mole fraction of added CO 2 increases. The chemical effects of added CO 2 suppress NO emissions, mainly because of the reduction of thermal NO. It is also stressed that the reaction N + CO 2 f NO + CO, which is represented herein as reaction step (R283), is a relatively important contributor to prompt NO production.

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