A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO 2 in a CH 4 -O 2 /N 2 /CO 2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO 2 and is also focused on chemical effects of recirculated CO 2 . Air-blown combustion and oxy-fuel combustion without recirculated CO 2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO 2 . The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO 2 , and the role of the recirculated CO 2 and its chemical effects are discussed. Importantly contributing reaction steps to the formation and destruction of NO are also estimated in oxy-fuel combustion in comparison to air-blown combustion.

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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.

SUMMARYHydrogen-blending effects in flame structure and NO emission behaviour are numerically studied with detailed chemistry in methane-air counterflow diffusion flames. The composition of fuel is systematically changed from pure methane to the blending fuel of methane-hydrogen through H 2 molar addition up to 30%. Flame structure, which can be described representatively as a fuel consumption layer and a H 2 -CO consumption layer, is shown to be changed considerably in hydrogen-blending methane flames, compared to pure methane flames. The differences are displayed through maximum flame temperature, the overlap of fuel and oxygen, and the behaviours of the production rates of major species. Hydrogen-blending into hydrocarbon fuel can be a promising technology to reduce both the CO and CO 2 emissions supposing that NO x emission should be reduced through some technologies in industrial burners. These drastic changes of flame structure affect NO emission behaviour considerably. The changes of thermal NO and prompt NO are also provided according to hydrogen-blending. Importantly contributing reaction steps to prompt NO are addressed in pure methane and hydrogen-blending methane flames.