Effects of Inert Placement (Z_st) on Soot and Radiative Heat Flux in Turbulent Diffusion Flames

Oxy-fuel combustion is one of the major strategies for reducing carbon dioxide emissions from combustion sources, as well as in applications requiring high temperature and stable flames. Currently in oxy-combustion, a large amount of flue gas (∼70%) is recycled to control the heat flux by reducing the flame temperature. This reduces the efficiency of the process and is unsuitable for processes requiring high temperature gases at the outlet of the combustor, such as gas turbines and industrial furnaces. Controlling the soot formation and growth at high temperatures could offer another means of controlling the heat flux without reducing the temperature. The separation of the inert from oxygen, in oxy-combustion, offers a unique advantage for burner design. Shifting the inert from the oxidizer to the fuel side in appropriate stoichiometric proportions increases the stoichiometric mixture fraction (Z_st) while maintaining the same adiabatic flame temperature. In this work, the impact of Z_st on the characteristics of a nonpremixed, turbulent-jet flame was studied. With increasing Z_st, soot inception is delayed, and the overall soot volume fraction and radiative heat flux are also drastically reduced. This is a consequence of not just the reduction in fuel concentration but also the change in the flame structure, which has a dramatic effect on the kinetics of soot formation. This method could provide an effective means of reducing the flue gas recycle in oxy-combustion, without resulting in drastically high heat flux.